Control of Magnetic Coercivity in epitaxial Ni/VO 2 /YSZ/Si(001) heterostructures by manipulation of Ni thin film growth
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Control of Magnetic Coercivity in epitaxial Ni/VO2/YSZ/Si(001) heterostructures by manipulation of Ni thin film growth modes Gabrielle M. Foley1, Srinivasa Rao Singamaneni1,2, John Prater1,2 and Jay Narayan1 1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA 2 Materials Science Division, Army Research Office, Research Triangle Park, North Carolina 27709, USA
ABSTRACT The control of ferromagnetic properties by external stimuli is of great interest in the electronics community. One method of producing such a control is through proximity of a ferromagnetic film with a material that has a semiconductor-to-metal transition (SMT). In order for these magnetic heterostructures to be beneficial, they must consist of high-quality, crystalline films. Epitaxial films increase the reproducibility of both devices and properties. We have investigated the trend in magnetic coercivity in epitaxial nickel films on VO2. We show that not only does the interaction between the Ni and VO2 change the normal coercivity trend found in Ni M-H curves with no proximity to VO2, but that the crystalline growth mode of the Ni film also impacts the magnetic coercivity as a function of temperature. INTRODUCTION VO2 is a strongly correlated oxide, which undergoes a first-order semiconductor-to-metal transition (SMT) and a structural phase transition (SPT) (monoclinic to rutile) well above room temperature at 340K [1]. This transition makes VO2 a potential material for high-temperature sensor applications. Previous works have shown that the stress associated with structural changes across the SMT in VO2 can produce significant changes in magnetic properties of overlayer ferromagnetic films such as Ni [2-4]. This control of the magnetic properties could be very important for many technological applications. However, the current use of r-sapphire and αAl2O3 as substrates can be restrictive in the microelectronics industry [2-4]. In addition, all the previous works focused their studies on polycrystalline Ni and VO2 films, which do not allow for precise control of the associated properties due to poor reproducibility of polycrystalline films [2-4]. To address the above issues, we have investigated the epitaxial growth conditions of thin (10nm) Ni films on VO2/YSZ/Si(001) epitaxial heterostructures and how the thin film growth modes affect the magnetic properties of the Ni. During thin film deposition, crystalline films can nucleate and grow under one of three different crystal growth modes [5]. The first mode is called the Volmer-Weber (or island) mode where islands of atoms nucleate and form on the substrate due to a stronger bond between the film atoms than between the atoms and the substrate [5]. As this growth continues, the islands increase in size, propagate out and eventually connect. The second mode is known as the Frank-van der Merwe (or 2D layer) mode [5]. This mode occurs when the atoms of the film have a stronger bond to the substrate and therefore desire to form a
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