Magnetic studies of 0.7(Fe₂O₃)/0.3(ZnO) nanocomposites in nanopowder form and dispersed in polymer matrix

In Fig. 2 temperature dependence of the DC magnetic susceptibility χ (defined as χ = M/H) in zero field cooled (ZFC) and field cooled (FC) modes of the 0.7(Fe₂O₃)/0.3(ZnO) nanopowder (Fig. 2a) and the 0.7(Fe₂O₃)/0.3(ZnO) dispersed in polymer (Fig. 2b) measured in two different magnetic fields (H = 100 Oe and 1000 Oe) is presented. In ZFC mode all magnetic moments in each single domain particle point along the nanoparticle easy axis when the nanoparticles are cooled down to very low temperature and magnetic anisotropy prevents switching of magnetization from the easy axis. As a result, the average magnetization is very small at low temperature because of random distribution of the easy axis directions. The magnetic anisotropy in some nanoparticles is overcome and the magnetization directions of these thermally activated nanoparticles start to align with the applied field as the temperature is increased and, as a result, the total magnetization initially increases with increasing temperature, reaching a maximum at the corresponding temperature T_max. For non-interacting particles this temperature is directly proportional to the average blocking temperature T_B. In case of our nanopowder sample T_max = 81 K and 36 K in magnetic field H = 100 Oe and 1000 Oe, respectively. For 0.7(Fe₂O₃)/0.3(ZnO) dispersed in polymer T_max = 70 K and 34 K in magnetic field H = 100 Oe and 1000 Oe, respectively. These temperatures are slightly lower in polymer sample indicating weaker anisotropy due to reduced magnetic interaction and probably smaller sizes of ZnFe₂O₄ nanoparticles.

Fig. 2

Interestingly, very similar results concerning the field dependence of T_max in concentrated and diluted samples of iron oxide nanoparticles were obtained by Knobel et al. [14]. They found that in low magnetic fields T_max is higher for the concentrated sample than for the diluted one and that the this difference in T_max vanishes in higher magnetic fields. This is exactly the same phenomenon as was observed in our samples. Yet another interesting effect of interparticle dipolar interaction was seen in the temperature dependence of FC magnetization below T_max [14]. While the FC curve of the diluted sample was kept increasing as T was decreasing below T_max, the FC of the concentrated sample remained almost constant. A similar effect can be found in FC magnetization in our samples (Fig. 2).

In FC mode of magnetization study all the magnetic moments of the nanoparticles are aligned along the external field direction, irrespective of the easy axis orientation of each individual nanoparticle and at low temperature the magnetization direction of each particle is frozen in the field direction. During temperature rise an increasing number of nanoparticles will be in a superparamagnetic state and the magnetization should decrease monotonically. Such behavior is observed in our samples for sufficiently large external magnetic fields (H ∼ 1 kOe), but for weaker fields non-monotonic M_FC(T) dependence is observed, especially for the nanopowder sample. This may be the result of magnetic interparticle interactions.

As can be observed in Fig. 2 the ZFC and FC curves diverge below a certain temperature T_irr called irreversibility temperature, where a magnetic irreversibility is observed. This temperature can be related to the blocking temperature of the largest in size particles. Therefore the difference between T_irr and T_max can be taken as the particle size distribution (in the absence of interparticle interactions). Closer inspection of Fig. 2 shows that T_irr strongly depends on external magnetic field H. For the nanopowder sample in H = 100 Oe T_irr ≈ 200 K, while in H = 1000 Oe the irreversibility temperature is very close to T_max. In case of the polymer sample, both temperatures are similar, T_irr ≈ T_max independent of external magnetic field. That dissimilar behavior of both investigated samples in the context of magnetic irreversibility could be attributed to non-negligible interparticle interaction in the nanopowder sample.

