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Clarifying the Mechanism behind the Induction of Ferromagnetism in the Partial Absence of a Rare-Earth Element (Press Release)

Release Date
02 Nov, 2011
- The strong interaction between the electrons of rare-earth elements and 3d transition elements induces ferromagnetism.


Key research achievements
• Clarification of the mechanism behind the induction of ferromagnetism using skutterudite, a cagelike compound
• Discovery of new behavior in the relationship between temperature and electronic state that is opposite the conventional behavior
• The above finding is of importance for both fundamental physics and industrial applications

Scientists of RIKEN (President, Ryoji Noyori) have clarified the mechanism by which skutterudite*1 (YbxFe4Sb12), a cagelike compound with a Yb filling rate of less than 100%, exhibits ferromagnetism.*2 This was achieved in joint research by the following scientists: Hitoshi Yamaoka (Senior Research Scientist) of Coherent X-ray Optics Laboratory, RIKEN SPring-8 Center (Director, Tetsuya Ishikawa); Ignace Jarrige (Research Scientist) of Japan Atomic Energy Agency (President, Atsuyuki Suzuki); Naohito Tsujii (Chief Scientist) of the National Institute for Materials Science (President, Sukekatsu Ushioda); Jung-Fu Lin (Assistant Professor) of the University of Texas at Austin (President, William Powers, Jr.); Tsuyoshi Ikeno (Graduate Student), Yosikazu Ishikawa (Professor), and Katsuhiko Nishimura (Professor) of University of Toyama (President, Shunro Endo); Hideyuki Sato (Professor) of Tokyo Metropolitan University (President, Fumio Harashimka); and colleagues.

Magnetism is a basic property of materials that is increasingly used in industrial applications; for example, the development of strong permanent magnets has led to the downsizing of electronic appliances. In general, 3d transition elements such as iron (Fe), cobalt (Co), and nickel (Ni) are easily magnetized and their magnetism is stabilized by adding 4f elements called rare-earth elements. Skutterudite, the material of interest in this research, also comprises 3d transition elements (Fe) and 4f elements (ytterbium, Yb); Yb is confined in a cagelike structure consisting of the elements Fe and antimony (Sb). Although skutterudite is known to exhibit ferromagnetism at low temperatures of less than -256 oC when Yb is partially absent, details of the mechanism of ferromagnetism have remained unclear.

The research group examined the relationship between temperature and pressure for each electronic state of Yb by resonant inelastic X-ray scattering (resonant X-ray emission spectroscopy*4) using the National Synchrotron Radiation Research Center (NSRRC, Taiwan) ID Beamline*3 (BL12XU) at SPring-8. It was found that the cause of ferromagnetism induced in the partial absence of Yb underlies the strong interaction between the electrons of Yb and Fe. It was also found that the magnetism is expressed on the Fe4Sb12 side rather than on the Yb side. This indicates that the magnetism is locally induced on the Fe4Sb12 side of the cagelike structure, where Yb is partially absent. Moreover, it was observed that the valence*5 of skutterudite in the partial absence of Yb sharply increased at low temperatures, despite the fact that the valence of Yb generally decreases at low temperatures. This is a completely new phenomenon that cannot be explained by conventional theoretical models.

The discovery in this research is a new finding related to the mechanism behind the induction of magnetism and is very important from the viewpoints of both fundamental physics and the industrial application of skutterudite to thermoelectric conversion materials.

The results of this study were published online in the American physical journal Physical Review Letters.

