Spin Caloritronics in Metallic Superlattices.

Spin caloritronics, a research field studying on the interconversion between a charge current (Jc) and a heat current (Jq) mediated by a spin current (Js) and/or magnetization (M), has attracted much attention not only for academic interest but also for practical applications. Newly discovered spin-caloritronic phenomena such as the spin Seebeck effect (SSE) have stimulated the renewed interest in the thermoelectric phenomena of a magnet, which have been known for a long time, e.g. the anomalous Nernst effect (ANE). These spin-caloritronic phenomena involving the SSE and the ANE have provided with a new direction for thermoelectric conversion exploitingJsand/orM. Importantly, the symmetry of ANE allows the thermoelectric conversion in the transverse configuration betweenJqandJc. Although the transverse configuration is totally different from the conventional longitudinal configuration based on the Seebeck effect and has many advantages, we are still facing several issues that need to be solved before developing practical applications. The primal issue is the improvement of conversion efficiency. In the case of ANE-based applications, a material with a large anomalous Nernst coefficient (SANE) is the key for solving the issue. This review article introduces the increase ofSANEcan be achieved by forming superlattice structures, which has been demonstrated for several kinds of materials combinations. The overall picture of studies on spin caloritronics is first surveyed. Then, we mention the pioneering work on the transverse thermoelectric conversion in superlattice structures, which was performed using Fe-based metallic superlattices, and show the recent studies for the Ni-based metallic superlattices and the ordered alloy-based metallic superlattices.

Room temperature chirality switching and detection in a helimagnetic MnAu2 thin film.

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control and detection methods have to be solved in practical applications. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu2 helimagnetic conductor. Moreover, we have succeeded in detecting the chirality at zero magnetic fields by means of simple transverse resistance measurement utilizing the spin Berry phase in a bilayer device composed of MnAu2 and a spin Hall material Pt. These results may pave the way to helimagnet-based spintronics.

Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation.

Transverse thermoelectric generation using magnetic materials is essential to develop active thermal engineering technologies, for which the improvements of not only the thermoelectric output but also applicability and versatility are required. In this study, using combinatorial material science and lock-in thermography technique, we have systematically investigated the transverse thermoelectric performance of Sm-Co-based alloy films. The high-throughput material investigation revealed the best Sm-Co-based alloys with the large anomalous Nernst effect (ANE) as well as the anomalous Ettingshausen effect (AEE). In addition to ANE/AEE, we discovered unique and superior material properties in these alloys: the amorphous structure, low thermal conductivity, and large in-plane coercivity and remanent magnetization. These properties make it advantageous over conventional materials to realize heat flux sensing applications based on ANE, as our Sm-Co-based films can generate thermoelectric output without an external magnetic field. Importantly, the amorphous nature enables the fabrication of these films on various substrates including flexible sheets, making the large-scale and low-cost manufacturing easier. Our demonstration will provide a pathway to develop flexible transverse thermoelectric devices for smart thermal management.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films.

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO2 amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu2 Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Microstructures and Interface Magnetic Moments in Mn2VAl/Fe Layered Films Showing Exchange Bias.

Heusler alloys are a material class exhibiting various magnetic properties, including antiferromagnetism. A typical application of antiferromagnets is exchange bias that is a shift of the magnetization curve observed in a layered structure consisting of antiferromagnetic and ferromagnetic films. In this study, a layered sample consisting of a Heusler alloy, Mn2VAl and a ferromagnet, Fe, is selected as a material system exhibiting exchange bias. Although the fully ordered Mn2VAl is known as a ferrimagnet, with an optimum fabrication condition for the Mn2VAl layer, the Mn2VAl/Fe layered structure exhibits exchange bias. The appearance of the antiferromagnetic property in the Mn2VAl is remarkable; however, the details have been unclear. To clarify the microscopic aspects on the crystal structures and magnetic moments around the Mn2VAl/Fe interface, cross-sectional scanning transmission electron microscope (STEM) observation, and synchrotron soft X-ray magnetic circular dichroism (XMCD) measurements were employed. The high-angle annular dark-field STEM images demonstrated clusters of Mn2VAl with the L21 phase distributed only around the interface to the Fe layer in the sample showing the exchange bias. Furthermore, antiferromagnetic coupling between the Mn- and Fe-moments were observed in element-specific hysteresis loops measured using the XMCD. The locally ordered L21 phase and antiferromagnetic Mn-moments in the Mn2VAl were suggested as important factors for the exchange bias.

