MAGNETIC ORDER IN NANOCRYSTALLINE CR AND SUPPRESSION OF ANTIFERROMAGNETISM IN BCC CR

Neutron-diffraction data and magnetization measurements of compacted nanophase Cr are presented. Two crystalline Cr phases were found, the majority phase having a bcc structure and the minority phase having an A15 structure. The 100 Bragg reflection seen in antiferromagnetically ordered coarse-grained bcc-Cr polycrystals was not observed in the nanocrystalline sample, suggesting that antiferromagnetism was suppressed in the bcc phase to at least 20 K. The suppression of antiferromagnetism may be due to small grain sizes, strain from grain boundaries or other defects, and/or concentrations of impurities, any of which may restrict the development of magnetic correlations in the bcc material. A magnetic diffraction peak corresponding to the 110 Bragg reflection of the Cr A15 lattice was observed at 20 K and was not observed at 149 or 300 K, suggesting that the minority phase of the nanocrystalline sample becomes antiferromapetically ordered at very low temperatures. Consistent with this observation, susceptibility measurements at several temperatures indicate that magnetic correlations develop below 100 K, representing at least momentary short-ranged magnetic order in the nanocrystalline sample.