Thin Film Magnetism Group, Cavendish Laboratory, University of Cambridge W. S. Cho and J. A. C. Bland Cavendish Laboratory, University of Cambridge, UK Spin transport using optically injected spin electrons Cavendish-KAIST Symposium, Spintronics, Daejeon 2006

Y. Ohno et al., Nature, 402, 790 (1999) R. Fiederling et al., Nature, 402, 787 (1999) Dilute Magnetic SC Advantages: No conduc- tivity mismatch, high spin injection efficiency has been demonstrated Drawbacks: Low Curie temperatures and/or high magnetic fields required FM metals Advantages: well-known Materials, operation at room temperature and low magnetic fields possible, low resistance Drawbacks: Spin injection and detection efficiencies observed so far are small Materials Challenges for Spintronic Devices B. T. Jonker and M. E. Flatté in Nanomagnetism: Ultrathin Films, Multilayers and Nanostructures, edited by D. L. Mills and J. A. C. Bland, Elsevier, Amsterdam (2006)

1.Can the electron spin be preserved upon transport across the FM/SC interface at RT.? 2.How important are the details (morphology, composition, defects, etc) of the interface structure? 3.Can FM metals act as efficient spin injectors/detectors (conductivity mismatch)? J. A. C. Bland et al. in Ultrathin Magnetic Structures IV: Applications of Nanomagnetism, Springer Verlag, Berlin (2005); E. I. Rashba, Phys. Rev. B 62, R16267 (2000) Direct Interface Tunnel Barrier Interface Critical Issues in Hybrid FM Metal/SC Structures

Injected electron spin polarisation S is determined from the circular polarisation of the electroluminescent light A. T. Hanbicki et al., Appl. Phys. Lett. 82, 4092 (2003) H. J. Zhu et al., Phys. Rev. Lett. 87, (2001) S ~ 2% for Fe/GaAs at RT S ~ 32% for Fe/AlGaAs at 4.5 K V. F. Motsnyi et al., Appl. Phys. Lett. 81, 265 (2002) S ~ 9% for CoFe/AlO x /AlGaAs at 80K Schottky Barrier Artificial Tunnel Barrier Spin LED Electrical spin injection and optical detection S ~ 55% for CoFe/MgO/AlGaAs at 100K R. Wang et al., Appl. Phys. Lett. 86, (2005)

D. T. Pierce et al. Phys. Rev. B, 13, 5484 (1976) The transition selection rule  m j = ± 1  For h > E g + , P decreases due to the mixture of P 3/2 states with P 1/2 states, which have an opposite sign of polarization to P 3/2 states.  Although the maximum polarization is expected to be 50% in theory, the maximum is experimentally observed to be ~40% at the threshold. mjmj mjmj hh lh hh -1/2+1/2 -3/2-1/2+1/2+3/2 -1/2+1/2 S 1/2 P 3/2 P 1/2 E g = 1.43 eV  = 0.34 eV h ++  - Optical Spin injection using circularly polarized light

NiFe 5.0 nm  True spin filtering signal  I SF =  I -  I ph  Bias and GaAs doping density dependence of  I SF suggest that electron tunneling is the relevant spin dependent transport process S. J. Steinmuller et al., Phys. Rev. B 72, (2005) Identifying the True Spin Filtering Signal in FM/GaAs Schottky Barrier Structures Identifying the True Spin Filtering Signal in FM/GaAs Schottky Barrier Structures

Future works Study of tunneling polarisation through optical injection Resonant tunneling through optical injection - High efficiency of spin-polarised electron transport into the MTJ structure; systematic investigation of tunneling process & relaxation mechanism - Observation of Quantum-Well states in FM layer Z. M. Zeng et al. Phys. Rev. Lett. 97, (2006) J. Mathon et al, Phys. Rev. B 71, (R) (2005) S. Yuasa et al. Science (2002) - Quantum oscillation along the voltage change - Comparison (QW in FM & NM layer) S. Van Dijken et al. Phys. Rev. Lett (2003) T. Nozaki et al. Phys. Rev. Lett. 96, (2006) FM Insulator (AlOx / MgO) FM Insulator (AlOx / MgO) GaAs / InP FM NM Insulator (AlOx / MgO) FM Insulator (AlOx / MgO) GaAs / InP

