Duong Van Thiet

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.

Current Development in Lead-Free -Based Piezoelectric Materials

The lead-free piezoelectric ceramics display good piezoelectric properties which are comparable with Pb(Zr,Ti)O3 (PZT) and these materials overcome the hazard to the environment and human health. The Bi0.5(Na,K)0.5TiO3 (BNKT) is rapidly developed because of good piezoelectric, ferroelectric, and dielectric properties compared to PZT. The origin of giant strain of BNKT piezoelectric materials was found at morphotropic phase boundary due to crystal change from tetragonal to orthorhombic and/or precipitation of cubic phases, in addition to domain switching mechanism. The dopants or secondary phases with ABO3 structure as solid solution are expected to change the crystal structure and create the vacancies which results in enhancement of the piezoelectric properties. In this work, we reviewed the current development of BNKT by dopants and secondary phase as solid solution. Our discussion will focus on role of dopants and secondary phase to piezoelectric properties of BNKT. This result will open the direction to control the properties of lead-free piezoelectric materials.

Strain effects in epitaxial Mn2O3 thin film grown on MgO(100)

We report on the epitaxial growth and magnetic properties of Mn2O3 thin films grown on MgO(001) substrate by molecular beam epitaxy. We observed the reduction in binding energy of Mn valence states, the increase in satellite separation up to 12.7 eV, and the smaller band gap of 3.32 eV. In addition, the antiferromagnetic ordering below 90 K in bulk changed to ferrimagnetic up to 175 K. The results were possibly to be explained by a lattice mismatch strain on Mn2O3 film on MgO(001) substrate.

Giant magnetocaloric effect of Mn0.92Ba0.08As thin film grown on Al2O3(0001) substrate

The epitaxial Mn0.92Ba0.08As thin film was grown on Al2O3(0001) substrate by molecular beam epitaxy. The Curie temperature (TC) around 350 K was enhanced with the addition of Ba, compared to that of bulk MnAs (TC ∼ 318 K). We have observed the linear resistivity versus the square of temperature and high negative magnetoresistance near Curie temperature. Moreover, the giant magnetocaloric effect was found with maximum magnetic entropy change, 65 J/kgK, around room temperature at 5 T.

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.