Pyroxenes: a new class of multiferroics

Pyroxenes with the general formula AMSi(2)O(6) ( A = mono- or divalent metal, M = di- or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi2O6 becomes ferroelectric in a magnetically ordered state below similar to 6 K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi2O6 ( T-C similar to 18 K) and LiCrSi2O6 ( T-C similar to 11 K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants ( and their dependence on magnetic field) are performed using a natural crystal of aegirine ( NaFeSi2O6) and synthetic crystals of LiFeSi2O6 and LiCrSi2O6 grown from melt solution. For NaFeSi2O6 a temperature versus magnetic field phase diagram is proposed. Exchange constants are computed on the basis of ab initio band structure calculations. The possibility of a spiral magnetic structure caused by frustration to be the reason for the origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity.