Magnetic Properties of Carbon-Coated Ni ...

NANO: Brief Reports and Reviews Vol. 10, No. 6 (2015) 1550089 (7 pages) © World Scienti¯c Publishing Company DOI: 10.1142/S1793292015500897

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Magnetic Properties of Carbon-Coated Ni Nanoparticles Prepared by Solid-Phase Pyrolysis of Nickel-Phthalocyanine Aram Manukyan* and Armen Mirzakhanyan† Laboratory of Solid State Physics Institute for Physical Research of NAS of Armenia Gitavan-2, Ashtarak 0203, Armenia *[email protected] †

[email protected]

Laszlo Sajti Department of Nanotechnology, Laser Zentrum Hannover e.V. Hollerithallee 8, 30419 Hannover, Germany [email protected]

Ruben Khachaturyan Laboratory of Solid State Physics Institute for Physical Research of NAS of Armenia Gitavan-2, Ashtarak 0203, Armenia [email protected]

Egor Kaniukov‡ and Leonid Lobanovsky§ Division of Cryogenic Research Scienti¯c-Practical Materials Research Centre NAS of Belarus, 19 P. BrovkaStr, Minsk BY-220072, Belarus ‡

[email protected] [email protected]

§

Eduard Sharoyan Laboratory of Solid State Physics Institute for Physical Research of NAS of Armenia Gitavan-2, Ashtarak 0203, Armenia [email protected]

Received 5 February 2015 Accepted 30 April 2015 Published 15 June 2015

Carbon-coated nickel nanoparticles with a mean diameter of 40 nm were synthesized via solidphase pyrolysis of nickel-phthalocyanine. The composition structure and morphology of samples were investigated by scanning and transmission electron microscopy, X-ray di®raction, Raman spectroscopy and energy-dispersive X-ray microanalysis. Magnetic characteristics of samples

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were measured with a vibrational magnetometer and a magnetic resonance spectrometer. The main mass of Ni ferromagnetic nanoparticles have a single-domain and a vortex (pseudo-single domain) structures. We also revealed superparamagnetic Ni nanoparticles (several percent) and carbon paramagnetic centers caused by defects in the carbon matrix. We determined the main magnetic characteristics of Ni nanoparticles, their temperature and ¯eld dependences as well as the parameters of ferromagnetic resonance spectra.


Keywords: Ni nanoparticles; solid-phase pyrolysis; carbon matrix; magnetic characteristics.

1. Introduction

2. Experimental Methods

Magnetic metal nanoparticles attract a great interest both from the scienti¯c point of view and in connection with possible applications. In particular, they can be used in sensors, magnetic storage, magnetic °uids, spintronics, catalysis, magnetic paints, biomedicine, etc.1–8 Of special interest are single-domain nanoparticles, which exhibit unusual magnetic properties due to size e®ects. Investigation of these phenomena gets complicated because they depend on the size distribution of nanoparticles, their shape, interaction between nanoparticles, on the properties of host matrix, and so on. As is known, there are several methods for preparation of metal nanoparticles, in particular, hydrothermal, arc discharge, laser ablation, chemical vapor deposition, pyrolysis, etc.1–3,9 Among them solid-phase pyrolysis is characterized by simplicity and possibility to control the properties of nanoparticles via the pyrolysis parameters. Earlier we have developed the method of solid-phase pyrolysis of metal-phthalocyanines used for synthesis of Ni, Cu nanoparticles and their nanoalloys Ni1x Cux in di®erent carbon matrices.10–12 The advantage of carbon matrix is in the fact that it is biocompatible, prevents the aggregation of nanoparticles, and protects them from oxidation. In this paper, we present the results of investigation of magnetic properties of Ni nanoparticles in carbon matrix, prepared by solid-phase pyrolysis of nickel-phthalocyanine. It is shown that the dependences of magnetization of Ni/C nanocomposite on the external ¯eld and temperature can be explained with allowance for the size distribution of Ni nanoparticles. A considerable increase in the coercive force, remnant magnetization and broadening of the ferromagnetic resonance (FMR) signal with lowering the temperature of the sample is revealed as well.

