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Atsufumi Hirohata, David C. Lloyd, Takahide Kubota, Takeshi Seki, Koki Takanashi, [Hiroaki Sukegawa](https://orcid.org/0000-0002-4034-7848), [Zhenchao Wen](https://orcid.org/0000-0001-7496-1339), [Seiji Mitani](https://orcid.org/0000-0002-1348-0774), Hiroki Koizumi

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[Antiferromagnetic Films and Their Applications](https://mdr.nims.go.jp/datasets/0a9148a2-cfee-49a9-a138-ef4ceb51be39)

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Antiferromagnetic Films and Their ApplicationsIEEE MAGNETICS SOCIETY SECTIONReceived 27 September 2023, accepted 17 October 2023, date of publication 20 October 2023, date of current version 27 October 2023.Digital Object Identifier 10.1109/ACCESS.2023.3326448Antiferromagnetic Films and Their ApplicationsATSUFUMI HIROHATA 1,2, (Senior Member, IEEE), DAVID C. LLOYD 1,TAKAHIDE KUBOTA 3, (Member, IEEE), TAKESHI SEKI 4,KOKI TAKANASHI2,4,5, (Senior Member, IEEE), HIROAKI SUKEGAWA 6, (Member, IEEE),ZHENCHAO WEN6, SEIJI MITANI6,7, AND HIROKI KOIZUMI 21School of Physics, Engineering and Technology, University of York, YO10 5DD York, U.K.2Center for Science and Innovation in Spintronics, Core Research Cluster, Tohoku University, Sendai 980-8579, Japan3Advanced Spintronics Medical Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-0845, Japan4Institute for Materials Research, Tohoku University, Sendai 980-8579, Japan5Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan6Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan7Graduate School of Science and Technology, University of Tsukuba, Tsukuba 305-8577, JapanCorresponding author: Atsufumi Hirohata (atsufumi.hirohata@york.ac.uk)This work was supported in part by EU-FP7 Heusler Alloy Replacement for Iridium (HARFIR) under Grant NMP3-SL-2013-604398; inpart by the Engineering and Physical Sciences Research Council (EPSRC) under Grant EP/K03278X/1, Grant EP/M02458X/1, and GrantEP/V007211/1; in part by the Japan Science and Technology Agency (JST) Core Research for Evolutional Science and Technology(CREST) under Grant JPMJCR17J5; and in part by the Global Institute for Materials Research Tohoku (GIMRT) of Tohoku University andMinistry of Education, Culture, Sports, Science and Technology (MEXT) Initiative to Establish Next-Generation Novel Integrated CircuitsCenters (X-NICS) under Grant JPJ011438.ABSTRACT Spintronic devices are expected to replace the recent nanoelectronic memories and sensors dueto their efficiency in energy consumption and functionality with scalability. To date, spintronic devices,namely magnetoresistive junctions, employ ferromagnetic materials by storing information bits as theirmagnetization directions. However, in order to achieve further miniaturization with maintaining and/orimproving their efficiency and functionality, new materials development is required: 1) increase in spinpolarization of a ferromagnet or 2) replacement of a ferromagnet by an antiferromagnet. Antiferromagneticmaterials have been used to induce an exchange bias to the neighboring ferromagnet but they have recentlybeen found to demonstrate a 100% spin-polarized electrical current, up to THz oscillation and topologicaleffects. In this review, the recent development of three types of antiferromagnets is summarized with offeringtheir future perspectives towards device applications.INDEX TERMS Antiferromagnetic materials, Hall effect, magnetoresistance, spintronics, spin polarizedtransport.I. INTRODUCTIONSpintronics is one of the emerging fields in condensedmatter physics in the view of replacing the recent nanoelec-tronic devices by improving their efficiency and functionality[1], [2]. In spintronic devices, further improvements arerequired to continue miniaturization to be comparable withthe Si-based semiconductor technology. With a ferromagnet(FM), the miniaturization may induce edge domains andcross-talk between junctions via stray fields, which may pre-vent fast and reliable operation. On the other hand, usingan antiferromagnet (AF), these obstacles can be avoided.The associate editor coordinating the review of this manuscript andapproving it for publication was Montserrat Rivas.Namely for the reduction in power consumption, highly effi-cient generation and detection of a spin-polarized electricalcurrent need to be developed using the spin-orbit torque, spincaloritronic and topological effects. AF materials and theirproperties were initially investigated by Néel [3] and havebeen utilized to exchange couple with the neighboring FMmagnetization [4]. This can be measured as a shift in thecorresponding magnetization curve as known as an exchangebias (EB) field Hex. Hex has been used to pin one of theFM magnetizations in a FM/non-magnet (NM)/FM trilayer,i.e., a spin-valve structure [5]. This is a basic structure for aread head of a hard disk drive (HDD). By replacing the NMlayer with an insulating barrier, a magnetic tunnel junction(MTJ) has been fabricated in a similar manner, which hasVOLUME 11, 2023 2023 The Authors. