Hundred years back, people dreamed of a personal fridge to preserve food and chill beverages in their apartment, whereas researchers struggled to reach temperatures several degrees above absolute zero (0 Kelvin or -273.15 oC) to observe novel interesting phenomena like superconductivity. Both tasks have been accomplished by the modern technology, and temperatures of 2-3 Kelvin are nowadays routinely available in every low-temperature lab. Approaching absolute zero even closer is the next challenge motivated by important technological prospects. Nascent quantum computers prepare to revolutionize numerical calculations and boost large-scale simulations, yet these new devices have to operate at temperatures as low as 0.02 Kelvin, or -273.13 oC, to avoid decoherence.

Ultra-low temperatures for a steep price

Reaching temperatures close to absolute zero has been possible since decades using the so-called dilution refrigeration where a mixture of helium isotopes is employed in a standard vapor compression cycle, the same principle as used by domestic refrigerators in nearly every household. Unlike kitchen appliances, dilution refrigerators are expensive – the prices are in the range of hundreds kEUR – but even more expensive is the helium gas. Its price grows continuously, as the world supplies dwindle. “Helium costs are an ever-increasing part of my research budget”, says Prof. Philipp Gegenwart who heads the Chair of Experimental Physics VI, the focal point of ultra-low-temperature research at the University of Augsburg. “This financial pressure compels us to look into alternative, helium-free cooling technologies.”

Drawbacks of magnetic cooling

In their search for alternative cooling techniques, researchers from Augsburg pursued another classical method, adiabatic demagnetization. Here, external magnetic field is used to align electron spins, microscopic magnetic moments residing on individual atoms. This alignment abates entropy, which is the gauge of system’s disorder. “Fundamental laws of thermodynamics relate temperature and entropy”, explains Dr. Yoshi Tokiwa, previously the group leader in Augsburg and now the senior scientist at Advanced Science Research Center in Tokai, Japan. “Removing magnetic field adiabatically – without any heat exchange with the environment – will keep entropy at its minimum and force the system to cool down. Very low temperatures can then be reached upon a simple sweep of the magnetic field.” The bottleneck in this process are residual interactions between magnetic moments. For a practical cooling below 1 Kelvin, magnetic atoms should be placed very far apart to reduce their interactions. Such paramagnetic salts known since 1920’s have been of limited use because of their poor durability and low density of magnetic ions. Both drawbacks go back to the abundant presence of water molecules that are incorporated into a solid in order to separate the magnetic ions and prevent them from interacting.

Insights from quantum physics

Latest concepts of quantum physics are used to design a water-free material for magnetic cooling. Researchers from Augsburg investigate special quantum states called spin liquids. Those are not liquid per se, but feature electron spins behaving as if they were particles in a liquid, dynamic and flowing. “This fundamental topic is very interesting, yet hard to justify when you are asked about possible applications”, reflects Dr. Alexander Tsirlin, group leader at the Chair of Experimental Physics VI. “But now we are finally in a position to make this research practical. Using competing and random magnetic interactions as typical ingredients of a spin liquid, we can also design a material, complex ytterbium borate, with a high density of magnetic ions and excellent cooling performance.” The paper published in Communication Physics demonstrates temperatures as low as 0.022 Kelvin achieved with this material upon a single reduction of magnetic field from 5 to 0 Tesla.

Technological potential

The discovery opens immediate lab applications for single-shot ultra-low-temperature experiments, but also has a broader potential if several pellets of the material could be used in a sequence for continuous cooling. Gegenwart’s group has filed a utility model and receives support from the project  knowledge-transfer region Augsburg (WiR) to approach collaboration with industrial partners for investigating the technological opportunities of the new material.

Publication:

Yoshi Tokiwa, Sebastian Bachus, K. Kavita, Anton Jesche, Alexander A. Tsirlin, and Philipp Gegenwart, Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures. Comm. Mater. 2, 42 (2021), DOI: 10.1038/s43246-021-00142-1.

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