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Magnetic and magnetostrictive properties in amorphous ( Tb 0.27 Dy 0.73 )( Fe 1−x Co x ) films N H Duc, K Mackay, J Betz, and D Givord Citation: Journal of Applied Physics 87, 834 (2000); doi: 10.1063/1.371950 View online: http://dx.doi.org/10.1063/1.371950 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/87/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Anisotropic magnetostriction in a 110 oriented crystal Tb 0.36 Dy 0.64 ( Fe 0.85 Co 0.15 ) after coaxial field annealing J Appl Phys 108, 043908 (2010); 10.1063/1.3467785 Stress influences on magnetization and magnetostriction in magnetically annealed Tb 0.36 Dy 0.64 ( Fe 0.85 Co 0.15 ) polycrystals J Appl Phys 105, 093915 (2009); 10.1063/1.3117184 Combining large magnetostriction and large magnetostrictive susceptibility in Tb Fe Co ∕ Y x Fe − x exchangespring-type multilayers Appl Phys Lett 85, 1565 (2004); 10.1063/1.1787156 Synthesis and magnetostriction of melt-spun Pr 1−x Tb x (Fe 0.6 Co 0.4 ) alloys J Appl Phys 91, 271 (2002); 10.1063/1.1420772 Magnetic, Mössbauer and magnetostrictive studies of amorphous Tb(Fe 0.55 Co 0.45 ) 1.5 films J Appl Phys 87, 7208 (2000); 10.1063/1.372970 [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 15 JANUARY 2000 Magnetic and magnetostrictive properties in amorphous „Tb0.27Dy0.73…„Fe1؊ x Cox … films N H Duca) Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334-Nguyen Trai, Thanh Xuan, Hanoi, Vietnam K Mackay, J Betz, and D Givord Laboratoire de Magne´tisme Louis Ne´el, CNRS, 38042 Grenoble, Cedex 9, France ͑Received May 1999; accepted for publication 16 September 1999͒ Magnetic and magnetostrictive properties have been investigated for amorphous (Tb0.27Dy0.73) (Fe1Ϫx Cox ) thin films An increase in the 3d magnetic moment due to the enhancement of T–T interactions in substituted ͑Fe, Co͒ alloys was found This leads to stronger R–͑Fe, Co͒ exchange energies and then to enhancements of R–sublattice magnetization as well as magnetostriction in these amorphous R͑Fe, Co͒ thin films In addition, a well-defined in-plane anisotropy is created by magnetic-field annealing for the Co-rich films A large magnetostriction of 480ϫ10Ϫ6 developed in low fields of 0.3 T was observed for films with xϭ0.47 after magnetic-field annealing The differing roles of Fe and Co atoms on the magnetization process have also been discussed © 2000 American Institute of Physics ͓S0021-8979͑99͒06624-4͔ I INTRODUCTION Over the past few years there has been a growing interest in magnetic thin films with large magnetostriction.1–3 This interest is motivated by the potential such films show for use in microsystems actuators R–Fe (Rϭrare earth) based alloys offer the possibility to develop very large magnetostriction at room temperature This is due to the highly aspherical f orbitals remaining oriented by the strong coupling between R and Fe moments In order to exploit this property at reasonably low fields, it is essential to have low macroscopic anisotropy A first route to low anisotropy is by using cubic compounds in which the second-order anisotropy constants vanish This is the case for the RFe2 laves phase compounds of which TbFe2 ͑terfenol͒, a ferrimagnet with T C ϭ710 K, is probably the best known,4 having ␭ s ϭ1753ϫ10Ϫ6 The anisotropy can be further decreased by substitution of Tb and Dy in these compounds This is due to Dy and Tb having opposite signs of the Steven’s ␤ J coefficient and thus their contribution to the fourth-order anisotropy being of opposite sign This leads to the magnetostriction, albeit less than in pure TbFe2, being saturated in much lower fields This is the case for the terfenol-D material, the crystalline ͑Tb0.27Dy0.73͒Fe2 compound, which has found many applications as high-power actuators An alternative route to low macroscopic anisotropy is by using amorphous materials In Fe-based amorphous alloys, both positive and negative exchange interactions exist5 leading to magnetic frustration in the Fe sublattice In amorphous a-YFe alloys, this results in a concentrated spin-glass behavior below room temperature In a-RFe alloys, where R is a magnetic rare earth, the additional contributions