m agent i chard drive

Overcoming the Trilemma Issues of Ultrahigh DensityPerpendicular Magnetic Recording Media

by L10-Fe(Co)Pt Materials

Fang WangKey Laboratory of Magnetic Molecules and Magnetic Information

Material of Ministry of EducationSchool of Chemistry and Materials Science of Shanxi Normal University

Linfen 041004, P. R. [email protected]

Hui XingDepartment of Physics, University at Bualo

SUNY, Bualo, NY 14260, [email protected]

Xiaohong Xu*

Key Laboratory of Magnetic Molecules and Magnetic InformationMaterial of Ministry of Education

School of Chemistry and Materials Science of Shanxi Normal UniversityLinfen 041004, P. R. China

[email protected]

Received 20 January 2015Accepted 6 April 2015Published 22 April 2015

L10-ordered FePt and CoPt (collectively called L10-Fe(Co)Pt in this review) have become po-tential materials for future ultrahigh density perpendicular magnetic recording (PMR) media dueto their high magnetocrystalline anisotropy, rendering small grains with high thermal stability.However, PMR media using such high anisotropy faces the well-known trilemma issues amongthermal stability, signal-to-noise ratio (SNR), and writability. This paper will provide an over-view of the impact of L10-Fe(Co)Pt on overcoming the superparamagnetic limit and balancingthe trilemma issues for ultrahigh density PMR media. Here the research and development ofL10-Fe(Co)Pt materials will be presented, from the perspectives of enhancing thermal stability,SNR and writability. Furthermore, we will provide some combined approaches to tackle thechallenges in balancing the trilemma issues, focusing on materials engineering.

Keywords: Perpendicular magnetic recording media; L10-Fe(Co)Pt; trilemma; thermal stability;signal-to-noise ratio; writability.

*Corresponding author.

SPINVol. 5, No. 1 (2015) 1530002 (26 pages) World Scientic Publishing CompanyDOI: 10.1142/S2010324715300029

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1. Introduction

The demand for data storage devices has never beenhigher due to the exponential growth of new infor-mation. Up to now, hard disk drive (HDD) is stillthe main storage device among various storagetechnologies for its cost per gigabyte of data al-though solid state memories have gained momen-tum in personal computers and electronic devicesmarket.1 On the other hand, the fast developingcloud storage market provides new demand forHDD. Over the past several decades, there wastremendous growth in the areal density of HDDs,especially since the introduction of the giant mag-netoresistance (GMR)2,3 head technology in 1996.However, the rate of increase started to drop since2003, partly due to the superparamagnetic behaviorof the longitudinal recording media, which made itdicult to break 100Gb/in.2.4 Thus CoCr-basedperpendicular magnetic recording (PMR) with themagnetic moment of the bit oriented along the lmnormal was introduced to overcome the super-paramagnetic eect.5 Moreover, both signal-to-noiseratio (SNR) and areal density can be improved bythis recording structure. The rst CoCrPt perpen-dicular recording HDD with areal density of133Gbit/in.2 was rst demonstrated by Toshiba in2004. The transition from longitudinal to perpen-dicular recording has resulted in rapid growth inareal density for nearly another decade. However,with the areal density approaching 1Tbit/in.2

today, the CoCrPt perpendicular recording media isstill facing the superparamagnetic limit. In order toovercome this limit on magnetic recording, the ex-ploitation of new technology or new materials isnecessary.

It is well known that increasing the anisotropyof the media can compensate for the super-paramagnetic eect caused by the reduction in grainsize because the anisotropy barrier of magnetizationreversal is proportional toKuV , whereKu and V arethe uniaxial magnetocrystalline anisotropy and grainvolume, respectively.6 Chemically orderedL10-alloyswith a face-centered-tetragonal (fct) structure, suchas L10-Fe(Co)Pt, have become one of the potentialmaterials due to their high magnetocrystalline an-isotropy.7 Thus, L10-Fe(Co)Pt grains with smallsizes can provide high SNR, which is determined bythe number of grains in each bit (SNR 10log(N),where N is the number of grains in a bit.).810 How-ever, the high anisotropy results in an increase in

coercivity (Hc) proportional to Ku/Ms, whereMs isthe saturation magnetization. Thus the writing eldwill have to be increased, which may exceed the strayeld that can be supplied by the write head. There-fore, thermal stability, SNR and writability areintertwined in such a way that the improvement ofone parameter may lead to deterioration of the other.These mutually restraining factors are commonlycalled the trilemma issues of the magneticrecording media, and the relations are shown inFig. 1. At present, it has become the major roadblockto the ever increasing areal density growth ofL10-Fe(Co)Pt perpendicular recording media.

11,12 Inorder to overcome and balance these issues, severaltechnologies have been proposed, including bit pat-terned media (BPM), heat assisted magnetic re-cording (HAMR), and microwave assisted magneticrecording (MAMR). In this paper, we only provide areview on the contributions of L10-Fe(Co)Pt onovercoming and balancing the trilemma issues forultrahigh density magnetic recording media. There-fore, we will focus on summarizing the structure andmagnetic manipulation of L10-Fe(Co)Pt materialson realizing the high thermal stability, high SNR, andhigh writability of perpendicular recording media.

In the past decade, several new schemes wereproposed to overcome the trilemma issues. In thefollowing sections, we review recent progress, fo-cusing on material engineering in L10-Fe(Co)Ptalloys. Section 2 discusses approaches to realizingphase transformation of Fe(Co)Pt alloy to enhancethe thermal stability. Section 3 discusses approachesto improving SNR, such as granular perpendicularmedia (GPM), percolated perpendicular media(PPM) and BPM. Section 4 discusses approaches topromoting writability, including texture-tilting-assisted media and domain-wall-assisted media.

Fig. 1. Trilemma issues of the PMR media.

F. Wang, H. Xing & X. Xu

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However, any single approach mentioned above canonly mitigate but not solve the trilemma problems.In the last section, we provide an overview of thepredicted combining approaches to tackle the tri-lemma challenges.

2. Approaches to EnhancingThermal Stability

The thermal stability depends on anisotropy barrier,which is proportional to the Ku value of the mate-rial. It is known that Ku of L10-ordered FePt andCoPt are 6:6 107 erg/cm3 and 4:9 107 erg/cm3,respectively, which are about 20 times higher thanthat of CoCrPt used widely in the commercial re-cording media.6,7 It is estimated that L10-FePt orL10-CoPt is thermally stable even for grain size assmall as 34 nm. If one can obtain such smallgrains and write the information on them, L10-Fe(Co)Pt-based media with an ultrahigh arealdensity can be readily achieved. Usually, FePt orCoPt alloy lms deposited at room temperatureare a disordered face-centered-cubic (fcc) phasethat is magnetically soft. A high temperaturetreatment, such as in situ substrate heating orpost-deposition annealing at temperatures as highas 550750C is necessary to obtain an orderedL10-Fe(Co)Pt phase.

1315 However, it is dicult toobtain (001) texture with perpendicular easy axisorientation because the (111) plane is the closestpacked plane with the lowest surface energy. Inaddition, such a high annealing temperature is notsuitable for practical applications. Therefore, manyattempts have been made to induce perfect fct(001) texture and reduce the ordering temperature.Here, two typical strategies are introduced to re-alize this goal, one is stress-assisted phase trans-formation, and the other is metal-doping-promotedphase transformation.

