F.J.A.M. Greidanus

Magnetic properties of PrCo2 and its ternary hydride PrCo2H4

Abstract Magnetization and susceptibility data on PrCo 2 and PrCo 2 H 4 are presented. The ac susceptibility of PrCo 2 measured in zero dc field displays a sharp and high peak at T c = (39.9 ± 0.2) K. The magnetization versus temperature curves show ferromagnetic behaviour for B >1 T, but display a maximum at lower values of the applied field. These results, together with the behaviour of the hysteresis loops at different temperatures below T c , indicate that PrCo 2 orders ferromagnetically, the magnetic hardness increasing strongly for T → 0. The saturation moment at 4.2 K equals 3.9 μ B per formula unit, as found from the magnetization curve measured in a pulsed-field magnet up to B = 30 T. Similar experiments on PrCo 2 H 4 provide evidence that the introduction of hydrogen in PrCo 2 not only destroys the long-range atomic order, but also considerably reduces the ferromagnetic interactions. Such an effect of the hydrogen is commonly observed in cobalt intermetallics. Part of the PrCo 2 H 4 is found to have decomposed into PrH 2 and free Co. The clusters of free Co atoms give rise to a maximum in the zero-field ac susceptibility versus temperature curves, similar as observed in spin glasses or magnetic glasses. By increasing the ac frequency, the maximum shifts to higher temperatures. The behavior can be explained in terms of the Neel model for superparamagnetic particles with randomly oriented local anisotropy axes.

Crystal field splittings of PrX2 compounds (X=Pt, Rh, Ir, Ru, Ni) studied by inelastic neutron scattering

Neutron inelastic scattering experiments have been performed on polycrystalline samples of the cubic Laves phase compounds PrX2(X=Pt, Rh, Ir, Ni). Measurements in the paramagnetic state yield LLW parameters 0.6<x<1 and W<0. In this region various levels cross at an x value 0.86 and as a consequence the electronic ground state in the paramagnetic regime is either the singlet Γ1, or the non-magnetic doublet Γ3. Measurements in the ferromagnetic state support these conclusions. The crystal-field parameters obtained can be used in model calculations of some macroscopic quantities, in particular the specific heat and the spontaneous magnetization. The variation of the x values in the present series of Laves phase compounds evidences the presence of a contribution by conduction electrons to the crystal field.

Hyperfine specific heats of PrX2 (X = Ir, Pt, Rh, Ru) laves phase compounds

Abstract Specific heat data below 1 K for the C-15 compounds PrX 2 (X = Ir, Pt, Rh, Ru) reveal Schottky-type anomalies, ascribed to hyperfine interactions. Apparently the 4f-moments are magnetically ordered. The values deduced for these moments are only ≈ 70% of that for J = 4, indicating that the Pr 3+ moment is partially quenched by the crystal field.

Field‐dependent specific heat study of the spin‐Peierls transition in TTF–AuS4C4(CF3)4

The compound TTF–AuS4C4(CF3)4 is one of the very few existing examples of the spin‐Peierls transition, in which a system of 1D antiferromagnetic chains dimerizes spontaneously as a consequence of magnetoelastic coupling with the surrounding lattice. We have studied the spectific heat anomaly accompanying the SP transition for this compound as a function of an applied magnetic field. Our H=0 data differ considerably from those obtained earlier by Wei et al., and in fact remove the discrepancy found by them between their results for TTF‐AuS4C4(CF3)4 and the isomorphous TTF–CuS4C4(CF3)4. The differences between the two experiments are probably due to thermal relaxation effects. The combination of the specific heat data now available confirms the conclusion drawn from our previous field‐dependent differential susceptibility studies on the Au compound, namely that the SP transition involves a considerable degree of short‐range order, starting to develop below T=1.2 Tsp. We have been able to follow the specific...

Magnetic properties of PrX2 compounds (X = Pt, Rh, Ru, Ir) studied by hyperfine specific heat, magnetization and neutron-diffraction measurements

Abstract Magnetic ordering phenomena in rare-earth intermetallic compounds can be unravelled most advantageously in the case of simple crystallographic structure and when a combination of microscopic techniques is applied. Here we shall present the temperature and magnetic field dependence of the magnetic moment of the cubic PrX2 compounds (X = Pt, Rh, Ru, Ir), as inferred from hyperfine specific-heat, magnetization and neutron-diffraction measurements. The results are compared with a mean-field calculation, taking crystalline electric field and bilinear (dipolar) exchange interactions into account. Adopting experimental values of the Lea, Leask and Wolf parameters x and W from inelastic neutron scattering results, we find satisfactory agreement between our magnetic data and the mean-field theory. An observed discrepancy of about 15% between the calculated and measured saturation values of the spontaneous magnetization can be explained by the presence of quadrupolar interactions.

