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Synthesis and surface modified hard magnetic properties in Co0.5Pt0.5 nanocrystallites from a rheological liquid precursor
Journal of Magnetism and Magnetic Materials 324 (2012) 3893–3898
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Synthesis and surface modified hard magnetic properties in Co0.5Pt0.5 nanocrystallites from a rheological liquid precursor S.S. Kalyan Kamal a,n, P.K. Sahoo a, L. Durai a, P. Ghosal a, M. Manivel Raja a, S. Ram b a b
Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India
a r t i c l e i n f o
abstract
Article history: Received 14 April 2012 Received in revised form 29 May 2012 Available online 30 June 2012
Small crystallites of a metastable phase Co0.5Pt0.5 are precipitated by heating a rheological liquid precursor of cobalt–hydrazine complex and platinum chloride H2PtCl6 xH2O in polymer molecules of poly(vinylpyrrolidone) (PVP) in ethylene glycol. The hydrazine co-reduces nascent atoms from the Co2 þ and Pt4 þ that recombine and grow as Co0.5Pt0.5. The PVP molecules cap a growing Co0.5Pt0.5 as it achieves a critical size so that it stops growing further in given conditions. X-ray diffraction pattern of a recovered powder reveals a crystalline Co0.5Pt0.5 phase (average crystallite size D 8 nm) of a wellknown Fm3m-fcc crystal structure with the lattice parameter a ¼ 0.3916 nm (density r ¼14.09 g/cm3). A more ordered L10 phase (r ¼ 15.91 g/cm3) transforms (D Z 25 nm) upon annealing the powder at temperature lesser than 700 1C (in vacuum). At room temperature, the virgin crystallites bear only a small saturation magnetization Ms ¼ 5.54 emu/g (D¼ 8 nm) of a soft magnet and it hardly grows on bigger sizes (D r 31 nm) in a canted ferromagnetic structure. A rectangular hysteresis loop is markedly expanded on an optimally annealed L10 phase at 800 1C for 60 min, showing a surface modified coercivity Hc ¼ 7.781 kOe with remnant ratio Mr/Ms ¼0.5564, and Ms ¼ 39.75 emu/g. Crystallites selfassembled in an acicular shape tailor large Hc from ideal single domains and high magnetocrystalline anisotropy of a hard magnet L10 phase. & 2012 Elsevier B.V. All rights reserved.
Keywords: Co–Pt Nanoparticle Polyol process Magnetic property Ferromagnetism
1. Introduction Special interest has been laid on synthesis of bimetallic magnetic nanocrystallites (NCs) like Co–Pt, Fe–Pd and Fe–Pt, with effective size (D) varied from 2 to 20 nm, because of their good chemical stability, long range chemical ordering, and high magnetocrystalline anisotropy (Ku) of 4 107 erg/cm3 [1–4]. An ordered crystalline L10 phase of a face centered tetragonal (fct) crystal structure is highly sought in Co100 x Ptx(xZ10) alloys as a candidate material. Markedly hard magnetic properties can be tailored in this alloy by proper nanostructuring in terms of average D-value, shape and surface topology. A high Ku value owned by this phase defers the emergence of superparamagnetic limit well into terabit/in2 and retains a steady magnetization against thermal fluctuations and demagnetizing effects in small NCs down to D ¼10 nm [3,4]. In particular, the parameters which govern a high performance magnetic recording system involve not only the composition and size of single magnetic domain particles but also a sharp particle size distribution because it plays a very important role in keeping
n
Corresponding author. Tel.: þ91 40 24586779; fax: þ91 40 24340884. E-mail address: [email protected] (S.S. Kalyan Kamal).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.041
the signal to noise ratio well within an acceptable limit. Several physical and chemical routes have been explored in synthesizing magnetic particles with desired qualities [5,6]. Thus far chemical methods being employed for producing Co–Pt NCs include reverse micelle routes employing sodium borohydride reductions [7,8], superhydride reduction [9], thermal decomposition of organometallic compounds in high boiling point liquids such as dioctyl ether [10], modified polyol procedures using 1,2-hexadecanediol and polyethylene glycol (PEG) [11,12], H2 gas (a reductant) in preparing magnetic films [13], hollow magnetic particles using one pot synthesis [14], core-shell NPs [15], and bio-templated routes [16]. Amongst all these methods, a polyol process is considered as one of the simplest and cost effective way for producing magnetic NPs of such alloys. Here, ethylene glycol (EG) molecules used as the reaction medium plays a tripartite role of a solvent, a stabilizer and a model reductant to derive Co–Pt alloys by reducing a mixed salt solution at low temperature. EG efficiently protects nascent metal species and alloy derivatives from hydrolyzing in an adverse reaction during the processing. Nevertheless, a classical polyol process is often too slow so as it prolongs 3–5 h to accomplish itself. In this article, we explore a modified polyol process in the presence of hydrazine with EG in order to produce surface stabilized NCs of Pt-based ferromagnetic alloys of an ordered
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fct–L10 phase. The hydrazine facilitates a controlled reduction reaction from a complex and helps the later (nascent atoms) to grow as an alloy of small NCs quickly, e.g., within 30 min of reaction in a precursor solution at moderate temperature. A sequence of the transient reaction processes used in a rheological medium is shown to affect the crystallite size, shape and their distribution in a sample. It affects the initial phase stability that is used to tailor hard magnetic properties by thermal annealing. The results are described with X-ray diffraction (XRD) and microstructure of a typical alloy of an equiatomic ratio Co0.5Pt0.5 before and after annealing in optimal conditions.
2. Experimental 2.1. Synthesis Analytical grade chemicals dihydrogen platinum hexachloro hydrate H2PtCl6 xH2O, ethylene glycol (EG), cobalt acetate tetrahydrate, hydrazine hydrate, polyvinylpyrrolidone (PVP) of molecular weight 40,000, and ethanol were used to synthesize Co0.5Pt0.5 NCs. The stock H2PtCl6 xH2O and PVP solutions each of 20.0 g/L in EG were used to execute a reaction alloying Co0.5Pt0.5 in two sequences of scheme-1 and scheme-2. In scheme-1, a 50 ml PVP solution was taken in a 250 mL threenecked round bottom flask fitted with a reflux condenser, a mechanical stirrer and a teflon blade. A 25 mL Pt4 þ -solution (supplies as much as 0.2 g platinum) was admixed and heated at 140 1C for 10 min by stirring on a heating mantle. Then, a cobalt–hydrazine complex {0.3 g cobalt (II) acetate tetrahydrate (0.06 M) complexed with 0.3 mL hydrazine hydrate (0.24 M)} in 25 mL EG, was added to supply Co2 þ species. On refluxing the mixture at 190–195 1C for 20 min, a Co2 þ -Co reaction ensues with a change in the initial color (pink) of a cobalt complex to a blackish color. Then, the heating was turned off but the stirring was continued for another 15 min before decanting a Co0.5Pt0.5 colloid. In scheme-2, the cobalt–hydrazine complex was refluxed first at 190–195 1C for 20 min in the reaction vessel followed by admixing the Pt4 þ solutions in the same ratios as in the scheme-1. Finally 50 ml ethanol was added to a colloidal alloy sample so obtained in order to extort it of Co0.5Pt0.5 NCs from byproduct species by using a SmCo5 magnet. Washing a recovered sample repeatedly in ethanol removes the free PVP with any unreacted ionic species. A powdered Co0.5Pt0.5 obtained in the two schemes in this way was dried in an inert atmosphere and annealed in vacuum at different temperatures 600–800 1C for 60 min to tailor grain-growth, phase transformation and hard magnetic properties. 2.2. Measurement and analysis The chemical compositions of the Co0.5Pt0.5 powders were determined using a JY ULTIMA inductively coupled plasma optical emission spectrometer (ICPOES), a carbon and sulfur analyzer (LECO CS-444), and an oxygen and nitrogen analyzer (LECO TC600). To analyze that PVP molecules form an effective surface bonding on the Co0.5Pt0.5 NCs, Fourier transform infrared (FTIR) spectra were studied for the alloy powders in comparison to a PVP sample in KBr pellets by using a Bruker TENSOR 27 instrument. The size and shape of various Co0.5Pt0.5 samples were studied with a transmission electron microscope (TEM) of FEI TECNAI G2. TEM images studied along with selected area electron diffraction (SAED) confer the specific alloy phase. A small drop of a sample dispersed in ethanol was placed onto a specific 3 mm copper grid with a continuous carbon film and dried under vacuum at room
temperature before the measurements. Differential thermal analysis (DTA) was carried out by heating as-prepared alloys over 300–850 1C with a TA-1600 instrument at a 10 1C/min heating rate in pure argon allowed studying phase transformation in the alloys prepared in the two reaction schemes. As-prepared Co0.5Pt0.5 powders were annealed, as per the phase transformation temperatures in the DTA curves, in order to induce a polymorphic Fm3m-fcc-L10 transformation with single magnetic NCs domains. The two phases were analyzed with XRD patterns recorded from the virgin and annealed samples in different conditions by using a Philips-PW3020 X-ray diffractometer with 0.15405 nm wavelength of a CuKa source. Coercivity Hc, remnant Mr/Ms, and saturation magnetization Ms were studied from hysteresis loops measured with a field H varied up to 20 kOe by using a vibrating sample magnetometer (model ADE EV9). A superconducting quantum interference device (SQUID) was used to measure the results especially at low temperature 5 K.
