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Magnetostrictive Properties Together with Resistivity and Corrosion Behaviors of CoFe2 and Its Composite with CoFe2N
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Magnetic/Magnetostrictive Properties Together with Resistivity and Corrosion Behaviors of CoFe2 and Its Composite with CoFe2N B.Q. Geng 1, Y.Q. Ma 1,*, M. Wang 1, Z.L. Ding 1, W.H. Song 2, B.C. Zhao 2 1 2
Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230601, China Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
A R T I C L E
I N F O
Article history: Received 27 February 2016 Received in revised form 23 April 2016 Accepted 18 May 2016 Available online Key words: CoFe alloy CoFe2N alloy Magnetostriction Resistivity Corrosion resistance
The CoFe2 alloy (CF) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere to produce the composite of CoFe2N and CoFe2. The magnetostriction, thermal expansion, resistivity and corrosion resistance of CF sample and the nitrided sample (CFN) at 1000 °C were investigated. The saturation magnetostriction coefficiency λs and thermal expansion coefficient α at 300 K for the nitrided CFN were 50 ppm and 10 ppm/K, respectively, approximately equal to those for the CF sample. However, compared with CF, CFN presents a decrease in temperature coefficient Rλ (300 K) of magnetostriction by ~11%. The smaller resistivity and improved corrosion resistance in the H2SO4 solution may expand the applications of the CoFe2 in the fields needing lower resistivity or in the acidic environment. Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
1. Introduction Binary cobalt–iron (Co–Fe) alloys have found many applications in magnetic recording (read/write heads) and ultra large scale integration devices. Moreover, the Co–Fe alloy has also been incorporated into microelectromechanical devices such as sensors, microactuators, microgears, and micromotors[1]. These broad applications of Co–Fe alloys are based on their highest saturation magnetization of all known magnetic alloys in the Slater–Pauling curve[2,3] as well as their high Curie temperature, low magnetic loss, low coercive force, and high permeability[4–6]. Besides, the Co–Fe alloy with face centered cubic (fcc) crystal structure is particularly important because of its interesting properties such as the invar anomaly, which exhibits unusual thermal properties typified by an anomalously low thermal expansion coefficient near room temperature, and the behavior known as reentrant spin glass[7–9]. Co–Fe alloys with molecular formula Co1-xFex also exhibit the magnetostrictive properties[10]. Their isotropic magnetostriction coefficient λs depends on the relative content Co and Fe. From pure Co to 4 at.% Fe λs was negative and of the order of 10−6; λs increased to 10−5 up to 25 at.% Fe and achieved 10−4 from 30 at.% Fe to 90 at.% Fe; λs decreased to 10−5 for pure Fe. The Co–Fe alloy with
* Corresponding author. Fax: +86 551 63861820. E-mail address: [email protected] (Y.Q. Ma).
positive magnetostriction is an important component in laminated magnetoelectric composites, which were made up of negative magnetostrictive/piezoelectric/positive magnetostrictive plates such as Ni/Pb(Zr,Ti)O3/FeCo[11], for the purpose of obtaining the large magnetoelectric voltage coefficients in order to meet the requirements for practical applications. However, the metals or alloys are prone to be oxided or corroded[12–14]. Recent work revealed that the nanocrystalline Co and CoFe showed active behavior in acidic condition[1]. In order to further expand their capabilities and future applications, their corrosion resistance is of great importance because corrosion not only changes the properties of Co–Fe alloys but also decreases their service life. Nitriding is a widely used thermochemical surface treatment for components made of iron-based alloys. Nitriding can greatly improve mechanical properties such as wear and the fatigue resistance, as well as chemical properties such as corrosion resistance[15,16]. By choosing different nitriding conditions, such as temperature or/ and gas types and components (N2, NH3, N2/N2 + Ar or NH3/H2), a nitrogen-diffusion zone[15] or metallic nitrides can be obtained. In the case of metallic nitrides, numerous efforts were focused on the Fe[17–21] and Fe–Me (Me = Al, Ti, V, Cr or Si) alloys. However the nitriding of Co–Fe alloys has seldom been considered as far as the authors know. The Co–Fe alloys were prepared by many methods[22–24]. In the case of the Co–Fe alloys synthesized by electrodeposition from a sulfate bath, the sulfide presented at grain boundaries so that Co–Fe
http://dx.doi.org/10.1016/j.jmst.2016.07.006 1005-0302/Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
Please cite this article in press as: B.Q. Geng, et al., Magnetic/Magnetostrictive Properties Together with Resistivity and Corrosion Behaviors of CoFe2 and Its Composite with CoFe2N, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.07.006
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had a lower corrosion resistance[1]. In this work, the CoFe2 alloy (denoted by CF below) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere at different temperatures to form the CoFe2N and CoFe2 composites. Finally the magnetostrictive, thermal expansion, resistivity and corrosion resistance of composites nitrided at 1000 °C (denoted by CFN below) were investigated in detail. 2. Experimental CoFe 2 O 4 powders were prepared by a conventional sol-gel method. Iron nitrate (Fe(NO3)3·9H2O), cobalt nitrate (Co(NO3)2·6H2O), citric acid, ethylene glycol, aqueous ammonia and distilled water (H2O) were used as the starting materials. All of them were of analytical grade and used without any further purification. Co(NO3)2·6H2O and Fe(NO3)3·9H2O with the mole ratio of Co2+ and Fe3+ being 1:2 were dissolved in H2O. Then, citric acid and ethylene glycol were added to the solution with continuously stirring at 60 °C. The pH value was adjusted by aqueous ammonia to 7. Finally the solution was dried in oven at 80 °C followed by calcinating at 800 °C for 4 h to obtain the CoFe2O4 (denoted by CFO hereafter) powders. The CoFe2 alloy was prepared by reducing the CFO powder in a flow of H2 and N2 gas mixture (4% H2 + 96 %N2, 500 sccm) at 800 °C for 4 h, and the produced sample was referred to as CF below. The CF alloy powders were pressed into pellets with the diameter of ca. 12 mm, and then treated in a pure NH3 ambient at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C. The sample treated at 1000 °C was denoted by CFN hereafter. The phase composition was determined by X-ray diffraction (XRD) using an X-ray diffractometer (XRD; DX-2000 SSC, Dandong Fangyuan Instrument Company, DanDong, Liaoning, China) with CuKα irradiation (λ = 0.15406 nm) in the scanning range 20°–80° with a step of 0.02°. High-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100, Tokyo, Japan) was used to observe lattice fringes, and obtain selected-area electron diffraction (SAED) patterns. Scanning electron microscopy (SEM, S-4800, Hitachi, Japan) was used to observe the surface morphologies. The chemical composition was determined by X-ray photoelectron spectroscopy (XPS; Escalab 250Xi, Thermo, USA). The magnetic properties, resistivity, thermal expansion and magnetostriction were carried out using a superconducting quantum interference device PPMS system (SQUID, PPMS EC-II, Quantum Design Inc., San Diego, CA, USA). The strain was measured using a strain gauge (BA120-05AA-150(11), Zemic, China).
