- Email: [email protected]
Mössbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co0.2Fe0.8)72.5Si12.5B15 glass coated micro-wires
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 311 (2007) 555–559 www.elsevier.com/locate/jmmm
Mo¨ssbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co0.2Fe0.8)72.5Si12.5B15 glass coated micro-wires I. Nowika,, I. Felnera, H. Garcia-Miquela,b a
b
The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Department of Electronic Engineering, Polytechnic University of Valencia, Valencia 46022, Spain Received 4 January 2006; received in revised form 12 June 2006 Available online 14 September 2006
Abstract Thermo-gravimetric, differential scanning calorimetry and comprehensive 57Fe Mo¨ssbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co0.2Fe0.8)72.5Si12.5B15 micro-wires have been recorded. The Curie temperature of the amorphous phase is TC(amorp) 730 K. The analysis of the Mo¨ssbauer spectra reveals that below 623 K the easy axis of the magnetization is axialalong the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mo¨ssbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly a-Fe(Si) and Fe2B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both a-Fe(Si) and Fe2B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating. r 2006 Elsevier B.V. All rights reserved. PACS: 78.70.Gq; 75.80.+q; 77.84.Lf; 76.80.+y Keywords: Magnetic micro-wires; Mo¨ssbauer spectroscopy; Spin reorientations
1. Introduction Several recent publications [1–3] discuss the magnetic properties of soft magnetic glass-covered amorphous Corich and Co–Fe micro-wires, because of their outstanding magnetic properties and reduced diameter. These materials were proposed in applications as sensing elements in sensor devices, as active elements in magnetic shielding or making use of their absorption characteristics [4]. Therefore, particular interest was focused on their properties such as magneto-impedance and microwave behavior. The wires are prepared in such a way that the insulating glass coating covers the metallic nucleus. The coating induces additional internal stress due to the difference between the thermal expansion coefficient of the glass coating and the metallic nucleus [2]. By using different thickness of glass coating, it Corresponding author. Tel.: +972 2 6584347; fax: +972 2 6586347.
E-mail address: [email protected] (I. Nowik). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.07.039
was shown that this stress affects the process of magnetization reversal and surface domain structure. The magnetic Fe-based wires are basically axially magnetized (along the wire), with a small fraction tangential (on the circumference). Amorphous wires are non-equilibrium metallic solids, formed by very rapid solidification from the molten to the amorphous state. Since the amorphous state is essentially metastable, it can easily transform into a more stable crystalline state. However, the most promising properties of the wires discussed above, have been found to deteriorate drastically upon crystallization. Mo¨ssbauer spectroscopy studies (MS) of 57Fe have been proved to be a powerful tool in the determination of the magnetic nature of Fe in its various locations. The analysis of the MS spectra also yields the orientation of the magnetization in the sample to be studied. Here we report MS, up to 800 K, of amorphous glass coated micro-wires of (Co0.2Fe0.8)72.5Si12.5B15 which are ferromagnetic (FM) at room temperature, their Curie temperature
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
being TC(amorp) 730 K. The purpose of this study was to reveal possible changes in the orientation of the magnetization in these glass-coated micro-wires as a function of temperature, in both the amorphous and crystalline phases.
(Co0.2Fe0.8)72.5Si12.5B15
2.70
mW
556
3.25
2. Experimental details Glass-coated amorphous micro-wires of (CoxFe1x)72.5Si12.5B15 have been obtained by quenching and drawing the molten alloy (Taylor-Ulitovskue technique), having a metallic nucleus of around 4 mm in diameter and a thickness for the Pyrex-loke coating of around 3 mm, Fig. 1. The analysis and the characterization of the wires were described in Ref. [4]. 57 Fe Mo¨ssbauer studies of magnetic micro-wires of (Co0.2Fe0.8)72.5Si12.5B15 were performed using a conventional constant acceleration drive in the transmission mode, in conjunction with a 30 mCi 57Co:Rh source. The velocity calibration and zero velocity reference are those obtained from an iron foil spectrum at room temperature. The experimental spectra were least square fitted with theoretical spectra, in which the magnetic hyperfine field distribution and the angle between the magnetization axis and the Mo¨ssbauer g ray were adjusted. The micro-wires were packed in the absorber holder so that the g rays were perpendicular to the wires. Thermo-gravimetric study (TGA) of the sample with and without external magnetic field was carried out at a heating rate of 10 1C/min under nitrogen flow by using a Mettler TGA/STDA 851 equipped with a small permanent magnet. Differential scanning calorimetry (DSC) measurements were carried out in nitrogen atmosphere on a Mettler DSC-30 instrument (Mettler TC11 TA processor). 3. Experimental results The thermo-gravimetric (TGA) measurement with a small magnetic field directed opposite to the gravitation
Fig. 1. Scanning electron microscope picture of a glass coated Fe-based micro-wire.
