Role of SiO2 coating in multiferroic CoCr2O4 nanoparticles M Kamran, Asmat Ullah, Y Mehmood, K Nadeem, and H Krenn Citation: AIP Advances 7, 025011 (2017); doi: 10.1063/1.4973732 View online: http://dx.doi.org/10.1063/1.4973732 View Table of Contents: http://aip.scitation.org/toc/adv/7/2 Published by the American Institute of Physics AIP ADVANCES 7, 025011 (2017) Role of SiO2 coating in multiferroic CoCr2 O4 nanoparticles M Kamran,1 Asmat Ullah,1 Y Mehmood,1 K Nadeem,1,a and H Krenn2 Nanomagnetism and Nanotechnology Laboratory, International Islamic University, Islamabad, Pakistan Institute of Physics, Karl-Franzens University, Universită atsplatz 5, A-8010 Graz, Austria (Received 25 September 2016; accepted 24 December 2016; published online 17 February 2017) Effect of silica (SiO2 ) coating concentration on structural and magnetic properties of multiferroic cobalt chromite (CoCr2 O4 ) nanoparticles have been studied The nanoparticles with average crystallite size in the range 19 to 28 nm were synthesised by sol-gel method X-ray diffraction (XRD) analysis has verified the composition of single-phase cubic normal spinel structure of CoCr2 O4 nanoparticles The average crystallite size and cell parameter decreased with increasing SiO2 concentration TEM image revealed that the shape of nanoparticles was non-spherical Zero field cooled/field cooled (ZFC/FC) curves revealed that nanoparticles underwent a transition from paramagnetic (PM) state to collinear short-range ferrimagnetic (FiM) state, and this PM–FiM transition temperature decreased from 101 to 95 K with increasing SiO2 concentration or decreasing crystallite size A conical spin state at Ts = 27 K was also observed for all the samples which decreased with decreasing average crystallite size Low temperature lock-in transition was also observed in these nanoparticles at 12 K for uncoated nanoparticles which slightly shifted towards low temperature with decreasing average crystallite size Saturation magnetization (Ms ) showed decreasing trend with increasing SiO2 concentration, which was due to decrease in average crystallite size of nanoparticles and enhanced surface disorder in smaller nanoparticles The temperature dependent AC-susceptibility also showed the decrease in the transition temperature (Tc ), broadening of the Tc peak and decrease in magnetization with increasing SiO2 concentration or decreasing average crystallite size In summary, the concentration of SiO2 has significantly affected the structural and magnetic properties of CoCr2 O4 nanoparticles © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4973732] I INTRODUCTION Multiferroic materials show simultaneously ferroelectric and ferromagnetic ordering that exhibit unprecedented physical properties due to coupling between magnetic and electric order parameters.1 Due to potential uses of multiferroic, they have recently gained much attention in dynamic random access memories, electromagnetic sensors, telecommunication systems, data storage media and spintronics.2–5 Despite their applications, multiferroic are few and exploration of new multiferroic materials is of great importance for the emerging technology Ferrimagnetic cobalt chromite (CoCr2 O4 ) is one of the most important members of multiferroic compound family.6 CoCr2 O4 exists in a normal cubic spinel structure having space group Fd3m in which Co2+ ions occupy the tetrahedral sites (A) and Cr3+ ions occupy the octahedral sites (B) The unit cell of CoCr2 O4 consists of 32 oxygen, 16 chromium and divalent transition metal ions It is a ferrimagnetic (FiM) system that undergoes long range order at lock in temperature state at TF = 14 – 15 K, spiral spin state at Ts = 27 K and collinear short range FiM state at Tc = 93-97 K.7–10 Nanoparticle’s surface spins play important role in controlling its magnetic properties for various applications and surface functionalization.8,10 Due to magnetic nature of chromites, they have high a Corresponding author’s e-mail: kashif.nadeem@iiu.edu.pk Phone # 0092-519019714 2158-3226/2017/7(2)/025011/7 7, 025011-1 © Author(s) 2017 025011-2 Kamran et al AIP Advances 7, 025011 (2017) tendency to agglomerate and one can get their collective magnetic response.11 To avoid agglomeration, nanoparticles can be coated or disperse in non-magnetic matrix to get separate individual nanoparticles Tsoukatos et al.12 reported that SiO2 acts as the best serving non-magnetic material than any other such type of materials as Al2 O3 and TiO2 12,13 There are various advantages of SiO2 such as its excellent stability, controlling particle size, controlling surface effects, and control of interparticle interactions through its shell thickness.14 SiO2 can be used to prepare nanoparticles with smaller size of single phase because it provides large number of nucleation sites during synthesis process, which finally restrict the growth of nanoparticles.15–17 Consequently, it can be used to control the magnetic properties of multiferroic nanoparticles which will finally affects the magneto-electric coupling.18 Therefore, it is interesting to study the effect of non-magnetic SiO2 coating on the structural and magnetic properties CoCr2 O4 nanoparticles In this article, we will focus on the effect of SiO2 coating concentration on the structural and magnetic properties of CoCr2 O4 nanoparticles II EXPERIMENT The SiO2 coated CoCr2 O4 nanoparticles (CoCr2 O4 /(SiO2 )y with y = 0, 45 and 80 wt.