Mössbauer study of the two-dimensional antiferromagnet (CH3NH3)2FeCl4

683 MOSSBAUER STUDY OF THE TWO-DIMENSIONAL ANTIFERROMAGNET (CH3NH3)2FeCI4* H. K E L L E R , W. KUNDIG Physics Institute, University of Zurich, Switzerland

and H. AREND Laboratory of Solid State Physics, ETH-Zurich, Switzerland M6ssbauer-effect measurements of the hyperfine interaction of S'Fe nuclei in the layered antiferromagnet (CH3NH3)2FeCL have been used to study the temperature dependence of the sublattice magnetization. Near the N6el temperature (2×lO-'
In the face-centred tetragonal structures of (C.H2,+,NH3)2MCL (M = Mn, Cu, Fe; n = 1, 2, 3 . . . ) the magnetic Fe E+ ions are arranged in perovskite-type layers separated by long organic chains of variable length. This class of compounds are ideal candidates in which to study the magnetic behaviour of quasi two-dimensional magnetic systems. 57Fe M6ssbauer spectra of powdered and single crystal samples were measured as a function of temperature in the range 6-300 K [1-3]. Extrapolation to 0 K yields the following parameters: the magnetic hyperfine field H ( 0 ) = 280 -+ 3 kOe, the quadrupole interaction ½eQVzz = 2.58 - 0.04 mm/s (the asymmetry parameter ,/ of the electric field gradient in this tetragonal structure is assumed to be zero), the angle 0 = ~.(Vzz, H) = 85 -+ 1°. The value of 0 indicates that the spins are almost parallel to the layers. Below the N6el temperature TN, V~z and 0 were found to be nearly temperature independent. TN was determined by two self-consistent methods: by an extrapolation of the measured hyperfine field H(T) to zero (fig. 1) and by the characteristic peak in the linewidth (fig. 2). The two methods give a value of TN =94.5-+0.1 K in agreement with susceptibility measurements [4]. If one assumes that the hyperfine field H(T) at the Fe 2+ nucleus is proportional to the sublattice magnetization M(T), one may write near

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Fig. 1. Hyperfine field as a function of temperature. The dashed line corresponds to the measured points and the solid line corresponds to the calculated power-law magnetization curve. I I I

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Physica 86-88B (1977) 683--684 © North-Holland

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Fig. 2. Temperature dependence of the linewidth near TN.

684 where H(0) is the hyperfine field at T = 0 K, D is a reduction factor matching this high-temperature formula to M(0), and /3 is the critical exponent. For most magnetic model systems, this power-law behaviour is limited to the temperature region 1 - T/TN < 4 x 10 2 [5]. A least square fit to the data in the range 2 × 10 4< 1 - T / T N < I O -2 yields (figs. 1 and 3.), D = 1.01 +0.02, 7", =94.46---0.02, /3 =0.146-+0.005.

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Table 1 Critical exponent /3 as predicted by various models

lsing XY

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0.125 0.13±0.03" -

0.31 0.33 0.36

a 2d-XY-model with quartic anisotropy (ref. [7]).

character is further supported by susceptibility measurements of Willet and Gerstein [4]. They found for the critical exponents, y = 1.67 and T' = 1.60 (2d-Ising: T = T' = 1.75). The present value f o r / 3 is smaller than those given earlier (/3 = 0.23 [2] and/3 = 0.22 [6]. This deviation may be explained by the fact that the earlier results were obtained for 1 - T I T , > 2 × 10-2, a temperature region not close enough to 7", to justify the use of the power law.

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A comparison of the experimentally determined value /3 and the theoretically predicted values given in table I indicates that (CH3NH3)2FeCI4 is a good example for a quasitwo-dimensional magnetic system with a dominant magnetic exchange within the layers. Since the spins are almost parallel to the layers, the 2d-XY-model is most applicable. The 2d

References [1] M.F. Mostafa and R.D. Willet, Phys. Rev. B4 (1971) 2213. [2] J.L. Schurter, R.G. Barnes and R.D. Willet, Proc. 20th Conf. on Magnetism and Magnetic Materials, San Francisco (1974). [3] H. Keller, W. Kiindig and H. Arend, Proc. of the M6ssbauer Conf., Vol. l, Cracow (1975). [4] R.D. Willet and B,G. Gerstein, Phys. Lett. 44A (1973) 153. [5] R.F. Wielinga, Progr. Low Temp. Phys., Vol Vl (NorthHolland, 1970) p. 333. [6] H. Keller, W. KiJndig and H. Arend, Helv. Phys. Acta 49 (1976) 148. [7l T. Schneider and E. Stoll, Phys. Rev. Lett. 36 (1976) 1501.