Enhancing the Ordering and Coercivity of L1

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Enhancing the Ordering and Coercivity of L10 FePt Nanostructures with Bismuth Additives for Applications Ranging from Permanent Magnets to Catalysts Frank M. Abel, Vasileios Tzitzios, Eamonn Devlin, Saeed M. Alhassan, David J. Sellmyer, and George C. Hadjipanayis ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00463 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Nano Materials

Enhancing the Ordering and Coercivity of L10 FePt Nanostructures with Bismuth Additives for Applications Ranging from Permanent Magnets to Catalysts Frank M. Abel†, Vasileios Tzitzios*,‡,¥, Eamonn Devlin¥, Saeed Alhassan‡, David J. Sellmyer§ and George C. Hadjipanayis† † Department

of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA

‡ Department

of Chemical Engineering, Khalifa University of Science and Technology,

Petroleum Institute, P.O. Box 2533, Abu Dhabi, 127788, United Arab Emirates ¥ Institute

of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, 15310, Greece

§ Department

of Physics and Astronomy and NCMN, University of Nebraska, Lincoln, NE

68588, USA

KEYWORDS: FePt, L10 ordering, chemical synthesis, bismuth, ferromagnetic nanostructures.

Corresponding Author * Vasileios Tzitzios, E-mail address: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT. L10 highly ordered FePt nanostructures were successfully synthesized following a direct one-step liquid phase chemical approach. The enhanced ordering was achieved with the use of bismuth additives in the reaction mixture. The as-made nanostructures are ferromagnetic, revealing high coercivity without any post annealing processing. The effect of bismuth addition was studied extensively and the synthesized nanostructures were characterized by a plethora of techniques including TEM, STEM, elemental mapping, XRD, and Mӧssbauer spectroscopy for the structural and morphological characterization, and VSM for the study of magnetic properties. The maximum room temperature coercivity of the directly synthesized FePt nanoalloys is 15.2 kOe, a value which is the highest, to the best of our knowledge, for analogous as-made liquid phase synthesized nanomaterials. The L10 FePt nanostructures with bismuth additives have promising applications in permanent magnets, ultra-high density recording media, and as highly durable Ptbased catalysis.

INTRODUCTION Chemically ordered (L10), bimetallic FePt nanoparticles have attracted great scientific and technological interest during the last few decades because of their potential applications in a variety of fields such as permanent magnets, high density magnetic storage media, catalysis, and biomedicine. The chemical stability, high magnetocrystalline anisotropy (Ku ≈ 70 Merg/cc)1,2, and high room temperature coercive field values, allow the reduction of particle size to below 10 nm with simultaneous stabilization of their magnetization against thermal fluctuations and demagnetizing effects. These properties are necessary for both exchange coupled permanent magnets and ultra-high density magnetic storage applications.1-6 On the other hand, FePt in the ordered L10 crystal structure serves as a highly active and durable electrocatalyst for various reactions such as Oxygen Reduction Reaction (ORR) in polymer electrolyte membrane fuel


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cells.7,8 Compared with fcc FePt nanoparticles, the L10 particles are more active and durable, with the activity and durability of Pt-based alloy nano-catalysts dependent not only on composition but also on their structure9,10,11 making the L10 nanoalloys very promising candidates as industrial catalysts for fuel cell applications.12,13 Previous studies have presented a plethora of chemical routes for the synthesis of ultra-fine, monodispersed FePt nanoparticles in the sub-10 nm range size.14- 22 In most of these studies, the as-made nanoparticles from liquid phase reactions have a disordered face centered cubic (fcc) crystal structure and are superparamagnetic at room temperature. To obtain the more interesting and magnetically hard face centered tetragonal (L10) crystal phase, a thermal treatment (annealing) of the as-made nanoparticles is required under an inert or reducing atmosphere at temperatures above 550 oC. However, annealing at these temperatures produces a complete decomposition of the protective organic layer surrounding the particles and leads to an undesirable aggregation and sintering of the nanoparticles. As a result, the nanoparticles lose their solubility, and most importantly their size and shape uniformity, which is essential for both the magnetic and catalytic applications. To decrease the fcc to L10 transformation temperature, or even better to eliminate the necessity for a post annealing procedure entirely, various methodologiess23-26 have been reported. Among these, the introduction of a third metal, such as Ag,27 Au28 and Sb29 into chemically synthesized FePt nanoparticles has succeeded in lowering the fcc to L10 transformation temperature from 550 oC