"Strong coupling between 4f valence instability and 3d ferromagnetism in YbXFe4Sb12 studied by resonant x-ray emission spectroscopy."
H. Yamaoka, I. Jarrige, N. Tsujii, J-F. Lin, T. Ikeno, Y. Isikawa, K. Nishimura, R. Higashinaka, H. Sato, N. Hiraoka, H. Ishii, and K-D. Tsuei.
Physical Review Letters107, 177203 (2011)

*1 Skutterudite

Skutterudite was named after Skutterud, northwest of Oslo in Norway, where CoAs3, a compound with the chemical formula of RXT4X12 (R, metal; T, transition metal; X, pnictogen), was discovered. Skutterudite is synthesized by a flux method or a high-pressure method; the latter is more frequently adopted to increase the Yb filling rate. Depending on the combination of elements, skutterudite exhibits various properties such as superconductivity, semiconductivity, and valence fluctuation.

*2 Ferromagnetism
Magnetism is induced when the spins of electrons are aligned in a material. In particular, Fe, Ni, and Co, all of which are 3d transition metals, are well known to easily become magnetic. The cases in which electron spins are aligned parallel and antiparallel are referred to as ferromagnetism and antiferromagnetism, respectively.

*3 National Synchrotron Radiation Research Center (NSRRC, Taiwan) ID Beamline
SPring-8 is a facility of RIKEN that produces synchrotron radiation from soft to hard X-rays and is located in Harima Science Park City, Hyogo Prefecture. Japan Synchrotron Radiation Research Institute is in charge of the operation and maintenance of SPring-8. The name SPring-8 is derived from "Super Photon ring-8 GeV." Synchrotron radiation refers to narrow and intense electromagnetic waves that are generated when electrons are accelerated to near the speed of light and their traveling direction is bent by electromagnets. At SPring-8, research on various fields, including materials science, life science, nanotechnology, biotechnology, and industrial applications, is carried out using synchrotron radiation in the X-ray region. Although Taiwan has its own synchrotron radiation facility, Taiwan Light Source (TLS), the energy region of the available light is lower than that of the light produced at SPring-8; therefore, NSRRC in Taiwan has constructed a beamline for hard X-rays at SPring-8. The beamline is under administration by NSRRC and open to not only researchers in Taiwan but also researchers worldwide including those in Japan.

*4 Resonant X-ray Emission Spectroscopy
Resonant X-ray emission spectroscopy is a method of examining the electronic state of a sample by tuning the incident energy of synchrotron radiation to the K- or L-absorption edge of a sample and measuring the emitted light by crystal spectroscopy. The signal intensity is enhanced by the resonant effect at the absorption edge. This method is also called resonant inelastic X-ray scattering, where the incident energy is different from the energy of the scattered X-rays.

*5 Valence
In compounds, electrons are frequently exchanged between the constituent atoms. When certain rare-earth elements, such as cerium (Ce), samarium (Sm), europium (En), and Yb, are contained in a compound, they donate their electrons to other elements in the compound and their valence fluctuates between two (the supply of two electrons) and three (the supply of three electrons), which is well known to determine the properties of the compounds. This phenomenon is referred to as valence fluctuation and has been studied for a few decades.


Fig. 1 Crystal structure of skutterudite and temperature dependence of Yb valence.
Fig. 1 Crystal structure of skutterudite and temperature dependence of Yb valence.

(Left) Skutterudite is the collective term for compounds with a molecular formula of RXT4X12. For the material in this study, R = Yb, T = Fe, and X = Sb, and Yb is confined in a cagelike structure. The filling rate of Yb is expressed by x. X atoms with a regular icosahedral structure, as shown in the center of the figure, are surrounded by T atoms with a cubic structure to form a cagelike structure, in which an R atom is confined at the center.

(Right) Magnetization rate of skutterudite and change in valence with temperature. If the filling rate is 88% (x = 0.88), both (a) the magnetization rate and (b) the Yb valence sharply increase at low temperatures.

Fig. 2 Diamond anvil cell and pressure dependence of Yb valence.
Fig. 2 Diamond anvil cell and pressure dependence of Yb valence.

(Left) Principle of diamond anvil cell.
(Right) When the pressure increases, the Yb valence sharply increases at a certain pressure.

For more information, please contact:
 Dr. Hitoshi Yamaoka (RIKEN)

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