Atomistic Spin Simulations of Heat Assisted Magnetic Recording Media

The continuous developing of magnetic recording requires to understand the physical properties of the magnetic media in detail in order to maximise the performance of this application. In this thesis, we investigated different paradigms of magnetic recording media by using
atomistic spin dynamics model.
The inter-granular exchange is important for maintaining the stability of stored information in magnetic recording media. Therefore, we investigated the exchange coupling between neighbouring magnetic grains where magnetic impurity atoms are assumed to migrate into the non-magnetic grain boundary. Tri-layer and multi-grain system have been proposed to be studied where we found that a lower concentration of magnetic impurity reduces the inter-granular exchange coupling, respectively the exchange energy between the grains. Different angular dependence of the exchange energy is found for the multi-grain system
compared to the tri-layer system, where an additional term of the exchange energy needs to be considered in order to describe the angular dependence. This additional term is called biquadratic term. The temperature dependence of both terms is found to follow a power law behaviour with the biquadratic exchange constant decaying faster than the bilinear. For increasing grain boundary space the intergranular exchange reduces and also decays more
quickly with temperature.
Another problem of magnetic recording particularly to the heat-assisted magnetic recording is given by the design of the recording grains. Exchange-coupled composite media is found to give optimal performance due to low energy barrier at elevated temperature demonstrated energetically using Monte-Carlo simulations. Dynamic simulations show an
acceleration of the switching due to spring effect being determined by several factors. One factor is the Gilbert damping which plays a significant role in magnetic reversal processes and determines the timescale of the switching. For a bilayer Fe/FePt medium we found an anomalous increase of the switching time with increasing soft layer damping constant. The reversal occurs via a high-temperature exchange spring, this phenomenon being delicately balanced in that the switching time increase occurs only in fields close to the coercivity.
Lastly, we investigated a new model of exchange interaction in FePt L1 0 following Ruderman-Kittel-Kasuya-Yosida function. An obvious similarity can be observed between
first principle calculations of the exchange constant as a function of the distance between neighbours atoms and the function proposed by Ruderman-Kittel-Kasuya-Yosida which
allows us to reproduce the magnetic properties of L1 0 FePt using the last mentioned function as first principle calculations require extremely long computational time.