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Magnetostrictive properties of titanate coupling agent treated Terfenol-D composites
Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetostrictive properties of titanate coupling agent treated Terfenol-D composites Xufeng Dong a,n, Min Qi a, Xinchun Guan b, Jinhai Li c, Jinping Ou b,c a
School of Materials Science and Engineering, Dalian University of Technology, 116024 Dalian, China School of Civil Engineering, Harbin Institute of Technology, 150090 Harbin, China c School of Civil Engineering, Dalian University of Technology, 116024 Dalian, China b
a r t i c l e i n f o
abstract
Article history: Received 6 February 2011 Available online 18 November 2011
As a kind of composites, the bond strength between the polymer matrix and the Terfenol-D particles affects the performance of magnetostrictive composites. By observing the fracture morphologies, the bond strength of the magnetostrictive composites prepared with untreated Terfenol-D was proved weak. Titanate coupling agent was used for particles to improve the bond strength. Contact angle analysis indicates the work of adhesion of the epoxy resin to the treated Terfenol-D is larger than that to the untreated Terfenol-D. Different magnetostrictive composites with 20%, 35% and 50% particle volume fractions were prepared with treated and untreated Terfenol-D particles. Their static and dynamic magnetostriction was tested without pre-stress at room temperature. The results indicate titanate coupling agent treating increases the magnetostrictive properties of magnetostrictive composites, that is probably because the bond strength improves due to the particle treating. & 2011 Elsevier B.V. All rights reserved.
Keywords: Magnetostrictive composite Terfenol-D Bond strength Titanate coupling agent
1. Introduction
2. Experimental
Magnetostrictive effect refers the phenomenon of the change in elastic state exhibited by certain ferromagnetic materials when subjected to changes in the magnetization state [1]. Terfenol-D composed of terbium, dysprosium and iron is a giant magnetostrictive material [2,3], and has been a commercially available material for application in many fields. Recently, magnetostrictive composites, composed of Terfenol-D particles dispersed in a polymer matrix, have attracted considerable interest due to their less eddy current losses, high tensile-strength, low cost and less size limit [4–16]. Magnetostrictive composites are a kind of particle reinforced polymer composite material. Their magnetostrictive behavior is strongly influenced by the bond strength between particles and polymer matrix. An ideal interface should ensure the fully transfer of the magnetostrictive effect from Terfenol-D to the polymer. One aim of this study is to evaluate the bond strength between untreated Terfenol-D and polymer matrix. Another aim, which is more important, is to find an effective method to improve the bond strength. Using titanate coupling agent treated Terfenol-D particles to fabricate magnetostrictive composites is chosen as a testing way.
Monolithic Terfenol-D and Terfenol-D particles, the chemical formula of which is Tb0.3Dy0.7Fe2, were purchased from Gansu Tianxing Rare Earth Functional Materials Co., Ltd. The shape of the particles is irregular, and the distribution of the particle size is mainly in the range of 30 mm to 500 mm. Epoxy resin was chosen as the polymer matrix. Titanate coupling agent, which is usually used to improve the interfacial strength of the traditional composites composed of organic and inorganic matters, was chosen as the treating material. The molecular of titanate coupling agent is divided into six functional zones, and each functional zone has its own effect in the coupling mechanism. The six mechanism zones are demonstrated in Fig. 1, where function zone (RO)m is the group to couple between inorganic substance and Titanium. Titanate coupling agent uses its alkoxy group to directly couple with a small amount of hydroxyl or carboxyl (through chemical action) absorbed on the surface of particles. The bond strength of the chemical bonding is larger than that of physisorption. Initially, the Terfenol-D particles were dispersed in sodium hydroxide aqueous solution for a few minutes to create a new surface with hydroxyl groups, and then washed several times using de-ionized water. Titanate in 1% of the weight of Terfenol-D particles was dispersed in an acetone solution to prepare a mixture. The washed Terfenol-D particles were then dispersed in the mixture. To ensure that all the particles were treated with titanate, ultrasonic wave was applied during the volatilization of
n
Corresponding author. Tel.: þ86 411 8470 8441; fax: þ 86 411 8470 9284. E-mail address: [email protected] (X. Dong).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.11.010
1206
X. Dong et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
Fig. 1. Mechanism zones of titanate coupling agent.
the acetone. After all the acetone volatilized, the titanate coupling agent treated Terfenol-D particles were obtained. Two kinds of particles, the untreated Terfenol-D particles and the treated Terfenol-D particles, were used as fillers to fabricate magnetostrictive composites. The particle volume fractions of the specimens were 20%, 35% and 50%. First, the particles and the epoxy resin were mixed by a stirrer bar for about 30 min at room temperature. After that, the even mixture was put into a duralium mold with a cavity of F 10 mm 25 mm. Then the mold was placed in a vacuum for degassing. Two hours later, the mold was sealed and placed between two permanent magnets, which provide an orientation magnetic field along the length of the cavity to align the particulates into chains to form a pseudo-fiber structure [17]. Finally, the mold with the magnets was placed in an oven and cured at 60 1C for 10 h. Scanning electron microscope (SEM, JSM-5600LV) was used to observe the fracture morphologies of the composites. The contact angle between the surface of the treated/untreated Terfenol-D and the epoxy resin was measured by a Drop Shape Analysis System (DSA100). The Fourier transform IR (FT-IR) transmission spectrum was recorded on an EQUINOX55 spectrometer. The magnetostriction of the two samples was measured by strain gauges.
Fig. 2. Fracture morphologies of the composites prepared with untreated Terfenol-D particles.
3. Results and discussion Fig. 2 shows fracture morphologies of the composites prepared with untreated Terfenol-D particles. The Obscission zone is observed in Fig. 2(a), and cracked section is observed between the boundary of the particle and the polymer in Fig. 2(b). It indicates the untreated particles and the polymer matrix have poor adhesion. The weak bonding prevents the effective transfer of the strain from the particles to the matrix, and results in the easy failure of the material. Fig. 3 gives the infrared spectra of untreated and treated Terfenol-D particles. For the treated particles, the narrow bands around 2960 cm 1, 2931 cm 1 and 2875 cm 1 are assigned to the C–H stretching vibration. The band around 1463 cm 1 is assigned to the C–H bending vibration. The small absorption tip at 1039 cm 1 is attributed to the stretching vibration frequency of C–O. To conclude, it proves that the Terfenol-D is really modified by the titanate coupling agent. Wettability is the ability of liquid to spread over a surface. Young described the trigonometric relations between the contact angle and the forces acting on a liquid drop in mechanical equilibrium on a solid surface
gsv gsl ¼ glv cos y
ð1Þ
where gsv is the surface tension of the solid, glv the surface tension of the liquid, gsl the solid–liquid interfacial tension and y the contact angle. Dupre described the relative adhesion of the liquid to itself as compared to the solid, in the form of W sl ¼ glv ð1þ cos yÞ where W sl is the work of adhesion of the liquid to the solid.
ð2Þ
Fig. 3. FT-IR spectra for untreated and treated Terfenol-D particles.
Formula (2) indicates the work of adhesion increases with decreasing contact angle. The droplets of epoxy resin were placed on the substrates of untreated and treated bulk Terfenol-D, as shown in Fig. 4. The contact angle between the resin and the untreated Terfenol-D is 451, while the angle between the resin and the treated Terfenol-D is 201. It indicates that the work of adhesion of the resin to the treated Terfenol-D is larger than that of the resin to the untreated Terfenol-D.
X. Dong et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
1207
Fig. 4. Contact states between (a) the untreated bulk Terfenol-D and the epoxy resin, and (b) the treated Terfenol-D surface and the epoxy resin.
Fig. 5(a) shows the dependence of the static magnetostriction
l of the treated and untreated Terfenol-D composites with different particles volume fractions on applied magnetic field. For all the specimens, their static magnetostriction is a function of the applied field. For the composites with the same particle volume fractions, at the same magnetic field, the one with treated particles presents higher static magnetostriction than that with untreated particles. However, when the applied field approaches the saturation value, their magnetostriction tends to be approximate to each other. The dependence of the dynamic magnetostriction of the specimens on applied magnetic field is shown in Fig. 5(b). With the same particle volume fraction, the maximum dynamic magnetostriction of the treated specimen is higher than that of the untreated one. The peak position shifts to low field levels with the addition of coupling agent. It indicates preparing magnetostrictive composites with coupling agent treated Terfenol-D particles can get a quick response material. The promotion of the magnetostrictive performance is probably caused by the improving bond strength between the polymer and the particles. Like most of the traditional composites, such as glass fiber reinforced plastic and silicon dioxide reinforced metal, the interface between the active phase and the matrix plays a crucial role on the performance of magnetostrictive composites. It is the media of transferring magnetostrictive effect from the Terfenol-D to the polymer, which leads to the elongation of the composites under magnetic field. If the bond strength is weak, slipping would happen on the interface, and cause delayed transfer of magnetostriction. Therefore, with increasing bond strength, the magnetostrictive effect is easier to transfer from the Terfenol-D particles to the matrix. As confirmed by the contact angle analysis, the work of adhesion of the epoxy resin to the particles is enhanced by the treatment, and the bond strength is improved. Fig. 5 also indicates the enhancement of the magnetostrictive performance due to the particle treating is more significant for the composites with larger particle volume fractions. The most
Fig. 5. Dependence of (a) static and (b) dynamic magnetostriction on applied magnetic field for magnetostrictive composites prepared with untreated TerfenolD and treated Terfenol-D.
