Tuning the surface anisotropy in Fe-doped NiO nanoparticles.

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DOI: 10.1039/C3NR04926D

K. O. Moura1, R. J. S. Lima2, A. A. Coelho1, E. A. Souza-Junior2, J. G. S. Duque3, C. T. Meneses3* 1

2

Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin, 13083-859, Campinas, SP, Brasil Universidade Federal de Sergipe, Campus Prof. José Aloísio de Campos, Departamento de Física, 49100-000, São Cristóvão, SE, Brasil

3

Universidade Federal de Sergipe, Campus Prof. Alberto Carvalho, Departamento de Física, 49500-000, Itabaiana, SE, Brasil

Abstract:

Ni1-xFexO nanoparticles have been obtained by the co-precipitation chemical route. Xray diffraction analyses using Rietveld refinement have shown a slightly decreasing in the microstrain and mean particle size as a function of the Fe. The Zero-Field-Cooling and Field-Cooling magnetization curves show a superparamagnetic behavior at high temperature and low temperature peak (at T = 11 K) which is enhanced with increasing of the Fe concentration. An unusual behavior of the coercive field at low temperature region and an exchange bias behavior were also observed. A decreasing in Fe concentration induces an increasing in exchange bias field. We argue that these behaviors can be linked with the strengthening of surface anisotropy caused by the incorporation of Fe ions. Keywords: Magnetic nanoparticle, superparamagnetism, surface anisotropy, X-ray diffraction, X-ray Photon Spectroscopy. *Corresponding author: [email protected] 1

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Published on 04 October 2013. Downloaded by University of South Carolina Libraries on 06/10/2013 15:33:35.

Tuning the Surface Anisotropy in Fe-doped NiO Nanoparticles

Nanoscale

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DOI: 10.1039/C3NR04926D

1. Introduction In the last decades the nanoscience have been an exciting area of research as

nanoparticles (NP’s) have been extensively studied mainly owing to their unique physical properties as compared to their bulk counterparts [1-6]. It is well known that below a critical size, magnetic NP’s behave as a single-domain, in contrast with the multi-domain structure observed in their bulk form. In a rough approximation, the energy states of the magnetization vector of the particles can be modeled as a two level system separated by barriers that depend on their size and magnetic effective anisotropy (Eb = keffV) [2-4, 7-11]. It is worth to comment that in most of cases it is very difficult to separate the contributions of the different kind of anisotropies, e. g., magnetocrystalline, magnetoelastic, shape and surface anisotropies to keff. However, in the particular case of small moment nickel oxide (NiO) the surface anisotropy appear be dominant [9, 12]. In some of these cases, the appearing of a peak in low temperature characterize a system with a large surface effect taken the surface anisotropy dominant on the others anisotropies. Some authors suggest that this effect is related to the uncompensated surface moments due the bond break of Ni ion on the surface [3, 9, 12, 13]. In this work we report structural and magnetic data that show a reinforcement of the surface anisotropy of NiO NP’s doped with iron. We discuss this strengthening on basis of the increase of surface anisotropy caused by the insertion of the Fe ions on the NiO crystalline structure.

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much of application point of view as of fundamental research. In this scenario, magnetic

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2. Experimental Details Ni1-XFeXO NP´s (x = 0, 0.01, 0.05 and 0.10) were prepared by annealing a precursor

The precursors were chemically obtained at room temperature by mixing aqueous solution of nickel nitrate Ni(NO3)2.6H2O (and iron nitrate Fe(NO3)2.3H2O, to the case of doped samples) and a solution of 1 mol/l of NaOH to kept pH 12. The resulting precipitated was washed several times in order to remove completely Na ions, centrifuged and dried in air at 80 °C. Finally, the precursors were synthesized at 350 oC during 3 hours. The crystalline structures of samples were investigated by X-ray diffraction (XRD) using a Rigaku powder diffractometer with Bragg-Brentano geometry mode θ - 2θ (with CuKα operated at 40 kV, 40 mA). Rietveld refinement was carried out by DBWS software using modified pseudo-voigt function as profile function. From these analyses we have also extracted information on the full width at half maximum (FWHM) for {1 1 1}, {0 0 2}, {0 2 2} crystallographic families. These analyses allow us to estimate the crystallite size and lattice distortion (microstrain) through Williamson-Hall equation [15], cos

