Iduru Shigeta

Spin-glass and antiferromagnetic transitions in Ru2−xFexCrSi

In this study the properties of the Heusler compounds Ru2−xFexCrSi (0< x < 0.1) are investigated. Ru1.9Fe0.1CrSi was revealed to exhibit two anomalies in the temperature dependence of magnetization M(T); one is a peak in M(T) at T*N ~ 30 K and the other is strong irreversibility in M(T) below T*N, the onset of which is defined as Tg. M(T) and specific heat CP(T) of Ru2CrSi are measured. A clear peak is observed in both M(T) and CP(T). This is in quite contrast to the case for Ru1.9Fe0.1CrSi, where no anomaly in CP(T) was observed at T*N or at any other temperatures. These indicate that an antiferromagnetic transition occurs at TN = 14 K in Ru2CrSi. For x = 0.02 the characteristic two anomalies in M(T) are also found at various magnetic fields, and their field dependence is basically the same as that for x = 0.1. These two anomalies are suggestive of successive spin-glass transitions. As for the dependence of the two anomalies on x, Tg does not change with x, whereas T*N decreases with decreasing x and seems to approach Tg at zero field.

Substitution effect on the magnetic transitions of Fe2MnSi

The Heusler compound Fe2MnSi exhibits a ferromagnetic transition at TC ~ 220 K, and, below, another magnetic transition at TR ~ 65 K, which is supposed to be a ferrimagnetic transition. In this study we investigate the effects of the substitution of Co and V for Fe and Mn, respectively, on these transitions. The temperature dependence of magnetization M(T) is measured for (Fe1−yCoy)2MnSi (y < 0.1) and Fe2Mn1−xVxSi (x < 0.2). Substitution for both the cases leads to the fairy rapid decrease and the disappearance of TR. However, for Co-substitution TC increases rapidly but for V-substitution TC increases only slowly. The electrical resistivity ρ(T) of Fe2Mn0.8V0.2Si is measured in magnetic field up to 9 T. From this it is found that TR is 6 K at 0 T and is absent at 7 T. The critical field of the transition for TR is estimated to be ~ 7 T.

Transport Properties of Heusler Compound Ru2−xFexCrSi under Pressure

We investigated transport properties of the full-Heusler compound Ru2−xFexCrSi (x = 0.1), which is an insulator, under pressure P up to 8 GPa. The temperature dependence of electric resistance R(T) for Ru1.9Fe0.1CrSi is insensitive to pressure. R(T) is not described by the simple activation-type but -logT in the wide T and all P range we measured. This might be due to atomic disorder in the crystalline.

Magnetic properties of the Heusler alloys Fea-xCoxMnSi

From first-principle band structure calculations the Heusler alloys Fe2MnSi and Co2MnSi are predicted to be half-metals, if the ferromagnetic state is assumed. In reality, however, Fe2MnSi exhibits a complex magnetic state with antiferromagnetic components, while Co2MnSi is ferromagnetic with high Curie temperature and probably a half metal. In order to search better half-metals the alloys Fe2-xCox MnSi are worth investigating. We have studied the magnetic properties of Fe2-xCo:cMnSi experimentally. With increasing Co concentration from x = 0, the antiferromagnetic phase rapidly disappears. The saturation magnetization increases with increasing x. For x≥ 1.0 the saturation magnetizations agree well with those calculated theoretically. The dependence of the saturation magnetization on x is well described by the Slater-Pauling rule. This result suggests that Fe2-xCoxMnSi is probably a half-metal for a wide range of x.

Smearing origin of zero-bias conductance peak in Ag-SiO-Bi2Sr2CaCu2O8+δ planar tunnel junctions: influence of diffusive normal metal verified with the circuit theory

Abstract.We propose a new approach of smearing origins of a zero-bias conductance peak (ZBCP) in high-Tc superconductor tunnel junctions through the analysis based on the circuit theory for a d-wave pairing symmetry. The circuit theory has been recently developed from conventional superconductors to unconventional superconductors. The ZBCP frequently appears in line shapes for this theory, in which the total resistance was constructed by taking account of the effects between a d-wave superconductor and a diffusive normal metal (DN) at a junction interface, including the midgap Andreev resonant states (MARS), the coherent Andreev reflection (CAR) and the proximity effect. Therefore, we have analyzed experimental spectra with the ZBCP of Ag-SiO-Bi2Sr2CaCu2O8+δ (Bi-2212) planar tunnel junctions for the {110}-oriented direction by using a simplified formula of the circuit theory for d-wave superconductors. The fitting results reveal that the spectral features of the ZBCP are well explained by the circuit theory not only excluding the Dyness broadening factor but also considering only the MARS and the DN resistance. Thus, the ZBCP behaviors are understood to be consistent with those of recent studies on the circuit theory extended to the systems containing d-wave superconductor tunnel junctions.

Magnetization and magnetic phase diagram of Heusler compounds Fe3−y (Mn1−x V x ) y Si (y = 1 and 1.5)

Fe2MnSi exhibits a ferromagnetic transition at TC and another transition to a phase with antiferromagnetic components at a low temperature of TA . By substituting V for Mn, so as to obtain Fe2Mn1−x V x Si, TA decreases with x and then vanishes around x ~ 0.2. In this study, the magnetic transitions are investigated by magnetization measurements for Fe2Mn1−x V x Si (y = 1, 0 ≤ x ≤ 0.2) in high magnetic fields up to ~70 T and for Fe1.5(Mn1−x V x )1.5Si (y = 1.5, 0 ≤ x ≤ 0.1) up to 5 T. For y = 1.5, with increasing x, TC increases and TA decreases as for y = 1, and the rate of the decrease in TA with x is slightly smaller than for y=1. In the low magnetic field region, TA for y = 1 does not significantly change due to the magnetic field but a distinct decrease is observed in high fields. The critical field at 0 K of this transition in Fe2MnSi is found to be larger than 70 T, and with increasing x, the critical field decreases corresponding to the decrease of TA at zero field.