of R–Fe exa͒ Author to whom correspondence should be addressed; electronic mail: duc@cryolab.edu.vn change and local crystalline electric-field interactions lead to the formation of sperimagnetic structures.5 The ordering temperatures are above room temperature ͓T C ϭ410 K for a-Tb0.33Fe0.66 ͑Refs and 7͔͒ It is, however, still rather low and is thus detrimental to large magnetostrictions being obtained in such materials at room temperature Actually, with a view to obtaining large magnetostrictions in the amorphous state, it is interesting to consider the equivalent a-RCo-based alloys Although crystalline RCo2 compounds order below 300 K as Co is merely paramagnetic,8 the amorphous state stabilizes a moment on the Co sublattice due to band narrowing These Co moments are strongly ferromagnetically coupled A sperimagnetic structure occurs as in a-RFe alloys but the ordering temperature is now raised up to 600 K ͑Ref 7͒ for Tb0.33Co66 Recently, we have studied a-Tbx Co1Ϫx and shown that large magnetostrictions of b ␥ ,2ϭ300ϫ10Ϫ6 at 300 K are obtained for xϳ0.33.9 In general, however, R–Fe exchange energies are larger than the equivalent R–Co interaction energies.10 This arises from the fact the Fe moment is significantly larger than the Co one, while the R–T intersublattice exchange constant (Tϭtransition metal) is approximately the same for TϭFe and Co In addition, the T–T interactions tend to be stronger in ͑FeCo͒- than in either Fe- or Co-based alloys.11 This results in an increase of T C for a given R:T ratio The stronger R–FeCo exchange energies should then lead to an enhancement of the R moment at room temperature and thus the magnetostriction in these amorphous alloys Recently, we have studied the magnetostriction in amorphous (Tb1Ϫx Dyx )(Fe0.45Co0.55) 2.1 thin films A magnetostriction of was obtained for amorphous 1020ϫ10Ϫ6 Tb͑Fe0.45Co0.55͒2.1 12 Indeed, this is much larger than that seen in other amorphous films of either TbFe or TbCo 0021-8979/2000/87(2)/834/6/$17.00 834 © 2000 American Institute of Physics [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 Duc et al J Appl Phys., Vol 87, No 2, 15 January 2000 835 In the present article, we have studied the influence of the Fe:Co ratio on the magnetization and magnetostriction of (Tb0.27Dy0.73)(Fe1Ϫx Cox ) We will show that the Fe:Co ratio of 50:50 responds approximately to the optimum composition for the giant magnetostriction II EXPERIMENT The films were prepared by rf magnetron sputtering The typical power during sputtering was 300 W and the Ar pressure was 10Ϫ2 mbar A composite target was used allowing a wide range of alloys to be made in a controllable way without a large cost of materials The target consisted of 18 segments of about 20°, of different elements ͑here, Tb, Dy, Fe, Co͒ These were made by spark cutting pure element disks They were then assembled and stuck to a Cu sample holder using silver paint It was verified by Rutherford backscattering spectroscopy ͑RBS͒ and X-ray energy-dispersive spectroscopy ͑XEDS͒ measurements that no Cu and Ag contamination has occurred The target–substrate distance was cm The substrates were glass microscope cover slips with a nominal thickness of 150 ␮m Both target and sample holder were water cooled The ratio of the deposition rates of RϭTb, Dy to TϭFe, Co is 0.85 Thus, for the (Tb0.27Dy0.73)(Fe1Ϫx Cox ) films made here, the Tb͑Dy͒ and Fe͑Co͒ concentrations could, in principle, be varied in steps of about 14% and 9%, respectively The resulting composition, contamination, and the composition homogeneity were measured using XEDS and RBS analyses The thicknesses were measured mechanically using an ␣-step and the sample mass was determined from the mass difference of the substrates before and after sputtering The typical film thickness was 1.2 ␮m X-ray ␪ – ␪ diffraction showed the as-deposited samples to be amorphous Samples were annealed at 150° and 250 °C for h under a magnetic field of 2.2 T in order to relieve any stress induced during the sputtering process and to induce a welldefined uniaxial in-plane anisotropy Subsequent x-ray ␪ – ␪ diffraction showed no evidence of recrystallization after annealing The magnetization