You shout in a room full of crooked mirrors, and instead of a crisp echo you hear a rumble from all sides. The same thing happens in a thin magnetic film: a wave launched by an antenna bounces off defects and roughness, and a microscope with a diamond tip picks up not the wave itself but a chaotic mix of reflections. People used to think that a map taken with such a microscope shows which waves are present in the film. But a new study has shown that near the surface, this map deceives you.

When the wave from the antenna and the waves reflected from defects add up, a false signal with zero wave vector appears, as if the wave were standing still, even though in reality it isn't there. The scientists tested this on an iron-yttrium garnet film, varying the distance from the tip to the surface. At a large distance the spectrum was clean, but as the tip got closer, an anomalous peak appeared near zero. The model showed that this peak arises from the addition of waves that cancel each other out in one direction but reinforce each other in another.

It's like the room of crooked mirrors: echoes from different walls add up into a rumble that doesn't match any of the original sounds. Now we know that even tiny roughness on the edge of the antenna (just 100 nanometers) produces noticeable false signals. For future magnetic devices, where spin waves carry information, this means defects have to be taken into account during design, not ignored. The scientists propose a way to detect such artifacts and correct the antennas so that the waves come out cleaner.