J Nanopart Res (2010) 12:1107–1116 DOI 10.1007/s11051-009-9639-5
RESEARCH PAPER
Synthesis and magnetic characterization of CoMoN2 nanoparticles Sayan Bhattacharyya Æ Sajith Kurian Æ S. M. Shivaprasad Æ N. S. Gajbhiye
Received: 15 September 2008 / Accepted: 10 April 2009 / Published online: 1 May 2009 Ó Springer Science+Business Media B.V. 2009
Abstract A new ternary nitride, CoMoN2, was prepared in the nanosize regime of 9.0 ± 2.0 nm, by nitridation of the precursor intermetallic nitride Co3Mo3N. XRD–Rietveld analysis revealed the presence of 0.60 (±0.02) mass % of Co impurity phase. The calculated space groups of CoMoN2 and Co are P63/mmc and Fm-3m, respectively. The N atoms lie at the interstitial sites and the 12 calculated nitrogen sites indicate the presence of a layered structure. The XPS studies indicated the presence of the nitride and surface oxynitride/oxide phases. CoMoN2 is an interstitial nitride with Co and Mo in the zero oxidation state. The room temperature susceptibility is estimated after subtracting the ferromagnetic contribution from cobalt and found to be 2.7 9 10-4 emu g-1 Oe-1, indicating the Pauli-paramagnetic nature. The ferromagnetic exchange interactions between the Co atoms in CoMoN2 are reduced due to the presence of Mo and N in the crystal lattice. The hysteresis loop shift 19 Oe is attributed to the demagnetizing dipolar fields created in the soft CoMoN2 phase by the hard Co phase.
S. Bhattacharyya S. Kurian N. S. Gajbhiye (&) Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, UP, India e-mail:
[email protected] S. M. Shivaprasad Surface Physics & Nanostructures Group, National Physical Laboratory, New Delhi 110012, India
Keywords Nitride Nanoparticle Magnetism
Introduction The ternary metal nitrides represent a rich class of fascinating compounds, because of their potential technological utilities (Oyama 1996). The metal nitrides have a wide range of applications due to distinctive properties such as strong hardness, chemical resistance to corrosion, high melting point, high electrical and thermal conductivities, and unique electronic properties (Chen et al. 2009; Li et al. 2009; Luo et al. 2009). In the recent years, there has been a lot of reports on ternary nitrides in the form of thin films, multilayers, single crystals, polycrystalline, and nanocrystalline powders (Arranz and Palacio 2008; Chevire´ and DiSalvo 2008; Dolique et al. 2008; Du et al. 2008; Ho¨glund et al. 2008; Krawiec et al. 2008; Vomiero et al. 2007). However, nitrides are relatively less stable as compared to oxides because of the low decomposition temperature of nitrides owing to the high bond energy of nitrogen (941 kJ mole-1). In the last decade, extensive research has been carried out on the intermetallic ternary transition metal nitrides like Fe3Mo3N, Co3Mo3N, and Ni3Mo3N (Bem et al. 1993; Jackson et al. 1999). These nitrides relate to the g-carbide type structure with general formula
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MxM0 12-xNy (M = group 4, 5, or 6 transition element, M0 = group 7 or 8 transition element). New layered ternary transition metal nitrides with the general formula MM0 N2 (M = Fe, Mn, Fe0.8Mo0.2; M0 = Mo, W) were also synthesized by ammonolysis of transition metal oxides and explored for their structural, electrical, and magnetic properties (Bem and zur Loye 1993; Bem et al. 1995, 1996; Panda and Gajbhiye 1998). The high temperature techniques have limited success in the preparation of ternary nitrides and hence low temperature routes are more suitable for synthesizing the metastable and thermodynamically stable phases. The low temperature routes provide a much better surface area for the catalytic activities of these nitride nanomaterials. In this article, nanoparticles of a new ternary nitride CoMoN2 are prepared, and the structure and morphology are studied using XRD–Rietveld analysis, electron microscopy techniques, and X-ray photoelectron spectroscopy (XPS). The room temperature magnetic properties are discussed.
