Enhanced Magnetic Behavior of Chemically Prepared Multiferroic

Article pubs.acs.org/JPCC

Enhanced Magnetic Behavior of Chemically Prepared Multiferroic Nanoparticles of GaFeO3 in (GaFeO3)0.50 (Ni0.4Zn0.4Cu0.2 Fe2O4)0.5 Nanocomposite K. Mukhopadhyay,† S. Sutradhar,† S. Modak,† S. K. Roy,‡ and P. K. Chakrabarti*,† †

Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan 713 104, West Bengal, India Department of Spectroscopy, IACS, Jadavpur, Kolkata-700 032, India

‡

ABSTRACT: Nanoparticles of GaFeO3 (GFO) and Ni0.4Zn0.4Cu0.2 Fe 2 O 4 (NZCF) and their nanocomposite [(GaFeO 3 ) 0 . 5 0 (Ni0.4Zn0.4Cu0.2Fe2O4)0.50, GFONZCF] were prepared by chemical route. Nanoparticles of GFO were synthesized by sol−gel route, and those of NZCF were prepared by chemical coprecipitation method. The nanoparticles of GFO were incorporated in the matrix of NZCF by coprecipitating the salts required for NZCF in the presence of GFO particles, followed by subsequent washing and heat treatment at 500 °C. X-ray diffractograms (XRDs) were recorded to confirm the formation of the desired crystallographic phases of the samples. The sizes of the nanoparticles were estimated from the broadening of the well-defined peaks using the Debye−Scherrer equation. The nanoparticle size and its distribution, crystallographic phase, nanocrystallinity, and so on were studied by a high-resolution transmission electron microscope (HRTEM), and the extracted results were in good agreement with those obtained from the XRD patterns. The static and dynamic magnetic measurements were carried out. The observations of field-cooled (FC), zerofield-cooled (ZFC) magnetizations, and hysteresis loops (M-H loop) in the temperature range of 300 to 2 K were carried out in the static measurements. The static magnetic data were analyzed to evaluate the particle size, nanocrystalline anisotropy, and so on, and the agreement of these evaluated data are quite satisfactory, so far as the extracted results obtained from XRD and HRTEM are concerned. The maximum magnetization of the GFO sample has been drastically enhanced by incorporating them in the matrix of NZCF. Also, the nature of variation of the magnetization in all cases of FC, ZFC, and M-H curves of the nanoparticles of GFO has been drastically modulated by the NZCF. The dynamic magnetic measurements include the measurements of ac magnetization versus excitation curves, hysteresis loops at different frequencies at room temperatures, and so on. The remarkable enhancement of magnetization of the multiferroic system of GFO by the encapsulation of NZCF would be quite interesting for various applications.

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TbMnO3,9 electric-field-induced spin flop in BiFeO3,10 magnetic-field-induced ferroelectric state in DyFeO3,11 and the ME memory effect in MnWO412 are quite interesting from both fundamental and technical perspectives. However, one of the striking problems in the field of multiferroics is their smaller number of room-temperature (RT) multiferroics. In general, the magnetism (either ferro or antiferro) requires partially filled 3d shells, and it becomes either ferromagnetic or antiferromagnetic depending on the intersite exchange energy J, whereas the ferroelectricity generally occurs only in the absence of d electrons.13 This apparent incompatibility for the simultaneous coexistence of ferroic properties in magneto-dielectric multiferroic has been overcome in materials viz. BiFeO3, GaFeO3, and FeAlO3 with general formula ABO3 including rare earth manganites of AMnO3 (A = Y, Tb, Gd, Ho), in which A and B serve the sources of ferroelectricity and ferromagnetism, respectively.14 In this direction, GaFeO3 has attracted much attention from various points of view. The magnetic and

INTRODUCTION Recently multiferroic materials are of particular interest for the simultaneous coexistence of two or three ferroic properties among ferromagnetism, ferroelectricity, ferroelasticity, and so on in the same crystallographic phase. Among these materials, the magneto-dielectric multiferroics represent the most interesting type where two ferroic properties viz. ferromagnetic and ferroelectric orderings are simultaneously present under coupled condition. These materials are normally classified as type-II multiferroic, whereas the type-I multiferroics do not exhibit the coupled behavior. Also, the type-II materials have been attracted much academic and technological attention in recent times.1 Multiferroic nanocomposite materials have already been proposed for various applications in solid oxide fuel cells, nonvolatile magnetic memories, such as magnetically tunable ferroelectric random access memories, electrically tunable magnetic random access mamories, high frequency filters, ultrasensitive magnetic read-heads, and so on.2,3 Also, the coupled behavior in the type-II multiferroics have enormous potential for their suitability in the design of new functional sensors and multistate devices.2−8 In addition, the observations of magnetic control of ferroelectric polarization in © 2012 American Chemical Society

