No abstract

Fully epitaxial, exchange-biased magnetic tunnel junctions ͑MTJs͒ were fabricated with a Co-based full-Heusler alloy Co 2 MnSi ͑CMS͒ thin film as a lower electrode, a MgO tunnel barrier, and a Co 50 Fe 50 upper electrode. The microfabricated CMS/ MgO / Co 50 Fe 50 MTJs exhibited relatively high tunnel magnetoresistance ratios of 90% at room temperature and 192% at 4.2 K. The bias voltage dependence of differential conductance ͑dI / dV͒ for the parallel and antiparallel magnetization configurations suggested the existence of a basic energy gap structure for the minority-spin band of the CMS electrode with an energy difference of about 0.4 eV between the bottom of the vacant minority-spin conduction band and the Fermi level. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2378397͔Employing spin-polarized electrons is essential for spintronic devices. Due to the existence of an energy gap at the Fermi level ͑E F ͒ for one spin direction, half-metallic ferromagnets are characterized by a complete spin polarization at E F , 1 so these are the most promising ferromagnetic electrode materials for spintronic devices. Co-based full-Heusler alloys have been extensively studied recently as candidates for use in half-metallic ferromagnetic electrodes 2,3 and regarding their application to magnetic tunnel junctions ͑MTJs͒. [4][5][6][7][8][9][10][11][12][13] This is because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys 14,15 and because of their high Curie temperatures, which are well above room temperature ͑RT͒. 16 One Co-based full-Heusler alloy, in particular, Co 2 MnSi ͑CMS͒, has attracted interest because of its half-metallic nature theoretically predicted, with a large energy gap of 0.42 eV ͑Ref. 14͒ to 0.81 eV ͑Ref. 15͒ for its minority-spin band, and its high Curie temperature of 985 K. 16 Sakuraba et al. reported a high tunneling magnetoresistance ͑TMR͒ ratio of 570% at 2 K ͑67% at RT͒ for MTJs consisting of an epitaxial CMS lower electrode, an amorphous AlO x tunnel barrier, and a highly oriented CMS upper electrode. 11 We recently developed fully epitaxial MTJs with a Co-based full-Heusler alloy ͑Co 2 YZ͒ thin film of either Co 2 Cr 0.6 Fe 0.4 Al ͑CCFA͒ ͑Refs. 7-9͒ or Co 2 MnGe ͑Refs. 8 and 10͒ as a lower electrode and a MgO tunnel barrier and have demonstrated a relatively high TMR ratio of 90% at RT ͑240% at 4.2 K͒ for fully epitaxial CCFA/ MgO / Co 50 Fe 50 MTJs. 9 We have developed fully epitaxial MTJs with a CMS thin film as a lower electrode and a MgO tunnel barrier in the present study and have investigated their spin-dependent tunneling characteristics.We will now describe the fabrication of fully epitaxial MTJs with a CMS thin film and a MgO tunnel barrier. Each layer in the MTJ structure was successively deposited in an ultrahigh vacuum chamber ͑with a base pressure of ϳ8 ϫ 10 −8 Pa͒ through the combined use of magnetron sputtering and electron beam ͑EB͒ evaporation.The CMS layer was deposited on a MgO buffer layer by magnetron sputtering at RT and subseq...

