Some motivations Key challenge of electronic materials – to control both electronic and magnetic properties – to process the full electronic states Prospects for new materials and properties in group IV elements –Compatible with integration onto Si-based CMOS platform –Potential long spin mean-free-path for both electrons and holes in strained Si –Novel device concepts to exploit spin-polarization, spin-diffusion length, and carrier-mediated magnetism Prospects for synthesis and study of half-metallic Heusler alloys –Ideal electronic spin filters –Materials difficulties: effects associated with stoichiometry and/or epitaxial constraints strongly influence magnetism and can suppress half-metallicity Systematic studies need combinatorial approach – to explore and tailor synthesis and properties

Structural Phase Boundaries Mn Co Ge

Highly Ordered Regions and Regions of Magnetic Measurements

Highly Ordered Regions and Regions of Magnetic Measurements

Saturation MOKE Intensity

Remanent MOKE Intensity

T C  393 K 343 K 293 K Magnetic Phase Diagram Correlation between high Curie temperature and ordering along Co to Mn ratio of 2:1 Ge MnCo

Ge MnCo Saturation Kerr Rotation vs. Composition High saturation values along Co to Mn ratio of 2:1 Heusler stoichiometry from

Ge MnCo Coercivity vs. Composition along one of Low coercive fields along Co to Mn ratio of 2:1 Distinct regions of magnetic anisotropy Systematic evolution along Co 2 Mn 1 (data shown in following pages are at red circles) Abrupt changes across (indicated by the dotted white line) from

Model for Magnetic anisotropy Six-fold vs. uniaxial magnetic anisotropy Magneto-crystalline anisotropy Anisotropic energy in cubic crystals Demagnetization energy 0 Additional term for the observed uniaxial magnetic anisotropy with coefficient K u In-plane magnetic anisotropy (IPMA) energy

Asymmetric magneto-optic response Second-order terms – contain transverse component of magnetization. Parameters a and b depend on optic geometry, polarization configurations, and dielectric tensor of the material. Matrix elements contain terms that are second order in magnetization Resulting second-order terms give rise to “anomalous” asymmetric features (arrow) in hysteresis loops The 2 nd -order terms become dominant in the case of perpendicular incidence * Osgood et al., Phys. Rev. B (1997) Intensity corresponds to Kerr rotation to the lowest order of  M can be expressed as [*]:

Evolution of Magnetic Anisotropy along Co to Mn Ratio of 2:1 systematic increase of K u as Ge concentration increases Six-fold (0  ) magnetic anisotropy + an uniaxial (30  ) component Uniaxial anisotropy increases with increasing Ge concentration Ge: 10 at. %Ge: 22 at. %

Ge: 35 at. %Ge: 44 at. %Ge: 25 at. % Stoichiometric Heusler alloy Evolution of Magnetic Anisotropy along Co to Mn Ratio of 2:1 – r increases systematically

Model Comparisons r = 0.1 Ge: 10 at. %; Co 2 Mn 1

Model Comparisons at Ge: 35 at. %; Co 2 Mn 1 r = 1

Ge MnCo Coercivity vs. Composition along one of Low coercive fields along Co to Mn ratio of 2:1 Distinct regions of magnetic anisotropy Systematic evolution along Co 2 Mn 1 (red circles) Abrupt changes across (white dotted line thru various red circles) – isotropic behavior near structural phase transition (Mn-rich) to sign reversal of uniaxial anisotropy (Co-rich) from

Abrupt change of magnetic anisotropy across the optimum Co to Mn ratio of 2:1 Ge at 10 at. % Co to Mn ratio > 2 r = 0 isotropic r < 0 Uniaxial ratio-negative Co to Mn ratio = 2Co to Mn ratio < 2 r ~ 0.1

Abrupt change of magnetic anisotropy across the optimum Co to Mn atomic ratio of 2:1 Ge at 22 at. % Co to Mn ratio > 2Co to Mn ratio = 2Co to Mn ratio < 2 r = 0 isotropic r < 0 Uniaxial ratio-negative r ~ 0.5

Abrupt change of magnetic anisotropy across the optimum Co to Mn atomic ratio of 2:1 Ge at 35 at. % Co to Mn ratio > 2Co to Mn ratio = 2Co to Mn ratio < 2 r = 0 isotropic r < 0 Uniaxial ratio-negative r ~ 1