Magnetic and magnetooptic effects under phase transitions in antiferromagnets

A complex study of magnetization change, optical Faraday effect, and light absorption spectrum has been carried out on uniaxial antiferromagnetic crystals of MnF2of FeCO3under the phase transitions induced by strong external magnetic field. The measurements were made at 4.2 and 20.4 K, which is lower than Néel temperature (Tn=68 and 38 K for MnF2and FeCO3, respectively). The field used was pulsed up to 400 kOe. MnF2: The magnetic properties have been studied in detail employing a pickup coil method in a narrow range of magnetic fields near the spin-flopping field (Hc=92 kOe) with a strict field orientation along the tetragonal axis of the crystal symmetry. The angular field deviation ψ from the c4axis is controlled to within 1.5´. In the narrow range of angles ψ < 30´, the differential magnetic susceptibility near the spin-flopping field Hcdepends on the sample form and the magnetization process is of a jump-like character. The measurements of the longitudinal (along the magnetic field) and transversal susceptibility components made using three pickup coils of mutually perpendicular axes and three coaxial coils shifted along the sample axis show that at ψ < 30´ and H=Hc± 200 Oe the sample magnetically cleaves into interchanging layers of 10^-5cm3volume which stretch along the symmetry axis of the sample and differ from each other by a longitudinal component of the magnetization vector.