Superparamagnetism and spin-glass like state for the MnFe2O4 nano-particles synthesized by the thermal decomposition method

Journal of Magnetism and Magnetic Materials 324 (2012) 2534–2538

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Superparamagnetism and spin-glass like state for the MnFe2O4 nano-particles synthesized by the thermal decomposition method Ruo-Rui Gao a, Yue Zhang a,b,n, Wei Yu a, Rui Xiong a,c, Jing Shi a,c,d a

Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, PR China Department of Electric Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China c Key Laboratory for the Green Preparation and Application of Functional Materials of Ministry of Education, Hubei University, Wuhan 430062, PR China d International Center for Material Physics, Shen Yang 110015, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2011 Received in revised form 24 February 2012 Available online 26 March 2012

MnFe2O4 nano-particles with an average size of about 7 nm were synthesized by the thermal decomposition method. Based on the magnetic hysteresis loops measured at different temperatures the temperature-dependent saturation magnetization (MS) and coercivity (HC) are determined. It is shown that above 20 K the temperature-dependence of the MS and HC indicates the magnetic behaviors in the single-domain nano-particles, while below 20 K, the change of the MS and HC indicates the freezing of the spin-glass like state on the surfaces. By measuring the magnetization–temperature (M–T) curves under the zero-field-cooling (ZFC) and field-cooling procedures at different applied fields, superparamagnetism behavior is also studied. Even though in the ZFC M–T curves peaks can be observed below 160 K, superparamagnetism does not appear until the temperature goes above 300 K, which is related with the strong inter-particle interaction. & 2012 Elsevier B.V. All rights reserved.

Keywords: MnFe2O4 nano-particle Superparamagnetism Spin-glass like state Thermal decomposition

1. Introduction In recent years, the studies based on the magnetic nanomaterials have raised an interesting concern in both the magnetism and nano-science field. This may be mainly associated with its sound scientific significance [1–3] and potential application foreground [4]. It is well known that, the magnetic structure and magnetic interaction of the nano-sized magnetic materials are quite different from that of the bulk ones in many aspects, such as the formation of the single-domain structure and the breaking of exchange bonds near the surfaces, which results in some special magnetic properties. For the nano-particles with the single-domain structure, an interesting phenomenon named superparamagnetism may be observed. This is mainly related with the competition between thermal energy and magnetocrystalline anisotropy energy (EA) [5]. EA is the energy barrier to the rotation of the magnetic moments between the directions parallel and anti-parallel on the easy-magnetization axis. Since for uniaxial anisotropy EA equals to the product of anisotropy constant (KA) and the volume of the single-domain nanoparticle, if the crystallite size is so small that EA is much smaller than the thermal energy [6], this energy barrier will be overcome, and a

n

Corresponding author. E-mail address: [email protected] (Y. Zhang).

0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.03.035

quick rotation of the magnetic moments in response to an applied field can be realized. As a result, hysteresis disappears in the magnetization–magnetic field loop, which indicates the superparamagnetic relaxation effect [7]. At low temperatures, due to the small thermal energy and the enhancement of KA, the magnetic moments will be ‘‘blocked’’ in the directions parallel or anti-parallel the easymagnetization axis and hysteresis will be observed [8–10]. Besides superparamagnetism, the enhancement of surface effect will also affect the magnetism for the nano-particles due to the increased atomic percentage of the surface atoms [11].The mostly concerned issue is the spin-glass like (SGL) state for the disordered canting spins near the surfaces, which is generally ascribed to the broken translation symmetry for the surface ions due to the imperfect coordination number [12]. At high temperatures, the surface spins will have multiple states with similar energies, and the surface spins can experience most of these states in a very short time, while at low temperatures, due to the great increase in relaxation time, the surface spins can now choose one of these states and not relax to other ones, which indicates the freezing of the SGL state [13]. Among magnetic materials, the ferrimagnetic spinel ferrites, MFe2O4 (M ¼Co2 þ , Ni2 þ , Mn2 þ , Mg2 þ , Fe2 þ , etc.), have some potential merits for technological applications due to their good properties, such as large resistivity and permeability, as well as moderate magnetization and coercivity [14]. Manganese ferrite (MnFe2O4) is a kind of such spinel ferrite material with a partial inverse spinel structure. It has moderate saturation magnetization

