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

We studied the exchange bias effect in an array of IrMn(3nm)/NiFe(3nm) circular dots(size 140nm and center-to-center distance 200nm,as revealed by microscopy analyses),prepared on a large area (3x3 mm2) by electron beam lithography and lift-off, using dc sputtering deposition. Hysteresis loops were measured by SQUID magnetometer at increasing value sof temperature T (in the 5–300K range) 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 T - 200K and, at each temperature, the exchange field H_EX 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 a low-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 H_EX and of the coercivity H_C are definitely smaller compared to those measured in a reference continuous film with the same stack configuration (at T = 5 K, H_EX = 90 Oe and H_C = 180 Oe in the dots and H_EX = 1270 Oe and H_C = 860 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 acollective freezing below T=100K. We propose, al so 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.