N. Takeshita

Metallization of magnetite (Fe3O4) under high pressure

Electrical resistivity measurements have been made on a good qualified single crystalline magnetite (Fe3O4) at temperatures from 300 down to 3.0 K under pressure up to 10 GPa. A steep change in resistivity at the Verwey transition temperature has been observed at pressure below 6.5 GPa, which shows a quite distinct result reported in prior work. Moreover, the Verwey transition temperature has been found to decrease nonlinearly with increasing pressure and it disappears at around 8 GPa. Above 8 GPa magnetite exhibits metallic behavior. The residual-resistivity ratio of the metallic state is observed to be more than 350. This finding of a metallic ground state in magnetite provides insight into the understanding of the Verwey transition in magnetite.

A newly developed high-pressure cell by using modified Bridgman anvils for precise measurements in magnetic fields at low temperatures

We have succeeded in developing a new high-pressure cell designed for measuring the absolute value of the electrical resistivity in magnetic fields at low temperatures. This pressure cell, based on a technique using modified Bridgman anvils with a Teflon capsule, can generate nearly hydrostatic pressure at least up to 6 GPa. The most important feature of this pressure cell is its compact body which can be easily combined with a dilution refrigerator in a superconducting magnet. One of its applications is for precise magneto-transport measurements under nearly hydrostatic pressure, which is demonstrated by the investigation into the pressure-induced superconducting state of the spin-ladder Sr14−xCaxCu24O41 (x=12) single crystal.

Metallization of magnetite at high pressures

We report on the first finding that an insulator to metal transition (i.e. Verwey transition) of magnetite (Fe 3 O 4 ) takes place at absolute zero temperature as a function of pressure. In order to investigate the Verwey transition, we measured the electrical resistivity on two single crystalline samples with distinct oxygen compositions of Fe 3 O 4.002 and Fe 3 O 4.007 at temperatures from 300 K down to 3 K under a pressure up to 9 GPa using a cubic anvil apparatus. The Verwey transition temperature of these samples suddenly disappears at a certain critical pressure P c in between 6 and 8 GPa. We confirmed that a metallic ground state is realized in magnetite under pressure above P c .

EFFECT OF PRESSURE ON THE MAGNETIZATION OF SINGLE CRYSTAL CERH2SI2

Abstract The effect of pressure on the magnetization and susceptibility of single crystals of CeRh 2 Si 2 in the pressure range up to 10xa0kbar is reported. The antiferromagnetic ordering temperature as a function of pressure is also given.

PHYSICAL PROPERTIES OF TM MONOCHALCOGENIDES UNDER PRESSURE

Abstract The resistivity of TmSe has been measured under pressure up to 8xa0GPa, below room temperature. At low temperatures below about 10xa0K, the pressure dependence of the resistivity shows two maxima; one occurring at about 1xa0GPa where the magnetic phase transition takes place, and the other at about 7xa0GPa. The resistivity at 7xa0GPa shows a remarkable temperature dependence, which increases with decreasing temperature rapidly below 8xa0K. The anomalous behavior in pressure and temperature dependent resistivity in Tm monochalcogenide may be caused by the change in Tm valence under high pressure.

A technique for precise magneto-transport measurements at low temperatures under pressure up to 8 GPa

Abstract A technique for a precise transport measurement under high pressure up to 8 GPa in a superconducting magnet has been developed. In order to obtain a quasi-hydrostatic pressure the direct compression method is applied to modified-Bridgman anvils with a Teflon capsule. This technique enables us to measure the absolute values of electrical resistivity, magneto-resistance and the Hall coefficient at various temperatures from 300xa0K down to 1.5xa0K in magnetic fields up to 20xa0T. We demonstrate some experimental results of magneto-transport measurements on single crystals of SmS and CeSb, showing extraordinary behavior in the temperature, pressure and magnetic field-dependent transport phenomena.

Pressure-induced transition from a spin glass to an itinerant ferromagnet in the half-doped manganite L0.5Ba0.5MnO3 (L=Sm and Nd) with quenched disorder

The effect of quenched disorder on the multiphase competition has been investigated by examining the pressure phase diagram of half doped manganite Ln0.5B0.5MnO3 (Ln = Sm and Nd) with A-site disorders. Sm0.5Ba0.5MnO3, a spin glass insulator at ambient pressure, switches to a ferromagnetic metal with increasing pressure, followed by a rapid increase of the ferromagnetic transition temperature Tc. The rapid increase of Tc was confirmed also for Nd0.5Ba0.5MnO3. These observations indicate that the unusual suppression of the multicritical phase boundary in the A-site disordered system, previously observed as a function of the averaged A-site ionic radius, is essentially controlled by the pressure and hence the band width. The effect of quenched disorder is therefore much enhanced with approaching the multicritical region.

The valence fluctuation state of Tm monochalcogenides under high pressure

Abstract The valence fluctuation between 4f 12 and 4f 13 of Tm monochalcogenides (TmX; X=Te, Se, S) has been found to play a dominant role in their unusual physical properties. We report here the pressure dependence of the electrical resistivity in TmX obtained up to 8xa0GPa. The temperature-dependent resistivity of TmTe with increasing pressure becomes to show just like that of TmSe at ambient pressure, implying that the application of pressure not only increases the valence but also suppresses the ferromagnetic exchange interaction in this system. We obtained a unified magnetic phase diagram as a function of lattice constant for all of TmX. These results indicate that the magnetic exchange interaction in TmX changes from ferromagnetic to antiferromagnetic with decreasing lattice constant.

Upper critical field of spin-ladder Sr2Ca12Cu24O41

Abstract We report the upper critical magnetic field, Hc2(T), of the spin-ladder Sr 14−x Ca x Cu 24 O 41 (x=12) single crystal, which becomes superconducting above ∼3.0xa0GPa. From the measurements of the temperature dependence of AC susceptibility at 4.0 GPa in magnetic fields applied along the a-axis (⊥ ladder), the b-axis (⊥ ladder-plane) and the c-axis (|| ladder), we have determined Hc2(T) for these three directions down to about 1.6xa0K. A clear difference is observed among these three curves, indicating that this system has an anisotropic superconducting ground state.

Magnetoresistance in PrInAg2 at 40 mK

Abstract The magneto- (electrical-) resistivity of a Heusler-type intermetallic paramagnetic compound PrInAg2 is measured under high fields up to 12xa0T at 40xa0mK (down to 50xa0mK under zero field). No evidence of any transition is detected under zero field in our sample and deviations of the resistivity can well fit as ∝+log T function below 1xa0K down to even 50xa0mK. The magnetoresistivity is positive at any measuring fields. It seems that the external magnetic field dissolves the anomaly at low temperature via breaking the cubic symmetry. Moreover, the magnetoresistivity varies with +log H from 0.01xa0T to 1xa0T and is scaleable to the electrical resistivity via the relation of T∝+|H n |(n≈ 2 3 ) . Then both of the anomalies between the magneto- and the electrical-resistivity must be originated from a common source. Thereon, the value of the exponent n does not take 1, i.e. the field effect on the resistivity is not Zeemanian. It has been already known that a Pr3+(4f2) ion in the compound is located in cubic symmetry and has a non-magnetic non-Kramers doublet Γ3 ground state. It is possible that the anomalies originate from the Γ3 ground state.