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.For more information, see https://creativecommons.org/licenses/by-nc-nd/4.0/ 117443https://orcid.org/0000-0001-9107-2330https://orcid.org/0000-0002-5774-214Xhttps://orcid.org/0000-0002-4981-1977https://orcid.org/0000-0003-3195-7051https://orcid.org/0000-0002-4034-7848https://orcid.org/0000-0002-2293-9428A. Hirohata et al.: Antiferromagnetic Films and Their Applicationsbeen commonly used as the latest HDD read head and a bitcell of a magnetic random access memory (MRAM). Forthese applications, an IrMn3 alloy has been predominantlyemployed due to its corrosion resistance and robustnessagainst nanofabrication processes.Recently, the studies on AF materials and devices havebeen revitalized after the demonstration of spin polariza-tion by simply flowing an electrical current in an AF layer,which has led to AF spintronics [6], [7], [8]. AF materialshave also been demonstrated to generate a spin current atTHz frequency [9]. Additionally with the exchange couplingwith a FM layer attached, interfacial interactions includingDzyaloshinskii-Moriya interaction (DMI) [10], [11] can becontrolled, resulting in the formation of a magnetic skyrmion[12], a semimetal [13] and topological states [14]. Thesephenomena can offer further improvement in the performanceand functionality of spintronic devices.In parallel, a search for new AF materials which can main-tain their AF properties with miniaturisation especially in afilm form has been made intensively. There are large vari-ety of AF materials investigated as summarized in Table 1:(i) cubic and (ii) hexagonal structures as shown in Figs. 1and 2, respectively. The cubic AF includes Group III-VI asappeared in the Periodic Table (e.g., FeO, CoO andNiO [15]),II-VI (e.g., MnO [16],MnS2, MnSe2 andMnTe2 [16]), GroupIII (e.g., Cr, FeMn [17] and NiMn [18] ) and III-V (e.g., FeN[19] andMnN [20]) compounds as well as ternary/quaternaryHeusler alloys (with some atomic substitutions) [21]. Thehexagonal AF contains Group III (e.g., IrMn3 [22]), I-III-VI(e.g., CuFeO2 [23], CuFeS2 [24], CuFeSe2 [25] and CuFeTe2[26]), tetragonal AF (e.g., CuMnAs [27], [28]), antiperovskitemanganites [29], [30], [31], [32], [33] and binary Heusleralloys (with some atomic substitutions).The cubic Heusler alloys crystallize in (i) L21 phase withX2YZ composition (full-Heusler) and (ii) C1b phase withXYZ composition (half-Heusler) [34]. The half-Heusler alloyshave an X -vacancy in the unit cell, making them susceptibleto atomic displacement. Even for the full-Heusler alloys,the perfectly-ordered L21 phase can be deformed into theB2 phase by atomically displacing Y -Z elements, the D03phase by X -Y displacements and the A2 phase by randomlyexchanging X -Y -Z elements. We have recently found thatRu2YZ [35], Ni2YZ [36], [37] and Mn2YZ [38], [39] Heusleralloys exhibit AF behavior in their L21, B2 and A2 crystallineordering phases. By attaching a FM Fe layer to these AFlayers, Hex of up to 600 Oe at 100 K, 90 Oe at 100 Kand 30 Oe at 100 K for Ru2MnGe, Ni2MnAl and Mn2Val,respectively, have been found. Mn2Val is found to maintainits AF properties at room temperature (RT). These differencesare found to be induced by the AF alignment of spin momentsat the Y site in unique ordered phases. In the ordered L21 typeRu2MnZ (Z = Si, Ge, Sn and Sb), the complex AF order-ing (2nd type) is a consequence of the frustrated exchangeinteraction between the Mn atoms. It is concluded that Néeltemperature TN sharply depends on the Z element and thatTN in Ru2MnGe can be increased by avoiding the disorder inthe Mn-Z sub-lattice. For Ni2MnAl, the (checkerboard-like)AF order only exists in the chemically disordered B2 phasedue to the large AF nearest neighbor Mn-Mn interactionas schematically shown in Fig. 1. Decreasing the atomicdisorder in the Mn-Al sublattice leads to non-zero total mag-netization (i.e., ferrimagnet; FI). The excess of Mn or Nidoes not improve the anisotropy of the AF state. From thedevice application point of view,Mn-basedAFHeusler alloysare ideal due to their robustness against atomic disordering,especially at the interfaces to neighboring layers.FIGURE 1. Schematic crystalline and spin structures of the pseudo-B2 (Iand II) phases for Ni2MnAl created by VESTA [40].As Mn-based AF Heusler alloys, binary Heusler alloyswith perpendicular anisotropy, such as the D019 Mn3Z(Z = Ga, Ge and Sn) have been studied to determinetheir structural and magnetic properties. In perpendicularlyanisotropic AF films, the effect of AF-coupled ‘‘domains’’can be minimized in the in-plane electron scattering as shownschematically in Fig. 2. The Mn3Ge binary alloy is takenas an example. It allows two stable crystalline structures ofthe tetragonal D022 and hexagonal D019 structures which isdistorted from the basic full-Heusler L21 structures alongthe <001> and <111> directions, respectively [41]. As aresult of different crystalline structures, the D022 or D019structure exhibits different magnetic anisotropy such as aFI with perpendicular magnetic anisotropy and low satura-tion magnetization [42] or AF with noncollinear magneticmoments in which the EB effect [43] appears. Recently,D019Mn2.8Ga1.2 films have been grown with exhibiting Hex upto 430 Oe at 120 K [44], which is almost comparable withrecent reports on IrMn with Hex = 688 Oe [45] and MnN(Hex = 3.6 kOe but with less corrosion resistance) [46].FIGURE 2. Schematic crystalline and spin structures of the D019 phase forMn3Ga from the top and side views created by VESTA [40].117444 VOLUME 11, 2023A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsTABLE 1. List of major AF materials and their properties. After Refs. [6] and [21]. The bulk and calculate (calc.) values are also included as references.VOLUME 11, 2023 117445A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsTABLE 1. (Continued.) List of major AF materials and their properties. After Refs. [6] and [21]. The bulk and calculate (calc.) values are also included asreferences.The major challenge in the development of AF Heusleralloys is that there are over 3000 known Heusler alloy com-positions [96]. The key parameter in an AF order is thespacing between planes where the magnetic spins are orderedferromagnetically in the (001) plane as shown in Figs. 1 and 2(with slight canting from the plane). Hence it is critical toengineer the composition, so that the right spacing is achievedwith typically B2 or D019 ordering. This can be confirmedusing our recently developed Q-factor analysis as shown inFig. 