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Magnetic SANS in FeNiP glasses above the Curie temperature
Physica B 156 & 157 (1989) North-Holland. Amsterdam
MAGNETIC A.R.
198-200
SANS IN FeNiP GLASSES
YAVARI’,
D. BIJAOUI’,
M.C.
ABOVE
THE CURIE TEMPERATURE
BELLISSENT-FUNEL’
and P. DESRE’
‘LTPCM-CNRS, UA29, Institut National Polytechniyue de Grenoble. BP 7-f. Domaine 38402 Saint Martin d’Ht?res Cedex. France ‘Laboratoire Lton Brillouin, CEA Saclay, 91191 Gif sur Yvette, France
Universitaire,
It is shown in two different FeNiP alloys that magnetic scattering of neutrons at small angles persists at temperatures 7 well above the Curie temperature T,.. This is taken as evidence of existence of zones of concentration fluctuations with higher magnetic moments that continue to produce coherent magnetic neutron scattering at T > T,. of the overall alloy composition.
1. Introduction We have measured coherent small-angle neutron scattering (SANS) in amorphous alloys due to various origins including magnetic scattering in ferromagnetic glasses [ 11, nuclear scattering due to concentration and density flucutations [2,3] and the surface scattering [4,5]. Scattering due to concentration fluctuations can be weak due to small contrast, Ab, between the average coherent scattering lengths b, and 6, of the particles and the surrounding matrix. When atoms such as transition metals carry a magnetic moment, the effective coherent scattering crosssection and length are significantly larger below the Curie temperature T,. However in this same T-range (T < T,), at sufficiently low angles, strong scattering occurs due to Bloch walls acting as magnetic structural defects [l] and although in general this scattering is independent of that due to atomic concentration fluctuations, the high angle tail of the Bloch wall scattering can mask their contribution. Both contributions disappear under strong saturating magnetic fields. Many papers have been published on phase separation phenomena and concentration fluctuations (see for example [6,7]). Small-angle X-ray (SAXS) scattering [S], Auger electron (AES spectroscopy [9], FIM-atom probe [lo] and high resolution electron microscopy and STEM [ll] have been used for such studies. In these systems, SAXS is often of low sen0921-4526/89/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
sitivity [6, 121, AES is well adapted for surface studies but less so for fluctuation zones in the bulk, FIM observations in amorphous alloys have so far remained controversial [12, 131 and STEM observations are most fruitful for amorphous particles larger than 20 A [II]. In this context we have measured coherent neutron scattering in some ferromagnetic amorphous alloys below and above T, because while Bloch walls disappear at T,., a contribution from lluctuation zones with local concentrations corresponding to higher T,.‘s may subsist above the overall Curie temperature. Such zones remain ferro- or supraparamagnetic at T > T, of the alloy. For this study we choose the FeNiP amorphous system both because of previous reports of phase separation in this system [S, 9, 141 and because of the strong concentration dependence of T, which varies sharply with the metaliphosphorous and Fe/Ni ratios. 2. Experimental Since Fe and Ni have nearly equal atomic radii and form similar crystal structures with P (with metallic trigonal prism motifs centered by Patoms), variation of the Fe/Ni ratio at constant P content is not expected to modify the atomic structure of the amorphous phase. We therefore prepared several FeNiP amorphous alloys from P-rich, Fe-P ingots provided by IMPHY S.A. and high purity nickel. The Fe-P alloy contained B.V.
A.R.
Yavari et al.
I Magnetic SANS in FeNiP glasses
residual Cr and V which were thus introduced in the alloys. In this paper we discuss results concerning amorphous Fe28Ni50P19(CrV)3 and 42.5P19(CrV)3). As expected, these alloys Fe,,.,Ni crystallise into the same structure (y(Fe-Ni) and (FeNi),P above 680 K) [15]. The Curie temperatures T,, of the amorphous phases are 265 K and 330 K, respectively, for the phases containing 29 and 35.5% Fe as measured by thermomagnetic balance [16]. 1 cm wide amorphous tapes were prepared by planar-flow casting under helium gas [17] and checked by X-ray diffraction on both faces. The neutron scattering experiments were conducted on the PACE spectrometer at CEASaclay with a wavelength A = 7 A and a detector-sample distance of 3 meters. 1 x 2 cm2 sheets of the 30 urn thick tapes were piled up to a thickness of 0.5 mm and placed perpendicular to the beam. The intensities were corrected for background due to sample holder and furnaces (under vacuum or inert gas), for non-magnetic incoherent scattering and for absorption. Normalised coherent scattering cross-sections, Z(Q), in barns were obtained using the incoherent scattering of water. For full details see [15]. 3. Results and discussion Fig. 1 shows Z(Q) versus Q = 41r sin 8/A for the two FeNiP alloys at 290 K. Consistent with the higher T, of the Fe-rich alloys, its low angle scattering is much stronger. Fig. 2 shows Z(Q) versus Q obtained for the alloy with 28 at.% Fe and T, = 265 K at various temperature T > T,. It is seen that Z(Q) continues to decrease with
Fig. 1. Coherent scattering intensity I(Q) versus wave vector Q for two as-quenched amorphous FeNiP compositions: (a) ferromagnetic and (b) paramagnetic at 290 K.
