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Dirac semimetals show nontrivial physical properties and can host exotic quantum states like Weyl semimetals and topological insulators under suitable external conditions. Here, by combining angle-resolved photoemission spectroscopy measurements (ARPES) and first-principle calculations, we demonstrate that Zintl-phase compound SmMg$_2$Bi$_2$ belongs to the close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone-center near the Fermi level ($E_\mathrm {F}$) which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg$_2$Bi$_2$ belongs to a $Z_2$ topological class and hosts spin-polarized states around the $E_\mathrm {F}$. Zintl's theory predicts that the valence state of Sm in this material should be Sm$^{2+}$, however we detect many Sm-4$f$ multiplet states (flat-bands) whose energy positions suggest the presence of both Sm$^{2+}$ and Sm$^{3+}$. It is also evident that these flat-bands and other dispersive states are strongly hybridized when they cross each other. Due to the presence of Sm$^{3+}$ ions, the temperature dependence of magnetic susceptibility $\chi(T)$ shows Curie-Weiss-like contribution in the low temperature region, in addition to the Van Vleck-like behaviour expected for the Sm$^{2+}$ ions. The present study will help in better understanding of the electronic structure, magnetism and transport properties of related materials., Comment: 11 pages, 7 figures

We have developed hard X-ray photoelectron spectroscopy (HAXPES) under an applied magnetic field of 1 kOe to study the electronic and magnetic states related to the MgO/Fe interface-induced perpendicular magnetic anisotropy (PMA). In this work, we used MgO (2 nm)/Fe (1.5 and 20 nm)/MgO(001) structures to reveal the interface-induced electronic states of the Fe film. Perpendicular magnetization of the 1.5-nm-thick Fe film without extrinsic oxidation of the Fe film was detected by the Fe 2p core-level magnetic circular dichroism (MCD) in HAXPES under a magnetic field, and easy magnetization axis perpendicular to the film plane was confirmed by ex situ magnetic hysteresis measurements. The valence-band HAXPES spectrum of the 1.5-nm-thick Fe film revealed that the Fe 3d electronic states were strongly modified from the thick Fe film and a reference bulk Fe sample due to the lifting of degeneracy in the Fe 3d states near the MgO/Fe interface. We found that the tetragonal distortion of the Fe film by the MgO substrate also contributes to the lifting of degeneracy in the Fe 3d states and PMA, as well as the Fe 3d-O 2p hybridization at the MgO/Fe interface, by comparing the valence-band spectrum with density functional theory calculations for MgO/Fe multilayer structures. Thus, we can conclude that the Fe 3d-O 2p hybridization and tetragonal distortion of the Fe film play important roles in PMA at the MgO/Fe interface. HAXPES with in situ magnetization thus represents a powerful new method for studying spintronic structures., GRAPHICAL ABSTRACT

We study IrCrMnZ (Z=Al, Ga, Si, Ge) systems using first-principles calculations from the perspective of their application as the electrode materials of MgO-based MTJs. These materials have highly spin-polarized conduction electrons with partially occupied $\Delta_1$ band, which is important for coherent tunneling in parallel magnetization configuration. The Curie temperatures of IrCrMnAl and IrCrMnGa are very high (above 1300 K) as predicted from mean-field-approximation. The stability of ordered phase against various antisite disorders has been investigated. We discuss here the effect of "spin-orbit-coupling" on the electronic structure around Fermi level. Further, we investigate the electronic structure of IrCrMnZ/MgO heterojunction along (001) direction. IrCrMnAl/MgO and IrCrMnGa/MgO maintain half-metallicity even at the MgO interface, with no interfacial states at/around Fermi level in the minority-spin channel. Large majority-spin conductance of IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa is reported from the calculation of ballistic spin-transport property for parallel magnetization configuration. We propose IrCrMnAl/MgO/IrCrMnAl and IrCrMnGa/MgO/IrCrMnGa as promising MTJs with a weaker temperature dependence of tunneling magnetoresistance ratio, owing to their very high Curie temperatures., Comment: 24 pages, 11 figures

