Magnetoelectric Effect in a Spin-State Transition System

Abstract

Magnetic, dielectric, and magnetoelectric properties in a spin-state transition system are examined, motivated by the recent discovery of a multiferroic behavior in a cobalt oxide. We construct an effective model Hamiltonian based on the two-orbital Hubbard model, in which the spin-state degrees of freedom in magnetic ions couple with ferroelectric-type lattice distortions. A phase transition occurs from the high-temperature low-spin phase to the low-temperature high-spin ferroelectric phase with accompanying an increase of the spin entropy. The calculated results are consistent with the experimental pressure-temperature phase diagram. We predict the magnetic-field induced electric polarization in the low-spin paraelectric phase near the ferroelectric phase boundary.Multiferroics are the coexistence phenomena of the ferromagnetic, ferroelectric, and ferroelastic orders. 1) The crosscorrelation effects between the magnetism and dielectrics are termed the magnetoelectric (ME) effect. 2-7) These issues have attracted recently much attention not only from the fundamental condensed matter physics, but also the wide potentiality for the technological applications. One of the predominant target materials of the multiferroics and the ME effect is the Mott insulating systems, in which the electronic charge degree of freedom is quenched and the spin degree of freedom dominates the low energy physics. The well studied examples are the systems showing non-collinear and non-coplanar spin orders owing to the competing exchange interactions. 8,9) The spontaneous electric polarizations are induced by the socalled inverse Dzyaloshinsky-Moriya interaction mechanism, and are controlled by an external magnetic field. 10-12) Now, the candidates of the multiferroics and ME effect are surveyed extensively in a wide variety of materials. [13][14][15][16][17] In some classes of the magnetic ions, multiple spin amplitudes (S ) are realized under different external conditions. Cooperative changes in the magnetic states induced by the interacting spin-state degree of freedom are termed the "spincrossover" or "spin-state transition" phenomena, which are often seen in correlated electron materials, [18][19][20] earth-inner mantels, 21-23) biomolecules, 24) and so on. These phenomena originate from competition between the crystalline electricfield (CEF) effect and the Hund coupling; the low (high)-spin state with small (large) S is stabilized, when CEF is larger (smaller) than the Hund coupling.One of the well-known materials in which the spincrossover phenomena are realized is the cobalt oxides with the perovskite structure, R 1−x A x CoO 3 (R: trivalent ion, A: divalent ion). 25) The nominal valence of the Co ion at x = 0 is 3+ and the number of the 3d electrons is six. There are three possible spin states, the low-spin (LS) state with S = 0 and the configuration of (t 2g ) 6 (e g ) 0 , the intermediate-spin state with S = 1 and (t 2g ) 5 (e g ) 1 , and the high-spin (HS) state with S = 2 and (t 2g ) 4 (e g ) 2 . When the LS (HS) states are ...