Diamonds to the rescue: Keeping track of spin transport inside a spintronic device is challenging. Du et al. came up with a method involving diamond nitrogen-vacancy (NV) centers, which can act like tiny, very sensitive magnetometers. The authors placed diamond nanobeams containing the NV centers in close proximity to the sample. This allowed them to measure the spin chemical potential of spin waves—so-called magnons—with nanometer resolution in the material yttrium iron garnet. Because NV centers are also sensitive to temperature, the method may be of use in spin caloritronics. Science , this issue p. 195

Yttrium iron garnet (YIG, Y ₃Fe₅O₁₂) films have been epitaxially grown on Gadolinium Gallium Garnet (GGG, Gd₃Ga₅O₁₂) substrates with (100) orientation using pulsed laser deposition. The films were single-phase, epitaxial with the GGG substrate, and the root-mean-square surface roughness varied between 0.14 nm and 0.2 nm. Films with thicknesses ranging from 17 to 200 nm exhibited low coercivity (<2 Oe), near-bulk room temperature saturation moments (∼135 emu cm⁻³), in-plane easy axis, and damping parameters as low as 2.2 × 10⁻⁴. These high quality YIG thin films are useful in the investigation of the origins of novel magnetic phenomena and magnetization dynamics.

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to $50\mu \mathrm{m}$ at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found.