Received: July 10, 2011 Revised: February 1, 2012 Published: February 1, 2012 4948

dx.doi.org/10.1021/jp2065216 | J. Phys. Chem. C 2012, 116, 4948−4956

The Journal of Physical Chemistry C

Article

was prepared in 200 mL of ethanol. This prepared solution was then sonicated for 1 h at ∼60 °C. During the sonication, equimolar amount of citric acid was added to the solution, and the sonication was continued for 1 h, keeping the temperature in the range of 60−80 °C. The temperature was varied due to continuous sonication. After the completion of sonication, the solution was vigorously stirred for ∼4 h at ∼60 °C, and finally a heavily dense form of the sol has been obtained. The sol was then put into the oven at 60 °C, and it was left for 10 h to get the dried form of the sol. Finally, the dried form of sol was annealed at 250 °C for 12 h and at 500 °C for 6 h, respectively. For the preparation of the sample of NZCF, the salts of ZnCl2, NiCl2·6H2O, CuCl2·2H2O, and FeCl3·6H2O were taken in a beaker, where the stoichiometric ratio of Ni, Zn, Cu and Fe was 0.4:0.4:0.2:2.0. The aqueous solution of these salts was prepared by adding 100 mL of triple-distilled water. The solution was then sonicated for 2 h at ∼80 °C. In another beaker, NaOH solution was prepared, and it was mixed with the previous solution dropwise until the pH became 10 and a precipitation occurred. This was done under stirring condition. The solution was further sonicated for another 2 h at ∼80 °C. The precipitate was then washed thoroughly by triple-distilled water to neutralize the pH as well as to remove extra ions. Asprepared sample was dried at 100 °C for 24 h and finally annealed at 500 °C for 6 h to obtain the nanoparticles of NZCF sample.22,24 For the preparation of the nanocomposite sample of GFONZCF, the salts required for the NZCF as stated above were taken in 100 mL of triple-distilled water, and the solution (say A) was then sonicated for ∼1 h. The nanoparticles of GFO required for the desired composition of the GFO-NZCF were then slowly added to the solution of A, and for well-dispersion of the GFO-nanoparticles, sonication was further continued for another 2 h. NaOH solution was then added dropwise to the solution of A plus GFO for coprecipitation of the salts required for NZCF, and the final pH was maintained at ∼10. Finally, the coprecipitated particles and GFO particles were vigorously stirred for another 2 h for complete digestion of the salts. The precipitate was then washed thoroughly by triple-distilled water to neutralize the pH as well as to remove extra chloride ions. The prepared sample was dried at 100 °C and finally annealed at 500 °C for 6 h to get the nanocomposite sample of GFONZCF. The XRD patterns of all samples were recorded in powder diffractometer, model D8, BRUKER AXS, using Cu Kα radiation (λ = 1.5405 Å). The quantitative amount of Ga and Fe in the sample of GFO was measured by ICP OCS. HRTEM pictures were taken in a JEOL JEM 2100 HRTEM, Japan (resolution = 1.9 Å). Static magnetic measurements were carried out in Quantum design SQUID magnetometer where the maximum applied field was 5 T. Digital hysteresis loops at different frequencies were observed by using a digital hysteresis loop tracer supplied by Metis Instruments and Equipments NV, Belgium.

