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ABSTRACT: Abstract–$rm Gd\_5({rm Si\_rm xrm Ge\_1-rm x\})_4\$ exhibits a first order phase transition for the compositions $0<rm x<0.575$ leading to a magnetic phase transition as well. It is not possible to measure the second order phase (magnetic) transition temperatures of the individual phases with direct measurements. This is because the first order phase transition occurs before the second order phase transition. With modified Arrott plots we have shown previously that it is possible to estimate the second order phase transition of the $rm Gd\_5rm Si\_4\$ -type orthorhombic phase. In this paper we have estimated the second order phase transition temperature of the $rm Gd\_5rm Si\_2rm Ge\_2\$-type monoclinic phase using a single crystal sample of $rm Gd\_5rm Si\_1.5rm Ge\_2.5\$ (0.375) which falls in the mixed phase region of the sample.
IEEE Transactions on Magnetics 11/2012; 48(11):4070-4073. · 1.36 Impact Factor
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ABSTRACT: Abstract–$rm Gd\_5({rm Si\_rm xrm Ge\_1-rm x\})_4\$ exhibits a first order phase transition for the compositions $0<rm x<0.575$ leading to a magnetic phase transition as well. It is not possible to measure the second order phase (magnetic) transition temperatures of the individual phases with direct measurements. This is because the first order phase transition occurs before the second order phase transition. With modified Arrott plots we have shown previously that it is possible to estimate the second order phase transition of the $rm Gd\_5rm Si\_4\$ -type orthorhombic phase. In this paper we have estimated the second order phase transition temperature of the $rm Gd\_5rm Si\_2rm Ge\_2\$-type monoclinic phase using a single crystal sample of $rm Gd\_5rm Si\_1.5rm Ge\_2.5\$ (0.375) which falls in the mixed phase region of the sample.
IEEE Transactions on Magnetics 11/2012; 48(11):4070-4073. · 1.36 Impact Factor
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ABSTRACT: The contrast between the saturation tetragonal magnetostriction, λγ,2 = (3/2)λ100, of Fe1−xGax and Fe1−yGey, at compositions where both alloys exhibit D03 cubic symmetry (second peak region), was investigated. This region corresponds to x = 28 at. % Ga and y = 18 at. % Ge or, in terms of e/a = 2 x + 3 y + 1, to an e/a value of ∼1.55 for each of the alloys. Single crystal, slow-cooled, ternary Fe1−x−y GaxGey alloys with e/a ∼1.55 and gradually increasing y/x were investigated experimentally (magnetostriction, elasticity, powder XRD) and theoretically (density functional calculations). It was found that a small amount of Ge (y = 1.3) replacing Ga in the Fe–Ga alloy has a profound effect on the measured λγ,2. As y increases, the drop in λγ,2 is considerable, reaching negative values at y/x = 0.47. The two shear elastic constants c′ = (c11− c12)/2 and c44 measured for four compositions with 0.06 ≤ y/x ≤ 0.45 at 7 K range from 16 to 21 GPa and from 133 to 138 GPa, respectively. Large temperature dependence was observed for c′ but not for c44, a trend seen in other high-solute Fe alloys. The XRD analysis shows that the metastable D03 structure, observed previously in slow-cooled Fe–Ga at e/a = 1.55, is replaced with two phases, fcc L12 and hexagonal D019, at just 1.6 at. % Ge. The two are the stable phases of the assessed Fe–Ga phase diagram at x ∼ 28. Notably, at y = 7.8, only the D03 phase (the equilibrium phase of Fe–Ge at e/a = 1.54) was found in the ternary alloy. The theory also shows that the D03 instability is removed for compositions with y ≥ 3.9, when D03 becomes the structure’s ground-state phase. Thus, the high, positive λγ,2 value for Fe–Ga at x = 28 could be the result of the high sensitivity of its metastable D03 structure.
