Renormalized Field Theory of Rhombohedral Antiferromagnets with Weak Spin Anisotropy

Abstract

Recently quenched magnetic mixtures with competing orthogonal spin anisotropies are under theoretical 1)~5) and experimenta16)~Il) investigations. Especially differences of the experimental results on Fel-XCoxCb· 2H206)~B) and anhydrated Fel_XCoxCb ) have yielded new problems. [1] According to the measurements by neutron scattering or the measurements of magnetic susceptibility and specific heat, both Fel-XCoxCb· 2H206),7) and Fel_XCoxCb ) exhibit considerably sharp PA(para-antiferro) transitions at A, Band C [Fig. l(a) and (b)]. To the contrary, A-OAF(antiferro-oblique antiferro) transition points A' and B' remain as sharp as P A transitions for Fel-XCoxCb·2H20, but get smeared for Fel-XCoxCb. [2] In Fel-XCoxCb·2H20, AB' crosses with BN at C, indifferently to each other, which means that C is a decoupled tetracritical point. On the other hand, CD line is not a smooth extention of BC line in Fel-XCoxCb ll ),*) so that C is considered to