Magnetic and thermal relaxation in (NiFe/CoFe)/PtMn and NiFe/NiMn bi-layers for spin valve heads

The magnetic and thermal relaxation of exchange coupling, H-ex in (NiFe/CoFe)/PtMn and NiFe/NiMn bi-layers has been studied as a function of temperature between 25 degrees C to 250 degrees C and magnetic field between 0 to 1000 Oe applied antiparallel to the original exchange coupling direction. The blocking temperature was about 380 degrees C for (NiFe/CoFe)/PtMn, and 410 degrees C for NiFe/NiMn. It was found that the H-ex of the bi-layers decreased gradually with increasing time, while the coercivities remained almost a constant. The H-ex, measured at a temperature of 250 degrees C and a reversal field of 1000 Oe, decreased from 205 Oe to 0 and then changed its sign. The relaxation time, defined as the time required for H-ex to reach zero, is about 3.1 h and 8.7 h for (NiFe/CoFe)/PtMn and NiFe/NiMn, respectively. The relaxation process for the bi-layers could be understood as the superposition of two exponential decays with different rates associated with a distribution of energy barriers. Spin valve heads using NiFe/NiMn bi-layers could be expected more stable, than those using (NiFe/CoFe)/PtMn bi-layers at an elevated operating temperature. (C) 2000 American Institute of Physics. [S0021-8979(00)85908-3].