Magnetoresistive Co/Cu multilayers: Hysteresis, polycrystallinity, and irreversible changes on magnetization

Magnetoresistive metal multilayers are known to undergo an irreversible decrease in the maximum value of their field-dependent resistance after exposure to a magnetic field. An explanation for this effect in terms of the creation of antiphase domain boundaries is considered and rejected on the basis of experimental tests that point instead to a strong correlation of irreversible loss of resistance with magnetoresistive hysteresis. The main features of the phenomenon are reproduced by a model that treats polycrystalline multilayers as assemblies of grains with random orientations of magnetic easy axes. The key feature of the model is that for each individual grain the magnetic state corresponds to an energy minimum that may be only local, rather than global. Trapping in local energy minima accounts for both magnetoresistive hysteresis and incomplete antiferromagnetic order at the maximum resistance that is attained after cycling. The model explains experimental observations, such as the relatively small hysteresis and resistance loss at the first antiferromagnetic maximum. It also predicts a strong Co-thickness dependence of the resistance loss in Co/Cu multilayers at the second antiferromagnetic maximum that is verified with experimental data for Co/Cu multilayers.