M. R. Wells

Magnetic interactions in gadolinium orthochromite, GdCrO3

The magnetic properties of the distorted perovskite GdCrO3 have been investigated by magnetic moment measurements in the temperature range below 100K and specific heat measurements below 10K. Measurements show that the canting of the Cr3+ ions from a simple antiferromagnetic alignment is due to antisymmetric exchange coupling between Cr ions. The value of the weak ferromagnetic moment Fz is 400 emu mol-1. The interaction between the Cr3+ and Gd3+ ions produces an effective field at the Gd sites of 5.5 kOe, acting in the opposite direction to Fz. At a temperature of approximately 7K the Cr-Gd interaction is sufficient to cause the Cr system to undergo a spontaneous spin reorientation from a GxFz to a GzFx mod. At 2.3K the interaction between the Gd3+ ions is sufficient to bring about a co-operative transition to an antiferromagnetic state.

Ferromagnetism in lithium holmium fluoride-LiHoF4. I. Magnetic measurements

Magnetic susceptibility measurements on LiHoF4 show that this compound undergoes a transition to a ferromagnetic state below Tc=1.53K, with the Ising-like Ho3+ moments oriented along the crystallographic c-axis. Analysis of the shape-dependent Curie-Weiss O indicates that magnetic dipole interaction dominates the ordering process.

Antiferrodistortive Jahn-Teller ordering in KDy(MoO4)2

KDy(MoO4)2 is known to undergo a structural phase transition at 14.3K. Spectroscopic and magnetic studies of the behaviour of this compound at low temperatures have revealed some striking properties which suggest that the phase transition is of the Jahn-Teller type and that ordering occurs in an antiferrodistortive mode. A two-sublattice molecular-field model has been developed to account qualitatively for the observed phenomena. This is thought to be the first reported instance of antiferrodistortive Jahn-Teller ordering in a rare-earth compound.

Further studies of the enhanced nuclear magnet HoVO4 - I. The crystal field and the Zeeman spectrum

A novel approach is adopted to fit the experimental results for the Van Vleck paramagnet HoVO4. Within the ground manifold 5I8, J = 8, the five parameters for a crystal field of tetragonal symmetry are adjusted to give values in agreement with the optical spectrum for the lowest energy levels: the ground singlet, the first excited doublet at 21 cm-1, and the (accidental) triplet at 47 cm-1. Within experimental error (of order 1 cm-1), this agreement is not impaired by a small modification in which all the crystal field parameters are multiplied by a factor 1.0225. This factor is introduced to give the correct value of the enhanced nuclear magnetic resonance frequency for the stable isotope 165Ho (I = 7/2), known to 0.3% (Bleaney et al. Proc. R. Soc. Lond. A 362, 179 (1978)). The optical Zeeman effect, calculated therefrom, is in good agreement with that observed experimentally for the lowest levels in magnetic fields up to 15 T, directed along the [100], [110] and [001] axes (Battison et al. Phys. Lett. A 55, 173 (1975); J. Phys. C 10, 323 (1977)).

The magnetic, spectroscopic and thermal properties of KDyMo2O8

The low-temperature behaviour of KDyMo2O8 has been investigated by magnetic and spectroscopic techniques. At 14K there is evidence for a crystallographic phase transition and at 1.1K magnetic ordering occurs.

Magnetic interactions in gadolinium orthoferrite

It has been shown that the Fe3+ ions in the distorted perovskite GdFeO3 become ordered antiferromagnetically at 650 K and the ordering arrangement is GxFz in Bertauts (1963) notation. The canted moment has been measured at temperatures below 77 K, and has been found to produce an effective field of 310 Oe at the gadolinium sites opposing the canted moment. The induced susceptibility of the Gd3+ ions in zero applied field is in direct opposition to the resultant Fe moment along the c axis. The specific heat has been measured in the temperature range 0.5 K<or=T<or=12 K and the result is a broad, rounded, Schottky curve with a lambda -type anomaly superposed at 1.5 K the Neel point of the Gd system. Susceptibility measurements below 4.2 K are presented for the orthorhombic a, b c axis.

The magnetic properties of holmium trifluoride HoF3

In the orthorhombic compound holmium trifluoride, HoF3, the trivalent holmium ion has a singlet ground state, with another singlet at 5.90 cm-1. Optical spectroscopy, magnetic and thermal measurements are used to investigate the behaviour of single crystals at 4.2 K and below. The results largely confirm the crystal-field analysis of Ram and Sharma (1985), but give a value of 6.59 cm-1 for the separation of the two singlets. It is found also that the system orders antiferromagnetically at TN=0.53 K, with the holmium moments aligned parallel to the a axis. The enhancement of the 165Ho NMR frequency is determined by a resonance experiment at 3.5 GHz. A simple theory of two electronic levels including hyperfine structure is used to interpret the results. Despite exceptionally large corrections required for the internal field, the magnetic and thermal measurements can be fitted using molecular-field theory.

Nuclear magnetic resonance in holmium vanadate, HoVO4

In HoVO4 the Ho3+ ion, 4f105I8, experiences a crystal field of tetragonal symmetry which produces a ground state singlet and a nearby excited doublet at 21 cm-1 and at low temperatures the material behaves as a Van Vleck paramagnet with high anisotropy. The nuclear Zeeman interaction is greatly enhanced through the hyperfine interaction and the nuclear Hamiltonian may be written HN=−γ1hB2I2−γ⊥h(BxIx+ByIy)+P[Iz2−13I(I+1)]. The parameters have been determined by nuclear magnetic resonance measurements on 165Ho and 51V at frequencies of 500 and 11 MHz respectively. The dependence of (γ┴/2π) in HoVO4 on sample shape is analysed, and shown to be consistent with the value 1526(3) MHz T-1 for 165Ho in YV04, while │P/h│= 25.9(3) MHz in HoVO4 and 25.3(2) MHz in YVO4 (T ≤ 2K). The origins of the line width and quadrupole interaction are discussed. The high degree of anisotropy prohibits a direct measurement of γ║ but it is extimated that (γ//γ┴) ˜ 10-2.

Energy levels in PrVO4

Investigations of the low-lying energy levels of PrVO4 using fluorescence, Raman and far infrared spectroscopy give the energies of the first excited singlet and doublet as about 35 cm-1 and 84 cm-1, respectively, above the ground singlet. Detailed measurements of the nuclear magnetic resonance spectrum of 141Pr show that the temperature variation up to 20K of the spin-Hamiltonian parameters gamma /sub ///, gamma perpendicular to and P can be accurately fitted by thermal averages over these three levels. The positions of the remaining levels of the 3H4 manifold are estimated by an extrapolation procedure based on properties of a crystal field Hamiltonian with tetragonal symmetry.

High‐sensitivity ultrasonic interferometer for the detection of magnetic phase transitions

An improved technique is described for the detection of magnetic phase transitions using acoustic waves with electrical interferometry. The method employs zinc oxide transducers sputtered directly onto the sample. These transducers operate efficiently at frequencies up to 3 GHz, corresponding to acoustic wavelengths of the order of 2 μm. By measuring the phase of high‐order multiple echoes, changes in sample thickness and/or velocity of parts in 109 can be detected. The method has been used to determine the phase diagram of the metamagnet dysprosium aluminum garnet for magnetic fields parallel to [001]. General agreement with earlier determinations based on thermodynamic measurements is obtained.