Wendl, A.MLZ*

2018

Abstract: In the cubic rare-earth pyrochlore Ho2Ti2O7, the magnetic holmium ions sit on a tetraedric rare-earth sublattice and experience strong crystalline electric field effects. The resulting pronounced local〈111-Ising anisotropy, in the combination with ferromagnetic coupling,creates a macroscopically degenerate ground state. This so-called spin ice state hosts magnetic excitations, which can be described in terms of magnetic monopoles. The mechanisms behind the monopole creation and diffusion are goals of ongoing research, where the spin dynamics in the system provide valuable insights. In this study MIEZE spectroscopy, a special neutron resonance spin echo technique, was applied to the high temperature dynamics of Ho2Ti2O7, where the crystal field spectrum dominates. The single-ion character of the excitations was confirmed by showing theQ-independence of the excitations down to 0.1 ̊A−1. The quasi-elastic linewidth was studied to access the spin-flip transitions in the crystal field ground state doublet. This linewidth was tracked over a large temperature range and an energy range was covered, which was previously only accessible by a combination of time-of-flight and spin echo spectroscopy. The line broadening is in excellent agreement with the Orbach model, including magneto-elastic coupling of phononic modes to crystal field transitions. In particular, this model delivers a microscopical explanation of spin flips over the large anisotropy barrier. The present study furthermore revealed crystal field transitions between excited states, which are not detected in classical neutron spin echo spectroscopy due to their large excitation energy of E0≈1.5 meV. This large energy transfer required new a data fitting method, going beyond the standard spin echo approximation.

Keyword(s): Magnetic Materials (1st) ; Magnetism (2nd)

1. RESEDA (RESEDA)
2. E21 (E21)
3. Technische Universität München (TUM)

1. RESEDA: Resonance spin echo spectrometer (NL5S)