Single crystal polarized neutron diffraction study of the magnetic structure of HoFeO₃

Abstract

Abstract. Polarised neutron diffraction measurements have been made on HoFeO 3 single crystals magnetised in both the [001] and [100] directions (P bnm setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of H = 9 T parallel to [001] at T = 70 K and with the lower field H = 0.5 T parallel to [100] at T = 5, 15, 25 K. A Fourier projection of magnetization induced parallel to [001], made using the hk0 reflections measured in 9 T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70 K, 9 T applied parallel to [001] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the x-y plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature Γ 2 structure with a lower field, 0.5 T [100] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the 180• domain stabilised by the field were determined relative to the distorted perovskite structure,. This relationship fixes the sign of the Dzyalshinski-Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.PACS numbers: 75.30.Gw HoFeO 3 , polarized neutrons,weak-antiferromagnetism/ Polarised Neutron diffraction study of HoFeO 3

Natural ferroelectric order near ambient temperature in the orthoferrite HoFeO3

“…We note that a G-type k = 0 AFM structure has been proposed 21,25,26 , around which ferroelectricity occurs in HoFeO 3 . Thus, the inverse Dzyaloshinskii-Moriya (DM) interaction does not contribute to the observed ferroelectricity.…”

“…We note that the R-factor improves from 3.96 to 3.76 for the refinement using P bn2 1 as a parent space group at 200 K, which has been considered in the reported neutron results. 25,26 Although the symmetry is lowered, here, the final refinement is stable and converges to reasonable values. More importantly, it explains the ferroelectricity.…”

“…The compound of our interest, the orthoferrite HoFeO 3 , exhibits a long range magnetic order governed by the Fe−Fe, R−Fe, and R − R interactions. The Fe spins of the compound order antiferromagnetically (T N ) around 647 K, which is followed by two more transitions driven by the R−Fe and R − R interactions with decreasing temperature [21][22][23][24][25][26] . As a result of the R−Fe interaction, the Fe spins reorient along the c-axis below ∼ 51 K and transform to a G z F x phase from G x F z in Bertaut's notation.…”

“…Out of four different Shubnikov space groups, such as P bn2 1 , P bn ′ 2 ′ 1 , P b ′ n2 ′ 1 and P b ′ n ′ 2 1 , the last one only reproduces the magnetic structure at 200 K 25,26 . Only two irreducible representations are needed to distort the P bn2 1 parent space group into the P b ′ n ′ 2 1 Shubnikov space group.…”

“…, the last one only reproduces the magnetic structure at 200 K 25,26 . Only two irreducible representations are needed to distort the P bn2 1 parent space group into the P b ′ n ′ 2 1 Shubnikov space group.…”