Effects of Ar/H₂ annealing on the microstructure and magnetic properties of CoO nanoparticles

Abstract

Wurtzite CoO nanoparticles were annealed at 200–400 °C for 1 h in Ar/H₂. The effects of the annealing temperature on the structure, morphology and magnetic properties were investigated in detail. As the annealing temperatures rise, the obtained samples change from the hcp-CoO/fcc-CoO complex phase to the fcc-CoO/fcc-Co complex phase to the fcc-Co single phase. Notably, the 300 °C- and 325 °C-annealed samples show the most distinct coexistence of ferromagnetic (FM) Co and antiferromagnetic (AFM) CoO components, and therefore display certain exchange bias (H_E = 284 and 250 Oe) and enhanced coercivity (H_C = 1583 and 1148 Oe) at 5 K. The magnetization M, H_C, H_E and Néel temperature T_N exhibit interesting change rules, which are determined by three factors: size effect, phase composition, and FM-AFM interface coupling effect. Obviously, as adjusting the annealing temperature can control the microstructure of CoO (or Co) particle samples, many superior magnetic properties can be expected.

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1. Introduction

Over the past few decades, the nanosized transition-metal oxides have attracted attention because of their unusual properties, in particular, magnetic and catalytic ones.^1,2 Among them, CoO, which as a bulk material is antiferromagnetic (AFM, T_N = 293 K) in its native rocksalt phase with an fcc structure,³ has gained special interest. So, as an example, it greatly optimizes the magnetic properties of small cobalt clusters by enclosing them with a shell of CoO,⁴ an exchange bias effect due to the magnetic coupling of ferromagnetic (FM) cobalt with antiferromagnetic CoO. Actually, the study of FM-AFM exchange interactions in fine particle systems has found interesting applications to improve the performance of permanent magnetic materials (by means of an enhancement of the coercivity which typically accompanies the hysteresis loop shift)^5,6 and to combat the superparamagnetic limit in magnetic recording media.^7,8 Hence, in fine particle systems exchange bias studies may be particularly interesting not only for the loop shift itself, but also for other exchange bias related phenomena.

In addition, the wurtzite CoO with hcp structure has been extensively prepared^9–12 and investigated as an “end-member” of the solid solution Zn_1−xCo_xO,¹³ which is of particular interest as such transition-metal-substituted semiconductors can exhibit ferromagnetism with up to high Curie temperatures.^14,15 This wurtzite-type CoO is metastable and could be prepared only in small clusters (<20 nm) containing, however, still impurities of cobalt.¹³ For the impurity free material, density functional theory (DFT) calculations including electron correlations, found antiferromagnetic spin ordering similar to rocksalt-type CoO.^16–18 In our previous works,^19–21 the pure fcc- and hcp-structured CoO nanoparticles with various sizes and shapes have been successfully synthesized by thermal decomposition of organic metal salt, and their magnetic properties have also been studied by using VSM, SQUID, PPMS and ESR tools.

Herein, based on the hcp-structured CoO nanoparticles (45 nm in size and hexagonal pyramidal in shape),²⁰ this sample was annealed at selected temperatures (200, 250, 275, 300, 325, 350, 375 and 400 °C) for 1 h in Ar/H₂. Effects of annealing temperature on structure, morphology and magnetic properties were investigated in detail. We made emphasis on the interpretation of magnetic parameters (magnetization M, coercivity H_C, exchange bias H_E, and Néel temperature T_N) in terms of size effect, phase composition, and FM-AFM interface coupling effect. As a consequence, the Co/CoO composite nanoparticles exhibit high H_E and enhanced H_C.

2. Experimental section

In our previous work,²⁰ the wurtzite CoO nanoparticles with pure hcp structure have been prepared by thermal decomposition of organometallic precursor Co(acac)₃ (acac≡acetylacetonate) in oleylamine. Herein this sample was annealed sequentially at selected temperatures (200, 250, 275, 300, 325, 350, 375 and 400 °C) for 1 h in Ar/H₂ (15 vol% H₂, 85vol% argon) flow. As shown in Table 1, the obtained eight powder samples were denoted as M1–M8, respectively.

