Hwachol Lee

Growth and characterization of 144 μm thick barium ferrite single crystalline film for microwave device application

Liquid phase epitaxy technique was used to grow 144 μm thick barium ferrite (BaFe12O19; BaM) single crystalline films on (111) Gd3Ga5O12 substrate. The growth rate of 72 μm/h was achieved with a flux system of Fe2O3–BaCO3–Na2CO3. The grown BaM films show single crystalline (000l) orientation that was confirmed by x-ray diffraction and magnetic torque curves. The saturation magnetization (4πMs) and the anisotropy field (Hk) were found to be 4.2 kG and 16.0 kOe, respectively. The ferrimagnetic resonance linewidth (ΔH) at 35 GHz was measured to be 0.1 kOe.

Magnetic and Microwave Properties of Sm-Doped SrFe

Samarium (Sm) doped M-type strontium ferrite single crystals have been successfully grown from melts using a flux system of SrCO3 and Na2CO3. The crystals exhibited the typical hexagonal ferrite growth habit with a plate-like geometry, and the largest crystals obtained are of 10 mm in length and 3-4 mm in thickness, respectively. It was observed that the saturation magnetization and anisotropy field are greatly affected by the doping concentration of Sm. A saturation magnetization (sigmas) of 69.8 emu/g and an anisotropy field (H k) of 25 kOe was achieved. Room temperature FMR measurements show 85 Oe of linewidth (DeltaH) at 53 GHz.

_{12}

Perpendicularly magnetized flat thin films of antiperovskite Mn67Ga24N9 were grown on an MgO(001) substrate by reactive sputtering using an argon/1% nitrogen gas mixture and a Mn70Ga30 target. The films showed a saturation magnetization of 80 –100 kA/m, an effective perpendicular magnetic anisotropy (PMA) energy of 0.1–0.2 MJ/m3, and a Curie temperature of 660–740 K. Upon increasing the N composition, the films transformed from ferromagnetic to antiferromagnetic as expected in the stoichiometric Mn3GaN phase. Point contact Andreev reflection spectroscopy revealed that the ferromagnetic MnGaN has a current spin polarization of 57%, which is comparable to D022-MnGa. These findings suggest that MnGaN is a promising PMA layer for future spintronics devices.

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We study the relationship between long range order parameters and the magnetocrystalline anisotropy of off-stoichiometric D022 Mn2.36Ga (MnGa) epitaxial films grown on MgO (001) and SrTiO3 (STO) (001) single crystalline substrates. MnGa films deposited on MgO (001) show rather large irregular variation in magnetization with increasing substrate temperature in spite of the improved long range order of total atomic sites. The specific site long range order of Mn-I site characterized in the [101] orientation revealed the fluctuation of the occupation fraction of two Mn atomic sites with elevated substrate temperature, which appears more relevant to the observed magnetization change than the long range order of the total atomic sites. In case of MnGa films grown on the lattice-matched STO (001), high long range order of the total atomic sites in spite of the existence of secondary phase represents that the lattice mismatch plays a crucial role in determining the atomic arrangement of Mn and Ga atoms in the of...

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We report the growth of (001)-textured polycrystalline D022 MnGa films with perpendicular magnetic anisotropy (PMA) on thermally oxidized Si substrates using an (Mg0.2Ti0.8)O (MTO) buffer layer. The ordered D022 MnGa film grown at the optimum substrate temperature of 530 °C on the MTO buffer layer shows PMA with magnetization of 80 kA/m, PMA energy density of 0.28 MJ/m3, and coercivity of 2.3 T. The scanning transmission electron microscope analysis confirms the formation of a highly (001)-textured structure and the elementally sharp interfaces between the MTO layer and the MnGa layer. The achieved D022 MnGa PMA films on an amorphous substrate will provide the possible pathway of integration of a Mn-based PMA film into Si-based substrates.

Single Crystals

We report the tunable magnetic properties and the smoothened surface morphology of epitaxial D022 Mn-Ga (Mn3Ga and Mn2.5Ga) films by N doping using reactive sputtering at 480 °C. The 50 nm thick Mn-Ga films grown with the N2/Ar gas flow rate (η) up to 0.66% showed 33%–50% reduction in the saturation magnetization compared to non-doped Mn-Ga. In particular, a single D022 phase was obtained in an optimal η range for Mn2.5Ga, resulted in the perpendicular magnetic anisotropy energy density of ∼1 MJ/m3 with 33% reduction in magnetization. Furthermore, the introduction of N provided the smoothened surface morphology at 50 nm thickness despite its high growth temperature, which is advantageous for thin film device applications.

Ferromagnetic MnGaN thin films with perpendicular magnetic anisotropy for spintronics applications

Although single-crystalline spinel (MgAl2O4)-based magnetic tunnel junctions (MTJs) are known to show a good bias voltage dependence of a tunnel magnetoresistance (TMR) ratio over MgO-based MTJs, no polycrystalline MgAl2O4-based MTJs exhibiting large TMR ratios have been grown previously due to the lack of crystallinity of the MgAl2O4 barrier. In this work, we demonstrate the growth of polycrystalline-based MTJs with large TMR ratios exceeding 240% and an improved bias voltage dependence compared to that of MgO-based MTJs. An ultra-thin CoFe/MgO seed layer on the amorphous CoFeB layer induced the growth of a highly (001)-textured MgAl2O4 barrier, which worked as a template layer for the solid epitaxy of CoFe grains during the crystallization of the CoFeB layers. High resolution scanning transmission electron microscopy shows lattice-matched epitaxy between the (001)-textured MgAl2O4 barrier and CoFe grains. This study demonstrates the industrial viability of MgAl2O4-based polycrystalline MTJs with an impr...

Order parameters and magnetocrystalline anisotropy of off-stoichiometric D022 Mn2.36Ga epitaxial films grown on MgO (001) and SrTiO3 (001)

We report on the tunnel magnetoresistance (TMR) effect of perpendicular magnetic tunnel junctions (p-MTJs) composed of a ferromagnetic antiperovskite MnGaN film with perpendicular magnetic anisotropy. The p-MTJ multilayer stack with MnGaN/Mg/MgO/Fe/CoFeB was used to control the MgO interface conditions by the Mg and Fe insertion layers. The Mg insertion under the MgO barrier enhanced the perpendicular anisotropy of the top Fe/CoFeB layers with post-annealing process, and thus nearly perfect perpendicular magnetization was realized. A perpendicular TMR ratio up to 3.7%

Perpendicularly magnetized (001)-textured D022 MnGa films grown on an (Mg0.2Ti0.8)O buffer with thermally oxidized Si substrates

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Tuning the magnetic properties and surface morphology of D022 Mn3-δGa films with high perpendicular magnetic anisotropy by N doping

% was obtained at room temperature. The obtained low TMR ratio is mainly attributed to the imperfect MgO barrier, which includes many lattice-misfit dislocations at the MnGaN/MgO interface due to the large lattice mismatch of 8%. This paper suggests that improving the interface state and reducing the lattice mismatch will be required to observe a higher perpendicular TMR ratio using an MnGaN electrode.