2.1. Driving L10-Fe(Co)Pt phasetransformation by stress-assistedgrowth

Epitaxial growth is a common means to induce (001)texture in L10-Fe(Co)Pt materials. For FePt orCoPt L10-alloys, people usually use single crystalsubstrates, such as MgO or introduce an additionalunderlayer, such as Ag and Cr, to induce the phasetransformation from fcc to fct and to obtain (001)orientation with the help of a small lattice mismatch

between FePt lms and substrates/underlayers.1629

Farrow et al. rst realized the control of chemicalordering and magnetic anisotropy in epitaxial FePtlms prepared on Pt/MgO (001) substrates.16

Figure 2 shows the specular X-ray diraction (XRD)patterns for FePt lms grown at dierent tempera-tures. It is found that the long-range order param-eter increases from near zero for lms grown at100C to a maximum of 0.93 in lms grown at500C. Over this range, the magnetic easy axischanges from in-plane to perpendicular. Further-more, Shima et al. prepared ordered L10-FePt lmswith large magnetic anisotropy by alternating Feand Pt monatomic layers on MgO (001) substratesat low temperatures.17

Among these underlayers, Ag is one of the mostpopular one. Figure 3 shows the lattice constants ofFePt and fcc-Ag crystal structures. Lattice a of thedisordered fcc-FePt is 3.82 and c=a 1, whilec (3.71) is shorter than a (3.86) for orderedL10-FePt and c=a 0:961. As shown in Fig. 3, Ag(001) plane has a slightly larger lattice than fct-FePt(001) plane, and the stress caused by a small mis-match (5.6%) between Ag (001) and FePt (001) canresult in the shrinkage of the FePt lattice along thelm normal direction and thus induce the orderingof FePt lms at a lower ordering temperature.

Fig. 2. XRD patterns for FePt/Pt(001)/MgO(001) lms atsubstrate temperatures of 100C, 200C and 550C.16

Overcoming the Trilemma Issues of Ultrahigh Density PMR

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Therefore, the stress-induced phase transformationshould be an eective method to control the per-pendicular orientation and reduce the orderingtemperature of L10-Fe(Co)Pt lms.

Several groups have prepared L10-FePt orL10-CoPt lms at low temperatures by using dif-ferent underlayers, and also investigated the de-pendence of the lattice mismatch on the ordering ofFe(Co)Pt lm and the corresponding magnetic an-isotropy. Hsu et al. reported that the epitaxialgrowth of FePt induced by the Ag underlayeris clearly improved with increasing substrate tem-perature from 75C to 300C.18,19 Xu et al. dem-onstrated the importance of Ag underlayer bycomparing the magnetic properties and structures ofFePt, FePt/Ag and FePt/Cu thin lms depositedby magnetron sputtering.20,21 Figure 4 shows thehysteresis loops of the FePt thin lms annealed at350C and 550C, respectively.21 It is found thatFePt/Ag thin lm has a high coercivity of about6.2 kOe at a relatively low temperature of 350C,

while the coercivities of FePt and FePt/Cu lmsare only 1.2 kOe and 0.1 kOe due to the presenceof fcc-FePt disordered phase. The coercivity ofFePt/Cu lm only reaches about 3 kOe afterannealing at 550C. This is because the latticeparameters of Cu underlayer are too small comparedwith that of FePt to induce the phase transforma-tion of FePt lms. Furthermore, the dependenceof Ag underlayer thickness on the orientation ofCoPt lms was investigated. The (001) texture ofL10-CoPt/Ag lms deposited by magnetron sput-tering can be improved signicantly by introducinga thicker Ag underlayer.22,23

Besides Ag underlayer, Suzuki et al. preparedordered FePt thin lms with perpendicular mag-netic anisotropy on Cr(100) underlayer/MgO seedlayer at low temperature of 450C due to theirproper mismatch.24 Based on the success of Cr (100)underlayer, Wang et al. prepared the ordered FePtthin lms with fct (001) texture on Cr100xRuxcomposite underlayer.25 Addition of Ru in Cr

Fig. 3. Lattice constants of FePt and fcc-Ag crystal structures.

(a) (b)

Fig. 4. In-plane hysteresis loops of the thin lms: (a) 350C and (b) 550C.21


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underlayer results in the formation of the FePt or-dered phase with c-axis orientation perpendicular tothe lm plane at a lower substrate temperature of350C. Figure 5 shows the out-of-plane and in-planehysteresis loops of Cr91Ru9/Pt/FePt lm depositedat 400C.25 A thin Pt intermediate layer betweenthe FePt layer and the CrRu underlayer is intro-duced to eectively resist the Cr diusion from theCrRu underlayer into the FePt layer. The out-of-plane loop shows a coercivity of 3.7 kOe with rem-anent magnetization squareness of 0.97, while thein-plane coercivity is only 190Oe. It indicates thatthe fct (001) texture has been achieved at a lowerdeposition temperature. They also found that acritical lattice mismatch near 6.3% to be the mostsuitable for improving the chemical ordering of theFePt lms. Recently (001) textured FePt lm wasobtained on MoC/CrRu/glass at 380C by usingmagnetron sputtering, in which the MoC conductiveintermediate layer was used to resist the Cr diusionat high deposition temperatures and promote theepitaxial growth of the (001) texture FePt lm.26 Inaddition, the FePt grains can be further separatedby excess carbon from MoC intermediate layer,resulting in small intergrain exchange interaction.27

Other than these approaches, Bi,28 PtMn,29 andAuCu30 underlayers were all used to induce thephase transformation and reduce the ordering tem-perature of L10-Fe(Co)Pt alloys. However, Hottaet al. recently conrmed that there is no markeddierence in the thickness dependence of Ku inL10-FePt (001) single-crystal lms grown epitaxi-ally on dierent substrates, although the latticemismatch between FePt and the substrates is

markedly dierent (from 1.4% to 9.1%).31 However,Ku decreased gradually as the lm thickness de-creased. It is likely that the lattice mismatchbetween FePt and these substrates was relaxed inthe rst 1 or 2 layers of FePt (001) lattices. There-fore, the lattice mismatch may not be the mostcritical factor to obtain high Ku, the lm thicknessalso plays an important role.

However, the usage of single crystal substrate orunderlayer is restricted in actual applications be-cause a soft underlayer is required under the re-cording layer. Zeng et al. obtained nearly perfect(001)-oriented CoPt and FePt lms with none-pitaxial growth by directly depositing lms on glassor thermally oxidized Si substrates and rapidthermal annealing.32,33 Figure 6 shows the typicalhysteresis loops for CoPt and FePt lms fabricatedby rapid thermal annealing.32 It is seen that theeasy axis is in the perpendicular direction for bothlms, and the perpendicular loops show large co-ercivity and high remanence ratio. The textureevolution mechanism was proposed to be the mis-match between the thermal expansion coecientsof the metallic Fe(Co)Pt and glass or SiO2, whichleads to large strain. This proposal was later con-rmed by works of other groups.34,35 Dang et al.found that FePt (001) texture is obtained onthermally oxidized Si substrates when lms thick-ness is less than 10 nm, whereas (111) orientationappears in the lms with the thickness larger than10 nm. This is similar to Hotta's results.31 Theminimizing surface energy is proposed to explainthe texture variation in the lms based on a theo-retical mode.35

Fig. 5. Hysteresis loops of Cr91Ru9/Pt/FePt lm deposited at400C.25

Fig. 6. Typical hysteresis loops for (a) CoPt annealed at750C for 300 s and (b) FePt annealed at 550C for 5 s.32


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2.2. Driving L10-Fe(Co)Pt phasetransformation by metal-doping

Another eective way to promote the ordering ofL10-Fe(Co)Pt and reduce the phase transformationtemperature is metal-doping. However, dierentdriving mechanisms were proposed for dierentadditives in L10-Fe(Co)Pt lms. Maeda et al. foundthat the addition of Cu into FePt alloy lm is veryeective in reducing the ordering temperature ofL10-FePt.