199
Fig. 2. I(Q) versus Q for amorphous FeNiP with 35.5 at.% Fe in the paramagnetic T range during cooling after relaxation annealing at T = 473 K.
increasing T. Fig. 3 shows Z(Q) versus Q for the amorphous tape with 35.5 at.% Fe and T, 2: 330 K at various temperatures T > T,. Again it is seen that Z(Q) continues to decrease with increasing T well above T,. To avoid any relaxation effects which results in AT, = 5 K increase in T,, the measurements of figs. 2 and 3 were taken after relaxation annealing at 480 K (the alloys crystallise above 680 K). The results presented in figs. 2 and 3 and other results (not shown) clearly indicate that although the magnetic domain structure disappears at T,, significant magnetic scattering subsists at T > T,. Figs. 4 and 5 show further details of the variation of Z(Q) with T for various scattering vectors Q. These results indicate that concentration fluctuation zones corresponding to compositions with higher T, and higher magnetic moments exist in these amorphous alloys. Since T, drops sharply
Fig. 3. I(Q) versus Q for amorphous FeNiP with 28 at.% Fe in the paramagnetic range after relaxation annealing at 473 K.
A.K.
200
Yavari et al. I Magnetic SANS in FeNiP glasses
150
h
Fig. wave Fig. wave
35.5at"dFe
:f
\
28at8
Fe
4. (Left) scattering intensity I(Q) versus 7‘ > T, u,,c = 333 K for annealed amorphous FeNiP with 35.5 at.‘% Fe at various vectors. 5. (Right) scattering intensity I(Q) versus 7‘ > r, ,,1,1 = 270 K for annealed amorphous FeNiP with 2X at.8 Fe at various vectors.
with increasing P and Ni, these zones are likely to be rich in Fe. We have obtained independent evidence for the presence of such zones by STEM observations but contrary to reports by other authors, these zones exist in the as-quenched amorphous phase and do not evolue with post-quench annealing. Consequently, they cannot be directly responsible for the thermal embrittlement of these alloys as we have discussed earlier (161. Apparently, they form during the quench in the supercooled liquid alloy at T > T,. References [l] A.R. Yavari. Rapidly Quenched Metals, V, S. Steeb and H. Warlimont, eds. (Elsevier Science Publ.. Amsterdam, 1985). pp. 445-500. [2] A.R. Yavari and M. Maret, C.R. Acad. Sci. 296 (1983) 165. [3] A.R. Yavari, .I. de Physique Lett. 46 (1985) 18Y. [4] A.R. Yavari. J.C. Joud and M.C. Bellissent-Funel. Phys. Lett. A 105 (19X4) XX.
151A.R.
Yavari, P. Desre and P. Chieux. Atomic Transport and Defects in Metals by Neutron Scattering. C. Janot et al., eds. (Springer. Berlin. 1986). pp. 42-49. 161A.R Yavari, K. Osamura, H. Okuda and Y. Amemia, Phys. Rev. B 37 (1988) 7750. 171 A.R. Yavari. Acta Metall. (July 1988). Mat. Sci. IXI J.L. Walter. P.G. Legrand and F.E. Luborsky. Eng. 29 (1977) 161. Mat. Sci. 191 J.L. Walter. F. Bacon and F.E. Luhorsky. Eng. 24 (1076) 239. [lOI J. Piller and P. Haasen. Acta Metall. 30 (19X2) 1. and H.R. Sinning, [ItI A.R. Yavari, S. Hamar-Thibault Scripta Metall. (August 1988). 1121A.R. Yavari, J. Mat. Res. 1 (1986) 746. Acta Metall. 33 (1YXS) 2185. [I31 T.N. Wu and F. Spaepen. [l41 H.S. Chen, Mat. Sci. Eng. 26 (1976) 79. II.51 D. Bijaoui. doctoral thesis. lnstitut National Polytechnique de Grenoble (1987). 114 A.R. Yavari. D. Bijaoui, M.C. Bellissent. P. Chteux and M. Harmelin, Magnetic Properties of Amorphous Metals, A. Hernando et al.. eds. (Elsevier Science Publ., Amsterdam. 1987). pp. 336-9. of I171 A.R. Yavari and P. Desre. Magnetic Properties Amorphous Metals. A. Hernando et al.. eds. (Elsevier Science Publ., Amsterdam. 19X7). pp. 6-14.
