Structure and magnetic properties of carbon encapsulated [email protected] and [email protected] nanoparticles

Structure and magnetic properties of carbon encapsulated [email protected] and [email protected] nanoparticles

Materials Letters 254 (2019) 202–205 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue S...

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Materials Letters 254 (2019) 202–205

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/mlblue

Structure and magnetic properties of carbon encapsulated [email protected] and [email protected] nanoparticles A.Ye. Yermakov a,b,c,⇑, M.A. Uimin a,b,c, I.V. Byzov a,c, A.S. Konev a, S.I. Novikov a, A.S. Minin a,b,c, V.S. Gaviko a,b, A.M. Murzakaev b,d, V.V. Maikov a a

M.N. Miheev Institute of Metal Physics of UB of RAS, Yekaterinburg, Russia Ural Federal University named after the first President of Russia B. N. Yeltsin, Yekaterinburg, Russia Institute of Ecology and Genetics of Microorganisms of UB of RAS, Perm, Russia d Institute of Electrophysics of UB of RAS, Yekaterinburg, Russia b c

a r t i c l e

i n f o

Article history: Received 16 July 2019 Accepted 17 July 2019 Available online 19 July 2019 Keywords: Nanocrystalline materials NiFe alloy CoFe alloy Magnetic properties

a b s t r a c t A comparative analysis of the structure and magnetic properties of nanoparticles of Fe-Ni and Fe-Co alloys without carbon and encapsulated in carbon obtained by gas-condensation synthesis is carried out. Particular attention is paid to the study of the formation of a solid solution in nanoparticles with carbon. X-ray analysis and magnetic properties convincingly demonstrate that a disordered supersaturated solid solution of carbon in [email protected] and [email protected] nanoparticles is formed. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Interest in magnetic nanomaterials based on 3d-metals encapsulated in carbon has increased due to the extensive use of these objects for different applications [1]. The magnetic nanoparticles attract attention in the field of medicine and biology as therapeutic and diagnostic agents [2–4]. The method of gas-condensation synthesis [5], along with other methods [6] provides not only a protective coating on nanoparticles, but it allows controlling the magnetic properties. It was shown in [7] that [email protected] and [email protected] nanoparticles have BCC and FCC lattice, respectively. But, the effect of carbon on the properties of the nanoparticles is still open. In this paper the more detailed study of magnetic properties of Fe-Co and Fe-Ni nanosystems with carbon was performed to clarify the carbon influence. TEM observation and X-ray analysis of the lattice parameters and the influence carbon on magnetic properties for [email protected] and [email protected] systems were undertaken. 2. Experimental Nanoparticles based on binary compounds Fe50Ni50 and Fe50Co50 and particles coated by carbon [email protected] and [email protected] ⇑ Corresponding author at: M.N. Miheev Institute of Metal Physics of UB of RAS, Yekaterinburg, Russia. E-mail address: [email protected] (A.Ye. Yermakov). https://doi.org/10.1016/j.matlet.2019.07.067 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

were obtained by the gas-condensation method, described in detail in [5]. To synthesize the nanoparticles, the wires of the Fe50Ni50 and Fe49Co50V1 alloys were fed into the melting zone. The mass fraction of carbon (30%) was determined using TG analysis [5]. The relative content of the 3d elements was determined using the ICP and EDAX methods and is given in Table 1. In the following notations, the symbol V in the sample designations will be omitted. X-ray diffractometer ‘‘Empyrean” with high-resolution in filtered copper radiation and HighScore Plus 4.1 software package were used to determine phase composition and lattice parameters. The temperature dependence of the susceptibility vðTÞ of the samples was studied in an alternating field 10 Oe with a frequency of 130 Hz. A transmission electron microscope (TEM) JEM2100 (JEOL) was used to determine the morphology and structure of nanopowders. 3. Results and discussion Fig. 1(a–f) shows TEM pictures of FeCo, [email protected] and FeNi, [email protected] nanoparticles. The average size of nanoparticles without carbon is (30–40) nm. The particles of carbon-free alloys FeNi and FeCo form chains as a result of the magnetostatic interactions between particles (Fig. 1(a and d)). The nanoparticles are coated with a disordered oxide layer, with unknown content (Fig. 1(b and e)). The electron diffraction pattern indicates only the BCC lattice reflections for

A.Ye. Yermakov et al. / Materials Letters 254 (2019) 202–205 Table 1 The chemical compositions of powders based on Fe50Ni50 and Co50Fe50 are shown. The content of Fe, Ni, Co and V was determined by the ICP and EDAX methods with an accuracy of 1%. Sample

Fe, wt%

Ni, wt%

Co, wt%

V, wt%

FeCo [email protected] FeNi [email protected]

58.5 55 59 58

– – 41 42

40.5 44 – –

1.0 1.0 – –

FeCo alloy (Fig. 1a FCC lattice reflections for FeNi (Fig. 1The [email protected] and [email protected] nanoparticles are coated by carbon shell (Fig. 1 The size of particles core is about (3–5) nm. On Fig. 2(a and b) X-ray patterns of FeNi, [email protected] and FeCo, [email protected] nanoparticles are shown. It is important to remark that the formation of nanoparticles occurs at high temperatures, followed by rapid quenching of nanoparticles (more than 105 =s) from the high-temperature region. It cannot be ruled out that nanoparticle synthesis can proceed via the direct vapor-solid formation mechanism proposed in [8]. Therefore, it is difficult to expect the appearance of other phases besides the disordered c-FeNi and FeCo solid solution existing in the high-temperature region of the phase diagram [9]. Due to the high cooling rate, the ordering of the FeNi phase, for example, of the L1o type [10] and the ordering of FeCo [11] should be suppressed. The size of coherent scattering regions for [email protected] nanoparticles correlates with the TEM data and is equal to 3–5 nm. In the samples of [email protected], as compared with a carbon-free alloy, a noticeable decrease in the lattice parameter is observed from a = 3.590 Å to