Weight (mg)
3.00 800
2.55
850 T (K)
2.40
450
600
750 900 Temperature (K)
1050
Fig. 2. Thermo-gravimetric measurement of (Co0.2Fe0.8)72.5Si12.5B15 with a small magnetic field directed opposite to the gravitation force. The DCS curve, shown in the insert, exhibits a strong exothermic peak around 835 K.
force for (Co0.2Fe0.8)72.5Si12.5B15 is displayed in Fig. 2. The strong exothermic peak, which appears at around 835 K, in the DSC curve (Fig. 2, inset) is due to the crystallization process. This implies that the as prepared wires are in the amorphous state. This result is consistent with the sharp drop in the TGA curve exhibited in Fig. 2 (main panel). The key points to be noted in understanding the observations shown in Fig. 2 are as follows. (1) Crystallization of the amorphous wires takes place at temperatures above 770 K. The crystallization is completed by formation of the predominantly two FM, a-(Fe, Si) and Fe2B, materials [5,6]. (2) The Curie temperature of the amorphous wires, TC(amorp) is lower than those for the crystalline materials, which are around 1000 K. As a result, the hyperfine magnetic parameters (see below) for crystalline materials are higher than those for the amorphous phase. (3) The atomic magnetic moment in the PM state is lower than the moments in the two ordered phases. Therefore the sample weight in the PM state is higher than those in the FM state. (4) The crystallization process is irreversible, upon cooling the formed FM crystalline phases remain stable. Bearing these points in mind, the interpretation of the TGA curve is straightforward. Upon increasing the temperature from room temperature (RT), the TGA curve exhibits a typical FM behavior with TC(amorp) 730 K. Any increase of the temperature should yield the paramagnetic amorphous state with almost a constant high sample weight. At higher temperatures, the crystallization rate is accelerated and the growth of the two FM a-(Fe, Si) and Fe2B phases dominates the TGA curve features. This is most apparent at 835 K, where the magnetic moment increases, and as a result the weight is sharply decreased. At further fast warming, the crystallization process ends (855 K) and the magnetization reaches a new maximum, and then drops toward the Curie temperatures of the crystalline phases, 980–1020 K.
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
557
Mo¨ssbauer spectra of (Co0.2Fe0.8)72.5Si12.5B15 amorphous micro-wires at various temperatures have been measured. In Figs. 3 and 4 we display some of the spectra obtained. The order of the measurements in the temperature range 620–675 K was 623 K-673 K (Fig. 3) 620 K-670 K (Fig. 4). The spectra were analyzed in terms of an asymmetric Gaussian distribution of hyperfine fields, which contained three free parameters (field at maximum, left and right width at half maximum). The field distribution obtained for the spectrum at RT shown in Fig. 5 is very similar to those observed in similar systems [7]. Similar distributions were obtained for all the spectra of the (Co0.2Fe0.8)72.5Si12.5B15 amorphous wires. In the crystalline form, the spectra were fitted with well-defined hyperfine fields. The Mo¨ssbauer spectra are very sensitive to the magnetization orientation. The relative intensities of the six Mo¨ssbauer absorption lines depend on the angle y between the magnetization axis and the Mo¨ssbauer g rays direction. This dependence is given by Ref. [8]: 3(1+/cos2 yS): 4(1/cos2 yS): (1+/cos2 yS): (1+/cos2 yS): 4(1/cos2 yS): 3(1+/cos2 yS), where
Fig. 4. Mo¨ssbauer spectra of (Co0.2Fe0.8)72.5Si12.5B15 above 620 K. The spectrum at 770 K after 23 h includes the first 4 hours. The crystalline 770 K spectrum was started after the absorber stayed 24 h at this temperature.
