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Temperature dependence of the coercive field of gas atomized Fe73.5Si13.5B9Nb3Cu1
Journal of Alloys and Compounds 536S (2012) S300–S303
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Temperature dependence of the coercive field of gas atomized Fe73.5 Si13.5 B9 Nb3 Cu1 A. García-Escorial a,∗ , M. Lieblich a , A. Hernando b , A. Aragón b , P. Marín b a b
CENIM-CSIC, Avda, Gregorio del Amo, 8, 28040 Madrid, Spain Instituto de Magnetismo Aplicado, IMA, P.O. Box 155, 28230 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 22 June 2011 Received in revised form 28 October 2011 Accepted 3 November 2011 Available online 11 November 2011 Keywords: Metals and alloys, Magnetic materials Powder metallurgy Rapìd solidification Quenching Microstructure Magnetic measurements Metallography Scanning electron microscopy SEM
a b s t r a c t In this work, the dependence of the coercive field of Fe73.5 Si13.5 B9 Nb3 Cu1 gas atomized powder with the temperature for different particle sizes has been studied, observing an anomalous behavior in the under 25 powder particle size fraction. This unusual behavior is related with the microstructure of the powder, and is attributed to the presence of a multiphase magnetic system, with non-magnetic regions decoupling the ferromagnetic domains. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The alloy Fe73.5 Si13.5 B9 Nb3 Cu1 has been the object of an enormous research interest due to its excellent soft magnetic behavior after partial crystallization [1,2]. This alloy has been produced and studied in many different forms, such as, melt spun ribbons, amorphous material, partially crystallized state, mechanically alloyed, ball milled and recently also gas atomization [3,4]. In a previous work, Fe73.5 Si13.5 B9 Nb3 Cu1 was obtained as powder particles by gas atomization and ball milling of melt spun ribbons to show the analogies and differences between both powders [3]. Furthermore, gas atomized Fe73.5 Si13.5 B9 Nb3 Cu1 and Fe93 Si7 were investigated to study the influence of the alloying elements [4]. In these works an increase in the coercive field, Hc, as particle size decreased was obtained in both alloys [5]. Moreover, it was observed that for the same powder particle size and similar grain size the coercive fields for the Fe73.5 Si13.5 B9 Nb3 Cu1 powder were almost one order of magnitude higher than those of the Fe93 Si7 alloy. This effect was attributed to Nb segregation, which could act as pinning center of domains walls.
∗ Corresponding author. Tel.: +34 91 5538900; fax: +34 91 5347425. E-mail address: [email protected] (A. García-Escorial). 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.11.015
This magnetic hardening is especially relevant nowadays due to the interest in obtaining hard magnetic materials without Rare Earths elements. Magnetic softening is also important since powders can be used as magnetic flux multiplier, forming nucleus of electromagnetic devices easy to form into different shapes. In this work, with the aim of deepening our understanding of the high coercive fields presented by the Fe73.5 Si13.5 B9 Nb3 Cu1 gasatomized powder particles, the dependence of the coercive field with the temperature from 4 to 350 K has been investigated. The smallest and largest particles were specifically used to simplify the study. The big difference in size between those of less than 25 m, the ones with higher cooling rate, and those in the range of 500–1000 m, with slower cooling rate, implies very different solidification rates that lead to different phases, microstructure and grain and particle size, strongly correlated to the coercivity and its thermal dependence as outlined below. 2. Experimental techniques Fe73.5 Si13.5 B9 Nb3 Cu1 , composition expressed in atomic per cent, was gas atomized at CENIM in a confined nozzle atomizer, already described in [3,4]. Gas atomization is a containerless process, where the liquid melt solidifies rapidly at high undercooling, with a cooling rate of the order of 103 –105 K s−1 . The Fe73.5 Si13.5 B9 Nb3 Cu1 at% alloy (83.4Fe–7.7 Si–1.2 B–5.66 Nb–1.3 Cu% in weight) was prepared from Fe–Si and Fe–Nb master alloys and Fe, B, and Cu elements. The resulting powder was allowed to cool down in the inert gas atmosphere of the atomizer. Afterwards, it was collected in air and sieved to achieve separation into different
A. García-Escorial et al. / Journal of Alloys and Compounds 536S (2012) S300–S303
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25
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20 500
15 10 5
0
20
40
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2θ (degree)
0 0
50
100
150
200
250
300
350 Fig. 2. XRD of as-atomized under 25 m powder, A.
Temperature (K) Fig. 1. Evolution of the coercive field with the temperature for under 25 m powder, A (full symbols, black) and for a 500–1000 m particle, B (open symbols, gray).