Fig. 1. XRD patterns of the samples CFO (a), CF (c) and the JCPDS cards for CoFe2O4 (No. 22–1086) (b) and CoFe2 (No. 65–4131) (d).
and Fe3N (No. 76–0091). As shown in Fig. 2b, the sample annealed at 400 °C retains the structure of CoFe2 alloy. For the sample annealed at 600 °C (Fig. 2c), a small diffraction peak appears at around 2θ = 43.2° besides the peaks from CoFe2 alloy. With the annealing temperature increasing to 900 °C, the diffraction intensity at 2θ = 43.2° gradually increases, accompanying with the appearance of the another weak peak at 2θ = 41.2° (as shown in Fig. 2f). The diffraction peaks at 2θ = 41.2° and 43.2°, close to those previously observed[18–20], can be indexed to the Fe3N phase according to the JCPDS card in Fig. 2i, which has a hexagonal symmetry with the space group P 31m (162) and lattice parameters of a = 0.4668 nm and c = 0.4362 nm. Further annealing in NH3 at higher temperature above 900 °C weakens the Fe3N diffraction due to the loss of N element. The diffraction intensity ratio between reflections of Fe3N (111) and CF (110) crystal planes was shown in Fig. 3, illustrating that the fraction of Fe3N phase reaches the maximum for the sample annealed at 850 °C. So the overall phase composition of the samples treated from 600 to 1000 °C can be considered as a mixture of CoFe2 and CoFe2N phase. The possible reaction for the formation of CoFe2N can be suggested as follows: first NH3 decomposes to H2 and N2 according to the Temkin–Pyzhev reaction; then N atoms enter into the lattice to form CoFe2N due to the reaction of CoFe2 + 1/2N2 = CoFe2N[26].
3. Results and Discussion 3.1. Structural evolution from CoFe2O4 to CoFe2 and its composite with CoFe2N The main diffraction peaks of the sample CFO in Fig. 1a can be indexed to the CoFe2O4 cubic spinel structure, compared with the powder diffraction file (PDF) of CoFe2O4 (JCPDS No.22–1086) in Fig. 1b. After CoFe2O4 was reduced in a H2/N2 ambient, the obtained product, i.e. the CF sample just exhibits the reflections (Fig. 1c of CoFe2 alloy according to the JCPDS card No.65–4131 in Fig. 1d, due to the reducing reaction: CoFe2O4 + 4H2→CoFe2 + 4H2O[25]. The lattice parameter a of CoFe2O4 and CoFe2 can be calculated from sin2θ = λ2(H2 + K2 + L2)/4a2, in which θ is the diffraction angle, HKL is the crystal plane index, and λ is the wavelength of the CuKα irradiation. The calculated lattice parameter a is equal to 0.835 nm and 0.285 nm for CoFe2O4 and CoFe2, respectively. Fig. 2 shows the XRD patterns of the samples that the CoFe2 alloy was annealed in NH3 at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C as well as the JCPDS cards of CoFe2 (No. 65–4131)
Fig. 2. XRD patterns of the samples after the CoFe2 alloy was annealed in NH3 at temperatures 400 (b), 600 (c), 800 (d), 850 (e), 900 (f), 950 (g) and 1000 °C (h) together with the JCPDS cards of CoFe2 (No. 65–4131) (a) and Fe3N (No. 76–0091) (i).
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Fig. 3. Diffraction intensity ratio between reflections of Fe3N phase (111) and CF (110) crystal planes for samples annealed in NH3 at temperatures of 600, 800, 850, 900, 950 and 1000 °C.
Following the XRD analysis, the composition of the CFO, CF and CFN was further characterized by X-ray photo electron spectroscopy (XPS). The XPS spectra around Co 2p core level were shown in Fig. 4a. For the CFO sample, several peaks locate at 781.1, 787.1 and 796.1 eV, similar to those in previous reports for CoFe2O4
samples[27,28]. Among them, Co 2p doublet with the binding energies of 781.1 and 796.1 eV implies the Co—O bond formation[29]. In the case of the CF sample, the XPS spectrum only shows a broad peak at 182.1 eV, above which it almost keeps constant, indicating the absence of the Co—O bond. For the CFN sample, two peaks at 781.1 and 796.1 eV may indicate the formation of Co—N bond. Fig. 4b shows the XPS spectrum of the Fe 2p region. The CFO sample exhibits three peaks at 711.1, 724.1 and 732.1 eV, consistent with the CoFe2O4[27,28] and Fe3O4 in literature[30]. For the CF sample, the peaks locate at 711.1, 725.1 and 733.1 eV with the latter two peaks being one electron volt higher than those in CFO. The XPS spectrum of the sample CFN is similar to that of CFO except a slight difference around 732.1 eV. The XPS spectrum in the N 1s region was plotted in Fig. 4c. It shows that the CFO and CF samples have constant spectra while the CFN sample peaks at the photon energy of 397 eV. The peak in the CFN sample confirmed the presence of N element[17,18]. As shown in Fig. 5a, the HRTEM image shows clear fringes with distance of 0.30 nm that correspond to the (220) crystalline planes of CFO. The SAED image in the inset exhibits distinct diffraction dots from the (400) and (440) crystal planes of CFO. After the CFO was reduced, the obtained CF sample shows clear (100) fringes and the (100) diffraction dot of CoFe2, as shown in Fig. 5b and its inset. In the case of the nitrided sample NFN, fringes with distances of 0.20
Fig. 4. XPS spectra of samples CFO, CF and CFN around Co 2p core level (a), Fe 2p core level (b) and N 1s core level (c).
Fig. 5. HRTEM image with the inset being the SAED patterns for the samples CFO (a) and CF (b); HRTEM image (c) and SAED pattern (d) for the CFN sample.
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Fig. 6. M(H) loops for the sample CF and the samples annealed in NH3 at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C.
and 0.28 nm correspond to the (110) and (100) crystalline planes of CoFe2, respectively (Fig. 5c). Meanwhile, the (110) fringes with a distance of 0.23 nm in CoFe2N phase were also observed, consistent with the XRD result. Correspondingly, the SAED in Fig. 5d also exhibits the diffraction circles from CoFe2 and CoFe2N phases. 3.2. Magnetic and magnetostrictive properties of samples 3.2.1. Magnetic properties Fig. 6 shows the dependence of magnetization (M) on the applied magnetic field (H), i.e. M(H) loops for the sample CF and the samples annealed in NH3 at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C, and the saturation magnetization (Ms) was also given in the figure for each sample. The CF sample has the large Ms value of 217 emu/g because the CoFe2 alloy is a strongly magnetic material[4–6]. Annealing in NH3 from 400 to 800 °C gradually reduces the Ms value, as a consequence of the increase in the Fe3N phase fraction supported by the results in Fig. 3. The samples annealed at 850, 900 and 950 °C possess the low Ms values of 163−169 emu/g while the Ms value of sample annealed at 1000 °C increases to 192 emu/g. The variation of Ms value in the composites can be assigned to the following aspects: (1) The relative content between CoFe2 and Fe3N phase changes the Ms value. (2) The magnetization of Fe3N phase is very sensitive to the number of the nearest N atoms around Fe atoms because the N atom has a significant influence on the exchange splitting of the Fe atoms. Due to the hybridization of Fe-3d and N-2p states, the magnetic moment of Fe atoms decreases with an increase in the number of nearest neighbor nitrogen atoms even though the samples retain very similar crystalline structures[19].
