Magnetostrictive Performance of Electrodeposited Tbxdy(1-X)

micromachines

Article Magnetostrictive Performance of Electrodeposited TbxDy(1−x)Fey Thin Film with Microcantilever Structures

Hang Shim 1,*, Kei Sakamoto 2, Naoki Inomata 1 , Masaya Toda 1, Nguyen Van Toan 1,3 and Takahito Ono 1,2,3,* 1 Department of Mechanical Systems Engineering, Tohoku University, Sendai 980-8579, Japan; [email protected] (N.I.); [email protected] (M.T.); [email protected] (N.V.T.) 2 Micro-Nanomachining Research Education Center, Tohoku University, Sendai 980-8579, Japan; [email protected] 3 Micro System Integration Center (µSiC), Tohoku University, Sendai 980-8579, Japan * Correspondence: [email protected] (H.S.); [email protected] (T.O.)

Received: 30 March 2020; Accepted: 17 May 2020; Published: 21 May 2020

Abstract: The microfabrication with a magnetostrictive TbxDy(1 x)Fey thin film for magnetic − microactuators is developed, and the magnetic and magnetostrictive actuation performances of the deposited thin film are evaluated. The magnetostrictive thin film of TbxDy(1 x)Fey is deposited on − a metal seed layer by electrodeposition using a potentiostat in an aqueous solution. Bi-material cantilever structures with the Tb0.36Dy0.64Fe1.9 thin-film are fabricated using microfabrication, and the magnetic actuation performances are evaluated under the application of a magnetic field. The actuators show large magnetostriction coefficients of approximately 1250 ppm at a magnetic field of 11000 Oe.

Keywords: magnetostriction; thin ﬁlm; Terfenol-D; TbxDy(1 x)Fey; electrodeposition −

1. Introduction Magnetostriction is a useful property of ferromagnetic materials that causes strain during the process of magnetization. The strain of the magnetostriction materials can be controlled by a magnetic field [1–6]. Also, the strain itself can generate a magnetic field, referred to as inverse-magnetostrictive effect or Villari effect [1–3]. Magnetostriction can be quantified by the magnetostrictive coefficient which can be positive or negative and is defined as the generated strain when a magnetic field causing magnetization saturation is applied. For example, Fe, Ni, and Co are well known as magnetic materials, which show small magnetostrictive coefficients of 14, 50, and 93, respectively [1–3]. It is known − − − that Fe alloys containing a rare earth material exhibit a large magnetostrictive coefficient at room temperature, and those alloys are referred to as “giant magnetostriction material” [1–4]. Among those, Terfenol-D (Tb0.3Dy0.7Fe2) exhibits a large magnetostrictive coefficient up to 1400 ppm at a magnetic field of ~2 kOe [1–4,7]. In addition, Galfenol, Fe0.8Ga0.2, is known as a material, which shows a large magnetostriction up to 400 ppm [5,6] and CoFe shows 260 ppm as well [8–12]. Since these discoveries, those materials have emerged as a smart material for microdevices, including actuators [13,14], wireless sensors, biosensors [15,16], energy harvesting devices [17], and atomic force microscopy [18]. Most of the devices based on the giant magnetostriction materials are made from bulk materials. However, the importance of the thin-film technology for those materials have gained for realizing miniaturized smart actuators and devices. Many methods have been reported for thin-film preparation, including pulsed laser deposition [7], sputtering [10,11,19–28], and electrochemical deposition [5,6,9,29]. For the film deposition of Terfenol-D, the sputtering method has been reported because of the simple

Micromachines 2020, 11, 523; doi:10.3390/mi11050523 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 523 2 of 13 approach, high film uniformity, and low roughness. However, the substrate must be heated at a temperature higher than 400 ◦C for crystallization, because the sputter-deposited Terfenol-D films are amorphous state at low temperatures, which show a low magnetostrictive performance [23–25]. Those sputtered films exhibit a magnetostriction coefficient approximately 1/3 (540 ppm) of the bulk value without annealing and 2/3 (920 ppm) of the bulk value with annealing at 450 ◦C[19–21]. Electrochemical deposition has advantages for simplicity, low cost, and compatibility with batch fabrication, etc., and some of the researches have been reported regarding magnetostrictive thin films including Galfenol [5,6], CoFe [9], and TbFe2 [29]. Rare earth atoms including Tb and Dy are generally difficult to deposit by electrodeposition using an aqueous solution because those materials have reduction potential < 2 V (eg., Tb+3 + 3e = − − Tb: 2.28 V, Dy3+ + 3e = Dy: 2.6 V); therefore, hydrogen evolution makes the aqueous solution − − − unstable [26]. Chemical additives add to reducing electrochemical deposition potential and improve film quality [29–33]. Gong et al. demonstrated the deposition of TbFe2 film in an aqueous solution with a rare earth metal complex [29]. In this work, Terfenol-D films are deposited by electroplating and the performances of the deposited films are evaluated using energy-dispersive X-ray spectroscopy (EDX) and vibrating-sample magnetometer (VSM) analysis. Microcantilever bi-material structures are fabricated, and the magnetostriction performances are evaluated.

