Structural, magnetic, and magnetocaloric properties of intermetallic Fe2CoAl Heusler nanoalloy
11 Structural, magnetic, and magnetocaloric properties of intermetallic Fe CoAl Heusler nanoalloy Aquil Ahmad , Srimanta Mitra , S. K. Srivastava , and A. K. Das Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur, India-721302 Space Applications Center, ISRO, Ahmedabad, India-380015 E-mail: [email protected]
ABSTRACT
Spherical nanoparticles (NPs) made of intermetallic Fe CoAl (FCA) Heusler alloy are synthesized via co-precipitation method and its structural, magnetic and magnetocaloric properties are explored, for the first time. The basic structural characterizations have revealed A2-disordered cubic Heusler structure. HRTEM with the SAED pattern analysis revealed crystalline nature of the FCA-NPs with a mean diameter of around 14 nm. Field and temperature dependent magnetization (M) study shows that the NPs are soft ferromagnetic with a high saturation magnetization (M s ) and Curie temperature (T c ). We also observed that FCA-NPs do not follow the Slater Pauling (SP) rule possibly because of the disorder present in this system. We further investigate its phase transition and magnetocaloric properties. The peak value of -∆S M vs T curve at a magnetic field change of 20 kOe corresponds to about 2.65 J/Kg-K, and the observed value of refrigeration capacity (RCP) was as large as 44 J/Kg, suggesting a large heat conversion in magnetic refrigeration cycle. To analyze the magnetic phase transition (MPT), magnetization property is studied in detail. The Arrott plot and the nature of the universal curve accomplish that the ferromagnetic (FM) to paramagnetic (PM) phase transition in FCA-NPs is of second order. Present study suggests that the Fe CoAl nanoscale system is proficient/useful/a good candidate for the spintronics application and opens a window for further research on full-Heusler based magnetic refrigerants. .
1. Introduction
A significant enhancement over conventional electronics is possible with spintronics, where aside from charge of the electrons, its spin also plays an important role in order to transfer and storage of the information [1]. Numerous Heusler compounds showing a half-metallic character, low gilbert damping together with the high T c and magnetic moment have proven their potential for spintronics applications [2-5]. Their unusual half-metallic property arises due to their unique electronic band structures at the fermi energy (E F ) of which one-spin band behaves as a metal while the other behaves as a semiconductor and unveils an energy-gap at the E F [3, 5]. Hence, Heusler alloys (HAs) may increase the performance of spin-based devices relying upon magnetic tunnel junction and giant magnetoresistance or spin transfer torque [6]. Quite a few materials earlier have shown up to 100% spin polarization (SP), e.g., CrO [7], Fe O [8], EuS [9] and EuO [10], but most of them are neither appropriate for a high spin transport effect nor suitable with Si platforms because of poor electrical contacts [1, 2]. Subsequently, hunt for novel materials exhibiting high SP is a consistently growing research field. In recent years, HAs have attracted enormous interest to the researchers due to their unique and multifunctional properties such as half-metallicity [11-14], magnetocaloric effects [15-18], thermoelectric behavior [19], catalytic behavior [20-23], shape memory effect [24], spin injection [25] and spin filtering [26]. Such alloys have not only proven their ability for spintronic devices but also suitable for efficient magnetic refrigerants. Physical properties of HAs can easily be tuned preserving their high magnetic moments and Curie temperatures, by altering their elemental compositions and/or partial substitution of other elements [27-30]. Heusler compounds having general formula X YZ (X/Y: transition metals; Z: main group element) may be synthesized in either cubic L2 (i.e. fully ordered) or in partial disorder of B2 type or in fully disorder i.e., A2 phases [6]. A significant advancement can be seen, especially in HAs due to new experimental approaches [31]. They are extensively studied in the form of thin