E. Yamamoto

Spin-glass behaviour with extended short-range ferromagnetic order in U2RhSi3

For U2RhSi3, which crystallizes in a disordered derivative of the hexagonal AlB2-type structure, we present a detailed investigation of magnetic properties by measurements of AC and DC susceptibility, magnetization, magnetic relaxation, specific heat and electrical resistivity. The results provide evidence for the formation of extended short-range ferromagnetic order at the temperature TC25 K, as indicated by a specific heat anomaly, and for a low-temperature spin-glass state with a freezing temperature Tf = 19.6 K. According to systematic specific heat experiments on isostructural spin-glass systems, short-range ferromagnetic correlations above the spin-freezing temperature are observed in U2RhSi3 (TC25 K, Tf = 19.6 K) and U2CoSi3 (TC10 K, Tf8 K), but not in U2PdSi3 (Tf = 13.5 K) and U2NiSi3 (Tf22 K). Different degrees of structural disorder of transition metal and silicon atoms, both on the same crystallographic site, seem to be the origin for the distinct different magnetic behaviour.

Fermi surface properties and de Haas–van Alphen oscillation in both the normal and superconducting mixed states of URu2Si2

Abstract We have succeeded in growing a high-quality single crystal of URu2Si2 with a residual resistivity ratio of 255 and performed magnetoresistance and de Haas–van Alphen (dHvA) experiments. The dHvA oscillations were observed clearly in both the normal and superconducting mixed states. From the magnetoresistance experiments, it is concluded that URu2Si2 is a compensated metal with equal carrier numbers of electrons and holes. In the dHvA experiments we observed three kinds of Fermi surfaces. We studied precisely the Fermi surface properties in both the normal and mixed states. The dHvA frequency does not change in magnitude between the normal and mixed states, while the cyclotron effective mass is reduced, and the corresponding Dingle temperature or scattering rate of the conduction electron increases in the mixed state.

Magnetic and Fermi Surface Properties of UCoGa5 and URhGa5

We succeeded in growing high-quality single crystals of UCoGa5 and URhGa5 by the self-flux method and measured the electrical resistivity, magnetic susceptibility, and de Hass-van Alphen(dHvA) oscillation. Three kinds of dHvA branches were observed, ranging from 8.6x 105 to 8.1 x 106Oe, which correspond to closed Fermi surfaces. UCoGa5 and URhGas are concluded to be semimetals. The cyclotron mass is small, ranging from 0.6 m0 to 2.9 m0.

Spin-glass behavior with short-range antiferromagnetic order in Nd2AgIn3

We present experimental results of complex susceptibility, static magnetization, magnetic relaxation, specific heat, and electrical resistivity measurements on an intermetallic compound Nd2AgIn3. The results indicated that Nd2AgIn3 undergoes a spin-glass transition at a freezing temperature Tf=12.6 K accompanied with the formation of short-range antiferromagnetic order. The mechanism of formation of spin-glass state in Nd2AgIn3 is different from that in diluted metallic spin-glass or uranium intermetallic compound. The existences of frustrated moments due to the triangles of nearest neighbors in Nd atomic layers and randomly distributed Ruderman–Kittle–Kasuya–Yosida interactions may be responsible for the spin glass state formed in Nd2AgIn3.

Fermi surface property of UPt3 studied by de Haas–van Alphen and magnetoresistance experiments

Abstract We have performed the de Haas–van Alphen (dHvA) and the magnetoresistance experiments in UPt3. The result of the magnetoresistance is consistent with the topology of the Fermi surface predicted from the 5f-itinerant band theory. The main Fermi surfaces detected by the dHvA are also well explained by the 5f-itinerant band model, although the cyclotron effective masses with 15–105m0 are 10–20 times larger than the corresponding band masses.

Specific-heat anomaly of metamagnetism on PrFe4P12 and UCoAl

Abstract The field dependence of specific heat has been measured at low temperatures in order to study two different types of f-electron metamagnetic systems. The antiferromagnet PrFe4P12 with Neel temperature of 6.4 K shows a sharp peak structure in C/T at the spin flip metamagnetic transition. In the high field state, the magnetic contribution Cm/T shows an anomalous huge value ∼1.4 J/K 2 mol . UCoAl shows the metamagnetic transition from paramagnetic ground state at ∼1 T, where C/T at 0.5 K shows an apparent decrease reflecting a change of the density of states. In the low-field state, ferromagnetic spin-fluctuation dominates in an anomalous temperature dependence of C.

Itinerant f-electron systems of cerium and uranium compounds

Abstract The f-electron systems of the cerium and uranium compounds have attracted attention in the context of heavy fermion and anisotropic superconductivity. We clarified an itinerant f-electron nature for UPt 3 by the de Haas–van Alphen experiments, detecting heavy conduction electrons with 100 m 0 . The dHvA oscillation was also observed clearly in both the normal and superconducting mixed states in CeRu 2 ,URu 2 Si 2 and UPd 2 Al 3 . An anisotropic energy gap with a line node for URu 2 Si 2 was discussed from the angular dependence of the dHvA amplitude in the mixed state.

Fermi Surface Properties of USi3

We succeeded in growing a single crystal of USi 3 with the cubic crystal structure and measured the transverse magnetoresistance and de Haas-van Alphen(dHvA) oscillation. Fifteen dHvA branches were observed, ranging from 7.37×10 6 to 2.33×10 8 Oe. These branches are in good agreement with theoretical ones based on 5 f -itinerant FLAPW band calculations. Cyclotron masses, which are in the range from 0.39 to 4.17 m 0 , are approximately the same as the corresponding band masses. These results indicate that the itinerant 5 f -electrons form wide bands. The magnetoresistance results confirmed the existence of open orbits, which can be well explained by the band 10-electron Fermi surface.

The de Haas–van Alphen effect in URu2Si2 under pressure

We carried out a de Haas–van Alphen (dHvA) experiment under pressure on a heavy-fermion superconductor, URu2Si2. The dHvA frequency, which corresponds to a nearly spherical Fermi surface, increases slightly with increasing pressure, while the corresponding cyclotron mass decreases considerably. Neither the dHvA frequency nor the cyclotron mass exhibit any abrupt change at the critical pressure of 1.5 GPa. The present result is thus inconsistent with a recent phase-separation proposal.

Magnetic, transport, and thermal properties of ternary intermetallic compound Nd2PtSi3

Abstract Physical properties of new non-magnetic atom disorder compound Nd2PtSi3 are investigated by dc magnetization, ac susceptibility, magnetic relaxation, specific heat and electrical resistivity measurements. The results establish that Nd2PtSi3 is a long-range magnetic exchange interactions dominated ferromagnet with Curie temperature T C =17.5 K . Irreversible magnetism and long time magnetic relaxation observed in this system are mainly resulted from the domain-wall pinning effects. The possibility of formation of spin-glass state in Nd2PtSi3 is discussed. The very small frequency shift of ac susceptibility peak position and distinct anomalies at TC in temperature dependent specific heat and electrical resistivity suggest that though more or less spins may be randomly frozen in Nd2PtSi3, the effects of spin-glass state are rather weak in this system.