  Using high resolution neutron diffraction and capacitance dilatometry we show
that the thermal evolution of the helimagnetic state in CoMnSi is accompanied
by a change in inter-atomic distances of up to 2%, the largest ever found in a
metallic magnet. Our results and the picture of competing exchange and strongly
anisotropic thermal expansion that we use to understand them sheds light on a
new mechanism for large magnetoelastic effects that does not require large
spin-orbit coupling.
