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Structural and magnetic study of mechanically alloyed Fe30Cr70 by neutron thermo-diffractometry and magnetization measurements
Journal of Non-Crystalline Solids 354 (2008) 5156–5158
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Structural and magnetic study of mechanically alloyed Fe30Cr70 by neutron thermo-diffractometry and magnetization measurements A. Fernández-Martínez a,b, D. Martínez-Blanco c, M.J. Pérez d, G.J. Cuello b, G.R. Castro e, P. Gorria d, J.A. Blanco d,* a
LGIT, University of Grenoble and CNRS, Maison des Geosciences, B.P. 53, 38041 Grenoble cedex 9, France Institut Laue-Langevin, BP 156, 6 rue Jules Horowitz, 38042 Grenoble cedex 9, France c Unidad de Magnetometría, SCT’s, Universidad de Oviedo, Julián Clavería 8, 33006 Oviedo, Spain d Departamento de Física, Universidad de Oviedo, Calvo Sotelo, s/n, 33007 Oviedo, Spain e European Synchrotron Radiation Facility, B.P. 220, 38043 Grenoble, France b
a r t i c l e
i n f o
Article history: Available online 22 October 2008 PACS: 61.05.fm 75.50.Bb 81.20.Ev Keywords: Neutron diffraction/scattering Magnetic properties
a b s t r a c t The temperature dependence of magnetization together with neutron powder thermo-diffraction show that nominal ball milled Fe30Cr70 for 110 h is formed of two mixed phases (Fe20Cr80 + Fe), both of them with body centered cubic crystal structure and very close values for the lattice parameter (2.87 Å). On heating above 900 K, the system exhibits an irreversible structural transformation, giving rise to the homogenization of the material, and then recovering the well-defined starting composition Fe30Cr70. Subsequent heating-cooling neutron thermo-diffraction cycles do not show any additional transformation, thus explaining the overlapping M(T) curves after the first heating. Ó 2008 Elsevier B.V. All rights reserved.
Binary FexCr100x solid solutions have a body centered cubic (bcc) crystal structure at room temperature in the whole compositional range, with a lattice parameter close to those of Fe (2.8665 Å) and Cr (2.8847 Å). These alloys display a complex magnetic phase diagram, especially in the Cr-rich compositional range where coexistence of spin–glass, ferro- or antiferro-magnetism can occur [1–3]. On the other hand, mechanical alloying technique allows synthesizing single phase Fe-TM (TM = Cr, Ni, Cu) binary compounds with nanometer grain size and displaying a rich variety of magnetic behaviors [4–6]. In the case of mechanically alloyed Fe100xCrx, the magnetic response can be largely influenced by the disordered intergranular zone, which is paramagnetic at room temperature [7–9]. In this paper we report on the structural and magnetic changes occurring in mechanically alloyed Fe30Cr70 in a large temperature range (2–1100 K), through the combination of magnetization vs. temperature measurements, M(T) curves, and neutron powder thermo-diffraction (NPTD). These techniques have revealed very useful in understanding the correlation between structural changes and magnetic behavior in metastable Fe-based alloys [4–6,10]. Besides that, preliminary X-ray absorption experiments were carried out at room temperature in the Cr K-edge, at * Corresponding author. Tel.: +34 985102950; fax: +34 985103324. E-mail address: [email protected] (J.A. Blanco). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.06.114
the BM25 beamline (SpLine) at the European synchrotron radiation facility (ESRF, Grenoble) in order to obtain structural information around Cr atoms through the analysis of the extended X-ray absorption fine structure (EXAFS) spectrum. Powders of elemental Cr and Fe (99.98% purity) were milled for 110 h to a nominal composition Fe30Cr70 using a RETSCH PM400/4 high-energy ball mill. M(T) curves were measured in the temperature range 2–1100 K, by means of a PPMS-14T magnetometer (2350 K) and a Faraday susceptometer (3001100 K) under an applied magnetic field, l0H = 0.1 T. NPTD experiments were performed at the D20 two-axis diffractometer of the Institut Laue– Langevin (Grenoble) using a neutron wavelength of 1.867 Å and a high take-off angle of 120° for the Ge(1 1 5) monochromator