Transition Temperature of Wurtzite CoO Nanocrystals as Revealed in

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Transition Temperature of Wurtzite CoO Nanocrystals as Revealed in Comprehensive Magnetic Characterization Xuemin He,† Wei Zhong,*,† Shiming Yan,† Chao Liu,‡ Huigang Shi,‡ Chak-Tong Au,§ and Youwei Du† †

National Laboratory of Solid State Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China § Department of Chemistry, Hong Kong Baptist University, Hong Kong 852, People’s Republic of China ABSTRACT: We conducted magnetic study over wurtzite CoO nanocrystals (about 45 nm in size). The blocking temperature TB and Néel temperature TN were confirmed by comprehensive magnetic characterization. Below TB of ∼7 K the nanocrystals exhibit coercivity of 400 Oe and exchange bias of 206 Oe ascribable to composition influences of uncompensated surface spins as well as to antiferromagnetic volume phase. The uncompensated magnetic sublattice and the spatial distribution of the anisotropy axis relative to the magnetic field are proposed to be responsible for the distinct electron spin resonance (ESR) line shape. Based on the temperature dependence of ESR intensity, the accurate TN is found to be 245 K. It is observed that there is anomalous change in resonance field and line width around TN. antiferromagnetic ground state.16,17 On the other hand, some experimental investigations concluded that no long-range magnetic ordering is present in wurtzite CoO.18,19 Obviously, the magnetic properties of wurtzite CoO nanocrystals still need to be further verified by comprehensive magnetic characterization. For a magnetic nanomaterial, the transition temperature is a very important parameter. Usually, the transition temperature of AF nanocrystals includes two types: blocking temperature TB and AF ordering temperature TN.20,21 It is well-known that the measurements of low-temperature hysteresis loop and zerofield-cooled/field-cooled magnetization curves can provide the information on the net moment, coercivity, exchange bias and average TB for the antiferromagnets equipped with uncompensated surface spins.22,23 Furthermore, electron spin resonance (ESR) can be used to study the correlation of static and dynamic magnetic behaviors at microscopic level. According to ESR analysis, researchers have gained better understanding on the magnetic phase and ordering temperature TN of many AF materials.24−26 It is hence hopeful to use the ESR technique for the clarification of the magnetic ordering and/or magnetic state of wurtzite CoO nanocrystals. As for the nanocrystals of transition-metal monoxide, MnO and FeO are extremely unstable,2,3 while the anomalous magnetic properties and transition temperature of MnO and NiO nanocrystals,27−31 as well as cubic CoO nanocrystals have been well studied by

1. INTRODUCTION Magnetic properties of nanosized materials are of great importance from fundamental research as well as technical applications point of view. As initially noted by Néel in 1962,1 small antiferromagnetic (AF) materials may exhibit superparamagnetism or weak ferromagnetism due to inexact compensation of the magnetic sublattices, an effect which increases with decreasing specimen dimension. More interestingly, the properties of these uncompensated spins usually dominate the net magnetic properties of AF materials, and their presence simultaneously facilitates and complicates a magnetic characterization. Given all this, in the following half-century, the anomalous magnetic properties such as large moment, enhanced coercivity, loop shift and size-dependent transition temperature were extensively studied in many AF materials, such as MO-type (M ≡ Mn, Fe, Co, Ni, Cu) nanoparticles,2−6 CoO layers,7,8 CoO thin films,9 and NiO/CoO superlattices.10 Among them, the monoxide AF nanoparticles are the most research subject based on the analyses of low-temperature magnetic property, AF ordering temperature TN and blocking temperature TB. CoO, as a transition-metal monoxide, typically crystallizes in one of two stable phases:11 rocksalt-type cubic (space group Fm3̅m) and wurtzite-type hexagonal (space group P63mc). There are many reports on the preparation, microstructures and anomalous magnetic properties of cubic CoO nanocrystals.12−15 However, few works were reported on the magnetic properties of wurtzite CoO nanocrystals; even in these reports, there are controversies. On the one hand, some electronic structure calculations predicted that wurtzite CoO has an © 2014 American Chemical Society