Epitaxial contact Andreev reflection spectroscopy of NbN/Co2FeSi layered devices

We investigated the spin polarization P of Co-based Heusler alloy Co2FeSi by epitaxial contact Andreev reflection (ECAR) spectroscopy using epitaxially grown superconductor NbN and Heusler alloy Co2FeSi layered devices. Ferromagnetic Co2FeSi possesses the highest Curie temperature (TC ≈ 1100 K) and the largest spontaneous magnetic moment (ps ≈ 6 μB) in the class of Heusler alloys. The ECAR measurements revealed that the P value of Co2FeSi was 54 ± 2% with a finite barrier parameter Z, indicating that an intrinsic P value in ECAR spectroscopy would exceed reported values in point-contact Andreev reflection spectroscopy. We therefore established not only the epitaxial integration of ferromagnetic Co2FeSi with superconductor NbN on an MgO substrate but also the fabrication and evaluation techniques of their ECAR devices. This highly versatile superconducting spintronic system enables fundamental superconducting spintronic studies, and it is also a candidate for practical superconducting spintronic devices.

Fabrication and Measurement of High-Temperature Superconductor YBa2Cu3O7−δ: Activity Report of Science Club

We report our activity of science club in a special program that provides intensive education in science and mathematics at the Faculty of Science in Kagoshima University. The special program of intensive education in science and mathematics was started in 2008 as a four-year project for undergraduate students who belonged to our Faculty of Science. This program was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The science clubs have been organized as one of its extracurricular activities for early research experiences in the first to third year undergraduate students. Our research group has fabricated and measured polycrystalline samples of high-temperature superconductors in the science club. We have chosen YBa2Cu3O7−δ (YBCO) from many high-temperature superconductors because YBCO is not sensitive for heat treatment temperatures in fabrication process and its superconducting transition temperature Tc is higher than the boiling point of nitrogen. Therefore we think that YBCO is the most suitable for teaching materials of superconductors in undergraduate student experiments. Polycrystalline YBCO samples were fabricated by the solid state reaction method. After mixing amounts of starting reagents (Y2O3, CuO and BaO) in a mortar using a pestle, mixed powder was heated in an electric furnace at 930◦C for three days. At this point, sintered YBCO was mixed again and powdered YBCO were pressed into pellets. The YBCO pellets were sintered again in the electric furnace at the same heating condition. We have evaluated the crystal structure of obtained YBCO samples by power X-ray diffraction (XRD) measurements. The resulting powder XRD patterns were analyzed by using the Rietveld refinements in order to determine lattice constants and other structural parameters. The superconductivity of obtained YBCO samples was checked by the Meissner effect. A small cylindrical neodymium magnet levitated over a piece of liquid nitrogencooled YBCO in a foam polystyrene plate. The electrical resistivity was measured by the standard dc four-probe method. The electrical resistivity was sharply dropped due to the superconducting transition with the decrease of temperature, then we have estimated the superconducting transition temperature to be Tc = 95.6 K. The magnetization was also measured by a commercial SQUID magnetometer (MPMS, Quantum Design) and the resulting onset of the perfect diamagnetism agreed with the Tc obtained from the electrical resistivity measurements. At the end of each term that lasts half a year, we have investigated their level of achievement for activity of the science club using several questionnaires. The most undergraduate students were able to be interested in superconductivity and to understand superconducting phenomena. We therefore conclude that the science club of intensive education in science and mathematics was useful for keeping students’ interest and motivation in science at high level. APPC12 The 12th Asia Pacific Physics Conference

Spin Polarization Measurements of Heusler Alloy

We report spin polarization P of Fe-rich Heusler alloy Ru_0.2Fe_1.8CrSi, which was determined by the Andreev reflection technique. Heusler alloy Ru_2-xFe_xCrSi was predicted to be half-metal by the first-principle band calculations. Fe-rich Ru_0.2Fe_1.8CrSi compounds are ferromagnetic and the saturation magnetization M₀ almost coincides with that expected from the Slater-Pauling rule. Two theoretical approaches in the determination of spin polarization by Andreev reflection are discussed and compared from an experimental perspective. The spin polarization more than P=0.60 is obtained for Ru_0.2Fe_1.8CrSi specimens and it is highest value in all Ru_2-xFe_xCrSi compounds. The difference in values of spin polarization as determined by two models is less than 0.02.

{\hbox{Ru}}_{0.2}{\hbox{Fe}}_{1.8}{\hbox{CrSi}}

The purpose of our present study is to understand the Superconductor-Insulator Transition. In this paper we report tunnel experiments on Bi2Sr2Ca1-xYxCu2O8+δ (Y-Bi2212) crystals with x = 0, 0.01, 0.05, 0.1, 0.11 and 0.12. The results are discussed about Copper pairs existing. Tunneling spectra were measured for Y-Bi2212 crystal-SiO-Ag planar junctions. For superconducting samples the V-shape gap structure and zero bias conductance peaks (ZBCP) were observed. While neither superconducting gap nor ZBCP was observed for insulating samples. These results suggest that Cooper pairs break up on the insulating side of SIT. In our spectra there were also several characteristic behaviors.