crystal in order to increase resolution [11], which is needed due to the very close values for the cell parameters of Fe and Cr. Two heating-cooling cycles from room temperature (RT) to 1100 K were performed using a vanadium furnace in order to monitor the temperature-induced structural transformations. The heating rate of the first heating was of 2 K/min in the region of temperatures up to 900 K and of 1 K/min in the region from 900 K to 1100 K. Diffraction patterns were collected every 5 min. The first cooling was done at a rate of 5 K/min and it was followed by a second heating at a rate of 10 K/min and a final cooling. Rietveld refinement of the diffraction patterns was performed using the FULLPROF package
A. Fernández-Martínez et al. / Journal of Non-Crystalline Solids 354 (2008) 5156–5158
[12]. The microstructural analysis was performed by assuming an anisotropic size broadening model, expressed in terms of spherical harmonics, to calculate the average coherent domain size and an anisotropic broadening model, based on the Stephens’ notation, accounting for strains within the crystallites [13]. It has been previously reported a value of 260 K for the Curie temperature, TC, of the Fe30Cr70, alloy [2]. In Fig. 1 the M(T) curves of the as-milled sample on the first heating and cooling processes are shown. On heating from 2 K, the magnetization exhibits a decrease of around 25% of its value above 40 K, followed by a slow decrease up to 900 K, where a clear change in the M(T) slope is observed, reaching a vanishing M value at around 1050 K (see Fig. 1). On cooling from 1100 K, the M(T) curve does not overlap the heating one at any temperature, the magnetization slowly increases its value, exhibiting a clear change in the slope below 400 K, and reaching a value at 2 K which is near double to that of the as-milled sample. Further heating–cooling cycles leads to overlapping M(T) curves displaying a similar trend to that of the cooling curve shown in Fig. 1. In order to correlate this irreversible M(T) behavior with possible high temperature-induced structural transformations NPTD experiments were performed. Neutron diffraction patterns of the as-milled sample show four broad intense peaks that can be ascribed to a bcc phase with a cell parameter around 2.88 Å. Besides that, the microstructural analysis of the as-milled sample yields an apparent coherent domain size of htiRT = 9 ± 1 nm, with a maximum strain of e = 1.0 ± 0.1%. Heating above 900 K, the strain is totally relaxed and the crystalline grain size increases above the diffraction detection limit. However, the peaks observed in the diffraction pattern corresponding to the as-milled sample present a noticeable asymmetry (see inset in Fig. 2a), suggesting that two different bcc crystalline phases coexist in the sample, with very close values for the cell parameter [2.8875(2) Å and 2.8679(5) Å]. This fact led us to assume that some of the initial bcc-Fe has not been alloyed. Rietveld refinement yields a percentage of 86(2)% of a bcc-FeCr phase and a 14(2)% of a bcc-Fe, hence, the composition of the initial FeCr phase in the as-milled alloy could be close to Fe20Cr80. The latter could explain the M(T) curve of the as-milled alloy in the following terms. In the temperature range between 2 K and 300 K, no signature for the TC around 260 K is observed (see Fig. 1), and the trend
Fig. 1. Magnetization vs. temperature curve measured by combining a Faraday susceptometer and a PPMS magnetometer under an applied magnetic field l0H = 0.1 T.
5157
Fig. 2. Powder neutron diffraction patterns of the nominal Fe30Cr70 sample at 300 K: (a) as-milled, and (b) after being heated up to 1100 K. Observed (points) and calculated (solid line) patterns, positions of the Bragg reflections represented by vertical bars, and the observed-calculated difference depicted at the bottom of the figure. In a (b) the first raw of marks below the Bragg peaks corresponds to Fe20Cr80 (Fe30Cr70), while the second one is associated with the bcc-Fe (an oxide-impurity phase [<5%]). The insets show the (2 1 1) reflection, with a shoulder in a) the asmilled sample evidencing the existence of a bcc-Fe phase.
