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 5nmthick NiFe layer. We have introduced an original protocol for the measurement of NiFe/IrMn sample magnetization as a function of temperature and time at different values of an external magnetic field Hinv, applied antiparallel with respect to the unidirectional exchange anisotropy [1]. This analysis probes the effective distribution of anisotropy energy barriers of the antiferromagnetic phase, as it is sensed by the coupled ferromagnetic layer. The NiFe/IrMn samples have been grown by DC magnetron sputtering at room temperature in a magnetic field of 400 Oe. At low temperature, T < 100 K, the barrier distribution features a peak, centered at T~20 K, and this value is nearly independent on tAFM and Hinv. For T > 100 K, a large peak is visible, and its position changes with tAFM and Hinv. These results are consistent with the existence of a low-temperature magnetic regime in which the interfacial IrMn spins are frozen in a disordered glassy state and collectively involved in the exchange coupling mechanism. 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 anisotropy energy barriers of bulk IrMn nanograins 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. This research was sponsored by MIUR-­‐FIRB2010 project RBFR10E61T/NANOREST.