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I (a.u.)
sizes ranges. This work has been focused on the small, less than 25 m, and larger 500–1000 m size ranges, labeled as A and B respectively. Hysteresis loops at 300 K for the different particle size fractions were measured by using a vibrating sample magnetometer PPMS-VSM Quantum Design at 300 K and with a maximum field of 4 T, on encapsulated samples. The dependence of the coercive field of the gas atomized powder with the temperature from 4 to 350 K as a function of the powder particle size was measured. The determination of the oxygen content was carried out in both size fractions by an infrared absorption method after fusion under inert gas of the powders. The smallest size fraction has an oxygen content of 0.2 wt% and the larger size 0.03 wt%, one order of magnitude less. The Fe73.5 Si13.5 B9 Nb3 Cu1 powder size fractions were studied by X-ray diffraction (XRD) using Cu-Ka radiation. The morphology and microstructure of particles as a function of size range were observed by scanning electron microscopy (SEM) equipped with an X-ray energy dispersive analysis unit (EDX). Electron channeling contrast images have been taken, that are produced from electrons which channel down the crystals planes, and thus, are sensitive to crystallographic orientation.
300
100
0
20
40
60 80 2θ (degree)
100
120
Fig. 3. XRD of as-atomized 500–1000 m particles, B.
3. Results and discussion Fig. 1 shows the influence of temperature on the coercive fields for both particle sizes, A and B, where an anomalous increase in the coercive field with temperature is observed in the smaller powder particles. In this graph, it can be seen that at room temperature the smaller powder is magnetically harder than the larger one, as has been reported in previous works [3,4]. However, this is not the case at temperatures below 180 K. For homogeneous ferromagnets the coercive field depends inversely on the exchange correlation length that generally increases with temperature [5]. In the present case, the observed thermal dependence indicates that an effect, different
from the one associated with size, is affecting the overall behavior of the small Fe73.5 Si13.5 B9 Nb3 Cu1 particles. A similar anomalous increase in coercivity with temperature, as shown in Fig. 1, was observed and reported for Fe–Cu alloys during its spinodal decomposition [6]. In heterogeneous microstructures some ferromagnetic phases could evolve during heating towards paramagnetic states so decreasing the exchange coupling between ferromagnetic regions which gives rise to a shortening of the exchange correlation length and consequently to an increase in the coercive field. To correlate this magnetic behavior with the powder microstructure, particles were studied through X-ray
Fig. 4. SEM micrographs of as-atomized under 25 m powder particles, A.
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A. García-Escorial et al. / Journal of Alloys and Compounds 536S (2012) S300–S303
Fig. 5. Backscattered SEM micrographs of the same area of a 500–1000 m particle, B, showing crystallographic contrast.
Fig. 6. SEM micrographs of 500–1000 m particles, B, at different magnifications.
diffractometry and scanning electron microscopy. Figs. 2 and 3 show the XRD patterns for the less than 25 m powder particles, A, and for the 500–1000 m particles, B. The main peaks of both patterns, 2 = 45, 65.5, 83, 99.5 and 117◦ , could be attributed to ˚ Fe3 Si, Im−3m, several phases as follows: bcc-Fe, Im−3m, a = 2.86 A; ˚ Fe3 Si, Fm−3m, a = 5.65 A; ˚ and/or the B6 CuFe73.5 Nb3 Si16.5 , a = 2.84 A; ˚ ordered phase identified by Mattern [7]. In this Fm−3m, a = 5.66 A, work, this undetermined phase has been denominated Fex (Si, B, Nb, Cu). The XRD pattern of the as-atomized < 25 m powder size fraction, A, Fig. 2, shows the presence of this Fex (Si, B, Nb, Cu) phase and a weak signal of the ternary Fe16 Nb6 Si7 phase (2 = 41.3◦ ).
I (a.u.)
1000
500
0 20
40
60
80
100
2θ (degree) Fig. 7. XRD of under 25 m powder, A, annealed 1 h at 973 K.
