Enhanced magnetic properties of yttrium iron nanoparticles Amir Aslani, Mohammadreza Ghahremani, Ming Zhang, LawrenceH Bennett, and Edward Della Torre Citation AIP Advances 7, 056423 (2017); doi 10 10[.]
Enhanced magnetic properties of yttrium-iron nanoparticles Amir Aslani, Mohammadreza Ghahremani, Ming Zhang, LawrenceH Bennett, and Edward Della Torre Citation: AIP Advances 7, 056423 (2017); doi: 10.1063/1.4975695 View online: http://dx.doi.org/10.1063/1.4975695 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics AIP ADVANCES 7, 056423 (2017) Enhanced magnetic properties of yttrium-iron nanoparticles Amir Aslani,1 Mohammadreza Ghahremani,2 Ming Zhang,3 Lawrence H Bennett,1 and Edward Della Torre1 Department of Electrical and Computer Engineering, George Washington University, Washington, DC 20052, USA Department of Computer Science, Mathematics, and Engineering, Shepherd University, Shepherdstown, West Virginia 25443, USA Department of Chemistry, George Washington University, Washington, DC 20052, USA (Presented November 2016; received 23 September 2016; accepted November 2016; published online February 2017) A systematic study of the size effect on the magnetic and structural properties of Y2 Fe17 nanoparticles has been performed We present new data to explain the enhanced magnetic properties of nanostructured yttrium-iron alloy synthesized through alkalide reduction chemical synthesis The properties of the particles were characterized by x-ray diffraction, electron microscopy, and magnetometer techniques As the size of the nanoparticles is reduced, there is an increase in magnetization per unit of applied magnetic field, a decrease in the coercivity and a substantial reduction in hysteresis © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4975695] I INTRODUCTION The research in magnetocaloric materials can be grouped into two categories: the search for higher performance materials and the reduction of materials cost The first grouping is usually concentrated on rare earth alloys, while the second is focused on substituting transition metals for rare earth elements In recent years, research on magnetocaloric materials has shifted toward finding the most economically advantageous magnetic refrigerant with the highest performance Magnetic nanoparticles have attracted a lot of interest and research attention due to their potential application in a variety of fields The complex magnetic behavior exhibited by nanoparticles is governed by many factors, including their size, composition, shape, morphology, and shell-core structure.1 The increased surface to volume ratio and tailored structure in nanoparticles introduces many size dependent phenomena which may be used to optimize the physical and chemical properties Sequence of phase transformation and magnetic properties may be effectively controlled by adjusting the particle size and atomic packing Nanoscale magnetic materials are good candidates for magnetic refrigeration due to a presence of a large magnetocaloric effect (MCE) in the superparamagnetic system.2–4 Work by5 and6 showed MCE enhancement in superparamagnetic gadolinium gallium iron garnet and an enhancement of magnetic entropy change in superparamagnetic iron nitride nanograins, respectively Work by7 showed that Gd0.85 Y0.15 nanopowders displayed 15% greater MCE when compared with a bulk alloy of the same stoichiometry due to the superparamagnetic behavior of the nanopowders in contrast to the ferromagnetic behavior of the bulk alloy An increased in MCE in nanostructured ribbons compared to polycrystalline alloy in Y2 Fe(14-13) Mn(3-4) was reported in.8 Nanostructured Y2 Fe17 ribbons studied in9 showed a considerable MCE property in nanoparticles The study of metallic alloys is one of the main fields of current research in Material Science.10 In this case, Fe-based alloys attract great interest because of the number of applications in which they can be used, and its commercial value11,12 and they are believed to be promising magnetic refrigerants.13 A wide family of magnetic intermetallic compounds arises from an alloy of rare-earth (R) and a 3d transition metals (M) Localized magnetism of rare-earth sublattice with an itinerant magnetism 2158-3226/2017/7(5)/056423/5 7, 056423-1 © Author(s) 2017 056423-2 Aslani et al AIP Advances 7, 056423 (2017) 3d sublattice, makes these magnetic intermetallics very attractive for commercial applications The unpaired 3d electrons of the transition metal component give rise to a net magnetic moment Moreover, these alloys can exhibit different magnetic behavior: ferromagnetism (e.g Pr2 Fe17 ), ferrimagnetism (e.g Tb2 Fe17 ), etc The isothermal magnetic entropy change, ∆SM , due to the change in magnetic field can be written as,14,15 Ng2 J(J + 1) µ2B H Ng2 J(J + 1)µ2B H ∆SM = − dH = − (1) 3kB (T − TC )2 6kB (T − TC )2 where N, J, g, µB , k B , and A stand for the number of magnetic atoms per unit volume, angular momentum, gyromagnetic ratio, Bohr magneton Boltzmann