(Bi_1-xBa_x)FeO₃ multiferroic thin films with ferromagnetism and ferroelectricity were fabricated and applied to create magnetic recordings using an electric field. The (001)-oriented (Bi_1-xBa_x)FeO₃ thin films of which electric polarization direction is perpendicular to the film plane were fabricated onto a non-single-crystalline substrate with a Ta seedlayer / (111)-oriented Pt underlayer at a low substrate temperature of 500 ℃. A very high frequency plasma irradiation was applied during sputtering deposition of (Bi_1-xBa_x)FeO₃ to accelerate the crystallization at the low substrate temperature. The fabricated (Bi_0.6Ba_0.4)FeO₃ film exhibited hysteresis curves indicating ferromagnetic and ferroelectric behavior. The saturation magnetization of the film was approximately 60 emu/cm³ and the coercivity was approximately 2.5 kOe, respectively. Magnetic Force Microscopy analysis of the (Bi_0.6Ba_0.4)FeO₃ film confirmed that the magnetization was reversed by applying only a local electric field. The multiferroic film described here is expected to be useful for electric field-driven magnetic devices.

  We discuss the writing field amplitude in heat-assisted magnetic recording (HAMR) employing a new model calculation. The grain magnetization direction is calculated using the magnetization reversal probability P for each attempt period, whose inverse is the attempt frequency. When the writing field H_w is low, P_- and P₊, where the magnetization and H_w change from antiparallel to parallel and vice versa, respectively, are close. Therefore, there are many grains whose magnetization turns in the direction opposite to the recording direction for a low H_w. The temperature dependence of the attempt frequency is calculated by employing the conventionally used micromagnetic calculation, and the attempt frequency decreases monotonically as the temperature increases and becomes zero at the Curie temperature T_c. Therefore, writing is difficult just below T_c since there is almost no opportunity for writing. Although the coercivity can be reduced by any amount during writing in HAMR, a relatively high writing field is necessary.

  The bulk coercivity and coercivity of the polished surface layer of a Nd-Fe-B-based sintered magnet were measured systematically in order to discuss the effectiveness of observing the magnetic domain on the surface of the magnet. It was revealed that the bulk coercivity and coercivity of the polished surface layer were in a proportional relationship even though the coercivity of the surface layer was much smaller than the bulk coercivity. This suggests that the magnetization reversal that occurred in the surface layer has a mechanism similar to that inside of the magnet. The relationship between the bulk coercivity and coercivity of the surface layer was not proportional when the degree of alignment decreased, which can be explained by the increase in the local demagnetizing field on the surface of the magnet.

  To clarify the difference between the degree of alignment dependence of coercivity and the angular dependence of coercivity, the crystal orientation distribution and demagnetization curve of Nd-Fe-B-based sintered magnets with different degrees of alignment were compared. It is suggested that the increase in coercivity due to a low degree of alignment cannot be explained only by the angular dependence of coercivity. A crystal orientation analysis and in-situ observation of magnetic domains that were performed in the same area clarified that the ratio of the grain boundary where the magnetization reversal stopped became larger when the misorientation angle between adjacent grains became larger. This suggests that a grain boundary having a larger misorientation angle is one of the factors that suppresses magnetization reversal.

  Fe, Co, and Ni films of 40 nm thickness are prepared on GaAs(111) single-crystal substrates at room temperature by using a radio-frequency magnetron sputtering system. The film growth behavior and the crystallographic properties are investigated by in-situ reflection high-energy electron diffraction and pole-figure X-ray diffraction. bcc single-crystals of (111) orientation are formed on the substrates for all the film materials, though the bcc structure is metastable for Co and Ni materials. The metastable structure is stabilized through hetero-epitaxial growth. Fe films possess bcc structure for the investigated thickness range. On the contrary, the bcc-Co and the bcc-Ni crystals, respectively, start to transform into hcp and fcc structures, as the thickness is increased beyond 2 nm. The phase transformations occur through atomic displacements from the close-packed planes of bcc(110), bcc(101), and bcc(011), which are perpendicular to the substrate surface, to hcp(0001) and fcc(111) close-packed planes. The crystallographic orientation relationships of hcp and fcc crystals with respect to bcc crystal are similar to the Kurdjumov-Sachs relationship.

