Introduction

Exchange anisotropy refers to the effect that an antiferromagnetic (AF) layer grown in contact with a ferromagnetic (FM) layer has on the magnetic response of the FM layer.[1] Exchange anisotropy is one of several magnetic interfacial interactions, which include interlayer coupling in multilayers, that have been intensively studied in recent years. The most notable changes in the FM hysteresis loop due to the surface exchange coupling are a coercivity enhanced over the value typically observed in films grown on a nonmagnetic substrate, and a shift in the hysteresis loop of the ferromagnet away from the zero field axis. The characteristics of the AF layer and the interface between the two layers that produce the strongest exchange bias are not well understood. Experimental studies and theoretical models[2-5] indicate that intrinsic magnetic properties of the AF such as the magnetocrystalline anisotropy, exchange stiffness and crystalline texture,[6-9] as well as extrinsic properties such as grain size, domain size and interface roughness[8,10,11] may influence the resulting response of the FM. Unfortunately, it is difficult to manipulate these properties independently, or to probe the magnetic structure of the bilayer interface directly.

Exchange couples which incorporate FeMn, NiMn, PdMn, IrMn, Pd-Pt-Mn, NiO, and NiCoO antiferromagnetic layers are currently under study for use in magnetoresistive sensors and magnetoresistive and spin-valve-based hard disk readback heads.[12] The exchange anisotropy is employed to achieve the optimum sensitivity in the sensor and to reduce noise by stabilizing domains.[13-15] In this paper we focus on the oxide AF materials which share the same rocksalt crystal structure. The AF spin configurations and exchange coupling properties of the Mn-based materials are significantly different from the oxide materials and thus must be considered separately. The oxide films proposed for applications are polycrystalline with relatively small grain sizes. Achieving a clearer understanding of how magnetocrystalline anisotropy and texture influence the exchange anisotropy, however, requires that films with a high degree of crystalline perfection be examined as well.

Typically NiFe is deposited on top of NiO to form a NiFe/NiO exchange couple. A field of 20 to 200 Oe is applied during deposition to induce a uniaxial anisotropy in the NiFe layer. The interaction of the aligned NiFe spins at the interface with the NiO during deposition influences the AF spins in the NiO since the applied field is too weak to induce ordering in the NiO spins directly. In turn the NiO spin arrangement stores the exchange bias information and induces a unidirectional surface anisotropy in the NiFe. Heating bilayers above the blocking temperature T_b and cooling in a field has been variously reported to increase[16] and to decrease[13,17] interfacial exchange field H_E relative to the as-deposited values. How the magnitude of the deposition field or the cooling field influences H_E has not been well established in the case of oxide antiferromagnets,[18] although a striking change of sign in the exchange bias field has been observed with large deposition fields in the Fe/FeF₂ system.[19]

The NiO spin structure is relatively simple, however the large number of domain configurations and domain walls in a multidomain sample make theoretical models of exchange anisotropy in NiO/NiFe bilayers considerably more challenging.[20-23] NiO has a cubic FCC NaCl crystal structure above its Néel temperature T_N. Below the Néel temperature there is a slight distortion of the NiO lattice in a (111) direction (Delta l/l = 4.5x10^-3).[24] A strong negative uniaxial anisotropy accompanies the contraction, resulting in an easy plane defined by K₁ approximately 10⁶ erg/cm³.[23] Sheets of ferromagnetically aligned spins form in the (111) planes defined by the contraction axis,[21] the Ni spins in neighboring sheets oppositely aligned. Within a (111) plane the direction of the spin axis is determined by a second 3-fold anisotropy (K₃) that is roughly three orders of magnitude weaker than K₁.[24,25]

The AF domain configurations in NiO have been studied both experimentally and theoretically. There are four possible (111) directions in a NiO crystal from which the contraction axis may choose, and 3 spin directions once the contraction axis is defined. Thus there are 4x3 = 12 distinct possible AF domain configurations in NiO below T_N. Since the four (111) directions in the cubic NiO are nominally equivalent, local inhomogeneities break the symmetry and determine which (111) axis becomes the contraction axis in different regions of the crystal. External applied magnetic fields and strain can make one (111) direction more kinetically favorable, and thereby influence the distribution of AF domains. The magnetic susceptibility of the NiO is largest parallel to the contraction axis (and perpendicular to the planes of spins), so this axis tends to align parallel to strong applied fields.[26] Once the sample temperature has been lowered below the Néel temperature, domain walls become strongly pinned[22,26,27] and extremely large fields are required to change the AF domain configuration.

In this study, we compare the magnetic properties of polycrystalline and epitaxial (001) NiO/NiFe bilayers deposited simultaneously. We also compare epitaxial (001) NiO/NiFe bilayers with the deposition bias field, H_b, aligned along different in-plane NiO crystalline axes. The results of these studies are interpreted in terms of induced anisotropies at the NiO/NiFe interface.