3. Results and discussion 3.1. Formation of polymer coated Co0.5Pt0.5 crystallites In scheme-1, on heating a Pt4 þ precursor solution with PVP molecules at 140 1C, a yellowish sample color turns to dark yellow and then to pale grayish. As soon as adding a cobalt–hydrazine complex, it exchanges the color to a pinkish grey, which turns to a dark blackish color within 20 min of the reaction on refluxing the mixture, ensuring a Co2 þ -Co conversion reaction. A mechanism of such reaction is described elsewhere [17]. In this process, nascent Pt reduced from Pt4 þ in a reaction with PVP molecules in a hot condition recombine with Co atoms from the surrounding and ultimately forms an alloy Co100 xPtx in a metastable fccphase of small NCs. In changing, mixing and reaction sequence of the precursors in scheme-2, a Co2 þ -Co reaction precedes first so as the nascent Co species recombine Pt atoms from a subsequent Pt4 þ -Pt reaction with PVP and hydrazine. As illustrated by a chemical analysis with three different methods in Table 1, an average alloy Co100 xPtx, x¼49.4, is formed in both the reaction schemes. The reported values are an average of three different readings within a standard deviation of 0.2 wt% in the metal parts. A byproduct of 1.9 wt% from C, O and N in scheme-1 is reduced to 1.4 wt% in the scheme-2 in part as a polymer PVP surface layer bonded on the alloy of NCs. Small oxygen residue r0.19 wt% adds no significant oxidation of alloy during the processing. Fig. 1A compares FTIR spectra in (a) a PVP powder and the Co0.5Pt0.5 alloy powders prepared in the (b) scheme-1 and (c) scheme-2. Evidently, the spectra from the alloy samples contain modified IR bands of PVP molecules in chemical bonding on the alloy surfaces of NCs in the form of a surface layer.
Table 1 Chemical composition of the Co–Pt alloy prepared from a reaction of PVP capped Pt4 þ species in EG with cobalt acetate in hydrazine in a liquid precursor Composition (wt%) ICPOES
Scheme-1 Scheme-2
CS-444
TC-600
Average formula
Pt
Co
C
O
N
75.3 (0.2) 74.9 (0.2)
23.2 (0.2) 23.8 (0.2)
1.6 (0.1) 1.1 (0.1)
0.19 (0.02) 0.15 (0.02)
0.14 (0.02) 0.12 (0.02)
The standard deviation in the values is given in parentheses.
Co49.5Pt50.5 Co50.6Pt49.4
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Fig. 1. (A) FTIR spectra of (a) PVP and (b, c) PVP protected Co0.5Pt0.5 NCs obtained from (b) scheme-1 and (c) scheme-2, (B) A deconvolution of the C ¼ O stretching band into two components in the PVP molecules.
A characteristic C¼O stretching band of 1652 cm 1 in virgin PVP molecules has an intensity diminished noticeably over other bands of 1500–500 cm 1. Also, its frequency is decreased by 15 cm 1 in sample (b), or 20 cm 1 in sample (c), in accordance to the C ¼O units coordinate a dangling bond to the Co0.5Pt0.5 via the surface atoms. A stable polymer surface layer thus deposits on a growing Co0.5Pt0.5 so as it stops growing above a critical value as per the experimental conditions. As briefed in Fig. 1B with a deconvolution, two overlapping C¼ O stretching bands are split more visibly in the PVP molecules bonded on a nascent metallic Co0.5Pt0.5 surface. Primary band-1 ascribes to bare C ¼O moieties in PVP molecules and the weaker band-2 displaced at lowered frequency to those bonding to the alloy surfaces. A detail vibrational analysis of other IR bands of PVP polymer molecules is available elsewhere [18,19]. The TEM images in a Co0.5Pt0.5 powder from scheme-1 in Fig. 2a give rectangular plates with 80% NPs of the average DE8.5 nm size in a sharp size distribution, as given in the inset. An SAED included in the right corner contains four rings in the reflections (111), (200), (220) and (311) of 0.2261, 0.1960, 0.1385 and 0.1181 nm interplanar spacings (dhkl) in an Fm3m-fcc lattice, with the lattice parameter a¼0.3916 nm, as observed also in the XRD in Table 2. Small cuboids (D¼10–20 nm) of Co0.5Pt0.5 NCs are self-assembled (Fig. 2b) in nearly spherical shapes, 25–50 nm diameters, in the alloy powder prepared as in the scheme-2. In magnified images in Fig. 2(c, d), rectangular Co0.5Pt0.5 plates are bridging chains (100–200 nm long) via bit thick polymer surface layers while the cuboids self-assemble in lacking off such diverse surface PVP layers. A polymer surface PVP layer grown on an alloy of NCs exhibits an intense peripheral ring of diffraction (radius rps ¼0.4215 nm) around the central spot in SAED in Fig. 2(a, e). A fairly matching rps value 0.4210 nm appears in PVP fibers (amorphous) [20], which exhibits two XRD halos, in a fact that the alloy template PVP molecules to grow over it in topotactic layers. A sample annealed (vacuum) at 300 1C for 1 h thus displays a distinctly grown surface layer (2–3 nm thickness) on round shaped Co0.5Pt0.5 NCs in Fig. 2f. Two endothermic peaks overlapping in a broad band of average peak value at 463 1C arise on heating a Co0.5Pt0.5 powder, processed in the scheme-1, in (a) DTA in Fig. 3A at 10 1C/min in argon. A weak exotherm emerges at 600 1C before a weak endotherm at 700 1C in a reversible fcc-L10 phase transition. Only a modified endotherm lasts at 721 1C (after the NCs are grown-up in the first heating cycle) on (b) reheating a sample heated up to 850 1C in the first run and then cooled down to room temperature. The first part of the endotherm at 451 1C arises
Fig. 2. TEM images in the Co0.5Pt0.5 NCs with a bonded PVP surface overlayer prepared from (a) scheme-1 and (b) scheme-2 with parts of a self-assembled structure of chains (c) and (d) respectively, and (e) SAED from (f) a core–shell structure from (b). The images (c) are displayed in reversed contrast. The insets brief the size distributions.
primarily as a bare PVP decomposes with other polymeric lingers, while a more stable surface PVP layer bonded on the NCs crumbles later in a reasonably weaker endothermic peak displaced at 475 1C. A bonded carbon yield, which often graphitizes and deposits in molecular layers on a metal surface [19,21] upon a carbon linger relieves, burns-up in an exothermic peak as seen at 600 1C in Fig. 3A. Further, the DTA peaks are modified in Fig. 3B as the alloy contains effectively bigger D ¼16 nm Co0.5Pt0.5 NCs in the scheme-2. Relative to the above sample (D ¼8 nm), the L10 phase transition is shifted considerably to higher temperature by 61 1C. Evidently, bigger NCs are more stable thermally with a thinner bonded surface layer.
