Magnon dispersion and magnetic phase diagram of MnWO4

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Physica B 234-236 (1997) 560-563

Magnon dispersion and magnetic phase diagramof MnWO4 H. E h r e n b e r g * , H. W e i t z e l , H. F u e s s Technische Hochschule Darmstadt, Fachbereich Materialwissenschaft D-64287 Darmstadt, Germany

Abstract The magnetic phase diagram of MnWO4 is derived for several field directions from the temperature and field dependence of magnetization, the specific heat in applied fields and by neutron diffraction. Furthermore, we report about magnon dispersion in MnWO4, determined by inelastic neutron scattering and calculated analytically in spin-wave approximation.

Keywords: Spin waves; Antiferromagnetism; Phase transitions; Magnetic anisotropy

For the compound MnWO4 three magnetic phases AF1, AF2 and AF3 are known in zero field [1]. The magnetic structure of the ground state AF1 is collinear, and the magnetic phase diagram for an external field parallel to the easy direction was previously reported up to 5.5 T [2]. MnWO4 is an interesting and promising model compound for the investigation of critical phenomena and the simulation of phase diagrams: On the one hand, at least three magnetically ordered phases exist with both temperature and field-induced transitions between them; on the other, the crystal structure is rather simple and all Mn-ions are situated on one crystallographic site of twofold rotational symmetry. Therefore, the investigation of magnetism in MnWO4 has been extended to higher fields, other field directions and dynamic properties. In Fig. 1 we present the phase diagram for a field parallel to the easy direction of AF1 up to 20 T, determined from magnetization studies, neutron diffraction and specific heat measurements. Another exposed field direction is [0 ! 0]: While in both phases AF1 and AF3 the magnetic structures are collinear with the same easy direction perpendicular to [0 1 0], a component of the ordered magnetic moment in * Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved PI! S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 11 8 0 - 5

[0 1 0]-direction exists only in the intermediate phase AF2. The magnetic phase diagram for this direction is shown in Fig. 2. It is of particular interest to study further in which way both topologies lead into each other by the variation of field direction within the plane spanned by [0 1 0] and the easy direction [3, 4]. The magnon dispersion was determined by inelastic neutron scattering for two pronounced directions of momentum transfer, q = (-t/,0,2t/) and (0, ~,0), preserving mirror planes and rotational axes, respectively. In spin-wave approximation the dispersion relations can be calculated analytically based on a model including Heisenberg coupling by superexchange and a uniaxial anisotropy. Exchange integrals and anisotropy constant are deduced by fitting calculated dispersion to observed excitations, see Fig. 3. This parameter set can further be used to compute the spin deviations at T = 0 K to 6S = 0.160 and to calculate the propagation vector k of the magnetic structure just below the ordering temperature. The derived value k=(-0.285,1,0.445) is in good agreement with the observed one k (AF3) = (-0.2145(30), 1,0.4580(35)). Furthermore, the temperature dependence of magnetic excitations at the F-point has been examined [3, 5].

H. Ehrenberg et al. / Physica B 234-236 (1997) 560-563

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We gratefully acknowledge financial support by the Bundesminister ftir Bildung und Forschung (03FU3DAR) and by the European Community (HCM) as well as allocated measuring time at 4F2, Laboratoire Lron Brillouin, Saclay, at the VSM of the Interdisciplinary Research Center in Superconductivity, Cambridge, and at the High Magnetic Field Laboratory, Grenoble. References

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Fig. 3. Observed excitations at T < 3.0 K and best fitting calculated dispersion in MnWO4 for q = (-r/, 0, 2r/) and (0, 4, 0), respectively.

[1] G. Lautenschl[iger, H. Weitzel, T. Vogt, R. Hock, A. Brhm, M. Bonnet and H. Fuess, Phys. Rev. B 48 (1993) 6087. [2] G. Lautenschl~iger, H. Weitzel, R. Hock and P. Burlet, ILLAnnual Report 1993, Grenoble, France (1994) p. 53. [3] H. Ehrenberg, Thesis, TH Darmstadt, Germany (1996). [4] H. Ehrenberg, H. Weitzel, C. Heid, H. Fuess, G. Wltschek, T. Kroener, J. van Tol and M. Bonnet, J. Phys.: Condens. Matter, in print. [5] H. Ehrenberg, H. Weitzel, H, Fuess and B. Hennion, Phys. Rev. B, in preparation.