Doctoral defence in Physics - Induced uniaxial magnetic anisotropy in polycrystalline, single crystal and superlattices of permalloy

WHEN:Thu, 12/12/2019 -13:30 to 15:30
WHERE: Aðalbygging - The Aula

Ph.D. student: Movaffaq Kateb

Dissertation title: Induced uniaxial magnetic anisotropy in polycrystalline, single crystal and superlattices of permalloy

Dr. Sigurður Ingi Erlingsson, Professor at Reykjavík University.
Dr. Victor Kuncser, Head of Laboratory of Magnetism and Superconductivity, The National Institute of Materials Physics, Bucarest, Romania.

Advisor: Dr. Snorri Þorgeir Ingvarsson, Professor at the Faculty of Physical Sciences, University of Iceland.

Doctoral committee: Dr. Jón Tómas Guðmundsson, Professor at the Faculty of Physical Sciences, University of Iceland.
Dr. Friðrik Magnus, Research Scientist at the Science Institute, University of Iceland.

Chair of Ceremony: Dr. Oddur Ingólfsson, Professor and the Head of Faculty of Physical Sciences, University of Iceland.

Permalloy Ni80Fe20 at. % (Py) is a well-known ferromagnet utilized in anisotropic magnetoresistance and planar Hall effect sensors as well as magnetic read heads in magnetic memories. These applications rely on the uniaxial magnetic anisotropy in permalloy thin films. Various methods have been developed to achieve uniaxial anisotropy such as post-annealing, in-situ growth and ion beam irradiation in the presence of a magnetic field, deposition under an angle with respect to the substrate normal and mechanical deformation. Among these we found tilt deposition as the most general case since most of ferromagnetic films are grown in co-deposition vacuum systems under an angle.
In the first stage we compared the effect of tilt deposition with that of applying in-situ magnetic field, in definition of the easy magnetization axis direction, and showed that in a competition of both effects the former has a major effect (paper I). It has been also shown that tilt deposition gives very well-defined uniaxial anisotropy in a wide range of pressures (paper II). Since the sputter flux might scatter off at higher pressures, utilizing high power impulse magnetron sputtering (HiPIMS) is required to provide higher adatom energy and maintain high mass density and magnetic softness of the Py films. Then it is shown that very well-defined uniaxial anisotropy can be achieved in a series of Py films, with variable thicknesses. This is achieved in the absence of self-shadowing and off-normal texture which have been mentioned in the literature to explain the origin of uniaxial anisotropy (paper III). None of these, however, explained the origin of induced uniaxial anisotropy in Py films. Thus in the second stage we grew single crystal Py films on (001) MgO using tilt deposition and compared conventional dc magnetron sputtering (dcMS) and HiPIMS (paper IV). Based on X-ray diffraction (XRD) and resistivity measurements it has been demonstrated that single crystal films prepared by dcMS present an ordered microstructure towards L12 Ni3Fe superlattice while the very high deposition rate of HiPIMS (more 50 times of dcMS during the pulse) gives a disordered single crystal. Surprisingly, the single crystal prepared by HiPIMS showed uniaxial behavior along the ⟨001⟩ orientation. The more ordered single crystal grown by dcMS presented biaxial anisotropy along the ⟨011⟩ orientation which is in agreement with magnetocrystalline anisotropy along the ⟨111⟩ orientation and being forced into the film plane by the demagnetization field (shape anisotropy). It is worth mentioning that previously post annealing and in-situ magnetic field failed to induce uniaxial anisotropy along the ⟨001⟩ orientation. The latter controversy can be explained by the fact that normally very low deposition rates are utilized for the growth of single crystals. Thus they were unable to achieve enough disorder required for uniaxial anisotropy to appear. Later it is shown that although it is very hard to detect the atomic order using XRD in polycrystalline films, one can utilize resistivity measurements to study the microscopic origin of uniaxial anisotropy (paper V). It has been shown that the resistivity of a film prepared by in-situ field is minimum along the hard axis and maximum along the easy axis. We explain this by atomic arrangement of Ni and Fe (or order) along the easy axis of the film.
In order to study interface or surface anisotropy, we prepared multilayers of Py using dcMS and HiPIMS. It was shown that a sharp interface can be achieved using the HiPIMS method. However, this is accompanied by large strain in the case of Py/Pt that increases coercivity and gives an open hard axis. Using Cu and CuPt present well defined uniaxial anisotropy.

About the doctoral candidate:

Movaffaq graduated from the Amirkabir University of Technology in Materials Science, where he began modelling and simulation of physical properties of nanoscale systems.

Then he attended Nanotechnology Engineering at Tarbiat Modares University with one of the pioneering groups in Nanotechnology worldwide. During his master studies, he solved the long-standing problem of simultaneous high contrast and ultrafast switching in electrochromic displays using an array of core-shell nanowires and designed a facile fabrication method for their synthesis.

Movaffaq is married to Sahar Safarian and they have 3 sons: Amirhossein, Taha and Yasin.

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