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Table 2 The values for the lattice parameters, lattice volume VL, lattice surface area SL, and density r in Co0.5Pt0.5 NCs prepared from a liquid precursor Structure/ D-value (nm)
Lattice parameters (nm)
VL (nm3)
DVL (%)
SL (nm2)
SL/VL (nm 1)
DSL (%)
r (g/cm3)
Scheme-1 1. As-prepared 2. 600 1C, 1 h 3. 700 1C, 1 h
fcc, 8 fcc, 15 fct, 26
a ¼0.3916 a ¼0.3819 a ¼0.3799 c ¼0.3688
0.0601 0.0557 0.0532
12.76 4.50 0.19
0.1534 0.1458 0.1401
2.5524 2.6176 2.6334
8.47 3.70 0.21
14.09 15.20 15.91
Scheme-2 4. As-prepared 5. 700 1C, 1 h 6. 800 1C, 1 h
fcc, 16 fcc, 27 fct, 40
a ¼0.3870 a ¼0.3798 a ¼0.3793 c ¼0.3691
0.0580 0.0548 0.0531
8.82 2.81 0.38
0.1497 0.1443 0.1399
2.5810 2.6332 2.6346
6.21 2.70 0.35
14.62 15.45 15.94
Sample
The DVL and DSL describe changes in the VL and SL from the bulk values respectively. Bulk Co0.5Pt0.5: a¼ 0.3800 nm, c¼ 0.3693 nm, c/a¼ 0.9720, VL ¼0.0533 nm3 and SL ¼ 0.1404 nm2.
Fig. 4. XRD patterns (a) before and after annealing Co0.5Pt0.5 prepared from scheme-1 at (b) 600 1C and (c) 700 1C in inducing phases and different crystallite sizes.
Fig. 3. DTA thermograms for the Co0.5Pt0.5 NCs prepared with a bonded PVP surface overlayer from (A) scheme-1 and (B) scheme-2. The curves (b) were measured in reheating the samples after measuring curves (a) at 10 1C/min heating rate in argon.
3.2. Phase transformation in Co0.5Pt0.5 crystallites A phase fcc-L10 reordering is studied in terms of XRD patterns in the Co0.5Pt0.5 powders (prepared in the two schemes) annealed at different temperatures 600–800 1C as per the transitions shown in the DTA thermograms. An as-prepared Co0.5Pt0.5
powder was draped in a titanium foil and sealed in a quartz tube in vacuum so as it picks-up no oxygen during annealing. After 60 min of isothermal heating at the set temperature with 10 1C/ min heating to reach the set point, the sample was quenched in the tube in cold water. A simple XRD pattern arises in Fig. 4 with only four (111), (200), (220) and (311) peaks in a virgin powder (scheme-1) in the 20_901 range of the diffraction angle 2y and those are sharpened and shifted over lower 2y-values in a graingrowth at temperature as high as 600 1C. It briefs an Fm3m-fcc phase of Co0.5Pt0.5 NCs. A characteristic XRD pattern of the fct–L10 phase turns-up on annealing at higher temperature such as 700 1C for 60 min. All the observed peaks describe a single phase with the lattice parameters a¼ 0.3799 nm, c¼0.3688 nm, and density r ¼15.91 g/cm3, i.e., a larger value relative to the bulk value 15.88 g/cm3 known in a similar compound Co0.5Pt0.5 [22–24]. Two separate peaks (200) and (002) are split-up from a single (200) peak in the fcc phase, including two superlattice peaks (001) and (110) allowed in the L10–fct phase. Other details on D-values, calculated from the peak broadening in the XRD peaks, lattice parameters, lattice volume VL, lattice surface area SL, changes in the VL and SL by DVL and DSL over the bulk values, and r-value estimated from the XRD are given in Table 2. A powder Co0.5Pt0.5 prepared in scheme-2 has a similar XRD pattern as in Fig. 4a, but the fct–L10 phase reorders at bit high
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temperature (800 1C) above those in the scheme-1 as compared in Table 2. This difference indicates that Co0.5Pt0.5 alloys differently in the two reaction schemes in terms of effective D-value, selfassembly, and a polymer PVP surface layer on the Co0.5Pt0.5 NCs. The r-value stands reasonably larger, as much as 9.7% in the fcc phase, or 0.2% in the fct–L10 phase, comparing to those after the scheme-1. An aspect c/a ratio 0.971 in the fct–L10 phase after scheme-1 is enhanced to 0.972 on a reasonably improved tetragonality in alloying it via the scheme-2. From Table 2 it could be observed that smaller crystallites in the scheme-1 bear a reasonably larger DSL ¼0.14% (significant lattice distortion) relative to bigger crystallites in the scheme-2.
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Table 3 Magnetic parameters Hc, Ms, and Mr/Ms in Co0.5Pt0.5 prepared in different crystallite sizes from a liquid precursor in two reaction schemes Sample Scheme-1 As-prepared 600 1C, 1 h 700 1C, 1 h Scheme-2 As-prepared 700 1C, 1 h 800 1C, 1 h
D (nm) Hc (kOe) Mh (emu/g) Ms (emu/g) Mr/Ms Phase
8 15 26
0.020 0.831 6.501
5.54 30.35 38.50
6.51 30.90 38.94
0.054 0.467 0.656
Fm3m-fcc Fm3m-fcc fct–L10
16 27 40
0.021 1.110 7.781
9.30 32.95 39.75
9.50 33.74 40.23
0.056 0.465 0.556
Fm3m-fcc Fm3m-fcc fct–L10
The values are reported at room temperature. Mh is reported at 20 kOe field.
3.3. Magnetic properties in Co0.5Pt0.5 crystallites A very narrow hysteresis loop characteristic of a weak ferromagnetic phase arises in the as-prepared Co0.5Pt0.5 powders in both the reaction schemes. As shown in Fig. 5a, the magnetization (M) hardly saturates-up completely at a field as high as 20 kOe used here, showing a maximum value Mh ¼5.54 emu/g at 20 kOe field, Hc ¼20 Oe and Mr/Ms ¼0.054 in powder from scheme-1. Corresponding values from a hysteresis loop on a powder obtained in scheme-2 are compared in Table 3. In this sample, despite any noticeable change in Hc and Mr/Ms values, the Mhvalue is almost doubled to 9.30 emu/g, i.e., very close to an Ms ¼ 9.50 emu/g estimated by extrapolating the curve over H-N. With grain-growth, D ¼16 nm obtained above the first sample of 8 nm, the effective Mh rises-up progressively in a superparamagnetic part (with uncompensated surface spins in small D r8 nm NCs) converts to a ferromagnetic phase in a core– shell structure (Fig. 2f). Large paramagnetic background and lack of saturation are a result of an amorphous alloy present along with a residual carbon. A shell that contains Co and Pt in an alloy possibly bonds carbon species in the form of a surface layer on individual Co0.5Pt0.5 NCs. As shown in Fig. 5b and c, the Mh-value increases more effectively along with Hc and Mr/Ms in a rectangular M-H loop which turns-up on annealing so as the fcc phase grows in to ordered structure before a hard magnetic phase fcc-L10 transforms at 700–800 1C, as per the endothermic peak in DTA (Fig. 3). An Ms-value of 30.90 emu/g (extends over H415 kOe) is obtained with Hc ¼0.831 kOe in annealed sample (D ¼15 nm) from
Fig. 5. M-H curves for Co0.5Pt0.5 prepared in scheme-1 and measured (a) before and after annealing at (b) 600 1C and (c) 700 1C in tailoring hard magnet properties in an ordered L10 phase. Small loop (a) is enlarged in the inset.
