Yuichi Hiranaka

Heavy Fermion State Based on the Kondo Effect in EuNi2P2

EuNi2P2 is known as a heavy fermion compound with an electronic specific heat coefficient γ=100 mJ/(K2 \(\cdot\)mol). We grew single crystals and studied their electronic and magnetic properties by measuring the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, and thermal expansion. The present heavy fermion state is clarified to be based on the Kondo effect as in CeRu2Si2, revealing an intensive shrinkage of the volume below about 100 K in the temperature dependence of thermal expansion. The temperature dependences of the \(4f\)-electron contribution to the volume thermal expansion Δ V/V)4f and the average Eu valence are found to show good scaling in EuNi2P2.

Fermi Surface and Magnetic Properties of Antiferromagnet EuBi3

EuBi3 with the AuCu3-type cubic structure is known to be a Eu-divalent antiferromagnet with the Neel temperature \(T_{\text{N}}\simeq 7.5\) K. We succeeded in growing a high-quality single crystal by the Bi self-flux method. The magnetization at 1.3 K for the magnetic field along the \(\langle 100\rangle\) direction increases linearly as a function of magnetic field, and saturates at a critical field \(H_{\text{c}}=225\) kOe, reaching a saturated magnetic moment of 7 µB/Eu. \(H_{\text{c}}\) is well explained by the magnetic exchange interaction based on a two-sublattice model, using the simple relation Hc = (kB/3µB)(TN-θp), namely, Hc [kOe]=4.9 (TN-θp) [K], where θp is the paramagnetic Curie temperature θp=-36 K. The present anti ferromagnetic state is found to be stable under pressures up to 8 GPa, where the Neel temperature increases with increasing pressure, being \(T_{\text{N}}=16.5\) K at 8 GPa. From the results of de Haas–van Alphen experiments on EuBi3 and energy band calculations for the non-\(4f\...

First-Order Antiferromagnetic Transition and Fermi Surfaces in Semimetal EuSn3

We grew high-quality single crystals of the antiferromagnet EuSn3 with the AuCu3-type cubic crystal structure by the Sn self-flux method and measured the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, thermal expansion, and de Haas–van Alphen (dHvA) effect, in order to study the magnetic and Fermi surface properties. We observed steplike changes in the electrical resistivity and magnetic susceptibility, and a sharp peak of the specific heat and thermal expansion coefficient at a Neel temperature TN = 36.4 K. The first-order nature of the antiferromagnetic transition was ascertained by the observation of thermal hysteresis as well as of latent heat at TN. The present antiferromagnetic transition is found to be not a typical second-order phase transition but a first-order one. From the results of dHvA experiment, we clarified that the Fermi surface is very similar to that of the divalent compound YbSn3, mainly consisting of a nearly spherical hole Fermi surface and...

Magnetic and Fermi Surface Properties of Antiferromagnet EuCd 11

We succeeded in growing a high-quality single crystal of EuCd11 with the BaCd11-type tetragonal structure by the Cd self-flux method, and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and de Haas–van Alphen (dHvA) oscillations, together with the electrical resistivity under high pressures up to 8 GPa. The antiferromagnetic ordering was confirmed to occur at the Neel temperature TN = 2.7 K, and the antiferromagnetic state was found to change into the field-induced ferromagnetic state with 7 μB/Eu above a critical field Hc \( \simeq \) 15 kOe for both the magnetic fields H || [100] and [001], revealing a typical Eu-divalent antiferromagnet. Reflecting the large lattice parameters of a = 11.93 A and c = 7.682 A with the caged structure, where each Eu atom is surrounded by a polyhedron of 22 Cd atoms, the Brillouin zone and the corresponding Fermi surfaces are small in volume. The detected dHvA branches are well explained by the full potential linear augmented pla...

