Clarifying the Hidden Magnetism of Gold (Au) (Press Release)
- Release Date
- 23 Jan, 2012
- BL39XU (Magnetic Materials)
Japan Synchrotron Radiation Research Institute (JASRI)
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Scientists of JASRI, in cooperation with those from Japan Advanced Institute of Science and Technology (JAIST) and other institutions, revealed a new magnetic property of the well-known metal gold (Au). Au has been a familiar precious metal throughout recorded history that has been used in jewelry and coins. Also, the use of Au in high-tech devices has been attracting recent attention. Au is currently used for wiring of semiconductors and other purposes because it is flexible, easy to fabricate, and has high conductivity. When combined with magnetic materials, such as iron and cobalt, the use of Au as a magnetic recording material is being discussed. Conventionally, Au has been considered as a typical diamagnetic material*1 and is not considered to have strong magnetism, meaning that it cannot act as a magnet by itself. However, the results of recent studies have indicated that when Au is fabricated in the form of nanoparticles, it has a strong magnetism, which has been attracting attention from both academic and practical viewpoints. In this study, it was clarified, for the first time in the world, that bulk Au also has a clearly recognizable magnetic property called paramagnetism.*2 Using conventional magnetometric methods, it was previously impossible to observe this paramagnetism, because the paramagnetic signals are faint compared with diamagnetic signals and were therefore unobservable. The research group succeeded in the first ever detection of these faint paramagnetic signals by X-ray magnetic circular dichroism (XMCD)*4 spectroscopy using high-brilliance circularly polarized X-rays produced by the Magnetic Materials Beamline (BL39XU) of SPring-8.*3 For this measurement, a new polarization control technology, which was developed jointly by JASRI and the RIKEN SPring-8 Center, and high-sensitivity XMCD measurement using this technology were indispensable. The comparison of the measurement results for bulk Au and Au nanoparticles indicated that the magnetism attributable to the orbital motion*5 of electrons (orbital component) was high in the two samples. Namely, the high magnetism of Au nanoparticles is attributable to the magnetic property of Au itself, which had not previously been clarified. The results of this study cast doubt on the statement in physics textbooks, “Au is a diamagnetic material.” The results of this study are expected to advance understanding of the magnetic mechanism of not only Au nanoparticles but also the nanoparticles of other precious metals such as platinum and lead to the development of ultrahigh-density magnetic recording materials for hard disks and other devices using such nanoparticles as recording units in the future. This study was carried out by Motohiro Suzuki (Senior Scientist) and Naomi Kawamura (Associate Senior Scientist) of the Research and Utilization Division of JASRI; Hayato Miyagawa (Associate Professor) of Kagawa University; Yoshiyuki Yamamoto (Associate Professor) of Akita University; Hidenobu Hori (Professor) of JAIST; and Jose S. Garitaonandia (Professor) of the University of the Basque County, UPV/EHU (Spain). The results of this study were published online in the American scientific journal Physical Review Letters on 24 January 2012. Publication: |
<Figures>
The research group succeeded in detecting, for the first time in the world, paramagnetic signals (blue line), which are fainter than the conventionally known diamagnetic signals (black dotted line), by XMCD measurement using synchrotron radiation (red circles: experimental data). These results indicate that Au has a paramagnetic state caused by electron spins and orbital motion. The great contribution of the orbital component, which is observed in other nanoparticles, was found to be the cause of the high magnetism of Au nanoparticles.
<Glossary>
*1 Diamagnetism
Diamagnetism is the phenomenon of a material inducing magnetism in opposition to an externally applied magnetic field. This has been explained as an apparent magnetization induced by electromagnetic induction. Diamagnetism is based on a mechanism different from that of ferromagnetism and paramagnetism, both of which are related to electron spins. Diamagnetic materials cannot be used as permanent magnets or magnetic recording materials.
*2 Paramagnetism and ferromagnetism
Paramagnetism and ferromagnetism are the phenomena of a material inducing magnetism in the same direction as an externally applied magnetic field. The magnitude of Pauli paramagnetism is proportional to the magnitude of the applied magnetic field and is independent of the sample temperature. This can be microscopically explained by an increase in the number of electron spins in a particular direction upon the application of the magnetic field. For ferromagnetic materials, a certain level of magnetization remains even after the removal of the external magnetic field. This is because the bonding between electron spins is strong and their direction is easily fixed. Ferromagnetic materials can be used as permanent magnets and magnetic recording materials.
*3 SPring-8
SPring-8 is a facility that generates the world’s highest-performance synchrotron radiation. It is located in Harima Science Garden City in Hyogo prefecture and is owned by RIKEN. JASRI is responsible for its operation, management, and support for users. The name SPring-8 is derived from Super Photon ring-8 GeV. Synchrotron radiation is the narrow and extremely powerful light that is obtained when the direction of electrons accelerated to close to the speed of light is bent using electromagnets. Research in a wide range of fields including nanotechnology, biotechnology, and their industrial applications has been carried out using the synchrotron radiation at SPring-8.
*4 Circularly polarized light and X-ray magnetic circular dichroism (XMCD) spectroscopy
X-rays are a type of electromagnetic wave, in common with light and radio waves. The waves of an electric field and magnetic field propagate along the direction of X-ray propagation. Circularly polarized light refers to the electromagnetic waves that propagate when electric and magnetic fields propagate in a spiral rotation, which is similar to the motion of the pole placed in front of barber shops. When the circularly polarized X-rays are absorbed by a magnetic material, the amount of absorption depends on the magnetic state of the electrons in the material. The amount of absorption is also affected by whether the rotation of the electric field is clockwise or counterclockwise. XMCD spectroscopy is a method of analyzing magnetic materials using this phenomenon.
*5 Electron spin and orbital motion
The motion of electrons in a material induces magnetism. Each electron has a property of a micromagnet, i.e., a spin. Electron spins, or the directions of micromagnets, in a material without magnetism have various directions, resulting in no overall magnetic property and magnetism is not expressed. In contrast, the magnetic properties of electrons are macroscopically observed in magnetic materials because a certain number of electron spins are aligned in the same direction. Electrons rotate around a nucleus; this orbital motion also contributes to the properties of micromagnets. When the contribution of the orbital component is great, the magnetism of the material tends to align in a certain direction. This property is called magnetic anisotropy and is an important characteristic of magnetic recording materials.
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