Characterization of CoFe2O4 Thin Films and Associated Multilayers 

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From Eu Chalcogenides to Complex Magnetic Oxides : The Diverse Spectrum of Spin Filter Materials

The first spin filter to be revealed by the Meservey-Tedrow technique was EuS [47]; hence the important number of theoretical and experimental studies based on this model material. As explained in the previous section, Moodera et al.’s pioneering SPT measurements with Al/EuS/Au tunnel junctions were particularly interesting because they revealed Zeeman splitting in the zero-field conductance curve, due to the exchange interaction between the conduction electrons in the SC Al electrode and the strongly ferromagnetic Eu2+ cations (μ = 7-8 μB). To determine the spin filter efficiency of the EuS barrier, the dI/dV curves in Figure 2.9 were fitted using Maki-Fulde theory [21], yielding a notably large value of PSF = 80 ± 5%. Also extracted from the 0.07 T curve in Figure 2.9 was the magnitude of the effective magnetic field in the Al layer induced by the EuS, given that the Zeeman splitting is equal to 2μ0H0. The result is an effective field H0 = 3.46 T as opposed to the applied field Happl = 0.07 T.
Evidence of the spin filter effect in EuS was also identified from the junction resistance, Rj , versus temperature curves. For T > TC, these display the increasing Rj(T) behavior typically found in semiconductors. On the other hand, when T < TC, the lowering of ↑ due to the onset of exchange splitting in the conduction band results in a significant drop in Rj with decreasing T.
In fact, Rj decreases as much as 65% with respect to its maximum value at Tc, confirming the significant exchange splitting and PSF expected at low T. Motivated by the work of Moodera et. al., LeClair et al. further proved the spin filtering capacity of EuS via TMR measurements in spin filter MTJs [49]. This work was again of major importance to the development of spin filters, as it was the first to successfully integrate a spin filter barrier into a MTJ. In their Al/EuS(5 nm)/Gd tunnel junctions, LeClair et al. measured TMR ∼ 100% at 2K which corresponds to PSF = 87%, in very good agreement with the previous Meservey-Tedrow measurements. A few degrees higher in temperature, TMR decreased significantly, disappearing completely above the Tc of EuS (16K). As may be seen in Figure 2.10, the MR signal contains significant amount of noise which was attributed to instabilities in the EuS magnetization, although this could potentially be due to instabilities in the Gd layer as well (see Figure 2.11-b for EuO below). A possible exchange coupling between the EuS and Gd layers could also explain the observed noise [50].

CoFe2O4 : A New Candidate for Room Temperature Spin Filtering

In this thesis, we endeavor to demonstrate that spin filtering at room temperature is indeed possible with another material from the spinel ferrite family : cobalt-ferrite or CoFe2O4. CoFe2O4 (CFO) is a very good candidate for room temperature spintronics applications thanks to its ferrimagnetic nature, high Curie temperature (793 K), and good insulating properties. As we will see in detail below, electronic band structure calculations from first principles methods predict CoFe2O4 to have a band gap, Eg, of 0.8 eV and an exchange splitting, 2Eex, of 1.28 eV between the minority (low energy) and majority (high energy) levels in the conduction band [57], thus confirming its high potential to be a very efficient spin filter, even at room temperature. We emphasize that CoFe2O4 is expected to have a negative spin polarization, as opposed to its Eu chalcogenide and perovskite counterparts (see Section 2.4.3). Recently, a tunneling spectroscopy study of CoFe2O4/MgAl2O4/Fe3O4 double barrier tunnel junctions revealed optimistic results for the spin-filter efficiency of CoFe2O4 [58]. However, the polarization (P) and TMR values obtained in this work were indirectly extracted from a complex model developed to fit experimental current-voltage curves rather than from direct Merservey-Tedrow or TMR measurements. In the present work, we will use both of these SPT techniques to unequivocally demonstrate the spin filter capabilities of CoFe2O4.

EXPERIMENTAL METHODS : FROM THIN FILM GROWTH TO SPIN-POLARIZED TUNNELING

In this chapter we will describe the ensemble of experimental techniques used throughout this thesis. We have granted this topic a chapter of its own, rather than integrating it into the forthcoming chapters, in order to avoid drowning out the important experimental results with technical details. The goal here is thus to introduce a sort of reference that the reader may refer back to if needed to understand the results and physical interpretations in oxides by molecular beam epitaxy (MBE), which is of course the heart of this thesis. Without the MBE, nothing else would have been possible. Next we will continue on to the numerous in situ and ex situ characterization techniques used to verify the structural, chemical and magnetic properties of our films. Finally, we will conclude with a detailed description of the different magneto-transport techniques used to measure the electronic properties and spin polarized tunneling in our CoFe2O4-based systems.

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Epitaxial Growth of Spinel Ferrite Thin Films

Nearly all of the films studied throughout this thesis have been grown by molecular beam epitaxy (MBE). The only exception, as we will see in by the sputtering technique at the Unit´e Mixte de Physique CNRS/Thales in Palaiseau, France. The fundamental basis of simple material deposition by MBE consists in generating a molecular flux of the metal or semiconductor to be deposited by evaporating a solid source of this material from a Knudsen effusion cell. The condensation of the evaporated metal on a crystalline substrate leads to the growth of a single crystalline film. This film is considered epitaxial when the crystalline lattice of the substrate and that of the deposited film are geometrically coherent. In other words, the crystalline substrate in essence imposes its lattice on the growing film such that, in addition to the other physical constraints imposed by the deposi tion conditions, only a geometrically compatible phase may form. In order to avoid the contamination of the deposited film during growth, the MBE chamber must imperatively be maintained under ultra high vacuum (UHV) conditions (base pressure ∼5.10−11 mbar). Not surprisingly, this constraint makes MBE a very delicate and complex deposition technique.