The magnetocaloric effect (MCE), magnetization, specific heat, and magnetostriction measurements were performed in both pulsed and steady high magnetic fields to investigate the magnetocaloric properties of Heusler alloys ${\mathrm{Ni}}_{50\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ $(x=9 \mathrm{and} 9.7)$. From direct MCE measurements for ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{9}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ up to 56 T, a steep temperature drop was observed for magnetic-field-induced martensitic transformation (MFIMT), designated as inverse MCE. Remarkably, this inverse MCE is apparent not only with MFIMT, but also in the magnetic-field-induced austenite phase. Specific heat measurements under steady high magnetic fields revealed that the magnetic field variation of the electronic entropy plays a dominant role in the unconventional magnetocaloric properties of these materials. First-principles based calculations performed for ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{9}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ and ${\mathrm{Ni}}_{45}{\mathrm{Co}}_{5}{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ revealed that the magnetic-field-induced austenite phase of ${\mathrm{Ni}}_{41}{\mathrm{Co}}_{9}{\mathrm{Mn}}_{31.5}{\mathrm{Ga}}_{18.5}$ is more unstable than that of ${\mathrm{Ni}}_{45}{\mathrm{Co}}_{5}{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ and that it is sensitive to slight tetragonal distortion. We conclude that the inverse MCE in the magnetic-field-induced austenite phase is realized by marked change in the electronic entropy through tetragonal distortion induced by the externally applied magnetic field.

Using first-principles calculations, we studied Mn$_2$RuZ (Z=Al, Ga, Si, Ge) and their heterojunctions with MgO along (001) direction. All these alloys possess Hg$_2$CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized $\Delta_1$ band, which is robust against atomic disorder. Next we studied the electronic structure of Mn$_2$RuAl/MgO and Mn$_2$RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn$_2$RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn$_2$RuAl/MgO junction, high spin polarization of bulk Mn$_2$RuAl is preserved independent of its termination., Comment: 15 pages, 7 figures

We investigated the effect of off-stoichiometry on the Gilbert magnetic damping constant (\ensuremath{\alpha}) of Heusler alloy $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnSi}$ (CMS) thin films by employing time-resolved magneto-optical Kerr effect (TR-MOKE) measurements along with first-principles calculations based on the linear-response theory for magnetic damping. Because of the contribution of extrinsic damping arising from two-magnon scattering, the effective \ensuremath{\alpha} (${\ensuremath{\alpha}}_{\mathrm{eff}}$) extracted from the TR-MOKE responses showed dependences on the in-plane magnetic-field angle (${\ensuremath{\theta}}_{\mathrm{H}}$) and the field strength ($H$). Then, we obtained the smallest ${\ensuremath{\alpha}}_{\mathrm{eff}}$ (${\ensuremath{\alpha}}_{0}$) for each sample under the most reduced contribution from two-magnon scattering realized through varying ${\ensuremath{\theta}}_{\mathrm{H}}$ and $H$ values, which is the closest value to \ensuremath{\alpha}. The thus obtained ${\ensuremath{\alpha}}_{0}$ values of epitaxially grown off-stoichiometric $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\beta}}\mathrm{S}{\mathrm{i}}_{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=0.82$) films with various (Mn + Si) compositions, $(\ensuremath{\beta}+\ensuremath{\gamma})$, decreased with increasing $(\ensuremath{\beta}+\ensuremath{\gamma})$ from ${\ensuremath{\alpha}}_{0}=0.0057$ for (Mn + Si)-deficient $(\ensuremath{\beta}+\ensuremath{\gamma})=1.44$ to ${\ensuremath{\alpha}}_{0}=0.0036$ for $(\ensuremath{\beta}+\ensuremath{\gamma})=1.90$ being close to the stoichiometric one of $(\ensuremath{\beta}+\ensuremath{\gamma})=2.0$ at 300 K. It was also demonstrated that a half-metallic (Mn+Si)-rich CMS film with $\ensuremath{\beta}=1.30$ and $\ensuremath{\gamma}=0.90$ showed a low ${\ensuremath{\alpha}}_{0}$ of 0.0035. The dependence of ${\ensuremath{\alpha}}_{0}$ on $(\ensuremath{\beta}+\ensuremath{\gamma})$ in $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\beta}}\mathrm{S}{\mathrm{i}}_{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=0.82$) was well explained by the first-principles calculations. Through the systematic investigations of off-stoichiometric CMS with various values of $(\ensuremath{\beta}+\ensuremath{\gamma})$, it was clarified that the total density of states (DOS) at the Fermi level, $D({E}_{\mathrm{F}})$, plays the key role for determining the damping constant of CMS. Furthermore, it was revealed that the reduced minority-spin DOS at ${E}_{\mathrm{F}}$, caused by decreasing harmful $\mathrm{C}{\mathrm{o}}_{\mathrm{Mn}}$ antisites, is essential for reducing the damping constant of CMS. These findings demonstrate that appropriately controlling off-stoichiometry and film composition is thus promising for achieving half-metallicity and a low \ensuremath{\alpha} simultaneously for CMS thin films.