constant, and atomic weight, respectively The magnetic entropy change, ∆SM , attains a maximum when the temperature, T, approaches the Curie temperature, T C , as seen in equation (1) Therefore, for applications around room temperature, MCE materials with T C close to room temperature should be selected The selection of a material with a large angular momentum, J, will also satisfy the need for sufficient change in entropy Alloys and compounds using rare-earth elements might be suitable for this purpose due to their large angular momentum But these materials are expensive for commercial use Low-cost Fe-rich materials would be a good choice as refrigerant in our daily life refrigerators because of the large g and J values in iron Therefore, attempt has been made to prepare and study the MCE in inexpensive iron-rich binary alloys R2 Fe17 (R = Y, Pr, Nd).16,17 The T C of these alloys is close to room temperature.18 These have motivated us to study the size-dependent magnetization behavior in yttrium-iron alloy as a function of both temperature and applied magnetic field II EXPERIMENT A sample of Y2 Fe17 alloy nanoparticle with gold coating was synthesized using alkalide reduction chemical synthesis The procedure is as follows Yttrium (III) chloride, YCl3 (99.99%, Alfa Aesar), Iron (III) chloride, FeCl3 (99.9%, Aldrich), and Gold (III) chloride, AuCl3 (99.99%, Aldrich) were used as purchased without further purification 100 ml of solvent tetrahydrofuran (THF) solution of YCl3 (1 mmol/L) and FeCl3 (8.5 mmol/L) was prepared and hand poured into a 100 ml THF solution of the reducing agent K+ (15-Crown-5)2 Na- (20 mmol/L) under vigorous magnetic stirring for a few seconds The mixture colloid was allowed to stir for minutes, and then 100 ml THF solution of AuCl3 (3.5 mmol/L) was slowly added in minutes The synthesis was performed in Nitrogen dry box The products were transferred into an air-tight solvent bottle for storage Gold coated Y2 Fe17 nanoparticles were extracted by vacuum removal of solvents, washed with water, centrifuged, and dried in the air The final product is a dark brown powder The particle size of nanoparticles can be controlled by systematically adjusting the reaction parameters, such as time, temperature, and the concentrations of reagents and stabilizing surfactants To obtain different-sized nanoparticles, the concentration of the salts was varied Nanoparticles are air sensitive, hence to prevent oxidation they were coated with gold Powder X-ray diffraction (Rigaku MiniFlex) measurement at room temperature was carried out to study the crystal structures using Cu-Kα radiation Surface morphology of the synthesized powder alloy was characterized by scanning electron microscopy (Raith PIONEER Two) technique The composition of the powders was determined from energy dispersive X-ray fluorescence (Shimadzu EDX-700) Magnetization measurements were performed using vector vibrating sample magnetometer (Lake Shore VVSM 7410) with standard zero field cooling (ZFC), field cool cooling (FCC), and field cool warming (FCW) techniques III RESULTS AND DISCUSSION The XRD patterns of synthesized samples showed some disordered (amorphous) phase To restore the crystallinity and log-range magnetic order, all samples were annealed at different temperatures and characterized to determine the optimal annealing temperature and time, which was 673 K for hours XRD measurements of the annealed samples, shown in Fig 1(a), fit the yttrium-iron hexagonal closed 056423-3 Aslani et al AIP Advances 7, 056423 (2017) FIG (a) XRD pattern of annealed synthesized nanoparticles (b) A sample SEM image of Y2 Fe17 nanoparticles pack structure and a phase prototype similar to Th2 Ni17 , with space group P63 /mmc(194) Annealing leads to an improvement in the magnetic properties of the synthesized nanoparticles as they become ordered The morphology of nanoparticles was observed with SEM and a sample image is shown in Fig 1(b) The average sizes of synthesized nanoparticles were observed to be 21, 28, 36 and 42 nm, in good agreement with crystallite size estimated from XRD data using well-known Scherrer relation.19 Figure 2(a) shows the magnetization as a function of applied field for the 21 nm sample; measured at discrete temperatures from 10 to 290 K Figure 2(b) shows the M(H) curves plotted from 8,000 to 10,000 Oe By increasing the temperature there is a reduction in magnetization Figure 2(c) shows the M(H) curves from -600 to 600 Oe For increased temperature, there is a reduction in the coercivity This occurs because as temperature increases, more thermal energy is supplied and individual electron spins become more likely to be in higher energy states, pointing randomly, opposite to their neighbors and less aligned, leading to a reduction in the total magnetization Additionally a smaller field is required to reduce remnant magnetization to zero, leading to a reduction in coercivity Figure 2(d) shows the coercivity as a function of temperature for the 21 nm sample