Table 1 Structure and magnetic parameters for the samples M1–M8 obtained at different annealing temperatures

Sample	T (°C)	Crystal structure	Mr (emu g^−1)		Ms (emu g^−1)		HC (Oe)
			300 K	5 K	300 K	5 K	300 K	5 K
M1	200	hcp CoO/fcc CoO	0	0.002	—	—	0	20
M2	250	fcc CoO/hcp CoO	0.002	0.016	—	—	24	71
M3	275	fcc CoO	0.006	0.066	—	—	75	138
M4	300	fcc CoO/fcc Co	17.61	26.80	69.14	70.02	506	1261
M5	325	fcc Co/fcc CoO	24.51	34.93	121.99	122.54	383	910
M6	350	fcc Co	28.45	37.92	126.51	126.96	314	817
M7	375	fcc Co	31.98	41.06	130.13	131.08	306	743
M8	400	fcc Co	34.33	44.71	139.36	141.42	302	610

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Firstly, the effect of Ar/H₂ annealing on the microstructure of CoO nanoparticles was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and high-resolution transmission electron microscope (HRTEM). Based on the superconducting quantum interference device (SQUID), the magnetic measurement mainly including the following three aspects: (i) room-temperature and 5 K hysteresis (M–H) loops under zero-field-cooling (ZFC); (ii) exchange bias displayed by 5 K loop shifts under field-cooling (FC, from 350 K in a field of H_FC = 30 kOe); and (iii) ZFC and 100 Oe FC magnetization (M–T) curves in the range 2–330 K.

3. Results and discussion

To investigate the effect of Ar/H₂ annealing on the phase composition, the XRD patterns of hcp-structured CoO nanoparticles annealed at different temperatures were measured, as shown in Fig. 1. Comparing with the standard cards, it is obvious that the crystal structure of samples changes from hcp-CoO/fcc-CoO complex phase to fcc-CoO/fcc-Co complex phase, and then to fcc-Co single phase with the rise of annealing temperature. Among them, the coexistence of Co and CoO phases is most obvious for the samples annealed at 300 and 325 °C; however, the difference is that the antiferromagnetic fcc-CoO occupies main phase in the former, while the latter is ferromagnetic fcc-Co. To obtain the ideal magnetic properties, especially the high exchange bias and the enhanced coercivity, the above two samples are to be preferred. Higher annealing temperatures (350, 375 and 400 °C) cause some pure Co particles in which grain size is increasing. The controllable and relatively large size for the three samples makes sure they have relatively large magnetization and low coercivity. These inferences will soon be confirmed by the follow-up magnetic analysis. However, low annealing temperatures (200 and 250 °C) lead to the formation of CoO complex phase, in which it contains two structures: hexagonal hcp and cubic fcc. Therefore, low temperatures can not effectively promote the reduction of wurtzite CoO. Anyhow, Ar/H₂ annealing led to the occurrence of phase transition of hcp-CoO to fcc-CoO and fcc-Co; this process was accompanied by a growth of grain.

Fig. 1 XRD patterns of hcp-structured CoO nanoparticles annealed at different temperatures.

