IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 4, AUGUST 2014 1800307

Use of PdFe and NiFeNb as Soft MagneticLayers in Ferromagnetic Josephson Junctions for

Nonvolatile Cryogenic MemoryBethany M. Niedzielski, Simon G. Diesch, Eric C. Gingrich, Yixing Wang,

Reza Loloee, William P. Pratt, Jr., and Norman O. Birge

(Invited Paper)

AbstractJosephson junctions containing ferromagnetic layersare under consideration as the basic elements for cryogenic ran-dom access memory. For memory applications, either the ampli-tude of the critical current or the phase shift across the junctionmust be controllable by changing the direction of magnetiza-tion of one or more of the ferromagnetic layers in the junction.We have measured the critical currents in large-area Josephsonjunctions containing three ferromagnetic layers. These junctionscarry spin-triplet supercurrent. This work addresses the choice ofmaterial and optimum thickness for the one soft magnetic layer insuch junctions. We have used either a PdFe or NiFeNb alloyfor the soft layer, and find hysteresis in the low-field Fraunhoferpatterns due to magnetic switching of the soft layer. The criticalcurrent is one order of magnitude smaller in the junctions contain-ing the NiFeNb alloy compared to those containing PdFe alloy,which is probably due to strong spin-memory loss in the former.While the large-area junctions studied here are not suitable formemory applications, these experiments lay the groundwork forfuture studies of submicron junctions where the magnetic state ofthe junction can be controlled by shape anisotropy.

Index TermsJosephson junctions, magnetic materials, mag-netic memory.

I. INTRODUCTION AND MOTIVATION

THERE is renewed interest in the development of super-conducting computers to help alleviate the problem ofpower dissipation in high-end computers based on conventionalsilicon technology [1]. Superconducting logic circuits basedon the propagation of single flux quanta [2] were inventedover 25 years ago, and several low-power descendants of that

Manuscript received January 10, 2014; revised February 10, 2014; acceptedMarch 3, 2014. Date of publication March 12, 2014; date of current versionApril 3, 2014. This work was supported by IARPA under SPAWAR contractN66001-12-C-2017. The work of S. Diesch was also supported in part by theGerman-Israeli-Project (DIP) from the Deutsche Forschungsgemeinschaft. Thispaper was recommended by A. Kleinsasser.

B. M. Niedzielski, E. C. Gingrich, Y. Wang, R. Loloee, W. P. Pratt, Jr., andN. O. Birge are with the Department of Physics and Astronomy, Michigan StateUniversity, East Lansing, MI 48824-2320 USA.

S. G. Diesch is with the Department of Physics and Astronomy, MichiganState University, East Lansing, MI 48824-2320 USA, and also with Universityof Konstanz, 78464 Konstanz, Germany (e-mail: birge@pa.msu.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TASC.2014.2311442

logic are currently under development [3][7]. One of the mainobstacles to the development of large-scale superconductingcomputing has been the lack of a high-density, low-powerform of superconducting memory. Recent advances in thedevelopment of Josephson junctions containing ferromagneticmaterials [8], [9], in conjunction with the commercial devel-opment of magnetic random access memory [10], have led toseveral new ideas for high-density, low-power, superconductingmemory [11][14].

We are working toward superconducting memory elementsbased on the Josephson Magnetic Random Access Mem-ory (JMRAM) concept developed by workers at NorthropGrumman Corporation [12]. A JMRAM memory cell containsone or more Josephson junctions whose primary characteristics(critical current or phase shift) can be controlled by the relativeorientations of the magnetizations of the ferromagnetic layersinside the junction. In principle, there are several flavors ofJosephson junctions that have this capability. The most basicdesign consists of two ferromagnetic layerscalled F andFinside an S/F/N/F/S Josephson junction. (S refers to a su-perconducting electrode, and N is a non-magnetic normal metalspacer layer that serves to decouple the magnetizations of F andF.) Typically F is a hard ferromagnet whose magnetizationstays fixed, while F is a soft ferromagnet whose magnetizationdirection can be switched with a modest external field. Recentwork by Baek et al. [15] and by Qader et al. [16] on thistype of junction demonstrates switching of the critical currentbetween high and low values by switching the direction of theF magnetization. Their work also provides indirect evidencethat it should be possible to switch the junction between 0 and states if the thickness of the F layer is chosen judiciously,although no phase-sensitive measurements were performed inthose works.

One potential drawback of the S/F/N/F/S junction describedabove is that the electron pair correlations oscillate and decayvery rapidly in the ferromagnetic layers. That occurs becausethe two electrons from the Cooper pair leaving S enter differentspin bands in F or F, which have different Fermi wave vectors.In the diffusive limit, the length scale governing both theoscillation and decay is F =

D/Eex, where Eex is the

exchange energy in the ferromagnet. (Typical values for F1051-8223 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.

See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

1800307 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 4, AUGUST 2014

are less than 1 nm for strong ferromagnetic materials [17].)Hence, the thicknesses of both F and F must be controlledvery preciselyespecially if the memory element relies oncontrolling the phase of the S/F/N/F/S junction.

