0.80(ZnO) nanocomposite

Journal of Magnetism and Magnetic Materials 361 (2014) 12–18

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Magnetic study of 0.20(Fe2O3)/0.80(ZnO) nanocomposite J. Typek a,n, K. Wardal a, G. Zolnierkiewicz a, N. Guskos a,b, D. Sibera c, U. Narkiewicz c a b c

Institute of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, K. Pulaskiego 10, 70-322 Szczecin, Poland

art ic l e i nf o

a b s t r a c t

Article history: Received 20 January 2014 Received in revised form 14 February 2014 Available online 25 February 2014

ZnO nanocrystals doped with Fe2O3 have been synthesized by the calcination method. Ferromagnetic resonance (FMR) and dc magnetization measurements of 0.2(Fe2O3)/0.8(ZnO) nanocomposite have been carried out in the 4–300 K range. The presence of agglomerated magnetic zinc ferrite ZnFe2O4 nanoparticles in 0.2(Fe2O3)/0.8(ZnO) nanocomposite with an average crystallite size of 8 nm was identified by XRD. Temperature dependence of the resonance field, linewidth and the integrated intensity calculated from FMR spectra has been determined. Magnetization measurements in ZFC and FC modes as well as study of hysteresis loops allowed calculating different magnetic characteristics – blocking/freezing temperature, magnetic moment, anisotropy constant and anisotropy field. The observed magnetic properties of 0.2(Fe2O3)/0.8(ZnO) nanocomposite were explained based on the core–shell model of ZnFe2O4 nanoparticles covered by ZnO layer. & 2014 Elsevier B.V. All rights reserved.

Keywords: Ferromagnetic resonance Magnetic nanoparticle Magnetic property Zinc ferrite

1. Introduction Nanocrystalline materials, among them ZnO semiconductor nanocrystals doped with 3d metal ions (Fe, Ni, Co, Mn), received considerable attention as potential components of future nanotechnology [1]. The so called diluted magnetic semiconductors will allow the control of both the spin and charge carriers. To be useful in spintronic devices these materials should be ferromagnets at room temperature (RT) [2]. Many reports on ferromagnetism at RT in ZnO based diluted magnetic semiconductors have been published but there are other reports also that did not evidence any ferromagnetism [3–6]. Thus the presence and the origin of magnetism in these compounds are still under discussion. In Fe2O3/ZnO system two kinds of magnetic nanoparticles could appear: zinc ferrite (ZnFe2O4) and maghemite (γ-Fe2O3). The zinc ferrite ZnFe2O4 (ZFO) is the only magnetic phase detected in our 0.2(Fe2O3)/0.8(ZnO) nanocomposite. Bulk ZFO exhibits normal spinel structure and is a weak antiferromagnet with low Neel temperature (TN ¼ 10.5 K) [7]. When deposited in nanocrystalline thin film form it shows ferrimagnetic behavior depending on grain size [8]. Although at RT bulk ZFO with the normal spinel structure is paramagnetic, nanocrystalline nonstoichiometric ZFO with partial inverted spinel structure is a ferrimagnet [9]. Additional occupation of tetrahedral A-sites by Fe and octahedral

B-sites by Zn leads to a strong superexchange coupling of the intra-sublattice Fe ions and is responsible for the appearance of ferrimagnetic phase. Superparamagnetism and interparticle interactions (dipolar and surface spins at low temperatures) have been also studied in ZFO nanocrystals [10–12]. The aim of this work was to determine magnetic characteristic of ZFO nanoparticles in the 0.2(Fe2O3)/0.8(ZnO) nanocomposite by using ferromagnetic resonance (FMR) and dc magnetization measurements. Previous ac magnetization studies of n(Fe2O3)/(1 n)(ZnO) nanocomposites (where the composition index n¼0.05–0.95) have shown that the relative shift of the peak temperature per decade shift in frequency indicates the existence of a spin-glass state in ZFO magnetic nanoparticles [13–15]. When the phenomenological Vogel–Fucher law was used it also confirmed the spin-glass like behavior in n(Fe2O3)/(1 n) (ZnO) samples synthesized by calcination process. Earlier FMR study of 0.2(Fe2O3)/0.8(ZnO) nanocomposite was carried out only at RT so it provided only limited information on the magnetic state of ZFO nanoparticles [16,17]. In this work we extend the range of temperature FMR investigations down to 4 K and attempt to correlate the obtained results with dc magnetization measurements to acquire more comprehensive magnetic characterization of 0.2(Fe2O3)/0.8(ZnO) nanocomposite.

