Ferromagnetic-Like Behavior in Nanosilica Glass Containing Iron Ions

Article pubs.acs.org/JPCC

Ferromagnetic-Like Behavior in Nanosilica Glass Containing Iron Ions and Giant Magnetodielectric Effect in Composites of these Glasses with Mesoporous Silica Dhriti Ranjan Saha,† Shilpi Banerjee,† Arun Kumar Nandi,‡ and Dipankar Chakravorty*,† †

MLS Professor’s Unit, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India Polymer Science Unit, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India

‡

ABSTRACT: Iron ion containing nanodimensional silica glass was grown within the 5 nm pores of mesoporous silica. The nanocomposite material showed ferromagnetic-like behavior at room temperature. This was ascribed to the presence of Fe2+ and Fe3+ ions within the nanodimensional glass phase and an antiferromagnetic superexchange interaction between them. The nanocomposite showed a large magnetodielectric coefficient (up to 51%) for a magnetic field of 1.7 T. The dielectric loss (tan δ) was found to be in the range 0.35 to 0.66. This was explained on the basis of Catalan’s model of space−charge polarization. From the theoretical fitting of experimental data, magnetoresistance of nanoglass phase was extracted to be 58% up to a magnetic field of 1.7 T.

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INTRODUCTION Multiferroic materials1,2 have the intriguing property of possessing both ferroelectric and ferromagnetic order. What is most interesting however is the coupled electric and magnetic order parameters referred to as magnetoelectric coupling. This has led to the concept of fabricating multiple-state memory devices with data written electrically and read magnetically or vice versa. Single-phase multiferroics being rare in nature, research efforts have concentrated on synthesizing nanostructured composites containing ferroelectric and ferromagnetic phases, respectively. Some of the examples are BaTiO3− CoFe2O4,3 PbTiO3−CoFe2O4,4 Pb(Zr,Ti)O3−CoFe2O4,5,6 BaTiO 3 −Ni, 7 and BaTiO 3 −Co. 8 In these systems on application of magnetic field the magnetostrictive strain induced in the ferromagnetic phase is transmitted to the ferroelectric phase resulting in a change in dielectric permittivity of the composite. Such a magnetodielectric (MD) effect has been sought to be used in practical applications. However the MD parameter defined as MD =

ε′(H ) − ε′(0) × 100 ε′(0)

conductivity orders of magnitude lower than that of the former have shown that a large MD effect can be observed in such composites.11 A negative magnetoresistance was assumed in these inhomogeneous materials.11 This physical mechanism has been exploited to synthesize composites with reasonable values of MD parameters recently.12 Composites comprised of alternate nanolayers with different conductivities in the CdSNa-4 mica system have shown MD parameters of 5.3% .13 In the present investigation three-dimensional mesoporous silica was used as a template in which a silica glass of nanodimensions containing iron ions was formed by soaking of a sol and subsequent heat treatment. Nanoglass based on silica has earlier been shown to exhibit properties different from that of its bulk counterpart.14 In this work nanoglass showed ferromagneticlike behavior. It would have been interesting to investigate the magnetoresistance characteristics of the nanoglass itself. But that would involve growing the glass of nanodimensions on a suitable substrate and then carrying out the measurements as a function of magnetic field. Such synthesis has been difficult so far. On the other hand the nanocomposite prepared here exhibited a large MD effect because of space charge polarization coupled with magnetoresistance of the glass. The details are reported in this paper.

(1)

where ε′(H) and ε′(0) are the dielectric constants measured at magnetic fields H and zero, respectively, which have not been found to be higher than a few percent.9 Nanoporous ferroelectric materials have been synthesized with a view to exploiting a large surface area generated with the associated oxygen vacancies for inducing ferromagnetic behavior.10 Recent theoretical investigations on inhomogeneous systems comprising a magnetic material with an interface having electrical © 2012 American Chemical Society

