We systematically investigated the possible magnetization reversal behavior in well-characterized, epitaxial, Fe/IrMn exchange-biased bilayers as a function of the antiferromagnetic ͑AF͒ layer thickness. Several kinds of multistep loops were observed for the samples measured at various field orientations. The angular dependence of the switching fields, observed using longitudinal and transverse magneto-optic Kerr effect, were shown to depend on the competition between the magnetocrystalline

more »

... otropy and the exchange bias ͑EB͒. A modified "effective field" model was applied to quantitatively describe the evolution of the magnetic behavior and correctly predict the occurrence of different magnetic switching processes. The dependence of the effective anisotropy fields on the AF layer thickness directly reflects the competing effects of the pinned and rotatable AF spins at the EB interface. The exchange bias ͑EB͒ ͑Ref. 1͒ effect, particularly in the form of a ferromagnetic ͑F͒/antiferromagnetic ͑AF͒ bilayer, has been widely studied due to its applications in magnetic storage technologies. 2 One of the fundamental issues in EB is the spin behavior at the F/AF interface. Recent studies using synchrotron radiation 3 revealed the interfacial AF spins are either pinned, providing a hysteresis loop shift ͑H eb ͒, or rotatable, resulting in a coercivity ͑H c ͒ enhancement. 4-6 The values of H eb and H c intrinsically depend on the thicknesses of the F and AF layers. 7 Previous studies have shown that H eb is roughly inversely proportional to the thickness of the F layer. 8 However, the dependence of H eb on the AF layer thickness, t AF , is complicated and largely depends on other parameters such as the material, 9 the setting field of EB, 10 and the temperature. 11 Magnetic anisotropy is the fundamental physical parameter that determines the magnetization reversal processes. 12 Considering an unidirectional anisotropy, K eb , and an induced uniaxial anisotropy, K u , the value of H eb and H c for the polycrystalline EB systems can be numerically fitted by the Stoner-Wohlfarth model. 13 However, as compared to the extensive investigations on polycrystalline EB systems, only few works have focused on epitaxial bilayers, 14,15 which are, in fact, ideal systems for investigating EB due to the better control of the spin configuration at the interface. 4, [16] [17] [18] [19] In epitaxial EB systems, the intrinsic magnetocrystalline anisotropy results in multistep hysteresis loops and a complex angular dependent behavior. 14,15,20 An "effective field" model, taking into account the unidirectional anisotropy field, H X , and the cubic F anisotropy field, H A , was proposed to quantitatively interpret the angular dependent switching fields. 15 To date, however, the dependence of the magnetization reversal on t AF in epitaxial EB systems has not been fully understood. In this work, we present a comprehensive study of the dependence of EB on t AF for epitaxial Fe/IrMn bilayers. Different magnetic switching processes were found at various field orientations by vector magneto-optic Kerr effect ͑MOKE͒, which offers a comprehensive understanding of the magnetization reversal of the film by probing both the longitudinal and transverse magnetization components. 21, 22 The evolutions of the angular dependent switching fields were interpreted by a modified effective field model. Peculiar dependence of both H X and H A on t AF was observed and interpreted in terms of the competition between the pinned and rotatable interfacial AF spins. A series of Fe/IrMn bilayers were grown on transparent MgO͑001͒ substrates by ultrahigh vacuum ion beam sputtering with deposition rate as low as 1 Å/s, which is well suited for growing epitaxial magnetic thin films. 17,18 The substrates were preannealed at 500°C for 1.5 h and held at 145°C for deposition. A permanent magnet generating a field of ϳ300 Oe was positioned along the Fe͓010͔ direction during growth. Samples with the structure of MgO/ Fe͑15 nm͒ / IrMn͑t IrMn ͒ / Ta͑3nm, cap͒ were deposited with the IrMn layer thickness, t IrMn =0,2,3,4,4.5,5,5.5,6,8, 10, 14 nm. The epitaxial relation of the samples was established using x-ray diffraction ͑XRD͒ with Cu K␣ radiation. In the -2 scan ͓Fig. 1͑a͔͒, the ͑002͒ peaks of Fe and IrMn indicated a good out-of-plane ͑002͒ texture. Furthermore, the x-ray in-plane ⌽Ϫscan ͓Fig. 1͑b͔͒ not only showed the fourfold symmetry but confirmed the epitaxial relationship of MgO͑001͓͒100͔ ʈ Fe͑001͒ ͓110͔ ʈ IrMn͑001͓͒100͔. 23 Magnetic properties were probed ex situ at room temperature by vector MOKE and by illuminating the back side of the sample through the transparent substrates. The anisotropy geometry and the magnetic switching routes used in this letter are summarized in Figs. 1͑c͒ and 1͑d͒. The EB gives rise to H X and a collinear uniaxial anisotropy field, H U , along the field cooling direction. Both of them are superimposed on H A . Various switching routes between the Fe easy axes were observed using longitudinal ͑ ʈ ͒ a͒ Electronic