Magnetic‐Instability‐Induced Giant Magnetoelectric Coupling

Abstract

Understanding the coupling between magnetism and ferroelectricity is of high fundamental and technological importance. [1][2][3][4][5] For example, a sufficiently strong coupling between magnetic and dielectric properties may enable switching of the magnetization by means of an electric field, thereby drastically reducing the thermal power needed for a magnetic memory. If electric writing and magnetic reading were to be used, one of the layers in spin valves composed of ferromagnetic and magnetoelectric layers would no longer have to be pinned. [6] Giant magnetoelectric materials facilitate highly efficient elastic interactions and a strong magnetic response to an electric field (or vice versa) via magnetostriction and piezoelectric effects. [7,8] Several materials possess both ferroelectricity and a co-operative magnetism; however, the coupling between these properties is not necessarily large. The structural and electronic circumstances that would favor a large coupling remain unexplored, partly due to the lack of materials with a large coupling. Here we report that such a large coupling occurs in BiCoO 3 , for which an external electric field can induce a strong magnetic response by changing the spin-state of cobalt from a magnetic high spin (HS) state to a nonmagnetic low spin (LS) state. The simultaneous presence of strong electron-electron interaction within the transition metal d manifold and a sizable hopping interaction strength between the transition metal d and oxygen p states are primarily responsible for a wide range of properties exhibited by perovskite-like oxides. Several ferroelectric perovskites undergo a phase transition from a high-temperature, high-symmetry phase that behaves as an ordinary dielectric to a spontaneously polarized phase at low temperature. Considerable research activities are focused on understanding the nature of and the driving force for such ferroelectric transitions. The electric and magnetic properties of cobaltites depend on the spin state of the Co ions, that is, whether they are in LS, intermediate spin (IS), or HS states (simplified pure ionic limit). For example, LaCoO 3 is a diamagnetic insulator at low temperature and transforms to the paramagnetic state at 90 K due to an LS-IS transition. [9,10] Metamagnetism from spin-state transitions may give rise to exciting phenomena such as giant magnetoresistance, [11] giant magnetocaloric effect, [12] shape memory effect, [13] etc. We show presently that metamagnetism can also lead to giant magnetoelectric coupling. Generally, spin-state transitions are induced by hole/electron doping, temperature, magnetic field, pressure, and/or lattice strain. For the first time we here show that spin-state transitions can also be induced by an electric field in the case of magnetoelectric materials that display magnetic instabilities. Materials with pure ionic bonding typically possess centrosymmetric structures owing to the minimizing of short-range Coulomb repulsions and they are hence nonferroelectric. The noncentrosymmetry required...

Ab initiostudy of the giant ferroelectric distortion and pressure-induced spin-state transition in BiCoO3

“…Plain GGA [or local-spin-density approximation (LSDA)] calculations 5,6,11,13 seem to qualitatively re- produce the C-type AF and insulating ground state of BiCoO 3 in the ambient phase, which was ascribed to the strong Hund-exchange stabilized HS state of the Co 3+ ions (and thus the AF order and narrow bands) and to the well split-off xy-singlet orbital. However, the band gap of 0.6 eV and the Co 3+ spin moment of 2.4 µ B given by the GGA/LSDA calculations are much smaller than the experimental values of 1.7 eV and 3.2 µ B .…”

“…6 We note that the prediction may simply be an artifact of the fixed-spin-moment calculations, as (1) LSDA or GGA (it was mentioned as a density-functional method in Ref 6 ) is not well suitable to describe this Mott insulator; (2) most probably those calculations were carried out in a wrong ferromagnetic metallic state; and (3) their metallic solutions blurred the distinction between the different spin and orbital multiplets of the concern and thus suppressed significantly their level splittings and particularly the HS/LS splitting: the fixed-spin-moment calculations showed that for the ambient structure, the energy preference of the HS state over the LS state is less than 0.15 eV/f.u. (see Fig.…”

“…BiCoO 3 was recently synthesized by a high-pressure (HP) technique, 4 and it has been suggested to be a promising multiferroic material by Uratani et al 5 and by Ravindran et al 6 both through first-principles Berry-phase calculations. BiCoO 3 has a giant tetragonal lattice distortion of c/a = 1.27 with remarkable off-center atomic displacements (see the inset of Fig.…”

First-Principles Study on the Magnetic Anisotropy in Multiferroic PbVO3 and BiCoO3

“…Our results have been also confirmed by the other first-principles studies. [25][26][27] The basic framework of the observed crystal structure is of perovskite-type, as the same as that in the typical ferroelectric oxide PbTiO 3 . However, the crystal structure shows quite large tetragonality (c=a ratio) of 1.229 for PbVO 3 and 1.267 for BiCoO 3 , compared with 1.06 for PbTiO 3 .…”

“…Since the details of electronic structure of PbVO 3 and BiCoO 3 including the mechanism of multiferroicity have been reported previously, [24][25][26][27] we briefly summarize their important aspects for the discussion of magnetic anisotropy in this section. The band structure reveals an insulating (or semiconducting) behavior with rather small energy gaps.…”