Critical Metamagnetic Field Research Articles

Extended stability range of the non-Fermi liquid phase in UCoAl

High pressure was used to investigate the stability of the non-Fermi liquid (NFL) state, observed in electrical resistivity of uranium-based band metamagnet UCoAl in a pure form (paramagnet) or with Fe substitution (ferromagnetic ground state), both in a single-crystal form. By combining the pressure variations of magnetization and resitivity in these materials the phase diagram for UCoAl had been constructed. The band metamagnet transforms into the ferromagnetic state as the critical metamagnetic field is reduced to zero by the lattice expansion analogous to the negative pressure. Within the same diagram, the increasing hydrostatic pressure drives the critical metamagnetic field upwards while reducing the magnetization increment at the transition. The NFL state persists to about 4–5 GPa. Although spin fluctuations play an important role in the character of UCoAl, they do not exhibit any criticality in the sense of divergence of parameters describing the resistivity around the Ferro-NFL phase transition, which is of the first order type.

Ferromagnetism in 5f-band metamagnet UCoAl induced by Os doping

UCoAl is a 5f-band metamagnet with a uniquely low paramagnetic-to-ferromagnetic transition field, 0.7 T, extremely sensitive to any perturbation such as elemental substitution. Here, we study variations of magnetic properties in the UCoAl-UOsAl system on single- and polycrystalline samples with different concentration of Os. We found that osmium can substitute Co in UCoAl up to 20%, while preserving the ZrNiAl structure type. Pure UOsAl was identified as a hexagonal Laves phase, MgZn2 type. It is a weak Pauli paramagnet. Even a 2%-substitution of Os for Co in the 5f band metamagnet stabilizes ferromagnetism with the Curie temperature TC = 26 K and uranium magnetic moment μU = 0.4 μB and shifts the critical metamagnetic field to zero. Higher Os concentrations enhance both TC and μU. All magnetic response is concentrated into the c-axis; the susceptibility for magnetic field perpendicular to c is low and practically temperature-independent. Our study reflects the decisive role of the 5f-5d hybridization in the magnetism of the UCoAl-UOsAl system. This work completes the study of the alloying of UCoAl with late transition metals and indicates that the non-magnetic phase exhibiting band metamagnetism is very limited in the concentration range.

Effects of pressure and magnetic field on transport properties of Y1−xRxCo2 alloys (R=Gd, Tb, Dy, Ho and Er)

Electrical resistivity ρ and thermopower S of Y1−xRxCo2 (R=Gd, Tb, Dy, Ho and Er) Laves phase alloy systems were measured at temperatures from 1.5 K to 300 K in magnetic fields up to 15 T and under hydrostatic pressure up to 2 GPa. We show that there is a universal linear relation between the pressure and magnetic field derivatives of the resistivity, dρ/dP and dρ/dB, with gradient, determined by pressure derivative of the critical metamagnetic field of the cobalt 3d electron system. A similar scaling behavior was found for the thermopower dependencies on pressure and alloy composition.

Magnetic and Transport Properties of PrRu2Si2Single Crystal under High Pressure

We report on magnetization and electrical resistivity with respect to temperature, magnetic field and hydrostatic pressure ( p ) up to 0.9 GPa. Moderate dependence of T C on pressure d T C /d p = -1.8 K/GPa was observed whereas the value of T N remains almost intact up to highest applied pressure. In the AF phase region ( T C < T < T N ) a metamagnetic transition occurs with rather low value of the critical metamagnetic field (µ 0 H C ∼50 mT at 15 K). The µ 0 H C value increases nearly twice when applying pressure of 0.8 GPa (the d H C /d p is almost constant yielding 4 ×10 -2 T/GPa).

High-pressure effect on magnetic phases in PrRu2Si2 single crystal

We report on magnetization and electrical resistivity with respect to temperature, magnetic field and hydrostatic pressure (p) up to 0.9 GPa. Moderate dependence of the Curie temperature, T C, on pressure was observed, whereas the value of the Neel temperature, T N, remains almost intact up to the highest applied pressure. In the antiferromagnetic phase region (T C<T<T N), a metamagnetic transition occurs, with a rather low value of the critical metamagnetic field (μ0 H C ∼ 50 mT at 15 K). The μ0 H C value increases nearly twice when applying pressure of 0.8 GPa.