2. Experimental n

Corresponding author. E-mail address: [email protected] (J. Typek).

http://dx.doi.org/10.1016/j.jmmm.2014.02.065 0304-8853 & 2014 Elsevier B.V. All rights reserved.

The investigated sample was synthesized by the traditional wet chemical method followed by calcination. A mixture of iron and

J. Typek et al. / Journal of Magnetism and Magnetic Materials 361 (2014) 12–18

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zinc hydroxides was obtained by adding ammonia solution to 20% solution of proper amounts of Zn(NO3)4  6H2O and Fe(NO3)3  4H2O in water. The obtained hydroxides were filtered, dried and calcined at 573 K for 1 h. The details of synthesis are presented elsewhere [14]. Magnetic resonance measurements were performed on a conventional Bruker E 500 spectrometer operating in the X-band (ν ¼ 9.4 GHz) with 100 kHz magnetic field modulation. The temperature measurements of FMR spectra were performed in 4–290 K temperature range by using an Oxford helium-flow cryostat. Magnetization studies were performed with the help of a Quantum Design Magnetic Property Measurements System MPMS XL-7 with a superconducting quantum interference device magnetometer in magnetic fields up to 70 kOe and in 2–300 K temperature range.

3. Results and discussion 3.1. XRD, SEM, TEM and XPS studies

Fig. 2. Magnetic susceptibility measured in FC and ZFC modes in two different magnetic fields H¼100 Oe, and H¼ 1000 Oe. The inset shows the temperature dependence of the inverse susceptibility in FC mode in H¼ 100 Oe.

As the results of XRD, SEM, TEM and XPS studies have been published previously, only a brief account will be given here. The obtained nanocomposite, designated as 0.2(Fe2O3)/0.8(ZnO), was characterized by means of X-ray diffraction which revealed the presence of only two phases: ZnO (with mean crystallite size of 69 nm) and ZFO (with nanocrystallites of average size 8 nm) [18]. Scanning electron microscopy images showed two types of grains: small grains of the ZFO agglomerates below 100 nm in size, and bigger hexagonal shaped monolithic grains of zinc oxide crystallites [18]. As an example the SEM image of 0.2(Fe2O3)/0.8(ZnO) nanocomposite is shown in Fig. 1. High-resolution TEM analysis indicated the presence of a core–shell structure of magnetic nanocomposites, ZFO nanoparticles covered by a ZnO layer [18]. Calculation of iron and zinc concentrations from X-ray photoelectron spectroscopy showed that the elemental iron concentration on the surface is much lower than the nominal concentration of iron in 0.2(Fe2O3)/0.8(ZnO) nanocomposite [18]. 3.2. dc magnetization study Fig. 2 presents the temperature dependence of dc magnetic susceptibility measured in FC and in ZFC modes in magnetic fields H¼ 100 and 1000 Oe. Large difference between ZFC and FC magnetization branches below certain temperature of irreversibility points out

Fig. 3. Dependence of Tmax determined from ZFC magnetization on applied magnet field.

on the presence of a strong magnetic anisotropy. ZFC branches exhibit pronounced maxima which shift to lower temperatures with increasing magnetic field. Magnetization measured in FC mode at low temperatures saturates or even decreases (for small magnetic fields). Generally, the ZFC susceptibility shows a peak for both superparamagnets and spin glasses. In contrast, it is usually seen that the temperature dependence of the FC susceptibility becomes saturated below the peak temperature (freezing temperature Tf) for spin glasses and continues to increase below that temperature (blocking temperature TB) for superparamagnets [19]. Separation of ZFC and FC curves in low temperature range indicates the presence of magnetic anisotropy that is removed only in high magnetic fields above 5 kOe. ZFC branch of magnetization curve has a maximum at a specific temperature Tmax that depends on applied magnetic field and this dependence is presented in Fig. 3. For glassy systems, in the mean-field approximation, Tmax(H) dependence could be given by equation [20]   α  H T max ¼ T f 1  Ha

Fig. 1. SEM image of 0.2(Fe2O3)/0.8(ZnO) nanocomposite. Large hexagonal shaped monolithic grains of zinc oxide crystallites and much smaller grains of the ZnFe2O4 agglomerates are visible.