measurements were carried out using a vibrating sample magnetometer in a field of up T from 4.2 to 800 K The magnetostriction was measured using an optical deflectometer ͑resolution of 5ϫ10Ϫ8 rad͒, in which the bending of the substrate due to the magnetostriction in the film was measured This allows the magnetoelastic coupling coefficient of film ͑b͒ to be directly determined13,14 using bϭ ␣ h s2 Es , L h f ͑ 1ϩ ␯ s ͒ ͑1͒ where ␣ is the deflection angle of the sample as a function of applied field, L is the sample length, and E s and ␯ s are the Young’s modulus and Poission’s ratio for the substrate which are taken to be 72 GPa and 0.21, respectively h s and h f are the thicknesses of the substrate and film, respectively L was typically of the order of 13 mm FIG Hysteresis loops at 4.2 K for several (Tb0.27Dy0.73)(Fe1Ϫx Cox ) thin films: ͑1͒ Ϫxϭ0, ͑2͒ xϭ0.31, and ͑3͒ Ϫxϭ1.0 b is proportional to the magnetostriction via the Young’s modulus (E f ) and Poisson’s ratio ( ␯ f ) of the film These cannot be reliably measured for thin films However, for comparison, we also give values of ␭ calculated using ␭ϭ Ϫb ͑ 1ϩ ␯ f ͒ , Ef ͑2͒ where E f and ␯ s are taken to be 80 GPa and 0.31, respectively We measured two coefficients at saturation, b ʈ and bЌ , which correspond to the applied field, always in the film plane, being, respectively, parallel and perpendicular to the sample length ͑i.e., the measurement direction͒ In addition, the perpendicular direction corresponds to the easy axis induced after field annealing The intrinsic material-dependent parameter b ␥ ,2 ͑or ␭ ␥ ,2͒ is just the difference b ʈ ϪbЌ ͑or ␭ ʈ Ϫ␭Ќ , respectively͒ III EXPERIMENTAL RESULTS A Magnetization Figure presents the hysteresis loops for several asdeposited (Tb0.27Dy0.73)(Fe1Ϫx Cox ) films at 4.2 K The coercive fields are very large for all samples and the magnetization does not completely saturate even at T Such large coercive fields are typical of amorphous RT alloys at low temperatures, where R is a non-S state rare earth They are related to the strong local anisotropy of the R atoms and their random distribution of easy axes present in such sperimagnetic systems The high-field susceptibility ( ␹ hf) is also typical of sperimagnetic systems and is associated with the closing of the cone distribution of R moments as the field is increased.5 The coercive fields ( ␮ H C ) reach their highest value of 3.4 T for xϭ0 With increasing Co concentration, coercivity decreases rapidly down to about 0.5 T for 0.67рxр1.0 ͓see Fig 2͑a͔͒ The ␹ hf also decreases with increasing Co concentration, to a minimum at xϭ0.47 and then slightly increases with further increasing x In all cases, ␮ H C also decreases with increasing temperature ͓see the inset in Fig 2͑a͔͒, while the ␹ hf is strongly enhanced This is due to the rapid decrease local anisotropy of the R atoms as the temperature is increased compared to [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 836 Duc et al J Appl Phys., Vol 87, No 2, 15 January 2000 FIG Hysteresis loops for the ͑Tb0.27Dy0.73͒Co2 ͑1͒ as-deposited film and ͑2͒ after annealing along induced easy axis and ͑3͒ hard axis FIG ͑a͒ Coercive field ␮ H c as a function of Co concentration at 4.2 K Inset shows the temperature dependence of ␮ H c for xϭ0.83 ͑b͒ Coercive field ␮ H c as a function of Co concentration at 300 K: ͑1͒ the asdeposited films, ͑2͒ after annealing at 150 °C, and ͑3͒ after annealing at 250 °C the exchange field In Fig 2͑b͒, we present ␮ H C at 300 K as function of x All the films are magnetically rather soft at room temperature and there is a maximum in ␮ H C at x ϭ0.63 The spontaneous magnetization values at 4.2 and 300 K for the as-deposited (Tb0.27Dy0.73)(Fe1Ϫx Cox ) films extrapolated to zero field are shown in Fig At 4.2 K there is a maximum at xϭ0.47 while at 300 K, within experimental errors, the magnetization is independent of the Co concentration This is in contrast with the behavior observed for the corresponding crystalline alloys where M s always shows a minimum in the middle of the composition range due to the enhancement of the 3d magnetic moment (M 3d ) In the amorphous case, however, an increase in M 3d will close the R-sperimagnetic