Received: June 19, 2012 Revised: September 5, 2012 Published: September 25, 2012 21679

dx.doi.org/10.1021/jp306017k | J. Phys. Chem. C 2012, 116, 21679−21684

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Article

EXPERIMENTAL SECTION

The mesoporous silica template referred to as KIT-6 was synthesized following the method reported earlier.15 p123 (1 g, triblock copolymer based on poly(ethylene glycol)-poly(propelene glycol)-poly(ethylene glycol)), the one-dimensional template, was dissolved in 36 g of distilled water and 1.96 g of concentrated HCl (35%) with constant stirring at 308 K for 1 h. Butanol (1 g) was added to the solution and stirred for another hour. Tetraethyl ortho silicate (2.15 g) was then added and the mixture stirred for 24 h at 308 K. This was transferred to a Teflon-lined stainless steel autoclave and treated hydrothermally at 393 K for 24 h. The product was then filtered and washed with distilled water and subsequently dried at 333 K for 24 h. The resulting white powder was calcined at 823 K for 5 h to remove the surfactant. The KIT-6 powder was then taken in a mold of 1 cm diameter and pressed into pellet form by applying a load of 5 tons. The pore surface area and pore volume of KIT 6 powder were determined from N2 adsorption isotherm measurements carried out by a Quantachrome surface area analyzer. A sol having a target glass composition 10Fe2O3·90SiO2 (mol %) was prepared with appropriate amounts of FeCl3 and tetraethyl ortho silicate (TEOS). The KIT-6 pellet prepared as above was dipped in the solution and kept for 9 h. The sample was then heat treated at 473 K for 2 h. Subsequently, it was air cooled and kept in a sealed box. X-ray diffractograms of the composite samples were taken with a Bruker D8 XRD SWAX Diffractometer using Cu Kα radiation. Magnetic measurements were carried out using an MPMS system (Quantum Design USA) having a SQUID magnetometer. For direct current electrical measurements both the faces of the pellet sample were coated with silver paint electrodes (supplied by M/S Acheson Colloiden B. V. Netherlands), and a Keithley 617 electrometer was used. The microstructure was studied by a JEOL 2010 transmission electron microscope. The MD effect was investigated by suspending the sample between the pole pieces of an electromagnet supplied by M/S Control Systems and Devices, Mumbai, India. The dielectric permittivity was measured as a function of magnetic field using Agilent E4980A precision LCR meter.

Figure 1. (a) Small angle X-ray diffraction pattern of KIT-6. (b) Wideangle X-ray diffraction pattern of the same.

taken from the composite. The elements present are oxygen, silicon, and iron, respectively, confirming the absence of any other impurity in the system. Table 1 summarizes the contents of these elements present in our composite. We have calculated the relative ratio of these elements as predicted by the starting compositions of the nanoglass and the silica matrix. The ratios of Fe:Si:O as inferred from the composition and EDAX analysis are found to be 1.0:9.5:20.5 and 1.0:8.7:13.1, respectively. Evidently, there were oxygen vacancies in our sample. This was expected because of a large surface to volume ratio present in the nanoglass. As a consequence, there would be a number of Fe2+ ions present in the nanoglass. To confirm the presence of Fe2+ ions, a method of titration was used to determine the fraction of Fe2+ ions in the glass. For this the composite was first of all dissolved in dilute HF. This was titrated with standard KMnO4 solution to find the amount of Fe2+ present. By reducing the solution with the treatment of (SnCl2 + HCl) and then repeating the titration as mentioned above, the total iron content was obtained. The ratio of [Fe2+]/[Fe]total was estimated to be 0.0092. Similar procedure was reported earlier.17 The presence of Fe2+ therefore induced semiconducting behavior to the nanoglass because of a polaron hopping mechanism between Fe2+ and Fe3+ sites.18 Also a superexchange antiferromagnetic interaction between Fe2+ and Fe3+ ions in the nanoglass would give rise to a ferromagnetic-like property in the system. The results in the following sections substantiate these conclusions. Figure 3a gives the magnetization as a function of magnetic field for the composite at room temperature. It can be seen that there is a hysteresis loop indicating the presence of ferromagnetic-like behavior with the magnetization being small. Figure 3b gives an amplified view of the near zero magnetic field. As discussed earlier we ascribe this behavior to an antiferromagnetic super exchange interaction between Fe2+ and Fe3+ ions in the nanoglass. The interaction envisaged occurs through the intervening oxygen ion such that the spins on the two iron ions are aligned in an antiparallel way. Because of unequal spin moments of Fe2+ and Fe3+ ions, respectively, a resultant spin moment occurs which contribute to the remanent magnetization of the system.19,20 With the number of such uncompensated spins being quite small in the present case, the saturation magnetization observed in the system is also rather low. In Figure 3c is shown the variation of magnetic susceptibility as a function of temperature over the range 2−

