Phase transitions in chiral magnets from Monte Carlo simulations

摘要

Motivated by the unusual temperature dependence of the specific heat in MnSi,comprising a combination of a sharp first-order feature accompanied by a broadhump, we study the extended Heisenberg model with competing exchange $J$ andanisotropic Dzyaloshinskii-Moriya $D$ interactions in a broad range of ratio$D/J$. Utilizing classical Monte Carlo simulations we find an evolution of thetemperature dependence of the specific heat and magnetic susceptibility withvariation of $D/J$. Combined with an analysis of the Bragg intensity patterns,we clearly demonstrate that the observed puzzling hump in the specific heat ofMnSi originates from smearing out of the virtual ferromagnetic second orderphase transition by helical fluctuations, which manifest themselves in thetransient multiple spiral state. These fluctuations finally condense into thehelical ordered phase via a first order phase transition as is indicated by thespecific heat peak. Thus the model demonstrates a crossover from a second-orderto a first-order transition with increasing $D/J$. Upon further increasing$D/J$ another crossover from a first-order to a second-order transition takesplace in the system. Moreover, the results of the calculations clearly indicatethat these competing interactions are the primary factor responsible for theappearance of first order phase transitions in helical magnets with theDzyaloshinskii-Moriya (DM) interaction.