MECHANISM OF EXCHANGE-ANISOTROPY AND THERMAL-STABILITY OF SPIN VALVESBIASED WITH ULTRATHIN TBCO LAYERS

A sharp transition from unidirectional to uniaxial behavior is observe d in magnetization measurements performed either at high temperatures or after annealing, on spin valves exchange biased by a thin TbCo laye r. Magneto-optic Kerr-effect measurements with in-plane or out-of-plan e magnetic fields were carried out on annealed samples to demonstrate that the weakening of the TbCo perpendicular anisotropy is responsible for this transition. The uniaxial characteristic is achieved when the exchange coupling between the in-plane and out-of-plane magnetization components of TbCo (J(\\)(perpendicular to)) overcomes the perpendicu lar anisotropy of the ToCo layer (K-perpendicular to(TbCo)). A modifie d spin-valve structure with reinforced perpendicular anisotropy, devel oped to shift the transition to high temperature, shows a blocking tem perature as high as 270 degrees C and an exchange field at room temper ature larger than 500 Oe. Unshielded sensors were implemented with thi s structure showing good thermal stability up to 280 degrees C. (C) 19 98 American Institute of Physics.