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Low temperature study of mechanically alloyed Fe67.5Ni32.5 Invar sample
Journal of Magnetism and Magnetic Materials 385 (2015) 83–87
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Low temperature study of mechanically alloyed Fe67.5Ni32.5 Invar sample J.L. Valenzuela a, J.F. Valderruten b, G.A. Pérez Alcázar a,n, H.D. Colorado a, J.J. Romero c, J.M. González d, J.M. Greneche e, J.F. Marco f a
Departamento de Física, Universidad del Valle, A. A. 25360, Cali, Colombia Departamento de Ingeniería, Universidad Cooperativa de Colombia, Bucaramanga, Colombia c Instituto de Microelectrόnica de Madrid, CNM, CSIC, C/Isaac Newton 8, Tres Cantos, 28760 Madrid, Spain d Unidad Asociada ICMM-IMA, Apdo. 155, Las Rozas, 28230 Madrid, Spain e LUNAM, Université du Maine, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, 72085 Le Mans, Cedex 9, France f Instituto de Química-Física ˝Rocasolano˝, CSIC, C/Serrano 119, 28006 Madrid, Spain b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 January 2015 Received in revised form 9 February 2015 Accepted 1 March 2015 Available online 3 March 2015
The study at low temperatures of powder of the Invar alloy, Fe67.5Ni32.5, produced by mechanical alloying, shows that the sample presents two structural phases, the Fe–Ni BCC and the Fe–Ni FCC. The 57Fe Mössbauer spectra obtained in this sample at different temperatures were fitted considering two hyperfine magnetic field distributions. The first one having the larger mean field and only one peak (at ca. 35 T, varying with T), is associated with the BCC phase, and the second one, presenting several broad peaks (distributed between 10 and 35 T), is associated to the FCC phase. A singlet, which is associated to low spin Fe sites of the FCC phase, was also considered. The mean hyperfine magnetic field of the BCC phase increases monotonically as temperature decreases, while that of the FCC phase presents an anomaly near 75 K. The real part of the ac magnetic susceptibility temperature scans presents a peak whose position increases from 31 to 39 K, when the ac field frequency increases from 100 to 5000 Hz. These results permit to associate the detected anomaly to the occurrence of a reentrant spin glass transition. & 2015 Elsevier B.V. All rights reserved.
1. Introduction One of the most studied binary alloys is the Fe–Ni system, as various compositions exhibit important thermal and magnetic properties [1–14]. One of these compositions is Invar one (65– 70 at% Fe). In this composition range, some of the structural and magnetic properties exhibit anomalous behavior. The Invar composition occurs over a small region of the phase diagram in which two crystalline phases coexist, a body-centered cubic (BCC) or αphase which is exclusively present in the Fe-rich alloys and a facecentered cubic (FCC) or γ-phase, which occurs in Ni-rich alloys. The compositional limits of this two phases region depends on the preparation method and the subsequent thermal treatments. For Invar samples composed of Fe–Ni alloys, the anomalous magnetic behavior gives rise to some controversial interpretations. The most widely accepted interpretation, is based on the two-γstate like model (described in [15] and references therein), which n
Corresponding author. E-mail address: [email protected] (G.A. Pérez Alcázar).
http://dx.doi.org/10.1016/j.jmmm.2015.03.001 0304-8853/& 2015 Elsevier B.V. All rights reserved.