Samples of Ni/C nanocomposites, prepared by solidphase pyrolysis of Ni-phthalocyanine, represent a black powder consisting of Ni nanoparticles embedded in carbon matrix. The pyrolysis process can be presented by the following reaction10: Tpyr ;tpyr ;p

NiðC32 N8 H16 Þ ! Ni þ 32C; 8H2 ;4N2

where Tpyr is the pyrolysis temperature (600– 1100 C), tpyr the pyrolysis time (3–450 min), and ppyr the autogenic pressure in a reaction ampoule. It is evident that the concentration of Ni in nanocomposites is about 3 at.% (13 wt.%). Changing the pyrolysis conditions, one can fabricate Ni nanoparticles with a mean diameter from 10 nm to 300 nm in different carbon matrices.10 For the present investigations, we chose the nanocomposites synthesized at Tpyr ¼ 700 C, tpyr ¼ 20 min and ppyr 0:8 MPa which contain Ni nanoparticles with a mean diameter of about 40 nm. The composition and morphology of nanocomposites were studied with use of scanning electron microscope (SEM) Vega Tescan 5130 MM with an energy dispersive X-ray microanalyzer INCA Energy-300, and transmission electron microscope (TEM) Tecnai G2 Spirit. The structure of nanocomposites was studied with an X-ray di®ractometer DRON-2 (radiation CoK) and a Raman spectrometer Renishaw operating at the argonion laser wavelength of 514.5 nm. Magnetic measurements were performed by using the methods of vibrational magnetometry and electron magnetic resonance. A vibrational magnetometer (Cryogenic Ltd) worked in the range of magnetic ¯elds 0–14 T and temperatures 4.3–300 K. Resonance measurements were done with a standard electron spin resonance spectrometer of X-band at the frequency of 9.37 GHz in the temperature range from 77 K to 300 K.

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3. Results and Discussion

More information on the structure of Ni/C nanocomposite was obtained from X-ray di®raction and Raman spectra (shown in Figs. 3 and 4). In particular, there are three narrow peaks at 2 58 , 72 , and 92 in X-ray di®raction spectra, which correspond to Ni nanoparticles with a fcc structure and a broad peak at 2 31 corresponding to graphite-like structures. In Raman spectra (shown in Fig. 4) there are two peaks at 1361 cm 1 and 1602 cm 1 corresponding to D- and G-bands, respectively characteristic for partially graphitized structures.13–15 Main magnetic characteristics of Ni/C nanocomposite are presented in Figs. 5–7 and in Table 1. The ¯eld-dependences of magnetization of Ni/C nanocomposite at 5 K and 300 K are shown in Fig. 5(a). From this ¯gure and the table it is seen that saturation magnetization Msat decreases weakly in the range of 5–300 K. This fact indicates that the main mass of nanoparticles consists of ferromagnetic nanoparticles. One can assume also the presence of single-domain superparamagnetic Ni nanoparticles from a weak increase in the magnetization at T < 50 K and H ¼ 1 T [shown in Fig. 5(b)]. It is evident that this is connected with the transition of superparamagnetic nanoparticles into the blocked ferromagnetic state at T < TB . There is relatively broad distribution of blocking temperature and average blocking temperature hTB i 50 K at H ¼ 1 T. The magnetization at 5 K and H > 1 T is caused by ferromagnetic and blocked superparamagnetic nanoparticles: the behavior of the magnetization curve with a monotonic saturation up to 10 T, which is observed at low temperatures can be explained by the presence of very small superparamagnetic Ni