Received: February 6, 2014 Revised: May 27, 2014 Published: June 2, 2014 13898

dx.doi.org/10.1021/jp501313r | J. Phys. Chem. C 2014, 118, 13898−13903

The Journal of Physical Chemistry C

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numerous characterization methods such as superconducting quantum interference device (SQUID) and ESR. However, to our knowledge, none of studies have been reported for the transition temperature of wurtzite CoO nanocrystals. With such a background, SQUID and ESR investigations of wurtzite CoO nanocrystals are meaningful. In this paper, we report the synthesis and microstructures of 45 nm wurtzite CoO nanocrystals. We focus on the low-temperature magnetic properties (i.e., net magnetization, coercivity and exchange bias) and the line shape, resonance field, line width, and the signal intensity of ESR spectra. The blocking temperature TB and Néel temperature TN are obtained as a result of comprehensive magnetic characterization.

2. EXPERIMENTAL SECTION The wurtzite CoO nanocrystals employed in this study were prepared by thermal decomposition of organometallic precursor Co(acac)3 (acac ≡ acetylacetonate) in Ar. First, a green slurry of Co(acac)3 and oleylamine was heated at 130 °C for 10 min in an Ar atmosphere. Then, decomposition was initiated by flash-heating to 220 °C. After the solution was annealed for 1 h, the reaction mixture was cooled down to room temperature. Finally, the CoO nanocrystals were separated by centrifugation and purified by washing with ethanol. X-ray diffraction (XRD) measurement was performed with a Rigaku D/Max-2400 powder diffractometer equipped with a rotating anode and Cu Kα radiation (λ = 0.15406 nm). For transmission electron microscopic (TEM) investigation, a few drops of dilute dispersion of nanocrystals was landed on a carbon-coated copper grid, and subject to the evaporation of solvent. High-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) measurements were performed using a JEOL JEM-2100 instrument equipped with an energydispersive X-ray (EDX) analyzer. The dc magnetization measurements were carried out at 500 Oe after zero-fieldcooling (ZFC) or field-cooling (FC) over a superconducting quantum interference device (SQUID, MPMS-XL) magnetometer with 70 kOe maximum field, and the hysteresis loops were measured at T = 5 K after ZFC and FC (from 350 K) in a field of HFC = 50 kOe. The electron spin resonance (ESR) spectra were recorded using a JEOL FA200 spectrometer at 8.98 GHz in the 80−300 K temperature range.

Figure 1. (a) XRD pattern and (b) EDX spectrum of the as-prepared wurtzite CoO nanocrystals. (a) Experimental XRD data shown as red dots; calculated pattern from Rietveld profile refinement, black line; background, blue line; short vertical bars, positions of the Bragg reflections of hexagonal CoO (h-CoO). (b) Inset TEM image corresponds to the region of EDX analysis, and inset table shows quantified results of EDX elemental analysis.

the copper and carbon signals from the carbon-coated copper grid, the average composition was found to be Co: 78.67 wt % (50.04 at%) and O: 21.32 wt % (49.95 at%), which is close to the stoichiometric composition of CoO. Furthermore, the morphology, particle size, and crystallinity of the as-prepared CoO nanocrystals were investigated by TEM, and the results are shown in Figure 2. The low-resolution TEM image in Figure 2a shows that all of the CoO nanocrystals were hexagonal pyramidal in shape and the aggregation of particles occurred. As shown in Figure 2b,c, the hexagonal pyramidal shape was confirmed by viewing both hexagons and triangles in the TEM images. The length of the side edge (corresponding to the triangle) was almost twice that of the basal edge (corresponding to the hexagon). According to statistical measurements of more than 100 particles, we obtained a histogram of the size distribution (of the side edge length), as shown in Figure 2d. It is clear that the average particle size of CoO nanocrystals was 44.96 ± 0.20 nm, consistent with the XRD results. We studied the lattice and crystallinity of the CoO nanocrystals based on HRTEM images collected over the hexagonal and triangular nanocrystals (Figure 2e,h). The lattice spacing (d) of 2.81 Å in the hexagon image corresponds to the interplanar separation between (100) lattice planes, whereas the triangle image shows the lattice fringes (d = 2.60 Å) of (002) planes. The clear and single type of lattice fringes indicates that the as-prepared CoO nanocrystals were free of dislocations and stacking faults. Correspondingly, the two-dimensional fast Fourier transforms (FFTs) of the hexagon and triangle images are shown in Figure 2f,g, depicting the [001] and [11̅0] zone axes of CoO, respectively. In addition, the regular dot matrix composed of many bright spots further confirms the hcp

