9 7og$y4- International Conference On Neutron Scattering, Toronto August Spin Dynamics of the reentrant spin glass Fe0.7A10.3.

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1 i.b? BNL CO-Nf- nternational Conference On Neutron Scattering, Toronto August og$y4- Spin Dynamics of the reentrant spin glass Fe0.7A10.3. S. Raymond a, W. Bao a, S.M. Shapiro a, K. Motoya b. a Brookhaven National Laboratory, Upton NY 11973, USA. b Science University of Tokyo, Noda 278, Japan. Abstract. nelastic neutron scattering experiments are reported to study magnetic excitations in a single crystal of the reentrant spin glass Fe0.7A10.3 near the (111) Bragg peak. n the ferromagnetic phase, the magnetic excitation spectrum is separated in two areas : at low 4,spin waves are propagating and at higher 4,the response is quasielasticlike. n the spin glass phase, the response is always quasielastic. Keywords : Spin glass, Fq.7Alo.3, Spin waves. Stephane Raymond Brookhaven National Laboratory, Physics Dpt., Bldg. 510B, PO BOX 5000 Upton, NY , USA Tel : (516) Fax : (516) raymond@mail.bnl.gov

2 4 The frozen spin configuration of spin glasses is a ground state often encountered in disordered magnetic systems. Beyond the prototype compounds studied during the past decades (e.g. CuMn, FeAu, FeCr [l]), interest has focused on new compounds and a continuing effort on the theoretical side [2]. n this framework, we reinvestigate the dynamics of a classical reentrant spin glass FeoJMO.3. This compound crystallizes in the Fe3A structure and is ferromagnetic below Tc=510 K and exhibits a reentrant spin glass phase at low temperatures. Previous studies of the spin dynamics were done near the forward direction [3] stressing the disappearance of spin waves when entering the spin glass phase. We extend this work to higher energy by studying the spin dynamics near the (111) Bragg peak using a single crystal. Experiments were performed on the triple axis instrument H8 of the Brookhaven High Flux Beam Reactor. Two spectrometer configurations were used in order to cover a wide range of Q-w space : (1) Constant initial energy of 4 mev with a beryllium filter in the incident beam and (2) Constant final energy of 14.7 mev with a PG filter in the scattered beam. The collimations were '-40 giving an incoherent width of respectively 0.09 and 0.8 mev for configurations (1) and (2) respectively. The sample was mounted in a displex with the [lo01 and [110] axis in the scattering plane. The dynamics were studied in the spin glass phase at 50 K and in the ferromagnetic phase at 300 K. Measurements performed with configuration (1) around Q=(l+q, l+q, l+q) allow us to extend the range of wavevectors q studied near the forward direction from 0.1 up to 0.15 A-1. n the ferromagnetic state, well defined excitations are observed in this region of q in agreement with the earlier study [3]. Peaks are observed either when doing constant E scans or constant Q scans. The situation is quite different in the spin glass state. Beside the appearance of a sharp central peak characteristic of the frozen order parameter, peaks are observed only when performing constant E scans. These peaks are located at the same position in 4 as the ones determined in the ferromagnetic state. However, the lineshape obtained in constant Q scans shows no peaks and is maximum at E=O. Constant Q scans 2 1

3 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, p mw, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

4 performed at Q=(0.94,0.94,0.94),using configuration (l), are shown in Fig. 1 at T=50 K and T=300 K. Using configuration (2), the measurements are extended to energies up to 10 mev. The striking point is that the measurements obtained at 50 K and 300 K are very similar. When performing a constant Q scan, the lineshape obtained is quasielastic for both temperatures. On another hand, peaks are obtained when performing constant E scans and again the peak position is the same at both temperatures. The energy scale is now ten times bigger that the one obtained at lowest Q. Typical constant E scans are shown comparatively in Fig.2 at T=50 K and T=300 K. All these results are summarized in Fig.3 which shows the "dispersion" of the spin excitations obtained at the lowest 4 in the ferromagnetic phase and the location of the maxima obtained when performing constant E scans in the different regions described. One key issue in this study is the description of the ferromagnetic state. The experimental situation is somewhat similar to studies on the dynamics of pure Fe and Ni [4]. n order to include the measurements performed with both configuration, two scenarios are considered. The first one considers that the low energy and high energy measurements probe the same dynamics with an extreme broadening of the spin waves with increasing 4.The second interpretation includes a cross section containing both spin waves and paramagnetic scattering. This is supported by diffuse scattering experiments [5] which show that only a fraction of the compound is ferromagnetic. More experiments are needed to conclude definitively. Acknowledgements Work at Brookhaven National Laboratory is supported by the Division of Material Sciences, U.S. D.O.E., under contract No. DE-AC0276CH

5 References : [l] For a review see S.M. Shapiro in Spin Waves and Magnetic Excitations 2, A S. Borovik-Romanov and S.K. Sinha eds, Elsevier Science [2] See e.g. references in S. Siillow et al., PRL 78 (354) 1997 and P. Gawiec and D. Grempel, Phys. Re. B 48 (71 14) [3] K. Motoya, S.M. Shapiro and Y.Muraoka, Phys. Rev. B 28 (6183) [4] J.P. Wicksted, P. Bani and G. Shirane, Phys. Rev. B 30 (3655) [5] J.W. Cable, L. David, R. Parra, Phys. Rev. B 16 (11132)

6 1 Figure caption : Fig.1 : Energy scans performed at Q=(0.94, 0.94, 0.94) at T=50 K (open circles) and T=300 K (solid circles). Lines are guides to the eyes. Fig.2 : Q scan performed at E=4 mev at T=50 K and T=300 K. The measurements performed at 50 K are enlarged for clarity. Lines are guide to the eyes. Fig. 3 : Location of the maxima obtained by performing constant q scans and constant E scans with configurations (1) and (2).

7 250 e(0.94, 0.94, 0.94) UH) SO Fig T= +T= -0.8 E i 4 mev 300K 50 K EN (mev) S. Raymond et al. 6

8 . 308 e 250 s 200 ; 1 EO 1 loo v 3 w * e z Fig. 2 o a q (r.1.u.) a2 a25 03 S. Raymond et al. 7

9 z w a05 ai q (r.1.u.) Fig. 3 S. Raymond et al. 8

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