likely reason is that the bond strength is much weaker for the untreated composites with larger particle volume fractions.
4. Conclusion To evaluate the interfacial strength of Terfenol-D magnetostrictive composites, SEM was used to observe the fracture morphologies of the failure material. To improve the bond strength between the matrix and the Terfenol-D particles, titanate coupling agent was used to treat the particles. Magnetostrictive properties of the composites with different particle volume fractions prepared with the treated and the untreated TerfenolD particles were compared. Following conclusions are obtained: (1) The bond strength between the untreated Terfenol-D particles and the epoxy resin is weak, and cause the failure of the magnetostrictive composites prepared with untreated particles. The weak bond strength is also harmful for the transfer of magnetostrictive effect from the particles to the matrix. (2) The droplets of epoxy resin were placed on the substrates of untreated and treated Terfenol-D. The contact angle between the resin and the treated Terfenol-D is less than that between the resin and the untreated Terfenol-D, indicating the enhancement of the bond strength due to the particle treating.
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X. Dong et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
(3) Magnetostrictive composites prepared with treated particles present higher magnetostrictive properties, especially at low field levels, with respect to those prepared with untreated particles. The reason for the phenomenon is the increase of the bond strength due to the titanate coupling agent treating.
Acknowledgments This research is financial funded by the National Basic Research Program of China under the Grant number of 2007CB714204, the China Postdoctoral Science Foundation under the Grant number of 20100471446, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] J.P. Joule, Annals of Electricity, Magnetism, and Chemistry 8 (1842) 219. [2] A.E. Clark, B.D. DeSavage, R. Bozorth, Physical Review 138 (1965) 216. [3] A.E. Clark, H. Belson, Physical Review B. 5 (1972) 3642.
[4] L. Sandlund, M. Fahlander, T. Cedell, A.E. Clark, J.B. Restorff, M. Wun-Fogle, Journal of Applied Physics 75 (1994) 5656. [5] T.A. Duenas, G.P. Carman, Journal of Applied Physics 90 (2001) 2433. [6] T.A. Duenas, G.P. Carman, Journal of Applied Physics 87 (2000) 4696. [7] W.D. Armstrong, Materials Science and Engineering A 285 (2000) 13. [8] S.W. Or, N. Nersessian, G.P. McKnight, G.P. Carman, Journal of Applied Physics 93 (2003) 8510. [9] J. Hudson, S.C. Busbridge, A.R. Piercy, Journal of Applied Physics 83 (1998) 7255. [10] G. Altin, K.K. Ho, C.P. Henry, G.P. Carman, Journal of Applied Physics 83 (2007) 033537-1. [11] S.H. Lim, S.R. Kim, S.Y. Kang, J.K. Park, J.T. Nam, D. Son, Journal of Magnetism and Magnetic Materials 191 (1999) 113. [12] C. Rodrı´guez, A. Barrio, I. Orue, J.L. Vilas, L.M. Leo´n, J.M. Barandiara´n, M.L. Fdez-Gubieda Ruiz, Sensors and Actuators A 142 (2008) 538. [13] S.W. Or, N. Nersessian, G.P. Carman, IEEE Transaction on Magnetics 40 (2004) 71. [14] X. Dong, M. Qi, X. Guan, J. Ou, Polymer Testing 29 (2010) 369. [15] X. Guan, X. Dong, J. Ou, Journal of Magnetism and Magnetic Materials 321 (2009) 2742. [16] X. Guan, X. Dong, J. Ou, Journal of Magnetism and Magnetic Materials 320 (2008) 158. [17] X. Dong, J. Ou, M. Qi, X. Guan, Journal of Magnetism and Magnetic Materials 322 (2010) 3648.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetostrictive properties of titanate coupling agent treated Terfenol-D composites Xufeng Dong a,n, Min Qi a, Xinchun Guan b, Jinhai Li c, Jinping Ou b,c a
School of Materials Science and Engineering, Dalian University of Technology, 116024 Dalian, China School of Civil Engineering, Harbin Institute of Technology, 150090 Harbin, China c School of Civil Engineering, Dalian University of Technology, 116024 Dalian, China b
a r t i c l e i n f o
abstract
Article history: Received 6 February 2011 Available online 18 November 2011
