Microstructure and magnetic properties of FePt and Fe/FePt ...

plain the gradual decrease of the coercivity. If grain size canbe kept small, higher coercivity may be achieved even in thethick films.The critical size of a single domain particle with a flatellipsoidal shape, d, is given by 22d 24AK u, 12NM swhere A is the exchange stiffness constant, K u is the uniaxialanisotropy constant, N is the demagnetization factor, and M sis the saturation magnetization. For L1 0 FePt, A110 6 erg/cm, 23 K u 710 7 erg/cc, 8 and M s1150 emu/cc. 8 In the 10 nm-thick film, N is about 7.81.Substituting the demagnetization factor for Eq. 1, the criticalparticles size of the single domain is estimated to beabout 200 nm. In the 10 and 20 nm thick films, the particlesize is smaller than the single domain size. Hence, the initialmagnetization curves observed in Figs. 4a and 4b are explainedby the rotation mechanism. In 60 and 100 nm thickfilms Figs. 4e and 4f, the morphology changes to a continuousstructure. The initial magnetization curve is characterizedas the nucleation type, although there are twins andgrain boundaries which can act as pinning sites for domainwall motion. This is because each of the grains can be easilymagnetized by the domain wall displacement from a multipledomain to a single domain state, because there are few pinningsites within the grains. Although the twins within thegrains can act as pinning sites, the number of twins alsodecreases with the increase of the film thickness.In the 30 and 38 nm thick films Figs. 4c and 4d, themagnetization increases rapidly at a low magnetic field andincreases slowly at a high magnetic field. From the TEMbright field images, the films consist of isolated polycrystallineparticles. The initial magnetization at the low magneticfield region is due to the nucleation-type magnetizationwithin the crystal grains. At this stage, the orientation of themagnetization within the grains will be aligned to the c axisof each grain by domain wall displacement within the grains.The second increase of the magnetization above 3 kOe isattributed to the rotation magnetization of randomly oriented