We have inserted nonmagnetic impurity layers of Au into sputtered AlOx-based magnetic tunnel junctions (F/I/F) and Meservey–Tedrow junctions (S/I/F) in order to study their effect on the tunneling magnetoresistance (TMR) and spin polarization (TSP). Both room temperature TMR and the TSP at 250  mK decay exponentially as an interfacial Au layer is introduced between the barrier and one Co electrode, with 1/e decay lengths λTMR=11±3  Å and λTSP=14±2  Å. We also inserted a 1  Å thick Au layer at a variable distance from the barrier/Co interface and find that both the TMR and TSP recover to the undoped value with the shorter exponential length scales of λTMR=7±4  Å and λTSP=6±2  Å

Courtois, H.

Dempsey, K.J.

Gabureac, M.S.

Marrows, C.H.

Porter, N.A.

Rajauria, S.

English

M. S. Gabureac

K. J. Dempsey

N. A. Porter

C. H. Marrows

S. Rajauria

H. Courtois

Crossref

Spin-polarized tunneling with Au impurity layers

White Rose Research Online

This is a repository copy of Spin-polarized tunneling with Au impurity layers .White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/3794/Article:Gabureac, M.S., Dempsey, K.J., Porter, N.A. et al. (3 more authors) (2008) Spin-polarized tunneling with Au impurity layers. Journal Of Applied Physics, 103 (7). 07A915. https://doi.org/10.1063/1.2839623eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse See Attached Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. promoting access to White Rose research papers    White Rose Research Online   Universities of Leeds, Sheffield and York http://eprints.whiterose.ac.uk/    This is an author produced version of a paper published in Journal of Applied Physics.   White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/3794/    Published paper Gabureac, M.S., Dempsey, K.J., Porter, N.A., Marrows, C.H., Rajauria, S. and Courtois, H.  (2008) Spin-polarized tunneling with Au impurity layers, Journal of Applied Physics, Volume 103 (7), 07A915.     eprints@whiterose.ac.uk  Spin-polarized tunneling with Au impurity layersM. S. Gabureac, K. J. Dempsey, N. A. Porter, and C. H. Marrows∗School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United KingdomS. Rajauria and H. Courtois†Institut Néel, CNRS/UJF, 25 Avenue des Martyrs, 38042 Grenoble Cedex 9, France(Dated: November 15, 2007)We have inserted non-magnetic impurity layers of Au into sputtered AlOx-based magnetic tunneljunctions (F/I/F) and Meservey-Tedrow junctions (S/I/F) in order to study their effect on thetunneling magnetoresistance (TMR) and spin-polarization (TSP). Both room temperature TMRand the TSP at 250 mK decay exponentially as an interfacial Au layer is introduced between thebarrier and one Co electrode, with 1/e decay lengths λTMR = 11 ± 3 Å and λTSP = 14 ± 2 Å. Wealso inserted a 1 Å thick Au layer at a variable distance from the barrier/Co interface and find thatboth the TMR and TSP recover to the undoped value with the shorter exponential lengthscales ofλTMR = 7 ± 4 Å and λTSP = 6 ± 2 Å.PACS numbers: 75.47.-m, 75.70.-iThe first report of a magnetic tunnel junction (MTJ)was made over 30 years ago by Julliere1. These struc-tures consist of an ultrathin insulating barrier separatingtwo ferromagnetic electrodes (a so-called F/I/F config-uration). In the past decade there has been a huge up-surge in interest in these structures (recently reviewed byTsymbal, Mryasov and LeClair2) as the improvement offabrication techniques has lead to values of room temper-ature tunneling magnetoresistance (TMR) that are largeenough for technological applications3. The magnitudeof the TMR, defined as the fractional change in junctionresistance ∆R/R on switching the junction from a par-allel (P) to an antiparallel (AP) magnetization state, de-pends on the degree of tunneling spin polarization (TSP)of the F electrode materials1. This can be measured inan independent way by forming a junction between theferromagnet in question and a superconductor (an S/I/Fstructure) by a means devised by Meservey and Tedrow4.The quality of the interfaces on either side of the tun-neling barrier is widely considered to be paramount inthese structures. Here we describe experiments wherewe have deliberately inserted thin layers of Au as an im-purity at this point. We have introduced the Au bothat the interface