Oomi Gendo

Pressure Effect on the Spontaneous Volume Magnetostriction of Fe-Ni and Fe-Pt Invar Alloys

Pressure effect on the spontaneous volume magnetostriction ω s of Fe-Ni and Fe-Pt Invar alloys is studied by measuring the pressure dependence of lattice constants usins X-rav diffraction technique. The pressure derivative of ω s and the magnetic contribution to the compressibility in Fe-Ni Invar alloy near 0 K are found to be largely different from those in Fe-Pt Invar alloy. The differences are interpreted phenomenologically in terms of the pressure dependence of T c and M 0 and suggested to relate with the stability of magnetic moment at low temperature.

Bulk Modulus Anomalies of Fe-Ni and Fe-Pt Invar Alloys

The bulk moduli of Fe 70 Ni 30 , Fe 65 Ni 35 and Fe 72 Pt 28 Invar alloys are obtained by measuring the lattice constants under high pressure up to about 100 kbar using X-ray diffraction technique. Large changes in bulk modulus are observed at ferromagnetic to paramagnetic phase transition. The temperature dependence of bulk modulus shows normal behavior both in the ferromagnetic and in the paramagnetic phases of Fe-Ni Invar alloys but shows anomalous behavior in the ferromagnetic phases of Fe 72 Pt 28 alloy. The different behaviors are discussed in connection with thermodynamical relation and the recent theoretical and experimental results.

Anisotropic Thermal Expansion of Heavy Fermion Material CeCu6

The thermal expansion of single-crystalline CeCu 6 and LaCu 6 has been measured by using the strain gauge method in the temperature range from 6 K to 300 K. A large anisotropy was found in the temperature dependence of thermal expansion (Δ l / l ) i ( i denotes the crystal axis, i = a , b and c ). The contribution of the 4f electron to thermal expansion is very large for the b -axis and extremely small for the a -axis. The thermal expansion coefficient α was also estimated from these data. An anomalous enhancement in the α l / T vs T 2 plot (α 1 =1/3 (α a +α b +α c )) was found at low temperature below about 20 K, which is considered to correspond to the large enhancement of the C / T value at low temperatures where C is the specific heat. These results are discussed in connection with other heavy fermion materials.

Pressure Collapse of Kondo Gap in Kondo Compound CeRhSb

The effect of pressure on the electrical resistivity of CeRhSb has been studied up to 80 kbar in order to elucidate the electronic Properties of CeRhSb at low temperature. It is found that the Kondo temperature increases with pressure and gap is quenched at P =80 kbar, showing a maximum around 30 kbar.

Thermal Expansion Coefficients of CeCu2 and YCu2 Single Crystals

Thermal expansions of CeCu 2 and YCu 2 single crystals have been measured in the temperature range from 2.0 K to 300 K. It is found that the linear thermal expansion coefficients α i ( i = a , b and c ) of CeCu 2 are strongly anisotropic among each crystal axis reflecting orthorhombic crystal structure. A well-defined anomaly due to antiferromagnetic ordering is observed near 3.5 K in the α i - T curves of CeCu 2 . A magnetic contribution to α i ( T ) due to the 4f electron is estimated by comparing the α i - T curve of CeCu 2 with that of YCu 2 .

High Pressure X-Ray Study of Anomalous Bulk Modulus of an Fe70Ni30 Invar Alloy

Pressure dependence of the lattice constant is measured for the pressure-stabilized fcc phase of an Fe 70 Ni 30 Invar alloy by high pressure X-ray diffraction method at 77.4 K. The bulk modulus B in the ferromagnetic phase is obtained to be (14 ±1) ×10 2 kbar. A discontinuous increase in B , Δ B , is found to be Δ B ≈700 kbar near 64 kbar. The magnitude Δ B at 77.4 K is the same as that observed at the ferromagnetic to paramagnetic phase transition at room temperature around 15 kbar.

Effects of Pressure on the Magnetic and Structural Properties of GdCu

The electrical resistivity down to 4.2 K and X-ray at room temperature have been measured under pressure in GdCu. At ambient pressure, this material undergoes a martensitic transformation when the temperature is decreased below T MS = 235 K. We found that at room temperature, the CsCl-type crystal structure remains stable up to 13 GPa. However, the martensitic transformation is very sensitive to pressure, being suppressed above 0.6 GPa.

Effect of Pressure on the Electrical Resistivity of Fe65(Ni0.84Mn0.16)35 Alloy

The electrical resistivity of Fe 65 (Ni 0.84 Mn 0.16 ) 35 alloy is measured under high pressure up to 30 kbar from 4.2 K to 300 K. A resistance minimum is found near 25 K at atmospheric pressure. The temperature of the resistance minimum is appeared not to depend on pressure. Isothermal resistivity shows anomalous behaviors against pressure below 150 K. These results suggest the existence of a pressure induced magnetic phase transition at low temperature.

Seebeck Coefficient of Concentrated Kondo Compound CePtSi2 under Hydrostatic Pressure

We measured the Seebeck coefficient and resistivity of the concentrated Kondo compound CePtSi 2 under hydrostatic pressure. At ambient pressure, S gradually increases with decreasing temperature and shows a maximum value S max = 28 µV/K around 86 K, followed by a minimum around 4.5 K, as is often observed in antiferromagnetic Ce-based heavy fermion compounds. With increasing pressure, S at high temperature increases, whereas S below 20 K does not show a significant change.

Low-Energy Magnetic Excitations in CeAl2 under Pressure Investigated by 27Al-Nuclear Magnetic Resonance Measurement

To microscopically elucidate the effect of applying pressure on the magnetic properties of antiferromagnetic CeAl 2 ( T N ∼3.8 K), we have carried out 27 Al nuclear magnetic resonance measurements. In the paramagnetic state, the spin–lattice relaxation rate 1/ T 1 T and the staggered susceptibility χ( Q AF ) exhibit the Curie Weiss-type behavior at high temperatures and show a tendency to be saturated at low temperatures. The application of pressure 2.5 GPa has the effect of reducing the effective magnetic moments of both χ(0) and χ( Q AF ) by about 20%. The important feature is that the Weiss temperature T N of χ( Q AF ) largely decreases from ≃20 K at P =0 to ≃12 K at P =2.5 GPa, in contrast with the near pressure-independent behavior of the Weiss temperature θ CW ≃-28 K of χ(0). This leads a conclusion that the depression of the antiferromagnetic ordering and the increase of the itinerant character of CeAl 2 with pressure is ascribed to the decrease of the AF spin fluctuations.