Success in Elucidation and Large Improvement of Spin Properties of Iron Oxide Surface

Improvement of Half-Metal Characteristics by Interfacial Control

2012.05.25
(2012.06.13 Update)

National Institute for Materials Science

The NIMS Nano Characterization Unit, in joint research with the University of York (U.K.) and the University of Science and Technology of China, showed that large improvement can be realized in the spin polarization of the outermost layer of the surface of the iron oxide (Fe3O4), which is known as a half-metal, by surface treatment, and obtained guidelines for achieving high spin polarization characteristics such as a high tunnel magnetoresistance (TMR) ratio, etc. by using half-metallic oxides.

概要

  1. Dr. Mitsunori Kurahashi, a Principal Researcher, and Dr. Yasushi Yamauchi, Group Leader, both of the Spin Characterization Group, Nano Characterization Unit (Unit Director: Daisuke Fujita) of the National Institute for Materials Science (NIMS; President: Sukekatsu Ushioda), in joint work with Dr. Andrew Pratt, a Senior Research Fellow at the University of York in the United Kingdom, and Associate Prof. Xia Sun of the University of Science and Technology of China, showed that large improvement can be realized in the spin polarization of the outermost layer of the iron oxide (Fe3O4), which is known as a half-metal, by surface treatment. The group also obtained guidelines for achieving high spin polarization characteristics such as a high tunnel magnetoresistance (TMR) ratio, etc. using half-metallic oxides.
  2. Half-metals are materials in which the spin direction of conduction electrons is 100% aligned, and are indispensable for the development of high performance magnetic sensors and other spintronic devices. Iron oxide (Fe3O4) is an half-metallic oxide with the distinctive features of a high Curie temperature (558°C) and simple composition. Because Fe3O4 consists of abundant elements, it is becoming more and more important in recent years to reduce the dependence on rare metals. The TMR ratios obtained with the use of Fe3O4 are, however, quite low and the origin of this poor performance is unclear at present.
  3. Using a newly-developed apparatus with a spin-polarized metastable helium beam, the group led by Dr. Kurahashi found that the spin polarization of the outermost surface of Fe3O4 is much smaller than that in the bulk, being substantially zero at the (100) surface and is unexpectedly of reverse polarity at the (111) surface. However, most importantly, this work showed that the spin polarization of the (100) surface can be recovered to at least -50%, even at room temperature, by adsorbing atomic hydrogen onto the surface. This phenomenon was supported by a numerical simulation indicating the spin polarization of nearly -100% at the hydrogen-adsorbed Fe3O4 surface.
  4. This research demonstrated that high spin polarization can be obtained at the interface between Fe3O4 and other materials by appropriately controlling the interface structure, and yielded guidelines for obtaining a high TMR ratio and for introducing a high spin polarization to organic molecules using half-metallic oxides.
  5. These research results were obtained as part of the NIMS 3rd Mid-Term Program Project “Development and Application of Advanced Materials Characterization Technologies” (Leader: Daisuke Fujita) and the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research / Challenging Exploratory Research topic “Recovery of Half-Metallicity of Outermost Surface of Oxides by Surface Modification and Application to Spin Injection” (Research Representative: Masanori Kurahashi). These results were published in the journal of the American Physical Society “Physical Review B (Rapid Communication).
"Fig : Spin polarization of the topmost layer of Fe3O4 (100), (111) surfaces and the hydrogen termination effect. The spin asymmetry of the (100) surface is significantly improved by hydrogen termination. The asymmetry around the sample voltage of 14eV is approximately equal to the opposite sign of the spin polarization of the conduction electrons." Image

Fig : Spin polarization of the topmost layer of Fe3O4 (100), (111) surfaces and the hydrogen termination effect. The spin asymmetry of the (100) surface is significantly improved by hydrogen termination. The asymmetry around the sample voltage of 14eV is approximately equal to the opposite sign of the spin polarization of the conduction electrons.