Fig. 2 shows the SEM images of hcp-structured CoO nanoparticles annealed at different temperatures. Obviously, these images truly reflect the change of particle size and shape with increasing annealing temperature. When an annealing temperature of 200 °C was used, pyramidal CoO nanoparticles with nanoscale can be seen clearly (Fig. 2a). Compared to the unannealed sample (namely the 45 nm wurtzite CoO nanoparticles),²⁰ low-temperature Ar/H₂ annealing does not change the morphology of particles. Actually, this is mainly because the hcp-CoO occupies main phase in the 200 °C-annealed sample. Further, the 250 °C- and 275 °C-annealed particles show a certain degree of passivation and aggregation (see Fig. 2b and c). When the annealing temperature reaches to 300 and 325 °C, some particles turn to quasi-spherical appearance, as shown in Fig. 2d and e. Herein, to some extent, the passivation or corrosion of some particles is due to the reduction of hcp-CoO to fcc-CoO and fcc-Co. With the occurrence of phase transition, the particle size has subjected to slight increase. As the annealing temperature further increased to 350–400 °C, the spherical particles get even bigger (Fig. 2f and g) and merge into large pieces (Fig. 2h). It can be speculated that these large Co particles will have large saturation magnetization. From the above SEM investigation, the change of particle size is consistent with the results of XRD analysis.

Fig. 2 SEM images of hcp-structured CoO nanoparticles annealed at (a) 200 °C, (b) 250 °C, (c) 275 °C, (d) 300 °C, (e) 325 °C, (f) 350 °C, (g) 375 °C, and (h) 400 °C.

The microstructure, crystallinity and phase composition of hcp-structured CoO nanoparticles annealed at 300 °C were further explored by TEM, as shown in Fig. 3. Firstly, the passivated pyramid-type nanoparticles with some degree of aggregation can be seen clearly (Fig. 3a). High-resolution lattice image of particle edge reveals the [111] and [200] orientations of CoO phase, as well as the [111] orientation of Co phase. The region bounded by red dash lines in Fig. 3b depicts a superimposed stripe consists of many lattices. These results confirm the coexistence of Co and CoO phases in the 300 °C-annealed sample. Finally, the SAED pattern (Fig. 3c) gives a further confirmation of fcc-CoO/fcc-Co complex phase and those perfect cubic symmetry shows that the 300 °C-annealed nanoparticles is a very good polycrystalline sample. From the above TEM investigation, both structure and shape is consistent with the results of XRD and SEM analyses. Considering the fact that the sample is of good Co/CoO interface, it will show excellent magnetic properties.

Fig. 3 Low magnification TEM image, HRTEM lattice fringe image and the corresponding SAED pattern of hcp-structured CoO nanoparticles annealed at 300 °C.

Since Ar/H₂ annealing not only changes the crystal structure of hcp-CoO nanoparticles but also has a big impact on the particle morphology. Then the Ar/H₂-annealed samples will have very big difference for magnetic properties. Based on this, the room-temperature and 5 K hysteresis loops of hcp-structured CoO nanoparticles annealed at different temperatures were measured by SQUID magnetometer, as shown in Fig. 4. For convenient comparison, the specific values of magnetic parameters such as remanent magnetization M_r, saturation magnetization M_s and coercivity H_C of all the samples are given in Table 1. For the 200 °C-annealed sample, the complete room-temperature paramagnetic behavior could be due to the absence of long-range magnetic ordering in hcp-CoO body.^22,23 At 5 K, however, this sample shows weak hysteresis (the inset in Fig. 4a corresponds to a coercivity of 20 Oe), which is due to the contribution of the uncompensated moments at the disordered surface of a small number of fcc-CoO nanoparticles.^24,25 And, more remarkable, the 275 °C-annealed sample displays a relatively large coercivity at 5 K (H_C = 138 Oe in Fig. 4c), which can be attributed to the numerous uncompensated surface spins contained in pure fcc-CoO phase. For the other five samples annealed at 300–400 °C, each exhibits strict ferromagnetic behavior at room temperature and 5 K. This is mainly because the presence of fcc-Co component in the five samples. Especially for the three samples annealed at 350, 375 and 400 °C, they displayed better ferromagnetic properties (such as higher saturation magnetization and smaller coercivity) because of their pure fcc-Co phase. Considering the 300 °C- and 325 °C-annealed samples, which are the Co/CoO composite nanoparticles, they shows relative high saturation magnetization accompany with large coercivity. Such interesting phenomenon is related to the exchange coupling interaction in the field of magnetism, and therefore the detailed analysis will be mentioned later.

Fig. 4 Room-temperature and 5 K hysteresis loops of hcp-structured CoO nanoparticles annealed at different temperatures.

Fig. 5a and b show the annealing temperature dependence of the main magnetic parameters at room temperature and 5 K, respectively. With increasing annealing temperature, the saturation magnetization M_s and remanent magnetization M_r increase monotonously. At room temperature and 5 K, the maximum value of M_s is 139.36 and 141.42 emu g⁻¹, respectively. Among them, 5 K loops reveal that the saturation magnetization (M_s = 70.02 emu g⁻¹) for the 300 °C-annealed sample is much lower than that (M_s = 122.54 emu g⁻¹) for the 325 °C-annealed sample. This is mainly because the antiferromagnetic CoO occupies main phase in the former, while the latter is ferromagnetic Co. As shown in Table 1, the magnetization at 5 K is slightly larger than that of room temperature due to the die out of thermal fluctuation at low temperatures.^26,27 Actually, the variations of M_r and M_s may partly depend on the change of phase composition of Ar/H₂-annealed samples; on the other hand, it indicates the influence of size effect. We already know from the previous analyses that the crystal structure of samples changes from hcp-CoO/fcc-CoO complex phase to fcc-CoO/fcc-Co complex phase to fcc-Co single phase with the rise of annealing temperature; and at the same time, the particle size increases monotonously. It is the two reasons that lead to monotone increase of magnetization.

Fig. 5 Variations of coercivity H_C, saturation magnetization M_s, and remanent magnetization M_r versus annealing temperature.

As for coercivity H_C, it increases at first and then decreases with increasing annealing temperature. In our opinion, three main factors relate to coercivity: the dual phase scale, the interface effect and the size effect. Firstly, size effect is especially pronounced on the last three samples M6–M8 annealed at 350, 375 and 400 °C. According to the results of XRD and SEM analyses, the three samples are pure Co particles. It is well known that H_C of ferromagnetic particles conforms to the rule of H_C ∝ 1/D,²⁸ where D denotes the average particle size. Therefore, for the Co samples M6–M8, the higher the annealing temperature, the larger the particle size, and the lower the coercivity. Secondly, the main phase of the 200 °C-annealed sample M1 is hcp CoO. Some experimental studies concluded that there is no long range magnetic ordering related to such wurtzite structure.^22,23 Thus, this sample shows entire room-temperature paramagnetic behavior with zero coercivity. At 5 K, however, the displayed tiny coercivity (H_C ≅ 20 Oe) can be attributed to the uncompensated surface spins in fcc CoO regarded as the second phase.^24,25,29 Finally, the 300 °C- and 325 °C-annealed samples M4 and M5 reveal relative large coercivity (H_C(T = 5 K) = 1261 and 910 Oe), mainly thanks to Co/CoO complex phase. Based on the previous XRD and TEM analyses, there are Co–CoO interfaces in this kind of composite nanoparticles. The coupling interaction between the ferromagnetic Co and antiferromagnetic CoO components allows a variety of reversal paths for the spins upon cycling the applied field, and thereby resulting in large coercivity.³⁰ As for the numerical difference between the two, it mainly depends on the reversal of main and second phases. Relative high content of antiferromagnetic phase in such Co/CoO samples could further enhance its coercivity.

It is generally known that field-cooling (FC) is needed to induce an exchange anisotropy in hybrid FM-AFM systems. In order to measure the exchange bias field, the samples annealed at 300 and 325 °C are cooled from 350 to 5 K in an applied magnetic field of 30 kOe. The magnetization is then measured as a function of an applied magnetic field at 5 K, as shown in Fig. 6. The exchange bias H_E is determined from the loop shift and calculated as H_E = H_C1 − H_C2/2, where H_C1 and H_C2 are the negative and positive coercive fields, respectively. For the two samples with Co/CoO complex phase, the field-cooled hysteresis loops differ from the zero-field-cooled loops (see Fig. 4d and e). The zero-field-cooled loops are perfectly symmetric; however, horizontal and vertical shifts are observed in the field-cooled hysteresis loops. This asymmetry proves existence of exchange biased nanoparticles in the two samples. To be specific, the samples annealed at 300 and 325 °C display certain exchange bias (H_E = 284 and 250 Oe) because of the coupling interaction between the FM Co and AFM CoO components. Further, such coupling effect allows a variety of reversal paths for the spins upon cycling the applied field,³⁰ and thereby resulting in the enhancement of coercivity (H_C = 1583 and 1148 Oe). In addition, the values of H_E and H_C for the former is greater than that of latter, this is strongly influenced by the percentage of AFM CoO phase.

Fig. 6 Hysteresis loops for the annealed samples at 5 K after field cooling (FC) from 350 K in a field of H_FC = 30 kOe; insets show the enlarged loops displaying the corresponding exchange bias.

Further, the temperature-dependent magnetization M(T) of the samples annealed at 300 and 325 °C was measured in a fixed field of 100 Oe. As can be seen from the ZFC/FC curves (Fig. 7), the Co component remains FM up to the Néel temperature T_N of AFM CoO. This indicates that an extra anisotropy is induced such that K_UV ≫ k_BT,⁴ where K_U is anisotropy, V is the particle volume and k_B is Boltzmann's constant. In this case, the FM Co moments are prevented from flipping over the energy barrier for all temperatures below T_N of CoO, and thus the Co/CoO composite nanoparticles remain magnetically stable below this temperature. For the 300 °C-annealed sample M4 (Fig. 7a), the peak value of ZFC curve gives a T_N of 295 K, slightly above bulk's T_N (293 K).³ On the one hand, the increase of T_N is usually ascribed to the size effect resulting from the nanoscale of AFM materials.³¹ On the other hand, in the exchange bias system the coupling interaction at the interfaces of ferromagnetic Co and antiferromagnetic CoO also promote the rise of T_N.^32–34 However, we do not see such a distinct AFM transition in the 325 °C-annealed sample (Fig. 7b). When the content of FM Co in FM/AFM system is high, its magnetic moment will dominate in the ZFC/FC curves and thereby masks the contribution of AFM CoO. Therefore no phase transition peak was observed in the ZFC curve of the Co/CoO sample M5 with tiny CoO content.

Fig. 7 100 Oe FC and ZFC magnetization curves of hcp-structured CoO nanoparticles annealed at (a) 300 °C, and (b) 325 °C.

4. Conclusions

Based on the hcp-structured CoO nanoparticles (45 nm in size), this sample was annealed at 200–400 °C for 1 h in Ar/H₂. Effects of annealing temperature on microstructure and magnetic properties were investigated by XRD, SEM, TEM and SQUID. The results indicate that the crystal structure of the annealed samples changes from hcp-CoO/fcc-CoO complex phase to fcc-CoO/fcc-Co complex phase to fcc-Co single phase with the rise of annealing temperature. Among them, the coexistence of Co and CoO phases is most obvious for the samples annealed at 300 and 325 °C. The above two samples display certain exchange bias (H_E = 284 and 250 Oe) because of the coupling interaction between ferromagnetic Co and antiferromagnetic CoO components, and such coupling effect resulting in large coercivity (H_C = 1583 and 1148 Oe). The magnetic properties, in terms of the values of M_r, M_s, H_C, H_E and T_N, show an interesting change with increasing annealing temperature, which are mainly determined by the size effect, the dual phase scale, and the interface effect.

Acknowledgements

The authors are grateful to the National Natural Science Foundation (Grant No. 11174132, 11474151 and U1232210), the National Key Project for Basic Research (Grant No. 2011CB922102 and 2012CB932304), the Innovation Program for Doctoral Research of Jiangsu Province (Grant No. CXZZ13_0035), and PAPD, the People's Republic of China, for financial support.

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