A perpendicularly magnetized ferromagnetic layer is an important building block for recent/future highdensity spintronic memory applications. This paper reports on the fabrication of perpendicularly magnetized Ni / Pt superlattices and the characterization of their structures and magnetic properties. The optimization of film growth conditions allowed us to grow epitaxial Ni / Pt (001) superlattices on SrTiO$_{3}$ (001) single crystal substrates. We investigated their structural parameters and magnetic properties as a function of the Ni layer thickness, and obtained a high uniaxial magnetic anisotropy energy of 1.9 x 10$^{6}$ erg/cm$^{3}$ for a [Ni (4.0 nm) / Pt (1.0 nm)] superlattice. In order to elucidate the detailed mechanism on perpendicular magnetic anisotropy for the Ni / Pt (001) superlattices, the experimental results were compared with the first-principles calculations. It has been found that the strain effect is a prime source of the emergence of perpendicular magnetic anisotropy., 33 pages, 12 figures

We report an enhancement of the anomalous Nernst effect (ANE) in Ni/Pt (001) epitaxial superlattices. The transport and magneto-thermoelectric properties were investigated for the Ni/Pt superlattices with various Ni layer thicknesses (${\it t}$). The anomalous Nernst coefficient was increased up to more than 1 ${\mu}$V K$^{-1}$ for 2.0 nm ${\leq}$ ${\it t}$ ${\leq}$ 4.0 nm, which was the remarkable enhancement compared to the bulk Ni. It has been found that the large transverse Peltier coefficient (${\alpha}$$_{xy}$), reaching ${\alpha}$$_{xy}$ = 4.8 A K$^{-1}$ m$^{-1}$ for ${\it t}$ = 4.0 nm, plays a prime role for the enhanced ANE of the Ni/Pt (001) superlattices., Comment: 22 pages, 4 figures

Tetragonally ordered Fe2Ni2N powder was successfully synthesized from L10-FeNi using a nitrogen topotactic reaction. The magnetic properties and structural stability of the tetragonally ordered Fe2Ni2N were examined using first principles calculations. The nitriding process gives 96 wt% of tetragonally ordered Fe2Ni2N phase and the rest is the more nitrided phase of FeNiN. Analysis of the crystal structure confirmed that the c/a is 0.989 for the tetragonally ordered Fe2Ni2N, which is consistent with the results of the first-principles calculations. Magnetization measurements reveal that the Curie temperature and thermal decomposition temperatures are 441 and 667 K, respectively. The high field magnetization processes at 10 K indicate that the sample contains a component of high magnetic anisotropy energy Ku. The Ku estimated by the law of approach to ferromagnetic saturation is 1.51 MJ/m3, which is in reasonably good agreement with that obtained by the first-principles calculations 1.96 MJ/m3.

MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes have attracted considerable attention for spintronics applications. However, there remain a couple of issues related to materials that should be resolved before practical use. Recently, quarterly equiatomic Heusler alloys have attracted attention as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be lattice-matched with the MgO barrier, which is advantageous compared to traditional Co2 Heusler alloys T. T [T. Roy et al., J. Magn. Magn. Mater. 498 (2020) 166092]. Here, we experimentally investigated the structure and magnetic properties of thin films composed of one of these alloys, CoIrMnAl, fabricated by sputtering deposition. We successfully obtained films with the B2 chemical ordering, even without a post-annealing process. The lattice constants for the films annealed at 500–600 ∘C were approximately equal to the predicted values. The magnetization at 10 K was close to 500 kA/m, and the Curie temperature was approximately 400 K, which were approximately 70% of the values predicted for the fully ordered structure. The magnetic properties observed in the B2-ordered films were well explained by ferrimagnetism that appeared in the B2-ordered CoIrMnAl with full-swap disorders of Co-Ir and Mn-Al and almost full-swap disorder of Co-Mn, which was predicted from the first-principles calculations.

Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal–dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter., Electric field control of magnetization is usually weak and this hampers its application for the ultralow-power-consumption spintronic devices. Here, the authors demonstrate a mechanism to enhance the control of magnetic anisotropy by voltage-induced electric quadrupole in a metal–dielectric interface.

The FeIr/MgO multilayer system was found to exhibit large interfacial perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy effects. We herein report the x-ray magnetic circular dichroism spectroscopy of both the Fe and Ir absorption edges in the FeIr/MgO system. Although Ir insertion at the Fe/MgO interface increased the PMA energy in the system, the orbital magnetic-moment anisotropy of Fe remained approximately constant. We found significant anisotropy in the orbital magnetic moment but not in the magnetic-dipole ${T}_{\mathrm{z}}$ term in the Ir atoms. Our experimental and theoretical results demonstrated that the large PMA of FeIr/MgO originates from second-order perturbation of the spin-orbit interaction energy of Ir. Moreover, the energy mainly originates from the spin-conserved virtual excitation process terms from the majority-spin bands of the Ir atoms located in the second atomic layer from MgO.

The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a $B2$-ordered phase. The saturation magnetization is $380\phantom{\rule{0.16em}{0ex}}{\mathrm{emu}/\mathrm{cm}}^{3}$, almost the same as the value given by the Slater--Pauling--like rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows that the MTJs have MgO interfaces with fewer dislocations. The temperature- and bias-voltage-dependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state possessing SGS characteristics. Ab initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed.

Nowadays advanced magnetic tunnel junction applications demand very high tunnel magnetoresistance at room temperature, thus it is quite important to explore high Curie temperature Tc half-metallic Heusler alloys. In this article rst-principles calculation unveiled that NiCrMnSi has Tc of 1200 K comparable to that of the traditional Co2MnSi Heusler alloys, even though it does not contain Co element. In addition, we examined whether NiCrMnSi Heulser phase lms can be obtained by a magnetron sputtering on MgO substrates. The results of the structural analysis and rst-principles calculations indicated that NiCrMnSi Heusler phase is metastable. A possible route to obtain metastable NiCrMnSi Heusler alloy is to utilize appropriate templates.

Tetragonal alloys, such as D022-Mn3Ga, are potential candidates for rare-earth free permanent magnets due to their high Curie temperature and uniaxial magnetic anisotropy. For high-performance permanent magnets, high saturation magnetisation is necessary. However, the saturation magnetisation of D022-Mn3Ga is small due to ferrimagnetic ordering. We investigated the possibility of developing ferromagnetic Heusler alloys with high magnetic anisotropy and saturation magnetisation using the first-principles calculation. We focused on the effects of Fe substitution for Mn in D022-Mn3Ga as well as the consequent volume expansion; the ferromagnetic tetragonal XA phase is stabilized in Fe2MnGa by an 8% volume expansion. This tetragonal XA-Fe2MnGa has desirable properties for a high-performance permanent magnet, such as high magnetisation (1350 emu cc−1), perpendicular magnetic anisotropy (2.12 MJ m−3), and Curie temperature (1047 K). In addition, the substitution of Sn and increasing the Ga composition in the Fe2MnGa alloy results in volume expansion, which stabilizes the ferromagnetic tetragonal XA phase.