As a kind of composites, the bond strength between the polymer matrix and the Terfenol-D particles affects the performance of magnetostrictive composites. By observing the fracture morphologies, the bond strength of the magnetostrictive composites prepared with untreated Terfenol-D was proved weak. Titanate coupling agent was used for particles to improve the bond strength. Contact angle analysis indicates the work of adhesion of the epoxy resin to the treated Terfenol-D is larger than that to the untreated Terfenol-D. Different magnetostrictive composites with 20%, 35% and 50% particle volume fractions were prepared with treated and untreated Terfenol-D particles. Their static and dynamic magnetostriction was tested without pre-stress at room temperature. The results indicate titanate coupling agent treating increases the magnetostrictive properties of magnetostrictive composites, that is probably because the bond strength improves due to the particle treating. & 2011 Elsevier B.V. All rights reserved.
Keywords: Magnetostrictive composite Terfenol-D Bond strength Titanate coupling agent
1. Introduction
2. Experimental
Magnetostrictive effect refers the phenomenon of the change in elastic state exhibited by certain ferromagnetic materials when subjected to changes in the magnetization state [1]. Terfenol-D composed of terbium, dysprosium and iron is a giant magnetostrictive material [2,3], and has been a commercially available material for application in many fields. Recently, magnetostrictive composites, composed of Terfenol-D particles dispersed in a polymer matrix, have attracted considerable interest due to their less eddy current losses, high tensile-strength, low cost and less size limit [4–16]. Magnetostrictive composites are a kind of particle reinforced polymer composite material. Their magnetostrictive behavior is strongly influenced by the bond strength between particles and polymer matrix. An ideal interface should ensure the fully transfer of the magnetostrictive effect from Terfenol-D to the polymer. One aim of this study is to evaluate the bond strength between untreated Terfenol-D and polymer matrix. Another aim, which is more important, is to find an effective method to improve the bond strength. Using titanate coupling agent treated Terfenol-D particles to fabricate magnetostrictive composites is chosen as a testing way.
Monolithic Terfenol-D and Terfenol-D particles, the chemical formula of which is Tb0.3Dy0.7Fe2, were purchased from Gansu Tianxing Rare Earth Functional Materials Co., Ltd. The shape of the particles is irregular, and the distribution of the particle size is mainly in the range of 30 mm to 500 mm. Epoxy resin was chosen as the polymer matrix. Titanate coupling agent, which is usually used to improve the interfacial strength of the traditional composites composed of organic and inorganic matters, was chosen as the treating material. The molecular of titanate coupling agent is divided into six functional zones, and each functional zone has its own effect in the coupling mechanism. The six mechanism zones are demonstrated in Fig. 1, where function zone (RO)m is the group to couple between inorganic substance and Titanium. Titanate coupling agent uses its alkoxy group to directly couple with a small amount of hydroxyl or carboxyl (through chemical action) absorbed on the surface of particles. The bond strength of the chemical bonding is larger than that of physisorption. Initially, the Terfenol-D particles were dispersed in sodium hydroxide aqueous solution for a few minutes to create a new surface with hydroxyl groups, and then washed several times using de-ionized water. Titanate in 1% of the weight of Terfenol-D particles was dispersed in an acetone solution to prepare a mixture. The washed Terfenol-D particles were then dispersed in the mixture. To ensure that all the particles were treated with titanate, ultrasonic wave was applied during the volatilization of
n
Corresponding author. Tel.: þ86 411 8470 8441; fax: þ 86 411 8470 9284. E-mail address: [email protected] (X. Dong).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.11.010
1206
X. Dong et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
Fig. 1. Mechanism zones of titanate coupling agent.