3. RESULTS AND DISCUSSION The phase purity of the as-prepared nanocrystals was investigated by powder X-ray diffraction (XRD) (Figure 1a). The peaks can be indexed to the characteristic peaks of wurtzite CoO phase with hexagonal close-packed (hcp) structure (P63mc, a = 3.253 Å, c = 5.209 Å, JCPDS no. 80-0075). We detected no difference in peak position between the experimental and standard XRD patterns in the 2θ = 20−80° range, indicating the absence of additional strain on the produced wurtzite CoO nanocrystals. After Rietveld profile refinement, it is concluded that the sample is phase-pure with cell parameters a = 3.259(4) Å and c = 5.210(7) Å. There is no detection of any other secondary phases, such as metallic Co, cubic CoO or Co3O4, confirming the phase purity of the synthesized sample. The strong and sharp XRD peaks suggest good crystallinity of the wurtzite CoO nanocrystals. Based on the Scherrer formula, the average crystallite size is estimated to be about 45 nm. In the EDX spectrum (Figure 1b), disregard 13899



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Figure 2. (a) Low-magnification TEM image, (d) size distribution histogram, and (i) SAED pattern of the as-prepared wurtzite CoO nanocrystals. TEM images of (b) single hexagonal and (c) trigonal nanocrystals. (f) HRTEM lattice fringe images and (g) corresponding FFT patterns of (e) hexagonal and (h) triangular nanocrystals. The red line in panel d denotes the results of Gaussian fitting.

blocked in the temperature range of dT around T. It is known that TB follows the relationship of TB = KaV/25kB, where Ka is the magnetocrystalline anisotropy constant, V is the magnetic volume of nanoparticles, and kB is the Boltzmann constant. Considering the temperature independence of Ka, TB depends only on the volume of individual nanoparticles.34 Therefore, a plot of d(ΔM)/dT represents a function of particle-volume density. It is appropriate to obtain the median TB from the lognormal density function.35,36 Shown in the inset of Figure 3b is a fit of −d(ΔM)/dT versus T to the log-normal distribution for the wurtzite CoO nanocrystals, yielding a value of 4.29 K as the median TB and 0.02 as the corresponding variance. Figure 3c shows the 5 K hysteresis loops of the wurtzite CoO nanocrystals obtained at 50 kOe. The ZFC loop is symmetric around the origin, whereas the FC loop is displaced from the origin and broadened. The value of the displacement defines directly the exchange bias field HE. A relatively large HE and enhancement of coercivity HC were found in the FC case, as can be seen in the inset of Figure 3c. To be specific, the HC(ZFC), HC(FC), and HE values were found to be 358, 400, and 206 Oe, respectively. In fact, the shape of hysteresis loops can be divided into curved and linear portions, corresponding to the ferromagnetic and antiferromagnetic contributions, respectively.37 Of them, the former is attributed to the increase in uncompensated moments at the disordered particle surface resulting from the reduced coordination of surface spins, whereas the latter is mainly due to the antiferromagnetic structure of the volume phase.38,39 As has been previously reported for NiO nanoparticles,40,41 observations of large HC and HE values are expected because of surface termination of antiferromagnetic structure. Herein, the broad and asymmetric FC loop is further explained as a result of multiple sublattice formation. According to the multisublattice model,40,42 the lower coordination of surface moments affects the overall antiferromagnetic structure of the entire nanoparticle. It can be deduced that, compared to their bulk counterpart, CoO nanocrystals are larger in the canting of spins and number of