to approximately 400 oC. Howard et al.30 reported the direct synthesis of L10 FePt nanoparticles

using Collman’s reagent, Na2Fe(CO)4, as a reducing agent for the Pt2+. The reaction occurs in hydrocarbon solvents at 330 oC in the presence of surfactants under inert atmosphere. The as-made FePt nanoparticles are partially ordered with an average particle size of 6-8 nm and reveal


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coercivities of ~1.3 kOe and 3.1 kOe at room temperature and 10 K, respectively. Kang et al.,31 synthesized partially ordered FePt nanoparticles by thermal decomposition of Fe(CO)5 and reduction of Pt(acac)2 in hexadecylamine at 360 oC in the presence of 1-adamantanecarboxylic acid. The particles exhibit coercive fields of 0.5 and 0.8 kOe in the perpendicular and parallel directions, respectively. A similar procedure has been reported by Jia et al.32 for FePtAu nanoparticles of different compositions, with coercivities varying from a few hundred to a few thousand Oe. Key factors for the L10 FePt formation seem to be the time of the reaction (long time) and the heating rate (slower rate).33 More recently, Wang et al.34 reported the liquid phase synthesis of L10 FePt nanoparticles with high coercivity using a high percentage of Ag in the reaction mixture. The reaction takes place in hexadecylamine at 340oC for 3 h and a composition with 29 at. % Ag yields a room temperature coercive field of 7.6 kOe. Similarly, Yongsheng Yu et al.35 reported the synthesis of high room temperature coercivity, (up to 12.1 kOe), L10 FePt nanoparticles using 32 at. % Au. The addition of Cu in the liquid synthesis of FePt was also recently studied using a similar methodology with the co-reduction of FeCl2 and Fe(acac)2 in hexadecylamine at 320oC. The maximum coercivity obtained was 4.8 kOe with 20 at. % Cu with varying of the Fe to Pt ratio.36 Along with third element doping, it has been recently shown that the use of halide ions can lead to the formation of L10 FePt at solution synthesis temperatures, with the most promising being the Cl- ions. In one study, the chloride anion was introduced to the reaction mixture by using K2PtCl6 as the platinum source with Fe(acac)3 as the iron source. The reaction was performed in oleylamine at a temperature of 350 oC, leading to a high coercivity of 10.50 kOe after an 8 h reaction.37 The use of halide ions in FePt synthesis was further studied by Lei et al. by using NH4Cl for the chloride anion source with Pt(acac)2 and Fe(acac)3 in oleylamine at a reaction temperature of 350 oC. The best magnetic properties were obtained with a Cl- /Pt2+


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ratio of 3:1, and a reaction time of 9 h resulting in a coercivity of 8.64 kOe and saturation magnetization of 64.21 emu/g at room temperature.38 In this study, we report the influence of Bi additives on the ordering and magnetic properties of FePt chemically synthesized nanostructures. The results indicate that even a very small amount of bismuth significantly enhances the ordering process in FePt bimetallic nanostructures. In the directly synthesized materials the room temperature coercive field values reach 15.2 kOe. To our knowledge, the above coercive field value is the highest published value to date for the liquid phase, directly synthesized L10 FePt nanomaterials. EXPERIMENTAL Synthesis of L10 FePt nanoparticles. The synthesis of the Bi-doped FePt nanostructures was performed using an equiatomic ratio of Fe(acac)3 and Pt(acac)2 as precursors with bismuth (III) acetate used as the Bi source. The inorganic salts were added in 20 ml of trioctylamine along with palmitic acid and oleylamine as co-surfactants and 1-2 hexadecanediol as reducing agent. The mixture was magnetically stirred under a constant flow of 5% H2/Ar in a sealed environment for 30 minutes at room temperature, then for 1 hour at approximately 60 oC, and finally were heated directly to a temperature in the range 260 oC to 360 oC for 1 hour. The bismuth additive was varied between 0 and 21 at. % nominal composition for reactions performed at 360 oC for 1 hour. Additional experiments were performed at 260 oC, 320 oC, and 340 oC for 1 hour with a fixed bismuth composition of 21 at. %. After one hour reaction the heating source was set to room temperature, and the crude black solution was cooled. The solution was then exposed to air, and the material precipitated by the addition of ethanol and then separated magnetically. The black (FePt)100-xBix product was washed with ethanol, and then with hexane to remove the excess organic material and reaction by-products. Additional washing was performed by ultrasonication using


