DSpace at VNU: Raman spectroscopy of Cu doping in Zn1-xCoxO diluted magnetic semiconductor

ISSN 1063 7850, Technical Physics Letters, 2009, Vol 35, No 11, pp 990–993 © Pleiades Publishing, Ltd., 2009 Original Russian Text © N.V Volkov, E.V Eremin, V.S Tsikalov, G.S Patrin, P.D Kim, Seong Cho Yu, Dong Hyun Kim, Nguyen Chau, 2009, published in Pis’ma v Zhurnal Tekhnicheskoі Fiziki, 2009, Vol 35, No 21, pp 33–41 Switching of Current Channels and New Mechanism of Magnetoresistance in a Tunneling Structure N V Volkov*, E V Eremin, V S Tsikalov, G S Patrin, P D Kim, Seong Cho Yu, Dong Hyun Kim, and Nguyen Chau Kirensky Institute of Physics, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia Siberian Federal University, Krasnoyarsk, 660041 Russia Siberian State Aerospace University, Krasnoyarsk, 660041 Russia Department of Physics, Chungbuk National Un Iniversity, Cheongju 361 763, Korea Center for Materials Science, National University of Hanoi, 334 Nguen Trai, Hanoi, Vietnam *e mail: volk@iph.krasn.ru Received April 28, 2009 Abstract—We have experimentally studied the transport properties of a planar La0.7Sr0.3MnO3 (LSMO)/Mn depleted LSMO/MnSi tunneling structure, in which the Mn depleted LSMO layer plays the role of a potential barrier between the conducting layers of LSMO and MnSi The measurements were per formed in geometry with the current direction parallel to the planes of interfaces in the tunneling structure It is established that the structure exhibits a nonlinear current–voltage characteristic and possesses a positive magnetoresistance, the value of which depends on the tunneling current It is suggested that specific features of the transport properties of this structure are related to the phenomenon of current channel switching between the conducting layers The switching mechanism is based on the dependence of the resistance of the tunneling junction between the conducting layers on the bias voltage and the applied magnetic field PACS numbers: 72.25 b, 73.40 c, 75.47 m DOI: 10.1134/S1063785009110054 Magnetic tunneling junctions and the phenomena of spin polarized tunneling from ferromagnetic layers have been extensively studied in recent years [1] This interest is related to both potential practical applica tions and a rich spectrum of new physical phenomena related to a relationship between the spin polarized electron transport and the magnetic subsystem of low dimensional structures Tunneling junctions have been traditionally studied in geometry with the current per pendicular to the planes of interfaces in the tunneling structure This geometry is simpler for a theoretical analysis and the interpretation of experimental results However, the current in plane (CIP) geometry, in which the current is parallel to the planes of interfaces, is sometimes preferred for the practical purposes, in particular, in the case of ferromagnet/semiconductor hybrid nanostructures compatible with the traditional CMOS technology In addition, this geometry can be expected to reveal new manifestations of the spin dependent electron transport This Letter presents the results of an investigation, in which the transport properties of a tunneling struc ture have been measured in the CIP geometry The tunneling structure was manufactured using the method of pulsed laser ablation of La0.7Sr0.3MnO3 (LSMO) and Si targets with deposition onto a (001) oriented SiO2 substrate According to this, nm thick Si layer and 500 nm thick LSMO layer were sequen tially deposited onto the substrate heated to 500°C The as grown structure was immediately annealed in an oxygen containing atmosphere at 800°C for h It was expected that this technological process must yield a structure, in which the lower layer of manga nese monosilicide (MnSi) and the upper LSMO layer are separated by an interfacial layer of manganese depleted LSMO (LSMδO) with a thickness of about nm (see the inset to Fig 1) This composition pro file must form due to a high coefficient of Mn diffu sion in Si [2] The LSMδO layer possesses dielectric properties and plays the role of a potential barrier between the conducting layers of LSMO and MnSi (electrodes) According to the results of magnetic measure ments, the manganite film exhibits a transition to the ferromagnetic (FM) state at a temperature of ~300 K X ray diffraction data show evidence for a predomi nant (110) orientation of the LSMO layer, but it is most probable that this layer is textured The presence of boundaries, which separate crystalline grains and play the role of potential barriers, is confirmed by a low conductivity of the LSMO film and a large contribu tion of tunneling to the sample resistance As for the 990 SWITCHING OF CURRENT CHANNELS AND NEW MECHANISM lower layer, it is known that MnSi is characterized by metallic conduction and exhibits a transition to the FM state at about 30 K [3] The transport properties of the samplers were stud ied using a Model 2400 SourceMeter (Keithley Instru ments) The current carrying contact pads were formed on the upper surface of the structure using a two component silver filled epoxide glue The experi mental geometry is schematically depicted in the inset to Fig The resistance was measured in the regime of stabilized current, while the current–voltage (I–V) curves were obtained in the current sweep regime The magnetic field was applied in the plane of the struc ture The sample temperature T was set and kept accu rate to within 0.1 K in the entire range studied The experimental data can be conditionally divided into two parts, which refer to (i) nonlinear transport