| The temperature derivative of the linear birefringence, d((DELTA)n)/dT, has been measured as a function of temperature in several pure and random systems: FeF(,2), which has a large single-ion uniaxial anisotropy; MnF(,2), which has a small dipolar uniaxial anisotropy; the mixed magnetic system Fe(,x)Mn(,1-x)F(,2); the magnetically diluted systems Fe(,x)Zn(,1-x)F(,2) and Mn(,x)Zn(,1-x)F(,2); and the diamagnetic crystals ZnF(,2) and MgF(,2).; For MnF(,2), the magnetic contribution to d((DELTA)n)/dT is shown to be proportional to the magnetic specific heat both in the critical region t = (VBAR)T/T(,N) - 1(VBAR) << 1, (T(,N) (TURN) 67K), and over a much larger temperature range 10 (LESSTHEQ) T (LESSTHEQ) 100K. The analysis of d((DELTA)n)/dT in FeF(,2), MnF(,2), and Fe(,.75)Mn(,.25)F(,2) in the critical region 10('-4) (LESSTHEQ) t (LESSTHEQ) 10('-2), reveals Ising-like behavior of the magnetic specific heat in each case.; The variation of T(,N)(x) for small x, and the x-dependence of the Ising-like critical amplitudes in the mixed magnetic Fe(,x)Mn(,1-x)F(,2) system are explained in terms of the scaling relations which describe the crossover from Heisenberg to Ising critical behavior.; The variation of T(,N)(x) for small x, and the x-dependence of the Ising-like critical amplitudes in the mixed magnetic Fe(,x)Mn(,1-x)F(,2) system are explained in terms of the scaling relations which describe the crossover from Heisenberg to Ising critical behavior.; In the diluted magnetic systems, the more rapid decrease of T(,N)(x)/T(,N)(1) in Mn(,x)Zn(,1-x)F(,2), as opposed to Fe(,x)Zn(,1-x)F(,2) is attributed to the more Ising-like character of FeF(,2) compared with MnF(,2).; In the diluted FeF(,2) and MnF(,2) samples, large depressions in the transition temperatures and dramatic changes in the shapes of transition peaks were observed when magnetic fields 0 (LESSTHEQ) H(,0) (LESSTHEQ) 20 kOe were applied parallel to the easy axis. These effects, which have not previously been observed, are in semi-quantitative agreement with predictions of recent "random field" theories.; The apparent rounding of the phase transition, observed in the birefringence data of many of the mixed and diluted magnetic crystals, is shown to be primarily the result of concentration gradients in the samples. The birefringence method, in contrast to conventional specific heat techniques, is shown to be particularly advantageous in regards to minimizing concentration gradient effects on critical phenomena. |