36,37 Figure 7 shows the coercivity of FePtand (FePt)8Cu15 lms as a function of the annealingtemperature.36 The coercivity of FePtCu lm isaround 5 kOe after annealing at 300C, whereasthat of FePt shows several hundred Oe. The for-mation of the ternary FePtCu alloy is considered tobe the main reason in reducing the ordering tem-perature. The lattice parameters of the L10-orderedphase suggest that Fe atoms are substituted by Cuatoms. The grain size increases by the addition ofCu, suggesting that the decrease of the annealingtemperature for ordering is due to the enhancedkinetics of ordering during the alloying process.38

Similarly, Wang et al. prepared CoPtCu/Ag lmsby magnetron sputtering and subsequent annealing,in which Ag underlayer plays a dominant role ininducing the (001) texture, while Cu dopant is usedto form CoPtCu ternary alloy. The CoPtCu/Aglms with perpendicular orientation start to order ata lower annealing temperature of 450C, which islower by 150C than the pure CoPt/Ag lms.39

The inuences of Cu, Ag and Au additives on theL10 ordering, texture and grain size of FePt thinlms are reported.4043 It is suggested that Au andAg additives tend to segregate at the FePt grainboundaries to inhibit FePt grain growth. However,Cu substitution in FePt increases the average grain

size and lm roughness. FePt lms with Au or Agadditive show 12 kOe higher coercivity comparedto that of pure FePt lm after annealing at 450C.The driving force of phase transformation comesfrom the vacancy defects during the diusion out ofthe FePt grains, resulting in an enhancement of L10ordering kinetics and reduction of the orderingtemperature.

Besides, Kitakami et al. studied the eects ofadditional elements Sn, Pb, Sb and Bi on the or-dering of L10-CoPt lms.

44 All these additives aredemonstrated to be very eective to promote theordering of the samples annealed at 400C, which is200C lower than that of pure CoPt. That is becausethese additives are easy to diuse and segregate ontothe lm surfaces by post-annealing due to their verylow surface free energy and extremely low solubilityin CoPt, leading to a lot of defects to drive phasetransformation. The results are similar to that of Auor Ag additives. In order to investigate this point,Fig. 8 shows the Auger electron spectroscopy depthproles of the CoPtSb lms before and afterannealing at 650C.44 Clearly, Sb tends to diusetowards the lm surfaces. Such surface segregationis caused by low surface free energy and limitssolubility of Sb in CoPt. Lee et al. found that Zr-doped FePt alloy lms could accelerate ordering

Fig. 7. In-plane coercivity Hc of FePt and (FePt)8Cu15 lmsas a function of the annealing temperature.36

(a)

(b)

Fig. 8. Compositional depth proles for (a) as-made and (b)annealed CoPt-Pb lms.44


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transformation kinetics while keeping small grains.45

It is thought that the point defects and lattice strainintroduced by Zr-doping activate the nucleation ofthe ordered phase. Therefore, various metal-dopantswith dierent mechanism contribute in inducing theordering of L10-Fe(Co)Pt thin lms.

Summarizing this section, many studies havebeen done in obtaining L10-Fe(Co)Pt with high Kuvalues, including stress-assisted growth and metal-doping. However, L10-Fe(Co)Pt with high Kuvalues is just one of the basic requirements forultrahigh density recording media. In the following,we will review strategies to improve the SNR andwritability.

3. Approaches to Improving SNR

Both stress-assisted growth and metal-doping canhelp to obtain L10-Fe(Co)Pt with large Ku valuesand perpendicular orientation at relatively low tem-peratures. However, there exists a large transitionnoise for any continuous medium. In order to reducethe transition noise and achieve high SNR,46,47 sev-eral common approaches have been adopted forL10-Fe(Co)Pt, such as GPM, PPM and BPM.

3.1. Granular perpendicular media

In GPM, nonmetal additions have been used todecouple magnetic grains. Several methods such ascosputtering or laminating FePt or CoPt withnonmagnetic materials have been attempted to de-crease the intergrain exchange interaction. For ex-ample, highly anisotropic fct-Fe(Co)Pt nano-particles have been prepared and embedded in Cmatrix by cosputtering deposition.48 Figure 9 is theTEM images of annealed FePt/C lms with dier-ent carbon thickness.48 It is shown that FePt par-ticles are embedded in C matrix and the particle sizevaries from below 3 nm to about 8 nm with de-creasing the carbon contents. This is because thenonmagnetic carbon atoms are easy to diuse intothe grain boundaries to isolate the FePt magneticgrains during the annealing process, resulting ina weak intergrain exchange coupling. Also theincrease of C contents plays a role in restrain-ing grain growth. Xu et al. also found that thegrain size and intergrain interaction of the FePt/Cmultilayer lms decreases with increasing C con-tent.49 The coercivity not only depends on the Ccontent, but also on the structure of the FePt/Cmutilayer. Moreover, L10-FePt particles with high

Fig. 9. TEM images of annealed FePt/C lms with dierent carbon thickness.48


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magnetocrystalline anisotropy and small particlesizes between 3 nm and 15 nm were fabricated byannealing FePt/BN multilayers at high tempera-tures.50 The BN layers are used to control the in-terparticle interactions of FePt grains.

Oxides are usually used as segregating materialsto achieve the required L10-Fe(Co)Pt microstruc-ture.51,52 Nanocomposite FePt:SiO2 lms have beenfabricated by annealing the as-deposited FePt/SiO2multilayers at high temperatures.53 These lmsconsist of high-anisotropy L10-FePt particles em-bedded in a SiO2 matrix. The coercivity in therange from 2 to 8 kOe and grain size of 10 nm or lessare highly dependent on the annealing temperatureand SiO2 concentration. The nanostructured FePt:B2O3 lms with average grain sizes from 4nm to17 nm were obtained by similar methods.54,55 Thec-axis of the FePt grains with a nearly perfect (001)orientation can be obtained, resulting in perpen-dicularly oriented nanocomposite lm with a highanisotropy constant of 3:5 107 erg/cm3. Figure 10shows the XRD patterns of [Fe/Pt/B2O3]n lmsannealed at 550C.55 One can see that the relativeintensity of the (111) peak decreases when theinitial B2O3 layer increases. The (111) peak nearlydisappears and the (00n) peaks become dominant at

x 8, indicating the alignment of the c-axis ofFePt grains along the normal direction. Furtherexperimental studies and ab initio calculations alsoindicate that the B2O3 matrix results in strain onFePt grains that changes the c=a ratio and thusmagnetic properties such as Curie temperature.56 Inaddition, Al2O3 and ZrO2 were used to controlthe grain size and intergrain interaction of L10-Fe(Co)Pt lms.57,58 Strong perpendicular anisotro-py, adjustable coercivity, and ne grain size suggestthat oxide addition can play a signicant role inreducing the exchange coupling interaction betweenmagnetic grains.