Experimental details The synthesis of ternary transition metal nitrides using transition metal molybdate precursors has been reported in the literature (Bem et al. 1993; Jackson et al. 1999). The preparation of CoMoN2 involves the synthesis of two successive precursor compounds. Preparation of CoMoO4 Ammonium molybdate, (NH4)2MoO4, was prepared by adding 25 mL of aqueous ammonia solution to 25 mL solution of 0.1 (M) ammonium heptamolybdate (NH4)6Mo7O24 4H2O (Aldrich 99.9%). The solution was boiled to remove the excess of ammonia. Cobalt molybdate, CoMoO4, was prepared by dropwise addition of an aqueous solution of cobalt nitrate, Co(NO3)2 6H2O (Merck 99.9%), to this boiling stirred solution for a period of 10–15 min. A purple precipitate immediately formed and the suspension was stirred for 1 h. The purple solid product was filtered, washed with water and methanol, and dried in air at 423 K overnight. The solid powder was XRD amorphous. The amorphous powder was heat treated at 923 K in air to give crystalline CoMoO4 (JCPDS 21-0868).
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Preparation of Co3Mo3N In NH3 (g) atmosphere, 0.5 g of CoMoO4 was heat treated at 1,023 K for 12 h (flow rate: 150 cm3 min-1) at a heating rate of 5 K min-1. At the completion of the nitridation process, the sample was cooled rapidly down to 723 K at 25 K min-1 and to room temperature at 10 K min-1 to obtain Co3Mo3N nanoparticles. Preparation of CoMoN2 Approximately 0.5 g of Co3Mo3N was heat treated in NH3 (g) (200 cm3 min-1) at 673 K for 1 h, followed by normal cooling to obtain CoMoN2 as the major phase in addition to a minority phase of metallic cobalt. CoMoN2 is not formed below 648 K and at 648–673 K, a mixture of Co3Mo3N, CoMoN2, Co-N, and Co phases were obtained. On increasing the nitridation temperature above 673 K, the concentration of the Co impurity phase also found to increase. The flow rate of NH3 (g) was maintained at an optimum 200 cm3 min-1, below which the nitridation process is incomplete, and results in the formation of oxynitrides. Again, the nitridation time \1 h leads to amorphous or semi-crystalline CoMoN2 phase formation, as monitored with control experiments for 30 and 45 min. With increase in nitridation time above 1 h gives more Co precipitate (evidenced from X-ray diffraction (XRD) patterns and magnetization values). The nitrogen content was determined with CHN elemental analyzer (Perkin-Elmer 240c) and Kjeldahl method (Vogel 1978). The cobalt and molybdenum content was quantitatively analyzed by inductively coupled plasma atomic emission spectroscopy (ICPAES; Spectroflame Module E). The structure of CoMoN2 and Co phases was examined by powder XRD (Rich Seifert Isodebyflex X-ray unit model 2002) using CuKa radiation and Ni filter, and the phases were analyzed using XRD–Rietveld refinement. The Rietveld refinement software used was DBWS-9006PC, which has a plot view program (version 3.41 beta) for Rietveld refinement method. The morphology was examined using scanning electron microscopy (SEM: JEOL JSM-840A) operated at 15.0 kV, atomic force microscopy (AFM: Molecular Imaging USA with model PICOSPM), transmission electron microscopy (TEM: Phillips Model E-301), and high resolution TEM (HRTEM, JEOL, 2010). The
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surface analysis was performed using XPS (PerkinElmer-1257) with AlKa (1486.6 eV) and a hemispherical section analyzer using a monochromatized source with *370 meV resolution. The room temperature magnetic measurements were performed using a vibrating sample magnetometer (Par-Model 150A) with maximum magnetic field of 11 kOe.
