A neutron diffraction study has been made of powdered samples of CrAs and Mn-substituted CrAs, namely, Cr0.7Mn0.3As, Cr0.4Mn0.6As, and Cr0.3Mn0.7As. Although CrAs shows a kink in susceptibility at 796°K, the anomaly was found to be due to a crystallographic transformation, from MnP-type structure (low-temperature side) to NiAs type. The liquid-nitrogen temperature diffraction pattern contains a number of magnetic peaks. An analysis of the data gives a double-layered helical structure, with the propagation vector along the c axis (the space group Pnma), like that found in MnP. The Néel temperature as determined from the temperature dependence of (101)− intensity is about 280°K. The magnetic structure of Cr0.7Mn0.3As is much the same as that of CrAs, but with the Néel temperature at 263°K. Cr0.4Mn0.6As is ferromagnetic with a Curie temperature at 160°K, and has a magnetic transition in the paramagnetic range, which separates the low-temperature low-spin state from the high-temperature intermediate-spin state. Cr0.3Mn0.7As is metamagnetic at low temperatures.

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We have been able to prepare amorphous oxides of Fe and Cr by sputter deposition. Magnetization measurements were made between 4.2 K and 300 K. Mössbauer spectra were obtained for the iron oxide samples in the same temperature range. Amorphous CrO2 films showed no evidence of magnetic order. For the amorphous Fe oxide films, the 4.2 K Mössbauer spectrum showed a broad symmetric six line pattern which collapsed to a doublet at T?100 K. The susceptibility of amorphous iron oxide shows a broad maximum near T=100 K and a paramagnetic moment of only 3μβ. We present arguments that this behavior cannot be described by superparamagnetism and therefore represents the intrinsic magnetic behavior of amorphous iron oxide.

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