In previous studies, Ag underlayer can induce(001)-oriented L10-Fe(Co)Pt lms and nonmagneticC-doping can adjust particle size and intergranularexchange coupling. It is well known that the idealPMR media should have the perfect perpendicularorientation, small and isolated grains, suitable co-ercivity, and low media noise. Based on the aboveideas, we proposed a novel triple material system of[CoPt/C]n/Ag/glass lms prepared by magnetronsputtering and post-annealing.59 It is found that theoriented growth of L10-CoPt lms is stronglyinuenced by both Ag underlayer thickness and Ccontent. A nearly perfect (001) texture and a highperpendicular magnetic anisotropy can be obtainedin the [CoPt/C]5/Ag lms. Xu et al. also prepared(001)-oriented [C/CoPt/Ag]5 lms

60 and furtherAg/[CoPt/C]5/Ag lms with Ag as the underlayerand top layer.61 Figure 11 shows the hysteresis loopsof the samples with dierent structures.61 Sample Cof Ag(5 nm)/[CoPt(3 nm)/C(3 nm)]5/Ag(5 nm) lmhas a very large perpendicular coercivity of856 kAm1 (10.7 kOe) and a very low parallel co-ercivity of 63 kAm1 (0.8 kOe), which is consistentwith the XRD result showing the high intensity(001) and (002) peaks of L10-CoPt. The strain en-ergy caused by the Ag underlayer together with thediusion of Ag and C atoms results in the en-hancement of the degree of chemical ordering andthe development of the (001) texture for L10-CoPtlms. Similarly, Chen et al. also prepared FePt-Clms with high coercivity, (001) texture, and smallgrain size on MgO/CrRu/glass substrate bycosputtering FePt and carbon at 350C, in whichMgO underlayer is used to induce the ordering ofFePt at low temperatures.62 It is clear that non-magnetic isolation combined with underlayer is oneof the eective methods to obtain L10-Fe(Co)Ptwith (001) texture.

Fig. 10. XRD patterns of [Fe(3)/Pt(4)/B2O3(x)]n lmsannealed at 550C for 30min. (a) x 2, n 10; (b) x 4,n 9; (c) x 8, n 7; and (d) x 12, n 6. Insets areXRD patterns of the corresponding as-deposited lms.55


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3.2. Percolated perpendicular media

Dierent from the GPM, a so-called PPM consistingof fully exchanged coupled grains with densely dis-tributed nonmagnetic pinning sites, is proposedtheoretically by Zhu et al. to reduce the transitionnoise.6365 Figure 12 shows the illustration of tran-sition boundaries for the GPM and PPM.64 Obvi-ously, transition jitter noise is dominated by thegrain size and size distribution for GPM. If the pin-ning sites are distributed more densely than that ofthe grains in GPM, the resulting recorded transitionboundaries would be signicantly less irregular, asshown in the right picture. Consequently, transition

jitter noise would be signicantly reduced in PPM,while the ferromagnetic exchange coupling betweenthe grains ensures sucient thermal stability. Acomprehensive micromagnetic simulation study hasshown that the medium transition noise can be op-timizedwith amoderate exchange coupling constant.The switching time of the percolated medium issmaller than 1 ns even with low switching eld andsmall damping constant.66 Therefore, PPMmay oerbetter recording properties over the present GPM inimproving the SNR of the perpendicular media.

Based on this model, the rst PPM has beenreported in hcp CoPt-SiO2 thin lms by the alter-nate sputtering of CoPt and SiO2 targets. Desired

Fig. 12. Illustration of transition boundaries for the present GPM and PPM. The white dots in the PPM medium indicatesnonmagnetic columnar grains which act as domain wall pinning sites.64

Fig. 11. Hysteresis loops of samples with dierent structures before (insets) and after annealing at 600C. Here represents out-of-plane hysteresis loops and represents in-plane ones.61


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CoPt-SiO2 PPM microstructure was obtained uponpost-deposition annealing.65,67 Figure 13 showsthe TEM images of CoPt-SiO2 and (FePt)(MgO)/Pt/Cr thin lms.67,68 The plan-view image of as-deposited CoPt-SiO2 lm [Fig. 13(a)] has a micro-structure similar to conventional GPM. The plan-view microstructure of the annealed sample is shownin Fig. 13(b). The magnetic grains are magneticallyinterconnected, while the oxide forms sphericalparticles in the grain boundaries of the lm. Theoxide phase pins the magnetic domain walls, hin-dering their motion and hence producing increasedcoercivity. Moreover, Sun et al. prepared thepercolated perpendicular FePtMgO lms by con-ventional magnetron sputtering.68 Magnetic mea-surements demonstrate that the coercivity of themagnetic lm drastically increases from 2.1 to3.6 kOe as the MgO content is increased from 0 vol.% to 0.15 vol.%. Here MgO is present as crystallinedots that are uniformly precipitated in the FePtmatrix, which can be conrmed by comparingFigs. 13(c) and 13(d). The MgO dots serve as pin-ning sites of the domain wall and enhance perpen-dicular coercivity.

In order to form the PPM with evenly distributedpinning centers, Lai et al. proposed a simple route tofabricate PPM, where the Co/Pt multilayers weredeposited onto anodized alumina oxide (AAO)substrates utilizing pores as pinning sites.6971

Coercivity, domain size, and switching eld canbe engineered by controlling pore density. Figure 14shows the MFM/SEM images and correspondinghysteresis loops of (Co/Pt)/Si lm and (Co/Pt)/AAO/Si lms with dierent pore density.71 It isfound that the perpendicular coercivity increaseslinearly with increasing pore density due to thepinning eect imposed by the pores, which is con-sistent with theoretical calculation for PPM. Abetter tolerance to switching-eld distributions canthus be expected, which may help to achieve a highSNR. In addition, Schulze et al. obtained a similarresult in the Co/Pt multilayers deposited ontonanoperforated ZrO2 membrane.

72

Taken together, these domain wall pinning sitescan be either nonmagnetic oxides or physical voidsor both of them. All previous theoretical studies onPPM were done on exchange coupled magneticlms with holes acting as pinning sites. However,

Fig. 13. Plan-view TEM image of CoPtSiO2 and FePtMgO thin lms. (a) As-deposited CoPtSiO2 lm; (b) annealed CoPtSiO2lm; (c) (FePt)(MgO)/Pt/Cr lm and (d) FePt/Pt/Cr trilayer lm.67,68


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magnetic nanodots may be present in the holes, andthus the magnetic interactions in such systems aremore complicated. Therefore, Schulze et al. carriedout a series of micromagnetic simulations to under-stand the modication of the pinning strength ofdomain walls due to the magnetic interaction be-tween nanodots and the surrounding lm.72 Acomparison of magnetic domain wall pinning inPPM systems with and without nanodots is given inFig. 15. The simulation data shows that the domainwall pinning behavior strongly depends on the ex-change coecient Aint. When Aint 0 (no exchange

coupling between lm and nanodots), the depinningeld (Hd is the same as the system without nano-dots. The magnetic state of the nanodots is not af-fected by the domain wall motion in the surroundinglm. As Aint is increased from 0 to 1 106 ergs/cm,a larger Hd is required as the nanodots stabilizes thepinned domain wall. The dierence in morphologyprohibits a domain wall motion, as shown in theright column of Fig. 15(b). The pinning strength isdetermined not only by the geometrical properties ofthe template but is also aected by the exchangecoupling between the lm and the nanodots in the

(a) (b) (c) (d)

Fig. 14. MFM and SEM images of (Co/Pt)/Si (a), (Co/Pt)/AAO/Si lms with pore density of 3:3 101 cm2 (b), and11:6 1010 cm2 (c), (d) is the corresponding out-of-plane and in-plane hysteresis loops of (a) and (c).71

Fig. 15. (a) Initial magnetization states for two types of PPM, (b) domain wall displacement in an increasing eld H. Themagnetization states in (b) correspond to the same eld step H H Hd.72


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system. However, the possibility of controlled pin-ning of magnetic domain walls at even smallerlength scales, as required for storage densities be-yond 10Tbit/in.2, remains an open question.