In Fig. 3 isothermal magnetization of nanopowder and polymer samples in magnetic field up to 70 kOe is presented. For the nanopowder sample (Fig. 3a) no saturation even in the strongest available field was reached. At temperature higher than the blocking temperature, in the superparamagnetic phase, magnetization M(H) is expected to follow the Langevin, M = M_S·L(x), or modified Langevin function, M = M_S·L(x) +αH, where L(x)=coth(x)−1x${\rm{L}}\left( {\rm{x}} \right) = \coth \left( {\rm{x}} \right) - {1 \over {\rm{x}}}$ is the Langevin function and x = μpHkT${{{{\rm{\mu }}_{\rm{p}}}{\rm{H}}} \over {{\rm{kT}}}}$. Here, M_S is the saturation magnetization, µ_p is the particle magnetic moment, k is the Boltzmann constant and α is linear susceptibility. For isothermal magnetization of nanopowder sample at T = 290 K it was found that the modified Langevin function is better than the classical Langevin in fitting experimental points, although even that fit is not quite satisfactory. One of the reasons might be a broad distribution of particle sizes and particle magnetic moments in 0.7(Fe₂O₃)/0.3(ZnO) nanopowder [15]. The effective magnetic moment of one Fe³⁺ ion calculated from the obtained value of M_S was only 0.5 Bohr magnetons what is well below for an expected value for spin-only magnetism of iron ion and thus it might be the evidence of the core-shell structure of investigated nanoparticles.

Fig. 3

Isothermal magnetization of the polymer sample (Fig. 3c and Fig. 3d is fairly different from the nanopowder sample. Due to a small concentration of ferromagnetic nanoparticles, the diamagnetic properties of the PEN-b-PTMO polymer matrix prevail in strong magnetic fields and at higher temperatures so the resulting magnetization becomes negative. Fig. 3b and Fig. 3d present magnetization in small magnetic fields (hysteresis loops) for a nanopowder and polymer sample, respectively. At temperature T = 8 K, well below T_max, in weak magnetic fields, the hysteresis loop in M(H) magnetization is observed for both samples (Fig. 3b and Fig. 3d). For the nanopowder sample the coercive field is 390 Oe, while for the polymer sample it is 480 Oe. Also the remanent magnetization is slightly bigger for the polymer sample than for the nanopowder one, 23 emu/g_Fe and 20 emu/g_Fe, respectively. For the nanopowder sample the hysteresis loop is only observed at the lowest temperatures (T < 10 K) indicating the existence of ferromagnetic state in that temperature range. Bearing in mind the M(T) curve for the nanopowder sample it could be deduced that the spin-glass state is formed at higher temperatures, up to T_max, depending strongly on applied magnetic field. Above T_max the superparamagnetic phase is expected.

Matrix effects on magnetic properties of γ-Fe₂O₃ (maghemite) nanoparticles dispersed in a multiblock copolymer have been investigated previously [16]. Behavior of two polymer samples with magnetic nanoparticles in two different dispersion forms (large agglomerates and separate nanoparticles) was compared. It was found that the shift of T_max in M_ZFC(T) curves towards higher temperatures as well as the broadening of the M_ZFC(T) peak and flattening of the M_ZFC/FC(T) curves above T_max is the sign of an increased interaction between nanoparticles. Exactly the same behavior was seen in our 0.7(Fe₂O₃)/0.3(ZnO) nanopowder sample in which strong interparticle interactions have been expected.

The magnetization M(H) of large aggregates was smaller than that of separate nanoparticles and exhibited a relatively slower increment at low fields in the study of maghemite samples [16]. This was also confirmed in our study of 0.7(Fe₂O₃)/0.3(ZnO) nanopowder and polymer samples. As the hysteresis loop is concerned the coercive field was bigger for the sample containing agglomerated maghemite nanoparticles than for the dispersed ones [16]. In case of our 0.7(Fe₂O₃)/0.3(ZnO) samples a reversed effect was registered: the coercive field was bigger for the polymer sample. While a significant decrease of both coercivity and remanence has been predicted for interacting single domain particles with dipolar forces, a more complex behavior may arise from the interplay of exchange interactions and magnetic anisotropy [17]. In case for our polymer sample the bigger coercive field could be attributed to interaction of ZnFe₂O₄ nanoparticles with the polymer matrix.

