Mamoru Ishizuka

Precise magnetization measurements under high pressures in the diamond‐anvil cell

A precise magnetization measurement system for extremely small samples in the diamond‐anvil cell has been developed. The system is based on position differentiating detection (PDD) combined with the use of a SQUID magnetometer. The PDD is a kind of vibrating coil magnetometer and is suitable for the case where high sensitivity detection is required for a sample around which there is little space for the detection coil. The present system enables one to make magnetization measurements at pressures up to about 12 GPa in the temperature range from 1.4 to 30 K. The sensitivity is about 10−9–10−10 emu. Three piezoelectric actuators of a bimorph type are used at liquid–helium temperature for stable vibration of the detection coil with an amplitude of 90 μm around the tips of the diamond anvils. The design and construction of the system is described in detail. The performance of the system is illustrated by magnetization measurements of EuTe under high pressures.

The pressure-induced superconductivity of iodine

The superconductivity of metallic iodine is observed at temperatures below 1.2 K under pressures above 28 GPa by magnetization and electrical resistance measurements. The pressure dependence of the superconducting transition temperature is studied up to 74 GPa. This is the first observation of the superconductivity of the molecular-dissociated monatomic metal.

High Field Magnetization of Quasi One Dimensional Ising Antiferromagnet CsCoCl3

Magnetization of quasi one dimensional Ising antiferromagnet CsCoCl 3 is measured at liquid 4 He temperatures in pulsed high magnetic fields. In the case of an applied field H parallel to the Ising c -axis, the magnetization M at liq. 4 He temperatures shows a metamagnetic transition which is characterized by the sharp double peaks of the field derivative of the magnetization d M /d H at 33.0 T and 43.9 T, respectively. The results are qualitatively explained by the Ising-like linear chain model with small antiferromagnetic inter-chain exchange interaction. The role of the inter-chain interaction is discussed within the framework of molecular field approximation. The temperature dependence and the transverse case of the magnetization process are also studied.

The pressure effect on the superconducting transition temperature of black phosphorus

We have measured the pressure effect on the superconducting transition temperature Tc of black phosphorus up to 160 GPa using a superconducting quantum interference device vibrating coil magnetometer. It was found that Tc had a maximum value of about 9.5 K at about 32 GPa, began decreasing with pressure and reached about 4.3 K at about 100 GPa.

Pressure-Induced Superconductivity of Iodine

The electrical resistance and the magnetization of solid iodine in the monatomic phase above 210 kbar are measured at temperatures down to 0.1 K. The negligibly small resistance and the diamagnetism observed below 1.2 K indicate the appearance of the superconductivity.

Magnetization measurements in very high pulsed fields produced by a single-turn coil system

Abstract Magnetization measurements are tried in megagauss fields produced by a single-turn coil system at temperatures down to 4.2 K. The measuring apparatus and techniques are described as well as the observed metamagnetic transition of the 1-D Ising antiferromagnet CsCoCl3. Application to other systems is also discussed.

Change of Magnetism in the Two-Dimensional Heisenberg Ferromagnet K2CuF4 Observed at High Pressures.

The mechanism responsible for determining the exchange interaction to be ferromagnetic or antiferromagnetic in Cu 2+ compounds is closely correlated with the antiferrodistortive (AFD) or ferrodistortive (FD) order of Cu 2+ hole orbitals. One of the representative two-dimensional Heisenberg ferromagnets, K 2 CuF 4 , has an ideal AFD orbital ordering. To investigate the possibility of the magnetic phase transition due to a change in the orbital ordering from AFD to FD at high pressures, susceptibility measurements of this compound have been made under various pressures up to 13 GPa over the temperature range from 1.5 to 18 K. A sudden decrease in the Curie temperature starting from 7 GPa was observed. Furthermore, a large decrease in the susceptibility was observed at pressures above 10 GPa. These results indicate a pressure induced ferromagnetic to antiferromagnetic phase transition occurring at about 8–9 GPa as a result of a change of orbital state from AFD to FD order.

Hall effect of iodine in high pressure

The Hall coefficient of metallic iodine is measured in pressures up to 74 GPa at room temperature and in 30 GPa at 4.2 K. The carrier of the pressure-induced metallic iodine is found to be a hole in agreement with the theoretical prediction.

Pressure-induced ferromagnetism in EuTe

Abstract The magnetic behavior of EuTe was investigated under various pressures up to 11.7 GPa and in the temperature range from 1.4 to 30 K by means of uniform magnetization measurements of extremely small samples in a diamond anvil cell. A pressure-induced transition from antiferromagnetic to ferromagnetic ordering was observed at a pressure of about 8 GPa. It was found that, at pressures below 8 GPa, the susceptibility x  M / H at high temperatures increased with pressure due to the development of the ferromagnetic interaction. At pressures just above 8 GPa, χ ( T ) showed anomalous behavior resulting from strong competition of magnetic interactions below the critical temperature at which spontaneous magnetization appeared. At pressures above 9 GPa, a ferromagnetic phase was established at low temperature and the Curie temperature increased with pressure.

Magnetic behaviour of the two-dimensional Heisenberg ferromagnet Cs2CuF4 under high pressure: a sensitive magnetic measurement in a diamond-anvil cell to 26 GPa

The isomorphous compounds A2CuF4 (A = K, Rb, Cs) are two-dimensional Heisenberg ferromagnets with the Curie temperatures of 6.25, 6.05 and 4.55 K, respectively. Previous studies showed that pressure-induced magnetic phase transitions occur in K2CuF4 and Rb2CuF4 at around 10 GPa, above which the ferromagnetic behaviour disappears as a result of the structural change from antiferrodistortive to ferrodistortive order of the CuF6 octahedra in the basal plane. For Cs2CuF4 it was revealed that the interlayer exchange interaction changes from ferromagnetic to antiferromagnetic at a certain pressure below 2.2 GPa. Motivated by this unexpected result, we have carried out magnetic measurements on Cs2CuF4 in a diamond-anvil cell at various pressures up to 26 GPa by using a superconducting quantum interference device vibrating coil magnetometer. It was found that the Neel temperature increases with pressure at a rate of 0.3 K GPa−1 up to 17 GPa, reaches a maximum of about 9 K at around 17 GPa and then decreases rapidly to zero. At pressures above 21 GPa the magnetic susceptibility decreases to the background level, strongly suggesting that a structural change similar to those observed for the other two compounds occurs for Cs2CuF4 at around this pressure.