120∘helical magnetic order in the distorted triangular antiferromagnetα-CaCr2O

Abstract

α-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr 3+ ions which have spin-3/2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below TN = 42.6 K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wavevector k = (0, ∼ 1/3, 0) and angles close to 120• between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the ac plane perpendicular to k. The magnetic structure is therefore that expected for an ideal triangular antiferromagnet where all nearest neighbor interactions are equal, in spite of the fact that α-CaCr2O4 is distorted with two inequivalent Cr 3+ ions and four different nearest neighbor interactions. By simulating the magnetic order as a function of these four interactions it is found that the special pattern of interactions in α-CaCr2O4 stabilizes 120• helical order for a large range of exchange interactions.

Multiferroicity in geometrically frustratedα-MCr2O4Zhao1, Lan2, Wang3 et al. 2012Phys. Rev. B10020View full textAdd to dashboardCite

“…In low temperature, the corresponding magnetic susceptibilities in H⊥a and H a have been measured (not shown). And both behave very similar as the bulk sample, which is consistent with the published data on the isostructural crystals of α-CaCr 2 O 4 by Toth et al [23] As shown in Fig. 6, in zero field, ǫ a (T) decreases gradually on cooling.…”

“…The antiferromagnetic humps in χ(T ) are clearly observed on both compounds, very similar to that of α-SrCr 2 O 4 . The neutron diffraction measurements on the powder and single crystal samples of α-CaCr 2 O 4 [22,23] have revealed a long-range incommensurate proper-screw-type helimagnetic ground state, which is almost identical to that observed in α-SrCr 2 O 4 . [24] An analogous helical spin picture in α-BaCr 2 O 4 was just proposed based on the recent neutron powder diffraction by Hardy et al [25] The high similarity of magnetic structures in the α-MCr 2 O 4 family accounts for their analogous magnetic properties, which comes from the basal structure of the quasi-2D triangular CrO 2 layers.…”

“…The fit to the high temperature χ(T ) (T changes from 300 to 1000K) in α-CaCr 2 O 4 has given a quite close value to the expected moment. [23] Therefore, for a better Curie-Weiss fitting, the measurement of χ(T ) well above room temperature is needed in our further work.…”

Interpretation of magnetoelectric phase states using the praphase concept and exchange symmetry

“…It possesses an orthorhombic crystal structure described by the space group P mmn (D 13 2h ), in which chromium ions form a distorted triangular lattice [62]. Below T N an incommensurate magnetic order appears with the wave vector k = (0, 0.3317, 0) and magnetic moments rotating in the ac plane [60,62].…”

“…Thus, two different IR's ∆ 1 and ∆ 2 condense at T N as a result of a weakly first order phase transition [60,61]. However, such transition is only possible if the coefficients at a ) and (0.4932, 1 4 , 0.0046) [62]. The permutational representation for both of them in the (0, 1 3 , 0) point of the Brillouin zone is given by ∆ 1 ⊕∆ 2 ⊕∆ 3 ⊕∆ 4 .…”