2. Experimental

A TbxDy(1 x)Fey thin film was deposited by electrodeposition using a potentiostat with three − conventional electrodes: working, counter, and reference electrodes. This aqueous electrolyte was prepared from rare earth sulfate salts and iron salts. In order to reduce the deposition potential, those ions are chelated by citric acid and tartaric acid. Thus, the electrolyte is formed by mixing deionized 150 mL water with following chemicals, i.e., Tb2(SO4)3 0.3 g, Dy2(SO4)3 0.7 g, FeCl3 0.5 g, FeSO4 1.8 g, tartaric acid 3 g, citric acid 0.5 g, KCl 33 g, NaOH 1.1 g [34]. Electrodeposition was proceeded on a 300 nm-thick Cr-Cu seed layer deposited on a silicon on insulator (SOI) wafer with a 100 nm-thick Si/300 nm-thick buried oxide layer/ 750 µm-thick Si handling layer. The electrodeposition was performed at 40 C for 2.5 h at a working electrode potential of ( 925)–( 950 mV). The typical ◦ − − deposition rate of the TbxDy(1 x)Fey film is approximately 100 nm/h. However, the deposition speed − was varied by deposition area, the resistance of the seed layer, the distance between working and counter electrode, and temperature of the electrolyte. The compositions of the deposited films are analyzed by energy-dispersive X-ray spectroscopy (EDX). The magnetization properties for the in-plane direction of the film are measured using a vibrating sample magnetometer (VSM). For the analysis of surface morphology, atomic force microscopy (AFM) and magnetic force microscopy (MFM) are used. The magnetostriction coefficients are measured from the bi-material cantilever actuation using an optical microscope with a special resolution of 4.4 µm corresponding to one imaging pixel. The sample preparation for the TbxDy(1 x)Fey cantilevers is performed by microfabrication. The details of the − fabrication process are described later. Figure1 shows the typical EDX result of a 300 nm-thick Tb xDy(1 x)Fey thin film prepared at 930 mV − − of potential, which formed on the SOI wafer with the Cr-Cu seed layer. For the analysis, the substrate and seed layer components are ignored. Generally, the composition of electrodeposited alloys can be adjusted by the applied potential because of the reduction potential difference of each component. However, applicable potential has a limited window; for the potential V > 700 mV, Fe atoms do not − deposit. For the case of the potential V < 1055 mV, hydrogen evolution happens, and the electrolyte is − degraded. To maximize the magnetostriction performance, the atomic concentration ratio of rare earth and iron atoms must satisfy 1:2 [1–4]. Thus, the ideal weight percentage of Fe atoms is approximately 40%, and the atomic percentage is approximately 66%. The Fe concentration can be controlled by the working electrode potential, as shown in Figure2, which shows approximately 1% to 2% of the atomic percentage change of Fe atoms with 1 mV of potential variation. The optimal electrochemical potential Micromachines 2020, 11, 523 3 of 13 Micromachines 2020, 11, x 3 of 14

compositionMicromachines 2020fractions, 11, x of Tb, Dy, and Fe were analyzed to be approximately 12.6, 22 and 65.4 atomic3 of 14 %, can be found at 930 mV for the ﬁlm deposition on the Cu seed layer. The composition fractions of Tb, respectively; −thus, the film composition is approximated to be Tb0.36Dy0.64Fe1.9. Dy, andcomposition Fe were analyzedfractions of to Tb, be Dy, approximately and Fe were analyzed 12.6, 22 to and be approximately 65.4 atomic %, 12.6, respectively; 22 and 65.4 thus,atomic the %, ﬁlm compositionrespectively; is approximated thus, the film composition to be Tb0.36 Dyis approximated0.64Fe1.9. to be Tb0.36Dy0.64Fe1.9.

Figure 1. Energy-dispersive X-ray spectroscopy (EDX) spectrum of the TbxDy(1−x)Fey film formed at an electrochemical potential of −930 mV. FigureFigure 1. Energy-dispersive 1. Energy-dispersive X-ray X-ray spectroscopy spectroscopy (EDX) spectrum spectrum of ofthe the Tbx TbDyx(1Dy−x)Fe(1y filmx)Fe formedy ﬁlmformed at an at − an electrochemicalelectrochemical potentialpotential of of −930930 mV. mV. − Tb TbDy DyFe 90 FeTotal Rare Earth 85 90 80 Total Rare Earth 85 75 80 70 75 6570 6065 5560 5055 4550 4045 Atomic % Atomic 3540

