Magnetization reversal with variation of the ratio of the anisotropy energies in exchange bias systems

Abstract

We systematically vary the ratio of the exchange and ferromagnetic anisotropies in a single multilayered system in finally unravelling the mysteries of magnetization reversal of exchange coupled systems. This is particularly possible due to increasing unidirectional exchange anisotropies in our multilayer system which we saw earlier from sequential switching of exchange coupled layers along the stack with increasing applied field strengths. Here, by introducing different directions ͑͒ of the applied field with respect to the unidirectional anisotropy direction, we have varied two different energy parameters: ͑i͒ the exchange anisotropy for each layer, ͑ii͒ the ferromagnetic anisotropy. Our polarized neutron measurements thus show a gradual crossover from layer flipping ͑domain-wall motion͒ for low torque regimes of the effective field to coherent rotation for high torque regimes. We explain these findings within a general and simple model where the angular dependencies for reversal are guided by the relative strengths of Zeeman, exchange, and anisotropy energies.

Microscopic origin of training in exchange bias system

“…1 Exchange biasing has a significant technological importance in magnetic sensorics and high-density magnetic data storage, which demands a complete under standing of various other related phenomena, such as coercivity enhancement, asymmetric hysteresis loops, and training effect. 2, 3 Within the domain state model, 4 the exchange field is provided by the remanent magnetization of domains in the AF during different field cooling procedures. This remanent magnetization is influenced by the magnetization state of the FM, i.e., if the FM is divided into almost equal proportion of domains ͑multidomain͒ or it is already in a saturated state ͑single domain͒.…”

“…Details on similar sample growth and sample characterization ͑structural and magnetic͒ has been published earlier. 3 The neutron scattering experiments were performed at the polarized neutron reflectometer with polarization analysis 12 TREFF at the FRM-II for a wavelength of 4.73 Å. In specular geometry the angle of incidence ␣ i equals the final angle ␣ f , and is the wavelength from the monochromator.…”

Depth-resolved investigation of the lateral magnetic correlations in a gradient nanocrystalline multilayer

“…Specular reflection of polarized neutrons has been shown to be a method of choice for the quantitative, depth-resolved determination of the amplitudes and directions of the magnetizations in magnetic multilayers, leading to precious informations, for example on multilayers with interlayer exchange coupling, 12,13 on exchange bias systems, [14][15][16] on exchange spring magnets, 17 on the magnetism of the interface between a ferromagnet and a superconductor, 18,19 or on dipolarly coupled magnetic multilayers. 20 In the specular reflection geometry the wave vector transfer Q of the radiation is perpendicular to the sample surface and only laterally averaged informations can be deduced.…”

Reversal mechanism and suppression of training in an exchange-coupled system

“…19 All four polarization channels of the specular reflectivities were recorded by a single 3 He detector at different external fields H a . The measurements were performed in time-of-flight ͑TOF͒ mode for two different incident angles 0.4°and 1.0°covering the normal wave vector transfer Q Ќ = 2 ͓sin͑␣ i ͒ + sin͑␣ f ͔͒ range from 0.005 to 0.1 Å −1 .…”

“…3 However, a comprehension of many other interesting aspects such as the distribution of uncompensated spins at the interface and the exchange interaction of the FM with the uncompensated spins 4,5 still remains elusive for half a century now. The phenomenon of exchange bias depends predominantly upon the state of the interface, at which the AF-FM spins are frozen in, as they are field cooled, provided that the AF domain size remains unchanged with respect to its initial state.…”