Surface and volume hysteresis of sputtered CoCr

Surface magnetic hysteresis, measured by Polar M.O. Kerr effect on RF and Magnetron sputtered CoCr (81/19 at%) films in the range 20 - 4000 nm is compared with volume hysteresis measured by VSM, both with external field perpendicular to the surface. The surface coercivity Hcswas found to decrease below the volume coercivity at a critical thickness of 125 nm for RF films [1] and at about 1100 and 800 nm for centre and off-centre Magnetron samples respectively. The slope of the Kerr loop at θk= 0 increased slightly again deviating from the VSM slope at a film thickness of about 100, 800 and 500 nm for RE, centre and off-centre Magnetron CoCr samples respectively. These results are consistent with the formation of small spikes from the surface for films thicker than a critical value. Order of magnitude calculations do not oppose the formation of such small reversed domains and the higher critical thickness for Magnetron samples is also in qualitative agreement with these calculations, due to a higher perpendicular anisotropy by a factor 2 for these films. If spike domains do exist in CoCr the reversal mechanism is most likely one in which the reversed domains grow at the expense of the main domains.