Fig. 7. FC (solid circles) and ZFC (empty circles) curves for the samples CFN (a), CF (b) and CFO (c).
Fig. 7 shows the zero field cooling (ZFC, empty circles) and field cooling (FC, solid circles) magnetization (M) curves recorded at 100 Oe from 10 K up to 390 K of the CFN, CF and CFO samples. With decreasing temperature from 390 K, the FC and ZFC curves exhibit the following features: (1) For the CFO sample, the FC and ZFC curves monotonically decrease untill to 32 K and then increase. (2) For the CF sample, the FC curve almost keeps constant as observed before[31,32] and the ZFC curve decreases rapidly as the temperature drops across 132 K as a result of the rapid freezing of particle moments. (3) For the CFN sample, the FC curve first shows a local minimum at 326 K, as observed before[33], then it increases with decreasing temperature, which resembles the behavior in non-interacting systems[34], and finally keeps constant below ~44 K[35,36]. The ZFC curve shows a peak at 56 K, i.e. the blocking temperature TB is 56 K, below which the particles anisotropy energy is overcome by the energy of thermal fluctuations and the particles get into the blocked state. The possible reasons for these features are suggested as follows: (1) The decreasing and constant FC behaviors result from the strong dipolar interaction among the magnetic particles, which leads to the freezing or blocking of the particle moments. (2) The increasing behavior in the FC curve for the CFN sample is similar to the superparamagnetic behavior observed in ZnFe2O4 nanoparticles[37]. However the FC and ZFC curves do not overlap above TB, indicating that there might be weak interparticle dipolar interaction in the sample. The local minimum at 326 K may result from the synergetic effects of anisotropy, dipolar interaction and the energy of thermal fluctuations. 3.2.2. Magnetostrictive properties and thermal expansion behaviors Fig. 8 shows the plots of magnetostriction (λ) as a function of applied magnetic field (H), measured at temperatures of 10, 50, 200,
Fig. 8. Magnetostriction as a function of magnetic field at different measuring temperatures of 10, 50, 200, 300 and 360 K for CF (a) and CFN (b).
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Fig. 9. Saturation magnetostriction λs for CF and CFN as a function of temperature with the solid lines being the fitting curves.
300 and 360 K for CF and CFN samples. The magnetostriction of both samples approaches saturation at H = 10 kOe. The saturation magnetostriction coefficiency λs has the maximum values of 67 and 61 ppm at low temperatures, and monotonically decreases to 54 and 50 ppm at 300 K, 48 and 45 ppm at 360 K for the samples CF and CFN, respectively, which are comparable to those reported before[10]. Our samples do not contain any resource-critical rareearth elements[38]. They exhibit larger λs value at 300 K than 34 ppm for Fe81Ga19[39] and are much lower in cost because Ga is very expensive[40]. The λs versus temperature can be fitted by
⎡ ⎛ T ⎞α ⎤ λ (T ) ≈ λ0 ⎢1 − ⎜ ⎟ ⎥ ⎣ ⎝ Tc ⎠ ⎦
2β
(1)
where α and β are the fitting parameters, λ0 is the saturation magnetostriction in the limit that the temperature approaches zero and the Curie temperature (Tc) was set as a constant[39]. The experimental (solid circles) and fitting (solid lines) curves are shown in Fig. 9. The values of α, β and Tc are 1.96, 1.73 and 1214 K for CF and 2.11, 1.46 and 1114 K for CFN. So the temperature coefficients Rλ(T) of magnetostriction at room temperature (300 K) can then be calculated by
Rλ (T ) ≈ −2αβ
⎛T⎞ ⎜⎝ ⎟⎠ Tc
α
⎡ ⎛ T ⎞α ⎤ T ⎢1 − ⎜ ⎟ ⎥ ⎣ ⎝ Tc ⎠ ⎦
(2)
The calculated Rλ(T) are −1.55 × 10−3 and −1.38 × 10−3 K−1 for CF and CFN, respectively. It can be noted that the CFN sample presents a decrease in temperature coefficient Rλ(300 K) of magnetostriction by ~11%. Fig. 10 shows the linear thermal expansion curves ΔL/L(360 K) for CF and CFN. They almost overlap in the high temperature region while the thermal expansion of CFN is slightly smaller than CF in the low temperature range. The derivative of ΔL/L(360 K) versus temperature, i.e. the thermal expansion coefficient α was shown in the inset of Fig. 10. Both samples exhibit the closed α values in the whole temperature range, and the α value is about 10 ppm/K at 300 K, close to that for skutterudites BaPt4Ge12 and PrFe4As12[41].
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Fig. 10. Linear thermal expansion ΔL/L for samples CF and CFN. Inset shows the thermal expansion coefficient α.
dependent on temperature. The resistivity variation with temperature can provide an effective method to identify the scattering mechanisms: including electron–phonon scattering (T dependence), electron–electron or electron–magnon scattering (T 2 dependence), and two-magnon scattering (T9/2 dependence). Considering the different scattering mechanism in the low and high temperature region[42], the resistivity in the temperature range of 10–100 K was fitted by 9
ρ = ρ0 + AT 2 + BT 2
(3)
The obtained fitting parameters are ρ 0 = 5.22 μΩ cm, A = 4.0 × 10−5 μΩ cm K−2 and B = 1.42 × 10−10 μΩ cm K−4.5 for CFN; ρ0 = 12.58 μΩ cm, A = 7.0 × 10−5 μΩ cm K−2 and B = 8.84 × 10−10 μΩ cm K−4.5 for CF. The low temperature resistivity behaviors of CF and CFN resemble those of half-metallic Fe3−xCoxSi (x = 0.5, 0.75, 0.9 and 1) alloys. However, the fitting parameters ρ0, A and B for CF and CFN are much smaller than those for Fe3-xCoxSi. Therefore, the nature of resistivity deserves further theoretical and experimental investigations. In the temperature region from 100 to 390 K, the resistivity was fitted by
ρ = ρ0 + CT n
(4)
The obtained fitting parameters are ρ 0 = 5.35 μΩ cm, C = 1.3 × 10−4 μΩ cm K-n and n = 1.81 for CFN; ρ0 = 12.80 μΩ cm, C = 5.5 × 10−4 μΩ cm K-n and n = 1.70 for CF. The fitting and experimental resistivity curves are shown in Fig. 12 in the low and high temperature region. The obtained n value indicates that the electron is more close to the T2 scattering in CFN than in CF, implying that the electron–phonon (lattice vibration) scattering is weaker in
3.3. Resistivity of CF and CFN samples Fig. 11 shows the temperature dependence of resistivity for CF and CFN samples. The resistivity of both samples monotonically increases with temperature, and a slow variation is seen in the low temperature region, indicating that the resistivity is weakly
Fig. 11. The temperature dependence of resistivity for samples CF and CFN.
