Magnetic field-dependent effective microwave properties of microwire-epoxy composites Appl. Phys. Lett. 101, 152905 (2012) Giant magnetoresistance effect in graphene with asymmetrical magnetic superlattices Appl. Phys. Lett. 101, 152404 (2012) Giant magneto-resistance estimated from direct observation of nanoscale ferromagnetic domain evolution in La0.325Pr0.3Ca0.375MnO3 J. Appl. Phys. 112, 053924 (2012) Electrical determination of relative chirality direction in a Co/Cu/Co ferromagnetic ring

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...

Fully epitaxial magnetic tunnel junctions ͑MTJs͒ were fabricated with full-Heusler alloy Co 2 MnSi thin films as both lower and upper electrodes and with a MgO tunnel barrier. The fabricated MTJs showed clear exchange-biased tunnel magnetoresistance ͑TMR͒ characteristics with high TMR ratios of 179% at room temperature ͑RT͒ and 683% at 4.2 K. In addition, the TMR ratio exhibited oscillations as a function of the MgO tunnel barrier thickness ͑t MgO ͒ at RT, having a period of 0.28 nm, for t MgO ranging from 1.8 to 3.0 nm. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2843756͔Highly spin-polarized electrons are essential for spintronic devices, in which both the charge and the spin of the electron are utilized as the information carrier. Half-metallic ferromagnets ͑HMFs͒ are characterized by a complete spin polarization at the Fermi level ͑E F ͒ due to the existence of an energy gap for one spin direction ͑usually minority spin͒ ͑Ref. 1͒. The potentially high spin polarization of HMFs is widely advantageous for ferromagnetic electrodes used in spintronic devices in terms of achieving high tunnel magnetoresistance ͑TMR͒ ratios in magnetic tunnel junctions ͑MTJs͒ and efficient spin injection from ferromagnetic electrodes into semiconductors.Co-based full-Heusler alloys ͑Co 2 YZ͒ ͑Ref. 2͒ have attracted much interest as a preferable ferromagnetic electrode material for spintronic devices. This is because of the HMF nature theoretically predicted for many of these alloys, and because of their high Curie temperatures, which are well above room temperature ͑RT͒. In particular, Co 2 MnSi ͑CMS͒ has especially attracted interest because of its half-metallic nature theoretically predicted, with a large energy gap of 0.42 eV ͑Ref. 3͒ to 0.81 eV ͑Ref. 4͒. Much effort has been dedicated to fabricating and characterizing CMS thin films [5][6][7] and also to fabricating MTJs with a CMS as a lower electrode or CMS as both lower and upper electrodes and with an amorphous AlO x barrier. [8][9][10] We recently developed fully epitaxial MTJs with a Co 2 YZ thin film ͓Co 2 Cr 0.6 Fe 0.4 Al ͑CCFA͒, Co 2 MnSi ͑CMS͒, or Co 2 MnGe ͑CMG͔͒ as a lower electrode, and a MgO ͑001͒ tunnel barrier. [11][12][13][14][15][16] The relatively small lattice mismatch between Co 2 YZ and MgO for a 45°in-plane rotation ͑e.g., about −3.7% for CCFA and −5.1% for CMS͒ enabled us to successfully fabricate fully epitaxial MTJ trilayers featuring highly smooth and abrupt interfaces. 12 16 However, there is much room for further enhancement of the TMR ratio of these fully epitaxial MTJs. Since Co 2 YZ thin films potentially have a high spin polarization value at RT, a promising approach would be to use these films as both the lower and upper electrodes. 10,17,18 In the present study, as an extension of our work on CMS/ MgO / Co 50 Fe 50 MTJs, 13 we fabricated fully epitaxial MTJs with CMS electrodes as both the lower and upper electrodes and with a MgO tunnel barrier and investigated their TMR characteristics.We fabricated exchange-biased MTJs. In order ...

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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...