Panel discussion on “3d-magnetism at finite temperatures”: How to picture and calculate spin correlations in ferromagnetic metals at finite temperatures

Abstract

The peculiar magnetic structure of pure iron particles with diameters between 2.6 and 6.8 nm, as a function of temperature and time, was described in our previous publications. We concluded that iron particles with diameters below 6.8 nm are in a “core+surface layer” inhomogeneous magnetization state. The surface layer posesses a non-colinear spin structure, a pronounced “magnetic rigidity” (as compared with the core), and exchange parameters larger than those corresponding to the core region. A thermodynamic treatment of the core subjected to the built-in magnetic field provided by the surface layer, predicts two metastable states “α” and “ϵ”, corresponding to anti-parallel and parallel coupling between “core” and “surface”. The kinetics of the new phase nucleation within the parent phase of both magnetically isotropic and anisotropic particles is the main topic of the present work. We conclude that the growth process of uncorrelated elementary magnetization fluctuations, together with macroscopic thermal fluctuations which collapse according to a Néel-Arrhenius law, could justify the empirically determined Avrami-type spontaneous nucleation rates for spin systems. Moreover, the excitation of a few correlated elementary fluctuations involved in a domain-wall-like (DWL) screw motion, could account for field-induced orientational phase transitions within nanoscopic particles with mixed magnetic anisotropies. Such an approach allows us to derive the temperature dependence of Néels “reptation phenomenon” in metallic, particulate, magnetic recording media. Due to the itinerant nature of both conduction and magnetic electrons in iron, the possibility of a metal-insulator transition in the same size range where the particles are in a peculiar magnetic state, is discussed.