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Superparamagnetic Behavior of Antiferromagnetic Cuo Nanoparticles G IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 3409 Superparamagnetic Behavior of Antiferromagnetic CuO Nanoparticles G. Narsinga RaoIYP,Y.D.YaoI, and J. W. ChenP Institute of Physics, Academia Sinica, Taipei 10617, Taiwan, R. O. C. Department of Physics, National Taiwan University, Taipei 10617, Taiwan, R.O.C. We report on the magnetic properties of CuO nanoparticles prepared by the sol-gel method. M (T) curves show a blocking temperature at f aIQSK and 160 K for 13 nm and 17 nm size particles, respectively, in measuring field raIHHOe. The M-H data for both the samples show a reversal magnetization at 300 K, whereas below Tf it exhibit symmetrical hysteresis loops with a coercive field of 200 Oe (155 Oe) and a remanence 0.306 emu/mole (0.171 emu/mole) for 13 nm (17 nm) at 5 K. Presence of superparamagnetism and net magnetic moments of CuO nanoparticles are attributed to the uncompensated CuPCions at the surface of the particles. Index Terms—Antiferromagnetism, blocking temperature, CuO nanoparticles, superparamagnetism. I. INTRODUCTION II. EXPERIMENT HE magnetic properties of antiferromagnetic nanoparti- CuO nanoparticles were prepared by the sol-gel method. The T cles (AFN) have been receiving an intensive interest in copper hydroxide, Cu(OH) used as a precursor was prepared by the last few years because of their potential due to the small in- reacting aqueous solutions of 0.1 M copper nitrate, Cu(NO ) trinsic magnetic moment for exhibiting magnetization reversal 2.5 H O and 0.9 M sodium hydroxide, NaOH solution whose by quantum tunneling [1]. The antiferromagnetic nanoparticle pH was at room temperature. The resulting blue gel was systems below their Néel temperature provide a particular inter- washed several times with distilled water until free of nitrate esting case due to uncompensated surface spins, which gives rise ions, later centrifuged and dried in air at 333 K for 12 h. CuO to ferromagnetic-like moment [2]. Temperature dependent mag- nanoparticles were synthesized by heat treating the dried gel in netic effects of the surface spins lead to several very interesting air for 3 h at the temperatures of 433 K and 473 K, in order to de- phenomena like superparamagnetism, magnetic hysteresis, ex- compose the hydrous copper oxide and crystallize it to the mon- change bias and interparticle interactions [2], [3]. The origin of oclinic CuO. The structure and phase purity of the CuO nanopar- the net magnetic moments of AFN is still a matter of investi- ticles were checked by powder X-ray diffraction (XRD) using gation due to the complicated surface effects. A detailed un- a Philips PW-1710 diffractometer with a Cu-K radiation at derstanding of these complex but interesting properties is very room temperature. The morphology of the samples was obtained essential to study the presence and role of magnetic nanoparti- using field-emission scanning electron microscope (FE-SEM). cles in several areas of science and technology including spin- The field-cooled (FC) and zero-field-cooled (ZFC) magnetiza- tronics, biomedical research, and catalysis. Most of the studies tion were performed in a SQUID magnetometer (Quantum De- are concentrated on understanding the magnetic properties of sign Model MPMS) with the temperature range from 4 K to 300 antiferromagnetic nanoparticle such as NiO [4]–[6], MnO [7], K and the hysteresis curves were measured for kOe H and -Fe O [8], which display a range of interesting size and kOe. The magnetization data were carried out on tightly surface effects. packed powder samples placed in a white straw. The value of Copper oxide, CuO, is a unique both structurally and magnet- diamagnetic susceptibility of the straw is around ically among the monoxides of the 3d transition elements. The emu/Oe. The background signal from the sample holder was crystal structure of CuO is a low symmetry monoclinic in con- corrected. trast to the cubic structure of other 3d transition metal monox- ides. This compound is low-dimensional antiferromagnet with a Neel temperature, K. The diverse results obtained III. RESULTS AND DISCUSSIONS by different authors have been attributed to the existence of in- The X-ray diffraction patterns of CuO bulk and nanoparti- trinsic defects like cation or anion vacancies [9], [10], particle cles are shown in Fig. 1. The XRD revealed single-phase pattern size effects [11], and uncompensated charges [9], [12]. In this without any observable traces of impurity phase and the patterns paper, we present our recent work on the preparation and mag- can be indexed to monoclinic structure with space group C2/c. netic properties of CuO nanoparticles. The lattice constants of CuO nanoparticles were obtained by re- finement of the XRD data and found to be , and , which is in consis- tent with bulk