Exchange bias properties of 140nm-sized dipolarly interacting circular dots with ultrafine IrMn and NiFe layers

Westudied the exchange bias effect in an array of IrMn(3nm)/NiFe(3nm) circular dots (size 140nm and center-to-centerdistance 200nm, as revealed by microscopy analyses), prepared on a large area (33 mm2) by electron beam lithography and lift-off, using dc sputtering deposition. Hysteresis loops were measured by SQUID magnetometer at increasing values of temperature T (in the5–300Krange) after cooling from 300K down to 5K in zero field (ZFC mode) and in a saturating magnetic field (FC mode). The exchange bias effect disappears above T200K and, at each temperature,the exchange field HEX measured in ZFC is substantially lower than the FC one. Micromagnetic calculations indicate that, at room temperature, each dot is in high-remanence ground state, but magnetic dipolar interactions establish alow remanence configuration of the array as a whole. Hence, at low temperature, following the ZFC procedure, the exchange anisotropy in the dot array is averaged out, tending to zero. However,even the FC values of HEX and of the coercivity HC are definitely smaller compared to those measured in a reference continuous film with the same stack configuration (at T=5 K, HEX 90 Oe and HC 180Oe in the dots and HEX1270Oe and HC860 Oe in the film). Our explanation is based on the proven glassy magnetic nature of the ultrathin IrMn layer, implying the existence of magnetic correlations among the spins, culminating in a collective freezing below T100K. We propose, also by the light of micromagnetic simulations, that the small dot size imposes a spatial constraint on the magnetic correlation length among the IrMn spins so that, even at the lowest temperature, their thermal stability, especially at the dot border, is compromised.