Using first principles calculations based on density functional theory, we study electronic and magnetic properties of CoIrMnZ (Z = Al, Si, Ga, Ge) Heusler systems. Based on the electronic structure calculations, CoIrMnSi and CoIrMnGe are predicted to be half-metallic. For the studied systems, the ground state magnetic configurations are found to be ferromagnetic with magnetic moment consistent with the Slater-Pauling rule. The calculated ferromagnetic transition temperatures are much higher than the room temperature for all the cases. The effect of spin orbit coupling has been discussed on the electronic structure for all the systems studied here. Furthermore, we construct junctions with MgO along (001) direction. The MnZ terminations are found to be energetically more favourable compared to the CoIr terminations. After discussing the tunnelling conductance, we find that CoIrMnSi/MgO/CoIrMnSi could be a promising magnetic tunnelling junction (MTJ) candidate.

Precise magnetization measurements of a half-metallic ferromagnet ${\mathrm{Co}}_{2}\mathrm{TiSn}$ with the Heusler type structure have been carried out under pressure up to 1.27 GPa. At ambient pressure, the spontaneous magnetic moment ${M}_{\mathrm{s}}^{\mathrm{exp}}$ and the Curie temperature ${T}_{\mathrm{C}}^{\mathrm{exp}}$ are experimentally obtained to be $1.993(3)\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$/f.u. and $376.76(7)$ K, respectively. The ${M}_{\mathrm{s}}^{\mathrm{exp}}$ is independent of applying pressure. The ${T}_{\mathrm{C}}^{\mathrm{exp}}$ decreases with the increase of pressure and the pressure derivative of the Curie temperature $d{T}_{\mathrm{C}}^{\mathrm{exp}}/dp$ is estimated to be $\ensuremath{-}3.15(9)$ K/GPa. Furthermore, the pressure effect on the electronic and magnetic properties of ${\mathrm{Co}}_{2}\mathrm{TiSn}$ has been theoretically investigated on the basis of first-principles density functional calculations by using the projector augmented wave method in order to evaluate the half metallicity of ${\mathrm{Co}}_{2}\mathrm{TiSn}$. Both the experimental and the theoretical results under pressure reveal that ${\mathrm{Co}}_{2}\mathrm{TiSn}$ is half metal. The experimental results of pressure-dependent magnetic properties are finally discussed on the basis of the spin fluctuation theory for itinerant electron magnetism.

In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in $3d$-ferromagnetic metals are revealed. Using in situ x-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole ${T}_{\mathrm{z}}$ terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of $0.013\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ was generated in the presence of electric fields of $\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{nm}$. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The induced change in the magnetic dipole ${T}_{\mathrm{z}}$ term only slightly contributed to the VCMA in $3d$-ferromagnetic metals.

A powder sample of body-centered tetragonal Ni2N in a single phase was prepared by a high rate-type triode dc reactive sputtering and the magnetic properties have been reinvestigated. It is confirmed that Ni2N is not a ferromagnet. However, the material is not a simple paramagnet either and shows a weak spin-glass-like behavior at low temperatures with a freezing temperature of about 8 K. Posible origins of the behavior are discussed. Band-structure calculations are reported to see the stability of the nonmagnetic ground state of bct Ni2N against lattice expansion and appearance of nitrogen vacancies.

Anisotropic magnetoresistance (AMR) effects in Co x (Mn0.44Ga0.56)100−x epitaxial thin films were investigated in the temperature range of 5–300 K by changing the current (I) direction to the crystal axis and the Co content (x). The AMR ratios were mostly positive for I // Co2MnGa[100] and negative for I // Co2MnGa[110] and showed peak values at x = 49.7 at% for both current directions. AMR ratios calculated from s–d scattering theory, including crystal field effects with density of states information from first-principles calculations, well reproduced experimental features.

