Magnetization reversal in exchange-biased Ni∕NiO layered structures (original) (raw)

Magnetization reversal processes in patterned exchange biased NiO/Ni samples

We have investigated the magnetization reversal of arrays of exchange biased NiO/Ni squares with SQUID magnetometry. The edges of the squares were 0.5, 1.5 and 3.0 µm long. The NiO/Ni structures exhibit hysteresis loops typical of vortex states and the exchange bias field behaves very unusual: for the arrays with structures of 1.5 µm edge length the sign of the exchange bias field changes, as compared to the same continuous NiO/Ni layer. We attribute this to the interplay between shape and unidirectional anisotropy. In the exchange biased nanostructures the unidirectional anisotropy causes a deformation of the whole hysteresis and not merely a uniform shift.

Direct observation of magnetization reversal behaviors in exchange-coupled NiO/Fe films

Journal of Applied Physics, 2010

We have investigated the magnetization reversal behavior in exchange-coupled NiO/Fe films with varying the NiO thicknesses using a magneto-optical microscope magnetometer capable of direct domain observation in real time. Interestingly enough, the magnetization reversal mechanism is gradually changed from a domain wall-motion process to a nucleation process as the NiO thickness increases. This result clearly demonstrates that the exchange coupling effect between the NiO and Fe layers increases the domain wall pinning effect of the Fe layer, resulting in the nucleation reversal mode.

Field dependent exchange coupling in NiO/Co bilayers

Physical Review B, 2003

Dynamic magnetization reversal measurements have revealed a strong dependence of the exchange-coupling strength on the maximum applied field for NiO/Co bilayers. Time-resolved Kerr measurements, performed at room temperature, show that the coercive field increases linearly with the maximum applied field between two reversals. If the maximum applied field is different in the positive and negative directions, an exchange bias is observed as well as an asymmetry in the magnetization reversal behavior between the two sides of the hysteresis loop. X-ray magnetic circular dichroism measurements at the Ni L 2,3 edges indicate that these effects are related to an increase of the net uncompensated Ni moment with increasing field. These effects should also be present in other low anisotropy antiferromagnetic/ferromagnetic systems.

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

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.

Field Cooling Induced Changes in the Antiferromagnetic Structure of NiO Films

Physical Review Letters, 2001

The magnetic anisotropy in antiferromagnetic 500 Å thick NiO films, before and after the establishment of an exchange bias field with Co 84 Fe 16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both ͗111͘ textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction.

Dynamical properties of magnetization reversal in exchange-coupled NiO/Co bilayers

Physical Review B, 2001

We present a dynamical study of hysteresis loops of a MoS,/[Au/Co/Au] sandwich performed by surface magnetooptical Kerr effect with a field variation rate up to 1.2 Moe/s. An interpretation of dynamical effects at room temperature is proposed, using a modelization of the magnetization reversal. We discuss simulations which describe two different processes of the magnetization reversal to interpret the evolution of the hysteresis loops for several rates of variation of the magnetic field. For a first range of field variation rates lower than 180 kOe/s, the predominant mechanism seems to be wall motion and beyond 180 kOe/s, an expression for the magnetization is given, which supposes micro-domains reversal as a prevailing process. Finally, the general behaviour of the relaxation time, depending on the magnetic field, is investigated. 0304~8853/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO304-8853(95)00515-3 L6 4. Raquet et al./Journal ofMagnetcsm and Magneiic Materiais 150 (1995) Ls-ti2 B. Raquet et al./Journal of Magnetism and Magnetic Materials 150 (1995) L5-L12 Lll

Magnetization reversal via symmetric rotation of layers in exchange biased multilayers

Journal of Applied Physics, 2007

We have investigated the magnetization reversal for exchange coupled polycrystalline [IrMn/CoFe]N multilayers. Polarized neutron reflectivity (PNR) data indicate a simultaneous coherent rotation of all ferromagnetic layers for a sample with N =10 and angles of 45° and 90° between the applied field and the exchange bias direction. On the other hand, magneto-optic Kerr effect (MOKE) measurements, which are sensitive mainly to the two topmost bilayers, reveal a variation of the strength of the exchange bias and the uniaxial anisotropy as a function of N for multilayers with N =1 up to 10. The MOKE data thus indicate the direction of the magnetization to vary from layer to layer for intermediate fields. PNR was found to be insensitive to this variation as the deviation of the layer magnetization directions from its mean value is relatively small (≈10°). These studies demonstrate how the complementary techniques PNR and MOKE can be used to obtain a layer-by-layer vector magnetometry of multilayer stacks.

Thermally assisted reversal of exchange biasing in NiO and FeMn based systems

Applied Physics Letters, 1998

The stability of the exchange bias field H eb has been studied for magnetron sputtered NiO/Ni 66 Co 18 Fe 16 and Ni 66 Co 18 Fe 16 /FeMn bilayers. A forced antiparallel alignment of the ferromagnetic magnetization to H eb results in a gradual decrease of H eb as a function of time for NiO as well as FeMn based samples. The observed decrease of H eb increases with temperature and is interpreted as a thermally assisted reversal of magnetic domains in the antiferromagnetic layer.