3 [37]. The Q-factor is defined as the peak intensitymeasured by X-ray diffraction (XRD) divided by full widthat half maximum, which offers a very simple measure toevaluate the crystallinity of Heusler alloys and beyond.II. ELECTROMAGNETIC CHARACTERISATIONSFor the characterization of AF materials, the followingtechniques have been traditionally used: Magnetization mea-surements with and without a FM layer attached, trans-port measurements with and without a magnetic field,and synchrotron-based measurements. The former two117446 VOLUME 11, 2023A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsFIGURE 3. Calculated Q-factors and the corresponding crystallization ofFe72.1V14.4Al13.5/Ru samples annealed at elevating temperatures.techniques are relevant for macroscopic analysis, whilethe last one is sensitive to microscopic characterization asdetailed below and Fig. 4.A. MAGNETIC SUSCEPTIBILITYIn a simplified picture, AF can be treated as two sets of FM-coupled magnetic moments antiparallelly aligned with eachother, MA = – MB (see Fig. 1). The temperature dependenceof this antiparallel alignment can be calculated similar to thatof a ferromagnet with the parallel alignment. The antiparallelalignment is stable up to the characteristic temperature, TN,above which the alignment becomes random due to thermalfluctuation, typically leading to paramagnetic phase [97].This magnetic phase transition becomes apparent by plottingthe temperature dependence of magnetic susceptibility χ .For a single crystal, by applying a magnetic field along themoments, χ increases linearly with increasing temperature T .By applying a field perpendicular to the moments, χ staysconstant. For polycrystalline AF, χ (T ) follows between thesetwo cases. Above TN, χ decreases almost inversely propor-tional with T , forming a kink in χ (T ) at TN (see Fig. 4). Thismethod can be useful for a bulk material to generate sufficientchange in the magnetic moments, especially in a single phase.B. EXCHANGE BIASAs AF does not produce intrinsic magnetization macro-scopically, no signal can be detected by a magnetizationmeasurement. Instead, by attaching a FM layer to inducean EB at the interface, a shift of the corresponding FMmagnetization curve (Hex) can be measured, the amplitudeof which is proportional to the interfacial exchange couplingbetween AF and FM. This is particularly useful for AF films.Hex can be evaluated by the York Protocol [98]. In an AF/FMbilayer, AF is first set at the setting temperature TSET for90 min., which is above TN of AF but below the Curietemperature TC of the FM film. The bilayer is then cooledto the thermally activated temperature TNA, followed by theheating to the activation temperature TACT for 30min. and themagnetization measurement at TNA. In the activation periodany activated FM grains reverse their magnetic moments totheir originally set direction. This procedure removes the firstloop training effect and any thermal activation that may occurduring the temperature rise and fall. In polycrystalline bilay-ers, individual grains have their own blocking temperatureTB, which can be determined by increasing TACT until theloop shift becomes zero, which represents the median valueof TB (< TB >). < TB > satisfies the reversed AF volume tobe the same with that of the initially set volume [98], whichcan be an indicative measure of TN.C. ELECTRICAL RESISTIVITY AND MAGNETORESISTANCESimilarly, the temperature dependence of electrical resistiv-ity ρ(T ) exhibits a kink in the gradient [21]. Below TN,antiparallelly-coupled magnetic moments in single-crystalAF can suppress electron scattering. Above TN, however,the moment alignment becomes random and changes thecorresponding resistivity. It should be noted that the changesin the resistivity are found to be 11% maximum, which canbe smaller than that due to electron scattering at grain bound-aries by over three orders. This is a powerful technique todetermine TN for epitaxial or highly-textured films as well.By applying an external magnetic field during the transportmeasurements, similar changes in anisotropic magnetoresis-tance (AMR) [99] and tunneling AMR (TAMR) [100] can beused to determine TN due to the magnetic phase transforma-tion. TAMRwas experimentally demonstrated by Gould et al.with a junction consisting of Ga0.94Mn0.06As/Al-O/Ti [101].A TAMR ratio was found to increase using an AF layer up to0.15% with a IrMn/MgO/Pt multilayer at room temperature[102]. Recently, AF materials has also been used for TAMR,demonstrating 10% TAMR in IrMn/MgO/Ta junctions [103]and 20% TAMR using CsCl-ordered FeRh magnetic phasetransition in FeRh/MgO/FeRh junctions [100].D. X-RAY MAGNETIC LINEAR DICHROISMFor microscopic evaluation, synchrotron radiation can beused. X-raymagnetic linear dichroism (XMLD) [104] utilizesa pair of linearly polarized soft X-ray with perpendicularpolarization. XMLD signals are proportional to the averagevalue of the magnetic moment squared in a domain <M2>.For an AF material, < M > is zero asMA = –MB within anAF domain but <M2> is a finite value, allowing AF domainimaging. Such domain imaging requires a relatively large uni-form domain (>a fewµm) due to the spatial resolution, whichmakes it difficult to be used for AF films. Recently, XMLDhas been combined with photoemission electron microscopy(PEEM) imaging to achieve sub-µm resolution [105].E. POLARISED NEUTRON REFLECTIVITYAnother synchrotron-based technique