Journal of the Korean Physical Society, Vol. 63, No. 5, September 2013, pp. 1055∼1059

Growth-temperature-dependent Ferrimagnetism in Mn3Ga Thin Films

Wuwei Feng

School of Materials Science and Engineering, China University of Geosciences, Beijing 100083, China

Yooleemi Shin and Sunglae Cho∗

Department of Physics, University of Ulsan, Ulsan 680-749, Korea

Dang Duc Dung

Department of General Physics, School of Engineering Physics,Ha Noi University of Science and Technology, Ha Noi, Vietnam

(Received 9 April 2013)

We report on the growth of and the modified ferrimagnetism in DO22-structured Mn3Ga thin filmsepitaxially grown on GaSb(001) substrate by using molecular beam epitaxy. We have observed thatthe magnetic properties strongly depend on the growth temperature. The net magnetic moments ofthe Mn3Ga films grown at 30 ◦C and 200 ◦C are 0.23 µB/Mn and 0.75 µB/Mn atom, respectively.The as-studied Mn3Ga film is found to exhibit a relatively small coercivity around 400 Oe, whichdiffers greatly from the hard magnetic properties of bulk Mn3Ga.

PACS numbers: 81.15.H, 81.15.A, 75.50GKeywords: MBE, Epitaxy, FerrimagneticsDOI: 10.3938/jkps.63.1055

I. INTRODUCTION

MnxGa (x = 2 - 3) has attracted recent research in-terest due to its potential applications in spintronic de-vices. The spin valves [1,2] and the magnetic tunnelingjunctions (MTJs) [3,4] made of perpendicular magneticanisotropic (PMA) films allow the realization of low-dimensional and highly-reliable spintronic devices withimproved performance [5, 6]. MnxGa (x = 2 - 3) witha tetragonal DO22 crystal structure has the potential tobe an appropriate candidate PMA film according to pre-vious theoretical and experimental results for polycrys-talline alloys [7–10]. In this structure, an atomic layerof Mn and a layer containing both Mn and Ga atomsare arranged periodically along the c-axis, demonstrat-ing a hard ferrimagnetic (FIM) ordering aligned alongthe c-axis with moments of −2.8 µB for MnI and 1.6µB for MnII . Therefore, a potential high PMA propertyhas been proposed [7] and has been experimentally ver-ified [6]. In addition, the ferrimagnetic Mn3Ga was alsoproposed to be an appropriate new material that couldachieve stable spin switching by using the spin transfertorque mechanism at significantly low current density inthe MTJs due to its high spin polarization (88%), highCurie temperature (TC), and low magnetic moment [9].

∗E-mail: slcho@ulsan.ac.kr

Epitaxial MnxGa (x = 2 - 3) single-crystal films arestrongly desired in order to demonstrate the comprehen-sive properties and high potential for practical applica-tions in spintronic devices. Several attempts have beenmade to grow crystalline, tetragonal ferromagnetic (FM)MnxGa (1.2 < x < 1.5) thin films on GaAs (100) semi-conductor substrate by using molecular beam epitaxy(MBE). MnxGa film were grown with the c-axis orientednormal to the (001) GaAs substrate and exhibited per-fect square hysteresis loops with a coercivity of 6.27 kOeand a saturation magnetization of 460 emu/cm3, con-firming the PMA properties [5,11]. The FM MnxGa (1.2< x < 1.5) thin films were also grown epitaxially onthe wide band-gap semiconductor GaN (0001) by usingrf N-plasma MBE, demonstrating both the face-centeredtetragonal structure of CuAu type-I (L10) ordering witha (111) orientation and the modified magnetic properties[12].

However, few studies have been conducted on the epi-taxial growth of MnxGa (x = 2 - 3) thin films. Wu etal. [6] achieved epitaxial growth of Mn2.5Ga thin filmswith DO22-ordered structure on Cr/MgO single crys-tal substrates by magnetron sputtering, where a giantPMA and low saturation magnetization were obtained.In addition, the Mn3Ga phase with hexagonal structurewas obtained in the metalorganic vapor phase epitaxy(MOVPE) growth of MnxGay thin films on AlP/GaPheterostructures [13]. In our earlier studies, we reported

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the structural and the magnetic properties of Mn-Gafilms epitaxially grown on GaSb(001) as a structure forMnxGa (1.2 < x < 1.5) films. The ferrimagnetic tetrag-onal DO22 structure is stable over a wide range of Mncontent, x = 1 ∼ 3, in MnxGa films [14]. We observednon-magnetic MnGa3 Ga-rich phase. We also reportedthe growth of CuAu type-I (L10) MnxGa (1.2 < x < 1.5)thin films on various substrates, such as Al2O3 (0001),GaSb(111), GaSb(100), and Si (111) [15]. The MnGafilm is a hard ferrimagnet when grown on GaSb(111), be-comes a soft ferrimagnet when grown on Al2O3 (0001),and exhibits an absence of a net magnetic moment whenstabilized on GaSb(100).