is based on the two-γ-state model that Weiss [16–18] proposed for the study of the properties of γ-Fe. In this model, the γ phase of a Fe–Ni Invar alloy exhibits two FCC lattices with the same crystal structure, but different compositions and magnetic behavior. One phase, which is rich in iron, exhibits a low moment (LM) and behaves as a paramagnetic material at room temperature (RT). The other phase is rich in Ni, exhibits a high moment (HM) and behaves as a ferromagnetic material at RT. The invar effect appears as a spinodal equilibrium between these two phases. The experimental evidence for the coexistence of Fe atoms in HM and LM states has been provided by magnetic neutron scattering measurements [19,20] and by Mossbauer experiments [17,21–23]. Nakamura [21] proposed that the single line and the six-line patterns obtained from 57Fe Mössbauer spectrometry (MS) are unambiguously attributed to the paramagnetic and ferromagnetic regions, respectively. Asano proposed a transition to an antiferromagnetic phase as a result of the linewidth of the single line being considerably broadened at low temperatures (below 77 K) [24]. The single line, which corresponds to the paramagnetic LM FCC phase, and was also obtained from the Mössbauer spectra of
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some meteorites, was determined by Rancourt and Scorzelli [25,26] to be a new mineral phase, which was named by them as antitaenite. Antitaenite exhibits the same crystal structure as taenite, the ferromagnetic HM FCC phase. Another interpretation is based on a mixed exchange model. In this model, Fe–Ni FCC alloys are strong local moment systems with an antiferromagnetic exchange interaction occurring between the next nearest Fe atoms (JFeFe o0) and ferromagnetic exchange interactions occurring between Ni atoms and between Fe and Ni atoms (JNiNi 4 0 and JFeNi 4 0). According to this model and theoretical predictions, obtained from Monte Carlo simulations by Rancourt and Dang [27], the antiferromagnetic low-Ni-content phase possesses a Fe atomic moment of μFe E0.5 μB. Additionaly, the ferromagnetic high-Ni-content phase exhibits a high Fe atomic moment of μFe E2.8 μB and exhibits predominantly ferromagnetic interactions. These theoretical works describe the Invar composition as a highly frustrated system due the competition between the ferromagnetic and antiferromagnetic bonds. Further work reported on by Lagarec et al. [28] presented experimental and theoretical evidence of a HM/LM transition controlled by the composition in the Fe–Ni FCC samples. The use of high energy ball mills allows for the preparation of nanostructured Fe–Ni alloys and other different equilibrium and non-equilibrium materials [29,30]. Their small grain size gives rise to a high number of atoms located at the grain boundaries compared to those within the crystalline grains. It is very well established that intermetallic compounds prepared by mechanical alloying (MA) have a high degree of structural disorder and are locked into a metastable state [31–33]. These factors imply unusual physical properties, which are very different from those observed in bulk-like microcrystalline materials. For this reason, several studies have been conducted investigating the structural properties of the Fe–Ni alloys prepared by MA [6,7]. Kaloshkin et al. showed that the concentration required for different phases shifted to the side of the phase diagram corresponding to less Ni and formed non-ferromagnetic alloys after annealing samples prepared by MA at 650 °C [12]. More recently, Valderruten et al. [34] reported on MS and XRD studies conducted at room temperature (RT) using MA samples from the Fe100 xNix system, with x varying between 22.5 and 40. They found that all the samples exhibited coexisting BCC and FCC Fe–Ni phases. The Mössbauer spectra were fitted using a new model that included two hyperfine magnetic field distributions (HMFDs) corresponding to the two ferromagnetic phases and a paramagnetic site attributed to the LM state Fe sites of the FCC phase. The Ni content dependence of the hyperfine field for the 57Fe nuclei of the FCC phase provides evidence of an anomaly occurring at approximately 32.5 at% Ni, which is attributed to Invar behavior. This anomaly was also detected in the Ni content dependence of the lattice parameter of the FCC phase. These results provide evidence that MA induces a nanostructuring shift in the Invar composition to low Ni contents, which is in accordance with the results from Kaloshkin et al. [12]. Finally, Rodriguez et al. [35] reported on a study of Fe65Ni35 alloy, prepared by MA using different milling times, ranging from 5 to 11 h. This composition corresponds to an Invar composition of the Fe–Ni melted alloy. Using XRD, the researchers found that three phases coexisted for all the milling times: one BCC and two FCC phases. The Mossbauer spectra, at different temperatures, of the sample milled for 10 h were fitted using two HMFDs. One with fields related to the BCC phase and; the second with a hyperfine magnetic field distribution involving low hyperfine fields related to the ferromagnetic HM FCC phase. A third component, a singlet related to the paramagnetic LM FCC, was necessary to be added to obtain the best fit. These fits are similar to that proposed by Valderruten [34]. The aim of the work described here is to report on the results
obtained from a low temperature study of the magnetic behavior of the Fe67.5Ni32.5 alloy prepared by MA. The preparation of the powder follows the same methodology as Valderruten et al. [34]. We intend to determine if the frustration reported to occur in molten and cast Invar alloys is also present in an Invar sample produced by MA.
2. Experimental method Pure carbonyl Fe powder (99.9%) and electrolytic Ni powder (99.9%) were used as the starting materials for the MA. A Fe67.5Ni32.5 sample was mechanically alloyed under a vacuum for 10 h in a planetary ball mill (Fritsch ‘Pulverisette 5’), using hardened chromium steel vials and balls and rotation frequency of 280 rev min 1. The ball mass-to-powder mass (BM/PM) ratio was approximately 20:1. The final powdered product was characterized with x-ray diffraction, using 57Fe Mössbauer spectrometry (MS) to collect the spectra at different temperatures with a conventional transmission spectrometer using a 57Co (Rh) source and a α-Fe foil as the calibration sample, and using ac magnetic susceptibility by means of a MAGLAB ac susceptibility system of Oxford Instruments. The ac magnetic susceptibility scans were collected with an ac field of 30 Oe at frequencies of 100, 1325, 2550, 3750 and 5000 Hz. The Mössbauer spectra were fitted using the MOSFIT program [36] and the values of isomer shift are quoted to that of α-Fe at 300 K.
