Journal of Magnetism and Magnetic Materials 81 (1989) 125-132 North-Holland, Amsterdam 125 zyxwvutsr MAGNETO-OPTICAL STUDIES OF H-T PHASE DIAGRAM FOR DyFeO, (H I( a) zyxwvutsrqponmlk S.L. GNATCHENKO, N.F. KHARCHENKO, P.P. LEBEDEV Insiitute for Low Temperature Physics and Engineering, Academy of Sciences of the Ukrainian SSR, Kharkov, USSR K. PIOTROWSKI, H. SZYMCZAK and R. SZYMCZAK Institute of Physics, Polish Academy of Sciences, Warsaw, Poland Received 17 April 1989 The sequence of phase transition induced in DyFeO, by an external magnetic field H 11 a has been studied in the temperature range near the Morin point. In this transition sequence, a first-order phase transition is followed by a second-order one. The transitions observed have been investigated with the help of visual observations of inhomogeneous magnetic states, as well as by means of magneto-optical methods (Faraday rotation and linear magnetic birefringence). As an outcome of these measurements, a magnetic phase H-T diagram has been established, and coordinates of the tetracriticat point have been determined. A phenomenological model describing the phase transitions studied is proposed. Some peculiarities noted in magneto-optical measurements near the tetracritical point have been explained by either assuming existence of a space phase in this temperature and field range, or by magnetic field deflection from the a-axis. 1. Introduction Recently, new magnetic field-induced phase transitions have been discovered in DyFeO, by means of a polarizing optical microscope [l] and using the antiferromagnetic resonance technique [2]. A prominent feature of DyFeO, is the ex- istence of a spontaneous spin-reorientation phase transition from the antiferromagnetic l?,(G,,) state to the weak ferromagnetic I?,(G,, F,) state. Here, G, represents the basic antiferromagnetic spin- arrangement along the i-axis; 5 is the ferromag- netic spin-arrangement along the j-axis, and I, are the irreducible representations, according to the notation given in ref. [3]. The spontaneous I, = I, phase transition (the Morin transition) takes place at TM = 50 K. In this material, an external magnetic field H applied parallel to the u-axis induces a spin reorientation r,(G,) + I,,(<, G,, G,) --$ I,(<, G,), for T< T,, or I,(&, G,) -, Iz4(Fx, F,, G,, G,) -, I,(c, G,), for T > TM. In both cases, spin reorientation occurs through continuous rotation of the G and F vec- tors and is completed by a second-order phase transition to the I, phase. We have previously shown [l], that in DyFeO, near TM, another type of spin reorientation is possible, namely the I, + r 24 -+ r12 -+ r2 transition. In this transition se- quence, the I,, + Ii2 transition is a first-order phase transition. The temperature range in which r24 -+ r,, takes place is rather narrow ( 1 TM - T,, 1 e TM). During this transition, the antiferromag- netic vector G reorients rapidly from the (a, c) plane to the (b, c) plane, while the ferromagnetic vector F reorients from the (a, c) plane to the u-axis. Using a phenomenological approach, it has been shown in ref. [l] that the line of the I,, e Ii2 first-order phase transition should end at a critical point. A theoretical analysis performed in ref. [4] led to the conclusion that this critical point is, in fact, a tetracritical point, at which I,, = r24, Ii2 0 I2 and I,, e I2 transitions should end. The main purpose of this paper is to report results of a careful study, performed using mag- neto-optical methods, of the H-T phase diagram of DyFeO,, with special attention paid to the multicritical point. Magneto-optical methods seem to be the most suitable ones for such studies, since 0304~8853/ 89/ $03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)