H. Amitsuka

Faraday force magnetometer for high-sensitivity magnetization measurements at very low temperatures and high fields

A Faraday force magnetometer has been developed for static magnetization measurements at very low temperatures down to 100 mK and in fields up to 9 T. The magnetic force acting on a specimen located in the adiabatic vacuum chamber of a dilution refrigerator is detected by a newly designed load-sensing variable capacitor. Use of a superconducting magnet with gradient coils enables accurate measurement of magnetic moment with a resolution of better than 1×10-7 Am2 ( 1×10-4 emu). An application to magnetization measurements on the heavy-electron antiferromagnet CeB6 is presented.

Magnetic Phase Diagram of CexLa1-xB6 Studied by Static Magnetization Measurement at Very Low Temperatures.

DC magnetization of single crystals of Ce x La 1- x B 6 , 0.5≤ x ≤1, have been measured at low temperatures ( T ) down to 40 mK in magnetic field H //[100], and the H - T - x phase diagrams were obtained. At zero field, the antiferro-quadrupolar (AFQ) transition temperature T Q rapidly decreases with decreasing x and vanishes abruptly between x =0.75 and 0.7. The Neel temperature T N , which is lower than T Q in CeB 6 , varies more slowly with x and exceeds T Q for \(x \gtrsim 0.8\). A clear peak observed in the susceptibility strongly suggests that a new antiferromagnetic (AF) state having no AFQ moment (phase IV) exists for \(0.8\gtrsim x\gtrsim 0.5\), in addition to the two ordered phases II (AFQ) and III (AF+AFQ). As H increases, a first-order phase IV-III transition is observed with a metamagnetic jump in the magnetization. Successive I (paramagnetic)-IV-III phase transitions are observable in a very narrow region of the H - T - x space.

Single uranium-site properties of the dilute heavy electron system UxTh1-xRu2Si2(x≤0.07)

Specific heat C , electrical resistivity ρ , magnetization M and magnetic susceptibility χ have been measured for the dilute tetragonal heavy-electron compounds U x Th 1- x Ru 2 Si 2 ( x ≤0.07), in the temperature range 100 mK≤ T ≤360 K and in the magnetic fields H up to 55 kOe. Non-Fermi-liquid behavior with unusual lnT dependence of C , ρ and χ has been observed at the low temperatures: as the temperature is lowered below about 10 K, C / T and χ increase logarithmically, and ρ decreases logarithmically. Systematical variations of these quantities with uranium concentration indicate that the observed non-Fermi-liquid behavior is ascribed to the properties of a single U ion embedded in the Th site of the ThRu 2 Si 2 metal. The magnetization curves M vs H / T indicate a strong uniaxial magnetic anisotropy along the tetragonal c -axis as well as a strong reduction of the 5 f moment of U ions. A possible interpretation based on the recent developments of the S =1/2 two-channel Kondo model is given.

Specific Heat Study on Sm-based Filled Skutterudite Phosphides SmT4P12(T=Fe, Ru and Os)

Sm-based filled skutterudite phosphides SmFe 4 P 12 , SmRu 4 P 12 and SmOs 4 P 12 synthesized at high temperatures and high pressures have been studied by specific heat measurements in magnetic fields. SmFe 4 P 12 shows an anomaly at 1.6 K attributed to a ferromagnetic transition. SmRu 4 P 12 shows a metal–insulator transition at 16 K. This transition consists of two successive transitions, which suggests orbital and antiferromagnetic orderings. SmOs 4 P 12 shows a sharp anomaly at 4.5 K due to an antiferromagnetic ordering. We discuss the characteristic behavior of those phase transitions and the crystalline electric field state in Sm T 4 P 12 ( T =Fe, Ru and Os). Interestingly, the magnetic entropy of SmFe 4 P 12 have only 0.16 R ln 2 below the ferromagnetic ordering temperature. This suggests a strong spin fluctuation effect. On the other hand, the variation of magnetic entropy in SmRu 4 P 12 and SmOs 4 P 12 reaches to nearly R ln 4 below the ordering temperature.

Magnetic properties and phase diagram of Ce(Ru1-xRhx)2Si2 (0≤x≤0.5)

Magnetic susceptibility, magnetization and specific heat measurements have been performed on single crystalline Ce(Ru 1- x Rh x ) 2 Si 2 in the Ru rich region(0≤ x 5 T). The AF ordering is destroyed above H C and the system further undergoes a metamagnetic transition at H M .

