Physics Student Captures National Award at AVS Symposium

Andrada Mandru

Physics & Astronomy graduate student Andrada Mandru won one of three national-level named student awards in October at the AVS (American Vacuum Society) International Symposium in San Jose, CA. She made two presentations at the symposium and also won a second award in the Magnetic Interfaces & Nanostructures Division.

Mandru received the Russell and Sigurd Varian award, considered by many as the top-most named student award, which recognizes and encourages excellence in continuing graduate studies in the sciences and technologies of interest to AVS. This award was established in 1982 to commemorate the pioneering work of Russell and Sigurd Varian in the field of vacuum science and technology. It is supported by Agilent Technologies Vacuum Products Division. In competing for this award, Mandru made a special presentation to a committee of experts in which she talked in general about her dissertation work at Ohio University.

Mandru is the first Ohio University student to win the Varian award. Her adviser is Dr. Arthur Smith, Professor of Physics & Astronomy.

In addition, Mandru received the Leo M. Falicov Student Award in the Magnetic Interfaces & Nanostructures Division. The Falicov Award was established in memory of Professor Leo M. Falicov to recognize outstanding research performed by a graduate student in areas of interest to the MIND, especially in the area of magnetism

Each of the two awards carries a cash prize and a certificate, and she received in addition travel support to attend the AVS symposium.

The AVS International Symposium and Exhibition addresses cutting-edge issues associated with materials, processing, and interfaces in the research and manufacturing communities. The symposium fosters a multidisciplinary environment that cuts across traditional boundaries between disciplines, featuring papers from AVS technical divisions, technology groups, and focus topics on emerging technologies. The equipment exhibition is one of the largest in the world and provides an opportunity to view the latest products and services offered by 200+ participating companies. More than 2,000 scientists and engineers attend from around the world.

Mandru AVS Symposium Abstract

Transitioning into the Ga-rich Regime of Ferromagnetic Manganese Gallium Films Grown on Gallium Nitride: Structure and Magnetism, Andrada-Oana Mandru*, J.P. Corbett, A.L. Richard, Ohio University, J.M. Lucy, Ohio State University, D.C. Ingram, Ohio University, F. Yang, Ohio State University, A.R. Smith, Ohio University.

Depositions of magnetic atoms such as Mn onto wide-gap semiconducting GaN surfaces give rise to various MnGa alloyed nanostructures, some having promising magnetic properties. Co-depositions of Mn and Ga result in ferromagnetic alloys that grow with high epitaxial quality on GaN. Such sharp interfaces undoubtedly make MnGa/GaN a very attractive spintronic system. Growth under slightly Mn-rich conditions (Mn:Ga composition ratio ~1.09) causes Mn atoms to incorporate at different rates; surfaces become highly Mn-rich, while the bulk retains a 1:1 stoichiometry. In addition, their magnetic properties could potentially be tailored by altering elemental composition and/or film thickness. Motivated by these intriguing observations and possibilities, we explore what happens when crossing the Mn:Ga 1:1 stoichiometric limit into the less studied Ga-rich side. We combine various techniques to investigate in detail the growth, structure and magnetism of MnGa alloys with different thicknesses and compositions, when coupled with GaN substrates.

Samples are prepared using molecular beam epitaxy with GaN/Sapphire used as starting substrate. Subsequent depositions involve a fresh film of GaN followed by thin (~30-50 nm) or ultra-thin (~3.3 nm) MnGa films. Manganese and gallium are co-evaporated from Knudsen cells while keeping the substrate temperature at ~250 °C. The growth is monitored in real time using a 20 keV reflection high energy electron diffraction system. In-situ room temperature scanning tunneling microscopy investigations reveal highly epitaxial films with smooth surfaces that exhibit a rich variety of reconstructions. The Mn:Ga composition ratios range from ~1 (stoichiometric) to ~0.42 (very Ga-rich), as determined by Rutherford backscattering spectrometry. For stoichiometric films, x-ray diffraction characterizations show primarily MnGa peaks; upon transitioning into the Ga-rich regime, we find a co-existence of Mn₃Ga₅ and Mn₂Ga₅ phases, with Mn₂Ga₅ becoming predominant for the highly Ga-rich samples. Magnetic investigations reveal that all films exhibit ferromagnetism, including the very Ga-rich ones. Vibrating sample magnetometry measurements performed on the thin samples show stepped hysteresis loops, along with a decrease in coercivity and magnetic moment values as the Ga concentration increases. Additional superconducting quantum interference device measurements performed on the ultra-thin samples show that large magnetic anisotropies are induced by decreasing the thickness of our films. Most recently, similar investigations applied to FeGa magnetostrictive alloys reveal very interesting surfaces and magnetic properties.