Transport and magnetic properties of encapsulated Ni-Ni-O/Zr-O nanostructures

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Transport and Magnetic Properties of Encapsulated Ni—Ni–O/Zr–O Nanostructures Bibhuti B. Nayak1 , Student Member, IEEE, Satish Vitta1 , Arun K. Nigam2 , and Dhirendra Bahadur1 Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay 400 076, India Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400 005, India The transport and magnetic properties of nickel (Ni) nanoparticles in the range 9–70 nm having a surface oxide layer of Ni or zirconium (Zr) have been studied. The oxide-encapsulated Ni has been prepared by chemical reduction of nickel and zirconium salts in an aqueous medium with sodium borohydride as a reducing agent. Both X-ray diffraction and transmission electron microscopy studies indicate the presence of amorphous Ni–O in the native state. With the addition of Zr–O, Ni crystalline peaks become strong, and amor0 10 where is the molar concentration of Zr salt phous Ni–O peaks become weak. The absolute resistivity decreases first (up to 0 15). The saturation magnetization increases with in the starting solution), and then increases with increasing addition of Zr–O ( the addition of Zr–O up to = 0 05 and then decreases. These results are in agreement with the microstructural results, which show 0 10) due to that addition of Zr–O promotes Ni formation by suppressing the Ni–O shell. The resistivity drops initially (up to better interparticle connectivity, whereas for 0 10, the resistivity increases as the interparticle connectivity is reduced, due to Zr–O encapsulation. The low field magnetization shows a superparamagnetic behavior, observed in nanoscale structures. Index Terms—Encapsulated nanoparticles (NPs), magnetic properties, transport properties.

I. INTRODUCTION

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ESEARCH in the synthesis and functionalization of nanoparticles (NPs) has received considerable interest in recent years, because of potential applications in high-density magnetic recording media, catalysts, drug-delivery systems, ferrofluids, medical diagnostics, solid fuels, and pigments in paints and ceramics [1]–[4]. The synthesis of nanoscale metallic materials with desired properties is difficult, as it is accompanied by a spontaneous oxidation of the surface. The high surface-to-volume ratio in NPs accelerates the oxidation tendency. This tendency for surface oxidation, however, results in the formation of an interesting class of materials called “core-shell structures,” which have unusual properties. In the case of ferromagnetic metal NPs, encapsulating the metal with either a paramagnetic or an antiferromagnetic oxide is found to lower the “superparamagnetic limit” due to exchange biasing [5], [6]. This provides an opportunity to tailor the magnetic and electrical transport characteristics by modifying the relative dimensions of the core and the shell. In this paper, the transport and magnetic properties of encapsulated Ni with either Ni–O or Zr–O are studied and correlated with the structure of the NP assemblies.

The three precursor solutions, NiCl 6H O, ZrOCl 8H O, and NaBH with different concentrations were prepared separately. The NiCl solution (1 M conc.) had an initial pH of 5.1, while a 1 M ZrOCl solution was highly acidic with a pH of 1.3. The 2 M NaBH solution, on the other hand, had a starting pH of 9.7. Mixing the NiCl and ZrOCl solutions results in an acidic , and addition of the NaBH solusolution with a pH of tion increases the pH to 5–6, depending on the concentration of the ZrOCl solution. The byproducts of the reduction reaction [2], NaCl and H BO , remain dissolved in the aqueous solution and were removed by repeated centrifuging and washing with distilled water. The centrifuged product was dried under a lamp. Pellets were made from the prepared powder at a constant pressure of 75 kg/cm and heat-treated at 450 C under an H atmosphere. The density and porosity of heat-treated pellets was determined using the standard ASTM technique [7], and it is found to be 16%–18% porosity in all the cases, indicating that it is independent of Zr–O content. Structural studies of the reduced powder were done by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The resistivity was measured by the standard four-probe technique, and the magnetization was studied using a vibrating sample magnetometer.

II. EXPERIMENTAL METHODS

III. RESULTS AND DISCUSSION

The encapsulated Ni particles with Ni–O/Zr–O were synthesized by a two-stage reduction process. The first stage involves fast nucleation of the particles from an aqueous salt solution of nickel chloride and zirconium oxychloride by a reducing agent, sodium borohydride, NaBH . In the second stage, these powders are heat-treated in controlled atmosphere under the flow of pure H at 450 C for 30 min.

