A STUDY OF THE MAGNETIC CRITICAL SCATTERING FROM FECO3 BY TIME-OF-FLIGHT NEUTRON-DIFFRACTION

The temperature dependences of the magnetic critical scattering above T-N and the staggered magnetization below T-N have been measured for t he antiferromagnet FeCO3 using time-of-flight neutron diffraction tech niques. FeCO3 undergoes a paramagnetic to antiferromagnetic phase tran sition at its Neel temperature and was expected to be characterized by the universality class of the d = 3 Ising model. The transition, whic h occurs at T-N = 37.77 +/- 0.02 K, appears to be that of a continuous second-order transition. From an analysis of the critical scattering data, the exponents nu = 0.61 +/- 0.03 and gamma = 1.17 +/- 0.06 have been deduced. These values are in general agreement with the theoretic al values for the d = 3 Ising model of nu = 0.63 and gamma = 1.24. Fro m the temperature dependence of the staggered magnetization below T-N the exponent beta = 0.32 +/- 0.01 has been determined, which is also c onsistent with the theoretical value of beta = 0.33 for the d = 3 Isin g model.