Chemical and magnetic interface properties of tunnel junctions with co2mnsi/co2fesi multilayer electrode showing large tunneling magnetoresistance

IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 Chemical and Magnetic Interface Properties of Tunnel Junctions With Co 2 MnSi/Co 2 FeSi Multilayer Electrode Showing Large Tunneling Magnetoresistance Jan Schmalhorst 1 , Daniel Ebke 1 , Marc D. Sacher 1 , Ning-Ning Liu 1 , Andy Thomas 1 , Gunter Reiss 1 , Andreas Hutten 1 ; 2 , and Elke Arenholz 3 Thin Films and Nano Structures, Department of Physics, Bielefeld University, 33501 Bielefeld, Germany Institute for Nano-Technology, Research Center Karlsruhe, 76021 Karlsruhe, Germany Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA Transport, as well as chemical and magnetic interface properties of two kinds of magnetic tunnel junctions (MTJs) with Co 2 FeSi electrode, Al-O barrier, and Co-Fe counter electrode, are investigated. For junctions with Co 2 FeSi single-layer electrodes, a tunnel mag- netoresistance of up to 52% is found after optimal annealing for an optimal Al thickness of 1.5 nm, whereas the room temperature bulk magnetization of the Co 2 FeSi film reaches only 75% of the expected value. By using a [Co 2 MnSi/Co 2 FeSi] 10 multilayer electrode, the magnetoresistance can be increased to 114%, corresponding to a large spin polarization of 0.74, and the full bulk magnetization is reached. For Al thickness smaller than 1 nm, the TMR of both kinds of MTJs decreases rapidly to zero. On the other hand, for 2- to 3-nm-thick Al, the TMR decreases only slowly. The Al thickness dependence of the TMR is directly correlated to the element-specific magnetic moments of Fe and Co at the Co 2 FeSi/Al-O interface for all Al thickness. Especially, for optimal Al thickness and annealing, the interfacial Fe moment of the single-layer electrode is about 20% smaller than for the multilayer electrode, indicating smaller atomic disorder at the barrier interface for the latter MTJ. Index Terms—Heusler alloys, Heulser compounds, magnetic interface properties, magnetic tunneling junctions, X-ray absorption spectroscopy. AGNETIC tunnel junctions (MTJs) consisting of fer- romagnetic electrodes separated by a thin insulating tunnel barrier are basic elements for spintronic devices. A very large tunnel magnetoresistance (TMR) is anticipated. The TMR amplitude is connected to the Julliere spin polarization of electrode and Ferromagnetic half-metals—having a gap in the minority or and, majority electron density of states at the Fermi energy %—are important to reach highest TMR ampli- thus, tudes. This property has been predicted theoretically for some half and full Heusler compounds [2]–[5], such as Co MnSi and Co FeSi. The Heusler alloy Co FeSi crystallized in L2 structure has a magnetic moment of 6 per formula unit [5] and a very high Curie temperature of 1100 K [5], making it especially attractive for applications. MTJs with Co FeSi single-layer electrodes will be discussed first. It will be shown that the atomic ordering of the Co FeSi film is not perfect after optimal post-annealing resulting in reduced magnetization and spin polarization. To increase both properties, an improved atomic order of the Co FeSi is required. This can be achieved multilayer electrodes by introducing [Co MnSi/Co FeSi] instead of the Co FeSi single layers to force the atomic ordering of the Co FeSi layers by the adjacent Co MnSi layers. DC- and RF-magnetron sputter deposition at RT from sto- ichiometric targets was used to prepare the MTJ stacks. On thermally oxidized Si(100) wafers we deposited a 40-nm-thick V buffer and subsequently a magnetically soft Co FeSi nm M Digital Object Identifier 10.1109/TMAG.2007.893475 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. single layer or a [Co MnSi nm /Co FeSi nm ] multilayer. This bottom electrode was coverd by a 0.5–3-nm-thick Al film and plasma oxidized for 150 s to form a tunnel barrier. This stack was in situ annealed at about 380 C for 60 min. To remove surface contamination the barrier was oxidized for another 50 s after annealing and covered with the top electrode (Co Fe nm /Ta nm /Cu nm /Ta nm /Au nm ). nm/Mn Ir Fi- nally, the layer stacks were ex situ vacuum annealed at 275 C in a magnetic field of 0.1 T to set the exchange bias of the pinned electrode and patterned by optical lithography and ion beam etching. “Half” junctions without top electrode were also fabricated for X-ray absorption (XAS), X-ray magnetic circular dichroism (XMCD) and Auger depth profiling probing the structural and magnetic properties of the Heusler com- pound-based electrodes and at the Co FeSi/AlO interfaces. These “half” junctions were ex situ annealed up to 550 C. XAS and XMCD was performed at beamline 4.0.2 and 7.3.1.1 of the Advanced Light Source, Berkeley, CA. Surface-sensitive total electron yield (TEY) [6], as well as bulk-sensitive fluorescence yield (FY) spectra [6] were recorded. Typical TMR majorloops (measured at low tempera- ture) of our Co FeSi/AlO /Co-Fe junctions with optimum Al thickness of 1.5 nm are shown in Fig. 1(a): Using a Co FeSi single electrode a maximum TMR amplitude of (here, 52% is found corresponding to we assumed for the Co-Fe electrode on top of the Al-O barrier [7]). The TMR amplitude of the optimal [Co MnSi/Co FeSi] /AlO /Co-Fe junctions is more than two times larger than for the Co FeSi single-layer-based MTJ, namely 114% at 17 K. The according low temperature spin polarization of the [Co MnSi/Co FeSi] /AlO interface is 0.74, which is comparable to the highest values reported for Co MnSi-based junctions so far [8], [9]. The Al thickness 0018-9464/$25.00 © 2007 IEEE