Uncorrelated magnetic domains in decoupled SrFe₁₂O₁₉/Co hard/soft bilayers

Abstract

Composites of magnetically hard and soft phases are present in multiple and diverse applications, ranging from bulk permanent magnets in motors and generators to state-of-the-art recording media devices. The nature of the magnetic coupling between the hard and soft phases is of great technological relevance, as the macroscopic properties of the functional composite material ultimately depend on the atomic-scale interactions between phases. In this work, the hard/soft bilayer system SrFe 12 O 19 /Co has been studied based on photoemission electron microscopy combined with x-ray absorption and magnetic circular dichroism. Our experiments show that the magnetization of the hard magnetic oxide has a direction perpendicular to the layer plane, whereas the magnetization of the soft metallic overlayer remains in-plane. As a consequence, the magnetic domain patterns observed for the hard and soft phases are very different and completely uncorrelated to one another, indicating that no soft spins align with the hard phase by pure magnetodipolar arguments. The results are understood as the consequence of an absence of exchange-coupling between phases, in a scenario in which the shape anisotropy of the soft layer overcomes the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite. Micromagnetic simulations of our system predict that low degrees of exchange-coupling effectively prevent substantial softening of the composite and lead to the alignment of soft and hard magnetic moments. A strategy thus emerges for the development of future hard-soft magnets, based on minimizing the degree of exchange-coupling while avoiding complete uncoupling.

Magnetic performance of SrFe12O19–Zn0.2Fe2.8O4 hybrid magnets prepared by spark plasma sintering

“…No observable difference in the H a of composites, when compared to the pure SFO, indicates that the exchange-coupling between the hard and the soft phase was not effective even though that particles were in close contact and that the size of the soft phase was within limits. This presumption is supported by the recently published paper by Soria et al [43] in which it was shown that for the effective coupling between the phases also a structural match must exist.…”

The Future of Permanent-Magnet-Based Electric Motors: How Will Rare Earths Affect Electrification?

“…So, to effectively exchange the hard and soft magnetic phases, the particle size of the soft phase should not exceed twice that of the domain-wall width of the hard-phase material [83,88]. Some recent studies even indicated that for an efficient exchange coupling, the phases also need to have some degree of structural matching [96,97]. Furthermore, to increase the (BH) max of the composite as much as possible, the particle size of the hard phase should be at the limit of the single-domain particle size (to obtain the maximum coercivity).…”

Permanent magnets based on hard ferrite ceramics

“…While recent studies have demonstrated new approaches to improve magnetic properties of hard hexaferrite powders (e.g., nanostructuring [16][17][18][19], chemical substitution [20][21][22][23], exchange spring composites [19,24,25]), manufacturing dense sintered pellets of sufficient structural integrity without degrading the optimized properties has proven a key challenge. In practice, this prevents the replacement of expensive and unsustainable REE PMs in a range of applications and is the reason why hard ferrites still generate great scientific interest [26].…”