Roles of grain size and strain on antiferromagnetic order in nanocrystalline chromium

Neutron-diffraction investigations of powder and consolidated ultrafine-grain-sized chromium samples indicate that antiferromagnetic order in the body-centered-cubic phase of this material can be suppressed to well below the Néel temperature of coarse-grained and single-crystal chromium. The suppression is correlated strongly with decreasing grain size. Antiferromagnetic order was not observed in powder or in consolidated samples with grain sizes less than 16 nm, indicating that free surfaces and grain boundaries play the same role in preventing antiferromagnetic order with the structure of bulk chromium. Antiferromagnetic order was observed in nanocrystalline samples with grain sizes greater than 19 nm at 20 K. No correlation is seen between the Néel temperature and the degree of long-range microstrain, or with contents of light-element impurities in the samples. Even in cases where antiferromagnetic order is detected, the transversely polarized ${\mathit{AF}}_1$ spin-density-wave magnetic phase is never seen. While this may suggest that spin-density-wave phases do not occur in nanocystalline chromium, the presence of the longitudinally polarized ${\mathit{AF}}_2$ spin-density-wave phase cannot be ruled out unambiguously.