Evidence for first-order nature of the ferromagnetic transition in Ni, Fe, Co, and CoFe₂O₄

Nearly all ferromagnetic transitions have been considered to be continuous or second order, and the most typical examples are the ferromagnetic transitions in Ni, Fe, Co, and CoFe2 O4. However, by precise measurement with electrical resistivity or impedance and differential scanning calorimetry, we show clear evidence for the first-order nature of these "second-order transitions"-a small thermal hysteresis and latent heat. Such first-order signatures are found to be the same as those for the well-recognized first-order transitions in the ferroelectric BaTiO3 and ferroelastic Ti50 Ni47 Fe3. These results question the existence of genuine second-order transition in ferromagnetic systems. By a phenomenological approach, we further show that the first-order nature of ferromagnetic transition may stem from a coupling of magnetic moment to other order parameter(s) like strain. Such a coupling may provide insight into developing highly magnetoresponsive materials.