Abrupt Transition from Ferromagnetic to Antiferromagnetic of

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week ending 18 APRIL 2014

PHYSICAL REVIEW LETTERS

Abrupt Transition from Ferromagnetic to Antiferromagnetic of Interfacial Exchange in Perpendicularly Magnetized L10 -MnGa/FeCo Tuned by Fermi Level Position 1

Q. L. Ma,1,* S. Mizukami,1 T. Kubota,1 X. M. Zhang,1 Y. Ando,2 and T. Miyazaki1

WPI Advanced Institute for Materials Research, Tohoku University, 2-1-1, Katahira, 980-8577 Sendai, Japan Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05, Aoba-yama, 980-8579 Sendai, Japan (Received 13 January 2014; published 18 April 2014)

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An abrupt transition of the interfacial exchange coupling from ferromagnetic to antiferromagnetic was observed in the interface of perpendicularly magnetized L10 -MnGa=Fe1−x Cox epitaxial bilayers when x was around 25%. By considering the special band structure of the MnGa alloy, we present a model explaining this transition by the spin-polarization reversal of Fe1−x Cox alloys due to the rise of the Fermi level as the Co content increases. The effect of interfacial exchange coupling on the coercive force (H c ) and the spin-dependent tunneling effect in perpendicular magnetic tunnel junctions (pMTJs) based on the coupled composite were also studied. Changes from the normal spin valve to inverted magnetoresistance loops corresponding to the coupling transition were observed in pMTJs with MnGa=Fe1−x Cox as an electrode. DOI: 10.1103/PhysRevLett.112.157202

PACS numbers: 75.30.Et, 75.47.−m, 75.70.Cn

Exchange coupling is one of the fundamental issues in magnetism. As material fabrication techniques enable various kinds of thin film structures to be achieved, the interfacial exchange interaction becomes a model system to study the physics. Tailoring interfacial interactions and providing a physical picture are desired not only for scientific study but also for their potential device applications, as exemplified by the interlayer exchange coupling and exchange spring as the basis for spintronics and the core structure of high-density magnetic recording media, respectively [1–5]. The occurrence of direct exchange coupling at the interface of two ferromagnets leads to two types of coupling: ferromagnetic (FM) and antiferromagnetic (AFM). For materials exhibiting in-plane magnetization, the direct interfacial exchange interaction has been reported in a wide range of structures, such as Ni/Fe and Co/Ni for the FM interface and Fe/Gd and Co/Mn for the AFM interface [6–9]. Compared with in-plane magnetized materials, tailoring of the interface exchange coupling in materials with perpendicular magnetic anisotropy (PMA) is a big challenge. In particular, the formation of the AFM interface is rare because of frustration in the spin structures at the interface. However, in trying to maintain thermal stability of spintronics devices as cells are minimized, magnetic materials with high anisotropy (K u ) are in high demand. Moreover, spintronics devices, based on PMA materials manipulated by spin transfer torque, have a smaller switching current compared with materials with an in-plane easy axis of magnetization [10–12]. In addition, antiferromagnetically coupled FM layers with PMA become increasingly interesting in the design of novel ferrimagnetic PMA superlattices and can potentially be applied in an all-optical manipulation of magnetization [13,14]. One of the obstacles is that the ferromagnetism in conventional PMA materials 0031-9007=14=112(15)=157202(5)

such as FePt(Pd) alloys and ½Co=Ptn multilayers is based on Fe and Co, which have relatively stable electronic properties. Beyond the conventional PMA materials, tetragonal Mn-based Heusler alloys have opened a new way for PMA material with high K u . Mn-Ga alloys with L10 and D022 structures are most interesting because they have a unique combination of large PMA with a K u of 10 Merg=cm3 , low magnetization, and a low Gilbert damping constant [15–19]. The magnetization of Mn-Ga alloys originates from the Mn atoms, which have half-filled d shells. This makes the exchange interaction involving Mn-Ga thin film more sensitive to the interfacial environment and more flexible for being tuned. In this Letter, using L10 -ordered Mn62 Ga38 alloy (MnGa) as the PMA material, we observed an abrupt transition of the exchange coupling from FM to AFM in the MnGa=Fe1−x Cox interface as the Co content (x) increases to 25%. Taking into consideration the band structure of L10 -MnGa alloy, we present a model explaining this transition by the spin-polarization reversal of Fe1−x Cox alloys stemming from the rise in the Fermi level as the Co content increases. Corresponding to the transition, we observed significant changes in the coercive force (Hc ) of the MnGa=Fe1−x Cox exchange spring that depended on exchange type and strength. Specifically, AFM interface enhances the H c , whereas the FM interface suppresses the Hc . The effect of interfacial coupling on the spin-dependent tunneling (SDT) property in perpendicular magnetic tunnel junctions (pMTJs) with MnGa=Fe1−x Cox as the electrode was also presented. The samples in this study were grown using an ultrahigh-vacuum sputtering system with a base pressure less than 1 × 10−7 Pa. The MnGa ð30 nmÞ=Fe1−x Cox (1.5 nm) bilayers, including a 40-nm Cr buffer layer, were deposited on a MgO(001) single-crystal substrate. The stacks were