In this paper, we focus on a different flavor of Josephsonjunction, namely one that carries spin-triplet supercurrent.Spin-triplet pair correlations with angular momentum projec-tion m = 1 along the magnetization axis consist of twoelectrons in the same spin band, so they behave as thoughthey were in a normal, nonmagnetic metal. In particular, theymaintain phase coherence over long distances (up to severalhundred nm at low temperatures), in contrast to the very shortdistances over which spin-singlet pairs lose phase coherencein ferromagnets. The motivation for working toward spin-triplet Josephson junctions for cryogenic memory is that thetolerances on the thicknesses of the various magnetic layers inthe junctions may be more forgiving than they are when usingspin-singlet junctions.

Superconducting materials with inherent spin-triplet pairingare rare in nature. However, the possibility to convert spin-singlet Cooper pairs to spin-triplet supercurrent in ferromag-netic Josephson junctions with conventional superconductingelectrodes was predicted in a series of theoretical papers startingwith a seminal paper by Bergeret, Volkov, and Efetov in 2001[18]. The basic physics of the conversion is nicely explainedby Eschrig [19]. When a spin-singlet Cooper pair leaves asuperconductor and enters a ferromagnet, the two electronsenter different spin bands with different Fermi wave vectors, asdescribed above, and rapidly accumulate a relative phase shift.A consequence of that phase shift is that the pair correlationfunction acquires a spin-triplet component with zero projec-tion along the quantization axis (m = 0). If the electron pairthen enters a second ferromagnet whose magnetization is non-collinear with the first, the m = 0 triplet component transformsinto m = 1 and m = 1 triplet components along the newquantization axis. Those components are long-range in thesecond ferromagnet because they correspond to both electronsof the pair residing in the same spin bandthe majority bandfor m = 1 or the minority band for m = 1.

Experimental evidence for spin-triplet supercurrent orspin-triplet pair correlations has been obtained by severalgroups using different approaches [20][26]. Our approachis based on Josephson junctions with the generic structure:S/F/N/F/N/F/S, where F, F, and F are all ferromagneticlayers. This geometry was proposed by Houzet and Buzdinin 2007 [27], but was not experimentally realized until ourwork appeared in 2010 [20]. Spin-singlet pairs coming fromthe left-hand S electrode (in the S/F/N/F/N/F/S structure) areconverted to triplet pairs in F when the magnetization of F isnon-collinear with that of F, and are re-converted back to spin-singlet pairs at the right-hand S electrode when the magneti-zation of F is non-collinear with that of F. The conversionefficiency from spin-singlet to spin-triplet pairs and back ismaximized when the magnetizations of adjacent layers in thestructure are orthogonal.

To achieve orthogonal magnetizations, we have exploredtwo experimental realizations. All of our Josephson junctionsamples have the supercurrent flowing in the vertical, out-

of-plane direction. In our earlier work, we used a Co/Ru/Cosynthetic antiferromagnet (SAF) for F, and found that applyinga strong in-plane magnetic field to the junctions led to a largeenhancement of the junction critical current, Ic, relative toits value in the as-grown state [28]. This enhancement occursbecause the applied field causes the magnetizations of the F andF layers to orient parallel to the field direction. Simultaneously,the Co magnetizations in the SAF undergo a spin-flop transi-tion, and end up oriented perpendicular to the field direction(but still in plane) after the field is removed. As a consequencethe magnetized samples have the magnetizations of adjacentlayers orthogonal, as desired. The second option we pursuedwas to use a [Ni/Co]n multilayer with perpendicular magneticanisotropy (PMA) for the F layer [29], with its magnetizationpointing out-of-plane. The spin-triplet supercurrent in thosesamples was already optimized in the as-grown state; subjectingthe samples to an in-plane field did not enhance the criticalcurrent, and in some samples it reduced the critical current.(The in-plane field should have magnetized the F and F layers,but apparently it was detrimental to the out-of-plane F-layermagnetization.)

The magnitude of the critical current in Josephson junctionsis conveniently characterized by the IcRN product, where RNis the normal-state resistance of the junction, because in mostsituations IcRN is independent of the junction cross-sectionalarea. In our previous work [20], [28], [29], the distinctive sig-nature of spin-triplet supercurrent was a much slower decay ofIcRN with increasing F layer thickness in samples containingthe F and F layers, compared to similar samples without thoseextra ferromagnetic layers. We obtained values of IcRN of 1to 2 V in spin-triplet junctions containing the Co/Ru/Co SAFafter they were magnetized [28], [30], and 3 to 5 V in junctionscontaining the multilayer with PMA [29].