Journal of Applied Physics 03/2011; 109(7):07A904-07A904-3. · 2.17 Impact Factor
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ABSTRACT: Both components of the tetragonal magnetoelastic constant b<sub>1</sub> : the saturation magnetostriction, λ<sup>γ,2</sup>=(3/2)λ<sub>100</sub> , and the magnetic-field saturated shear elasticity, c<sup>′</sup>=(c<sub>11</sub>-c<sub>12</sub>)/2 , were investigated over a wide temperature range for the magnetostrictive Fe <sub>1-x-y</sub> Ga <sub>x</sub> Ge <sub>y</sub> alloys, ( x+y≅0.125 , 0.185, and 0.245; x/y≅1 and 3). The magnetostriction was measured from 77 to 425 K using standard strain gage techniques. Both shear elastic constants ( c<sup>′</sup> and c<sub>44</sub> ) were measured from 5 to 300 K using resonant ultrasound spectroscopy. Six alloy compositions were prepared to cover three important regions: (I) the disordered solute α-Fe region, (II) a richer solute region containing both disordered and ordered phases, and (III) a rich solute region containing ordered multiphases. Our observations reveal that, when the data is presented versus the total electron/atom (e/a) ratio, the above regions for both the ternary and binary alloys are in almost perfect alignment. Following this analysis, we find that the magnetoelastic coupling, b<sub>1</sub> , peaks for both the binary and the ternary alloys at e/a∼1.35 . The values of c<sup>′</sup> as well as of λ<sup>γ,2</sup> in region I of the ternary alloys, when plotted versus e/a , fall appropriately between the binary limits.
Journal of Applied Physics 06/2010; · 2.17 Impact Factor
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ABSTRACT: Temperature dependent magnetoelastic properties of Fe <sub>100</sub> <sub>-</sub> <sub>x</sub>Ge<sub>x</sub>(5 < x < 18) single crystals have been measured. Tetragonal magnetostriction (3/2) lambda<sub>100</sub> measurements at x = 5.7, 12.1, 14.9, and 17.7 were performed between 78 K and 426 K and resonant ultrasound spectroscopy measurements were used to determine the shear elastic constant c' from 5 K to 300 K for x = 6.4, 7.2, 10.8, 14.6, 17.7 , and 17.9 . A clear distinction was observed between the temperature dependencies of lambda<sub>100</sub> for the A2 and D0<sub>3</sub> phases of Fe<sub>100-x</sub>Ge x . The elastic constant c' displays a monotonic decrease with concentration through the different phases (6 < x < 18) and at all temperatures. Experimental values of the tetragonal magnetoelastic coupling constant - b <sub>1</sub> at 81 K were remarkably consistent with theoretical values determined by density functional calculations at 0 K.
IEEE Transactions on Magnetics 11/2009; · 1.36 Impact Factor
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ABSTRACT: Despite the huge magnetic-field-induced strain (MFIS) of up to 9.5% exhibited by certain Ni–Mn–Ga alloys, their usefulness in applications is severely hindered by the electromagnet device needed for driving the alloys with a magnetic field and orthogonal loading stress. In this paper we present macroscopic measurements obtained from a single crystal of Ni <sub>50</sub> Mn <sub>28.7</sub> Ga <sub>21.3</sub> which demonstrate a large reversible MFIS of -4100 ppm when the alloy is driven with quasistatic magnetic fields and fixed compressive stresses applied collinearly along the [001] axis. This collinear configuration marks a fundamental difference with prior research in the field and points to the existence in this alloy of stable bias or residual stresses—likely associated with pinning sites in the alloy—which provide the energy necessary to restore the original variant state when the field is reversed. We present macroscopic magnetomechanical measurements which show a decrease of the MFIS with increasing stress loading and a stiffening of the alloy with increasing dc fields. The latter behavior is phenomenologically similar to the ΔE effect in magnetostrictive materials. The large reversible MFIS and tunable stiffness properties exhibited by this alloy could enable practical Ni–Mn–Ga actuators for high-deflection, low-force applications which due to being driven by a solenoid transducer are more compact, energy efficient, and faster than their electromagnet counterpart. A thermodynamic model is presented which qualitatively characterizes the decay in MFIS with increasing compressive external load and provides a starting point for the characterization, design, and control of the proposed Ni–Mn–Ga devices.