4 sin

where β is the FWHM of the XRD peak, θ is the diffraction angle, k is a constant (close to 1 for cubic structure and spherical crystallites), λ is the incident X-ray wavelength, t is the crystallite size and ε is the microstrain. The particle shape and size were measured by

means

of

transmission

electron

microscopy

(HR-TEM

JEOL

3010,

LNNano/LNLS/CNPEM). X-ray Photon Spectroscopy (XPS) measurements were recorded with SPECSLAB II (Phoibos-Hs 3500 150 analyzer, SPECS, 9 channels) using non-monochromatic Al Kα (hν = 1486.6 eV) Soft X-ray Spectroscopy beamline at 3



powder obtained by co-precipitation method following steps of our previous work [14].

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the LNLS/CNPEM (Brazilian Synchrotron Light Laboratory, Brazil). The samples were analyzed as a function of emission angle (15o, 45o and 75o) to investigate surface and

temperature were carried out using a SQUID magnetometer (Quantum Design MPMS evercool system).

3. Results and Discussion 3.1 Structural Properties Figure 1 present X-ray diffraction patterns to Ni1-xFexO (x = 0, 0.01, 0.05 and 0.10) nanoparticles synthesized at 350oC obtained via co-precipitation method. We also show the difference between the experimental and calculated patterns which have been evaluated via Rietveld method. Unlike the results reported by Mallik et al. [16], all XRD patterns showed in Fig. 1 have the same structure of cubic NiO without the presence of spurious phase to Fe concentrations in the range from 0 to x = 0.10. As one can see there is a broadening in FWHM of Bragg peaks for increasing the doping concentration which is associated to the growth process of particles. Part of this broadening is due to the slightly decrease in the average crystallite sizes from pure to doped samples (see Table 1). However, we observed that the change in the FWHM broadening can also be associated to the crystallinity of doped samples once we have used the same synthesis conditions. Indeed, we have observed an increasing in the microstrain of the doped samples as a function of the iron substitution. In this scenario, we attributed the FWHM broadening to that X-ray reflection occurring at planes families where likely there is larger iron incorporation. Another important feature produced by iron incorporation is the disorder of the crystalline structure created by stacking faults in the crystal due the difference between Fe3+ (0.64 Å) and Ni2+ (0.69 Å)

4



internal regions of the particles. Magnetic measurements as a function of the field and

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ionic radii. Finally, in agreement with our previous statements, our XRD data show an decrease in the lattice parameter as a function iron doping (see Table 1).

resolution mode of the Fe-doped NiO with 1% (Fig. 2a) and 10% (Fig. 2b). The main planes of NiO structure (111), (200) and (220) are readily seen in the images with lattice fringes shown in both samples. Although the sample doped with 1% of Fe present particles with sphere-like (Fig 2a) we have observed that some nanoparticles present a rod-like shape similar to the found in doped sample with 10% of Fe. These results show that mean size are of 17x6 nm2 for sample with 1% of Fe and 13x5 nm2 for sample with 10% of Fe. The histograms presented in the Figure 2 show that the mean particles size present difference between the results estimated by Williamson-Hall equation and TEM analyses. This difference is close agreement due XRD analyses calculate the crystallite size considering a spherical morphology. Figure 3 displays the experimental and calculated XPS spectrum for 10%Fe-doped sample of the region corresponding to the binding energy range of 704-730 eV, which includes the Fe 2p3/2 and Fe 2p1/2 peaks recorded at 15°, 45° and 75°. The peaks shown, including the shake-up peaks are a convolution of the Fe with different oxidation state (Fe2+ and Fe3+). The calculated areas after fitting for each peak are presented in Table 2. These analyses show an increasing of the amount of Fe3+ with increasing of emission angle, indicating that the surface particles present an amount of Fe3+ more than the Fe2+. Based on the magnetizations results, the core particles present a spin configuration likeantiferromagnetic and the composition on the particle surface close to ferrimagnetic structure. This disordering on the surface spin can leave to a spin-glass-state in low temperature region.