MAGNETIC A.R.
198-200
SANS IN FeNiP GLASSES
YAVARI’,
D. BIJAOUI’,
M.C.
ABOVE
THE CURIE TEMPERATURE
BELLISSENT-FUNEL’
and P. DESRE’
‘LTPCM-CNRS, UA29, Institut National Polytechniyue de Grenoble. BP 7-f. Domaine 38402 Saint Martin d’Ht?res Cedex. France ‘Laboratoire Lton Brillouin, CEA Saclay, 91191 Gif sur Yvette, France
Universitaire,
It is shown in two different FeNiP alloys that magnetic scattering of neutrons at small angles persists at temperatures 7 well above the Curie temperature T,.. This is taken as evidence of existence of zones of concentration fluctuations with higher magnetic moments that continue to produce coherent magnetic neutron scattering at T > T,. of the overall alloy composition.
1. Introduction We have measured coherent small-angle neutron scattering (SANS) in amorphous alloys due to various origins including magnetic scattering in ferromagnetic glasses [ 11, nuclear scattering due to concentration and density flucutations [2,3] and the surface scattering [4,5]. Scattering due to concentration fluctuations can be weak due to small contrast, Ab, between the average coherent scattering lengths b, and 6, of the particles and the surrounding matrix. When atoms such as transition metals carry a magnetic moment, the effective coherent scattering crosssection and length are significantly larger below the Curie temperature T,. However in this same T-range (T < T,), at sufficiently low angles, strong scattering occurs due to Bloch walls acting as magnetic structural defects [l] and although in general this scattering is independent of that due to atomic concentration fluctuations, the high angle tail of the Bloch wall scattering can mask their contribution. Both contributions disappear under strong saturating magnetic fields. Many papers have been published on phase separation phenomena and concentration fluctuations (see for example [6,7]). Small-angle X-ray (SAXS) scattering [S], Auger electron (AES spectroscopy [9], FIM-atom probe [lo] and high resolution electron microscopy and STEM [ll] have been used for such studies. In these systems, SAXS is often of low sen0921-4526/89/$03.50 @ Elsevier Science Publishers (North-Holland Physics Publishing Division)
sitivity [6, 121, AES is well adapted for surface studies but less so for fluctuation zones in the bulk, FIM observations in amorphous alloys have so far remained controversial [12, 131 and STEM observations are most fruitful for amorphous particles larger than 20 A [II]. In this context we have measured coherent neutron scattering in some ferromagnetic amorphous alloys below and above T, because while Bloch walls disappear at T,., a contribution from lluctuation zones with local concentrations corresponding to higher T,.‘s may subsist above the overall Curie temperature. Such zones remain ferro- or supraparamagnetic at T > T, of the alloy. For this study we choose the FeNiP amorphous system both because of previous reports of phase separation in this system [S, 9, 141 and because of the strong concentration dependence of T, which varies sharply with the metaliphosphorous and Fe/Ni ratios. 2. Experimental Since Fe and Ni have nearly equal atomic radii and form similar crystal structures with P (with metallic trigonal prism motifs centered by Patoms), variation of the Fe/Ni ratio at constant P content is not expected to modify the atomic structure of the amorphous phase. We therefore prepared several FeNiP amorphous alloys from P-rich, Fe-P ingots provided by IMPHY S.A. and high purity nickel. The Fe-P alloy contained B.V.
A.R.
Yavari et al.
I Magnetic SANS in FeNiP glasses
residual Cr and V which were thus introduced in the alloys. In this paper we discuss results concerning amorphous Fe28Ni50P19(CrV)3 and 42.5P19(CrV)3). As expected, these alloys Fe,,.,Ni crystallise into the same structure (y(Fe-Ni) and (FeNi),P above 680 K) [15]. The Curie temperatures T,, of the amorphous phases are 265 K and 330 K, respectively, for the phases containing 29 and 35.5% Fe as measured by thermomagnetic balance [16]. 1 cm wide amorphous tapes were prepared by planar-flow casting under helium gas [17] and checked by X-ray diffraction on both faces. The neutron scattering experiments were conducted on the PACE spectrometer at CEASaclay with a wavelength A = 7 A and a detector-sample distance of 3 meters. 1 x 2 cm2 sheets of the 30 urn thick tapes were piled up to a thickness of 0.5 mm and placed perpendicular to the beam. The intensities were corrected for background due to sample holder and furnaces (under vacuum or inert gas), for non-magnetic incoherent scattering and for absorption. Normalised coherent scattering cross-sections, Z(Q), in barns were obtained using the incoherent scattering of water. For full details see [15]. 3. Results and discussion Fig. 1 shows Z(Q) versus Q = 41r sin 8/A for the two FeNiP alloys at 290 K. Consistent with the higher T, of the Fe-rich alloys, its low angle scattering is much stronger. Fig. 2 shows Z(Q) versus Q obtained for the alloy with 28 at.% Fe and T, = 265 K at various temperature T > T,. It is seen that Z(Q) continues to decrease with
Fig. 1. Coherent scattering intensity I(Q) versus wave vector Q for two as-quenched amorphous FeNiP compositions: (a) ferromagnetic and (b) paramagnetic at 290 K.
