Shuhei Kamikawa

Successive Phase Transitions and Anisotropic Magnetic Field–Temperature Phase Diagram in NdRu2Al10

To investigate the successive phase transitions in NdRu 2 Al 10 , we carried out ultrasonic measurements on single-crystalline samples. We found that the elastic moduli show an abrupt hardening at 2.4 K without hysteresis and a two-step softening at 1.3 and 1.0 K with clear hysteresis in the temperature sweep. The hardening at 2.4 K is very similar to that observed in CeRu 2 Al 10 and CeOs 2 Al 10 , suggesting that the second-order transition at 2.4 K originates from a similar mechanism of the transitions in CeRu 2 Al 10 and CeOs 2 Al 10 . Both transitions at 1.3 and 1.0 K would originate from an antiferromagnetic first-order spin ordering. We also clarified the anisotropic magnetic field H dependences of the transition temperatures in H along the a -, b -, and c -axes, and found a new phase above 2.5 T in H ∥ a .

Elastic Softening in HoFe2Al10 due to the Quadrupole Interaction under an Orthorhombic Crystal Electric Field

To investigate 4f electronic states in HoFe2Al10 under an orthorhombic crystal electric field (CEF), we measured the specific heat, magnetic susceptibility, magnetization, and elastic modulus of single-crystalline samples. We found elastic softening of the transverse elastic moduli C55 and C66 below 20 and 130 K, respectively. With further decreasing temperature, C66 shows further elastic softening below 5 K. We observed two Schottky peaks in the specific heat at 2.2 and 20 K and small anisotropy of the magnetic susceptibility and magnetization in the paramagnetic region. By analyzing these experimental data, we obtained the CEF parameters of HoFe2Al10. From the analysis, we clarified that the softening of C55 and C66 originates from indirect quadrupole interactions of Ozx and Oxy, and propose that the overall CEF splitting is about 85 K.

Anomalous Elastic Softening due to the Crystal Electric Field Effect and Successive Phase Transitions in the Yb-Based Heavy-Fermion Antiferromagnet YbIrGe

To investigate the crystal electric field effect and antiferromagnetic transitions at 2.4 and 1.4 K, we carried out ultrasonic measurements on the Yb-based heavy-fermion antiferromagnet YbIrGe. We found characteristic softening of the elastic moduli originating from a quadrupole interaction. By theoretical analyses of the transverse elastic moduli, the magnetic susceptibility, and the Schottky specific heat between 10 and 300 K, we determined the crystal electric field: the ground doublet and an excited doublet separated by 138 K. The moduli exhibit elastic hardening below both 2.4 and 1.4 K, suggesting a strong coupling between a strain and magnetic order parameters. We estimated the strain dependence of the transition temperature at 2.4 K along the a- and c-axes from the Ehrenfest relation for the first time.

The Crystal Electric Field Effect on 4f-Hole and Magnetic Phase Diagram in Heavy-Fermion Compound YbPtGe Determined from Ultrasonic Measurements

YbPtGe with the orthorhombic e-TiNiSi-type structure is reported as a heavy-fermion ferromagnet with the magnetic transition temperature TC = 5.4 K. Recent research suggested that its magnetic anisotropy can be ascribed to the crystal electric field (CEF) effect. In order to clarify the CEF effect and to investigate the magnetic phase transition in this compound, ultrasonic measurements of YbPtGe and related theoretical calculation have been performed. The transverse elastic moduli C44, C55, and C66 exhibit a softening characteristic with a peak around 150 K while the longitudinal moduli C11, C22, and C33 show a continuous hardening in the temperature (T) range above TC. Theoretical strain-susceptibility fitting indicates that the softening results from a quadrupole interaction between the ground and excited Kramers doublets of CEF. For the elastic moduli below TC, abrupt elastic hardening is observed in C22, C33, and C55 while C11, C44, and C66 exhibit a step-like softening. The difference between the so...

Exotic Ground State and Elastic Softening under Pulsed Magnetic Fields in PrTr2Zn20 (Tr = Rh, Ir)

To investigate a field-induced level crossing of the ground-state doublet in PrTr2Zn20 (Tr = Rh, Ir), we performed ultrasonic measurements in pulsed magnetic fields applied along the [110] and [001] directions and analyzed the results in the framework of the strain-susceptibility approach. Above 40 T for H || [110], we observed an elastic softening of the transverse modulus (C11 − C12)/2 corresponding to the ground-state doublet. In both compounds the softening is followed by a minimum at about 47 T at low temperatures. We predict the presence of a new field-induced phase boundary in PrTr2Zn20 at this field with two possible cases. The magnetic field of the minimum cannot be explained by only the quadrupole interaction.

Elastic anomalies of YbIrGe in magnetic fields

The Yb-based heavy-fermion compound YbIrGe, which has the orthorhombic structure, shows the crystal electric field effect at high temperatures and antiferromagnetic orderings at TN = 2.4 K and Tm = 1.4 K. We previously found anomalous elastic softening in zero magnetic field originating from an indirect quadrupole interaction between the ground doublet and the excited doublets, and determined the crystal electric field level scheme: the ground doublet and an excited doublet at 138 K. To investigate the antiferromagnetic orderings at TN and Tm, we performed ultrasonic measurements under magnetic fields along the a-axis on YbIrGe single crystals. Temperature dependence of elastic modulus C11 exhibits elastic hardening below both TN and Tm at 0 T. As increasing the magnetic field along the a-axis, both TN and Tm decrease monotonically, suggesting that both transitions are antiferromagnetic ordering. TN closes around 2 T, and Tm disappears above 1 T. We clarified the magnetic field- temperature phase diagram along the a-axis.