ð1Þ

where Tf is the freezing temperature in H¼ 0, Ha is the anisotropy field at zero temperature, and α is a parameter that depends on the interaction strength between nanoparticles. For non-interacting nanoparticles α ¼2 while for interacting, α ¼ 2/3. The anisotropy field Ha at

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T¼0 K can be calculated also from the following equation [19]: 2K ef f H a ðT ¼ 0Þ ¼ ρM S

ð2Þ

where Keff is the effective anisotropy constant, ρ is sample density, and MS is the saturation magnetization. The solid line in Fig. 3 is the least-square fit to Eq. (1) for the experimental points in 0.30–2 kOe range. The following values were obtained: Tf ¼ 33.8 K, Ha ¼3208 Oe, and α ¼1.12. As expected, the calculated value of α is close to that of interacting nanoparticles. Because Eq. (1) is fulfilled for Ho1500 Oe, it means that collective behavior exists in this range of magnetic field and for higher fields the collective behavior is suppressed. This limiting field is much smaller than the calculated value of the anisotropy field Ha which is an intrinsic characteristic of a singledomain nanoparticle. Knowledge of density of n¼0.20 sample ρ ¼4.5 g/cm3 and saturation magnetization MS(T¼0)¼24.1 emu/g (see Fig. 6) allows calculating the value of the effective anisotropy constant Keff ¼ 1.74  105 erg/cm3. Keff values between 7  105 and 4  103 erg/cm3 have been estimated from Mossbauer spectroscopy for particle sizes between 3 and 20 nm [21]. The Keff value is influenced by several anisotropy factors but its variation mainly depends on the surface anisotropy that increases when the particle size decreases and the crystalline anisotropy that increases when the inversion parameter increases. In case of our nanocomposite a strong agglomeration of nanoparticles and the core–shell structure of a nanoparticle can play dominating roles in establishing this large value of the anisotropy constant. The inset in Fig. 2 presents temperature dependence of inverse magnetic susceptibility measured in FC mode in H¼ 100 Oe. It could be noticed that in the high temperature range (T4 250 K) the Curie–Weiss law, χ(T)¼C/(T  T0), is fulfilled. Least-square fit gave the following values of the parameters in that law: Curie constant C ¼3.71(emu K)/(mol Oe) and Curie–Weiss temperature T0 ¼ 210.8 K. The positive sign of T0 constant indicates a predominance of strong ferromagnetic interactions in the spin system. Effective magnetic moment (for a single Fe3 þ ion) calculated from the value of Curie constant is μeff ¼ 5.45μB, where μB is Bohr magneton. This value is slightly smaller than what is expected for a spin-only S ¼5/2 magnetic moment of Fe3 þ ion (5.9μB). Theoretical investigation of magnetization in core–shell nanoparticles using the Monte Carlo technique showed that the magnetic order in the core is disturbed by the surface and does not reach the saturation value of a bulk material [22]. Fig. 4 presents the hysteresis loops registered at two different temperatures for 0.2(Fe2O3)/0.8(ZnO) nanocomposite. The inset displays in more detail the region of low magnetic fields to show

Fig. 4. Hysteresis loops registered at T ¼ 3 K and 290 K. The inset shows the enlarged view of hysteresis loops near the center.

the remanent magnetization and the coercive field. It can be seen that magnetization does not saturate even in magnetic field of 70 kOe. This may be due to the existence of the core–shell structure in investigated nanoparticles and thus also explains their large magnetic anisotropy. In Fig. 5 the temperature dependence of the coercive field is presented. Its value decreases abruptly with temperature increase in the low temperature range and shows fairly constant values at temperatures above 30 K. There is also an anomalous dip in HC(T) curve visible around T 0f ¼20 K. According to the Monte Carlo simulation, the exchange interaction between the ferrimagnetic core and the spin-glass surface layer of a nanoparticle with a magnetic core–shell structure is essential to yield such an anomalous behavior in Hc(T) [23,24]. Temperature T 0f , is identified as the freezing temperature of surface spins in the spin-glass phase. Below T 0f , core–shell coupling exerts a strong pinning effect on the core ferrimagnetic phase to cause an increase in the coercivity. Assuming that below T 0f the majority of nanoparticles are blocked, the following equation could be used [25]: H c ðTÞ ¼ H c ð0Þð1  AT k Þ

