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Superpoissonian shot noise in organic magnetic tunnel junctions Juan Pedro Cascales,1 Jhen-Yong Hong,2 Isidoro Martinez,1 Minn-Tsong Lin,2,a) Tomasz Szczepan´ski,3 Vitalii K. Dugaev,3 Józef Barna´s,4 and Farkhad G. Aliev*1,b) 1)Dpto. Fisica Materia Condensada C3, Instituto Nicolas Cabrera (INC), Condensed Matter Physics Institute (IFIMAC), Universidad Autonoma de Madrid, Madrid 28049, Spain 2)Department of Physics, National Taiwan University, Taipei 10617, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan 3)Department of Physics, Rzeszów University of Technology, al. Powstanćów Warszawy 6, 35-959 Rzeszów, Poland 4)Faculty of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614 Poznan´, and Institute of Molecular Physics, Polish Academy of Sciences, ul. Smoluchowskiego 17, 60-179 Poznan´, Poland 5 Organicmoleculeshaverecentlyrevolutionizedwaystocreatenewspintronicdevices. Despiteintensestudies, 1 thestatisticsoftunnelingelectronsthroughorganicbarriersremainsunclear. Hereweinvestigateconductance 0 andshotnoiseinmagnetictunneljunctionswithPTCDAbarriersafewnmthick. Forjunctionsintheelectron 2 tunnelingregime, with magnetoresistance ratiosbetween10 and40%, weobservesuperpoissonianshot noise. n The Fano factor exceeds in 1.5-2 times the maximum values reported for magnetic tunnel junctions with a inorganic barriers, indicating spin dependent bunching in tunneling. We explain our main findings in terms J of a model which includes tunneling through a two level (or multilevel) system, originated from interfacial 2 2 bonds of the PTCDA molecules. Our results suggest that interfaces play an important role in the control of shot noise when electrons tunnel through organic barriers. ] l l PACS numbers: 72.25.Mk; 72.70.+m; 78.47.-p a h - s Organic elements in electronic devices have some ad- study, the 1/f noise and DC leakage measurements were e vantages over inorganic ones, like the ability to chem- used as a diagnostic tool for OLED reliability in a pro- m ically adjust their electronic properties, their mechani- duction line19. Current 1/f noise measurements have t. cal flexibility, and the capability to form self-assembled been also used to identify individually contacted organic a layers. Exploring the unique properties of the organic molecules20,21. m world to improve and create new functionalities in spin Earlier noise measurements in organic spintronic de- d- relatedoptics,electronicsandmemoryelementshasbeen vices were carried out at large applied voltages, where n attracting considerable attention in the past decade1–7. the1/f noiseisdominant. Therefore,theywerenotable o Organic spintronics may lead to unique devices, for in- to unveil the role of shot noise, the most fundamental c stance, organic light emitting diodes (OLEDs) based on noisesourceinnanodevices. Apartfrombeingimportant [ magnetically controlled luminescence8. A key limiting fromthepointofviewofapplications, preciseknowledge 1 factor for the operation of these and related devices is ofSNcanprovideavaluableinformationonelectroncor- v their signal to noise ratio. Thus, the investigation of relations near the interfaces with organic barriers, espe- 4 noise sources in organic tunnel junctions and spin valves cially in the regime of direct tunneling. In fact, the role 5 is of fundamental and technological interest, as the noise of interfaces remains one of the central issues in organic 4 ultimately determines their practical applications. spintronics22. 5 Low frequency noise, and shot noise (SN) mea- 0 Here, we analyze the tunneling statistics in organic surements have been systematically used to character- . magnetic tunnel junctions (O-MTJs) by measuring shot 1 ize the electronic transport mechanisms in inorganic 0 spintronics9–16. On the other hand, noise in organic- noise, which is known to be an excellent tool to in- 5 vestigate the correlations and other details of electron based devices, which could have 1/f and shot noise con- 1 tunneling, well beyond the capabilities of transport : tributions,remainspoorlyunderstood. Forexample,1/f measurements23–33. Being a consequence of the dis- v noise measurements have been used to determine de- i crete nature of charge carriers, SN is the only contri- X vice quality17, or transport features in graphene-based bution to the noise which survives down to low temper- devices (including one or several layers)18. In another r atures. The normalized shot noise (or Fano factor F) a indicates23 whether the tunneling is uncorrelated (poissonian, F = 1), anti-bunched (sub-poissonian, typically due to negative correlations, F < 1) or bunched (super- a)Electronicmail: [email protected] b)Electronic mail: (*) Corresponding