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RESULTS AND DISCUSSION Figure 1a shows the small-angle X-ray diffraction pattern of KIT-6 silica powder which indicated an ordered mesoporous structure. The wide-angle X-ray diffractogram characteristic of an amorphous phase can be seen in Figure 1b. The composite samples also showed an amorphous structure. This confirmed that all the phases present were amorphous. Figure 2a shows the transmission electron micrograph of the composite sample. The pore size is seen to be around 5 nm. This is estimated from the width of the dark phase marked in the figure. Because the iron-containing silica glass has an electron density higher than that of pure silica, the former will appear dark in the electron micrograph. The pore size was also extracted from the specific pore volume Vp = 0.9725 mL/g and specific pore area Sp = 617 m2/g as determined from N2 adsorption measurements. From these data the mean diameter w of the pores was calculated to be 6.3 nm using the relation w = 4Vp/Sp.16 Figure 2b is the electron diffraction pattern obtained from Figure 2a. It is evident that no crystalline phase is present in the system. Figure 2c is the energy-dispersive X-ray analysis (EDAX) spectrum 21680

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Figure 2. (a) Transmission electron micrograph of the mesoporous silica−nanoglass composite. (b) Electron diffraction pattern of the same (c) EDAX of the nanocomposite.

dipole moment which is reflected in an increase of dielectric constant. In the case of a magnetically active conductor if an applied magnetic field lowers the electrical resistivity due to the above-mentioned effect the dielectric constant increases. We have therefore analyzed the dielectric behavior in terms of two lossy capacitors connected in series. These consist of iron ion containing silica nanoglass and the mesoporous silica, respectively. Because of the difference in electrical conductivities of these two phases space charge polarization occurs at the interface between them. The real and imaginary parts of the dielectric permittivity have been shown to be given by11

Table 1. Atomic Percent of Fe, Si, and O from EDAX Analysis element

atomic %

Fe Si O

4.38 38.20 57.42

300 K under both field-cooled (FC) and zero-field-cooled (ZFC) conditions. Both these data show a sharp rise as the temperature is lowered below ∼25 K. This could be due to the two-dimensional nature of the nanoglass phase in which a ferromagnetic behavior results from antiferromagnetic interaction between Fe2+ and Fe3+ ions. A Heisenberg ferromagnet model has been used earlier to explain such a result21 in systems having spin moments arranged in a two−dimensional lattice. It may be mentioned that a bulk glass with an identical composition to that of the nanoglass used here did not show any ferromagnetic behavior like the one given in Figure 3a. We believe that the small volume of the nanoglass made it possible for the Fe3+ and Fe2+ ions to come close to each other in the silica glass network to ensure the super exchange interaction envisaged. Parts a and b of Figure 4 show the variation of real and imaginary parts of relative dielectric permittivity ε′ and ε″, respectively, as a function of applied magnetic field at different frequencies. It is evident that the dielectric constant increases as a function of magnetic field. This increase is in the range 30.6% to 51.1% depending on the frequency of measurement. The dissipation factor tan δ (ε″/ε′) is found to have values in the range of 0.36−0.66. Table 2A summarizes these results. The large value of the dielectric constant reported in this work owes its origin to the space charge polarization occurring in the nanocomposite. When two phases having widely different electrical conductivities form an interface, charge accumulation at the latter takes place on application of an electrical field to the composite. This gives rise to an electric

ε′(ω) =

τi + τb − τ + ω 2ττ 1 i bτ C0(R i + R b) 1 + ω 2τ 2

(2)

2 1 − ω 2ττ 1 i b + ω τ(τi + τb) ωC0(R i + R b) 1 + ω 2τ 2

(3)

and ε″(ω) =

where Ri is the resistance of the insulating layer made up of the silica glass in mesoporous silica, Rb the resistance of the iron ion containing nanoglass, ω the angular frequency τi = CiRi, Ci the capacitance of the silica glass in mesoporous silica τb = CbRb, Cb the capacitance of the iron ion containing silica nanoglass, τ = (τiRb + τbRi)/(Ri + Rb) and C0 = ε0A/t, A the area of the specimen, t its thickness, and ε0 the free space dielectric permittivity. The experimental data in parts a and b of Figure 4 were fitted to eqs 2 and 3, respectively, by considering the presence of a negative magnetoresistance in the nanoglass described by R(H) = R0 + R1 exp(−H/Hs), where R0, R1, Hs were fitting parameters. The solid lines in these figures represent the theoretical fits. It must be noted here that there are two regions in the figures described by the magnetic field ranges 0−4000 Oe and 7000−17000 Oe, respectively, for which different sets of parameters were used. This is explained as arising from two different configurations of the Maxwell− 21681

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Figure 3. (a) M−H loop at room temperature of iron ion containing nanoglass within mesoporous silica nanocomposite. (b) Expanded view of M− H for small magnetic field. (c) FC−ZFC curve of the nanocomposite .