3.3. Bit patterned media

To achieve ultrahigh areal density, BPM have beenproposed by Chou and Krauss73,74 to reduce oreliminate transition noise without a loss of thermalstability. Figure 16 shows the comparison betweenconventional media and BPM.75 Compared to theconventional granular media, BPM consist of peri-odic magnetic nanodot arrays, where each dot canbe regarded as a separately recorded magnetic bit,which can eectively enhance the SNR of perpen-dicular recording media.76,77 However, one of themain challenges is to obtain the smallest possiblefeature size for ultrahigh areal density. To date,there are several methods to realize BPM. The mostcommon approach is top-down physical fabricationtechnologies,78,79 in which the magnetic nanoarraysare obtained by lithographic patterning, ion milling,or focused ion beam (FIB). However, there are somelimitations of such techniques, including thehigh production cost and low throughput, as well asthe maximum achievable areal density.

In view of the drawbacks of top-down approa-ches, some bottom-up chemical template methodswere used to prepare BPM,80,81 for which the bitsare formed by electrochemical deposition or sput-tering using the self-organized templates. A pio-neering work was done by Kim et al. to push thelimit of the areal density of the magnetic nanodotarray.82 Ordered FePt nanodot arrays with a per-fect perpendicular easy axis were deposited bymagnetron sputtering into AAO templates fol-lowed by a rapid thermal annealing. Figure 17shows the SEM images, XRD pattern, and hys-teresis loops of annealed FePt nanodots array.82

FePt nanodots with diameter of 18 nm and peri-odicity of 25 nm have been fabricated, resultingin an areal density exceeding 1Tbit/in.2. Rapidthermal annealing converts the disordered fcc to(001)-oriented L10-FePt nanodot arrays with per-pendicular anisotropy and large coercivity. L10-Fe(Co)Pt nanowires and nanorods were also pre-pared in AAO templates by electrochemistryfollowed by annealing.83,84 These studies show thatself-organized templates are low-cost with highuniformity and easily controllable structural para-meters. In particular, for AAO templates, not onlythe diameter and density can be adjusted, but alsothe template can withstand high temperature of

Fig. 16. Comparison between conventional media and BPM.75


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650C,85 which is favorable in fabricating L10-Fe(Co)Pt patterned media. However, it is hard toobtain templates with small pore sizes less than10 nm and thin pore wall less than 5 nm. It is thusunfavorable to realize ultrahigh density L10-FePt-based BPM.

In addition, the block copolymer template is usedto prepare BPM. Naito et al. combined a diblockcopolymer template to fabricate a long-range or-dered CoPt patterned media with 40 nm sized dots,where single magnetic domains with an almostperpendicular orientation were obtained in each

magnetic dot.81,86 Recently, Zhu et al. obtained thenanoscale CoPtSiO2 magnetic media with high-coercivity using self-assembling block copolymers asan etch-mask.87 Figure 18 is the TEM image oftemplate CoPtSiO2 magnetic media and EDS in-tegrated intensity maps of Co, Pt and Ru for onegrain. Clearly, the CoPt magnetic grains are sur-rounded by the lighter-appearing amorphous SiO2.The average grain size of the CoPt grains is found tobe 16.2 nm with a standard deviation of 11%. Asseen from EDS mapping, the CoPt grain is alsoclearly outlined, and it has grown on top of the

Fig. 17. SEM images, XRD pattern, and hysteresis loops of annealed FePt nanodots array with dierent templates.82

Fig. 18. TEM image of template CoPtSiO2 magnetic media and EDS integrated intensity maps of Co, Pt and Ru for one grain.87


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dome. The SiO2 is in-between the CoPt grains andin the trenches dened by the Pt domes.

Recently, Wang et al. proposed a nanopatterningprocess named as the embedded mask patterning(EMP) to prepare FePt recording media, as shownin Fig. 19.88 The mean size of FePt grain with 4.6 nmcan be obtained, and the center to center distance isonly 6.3 nm. The FePt grain size and packing den-sity can be adjusted and optimized by changing thesputtering conditions of the embedded mask layer.This EMP process is compatible with today's mediamanufacturing line because each step could becompleted in vacuum without taking the disc out ofthe chamber. Therefore, it provides an eectivemethod to fabricate FePt BPM with low cost.

Comparing the advantages and disadvantagesamong the above-mentioned GPM, PPM and BPM,it is clear that BPM is a better choice to obtain highSNR due to the weak transition noise than GPMand PPM. However, there are some diculties andchallenges to realize BPM, in particular the di-culty in fabricating ordered magnetic nanoarrayswith ultrahigh density at a reasonable cost. Thisshould be the future direction if BPM is to be used inthe next generation recording media.

4. Approaches to Promoting Writability

Even if L10-Fe(Co)Pt media could be tailored toachieve the desired grain size, distribution, and

exchange decoupling, writing information on L10-Fe(Co)Pt media is still a challenge due to its largeHc and therefore requires a very high writing eld.To overcome this problem, researchers are focusingon the development of new recording paradigm.A number of advanced approaches such as texture-tilting-assisted media, domain-wall-assisted media,and energy-assisted magnetic recording were pro-posed to reduce writing eld of L10-Fe(Co)Pt-basedPMR media. Here we will briey review the progressof texture-tilting-assisted magnetic recording mediaand domain-wall-assisted magnetic recording mediaby manipulating the structures and magneticproperties of L10-Fe(Co)Pt materials.

4.1. Texture-tilting-assisted magneticrecording

Texture-tilting-assisted magnetic recording is a re-cording scheme, in which the magnetic recording isaccomplished by tilting the easy magnetization axisthat depends on the crystallographic orientation ofthe magnetic material comprising the recordingmedium. Tilted magnetic media with easy axis tilt-ing of 45 was rst proposed by Gao et al.89,90 andrealized by experiments by Wanget al.91,92 It allowsreducing the switching eld of high Ku media,thereby leading to an improvement in the writ-ability. Figure 20 shows the schematic of the tiltedmagnetic media with a single-pole writing head, anda plot of the normalized switching eld versus thetilting angle.93 Obviously, three advantages can beobtained by this design. First, the minimumswitching eld (Hs) resulted from titling the easy-axis direction is only a half of what is required inperpendicular media with a tilting angle of zero.Second, a much better tolerance of switching-elddistribution can be achieved. Third, a much fastermagnetization switching speed can be realized whencompared to the untilted perpendicular media.94

The areal density of tilted media could be more than62% higher than that of traditional perpendicularmedia if Ku 7 106 erg/cm3. Therefore, tiltingthe easy magnetization axis in magnetic recordingmedia is becoming one of the eective methods toimprove the writability without compromising on itsthermal stability.