followed by the M(T) curve below 100 K resembles that for a spinglass regime. It is worth noting that Cr-rich alloys with a percentage around 80% in Cr exhibit spin-glass behavior [3], hence, these findings support the existence of Fe20Cr80 spin-glass phase, even comparison between results from ball milled and melted samples need to be done with care. However, a significant magnetization value is measured above 200 K, which comes from the bcc-Fe phase. Further heating up to around 900 K shows a slow decrease of the magnetization, and then, a clear change of slope is observed, reaching to a vanishing value of M at around 1040 K (see Fig. 1). On the other hand, neutron diffraction patterns above 875 K show the progressive decrease of the amount of bcc-Fe phase. This fact suggests that together with the thermally-induced microstructural relaxation, the structural transformation, Fe20Cr80 + bcc-Fe ) Fe30Cr70, takes place. This feature is not surprising, owing that at least 400 h of milling time is needed to reach a homogeneous a-phase solid solution. Hence, lower milling times would give rise to heterogeneous samples, with intra granular composition fluctuations 10% in a scale of a few nanometers [7,9]. On the other hand, the origin of the linear decrease of M between 900 K and 1040 K cannot be attributed to the existence of any ferro-to-paramagnetic phase transition. On cooling from 1100 K, M(T) curve looks like the typical one for a ferromagnetic material with a value of TC = 275 ± 10 K (see Fig. 1). Subsequent heating–cooling cycles exhibit reversible M(T) curves, suggesting no further structural transformations in the material. The latter is confirmed by NPTD experiments.
5158
A. Fernández-Martínez et al. / Journal of Non-Crystalline Solids 354 (2008) 5156–5158
Preliminary analysis of the EXAFS spectrum corresponding to the as-milled sample indicates a decrease in the coordination number, which could be explained as a result of a high density of defects induced by the mechanical alloying process [14] and/or the existence of ill-coordinated atoms at the nanograins’ surfaces [15]. Additional absorption experiments in the heat-treated samples as well as in the Fe K-edge are needed in order to elucidate the decrease in the coordination number upon milling. In summary, the combination of neutron powder thermo-diffraction and magnetization vs. temperature measurements shows that: (i) in the as-milled powders, with nominal composition Fe30Cr70, a Cr-rich Fe20Cr80 together with a bcc-Fe phases coexist. (ii) heating the as-milled alloy up to 1100 K gives rise to a thermal relaxation of the microstructure and to the Fe20Cr80 + bcc-Fe ) Fe30Cr70 reversal phase transformation. (iii) the Fe30Cr70 high temperature stabilized phase does not exhibit any further structural transformation in subsequent heating–cooling cycles. A more detailed study using Mössbauer spectroscopy at different temperatures with the aim of elucidating the above questions
about the heterogeneity of the samples and the existence of a-Fe phase in the as-milled powders is under progress. Acknowledgments We are grateful to Spanish MEC for financial support under project MAT2005-06806-C04-01. SCT’s (Univ. Oviedo) are acknowledged for assistance in magnetic measurements. We thank ILL for neutron beam time allocation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
B. Loegel, J. Phys. F: Metal Phys. 5 (1975) 497. R. Nemanich et al., Phys. Rev. B 16 (1977) 124. S.K. Burke et al., J. Phys. F 13 (1983) 451. P. Gorria et al., Phys. Rev. B 69 (2004) 214421. P. Gorria et al., Phys. Rev. B 72 (2005) 014401. P. Gorria et al., J. Magn. Magn. Mater. 294 (2005) 159. T. Koyano et al., J. Appl. Phys. 73 (1993) 429. A. Fnidiki et al., Phys. B 357 (2005) 319. P. Delcroix et al., Mater. Sci. Forum 360-362 (2001) 329. A. Fernández-Martínez et al., J. Non-Cryst. Solids 353 (2007) 855. T.C. Hansen et al., Meas. Sci. Technol. 19 (2008) 034001. J. Rodríguez-Carvajal, Phys. B 192 (1993) 55. D. Martinez-Blanco, P. Gorria, J.A. Blanco, M.J. Perez, J. Campo, J. Phys.: Condens. Mat. 20 (2008) 335213. [14] P.J. Schilling et al., Appl. Phys. Lett. 68 (1996) 767. [15] A. Di Cicco et al., Phys. Rev. B 50 (1994) 12386.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Structural and magnetic study of mechanically alloyed Fe30Cr70 by neutron thermo-diffractometry and magnetization measurements A. Fernández-Martínez a,b, D. Martínez-Blanco c, M.J. Pérez d, G.J. Cuello b, G.R. Castro e, P. Gorria d, J.A. Blanco d,* a