Wagner capacitor in our system. The first consists of mesoporous silica and nanoglass interfaces, whereas the second comprises nanoglass and air interface within a channel. The latter situation arises because nanochannels of the mesoporous silica cannot be completely filled with the nanoglass by the soaking method used. The configurations mentioned above are schematically represented by Figure 5. Because of random alignment of the mesoporous silica grains there will be a substantial number of the above-mentioned nanochannels being subjected to an electrical field parallel to them. The latter would then cause space charge polarization at the nanoglass air interface. A higher value of the conductivity ratio in the latter case would give rise to a larger value of the MD coefficient as has been borne out by our results shown in Figure 4a. The extracted values of the Rb and Ri for the two different mechanisms discussed above are also consistent with this model. The MD effect in the two regions of magnetic field, viz., 0−8000 Oe and 8000−17000 Oe, respectively, has been summarized in Table 2B. It is evident that the MD parameter is larger in the first range than that in the second one. The variation of resistance as a function of magnetic field of the nanoglass as extracted from the above fitting is shown in Figure 6. It can be seen that the magnetoresistance over the entire magnetic field changes by ∼58%. However, there is a difference in the rate of change of resistance vs magnetic field in the two regions of magnetic field. In the magnetic field range of 0−8000 Oe the rate is higher than that in the range above this field. This indicates that there is anisotropy in magnetoresistance, viz., the change with magnetic field is higher when the field direction is parallel to the nanoglass and is smaller

when the field is perpendicular to the same. This is to be expected because of the large aspect ratio of the nanoglass formed by the present synthesis method. This can be rationalized as follows. The magnetoresistance is controlled by electron phonon scattering.22 For the magnetic field perpendicular to the nanoglass (Figure 5b) because of the dimensional constraint there will be phonon confinement thereby reducing the effect of magnetic field on the hopping conduction of electrons. This will result in a lower rate of change of resistance as a function of applied magnetic field. To delineate the physical mechanism of magnetoresistance in the present nanoglass system it would be necessary to make resistance measurements on them directly. For this purpose a percolative configuration of the nanoglass will need to be generated using the mesoporous silica template. Resistance measurements could then be carried out under different magnetic fields with the latter aligned parallel or perpendicular to the nanoglass specimen. We hope to take up such work shortly.

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CONCLUSIONS

In summary, iron ion containing silica nanoglasses were grown within nanopores with 5 nm diameters of mesoporous silica by a soaking technique. The nanocomposite showed ferromagnetic-like behavior at room temperature. From the lowering of oxygen content in the nanoglass, the presence of Fe2+ and Fe3+ ions could be concluded, and a super exchange antiferromagnetic interaction gave rise to the ferromagnetic property. The composite exhibited a large MD coefficient (30.6−51.1%) for a magnetic field of 1.7 T. This was explained on the basis of 21682

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Table 2B. MD Parameter at Different Frequencies in the Two Ranges of Magnetic Field magnetic field ranges (Oe) 0−8000

8000−17000

frequency (kHz)

MD %

13.37 22.44 33.58 13.37 22.44 33.58

16.4 20.2 24.0 12.2 16.9 21.9

Figure 6. Magnetoresistance as extracted by the fitting of dielectric data at different magnetic fields within the ranges 0−4000 and 7000− 17000 Oe.

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Figure 4. (a) Variation of the real part of the relative dielectric permittivity, ε′, of the nanocomposite with external magnetic field and theoretical fitting of the same. (b) Variation of the imaginary part of the relative dielectric permittivity, ε″, of the nanocomposite with external magnetic field and theoretical fitting of the same.