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then capped by a 5-nm Ta layer. All of the layers were deposited at room temperature and annealed in situ after the deposition of Cr and MnGa at 700 °C and 500 °C, respectively [17,18,20]. The Fe1−x Cox films were deposited by cosputtering of Fe and Co elemental targets at a controlled rate to form alloys with a different composition. In situ annealing was performed after deposition of the Fe1−x Cox layer at 350 °C for 30 min. The Fe1−x Cox layers were deduced to have body-centered cubic (bcc) structure after the in situ annealing [20–23]. The magnetic properties were determined using a superconducting quantum interference device system with magnetic fields of 9 T. The MTJ stacks with a core structure of MnGa ð30nmÞFe1−x Cox ð1.5nmÞ=MgO ð2.2nmÞ=Fe1−x Cox ð0.1 nmÞ=Co40 Fe40 B20 (1.2 nm) were deposited using the same conditions as the bilayer samples. The junctions were fabricated with a lateral size ranging from 10 × 10 to 100 × 100 μm2 using a conventional UV lithography combined with Ar-ion etching. After that, the MTJs were annealed in vacuum at 300 °C for 10 min. Spin-dependent transport properties were measured using the four-probe technique on a physical property measurement system [24]. Figures 1(a) and 1(b) show the magnetization per unit area as a function of the applied magnetic field [MðHÞ loops] of MnGa=Fe1−x Cox in the perpendicular direction with x being equal to 20% and 60%, respectively. The MðHÞ loops in the low-field regime are shown as insets; the gray line is the MðHÞ loop of 30-nm MnGa single film included for comparison. At zero field, the magnetization in unit area (Mr =A) of MnGa=Fe80 Co20 is 1.41 emu=cm2 , which is 0.21 emu=cm2 larger than a single 30-nm-thick

FIG. 1 (color online). MðHÞ loops of MnGa ð30 nmÞ= Fe1−x Cox (1.5 nm) with x equal to 20% (a) and 60% (b). The gray line is the MðHÞ loop of 30-nm MnGa single films included for comparison. (c) and (d) are the corresponding MRðHÞ loops of pMTJs tested in a perpendicular direction at room temperature. The solid lines in (c) and (d) are the fits based on the FM and AFM J ex in MnGa=Fe80 Co20 and MnGa=Fe40 Co60 interfaces, respectively.