the acetone. After all the acetone volatilized, the titanate coupling agent treated Terfenol-D particles were obtained. Two kinds of particles, the untreated Terfenol-D particles and the treated Terfenol-D particles, were used as fillers to fabricate magnetostrictive composites. The particle volume fractions of the specimens were 20%, 35% and 50%. First, the particles and the epoxy resin were mixed by a stirrer bar for about 30 min at room temperature. After that, the even mixture was put into a duralium mold with a cavity of F 10 mm 25 mm. Then the mold was placed in a vacuum for degassing. Two hours later, the mold was sealed and placed between two permanent magnets, which provide an orientation magnetic field along the length of the cavity to align the particulates into chains to form a pseudo-fiber structure [17]. Finally, the mold with the magnets was placed in an oven and cured at 60 1C for 10 h. Scanning electron microscope (SEM, JSM-5600LV) was used to observe the fracture morphologies of the composites. The contact angle between the surface of the treated/untreated Terfenol-D and the epoxy resin was measured by a Drop Shape Analysis System (DSA100). The Fourier transform IR (FT-IR) transmission spectrum was recorded on an EQUINOX55 spectrometer. The magnetostriction of the two samples was measured by strain gauges.
Fig. 2. Fracture morphologies of the composites prepared with untreated Terfenol-D particles.
3. Results and discussion Fig. 2 shows fracture morphologies of the composites prepared with untreated Terfenol-D particles. The Obscission zone is observed in Fig. 2(a), and cracked section is observed between the boundary of the particle and the polymer in Fig. 2(b). It indicates the untreated particles and the polymer matrix have poor adhesion. The weak bonding prevents the effective transfer of the strain from the particles to the matrix, and results in the easy failure of the material. Fig. 3 gives the infrared spectra of untreated and treated Terfenol-D particles. For the treated particles, the narrow bands around 2960 cm 1, 2931 cm 1 and 2875 cm 1 are assigned to the C–H stretching vibration. The band around 1463 cm 1 is assigned to the C–H bending vibration. The small absorption tip at 1039 cm 1 is attributed to the stretching vibration frequency of C–O. To conclude, it proves that the Terfenol-D is really modified by the titanate coupling agent. Wettability is the ability of liquid to spread over a surface. Young described the trigonometric relations between the contact angle and the forces acting on a liquid drop in mechanical equilibrium on a solid surface
gsv gsl ¼ glv cos y
ð1Þ
where gsv is the surface tension of the solid, glv the surface tension of the liquid, gsl the solid–liquid interfacial tension and y the contact angle. Dupre described the relative adhesion of the liquid to itself as compared to the solid, in the form of W sl ¼ glv ð1þ cos yÞ where W sl is the work of adhesion of the liquid to the solid.
ð2Þ
Fig. 3. FT-IR spectra for untreated and treated Terfenol-D particles.
Formula (2) indicates the work of adhesion increases with decreasing contact angle. The droplets of epoxy resin were placed on the substrates of untreated and treated bulk Terfenol-D, as shown in Fig. 4. The contact angle between the resin and the untreated Terfenol-D is 451, while the angle between the resin and the treated Terfenol-D is 201. It indicates that the work of adhesion of the resin to the treated Terfenol-D is larger than that of the resin to the untreated Terfenol-D.
X. Dong et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
1207
Fig. 4. Contact states between (a) the untreated bulk Terfenol-D and the epoxy resin, and (b) the treated Terfenol-D surface and the epoxy resin.
Fig. 5(a) shows the dependence of the static magnetostriction
l of the treated and untreated Terfenol-D composites with different particles volume fractions on applied magnetic field. For all the specimens, their static magnetostriction is a function of the applied field. For the composites with the same particle volume fractions, at the same magnetic field, the one with treated particles presents higher static magnetostriction than that with untreated particles. However, when the applied field approaches the saturation value, their magnetostriction tends to be approximate to each other. The dependence of the dynamic magnetostriction of the specimens on applied magnetic field is shown in Fig. 5(b). With the same particle volume fraction, the maximum dynamic magnetostriction of the treated specimen is higher than that of the untreated one. The peak position shifts to low field levels with the addition of coupling agent. It indicates preparing magnetostrictive composites with coupling agent treated Terfenol-D particles can get a quick response material. The promotion of the magnetostrictive performance is probably caused by the improving bond strength between the polymer and the particles. Like most of the traditional composites, such as glass fiber reinforced plastic and silicon dioxide reinforced metal, the interface between the active phase and the matrix plays a crucial role on the performance of magnetostrictive composites. It is the media of transferring magnetostrictive effect from the Terfenol-D to the polymer, which leads to the elongation of the composites under magnetic field. If the bond strength is weak, slipping would happen on the interface, and cause delayed transfer of magnetostriction. Therefore, with increasing bond strength, the magnetostrictive effect is easier to transfer from the Terfenol-D particles to the matrix. As confirmed by the contact angle analysis, the work of adhesion of the epoxy resin to the particles is enhanced by the treatment, and the bond strength is improved. Fig. 5 also indicates the enhancement of the magnetostrictive performance due to the particle treating is more significant for the composites with larger particle volume fractions. The most
Fig. 5. Dependence of (a) static and (b) dynamic magnetostriction on applied magnetic field for magnetostrictive composites prepared with untreated TerfenolD and treated Terfenol-D.