Specific heat, resistivity, and AC susceptibility of the cubic PrX2 compounds (X = Pt, Ru, Ir, Rh)

Abstract Specific-heat, differential-susceptibility and electrical-resistivity measurements on PrX2 (X = Ir, Pt, Rh, Ru) compounds reveal phase transitions at Tc = 11.2±0.5 K, 7.7±0.5 K, 7.9±0.5 Kand 33.9±0.5 K for X = Ir, Pt, Rh, and Ru, respective ly. From earlier neutron inelastic scattering experiments, the crystalline electric field levels of these compounds have been determined. The specific-heat results are compared with the results of a mean-field calculation, assuming bilinear exchange interactions. The presence of broad secondary maxima in the temperature dependence of the specific heat of PrRh2, PrRu2 and especially PrIr2 can qualitatively be explained by the presence of biquadratic (quadrupolar) interactions. The behaviour of the susceptibility is in agreement with ferromagnetic ordering. The electrical resistivity drops markedly below Tc, and the dϱ/dT versus T curve is similar to that of the specific heat.

Magnetic Properties and Neutron Spectroscopy of Intermetallic Praseodymium Compounds

Elastic and inelastic neutron scattering, specific heat, susceptibility and resistivity experiments on the series of cubic Laves phase compounds PrX2 (X = Mg, Al, Co, Ni, Ru, Rh, Pt, Ir) are discussed. In all these compounds, magnetic phase transitions are found. The electronic ground state in the paramagnetic regime is either the singlet Г1, or the non-magnetic doublet Г3. Furthermore the ratio of the magnetic interaction to the crystal field splitting varies considerably through the series. In PrNi2 the hyperfine interaction determines the nature of the phase transition.

Specific heat and frequency-dependent ac-susceptibility of PrNi2 below 1 K

Abstract Ac-susceptibility measurements on PrNi2 show broad anomalies, the temperature of the susceptibility maximum shifting from 0.3–0.5 K as the frequency is varied in between 3 Hz and 3 kHz. On the other hand the specific heat is featureless in the same temperature range. The behaviour is interpreted in terms of a mixed electronic-nuclear phase transition. Strong relaxation effects are observed in the susceptibility at temperatures close to the maxima.

Hyperfine specific heats of two paramagnetic, octahedrally coordinated Co2+ compounds

The specific heats of Co(C3H4N2)6(NO3)2 and Co[C11H12ON2]6(ClO4)2 have been studied in the range 0. 04 < T < 1 K. No magnetic ordering is observed, indicating that the magnetic interactions are of the order of 10-2 K or less. The observed specific heats are found to be mainly due to the hyperfine (h.f.) interactions between the Co2+ electronic and nuclear spins. The h.f. specific heats are calculated — assuming zero magnetic exchange between the electron spins — from the h.f. constants known from E.S.R. experiments. The observed data can be accounted for within the errors by the calculated h.f. specific heats, plus a (weaker) contribution due to the magnetic dipolar interactions.

Magnetic ordering in [Co(γ-CH3C5H4NO)6](ClO4)2: II. Analysis of susceptibilities and specific heat in terms of the two-dimensional rectangular Ising model; evidence for methyl-group rotational tunneling

Abstract The single-crystal susceptibilities of [Co(γ-CH3C5H4NO)6](ClO4)2 have been measured over the temperature interval 40 mK-4.2 K, and antiferromagnetic ordering is found to occur at 0.49±0.02 K. Specific heat data (50 mK-1.2 K) exhibit a λ-anomaly at 0.480±0.015 K. Both sets of measurements have been analyzed in terms of the rectangular 2d Ising lattice for S = 1 2 , with the parameters |Ja/kB| = (0.55±0.02)K, Jb/Ja = 0.6±0.2 from the specific heat, and g c′ = 5.48±0.03, Ja/kB = (−0.54±0.05) K, Jb/Ja = 0.6±0.2 from the susceptibility. At the lowest temperatures reached (0.05–0.3 K), the specific heats of both [Co(γ-CH3C5H4NO)6](ClO)4)2 and the Cu2+-doped related Zn compound, [Zn(γ-CH3C5H4NO )6](ClO4)2 show contributions which probably arise from rotational tunneling of the CH3 groups attached to the pyridine N-oxide rings. Assuming a 3-fold rotational barrier, the height of the potential barriers opposing CH3-rotations is estimated as V 3 ⋍ 430 K .