Fig. 3. Mo¨ssbauer spectra of (Co0.2Fe0.8)72.5Si12.5B15 up to 673 K.
/cos2 yS is the average of all values of cos2 y in the given absorber, it’s experimental value is obtained from the relative intensities of the absorption lines in the spectra. (i) In the axial case, where the magnetization is along the wires which lie perpendicular to the g rays (y ¼ 901), one obtains /cos2 yS ¼ 0 and the relative intensities are 3:4:1:1:4:3. (ii) For the case of complete random orientation (always valid for powder samples) /cos2 yS ¼ 1/3, and the relative intensities are 3:2:1:1:2:3. (iii) For the case of tangential or radial direction of magnetization /cos2 yS ¼ 1/2, and the relative intensities are 3:4/3:1:1:4/3:3.
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
558
1.0
Relative Probability
(Co0.2Fe0.8) 72.5 Si12.5 B15
0.8 298K
0.6
0.4
0.2
0.0 15
20
25
30
Hyperfine Field (T) Fig. 5. Magnetic hyperfine field distribution in (Co0.2Fe0.8)72.5Si12.5B15 at room temperature.
Analyzing our Mo¨ssbauer spectra of the micro-wires, in their amorphous and crystalline state (Figs. 3, 4), we can divide the temperature dependence of /cos2 yS into five regions (see Fig. 6). (1) Up to 623 K the intensity ratio (3:4:1:1:4:3) is well observed (Fig. 3). Therefore, our conclusion is that up to 623 K, y ¼ 901, the magnetization is along the amorphous wires. (2) Within 623–670 K a sharp spin reorientation transition occurs. The spectra at 670 K and 673 K show definitely that /cos2 yS ¼ 1/2, namely the magnetization orientation in the wires is tangential and/or radial. This spin reorientation in the amorphous wires is reversible, as already previously indicated, the order of measurements was 623 K673 K-620 K-670 K. (3) In the PM range (at 770 K) the spectrum observed within the first 4 hours is a pure paramagnetic quadrupole doublet, Fig. 4. (4) However, at this temperature the amorphous wires slowly start to crystallize, and after 1 day, the Mo¨ssbauer spectrum displays well-defined absorption lines of two magnetically ordered different iron components. This indicates that the wires are crystalline as a whole and with TC higher than TC(amorp). The relative intensities of the absorption lines (Fig. 4), indicate that /cos2 yS ¼ 1/3, namely the direction of magnetization in the crystalline state, is completely random. Note that when the heating rate is fast the crystallization starts at about 855 K, observed in Fig. 2. (5) Cooling the sample back to RT, the crystalline spectrum remains quite similar to that at 770 K. The magnetic hyperfine field values in the crystalline components are higher than those for the amorphous wires (Figs. 4 and 7). On the other hand, the cooling to RT leads to reorientation of the magnetization direction back to /cos2 yS ¼ 0 (y ¼ 901), indicating again axial orientation, like in the amorphous wires below 623 K. This proves that also the crystalline wire undergoes a spin reorientation transition in between 770 and 300 K. The spin reorientation transitions in both the amorphous and crystalline wires
Fig. 6. Temperature dependence of /cos2 yS in (Co0.2Fe0.8)72.5Si12.5B15. The error bars on the /cos2 yS values are less than 0.05.