likely reason is that the bond strength is much weaker for the untreated composites with larger particle volume fractions.
4. Conclusion To evaluate the interfacial strength of Terfenol-D magnetostrictive composites, SEM was used to observe the fracture morphologies of the failure material. To improve the bond strength between the matrix and the Terfenol-D particles, titanate coupling agent was used to treat the particles. Magnetostrictive properties of the composites with different particle volume fractions prepared with the treated and the untreated TerfenolD particles were compared. Following conclusions are obtained: (1) The bond strength between the untreated Terfenol-D particles and the epoxy resin is weak, and cause the failure of the magnetostrictive composites prepared with untreated particles. The weak bond strength is also harmful for the transfer of magnetostrictive effect from the particles to the matrix. (2) The droplets of epoxy resin were placed on the substrates of untreated and treated Terfenol-D. The contact angle between the resin and the treated Terfenol-D is less than that between the resin and the untreated Terfenol-D, indicating the enhancement of the bond strength due to the particle treating.
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X. Dong et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1205–1208
(3) Magnetostrictive composites prepared with treated particles present higher magnetostrictive properties, especially at low field levels, with respect to those prepared with untreated particles. The reason for the phenomenon is the increase of the bond strength due to the titanate coupling agent treating.
Acknowledgments This research is financial funded by the National Basic Research Program of China under the Grant number of 2007CB714204, the China Postdoctoral Science Foundation under the Grant number of 20100471446, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] J.P. Joule, Annals of Electricity, Magnetism, and Chemistry 8 (1842) 219. [2] A.E. Clark, B.D. DeSavage, R. Bozorth, Physical Review 138 (1965) 216. [3] A.E. Clark, H. Belson, Physical Review B. 5 (1972) 3642.
[4] L. Sandlund, M. Fahlander, T. Cedell, A.E. Clark, J.B. Restorff, M. Wun-Fogle, Journal of Applied Physics 75 (1994) 5656. [5] T.A. Duenas, G.P. Carman, Journal of Applied Physics 90 (2001) 2433. [6] T.A. Duenas, G.P. Carman, Journal of Applied Physics 87 (2000) 4696. [7] W.D. Armstrong, Materials Science and Engineering A 285 (2000) 13. [8] S.W. Or, N. Nersessian, G.P. McKnight, G.P. Carman, Journal of Applied Physics 93 (2003) 8510. [9] J. Hudson, S.C. Busbridge, A.R. Piercy, Journal of Applied Physics 83 (1998) 7255. [10] G. Altin, K.K. Ho, C.P. Henry, G.P. Carman, Journal of Applied Physics 83 (2007) 033537-1. [11] S.H. Lim, S.R. Kim, S.Y. Kang, J.K. Park, J.T. Nam, D. Son, Journal of Magnetism and Magnetic Materials 191 (1999) 113. [12] C. Rodrı´guez, A. Barrio, I. Orue, J.L. Vilas, L.M. Leo´n, J.M. Barandiara´n, M.L. Fdez-Gubieda Ruiz, Sensors and Actuators A 142 (2008) 538. [13] S.W. Or, N. Nersessian, G.P. Carman, IEEE Transaction on Magnetics 40 (2004) 71. [14] X. Dong, M. Qi, X. Guan, J. Ou, Polymer Testing 29 (2010) 369. [15] X. Guan, X. Dong, J. Ou, Journal of Magnetism and Magnetic Materials 321 (2009) 2742. [16] X. Guan, X. Dong, J. Ou, Journal of Magnetism and Magnetic Materials 320 (2008) 158. [17] X. Dong, J. Ou, M. Qi, X. Guan, Journal of Magnetism and Magnetic Materials 322 (2010) 3648.