M Krishnan

Effect of coherent to incoherent structural transition on magnetic anisotropy in Co/Pt multilayers

The crystallographic structure and magnetic anisotropy have been investigated in Co/Pt multilayers deposited by direct current magnetron sputtering on two different buffer layers, Ta and Pt. Detail theoretical and experimental investigations reveal the presence of three effects: magneto-elastic, interface and shape anisotropies, and their competition results in three distinct regions based on the Co layer thickness (tCo). In the region I, with tCo< 6 A, the coherent lattice strain modifies the volume anisotropy through magneto-elastic contribution and it leads to overall positive volume anisotropy energy. With further increase in tCo, lattice mismatch initiates and this results in a decrease in magneto-elastic effect and it enhances the interface anisotropy. The presence of both diminished magneto-elastic effect and enhanced interface anisotropy results in an intermediate region (region II) with negative volume anisotropy energy which is relatively wider (6 A

Magnetic properties of ordered bi-layer nanowire arrays with different Co crystallographic structures

Cobalt nanowire arrays with varying length were grown by pulse electrochemical route into the pores of alumina template. The structural investigation reveals that at the bottom of the nanowire, it grows with hcp (002) structure up to few hundred nanometer in length and then changes its crystal orientation to hcp (100) phase as it further grows along the wire axis. The variation of measured coercivity and the remanent ratio as a function of nanowire length indicate that each nanowire in array grows with three different segments. The micromagnetic simulations carried out on these structures are then correlated with experimental observations.

Anisotropic magnetic properties of bi-layered structure of ordered Co nanowire array: Micromagnetic simulations and experiments

The angular dependence of magnetic properties were investigated in Co nanowire (NW) arrays with varying length grown by pulse electrochemical route into pore of alumina template. In these investigations, three different NW lengths, 200, 400 and 1000 nm were considered. The structural investigations reveals that shorter NW of length 200 nm have hcp (002) structure, however the existence of both hcp (002) and hcp (100) was observed in longer wire length of 1000 nm, though these phases were well separated in lower and upper segments along the NW axis, respectively. Micromagnetic simulation has been carried out considering the different crystallographic structures as a function of NW length and with the variation of angle between the applied field and the NW axis. Both using experimentally measured magnetization data and micromagnetic simulations, we have inferred that in the nanowire with only hcp (002) structure, the magnetization reversal occurs by coherent rotation, whereas in the case of hcp (100) structure, the reversal phenomena occurs via vortex state and in bilayer structure, the interaction among them leads to a gradual crossover from a coherenet state to vortex along the nanowire axis. Irrespective of the NW length, the angular dependence of H

Fabrication of magnetic nanodot arrays using ultrathin alumina membrane (UTAM)

_c

Effect of Pt layer thickness on perpendicular magnetic anisotropy in ultrathin Co/Pt multilayers

was explained qualitatively using the micromagnetic simulation.

Effect of process parameters on growth rate and diameter of nano-porous alumina templates

Fabrication of ultrathin alumina membranes were carried out using two step anodization from high purity aluminum foil. This membrane provides highly ordered nano pores with diameter of 80 nm and intra-pore distance of the order 110 nm. The membrane with thickness < 100 nm was used as a evaporation mask for the growth of magnetic nanodot arrays of Co, SmCo5 and CoPt on Si substrates. The structural investigations were carried out using Field Emission Scanning Electron Microscopy (FESEM). The elemental composition of nanodot arrays was confirmed using Energy Dispersive X-Ray (EDX) analysis. Magnetic characterizations were carried out to study the in-plane and out of-plane hysteresis behaviors and the coercive fields. The present novel two-step anodization approach is simple, efficient, and provides an inexpensive non-lithographic method to produce high density nano-structured materials for wide range of applications in nanotechnology and spintronics.

In situ electrochemical thinning of barrier oxide layer of porous anodic alumina template

Co/Pt multilayer structures with varying Pt layer thicknesses from 1.5 to 12 A were deposited on Ta buffer layer by DC magnetron sputtering technique and both of their magnetic and structural properties were investigated. All the multilayers showed perpendicular magnetic anisotropy (PMA) and the enhancement in PMA is observed with increase in the Pt layer thickness. The structural investigation using XRD reveals the presence of single peak in all these multilayers, which indicate the formation of coherent Co/Pt (111) structure. The origin of PMA in these multilayer structure is due to the strain induced in the Co layer. From the full-width-half-maximum (FWHM) of the XRD patterns it is observed that the grain size is increasing with increasing the Pt layer thickness. The influence of the enhancement of the grain size is then correlated with the measured M-H hysteresis behavior and subsequently the anisotropy energy.