cone The maximum in M s at xϭ0.47 reflects that, at low temperature, the enhancement of M 3d is smaller than the associated increase in the magnetization of the R sublattice ( ͗ M R͘ ) Samples were annealed at temperatures between 150 and 250 °C in an applied magnetic field of 2.2 T The field dependences of the magnetization before and after annealing are shown in Fig for xϭ1 For the as-deposited samples, the magnetization reversal process is progressive and isotropic with a rather large coercive field This property is often observed in sperimagnetic systems where domains of correlated moments are formed due to the competition between exchange interactions and random local anisotropy These domains, termed Imry and Ma domains,15,16 are oriented more or less at random in zero field but can be reoriented relatively easily under applied field After annealing, there are a number of clear differences in the magnetization process First, the coercive field is strongly reduced Figure 2͑b͒ shows the coercive field as a function of composition before and after annealing After annealing at 250 °C, ␮ H C is less than 0.002 T for samples with xϭ0.0 and 1.0 A slight maximum of ␮ H C around the middle of the composition range is still observed, however, with ␮ H C ϳ0.006 T only Second, for this sample, there is now a well-defined easy axis with an increased low-field susceptibility These properties are characteristic of systems which show uniaxial anisotropy This field-annealing induced anisotropy suggests that a process of single-ion directional ordering17 has occurred, in which there is a local reorientation of the Tb easy axes along the field direction The composition dependence of this uniaxial anisotropy is, however, more complex and will be discussed further in connection with the magnetostriction data The field annealing also causes a reduction in ␹ hf , indicating that the cone distribution of the Tb moments is somewhat closed B Magnetostriction In general, the comparison of b ʈ and bЌ indicates clearly the anisotropy state of the sample If the zero-field state is fully isotropic, then b ʈ ϭϪ2bЌ , and if it is isotropic in the plane, then b ʈ ϭϪbЌ 18 For a well-defined in-plane, uniaxial system, magnetization reversal under a field applied along the easy axis, occurs by 180° domain-wall displacement Ne- FIG Variation of spontaneous magnetization as a function of x at 4.2 and 300 K for (Tb0.27Dy0.73)(Fe1Ϫx Cox ) thin films [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 Duc et al J Appl Phys., Vol 87, No 2, 15 January 2000 FIG ͑a͒ Magnetostriction for xϭ0.83: ͑1͒ as-deposited film, ͑2͒ annealing at 150 °C, and ͑3͒ 250 °C ͑b͒ Magnetostriction for xϭ0: ͑1͒ asdeposited film, and ͑3͒ 250 °C glecting domain-wall contributions, no magnetostriction is associated with this process Thus, bЌ should be zero and b ʈ ϭb ␥ ,2 Figure shows the effect of annealing on the magnetostriction for two alloys with xϭ0.83 and xϭ0 For xϭ0.83 ͓see Fig 5͑a͔͒, we see that annealing increases the ratio of b ʈ to bЌ while b ␥ ,2 rests roughly constant This is due to the creation of an in-plane uniaxial anisotropy as seen from magnetization measurements In addition, we see that this anisotropy is completely induced after annealing at 150 °C and is accompanied by a reduction in the saturation field Subsequent annealing at 250 °C simply further reduces the saturation field For the xϭ0 sample ͓see Fig 5͑b͔͒, we see a different behavior Before annealing, the approach to saturation is rather slow and the ratio of b ʈ to bЌ indicates an initial anisotropy After annealing, the saturation field is reduced and this initial anisotropy is destroyed, leaving the sample almost isotropic However, b ␥ ,2 ͑measured at 1.8 T͒ actually increases after annealing probably due to the reduction in the saturation field These differences are reflected across the whole composition range and the results obtained are summarized in Fig 6͑a͒ As outlined above, it is clear that the annealing affects very differently the Fe-rich alloys compared to the Co-rich ones For the Co-rich alloys, b ʈ increases significantly after annealing while b ␥ ,2 rests virtually unchanged For the Ferich alloys, we