In a new study reported in the journal Science, Princeton University physicists tested the frustrated magnets – so-named because they should be magnetic at low temperatures but aren’t – to see if they exhibit the so-called Hall effect.
Visualization of magnetic fields. Image credit: Windell Oskay / CC BY 2.0.
In 1879, the famed American physicist Edwin H. Hall observed that when an electrical current passes through a sample placed in a magnetic field, a potential proportional to the current and to the magnetic field is developed across the material in a direction perpendicular to both the current and to the magnetic field.
This effect is known as the Hall effect, and is the basis of many practical applications and devices such as computer printers and automobile anti-lock braking systems.
Because the effect happens in charge-carrying particles, most researchers thought it would be impossible to see such behavior in non-charged particles like those in frustrated magnets.
Nevertheless, some scientists speculated that the non-charged particles in frustrated magnets might bend to the Hall rule under extremely cold conditions, where particles behave according to the laws of quantum mechanics rather than the classical physical laws we observe in our everyday world. Harnessing quantum behavior could enable game-changing innovations in computing and electronic devices.
Prof N. Phuan Ong and his colleagues at Princeton decided to see if they could settle the debate and demonstrate conclusively that the Hall effect exists for frustrated magnets. To do so, they turned to a class of the magnets called pyrochlores.
Pyrochlores contain magnetic moments that, at temperatures near absolute zero, should line up in an orderly manner so that all of their ‘spins’ point in the same direction. Instead, experiments have found that the spins point in random directions. These frustrated materials are also referred to as ‘quantum spin ice.’
The experiments were performed at temperatures of 0.5 degrees Kelvin, and required the Prof Ong’s team to resolve temperature differences as small as a thousandth of a degree between opposite edges of a crystal.
The scientists first synthesized the pyrochlore Tb2Ti2O7 from terbium oxide and titanium oxide in a lab furnace, and then obtained thin transparent or orange slabs about the size of a sesame seed.
To test each crystal, they attached tiny gold electrodes to either end of the slab, using microheaters to drive a heat current through the crystal. At the same time, they applied a magnetic field in the direction perpendicular to the heat current. To their surprise, they saw that the heat current was deflected to one side of the crystal. They had observed the Hall effect in a non-magnetic material.
“All of us were very surprised because we work and play in the classical, non-quantum world. Quantum behavior can seem very strange, and this is one example where something that shouldn’t happen is really there. It really exists,” said Prof Ong, who is the senior author of the paper.
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Max Hirschberger et al. 2015. Large thermal Hall conductivity of neutral spin excitations in a frustrated quantum magnet. Science, vol. 348, no. 6230, pp. 106-109; doi: 10.1126/science.1257340