Digital Object Identifier 10.1109/TMAG.2005.854909

Fig. 1 shows the XRD pattern of Ni: M Zr–O encapsulated structures, where corresponds to molar concentration of Zr salt in the precursor solution. All the structures clearly show the presence of Ni crystalline peaks which grow in intensity with increasing Zr–O. The Ni particles are covered with an amorphous native oxide, Ni–O, as seen from the broad peaks in diffraction pattern. With the addition of Zr–O, the the broad amorphous Ni–O peaks progressively disappear, and the Ni crystalline becomes strong, indicating that Zr–O stabilizes Ni formation and also protects Ni from getting oxidized. In the

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NAYAK et al.: TRANSPORT AND MAGNETIC PROPERTIES OF ENCAPSULATED Ni—Ni–O/Zr–O NANOSTRUCTURES

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Fig. 2. TEM micrographs showing presence of amorphous Ni–O encapsulate on Ni NPs for (a) sample x = 0, whereas for (b) x = 0:10, it decreases. Diffraction pattern indicates the (c) diffuse Ni–O ring, which is replaced by (d) sharp rings of Ni (FCC structure) for 0.1 Zr–O. Scale corresponds to 100 nm.

Fig. 1. XRD from Ni: x M Zr–O nanostructures show that Ni (111) peak increases with increasing Zr–O content and also becomes sharp.

Fig. 3. (a) Electrical resistivity as a function of temperature. (b) Resistivity at three different temperatures as a function of Zr–O content.

TABLE I STRUCTURAL, ELECTRICAL, AND MAGNETIC PARAMETERS OF ENCAPSULATED Ni–Ni–O/Zr–O NANOSTRUCTURES

case of Zr–O composite, only Ni peaks could be ob, a broad amorphous peak corresponding served, and for to Zr–O is seen, showing clearly that this is the range of composition over which a transition from the Ni–O shell to the Zr–O shell takes place. The average size of the Ni crystallites can be determined from the half-width of peaks using the Scherrer relation, and is given in Table I. The average size increases from 9 to 72 nm, with the addition of Zr–O showing clearly that it not only protects Ni from oxidation, but also catalyzes the aggregation of Ni atoms into large crystallites. The TEM micrograph and the diffraction pattern from Ni particles with Ni–O as the shell, Fig. 2, show that Ni–O is amorphous and encapsulates the Ni core. The diffuse ring from amorZr–O, supporting phous NiO [Fig. 2(c)] is absent for the XRD results. The diffraction pattern is indexed with the faceZr–O, centered cubic (FCC) structure of Ni for sample as shown in Fig. 2(d). The electrical resistivity, as a function of temperature as well as Zr–O content, is shown in Fig. 3(a) and (b), respectively. The electrical resistivity of composites with Zr–O was very high, insulating the regime, and could not be measured. It can be clearly seen that the absolute resistivity decreases first ) and then increases with the addition of Zr–O (up to

Fig. 4. (a) Magnetization as a function of field at room temperature. Inset: magnetization as a function of field at 5 K. (b) Magnetization as a function of temperature.

. Also, up to , the resistivity has a very low , the resistivity shows temperature dependence, and for a strong temperature dependence, which is typical of metals [8], [9]. These results are in agreement with the microstructural results which show that initially, the addition of Zr–O promotes Ni formation leading to a better interparticle connectivity, while it due to Zr–O encapsulation, which is an is reduced for insulator.

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Fig. 5. Size of Ni crystallites obtained from XRD and the weight fraction of Ni determined from magnetization show an opposite dependence on Zr–O content.

IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

, NP system is cooled in an external magnetic field to T the Néel temperature of the antiferromagnetic material, the antiferromagnetic moment exchange couples to the magnetization in the ferromagnetic layer to minimize the interaction energy of the system. In the case of Ni: Ni–O, the Néel temperature of antiferromagnetic NiO is 523 K, which is below the Curie temperature of Ni, 627 K. Hence, the system behaves as an 300 K exchange-coupled system in the temperature range that has been investigated in this paper. The large coercivity at 5 K supports the presence of exchange coupling in these encapsulated particles [5]. IV. CONCLUSION

