Increasing Magnetic Response of Ferromagnetic Semiconductors under High Pressure
FEBRUARY 26, 2009
When squeezed, electrons increase their ability to move around. In compounds such as semiconductors and electrical insulators, such squeezing can dramatically change the electrical and magnetic properties.
Under ambient pressure, europium oxide (EuO) becomes ferromagnetic only below 69K, limiting its applications. However, its magnetic ordering temperature is known to increase with pressure, reaching 200K when squeezed by 150,000 atm. The relevant changes in electronic structure responsible for such dramatic changes, however, remained elusive until recently.
Now scientists at the U.S. Department of Energy's (DOE’s) Argonne National Laboratory have manipulated electron mobility and pinpointed the mechanism controlling the strength of magnetic interactions and, hence, the material's magnetic ordering temperature.
"EuO is a ferromagnetic semiconductor and is a material that can carry spin polarized currents, which is an integral element of future devices aimed at manipulating both the spin and the charge of electrons in new generation microelectronics," said Argonne postdoctoral researcher Narcizo Souza-Neto.
Using powerful x-rays from X-ray Operations and Research (XOR) beamline 4-ID-D at the DOE’s Advanced Photon Source to probe the material's electronic structure under pressure, Souza-Neto, Argonne physicist Daniel Haskel, graduate student Yuan-Chieh Tseng (Northwestern University), and Gerard Lapertot (CEA-Grenoble) reported in the February 6 issue of Physical Review Letters that localized, 100% polarized Eu 4f electrons become mobile under pressure by hybridizing with neighboring, extended electronic states. The increased mobility enhances the indirect magnetic coupling between Eu spins resulting in a three-fold increase in the ordering temperature.
While the need for large applied pressures may seem a burden for applications, large compressive strains can be generated at interfacial regions in EuO films by varying the mismatch in lattice parameter with selected substrates. By pinpointing the mechanism the research provides a road map for manipulating the ordering temperatures in this and related materials, e.g., through strain or chemical substitutions with the ultimate goal of reaching 300K (room temperature).
"Manipulation of strain," Haskel said, "adds a new dimension to the design of novel devices based on injection, transport and detection of high spin-polarized currents in magnetic/semiconductor hybrid structures."