Magnetodielectric effect in composites of nanodimensional glass and CuO nanoparticles Dhriti Ranjan Saha a , Manabendra Mukherjee b , Dipankar Chakravorty a,n a MLS Professor’s Unit, Indian Association for the Cultivation of Science, 2A and 2B Raja S.C. Mullick Road, Kolkata- 700032, India b Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF BidhanNagar, Kolkata- 700064, India. article info Article history: Received 11 May 2012 Available online 21 July 2012 Keywords: Nanocomposites Ferromagnetism Nanodimensional glass Magnetoresistance abstract Nanocomposites comprising CuO particles of average diameter 21 nm coated with 5 nm silica glass containing iron ions were synthesized by a chemical route. An ion exchange reaction at the nanoglass/ CuO interface produced iron-doped CuO with copper ion vacancies within the nanoparticles. Room temperature ferromagnetic-like behavior was observed in the nanocomposites. This was ascribed to uncompensated spins contributed by Fe ions with associated copper ion vacancies. A rather high value of magnetodielectric parameter in the range 16–26% depending on the measuring frequency was exhibited by these nanocomposites at a magnetic field of 10 KOe. This was caused by a magnetore- sistance of 33% in the iron doped CuO nanoparticles. The experimental results were fitted to the Maxwell–Wagner Capacitor model developed by Catalan. These materials will be suited for magnetic sensor applications. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Magnetodielectric effect in materials has attracted consider- able attention in recent years because of both basic physics as well as potential device applications such as magnetic field sensors, capacitive resonators, memory devices etc. [1–15]. Most of these materials fall into the category of multiferroics—a single phase material possessing both ferroelectric and ferromagnetic order. Some of them are composites of ferroelectric and ferro- magnetic phases. High values of magnetodielectric parameters were reported only at low temperatures [16,17]. Recently room temperature magnetodielectric effect was reported in graphene based polymer nanocomposite film [18]. In this paper we report on large magnetodielectric effect observed in the case of unusual composites made up of nanoglass and a semiconducting nano- particle. Nanodimensional glasses have attracted the attention of materials scientists in recent years because of novel physical properties expected from such systems. Most of the work carried out so far has been concerned with metallic glass nanoparticles produced by the inert gas condensation method and subsequently consolidated under high vacuum [19–22]. Synthesis of nanoglass in the lithia silica system was reported recently using the nanopores of pellets consisting of CuO nanoparticles [23]. In the present work the nanodimensional glass filling up the pore volume in a pellet comprising CuO nanoparticles provided stability to the nanocomposite. Since the silica glass used had Fe 3 þ ions in the matrix, an ion exchange reaction of the type 3Cu 2 þ 22Fe 3 þ produced iron doped CuO nanoparticles. The latter showed ferromagnetic like characteristics. Preparation of iron doped CuO nanocrystals with ferromagnetic properties was reported earlier in which a combustion synthesis technique was used [24]. In this study the system was a heterogeneous one with different conductivities of the nanoglass and the doped CuO nanophase accordingly. As a result it showed a giant magnetodi- electric effect and the analysis [25] of the experimental data led to the conclusion that the doped CuO nanocrystals had a large magnetoresistance. The results are reported in this paper. 2. Experimental Section A sol with target glass composition 10 Fe 2 O 3 .90 SiO 2 (mole %) was prepared. The precursors used were FeCl 3 and Si(OC 2 H 5 ) 4 . 1 ml of tetra ethyl orthosilicate (TEOS) and 2 ml of ethanol were mixed in a beaker and stirred for an hour. In another beaker 0.36 g FeCl 3 salt was dissolved in a mixture of 1.2 ml water and 2.8 ml ethanol. The latter solution was sonicated for 30 s. 2 ml of this salt solution was poured into the TEOS solution prepared as men- tioned above. The resultant sol was stirred for ð1=2Þ h. A part of this sol was poured into a plastic dish for gellation so that a bulk glass of the composition stated before could be formed. A pellet formed of nanosized CuO particles heat treated at 373 K for 1ð1=2Þ h was dipped into the iron containing sol just prepared. The preparation of CuO nanoparticles has been given in detail in Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.07.021 n Corresponding author. Fax: þ913324732805. E-mail address: [email protected] (D. Chakravorty). Journal of Magnetism and Magnetic Materials 324 (2012) 4073–4077
Transcript
Journal of Magnetism and Magnetic Materials 324 (2012) 4073–4077
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
0304-88
http://d
n Corr
E-m
journal homepage: www.elsevier.com/locate/jmmm
Magnetodielectric effect in composites of nanodimensional glassand CuO nanoparticles
Dhriti Ranjan Saha a, Manabendra Mukherjee b, Dipankar Chakravorty a,n
a MLS Professor’s Unit, Indian Association for the Cultivation of Science, 2A and 2B Raja S.C. Mullick Road, Kolkata- 700032, Indiab Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF BidhanNagar, Kolkata- 700064, India.