Journal of Applied Physics 04/2006; · 2.17 Impact Factor
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ABSTRACT: The complex dielectric functions of single crystals of Gd5Si2Ge2 were obtained using spectroscopic ellipsometry (SE) in the photon energy range of 1.5–5.0 eV at room temperature. Reflectance difference (RD) spectra for the a‐b and b‐c planes of single crystals of Gd5Si2Ge2 were derived from these dielectric functions and compared to those obtained from reflectance difference spectroscopy (RDS) at near-normal incidence. The two experimental RD spectra from SE and RDS agreed well. The in-plane optical anisotropy of the sample is mainly due to intrinsic bulk properties because of its larger magnitude (4×10−2) compared to surface-induced optical anisotropies, with a magnitude of only about 10−3 for a typical cubic material.
Phys. Rev. B. 01/2006; 73(3).
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ABSTRACT: The Tb <sub>5</sub>( Si <sub>x</sub> Ge <sub>4-x</sub>) alloy system has many features in common with the Gd <sub>5</sub>( Si <sub>x</sub> Ge <sub>4-x</sub>) system although it has a more complex magnetic and structural phase diagram. This paper reports on the magnetic anisotropy and magnetic phase transition of single-crystal Tb <sub>5</sub>( Si <sub>2.2</sub> Ge <sub>1.8</sub>) which has been investigated by the measurements of M-H and M-T along the a , b , and c axes. The variation of 1/χ vs T indicates that there is a transition from paramagnetic to ferromagnetic at T<sub>c</sub>=110 K . Below this transition temperature M-H curves show very strong anisotropy, and it is believed that this is due to the complex spin configuration. M-H measurements at T=110 K show that the a axis is the easy axis, and that the saturation magnetization is 200 emu / g . The b axis is the hard axis, which needs an external magnetic field much higher than 2 T to saturate the magnetization in that direction, indicating a high magnetocrystalline anisotropy. The c axis is of intermediate hardness. The magnetic properties of this material are therefore very different from those of the related Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub> system, in which the b axis was found to be the easy axis and the magnitude of the anisotropy wa-
s smaller.
Journal of Applied Physics 06/2005; · 2.17 Impact Factor
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ABSTRACT: Elastic shear moduli measurements on Fe100−xGax (x = 12–33) single crystals (via resonant ultrasound spectroscopy) with and without a magnetic field and within 4–300 K are reported. The pronounced softening of the tetragonal shear modulus c′ is concluded to be, based on magnetoelastic coupling, the cause of the second peak in the tetragonal magnetostriction constant λ100 near x = 28. Exceedingly high ΔE effects ( ∼ 25%), combined with the extreme softness in c′ (c′<10 GPa), suggest structural changes take place, yet, gradual in nature, as the moduli show a smooth dependence on Ga concentration, temperature, and magnetic field. Shear anisotropy (c44/c′) as high as 14.7 was observed for Fe71.2Ga28.8.
