Probing magnetostructural correlations in multiferroicHoAl3(BO3)4

Abstract

The system HoAl 3 (BO 3 ) 4 has recently been found to exhibit a large magntoelectric effect. To understand the mechanism, macroscopic and atomic level properties of HoAl 3 (BO 3 ) 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 HoO 6 polyhedra is seen to occur with magnetic field. However, the magnetic field dependent structural measurements reveal enhanced correlation between neighboring HoO 6 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 RAl 3 (BO 3 ) 4 systems (R= rare earth) is developed.

Structural features associated with multiferroic behavior in the RX3(BO3)4 system

“…This is in good agreement with the published results that the electric field induced polarization of HoFe 3 (BO 3 ) 4 , GdFe 3 (BO 3 ) 4 and EuFe 3 (BO 3 ) 4 are much lower than systems like SmFe 3 (BO 3 ) 4 and NdFe 3 (BO 3 ) 4 . Meanwhile, our previous research indicated that HoAl 3 (BO 3 ) 4 is much stiffer along the c axis [29], which would result in much larger couplings between the ferroelectric and magnetic units as suggested by the above theory. In this system, the polyhedra and BO 3 planes are less likely to tilt.…”

“…Note the right y -axis scale is for the HABO system only and the left y -axis scale is for the iron borate systems. The a/c ratio for HABO is much larger than any of the iron-based systems, indicating that the former is more closely packed in the c direction [29]. Meanwhile, the phase transitions in the GFBO and EFBO (indicated as vertical arrows in figure 3(a)) appear as jumps (vertical arrows) in the trends of a/c versus temperature.…”

“…The preparation details can be found in [28]. Detailed x-ray characterization of the HoAl 3 (BO 3 ) 4 system can be found in [29].…”

“…Addenda measurements (grease without sample) were collected and subtracted from the sample measurements. The heat capacity data were fitted with a double Debye model above the magnetic transitions (see [29]):…”