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The XRD pattern of the as-atomized 500–1000 m powder size fraction, B, shows the presence of the Fex (Si, B, Nb, Cu) phase, and signal of the ternary Fe16 Nb6 Si7 phase, Fig. 3, the latter stronger than in the <25 m powder. Fig. 4 presents backscattered SEM micrographs of as-atomized <25 m powder, A, showing a homogeneous and fine microstructure of grains, of around 5 m in size, of different contrasts. Inside the grains a very fine cellular-dendrite network of brighter contrast is observed. Some 0.5 m size rhombohedra crystals are also present. EDX analysis indicates that these rhombohedra crystals are very Nb enriched. The remaining matrix must have a composition close to the nominal one, although EDX was unable to analyse the boron content. Fig. 5 presents backscattered SEM micrographs of the same area of a 500–1000 m particle showing crystallographic contrast and a structure of dendrite grains of around 25 m in size. The dendrite microstructure is coarse with a dendrite arms spacing of several micrometers, Fig. 6. The dendrite arms are bright, indicating the presence of heavier elements than in the matrix. EDX analysis points to a Fe, Nb and Si composition, most probably corresponding to the Fe16 Nb6 Si7 phase. Some other precipitates, very rich in Fe, also appear in this powder size, as shown in Fig. 6a (arrowed). Some dendrite-cells have a fine cellular superimposed structure, similarly to the one that appeared in the under 25 m powder size fraction, Fig. 6b. Several 1 m size rhombohedra Nb rich crystals are also observed in Fig. 6. By combining XRD and SEM-EDX results, it follows that, apart from the Fex (Si, B, Nb, Cu) phase, the small particles, A, give a weak signal of the ternary Fe16 Nb6 Si7 phase and 0.5 m Nb crystals, whereas the large particles, B, show a stronger signal of the ternary phase, 1 m Nb crystals and Fe precipitates.
A. García-Escorial et al. / Journal of Alloys and Compounds 536S (2012) S300–S303
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Fig. 8. SEM micrographs of under 25 m powder, A, annealed 1 h at 973 K.
From what was previously stated, as the temperature rises, a heterogeneous microstructures may be behind a shortening of the exchange correlation length and consequently an increase in the coercive field. To carry out further research into the existence of such a heterogeneous microstructure a sample of the under 25 m powder was submitted to heat treatment under argon atmosphere at 973 K for 1 h. Fig. 7 shows the XRD pattern of the treated powder. Peaks of three phases are present: the Fex (Si, B, Nb, Cu) phase, the ternary Fe16 Nb6 Si7 phase and pure Cu, on 2 = 43.3◦ . Fig. 8 shows backscattered SEM micrographs of the heat treated powder. The grain microstructure is similar to that of the untreated powder, the main differences being a bright segregation at the grain boundaries with the formation of small bright precipitates. In both cases, EDX analysis indicates a clear enrichment of Cu. The presence of this segregated Cu at the grain boundaries suggests the possible existence of modulated compositions, with a broad distribution of Curie temperatures, that gives rise to the shortening of the correlation length as they are reached. Measurement of the coercive field of the treated powder is in progress. The exchange correlation length depends on the grain size that determines the correlation length of the anisotropy axes, and the exchange coupling between grains [5]. For particles with grains of the same composition and size, the small particles that have less grains and fewer boundaries are expected to be harder [3,4]. However for grains of different compositions, the influence of particle size is less important since the degree of exchange coupling between grains becomes the most relevant parameter for determining the coercivity. It should also be noted that the
anomalous thermal dependence, shown in Fig. 1, indicates that tailoring microstructure and size, by controlling the cooling rate of more adequate multiphase systems, could be a promising procedure to develop soft or hard magnets, avoiding Rare Earths metals that is nowadays an important target for the engineering of magnetic materials. 4. Conclusions In this work, gas atomized Fe73.5 Si13.5 B9 Nb3 Cu1 powder particles in the size range of less than 25 m and 500–1000 m were studied. It was found that the coercive field of the particles under 25 m increases with temperature. To inquire into this anomalous thermal dependence microstructural studies were carried out and as a result it is proposed that Cu rich regions at inter-grain boundaries could act as exchange decoupling regions contributing to the thermal increase of coercivity. References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [3] P. Marin, M. López, A. García-Escorial, M. Lieblich, Mater. Sci. Eng. 449 (2007) 414. [4] A. García-Escorial, M. Lieblich, M. López, P. Marin, J. Alloy Compd. 509 (2011) 239–242. [5] A. Hernando, P. Marin, M. López, T. Kulik, L.K. Varga, G. Hadjipanayis, Phys. Rev. B 69 (2004) 52501. [6] P. Crespo, A. Hernando, A. García-Escorial, Phys. Rev. B 49 (18) (1994) 13227. [7] N. Mattern, Mater. Sci. Forum. 79 (1991) 607.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Temperature dependence of the coercive field of gas atomized Fe73.5 Si13.5 B9 Nb3 Cu1 A. García-Escorial a,∗ , M. Lieblich a , A. Hernando b , A. Aragón b , P. Marín b a b