3. Experimental results and discussion Fig. 1 presents the XRD pattern of the Fe67.5Ni32.5 powdered sample produced by MA. The pattern was refined with the line sequences of the BCC and FCC phases, with space groups Im3m and Fm3m, respectively. This refinement allows for estimation of the lattice parameters of the BCC and FCC phases, which are 2.872 70.005 and 3.597 70.005 Å, respectively, and of the mean crystallite sizes, which are 10 72 and 157 2 nm, respectively. These results indicate that the powdered sample presents a nanostructured character. Fig. 2 illustrates the Mössbauer spectra of the Fe67.5Ni32.5 powdered sample recorded at different temperatures. The fitting procedure of the spectra, which is the same used by Valderruten et al. [34] at room temperature (RT), was conducted with two independent hyperfine magnetic field distributions (HMFDs) and
Fig. 1. XRD pattern of the mechanically alloyed Fe67.5Ni32.5 powdered sample.
J.L. Valenzuela et al. / Journal of Magnetism and Magnetic Materials 385 (2015) 83–87
Fig. 2. Mössbauer spectra of the Fe67.5Ni32.5 alloy at different temperatures.
one single line. The HMFD with larger hyperfine fields and mean isomer shift values is associated with the BCC ferromagnetic and disordered crystalline grains (tetrataenite), and its spectral area, as obtained at room temperature (27.4%), was assumed constant for lower temperatures. The second HMFD, with smaller hyperfine fields and mean isomer shift values, is assigned to the FCC ferromagnetic and disordered crystalline grains (taenite). The isomer shift value allows for the conclusion that the single line is due to Fe sites of the FCC phase which present a surrounding rich in Fe atoms, which, in accord with the proposal of Rancourt and Scorzelli, can be attributed to antitaenite, a paramagnetic low spin FCC γ-LS-FeNi phase [25,26]. The small asymmetry or differences observed between the intensities of lines 2 and 3 of the spectra respect the corresponding intensities of the lines 4 and 5, was associated to a distribution of the isomer shifts of the HMFD related to the FCC ferromagnetic phase. Fig. 3 compares the corresponding HMFDs established from the refinement of the Mössbauer spectra at different temperatures. It can be noted that the HMFDs of the BCC grains (right) vary
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between 28 and 40 T and present basically one maximum near 35 T which increases with decreasing temperature. On the other hand, the FCC HMFDs (left) are ranged from 3 to 37 T with a prevailing peak at approximately 30–33 T. Fig. 4 shows the variation with temperature of the BCC and FCC ferromagnetic phase proportions corresponding to the respective absorption area as the Lamb-Mössbauer f recoilless factors are assumed to be the same for both Fe species. The inset corresponds to the variation with temperature of the paramagnetic fraction. It can be noted that the proportions of the BCC phase remains nearly temperature independent, whereas those of the ferromagnetic FCC phase slowly increases between room temperature down to 61 K. Indeed, at this temperature a kink occurs with a faster increase of the FCC phase proportion with decreasing temperature. These results and those of DRX permit the conclusion that the paramagnetic sites are clearly attributed to Fe species of the FCC phase. In this way, as temperature decreases, more and more paramagnetic FCC sites become ferromagnetic, increasing the ferromagnetic spectral area and decreasing the paramagnetic spectral area of this phase. Below 61 K, this transformation runs more rapidly, indicating an anomaly or magnetic phase transition in the FCC phase. Fig. 5 shows the variation of the mean isomer shifts (δ) of the BCC and FCC HMFDs with the temperature. It can be noted that the δ values of the BCC phase continuously increases with the decreasing of temperature. This is the expected behavior of a sample which does not present phase transitions. However, the δ values of the FCC phase present a quick and anomalous increase below 75 K in accordance with the anomalies presented by the paramagnetic and ferromagnetic FCC spectral areas. To explain the origin of the anomaly detected in the FCC phase, ac magnetic susceptibility measurements were conducted. Fig. 6 shows the real part of the ac susceptibility curves taken at 100, 1325, 2550, 3750 and 5000 Hz. It can be noted that these scans present peaks, the positions of which increase monotonically from 31 to 39 K, when frequency increases from 100 to 5000 Hz. This variation of the ac peak temperatures is small compared to that expected from the superparamagnetic blocking process, indicating that the associated transition could corresponds to a spin glass (SG) freezing [37]. Otherwise, for temperatures higher than that of the peak, the signal increases