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Spin dependent transport in half-metallic nanostructures  

Spin dependent transport in half-metallic nanostructures  . Indranil Das, Soumik Mukhopadhyay ECMP Division Saha Institute of Nuclear Physics, Kolkata E-Mail: indranil.das@saha.ac.in. Acknowledgements :. S. P. Pai — help in device fabrication. Plan of talk :.

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Spin dependent transport in half-metallic nanostructures  

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  1. Spin dependent transport in half-metallic nanostructures   Indranil Das, Soumik Mukhopadhyay ECMP Division Saha Institute of Nuclear Physics, Kolkata E-Mail: indranil.das@saha.ac.in

  2. Acknowledgements : S. P. Pai —help indevicefabrication Plan of talk : Anomalous bias dependence of tunnel magnetoresistance Inversion of magnetoresistance in magnetic tunnel junction : Effect of pinhole nanocontacts Magnetic field sensitive non-tunneling emission of localized charge & its influence of on low field magnetoresistance

  3. Tunnelling VB VB-Ex Ex d Metal-Insulator-Metal tri-layers metal insulator metal

  4. Spin polarisation P ~ 40% - 50% P ~ 80% - 100% P=0 E EF N(E) N(E) N(E) Half-metals: La0.7Sr0.3MnO3 Sr2FeMoO6 Fe304, Cr02 FM Normal M Fe, Co, Ni La0.7Sr0.3MnO3 (LSMO) P = 95 % at 5K Bowen et al, APL 82, 233 (2003) CrO2 P> 96% at 77 K Y.Ji, PRL 86, 5585 (2001)

  5. Magnetic Tunnel junctions - MTJ Meservey and Tedrow showed Spin is conserved in tunneling Tunneling current  DOS of the electrodes Parallel M Anti Parallel M M M FM1 FM1 Insulator M M FM2 FM2 Large tunnel current Smaller tunnel current Julliere Model (1975)  TMR R/RP= (RAP – RP)/ RP =2P1P2/(1-P1P2) TMR > 400% at RT

  6. Tunnel magnetoresistance in MTJ TMR decreases with increasing bias 1) Higher order spin independent Tunneling 2) Hot electron creates more magnons Bias dependence of TMR

  7. Zero bias anomaly in the Bias dependence of Tunnel Magnetoresistance • La0.67Sr0.33MnO3(LSMO) (50 nm)/ Ba2LaNbO6 (BLNO) (4 nm) / LSMO (100 nm) substrate SrTiO3 using pulsed laser deposition. • Junction area 5050 m2 • The micro-fabricationby photo-lithography and ion-beam milling.

  8. Spin is conserved in tunneling • Conductance proportional to the DOS at the Fermi level of two electrodes • Tunneling probability dependent on magnetic orientation of electrodes R/RP= (RAP – RP)/ RP =2P1P2/(1-P1P2) • Tunnel Magnetoresistance (TMR) positive when P1 & P2 have same sign • Inverse TMR when P1 & P2 have opposite sign Low field (100 Oe) switching

  9. TMR expected to decrease with increasing bias — higher order tunneling via defect states — tunneling of hot electrons emitting magnons In contrast, near zero bias, TMR increases with increasing voltage Zero bias anomaly : evidence of minority spin Tunneling states Soumik Mukhopadhyay, I. Das et al. Appl. Phys. Lett. 86, 152108 (2005) V. J. Nano. Sc. Tech (2005)

  10. Inverse TMR • The Co/SrTiO3 interface is negatively spin polarized • The Co ‘d’ band is selected for tunneling • Bands comparable with s symmetry can tunnel across Al2O3, whereas bands comparable with both s and d symmetry can tunnel across SrTiO3

  11. LSMO/STO/Co 300 6 4.2 10 -6 -30 280 Co -4 6 4.1 10 LSMO 260 -2 -20 6 4 10 0 TMR (%) 240 6 Resistance (Ohms) -10 3.9 10 2 Resistance (Ohms) TMR (%) 6 4 3.8 10 220 0 (a) 6 6 3.7 10 200 8 10 6 3.6 10 10 -100 -50 0 50 100 6 3.5 10 Magnetic field (mT) -150 -100 -50 0 50 100 150 Magnetic Field (mT) Co SrTiO3 (STO) LSMO 5 4.6 10 -4 5 4.4 10 0 TMR (%) 5 4 4.2 10 8 5 4 10 Resistance (Ohm) 12 5 3.8 10 16 5 3.6 10 20 -100 -50 0 50 100 Magnetic Field (mT) Interfaces play a dominant role Co Co Al2O3(ALO) Al2O3 (ALO) SrTiO3(STO) LSMO LSMO LSMO/STO 2.5nm/Co LSMO/ALO 30nm/Co LSMO/STO 1nm/ALO 1.5nm/Co TMR inverse  SPCo <0 TMR normale  SPCo >0 Hybridization at interface decides the spin polarization ★ de Teresa et al., Science 286, 509 (1999)

  12. Inverse TMR due to ballistic transport through nanocontact Observation of Inverse TMR in symmetric MTJ – a novel phenomenon Metallic junction resistance  Existence of pinholes • TMR undergoes change of sign at higher temperature Soumik Mukhopadhyay and I. Das Phys. Rev. Lett. 96,026601 (2006) V. J. Nano Sc. Tech. (2006)