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both hexane and ethanol. The bismuth phases dissolution in the (FePt)100-xBix nanomaterial was performed by a room temperature treatment with concentrate sulfuric acid for 6 hours in which 10 mL of H2SO4 was added to approximately 20 mg of the (FePt)91Bi9 powder, followed by washing with distilled water, ethanol and, finally, acetone; the sample was magnetically separated at each step of the washing. The overall synthetic procedure is illustrated in Scheme 1.

Scheme 1. Illustration of the overall methodology for the synthesis of L10 FePt nanoparticles. Material Characterization. The crystal structure of the materials was determined using X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Kα radiation, additional structural characterization and phase analysis was determined by Mӧssbauer spectroscopy on selected samples. The size and the morphology of the particles was studied using transmission electron microscopy (TEM, JEOL JEM-3010) and the magnetic properties measured with a 3 Tesla vibrating sample magnetometer (VSM, Quantum Design); additional high field measurements of selected samples were performed with a 9 Tesla physical property measurement system (PPMS, Quantum Design). Scanning electron microscopy (SEM, JEOL JSM-7400F) and energy dispersive spectroscopy (EDS, JEOL JSM-7400F) was used to determine the approximate composition, the quoted values being rounded to the nearest whole numbers. Elemental mapping was carried out in a scanning transmission electron microscope (STEM, JEOL JEM-2010F).


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RESULTS AND DISCUSION The crystal structure of the as-made FePt samples was studied using powder X-ray diffraction (XRD). The XRD patterns in Figure 1(a) clearly show in all samples the superlattice reflections (001) and (110), which, together with the appearance of the (200)/(002) splitting, verify the formation of the L10 FePt phase synthesized at 360 oC for 1 hour with different atomic percent bismuth. The reflections denoted with blue asterisks, main peak at ~27o, coincide with the peaks of metallic bismuth, while the peaks at ~29o and ~33o (red asterisks) belong to Bi2Pt (cP12), which form separately from the magnetic L10 FePt phase. To evaluate the degree of long-range ordering of the L10 phase, the order parameter S was calculated according to the approximate relation between the c/a ratio and S shown below: S2 =

1 ― (c/a) 1 ― (c/a)f

where (c/a)f is the axial ratio for the fully ordered phase, and c/a the experimentally determined ratio for the partially ordered phase.39 S=0 corresponds to the fully disordered phase and S=1 to the fully ordered phase. In Figure 1(b), the order parameter S is presented for the as-made FePt nanostructures, synthesized with and without bismuth. The S value for the as-made bimetallic FePt particles without bismuth is approximately 0.59, increasing to 0.89 with only 5 at. % bismuth. With increasing bismuth content, the order parameter gradually increases reaching a maximum of 0.91 with 20 at. % bismuth. This clearly indicates that the presence of bismuth leads to an enhancement of the L10 chemical ordering of the FePt samples.


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0.0

x Bismuth (at. %) Figure 1. XRD patterns of the as-made (FePt)100-xBix nanostructures with different bismuth additions synthesized at 360 oC; compositions are determined by SEM EDS (Figure S1) except for dagger (†) indicating nominal composition (a), corresponding ordering parameter and c/a ratios plotted as a function of at. % bismuth (b). Lattice parameters were determined using the MAUD diffraction refinement program, with errors estimated by the refinement software.40 The reference (c/a)f ratio is that of the ordered bulk structure of the equiatomic alloy.41 Figure 2 shows the room temperature hysteresis loops for bismuth doped and pure bimetallic asmade FePt particles. In all cases the samples show ferromagnetic behavior with coercive field values varying from approximately 1.0 kOe to 15.2 kOe for bimetallic FePt and 20 at. % Bi