199
Fig. 2. I(Q) versus Q for amorphous FeNiP with 35.5 at.% Fe in the paramagnetic T range during cooling after relaxation annealing at T = 473 K.
increasing T. Fig. 3 shows Z(Q) versus Q for the amorphous tape with 35.5 at.% Fe and T, 2: 330 K at various temperatures T > T,. Again it is seen that Z(Q) continues to decrease with increasing T well above T,. To avoid any relaxation effects which results in AT, = 5 K increase in T,, the measurements of figs. 2 and 3 were taken after relaxation annealing at 480 K (the alloys crystallise above 680 K). The results presented in figs. 2 and 3 and other results (not shown) clearly indicate that although the magnetic domain structure disappears at T,, significant magnetic scattering subsists at T > T,. Figs. 4 and 5 show further details of the variation of Z(Q) with T for various scattering vectors Q. These results indicate that concentration fluctuation zones corresponding to compositions with higher T, and higher magnetic moments exist in these amorphous alloys. Since T, drops sharply
Fig. 3. I(Q) versus Q for amorphous FeNiP with 28 at.% Fe in the paramagnetic range after relaxation annealing at 473 K.
A.K.
200
Yavari et al. I Magnetic SANS in FeNiP glasses
150
h
Fig. wave Fig. wave
35.5at"dFe
:f
\
28at8
Fe
4. (Left) scattering intensity I(Q) versus 7‘ > T, u,,c = 333 K for annealed amorphous FeNiP with 35.5 at.‘% Fe at various vectors. 5. (Right) scattering intensity I(Q) versus 7‘ > r, ,,1,1 = 270 K for annealed amorphous FeNiP with 2X at.8 Fe at various vectors.
with increasing P and Ni, these zones are likely to be rich in Fe. We have obtained independent evidence for the presence of such zones by STEM observations but contrary to reports by other authors, these zones exist in the as-quenched amorphous phase and do not evolue with post-quench annealing. Consequently, they cannot be directly responsible for the thermal embrittlement of these alloys as we have discussed earlier (161. Apparently, they form during the quench in the supercooled liquid alloy at T > T,. References [l] A.R. Yavari. Rapidly Quenched Metals, V, S. Steeb and H. Warlimont, eds. (Elsevier Science Publ.. Amsterdam, 1985). pp. 445-500. [2] A.R. Yavari and M. Maret, C.R. Acad. Sci. 296 (1983) 165. [3] A.R. Yavari, .I. de Physique Lett. 46 (1985) 18Y. [4] A.R. Yavari. J.C. Joud and M.C. Bellissent-Funel. Phys. Lett. A 105 (19X4) XX.
151A.R.
Yavari, P. Desre and P. Chieux. Atomic Transport and Defects in Metals by Neutron Scattering. C. Janot et al., eds. (Springer. Berlin. 1986). pp. 42-49. 161A.R Yavari, K. Osamura, H. Okuda and Y. Amemia, Phys. Rev. B 37 (1988) 7750. 171 A.R. Yavari. Acta Metall. (July 1988). Mat. Sci. IXI J.L. Walter. P.G. Legrand and F.E. Luborsky. Eng. 29 (1977) 161. Mat. Sci. 191 J.L. Walter. F. Bacon and F.E. Luhorsky. Eng. 24 (1076) 239. [lOI J. Piller and P. Haasen. Acta Metall. 30 (19X2) 1. and H.R. Sinning, [ItI A.R. Yavari, S. Hamar-Thibault Scripta Metall. (August 1988). 1121A.R. Yavari, J. Mat. Res. 1 (1986) 746. Acta Metall. 33 (1YXS) 2185. [I31 T.N. Wu and F. Spaepen. [l41 H.S. Chen, Mat. Sci. Eng. 26 (1976) 79. II.51 D. Bijaoui. doctoral thesis. lnstitut National Polytechnique de Grenoble (1987). 114 A.R. Yavari. D. Bijaoui, M.C. Bellissent. P. Chteux and M. Harmelin, Magnetic Properties of Amorphous Metals, A. Hernando et al.. eds. (Elsevier Science Publ., Amsterdam. 1987). pp. 336-9. of I171 A.R. Yavari and P. Desre. Magnetic Properties Amorphous Metals. A. Hernando et al.. eds. (Elsevier Science Publ., Amsterdam. 19X7). pp. 6-14.