Text Box: Department of Physics

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 Highlights

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).


Giant Coercive Field in Rare-earth-free MnxGa 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 MnxGa films was found to exhibit remarkably large coercive fields as high as μoHC=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 MnxGa 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).

Topological Insulators - We have synthesized and studied MBE-grown epitaxial films of Topological Insulators including Bi2Te2Se and SnTe. We discovered that ferromagnetism could be induced in the surface of Bi2Se3 through proximity-coupling with EuS. In Bi2Te2Se we observed linear magnetoresistance (MR) in thin films and modeled it with a modified Hikami, Larkin and Nagaoka (HLN) quantum interference model. In addition to the weak antilocalization (WAL) cusp in the MR at low fields, at high fields we observe the MR increasing linearly up to B = 14 T. We can fit MR(B) over the entire range of fields and temperatures as shown in the figure. Our modified model simultaneously accounts for the quantum phase interference cusp at low fields as well as the linear-like MR at high fields. Nevertheless, the lower figure shows that even at high temperatures the quantum interference dominates, indicating that electrons retain quantum phase coherence at high temperatures, possibly up to room temperature.

- B.A. Assaf, T. Cardinal, P. Wei, F. Katmis, J.S. Moodera and D. Heiman, Appl. Phys. Lett. 102, 012102 (2013). P. Wei, F. Katmis, B.A. Assaf, P. Jarillo-Herrero, D. Heiman, and J.S. Moodera, Phys. Rev. Lett. 110, 186807 (2013).