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Nd-Fe-B-based permanent magnets are widely used for energy conversion applications. However, their usage at elevated temperatures is difficult due to the relatively low coercivity (Hc) with respect to the anisotropy field (HA) of the Nd2Fe14B compound, which is typically 0.2HA. In this work, we found that the coercivity of an (Nd0.8Dy0.2)-Fe-B sintered magnet could reach 0.4HA, which was twice as high as the Hc/HAof its Dy-free counterpart. Detailed microstructural characterizations, density functional theory and micromagnetic simulations showed that the large value of coercivity, Hc= 0.4HA, originated not only from the enhanced HAof the main phase (intrinsic factor) but also from the reduced magnetization of the thin intergranular phase (extrinsic factor). The latter was attributed to the dissolution of 4 at.% Dy in the intergranular phase that anti-ferromagnetically coupled with Fe. The reduction in the magnetization of the intergranular phase resulted in a change in the angular dependence of coercivity from the Kondorsky type for the Dy-free magnet to the Stoner–Wohlfarth-like shape for the Dy-containing magnet, indicating that the typical pinning-controlled coercivity mechanism began to show nucleation features as the magnetization of the intergranular phase was reduced by Dy substitution.

Abstract

Spin-transfer-torque magnetoresistive random-access memory (STT-MRAM) is an emerging nonvolatile memory that uses magnetic tunnel junctions (MTJs) to store information. MTJs with a crystalline MgO(001) tunnel barrier sandwiched between ferromagnetic layers, such as CoFeB, exhibit giant tunnel magnetoresistance, which is used to readout the STT-MRAM. Writing of STT-MRAM is based on current-induced magnetization reversal, called STT switching. STT-MRAM with perpendicular magnetization is especially important for high-density and low-power-consuming memory applications such as embedded memory for large-scale integrated circuit. For STT-MRAM to replace ultrahigh-density dynamic random-access memory, however, there are still technological challenges concerning the materials and fabrication processes of MTJs. This article reviews the physics and materials science of MTJs for STT-MRAM. We also discuss the importance of new MTJ materials and processes for next-generation ultrahigh-density MRAM.

This article reviews progress in magnetoresistive (MR) read sensor technology for hard-disk drives (HDDs). MR reader technology has progressed from the anisotropic magnetoresistance sensor, to the current-in-plane giant magnetoresistive (CIP-GMR) sensor, to today’s current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) sensor. This evolution has driven the continuous growth of the areal density of HDDs from 2 Gbpsi (gigabits/in2) in early longitudinal recording to ∼1 Tbpsi (terabits/in2) currently in perpendicular magnetic recording. For further increases in the areal density, a transition to energy-assisted recording is expected in the near future. Further technical challenges for the read sensor technology toward 2 Tbpsi and then 5 Tbpsi areal densities are discussed based on recent promising experimental work on CPP-GMR using Heusler alloys, and CPP-GMR’s laterally expanded version, the lateral spin valve (LSV). To realize large MR output and narrow shield-to-shield spacing requirements for higher density recording, materials selection and optimization of interface structures of CPP-GMR and LSV devices are critical.

Abstract

In the long history of permanent magnet research for more than 100 years, three-dimensional magnetic microscopy has been eagerly awaited to elucidate the origin of the magnetic hysteresis of permanent magnets. In this study, we succeeded in observing the three-dimensional magnetic domain structure of an advanced high-coercivity Nd-Fe-B-based permanent magnet throughout the magnetic hysteresis curve using a recently developed hard X-ray magnetic tomography technique. Focused-ion-beam-based three-dimensional scanning electron microscopy was employed to study the relationship between the observed magnetic domains and the microstructure of the magnet for the same observing volume. Thermally demagnetized and coercivity states exhibit considerably different magnetic domain structures but show the same periodicity of 2.3 μm, indicating that the characteristic length of the magnetic domain is independent of the magnetization states. Further careful examination revealed some unexpected magnetic domain behaviors, such as running perpendicular to the magnetic easy axis and reversing back against the magnetic field. These findings demonstrate a wide variety of real magnetic domain behaviors along the magnetic hysteresis inside a permanent magnet.

Abstract

Large saturation magnetization in the SmFe12-based compounds is realized by substitution of scarce Co for Fe site. However, reduction of Co is necessary for their potential practical applications. In this work, we successfully decrease the concentration of Co, from 17.3 at% to 8.7 at% while maintaining large μ0Msof 1.41 T at 300 K in Ti reduced Sm0.8Y0.2(Fe0.9Co0.1)11.25Ti0.75compound. In addition, we report the highest μ0Msfor bulk Sm0.8Y0.2(Fe0.8Co0.2)11.25Ti0.75with μ0Ms= 1.49 T, μ0Ha= 9.14 T at 300 K and Tc= 831 K. We demonstrate (Sm0.8Y0.2)Fe12-based compounds exhibit superior intrinsic magnetic properties than Nd2Fe14B above 473 K.