FMR spectra of nanopowder and nanopowder dispersed in PEN-b-PTMO polymer taken at different temperatures in the 4 K to 423 K range are presented in Fig. 4. In Fig. 4a and Fig. 4b FMR spectra of nanopowder in low and high temperature ranges are shown, respectively. Similarly, in Fig. 4c and Fig. 4d the spectra of 0.7(Fe₂O₃)/0.3(ZnO) nanocomposite dispersed in polymer matrix are displayed. Above room temperature (RT) a single, nearly symmetrical and narrow line is registered that decreases in intensity with temperature increase in both samples. On cooling the samples from RT the amplitude of the resonance line decreases, the apparent resonance field decreases, the apparent linewidth increases and the lineshape becomes visibly asymmetrical (the apparent parameters are measured directly from the observed spectra). Such behavior is typical of FMR spectra of magnetic nanoparticles in the superparamagnetic phase [12, 18]. The values of apparent resonance fields (Fig. 5, right axis) and linewidths are similar for both types of the samples.

Fig. 4

Fig. 5

Closer inspection of the temperature dependence of the apparent resonance fields (Fig. 5, right axis) reveals that in the case of polymer sample it increases slightly on cooling from RT down to about 150 K but decreases on further cooling, especially strongly below 50 K. The apparent resonance field is slightly smaller in the nanopowder sample what can be explained by the existence of bigger internal field in that stronger magnetic material. The shift of the resonance field toward lower magnetic field could be explained by the core-shell model [19]. The core is assumed to be in the ferromagnetic (superparamagnetic) state and the surface layer in the paramagnetic state. The nonuniform surface magnetization produces unidirectional field seen by the bulk spins which causes temperature-dependent shift of the FMR line towards lower fields. Strong surface anisotropy results in an increase of the average resonance frequency of the surface spins. Due to exchange interactions, this frequency shift is partly transferred to the bulk spin system, leading to the corresponding shift of the FMR spectrum toward lower fields [19].

Fig. 5 (left axis) presents also the temperature dependence of the integrated intensity. The integrated intensity, calculated as an area under the absorption curve (not the first derivative shown in Fig. 4), is proportional to the magnetic susceptibility of the spin system at a microwave frequency. As seen in Fig. 5 it behaves similarly in both samples. It increases with temperature decrease from RT and reaches a maximum at about 45 K in the polymer sample (35 K in the nanopowder sample) followed by a strong decrease down to 12 K in the polymer sample (18 K in the nanopowder sample). Below that minimum in the integrated intensity, it starts to increase again with temperature decrease. As can be easily noticed the temperature difference between the maximum and minimum in integrated intensity is much narrower in the nanopowder sample and is shifted towards lower temperatures. It might be connected with stronger magnetic interactions in that sample. The observed maximum in the temperature dependence of the integrated intensity curve is often identified with the so called blocking temperature below which most of nanoparticles have fixed in space magnetic moment that could not be reversed by thermal motion. Another, more plausible explanation is related to an effective anisotropy field of a nanoparticle becoming larger at that temperature than the characteristic microwave field of the spectrometer [20, 21]. A steep increase of the integrated intensity with decreasing temperature at the lowest temperatures (below 19 K and 12 K for the nanopowder and polymer samples, respectively) can be attributed to the existence of a spurious paramagnetic phase or paramagnetic component in the core-shell model of ZnFe₂O₄ nanoparticles [18].

All the registered FMR spectra could be very well fitted with the sum of two Landau-Lifshitz (LL) lines (Fig. 6). The LL lineshape function is given by the following equation:

Fig. 6

where H_r is the true resonance field and ∆_H is the true linewidth. Usage of the two LL components can be explained by assuming two anisotropy axes of a nanoparticle which are arranged perpendicularly to each other. Because each registered FMR spectrum is produced by an array of many individual nanoparticles with the anisotropy axes randomly oriented in respect to an external magnetic field, the decomposition of FMR spectrum on only two component lines is just a fairly coarse approximation. However, it could provide an estimation of the value of magnetic anisotropy in the nanoparticles system. Each registered spectrum was fitted with two LL lines that corresponded to two orientations – parallel and perpendicular – of an external magnetic field with respect to the effective nanoparticle anisotropy axis. In Fig. 6, as an example, the experimental and fitted spectra taken at low and high temperatures in both types of samples are presented. Additionally, each component is also drawn. It is easily to notice that the experimental spectrum could be very well fitted with only these two components.

