Breaking the Magnetic Symmetry by Reorientation Transition Near 50 K in Multiferroic Magnetocaloric HoFeO3

Using the new polarized neutron diffraction (PND) setup at Heinz Maier-Leibnitz Zentrum (MLZ) in Garching Germany (MLZ), the spin reorientation transition in the magnetocaloric orthoferrite HoFeO
₃
was studied at different wavelengths. Various experiments provided reproducible results demonstrating the high reliability of the used setup. We show that during the phase transition at T
_SR
= 53 K in an external magnetic field applied along the crystal
$c$
-axis, the ordered magnetic moment of the Fe sublattice rotates from the crystallographic direction
$b$
to a not just in the
ab
plane, but through the
$z$
-axis. This means that the applied field breaks the orthorhombic symmetry allowing some magnetization parallel to
$z$
within a short temperature region. Interestingly, this is the same temperature region where a large magnetocaloric effect for HoFeO
₃
was previously reported. A general model of the magnetic structure of HoFeO
₃
, unconstrained by the orthorhombic symmetry, would allow the magnitudes and directions of the moments on each of the eight magnetic sublattices in the unit cell to be independent of one another, leading to 24 independent magnetic parameters. PND measurements were used to determine the absolute sign of the Dzyaloshinskii–Moriya interaction (DMI) in the
ab
plane for the Fe magnetic sublattice at 65 K. DMI plays an important role in the energy balance of the system.