Pv Paresh Paluskar

Correlation between magnetism and spin-dependent transport in CoFeB alloys

We report a correlation between the spin polarization of the tunneling electrons and the magnetic moment of amorphous CoFeB alloys. Such a correlation is surprising since the spin polarization of the tunneling electrons involves s-like electrons close to the Fermi level (E_{F}), while the magnetic moment mainly arises due to all the d electrons below E_{F}. We show that probing the s and d bands individually provides clear and crucial evidence for such a correlation to exist through s-d hybridization, and demonstrate the tunability of the electronic and magnetic properties of CoFeB alloys.

Spin tunneling in junctions with disordered ferromagnets

We provide compelling evidence to establish that, contrary to ones elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is larger than that of fcc CoFeB. First-principles atomic and electronic structure calculations reveal striking agreement between the measured TSP and the predicted s-electron spin polarization. Given the disordered structure of the ternary alloy, not only do these results strongly endorse our communal understanding of tunneling through AlO(x), but they also portray the key concepts that demand primary consideration in such complex systems.

Magnetism in Co80-xFexB20: Effect of crystallization

We report on the change in the structural and magnetic properties of magnetically soft ternary Co80-xFexB20 alloys as a function of composition, thickness, and annealing temperature. Compositions high in cobalt show a significant change in coercivity after annealing. This is explained using the random anisotropy model by relating the magnetic exchange length to the grain size of the crystallites. The presented results are a systematic study explaining trends seen in the transition from soft to hard magnetic behavior, providing insight into why the soft CoFeB alloys have been so successful recently in spintronic devices.

Co72Fe20B8 : structure, magnetism, and tunneling spin polarization

We present measurements of tunneling spin polarization of Co72Fe20B8 in Al∕AlOx∕CoFeB tunnel junctions using superconducting tunneling spectroscopy. We consistently observe high spin polarization (above 53%), higher than that measured for comparable CoFe alloys, indicating a crucial role played by the 8% atomic boron. X-ray diffraction measurements on 700A thick films reveal an amorphous structure for as-deposited films, while anneals above 250°C initiate a gradual crystallization. Superconducting quantum interference device magnetometery shows that the magnetic moment exhibits minor changes after anneals, but there is a dramatic change in the coercivity after anneals above 400°C. However, these structural and magnetic changes induce only a slight change in the tunneling spin polarization of CoFeB.

Mn diffusion and the thermal stability of tunneling spin polarization

We examine the role of Mn diffusion in the thermal stability of tunneling spin polarization P by directly measuring P of Al∕AlOx∕Co∕FeMn and Al∕AlOx∕Co90Fe10∕FeMn junctions using superconducting tunneling spectroscopy (STS). We confirm Mn diffusion in our junctions using x-ray photoelectron spectroscopy after an ultrahigh vacuum 500°C anneal. Surprisingly, and in contrast to the current belief, no drop in P is observed using STS. Therefore, though Mn diffuses significantly, it cannot be solely responsible for the drop in tunneling magnetoresistance observed after postdeposition anneals above 300°C.

Influence of interface structure on the tunnelling spin polarization of CoFeB alloys

We present measurements of tunnelling spin polarization (TSP) of CoFeB alloys in Al/AlOx/CoFeB tunnel junctions using superconducting tunnelling spectroscopy. We observe that though Co72Fe20B8 (at.%) layers crystallize in the fcc structure, in junctions annealed above 300 °C the measured TSP degrades, indicating that in these junctions the TSP of amorphous CoFeB is higher than its crystalline counterpart.

Impact of interface crystallization on inelastic tunneling in Al/AlOx/CoFeB

We report the change in inelastic electron tunneling spectra (IETS) for Al∕AlOx∕CoFeB∕Al junctions when the structure of CoFeB at its interface with AlOx is intentionally changed from quasiamorphous to highly textured fcc. While for the quasiamorphous interface there are signs of the size quantization of magnons, the spectra for the fcc interface show distinct excitations at bias voltages associated with known surface magnon modes in fcc Co. These results demonstrate that IETS can be used as a tool to probe distinct structural changes of the magnetic electrode in tunnel junctions.