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Topic Name: Scientists have Discovered that the Magnetic Strength of Magnetite Loses Under Pressure
Category: Nuclear Magnetic Resonance
Research persons: Yang Ding
Location: Carnegie Institution’s Geophysical Laboratory, Uruguay
Details
Scientists have discovered that the magnetic strength of magnetite—the most
abundant magnetic mineral on Earth—declines drastically when put under
pressure. Researchers from the Carnegie
Institution’s Geophysical Laboratory, together with colleagues at the Advanced
Photon Source of Argonne National Laboratory, have found that when magnetite
is subjected to pressures between 120,000 and 160,000 times atmospheric pressure
its magnetic strength declines by half. They discovered that the change is due
to what is called electron spin pairing.
Magnetism comes from unpaired electrons in magnetic materials. The strength
of a magnet is a result of the spin of unpaired electrons and how the spins of
different electrons are aligned with one another. This research showed that the
drop in magnetism was due to a decrease in the number of unpaired electrons.
“Magnetite is found in small quantities in certain bacteria, in brains of
some birds and insects, and even in humans,” commented Yang Ding, the
study’s lead author with the Carnegie-led High-Pressure Synergetic Consortium.
“Early navigators used it to find the magnetic North Pole and birds use it for
their navigation. And now it is used in nanotechnology. There is intense
scientific interest in its properties. Understanding the behavior of magnetite
is difficult because the strong interaction among its electrons complicates its
electronic structure and magnetic properties.”
To study the mineral, the researchers developed and applied a novel
technique, called X-ray Magnetic Circular Dichroism (XMCD) at the Advanced
Photon Source, a high-energy synchrotron facility. The technique uses
high-brilliance circularly polarized X-rays to probe the magnetic state of
magnetite as a diamond anvil cell subjects a sample to many hundreds of
thousands of atmospheres. The researchers combined their experimental results
with theoretical calculations by collaborators* to pinpoint why the magnetic
strength changes. The study, to be published in February in Physical Review
Letters, reveals the electron-spin configuration in the iron sites of the
mineral to be the origin of the phenomenon.
This discovery not only shows the profound effects of pressure on magnetism,
it also discloses, for the first time, that pressure induced a spin pairing
transition that results in changes in the electron mobility and structure.
“The discovery is important,” Ding said. “It advances our understanding
of the correlation of magnetism, electron transport, and structural stability in
materials with strong electron interactions, like magnetite.”
“It is not surprising to see that a new phenomenon has been trigged by
pressure in the oldest magnet. Pressure can directly change electron-electron
interactions by squeezing the spacing between them,” said Ho-kwang Mao, the
director of the High-Pressure Synergetic Consortium and the High-Pressure
Collaborative Access Team. “In the future, the integration of high
pressure with novel synchrotron techniques will no doubt lead to more new
discoveries.”
Note for Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron(II,III) oxide and the common chemical name ferrous-ferric oxide. The formula for magnetite may also be written as FeO.Fe2O3, which is one part wüstite (FeO) and one part hematite (Fe2O3). This refers to the different oxidation states of the iron in one structure, not a solid solution.
The Curie temperature of magnetite is 858 K. Magnetite is the most magnetic of all the naturally occurring minerals on Earth, and these magnetic properties led to lodestone being used as an early form of magnetic compass. Magnetite typically carries the dominant magnetic signature in rocks, and so it has been a critical tool in paleomagnetism, a science important in discovering and understanding plate tectonics. The relationships between magnetite and other iron-rich oxide minerals such as ilmenite, hematite, and ulvospinel have been much studied, as the complicated reactions between these minerals and oxygen influence how and when magnetite preserves records of the Earth's magnetic field.
Magnetite has been very important in understanding the conditions under which rocks form and evolve. Magnetite reacts with oxygen to produce hematite, and the mineral pair forms a buffer that can control oxygen fugacity. Commonly igneous rocks contain grains of two solid solutions, one between magnetite and ulvospinel and the other between ilmenite and hematite. Compositions of the mineral pairs are used to calculate how oxidizing was the magma (i.e., the oxygen fugacity of the magma): a range of oxidizing conditions are found in magmas and the oxidation state helps to determine how the magmas might evolve by fractional crystallization.
