Amitesh Paul

Microscopic origin of training in exchange bias system

The microscopic origin of training in exchange coupled systems has been identified from our experimentally observed microscopic suppression of training. It is an interplay of uniaxial anisotropy and uncompensated spins in the antiferromagnet grains that are rotatable in polycrystalline antiferromagnetic layer similar to spin-glass-like behavior.

Analysis of randomly oriented structures by grazing-incidence small-angle neutron scattering

A formalism is presented which allows the quantitative evaluation of data from grazing-incidence small-angle neutron and X-ray scattering – GISANS and GISAXS – in the framework of the distorted wave Born approximation. While several aspects have been reported previously, this formalism combines solutions for scattering intensities in both reflection and transmission hemispheres, taking into account instrumental resolution effects. This formalism is applied to the case of GISANS from self-organized diblock copolymers, ordered in perpendicular lamellar structures on an Si wafer in randomly oriented short-range-ordered regions. The periodicity of D = 85u2005(9)u2005nm found for deuterated polystyrene–polybutadiene of molecular weight xa0Mw = 165u2005kgu2005mol−1 and a molecular weight fraction of the deuterated polystyrene block of 52% is consistent with atomic force microscopy and specular neutron reflectivity results.

Can uniaxial anisotropy be responsible for training in exchange coupled system

Training in exchange bias can be related to the field cooled state of the ferromagnet—multidomain or single domain. In a system of coexistent states, realized by coercive field cooling, we could observe usual training for the single domain portion while the multidomain portion remains untrained. This crossover state indicates to the fact that antiferromagnetic uniaxial anisotropy can be made responsible for the training in exchange coupled system where no biaxial anisotropy could exist.

Magnetization reversal and domain correlation for a non-collinear and out-of-plane exchange-coupled system

We have investigated the impact of out-of-plane ferromagnetic (FM) anisotropy (which can be coincident with the direction of unidirectional anisotropy), where antiferromagnetic (AF) anisotropy is along the film plane. This provides a platform for non-collinear exchange coupling in an archetypal exchange coupled system in an unconventional way. We probe the in-plane magnetization by the depth-sensitive vector magnetometry technique. The experimental findings reveal a magnetization reversal (i) that is symmetric for both the branches of the hysteresis loop, (ii) that is characterized by vertically correlated domains associated with a strong transverse component of magnetization and (iii) that remains untrained (suppression of trained state) with field cycling. This scenario has been compared with in-plane magnetization reversal for a conventional in-plane unidirectional anisotropic case in the same system that shows usual asymmetric reversal and training for vertically uncorrelated domains. We explain the above observations for the out-of-plane case in terms of inhomogeneous magnetic states due to competing perpendicular anisotropies that result in non-collinear FM–AF coupling. This study provides direct evidence for the vertical correlation of domains mediated by out-of-plane exchange coupling.

Microscopic picture of magnetic correlation with loss of uniaxial anisotropy upon swift ion beam irradiation in an interlayer coupled system

Exclusive evidence of magnetic structure modification has been revealed by depth-sensitive polarized neutron scattering measurements in an antiferromagnetically coupled specimen upon swift heavy ion irradiation. The effects are independent of any structural modifications within the system (such as intermixing or roughness) that may normally have contributed to the changes in magnetic structure as well. The modifications include (a) the loss of uniaxial anisotropy, (b) changes in the variation of the mean magnetization angle and (c) changes in the variation of the lateral and longitudinal magnetic correlation lengths. We explain the changes within the thermal spike model. The possibility of controlling local magnetization can be foreseen from these results, which can be done even without damaging the structure.

Chemical and magnetic profile of magnetic semiconductors as probed by polarized neutron reflectivity

We investigate the depth dependence of magnetic species in co-doped dilute magnetic semiconductor (DMS) specimens. We also compare the reliability of the nuclear and magnetization depth profile of these specimens on different polarized neutron reflectivity instruments. These reflectometers were not only angle dispersive instruments but also wavelength dispersive instruments. Measurements indicate a segregation of DMS species onto the top into the cap layer for lower Fe doping. However, for higher Fe doping, as we alter the doping concentrations of magnetic species, no segregation and lower clustering tendencies are indicated. Thus sensitive depth profiling, for specimens with such low magnetic-ion concentrations, may not be a challenge for brighter sources and careful measurements. Our results show that homogeneity can be achieved with Fe co-doping within a Ge matrix which can make it a potential candidate for spintronic applications.