films and bulk materials [32-34], although, their study and production at the nanoscale still stay challenging [35]. Size and shape modulated nanostructures may preserve an ingenious role in multiple technological areas for example spintronics, topological insulators and skyrmionic structures [36]. As a growing research field, much of the attention was given to their synthesis, structural and magnetic properties; hence, it is vital to see their compatibility for energy and microactuators applications. Among all, Fe-based HAs have shown great interest due to their high M s and T c [13]. The structural and electronic properties of Fe XAl type HAs have been intensively investigated [37-42] but their studies are very limited in nano regime, especially in Fe CoAl. Recently, thermoelectric properties of Fe CoAl ribbons sample was explored revealing its suitability as good thermoelectrics [43], which showed a high M s of 135 emu/g and T c of around 1000 K along with a negative seebeck coefficient of -20 µV/K. Here, we explore the structural and magnetic properties of FCA-NPs, as described in section III. A. The powder x-ray diffraction (XRD) study suggests that Fe CoAl nanoparticles are crystallized in single phase with A2 disordered Heusler phase, and the observed lattice constant is to be 5.74 Å, which closely matches with its bulk value. Morphological and microstructural studies reveal that the particles are crystalline and agglomerated due to their highly magnetic nature. A relatively large M s and high T c are observed in the present system; moreover, the M s value is very stable even up to room temperature, which are desired for spintronics application. We further systematically explore its magnetic phase transition (MPT) and magnetocaloric properties across the T c , as described in section III. B. A large value of magnetic entropy change (-∆S M ) peak together with the high refrigeration capacity (RCP) and a broad working temperature range make FCA-NPs system a potential candidate not only for spintronics but also for multistage magnetic refrigeration technology.
2. Experimental details Fe CoAl Heusler alloy nanoparticles were synthesized using co-precipitation method as described in ref. [44] with slight changes. We have used Fe (NO ) ·9H O, CoCl ·6H O, and Al (NO )·18H O as a precursor salts, which were directly purchased from the Sigma-Aldrich company. In a typical preparation of the FCA-NPs, all the precursors with an appropriate molar ratio had dissolved in 50 ml CH OH and dried for 10 hrs. at 100 °C. Thereafter, the dried powder was placed inside a tubular furnace and heated up to around 850 °C for the 5 hrs., in presence of H environment. Crystalline phase was identified by high-resolution (HR) x-ray diffraction technique. Microstructural studies were performed using field-emission scanning electron microscope (MERLIN), and HR-transmission electron microscope (TEM).
The purity and composition of the elements were determined from the energy dispersive x-ray analysis (EDAX) analysis. To study magnetic and magnetocaloric properties of FCA-NPs, physical property measurement system (PPMS), and vibrating sample magnetometer (VSM) with working temperature range of 5-300 K and 300-1273 K respectively, were used.
3. Results and discussion 3.1. Microstructural, morphological and compositional analysis of Fe CoAl Heusler alloy nanoparticles
The experimental and simulated X-ray diffraction patterns of FCA-NPS is shown in figure 1. The absence of (111) and (200) reflections in experimental curve reveal that L2 phase was not Figure 1.
Experimental and simulated x-ray diffraction pattern of
FCA-NPs annealed at 850 °C for five hrs. observed in
FCA-NPs. The Bragg peaks (220), (400), (422) and (440), observed at = , , , and respectively, conclude that the sample was crystallized in A2- Figure 2. (a) FESEM image of the FCA-NPs annealed at 850 °C for five hours and (b) EDAX spectrum; the inset depicts elemental composition of Fe CoAl nanoparticles. disordered phase [45]. The calculated lattice constant from the main peak of the XRD spectrum was found to be a = . This closely matches with the theoretical value of the bulk Fe CoAl [13].
