Positive exchange bias in epitaxial permalloy/MgO integrated with Si (100)

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (100) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni82.5Fe17.5 (permalloy, Py) on Si (100) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (100). 2014 Elsevier Ltd. All rights reserved. Motivated by the proposed racetrack memory patented [1] by Parkin et al, a great deal of research activity has been devoted to Permalloy (Py) magnetic nanostructures for high frequency applications. Py usually refers to Ni1 xFex alloys that have a vanishingly small magnetocrystalline anisotropy and magnetostriction but extremely large magnetic permeability. These unique properties make Py one of the most important soft magnets for a variety of applications, such as free layers in spin-valve magnetic-reading heads [2,3]. The Py/MgO combination has been widely used in magnetic tunnel junctions (MTJs) devices. More recently, Py is being extensively used [4,5] to demonstrate electric field control of ferromagnetism (FM) in multiferroic based devices. Hence, exploring and understanding the magnetic properties of Py when interfaced with magnetic/non-magnetic oxide materials is of primary importance. To date, epitaxial Py layers have been deposited [6] on MgO bulk substrates using molecular beam epitaxy, e-beam evaporation and magnetron sputtering deposition techniques. However, MgO substrates are unsuitable from a viewpoint of practical applications in the magnetic recording industry because of its high single-crystal cost, limited wafer size, hygroscopic nature, and inferior mechanical properties. The unique feature in the present study stems from the usage of single crystal Si (100) substrates. Silicon is inexpensive and readily available, but most importantly, by using Si (100) we can achieve integration with present day microelectronic devices. The magnetic properties of Py have been extensively reported for the case where it is interfaced with anti-ferromagnetic (AFM) http://dx.doi.org/10.1016/j.cossms.2014.02.001 1359-0286/ 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA. Tel.: +1 919 515 7010. E-mail address: [email protected] (S.S. Rao). Current Opinion in Solid State and Materials Science 18 (2014) 140–146