Overview

The Horii Laboratory was established in April 2025 in the Faculty of Engineering and Design at Kagawa University. Our research focuses on the development of molecular materials with unprecedented functionalities, based on metal complexes that are hybrids of organic and inorganic components. The physical properties of interest span a wide range, including magnetism, electrical conductivity, and mechanical motion. Representative research achievements are summarised below.

Crystalline Thermal Engine

When you hear the word “crystal,” what kind of image comes to mind? Familiar crystals such as salt or rock sugar are hard and brittle, and they break easily when excessive force is applied. In our laboratory, however, we study “soft crystals” that can be bent by hand despite being crystalline. These materials are known as elastic crystals: they can be deformed by an applied force without breaking, and return to their original shape once the force is removed, exhibiting rubber-like behaviour. Elastic crystals are a relatively new class of materials, first reported by an Indian research group in 2012. Because they can withstand large deformations while retaining their crystal structure, they have attracted considerable attention as crystals that overturn conventional notions of what crystalline materials can be.

The crystal exhibiting high flexibility

In our laboratory, we have successfully developed a “thermal engine” that continuously operates autonomously using elastic crystals, driven solely by a temperature difference near room temperature—approximately 0 °C and 30 °C. The structure of the thermal engine is remarkably simple: a small weight is attached to the tip of an elastic crystal using an adhesive. When the thermal engine is placed in an environment with a temperature gradient, the crystal spontaneously begins to vibrate, with the vibration being amplified, and continues operating for more than 160 hours without interruption or degradation (a video showing the amplification of the crystal’s vibration induced by the temperature difference is presented). This behaviour indicates that a static temperature difference is converted into the kinetic energy of the weight—that is, the organic crystal functions as a thermal engine. To the best of our knowledge, this represents the world’s first thermal engine based on organic crystals [J. Am. Chem. Soc. 2025].

By further developing this research, it will become possible to create materials that move autonomously simply by being placed in a mild environment. In the future, such motion could be exploited as a driving source for miniature robots, or applied to power generation by converting the crystal’s mechanical motion into electrical energy, opening up a wide range of potential applications.

Note: This elastic crystal can be synthesised on the gram scale. Samples can be provided to interested researchers; please feel free to contact Horii for further information.

Two-Dimensional Nanosheets of Single-Molecule Magnets

Single-molecule magnets are molecules that exhibit magnetic bistability on their own. In conventional magnetic materials, magnetism emerges from the collective ordering of spins across a bulk solid. In contrast, a single-molecule magnet possesses a large spin ground state and strong magnetic anisotropy, which together stabilise the spin orientation within an individual molecule. As a result, the molecular spin can remain trapped in one of two preferred orientations for an extended period, allowing the molecule to behave as a nanoscale magnet.

Because each single-molecule magnet can, in principle, represent two distinct states—spin up and spin down—they are regarded as promising building blocks for ultra-high-density magnetic memory. In addition, the well-defined and addressable molecular spin states make single-molecule magnets attractive candidates for spin-based quantum bits (spin qubits) in emerging quantum technologies.

Reading and Writing

However, to write and read information at the level of individual single-molecule magnets, it is essential to arrange them in a well-ordered two-dimensional array. To achieve this, we fabricated ultrathin, sheet-like structures by assembling the molecules at the surface of water and linking them with metal ions. A key advantage of this approach is that it does not require complex or large-scale equipment, enabling the formation of uniform thin films through an energy-efficient solution-based process.

There are two principal types of magnetic alignment in magnetic recording media: in-plane magnetisation, in which magnetic moments lie parallel to the two-dimensional surface, and perpendicular magnetisation, in which they are oriented normal to the surface. Of these, perpendicular magnetisation is considered more desirable for recording media because it minimises stray magnetic fields between neighbouring magnets.

Using experiments conducted at the synchrotron radiation facility SPring-8, we demonstrated that the thin films prepared in this study exhibit perpendicular magnetic alignment, highlighting their potential as magnetic recording media [J. Mater. Chem. C 2023].

Furthermore, by introducing steric repulsion between neighbouring molecules through chemical modification of the single-molecule magnets, we found that the molecular arrangement within the film changes from a dense to a more dilute structure. This structural modulation leads to a further enhancement of perpendicular magnetic anisotropy [J. Mater. Chem. C 2024].

These results demonstrate the effectiveness of an approach based on molecular self-assembly and clearly show that both the structure and magnetic properties of thin films can be tuned in a stepwise manner through molecular design. Looking ahead, the strategy presented here—combining an energy-efficient solution-based process with rational molecular design—is expected to provide a viable pathway towards the development of next-generation ultra-high-density magnetic recording media and molecular spintronic materials.

News

2026-01-05 | The laboratory website has been launched.

2025-11-01 | Three third-year undergraduate students have joined the laboratory.