Here, we explore the next step toward control of the su-percurrent in spin-triplet junctions. Theory predicts that theground-state phase across a spin-triplet junction can be either0 or , depending on the relative orientations of the threeferromagnetic layer magnetizations [27]. Assuming that themagnetizations of the F and F layers are non-collinear, thejunction will be a junction if the F magnetization is parallelto the F magnetization, or a 0 junction if those two are antipar-allel. Verification of that theoretical prediction will require thefabrication of junctions containing a combination of hard andsoft magnetic materials, so that their magnetization directionscan be controlled independently. The goals of this work are:1) to assess the viability of various soft magnetic materialsfor the F layer, and 2) to determine the optimum thicknessof those materials for producing spin-triplet supercurrent. Wedo not perform phase-sensitive measurements in this work. Inthe junctions studied here, we have utilized 1.2-nm thick Nifor the hard F layer, with a high coercive field of 0Hc 0.1 T. (Of the various alloys weve used for the F layer, Niis the most effective at converting spin-singlet to spin-tripletsupercurrent [31].) For the soft F layer, we have utilized eithera Pd1xFex alloy with x = 1.4%, or a Nb-doped Permalloyalloy of approximate composition Ni76Fe15Nb9. The PdFealloy is known to have low spin-orbit coupling and hence lowmagneto-crystalline anisotropy; in addition, it has been used as

NIEDZIELSKI et al.: PdFe AND NiFeNb IN FERROMAGNETIC JOSEPHSON JUNCTIONS 1800307

Fig. 1. Cross-sectional schematic of Josephson junction multilayered struc-ture (not to scale). Arrows indicate magnetization directions of the ferromag-netic layers. The lateral dimension of the ion-milled section between the SiOxinsulators is between 3 and 12 m for the junctions in this study. (Patterningonly the top S electrode is sufficient to define the junction geometry becausethe supercurrent tends to take the shortest path through the multilayer.)

a soft magnetic layer in a different style of memory elementby the Hypres-Chernogolovka collaboration [14]. Nb-dopedPermalloy alloys were shown recently by the NIST Bouldergroup to have very low coercive field when incorporated intoJosephson junctions [15]. Those authors had less success usingPdFe than did the Hypres groupa result that foreshadowssome of the results of our work reported here. For the F layer,we have utilized both the Co/Ru/Co SAF and the [Ni/Co]nmultilayer with PMA. The F layers in these junctions have beenchosen to be thick enough so that the spin-singlet supercurrentis suppressed to a level no more than 10% of the spin-tripletsupercurrent.

II. SAMPLE FABRICATION AND MEASUREMENT

Josephson junction samples were fabricated in the same man-ner as in our previous work [20]. The entire S/F/N/F/N/F/Smultilayer is sputtered in one run onto 0.5 0.5 square,unoxidized Si chips held between 30 C and 0 C, withoutbreaking vacuum. The sputtering chamber has a base pressureof 2 108 Torr, and sputtering is carried out in 3 mTorr ofpurified Ar gas. Layer thicknesses are indicated in Fig. 1. Thetop Nb (S) layer in the initial deposition has a thickness of only20 nm, and is protected by Au to prevent oxidation. Circularjunctions of diameters between 3 and 12 m are defined byimage-reversal photolithography and subsequent ion milling.For this work we milled through the 20-nm Nb layer andpart-way through the Cu (N) layer below it, leaving all ofthe ferromagnetic layers unpatterned. Immediately followingthe ion milling, a layer of insulating SiOx is deposited bythermal evaporation, then the photoresist protecting the circularjunctions is removed. Finally, the thick top Nb electrodes(coated by Au again) are sputtered either through a mechanicalmask or a photolithographic stencil. The thin Au layer betweenthe 20-nm Nb and thick Nb top electrode effectively becomessuperconducting by proximity.

Fig. 2. Critical current versus applied external magnetic field for a Josephsonjunction of diameter 10 m with F = Ni(1.2), F = Ni(0.4)/[Co(0.2)/Ni(0.4)]10 multilayer and F = Pd Fe(30) alloy, where all thicknesses aregiven in nm. Colors and symbols indicate the direction of increasing field withblack squares sweeping from negative to positive field values and red circlessweeping from positive to negative field values. No hysteresis is observed.Inset: Current-voltage characteristic of one of our junctions. The data followthe theoretical expectation for overdamped junctions given in the text.

Because our Josephson junction samples have wide lateraldimensions and relatively thin interlayers, their normal-stateresistances are lowabout 1 m for a 3-m wide junction.To achieve high signal-to-noise ratio while measuring junc-tions of such small resistance, we measure the current-voltage(IV ) characteristics of the samples using a current-comparatorcircuit based on a commercial rf SQUID null detector [32].All of the IV characteristics measured in this work have thestandard form for overdamped Josephson junctions: V = 0 for|I| < Ic, and |V | = RN (I2 I2c )1/2 for |I| > Ic. (See theinset to Fig. 2.)

An excellent way to characterize Josephson junctions is tomeasure Ic as a function of an external magnetic field Happlied in the plane of the junctioni.e., perpendicular to thedirection of current flow. The shape of the resulting Fraunhoferpattern in Ic(H) provides information about the supercurrentdistribution in the junction [33], and deep minima indicate thatthere are no short-circuits in the surrounding oxide. Junctionscontaining strong ferromagnetic materials do not always showtextbook-like Fraunhofer patterns due to the complex patternof intrinsic magnetic flux inside the junctionssee our earlierwork for a discussion of that issue [34], [35]. In this paper,we show data only from those samples that exhibit reasonablygood Fraunhofer patternsabout 75% of the total samplesmeasured.