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samples, respectively; note that the coercivity increases by a small amount from 13.6 kOe to the maximum of 15.2 kOe when the Bi increases from 9 at. % to 20 at. % . The increase in coercivity is consistent with the increase in the degree of ordering of the L10 phase obtained from the XRD patterns. The development of L10 ordering primarily influences the magneocrystalline anisotropy which leads to enhanced coercivity. However, the magneocrystalline anisotropy is only one factor affecting the coercivity, the other being the particle/grain size. As it will be discussed later, this becomes clear in the examination of the particle and grain sizes estimated from TEM images and Scherrer broadening of the XRD data. The maximum hypothetical energy product based on the powder samples was estimated using 9 Tesla demagnetization curves for the highest coercivity material, Figure S2(a-c). The energy product of the 5 at. % Bi sample was estimated to be 6.7 (MGOe) increasing slightly to 6.9 (MGOe) for the 9 at. % Bi sample, and decreasing to 5.6 (MGOe) in the 20 at. % Bi sample due to reduction in remanence because of the significant increase in diamagnetic bismuth based phases.

Magnetization (emu/g)

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40 20 0

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Magnetic Field (kOe) Figure 2. Room temperature hysteresis loops of the as-made (FePt)100-xBix particles, with x=0 (red), x=3† (blue), x=5 (yellow), x=9 (green) and x=20 (purple), showing increasing coercivity with bismuth addition.


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Transmission electron microscopy (TEM) was used to study the size and morphology of the asmade samples, and the sample after H2SO4 treatment, Figure 3(a-j). With no bismuth present, the particles aggregate into clusters of small particles with an approximate particle size of 9 nm. With 5 at. % and 9 at. % bismuth the particles form large interconnected networks, but appear less tightly clustered compared to the bismuth free case. The average particle size estimated from TEM statistical analysis increases from 13 nm (5 at. %) to 16 nm (9 at. %). In the highest bismuth case, 20 at. %, the particles still aggregate and form large irregularly shaped crystals with average size of 21 nm. The TEM images thus indicate that the particle size increases with increasing bismuth content. Figure 4(a) shows the size estimated from TEM and the crystal size estimated from XRD data using the Scherrer equations, both showing an increasing size trend with increasing bismuth. The error bars derived from the standard deviation of the particle/grain size distributions are given in the supplemental data, Figure S3(a-d).


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Figure 3. TEM of as-made (FePt)100-xBix particles with x=0 (a,b), x=5 (c,d), x=9 (e,f), x=9 after H2SO4 treatment (g,h), and x=20 (i,j) bismuth content.


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Figure 4. Comparison of average particle/grain size estimated by analysis of TEM images and crystal size using the Scherrer equation, determined by fitting the (111) peak with Voigt function (a), coercivity and crystal size (Scherrer) versus at. % bismuth (b), and coercivity as a function of crystal size (Scherrer) for (FePt)100-xBix samples synthesized at 360 oC (c). As mentioned before, the development of large coercivity in L10 structures is related to three primary factors; the relative Fe-Pt composition, the ordering parameter which is directly related to the magnetocrystalline anisotropy, and the particle/grain size. In the case of the (FePt)100-xBix


