ARTICLE IN PRESS
Physica B 404 (2009) 4155–4158
Contents lists available at ScienceDirect
Physica B
0921-45
doi:10.1
� Corr
E-m
journal homepage: www.elsevier.com/locate/physb
Effect of heat treatment on structure, magnetization andmagnetostriction of Fe81Ga19 melt-spun ribbons
Jingjing Zhang, Tianyu Ma, Mi Yan �
State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
Article history:
Received 13 January 2009
Received in revised form
18 June 2009
Accepted 24 July 2009
PACS:
75.80.+q
75.50.Bb
75.60.Ej
Keywords:
Magnetostriction
Magnetization
Fe–Ga alloy
26/$ - see front matter & 2009 Elsevier B.V. A
016/j.physb.2009.07.185
esponding author.
ail address: [email protected] (M. Yan).
a b s t r a c t
Structure, magnetization and magnetostriction of melt-spun Fe81Ga19 ribbons were investigated both
before and after heat treatment. The matrix of melt-spun Fe81Ga19 ribbons kept a body-centered-cubic
(bcc) structure (A2) at room temperature. [10 0] preferred orientation was formed during melt-spinning
process and became stronger with the increase of the ribbon thickness. For the ribbons with a thickness
of 110mm, maximum saturation magnetostrictive strain of �189 ppm along ribbon length was obtained
in the samples heat treated at 800 1C for 3 h and then quenched into water. This value was about
16% larger than that of melt-spun ones, which could be contributed to the single disordered A2 structure
and the enhancement of [10 0]-oriented texture. However, when the ribbon samples were cooled at
2 and 0.5 1C/min after heat treatment at 800 1C for 3 h, a minor quantity of ordered D03 and L12 phase
was found to precipitate in the A2 matrix, respectively, which resulted in the reduction of both
magnetization and magnetostrictive strain.
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
Fe–Ga alloys (Galfenol) have attracted much attention asmagnetostrictive materials due to their ductility and largemagnetostriction at a low saturation field [1]. A maximummagnetostriction of about �400 ppm has been achieved inquenched Fe81Ga19 single crystal with a disordered bcc (A2)structure [2]. It is commonly believed that the local short-rangordering of Ga atoms along [10 0] directions can be formed duringquenching, which enhances the magnetoelastic energy densityand results in a larger magnetostriction [3].
According to the metastable phase diagram for Fe–Ga alloy, A2phase is in metastable equilibrium with Fe3Ga D03 ordered phaseat low temperatures [4]. During the melt-spinning process, liquidalloy is solidified with an extreme cooling rate and the D03
ordering could be suppressed. As a result, more short-rangeordering of Ga atoms with the disordered A2 phase would bepreserved at room temperature and a larger magnetostriction isexpected. Grain morphology and texture evolution of melt-spunFe85Ga15 and Fe83Ga17 ribbons before and after annealing havebeen investigated and it was found that the short-time heattreatment was helpful for the improvement of magnetostrictiondue to the enhancement of the [10 0]-oriented texture [5,6].
ll rights reserved.
Based on previous studies, the intrinsic magnetostriction ofFe81Ga19 alloy is very large and has a significant dependence onthe phase composition and their distribution [7]. In addition, it isconsidered that the phase transformation in Fe1�xGax alloys withx around 19 is related to the vacancy concentration in theA2 phase [8], which means the phase transformation in the melt-spun ribbons may be different from the conventional melt-worked bulk samples due to their great difference in cooling rate.Thus, in this work, the phase structure, magnetization andmagnetostriction of Fe81Ga19 ribbons before and after differentheat treatments have been investigated.
2. Experiments
The precursor ingot of Fe81Ga19 was prepared from high purityFe (99.9%) and Ga (99.99%) metals by induction melting under theprotection of argon atmosphere, followed by homogenizing for72 h at 1000 1C. Ribbon samples with the thickness of 35–110mmand the width of 4 mm were melt-spun at a linear velocity of acopper wheel from 8 to 30 m/s. Heat treatment was carriedout in encapsulated quartz tubes under vacuum in the followingmanner: 800 1C for 3 h and then quenched into water(as-quenched), 800 1C for 3 h and then cooled at 2 1C/min, and800 1C for 3 h and then cooled at 0.5 1C/min. Microstructures ofthe Fe81Ga19 ribbons were identified by a Rigaku D/max 2400 pcX-ray diffractometer with CuKa radiation on the wheel side of theribbons using a step size of 0.021. The XRD wavelength of CuKa
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J. Zhang et al. / Physica B 404 (2009) 4155–41584156
was 1.5406 A. The high power X-ray generator was operated at45 kV and 250 mA in order to improve resolution of peaks.Magnetization curves were measured by a Lakeshore 7407vibrating sample magnetometer when the magnetic filed wasapplied along different directions. Magnetostrictive strain alongthe ribbon length was measured by standard strain gauge method.Schematic diagram of strain measurements in our study wasshown in Fig. 1(a). The ribbon samples were fixed by beingattached on upper and bottom bakelite with glue, so that thebottom plate could move only along vertical direction with aweight about 12 MPa. Compared with the measure methodemployed in Ref. [9], as shown in Fig. 1(b), it can avoid theribbons bending towards the field direction due to the largedemagnetizing factor.
3. Results and discussion
3.1. Structure evolution
Fig. 2 shows X-ray diffraction spectra of the wheel side of melt-spun Fe81Ga19 ribbons with various thicknesses. The samplesexhibited mainly a disordered A2 structure at room temperature,which is normally stable only at temperatures above 550 1C [4].Due to the extreme cooling rate in the melt-spinning process, thehigh-temperature disordered A2 phase can be retained to roomtemperature, which is considered to be responsible for theimproved magnetostriction of Fe–Ga ribbons [10].
In comparison with the intensity ratio among the threestrongest peaks of A2 structure in Fig. 2, it could be found thatthe intensity of [10 0] orientation on the wheel side grosslyincreased with the increase of ribbon thickness. X-ray pole figureexaminations indicated that the [10 0]-orientated texture of melt-spun ribbons was almost parallel to the thickness direction [5,11],which coincides well with the direction of temperature gradientsand internal stress gradients during crystallization. With thelinear velocity increasing in melt-spinning process, the ribbonthickness decreases and the solidification rate increases, which
Fig. 1. (a) Schematic diagram of magnetostrictive strain measurements in our
study. (b) Stacked ribbon samples with the configuration of applied field and strain
gauges [9].
restrains the growth of grains and is considered to be responsiblefor the weakened of [10 0] orientation.
Fig. 3 shows X-ray diffraction spectra of the wheel side ofFe81Ga19 ribbons with the thickness of 110mm after different heattreatments. The peak intensity ratio I(2 0 0)/I(110) of A2 structurein heat-treated samples was higher than that in the as-melt-spunones, indicating that the initial [10 0]-orientated texture of ribbonsamples became stronger after heat treatments. Moreover, the as-quenched ribbons have the strongest [10 0] texture, but it becameweaker with the decrease of the cooling rate.
Fig. 4 shows the enlargement of the low intensity parts ofFig. 3. The as-quenched ribbons preserved the same disorderedA2 structure at room temperature as the melt-spun ones, asshown in Figs. 4(a) and (b). However, when the ribbon sampleswere cooled from 800 1C at a cooling rate of 2 1C/min [Fig. 4(c)],besides the primary reflection from A2 structure, two small extrapeaks emerged at 30.871 and 52.311, which could be indexed as(2 0 0) and (311) superlattice reflection from Fe3Ga D03 structure.
Fig. 2. X-ray diffraction spectra of the wheel side of Fe81Ga19 ribbons with various
thicknesses.
Fig. 3. X-ray diffraction spectrum of the wheel side of Fe81Ga19 ribbons with the
thickness of 110mm after different heat treatments: (a) as-melt-spun, heat heated
at 800 1C for 3 h and then (b) quenched in to water, (c) slow cooled at 2 1C/min, and
(d) slow cooled at 0.5 1C/min.