5



Figure 2 displays representative transmission electron microscope images in high

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3.2 Magnetic Properties

xFexO

(x = 0, 0.01, 0.05 and 0.10) nanoparticles synthesized at 350 oC. It is evident that

the ZFC-FC curves display to all samples a typical superparamagnetic (SPM) behavior in the high temperature regime and a blocked state to temperatures lower than 160 K. It is known that the blocking temperature is associated to the mean particles size and anisotropy. But, when a large surface effect appears an effective uniaxial anisotropy must be considered [17] as described by this phenomenological expression:

K eff = KV +

6 KS D

where, is the particle diameter and KV and Ks are the core and surface uniaxial anisotropy, respectively. In this case, the constant K in the expression to calculate the average blocking temperature

TB =

KV0 25k B

where, V0 is the particle volume and kB is Boltzmann constant. Although, our results show that all samples present an average blocking temperature near to 160 K, once the average particle for pure sample is larger than the Fe-doped samples a decreasing in anisotropy is observed. On the other hand, the broadening of the ZFC-FC curves is a consequence of a large distribution of energies barriers which can be associated to the particles sizes distribution. In this sense, our results show that the irreversibility temperatures of Fe-doped samples vary into lower temperature regions for increasing the doping concentration indicating likely one reduction of particles size distribution. Besides, the magnetization measurements display to all samples an unusual increasing of magnetization (at T = 11 K) which is enhanced for increasing the doping 6



Figure 4 shows the ZFC-FC magnetization curves measured at H = 100 Oe to Ni1-

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concentration. This unusual behavior has been interpreted as an effect of finite size of particles, that is, the breaking of a large number of exchange bonds on particle surface

the particle size increase the surface to volume ratio, this decrease in the size is not sufficient to change significantly the magnetic properties, mainly the peak at low temperature. In this sense, if the origin of blocking effects and the low temperature peak on the Ni1-xFexO nanoparticles comes from core and surface particle, respectively. Our results show that most of iron ions can be incorporated on surface particle. One must note for analyzing Fig. 4 that while the maximum value of ZFC curves are almost constant (Mmax ∼ 0.010 emu/g) the intensity of the low temperature peak increase with the iron concentration. The field dependence of ZFC-FC curves to the Ni0.9Fe0.1O sample is displayed in the Fig. 5. Our MvsH loops measured at T = 5 K remain separated for field 50 kOe to 10% Fe-doped sample as one can see in the insert Fig. 5. This splitting means that magnetic moments can present a switching field of 50 kOe which can be indicating “high field irreversibility” [2] in good according with results shown in the insert (left) Figure 6c. This behavior has been associated with frozen magnetically ordered regions in the surface shell [4, 9]. In this scenario, results showed in the Fig. 4 provide strong evidences about the change in spins state on particle surface as a function of iron doping. Unlike the results reported by Winkler et al. [9], to NiO nanoparticles we have noted that at H = 50 kOe the Ni0.9Fe0.1O sample still remain splitted, indicating a strengthening of surface anisotropy with the Fe doping. Figure 6 shows MvsH curves recorded at temperatures of 300 K (Fig 6a) and 5 K (Fig 6b) for all samples studied in this work. At T = 5 K the MvsH loops present a hysteretic behavior to all samples, however the magnetization never reaches the saturation value. On the other hand, the high field behavior shows a linear contribution 7



drives the spins to a strongly frustrated state [4, 9, 12, 13]. Although, the decreasing in