Heusler alloys for spintronic devices: review on recent development and future perspectives.

Heusler alloys are theoretically predicted to become half-metals at room temperature (RT). The advantages of using these alloys are good lattice matching with major substrates, high Curie temperature above RT and intermetallic controllability for spin density of states at the Fermi energy level. The alloys are categorised into half- and full-Heusler alloys depending upon the crystalline structures, each being discussed both experimentally and theoretically. Fundamental properties of ferromagnetic Heusler alloys are described. Both structural and magnetic characterisations on an atomic scale are typically carried out in order to prove the half-metallicity at RT. Atomic ordering in the films is directly observed by X-ray diffraction and is also indirectly probed via the temperature dependence of electrical resistivity. Element specific magnetic moments and spin polarisation of the Heusler alloy films are directly measured using X-ray magnetic circular dichroism and Andreev reflection, respectively. By employing these ferromagnetic alloy films in a spintronic device, efficient spin injection into a non-magnetic material and large magnetoresistance are also discussed. Fundamental properties of antiferromagnetic Heusler alloys are then described. Both structural and magnetic characterisations on an atomic scale are shown. Atomic ordering in the Heusler alloy films is indirectly measured by the temperature dependence of electrical resistivity. Antiferromagnetic configurations are directly imaged by X-ray magnetic linear dichroism and polarised neutron reflection. The applications of the antiferromagnetic Heusler alloy films are also explained. The other non-magnetic Heusler alloys are listed. A brief summary is provided at the end of this review.

Spin-charge conversion in NiMnSb Heusler alloy films.

Half-metallic Heusler alloys are attracting considerable attention because of their unique half-metallic band structures, which exhibit high spin polarization and yield huge magnetoresistance ratios. Besides serving as ferromagnetic electrodes, Heusler alloys also have the potential to host spin-charge conversion. Here, we report on the spin-charge conversion effect in the prototypical Heusler alloy NiMnSb. An unusual charge signal was observed with a sign change at low temperature, which can be manipulated by film thickness and ordering structure. It is found that the spin-charge conversion has two contributions. First, the interfacial contribution causes a negative voltage signal, which is almost constant versus temperature. The second contribution is temperature dependent because it is dominated by minority states due to thermally excited magnons in the bulk part of the film. This work provides a pathway for the manipulation of spin-charge conversion in ferromagnetic metals by interface-bulk engineering for spintronic devices.

Spin-Orbit-Torque Memory Operation of Synthetic Antiferromagnets.

In this Letter, we show the demonstration of a sequential antiferromagnetic memory operation with a spin-orbit-torque write, by the spin Hall effect, and a resistive read in the CoGd synthetic antiferromagnetic bits, in which we reveal the distinct differences in the spin-orbit-torque and field-induced switching mechanisms of the antiferromagnetic moment, or the Néel vector. In addition to the comprehensive spin torque memory operation, our thorough investigations also highlight the high immunity to a field disturbance as well as a memristive behavior of the antiferromagnetic bits.

Chemical and structural analysis on magnetic tunnel junctions using a decelerated scanning electron beam.

Current information technology relies on the advancement of nanofabrication techniques. For instance, the latest computer memories and hard disk drive read heads are designed with a 12 nm node and 20 nm wide architectures, respectively. With matured nanofabrication processes, a yield of such nanoelectronic devices is typically up to about 90%. To date the yield has been compensated with redundant hardware and software error corrections. In the latest memories, approximately 5% redundancy and parity bits for error corrections are used, which increase the total production cost of the devices. This means the yield directly affects the device costs. It is hence critical to increase the yield in nanofabrication. In this paper, we have applied our recently developed method to image buried interfaces in combination with chemical analysis to evaluate magnetic tunnel junctions and have revealed their different magnetoresistance ratios caused by the presence of materials formed at the junction edges. The formation of these materials can be avoided by optimising the junction patterning process to remove residual carbon introduced from resist. Our imaging method with chemical analysis have demonstrated a significant potential for the improvement of junction performance, resulting in higher yields. This can be used as a quality assurance tool in a nanoelectronic device production line.

