Spintronics makes microscopic maser possible
A maser is a device that operates exactly like a laser, but it produces microwave radiaton instead of visible light. Researchers at the University of Groningen and the FOM-Foundation have demonstated the operating principle of a maser at a microscopic scale, based on a new type of electronics: spintronics. They will publish their results in Physical Review Letters on 15 September 2006.
Spintronics is a type of electronics that does not only use the fact that electrons have a charge, but that they also have a spin. It looks like an electron has either a left-orientated spin around its axis or a right-orientated spin. In the one case the spin is depicted as an upward arrow, in the other case, as a downward arrow. Spins that actually behave like little magnets, are directing in an external magnetic field. Spintronics will gratefully make use of this fact. A well-known, commercially applied spintronics component is the Giant MagnetoResistance (GMR) sensor of the reader head in computer hard drives.
FOM-postdocs Steven Watts and Bart van Wees (University of Groningen) have now been working out that it is possible to create a spintronic component that operates like a maser (Microwave Amplification by Stimulated Emision of Radiation). Whereas a laser produces visible light, a maser creates microwave radiation. Just like a laser, a maser shares the practical quality of transmitted radiation having an exact frequency. The researchers have found a way to use a small piece of paramagnetic material (material that has free electron spins, like a metal or a semiconductor) as a maser medium.
Sandwich of thin layers
The intended maser exists of a thin layered-sandwich, containing a slice of paramagnetic material below, a slice of ferromagnetic material on top and in between an insulated layer. When an external magnetic field is laid upon the paramagnetic material, two energy levels will come into being: a ground state in which all electrons have a pin downwards, and an excited state ( a state with a higher energy), in which all electrons have a spin upwards. It is now possible for microwaves to pump electrons from a ground state to an excited state. However, just one part of the energy from the microwaves is remaining in this pumping action. Another part in the magnetic material is lost to heat, and a small part comes out of the material as microwave radiation.
The researchers have been demonstrating for the first time that it is theoretically possible to get the sandwich in a similar condition by supplying a current flow of electrons with the same spin, so that the paramagnetic layer will be transmitting more microwave radiation than is being put in. Moreover, this process is able to preserve itself. The result is a maser at a microscopic scale.
The upper, ferromagnetic layer of the sandwich only serves to provide the lower, paramagnetic layer of an extra current flow of electrons in the same spin condition. By installing an electric tension on the sandwich, this current flow will run from the ferromagnet to the paramagnet. There, it provides for the population-inversion that is necessary for a maser (as well as for a laser): the greater part of the electrons is in the excited state. Thereupon, the electrons collectively fall into the ground state , while transmitting microwave radiation of a closely determined frequency. However, the population-inversion remains intact, because the spin current time and again provides for more electrons in the excited state than in the ground state.
Microwave power
Watts and Van Wees have been working out that the maser at a typical supply power of 100,000 microwatt may produce a microwave power of one microwatt for a paramagnetic layer the size of 500 to 500 to 50 cubic nanometre. The relatively low efficiency of 0.001 percent has been, as yet, a technological restriction that is the result of using an insulated layer of alumina. The description of this maser in a metallic sandwich may also be completely analogously applied to other materials, like semiconductors, and to other ways of injecting spins.
As yet, there are not any practical applications of the microscopic maser, but in view of the increasing importance of spintronic components and considering the fact that microwaves, for example, have already been applied in electronic devises like mobile phones, the researchers believe that the applications may arise. As a next step the researchers at the University of Groningen want to set up a demonstration model of the maser.
The article is entitled:
An electron spin injection driven paramagnetic solid state maser device,
by S.M. Watts en B.J. van Wees
For more information, please contact: Professor Bart van Wees, University of Groningen, phone (50) 363 49 33, or Dr. Steven Watts, University of Groningen, phone (050) 363 49 19.