LGIT, University of Grenoble and CNRS, Maison des Geosciences, B.P. 53, 38041 Grenoble cedex 9, France Institut Laue-Langevin, BP 156, 6 rue Jules Horowitz, 38042 Grenoble cedex 9, France c Unidad de Magnetometría, SCT’s, Universidad de Oviedo, Julián Clavería 8, 33006 Oviedo, Spain d Departamento de Física, Universidad de Oviedo, Calvo Sotelo, s/n, 33007 Oviedo, Spain e European Synchrotron Radiation Facility, B.P. 220, 38043 Grenoble, France b
a r t i c l e
i n f o
Article history: Available online 22 October 2008 PACS: 61.05.fm 75.50.Bb 81.20.Ev Keywords: Neutron diffraction/scattering Magnetic properties
a b s t r a c t The temperature dependence of magnetization together with neutron powder thermo-diffraction show that nominal ball milled Fe30Cr70 for 110 h is formed of two mixed phases (Fe20Cr80 + Fe), both of them with body centered cubic crystal structure and very close values for the lattice parameter (2.87 Å). On heating above 900 K, the system exhibits an irreversible structural transformation, giving rise to the homogenization of the material, and then recovering the well-defined starting composition Fe30Cr70. Subsequent heating-cooling neutron thermo-diffraction cycles do not show any additional transformation, thus explaining the overlapping M(T) curves after the first heating. Ó 2008 Elsevier B.V. All rights reserved.
Binary FexCr100x solid solutions have a body centered cubic (bcc) crystal structure at room temperature in the whole compositional range, with a lattice parameter close to those of Fe (2.8665 Å) and Cr (2.8847 Å). These alloys display a complex magnetic phase diagram, especially in the Cr-rich compositional range where coexistence of spin–glass, ferro- or antiferro-magnetism can occur [1–3]. On the other hand, mechanical alloying technique allows synthesizing single phase Fe-TM (TM = Cr, Ni, Cu) binary compounds with nanometer grain size and displaying a rich variety of magnetic behaviors [4–6]. In the case of mechanically alloyed Fe100xCrx, the magnetic response can be largely influenced by the disordered intergranular zone, which is paramagnetic at room temperature [7–9]. In this paper we report on the structural and magnetic changes occurring in mechanically alloyed Fe30Cr70 in a large temperature range (2–1100 K), through the combination of magnetization vs. temperature measurements, M(T) curves, and neutron powder thermo-diffraction (NPTD). These techniques have revealed very useful in understanding the correlation between structural changes and magnetic behavior in metastable Fe-based alloys [4–6,10]. Besides that, preliminary X-ray absorption experiments were carried out at room temperature in the Cr K-edge, at * Corresponding author. Tel.: +34 985102950; fax: +34 985103324. E-mail address: [email protected] (J.A. Blanco). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.06.114
the BM25 beamline (SpLine) at the European synchrotron radiation facility (ESRF, Grenoble) in order to obtain structural information around Cr atoms through the analysis of the extended X-ray absorption fine structure (EXAFS) spectrum. Powders of elemental Cr and Fe (99.98% purity) were milled for 110 h to a nominal composition Fe30Cr70 using a RETSCH PM400/4 high-energy ball mill. M(T) curves were measured in the temperature range 2–1100 K, by means of a PPMS-14T magnetometer (2350 K) and a Faraday susceptometer (3001100 K) under an applied magnetic field, l0H = 0.1 T. NPTD experiments were performed at the D20 two-axis diffractometer of the Institut Laue– Langevin (Grenoble) using a neutron wavelength of 1.867 Å and a high take-off angle of 120° for the Ge(1 1 5) monochromator crystal in order to increase resolution [11], which is needed due