Effects of cation size disorder and lattice distortion on metamagnetism in phase-separated manganites

The effects of A-site cation size disorder in ABO 3 type charge-ordered and antiferromagnetic Pr 0.5Ca 0.5MnO 3 system have been studied by substituting La 3+, Sr 2+ or Ba 2+, while keeping the valency of Mn ions and the tolerance factor ( t=0.921) constant in the substituted compounds. We find that the substitutions by these larger cations induce successive sharp step-like metamagnetic transitions at 2.5 K. The critical field for metamagnetism is the lowest for 3% Ba substituted compound, which has the largest A-site cation size disorder and the least distorted MnO 6 octahedra, among the compounds reported here. These cation substitutions give rise to ferromagnetic clusters within antiferromagnetic matrix, indicating phase-separation at low temperatures. The growth of the clusters is found to vary with the substitution amount. The local lattice distortion of MnO 6 octahedra enhances the charge ordering temperature and reduces the magnetization at high fields (>1 T) in these manganites.

Transport and thermodynamic properties ofSr3Ru2O7near the quantum critical point

The specific heat and electrical resistivity of ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ single crystals are measured in several magnetic fields applied along the $c$ axis for temperatures below 2 K and at fields up to 17 T. Near the critical metamagnetic field at ${B}_{1}^{*}\ensuremath{\sim}7.8\mathrm{T},$ the electronic specific heat divided by temperature increases logarithmically as the temperature decreases, over a large range of T, before saturating below a certain ${T}^{*}$ (which is sample dependent), indicating a crossover from a non-Fermi liquid (NFL) region dominated by quantum critical fluctuations to a Fermi liquid (FL) region. This crossover from a NFL to a FL state is also observed in the resistivity data near the critical metamagnetic field for $I\ensuremath{\parallel}c$ and $B\ensuremath{\parallel}c.$ The coefficient of electronic specific heat, \ensuremath{\gamma}, plotted as a function of field shows two peaks, consistent with the two metamagnetic transitions observed in magnetization and magnetic torque measurements. At the lowest temperatures, a Schottky-like upturn with decreasing temperature is observed. The coefficient of the Schottky anomaly exhibits a field dependence similar to that of \ensuremath{\gamma}, implying an influence by the electrons near the Fermi surface on the Schottky level splitting.

Magnetism in UCO1-xTxAl (T = Fe, Ni) single crystals

UCoA1 has a paramagnetic ground state, although the c-axis susceptibility shows a maximum at Tmax ≈ 20 K. The low-T state is easily modified to ferromagnetism by application of magnetic field along the c-axis or by doping by suitable elements. The field and temperature dependences of the magnetization and specific heat of single crystals of the UCo1-xTxAl (T = Fe, Ni) solid solutions are presented. All the compounds exhibit a strong uniaxial anisotropy with the dominant magnetic response along the oaxis of the hexagonal ZrNiAl-type structure. Ni substitution yields an increase of the critical field for metamagnetism which disappears at x = 0.10. UCo0.95Fe0.05Al is ferromagnetic below 30 K. UCo0.98Fe0.02Al and UCo0.95Fe0.05Ni0.05Al exhibit ferromagnetism with a significant trace of metamagnetism. An analysis of specific-heat data for UCoAl and UCo0.95Ni0.05Al in 0 and 10 T allows the entropy contributions due to spin fluctuations to be extracted.

Magnetization and Specific Heat of UCo0.95T0.05Al (T = Fe, Ni) Single Crystals

We present the first single-crystal study of Fe and Ni doped U(Co1-x T x )Al compounds in comparison to the parent compound, the itinerant 5f-electron metamagnet UCoAl. Magnetization and specific-heat data measured in the temperature range of (1.8-300) K and in fields up to 10 T are analyzed and discussed. Both compounds exhibit a strong uniaxial anisotropy with the dominating magnetic response along the c-axis of the hexagonal ZrNiAl-type structure. Ni substitution yields an increase of the critical field for metamagnetism; UCo0.95Fe0.05Al is ferromagnetic below 30 K. Temperature evolution of the entropy change between 0 and 10 T points to involvement of magnetic correlations or spin fluctuations in the paramagnetic compounds UCoAl and UCo0.95Ni0.05Al.