The electron spin degree of freedom can provide the functionality of “nonvolatility” in electronic devices. For example, magnetoresistive random access memory (MRAM) is expected as an ideal nonvolatile working memory, with high speed response, high write endurance, and good compatibility with complementary metal-oxide-semiconductor (CMOS) technologies. However, a challenging technical issue is to reduce the operating power. With the present technology, an electrical current is required to control the direction and dynamics of the spin. This consumes high energy when compared with electric-field controlled devices, such as those that are used in the semiconductor industry. A novel approach to overcome this problem is to use the voltage-controlled magnetic anisotropy (VCMA) effect, which draws attention to the development of a new type of MRAM that is controlled by voltage (voltage-torque MRAM). This paper reviews recent progress in experimental demonstrations of the VCMA effect. First, we present an overview of the early experimental observations of the VCMA effect in all-solid state devices, and follow this with an introduction of the concept of the voltage-induced dynamic switching technique. Subsequently, we describe recent progress in understanding of physical origin of the VCMA effect. Finally, new materials research to realize a highly-efficient VCMA effect and the verification of reliable voltage-induced dynamic switching with a low write error rate are introduced, followed by a discussion of the technical challenges that will be encountered in the future development of voltage-torque MRAM.

Electric control of magnetism has been a topic of interest for various spintronic applications. It is known that monoatomic Pt layer insertion at the Fe/MgO interface increases voltage-controlled magnetic anisotropy (VCMA). However, the reason for the optimality of this thickness has not been explained thus far. In this study, we observed the changes in the electronic states at the Fe/MgO interface using tunneling spectroscopy on an epitaxial Fe(001)/Pt/MgO(001) structure to characterize the density of states around the Fermi level. We found that a surface resonant state is formed at the Fermi level by the insertion of a monoatomic Pt layer, which is consistent with our first principles study. In addition, the VCMA enhancement owing to the formation of this surface resonance state agrees with the recently proposed microscopic theory.

The metastable phases of Fe1− x Sn x alloys were stabilized by their epitaxial growth on MgO(100). The Fe1− x Sn x alloys with a body-centered-cubic-based structure were realized in the range of 0 ≤ x ≤ 0.4. For x = 0.25, the B2 structure was observed by transmission electron microscopy and X-ray diffraction analysis, although the hexagonal D019 structure was a thermodynamically stable phase. With respect to magnetic properties, all the compositions show similar hysteresis curves except for x = 0.25. According to ab initio calculations, D03-Fe3Sn and B2-Fe3Sn have band structures similar to that of Fe, which results in a large tunnel magnetoresistance by coherent tunneling.

Magnetic materials with low Gilbert damping and low magnetization are necessary for the realization of faster or more energy-efficient spintronic devices based on spin-transfer-torque. Here, we report Gilbert damping in epitaxially grown equiatomic quaternary CoFeMnSi Heusler alloy films. The 10 nm-thick films show a saturation magnetization of M S = 630 emu cm−3 and a Gilbert damping constant of , which are relatively small values among transition metal ferromagnets, in addition to its soft magnetic properties. The physical origin of the relatively low damping and the possibility of a further reduction of α to the ultra-low damping regime ~10−4 are discussed in terms of the spin-gapless-like electronic structure and the effect of the chemical order computed from first principles.

We used x-ray absorption spectroscopy and x-ray magnetic circular dichroism to investigate the effects of inserting Cu into Co/Pt interfaces, and found that a 0.4-nm-thick inserted Cu layer showed perpendicularly magnetized properties induced by the proximity effect through the Co and Pt layers. The dependence of the magnetic properties on the thickness of the Cu layers showed that the proximity effects between Co and Pt with perpendicular magnetic anisotropy can be prevented by the insertion of a Cu layer with a nominal threshold thickness of 0.7 nm. Element-specific magnetization curves were also obtained, demonstrating that the out-of-plane magnetization is induced in the Cu layers of the Co/Cu/Pt structures.