to the very close values for the cell parameters of Fe and Cr. Two heating-cooling cycles from room temperature (RT) to 1100 K were performed using a vanadium furnace in order to monitor the temperature-induced structural transformations. The heating rate of the first heating was of 2 K/min in the region of temperatures up to 900 K and of 1 K/min in the region from 900 K to 1100 K. Diffraction patterns were collected every 5 min. The first cooling was done at a rate of 5 K/min and it was followed by a second heating at a rate of 10 K/min and a final cooling. Rietveld refinement of the diffraction patterns was performed using the FULLPROF package
A. Fernández-Martínez et al. / Journal of Non-Crystalline Solids 354 (2008) 5156–5158
[12]. The microstructural analysis was performed by assuming an anisotropic size broadening model, expressed in terms of spherical harmonics, to calculate the average coherent domain size and an anisotropic broadening model, based on the Stephens’ notation, accounting for strains within the crystallites [13]. It has been previously reported a value of 260 K for the Curie temperature, TC, of the Fe30Cr70, alloy [2]. In Fig. 1 the M(T) curves of the as-milled sample on the first heating and cooling processes are shown. On heating from 2 K, the magnetization exhibits a decrease of around 25% of its value above 40 K, followed by a slow decrease up to 900 K, where a clear change in the M(T) slope is observed, reaching a vanishing M value at around 1050 K (see Fig. 1). On cooling from 1100 K, the M(T) curve does not overlap the heating one at any temperature, the magnetization slowly increases its value, exhibiting a clear change in the slope below 400 K, and reaching a value at 2 K which is near double to that of the as-milled sample. Further heating–cooling cycles leads to overlapping M(T) curves displaying a similar trend to that of the cooling curve shown in Fig. 1. In order to correlate this irreversible M(T) behavior with possible high temperature-induced structural transformations NPTD experiments were performed. Neutron diffraction patterns of the as-milled sample show four broad intense peaks that can be ascribed to a bcc phase with a cell parameter around 2.88 Å. Besides that, the microstructural analysis of the as-milled sample yields an apparent coherent domain size of htiRT = 9 ± 1 nm, with a maximum strain of e = 1.0 ± 0.1%. Heating above 900 K, the strain is totally relaxed and the crystalline grain size increases above the diffraction detection limit. However, the peaks observed in the diffraction pattern corresponding to the as-milled sample present a noticeable asymmetry (see inset in Fig. 2a), suggesting that two different bcc crystalline phases coexist in the sample, with very close values for the cell parameter [2.8875(2) Å and 2.8679(5) Å]. This fact led us to assume that some of the initial bcc-Fe has not been alloyed. Rietveld refinement yields a percentage of 86(2)% of a bcc-FeCr phase and a 14(2)% of a bcc-Fe, hence, the composition of the initial FeCr phase in the as-milled alloy could be close to Fe20Cr80. The latter could explain the M(T) curve of the as-milled alloy in the following terms. In the temperature range between 2 K and 300 K, no signature for the TC around 260 K is observed (see Fig. 1), and the trend
Fig. 1. Magnetization vs. temperature curve measured by combining a Faraday susceptometer and a PPMS magnetometer under an applied magnetic field l0H = 0.1 T.
5157
Fig. 2. Powder neutron diffraction patterns of the nominal Fe30Cr70 sample at 300 K: (a) as-milled, and (b) after being heated up to 1100 K. Observed (points) and calculated (solid line) patterns, positions of the Bragg reflections represented by vertical bars, and the observed-calculated difference depicted at the bottom of the figure. In a (b) the first raw of marks below the Bragg peaks corresponds to Fe20Cr80 (Fe30Cr70), while the second one is associated with the bcc-Fe (an oxide-impurity phase [<5%]). The insets show the (2 1 1) reflection, with a shoulder in a) the asmilled sample evidencing the existence of a bcc-Fe phase.