Results and discussion Structural characterization Figure 1 shows the XRD patterns of the precursor Co3Mo3N and the resultant product CoMoN2. The XRD pattern of Co3Mo3N was indexed according to a previous report (Jackson et al. 1999). Co3Mo3N crystallizes in cubic g-Fe3W3C structure, lattice ´˚ parameter, a = 11.026 A . The XRD peaks are broad and indicate the nanocrystalline nature of Co3Mo3N having crystallite size of 30 nm, calculated using the Scherrer’s formula. Upon nitridation at 673 K for 1 h, the Co3Mo3N phase changes to CoMoN2 phase having similarity to hexagonal crystal structure. The crystallite size obtained from the line broadening of the 100% peak is 8 nm. However, Scherrer’s equation does not distinguish between size and crystallinity and rather
Fig. 1 XRD patterns of the precursor Co3Mo3N and the product CoMoN2. The asterick shows the Co phase
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assumes that the peak broadening is entirely due to size effects. In this low temperature synthesis, the polycrystalline nature of the samples affects the particle size calculation, since at 673 K there might be crystallites of varying size with an additional unavoidable amorphous content that cannot be determined with XRD analysis. Rietveld refinement of the XRD pattern was performed with many possibilities and the best refinement was obtained with a mixture of CoMoN2 and Co phases. The refinement is carried out taking four unit cells into consideration. The XRD pattern and Data Master Plot (DMPLOT) are shown in Fig. 2 and the data obtained from XRD–Rietveld refinement is given in Table 1. The determined parameters such as lattice parameters, interaxial angles, atomic positions, and occupation number of the lattice sites are given in Table 2. The standard deviations (SD) for the atomic positions varied from 0.00 to 0.05. The space groups generated by the XRD–Rietveld refinement are P63/mmc for CoMoN2 and Fm-3m for Co phase. The relative mass percentages of CoMoN2 and Co phases are 99.40 (±1.46) and 0.60 (±0.02) and the molar percentages are 93.08 (±0.00) and 6.92 (±0.00), respectively. The lattice parameters of ˚ and c = 5.409 A ˚ and the CoMoN2 are a = 5.623 A interaxial angles are a = b = 90° and c = 120°. For the minority Cobalt phase, the lattice parameter ˚ is slightly lower than the reported value a = 3.535 A
Fig. 2 XRD–Rietveld analysis shows the calculated (solid line) and observed (dots) XRD patterns (upper), the allowed Bragg’s reflections (tick mark in middle; upper row: CoMoN2 and lower row: Co) and difference of calculated and observed XRD patterns (lower)
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Table 1 Observed/recorded XRD parameters using the space group P63/mmc for CoMoN2 and Fm-3m for Co phases Calculated data (2h)
Observed data (2h)
CoMoN2
18.113
–
10
010
24.570
–
3
011
31.689
–
7
110
33.179
33.083
429
002
36.821
36.821
999
020
37.980 40.464
– –
0 7
012 021
46.424
–
0
112
49.238
–
0
120
50.397
50.397
52.219
Co
Relative intensity (I/I0)
Miller indices (hkl)
Phases
749
022
–
0
121
54.205
–
0
013
56.523
56.523
1
030
59.172
–
0
031
60.662
–
0
122
64.139
–
1
023
66.126
66.126
289
220
67.119
–
0
032
69.272
–
0
130
69.437
69.106
71.755
13
004
–
0
131
72.583 73.245
– –
0 0
014 123
75.894
75.894
224
222
78.212
78.046
110
040
78.377
–
0
114
78.874
–
0
132
79.205
–
0
033
80.530
–
0
041
81.358
81.026
86.656
–
87.649
87.026
88.974
–
89.801
–
44.437
44.603
44
111
51.722
–
21
002
70.060
–
11
022
74
024
0
230
138
042
0
231
0
124