Up to now, two main methods have been pro-posed to fabricate the tilted media: one is the arti-cial-tilted easy magnetization axis obtained byoblique surface fabrication method; another is

Fig. 19. The schematic drawing of EMP process: (1) L10-FePtcontinuous layer was deposited on the substrate, (2) mask layerRuSiO2 with a ne granular structure was deposited on theFePt layer, (3) the SiO2 of mask layer was removed using areactive ion etching process (RIE), (4) the Ru dots array pat-tern was transferred to FePt layer using a RIE process.88


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natural-tilted easy magnetization axis achieved bycontrolling the relative textures. The rst articial-tilted CoCrPt recording media was fabricated bycombining the oblique sputtering and collimatedsputtering by Wang's group.91 Similarly, L10-FePtnanoparticles with articial-tilted c-axis wereassembled onto MgO (110) substrates with self-organized grooves, where the c-axis of L10-FePtnanoparticle is tilted an angle of 45 from the sub-strate normal.95 L10-FePt lms with dierent easy-axis orientations also can be deposited onto thepyramid-type Si substrates.96 Therefore, the use ofpreprocessed substrates and oblique sputteringmethods are favorable for the fabrication of arti-cial-tilted recording media. However, the obliqueincidence approach may cause a large angular dis-persion around the tilted preferred orientation.Albrecht et al. fabricated Co/Pd magnetic multi-layers with articial-tilted easy-axis through thecurved surface of spherical nanoparticles.97 Theminimum Hc appears at 45

between the appliedeld and lm normal, which is similar to the simu-lated results of tilted media.89 The nanostructuresfabricated by this method are monodisperse, singledomain, and uniform magnetic anisotropy, whichare expected to provide higher density, higherthermal stability, and faster switching when com-pared to conventional PMR media.

To overcome the large angular dispersion causedby the preparation process of articial-tilted media,

natural-tilted textures with tilted easy magnetiza-tion axis, such as (101) or (111) textures ofL10-FePt, can be used as tilted media, whereinthe easy magnetization axis is directly oriented by3645 with respect to the medium surface normal.Natural-tilted media has the similar advantageswith the articial-tilted media.98,99 High-anisotropyL10-CoPt or L10-FePt lms having natural-tilted(111) texture have been reported.100,101 Room-tem-perature angular remanence measurements (ARM)of L10-CoPt were carried out in order to determinethe geometrical arrangement of the easy axis in thelm, as shown in Fig. 21.100 The ARM curves pro-vided the evidence of four out-of-plane maxima at 36 and 144 , within both the (110)and the (110) planes. The maximum coercivities( 4.8 kOe) were observed, when the eld was ap-plied along each maxima direction of the ARMcurves [Fig. 21(c)]. The observed behavior is con-sistent with the presence of four easy axes withmutually orthogonal in-plane projections, symmet-rically tilted at an angle of 36 with respect to thelm plane. Such methods can result in approxi-mately 75% reduction of the writing eld without aloss of thermal stability.

Therefore, both the articial and naturally-tiltedmedia play a certain role in improving the writ-ability of PMR without decreasing its thermalstability. However, the articial-tilted media cannegatively inuence the magnetic properties due to

(a) (b)

Fig. 20. (a) Schematic illustration of the tilted magnetic media with a single-pole writing head, (b) normalized switching eld(Hs=Hk; Hk, magnetic anisotropy eld) versus the angle () between the external eld and easy axis of a grain exhibiting uniformmagnetization reversal.93


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the increased roughness, stress and shape anisotropyeects, and the naturally-tilted media has an axialdistribution of the easy magnetization directions. Itis therefore a great challenge to design a head ca-pable of generating a uniform tilted eld, whichlimits their practical applications in magnetic re-cording media. Therefore, some new technologiesare still required to realize the high writability ofL10-FePt-based recording media.

4.2. Domain-wall-assisted recording

Another way to reduce the switching eld is domain-wall-assisted recording, including exchange-spring(ES), exchange coupled composite (ECC), and ex-change coupled graded (ECG) recording media. Allof them are composed of hard magnetic layer and

soft magnetic layer, which are coupled by sharp orgraded interfaces, as shown in Fig. 22.102 The hardlayer with high Ku provides a high energy barrier tomaintain high thermal stability, while the soft layerwith high saturation magnetization switches at lowapplied eld. A domain wall at the hard/soft inter-face provides an additional exchange eld on thehard magnetic layer, which helps to reduce theswitching eld of the recording media. Thus, thesoft/hard composite media can improve the writ-ability, while still maintaining a high thermal sta-bility. This idea is very attractive because themethod for preparing the media is relatively easywhen compared to that of the above-mentionedtexture-tilting-assisted media.

The exchange spring scheme was introduced inPMR media in 2005 by Suess.103,104 The soft

(a) (b)

(c)

Fig. 21. ARM curves in the planes of the MgO substrate: (a) (110) and (b) (001), (c) schematic illustration of the four tilted easyaxis model.100


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magnetic layer is directly coupled with the hardmagnetic layer by the soft/hard interface in ex-change-spring media. The simulated magnetic re-versal process and hysteresis loop are shown inFig. 23.105 It can be seen that the soft layer can helpthe hard layer to reverse its magnetization directionby exerting an additional demagnetization eld. Theswitching eld of the soft/hard bilayer is determinedby the pinning eld at the soft/hard interface,

Hp 1

4 2Khard Ksoft

Jhard;

where Khard and Ksoft are the anisotropies of hardlayer and soft layer, respectively. Js is the saturationmagnetization of the hard layer. The switching eldcan be reduced to one quarter of that of the hardlayer whenKsoft is taken to be zero. When soft-layeranisotropy is about one fth of the hard-layer an-isotropy, the smallest switching eld, which is aboutone fth of that of the hard layer, can beobtained.106 The inuences of soft layer thickness onnucleation eld, coercivity and the magnetizationreversal mechanism of exchange spring media wereanalyzed by micromagnetic simulations.107109 It isan eective method to improve the writability ofperpendicular recording media.

Based on the above-mentioned theoretical stud-ies, L10-FePt/Fe exchange spring lms were suc-cessfully prepared by Casoli et al.110,111 In order toobtain an ordered L10-FePt hard layer, FePt layerwas rst deposited onto MgO (001) substrate athigh temperature, and then Fe soft layer was de-posited onto the L10-FePt hard layer at room tem-perature to directly form L10-FePt/Fe exchangespring lms. High anisotropy perpendicular systemswith moderate coercivity can be easily obtained bycontrolling the thickness of the Fe soft layer.112

Moreover, the control of the interface morphologycan adjust the magnetic regime from rigid magnet toexchange-spring magnet because of the hard/softinterlayer coupling.

