J Supercond Nov Magn (2011) 24: 629–633 DOI 10.1007/s10948-010-0958-7
O R I G I N A L PA P E R
Magnetic Phase Transitions in Cobalt Chromite Nanoparticles Chandana Rath · P. Mohanty
Received: 12 September 2010 / Accepted: 14 September 2010 / Published online: 9 October 2010 © Springer Science+Business Media, LLC 2010
Abstract Cobalt chromite (CoCr2 O4 ), an insulating normal spinel compound, is a potential multiferroic material. We report that pure, nanosize CoCr2 O4 particles are synthesized through a conventional coprecipiation technique by controlling the pH of the precipitation. Both single-crystal and polycrystalline samples develop long-range ferrimagnetic order below the Curie temperature, T c (97 K), and a sharp phase transition at T s ∼ 31 K, attributed to the onset of long-range spiral magnetic order. However, we observed a transition from paramagnetic to superparamagnetic phase at T c . Further lowering the temperature below T c (97 K), the superparamagnetic phase transforms to ferrimagnetic phase at blocking temperature, T b , which is found to be between 50 and 60 K. This intermediate superparamagnetic phase in between paramagnetic and long-range ferrimagnetic phases is attributed to a nanosize effect.
1 Introduction The nanomagnetic materials have generated renewed research interest due to applications in nanoscience and technology [1–4]. For example, chromites, cubic normal spinel compounds, exhibit many unusual properties such as ferrimagnetism, colossal magnetoresistance and have recently attracted much attention as multiferroic materials [5]. In cobalt chromite (CoCr2 O4 ) spinel, Co2+ ions occupy tetrahedral A site and Cr3+ occupy octahedral B sites [6].
C. Rath () · P. Mohanty School of Materials Science and Technology Institute of Technology, Banaras Hindu University, Varanasi 221005, India e-mail:
[email protected]
Like ferrites, chromites also exhibit ferrimagnetism. In the collinear ferrimagnetic structure of spinel, A–O–B and B– O–B bond angles are 125 and 90°, respectively. For other configurations the distance between the oxygen ions and cations are too large to give rise to a strong JAB superexchange interaction. In chromites, for example, the negative JBB (Cr–O–Cr) interactions are stronger and they control the magnetic properties. As a result, CoCr2 O4 shows both ferrimagnetic and spiral magnetic ordering below Curie temperature, T c (97 K) and T s (31 K), respectively [7, 8]. The ferrimagnetic component which forms long-range order below T c causes a spontaneous magnetization of about 0.3 μB /formula and it is in fact much less than the expected value (3 μB /formula) [7, 9]. There is no study on the effect of size on magnetic phases below and above the transition temperatures to the best of our knowledge. Therefore, the present work was undertaken to study the effect of size reduction on the magnetic properties of cobalt chromite. For the first time we synthesized pure cobalt chromite through a simple coprecipitation technique by carefully controlling the pH of precipitation. The particles are of ∼8 nm observed from TEM as well as XRD line width. While bulk cobalt chromite shows a transition from paramagnetic to ferrimagnetic phase, we observed a transition from paramagnetic to superparamagnetic (SPM) phase at the Curie temperature, T c . However, T c and T s remain unchanged even by reducing the size from bulk to nanoscale range in contrast with the large increase in T c ’s reported by reducing the size to nanoscale range in similar spinel compounds like ferrites [10].
2 Experimental Cobalt chromite powders were synthesized by a coprecipitation technique using stock solutions of cobalt nitrate and
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chromium nitrate. The desired quantity of cobalt nitrate was added slowly to the chromium nitrate solution under continuous stirring. Diluted aqueous ammonia (30%) solution was added drop wise to the mixed solution to maintain the pH between 8 and 10. The precipitates were filtered, washed and dried in oven at 120 °C for 16 h. Due to the amorphous nature of the as-synthesized particles, samples are further calcined at 600 °C for 4 hours. The calcined powders were characterized by X-ray Diffraction (XRD) using an 18 kW rotating anode (CuKα ) based Rigaku powder Diffractometer operating in the Bragg–Brentano geometry and fitted with a graphite monochromator in the diffracted beam. Transmission Electron Microscopy (TEM) was used to measure the particle size of the calcined sample. The X-ray photoelectron spectra (XPS) were recorded for identification of the oxidation state of cobalt and chromium species on X-ray electron spectrometer using AlKα radiation (1486.6 eV). Temperature- and field-dependent dc magnetization was measured by using Physical Properties Measurement System (PPMS Model No. 6000) of Quantum design operating between 2 K to 350 K.