In this work, our attempts focused on Ge effects on magnetic-microstructural relationships in Nd-Fe-Co-B nanocrystalline ribbons. Based on Transmission Electron Microscopy (TEM) and three dimensional atom probe (3DAP) analysis, it was observed that Ge containing alloys have a new behavior that was not observed yet. The increase of Ge content up to 0.36at% caused the increase of Brand ( BH) maxdue to increase of Co content of Nd 2Fe 14B phase. However, the magnetic properties deteriorated for Ge bearing samples beyond 0.36at% due to the decrease of volume fraction of Nd 2Fe 14B phase. The comparison of Ge and Ga (another M1 dopant) effects on coercivity was reported and microstructural studies (using TEM and 3DAP) were shown the Nd-rich phase morphology and its alloying elements distribution in the nanometer sized intergranular phase.

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The compositional and structural investigations of threading dislocations (TDs) in InGaN/GaN multiple quantum wells were carried out using correlative transmission electron microscopy (TEM) and atom probe tomography (APT). The correlative TEM/APT analysis on the same TD reveals that the indium atoms are diffused along the TD and its concentration decreases with distance from the InGaN layer. On the basis of the results, we directly observed that the indium atoms originating from the InGaN layer diffuse toward the epitaxial GaN surface through the TD, and it is considered to have occurred via the pipe diffusion mechanism induced by strain energy relaxation.

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GP zones in A1-1·7at.%Cu alloy aged at room temperature for 760 h, and 373 and 403 K for less than 1000min, have been observed by field-ion microscopy to elucidate the structure of the zones. A typical zone structure was found to be the well-known single layer of Cu atoms on (001) planes. On the other hand, another type of zone, i.e. multi-layer GP zones, was found to co-exist with the single-layer GP zones. Detailed observation on (022) and (002) planes showed that the multilayer GP zones consist of a few Cu-rich layers on (002) planes.

Progress in theory and experiment on the early stage of decomposition of alloys is reviewed. In the first half of this paper, theoretical treatments on decomposition of alloys by spinodal and nucleation mechanism and also continuum theories and the discrete lattice theories are comparatively reviewed. In the latter half of this paper, experimental results on Al-Zn, Ni-Al, Cu-Co and Al-Ag alloys are discussed.

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We investigated laser-assisted field evaporation phenomena of Al x Ga1[?]x N alloys with various Al site fractions x (0.05, 0.10, 0.15, 0.20, and 0.25) using an ultraviolet (UV, 343 nm) femtosecond laser to clarify the role of the photoexcitation process. While the measured Al site fractions largely deviate from the corresponding nominal fractions when the band gap energy is higher than the photon energy, these deviations of the measured Al site fractions decreased for band gap energies smaller than the photon energy. We also observed notable improvements in the artifacts on 3D image reconstruction and the measured Al compositions using a deep UV (257 nm) laser, which offers a photon energy that is higher than the band gap and the electron affinity. These tendencies indicate that both the surface electric field and the photoexcitation process significantly influence the field evaporation processes.

InGaN/GaN quantum wells (QWs) on GaN nanowire are expected for applications in high efficiency optoelectronic devices. To improve efficiency, it is important to clarify the relation between optical and structural properties. A systematical optical and structural characterization of QWs in core-shell GaN nanowires is performed by cathodoluminescence and scanning transmission electron microscopy. The QWs grown on the m-facet show varied optical behavior with respect to height position: main-body region (2.82 ? 3.10 eV), upper region (3.26 eV), and corner region (2.88 ? 3.20 eV). The variant luminescence behaviors are attributed to interface quality and In diffusion.

Refinement of the FePt grain size for heat-assisted magnetic recording (HAMR) media was explored by utilizing high melting point metals Pt and W as seed layers. Unlike the W, the Pt seed layer promotes a good epitaxial growth and leads to a higher degree of L10-ordering in FePt films. It also substantially refines the FePt grains from big islands into smaller particles. For the FePt-C nanogranular film, the Pt seed layer improves the wettability and favors a single-columnar growth. Thus, the Pt seed layer provides a new valid approach to optimize the microstructure and magnetic properties for next-generation FePt-based HAMR media.

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−2was 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,0and 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.

Mg-Zn-Zr-Ca (ZKX600) alloy specimens with various fully recrystallized grain sizes ranging from 0.77 um to 23.3 um were fabricated by high pressure torsion (HPT) followed by subsequent annealing. Tensile tests carried out at room temperature revealed that ultrafine grained (UFG) specimens exhibited enhanced combinations of strength and ductility compared to their coarse-grained counterparts. Observation of deformation microstructures suggested that basal slip and {10-12} extension twinning were the dominant mechanisms during deformation of coarse grained specimens, while deformation twinning was significantly inhibited in the UFG specimens. Based on the observations it is proposed that non-basal slip activity is responsible for the enhanced mechanical properties, and should be the focus of further investigations.