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Fig. 12. The fitting and experimental resistivity curves in the low (a) and high (b) temperature regions for samples CF and CFN.
CFN than in CF because of the improved grain boundaries by CoFe2N produced among CoFe2 particles, as discussed below. It is noticed that the nitrided sample CFN has the smaller resistivity than the CF sample. The resistivity of CF ranges from 12.3 to 28.0 μΩ cm, being slightly smaller than 20–40 μΩ cm for the electrodeposited nanocrystalline CoFe thin films[43] as a result of the stronger electron scattering at grain boundaries for the nanocrystalline CoFe thin film. The resistivity of CFN is 5.1– 11.8 μΩ cm. The possible reason for the resistivity reduction of CFN can be suggested as follows. When the CoFe2 alloy was treated in NH3, CoFe2N was first formed at the grain boundary of CoFe2. CoFe2N acts as a bridge to link the adjacent CoFe2 grains, opening an electron passage and consequently leading to the resistivity reduction of the nitrided sample. This situation is somewhat similar to that reported recently[44], i.e. NbC grows as a second-phase on the grain boundary of Fe–Ga alloy.
presents a decrease in temperature coefficient Rλ (300 K) of magnetostriction by ~11%. Furthermore the CF and CFN samples have the closed thermal expansion coefficient α, ca. 10 ppm/K at 300 K. Interestingly the nitrided sample CFN has the smaller resistivity than the CF sample. The possible reason is that CoFe2N formed at the grain boundary of CoFe2 acts as a bridge to link the adjacent CoFe2 grains and opens an extra electron passage, consequently leading to the resistivity reduction.
3.4. Corrosion in a H2SO4 solution To test the corrosion resistance, the samples CF and CFN were respectively immersed in a solution of 0.1 mol/L H2SO4. The CF sample was completely dissolved in 20 h because the Co–Fe alloys showed active behavior in acidic condition[1]. For the nitrided sample CFN, compared with the SEM micrograph of the CFN before corrosion in Fig. 13a, the surface appears some small holes 24 h later, as shown in Fig. 13b. The SEM micrograph with higher magnification is shown in Fig. 13c. Clearly, the sample surface consists of coherent sheet-like and sponge-like matters. Such matters may be composed of CoFe2N phase because the CoFe2 alloys is more easily dissolved in the H2SO4 solution. The SEM result in Fig. 13c also supports the above suggestion that the CoFe2N phase was first formed at the grain boundary of CoFe2 and the dissolution of CoFe2 leaves the CoFe2N framework. 4. Conclusions The CoFe2 alloy (CF) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C, and the CoFe2N phase was crystallized above 600 °C which coexists with CoFe2 to produce composites. The relative content in CF and CoFe2N composites was changed with the treatment temperature, which affects the magnetic properties. The CF sample and the nitrided sample (CFN) at 1000 °C were investigated in detail, including their magnetostrictive, thermal expansion, resistivity and corrosion resistance. The saturation magnetostriction coefficiency λs has the maximum values of 67 and 61 ppm at 10 K, and monotonically decreases to 48 and 45 ppm at 360 K for the samples CF and CFN, respectively. By fitting λs versus temperature, it can be noted that the CFN sample
Fig. 13. SEM images of CFN before corrosion in H2SO4 (a), and low (b) and high (c) magnification for corroded CFN in H2SO4 for 24 h.
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The corrosion resistance of samples CF and CFN was tested by immersing samples in the 0.1 mol/L H2SO4 solution. The CoFe2 alloy has completely dissolved in 20 h while the surface of CFN sample only shows some small holes because the dissolution of CoFe2 left the CoFe2N framework. The nitrided CoFe2 may have the expanded applications due to its lower temperature coefficient Rλ(300 K) of magnetostriction, lower resistivity and improved corrosion resistance. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51471001, 11174004 and 11304001) and Key Project of the Foundation of Anhui Educational Committee (No. KJ2013A035). References [1] N.M. NikRozlin, A.M. Alfantazi, J. Appl. Electrochem. 43 (2013) 721–734. [2] X. Sun, Y.Q. Ma, Y.F. Xu, S.T. Xu, B.Q. Geng, Z.X. Dai, G.H. Zheng, J. Alloys Compd. 645 (2015) 51–56. [3] T. Sourmail, Prog. Mater. Sci. 50 (2005) 816–880. [4] F.L. Zan, Y.Q. Ma, Q. Ma, G.H. Zheng, Z.X. Dai, M.Z. Wu, G. Li, Z.Q. Sun, X.S. Chen, J. Alloys Compd. 553 (2013) 79–85. [5] F.L. Zan, Y.Q. Ma, Q. Ma, Y.F. Xu, Z.X. Dai, G.H. Zheng, M.Z. Wu, G. Li, J. Am. Ceram. Soc. 96 (10) (2013) 3100–3107. [6] F.L. Zan, Y.Q. Ma, Q. Ma, Y.F. Xu, Z.X. Dai, G.H. Zheng, J. Alloys Compd. 581 (2013) 263–269. [7] F. Ortiz-Chi, A. Aguayo, R.D. Coss, J. Phys. Condens. Matter 25 (2013) 026001, 9pp. [8] S. Khmelevskyi, A.V. Ruban, Y. Kakehashi, P. Mohn, B. Johansson, Phys. Rev. B 72 (2005) 64510–64516. [9] S. Khmelevskyi, P. Mohn, Phys. Rev. B 71 (2005) 144423–144427. [10] V. Madurga, C. Favieres, J. Vergara, J. Non Cryst. Solids 353 (2007) 941–943. [11] J.H. Cheng, Y.G. Wang, D. Xie, Mater. Lett. 143 (2015) 273–275. [12] S. Peng, C. Wang, J. Xie, S.H. Sun, J. Am. Chem. Soc. 128 (2006) 10676–10677. [13] I. Tabakovic, S. Riemer, K. Tabakovic, M. Sun, M. Kief, J. Electrochem. Soc. 153 (8) (2006) C586–C593. [14] L. Liu, Y. Li, F.H. Wang, J. Mater. Sci. Technol. 26 (1) (2010) 1–4. [15] B. Schwarz, S.R. Meka, R.E. Schacherl, E. Bischoff, E.J. Mittemeijer, Acta Mater. 76 (2014) 394–403. [16] J.C. Díaz-Guillén, G. Vargas-Gutiérrez, E.E. Granda-Gutiérrez, J.S. Zamarripa-Piña, S.I. Pérez-Aguilar, J. Candelas-Ramírez, L. Álvarez-Contreras, J. Mater. Sci. Technol. 29 (3) (2013) 287–290.
