ИСТИНА

Magnetoresistance in sub-millimeter-size spin-valves was investigated. Spin-valve structures Si/Ta(22)/NiFe(12)/Cu(tCu)/NiFe(7)/IrMn(tAF)/Ta(22), with the lateral size of 200 x 500 micrometers and with the thicknesses of the layers given as a number (in nm) in brackets, tCu = 0,75 to 3 nm and tAF = 1 to 15 nm were prepared by DC magnetron sputtering with the magnetic field of 400 Oe applied during the depositions. The change in magnetoresistance was negligibly small for spacer thickness tCu smaller than 1.5 nm, saturated at tCu >2.0 nm and decayed at tCu >3.5 nm, vanishing at tCu >7.0 nm. Two types of asymmetry were observed in magnetoresistance-vs-applied field, R(Hext), curves in spin-valve structures Si/Ta(22)/NiFe(12)/Cu(tCu)/NiFe(7)/IrMn(tAF)/Ta(22), where thicknesses of the layers are given as a number (in nm) in brackets, tCu = 0,75 to 3 nm and tAF = 1 to 15 nm. The first type is a shape asymmetry when the ascent is smoother than the descent in the R(Hext). The second one is a difference in the height of the resistance peaks in the direct and reverse ramping of the magnetic field. We note that this effect is not a training effect, since it is reproducible in multiple field scans. The effect indicates that the upward and downward reversals of magnetisation in the pinned and free layers are different.