Cascades of phase transitions in spiral magnets caused by dipolar forces

Abstract

We present a mean-field theory describing the influence of long-range dipolar forces on the temperature transition from the paramagnetic to ordered phases in frustrated Heisenberg spiral magnets. It is shown that the dipolar interaction produces a cascade of first- and second- order phase transitions between the paramagnetic and the spiral states upon temperature decreasing. Depending on system parameters, the following intermediate phases can arise: an incommensurate and a commensurate sinusoidally modulated states, spiral phases in which perpendicular spin components have different amplitudes and are modulated with the same and with different wave vectors. We distinguish six possible sequences of phase transitions upon temperature decreasing at least four of which were observed before experimentally in specific compounds. It is found that the action of dipolar forces cannot always be modeled even qualitatively by small one-ion anisotropic spin interactions. We demonstrate that the dipolar interaction is responsible for successive phase transitions in the triangular-lattice multiferroic MnI$_2$: almost all available experimental findings are described quantitatively within the mean-field theory by taking into account the exchange, the dipolar and small symmetry-allowed anisotropic spin interactions.Comment: 12 pages, 5 figure

Spiral plane flops in frustrated helimagnets in external magnetic field

“…Terms linear in Bose-operators H 1an arise in Hamiltonian (14) because we assume in derivation of Eqs. (12) and (13) that the spiral ordering is undisturbed (see Eq. (6)).…”

“…In particular, magneto-dipolar interaction was shown to be arXiv:1809.01680v1 [cond-mat.str-el] 5 Sep 2018 important for the description of transitions in many multiferroics of spin origin. 12 Then, we discuss in Sec. III the spiral plane flops in frustrated Heisenberg magnets with dipolar interaction.…”

Mean-field approach for skyrmion lattice in hexagonal frustrated antiferromagnets

“…Near the ordering temperature |s i | S, and one can expand the free energy of the mean-field approach in powers of the order parameters as follows (see, e.g., Refs. 24,28 for details):…”

“…Magneto-dipolar interaction, despite being small, can play significant role in helimagnets properties including temperature-and magnetic-field-induced phase transitions (see, e.g., Refs. [22][23][24][25][26][27][28][29][30] ). Noteworthy, for magnetic ions in spherically-symmetrical L = 0 state (as, e.g., Gd 3+ in Gd 2 PdSi 3 31 ) the dipolar forces are of particular importance, since the strength of other anisotropic interactions is governed by the spin-orbit coupling 32 .…”

Control of multiferroic order by magnetic field in frustrated helimagnet MnI2. Theory

“…By minimizing the mean-field free energy, we found in Ref. 7 a set of parameters of the model (1) which successfully describes the cascade of phase transitions in MnI 2 at h = 0. We found also that a lot of distinct set of parameters can be suggested to describe by model (1) the proper screw spiral ordering with q = q sp = (0.181, 0, 0.439) observed experimentally at small T .…”