Fig. 7. Temperature dependence of the magnetic hyperfine fields in (Co0.2Fe0.8)72.5Si12.5B15. The dotted lines follow the expected temperature dependence for a-Fe(Si) and Fe2B.
clearly observed in the Mo¨ssbauer spectra and displayed in Fig. 6 are due to the glass coating. These transitions do not appear in non-coated wires. The transitions are caused by the change with temperature of the glass coating stressinduced magnetic anisotropy. The pressure change is caused by the different thermal expansion coefficients of the glass and the metallic wire. From the analysis of the hyperfine interaction parameters (isomer shift, quadrupole interaction and magnetic hyperfine field) of the RT spectrum of the crystalline wire, we conclude that the two crystalline components observed are predominantly a-Fe(Si) and Fe2B, as previously reported [5,6], with magnetic hyperfine fields of 32.6(5) T and 22.5(5) T [5]. Both materials have Curie temperatures well above 730 K, Figs. 7 and 2. 4. Conclusions Our MS indicate that the magnetization is along the wire at temperatures up to at least 623 K. Above 670 K the
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
magnetization direction is tangential or/and radial. At 770 K, above TC(amorp) the MS spectrum exhibits pure paramagnetic behavior for several hours. Within one day at 770 K slow crystallization occurs, the pure crystalline wires are composed of two magnetic phases, predominantly a superposition of various contributions that correspond to a-Fe(Si) and Fe2B, both with complete random orientation of the magnetic domains. After cooling back to ambient temperature, the MS is still composed of the same two Fe components, but again pure axial magnetic orientation of the magnetization is observed. The changes in orientation of magnetization as a function of temperature are due to changes in the glass coating stress induced magnetic anisotropy. Acknowledgment We gratefully acknowledge the support from the Israel Science Foundation (ISF, 2004 Grant number: 618/04). H.
559
Garcia-Miquel is grateful to the Polytechnic University of Valencia for financial support (Programa de Incentivo a la investigacion de la U.P.V./PPI-00-05).
References [1] H. Garcia-Miquel, M. Vazquez, Physica B 299 (2001) 255. [2] V. Madurga, J.L. Costa, A. Inoue, K.V. Rao, J. Appl. Phys. 68 (1990) 1164. [3] H. Chiriac, Mater. Sci. Eng. A 304–306 (2001) 166. [4] S.E. Lofland, H. Garcia-Miquel, M. Vazquez, S.M. Bhagat, J. Appl. Phys. 92 (2002) 2058. [5] B. Yao, S. Liu, L. Liu, W. Su, Y. Li, J. Appl. Phys. 90 (2001) 1650. [6] I. Nowik, I. Felner, Y. Wolfus, Y. Yeshurun, J. Phys. F: Met Phys. 18 (1988) L181. [7] A. Gupta, S.N. Cane, N. Bhagat, T. Kulik, J. Magn. Magn. Mater. 254 (2003) 492. [8] D. Atkinson, M.R.J. Gibbs, P.T. Squire, Q. Pankhurst, J. Mag. Mat. 131 (1994) 19.
Journal of Magnetism and Magnetic Materials 311 (2007) 555–559 www.elsevier.com/locate/jmmm
Mo¨ssbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co0.2Fe0.8)72.5Si12.5B15 glass coated micro-wires I. Nowika,, I. Felnera, H. Garcia-Miquela,b a
b
The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Department of Electronic Engineering, Polytechnic University of Valencia, Valencia 46022, Spain Received 4 January 2006; received in revised form 12 June 2006 Available online 14 September 2006
Abstract Thermo-gravimetric, differential scanning calorimetry and comprehensive 57Fe Mo¨ssbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co0.2Fe0.8)72.5Si12.5B15 micro-wires have been recorded. The Curie temperature of the amorphous phase is TC(amorp) 730 K. The analysis of the Mo¨ssbauer spectra reveals that below 623 K the easy axis of the magnetization is axialalong the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mo¨ssbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly a-Fe(Si) and Fe2B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both a-Fe(Si) and Fe2B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating. r 2006 Elsevier B.V. All rights reserved. PACS: 78.70.Gq; 75.80.+q; 77.84.Lf; 76.80.+y Keywords: Magnetic micro-wires; Mo¨ssbauer spectroscopy; Spin reorientations
1. Introduction Several recent publications [1–3] discuss the magnetic properties of soft magnetic glass-covered amorphous Corich and Co–Fe micro-wires, because of their outstanding magnetic properties and reduced diameter. These materials were proposed in applications as sensing elements in sensor devices, as active elements in magnetic shielding or making use of their absorption characteristics [4]. Therefore, particular interest was focused on their properties such as magneto-impedance and microwave behavior. The wires are prepared in such a way that the insulating glass coating covers the metallic nucleus. The coating induces additional internal stress due to the difference between the thermal expansion coefficient of the glass coating and the metallic nucleus [2]. By using different thickness of glass coating, it Corresponding author. Tel.: +972 2 6584347; fax: +972 2 6586347.