for the characteriza-tion of AF materials is polarized neutron reflectivity (PNR).PNR can determine magnetic properties of bulk and layeredVOLUME 11, 2023 117447A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsFIGURE 4. List of characterization techniques for AF materials and devices. For AF only, magnetic susceptibility, electricalresistivity and (tunneling) anisotropic magnetoresistance, X-ray magnetic linear dichroism, and polarized neutronreflectivity can be used for characterization as detailed in Sections II-A, C, D and E. By attaching FM and a heavy metal(HM), exchange bias, and (inverse) spin Hall and spin caloritronic effect can indirectly show the AF properties as discussedin Sections II-B and III, respectively. Using a trilayer consisting of AF/NM/HM, spin pumping and ferromagnetic resonancecan be used for characterization as described in Section IV.materials [106]. Due to the magnetic moment of neutronbeam interacting with magnetic materials to be imaged, notonly layer structures, such as thickness, density, composi-tion and interfacial roughness as similar to X-ray reflectivity(XRR), but also in-plane magnetic moments can be deter-mined. PNR has higher accuracy in a shorter scanning period(<1 min.). The latter magnetic information can be obtainedby detecting the neutron reflection with its spins interactedwith those in an AF and/or FM layers.III. SPIN-ORBIT TORQUEIn AF spintronics, three key phenomena can be used fordevice applications: spin-orbit torque (SOT), spin dynam-ics and interfacial effects as schematically listed in Fig. 5.These phenomena are discussed in the following sections.In this section, (inverse) spin Hall effects and spin caloritroniceffects are reviewed, both of which are induced by SOT.A. ANOMALOUS HALL AND SPIN HALL EFFECTSThe Hall effect is induced by the Lorentz force under anexternal magnetic field (ordinary Hall effect). In a magneticmaterial, anomalous Hall effect (AHE) can be induced dueto the spin-orbit interaction, where an effective magneticfield exists by the presence of an intrinsic magnetization.Large AHE was reported in Mn3Sn [107]. This is due to aweak ferromagnetism induced in the non-colinear AF align-ment (∼0.002 µB/Mn [87]). Note that additional topologicalcontribution to the Hall signals may need to be consid-ered in a noncolinear magnet [108]. This was employed toFIGURE 5. Concept of antiferromagnetic spintronics, showing spin Halleffects, spin caloritronics, THz oscillation, magnetic skyrmions,topological effects, Heusler alloys and exchange bias (from left to right).develop an AF memory with the writing capability at THzfrequency [109].Spin accumulation in a semiconductor was theoreticallypredicted by Averikev and Dyakonov under the flow of anelectron charge current, introducing the resultant spin currentperpendicular to the charge current [110]. This is induced byspin-dependent scattering by an impurity and intrinsic spin-orbit interactions of the material. Averikev and Dyakonovalso proposed the inverse effect by aligning the spins byan electromagnetic wave to generate a charge current [111].These predictions were revisited by Hirsch and named asspin Hall and inverse spin Hall effects (SHE and ISHE),respectively (see Fig. 6) [112]. The relationship between thecharge and spin currents can be defined as(Spin current) = θSH × (Charge current), (1)117448 VOLUME 11, 2023A. Hirohata et al.: Antiferromagnetic Films and Their Applicationswhere the coefficient θSH is the spin Hall angle specificto a material used. The corresponding Hamiltonian can bedetermined asH =ℏ2k22m+ λSO (σ × k) , (2)where ℏ: Planck constant divided by 2π , k: wave vector, m:electron mass, λSO: spin-orbit coupling constant and σ : spinmatrix.Experimentally, magneto-optical Kerr effect (MOKE)imaging was used to detect the spin accumulation at the edgesof GaAs at 30 K, resulting in a spin current of the order of10 nA/µm2 [113]. In a similar system, a spin current was alsoelectrically detected [114].FIGURE 6. Schematic diagram of the (a) SHE and (b) ISHE measurements.SHE and ISHE do not require an external magnetic fieldunlike the original Hall effect. When a large magnetic fieldis applied perpendicular to the material, the accumulatedspins start to precess and diminish SHE and ISHE. Thisinduces the corresponding resistance changes with respectto the field, spin Hall magnetoresistance (SHMR) [115].SHMR was experimentally measured in Y3Fe5O12 (YIG)/Ptbilayer [116].In AF materials, large (I)SHE signal has been shown, e.g.,(5.3 ± 2.4)% for Mn3Sn [117], as listed in Table 2. Thesesignals are induced by the spin Hall angles θSH as listed inTable 2, which are almost one order of magnitude smallerthan those for heavy-metals, e.g., −35 (−50)% for W [118](WOx [119]) and 5.6% for Pt [120].B. SPIN CALORITRONIC EFFECTSSpin caloritronic effects, namely spin Seebeck and Nernsteffects (SSE and SNE, respectively), can induce a spin currentas originally demonstrated in Ni80Fe20/Pt [133] and YIG/Pt(see Fig. 7) [134]. InAFmaterials, a pioneeringworkwas per-formed with a Cr2O3/Pt bilayer by Seki et al. [135] as listedin Table 3. These effects have been intensively investigatedfor energy harvesting. For SSE, the figure of merit ZT can bedetermined as [136]ZT (SSE) =σS2κT , (3)where T : temperature, σ : electrical conductivity, S: Seebeckcoefficient and κ: thermal conductivity. ZT > 1 is needed forpractical device applications. Similarly for SNE, ZT can beobtained using the Nernst coefficient N as follows:ZT (SNE) =σN 2κT . (4)An anomalous Nernst effect (ANE) is normally propor-tional to the intrinsic magnetization of the material underTABLE 2. List of spin Hall angle and SMR