structure of wurtzite CoO nanocrystals. The SAED pattern in Figure 2i shows clear lattice planes, again confirming the pure crystallinity of the as-synthesized nanocrystals. The spotty rings in the SAED pattern can be attributed to the random orientation of CoO nanocrystals. ZFC and FC measurements are commonly used to determine the blocking temperatures, TB, of magnetic nanomaterials. The temperature dependence of the corresponding magnetizations, MZFC and MFC, is shown in Figure 3a. It is apparent that MFC increases monotonically with decreasing temperature whereas MZFC shows a peak at Tmax ≈ 7 K (see inset). It can be seen that, when the temperature was less than TB, the magnetic moment of each nanoparticle was blocked along one of the anisotropy directions and did not respond to a weak applied field. It was hence considered that magnetization is dependent on magnetic history, which explains the difference in FC and ZFC magnetizations. One can see that the FC curve rises continuously whereas the ZFC curve declines for temperatures below 7 K. As shown in the inset of Figure 3a, the branching of MZFC from MFC started between 10 and 15 K, and magnetic blocking progressed gradually with decreasing temperature. Bulk CoO is known to undergo an antiferromagnetic transition at around 300 K.32 However, in the present study, we did not find such a distinct transition over our nanoscale CoO sample in the MZFC dc magnetization curve. The antiferromagnetic transition was eliminated because of the decrease of particle size, plausibly a result of ferromagnetic interactions in nanodimensions. Analogous results were also observed over cubic CoO nanoparticles by Ghosh et al.15 and Dutta et al.33 At any temperature T below the temperature where branching starts (expressed as branching temperature hereafter), the difference between MFC and MZFC (i.e., ΔM ≡ MFC − MZFC) gives the extent of blocked magnetization. A plot of ΔM versus T gives a branching temperature of ∼13 K (corresponding to ΔM = 0), as can be seen in Figure 3b. Furthermore, d(ΔM)/dT is the magnetization of nanoparticles 13900



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enhancement of the spectral asymmetry. In the above analysis, anomalous magnetic properties such as relatively large HC and HE values are difficult to understand in terms of two-sublattice antiferromagnetic ordering, whereas it is acceptable for bulk CoO. Consequently, the uncompensated magnetic sublattice becomes another factor that determines the ESR line shape. As shown in Figure 4b, there is a marked decrease in signal amplitude when the temperature diminishes within a certain range. This behavior can be related to the antiferromagnetic ordering temperature of wurtzite CoO nanocrystals. It is known that the resonance field, Hr, is a typical characteristic of the resonance spectrum. For simplicity, we take the observed zero level in the ESR absorption spectrum as Hr. The temperature dependence of Hr for the wurtzite CoO nanocrystals is shown in Figure 4c. It can be seen that Hr increased slowly at first and then rapidly with increasing temperature. Another important characteristic of the ESR spectrum is the line width, ΔHpp. Herein, the peak-to-peak width of the ESR absorption spectrum is defined as ΔHpp and can be obtained from the point at which the peaks are farthest apart. Figure 4d shows the variation of ΔHpp as a function of temperature. It is clear that ΔHpp exhibited a minimum at a certain transition temperature and then remained unchanged with the increase of temperature, which can be attributed to the increase in critical spin fluctuations near TN. Analogous changes in Hr and ΔHpp were also observed for MnO nanoparticles.27,29 Because the transition temperature is the temperature at which Hr and ΔHpp exhibit anomalous change, we preliminarily considered the TN value of 45-nm wurtzite CoO nanocrystals to be ∼245 K. The deduction was further confirmed in a later analysis, as discussed below. From the room-temperature (298 K) ESR spectrum, the g value was calculated from the peak-to-peak line width ΔHpp according to the relation g = hν/μBHr,44 where h is the Planck constant, ν is the microwave frequency, and μB is the Bohr magneton. Substituting the related parameters (h = 6.62 × 10−34 J·s, μB = 9.27 × 10−28 J/Oe,45 ν = 8.98 × 109 Hz, Hr = 2.99 × 103 Oe) into this equation, the g value was determined to be 2.14. The result clearly indicates the presence of high-spin Co2+ with S = 3/2. As mentioned in some literature reports,24,26 the ESR technique is sensitive to magnetic heterogeneity, even if it involves a relatively small number of spins, and the ESR intensity is an important parameter in determining the resonant magnetic ions. According to Rubinstein et al.,31 the ESR intensity is a measure of the number of spins, and the ordering temperature of antiferromagnetic nanoparticles can be deduced from the temperature dependence of the ESR intensity, IESR. To obtain the exact antiferromagnetic order temperature of wurtzite CoO nanocrystals, we normalized the ESR intensity IESR with respect to the peak value of the integrated ESR spectrum (see Figure 4b). The temperature dependence of IESR for the 45-nm wurtzite CoO nanocrystals is shown in Figure 4e. As the testing temperature increased from 80 to 300 K, IESR increased monotonically at first, reached a maximum at a particular transition temperature, and then decreased quickly. The transition temperature as described here, namely, the Néel temperature TN, can be accurately considered to be 245 K.

