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Abstract
BiMnO3 is a promising multiferroic material but it’s ferromagnetic TC is well below room temperature and the magnetic phase diagram is unknown. In this work, the relationship between magnetic transition temperature (TC) and the substrate induced (pseudo-) tetragonal distortion (ratio of out-of-plane to in-plane lattice parameters, c/a) in BiMnO3 thin films, lightly doped to optimize lattice dimensions, was determined. For c/a > 0.99, hidden antiferromagnetism was revealed and the magnetisation versus temperature curves showed a tail behaviour, whereas for c/a < 0.99 clear ferromagnetism was observed. A peak TC of up to 176 K, more than 70 K higher than for bulk BiMnO3, was achieved through precise strain tuning. The TC was maximised for strong tensile in-plane strain which produced weak octahedral rotations in the out-of-plane direction, an orthorhombic-like structure, and strong ferromagnetic coupling.
Highlights
BiMnO3 is a promising multiferroic material but it’s ferromagnetic TC is well below room temperature and the magnetic phase diagram is unknown
Recent theoretical and experimental studies have shown that bulk BMO has a centrosymmetric monoclinic structure (C2/c) which means the ground state of BMO is not ferroelectric (FE)[3]
Solovyev et al proposed that hidden antiferromagnetic (AFM) ordering is responsible for the ferroelectricity[4]. These studies have shown the potential of BMO to be a multiferroic, but the combination of strong FM and FE are difficult to achieve
Summary
BiMnO3 is a promising multiferroic material but it’s ferromagnetic TC is well below room temperature and the magnetic phase diagram is unknown. Solovyev et al proposed that hidden antiferromagnetic (AFM) ordering is responsible for the ferroelectricity[4] These studies have shown the potential of BMO to be a multiferroic, but the combination of strong FM and FE are difficult to achieve. In BMO, the interaction between the dz[2] and dx2−y2 orbitals happens along both in-plane and out-of-plane directions and leads to three-dimensional (3D) ferromagnetic interactions (see Fig. 1b). The 3D magnetic interactions in BMO are very different to LMO where ferromagnetic planes alternate antiferromagnetically along out-of-plane (A-type AFM). Strain in the structure (whether induced isostatic pressure applied to bulk samples or biaxial pressure applied in thin films by heteroepitaxial growth) can cause octahedral rotations that change the Mn-O-Mn bond lengths and angles. That when the FM properties are optimised, the FE properties are not necessarily optimised[1,11,13,15,16,17]
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