III. JOSEPHSON JUNCTIONS WITH PdFe

As discussed earlier, we decided to use PdFe alloy as thesoft magnetic materials for the F layer because we had pre-vious experience with that material [36], and because anothergroup had successfully used it inside Josephson junctions [14].For the choice of F layer material, we initially decided to use

1800307 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 4, AUGUST 2014

Fig. 3. Semi-log plot of IcRN versus thickness of the F layer for Josephsonjunctions containing F = Ni/Co multilayer and F = Pd Fe alloy. Eachcircle represents a single junction; multiple junctions with the same PdFethickness may be on different chips.

the Ni/Co multilayer for this work, since it provided largervalues of IcRN than did the Co/Ru/Co SAF in our earlierwork [28][30].

A. Junctions Containing Ni/Co Multilayers

Fig. 2 shows a typical Fraunhofer pattern for anS/F/N/F/N/F/S Josephson junction of diameter 10 mwith F = Ni(1.2), F = [Ni(0.4)/Co(0.2)]10/Ni(0.4), andF = Pd Fe(30), where all thicknesses are in nm. Severalfeatures are immediately apparent. First, the shape of the pat-tern is reasonable, but is certainly not excellent. This is nottoo worrisome, because we found previously that many of ourjunctions containing Ni/Co multilayers had strangely-shapedFraunhofer patterns [29]. At least there is a clear central peakfor applied fields between 0 and 1.5 mT, giving us a reasonableexpectation that the supercurrent is nearly uniform in that fieldrange. A more puzzling feature of the data shown in Fig. 2 isthat there is no visible hysteresis between the data taken whilethe field was ramped in the positive direction (black squaresand curve) and the data taken while the field was ramped inthe negative direction (red circles and curve). Due to the softmagnetic properties of the PdFe alloy, we expected to seehysteresis in the Fraunhofer patterns at low fields, indicative ofthe magnetic configuration of the PdFe changing with appliedfield.

One of the goals of this work was to determine the optimumthickness of the PdFe alloy for converting the spin-singletCooper pairs from the Nb electrodes into spin-triplet supercur-rent inside the F layer. To achieve that goal, we fabricated andmeasured junctions with a wide range of PdFe thicknesses.One expects to see first an increase, and then a decrease inIcRN as the PdFe thickness increased [20], [27], [28]. Fig. 3shows a summary of the results on all the samples, in a plot ofIcRN as a function of PdFe thickness. There is considerablescatter in the data, but we can conclude that there is littlevariation in IcRN as the PdFe thickness varies between 12

Fig. 4. Critical current versus applied magnetic field for a Josephson junctionof diameter 3 m with F = Co(6)/Ru(0.6)/Co(6) and F = Pd Fe(15)alloy. The black squares and red circles represent data taken with the externalfield increasing in the positive and negative directions, respectively. At low fieldvalues, hysteresis is observed.

and 30 nm. That is a very wide thickness range compared towhat we have seen with other F or F materials, but this 1.5%PdFe alloy is by far the weakest ferromagnetic material wehave studied in S/F/N/F/N/F/S or S/F/N/F/N/F/S junctions.The highest values of IcRN in these junctions is only about1 V, which is somewhat lower than the highest values of 4 to5 V we observed in similar junctions where both F and Fwere Ni [29]. Apparently PdFe is not as efficient as Ni is atconverting singlet supercurrent to triplet supercurrent and vice-versa.

B. Junctions Containing Co/Ru/Co SAF

The lack of hysteresis in the Fraunhofer patterns of thesamples discussed above led us to try substituting the Co/Ru/CoSAF for the Ni/Co multilayer as the F layer in our junctions.Fig. 4 shows a Fraunhofer pattern for a Josephson junctionof diameter 3 m containing F = Ni and F = Pd Fe alloyof thickness 15 nm, but now with F = Co(6)/Ru(0.6)/Co(6),where the thicknesses are in nm. The sample was magnetized inan in-plane field of 260 mT in the positive direction before thesedata were acquired. That field is sufficient to fully magnetizethe Ni F layer, and also to trigger the spin-flop transition inthe Co/Ru/Co SAF. Several features of the data in Fig. 4 jumpout. First and foremost, the quality (shape) of the Fraunhoferpattern in Fig. 4 is much better than the one shown in Fig. 2.Apparently the Co/Ru/Co SAF does a better job of eliminatingintrinsic magnetic flux in the junction than does the Ni/Comultilayer with PMA. Second, and equally important, thereis hysteresis in the Fraunhofer data at low field. The datataken with the field ramping in the positive direction showa nearly textbook-quality Fraunhofer pattern with the centralpeak slightly shifted to negative field. That is due to the Nimagnetization, which points in the positive direction. The cen-tral peak in the Fraunhofer pattern occurs when the flux in thejunction due to the external field exactly cancels the net flux

NIEDZIELSKI et al.: PdFe AND NiFeNb IN FERROMAGNETIC JOSEPHSON JUNCTIONS 1800307

Fig. 5. Semi-log plot of IcRN versus thickness of the F layer for Josephsonjunctions with F = Co/Ru/Co and F = Pd Fe alloy.

due to the internal magnetizations [34]. The data taken withthe field ramping in the negative direction do not show suchan ideal Fraunhofer pattern. That is because the magnetizationof the soft PdFe layer is already starting to switch directionsbefore the central peak of the pattern has been reached. The hys-teresis is apparent already when the field decreases to +2 mT,and is well-pronounced when the field reaches zero. The largedrop in critical current compared to the data taken whileramping the field upward is indicative of the complex domainstructure in the PdFe alloy that occurs while its magnetizationis switching direction. Note that the order in which the two datasets are acquired is not relevant.