  We study spin-dependent transport properties in magnetic tunneling junctions (MTJs) with semiconductor barriers, Fe/CuInSe₂/Fe(001) and Fe/CuGaSe₂/Fe(001). By analyzing their transmittances at zero bias voltage on the basis of the first-principles calculations, we find that spin-dependent coherent tunneling transport of Δ₁ wave functions yields a relatively high magnetoresistance (MR) ratio in both the MTJs. We carry out a detailed analysis of the band-resolved transmittances in both the MTJs and find an absence of the selective transmission of Δ₁ wave functions in some energy regions a few eV away from the Fermi level due to small band gaps in CuInSe₂ and CuGaSe₂.

  Medical applications such as those using magnetic nanoparticles (MNPs) for hyperthermia and magnetic particle imaging (MPI) require suitably designed particles with distinct characteristics. However, it is challenging to develop such particles with a high degree of biocompatibility. In this study, a cationic diblock copolymer (PMPC₁₀₀-b-MMAPTAC₁₀₀: P₁₀₀M₁₀₀) composed of poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) and poly(3-(methacryloylamino)propyl trimethylammonium chloride) (PMAPTAC) was synthesized via a controlled radical polymerization technique to obtain particles with high biocompatibility and antithrombogenicity. Magnetic polyion complex (PIC) aggregate (M-300/P₁₀₀M₁₀₀) is an aggregate of magnetic Fe₃O₄ nanoparticles (M-300), in which their anionic surface is electrostatically coated with cationic PMPC₁₀₀-b-MMAPTAC₁₀₀ (P₁₀₀M₁₀₀). We investigated the stability of the magnetic PIC aggregate in an ionic solution by evaluating the relationship between the particle diameter and salt concentration. We then estimated the intrinsic loss power (ILP) from the areas of the alternating current (AC) hysteresis loops and measured the AC magnetization of the magnetic PIC aggregate. The peak frequencies from the Brownian relaxation of M-300 and M-300/P₁₀₀M₁₀₀ were 9 kHz and 245 Hz, respectively. When the third harmonic was evaluated for use in MPI, the signal intensity was found to be comparable to that of M-300 in a fixed state.

  Implantable medical devices are utilized in the human body for maintaining good health. As these devices are becoming increasingly functionalized, supplying power to them has become very important. Instead of batteries, technologies for wireless power supplies are being developed, such as inductive coupling or piezoelectric elements. However, we propose the use of magnetic wires for this purpose. Inside these wires, a fast magnetization reversal, called a “large Barkhausen jump,” occurs due to an applied magnetic field. This reversal induces a large pulse voltage in pick-up coils, which is called a “Wiegand pulse.” By applying this pulse as an electric source, a higher electric power is expected compared with the conventional method using a sinusoidal excitation field. A wire core coil was prepared, and open-circuit voltage was measured. In addition, DC voltage and electric power were measured by connecting the wire core coil to a rectifier. The experimental results confirmed the superiority of using a Wiegand pulse at an applied magnetic field intensity of 60 Oe and a frequency of lower than 10 kHz.

  Magnetic particle imaging (MPI) is a novel diagnostic imaging technique based on the use of magnetic nanoparticles (MNPs). Investigating the magnetic properties of magnetic nanoparticles is important for achieving a high spatial and temporal resolution in MPI. In this study, γ-Fe₂O₃ nanoparticles (core diameter: d_c = 4 nm), Fe₃O₄ nanoparticles (d_c = 20–30 nm), and Resovist^® were immobilized in a DC magnetic field with their easy axes aligned. DC and AC magnetization curves were measured for the prepared MNPs. The measurements were performed by applying fields parallel and perpendicular to the easy axis and evaluating the magnetic properties of the MNPs for the easy and hard axes. The direction of magnetic moments under the AC magnetic field applied to the direction of the easy axis or hard axis was evaluated by using both experimental results and numeric simulation using the Landau–Lifshitz–Gilbert equation to reveal magnetic relaxation property at wide frequency range and the effect of core size distribution of oriented MNPs. The effect of anisotropy in superparamagnetic nanoparticles, the relaxation property depending on the anisotropy energy barrier, and the fast magnetization process of Néel relaxation were successfully observed.