In this study, a fully recrystallized ultrafine grained (UFG) Mg-Zn-Zr-Ca alloy was successfully fabricated by a process including high pressure torsion (HPT) and subsequent rapid annealing. Room temperature tensile test revealed that the UFG Mg alloy with a mean grain size of 0.98 um exhibited simultaneously enhanced strength and ductility compared to those of the coarse grained counterpart (grain size 57 um). Observation of deformation microstructures revealed that {10-12} deformation twinning and basal slip were the dominant deformation mechanisms in the coarse grained specimen, while deformation twinning was significantly inhibited but non-basal slip systems seemed activated in the UFG specimen. The reason for the enhanced mechanical properties in the UFG specimen was discussed based on the change of deformation mechanisms observed.

The interfacial atomic structure of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures, which is related to the interface-induced perpendicular magnetic anisotropy (PMA), was investigated using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray magnetic circular dichroism. Al atoms from the Co2FeAl layer significantly interdiffused into MgO, forming an Al-deficient Co-Fe-Al/Mg-Al-O structure near the Co2FeAl/MgO interface. This atomic replacement may have enhanced the PMA, which is consistent with the observed large perpendicular orbital magnetic moments of Fe atoms at the interface. This work suggests that control of interdiffusion at ferromagnet/barrier interfaces is critical for designing an interface-induced PMA system.

Magnetoresistance (MR) and its bias-voltage dependence were investigated in magnetic tunnel junctions (MTJs) with a 2-nm-thick Cu(In0.8Ga0.2)Se2semiconducting barrier. A relatively high MR ratio of 47% was observed with a low resistance-area product RAof 0.14 ?·µm2at 300 K. By increasing the bias voltage, a high output voltage (as high as 24 mV) was achieved; this value is significantly higher than those ever reported for MR devices with RAvalues less than 0.5 ?·µm2. These MR performance characteristics of the MTJs with Cu(In0.8Ga0.2)Se2are suitable for high-sensitivity read head sensors for hard disk drives with a recording density higher than 2 Tbit/in.2.

Epitaxial magnetic tunnel junctions (MTJs) with a Co2FeAl/CoFe (0.5 nm)/MgAl2O4/Co2FeAl(001) structure were fabricated by magnetron sputtering. High-temperature in situ annealing led to a high degree of B2-order in the Co2FeAl layers and cation order of the MgAl2O4barrier. Large tunnel magnetoresistance (TMR) of up to 342% was obtained at room temperature (616% at 4 K), in contrast to the TMR ratio (%) suppressed by the band-folding effect in Fe/cation-ordered MgAl2O4/Fe MTJs. The present study reveals that the high degree of B2-order and the resulting high spin polarization in the Co2FeAl electrodes enable us to bypass the band-folding problem in spinel barriers.

Thin Ga1[?] x In x N layers with various indium compositions x $(0.08,0.15,0.25)$ were examined by high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Although nanoscale compositional fluctuation was observed in all Ga1[?] x In x N layers, no appreciable phase separation was observed. Since Ga1[?] x In x N layers were coherently grown on underlying GaN layers, it is considered that the elastic strain constrains phase separation in the Ga1[?] x In x N layers and that compositional fluctuations develop in order to minimize the local strain energy. The measured period of compositional fluctuations ranged from 3.4 to 4.9 nm and decreased with growth temperature (T growth). This tendency is suggested to be dominated by the surface migration length of the adatom during the crystal growth.

To clarify the magnetism of an amorphous grain boundary (GB) phase in Nd-Fe-B sintered magnets, the magnetic properties of Nd100?xFexfilms with various compositions have been investigated both experimentally and theoretically. The model films exhibit ferromagnetism at room temperature when the Nd content is lower than 70 at. %. First-principles calculations indicate ferromagnetism in fcc Nd-Fe alloys for x< 70, which supports recent experimental works on the intergrain exchange coupling of Nd-Fe-B sintered magnets.

We have successfully developed a technique to obtain atomically flat ultrathin Fe(001) layers on MgO(001). In contrast to the usual island morphology of ultrathin Fe layers grown directly on MgO(001), the insertion of an ultrathin ?-Fe2O3layer improves the crystalline quality and interface flatness of the ultrathin Fe layer of less than 1 nm thick. Owing to the continuous layered structure, even 0.6-nm-thick Fe layers exhibited a ferromagnetic hysteresis curve with high remanent magnetization. The technique enables flexible structural design of spintronic devices with ultrathin ferromagnetic electrodes.

