Abstract
We have investigated the temperature evolution of the magnetic structures of HoFeO3 by single crystal neutron diffraction. The three different magnetic structures werevfound as a function of temperature for HoFeO3. In all three phases the fundamental coupling between the Fe sub-lattices remains the same and only their orientation and the degree of canting away from the ideal axial direction varies. The magnetic polarisation of the Ho sub-lattices in these two higher temperature regions, in which the major components of the Fe moment lie along x and y, is very small. The canting of the moments from the axial directions is attributed to the antisymmetric interactions allowed by the crystal symmetry. In the low temperature phase two further structural transitions are apparent in which the spontaneous magnetisation changes sign with respect to the underlying antiferromagnetic configuration. In this temperature range the antisymmetric exchange energy varies rapidly as the the Ho sub-lattices begin to order. So long as the ordered Ho moments are small the antisymmetric exchange is due only to Fe-Fe interactions, but as the degree of Ho order increases the Fe-Ho interactions take over whilst at the lowest temperatures, when the Ho moments approach saturation the Ho-Ho interactions dominate. The reversals of the spontaneous magnetisation found in this study suggest that in HoFeO3 the sums of the Fe-Fe and Ho-Ho antisymmetric interactions have the same sign as one another, but that of the Ho-Fe terms is opposite.
Highlights
Rare-earth orthoferrites RFeO3 where R is a rare-earth element constitute an important family of magnetic compounds intensively studied over several decades starting from the early forties of the last century
Due to the strong Fe-Fe exchange interaction the Fe subsystem usually orders with a Neel temperature TN ∼ 620 − 740 K as a slightly canted antiferromagnetic structure with weak ferromagnetism
The interactions leading to the weak ferromagnetism are small they still play an important role in determining the magnetic properties and spin reorientation transitions at lower temperatures
Summary
Spin reorientation transitions which have been studied in both bulk materials and thin films.[1,2,3,9,10,11,12]. HoFeO3 is very similar in both structure and magnetic properties to the aforementioned three compounds, so one may conjecture that it will show similar multiferroic behaviour It is for this reason that we have undertaken a more detailed study of the temperature variation of its magnetic structure. The distorted perovskite crystal structure of HoFeO317 is described using space group Pbnm, its magnetic structures were determined in early neutron diffraction measurements on powder samples.[18,19] It was found that the Fe sub-lattices order anti-ferromagnetically in the G-type configuration[20] with a Neel temperature of 647 K. In the present investigation we have studied the temperature evolution of the magnetic structure of HoFeO3 using single crystals in zero field.
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