Fig. 5 shows a summary of all of the junctions with this struc-ture, again as a plot of IcRN versus PdFe thickness. There issome scatter in the data, but the data do show a broad maximumin IcRN for PdFe thicknesses in the range of 1524 nm,consistent with the plot shown in Fig. 3. Note that the maximumvalues of IcRN are about 0.5 Vonly about half as largeas in the junctions with the Ni/Co multilayer as F. The factthat the Ni/Co multilayer produces somewhat higher valuesof IcRN than does the Co/Ru/Co SAF is consistent with theresults of our earlier work with S/F/N/F/N/F/S junctions [28][30]. Nevertheless, the higher quality of the Fraunhofer patternsand the smaller data scatter in the junctions containing F =Co/Ru/Co suggest that it is more promising than the Ni/Comultilayer for future applications.

An astute reader is undoubtedly curious as to why we ob-serve hysteresis in junctions containing F = Pd Fe alloy andF = Co/Ru/Co, but not in junctions with the same F layer butwith the Ni/Co multilayer as F. The most plausible explanationis that stray fields from the Ni/Co domain structure tend topin the magnetic configuration of the soft PdFe alloy, so thatthe latter does not change as the external field ramps throughzero. Substantial stray fields are expected in a material withPMA when it breaks up into upward and downward pointingdomains. In the Co/Ru/Co SAF, in contrast, most of the straymagnetic field generated at the domain boundaries stays insidethe trilayer. This is an important lesson, as it reinforces thenotion that the magnetic properties of any thin magnetic film

Fig. 6. Critical current versus applied field for a Josephson junction of di-ameter 3 m with F = Co(6)/Ru(0.6)/Co(6) and F = Ni Fe Nb(1.5)alloy. The black squares and red circles indicate positive and negative fieldsweep directions, respectively. Again, hysteresis is observed at low field values.

may depend as much on the geometrical configuration and thesurrounding materials as they do on the film itself.

IV. JOSEPHSON JUNCTIONS WITH NiFeNb

While this work was being carried out, we learned about thework by Baek et al. mentioned in the Introduction [15]. Thoseauthors were not interested in spin-triplet supercurrent; never-theless, their experience fabricating and measuring Josephsonjunctions containing both hard and soft magnetic ferromaterialsis clearly relevant to our work. In particular, those authors foundthat S/F/N/F/S junctions containing Ni as the hard F layer andPdFe as the soft F layer did not switch cleanly between twostates, in spite of the fact that isolated PdFe films showed cleanmagnetization switching with low coercivity. Apparently theincorporation of the PdFe films into the Josephson junctionschanged their magnetic behavior substantially. That could bedue to stray fields from the domains of the nearby Ni layer,or it could be due to different growth conditions for the film,which can result in strain and/or disorder. Baek et al. didfind an alternative soft material with better magnetic switchingbehavior, namely a Nb-doped Permalloy (NiFeNb) alloy withNb concentration in the range of 9 to 13%.

Following the work by Baeket al., we decided to try using9% Nb-doped Permalloy in our spin-triplet junctions. We chosethe lower Nb concentration because the resistivity of Nb-dopedPermalloy grows very rapidly with Nb concentration [37]. Theincrease in the resistivity usually signals a decrease in the meanfree path. That, in turn, leads to decreases in other significantlength scales, including the spin-diffusion length, lsf , which isthe length scale over which electron spin information is lost. Iflsf in a ferromagnetic layer is small compared to the thicknessof that layer, then one expects the supercurrent to be suppressedexponentially with thickness.

Fig. 6 shows a typical Fraunhofer pattern for anS/F/N/F/N/F/S junction of diameter 3 m with F = Ni, F =Co/Ru/Co and F = Ni Fe Nb alloy of thickness 1.5 nm.

1800307 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 4, AUGUST 2014

Fig. 7. Semi-log plot of IcRN dependence on F layer thickness for Josephsonjunctions with F = Co/Ru/Co and F = Ni Fe Nb alloy.

The sample was magnetized in an in-plane field of +260 mTprior to taking these measurements. The data are very similarto those shown in Fig. 4, except with lower values of IcRN .The quality of the Fraunhofer pattern is quite good when thefield is ramped in the positive direction, but not when the fieldis ramped in the negative direction. Again, this is because thecentral peak in the Fraunhofer pattern is slightly shifted tonegative field to compensate the magnetization of the Ni layer.When the field reaches zero while ramping downward, beforereaching the peak in the Fraunhofer pattern, the NiFeNb Flayer magnetization is already starting to switch. Hysteresis inthe Fraunhofer pattern is apparent for applied fields between 7and +7 mTa slightly wider range than observed in the caseof the PdFe alloy.