Fig. 6. High field M-H loops (measured on a SQUID) in an ordered L10 phase of a hard magnet at (a) 300 K and (b) 5 K after optimally annealing Co0.5Pt0.5 from scheme-1.
scheme-1 at 600 1C for 1 h in the fcc Co0.5Pt0.5 phase. This phase extends up to 700 1C, if prepared in scheme-2, with larger Ms ¼33.74 emu/g and Hc ¼1.110 kOe (D ¼27 nm). Still larger Ms ¼38.94 emu/g (Hc ¼6.501 kOe) evolves on similar sized NCs (D ¼ 26 nm) annealed out from scheme-1 but of a L10 phase. The fcc Co0.5Pt0.5 in this case pertains smaller D r20 nm at temperatures below 650 1C. Mr/Ms ¼ 0.47 in both the fcc alloys. Markedly larger Ms and Hc in Co0.5Pt0.5 NCs of a bit bigger size obtained from scheme-2 confer a thinner surface layer (less magnetic compared to the core) built-up on the NCs. As given in Table 3, both Hc and Mr/Ms are enhanced abruptly in the fct–L10 phase annealed at 700–800 1C for 1 h. A maximum Hc ¼7.781 kOe is achieved with Mr/Ms ¼0.56 and Ms ¼40.23 emu/ g at room temperature in sample (D ¼40 nm) after the reaction scheme-2. Similar Co0.5Pt0.5 NCs (D ¼26 nm) processed in reaction scheme-1 adapt larger Mr/Ms ¼0.66 but smaller Hc ¼6.501 kOe and Ms ¼38.94 emu/g. Such small Co0.5Pt0.5 NCs are ideal to exchange mutual interactions in ideal single domains, promote Mr/Ms Z0.55 in a rectangular M-H loop [25]. In thermally stable values, a sample cooled down to 5 K exhibits no change in Ms except the loop simply widens, Hc ¼9.110 kOe (Mr/Ms ¼0.59) after scheme-1 (see M-H loops at 300 K and 5 K in Fig. 6, with Hr50 kOe), or Hc ¼11.310 kOe (Mr/Ms ¼0.61) after scheme-2 (Table 3). Optimally annealed Co0.5Pt0.5 NCs present larger Hc and Ms from a sample produced by co-reducing cobalt and platinum acetylacetonates in trimethylene followed by annealing in H2/N2
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at 600 1C for 1 h [26]. Hc ¼7.57 kOe, Ms ¼37.50 emu/g and Mr/ Ms ¼0.71 were reported in an oblate shaped M-H loop at room temperature. Another powder co-reduced from acetates of cobalt and platinum by PEG and annealed at 700 1C in argon for 4 h gives an asymmetric loop and inferior Hc 6.0 kOe and Ms ¼36.0 emu/g in lack of a single phase alloy that had formed [12]. Co-reducing acetates by a diol in the presence of PVP (surfactant) gives a single phase Co–Pt alloy but still smaller Hc ¼3.40 kOe. As-prepared alloy is superparamagnetic (Hc ¼0) as thermal energy dominates over the anisotropy energy in small NCs, D ¼5.3 nm. Bimetallic Co–Pt NCs embedded in biomolecules [27,28], or those of a polymer composite [9], often reveal no ferromagnetism at room temperature. Only small Hc r300 Oe develops when coupled with annealing NCs (D 25 nm). Unlike, a bulk ferromagnetic Co–Pt alloy of multidomains, the NCs exist as single magnetic domains with a native surface layer (less magnetic) binding the domains. As a stable pinning barrier it pins down the spins opposite to those in the domain in a self-confined core–shell structure. As a result, such NCs display only part of the bulk Ms ¼ 50.3 emu/g a theoretical value [24] and 54 emu/g found experimentally [29] in a bulk Co0.5Pt0.5 alloy. A high Ms agrees with the presence of a minority phase Co NCs [29]. Obviously, surface modified Co0.5Pt0.5 NCs with a PVP surface stabilizer promptly tailor reasonably superior Hc and Mr/Ms by only a small sacrifice of Ms than ever reported so far. Such large variations have open scope of optimizing functional magnetic properties in such ferromagnetic alloys and composites for diverse applications.
4. Conclusion A simple liquid precursor route is explored for synthesizing a bimetallic Co–Pt alloy of single magnetic domains (NCs) at moderate temperature in ambient air. Two precursor solutions H2PtCl6 xH2O in EG with PVP molecules and cobalt–hydrazine complex are reacted in two sequences (scheme-1 and scheme-2) in alloying Co and Pt co-reduced in this reaction in the form of NCs with a protective surface PVP layer with controlled size and/ or shape. For example, Co0.5Pt0.5 NCs produced by adding a Co2 þ precursor into a Pt4 þ sample capped in PVP followed by refluxing the later at 190–195 1C (scheme-1) has an average D ¼8 nm size. In scheme-2, PVP capped Pt4 þ species are added to the Co2 þ precursor and refluxed at 190–195 1C to give an alloy. Reasonably bigger NCs, D ¼16 nm, form in a faster reaction. Both the schemes yield a single fcc Co0.5Pt0.5 phase with a narrow ferromagnetic loop, Hc 20 Oe and Ms as large as 9.50 emu/g at room temperature. In a range of synthetic conditions, a carbon surface layer bonds on the NCs unless heating above 600 1C. Annealing improves a maximum Ms ¼33.74 emu/g with Hc ¼1.110 kOe in the alloy-2, while Ms ¼30.90 emu/g and Hc ¼0.831 kOe in the alloy-1. An order of smaller Ms lasts in biomimetics or other methods and it hardly gives room temperature ferromagnetism. Only small Hc r300 Oe arises on annealing [9,27,28]. The Hc value increased abruptly along with Mr/Ms in a hard magnetic fct–L10 phase reorders on annealing Co0.5Pt0.5 NCs at temperatures above 700 1C (vacuum). At room temperature, a maximum Hc ¼7.781 kOe aroused with Mr/Ms ¼ 0.56 and Ms ¼40.23 emu/g in an 800 1C annealed alloy-2 (D ¼40 nm). A larger Mr/Ms ¼0.66 but adversely smaller Hc ¼6.501 kOe and Ms ¼38.94 emu/g turn-up in the alloy-1 (D ¼26 nm). Uniquely, these values are highly stable with temperature. As a result, despite the Ms does not increase apparently, the Hc has enhanced by 3% with a final value 11.310 kOe in alloy-2, or Hc ¼9.110 kOe
in alloy-1, on cooling down from 300 K to 5 K. These are superior values than ever reported in such alloys. This confects utility of this simple method of devising surface modified NCs to aid synthesis of single magnetic domains of such alloys or composites with well-defined structure–property relationships.