between a Co electrode and an AlOxbarrier, and also as a δ-layer, which we can place at avariable distance from that interface. This second casemirrors a previous study of the giant magnetoresistance(GMR) in Co/Cu/Co spin-valves5. There are reports ofprevious experiments on MTJs similar to our interfacelayer studies. Moodera et al. deposited Au layers ontoCo electrodes before covering them with plasma-oxidizedalumina, and found that the TMR almost vanished forAu layers thicker than about 1 nm, in addition to a weakquantum well oscillation6. Other spacer materials havealso been studied, such as Cu7–9, Cr10, and Ru11,12. Avariety of different effects, some pointing to the presenceof quantum well states, were found8,11. On the otherhand, we know of no comparable studies in the tunnelingregime for the δ-layer experiments.Our junctions were of the cross-strip form, depositedby dc magnetron sputtering through shadow masks,which were changed in situ to give an active area 50 µmsquare. The substrates were pieces of Si wafer with ∼ 100nm thermal oxide at the surface. The chamber base pres-sure was ∼ 2× 10−8 Torr, whilst the working pressure ofAr was 2.5 mTorr. Typical deposition rates, calibrated bylow-angle x-ray reflectometry on test films of each mate-rial, were 2.5 Å/s for the metal layers. The barriers wereformed by dc plasma oxidation in 55 mTorr of O2 for 30 sat a power of 100 W. Two types of junctions were grown:the first were F/I/F MTJs based on the following layerstack sequence: substrate / Al (150 Å) / Co68Fe22B10(40 Å) / AlOx (13 Å) / Co (30 Å) / Ir20Mn80 (60 Å)/ Al (150 Å). The IrMn layer pins the Co through theexchange bias effect to control the switching between theP and AP states. The others were S/I/F junctions basedon the stacking sequence Al96Si4 (40 Å) / AlOx (13 Å)/ Co (200 Å). The superconducting Al96Si4 layer had acritical temperature TC of ∼ 3 K and an in-plane criticalfield, measured at 1.4 K, exceeding 5 T; both quantitieswere determined from two point resistance measurementsof a bottom electrode strip. The junctions were doped byintroducing a thin layer of Au either at the barrier/Co in-terface, or at a point within the Co layer a distance x fromthat interface. It is also important to emphasize that thegrowth protocol used here has been demonstrated to givehighly reproducible results: as series of several nominallyidentical undoped MTJs had TMR ratios and resistance-area products that were the same to within only a fewper cent. This means that sample-to-sample variationsshould not mask any changes due to the introduction ofAu above this level. The good tunnelling I-V propertiesof our junctions (not shown), along with x-ray reflectom-etry of comparable sheet film test samples, confirms thatinterfacial roughnesses are all well below 1 nm, and ourbarrier is smooth and pinhole-free.The TMR of the MTJs was measured by a conventionalfour-probe dc technique at room temperature: measure-202.55.07.510.012.5-500 -250 0 250 500no Au8 Å AuH (Oe)TMR (%)FIG. 1: (Color online) The TMR with field of a undopedMTJ, and a similar junction with 8 Å of Au at the pinned Colayer interface, measured at room temperature and a voltagebias of 10 mV.ments were taken under variable bias, but we only showdata here for a 10 mV applied voltage bias. All junc-tions were measured in their as-grown, unannealed state,to prevent any migration of the Au within the structure,for instance along grain boundaries. The differential con-ductance (dI/dV ) as a function of applied bias V of theS/I/F junctions were measured at 250 mK in a 3He re-frigerator, under a constant applied field of 2 T suppliedby a superconducting magnet. A separate transverse coilwas used to null off any small component of field normalto the junction surface due to sample misalignment, thatwould otherwise give rise to vortices in the superconduc-tor.In Fig. 1 we show an example of some TMR data froma pair of selected F/I/F samples: an undoped one, whichconforms exactly to the standard stacking sequence givenpreviously, and one in which a Au doping layer of 8 Åthickness has been introduced at the interface betweenthe Co pinned layer and the alumina barrier. The un-doped junction has a TMR of 11%, whilst the introduc-tion of the Au barrier has reduced the TMR to 8 %. Itcan be