Nanocomposites comprising CuO particles of average diameter 21 nm coated with 5 nm silica glass
containing iron ions were synthesized by a chemical route. An ion exchange reaction at the nanoglass/
CuO interface produced iron-doped CuO with copper ion vacancies within the nanoparticles. Room
temperature ferromagnetic-like behavior was observed in the nanocomposites. This was ascribed to
uncompensated spins contributed by Fe ions with associated copper ion vacancies. A rather high value
of magnetodielectric parameter in the range 16–26% depending on the measuring frequency was
exhibited by these nanocomposites at a magnetic field of 10 KOe. This was caused by a magnetore-
sistance of 33% in the iron doped CuO nanoparticles. The experimental results were fitted to the
Maxwell–Wagner Capacitor model developed by Catalan. These materials will be suited for magnetic
sensor applications.
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
Magnetodielectric effect in materials has attracted consider-able attention in recent years because of both basic physics aswell as potential device applications such as magnetic fieldsensors, capacitive resonators, memory devices etc. [1–15]. Mostof these materials fall into the category of multiferroics—a singlephase material possessing both ferroelectric and ferromagneticorder. Some of them are composites of ferroelectric and ferro-magnetic phases. High values of magnetodielectric parameterswere reported only at low temperatures [16,17]. Recently roomtemperature magnetodielectric effect was reported in graphenebased polymer nanocomposite film [18]. In this paper we reporton large magnetodielectric effect observed in the case of unusualcomposites made up of nanoglass and a semiconducting nano-particle. Nanodimensional glasses have attracted the attention ofmaterials scientists in recent years because of novel physicalproperties expected from such systems. Most of the work carriedout so far has been concerned with metallic glass nanoparticlesproduced by the inert gas condensation method and subsequentlyconsolidated under high vacuum [19–22]. Synthesis of nanoglassin the lithia silica system was reported recently using thenanopores of pellets consisting of CuO nanoparticles [23]. In thepresent work the nanodimensional glass filling up the porevolume in a pellet comprising CuO nanoparticles provided
ll rights reserved.
).
stability to the nanocomposite. Since the silica glass used hadFe3þ ions in the matrix, an ion exchange reaction of the type3Cu2þ22Fe3þ produced iron doped CuO nanoparticles. Thelatter showed ferromagnetic like characteristics. Preparation ofiron doped CuO nanocrystals with ferromagnetic properties wasreported earlier in which a combustion synthesis technique wasused [24]. In this study the system was a heterogeneous one withdifferent conductivities of the nanoglass and the doped CuOnanophase accordingly. As a result it showed a giant magnetodi-electric effect and the analysis [25] of the experimental data led tothe conclusion that the doped CuO nanocrystals had a largemagnetoresistance. The results are reported in this paper.
2. Experimental Section
A sol with target glass composition 10 Fe2O3.90 SiO2 (mole %)was prepared. The precursors used were FeCl3 and Si(OC2H5)4.1 ml of tetra ethyl orthosilicate (TEOS) and 2 ml of ethanol weremixed in a beaker and stirred for an hour. In another beaker 0.36 gFeCl3 salt was dissolved in a mixture of 1.2 ml water and 2.8 mlethanol. The latter solution was sonicated for 30 s. 2 ml of this saltsolution was poured into the TEOS solution prepared as men-tioned above. The resultant sol was stirred for ð1=2Þ h. A part ofthis sol was poured into a plastic dish for gellation so that a bulkglass of the composition stated before could be formed. A pelletformed of nanosized CuO particles heat treated at 373 K for1ð1=2Þ h was dipped into the iron containing sol just prepared.The preparation of CuO nanoparticles has been given in detail in
D. Ranjan Saha et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4073–40774074
our earlier paper [19]. In brief, the precursor solutions of CuCl2and NaHCO3 were mixed and stirred at 278 K for ð1=2Þ h. Theprecipitate was filtered out and washed with distilled waterseveral times so that the pH of water was 7.0. After drying theprecipitate at 333 K for 12 h. the resulting powder was given aheat treatment at 613 K for ð1=2Þ h and then cooled to roomtemperature.