with T showing a ferromagnetic behavior which is stable above room temperature, as evidenced by the Mössbauer results. Consequently the transition present in the sample is of the reentrant SG type in a ferromagnetic matrix or RSG-F [38]. The facts that the phase proportion of the BCC phase do not exhibit any anomaly, and that their corresponding HMFDs are narrowly distributed from 28 to 40 T allow competitive interactions to be discarded inside this phase (that is, only ferromagnetic interactions are present) and consequently, the presence of the RSG-F phenomenology is not expected in this case. Otherwise, in the FCC phase the two necessary ingredients to stabilize the SG inside the ferromagnetic FCC phase are present: atomic disorder originated from the preparation method (MA), and competitive interactions which appear in the regions which separate the ferromagnetic Fe sites (those sites rich in Ni neighbors or high spin sites) from the antiferromagnetic Fe sites (those sites rich in Fe neighbors or low spin sites) [39]. These results are in agreement with the Monte Carlo simulation results corresponding to molten and cast (close to equilibrium) Invar alloys, which indicates that this magnetic alloy is highly topologically frustrated [27], as well as with more recent calculations based on first principles, which obtain competition between the magnetic states found in the Invar concentration [40]. With the previous results it is now possible to propose a local model to interpret the magnetic behavior of the FCC phase. At
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Fig. 3. HMFDs of the FCC (left) and BCC (right) phases obtained at different temperatures.
Fig. 4. Magnetic phase proportions of the BCC phase (below) and of the FCC phase (above) as a function of temperature. The inset corresponds to the paramagnetic fraction.
Fig. 5. Isomer shift vs. T for the BCC (up), and FCC (down), ferromagnetic phases.
room temperature, three types of Fe sites are present: those Fe sites rich in Ni atoms, which are the majority and behave as ferromagnetic because they present high spin values (these sites account for the HMFD); those Fe sites rich in Fe atoms which behave as paramagnetic for all temperatures; and those which
Fig. 6. Real part of the AC susceptibility vs. T curves of the sample for different frequencies.
present intermediate numbers of Fe and Ni atoms, which originate frustration due to the disorder and the competition between low spin and high spin sites. The latter sites behave as paramagnetic at room temperature. These sites and those rich in Fe atoms account for the singlet of the spectra. When temperature decreases, the mean hyperfine magnetic field increases as a consequence of the increase of the ferromagnetic coupling of the first sites and of the decrease of the number of paramagnetic sites (see inset of Fig. 4) which now are behaving ferromagnetic. This process continues up to 61 K. At this temperature, some of the remaining frustrated sites are freezing, but due they are in the presence of a ferromagnetic matrix, they present a preferential freezing orientation, of the matrix. In this way, the mean hyperfine magnetic field presents the additional contribution of the frustrated sites. This process continues as temperature decreases. The behavior of the δ values of the FCC ferromagnetic phase is in accordance with this model. Below 61 K, the δ values increase quickly due to the contribution of new sites which are freezing and then contribute to the ferromagnetic ones with different surroundings, causing the mean density of s electrons in the Fe nucleus to decrease. The different values of the anomalous temperature detected by the Mossbauer and ac susceptibility techniques are due to their different measurement times. Finally, our results and interpretation indicates that the Invar alloy obtained by MA behaves in according with the second model proposed for the Invar Fe–Ni samples, i. e. a mixed exchange model.
J.L. Valenzuela et al. / Journal of Magnetism and Magnetic Materials 385 (2015) 83–87
4. Conclusions Our results which give clear evidence for the anomalous behavior of the mean hyperfine magnetic field and of the lattice parameter, detected at room temperature for the mechanically alloyed Fe67.5Ni32.5 [34], can be assigned to a RSG behavior of the FCC Fe–Ni phase. This phenomenon was evidenced by anomalies from the magnetic and paramagnetic phase proportions, obtained by Mössbauer spectrometry, approximately 61 K, and by the peaks of the real part of the ac magnetic susceptibility scans of the sample, observed to appear between 31 and 39 K as a function of the applied frequency. These results, specially the presence of a RSG transition, are evidence that our sample behaves like the mixed bond model proposed for Invar alloys.
