Haiwen Xi

Study of the angular-dependent exchange coupling between a ferromagnetic and an antiferromagnetic layer

The angular dependence of the exchange coupling between a Permalloy film and an antiferromagnetic Cr45.5Mn45.5Pt9 film has been measured. We show that this complex angular dependence of the exchange coupling can be understood by a simple Stoner–Wohlfarth model involving only a uniaxial anisotropy and a unidirectional exchange coupling at the interface. Numerical results are in excellent agreement with the experimental data.

Angular dependence of exchange anisotropy in Ni81Fe19/CrMnPtx bilayers

We have investigated the unidirectional exchange anisotropy between a ferromagnetic (FM) Ni81Fe19 film and a disordered antiferromagnetic (AF) CrMnPtx (x=3, 6, or 9) film. The angular dependence of the exchange coupling in these FM/AF bilayers has been measured. We show that the complex angular dependence of this exchange coupling can be understood by a simple Stoner–Wohlfarth model involving only a uniaxial anisotropy and a unidirectional exchange coupling. Numerical results are in excellent agreement with the experimental data.

Ferromagnetic resonance studies of exchange biasing in Ni81Fe19/Pt10Mn90 bilayers

In-plane ferromagnetic resonance (FMR) has been used to study exchange anisotropy in Ni81Fe19/Pt10Mn90 bilayers at room temperature. A theoretical calculation for the FMR resonance field was carried out based on a simple model of the exchange bias in a polycrystalline bilayer that incorporates reversible as well as irreversible behavior of the antiferromagnetic layer. In addition to the expected cos φH contribution to the resonance field, where φH is the direction of the applied field, there is a negative shift of the resonance field. It is argued that the negative shift of the resonance related to the irreversible behavior of the antiferromagnetic moments is not isotropic. The linewidth broadening of in-plane resonance for the exchange biased bilayers is also discussed.

Interface coupling and magnetic properties of exchange-coupled Ni81Fe19/Ir22Mn78 bilayers

Hysteresis loop measurements using magnetooptic Kerr effect magnetometry in the low frequency region and magnetization dynamics measurements using high frequency permeametry are carried out to study the exchange anisotropy in Ni81Fe19/Ir22Mn78 bilayers. These two measurement techniques provide different exchange anisotropies for bilayers with thin Ir22Mn78 films. The observations can be understood by assuming that the AF grains break into domains due to the interface random field. By analysing the results from these two techniques, the interface coupling strength and the magnetic properties of the bilayers can be quantitatively determined.

Annealing effect on exchange bias in Ni/sub 81/Fe/sub 19//Cr/sub 50/Mn/sub 50/ bilayers

The effect of annealing on the exchange bias in Ni/sub 81/Fe/sub 19//Cr/sub 50/Mn/sub 50/ bilayers was investigated. A large increase of the exchange field and coercivity was observed in the samples annealed at temperatures higher than 310/spl deg/C. The annealing causes the Mn atoms in the Cr/sub 50/Mn/sub 50/ layer to diffuse into the Ni/sub 81/Fe/sub 19/ layer and form an antiferromagnetic NiFeMn phase, which coats on the Ni/sub 81/Fe/sub 19/ grain surface and contributes to the improvement of exchange bias. An increase of interface roughness due to the Mn diffusion also results in the exchange bias enhancement. The enhanced exchange field and coercivity were explained by the reversible and irreversible motions of NiFeMn moments of the inhomogeneous grains.

Exchange biasing in sputtered NiFe/PtMn bilayers

The exchange coupling between ferromagnetic Ni81Fe19 and antiferromagnetic (AF) PtxMn1−x films prepared by rf and dc magnetron sputtering has been investigated. The Pt content in the PtxMn1−x film is in the range of 0 at. %<x<20 at. %. The exchange field and coercivity were found to depend strongly on the deposition conditions and the AF film composition. X-ray diffraction measurements and transmission electron microscopy measurement showed a γ-PtMn phase with a disordered fcc structure when the PtxMn1−x was deposited on top of the Ni81Fe19 layer. The exchange bias was found to depend on the texture and film composition of the γ-PtMn layers.

