Interfacial exchange coupling is known to improve the permanent magnetic performance (i.e., maximal energy product) in composites of magnetically hard and soft particles. The prevailing strategy, employed in a plethora of compositions, consists in maximizing the coupling between the hard and soft phases and optimizing material parameters such as particle size or phase composition. In CoFe2O4–FeCo nanocomposites, it is experimentally shown that interparticle uncoupling in combination with the sizes of the soft phase grains below the single‐domain threshold leads to enhanced magnetic properties at room temperature, while maximizing exchange coupling implies a collapse in coercivity and hence in the maximal energy product. The results are corroborated by micromagnetic calculations and the origin of the exchange‐induced softening is discussed. It is emphasized that engineering interfaces in order to optimize, rather than maximize, the degree of exchange coupling are a necessary requirement to improve the energy product in nanocomposite magnets and to successfully develop advanced rare‐earth‐free permanent magnets.

A vectorial magneto-optic Kerr effect (v-MOKE) setup with simultaneous and quantitative determination of the two in-plane magnetization components is described. The setup provides both polarization rotations and reflectivity changes at the same time for a given sample orientation with respect to a variable external magnetic field, as well as allowing full angular studies. A classical description based on the Jones formalism is used to calculate the setup's properties. The use of different incoming light polarizations and/or MOKE geometries, as well as the errors due to misalignment and solutions are discussed. To illustrate the capabilities of the setup a detailed study of a model four-fold anisotropy system is presented. Among others, the setup allows to study the angular dependence of the hysteresis phenomena, remanences, critical fields, and magnetization reversal processes, as well as the accurate determination of the easy and hard magnetization directions, domain wall orientations, and magnetic anisotropies.

The Stoner-Wohlfarth (SW) model is the simplest model that describes adequately the magnetization reversal of nanoscale systems that are small enough to contain single magnetic domains. However for larger sizes where multi-domain effects are present, e.g., in thin films, this simple macrospin approximation fails and the experimental critical curve, referred as SW astroid, is far from its predictions. Here we show that this discrepancy could vanish also in extended system. We present a detailed angular-dependent study of magnetization reversal dynamics of a thin film with well-defined uniaxial magnetic anisotropy, performed over 9 decades of applied field sweep rate (dH/dt). The angular-dependent properties display a gradual transition from domain wall pinning and motion-like behaviour to a nucleative single-particle one, as dH/dt increases. Remarkably, in the high dynamic regime, where nucleation of reversed domains is the dominant mechanism of the magnetization reversal (nucleative regime), the magnetic properties including the astroid become closer to the ones predicted by SW model. The results also show why the SW model can successfully describe other extended systems that present nucleative regime, even in quasi-static conditions.