Manifestation of higher-order inter-granular exchange in magnetic recording media

Abstract

Exchange coupling between magnetic grains is essential for maintaining the stability of stored information in magnetic recording media. Using an atomistic spin model, we have investigated the coupling between neighbouring magnetic grains where magnetic impurity atoms have migrated into the non-magnetic grain boundary. We find that when the impurity density is low a biquadratic term in addition to the bilinear term is required to properly describe the intergranular exchange coupling. The temperature dependence both terms is found to follow a power law behaviour with the biquadratic exchange constant decaying faster than the bilinear. For increasing grain boundary thickness the intergranular exchange is lower and decays faster with temperature. Further simulations of a grain at a bit boundary show an unexpected energy minimum for in-plane magnetisation which can only be reproduced using a biquadratic exchange term.The coupling of magnetic grains in recording media is an important challenge for future high-density magnetic recording devices. To achieve suitably high areal densities the grain size and pitch distance are being pushed to a few nanometers 1,2 . The properties of magnetic recording media, including the dynamic behaviour during the recording process 3 and the long-term thermal stability 4 , are dependent on the the inter-granular interactions comprised of long-range magneto-static and short-ranged exchange coupling. However, the origin and nature of this short-ranged exchange coupling until recently was unexplored. As the design of recording media becomes more complex an understanding of the nature and magnitude of the exchange coupling and the development of realistic representation of the exchange for use in recording models becomes increasingly important.Initial experimental measurements of inter-granular exchange by Sokalski et al. 5 simplified the system to two thin films of magnetic media separated by a non-magnetic interlayer giving a well-characterized system to investigate the physics of the exchange coupling. From these and other experimental measurements 6,7 the inter-granular exchange is observed to decay exponentially with grain boundary thickness and linearly with temperature.A model for this interaction is proposed to be magnetic impurities present in the grain boundary which provides a channel for the grains to interact directly 8 . Simulations using the proposed impurity model by Evans et al. 8 on a multilayer system similar to Sokalski's showed the same exponential dependence on grain boundary thickness as observed experimentally and also extracted the temperature dependence of the inter-granular exchange parameter which is important for future heat assisted recording schemes.In addition to its origin, the particular angular form of the inter-granular exchange is known to have a large impact on the modelled bulk properties of granular media and on the recording performance. [9][10][11] The inter-granular exchange is typically written in the form of a Heisenberg exchange, often referred t...

Magnetic stray fields in nanoscale magnetic tunnel junctions

“…Defects present a particular challenge considering dipole fields, since the film morphology can influence the specific characteristics at the nanoscale. In particular orange-peel coupling effects can become important [33] and even percolated exchange coupling [34][35][36]. Future devices utilizing shape anisotropy to enhance thermal stability for sub-20 nm lateral dimensions [37][38][39] rely on a full understanding of dipole interactions at the nanoscale, and so similar atomistic calculation methods presented here can be used to model the role of different physical defects on the effective thermal stability and in particular their switching dynamics.…”

Models of Advance Recording Systems: A Multi-timescale Micromagnetic code for granular thin film magnetic recording systems

“…Ellis et.al. [11] found a similar relation using an atomistic model based on the presence of ferromagnetic impurities in the oxide layer. This study also showed the presence of higher order (biquadratic) exchange and importantly demonstrated that the intergranular exchange decayed to zero rapidly with increasing temperature, suggesting that intergranular exchange does not play a major role in the HAMR recording process.…”

“…Atomistic models provide this level of detail and have been used to provide temperature dependent magnetic properties and reversal mechanisms in recording media [5,6,7,8,9,10,11,12,13,14]. Although atomistic simulations can provide exceptional detail of the underlying physical processes which govern their macroscopic properties these simulations carry a significant computational cost.…”