seen that there are no other significant changes tothe form of the TMR loop apart from a reduction in itsamplitude.We also measured the effects of introducing a Au in-terface layer on the TSP of the Co electrode by theMeservey-Tedrow method. Representative data mea-sured on an undoped S/I/F junction, and one contain-ing an 8 Å thick Au layer at the barrier/Co interfaceare shown in Fig. 2. The four peaks corresponding tothe Zeeman split edges of the BCS (Bardeen-Cooper-Schrieffer) gap in the Al96Si4 electrode are clearly visible.These data were modeled using the theory of Alexan-der, Orlando, Rainer, and Tedrow (AORT)13,14. In thismodel there are four input parameters: the BCS gap∆, the spin-orbit parameter b (which accounts for all themechanisms that mix spin up and down without destroy-ing the Cooper pairs), the spin-flip or depairing param-eter ξ (which includes all mechanisms which break thetime-reversal symmetry and hence break Cooper pairs),00.51.01.52.0-1.5 -1.0 -0.5 0 0.5 1.0 1.5no Au 8 Å Au 8 Å AuTSP = 21 %∆ = 0.435 mVξ = 0.02b = 0.03no AuTSP = 39 %∆ = 0.435 mVξ = 0.02b = 0.03V (V)Normalized dI/dVFIG. 2: (Color online) The differential conductance with biasfor an undoped Al96Si4/AlOx/Co junction and a similar junc-tion with 8 Å of Au at the barrier/Co interface, normalized tothe high bias value. Simulations of the data using the AORTmodel are plotted as black solid lines. The model parametersfor each curve are given in the plot.and the TSP itself. The fits to the data are shown in theplot as solid lines, along with the values of these param-eters. Both samples are well described by similar valuesfor ∆, b, and ξ. The TSP of our undoped sample is 39%,a typical value for a transition metal15. This is roughlyhalved, to 21 %, on the introduction of the Au interfacelayer.Having shown representative data for both F/I/F andS/I/F junctions, we turn our attention to a summary ofall our results, shown in Fig. 3. In panel (a) of thatfigure we show the data for the introduction of a Auinterface layer of variable thickness between the barrierand Co electrode for both types of samples. There isa drop in TMR when the Au is first introduced, but,remarkably, an easily measurable TMR of a few % isstill present for Au thicknesses exceeding 20 Å. We wereable to fit our data (excluding the first point) with aphenomenological exponential decay, which returns a 1/elengthscale of λTMR = 11 ± 3 Å. The TSP is seen todecay in a similar manner to the TMR. There is a ratherabrupt drop in TSP on introduction of the first Å of Aufrom about 40 % to 30 %, but then the TSP decays moregently: again there is a finite and substantial TSP for 20or 30 Å of Au between the barrier and the Co. Again anexponential decay fits the data (excluding the first datapoint), yielding λTSP = 14 ± 2 Å. No sign of quantumwell oscillations is present in these data.In Fig. 3(b) we show similar data for the case where1 Å thick Au δ-layers have been introduced into the Colayer at a variable distance from the barrier/Co interface.When the Au is at the interface both TMR and TSPare suppressed in line with what is seen in Fig. 3(a).As the Au moves into the Co layer both the TMR andthe TSP rise smoothly and rapidly towards the 39 %value for an undoped Co layer. Exponential fits yieldsubstantially shorter 1/e lengthscales than those foundabove, λTMR = 6 ± 4 Å, whilst λTSP = 7 ± 2 Å: again,324681012140 20 40 600153045TSPTMRλTMR = 11±3 ÅλTSP = 14±2 Å(a)Au thickness (Å)TMR (%)TSP (%)91011120 2 4 6 8 1028323640TSPTMRλTMR = 7±4 ÅλTSP = 6±2 Å(b)Co thickness (Å)TMR (%)TSP (%)FIG. 3: (Color online) (a) The dependence of TMR and TSPon Au interface layer thickness. (b) The dependence of TMRand TSP on the distance of a 1 Å Au δ-layer from the bar-rier/Co interface. TMR was measured at 300 K, TSP at 250mK. The solid lines are the results of fitting exponential de-cays to the data, and in each plot we report the decay length-scales, λ, extracted from the fits.the two fitted values are identical to within the errorbar. We interpret this result as showing the lengthscaleon which the Au atoms at the interface are replaced withCo: once the entire interface consists of Co the full TSPis recovered. This result is reminiscent of those obtainedin GMR spin-valve systems16,17.The most interesting