Chromium nitride, crystallizing with the rock salt structure, has been found to be antiferromagnetic with a Neel point of O'C. The magnetic ordering scheme, as determined by neutron diftraction, is that of the fourth kind, with moments arranged in double ferromagnetic sheets which alternate along a line parallel to a face-diagonal. The structure is orthorhombic below the transition and the space group Pnme has been tentatively assigned. The atomic displacements accompanying the distortion have been partially determined from the diftraction data and are discussed in relation to the magnetic structure. A value of 2.4 le p has been assigned to the Cr moment; the moment direction is parallel to the b axis of the orthorhombic cell (facediagonal of the cube). INTRODUCTIOÑ

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An investigation of the magnetic structure and properties of MnCr 2 04 has been carried out by means of neutron diffraction and an evaluation of the suitability of various proposed theoretical models has been made. From room-temperature diffraction patterns it is established that MnCr 2 04 is a normal spinel with a u parameter of 0.3892=1=0.0005 and with less than one percent of the Mn ++ ions present on B sites. The Curie temperature, as determined from diffraction data, is ^-43°K. Below this temperature the magnetic contribution to the fundamental spinel peaks arising from aligned spins increases as the temperature is lowered and is effectively saturated at about 20°K. At 18°K, additional sharp peaks appear at positions which cannot be indexed either on the original unit cell or on any reasonably enlarged cell. These extra reflections persist with unchanged intensities down to 4.2 °K. No change in either the positions or intensities of the fundamental lines is observed in going through the transition. Above 18°K, a diffuse peak is present in the region where the principal extra lines develop. This diffuse peak decreases with increasing temperature, but is still observable above the Curie point. Application of a magnetic field along the neutron scattering vector decreases the magnetic contributions to the fundamentals, but increases the intensities of the extra reflections.The Neel model and the Yafet-Kittel model fail to account for major qualitative features of the diffraction results. On the other hand, the ferrimagnetic spiral model of Lyons, Kaplan, Dwight, and Menyuk is in good qualitative accord with all the observations. A detailed comparison with the theoretical predictions is presented and attention is called to broad areas of agreement as well as to certain discrepancies. The general conclusion reached is that the spiral model is a good first approximation to the groundstate spin configuration.

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Pratt and Bailey have recently explained the optical and anomalous magnetic properties of Cr2O3 by means of a model consisting of the basic Cr2O3 antiferromagnetic structure with moments canted away from the c axis and forming some kind of spiral. Our neutron powder-diffraction data confirm the original Brockhouse moment configuration but indicate that the c-axis component of the Cr moment is 8% lower than the spin-only value. This result can be interpreted as supporting the canted model of Pratt and Bailey. A careful search has been made, using single crystals, to find evidence for an ordered or spiraling perpendicular component. Polarization studies establish that there is no magnetic contribution to the fundamental reflections other than that arising from the c-axis components. A survey of reciprocal space revealed extra spots at forbidden positions, but these could be accounted for semiquantitatively in terms of double-Bragg scattering. We conclude, therefore, that either the Cr moment is intrinsically lower, or, if canting exists, a perpendicular component of the magnitude required by the canted model does not exhibit long-range order.

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