斯格明子相关的螺旋磁有序体系的临界行为

Critical behaviors of helimagnetic ordering systems relating to skyrmion

介绍了与斯格明子相关的螺旋磁有序体系的临界行为.首先阐述了连续相变中的临界现象、临界指数、标度律、普适性等概念;随后介绍了磁相变体系中几种临界指数的获得方法,包括直流磁性迭代法、磁熵变法;进而,分析了几类与斯格明子相关的螺旋磁有序体系的临界行为.MnSi是典型的斯格明子材料,临界指数显示其磁性行为符合三重临界行为.MnSi的临界行为揭示:外磁场可以抑制这一体系在零场下的一级相变,使其转变为二级相变,从而在螺旋磁有序、锥形磁有序、顺磁相的三相交汇点形成三重临界点.斯格明子体系FeGe和Cu_2OSeO_3的临界行为符合三维海森伯相互作用,表明它们的磁性行为主要是由近邻的各向同性的自旋耦合作用所决定;而Fe_(1-x)Co_xSi和新发现的斯格明子体系Fe_(1.5-x)Co_xRh_(0.5)MoN的临界行为显示Co掺杂可以有效地调制其中的磁性耦合.对螺旋磁有序体系的临界行为研究表明,尽管这些体系都表现出类似的斯格明子态,但是它们的磁性耦合机制却大不相同,并且其耦合机制可以受到外界手段的调制.最后,根据普适性原理和标度方程,阐述了一种构建磁场诱导相变体系在临界温度附近H-T相图的方法.

Study of critical phenomena plays a key role in developing the theory of phase transition. In this article, we mainly review some new experimental results about the critical phenomena reported recently in the helimagentic ordering materials. These materials exhibit a kind of a vortex-like spin texture so-called skyrmion phase. The skyrmion phase has great potential applications in the new spin-based storage due to the topologically protected stability, nanometric size, and current-driven motion. Generally, the skyrmion state exists in a helimagentic system due to the DzyaloshinskiiMoriya（DM） interaction which forms in the crystal structure without inversion symmetry. It usually emerges just below the helimagentic phase transition temperature TCunder a certain temperature and magnetic field. In this review article,firstly, we introduce some basic concepts about the phase transition, such as critical phenomenon, critical exponents,scaling law, and universality. Secondly, we discuss two different methods which can help us to obtain the critical exponents, i.e., the iteration method based on the isothermal dc-magnetization and the fitting technique based on the magnetic entropy change. Both methods are extensively used in the current study of critical phenomena Thirdly, we analyze and outline some latest studies of critical behaviors and critical exponents for several typical helimagnetic systems with skyrmion state, such as MnSi, FeGe, Cu_2OSeO_3, Fe_（1-x）Co_xSi, and Fe_（1.5-x）Co_xRh_（0.5） Mo N. The B20 compound Mn Si is a typical skyrmion material, which undergoes a paramagnetic-to-helimagnetic phase transition at ～ 30.5 K and the skyrmion phase appears just below TCas an appropriate external magnetic field is applied. Investigations show that critical exponents of Mn Si belong in the universality class of a tricritical mean-field model, implying the existence of a long-rang magnetic interaction in this system. The critical behavior of Mn Si reveals that its first-order phase transition can be driven into a second-order phase transition by the action of external magnetic field, where a field-induced tricritical point is found among the helimagnetic, conical, and paramagnetic phases in Mn Si system. Unlike Mn Si, the critical exponent of the near-room-temperature skyrmion system Fe Ge, which undergoes a helimagentic phase transition at ～278 K, belong to the three-dimensional Heisenberg model. The critical behavior of Cu_2OSeO_3 is similar to that of Fe Ge, which indicates that the magnetic interactions in these two systems are dominated by the short-range nearestneighbor isotropic magnetic coupling. In addition, studies revealed that magnetic interaction and critical behavior of the skyrmion system can be effectively modulated by doping. The critical exponents of Fe_（1-x）Co_xSi and the newly founded skyrmion system of Fe_（1.5-x）Co_xRh_（0.5）MoN indicated that the doping concentration of Co can change and affect their critical behaviors. In addition, it was demonstrated that the doping of Co enhanced the anisotropic magnetic coupling in Fe_（1-x）Co_xSi while it suppressed that in Fe_（1.5-x）Co_xRh_（0.5）MoN. Fourthly, according to the universality and the scaling equations, we proposed a method to construct the detailed H-T phase diagram around the phase transition temperature in the system exhibiting field-induced phase transition. Finally, we make a brief summary and suggest our perspectives of the study of critical phenomena in helimagentic system. The results of critical behaviors indicate that although all these helimagentic systems exhibit a similar skyrmion phase, their essential magnetic interactions belong in different universality classes, indicating different types of magnetic coupling in these systems. Furthermore, the results also suggest that magnetic coupling can also be effectively tuned by the external modulation.