Morphotropic Phase Boundaries in Tb_1-xDy_xFe₂ Alloys

Abstract

Magnetostrictive alloys, materials that change in dimension under an applied magnetic field, are desired candidates for transducers. Unfortunately, common magnetostrictive metals, alloys, and oxides produce such small strains that they are not a viable option. In the early 1960's rare earths were found to possess extraordinary magnetostriction values at cryogenic temperatures. When alloyed with traditional transition metals they form a Laves phase compound of the form AB₂. These Laves phase compounds have shown large magnetostriction values, up to 2500με in TbFe₂. A major drawback to using these materials as transducers is their huge magnetocrystalline anisotropy constants, K₁ and K₂. However, it was found that TbFe2 and DyFe2 have opposing signs of K₁ and K₂. A pseudo-binary alloy, Tb_1-xDy_xFe₂ (Terfenol-D) TDFx, was formed to decrease the total magnetocrystalline anisotropy. The anisotropy reached a room temperature minimum for TDF73. It is suspected that this minimum of the anisotropy is accompanied by a morphotropic phase boundary (MPB) at which the crystal structure changes from tetragonal to rhombohedral. Unraveling the nature of the temperature and composition dependence of the magnetic and crystalline properties along this MPB is the primary focus of this thesis.

The structure of the TDF alloys was probed through macroscopic and microscopic techniques. The maximum in the DC magnetization at the transition temperature from tetragonal to rhombohedral broadens as the transition temperature is increased. This is attributed to decreasing anisotropy at increased temperature.

Synchrotron and neutron powder diffraction are utilized to elucidate the microscopic changes in the structure and magnetism. Neutron powder diffraction results were somewhat inconclusive but were sufficient to produce magnetic moments that were invariant, within experimental error, across the transition region. Synchrotron powder diffraction was used to probe the structure at temperatures across the MPB. Reitveld refinement of the structure in TDF65 reveals that large strain gradients exist across the MPBs. This was supplemented by temperature dependent scans of various TDF alloys showing a broadening of the phase transition with increasing temperature which we attribute a widening of the meta-stable [100] and [111] easy directions.