ð3Þ

where Hc(0) is coercivity at T¼0 K and A is a constant, while k¼ 0.77 for randomly oriented and blocked particles. The constant A ¼ ðβ kB =K ef f V Þk , where β ¼ ln ðτm =τ0 Þ is a coefficient that depends both on the measuring time of experiment τm and on the prefactor τ0 that governs the superparamagnetic relaxation through the relaxation time τ of the Arrhenius law: τ ¼ τ0 expðK ef f V=kB TÞ. It is customary to take β value in the range 25–34. The fitting of the experimental Hc values with Eq. (3) leads to Hc(0)¼ 2.9 kOe and A¼0.24 K  0.77. The straight line in Fig. 5 represents Eq. (3) in the temperature range where it is valid. Taking the experimental value of A and assuming β ¼30, the product KeffV¼ 2.64  10  14 erg could be calculated, which in turn gives the size of ZFO nanoparticle d¼ 6.6 nm. This is smaller than the size obtained from XRD measurements (8 nm), but supports the core–shell model of these nanoparticles with smaller ZFO magnetic core. In Fig. 6 the dependence of the saturation magnetization is shown. Although magnetization does not saturate even in an external magnetic field of 70 kOe (see hysteresis loops in Fig. 3), saturation magnetization can be calculated from linear extrapolation of M(1/H) data, when H-1. Obtained MS(T) values, in the temperature range 3–60 K, are presented as full squares in Fig. 6. The common practice in describing experimental data on nanoparticles is to use a modified Bloch formula [26] M S ðTÞ ¼ M 0 ð1  BT b Þ

ð4Þ

Fig. 5. Temperature dependence of coercivity obtained from the hysteresis loops measured at different temperatures. The solid line is the fitting to Eq. (3).

J. Typek et al. / Journal of Magnetism and Magnetic Materials 361 (2014) 12–18

Fig. 6. The temperature dependence of the saturation magnetization obtained by the method of extrapolation. The solid line is the fit to Eq. (4).

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Fig. 8. Experimental (symbols) and fitted (solid line) FMR spectra of 0.2(Fe2O3)/0.8 (ZnO) nanocomposite at 8 K. FMR spectrum was fitted with two LL lines. These two components are shown separately as dotted and dashed lines.

temperature range, above 50 K, and low temperature range, below 50 K. In the high temperature range, the registered FMR line could be very well fitted with a single Landau–Lifshitz (LL) line, while in the low temperature range satisfactory results were achieved by using two LL lines. The LL function is given by [28]

χ ″ðHÞ ¼

Fig. 7. Registered FMR spectra of 0.2(Fe2O3)/0.8(ZnO) nanocomposite at different temperatures in the 4–300 K range.

where M0 is magnetization at T¼ 0 K; B and b are the Bloch constant and Bloch exponent, respectively. The three parameters fitted to the observed values are strongly size dependent and approach the bulk limit for larger samples. The obtained values are B ¼0.0058 K  0.72 and b¼ 0.72. From the modified Bloch equation it follows that MS(0)¼ 24.1 emu/g and this value is used in calculation of the effective anisotropy constant. For bulk ferromagnets b¼ 3/2, while for nanoparticles it turns out that values of b show little regularity with size variation, and can be both larger and smaller than 3/2, depending on their preparation technology [27]. 3.3. FMR study FMR spectra taken at different temperatures in the 4–300 K range are presented in Fig. 7. At temperatures near RT a strong, single, slightly asymmetric line centered at g  2.0, with peak-topeak linewidth ΔHpp ¼210 Oe, is recorded. With temperature decrease the line shifts towards lower magnetic fields, its linewidth increases and the line becomes highly asymmetric. Such a behavior is typical for magnetic nanoparticles in the superparamagnetic phase. The evolution of the FMR spectrum could be conveniently described by dividing it in two temperature ranges: high