Atomic % 30Atomic 35 2530 2025 1520 1015 105 05 0 920 925 930 935 940 945 950 920 925 930Potential 935 (mV) 940 945 950 Potential (mV) Figure 2. Composition dependence on the applied working electrode potential on a Cu seed layer. Figure 2. Composition dependence on the applied working electrode potential on a Cu seed layer. The in-planeFigure 2. Composition magnetization dependence analysis on the of theapplied 200 working nm-thick electrode Tb0.36 potentialDy0.64 Feon 1.9a Cusample seed layer. with the SOI wafer wasThe proceeded in-plane magnetization using VSM, analysis as a result of the is 200 shown nm-thick in Figure Tb0.363Dy.0.64 ItFe is1.9 found sample that with the the coercive SOI magneticwafer The fieldwas in-plane proceeded is approximately magnetization using VSM, 285 analysis Oe.as a Theresult of magnetizationthe is 200shown nm-thick in Figure is Tb saturated0.36 3.Dy It0.64 isFe atfound1.9 approximatelysample that with the thecoercive 5000SOI Oe, wafer was proceeded using VSM, as a result is shown in Figure 3. It is found that the coercive andmagnetic magnetization field is starts approximately to decrease. 285 Thus,Oe. The the magnetization effective magnetic is saturated field for at magnetostrictiveapproximately 5000 actuation Oe, magnetic field is approximately 285 Oe. The magnetization is saturated at approximately 5000 Oe, can beand regarded magnetization to be instarts the to range decrease. of 285–5000 Thus, the Oe. effe Itctive is reported magnetic that field the for bulk magnetostrictive Terfenol-D shows actuation 63 Oe of canand be magnetization regarded to startsbe in tothe decrease. range of Thus, 285–5000 the effe Oe.ctive It magneticis reported field that for the magnetostrictive bulk Terfenol-D actuation shows coercivecan magneticbe regarded field to [be27 ],in 2000 the range Oe of saturationof 285–5000 magnetic Oe. It is reported field and that 1 T ofthe saturation bulk Terfenol-D magnetization shows [1]. 63 Oe of coercive magnetic field [27], 2000 Oe of saturation magnetic field and 1 T of saturation Compared63 Oe of with coercive bulk value,magnetic the field saturation [27], 2000 magnetization Oe of saturation of themagnetic electrodeposited field and 1 Tfilm of saturation is 40% lower magnetization [1]. Compared with bulk value, the saturation magnetization of the electrodeposited than thatmagnetization of the bulk [1]. value. Compared There with are bulk several value, suspected the saturation reasons magnetization for this degradation. of the electrodeposited One is that the film composition ratio is slightly different from the ideal value of Terfenol-D. The different composition ratio shows different magnetization characteristics [1]. Another reason is the magneto crystalline Micromachines 2020, 11, x 4 of 14

film is 40% lower than that of the bulk value. There are several suspected reasons for this degradation. Micromachines 2020, 11, 523 4 of 13 One is that the film composition ratio is slightly different from the ideal value of Terfenol-D. The different composition ratio shows different magnetization characteristics [1]. Another reason is the anisotropymagneto crystalline effect [35]. anisotropy The magneto effect crystalline [35]. The anisotropymagneto crystalline plays important anisotropy roles plays in magneticimportant domainroles rotation,in magnetic magnetization domain and rotation, magnetostriction. magnetization An electrodeposited and magnetostriction. magnetostriction An electrodeposited GaFe film shows uncontrolledmagnetostriction lattice orientationGaFe film shows as reported uncontrolled [6]. These latti randomce orientation lattice structures as report aedffect [6]. low These magnetization random lattice structures affect low magnetization and magnetostriction characteristics [6,9]. The possible and magnetostriction characteristics [6,9]. The possible other reason for this is the oxide impurity of other reason for this is the oxide impurity of the film, which pins the magnetic domains [21], decreases the film, which pins the magnetic domains [21], decreases saturation magnetizations and increases the saturation magnetizations and increases the coercive magnetic field [21,22,26]. coercive magnetic field [21,22,26].

0.8

0.6

0.4

0.2

0.0

-0.2 Magnetization (T)

-0.4

-0.6

-0.8 -1.0 -0.5 0.0 0.5 1.0 Magnetic field (T)

Figure 3. In-plane magnetization measurement of the TbxDy(1 x)Fey ﬁlm using vibrating sample Figure 3. In-plane magnetization measurement of the TbxDy(1−−x)Fey film using vibrating sample magnetometermagnetometer (VSM). (VSM).