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Magnetic/Magnetostrictive Properties Together with Resistivity and Corrosion Behaviors of CoFe2 and Its Composite with CoFe2N B.Q. Geng 1, Y.Q. Ma 1,*, M. Wang 1, Z.L. Ding 1, W.H. Song 2, B.C. Zhao 2 1 2
Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230601, China Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
A R T I C L E
I N F O
Article history: Received 27 February 2016 Received in revised form 23 April 2016 Accepted 18 May 2016 Available online Key words: CoFe alloy CoFe2N alloy Magnetostriction Resistivity Corrosion resistance
The CoFe2 alloy (CF) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere to produce the composite of CoFe2N and CoFe2. The magnetostriction, thermal expansion, resistivity and corrosion resistance of CF sample and the nitrided sample (CFN) at 1000 °C were investigated. The saturation magnetostriction coefficiency λs and thermal expansion coefficient α at 300 K for the nitrided CFN were 50 ppm and 10 ppm/K, respectively, approximately equal to those for the CF sample. However, compared with CF, CFN presents a decrease in temperature coefficient Rλ (300 K) of magnetostriction by ~11%. The smaller resistivity and improved corrosion resistance in the H2SO4 solution may expand the applications of the CoFe2 in the fields needing lower resistivity or in the acidic environment. Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
1. Introduction Binary cobalt–iron (Co–Fe) alloys have found many applications in magnetic recording (read/write heads) and ultra large scale integration devices. Moreover, the Co–Fe alloy has also been incorporated into microelectromechanical devices such as sensors, microactuators, microgears, and micromotors[1]. These broad applications of Co–Fe alloys are based on their highest saturation magnetization of all known magnetic alloys in the Slater–Pauling curve[2,3] as well as their high Curie temperature, low magnetic loss, low coercive force, and high permeability[4–6]. Besides, the Co–Fe alloy with face centered cubic (fcc) crystal structure is particularly important because of its interesting properties such as the invar anomaly, which exhibits unusual thermal properties typified by an anomalously low thermal expansion coefficient near room temperature, and the behavior known as reentrant spin glass[7–9]. Co–Fe alloys with molecular formula Co1-xFex also exhibit the magnetostrictive properties[10]. Their isotropic magnetostriction coefficient λs depends on the relative content Co and Fe. From pure Co to 4 at.% Fe λs was negative and of the order of 10−6; λs increased to 10−5 up to 25 at.% Fe and achieved 10−4 from 30 at.% Fe to 90 at.% Fe; λs decreased to 10−5 for pure Fe. The Co–Fe alloy with
* Corresponding author. Fax: +86 551 63861820. E-mail address: [email protected] (Y.Q. Ma).
positive magnetostriction is an important component in laminated magnetoelectric composites, which were made up of negative magnetostrictive/piezoelectric/positive magnetostrictive plates such as Ni/Pb(Zr,Ti)O3/FeCo[11], for the purpose of obtaining the large magnetoelectric voltage coefficients in order to meet the requirements for practical applications. However, the metals or alloys are prone to be oxided or corroded[12–14]. Recent work revealed that the nanocrystalline Co and CoFe showed active behavior in acidic condition[1]. In order to further expand their capabilities and future applications, their corrosion resistance is of great importance because corrosion not only changes the properties of Co–Fe alloys but also decreases their service life. Nitriding is a widely used thermochemical surface treatment for components made of iron-based alloys. Nitriding can greatly improve mechanical properties such as wear and the fatigue resistance, as well as chemical properties such as corrosion resistance[15,16]. By choosing different nitriding conditions, such as temperature or/ and gas types and components (N2, NH3, N2/N2 + Ar or NH3/H2), a nitrogen-diffusion zone[15] or metallic nitrides can be obtained. In the case of metallic nitrides, numerous efforts were focused on the Fe[17–21] and Fe–Me (Me = Al, Ti, V, Cr or Si) alloys. However the nitriding of Co–Fe alloys has seldom been considered as far as the authors know. The Co–Fe alloys were prepared by many methods[22–24]. In the case of the Co–Fe alloys synthesized by electrodeposition from a sulfate bath, the sulfide presented at grain boundaries so that Co–Fe
http://dx.doi.org/10.1016/j.jmst.2016.07.006 1005-0302/Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
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had a lower corrosion resistance[1]. In this work, the CoFe2 alloy (denoted by CF below) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere at different temperatures to form the CoFe2N and CoFe2 composites. Finally the magnetostrictive, thermal expansion, resistivity and corrosion resistance of composites nitrided at 1000 °C (denoted by CFN below) were investigated in detail. 2. Experimental CoFe 2 O 4 powders were prepared by a conventional sol-gel method. Iron nitrate (Fe(NO3)3·9H2O), cobalt nitrate (Co(NO3)2·6H2O), citric acid, ethylene glycol, aqueous ammonia and distilled water (H2O) were used as the starting materials. All of them were of analytical grade and used without any further purification. Co(NO3)2·6H2O and Fe(NO3)3·9H2O with the mole ratio of Co2+ and Fe3+ being 1:2 were dissolved in H2O. Then, citric acid and ethylene glycol were added to the solution with continuously stirring at 60 °C. The pH value was adjusted by aqueous ammonia to 7. Finally the solution was dried in oven at 80 °C followed by calcinating at 800 °C for 4 h to obtain the CoFe2O4 (denoted by CFO hereafter) powders. The CoFe2 alloy was prepared by reducing the CFO powder in a flow of H2 and N2 gas mixture (4% H2 + 96 %N2, 500 sccm) at 800 °C for 4 h, and the produced sample was referred to as CF below. The CF alloy powders were pressed into pellets with the diameter of ca. 12 mm, and then treated in a pure NH3 ambient at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C. The sample treated at 1000 °C was denoted by CFN hereafter. The phase composition was determined by X-ray diffraction (XRD) using an X-ray diffractometer (XRD; DX-2000 SSC, Dandong Fangyuan Instrument Company, DanDong, Liaoning, China) with CuKα irradiation (λ = 0.15406 nm) in the scanning range 20°–80° with a step of 0.02°. High-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100, Tokyo, Japan) was used to observe lattice fringes, and obtain selected-area electron diffraction (SAED) patterns. Scanning electron microscopy (SEM, S-4800, Hitachi, Japan) was used to observe the surface morphologies. The chemical composition was determined by X-ray photoelectron spectroscopy (XPS; Escalab 250Xi, Thermo, USA). The magnetic properties, resistivity, thermal expansion and magnetostriction were carried out using a superconducting quantum interference device PPMS system (SQUID, PPMS EC-II, Quantum Design Inc., San Diego, CA, USA). The strain was measured using a strain gauge (BA120-05AA-150(11), Zemic, China).
