Enhanced exchange bias in ferromagnet/antiferromagnet multilayers (original) (raw)
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Structural and magnetic properties of (Fe/Mn) exchange-biased multilayers
Physica B: Condensed Matter, 2013
Exchange-biasing of ferromagnetic (F) Fe layers by adjacent antiferromagnetic (AF) Mn layers has been investigated in (Fe/Mn) 10 multilayered films. This study has been focused on the relationship between the evolution of the exchange-bias field and the evolution of the film microstructure as a function of the deposition temperature. The increase of the deposition temperature results in the formation of an Fe-Mn alloy at the interfaces and columnar features whose size increases with the deposition temperature. In parallel, the exchange-bias field decreases significantly, due to interface roughness. (C. Bordel). Physica B 416 (2013) 45-50
Dual behavior of antiferromagnetic uncompensated spins in NiFe/IrMn exchange biased bilayers
Physical Review B, 2010
We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias. PACS numbers: 75.60.Jk, 75.70.Cn, 61.12.Ha The tremendous advances of spintronics research initiated by the discovery of interlayer exchange coupling [1] and giant magnetoresistance [2, 3] uses extensively the exchange bias (EB) effect to control the magnetization of ferromagnetic components. This is a consequence of the direct exchange at the interface between feromagnetic and antifereomagnetic layers and/or nanoscale heterostructures which causes a shift and a broadening of the hysteresis loop of the ferromagnet. This effect which was engineered by nature a few billion years ago [4], was experimentally discovered 60 years ago by Meiklejohn and Bean (M&B) [5] when studying Co particles embedded in their natural oxide (CoO) matrix. Extensive experimental and theoretical studies of the EB effect provide now sufficient understanding for utilizing it as a probe for further fundamental research .
Exchange bias in ferromagnetic/antiferromagnetic bilayers with imperfect interfaces
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.
Coercivity and exchange bias of ferromagnetic/antiferromagnetic multilayers
Physical Review B, 2005
For a model system consisting of a ferromagnetic layer exchange coupled to an antiferromagnetic layer with a compensated interface detailed mean field type calculations are performed. Both the coercive field and the exchange bias field are calculated. For the coercive field a rather broad enhancement around the Néel temperature TN of the antiferromagnetic layer is found irrespectively of whether the antiferromagnetic layer is structurally disordered or not, while exchange bias is only found for disordered systems. We show that the observed enhancement of the coercivity around TN also found experimentally and the occurrence of exchange bias are of different origin.
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.
Critical thickness investigation of magnetic properties in exchange-coupled bilayers
2011
We present a systematic investigation of the magnetic properties of two series of polycrystalline ferromagneticantiferromagnetic bilayers (FM-AF) of Ni 81 Fe 19 (10nm)/Ir 20 Mn 80 (t AF) grown by dc magnetron sputtering. One series was grown at an oblique angle of 50 • and the other one was grown at 0 •. Ferromagnetic resonance (FMR) was used to measure the exchange bias field H E , the rotatable anisotropy field H RA , and the FMR linewidth H as a function of the antiferromagnetic layer thickness t AF. Three relaxation channels due to isotropic Gilbert damping, anisotropic two-magnon scattering, and mosaicity effects are simultaneously distinguished through the angular dependence of the FMR linewidth. In the regime of small IrMn layer thicknesses, not enough to establish the exchange bias anisotropy, the FMR linewidth shows a sharp peak due to the contribution of the two-magnon scattering mechanism. The results presented here are of general importance for understanding the dynamics of magnetization in the FM-AF structures.
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.
Ferromagnetic resonance study of the exchange bias field in NiFe∕FeMn∕NiFe trilayers
Journal of Applied Physics, 2006
The ferromagnetic resonance ͑FMR͒ technique is used to study the exchange bias field in asymmetrical NiFe/ FeMn/ NiFe trilayers produced by dc magnetron sputtering under different working pressures. The FMR spectra give evidence of two resonance modes attributed to the two asymmetrical noninteracting NiFe layers. The study of the in-plane angular dependence of the absorption field allows the measurement of the exchange bias field at both bottom ferromagnetic ͑FM͒/antiferromagnetic ͑AFM͒ and top AFM/FM interfaces.
Positive and negative exchange bias in IrMn/NiFe bilayers
Thin Solid Films, 2010
We present the observation of double shifted hysteresis loops in IrMn/NiFe bilayer structures. The bilayer structures were fabricated using high vacuum DC magnetron sputtering system. The hysteresis loops of the as deposited samples show the double shifted loops at NiFe layer thicknesses 5 nm and 6 nm, whereas the IrMn layer thickness was kept constant at 15 nm. The results were interpreted as the contribution of both positive and negative exchange bias fields. We suppose that this phenomenon is occurring due to the ferromagnetic (FM) layer exchange coupled with the antiferromagnetic (AFM) layer in two different magnetization directions. The ferromagnetic coupling of the interface spins in some regions of the film generates the hysteresis loop shift toward negative fields and antiferromagnetic coupling toward positive fields in the other regions. The double shifted hysteresis loops disappeared after magnetic field annealing of the samples above Neel temperature of the AFM layer. The X-ray diffraction patterns of the sample show the IrMn (111) crystalline growth necessary for the development of exchange bias field in this system. The correlation between the Magnetic Force Microscopy (MFM) domain structures of the as deposited sample and the magnetization reversal process of the double shifted hysteresis loops were discussed. The results suggest that the larger multidomain formation in the AFM layer with different magnetization directions was responsible for the positive and negative exchange bias fields in IrMn/NiFe bilayer samples.