Magnetic Phase Diagram and Chiral Soliton Phase of Chiral Antiferromagnet [NH4][Mn(HCOO)3].

We synthesize herein millimeter-sized single crystals of the molecule-based chiral antiferromagnet, [NH₄][Mn(HCOO)₃]. The crystal is optically transparent, and has a hexagonal crystal structure that belongs to the space group P6₃22. We study the magnetic phase diagram (i.e., H–T phase diagram) under the magnetic field perpendicular to the helical axis by measuring the dc magnetic susceptibility, magnetization curve (M–H curve), M–T curve, ac magnetic response, and heat capacity. We find a continuous magnetic phase transition at T^∗≃8 K, which weakly depends on magnetic fields up to 5 × 10⁴ Oe. The saturation field for T = 0 K is estimated as H_st(T = 0) = 1.41 × 10⁵ Oe from the extrapolation of the M–H curve. Below T*, we find two magnetic phases separated by the critical field H_c(T) of the order of hundreds Oe. A sharp peak in the out-of-phase component of the ac linear response as a function of T and H is suggestive of the discontinuous transition at H = H_c(T). We find a downward convexity in the M–H curve (i.e., d²M/dH² > 0) in H < H_c(T) and a nonzero third-order harmonic ac response around H≃H_c(T). On these observations, we argue that the antiferromagnetic chiral solitons are condensed and form lattice or liquid in the magnetic phase at H < H_c(T) while the magnetic phase at H_c(T) < H < H_st(T) is a canted antiferromagnetic phase. We compare the experimental results with the mean-field theory on a spin model for the monoaxial chiral antiferromagnet and find a qualitative agreement on the M–H curve, M–T curve, and phase diagram. Our calculation implies that the discontinuous transition between the two magnetic phases is caused by the condensation of the chiral solitons with an attractive interaction. The period of the antiferromagnetic helical structure at the zero field is estimated to be the order of H_st(T)/H_c(T) × [lattice constant], which amounts to a submicron scale. [ABSTRACT FROM AUTHOR]