Temperature dependence of the true resonance fields H_r and true linewidths ∆_H calculated from equation 1 for two LL components and for two investigated samples is presented in Fig. 7 and Fig. 8. The linewidths of two components in each sample increase with cooling down the samples from RT. This is typical of FMR spectra of magnetic nanoparticles. The broadening is especially strong in the low temperature range, below ∼40 K (Fig. 8) and is correlated with a rapid decrease of the resonance field. This sudden broadening might be interpreted in terms of the spin-glass freezing of the particle surface layer. This effect seems to be stronger in polymer sample, probably due to interaction with the surrounding matrix, as evidenced by broader linewidths registered at the lowest temperature.

Fig. 7

Fig. 8

According to the procedure presented in the literature [21], the difference of the resonance fields of both components in each sample can be correlated with the effective anisotropy field H_a of that sample [22]. As the linewidths of the two components are not very small, the relaxation effects influencing the resonance fields must be taken into account. If H_r is the observed resonance field with relaxation effects and Hr0${\rm{H}}_{\rm{r}}^0$ is the resonance field without relaxation, the following relation holds:

where ∆H_pp is the observed peak-to-peak linewidth. The effective uniaxial anisotropy field H_a could be calculated from the resonance fields of both LL components without relaxation as [23]:

The obtained from equation 3 Ha values as a function of temperature are presented in Fig. 9. In general, the anisotropy field is slightly stronger in the polymer sample and increases with temperature decrease (for T < 250 K and T < 350 K for the polymer and nanopowder samples, respectively) for both studied samples. It is also evident that temperature change of the anisotropy field is more accentuated in the polymer sample and that points to a stronger contribution of the surface anisotropy in that sample caused probably by the surrounding matrix. Similar results were obtained in the investigation of magnetite (Fe₃O₄) nanoparticles with different sizes [23].

Fig. 9

PTMO-based poly(ether-esters) copolymers are sensitive to oxidative degradation due to the ether soft block. It is well known that the mechanism of the thermo-oxidative degradation of these copolymers involves a radical chain process with formation of hydroperoxides at the carbon adjacent to the ether oxygen [24]. The effect of the presence of 0.7(Fe₂O₃)/0.3(ZnO) in PTT-PTMO matrix on thermal and oxidative stability of the composite was studied by thermogravimetric analysis (TGA). The mass loss (TG) and the DTG (derivative of TG) curves for the nanocomposite and the neat copolymer (without nanofiller) under oxidative (air) and inert (argon) atmosphere are presented in Fig. 10. From the shape of the TG and DTG curves for the nanocomposite and the neat PEN-b-PTMO copolymer, it can be observed that in air and in an inert atmosphere (argon) the thermal behavior of the composite and neat copolymer is very similar. The PEN-PTMO copolymer in oxidative atmosphere has two stages of degradation, which appear in 250 °C to 450 °C and 450 °C to 600 °C ranges. The first step of decomposition is attributed to the decomposition of flexible polyether and rigid polyester segments and the second step is ascribed to the decomposition of the residue [25]. The temperatures at the maximum decomposition rate (maximum on DTG peak) of the main decomposition step in air (at 405 °C) and in argon (at 400 °C) are not induced by the presence of the nanofiller in the copolymer matrix. In air, the first stage of decomposition is slightly influenced by the presence of the nanofiller. The value of temperature corresponding to the 10 % weight loss is shifted to lower temperatures. This indicates that the nanocomposite has lower thermo-oxidative stability than the neat PEN-b-PTMO copolymer. This can be explained by the catalysing of the decomposition reaction by the presence of metal oxide or the increasing of molecular weight polydispersity of the block copolymer in the composite which results in higher concentration of the functional groups (OH, COOH).

Fig. 10