Ferromagnetism and lattice distortions in the perovskiteYTiO3

Abstract

The thermodynamic properties of the ferromagnetic perovskite ${\text{YTiO}}_{3}$ are investigated by thermal expansion, magnetostriction, specific-heat, and magnetization measurements. The low-temperature spin-wave contribution to the specific heat, as well as an Arrott plot of the magnetization in the vicinity of the Curie temperature ${T}_{C}\ensuremath{\simeq}27\text{ }\text{K}$, is consistent with a three-dimensional Heisenberg model of ferromagnetism. However, a magnetic contribution to the thermal expansion persists well above ${T}_{C}$, which contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a comparison with the corresponding model system EuS. The pressure dependences of ${T}_{C}$ and of the spontaneous moment ${M}_{s}$ are extracted using thermodynamic relationships. They indicate that ferromagnetism is strengthened by uniaxial pressures $\mathbf{p}\ensuremath{\parallel}\mathbf{a}$ and is weakened by uniaxial pressures $\mathbf{p}\ensuremath{\parallel}\mathbf{b},\mathbf{c}$ and hydrostatic pressure. Our results show that the distortion along the $a$ and $b$ axes is further increased by the magnetic transition, confirming that ferromagnetism is favored by a large ${\text{GdFeO}}_{3}$-type distortion. The $c$-axis results, however, do not fit into this simple picture, which may be explained by an additional magnetoelastic effect, possibly related to a Jahn-Teller distortion.