Charge Density Wave Transition in EuAl4

EuGa4 and EuAl4 crystallize in the BaAl4-type tetragonal structure. The antiferromagnetic order was observed at 15 and 16.5K in EuGa4 and EuAl4, respectively.1,2) The magnetic susceptibilities in EuGa4 and EuAl4 well follow the Curie–Weiss law with effective moments of 7.97 and 8.02 B, respectively, which are consistent with the theoretical value of 7.94 B for a Eu-divalent electronic state.1,3) In the electrical resistivity and thermoelectric power measurements under pressure, a shoulderlike change was observed below about 100K at 1GPa in EuGa4. Moreover, a similar resistivity hump was observed below TCDW 1⁄4 140K at ambient pressure in EuAl4. The authors in Ref. 2 considered that this transition corresponds to the emergence of a charge density wave (CDW). In this study, we measured the magnetoresistance and Hall effect to clarify whether this is indeed based on CDW transition. Single crystals of EuAl4 were grown by the Al self-flux method. The transverse magnetoresistance and Hall resistivity were measured with an AC resistance bridge. The magnetic field H was applied along the [010] and [001] directions. The electrical current flow J was parallel to the [100] direction. Figure 1 shows a Kohler plot for the transverse magnetoresistance in the configurations of H k 1⁄2001 and J k 1⁄2100 . Kohler’s rule, i.e., = 1⁄2 ðHÞ ð0Þ = ð0Þ 1⁄4 fðH= ð0ÞÞ, holds when the temperature dependences of the scattering relaxation time are the same for all carriers. The = above 150K, namely, at 150, 160, and 200K, is well scaled on a single curve, which indicates that Kohler’s rule is satisfied above TCDW. On the other hand, the = below 140K deviates from Kohler’s rule, indicating that the carrier density and/or the scattering mechanism change below TCDW. In the two-band model, the transverse magnetoresistance and Hall resistivity are expressed as xx 1⁄4 ex hxð ey þ hyÞ þ ð exR 2 h þ hxReÞH ð ex þ hxÞð ey þ hyÞ þ ðRe þ RhÞH2 ; ð1Þ yx 1⁄4 ðRe hx hy þ Rh ex eyÞ þ ReRhðRe þ RhÞH 2 ð ex þ hxÞð ey þ hyÞ þ ðRe þ RhÞH2 H; ð2Þ where ei ( hi) and Re (Rh) are the resistivity along the i 1⁄4 xor y-direction and the Hall coefficient of the electron (hole) band, respectively.4) The Hall coefficient of each band is inversely proportional to the carrier density as Re 1⁄4 1=nee and Rh 1⁄4 1=nhe, where ne and nh are the electron and hole densities, respectively. The resistivity of each band is ei 1⁄4 1=ne eie ( hi 1⁄4 1=nh hie), where ei ( hi) is the mobility of the electron (hole). The transverse magnetoresistance increases quadratically, = H, for a compensated metal (ne 1⁄4 nh) at a high field limit (!c 1), whereas it tends to saturate for an uncompensated metal (ne 61⁄4 nh). Here, !c is the cyclotron frequency and is the scattering relaxation time. !c is roughly estimated using the relation = ð!c Þ2, which is obtained from the free-electron model. There is one EuAl4 unit in a body-centered tetragonal primitive cell. Considering that Eu is divalent, as proved by the magnetic susceptibility data,3) EuAl4 is a compensated metal with an even number of valence electrons. The experimentally obtained H-dependent magnetoresistance of up to = 2:5 10 2 (!c 0:16), shown as the solid curve in Fig. 1, reveals that EuAl4 is a compensated metal above TCDW. On the other hand, the = below TCDW gradually deviates at higher fields from the initial H dependence, as shown by the broken curve in Fig. 1 and the broken line in the inset of Fig. 1, at 100K. The deviation at 100K occurs at = 8 10 3 (!c 0:09). This behavior corresponds to the tendency toward saturation at higher fields, although the high field limit (!c 1) is not satisfied. Therefore, the transverse magnetoresistance indicates that EuAl4 is a compensated metal above TCDW and changes into an uncompensated metal below TCDW. The present experimental result and the hump of μ just below TCDW indicate that ne or nh starts to decrease below TCDW. Figure 2 shows the temperature dependence of the Hall coefficient RH determined at 0 T, shown by solid circles for H k 1⁄2010 and by solid squares for H k 1⁄2001 . The phase transitions at TCDW 1⁄4 140K and TN 1⁄4 14K are well observed. RH is temperature independent and negative above TCDW for both field directions. This means that the anomalous Hall coefficient, which is related to the magnetic susceptibility, is negligible in EuAl4. The negative RH for the compensated metal indicates e > h. The absolute value of RH increases with decreasing temperature below TCDW. The Hall resistivity is proportional to the magnetic field above TCDW, for example, at 200K, as shown in the inset of Fig. 2. On the other hand, the Hall resistivity gradually deviates downward from the linear field dependence with decreasing 0.06

Pressure and Substitution Effects on Transport and Magnetic Properties of Y1-xRxCo2 Systems with Static Magnetic Disorder

The electrical resistivity and thermopower of light- and heavy-rare-earth-based pseudo-binary Y1-xRxCo2 (R = Nd, Gd, and Tb) alloys are measured at temperatures from 2 to 300 K under pressures up to 3.5 GPa. The resistivity and thermopower of Y1-xRxCo2 show unusual large variations with atomic substitution and pressure in the range of x<xm, where an inhomogeneous magnetization of the Co 3d electron subsystem is observed. These results indicate that the low-temperature transport properties of Y1-xNdxCo2, as well as of Y1-xR\text{HCo2 (R\text{H = heavy rare earth) alloys, are related to conduction electron scattering due to the static magnetic disorder in the itinerant Co 3d electron subsystem. We found that there is a universal relationship between \(\mathrm{d}\ln T_{\text{C}}/\mathrm{d}P\) and \(x/x_{\text{m}}\) in Y1-xRxCo2 alloys, where \(x_{\text{m}}\) is the boundary composition, which separates the alloy phase diagram into regions with uniform and nonuniform magnetizations of the Co-3d electron subsy...

Effect of Pressure on the Electronic state in Eu-Divalent EuTIn4 (T: Ni, Pd, Pt, Au) Compounds

We grew single crystals of Eu-divalent antiferromagnets EuTIn4 (T: Ni, Pd, Pt, Au) by the In-self flux method, with the constitution of Eu:T:In = 1.1:1:30. Single crystals are characteristic in shape, being long along the orthorhombic [100] direction. We measured the electrical resistivity, specific heat, magnetic susceptibility, and de Haas-van Alphen effect for these compounds, together with the electrical resistivity under pressure for EuTIn4 (T: Ni, Pd, Pt). Under pressure, the Eu-divalent electronic state is often changed into the Eu-trivalent state, revealing the valence transition. In the present experiments, the valence transition was, however, not observed even at high pressures up to 8 GPa for these compounds, but a sharp resistivity drop was observed just below the Neel temperature under pressure. This is most likely due to a change of the magnetic structure.