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(about 80 emu/g) and small HC (smaller than 100 Oe) [15]. In recent years, many studies have been reported on both theory and application for the MnFe2O4 nano-particles. For example, for the MnFe2O4 nano-particles with the size between 5 and 15 nm, the Ne´el temperature (TN) is observed to increase with the decrease of the particle size [16]. In its biological applications, it seems that the MnFe2O4 nano-particles may have some potential merits in the medical technology such as MRI [17]. Till now, many synthesis technologies such as sol–gel [14], autocombustion [18–20], thermal decomposition [21,22], and hydrothermal [23–28] have been developed to prepare the single-domain MnFe2O4 nanoparticles. Among these methods, thermal decomposition is often used to synthesize the MnFe2O4 nanoparticles with small size and regular shape [29]. However, the detailed study on the magnetic properties for the MnFe2O4 nanoparticles synthesized by thermal decomposition has not yet been reported. In the present work, the MnFe2O4 nanoparticles with an average size of about 7 nm are prepared by the thermal decomposition method. Based on a detailed magnetic measurement, some special magnetic properties such as the surface SGL state and the superparamagnetism have been studied in detail.

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Fig. 1. The XRD pattern of the MnFe2O4 nano-particles synthesized by the thermal decomposition method.

2. Experimental methods Firstly, 20 ml solutions were prepared by dissolving 0.4805 g iron(III) acetylacetonate and 0.1688 g manganese(II) acetylacetonate in diphenyl ether. At 40 1C, 3 ml oleic acid and 3 ml oleylamine were added into the solution under drastic stirring. Then, the mixture was heated to 200 1C for 0.5 h, and then heated to reflux (250 1C) for another 0.5 h. Finally, the obtained black powders were centrifugalized with cyclohexane and ethanol several times and dried in air. The phase analysis was performed by using an X-ray diffraction (XRD, D8-Advanced) with the CuKa radiation in a 2y scanning from 201 to 701 (11/min) at a step of 0.021. The morphologies were observed by using a transmission electron microscope (TEM) (JEM2010). The magnetic measurements were carried out by using a vibrating sample magnetometer (VSM) on a physical property measure system (PPMS-9 Quantum Design). The hysteresis loops were collected at 300, 250, 200, 150, 100, 50, 20, 15, 10 and 5 K, respectively. The magnetization–temperature curves (M–T) were measured with the zero-field cooling (ZFC) and the field cooling (FC) procedures in different fields: 100, 150, 200, 300 and 500 Oe. Firstly, the samples were cooled to 10 K without the application of magnetic field, then different magnetic fields (100, 150, 200, 300 and 500 Oe) were applied and the samples were warmed to 300 K, finally, under the same fields, the samples were cooled to 10 K again.

3. Results and discussion The XRD pattern is shown in Fig. 1. All the observed peaks can be indexed from the JCPDS card (no. 73-1964) for MnFe2O4. Through the position and full-width at half maximum (FWHM) of the strongest (311) peak, the average crystallite size, about 6.8 nm70.2 nm, was determined by using the Scherrer formula with the deduction of broadening from instrument. The TEM image is shown in Fig. 2(a). It can be seen that the powders are composed of small nano-particles in special shapes, which are not aggregated, and that even though the sizes have a wide distribution, most particles have a size smaller than 10 nm. This may be related with the low reaction temperature. We have

Fig. 2. (a) The TEM image and (b) the size distribution for the MnFe2O4 nanoparticles synthesized by the thermal decomposition method.