CENIM-CSIC, Avda, Gregorio del Amo, 8, 28040 Madrid, Spain Instituto de Magnetismo Aplicado, IMA, P.O. Box 155, 28230 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 22 June 2011 Received in revised form 28 October 2011 Accepted 3 November 2011 Available online 11 November 2011 Keywords: Metals and alloys, Magnetic materials Powder metallurgy Rapìd solidification Quenching Microstructure Magnetic measurements Metallography Scanning electron microscopy SEM
a b s t r a c t In this work, the dependence of the coercive field of Fe73.5 Si13.5 B9 Nb3 Cu1 gas atomized powder with the temperature for different particle sizes has been studied, observing an anomalous behavior in the under 25 powder particle size fraction. This unusual behavior is related with the microstructure of the powder, and is attributed to the presence of a multiphase magnetic system, with non-magnetic regions decoupling the ferromagnetic domains. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The alloy Fe73.5 Si13.5 B9 Nb3 Cu1 has been the object of an enormous research interest due to its excellent soft magnetic behavior after partial crystallization [1,2]. This alloy has been produced and studied in many different forms, such as, melt spun ribbons, amorphous material, partially crystallized state, mechanically alloyed, ball milled and recently also gas atomization [3,4]. In a previous work, Fe73.5 Si13.5 B9 Nb3 Cu1 was obtained as powder particles by gas atomization and ball milling of melt spun ribbons to show the analogies and differences between both powders [3]. Furthermore, gas atomized Fe73.5 Si13.5 B9 Nb3 Cu1 and Fe93 Si7 were investigated to study the influence of the alloying elements [4]. In these works an increase in the coercive field, Hc, as particle size decreased was obtained in both alloys [5]. Moreover, it was observed that for the same powder particle size and similar grain size the coercive fields for the Fe73.5 Si13.5 B9 Nb3 Cu1 powder were almost one order of magnitude higher than those of the Fe93 Si7 alloy. This effect was attributed to Nb segregation, which could act as pinning center of domains walls.
∗ Corresponding author. Tel.: +34 91 5538900; fax: +34 91 5347425. E-mail address: [email protected] (A. García-Escorial). 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.11.015
This magnetic hardening is especially relevant nowadays due to the interest in obtaining hard magnetic materials without Rare Earths elements. Magnetic softening is also important since powders can be used as magnetic flux multiplier, forming nucleus of electromagnetic devices easy to form into different shapes. In this work, with the aim of deepening our understanding of the high coercive fields presented by the Fe73.5 Si13.5 B9 Nb3 Cu1 gasatomized powder particles, the dependence of the coercive field with the temperature from 4 to 350 K has been investigated. The smallest and largest particles were specifically used to simplify the study. The big difference in size between those of less than 25 m, the ones with higher cooling rate, and those in the range of 500–1000 m, with slower cooling rate, implies very different solidification rates that lead to different phases, microstructure and grain and particle size, strongly correlated to the coercivity and its thermal dependence as outlined below. 2. Experimental techniques Fe73.5 Si13.5 B9 Nb3 Cu1 , composition expressed in atomic per cent, was gas atomized at CENIM in a confined nozzle atomizer, already described in [3,4]. Gas atomization is a containerless process, where the liquid melt solidifies rapidly at high undercooling, with a cooling rate of the order of 103 –105 K s−1 . The Fe73.5 Si13.5 B9 Nb3 Cu1 at% alloy (83.4Fe–7.7 Si–1.2 B–5.66 Nb–1.3 Cu% in weight) was prepared from Fe–Si and Fe–Nb master alloys and Fe, B, and Cu elements. The resulting powder was allowed to cool down in the inert gas atmosphere of the atomizer. Afterwards, it was collected in air and sieved to achieve separation into different
A. García-Escorial et al. / Journal of Alloys and Compounds 536S (2012) S300–S303
S301
40 1500
35
I (a.u)
30
Hc (Oe)
25
1000
20 500
15 10 5
0
20
40
60
80
100
120
2θ (degree)
0 0
50
100
150
200
250
300
350 Fig. 2. XRD of as-atomized under 25 m powder, A.
Temperature (K) Fig. 1. Evolution of the coercive field with the temperature for under 25 m powder, A (full symbols, black) and for a 500–1000 m particle, B (open symbols, gray).