Acknowledgments This work was supported by the Defence Research and Development Organization (DRDO), Ministry of Defence, Government of India. We extend our thanks to the Director, DMRL, Hyderabad, for his keen interest in this work and also permitting us to publish these results. The authors are also thankful to Dr. M. Vijaya Kumar and Dr. A.K. Singh for their support while carrying out this work. References [1] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989–1992. [2] Y. Kobayashi, H. Kakinuma, D. Nagao, Y. Ando, T. Miyazaki, M. Konno, Journal of Sol–Gel Science and Technology 47 (2008) 16–22. [3] S.S.K. Kamal, P.K. Sahoo, L. Durai, P. Ghosal, S. Ram, M. Raja, Journal of Alloys and Compounds 501 (2010) 297–300. [4] J. Tuaillon-Combes, E. Bernstein, O. Boisron, P. Melinon, Journal of Physical Chemistry C 114 (2011) 13168–13175. [5] F. Tournus, N. Blanc, A. Tamion, M. Hillenkamp, V. Dupuis, Journal of Magnetism and Magnetic Materials 323 (2011) 1868–1872. [6] N. Blanc, F. Tournus, V. Dupuis, T. Epicier, Physical Review B 89 (2011) 092403. [7] H.L. Wang, Y. Zhang, Y. Huang, Q. Zeng, G.C. Hadjipanayis, Journal of Magnetism and Magnetic Materials 272 (2004) e1279–e1280. [8] X. Du, M. Inokuchi, N. Toshima, Journal of Magnetism and Magnetic Materials 299 (2006) 21–28. [9] J. Fang, L.D. Tung, K.L. Stokes, J. He, D. Caruntu, W.L. Zhou, C.J. O’Connor, Journal of Applied Physics 91 (2002) 8816–8818. [10] M. Chen, D.E. Nikles, Journal of Applied Physics 91 (2002) 8477–8479. [11] C. Frommen, H. Rosner, Materials Letters 58 (2003) 123–127. [12] V. Tzitzios, D. Niarchos, G. Margariti, J. Fidler, D. Petridis, Nanotechnology 16 (2005) 287–291. [13] Y. Sui, L. Yue, R. Skomski, X.Z. Li, J. Zhou, D.J. Sellmyer, Journal of Applied Physics 93 (2003) 7571–7573. [14] Y. Vasquez, A.K. Sra, R.E. Schaak, Journal of the American Chemical Society 127 (2005) 12504–12505. [15] J. Dai, Y. Du, P. Yang, Zeitschrift Fur Anorganische Und Allgemeine Chemie 632 (2006) 1108–1111. [16] M.T. Klem, D. Willits, D.J. Solis, A.M. Belcher, M. Young, T. Douglas, Advanced Functional Materials 15 (2005) 1489–1494. [17] S.S.K. Kamal, P.K. Sahoo, M. Premkumar, N.V.R. Rao, T.J. Kumar, B. Sreedhar, A.K. Singh, S. Ram, K.C. Sekhar, Journal of Alloys and Compounds 474 (2009) 214–218. [18] A. Mishra, S. Ram, Journal of Physical Chemistry A 113 (2009) 14067–14073. [19] S. Ram, H.J. Fecht, Journal of Physical Chemistry C 115 (2011) 7817–7828. [20] D.G. Lu, L.D. Gao, K. White, C.B. White, W.Y. Lu, L.M. Zhu, Pharmaceutical Research 27 (2010) 2466. [21] K.K. Bhargav, S. Ram, S.B. Majumder, Materials Express 1 (2011) 210–218. [22] JCPDS-International Center for Diffraction Data (1997); Co0.5Pt0.5, card number 43-1358. [23] W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys Pergammon, New York, in: P. Villars, L.D. Culvert (Eds.), Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, ASM, Materials Park, 1967in: P. Villars, L.D. Culvert (Eds.), Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, ASM, Materials Park, 1991. [24] N.I. Vlasova, G.S. Kandaurova, N.N. Shchegoleva, Journal of Magnetism and Magnetic Materials 222 (2000) 138–158. [25] V. Singh, S. Ram, M. Ranot, Je-G. Park, V. Srinivas, Applied Physics Letters 92 (2008) 253104–253106. [26] C.N. Chinnasamy, B. Jeyadevan, K. Shinoda, K. Tohji, Journal of Applied Physics 93 (2003) 7583–7585. [27] E. Mayes, A. Bewick, D. Gleeson, J. Hoinville, R. Jones, O. Kasyutich, A. Nartowski, B. Warne, J. Wiggins, K.K.W. Wong, IEEE Transactions on Magnetics 39 (2003) 624–627. [28] B.H. An, J.H. Wu, H.L. Liu, S.P. Ko, J.-S. Ju, Y.K. Kim, Engineering Aspects 313 (2008) 250–253. [29] C.S. Yoo, S.K. Lim, C.S. Yoon, C.K. Kim, Journal of Magnetism 8 (2003) 113–117.
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis and surface modified hard magnetic properties in Co0.5Pt0.5 nanocrystallites from a rheological liquid precursor S.S. Kalyan Kamal a,n, P.K. Sahoo a, L. Durai a, P. Ghosal a, M. Manivel Raja a, S. Ram b a b
Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India
a r t i c l e i n f o
abstract
Article history: Received 14 April 2012 Received in revised form 29 May 2012 Available online 30 June 2012
Small crystallites of a metastable phase Co0.5Pt0.5 are precipitated by heating a rheological liquid precursor of cobalt–hydrazine complex and platinum chloride H2PtCl6 xH2O in polymer molecules of poly(vinylpyrrolidone) (PVP) in ethylene glycol. The hydrazine co-reduces nascent atoms from the Co2 þ and Pt4 þ that recombine and grow as Co0.5Pt0.5. The PVP molecules cap a growing Co0.5Pt0.5 as it achieves a critical size so that it stops growing further in given conditions. X-ray diffraction pattern of a recovered powder reveals a crystalline Co0.5Pt0.5 phase (average crystallite size D 8 nm) of a wellknown Fm3m-fcc crystal structure with the lattice parameter a ¼ 0.3916 nm (density r ¼14.09 g/cm3). A more ordered L10 phase (r ¼ 15.91 g/cm3) transforms (D Z 25 nm) upon annealing the powder at temperature lesser than 700 1C (in vacuum). At room temperature, the virgin crystallites bear only a small saturation magnetization Ms ¼ 5.54 emu/g (D¼ 8 nm) of a soft magnet and it hardly grows on bigger sizes (D r 31 nm) in a canted ferromagnetic structure. A rectangular hysteresis loop is markedly expanded on an optimally annealed L10 phase at 800 1C for 60 min, showing a surface modified coercivity Hc ¼ 7.781 kOe with remnant ratio Mr/Ms ¼0.5564, and Ms ¼ 39.75 emu/g. Crystallites selfassembled in an acicular shape tailor large Hc from ideal single domains and high magnetocrystalline anisotropy of a hard magnet L10 phase. & 2012 Elsevier B.V. All rights reserved.
Keywords: Co–Pt Nanoparticle Polyol process Magnetic property Ferromagnetism
1. Introduction Special interest has been laid on synthesis of bimetallic magnetic nanocrystallites (NCs) like Co–Pt, Fe–Pd and Fe–Pt, with effective size (D) varied from 2 to 20 nm, because of their good chemical stability, long range chemical ordering, and high magnetocrystalline anisotropy (Ku) of 4 107 erg/cm3 [1–4]. An ordered crystalline L10 phase of a face centered tetragonal (fct) crystal structure is highly sought in Co100 x Ptx(xZ10) alloys as a candidate material. Markedly hard magnetic properties can be tailored in this alloy by proper nanostructuring in terms of average D-value, shape and surface topology. A high Ku value owned by this phase defers the emergence of superparamagnetic limit well into terabit/in2 and retains a steady magnetization against thermal fluctuations and demagnetizing effects in small NCs down to D ¼10 nm [3,4]. In particular, the parameters which govern a high performance magnetic recording system involve not only the composition and size of single magnetic domain particles but also a sharp particle size distribution because it plays a very important role in keeping
n
Corresponding author. Tel.: þ91 40 24586779; fax: þ91 40 24340884. E-mail address: [email protected] (S.S. Kalyan Kamal).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.041
the signal to noise ratio well within an acceptable limit. Several physical and chemical routes have been explored in synthesizing magnetic particles with desired qualities [5,6]. Thus far chemical methods being employed for producing Co–Pt NCs include reverse micelle routes employing sodium borohydride reductions [7,8], superhydride reduction [9], thermal decomposition of organometallic compounds in high boiling point liquids such as dioctyl ether [10], modified polyol procedures using 1,2-hexadecanediol and polyethylene glycol (PEG) [11,12], H2 gas (a reductant) in preparing magnetic films [13], hollow magnetic particles using one pot synthesis [14], core-shell NPs [15], and bio-templated routes [16]. Amongst all these methods, a polyol process is considered as one of the simplest and cost effective way for producing magnetic NPs of such alloys. Here, ethylene glycol (EG) molecules used as the reaction medium plays a tripartite role of a solvent, a stabilizer and a model reductant to derive Co–Pt alloys by reducing a mixed salt solution at low temperature. EG efficiently protects nascent metal species and alloy derivatives from hydrolyzing in an adverse reaction during the processing. Nevertheless, a classical polyol process is often too slow so as it prolongs 3–5 h to accomplish itself. In this article, we explore a modified polyol process in the presence of hydrazine with EG in order to produce surface stabilized NCs of Pt-based ferromagnetic alloys of an ordered
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fct–L10 phase. The hydrazine facilitates a controlled reduction reaction from a complex and helps the later (nascent atoms) to grow as an alloy of small NCs quickly, e.g., within 30 min of reaction in a precursor solution at moderate temperature. A sequence of the transient reaction processes used in a rheological medium is shown to affect the crystallite size, shape and their distribution in a sample. It affects the initial phase stability that is used to tailor hard magnetic properties by thermal annealing. The results are described with X-ray diffraction (XRD) and microstructure of a typical alloy of an equiatomic ratio Co0.5Pt0.5 before and after annealing in optimal conditions.
