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We report the magnetic properties of Dy pyrochlore oxides, Dy2Sn2O7 and Dy2Ir2O7. We show that the temperature dependence of the relaxation time τ(T) in the case of Dy2Sn2O7 is very similar to that in the case of a typical spin ice, Dy2Ti2O7. However, the time scale of Dy2Sn2O7 is 10-100 times slower than that of Dy2Ti2O7 over the entire temperature range. Dy2Ir2O7 exhibits a metal-insulator transition at 134 K; it has a broad peak at 4.5 K in the DC magnetic susceptibility, indicating an antiferromagnetic correlation. However, the specific heat has no sharp anomaly at 4.5 K. We show that the Dy moments in Dy2Ir2O7 have no long-range ordering down to 100 mK. We elucidate the anomalous slow dynamics that τ(T) decreases and becomes inhomogeneous below 12 K.

We carried out inelastic neutron scattering measurements to identify crystalline electric field (CEF) states of Nd3+ (a total angular momentum J = 9/2) in Nd2Ir2O7, which exhibits a metal-insulator transition at TMI = 36 K. Excitation peaks observed around 26 and 41 meV are interpreted as CEF excitations within five Kramers doublets. It is notable that splitting of the ground doublet by 1.3 meV was microscopically observed at 3 K for the first time, indicating emergence of magnetic long-range order despite geometrical frustration. The splitting width decreases with increasing the temperature towards TMI like an order parameter of the transition. We discuss the results in light of Nd-Ir interactions.

The ceramic YBa2Cu4O8 composed of sub-micron size grains is considered as random Josephson-coupled networks of 0 and π junctions and shows successive phase transitions. The upper transition occurs inside each grain at Tc1 and the lower transition occurs among the grains at Tc2 (< Tc1), where a negative divergence of nonlinear susceptibility is found. This critical phenomenon at Tc2 suggests the onset of the chiral-glass phase predicted by Kawamura and Li. We investigated the magnetic memory phenomena in the temperature dependence of thermoremanent magnetization (TRM). The memory effects are observed in TRM measured on heating after the cooling process with a halt at the temperature Ts below Tc2. The results indicate the existence of a glassy phase such as the chiral-glass one.

The d-wave ceramic YBa2Cu4O8 superconductor composed of sub-micron size grains is considered as random Josephson-coupled network of 0 and π junctions and shows successive phase transitions. The upper transition occurs inside each grain at Tc1 = 82 K and the lower transition occurs among the grains at Tc2 = 66 K. We measured the temperature dependence of the current-voltage characteristics of the ceramic YBa2Cu4O8 and derived the linear and nonlinear resistivity. The nonlinear resistivity ρ2 and ρ4 have finite values between Tc1 and Tc2 and have the peak at the same temperature Tp = 70 K above Tc2. The result agrees with the theoretical one obtained by Li and Dominguez. They interpreted Tp as the crossover temperature from the normal state phase to a chiral paramagnetic one.

The second nonlinear susceptibility, χ(2), reflects magnetic domain formation. At zero DC field, the temperature dependence of χ(2) can be obtained through that of the third-harmonic AC susceptibility, χ3ω. Generally, the magnitude of χ3ω is at most a few percent of the linear component, χ1ω, connected with the linear susceptibility, χ(0). At GPa-level pressures generated using a diamond anvil cell, the detection of χ3ω becomes more difficult, because of the small amount of measurement sample. We propose an effective method of measuring χ3ω using a superconducting quantum interference device magnetometer and a statistics average technique. The experimental results for an inorganic chiral magnet, Cr1/3NbS2, indicate the efficiency of this technique and demonstrate that magnetic signals of the order of 10−9 emu can be detected.

The size effects on magnetic properties of FePt nanoparticles were investigated through ESR experiment in X-band at room temperature. For particle size (D) of ≥ 2.6 nm, the absorption spectra were the Gaussian type, whereas for D = 2.0 nm, it was reproduced with the summation of the Gaussian and Lorentzian types. It has been well known that a face-centered-tetragonal (fct) structure becomes unstable for D of below 2-4 nm. Two kinds of magnetic components for FePt nanoparticles with D = 2.0 nm, observed in the ESR spectrum at room temperature, are considered to be attributed to the coexistence of the particles with fct structure and those with face-centered cubic structures.

Raman scattering spectra of iridium pyrochlore oxides R2Ir2O7 (R=Nd, Sm, and Eu) have been measured. These three compounds undergo metal-insulator transition. Below the transition temperature TMI, new peaks appear for R = Sm and Eu, but no remarkable change for R = Nd. Therefore, Sm2Ir2O7 and Eu2Ir2O7 accompany structural change with the transition, but not the case for Nd. Even if Nd2Ir2O7 deforms, the distortion is different from that of Sm2Ir2O7 and Eu2Ir2O7, because the spectra of Nd are clearly different from those of Sm and Eu below TMI.

The magnetic properties of FePt magnetic nanoparticles with the particle size of 2.0 ± 0.6 nm were investigated through AC magnetic susceptibility measurements under hydrostatic pressure. At a frequency of 1 Hz, a blocking temperature increases linearly from 42.9 to 44.6 K by increasing pressure (P) up to 1.43 GPa. The X-ray diffraction experiments under hydrostatic pressure reveal that both the particle size and lattice parameter hardly change in the considered pressure region. These experimental results suggest that the effective magnetocrystalline anisotropy increases under hydrostatic pressure.

