X-ray resonance scattering can reveal the magnetic properties of transition metal oxides made out of heavy elements

Dec 07, 2012
The scattering of x-rays with a wavelength 'in resonance' with an ion’s absorption edge depends on the direction of the ion’s magnetic moment (red and blue arrows). The researchers used x-rays (pink) with a wavelength of 0.11 nm—close to two absorption edges in iridium—to study the onset of magnetic order in Sr2IrO4. X-rays scattered at the blue arrows are out of phase with those scattered at the red arrows. Credit: 2012 The American Physical Society

Transition metal oxides are known for their interesting properties, including high-temperature superconductivity and resistance that can be tuned with a magnetic field. Researchers have mainly focused on oxides made from '3d' transition metals—the elements from scandium to zinc—but they are starting to uncover new material properties in oxides containing the much heavier '5d' transition metal elements found between hafnium and mercury.

Unfortunately, scientists have not been able to rely on their usual tool, neutron scattering, to study in these materials because the samples are often too small and 5d elements strongly absorb neutrons. Now, Shigeki Fujiyama at the RIKEN Advanced Science Institute and his colleagues have shown they can use x-rays to study magnetism in the 5d oxide Sr2IrO4 over a wide temperature range. "The sample size needed is three orders of magnitude smaller than what is needed for conventional neutron scattering experiments," explains Fujiyama, who says the technique will be important for studying other 5d .

Atoms in a solid are magnetic if their valence electrons have a net (non-zero) angular momentum. The valence electrons' total angular momentum is a combination of their orbital motion about the nucleus and their 'spin'. In heavy elements, like iridium (Ir), a relativistic effect called the spin-orbit interaction causes the electrons' orbital momentum to drag their spin momentum with it. In materials where the spin-orbit effect is large, the electronic motion can be controlled to affect the magnetic properties (and vice versa).

In Sr2IrO4, a large spin-orbit interaction makes the material a '', similar to La2CuO4, a 3d metal oxide that can be chemically modified to become a superconductor. In both materials, the (iridium and copper) also form the same magnetic structure—an antiferromagnet—at low temperature. It has not been clear if both magnetic structures can be described by the same models.

Fujiyama and his team therefore used resonance x-ray scattering, where the x-ray wavelength matches an absorption edge of the iridium ion so it is sensitive to the ion's magnetic state, to study the onset of magnetic order in Sr2IrO4. Their measurements, performed at the RIKEN SPring-8 synchrotron, show that magnetic order in Sr2IrO4 develops in two-dimensional planes first and only becomes three-dimensional near the anti-ferromagnetic transition, similar to La2CuO4. Similar oxides containing iridium may exhibit superconductivity or a 'quantum spin liquid' and Fujiyama's group is examining these possibilities.

Explore further: Cold Atom Laboratory creates atomic dance

More information: Fujiyama, S., Ohsumi, H. Komesu, T., Matsuno, J., Kim, B.J., Takata, M., Arima, T. & Takagi, H. Two-dimensional Heisenberg behavior of Jeff = ½ isospins in the paramagnetic state of the spin-orbital Mott insulator Sr2IrO4. Physical Review Letters 108, 24721 (2012). prl.aps.org/abstract/PRL/v108/i24/e247212

Related Stories

An electronic dance of spins and orbits

Dec 13, 2010

Because of their potential application in spintronic devices such as next-generation spin-based transistors, the quest for new materials with significant spin-orbit interactions in the electronic ground state ...

Mediating magnetism

May 04, 2011

(PhysOrg.com) -- Titanium oxide doped with cobalt produces magnetic properties at room temperature via a newly discovered mechanism.

Recommended for you

Cold Atom Laboratory creates atomic dance

5 hours ago

Like dancers in a chorus line, atoms' movements become synchronized when lowered to extremely cold temperatures. To study this bizarre phenomenon, called a Bose-Einstein condensate, researchers need to cool ...

Scientists create possible precursor to life

12 hours ago

How did life originate? And can scientists create life? These questions not only occupy the minds of scientists interested in the origin of life, but also researchers working with technology of the future. ...

Superconducting circuits, simplified

Oct 17, 2014

Computer chips with superconducting circuits—circuits with zero electrical resistance—would be 50 to 100 times as energy-efficient as today's chips, an attractive trait given the increasing power consumption ...

User comments : 0