E-mail address: [email protected] (I. Nowik). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.07.039
was shown that this stress affects the process of magnetization reversal and surface domain structure. The magnetic Fe-based wires are basically axially magnetized (along the wire), with a small fraction tangential (on the circumference). Amorphous wires are non-equilibrium metallic solids, formed by very rapid solidification from the molten to the amorphous state. Since the amorphous state is essentially metastable, it can easily transform into a more stable crystalline state. However, the most promising properties of the wires discussed above, have been found to deteriorate drastically upon crystallization. Mo¨ssbauer spectroscopy studies (MS) of 57Fe have been proved to be a powerful tool in the determination of the magnetic nature of Fe in its various locations. The analysis of the MS spectra also yields the orientation of the magnetization in the sample to be studied. Here we report MS, up to 800 K, of amorphous glass coated micro-wires of (Co0.2Fe0.8)72.5Si12.5B15 which are ferromagnetic (FM) at room temperature, their Curie temperature
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
being TC(amorp) 730 K. The purpose of this study was to reveal possible changes in the orientation of the magnetization in these glass-coated micro-wires as a function of temperature, in both the amorphous and crystalline phases.
(Co0.2Fe0.8)72.5Si12.5B15
2.70
mW
556
3.25
2. Experimental details Glass-coated amorphous micro-wires of (CoxFe1x)72.5Si12.5B15 have been obtained by quenching and drawing the molten alloy (Taylor-Ulitovskue technique), having a metallic nucleus of around 4 mm in diameter and a thickness for the Pyrex-loke coating of around 3 mm, Fig. 1. The analysis and the characterization of the wires were described in Ref. [4]. 57 Fe Mo¨ssbauer studies of magnetic micro-wires of (Co0.2Fe0.8)72.5Si12.5B15 were performed using a conventional constant acceleration drive in the transmission mode, in conjunction with a 30 mCi 57Co:Rh source. The velocity calibration and zero velocity reference are those obtained from an iron foil spectrum at room temperature. The experimental spectra were least square fitted with theoretical spectra, in which the magnetic hyperfine field distribution and the angle between the magnetization axis and the Mo¨ssbauer g ray were adjusted. The micro-wires were packed in the absorber holder so that the g rays were perpendicular to the wires. Thermo-gravimetric study (TGA) of the sample with and without external magnetic field was carried out at a heating rate of 10 1C/min under nitrogen flow by using a Mettler TGA/STDA 851 equipped with a small permanent magnet. Differential scanning calorimetry (DSC) measurements were carried out in nitrogen atmosphere on a Mettler DSC-30 instrument (Mettler TC11 TA processor). 3. Experimental results The thermo-gravimetric (TGA) measurement with a small magnetic field directed opposite to the gravitation
Fig. 1. Scanning electron microscope picture of a glass coated Fe-based micro-wire.
Weight (mg)
3.00 800
2.55
850 T (K)
2.40
450
600
750 900 Temperature (K)
1050
Fig. 2. Thermo-gravimetric measurement of (Co0.2Fe0.8)72.5Si12.5B15 with a small magnetic field directed opposite to the gravitation force. The DCS curve, shown in the insert, exhibits a strong exothermic peak around 835 K.