followed by the M(T) curve below 100 K resembles that for a spinglass regime. It is worth noting that Cr-rich alloys with a percentage around 80% in Cr exhibit spin-glass behavior [3], hence, these findings support the existence of Fe20Cr80 spin-glass phase, even comparison between results from ball milled and melted samples need to be done with care. However, a significant magnetization value is measured above 200 K, which comes from the bcc-Fe phase. Further heating up to around 900 K shows a slow decrease of the magnetization, and then, a clear change of slope is observed, reaching to a vanishing value of M at around 1040 K (see Fig. 1). On the other hand, neutron diffraction patterns above 875 K show the progressive decrease of the amount of bcc-Fe phase. This fact suggests that together with the thermally-induced microstructural relaxation, the structural transformation, Fe20Cr80 + bcc-Fe ) Fe30Cr70, takes place. This feature is not surprising, owing that at least 400 h of milling time is needed to reach a homogeneous a-phase solid solution. Hence, lower milling times would give rise to heterogeneous samples, with intra granular composition fluctuations 10% in a scale of a few nanometers [7,9]. On the other hand, the origin of the linear decrease of M between 900 K and 1040 K cannot be attributed to the existence of any ferro-to-paramagnetic phase transition. On cooling from 1100 K, M(T) curve looks like the typical one for a ferromagnetic material with a value of TC = 275 ± 10 K (see Fig. 1). Subsequent heating–cooling cycles exhibit reversible M(T) curves, suggesting no further structural transformations in the material. The latter is confirmed by NPTD experiments.
5158
A. Fernández-Martínez et al. / Journal of Non-Crystalline Solids 354 (2008) 5156–5158
Preliminary analysis of the EXAFS spectrum corresponding to the as-milled sample indicates a decrease in the coordination number, which could be explained as a result of a high density of defects induced by the mechanical alloying process [14] and/or the existence of ill-coordinated atoms at the nanograins’ surfaces [15]. Additional absorption experiments in the heat-treated samples as well as in the Fe K-edge are needed in order to elucidate the decrease in the coordination number upon milling. In summary, the combination of neutron powder thermo-diffraction and magnetization vs. temperature measurements shows that: (i) in the as-milled powders, with nominal composition Fe30Cr70, a Cr-rich Fe20Cr80 together with a bcc-Fe phases coexist. (ii) heating the as-milled alloy up to 1100 K gives rise to a thermal relaxation of the microstructure and to the Fe20Cr80 + bcc-Fe ) Fe30Cr70 reversal phase transformation. (iii) the Fe30Cr70 high temperature stabilized phase does not exhibit any further structural transformation in subsequent heating–cooling cycles. A more detailed study using Mössbauer spectroscopy at different temperatures with the aim of elucidating the above questions
about the heterogeneity of the samples and the existence of a-Fe phase in the as-milled powders is under progress. Acknowledgments We are grateful to Spanish MEC for financial support under project MAT2005-06806-C04-01. SCT’s (Univ. Oviedo) are acknowledged for assistance in magnetic measurements. We thank ILL for neutron beam time allocation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
B. Loegel, J. Phys. F: Metal Phys. 5 (1975) 497. R. Nemanich et al., Phys. Rev. B 16 (1977) 124. S.K. Burke et al., J. Phys. F 13 (1983) 451. P. Gorria et al., Phys. Rev. B 69 (2004) 214421. P. Gorria et al., Phys. Rev. B 72 (2005) 014401. P. Gorria et al., J. Magn. Magn. Mater. 294 (2005) 159. T. Koyano et al., J. Appl. Phys. 73 (1993) 429. A. Fnidiki et al., Phys. B 357 (2005) 319. P. Delcroix et al., Mater. Sci. Forum 360-362 (2001) 329. A. Fernández-Martínez et al., J. Non-Cryst. Solids 353 (2007) 855. T.C. Hansen et al., Meas. Sci. Technol. 19 (2008) 034001. J. Rodríguez-Carvajal, Phys. B 192 (1993) 55. D. Martinez-Blanco, P. Gorria, J.A. Blanco, M.J. Perez, J. Campo, J. Phys.: Condens. Mat. 20 (2008) 335213. [14] P.J. Schilling et al., Appl. Phys. Lett. 68 (1996) 767. [15] A. Di Cicco et al., Phys. Rev. B 50 (1994) 12386.