The nickel concentration in prepared samples was controlled by the energy dispersive X-ray analyzer. Distribution of Ni nanoparticles in a carbon matrix is not ordered but su±ciently uniform. The mean value of Ni concentration over di®erent regions of the sample is 2.8 at.%, which considers with the nominal concentration 3 at.% (13 wt.%) with an accuracy to 10 at.%. Figure 1 shows SEM and TEM images of Ni/C nanocomposite prepared at 700 C/ 20 min. As seen, Ni nanoparticles are coated by graphite-like shells with a thickness of 5–20 nm and have some shape anisotropy. The size distribution of Ni nanoparticles, obtained by consideration of about 200 nanoparticles in SEM and TEM images, is shown in Fig. 2(a). This distribution is su±ciently well described by a log-normal function with a maximum at 30 nm and a mean diameter 40 nm. These values are lower than the critical diameter dcr 55 nm for single-domain ferromagnetic Ni nanoparticles.1,2 Figure 2(b) presents the corresponding volume distribution of nanoparticles which is important in consideration of magnetic characteristics. As seen, this distribution essentially di®ers from that presented in Fig. 2(a). It is clear that about 50 wt.% of Ni nanoparticles have a single-domain structure and other nanoparticles have pseudo-single domain structure — vortex state.2 Contribution of superparamagnetic nanoparticles at room temperature (with d < 20 nm) is less than several percent. Figure 2 shows that it is important to take into account not only the size distribution of metal nanoparticles, but also their volume (mass) distribution. (a)

(b)

Fig. 1.

SEM (a) and TEM (b) images of Ni/C nanocomposite. 1550089-3


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(a)

(b)

Fig. 2. (a) Size distributions of Ni nanoparticles. (b) Dependence of relative volumes of Ni nanoparticles on their diameters with allowance for the size distribution. dm is the mean diameter of nanoparticles.

nanoparticles and paramagnetic Ni clusters as well as of carbon paramagnetic centers in the samples. We reported on the observation of an intense electronic spin resonance signal ( 10 20 spin/g) from a carbon matrix in Ref. 16, where nanographite structures in carbon microspheres were synthesized by solid-phase pyrolysis of metal-free phthalocyanine. It should be noted, that the saturation of magnetization in our samples at T ¼ 5 K is two times less than in bulk Ni samples (see the table). The decrease in Msat as compared to Msat of bulk Ni is a characteristic of carbon-coated nanoparticles.17–19 Commonly, a decrease in magnetization of nanoparticles as compared with bulk samples is connected with a number of reasons. It is often caused by the oxidation leading to decrease of core or by existence of thin layer with noncolinear magnetic moments on the surface of crystallites.20–23 Formation of the

metastable solid solution of Ni/C, which was studied in Refs. 24 and 25 would also lead to a decrease in saturation magnetization. However, in our experiments, at the pyrolysis temperature 700 C Ni practically is not dissolved in graphite and solubility of C in Ni is negligible (0.4 at.%). We speculate that in the case of Ni/C nanocomposites the reduction of magnetization is a result of interaction of Ni nanoparticles with the nanographite matrix. Probably, this is conditioned by the change in the band structure of Ni nanoparticles due to their interaction with graphite-like shells. A ¯nal estimation of this e®ect, probably, will be made after investigation of dependences of the magnetization on the sizes of Ni nanoparticles, on the thickness of nanographites

Fig. 3. X-ray di®raction spectrum of Ni/C nanocomposite (radiation CoK , ¼ 1:7902 Å).

Fig. 4. Raman spectrum of Ni/C nanocomposite (exciting wavelength 514.5 nm).

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(a)

(b)

Fig. 5. Dependences of magnetization of Ni/C nanocomposite on the external magnetic ¯eld (a) and on the temperature (b). The arrows correspond to the cases of zero-¯eld and ¯eld cooling.

coating, etc. At present we are performing these experiments. Figure 6 presents the hysteresis loops of the Ni/C nanocomposite at di®erent temperatures and the temperature dependence of the coercive force. The magnitudes of Hc have intermediate values: they are larger than Hc of multidomain nanoparticles but lower than that of single-domain Ni. Evidently, this is caused by the presence of intermediate vortex structure.2 This fact is also in agreement with the distribution in Fig. 2(b). The value of Mrem = Msat ¼ 0:51 at 5 K, which practically coincides with the value of remnant magnetization in the Stoner–Wohlfarth model calculated for the system of noninteracting single-domain nanoparticles with

a uniaxial asymmetry.26 However, according to this model, in the case of single domain Ni nanoparticles, which have fcc-structure and four-fold magnetic symmetry (four easy axis), we should wait the ratio Mr =Ms ¼ 0:86 (restricted cubic magnetic anisotropy). It is evident that in our case, for Ni/C nanocomposites one should take into account both the uniaxial anisotropy caused by the nonspherical shape of nanoparticles (elongated particles), and crystalline anisotropy. In Refs. 27 and 28 a combined Stoner–Wohlfarth model has been developed where the total energy of magnetic anisotropy of a single-domain particle consists of cubic and uniaxial components. A dependence of the remnant magnetization on the contribution of the uniaxial

