“Magnetic shape memory effect”, the phenomenon that deformed materials returning to their original shape upon exceeding a certain magnetic field, can be adopted for applications such as field-tuned actuators, as it offers much faster controllability than the temperature-driven shape memory effect. The magnetic shape memory effect has been studied predominantly in Heusler alloys composed of transition metals using magnetic and structural properties of martensitic and parent phases. Similar effects are well studied for the $f''-''$electron compounds $R$Cu₂ ($R$: rare-earth elements) [1]. Applying a magnetic field along the hard magnetization axis causes a rapid increase in magnetization that results in the direction of the field becoming the easy magnetization axis. However, such memory effects in $R$Cu₂ are stable only at low temperatures, and no $f''-''$electron compounds that maintain memory at room temperature have been identified. This study revealed that the heavy-fermion compound CeSb₂ exhibited easy-axis switching accompanied by crystal-axis conversion under a magnetic field. Remarkably, this magnetic shape memory effect remains stable, even at room temperature.

As shown in Fig. 1(a), CeSb₂ exhibits a sharp metamagnetic-like increase in magnetization near 34 T, and significant hysteresis is observed during demagnetization when a magnetic field is applied along one of the in-plane principal axes [2]. Subsequent measurements revealed a memory effect, that is, the magnetization approached its previous maximum. The measurements obtained by applying fields along the other in-plane axis revealed a reduction in magnetization, indicating that the direction perpendicular to the field becomes the hard axis. These observations were surprising because the applied field direction was changed by rotating the sample by 90° after heating the sample to room temperature. Moreover, the change from the hard to easy axis is reproducibly observed through the higher-field magnetization measurements. These findings demonstrate that the “magnetic memory effect” remains stable up to at least room temperature. This easy-axis switching accompanies crystallographic axis conversion, which can be confirmed as a domain rearrangement through a polarizing light microscope. Thus, this phenomenon constitutes a “magnetic shape memory effect” that is remarkably stable at room temperature.

CeSb₂ crystallizes in a nearly tetragonal orthorhombic lattice that consists of Sb layers and Ce-Sb layers stacked along the orthorhombic $c''-''$axis. Ce atoms in the $ab''-''$plane form a distinctive “pantograph” structure [Fig. 2]. Adjusting the pantograph angles allows the lattice constants to be swapped between the $a''-''$ and $b''-''$axes. The Sb₂ sandwiched between the Ce pantographs forms dimers and acts as hinges during axis transformations. The magnetic anisotropy inherent in the pantograph structure is a key to the magnetic shape memory effect of CeSb₂.

Although the relationships between the magnetic shape memory effect and heavy electron states remain unclear, the characteristic feature of CeSb₂, which memorizes the magnetization values corresponding to the maximum applied field, indicates its potential as a magnetic memory material. This discovery opens new avenues for exploring materials for practical applications under moderate magnetic fields and temperatures.

References

• [1] see for example, K. Sugiyama et al., J. Magn. Magn. Mater. 262, 389 (2003).
• [2] A. Miyake et al., J. Phys. Soc. Jpn. 94, 043702 (2025).

Authors

• A. Miyake^a, R. Hayasaka^a, H. Fukuda^a, M. Kondo, Y. Kinoshita, D. Li^a, A. Nakamura^a, Y. Shimizu^a, Y. Homma^a, F. Honda^b, M. Tokunaga, and D. Aoki^a
• ^aIMR, Tohoku University
• ^bRI center, Kyushu University