Shortly after our first entry, a news article in DER STANDARD from 9th of April probably went unnoticed by many. It reported the sad demise of the German physicist Peter Grünberg, the man who alongside the French physicist Albert Fert, received the Physics Nobel Prize for the discovery of the giant magnetoresistance in 2007. A discovery that gave birth to spintronics.
Peter Grünberg and Albert Fert provided the keystone for the scientifically fascinating and technologically significant field of magnetoelectronics or spintronics. The discovery of the phenomenon of giant magnetoresistance (GMR) paved the way for the unthinkable inflation in the size of memory devices and recording media. Aided by the advances in nanotechnology, the storage capacities in our computers, laptops, et cetera, scaled from mere few hundreds of megabytes in the 90s to whopping terabytes range in the present decade. But what is the GMR and spintronics?
Quick guide through magnetism
Magnetism is a complex subject, as it is often in physics, which can only be properly described in terms of quantum mechanics. Nevertheless we use a rather simple and classical point of view to explain it. The theoreticians among our readers will hopefully forgive us for this. An atom can be described in a simple way as a central positive nucleus surrounded by negatively charged electrons orbiting around it in well-defined closed tracks known as the atomic orbitals. But the electrons do not only orbit around the nucleus, they also orbit or spin around themselves. This important property is called the spin. The spin of the electrons describes the orbiting motion of the electron around itself and can therefore have two directions: spin-up so that the electrons spin clockwise, and spin-down so that they spin counterclockwise. This spin is also a magnetic dipole, this means a small magnet with north and south pole. Now it gets a bit complicated, because each atomic orbital can only be filled with a pair of electrons with a complementary spin direction, so that the magnetic dipoles in this orbital cancel each other out. In atoms containing an uneven number of electrons, there will always be an unpaired electron with a missing partner. And exactly the order of the spins of these unpaired electrons determine the magnetic properties of a material.
A material where the unpaired electrons spins are randomly oriented is called a paramagnet. If the spins of neighbouring atoms point all to the same direction, we have a ferromagnet. These are the materials that we know commonly as magnets and are sticking on our fridge. Now, if in the material the spins are ordered and pointing alternatingly up and down, we have an antiferromagnet. This is depicted in the image shown below:
As mentioned above, the spins are small magnets and can be therefore align themselves in a magnetic field, like the needle of a compass in the earth magnetic field. In such an external magnetic field, all the spins of a paramagnet or a ferromagnet will align in the direction of that applied field. When the external field is withdrawn, the spins in the paramagnet will get disoriented and return to their initial state, before the field was applied. In ferromagnets, however, the individual spins stay oriented, because this is the natural state – in physics we call it the lowest energy state, because nature doesn’t like to spend energy if it is not needed – of a ferromagnet. Antiferromagnets on the other hand, remain unchanged in the presence of an external magnetic field, unless the magnetic field is extremely high, because the unpaired electrons have found a partner – even if it is in the neighbouring atom. This means, that antiferromagnets cannot be easily manipulated by magnetic fields. This is an advantage over ferromagnets, which is actually used to fix (the scientific term is "pin") the spin orientation in a ferromagnet by placing it in contact with an antiferromagnet.
The discovery of the giant magnetoresistance: the birth of spintronics
Magnetoelectronics or Spintronics, is the coalition of two entities: magnetism and electronics. Electronics is based on the conduction or flow of electrons through a conducting material in complex circuits with copper wires, resistors, condenser or capacitors, diodes, transistors, microprocessors and off course switches. In electronics the electrons are only influenced by their charge, this means they move from A to B due to an electrical voltage difference. Now, there are materials, where the electrical resistance – the property of the material to oppose the electron flow – will depend of the spin orientation of the flowing electrons. So that one can alter the electrical resistance by applying a magnetic field. And this is the GMR. In this way one can not only use the charge of an electron, but also its spin, to move the electrons in a material. This approach helps us to read, write and also store data and build memory devices. That’s exactly why the discovery of Grünberg and Fert was worth a Nobel Prize.
Let's see what they did precisely: they fabricated a device structure out of two ferromagnets separated by a non-magnetic conductor and moved electrons from one side to the other. Imagine this like two dancefloors separated by a corridor where electrons can move freely. In both dancefloors there are unpaired electrons with oriented spins (ferromagnets). If the music in both dancefloors is the same, then all the electrons spin in the same direction and can pass easily from one dancefloor to the other through the corridor and keep dancing. This gives a low resistance. However, if in the second dance floor a different music is being played, so that the electrons spin in the opposite direction to those in the first dance floor, then the unpaired electrons will not be able to move in the second dancefloor easily. This leads to a high resistance state. This is the GMR. By changing the music in one of the dancefloors (applying an external magnetic field) we can determine the resistance we want to have: either high (1) or low (0). This is exactly what is needed to save binary data and the GMR principle used in hard drives. The high stability of oriented ferromagnets in combination with the advances in nanotechnology, led to more compact storage technology and soon the hard drives of two GB were replaced by 200 GB and later by two TB!
Spintronics and its perspectives
Not only did the GMR boost the computer technology, but soon new sensors based on GMR started finding their way out from the laboratories into the real world and markets. GMR sensors are applied today successfully in many different fields: from car parking systems to detection of biological materials. And this is exactly what spintronics is: a research field dedicated to the development of such systems, where not only the charge of the electron, but also its spin, can be used to build more effective and faster devices. Shortly after the discovery of the GMR effect, a new effect known as the tunnel magnetoresistance (TMR) was demonstrated by an IBM team led by S. P. Parkin. The TMR soon replaced the GMR in the memory device technology, for example in magnetic random-access memories (MRAMs).
Physicists and material scientists nowadays research on many different directions of spintronics, like for example antiferromagnetic spintronics. In this research field, one uses the antiferromagnets as the primary elements (the two dancefloors) and the fact that one cannot change the magnetic state of an antiferromagnet by an external field. This makes the approach interesting and usable for applications such as magnetic cards, because a magnetic card is always at risk of erasing the stored data near the vicinity of a magnet due to the ferromagnetic elements in it. Our research group (Qmag) is actively engaged in finding new materials for spintronic applications and also for antiferromagnetic spintronics. Currently a project to integrate antiferromagnetic nanostructures with nitride-based semiconductors for functional devices is being carried out.
The research group of Albert Fert is still actively engaged in the study of new routes for storage technology, which can be protected against data manipulation. He believes that skyrmion based storage devices will be a reality in about a decade or so. But what skyrmions are, that’s another story to be told. (Andrea Navarro-Quezada, Rajdeep Adhikari, 22.5.2018)
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