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which increase with the iron doping, suggesting an increase of the surface contribution to the magnetization. It is also evident that, to 10% Fe-doped sample, the hysteresis

magnetization (MR) are slowly narrowed. In fact, these results are corroborated to the uncompensated spins systems either on the surface or in the core of the NiO nanoparticles. It is more evident in doped samples with high concentration of doping (5% and 10%), which the surface effect is large. At room temperature the MvsH loops also display a hysteretic which can be attributed to the contribution of larger particles. However, the coercive field and reversibility temperature decreases with the iron doping indicating a change in the particle size distribution. Furthermore, we have observed in MvsH loops the exchange bias phenomenon for doped systems, which increasing with the decrease in the amount of doping as can be observed in inset of the Fig. 6b. To illustrate shift of the coercive field, we present in Figure 6c MvsH loops measured in ZFC and FC (with applied field of 50 kOe) mode taken at 5K. Recent studies performed by Punnoose et al. have shown this phenomenon for CuO nanoparticles [18]. They have suggested that uncompensated exchange couplings of the surface spins leads to significant magnetic moment per particles even through the spins in the core are antiferromagnetically ordered. Sharma et al. have reported exchange bias in Ni-NiO nanoparticles and they have associated the decreasing in the exchange bias field (HEB) with increasing of the ferromagnetic phase on the particle surface [19]. So, our results suggest a magnetic behavior similar where the origin of coupling between the ferromagnetic surface spins with antiferromagnetic (core particle) result in a shifted hysteresis loops of the ferromagnetic surface spins. These anomalous behaviors are further confirmed from the dependence of coercive field (HC) and remanent magnetization (MR) as functions of temperature (see Figure 7). First of all we can see that HC does not decay with the square root of temperature 8



curves measured at low temperature, the coercive field (HC) and remanent

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following a Néel relaxation and the Bean-Livingston approaches [1]. Instead the coercive field and remanent magnetization decrease in the temperature range where the

maximum at T = 15 K and then decrease to zero to higher temperatures. The significant decrease in HC observed close to T = 10 K can be related to a competition between the blocked and unblocked particles which is very usual behavior of superparamagnetic systems. On the other hand, the increment in HC in the temperature ranges of 15 < T < 25 K is a consequence of the competition between the magnetic interactions existing on the blocked particles. In fact, one can infer that the surface effects become dominant over the particle core and it is more difficult to align the uncompensated spins on the particle shell. Therefore, for decreasing the temperature, a strong contribution of the blocked particles increases the HC values and consequently the MR [5]. Besides, it is not unreasonable to state that this increase the total magnetization can be related with the formation of the spin clusters of short-range on the particle surface [12, 13, 20].

4. Conclusion In conclusion, single phases of Ni1-xFexO (x = 0, 0.01, 0.05 and 0.10) nanoparticles were synthesized via co-precipitation method. XRD analysis indicate that there is a slightly decrease in the average particles size and an increase in the particle microstrain for increasing the iron concentration. The ZFC-FC measurements display two different magnetic behaviors: i) a progressive blocking process of the core particle moments at around 160 K and then ii) below 20 K, an anomalous magnetic behavior with an increase of the magnetic moment which can be associated with spin clusters at the particle surface driving the system into a collective freezing of spins. This latter effect has been enhanced for increasing the iron concentration. Exchange bias effects were observed in doped samples and bias field increase with decrease of Fe amount. These 9



surface effects are stronger, reaching a minimum at T = 10 K. After that HC reaches a

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results is attributed the interactions between ferromagnetic (particle surface) and antiferromagnetic (core). These facts together with the increase structural results allows

in the surface anisotropy as a function of iron doping. Finally, the anomalous behavior of Mr and HC at low temperature can also linked with the strengthening of the surface effects of Fe-doped NiO nanoparticles.

Acknowledgments This research was supported by CNPq funding agency (Project 577512/2008-0, 477114/2008-3) and FAPITEC.