˚ [JCPDS 15-0806]. This might occur due of 3.545 A to the strain factor involved from the coexistence of two phases. For Co, the interaxial angles are
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a = b = c = 90° and possess a face-centered cubic (fcc) crystal structure. This is in contrast to the widespread notion of a hexagonal crystal system for cobalt with space group P63/mmc [JCPDS 89-4308]. The unit cell volume of CoMoN2 and Co phases are ˚ 3 and 44.19 (±0.00) A ˚ 3, respec148.57 (±0.04) A tively. CoMoN2 has a hexagonal WC-type crystal structure. The observed XRD peaks are broader than the calculated peaks because of the nanostructured nature of the material. The N atoms lie at the interstitial sites, and the 12 calculated nitrogen sites indicate the presence of a layered structure. This structure is similar to the other two layered ternary nitrides FeWN2 and MnWN2 (Bem et al. 1996). The structure consists of alternating layers of edge-shared CoN6 octahedra and MoN6 trigonal prisms. The XRD pattern of CoMoN2 bears close structural resemblance to d-MoN [JCPDS 25-1367]. Recently, dMoN was prepared in a similar way by nitridation of a nitride precursor c-Mo2N at 573 K (Gomathi et al. 2007). The composition of the product was counter checked with ICP analysis which shows Co:Mo ratio = 1.02:1, and amounts to 0.8 mass % of Co minority phase. This is close to 0.6% Co, as determined by Rietveld analysis. Based on the nitrogen content as determined by Kjeldahl technique, the stoichiometry of the product is CoMoN2.02. The surface area measured from BET surface area measurement (Micrometrics Pulse Chemisorb 2700) is 12.0 m2 g-1. He gas was used as the carrier and N2 as the adsorbate. The particle size of CoMoN2 (=45 nm) was determined using the BET surface area using the relation, R = 3/qS, where r = radius of the CoMoN2 particles, q = density, and S = BET surface area. The FTIR spectrum shows a band at 650–850 cm-1 and indicates the M–N stretching frequency (where M = Co, Mo). The M–N stretching vibration appears in the low frequency region because of the relatively heavy mass of the metal and the low bond order of the M–N bond in CoMoN2. Morphological studies The SEM image (Fig. 3) shows that the particles are in the nano-size regime and spherical in shape. The particle diameter is 13–30 nm and the particles are found to stick to each other. The *100 nm white spots appear from the agglomerated nanoparticles at different heights with respect to the focus of the
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1111
Table 2 Rietveld refinement parameters for the XRD pattern using the space groups P63/mmc and Fm-3m for CoMoN2 and Co
phases, respectively, are calculated. R-p = 16.4, R-expected = 14.06, and Goodness of fit = 1.59
Phases
˚) Lattice parameters (A
Angles (degrees)
CoMoN2
a = b = 5.623(4), c = 5.409(5)
a = b = 90.0(0), c = 120.0(2)
Co
a = 3.535
a = b = c = 90.0
Atomic positions (x, y, z)
Occupation number
Co1 (0.0, 0.0, 0.21)
Co1 = 1.0
Co2 (0.50, 1.00, 0.21)
Co2 = 3.0
Mo1 (0.0, 0.0, 0.21)
Mo1 = 1.0
Mo2 (0.50, 1.00, 0.21)
Mo2 = 3.0
N1 (0.16, 0.33, 0.41)
N1 = 0.67
N2 (-0.33, -0.16, 0.41)
N2 = 0.67
N3 (0.16, -0.16, 0.41)
N3 = 0.67
N4 (-0.16, -0.33, 0.82)
N4 = 0.67
N5 (0.33, 0.16, 0.82)
N5 = 0.67
N6 (0.16, -0.16, 0.82)
N6 = 0.67
N7 (-0.16, -0.33, -0.41)
N7 = 0.67
N8 (0.33, 0.16, -0.41)
N8 = 0.67
N9 (-0.16, 0.16, -0.41)
N9 = 0.67
N10 (0.16, 0.33, -0.82)
N10 = 0.67
N11 (-0.33, -0.16, -0.82) N12 (-0.16, 0.16, -0.82)
N11 = 0.67 N12 = 0.67
Co1 (0.0, 0.0, 0.0)