magnetoelectric properties of GaFeO3 was a topic of intensive research in 1960 since Remeika et al.15 reported the occurrence of piezoelectricity at RT above the transition temperature Ga2−xFexO3 (x = 1). GaFeO3 has an orthorhombic crystal structure (space group Pc21n) with four different cation sites labeled Ga1, Ga2 (mostly occupied by gallium) and Fe1, Fe2 (mostly occupied by iron). This material has a spontaneous polarization along the b axis, and a ferrimagnetic structure below RT results from unequal distribution of Fe spins of nearly equal magnitude on the sublattices with a magnetic moment of the spin along the c axis.16,17 Various investigations viz., Mö ssbauer study by Nakamura et al.,18 structural, magnetic, dielectric properties of multiferroic GaFeO3 by Mohamed et al.,17 have been carried out. Apart from these investigations, divalent- or trivalent-metal-ion-doped GaFeO3 has been investigated in recent times.19−21 Although the magnetization has been enhanced by doping of various divalent or trivalent (metal or transition metal) ion-doped GFO, but no attempt has been made to enhance the magnetization of the multiferroic system like GFO by using some other strong magnetic host of ferrite family in the nanocomposite state. In the present Article, we report the preparation and characterization of the nanocrystalline GaFeO3, Ni0.4Zn0.4Cu0.2 Fe2O4, and nanocomposite of them, that is, [(GaFeO 3 ) 0 .5 (Ni0.4Zn0.4Cu0.2 Fe2O4)0.5] by coprecipitation method. Here X-ray diffractograms (XRDs) and high-resolution transmission electron microscopy (HRTEM) of the sample were observed to investigate the crystallographic phase and morphology of the samples. The main aim of this Article is to enhance the magnetization of the GFO in the nanocomposite system by the soft magnetic nanocrystalline NZCF, whose saturation magnetization is quite high compared with that of GFO.22 In our previous study,22 we have observed quite a high value of saturation magnetization (∼61.5 emu/g) of NZCF nanoparticles, even when the average particle size was not too high (∼28 nm). Also, it is observed that the magnetization of Niferrite in the bulk state is substantially enhanced when a major fraction of cations (40% of Ni) is substituted by Zn cations.23 Besides, the enhancement of magnetic property was further improved by the substitution of Cu-cations in the Ni−Znferrite. This fact has also been observed in the magnetic properties of various nanoparticle systems of NZCF.22,24 Therefore, to enhance the magnetic property of the nanoparticles of GFO, NZCF would be a good candidate. The details of static magnetic measurements by SQUID magnetometer and their analysis have also been included in the present investigation. Dynamic magnetic measurements were carried out to characterize the nanocrystalline and nanocomposite systems. Static and dynamic magnetic measurements suggest the presence of superparamagnetic (SPM) relaxations of all of the samples. Different quantities related to nanocrystalline/ nanocomposite systems viz. particle sizes and their distributions, blocking temperature, and so on were extracted from the systematic analysis of the magnetic data.

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RESULTS, ANALYSIS, AND DISCUSSION XRD Analysis. XRD patterns of all samples were shown in Figure 1. All peaks of the patterns were duly assigned with the help of the JCPDS files (nos. 260673 for GFO, 080234 for NZCF, and 080234 for GFONZCF). It is interesting to note that no extra peak due to any other impurity phase has been found in the patterns. Therefore, the samples are free from impurity phase like α-Fe2O3, and so on. Hence all samples are in the desired phase/phases only. The average particle sizes of all samples were calculated from the broadening of 100%

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EXPERIMENTAL METHODS Samples of GaFeO3 (GFO) were prepared by sol−gel method, and samples of Ni 0.4 Zn 0.4 Cu 0.2 Fe 2 O 4 (NZCF) and (GaFeO3)0.50 (Ni0.4Zn0.4Cu0.2 Fe2O4)0.5 (GFONZCF) were prepared by chemical coprecipitation method. The precursor materials for GFO were Ga(NO3)3, Fe(NO3)3·9H2O. At first, the stoichiometric amount of Ga(NO3)3 and Fe(NO3)3·9H2O were taken in a beaker, and a complete homogeneous solution 4949



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Figure 1. XRD patterns of (a) GFO, (b) NZCF, and (c) GFONZCF, where * represents the peaks of NZCF in panel c.

intense peak (311) of each sample by using the Debye− Scherrer equation = 0.89 λ/(β1/2 cosθ), where is the average particle size, λ is the wavelength of the incident X-ray radiation, and θ is the Bragg angle. Here β1/2 is the full-width at half maximum (fwhm) of the XRD peak.25 We have calculated the particle size of each sample using the 100% peak in the XRD pattern. The estimated particle sizes of GFO and NZCF are, respectively, 18 and 9 nm. The average particle sizes of GFO and NZCF in the nanocomposite sample are, respectively, 19 and 13 nm. Although the average particle size of the GFO particle in the nanocomposite sample is more-or-less the same as that of the pure GFO particles, the average particle size of bare NZCF sample has been increased from 9 to 13 nm in the nanocomposite sample. The calculated values of the average particle sizes of GFO, NZCF, and GFONZCF along with annealing temperatures are listed in Table 1. The average

Figure 2. Results of HRTEM observations of GFONZCF: (a,b) Micrographs, (c) histogram of particle size distributions, (d,e) lattice fringe patterns, and (f) SAED pattern. Here * represents the rings of (NZCF).

