ABSTRACT
Contents 9.1 Introduction to Landau eory ................................................................ 257 9.2 Landau eory of Ferroic Materials ........................................................ 259
9.2.1 Classication of Ferroic Materials .............................................. 259 9.2.2 Landau eory of Ferroelectrics .................................................260 9.2.3 Landau eory of Magnets .......................................................... 263
9.3 Multiferroics ................................................................................................ 263 9.3.1 Magnetoelectric Coupling and Multiferroics ............................ 263 9.3.2 Landau eory of Type-I Multiferroics ......................................266 9.3.3 Landau eory of Type-II Multiferroics:
RMnO3 and Spiral Magnets ......................................................... 270 9.4 Conclusions and Outlook .......................................................................... 273 References ............................................................................................................... 275
phases. e total free energy in the vicinity of the transition can be expressed as a power series in the order parameter, and the stable state can be determined by minimizing the free energy with respect to the order parameter. Specic thermodynamic function/quantities can be obtained by dierentiating the free energy. e Landau approach establishes straightforwardly the relationship between crystal symmetry and physical properties [2]. In a word, the Landau theory is a symmetry-based analysis of equilibrium physical behaviors in the vicinity of phase transitions.
