Correlation between antiferromagnetic interface coupling and positive exchange bias

Journal of Physics D: Applied Physics, 2006

The influence of an imperfect interface on exchange bias (EB) properties is investigated. Within the framework of the domain state model, the EB field H EB and the coercive field H C are determined using computer simulations, and they are found to depend strongly on the details of the interface structure. This dependence is sensitive to the dilution of the antiferromagnet (AFM) with non-magnetic defects in the bulk. For the optimal interface structure, giving greatest EB, the optimal dilution is found to be much less than that for an ideal-interface system, taking a value in better agreement with experimental results. Even without any defects in the bulk of the AFM the interface roughness leads to EB for thin antiferromagnetic layers, in accordance with the model by Malozemoff. Finally, the thickness dependence of rough-interface systems is found to differ significantly from that of ideal-interface systems.

Low Temperature Physics, 2012

The influence of magnetic anisotropy of ferromagnetic film on the phenomenon of exchange bias is studied. Hysteresis behavior in the 2-spin model of a ferro/antiferromagnet (FM/AFM) bilayer with exchange bias has been investigated in detail. In this model a half-space of AFM with fixed magnetic configuration contacts with a 2-layer FM film. Twelve different types of magnetization curves M(H) (both with and without hysteresis) have been found. Some of the M(H) curves demonstrate unusual features, such as plateaus and inclined segments. The hysteresis loop becomes asymmetric if the surface anisotropy is taken into account. 0 2 2 0 J J J J H .

Solid State Communications, 2000

Positive exchange bias (PEB) is a remarkable phenomenon, which was recently observed experimentally. Normal (negative) exchange bias (NEB) was discovered more than 40 years ago. Its signature is the shift of the hysteresis loop along the applied field axis by H E Ͻ 0; in systems where a ferromagnet (FM) is in close contact with an antiferromagnet (AFM). This occurs after the system is cooled below the Néel temperature in an external field H cf of a few kOe. As H cf is substantially increased H E adopts positive values. Here we explain this rather unexpected behavior on the basis of an incomplete domain wall model that develops in the FM, for Fe/FeF 2 and Fe/MnF 2 systems. A consistent and unified picture of both NEB and PEB, and satisfactory quantitative agreement with experimental results are obtained on the basis of our theory.

Low Temperature Physics, 2009

Field dependences of the magnetization and exchange bias in ferro/antiferromagnetic systems. II. Continuum model of a ferromagnetic layer

Journal of Applied Physics, 2004

Journal of Applied Physics, 2003

Exchange bias in polycrystalline IrMn/NiFe was found at perfectly compensated interfaces. The energy associated with unidirectional anisotropy is stored in lateral domain walls in the antiferromagnet. In addition to exchange bias, this mechanism leads to a training effect. The bias field shows a maximum of 0 H b ϭ4 mT at an antiferromagnetic layer thickness of 22 nm. The coercivities are on the order of 0 H c ϭ10 mT. The coercive field increases with decreasing intergrain exchange interactions within the ferromagnet.

Physical Review B, 2000

Analytical expressions have been derived for the exchange bias field, coercivity, and effective anisotropy field in ferromagnetic/antiferromagnetic bilayers in the framework of a model assuming the formation of a planar domain wall at the antiferromagnetic side of the interface with the reversal of the ferromagnetic orientation. It is shown that there are five different sets of analytical expressions for the hysteresis loop displacement and coercivity, which depend on the interfacial exchange coupling strength and ferromagnetic anisotropy, and only one expression for the effective anisotropy field. These expressions are compared with the previously reported theoretical results, and the validity of the latter is discussed. It is shown that in the framework of the present model, the hysteresis loop, ac susceptibility, and ferromagnetic resonance measurements of exchange anisotropy should give the same values for the exchange bias field. The difference between the exchange bias field values, estimated experimentally by ac susceptibility and through hysteresis loop measurements for Co/CoO bilayers, is explained as well.

2009

We report a theoretical investigation of thermal hysteresis of fourfold anisotropy ferromagnetic ͑FM͒ film exchange coupled to a compensated antiferromagnetic substrate. Thermal hysteresis occurs if the temperature interval includes the reorientation transition temperature, below which the frustration of the interface exchange coupling leads to a 90°rotation of the magnetization of the ferromagnetic layer. The temperature width of the thermal hysteresis is tunable by external magnetic fields of modest magnitude, with values of 43 K for an external field of 110 Oe and of 14 K for a field of 210 Oe, for a Fe͑12 nm͒ / MnF 2 ͑110͒ bilayer. For a Fe͑3 nm͒ / FeF 2 ͑110͒ bilayer the width of the thermal hysteresis is 23 K at 110 Oe and 13 K at 300 Oe. We discuss how the thickness of the iron film affects the field tuning of the thermal hysteresis width, and also how the thermal loops may be used to identify the nature of the interface exchange energy.

Journal of Physics-Condensed Matter, 2013

The magnetothermal behavior of antiferromagnetic IrMn layers of different thickness (3, 6, 10 nm) has been studied by exploiting the exchange coupling with a ferromagnetic 5 nm-thick NiFe layer. A procedure has been devised for the measurement of the magnetization of the NiFe/IrMn bilayers as a function of temperature and time at different values of an external magnetic field, H inv , antiparallel to the unidirectional exchange anisotropy. This analysis allows one to probe the effective distribution of anisotropy energy barriers of the antiferromagnetic phase, as sensed by the ferromagnetic layer. Two magnetic regimes have been distinguished. At temperature T < 100 K, the interfacial IrMn spins are frozen in a glassy state and are collectively involved in the exchange coupling with the NiFe spins. At T ∼ 100 K the collective state breaks up; thus, above this temperature, only the interfacial IrMn spins which are tightly polarized by the IrMn nanograins, forming the bulk of the layer, are effectively involved in the exchange coupling mechanism. Due to that, for T > 100 K the exchange coupling is ruled by the anisotropy energy barriers of the bulk IrMn nanograins, namely by the layer thickness. The thermal evolution of the exchange field and of the coercivity in the three samples is coherently explained in the framework of this description of the dynamic magnetic behavior of the IrMn phase.

Journal of Magnetism and Magnetic Materials, 2019

The M-H hysteresis curves of field cooled CoFe /FeMn bilayers and CoFe /FeMn /CoFe trilayers were studied to understand the exchange bias phenomena in these systems. The measured data revealed that the values of the exchange bias corresponding to a bottom CoFe layer reduced by about 23.5 % with an addition of another CoFe layer at the top of bilayer stack. It was also observed that, while this reduction in exchange bias of a bottom CoFe layer (calculated in %) depends on thicknesses of a top CoFe layer and an antiferromagnetic FeMn layer, it is independent of the thickness of bottom CoFe layer. As the strength of exchange bias depends on the presence of pinned uncompensated moments in an antiferromagnetic layer, our observations indicate that the FeMn layer consists comparatively lower amount of pinned uncompensated moments in trilayers. This reduction in pinned uncompensated moments of FeMn layer in trilayers is then co-related with the domain wall suppression in the FeMn layer in CoFe /FeMn /CoFe trilayers.