Physica 126B (1984) 469- 470 469 North-Holland, Amsterdam MAGNETIC PHASE DIAGRAM OF Cd1_xMnxSeBELOWTHE NEARESTNEIGHBORPERCOLATIONLIMIT M.A. NOVAK, O.G. SYMKO, D.J. ZHENG, and S. OSEROFF + Department of Physics, University of Utah, Salt Lake City, UT 84112, + Department of Physics, San Diego State University, San Diego, CA 92180. Magnetization measurements down to 10 mK of Cdl-xMnxSe at Mn concentrations below the nearest- neighbor percolation limit show spin glass behavior. The previously accepted phase diagram is revised, showing a x2 dependence at low x. Such behavior is attributed to neighbors further than nearest-neighbors and Bloembergen-Rowland type of indirect exchange. .08 Cd1_xMnxSe belongs to a group of materials known as dilute magnetic semiconductors, where above a Mn concentration x of ~ 0.20, spin glass behavior is observed(1, 2) below a spin i.o0 freezing temperature Tq. Most of the suscepti- bility data have been ~btained at temperatures above 1.5 K where a linear extrapolation of Tg intersects the x-axis at a value near the per- ,~.o4 colation critical point of 0.20 calculated for nearest-neighbors (n.n.). This finding, together o with the fact that the interaction between n.n. ~o2 is known to be antiferromagnetic and much lar- ger than the interaction with further neighbors, led to the suggestion~~, 3) that dilute magne- tic semiconductors behave like spin-glasses o~ above x = 0.20. Such behavior was presented on a magnetic phase diagram where below 0.20 the system was believed to be paramagnetic down to 0 K. Also, measurements of the specific heat and susceptibility at low x could not fit in terms of a simple spin-cluster model when a random distribution of Mn with only n.n. inter- actions was assumed (3). Hence we have investi- gated for this system the low concentration re- 2o gion at very low temperatures. Powder samples for x = 0.15, 0.10, 0.05, and 0.01 were studied between 4 K and I0 mK in a I~ 3He-4He dilution refrigerator using a SQUIDmag- netometer; they were cooled in a field of i Oe. Figure 1 shows the behavior of the magnetization as a function of temperature, the arrows indi- ~Q cating T a for each concentration. Fig, 2 shows the variation of Tg with x, joining in to the generally accepted phase diagram based on n.n. percolation. The inset in Fig. 2 shows the de- 5 pendence of Tg.on x2. The new complete magnetic phase diagram in Fig. 2 s h ~ ~ e r mecha- nisms than the proposed short range antiferro- magnetic n.n. interaction should be invoked, o such as the inclusion of further neighbors be- yond the nearest-neighbors. Calculations(4) for a f.c.c, lattice have shown that for second n.n. the percolation concentration becomes 13.6 % while for third n.n. it is 6.1%, decreasing as the number of sites considered is increased. \, \ \.,\ ".,., ............ .__...j'~, ,__.~ 0.4 O.a T(K) I 4°_.., ~,~,~,~. • X =0.15 ............... ~'~ • X =0.10 "~'~'-'~ ~ ......... " o.5 1.o 1.5 T(K) FIGURE 1 Magnetization as a function of temperature for various x. i ,~. ~ Cd,_~M.~(Se 'lM,x.d 2"~1 / ~C, tystal '~ "~/ ~ °°J o,, 0o, x" .o.°o,.,,o. sa , r ' , 0-2 0.4 o~e x FIGURE 2 New Magnetic Phase Diagram of Cdl-xMnxSe. Inset shows x2 dependence of Tg below x = 0.20. The data above x = 0.20 is from Ref. 2. 0378-4363/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)