Yooleemi Shin

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.

Formation and ferromagnetic properties of FeSi thin films

In this work, the growth and ferromagnetic properties of e-FeSi thin film on Si(100) substrate prepared by molecular beam epitaxy are reported. The inter-diffusion of Fe layer on Si(100) substrate at 600 °C results in polycrystalline e-FeSi layer. The determined activation energy was 0.044 eV. The modified magnetism from paramagnetic in bulk to ferromagnetic states in e-FeSi thin films was observed. The saturated magnetization and coercive field of e-FeSi film are 4.6 emu/cm3 and 29 Oe at 300 K, respectively.

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.

Perpendicular ferrimagnetism in strained Mn2As film

Ferrimagnetic Mn2As thin films with perpendicular magnetic anisotropy were successfully grown on Si(100) by molecular-beam epitaxy. From the reflection high-energy electron diffraction and X-ray diffraction patterns, the orientation of the Mn2As film on Si was along the c-axis in the tetragonal crystal structure. Mn2As film exhibited ferrimagnetic ordering at temperatures greater than 300 K, which differs from antiferromagnetic or paramagnetic behaviors in the bulk form. The magnetic moment of Mn2As determined by saturated magnetization was 0.51 µB per unit cell.

Magnetism and transport properties of epitaxial Fe-Ga thin films on GaAs(001)

Epitaxial Fe–Ga thin films in disordered bcc α-Fe crystal structure (A2) have been grown on GaAs(001) by molecular beam epitaxy. The saturated magnetization (MS) decreased from 1371 to 1105 kA/m with increasing Ga concentration from 10.5 to 24.3 % at room temperature. The lattice parameter increased with the increase in Ga content because of the larger atomic radius of Ga atom than that of Fe. The increase in carrier density with Ga content caused in lower resistivity.

New synthesis of MnSi2 thin film and its thermoelectric properties

The authors report the formation of a new manganese silicide phase, MnSi2, using an intermixing method that involves evaporating a manganese thin film on a silicon substrate at a high temperature of 600 °C. The crystal structure of MnSi2 is tetragonal with lattice constants of a = 5.518 A and c = 17.449 A, identical to that of Mn4Si7. Analysis of the film using high-angle annular dark-field imaging confirmed that the composition ratio of Mn and Si was 1:2. The electric resistivity and Seebeck coefficient of the film were 2.08 × 10−3 Ω cm and 107.84 μV/K at 410 K, respectively, which resulted in a maximum power factor of 5.60 × 10−4 W/K2m.

Magneto-transport and thermoelectric properties of epitaxial FeSb2 thin film on MgO substrate

We report magneto-transport and thermoelectric properties of FeSb2 thin film epitaxially grown on the MgO substrate using molecular beam epitaxy. The film exhibits compressive strain of 1.74% owing to large lattice mismatch, whose physical consequences are nontrivial. Magnetic phase has been changed from diamagnetic in bulk, as evidenced by anomalous Hall effect (AHE) and negative magneto-resistance (MR). The FeSb2 film is semiconducting without any metallic transition unlike the bulk counterpart. In particular, hysteresis in MR with distinct feature of AHE is evident with coercive field of 500 and 110 Oe for T = 20 and 50 K, respectively. Furthermore, from the Seebeck coefficients and temperature dependence of the resistivity, it is evident that the film is semiconducting with small band gap: 3.76 meV for T   40 K, respectively, where maximum thermoelectric power factor of 12 μV/cm·K at T = 50 K.

Influence of Ga content on the structure and anomalous Hall effect of Fe1−xGax thin films on GaSb(100)

The Fe1−xGax thin films (x = 0.4, 0.5) have been grown on GaSb(100) substrate using molecular beam epitaxy. An epitaxial film with bcc α-Fe crystal structure (A2) is observed in Fe0.6Ga0.4 film, while an impure Fe3Ga phase with DO3 structure is appeared in Fe0.5Ga0.5 film. The saturated magnetizations at room temperature are observed to be 570 emu/cm3 and 180 emu/cm3 and the coercivities to be 170 and 364 Oe for Fe0.6Ga0.4 and Fe0.5Ga0.5, respectively. A hysteresis trend in Hall resistance vs. magnetic field is observed for Fe0.5Ga0.5 film. However, there is a weak hysteresis noticed in Fe0.4Ga0.6 thin film.