Lawrence Berkeley National Laboratory
Growth, structural and magnetic properties of high coercivity Co/Pt multilayers
- Author(s): Thomas, G.
- et al.
Co/Pt multilayer films ([Co (tCo nm)/Pt(1 nm)]10, 0.2&60;tCo&60;2 nm) with perpendicular magnetic anisotropy and room temperature coercivity Hc = 12-15 kOe are fabricated using electron beam evaporation at elevated growth temperatures onto Si(001)/SiNx(40nm)/Pt(20nm) substrate/seed structures. Hysteresis and magnetic force microscopy (MFM) studies indicate changes in the magnetization reversal mechanism along with a sharp increase in coercivity for growth temperatures higher TG ~ 230 - 250 degrees C. Films grown at TG &60; 230 degrees C (tCo = 0.2 - 0.4 nm) show large micrometer size magnetic domains and rectangular hysteresis loops indicating magnetization reversal dominated by rapid domain wall motion throughout the film following nucleation at Hn ~ Hc. Films grown at TG &62; 250 degrees C show fine-grained MFM features on the sub-100 nm length scale and hysteresis studies indicate reversal dominated by localized switching of small clusters. The hysteresis curves for the highest coercivity films are sheared with with a hysteresis slope alpha=4(pi)dM/dH|Hc approximately equal 1.5, which is close to the ideal value for completely decoupled grains of alpha =1. High resolution cross-sectional TEM with elemental analysis shows columnar grains extending throughout the multilayer stack. Sharp Co/Pt interfaces are found from TEM and grazing incidence X-ray diffraction. At higher TG, Co depletion and structural defects at the grain boundaries provide a mechanism for exchange decoupling of adjacent grains, which may result in the high coercivities observed. Anisotropy and magnetization values are estimated as Ku ~ 8x106 erg/cc and MS ~ 450 emu/cc (per total volume), hence Hk = 2Ku/MS ~ 17.5 kOe for the highest coercivity Hc ~ 15 kOe films.