  Materials exhibiting zero thermal expansion (ZTE), namely, volume invariance
during temperature change, can resist thermal shock and are highly desired in
modern industries as high-precision components. However, pure ZTE materials are
rare, especially those that are metallic. Here, we report the discovery of a
pure metallic ZTE material: an orthorhombic Mn1-xNixCoSi spiral magnet. The
introduction of Ni can efficiently enhance the ferromagnetic exchange
interaction and construct the transition from a spiral magnetic state to a
ferromagnetic-like state in MnCoSi-based alloys. Systematic in situ neutron
powder diffraction revealed a new cycloidal spiral magnetic structure in bc
plane at ground state which would transform to the helical spiral in the ab
plane with increasing temperature. Combined with Lorentz transmission electron
microscopy techniques, the cycloidal and helical spin order coherently rotated
at varying periods along the c axis during the magnetic transition. This spin
rotation drove the continuous movement of the coupled crystalline lattice and
induced a large negative thermal expansion along the a axis, eventually leading
to a wide-temperature ZTE effect. Our work not only introduces a new ZTE alloy
but also presents a new mechanism by which to discover or design ZTE magnets.