Exchange coupling of NiFe/CrMnPtx bilayers prepared by a substrate bias sputtering method

We have investigated the unidirectional exchange anisotropy between a ferromagnetic Ni81Fe19 film and a disordered antiferromagnetic CrMnPtx (x=3, 6, or 9) film prepared by substrate bias sputtering. A hysteresis loop with the shape predicted by the planar domain-wall model was found in the sample without substrate bias, suggesting that the interfacial exchange coupling energy between the Ni81Fe19 and CrMnPtx layers is comparable to the antiferromagnetic domain-wall energy. The substrate bias on the Ni81Fe19 underlayers improved the texture of the CrMnPtx layers but also increased the interface roughness. The substrate bias effect on the exchange bias can be understood in terms of the planar domain-wall model. The interface roughness reduces the interfacial exchange coupling by introducing defects and disordering the antiferromagnetic moments at the interface. This leads to a triangular hysteresis loop. The texture improvement increases the exchange bias by increasing the domain-wall energy of the antiferromagnet. The differences between these substrate bias effects and those observed in the Ni81Fe19/Fe50Mn50 bilayer system are also discussed.We have investigated the unidirectional exchange anisotropy between a ferromagnetic Ni81Fe19 film and a disordered antiferromagnetic CrMnPtx (x=3, 6, or 9) film prepared by substrate bias sputtering. A hysteresis loop with the shape predicted by the planar domain-wall model was found in the sample without substrate bias, suggesting that the interfacial exchange coupling energy between the Ni81Fe19 and CrMnPtx layers is comparable to the antiferromagnetic domain-wall energy. The substrate bias on the Ni81Fe19 underlayers improved the texture of the CrMnPtx layers but also increased the interface roughness. The substrate bias effect on the exchange bias can be understood in terms of the planar domain-wall model. The interface roughness reduces the interfacial exchange coupling by introducing defects and disordering the antiferromagnetic moments at the interface. This leads to a triangular hysteresis loop. The texture improvement increases the exchange bias by increasing the domain-wall energy of the antifer...

Measurement dependence of the exchange bias

Recently, in addition to the conventional hysteresis loop measurement, several different measurement techniques, such as ac susceptibility, ferromagnetic resonance (FMR), and Brillouin light scattering, have been used to study the unidirectional exchange coupling induced in a ferromagnetic film in proximity to an antiferromagnet. In this article, we theoretically investigate how interface exchange coupling manifests itself in different measurements. The magnetic degrees of freedom of the antiferromagnet are incorporated in this analysis. Anisotropies from hysteresis loop measurements, ac susceptibility measurements, and FMR measurements are obtained. These results show that the measured exchange anisotropies differ with measurement techniques.

Theory of the blocking temperature in polycrystalline exchange biased bilayers based on a thermal fluctuation model

It is well known that the exchange bias between an antiferromagnet (AF) and a ferromagnet (FM) vanishes at a temperature called the “blocking” temperature. The blocking temperature of polycrystalline exchange biased bilayers is theoretically studied on the basis of a thermal fluctuation model, from which an analytic expression for the blocking temperature as a function of the AF thickness and the AF grain size is obtained for a thin AF layer. This study also gives a quantitative understanding of the relationship of the blocking temperature to the Neel temperature of the AF and the Curie temperature of the FM.

A theoretical study of interfacial spin flop in exchange-coupled bilayers

A variational approach is used to study the role of the spin-flop configuration on exchange bias in a ferromagnetic/antiferromagnetic bilayer with a compensated interface. Both irreversible and reversible behavior of the antiferromagnet is predicted as the ferromagnetic magnetization rotates in the plane of the film. The analysis shows that the spin flop does not introduce exchange bias but it does lead to a coercivity in either the irreversible or reversible regimes.