Turning round with electron spin
(appeared on 30th Oct 2013)

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New materials may help electron spin enter the world of communications and computing, says S.Ananthanarayanan.

The electron has been at the centre of science since the 1890s, when it was discovered. We know it is as the particle whose motion is the meaning of the electric current, and electricity has transformed the world in the last century. With more of its many properties being discovered, the electron also ushered in the world of electronics, with the telephone, radio, television, the computer and all else.

But these are applications of the electron as a charged particle in motion. As a particle that exists in all atoms, and in a relatively free way in metals, the electron can be free of the atom and move, to carry current, run motors and boil water. As a charged particle, its movement can be blocked or promoted by other electric charges, or controlled by magnetic fields and this is the field of electronics. But apart from its mass and its charge, another remarkable property of the electron is that it seems to have a spin, which makes it behave as a tiny, moving magnet too. This additional property of the electron has given rise to a separate field of applications, over and above the field of electronics.

A group of scientists at London and Vancouver report in the journal, Nature, that they have developed a low cost, versatile organic semiconductor material which is able to conserve a state of electron spin for longer period of time, which would increase its field of possible applications.

Electron spin

Electron spin was discovered while investigating why the emission spectra alkali metals, which have one electron in their outermost shell, split into two closely separated frequencies when a magnetic field was switched on.

Wolfgang Pauli first suggested that this was because the energy levels of an electron in an atom existed not as single state, but as a pair of closely separated states. Ralph Cronig suggested that the two states could be understood by associating a spin with the electron. Pauli did examine the idea, but found that given the size of the electron, it would need to spin so fast that its surface would move faster than light! Cronig seems to have let the idea drop, but it was soon found that there was a way round the objection and Pauli came back to formalize a theory of electron spin, combining the principles of motion of the spinning top with quantum effects, which would be relevant at the scale of the electron. That electrons have spin had also been dramatically shown by an experiment by Otto Stern and Walther Gerlach where a beam of electrons was split into two when it passed through a powerful magnetic field.

The spin of the electron is also the reason for the property of magnetism. As the electron is a spinning charge, it has magnetic effect and behaves like an elemental bar magnet. In atoms that have an even number of electrons, the spins are opposite and matched and the atom has no net magnetic effect.

But where there is an ‘unpaired’ electron, the atom itself is an elemental magnet. In many metals, called ferromagnetics, the crystal structure is such that atoms tend to orient themselves, either spontaneously or by the effect of a magnetic field, so that the electron spins are aligned and there is a powerful net magnetic effect.

Electrons thus create transient or permanent magnetic effects, which then impel or control the movement of other electrons in the form of electric currents, and hence the great number of applications of electricity in modern times. But these applications, including electronics, are based on linear, or translational, movement of electrons. The hugely significant fact that the electron has spin is not exploited. This kind of application, whose development perhaps needed the support of conventional electronics, appeared in the 1980s, with the discoveries of how spin of electrons could also affect the passage of electric current.

The effect used was that passing a current through a ferromagnetic material resulted in the current getting ‘spin polarised’, or that the spin axes of the electrons were turned so that they all pointed the same way. This effect makes possible the effect of giant magnetoresistance (GMR), which comes about with a conductor that consists of at least two layers of ferromagnetic material separated by a spacer layer. When the directions of magnetization of both layers are the same, the electrical resistance is lower, which is to say that a larger current flows, than when the directions are not aligned. As the direction of magnetization can be readily controlled by en external field, the resistance of the layer becomes a sensitive sensor of magnetic fields.

The effect has been used to great advantage in the read heads of modern computer hard drives. The head consists of a GMR device that is exposed to the fast moving hard drive surface. The magnetization of the drive sectors, which represents the data recorded, affects the electrical resistance of the device and transfers the data from the disc to the current which flows though the device. GMR very sensitive to slightest changes in magnetization and the device is hence fast and reliable, for reading disc data.

Information processing

But apart from such applications of retrieval or information from magnetic storage, for processing in the ordinary way, future applications of interest are for processing of information itself. This idea involves the spin of an electron, viz, ‘up’ or ‘down’, being considered a unit of information, to be processed, for example by being admitted or blocked in a ‘spin gate’, like conventional currents are managed by diodes or transistors. This would involve both the generation as well as the filtering of spin polarized currents.

The existing ferromagnetic or metal based devices do function as filters, but they cannot amplify, or strengthen a signal and also cannot easily integrate with existing, semiconductor-based electronics of diodes and transistors. This would not be true of semiconductor based devices, which could be multifunctional. Transfer of spin information across transitions may also bring in new technologies, including use of optical interfaces, which may overcome the size limitations that electronics is now grappling with. But in these possible applications, an important consideration is the persistence of the spin information for a reasonable length of time. This property is important both for conventional computing, where the spin state would represent a value, and also for the ambitious quantum computing, where a object like an electron, could represent both its possible states at once, and could participate in massive, parallel computation tasks so long as its condition of being in ‘both states’ is not destroyed by interactions with the environment, which would amount to a ‘measurement’ of its state.

The authors of the paper in Nature note that solid state, inorganic materials were first considered and then more exotic, large, complex molecules that acted as single molecule magnets entered the field. The complexity of the molecules gives rise to disturbances and they necessitate very low temperatures and isolation, in dilute solution, to sustain electron spin. In contrast, the material that the group has worked with is a common, low cost and simpler, organic blue pigment molecule – copper phthalocyanine (CuPc), which is easily processed in thin film form, which is useful for inclusion in devices. CuPc has been found to maintain electron spin states for a whole 59 milliseconds and stay undisturbed for quantum computing for 2.6 micro seconds, which are time spells that compare well with other materials, but at a practical temperature of 5°C above absolute zero. Even at 80°C above absolute zero, which can be maintained by use of liquid nitrogen, the time spells are ten and one microseconds.

As this material is robust and easy to produce and handle, the authors believe its use would lead to effective applications where the property of spin is exploited by semiconductor technology.

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