It can clearly be seen from the FESEM micrograph (figure 2(a)) of FCA-NPs that particles are densely agglomerated in the form of big clusters, indicating highly magnetic nature of the NPs. To check the purity and composition of the sample, we have done the EDAX analysis on quite a few NPs, as shown in figure 2 (b). This confirms that atomic percentage ratio of Fe:Co:Al is nearly 2:1:1 and pure in phase. The extra peaks are from the Si-substrate and gold coating, used at the time of sample preparation. For further insights of the particles, we have performed the high-resolution TEM imaging (figure 3(a)). From this figure, formation of the spherical NPs can be seen. The particles are analyzed by using Image J programme and their size distribution is shown in figure 3(b). From Gaussian fitting of the particles’ size distribution, the average size was found to be 14 ± 7 nm. Selected area electron diffraction (SAED) pattern encompasses concentric rings with dots specifying that particles are highly-crystalline but randomly oriented in the powder form. We have indexed initial four rings of SAED pattern, figure 3(c). These (hkl) values agree well with the x-ray diffraction results. Higher resolution image of the NPs, as shown in figure 3(d), approves its crystallinity, and the measured interplanar spacing was found to be around 2.07 Å corresponds to the (220) plane of FCA-NPs. On focusing at an edge of a single nanoparticle (figure 3(f), it can clearly be distinguished between crystalline fringes (characteristic of the particle) and liquid (amorphous) film upon which the FCA-NPs were dispersed. It further suggests that no capping layer is present in these particles. CoAl Heusler alloy nanoparticles
The field-dependent magnetization M (H) curves at different temperatures started from 5 to 300 K are shown in figure 4(a). The low-temperature (5 K) saturation magnetization (M s ) was found to be 127.6 emu/g and converted to 4.5 µ B /f.u. This is larger than the Slater Pauling (SP) value of 4 µ B /f.u. [46], and is slightly less than the bulk value as reported by V. Jain et al. [47]. The reason of deviation from the SP value might be due to the disorder present in this nanometric sample. As evident from the inset of M (H) curve, the finite value of the coercivity (H c ) and the remanence (M r ) is indicating ferromagnetic behavior of the nanoparticles. A comparison of all the values of M s , M r , and H c is tabulated in table 1. The temperature dependence of the saturation magnetization (M s ) curve, lower inset of figure 4(a), reveal that M s is stable even up to room temperature, which is good for spintronics application. Figure 3. (a) Transmission electron microscopy (TEM) image of the Fe CoAl Heusler alloy nanoparticles (b) the particle size distribution (c) SAED pattern with indexing of initial four rings (d, e) High-resolution image displaying the lattice planes and crystallinity and (f) its magnified view.
The temperature dependent magnetization, M (T) curve at 100 Oe applied field, under zero field-cooled condition (ZFC) is shown in figure 4(b). M increases with respect to the T and reaches maximum at 650 K. ZFC curve of FCA-NPs was smooth and show a clear FM to PM phase
Figure 4. (a-b) Temperature and field dependent magnetization curves of FCA-NPs; the upper and lower insets of figure 4(a) show the magnified version near the zero field region and the variation of saturation magnetization with temperature, respectively. Inset of figure 4(b) shows the dM/dT vs T curve. transition near the T c . We have also measured the M (T) curve under field cooled (FC) condition (not shown here) and found no significant thermal hysteresis in ZFC-FC curves of FCA-NPs, which suggests that the phase transition is of second-order [48]. The T c was calculated from the first derivative of M (T) versus T curve (see inset of figure 4(b)), and was found to be 830 K. C. Magnetocaloric properties of Fe CoAl Heusler alloy nanoparticles
To analyze the magnetocaloric (MC) properties of FCA-NPs, isothermal magnetization curves in field increasing (0 Oe-20 kOe) mode, were recorded at various temperatures from 795-851 K with a temperature interval of 4 K, across the T c , figure 5(a). The increasing nature of the magnetization up to 5 kOe subsequently becomes constant, then decreases with respect to the Temperature (K) Sat. magn. M s (emu/g) Remanence M r (emu/g) Coercivity H c (Oe) Curie temperature (K)
5 127.6 9.2 259 830 [this work] 990 [43] 50 126.8 8.2 229 100 125.9 7.3 200 200 123.2 5.9 171 300 118.6 5.0 150
Bulk Sat. magn. B /f.u. [47] Slater Pauling M s value B /f.u. [46] Table 1.
