Magnetothermal behavior of the antiferromagnet in exchange-coupled NiFe/IrMn bilayers

The magnetothermal behavior of antiferromagnetic IrMn layers of different thickness (tAFM = 3, 6, 10 nm) has been studied exploiting the exchange coupling with a ferromagnetic 5nm-thick NiFe layer [1]. The NiFe/IrMn samples have been grown by DC magnetron sputtering at room temperature in a magnetic field of 400 Oe. A specific procedure has been devised for the measurement of the magnetization of the NiFe/IrMn bilayers as a function of temperature (5-400 K range) and time at different values of an external magnetic field Hinv, applied antiparallel with respect to the unidirectional exchange anisotropy. This analysis allows one to probe the effective distribution of anisotropy energy barriers of the antiferromagnetic phase, as it is sensed by the coupled ferromagnetic layer. At temperature T < 100 K, a very weak magnetic relaxation is experienced and the barrier distribution features a peak, centered at T ~ 20 K, which does not depend significantly on tAFM and on Hinv; for T > 100 K, a large peak is visible in the barrier distribution, whose position depends on tAFM (it is centered at T ~ 230 K and at T ~ 400 K for tAFM = 3 nm and 10 nm, respectively) and shifts to lower temperature with increasing Hinv. These results are consistent with the existence of a low-temperature magnetic regime (T < 100 K) in which the interfacial IrMn spins are frozen in a disordered glassy state and are collectively involved in the exchange coupling mechanism with the NiFe spins. At T ~ 100 K, the collective state breaks up and only the interfacial IrMn spins which are tightly polarized by the IrMn nanograins, forming the bulk of the layer, are effectively involved in the exchange coupling. Therefore, for T > 100 K, the coupling mechanism is ruled by the bulk IrMn nanograins, whose anisotropy energy barriers mainly give rise to the large peak in the distribution. The thermal evolution of the exchange field and of the coercivity in the three samples is discussed and coherently explained in the framework of this description of the dynamic magnetic behavior of the IrMn phase. Research work sponsored by MIUR under project FIRB2010 ‘Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices - NANOREST’