FiguresFigures4 and 4 and5 show 5 show the the atomic atomic force force microscopy microscopy and and magneticmagnetic force microscopy microscopy images images of of the the depositeddeposited film film with with a thicknessa thickness of of 300 300 nm. nm. In In the the atomic atomic forceforce microscopymicroscopy image,image, the Tb 0.34DyDy0.650.65FeFe1.9 1.9 filmfilm has has a finea fine grain grain microstructure microstructure with a a diameter diameter of of ~170 ~170 nm, nm, which which is larger is larger than than the grain the grain size of size ofreported reported sputtered sputtered films films ~50–55 ~50–55 nm nm[19,20]. [19, 20Generally]. Generally,, the grain the grainsize of sizeelectrodeposited of electrodeposited films is much films is muchbigger bigger than than that thatof the of as-deposited the as-deposited sputtered sputtered film. From film. Fromthe magnetic the magnetic force microscopy force microscopy images, images, the theTb Tb0.340.34DyDy0.65Fe0.651.9Fe film1.9 showsfilm shows a large a magnetic large magnetic domain domainand grain and boundary. grain boundary. This is one Thisof the is evidences one of the evidencesthat electrodeposited that electrodeposited Tb0.34Dy0.65 TbFe0.341.9 filmDy0.65 possessesFe1.9 film a polycrystalline possesses a polycrystalline structure. Sputtered structure. Terfenol-D Sputtered Terfenol-Dfilms without films annealing without annealingshows amorphous shows amorphous state crystallinity. state crystallinity. The amorphous The Terfenol-D amorphous film Terfenol-D show filmmaze show shape maze magnetic shape magneticdomain image domain from image magnetic from force magnetic microscopy. force microscopy. Polycrystalline Polycrystalline state Terfenol- state Terfenol-D,D,Micromachines however, however, 2020 shows, 11, xmagneticshows magnetic domain domainstructure structure sillier to crystal sillier tograin crystal structure grain [21]. structure [21]. 5 of 14

FigureFigure 4. 4.Atomic Atomic forceforce microscopymicroscopy image of of the the deposited deposited film. ﬁlm.

Figure 5. Magnetic force microscopy the deposited film.

The magnetostriction coefficient is generally defined by generated strain under the application of a magnetic field. However, in the case of thin films, it is difficult to measure the strain of the films directly. Thus, an analytical model using the displacements of bi-material cantilevers is employed to evaluate the magnetostriction coefficients [9,36,37], in which the effective magnetostriction coefficient

value can be calculated from the displacements of the bi-material cantilever structures with an application of magnetic fields in parallel and perpendicular against the longitudinal direction of the cantilever, as given by [37],

2(∥ −) (1 + ) = (1) 9 (1 + )

where ∥ is the displacement in parallel to the magnetic field, is the displacement in perpendicular to the magnetic field, and are Young’s modulus and Poisson ratio of the

Micromachines 2020, 11, x 5 of 14

Micromachines 2020, 11, 523 Figure 4. Atomic force microscopy image of the deposited film. 5 of 13

Figure 5. Magnetic force microscopy the deposited film. Figure 5. Magnetic force microscopy the deposited film. The magnetostriction coefficient is generally defined by generated strain under the application of a magneticThe magnetostriction field. However, coefficient in the case is ofgenerally thin films, defined it is di byffi generatedcult to measure strain theunder strain the ofapplication the films directly.of a magnetic Thus, field. an analytical However, model in the using case theof thin displacements films, it is difficult of bi-material to measure cantilevers the strain is employed of the films to evaluatedirectly. theThus, magnetostriction an analytical model coeffi cientsusing [the9,36 displa,37], incements which theof bi-material effective magnetostriction cantilevers is employed coefficient to valueevaluateλe f the f can magnetostriction be calculated from coeffi thecients displacements [9,36,37], in of which the bi-material the effective cantilever magnetostriction structures coefficient with an applicationvalue of can magnetic be calculated fields from in parallel the displacements and perpendicular of the againstbi-material the longitudinalcantilever structures direction with of the an cantilever,application as of given magnetic by [37 fields], in parallel and perpendicular against the longitudinal direction of the 2 cantilever, as given by [37], 2 D D Ests 1 + v f k − ⊥ λe f f = (1) 9l2E t (1 + v ) f f s 2(∥ −) (1 + ) where D is the displacement in parallel = to the magnetic field, D is the displacement in perpendicular(1) k 9 (1 + ⊥) to the magnetic field, Es and vs are Young’s modulus and Poisson ratio of the substrate material, respectively, E f and v f are the Young’s modulus and Poisson ratio of the magnetostrictive material, where is the displacement in parallel to the magnetic field, is the displacement in t f and ts are∥ the film thicknesses of the magnetostrictive layer and the substrate, respectively, and l is perpendicular to the magnetic field, and are Young’s modulus and Poisson ratio of the the length of the cantilever. The spring constant k cantilever of the cantilever structure, and the force F generated by the magnetostrictive film can be approximated as given by following equations [38],

!2 !3 t f t f E f t f E f ts kcantilever = 4 + 6 + 4 + + , (2) ∗ ts ∗ ts Es ∗ ts Es ∗ t f

F = kcantileverD (3) where D is the displacement of the cantilever. The elastic properties of thin-films and microstructures are generally almost same with that of the bulk [39]; thus, for this calculation, the Young’s modulus of each layer is supposed to be the bulk value of silicon and Terfenol-D, i.e., 179 109 Pa and 50 109 Pa, × × respectively, and the Poisson ratios of the silicon substrate and the Tb0.34Dy0.65Fe1.9 film are supposed to be 0.22 and 0.3, respectively [28,35]. Table1 shows the typical dimensions of the fabricated cantilever. From Table1 and Equation (2), the effective spring constant of the composite cantilever is calculated to be 26.4 N/m. Micromachines 2020, 11, x 6 of 14

substrate material, respectively, and are the Young’s modulus and Poisson ratio of the magnetostrictive material, and are the film thicknesses of the magnetostrictive layer and the substrate, respectively, and is the length of the cantilever. The spring constant kcantilever of the cantilever structure, and the force F generated by the magnetostrictive film can be approximated as given by following equations [38],