Table of contents :

1. Introduction
2. Spin-Polarized Tunneling and Spin Filtering
2.1 Spin-Polarized Tunneling
2.1.1 The Basics
2.1.2 Detecting Spins
2.1.3 Tunneling Magnetoresistance
2.2 Spin Filtering
2.2.1 Phenomenological Origin
2.2.2 Measurement Techniques
2.3 The Diverse Spectrum of Spin Filter Materials
2.3.1 Eu Chalcogenides
2.3.2 Perovskites
2.3.3 Ferrites
2.4 CoFe2O4 : A New Candidate for Room Temperature Spin
2.4.1 Structure
2.4.2 Magnetism
2.4.3 Electronic Band Structure
3. Experimental Methods : From thin film growth to spin-polarized tunneling
3.1 Epitaxial Growth of Spinel Ferrite Thin Films
3.1.1 Molecular Beam Epitaxy
3.1.2 Growth Conditions
3.2 In situ Characterization
3.2.1 Reflection High Energy Electron Diffraction
3.2.2 Electron Spectroscopies
3.3 Structural and Chemical Characterization Electron Microscopies
3.3.1 Transmission Electron Microscopy
3.3.2 Geometric Phase Method
3.3.3 Electron Energy Loss Spectroscopy
3.4 Magnetic Characterization
3.4.1 Vibrating Sample Magnetometry
3.4.2 Polarized Neutron Reflectometry
3.5 Electronic Transport and Spin-Polarized Tunneling
3.5.1 In-plane electronic transport
3.5.2 Two Terminal Versus Four Terminal Measurements
3.5.3 Sample Preparation for TMR Experiments : Optical Lithography
3.5.4 Out-of-plane Electronic Transport
3.5.5 Sample Preparation for Meservey-Tedrow Experiments
3.5.6 The Meservey-Tedrow Experiment
4. Characterization of CoFe2O4 Thin Films and Associated Multilayers 
4.1 CoFe2O4 Single Layers
4.1.1 Epitaxial Growth : RHEED In situ
4.1.2 In situ Chemical Characterization by XPS
4.1.3 X-ray Diffraction and Reflectivity
4.1.4 X-ray Absorption and X-ray Magnetic Circular Dichroism
4.1.5 Growth on a Pt(111) buffer layer
4.2 CoFe2O4/Fe3O4 Bilayers
4.2.1 RHEED and XPS
4.2.2 Microscopy studies of CoFe2O4/Fe3O4
4.3 CoFe2O4/γ-Al2O3/Co trilayers and their variants
4.3.1 Epitaxial growth and RHEED
4.3.2 In situ spectroscopies : XPS and AES
4.3.3 TEM
5. CoFe2O4 single layer spin-filter tunnel barriers
5.1 Magnetic Properties of CoFe2O4 single layers
5.1.1 Magnetism in “thick” CoFe2O4 films
5.1.2 Magnetism in ultra-thin CoFe2O4 films
5.1.3 Optimization with a Pt(111) buffer layer
5.1.4 Magnetic properties of CoFe2O4/γ-Al2O3 double tunnel barriers
5.1.5 Low temperature SQUID measurements
5.2 In-plane Electronic Transport Measurements
5.3 Spin-polarized Tunneling in CoFe2O4 : Meservey-Tedrow Technique
5.3.1 The initial Meservey-Tedrow measurement
5.3.2 Optimizing the SPT results : Effect of oxidation
5.3.3 Junction Resistance Temperature Dependence
5.3.4 Discussion
6. CoFe2O4/Fe3O4 bilayers for spinel-based tunnel junctions
6.1 Magnetic Properties of the CoFe2O4/Fe3O4 system
6.1.1 Room temperature magnetization curves
6.1.2 Low temperature magnetization curves
6.1.3 Insertion of a thin γ-Al2O3 spacer
6.1.4 In-plane Magnetoresistance Measurements
6.1.5 Polarized Neutron Reflectivity
6.2 Discussion of the Exchange Coupling Mechanism
6.2.1 Switching Order
6.2.2 Nature of the Exchange Interaction
6.2.3 Local Magnetic Configuration at the Interface
7. CoFe2O4-based Magnetic Tunnel Junctions with cobalt electrodes . 
7.1 Magnetic Characterization
7.1.1 CoFe2O4/Co bilayers
7.1.2 CoFe2O4/γ-Al2O3/Co trilayers
7.2 Tunneling Experiments
7.2.1 Resistance Measurements and TMR
7.2.2 Current-Voltage Characteristics
7.2.3 TMR versus Bias Voltage
7.2.4 Discussion
8. Conclusions and Future Work
8.1 Conclusions
8.1.1 Growth and materials characterization
8.1.2 Magnetism
8.1.3 Spin-polarized tunneling
8.2 Short-term Future Work
8.2.1 Meservey-Tedrow experiments on CoFe2O4 single layers
8.2.2 CoFe2O4/Fe3O4 bilayers and MTJs
8.2.3 Pt/CoFe2O4/γ-Al2O3/Co MTJs
8.3 Long-term Perspectives and Applications
8.3.1 Double spin filter tunnel junctions
8.3.2 Spin injection into semiconductors and organics
14 Contents
Appendix
A. Crystalline Co/α-Al2O3(0001) bilayers for fully epitaxial magnetic tunnel junctions
A.1 Epitaxial Growth and Materials Characterization
A.2 Spin-Polarized Tunneling Experiments
A.3 Conclusion

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