Spatially Resolved Infrared Spectroscopy For Spintronics

Magnetoresistance, a change in the resistance of a material in an externally applied magnetic field, is an extremely important property of magnetic materials. The discovery of giant magnetoresistance has led to a revolution in computing, driving increases in storage density of hard disks and paving the way for commerical spintronic devices. Conventionally, magnetoresistance is measured by sourcing a current through a material and measuring the voltage. Ohm’s law is used to calculate the resistance of the material with and without an external magnetic field, the difference between these results being the magnetoresistance. This technique is limited as it does not offer spatial resolution, so variations in magnetoresistance in a material can not be detected. Electrical contact must also be made to the material, which can cause damage to the material being measured. The magnetorefractive effect, the change in the reflection spectrum of a material in an external magnetic field, can be used as an alternative to the electrical measurement of magnetoresistance. The magnetorefractive effect allows non-contact measurements of magnetoresistance to be made, so the material remains undamaged, whilst also offering the possibility of spatial resolution. Modelling the spectral magnetorefractive effect can also aid in understanding the underlying physical mechanism behind the magnetoresistance, which is impossible with an electrical measurement. Infrared reflection microspectroscopy was used to observe variations in reflectivity across Fe3O4 thin films. By modelling these variations, it was possible to estimate the chemical composition of the samples as well as observe any variations in composition across them. A spatial variation in magnetoresistance was observed across a CoFe/Cu multilayer using the magnetorefractive effect, whilst also obtaining the spectral resolution necessary to model the system, the first time such a measurement has been performed. The correlation between the magnetorefractive effect and magnetoresistance had been predicted to be strong in the far-infrared by previous theoretical work. The magnetorefractive effect in the far-infrared was measured for a series of spin valves, demonstrating this strong correlation in the far-infrared for the first time, providing long awaited experimental confirmation of this theoretical prediction.