Light Flips Spin States in Manganese Ions

Light Flips Spin States in Manganese Ions

Researchers across the globe are leveraging the unique magnetic properties of manganese to advance fields ranging from digital data storage and high-speed computing to solar energy conversion and photochemistry. By manipulating the spin states of manganese ions—the magnetic moment of electrons that act like bar magnets—scientists are overcoming previous thermal and efficiency limits associated with other materials.

Breakthroughs in Molecular Data Storage

At Johannes Gutenberg University Mainz (JGU), a research group led by Prof. Dr. Katja Heinze has developed a manganese-based material that functions as a tiny data storage device. This approach targets the "writing" of memory through spin states: parallel or antiparallel alignments correspond to "1" or "0," known as high-spin and low-spin states. Previously, such molecular storage relied on iron-containing materials. Those devices required extreme cooling, typically to a maximum of 100 Kelvin (approximately minus 173 degrees Celsius), though some reached 130 Kelvin (minus 143 degrees Celsius). These low temperatures created high energy requirements for cooling. The JGU team, including doctoral student Sandra Kronenberger and Dr. Luca Carrella, achieved a temperature jump by combining manganese with ligands derived from N-heterocyclic carbenes. This strong bond stabilizes the low-spin state and creates a high energy barrier between spin states. On their first attempt, the team raised the operating temperature to around minus 132 degrees Celsius. The process is visually detectable. When irradiated with light, the manganese ions change spin states, causing the material to shift in color from dark red in the low-spin state to light yellow in the high-spin state. According to Heinze, both the magnetic properties and the color persist after the light is deactivated.

Quantum Dot Energy Harvesting

While JGU focuses on storage, scientists at Los Alamos National Laboratory are using manganese to improve how solar cells and photodetectors capture energy. The team, led by principal investigator Victor Klimov and lead chemist Jungchul Noh, introduced magnetic manganese impurities into core-shell quantum dots. These quantum dots feature an inverted band structure where the shell has a lower bandgap than the core, allowing for the efficient localization of electrons and holes. The researchers utilized ultrafast spin-exchange interactions mediated by manganese ions to capture the energy of "hot" carriers generated by photons. This process converts that energy into additional excitons (electron-hole pairs). Unlike conventional carrier multiplication, which is often limited by the rapid cooling of carriers via phonon emission, this spin-exchange method uses bidirectional energy transfer between manganese dopants and the intrinsic states of the quantum dots. Photocurrent measurements in real-world devices confirmed a sharp increase in generation above the spin-flip transition threshold of the manganese ion. Simulations suggest this could increase power conversion efficiency by up to 41%. Noh stated that this has implications for light-driven chemical synthesis, including energy-intensive processes like ammonia production via nitrogen fixation, which currently uses more than 2% of global energy.

Ultrafast Switching in Antiferromagnets

In a separate effort at the University of Tokyo, a team led by Ryo Shimano observed electron spins flipping within an antiferromagnet, specifically Mn3Sn (manganese three tin). In these materials, opposing spins cancel each other out, making them appear magnetically neutral. By using a thin film of Mn3Sn and applying brief electrical pulses combined with ultrafast flashes of light, Shimano's team captured a frame-by-frame view of the switching process. They discovered two distinct mechanisms:

• Heat-driven: Occurs when a strong current is applied.
• Heat-free: Occurs under weaker current conditions, allowing spins to flip without wasting energy as heat.

This heat-free pathway suggests a route toward non-volatile magnetic memory and logic devices. While the current observation limit is 140 picoseconds due to device setup, Shimano believes the material may be capable of switching even faster.

Low-Energy Photocatalysis

Further expanding the utility of the element, researchers have demonstrated the use of the complex [Mn(dgpy)2]4+, which utilizes the tridentate 2,6-diguanidylpyridine ligand (dgpy). This Earth-abundant manganese complex can be excited by low-energy near-infrared light. The complex evolves into a luminescent doublet ligand-to-metal charge transfer (2LMCT) excited state with a lifetime of 1.6 ns, which can oxidize naphthalene. Additionally, substrates with oxidation potentials up to 2.4 V enable photoreduction via a high-energy quartet 4LMCT excited state with a lifetime of 0.78 ps. This process allows for the oxidation of benzene and nitriles using low-energy light and abundant elements.