Table 1. Annealing Temperatures of GFO, NZCF, and GFONZCF, Particle Sizes Obtained from the XRD, TEM, and Analysis of Static Magnetic Data samples

TA (°C)

GFO NZCF GFONZCF

800 500 500

XRD (DXRD ) error (±1 nm) (nm) 18 9 19 (For GFO) 13 (For NZCF) 16 (average of GFO and NZCF)

TEM (DTEM ) error (±0.1 nm) (nm)

Dmg error (±0.1 nm) (nm)

16 ± 0.5

12.5 ± 0.1 7.1 ± 0.2 13.0 ± 0.1

measured from different micrographs. The measured diameters of each nanoparticle in different directions were almost equal, which indicates that the nanoparticles are nearly spherical. The average particle size of GFONZCF is ∼16 nm. Therefore, the average particle size obtained from the TEM micrographs is in good agreement with that obtained from the XRD pattern (16 nm). From the micrographs, it is difficult to conclude the perfect encapsulation of the GFO nanoparticles by NZCF in the GFONZCF, and for this we could not measure the thickness of the matrix separately, but careful observations revealed that in many portions of the micrographs the bigger size particles are encapsulated by smaller size particles. Also, the particle size of GFO is not remarkably increased in the nanocomposite sample, although it is heat-treated twice: one is during the phase formation of the GFO and the other is after the coating of GFO by NZCF. Therefore, the restriction of the growth of GFO particles may be attributed to the fact that the GFO particles are mostly encapsulated by the smaller group of NZCF particles. The distribution of particle sizes obtained from the TEM micrographs is shown in Figure 2c. The distribution of sizes was tried to fit using the log-normal function, and the fitting is shown in Figure 2c. It is clear from the Figure 2c that the distribution of particle sizes is fitted well in log-normal function (solid line of Figure 2c). Although the measured particle sizes in the TEM micrographs are in the range of 10− 40 nm, the majority of the particles are lying in the range of 15−20 nm, which means that size distribution has been

particle size of all samples lies in the range of 9−19 nm. The particle size of GFO is not remarkably increased in the nanocomposite sample, although it is heat-treated twice during the annealing of the nanocomposite sample. Therefore, the growth of GFO particles is restricted in the nanocomposite sample due to coating with NZCF particles.26 TEM Analysis. The results of HRTEM observations of the nanocomposite sample (GFONZCF) are shown in Figure 2. Among the various micrographs of GFONZCF recorded during the TEM observations, some representatives pictures are displayed in Figure 2a,b, respectively. The particles displayed in the micrographs are well-dispersed; that is, the particles are not yet agglomerated. To estimate the average particle size of the sample, the average diameter of ∼300 nanoparticles was 4950



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curves were bifurcated at different temperatures, which are attributed to the presence of the SPM relaxation of the nanoparticles of samples. The temperature at which the curves are bifurcated is called the blocking temperature, TB. The blocking temperature of the pure multiferroic system of GFO is the highest (just above 300 K), whereas the value of TB for the pure system of NZFC is the lowest (∼290 K), but the blocking temperature of the nanocomposite system is lying between these two, and its value is very close to RT (∼298 K). Hence introducing the GFO nanoparticles in the matrix of NZCF, it is possible to obtain the SPM relaxation at RT. The variation of ZFC data of the GFO is more-or-less flat, whereas the other two curves of NZCF and GFONZCF show prominent peaks in their variations. Also, the nature of variation of the zero-fieldcooled curve of the sample of GFO (Figure 3) has been modulated by the presence of NZCF in the nanocomposite system. Besides, the nature of variation in the nanocomposite system is mostly controlled by the NZCF system. Another striking feature is that the values of the magnetization of the nanocomposite system in both the FC and ZFC curves are higher than the other two pure samples, although the saturation magnetization of the NZCF sample is the highest, which is discussed in the next section. This is due to the fact that the rate of increase of the magnetization in the lower field region is more than that in the nanocomposite system. This result also indicates that the growth rate has been changed in the nanocomposite mixture of the two. Actually, the ZFC magnetization shows a peak at a temperature TP, which is lower than the blocking temperature. In the case of nanoparticle system having a size distribution, the temperature TB can be regarded as the highest temperature at which the ZFC and FC magnetizations bifurcated corresponding to the larger group of particles in the system, and the ZFC magnetization decreases below the temperature TP due to the smaller group of particles in the system.27−30 Therefore, the different values of TB and TP are due to the particle size distribution of the SPM particles, which is also in agreement with the findings of TEM analysis. The extracted values of TB and TP are listed in Table 2. The variation of magnetization as a function of applied field of the samples of GFONZCF, NZCF, and GFO recorded at two temperatures viz. 300 and 2 K is shown in Figure 4a,b. From the RT magnetization curves, it is clear that the magnetization of each sample increases with the increase in applied magnetic field. In the case of GFO, the magnetization curve observed at RT is nearly saturated at the maximum applied field of 3.5 T, but the magnetization curve of the NZCF sample is far from saturation; rather the magnetization curve is the typical characteristic feature of SPM behavior, and this is in agreement with the blocking temperature, but the mixed behavior is clearly reflected in the nanocomposite state of the GFONZCF sample, as in this case the magnetization is not yet

