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

Abstract

The electronic and magnetic states of an epitaxially grown full-Heusler alloy Co 2 MnSi ͑CMS͒ 1.1 nm ͑4 ML͒ thick ultrathin film and a CMS 50-nm-thick film both facing an epitaxial MgO͑001͒ tunnel barrier were element-specifically studied by means of x-ray absorption spectroscopy ͑XAS͒ and x-ray magnetic circular dichroism ͑XMCD͒. The observed XAS and XMCD spectra revealed that both the CMS films were not oxidized. The Co and Mn spin magnetic moments for the 4 ML thick CMS film obtained by applying the sum rules were close to those for the 50-nm-thick one and the theoretical values for bulk CMS with the L2 1 structure. These results indicate that 4 ML thick CMS ultrathin films facing a MgO barrier still retain the electronic and magnetic states for the L2 1 structure. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2836676͔Co-based Heusler alloys ͑Co 2 YZ͒ are promising ferromagnetic electrode materials for spintronic devices. [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. [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.The sample layer structure was grown on a MgO͑001͒ single-crystal substrate and, from the substrate side, consisted of MgO buffer ͑10 nm͒ / Fe underlayer ͑50 nm͒/CMS ultrathin film ͑4 ML͒/MgO barrier ͑2 nm͒ / Ru cap ͑2 nm͒. A monolayer of Co 2 MnSi consists of a Co plane and a Mn-Si plane, and a unit cell of CMS, whose lattice parameter is 0.5654 nm ͑Ref. 13͒, corresponds to 2 ML of CMS. We grew the 4-ML-thick CMS ultrathin film on the Fe underlayer to stabilize its ferromagnetism at RT. 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...

Magnetic states of Mn and Co atoms atCo2MnGe/MgOinterfaces seen via soft x-ray magnetic circular dichroism

“…This consideration is consistent with that the spin polarization of Co-rich CMG estimated from the TMR ratio at 4.2 K for CMG/MgO/ CoFe MTJs assuming Jullière's model, P CMG , was as low as 0.74. 6 It was reported that the m spin ͑Co͒ of Co-rich CMS/MgO was 1.16 B for a sample with t CMS = 50 nm, and 1.25 B for a sample with t CMS = 1.1 nm ͑4 ML͒, 43 both being larger than a theoretical value of 1.06 B . 27 The CMS film composition of Co: Mn: Si= 2 : 0.91: 0.93 was Co-rich similar to the Co-rich CMG that we have studied here.…”

Antiferromagnetic exchange interactions in the Ni2Mn1.4In0.6ferr

“…4 In 0.6 is more than that in Ni 2 MnIn in agreement with experiment. This indicates an increased hybridization between Ni 3d and Mn [28][29][30] . These features are considered to be a signature of localized 3d electrons 28,31,32 .…”

“…This indicates an increased hybridization between Ni 3d and Mn [28][29][30] . These features are considered to be a signature of localized 3d electrons 28,31,32 . Alternatively, a selective oxidation of Mn atoms can also result in multiplet structures 29,30 .…”

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

“…To date several studies have utilized these techniques to examine the interfacial properties of materials used in MTJs, [16][17][18][19][20][21][22][23][24] and some of them have focused on Heusler-alloy-based MTJs. Electronic and magnetic properties at interfaces have been investigated for various thicknesses in Co-rich Co 2 MnSi 25,26 and Co 2 MnGe 27 thin films. However, so far, element-specific studies of the effects of off-stoichiometry in Heusler-alloy thin films on their electronic and magnetic properties have not been carried out.…”