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which go beyond Landau-Ginzburg theory and which do not have this restriction show that the change in the character of the phase transition from second to first order with decreasing temperature occurs as long as K <0,4. Here K is the ratio XQ{T)/i{T), When /c> 0.7 the phase transition is always second order. When 0.4

Far-infrared transmission measurements have been made on pure and Fe2+-doped MnF2 at 4.2°K using magnetic fields up to 105 kOe. By means of Fourier-transform spectroscopy we have investigated the behavior of the spin-wave modes near the spin-flop transition. Both the AFMR and the two-magnon absorption with the E field perpendicular to the c axis have been measured. In Fe2+-doped MnF2 the spin flop occurs at higher fields depending on the Fe2+-concentration. The localized mode which splits linearly in low fields no longer shows a linear dependence in the spin-flop region.

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