Correlation between micromagnetism and tunnel magnetoresistance in magnetic tunnel junctions using artificial ferrimagnets

The impact of the micromagnetic configuration on transport properties of magnetic tunnel junctions is presented. An artificial ferrimagnet system (AFi), made of two ferromagnetic layers with different magnetic moments antiferromagnetically coupled by exchange interactions through a Ru spacer layer, is used as a hard magnetic subsystem of the magnetic junction. Magnetic-force microscopy studies have shown that the magnetization reversal in artificial ferrimagnet systems occurs through the formation of Neel-type 360 degrees domain walls. The stability of these walls is demonstrated to be different in the two magnetic layers of the AFi. These aspects have major effects in magnetic tunnel junction devices, due to the extreme sensitivity of spin tunneling to spatial variations in local magnetic order. The large tunnel magnetoresistance (TMR) of our samples, in the range of 20-30 % at room temperature, makes the tunnel junctions highly sensitive to magnetic fluctuations. A sharp switching of the soft magnetic layer upon field reversal prevents a domain structure from occurring in the soft magnetic layer, in the field window used for our micromagnetic studies. The tunnel device can thus be used as a sensitive probe for measuring small magnetic fluctuations associated with micromagnetic defects, domains, and walls in the AFi system. These fluctuations modulate the resistance of the tunnel junction and are fully reflected in the shape and the amplitude of the TMR signal.