The effects of Ti substitution for Fe on octahedral sites in magnetite has been investigated through electrical resistivity measurements. In a manner analogous to zinc ferrites, the Verwey transition temperature is depressed with increasing Ti content. The Verwey transition is of first order in Fe3~Ti~04 in the concentration range O~y~y, =0. 012; for y, y 3y, the transition is of second order, and for y) 3y, the phase transformation is completely eliminated. The data are correlated with comparable work on dilute zinc ferrites and nonstoichiometric magnetite, and the implications of the findings are discussed.

Magnetic susceptibility, paramagnetic resonance, and antiferromagnetic resonance have been measured in MnTi0 3 , FeTi0 3 , CoTi0 3 , and NiTi0 3 . N6el temperatures T N of 65, 56, 37, and 23±2°K, respectively, have been measured. The molar susceptibilities appear to obey a Curie-Weiss law with C m =4.36, 3.37, 3.72, and 1.24°K, and with paramagnetic Curie temperatures 9 P =-219, +32, -9.3, and -11°K, respectively. The effective spectroscopic splitting factors consistent with this law and with paramagnetic resonance are gef f =2.01, 2.13,2.83,2.22±0.01. The susceptibility of MnTi0 3 exhibits a broad peak near 100°K rather than a well-defined peak at TV. No antiferromagnetic resonance was observed in FeTi0 3 , and no paramagnetic resonance was observed in FeTi0 3 or CoTi0 3 . The antiferromagnetic resonant frequencies, extrapolated to 0°K, are 156,145, and 187 kMc/sec for MnTi0 3 , CoTi0 3 , and NiTi0 3 , respectively. A low-frequency mode of 19.1 kMc/sec was also detected in NiTi0 3 . In the system Mni-^Mg^TiOs, both a R and T N decrease roughly linearly with x in the range 0<#<0.075.

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Experimental quantum effect spectra have been observed in germanium and silicon at liquid helium temperatures and a frequency of 136 kMc/sec. Detailed spectra are presented for the applied magnetic field in the [100], [111], and [110] directions. Anisotropy data of some of the more intense lines are also presented. Using the theory of Luttinger and Kohn with the assumption of thermal equilibrium, effective masses and line intensities were computed for an applied magnetic field in the [111] direction. By adjusting the parameters of the theory, good agreement was obtained between the theoretical and experimental spectra. The experimentally determined parameters are: Yl 72 73 * Ge 13.2 4.10 5.62 3.29 Si 4.22 0.50 1.35 -0.39 These lead to parameters A =* 13.2, | B \ = 8.2, and | C | = 13.3 for germanium and A = 4.22, | B | = 1.0, and | C\ =4.34 for silicon. (See note added in proof.)

The effective internal field in NiTiO3 as a function of temperature has been determined from zero-field resonance absorption in a powder sample at millimeter wavelengths. Assuming a g value of 2, the resonant frequency (187 kMc) leads to an effective field at 4.2°K of approximately 67 kG. The temperature dependence fits within experimental error a Brillouin function for S = 1, with a Néel temperature of 22°K. From dc susceptibility data, a Néel temperature of 23°K is obtained and an exchange field of approximately 270 kG is computed. These data yield an out-of-plane anisotropy of about 8.3 kG.

Owing to their high internal fields, antiferromagnetic materials have natural resonant frequencies in the millimeter and submillimeter wavelength region of the electromagnetic spectrum. Since these normal modes are circularly polarized and can be tuned by an applied magnetic field, devices similar to the usual ferrite devicesare possible at these high frequencies with the application of relatively low fields. The dynamics of a simple antiferromagnetic system are briefly reviewed and the important quantities which characterize antiferromagnetic devices are discussed. The figures of merit for antiferromagnetic resonance isolators and phase shift devices are derived. Experimental data on resonant frequency and linewidth, as well as a typical nonreciprocal resonance absorption trace showing a reverse to forward loss of 10 to 1 in chromic oxide at 140 kMc and at 77°K are presented.

Microwave resonance has been observed in MnCr2O4 and CoCr2O4 (ferrimagnetic spirals), ZnCr2Se4 and MnI2 (flat antiferromagnetic spirals), and ZnCr2S4 (a complex spiral). Spectra were observed in powders and some single crystals, as a function of frequency and temperatures down to 4°K or lower. MnCr2O4 shows a uniform-mode ferrimagnetic resonance with g=2. But CoCr2O4, with a small net moment, shows complex spectra in powders and single crystals, which mirror roughly the phase transitions observed in neutron diffraction, and predicted by molecular field theory. Powder spectra of ZnCr2S4 and ZnCr2Se4 yield effective internal fields which vary with temperature qualitatively as the average moment per magnetic ion. Resonance has been observed in single crystals of MnI2, from the transition temperature of ∼3.4°K to ∼1.5°K. Theoretical models of the spectra are discussed.