MnGa layer. The increase in Mr =A in the perpendicular direction for Fe80 Co20 deposited on MnGa film suggests a parallel alignment of the magnetizations of the MnGa and Fe80 Co20 layers. In addition, it shows an Hc of 400 mT, which is smaller than that of a single MnGa layer drawn with gray lines. These experimental observations are consistent with typical phenomena observed in magnetic spring bilayers with FM coupling at the interface, such as the FePt/Fe system [5,25]. The behavior for MnGa=Fe40 Co60 is the complete opposite. The M r =A is 1.02 emu=cm2 , which is 0.18 emu=cm2 smaller than for the single 30-nm-thick MnGa layer. Moreover, the Hc of MnGa=Fe40 Co60 bilayer is 525 mT, which is larger compared with the single MnGa layer. The decrease in Mr =A reveals an antiparallel alignment of the magnetizations of Fe40 Co60 and MnGa when the applied field is zero, which suggests an AFM coupling at the MnGa=Fe40 Co60 interface [24]. As the field increases to 5 T, which is large enough to overcome the AFM coupling between MnGa and Fe40 Co60 , the magnetizations of Fe40 Co60 and MnGa align parallel, giving a saturation magnetization of 1.32 emu=cm2 , which is 0.12 emu=cm2 larger than the values of single MnGa film. Thus, the increase in Hc originates from the magnetization frustration at the MnGa=Fe40 Co60 interface because of the AFM interfacial coupling as Leighton et al. demonstrated for the MnF2 =Fe bilayer [26]. Beyond the magnetic properties, the interfacial exchange coupling can be identified by the field dependence of the magnetoresistance [MRðHÞ loops] of MTJs using the coupled structure as electrode [20,27,28]. Here, the MR for a given field is defined as MR ¼ ½ðRðHÞ − Rmin Þ=Rmin × 100%, where RðHÞ is the resistance at field H and Rmin is the minimum resistance corresponding to the parallel configuration of magnetizations of electrodes in contact with the barrier layer of the MTJ. The room-temperature MRðHÞ loops tested in the perpendicular direction for the MTJ samples with MnGa ð30 nmÞ=Fe80 Co20 (1.5 nm) and MnGa ð30 nmÞ= Fe40 Co60 (1.5 nm) as the bottom electrodes are shown in Figs. 1(c) and 1(d), respectively. A typical pseudo-spinvalve (PSV) MRðHÞ loop was observed in MTJs with MnGa=Fe80 Co20 as the electrode, which further confirms the FM nature of the interface of MnGa=Fe80 Co20 [21,28]. Instead of normal PSV MRðHÞ loops, the MTJs with MnGa=Fe40 Co60 as the electrode exhibited an inverted loop in the low-field range (1 T) and saturated to a lowresistance state when the field is up to 5 T. The loop here has been demonstrated to correspond to the AFM interface in the hybrid electrode [24,27]. The different behaviors in the SDT properties of MTJs with MnGa=Fe1−x Cox bottom electrode with different Fe1−x Cox composition provide further evidences of interfacial exchange coupling in MnGa=Fe1−x Cox depending on x. The composition dependence of M r =A and Hc for the MnGa=Fe1−x Cox bilayers is shown in Fig. 2(a). When x is less than 25%, the M r =A values are around 1.4 emu=cm2 ,

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FIG. 2 (color online). (a) Fe1−x Cox composition dependence of the M r =A (diamond) and Hc (circle) values of the MnGa ð30 nmÞ=Fe1−x Cox (1.5 nm) bilayer structure; a transition appears when x is approximately 25%. (b) MR ratios at room temperature (solid square) and 5 K (open square) of pMTJs with MnGa=Fe1−x Cox as bottom electrode. A transition from the normal PSV type to an inverted curve of perpendicular MRðHÞ occurs when x is approximately 25%.

which are larger than that of single MnGa film, indicated by the dashed line in the figure. When x increases to 25%, the Mr =A decreases abruptly to around 1.0 emu=cm2 . Over the same composition range, the Hc values change from 400 10 to 525 20 mT, which are respectively smaller and larger than the Hc of the single MnGa film (475 mT). The values of M r =A and Hc of the perpendicular exchange spring are directly related to the coupling type at the interface. As a reference to the hard PMA single layer, an increase in M r =A and a decrease in H c correspond to FM coupling, a consequence of the small switching field of the soft layer [25]. In contrast, a decrease in Mr =A and an increase in Hc correspond to AFM coupling resulting from frustration within the magnetic structure at the interface [24,26]. Thus, the significant changes in M r =A and Hc values imply that the coupling changes from FM to AFM when x is approximately 25%, as exhibited in the figure. To further confirm the coupling transition, MTJs based on MnGa=Fe1−x Cox with different Fe1−x Cox compositions were fabricated and tested. The MR ratios at 5 and 300 K are shown in Fig. 2(b). A transition from a normal PSV curve to an inverted curve was observed when the Co content of x is ∼ 25%, corresponding to the transition regime in M r =A and H c . In addition, maximum MR ratios of 60% at room temperature and 120% at 5 K were obtained; these values are the highest MR ratios in MnGa-based MTJs. The FeCo alloy composition dependence of the MR ratio has been reported in in-plane FeCo/ MgO/FeCo and CoFeB/MgO/CoFeB MTJs, and a maximum MR ratio was observed in both experiments when the Co content is 25% [29,30]. However, in our experiment, the MR shows a minimum value when the Co content is 20%.


