Multiferroics: Different ways to combine magnetism and ferroelectricity

Abstract

Multiferroics -materials which are simultaneously (ferro)magnetic and ferroelectric, and often also ferroelastic, attract now considerable attention, both because of the interesting physics involved and as they promise important practical applications. In this paper I give a survey of microscopic factors determining the coexistence of these properties, and discuss different possible routes to combine them in one material. In particular the role of the occupation of d-states in transition metal perovskites is discussed, possible role of spiral magnetic structures is stressed and the novel mechanism of ferroelectricity in magnetic systems due to combination of site-centred and bond-centred charge ordering is presented. Microscopic nature of multiferroic behaviour in several particular materials, including magnetite Fe 3 O 4 , is discussed.

Magnetic properties and magnetic structures ofTbBaMn2O5.75: Possible observation of unconventional polaron trimers

“…If S = 15/2 tetramers were formed by 4 Mn sharing 3 e g electrons then a theoretical moment of 21.07 μB would be obtained, which is much larger than the actual moment. These results suggest that the CM to CO phase transition does not involve the formation of 3 Mn 3+ per 1 Mn 4+ but instead involves the formation of (Mn 3.33+ ) 3 polarons, leaving some Mn 3+ . The CM to CO transition could be described as a switch from the e g electrons on the Mn being completely delocalized to pairs of them becoming localized on trimers of Mn ions and single e g electrons localizing on Mn 3+ .…”

“…These measurements both indicate that the nature of the charge ordering is different than originally proposed. The evidence indicates that the charge ordering likely consists of ferromagnetically coupled (Mn 3.33+ ) 3 trimer polarons and Mn 3+ in a 1:1 ratio.…”

“…While this is an improvement it is still somewhat less than the observed moment. If instead it is assumed that 3 Mn ions share 2 e g electrons to form (Mn 3.33+ ) 3 trimers then an even larger moment would be obtained. The theoretical moment for 2 Tb 3+ , 1 Mn 3+ and 1 S= 11/2 trimer is 18.87 μB.…”

“…Multiferroic materials are uncommon since the type of cations which tend to undergo the offcenter displacements which produce ferroelectricity (highly charge d 0 transition metals such as Ti 4+ and Nb 5+ or heavy lone pair cations such as Pb 2+ and Bi 3+ ) lack the unpaired electrons necessary for magnetism. [1][2][3][4][5] Multiferroic materials which have a strong coupling between the electronic and magnetic order parameters are even less common. Many of the known multiferroics use two different cations, one which produces the ferroelectricity and the other which produces the magnetism, but in these cases there is no mechanism to couple these orders.…”

“…11 The symmetry of the crystal structure of this compound, which is non-centrosymmetric (P11m), and its magnetic properties seemed to indicate multiferroic behavior. Subsequent studies have experimentally evidenced that charge-ordered rare earth manganites such as Nd 0.5 Ca 0.5 MnO 3 , Pr 0.7 Ca 0.3 MnO 3 and Gd 0.5 Sr 0.5 MnO 3 12, 13 possess multiferroic and magnetoelectric properties. These results clearly point out that charge-ordering provides a novel route to multiferroic properties especially in the case of the manganites.…”