Neutron-diffraction experiments on ceramic powders of YBa2Cu306+x (nonsuperconducting, with x ~0.0 and 0.15) have confirmed the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins. The structure determination was aided by the use of uniaxially oriented powders. The ordering wave vector within a Cu02 plane is (j j ) and the planes are coupled antiferromagnetically along the c axis. The Neel temperature is 400 ± 10 K for the x =0.15 ± 0.05 sample and S;500 K for jc=0.0.

A neutron study of the tetragonal antiferromagnet FeGe2 has shown the existence of two continuous magnetic transitions at temperatures of -263 and -289 K. The upper temperature corresponds to a transition from paramagnetism to a basal-plane spiral structure propagating along the cell edges in that plane. At the lower temperature the spiral structure is transformed into the simple collinear structure previously reported in the literature. Typical critical behavior is observed at the upper temperature for individual satellite peaks. T' he spiral propagation vector decreases continuously to zero at the lower critical point, exhibiting power-law behavior with an exponent of 0.407+0.005. Heat-capacity measurements reveal two A.-type anomalies with critical exponents in the expected range. The phase diagram has been analyzed using mean-field and renormalizationgroup considerations. A model based on zero basal-plane spin anisotropy yields a magnetic structure which agrees with the observed structure of the intermediate phase. The effect of an external field has also been treated theoretically.

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