feature of these results is howlong the exponential decay lengths in Fig. 3(a) are whencompared to those in Fig. 3(b) and to most of the resultscited in the introduction. One possibility is that the Augrows in islands and the data simply represent the frac-tion of the barrier which is still in contact with some Co.We have performed some atomic force microscopy (AFM)to try to gain information about the Au layer. Fig. 4 (a)shows an AFM image of 7 Å of Au grown on an AlOxbarrier layer, part of an incomplete MTJ stack. Smallgrains ∼ 10 nm in diameter are visible, which are likelyto be made of Au. Having performed an image floodinganalysis, we can say that only 11 % of the image arealies more than the nominal 7 Å thickness below the aver-age image height. Hence, if this topography were coveredwith a Co layer, we would expect that Co would be indirect contact with the barrier only in this area fraction,and we ought to observe no more 11 % of the TMR orTSP of a junction without Au. Nevertheless, we can seefrom Fig. 3 that the observed values are rather higherthan this.Moreover, if the Au hillocks were as high as required(a) (b)FIG. 4: (Color online) Atomic force micrograph of (a) a Au (7Å) / AlOx / SiO2 sample (σrms = 7.9 Å), and (b) a completedMTJ stack including 7 Å Au (σrms = 11.5 Å).by to explain the reduction in TMR and TSP using thisisland-growth hypothesis, the huge additional roughnessfor the thicker Au layers, a few nm in amplitude, wouldlead to a substantial increase in the orange-peel couplingfield for the free layer,18, whereas we see no observablechange in Fig. 1. Moreover, this enormous roughnessshould also be present at the top interface of the Co layer,with the IrMn, which should give rise to a drastic reduc-tion in exchange bias, which is also absent in our experi-ment. Such rough layers might also be expected to havea very much higher coercivity - again this is not observedin the TMR loops. Fig. 4(b) shows an AFM image ofa completed MTJ stack including 7 Å of Au, which willgive some idea of the roughness in these layers, as con-formal roughness is generally accumulated through thestack in sputtered samples: the rms roughness σrms isbarely more than ∼ 1 nm.Hence, a substantial excess TMR and TSP remains tobe explained for the case of the Au interfacial layers. Aninteresting possibility is that our results are due to somespin-polarization in the Au. This could arise either bya proximity effect due to the contact with the Co, orby spin injection when a current is driven by a voltagebias. This will give rise to a weak spin-polarization atthe barrier which is detected by the tunnel junction.To summarize, our measurements show that a substan-tial TSP can penetrate a few nanometers of Au from aninterface with Co, measured both directly at 250 mKand by TMR measurements at room temperature. Onthe other hand both the TMR and TSP recover fromtheir suppression by the Au δ-layer much more quicklywhen it is withdrawn into the Co, with the 1/e length-scales < 1 nm. It seems that the TMR and TSP observedfor the thicker Au layers cannot be explained entirely bydiscontinuities in the Au layer. Further modelling andcharacterization to investigate this point is ongoing atthe time of writing, as well as experiments using otherdopant species.We would like to thank A. T. Hindmarch for help withthe coding of the AORT model, and the EU (via projectNMP2-CT-2003-505587 “SFINx”) and the EPSRC forfunding.4∗ Electronic mail: c.h.marrows@leeds.ac.uk† Also at Institut Universitaire de France1 M. 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B 64,100406 (2001).12 T. Nozaki, Y. Jiang, Y. Kaneko, A. Hirohata, N. Tezuka,S. Sugimoto, and K. Inomata, Phys. Rev. B 70, 172401(2004).13 J. A. X. Alexander, T. P. Orlando, D. Rainer, and P. M.Tedrow, Phys. Rev. B 31, 5811 (1985).14 D. C. Worledge and T. H. Geballe, Phys. Rev. B 62, 447(2000).15 R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173(1994).16 S.S.P. Parkin, Phys. Rev. Lett. 71, 1641 (1993).17 F. E. Stanley, C. H. Marrows, and B. J. Hickey, J. Appl.Phys. 87, 4864 (2000).18 J. C. S. Kools and W. Kula, J. Appl. Phys 85, 4466 (1999).

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Spin-polarized tunneling with Au impurity layers

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