Journal of Applied Physics 05/2005; 97(10):10M315-10M315-3. · 2.17 Impact Factor
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ABSTRACT: Magnetostriction measurements from 77 K to room temperature on oriented (100) and (110) disk samples of Fe <sub>93.9</sub> Be <sub>6.1</sub> and Fe <sub>88.7</sub> Be <sub>11.3</sub> reveal substantial increases in λ<sub>100</sub> compared to iron. For the 11.3% alloy, λ<sub>100</sub>=110 ppm , a sixfold increase above that of α-Fe. For the 6.1% alloy, λ<sub>100</sub>=81 ppm , ∼40% and ∼170% greater than λ<sub>100</sub> of comparable Fe–Ga and Fe–Al alloys, respectively, for H=15 kOe . Large differences exist between the values of λ<sub>100</sub> and λ<sub>111</sub> (λ<sub>100</sub>≫0, λ<sub>111</sub>≪0) and their temperature dependencies. Elastic constants, c<sub>11</sub>, c<sub>12</sub>, and c<sub>44</sub>, from 4 to 300 K were obtained on the same Fe–Be alloys. From these measurements, the magnetoelastic energy coefficients b<sub>1</sub> and b<sub>2</sub> were calculated. While the magnitudes of the magnetostrictions λ<sub>100</sub> and λ<sub>111</sub> are widely different, the magnitudes of b<sub>1</sub> and b<sub>2</sub> are within a factor of 2. The Fe–Be alloys are highly anisotropic magnetostrictively, elastically, and magnetoelastically. For Fe <sub>88.7</sub> Be <sub>11.3</sub> at room temperature λ<sub>100</sub>/λ<sub>111</sub>, 2c<sub>44</sub>/(c 11</sub>-c<sub>12</sub>), and b<sub>1</sub>/b<sub>2</sub> are -6.6, 3.55, and -1.86, respectively. © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; · 2.17 Impact Factor
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ABSTRACT: The system Gd <sub>5</sub>( Si <sub>x</sub> Ge <sub>1-x</sub>)<sub>4</sub> for 0.4≤x≤0.5 has been shown to have an unusual first order, coupled magnetic-structural phase transformation at the Curie temperature. Above the transformation temperature T<sub> c </sub>, the material is paramagnetic with a monoclinic structure; below T<sub> c </sub>, it is ferromagnetic with an orthorhombic structure. Another unusual feature of this phase transformation is that an applied magnetic field can increase T<sub> c </sub> by 5 K per tesla. In this study, the magnetic-structural transformation in single crystal Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub> was triggered by holding the sample at a temperature just above T<sub> c </sub>, then using an applied field to increase T<sub> c </sub> beyond the sample temperature, thereby inducing the magnetic-structural transformation. The dynamics of this field-induced phase transformation at various temperatures just above T<sub> c </sub> were observed by measuring the magnetization as a function of time. This magnetization change is caused by the first order phase transformation which is distinctly different from the magnetization reversal which one observes in conventional magnetic relaxation experiments. The transformation could be modeled as a thermal activation process with a single energy barrier of height 4.2±0.2 eV . © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; · 2.17 Impact Factor
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ABSTRACT: We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single crystal sample of Gd <sub>5</sub>( Si <sub>0.5</sub> Ge <sub>0.5</sub>)<sub>4</sub> prepared by the Bridgman method. TE and MS were measured along the c axis by the strain gauge method and the temperature was controlled using a closed cycle helium refrigerator. From the TE measurements the magnetic structural phase transition temperature was found to be 259.5 K on cooling and 261.5 K on heating. The abrupt change in strain and the temperature hysteresis indicate that it is a first order transition. MS measurements were conducted at 15, 258, and 265 K. At 15 K, the magnetostriction amplitude was 3–4 ppm, whereas at 258 K it was 100 ppm. At 265 K, which is just above the Curie temperature, a giant magnetostriction of 2000 ppm was found. This unusual behavior is due to the fact that the external magnetic field can increase the transition temperature above 265 K, resulting in a first order magnetic/structural phase transition. The results reveal that giant magnetostriction in Gd <sub>5</sub>( Si <sub>0.5</sub> Ge <sub>0.5</sub>)<sub>4</sub> only occurs as a result of the magnetic/structural transformation. © 2004 American Institute of Physics.
Journal of Applied Physics 07/2004; · 2.17 Impact Factor
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ABSTRACT: Reflectance difference (RD) spectra for the a–b plane of the single crystals of Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub> and b–c planes of Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub> and Tb <sub>5</sub> Si <sub>2.2</sub> Ge <sub>1.8</sub> were obtained in the photon energy range of 1.5–5.5 eV. Several peaks were observed for these crystals in the measured spectrum range. Similar features were observed in the RD spectra for the b–c planes of Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub> and Tb <sub>5</sub> Si <sub>2.2</sub> Ge <sub>1.8</sub>, while different features were observed for the a–b plane and b–c plane of Gd <sub>5</sub> Si <sub>2</sub> Ge <sub>2</sub>. The RD spectra for the crystals arise not only from the surface anisotropy but also from the bulk anisotropy due to the monoclinic structure of the bulk crystal. © 2004 American Institute of Physics.
