  Materials with negative thermal expansion (NTE), which contract upon heating,
are of great interest both technically and fundamentally. Here, we report giant
NTE covering room temperature in mechanically milled antiperovksite GaNxMn3
compounds. The micrograin GaNxMn3 exhibits a large volume contraction at the
antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition within a
temperature window ({\Delta}T) of only a few kelvins. The grain size reduces to
~ 30 nm after slight milling, while {\Delta}T is broadened to 50K. The
corresponding coefficient of linear thermal expansion ({\alpha}) reaches ~ -70
ppm/K, which is almost two times larger than those obtained in chemically doped
antiperovskite compounds. Further reducing grain size to ~ 10 nm, {\Delta}T
exceeds 100 K and {\alpha} remains as large as -30 ppm/K (-21 ppm/K) for x =
1.0 (x = 0.9). Excess atomic displacements together with the reduced structural
coherence, revealed by high-energy X-ray pair distribution functions, are
suggested to delay the AFM-PM transition. By controlling the grain size via
mechanically alloying or grinding, giant NTE may also be achievable in other
materials with large lattice contraction due to electronic or magnetic phase
transitions.