FIG (a) M(H) of the 21 nm sample (b) M(H) plotted from 8,000 to 10,000 Oe (c) M(H) plotted within + − 600 Oe (d) Coercivity as a function of temperature for the 21 nm sample 056423-4 Aslani et al AIP Advances 7, 056423 (2017) Figure 3(a) shows M(T) measurement of the synthesized nanoparticles in ZFC, FCC, and FCW sequences from 10 to 316 K under a magnetic field of 1000 Oe Magnetization of the nanoparticles increases as their size reduces For instance, the magnetization of the 21 nm sample is about five times than that of the 42 nm sample Figure 3(b) shows the size-dependent magnetization from 10 to 316 K under an applied field of 1000 Oe At any given temperature there is an inverse correlation between the nanoparticles size and the magnetization By reducing the size, the magnetization increases For a given size the magnetization reduces by increasing the temperature When domains are formed, the magnetostatic energy decreases, and the wall energy and the magnetocrystalline anisotropy energy increases The transition point from superparamagnetic to single-domain to multi-domain for each type of nanoparticle depends upon the size and geometry of the nanoparticles In addition, temperature can influence the transition Even though there is no definite point between each transition region, these can be approximated As particle size becomes smaller, the role of particle’s core and surface region becomes comparable Now the surface region essentially modifies magnetization configuration of the particles and has an effect on its magnetic characteristics For ferromagnetic nanoparticles, pure finite-size effects are expected to enhance the Ms value with respect to the bulk Metal atoms at the surface present a higher magnetic moment due to the band narrowing caused by the lack of orbital overlap On the other hand, the surface anisotropy makes the surface layer magnetically harder than the core of the particle At low temperatures, this can result in the magnetization enhancement The M(T) data of the 28 nm sample showed a magnetization increase of about 19% at room temperature compared to the bulk Y2 Fe17 magnetization that was reported in.14 Hysteresis is an important parameter in evaluating the performance of refrigerant materials Hysteresis losses must be minimized to achieve a higher refrigeration capacity which is a key criterion in distinguishing a suitable refrigerant material Figure 4(a) shows M(H) of the four different-sized Y2 Fe17 nanoparticle samples at room temperature (292 K) Coercivity reduces as particle size gets smaller The coercivity (Hc ) is reduced by decreasing the nanoparticles size, which leads to the reduction in magnetic hysteresis losses of the samples [Fig 4(a)] This phenomenon occurs because as the size of magnetic nanoparticles increases, the nanoparticles become pseudo single domain and then multi domain structures in which the moments of the domains may not be aligned After applying a magnetic field some of the non-parallel vector magnetic moments cancel, leading to a reduction in the coercive field required to reduce the magnetization to zero For small nanoparticles hysteresis losses are absent or negligible Kneller 20 related the decrease in coercivity with decreasing particle size to thermal effect in superparamagnetic particles As shown in Fig 4(b), coercivity increases for the 21 nm sample Two main factors relate to coercivity: particle size and shape anisotropy The relationship of H C ∼ ∆NMS (where ∆N is the diversity of demagnetization factor in different direction) may explain why the 21 nm sample has higher coercivity In addition, as shown by,21 coercivity depends on the size of nanoparticles involved, and that for series of magnetic nanoparticles over a range of sizes, nanoparticles go through two maxima in the two separate single-domain and multi-domain regimes FIG (a) M(T) under 1000 Oe applied field for different-sized Y2 Fe17 nanoparticles (b) Size-dependent magnetization of Y2 Fe17 nanoparticles from 10 to 316 K under 1000 Oe applied field 056423-5 Aslani et al AIP Advances 7, 056423 (2017) FIG (a) M(H) of Y2 Fe17 nanoparticles measured at 292 K, (b) Coercivity of Y2 Fe17 nanoparticles as a function of size IV CONCLUSIONS Y2 Fe17 nanoparticles were successfully synthesized through chemical synthesis XRD analysis revealed the transformation of disordered crystal structure to Y2 Fe17 structure during the postannealing process SEM analysis showed a formation of nanoparticles with an average size of 21, 28, 36 and 42 nm, in close agreement with crystallite size estimation from XRD The effects of temperature and size on the magnetic properties revealed a significant improvement in the magnetic properties of the nanoparticles Magnetization increased with reduction in particle size Hysteresis first decreased by reducing the size and then increased for the smallest sample Thus we have shown that nanoparticles as 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through two maxima in the two separate