Applied Physics Letters 04/2004; · 3.84 Impact Factor
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ABSTRACT: Single crystal Gd<sub>5</sub>(Si<sub>∼0.5</sub>Ge<sub>∼0.5</sub>)<sub>4</sub> was studied using thermal expansion measurements in the vicinity of the magnetic-structural phase transition. A strain gauge was attached on the a-b plane oriented to measure the strain along the b axis and a magnetic field of H=0.8×10<sup>6</sup> A/m (B=1 Tesla) was applied along this same b axis. The sample was then rotated through 45°, 90°, 120°, 165°, and 180° from the direction of the applied magnetic field and the thermal expansion measurements were repeated. It was found that there was an unexpected variation in the temperature at which the sudden discontinuity in strain occurred. The results seem to suggest that the transition temperature changes with direction of applied field relative to the crystal lattice. This surprising result is attributed to the combined effects of the first order phase transformation and the presence of an anisotropy "field," which differs with angle. This combination of factors, therefore, results in a different strength of effective field being experienced by the magnetic moments on the Gd atoms depending on their orientation. Comparing this with the previously observed field dependence of the Curie temperature in this material provides a possible explanation for this highly unusual observation.
IEEE Transactions on Magnetics 10/2003; · 1.36 Impact Factor
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ABSTRACT: Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. λ<sub>100</sub> exhibits two peaks as a function of Ga content. At room temperature, λ<sub>100</sub> reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, λ<sub>100</sub> drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling |b<sub>1</sub>| of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c<sub>11</sub>-c<sub>12</sub>) which approaches zero near 27% Ga. λ<sub>111</sub> is significantly smaller in magnitude than λ<sub>100</sub> over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%. © 2003 American Institute of Physics.
Journal of Applied Physics 06/2003; · 2.17 Impact Factor
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ABSTRACT: Textured polycrystalline Fe-Ga alloys exhibit magnetostrictive strains of 100 ppm or greater and may function as a mechanically robust actuator/sensing material. Current efforts seek to combine the 300+ ppm magnetostrictive strain performance of [100] oriented single crystals with the mechanical properties of polycrystalline forms. One approach to combining these properties is to control the crystallographic texture through deformation processing such as rolling. To determine the relationship between saturation magnetostriction, degree of texturing, and grain morphology we compare the results of three-dimensional finite element simulations with the analytical solution for a random polycrystal and the experimental responses of rolled polycrystalline Fe83Ga17. Textured specimens were produced through rolling reductions up to 99% of an as-cast ingot and a subsequent 1100 or 590 °C anneal. The high temperature anneal produced a recrystallized grain structure having a wide variation in crystal orientation as determined by orientation imaging microscopy. This recrystallized specimen exhibited a net magnetostriction of ∼170 ppm in the rolling direction and was well correlated with the finite element model result. The low temperature annealed specimen possessed fine elongated grains having dispersed {001}〈110〉 and {111}〈211〉 textures. Net magnetostrictions of 30 and 37 ppm were measured in the rolling direction and 45° off the rolling direction, respectively. The low magnetostriction value in the 45° direction disagrees substantially with the finite element solution of 157 ppm and suggests that unknown factors are dominating the response. © 2003 American Institute of Physics.
Journal of Applied Physics. 05/2003; 93(10):8495-8497.
