Text Box: Department of Physics

Research Highlights

NANOELECTRONICS & MAGNETICS GROUP

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 (see figure) 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, Large Coercivity in Nanostructured Rare-earth-free MnXGa Films, Appl. Phys. Lett. 99, 252506 (2011).

Topological (Crystalline) Insulators

We have synthesized and studied MBE-grown epitaxial films of Topological Insulators (TI), including BiTeSe and SnTe. We found: (1) that quantum coherent magnetotransport measurements reveals a multiplicity of Dirac surface states in SnTe that are unique to TCI; (2) ferromagnetism can be induced on the surface of Bi2Se3 and SnTe through proximity-coupling with the ferromagnetic insulator EuS; (3) observed linear magnetoresistance in Bi2Te2Se thin films and modeled it with a modified Hikami, Larkin and Nagaoka quantum interference model (shown); (4) demonstrated that quantum interference dominates up to 140 K, indicating that electrons retain quantum phase coherence at high temperatures, possibly up to room temperature.

 

B.A. Assaf, F. Katmis, P. Wei, B. Satpati, Z. Zhang, S.P. Bennett, V.G. Harris, J.S. Moodera and D. Heiman, Quantum Coherent Transport in SnTe Topological Crystalline Insulator Thin Films, Appl. Phys. Lett. (2014).

P. Wei, F. Katmis, B.A. Assaf, P. Jarillo-Herrero, D. Heiman, and J.S. Moodera, Magnetic proximity-induced symmetry breaking in topological insulators, Phys. Rev. Lett. 110, 186807 (2013).

B.A. Assaf, T. Cardinal, P. Wei, F. Katmis, J.S. Moodera and D. Heiman, Linear magnetoresistance in topological insulators: Quantum phase coherence effects at high temperatures, Appl. Phys. Lett. 102, 012102 (2013).

Spin Gapless Semiconductors and Half-metallic Antiferromagnets

A novel class of inverse-Heusler materials has recently been predicted that merge the properties of half-metallic magnets and semiconductors. Theoretical band structure calculations show that these inverse Heusler materials have a Fermi energy lying in a gap for one direction of electron spin, but for the other direction of electron spin the valence and conduction band edges meet at the Fermi energy. One of the great advantages of these spin gapless semiconductors (SGS) for devices relies on the property where a simple gate voltage can tune the spin properties. Furthermore, these inverse Heusler materials encompass half-metallic antiferromagnets (HMAF) that are spin-polarized but nonmagnetic. We have synthesized several of these materials by MBE and arc-melting, including several gapless AF compounds. Oriented SGS Mn2CoAl has been grown epitaxially on GaAs and we have studied the magnetic and electrical properties.

 

Michelle E. Jamer, Badih A. Assaf, Trithep Devakul and Don Heiman, Magnetic and transport properties of Mn2CoAl oriented films, Appl. Phys. Lett. 103, 142403 (2013). arXiv:1309.6660.