First-principles investigation of spin-dependent thermoelectric transport and spin Seebeck in Fe(110)/Co( $$11\bar{2}0$$ ) heterostructures

Turning Heat into Spin Signals

Modern electronics waste a lot of energy as heat, but that heat can sometimes be recycled into useful electrical signals. This study explores a more exotic version of that idea: using heat to push not just electric charge, but electron spin – a tiny magnetic property – through a specially engineered iron–cobalt thin film. Understanding how heat drives spin currents in such simple metallic stacks could help design more efficient sensors, memory devices, and energy-harvesting technologies that work hand in hand with conventional electronics.

Why Iron and Cobalt Make an Interesting Pair

The researchers focused on a sandwich-like structure made of iron (Fe) and cobalt (Co), two familiar magnetic metals often found in hard drives and magnetic sensors. Unlike most previous work, which looked at a magnetic metal next to a non-magnetic “detector” metal, this study examines an all-ferromagnet stack: Fe(110)/Co(1120). In this geometry, both layers are magnetized, and their crystal lattices are carefully aligned so that the interface resembles realistic thin-film experiments. By building detailed computer models of the bulk materials, their exposed surfaces, and the final combined stack, the team ensured that the structure they studied is both physically reasonable and representative of real devices.

How the Properties Were Calculated

To probe how this Fe/Co stack responds to a temperature gradient, the authors used first-principles methods, meaning they started from the fundamental laws of quantum mechanics rather than fitting to experiment. They calculated the electronic structure – the allowed energy levels and velocities of electrons – with spin-polarized density functional theory, which handles the magnetic nature of iron and cobalt. These results were then fed into a transport code that solves a semiclassical equation describing how electrons flow under an applied temperature difference. The approach separates electrons into spin-up and spin-down channels, so the conventional voltage that builds up from heat and the additional “spin voltage” can be extracted in parallel.

What Happens to Charge and Spin Under Heat

The computed thermoelectric response looks metallic: the ordinary Seebeck coefficient (the voltage per unit temperature difference) is small, negative, and changes only gradually from zero to 500 kelvin, indicating that electrons dominate conduction. Both spin-up and spin-down channels contribute, but not equally – the spin-down channel shows a stronger response, reflecting a sharper variation of its conductivity around the Fermi level, the energy where electrons can move most easily. The team also evaluated the electrical conductivity and found that it depends strongly on in-plane direction: current flows more easily along one in-plane axis (labeled y) than the other (x), an effect tied to differences in the underlying band velocities and effective masses of the electrons in those directions. This built-in anisotropy imprints itself on both charge and spin signals.

Estimating How Often Electrons Scatter

Because their transport method naturally produces conductivity divided by a characteristic lifetime, the authors needed to estimate how long electrons travel before they scatter. They did this in two complementary ways. One model is based on how electrons interact with gentle ripples of the crystal lattice (acoustic phonons) and uses elastic constants, effective masses, and how sensitive band edges are to strain. This yields relatively long lifetimes in the sub-picosecond to picosecond range and represents an optimistic limit. The second model infers a shorter, more conservative lifetime directly from the size of the Seebeck coefficient using an empirical “Planckian”-type formula, producing values of only a few tens to a few hundred femtoseconds. Together, these two estimates bracket a realistic window for how strongly scattering limits electron motion in the Fe/Co stack.

How Strong Is the Spin Signal?

Combining the spin-resolved voltages with the conductivities in a two-current picture, the team extracted an effective spin Seebeck coefficient, which measures how strongly a temperature gradient drives a difference between spin-up and spin-down currents. With the optimistic, phonon-limited lifetimes, this spin thermopower can reach a few microvolts per kelvin, providing an upper bound on the intrinsic electronic response. When the shorter, Seebeck-derived lifetimes are used, the spin Seebeck signal shrinks by one to two orders of magnitude, giving a directional average near minus 0.15 microvolts per kelvin at room temperature. That value is comparable to spin Seebeck signals measured in related ferromagnet/heavy-metal devices, suggesting that the purely electronic contribution inside the Fe/Co stack is already of the right order, even before including additional magnon or interface effects present in experiments.

What This Means for Future Spin-Heat Devices

For non-specialists, the key takeaway is that an all-metal iron–cobalt thin film can turn a temperature difference directly into a tiny spin imbalance whose size and direction depend on crystal orientation and details of electron scattering. The study does not yet include every real-world complication – such as interfacial roughness, magnon-driven transport, or the conversion of spin current into measurable voltage in an attached heavy metal – but it establishes a solid first-principles baseline for the electronic part of the spin Seebeck effect. This foundation can guide the design of future spin-caloritronic devices that seek to recycle waste heat into information-rich spin signals, potentially improving the efficiency and functionality of next-generation magnetic technologies.

Citation: Waritkraikul, P., Ektarawong, A., Busayaporn, W. et al. First-principles investigation of spin-dependent thermoelectric transport and spin Seebeck in Fe(110)/Co(\(11\bar{2}0\)) heterostructures. Sci Rep 16, 7686 (2026). https://doi.org/10.1038/s41598-026-37860-w

Keywords: spin Seebeck effect, spin caloritronics, thermoelectric transport, Fe/Co thin films, spintronics