The CuO pellet was kept in the sol containing the target iron-containing silica glass composition for 2ð1=4Þ h and then taken out.Both the faces of the pellet were washed with ethanol. It was keptat room temperature for three days for gellation of the sol withinits nanopores and then transferred to an oven kept at a tempera-ture of 343 K. The composite was then heat treated at 403 K for 2 h,and subsequently air cooled to 333 K. The sample was then kept ina sealed box. For preparation of the bulk glass of the samecomposition, an identical heating schedule as above was followed.
Fig. 1. X-ray diffractogram of nanoglass–CuO nanocomposites.
Fig. 2. (a) Transmission electron micrograph of iron containing nanoglass–CuO nanoco
phase around CuO nanoparticles and (e) (0 0 2) crystallographic planes of CuO nanopa
X-ray diffractograms of the composite samples were takenwith a Bruker D8 XRD SWAX Diffractometer using Cu Ka radia-tion. Magnetic measurements were carried out in the tempera-ture range 2–300 K using a Quantum design MPMS system havinga SQUID magnetometer. For dc electrical measurements both thefaces of the pellet were coated with silver paint electrodes(supplied by M/S Acheson Colloiden B.V. Netherland) and aKeithley 617 electrometer was used. The microstructure wasstudied by a JEOL 2010 Transmission Electron Microscope. Fordelineating the valence state of the iron ions present in the glasssynthesized here XPS (X-ray Photoelectron Spectroscopy) corelevel spectra were taken with an Omicron Multiprobe (OmicronNanotechnology GmbH, UK) spectrometer. A monochromated AlKa X-ray source operated at 150 W was used. Magneto dielectriceffect was studied by suspending the sample between the polepieces of an electromagnet supplied by M/S Control Systems andDevices, Mumbai, India and measuring the dielectric permittivityas a function of magnetic field using an Agilent E4980A precisionLCR meter.
3. Results and Discussion
Fig. 1 shows the X-ray diffractogram of the specimen. Thepeaks correspond to those of CuO phase. The miller indices aregiven in the figure. From the X-ray line broadening usingScherrer’s formula the size of crystalline domains was determinedto be 17 nm [23]. Fig. 2(a) shows the electron micrograph of thenanocomposite. From several electron micrographs includingFig. 2(a) we have obtained the CuO particle size distribution asshown in Fig. 2(b). The histogram was fitted to a log normaldistribution function from which a mean diameter of 21 nm wasextracted. Fig. 2(c) gives the selected area electron diffractionpattern obtained from Fig. 2(a). The diffraction spots correspondto the planes of CuO phase. In Fig. 2(d) is shown a CuO particlewith the glass layer around it. The thickness of the latter can beseen to be 5 nm. Fig. 2(e) is the high resolution electron micro-graph which shows the planes (0 0 2) of CuO with an interplanarspacing of 0.25 nm.
Fig. 4. (a) X-ray photoelectron spectrum of iron containing glass. (b) UV–vis
absorption spectra of iron containing glass dispersed in acetone.
D. Ranjan Saha et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4073–4077 4075
Fig. 3 gives the variation of log resistivity as a function ofinverse temperature for the nanocomposite being studied here.In the same figure we have plotted the resistivity data obtainedfor a pellet containing CuO nanoparticles [23]. It is evident thatthe composite has a resistivity which is more than three orders ofmagnitude higher than that of CuO nanoparticles. This shows thatthe nanoglass forms the matrix phase in this composite and hencedetermines the resistivity variation with temperature for thenanocomposite system.
