Investigation and tailoring the strengths of positive and negative exchange bias in the ion-beam sputtered Ni81Fe19/Ir7Mn93 bilayers by tuning the grain size of the antiferromagnetic Ir7Mn93 layer

Abstract

We report the investigation of large and tunable exchange bias in a series of top-pinned Ni81Fe19/Ir7Mn93 polycrystalline bilayer samples fabricated at room temperature in the presence of in situ magnetic field of 1000 Oe followed by magnetic annealing at 250°C in the presence of 3500 Oe by tuning the grain size of the antiferromagnetic (AF) Ir7Mn93 (IrMn) layer. These bilayers exhibit robust positive exchange bias (PEB) at room temperature, with a reasonably large positive shift of the center of MH-loop (i.e., exchange bias field, HEB) by ∼30 Oe observed in bilayers having the largest median grain size of ∼7.2 nm. However, on field cooling to 15 K in the presence of 3000 Oe, the magnetization hysteresis loops exhibited the conventional negative exchange bias (NEB). The PEB and NEB are found to be tailored in a controlled manner by a factor of ∼2.5 and ∼2, respectively, by systematically varying the grain size of the AF. In the training measurements, a relatively slower decay is observed in the magnitude of HEB on field cycling for the samples which possessed the largest-sized AF grains. This decay could not be understood within the framework of the thermal relaxation model. However, the decay in HEB is satisfactorily fitted by the spin relaxation model which considers decay in the metastable magnetic disorder at the magnetically frustrated interface during magnetization reversals.