Figure 3. (a) Temperature dependence of FC and ZFC dc magnetization M for wurtzite CoO nanocrystals measured in a field of H = 500 Oe; inset shows the detail of the low-temperature maximum in MZFC. (b) Temperature dependence of MFC − MZFC (i.e., ΔM); inset shows a log-normal fit to the −d(ΔM)/dT versus T data. (c) Hysteresis loops for wurtzite CoO nanocrystals at 5 K after ZFC and FC from 350 K in a field of HFC = 50 kOe; inset shows the enlarged loops displaying the corresponding coercivity and loop shift.

magnetic sublattices. As a result, the relatively weak coupling between the sublattices allows a variety of reversal paths for spins upon cycling of the applied field, resulting in enhanced HC and loop shift. To gain further insight into the microscopic magnetic properties, especially the antiferromagnetic ordering temperature, we performed ESR analysis. Panels a and b of Figure 4 show the ESR absorption spectra and integrated ESR spectra, respectively, of 45-nm wurtzite CoO nanocrystals. It can be observed that the resonance line is asymmetric even at room temperature (in the paramagnetic phase). This behavior is consistent with a noncubic-symmetry system. According to paramagnetic resonance theory,43 the ESR spectrum of a powder sample is the sum of ESR spectra of nanoparticles where the principal symmetry axes are different in orientation relative to the magnetic field. In other words, the spatial distribution of orientation is a factor influencing the line shape of the ESR spectrum. This is because, when the particle size is reduced to nanoscale, there is larger spatial distribution of the anisotropy axis (relative to the magnetic field), and the result is

4. CONCLUSIONS In summary, we performed a systematic magnetic study of wurtzite CoO nanocrystals based on SQUID and ESR characterizations. The 45-nm wurtzite CoO nanocrystals were 13901



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Figure 4. (a) ESR absorption spectra and (b) integrated ESR spectra obtained at different temperatures over the as-prepared wurtzite CoO nanocrystals. (c−e) Temperature dependences of (c) resonance field Hr, (d) peak-to-peak line width ΔHpp, and (e) normalized ESR intensity IESR.

found to have a blocking temperature of ∼7 K and to exhibit hysteresis at 5 K. The large coercivity (400 Oe) and exchange bias (206 Oe) confirmed the existence of uncompensated spins. The spatial distribution of the anisotropy axis and the uncompensated magnetic sublattice are important factors influencing the ESR line shape. The temperature dependence behaviors of the resonance field, line width, and ESR intensity provide evidence for the existence of antiferromagnetic ordering temperature, and thus, the accurate Néel temperature of 45-nm wurtzite CoO nanocrystals is 245 K.

