COMPETITION BETWEEN COMMENSURATE AND INCOMMENSURATE PHASES IN RARE-EARTH SYSTEMS - EFFECTS ON H-T MAGNETIC PHASE-DIAGRAMS

Some features common to many rare-earth intermetallic compounds with complex magnetic phase diagrams are explained using a realistic mean-field model, which takes into account the periodic-exchange-field and crystal-field effects which favor an easy magnetization axis, i.e.: (i) the change of an incommensurate or long-period commensurate structure near T(N) toward a simple commensurate one at low temperature, with and without intermediate structures of intermediate propagation vectors; (ii) the low-temperature metamagnetic process during which the high-temperature propagation vector(s) tend to be recovered; (iii) the trend for the propagation vectors to lock onto commensurate values, except near T(N). Exchange interactions alone are enough to account for these properties, the exact boundaries of the magnetic phase diagram being determined by the real variation of J(q) and crystal-field effects.