A uniaxial pressure technique is developed to study the dependence of the magnetization up to 12 kbar using a SQUID magnetometer. The technique has been tested for the magnetic properties of a Metal-Organic Framework, Co(H2O)6(C10H4O8), where C10H4O8 is benzene-1,2,4,5-tetracarboxylate, along three orthogonal axes of single crystals. It displays anisotropic paramagnetic properties between 2 and 300 K without any phase transition. The isothermal magnetizations at 5 K show decrease with pressure when the field is applied along the b-axis but an increase along the a-and c*-axes. The results are interpreted as a change in the g-value tensor as a function of pressure due to compression along the Jahn-Teller distorted axis.

We performed AC magnetic susceptibility measurement for high frequency (f) range, based on a superconducting quantum interference device (SQUID) magnetometer, MPMS (Quantum Design Inc.). Original system of MPMS uses an A/D converter with 12 bit or 16 bit, and there the frequency of AC field is limited to less than 1 kHz by a software. In the present system, we transformed an analog signal of SQUID output to a digital one via an A/D converter with 24 bit, and generated AC field by a signal generator outside MPMS. The maximum f was enlarged from 1 kHz to 10 kHz. However, it has been confirmed that amplitude of AC field (HAC), at which the rf-SQUID works as a current/voltage transformer, decreases with increasing f. The value of HAC was restricted below 0.02 Oe at f = 10 kHz. The performance of the present system has been confirmed via the measurements of a paramagnetic material Dy2O3 and a rare-earth ferromagnet Ho. The normalization constant for the magnitude of the SQUID output, to calibrate the effect of eddy current, is presented as a function of frequency.

Ceramic high-Tc superconductor has intra-grain superconducting transition at TC1 and the inter-grain one at TC2. We investigated the effects of pressure on the ceramic superconductor YBa2Cu3O7-y in the pressure region up to 5.3 GPa, through the AC susceptibility measurements using a miniature diamond anvil cell (mDAC). The TC1 increases against the initial pressure, whereas it turns to decrease at around 2.5 GPa. The TC2 also exhibits similar pressure dependence. The up-and-down change of TC1 has been already observed in the previous single crystal experiment, whereas that of Tc2 was observed through the present study for the first time. From the viewpoint of measurement technique, we also point out some problems on the high pressure AC susceptibility measurements using a SQUID magnetometer.

Ceramic YBa2Cu4O8 samples composed of sub-micron size grains are considered as random Josephson-coupled networks of 0 and π junctions, and they show successive phase transitions. The first transition occurs inside each grain at Tc1 and the second transition occurs among the grains at Tc2 (> Tc1), where a negative divergence of nonlinear susceptibility is found. This critical phenomenon at Tc2 suggests the onset of the chiral-glass phase, as predicted by Kawamura and Li. We measured the temperature dependencies of the current-voltage characteristics of the samples and derived the linear and nonlinear resistivities. With decreases in temperature, linear resistivity decreased monotonously and remained at a finite value at temperatures less than Tc2, while nonlinear resistivity diminished continuously for temperatures moving towards Tc2. These results are consistent with the theoretical predictions., Proceedings of the 25th International Conference on Low Temperature Physics (LT 25), August 6-13, 2008, Amsterdam, Netherlands

Nanoparticles of multiferroic material BiMnO3 were synthesized in the pores of the mesoporous silica SBA-15 and their magnetic properties were investigated. The powder X-ray diffraction pattern for the nanoparticles at room temperature was similar to that for bulk crystals with monoclinic symmetry. The particle size of the nanocrystals was estimated to be about 14 nm using Scherrer's equation. The temperature dependence of the DC susceptibilities for the nanoparticles showed superparamagnetic behavior. The susceptibility and thermoremanent magnetization exhibited the ferromagnetic feature and the ferromagnetic transition temperature was almost same to that for bulk crystals, approx 100 K. However, the Weiss temperature Θ was evaluated as the negative value and the magnetization curve showed antiferromagnetic behavior. The results suggest the antiferromagnetic properties appeared prominently in the nanoparticles., Proceedings of the 25th International Conference on Low Temperature Physics (LT 25), August 6-13, 2008, Amsterdam, Netherlands

The effects of pressure on iron oxide hydroxide ferrihydrite (FeOOHnH2O) nanoparticles with particle size of 4.7 ± 0.2 nm were investigated through DC magnetic measurements in the pressure region up to P = 13.5 kbar. The magnetization curve up to 50 kOe exhibits the coexistence of saturation and linear contributions. The value of high field susceptibility, the latter contribution, increases with increasing pressure for P ≤ 10.0 kbar. In contrast, the former contribution, associated to the uncompensated moment, decreases for P ≤ 10.0 kbar. At further pressure, there is no distinct pressure response. These effects of pressure are probably related to changes in the anisotropy energy due to the volume shrinkage.

We report on the low temperature magnetism of pyrochlore oxide Pr2Zr2O7. The crystal electric field ground state in Pr3+ (4f2, J = 4) ions has non-Kramers doublet with local Ising magnetic anisotropy. The negative Curie-Weiss temperature below 10 K (-0.55 K) indicates an antiferromagnetic coupling between the spins. The AC magnetic susceptibility does not exhibit any long range ordering at least down to 76 mK. Instead, a frequency dependence of AC magnetic susceptibility is observed below 0.3 K, indicating a spin freezing behavior. Furthermore, recently we have succeeded for the first time in growing a single crystal of Pr2Zr2O7 by the floating-zone method using an infrared furnace equipped with four Xe lamps. We will also comment on the crystal growth of Pr2Zr2O7.