Role of Ga Polarization & Defects in Ferromagnetism of GaMnN

1. Role of Ga Polarization & Defects inFerromagnetism of GaMnN Lian Li, University of Wisconsin, Milwaukee, DMR-0094105 • Using XAS (x-ray absorption spectroscopy) & XMCD (magnetic circular dichroism), we find: • two distinct Mn sites, & • a Ga moment antiparallel to Mn • These results show that: • defects may strongly affect Mn ordering and magnetic properties of Ga1-xMnxN. • polarization of the valence band of the host is significant, providing amechanism for long-range ferromagnetic ordering in Ga1-xMnxN. Figure 1 XAS (blue) and XMCD (red) at the Ga L3 edge in a 5T field at 4.2K. The sign of the initial dichroism suggests a Ga moment that is antiparallel to the Mn 3d moments. The dashed line is an estimate of the area under the XMCD curve based on a fit to two Gaussians of opposite sign.

2. N Mn Ga Role of Ga Polarization & Defects inFerromagnetism of GaMnN Lian Li, University of Wisconsin, Milwaukee, DMR-0094105 First-principles spin-polarized LDA electronic structure calculations of the Mn local structure, compared to XAS and XMCD measurements, and the nature of Ga moments in Ga1-xMnxN show that: • Most configurations (Mn substitutions, double Mn substitutions, Mn interstitials, Ga vacancies, etc.) lead to small Ga moments of ~ 0.05 uB/Ga parallel to Mn, inconsistent with experimental result. • Only Mn substitutions with split N interstitial produce antiparallel Ga moment. It is favored by 2 eV/Mn, with a moment of ~3 B/Mn. • Magnetic moments of other defects: • Split-interstitial with double Mn substitution: favored by 0.7 eV/Mn, moment ~3 B/Mn. • Single Mn substitutions: 4 B/Mn • most other defects: smaller and non-integral valued (more “band-like”) moments Figure 2. Upper left: Calculated local structure of a Mn atom next to an N split-interstitial defect. (a) the calculated (ground state) Mn L3 absorption spectra for different defects. (b) Experimental XAS spectra (colored) for different Mn concentrations, compared to calculations in the presence of core holes for substitutional MnGa, for one and two Mn atoms next to a N split-interstitial (bottom three spectra), and for different superpositions of these spectra (top two calculated).

3. Role of Ga Polarization & Defects in Ferromagnetism of GaMnN Lian Li, University of Wisconsin, Milwaukee, DMR-0094105 NSF-RET site at UWM-Physics The Physics Department at the University of Wisconsin-Milwaukee houses an NSF Research Experiences for Teachers (RET) site, which provides opportunities for high school physics teachers from southeastern Wisconsin and Milwaukee Public Schools. RET provides summer internships for these teachers to work directly with faculty on various research projects of their choice, which enrich both their background of the subject and their teaching. RET Project - Phase I: July – August ’05 Tim Moeller and Susan Navarro, spent eight weeks in PI’s lab learning the operation of scanning tunneling microscope (STM), electro-chemical etching of STM tips, and had successfully obtained atomic resolution image of Si(111)-(7x7) (Insert is an SEM image of a tip made by the teachers). Aside from educating the teachers with current research, the program also puts them in the students’ shoes, as Tim puts it: “I was clearly reminded of how learning new things/techniques can be a real struggle for students… This single lesson alone makes an RET program worth it because it reminds teachers that learning new things can be hard, and our job is to help kids grow and learn in spite of that. We as teachers need to work equally hard to help them ‘get it’.”   RET Project - Phase II: October ’05 With Supplemental funding, laptops, hardware upgrades, and software including PC Anywhere have been purchased in preparation for the second phase of the project, where the teachers bring the Nobel Prize winning technology STM back to their classrooms for real time demonstration in their high schools though internet using PC Anywhere. An onsite visit of the PI’s lab for the students will be scheduled in October for the students to familiarize themselves with the microscope.