Temperature dependence of M s , M r , H c , and T c of FCA-NPs; other theoretical and experimental values are also tabulated for comparison. temperature increment indicating a magnetic transition from FM-PM phase. Moreover, a huge change of the magnetization in the temperature range of 827-831 K suggests a large MC effect of FCA-NPs. Here we did not find any significant magnetic hysteresis while field decreasing (20 kOe - 0 Oe) mode (not shown here). This reversible nature of magnetic phase transition is vital for magnetic refrigeration. Arrott plot or M versus H/M curve [49] was also constructed, to check if the Landau mean field theory for the MPT is suitable for FCA-NPs, as shown in figure 5(b). Figure 5. (a) M (H) curves of Fe CoAl Heusler alloy nanoparticles measured around the phase transition and (b) Arrott curve of the isotherms near the T c. It is widely accepted that conventional Arrott curves would exhibit parallel straight lines for the critical exponents of γ = 1 and β = 0.5 , and critical temperature (T c ) will be defined exactly at the line which passes through the origin. The downward curvature along with the nonlinear behavior even at high-field region was observed in FCA-NPs. Apparently, we conclude that mean field theory of phase transition is not valid in such system and a complex behavior of the critical exponents near T c is expected. Moreover, positive slope of Arrott plot following Banerjee’s criterion [50] indicates FM to PM phase transition is of second order. The change in magnetic entropy (-ΔS M ) was calculated using Maxwell equation [51]: ∆𝑆 𝑀 = S(T, H) − S(T, O) = ∫ (𝜕𝑀𝜕𝑇 ) 𝐻 dH (1) 𝐻0 The T dependence of magnetic entropy change (-ΔS M ) is presented in figure 6. A positive anomaly in -ΔS M vs T curve was observed at 830 K, which is much closer to its T c . The maxima in the change in magnetic entropy vs temperature curve at a magnetic field of 20 kOe corresponds to Figure 6. - ∆S M versus temperature curve for Fe CoAl Heusler alloy nanoparticles; inset shows the change of ( - ∆S M ) peak value with respect to the changing field ( ∆H ). about 2.65 J/Kg-K. It is clear from the inset of figure 6 that the peak value of the magnetic entropy change, (-ΔS M ) peak increases linearly with temperature, therefore, a large value of -ΔS M is expected for higher fields but we could not measure it due to our measuring system limitations. The usefulness of magnetic refrigerants can be evaluated by a parameter: relative cooling power (RCP), it measures the amount of heat transferred between hot and cold reservoirs which is defined as RCP = - (∆ S M ) peak x ∆ T FWHM , where ∆ T FWHM denotes the full-width at half-maxima of the change in magnetic entropy (-∆ S M ) vs temperature curve [52]. ∆T FWHM also represents the span of working-temperature in present system, which was found to be around 17 K; such broad working temperature range is highly desired for magnetic refrigeration. As Engelbrecht et al. [53] previously suggested that a material having a broad peak of -∆ S M is much better than that of materials with a sharp peak of magnetic entropy change for cooling applications and therefore, such materials with broad temperature distribution, are much attractive for MC application. FIG. 7.
Field dependence of the relative cooling power (RCP) across the phase transition (T c ) The observed value of RCP was as large as 44 J/Kg at the magnetic field change of 20 kOe, suggesting a large heat conversion in magnetic refrigeration cycle. As observed from the inset of figure. 6 and figure. 7, both the change in magnetic entropy and RCP values have shown a linear dependence with the applied magnetic field in the range of 0-20 kOe. This increasing nature of (-∆S M ) peak and RCP with ∆H along with the broader working temperature range would be suitable for Ericsson-cycle refrigeration application [54]. Thus exploitation of magnetic and magnetocaloric properties in Fe -based Heusler nanoalloys may be useful for not only spintronics but also for multistage magnetic refrigeration technology [28]. A comparison of these values with previous high temperature magnetic refrigerants is presented in table 2. Heusler alloy nanoparticles References Entropy change (-∆S M ) peak in J/Kg-K RCP (J/Kg) Working temperature span, ∆T FWHM (K) Curie temperature, T c (K) Fe CoAl This work 2.65 (at 20 kOe) 44 (at 20 kOe) 17 830 Co Cr Mn Al [28] 3.8 (at 90 kOe) ~285 (at 90 kOe) - 720 Co FeAl [55] 15 (at 14 kOe) 89 (at 14 kOe) ~6 1261
Table 2.