It is an established fact [26] that semiconducting behavior canbe induced in a silica glass by incorporating within it variablevalence transition metal ions e.g., Fe2þ/Fe3þ [27]. In Fig. 4(a) isshown the XPS spectra obtained from the glass synthesized in thiswork. The peak position at 711.9 eV matches well with that ofFe3þ . The presence of a satellite peak at around 717 eV alsosupports the presence of Fe3þ state. However the peak corre-sponding to Fe2þ could not be detected because of a smallamount being present in the glass. We have confirmed thepresence of Fe2þ ions from the optical absorption spectra asshown in Fig. 4(b). Absorption peaks at around 370 nm and1116 nm arise due to Fe3þ and Fe2þ ions, respectively [28]. Theinset in the figure gives the amplified view of absorption peak ataround 1116 nm. From the ratio of the absorption peak areas weestimate a Fe2þ/Fe3þ ratio as �0.003. Such a low value of Fe2þ/Fe3þ ratio causes a high value of resistivity of nanoglass matrix inour system.
Fig. 5(a) shows the magnetization–magnetic field hysteresisloop obtained for the nanocomposite at room temperature. It isevident that the sample exhibits a ferromagnetic-like behavior.The magnetization value obtained was rather small. In order to beable to delineate any possible role of copper in the magnetizationbehavior of these nanocomposites, we prepared nanoparticles ofCuO by a method described earlier. These were then subjected toa 3Cu2þ22Fe3þ ion exchange reaction by dispersing them in aFeCl3 solution kept in a teflon tube used in a centrifuge.The reaction was carried out at 403 K for 2 h in an autoclave.The nanoparticles were obtained after subjecting the dispersionto centrifugation. The former were dried by keeping in an oven at333 K for 12 h and then subjected to a heat treatment at 453 K for2 h. Magnetization measurements were carried out at roomtemperature using MPMS SQUID magnetometer. Fig. 5(b) showsthe data. It can be seen that the ion exchanged CuO samplesexhibit a ferromagnetic-like behavior at room temperature.
Fig. 3. Variation of log resistivity of nanoglass–CuO nanocomposite and CuO
nanoparticles as a function of inverse temperature.
It may be pointed out here that CuO nanoparticles of identicaldiameter without being subjected to an ion exchange reaction didnot show any hysteresis behavior in its magnetization. Wetherefore explain our results as follows. Due to the ion exchangereaction 3Cu2þ22Fe3þ at the interface between the nanoglassand CuO nanoparticles iron-doped CuO phase was formed. Thisled to the formation of copper ion vacancies. In Fig. 5(c) is shownthe variation of magnetic susceptibility w as a function oftemperature for a zero field cooled (ZFC) sample. It is evidentthat w increases as the temperature is lowered. A field cooled (FC)sample exhibits an identical variation. This is explained as thatcaused by a two-dimensional Heisenberg ferromagnet [29]. In thepresent system the surface layers containing most of the Fe3þ
ions will contribute to this behavior. The inset in Fig. 5(c) shows amagnified view of ZFC curve. It is evident that the Neel tempera-ture for CuO nanoparticles in our system is �240 K. This is inreasonable agreement with that of bulk CuO (230 K) which isantiferromagnetic [30]. The room temperature ferromagnetic likebehavior is believed to arise due to uncompensated spins con-tributed by Fe–O–& entity where & represents a cation vacancy.The low value of magnetization obtained in our measurements isconsistent with the small number of Fe3þ ions introduced in CuOby our method of synthesis.
Fig. 6(a) gives the variation of dielectric constant e0 of thenanocomposite as a function of applied magnetic field at differentfrequencies ranging from 25.18 to 100.2 kHz. In Fig. 6(b) is shownthe corresponding variation of the imaginary part e0 0 of permittivity
Fig. 5. (a) M–H loop at room temperature of nanoglass–CuO nanocomposite.
(b) M–H loop at room temperature of Fe doped CuO nanoparticles. (c) ZFC and FC
curves for nanoglass–nanocomposite. Neel temperature of CuO is shown in
the inset.
