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To realize high areal density, hard disk drives larger than 4 Tbit in−2, ultrafine FePt grains of less than 5 nm and grain density larger than 24 T in−2 are required. Although there have been many investigations to reduce the grain size of FePt, there are only a few reports on the control of grain density. To increase the grain density, we focused on three aspects of the surface morphology and grain density: nucleation sites on the substrate surface, surface free energy, and lattice mismatch. We achieved 14 T in−2 by maximizing the number of nucleation sites in the FePt-C granular film and found that the surface free energy and lattice mismatch are crucial parameters for controlling the grain density.

To realize a sustainable society, ‘green technology’ with low (or even zero) CO2 emissions is required. A key material in such technology is a permanent magnet because it is utilized for electric-power conversion in several applications including electric vehicles (EVs), hybrid EVs (HEVs), and turbines for wind power generation. To realize highly efficient electric-power conversion, a stronger permanent magnet than Nd–Fe–B is necessary. One potential candidate is a Fe-rich SmFe12-based compound with a ThMn12 structure. In this paper, the phase stability, structure, and intrinsic and extrinsic magnetic properties in both film and bulk forms are reviewed. Based on these results, a possible way to realize a strong SmFe12-based permanent magnet in bulk form is discussed., Graphical abstract

We report the magnetotransport properties and the microstructure of polycrystalline current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) pseudospin valves (PSVs) using $\langle 001\rangle $ textured Co2Fe(Ga0.5Ge0.5) (CFGG) ferromagnetic layers grown on a conductive (Mg0.5Ti0.5)O (MTO) buffer layer. The PSV with a 10 nm CFGG on 2 nm MTO buffer layers annealed at 400 °C shows a resistance change–area ( $\Delta RA$ ) product of 6.6 $\text{m}\Omega ~\mu \text{m}^{2}$ . Interfacial roughness of the PSV multilayer has been substantially reduced (0.2 nm) compared with those of our previously reported MgO-buffered CFGG/Ag/CFGG PSVs. A relatively large magnetoresistance output and favorable interfacial smoothness make MTO a promising choice to fabricate a $\langle 001\rangle $ textured Heusler-alloy-based CPP-GMR device.

A magnetic tunnel junction (MTJ) spin-filter device of Pt/Co ferrite/MgO/Co demonstrated -19.8 % tunnel magnetoresistance (TMR) and 27.5% spin-filter efficiency at 10 K. By increasing the bias voltage, the TMR at 10 K first increased and then decreased, exhibiting a maximum at 0.17 V. This anomalous dependence of TMR on bias was interpreted as tunneling via the spin-down conduction band in the Co ferrite. At room temperature (RT), the tunneling mechanism was thought to originate from hopping in the defect states due to structural and chemical defects in the Co ferrite. To achieve higher spin-filter effect at RT, it is important to reduce the defects in the Co ferrite barrier.

For the development of room temperature spin-filtering devices with a ferromagnetic barrier, very thin ferromagnetic barrier films were required. Co-ferrite thin films are prepared by the surface plasma oxidization of a CoFe2 underlayer deposited on MgO (001) single crystal substrates. During oxidization, the substrate temperature was varied from 523 K to 673 K. The plasma oxidized CoFe2 films had a spinel structure, 4-fold symmetry in the plane, and an epitaxial relationship with the CoFe2 underlayer. The lattice parameter of Co-ferrite thin film was 0.828 nm, which is 1.2% less than that of bulk. The interface between the CoFe2 and its oxide was relatively smooth. When the substrate temperature during plasma oxidization was lower than 623 K, magnetization of CoFe2 and Co-ferrite rotated independently. XPS and MOKE measurements identified that the hard and soft phases as Co-ferrite and CoFe2, respectively. Measurements of the magnetic properties showed that the magnetization of the Co-ferrite thin films was about 1100 emu/cm3, which is 2.6 times more than that of bulk (420 emu/cm3). This phenomenon may be explained by the exchange of Co and Fe cation distribution in our Co-ferrite films.

A detailed study of the Fe2-xCoxMnSi (0 = 0.2 as revealed by the magnetization and resistivity data. The Curie temperature and saturation magnetization values increase with increase in the Co concentration. Anomalous, non metallic-like behaviour is observed for x = 0.4. Current spin polarization values of 0.61 +/- 0.01 and 0.66 +/- 0.01 were deduced for x = 0.2 and x = 0.4 respectively, using the point contact Andreev reflection spectroscopy. Considering the high spin polarization and Curie temperature, materials with x >= 0.2 appear to be very promising for spintronic devices.

The compositional dependence on spin polarization of Co100−xFex alloys has been studied by point contact Andreev reflection (PCAR) and tunneling magnetoresistance (TMR) measurements. The intrinsic spin polarization for bcc Co75Fe25 alloy is P=0.58±0.03 at 4.2K contrary to the pure Fe (P=0.46±0.03) and Co (P=0.45±0.03). The tunneling spin polarization values of Co75Fe25 (110) textured polycrystalline electrode and (001) epitaxially grown electrode was estimated to be PT=0.5±0.01 and PT=0.57±0.01 at 8K from the TMR ratios using Julliere’s model for the MTJs prepared on oxidized Si and MgO (001) substrates. The spin polarization obtained from the tunneling junctions and PCAR experiments have been discussed.

We have investigated the structure and the spin polarization of Co2MnAl, Co2MnSn, and Co2MnAl0.5Sn0.5 alloys. The structures of the Co2MnAl and Co2MnSn alloys that were annealed for four days at 450°C were B2 and L21, respectively. Co2MnAl0.5Sn0.5 had a dendrite microstructure that was formed by the phase separation into Al-enriched B2 and Sn-enriched L21 phases during solidification. The saturation magnetization values of Co2MnAl, Co2MnAl0.5Sn0.5, and Co2MnSn were 3.66μB, 4.3μB, and 4.9μB, respectively, which are slightly lower than the theoretical values of 4μB, 4.5μB, and 5μB due to the low degree of L21 ordering. The spin polarization P measured by the point contact Andreev reflection technique were 0.56 for B2-Co2MnAl, 0.6 for L21-Co2MnSn, and interestingly, a higher value of 0.63 for the two phase Co2MnAl0.5Sn0.5 alloy.