A comparison of the magnetic entropy change (-∆S M ) peak , RCP, ∆T FWHM , and Curie temperature with other high-temperature magnetocaloric materials. In our previous studies on Co FeAl Heusler nanoparticles [55], we have observed a giant magnetocaloric effect around the magnetic phase transition at 1252 K. In principle, such a high T c value and sharp peak in (-∆S M ) peak vs temperature curve make it less efficient for magnetic refrigeration. However, it suggests that a giant magnetocaloric effect may also be observed in X YZ type full-Heusler nanoalloys; this further opens a way of hunting the new Heusler compounds, especially, in nano regime. Our (-∆S M ) peak , value is higher than that of the Co Cr Mn Al (see table 2), and RCP value is at least comparable on the same field scale. Present values are also considerably larger than that of the other Co -based full Heusler alloys [56]. On the contrary, a huge RCP value of 400 J/Kg at 50-kOe magnetic field was reported in Gadolinium [57]. Nevertheless, it is not suitable for commercial purposes due to its high cost, and therefore Ni-based Heusler compounds [58-61] are extensively investigated for better performing magnetocaloric effect in recent years. Though, most of them exhibit a first-order phase transition causing a thermal and magnetic hysteresis near the T c . The present MC study suggests that the Fe CoAl nanoalloy might be suitable for multistage magnetic refrigeration technology and further opens a window of research on how its magnetic and magnetocaloric properties can be tuned by size and shape modulation of the nanoparticles, which we aim in our future work. A master curve or universal curve was proposed by Franco et al. [62] for the magnetocaloric materials exhibiting Figure. 8.
The magnetic entropy ( ∆𝑆 𝑀′ ) a s a function of rescaled temperature (θ) near T c for Fe CoAl Heusler alloy nanoparticles. second-order phase transition near T c . The curve was defined as the normalized entropy change ∆𝑆 𝑀′ with respect to rescaled temperature ( θ ) where ∆𝑆 𝑀′ = ∆𝑆 𝑀 /∆𝑆 𝑀𝑝𝑒𝑎𝑘 and θ defined as: 𝜃 = { −(T−T c )(T r1− T c ) , for T ≤ T C ; (T−T c )(T r2− T c ) , for T > T C } (2) In above equation, Curie temperature (T c ) denotes the temperature at the peak value of the magnetic entropy change ( ∆S Mpeak ) . Reference temperatures T r1 and T r2 were taken as the temperatures analogous to ∆S Mpeak below and above the Curie temperature (T C ). As seen from figure 8, ∆𝑆 𝑀′ vs θ curves of Fe CoAl HA-NPs, taken at different applied fields have been merged into a single universal curve, which confirms a second order magnetic phase transition across the T c . Such behavior of phenomenological curves was observed previously in second-order phase transition materials [55, 62-63]. This study recommends of using this large magnetocaloric material with a broad working temperature range in high temperature refrigeration application, as in multi-stage magnetic refrigeration where cooling from high temperature is desired in more than one stage [64]. Such multi-stage magnetic refrigerators are highly appropriate for industrial application, where the low temperature of one stage acts as upper temperature for the next stage [64, 65].
4. Concluding Remarks Fe CoAl Heusler nanoparticles with mean diameter of 14 nm have been grown using co-precipitation method. Structural x-ray diffraction, high resolution TEM together with compositional EDAX analysis and magnetic characterizations have confirmed uniform chemical distribution of the elements and the crystalline nature of particles with A2-disordered structure. Magnetic characterization carried out on the FCA-NPs supports their soft ferromagnetic character with high M s of about 127.6 emu/g (or 4.5 µ B /f.u.) and T c of 830 K. The peak value of the change in magnetic entropy (-∆S M ) vs temperature curve at a magnetic field of 20 kOe corresponds to about 2.65 J/Kg-K, and the observed value of refrigeration capacity (RCP) was as large as 44 J/Kg. To analyze the MPT, a detailed study of its magnetization is performed. The Arrott plot and the nature of the universal curve accomplish that the FM-PM phase transition is of second order. Acknowledgements:
A. Ahmad acknowledges the University Grant Commission, Delhi and Ministry of Human and Resource Development India, for providing Ph.D. fellowship. A. K. D. acknowledges the financial support of Department of Science and Technology, India (project no. EMR/2014/001026).
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