Future works Direct measurement of true spin filtered current in window type samples - True spin filtered photocurrents from window type sample; estimate of MCD factor, real spin device

Thin Film Magnetism Group, Cavendish Laboratory, University of Cambridge W. S. Cho, K. Y. Lee, S. J. Steinmuller, T. Trypiniotis and J. A. C. Bland Cavendish Laboratory, University of Cambridge, UK K. H. Shin KIST, Korea Room temperature spin detection in ferromagnetic metal/Al 2 O 3 /GaAs heterostructures Cavendish-KAIST Symposium, Spintronics, Daejeon 2006

 Different charge transport mechanisms contribute to the net measured photocurrent, depending on the applied bias  Magnetic circular dichroism (MCD) in the FM layer could obscure or even mimic any real spin filtering effect I D : Transport of photoexcited electrons into bulk semiconductor I T : Quantum mechanical tunneling of electrons I TE : Thermionic emission/Ballistic transport I HD : Electron transport beneath Fermi level I TE IDID I HD ITIT ∆I=p(i+-i-)∆I=p(i+-i-) i+i+ i-i- Transport mechanism of in Hybrid FM/SC Structures

Spin Polarized Electron Transmission in Schottky vs Artificial Tunnel Barrier Structures FM V ≥ 0 n-type SC p-type SC

The Tunnel device of NiFe/Al 2 O 3 /p-typeGaAs Structure The Tunnel device of NiFe/Al 2 O 3 /p-typeGaAs Structure  Well defined interface control and high efficient spin transport  Micro-fabrication process He-Ne laser energy: 1.96 eV Energy gap of GaAs: 1.43 eV Tunnel barrier height: ~1.2 eV

Optical Spin Injection and Electrical Detection Set Up

Interface characteristics in NiFe/Al 2 O 3 /p-type GaAs Structures Interface characteristics in NiFe/Al 2 O 3 /p-type GaAs Structures  The electron transport mechanism change from tunneling process to thermionic emission with applied forward bias (with respect to NiFe layer)  The contribution of the electron and hole current generated by photoexcitation  Well defined interface due to Al 2 O 3 tunnel barrier Tunneling process Thermionic emission

Spin filtering in NiFe/Al 2 O 3 /p-type GaAs Structures   I is proportional to the magnetisation of NiFe layer  Saturated ∆I due to thermionic emission under high applied forward bias. MCD contribution ∆I = p(i + -i - ) under circularly polarized light

  I SF peak heights around 0.55 V due to the optimised electron tunneling process. Evaluation of MCD and True Spin filtering in NiFe/Al 2 O 3 /p-type GaAs Structures Evaluation of MCD and True Spin filtering in NiFe/Al 2 O 3 /p-type GaAs Structures  I SF =  I -  I MCD  I MCD =  I ph  MCD factor  and  I SF curves reveal that tunneling effect is strongly dependent on the magnetisation of NiFe layer. the obstacle to develop a real spin device

Summary and Outlook  Spin filtering of electrons transmitted across the NiFe/Al 2 O 3 /p-type GaAs interface is observed at forward bias at room temperature  Only tunneling electrons show significant spin dependent transmission as in earlier experiments on n-type GaAs  in the structure based on p- type GaAs the depletion region should be modulated to realize high efficiency of spin tunneling.  Critical issues for the exploitation of the observed effect in spintronic devices are  a deeper understanding of the underlying theory  control of the FM/SC interface properties (incl. barrier layer)  optimisation of the details of the SC structure  new structure to exclude MCD from measured helicity dependent photocurrent

Spin Filtering in FM/AlGaAs Barrier/GaAs Structures Bias (V)  I (nA) S. E. Andresen et al., Phys. Rev. B 68, (2003) i + & i - due to the spin split DOS in the FM  Electron tunneling is the spin dependent transport process at the FM/GaAs interface i+i+ i-i-  = 0.112% P eff = 0.019%  = 0.149% P eff = 0.070%  = 0.171% P eff = 0.086% P eff = 0.107%  = 0.185%