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

Abstract

a b s t r a c tIn 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 (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni 82.5 Fe 17.5 (permalloy, Py) on Si (1 0 0) 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 (1 0 0). Ó 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 Ni 1Àx Fe x 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...

Multifunctional heterostructures integrated on Si (100)

“…The authors believe that this is not the case here as our STEM-Z images show clean and sharp interfaces with no NiO present. Such positive exchange bias has been observed in the authors' earlier work [46][47][48] on Ni 82·5 Fe 17·5 /MgO/TiN/Si (100) when the sample was zero field cooled. The observed variation in the nature of exchange bias may be assigned to the morphologyassociated residual strain and magnetic anisotropy.…”

“…The authors have epitaxially integrated Py with Si (100), as would be appropriate for integration with present-day microelectronic devices. [46][47][48] From the OOP and IP XRD measurements, it was found that Py films were grown epitaxially. The SEM and AFM characterisations of the film morphology revealed that the films of 6000 and 12 000 pulses were surprisingly similar, which may indicate that ablation rates from the Py target were dropping significantly over time.…”

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