reported for AF and Weylmaterials. After Ref. [7]. SP, SSE and FMR represent spin pumping, spinSeebeck effect and ferromagnetic resonance measurements, respectively.investigation as similar to AHE. Even so, a Nernst signal of∼0.35 µV/K was reported at RT [135], which is over twoorders of magnitude greater than that expected from the weakferromagnetism [87]. This increase is due to the fact thatthe transverse thermoelectric conductivity is determined bythe Berry curvature in the vicinity of the Fermi level (EF)offering adiabatic electron motion, while the anomalous HallVOLUME 11, 2023 117449A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsFIGURE 7. Schematic diagram of the (a) SSE and (b) SNE measurements.TABLE 3. List of spin caloritronic properties reported for AF materials.conductivity is defined as the sum of the Berry curvature forall the occupied bands. Hence, a Weyl metal can be advan-tageous for spin caloritronic applications due to the uniqueBerry curvature at Weyl points near EF [see Fig. 10(b)] Thedetailedmodel to calculate the corresponding spin current canbe found in Ref. [137].IV. DYNAMICSA. SPIN PUMPINGIn a FM/AF bilayer, a spin current can be introduced byspin pumping (SP) from FM to AF by precessing the FMmagnetization (see Fig. 8). The spin current may be dampedin AF and may reduce the reflected spin current into FM,which accordingly increases the damping constant of FM.Using ISHE, the spin current JSPS satisfies the followingrelationship [145].VISHE = wRθSH(2eℏ)JSPS , (5)where w and R are the width and resistance of the bilayeredHall bar and e is the electron charge. VISHE takes the max-imum at the resonant magnetic field, where the maximumprecession is achieved and the resulting maximum JSPS isintroduced to AF. Using this condition, θSH can be estimatedas listed in Table 2. This is sensitive to a small spin currentFIGURE 8. Schematic diagram of the spin pumping (SP) measurement ina trilayered structure with ferromagnet (FM)/nonmagnet (NM)/antiferromagnet (AF). Spin current (Js) generated by magnetizationprecession is converted to charge current (Jc) via inverse spin Hall effectin the AF.to be introduced by increasing R of the Hall bar sample. Thistechnique can also be applied to characterize a spin currentgenerated optically and thermally.B. FERROMAGNETIC RESONANCESimilar to SP, a FM/AF bilayer is used for ferromag-netic resonance (FMR) as schematically shown in Fig. 9(a).A high-frequency current I (typically at 10 GHz) is appliedto a FM/AF bilayer to generate an in-plane radio-frequency(rf) magnetic field perpendicular to the current in the AFlayer accompanying with the spin current from the AF layer.This experimental technique is called ‘‘spin-torque FMR(ST-FMR)’’, which was originally employed to evaluate θSHin the bilayer consisting of FM and NM [146], and haswidely been used for a variety of materials [147], [148].The in-plane rf magnetic field exerts a torque and inducesthe magnetization precession in the FM layer. At the sametime, a spin current can be generated by SHE in the AFand/or at the FM/AF interface, which diffusively flows intothe FM layer and exerts a torque perpendicular to the layer.Here, the in-plane torque is in-phase with the high-frequencycurrent, while the perpendicular spin-current torque is shiftedbyπ /2 from the current frequency. By fitting a FMR spectrumto antisymmetric and symmetric contributions due to thein-plane and perpendicular torques, respectively, JSFMRcanbe estimated from the latter contribution. VFMRsym andVFMRantisym are given as follows [146] and [149]:V symFMR =14dRdθγ I cos θ2π(df/dH)H=H0×[ℏJFMRS2eµ0MStFM1(H − H0)2](6)V antisymFMR =14dRdθγ I cos θ2π(df/dH)H=H0×[JCtAF2(1+eµ0MSℏ)1/2 (H − H0)12 + (H − H0)2](7)117450 VOLUME 11, 2023A. Hirohata et al.: Antiferromagnetic Films and Their Applicationswhere dR/dθ : resistance change by the precession, θ : anglebetween the external magnetic field (H ) and I , γ : gyromag-netic constant, f : frequency of I ,H0: FMRfield,µ0: magneticpermittivity in a vacuum (4π × 10−7 H/m), MS: saturationmagnetisation of FM, tFM: FM thickness, 1 is half width ofhalf maximum of FMR spectrum, tAF: AF thickness and JC:charge current.Using the symmetric and antisymmetric FMR spectrumcomponents, θSH can be calculated as follows [146]:θSH =JFMRSJC= tFMtAFV symFMR2VFMRantisymeµ0MSℏ√1 +(4πM effH)(8)The literature values of θSH measured by ST-FMR are listedin Table 2. An example of ST-FMR for the FM/AF bilayeris shown in Fig. 9, in which the Ni81Fe19 and Ir22Mn78(IrMn3.55) were chosen as FM and AF materials, respec-tively. Fig. 9(b) displays the optical microscope image of thecoplanar-waveguide-shaped device with the sputter depositedNi81Fe19 (3 nm)/IrMn3.55 (10 nm) bilayer together with themeasurement setup for ST-FMR. The representative ST-FMRspectra are shown in Fig. 9(c), in which the rf current with thefrequency of 8 GHz was applied and the in-plane H angleswere set at 45◦ and 225◦. The spectra were well fitted withthe summation of antisymmetric and symmetric Lorentzianfunctions. Using Eq. (8), θSH was evaluated to be 3% for theIrMn3.55 alloy.FIGURE 9. (a) Schematic diagram of the spin torque ferromagneticresonance (ST-FMR) measurement in a bilayer with ferromagnet (FM)/antiferromagnet (AF). (b) optical microscope image of thecoplanar-waveguide-shaped device together with the measurementsetup. (c) ST-FMR spectra for the Ni81Fe19 (3 nm)/IrMn3.55 (10 nm)bilayer. The rf current with the frequency of 8 GHz was applied, and thein-plane magnetic field angles were set at 45◦ and 225◦.C. THz OSCILLATIONBy overlaying a direct current (dc) on I in the FMR techniqueas described in Section IV-B, an effective damping