2. Experimental 2.1. Synthesis Analytical grade chemicals dihydrogen platinum hexachloro hydrate H2PtCl6 xH2O, ethylene glycol (EG), cobalt acetate tetrahydrate, hydrazine hydrate, polyvinylpyrrolidone (PVP) of molecular weight 40,000, and ethanol were used to synthesize Co0.5Pt0.5 NCs. The stock H2PtCl6 xH2O and PVP solutions each of 20.0 g/L in EG were used to execute a reaction alloying Co0.5Pt0.5 in two sequences of scheme-1 and scheme-2. In scheme-1, a 50 ml PVP solution was taken in a 250 mL threenecked round bottom flask fitted with a reflux condenser, a mechanical stirrer and a teflon blade. A 25 mL Pt4 þ -solution (supplies as much as 0.2 g platinum) was admixed and heated at 140 1C for 10 min by stirring on a heating mantle. Then, a cobalt–hydrazine complex {0.3 g cobalt (II) acetate tetrahydrate (0.06 M) complexed with 0.3 mL hydrazine hydrate (0.24 M)} in 25 mL EG, was added to supply Co2 þ species. On refluxing the mixture at 190–195 1C for 20 min, a Co2 þ -Co reaction ensues with a change in the initial color (pink) of a cobalt complex to a blackish color. Then, the heating was turned off but the stirring was continued for another 15 min before decanting a Co0.5Pt0.5 colloid. In scheme-2, the cobalt–hydrazine complex was refluxed first at 190–195 1C for 20 min in the reaction vessel followed by admixing the Pt4 þ solutions in the same ratios as in the scheme-1. Finally 50 ml ethanol was added to a colloidal alloy sample so obtained in order to extort it of Co0.5Pt0.5 NCs from byproduct species by using a SmCo5 magnet. Washing a recovered sample repeatedly in ethanol removes the free PVP with any unreacted ionic species. A powdered Co0.5Pt0.5 obtained in the two schemes in this way was dried in an inert atmosphere and annealed in vacuum at different temperatures 600–800 1C for 60 min to tailor grain-growth, phase transformation and hard magnetic properties. 2.2. Measurement and analysis The chemical compositions of the Co0.5Pt0.5 powders were determined using a JY ULTIMA inductively coupled plasma optical emission spectrometer (ICPOES), a carbon and sulfur analyzer (LECO CS-444), and an oxygen and nitrogen analyzer (LECO TC600). To analyze that PVP molecules form an effective surface bonding on the Co0.5Pt0.5 NCs, Fourier transform infrared (FTIR) spectra were studied for the alloy powders in comparison to a PVP sample in KBr pellets by using a Bruker TENSOR 27 instrument. The size and shape of various Co0.5Pt0.5 samples were studied with a transmission electron microscope (TEM) of FEI TECNAI G2. TEM images studied along with selected area electron diffraction (SAED) confer the specific alloy phase. A small drop of a sample dispersed in ethanol was placed onto a specific 3 mm copper grid with a continuous carbon film and dried under vacuum at room
temperature before the measurements. Differential thermal analysis (DTA) was carried out by heating as-prepared alloys over 300–850 1C with a TA-1600 instrument at a 10 1C/min heating rate in pure argon allowed studying phase transformation in the alloys prepared in the two reaction schemes. As-prepared Co0.5Pt0.5 powders were annealed, as per the phase transformation temperatures in the DTA curves, in order to induce a polymorphic Fm3m-fcc-L10 transformation with single magnetic NCs domains. The two phases were analyzed with XRD patterns recorded from the virgin and annealed samples in different conditions by using a Philips-PW3020 X-ray diffractometer with 0.15405 nm wavelength of a CuKa source. Coercivity Hc, remnant Mr/Ms, and saturation magnetization Ms were studied from hysteresis loops measured with a field H varied up to 20 kOe by using a vibrating sample magnetometer (model ADE EV9). A superconducting quantum interference device (SQUID) was used to measure the results especially at low temperature 5 K.
3. Results and discussion 3.1. Formation of polymer coated Co0.5Pt0.5 crystallites In scheme-1, on heating a Pt4 þ precursor solution with PVP molecules at 140 1C, a yellowish sample color turns to dark yellow and then to pale grayish. As soon as adding a cobalt–hydrazine complex, it exchanges the color to a pinkish grey, which turns to a dark blackish color within 20 min of the reaction on refluxing the mixture, ensuring a Co2 þ -Co conversion reaction. A mechanism of such reaction is described elsewhere [17]. In this process, nascent Pt reduced from Pt4 þ in a reaction with PVP molecules in a hot condition recombine with Co atoms from the surrounding and ultimately forms an alloy Co100 xPtx in a metastable fccphase of small NCs. In changing, mixing and reaction sequence of the precursors in scheme-2, a Co2 þ -Co reaction precedes first so as the nascent Co species recombine Pt atoms from a subsequent Pt4 þ -Pt reaction with PVP and hydrazine. As illustrated by a chemical analysis with three different methods in Table 1, an average alloy Co100 xPtx, x¼49.4, is formed in both the reaction schemes. The reported values are an average of three different readings within a standard deviation of 0.2 wt% in the metal parts. A byproduct of 1.9 wt% from C, O and N in scheme-1 is reduced to 1.4 wt% in the scheme-2 in part as a polymer PVP surface layer bonded on the alloy of NCs. Small oxygen residue r0.19 wt% adds no significant oxidation of alloy during the processing. Fig. 1A compares FTIR spectra in (a) a PVP powder and the Co0.5Pt0.5 alloy powders prepared in the (b) scheme-1 and (c) scheme-2. Evidently, the spectra from the alloy samples contain modified IR bands of PVP molecules in chemical bonding on the alloy surfaces of NCs in the form of a surface layer.
Table 1 Chemical composition of the Co–Pt alloy prepared from a reaction of PVP capped Pt4 þ species in EG with cobalt acetate in hydrazine in a liquid precursor Composition (wt%) ICPOES
Scheme-1 Scheme-2
CS-444
TC-600
Average formula
Pt
Co
C
O
N
75.3 (0.2) 74.9 (0.2)
23.2 (0.2) 23.8 (0.2)
1.6 (0.1) 1.1 (0.1)
0.19 (0.02) 0.15 (0.02)
0.14 (0.02) 0.12 (0.02)
The standard deviation in the values is given in parentheses.
Co49.5Pt50.5 Co50.6Pt49.4
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Fig. 1. (A) FTIR spectra of (a) PVP and (b, c) PVP protected Co0.5Pt0.5 NCs obtained from (b) scheme-1 and (c) scheme-2, (B) A deconvolution of the C ¼ O stretching band into two components in the PVP molecules.
A characteristic C¼O stretching band of 1652 cm 1 in virgin PVP molecules has an intensity diminished noticeably over other bands of 1500–500 cm 1. Also, its frequency is decreased by 15 cm 1 in sample (b), or 20 cm 1 in sample (c), in accordance to the C ¼O units coordinate a dangling bond to the Co0.5Pt0.5 via the surface atoms. A stable polymer surface layer thus deposits on a growing Co0.5Pt0.5 so as it stops growing above a critical value as per the experimental conditions. As briefed in Fig. 1B with a deconvolution, two overlapping C¼ O stretching bands are split more visibly in the PVP molecules bonded on a nascent metallic Co0.5Pt0.5 surface. Primary band-1 ascribes to bare C ¼O moieties in PVP molecules and the weaker band-2 displaced at lowered frequency to those bonding to the alloy surfaces. A detail vibrational analysis of other IR bands of PVP polymer molecules is available elsewhere [18,19]. The TEM images in a Co0.5Pt0.5 powder from scheme-1 in Fig. 2a give rectangular plates with 80% NPs of the average DE8.5 nm size in a sharp size distribution, as given in the inset. An SAED included in the right corner contains four rings in the reflections (111), (200), (220) and (311) of 0.2261, 0.1960, 0.1385 and 0.1181 nm interplanar spacings (dhkl) in an Fm3m-fcc lattice, with the lattice parameter a¼0.3916 nm, as observed also in the XRD in Table 2. Small cuboids (D¼10–20 nm) of Co0.5Pt0.5 NCs are self-assembled (Fig. 2b) in nearly spherical shapes, 25–50 nm diameters, in the alloy powder prepared as in the scheme-2. In magnified images in Fig. 2(c, d), rectangular Co0.5Pt0.5 plates are bridging chains (100–200 nm long) via bit thick polymer surface layers while the cuboids self-assemble in lacking off such diverse surface PVP layers. A polymer surface PVP layer grown on an alloy of NCs exhibits an intense peripheral ring of diffraction (radius rps ¼0.4215 nm) around the central spot in SAED in Fig. 2(a, e). A fairly matching rps value 0.4210 nm appears in PVP fibers (amorphous) [20], which exhibits two XRD halos, in a fact that the alloy template PVP molecules to grow over it in topotactic layers. A sample annealed (vacuum) at 300 1C for 1 h thus displays a distinctly grown surface layer (2–3 nm thickness) on round shaped Co0.5Pt0.5 NCs in Fig. 2f. Two endothermic peaks overlapping in a broad band of average peak value at 463 1C arise on heating a Co0.5Pt0.5 powder, processed in the scheme-1, in (a) DTA in Fig. 3A at 10 1C/min in argon. A weak exotherm emerges at 600 1C before a weak endotherm at 700 1C in a reversible fcc-L10 phase transition. Only a modified endotherm lasts at 721 1C (after the NCs are grown-up in the first heating cycle) on (b) reheating a sample heated up to 850 1C in the first run and then cooled down to room temperature. The first part of the endotherm at 451 1C arises
Fig. 2. TEM images in the Co0.5Pt0.5 NCs with a bonded PVP surface overlayer prepared from (a) scheme-1 and (b) scheme-2 with parts of a self-assembled structure of chains (c) and (d) respectively, and (e) SAED from (f) a core–shell structure from (b). The images (c) are displayed in reversed contrast. The insets brief the size distributions.