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a = 3.563 Å with D ¼ 0:027 Å. We assume that the difference lattice parameters for [email protected] and FeNi is due to the formation of a supersaturated solid solution of carbon in the FeNi lattice but not influence of the particle size which affects the width of the line (see Fig. 2b (curve 1 and 2)). So, common features for FeCo and FeNi nanocrystalline systems are observed. For the [email protected] alloy in nanostate, the lattice parameter is equal to 2.857 Å it is somewhat less than for the undoped FeCo sample where a = 2.865 Å with difference D ¼ 0:008 Å . Taking into account that the lattice parameter of FeCo nanopowders does not change with a change in particle size (see Fig. 2b), we believe that the only reason for the decrease in the lattice parameter of the [email protected] powder is the formation of a solid solution of carbon in the metal core. The magnetization of nanoparticles of the FeCo differs from the values of the bulk magnetization (Fig. 3a (curves 1, 3)). The difference reaches almost 20%, and it can be explained by, e.g., the existence of a nonmagnetic oxide based on Fe-Co. In [email protected] nanoparticles, the difference is more significant and reaches 45% (Fig. 3a (curves 1, 3)). The total carbon content (30 wt%) is too small to explain such a large difference. The substantial decrease of magnetization, hypothetically, can be a result of the hybridization of electron states of carbon and Fe, Co at the formation of the supersaturated solid solutions FeCo-C. We are have to assume that in the [email protected] nanoparticles, the core contains much more carbon than it is possible according to the equilibrium phase diagram of Fe-C and Co-C [9]. As a result of the formation of a solid solution of carbon in the FeCo alloy, the Curie temperature Tc should be changed. Unfortunately, we could not detect this change since our device allows measurements to be no higher than 800 °C (Fig. 3b (curves 2,3)).

Fig. 1. (a) TEM images of samples the TEM images of FeCo and FeCo (b) (higher magnification) and (c) [email protected]; and (d) TEM images of FeNi; (e) FeNi (high magnification) and (f) [email protected]

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Fig. 2. (a) X-ray diffraction patterns of FeNi (1) and FeNi @C (2). The FCC lattice parameter for FeNi is a = 3.590 Å, and for the system [email protected] a = 3.563 Å. (b) X-ray diffraction patterns of FeCo (1–38 nm, 2–22 nm) and [email protected] (3) samples. The BCC lattice parameters for nanoparticles (1–38 nm) of FeCo are a = 2.865 Å for nanoparticles (2–22 nm) a = 2.863 Å, and for an alloy with carbon [email protected] a = 2.857 Å.

Fig. 3. (a) Demagnetization curves of bulk sample FeCo (1); (2) FeCo nanoparticles and (3) [email protected]; (b) vðTÞ for the bulk FeCo (1), for FeCo nanoparticles (2) and [email protected] nanoparticles (3); (c) Demagnetization curves of FeNi bulk sample (1); for FeNi nanopowders (2) and [email protected] (3); (d) vðTÞ for FeNi nanopowders (2) and for FeNi nanopowders annealed in H2 at 800 °C (2a) and [email protected] nanoparticles(3).

The appearance of the oxide layer on the particles surface, admittedly, with enriched Fe content (e.g., a-Fe2O3), evidently, can change the core composition. On Fig. 3d (curve 2) on the sample FeNi with surface oxide, the decrease of v in temperature range (300–400) °C is revealed, probably, due to the existence of Feenriched oxides. The second phase has the extrapolated Tc = 580 °C, which presumably, can be attributed to c-FeNi with Ni content  (56 ± 2) wt% (Fig. 3d (curve 2)). After reducing of oxides at annealing in hydrogen (800 °C) and redistribution of Fe and Ni atoms in the sample an only single phase with Tc  450 °C was observed (Fig. 3d(2a)). This Tc corresponds to c-FeNi phase with Ni content which is equal (45 ± 2) wt% [9]. Carbon-containing FeNi samples radically change the value of the magnetization and the character of vðTÞ. The magnetization of samples with carbon is much less than expected in the case of bulk sample. The correction for carbon content (30%) cannot explain the high decrease of magnetization (Fig. 3c, curves 1 and 3). The monotonous rapid decrease for vðTÞ for [email protected] up to 400 °C, probably, indicates a set of local Curie temperatures in the different particles. This assumption allows us to explain a significant decrease in vðTÞ in a wide temperature range. Thus, for [email protected] and [email protected] nanoparticles, the magnetization and TC values are changed, admittedly, as a result of the hybridization of electron states of carbon and transition metals at formation of solid solution.

4. Conclusions The nanoparticles on the base of FeNi and FeCo systems with and without carbon were obtained by the non-equilibrium gascondensation method. X-ray analysis demonstrates a noticeable change in the lattice parameters upon carbon doping. The revealed drastic change of [email protected] and [email protected] magnetization could be reasonably explained by assuming the decrease of magnetic moments of the Fe, Co and Ni atoms at doping by carbon as a result of formation a non-equilibrium supersaturated solid solution in the process of synthesis. The anomalous variation of vðTÞ for the [email protected] system should be considered in the model of the set of nanoparticles with different carbon contents. Declaration of Competing Interest None. Acknowledgement This work was financially supported by Russian Science Foundation Grant No 17-15-01116. X-ray studies was carried out using the equipment of the Collaborative Access Center ‘‘Testing Center of Nanotechnology and Advanced Materials” IMP.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, athttps://doi.org/10.1016/j.matlet.2019.07.067.

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