Kunihiro Koike

High perpendicular magnetic anisotropy in D022-Mn3+xGe tetragonal Heusler alloy films

We prepared D022-Mn3+xGe (−0.67 ≤ x ≤ 0.35) epitaxial thin films on MgO(001) substrates with Cr(001) buffer layers and systematically investigated the dependence of their perpendicular magnetic anisotropy constant, saturation magnetization, coercivity, and tetragonal axial ratio (c/a) on their composition and substrate temperature. Single-phase D022 crystal structures were formed in films with compositions of 0 ≤ x ≤ 0.35, prepared at 400 °C. The D022-Mn3Ge films exhibited perpendicular magnetization with a magnetic squareness close to unity. Performing magnetic torque measurements at an applied field of 140 kOe, we estimated a perpendicular magnetic anisotropy constant of 11.8 ± 0.5 Merg/cm3, the highest and the most reliable value yet reported.

Reduction of Surface Resistance of ErBa2Cu3O7-δ Films by BaZrO3 Nano-Particle Inclusion

Low surface resistance is achieved by introducing BaZrO3 nano-particles in high crystalline quality ErBa2Cu3O7-δ films. The surface resistance of a ErBa2Cu3O7-δ film with BaZrO3 nano-particles grown by pulsed laser deposition is 55% of that without BaZrO3 nano-particles near 65 K. This result suggests that the ErBa2Cu3O7-δ films with BaZrO3 nano-particles have potential as high power microwave emitting devices. A new direction is shown for reducing the surface resistance of superconducting films.

Effect of rare-earth overlayer on the coercivity in sputter-deposited Nd-Fe-B films

Nd2Fe14B films with the c-axis perpendicular to the film plane were deposited by introducing Mo (110) underlayer on the SiO2 substrates. When the temperature of the substrate during the deposition of the Nd-Fe-B layer was 600 °C, the demagnetization curve exhibited the coercivity of Hc=11 kOe. We observed an Hc increase of up to 14 kOe by capping the film with the Nd overlayer and post-annealing at 500°C. Cross-sectional TEM observation and NBD analysis suggest that the Nd in the overlayer is in the oxide state with either cubic-NdO2 or hcp-Nd2O3 structures.

Large difference between the magnetic properties of Ba and Ti co-doped BiFeO3 bulk materials and their corresponding nanoparticles prepared by ultrasonication

The ceramic pellets of the nominal compositions Bi0.7Ba0.3Fe1−x Ti x O3 (x = 0.00–0.20) were prepared initially by standard solid state reaction technique. The pellets were then ground into micrometer-sized powders and mixed with isopropanol in an ultrasonic bath to prepare nanoparticles. The x-ray diffraction patterns demonstrate the presence of a significant number of impurity phases in bulk powder materials. Interestingly, these secondary phases were completely removed due to the sonication of these bulk powder materials for 60 minutes. The field and temperature dependent magnetization measurements exhibited significant difference between the magnetic properties of the bulk materials and their corresponding nanoparticles. We anticipate that the large difference in the magnetic behavior may be associated with the presence and absence of secondary impurity phases in the bulk materials and their corresponding nanoparticles, respectively. The leakage current density of the bulk materials was also found to suppress in the ultrasonically prepared nanoparticles compared to that of bulk counterparts.

Fabrication of a/c axes oriented grain boundaries in YBCO films by selective growth

The a/c axes grain boundaries of YBCO films are very interesting from the viewpoints of Josephson junction technology and the magnetic flux pinning. In this paper, we intend to flow superconducting current along the grain boundary and to prevent the magnetic flux motion from passing across the grain boundary. The a/c axes grain boundaries were fabricated on SrLaGaO/sub 4/ (100) substrates by pulsed laser deposition on Gd/sub 2/CuO/sub 4/ (GCO) buffer layers. The preferred orientation and in-plane orientation of the films were characterized by X-ray /spl theta//2/spl theta/ diffraction and /spl phi/-scan spectroscopy. 4-micrometer pitch GCO patterns were fabricated by a lithography technology. Subsequently, the YBCO films were deposited on the pattern. A TEM image showed that a-axis grains grew over c-axis grains.

Negative exchange coupling in Nd2Fe14B(100)/α-Fe interface

A model sample with an interface of the Nd2Fe14B(100) plane and the α-Fe phase was fabricated in order to evaluate the exchange-coupling constant Jex between these two ferromagnetic phases. By measuring the ferromagnetic resonance fields and the Kerr loops, the Jex was confirmed to be negative with the value of Jex = −6.5 × 10−4 J/m2, in striking contrast to the positive Jex value for the Nd2Fe14B(001)/α-Fe(100) interface reported previously.

Modification of the Interface Nanostructure and Magnetic Properties in Nd-Fe-B Thin Films

The effects of Nd2Fe14B grain size and Nd coating on the coercivity in sputter-deposited Nd-Fe-B/Nd thin films have been investigated in order to gain an insight into the coercivity mechanism of Nd-Fe-B magnets. Highly textured Nd2Fe14B particles were grown successfully on the MgO(100) single-crystal substrate with the Mo underlayer. As the Nd-Fe-B layer thickness tNFB was decreased from 70 to 5 nm, the coercivity Hc increased gradually from 6.5 to 16 kOe. By depositing the Nd overlayer onto these films and post-annealing at 500 °C, the Hc value further increased from 17.5 kOe (tNFB=70 nm) to 26.2 kOe (tNFB=5 nm). The amount of Hc increase by the combination of the Nd coating and post-annealing was about 10 kOe irrespective of the tNFB value. These results therefore suggest an independence of size and interface effects on the coercivity of Nd-Fe-B magnets.

Microstructure and coercivity in La-coated Nd2Fe14B thin films

We investigated La-coated Nd2Fe14B thin film magnets by scanning/transmission electron microscopy to clarify the coercivity enhancement mechanism in Nd-Fe-B permanent magnets. The coercivity of La-coated film magnets was increased from 8.5 to 15 kOe by post-annealing. The fcc-LaOx layer was epitaxially grown on [001]-oriented Nd2Fe14B fine grains with a crystallographic orientation relation of Nd2Fe14B (001)[110] || LaOx (111)[112¯], both in the as-deposited and post-annealed films. STEM-EDS elemental analysis revealed that the only difference caused by post-annealing was the O content in the LaOx layer, which increased from 15 to 35 at.%. These results suggest that the magnetocrystalline anisotropy of the Nd ions, which were reduced at the surface of Nd2Fe14B, would be recovered by the presence of O, leading to a remarkable increase in the coercivity.