Dr. Pratap K. Sahoo, Reader – F , School of Physical Science, National Institute of Science Education and Research (NISER) Bhubaneswar, Jatni, India

Dr. Pratap K. Sahoo is a Reader, School of Physical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, India. He has completed his Ph.D in 2004 from Department of Physics, Indian Institute of Technology Kanpur, India in the field of Ion Beam Physics. After completed his Ph.D., he worked as a Post-doc/visiting scientist fellow at II-Physics Institute, University of Göttingen, Germany, IKS-KU Leuven, Leuven, Belgium, Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, Switzerland, and Physikalisch- Technische Bundesanstalt, Braunschweig, Germany. He has published more than 100 papers in international reputed journals. He has presented his research work as invited talk at different international professional conferences around all over the world. He is serving as a member of board in many of international scientific societies, conferences and workshops. His current research interests are ion beam physics, new phase synthesis, materials synthesis for light emitting devices, Silicon nanostructures, Metal nanoparticles/nanodots/nanowires, Phonon tunnel devices, and electron-phonon related phenomena in nanomaterials.

Speech Title: Ion irradiation induced phase transition and magnetic softening of Co in Co/Au multilayers

Aims: The magnetic properties of individual magnetic layers in multilayers films are dependent on magneto crystalline anisotropy and exchange interactions, which dominate in the regime of higher and lower grain sizes. Here our aim is to understand structural and magnetic behavior of top layer in a ferromagnetic-nonmagnet (Co-Au) multilayer system by defect introduction by ion irradiation.

Methods: One to ten bilayers of Co and Au thin films of thickness 5 nm each were deposited on clean Si substrates using e-beam evaporation technique. 1.5 MeV Au ions were irradiated at different fulences which has synergetic influence of ballistic and thermal spike effects to modify the morphology. The temperature dependent resistivity study were performed to understand the phase changes in the system.

Results: As a function of the irradiation fluence, a gradual crossover to the hcp phase was observed in X-ray diffraction data. Rutherford back scattering and atomic force microscopy indicated some amount of ion-beam induced surface and volume segregation of Co. Bloch-Gruneisen fits to the four contact resistivity data showed a decrease in Debye temperature with evolution of hcp-phase Cobalt, consistent with the reduced modal freedom for phonons in the hcp structure compared to the fcc structure. This magnetic softening has been discussed in the light of Herzer’s random anisotropy model.

Conclusions: In conclusion we have shown that medium energy (1.5 MeV) ion irradiation is a suitable way to obtain fcc to hcp phase transition in Co thin films. Debye temperatures of Co as a function of fluence, extracted from resistivity data also corroborated to the observed fcc to hcp phase transformation in Co. This magnetic softening has been discussed in the light of Herzer’s random anisotropy model. Micromagnetic simulations of the multilayer system also corroborated these observations.

Acknowledgments: The authors acknowledge Kartik Senapati for in depth discussion, Maheswar Nayak and P. C. Pradhan for sample preparation.