Exchange bias like effect induced by domain walls in FeGd/FeSn bilayers

A study of exchange bias phenomenon in ferrimagnetic /ferromagnetic FeGd/ FeSn bilayers is presented. The amorphous FeSn and FeGd alloys have been grown by co-evaporation. Specific growth conditions allow to induce an uniaxial anisotropy in both alloys in a parallel direction. After saturation of the bilayers under a positive field, the hysteresis loop of one of the layer is shifted towards a positive field H _E . The sign of the exchange bias field H _E is shown to be due to the antiferromagnetic coupling between the net magnetizations of both alloys. The field H _E is studied as a function of the thickness of each layer and of the temperature. Using ac-susceptibility measurements and polarized neutron reflectometry, it is shown that the reversal of magnetization of the bilayers is dominated by the presence of a domain wall at the interface. This exchange bias system is shown to act as a potential well for the magnetic domain wall. Within this assumption and thanks to a precise magnetic characterization of each alloy, the evolution of H _E with the thickness of the layers is well reproduced using simple one-dimensional analytical models for the domain wall or a more elaborate numerical approach.Received: 20 February 2003, Published online: 9 September 2003PACS: 75.60.Ch Domain walls and domain structure - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces - 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.)     

Diluted antiferromagnets in exchange bias: proof of the domain state model

The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO layers can intentionally be increased by a factor of up to 3 if the antiferromagnetic CoO layer is diluted by nonmagnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model of a ferromagnetic layer on a diluted antiferromagnet show exchange bias and explain qualitatively its dilution and temperature dependence. These investigations reveal that diluting the antiferromagnet leads to the formation of volume domains, which cause and control exchange bias.     

Depth profile of uncompensated spins in an exchange bias system

We have used the unique spatial sensitivity of polarized neutron and soft x-ray beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet and an antiferromagnet. The net uncompensated magnetization near the interface responds to applied field, while uncompensated spins in the antiferromagnet bulk are pinned, thus providing a means to establish exchange bias.     

Symmetry theory of the flexomagnetoelectric effect in the magnetic domain walls

A local flexomagnetoelectric (A.P. Pyatakov, A.K. Zvezdin, 2009) effect in the magnetic domain walls (DWs) of the cubic hexoctahedral crystal has been investigated on the basis of a symmetry analysis. The strong connection between magnetic symmetry of the DW and the type of the distribution of the electric polarization was shown. Results were systemized in the scope of the DW chirality. It was shown, that new type of the local flexomagnetoelectric coupling corresponds to the presence of the coupled electric charge in the DW. It was found that all time-noninvariant chiral DWs have identical type of spatial distribution of the magnetization and polarization. There are coincidence between the symmetry predictions and results obtaining from the known term of the flexomagnetoelectric coupling for transverse polarization components.     

Uncompensated moments in the MnPd/Fe exchange bias system

The element-specific magnetic structure of an epitaxially grown Mn_52Pd_48/Fe bilayer showing exchange bias was investigated with atomic-layer depth sensitivity at the antiferromagnet/ferromagnet interface by soft-x-ray magnetic circular dichroism and magnetic reflectivity. A complex magnetic interfacial configuration, consisting of a 2-monolayer-thick induced ferromagnetic region, and pinned uncompensated Mn moments that reach far deeper (approximately 13 A), both in the antiferromagnet, were found. For the latter, a direct relationship with the magnitude of the exchange bias is verified by similar measurements perpendicular to the field cooling direction.     

Equations for domain walls in a coordinate system of the ferroelectric phase

Two methods are suggested for writing equations for domain walls in a coordinate system of the ferroelectric phase in ferroelastics and multiaxial ferroelectrics. The equations for domain walls in ferroelectric barium titanate and ferroelastic lead orthophosphate are derived. It is shown that suborientation states are possible in these crystals. The suggested methods make it possible to find the matrices of the transformation from the coordinate system of the paraelectric phase to a coordinate system of the ferroelectric phase for each orientation state.     

The effect of friction on nonlinear oscillations of interacting domain walls in an external periodic field

The effect of dissipation on nonlinear oscillations in a system of domain walls experiencing an external harmonic field is studied numerically. The problem is formulated for uniaxial ferromagnet films, with the easy magnetic axis being perpendicular to the surface and with the harmonic field being aligned with the axis. Account is taken of the dynamic redistribution of magnetic poles on the film surface, which enables one to derive, in a natural way, an expression for a restoring force acting on the domain walls. The force is a nonlinear function of domain-wall displacement from the equilibrium position. It is found that the domain walls may execute complicated steady-state quasi-periodic oscillations and long-term chaotic oscillations. Attractors in the phase space of the system are determined.     