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noticed that increasing reaction temperature can narrow the size distribution, which will be discussed in detail elsewhere. By measuring the diameters of about 300 particles, the size distribution was determined, as shown in Fig. 2(b). It can be seen that the size distribution can be fitted by a lognormal distribution, and the average size is determined to be 7.5 nm, with a standard deviation of 0.24. It is noticed that the average diameter by TEM is 0.7 nm larger than that by XRD, this may indicate the existence of amorphous layer on particle surface, with the thickness between 0.3 and 0.4 nm. Fig. 3 depicts the representative magnetic hysteresis loops measured at 5 K and 300 K, with the inset figure showing the magnified ones. It is shown that at 300 K, the HC is smaller than 30 Oe. At 5 K, the HC increases to about 420 Oe, which indicates a clear blocking behavior for the single-domain nano-particles. In addition, it is shown that the magnetization at 5 K is clearly larger than that of 300 K. This is because at 5 K the thermal energy is much weaker than that of 300 K. Thus, at 5 K, the magnetic moments can be aligned more orderly due to exchange interaction, and the projection of the moments in the direction of the applied field is larger. By measuring the hysteresis loops at different temperatures between 5 K and 300 K, the temperature dependence of the magnetic properties including HC and MS is also studied in detail, as depicted in Fig. 4. From Fig. 4(a), it can be seen that the HC increases with the decrease of temperature, and below 20 K, the HC shows a great enhancement. For single-domain nano-particles, the relationship between temperature and HC can be described by the Kneller law, i.e., HC and T1/2 satisfy a linear relationship [30]. As shown in the inset of Fig. 4, the data of HC between 20 K and 180 K indeed satisfy the Kneller law, which indicates that in this temperature range, the increase of HC can be related with the blocking behaviors due to EA. While below 20 K, deviation from this law is clearly observed. Fig. 4(b) shows the curve depicting the temperature-dependence of MS, which was obtained by using the law of approach to saturation: M ¼ M S a=H2 = þ wp H

ð1Þ

where M is the magnetization, MS is the saturation magnetization, H is the strength of applied magnetic field, and a is a constant related with the magnetic anisotropy constant, and wp is the highfield paramagnetic susceptibility [31].

Fig. 3. The M–H curves at 5 K and 300 K for the MnFe2O4 nano-particles synthesized by the thermal decomposition method.

Fig. 4. The temperature dependence of (a) HC and (b) MS for the MnFe2O4 nanoparticles synthesized by the thermal decomposition method, the inset figure in (a) shows the fitting result by the Kneller law. (c) Fitting result for the temperature dependence MS.

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From Fig. 4(b), we see that the MS between 5 K and 300 K is in the range of 44–53 emu/g. With the decrease in temperature, MS gradually increases, and the change of MS becomes smaller and smaller. At temperatures lower than 20 K, a clear turning point is observed, and the increasing rate for MS becomes greater, while at temperatures higher than 20 K, the thermal effect on the exchange bonding of the inner magnetic moment plays a major role on the change of MS. Basically, the applied field offers the energy to make the moments ordering, while the thermal energy destroys such order. With the decrease in temperature, the effect of thermal energy becomes weaker, thus making it easier for the magnetic moments to flip to the direction of the applied field. The abnormal change of MS below 20 K may be related with the additional contribution from the frozen spin-glass-like (SGL) state on the surfaces of the nano-particles. Since at high temperatures, the surface spins can experience many disorder states with similar energies in a short time, which weakens their response to the external field and thus lowers the MS in comparison with that of the bulk materials. While at temperatures lower than a certain value (the freezing temperature), the surface spins may freeze to a certain state, which increases the projection of the moments in the direction of external field, and thus enhances MS. In addition, extra energy is needed for the flip of the frozen surface spins, which induces the enhancement of surface anisotropy and may also cause the great increase of HC below 20 K, as shown in Fig. 4(a). For a quantitative analysis of the change of MS with temperature, the MS(T) curve in Fig. 4(b) is fitted by [32,33]   M S ðTÞ T ¼ ð1BT a Þ þ A0 exp  ð2Þ M S ð0Þ Tf where MS(0) is the saturation magnetization as T tends to zero, B is the Bloch constant, a is a exponent, A0 is a coefficient, and Tf represents the freezing temperature. In Eq. (2), the former term represents the change of MS contributed by the thermal effect on the inner moments, while the latter term reflects the contribution of the surface spins. The fitting curve is shown in Fig. 4(c). It can be seen that the MS(T) curve fits this equation quite well, which clearly proves the contribution of the surface frozen SGL spins to the magnetization. The MS(0) is 52.75 emu/g, a is about 2.3, and Tf is fitted to be about 9.2 K. It seems that this MS(0) is about 30 emu/g smaller than that in the bulk-sized MnFe2O4. This is usually attributed to the weak response from the ‘‘dead-magnetization layer’’ on particle surface. For the bulk-sized magnetic materials, the contribution from this dead layer can be neglected, but for the particles with very small size ( o10 nm), this surface effect can cause a considerable decrease of MS. Based on MS(0) of the bulk-sized MnFe2O4 ( 80 emu/g), the thickness of spindisordered surface layer is estimated [34] to be 0.5 nm, which seems to be close to the thickness of surface amorphous layer, which, as shown above, is determined by the difference of the particle size by XRD and by TEM. In addition, based on the value of the fitted a, the relationship between MS and T at high temperature deviates from the classical bloch T2/3 law. This may be attributed to the size-effect on the structure of the energy band in spin-wave spectrum [33]. Fig. 5(a) shows the M–T curves under the ZFC and the FC cooling procedures were measured in different applied fields. It can be seen that the ZFC curves have wide peaks especially for those measured under smaller fields. To determine the peak position in the ZFC curves, the differentiate analysis is carried out and the temperature corresponding to the maximum magnetization (Tm) is determined as dM/dT becomes zero. It is noticed that the peaks in the ZFC curves shift toward lower temperatures with increasing H. As a function of H, the Tm(H) in the ZFC curves

