NiFe2O4 nanocomposites

JOURNAL OF APPLIED PHYSICS 109, 07D711 (2011)

Exchange bias effect in NiO/NiFe2O4 nanocomposites W. J. Gong, W. Liu,a) D. Li, S. Guo, X. H. Liu, J. N. Feng, B. Li, X. G. Zhao, and Z. D. Zhang Shenyang National Laboratory for Materials Science and International Centre for Materials Physics, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China

(Presented 18 November 2010; received 21 September 2010; accepted 8 November 2010; published online 24 March 2011) A series of (100x)NiO/(x)NiFe2O4 nanocomposites (x ¼ 0, 2.5, 5, 8.3, 12.5, 25) synthesized by a chemical coprecipitation method have been investigated. The exchange bias field HE of the nanocomposites reaches a maximum at x ¼ 2.5, and then decreases with increasing x. The decrease of HE is attributed to the formation of isolated ferrimagnetic NiFe2O4 clusters, which is confirmed by observation with the use of high resolution transmission electron microscopy. The temperature dependence of HE and the coercivity HC for pure NiO is different from those with other samples, which is due to the exchange coupling between the uncompensated antiferromagnetic core and C 2011 American Institute of Physics. disordered surface shell of NiO nanoparticles. V [doi:10.1063/1.3544505]

I. INTRODUCTION

Exchange bias (EB) in nanoscale particles has attracted great interest because of the unique properties of nanoparticles and their potential technological applications.1 This effect was first discovered by Meiklejohn and Bean in oxidecoated Co particles in 1956,2 which referred to the shift of a hysteresis loop along the magnetic field axis. The magnetic properties are of particular interest in nanoparticles, such as superparamagnetism, disordered surface shell of antiferromagnetic (AFM) nanoparticles, since the spins and their environment are different from bulk materials.3 With numerical modeling of the spin configuration, Kodama et al.4 pointed out that AFM NiO nanoparticles exhibit anomalous magnetic behavior at low temperature and present finite size effects. Most of the previous studies have been focused on FM metal particles embedded in their AFM oxides,5 or FM/AFM bilayers. Although there are a few systematic studies of the EB effect of the FM/AFM ratios in nanocomposites,6–9 little attention has been given to the EB in AFM/ferrimagnetic (AFM/FI) NiO/NiFe2O4 nanocomposites. In this work, we report on the EB effect in the system of AFM/FI NiO/ NiFe2O4 nanocomposites with different NiO/NiFe2O4 ratios. II. EXPERIMENT

The (100x)NiO/(x)NiFe2O4 nanocomposites with x ¼ 0, 2.5, 5, 8.3, 12.5, 25 were prepared by a chemical coprecipitation method.10 High-pure NiCl26H2O, FeCl36H2O, and NH4HCO3 were employed as starting raw materials. Mixed metal hydroxides were obtained by the addition of a NH4HCO3 solution to nominal amounts of a NiCl2 þ FeCl3 solution under constant stirring. The hydroxide coprecipitations were filtered and dried at 120 C to obtain the precursor powders. Then the powders were sintered at 600 C for 3 h to obtain the (100x)NiO/(x)NiFe2O4 nanocomposites. The a)

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0021-8979/2011/109(7)/07D711/3/$30.00

analysis of the microstructure of the samples was performed by x-ray diffraction (XRD) using Cu Ka radiation and a highresolution transmission electron microscope (HRTEM) JEOL2100. The magnetic properties were measured using a superconductor quantum interference device. The hysteresis loops were measured at different temperatures in an applied field up to 50 kOe after field cooling (FC) from 295 K. III. RESULTS AND DISCUSSION

Figure 1(a) shows XRD patterns of all the (100x)NiO/ (x)NiFe2O4 nanocomposites with x ¼ 0, 2.5, 5, 8.3, 12.5, 25. For the composites with x ¼ 2.5, NiFe2O4 ferrite has not been detected by XRD in the measuring range. The AFM NiO and FI NiFe2O4 coexist in the nanocomposites with x > 2.5. The average particle size of 12 nm for all of the samples is obtained from the XRD pattern using the Scherrer formula,11 which is in agreement with the TEM observation as shown in Fig. 1(b). The typical hysteresis loop of (100x)NiO/ (x)NiFe2O4 nanocomposites with x ¼ 8.3 at 10 K after FC is shown in Fig. 1(c). The hysteresis loop shift is quantified by the EB field parameter HE ¼ (Hright þ Hleft)/2, whereas the coercivity (HC) is defined as HC ¼ (HrightHleft)/2, Hright and Hleft being the points where the loop intersects the field axis.12 Figure 2 shows the dependence of HE and HC at 10 K on the FI NiFe2O4 ratio (x). It can be seen that HE of (100x)NiO/(x)NiFe2O4 nanocomposites reaches a maximum at x ¼ 2.5, and then decreases with further increasing x. The EB field HE of pure NiO (i.e., x ¼ 0) is 1.1 kOe, which is attributed to the exchange coupling between the spins of the AFM core and the uncompensated surface moments of NiO nanoparticles.13 Neél first investigated the properties of AFM nanoparticles and suggested that they were likely to possess induced permanent magnetic moments due to the lack of an internal structural perfection and/or surface spin unbalance.14 The transition from two-sublattice to multisublattice of the AFM NiO nanoparticles, caused by the finite size effect, may explain the large loop shift and coercivity

109, 07D711-1

C 2011 American Institute of Physics V

Downloaded 24 Mar 2011 to 210.72.130.116. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

07D711-2

Gong et al.

observed in AFM nanoparticles.4,15 The maximum HE of 10.8 kOe is found in (100x)NiO/(x)NiFe2O4 nanocomposites with x ¼ 2.5, which originates from the strong interfacial exchange coupling between NiO and NiFe2O4 particles.16 For the samples with x > 2.5, the decrease of HE with increasing x is attributed to the formation of isolated FI NiFe2O4 clusters, as confirmed by HRTEM observation and shown in Fig. 1(d), which reduces the interface coupling between NiO and NiFe2O4. The inset in Fig. 1(d) is a fast Fourier transform (FFT) of Fig. 1(d), which reveals that the electron beam used for imaging is parallel to the ½ 112 direction of the face-centered-cubic NiFe2O4 structure. There is a critical value of the NiFe2O4 content above which the FI clusters are formed in the composites. Similar behavior was also observed in a previous report for Ni/NiO nanocomposites,17 in which there is a percolation thresholdlike value of NiO molar fraction, above which the EB is weakened. Generally, HE is inversely proportional to FM thickness (HE ! 1/tFM), here, with increasing x above 2.5, more FI NiFe2O4 clusters are formed, and the effective thickness of the FI NiFe2O4 increases, resulting in a decrease of HE. Similarly, too thin FI NiFe2O4 for x ¼ 2.5 leads to the increase of HE. If minor loop effects are considered in the nanocomposites,18,19 the observed value of HE will be smaller than the true value. Here only one FC loop of the nanocomposites with x ¼ 2.5 at 10 K is unsaturated completely in a field of 50 kOe, while loops of other samples are saturated. So the minor loop effects will not influence the variation trend of HE with x, but the value of HE. The variation of HE and HC with temperature for the nanocomposites are shown in Fig. 3. The temperature dependence of HE and HC for pure NiO and (100x)NiO/

FIG. 1. (Color online) (a) XRD patterns of the (100x)NiO=(x)NiFe2O4 nanocomposites with x, (b) TEM image of the nanocomposites with x ¼ 8.3, (c) typical hysteresis loop of the nanocomposites at 10 K with x ¼ 8.3 after 50 kOe field cooling, (d) HRTEM image of the FI NiFe2O4 clusters of the nanocomposites with x ¼ 8.3. The inset in (d): the FFT image of the FI NiFe2O4 grain in (d).

J. Appl. Phys. 109, 07D711 (2011)

FIG. 2. (Color online) Variation of HE and HC of (100x)NiO=(x)NiFe2O4 nanocomposites with x at 10 K.

(x)NiFe2O4 with x ¼ 2.5 is different from those with high x. It is typical for HE and HC to decrease with increasing temperature in AFM/FM or AFM/FI systems because the excess thermal energy decreases the exchange coupling between AFM/FM or AFM/FI.20 For pure NiO, both HE and HC reach a maximum at 50 K and then decrease with further increasing temperature. To understand this behavior the temperature dependence of magnetization of the pure NiO sample under ZFC and FC processes by applying a field of 100 Oe, has been presented. The ZFC magnetization curve as the inset in Fig. 4(a) shows a very broad maximum at 150 K and a minimum at 65 K, which can be explained in the spin core/shell model where an AFM NiO nanoparticle is considered as a system composed of an uncompensated AFM core and a disordered surface shell.21 The high-temperature magnetic behavior is attributed to an uncompensated particle core that thermally fluctuates at high temperature (superparamagnetic

FIG. 3. (Color online) Temperature dependence of HE and HC of (100x)NiO=(x)NiFe2O4 nanocomposites with different x.


07D711-3

Gong et al.

J. Appl. Phys. 109, 07D711 (2011)

IV. CONCLUSION

In conclusion, the EB effect in AFM/FI (100x)NiO/ (x)NiFe2O4 nanocomposites with average particle sizes of 12 nm has been investigated. The largest EB field HE of the nanocomposites with x ¼ 2.5 is due to the existence of FI NiFe2O4 nanoparticles separated by the NiO matrix, which is the percolation threshold of the nanocomposites. The pure NiO nanoparticles exhibit peculiar magnetic properties which are due to the uncompensated AFM ordered core and a disordered surface shell evidenced by the ZFC-FC measurement and temperature dependence of HE. ACKNOWLEDGMENTS

This work has been supported by the National Basic Research Program (Grant No.2010CB934603) of China, Ministry of Science and Technology of China and the National Nature Science Foundation of China under project nos. 50931006 and 50971123. FIG. 4. (Color online) The ZFC-FC magnetization curves of (a) pure NiO and (b) (100x)NiO=(x)NiFe2O4 nanocomposites with x ¼ 2.5, ZFC (open circle) and FC (solid circle). The inset in Fig. 4(a): the enlarged view of the high temperature region.

regime). With decreasing temperature, the progressive moments’ block is due to their effective anisotropy-energybarrier distribution, resulting in the maximum of ZFC magnetization at 150 K.14,21 The magnetic moment of the AFM NiO core at 10 K is about 4.9 emu/g, which was obtained from the extrapolation to H ¼ 0 of the high-field linear component of the hysteresis loop.22 The large value of the NiO moment can be understood by considering the finite size effect and the surface anisotropy effect. Numerical calculations and ferromagnetic resonance experiments confirmed this peculiar magnetic behavior.4,21 Meanwhile, the spins of the disordered surface shell, giving rise to a paramagneticlike contribution to the magnetization, can fluctuate thermally even below 150 K. The exchange coupled surface spin clusters are formed due to a magnetic correlation (dipoledipole and interparticle exchange interactions) as the temperature decreases below 65 K, leading to a superparamagnetic Langevin behavior.23 The size of the spin clusters increases with decreasing temperature as well. The superparamagnetic spin clusters are responsible for the reduction of HE and HC below 50 K. The ZFC-FC magnetization curves of (100x)NiO/ (x)NiFe2O4 with x ¼ 2.5 are shown in Fig. 4(b), which is a quite different behavior compared to the pure NiO. The ZFC-FC curves show a distinct irreversibility behavior up to room temperature and the magnetization is also larger than that of the pure NiO, due to the existence of FI NiFe2O4. The trace amount of FI NiFe2O4 in this sample is separated by the AFM NiO matrix, which causes the strongest exchange coupling between AFM/FI and correspondingly leads to the largest HE and HC.

1

V. Shumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, and J. Nogue´s, Nature 423, 850 (2003). 2 W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1956). 3 H. Ohldag, A. Scholl, F. Nolting, E. Arenholz, S. Maat, A. T. Young, M. Carey, and J. Sto¨hr, Phys. Rev. Lett. 91, 017203 (2003). 4 R. H. Kodama, S. A. Makhlouf, and A. E. Berkowitz, Phys. Rev. Lett. 79, 1393 (1997). 5 J. Nogue´s, J. Sort, V. Langlais, V. Skumryev, S. Surinãch, J. S. Munõz, M. D. Baro´, Phys. Rep. 422, 65 (2005). 6 S. R. Mishra, I. Dubenko, J. Losby, S. Roy, N. Ali, and K. Marasinghe, IEEE Trans. Magn. 40, 2716 (2004). 7 J. Sort, J. Nogue´s, X. Amils, S. Surinãch, J. S. Munõz, M. D. Baro´, J. Magn. Magn. Mater. 219, 53 (2000). 8 J. Ding, W. F. Miao, E. Pirault, R. Street, P. G. McCormick, J. Alloys Compd. 267, 199 (1998). 9 J. Y. Yi, C. L. Platt, M. L. Rudee, A. E. Berkowitz, and T. L. Cheeks, J. Appl. Phys. 79, 5072 (1996). 10 J. F. Wang, J. N. Cai, Y. H. Lin, and C. W. Nan, Appl. Phys. Lett. 87, 202501 (2005). 11 H. P. Klug and L. E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed. (Wiley, New York, 1974), Chap. 9. 12 X. H. Liu, W. B. Cui, X. K. Lv, W. Liu, X. G. Zhao, D. Li, and Z. D. Zhang, J. Appl. Phys. 103, 103906 (2008). 13 E. Winkler, R. D. Zysler, M. V. Mansilla, and D. Fiorani, Phys. Rev. B 72, 132409 (2005). 14 L. Neél, in Low Temperature Physics, edited by C. Dewitt, B. Dreyfus, and P. D. de Gennes (Gordon and Breach, New York 1962), p. 413. 15 R. H. Kodama and A. E. Berkowitz, Phys. Rev. B 59, 6321 (1998). 16 Z. M. Tian, S. L. Yuan, S. Y. Lin, L. Liu, J. H. He, H. N. Duan,P. Li, and C. H. Wang, Appl. Phys. Lett. 93, 222505 (2008). 17 L. Del Bianco, F. Boscherini, A. L. Fiorini, M. Tamisari, F. Spizzo, M. V. Antisari, E. Piscopiello, Phys. Rev. B 77, 094408 (2008). 18 J. Geshev, J.Magn. Magn. Mater. 320, 600 (2008). 19 G. Salazar-Alvarez, J. Sort, S Surinãch, M. D. Baro´, and J. Nogue´s, J. Am. Chem. Soc. 129, 9102 (2007). 20 U. Nowak, K. D. Usadel, J. Keller, P. Milteńyi, B. Beschoten, and G. Gu¨ntherodt, Phys. Rev. B 66, 014430 (2002). 21 E. Winkler, R. D. Zysler, M. Vasquez Mansilla, D. Fiorani, D. Rinaldi, M. Vasilakaki, and K. N. Trohidou, Nanotechnol. 19, 185702 (2008). 22 S. Mandal, S. Banerjee, and K. S. R. Menon, Phys. Rev. B 80, 214420 (2008). 23 R. D. Zysler, C. A. Ramos, E. De Biasi, H. Romero, A. Ortega, D. Fiorani, J. Magn. Magn. Mater. 221, 37 (2000).