200
I (a.u.)
sizes ranges. This work has been focused on the small, less than 25 m, and larger 500–1000 m size ranges, labeled as A and B respectively. Hysteresis loops at 300 K for the different particle size fractions were measured by using a vibrating sample magnetometer PPMS-VSM Quantum Design at 300 K and with a maximum field of 4 T, on encapsulated samples. The dependence of the coercive field of the gas atomized powder with the temperature from 4 to 350 K as a function of the powder particle size was measured. The determination of the oxygen content was carried out in both size fractions by an infrared absorption method after fusion under inert gas of the powders. The smallest size fraction has an oxygen content of 0.2 wt% and the larger size 0.03 wt%, one order of magnitude less. The Fe73.5 Si13.5 B9 Nb3 Cu1 powder size fractions were studied by X-ray diffraction (XRD) using Cu-Ka radiation. The morphology and microstructure of particles as a function of size range were observed by scanning electron microscopy (SEM) equipped with an X-ray energy dispersive analysis unit (EDX). Electron channeling contrast images have been taken, that are produced from electrons which channel down the crystals planes, and thus, are sensitive to crystallographic orientation.
300
100
0
20
40
60 80 2θ (degree)
100
120
Fig. 3. XRD of as-atomized 500–1000 m particles, B.
3. Results and discussion Fig. 1 shows the influence of temperature on the coercive fields for both particle sizes, A and B, where an anomalous increase in the coercive field with temperature is observed in the smaller powder particles. In this graph, it can be seen that at room temperature the smaller powder is magnetically harder than the larger one, as has been reported in previous works [3,4]. However, this is not the case at temperatures below 180 K. For homogeneous ferromagnets the coercive field depends inversely on the exchange correlation length that generally increases with temperature [5]. In the present case, the observed thermal dependence indicates that an effect, different
from the one associated with size, is affecting the overall behavior of the small Fe73.5 Si13.5 B9 Nb3 Cu1 particles. A similar anomalous increase in coercivity with temperature, as shown in Fig. 1, was observed and reported for Fe–Cu alloys during its spinodal decomposition [6]. In heterogeneous microstructures some ferromagnetic phases could evolve during heating towards paramagnetic states so decreasing the exchange coupling between ferromagnetic regions which gives rise to a shortening of the exchange correlation length and consequently to an increase in the coercive field. To correlate this magnetic behavior with the powder microstructure, particles were studied through X-ray
Fig. 4. SEM micrographs of as-atomized under 25 m powder particles, A.
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Fig. 5. Backscattered SEM micrographs of the same area of a 500–1000 m particle, B, showing crystallographic contrast.
Fig. 6. SEM micrographs of 500–1000 m particles, B, at different magnifications.
diffractometry and scanning electron microscopy. Figs. 2 and 3 show the XRD patterns for the less than 25 m powder particles, A, and for the 500–1000 m particles, B. The main peaks of both patterns, 2 = 45, 65.5, 83, 99.5 and 117◦ , could be attributed to ˚ Fe3 Si, Im−3m, several phases as follows: bcc-Fe, Im−3m, a = 2.86 A; ˚ Fe3 Si, Fm−3m, a = 5.65 A; ˚ and/or the B6 CuFe73.5 Nb3 Si16.5 , a = 2.84 A; ˚ ordered phase identified by Mattern [7]. In this Fm−3m, a = 5.66 A, work, this undetermined phase has been denominated Fex (Si, B, Nb, Cu). The XRD pattern of the as-atomized < 25 m powder size fraction, A, Fig. 2, shows the presence of this Fex (Si, B, Nb, Cu) phase and a weak signal of the ternary Fe16 Nb6 Si7 phase (2 = 41.3◦ ).
I (a.u.)
1000
500
0 20
40
60
80
100
2θ (degree) Fig. 7. XRD of under 25 m powder, A, annealed 1 h at 973 K.