primarily as a bare PVP decomposes with other polymeric lingers, while a more stable surface PVP layer bonded on the NCs crumbles later in a reasonably weaker endothermic peak displaced at 475 1C. A bonded carbon yield, which often graphitizes and deposits in molecular layers on a metal surface [19,21] upon a carbon linger relieves, burns-up in an exothermic peak as seen at 600 1C in Fig. 3A. Further, the DTA peaks are modified in Fig. 3B as the alloy contains effectively bigger D ¼16 nm Co0.5Pt0.5 NCs in the scheme-2. Relative to the above sample (D ¼8 nm), the L10 phase transition is shifted considerably to higher temperature by 61 1C. Evidently, bigger NCs are more stable thermally with a thinner bonded surface layer.
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Table 2 The values for the lattice parameters, lattice volume VL, lattice surface area SL, and density r in Co0.5Pt0.5 NCs prepared from a liquid precursor Structure/ D-value (nm)
Lattice parameters (nm)
VL (nm3)
DVL (%)
SL (nm2)
SL/VL (nm 1)
DSL (%)
r (g/cm3)
Scheme-1 1. As-prepared 2. 600 1C, 1 h 3. 700 1C, 1 h
fcc, 8 fcc, 15 fct, 26
a ¼0.3916 a ¼0.3819 a ¼0.3799 c ¼0.3688
0.0601 0.0557 0.0532
12.76 4.50 0.19
0.1534 0.1458 0.1401
2.5524 2.6176 2.6334
8.47 3.70 0.21
14.09 15.20 15.91
Scheme-2 4. As-prepared 5. 700 1C, 1 h 6. 800 1C, 1 h
fcc, 16 fcc, 27 fct, 40
a ¼0.3870 a ¼0.3798 a ¼0.3793 c ¼0.3691
0.0580 0.0548 0.0531
8.82 2.81 0.38
0.1497 0.1443 0.1399
2.5810 2.6332 2.6346
6.21 2.70 0.35
14.62 15.45 15.94
Sample
The DVL and DSL describe changes in the VL and SL from the bulk values respectively. Bulk Co0.5Pt0.5: a¼ 0.3800 nm, c¼ 0.3693 nm, c/a¼ 0.9720, VL ¼0.0533 nm3 and SL ¼ 0.1404 nm2.
Fig. 4. XRD patterns (a) before and after annealing Co0.5Pt0.5 prepared from scheme-1 at (b) 600 1C and (c) 700 1C in inducing phases and different crystallite sizes.
Fig. 3. DTA thermograms for the Co0.5Pt0.5 NCs prepared with a bonded PVP surface overlayer from (A) scheme-1 and (B) scheme-2. The curves (b) were measured in reheating the samples after measuring curves (a) at 10 1C/min heating rate in argon.
3.2. Phase transformation in Co0.5Pt0.5 crystallites A phase fcc-L10 reordering is studied in terms of XRD patterns in the Co0.5Pt0.5 powders (prepared in the two schemes) annealed at different temperatures 600–800 1C as per the transitions shown in the DTA thermograms. An as-prepared Co0.5Pt0.5
powder was draped in a titanium foil and sealed in a quartz tube in vacuum so as it picks-up no oxygen during annealing. After 60 min of isothermal heating at the set temperature with 10 1C/ min heating to reach the set point, the sample was quenched in the tube in cold water. A simple XRD pattern arises in Fig. 4 with only four (111), (200), (220) and (311) peaks in a virgin powder (scheme-1) in the 20_901 range of the diffraction angle 2y and those are sharpened and shifted over lower 2y-values in a graingrowth at temperature as high as 600 1C. It briefs an Fm3m-fcc phase of Co0.5Pt0.5 NCs. A characteristic XRD pattern of the fct–L10 phase turns-up on annealing at higher temperature such as 700 1C for 60 min. All the observed peaks describe a single phase with the lattice parameters a¼ 0.3799 nm, c¼0.3688 nm, and density r ¼15.91 g/cm3, i.e., a larger value relative to the bulk value 15.88 g/cm3 known in a similar compound Co0.5Pt0.5 [22–24]. Two separate peaks (200) and (002) are split-up from a single (200) peak in the fcc phase, including two superlattice peaks (001) and (110) allowed in the L10–fct phase. Other details on D-values, calculated from the peak broadening in the XRD peaks, lattice parameters, lattice volume VL, lattice surface area SL, changes in the VL and SL by DVL and DSL over the bulk values, and r-value estimated from the XRD are given in Table 2. A powder Co0.5Pt0.5 prepared in scheme-2 has a similar XRD pattern as in Fig. 4a, but the fct–L10 phase reorders at bit high
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temperature (800 1C) above those in the scheme-1 as compared in Table 2. This difference indicates that Co0.5Pt0.5 alloys differently in the two reaction schemes in terms of effective D-value, selfassembly, and a polymer PVP surface layer on the Co0.5Pt0.5 NCs. The r-value stands reasonably larger, as much as 9.7% in the fcc phase, or 0.2% in the fct–L10 phase, comparing to those after the scheme-1. An aspect c/a ratio 0.971 in the fct–L10 phase after scheme-1 is enhanced to 0.972 on a reasonably improved tetragonality in alloying it via the scheme-2. From Table 2 it could be observed that smaller crystallites in the scheme-1 bear a reasonably larger DSL ¼0.14% (significant lattice distortion) relative to bigger crystallites in the scheme-2.
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Table 3 Magnetic parameters Hc, Ms, and Mr/Ms in Co0.5Pt0.5 prepared in different crystallite sizes from a liquid precursor in two reaction schemes Sample Scheme-1 As-prepared 600 1C, 1 h 700 1C, 1 h Scheme-2 As-prepared 700 1C, 1 h 800 1C, 1 h
D (nm) Hc (kOe) Mh (emu/g) Ms (emu/g) Mr/Ms Phase
8 15 26
0.020 0.831 6.501
5.54 30.35 38.50
6.51 30.90 38.94
0.054 0.467 0.656
Fm3m-fcc Fm3m-fcc fct–L10
16 27 40
0.021 1.110 7.781
9.30 32.95 39.75
9.50 33.74 40.23
0.056 0.465 0.556
Fm3m-fcc Fm3m-fcc fct–L10
The values are reported at room temperature. Mh is reported at 20 kOe field.