Half-metallic ferromagnets are promising candidates for electrode materials of magnetic tunnel junctions (MTJ). Extensive studies have been carried out for MTJ using Co-based full Heusler alloys, such as Co 2 MnSi and Co 2 FeSi, with L21 structure. On the other hand, experimental attempt was limited for MTJ using Heusler alloys with $C1_{b}$ structure such as NiMnSb and PtMnSb. First-principles calculations indicated that atomic disorder gives rise to electronic states in the minority-spin gap and easily degrades the spin polarization of NiMnSb [1]. In this study, we performed density-functional calculations of electronic structure in NiMnSb/MgO and CoMnSb/MgO junctions to assess the feasibility of these Heusler alloys as electrode materials of MTJ. We adopted the Vienna ab initio simulation package (VASP) with use of the projector augmented-wave (PAW) potential. The exchange-correlation energy is considered with a generalized gradient approximation (GGA). First, we examined stable interfacial structure of NiMnSb/MgO and CoMnSb/MgO junctions by evaluating formation energy of each structure. As a result, the MnSb-terminated interface, where Mn and Sb atoms are located on top of O atoms, is preferable energetically for both junctions. While the magnetic moment of Mn atoms at the interface increases compared to that in bulk, the Ni or Co magnetic moment on the sub-interface layer remarkably decreases for both junctions. Note that the Co magnetic moment of Co 2 MnSi is almost unaltered even at the sub-interface layer in the MnSi-terminated Co 2 MnSi/MgO junction [2]. The reduction of the Ni or Co magnetic moment in the interfacial region enhances thermal fluctuation of the magnetic moment, which leads to significant reduction of tunnelling magnetoresistance (TMR) ratio with increasing temperature [3]. We also found the reduction of the spin polarization in the interfacial region of the NiMnSb/MgO and CoMnSb/MgO junctions due to the presence of interfacial resonant states in the minority-spin gap. The reduction of the spin polarization and the magnetic moment in the interfacial region of CoMnSb/MgO junction can be recovered by Co or Mn atoms occupied at the vacancy sites in the interfacial region.

Tunnel magnetoresistance (TMR) in MgO-based magnetic tunnel junctions (MTJs) with equiatomic quaternary CoFeMnSi Heusler and CoFe alloy electrodes is studied. The epitaxial MTJ stacking structures were prepared using ultrahigh-vacuum magnetron sputtering, where the CoFeMnSi electrode has a full B2 and partial L21 ordering crystal structure. Maximum TMR ratios of 101% and 521% were observed at room temperature and 10 K, respectively, for the MTJs. The large bias voltage dependence of the TMR ratio was also observed at low temperature (LT), as similarly observed in Co2MnSi Heusler alloy-based MTJs in the past. The physical origins of this relatively large TMR ratio at LT were discussed in terms of the half-metallicity of CoFeMnSi.

We have performed high-resolution angle-resolved photoemission spectroscopy of oxygen-adsorbed Fe thin films on W(110) to elucidate the evolution of electronic states upon surface oxidation. After oxygen adsorption of 0.25 monolayer onto Fe films, we found a surface state in the vicinity of the Fermi level which exhibits an undulating energy band dispersion. Our first-principles band-structure calculation indicates that this state is attributed to the spin-polarized minority band arising from the hybridization between Fe 3d and O 2p orbitals. The present result suggests that the observed surface state is related to the unusual magnetic properties of oxidized Fe surfaces.