Interface Tailoring Effect for Heusler Based CPP-GMR with an L12-Type Ag3Mg Spacer.

Current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) effects are of interest in a possible application of magnetic sensor elements, such as read-head of hard disk drives. To improve the junction performance, the interface tailoring effects were investigated for the Heulser alloy, Co2Fe0.4Mn0.6Si (CFMS), based CPP-GMR junctions with an L 1 2 -Ag3Mg ordered alloy spacer. Ultra-thin Fe or Mg inserts were utilized for the CFMS/Ag3Mg interfaces, and CPP-GMR at low bias current density, J and the J dependence were evaluated for the junctions. Although, at low bias J, MR ratio decreased with increasing the inserts thickness, the device output at high bias J exhibited quite weak dependence on the insert thickness. The output voltages of the order of 4 mV were obtained for the junctions regardless of the insert at an optimal bias J for each. The critical current density J c was evaluated by the shape of MR curves depending on J. J c increased with the insert thicknesses up to 0.45 nm. The enhancement of J c suggests that spin-transfer-torque effect may reduce in the junctions with inserts, which enables a reduction of noise and can be an advantage for device applications.

Synthesis of single-phase L10-FeNi magnet powder by nitrogen insertion and topotactic extraction.

Tetrataenite (L10-FeNi) is a promising candidate for use as a permanent magnet free of rare-earth elements because of its favorable properties. In this study, single-phase L10-FeNi powder with a high degree of order was synthesized through a new method, nitrogen insertion and topotactic extraction (NITE). In the method, FeNiN, which has the same ordered arrangement as L10-FeNi, is formed by nitriding A1-FeNi powder with ammonia gas. Subsequently, FeNiN is denitrided by topotactic reaction to derive single-phase L10-FeNi with an order parameter of 0.71. The transformation of disordered-phase FeNi into the L10 phase increased the coercive force from 14.5 kA/m to 142 kA/m. The proposed method not only significantly accelerates the development of magnets using L10-FeNi but also offers a new synthesis route to obtain ordered alloys in non-equilibrium states.

Voltage control of magnetic anisotropy in epitaxial Ru/Co2FeAl/MgO heterostructures.

Voltage control of magnetic anisotropy (VCMA) in magnetic heterostructures is a key technology for achieving energy-efficiency electronic devices with ultralow power consumption. Here, we report the first demonstration of the VCMA effect in novel epitaxial Ru/Co2FeAl(CFA)/MgO heterostructures with interfacial perpendicular magnetic anisotropy (PMA). Perpendicularly magnetized tunnel junctions with the structure of Ru/CFA/MgO were fabricated and exhibited an effective voltage control on switching fields for the CFA free layer. Large VCMA coefficients of 108 and 139 fJ/Vm for the CFA film were achieved at room temperature and 4 K, respectively. The interfacial stability in the heterostructure was confirmed by repeating measurements. Temperature dependences of both the interfacial PMA and the VCMA effect were also investigated. It is found that the temperature dependences follow power laws of the saturation magnetization with an exponent of ~2, where the latter is definitely weaker than that of conventional Ta/CoFeB/MgO. The significant VCMA effect observed in this work indicates that the Ru/CFA/MgO heterostructure could be one of the promising candidates for spintronic devices with voltage control.

Fully epitaxial C1b-type NiMnSb half-Heusler alloy films for current-perpendicular-to-plane giant magnetoresistance devices with a Ag spacer.

Remarkable magnetic and spin-dependent transport properties arise from well-designed spintronic materials and heterostructures. Half-metallic Heusler alloys with high spin polarization exhibit properties that are particularly advantageous for the development of high-performance spintronic devices. Here, we report fully (001)-epitaxial growth of a high-quality half-metallic NiMnSb half-Heusler alloy films, and their application to current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with Ag spacer layers. Fully (001)-oriented NiMnSb epitaxial films with very flat surface and high magnetization were prepared on Cr/Ag-buffered MgO(001) single crystalline substrates by changing the substrate temperature. Epitaxial CPP-GMR devices using the NiMnSb films and a Ag spacer were fabricated, and room-temperature (RT) CPP-GMR ratios for the C1b-type half-Heusler alloy were determined for the first time. A CPP-GMR ratio of 8% (21%) at RT (4.2 K) was achieved in the fully epitaxial NiMnSb/Ag/NiMnSb structures. Furthermore, negative anisotropic magnetoresistance (AMR) ratio and small discrepancy of the AMR amplitudes between RT and 10 K were observed in a single epitaxial NiMnSb film, indicating robust bulk half metallicity against thermal fluctuation in the half-Heusler compound. The modest CPP-GMR ratios could be attributed to interface effects between NiMnSb and Ag. This work provides a pathway for engineering a new class of ordered alloy materials with particular emphasis on spintronics.