Fig. 7 summaries all of the data from the samples with thisstructure. The largest value of IcRN observed in these samplesis only 50 nV, about 10 times smaller than in similar junctionscontaining PdFe alloy as the F layer in the place of theNiFeNb alloy. In addition, the values of IcRN decreaserapidly with increasing thickness of the F layer. In fact,we measured a few samples with larger F layer thicknesses(4.0 and 5.0 nm), but we were not able to detect any super-current in those samples. We conclude that the spin-diffusionlength is very short in this alloy, which suppresses the supercur-rent rapidly as the NiFeNb thickness increases. (The data donot extend over a wide enough range to permit us to determinethe value of lsf , but they suggest that it is very short, possiblein the range of 1 nm.)

V. CONCLUSION

Several conclusions can be drawn from the data presentedhere. The main conclusion is that both the PdFe and NiFeNballoys used in this work may be suitable as soft ferromagneticlayers inside controllable ferromagnetic Josephson junctions.For PdFe, the optimal thickness is in the range 1524 nm,while for NiFeNb, the optimal thickness is no greater than1.5 nm. Two warnings accompany that recommendation, how-

ever. First, the PdFe alloy behaved as a soft ferromagnet onlywhen the magnetic layer immediately below it was a Co/Ru/CoSAF, but not when the layer below it was a Ni/Co multilayerwith perpendicular magnetic anisotropy. The stray fields fromthe PMA layer evidently destroy the soft characteristic of thePdFe, suggesting that ferromagnetic layers with PMA shouldbe avoided in future work aimed at controlling the magneticstate of a Josephson junction. Second, The NiFeNb alloywith 9% Nb strongly suppresses the critical current in thesejunctions. That could be due to strong spin-memory loss. It isknown from earlier experimental [38] and theoretical [39], [40]work that spin-flip or spin-orbit scattering suppress spin-singletsupercurrent, hence any such scattering taking place withinthe F or F layers within our samples will suppress Ic. Thesuppression caused by the NiFeNb appears to be about oneorder of magnitude, even when the NiFeNb layer thickness isonly 1.5 nm. (We did not fabricate samples with layers thinnerthan that.)

In the future, we plan to control the magnetic state of thejunctions in a bi-stable manner by fabricating junctions with atleast the F layer patterned into an elliptical nanomagnet. If thesmall magnetic layer is single-domain, then shape anisotropyshould restrict the magnetic configuration to one of two possiblestates. Such a device would then become a potential candidatefor use in cryogenic random access memory.

ACKNOWLEDGMENT

The authors thank B. Bi for technical assistance, and useof the Keck Microfabrication Facility. N.O.B. acknowledgesnumerous discussions with A. Herr, Q. Herr, D. Miller,O. Naaman, and N. Newman.

REFERENCES[1] D. S. Holmes, A. L. Ripple, and M. A. Manheimer, Energy-efficient

superconducting computingPower budgets and requirements, IEEETrans. Appl. Supercond., vol. 23, no. 3, p. 1701610, Jun. 2013.

[2] K. K. Likharev and V. K. Semenov, RSFQ logic/memory family: A newJosephson-junction technology for sub-terahertz-clock-frequency digi-tal systems, IEEE Trans. Appl. Supercond., vol. 1, no. 1, pp. 328,Mar. 1991.

[3] Q. P. Herr, A. Y. Herr, O. T. Oberg, and A. G. Loannidis, Ultra-low-powersuperconductor logic, J. Appl. Phys., vol. 109, no. 10, pp. 103903-1103903-8, May 2011.

[4] D. E. Kirichenko, S. Sarwana, and A. F. Kirichenko, Zero static powerdissipation biasing of RSFQ circuits, IEEE Trans. Appl. Supercond.,vol. 21, no. 3, pp. 776779, Jun. 2011.

[5] M. Tanaka, M. Ito, A. Kitayama, T. Kouketsu, and A. Fujimaki, 18-GHz,4.0-aJ/bit operation of ultra-low-energy single-flux-quantum shiftregisters, Jpn. J. Appl. Phys., vol. 51, no. 5, pp. 053102-1053102-4,May 2012.

[6] M. H. Volkmann, A. Sahu, C. J. Fourie, and O. A. Mukhanov, Imple-mentation of energy efficient single flux quantum digital circuits withsub-aJ/bit operation, Supercond. Sci. Technol., vol. 26, no. 1, p. 015002,Jan. 2013.

[7] Y. Yamanashi, T. Nishigai, and N. Yoshikawa, Study of LR-loadingtechnique for low-power single flux quantum circuits, IEEE Trans. Appl.Supercond., vol. 17, no. 2, pp. 150153, Jun. 2007.