Here, we report on the growth of and the modified fer-romagnetism in DO22-structured Mn3Ga thin films epi-taxially grown on GaSb(001) substrates by using molecu-lar beam epitaxy. We observed that the magnetic proper-ties strongly depended on the growth temperature. Thenet magnetic moments of the 30 ◦C and the 200 ◦C-grown Mn3Ga films were 0.23 µB/Mn and 0.75 µB/Mnatom, respectively, which is possibly due to the strain.

II. EXPERIMENTS

Semi-insulating Si (001) substrates were used to growMn3Ga films by using standard solid-source MBE (VGSemicon model V80). The Si substrate was first etchedin dilute hydrofluoric acid (HF), followed by the desorp-tion of impurities on the substrate surface by heating thesubstrate to 600 ◦C and maintaining that temperaturefor 30 min. A GaSb buffer layer with a thickness of 100nm was grown at 560 ◦C. Whereafter, the 50 nm-thickMn3Ga films were deposited at a growth temperaturesof 30 ◦C and 200 ◦C under an ultrahigh vacuum of 10−9

Torr, followed by the growth of a 6-nm GaSb cap to avoidthe oxidization of the inner layer of Mn3Ga.

Reflection high-energy electron diffraction (RHEED)was applied to monitor the growth process of the Mn3Gafilms. The crystal structure was studied using X-raydiffraction (XRD) with Cu Kα radiation. The sur-face morphology of the films was studied using a field-emission scanning electron microscope (FE-SEM). Thecomposition of Mn3Ga was derived from the evaporationratio of the Mn effusion cell to Ga effusion cell (Effu-cell, Inc.) measured by using a quartz thickness monitorand was rectified as Mn3.1Ga by using FE-SEM energydispersive spectrometry (EDS) measurement. Hall mea-surements were carried out by using a transport prop-erty measurement system with a four terminal Van derPauw configuration, which was used to characterize themagneto-transport properties. The magnetic propertieswere characterized by superconducting quantum interfer-ence device (SQUID) magnetometry (Quantum Design,Inc.).

Fig. 1. (Color online) RHEED patterns for the Mn3Gafilms grown at (a) 200 ◦C and (b) 30 ◦C with the electronbeam azimuth along the [110] direction of GaSb (001).

Fig. 2. Surface morphologies of the Mn3Ga films grown at(a) 200 ◦C and (b) 30 ◦C as measured by using FE-SEM.

III. RESULTS AND DISCUSSION

An epitaxial Sb-stabilized GaSb buffer with a sharp (1× 3) surface reconstruction was obtained in our exper-iment, as shown in Fig. 1, in agreement with the sur-face reconstruction phase diagram for GaSb [16]. Mn3Gafilms could be epitaxially stabilized on GaSb (001) sub-strates, as seen from the RHEED pattern with the al-tered surface reconstruction. The spotty RHEED pat-

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Fig. 3. (Color online)θ − 2θ XRD patterns of the Mn3Gafilms grown at 200 ◦C and 30 ◦C.

Fig. 4. (Color online) (a) Schematic growth orientation be-tween the DO22 structure of the Mn3Ga and the zinc-blendestructure of GaSb. (b) Schematic lattice mismatch betweenthe Mn3Ga (114) and the GaSb (001).

tern of Mn3Ga also indicated a rough surface morphol-ogy, which was verified by the FE-SEM images shownin Fig. 2. The 200 ◦C-grown Mn3Ga was composedof mutually-connected large masses and irregular poresdue to the increased mobility of adatoms with increasinggrowth temperature. In comparison, when the growthtemperature was decreased to room temperature, theMn3Ga film consisted of small, compact particles.