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samples the Fe-Pt composition is slightly Fe rich, but close to the equiatomic composition in all cases, as confirmed by EDS (Figure S1); this suggests that the main causes of the coercivity increase are the ordering parameter and particle/grain size. Although the ordering parameter increases with the addition of bismuth (Figure 1b), it does not change significantly from x=3 to x=20, increasing from 0.87 to 0.91, respectively. In comparison, the TEM estimates of particle/grain size increases from 13 nm to 21 nm as the bismuth concentration goes from 5 at. % to 20 at. %. Figure 4(b) shows the crystal size estimated from the Scherrer equation along with the corresponding coercivity as a function of bismuth percentage. Size and coercivity show a nearly identical behavior with increasing bismuth, with a nearly linear correlation of coercivity with size, Figure 4(c). The increase in coercivity from x=0 to x=3 is most likely related to both the size and ordering of the particles. However, the large jump in coercivity between the x=5 and x=9 samples is most likely related to the sharp increase in grain size and to a much smaller degree to the increase in ordering, with the ordering parameter changing slightly from 0.89 to 0.90, respectively. Since there is more than one phase in the nanomaterials, it is crucial to determine whether the FePt and the Bi phases grow separately or together. Figure 5(a-e) shows elemental mapping of Fe (b, blue), Pt (c, green), and Bi (d, red) for the as-made FePt particles synthesized with 20 at. % Bi. The color distributions indicate that Fe, Pt, and Bi elements are uniformly distributed within each of the nanoclusters. The mapping suggests that a portion of the bismuth nucleates with the Fe and Pt phase, while the remaining bismuth nucleates separately into either metallic Bi or Bi-Pt phases, which are clearly observed by XRD. The Bi and Bi-Pt phases observed in XRD must exist separately to the Fe-Pt-Bi observed in the mapping due to more narrow peaks of these phases in comparison to the L10 FePt phase. TEM and HR-TEM confirm this, Figure S4(a-c) in the supplemental data, showing grain/particle size differences between the FePt and Bi-Pt phases.


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Additional mapping of a sample with a nominal composition of (FePt)94Bi6, presented in the supplemental data in Figure S5(a-e), shows the same behavior i.e. a uniform bismuth distribution throughout the FePt nanoalloy. Corresponding hysteresis loop and XRD data is given for this sample in Figure S6(a,b). HR-TEM of as-made FePt particles with 20 at. % Bi, Figure 5 (f,g), further confirms that the bismuth observed in the mapped data resides on the surface of the FePt, which can either be Bi or Bi-Pt phases most likely depending on the surface chemistry of the FePt grains.

Figure 5. STEM elemental mapping of a (FePt)80Bi20 nanostructure synthesized at 360 oC for 1 hour. Image corresponding to mapping region (a) mapping of Fe (b, blue), mapping of Pt (c, green),


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mapping of Bi (d, red), and STEM image of mapped region (e). HR-TEM of (FePt)80Bi20 nanostructure showing FePt grain with (111) diffraction plane with crystal lattice on the surface with d-spacing of 0.33 nm close to the (012) diffraction plane of Bi (f), and HR-TEM showing FePt grains with (201) and (111) diffraction planes with crystal lattice on the surface with dspacing of 0.30 nm close to the (210) diffraction plane of Bi2Pt. Figure 6(a,b), shows XRD patterns (a), and corresponding room temperature hysteresis loops (b), of FePtBi with nominal composition of (FePt)79Bi21 synthesized at different reaction temperatures. The data reveals that the ordering occurs even at the low reaction temperature of 260 oC,

with a coercive field of 0.7 kOe, increasing to 15.2 kOe at a reaction temperature of 360 oC.

Mӧssbauer Spectroscopy (MS) was performed at room temperature on (FePt)79Bi21 samples synthesized at 320 oC and 360 oC. An additional spectrum was obtained at 80 K on the 360 oC sample. The spectra and fitted data are given in the supplemental data, Figure S7(a-c). The tetragonal L10 phase is readily identified in all spectra by its large quadrupole interaction, eQ, 0.30±0.01. For the sample prepared at 320 oC, this tetragonal phase sub-spectrum makes up approximately 60% of the total spectral area of the room temperature spectrum, while the cubic site accounts for 30% of the spectral area. For the sample prepared at 360 oC the tetragonal has increased to 74% of the total spectral area, with the cubic site now accounting for 16% of the spectral area. A third sub-spectrum, a nonmagnetic doublet, with just 10% of the spectral area, is also required for an acceptable fit of the experimental data in both samples at room temperature. In the spectrum of the 360 oC sample obtained at 80 K we see that the non-magnetic component has decreased to just 5% of the spectral area with a corresponding increase in the area of the cubic phase, while the tetragonal phase area remains nearly constant. This is consistent with the low magnetic anisotropy (fcc phase) nanoparticles, being superparamagnetic at room temperature and becoming ferromagnetic at low temperature. The Mӧssbauer Spectroscopy thus confirms the formation of the L10 phase at low reaction temperatures using a bismuth additive.