Fig. 1. XRD patterns of the samples CFO (a), CF (c) and the JCPDS cards for CoFe2O4 (No. 22–1086) (b) and CoFe2 (No. 65–4131) (d).
and Fe3N (No. 76–0091). As shown in Fig. 2b, the sample annealed at 400 °C retains the structure of CoFe2 alloy. For the sample annealed at 600 °C (Fig. 2c), a small diffraction peak appears at around 2θ = 43.2° besides the peaks from CoFe2 alloy. With the annealing temperature increasing to 900 °C, the diffraction intensity at 2θ = 43.2° gradually increases, accompanying with the appearance of the another weak peak at 2θ = 41.2° (as shown in Fig. 2f). The diffraction peaks at 2θ = 41.2° and 43.2°, close to those previously observed[18–20], can be indexed to the Fe3N phase according to the JCPDS card in Fig. 2i, which has a hexagonal symmetry with the space group P 31m (162) and lattice parameters of a = 0.4668 nm and c = 0.4362 nm. Further annealing in NH3 at higher temperature above 900 °C weakens the Fe3N diffraction due to the loss of N element. The diffraction intensity ratio between reflections of Fe3N (111) and CF (110) crystal planes was shown in Fig. 3, illustrating that the fraction of Fe3N phase reaches the maximum for the sample annealed at 850 °C. So the overall phase composition of the samples treated from 600 to 1000 °C can be considered as a mixture of CoFe2 and CoFe2N phase. The possible reaction for the formation of CoFe2N can be suggested as follows: first NH3 decomposes to H2 and N2 according to the Temkin–Pyzhev reaction; then N atoms enter into the lattice to form CoFe2N due to the reaction of CoFe2 + 1/2N2 = CoFe2N[26].
3. Results and Discussion 3.1. Structural evolution from CoFe2O4 to CoFe2 and its composite with CoFe2N The main diffraction peaks of the sample CFO in Fig. 1a can be indexed to the CoFe2O4 cubic spinel structure, compared with the powder diffraction file (PDF) of CoFe2O4 (JCPDS No.22–1086) in Fig. 1b. After CoFe2O4 was reduced in a H2/N2 ambient, the obtained product, i.e. the CF sample just exhibits the reflections (Fig. 1c of CoFe2 alloy according to the JCPDS card No.65–4131 in Fig. 1d, due to the reducing reaction: CoFe2O4 + 4H2→CoFe2 + 4H2O[25]. The lattice parameter a of CoFe2O4 and CoFe2 can be calculated from sin2θ = λ2(H2 + K2 + L2)/4a2, in which θ is the diffraction angle, HKL is the crystal plane index, and λ is the wavelength of the CuKα irradiation. The calculated lattice parameter a is equal to 0.835 nm and 0.285 nm for CoFe2O4 and CoFe2, respectively. Fig. 2 shows the XRD patterns of the samples that the CoFe2 alloy was annealed in NH3 at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C as well as the JCPDS cards of CoFe2 (No. 65–4131)
Fig. 2. XRD patterns of the samples after the CoFe2 alloy was annealed in NH3 at temperatures 400 (b), 600 (c), 800 (d), 850 (e), 900 (f), 950 (g) and 1000 °C (h) together with the JCPDS cards of CoFe2 (No. 65–4131) (a) and Fe3N (No. 76–0091) (i).
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Fig. 3. Diffraction intensity ratio between reflections of Fe3N phase (111) and CF (110) crystal planes for samples annealed in NH3 at temperatures of 600, 800, 850, 900, 950 and 1000 °C.
Following the XRD analysis, the composition of the CFO, CF and CFN was further characterized by X-ray photo electron spectroscopy (XPS). The XPS spectra around Co 2p core level were shown in Fig. 4a. For the CFO sample, several peaks locate at 781.1, 787.1 and 796.1 eV, similar to those in previous reports for CoFe2O4
samples[27,28]. Among them, Co 2p doublet with the binding energies of 781.1 and 796.1 eV implies the Co—O bond formation[29]. In the case of the CF sample, the XPS spectrum only shows a broad peak at 182.1 eV, above which it almost keeps constant, indicating the absence of the Co—O bond. For the CFN sample, two peaks at 781.1 and 796.1 eV may indicate the formation of Co—N bond. Fig. 4b shows the XPS spectrum of the Fe 2p region. The CFO sample exhibits three peaks at 711.1, 724.1 and 732.1 eV, consistent with the CoFe2O4[27,28] and Fe3O4 in literature[30]. For the CF sample, the peaks locate at 711.1, 725.1 and 733.1 eV with the latter two peaks being one electron volt higher than those in CFO. The XPS spectrum of the sample CFN is similar to that of CFO except a slight difference around 732.1 eV. The XPS spectrum in the N 1s region was plotted in Fig. 4c. It shows that the CFO and CF samples have constant spectra while the CFN sample peaks at the photon energy of 397 eV. The peak in the CFN sample confirmed the presence of N element[17,18]. As shown in Fig. 5a, the HRTEM image shows clear fringes with distance of 0.30 nm that correspond to the (220) crystalline planes of CFO. The SAED image in the inset exhibits distinct diffraction dots from the (400) and (440) crystal planes of CFO. After the CFO was reduced, the obtained CF sample shows clear (100) fringes and the (100) diffraction dot of CoFe2, as shown in Fig. 5b and its inset. In the case of the nitrided sample NFN, fringes with distances of 0.20
Fig. 4. XPS spectra of samples CFO, CF and CFN around Co 2p core level (a), Fe 2p core level (b) and N 1s core level (c).
Fig. 5. HRTEM image with the inset being the SAED patterns for the samples CFO (a) and CF (b); HRTEM image (c) and SAED pattern (d) for the CFN sample.
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Fig. 6. M(H) loops for the sample CF and the samples annealed in NH3 at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C.
and 0.28 nm correspond to the (110) and (100) crystalline planes of CoFe2, respectively (Fig. 5c). Meanwhile, the (110) fringes with a distance of 0.23 nm in CoFe2N phase were also observed, consistent with the XRD result. Correspondingly, the SAED in Fig. 5d also exhibits the diffraction circles from CoFe2 and CoFe2N phases. 3.2. Magnetic and magnetostrictive properties of samples 3.2.1. Magnetic properties Fig. 6 shows the dependence of magnetization (M) on the applied magnetic field (H), i.e. M(H) loops for the sample CF and the samples annealed in NH3 at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C, and the saturation magnetization (Ms) was also given in the figure for each sample. The CF sample has the large Ms value of 217 emu/g because the CoFe2 alloy is a strongly magnetic material[4–6]. Annealing in NH3 from 400 to 800 °C gradually reduces the Ms value, as a consequence of the increase in the Fe3N phase fraction supported by the results in Fig. 3. The samples annealed at 850, 900 and 950 °C possess the low Ms values of 163−169 emu/g while the Ms value of sample annealed at 1000 °C increases to 192 emu/g. The variation of Ms value in the composites can be assigned to the following aspects: (1) The relative content between CoFe2 and Fe3N phase changes the Ms value. (2) The magnetization of Fe3N phase is very sensitive to the number of the nearest N atoms around Fe atoms because the N atom has a significant influence on the exchange splitting of the Fe atoms. Due to the hybridization of Fe-3d and N-2p states, the magnetic moment of Fe atoms decreases with an increase in the number of nearest neighbor nitrogen atoms even though the samples retain very similar crystalline structures[19].
