Nonlinear exchange coupling and magnetic domain asymmetry in ferromagnetic/IrMn thin films Jeffrey McCord,* Christine Hamann, Rudolf Schäfer, and Ludwig Schultz Institute for Metallic Materials, IFW Dresden, Postfach 270116, 01171 Dresden, Germany Roland Mattheis Institute of Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena, Germany Received 26 July 2008; revised manuscript received 19 August 2008; published 19 September 2008 Noncollinear uniaxial and unidirectional exchange anisotropy contributions are discovered and identified as a cause of loop asymmetry in exchange-coupled ferromagnetic/antiferromagnetic thin films. Adjusting the magnetic reversal field axis to compensate for the tilted anisotropies eliminates the loop and magnetic domain reversal asymmetry. The deviation from collinearity of exchange coupling is suggested to originate from antiferromagnetic-layer-induced interfacial magnetic frustration. The effects are independent of the occurrence of exchange bias, existing below and above the onset of exchange bias. The additional anisotropy contributions add another mechanism to the occurrence of exchange coupling. DOI: 10.1103/PhysRevB.78.094419 PACS numbers: 75.60.d, 75.70.i I. INTRODUCTION Exchange interaction at the interfaces of layered ferro- magnetic FM and nonmagnetic NM or antiferromagnetic AF films is a major contribution to the effective magnetic properties of thin-film nanostructures. In particular, exchange coupling across a NM transition-metal layer is observed in FM/NM/FM sandwich structures. There, the sign of coupling depends on the NM layer thickness and may result in a pre- ferred parallel, respectively, antiparallel alignment of the spins of the FM layers in the magnetic ground state. 1,2 For certain materials systems a biquadratic coupling, 3–5 favoring an orthogonal orientation of magnetization, becomes the dominating contribution. It may originate from local thick- ness variations, which result in spatial fluctuations of the bilinear interlayer coupling terms, inhomogeneities of the in- terlayer spin structure, FM impurities in the NM interlayer matrix, or magnetic proximity effects. On the other hand, direct exchange coupling is used to control the magnetic properties in exchange spring hard/soft FM/FM Ref. 6 or exchange biased FM/AF bilayers. 7 In the FM/FM systems the FM layers are accessible to traditional magnetic measure- ment techniques and it has been shown that by introducing a fluctuating spin structure in the FM hard layer, an effective biquadratic coupling across the interface is introduced. 8–10 For exchange biased systems, however, the magnetization of the AF layer can neither be adjusted nor probed easily. In such systems a stabilization of the FM magnetization along a preferred direction is achieved by the exchange bias effect, which for FM/AF structures manifests itself in a ferromag- netic loop shift 7,11–14 after the setting of unidirectional aniso- tropy during film deposition or by a field cooling process. Various possible coupling mechanisms have been proposed to describe the exchange effect in agreement with experi- mentally obtained data. 15–18 Biquadratic or spin-flop cou- pling across the FM/AF layer has been proven to exist in single-crystalline FeF 2 -based systems with well-defined and high uniaxial AF anisotropy. 19 The occurrence of positive exchange bias in such systems 20 has been found to depend on the magnitude of the applied magnetic field 21 during the field cooling process. For polycrystalline FM/AF layers, where the in-plane AF anisotropy axes of the grains are equally distributed, 22 the existence of pinned uncompensated spins at the interface is found to be responsible for the exchange bias effect. 23 For such systems, applying a magnetic field H dep during AF film deposition on top of a saturated FM layer predominately results in an exchange bias field H eb or unidi- rectional anisotropy K ud,FM-AF along and parallel to the ini- tially applied magnetic-field direction. In addition, exchange biased FM/AF systems evidence loop asymmetry effects, 24–27 which are explained by irrevers- ible magnetization processes in the antiferromagnetic layer 25,28 or by a misaligned external magnetic field 28,29 dur- ing reversal of an otherwise collinear coupled FM/AF sys- tem. In the latest coherent rotation models the reversal de- pends strongly on the ratio between FM film uniaxial anisotropy and unidirectional anisotropy. 30 Yet, the proposed models are not able to explain all aspects of the symmetry breaking phenomena. Especially, asymmetric ferromagnetic domain formation and incoherent rotation processes, 25,31–33 where the magnetic switching field is aligned along the ex- pected direction of exchange bias, are not understood. In this paper we show evidence for the existence of tilted uniaxial and unidirectional exchange anisotropies, deviating from col- linearity, in FM/AF systems, which lead to asymmetric mag- netization reversal phenomena. II. EXPERIMENT To investigate the asymmetry effects, Ta5 nm / FM20 nm / Ir 23 Mn 77 / Ru3 nmFM=Co 90 Fe 10 or Ni 81 Fe 19  films with varying AF thickness t IrMn =0–9 nm were prepared by dc-magnetron sputtering on Si / SiO 2 substrates. The Ta-seed layer ensures a 111-fiber texture of the polycrystalline films 22,25 and no signs of in- plane texture were detected. 22 The uniaxial anisotropy K u,FM of the FM layer and the unidirectional anisotropy K ud,FM-AF transmitted from the AF layer grown on top of the FM layer were set in a magnetic in-plane field of H dep =4.0 kA / m during the FM and AF film deposition. By this procedure an exact parallel alignment of the uniaxial anisotropy of the FM PHYSICAL REVIEW B 78, 094419 2008 1098-0121/2008/789/0944198 ©2008 The American Physical Society 094419-1