=4+6∗ +4∗ + ∗ + ∗ , (2)

= (3)

where D is the displacement of the cantilever. The elastic properties of thin-films and microstructures are generally almost same with that of the bulk [39]; thus, for this calculation, the Young’s modulus of each layer is supposed to be the bulk value of silicon and Terfenol-D, i.e., 179 10 and 50 Micromachines 2020 , 11, 523 6 of 13 10 , respectively, and the Poisson ratios of the silicon substrate and the Tb0.34Dy0.65Fe1.9 film are supposed to be 0.22 and 0.3, respectively [28,35]. Table 1 shows the typical dimensions of the fabricated cantilever. From Table 1 and Equation (2), the effectiveTable spring 1. constantTypical of dimensions the composite of cantilever the fabricated is calculated cantilevered to be 26.4 N/m. structure.

TableCantilever 1. Typical Length dimensions of the fabricated500 µ mcantilevered to 1.1 mm; structure. 100 µm Step Cantilever width 100 µm Cantilever Length 500 µm to 1.1 mm; 100 µm Step SOI wafer dimension (Si/SiO2/Si) 1.5 µm/3 µm/550 µm Cu SeedCantilever layer thicknesswidth 100 300µm nm TbxDySOI(1 waferx)Fey dimensionthickness (Si/SiO2/Si) 1.5 µm/3 µm/550 250 nm µm − Cu Seed layer thickness 300 nm

TbxDy(1−x)Fey thickness 250 nm The fabrication process of the bi-material cantilever is shown in Figure6. The Tb 0.34Dy0.65Fe1.9 (Terfenol-D) filmThe is depositedfabrication process on the of SOI the bi-material wafer with cantilever a Cu seedis shown layer in Figure by electrodeposition 6. The Tb0.34Dy0.65Fe1.9 and patterned (Terfenol-D) film is deposited on the SOI wafer with a Cu seed layer by electrodeposition and by ion beam milling with a photoresist mask. To prevent the oxidation of the Tb Dy Fe film, patterned by ion beam milling with a photoresist mask. To prevent the oxidation of the Tb0.36Dy0.360.64Fe1.90.64 1.9 a 25 nm-thickfilm, Si3 Na 254 thinnm-thick film Si is3N deposited4 thin film is ondeposited the magnetostrictive on the magnetostrictive film film by by sputtering. sputtering. After etching the handling Si layeretching from the handling the backside, Si layer from the the cantilever backside, th structurese cantilever structures are released are released by etching by etching the the buried oxide buried oxide using vapor HF etching. The SEM images of the fabricated bi-material microcantilever using vaporstructures HF etching. are shown The in SEM Figure images 7. Owing of to the the fabricatedstress of the films, bi-material the cantilevers microcantilever are slightly bent structures are shown in Figureupward.7. Owing to the stress of the films, the cantilevers are slightly bent upward.

MicromachinesFigure 2020 6., Fabrication11, x process of the magnetostrictive bi-material cantilevers.7 of 14 Figure 6. Fabrication process of the magnetostrictive bi-material cantilevers.

(a)

(b)

Figure 7. SEM images of fabricated Tb0.34Dy0.65Fe1.9 bi-material cantilevers. (a) Low magnification of Figure 7. SEM images of fabricated Tb Dy Fe bi-material cantilevers. (a) Low magniﬁcation of bi-material cantilevers; (b) High magnification0.34 0.65 of bi-material1.9 cantilevers. bi-material cantilevers; (b) High magniﬁcation of bi-material cantilevers. 3. Result and Discussion Using an electromagnet, a magnetic field of 0–11 kOe is applied to the fabricated cantilevers along the parallel direction of the cantilever. The actuation is observed by a microscope, as the typical result is shown in Figure 8, where the magnetic field was applied along horizontal direction in parallel to the cantilever length direction. The magnetostrictive film on the Si cantilever has 91 MPa tensile stress as observed from the initial bending. Theoretically, Terfenol-D is known as positive magnetostriction material. The Terfenol-D material will extend toward the magnetic field direction. When a magnetic field is applied to the cantilever, the cantilever will be bent downward because of the magnetostriction effect.

