M J M Leask

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.

Optical study of the cooperative Jahn-Teller transition in thulium vanadate, TmVO4

The optical absorption spectrum of Tm3+ in TmVO4 has been studied to obtain information on the cooperative Jahn-Teller phase transition which occurs in this material at 2.14+or-0.04 K. The temperature dependence of the absorption lines below 2.14 K was in good agreement with the predictions of molecular field theory along the crystallographic c axis the field dependence of the spectra at 1.4 K provided direct evidence for the reduction of the Jahn-Teller distortion to zero over the field range 0 to approximately 5kOe, an important consequence of the theoretical treatment of the problem. The observation of the optically uniaxial to biaxial transition associated with the crystallographic phase transition provided additional evidence for the proposed model of behaviour, and showed that the distortion is of the symmetry type Gamma 3+.

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.

Spectroscopic investigation of holmium vanadate, HoVO4

Spectroscopic investigation into the properties of HoVO4 has revealed interesting and unusual anomalies. These centre on the fact that the first excited doublet (at 21 cm-1) shows a small (2.3 cm-1) zero-field splitting at low temperatures. Although no phase transition has been detected in HoVO4, it is concluded from the experimental data that the splitting is real enough and that the symmetry at the Ho3+ site must be lower than tetragonal. A number of effects which might be responsible for this splitting are investigated, but the origin of the splitting remains obscure.

Ferromagnetism in lithium holmium fluoride-LiHoF4. II. Optical and spectroscopic measurements

For pt.I see ibid., vol.8, p.4083 (1975). The absorption spectrum of LiHoF4 has been studied, and the lowest-lying crystal-field levels of Ho3+ in LiHoF4 have been determined. The ground state is a crystal-field doublet with a uniaxial g-factor of gc=13.8+or-0.5. The spectroscopic measurements and the direct optical observation of domain structure both indicate that LiHoF4 orders ferromagnetically parallel to the crystallographic c-axis at Tc=1.52+or-0.01K. The ordering is caused by magnetic dipole interaction, and the formation of domains of small demagnetizing factor is essential.

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.

High field Zeeman effects in holmium vanadate

Abstract Zeeman effect measurements on HoVO 4 in magnetic fields up to 15 Tesla indicate that it could be a suitable candidate for enhanced nuclear cooling experiments.

Jahn-Teller induced magnon side bands in thulium vanadate

Abstract Weak absorption lines in the Tm 3+ spectrum are attributed to exciton-magnon processes allowed through Jahn-Teller coupling. Energy and intensity variation with field are explained in terms of a many-body treatment of the problem.

Fibre optic device for Zeeman spectroscopy at liquid helium temperatures

Using commercially available fibre optic material, a simple but effective device has been constructed which allows high resolution optical spectroscopy on rare earth compounds at liquid helium temperatures, in magnetic fields up to 15 T. The advantage of this device is that it allows an optical experiment to be performed in a helium Dewar without windows.