Condensed Matter Physics • IFF Scientific Report 2006 52 I 53
Influence of MgO Overlayers on the Electronic States of bct Co(001)
F. Matthes
1, M. Müller
1, C. M. Schneider
1, L.-N. Tong
2, C.-L. Deng
1, and C.-G. Lee
31Institut für Festkörperforschung IFF-9 “Elektronische Eigenschaften”
2Inst. Mat. Sci. Engin., Anhui University of Technology, Ma-An-Shan, 243002, Anhui, China
3School of Nano and Advanced Materials Engineering, Changwon National University, Changwon, Korea
Magnetic tunnel junctions involving single- crystalline MgO barriers and Co-based ferro- magnetic electrodes exhibit very large spin- dependent tunneling effects at room tempera- ture. This technologically very important effect is attributed to specific details in the band structure of Cobalt in the bcc structural modification. Lit- tle is known yet about the electronic states at the interface between Cobalt and MgO, which should crucially influence the spin-dependent transport properties. In order to shed light on the interfa- cial electronic properties in this system, we have performed spin-polarized photoemission experi- ments on bct-Co(001) films covered by MgO over- layers of various thicknesses. Beside a strong reduction of the spectral weight originating from the minority∆5band in the photoemission spec- tra, we find that the electronic structure of the bct-Co films is not altered by stoichiometric MgO overlayers.
Spin-dependent transport mechanisms are at the heart of spinelectronic functionality, as they pro- vide a control of the flow of the polarized charge carriers by means of the magnetic state of the electrodes. The most popular spin transport ef- fects, which are already being exploited in commer- cially available devices are giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) [1].
Recently, an epitaxial single-crystalline model system – Fe/MgO/Fe – has attracted great attention due to its very high magnetoresistance effect [2, 3]. The origin of the high TMR ratio in MgO-based single- crystal magnetic tunneling systems is attributed to a coherent spin-polarized tunneling process, involving a particular matching of the∆1 symmetry electronic states in the ferromagnet and the tunneling barrier.
In this context the electronic structure of the entire magnetic tunneling junction (MTJ) including the spin properties at the interfaces between the ferromag- netic electrode and the oxide barrier plays an im- portant role. However, the nature of the electronic states in the electrode and at the interface, and their role on the spin-polarized tunneling process is not well understood yet. Therefore, we have started a program addressing the interfacial ground-state elec- tronic properties of MgO-based TMR systems by means of spin-polarized, angle-resolved photoelec- tron spectroscopy. In the following, we focus on the
Co/MgO(001) system. Theoretical calculations per- formed by Zhang and Butler [4] predicted TMR val- ues for bcc-Co based MTJ’s that significantly exceed those in the Fe/MgO/Fe system.
Besides theoretical predictions, there are to our knowledge, no experimental band structure data available in literature that can give experimental evi- dence for the electronic structure of bcc Co(100) and its possible changes introduced by a MgO overlayer.
partialintensities/arbitraryunits
18 eV 22 eV 28 eV 32 eV 36 eV 40 eV 45 eV 60 eV hv
Spin up Spin down
binding energy/eV 0 -1 -2 -3
a)
spinpolarization/%
20 -60 20 -60 20 -60 20 -60 20 -60 20 -60 20
20 -60
-60
18 eV 22 eV 28 eV 32 eV 36 eV 40 eV 45 eV 60 eVhv
binding energy/eV 0 -1 -2 -3
b)
FIG. 1: Spin-resolved photoemission spectra for the 8 ML bct-Co film for different photon energies. (a) Partial intensi- ties with majority (green) and minority spin character (red).
(b) Corresponding intensity asymmetry spectra (blue).
The spin- and angle-resolved photoemission experi- ments were performed at the undulator beamline U- 125-1 PGM at BESSY (Berlin). The measurements were carried out in magnetic remanence with the p- polarized light impinging under45◦to the surface nor- mal. Analyzing only the photoelectrons along the sur- face normal, the non-relativistic dipole selection rules allowing direct transitions from initial bands with∆1
and∆5spatial symmetry are exploited. In a first step, we determined the dispersion of the electronic states of the uncovered Co-film for a wide range of photon energieshν, covering theΓ−∆−Hline in the Bril- louin zone.
