Thermally assisted reversal of exchange biasing in NiO and FeMn based systems (original) (raw)
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Magnetization reversal in exchange-biased Ni∕NiO layered structures
Physical Review B, 2007
The dependence of the asymmetric magnetization reversal, exchange field, and coercive field on the NiO microstructure and thickness is studied in polycrystalline Ni/ NiO. Probing structural properties in real and reciprocal space, we show that the modification of the NiO growth conditions results in nonstoichiometric NiO. Exchange and coercive fields are strongly enhanced in nonstoichiometric Ni/ NiO bilayers. Probing exchange properties in trilayers, we show that the coercive and exchange field enhancements are of different origins but depend on the NiO domain state. Modifying the internal part of the antiferromagnet allows a control of the exchange properties. Studying the Ni magnetization reversal, we reveal the contribution of nonuniform reversal modes and coherent rotation as a function of the angle between the applied field and the Ni easy axis. We observe two distinct critical angles revealing the significant role of nonuniform magnetization reversal in the asymmetry and its angular dependence.
The European Physical Journal B, 2005
We report on the magnetization reversal in series of exchange-biased multilayers NiFe(10.0 nm)/[Ir20Mn80(6.0 nm)/Co80Fe20(3.0 nm)]N studied by specular reflection and off-specular scattering of polarized neutrons. All specimens are sputtered and post-annealed at 530 K (i.e. above the IrMn Néel temperature of 520 K) in Ar atmosphere before cooling to room temperature in the presence of a field of 130 Oe which induces the unidirectional anisotropy. We find HEB is dependent upon the number of bilayers N as it gradually increases from 0.33 kOe for N = 1 to a considerably higher value of upto ≈0.9 kOe for N = 10. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N . Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the ferromagnetic domain structure. The enhancement of the exchange bias effect is presumably related to the shrinking of the related domain size in the antiferromagnet due to the structural evolution in the multilayers. Polarized neutron reflectometry (PNR) measurements are done at different applied fields sweeping both branches of the hysteresis loop. The spin-flip (SF) cross section of both the N = 10 and 3 samples show diffusely scattered intensity appears gradually as the field approaches HEB and is most intense where the net magnetization vanishes. The disappearance of diffuse scattering in saturation indicates that the off-specular intensity is related to the reversal process. The reversal proceeds sequentially starting with the bottom (top) CoFe layer for decreasing (increasing) field and is related to the evolution of the grain size along the stack. The reversal of each CoFe layer is for both field branches due to domain wall motion. Thus as a main result, we observe a sequential and symmetric magnetization reversal in exchangebiased multilayers. The concomitant in-plane magnetization fluctuations revealed by off-specular spin-flip scattering indicate a more complex reversal mechanism than hitherto considered. Moreover, although the grain size decreases from N = 3 to 10 by a factor of about four the reversal mechanism remains similar.
Journal of Magnetism and Magnetic Materials, 2003
We have investigated the magnetic and the structural characteristics of bi-layer exchange biasing (EB) systems NiMn-Co as function of the antiferromagnetic (AFM) film deposition parameters and of the post-deposition annealing and field-cooling procedures. The effects of sputtering pressure, growth rate, seed layer and of the annealing parameters were studied by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Magneto-Optical Kerr (MOKE) effect measurements. The results yielded optimised deposition conditions and permitted us to establish efficient annealing and field-cooling procedures. The best obtained EB systems with 5 nm Co film reveal coercive field H c X35 Oe and exchange bias field H EB X95 Oe and have good stability in air up to at least 2001C.
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 .
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.
Physical Review B, 2001
Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization vector of polycrystalline ferromagnetic ͑F͒ Fe films exchange coupled to twinned (110) MnF 2 antiferromagnetic ͑AF͒ layers. We observe a strong correlation between the temperature dependencies of the net sample magnetization perpendicular to the applied field at coercivity and exchange bias. For cooling field and measurement conditions involving magnetization reversal via rotation, we find a range of temperature dependencies. For the smoothest F-AF interface, the temperature dependence of exchange bias compares well to a Sϭ 5 2 Brillouin function-an observation predicted by some theoretical models. This temperature dependence is expected for the sublattice magnetization and the square root of the anisotropy constant ͱK 1 of bulk MnF 2. In contrast, for a rough F-AF interface the magnetization reversal process ͑and exchange bias͒ showed little temperature dependence up to temperatures approaching the AF Néel point-a clear consequence of increasing interfacial disorder in a F-AF epitaxial system.
2011
We report an investigation on the antiferromagnetic layer thickness dependence of magnetization reversal in c-axis oriented MnPd/Fe epitaxial exchange biased bilayers. Several kinds of multistep loops were observed for different samples measured at various field orientation. The evolution of the angular dependent magnetic behavior evolving from a representative Fe film to the exchange biased bilayers was revealed. With increase of the thickness of the antiferromagnetic layers, asymmetrically shaped loops and biased two-step loops are induced by exchange bias. Including the unidirectional anisotropy, a model based on the domain nucleation and propagation was developed, which can nicely describe the evolution of the magnetic behaviors for MnPd/Fe bilayers and correctly predicts the critical angles separating the occurrence of different magnetic switching processes. For fields applied along the bias direction, the 180 • magnetic reversal changes from two successive 90 • domain wall nucleations to a single 180 • domain wall nucleation at the critical thickness of the MnPd layer.
Enhanced exchange bias in ferromagnet/antiferromagnet multilayers
Journal of Magnetism and Magnetic Materials, 2005
We study a series of NiFe ð10:0 nmÞ=½Ir 20 Mn 80 ð6:0 nmÞ=Co 80 Fe 20 ð3:0 nmÞ N multilayers with different numbers N of bilayers grown by DC magnetron sputtering. After field-cooling, SQUID and MOKE measurements show a sizable increase of the exchange bias field with N. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N. Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the magnetic domain structure. We conclude that the enhancement of the exchange bias effect is related to the shrinking of the domain size in the antiferromagnet due to the structural evolution in the multilayers. r
Study of Magnetization Reversal by Minor Loops in IrMn/CoFe Exchange-Biased Bilayers
Journal of Nanoscience and Nanotechnology, 2012
We report a detailed investigation of the magnetization reversal by minor loops in Co 75 Fe 25 (t) single layer and Ir 22 Mn 78 (10 nm)/CoFe(t) exchange-biased bilayers with different CoFe thicknesses. With increasing CoFe layer thickness in IrMn/CoFe bilayers, the magnetization reversal process shows a transition from the coherent rotation to the domain-wall motion, which is attributed to the competition among the antiferromagnetic domain wall energy, ferromagnetic domain wall energy, and the interface coupling between antiferromagnetic and ferromagnetic layers.