Journal of Magnetism and Magnetic Materials 1114-11)7(1992) 1607-16118 North-Holland ,,, ,,,, ,,, ::: Equilibrium and nonequilibrium magnetic behaviour of a reentrant Ising spin glass K. Gunnarsson %J.-O. Andersson % P. Svedlindh % P. Nordblad ~', L. Lundgren % H. Aruga Katori b and A. lto ~' " Uppsaha Unicersity, Department of Technoh~gy, Ba~ 534, S-751 21 Uppsahl, Sweden t, Ochanomizu Unicersit3,, Department of Physics, Bunkyo-ku, Tokyo 112, Japan The reentrant lsing spin glass Fe,.,_,Mn0.3sTiO 3 has been investigated using SQUID magnetometry. The system is antiferromagnetic below TN(H = 0) = 33 K, and enters a mixed antiferromagnedc/spin glass phase below Tg( H = 0) = 25.8 K. Within the concept of a recent mean field model, a ratio TN(O) / T g(0) close to unity indicates both intra- and interlayer frustration. The existence of an ocerlap length, la. r, below waich equilibrium spin correlations at temperatures T and T + AT are indistinguishable has been verified by magnetic relaxation experiments on this spin glass system. Magnetic systems where order and disorder coexist, e.g. the reentrant spin glasses (RSG), have been in focus the last years. The Ising systems Fc0.55Mg,.45C! 2 [1] and Fe.~Mn._xTiO3 [2] arc cxamplcs of systems with mixed antifcrromagr, cfic (AF) and spin glass (SG) order. Here, we describe the results of our experimen- tal work on the RSG Fc..~,2Mn,.3sTiO 3 (FMTO), which includes SQUID measurements of the ac susceptibility (g(to)), zero field cooled (ZFC) and field cooled (FC) magnetizations. The transition tcmpcraturc from AF to AF/SG order is Tg = 26.6 K. The magnetic structure of Fc,Mn t .,TiO~ [2] can bc dcscribcd by honeycomb layers containing the Fc 2' and Mn 2+ ions. The spins arc directed parallel to the hexagonal c-axis, thus forming an lsing like system. The random distribution of exchange interactions within the layers gives rise to a SG phase. For 0.38 < x < 0.58, the compound shows pure SG behaviour, while x > 0.58 yields a RSG system. The sample, a single crystal of FMTO, was in the shape of a rectangular parallclcpiped (2 x 2 × 4 mm3). For the X(to) mcasurc- mcnts, a small time varying field, h = h. sin wt(h o = 0.1 G), was applied along the c-axis. X(to)=X'(to)+ ix"(to) was rcgistcred vs. temperature in the interval 5 X 10 -3 Hz <to/2w < 5 x 10 3 Hz. The zero field coolcd magnetization, Mzw,(T, H), was obtained by cooling the sample in zero field from T = 50 to x K. A magnetic field was then applied and the tcnt~;~,aturc stcpwisc increased; at each temperature Mzvc(T, H) was recorded. After reaching 50 K. the temperature was decreased and the field cooled magnetization, Mvc(T, H), was measured. This cxpcrimcnt was pcr- formed for ficlds in the interval 0 <H < 12 kG. In another ZFC experiment the sample was quenched to a tcmpcraturc" T,, below Tg and kept at constant temperature for a wait time, t,,. It was then subjected to a temperature cycling AT, and when Tm was rccov- ered a small magnetic field (3 G) was applied and the relaxation MzFc(Tm, t) was recorded. Figs. la and b show X'(to) and g"(to) vs. tempera- ture; the different curves correspond to different fre- quencies of the probing field, h. From the data shown in fig. la, we estimate TN = 32.8 K. The SG behaviour is reflected by the frequency dependence of X'(to) as well as the increase of X"(to) (fig. lb). The X"(w) vs. T curve exhibits two maxima, one, ~.,~, at T = 30 K and another at T = 10 K. According to results from neutron scattering experiments by Yoshizawa et al. [2], some spins will remain ordered, while others will form a SG phase at Tg. By combining results from Bragg and Z (a) ~ 56 top to bottom 560 5 I00 5 10 15 20 25 30 35 T(K) '2,°,° o .... . . . . . 5 10 15 20 25 30 35 T (K) Fig. 1. t'(w) and X"(w) vs. temperature. The different curves correspond to different frequencies of the applied ac field, h 0 = 0.1 G. TN = 32.8 K and T~ --- 25.8 K. (a) ~'(w) vs. T: (b) ~,"(to) vs. T. 11312-8853/92/$05.00 g~ 1992 - Elsevier Science Publishers B.V. All rights reserved