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us to state that Fe ions are located on the particle surface once there is a strong change

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References [1] Bean, C. P.; Livingston, J. D. The Anisotropy of Very Small Cobalt Particles. J.

[2] Kodama, R. H.; Berkowitz, A. E.; McNiff Jr, E. J.; Foner, S. Surface spin disorder in NiFe2O4 nanoparticles. Phys. Rev. Lett. 1996, 77, 394-397. [3] Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Finite Size Effects in Antiferromagnetic NiO Nanoparticles. Phys. Rev. Lett. 1997, 79, 1393-1396. [4] De Biasi E.; Ramos, C. A.; Zysler, R. D.; Romero, H. Large Surface Magnetic Contribution in Amorphous Ferromagnetic Nanoparticles Phys. Rev. B, 2002, 65, 144416. [5] Nunes, W. C.; Folly, W. S. D.; Sinnecker, J. P.; Novak, M. Temperature Dependence of the Coercive Field in Single-Domain Particle Systems. Phys. Rev. B. 2004, 70, 014419. [6] Allia, P.; Coisson, M.; Tiberto, P.; Vinai, F.; Knobel, M.; Novak, M. A.; Nunes, W. C. Granular Cu-Co Alloys as Interacting Superparamagnets. Phys. Rev. B 2001, 64, 144420. [7] Dormann, J. L., Fiorani, D.; Tronc, E. On the Models for Interparticle Interactions in Nanoparticle Assemblies: Comparison with Experimental Results. J. Magn. Magn. Mat. 1999, 202, 251-267. [8] Dormann, J. L.; Fiorani, D.; Tronc, E. Magnetic Relaxation in Fine-Particle Systems. Adv. Chem. Phys. 1997, XCVIII, 283-494. [9] Winkler, E.; Zysler, R. D.; Mansilla, M. V.; Fiorani, D. Surface Anisotropy Effects in NiO Nanoparticles. Phys. Rev. B 2005, 72, 132409. [10] De Biasi E.; Ramos, C. A.; Zysler, R. D. Size and Anisotropy Determination by Ferromagnetic Resonance in Dispersed Magnetic Nanoparticle Systems. J. Magn. Magn. Mat. 2003, 262, 235-241. 11



Phys. Rad. 1959, 30, S120.

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[11] Tronc, E.; Fiorani, D.; Nogues, M.; Testa, A. M.; Lucari, F.; D'Orazio, F.; Greneche, J. M.; Wernsdorfer, W.; Galvez, N.; Chaneac, C.; Mailly, D.; Jolivet, J. P.

Magn. Magn. Mat. 2003, 262, 6-14. [12] Meneses, C. T.; Duque, J. G. S.; De Biasi, E.; Nunes, W. C.; Sharma, S. K.; Knobel, M. Competing Interparticle Interactions and Surface Anisotropy in NiO Nanoparticles. J. Appl. Phys. 2010, 108, 013909. [13] Duan, W.; Lu, S. H.; Wu, Z. L.; Wang, Y. S. Size Effects on Properties of NiO Nanoparticles Grown in Alkalisalts J. Phys. Chem. C 2012, 116, 26043-26051. [14] Meneses, C. T.; Flores, W. H.; Garcia, F.; Sasaki, J. M. A Simple Route to the Synthesis of High-Quality NiO Nanoparticles. J. Nanoparticle Res. 2007, 9, 501-505. [15] Williamson, G.; Hall, W. X-Ray Line Broadening From Filed Aluminium and Wolfram. Acta Metalurgica 1953, 1, 22-31. [16] Mallick, P.; Rath, C.; Biswal, R.; Mishra, R. C. Structural and Magnetic Properties of Fe-doped NiO Indian Journ. Phys. 2009, 83, 517-.523 [17] Knobel, M.; Nunes, W. C.; Socolovsky, L. M.; De Biasis, E.; Vargas, J. M.; Denardin, J. C. J. Nanosc. Nanotech. 2008, 8, 1-20. [18] Punnoose, A.; Magnone, H.; Seehra, M. S.; Bonevich, J. Bulk to Nanoscale Magnetism and Exchange Bias in CuO Nanoparticles. Phys. Rev. B 2001, 64, 174420. [19] Sharma, S. K.; Vargas, J. M.; De Biasi, E.; Beron, F.; Knobel, M.; Pirota, K. R.; Meneses, C. T.; Kumar, S.; Lee, C. G.; Pagliuso, P. G.; Rettori, C. The nature and enhancement of magnetic surface contribution in model NiO nanoparticles. Nanotechlogy 2010, 21, 035602. [20] Berger, R.; Bissey, J. C.; Kliava, J.; Daubric, H.; Estournes, C. Temperature dependence of superparamagnetic resonance of iron oxide nanoparticles J. Magn. Magn. Mat. 2001, 234, 535-544. 12



