RELAXATION IN NIFE AG GIANT MAGNETORESISTIVE DEVICES/

Giant magnetoresistance was measured as a function of time and device size for patterned NiFe/Ag multilayer films. The sputtered NiFe/Ag mul tilayers were postannealed at 340 degrees C for 5 min in order to prod uce a change in resistivity Delta rho/rho of 5% in a saturating field of 4 kA/m (50 Oe). The microstructure of these films is believed to be discontinuous due to Ag bridging through the NiFe grain boundaries af ter the anneal. The films were fabricated into rectangular stripes wit h Au current lead, and then exposed to a magnetic field pulse to measu re the time response of the resistance, characterized by a time consta nt tau, from the relation Delta R(t)=Delta R(o)e(-t/tau). An apparatus was developed to produce a magnetic field pulse up to 8 kA/m (100 Oe) with a turn-on/off time constant of 10 mu s. The response of the NiFe /Ag devices saturated quickly with the turn-on step with a time consta nt nearly equal to that of the field pulse. The response to the turn-o ff step, however, had a time constant of nearly 300 mu s. When the fie ld is first applied, the torque on the magnetic moments quickly aligns the magnetization. When the field is shut off, however, the torque du e to the field drops to 0, so interacting magnetostatic fields from th e grains and thermal energy dominate the relaxation process. The avera ge time constant depends on the device size and the applied current de nsity. Relaxation may be detrimental for using this type of material f or wad heads where very high data rates are required.