force for (Co0.2Fe0.8)72.5Si12.5B15 is displayed in Fig. 2. The strong exothermic peak, which appears at around 835 K, in the DSC curve (Fig. 2, inset) is due to the crystallization process. This implies that the as prepared wires are in the amorphous state. This result is consistent with the sharp drop in the TGA curve exhibited in Fig. 2 (main panel). The key points to be noted in understanding the observations shown in Fig. 2 are as follows. (1) Crystallization of the amorphous wires takes place at temperatures above 770 K. The crystallization is completed by formation of the predominantly two FM, a-(Fe, Si) and Fe2B, materials [5,6]. (2) The Curie temperature of the amorphous wires, TC(amorp) is lower than those for the crystalline materials, which are around 1000 K. As a result, the hyperfine magnetic parameters (see below) for crystalline materials are higher than those for the amorphous phase. (3) The atomic magnetic moment in the PM state is lower than the moments in the two ordered phases. Therefore the sample weight in the PM state is higher than those in the FM state. (4) The crystallization process is irreversible, upon cooling the formed FM crystalline phases remain stable. Bearing these points in mind, the interpretation of the TGA curve is straightforward. Upon increasing the temperature from room temperature (RT), the TGA curve exhibits a typical FM behavior with TC(amorp) 730 K. Any increase of the temperature should yield the paramagnetic amorphous state with almost a constant high sample weight. At higher temperatures, the crystallization rate is accelerated and the growth of the two FM a-(Fe, Si) and Fe2B phases dominates the TGA curve features. This is most apparent at 835 K, where the magnetic moment increases, and as a result the weight is sharply decreased. At further fast warming, the crystallization process ends (855 K) and the magnetization reaches a new maximum, and then drops toward the Curie temperatures of the crystalline phases, 980–1020 K.
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
557
Mo¨ssbauer spectra of (Co0.2Fe0.8)72.5Si12.5B15 amorphous micro-wires at various temperatures have been measured. In Figs. 3 and 4 we display some of the spectra obtained. The order of the measurements in the temperature range 620–675 K was 623 K-673 K (Fig. 3) 620 K-670 K (Fig. 4). The spectra were analyzed in terms of an asymmetric Gaussian distribution of hyperfine fields, which contained three free parameters (field at maximum, left and right width at half maximum). The field distribution obtained for the spectrum at RT shown in Fig. 5 is very similar to those observed in similar systems [7]. Similar distributions were obtained for all the spectra of the (Co0.2Fe0.8)72.5Si12.5B15 amorphous wires. In the crystalline form, the spectra were fitted with well-defined hyperfine fields. The Mo¨ssbauer spectra are very sensitive to the magnetization orientation. The relative intensities of the six Mo¨ssbauer absorption lines depend on the angle y between the magnetization axis and the Mo¨ssbauer g rays direction. This dependence is given by Ref. [8]: 3(1+/cos2 yS): 4(1/cos2 yS): (1+/cos2 yS): (1+/cos2 yS): 4(1/cos2 yS): 3(1+/cos2 yS), where
Fig. 4. Mo¨ssbauer spectra of (Co0.2Fe0.8)72.5Si12.5B15 above 620 K. The spectrum at 770 K after 23 h includes the first 4 hours. The crystalline 770 K spectrum was started after the absorber stayed 24 h at this temperature.
Fig. 3. Mo¨ssbauer spectra of (Co0.2Fe0.8)72.5Si12.5B15 up to 673 K.
/cos2 yS is the average of all values of cos2 y in the given absorber, it’s experimental value is obtained from the relative intensities of the absorption lines in the spectra. (i) In the axial case, where the magnetization is along the wires which lie perpendicular to the g rays (y ¼ 901), one obtains /cos2 yS ¼ 0 and the relative intensities are 3:4:1:1:4:3. (ii) For the case of complete random orientation (always valid for powder samples) /cos2 yS ¼ 1/3, and the relative intensities are 3:2:1:1:2:3. (iii) For the case of tangential or radial direction of magnetization /cos2 yS ¼ 1/2, and the relative intensities are 3:4/3:1:1:4/3:3.
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
558
1.0
Relative Probability
(Co0.2Fe0.8) 72.5 Si12.5 B15
0.8 298K
0.6
0.4
0.2
0.0 15
20
25
30
Hyperfine Field (T) Fig. 5. Magnetic hyperfine field distribution in (Co0.2Fe0.8)72.5Si12.5B15 at room temperature.