Corresponding Author

*E-mail: [email protected]. Phone: 0332473-4971 ext 1580. Fax: 913324732805. Notes

Table 2A. Summary of Overall MD Parameter and Dielectric Loss at Different Frequencies for the Nanocomposite frequency (kHz)

MD %

tan δ

% change of tan δ

13.37 22.44 33.58

30.6 40.5 51.1

0.56−0.35 0.60−0.37 0.66−0.4

−37.5 −38.3 −39.39

AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Science and Technology, New Delhi, under an Indo- Australian Project on Nanocomposites. D. R. Saha and S. Banerjee thank the Council of Scientific and Industrial Research, New Delhi, for the award of Senior Research Fellowships. D. Chakravorty thanks Indian National Science Academy, New Delhi, for giving him an Honorary Scientist’s position.

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REFERENCES

(1) Spaldin, N. A.; Fiebig, M. Science 2005, 309, 391. (2) Hill, N. A. J. Phys. Chem. B 2000, 104, 6694. (3) Zheng, H.; Wang, J.; Lofland, S. E.; Ma, Z.; Mohaddas-Ardabili, L.; Zhao, T.; Salamanca-Riba, L.; Shinde, S. R.; Ogale, S. B.; Bai, F.; Viehland, D.; Jia, Y.; Schlom, D. G.; Wuttig, M.; Roytburd, A.; Ramesh, R. Science 2004, 303, 661. (4) Murakami, M.; Chang, K.-S.; Aronova, M. A.; Lin, C.-L.; Yu, M. H.; Simpers, J. H.; Wutting, M.; Takeuchi, I. Appl. Phys. Lett. 2005, 87, 112901. (5) Liu, M.; Li, X.; Lou, J.; Zheng, S.; Du, K.; Sun, N. X. Appl. Phys. Lett. 2005, 87, 112901. (6) Gao, X.; Rodrigue, B. J.; Liu, L.; Birajdar, B.; Pantel, D.; Ziese, M.; Alexe, M.; Hesse, D. ACS Nano 2010, 4, 1099. (7) Castle, V.; Brosseau, C. Appl. Phys. Lett. 2008, 92, 233110. (8) Narayanan, T. N.; Mandal, B. P.; Tyagi, A. K.; Kumarasiri, A.; Zhan, X.; Hahm, M. G.; Anantharaman, M. R.; Lawes, G.; Ajayan, P. M. Nano Lett. 2012, 12, 3025.

Figure 5. Schematic representation of two different configurations with respect to electrodes of nanoglass and mesoporous silica contributing to ε′ variation as a function of magnetic field because of random alignment of the nanocomposite grains.

Catalan’s model of space charge polarization in a two component system having different conductivities in the two phases. A magnetoresistance change of 58% in the nanoglass phase was extracted from the fitting of experimental data to the theoretical model. 21683

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(9) GuO, Y.; Liu, Y.; Wang, J.; Withers, R. L.; Chen, H.; Jin, L.; Smith, P. J. Phys. Chem. C 2010, 114, 13861. (10) Banerjee, S.; Datta, A.; Bhaumik, A.; Chakravorty, D. J. Appl. Phys. 2011, 110, 064316. (11) Catalan, G. Appl. Phys. Lett. 2006, 88, 102902. (12) Banerjee, S.; Hajra, P.; Bhaumik, A.; Chakravorty, D. Mater. Lett. 2012, 79, 65. (13) Mandal, A.; Mitra, S.; Datta, A.; Banerjee, S.; Chakravorty, D. J. Appl. Phys. 2012, 111, 074303. (14) Saha, D. R.; Datta, A.; Mandal, S.; Mukherjee, M.; Nandi, A. K.; Chakravorty, D. Solid State Ionics 2011, 186, 14. (15) Zhang, Z.; Zuo, F.; Feng, P. J. Mater. Chem. 2010, 20, 2206. (16) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1997, 101, 583. (17) Brahma, P.; Mitra, S; Chakravorty, D. Eur. J. Solid State Inorg. Chem. 1991, 28, 1139. (18) Murawski, L. J. Mater. Sci. 1982, 17, 2155. (19) Takahashi, M. Phys. Rev. Lett. 1987, 58, 168. (20) Jagiello, J.; Oliver, J. P. J. Phys. Chem. C 2009, 113, 19382. (21) Smith, J.; Wijn, H. P. J. Ferrites; John Wiley & Sons: New York, 1959; pp 84. (22) Ansermet, J.-Ph. J. Phys.: Condens. Matter 1998, 10, 6027.

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