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Figure 1 shows the X-ray spectra of CoCr2 O4 samples prepared at pH 8.9, 9.1 and 9.3. Except at pH 8.9, the other two samples prepared at pH 9.1 and 9.3 show the presence of Cr2 O3 phase (denoted by ). It has been shown in some earlier reports that impurity phases like NaCl and Cr2 O3 remain present in the solid by precipitating CoCr2 O4 from NaOH and ammonia solution [11–13]. For the first time we have been able to synthesize pure cobalt chromite through
a simple coprecipitation technique by controlling the pH of precipitation. The crystallite size calculated using the Scherrer equation is found to be ∼8 nm. The particle size measured from transmission electron micrography (Fig. 2) is well matched with the size calculated from the Scherrer formula [14]. CoCr2 O4 is ferrimagnetic, Co2+ ion occupies tetrahedral A-site and two Cr3+ ions occupy the octahedral B-site [15]. Theoretically the net magnetic moment per formula is due to one Cr3+ ion. However, the experimental value may change depending upon the valency and site occupancy of chromium as well as cobalt ions. From X-ray photoelectron spectroscopy, we observed that the binding energies of sublevels 2p1/2 and 2p3/2 of Co 2P are 797.58 eV and 781.71 eV, respectively. The observed difference in binding energy of two sublevels (15.87 eV) indicates that the Co ion present in the compound is in the 2+ valency state [16]. Chromium 2P photoelectron spectra show the binding energy of sublevel 2p1/2 and 2p3/2 to be at 586.61 eV and 576.76 eV, respectively. The observed difference in the binding energy (9.85 eV) between two sublevels confirms the +3 valency state of chromium [16] (figures are not shown here). Figure 3 shows the temperature dependence of the magnetization studied after zero field cooling (ZFC) and field cooling (FC). To measure M ZFC and M FC , we cooled the sample from room temperature to 10 K without and with field of 10 Oe, respectively. Then we measured the magnetization with increasing temperature by applying 10 Oe field. The M ZFC curve does not follow the same path as the M FC . The bifurcation in magnetization in M ZFC and M FC starts at temperature 96 K and shows a peak at 66 K in FC and 75 K in ZFC curves. The extrapolation of the linear
Fig. 1 XRD spectra of pure CoCr2 O4 nanoparticles synthesized at pH 8.9, 9.1 and 9.3. (‘ ’ denotes Cr2 O3 phase)
Fig. 2 Transmission electron micrograph of calcined CoCr2 O4 nanoparticles
3 Results and Discussion
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Fig. 3 (a) Temperature-dependent zero field cooled (ZFC) and field cooled (FC) dc magnetization measured at H = 10 Oe and (b) specific heat versus temperature plot of CoCr2 O4 nanoparticles
part intercepts at 97 K on the temperature axis which is, as is known, the Curie temperature, T c . T c observed in our case is well matched with the T c of bulk and single crystals of CoCr2 O4 [5, 8]. This suggests that T c remains unaffected by reducing the size to nanoscale range. This is in contradiction with results reported in case of normal spinel ferrites, i.e. T c increases with reduction of size [10, 17]. Below T c , magnetization shows a maximum at 75 K and 66 K in ZFC and FC curves, respectively. This could be due to the polydispersivity of the nanoparticles. The maximum magnetization in ZFC and FC further shifts to lower temperature by increasing the applied field to 500 Oe (figure is not shown here). The shifting in peak in M ZFC and M FC with field has been interpreted as a consequence of the non-linear field dependence of magnetization of the unblocked particles by Hanson et al. [18]. Further, magnetization decreases with decrease in temperature in both curves (Fig. 3). T s is not clearly observed from magnetization measurement. Specific heat versus temperature graph is shown as Fig. 3b. Both T c at 97 K and T s at 21 K are observed clearly and are indicated by arrows. In bulk and single crystals of CoCr2 O4 , T s is observed at 31 K and 24 K, respectively [8, 14]. The peak in M ZFC is a typical signature of SPM, superferromagnet (SFM) and spin glass state. However, at this stage it is difficult to distinguish between SPM, spin glass and (SFM). We further carried out the magnetiza-
tion versus external magnetic field measurement to clarify the scenario. Figure 4 depicts the magnetization vs. external applied field curve after cooling in zero field at temperature above T c and in between T c and T s . The magnetization linearly increases with field while measured at 100 K which clearly shows the paramagnetic phase (Fig. 4a). We do not observe any hysteresis loop at 60 and 80 K (Fig. 4b). Magnetization readily increases at low field and then linearly increases up to maximum applied field. The magnetization measured at maximum applied field of 60 kOe is 0.152 and 0.148 μB /formula for 60 and 80 K, respectively, which is half of the value observed earlier (0.3 μB /formula) below T c [9]. At 50 K, non-saturated hysteresis loop appears showing a coercivity of ∼1.14 kOe, which is more than an order of magnitude higher than reported H c in single crystals (∼50 Oe) (Fig. 4c) [8]. The non-saturation of magnetization after applying 60 KOe field and an order of magnitude higher H c could be due to the disordered spin configuration existing at the surface of magnetic nanoparticles [19]. The reduction in magnetization could be due to the existence of random canting of spins at the surfaces [20] or could be due to disordered cation distributions among A and B sites [21, 22]. The appearance of a loop at 50 K and its disappearance at 60 and 80 K clearly suggest that the paramagnetic phase transforms to superparamagnetic phase at Curie temperature and
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(c) Fig. 4 Magnetization vs. external applied field measured at (a) 100 K, (b) 60 and 80 K, (c) 50 K of pure CoCr2 O4 nanoparticles
the blocking temperature, T b , lies between 50 and 60 K. Below T b , the superparamagnetic phase transforms to a longrange ferrimagnetic phase.
