Micro-mechanism Of FePt:Ag Nanocomposite Thin Films And α-Fe/Nd₂Fe₁₄B Nanocomposite Magnets

The noise reduction at recording areal densities approaching1Tb/in2requires the medium tohave decoupled magnetic grains that are in the range of4–5nm with a narrow size distribution.L10-ordered FePt:X (X is nonmagnetic material) nanocomposite thin films, in which FePtnanoparticles are embedded into non-magnetic matrix X，have attracted much attention since theyhave great potential as ultrahigh density magnetic recording media. FePt:X nanocomposite thinfilms have potential for this application because structural parameters, e.g., grain size, sizedistribution, texture, and grain isolation can be controlled simultaneously in these films. Althougha great progress has been made in synthesizing these nanocomposite thin films and controllingtheir microstructures, coating the FePt particles with very thin non-magnetic layers at high packingfractions remains a challenge.The co-sputtering approach is ideally suited for this application because the formation ofnon-magnetic layers surrounding FePt grains, which is dependent on the diffusion of non-magneticatoms out of FePt lattice, may be controlled well if a detailed understanding of kinetics and atomicprocesses involved with the L10ordering transition has been achieved. In the present study, theeffects of Ag atoms on the magnetic properties of FePt:Ag nanocomposite films have beendiscussed. Ag atoms diffuse out of FePt lattice and migrate into grain boundaries, and thusseparate FePt grains. The kinetic, activation volume and atomic processes of L10orderingtransition of FePt crystals in FePt:Ag nanocomposite thin films have been studied by employingtemperature-and pressure-dependence of the rate constant of L10phase transition. Based on thesemeasurements, a thermal-vacancy-assisted L10ordering transition in FePt:Ag nanocomposite thinfilms is revealed. The diffusion of Ag atoms out of FePt lattice prompts the nucleation of L10ordered domains in the thin films. The growth of L10ordered domains is predominantly dependenton the diffusion of Fe atoms in FePt crystals. These findings have a direct implication fordesigning nanocomposite thin films with FePt crystals coated with thin non-magnetic molecularlayers at high packing fractions, and thus are of wide interest for the study of ultrahigh densitymagnetic recording media.Nanocomposite permanent magnets, composed of soft and hard magnetic grains on a nanometer scale，have attracted considerable attention because a high maximum energy product above100MGOe would be expected in the magnets according to micromagnetic calculation. At present,however, the experimentally achieved maximum energy product (BH)maxis far less than the valueof theoretical calculations. This has been attributed to the difficulty in obtaining the optimummicrostructure used in theoretical models. Some studies indicate that nanocomposite magnets areunable to generate strong coercivity because of the existence of soft magnetic phase. Therefore, tounderstand thoroughly the coercivity generation mechanism is important for development ofhigh-performance nanocomposite magnets.α-Fe/Nd2Fe14B nanocomposite magnets with strong crystal texture, small and uniform grain sizewere prepared by controlled melt spinning. The change of the anisotropy of Nd2Fe14B grains withthe cooling rate is attributed to both the directional solidification in a thermal gradient and theseeding effect of the-Fe phase. The alignment of hard grains and the distribution of grain sizesare crucial for improvement of uniformity in magnetization reversal, which results in theimprovement of squareness in hysteresis loop. The coercivity of α-Fe/Nd2Fe14B nanocompositemagnets is mainly determined by inhomogeneous grain-boundary pinning. The size of grain ismeaningful to the strength and distribution of pinning field due to the influence of the interfacesamong nanocrystals.The aim of Nb addition is to form an intergranular amorphous phase and modify the interfacialstructure of α-Fe/Nd2Fe14B nanocomposites. The coercivity and exchange coupling betweengrains are enhanced simultaneously due to the existence of intergranular amorphous phase..In this study, the mechanism of coercivity enhancement in the α-Fe/Nd2Fe14B nanocompositeswith an intergranular amorphous phase has been studied. The homogeneity and strength ofdomain-wall pinning in the magnets are enhanced by the existence of intergranular amorphousphase. The composition, suitable exchange constant and thickness of amorphous interface arefavorable for simultaneously obtaining high coercivity and strong exchange coupling between hardand soft grains. The present work provides a way to achieve high maximum energy product innanocomposite magnets by the modification of interfacial structure.