author: poissonian,typicallyduetopositivecorrelations,F >1). [email protected] We have investigated the conductance and shot noise Typeset by REVTEXand AIP 1 ofO-MTJswithPTCDAmolecularbarriersinthedirect a ) b ) tunneling regime34. In contrast to MTJs with inorganic 100 barriers10,11,tunnelingthroughmolecularbarriersshows ) super-poissonian shot noise which additionally depends W 10 on the relative alignment of the electrodes’ magnetiza- (k R 1 tion. Our observations are accounted for qualitatively within a model based on spin dependent electron tun- 0.1 0 1 2 3 4 5 neling through an interacting two-level (or multi-level) P T C D A (n m ) c ) d ) sgyasnTtiehcme. splainyer valsveeqsuenscteudieidn inthe thiPsTCleDttAer oirs-: (%)112050 TTT===016.003KKK S)4600 GG APP TM R 2300 (%) NiFe(25nm)/CoFe(15nm)/AlxO(0.6nm)/PTCDA(1.2- MR 5 mG( 10 MR 5nm)/AlxO(0.6nm)/CoFe(30nm). The structure was T 20 T deposited onto a glass substrate, and the junctions were 0 0 -3 -2 -1 0 1 2 3 -100 -50 0 50 100 prepared in a high-vacuum environment with a base H (k O e ) V (m V ) pressurelowerthan10−8 mbar. Themetalliclayerswere deposited by sputtering with an Ar working pressure of 5 × 10−3mbar. The PTCDA layers were grown by FIG.1. (a)Sketchofthesamplestructure. (b)Experimental thermal evaporation at 10−8 mbar, with a deposition dependence of the resistance on the PTCDA thickness. (c) rate of 0.1 nm/s. Thin AlO layers were grown between TMRcurvesatdifferenttemperaturesforasamplewith2nm x ofPTCDA.(d)DependenceoftheTMRanddifferentialcon- the PTCDA layer and both ferromagnetic layers by ductance on the bias voltage in the P and AP states for a partially oxidizing Al in oxygen plasma for 5 s. Figure 4nm PTCDA O-MTJ. 1(a) shows a sketch of the investigated O-MTJs. The voltage noise was measured using a cross- a)c b) 80 correlation technique, described elsewhere10,11. The cor- S (P) S (P) 2.0 exp full rect calibration of our setup has been confirmed by in- 60 Sexp(AP) Sfull(AP) dependent studies14,15. The current noise power in the Hz) o 1.5 absenceofcorrelationsisPoissonian(fullshotnoise)and 2V/ 40 an n F is given by SI = 2eI, where I is the average current S(V 20 1.0 and e the electron charge. The voltage full SN is then P 0 AP S =2eIR2,withR beingthedynamicresistanceob- 0.5 full d d -10 0 10 -10 0 10 tainedfromthecorrespondingI−V curves. Wehaveob- V(mV) V(mV) tained the experimental SN, Sexp, by fitting a Gaussian c)c1500 Sexp(P) Sfull(P) d) 2.0 peak to the histogram of the part of the spectra inde- S (AP) S (AP) pTehFnedigFeunartneoo1ff(afbrc)etqosurheoFnwcisyst(thshaeetenttchhaeelcrSueulsapistpteladenmaceesnFotfa=lthMSeaejxtuepnr/icSatlfi3ou5nl)ls.. 2S(nV/Hz)V1050000 exp full Fano 11..05 increasesexponentiallywiththethicknessofthePTCDA P barrier. This indicates that the insulating PTCDA layer 0 0.5 AP -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 actsasabarrierinsinglesteptunnelingregime36. Figure V(mV) V(mV) 1(c) shows the tunneling magnetoresistance (TMR) for three different temperatures in a 2nm PTCDA O-MTJ, where the parallel (P) and antiparallel (AP) magnetic FIG. 2. Voltage dependence of the noise at T=0.3 K in the PandAPstatesofthe(a)experimental(dots)andexpected alignmentoftheelectrodesareindicatedbyarrows. The fullshotnoise(lines). (b)Fanofactorfora2nmPTCDAO- TMR decreases when the bias reaches 100 mV (see Fig. MJT. (c) and (d) present similar results for a 5 nm PTCDA 1(d)). Figure 1(d) also presents the differential conduc- O-MJT. tance in the P and AP states as a function of the bias voltageatT=0.3Kfora4nmPTCDAO-MTJ.Wefound that the magnetic tunnel junctions with PTCDA barri- F factor ranges from F = 1 at low voltages to F (cid:39) 2 at ers were more robust than conventional inorganic MTJs, higher voltages. All the O-MTJ samples measured dis- and typically did not experience dielectrical breakdown playedaqualitativelysimilarvariationoftheFanofactor as readily. Out of 14 samples studied, only 3 have de- with the bias voltage. The shot noise could be obtained graded during multiple bias sweeps up to 500mV. for voltages up to a few tens of mV only. The maximum The experimental SN and S at T = 0.3K for the voltage for which the shot noise is measured corresponds full 2 nm PTCDA junction from Fig. 1(c) are shown in Fig. to the energy at which the 1/f noise becomes dominant 2(a)forthePandAPstates. Fig. 2(b)showsthedepen- andobscuresthefrequencyindependentpartofthenoise denceoftheFanofactorwiththebiasforanothersamples spectrum. Eventhoughthespectracouldbeobtainedup with a 5 nm thick PTCDA barrier. As can be seen, the to 100 kHz, filtering due to the capacitance of the sam- 2 ples (dependent on the PTCDA thickness) allowed shot a)ccc2ccccccccccccccccccccccccccccccccccccccccccccccccccccccccb) 