[8] V. V. Ryazanov, V. A. Oboznov, A. Y. Rusanov, A. V. Veretennikov,A. A. Golubov, and J. Aarts, Coupling of two superconductors through aferromagnet: Evidence for a junction, Phys. Rev. Lett., vol. 86, no. 11,pp. 24272430, Mar. 2001.

[9] T. Kontos, M. Aprili, J. Lesueur, F. Genet, B. Stephanidis, andR. Boursier, Josephson junction through a thin ferromagnetic layer: Neg-ative coupline, Phys. Rev. Lett., vol. 89, no. 13, pp. 137007-1137007-4,Sep. 2002.

NIEDZIELSKI et al.: PdFe AND NiFeNb IN FERROMAGNETIC JOSEPHSON JUNCTIONS 1800307

[10] M. Durlam, D. Addie, J. Akerman, B. Butcher, P. Brown, J. Chan,M. DeHerrera, B. N. Engel, B. Feil, G. Grynkewich, J. Janesky,M. Johnson, K. Kyler, J. Molla, J. Martin, K. Nagel, J. Ren, N. D. Rizzo,T. Rodriguez, L. Savtchenko, J. Salter, J. M. Slaughter, K. Smith,J. J. Sun, M. Lien, K. Papworth, P. Shah, W. Qin, R. Williams, L. Wise,and S. Tehrani, A 0.18m 4Mb toggling MRAM, in IEDM, Washington,DC, USA, 2003, pp. 34-6.134.6.3.

[11] C. Bell, G. Burnell, C. W. Leung, E. J. Tarte, D. J. Kang, and M. G. Blamire,Controllable Josephson current through a pseudospin-valve structure,Appl. Phys. Lett., vol. 84, no. 7, pp. 11531155, Feb. 2004.

[12] A. Y. Herr and Q. P. Herr, Josephson magnetic random access memorysystem and method, U.S. Patent 8 270 209 B2, Sep. 18, 2012.

[13] J. F. Bulzacchelli, W. J. Gallagher, and M. B. Ketchen, Hybridsuperconducting-magnetic memory cell and array, U.S. Patent 8 208 288B2, Jun. 26, 2012.

[14] I. V. Vernik, V. V. Bolginov, S. V. Bakurskiy, A. A. Golubov,M. Y. Kupriyanov, V. V. Ryazanov, and O. A. Mukhanov, MagneticJosephson junctions with superconducting interlayer for cryogenic mem-ory, IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 1701208, Jun. 2013.

[15] B. Baek, W. H. Rippard, S. P. Benz, S. E. Russek, and P. D. Dresselhaus,Hybrid superconducting-magnetic memory device using competing or-der parameters, arXiv:1310.2201.

[16] M. A. E. Qader, R. K. Singh, S. Galvin, L. Yu, J. M. Rowell, andN. Newman, Switching at small magnetic fields in Josephson junctionsfabricated with ferromagnetic barrier layers, Appl. Phys. Lett., vol. 104,no. 2, pp. 022602-1022602-5, Jan. 2014.

[17] J. W. A. Robinson, S. Piano, G. Burnell, C. Bell, and M. G. Blamire,Critical current oscillation in strong ferromagnetic junctions, Phys.Rev. Lett., vol. 97, no. 17, pp. 177003-1177003-4, Oct. 2006.

[18] F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Long-range proximity ef-fects in superconductor-ferromagnet structures, Phys. Rev. Lett., vol. 86,no. 18, pp. 40964099, Apr. 2001.

[19] M. Eschrig, Spin-polarized supercurrents for spintronics, Phys. Today,vol. 64, no. 1, pp. 4349, Jan. 2011.

[20] T. S. Khaire, M. A. Khasawneh, W. P. Pratt, Jr., and N. O. Birge, Obser-vation of spin-triplet superconductivity in co-based Josephson junctions,Phys. Rev. Lett., vol. 104, no. 13, pp. 137002-1137002-4, Mar. 2010.

[21] J. W. A. Robinson, J. D. S. Witt, and M. G. Blamire, Controlled injectionof spin-triplet supercurrents into a strong ferromagnet, Science, vol. 329,no. 5987, pp. 5961, Jul. 2010.

[22] D. Sprungmann, K. Westerholt, H. Zabel, M. Weides, and H. Kohlstedt,Evidence for triplet superconductivity in Josephson junctions with barri-ers of the ferromagnetic Heusler alloy Cu2MnAl, Phys. Rev. B, Condens.Matter, vol. 82, no. 6, pp. 060505-1060505-4, Aug. 2010.

[23] M. S. Anwar, F. Czeschka, M. Hesselberth, M. Porcu, and J. Aarts,Long-range supercurrents through half-metallic ferromagnetic CrO2,Phys. Rev. B, Condens. Matter, vol. 82, no. 10, pp. 100501-1100501-4,Sep. 2010.

[24] P. V. Leksin, N. N. Garifyanov, I. A. Garifullin, Y. V. Fominov,J. Schumann, Y. Krupskaya, V. Kataev, O. G. Schmidt, and B. Bchner,Evidence for triplet superconductivity in a superconductor-ferromagnetspin valve, Phys. Rev. Lett., vol. 109, no. 5, pp. 057005-1057005-5,Aug. 2012.