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ABSTRACT: Measurements of thermal expansion of single-crystal Gd<sub>5</sub>(Si<sub>1.95</sub>Ge<sub>2.05</sub>) during cooling and heating were conducted for the first time. A very steep change in strain with temperature was observed when the material underwent a phase transformation. This was an unusual simultaneous magnetic and structural phase transformation from a ferromagnet with an orthorhombic crystal structure below the transition temperature T<sub>c</sub> to a paramagnet with a monoclinic crystal structure above T<sub>c</sub>. This transition temperature T<sub>c</sub> was found to depend on the magnetic field, and to exhibit hysteresis depending on whether the material was being cooled or heated. In the absence of a magnetic field, T<sub>c</sub> was 267 K on cooling and 269 K on heating. However, when the material was subjected to a magnetic induction B in the range 0-2.5 tesla (T), the transition temperatures, on both cooling and heating, were found to increase linearly with temperature by about 4.8 K/T. This rate of change of transition temperature with magnetic field was in good agreement with calculations based on the assumption that the additional energy due to the magnetic field can suppress the thermal vibration of Gd atoms and that the additional thermal energy per Gd atom needed to cause the phase transition to occur is equal to the additional magnetic energy of each Gd atom caused by the magnetic field.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: Gd<sub>5</sub>(Si<sub>2</sub>Ge<sub>2</sub>) and related compounds with similar (nearly equal Si-to-Ge ratio) composition exhibit large magnetoresponsive properties including a giant magnetocaloric effect, colossal magnetostriction, and giant magnetoresistance near a structural-magnetic phase transition that occurs close to ambient temperature. Magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) measurements on single-crystal samples of these materials indicate that the easy magnetization axis is the b-axis of the orthorhombic magnetic phase-perpendicular to the slabs. In fact, the MFM image of a surface perpendicular to the b-axis is quite similar to domain patterns perpendicular to the easy axis of Co and other highly anisotropic magnetic materials. Therefore, it appears that Gd<sub>5</sub>(Si<sub>x</sub>Ge<sub>1-x</sub>)<sub>4</sub> may require modeling similar to other multilayers and superlattices of rare-earth metals with one or more nonmagnetic constituents that exhibit long-range magnetic order across nonmagnetic layers. Many of the important phenomena of these Gd compounds could be explained by the interaction of localized Gd magnetic moments across the covalent bonding between atomic slabs, adapting models already suggested for other similar materials.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: It was recently reported that the addition of nonmagnetic Ga increased the saturation magnetostriction (λ100) of Fe over tenfold while leaving the rhombohedral magnetostriction (λ111) almost unchanged. To determine the relationship between the magnetostriction and the magnetization we measured the temperature and stress dependence of both the magnetostriction and magnetization from −21 °C to +80 °C under compressive stresses ranging from 14.4 MPa to 87.1 MPa. For this study a single crystal rod of Fe0.81Ga0.19 was quenched from 800 °C into water to insure a nearly random distribution of Ga atoms. Constant temperature tests showed that compressive stresses greater than 14.4 MPa were needed to achieve the maximum magnetostriction. For the case of a 45.3 MPa compressive stress and applied field of 800 Oe, the maximum magnetostriction at 80 °C decreases from its value at −21 °C by 12.9%. This small magnetostrictive decrease is consistent with a correspondingly small 3.6% decrease in magnetization over the same temperature range. This well-behaved temperature response makes this alloy particularly valuable for industrial and military smart actuator, transducer, and active damping applications. The measured value of Young’s modulus is low (∼55±1 GPa) and almost temperature independent. The large magnetostriction over a wide temperature range combined with the nonbrittle nature of the alloy is rare. © 2002 American Institute of Physics.
Journal of Applied Physics 05/2002; 91(10):7821-7823. · 2.17 Impact Factor
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ABSTRACT: The magnetostriction (λ<sub>100</sub>) of b.c.c. Fe is
increased over 10-fold at room temperature by the substitution of ~20%
gallium for Fe. Fe<sub>1-x</sub>Ga<sub>x</sub> alloys with x between
0.19 and 0.214 that are quenched from 800°C exhibit
magnetostrictions ~25% higher than those furnace-cooled at 10°/min.
We propose that this great increase of magnetostriction above that of Fe
in Fe-Ga alloys is not due to conventional magnetoelastic effects but
due to the substitutive presence of asymmetrically shaped clusters of
the Ga atoms. As the concentration of solute atoms approaches 25%, the
lattice becomes relaxed with formation of a more ordered structure and
the magnetostriction decreases in value
IEEE Transactions on Magnetics 08/2001; · 1.36 Impact Factor
