Hidenobu Hori

Molecular beam epitaxy of wurtzite (Ga,Mn)N films on sapphire(0 0 0 1) showing the ferromagnetic behaviour at room temperature

Wurtzite (Ga,Mn)N films showing ferromagnetic behaviour at room temperature were successfully grown on sapphire(0001) substrates by molecular beam epitaxy using ammonia as nitrogen source. Magnetization measurements were carried out by a superconducting quantum interference device at the temperatures between 1.8K and 300K with magnetic field applied parallel to the film plane up to 7T. The magnetic-field dependence of magnetization of a (Ga,Mn)N film at 300K were ferromagnetic, while a GaN film showed Pauli paramagnetism like behaviour. The Curie temperatures of a (Ga,Mn)N film was estimated as 940K.

Direct Observation of Ferromagnetic Spin Polarization in Gold Nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.

Magnetic and transport characteristics on high Curie temperature ferromagnet of Mn-doped GaN

Mn-doped GaN films on sapphire (0001) substrates were grown by molecular beam epitaxy system using ammonia as nitrogen source. The result of magnetization measurement gives Curie temperature as high as 940 K. The field and temperature dependencies of the magnetization show coexistence of ferromagnetic and paramagnetic phases. In addition, the temperature dependencies of electrical resistance and carrier concentration were measured to investigate the relation between the ferromagnetism and transport property. Below about 10 K, a similar anomalous increase of magnetization and resistance is observed.

Anomalous magnetic polarization effect of Pd and Au nano-particles

Abstract The magnetization of Pd and Au nano-particles with a diameter of around 3 nm was measured down to 4.2 K. The data of magnetization of both nano-particles show an unexpectedly large magnetic moment of about 20 spins per particle. This result can not be understood by so-called odd/even electron number effect and Stoners enhancement model. The result might suggest the existence of some common and characteristic spin correlation mechanism in the nano-particles. The experimental result of size dependence on Pd nano-particles shows existence of a critical size of about 3 nm from a atomic spin correlation region to a metallic spin correlation region.

PHENOMENA IN RESONANT TUNNELING THROUGH DEGENERATED ENERGY STATES WITH ELECTRON CORRELATION

We introduce a general analysis method, which allows us to simulate the operation of high-performance molecular nano-devices and to design the expected function of a wide range of devices in nano-scale size. The method is based on the use of a resonant tunneling phenomenon, admitting strong electron correlation in a quantum dot with degenerated states. Three examples of the application of this method are given: Coulomb repulsion, uncorrelated resonant tunneling, and electron-phonon interaction. It is shown that there is a good agreement with experimental data in all three cases.

Evaluation of the Figure of Merit on Thermoelectric Materials by Harman Method

The figure of merit ZT of thermoelectric materials has been evaluated by the Harman method in the temperature region below room temperature. In this method only resistance measurements by both dc and ac methods are required to obtain the ZT values. ZT is given by ZT=(Rdc/Rac-1)/x, where Rdc, Rac and x are the resistance value by the ac and dc methods and the rate of the heat to the heat bath, respectively. The heat effect is experimentally confirmed to be negligibly small and we can use x=1 which corresponds to a sufficient adiabatic condition. Because an ambiguity due to experimental errors such as the length between the measurement terminals in the resistivity and the thermal conductivity measurements is removed, ZT can be determined very simply and precisely by the Harman method.

MAGNETIC PROPERTIES OF NANOPARTICLES IN PD/NI ALLOYS

Abstract In order to investigate the alloying effect in Ni Pd nanoparticles, a special chemical reaction method has been developed to generate a sufficient number of well-conformed Pd ultra-fine particles. Ni concentration dependence on magnetization reveals the existence of a giant magnetic moment effect, where the critical concentration of 6.3 at% is higher than the bulk state one. The higher harmonics intensity of ESR is remarkably enlarged in the alloying particles. The enhanced spectra with ΔS = 2 and the broad spectra arise from the nonlinear effect of the isolated nanoparticles with a long spin-lattice relaxation time.

Magnetization Measurement of NENP and NINO in High Magnetic Field

Magnetization measurements have been performed on the single crystal samples of Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ), NENP, and Ni(C 3 H 10 N 2 ) 2 NO 2 (ClO 4 ), NINO in pulsed high magnetic fields. No magnetization appears up to about 10 T reflecting the presence of the Haldane gap and the linear magnetization appears in both compounds above this field. The observed anisotropy of the transition field is explained by the Haldane gap energy E g and the crystalline field constants D and E in the lowest excited triplet. The parameters are estimated as E g =16.8 K, D =-16.1 K, and E =-1.3 K and E g =14.2 K, D =-11.5 K and E =2.1 K for NENP and NINO, respectively.

Multistep magnetization of single crystal TbNiSn and DyNiSn in high fields

Abstract Magnetic and transport properties are reported on TbNiSn and DyNiSn single crystals with the orthorhombic TiNiSi-type structure. Thermal variation of the electrical resistivity and magnetic susceptibility indicates the existence of three successive magnetic transitions at T 1 = 6.0K, T 2 = 7.6K and T N = 18.5 K in TbNiSn and two magnetic transitions at T 1 = 5.4K and T N = 7.3 K in DyNiSn. Magnetization up to 15 T along the b -axis at 1.6 K exhibits a four-step metamagnetic transition for TbNiSn and a three-step one for DyNiSn. High-field magnetization curves for TbNiSn in fields up to 30 T at 1.4K show unexpected small jumps, which may suggest a switching of the magnetic axis.

Magnetization process of haldane materials TMNIN and NINAZ

Magnetization measurements have been performed on the Haldane gap materials (CH 3 ) 4 NNi(NO 2 ) 3 (TMNIN) and Ni(C 3 H 10 N 2 ) 2 N 3 (ClO 4 ) (NINAZ) in pulsed high magnetic fields. TMNIN shows a typical magnetization profile of the Haldane material with the gap energy of 4.1 K. Quenching of the gap appears around 2.7 Tesla(T) and magnetic saturation is obtained above 30 T. In NINAZ, however, the quenching appears at around 30 T reflecting a large Haldane gap energy with the magnitude of 44.3 K.