Figure 3 shows the θ− 2θ XRD pattern, a logarithmicscale, of the diffraction intensities for the Mn3Ga filmsgrown at 30 ◦C and 200 ◦C. With the exception of thediffraction peaks from the substrates, there was only oneresolved peak which were located at the 2θ position of∼62.15◦ for the 200 ◦C-grown Mn3Ga film. Note thatthe stable structure of MnxGa (x = 2 - 3) is a tetragonalDO22 phase (Al3Ti-type) with ferrimagnetic momentsaligned along the c-axis [6,8,10,14]. The diffraction peaklocated at the 2 position of ∼62.15◦ could be assigned tothe (114) plane of the tetragonal DO22 structure. Thecalculated lattice mismatch between the Mn3Ga (114)plane and the GaSb (001) plane was ∼4.5%, as shown in

Fig. 5. (Color online) Magnetic field dependences of theHall resistance for the Mn3Ga films grown at (a) 200 ◦C and(b) 30 ◦C.

Fig. 4. For the 30 ◦C-grown Mn3Ga film, we could notresolve any distinct diffraction peak from the film, whichwas mainly due to a shift of the diffraction peak to lowerangle, indicating a stronger compressive strain than thatin the film grown on 200 ◦C.

The anomalous Hall effect (AHE) has historically beenan important tool to study magnetization processes inferromagnetic materials [17]. The Hall resistivity is com-monly expressed as ρxy = Roµ0H + Rsµ0M . It com-prises the ordinary Hall effect (OHE) term, where Ro

is the ordinary Hall coefficient, and the anomalous Halleffect (AHE) term, which is proportional to both theperpendicular component of magnetization (M) and theanomalous Hall coefficient (Rs) [18]. The AHE hystere-sis obtained from the Hall effect measurement is gen-erally believed to be consistent with the magnetometrymeasurement data [19]. Figure 5 shows the magneticfield dependence of the Hall resistance for the Mn3Gafilms grown at 30 ◦C and 200 ◦C. The distinct AHE hys-teresis loops indicate the presence of magnetic ordering,in accordance with the previously-reported ferrimagneticordering of Mn3Ga. The Curie temperature of Mn3Gafilms is high, and, judging from the persistence of AHEhysteresis at 380 K, is most likely much higher than 380K.

Figure 6 shows the room temperature magnetic-field-dependent magnetization curves for the Mn3Ga filmsgrown at 30 ◦C and 200 ◦C. It was determined that the

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Fig. 6. (Color online) Magnetization curves for the Mn3Gafilms grown at 200 ◦C and 30 ◦C as functions of the magneticfield in both the in-plane and the out-of-plane directions.

Table 1. Magnetic properties of Mn3Ga films grown at 200◦C and 30 ◦C: the coercive field (HC), remanence (Br), andspecific energy product (E = HC ×Br) at room temperature.

Direction Br (Oe) HC (kAm−1) E (kJm−3)

Mn3Ga, in-plane 2860 27.69 7.92

200 ◦C out-of-plane 540 32.23 1.74

Mn3Ga, in-plane 1040 47.75 4.97

30 ◦C out-of-plane 130 25.46 0.33

film was a relatively soft magnetic material with a coer-cive field (HC) value as low as around 400 Oe. The mag-netization could be saturated under an applied magneticfield of 5,000 Oe. These results deviated greatly from thetypical hard magnetic properties of Mn3Ga bulk speci-men or films, where the HC is usually higher than 0.4 to1 T and the magnetization along the hard axis cannot besaturated even at 5 T or 10 T [6,8–10]. Table 1 summa-rizes the values of the coercive field, the emanence (Br),and the specific energy product (E = HC × Br) mea-sured at room temperature. The values of the energyproduct, E, ranged from 0.33 to 7.92 kJm−3 at roomtemperature, which is one order of magnitude lower thanthe previously-reported values for the hard ferrimagnetMn3Ga [9,10]. The transition from the hard magnetic tothe “soft” magnetic Mn3Ga film in our experiment couldprobably be attributed to the compressive strain. It canalso be noted from Fig. 6 that the in-plane direction wasthe easy magnetocrystalline orientation due to the domi-nant shape anisotropy in the thin films, which was under-standable in that we had placed Mn3Ga (114), insteadof Mn3Ga (001), parallel to GaSb (001) even though themagnetic moment of Mn atoms is aligned along the c-axisin the tetragonal DO22 structure.

