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Research into electrical resistivity anomalies linked with magnetism in RECo2 (RE = Nd, Tb, Er, Ho) compounds in pressures of up to 8 GPa is presented. At ambient pressure, Nd and Tb show a second order magnetic phase transition at Tc, while a first order magnetic phase transition is seen in Er and Ho.

The temperature dependencies of the electrical resistivity Rho for current along and perpendicular to the c axis were measured on single crystalline UNiA1 under various hydrostatic pressures. The results are discussed in terms of the instability of the itinerant 5f-electron antiferromagnetism in UNiA1.

We have studied electrical resistivity anomalies connected with magnetism in RECo[sub 2] (RE=Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at T[sub C,] whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although T[sub C] decreases with pressure in all four compounds, the T[sub C](P) dependence for NdCo[sub 2] and TbCo[sub 2] differs considerably from this for HoCo[sub 2] and ErCo[sub 2]. For the latter two, T[sub c] vs P data deviate dramatically from the initial linear dependence above a critical pressure P[sub c] to become almost pressure independent at higher pressures. We propose that this is reflecting the loss of Co metamagnetism that is also indicated by the vanishing resistivity drop at T[sub c] for P>4 GPa and by the change from a FOMPT to a SOMPT. A scenario is discussed assuming that for P>P[sub c] the projected Co 3d! density of states at E[sub F] decreases. Hence the Co moment collapses because the RE-Co-RE exchange channel becomes ineffective to induce the itinerant 3d electron metamagnetism. The localized RE moments, however, order at a "residual" T[sub C] due to the persisting Ruderman-Kittel-Kasuya-Yosida-type exchange interaction. For NdCo[sub 2] and TbCo[sub 2] the T[sub c] values decrease with pressure by an exponential law but remain rather high in the highest applied pressures. Results of first-principles electronic structure calculations using the full-potential linearized augmented plane wave method are presented for HoCo[sub 2] and NdCo[sub 2] compounds, as well. © 2001 American Institute of Physics. We have studied electrical resistivity anomalies connected with magnetism in RECo[sub 2] (RE=Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at T[sub C,] whereas a firs! t order magnet... [ABSTRACT FROM AUTHOR]

We report on low-temperature measurements of magnetization, magnetoresistivity, Hall resistivity, magnetostriction, and elastic moduli on a UNiAl single crystal in magnetic fields up to 14 T applied along the c axis of the hexagonal structure. UNiAl is an itinerant 5f-electron antiferromagnet below T[sub N]=19.3 K. At temperatures below 7 K (∼0.3 T[sub N]), it exhibits a first-order metamagnetic transition (MT) at a critical field μ[sub 0]H[sub c]approx. 11.35 T between the low-field antiferromagnetic state and the high-field ferromagnetic one. The transition is accompanied by a sharp anomaly in transport and lattice properties. A striking observation is that the "virgin" curves expressing the field dependence of the various electronic properties clearly differ from those observed on a sample that has once appeared in the high-field state. This irreversibility in bulk properties corresponds to an irreversible change of the magnetic-structure propagation vector wi! thin the basal plane observed by neutron diffraction. The different shapes of M(H), ρ(H), and ρ[sub H](H) curves are strongly suggestive of field-induced changes in the spectrum of spin fluctuations, which play a substantial role in the physics of UNiAl. This seems to be corroborated by the effect of "high-field annealing" on the low-temperature specific-heat data. The irreversibility vanishes at temperatures T>=7 K where the MT loses its first-order character. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a magnetization, magnetostriction, electrical resistivity, specific heat and neutron scattering study of a UNi 2/3 Rh 1/3 Al single crystal, a solid solution of an antiferromagnet UNiAl and a ferromagnet URhAl. The huge uniaxial magnetic anisotropy confining the principal magnetic response to the c axis in the parent compounds persists also for the solid solution. The magnetization curve at 1.6 K has a pronounced S shape with an inflection at 12 T. The temperature dependence of magnetic susceptibility exhibits a maximum around 10 K and is magnetic history dependent at lower temperatures where the resistivity increases linearly with decreasing temperature. The low-temperature ρ(T) anomaly is removed in a magnetic field applied along c , which yields a large negative magnetoresistance amounting to -46 Ω in 14 T (at 2 K). The C/T values exhibit a minimum around 12 K and below 8 K they become nearly constant (about 250 mJ mol -1 K -2 ), which is strongly affected by magnetic fields. Neutron scattering data confirm a non-magnetic ground state of UNi 2/3 Rh 1/3 Al. The bulk properties at low temperatures are tentatively attributed to the freezing of U magnetic moments with antiferromagnetic correlations. The additional intensities detected on top of nuclear reflections in neutron diffraction in a magnetic field applied along c are found to be proportional to the field-induced magnetization, which reflects field-induced ferromagnetic coupling of U magnetic moments. This scenario is corroborated also by finding low-temperature magnetostriction data that also scale with the square of magnetization. [ABSTRACT FROM AUTHOR]

UNi2/3Rh1/3Al, a solid solution of UNiAl (Antiferromagnet (AF) with TN−19.3 K) and URhAl (Ferromagnet (F) with TC−27 K) exhibits no magnetic order due to a delicate balance of AF and F interactions. We report on a single-crystal susceptibility χ, magnetization M and resistivity ρ study. The huge uniaxial magnetic anisotropy typical for the parent compounds is preserved in UNi2/3Rh1/3Al. The χc vs. T (ρ vs. T) data exhibit a maximum (minimum) at 10 K. The Mc(B) curve at 1.6 K has a pronounced S-shape with an inflection around 12 T. The increase of ρ with decreasing T below 10 K is removed by magnetic fields. The low-T anomalies are attributed to proximity of magnetic order and AF correlations. The NFL features indicated earlier on polycrystals are not fully confirmed by single-crystal data. [Copyright &y& Elsevier]

Torsional oscillator experiments on solid 4He show frequency changes which suggest mass decoupling, but the onset is broad and is accompanied by a dissipation peak. We have measured the elastic shear modulus over a broad frequency range, from 0.5 Hz to 8 kHz, and observe similar behavior-stiffening and a dissipation peak. These features are associated with a dynamical crossover in a thermally activated relaxation process in a disordered system rather than a true phase transition. If there is a transition to a dc response, e.g., to a supersolid state, it must occur below 55 mK.

Torsional oscillator experiments show evidence of mass decoupling in solid 4He. This decoupling is amplitude dependent, suggesting a critical velocity for supersolidity. We observe similar behavior in the elastic shear modulus. By measuring the shear modulus over a wide frequency range, we can distinguish between an amplitude dependence which depends on velocity and one which depends on some other parameter such as displacement. In contrast with the torsional oscillator behavior, the modulus depends on the magnitude of stress, not velocity. We interpret our results in terms of the motion of dislocations which are weakly pinned by 3He impurities but which break away when large stresses are applied.