Antiferro-Quadrupolar Ordering and Multipole Interactions in PrPb3

Antiferro-quadrupolar (AFQ) ordering in the cubic Γ 3 ground state system PrPb 3 has been studied by means of magnetization and specific heat measurements under magnetic fields H . Field variation of the AFQ transition temperature T Q (0.4 K at H =0) has been determined for the three crystallographic directions , and . The obtained T Q ( H ) is strongly anisotropic, with T Q ( H ) the largest (∼0.7 K at 6 T). The AFQ phase diagram and the magnetization behavior observed for H ∥ are well explained by a model that incorporates both antiferromagnetic and antiferroquadrupolar interactions. The interactions between field-induced staggered magnetic moments stabilize the AFQ order. This simple model, however, seriously fails to explain the observed anisotropy: T Q ( H )> T Q ( H )> T Q ( H ). Reconsidering the field-induced multipole moments in the AFQ phase, we propose that inclusion of a weak ferro-octupolar interaction between Γ 5 type octupole moments is importa...

Field-Angle-Dependent Specific Heat Measurements and Gap Determination of a Heavy Fermion Superconductor URU2Si2

To identify the superconducting gap structure in URu2Si2, we perform field-angle-dependent specific heat measurements for the two principal orientations in addition to field rotations, and a theoretical analysis based on microscopic calculations. The Sommerfeld coefficient gamma(H)s in the mixed state exhibit a distinctly different field dependence. This comes from point nodes and the substantial Pauli paramagnetic effect of URu2Si2. These two features combined give rise to a consistent picture of superconducting properties, including a possible first order transition of Hc2 at low temperatures.

Neutron Scattering Study on Competition between Hidden Order and Antiferromagnetism in U(Ru1-xRhx)2Si2 (x ≤0.05)

We have performed elastic and inelastic neutron scattering experiments on solid solutions of U(Ru 1- x Rh x ) 2 Si 2 with Ru-rich concentrations: x =0, 0.01, 0.02, 0.025, 0.03, 0.04 and 0.05. Hidden order is suppressed with increasing x , and correspondingly, the onset temperature T m (∼17.5 K at x =0) of weak antiferromagnetic (AF) Bragg reflection decreases. For x =0.04 and 0.05, no magnetic order is detected in the investigated temperature range down to 1.4 K. In the intermediate range, 0.02 ≤ x ≤0.03, we found that the AF Bragg reflection is strongly enhanced. At x =0.02, this enhancement takes place at ∼7.7 K (≡ T M ), which is significantly lower than T m (∼ 13.7 K). T M increases with increasing x , and seems to merge with T m at x =0.03. If the AF state is assumed to be homogeneous, the staggered moment µ o estimated at 1.4 K increases from 0.02(2) µ B /U ( x =0) to 0.24(1) µ B /U ( x =0.02). The behavior is similar to that observed under hydrostatic pressure (µ o increases to ∼0.25 µ B /U at 1.0 ...

Inhomogeneous magnetism in URu2Si2 studied by muon spin relaxation under high pressure

Abstract We have studied the low-temperature phase below 17.5 K in the heavy-fermion superconductor URu 2 Si 2 , using the zero-field μSR technique under hydrostatic pressures P up to 0.85 GPa . The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic ‘hidden order’. The AF volume fraction, however, develops abruptly at around a pressure of 0.6 GPa , in contrast to the P -linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17.5 K , whereas the nominal Neel temperature seems to be about 20 K . These experimental results suggest that the AF phase exists as a metastable state.

Evidence of unusual Kondo effects in dilute uranium compounds UxTh1−xRu2Si2(x⩽0.07)

Abstract Single-site properties of 5f-electrons in URu 2 Si 2 have been studied by means of magnetic susceptibility ( χ ), specific-heat ( C ) and electrical resistivity (ϱ) measurements on U x Th 1− x Ru 2 Si 2 single-crystals in the uranium dilute region ( x ⩽0.07). Anomalous In T -dependence has been found in χ , C and ϱ at low temperatures, suggesting an unusual Kondo effect.