Article:Beevers, J. E., Love, C. J., Lazarov, V. K. orcid.org/0000-0002-4314-6865 et al. The magnetoelectric effect in M-type Ti-Co doped strontium hexaferrite has been studied using a combination of magnetometry and element specific soft X-ray spectroscopies. A large increase (> x30) in the magnetoelectric coefficient is found when Co 2+ enters the trigonal bi-pyramidal site. The 5-fold trigonal bi-pyramidal site has been shown to provide an unusual mechanism for electric polarization based on the displacement of magnetic transition metal (TM) ions. For Co entering this site, an offcentre displacement of the cation may induce a large local electric dipole as well as providing an increased magnetostriction enhancing the magnetoelectric effect. INTRODUCTIONThe control of magnetism using applied electric fields offer the possibility of a new generation of ultralow power, high density storage. In this respect, magnetoelectric (ME) multiferroic materials are intensely studied in order to understand how different symmetry breaking orders exist in the same material and how these orders can be coupled. Among the few room temperature single-phase ME multiferroics reported, hexaferrites show potential for device applications as they exhibit a low field ME effect at room temperature [1]. M-type hexaferrites are arranged in different repeating sequences of basic building blocks; the R and S layers [2]. Fe 3+ cations occupy both octahedral (Oh) and tetrahedral (Td) co-ordinated sites in the S block (Wyckoff positions 2a and 4f1) and octahedral sites (12k and 4f2) in the R block, see online supplementary materials. The Ba ion located at positions 2d strongly distorts the octahedral site located at the 2b positions giving rise to a bi-pyramidal 5 fold co-ordination which induces a large uniaxial magnetic anisotropy parallel to the c-axis [3]. However, Co 2+ and Ti 4+ substitutions for Fe 3+ dramatically alters the magnetic properties [4]. Ti substitutions at the 12k sites decrease the exchange coupling between spins in the R and S blocks whilst Co substitutions change the magnetic anisotropy from uniaxial to an easy cone of magnetization tilted away from the c-axis. The result is to stabilize a non-collinear conical magnetic structure [2,5] which is of high interest in the field of multiferroics [6]. The ME effect at room temperature in SrFe8Ti2Co2O19 was first reported in bulk [7] and, thereafter, in thin films [8]. Co 2+ substitutions in ferrite structures are a well-known source of magnetoelastic coupling due to the large orbital moment of Co 2+ [9]. The linear ME coupling, , which is the change in magnetization with an applied electric field is directly proportional to the magnetoelastic coupling, or magnetostriction, , implying that an increase in  increases  [10]. However, a mechanism such as the piezoelectric effect or electrostriction for coupling the applied electric field into strain is also required. Several mechanisms for magnetically induced ferroelectricity have been proposed in recent years [11] with the sp...

The size of the orbital moment in Fe3O4 has provided a long standing and contentious debate. In this paper we make use of ferromagnetic resonance (FMR) spectroscopy and x-ray magnetic circular dichroism (XMCD) to provide complementary determinations of the size of the orbital moment in "bulk-like" epitaxial Fe3O4 films grown on Yttria-stabilized zirconia (111) substrates. Annealing the 100 nm as-grown films to 1100 C in a reducing atmosphere improves the stoichiometry and microstructure of the films allowing for bulk like properties to be recovered as evidenced by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). In addition, in-plane angular FMR spectra exhibit a cross over from a 4-fold symmetry to the expected 6-fold symmetry of the (111) surface, together with an anomalous peak in the FMR linewidth at ~10 GHz; indicative of low Gilbert damping in combination with two-magnon scattering. For the bulk-like annealed sample, a spectroscopic splitting factor g ≈ 2.18 is obtained using both FMR and XMCD techniques, providing evidence for the presence of a finite orbital moment in Fe3O4.