Victora et al. theoretically proposed the ECCmedia for the perpendicular recording during the

Fig. 22. Schematic illustration of the domain-wall-assistedrecording.102

Fig. 23. Magnetic reverse process and hysteresis loop of the exchange spring media.105


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same period ES media were investigated.113,114 ECCmedia is based on a structure with nonmagneticinterlayer between the soft layer and the hard layer,by which the exchange interaction between thelayers is coupled indirectly. The switching eld isone half of that of the hard layer according to theratio between the thermal barrier of the media andits switching eld 2E=Hs Ms V ,114 inwhich E and Hs are the thermal barrier E 12Kh V is the switching eld, respectively. Wanget al. rst carried out the experimental work on suchmedia with CoPd or CoCrPt as hard layers.115,116

Figure 24 shows the exchange coupling dependenceof remnant coercivity and thermal stability of the[Co/PdSiO]16/PdSi/FeSiO lms on dierent PdSiinterlayer thickness.115 Adding nonmagnetic PdSilayer between hard and soft layers is very helpful forfurther reducing the coercivity. However, if it is toothick, the coercivity would increase again because ofthe absence of exchange coupling between soft andhard layers. They also found that the thermal sta-bility factor KuV =KBT remains nearly unchangedwith changing the thickness of the intermediatelayer. Clearly, the coercivity can be tuned by con-trolling the interlayer coupling strength, while theirthermal stability is still maintained. Furthermore,similar results were obtained in hcp-CoCrPt ECCmedia with nonmagnetic Pt and Pd layers asinterlayers.117120 In addition, Tang et al. employedmagnetic interlayers to adjust the exchange cou-pling strength between the soft and hard layers.Besides thickness, the saturation magnetization ofthe interlayer can be used to control the exchange

coupling strengthen between the two magnetic lay-ers, leading to the reduction in coercivity of the ECCmedia.121,122 In a word, ECC media can furtherimprove the writability compared to the corre-sponding ES media.

On the basis of the studies of ES and ECC media,researchers proposed theoretically ECG media witha multilayer structure,123 where the anisotropyvaries layer by layer from the hard to soft layer.Thermal stability of graded media is dependent onthe anisotropy of the hardest layer. If the number oflayers in exchange spring media is increased fromtwo layers to N layers to form the multilayerstructure of ECG media, the anisotropy dierence ofadjacent layers will be decreased, resulting in asmaller pinning eld at the interface,

Hp 1

4

2Kn1 KnJs

14N 1

2KhardJs

;

where Kn1, Kn and Khard are the anisotropies ofthe two adjacent layers and hardest layer, respec-tively. Js is the saturation magnetization of thehardest layer. The switching eld is decreased withincreasing the numbers of layers. As the multilayersare extended to graded media with continuouslyvarying anisotropy, the pinning eld can be rewrit-ten as: Hp 2=Js

AKhard=tG

p, in which A and tG

are the exchange coecient and gradient thickness,respectively. Obviously, the switching eld can bedecreased to an arbitrarily small value if the gradi-ent thickness tG is large enough.

The rst epitaxial L10-FePt/Fe graded mediawere fabricated by depositing a part of the Fe layerat elevated temperature.124,125 A graded interface isformed between L10-FePt phase and Fe phase dueto the interdiusion between layers at high tem-perature, which in turn results in a continuouschange in magnetocrystalline anisotropy. This is themain reason for coercivity reduction compared withthe corresponding one with sharp interface. More-over, L10-CoPtTa2O5 and FePtTiO2 exchangecoupled multilayer media with well isolated mag-netic grains were fabricated by adjusting Ta2O5 orTiO2 content layer-by-layer.

126,127 In order tocontrol the ordering degree and anisotropy gradi-ent, the FePtC graded lms can be fabricatedby varying the substrate temperature layer bylayer.128,129

It is well known that higher substrate tempera-ture provides kinetic energy for the interdiusionbetween the soft and hard layers.130 Therefore, we

Fig. 24. Exchange coupling dependence of remnant coercivityand thermal stability of [Co/PdSiO]16/PdSi/FeSiO lms withdierent PdSi interlayer thickness.115


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proposed using post-annealing method to formexchange coupled graded media, which can be a veryeective method for realizing interdiusion betweendierent layers.131 Graded lms should be easilyobtained by changing the annealing parameters,such as temperature, heating rate, and time.132,133

Our group combined sputtering and post-annealingtreatment to fabricate a series of L10-FePt gradedlms with continuously varying anisotropy on glasssubstrates. Figure 25 shows the schematic of threestructures from bilayer to multilayers before andafter annealing.102 The graded thickness can betuned by the structure design and annealing tem-perature of the multilayers, in which the nonmag-netic layers are used to adjust the anisotropydistribution.

Figures 26(a) and 26(b) show the compositionaldepth proles of FeAu/FePt lms before and afterannealing at 550C. Clearly, the graded thicknesscan be increased due to the interdiusion when post-annealing was carried out.134 In comparison to thelms with sharp interfaces, the graded interface ismore favorable for coercivity reduction.135 More-over, the graded L10-FePt:C/Fe lms with a con-tinuous variation in anisotropy were realizedexperimentally by varying the C concentration inthe FePt hard layer. Nonmagnetic C layer playsan important role in tailoring the gradient ofanisotropy and weakening the intergranular ex-change interaction.136,137 We also simulated themagnetization reversal process of the graded lm bythe object oriented micromagnetic framework(OOMMF) software.138 Figures 26(c) and 26(d)show the simulated hysteresis loops and magneti-zation reversal process of the multilayer gradedlms. The coercivity of the multilayer graded lmdecreases gradually with the decrease of the Ku

dierence between Fe soft layer and FePt hardlayer. The thermal stability of the graded media isonly determined by the anisotropy of the hardestpart of the media. The magnetization reversal pro-cess agrees with the domain-wall-assisted reversalmechanism. Thereafter, L10-FePt(hard)/CoPt(soft)graded lms were fabricated by post-annealingbased on the dierent ordering temperatures be-tween L10-FePt and L10-CoPt.

139 Zha et al. pre-pared a series of FePtCu graded lm by adoptingpost-annealing method.140,141 Lee et al. also dem-onstrated that the pinning eld is proportional tothe Ku dierence of the hard and soft layers,

142

which is in agreement with our results.138 Further-more, the pinning eld can be eciently decreasedafter an additional annealing step, which is due to aheat-induced phase transformation of iron oxidepresent at the interface between the hard and softlayers. Therefore, post-annealing treatment is con-sidered to be a feasible approach to prepare ECGmedia with continuously varying anisotropy. Suesset al. predicted that the graded media should becapable of ultrahigh-density recording of up to510Tbit/in.2, if the grain size is assumed to be3.2 nm.7 In short, graded media should be a moreeective approach than ECC and ES to improvewritability of L10-FePt recording media.

Compared with texture-tilting-assisted media,the domain-wall-assisted media can control the co-ercivity more easily by optimizing the structures ofthe lms. Also the anisotropy gradient and inter-grain interactions can be eectively adjusted bynonmagnetic additives to improve the SNR. Giventhese considerations, the domain-wall-assistedmedia, especially ECG media, is more eective forpromoting the writability without the loss of ther-mal stability.

(a) Bilayer lm (b) Sandwich-like lm (c) Multilayer lm

Fig. 25. Schematic of the three structures before and after annealing.102


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5. Balance of the Trilemma Issues forL10-Fe(Co)Pt PerpendicularRecording Media

To achieve ultrahigh areal density for L10-Fe(Co)Ptmedia, several proposals have been addressed so farto solve the trilemma problems, including the above-mentioned PPM, BPM, tilted media, domain-wall-assisted media. However, either of them alone maynot be enough to address completely the trilemmaissues of L10-Fe(Co)Pt. Fortunately, the abovereviewed approaches do not necessarily have to bemutually exclusive. In order to write information onsuch high anisotropy L10-Fe(Co)Pt, some form ofassisted recording that can be switched at a su-ciently low applied eld have been used on BPM tobalance the trilemma issues. If L10-Fe(Co)Pt lmswith ECC structure is combined with BPM to formECC/BPM system, L10-Fe(Co)Pt hard section andsoft section in ECC structure can ensure the high

thermal stability and low write eld, while the bitpatterning can provide the high signal-to-noiseratio.

