Enhanced magnetoelectric effect in M-type hexaferrites by Co substitution into trigonal bi-pyramidal sites

Abstract

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...

Broadband transverse susceptibility in multiferroic Y-type hexaferrite Ba0.5Sr1.5Co2Fe2O22

“…TS measurements have been carried out on Y type hexaferrites with composition Ba 0.5 Sr 1.5 Co 2 Fe 2 O 22 , (BSCFO) optimal to exhibit multiferroic properties [14,15]. In addition, Co substitution can enhance the magnetoelectronic effect in hexaferrites [16]. It is expected that the different spin configurations modify the magnetic properties detected with TS measurement, so that it can be a versatile tool to establish the different temperature regions in which the magnetoelectric effect occurs.…”

Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

“…Thus, we have used the same 10Dq parameter (=1.1 eV) for all octahedral sites (2a, 4 f 2 , 12k). In addition, we have modeled the tetrahedral sites(4 f 1 ) with 10Dq = −0.5 eV, and the bipyramidal one(2b) with 10Dq = 0.8 eV, D τ = −0.08 eV and D σ = 0.01 eV using literature values 22 . The Slater parameters were reduced to 72% of the Hartree Fock values following ref.…”

“…The Slater parameters were reduced to 72% of the Hartree Fock values following ref. 22 . The Gaussian broadening value used for each peak in the simulated spectrum was 0.09 eV, in order to match the experimental resolution.…”

“…To the best of our knowledge this is the first time that the x-ray absorption spectra at the Fe L 2,3 edges of this material in its pure form have been reported. The experimental results have been complemented with multiplet calculations 21 aimed at reproducing the observed XAS and XMCD spectra at the Fe L 2,3 absorption edge 22,23 , and by density functional theory (DFT) calculations to reproduce the oxygen K-absorption edge as well as to estimate the iron magnetics moments. Finally the domain pattern measured in remanence is compared with micromagnetic simulations.…”

Study of the Structural and Magnetic Properties of Co-Substituted Ba2Mg2Fe12O22 Hexaferrites Synthesized by Sonochemical Co-Precipitation

“…Thus, it was reported that the Co 2+ substitution in M-type and W-type hexaferrites raises the magnetic saturation and the coercivity [20,21]. In addition, the Co substitution can enhance the magnetoelectric effect in hexaferrites, as established by Beevers et al [20].…”