Magnetic Characterizations of Cobalt Oxide ... - Springer Link

J Supercond Nov Magn (2012) 25:2783–2787 DOI 10.1007/s10948-011-1265-7

O R I G I N A L PA P E R

Magnetic Characterizations of Cobalt Oxide Nanoparticles Yuksel Koseoglu · Figen Kurtulus · Hakan Kockar · Halil Guler · Oznur Karaagac · Sinan Kazan · Bekir Aktas

Received: 6 July 2011 / Accepted: 14 July 2011 / Published online: 10 September 2011 © Springer Science+Business Media, LLC 2011

Abstract Cobalt oxide (Co3 O4 ) nanoparticles were synthesized by a novel microwave-assisted decomposition reaction of the cobalt nitrate hexahydrate, Co(NO3 )2 ·6H2 O. While most of the traditional methods for the preparation of Co3 O4 are at relatively high temperature, microwave-assisted decomposition was adapted to have better control in the production of Co3 O4 nanoparticles. The temperature dependence of the magnetic properties for the Co3 O4 was investigated by vibrating sample magnetometer (VSM) and electron spin resonance (ESR) techniques. VSM and ESR measurements have shown a phase transition occurring at around 31 K, as the antiferromagnetic transition temperature for the bulk Co3 O4 crystal exhibits almost the same value. The average particle size of the sample at around the transition temperature is estimated as 2.015 nm. The title compound was characterized and identified by an x-ray powder diffraction (XRD). Keywords Magnetic properties of nanostructures · Electron spin resonance (ESR) · Antiferromagnetics

Y. Koseoglu Physics Department, Science & Literature Faculty, Fatih University, 34500, Istanbul, Turkey F. Kurtulus · H. Guler Chemistry Department, Science & Literature Faculty, Balikesir University, 10145 Balikesir, Turkey H. Kockar () · O. Karaagac Physics Department, Science & Literature Faculty, Balikesir University, 10145 Balikesir, Turkey e-mail: [email protected] S. Kazan · B. Aktas Physics Department, Science Faculty, Gebze Institute of Technology, 41410 Gebze, Kocaeli, Turkey

1 Introduction In recent years, the study of magnetic properties of nanosized particles has become of great importance due to their mesoscopic properties and applications [1, 2]. Nanostructural materials such as nanorods, nanowires, nanofibers and nanotubes have many potential applications for nanodevices [3, 4]. Black cobalt (Co3 O4 ) nanoparticles have also undergone many studies [5–9] and potential applications in nanostructural material science because of its particle size and surface effects on magnetic properties [10]. Although there are quite a lot of methods to obtain the Co3 O4 , such as chemical vapor deposition [11], sol-gel [12], chemical precursor routes [13], spray pyrolysis [14], electrochemical, sonochemical synthesis [15] and a simple reduction oxidation method [16], a relatively high temperature is required in the most of the techniques. In this study, Co3 O4 nanoparticles were synthesized by using a novel microwave method. The measurements of magnetic properties of the resultant material were carried out depending on the temperature and presented here. The phase transition was observed at around 31 K, while the bulk Co3 O4 crystal exhibits antiferromagnetic transition temperature at TN = 33 K [1, 14] and the particle size of the Co3 O4 nanoparticles was found as 2.015 nm at around transition temperature.

2 Experimental In order to synthesize the Co3 O4 nanoparticles, high-purity powder forms of cobalt(II) nitrate hexahydrate, Co(NO3 )2 ·6H2 O (Merck, purity 99.9%) were exposed to the microwave radiation in the microwave oven (Arcelik MD560).

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Magnetization measurements were made by using the Quantum Design Model 6000 vibrating sample magnetometer (VSM), and electron spin resonance (ESR) measurements of the samples were investigated by using the commercial X-band (f ≈ 9.7 GHz) Bruker EMX model spectrometer. To obtain the intensity of microwave power absorption, P, digital integration of the ESR curves was performed by using Bruker WinEPR software. Also, an Oxford continuous helium gas flow cryostat has been used, allowing the X-band microwave cavity to remain at ambient temperatures during ESR measurements at low temperatures. The temperature was stabilized by a conventional Lakeshore 340 temperature controller between 10 K–300 K. The x-ray diffraction (XRD) data were collected by using Philips X Pert-Pro X-ray diffractometer with position sensitive detector, graphite monochromator and CuKα radiation (30–40 kV, 10–20 mA, λ = 1.54056 Å).

J Supercond Nov Magn (2012) 25:2783–2787

Fig. 1 The room-temperature M–H curve of the sample

3 Results and Discussion As the starting material, high-purity powder form of cobalt(II) nitrate hexahydrate, Co(NO3 )2 ·6H2 O was used to synthesize Co3 O4 . Samples of 5 g were transferred into a ceramic type crucible and exposed to microwave radiation of 2.45 GHz frequency in a domestic type microwave oven at 750 W for 10 minutes. At the end of the experiment, the sample was allowed to cool inside the oven. The resulting powder product was subjected to XRD analysis. The XRD analysis showed that peaks can be attributed to the formation of the Co3 O4 as a main and pure phase (data not shown). The details of the production, chemical and structural analysis of Co3 O4 nanoparticles are out of scope of this investigation and can be found elsewhere [13]. In order to study the magnetic properties of the Co3 O4 nanoparticles, the M–H curve of the sample were measured at room temperature and is shown in Fig. 1. This is followed by zero field cooling (ZFC) and field cooling (FC) hysteresis curves measured at 5 K, shown in Fig. 2. As can be seen in Fig. 1, the room-temperature M–H curve is linear with the field and have no coercivity and remanence values were measured. In Fig. 2, the M–H curves were measured at ±5 kOe. Samples cannot reach saturation even in the presence of 5 kOe magnetic field. This is because it is known that Co3 O4 has normal spinel structure with antiferromagnetic exchange interaction between ions occupying tetrahedral A sites and octahedral B sites. The Co3+ ions have no moment on B sites, and Co2+ ions on A sites have a permanent moment of 3.25 μB from the neutron diffraction experiments [12, 14]. Temperature dependence magnetization of the sample at 1 kOe field-cooled (FC) case is shown in Fig. 3. In FC curve, the high-temperature magnetization of the sample is