Fig. 6. (a) Variation of real part of dielectric permittivity, e0 of nanocomposite with
external magnetic field and theoretical fitting of the same. (b) Variation of
imaginary part of dielectric permittivity, e00of nanocomposite with external
magnetic field and theoretical fitting of the same. Symbols represent K
Table 1Summary of magnetodielectric parameter and dielectric loss at different frequen-
cies for the nanocomposite.
Frequency (kHz) MD (%) tand % Change of tand
25.18 26 0.94–1.04 10.6
50.24 19.6 0.70–0.82 17
75.17 17.4 0.67–0.69 2.9
100.2 16 0.51–0.61 19.6
D. Ranjan Saha et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4073–40774076
as a function of magnetic field. We have estimated the magnetodi-electric (MD) parameter defined as
M:D:¼e0ðHÞ�e0ð0Þ
e0ð0Þ X100
where e0(H) and e0(0) are the dielectric constants measured atmagnetic fields H and zero, respectively, for different frequencies.Also the values of the percentage change of dissipation factor tand
were calculated from these data. These are summarized in Table 1.It can be seen that magnetodielectric parameter has values in therange 16–26% which are rather high. The dielectric loss tand hasvalues in the range 0.51 to 0.94. We have analyzed the results onthe basis of a heterogeneous system described by the Maxwell–Wagner capacitor model. The electrical behavior of our nanocom-posite can be described in terms of two leaky capacitors in series.The real and imaginary parts of dielectric permittivity e0 and e0 0
Fig. 7. Magnetoresistance as extracted by the fitting of dielectric data at different
magnetic fields.
D. Ranjan Saha et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4073–4077 4077
have been shown to be given by [25]
e0ðoÞ ¼ 1
C0ðRiþRbÞ
tiþtb�tþo2titbt1þo2t2
ð1Þ
and
e00ðoÞ ¼ 1
oC0ðRiþRbÞ
1�o2titbþo2tðtiþtbÞ
1þo2t2ð2Þ
where Ri is the resistance of the interfacial layer formed by thenanoglass, Rb is the resistance of the iron doped CuO, o is theangular frequency, ti¼CiRi, Ci being the capacitance of the interfaciallayer, tb¼CbRb, Cb being the capacitance of the iron-doped CuO,t¼(tiRbþtbRi)/(RiþRb) and C0¼e0A/t, A being the area of thespecimen, t the thickness and e0 the free space dielectric permittiv-ity. The experimental data in Fig. 6 were fitted to Eqs. (1) and (2) byconsidering the presence of a negative magnetoresistance in thedoped CuO described byRðHÞ ¼ R0þR1expð�H=HsÞ, where R0, R1, Hs
are fitting parameters. The theoretically fitted curves are also shownin the figure. It is evident that the above model explains our resultssatisfactorily. From the fitting described we find a magnetoresis-tance with 33% decrease at a magnetic field 10 KOe, shown in Fig. 7.It is thought to be due to spin polarized tunneling across grainboundaries of iron-doped CuO. The mechanism is similar to thatdiscussed in the cases of oxide materials [31,32].
4. Conclusions
In summary, composites of CuO nanoparticles of averagediameter 21 nm and an interfacial glass of system Fe2O3–SiO2
having thickness 5 nm were synthesized by a chemical route. Thenanocomposites showed a ferromagnetic-like behavior at roomtemperature. This was ascribed to uncompensated spins of Fe3þ
ions incorporated into CuO by iron/copper ion exchange reactionduring the synthesis process. A rather high magnetodielectricparameter in the range 16–26% depending on the frequency ofmeasurement was exhibited by these nanocomposites at a mag-netic fied of 10KOe. This was explained as that caused by acolossal magnetoresistance of 33% in the iron doped CuO nano-particles. The experimental results were satisfactorily fitted to
Maxwell–Wagner capacitor model developed by Catalan [25]. Thelarge value of magnetodielectric parameter obtained in thesenanocomposites will make them ideally suited for magneticsensor applications.
Acknowledgment
The work was supported by the Department of Science andTechnology, New Delhi, under an Indo–Australian project on Nano-composites. D. R. Saha thanks Council of Scientific and IndustrialResearch, New Delhi for the award of a Senior Research Fellowship.D. Chakravorty thanks Indian National Science Academy, New Delhi,for giving him an Honorary Scientist’s position.
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