concentrated in the narrow region by the application of sonication during the course of preparation.25 Various nanocrystalline fringe patterns were recorded, and out of these two fringe patterns of the nanocomposite sample of GFONZCF are shown in Figure 2d,e. The patterns indicate that the samples are in the nanocrystalline state with no defect. The assigned lattice planes are in good agreement corresponding to the crystallographic phase of GFO and NZCF; that is, lattice planes are also in agreement with those obtained from the XRD pattern. The estimated separations of the adjacent bright fringes are 0.49 and 0.47 nm, respectively (shown in the Figure 2d,e). These values correspond to the (111) crystallographic plane of both systems of GFO and NZCF in the nanocomposite systems. The selected area electron diffraction (SAED) pattern of GFONZCF is displayed in Figure 2f. The rings with different diameters in the pattern are duly assigned corresponding to the different lattice planes of the individual component of the nanocomposite system. The assigned planes are in agreement with those obtained from the XRD observations. Static Magnetic Properties. To record the FC magnetization versus temperature curves in the temperature range of 300 to 2 K, the sample was cooled in the presence of an applied field of 39.81 × 103 A/m (500 Oe), and the data were recorded during the rise of the temperature. The sample is again cooled to 2 K in the absence of field, and the data for ZFC curve were recorded during the increase in temperature. The FC and ZFC curves of three samples are shown in Figure 3. A relatively high

Figure 3. FC and ZFC magnetization curves: (a−c) FC and (d−f) ZFC curves of GFONZCF, NZCF, and GFO samples, respectively.

field of 500 Oe has been applied to get an appreciable difference between the FC and ZFC magnetizations. This will also minimize the error in the determination of particle size as the differences of these data were utilized to evaluate the particle size of the samples. The FC and ZFC magnetization

Table 2. Anisotropies and Particle Sizes of the Samples GFO, NZCF, and GFONZCF Obtained from Difference of FC and ZFC Magnetizations along with Saturation Magnetization and Different Frequency Factor Ka × 10−5 (erg/cm3) and errors

particle sizes (nm) estimated from blocking temperature with different frequency factors f 0 (s−1) and estimated errors

samples

TA (o C)

TP (K)

TB (K)

from fitting

from TB

109

1010

1011

1012

1013

GFO NZCF GFONZCF

800 500 500

255.5 200.3 195.5

>300 250.6 297.2

0.19 ± 0.006 8.84 ± 0.008 3.88 ± 0.003

4.68 ± 0.3 30.28 ± 0.3 6.60 ± 0.4

16.2 ± 0.004 8.2 ± 0.002 14.4 ± 0.012

16.7 ± 0.004 8.4 ± 0.002 14.8 ± 0.014

17.3 ± 0.004 8.7 ± 0.002 15.3 ± 0.017

17.6 ± 0.004 8.9 ± 0.002 15.6 ± 0.014

18 ± 0.004 9.1 ± 0.002 16 ± 0.012

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Figure 5. M-H loops observed at temperatures 300 and 2 K of the samples (a) GFONZCF, (b) NZCF, and (c) GFO, respectively. Figure 4. Magnetization curves of the samples GFONZCF, NZCF, and GFO at (a) RT and at (b) 2 K, respectively.

have found that the ratio between Fe and Ga (x) is greater than 1 (1.455). Thus, one of the main causes behind the high value of Tc is due to the larger fraction of Fe3+ compared with that of Ga3+, and this result is also in agreement with the previous observed results.31 Apart from this factor, there is also another cause that may be the redistribution of cations among the Fe and Ga in the nanoparticle state. The larger fraction of Fe in the nanoparticle state may be the cause of redistribution or mislocation of cations. Also, from the observed high value of Tc, it may be further concluded that the redistribution of cations is enhanced in the nanoparticle state. In the reported work of Kim et al.,32 it was observed that the magnetic anisotropy of the uniform and stoichiometric GFO is quite high, but in the present composition with x > 1, the recorded MH loops suggest the high anisotropy, and this high value is mainly attributed to the nanoparticle state of the sample. Actually, the anisotropy in the nanoparticle state is normally high compared with that of the bulk value, and this high value is usually explained by the surface and shape anisotropy contribution.24 The high value of magnetic anisotropy is also estimated from the analysis of the static magnetic data discussed in the next section. For the detail investigations of the loops of all samples recorded at different temperatures, we have extracted various properties viz. coercive field, saturation-to-remanence ratio, maximum saturation magnetization, and so on (Table 3). The coercive field (Hc) at RT is lowest in the case of NZCF, which is only 10.8 Oe, but this value increases to ∼395.0 Oe at 2 K. The low value of coercive field of NZCF is attributed to the presence of a very small number of soft magnetic ferrite nanoparticles (NZCF), which are not in SPM state. The large increase in coercive field at 2 K also established that at this temperature almost all particles are in the blocked state instead of SPM state. The magnetization of this soft ferrite is not yet saturated even above 3 T, which is not happened in its bulk