We have successfully developed a technique to obtain atomically flat ultrathin Fe(001) layers on MgO(001). In contrast to the usual island morphology of ultrathin Fe layers grown directly on MgO(001), the insertion of an ultrathin g-Fe2O3 layer improves the crystalline quality and interface flatness of the ultrathin Fe layer of less than 1 nm thick. Owing to the continuous layered structure, even 0.6-nm-thick Fe layers exhibited a ferromagnetic hysteresis curve with high remanent magnetization. The technique enables flexible structural design of spintronic devices with ultrathin ferromagnetic electrodes.

The spin polarization ($P$) of alternate monatomic layered (AML) epitaxial [Fe/Co]nsuperlattices grown on MgO(001) substrates by electron beam (EB) evaporation has been measured by the point contact Andreev reflection (PCAR) method. The intrinsic transport $P$ of 0.60 was obtained for the AML epitaxial [Fe/Co]nsuperlattice grown at 75 °C, which is comparable to that of half-metallic Heusler alloys measured by PCAR. The AML epitaxial [Fe/Co]nsuperlattices on MgO(001), which are expected to possess the B2 ordered structure, show the highest spin polarization of metallic Fe--Co alloy films.

We observed a giant Peltier effect in a submicron Cu-Ni/Au junction. The Peltier coefficient was evaluated to be 480 mV at room temperature from the balance between Joule heating and the Peltier cooling effect in the junction, which is 40 times that expected from the Seebeck coefficients of bulk Au and Cu-Ni alloy. This giant cooling effect lowered the inner temperature of the junction by 160 K. Microstructure analysis with a three-dimensional atom probe suggested that the giant Peltier effect possibly originated from nanometer-scale phase separation in the Cu-Ni layer.

We observed a giant Peltier effect in a submicron Cu-Ni/Au junction. The Peltier coefficient was evaluated to be 480 mV at room temperature from the balance between Joule heating and the Peltier cooling effect in the junction, which is 40 times that expected from the Seebeck coefficients of bulk Au and Cu-Ni alloy. This giant cooling effect lowered the inner temperature of the junction by 160 K. Microstructure analysis with a three-dimensional atom probe suggested that the giant Peltier effect possibly originated from nanometer-scale phase separation in the Cu-Ni layer.

The magnetic and transport properties of bulk polycrystalline samples of double perovskite SrLaVMoO6have been investigated. Structural analyses have shown that the samples are almost in single phase with an ordered double perovskite structure. Magnetization versus temperature measurements have indicated that the SrLaVMoO6compound exhibits a clear cusp at 125 K, which can be attributed to an antiferromagnetic transition. Electrical resistivity ? for the compound showed metallic temperature dependence from 10 to 300 K, and the resistivity ? value at 10 K was as low as 450 µ?·cm. A spin polarization Pvalue as high as 0.50 at 4.2 K was measured using the point-contact Andreev reflection (PCAR) technique. These results were discussed in terms of possible half metallic antiferromagnetism.

We report the study of L10 FePt-C nanogranular perpendicular anisotropy films fabricated on oxidized Si substrates with a (100) textured MgO intermediate layer. The addition of a small amount of C (<12%) to a FePt (4 nm) film results in the formation of interconnected FePt particles, while a higher C addition leads to the formation of well-isolated L10 FePt nanoparticles with a c-axis texture. The FePt particle size can be reduced to 5.5 nm with a size distribution of 2.3 nm in variance by adjusting FePt thickness. Perpendicular coercivity is controllable between 8 and 15 kOe with high squareness. These results demonstrate that the FePt-C system can accomplish a nanogranular structure suitable for ultrahigh density perpendicular recording media.

We report the study of L10FePt-C nanogranular perpendicular anisotropy films fabricated on oxidized Si substrates with a (100) textured MgO intermediate layer. The addition of a small amount of C (<12%) to a FePt (4 nm) film results in the formation of interconnected FePt particles, while a higher C addition leads to the formation of well-isolated L10FePt nanoparticles with a c-axis texture. The FePt particle size can be reduced to 5.5 nm with a size distribution of 2.3 nm in variance by adjusting FePt thickness. Perpendicular coercivity is controllable between 8 and 15 kOe with high squareness. These results demonstrate that the FePt-C system can accomplish a nanogranular structure suitable for ultrahigh density perpendicular recording media.

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Sharp needle-like specimens for atom-probe field-ion microscope (APFIM) analysis have been prepared successfully from mechanically milled pearlitic steel powders. Mechanically milled powders were packed in a stainless steel tube with carbonyl steel powders and then consolidated by sheath cold-rolling to obtain a bulk composite. A small square rod was cut out from the bulk and electropolished to a blunt needle. It was then fabricated to a nanometre-sized needle by a focused ion beam technique using an annular gallium ion beam. The tip radius of the specimen was <50 nm, which is suitable for use in APFIM.