CuO [13]. The average crystallite sizes of CuO samples prepared by heating Cu(OH) at 433 K and 473 K were determined to be 13 nm and 17 nm, respectively, from the XRD Digital Object Identifier 10.1109/TMAG.2005.855214 peak width employing the Scherrer relation with instrumental 0018-9464/$20.00 © 2005 IEEE 3410 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 Fig. 1. XRD patterns of CuO nanoparticles and bulk. Fig. 3. Temperature dependence of FC and ZFC magnetization measured under an magnetic field of 100 Oe for 13 nm and 17 nm CuO particles. confirms the purity of CuO nanocrystals. The FE-SEM images of CuO particles prepared by heating Cu(OH) at (a) 433 K and (b) 473 K are depicted in Fig. 2. The CuO nanoparticles are agglomerate in to uniform shuttle like morphology, which were produced in large quantities. This is because the small nanocrys- tals posses large surface energy, which leads the nanocrystals to aggregate in order to lower their surface energy during crystal growth. Fig. 3 shows the temperature dependence of FC and ZFC magnetization measured in an applied magnetic filed of 100 Oe for 13 and 17 nm CuO particles. The temperature below which the FC and ZFC magnetization curves separate (bifur- cation point), which is associated with a blocking temperature related to the larger particle size in the sample. This tempera- ture is decreased with decreasing particle size and increasing magnetic field. The significance of this temperature is that the whole sample becomes superparamagnetic above this temper- ature; the particles are free to align with the field during the measuring time. Below this temperature the FC magnetization increases monotonically. The ZFC curve exhibits a maximum and shifts downwards with decreasing particle size as well as with increasing magnetic field, which defines a typical blocking process of an assembly of superparamagnetic particles. The av- erage blocking temperature, T found to be 135 K and 165 K for 13 nm and 17 nm particles, respectively. The ZFC peak is broadened hinting at a distribution of relaxation times because of a distribution of particle environments. This indicates that the CuO nanoparticles exhibit weak ferromagnetic character in spite of the antiferromagnetic for bulk CuO. Further evidence of this behavior was inferred from hysteresis loops shown in Fig. 4 Fig. 2. FE-SEM images of CuO nanoparticles prepared by heating copper for both samples. The variation of magnetization with magnetic hydroxide at (a) 433 K and (b) 473 K. filed H data was collected after ZFC at 5 K (well below T ) and 300 K (well above T ). The M-H data for both samples ex- correction. For further evidence the purity and composition of hibit features of superparamagnetic particles, such as symmet- the product was obtained by ESCA. It indicates the correct com- rical hysteresis loops with a coercive field of 200 Oe (155 Oe) position and no other peaks of impurities were observed, which and a remanence 0.306 emu/mole (0.171 emu/mole) at 5 K for RAO et al.: SUPERPARAMAGNETIC BEHAVIOUR OF ANTIFERROMAGNETIC CUO NANOPARTICLES 3411 two contributions clearly. The two contributions to magnetiza- tion: , where the first term, M , is responsible for the large magnetization value observed due to the non compensation of the surface spins and second term, H, is antiferromagnetic contribution. The fitting parame- ters of the high field region at 5 K emu/mole (0.805 emu/mole) and emu/mole Oe ( emu/mole Oe); at 300 K emu/mole (0.481 emu/mole) and emu/mole Oe ( emu/mole Oe) for particle size of 13 nm (17 nm). The magnetic contributions of both antiferromagnetic and the non compensation of surface spins are larger for particle size of 13 nm. In summary, we observed coexistence of paramagnetic surface contribution and antiferromagnetic behavior in CuO Fig. 4. Isothermal M (H) curves for 13 and 17 nm CuO particles measured under ZFC at 5 K. Inset: expansion of lower field data at 5 K (open) and 300 K nanoparticles. The magnetic measurements clearly indicate the (solid) for both the samples. presence of a superparamagnetism in CuO nanoparticles, which may be arising from the presence of uncompensated Cu ions 13 nm (17 nm) particles, whereas the coercivity and remanence at the surface of the particles. at 300 K are nearly negligible (inset of Fig. 4). Magnetic measurements performed on CuO nanoparticles ACKNOWLEDGMENT revealed that the energy barrier prohibiting spins rotation could overcome by thermal energy. At room temperature and any This work was supported by the National Science Council, temperature above T , the particles have sufficient thermal en- Taiwan, R.O.C. under Grants NSC93-2811-M-001-015 and ergy to overcome the energy barrier and thus the magnetic spins NSC93-2112-M-002-038. are free to fluctuate between orientations. This process is called superparamagnetic relaxation.
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