At 4.2°K the R lines of chromium in ruby and in MgO exhibit an isotope shift of 0.13 and 0.17 cm -1 per unit mass, respectively. This is a much larger value than is found for free atoms of intermediate mass. We consider the possibility that this isotope shift is caused by the interaction of the ion with the vibrating lattice. The ion-phonon interaction causes a frequency shift in these R lines, and this shift depends on the amplitude of vibration of the chromium ion. The lighter chromium isotopes, however, have a larger amplitude of vibration than the heavier isotopes, and consequently the frequency shift varies from isotope to isotope. This leads to an isotope shift. For the R lines of ruby and MgO :Cr 3+ the isotope shift due to the interaction with the zero-point vibrations is calculated, and is in very good agreement with the experimental values, both in magnitude and direction.

A new approach to critical dynamic scaling of g" is presented wherein P/zv, zv, and T, may all be separately determined independent of each other. Application is made to Cdo.6Mn0. 4Te where by use of this new procedure it is shown that the data in the critical region above T, are inconsistent with activated dynamics. This new procedure applied to other systems, reveals frequent underestimates of zv and overestimates of T,. Apparent systematic deviations from scaling which are found near and below T, are brie8y discussed.Dynamic scaling of the ac susceptibility Z'(tu, T) +iZ"(m, T) has been used extensively as supporting evidence for critical behavior associated with phase transitions in random magnetic systems. In conventional dynamic scaling the relaxation time z is related to the correlation length g-( t ( " via r-g'-)

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The dynamics of magnetic fluctuations have been investigated in the random frustrated antiferromagnet Cd1−xMnxTe. The frequency-dependent susceptibility, χ′(ω,T) has been measured by Faraday rotation for x=0.4 and 0.65. The departure of the susceptibility from equilibrium Δχ′=[χeq(T)−χ′(ω,T)]/χeq(T) is found to obey activated dynamic scaling similar to that seen in random field systems. These results in conjunction with neutron measurements by others which revealed the abrupt saturation of the antiferromagnetic correlation length for x=0.65 at the onset of slow dynamics near Tc suggest a dynamically inhibited transition to a type-III AF state rather than a spin-glass transition.