Half-metallic ferromagnets ͑HMFs͒ are expected to be a key material for ferromagnetic electrodes that can provide highly spin-polarized currents. This is because HMFs are characterized by an energy gap at the Fermi level ͑E F ͒ for the minority-spin band, leading to complete spin polarization at E F . 1 Cobalt-based full-Heusler alloys, whose composition is represented by Co 2 YZ, have attracted much interest due to the half-metallic nature theoretically predicted for some of these alloys 2,3 and their high Curie temperatures, which are well above room temperature ͑RT͒. 4,5 The potentially high spin polarization of Co-based full-Heusler alloys is very advantageous for obtaining high tunnel magnetoresistance ͑TMR͒ ratios in magnetic tunnel junctions ͑MTJs͒ according to Jullière's model. 6 Inomata et al. first demonstrated a relatively high TMR ratio of 16% at RT for MTJs using a Cobased full-Heusler alloy ͑Co 2 YZ͒ thin film, where they used a polycrystalline Co 2 Cr 0.6 Fe 0.4 Al ͑CCFA͒ thin film as a lower electrode and an amorphous AlO x tunnel barrier. 7 Sakuraba et al. reported a high TMR ratio of 570% at 2 K ͑67% at RT͒ for MTJs consisting of a lower electrode made of epitaxially grown Co 2 MnSi ͑CMS͒ ͑which is a full-Heusler alloy͒, an amorphous AlO x tunnel barrier, and a highly oriented CMS upper electrode. 8 We recently developed fully epitaxial MTJs that have a Co 2 YZ thin film of CCFA, 9-12 Co 2 MnGe, 11,13 or Co 2 MnSi ͑Ref. 14͒ as a lower electrode, and a MgO tunnel barrier, and have demonstrated a relatively high TMR ratio of 90% at RT ͑240% at 4.2 K͒ for CCFA/ MgO / Co 50 Fe 50 MTJs ͑Ref. 12͒ and a TMR ratio of 90% at RT ͑192% at 4.2 K͒ for Co 2 MnSi/ MgO / Co 50 Fe 50 MTJs. 14 A high tunneling spin polarization of 0.79 at 4.2 K was estimated from the TMR ratios for the epitaxial CCFA films with the B2 structure. 12 For these CCFA/ MgO / Co 50 Fe 50 MTJs, however, the parallel and antiparallel magnetization configurations were controlled by using the difference in the coercive forces between the CCFA lower electrode and the Co 50 Fe 50 upper electrode. This resulted in peaked magnetoresistance versus magnetic field characteristics, 12 which probably led to TMR ratios lower than they potentially could be. Exchange biasing is favorable for realizing high degrees of the parallel and antiparallel magnetization configurations. 15 Our purpose in the present study was to demonstrate the potentially high tunneling spin polarization of a Co-based full-Heusler alloy of CCFA. To do this, we fabricated fully epitaxial MTJs with exchange biasing that consisted of a CCFA thin film and a MgO tunnel barrier, and then investigated the TMR characteristics of the fabricated MTJs. Our approach was to use an upper electrode of Co 50 Fe 50 film in an antiferromagnetically coupled ͑i.e., synthetic ferrimagnetic͒ Co 50 Fe 50 /Ru/Co 90 Fe 10 trilayer exchange-biased by an IrMn antiferromagnetic layer through the Co 90 Fe 10 / IrMn interface to obtain a high exchange-bias field value ͑H ex ͒ for epitaxial Co 50 Fe 50 el...

Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co2Cr0.6Fe0.4Al (CCFA) and Co2MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600 °C. X-ray pole figure measurements of the CCFA films indicated that these films were epitaxial and crystallized in the B2 structure. X-ray pole figure measurements of the annealed CMG films showed (111) peaks with four-fold symmetry, which provides direct evidence that these films were epitaxial and crystallized in the L21 structure. Furthermore, cross-sectional transmission electron microscope images of a fabricated CMG film indicated that it was single-crystalline. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.23 nm rms for 100 nm thick CCFA films and 0.26 nm rms for 45 nm thick CMG films. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier and a Co50Fe50 (CoFe) upper electrode. All layers were successively deposited in an ultrahigh vacuum chamber through the combined use of magnetron sputtering and electron beam evaporation. Reflection high-energy electron diffraction patterns observed in situ for each layer during preparation clearly indicated that all layers grew epitaxially in both the CCFA/MgO/CoFe and CMG/MgO/CoFe MTJ layer structures. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios, for MTJs using a full-Heusler alloy, of 42% at RT and 74% at 55 K. On the other hand, the microfabricated epitaxial CMG/MgO/CoFe MTJs showed strongly temperature-dependent TMR characteristics with typical TMR ratios of 14% at RT and 70% at 7 K. These results confirm the promise of epitaxial MTJs as a key device structure for clarifying and utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films.

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated using a Co-based full-Heusler alloy Co2Cr0.6Fe0.4Al (CCFA) thin film and a MgO tunnel barrier. The fabricated MTJs had the following layer structure: MgO buffer layer/CCFA lower electrode/MgO tunnel barrier/CoFe upper electrode, grown on a MgO single-crystal substrate. All layers were successively deposited in an ultrahigh vacuum chamber through the combined use of magnetron sputtering and electron beam evaporation. RHEED patterns observed in situ for each layer during preparation clearly indicated that all layers grew epitaxially with the (001) basal plane. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance ratios, for MTJs using a full-Heusler alloy, of 42% at room temperature and 74% at 55 K. These results confirm the promise of an epitaxial MTJ using a Co-based full-Heusler alloy as a key device structure utilizing the potentially high spin polarization of this material system.