Increasing temperatures to higher than the surrounding temperature is easier compared with decreasing temperatures to lower than the surrounding temperature. Many refrigerators and air conditioners use latent heat when liquid evaporates. Additionally, the Joule-Thomson effect and thermoelectric effect, termed the Peltier effect, have been used for cooling. Aligning magnetic moments using an external magnetic field is another useful method for controlling entropy. This magnetocaloric effect is useful in magnetic refrigeration, which is the most important technology for achieving extremely low temperatures.

In principle, an electric field can be used directly for cooling as the magnetic refrigeration. However, to date, no practical electric refrigerators exist, because the reported electrocaloric effect is much weaker than the magnetocaloric effect.

The strong correlation between different degrees of freedom in condensed matter allows an external field, such as a magnetic field, an electric field, and stress, to modulate the states in multiple degrees of freedom. For example, magnetism and electricity are tightly coupled in ferromagnetic ferroelectric materials, also known as multiferroics. Magnetic moments and atomic displacements are simultaneously controlled by an electric field, thus resulting in significant entropy change.

In this study, the electrocaloric effect of GdFeO₃ is investigated. The Fe moments are aligned to host weak ferromagnetism below 661 K. Addition, antiferromagnetic ordering of Gd moments emerges below 2.5 K. The coupling between Fe and Gd moments induces parasitic ferroelectric polarization. In other words, the Gd moments and ferroelectric polarization are strongly coupled. Therefore, if an external electric field is applied (removed) under adiabatic conditions, then the ordering of the Gd magnetic moments is expected to be enhanced (weakened). Hence, the Gd moment system loses (gains) some entropy, as shown in the lower-left panel. Entropy must be transferred to (from) lattice vibrations, thus resulting in an increase (decrease) in temperature.

This effect was successfully observed using the measurement system shown schematically in the top-right panel. A typical experimental result is shown in the lower middle panel. As shown, the temperature of GdFeO₃ increased and decreased owing to the application and removal of an external electric field, respectively, as predicted. The magnitude of the temperature change was determined by the initial temperature and external magnetic field, as shown in the two-dimensional color plot (bottom right panel). Furthermore, the energy efficiency was higher than that of typical magnetic caloric effects and adiabatic nuclear demagnetization.

(Written by T. Arima on behalf of all the authors)

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