properties and (ii) the effect of a magnetic field on the conduction Let us first consider features of the conduction in the absence of an external mag netic field Figure presents a series of I–V curves measured at various temperatures As can be seen, all curves exhibit an almost linear initial portion Then, at a certain threshold current (Ith), the slope sharply changes and, as the current I grows further, the voltage increases at a slow rate This behavior is especially clearly pronounced at T < 30 K We propose the following model to interpret the obtained I–V curves In the given sample structure, the upper layer (LSMO) possesses a higher resistance than the lower (MnSi) layer However, since the tact pads are formed on the upper surface and the lower layer is separated from the upper by a potential barrier, the current passes predominantly via the LSMO layer This is confirmed by the linear V(I) dependence observed at small currents, in agreement with what has to be expected for the manganite An increase in the current I and, hence, in the voltage drop V between the contacts, leads to a redistribution of charges in the lower conducting layer This, in turn, gives rise to a bias voltage Vb (Vb Ӷ V) on the tunneling junctions under the current carrying contacts and the related increase in the tunneling current IT through the potential barrier separating the upper and lower layers of the structure Thus, the resistance RT of the tunneling barrier decreases and the current begins to flow predominantly via the lower (silicide) layer, the resistance of which (RS) is small compared to that (RM) of the manganite film The upper inset in Fig shows an equivalent electric scheme of the tunneling structure in the CIP geometry The current through this circuit is determined by the parallel connection of RM and the series of RT and RS According to this scheme, we have approximately described the I–V curves by assuming that RS is small compared to RT so that the charge transfer via the TECHNICAL PHYSICS LETTERS Vol 35 No 11 991 Voltage, V T = 10 K 30 I+ 15 LSMO LSMδO MnSi I− V+ V− 22 K Vb SiO2 30 K 90 K 160 K −15 H=0 −30 −100 −75 200 K 250 K −50 −25 25 50 75 100 Current, µA Fig I–V curves of the tunneling structure measured at various temperatures in the CIP geometry without an external magnetic field The inset shows a schematic dia gram of the sample structure (arrows indicate the possible current pathways) lower layer is fully determined by the tunneling cur rent (IT) This current was approximately described using the Simmons formula [4] obtained in the approximation of elastic electron tunneling through the potential barrier: 1/2 IT = I0{ϕ0exp(–A ϕ ) – (ϕ0 + eV)exp(–A(ϕ0 + eV)1/2)}, where ϕ0 is the potential barrier height, A is a coeffi cient proportional to the barrier width Δx The current through the upper layer was described according to the Ohm law as IM = V/RM, where RM was determined from the temperature dependence of the sample resis tance measured at I = Ith, that is, for the current pass ing almost entirely through the manganite film The best fit was obtained for the following parameters: potential barrier width Δx = nm (this value well agrees with the proposed structure composition); bias voltage Vb ≅ × 10–2 V; and the average potential bar rier height varying from ϕ0 ≅ 0.3 eV at T = 250 K to ϕ0 ≅ 0.8 eV at T = K Figure gives an example of the experimental I–V curve approximated using the proposed model The lower inset shows the temperature dependence of ϕ0, which can be determined by changes in the electron structures of the tunneling junction components in the course of FM ordering [5] Some manganites [6] exhibit an anomalously large change in the chemical potential below TC, which is proportional to the square of the sample magnetization Thus, the work 2009 992 VOLKOV et al Voltage, V 150 (a) 100 T = 10 K 50 Voltage, V 15 10 RM I+ RT IM IT I− RT H=0 −50 ϕ0, eV 0.9 T = 30 K −100 −150 MR magnitude 0.6 −10 0.3 −15 −50 10 kOe RS −5 kOe 100 (b) 200 T, K kOe −25 25 50 Current, µA Fig Example of the experimental I–V curve of the tun neling structure in the CIP geometry at T = 30 K, approx imated within the framework of the proposed model (with an equivalent scheme in the upper inset): (1) experimental points; (2) approximating curve; (3) I–V curve of the tun neling junction; (4) I–V curve of the manganite layer The lower inset shows the temperature dependence of the aver age potential barrier height ϕ0 in the structure, obtained for the tunneling current approximated by the Simmons formula 10 kOe −100 function of the LASMO layer upon the transition to the FM state increases as compared to that of the interfacial LSMδO layer (remaining in the nonmag netic state), which plays the role of the potential bar rier As a result, ϕ0 increases with the magnetization of LSMO, which explains the behavior observed at high temperatures The growth in ϕ0 at temperatures below 30 K is naturally explained by an increase in the work function of the MnSi layer upon its transition to the FM stage at T ~ 30 K The results of measurements of the I–V curves in an external magnetic field H showed that the influence of this field at T > 30 K is manifested only for I < Ith The effect of H in this interval of currents is fully deter mined by the magnetoresistance (MR) of the LSMO film The MR is negative and its absolute value is inde pendent of the probing current, which is typical of manganites For I > Ith, the sample exhibits switching so that the current begins to flow predominantly via the lower (silicide) layer Since MnSi does not