  13. Bias induced inversion of tunnel magnetoresistance • MTJ1 shows inverse TMR at bias current I = 200A while at I = 1mA, exhibits positive TMR. • However, at a lower temperature 100 K, there is no evidence of such inversion with increasing bias Present system is equivalent to two ferromagnetic metal electrodes connected by ballistic nano-scale metallic channels along with a tunneling conduction channel connected in parallel

  14. T+ , T- transmittivity for majority and minority spins There is an upper bound (10 %) to allowed values of inverse TMR Larger the imbalance between T+ and T- , higher the value of Inverse TMR. Condition for inverse TMR : transmittivityclose to unity Allowed values of inverse TMR & corresponding values of T+ and T-: Theoretical and experimental

  15. Spin polarized non-tunneling emission of localized charge • Low temperature rise in resistance • Non-ohmic transport below 6 K  Enhancement of conductivity with applied bias Unusually strong bias & temperature dependence of MR in non-ohmic regime

  16. Low temperature resistivity rise in granular ferromagnets S1:LSMO nanoparticles S2, S3:LSMO/ALO nanocomposites S4: LSMO film on LAO substrate S5: NSMO film on LAO substrate S6:LSMO film on STO substrate Low temperature resistivity minima • Coulomb charging • Quantum interference • Spin dependenttunneling ?

  17. Low field magnetoresistance at low temperature T = 3 K S1:LSMO nanoparticles S2, S3:LSMO/ALO nanocomposites S4: LSMO film on LAO substrate S5: NSMO film on LAO substrate S4: LSMO film on LAO substrate

  18. Observed universality of transport in granular ferromagnets T = 3 K The voltage dependence of conductivity is described by universal scaling function G(V ) = G0 exp(V/V0)1/2, V0 a parameter sensitive to magnetic field. Deviation in the lower electric field regime Poole-Frenkel effect ? (disordered semiconductors) • Granular metal !! • Sensitive to magnetic field !! Soumik Mukhopadhyay and I. Das (Communicated) New transport mechanism

  19. Cross-over from G ~ exp(-A/V) to G ~ exp(BV1/2) with increasing voltage LSMO film on LAO (2 K) • Resistance minima at 6 K • Conductance slope and Vth sensitive to magnetic field • Distinct cross-over from tunneling to non-tunneling regime • Em/<ij>= 0.79 NSMO film on LAO (3 K) • Resistance minima at 20 K • Conductance slope and Vth not so sensitive to magnetic field • Cross-over from tunneling to non-tunneling regime not so distinct • Em/<ij>= 0.07

  20. LSMO/ALO nanocomposites (S1, S2, S3): Microstructure completely different from thin films T = 3K

  21. Consider a granular metallic system. In each grain, electrons can be assumed to be localizedat sufficiently low temperaturewith minimal leakage of wave function outside the grain. The electric field will affect two important parameters of the system: 1) The shape of the potential barriers and hence the tunneling probability 2) The activation energy Ea required for one electron to hop from a neutral grain to another. Transport is in the moderate electric field regime: electron emitted as free particle into the insulator over a path much shorter than the total distance between the grains.

  22. The tunneling probability P(Rij) between two grains located at sites ~Ri and ~Rj separated by distance Rij = |Ri-Rj| can be written as, using WKB approx. The activation energy required to transfer an electron from site i to site j in presence of electric field E is Ea . Hence the activation probability is, The electrical conductance Gijbetween sites i and j is the product of the tunneling probability across the barrier and the activation probability over the barrier.

  23. The model can explain:universality in voltage dependence of conductance Cannot explain:the varying response to magnetic field in different samples Prescription: 1) Additional inter-grain magnetic exchange energy (Em) term in the model. 2)The magnetic exchange arises due to the relative spin orientation of two adjacent localized sites when the spin is conserved in hopping from one site to another. 3) Will also depend on the inter-grain overlap of electronic wave functions. If the spin orientation is anti-parallel an additional energy +Emis necessary for charge carrier emission while less energy -Emis required if parallel Replace the effective potential barrier And the activation energy

  24. Expressing the magnetic exchange energy in the unit of ijas Em = ξmεij , where ξm is a dimensionless quantity. • Tunneling conduction term exp(-A/E), the potential barrier width being inversely proportional to the electric field. • Non-tunneling term exp(BE1/2/KT), effect of barrier lowering at higher electric field (and enhancement in thermal activation over the barrier). • Cross-over from tunneling to non-tunneling regime is possible. B↑↓,↑↑ (1ξm)1/2 for a given system, with ↑↓ and ↑↑being the anti-parallel and parallel magnetic configuration between nearest neighboring grains, respectively. Value of B is higher in anti-parallel configuration compared to the parallel ξm= Em/<ij>=(B↑↓2- B↑↑2)/(B↑↓2- B↑↑2) Gives vital information about the microstructure of the system

  25. New findings : • Zero bias anomaly in the bias dependence of TMR  Evidence of minority spin tunneling states in manganites • Inverse tunnel magnetoresistance in magnetic tunnel junction with pinhole nanocontact  Opposite contributions from elastic tunneling and ballistic transport to TMR  Ballistic spin conserved transport through pinhole at lower temperature  transmittivity close to unity  Inverse TMR • Unified description of spin dependent transport in granular ferromagnets:  Spin polarized emission  Universality of transport property in granular ferromagnets  New origin of low field magnetoresistance

  26. M T R Thank You

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