Spin filtering and resistive switching in all-oxide tunnel junctions

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Laura Äkäslompolo Name of the doctoral dissertation Spin filtering and resistive switching in all-oxide tunnel junctions Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 46/2015 Field of research Engineering Physics Manuscript submitted 20 January 2015 Date of the defence 4 May 2015 Permission to publish granted (date) 9 March 2015 Language English Monograph Article dissertation (summary + original articles) Abstract In tunnel junctions, electrons quantum-mechanically tunnel through a thin insulating barrier between two electrodes. Different types of tunnel junctions are used in a variety of solid-state nanoelectronic devices, including transistors, diodes, memories, and magnetic field sensors. In most of these devices, the tunnel barrier acts as a passive element whose properties cannot be modified by external actuation. The use of oxide tunnel barriers with magnetic or ferroelectric order would enable active control of the tunnelling conductance in a magnetic or electric field. Moreover, the physics of oxide electrode-barrier interfaces is very rich and its exploration could lead to new functionalities. In this thesis, two types of all-oxide epitaxial tunnel junctions are studied. First, the growth of tunnel junction with a ferrimagnetic CoFe2O4 barrier is discussed. In this realisation, the tunnel barrier acts as a spin filter. As electrode materials, ferromagnetic La2/3Sr1/3MnO3 and SrRuO3 are used. Different growth sequences are examined on single-crystalline SrTiO3(001) and MgO(001) substrates. X-ray diffraction and high-resolution transmission electron microscopy reveal that the quality of epitaxial growth is compromised by the large lattice mismatches between the tunnel barrier and the two electrodes. The best results are obtained for SrRuO3/CoFe2O4/La2/3Sr1/3MnO3 trilayers on SrTiO3 substrates. In these structures, the magnetization of the CoFe2O4 barrier and the La2/3Sr1/3MnO3 layer switch at different magnetic field, which is a prerequisite for spin-filter tunnel junctions. Tunnelling magnetoresistance measurements, however, only show a high-field effect, which can be attributed to the La2/3Sr1/3MnO3 electrode. In the second part of this thesis, resistive switching in tunnel junctions with a ferroelectric PbZr0.2Ti0. 8O3 or BaTiO3 tunnel barrier and two La2/3Sr1/3MnO3 electrodes is investigated. Despite the nominally symmetric trilayer structure, very large resistive switching effects up to 107 % are obtained at low temperatures. The origin of this phenomenon is discussed in terms of a polarization-induced metal-to-insulator transition in La2/3Sr1/3MnO3 and electric-field driven migration of oxygen vacancies. Transmission electron microscopy measurements reveal clear differences in structural roughness and atomic mixing at the two electrode-barrier interfaces. This breaks the symmetry of the junction and enables hysteretic resistive switching. Model fits to electrical transport data indicate that the potential energy profile of the barrier is asymmetric in the high resistance state and that an insulating interface layer is formed in the La2/3Sr1/3MnO3 bottom electrode. This observation is independently verified by in-plane transport measurements on Hall bar structures.In tunnel junctions, electrons quantum-mechanically tunnel through a thin insulating barrier between two electrodes. Different types of tunnel junctions are used in a variety of solid-state nanoelectronic devices, including transistors, diodes, memories, and magnetic field sensors. In most of these devices, the tunnel barrier acts as a passive element whose properties cannot be modified by external actuation. The use of oxide tunnel barriers with magnetic or ferroelectric order would enable active control of the tunnelling conductance in a magnetic or electric field. Moreover, the physics of oxide electrode-barrier interfaces is very rich and its exploration could lead to new functionalities. In this thesis, two types of all-oxide epitaxial tunnel junctions are studied. First, the growth of tunnel junction with a ferrimagnetic CoFe2O4 barrier is discussed. In this realisation, the tunnel barrier acts as a spin filter. As electrode materials, ferromagnetic La2/3Sr1/3MnO3 and SrRuO3 are used. Different growth sequences are examined on single-crystalline SrTiO3(001) and MgO(001) substrates. X-ray diffraction and high-resolution transmission electron microscopy reveal that the quality of epitaxial growth is compromised by the large lattice mismatches between the tunnel barrier and the two electrodes. The best results are obtained for SrRuO3/CoFe2O4/La2/3Sr1/3MnO3 trilayers on SrTiO3 substrates. In these structures, the magnetization of the CoFe2O4 barrier and the La2/3Sr1/3MnO3 layer switch at different magnetic field, which is a prerequisite for spin-filter tunnel junctions. Tunnelling magnetoresistance measurements, however, only show a high-field effect, which can be attributed to the La2/3Sr1/3MnO3 electrode. In the second part of this thesis, resistive switching in tunnel junctions with a ferroelectric PbZr0.2Ti0. 8O3 or BaTiO3 tunnel barrier and two La2/3Sr1/3MnO3 electrodes is investigated. Despite the nominally symmetric trilayer structure, very large resistive switching effects up to 107 % are obtained at low temperatures. The origin of this phenomenon is discussed in terms of a polarization-induced metal-to-insulator transition in La2/3Sr1/3MnO3 and electric-field driven migration of oxygen vacancies. Transmission electron microscopy measurements reveal clear differences in structural roughness and atomic mixing at the two electrode-barrier interfaces. This breaks the symmetry of the junction and enables hysteretic resistive switching. Model fits to electrical transport data indicate that the potential energy profile of the barrier is asymmetric in the high resistance state and that an insulating interface layer is formed in the La2/3Sr1/3MnO3 bottom electrode. This observation is independently verified by in-plane transport measurements on Hall bar structures.