(a)

(b)

Fig. 6. (a) Hysteresis loops of the Ni/C nanocomposite at di®erent temperatures; (b) temperature dependence of the coercive force of Ni/C nanocomposite. 1550089-5

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Fig. 7. Spectra of FMR of Ni/C nanocomposite at 77 K (1) and 300 K (2).

component into the sum anisotropy was studied. When the contribution of the uniaxial anisotropy is in the range of 60–100%, the value of Mrem =Msat does not practically di®er from values characteristic for the models considering only the uniaxial magnetic anisotropy, i.e. Mrem =Msat ¼ 0:5.26 Signi¯cant contribution of uniaxial sharp anisotropy is observed not only in the hysteresis parameters but also in FMR spectra. There are interesting features in the FMR spectra of Ni/C nanocomposite (shown in Fig. 7). As known, the g-factor of bulk Ni has been found to be 2:22 0:02.29 A deviation of the g-factor in FMR spectra of our sample from the bulk value is caused by the in°uence of e®ective magnetic ¯eld: Beff ¼ Bapp þ Bd þ Ba ; where Bapp is the external magnetic ¯eld, Bd the demagnetization ¯eld, Ba the ¯eld of magnetocrystalline anisotropy. Then from the resonance condition it is clear that in view of contributions from Bd and Ba the values of Beff increase which leads to the decrease in g-factor.

Table 1. Magnetic characteristics of Ni/C nanocomposite at di®erent temperatures. T[K] Msat [emu/g] Mrem [emu/g] Mrem =Msat Msat =Mbulk Hc [Oe] geff H[Oe]

5

77

300

29.0 14.8 0.51 0.5 400 — —

28.0 11.1 0.39 — 260 2.15 1100

27.3 6.8 0.25 —110 2.14 850

Note that a relatively broad FMR line (850 Oe at 300 K) of our sample is determined both by the anisotropy of nanoparticles shape and the magnetocrystalline anisotropy. A strong temperature dependence of the magnetocrystalline anisotropy causes the broadening of FMR line with a decrease in temperature (H ¼ 1100 Oe) at 77 K. This broadening corresponds to the increase in Hc with lowering of temperature, since both parameters are proportional to Ba .30 Finally it should also be noted that we did not observe a natural resonance in zero magnetic ¯eld, which can be expected in the case of the presence of two-domain Ni nanoparticles.

4. Conclusions Carbon-coated nickel nanoparticles with a mean diameter 40 nm were successfully synthesized using solid-phase pyrolysis of nickel-phthalocyanine. The X-ray di®raction data, Raman spectrum, SEM and TEM images show that Ni nanocrystallites have fcc structure and are su±ciently uniformly distributed in the carbon matrix. They are coated by graphite-like shells with a thickness of 5–20 nm. TEM images as well as analysis of parameters of hysteresis and FMR indicate the presence of a sharp uniaxial anisotropy. The main mass of Ni nanoparticles (from 20 nm to 100 nm) consists of single-domain and pseudo-single domain (having vortex structure) ferromagnetic nanoparticles. The volume distribution of Ni nanoparticles and temperature measurements of magnetization show that there are also superparamagnetic Ni nanoparticles and carbon paramagnetic centers caused by defects in the carbon matrix. Magnetic characteristics of Ni/C nanocomposites at di®erent T are in agreement with the data of FMR.

Acknowledgments Authors are grateful to G. Badalyan and V. Mekhitarian for assistance in the work. This work was supported by the SCS MES RA, within the frames of joint Armenian–Belarusian research Project No. 13RB-050, as well as by the grant from the International Innovative Nanotechnology Center (ININC) CIS No. 080–193 and the FP7 Project of the European Commission with Grant No. 608906– NANOMAT–EPC. 1550089-6



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