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Magnetic Field (kOe) Figure 6. XRD patterns of as-made FePtBi nanostructures synthesized at different reaction temperatures with nominal composition of (FePt)79Bi21 (a), and corresponding room temperature hysteresis loops (b). One of the primary difficulties in using a third element as a dopant to produce highly ordered L10 FePt is the development of impurity phases during the synthesis. Through treatment with concentrate sulfuric acid following the reaction shown below,


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2Bi + 6H2SO4→Bi2(SO4)3 + 3SO2 + 6H2O the diamagnetic bismuth phases can be dissolved. The XRD patterns of the FePt91Bi9 nanomaterial as well as the room temperature hysteresis loops are presented in Figure 7(a,b). After the H2SO4 treatment the diffraction peaks corresponding to the bismuth-rich phases are removed while the magnetic properties remain unchanged, suggesting the segregated excess bismuth-rich phases are dissolved without any negative effects in both the structure and magnetic properties. TEM of the H2SO4 treated sample is shown in Figure 3(g,h) showing similar morphology before and after washing. Although the magnetic properties are unchanged, an enhancement in magnetization is not observed even with dissolution of the diamagnetic crystal phases; this may be due to a remaining amorphous residue created from the H2SO4 that was not completely removed with the washing and possibly to some Fe-leaching. This methodology allows the formation of additivefree L10 FePt nanomaterials, something which is impossible for the case of Au addition, as Au is soluble only in aqua regia, which also completely dissolves the ferromagnetic phase. In FePtAg nanoparticles a selective dissolution of the Ag additives has been reported using nitric acid to produce pure L10 FePt nanoparticles32. While the authors write that the nitric acid treatment doesn’t affect the magnetic properties, and the XRD patterns clearly show the disappearance of the Ag diffraction peaks, the magnetic properties aren’t presented in the manuscript.


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56.8

Unit Cell Volume (Å3)

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x Bismuth (at. %) Figure 8. Unit cell volume of as-made (FePt)100-xBix as a function of at. % bismuth, blue point shows bulk unit cell volume for comparison. Our data indicate that Fe, Pt and Bi nucleate together in the first steps of the reaction leading to the formation of a ternary disordered Fe-Pt-Bi fcc alloy. At high temperatures and as the reaction progresses the Bi atoms segregate from the fcc FePt lattice to the surface of the particles. Calculation of the unit cell volume for (FePt)100-xBix synthesized at 360oC, Figure 8, shows that the unit cell decreases with increasing bismuth. If bismuth was incorporated into the lattice after the reaction is complete an expanded unit cell volume would be expected due to larger atomic radius of Bi compared to Fe and Pt. These findings suggest the bismuth observed in the elemental mapping resides on the surface of the particles, a result of surface segregation. This segregation may create vacancies which increase the mobility of the Fe, and Pt atoms allowing their rearrangement into the L10 structure. This mechanism is generally accepted in the literature, and from the experimental results from similar liquid phase methodologies, the ordering enhancement, based on the coercivity values, follows the relationship Bi > Au35 > Ag34> Cu36. This order, assuming that the segregation of the third element is the crucial step, seems to follow the difference