Fe-Ni composition dependence of magnetic anisotropy in artificially fabricated L1 0-ordered FeNi films.

We prepared L10-ordered FeNi alloy films by alternate deposition of Fe and Ni monatomic layers, and investigated their magnetic anisotropy. We employed a non-ferromagnetic Au-Cu-Ni buffer layer with a flat surface and good lattice matching to L10-FeNi. An L10-FeNi film grown on Au6Cu51Ni43 showed a large uniaxial magnetic anisotropy energy (Ku = 7.0 × 10(6) erg cm(-)3). Ku monotonically increased with the long-range order parameter (S) of the L10 phase. We investigated the Fe-Ni composition dependence by alternating the deposition of Fe 1 − x and Ni 1 + x monatomic layers (− 0.4 < x < 0.4). Saturation magnetization (Ms) and Ku showed maxima (Ms = 1470 emu cm(-3), Ku = 9.3 × 10(6) erg cm(-3)) for Fe60Ni40 (x = -0.2) while S showed a maximum at the stoichiometric composition (x = 0). The change in the ratio of lattice parameters (c/a) was small for all compositions. We found that enrichment of Fe is very effective to enhance Ku. The large Ms and Ku of Fe60Ni40 indicate that Fe-rich L10-FeNi is promising as a rare-earth-free permanent magnet.

Spin wave-assisted reduction in switching field of highly coercive iron-platinum magnets.

Recent rapid progress in spintronic and magnetic storage nanodevices has required nanomagnets to balance competing goals for high coercive field and low switching field. However, a decisive route for highly efficient magnetization switching has not been established yet. Here we propose a novel switching method using a spin wave of magnetic structures twisted in a nanometre scale. We have experimentally demonstrated extremely low field-magnetization switching in a highly coercive FePt by using a spin wave excited in a soft magnetic permalloy (Ni81Fe19), where permalloy is exchange-coupled to FePt through the interface. We can tune the switching field by varying the magnitude and frequency of the radio frequency magnetic field, and a significant decrease in switching field by one order of magnitude is achieved under the optimum conditions. The spin wave-assisted magnetization switching is a promising technique for ultralow-energy magnetization manipulation.

Giant spin Hall effect in perpendicularly spin-polarized FePt/Au devices.

Conversion of charge current into pure spin current and vice versa in non-magnetic semiconductors or metals, which are called the direct and inverse spin Hall effects (SHEs), provide a new functionality of materials for future spin-electronic architectures. Thus, the realization of a large SHE in a device with a simple and practical geometry is a crucial issue for its applications. Here, we present a multi-terminal device with a Au Hall cross and an FePt perpendicular spin injector to detect giant direct and inverse SHEs at room temperature. Perpendicularly magnetized FePt injects or detects perpendicularly polarized spin current without magnetic field, enabling the unambiguous identification of SHEs. The unprecedentedly large spin Hall resistance of up to 2.9 mOmega is attributed to the large spin Hall angle in Au through the skew scattering mechanism and the highly efficient spin injection due to the well-matched spin resistances of the chosen materials.

Enhanced spin accumulation and novel magnetotransport in nanoparticles.

Spin injection and accumulation are key phenomena supporting a variety of concepts for spin-electronic devices. These phenomena are expected to be enhanced in nanoparticles over bulk structures due to their discrete energy levels and large charging energies. In this article, precise magnetotransport measurements in the single-electron tunnelling regime are performed by preparing appropriate microfabricated devices containing cobalt nanoparticles. Here we provide experimental evidence for characteristic features of spin accumulation in magnetic nanoparticles, such as oscillations of the magnetoresistance with a periodical sign change as a function of bias voltage. Theoretical analysis of the magnetoresistance behaviour clearly shows that the spin-relaxation time in nanoparticles is highly enhanced in comparison with that in the bulk.