Acknowledgements The authors would like to thank the support given by the Universidad del Valle. G.A.P.A., the Sabbatical Year Grants provided by the Spanish Science Ministry and Universidad del Valle.
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Low temperature study of mechanically alloyed Fe67.5Ni32.5 Invar sample J.L. Valenzuela a, J.F. Valderruten b, G.A. Pérez Alcázar a,n, H.D. Colorado a, J.J. Romero c, J.M. González d, J.M. Greneche e, J.F. Marco f a
Departamento de Física, Universidad del Valle, A. A. 25360, Cali, Colombia Departamento de Ingeniería, Universidad Cooperativa de Colombia, Bucaramanga, Colombia c Instituto de Microelectrόnica de Madrid, CNM, CSIC, C/Isaac Newton 8, Tres Cantos, 28760 Madrid, Spain d Unidad Asociada ICMM-IMA, Apdo. 155, Las Rozas, 28230 Madrid, Spain e LUNAM, Université du Maine, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, 72085 Le Mans, Cedex 9, France f Instituto de Química-Física ˝Rocasolano˝, CSIC, C/Serrano 119, 28006 Madrid, Spain b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 January 2015 Received in revised form 9 February 2015 Accepted 1 March 2015 Available online 3 March 2015
The study at low temperatures of powder of the Invar alloy, Fe67.5Ni32.5, produced by mechanical alloying, shows that the sample presents two structural phases, the Fe–Ni BCC and the Fe–Ni FCC. The 57Fe Mössbauer spectra obtained in this sample at different temperatures were fitted considering two hyperfine magnetic field distributions. The first one having the larger mean field and only one peak (at ca. 35 T, varying with T), is associated with the BCC phase, and the second one, presenting several broad peaks (distributed between 10 and 35 T), is associated to the FCC phase. A singlet, which is associated to low spin Fe sites of the FCC phase, was also considered. The mean hyperfine magnetic field of the BCC phase increases monotonically as temperature decreases, while that of the FCC phase presents an anomaly near 75 K. The real part of the ac magnetic susceptibility temperature scans presents a peak whose position increases from 31 to 39 K, when the ac field frequency increases from 100 to 5000 Hz. These results permit to associate the detected anomaly to the occurrence of a reentrant spin glass transition. & 2015 Elsevier B.V. All rights reserved.
1. Introduction One of the most studied binary alloys is the Fe–Ni system, as various compositions exhibit important thermal and magnetic properties [1–14]. One of these compositions is Invar one (65– 70 at% Fe). In this composition range, some of the structural and magnetic properties exhibit anomalous behavior. The Invar composition occurs over a small region of the phase diagram in which two crystalline phases coexist, a body-centered cubic (BCC) or αphase which is exclusively present in the Fe-rich alloys and a facecentered cubic (FCC) or γ-phase, which occurs in Ni-rich alloys. The compositional limits of this two phases region depends on the preparation method and the subsequent thermal treatments. For Invar samples composed of Fe–Ni alloys, the anomalous magnetic behavior gives rise to some controversial interpretations. The most widely accepted interpretation, is based on the two-γstate like model (described in [15] and references therein), which n
Corresponding author. E-mail address: [email protected] (G.A. Pérez Alcázar).
http://dx.doi.org/10.1016/j.jmmm.2015.03.001 0304-8853/& 2015 Elsevier B.V. All rights reserved.