Co1 = 0.3
is *30 nm. The bright field TEM image (Fig. 5a) shows the spherical particle diameter D = 9.0 ± 2.0 nm (Fig. 5b). The nanoparticles stay one over the other in contact with each other. The inter-layer spacing of the lattice planes of CoMoN2 is observed to be 0.244 nm and is shown with arrows in Fig. 5a (inset). The different techniques afford different particle sizes due to the agglomeration of the nanoparticles and this is typical in the case of magnetic nanoparticles (Gajbhiye and Bhattacharyya 2006). XPS studies
Fig. 3 SEM image of CoMoN2 nanoparticles
electron beam. The 2D AFM image (Fig. 4a) shows a wide area of 4,000 nm 9 4,000 nm, where the particles are found to be well dispersed. The nanoparticles of CoMoN2 are clearly visible in the 3D AFM image (Fig. 4b) and hence corroborate the SEM image. In the AFM image, the particles are observed in the range 30–35 nm. The height of the nanoparticles measured with AFM section analysis software
The surface compositional analysis was performed with XPS measurement and the XPS spectra of the Co 2p3/2, Mo 3d, N 1s, and O 1s levels are shown in Fig. 6. The XPS parameters are presented in Table 3. The sample powder was analyzed by mounting them on a double-sided adhesive tape. The constant charging of the sample was removed by referencing all the energies with C 1s (284.6 eV). The Co 2p3/2 spectrum is deconvoluted into two peaks at binding energy (BE) values of 778.3 and 781.9 eV which corresponds to Co0 and Co–O–N/Co–O, respectively (Parola et al. 2002; Ye et al. 2005). The Co0 state is
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Fig. 5 Bright field TEM image of CoMoN2 nanoparticles. (Inset) HRTEM image
Fig. 4 a 2D and b 3D AFM images of CoMoN2 nanoparticles
due to the cobalt in the interstitial nitride CoMoN2 and also the additional cobalt phase. The relative percentages of Co0 and Co–O–N/Co–O states are 45% and 55%, respectively. The triplet in Mo 3d spectrum is deconvoluted to three peaks at 227.6, 230.4, and 233.1 eV and corresponds to Mo0, Mo4?, and Mo6? states, respectively (Parola et al. 2002). The Mo0 state represents the molybdenum in CoMoN2, whereas the Mo4? and Mo6? states are due to the Mo–O–N/Mo–O states. In this case, the Mod? state at 228.4 eV for Mo–N is not observed and hence any chance of ionic character in the Mo–N
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bond is ruled out (Kim et al. 1999). In the N 1s spectrum, the two peaks at 396.9 and 398.0 eV of relative intensities 40% and 60% correspond to nitride and oxynitride phases, respectively. The O 1s spectrum is fitted with a single peak centered at 530.5 eV and FWHM 3.1 eV, and has contribution from both oxide and oxynitride surface layers. The oxide and oxynitride states are not separately deconvoluted since the exact FWHM of the oxynitride phase is not certain. Hence, the XPS studies provide evidence of an oxynitride/oxide layer on the surface of CoMoN2 nanoparticles. Also, both Co and Mo are in the zero oxidation state in the nitride material, which confirms CoMoN2 to be an interstitial ternary nitride. The Co0 and Mo0 states, apart from being
J Nanopart Res (2010) 12:1107–1116
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Fig. 6 XPS spectra showing a Co 2p3/2, b Mo 3d, c N 1s, and d O 1s levels
Table 3 Deconvoluted XPS Profile Parameters for Co 2p3/2, Mo 3d, N 1s, and O 1s levels Level
State
Co 2p3/2
Co, nitride
778.3
3.2
45
Mo 3d
Oxynitrides Mo0
781.9 227.6
7.5 2.0
55 25
Mo4?
230.4
2.1
45
Mo6?