Small grains of magnetite occur in almost all igneous rocks and metamorphic rocks. Magnetite also occurs in many sedimentary rocks, including banded iron formations. In many igneous rocks, magnetite-rich and ilmenite-rich grains occur that precipitated together from magma. Magnetite also is produced from peridotites and dunites by serpentinization.
Note for Electron-Spin Resonance
Electrons, and other particles, have an intrinsic angular momentum, known as spin. This creates a magnetic dipole moment. When the electron is placed in a magnetic field, the intrinsic magentic dipole can align in one of two ways, parallel or anti-parallel to the field. The anti-parallel state is of lower energy. However, applying radiation of a certain frequency to the electron can raise it to the higher energy state, in which its magnetic dipole is parallel to the applied magnetic field. It will then fall back to the lower energy state, emitting a photon. If radiation continues to be applied, then the electron will "resonate" between the two energy states. This is known as electron spin resonance, and is used to identify compounds, which each have a unique spectrum of radiation absorption. This occurrence is used in both NMR and MRI.
Note for Nanotechnology
Nanotechnology refers broadly to a field of applied science and technology whose unifying theme is the control of matter on the atomic and molecular scale, normally 1 to 100 nanometers, and the fabrication of devices with critical dimensions that lie within that size range.
It is a highly multidisciplinary field, drawing from fields such as applied physics, materials science, interface and colloid science, device physics, supramolecular chemistry (which refers to the area of chemistry that focuses on the noncovalent bonding interactions of molecules), self-replicating machines and robotics, chemical engineering, mechanical engineering, biological engineering, and electrical engineering. Much speculation exists as to what may result from these lines of research. Nanotechnology can be seen as an extension of existing sciences into the nanoscale, or as a recasting of existing sciences using a newer, more modern term.
Two main approaches are used in nanotechnology. In the "bottom-up" approach, materials and devices are built from molecular components which assemble themselves chemically by principles of molecular recognition. In the "top-down" approach, nano-objects are constructed from larger entities without atomic-level control. The impetus for nanotechnology comes from a renewed interest in Interface and Colloid Science, coupled with a new generation of analytical tools such as the atomic force microscope (AFM), and the scanning tunneling microscope (STM). Combined with refined processes such as electron beam lithography and molecular beam epitaxy, these instruments allow the deliberate manipulation of nanostructures, and led to the observation of novel phenomena.
Examples of nanotechnology in modern use are the manufacture of polymers based on molecular structure, and the design of computer chip layouts based on surface science. Despite the great promise of numerous nanotechnologies such as quantum dots and nanotubes, real commercial applications have mainly used the advantages of colloidal nanoparticles in bulk form, such as suntan lotion, cosmetics, protective coatings, drug
delivery, and stain resistant clothing.
Note for X-ray Magnetic Circular Dichroism
X-ray magnetic circular dichroism (XMCD) is a difference spectrum of two x-ray absorption spectra (XAS), one taken with left circularly polarized light, and one with right circularly polarized light. By closely analyzing the XMCD spectrum, information can be obtained on the magnetic properties of the atom, such as its spin and orbital magnetic moment.
In the case of transition metals such as iron, cobalt, and nickel, the absorption spectra for XMCD are usually measured at the L-edge. This corresponds to the process in the iron case: with iron, a 2p electron is excited to a 3d state by an x-ray of about 700 eV. Because the 3d electron states are the origin of the magnetic properties of the elements, the spectra contain information on the magnetic properties.
*Collaborators are at the Kirensky Institute of Physics (Russia). Other
authors in the paper are Daniel Haskel, Sergei G. Ovchinnikov, Jonathan C. Lang,
Yuan-Chieh Tseng, and Yuri S. Orlov.
This work was supported by the U.S.
Department of Energy (Basic Energy Sciences and NNSA), the National
Science Foundation, the W.M.
Keck Foundation and the Carnegie
Institution.
In figure, Magnetite is an abundant magnetic mineral. It was used by early
navigators to find the magnetic North Pole and birds use if for their
navigation.There is intense scientific interest in its properties.
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