is based on the two-γ-state model that Weiss [16–18] proposed for the study of the properties of γ-Fe. In this model, the γ phase of a Fe–Ni Invar alloy exhibits two FCC lattices with the same crystal structure, but different compositions and magnetic behavior. One phase, which is rich in iron, exhibits a low moment (LM) and behaves as a paramagnetic material at room temperature (RT). The other phase is rich in Ni, exhibits a high moment (HM) and behaves as a ferromagnetic material at RT. The invar effect appears as a spinodal equilibrium between these two phases. The experimental evidence for the coexistence of Fe atoms in HM and LM states has been provided by magnetic neutron scattering measurements [19,20] and by Mossbauer experiments [17,21–23]. Nakamura [21] proposed that the single line and the six-line patterns obtained from 57Fe Mössbauer spectrometry (MS) are unambiguously attributed to the paramagnetic and ferromagnetic regions, respectively. Asano proposed a transition to an antiferromagnetic phase as a result of the linewidth of the single line being considerably broadened at low temperatures (below 77 K) [24]. The single line, which corresponds to the paramagnetic LM FCC phase, and was also obtained from the Mössbauer spectra of
84
J.L. Valenzuela et al. / Journal of Magnetism and Magnetic Materials 385 (2015) 83–87
some meteorites, was determined by Rancourt and Scorzelli [25,26] to be a new mineral phase, which was named by them as antitaenite. Antitaenite exhibits the same crystal structure as taenite, the ferromagnetic HM FCC phase. Another interpretation is based on a mixed exchange model. In this model, Fe–Ni FCC alloys are strong local moment systems with an antiferromagnetic exchange interaction occurring between the next nearest Fe atoms (JFeFe o0) and ferromagnetic exchange interactions occurring between Ni atoms and between Fe and Ni atoms (JNiNi 4 0 and JFeNi 4 0). According to this model and theoretical predictions, obtained from Monte Carlo simulations by Rancourt and Dang [27], the antiferromagnetic low-Ni-content phase possesses a Fe atomic moment of μFe E0.5 μB. Additionaly, the ferromagnetic high-Ni-content phase exhibits a high Fe atomic moment of μFe E2.8 μB and exhibits predominantly ferromagnetic interactions. These theoretical works describe the Invar composition as a highly frustrated system due the competition between the ferromagnetic and antiferromagnetic bonds. Further work reported on by Lagarec et al. [28] presented experimental and theoretical evidence of a HM/LM transition controlled by the composition in the Fe–Ni FCC samples. The use of high energy ball mills allows for the preparation of nanostructured Fe–Ni alloys and other different equilibrium and non-equilibrium materials [29,30]. Their small grain size gives rise to a high number of atoms located at the grain boundaries compared to those within the crystalline grains. It is very well established that intermetallic compounds prepared by mechanical alloying (MA) have a high degree of structural disorder and are locked into a metastable state [31–33]. These factors imply unusual physical properties, which are very different from those observed in bulk-like microcrystalline materials. For this reason, several studies have been conducted investigating the structural properties of the Fe–Ni alloys prepared by MA [6,7]. Kaloshkin et al. showed that the concentration required for different phases shifted to the side of the phase diagram corresponding to less Ni and formed non-ferromagnetic alloys after annealing samples prepared by MA at 650 °C [12]. More recently, Valderruten et al. [34] reported on MS and XRD studies conducted at room temperature (RT) using MA samples from the Fe100 xNix system, with x varying between 22.5 and 40. They found that all the samples exhibited coexisting BCC and FCC Fe–Ni phases. The Mössbauer spectra were fitted using a new model that included two hyperfine magnetic field distributions (HMFDs) corresponding to the two ferromagnetic phases and a paramagnetic site attributed to the LM state Fe sites of the FCC phase. The Ni content dependence of the hyperfine field for the 57Fe nuclei of the FCC phase provides evidence of an anomaly occurring at approximately 32.5 at% Ni, which is attributed to Invar behavior. This anomaly was also detected in the Ni content dependence of the lattice parameter of the FCC phase. These results provide evidence that MA induces a nanostructuring shift in the Invar composition to low Ni contents, which is in accordance with the results from Kaloshkin et al. [12]. Finally, Rodriguez et al. [35] reported on a study of Fe65Ni35 alloy, prepared by MA using different milling times, ranging from 5 to 11 h. This composition corresponds to an Invar composition of the Fe–Ni melted alloy. Using XRD, the researchers found that three phases coexisted for all the milling times: one BCC and two FCC phases. The Mossbauer spectra, at different temperatures, of the sample milled for 10 h were fitted using two HMFDs. One with fields related to the BCC phase and; the second with a hyperfine magnetic field distribution involving low hyperfine fields related to the ferromagnetic HM FCC phase. A third component, a singlet related to the paramagnetic LM FCC, was necessary to be added to obtain the best fit. These fits are similar to that proposed by Valderruten [34]. The aim of the work described here is to report on the results
obtained from a low temperature study of the magnetic behavior of the Fe67.5Ni32.5 alloy prepared by MA. The preparation of the powder follows the same methodology as Valderruten et al. [34]. We intend to determine if the frustration reported to occur in molten and cast Invar alloys is also present in an Invar sample produced by MA.