IFF Scientific Report 2006 • Condensed Matter Physics
52 I53
-3.0 -2.0 -1.0 0.0
binding energy / eV a)
b)
-40 40 40
0
0
-40
spinpolarization/%spinpolarization/%
partialintensities/arbitraryunitspartialintensities/arbitraryunits
FIG. 2: Comparison of spin-polarized photoemission from a clean Co film (a) to that from a film with a MgO overlayer (b). Given are the spin polarization distribution (blue, right scale), and the majority (green) and minority spin (red) par- tial intensities (left scale).
From the measured total intensity the spin polariza- tion (Fig.1b) and the spin-resolved intensity contri- butions (partial intensities) of spin-up (majority spin) and spin-down character (minority spin) can be de- termined. The dispersion of these spectral features is interpreted on the basis of a calculated electronic band structure for bct-Co, the structural parameters (a=0.287 nm, c=0.2793 nm) being taken from [5]. For the calculation we employed the Munich SPRKKR package [6]. On this basis in the minority channel of the photoemission spectra we can identify the dis- persion of the∆↓5 band (marked by vertical bars in Fig.1a). The band intersects the Fermi level for tran- sitions at 22 eV excitation energy and approaches Γat 60 eV, having a binding energy of−1.1±0.2 eV. This band dispersion is accompanied by a pro- nounced minimum in the spin polarization spectra of the detected photoelectrons. The distinct appear- ance and dispersion of the∆↓5band indicates that the 8 ML Co film has indeed a three-dimensional elec- tronic structure. Another pertinent feature is the ma- jority spin peak around -0.8 eV belowEFarising from the∆↑1initial state band.
For the central issue of the study, the pronounced mi- nority spin peak at−0.2±0.2eV binding energy is of further importance. Scanning the excitation energy does not lead to noticable peak dispersions within our energy resolution, but the spin polarization spectra display a minimum of -55 % spin polarization at 60 eV excitation energy. This minority spin feature atEF, however, does not have a direct counterpart in the band structure of bcc-Co (lattice constant a=0.282 nm), because of a band gap in the∆↓1 band. The-
oretical calculations for tetragonal distorted bcc Co move this band closer to EF, having its maximum located only 0.1 eV above Fermi energy with small contributions even belowEF. We also note that de- pending on the underlying structure and also to some extend on the growth conditions different values for the tetragonal distortion have been reported in litera- ture. It is clear that altered lattice parameters lead to a shift of the individual band positions. The important finding in our studies is the occurrence of∆↓1 band contributions atEF. This may strongly influence the achievable TMR values, because the spin dependent tunneling conductivity can be increased in the case of antiparallel alignment of the Co electrodes, if the
∆↓1band shifts below the Fermi level.
In the presence of an ultrathin MgO overlayer, the spin-resolved spectra change in a distinct manner, an example being given in Fig. 2 forhν = 40 eV.
On the one hand, we note that the photoelectrons are still significantly polarized, i.e. the bonding at the Co/MgO interface does not significantly affect the magnetism in the interfacial Co layers. The same is found for the other photon energies as well. On the other hand, the spectral shapes change significantly.
The spectral weight originating from direct transitions of initial∆↓5 minority band is suppressed when com- pared to all other peaks in the photoemission spec- tra. This phenomenon is clearly reflected in the spin polarization spectra of the measured photoelectrons.
Fig. 2 shows that the minimum located at -1.0 eV is smeared out upon coverage with MgO. We reported an analogue effect for transitions originating from∆↓5 minority band in the case of bcc Fe/GaAs(001) upon coverage with MgO [7]. The origin of the selective suppression is still under discussion. It cannot be ex- cluded yet, that the introduced changes are only rel- evant for processes occurring only during photoelec- tron emission. More information on this topic may be found in Ref. [8].
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