saturated or not rising like the NZCF sample. The maximum magnetizations (@3.5 T) of GFO, NZCF, and GFONZCF are, respectively, 7.04, 45.70, and 32.12 emu/g. Therefore, the magnetization measured at RT of the nanocomposite sample has been drastically enhanced by the NZCF matrix. This enhancement of magnetization would be quite interesting for the multiferroic system of GFO. The magnetization versus field curve measured at 2 K are shown in Figure 4b. In all curves, the maximum magnetization has been increased at low temperature. From the magnetization curve of GFO and GFONZCF, it is clear that the magnetizations are very close to saturation, but this is not so in the case of NZCF sample. In NZCF, the magnetization curve is far away from saturation. The values of maximum saturation magnetization of GFO, NZCF and GFONZCF are, respectively, 28.73, 70.07, and 52.27 emu/g. Therefore, in all of the cases, the magnetization has been increased highly at 2 K from the values measured at RT. In Figure 5, we have displayed the variations of dc MH loops of the samples GFO, NZCF, GFONZCF recorded at various temperatures from 300 to 2 K. The displayed loop of GFO at 300 K established the fact that the value of Tc is quite high compared with the reported values of bulk GFO.31,32 Actually, there are various factors behind the variation of Tc in the bulk system of GFO, and these causes were explained in various papers.31,32 From these reported works, it is clear that the value of T c depends on the stoichiometry, mislocation, or redistribution of Ga and Fe cations from their respective sites along with the method of preparation. In the case of bulk system, the high value of Tc was observed in the case of GFO, where the ratio of Fe and Ga is greater than 1; that, is the quantitative amount of Fe is more than that of Ga. To substantiate this fact, we have measured the quantitative amount of Fe and Ga by ICP OCS, and in this measurement we 4952



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of particle sizes in the other two samples, in these cases we have also considered the same distribution function for the calculation of particle sizes and so on. Therefore, we can now write the distribution function for the blocking temperature as26

Table 3. Coercive Fields, Maximum Magnetizations, Remanence Magnetization, and Hysteresis Loss of GFO, NZCF, and GFONZCF at Different Frequencies frequency (Hz) 50

100

150

200

300

375

a

HC (A/m) a

6273 5663b 3297c 7638a 6917b 5686c 7817a 7104b 5968c 8980a 8107b 6655c 7066a 6764b 6073c 6604a 6412b 5839c

Bm (T) a

0.12 0.20b 0.10c 0.11a 0.18b 0.18c 0.09a 0.16b 0.17c 0.07a 0.13b 0.14c 0.07a 0.12b 0.13c 0.06a 0.11b 0.11c

Br (T)

Bm/Br

a


a


hysteresis loss (T·Am−1)

f (TB) =

a


⎧ [ln(T /T )]2 ⎫ ⎪ ⎪ 1 B B0 ⎬ exp⎨ − ⎪ ⎪ 2π TBλB 2λ2B ⎩ ⎭

(1)

33

The symbols have their usual meanings. If f(TB) dTB represents the fraction of particles having blocking temperatures lying in the interval TB to TB + dTB, then the difference between ZFC and FC curves is given by34 ΔM =

⎡ ln(T /TB0) 29MS2H ⎧ λ ⎤⎫ ⎨1 − erf⎢ − B ⎥⎬ 6Ka ⎩ 2 λB 2 ⎦⎭ ⎣ ⎪

⎪

⎪

⎪

(2)

Here we consider TB = TB0 exp( 2 λBx) and x =

ln(TB/TB0) 2 λB

The error function is introduced as ∞ −t 2 e dt = ( π /2) erfc(T )

∫T

For GFO. bFor NZCF. cFor GFONZCF.