In the pMTJs with MnGa=Fe1−x Cox bottom electrodes, the exchange coupling at the MnGa=Fe1−x Cox interface is an additional parameter affecting the MR ratio. Because the perpendicular magnetization of the Fe1−x Cox layers originates from the coupling and the PMA of MnGa, smaller coupling strength (Jex ) causes a larger in-plane component of Fe1−x Cox magnetization. Thus, a low MR ratio when x is equal to 20% may be related to an incomplete antiparallel configuration owing to the small J ex . The J ex for the interface of MnGa=Fe1−x Cox was estimated by fitting the MRðHÞ curves of the MTJs with MnGa=Fe1−x Cox as bottom electrodes, assuming the relative MR ratio is dependent on the angle between the directions of the magnetizations of the two FM layers in contact with the MgO barrier, MR ∝ 1- cos θ [20,27]. Here, the angle θ between directions of the magnetizations of the Fe1−x Cox and CoFeB layers can be evaluated simply based on the minimum condition of the free energy for Fe1−x Cox, given by EFeCo ¼ tFeCo ð−MFeCo H cos ϑFeCo − K FeCo cos2 ϑFeCo Þþ Jex cosðϑMnGa − ϑFeCo Þ, where tFeCo and MFeCo are the thickness and saturation magnetization of Fe1−x Cox layer. ϑFeCo;MnGa denotes the angle between the Fe1−x Cox (MnGa) magnetization and the normal film. The fits for typical PSV and inverted MRðHÞ curves shown in Figs. 1(c) and 1(d) indicate that the magnetization of Fe1−x Cox can be well described by considering the interfacial exchange coupling. Since the macrospin was used to describe the magnetizations of Fe1−x Cox and MnGa layers, there is divergence between the experimental data and fitting lines in the high field range. This difference comes from a noncoherent rotation of the magnetizations of Fe1−x Cox and MnGa layers due to their spatial distribution near the interface. Figure 3 shows the Jex values evaluated from the fitting of MRðHÞ as a function of Fe1−x Cox composition. The Jex strength of MnGa/Fe is þ2.04 erg=cm2 and shows a slight decrease as the Co content increases. With a Co content of ∼ 25%, the Jex changes sign, marking the transition to an AFM interfacial coupling. The J ex values are around 3 erg=cm2 and show a slight increase in magnitude as the Co content increases. It is noted that the transition from FM to AFM coupling when x is approximately 25% is abrupt rather than gradual. Earlier work on MnGa/Fe and MnAl/Co indicates MnGa (or MnAl) bonded with the 3d metals (Fe, Co) in Mn terminated surfaces during postannealing because of the lower interface energy [13,20]. From the point of view of interatomic exchange coupling, the 3d FM atoms with nearly filled or nearly empty bands tend to be ferromagnetically coupled, while exchange between two atoms with roughly half-filled bands tends to be coupled antiferromagnetically. Thus, the Fe is expected antiferromagnetically couples more easily with Mn in MnGa than Co because Fe is next to Mn. However, the experimental results demonstrate AFM interfacial coupling in MnGa=Fe1−x Cox for the Co-rich regime. In addition, based on an assumption of chemically random of Fe1−x Cox alloys, interatomic

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FIG. 3 (color online). J ex in the MnGa=Fe1−x Cox interface as a function of Fe1−x Cox composition. A transition from FM to AFM coupling occurs when x is approximately 25%.

