Symmetry of secondary ferroics. I

Although at present there are only a few well documented examples, secondary ferroic phenomena may prove to be rather common, since the symmetry requirements are not especially stringent. In this paper, the conditions for ferrobielectricity, ferrobimagnetism, ferromagnetoelasticity and ferromagnetoelectricity are discussed. It is shown that ferrobielectricity can be expected in antiferroelectric ferroelastic classes with substantial dielectric anisotropy. Nickel oxide and other antiferromagnetic ferroelastics with anisotropic magnetic susceptibility show ferrobimagnetism. Evidence for ferromagnetoelastic domains is found in CoF₂ and FeCO₃, while Cr₂O₃ shows ferromagnetoelectric switching. Of the 90 magnetic point groups, 35 are potentially ferromagnetoelastic and 40 potentially ferromagnetoelectric. In a companion paper (Part II) to be published, symmetry restrictions for ferrobielasticity and ferroelastoelectricity will be considered, and the conditions which determine the orientation of domain walls in secondary ferroics examined.