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(5) Winkler, E.; Zysler, R. D.; Vasquez Mansilla, M.; Fiorani, D. Surface Anisotropy Effects in NiO Nanoparticles. Phys. Rev. B 2005, 72, 132409. (6) Zheng, X. G.; Xu, C. N.; Nishikubo, K.; Nishiyama, K.; Higemoto, W.; Moon, W. J.; Tanaka, E.; Otabe, E. S. Finite-Size Effect on Néel Temperature in Antiferromagnetic Nanoparticles. Phys. Rev. B 2005, 72, 014464. (7) Ambrose, T.; Chien, C. L. Finite-Size Effects and Uncompensated Magnetization in Thin Antiferromagnetic CoO Layers. Phys. Rev. Lett. 1996, 76, 1743−1746. (8) Tang, Y. J.; Smith, D. J.; Zink, B. L.; Hellman, F.; Berkowitz, A. E. Finite Size Effects on the Moment and Ordering Temperature in Antiferromagnetic CoO Layers. Phys. Rev. B 2003, 67, 054408. (9) Alaria, J.; Cheval, N.; Rode, K.; Venkatesan, M.; Coey, J. M. D. Structural and Magnetic Properties of Wurtzite CoO Thin Films. J. Phys. D: Appl. Phys. 2008, 41, 135004. (10) Abarra, E. N.; Takano, K.; Hellman, F.; Berkowitz, A. E. Thermodynamic Measurements of Magnetic Ordering in Antiferromagnetic Superlattices. Phys. Rev. Lett. 1996, 77, 3451−3454. (11) Rao, C. N. R.; Subba Rao, G. V. Transition Metal Oxides: Crystal Chemistry, Phase Transitions, and Related Aspects; National Standard Reference Data System; National Bureau of Standards: Washington, D.C., 1974. (12) Yin, J. S.; Wang, Z. L. Ordered Self-Assembling of Tetrahedral Oxide Nanocrystals. Phys. Rev. Lett. 1997, 79, 2570−2573. (13) Zhang, L. Y.; Xue, D. S.; Gao, C. X. Anomalous Magnetic Properties of Antiferromagnetic CoO Nanoparticles. J. Magn. Magn. Mater. 2003, 267, 111−114. (14) Zhang, H. T.; Chen, X. H. Controlled Synthesis and Anomalous Magnetic Properties of Relatively Monodisperse CoO Nanocrystals. Nanotechnology 2005, 16, 2288. (15) Ghosh, M.; Sampathkumaran, E. V.; Rao, C. N. R. Synthesis and Magnetic Properties of CoO Nanoparticles. Chem. Mater. 2005, 17, 2348−2352. (16) Risbud, A. S.; Snedeker, L. P.; Elcombe, M. M.; Cheetham, A. K.; Seshadri, R. Wurtzite CoO. Chem. Mater. 2005, 17, 834−838. (17) Han, M. J.; Yu, J. Electronic Structure and Magnetic Properties of Wurtzite CoO. J. Korean Phys. Soc. 2006, 48, 1496. (18) Seo, W. S.; Shim, J. H.; Oh, S. J.; Lee, E. K.; Hur, N. H.; Park, J. T. Phase- and Size-Controlled Synthesis of Hexagonal and Cubic CoO Nanocrystals. J. Am. Chem. Soc. 2005, 127, 6188−6189.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation (Grants 50801033 and 11174132); the National Key Project for Basic Research (Grants 2011CB922102 and 2012CB932304); the Innovation Program for Doctoral Research of Jiangsu Province (Grant CXZZ13_0035); and PAPD, the People’s Republic of China, for financial support.

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Transition Temperature of Wurtzite CoO Nanocrystals as Revealed in

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