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Fig. 4. Enlargement of the low intensity parts of Fig. 3.
Fig. 5. (a) Dependence of hysteresis loops on direction y for melt-spun Fe81Ga19
ribbons with the thickness of 110mm. (b) Magnetization curves of Fe81Ga19 ribbons
with the thickness of 110mm after different heat treatments at y ¼ 01. The inset is
the enlargement of magnetization curves between 80 and 800 kA/m.
J. Zhang et al. / Physica B 404 (2009) 4155–4158 4157
It indicates the presence of long-range ordered D03 phase. Whenthe cooling rate decreased to 0.5 1C/min, a small amount ofordered Fe3Ga L12 phase with a fcc structure was detectedcoexisting with the matrix A2 phase in room-temperatureFe81Ga19 ribbon samples, as shown in Fig. 4(d), but no extrareflection peak corresponding to the ordered D03 phase wasobserved. According to the equilibrium phase diagram for Fe–Gaalloys [4], Fe81Ga19 alloy exhibits a mixed constitution of A2 andL12 phases at temperatures below 550 1C. However, because L12
phase has a larger precipitation activation energy than theordered D03 metastable one [8], D03 phase can more easily formin the A2 phase matrix in the composition range of the stableequilibrium L12 phase region. In addition, the phasetransformation from A2 to D03 or L12 is a precipitation reaction,which is controlled by the Ga-atoms diffusion in the coolingprocess. Therefore, it is considered that the room-temperaturephase constitution of heat-treated Fe81Ga19 ribbons has aclose relationship with the cooling rates. The precipitation of L12
phase in A2 matrix needs slower cooling rate than that of orderedD03 phase.
3.2. Magnetization and magnetostriction
Fig. 5(a) shows the magnetic hysteresis loops of melt-spunFe81Ga19 ribbons with the thickness of 110mm along differentdirections. The rotation angle y between the applied field and theribbon plane was 01, 451, 751 and 901 clockwise, respectively.Saturation magnetization was quickly reached for y ¼ 01 in arelatively low magnetic field. However, it became harder tosaturate the ribbons as y increased. It indicates that the strongshape anisotropy of ribbons forced magnetic moments stayingwithin the ribbon plane, resulting in a strong demagnetizing fieldalong the ribbon thickness direction (y ¼ 901). The saturationmagnetization of the melt-spun samples was 171.78 A m2/kg fory ¼ 01.
Magnetization curves of Fe81Ga19 ribbons with the thickness of110mm after different heat treatments are presented in Fig. 5(b),which was measured as y ¼ 01. The inset shows the enlargementof magnetization curves between 80 and 800 kA/m. It could beobserved that the as-quenched ribbons have almost the samesaturation magnetization of 171.70 A m2/kg with the as-melt-spunones due to the same room-temperature phase structure, asshown in Fig. 4. However, the value decreased to 168.18 and
165.76 A m2/kg when the ribbon samples were cooled at 2 and0.5 1C/min, respectively.
According to the X-ray diffraction spectra shown in Fig. 4, thephase constitution of melt-spun Fe81Ga19 ribbons, a single A2phase, transformed to a mixture of A2/D03 and A2/L12 aftercooling at 2 and 0.5 1C/min, respectively. The saturation magne-tization of A2 phase in Fe–Ga alloy increases with decreasing Gacontent [12]. So the saturation magnetization of A2 matrix willhave a slight increase due to the enrichment of Ga atoms in theordered D03 and L12 phases after slow cooling. However, both theD03 and L12 phase have a saturation magnetization lower thanthat of A2 phase [12]. Thus, the formation of the two phasesresults in a reduction of the saturation value of slow-cooledFe81Ga19 ribbons, as shown in Fig. 5(b).
