All Volumes (2001-2008)


Volume I, 2001

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The new layered III-VI Diluted Magnetic Semiconductors (DMS) are 2-D systems containing transition metal ions (e.g. Mn, Fe, Co, etc.) in a III-VI semiconducting host (e.g. GaSe, GaS, etc.). The III-VI DMS Ga1-xMnxSe exhibits a strong red emission at 1.804 eV attributed to the Mn ions. The III-VI semiconductors are known for their remarkable nonlinear optical properties and are promising materials for photoelectronic applications. This work also complements the enormous progress in the II-VI DMS and the more recent efforts in the Mn doped III-V DMS systems. In a manuscript published last summer, Pekerak et al present magnetization data on the III-VI DMS Ga1-xMnxSe that is strikingly different from any of the II-VI DMS. A key feature is a broad peak in the magnetization versus temperature data between 120 and 195 K that is ascribed to direct Mn-Mn pairs. This is a fundamentally different behavior that that observed in the heavily studied II-VI DMS. Except for this single publication, no previous magnetic or calorimetric measurements on III-VI DMS have been reported. Recently, we conducted magnetic measurement Ga1-xMnxS. Its magnetic behavior was remarkably different from Ga1-xMnxSe and II-VI DMS. The prominent broad peak between 120 and 195 K in the magnetization of Ga1-xMnxSe, ascribed to direct Mn-Mn pairs, is absent in the Ga1-xMnxS data. This suggests there are no direct Mn-Mn pairs in the GaS system. However, the magnetization of Ga1-xMnxS does show a sharp cusp at 11.3 K (an order of magnitude higher than the spin-glass transition in Cd1-xMnxS) suggesting that a similar mechanism with Mn-Se-Mn pairs may be present in Ga1-xMnxS. The exchange interactions in Ga1-xMnxSe and Ga1-xMnxS (with lower symmetry than the II-VI and III-V DMS) are more complex and exhibit significantly different magnetic properties. The magnetic and calorimetric measurements will provide key information for unraveling some of the observed novel magnetic effects. Calibration was done on the computer-controlled ac-temperature calorimeter, which was just constructed at the University of North Florida for use down to 0.5 K using liquid He in Ii pumped 3He Cryostat. This will help to determine how the Mn ions behave individually, as pairs in different configurations (e.g. Mn-Mn, Mn-Se-Mn, Mn-Ga-Se-Mn, etc.), as well as long-range cooperative interactions in the bulk crystals. Measurements were conducted for a week at the National High Magnetic Field Laboratory (NHMFL) to study the magnetic properties of Ga1-xMnxS at temperatures down to 0.5 K in fields up to 30+ Tesla. Initial measurements at the NHMFL have already been conducted on Ga1-xMnxSe for comparison.