Spin polarized coulomb blockade in metallic magnetic single electron transistors
Final Report Abstract
The aim of this project was the theoretical and experimental investigation of spin polarized coulomb blockade in metallic magnetic single electron transistors (magSET). From the theoretical side this problem was thoroughly investigated. First a diagrammatic real-time technique for spin-dependent transport through magSETs was derived. Based on this the linear and non-linear transport characteristics of the magSET were calculated. As a further step current fluctuations in magSETs were investigated. For transport through a spin polarized single-level quantum dot it was found that the interplay of Coulomb interaction and finite spin polarization implies spin-correlation induced charge correlations that gives rise to super-Poissonian transport behaviour. These effects should be accessible experimentally e.g. by noise measurements on spin polarized QD systems. In contrast to spin polarized QD systems the transport behaviour of magSETs remains sub-Poissonian all the time due to the dense level spectrum inhibiting strong spin blockade. However it was shown that both the charging energy and the exchange field need to be taken into account in order to properly describe the Fano factor of magSET transport. From the experimental side the aim was to fabricate metallic magSET from high quality double barrier magnetic tunnel junctions (DBMTJ) patterned into nanopillars to observe Coulomb blockade at mK temperatures. The growth of DBMTJ stacks were optimized with respect to middle layer thickness and annealing temperatures. Furthermore processing was optimized to pattern these devices into MTJ nanopillars. The investigation of such optimized single and double barrier MTJ nanopillar devices lead to a number of important discoveries. Among them the first observation of memristive switching, the study of magnon excitation, and the observation of a large tunneling magneto thermo power in the presence of thermal gradients across the barrier. Despite of the above sample optimization no signature of Coulomb blockade has been observed in any of the samples in the course of the project. Therefore the experimental studies were shifted to the effects of the (single) spin polarization in dynamic semiconductor QDs. These experiments lead to the first observation of a dynamic initialization and separation of spin-singlet and spin-triplet electron pairs. The observed initialization occurs by dynamic state selection of states with exponentially different tunnel coupling to a common lead. In this approach energy differences are translated into decay rates differences which allow the dynamical separation of the target states. This process can only occur when the dynamic selection process is faster than the singlet triplet relaxation time. Hence the dynamic spin state selection demonstrated here could allow a new path towards fast (spin) state initialization of solid state quantum states with various promising applications in fundamental solid state quantum experiments and solid state quantum technology.
Publications
- Influence of annealing temperature and thickness of a CoFeB middle layer on the tunnel magnetoresistance of MgO based double barrier magnetic tunnel junctions, J. Appl. Phys. 105, 014510 (2009)
A. Reinartz, J. Schmalhorst, G. Reiss
- Memristive switching of MgO based magnetic tunnel junctions, Appl. Phys. Lett. 95, 112508 (2009)
P. Krystezko, G. Reiss, A. Thomas
- Spin-induced charge correlations in transport through interacting quantum dots with ferromagnetic leads. Phys. Rev. B 79, 245303 (2009)
S. Lindebaum, D. Urban, and J. König
- Magnetoresistance and transport properties of CoFeB/MgO granular systems, J. Appl. Phys. 107, 113718 (2010)
K.M. Bhutta, G. Reiss
- Tunneling spectroscopy probing magnetic and nonmagnetic electrodes in tunnel junctions
Volker Drewello, Zoë Kugler, Günter Reiss, Andy Thomas
- Theory of transport through noncollinear single-electron spinvalve transistors, Phys. Rev. B 84, 235409 (2011)
S. Lindebaum and J. König
- Tunneling magneto thermo power in magnetic tunnel junction nanopillars. Phys. Rev. Lett. 107, 177201 (2011)
N. Liebing, S. Serrano-Guisan, K. Rott, G. Reiss, J. Langer, B. Ocker, and H. W. Schumacher
- Current fluctuations in noncollinear single-electron spin-valve transistors, Phys. Rev. B 86, 125306 (2012)
S. Lindebaum and J. König
(See online at https://doi.org/10.1103/PhysRevB.86.125306) - Magnon excitation and temperature dependent transport properties in magnetic tunnel junctions with Heusler compound electrodes, J. Appl. Phys. 111, 07C701 (2012)
V. Drewello, D. Ebke, M. Schäfers, Z. Kugler, G. Reiss, A. Thomas
(See online at https://doi.org/10.1063/1.3669913) - The influence of individual lattice defects on the domain structure in magnetic antidot lattices
X.K. Hu, S. Sievers, A. Müller, V. Janke, and H.W. Schumacher
(See online at https://doi.org/10.1063/1.4795147)
