DC 마그네트론 스퍼터링 방법을 이용한 하프메탈 Fe_(3)O_(4) 제작
Fabrication of Half-Metallic Fe_(3)O_(4) Thin Film using DC Magnetron Sputtering System
마그네트론 스퍼터링 하프메탈;
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Tunneling magnetoresistance(TMR), giant magnetoresistance (GMR), and magnetic random access memory(MRAM) are currently active areas of research. The magnetoresistance ratio is related to the spin polarization(P) of electrons in the ferromagnetic electrodes of tunnel junctions or intermediate layers. The most attractive materials for high P are the so-called half-metallic ferromagnets, which are metallic for one spin direction and insulating for the other. This means a P of 100%. Half metallic is known for CrO_(2), Fe_(3)O_(4) (magnetite), and La_(0.7)Sr_(0.3)MnO_(3) (LSMO). Application to TMR, GMR, and MRAM devices requires a high Curie temperature(Tc). Hence, the half metallic ferromagnetic material is limited to Fe_(3)O_(4) in terms of its high Tc=858 K, compared to LSMO (Tc=360 K) or CrO_(2)(Tc=395 K). The bulk Fe_(3)O_(4) saturation magnetization (MS) at room temperature is 477 emu/cc. Fe_(3)O_(4) has a cubic inverse spinel structure, in which the Fe cations occupy interstices of a face-centered-cubic closed packed frame of oxygen ions (lattice parameter a=8.396 ). The 8 tetrahedral (A) sites are solely occupied by Fe^(3+), while the 16 octahedral(B) sites are equally shared by Fe^(3+) and Fe^(2+) ions. The rapid hopping of electrons between Fe^(2+) and Fe^(3+) ions in the B sites results in good RT conductivity. Upon cooling, Fe_(3)O_(4) undergoes a metal-insulator transition which is defined as Verwey transition at a temperature of T_(V)=∼120 K (T_(V) : Verwey point) associated with the freezing of the electron hopping. In this study, Fe_(3)O_(4) thin films have been directly DC magnetron sputter deposited from a target consisting of Fe at room temperature(RT). The films were prepared by reactive sputtering and effects of film thickness, oxygen flow rate were examined. X-ray diffractometry showed that Fe_(3)O_(4) film was polycrystalline grown onto SiO_(2), Si and glass substrates. The saturation magnetization, resistivity, and Verwey transition, magneto-resistance were respectively, 300 emu/cc, ρ=4.0×10^(-2) Ωcm, T_(V)=125K, 1.6%. These values were comparable to those of the Fe_(3)O_(4) bulk. Our experimental results suggested that a high-quality Fe_(3)O_(4) film could be obtained even under the crucial conditions of deposition temperature being room temperature and the film being ultrathinned 10 nm. We have succeeded in preparing Fe_(3)O_(4) films by dc reactive magnetron sputtering system. To apply the Fe_(3)O_(4) film to the aforementioned devices, a RT preparation for the Fe_(3)O_(4) film and its thickness (≤∼300 nm, if possible ∼10 nm) are required.