There was a large difference in the saturated magne-tizations for the Mn3Ga films grown at 30 ◦C and 200◦C. The 200 ◦C-grown Mn3Ga film exhibited a satura-

Fig. 7. (Color online) In-plane MR curves for the 200 ◦C-grown Mn3Ga film [(a) transverse and (b) longitudinal] andthe 30 ◦C-grown Mn3Ga film [(c) transverse and (d) longitu-dinal].

tion moment of 380 emu/cm3, larger than the maximumof 250 emu/cm3 reported by Wu et al. [6]. The 30 ◦C-grown Mn3Ga film exhibited a saturation moment of 110emu/cm3. The differences in the HC , Br, and E valuesfor the Mn3Ga films grown at different temperatures arealso evident in Table 1. The net magnetic moments of the30 ◦C and 200 ◦C-grown Mn3Ga films were 0.23 µB/Mnand 0.75 µB/Mn atom, respectively, as calculated fromthe room temperature M-H data. The lower magneticmoment of Mn3Ga films grown at 30 ◦C was probablydue to the stronger compressive strain.

The in-plane magnetoresistance (MR) was measuredas a function of magnetic field in order to further studythe magnetic ordering of Mn3Ga. A positive transverseMR was evident when the magnetic field was perpendic-ular to the applied current for the 200 ◦C-grown Mn3Gafilm, as shown in Fig. 7(a), where the symmetrical pairof peaks around 600 Oe at 20 K corresponds to the co-ercivity. The positive MR could be attributed to thecontribution of the ordinary magnetoresistance (OMR).The so-called OMR caused by a Lorentz force that af-fects the carrier trajectories in conducting materials isusually positive [20,21]. However, the Lorentz force de-pends on the relative orientation of the field and thecharge velocity; therefore, the OMR is anisotropic andwill be diminished when a magnetic field is applied par-allel to the current. The transverse MR was dominatedby the positive MR, most likely indicating that the mag-netic moment was relatively low at that orientation. Anegative longitudinal MR with a slightly increased sym-metrical peak position of 900 Oe at 20 K was observedwhen the magnetic field was parallel to the applied cur-rent, as shown in Fig. 7(b), which probably resultedfrom the magnetocrystalline anisotropy towards differ-ent crystal orientations. The negative MR was due tothe decreased number of scattering centers caused by anincrease in magnetic domain size with increasing mag-

Growth-temperature-dependent Ferrimagnetism in Mn3Ga Thin Films – Wuwei Feng et al. -1059-

netic field [17], which is also evidence of ferrimagneticordering in the Mn3Ga film.

For the Mn3Ga film grown at 30 ◦C, a positive trans-verse MR with a coercivity of 600 Oe was exhibited attemperatures above 100 K, as shown in Fig. 7(c). How-ever, the positive MR gradually turned negative whenthe temperature was further decreased. This result couldbe explained by the fact that the positive OMR wasoverwhelmed by the negative MR. A negative longitu-dinal MR with the same coercivity of 600 Oe was alsoobserved at high temperatures, as shown in Fig. 7(d).However, the MR curve at 20 K exhibited an abnormaltransition from negative to positive with increasing mag-netic field to a value above 1.5 kOe. This result cannotbe well explained at the moment, but is probably re-lated to a magnetic alignment variation. The abnormalMR phenomenon at low temperatures might indicate adifference in the magnetic properties of the Mn3Ga filmat low temperature.

IV. CONCLUSION

In summary, we report the structural and the magneticproperties of Mn3Ga films epitaxially grown on GaSb(001) at 30 ◦C and 200 ◦C. Mn3Ga films with tetrag-onal DO22 structure were epitaxially stabilized at thegrowth orientation of Mn3Ga (114)//GaSb (001). Theas-studied Mn3Ga film was found to exhibit a relativelysmall coercivity around 400 Oe, which differed greatlyfrom the hard magnetic properties of bulk Mn3Ga. Thenet magnetic moments of the 30 ◦C and 200 ◦C-grownMn3Ga films were 0.23 µB/Mn and 0.75 µB/Mn atom,respectively. These results indicate that the epitaxialmagnetic films are strongly affected by the strain remain-ing in the film. How to relax or manipulate the strainwill be a key issue in epitaxial magnetic film.

ACKNOWLEDGMENTS

This work was supported by the 2013 Research Fundof University of Ulsan.

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