constantof FMMeff can be modified as1αeff =sin θ(H +Meff/2) ℏJFMRS2eµ0MStFM. (9)TABLE 4. List of spin dynamics reported for AF materials.FIGURE 10. Schematic band diagram of the (a) Dirac and (b) Weylsemimetals.By controlling I , the in-plane and perpendicular torques canbe cancelled out, resulting in 1αeff = 0. This allows oscilla-tion of the FM magnetisation.THz oscillation can be observed in AF due to strongexchange interactions between two sublattices with MA andMB [150], [151]. In NiO, the first demonstration of THz oscil-lation was achieved [151]. Since then, a great deal of researchhas been made to achieve higher oscillation frequency in AFmaterials as listed in Table 4.V. TOPOLOGICAL EFFECTS AND BEYONDTopological and interfacial phenomena in AF materials havebeen intensively investigated recently [154]. For example,Dirac and Weyl semimetals can be formed with AF nature asschematically shown in Fig. 10. The Dirac semimetals form apoint connection between the valence and conduction bandsat EF, which is known as the Dirac point. The Dirac pointcan demonstrate the ideal conductance of 2G0 (G0 = e2/h,where e is the electron charge and h is the Planck con-stant). The Weyl semimetals contain the chirality at the Diracpoint as schematically shown in Fig. 10(b). Weyl semimetalsshow large AHE, e.g., anomalous Hall angle and conduc-tivity of 0.23 and 60 �−1cm−1 for GdPtBi [155], 0.11 and1258.9 for Co2MnGa, and 0.08 and 1421.6 for Co2MnAl,respectively [156]. The chiral topological semimetal CoSishows a small spin Hall angle of ∼ 0.03 due to the uniqueelectronic structure [157].A. MAGNETIC SKYRMIONSNéel-type magnetic skyrmions were stabilized by attachingIrMn3 underneath Co20Fe60B20 at RT [12], which has beensupported by theoretical calculations [158]. According toVOLUME 11, 2023 117451A. Hirohata et al.: Antiferromagnetic Films and Their Applicationstheoretical prediction, skyrmions in AF can be displacedfaster by a smaller critical current density (106∼107 A/cm2)than those in conventional FM materials [159], [160].Although AF skyrmions were stabilized and imaged in asynthetic AF [161] and FI [162], no report has been madeto date in AF materials.B. TOPOLOGICAL EFFECTSSHE can be induced in an individual layer in two-dimensionalMnBi2Te4, achieving layer Hall effect with up and downspins to be generated at the edges of the top and bottom layersat 1.7 K [163]. This is induced by the layer-locked Berrycurvature, which can open a new research field of topologicalAF spintronics.C. ORBITAL FERRIMAGNETISM‘‘Orbital Ferrimagnetism’’ was firstly termed for FI CoMnO3by Bozorth et al. [164]. Orbital FI is defined as a systemwhere the net magnetic moment is only attributed to theorbital magnetic momentum. To date, CoMnO3 is the onlymaterial known to exhibit orbital FI, consisting of Co2+and Mn4+, which possess S = 3/2, respectively. Since thesecations are antiferromagnetically coupled, the spin angularmomenta cancel each other out. This causes the spinmomentato be compensated, while the orbital angular momentum ofCo2+ in the crystal field to be conserved. Consequently, thenet magnetic moment is proportional to the orbital angu-lar momentum [165], [166]. In other words, an orbital FIhas the properties of antiferroic-spin momenta and ferroic-orbital momentum. Therefore, one can expect that the orbitalFI would become a bridge between AF spintronics andorbitronics.D. ALTERMAGNETISMRecently, a new class of collinear antiferromagnet is the-oretically predicted by classifying magnetic material basedon spin-group formalism, which is termed as altermagnetism[167], [168]. Altermagnetism is induced by the anisotropicband structure that non-degenerate but equally populatedspin-up and spin-down energy isosurfaces. Altermagnetismis predicted to exhibit various spintronics phenomena [169],[170], [171] such as AHE and TMR similar to AF, and isexpected to be a functional material in a novel region of anti-ferromagnetic spintronics. For example, field-free switchingwas demonstrated in the heterostructure using RuO2 [172],[173]. Some other altermagnets, e.g., MnTe [174], [175] andMn5Si3 [176], have also been predicted and characterized.VI. CONCLUSION AND FUTURE PERSPECTIVESWe have reviewed the recent development and characteri-zation of AF materials and devices. In general, hexagonal(or non-colinear) AF exhibits larger magnetic anisotropy con-stants of the order of 10 Merg/cm3, while cubic AF showssmaller constants as summarized in Fig. 11. They corre-spond to the spin Hall angle θSH and spin Seebeck/NernstFIGURE 11. Correlations between the magnetic anisotropy constant andthe spin Hall angle θSH (closed symbols) as well as the spinSeebeck/Nernst coefficient (open symbols). After [190], new data addedon Mn3Ga [132] and NiO [137]. Open and closed symbols represent cubicand non-colinear spin configurations, respectively. Blue and red datashow θSH and spin Seebeck and Nernst coefficients, respectively. Targetranges are highlighted as broad lines in the corresponding axes.TABLE 5. List of abbreviations used in this review.coefficients. In order to develop efficient AF spintronicdevices, higher anisotropy without atomic disordering isneeded as highlighted by broad lines. Such materials canbe used for an electric-field controlled device [177], SOT-MRAM [178] and energy harvesting [136]. Hall memoryconcept has also demonstrated with CuMnAs [109] andMn2Au [179].117452 VOLUME 11, 2023A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsBy antiferromagnetically coupling two FM layers througha non-magnetic spacer, a synthetic AF (SyAF) can be formed,which has been commonly used to pin the magnetizationof the neighboring FM layer in perpendicularly-magnetizedMRAM [180]. Such SyAF has also been used to demonstratememory operation [181], [182], [183] similar to AF as dis-cussed in Sec. III-A. SyAF can offer broad controllability inthe quantization axis in the system, offering design flexibilityin AF spintronic devices.Recently, AF-based magnetic tunnel junctions (MTJs)have been fabricated by showing about a 100% TMRratio with Mn3Pt/MgO/Mn3Pt [184] and 2% ratio withMn3Sn/MgO/Mn3Sn [185]. By enhancing the TMR ratio,such an AF-based MTJ may offer a new architecture forspintronic devices. 