Effect of defects in Heusler alloy thin films on spin-dependent tunnelling characteristics of Co₂MnSi/MgO/Co₂MnSi and Co₂MnGe/MgO/Co₂MnGe magnetic tunnel junctions

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

“…The preparation of the samples has been described in detail elsewhere. 15 Reflection high-energy electron diffraction measurements clearly showed that both the cosputtered CMG electrode and the MgO barrier in the CMG/MgO halfjunction grew epitaxially. 14 Microbeam electron diffraction patterns indicated that a Mn-rich CMG film with a film composition of Co 2 Mn 1.20 Ge 0.38 prepared by cosputtering had the B2 structure.…”

“…The calculation has also predicted that Mn Co is coupled antiferromagnetically with neighboring Co atoms and that Co Mn has a larger magnetic moment than Co atoms at the regular sites. Ishikawa et al 14 and Yamamoto et al 15 have made systematic studies of the effect of nonstoichiometry of Heusler alloy thin films on the spin-dependent tunneling characteristics of Co 2 MnZ/MgO/Co 2 MnZ (Z = Si, Ge) magnetic tunnel junctions (MTJs) and found that a higher TMR ratio is obtained for Mn compositions α > 1.0 in Co 2 Mn α Si electrodes and, similarly, for Mn compositions β > 1.0 in Ge-deficient Co 2 Mn β Ge δ electrodes (where δ < 1). The observed lower TMR ratios for MTJs with Mn-deficient Co 2 Mn α Si or Co 2 Mn β Ge δ electrodes were explained by the formation of Co Mn antisites that lead to minority-spin in-gap states around E F as theoretically predicted by Picozzi et al 7 Furthermore, the observed higher TMR ratio for MTJs with Mn-rich Co 2 Mn α Si or Co 2 Mn β Ge δ electrodes was explained by suppressed Co Mn antisites, which caused a reduced density of minority-spin in-gap states around E F .…”

“…The effect of defects in the Heusler alloys Co 2 Y Z introduced by off-stoichiometry on the spin-dependent electronic structure has been investigated theoretically [7][8][9][10][11][12] as well as experimentally. [13][14][15] Picozzi et al 7 have predicted from a first-principles calculation that halfmetallicity in Co 2 MnSi and Co 2 MnGe is lost by Co Mn antisites through the appearance of minority-spin in-gap states around E F , while half-metallicity is robustly retained by Mn Co antisites. The calculation has also predicted that Mn Co is coupled antiferromagnetically with neighboring Co atoms and that Co Mn has a larger magnetic moment than Co atoms at the regular sites.…”

“…In this paper, we have studied element specifically how the electronic and magnetic states of Mn and Co atoms in Ge-deficient Co 2 Mn β Ge 0.38 (CMG) facing an MgO barrier are affected by nonstoichiometry by using XAS and XMCD and a density-functional calculation. The calculation was made for a wide range of nonstoichiometry corresponding to the real material Co 2 Mn β Ge δ with 0.6 < β < 1.8 and δ ∼ 0.4, 15 which exceeds the range studied by Piccozi et al, 7 where the properties of a single antisite defect are dominant.…”

Bias-voltage-dependence of magnetoresistance for epitaxial Fe/MgO/Co2MnSn tunnel junctions

“…Half metallic materials, which exhibit perfect spin polarization at the Fermi energy, are expected to be ideal materials to achieve large TMR ratio. Recently, TMR ratio of above 300 % at RT [4] was achieved in MTJs using Co-based Heusler alloys [4][5][6][7] which are theoretically predicted as half metallic materials [8][9]. On the other hand, we have reported that Co 2 MnSn Heusler alloy films with relatively uniform local magnetism can be prepared when the films are grown by atomically controlled alternate deposition at 400 o C [10].…”