Micromachines 2020, 11, 523 7 of 13

3. Result and Discussion Using an electromagnet, a magnetic field of 0–11 kOe is applied to the fabricated cantilevers along the parallel direction of the cantilever. The actuation is observed by a microscope, as the typical result is shown in Figure8, where the magnetic field was applied along horizontal direction in parallel to the cantilever length direction. The magnetostrictive film on the Si cantilever has 91 MPa tensile stress as observed from the initial bending. Theoretically, Terfenol-D is known as positive magnetostriction material. The Terfenol-D material will extend toward the magnetic field direction. When a magnetic field is applied to the cantilever, the cantilever will be bent downward because of the magnetostrictionMicromachinesMicromachines 20202020 e,, ff 1111ect.,, xx 8 of8 of 14 14

Figure 8. Optical images of side view for the typical magnetostrictive actuation of the Si- FigureFigure 8. Optical 8. Optical images images of side of view side for view the typical for the magnetostrictive typical magnetostrictive actuation ofactuation the Si- Tb of0.34 theDy 0.65Si-Fe 1.9 Tb0.34Dy0.65Fe1.9 bi-material cantilever for the cases without magnetic field and with a magnetic field of bi-materialTb0.34Dy cantilever0.65Fe1.9 bi-material for the casescantilever without for the magnetic cases without field and magnetic with afield magnetic and with field a magnetic of 11 kOe field along of the 11 kOe along the cantilever direction. cantilever11 kOe direction.along the cantilever direction. With an optical microscope, this displacement could be observed, as shown in Figure 8. An WithWith an optical an optical microscope, microscope, this displacementthis displacement could could be observed, be observed, as shown as shown in Figure in 8Figure. An application 8. An application of the magnetic field actuates the cantilever downward with displacement ∥. Figure 9 application of the magnetic field actuates the cantilever downward with displacement . Figure 9 of theshows magnetic the observed field actuates displacements the cantilever as a function downward of applied with displacement magnetic fieldD for. Figure three 9cantilevers shows∥ the with observed k displacementsshowsdifferent the lengths, observed as a function and displacements Figure of applied10 shows as magnetica thefunction magnet fieldof ostrictionapplied for three magnetic coefficients cantilevers field calculated for with three different using cantilevers Equation lengths, with and Figuredifferent(1). 10 Figure shows lengths, 11 theshows magnetostrictionand the Figure generated 10 shows forces coefficients the of eachmagnet cantilever calculatedostriction calculated usingcoefficients Equation from calculated the (1).cantilever Figure using deflection 11Equation shows the generated(1).under Figure forcesvarious 11 ofshows eachmagnetic the cantilever generated fields calculatedusing forces Equation of from each the (3).cantilever cantileverThe maximum calculated deflection force from undercan the becantilever various estimated magnetic deflection to be fields under various magnetic fields using Equation (3). The maximum force can be estimated to be usingapproximately Equation (3). 65 The mN. maximum force can be estimated to be approximately 65 mN. approximately 65 mN.

1100 μm 1000 1100 μ μmm 700 1000 μm μm 250 700 μm 250

200

m) 200 μ m) μ 150 150

100

Displacement ( Displacement 100 Displacement ( Displacement 50 50

0 0 2000 4000 6000 8000 10000 12000 0 0 2000 4000Magneticfield 6000 (Oe) 8000 10000 12000 Magneticfield (Oe) Figure 9. Observed displacements of the cantilevers with diﬀerent lengths (700, 1000, 1100 µm). Figure 9. Observed displacements of the cantilevers with different lengths (700, 1000, 1100 µm). Figure 9. Observed displacements of the cantilevers with different lengths (700, 1000, 1100 µm).

Micromachines 2020, 11, x 9 of 14

Micromachines 2020, 11, 523 8 of 13 Micromachines 2020, 11, x 9 of 14 1100 μm 1000 μm 1400 μ 1100 700 μmm 1000 μm 1400 1200 700 μm

1200 1000

1000 800

800 600

600 400

400 200 Magnetostriction Coefficient(ppm) Magnetostriction

200 Magnetostriction Coefficient(ppm) Magnetostriction 0 0 2000 4000 6000 8000 10000 12000 0 Magnetic field (Oe) 0 2000 4000 6000 8000 10000 12000 Magnetic field (Oe)

Figure 10. MagnetostrictionMagnetostriction coecoefficientsﬃcients ofof thethe TbTb0.360.36Dy0.64FeFe1.91.9 filmﬁlm obtained obtained from from three three cantilevers with lengths 700, 1000, 1100 µµm.m. Figure 10. Magnetostriction coefficients of the Tb0.36Dy0.64Fe1.9 film obtained from three cantilevers with lengths 700, 1000, 1100 µm. 1100 μm μ 1100 1000 μ mm 700 μm 0.006 1000 μm 700 μm 0.006 0.005