2. Experimental method Pure carbonyl Fe powder (99.9%) and electrolytic Ni powder (99.9%) were used as the starting materials for the MA. A Fe67.5Ni32.5 sample was mechanically alloyed under a vacuum for 10 h in a planetary ball mill (Fritsch ‘Pulverisette 5’), using hardened chromium steel vials and balls and rotation frequency of 280 rev min 1. The ball mass-to-powder mass (BM/PM) ratio was approximately 20:1. The final powdered product was characterized with x-ray diffraction, using 57Fe Mössbauer spectrometry (MS) to collect the spectra at different temperatures with a conventional transmission spectrometer using a 57Co (Rh) source and a α-Fe foil as the calibration sample, and using ac magnetic susceptibility by means of a MAGLAB ac susceptibility system of Oxford Instruments. The ac magnetic susceptibility scans were collected with an ac field of 30 Oe at frequencies of 100, 1325, 2550, 3750 and 5000 Hz. The Mössbauer spectra were fitted using the MOSFIT program [36] and the values of isomer shift are quoted to that of α-Fe at 300 K.
3. Experimental results and discussion Fig. 1 presents the XRD pattern of the Fe67.5Ni32.5 powdered sample produced by MA. The pattern was refined with the line sequences of the BCC and FCC phases, with space groups Im3m and Fm3m, respectively. This refinement allows for estimation of the lattice parameters of the BCC and FCC phases, which are 2.872 70.005 and 3.597 70.005 Å, respectively, and of the mean crystallite sizes, which are 10 72 and 157 2 nm, respectively. These results indicate that the powdered sample presents a nanostructured character. Fig. 2 illustrates the Mössbauer spectra of the Fe67.5Ni32.5 powdered sample recorded at different temperatures. The fitting procedure of the spectra, which is the same used by Valderruten et al. [34] at room temperature (RT), was conducted with two independent hyperfine magnetic field distributions (HMFDs) and
Fig. 1. XRD pattern of the mechanically alloyed Fe67.5Ni32.5 powdered sample.
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Fig. 2. Mössbauer spectra of the Fe67.5Ni32.5 alloy at different temperatures.
one single line. The HMFD with larger hyperfine fields and mean isomer shift values is associated with the BCC ferromagnetic and disordered crystalline grains (tetrataenite), and its spectral area, as obtained at room temperature (27.4%), was assumed constant for lower temperatures. The second HMFD, with smaller hyperfine fields and mean isomer shift values, is assigned to the FCC ferromagnetic and disordered crystalline grains (taenite). The isomer shift value allows for the conclusion that the single line is due to Fe sites of the FCC phase which present a surrounding rich in Fe atoms, which, in accord with the proposal of Rancourt and Scorzelli, can be attributed to antitaenite, a paramagnetic low spin FCC γ-LS-FeNi phase [25,26]. The small asymmetry or differences observed between the intensities of lines 2 and 3 of the spectra respect the corresponding intensities of the lines 4 and 5, was associated to a distribution of the isomer shifts of the HMFD related to the FCC ferromagnetic phase. Fig. 3 compares the corresponding HMFDs established from the refinement of the Mössbauer spectra at different temperatures. It can be noted that the HMFDs of the BCC grains (right) vary
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between 28 and 40 T and present basically one maximum near 35 T which increases with decreasing temperature. On the other hand, the FCC HMFDs (left) are ranged from 3 to 37 T with a prevailing peak at approximately 30–33 T. Fig. 4 shows the variation with temperature of the BCC and FCC ferromagnetic phase proportions corresponding to the respective absorption area as the Lamb-Mössbauer f recoilless factors are assumed to be the same for both Fe species. The inset corresponds to the variation with temperature of the paramagnetic fraction. It can be noted that the proportions of the BCC phase remains nearly temperature independent, whereas those of the ferromagnetic FCC phase slowly increases between room temperature down to 61 K. Indeed, at this temperature a kink occurs with a faster increase of the FCC phase proportion with decreasing temperature. These results and those of DRX permit the conclusion that the paramagnetic sites are clearly attributed to Fe species of the FCC phase. In this way, as temperature decreases, more and more paramagnetic FCC sites become ferromagnetic, increasing the ferromagnetic spectral area and decreasing the paramagnetic spectral area of this phase. Below 61 K, this transformation runs more rapidly, indicating an anomaly or magnetic phase transition in the FCC phase. Fig. 5 shows the variation of the mean isomer shifts (δ) of the BCC and FCC HMFDs with the temperature. It can be noted that the δ values of the BCC phase continuously increases with the decreasing of temperature. This is the expected behavior of a sample which does not present phase transitions. However, the δ values of the FCC phase present a quick and anomalous increase below 75 K in accordance with the anomalies presented by the paramagnetic and ferromagnetic FCC spectral areas. To explain the origin of the anomaly detected in the FCC phase, ac magnetic susceptibility measurements were conducted. Fig. 6 shows the real part of the ac susceptibility curves taken at 100, 1325, 2550, 3750 and 5000 Hz. It can be noted that these scans present peaks, the