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The XRD pattern of the as-atomized 500–1000 m powder size fraction, B, shows the presence of the Fex (Si, B, Nb, Cu) phase, and signal of the ternary Fe16 Nb6 Si7 phase, Fig. 3, the latter stronger than in the <25 m powder. Fig. 4 presents backscattered SEM micrographs of as-atomized <25 m powder, A, showing a homogeneous and fine microstructure of grains, of around 5 m in size, of different contrasts. Inside the grains a very fine cellular-dendrite network of brighter contrast is observed. Some 0.5 m size rhombohedra crystals are also present. EDX analysis indicates that these rhombohedra crystals are very Nb enriched. The remaining matrix must have a composition close to the nominal one, although EDX was unable to analyse the boron content. Fig. 5 presents backscattered SEM micrographs of the same area of a 500–1000 m particle showing crystallographic contrast and a structure of dendrite grains of around 25 m in size. The dendrite microstructure is coarse with a dendrite arms spacing of several micrometers, Fig. 6. The dendrite arms are bright, indicating the presence of heavier elements than in the matrix. EDX analysis points to a Fe, Nb and Si composition, most probably corresponding to the Fe16 Nb6 Si7 phase. Some other precipitates, very rich in Fe, also appear in this powder size, as shown in Fig. 6a (arrowed). Some dendrite-cells have a fine cellular superimposed structure, similarly to the one that appeared in the under 25 m powder size fraction, Fig. 6b. Several 1 m size rhombohedra Nb rich crystals are also observed in Fig. 6. By combining XRD and SEM-EDX results, it follows that, apart from the Fex (Si, B, Nb, Cu) phase, the small particles, A, give a weak signal of the ternary Fe16 Nb6 Si7 phase and 0.5 m Nb crystals, whereas the large particles, B, show a stronger signal of the ternary phase, 1 m Nb crystals and Fe precipitates.
A. García-Escorial et al. / Journal of Alloys and Compounds 536S (2012) S300–S303
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Fig. 8. SEM micrographs of under 25 m powder, A, annealed 1 h at 973 K.
From what was previously stated, as the temperature rises, a heterogeneous microstructures may be behind a shortening of the exchange correlation length and consequently an increase in the coercive field. To carry out further research into the existence of such a heterogeneous microstructure a sample of the under 25 m powder was submitted to heat treatment under argon atmosphere at 973 K for 1 h. Fig. 7 shows the XRD pattern of the treated powder. Peaks of three phases are present: the Fex (Si, B, Nb, Cu) phase, the ternary Fe16 Nb6 Si7 phase and pure Cu, on 2 = 43.3◦ . Fig. 8 shows backscattered SEM micrographs of the heat treated powder. The grain microstructure is similar to that of the untreated powder, the main differences being a bright segregation at the grain boundaries with the formation of small bright precipitates. In both cases, EDX analysis indicates a clear enrichment of Cu. The presence of this segregated Cu at the grain boundaries suggests the possible existence of modulated compositions, with a broad distribution of Curie temperatures, that gives rise to the shortening of the correlation length as they are reached. Measurement of the coercive field of the treated powder is in progress. The exchange correlation length depends on the grain size that determines the correlation length of the anisotropy axes, and the exchange coupling between grains [5]. For particles with grains of the same composition and size, the small particles that have less grains and fewer boundaries are expected to be harder [3,4]. However for grains of different compositions, the influence of particle size is less important since the degree of exchange coupling between grains becomes the most relevant parameter for determining the coercivity. It should also be noted that the
anomalous thermal dependence, shown in Fig. 1, indicates that tailoring microstructure and size, by controlling the cooling rate of more adequate multiphase systems, could be a promising procedure to develop soft or hard magnets, avoiding Rare Earths metals that is nowadays an important target for the engineering of magnetic materials. 4. Conclusions In this work, gas atomized Fe73.5 Si13.5 B9 Nb3 Cu1 powder particles in the size range of less than 25 m and 500–1000 m were studied. It was found that the coercive field of the particles under 25 m increases with temperature. To inquire into this anomalous thermal dependence microstructural studies were carried out and as a result it is proposed that Cu rich regions at inter-grain boundaries could act as exchange decoupling regions contributing to the thermal increase of coercivity. References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [3] P. Marin, M. López, A. García-Escorial, M. Lieblich, Mater. Sci. Eng. 449 (2007) 414. [4] A. García-Escorial, M. Lieblich, M. López, P. Marin, J. Alloy Compd. 509 (2011) 239–242. [5] A. Hernando, P. Marin, M. López, T. Kulik, L.K. Varga, G. Hadjipanayis, Phys. Rev. B 69 (2004) 52501. [6] P. Crespo, A. Hernando, A. García-Escorial, Phys. Rev. B 49 (18) (1994) 13227. [7] N. Mattern, Mater. Sci. Forum. 79 (1991) 607.