233.1
2.4
30
Nitride
396.9
2.9
40
Oxynitrides
398.0
6.4
60
Oxynitride
530.5
3.1
100
N 1s O 1s
BE (eV)
FWHM (eV)
Relative area %
representative of the bulk CoMoN2 phase, might have a contribution from the non-oxidized portions of the surface layer as well. Magnetic properties The observed magnetic properties of CoMoN2 are expected to be largely influenced by 0.60 (±0.02) mass % of ferromagnetic cobalt. Figure 7 (inset) presents the temperature-dependent magnetic susceptibility (v-1), measured up to 634 K with an applied
field of 5 kOe. The increase of susceptibility values with decreasing temperature, v294 K/v634 K = 1.25 is quite small. The true susceptibility of CoMoN2 is estimated in the high field region (5–11 kOe) from the relation: Total Magnetization (r) = Paramagnetic contribution (rp) ? ferromagnetic contribution (rf): r = v0 H ? (rf0 cf) where v0 = paramagnetic susceptibility of CoMoN2 and hence r/H = v0 ? rf/H; rf = rf0 cf, rf0 = saturation magnetization of Co impurity, cf = fraction of Co impurity phase (Navarro et al. 2001; Panda and Gajbhiye 1998). The r/H versus 1/H is plotted and extrapolation of the resulting straight line to the infinite field (1/H = 0) gives v0 , while the slope of the line represents rf (Fig. 7). The room temperature magnetic susceptibility (v0 ) of CoMoN2 is found to be 2.7 9 10-4 emu g-1 Oe-1, indicating the Pauli-paramagnetic nature. The ferromagnetic contribution (rf) is estimated to be 31.4 emu g-1. Thus, based on the 0.60 (±0.02) mass % of the Co phase, the saturation magnetization of additional Co impurity phase is calculated to be 52.3 emu g-1. This value is lower than the bulk saturation magnetization of fcc Co (165.5 emu g-1) and hcp Co (162 emu g-1) (Nishikawa et al. 1993), due to the presence of superparamagnetic fractions of
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60
-4
-1
σ/H x 10 (emu g Oe )
-1
χ X 10 (emu g)
70 0.019 0.018 0.017 0.016
-1
-1
50
0.020
40
0.015
300
400
500
600
T (K)
4
30 20 10 0 0.0
0.4
0.8
1.2 4
1.6
2.0
-1
1/H x 10 (Oe ) Fig. 7 Plot of r/H as a function of 1/H. (Inset) Plot of magnetic susceptibility (v-1) versus temperature under 5 kOe applied magnetic field
the nano-sized cobalt impurity phase particles. Also the data in Fig. 7 (inset) fit well with a simple Curie–Weiss law, v = C/(T - h) with Curie temperature, h = -572.4 K and Curie constant C = 6.08 emu g-1. In the CoMoN2 crystal lattice, Mo hinders the Co–Co nearest neighbor exchange interactions and hence reduces the total moment per formula weight. Moreover, similar to the previously reported FeMoN2 (Panda and Gajbhiye 1998), the distance between the neighboring Co atoms is large, reducing the ferromagnetic exchange interactions. The presence of N atoms in the crystal lattice magnetically dilutes the system and hence the intrinsic magnetic moment of Co is drastically reduced. The N-2p and Co-3d states intermixes due to their similar density of states and hence spin-pairing occurs, reducing the number of unpaired electrons responsible for the Co-moment in the Co-3d level. In addition, the d-orbital coupling of Co and Mo may decrease the Co moments and hence the effective ferromagnetic ordering of Co atoms in CoMoN2 lattice is reduced. The field-dependent magnetization characteristics are shown in Fig. 8, and the isotherm shows a curvilinear dependence. Hysteresis is observed with coercivity Hc = 172 Oe. The coercivity is due to the presence of ferromagnetic cobalt as a separate phase. The cobalt particles are in the nanometer size range and the mass % is also low; hence they are expected to show superparamagnetism. Due to the distribution in particle size, a fraction of the particles remain in
