Magnetic properties of HoDyTi2O7

Abstract

Frustrated spin systems give rise to a host of interesting magnetic property Among them lies the spin ice compound whereby spins arrange in a manner similar to ice-rule configuration found in water ice In this thesis we present a study on the mixture compound of two canonical spin ice candidates HoDyTi2O7 (HDTO) The parent compounds Ho2Ti2O7 (HTO) and Dy2Ti2O7 (DTO) are similar in so many ways such as having large moment on the magnetic sites Ho and Dy respectively and due to crystal electric field properties the spins are essentially Ising However a few publications have highlighted their subtle differences which include the slightly different spin freezing temperature and dynamic properties In this thesis the proposed HDTO is grown in both polycrystalline and single crystal form and measured for various low temperature physical properties and compared against their parent counterpart We present results from magnetization heat capacity measurements on polycrystalline samples and heat capacity measurements on single crystal samples respectively as well as neutron scattering experiments Temperature dependent magnetization reveals no transition down to 2 K High temperature paramagnetic phase was fitted using Curie-Weiss law and an effective moment of 11 5 uB High field magnetization yield a saturation moment of 11 2 uB for powder and 11 6 uB for single crystal along [111] direction The higher saturation moment compared to parent compound points to a softening in the Ising anisotropy constraint or slight tilt in spin easy axis AC susceptibility at high field present a frequency dependent peak related to single ion property of Dy Fitting with Arrhenius law obtained a energy barrier 183 k <EB <231 K Specific heat measurement on single crystal samples down to 50 mK showed a spin freezing peak at 1 2 K as well as a novel peak at 0 6 K The novel peak may point towards a new ordering in our system however its origin is currently unknown Under applied field along [111] direction both peaks experience a shift which suggest that it is magnetic Aside from above mentioned peaks a field induced peak was also seen at 0 4 K which may be related to the transition between kagome ice state and fully saturated state Interestingly nuclear hyperfine signal measured in our system is much smaller than typical Ho containing compound Powdered diffraction measurement yield a diffuse scattering profile at the low Q region at temperature below 5 K similar to other dipolar spin ice materials Inelastic neutron scattering measurement on HDTO powder showed two inelastic peaks at low temperature The energy transfer are at 0 26 meV and 0 5 meV respectively These inelastic signal hints at a splitting of ground state doublet of Holmium ion and Dysprosium ion respectively More work to study the microscopic behavior in our system need to be carried out to explain the changes brought in by mixing two spin ice compounds

Date of Award	2016 Aug 1
Original language	English
Supervisor	Lieh-Jeng Chang (Supervisor)