Wuwei Feng

Substrate-modified ferrimagnetism in MnGa films

We report the substrate-modified magnetic properties of the CuAu type-I (L10) structure of MnxGa (1.2<x<1.5) films. The magnetic properties of the MnGa films differed greatly due to the influence of the substrate. The MnGa film is a hard ferrimagnet when grown on GaSb (111), becomes a soft ferrimagnet when grown on Al2O3 (0001), and exhibits an absence of a net magnetic moment when stabilized on a GaSb (100) substrate. This difference was attributed to the substrate, which forces MnGa film to be two-dimensionally stabilized in a different orientation and thus leads to the modified crystal symmetry and a change in the magnetic property. The results may be helpful for forming a comprehensive understanding of MnGa and for finding new applications in spintronic devices.

Epitaxial growth and ferrimagnetic properties of Mn film on GaSb(100)

We report on the epitaxial stabilization and magnetic properties of Mn films on GaSb(100) using molecular beam epitaxy, a follow-up to our previous work on the growth of Mn films on GaAs(100) [Phys. Rev. B 79, 045309 (2009)]. A strong ferrimagnetic ordering was observed which was attributed to the largely expanded lattice parameter resulting from the lattice mismatch with substrate and the enhanced thermal energy with increasing growth temperature. The observed magnetic moment of 1.16μB/Mn atom was several times larger than that observed in the Mn/GaAs(100) films due to the lattice constant difference of substrates. The in-plane magnetoresistance behavior demonstrated the magnetic anisotropy that might result from the slightly distorted cubic structure of α-Mn phase under low-dimensional growth.

Magnetism and transport properties of α-Mn structure Mn3Ge thin film

We have newly synthesized the α-Mn structured Mn3Ge thin films on GaAs(001) substrate. The single phase Mn3Ge film was grown at a growth temperature of 150 °C, while the secondary phase was observed above a growth temperature of 250 °C. The Mn3Ge films exhibited ferrimagnetism with a Curie temperature of 334 K. The saturation magnetizations were 255.9 and 313.8 emu/cm3 and the corresponding coercive fields were 453 and 1166 Oe at 10 K for the samples grown at 150 and 300 °C, respectively. The ferrimagnetism was confirmed by the anomalous Hall effect data.

Electron mediated/enhanced ferromagnetism in a hydrogen-annealed Mn:Ge magnetic semiconductor

We report on the carrier type changes of the p-type for as-grown Mn:Ge films to n-type for post-annealed samples in a hydrogen ambient. The hydrogen-annealed samples exhibit the increased Curie temperature, from 165 to 198 K, and the enhanced magnetic moment, from 0.78 to 1.10 μB/Mn. The first principles calculation using the all-electron full-potential linearized augmented plane wave method indicates that the addition of an electron carrier strengthens the ferromagnetic coupling between the Mn atoms, while the hole carrier caused it to weaken.

Ferromagnetism and magneto-transport properties of Mn0.92Ca0.08As thin film grown on Al2O3(0001) substrate

The epitaxial Mn0.92Ca0.08As thin film was grown on Al2O3(0001) substrate by molecular beam epitaxy. The Curie temperature (TC) around 340 K was enhanced with the addition of Ca, compared to that of bulk MnAs (TC ∼ 318 K). The maxima magnetoresistance, ∼2.08% at 0.7 T, was observed near the critical magnetic transition temperature. Moreover, the giant magnetocaloric effect was found with the maximum magnetic entropy change, ∼200 J/kgK, around 330 K at 5 T.

Strain modified/enhanced ferromagnetism in Mn3Ge2 thin films on GaAs(001) and GaSb(001)

Ferromagnetic Mn3Ge2 thin films were successfully grown on GaAs(001) and GaSb(001) substrates using molecular beam epitaxy. The results of our work revealed that the substrate facilitates to modify magnetic and electrical properties of Mn3Ge2 films due to tensile/compressive strain effect between films and substrates. The characteristic spin-flopping transition at around 150 K for the bulk Mn3Ge2 disappeared completely for both samples. The antiferromagnetism below 150 K changed to ferromagnetism and retained above room temperature. The saturation magnetization was found to be 0.23 and 1.32 μB/Mn atom at 10 K for the samples grown on GaSb(001) and GaAs(001), respectively.

Structural and magnetic phase diagrams of epitaxial Cr–Mn alloy thin films

We report here a systematic study of Cr-Mn alloy films that have been epitaxially stabilized on GaSb (100) using molecular beam epitaxy. The crystal structural transition between the alpha-Cr-type and the alpha-Mn-type for the Cr-Mn alloy films is observed along with changes in growth temperature, film thickness, and the ratio of Cr to Mn. Ferrimagnetism is observed in the Cr-Mn films containing the alpha-Mn-type phase based on the magnetic field-dependent anomalous Hall effect hysteresis and is corroborated by the magnetization hysteresis. The alpha-Mn-type CrMn phase at the expanded lattice parameter induces the observed ferrimagnetic ordering. The magnetic moments of Cr-Mn films can be tuned by adjusting the growth temperature, film thickness, and the ratio of Cr to Mn. Eventually, new structural and magnetic phase diagrams of the epitaxial Cr-Mn alloy films are established. The results of this study can prove helpful in both forming a comprehensive understanding of Cr-Mn alloys and in finding new applications for it in spintronic devices