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the superparamagnetic size regime while the larger particles demonstrate ferromagnetism. The saturation magnetization (rs = 35.4 emu g-1) is measured from the r versus 1/H plots and by extrapolating the magnetization to infinite field according to the Langevin function (Cullity 1972). This rs value includes the magnetic moment of paramagnetic CoMoN2, the superparamagnetic and ferromagnetic fractions of the cobalt phase. It is observed from Fig. 8 (inset a) that the hysteresis loop is shifted in the positive direction of the field axis by 19 Oe. The presence of ferromagnetic (Co) and paramagnetic (CoMoN2) regions results into relatively more magnetic (hard) and less magnetic (soft) phases, respectively. At 300 K, the hard Co phase creates stray fields and produces significant changes in the local internal field of soft CoMoN2 phase (Gonza´lez et al. 2000). Hence, demagnetizing dipolar fields are created in the CoMoN2 phase. The different random distribution of ferromagnetic cobalt around CoMoN2 accounts for the inhomogeneities in the dipolar interaction fields resulting the lack of coincidence between the ferromagnetic and paramagnetic easy magnetization directions and gives rise to the hysteresis loop shift. In general, in a coupled antiferromagnetic–ferromagnetic system, the low temperature hysteresis loop is offset from zero field axis due to a phenomena called exchange bias (Nogue´s and Schuller 1999). Exchange bias spin coupling is also expected in this system at lower temperatures, due to the presence of ferromagnetic Co, paramagnetic CoMoN2, and antiferromagnetic surface oxide/oxynitride layer. When, Co3Mo3N is nitrided at 673 K for longer duration, the concentration of the cobalt phase also increases. This is evident from the increase in rs value with nitridation time (Fig. 8 inset b). The rs values are 35.4, 40.6, 40.85, 40.9, 43.0, and 43.3 emu g-1 for the increasing nitridation time of 1–6 h. Interestingly, Co atoms are released out of the CoMoN2 lattice with increased reaction time and temperature. In fact, to maintain the stoichiometry of CoMoN2, Mo atoms will also be released from the lattice along with Co. However, the presence of separate Mo phase is not evident from the XRD pattern and neither from the magnetic studies, due to the negligible influence of a small percentage of separate diamagnetic Mo phase on the total magnetic moment. Moreover, the magnetic exchange
J Nanopart Res (2010) 12:1107–1116 40 σ (emu/g)
20
attributed to the demagnetizing dipolar fields created in the soft CoMoN2 phase by the hard Co phase. The percentage of the cobalt phase increases with the increase in nitridation time. The presence of Mo and N reduces the ferromagnetic exchange interactions between the Co atoms in CoMoN2.
0.4 0.3
30
1115
(a)
0.2 0.1 0.0 -0.1 -0.2 -0.4
-0.02 -0.01
0.00
0.01
0.02
-4
H x 10 (Oe)
0
44
σ (emu/g)
σ (emu/g)
-0.3
10
-10 -20 -30
42
(b)
Acknowledgments The authors SB, SK, and NSG acknowledge DST, New Delhi, for the financial support in carrying out the research work.
40 38 36 34
1
2
3
4
5
6
Nitridation time (hours)
-40 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-4
H x 10 (Oe) Fig. 8 r-H hysteresis loop. (Inset a) Enlarged view of the hysteresis loop shift. (Inset b) The variation of rs with nitridation time
interactions arise between CoMoN2 and Co phases, and the nature of these interactions depends on the direct Co–Co or Mo–Mo and indirect Co–Mo–Co, Co–N–Co, Mo–N–Co exchange mechanisms. Such studies could be carried out at lower temperatures, where the thermal disordering factor is eliminated leaving only the magnetic ordering in the frozen crystal lattice.
Conclusions The nanoparticles of a new ternary nitride, CoMoN2, was prepared by nitridation of the precursor intermetallic nitride Co3Mo3N at 400 °C for 1 h. Rietveld refinement of the XRD pattern reveals 0.60 (±0.02) mass % of Co impurity phase. The space groups of CoMoN2 and Co are P63/mmc and Fm-3m, respectively. The N atoms lie at the interstitial sites, and the 12 calculated nitrogen sites indicate the presence of a layered structure. The XPS studies of the as-prepared nitride material indicate the presence of the nitride and oxynitride/oxide phases at the surface. Co and Mo are found in the zero oxidation state indicating CoMoN2 to be an interstitial nitride, without any ionic contribution. The true susceptibility of CoMoN2 is estimated after subtracting the ferromagnetic contribution from cobalt and found to be 2.7 9 10-4 emu g-1 Oe-1, indicating the Pauli-paramagnetic nature. The hysteresis loop shift 19 Oe is
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