william thomson

AMR – The Start of Something Special

William Thomson – The Father of Spintronics?

This paper from 1856 helped trigger the entire field of spintronics – but you’d be forgiven for not spottting it. The author William Thomson went on to become Lord Kelvin – now well known for the temperature unit named after him, could well be regarded as the father of spintronics.

Reads Like A Jules Verne Novel

The effect Thomson describes is now known as anisotropic magnetoresistance – or AMR.

This mouthful of a term describes the fact that the electrical resistance of a ferromagnetic material depends on the orientation of its magnetisation to the direction of current flow. Or as Thomson puts it, in  the language of his time:

iron, when subjected to magnetic force, acquires an increase of resistance to the conduction of electricity along, and a diminution of resistance to the conduction of electricy across, the lines of magnetization

The equipment list reads like something from a Jules Verne novelRuhmkorff electromagnets, Daniell cells (for generating current) and Galvanometers (for measuring voltage). But once you get beyond the unusual language you’re left with a rather clear description of an experiment that wouldn’t look out of place in a lab today.

Unfortunately figures do not seem to have been possible in 1856, so I’ve attempted to put together my interpretation of William Thomson’s 1856 experiment.

william thomson

William Thomson’s experiment for investigating the effect of magnetic field direction on the resistance of magnetic metals.

 

What William Thomson couldn’t have known is that it is not the direction of the applied magnetic field which is important. Rather, it is the direction of the magnetisation of the Ni relative to the direction of current that determines the resistance of the test piece.

To get an idea for what the measurements would have involved, here’s a ‘Ruhmkorff Electromagnet‘ similar to that used by Thomson in action:

Humble Beginnings

From these humble beginnings this study of the magnetic properties of metals helped bring on the field of “spintronics” today. The most successful and well known spintronic device is the hard-drive read head, which relies on an effect very similar to that first observed by William Thomson over 150 years ago.

Thomson W. (1856). On the Electro-Dynamic Qualities of Metals:–Effects of Magnetization on the Electric Conductivity of Nickel and of Iron, Proceedings of the Royal Society of London, 8 546-550. DOI: http://dx.doi.org/10.1098/rspl.1856.0144