positions of which increase monotonically from 31 to 39 K, when frequency increases from 100 to 5000 Hz. This variation of the ac peak temperatures is small compared to that expected from the superparamagnetic blocking process, indicating that the associated transition could corresponds to a spin glass (SG) freezing [37]. Otherwise, for temperatures higher than that of the peak, the signal increases with T showing a ferromagnetic behavior which is stable above room temperature, as evidenced by the Mössbauer results. Consequently the transition present in the sample is of the reentrant SG type in a ferromagnetic matrix or RSG-F [38]. The facts that the phase proportion of the BCC phase do not exhibit any anomaly, and that their corresponding HMFDs are narrowly distributed from 28 to 40 T allow competitive interactions to be discarded inside this phase (that is, only ferromagnetic interactions are present) and consequently, the presence of the RSG-F phenomenology is not expected in this case. Otherwise, in the FCC phase the two necessary ingredients to stabilize the SG inside the ferromagnetic FCC phase are present: atomic disorder originated from the preparation method (MA), and competitive interactions which appear in the regions which separate the ferromagnetic Fe sites (those sites rich in Ni neighbors or high spin sites) from the antiferromagnetic Fe sites (those sites rich in Fe neighbors or low spin sites) [39]. These results are in agreement with the Monte Carlo simulation results corresponding to molten and cast (close to equilibrium) Invar alloys, which indicates that this magnetic alloy is highly topologically frustrated [27], as well as with more recent calculations based on first principles, which obtain competition between the magnetic states found in the Invar concentration [40]. With the previous results it is now possible to propose a local model to interpret the magnetic behavior of the FCC phase. At
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Fig. 3. HMFDs of the FCC (left) and BCC (right) phases obtained at different temperatures.
Fig. 4. Magnetic phase proportions of the BCC phase (below) and of the FCC phase (above) as a function of temperature. The inset corresponds to the paramagnetic fraction.
Fig. 5. Isomer shift vs. T for the BCC (up), and FCC (down), ferromagnetic phases.
room temperature, three types of Fe sites are present: those Fe sites rich in Ni atoms, which are the majority and behave as ferromagnetic because they present high spin values (these sites account for the HMFD); those Fe sites rich in Fe atoms which behave as paramagnetic for all temperatures; and those which
Fig. 6. Real part of the AC susceptibility vs. T curves of the sample for different frequencies.
present intermediate numbers of Fe and Ni atoms, which originate frustration due to the disorder and the competition between low spin and high spin sites. The latter sites behave as paramagnetic at room temperature. These sites and those rich in Fe atoms account for the singlet of the spectra. When temperature decreases, the mean hyperfine magnetic field increases as a consequence of the increase of the ferromagnetic coupling of the first sites and of the decrease of the number of paramagnetic sites (see inset of Fig. 4) which now are behaving ferromagnetic. This process continues up to 61 K. At this temperature, some of the remaining frustrated sites are freezing, but due they are in the presence of a ferromagnetic matrix, they present a preferential freezing orientation, of the matrix. In this way, the mean hyperfine magnetic field presents the additional contribution of the frustrated sites. This process continues as temperature decreases. The behavior of the δ values of the FCC ferromagnetic phase is in accordance with this model. Below 61 K, the δ values increase quickly due to the contribution of new sites which are freezing and then contribute to the ferromagnetic ones with different surroundings, causing the mean density of s electrons in the Fe nucleus to decrease. The different values of the anomalous temperature detected by the Mossbauer and ac susceptibility techniques are due to their different measurement times. Finally, our results and interpretation indicates that the Invar alloy obtained by MA behaves in according with the second model proposed for the Invar Fe–Ni samples, i. e. a mixed exchange model.
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4. Conclusions Our results which give clear evidence for the anomalous behavior of the mean hyperfine magnetic field and of the lattice parameter, detected at room temperature for the mechanically alloyed Fe67.5Ni32.5 [34], can be assigned to a RSG behavior of the FCC Fe–Ni phase. This phenomenon was evidenced by anomalies from the magnetic and paramagnetic phase proportions, obtained by Mössbauer spectrometry, approximately 61 K, and by the peaks of the real part of the ac magnetic susceptibility scans of the sample, observed to appear between 31 and 39 K as a function of the applied frequency. These results, specially the presence of a RSG transition, are evidence that our sample behaves like the mixed bond model proposed for Invar alloys.
Acknowledgements The authors would like to thank the support given by the Universidad del Valle. G.A.P.A., the Sabbatical Year Grants provided by the Spanish Science Ministry and Universidad del Valle.
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