exchange causes a gradual change of Jex from 2.1 to −3.2 erg=cm2 with the transition regime when x is approximately 40%. Thus, the interpretation based on interatomic exchange coupling cannot account for the experimental observations. The crystal and electronic structures of the sample must be taken into account to explain this FM/AFM transition. Bcc-textured Fe1−x Cox and L10 -MnGa alloys are 3d transition alloys exhibiting itinerant magnetism. The electron itinerancy makes the magnetic properties sensitive to the local environment. For bcc Fe1−x Cox alloys, the composition variation changes several parameters, including the magnetic moment per atoms, lattice constant, and electronic structure [31,32]. Among these parameters, the sign of the direct exchange coupling depends principally on band occupancy and then on the interatomic spacing [33]. Because the 1.5-nm Fe1−x Cox on the MnGa film has the same in-plane lattice constant as MnGa and the difference in the atomic radius of Fe and Co is less than 5%, it is reasonable to treat the distance between MnGa and Fe1−x Cox as independent of x. Here based on the band theory of ferromagnetism, we introduce a model to describe the abrupt transition in the MnGa=Fe1−x Cox interface. The coupling between 3d metals mainly depends on the Coulomb repulsion and Pauli exclusion, which are dependent on the band structure and occupancy. The density of states (DOSs) of the Fe1−x Cox and MnGa alloys with bcc and L10 structures are shown in Fig. 4(a) [16,32]. Fe1−x Cox shows two major peaks in both spin-up (↑) and spin-down (↓) channels. The ↑ and ↓ bands of bcc Fe1−x Cox alloys have a similar shape, with a shift in energy from exchange splitting. Because of the sharp peak in the Fermi level, the exchange coupling between these 3d alloys mainly depends on Coulomb repulsion [33]. However, the band of L10 -ordered MnGa shown in Fig. 4(a) has a different structure. The ↑ spin band has a sharp peak located below the Fermi level. The ↓ band shows a broad distribution of the DOS number (N) across the Fermi level. Thus, the coupling is mainly dependent on the Pauli exclusion effect, which suggests electrons near the Fermi level with antiparallel spins coupled together [33,34]. This gives a FM coupling of magnetization of

FIG. 4 (color online). (a) Illustration of the band structure of bcc Fe1−x Cox and L10 MnGa alloys. (b) Spin-up (black up triangle) and spin-down (gray down triangle) density of states at the Fermi level of Fe1−x Cox alloy with bcc structure (from Ref. [35]).

MnGa and Fe layers in the MnGa/Fe interface because the Fe ↑ band is coupled with the ↓ band of MnGa alloys near the Fermi level. As the Co content increases in the Fe1−x Cox alloy, the electron number increases, and the Fermi level gradually moves up, as illustrated in Fig. 4(a). At the same time, the DOS number at the Fermi surface [NðEF Þ] for ↑ and ↓ bands decreases and increases, respectively. For details, the typical NðEF Þ as a function of Fe1−x Cox composition calculated based on the first-principles linear combination of atomic orbitals theory are shown in Fig. 4(b) [31,32,35]. NðEF Þ↑ and NðEF Þ↓ at the Fermi level are shown with an up triangle and a down triangle, respectively. To directly show the reversal of the spin polarization, NðEF Þ↑ − NðEF Þ↓ is also drawn in the figure. A sign change occurs when the Co content of x is approximately 25%, which implies the sign reversal of spin polarization at the Fermi level. It is noted that the reversal is also corresponding to the weak or strong ferromagnetism transition according to the Slater-Pauling curve [33]. As a basic parameter for the ferromagnet, the spin polarization of the Fe1−x Cox alloy was insensitively studied in early works by electron spectrum and the Meservey-Tedrow method, and the sign reversal when x is around 25% was evidenced experimentally in Fe1−x Cox composition investigation [30,36,37]. Here the positive sign means that the majority spin in the Fermi level has the same spin direction as that of magnetization. Since the interfacial exchange coupling in MnGa=Fe1−x Cox directly depends on the majority spin of the Fe1−x Cox alloys at the Fermi level, which is reflected by the spin polarization. Thus, the reversal of the spin polarization of the Fe1−x Cox alloy when x is approximately 25% is the origin of the abrupt transition from FM to AFM of the interfacial exchange coupling. The Jex reversal in MnGa=Fe1−x Cox suggests a way to tune the

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interfacial exchange coupling via the spin polarization in FM materials with broad distributed DOS in the up or down spin channels. In summary, we experientially demonstrated an abrupt transition of interfacial exchange coupling from FM to AFM in L10 -MnGa=Fe1−x Cox perpendicularly magnetized bilayer when x is approximately 25%. The transition was explained by the spin-polarization reversal for the bcc Fe1−x Cox alloys originating from the rise in the Fermi level as the Co content increases in the Fe1−x Cox alloy. Our experiment evidenced an interfacial exchange coupling transition in MnGa=Fe1−x Cox tuned via spin polarization. In addition, the effect of J ex , a critical parameter for exchange spring, on switching field and the spin-dependent tunneling effect of MTJs with an exchange-spring-type electrode was also systematically presented. This work was partially supported by the Strategic Japanese-German Cooperative Program ASPIMATT (JST), a Grant-in-Aid for Scientific Research (No. 25600070), and the Asahi Glass Foundation. Q. M. is grateful for the fusion research grant from WPI Advanced Institute for Materials Research.

*

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