We measured the high-field magnetization and de Haas–van Alphen (dHvA) oscillations for YbIr 2 Zn 20 with a cubic crystal structure, together with the electrical resistivity and magnetic susceptibility. The magnetic susceptibility χ with an effective magnetic moment of Yb 3+ becomes temperature-independent at low temperatures, with a broad peak at T χmax =7.4 K for H ∥<110>. The corresponding magnetization indicates a metamagnetic transition at H m =120 kOe, consistent with a T χmax vs H m relation in the Ce- and U-based heavy fermion compounds. The large cyclotron masses of 4–27 m 0 are detected in the dHvA experiment, and are found to be reduced at magnetic fields higher than H m =120 kOe. The resistivity follows the Fermi liquid relation ρ=ρ 0 + A T 2 under magnetic field, and the \(\sqrt{A}\) value is also found to have a maximum at H m as a function of magnetic field. From the present experimental results, together with the results of 4 f -itinerant energy band calculations, the 4 f electrons are found to contribute to the heavy fermion state in YbIr 2 Zn 20 .
We succeeded in growing single crystals of cage-structure compounds RCd 11 (R: La, Ce, and Pr) and precisely studied their low-temperature magnetic and electronic properties by measuring electrical resistivity, magnetic susceptibility, magnetization, specific heat, and the de Haas–van Alphen (dHvA) effect. We found antiferromagnetic ordering at 0.44 and 0.39 K in CeCd 11 and PrCd 11 , respectively, and clarified the magnetic phase diagrams of the compounds. In addition, low-lying crystalline electric field (CEF) schemes were proposed from the specific heat results of both compounds. From the present study, the antiferromagnetic ordering in PrCd 11 is found to be of the exchange-induced type with a singlet ground state. From the dHvA experiment, we detected small dHvA branches ranging from 7×10 5 to 2×10 7 Oe, which correspond to small Fermi surfaces. This is mainly due to a small Brillouin zone based on a large unit cell. Moreover, the dHvA frequencies and cyclotron masses are approximately the same among RCd 11 , revealing a localized character of 4 f electrons in CeCd 11 and PrCd 11 .
We studied an effect of pressure on the heavy fermion state of YbCo 2 Zn 20 under magnetic fields. The critical pressure P c , where the electronic state changes from the heavy fermion state to the antiferromagnetic state, is estimated to be P c ≃1.8 GPa. The super-heavy fermion state in the P c region is found to be strongly destroyed in magnetic fields for H ∥<100>. The field-induced quadrupolar phase, which is most likely realized only for H ∥<111>, is found to be robust even in the antiferromagnetic pressure region.
We measured the magnetic susceptibility, high-field magnetization, magnetoresistance, specific heat, and Hall coefficient, together with the electrical resistivity under high pressures and magnetic fields, for the heavy-fermion compound YbRh2Zn20 with the cubic CeCr2Al20-type structure. The metamagnetic behavior was observed at Hm = 65 kOe for the magnetic field along the <100> direction below Tχmax = 5.3 K, at which the temperature of the magnetic susceptibility indicates a broad maximum. The coefficient A of the T2 dependence of electrical resistivity ρ=ρ0 + AT2 and the electronic specific heat coefficient C/T possess a broad maximum at Hm. The metamagnetic field Hm is found to decrease with increasing pressure and to become zero at a critical pressure Pc ≃5.2 GPa. Correspondingly, the A value increases drastically in magnitude, and the Fermi liquid relation is no longer satisfied at 5.2 GPa in zero magnetic field, implying a non-Fermi liquid state. On the other hand, at magnetic fields of H > 20 kOe, the low-temperature resistivity exhibits a T2 dependence even at 5.2 GPa. The A coefficient decreases rapidly with increasing magnetic field. These results indicate that an electronic state at 5.2 GPa corresponds to the quantum critical point.
We studied an effect of pressure on the electronic state of YbIr 2 Zn 20 under magnetic fields. The heavy fermion state at ambient pressure is found to be changed into an antiferromagnetic state at pressures larger than a critical pressure P c ≃5.2 GPa, where the electronic state indicates a super-heavy fermion state with an order of the electronic specific specific heat coefficient γ= 20 J/(K 2 ·mol). The temperature dependence of the electrical resistivity shows a characteristic two-peak structure above 5.0 GPa, implying the combined phenomena between the Kondo effect and the crystalline electric field effect. At 7.6 GPa, the magnetoresistance shows a broad maximum at a critical field H c , corresponding to a pressure-induced antiferromagnetic phase with the Néel temperature T N = 1.4 K. T N increases with further increasing pressure.
We studied the heavy fermion compound YbCo2Zn20 with an electronic specific heat coefficient γ \( \simeq \) 8000 mJ/(K2·mol) by measuring the de Haas–van Alphen (dHvA) oscillation, Hall effect, magnetic susceptibility, and magnetization at ambient pressure, as well as the electrical resistivity in magnetic fields of up to 320 kOe and at pressures of up to 5 GPa. The detected Fermi surfaces are small in volume, reflecting the small Brillouin zone based on the large cubic lattice constant a = 14.005 Å. The cyclotron effective masses, which were determined from the dHvA experiment, are found to be markedly reduced in magnetic fields. In other words, the detected cyclotron masses of 2.2–8.9 m0 (m0: the rest mass of an electron) at Hav = 117 kOe are enhanced to 100–500 m0 at 0 kOe. By applying pressure, the heavy fermion state disappears at Pc \( \simeq \) 1.8 GPa and orders antiferromagnetically for P > Pc. The field-induced antiferroquadrupolar phase, which is observed only for \(H\parallel \langle 111\rangle \) in the magnetic field range from HQ = 60 kOe to \(H'_{\text{Q}} = 210\) kOe, is found to shift to lower magnetic fields and merge with theantiferromagnetic phase at 4.5 GPa.