1

π

Δ Δ

Δ

2 H 2r ½ðH 2r þ H ÞH 2 þ H 4r  H 2 2 2 2 ½H r ðH  H r Þ þH H ½H 2r ðH þ H r Þ2 þH 2

Δ2H 

ð5Þ

where Hr is the true resonance field and ΔΗ is the true linewidth. In Fig. 8, as an example, the observed (points) and fitted (solid line) spectra at low temperature (T¼8 K), are presented. The fitted spectrum is the sum of two LL components. The fitting enabled determining the temperature dependence of the true magnetic resonance spectra parameters: resonance field, linewidth and the integrated intensity. The integrated intensity can be calculated equivalently as the product of the FMR derivative line amplitude and the square of the linewidth or just by calculating the area under the FMR absorption curve. This quantity is proportional to the magnetic susceptibility of the spin system at microwave frequency. Fig. 9 shows temperature dependence of the apparent resonance field (measured directly from FMR spectrum as the center of the resonance line) and the integrated intensity in the hightemperature range. As temperature is decreased from RT the resonance field does not change noticeably down to 100 K but at lower temperatures a considerable shift towards smaller magnetic fields is recorded. This behavior of the resonance field below RT is typical for the superparamagnetic phase of non-interacting or weakly interacting magnetic nanoparticles and has been observed in many systems. The shift of the FMR spectrum to lower magnetic fields upon cooling is usually assigned to surface phenomena, particularly to the exchange anisotropy arising at the interface between ferro- and antiferromagnetic (or spin-glass) layers. A simpler approach applicable to relatively high temperatures and spherical shapes without any distortion was proposed by Noginova et al. [29]. In this approach, the core and the surface were considered separately and the effect of their interaction was introduced semi-phenomenologically. The core was assumed to be in the ferromagnetic (superparamagnetic) state and the surface layer in the paramagnetic state. The non-uniform magnetization on the surface produces a unidirectional field seen by the bulk spins and leads to temperature-dependent shift of the FMR line to lower fields. Thus strong surface anisotropy results in an increase of the average resonance frequency of the surface spins. Due to

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Fig. 9. Temperature dependence of the apparent resonance field (right-hand side axis, open circles) and the integrated intensity (left-hand side axis, open squares) in the high temperature range.

exchange interaction, this frequency shift is partly transferred to the bulk spin system, leading to the corresponding shift of the FMR spectrum toward lower fields [29]. The FMR integrated intensity increases with temperature decrease from 270 K and reaches a maximum at about 50 K (Fig. 9). Below that a strong decrease of the integrated intensity with decreasing temperature is observed. This maximum is often identified with the so called blocking temperature TB, below which most of nanoparticles have fixed in space magnetic moments that cannot be reversed by thermal motion. Blocking temperature depends on the frequency of the measuring method and is thus determined by static techniques (e.g. dc magnetization) and is usually smaller than that measured by high frequency methods (e.g. magnetic resonance). Another explanation for the peak in the temperature dependence of the integrated intensity has also been proposed and is based on an effective anisotropy field becoming larger than the characteristic microwave field [30,31]. As this could be a plausible explanation in case of our sample this approach will be discussed later. Above 270 K the integrated intensity begins to increase with temperature rise and this could be explained by assuming that the spin dimers with a spin singlet ground state and magnetic excited state exist in the sample. These dimers are supposed to be short-lived as the temperature dependence of dc magnetization did not evidence their presence above 270 K. Temperature dependence of the apparent linewidth Δ (measured directly from FMR spectra) in the high-temperature range is presented in Fig. 10. Except for temperatures above  270 K, the linewidth increases with decrease in temperature. The rate of linewidth change varies and two temperature regions could be recognized. They are easily seen when the linewidth is presented as a function of the reciprocal temperature (Fig. 10). Two straight lines could be fitted to the experimental points in appropriate temperature ranges. At  110 K a change of the d(Δ)/dT rate is discernible in Fig. 10. This is roughly the same temperature at which the resonance field starts to shift towards lower magnetic fields with decrease in temperature. Apparently around this temperature a change of the relaxation type takes place that might be connected with the core–shell spin structure of the ZFO nanoparticle. As could be seen in Figs. 7–10, the thermal evolution of the FMR spectra in the high-temperature, superparamagnetic phase of ZFO nanoparticles could be explained in terms of the core–shell model [29]. For high temperatures, T450 K, the behavior of FMR spectra is governed by core anisotropy and thermal fluctuations. As the temperature decreases from RT, the shell spins increase their magnetic susceptibility and produce an effective field on the core. This leads to a decrease of the resonance field from its high temperature value. In the 110–50 K range an internal field increases further, probably due to the