2 2nmcPTCDA noisemeasurementsonlybetween1-10kHz. Theappear- o n o astnucdeieodf 1sa/mfpnloeiss,eersepsetcriiacltleydinSNthme AeaPsusrteamtee.nts in all the Fa 1 InorganiccMTJs Fan1.5 5nmFcPPteToCrDAFPexp 2nmcPTCDA 0 Fteor Fexp Figure 3(a) presents the average saturation value of AP AP 1 theFanofactorinthePstateforthesampleswhichpre- %)40 %)30 TMRteor sented the frequency-independent spectra. Figure 3(a) MR(20 MR(20 TMRexp 5nm 2nm alsoshowsthevariationofTMRwiththePTCDAthick- T T10 0 2nm ness. Control junctions, with only a 1.2nm AlO layer, 0 x 0 1 2 3 4 5 1 1.5 2 show TMR below 1%, and a metallic-like electron trans- PTCDA(nm) β port (see35). This points to diffusive electron transport, for which the theory23 predicts the Fano factor equal to FIG.3. (a)MaximumFanofactorinthePstateandthezero 1/3. Thus, control measurements prove that the super- bias TMR vs the PTCDA thickness. The shadowed region corresponds to the range of Fano values for MTJs with inor- poissonian SN is due to the PTCDA barriers. Our O- ganicbarriers. (b)Fitofthetheorytoexperimentalvaluesof MTJs with PTCDA thicknesses between 1.2 and 5 nm F andTMRforthesampleswith2and5nmofPTCDA.The show relatively high TMR and super-poissonian tunnel- Fano points are obtained from the average saturation value ingstatisticswiththeFanofactorapproaching2, indica- in the Fano factor vs bias plots, and the error bars indicate tive of co-tunneling or tunneling with bunching. Eight the dispersion of the average. O-MTJs of different barrier thicknesses, from four sam- plesets,haveshownqualitativelysimilarSNvalues(Fig. 3(a)). could be a charge neutrality level42, or gap states43, A number of electron tunneling mechanisms (Kondo whichappearduetothealignmentoftheenergylevelsat effect37, co-tunneling38,39, and others40,41) are capable metal/organic interfaces44. The bias dependence of the of producing super-poissonian SN. However, they are interfacialdensityofstatescouldexplainthesuppression mostly relevant for small quantum dots. The observed of the Fano factor at large voltages (Fig. 2(b)). SNhasbeenaccountedforintermsoftheapproachdevel- In conclusion, super-poissonian statistics in tunneling oped by Belzig29, extended to spin dependent transport. eventsthroughthePTCDAmolecularbarriershavebeen Thecorrespondingmodelisbasedontunnelingthrougha unveiled by shot noise measurements. A superpoisso- two-levelsystem(ormulti-levelsysteminamoregeneral nian shot noise has been found, which is likely due to case), with remarkably different tunneling rates through localized states originated from interfacial bonds of the the two levels. Moreover, these tunneling rates are also PTCDA molecules. For a technological application, the spindependent. Thestatisticsofthetransportprocessis shot noise could be reduced or controlled, for instance, described as a sum of independent Poissonian processes by the growth of double-barrier45 O-MTJs. Challenges transferring bunches of electrons of different size. This for further work include extending the bias range where arises from the difference in tunnel rates between the the shot noise could be investigated and comparing the two levels and leads to an enhanced noise. Details of the role of the organic layers in the superpoissonian SN by model and description will be presented elsewhere. In the study of O-MTJs with different organic layers. Fig. 3(b) we show the TMR and Fano factor in the P The authors acknowledge support by the Spanish and AP states as a function of the parameter β, which MINECO (MAT2012-32743), UAM-SANTANDER, Co- describes the spin asymmetry in tunneling rates. The munidad de Madrid (P2013/MIT2850) and NSC 102- solid and dashed lines are the theoretical results, while 2120-M-002-005 (Taiwan) grants. This work is also the points correspond to the experimental data. Note partlysupportedbytheNationalCenterofResearchand that for each sample the TMR and Fano factors have Development in Poland in the frame of the EU project been fitted with the same parameter β, which justifies Era.Net.Rus “SpinBarrier”. the validity of our approach. Physically, the two or more levels with different cou- plings, which are responsible for the observed super- poissonian shot noise, can have their origin in localized REFERENCES statesarisingfrominterfacialbondsbetweenthePTCDA molecules and the AlO buffer layers. The following ar- 1V. Dediu, M. Murgia, F. Matacotta, C. Taliani, and x guments suggest that the localized states in the model S. Barbanera, Solid State Communications 122, 181 have an interfacial nature: (i) the exponential depen- (2002), ISSN 0038-1098. dence of the tunneling resistance on PTCDA thickness 2Z. H. Xiong, D. Wu, Z. Valy Vardeny, and J. 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