The magnetic characteristics of the Tb(Ni 1− xCo x) 2Ge 2 system

The compounds TbNi 2Ge 2 and TbCo 2Ge 2 both crystallize with the ThCr 2Si 2 structure. TbNi 2Ge 2 has a complex H– T phase diagram while TbCo 2Ge 2 has a much simpler, though disputed phase diagram. In order to investigate the effects on the magnetism of TbNi 2Ge 2 due to cobalt substitution, single crystals of Tb(Ni 1− x Co x ) 2Ge 2 with x ranging from 0 to 1 have been grown from a psuedoternary melt. We have traced the Néel and Weiss temperatures as a function of cobalt concentration, x. Close to x=0.5, the Weiss temperature is positive, indicating predominately ferromagnetic interactions (in contrast to x<0.4 and x>0.6 for which the Weiss temperatures are negative), χ( T≈ T N) is a maximum and the critical field for metamagnetism is a minimum. Nevertheless, the compound orders antiferromagnetically, similar to all other members of the dilution series.

Stability of the non-Fermi liquid state in UCoAl

A non-Fermi liquid (NFL) power law behaviour of the resistivity with exponent n= 5 3 was found in the low-field state of the band metamagnet UCoAl. This characteristic is rather robust, with weak dependence on magnetic field lower than the critical metamagnetic field. The critical field increases, while the anomaly at the metamagnetic transition is gradually reduced, with applied pressure.

Influence of 5d-metal on magnetic properties of UNi1−T Al compounds

We have studied structure and magnetic properties of UNi1−xTxAl (T=Ir, Pt) alloys. UNiAl does not form continuous solid solutions with either UPtAl or UIrAl. The solubility ranges extend only to 20% from each side in both systems. The alloys with 0.20<x <0.80 contain two isostructural phases with the ZrNiAl-type structure. In both systems, the compounds with x≥0.80 order ferromagnetically, whereas those with x≤0.20 exhibit metamagnetic behaviour characteristic for antiferromagnets like UNiAl. The initial Pt (Ir) substitutions lead to an increase (decrease) of the Neel temperature whereas the critical field for metamagnetism becomes gradually reduced in both cases. The electronic specific-heat coefficient γ is strongly reduced in UNi1−xPtxAl (from 164 to 113 mJ mol−1 K−2 for x increasing from 0 to 0.2) and enhanced in UNi1−xIrxAl (up to 198 mJ mol−1 K−2 for x=0.2). The obtained results are discussed in the context of the effects induced by substitutions of other transition metals in UNiAl considering the evolution of the 5f–d hybridization.

Magnetic structures of UNiAl in magnetic fields

UNiAl orders antiferromagnetically (AF) below T N=19.3 K with a propagation vector q =(0.1, 0.1, 0.5). U moments oriented along the c-axis of the hexagonal structure are modulated sinusoidally within the basal plane with an amplitude of 1.24 μ B/U. The magnetic structure consists of three U moments ( U 1, U 2, U 3 ) coupled ferromagnetically within the basal plane. U 1 and U 2 are substantially larger than U 3 and invariant with field, whereas U 3 becomes enhanced in a field along c. The vector q remains intact in fields lower than the critical metamagnetic field ( B c=11.4 T at 1.5 K). Above B c, UNiAl orders ferromagnetically with equal U moments of 0.91 μ B in 11.8 T at 1.7 K. When decreasing field below B c, the original propagation vector is lost and a new AF structure with q′=(0.17, 0.03, 0.5) is established. The original q is recovered only by warming far above T N and subsequent zero-field cooling.

Evidence for an extended critical region near the metamagnetic transition of UCoAl

Electrical resistivity and ac magnetic susceptibility studies of the 5f-metamagnet UCoAl reveal the presence of strong spin fluctuation effects for temperatures below 30 K and magnetic fields below the critical metamagnetic field μ0Hc≈1 T, where a non-Fermi liquid term ∼T3/2 is prominent in the electrical resistivity. A step-wise increase of the residual resistivity and an increasingly prominent Fermi liquid T2 term of the resistivity is observed as the field is increased though the metamagnetic transition.

Magnetoresistance anomalies in metamagnetic single crystals and epitaxial films

The magnetoresistance of rare-earth metamagnetic silicides RSi x (R = Gd, Tb, Dy, Er; x ≈ 1.67) and pnictides ErP, ErAs 0.4P 0.6 reveals some unusual features, which are explained by the increase of spin-disorder scattering as the field approaches the critical field for metamagnetism