shape memory effect and superelasticity have been known to originate from a diffusionless structural transformation called mar

A Change in a Century-old Belief

- Ferromagnetic transitions are no longer continuous

Phase transition is the origin of many important phenomena like ferromagnetism, piezoelectricity, shape memory effect and superconductivity. Thermodynamically phase transitions are classified into continuous (2nd-order) and discontinuous (1st-order) ones. Although most phase transitions are found to be of 1st-order, for more than a century nearly all ferromagnetic transitions have been believed to be of 2nd-order. The typical examples include magnetic elements of Ni, Fe, Co and magnetic compounds like CoFe₂O₄. Such a 2nd-order ferromagnetic transition has been a foundation of our present-day understanding of ferromagnetism. However, this century-old belief is challenged by our highly precise measurement with electrical resistivity/impedance and calorimetry. We showed the evidence for the 1st-order nature of these typical “2nd-order” ferromagnetic transitions”- a small thermal hysteresis (see the right side of Figure 1) and latent heat. Such 1st-order signatures are found to be the same as those for the well-recognized 1st-order transitions in the ferroelectric and ferroelastic systems. By a phenomenological approach, we further showed that the 1st-order nature of ferromagnetic transition stems from an inevitable coupling of magnetic moment to other order parameter(s) like strain, which is consistent with our previous research (see the left side of Figure 1, Yang and Ren, PRB, 77, 014407). Our finding may cause a change in the foundation of the theories of ferromagnetism, as most of them are based on continuous ferromagnetic transition. Furthermore it may provide new insight into developing highly magneto-responsive materials.

See Sen Yang, Xiaobing Ren, and Xiaoping Song, PHYSICAL REVIEW B 78, (2008), for details. (PDF)

Figure 1 Lattice distortion and crystal symmetry for several typical pseudocubic ferromagnetic/ferroelectric materials (left side) and the corresponding evidence of thermal hysteresis for first-order transition (right side).