Fig. 6(a) shows the magnetostrictive strain measured along thelength direction of melt-spun Fe81Ga19 ribbons with the thicknessof 110mm. When the magnetic field was applied perpendicular tothe ribbon plane, i.e. y ¼ 901 (see Fig. 1), the magnetostrictivestrain reached �163 ppm at H ¼ 1150 kA/m. However, when thefield was applied parallel to the ribbon plane, i.e. y ¼ 01, ribbonsonly had a saturation value of �21 ppm in a rather weak field of98 kA/m. For the ribbon samples, due to the strong shapeanisotropy, all magnetic moments stay in the ribbon plane.
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Fig. 6. (a) Magnetostrictive strain along the length direction of melt-spun Fe81Ga19
ribbons with the thickness of 110mm at y ¼ 01 and 901, respectively. (b)
Magnetostrictive strain along the length direction of Fe81Ga19 ribbons with the
thickness of 110mm after different heat treatments at y ¼ 901.
J. Zhang et al. / Physica B 404 (2009) 4155–41584158
When y ¼ 01, only non-1801 moment rotation contributed tothe magnetostrictive strain. But when y ¼ 901, all moment hadnon-1801 rotation and contributed to the strain, which leads to amuch larger saturation strain value, shown in Fig. 6(a). In addition,the larger saturation field of 1150 kA/m when y ¼ 901 is related tothe strong demagnetizing field along the ribbon thicknessdirection.
Fig. 6(b) shows the room-temperature magnetostrictive strainalong the length direction of Fe81Ga19 ribbons with the thicknessof 110mm after different heat treatments at y ¼ 901. The as-quenched ribbons had the largest saturation magnetostrictivestrain of �189 ppm, even 26 ppm larger than that of the as-melt-spun ones, which might be attributed to the enhancement of the[10 0]-oriented texture after annealing. However, for the ribbonsamples cooled at 2 and 0.5 1C /min, the saturation strain valuedecreased to �143 and �125 ppm, respectively. It has beenconsidered that the larger magnetostriction in Fe–Ga alloycompared with a-Fe came from a directional short-rang orderingof Ga atoms, which has a strong effect on the magnetostriction of
[10 0] direction [2,3]. Strong [10 0]-oriented texture in the slow-cooled Fe81Ga19 ribbons is helpful for the enhancement ofmagnetostrictive strain along ribbon length. However, orderedD03 or L12 phase precipitated during the slow cooling process, andthe short-rang ordering of Ga atoms in A2 phase cannot beretained to room temperature, which results in the decrease ofroom-temperature magnetostrictive strain of slow-cooledFe81Ga19 ribbons. Moreover, L12 phase has an opposite directionmagnetostriction compared with that of the A2 phase [13], whichis also considered to be responsible for the decrease of thesaturation strain value of the sample cooled at 0.5 1C/min.
In addition, the maximum strain obtained in our Fe81Ga19
samples is much smaller than that measured in a Fe85Ga15 stackedribbons [9] by the method shown in Fig. 1(b). It has beenconfirmed that the ribbons tend to bend towards the fielddirection for the large demagnetizing factor, which can lead to alarge signal in the strain gauge of more than 73000 ppm [14].Bending can be avoided by the improved measurement method inthis study, as exhibited in Fig. 1(a), where a tension stress alongthe ribbon length direction is applied, and the measurementvalues are more close to the intrinsic magnetostrictive strain ofFe–Ga ribbons.
4. Conclusions
In this work, we have investigated the evolution of structure,magnetization and magnetostriction of melt-spun Fe81Ga19
ribbons with different thermal history. It is found that the heat-treated ribbons have a stronger [10 0]-oriented texture comparedwith the as-melt-spun samples. In addition, the room-tempera-ture phase constitution of the heat-treated ribbon samples showsa great dependence on the cooling rate. As-quenched Fe81Ga19
ribbons have the same disordered A2 structure with the as-melt-spun ones, however, ribbon samples cooled from 800 1C at 2 and0.5 1C/min have a phase constitution of A2/D03 and A2/L12,respectively. The formation of the ordered D03 and L12 phase inslow-cooled Fe81Ga19 ribbons leads to a decrease in bothmagnetization and magnetostrictive strain.
Acknowledgment
This work was supported by NSFC-50701039, SKL2009-6, NCET05-0526, and IRT-0651.
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