possess significant MR, while the current via tunneling junc tions at T > 30 K is independent of H, the MR effect for I > Ith at T > 30 K is not observed At T < 30 K, the silicide layer exhibits magnetic ordering and the entire −50 50 100 Current, µA Fig LSMO/LSMδO/MnSi tunneling structure: (a) I–V curves measured in the CIP geometry at T = 10K in the absence of an external magnetic field (H = 0) and with an applied magnetic field of H = and 10 kOe; (b) plots of the MR magnitude versus bias current for H = and 10 kOe structure represents a magnetic tunneling junction The current through this junction depends on the mutual orientation of magnetizations (MM and MS, respectively) in the LSMO and MnSi layers As can be seen from the data in Fig 3a, the effect of the negative MR at T < 30 K for I < Ith is still retained, but a strong influence of H is additionally manifested in the I–V curves for I > Ith Indeed, at T = 10 K, this dependence already becomes linear in a field of H = kOe, which can be interpreted as the reverse switch ing of the current channel from the lower to upper layer of the structure as a result of increase in the resis tance RT of the tunneling junction in the applied mag netic field Indirect evidence for this scenario is the negative MR (typical of the manganite film) observed for I > Ith (see the MR curves for H = and 10 kOe in Fig 3b) Thus, there are several possibilities to control the switching of current channels in a magnetic tunneling structure in the CIP geometry The bias voltage (cur TECHNICAL PHYSICS LETTERS Vol 35 No 11 2009 SWITCHING OF CURRENT CHANNELS AND NEW MECHANISM rent) produces the current channel switching from the upper to lower layer of the structure, while the external magnetic field produces the reverse switching The lat ter factor determines the positive MR effect in the magnetic tunneling structure according to an abso lutely new mechanism, which has never been consid ered until now The proposed mechanism accounts for the main features of manifestation of the MR effect; a positive MR is induced by the bias current and its mag nitude depends on this current As can be seen from Fig 3b, the MR magnitude at T = 10 K and I = 100 μA exceeds 300% and shows no tendency to saturation Considering the dependence of the tunneling cur rent on the on the mutual orientation of magnetiza tions in the FM layers (electrodes), we must take into account that FM materials can be of the two types In the first case, charge carriers possess a preferred spin orientation parallel to the magnetization and these materials are referred to as majority spin carrier (MASC) ferromagnets In the second case, the spins are predominantly oriented antiparallel to the magne tization and these materials are referred to as minor ity spin carrier (MISC) ferromagnets In a tunneling junction with one FM electrode of the MASC type and the other electrode of the MISC type, the junction resistance is greater for the parallel orientation of mag netizations than for their antiparallel orientation [7] Apparently, this situation takes place in the structure under consideration In the absence of a magnetic field, the magnetostatic interaction results in the anti parallel orientation of MM and MS, the junction resis tance RT is at minimum, and the current at I > Ith flows via the lower layer An applied magnetic field tends to orient MM and MS parallel to each other, RT increases to become grater than RM, an the current even at I > Ith passes predominantly via the upper layer with a linear I–V curve TECHNICAL PHYSICS LETTERS Vol 35 No 11 993 In conclusion, we studied the transport properties of a tunneling structure in the CIP geometry and revealed the phenomenon of current channel switch ing between layers of the structure, which is controlled by the bias current At low temperatures, both elec trodes in this structure occur in the FM state, in which case the tunneling resistance can be controlled by an applied magnetic field, which produces current chan nel switching in the tunneling structure These phe nomena determine the new mechanism of MR, the magnitude of which depends on the bias current in the structure Acknowledgments This study was supported by the Russian Foundation for Basic Research (project nos 08 02 00259 and 08 02 100397) and the Minis try of Education and Science of the Russian Federation (program “Development of Scientific Potential of Higher Education 2009–2010,” project no 2.1.1/6038) REFERENCES J C Moodera and R H Meservey, Spin Polarized Tunneling, in Magnetoelectronics, Ed by M Johnson (Elsevier, 2004) G Ctistis, U Deffke, K Schwinge, et al., Phys Rev B 71, 035 431 (2005) Y Ishikawa, G Shirane, J A Tarvin, and M Kohgi, Phys Rev B 16, 4956 (1977) J G Simmons, J Appl Phys 34, 1793 (1963) J Klein, C Hofener, S Uhlenbruck, et al., Europhys Lett 47, 371 (1999) N Furukawa, J Phys Soc Jpn 66, 371 (1997) C Mitra, P Raychaudhuri, K Dörr, et al., Phys Rev Lett 90, 017 202 (2003) Translated by P Pozdeev 2009 ... scheme in the upper inset): (1) experimental points; (2) approximating curve; (3) I–V curve of the tun neling junction; (4) I–V curve of the manganite layer The lower inset shows the temperature... magnetization of LSMO, which explains the behavior observed at high temperatures The growth in ϕ0 at temperatures below 30 K is naturally explained by an increase in the work function of the MnSi... schematically depicted in the inset to Fig The resistance was measured in the regime of stabilized current, while the current–voltage (I–V) curves were obtained in the current sweep regime The magnetic