Ritesh K. Das

Structure and magnetism of cobalt-doped ZnO thin films

The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions. Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30?at.%. The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice. Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry. Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD. These particles are well oriented with the ZnO crystal structure. By increasing the base pressure of the vacuum chamber to pressures above 1?10?5?Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ~0.08??B/Co. Depositions in the presence of oxygen background gas at 0.02?mTorr decreases the magnetization. The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO. Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation. Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out.

Ferromagnetism in pseudocubic BaFeO3 epitaxial films

Growth and properties of epitaxial BaFeO3 thin films in the metastable cubic perovskite phase are examined. BaFeO3 films were grown on (012) LaAlO3 and (001) SrTiO3 single crystal substrates by pulsed-laser deposition. X-ray diffraction shows that films grown between 650 and 850°C yield an oxygen-deficient BaFeO2.5+x pseudocubic perovskite phase that decreases in lattice spacing with increasing growth temperature. Magnetization measurements on as-deposited BaFeO3 films indicate weakly ferromagnetic behavior. Annealing in 1atm oxygen ambient converts them into conductive and robustly ferromagnetic pseudocubic BaFeO3 phase with TC=235K. Observation of ferromagnetism with increasing oxygen content is consistent with superexchange coupling of Fe+4–O–Fe+4.

Finite size effects with variable range exchange coupling in thin-film Pd/Fe/Pd trilayers

Abstract The magnetic properties of thin-film Pd/Fe/Pd trilayers in which an embedded ∼ 1.5 A - thick ultrathin layer of Fe induces ferromagnetism in the surrounding Pd have been investigated. The thickness of the ferromagnetic trilayer is controlled by varying the thickness of the top Pd layer over a range from 8 to 56 A . As the thickness of the top Pd layer decreases, or equivalently as the embedded Fe layer moves closer to the top surface, the saturated magnetization normalized to area and the Curie temperature decreases, whereas the coercivity increases. These thickness-dependent observations for proximity-polarized thin-film Pd are qualitatively consistent with finite size effects that are well known for regular thin-film ferromagnets. The critical exponent β of the order parameter (magnetization) is found to approach the mean field value of 0.5 as the thickness of the top Pd layer increases. The functional forms for the thickness dependences, which are strongly modified by the nonuniform exchange interaction in the polarized Pd, provide important new insights to understand nanomagnetism in two-dimensions.

A collective dynamics description of dipolar interactions and the coercive field of magnetic nanoparticles

The effect of dipolar interactions on the coercive field of Ni nanoparticles embedded as layers in a Al2O3 host matrix is discussed. The results for two sets of 5 layer samples with different interlayer spacings and a set of single layer samples are compared for samples with particle size varying from 3 nm (single domain) to 60 nm (multiple domain). The dipolar interactions are strongest in the samples with shorter interlayer distances and weakest for the single layer samples. Our observation that dipolar interactions increase the coercive field and decrease the critical diameter separating single domain from multiple domain behavior reinforces a description based on collective dynamics.