3.3. Magnetic properties in Co0.5Pt0.5 crystallites A very narrow hysteresis loop characteristic of a weak ferromagnetic phase arises in the as-prepared Co0.5Pt0.5 powders in both the reaction schemes. As shown in Fig. 5a, the magnetization (M) hardly saturates-up completely at a field as high as 20 kOe used here, showing a maximum value Mh ¼5.54 emu/g at 20 kOe field, Hc ¼20 Oe and Mr/Ms ¼0.054 in powder from scheme-1. Corresponding values from a hysteresis loop on a powder obtained in scheme-2 are compared in Table 3. In this sample, despite any noticeable change in Hc and Mr/Ms values, the Mhvalue is almost doubled to 9.30 emu/g, i.e., very close to an Ms ¼ 9.50 emu/g estimated by extrapolating the curve over H-N. With grain-growth, D ¼16 nm obtained above the first sample of 8 nm, the effective Mh rises-up progressively in a superparamagnetic part (with uncompensated surface spins in small D r8 nm NCs) converts to a ferromagnetic phase in a core– shell structure (Fig. 2f). Large paramagnetic background and lack of saturation are a result of an amorphous alloy present along with a residual carbon. A shell that contains Co and Pt in an alloy possibly bonds carbon species in the form of a surface layer on individual Co0.5Pt0.5 NCs. As shown in Fig. 5b and c, the Mh-value increases more effectively along with Hc and Mr/Ms in a rectangular M-H loop which turns-up on annealing so as the fcc phase grows in to ordered structure before a hard magnetic phase fcc-L10 transforms at 700–800 1C, as per the endothermic peak in DTA (Fig. 3). An Ms-value of 30.90 emu/g (extends over H415 kOe) is obtained with Hc ¼0.831 kOe in annealed sample (D ¼15 nm) from
Fig. 5. M-H curves for Co0.5Pt0.5 prepared in scheme-1 and measured (a) before and after annealing at (b) 600 1C and (c) 700 1C in tailoring hard magnet properties in an ordered L10 phase. Small loop (a) is enlarged in the inset.
Fig. 6. High field M-H loops (measured on a SQUID) in an ordered L10 phase of a hard magnet at (a) 300 K and (b) 5 K after optimally annealing Co0.5Pt0.5 from scheme-1.
scheme-1 at 600 1C for 1 h in the fcc Co0.5Pt0.5 phase. This phase extends up to 700 1C, if prepared in scheme-2, with larger Ms ¼33.74 emu/g and Hc ¼1.110 kOe (D ¼27 nm). Still larger Ms ¼38.94 emu/g (Hc ¼6.501 kOe) evolves on similar sized NCs (D ¼ 26 nm) annealed out from scheme-1 but of a L10 phase. The fcc Co0.5Pt0.5 in this case pertains smaller D r20 nm at temperatures below 650 1C. Mr/Ms ¼ 0.47 in both the fcc alloys. Markedly larger Ms and Hc in Co0.5Pt0.5 NCs of a bit bigger size obtained from scheme-2 confer a thinner surface layer (less magnetic compared to the core) built-up on the NCs. As given in Table 3, both Hc and Mr/Ms are enhanced abruptly in the fct–L10 phase annealed at 700–800 1C for 1 h. A maximum Hc ¼7.781 kOe is achieved with Mr/Ms ¼0.56 and Ms ¼40.23 emu/ g at room temperature in sample (D ¼40 nm) after the reaction scheme-2. Similar Co0.5Pt0.5 NCs (D ¼26 nm) processed in reaction scheme-1 adapt larger Mr/Ms ¼0.66 but smaller Hc ¼6.501 kOe and Ms ¼38.94 emu/g. Such small Co0.5Pt0.5 NCs are ideal to exchange mutual interactions in ideal single domains, promote Mr/Ms Z0.55 in a rectangular M-H loop [25]. In thermally stable values, a sample cooled down to 5 K exhibits no change in Ms except the loop simply widens, Hc ¼9.110 kOe (Mr/Ms ¼0.59) after scheme-1 (see M-H loops at 300 K and 5 K in Fig. 6, with Hr50 kOe), or Hc ¼11.310 kOe (Mr/Ms ¼0.61) after scheme-2 (Table 3). Optimally annealed Co0.5Pt0.5 NCs present larger Hc and Ms from a sample produced by co-reducing cobalt and platinum acetylacetonates in trimethylene followed by annealing in H2/N2
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at 600 1C for 1 h [26]. Hc ¼7.57 kOe, Ms ¼37.50 emu/g and Mr/ Ms ¼0.71 were reported in an oblate shaped M-H loop at room temperature. Another powder co-reduced from acetates of cobalt and platinum by PEG and annealed at 700 1C in argon for 4 h gives an asymmetric loop and inferior Hc 6.0 kOe and Ms ¼36.0 emu/g in lack of a single phase alloy that had formed [12]. Co-reducing acetates by a diol in the presence of PVP (surfactant) gives a single phase Co–Pt alloy but still smaller Hc ¼3.40 kOe. As-prepared alloy is superparamagnetic (Hc ¼0) as thermal energy dominates over the anisotropy energy in small NCs, D ¼5.3 nm. Bimetallic Co–Pt NCs embedded in biomolecules [27,28], or those of a polymer composite [9], often reveal no ferromagnetism at room temperature. Only small Hc r300 Oe develops when coupled with annealing NCs (D 25 nm). Unlike, a bulk ferromagnetic Co–Pt alloy of multidomains, the NCs exist as single magnetic domains with a native surface layer (less magnetic) binding the domains. As a stable pinning barrier it pins down the spins opposite to those in the domain in a self-confined core–shell structure. As a result, such NCs display only part of the bulk Ms ¼ 50.3 emu/g a theoretical value [24] and 54 emu/g found experimentally [29] in a bulk Co0.5Pt0.5 alloy. A high Ms agrees with the presence of a minority phase Co NCs [29]. Obviously, surface modified Co0.5Pt0.5 NCs with a PVP surface stabilizer promptly tailor reasonably superior Hc and Mr/Ms by only a small sacrifice of Ms than ever reported so far. Such large variations have open scope of optimizing functional magnetic properties in such ferromagnetic alloys and composites for diverse applications.
4. Conclusion A simple liquid precursor route is explored for synthesizing a bimetallic Co–Pt alloy of single magnetic domains (NCs) at moderate temperature in ambient air. Two precursor solutions H2PtCl6 xH2O in EG with PVP molecules and cobalt–hydrazine complex are reacted in two sequences (scheme-1 and scheme-2) in alloying Co and Pt co-reduced in this reaction in the form of NCs with a protective surface PVP layer with controlled size and/ or shape. For example, Co0.5Pt0.5 NCs produced by adding a Co2 þ precursor into a Pt4 þ sample capped in PVP followed by refluxing the later at 190–195 1C (scheme-1) has an average D ¼8 nm size. In scheme-2, PVP capped Pt4 þ species are added to the Co2 þ precursor and refluxed at 190–195 1C to give an alloy. Reasonably bigger NCs, D ¼16 nm, form in a faster reaction. Both the schemes yield a single fcc Co0.5Pt0.5 phase with a narrow ferromagnetic loop, Hc 20 Oe and Ms as large as 9.50 emu/g at room temperature. In a range of synthetic conditions, a carbon surface layer bonds on the NCs unless heating above 600 1C. Annealing improves a maximum Ms ¼33.74 emu/g with Hc ¼1.110 kOe in the alloy-2, while Ms ¼30.90 emu/g and Hc ¼0.831 kOe in the alloy-1. An order of smaller Ms lasts in biomimetics or other methods and it hardly gives room temperature ferromagnetism. Only small Hc r300 Oe arises on annealing [9,27,28]. The Hc value increased abruptly along with Mr/Ms in a hard magnetic fct–L10 phase reorders on annealing Co0.5Pt0.5 NCs at temperatures above 700 1C (vacuum). At room temperature, a maximum Hc ¼7.781 kOe aroused with Mr/Ms ¼ 0.56 and Ms ¼40.23 emu/g in an 800 1C annealed alloy-2 (D ¼40 nm). A larger Mr/Ms ¼0.66 but adversely smaller Hc ¼6.501 kOe and Ms ¼38.94 emu/g turn-up in the alloy-1 (D ¼26 nm). Uniquely, these values are highly stable with temperature. As a result, despite the Ms does not increase apparently, the Hc has enhanced by 3% with a final value 11.310 kOe in alloy-2, or Hc ¼9.110 kOe
in alloy-1, on cooling down from 300 K to 5 K. These are superior values than ever reported in such alloys. This confects utility of this simple method of devising surface modified NCs to aid synthesis of single magnetic domains of such alloys or composites with well-defined structure–property relationships.
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