[25] Y. Kalcheim, O. Millo, M. Egilmez, J. W. A. Robinson, andM. G. Blamire, Evidence for anisotropic triplet superconductor orderparameter in half-metallic ferromagnetic La0.7Ca0.3Mn3O proximitycoupled to superconducting Pr1.85 Ce0.15CuO4, Phy. Rev. B, Condens.Matter, vol. 85, no. 10, pp. 104504-1104504-5, Mar. 2012.

[26] C. Visani, Z. Sefrioui, J. Tornos, C. Leon, J. Briatico, M. Bibes,A. Barthlmy, J. Santamara, and J. E. Villegas, Equal-spin Andreevreflection and long-range coherent transport in high-temperaturesuperconductor/half-metallic ferromagnet junctions, Nat. Phys., vol. 8,no. 7, pp. 539543, Jul. 2012.

[27] M. Houzet and A. I. Buzdin, Long range triplet Josephson effect througha ferromagnetic trilayer, Phys. Rev. B, Condens. Matter, vol. 76, no. 6,pp. 060504-1060504-4, Aug. 2007.

[28] C. Klose, T. S. Khaire, Y. Wang, W. P. Pratt, Jr., N. O. Birge,B. J. McMorran, T. P. Ginley, J. A. Borchers, B. J. Kirby, B. B. Maranville,and J. Unguris, Optimization of spin-triplet supercurrent in ferromag-netic Josephson junctions, Phys. Rev. Lett., vol. 108, no. 12, pp. 127002-1127002-5, Mar. 2012.

[29] E. C. Gingrich, P. Quarterman, Y. Wang, R. Loloee, W. P. Pratt, Jr., andN. O. Birge, Spin-triplet supercurrent in Ni/Co multilayer Josephsonjunction with perpendicular anisotropy, Phys. Rev. B, Condens. Matter,vol. 86, no. 22, pp. 224506-1224506-4, Dec. 2012.

[30] Y. Wang, W. P. Pratt, Jr., and N. O. Birge, Area-dependence of spin-triplet supercurrent in ferromagnetic Joesphson junctions, Phys. Rev. B,Condens. Matter, vol. 85, no. 21, pp. 214522-1214522-7, Jun. 2012.

[31] M. A. Khasawneh, T. S. Khaire, C. Klose, W. P. Pratt, Jr., and N. O. Birge,Spin-triplet supercurrent in co-based Josephson junctions, Supercond.Sci. Technol., vol. 24, no. 2, pp. 024005-1024005-7, Feb. 2011.

[32] D. Edmunds, W. Pratt, and J. Rowlands, 0.1 ppm four-terminal resistancebridge for use with a dilution refrigerator, Rev. Sci. Instrum., vol. 51,no. 11, pp. 15161522, Nov. 1980.

[33] R. C. Dynes and T. A. Fulton, Supercurrent density distribution inJosephson junctions, Phys. Rev. B, Condens. Matter, vol. 3, no. 9,pp. 30153023, May 1971.

[34] T. S. Khaire, W. P. Pratt, Jr., and N. O. Birge, Critical current behaviorin Josephson junctions with the weak ferromagnet PdNi, Phy. Rev. B,Condens. Matter, vol. 79, no. 9, pp. 094523-1094523-11, Mar. 2009.

[35] M. A. Khasawneh, W. P. Pratt, and N. O. Birge, Josephson junctions witha synthetic antiferromagnetic interlayer, Phy. Rev. B, Condens. Matter,vol. 80, no. 2, pp. 020506-1020506-4, Jul. 2009.

[36] H. Arham, T. S. Khaire, R. Loloee, W. P. Pratt, and N. O. Birge, Mea-surement of spin memory lengths in PdNi and PdFe ferromagnetic alloys,Phys. Rev. B, Condens. Matter, vol. 80, no. 17, pp. 174515-1174515-7,Nov. 2009.

[37] M. M. Chen, N. Gharsallah, G. L. Gorman, and J. T. Latimer, NiFeXas soft biasing film in a magnetoresistive sensor, J. Appl. Phys., vol. 69,no. 8, pp. 56315633, Apr. 1991.

[38] V. A. Oboznov, V. V. Bolginov, A. K. Feofanov, V. V. Ryazanov, andA. I. Buzdin, Thickness dependence of the Josephson ground states ofsuperconductor-ferromagnet-superconductor junctions, Phys. Rev. Lett.,vol. 96, no. 19, pp. 197003-1197003-4, May 2006.

[39] M. Faure, A. I. Buzdin, A. A. Golubov, and M. Y. Kupriyanov, Proper-ties of superconductor/ferromagnet structures with spin-dependent scat-tering, Phys. Rev. B, Condens. Matter, vol. 73, no. 6, pp. 064505-1064505-12, Feb. 2006.

[40] F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Scattering by magneticand spin-orbit impurities and the Josephson current in superconductor-ferromagnet-superconductor junctions, Phys. Rev. B, Condens. Matter,vol. 75, no. 18, pp. 184510-1184510-8, May 2007.

Authors biographies not available at the time of publication.

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice