Probing magnetostructural correlations in multiferroic HoA l3(B O3)4

The system HoAl₃(BO₃)₄ has recently been found to exhibit a large magnetoelectric effect. To understand the mechanism, macroscopic and atomic level properties of HoAl₃(BO₃)₄ were explored by temperature and magnetic field dependent heat capacity measurements, pressure and temperature dependent x-ray diffraction measurements, as well as temperature and magnetic field dependent x-ray absorption fine structure measurements. The experimental work was complemented by density functional theory calculations. An anomalous change in the structure is found in the temperature range where large magnetoelectric effects occur. No significant structural change or distortion of the HoO₆ polyhedra is seen to occur with magnetic field. However, the magnetic field dependent structural measurements reveal enhanced correlation between neighboring HoO₆ polyhedra. This observed response is seen to saturate near 3 T. A qualitative atomic level description of the mechanism behind the large electric polarization induced by magnetic fields in the general class of RAl₃(BO₃)₄ systems (R= rare earth) is developed.