Takashi Nishioka

Magnetic properties (Communication arising): Parasitic ferromagnetism in a hexaboride?

Surprisingly for a compound with no magnetic element, Young et al. have observed ferromagnetism in calcium hexaboride (CaB6) doped with lanthanum (La) — the system has a ferromagnetic Curie temperature as high as 600 K, which is comparable to that of transition-metal ferromagnets such as iron (Fe). Here we show that high-temperature ferromagnetism in this CaB6 system is not intrinsic but that it is instead due to alien phases of iron and boride.

Anomalous low temperature specific heat of UCu4+x Al8−x

Specific heats (C) of heavy-fermion system UCu4+xAl8−x (0<x<2) were measured systematically down to 0.15K in magnetic fields up to 5.2T. For 1.5≤x≤1.75, anomalous temperature dependences of specific heat at low temperatures were observed:C/T was proportional to −logT (x=1.5) orT−0.3 (x=1.75) in the temperature range from 0.15K to 10K. The specific heats were hardly influenced by magnetic fields of 5.2T. These anomalous behaviors suggest the possibility of non-Fermi liquid ground state in UCu4+xAl8−x.

Extrinsic origin of high-temperature ferromagnetism in CaB6

The discovery of ferromagnetism in CaB 6 doped with La was surprising, because there is no magnetic element in the compound nevertheless the system involved a ferromagnetic Curie temperature as high as 600 K. In order to resolve the origin of the ferromagnetism, we have made magnetization measurements and chemical analysis of CaB 6 and LaB 6 samples synthesized under various conditions. We have found that the ferromagnetism disappears almost completely by the acid etching, and that there is strong correlation between the magnitude of reduced magnetization and the iron concentration involved in the acid. Therefore we conclude that the ferromagnetism is of extrinsic origin and can be ascribed to iron impurity phases, probably FeB and Fe 2 B that have nearly the same Curie temperature as CaB 6 . We suggest that these iron impurities originate from the crucible used in the synthesis. Furthermore we have measured the thermoelectric power at high temperatures including the Curie temperature. There seems to be n...

Thermal expansion measurement under pressure of UGe2

Abstract Volume expansion measurements of single crystalline UGe 2 samples have been made under pressure. The linear coefficient α V ( T ) at ambient pressure shows a broad hump at around T x ∼25 K and a sharp transition at T C =52 K. We have observed that the anomaly in Δ α V at T x becomes prominent and increases in magnitude with increasing pressure.

Magnetic properties of CeCuxAl4−x and CeCuxGa4−x single crystals

Abstract Magnetic properties of CeCuxAl4−x (x=0.8, 0.9, 1.0 and 1.1) and CeCuxGa4−x (x=0.7 and 1.0) systems have been investigated by magnetic susceptibility, high-field magnetization, specific heat and electrical resistivity measurements using single crystal samples. In CeCuxAl4−x the susceptibility and magnetization showed a large anisotropy, the component parallel to the a-axis being larger than that parallel to the c-axis. With decreasing x from x=1, especially χ(H‖a) showed remarkable increase at low temperatures (development of ferromagnetic correlation), while TN showed a slight increase. Similar behavior was also observed in CeCuxGa4−x. These results including NMR measurements are discussed in terms of TK, TRKKY and CEF.

Structural and magnetic properties of U-Au compounds : U14Au51 and UAu3

The structural and magnetic properties of U-Au compounds have been investigated by X-ray diffraction, magnetic susceptibility, electrical resistivity and specific heat measurements on UAu3, UAu3.3 and UAu3.64. It is suggested that besides the hexagonal U14Au51, phase the orthorhombic TiCu3-type UAu3 phase exists as a high-temperature phase.

NMR and NQR studies on UCu4 + xAl8 - x (0 < x < 2) system

We have studied the microscopic properties of the ThMn12-type UCu4 + xAl8 - x (0 < x < 2) system by 27Al NMR and 63,65Cu NQR. With increasing x from x = 0, the Neel temperature TN decreased and disappeared abruptly at xc = 1 1.15. No magnetic ordering was detected down to 1.3 K for x ⩾ xc by the NMR, suggesting that the effect of short-range magnetic ordering is not a major origin of the heavy-fermion-like (i.e. large C/T) behavior observed for x ⩾ xc. From the K versus χ plot in UCu4 + xAl8 - x, the transferred hyperfine field Hhf for Al was determined to be 0.80, 0.77, 0.45 and 1.1 kOe/μB-Ce for x = 0.25, 0.50, 1.10 and 1.50, respectively. By NQR measurements at 1.3–4.2 K for x = 0.25 no resultant hyperfine field was observed at Cu sites in the AF ordered state, indicating a cancellation of Hhf at the 8(f) Cu sites in the type I AF. The relaxation rate 1/T1 by 63Cu NQR for x = 1.5 at 1.3–4.2 K revealed the Korringa-like behavior.

NMR studies on heavy fermion materials CeCuAl3 and CeCuGa3

Microscopic magnetic properties of heavy fermion antiferromagnets CeCuAl3 and CeCuGa3 of BaAl4 type have been studied by NMR measurements at 0.4∼300K. The NMR spectra for Al and Cu in thec-plane|H oriented CeCuAl3 were composed of well resolvede2qQ peaks, indicating good sample quality. From theK vs χ plot the transferred hyperfine fields for Al and Cu in CeCuAl3 and those for Cu and Ga in CeCuGa3 were determined to be 3.2, 4.8, 2.6 and 5.9kOe/μB-Ce, respectively. The relaxation rate 1/T1 vsT for Al in CeCuAl3 behaved similarly as in CeAl2, i.e. local 4f-moment type forT>TN and gap type forT

Insulator–metal transition studied by heat capacity measurements on SmS

Abstract We have measured the specific heat of SmS at pressures up to ∼7.5 kbar . We have observed that the electronic specific heat coefficient γ shows a steep increase at around P c ∼3.5 kbar , which corresponds to an insulator–metal phase transition. In the metal phase above Pc, we have found that γ reaches 100 mJ/mol K 2 , indicating a strongly correlated metallic state. In the insulator phase, on the other hand, we have observed an anomaly in the temperature dependence of the specific heat around 1 K , which seems to survive in the metal phase.

Anomalous low-temperature behavior in Th2Zn17-type U2CuxAl17 − x

The specific heat (C) and AC magnetic susceptibility (X AC ) of Th 2 Zn 17 -Zype U 2 Cu x Al 17-x (8 ≤ x ≤ 12) have been measured down to 0.1 K in magnetic fields up to 52 kOe. For all the samples studied, C/T increases with decreasing temperature through a kink at a characteristic temperature, T 0 , and attains a large value of about 1 J/K 2 mol U. The AC susceptibility shows a cusp just at that temperature. As magnetic field increases, the kink in C/T and the cusp in X AC is suppressed, but C/T and X AC below 0.5 K remain unchanged. Furthermore, the temperature dependence of C/T tends to vary as - log T in magnetic fields. These facts indicate that non-Fermi-liquid behavior and a spin-glass ordering coexist at zero field, but an application of fields suppresses the former but not the latter.