Universal properties of linear magnetoresistance in strongly disordered MnAs-GaAs composite semiconductors - Linear magnetoresistance (LMR) occurs in semiconductors as a consequence of strong electrical disorder and is characterized by nonsaturating magnetoresistance that is proportional to the applied magnetic field. By investigating a disordered MnAs-GaAs composite material, it is found that the magnitude of the LMR is numerically equal to the carrier mobility over a wide range and is independent of carrier density. This behavior is complementary to the Hall effect that is independent of the mobility and dependent on the carrier density. Moreover, the LMR appears to be insensitive to the details of the disorder and points to a universal explanation of classical LMR that can be applied to other material systems.. H.G. Johnson, S.P. Bennett, R. Barua, L.H Lewis, and D. Heiman, Phys. Rev. B 82, 085202 (2010).

Research

Determining magnetic nanoparticle size distributions from thermomagnetic measurements -  Thermomagnetic measurements are used to obtain the size distribution and anisotropy of magnetic nanoparticles. An analytical transformation method is described which utilizes temperature dependent zero-field cooling magnetization data to provide a quantitative measurement of the average diameter and relative abundance of superparamagnetic nanoparticles. Applying this method to self-assembled MnAs nanoparticles in MnAs-GaAs composite films reveals a log-normal size distribution and reduced anisotropy for nanoparticles compared to bulk materials. This analytical technique holds promise for rapid assessment of the size distribution of an ensemble of superparamagnetic nanoparticles. - R.S. DiPietro, H.G. Johnson, S.P. Bennett, T.J. Nummy, L.H. Lewis, and D. Heiman, Appl. Phys. Lett. 96, 222506 (2010).

Spin Injection into Ferromagnetic Co₂MnAl - Spin-oriented electrons were injected into ferromagnetic Co2MnAl by optical absorption in GaAs. Schottky-barrier diode heterostructures were fabricated by MBE growth of the Heusler ferromagnet Co2MnAl epitaxially on GaAs. Illumination near the bandgap of GaAs by circularly polarized light generated spin polarized electrons in the GaAs. In this technique, the optically pumped semiconductor is the source of spin polarized electrons, while the ferromagnet is the detector of spin polarized electrons. A spin-polarized photocurrent is observed only for forward biased conditions where electron tunneling is dominant. Similar results on Fe/MgO/GaAs structures were compared to calculations. - Samih Isber, Don Heiman, Young Ju Park and Jagadeesh S. Moodera, J. Appl. Phys. 103, 07D713 (2008).

 

MnAs and GaAs Nanowires - The goal of this project is the synthesis of nanowires of GaAs and MnAs. We use MBE and the vapor-liquid-solid (VLS) technique. MnAs nanowires were grown by MBE and were found to grow best on the (111) surface of GaAs. Nanowires were found to lie on the GaAs surface at 120 degree angles, in registration with the (111) planes. The SEM image on the right shows a MnAs nanowire growing out of the surface of the GaAs substrate.

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Giant Coercive Field in Rare-earth-free Mn_xGa Structured Films - Rare-earth-based magnets provide the backbone of many products, from computers and mobile phones to electric cars and wind-powered generators. But because of the high cost and limited availability of rare-earth and precious elements, which are expensive to mine and process, there is a growing interest in developing new magnetic materials without these elements. The magnetic hysteresis of Mn_xGa films was found to exhibit remarkably large coercive fields as high as μ_oH_C=2.5 T when fabricated with nanoscale particles of a suitable size and orientation. This coercivity is an order of magnitude larger than in well-ordered epitaxial film counterparts and bulk materials. The enhanced coercivity is attributed to the combination of large magnetocrystalline anisotropy and ~50-100 nm size nanoparticles. The large coercivity is also replicated in the electrical properties through the anomalous Hall effect. These results suggest that Mn_xGa is a good candidate for producing materials with enhanced coercive fields aimed at replacing some rare-earth-based magnets in use today. - T.J. Nummy, S.P. Bennett, T. Cardinal, and D. Heiman, Appl. Phys. Lett. 99, 252506 (2011).