Abstract
Magnetic susceptibility measurements on La2CuO4−y have revealed maxima in temperature indicative of antiferromagnetic ordering. Powder diffraction measurements have demonstrated the existence of superlattice peaks at temperatures below those corresponding to the susceptibility maxima, and polarized neutron measurements have confirmed that the peaks are indeed of magnetic origin. The deduced antiferromagnetic structure in the orthorhombic (Cmca) phase of La2CuO4−y consists of ferromagnetic sheets of Cu atoms in the b-c plane which are alternating in sign along the a axis [100]. The moment is parallel to the c axis. The Neel temperature is extremely sensitive to the oxygen deficiency y and decreases rapidly to zero as y goes from 0.03 to 0. The saturated moment is ≂0.4 μB/Cu atom for samples with the highest TN (290 K). Antiferromagnetic spin fluctuations have also been observed by polarized neutron scattering in the paramagnetic phase above TN. Recent results obtained on magnetic ordering in the YBa2Cu3O6−y family of compounds are also discussed, together with the possible implications for novel superconductivity mechanisms.
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