In their experiment, the researchers implanted muons into their device. Muons are elementary particles that act like small magnets, and can thus show up the magnetic fields in their surroundings. The muons for this experiment were generated in the particle accelerator at PSI and subsequently subjected to heavy deceleration – PSI is the only location world-wide where this process is available. In the long term, this type of experiment will help us to understand the processes going on inside the read head in greater detail, so that engineers can see where they need to concentrate their efforts to optimise the heads.
The fact that computers can store more data and MP3 players have become so much smaller in the past decade is largely due to an effect that physicists call giant magnetoresistance. In 2007, the Nobel Prize for physics was awarded for the discovery of this effect, which makes it possible to produce electronic components with an electrical resistance extremely sensitive to external magnetic fields. By using the effect in hard drive read heads, magnetically coded information can be packed together very densely, and the hard disc can then be extremely small. Without this effect, it would be impossible for a device half the size of a cigarette packet to store all the information contained in 100 CDs and more.
Spintronics – electronics with spin
“Unlike in most electronic components, where the electron’s intrinsic charge is used to carry the signal, magnetic read-heads also use the electron’s ‘spin’ that makes the electron into a tiny magnet. Thus the coining of the term ‘spintronics’” explains the initiator of this research project, Dr Alan Drew of the University of Fribourg (Switzerland) and Queen Mary, University of London. The technical term for the relevant component is a “Spin Valve”, consisting of at least three layers: two magnetic outer layers separated by a non-magnetic spacer layer. One of the magnetic layers is magnetised in a fixed direction, while the magnetisation of the other adjusts itself to the external magnetic field. Currents flowing between the two magnetic layers have a smaller resistance if both layers are magnetised in the same direction. However, if they are magnetised in opposite directions, the electron’s progression is inhibited at the second layer because it arrives with the “wrong” spin orientation.
Unstable particles tell us about magnetic fields
This only works as long as the spins do not flip in their journey through the middle layer, resulting in the arrival of electrons with a random spin direction at the second magnetic layer. Dr Drew wanted to understand this process in the read heads and undertook a series of experiments in collaboration with his colleagues at PSI. They wanted to see how far the spins penetrate into the spacer layer before they flipped. They relied on the fact that the ensemble of oriented spins create a magnetic field that becomes stronger with the uniformity of orientation. They employed muons – unstable elementary particles, similar to electrons but much heavier – as probes to measure the internal magnetic fields. If a muon is inserted into a magnetic field, it’s own spin begins to rotate, with a speed of rotation that depends on the strength of the field. After a few millionths of a second, the muon disintegrates into a number of particles, one of which travels in the direction of muon’s spin which is then detected. By observing several million decays, one can observe the speed of the rotation and finally determine the local field.
For their experiments, the researchers built a special kind of spin valve with an organic spacer layer – a conducting plastic. “These plastics are very flexible and easy to process. They could revolutionise electronic or spintronic devices in the future” explains Dr Drew “In addition to that, in organic materials, the electron spin flips at a much lower rate than in many other materials – up to a million times lower. This property of organic materials was one of the key facts that made the muon measurements possible.”
Slow enough, but only in Switzerland
It sounds easy enough in principle, but all this calls for an enormous technical effort. “Such experiments can only be undertaken at PSI, because only we can generate the very slow muons that stay within the thin layers of our read heads. The muons in other experiments are so fast that they would simply fly straight through our sample” explains the physicist Dr Elvezio Morenzoni, who runs the muon facility at the Paul Scherrer Institute. Even at PSI, the muons are too fast at first. They are produced in the proton accelerator at the Paul Scherrer Institute, where a beam of protons is initially accelerated to three quarters the speed of light, after which they strike a “carbon target”. This creates particles that subsequently decay into muons. The muons are decelerated at PSI by a unique process in a thin layer of frozen inert gas, and then the required speed can be selected. It is possible to establish the depth at which the muons stop within the sample by varying this speed, thus allowing the magnetic fields at various depths to be determined.
These experiments have demonstrated that the extent to which the electrons lose their spin orientation determines how well a read head will work, i.e. the extent to which electrical resistance depends on the magnetic field. Most importantly, however, it is now clear that experiments with muons can help us to understand the processes involved in spintronic components. “This will probably never become a standard method for the investigation of industrial components, as the process is far too expensive” stressed Dr Morenzoni, “But it will definitely help us to understand the basic issues that are involved, thus providing the industry with information on how to develop the components further”.
The Paul Scherrer Institute develops, builds and operates large and complex research facilities, and makes them available to the national and international scientific community. Its own work concentrates on solid-state research and material sciences, elementary particle physics, biology and medicine, energy research and environmental research. With a staff of 1300 and an annual budget of approximately CHF260 million, it is Switzerland’s largest research institution.
Queen Mary is one of the largest colleges of the University of London with some 15,000 undergraduate and postgraduate students and research across 21 academic departments and institutes, within three sectors: Science and Engineering; Humanities, Social Sciences and Laws; and the School of Medicine and Dentistry.
The University of Fribourg, Switzerland - Some 10.000 students and over 200 professors coming from 100 different countries, study teach and research the five faculties of the University of Fribourg. The Faculty of science has a long and successful tradition of research in the field of nano-materials of which the recently created Adolphe Merkle Institute is the flagship.