Suman Mandal

Finite size versus surface effects on magnetic properties of antiferromagnetic particles

The observation of finite magnetic moment in antiferromagnetic materials is quite unusual and has been immensely investigated in nanoparticle systems. Here, the structural and magnetic properties of NiO particles are explored by x-ray diffraction, extended x-ray absorption fine structure, and magnetization measurements. Using similar-sized particles with different surface defect structure, we show that the observed magnetic enhancement, which is present even beyond finite-size limit, is due to the surface effects. However, the well known spin glass freezing is found to occur only in nano-regime.

Direct view at colossal permittivity in donor-acceptor (Nb, In) co-doped rutile TiO2

Topical observations of colossal permittivity (CP) with low dielectric loss in donor-acceptor cations co-doped rutile TiO2 have opened up several possibilities in microelectronics and energy-storage devices. Yet, the precise origin of the CP behavior, knowledge of which is essential to empower the device integration suitably, is highly disputed in the literature. From spectromicroscopic approach besides dielectric measurements, we explore that microscopic electronic inhomogeneities along with the nano-scale phase boundaries and the low temperature polaronic relaxation are mostly responsible for such a dielectric behavior, rather than electron-pinned defect-dipoles/grain-boundary effects as usually proposed. Donor-acceptor co-doping results in a controlled carrier-hopping inevitably influencing the dielectric loss while invariably upholding the CP value.

High photon energy spectroscopy of NiO: Experiment and theory

We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method in conjunction with the standard fully localized limit (FLL) and around mean field (AMF) double-counting alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO. We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.

Microscopic investigation of surface and interfacial magnetic domain structure of Fe–NiO(1 0 0) system

We report our experimental observation of microscopic modification, reconstruction and evolution of the antiferromagnetic domain structure of the NiO(1?0?0) surface in view of the exchange bias effect. Some domain patterns (called non-equilibrium domains) as observed on the as-cleaved surface do not follow the well-known bulk symmetry traces on the (1?0?0) surface. But, bulk-terminated domains (called equilibrium domains) are found to be renucleated up on cooling the sample from above the N?el temperature, unless domains are strongly pinned by crystalline defects. We also observe certain domain evolution after annealing the crystal at various temperatures above the N?el temperature. Influence of growth condition on the ferromagnetic domain structure of the Fe film deposited on NiO(1?0?0) has been followed in situ. While for thin film case (7?ML), no influence of growth has been observed, for thicker film (21?ML) the Fe domain structure is found to be determined by both growth-induced and exchange anisotropy. Thus, our observations depict a thickness-dependent interplay between growth-induced and exchange anisotropy in ferromagnetic film grown on the antiferromagnetic substrate.

Surface spin orientation of NiO(100) and interfacial coupling of Fe/NiO(100) revisited with soft X-ray spectromicroscopy

Accurate retrieval of spin-axis orientation at the antiferromagnetic (AFM) surface requires involved consideration of the crystal field effect in X-ray magnetic linear dichroism (XMLD), which was neglected until recently. Here, we present a unique determination of surface spin-axes of the prototype antiferromagnet NiO(100) from detailed angular-dependent measurements using different polarizations of incident light by considering the recently developed angular dependence of the XMLD effect. The bulk twin domains terminating on the (100) surface have also been determined from the angular dependence of the experimental contrast at the oxygen K edge. The effect of Fe deposition on the AFM domain pattern was followed in situ and the interfacial exchange coupling of as-deposited Fe/NiO(100) has been explored using the recent formalism of XMLD. Unlike Co/NiO(100), we realize only the rough perpendicular coupling between Fe moments and compensated Ni spin-axes. The uncompensated spins (UCS) at the interface were also characterized and a mechanism of interfacial coupling is suggested.

Magnetic skin layer of NiO(100) probed by polarization-dependent spectromicroscopy

Using polarization-dependent x-ray photoemission electron microscopy, we have investigated the surface effects on antiferromagnetic (AFM) domain formation. Depth-resolved information obtained from our study indicates the presence of strain-induced surface AFM domains on some of the cleaved NiO(100) crystals, which are unusually thinner than bulk AFM domain wall widths (∼150 nm). Existence of such magnetic skin layer is substantiated by exchange-coupled ferromagnetic Fe domains in Fe/NiO(100), thereby evidencing the influence of this surface AFM domains on interfacial magnetic coupling. Our observations demonstrate a depth evolution of AFM structure in presence of induced surface strain, while the surface symmetry-breaking in absence of induced strain does not modify the bulk AFM domain structure. Realization of such thin surface AFM layer will provide better microscopic understanding of the exchange bias phenomena.

Theoretical Investigation Of The Spin Reorientation Transition In Epitaxial Films Of NiO

We study theoretically the spin reorientation transition in thin epitaxial films of NiO grown on Ag and MgO. Experimentally, the Ni spins point out‐of‐plane for films grown on the MgO substrate and in‐plane for those on the Ag substrate. We successfully explain these opposite trends in the ground state moments for the two cases and simulate the isotropic x‐ray absorption spectrum (XAS) for bulk NiO, in good agreement with experiments, on the basis of cluster configuration interaction calculations based on a NiO6 cluster.We study theoretically the spin reorientation transition in thin epitaxial films of NiO grown on Ag and MgO. Experimentally, the Ni spins point out‐of‐plane for films grown on the MgO substrate and in‐plane for those on the Ag substrate. We successfully explain these opposite trends in the ground state moments for the two cases and simulate the isotropic x‐ray absorption spectrum (XAS) for bulk NiO, in good agreement with experiments, on the basis of cluster configuration interaction calculations based on a NiO6 cluster.