where erfc(T) = 1 − erf(T). To fit the observed values of ΔM by varying the parameters, viz. TB0, Ka, and λB in eq 2, it is necessary to put the value of MS. In the fitting of ΔM values we have taken our observed values of the maximum magnetizations obtained in the SQUID measurements, although strictly speaking these are not the saturated values. During the fitting of the observed values of ΔM we have systematically varied the parameters TB0, Ka, and λB. All values of ΔM were successfully fitted, and one representative fitted curve along with the observed data is shown in Figure 6. From the fitted curve, it is clear that the

counterpart. This rise is due to the large value of nanocrystalline magnetic anisotropy, which is usually obtained in nanoparticle system. This rise of magnetization at low temperature with high field may be due to the small group of lower size particles, which are still in the SPM state. From the overall inspection of the loops, it is also clear that the shape of the loop of NZCF is almost square-type, whereas the loops of GFO are almost flat-type, but the loop of the nanocomposite sample is almost dominated by the NZCF particle, as in this case the loop is nearly square-type. The coercive field of GFO at RT is 1808.0 Oe, which increases to 5030.0 at 2 K, but the extracted value of coercive field of the nanocomposite sample (GFONZCF) is surprisingly low, which is only 18.0 Oe, and this value increases to 70.0 Oe at 2 K. Therefore, the hysteretic behavior of the GFO nanoparticles has been drastically reduced by the NZCF particles. This is mainly due to the encapsulation of GFO particles by the small particles of NZCF. Also, the maximum magnetization of GFO measured at RT is 6.90 emu/ g, and the magnetic moment estimated per Fe is 0.21 μB; this value is a bit less compared with that of bulk value.31 This relatively low value may be attributed to the nanoparticle state, where the anisotropy is quite high, and due to this the particles are not yet perfectly saturated. Analysis of Static Magnetic Data. To estimate the average particle size and the nanocrystalline anisotropy of the samples of GFO, NZCF, GFONZCF, respectively from the static magnetic data, at first we have calculated the difference of FC and ZFC magnetizations. These calculated values were utilized to calculate the blocking temperature as well as the particle size of three samples. As the size distribution in the sample of GFONZCF has been found in the TEM analysis (Figure 2c), it is quite obvious that the blocking temperature will also have a distribution, rather than a particular value. The distribution function, that is, log-normal type (which fit the observed particle size distribution obtained in the TEM) has been used in the calculation of distribution of blocking temperatures. Although, we could not measure the distribution

Figure 6. Variation in ΔM = MFC − MZFC with T, symbol (●) for the experimental while the solid line is theoretical fit of GFONZCF. Inset shows the distribution of blocking temperature with TB of GFONZCF.

observed data were successfully fitted by the analysis. It is further noted that the anisotropies of the present nanoparticle samples (Table 2) are quite high and this high value is mainly attributed to the shape and surface anisotropy contribution that is normally observed in the magnetic nanoparticle state. 4953



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The blocking temperature was estimated from the analysis of this magnetization data.35 Using the parameters extracted from the fitting of eq 2, the distribution of blocking temperature has been calculated using eq 1, and this distribution is shown in the inset of Figure 6. The average diameter is obtained from the relation ⎡ 6k T ⎤1/3 2 ⟨DTB⟩ = ⎢ B B0 e λB/6 ln(f0 τ)⎥ ⎣ πKa ⎦

(3) λB2/2

Here we have assumed that ⟨DTB⟩ = TB0 e and 1 = f0 e−KaV / kT (4) τ where TB0, B, and Ka are the parameters obtained from the fitting of eq 2 and f 0 is the characteristic relaxation frequency, ranging typically from 109 to 1013 s−1 for SPM nanoparticle.26 In eq 4, both the particle volume (V) and temperature (T) are in the exponent, and for this τ is strongly dependent on these quantities. The extracted values of λB, Ka, and estimated particle sizes with different frequency factors are shown in Table 2. From Table 2, it is clear that the anisotropy constant is large compared with those from their value in the bulk state. This increase in anisotropy constant from that of their bulk counterpart may be due to the nanoparticle state of the sample. In the nanoparticle state, various factors viz. particle shape, size, surface-to-volume ratio, grain boundary, and so on are mainly responsible for the large value of anisotropy compared with those of their bulk counterparts. We have also estimated the particle size for different frequency factors ranging from 109 to 1013. From the computed values of particle size, it is evident that the particle size steadily but slowly increases with the increase in frequency factor. The estimated particle sizes with frequency factor of 1013 are in good agreement with these obtained from the XRD pattern. ac Magnetic Properties. To investigate the dynamic magnetic behavior of all samples, we have observed the ac hysteresis loops, ac magnetization curves as a function of induction, and so on of all of the samples in the frequency range of 50−300 Hz. Various magnetic quantities were extracted from the ac hysteresis loops observed at different frequencies. The observed ac magnetic behaviors of the samples are depicted in Figures 7−10. AC magnetization curves of the

Figure 8. Variation in ac induction (Bm) as a function of ac magnetic field (Hm) observed at 50 Hz of the samples (a) GFONZCF, (b) NZCF, and (c) GFO.

Figure 9. ac hysteresis loops observed at 50 Hz of the samples (a) GFONZCF, (b) NZCF, and (c) GFO.

