Increasing Magnetic Response of Ferromagnetic Semiconductors under High Pressure

FEBRUARY 26, 2009

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Diagram: A ferromagnetic-semiconductor Europium oxide sample is subjected to high pressures in a diamond anvil cell. The electronic structure is simultaneously probed with circularly-polarized x-rays at the Advanced Photon Source, revealing the mechanism responsible for the strengthening of magnetic interactions under pressure. (Courtesy of Argonne National Laboratory)

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."

Left to right: Yuan-Chieh Tseng (Northwestern University), Daniel Haskel (X-ray Science Division, Argonne), and Narcizo Souza-Neto (X-ray Science Division, Argonne) in the XOR 4-ID-D research station. (Courtesy of Argonne National Laboratory)

Narcizo Souza-Neto (Argonne) places a diamond anvil cell (DAC) in a cryostat inside the 4-ID-D station. The DACo generates high pressures; the cryostat is used to control the sample temperature. The outer coils are part of an electromagnet that generates a magnetic field. The cryostat/DAC moves into the gap of the electromagnet for x-ray magnetic circular dichroism measurements of the sample under pressure, magnetic field, and low temperature. (Courtesy of Argonne National Laboratory)

Contacts: *narcizo@aps.anl.gov and ** haskel@aps.anl.gov

See: Narcizo M. Souza-Neto*, Daniel Haskel**, Yuan-Chieh Tseng, and Gerard Lapertot, “Pressure-Induced Electronic Mixing and Enhancement of Ferromagnetic Ordering in EuX (X=Te, Se, S, O) Magnetic Semiconductors,” Phys. Rev. Lett. 102, 057206 (2009). DOI: 10.1103/PhysRevLett.102.057206

The original press release can be found here.

Funding for this research was provided by the U.S. Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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