EuS based thin film layered systems : magnetoresistance and coupling phenomena

This thesis describes the results of a feasibility study on the use of the magnetic semiconductor europium sulfide (EuS) as a so-called "spin filter". A spin filter is a tunnel barrier with a barrier height that depends on the spin orientation of the tunneling electron. A whole myriad of applications exists for such a layer, in particular in the field of spintronics. Spintronics is an extension of current electronics, in which not only the electron charge but also its spin is exploited. This allows for many new applications, such as nonvolatile fast memories and reprogrammable logic, which is a type of electronics that can change functionality if a magnetic field is applied. For many applications, however, it is necessary to inject a current that consists of unequal amounts of spin-up and spin-down electrons into a semiconductor. In order to create such a current, a high and spin-dependent resistance is needed. A spin filter meets these requirements and therefore constitutes a challenging, but highly interesting solution. In this thesis several properties of spin filters will be treated that are of importance for their further development and implementation in electronics. An optimization study for a EuS spin filter tunnel barrier is presented and the magnetic properties of a spin filter will be discussed. Special attention is given to magnetic coupling effects: Both coupling between a spin filter and a neighboring magnetic electrode as well as coupling between two spin filter layers that are only separated by a thin nonmagnetic spacer layer are treated. Moreover, we will describe how a magnetoresistance resulting from two such spin filters, in a polarizer - analyzer fashion, depends on their resistance and on the spacer layer. The thesis is organized as follows. First, chapter 2 addresses the theory that is at the basics of the physics that is described in this thesis. This chapter is followed by a brief description in chapter 3 of the experimental techniques that were used. In the subsequent chapters the aforementioned issues, regarding the application of EuS as a spin filter, are addressed. Chapter 4 reports on the results of an optimization study on sputter-grown EuS tunnel barriers. Barriers grown at a temperature of 200±C and subsequently annealed at 400-450±C are best, both from magnetic and chemical point of view, as measured by SQUID an XPS, respectively. Combination of EuS as a tunnel barrier with a ferromagnetic electrode in principle leads to a magnetoresistance effect. Magnetoresistance is the dependence of the electrical resistance of a sys- tem on an applied magnetic field. In this case the magnetoresistance is given by the relative change in resistance resulting from a change in the mutual alignment of the magnetizations of the two magnetic layers, which can be modified by the application of a magnetic field. For the practical realization of such a magnetoresistance device aluminum (Al) appears to be most suitable as a nonmagnetic bottom electrode and gadolinium (Gd) appears to be the best magnetic top electrode. This configuration allows for magnetoresistances of more than 100 percent. However, it is not possible to reliably produce such magnetoresistance devices, due to the roughness of the EuS layer. Other combinations of spin filtering EuS with a magnetic electrode only led to small resistance changes as a function of the applied magnetic field (<1%), of which the origin is not clear. It is therefore not possible to use sputter-grown EuS for the reliable fabrication of spin-filter structures. Although the EuS spin filter is generally in direct contact with a ferromagnetic electrode (like Gd) for the systems mentioned above, it appears that the magnetic moment of both magnetic layers can be switched independently by application of a magnetic field. Chapter 4 also discusses the magnetic coupling between these two layers. Various detailed measurements show that the magnetic moments of EuS and Gd are weakly antiferromagnetically coupled to each other. The exact mechanism behind this coupling is not known. However, the interfaces of both materials can only be magnetized with very high magnetic fields, resulting in the presence of a nonmagnetic interface layer between EuS and Gd. This interface layer can in principle explain the absence of a strong direct coupling between the two magnetic layers. In chapter 5 a proposal is presented to produce a magnetoresistance device based on two EuS spin filters, separated by a lead sulfide (PbS) spacer. The resistance of this system was determined within a two-current model, which means that a simple resistor model was used for electrons of each spin orientation separately. Such a calculation suggests that for realistic materials high magnetoresistance ratios (>100%) can be obtained, as long as the resistance of the spin filter layer is between one and one hundred times higher than that of the spacer layer. The magnetic switching field of EuS can be adjusted by making use of magnetic interlayer coupling. The magnetic coupling between two EuS layers, separated by a nonmagnetic PbS layer, is discussed in chapter 6. In contrast to what is commonly observed in metallic structures, the coupling is always of antiferromagnetic nature in this semiconductor system. The magnetization of EuS was varied by performing measurements in a range of temperatures up to its Curie temperature. The interlayer coupling energy was found to depend strongly (as a fourth power) on the EuS magnetization. This is explained by the influence of the EuS magnetization on the band structure in the PbS spacer layer. Finally, chapter 7 discusses the effect of the addition of charge carriers on the magnetic properties of a magnetic semiconductor. Europium telluride (EuTe), a material that is chemically comparable to EuS, was n-doped with Gd (substitutional at Eu sites in the lattice). The originally antiferromagnetic EuTe becomes ferromagnetic upon the substitution of 1% of the Eu atoms with Gd, and can reach a Curie temperature of 15 K. This effect can be explained by magnetic coupling of the localized magnetic moments in the material to the free carriers that were added by the Gd doping. The investigated Gd-doped EuTe layers showed, apart from a ferromagnetic phase, also either a paramagnetic or an antiferromagnetic phase, probably due to a nonuniform distribution of the charge carriers in the layer, which could be an interesting subject for further research