Figure 10. RT ac hysteresis loops of the sample GFONZCF at different frequencies.

samples as a function of applied ac magnetic field, with maximum applied field, Hm = 70 kA/m, are displayed in Figure 8. In all samples, the ac magnetizations slowly increase with the increase in ac excitation. The AC magnetization curve of GFO is almost linear, which is the characteristic feature of SPM

Figure 7. Variations in saturation magnetization as a function of temperature of (a) GFONZCF, (b) NZCF, and (c) GFO. 4954



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and HRTEM studies. The coercive fields, saturation-toremanence ratio, and shape of the hysteresis loops have also been modified in the nanocomposite state. The flat hysteresis loop of the nanoparticles of GFO is being converted into a square loop in the mixed state of the two types of nanoparticles, and this would lead to the minimization of the hysteresis loss of the GFO. This will be also interesting for applications of the multiferroic nanocomposite in electromagnetic devices. Overall, inspection of all static magnetic data revealed that the magnetic behavior of the nanoparticles of GFO is mostly controlled by the mixed spinel nanoparticles of NZCF. SPM behavior of the nanoparticles all the samples is also confirmed by the magnetic measurements. Interestingly, the blocking temperature of the nanoparticles of GFO has been tuned in the nanocomposite state.

particles of this sample, but in the case of NZCF and GFONZCF, the magnetization curves are not linear, rather the magnetization curves indicate the presence of dominating phase of long-range ordered particles. Interestingly, the rate of increase in magnetization of the nanocomposite sample has been drastically enhanced by NZCF compared with that of the pure GFO sample. Also, in the low region of the ac field, the rate of increase in ac magnetization of the nanocomposite sample (GFONZCF) is a bit more compared with that of NZCF sample. Therefore, the ac response of the GFO particles has been modulated by the NZCF particles and vice versa. The ac hysteresis loops observed at 50 Hz of all samples are shown in Figure 9. Various magnetic properties viz. coercive field, hysteresis loss, maximum magnetization-to-remanence ratio, and so on were extracted from the ac hysteresis loops recorded at 50 Hz frequency. The value of coercive field of GFO is the maximum (6317 A/m), but this value is almost half in the case of GFONZCF sample (3290 A/m). Also, the value of the coercive field of the nanocomposite sample is the lowest among all samples. Although the sample of NZCF is a well-known soft magnetic system, but its coercive field (4858 A/m) is high compared with that of the nanocomposite sample. Alternatively it can also be noted that the soft magnetic property of NZCF is tuned by the GFO sample in the nanocomposite state. The ac hysteresis loops of all samples were also recorded at different frequencies (50−300 Hz), and among these one representative loops for the nanocomposite sample is shown in Figure 10. For a given sample, the coercive field increases with the increase in frequency. This is due to the lowering of measuring time window. Once we increase the frequency, the measuring time window decreases. For this, more and more particles that behave as SPM particle at lower frequency (say 50 Hz), will behave as long-range particle at higher frequency (say 250 Hz). This is due to the fact that the other factors viz. the magnetocrystalline anisotropy, particle volume, characteristic relaxation time, and so on, which are responsible for the onset of SPM relaxation in the nanoparticle state, are not changed due to the variation of measuring time window. Different magnetic quantities viz. coercive field (Hc), maximum magnetization (Bm), remanence magnetization (Br), ratio of Bm and Br and hysteresis loss, and so on were extracted from the different loops recorded at different frequencies. Although, there is no fixed variation of the different quantities, but overall inspection of the data shows that the magnetic property of the nanoparticles of GFO has been tuned by the NZCF in all respect.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 0091 342 2634200. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the DST, Govt. of India for the financial assistance of the work (project file no. SR/S2/CMP/0058/2010).

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REFERENCES

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CONCLUSIONS Nanoparticles of GFO were encapsulated by the mixed spinel soft magnetic system of NZCF. The magnetic behavior observed in the M-T curves (FC and ZFC) and in the hysteresis loops of the GFO was modulated in the presence of the NZCF in the nanocomposite state. The rate of growth of magnetization of GFO has been sharply increased in the nanocomposite state. Also, the encapsulation successfully enhanced the magnetization of the weekly magnetic sample of GFO in the nanocomposite state. This enhancement of the magnetization would be fruitful for applications in many devices. For the first time, the static magnetic data of GFO, NZCF, and GFONZCF have been satisfactorily explained to evaluate the particle size, its distribution, and magnetonanocrystalline anisotropy. The results obtained from the analysis are in good agreement with those extracted from XRD 4955



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Enhanced Magnetic Behavior of Chemically Prepared Multiferroic

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