Fig. 7. FC (solid circles) and ZFC (empty circles) curves for the samples CFN (a), CF (b) and CFO (c).
Fig. 7 shows the zero field cooling (ZFC, empty circles) and field cooling (FC, solid circles) magnetization (M) curves recorded at 100 Oe from 10 K up to 390 K of the CFN, CF and CFO samples. With decreasing temperature from 390 K, the FC and ZFC curves exhibit the following features: (1) For the CFO sample, the FC and ZFC curves monotonically decrease untill to 32 K and then increase. (2) For the CF sample, the FC curve almost keeps constant as observed before[31,32] and the ZFC curve decreases rapidly as the temperature drops across 132 K as a result of the rapid freezing of particle moments. (3) For the CFN sample, the FC curve first shows a local minimum at 326 K, as observed before[33], then it increases with decreasing temperature, which resembles the behavior in non-interacting systems[34], and finally keeps constant below ~44 K[35,36]. The ZFC curve shows a peak at 56 K, i.e. the blocking temperature TB is 56 K, below which the particles anisotropy energy is overcome by the energy of thermal fluctuations and the particles get into the blocked state. The possible reasons for these features are suggested as follows: (1) The decreasing and constant FC behaviors result from the strong dipolar interaction among the magnetic particles, which leads to the freezing or blocking of the particle moments. (2) The increasing behavior in the FC curve for the CFN sample is similar to the superparamagnetic behavior observed in ZnFe2O4 nanoparticles[37]. However the FC and ZFC curves do not overlap above TB, indicating that there might be weak interparticle dipolar interaction in the sample. The local minimum at 326 K may result from the synergetic effects of anisotropy, dipolar interaction and the energy of thermal fluctuations. 3.2.2. Magnetostrictive properties and thermal expansion behaviors Fig. 8 shows the plots of magnetostriction (λ) as a function of applied magnetic field (H), measured at temperatures of 10, 50, 200,
Fig. 8. Magnetostriction as a function of magnetic field at different measuring temperatures of 10, 50, 200, 300 and 360 K for CF (a) and CFN (b).
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Fig. 9. Saturation magnetostriction λs for CF and CFN as a function of temperature with the solid lines being the fitting curves.
300 and 360 K for CF and CFN samples. The magnetostriction of both samples approaches saturation at H = 10 kOe. The saturation magnetostriction coefficiency λs has the maximum values of 67 and 61 ppm at low temperatures, and monotonically decreases to 54 and 50 ppm at 300 K, 48 and 45 ppm at 360 K for the samples CF and CFN, respectively, which are comparable to those reported before[10]. Our samples do not contain any resource-critical rareearth elements[38]. They exhibit larger λs value at 300 K than 34 ppm for Fe81Ga19[39] and are much lower in cost because Ga is very expensive[40]. The λs versus temperature can be fitted by
⎡ ⎛ T ⎞α ⎤ λ (T ) ≈ λ0 ⎢1 − ⎜ ⎟ ⎥ ⎣ ⎝ Tc ⎠ ⎦
2β
(1)
where α and β are the fitting parameters, λ0 is the saturation magnetostriction in the limit that the temperature approaches zero and the Curie temperature (Tc) was set as a constant[39]. The experimental (solid circles) and fitting (solid lines) curves are shown in Fig. 9. The values of α, β and Tc are 1.96, 1.73 and 1214 K for CF and 2.11, 1.46 and 1114 K for CFN. So the temperature coefficients Rλ(T) of magnetostriction at room temperature (300 K) can then be calculated by
Rλ (T ) ≈ −2αβ
⎛T⎞ ⎜⎝ ⎟⎠ Tc
α
⎡ ⎛ T ⎞α ⎤ T ⎢1 − ⎜ ⎟ ⎥ ⎣ ⎝ Tc ⎠ ⎦
(2)
The calculated Rλ(T) are −1.55 × 10−3 and −1.38 × 10−3 K−1 for CF and CFN, respectively. It can be noted that the CFN sample presents a decrease in temperature coefficient Rλ(300 K) of magnetostriction by ~11%. Fig. 10 shows the linear thermal expansion curves ΔL/L(360 K) for CF and CFN. They almost overlap in the high temperature region while the thermal expansion of CFN is slightly smaller than CF in the low temperature range. The derivative of ΔL/L(360 K) versus temperature, i.e. the thermal expansion coefficient α was shown in the inset of Fig. 10. Both samples exhibit the closed α values in the whole temperature range, and the α value is about 10 ppm/K at 300 K, close to that for skutterudites BaPt4Ge12 and PrFe4As12[41].
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Fig. 10. Linear thermal expansion ΔL/L for samples CF and CFN. Inset shows the thermal expansion coefficient α.
dependent on temperature. The resistivity variation with temperature can provide an effective method to identify the scattering mechanisms: including electron–phonon scattering (T dependence), electron–electron or electron–magnon scattering (T 2 dependence), and two-magnon scattering (T9/2 dependence). Considering the different scattering mechanism in the low and high temperature region[42], the resistivity in the temperature range of 10–100 K was fitted by 9
ρ = ρ0 + AT 2 + BT 2
(3)
The obtained fitting parameters are ρ 0 = 5.22 μΩ cm, A = 4.0 × 10−5 μΩ cm K−2 and B = 1.42 × 10−10 μΩ cm K−4.5 for CFN; ρ0 = 12.58 μΩ cm, A = 7.0 × 10−5 μΩ cm K−2 and B = 8.84 × 10−10 μΩ cm K−4.5 for CF. The low temperature resistivity behaviors of CF and CFN resemble those of half-metallic Fe3−xCoxSi (x = 0.5, 0.75, 0.9 and 1) alloys. However, the fitting parameters ρ0, A and B for CF and CFN are much smaller than those for Fe3-xCoxSi. Therefore, the nature of resistivity deserves further theoretical and experimental investigations. In the temperature region from 100 to 390 K, the resistivity was fitted by
ρ = ρ0 + CT n
(4)
The obtained fitting parameters are ρ 0 = 5.35 μΩ cm, C = 1.3 × 10−4 μΩ cm K-n and n = 1.81 for CFN; ρ0 = 12.80 μΩ cm, C = 5.5 × 10−4 μΩ cm K-n and n = 1.70 for CF. The fitting and experimental resistivity curves are shown in Fig. 12 in the low and high temperature region. The obtained n value indicates that the electron is more close to the T2 scattering in CFN than in CF, implying that the electron–phonon (lattice vibration) scattering is weaker in
3.3. Resistivity of CF and CFN samples Fig. 11 shows the temperature dependence of resistivity for CF and CFN samples. The resistivity of both samples monotonically increases with temperature, and a slow variation is seen in the low temperature region, indicating that the resistivity is weakly
Fig. 11. The temperature dependence of resistivity for samples CF and CFN.