see the opposite effect in that b ␥ ,2 increases significantly after annealing while b ʈ rests virtually unchanged The annealing seems to destroy the initial asdeposited anisotropy and does not induce an in-plane uniaxial anisotropy These differences in anisotropy are also 837 FIG ͑a͒ Magnetostriction ␭ ␥ ,2(1.8 T) and ␭ ʈ (0.06 T) for the (Tb0.27Dy0.73)(Fe1Ϫx Cox ) as-deposited thin films ͑1͒ and ͑1͒, films annealed at 150 °C ͑2 and Ј ͒ and at 250 °C ͑3 and Ј ͒ ͑b͒ Ratio b ʈ /bЌ as a function of x before and after annealing reflected in Fig 6͑b͒, which shows the ratio of b ʈ to bЌ before and after annealing This will be discussed later The largest magnetostriction of ␭ ␥ ,2ϭ480ϫ10Ϫ6 and ␭ ʈ ϭ250 ϫ10Ϫ6 is found in the middle of the composition range at xϭ0.47 and can be obtained in very low applied magnetic fields of 0.06 T IV DISCUSSION The magnetic properties of these alloys are rather complex but it is important to attempt to understand them in order to better optimize the magnetostrictive properties of such alloys with respect to potential applications One of the main differences between the magnetic properties of amorphous RT2 alloys and their crystalline counterparts is the sperimagnetic distribution of R and Fe moments in the amorphous case.12 This sperimagnetic structure arises from the competition between exchange interactions and random local anisotropy and leads to the formation of domains of correlated moments These domains are oriented more or less at random in zero field and the macroscopic anisotropy energy, which determines the coercive field, is an average of the random local anisotropy over the volume of each domain.19 At low temperature, these domains are small and this explains the large coercive fields found in these alloys The sperimagnetic cone, within which the Tb and Dy moments lie, can be somewhat closed due to an increase in the molecular field of the T sublattice acting on them and this could account for the maximum seen in M s and the minimum in ␹ hf for xϭ0.47 At room temperature, however, this enhancement of the T sublattice moment is less clear The mag- [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 838 Duc et al J Appl Phys., Vol 87, No 2, 15 January 2000 FIG Calculated variation of ͗ M R ͘ and M 3d from magnetostriction data as a function of x netostriction is, on the other hand, much more sensitive to changes in the R–sublattice magnetization and we will now discuss this effect Assuming that the R moments have the same value as in the crystalline laves phase, we can estimate the magnetostriction of a sperimagnetic system with respect to a collinear ferrimagnetic one using ␥ ,2 b ␥ ,2ϭ 23 b int ͑ ͗ ␣ z ͘ Ϫ 31 ͒ , where ␣ z is the direction cosine for each rare-earth moment ␥ ,2 is the intrinstic with respect to the field direction and b int magnetoelastic coupling coefficient ͑i.e., that of the collinear ferrimagnet͒ Here, we take b ␥int,2ϭ127 MPa, the roomtemperature value of b ␥ ,2 in isotropic polycrystalline crystalline ͑Tb0.27Dy0.73͒Fe2 20 Assuming a uniform probability distribution of easy axes within a cone, we can deduce the characteristic sperimagnetic cone angle ͑␪͒ For the films under consideration, this gives values of between 48° and 53°, which are typical of those reported in the literature.5,21 This variation in ␪ implies that there is a variation in the average ͑Tb, Dy͒ moment as a function of x Using M ͑Tb, Dy͒ ϭ7.27 ␮ B , the room-temperature value in ͑Tb0.27Dy0.73͒Fe2, we can deduce ͗ M TbDy͘ ϭM ͑TbDy͒͗ ␣ z ͘ , as a function of x, and this is plotted in Fig From the measured magnetization data, we can now deduce M 3d as a function of x ͑Fig 7͒ The values thus determined are in good agreement with those found for M 3d in ‘‘pure’’ a-TbCo2 and a-TbFe2 alloys6 at room temperature This clearly indicates that there is an enhancement in M 3d for the substituted a-R͑Fe, Co͒2 alloys and a maximum is reached for xϭ0.47 where there is sufficient Co to ensure good ferromagnetic T–T coupling as well as sufficient Fe giving the larger magnetic moment We have, of course, neglected the variation in ordering temperature, and hence, the intrinsic R-moment value at room temperature