Fig. 10. Temperature dependence of apparent linewidth in the high-temperature range (solid squares, bottom scale). Linewidth dependence is also presented as a function of reciprocal temperature (open circles, top scale). The continuous straight lines are the least-square fits in appropriate temperature ranges.

contribution from large ferromagnetic spin clusters located on the surfaces of nanoparticles. In the low temperature regime (To50 K) the FMR line is very broad and strongly asymmetric. The observed FMR spectrum is created by an assembly of many individual nanoparticles with randomly oriented anisotropy axes. The theory of Raikher and Stepanov [32] of magnetic resonance spectra in the superparamagnetic regime in low temperature limit predicts a highly asymmetric lineshape typical for FMR in powdered samples. We have approximated the overall spectrum with only two LL lines that could relate to two orientations (parallel and perpendicular) of an external magnetic field with respect to the effective nanoparticle anisotropy axis (Fig. 8). This is a fairly coarse approximation of a rather complicated situation but it is believed that this approach could provide information on the degree of magnetic anisotropy in a nanoparticles system [33]. The fitting of the registered spectra of ZFO nanoparticles with two component lines at different temperatures allowed determining the temperature dependence of the true resonance fields and true linewidths. In Fig. 11 the temperature dependence of the resonance fields Hr1 and Hr2 of both components is presented. The lines are shifted in an external magnetic field due to different orientations of nanoparticle anisotropy axes with respect to magnetic field. The resonance field Hr1 increases sharply with temperature decrease in the low temperature range while the temperature behavior of Hr2 shows opposite behavior. The difference of both resonance fields could be correlated with the effective anisotropy field Ha and we will follow the procedure presented by Zins et al. [34] applied for mixed ferrite nanoparticles Mn1 xZnxFe2O monodispersed in glycerol. Because the linewidths of the two components are not negligible, the relaxation effects influencing the resonance fields have to be taken into account. If Hr is the resonance field with relaxation and H 0r is the resonance field without relaxation, the following relation holds: H r ¼ H 0r  ð3=4ÞðΔH pp Þ2 g ef f μB =hν, where ΔHpp is the peak-to-peak linewidth, geff is the effective Lande factor, μB the Bohr magneton, and ν is the frequency of the microwave field [32]. The uniaxial anisotropy field could be calculated from the resonance fields without relaxation as [35,36] 2 H a ¼ ðH 0r1  H 0r2 Þ 3

ð6Þ

Uniaxial anisotropy changes with temperature and the dependence is presented in the inset of Fig. 11.

J. Typek et al. / Journal of Magnetism and Magnetic Materials 361 (2014) 12–18

Fig. 11. Temperature dependence of the true resonance fields Hr1 and Hr2 of both components determined from Eq. (5). The inset shows temperature dependence of the uniaxial anisotropy field in the high temperature range calculated from Eq. (6). The solid lines are linear regressions in appropriate temperature ranges.

Fig. 12. Temperature dependence of the true linewidths ΔH1 and ΔH2 of both components in the low temperature range (T o 50 K) calculated from Eq. (5). The inset shows this dependence in a double logarithmic scale.

Fig. 12 presents temperature dependence of the true linewidths

ΔH1 and ΔH2 of both components in the low temperature regime calculated from Eq. (5). Both linewidths increase with decrease of temperature, but the change of ΔH1 is much greater than that of ΔH2. The inset in Fig. 12 shows temperature dependence of both linewidths in a double logarithmic scale. The temperature change of both linewidths could be described by a power law ΔH(T)  T  n, with n ¼1.5(2) and 0.44(2) for components 1 and 2, respectively. It is interesting to notice that the linewidth of the component 1, ΔH1, depends linearly on the difference between resonance fields of both components (see Fig. 13). Thus it can be assumed that this linewidth is mostly determined by a random distribution of the nanoparticles' magnetic easy axes. In Fig. 14 temperature evolution of the integrated intensity of FMR is presented. The Iint(T) curve shows a clear maximum at T  50 K. When the effective anisotropy field Haeff (it is anisotropy field modified by thermal fluctuations) becomes larger than H0 ¼ ω0/γ the integrated intensity decreases because the resonance condition cannot be achieved [30]. In general, as Haeff increases with decreasing temperature, it is natural to presume a loss of Iint when Haeff becomes larger than H0. Furthermore, a decrease in the average resonance fields is expected with a simultaneous increase of the dispersion in the resonance fields echoed in a large increase of the FMR linewidth. Thus not only nanoparticles with easy axis

17

Fig. 13. Dependence of ΔH1 linewidth on the difference of resonance fields of both components.