Please cite this article in press as: B.Q. Geng, et al., Magnetic/Magnetostrictive Properties Together with Resistivity and Corrosion Behaviors of CoFe2 and Its Composite with CoFe2N, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.07.006
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Fig. 12. The fitting and experimental resistivity curves in the low (a) and high (b) temperature regions for samples CF and CFN.
CFN than in CF because of the improved grain boundaries by CoFe2N produced among CoFe2 particles, as discussed below. It is noticed that the nitrided sample CFN has the smaller resistivity than the CF sample. The resistivity of CF ranges from 12.3 to 28.0 μΩ cm, being slightly smaller than 20–40 μΩ cm for the electrodeposited nanocrystalline CoFe thin films[43] as a result of the stronger electron scattering at grain boundaries for the nanocrystalline CoFe thin film. The resistivity of CFN is 5.1– 11.8 μΩ cm. The possible reason for the resistivity reduction of CFN can be suggested as follows. When the CoFe2 alloy was treated in NH3, CoFe2N was first formed at the grain boundary of CoFe2. CoFe2N acts as a bridge to link the adjacent CoFe2 grains, opening an electron passage and consequently leading to the resistivity reduction of the nitrided sample. This situation is somewhat similar to that reported recently[44], i.e. NbC grows as a second-phase on the grain boundary of Fe–Ga alloy.
presents a decrease in temperature coefficient Rλ (300 K) of magnetostriction by ~11%. Furthermore the CF and CFN samples have the closed thermal expansion coefficient α, ca. 10 ppm/K at 300 K. Interestingly the nitrided sample CFN has the smaller resistivity than the CF sample. The possible reason is that CoFe2N formed at the grain boundary of CoFe2 acts as a bridge to link the adjacent CoFe2 grains and opens an extra electron passage, consequently leading to the resistivity reduction.
3.4. Corrosion in a H2SO4 solution To test the corrosion resistance, the samples CF and CFN were respectively immersed in a solution of 0.1 mol/L H2SO4. The CF sample was completely dissolved in 20 h because the Co–Fe alloys showed active behavior in acidic condition[1]. For the nitrided sample CFN, compared with the SEM micrograph of the CFN before corrosion in Fig. 13a, the surface appears some small holes 24 h later, as shown in Fig. 13b. The SEM micrograph with higher magnification is shown in Fig. 13c. Clearly, the sample surface consists of coherent sheet-like and sponge-like matters. Such matters may be composed of CoFe2N phase because the CoFe2 alloys is more easily dissolved in the H2SO4 solution. The SEM result in Fig. 13c also supports the above suggestion that the CoFe2N phase was first formed at the grain boundary of CoFe2 and the dissolution of CoFe2 leaves the CoFe2N framework. 4. Conclusions The CoFe2 alloy (CF) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere at temperatures of 400, 600, 800, 850, 900, 950 and 1000 °C, and the CoFe2N phase was crystallized above 600 °C which coexists with CoFe2 to produce composites. The relative content in CF and CoFe2N composites was changed with the treatment temperature, which affects the magnetic properties. The CF sample and the nitrided sample (CFN) at 1000 °C were investigated in detail, including their magnetostrictive, thermal expansion, resistivity and corrosion resistance. The saturation magnetostriction coefficiency λs has the maximum values of 67 and 61 ppm at 10 K, and monotonically decreases to 48 and 45 ppm at 360 K for the samples CF and CFN, respectively. By fitting λs versus temperature, it can be noted that the CFN sample
Fig. 13. SEM images of CFN before corrosion in H2SO4 (a), and low (b) and high (c) magnification for corroded CFN in H2SO4 for 24 h.
Please cite this article in press as: B.Q. Geng, et al., Magnetic/Magnetostrictive Properties Together with Resistivity and Corrosion Behaviors of CoFe2 and Its Composite with CoFe2N, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.07.006
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The corrosion resistance of samples CF and CFN was tested by immersing samples in the 0.1 mol/L H2SO4 solution. The CoFe2 alloy has completely dissolved in 20 h while the surface of CFN sample only shows some small holes because the dissolution of CoFe2 left the CoFe2N framework. The nitrided CoFe2 may have the expanded applications due to its lower temperature coefficient Rλ(300 K) of magnetostriction, lower resistivity and improved corrosion resistance. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51471001, 11174004 and 11304001) and Key Project of the Foundation of Anhui Educational Committee (No. KJ2013A035). References [1] N.M. NikRozlin, A.M. Alfantazi, J. Appl. Electrochem. 43 (2013) 721–734. [2] X. Sun, Y.Q. Ma, Y.F. Xu, S.T. Xu, B.Q. Geng, Z.X. Dai, G.H. Zheng, J. Alloys Compd. 645 (2015) 51–56. [3] T. Sourmail, Prog. Mater. Sci. 50 (2005) 816–880. [4] F.L. Zan, Y.Q. Ma, Q. Ma, G.H. Zheng, Z.X. Dai, M.Z. Wu, G. Li, Z.Q. Sun, X.S. Chen, J. Alloys Compd. 553 (2013) 79–85. [5] F.L. Zan, Y.Q. Ma, Q. Ma, Y.F. Xu, Z.X. Dai, G.H. Zheng, M.Z. Wu, G. Li, J. Am. Ceram. Soc. 96 (10) (2013) 3100–3107. [6] F.L. Zan, Y.Q. Ma, Q. Ma, Y.F. Xu, Z.X. Dai, G.H. Zheng, J. Alloys Compd. 581 (2013) 263–269. [7] F. Ortiz-Chi, A. Aguayo, R.D. Coss, J. Phys. Condens. Matter 25 (2013) 026001, 9pp. [8] S. Khmelevskyi, A.V. Ruban, Y. Kakehashi, P. Mohn, B. Johansson, Phys. Rev. B 72 (2005) 64510–64516. [9] S. Khmelevskyi, P. Mohn, Phys. Rev. B 71 (2005) 144423–144427. [10] V. Madurga, C. Favieres, J. Vergara, J. Non Cryst. Solids 353 (2007) 941–943. [11] J.H. Cheng, Y.G. Wang, D. Xie, Mater. Lett. 143 (2015) 273–275. [12] S. Peng, C. Wang, J. Xie, S.H. Sun, J. Am. Chem. Soc. 128 (2006) 10676–10677. [13] I. Tabakovic, S. Riemer, K. Tabakovic, M. Sun, M. Kief, J. Electrochem. Soc. 153 (8) (2006) C586–C593. [14] L. Liu, Y. Li, F.H. Wang, J. Mater. Sci. Technol. 26 (1) (2010) 1–4. [15] B. Schwarz, S.R. Meka, R.E. Schacherl, E. Bischoff, E.J. Mittemeijer, Acta Mater. 76 (2014) 394–403. [16] J.C. Díaz-Guillén, G. Vargas-Gutiérrez, E.E. Granda-Gutiérrez, J.S. Zamarripa-Piña, S.I. Pérez-Aguilar, J. Candelas-Ramírez, L. Álvarez-Contreras, J. Mater. Sci. Technol. 29 (3) (2013) 287–290.
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