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Mater., vol. 2, no. 9, Sep. 2018, Art. no. 094404, doi:10.1103/PhysRevMaterials.2.094404.[188] J. Wang, T. Seki, Y.-C. Lau, Y. K. Takahashi, and K. Takanashi, ‘‘Ori-gin of magnetic anisotropy, role of induced magnetic moment, andall-optical magnetization switching for Co100-x Gdx /Pt multilayers,’’APL Mater., vol. 9, no. 6, Jun. 2021, Art. no. 061110, doi: 10.1063/5.0050985.[189] L. Šmejkal, J. Sinova, and T. Jungwirth, ‘‘Emerging research landscapeof altermagnetism,’’ 2022, arXiv:2204.10844.[190] A. Hirohata andD. C. Lloyd, ‘‘Heusler alloys for metal spintronics,’’MRSBull., vol. 47, no. 6, pp. 593–599, Jun. 2022, doi: 10.1557/s43577-022-00350-1.ATSUFUMI HIROHATA (Senior Member, IEEE)was born in Tokyo, Japan, in 1971. He received theB.Sc. andM.Sc. degrees in physics fromKeio Uni-versity, Japan, in 1995 and 1997, respectively, andthe Ph.D. degree in physics from the University ofCambridge, U.K., in 2001.From 2001 to 2002, he was a Postdoctoral Asso-ciate with The Cavendish Laboratory, Universityof Cambridge. In 2002, he moved to the FrancisBitter Magnet Laboratory, Massachusetts Instituteof Technology, Cambridge, MA, USA, as a Postdoctoral Associate. He thenbecame a Researcher with the Department of Materials, Tohoku University,Japan, in 2003, and the Frontier Research System, RIKEN, Japan, in 2005.He became a Lecturer with the Department of Electronics (now Departmentof Electronic Engineering), University of York, in 2007, and was promotedto a Reader, a Professor, and a Senior Professor, in 2011, 2014, and 2017,respectively. From 2015 to 2016, he was a Guest Professor (Global) withKeio University. From 2009 and 2017, he was a Visiting Professor withTohoku University. Since October 2023, he is a Professor at the Center forScience and Innovation in Spintronics, Tohoku University, Japan. He editedthree books and published more than 160 articles and 35 inventions. Hisresearch interests include spintronics and magnetic materials.Prof. Hirohata is the President-Elect of the IEEE Magnetics Society and amember of the Administrative Committee of the Conference on Magnetismand Magnetic Materials. He was awarded an Industry Fellowship by theRoyal Society, in 2013. He was a recipient of an Outstanding PresentationAward, in 2006, and a Young Scientist Award from TheMagnetics Society ofJapan, in 2005. He gave a Wohlfarth Memorial Lecture, in 2014. He is also aDeputy Editor of Science and Technology of Advanced Materials, an Editorof Journal of Magnetism and Magnetic Materials, Spin and Magnetochem-istry, an Associate Editor of Frontiers in Physics, and an Editorial BoardMember of Journal of Physics D: Applied Physics.DAVID C. LLOYD was born in 1991. He receivedthe M.Phys., M.Sc., and Ph.D. degrees in physicsfrom the University of York, U.K., in 2013, 2015,and 2019, respectively.From 2019 to 2020, he was with the OkinawaInstitute of Science and Technology Graduate Uni-versity, Japan, as a Postdoctoral Fellow. In 2021,he returned to the University of York as a Postdoc-toral Research Associate. His research interestsinclude in-situ electron microscopy, characteriza-tion of nanoscale materials, and the growth of functional magnetic materials.TAKAHIDE KUBOTA (Member, IEEE) was bornin Chiba, Japan, in 1982. He received the B.Eng.,M.Eng., and Ph.D. degrees from Tohoku Univer-sity, Japan, in 2005, 2007, and 2010, respectively.From 2010 to 2013, he was a Research Asso-ciate with the Advanced Institute for MaterialsResearch, Tohoku University. In 2013, he movedto the Institute for Materials Research, TohokuUniversity, as an Assistant Professor. Since 2022,he has been a specially appointed Associate Pro-fessor with the Advanced SpintronicsMedical Engineering, Graduate Schoolof Engineering, Tohoku University. His research interests include spintron-ics and magnetic materials. He was awarded the 28th Tokin Science andTechnology Award and the Special Award of Tokin Foundation by the TokinFoundation for Advancement of Science and Technology, in 2018, and the58th Harada Young Research Award by the Honda Memorial Foundation,Japan, in 2018.117458 VOLUME 11, 2023http://dx.doi.org/10.1103/PhysRevLett.130.036702http://dx.doi.org/10.1002/adma.201905603http://dx.doi.org/10.1109/TMAG.2021.3078583http://dx.doi.org/10.1038/s41467-017-02780-xhttp://dx.doi.org/10.1103/PhysRevLett.121.167202http://dx.doi.org/10.1103/PhysRevLett.121.167202http://dx.doi.org/10.1103/PhysRevB.101.224413http://dx.doi.org/10.1063/5.0140328http://dx.doi.org/10.1038/s41586-022-05461-yhttp://dx.doi.org/10.1038/s41586-022-05463-whttp://dx.doi.org/10.1103/PhysRevLett.128.197201http://dx.doi.org/10.1103/PhysRevLett.128.197201http://dx.doi.org/10.1103/PhysRevMaterials.2.094404http://dx.doi.org/10.1063/5.0050985http://dx.doi.org/10.1063/5.0050985http://dx.doi.org/10.1557/s43577-022-00350-1http://dx.doi.org/10.1557/s43577-022-00350-1A. Hirohata et al.: Antiferromagnetic Films and Their ApplicationsTAKESHI SEKI was born in Shizuoka, Japan,in 1980. He received the B.Eng., M.Eng.,and Ph.D. degrees from Tohoku University,Japan, in 2002, 2003, and 