W.J. Huiskamp

Investigation of structural phase transitions in some [(C[2H5)4N]2MX4 compounds, with M = Co, Zn, Mn, and X = Cl, Br

Abstract Dielectric and NMR measurements are presented on a series of compounds containing the [(C 2 H 5 ) 4 N] + ion. Two phase transitions with large dielectric effects, indicating a ferroelectric phase, are found to occur in the manganese and zinc compounds. In the case of the zinc compound, however, the two transitions are found only upon lowering the temperature. Upon raising the temperature the zinc compound shows only one phase transition with but a small increase in the dielectric permittivity, quite similar to the behaviour of the cobalt compound. This surprising result is argued to be due to the large hysteresis of the lowest phase transition. We propose a phase diagram of these compounds and relate the two transitions that occur to the liberations of the tetrahedrons constituting the compound.

Specific heat and susceptibility of the 1-dimensional S = 12 Heisenberg antiferromagnet Cu(Pyrazine) (NO3)2. evidence for random exchange effects at low temperatures

Abstract Magnetic susceptibility and specific heat data are presented on the S = 1 2 antiferromagnetic Heisenberg chain Cu(pyrazine) (NO 3 ) 2 in the range 0.05 K T The intrachain exchange is obtained as J / k B = -5.20(5) K by a theoretical fit to data. No evidence for a transition to a 3-d ordered state could be detected down to the lowest temperatures (corresponding to k B T /| J | ≈ 0.01). The specific heat data provide a beautiful demonstration of the predicted linear chain low-temperature behaviour, c / R = k B T /3| J |, over one decade in temperature. Superimposed upon the behaviour for the uniform antiferromagnetic chain, the data show contributions that can be interpreted in terms of a random exchange model. It is argued that these arise from random lattice imperfections (defects) which apparently break up the chains into weakly coupled finite segments with an average length of about 200 lattice sites. The segments with an odd number of spins act as a system of highly diluted, weakly interacting, spins 1 2 . The contributions due to defects may predominate the behaviour of the susceptibility and (to a lesser extent) the specific heat at very low temperatures, in case the interchain interactions are sufficiently small. In particular, the random defects will suppress the occurence of 3-d ordering between the chains. Furthermore, they lead to random exchange phenomena similar as observed in other quasi 1-d antiferromagnetic systems. It is suggested that many of the known examples of random exchange antiferromagnets can be explained in terms of the random defect model.

Field dependent specific heat study of the dipolar Ising ferromagnet LiHoF4

Abstract We present specific heat measurements on a spherically shaped single crystal of the dipolar Ising ferromagnet LiHoF4 for various values of a magnetic field applied along the c-axis (easy axis). The measurements in zero field cover the range 0.06– K, enabling the separation of the contributions to the specific heat due to the hyperfine interactions, to the T2 singlet state at about 10 K, and to the phonons, from the longe-range magnetic anomaly at Tc = 1.530(5) K. The zero-field specific heat anomaly is analysed assuming both dipolar and exchange interactions between Ho3+ ions to be present. The zero-field specific heat appears to have the same value when measured for different sample shapes, in agreement with the theoretical prediction by R.B. Griffiths that the free energy in zero field of dipolar magnets should be independent of the shape of the sample in the thermodynamic limit. The shape of the specific heat curve is discussed, and a comparison with the dipolar Ising ferromagnet LiTbF4 is made. The in-field specific heat measurements show that upon increasing the applied field Ha the ordering temperature Tc(Ha) decreases, as does the height of the specific heat anomaly. Above a critical field Hc = Hdem, where Hdem is the demagnetizing field, magnetic ordering can no longer be discerned. Below Tc(Ha) the specific heat appears to be independent of the applied field Ha up to Ha = Hc, confirming another prediction by Griffiths.

Heat capacity of Cs2CoCl4 below 1 K, compared with the S = 12 linear chain XY model

Abstract Heat-capacity data on Cs2CoCl4 below 1 K are reported. A magnetic exchange contribution is found, showing a broad, finite maximum at T ≃ 0.9 K, and a small λ-type anomaly at Tc = 0.222 ± 0.005 K. The data agree quite well with the prediction for the heat capacity of the S = 1 2 , linear chain, XY model. The applicability of the S = 1 2 , XY model is shown to follow from crystal-field arguments. Furthermore, it is argued that the existence of magnetic chains can be understood by considering the various superexchange paths occurring in the structure.

Magnetic properties of TmAl3 and TmGa3

We present measurements of the specific heat between 75 mK and 30 K, of the a.c. susceptibility from 1.1 K to 149 K and of the magnetization for the cubic intermetallic compounds TmAl3 and TmGa3. The crystal field ground state of TmAl3 appears to be nonmagnetic, while the exchange interaction is too small for the occurrence of an induced moment ordering. An analysis of the specific heat data allows the selection of a set of crystal field parameters from the various proposed values in the literature. The reported low temperature specific heat of TmGa3, suggests the presence of a small distortion from cubic symmetry in this compound. This agrees with results from available Mossbauer data.

Determination of the nuclear magnetic moments of 145–148Eu by low-temperature nuclear orientation

Abstract Nuclear orientation measurements down to 2 mK have been performed on sources of 145–148Eu in SmFe2. The gamma-ray anisotropy yields the nuclear magnetic moments of the europium nuclei with A = 145, 146, 147and148 to be 1.1 ± 0.3 μN, 0.70 ± 0.07 μN, 1.0 ± 0.1 μN and 1.33 ± 0.06 μN, respectively.

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.

Induced-moment ferromagnetic ordering in the singlet-ground state compound TmNi2

Abstract We present measurements of the magnetization in very high fields, the low temperature specific heat and the AC susceptibility, both in zero and nonzero magnetic field, of the intermetallic cubic Laves phase compound TmNi 2 . This material appears to be a singlet-singlet system with an inter-ionic exchange which just exceeds the critical value for magnetic ordering. The absence of a clear anomaly in the specific heat near the ferromagnetic transition temperature T c = 1.1 K is discussed in a molecular field approximation including crystal field effects. The results of the experiments can qualitatively be described in terms of a set of crystal field parameters corresponding to a Hamiltonian for a system with a distortion from cubic symmetry.

Low temperature magnetic properties of the bimetallic compounds [Co(C5H5NO)6](MX4) (M = Co2+, Zn2+; X = Cl-, Br-)

Abstract We have measured the specific heat and suceptibility of the compounds [Co(C 5 H 5 NO) 6 ] (MX 4 ) (M = Co, Zn; X = Cl, Br), which are all isostructural and monoclinic. The data show that two magnetic subsystems are present in the Co/Co compounds. The Co 2+ ions on the octahedral positions of the chlorine compounds exhibit antiferromagnetic ordering at T c = 0.95(1) K, the Co 2+ ions on the tetrahedral positions remaining paramagnetic down to at least 0.04 K. The paramagnetic ions appear to experience two different molecular fields from the ordered Co 2+ ions. The behaviour of the bromine compounds is similar, the ordering temperature being about 0.65 K. Both chlorine compounds are apparently canted antiferromagnets. The bromine compounds show compensated antiferromagnetism.