Frequency-dependent exchange bias in NiFe/NiO films

Abstract

Ferromagnetic ͑FM͒ resonance and magnetization curve measurements were performed at room temperature for a polycrystalline Ni 81 Fe 19 coupled to NiO. It was observed that the shape of the angular variation of the resonance field is frequency dependent, with the curve at 9.65 GHz typical for a strongly exchange-coupled bilayer, while the 34.0 GHz curve is characteristic for relatively weak interactions. Numerical simulations of the resonance field and of the hysteresis loop shift, carried out through the domain wall formation model, as well as the resonance linewidth data indicated that there must be two fractions in the antiferromagnetic part of the interface, with stable and unstable grains. Only the stable grains contribute to the exchange bias. In our sample, whether an interfacial antiferromagnetic grain is stable or not is predominantly determined by the strength of the exhange coupling between this grain and the adjacent FM domain. The stable antiferromagnetic grains, whose contribution is sensed by the resonance experiment, are the smaller ones, which are more strongly coupled to the ferromagnet than the larger grains. The exchange-bias phenomenon, 1 which results from the interfacial coupling between ferromagnetic ͑FM͒ and antiferromagnetic ͑AF͒ materials, has been extensively studied in the last decade, motivated by fundamental and practical interests. One of the reasons for the continued interest is the fact that different experimental techniques may yield different exchange anisotropy values. [2][3][4][5][6][7][8][9] In some cases, this was assigned to the fact that the measurements are performed on different sets of samples. 4 Another source for the discrepancy could be the reliability of the model used in the experimental data interpretation. 8 It has also been shown 9 that the exchange-bias field values derived from magnetization and ferromagnetic resonance ͑FMR͒ measurements, must, in general, give different values in the framework of the domain-wall formation ͑DWF͒ model which assumes formation of a planar domain wall at the AF side of the interface with the reversal of the FM orientation. 8,10 Recent FMR studies on exchange-coupled NiFe films showed the importance of the magnetic state of the AF part of the interface. McMichael et al.11 used FMR to study the relaxation behavior of polycrystalline AF layer and found that the stability of the AF order is critical to the existence of the exchange bias. Relaxation related to thermally driven reversal of the AF grains in such systems has also been observed. 12,13The aim of the present work was to apply the phenomenological approach for the DWF model recently developed by our group 9 on a real exchange coupled system. The paper describes FMR and magnetization measurements of a bilayer of Permalloy ͑Py͒ ͑here, Ni 81 Fe 19 ) exchange-coupled to NiO. The striking frequency dependence of the FMR angular variation has been related to the relaxation behavior of the interfacial AF grains.The sample under investigation was deposited by rf magnetron sputterin...

Discrimination between coupling and anisotropy fields in exchange-biased bilayers

“…Among them is the fact that different measurement techniques may yield distinct values [8][9][10][11][12][13][14][15][16][17][18][19][20] for the FM/AF exchange coupling constant J E , being these differences of up to one order of magnitude. This has lead some authors to classify the techniques in two categories: reversible and irreversible.…”

“…In the framework of this model, by finding the equilibrium directions of M FM and M AF , one can numerically simulate magnetization curves, 14,16,[18][19][20]23 transverse-biased ac susceptibility 20 t , FMR field 16,18,19 H R , and anisotropic magnetoresistance ͑AMR͒ ͑Ref. 24͒ for any in-plane dc field direction given by H .…”

Magnetic characteristics of a high-layer-number NiFe/FeMn multilayer

“…9,10 FMR has been used to study exchange bias as an effective bulk field in a number of different systems. [11][12][13][14][15][16][17][18] Standing spin waves have also been used to study exchange bias as an interface pinning effect. 19 In this work, we investigate primarily using FMR the magnetic properties of a Ni 0.8 Fe 0.2 /Fe 0.5 Mn 0.5 multilayer test structure.…”

The angular dependence of the exchange bias under the planar domain wall model

“…It can be conceivable that when σ W approaches infinity, i.e., γ ≈ 0, the simple relation α ≈ 0 o can be deduced, which demonstrates that PDW model will be reduced to RAF model as reported by other authors. 26,29,34 Whereas, when γ is larger, J E will be dominant in the competition. In this case, M AF will follow the rotatation of M F closely to generate a smaller angular deviation θ F -α, which is beneficial for minimizing the interfacial exchange coupling energy −J E cos(θ F − α).…”

“…27,28,[42][43][44][45][46] On the basis of the PDW model, Geshev et al have performed a series of researches on the exchange bias effect. [29][30][31][32][33][34][35][36][37] The distribution of the AFM axis, the enhancement of the coercivity, the dependence of the exchange bias on the measurement techniques, temperature, and the rotatable anisotropy have been discussed systematically in their works. These studies suggest that only using the PDW model, the magnetic behaviors of these exchange-biased systems can be well understood.…”