2006, respectively.From 2006 to 2008, he was a PostdoctoralResearcher with the Institute for MaterialsResearch, Tohoku University. Then, he movedto the Graduate School of Engineering Sci-ence, Osaka University, Japan, as a PostdoctoralResearcher. In 2010, he became an Assistant Pro-fessor with the Institute for Materials Research, Tohoku University, and waspromoted to an Associate Professor, in 2016. His research interests includethe materials development for spintronics and nanomagnetism, the physicsof spin transfer phenomena and spin current, and control of magnetizationreversal and spin dynamics. Hewas awarded the Japan Institute ofMetals andMaterials 22nd Young Researcher Award, in 2012, The Magnetics Societyof Japan Outstanding Research Award, in 2016, the 40th Honda MemorialYoung Researcher Award, in 2019, and the Commendation for Science andTechnology by the Minister of Education, Culture, Sports, Science andTechnology, The Young Scientists’ Prize, in 2019.KOKI TAKANASHI (Senior Member, IEEE) wasborn in Tokyo, Japan, in 1958. He received theB.S., M.S., and Ph.D. degrees in physics from TheUniversity of Tokyo. From 1994 to 1995, he wasan Alexander von Humboldt Research Fellowwith Forschunszentrum Jülich, Germany. Then,he joined the Institute for Materials Research(IMR), Tohoku University, and was a Professor,from 2000 to 2022. He was the Director of IMR,from 2014 to 2020, and the Vice President withTohoku University, from 2018 to 2020. He is currently the Director Generalof the Advanced Science Research Center, Japan Atomic Energy Agency.Hewas the leader of a national project in Japan ‘‘Creation andControl of SpinCurrent,’’ from 2007 to 2011. He has published more than 500 original andreview papers. He has received numerous awards, including the OutstandingResearch Award from The Magnetic Society of Japan (MSJ), in 2004, theOutstanding Paper Award from the Japan Society of Applied Physics (JSAP),in 2009, the Masumoto Hakaru Award from the Japan Institute of Metalsand Materials (JIMM), in 2011, the Prize for Science and Technology of theCommendation from theMinister of Education, Culture, Sports, Science andTechnology (MEXT), in 2018, the MSJ Society Award, in 2019, the JSAPFellowAward, in 2019, and theMurakamiMemorial Award, in 2021. He wasalso a Distinguished Lecturer of the IEEE Magnetic Society, in 2013, thePresident of MSJ, from 2017 to 2019, the President of the Asian Unionof Magnetics Societies, from 2018 to 2019, and the President of JIMM,from 2020 to 2021.HIROAKI SUKEGAWA (Member, IEEE) wasborn in Tokyo, Japan, in 1981. He received theM.Eng. and Ph.D. degrees in materials sciencefrom Tohoku University, Japan, in 2004 and 2007,respectively. He became a Researcher with theNational Institute for Materials Science, in 2007,a Senior Researcher, in 2014, and a PrincipalResearcher, in 2018. He is currently the GroupLeader of the Spintronics Group, Research Centerfor Magnetic and Spintronic Materials, NationalInstitute for Materials Science. His research interests include magnetic thinfilms and spintronics devices. He was awarded the Outstanding ResearchAward by The Magnetics Society of Japan, in 2017, the Commendationfor Science and Technology by the Minister of Education, Culture, Sports,Science and Technology, The Young Scientists’ Prize, in 2020, and theAUMS Young Researcher Award by the Asian Union of Magnetic Societies,in 2020.ZHENCHAO WEN was born in Hebei, China,in 1981. He received the B.S. degree in physicsfrom Lanzhou University, China, in 2005, andM.S. and Ph.D. degrees in condensed matterphysics from the Institute of Physics, ChineseAcademy of Sciences, in 2007 and 2010, respec-tively. From 2010 to 2015, he was a PostdoctoralResearcher with the National Institute for Materi-als Science (NIMS), Japan, where he has been aSenior Researcher, since 2018. In 2015, he movedto the Institute for Materials Research, Tohoku University, as a PostdoctoralResearcher, and a specially appointed Assistant Professor, in 2016. Hisresearch interests include spintronics and magnetic materials, especially,magnetic tunnel junctions, Heusler alloys, and topological materials.SEIJI MITANI was born in Kochi, Japan, in 1965.He received the M.Eng. and Ph.D. degrees inmaterials science from Nagoya University, Japan,in 1990 and 1993, respectively.In 1993, he became a Research Associate withthe Institute for Materials Research and was pro-moted to an Associate Professor, in 2001. In 2008,he moved to theMagneticMaterials Unit, NationalInstitute for Materials Science (NIMS), as aResearcher, and became the Group Leader of theSpintronics Group. Since 2010, he has been an Adjunct Professor with theGraduate School of Pure and Applied Sciences, University of Tsukuba. Since2021, he has also been the Director of the Research Center for Magnetic andSpintronic Materials, NIMS. His research interests include spintronics andmagnetic materials.Dr. Mitani was a recipient of the Japan Institute of Metals and MaterialsYoung Researcher Award, in 1997, and the Outstanding Presentation Awardfrom The Magnetic Society of Japan (MSJ), in 2005.HIROKI KOIZUMI was born in Ibaraki, Japan,in 1994. He received the B.S. degree in physicsfrom the Tokyo Institute of Technology, Japan,in 2017, and the M.Eng. and Ph.D. degreesin applied physics from the University ofTsukuba, Japan, in 2019 and 2022, respectively.From 2022 to 2023, he was a PostdoctoralResearcher with the National Institute for Mate-rials Science (NIMS), Japan. In 2023, he movedto the Center for Science and Innovation in Spin-tronics (CSIS), Tohoku University, as an Assistant Professor. His researchinterests include spintronics and magnetic materials.VOLUME 11, 2023 117459