Fig. 2 ZFC and FC hysteresis curves at 5 K

strongly increased by a decrease of temperature down to 31 K and reaches a peak value of 5.7 × 10−4 emu/g at around 31 K. Below 31 K, the magnetization sharply decreases by decreasing temperature. For higher temperature region, T ≥ 31 K, the plot of 1/M vs. T is linear, indicating that the magnetization obeys the Curie–Weiss law with a negative θ value about 65 K, while the Néel temperature of bulk Co3 O4 crystal was known to be T N = 33 K [1, 14]. ESR measurements were done in order to gain further information about the magnetic properties of the samples. The sample was placed at the maximum magnetic field in the cavity of the ESR. A cylindrical quartz tube was used to mount the sample in the cavity. The field derivative of microwave power absorption, dP /dH , was registered as a function of DC magnetic field H . Figure 4 shows some selected ESR spectra taken at various temperatures. Single and very broad ESR signals were seen at all temperatures. While the line width is slowly decreasing, the intensity of the ESR spectra is increasing and the resonance field is almost the


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Fig. 3 M–T curve of sample with 1 kOe applied field. The inset shows the 1/M curve

Fig. 4 ESR spectra for some selected temperatures

same down to the transition temperature. At room temperature, the resonance field and the line width values are observed as 3050 G and 2540 G, respectively. Temperature variation of the resonance field values were also measured and were shown in Fig. 5. While decreasing the temperature, the resonance field values remain almost constant at high temperatures. As there is no significant change between 300 K and 50 K, the most striking change in the resonance values occurred between 50 K and 25 K.

At this temperature, the resonance field value decreases suddenly from 3065 G to 2924 G indicating the phase transition. Below this temperature, the resonance field values continue to decrease. Further ESR measurements were also done, and the temperature dependence of the peak-to-peak line width of the ESR spectra is illustrated in Fig. 6. As seen from the figure, the line width slowly decreases from room temperature to 50 K, then it sharply increases by decreasing the temper-

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Fig. 5 Temperature dependence of the resonance field values Fig. 7 Intensity values of the ESR spectra versus temperature

Fig. 6 Line width of the ESR spectra against the temperature

ature down to 25 K. The most striking change in the ESR line width is observed at around 50 K. This decrease in the line width values also indicates the phase transition. Below the transition temperature, the line width increases down to 25 K. The striking change at around 50 K in all ESR parameters mentioned above indicates that the magnetic phase changes from paramagnetic to antiferromagnetic phase. However, the temperature strongly influences the ESR signals. The values for the intensity obtained from second integration of the ESR signal, which corresponds to DC susceptibility, are plotted as a function of temperature in Fig. 7. As is well known [6], the field dependence of microwave power absorption (first integral of ESR spectra) by the sample is proportional to the imaginary component of transverse (perpendicular to the strong DC field) AC susceptibility of the sample. The intensity, which is proportional to the DC magnetization, slightly increases with the increase of the temperature. It has a step like behavior at around 180 K and con-

tinues to increase down to 80 K. Below 80 K, the intensity slowly decreases and it sharply decreased by further cooling. A remarkable feature of the intensity curve is at around 31 K, at which the intensity sharply increases indicating the antiferromagnetic phase transition as seen in resonance field and line width curves. The bulk Co3 O4 exhibits antiferromagnetic transition temperature at T N = 33 K [1, 14] and Makhlouf [1] found as 25 K for the samples synthesized by co-precipitation method. Also Ichiyanagi et al. [9] have found the transition temperature to be 10 K for Co3 O4 nanoparticles surrounded by amorphous SiO2 . In our study, antiferromagnetic transition temperature was found to be 31 K, which is close to that of the bulk form of Co3 O4 . The reason for this difference between the transition temperatures could be found on their particle sizes. Simply applying the relation between anisotropy energy and thermal fluctuations at around the transition temperature as KV = k B T to this case, the particle size near T t ∼ 31 K could be estimated as about 2.015 nm, assuming K ∼ 105 J/m3 though the exact anisotropy constant is unknown [14].

4 Conclusion The Co3 O4 nanoparticles were synthesized by a novel microwave-assisted decomposition reaction of Co(NO3 )2 · 6H2 O compound. From magnetization and ESR measurements the transition temperature is found to be around 31 K, which is almost the same value as the antiferromagnetic transition temperature for the bulk Co3 O4 crystal exhibits. The average particle size near T t ∼ 31 K was estimated to be about 2.015 nm. Acknowledgement This work was partly supported by Balikesir University, Turkey under Grant no BAP 2001/02.


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