Fig. 14. Temperature dependence of the FMR integrated intensity.

oriented close to the applied field participate in the resonance but also those with axes nearly perpendicular to the external field must be taken into account [30]. Closer inspection of thermal behavior of the FMR parameters in the low temperature range (Figs. 11–13) allows dividing that temperature interval (T o50 K) into two subranges: one between 50 K and 20 K, the other below 20 K. In the former subrange the integrated intensity decreases with decrease in temperature and there is insignificant thermal variation of resonance fields and linewidths. Such a behavior could be explained by taking into account the dipolar interactions between particles cores [30]. This coupling of nanoparticles decreases the effective anisotropy seen by each magnetic core. In the temperature range below  20 K the FMR integrated intensity decreases further and the anisotropy field increases significantly with decrease in temperature. This could be explained by assuming that freezing occurs in the system of coupled nanoparticles, including both magnetic cores and shells. 3.4. Comparison of FMR and dc magnetization studies In Fig. 15 schematic presentation of different magnetic phases and characteristic temperatures revealed by FMR and dc magnetization in 0.2(Fe2O3)/0.8(ZnO) nanocomposite is shown. Above  250 K dc magnetic susceptibility follows the Curie–Weiss law appropriate for a paramagnetic material. On the other hand FMR measurements showed that above  270 K the integrated intensity

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References

Fig. 15. Schematic presentation of different magnetic phases and characteristic temperatures revealed by FMR and dc magnetization in 0.2(Fe2O3)/0.8(ZnO) nanocomposite.

starts to increase which could be interpreted in terms of the presence of spin pairs with a non-magnetic ground state and magnetic excited states. As these pairs are not visible in static magnetization study it could be deduced that they are dynamic, short-lived and thus are not detected by a static method. Between 250 K and 30 K the superparamagnetic phase of ZFO nanoparticles is consistently observed by the two methods used. FMR spectra have shown changes at  110 K that can be explained by invoking the core–shell structure of ZFO nanoparticles. In the temperature range below 30 K both methods indicate the freezing of surface and core spins and formation of the spin-glass phase. Although the core–shell model is very often invoked for magnetic characterization of ferrites, there are many mechanisms that could influence the core–shell interface (vacancies, cation substitution, changes in the degree of inversion) and make the predictions difficult [37–39]. Thus the explanation of the overall magnetism of the core–shell nanoparticle subjected to intrinsic, surface, interface, and disorder effects remains an open question. Even if the shell is non-magnetic, like in case of Cu covering the γ-Fe2O3 nanoparticles, it has had a significant effect on the surface Fe magnetism [40]. Moreover, the Cu coating modified the Fe (octahedral)–O–Fe (tetrahedral) main superexchange in the core of a nanoparticle. Although the existence of surface layer in ZFO nanoparticles is consistently indicated by magnetometry and FMR measurements the nature of the intermixed layer could not be clearly established as we observe only the sum total of its intrinsic magnetic properties. 4. Conclusions The FMR and dc magnetization studies of magnetic properties of 0.2(Fe2O3)/0.8(ZnO) nanocomposite were interpreted in terms of a simple model in which each single-domain ZFO nanoparticle is considered as a core–shell system with magnetocrystalline anisotropy on the core and surface anisotropy on the shell. Three main temperature ranges of magnetic behavior have been found, representing paramagnetic, superparamagnetic and spin-glass phases. In the 110– 250 K temperature interval the behavior typical for a system of isolated superparamagnetic nanoparticles is registered. Magnetic anisotropy and interactions are smoothed out by thermal fluctuations. In the 110–50 K range, surface ferromagnetic clusters grow and have an effect on the core spins while at temperature below  30 K the freezing of cores and shells greatly increases anisotropy and diminishes the FMR signal. Acknowledgments Thanks are due to Adam Presz from the Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw, for SEM measurements.

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