Composition-controlled exchange bias training effect in FeCr/IrMn bilayers

For FeCr/IrMn bilayers, the exchange bias training effect and the magnetization reversal mechanism are correlated to each other and depend on the composition of the ferromagnetic layer. For high Fe contents, the asymmetric magnetization reversal is observed. During the training effect, both exchange field and coercivity decrease monotonically, suggesting a type I training effect. For low Fe contents, the domain wall depinning takes place for the two hysteresis loop branches. Only exchange field diminution happens in the training effect. The coercivity almost does not change in the process, corresponding to a type II training effect. It is suggested that the motion of antiferromagnetic spins is modified by the magnetization reversal mechanism in the ferromagnetic layer.     

Thermal relaxation of exchange bias field in an exchange coupled CoFe/IrMn bilayer

This paper reports that a CoFe/IrMn bilayer was deposited by high vacuum magnetron sputtering on silicon wafer substrate; the thermal relaxation of the CoFe/IrMn bilayer is investigated by means of holding the film in a negative saturation field at various temperatures. The exchange bias decreases with increasing period of time while holding the film in a negative saturation field at a given temperature. Increasing the temperature accelerates the decrease of exchange field. The results can be explained by the quantitative model of the nucleation and growth of antiferromagnetic domains suggested by Xi H W et al. [2007 Phys. Rev. B 75 014434], and it is believed that two energy barriers exist in the investigated temperature range.     

Asymmetric magnetoresistance in an exchange bias Co/CoO bilayer

Magnetoresistance (MR) measurements are carried out on a Co(8 nm)/CoO(3.5 nm) bilayer in the exchange bias (EB) state prepared by molecular beam epitaxy. With the applied magnetic field parallel to the current, the EB MR curves show an asymmetric behavior about the minimum, in contrast to the symmetric one for non-EB systems. We generalize a well-known analytical expression used for the field dependence of the MR of paramagnets. Our generalization incorporates coercivity and EB in a new phenomenological MR expression. Excellent fits of the latter to the experimental MR data are achieved, showing the way to use MR techniques for the quantitative characterization of EB systems. Furthermore, the temperature dependence of the EB field obtained from MR loops can be described with a power law, which yields a value of 96.6 K for the EB blocking temperature, which is significantly below the Néel temperature of 293 K for bulk CoO.     

The synthesis and exchange bias effect of monodisperse NiO nanocrystals

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.     

The dynamics of domain walls in an easy-plane magnet in the field of an acoustic wave

The drift of a 180° domain wall is studied in an easy-plane weak two-sublattice ferromagnet subject to an elastic-stress field generated by an acoustic wave. The dependences of the drift velocity on the amplitude and polarization of the acoustic wave are found. The conditions of the drift of a stripe domain structure are determined.     

Changing exchange bias in spin valves with an electric current

We show that a high-density electric current, injected from a point contact into an exchange-biased spin valve, systematically changes the exchange bias. The bias can either increase or decrease depending upon the current direction. This observation is not readily explained by the well-known spin-transfer torque effect in ferromagnetic metal circuits, but could be evidence for the recently predicted current-induced torques in antiferromagnetic metals.     

Edge pinning and internal phase transitions in a system of domain walls

A model consisting of an array of flexible self-avoiding domain walls extending across a two-dimensional medium is considered. Adsorption phenomena in the presence of edgepinning forces and rupture, segregation, and order-disorder transitions due to shortrange attractive and repulsive interactions between the domain walls are studied using fermion transfer-matrix methods.Heisenberg Fellow of the Deutsche Forschungsgemeinschaft     

Nonuniform and sequential magnetization reversal via domain structure formation for multilayered system with grain size induced enhanced exchange bias

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 exchange-biased 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.     

Origin of the training effect and asymmetry of the magnetization in polycrystalline exchange bias systems

The training effect and asymmetry in exchange-coupled polycrystalline CoO/Co bilayers with in-plane magnetization has been investigated. This system is selected for its large training effect and initial asymmetry of the magnetic hysteresis after field cooling, which is removed after training. Applying an in-plane magnetic field perpendicular to the cooling field largely restores the untrained state with its pronounced asymmetry. The possibility to reinduce the asymmetry strongly depends on the magnitude of the perpendicular field, providing the key to identify the physical origin of training and removal of the asymmetry. These effects result from misalignment between the ferromagnetic magnetization and the uncompensated magnetization of the granular antiferromagnet.     

Effect of the micromagnetic structure of domain walls on the properties of an isolated domain in a thin magnetic film

The effect of the inner structure of domain walls on the time-independent parameters of an isolated stripe domain in a thin ferromagnetic film is studied. The adopted variant of the perturbation theory makes it possible to account, within a unified approach, for the contributions of the magnetostatic and exchange interactions. Zh. éksp. Teor. Fiz. 115, 1315–1325 (April 1999)