ther decrease in temperature transforms the superparamagnetic phase to a long-range ferrimagnetic phase keeping T c and T s the same as in the bulk phase.
4 Conclusion
Acknowledgement UGC-DAE Consortium for Scientific Research (CSR), Indore, is acknowledged for the support. DST, Govt. of India is acknowledged for funding the 14T-PPMS-VSM for magnetic measurement at CSR, Indore.
In summary, our investigations report the synthesis of nanoparticle of cobalt chromite through a coprecipitation technique by meticulously controlling the pH of precipitation. These nanoparticles undergo a paramagnetic to superparamagnetic phase transition at T c in contrast with the paramagnetic to ferrimagnetic phase transition in bulk. Fur-
References 1. Krupika, S., Novak, P.: In: Wolfarth, E.P. (ed.) Ferromagnetic Materials, vol. 3, p. 189. North-Holand, Amsterdam (1982)
J Supercond Nov Magn (2011) 24: 629–633 2. Bedanta, Subhankar, Kleemann, Wolfgang: J. Phys. D, Appl. Phys. 42, 013001 (2009) 3. Berry, C.C.: J. Phys. D, Appl. Phys. 42, 224003 (2009) 4. Iglesias, O., Labarta, A., Batlle, X.: J. Nanosci. Nanotechnol. 8, 2761 (2008) 5. Yamasaki, Y., Miyasaka, S., Kaneko, Y., He, J.P., Arima, T., Tokura, Y.: Phys. Rev. Lett. 96, 207204 (2006) 6. Broese Van Groenou, A., Bergers, P.F., Stuyts, A.L.: Mater. Sci. Eng. 3, 317 (1969) 7. Menyuk, N., Dwight, K., Wold, A.: J. Phys. 25, 528 (1964) 8. Tomiyasu, K., Fukunaga, J., Suzuki, H.: Phys. Rev. B 70, 214434 (2004) 9. Tomiyasu, K., Kamazawa, K.: Physica B 392, 16 (2007) 10. Rath, C., Anand, S., Das, R.P., Sahu, K.K., Kulkarni, S.D., Date, S.K., Mishra, N.C.: J. Appl. Phys. 91, 4 (2002) 11. Li, S., Bi, H., Tian, Z., Xu, F., Gu, B., Lu, M., Du, Y.: J. Magn. Magn. Mater. 281, 11 (2004) 12. Wang, Y., Zhou, Z., Jia, M., Zhu, X., Zhang, W., Jiang, D.: Catal. Lett. 104, 1 (2005)
633 13. Kim, D., Ihm, S.: Environ. Sci. Technol. 35, 222 (2001) 14. Patterson, A.L.: Phys. Rev. 56, 978 (1939) 15. Lawes, G., Melot, B., Page, K., Ederer, C., Hayward, M.A., Proffen, Th., Seshadri, R.: Phys. Rev. B 74, 024413 (2006) 16. Okamoto, Y., Nakano, H., Imanaka, T., Teranishi, S.: Bull. Chem. Soc. J. 48, 1163 (1975) 17. Rath, C., Sahu, K.K., Anand, S., Date, S.K., Mishra, N.C., Das, R.P.: J. Magn. Magn. Mater. 202, 77 (1999) 18. Hanson, M., Johansson, C., Morup, S.: J. Phys., Cond. Matter 7, 9263 (1995) 19. Kodama, R.H., Berkowitz, A.E., McNiff, Jr. E.J., Foner, S.: J. Appl. Phys. 81, 5552 (1997) 20. Coey, J.M.D.: Phys. Rev. Lett. 27, 1140 (1971) 21. Vollath, D., Szabu, D.V., Taylor, R.D., Willis, J.U.: J. Magn. Magn. Mater. 12, 2175 (1997) 22. Morrish, A.H., Li, Z.W., Zhou, Z.X.: J. Phys. (Paris) C1, 513 (1997)
