Quantum spins mimic refrigerator magnets

By Joseph Bernstein • October 11, 2012

The behavior of magnetic moments in metal oxides such as layered iridium is dominated by strong spin-orbit coupling effects. In layered compounds such as Sr3Ir2O7 (shown on the left), the direction of these moments (blue arrows) is controlled at the quantum level by dipolar interactions that are akin to those of classical bar magnets. Another outcome is an unprecedented 'magnon gap' (shown at right), which was measured at the Argonne Advanced Photon Source and reveals that these underlying dipolar magnetic interactions are extremely strong.

Current electronic devices depend on manipulating charge. Alternative approaches may rely on not only charge but also the spin of electrons.  This approach is known within scientific circles as “spintronics.” One of the significant hurdles to future spintronic devices is finding ways to manipulate the orientation of an electron spin without using magnetic fields. Argonne National Laboratory researchers from the Materials Science Division and Advanced Photon Source have identified a new design path for doing just that.

At the heart of this particular result is an advancement in magnetic structure engineering. Argonne researchers discovered a route for tuning magnetic interactions at the atomic level so that the interactions behave in the quantum realm similarly to classical bar magnets. Much like planets orbiting the sun, electrons surrounding an atomic nucleus possess both a top-like spin and an orbital momentum. When these shrink to the quantum limit, the spin and orbital momenta interact with those on neighboring atoms through an ‘exchange’ mechanism residing purely in the quantum realm.

However, in heavy transition elements like iridium, spin and orbit can lose individuality and merge into a composite ‘spin-orbit’ coupled state. X-ray measurements at the Advanced Photon Source show that when this happens in a class of layered iridium oxides, the coupled states can behave as if they have north and south poles, completely analogous to that of classical bar magnets. As a result, the direction of the spin was controlled simply by the number of stacked layers.

The result demonstrates that the conventional view of charge interactions dominating over spin interactions need not apply when spin-orbit coupling is strong. From a functional standpoint, the findings suggest novel routes toward engineered structures that allow manipulation of spin without magnetic fields, a general strategy for future low-power electronic devices.

Relevant publications:

Large Spin-Wave Energy Gap for the Bilayer Iridate Sr3Ir2O7 : Evidence for Enhanced Dipole-like Interactions near the Mott Metal-Insulator Transition
Jungho Kim, A. H. Said, D. Casa, M. H. Upton, T. Gog, M. Daghofer, G. Jackeli, J. van den Brink, G. Khaliullin, B. J. Kim
Phys. Rev. Lett 109 157402 (2012)

Dimensionality-Driven Spin Flop Transition in Layered Iridates
J. W. Kim, Y. Choi, Jungho Kim, J. F. Mitchell, G. Jackeli, M. Daghofer, J. van den Brink, G. Khaliullin, and B. J. Kim
Phys. Rev. Lett. 109 037204 (2012)