associated with such an enhancement of the T–T interactions However, this simple analysis illustrates the importance of considering the influence of the sperimagnetic structure on the magnetostriction and the magnetic properties of such alloys An intriguing aspect in this study is the variation of the anisotropy state as a function of T composition, before and after annealing The comparison of b ʈ to bЌ is a useful tool for understanding the role of Co in these alloys ͓Fig 6͑b͔͒ For the Fe-rich alloys before annealing, b ʈ /bЌ is large indi- cating a well-defined initial anisotropy After annealing, b ʈ /bЌ ϷϪ2 suggests that the zero-field magnetization state is isotropic The as-deposited material is not completely saturated at 1.8 T, while after annealing saturation is achieved at around T This leads to the measured increase in (b ʈ ϪbЌ ) at 1.8 T after annealing For the as-deposited Co-rich alloys, b ʈ /bЌ ϷϪ1 indicates that the film is isotropic in the plane After annealing at 250 °C, this ratio is significantly increased showing that a well-defined in-plane anisotropy direction has been induced Figure 6͑b͒ shows the variation of b ʈ /bЌ as a function of Co concentration It clearly indicates that after annealing the easy axis becomes better defined with increasing Co content This may be accounted for as follows During the annealing process, it is the local internal molecular field that is responsible for the reorientation of the R moments The external field merely saturates the material in a given direction For the Fe-rich alloys, the sperimagnetic nature of the Fe-sublattice distribution is conveyed to the R sublattice and gives no net anisotropy However, the strongly ferromagnetically coupled Co sublattice is well ordered and its molecular field acts to orient the R sublattice in one direction, giving rise to the observed uniaxial anisotropy The differing anisotropies seen in the asdeposited state are more difficult to account for precisely, but it has often been noted that Fe-based RT compounds have a different anisotropy state compared to their Co-based counterpart We can further illustrate this variation in anisotropy by associating the field dependence of the magnetostriction with different types of magnetization processes For a system of randomly oriented spin and random distribution of domain walls, the magnetization process takes place in two steps.22 First, the motion of 180° domain walls leads to a magnetization of M without any contribution to magnetostriction In the second step, the spins rotate into the direction of the applied magnetic field leading to the change of both magnetization and magnetostriction For the case M ϭM max/2, the relation between magnetostriction and magnetization is given as18 ␭ ͑ H ͒ /␭ maxϭ ͓ 2M ͑ H ͒ /M maxϪ1 ͔ 3/2 ͑3͒ For the rotation of magnetization out of the easy axis, the magnetostriction is related to magnetization as follows:22 ␭ ͑ H ͒ /␭ maxϭ ͓ M ͑ H ͒ /M max͔ ͑4͒ The results of this analysis are presented in Fig The experimental data for the ͑Tb, Dy͒Fe2 film are rather well described by Eq ͑3͒ With increasing Co concentration, the ␭/␭ max vs M /M max curves shift towards the line described by Eq ͑4͒ This further confirms that Co substitution is advantageous to the creation of a well-defined easy axis in this system Finally, the room-temperature magnetostriction is strongly influenced by the Curie temperature of the investigated alloys It is worth reporting here that one has found the T C value of 440 K for the a-͑Tb0.27Dy0.73)(Fe1Ϫx Cox ) film with xϭ0.63 Indeed, this T C value is much higher than that reported for a-͑Tb0.27Dy0.73͒Fe2 (T C ϭ370 K, see also, e.g., Ref 23͒ The larger T C is associated also to the stronger [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 Duc et al J Appl Phys., Vol 87, No 2, 15 January 2000 839 ACKNOWLEDGMENTS The authors thank Dr E du Tre´molet de Lacheisserrie for helpful discussions This work was carried out as part of the E C funded ‘‘MAGNIFIT’’ project ͑Contract No BRE2-0536͒ The work of one of the authors ͑N H D.