Photo-Induced Hall and Magnetoresistance Effect in Metals

金屬中的光致霍爾及磁阻效應研究

Student thesis: Doctoral Thesis

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Award date28 Nov 2018

Abstract

Computer-based information technology has thoroughly changed the society. A computer is an electronic system with strong computing ability. A sensor is a device that can transfer changes in its environment (like temperature, position, force, etc.) to other electronics. By using sensors, the changes in the physical world could be transmitted to computers, which leads a tight connection between the cyber world and the physical world. By receiving the data from sensors, computers will be able to monitor the real-time changes in the physical world. Modern computing techniques, like big data and artificial intelligence, allow quick analysis of those changes. Finally, computers could send the instruction to control the physical world based on analysis. Such a ‘sense-analysis-control’ cycle creates a smart world. Sensors are of great importance in this cycle.

Sensors are not always based on standard electronics, which exploit the electric charge. The largest part of mechanical sensors is based on the spin of the charge. This sub-field of electronics, called spintronics, has given a great contribution to modern sensing and computing. An example above all is the read-head of magnetic hard-disks that has allowed storage densities of Terabytes. The spintronic sensors used so far are based on charge-current that is spin-polarized. This means that a spin-current is still carried by a charge-current. A new paradigm has emerged in recent years which tries to disentangle the spin-current from a charge-current. An electronic based on pure-spin currents would be energy efficient, because free of loss from Joule dissipation.

In this thesis, we focused on the research of photo-electrical sensing devices, with particular emphasis on sensors that do not dissipate Joule energy, either because they operate in open-circuit conditions or because they exploit pure spin-currents. In either case, the sensors convert light into an electrical signal. Like other photo-electrical sensing devices, our sensors are based on the result of two concurrent physical processes: photo-generation and separation of carriers. Yet, in our sensors, separation is based on spin-Hall effect. In other words, carriers are discriminated based on the spin, rather than the charge.

Two different systems were systematically investigated: semiconductor/transparent-metal bilayers and yttrium iron garnet Y3Fe5O12/Platinum (YIG/Pt) bilayers under magnetic field and light illumination. The mechanisms for the photo-voltaic conversion and the potential of the device sensors were studied.

In the first chapter of this thesis, an overview of the Hall effects will be given, with particular emphasis on those that are non-conventional, such as spin-Hall and inverse spin-Hall effects. The spin-Hall effect is at the core of the generation of a pure spin-current, whereas the inverse spin-Hall effect, which is based on spin-orbit coupling, allows conversion of a pure spin-current to a charge current. The inverse spin-Hall effect is the mechanism that allows integration of spintronic devices into standard electronic devices.

Since metals in contact with semiconductors tend to form Schottky junctions, in the second chapter, an overview is given of the state of art of Schottky junctions for photo-voltaic sensing. Particular emphasis will be given to Schottky photo-diodes and Schottky magneto-diodes. The theory of image forces, which is used in chapter 6 to account for the rounding of the Schottky barrier under light illumination, is also presented.

A magnetic order arises in a paramagnetic metal in contact with a ferromagnet. This effect, called “ferromagnetic proximity effect” will be discussed in chapter 4. This proximity effect can lead to a new type of magnetoresistance based on pure spin-currents, called spin-Hall magnetoresistance, and to a novel photo-voltaic effect called photo-spin voltaic effect. These, recently discovered effects will be described.

In chapter 4, we report the details of the experiments. This includes the techniques for the deposition of the thin films, the measurement setups that were assembled to carry out the investigations and the Labview software that was written to automatize the measurements.

The results of our work will be presented and discussed in the last two chapters. In particular, in chapter 5, we present our study on photo-spin voltaic and photo-magnetoresistive effects. We prove that those effects can exist under illumination with light in the visible range. They are due to photo-excitation of carriers in the proximized layer. We developed a magneto-transport model to describe the change in magnetoresistance as a function of the light intensity. This model is presented at the end of chapter 5.

In chapter 6, the discovery of a new Hall effect, that we named “photo-induced Hall effect in metals” will be presented. We show that a significant Hall voltage can be photo-induced in metals and used for bias-free magnetic sensing. The system is based on the photo-generation and injection of charge through the space charge region of a Schottky barrier that undergoes significant rounding due to image potential. Since the charge is injected into the metal at high velocity, a magnetic field applied into the plane can generate large Lorentz forces and an open-circuit voltage appears at the metal edges. This voltage is proportional to the magnetic field, as well as light intensity. We show that the system offers sensitivities that are comparable to commercial Hall-sensors but no net current flows, therefore its linearity range is not affected by Joule heating.

Finally, some conclusions are drawn and some future perspectives are given.