首次在不同结构的DyCrO₄中，利用外加磁场实现了将常压锆石型z-DyCrO₄的热膨胀系数由负变为正，高压白钨矿型s-DyCrO₄的热膨胀系数由正变负再变成正的转变，为调控材料的热膨胀系数提供了一种全新策略。

(Courtesy: iStock/enot-poloskun)

Most materials expand when heated. A few, such as water just above freezing, contract. Now, for the first time, physicists have found a material that switches from expanding to contracting in the presence of an applied magnetic field. The discovery of this field-induced sign change could offer a new way of controlling a material’s thermal expansion – a prospect that would have many industrial applications as well as interest for fundamental research.

In devices made from many different materials, any mismatch in how these materials behave when heated – their positive or negative coefficients of thermal expansion (CTEs) – can have important and sometimes unwanted consequences. For example, a component that combines materials with very different CTEs may be prone to deforming, cracking or otherwise failing when the temperature changes.

Since this effect is ultimately due to different atoms vibrating at different frequencies, it is sometimes possible to tune the size of the CTE by substituting one element for another in the material’s chemical formula. However, for most materials, this chemical substitution process is very limited in its scope.

Using external variables such as magnetic or electric fields to tune a material’s CTE would be much more flexible than chemical substitution, and researchers had previously shown that this was possible with certain magnetic materials. In those studies, however, only the magnitude of the CTE had changed with magnetic field, not its sign.

In the new work, a team led by Youwen Long prepared a rare-earth chromate, DyCrO₄, in two isomorphic phases: a zircon-type phase and a scheelite-type phase. The first of these phases was created using standard solid-state annealing at ambient pressure, while the second used high-pressure annealing. To their surprise, the researchers found that for both phases, the sign of the CTE changes when a magnetic field is applied.

This figure shows the average linear coefficient of thermal expansion of (a) z-type DyCrO₄ at 3-23 K and (b) s-type DyCrO₄ at 3-24 K in a series of magnetic fields applied along the direction of sample length. Also included are a schematic diagram of the crystal structure of (c) z-type DyCrO₄ and (d) s-type DyCrO₄. (Courtesy: Y Long)

At zero magnetic field, Long explains that zircon-type DyCrO₄ exhibits a negative CTE at temperatures below the ferromagnetic order temperature of 23 K. When they increased the magnetic field to 1.0 T, however, the CTE turned positive. In the scheelite phase, a magnetic field of up to 2.0 T can switch the initially positive CTE to negative. What is more, a “reentrant positive” CTE can be induced by increasing the field further, up to and over 3.5 T.

The researchers say that this is the first time anyone has observed a magnetic-field-induced change in the sign of a material’s CTE. “Our study provides the first example where external magnetic fields can significantly change the thermal expansion, including the magnitude and especially the sign, opening up a new avenue to readily control the thermal expansion beyond conventional chemical substitution,” they report. “We believe that our work will be of broad interest in fundamental and applied material sciences.”

According to Long, the anomalous effect stems from the unusually strong spin-lattice coupling in DyCrO₄, and it could have broad applications in applied materials science. “One immediate application area, for example, might be to control the CTE of permanent magnet motors,” he tells Physics World.

The researchers are now exploring the possibility of using magnetic fields to tune the CTE in other magnetic functional materials, to see whether this could be a universal method for regulating their CTEs. They detail their present work in Chinese Physics Letters.

CPL Express Letters栏目简介