0.005 0.004

0.004 0.003

0.003 (N) Force

Force (N) Force 0.002

0.002 0.001

0.001 0.000 0 2000 4000 6000 8000 10000 12000 0.000 Magnetic field (Oe) 0 2000 4000 6000 8000 10000 12000 Magnetic field (Oe) Figure 11. Generated forces of TbTb0.360.36DyDy0.640.64FeFe1.91.9 obtainedobtained from from three three cantile cantileversvers with with lengths lengths 700, 700, 1000, 1000, 1100 µµm.m. Figure 11. Generated forces of Tb0.36Dy0.64Fe1.9 obtained from three cantilevers with lengths 700, 1000, 1100The µm. magnetostriction coefficient λ can be calculated using Equation (1), as shown in The magnetostriction coefficient e f f can be calculated using Equation (1), as shown in Figure Figure 10, where D is supposed to be negligible. The actuation is saturated at approximately 10, where D⊥is supposed⊥ to be negligible. The actuation is saturated at approximately 5000 Oe. This 5000The Oe. magnetostriction This actuation coefficient characteristic seems can be to calculated be reasonable using with Equation the VSM (1), as result. shown At in 11000 Figure Oe, actuation characteristic seems to be reasonable with the VSM result. At 11000 Oe, the Tb0.36Dy0.64Fe1.9 10,the where Tb0.36 DDy⊥is0.64 supposedFe1.9 film to shows be negligible. a magnetostrictive The actuation coe isffi saturatedcient of approximately at approximately 1250 5000 ppm Oe. in This strain, actuation characteristic seems to be reasonable with the VSM result. At 11000 Oe, the Tb0.36Dy0.64Fe1.9

Micromachines 2020, 11, 523 9 of 13 which is comparable to 1400 ppm of the magnetostriction coefficient of the bulk Terfenol-D [1,2,7]. This is the highest value among reported magnetostriction coefficients of the TbxDy(1 x)Fey films [19–28]. − Also, from the displacement data and deflection data, the energy density of the thin film actuator can be calculated from the stored elastic energy Wel in the cantilever [37]. The stored elastic energy is given by 2 Es 1 2 Wel = z Lldz (4) 1 vs ∗ R − where R is the radius of the curvature of the cantilever, L is the length of the cantilever, l is the width of the cantilever and Es, vs are the Young’s modulus and Poisson ratio of the cantilever, respectively. Specific dimensions and parameters used are shown in Table1. The radius R of the curvature R and deflection D of the cantilever is given by

h h 1 β f 1 vs f 1 vs R− = σint − = 6σint − (5) −β2 β + 1 h2 Es − h2 Es − 3 s s

h f 1 v D = 3σ s L2 (6) int 2 − − hs Es where σint is the initial stress of the cantilever, and β is the constant of the neutral plane of the cantilever. The Young’s modulus of silicon is approximately three times larger than that of Terfenol-D, also the thickness of the Tb0.34Dy0.65Fe1.9 film is very thin in comparison with the Si layer; thus β = 1/2 is the proper assumption in this model. As a consequence, the stored elastic energy in the cantilever is given by βhs Z 2 2 Es 1 2 Es 1 2 3 2 1 Wel = z Lldz = z Llhs β β + (7) 1 vs R 1 vs R − 3 (β 1)hs − − − The energy density Edensity of the magnetostrictive film is obtained by dividing the stored elastic energy by the volume Vf of the magnetostrictive film, as given by

Edensity = Wel/Vf (8)

In the actual calculation, the maximum energy density is calculated from the actuated cantilever deﬂection using Equation (8). The calculated and reported energy densities of the ﬁlm and bulk [1,6] are summarized in Table2. The variation of the calculated energy density seems to be large for three cantilevers, it may come from the cantilever dimension errors (possibly 50 µm) caused by alignment ± error and side etching in microfabrication.

Table 2. Energy density of bulk Terfenol-D and electrochemical deposited Tb0.36Dy0.64Fe1.9.

Materials Power Density Bulk Terfenol-D 5000 to 25,000 J/m3 3 Tb0.36Dy0.64Fe1.9 (700 µm-long cantilever at 11 kOe) 129,000 J/m 3 Tb0.36Dy0.64Fe1.9 (1000 µm-long cantilever at 11 kOe) 169,000 J/m 3 Tb0.36Dy0.64Fe1.9 (1100 µm-long cantilever at 11 kOe) 100,000 J/m

Figure 12 shows the comparison of the energy density of the Tb0.36Dy0.64Fe1.9 ﬁlm with another actuator. It is found that this magnetostrictive ﬁlm can produce very high energy density for actuation. It is found that the energy density can be higher than that of piezoelectric material (PZT) that is widely used for microelectromechcanical devices. Micromachines 2020, 11, x 11 of 14 Micromachines 2020, 11, 523 10 of 13

107

106 This work ) 3

105

104

103 Energy Density (J/m Density Energy

102

101

c c c e e on i l nge si ctr bbl SMA Fe1.9 e i an 4 Musc 6 e cha xp . zoel crobu Ni-T E y0 ie has ElectrostatiD P Mi p d Electromagneti Thermopneumatic iqui Thermal Tb0.36 id l ol S

Figure 12. Comparison of energy densities for Tb0.36Dy0.64Fe1.9 and another types of actuators [40].

