Spin-dependent tunneling characteristics of fully epitaxial magnetic tunneling junctions with a full-Heusler alloy Co2MnSi thin film and a MgO tunnel barrier

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...

Effects of off-stoichiometry on the spin polarization at the Co2MnβGe0.38/MgO interfa

“…Materials which exhibit half-metallic ferromagnetism have recently attracted great interest because a high efficiency is expected for spintronic device applications, including tunnel magnetoresistance (TMR) devices [1][2][3] and giant magnetoresistance devices. 4 In these materials the majority spin states have a metallic character with a nonzero density of states (DOS) at the Fermi level, while the minority spin states have a band gap at the Fermi level, resulting in 100% spin polarization at the Fermi level (E F ).…”

Electronic and magnetic properties of Heusler alloy Co2MnSi epitaxial ultrathin films facing a MgO barrier studied by x-ray magnetic circular dichroism

“…[1][2][3][4][5][6][7][8] This is because of their potentially high spin polarization arising from 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 ͑RT͒. We have recently developed fully epitaxial magnetic tunnel junctions ͑MTJs͒ with a Co 2 YZ thin film and a MgO͑001͒ tunnel barrier, and demonstrated a relatively high tunnel magnetoresistance ratio at RT.…”

“…We have recently developed fully epitaxial magnetic tunnel junctions ͑MTJs͒ with a Co 2 YZ thin film and a MgO͑001͒ tunnel barrier, and demonstrated a relatively high tunnel magnetoresistance ratio at RT. [3][4][5][6][7][8] The spin-dependent tunneling characteristics in MTJs are very sensitive to the electronic and magnetic states of the interfacial region of ferromagnetic electrodes with a tunnel barrier.Recently, x-ray absorption spectroscopy ͑XAS͒ and x-ray magnetic circular dichroism ͑XMCD͒ have proved to be effective techniques for obtaining microscopic information about the element-specific electronic and magnetic states in the interfacial region of MTJs. [9][10][11][12] In this work, we fabricated an epitaxial Co 2 MnSi ͑CMS͒ 1.1 nm ͑4 ML͒-thick ultrathin film and a CMS 50-nm-thick film with an epitaxial MgO barrier in order to investigate the interfacial region selectively by XAS and XMCD measurements at the Mn-L 2,3 and Co-L 2,3 edges.…”

“…We have recently developed fully epitaxial magnetic tunnel junctions ͑MTJs͒ with a Co 2 YZ thin film and a MgO͑001͒ tunnel barrier, and demonstrated a relatively high tunnel magnetoresistance ratio at RT. [3][4][5][6][7][8] The spin-dependent tunneling characteristics in MTJs are very sensitive to the electronic and magnetic states of the interfacial region of ferromagnetic electrodes with a tunnel barrier.…”

“…Each layer in the sample layer structure was successively deposited in an ultrahigh vacuum chamber ͑base pressure: about 6 ϫ 10 −8 Pa͒ through the combined use of magnetron sputtering ͑for Fe, CMS, and Ru͒ and electron beam evaporation ͑for MgO͒; the fabrication procedure was the same as in previous work. [3][4][5][6][7][8]14 The CMS film deposited at RT were annealed in situ at 325°C for 15 min. The composition of the CMS film used in this study was determined as Co 2 Mn 0.91 Si 0.93 through inductively coupled plasma analysis with an accuracy of 2%-3% for the composition of each element.…”