Several groups have carried out relevant works onECC/BPM. McCallum et al. prepared L10-FePt-based ECC/BPM with 180 nm pillars on a 300 nmpitch representing a density of about 8Gbit/in.2 bye-beam lithography into a hard mask and subse-quent ion milling.143 A 2.5-fold Hc reduction isobtained in this combination system. Moreover, theswitching eld distribution (SFD) is signicantlyreduced in ECC/BPM structures compared to thatof the FePt single layer BPM system. The reductionin Hc and SFD observed experimentally in thesestructures is consistent with micromagnetic simu-lations that conrm a vertically incoherent pillarreversal from the top to the bottom. The magneti-zation reversal of an areal density of 1.5Tbit/in.2

ECC/BPM was simulated.144 The magnetic an-isotropy distribution of ECC/BPM has a direct

(a) (b)

(c) (d)

Fig. 26. (a) and (b) Compositional depth proles of FeAu/FePt lms before and after annealing at 550C, (c) and (d) simulatedhysteresis loops and magnetization reversal process of the multilayer graded lms.134,138


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inuence on the switching eld distribution.L10-FePt ECC/BPM nanopillar media were alsofabricated by continuously varying the substratetemperature and then followed by electron-beamlithography and ion milling.145 Moreover, FePt-based ECC/BPM with 31 nm bit size and 37 nmpitch size were fabricated using diblock copolymerlithography on 3 inch wafer by Wang et al.146,147

L10-CoPt/Ni composite nanowires with the diame-ter of 25 nm and the length of 80 nm were fabricatedsuccessfully on AAO templates by electrochemicaldeposition and post-annealing.148L10-CoPt/Ni soft/hard composite nanowires exhibit an intermediatecoercivity of 1.96 kOe between those of CoPt array(10.97 kOe) and Ni array (242Oe). Such a largereduction in coercivity leads to easier data writing,showing a potential application of AAO templatesin self-assembled media. Similarly, well-coupled L10-FePt/Fe and L10-CoPt/FeCo composite nanotubeshave been prepared in AAO templates.149,150

Furthermore, Goll et al. reported the large-areacomposite L10-FePt/A1-FePt patterns fabricatedby ultraviolet nanoimprint lithography in combi-nation with ICP reactive Ar-ion etching ap-proach.142,151 It is clear that combination of ECCand BPM is an attractive way to balance thetrilemma of PMR.

By now the exchange coupled graded media isvery eective in reducing the write eld among do-main-wall-assisted media. Moreover, L10-Fe(Co)Pt-based ECC/BPM combinations have made some

progress in recent years. Due to the advantages interms of ECG and ECC/BPM, introducing ECGstructure into BPM to form ECG/BPM structurewould be a promising media to pursue ultrahighareal density in the future. This can provide opti-mum balance for the trilemma of L10-based mag-netic recording media. In fact, Krone et al. havesimulated that the switching eld of graded patternscan be successively decreased with increasing num-ber of layers in the ECC stack.152 A route for nar-rowing the switching eld distribution of the bitarray is provided as well, which is vital for the ap-plicability of the BPM concept in magnetic datastorage. Skomski et al. have theoretically investi-gated how the magnetization reversal processes ingraded recording media with columnar structureaect the write eld and the areal density.153 Byusing longer pillars, the write eld can be madearbitrarily small. However, there is an optimumlength, beyond which writing becomes dicultagain.

Up to now, there is little attempt to make ECG/BPM in experiments, due to the great challengeassociated with its fabrication. Combining ECGwith self-organized media might be a new routeto overcome such challenges. Recently, Goll andBublat provided a review on L10-FePt-based ECC/BPM, and the development of the recording densityin conventional and advanced magnetic hard diskdrives are shown in Fig. 27.154,155 It is unlikely thatthe trilemma issue can be addressed by the material

Fig. 27. Development of the recording density in conventional and advanced magnetic hard disk drives.155


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engineering of the media alone. Besides ECC, energyassisted recording such as HAMR and MAMR156158

is one of the promising schemes to combine withBPM in order to overcome the writing eld limitof L10-Fe(Co)Pt-based media. These combinedapproaches may very well be what are ultimatelyneeded to push the areal density towards 10Tbit/in2.However, there are still some signicant engineeringchallenges that need to be resolved.159,160 Althoughmany attempts have been made to balance the tri-lemma of L10-Fe(Co)Pt, there is still some technicalproblems to restrict their industrial application asthe next generation of recording media, especiallythe grain size. In short, coordinated eorts fromboth the materials engineering and technologies areneeded to balance the trilemma issues for futurePMR.

6. Summary

In summary, many proposals have been made onmedia materials engineering for L10-Fe(Co)Pt inorder to balance the trilemma of perpendicular re-cording media. For the thermal stability, stress-assisted growth and metal-doping methods are usedto reduce the ordering temperature of L10-Fe(Co)Pt lm and obtain the perfect fct (001) texture thatensure high thermal stability of the media. For theSNR, GPM, PPM and BPM were designed to en-hance it from dierent levels. Among them, BPM isconsidered to be the most promising scheme to re-alize high SNR without a loss of thermal stability.For the writability, both texture-tilting-assistedmedia and domain-wall-assisted media can realizeits improvement on materials engineering. In con-trast, the domain-wall-assisted media, especially forECG media, is thought to be a more eectiveapproach. However, it is necessary to combinesome alternatives to balance the trilemma forL10-Fe(Co)Pt perpendicular recording media dueto the shortages of single technology. Based on theprogress of ECG and BPM, it is predicted thatL10-Fe(Co)Pt based ECG/BPM should be one ofthe most eective paths to balance the trilemmafrom the materials design, which would open upa new avenue to realize an areal density of510Tbit/in.2 in the coming years. Certainly, thereexists still a great challenge in production technol-ogy, needing a synergic advance on the key tech-nologies of media and heads.

Acknowledgments

The authors would like to acknowledge the usefuldiscussions with Prof. Hao Zeng of University atBualo-SUNY and Prof. Dan Wei of TsinghuaUniversity. The work was supported by the Na-tional Natural Science Foundation of China (GrantNos. 51025101, 51101095, 11274214, 61434002), the863 Program (Grant No. 2014AA032904), Founda-tions from the Ministry of Education of China(Grant Nos. IRT1156, 20121404130001), ShanxiProvince Foundations (Grant Nos. [2012]12, [2012]10, [2013]9).

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Overcoming the Trilemma Issues of Ultrahigh Density Perpendicular Magnetic Recording Media by L10-Fe(Co)Pt Materials1. Introduction2. Approaches to Enhancing Thermal Stability2.1. Driving L10-Fe(Co)Pt phase transformation by stress-assisted growth2.2. Driving L10-Fe(Co)Pt phase transformation by metal-doping

3. Approaches to Improving SNR3.1. Granular perpendicular media3.2. Percolated perpendicular media3.3. Bit patterned media

4. Approaches to Promoting Writability4.1. Texture-tilting-assisted magnetic recording4.2. Domain-wall-assisted recording

5. Balance of the Trilemma Issues for L10-Fe(Co)Pt Perpendicular Recording Media6. SummaryAcknowledgmentsReferences

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