Surface Effects in Noninteracting and Interacting Gamma-Fe(2)O(3) Nanoparticles. J.

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Figures Caption

0.05 and 0.10) nanoparticles synthesized at 3500C. Figure 2. HR-TEM images of samples Fe-doped NiO nanoparticles , with (a) 1% of Fe and (b) 10% of Fe. Insets show lattice fringes corresponding to the crystalline planes of NiO and histograms of the particle size distribution calculated from TEM. Figure 3. Experimental and calculated Fe2p XPS spectra for 10%Fe-doped NiO nanoparticles for different angles. The deconvolution of each spectrum is also included. Figure 4. ZFC-FC magnetization curves at H = 100 Oe to Ni1-xFexO (x = 0, 0.01, 0.05 and 0.10) nanoparticles synthesized at 3500C, the insets show in detail ZFC region for observation of TB to Fe-doped samples with 5 and 10%. Figure 5. ZFC-FC magnetization curves to the Ni0.90Fe0.10O sample taken at different applied fields, the inset shows the split between the ZFC-FC curves at low temperature. Figure 6. Hysteresis curves measured at (a) 300 K and (b) 5 K taken in the ZFC mode to Fe-doped NiO nanoparticles and the insets show the doping dependence on the exchange bias field (HEB). (c) Hysteresis loops taken in ZFC and FC modes at 5 K for Ni0.90Fe0.10O sample and insets show original loops in low field region. Figure 7. Coercive field and remanent magnetization as a function of the temperature to Ni0.90Fe0.10O sample.

Tables Caption Table 1. Lattice parameters, particles size and microstrain to Ni1-xFexO (x = 0, 0.01, 0.05 and 0.10) nanoparticles synthesized at 3500C. Table 2. The results of deconvolution of Fe 2p3/2 peak: area rate of the (AFe3+/A Fe2+) oxidation state as function of emission angle for 10% Fe-doped sample. 13



Figure 1. X-ray diffraction and Rietveld refinement patterns to Ni1-xFexO (x = 0, 0.01,

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Sample

a (Å)

Size (nm)

Microstrain

Pure 1% Fe 5% Fe 10% Fe

4.1852(5) 4.1827(5) 4.1818(6) 4.1706(9)

13(2) 10(1) 10(1) 8(1)

0.0073 0.0073 0.0080 0.0095



Table 1

Table 2 Emission angle +3

AFe / AFe

+2

15°

45°

75°

1,8

2,1

2,3

14



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Graphical Abstract

increasing the Fe-doping concentration. ► We have observed in MvsH loops the exchange bias phenomenon for Fe-doped systems, which increasing with the decrease in the amount of doping. ► An increasing in the microstrain of the Fe-doped samples has been observed as a function of the iron substitution.



► An unusual increasing of magnetization (at T = 11 K) which is enhanced for

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An unusual increasing of magnetization (at T = 11 K) which is enhanced for increasing the Fe-doping concentration



DOI: 10.1039/C3NR04926D

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