͒ is partly supported by the National University of Hanoi within Project No QG.99.08 E Quandt, J Alloys Compd 258, 126 ͑1997͒ E Tre´molet de Lacheisserise, K Mackey, J Betz, and J C Peuzin, J Appl Phys 275–277, 685 ͑1998͒ N H Duc, in Handbook on the Physics and Chemistry of Rare Earths, edited by K A Gschneidner, Jr and L Eyring ͑North-Holland, Amsterdam͒, Vol 28 ͑to be published͒ A E Clark, in Ferromagnetic Materials, edited by E P Wohlfarth, Vol ͑North-Holland, Amsterdam, 1980͒, Vol 1, p 531 J M D Coey, D Givord, A Lie´nard, and J P Rebouillat, J Phys F 11, 2707 ͑1981͒ P Hansen, G Much, M Rosenkranz, and K Witter, J Phys 66, 756 ͑1989͒ K Lee and N Heiman, AIP Conf Proc 18, 108 ͑1973͒ R Lemaire R., Cobalt ͑Engl Ed.͒ 1968, 33 J Betz, Thesis, University Joseph Fourier of Grenoble ͑1997͒ 10 J P Liu, F R de Boer, P F de Chaˆtel, R Coehoorn, and K H J Buschow, J Magn Magn Mater 134, 159 ͑1994͒ 11 J P Gavigan, D Givord, H S Li, and J Voiron, Physica B 149, 345 ͑1988͒ 12 N H Duc, K Mackay, J Betz, and D Givord, J Appl Phys 79, 973 ͑1996͒ 13 E Tre´molet de Lacheisserise and J C Peuzin, J Magn Magn Mater 136, 189 ͑1994͒ 14 J Betz, E du Tre´molet de Lacheisserise, and L T Baczewski, Appl Phys Lett 68, 132 ͑1996͒ 15 Y Imry and S Ma, Phys Rev Lett 35, 1399 ͑1975͒ 16 B Boucher, A Lie´nard, J P Rebouillat, and J Schweizer, J Phys F 9, 1421 ͑1979͒ 17 L Ne´el, Compte Rendu 273, 1468 ͑1953͒; J Phys Radium 15, 225 ͑1954͒ 18 F Schatz, M Hirscher, M Schnell, G Flik, and H Kroămuller, J Appl Phys 76, 5380 1994 19 R Alben, J I Bundrik, and G S Cargill, Metallic Glasses ͑American Society for Metals, Metals Park, OH, 1978͒, Chap 12 20 The values given in Ref are for somewhat textured samples Here, we calculate b ␥ ,2 for an isotropic polycrystalline sample of Tb0.27Dy0.73Fe2, using single-crystal data b ␥ ,2ϭ3G␭ S with 1/(2G)ϭ2/5S ␥ ϩ3/5S ⑀ and ␭ S ϭ0.6␭ 111 E du Tre´molet de Lacheisserise ͑private communication͒ 21 P Hansen, in Ferromagnetic Materials, edited by K H J Buschow ͑North-Holland, Amsterdam, 1991͒, Vol 6, p 289 22 S Chikazumi, Physics of Magnetism ͑Willey, New York, 1964͒ 23 K Ried, M Schnell, F Schatz, M Hirscher, B Ludescher, W Sigle, and H Kroămuller, Phys Status Solidi A 167, 195 ͑1998͒ FIG Experimental and theoretical relations between normalized magnetostriction and magnetization for amorphous (Tb0.27Dy0.73)(Fe1Ϫx Cox ) thin films R–FeCo exchange energies This is one of the reasons why the room-temperature magnetostriction was enhanced in amorphous ͑Tb, Dy͒͑Fe, Co͒ films V CONCLUDING REMARKS In conclusion, we would like to point out that larger magnetostrictions are obtained in amorphous ͑Tb, Dy͒͑Fe, Co͒ films as compared to their parent amorphous films of either ͑Tb, Dy͒Fe or ͑Tb, Dy͒Co This has been explained in terms of an increase in the ferromagnetic coupling strength within the ͑Fe, Co͒ sublattice In addition, a well-defined uniaxial anisotropy can be induced by magnetic-field annealing for Co-rich films It is well known that the substitution of Dy for Tb gives rise to the increase of the magnetostriction at low magnetic fields, through the reduction of the saturation field However, it is also accompanied by a reduction in the saturation magnetostriction In this study, we have shown that Co substitution, coupled with the effects of annealing, results in an enhancement of both the low-field and saturation magnetostriction Thus, we can expect a further enhancement of the magnetostriction in these alloys by increasing the Tb concentration Indeed, we have obtained a giant magnetostriction of ␭ ␥ ,2ϭ1020ϫ10Ϫ6 at 1.8 T with ␭ ʈ ϭ585ϫ10Ϫ6 at 0.1 T in amorphous Tb͑Fe0.55Co0.45͒2 12 [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 128.42.202.150 On: Mon, 24 Nov 2014 01:15:35 ... with increasing Co concentration, to a minimum at xϭ0.47 and then slightly increases with further increasing x In all cases, ␮ H C also decreases with increasing temperature ͓see the inset in Fig... approach to saturation is rather slow and the ratio of b ʈ to bЌ indicates an initial anisotropy After annealing, the saturation field is reduced and this initial anisotropy is destroyed, leaving the... exist5 leading to magnetic frustration in the Fe sublattice In amorphous a-YFe alloys, this results in a concentrated spin-glass behavior below room temperature In a-RFe alloys, where R is a magnetic