Fig. 5. (a) M–T curves measured by ZFC and FC procedures under different applied fields and (b) the curves of M/H vs. H obtained from the ZFC M–T curves.

show good agreement with the well known de Almeida–Thouless (AT) line [10,12], i.e., Tm is proportional to the applied field raised to 2/3 power, which is shown in the inset of Fig. 5(a). By extrapolating H to zero, the Tm(0) (the spontaneous peak temperature) is fitted to be about 160 K. The physical significance of Tm is often interpreted as the blocking temperature of the superparamagnetic nano-particles. However, some authors think that if a distribution of sizes is present, this peak temperature should deviate from the true blocking temperature (TB) [35]. Quantitatively, as T bTm the MZFC curve can be expressed by an integral equation based on the known distribution functions

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for size and magnetic moment [36]. Approximately, the hightemperature MZFC values can also be expressed as [37] M ¼ aH þ bH3 þ cH5 þ . . .

ð3Þ

Where M and H are magnetization and the strength of magnetic field, a, b, c, etc. are the expansion coefficients. If a perfect superparamagnetism is present, the high-order terms bH3, cH5, etc. can be negligible, and the values of M/H should be fieldindependent, otherwise, if a strong inter-particle interaction exist, these high-order terms should be taken into account [37]. Fig. 5(b) depicts the curves of M/H vs. H at different temperatures above Tm, it can be seen that the M/H values are field-dependent even as temperature becomes as high as 300 K. Thus, as T4Tm, superparamagnetism is not really present due to the inter-particle interaction, which may enlarge the anisotropy energy barrier [36] and introduce the small room-temperature coercivity. Fig. 5(a) also shows the FC curves, a low-temperature flattening is shown, which also provides a clear evidence for inter-particle magnetic interactions [36,38,39]. In addition, at temperatures higher than a critical value (named as Tspilt), the ZFC and FC curves seem to overlap. From Fig. 5(a), it is shown that the overlap shift to a higher temperature as the applied field is reduced, and as the field is 100 Oe, this overlap occurs at about 300 K. Therefore, it can be concluded that if field is reduced to zero, the Tspilt should shift to be higher than 300 K. This divergence in the ZFC and FC M–T curves clearly indicates the ferrimagnetic behaviors at room temperature, as is proved by the small coercivity at room temperature.

4. Conclusion The MnFe2O4 nano-particles with an average size of about 7 nm were synthesized by the thermal decomposition method. The magnetic properties of the products manifest strong temperature dependence. At temperatures lower than 20 K, the freezing of the surface spin-glass like state occurs, causing an enhancement in MS and HC. At temperatures lower than 160 K, peaks appear in ZFC M–T curves. However, due to the strong inter-particle interactions, superparamagnetism does not appear until the temperature goes above 300 K.

Acknowledgments The authors would like to acknowledge the financial support from the Chinese National Foundation of Natural Science (nos. 10974148 and 51172166), the National Science Fund for Talent Training in Basic Science (no. J0830310), the 973 Program (2009CB939705 and 2009CB724505), and the funding of Wuhan University (5082003). References [1] D. Fiorani, J.L. Dormann, Magnetic properties of fine particles, in: Proceedings of the Workshop on Studies of Magnetic Properties of Fine Particles and Their Relevance on Materials Science, 1991. [2] S. Morup, M.F. Hansen, C. Frandsen, Magnetic Nanoparticle, Elsevier, 2011.

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