Voltage control of spin enables both a zero standby power and ultralow active power consumption in spintronic devices, such as magnetoresistive random-access memory devices. A practical approach to achieve voltage control is the electrical modulation of the spin–orbit interaction at the interface between 3d-transition-ferromagnetic-metal and dielectric layers in a magnetic tunnel junction (MTJ). However, we need to initiate a new guideline for materials design to improve both the voltage-controlled magnetic anisotropy (VCMA) and perpendicular magnetic anisotropy (PMA). Here we report that atomic-scale doping of iridium in an ultrathin Fe layer is highly effective to improving these properties in Fe/MgO-based MTJs. A large interfacial PMA energy, Ki,0, of up to 3.7 mJ m−2 was obtained, which was 1.8 times greater than that of the pure Fe/MgO interface. Moreover, iridium doping yielded a huge VCMA coefficient (up to 320 fJ Vm−1) as well as high-speed response. First-principles calculations revealed that Ir atoms dispersed within the Fe layer play a considerable role in enhancing Ki,0 and the VCMA coefficient. These results demonstrate the efficacy of heavy-metal doping in ferromagnetic layers as an advanced approach to develop high-density voltage-driven spintronic devices. Researchers from Japan's AIST demonstrated a new approach to reduce the energy consumption of spintronic devices. Magnetic random-access memory requires approximately 10,000 times more energy to record data than to safely maintain it — a discrepancy that arises due to the wastefulness of electric-current-based switching of magnetic bits. Takayuki Nozaki and colleagues now report a device that enables us to write magnetic memory using electric fields, a more energy-efficient control mechanism. The team introduced an iridium-doped ultrathin iron film in magnesium oxide-based magnetic tunnel junctions, and found that the heavy-metal dopants provoked a strong voltage-controlled magnetic anisotropy change with high-speed response. Physical role of heavy metal dopants was unveiled by first-principles calculations. The developed technique can lead to a new type of non-volatile memory with ultra-low energy consumption. Highly efficient voltage control of magnetic anisotropy has been demonstrated utlizing an ultrathin Ir-doped Fe layer in MgO-based magnetic tunnel junctions. Ir adoms are dispersed inside the ultrathin Fe layer through the interdiffusion process. Large spin–orbit interaction of Ir atoms having proximity-induced magnetism is attributed to the enhancement of the voltage-controlled magnetic anisotropy (VCMA) effect. High speed response of the VCMA effect was also confirmed by voltage-induced ferromagnetic resonance. The achieved properties first satisfy the required specification for the new type of magnetoresistive random access memory (MRAM) driven by voltage.

Structures and magnetic properties were systematically investigated for the monolayer (ML)-controlled Co | Ni multilayers with various Ni layer thicknesses, where the Co and Ni layers were epitaxially grown on a Al2O3 (\(11\bar{2}0\)) single crystal substrate or deposited on a thermally oxidized Si substrate. The epitaxial Co | Ni multilayers exhibited high uniaxial magnetic anisotropy energies (Ku) and low effective magnetic damping constants (αeff), resulting in the perpendicular magnetization with low damping even for the 1 ML-Ni | 1 ML-Co multilayer. By comparing the experimental results with the first principles calculation, we found that the Ni atoms largely contributed to the perpendicular anisotropy for the Co | Ni multilayers. We also discussed the possible mechanism dominating the magnitude of αeff for the Co | Ni multilayers based on the experiments and the calculations. The present results suggest that the epitaxial Co | Ni multilayer is a promising material to achieve high Ku and low magnetic...

Current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) effects in devices including Co2Fe0.4Mn0.6Si (CFMS)/Ag100−x Mg x /CFMS structures were investigated theoretically and experimentally. First-principles transport calculation revealed that the Fermi surface matching between CFMS and L12 Ag3Mg is better than that between CFMS and fcc-Ag. In the experiments the Mg composition, x was changed from 0 to 26 at.%, in which both face centered cubic phase and L12 phase of Ag–Mg alloys are included depending on the Mg composition. It was confirmed by a cross-sectional high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image that the Ag–Mg spacer layer with L12 ordered phase was successfully fabricated for x = 22 at.%. The maximum CPP–GMR ratio and the change of the areal resistance () were 56% and 20 mm2, respectively, for x = 22 at.% at room temperature, which is much higher than that of the conventionally used pure Ag spacer devices. It was suggested from the HAADF-STEM images and the results of the temperature dependence of CPP–GMR effects that the diffusion of Mn atoms occurred less at the CFMS/Ag–Mg interfaces for the L12 ordered Ag–Mg spacer devices than the Ag spacer devices, which might be a key factor for the enhancement of the value. The newly developed L12 Ag–Mg spacer is a promising material for realizing large of the CPP–GMR devices.

金沢大学大学院自然科学研究科計算科学, 金沢大学理学部, We have developed a fully relativistic pseudopotential in the planewave method. The pseudopotential shows the sufficient transferability that the error is much smaller than the energy splitting of spin-orbit coupling. This pseudopotential scheme has been implemented for bulk systems, surfaces, and the thin magnetic film on the metal. © 2006 Elsevier B.V. All rights reserved.