Analyzing our Mo¨ssbauer spectra of the micro-wires, in their amorphous and crystalline state (Figs. 3, 4), we can divide the temperature dependence of /cos2 yS into five regions (see Fig. 6). (1) Up to 623 K the intensity ratio (3:4:1:1:4:3) is well observed (Fig. 3). Therefore, our conclusion is that up to 623 K, y ¼ 901, the magnetization is along the amorphous wires. (2) Within 623–670 K a sharp spin reorientation transition occurs. The spectra at 670 K and 673 K show definitely that /cos2 yS ¼ 1/2, namely the magnetization orientation in the wires is tangential and/or radial. This spin reorientation in the amorphous wires is reversible, as already previously indicated, the order of measurements was 623 K673 K-620 K-670 K. (3) In the PM range (at 770 K) the spectrum observed within the first 4 hours is a pure paramagnetic quadrupole doublet, Fig. 4. (4) However, at this temperature the amorphous wires slowly start to crystallize, and after 1 day, the Mo¨ssbauer spectrum displays well-defined absorption lines of two magnetically ordered different iron components. This indicates that the wires are crystalline as a whole and with TC higher than TC(amorp). The relative intensities of the absorption lines (Fig. 4), indicate that /cos2 yS ¼ 1/3, namely the direction of magnetization in the crystalline state, is completely random. Note that when the heating rate is fast the crystallization starts at about 855 K, observed in Fig. 2. (5) Cooling the sample back to RT, the crystalline spectrum remains quite similar to that at 770 K. The magnetic hyperfine field values in the crystalline components are higher than those for the amorphous wires (Figs. 4 and 7). On the other hand, the cooling to RT leads to reorientation of the magnetization direction back to /cos2 yS ¼ 0 (y ¼ 901), indicating again axial orientation, like in the amorphous wires below 623 K. This proves that also the crystalline wire undergoes a spin reorientation transition in between 770 and 300 K. The spin reorientation transitions in both the amorphous and crystalline wires
Fig. 6. Temperature dependence of /cos2 yS in (Co0.2Fe0.8)72.5Si12.5B15. The error bars on the /cos2 yS values are less than 0.05.
Fig. 7. Temperature dependence of the magnetic hyperfine fields in (Co0.2Fe0.8)72.5Si12.5B15. The dotted lines follow the expected temperature dependence for a-Fe(Si) and Fe2B.
clearly observed in the Mo¨ssbauer spectra and displayed in Fig. 6 are due to the glass coating. These transitions do not appear in non-coated wires. The transitions are caused by the change with temperature of the glass coating stressinduced magnetic anisotropy. The pressure change is caused by the different thermal expansion coefficients of the glass and the metallic wire. From the analysis of the hyperfine interaction parameters (isomer shift, quadrupole interaction and magnetic hyperfine field) of the RT spectrum of the crystalline wire, we conclude that the two crystalline components observed are predominantly a-Fe(Si) and Fe2B, as previously reported [5,6], with magnetic hyperfine fields of 32.6(5) T and 22.5(5) T [5]. Both materials have Curie temperatures well above 730 K, Figs. 7 and 2. 4. Conclusions Our MS indicate that the magnetization is along the wire at temperatures up to at least 623 K. Above 670 K the
ARTICLE IN PRESS I. Nowik et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 555–559
magnetization direction is tangential or/and radial. At 770 K, above TC(amorp) the MS spectrum exhibits pure paramagnetic behavior for several hours. Within one day at 770 K slow crystallization occurs, the pure crystalline wires are composed of two magnetic phases, predominantly a superposition of various contributions that correspond to a-Fe(Si) and Fe2B, both with complete random orientation of the magnetic domains. After cooling back to ambient temperature, the MS is still composed of the same two Fe components, but again pure axial magnetic orientation of the magnetization is observed. The changes in orientation of magnetization as a function of temperature are due to changes in the glass coating stress induced magnetic anisotropy. Acknowledgment We gratefully acknowledge the support from the Israel Science Foundation (ISF, 2004 Grant number: 618/04). H.
559
Garcia-Miquel is grateful to the Polytechnic University of Valencia for financial support (Programa de Incentivo a la investigacion de la U.P.V./PPI-00-05).
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