Tuning exchange bias

Physical Review B, 1999

We consider three mechanisms of hysteresis phenomena in alternating magnetic field: the domain wall motion in a random medium, the nucleation and the retardation of magnetization due to slow (critical) fluctuations. We construct quantitative theory for all these processes. The hysteresis is characterized by two dynamic threshold fields, by coercive field and by the so-called reversal field. Their ratios to the static threshold field is shown to be function of two dimensionless variables constituted from the frequency and amplitude of the ac field as well as from some characteristics of the magnet. The area and the shape of the hysteresis loop are found. We consider different limiting cases in which power dependencies are valid. Numerical simulations show the domain wall formation and propagation and confirm the main theoretical predictions. Theory is compared with available experimental data. 75.70.Ak,75.60.Ej, 75.60.Ch

Physical Review B, 2004

The strength of exchange bias and rotatable anisotropy in polycrystalline NiFe− IrMn ferromagnet/ antiferromagnet systems is quantified from dc down to the picosecond time scale by regular quasistatic and microwave magnetometry, as well as magnetic domain observation. A transition from superparamagnetic to antiferromagnetic behavior with increasing IrMn thickness is derived from the magnetic resonance frequency and the effective magnetic damping parameter. A discrepancy between magnetic loop shift and dynamically obtained exchange bias strength is explained by asymmetric rotatable anisotropy contributions with different relaxation times in the antiferromagnetic layer. The time-dependent relaxation is directly confirmed by magnetic domain observations. Partially switching in the IrMn layer even with strong exchange bias is concluded. The increase of coercivity rises solely from the rotatable anisotropy contribution.

Low Temperature Physics, 2018

This study offers an explanation for the occurrence of magnetization exchange bias in antiferromagnets with ferromagnetic inclusions during pre-cooling the system in a magnetic field. The ferromagnetic (FM) subsystem ordered in this field at the Néel temperature leads to an inhomogeneous state of the antiferromagnetic (AFM) matrix with the finite mean effective field at the FM/AFM interface. This field causes exchange bias in the dependence M = M(H) during further remagnetization of the heterogeneous system. To describe the proposed scenario for such an effect, a simple model of a two-dimensional system with round inclusions of the FM phase was considered. Using numerical calculations and previously obtained analytical results, the study determines magnetization dependencies on the external field, which qualitatively explains the features of exchange bias in experimentally studied heterogeneous systems.

APS, 1998

A theory of the hysteresis loop in ferromagnets controlled by the domain wall motion is presented. Domain walls are considered as plane or linear interfaces moving in a random medium under the action of the external ac magnetic field H = H 0 sin ωt. We introduce important characteristics of the hysteresis loop, such as dynamic threshold fields, reversal field etc. together with well known

Physical Review B, 2005

The hysteresis loop shift H E of sub-100-nm ferromagnetic-͑FM-͒ antiferromagnetic ͑AFM͒ nanostructures is found to be strongly influenced by thermal activation effects. These effects, which tend to reduce H E , are more pronounced in the nanostructures than in continuous films with the same composition, particularly for thin AFM layers. In addition, the reduced dimensions of the nanostructures also impose spatial constraints to the AFM domain size, particularly for thick AFM layers. This favors an enhancement of H E . Due to the interplay between these two competing effects, the loop shift in the dots can be either larger or smaller than in the continuous films with the same composition, depending on both the AFM thickness and temperature. A temperature-AFM thickness phase diagram, separating the conditions resulting in larger or smaller H E in the nanostructures with respect to continuous film is derived.

Dynamic magnetization reversal measurements at room temperature have been performed by magneto-optical effect on Au/Co/Au sandwiches with perpendicular anisotropy. Domain wall displacement and domain nucleation regimes govern the magnetization reversal at low and high-applied field sweep rates, respectively. The transition between the two regimes occurs at 200 Oe/s.

Physical Review B, 2016

The exchange bias effect is usually defined as horizontal shift of the field-cooled magnetization loop when an antiferromagnet is directly coupled to a ferromagnet. Uncompensated spins at the interface between the two layers are believed to cause this phenomenon. The presence of such, on the other hand, would infer a vertical, i.e., a magnetization-like shift stemming from the antiferromagnet. Observations of this effect are sparse, especially in the absence of a ferromagnet. We present a model system based on extremely Co doped ZnO in which the uncompensated spins of antiferromagnetic CoO Co . .. configurations lead to this vertical shift and therefore to a field-resistant magnetization. A simple Stoner-Wohlfarth-like model based on configurations of different sizes is used to explain the occurrence of this exchange-bias-like shift and a narrow opening of the magnetization curves.

Journal of Applied Physics, 1983

We have obtained theoretical expressions for the ferromagnetic magnetization curve and hysteresis loop using an extension of the general ideas of the Globus model for polycrystalline ferrimagnets. In this work we take into account the force which resul ts from the variation of the total energy (magnetic energy plus surface energy) in order to find the value of the critical field . Our theoretical magnetization curve agrees well with the experimental curve and our hysteresis loop has the general qualitative features of the corresponding experimental loops.

Journal of Magnetism and Magnetic Materials, 2007

We propose a method for determination of the distribution function P(j) of the coupling energy density j in polycrystalline textured ferromagnetic (F)/antiferromagnetic (AF) film systems. P(j) governs the entire film coupling J and the exchange bias field H e and was not measurable until now. The method is verified by torquemetry in a high magnetic field and by reversing its rotation sense. The transition to a new magnetic steady state after rotation reversal is analyzed within a Stoner-Wohlfarth model including thermal relaxation. This transition is completed earlier for strongly coupled grains than for grains with smaller j, which is reflected in the torque curves. We determined P(j) for a sputtered NiFe(16 nm)/IrMn(0.8 nm) film at T ¼ 50 K in the hysteretic range of coupling energies and found that P strongly decreases for increasing j.

Applied Physics Letters, 2003

In (Pt/Co) n /FeMn multilayers, the magnitude of exchange bias, H E , can be considerably enhanced by placing an ultrathin nonmagnetic Pt spacer between the multilayer ͑ML͒ and the antiferromagnetic ͑AFM͒ layer. The bias is maximum for a spacer layer thickness, t, of a few angstroms and it decreases progressively as t is further increased. This bias enhancement is accompanied by an increase of coercivity, H C . This behavior is due to the role of the Pt spacer in enhancing the perpendicular effective anisotropy of the last Co layer in the ML, which has the effect of increasing the net ferromagnetic ͑FM͒/AFM spin projection, thus leading to the H E and H C enhancements. The decrease of H E and H C for thicker spacer layers is due to the limited range of the FM-AFM proximity effect.