The magnetic properties of the encapsulated Ni particles were studied both as a function of field and temperature, and the results are shown in Fig. 4(a) and (b), respectively. The room temperature magnetization at a field of 20 KOe, M is given in Table I, and is found to increase with the addition of Zr–O up to 0.05, and then decrease. The absolute value of M , however, is low, compared with that of pure Ni, 55 emu/g, and the amount of Ni in each of these composites can be determined based on the M of pure Ni, as the magnetization is proportional to concentration. The weight fraction of Ni in the nanocomposites is given in Table I, and is shown in Fig. 5, together with the size of the crystallites. It can be seen that the weight fraction of Ni obtained from magnetization and the crystallite size obtained from XRD vary exactly the opposite of the Zr–O content. The Ni crystallites, although grown with the addition of Zr–O, decrease in weight fraction and become increasingly isolated, leading to lack of electrical transport. At 5 K, the magnetization does not saturate, even in fields as high as 15 KOe (Fig. 4(a) inset), due to the presence of an oxide surface on the Ni particles. The encapsulated particles exhibit remanence and the coercivity is found Oe, far higher than that of bulk pure Ni. The large to be coercivity coupled with lack of saturation clearly indicates the presence of exchange interaction between the antiferromagnetic Ni–O cover and the Ni core, as seen in the TEM micrograph, Fig. 2. The temperature-dependent magnetization in a field of 5% of the field required for saturation, shows a 100 Oe, superparamagnetic behavior with the blocking temperature close to room temperature [Fig. 4(b)]. The zero field cooled (ZFC) magnetization in all the cases does not exhibit a clear rounded maximum, indicating a narrow size distribution of the Ni NPs [10]. The magnetization shows a antiferromagnetic transition at low temperatures, 10–25 K, which is possibly due to adsorbed oxygen molecules and is further investigated. The field cooled (FC) and ZFC magnetizations diverge for T as the system regains its coercivity and remanence, seen from the hysteresis loops at 5 K, shown in Fig. 4(a), inset. The below is still small, compared with the thermal energy magnetostatic or magnetic anisotropy energy of the NPs, and hence, the system exhibits a net ferromagnetic behavior. In NP systems with ferromagnetic/antiferromagnetic interfaces, exchange coupling takes place across the interface, leading to a reduction of the effective superparamagnetic limit [6], and also biasing of the magnetic hysteresis loop [11]. If the

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Encapsulation of Ni–O/Zr–O on an Ni core has been made successfully by a chemical reduction process using a reducing agent (NaBH ). There is no variation of measured density or porosity with the Zr–O content. Both XRD and TEM study indicates the presence of Ni–O in the absence of Zr–O, and the size of Ni particles ranges from 9–70 nm. The addition of Zr–O is found to promote formation of large Ni grains, and also prevent oxidation by encapsulation. Electrical transport behavior shows , while for better interparticle connectivity for sample , the interparticle connectivity is reduced due to Zr–O encapsulation. The magnetic studies indicate a superparamagnetic behavior of the nanocomposite system. The microstructural results, together with the transport and magnetic properties of the NP system, clearly show the potential of this technique to obtain size-controlled property tuning. REFERENCES [1] H. Kisker, T. Gessmann, R. Wiirschum, H. Kronmiiller, and H.-E. Schaefer, “Magnetic properties of high purity nanocrystalline nickel,” Nanostruct. Mat., vol. 6, pp. 925–928, 1995. [2] G. N. Glavee, K. J. Klabunde, C. M. Sorensen, and G. C. Hadjipanayis, “Borohydride reduction of nickel and copper ions in aqueous and nonaqueous media. Controllable chemistry leading to nanoscale metal and metal boride particles,” Langmuir, vol. 10, pp. 4726–4730, 1994. [3] J. Legrand, A. Taleb, S. Gota, M. J. Guittet, and C. Petit, “Synthesis and XPS characterization of nickel boride nanoparticles,” Langmuir, vol. 18, pp. 4131–4137, 2002. [4] S. Ram and P. S. Frankwickz, “Granular GMR sensors of Co–Ag and Co–Cu nanoparticles synthesized through a chemical route using NaBH ,” Phys. Stat. Sol. (a), vol. 188, pp. 1129–1140, 2001. [5] X.-C. Sun and X.-L. Dong, “Magnetic properties and microstructure of carbon encapsulated Ni nanoparticles coated with NiO layer,” Mater. Res. Bull., vol. 37, pp. 991–1004, 2002. [6] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, and J. Nogués, “Beating the superparamagnetic limit with exchange bias,” Nature, vol. 423, pp. 850–853, 2003. [7] Annual Book of ASTM Standards. Philadelphia, PA: ASTM, 1989, vol. C373–88, pp. 109–110. [8] J. E. Sundeen and R. C. Buchanan, “Electrical properties of nickel-zirconia cermet films for temperature- and flow-sensor applications,” Sens. Actuators A., vol. 63, pp. 33–40, 1997. [9] E. Sundeen and R. C. Buchanan, “Thermal sensor properties of cermet resistor films on silicon substrates,” Sens. Actuators A., vol. 90, pp. 118–124, 2001. [10] M. Hanson, C. Johansson, M. S. Pedersen, and S. Morup, “The influence of particle size and interactions on the magnetization and susceptibility of nanometer-size particles,” J. Phys.: Condens. Matter, vol. 7, pp. 9269–9277, 1995. [11] W. H. Meiklejohn, “Exchange anisotropy—A review,” J. Appl. Phys., vol. 33, pp. 1328–1325, 1962. Manuscript received February 5, 2005; revised March 25, 2005.