Structural and chemical order impact magnetic properties of solids, which are governed by spin-orbit coupling and exchange interaction. The ordered L1₀ phase of FePt is a key material to heat-assisted magnetic recording; to enable high storage density, a solid understanding is needed of structural and chemical disorder at small length scales, as well as associated modifications of the electronic band structure. Here, we investigate the effect of boron and copper additions (≲6 mol% Cu) on structural and magnetic properties of L1₀ FePt granular media. Two copper-driven mechanisms, although competing, can lead to improvements in both structural and magnetic properties. In particular, the Cu substitution on the Fe-site leads to a degradation of magnetic properties due to the delocalized electron orbitals originating from a larger Cu d-orbital occupancy. At the same time, Cu substitution leads to an enhanced crystallographic order and consequently magneto-crystalline anisotropy, which offsets the former effect to a large extent. Our study is based on magnetometry, x-ray absorption spectroscopy, ab-initio calculations and a phenomenological theory of disordered FePt granular media. We do not observe a sizable modification to Fe moments and electronic configuration; Cu reveals two different resonances associated with the presence and absence of Cu-B bonds that vary with total Cu concentration.

We study the effect of carbon segregants on the spin and orbital moments of L10 FePt granular media using x-ray magnetic circular dichroism (XMCD) spectroscopy and report an effective decoupling of the structural film properties from the magnetic parameters of the grains. The carbon concentration reduces the grain size from (200 ± 160) nm2 down to (50 ± 20) nm2 for 40 mol. %C and improves sphericity and the order of grains, while preserving the crystalline order, spin and orbital moments, and perpendicular magnetocrystalline anisotropy. We identify the primary cause of enhanced saturation and coercive fields as the reduced demagnetization fields of individual grains. The ability to shrink grains without impairing their magnetic properties is a critical requirement for the commercialization of Heat-Assisted Magnetic Recording.

FePt granular films with grain size smaller than 10 nm are promising candidates for storage media used in the next generation heat-assisted magnetic recording technology. However, FePt films show degraded magnetic properties when the grain size is reduced to this scale, which cannot be explained solely by the finite size theory. In this study, we explored the structural cause of property degradation by employing advanced electron microscopy and atomistic modeling. Structural features unique to the nanostructured FePt granular films at significantly reduced grain sizes of 2∼8 nm were studied by high-resolution scanning transmission electron microscopy with geometric aberrations corrected up to the third order. Two critical structural parameters, the threshold grain size corresponding to the upper size limit of the FePt grains with zero chemical ordering and the sub-nanometer thin interfacial impurity at grain boundaries, were identified. A new atomistic model was developed to correlate these structural characteristics with key magnetic properties such as Curie temperature, saturation magnetization, magnetocrystalline anisotropy, and their grain-to-grain variation. The model shows good agreement with the experimental magnetic data and explains the gap in magnetic properties between the bulk and nanostructured FePt.