Microstructure effects on the magnetic properties of nano-sized $L1_{0} CoPt$ were investigated by intercalating the carbon layer in the form of $CoPt/C_{n}$ (n=0, 1, 4) using RF and DC magnetron sputter deposition. The carbon was observed to dissolve into the ordered CoPt lattice and reduce the ordering kinetics, which results in the reduction of effective crystalline anisotropy constant. The growth of CoPt grains was drastically hindered by the carbon dissolution. The amount of dissolution increases with decreasing carbon single layer thickness due to the capillarity effect. It was found that the grain growth kinetics has a close relationship with the volume fraction of disorderd fcc phase. The grain size of annealed CoPt film was ranging from 30 to 55 nm but CoPt films with additive C showed about 10 nm at n=4 films.
CoPt-Ag films deposited at $T_{sub}$=650℃ showed a distinctive microstructure. It was observed that the island-like and discontinuous grains with (110) texture were formed. The (110) texture formation was due to the changes of surface energy and elastic energy upon in-situ ordering transformation. The (110) texture is attributed to the isotropic coercivity.
In order to investigate the grain-size distribution, Monte Carlo grain growth simulation and the Weibull function were adapted. It was found that the Weibull function is very useful to know the evolution of microstructure of thin film. When the grain growth occurs as like a 2-D growth system, the Weibull β gives a low value below 3.0 and at this stage the best-fit function of grain-size distribution is not the lognormal function. When the growth occurs as like film thickening, i.e., grains usually grow into film normal direction rather than into in-plane direction, the Weibull β is over 3.0 and the grain-size distribution is following the lognormal distribution.
Through temperature dependence (TDC) and angular dependence of coercivity (ADC) of CoPt films and micromagnetic simulations, we have tried to figure out the reversal mechanism of magnetization. TDC curves of nano-sized CoPt films seemed to indicate that the magnetization reversal occurs by the domain wall motion hindered by weak pinning sites. However, it was found that the magnetization reversal of CoPt film occurs by incoherent spin rotation rather than domain wall motion from the prediction of critical single domain size and the TDC results of micromagnetics including thermal fluctuation. The comparison of measured ADC with simulated ADC curves clearly shows that the reversal mechanism of nano-sized CoPt film is incoherent spin rotation. In addition, The ADC curve is found to be strongly dependent on the magnetic easy-axis distribution, 2-D, 3-D random and (111) texture.
Micromagnetic simulation of hysteresis loops of nano-sized $L1_{0}CoPt$ thin film with two-phase (magnetically hard(fct) and soft(fcc)) microstructure was performed. Comparing the shapes of simulated hysteresis with measured ones, it can be concluded that the ordered region of CoPt is not fully ordered. It is reasonable that the effective magnetocrystalline anisotropy constant is useful for micromagnetic simulation because the anisotropy constant is dependent on the annealing time and the degree of ordering parameter.