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between the Fe and Pt surface energy, γ, and the surface energy of the third element. Bismuth has the lowest surface energy value, based on calculated values42, (0.537 J/m2) in comparison with Ag, Au and Cu which have 1.172, 1.283 and 1.952 J/m2 respectively. On the other hand, Fe and Pt have surface energy values of 2.430 and 2.299 J/m2, respectively. With increasing γFe,Pt-γthird element the third element segregation accelerates and takes place at lower temperatures, thus enhancing the L10 ordering procedure.42,43 It should be noted in the case of Cu36 doping the authors suggest that Cu incorporates into the FePt structure, unlike in the case of Bi, Ag34 and Au35. The miscibility of Cu36 in the FePt structure is consistent with its high surface energy and an inability for significant diffusion to occur at low reaction temperatures, which results in lower coercivities in comparison to Bi, Ag34, and Au35. In contrast, the significantly lower surface energy of Bi allows for diffusion and ordering to occur as low as 260 oC, resulting in an observable coercivity. CONCLUSIONS In conclusion, we introduce a chemical approach for the direct synthesis of highly ordered FePt nanostructures/ nanoparticles. The results show a superior enhancement of the ordering procedure with bismuth present in the reaction mixture even at very low bismuth concentrations. The low surface energy of bismuth is suggested as the cause of enhanced surface segregation of the dopant. Additionally, the increase in grain/particle size with increasing bismuth is also responsible for the high coercivity values obtained. The as-made FePt reveal room temperature coercivities up to 15.2 kOe, the highest value reported to date for similar materials. Furthermore, the use of bismuth as a dopant allows for selective dissolution of the diamagnetic bismuth phases by a simple room temperature treatment with concentrated H2SO4, which produces no changes to the structural and magnetic properties of the FePt nanoalloys, leading to the formation of pure ferromagnetic phases.


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These, additive free, L10 FePt nanostructures are very promising candidates for a variety of applications from exchange coupled permanent magnets5,6,44 to catalysts.10,11,12,45,46

ASSOCIATED CONTENT Supporting Information The supporting information includes the size distribution histograms, composition analysis data with EDS spectrum, additional elemental mapping with accompanying XRD patterns and VSM measurements of that sample, Mӧssbauer Spectrum of samples synthesized at 320 oC and 360 oC, additional TEM and HR-TEM of sample synthesized at 360 oC with (FePt)80Bi20 composition, and high field demagnetization curves, and magnetic induction curves of (FePt)95Bi5, (FePt)91Bi9, and (FePt)80Bi20 samples synthesized at 360 oC from which the energy product was estimated

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ORCID Vasileios Tzitzios: 0000-0002-5687-081X Saeed Alhassan: 0000-0002-5148-3255 David J. Sellmyer: 0000-0002-0130-6488 Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS This work has been supported by our joint DOE-BES program under the grants: University of Delaware DE-FG02-90ER45413 and University of Nebraska DE-FG02-04ER46152. Part of the work was done in the Nebraska Nanoscale Facility, Nebraska Center for Materials and Nanoscience, which is supported by the National Science Foundation under Award ECCS: 1542182, and the Nebraska Research Initiative. We also acknowledge support of this work by the project MIS 5002567, implemented under the “Action for the Strategic Development on the Research and Technological Sector”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund) REFERENCES 1. Weller, D.; Parker, G.; Mosendz, O.; Lyberatos, A.; Mitin, D.; Safonova, N. Y.; Albrecht, M. Review Article: FePt Heat Assisted Magnetic Recording Media. Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 2016, 34, 060801. 2. Dong, Y; Liu, F; Yang, W; Zhu, J; Yu, J; and Hou, Y. Layer-by-layer assembly of L10-FePt nanoparticles with significant perpendicular magnetic anisotropy. Cryst. Eng. Comm, 2014, 16, 9430-9433. 3. Hyeon, T. Chemical Synthesis of Magnetic Nanoparticles. Chem. Comm. 2003, 34. 4. Sun, S. Recent Advances in Chemical Synthesis, Self-Assembly, and Applications of FePt Nanoparticles. Adv. Mater. 2006, 37. 5. Yang, W.; Lei, W.; Yu, Y.; Zhu, W.; George, T. A.; Li, X.-Z.; Sellmyer, D. J.; Sun, S. From FePt–Fe3O4 to L10-FePt–Fe Nanocomposite Magnets with a Gradient Interface. Journal of Materials Chemistry C 2015, 3(27), 7075–7080. 6. Zhu, K.; Ju, Y.; Yang, Z.; Gao, S.; Hou, Y. Magnetic Nanomaterials: Chemical Design, Synthesis, and Potential Applications. Accounts of Chemical Research 2018, 51, 404–413. 7. Xu, Y.; Ruban, A. V.; Mavrikakis, M. Adsorption and Dissociation of O2 on Pt−Co and Pt−Fe Alloys. J. Am Chem. Soc. 2004, 126, 4717–4725.


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82x44mm (300 x 300 DPI)


Enhancing the Ordering and Coercivity of L1

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