Figure 12. Comparison of energy densities for Tb0.36Dy0.64Fe1.9 and another types of actuators [40]. In Table3, the magnetostriction coe fficient of this Tb0.36Dy0.64Fe1.9 film is compared with that of bulk materials and reported magnetostrictive thin films. The electrodeposited Galfenol and CoFe filmsIn show Table a 3, relatively the magnetostriction low performance coefficient in comparison of this Tb with0.36Dy the0.64Fe sputtered1.9 film is films.compared It is with considered that of thatbulk thismaterials low performance and reported possibly magnetostrictive comes from thin random films. lattice The electrodeposited orientation [6,9]. Galfenol The sputtered and CoFe and annealedfilms show Terfenol-D a relatively and low CoFe performance show better inthan comparison non-annealed with the film sputtered [11,19,20 films.]. This It better is considered performance that comesthis low from performance improved possibly crystallinity comes and from grain random size. It lattice can be orientation concluded that[6,9]. the The electrodeposited sputtered and annealed Terfenol-D and CoFe show better than non-annealed film [11,19,20]. This better Tb0.36Dy0.64Fe1.9 film shows excellent magnetostriction performance than that of other types of magnetostrictiveperformance comes thin filmsfrom andimproved has high crystallinity potential ability and forgrain the applicationsize. It can to be microelectromechanical concluded that the systemselectrodeposited (MEMS) Tb including0.36Dy0.64Fe magnetic1.9 film shows actuators, excellent energy magnetostriction harvesters, and performance microsensors. than that of other types of magnetostrictive thin films and has high potential ability for the application to microelectromechanicalTable3. Comparison of systems the magnetostrictive (MEMS) coefficientsincluding among magnetic bulk values, actuators, sputtered, energy and electrochemical harvesters, and microsensors.deposited films.

Table 3. ComparisonMaterials of the magnetostrictive coeffi Magnetostrictioncients among bulk Coe ffivalues,cient (ppm)sputtered, and Refs electrochemicalBulk deposited Terfenol-D films. 1400 [1,2,7] Electrodeposited Tb0.36Dy0.64Fe1.9 at 11 kOe 1250 This work Magnetostriction Coefficient Sputtered Terfenol-DMaterials at 6 kOe 450Refs [24] Sputtered Terfenol-D at 10 kOe(ppm) 540 [23] Sputtered Terfenol-DBulk annealed Terfenol-D 400 ◦C at 740 emu/cc 1400910 [1,2,7] [20] Sputtered Terfenol-D annealed 450 ◦C at 700 emu/cc 880This [19] Electrodeposited Tb0.36Dy0.64Fe1.9 at 11 kOe 1250 Electrodeposited Galfenol at 628 Oe 96work [6] SputteredBulk Terfenol-D Galfenol at 6 kOe 320~400 450 [24][5,6 ] SputteredSputtered Co0.66 Terfenol-DFe0.34 annealed at 10 kOe 800 ◦C 260 540 [23] [11] Electrodeposited Co0.65Fe0.35 1.5 [9] Sputtered Terfenol-D annealed 400 °C at 740 emu/cc 910 [20] Bulk TbFe2 2630 [1,2] Sputtered Terfenol-D annealed 450 °C at 700 emu/cc 880 [19] Electrodeposited Galfenol at 628 Oe 96 [6] 4. Conclusions Bulk Galfenol 320~400 [5,6] Sputtered Co0.66Fe0.34 annealed 800 °C 260 [11] This paper reported the performance of a Tb0.36Dy0.64Fe1.9 film deposited by electrodeposition at Electrodeposited Co0.65Fe0.35 1.5 [9] 40 ◦C. The deposited Tb0.36Dy0.64Fe1.9 film shows the coercive magnetic field 285 Oe and the saturation Bulk TbFe2 2630 [1,2] magnetic field 5000 Oe. From AFM and MFM analysis, the film has ~170 nm grain size. At 11 kOe

Micromachines 2020, 11, 523 11 of 13

magnetic field, the Tb0.36Dy0.64Fe1.9 film shows approximately 1250 ppm of magnetostriction coefficient. Moreover, the energy density of the film is calculated to be 100,000~165,000 J/m3. These performances are almost the same to those of bulk Terfenol-D. As a consequence, the electrodeposited Tb0.36Dy0.64Fe1.9 film has a high potential ability for magnetic actuator, energy harvesting, and sensor applications.

Author Contributions: Conceptualization, T.O. and H.S.; support to film deposition, K.S.; validation, formal analysis, investigation, and data curation, H.S.; writing—original draft preparation, H.S.; writing—review and editing, T.O.; supervision, N.I., M.T., N.V.T., and T.O.; funding acquisition, T.O. All authors have read and agreed to the published version of the manuscript. Funding: A part of this work was funded by The New Energy and Industrial Technology Development Organization, NEDO. Acknowledgments: This work is partly done at Micro/nanomachining education center, Tohoku University, and Micro System Integration Center (µSiC), Tohoku University. This work was partly supported by Cabinet Office, Government of Japan, Cross-ministerial Strategic Innovation Promotion Program (SIP). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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