New material routes heat on demand and keeps its setting when the power cuts

A new thin-film device can be programmed to send heat one way and emit it from another, then keep doing it after the power is cut. Researchers at Osaka Metropolitan University built it by stacking a magneto-optical layer with GST, a phase-change alloy (germanium-antimony-tellurium) that flips between two solid states, and used the combination to break two long-standing rules of thermal radiation in a single platform (press release).

The result, [published 25 June 2026 in Laser & Photonics Reviews](https://onlinelibrary.wiley.com/doi/10.1002/lpor.71438), combines non-volatile memory with non-reciprocal heat flow at near-normal incidence, where heat strikes the surface at a near-perpendicular angle. Earlier non-reciprocal thermal devices needed extremely steep angles to work, and efficiency dropped off as the angle flattened.

The two rules it breaks govern how any surface exchanges heat with its surroundings. Kirchhoff's law of thermal radiation, laid out in 1862, says a surface's absorption and emission at a given wavelength are linked: a good absorber is a good emitter, and a poor absorber is a poor emitter, at the same wavelength. Lorentz reciprocity says the path heat takes in one direction is the same as the path it takes back. Together, those rules have kept thermal engineers from building a device that absorbs heat from one side and radiates it out the other, or that routes heat asymmetrically across a surface.

The Osaka team separated absorption from emission by combining a magneto-optical material, one whose optical response shifts in a magnetic field, with a thin GST layer. A short electrical pulse switches GST between an amorphous (glassy) and a crystalline (ordered) state. In one state, the stack obeys the Kirchhoff limit, absorbing and emitting at the same wavelength. In the other, the magneto-optical layer tilts the emission direction, decoupling absorption from emission and breaking reciprocity. Because the GST state is non-volatile, the device keeps its setting after the power is removed. "We made heat radiation behave in a smarter way," said co-author Dr. Shunsuke Murai.

The team is led by Prof. Koichi Okamoto and Dr. Murai at Osaka Metropolitan University's Graduate School of Engineering, with collaborators from China and Singapore. The paper lists five application classes: radiative cooling, thermophotovoltaics (heat-to-electricity cells that sit next to a hot surface and convert infrared radiation into current), infrared sensing, thermal communication, and photonic memory.

Wire coverage of this work has leaned on the AI-chip-cooling angle (Tom's Hardware, SciTechDaily). Dense AI accelerators and silicon-photonics stacks are thermally constrained, and most thermal-routing approaches need constant power or extreme angles to work. A surface that can be programmed once and then route heat asymmetrically for as long as the GST state holds is a new tool for thermal management, sitting alongside existing approaches like cold plates and two-phase liquid cooling rather than replacing them.

The next milestones are quantitative: specific switching ratios, operating wavelength bands, and endurance across many GST cycles. Okamoto's stated next step is tunable spectral bands and integration with on-chip photonic circuits. "Our ultimate goal is to develop compact devices that can actively control heat radiation, much like electronic circuits control the flow of electricity," he said.