%) were synthesized by using sol-gel method The chemical reagents like cobalt nitrate (Co(NO3 )2 6H2 O), citric acid (C6 H8 O7 H2 O), chromium nitrate (Cr(NO3 )3 9H2 O), ammonia and tetraethyl orthosilicate (TEOS, as a precursor for SiO2 ) of analytical grade were used in stoichiometric ratios All the reagents were obtained from Sigma-Aldrich and used without further purification Initially (Cr(NO3 )3 9H2 O) and (Co(NO3 )2 6H2 O) in their stoichiometry were mixed in 30 ml ethanol under constant stirring to get first homogeneous solution The second solution was prepared by using TEOS (0, 45, and 80 wt.% of total nitrates), citric acid and distilled water The molar ratio of citric acid and nitrates was 1:1 Later a homogeneous combined solution was obtained by mixing two solutions Furthermore ammonia was dropped in to the combined solution until a pH value of was obtained and heated at 70o C till the formation of gel The gel was dried in an electric oven at 100o C for 12 h and then grinded to acquire the powder which was later annealed at 900o C for h to get the desired SiO2 coated CoCr2 O4 nanoparticles PANalytical X-ray diffractometer (XRD) was used for structural determination of samples using CuKα having wavelength λ = 0.1541 nm Transmission electron microscopy (TEM) was used to analyse particle size and shape Superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS-XL-7) was used for magnetic measurements The same magnetometer was used for the AC-susceptibility measurements in the range of temperature from 4.2–120 K, at frequency of Hz and AC field of Oe III RESULTS AND DISCUSSION XRD is a powerful technique to determine the crystal structure of the material Fig 1(a) shows the XRD diffraction patterns of CoCr2 O4 /(SiO2 )y nanoparticles with different SiO2 concentration (y) = 0, 45 and 80 wt.% The data was analysed by using PANalytical X pert High Score software, which was in good agreement with the standard pattern JCPDS No 22-1084 All the diffraction peaks are well indexed and correspond to single phase spinel CoCr2 O4 structure No other impurity phases are found The Debye-Scherrer’s formula is used to calculate average crystallite size from the most intense diffraction peak (311) for all the samples The Debye-Scherrer’s formula is given by, Average crystallite size (t) = 0.9λ β Cosθ (1) where ‘θ’ is angle of diffraction, ‘ β’ is the full width at half maximum (FWHM) in units of radian and ‘λ’ depicts the wavelength (λ) = 0.1541 nm of the CuKα radiation used to obtain the XRD pattern Fig 1(b) shows the variation of average crystalline size with increasing SiO2 concentration The average crystallite size of samples with y = 0, 45, and 80 wt.% come out to be 28, 22, and 19 nm, respectively The crystallite size reduces with increasing the SiO2 concentration The decreasing crystallite size with increasing SiO2 concentration is due to the formation of large number of nucleation sites during synthesis process which restrict the further growth of nanoparticles.19 The lattice parameter “a” was calculated by using Bragg’s equation which came out to be 8.325 Å, 8.314 Å, 025011-3 Kamran et al AIP Advances 7, 025011 (2017) FIG (a) X-ray diffraction patterns of CoCr2 O4 /(SiO2 )y nanoparticles with y = 0, 45 and 80 wt.%, (b) variation of average crystallite size with SiO2 concentration, (c) variation of lattice parameter with SiO2 concentration Dashed lines just show the trends 8.299 Å for samples with y = 0, 45 and 80 wt.%, respectively Fig 1(c) demonstrate the variation of lattice parameter (a) with SiO2 concentration, which proves the lattice contraction of nanoparticles with increasing SiO2 concentration or decreasing crystallite size Transmission electron microscopy (TEM) was used to analyse the shape and size of nanoparticles Fig shows the TEM image of uncoated CoCr2 O4 /(SiO2 )y nanoparticles (y = 0) at 50 nm scale It is observed that the nanoparticles are non-spherical and quite well dispersed However, nanoparticles show some degree of agglomeration due to magnetic interactions The nanoparticles looks larger than their average crystallite size as obtained by XRD analysis FIG TEM image of uncoated (y = %) CoCr2 O4 /(SiO2 )y nanoparticles at 50 nm scale 025011-4 Kamran et al AIP Advances 7, 025011 (2017) Fig shows the zero field cooled (ZFC) and field cooled (FC) curves of CoCr2 O4 /(SiO2 )y nanoparticles with y = 0, 45 and 80 wt.% under magnetic field of 50 Oe The ZFC curve of uncoated nanoparticles exhibits negative magnetization which is very similar to the data obtained by Lawes et al.9 and Kahn et al.20 for polycrystalline material The ZFC negative magnetization decreases for 45 and 80% SiO2 coated nanoparticles In nanoparticles, broken surface bonds alter exchange interaction and form different surface spins structure as compared to bulk material The negative magnetization also refers to the existence of uncompensated spin at the grain boundaries Molecular based ferrimagnets also exhibits negative magnetization due to direction reversal of ferrimagnet components at a certain temperature known as compensation temperature.21 Panel (b) shows the variation of Tc , Ts and TF with increasing SiO2 concentration or decreasing crystallite size The transition temperatures were obtained from FC curves The uncoated (y = 0%) nanoparticles exhibit a transition from paramagnetic to ferrimagnetic and conical spin state at Tc = 101 K and Ts = 27 K, respectively, which are very near to Tc = 99 K and Ts = 26 K values as obtained by Plocek et al.22 The CoCr2 O4 nanoparticles with y = 45% SiO2 coating concentration have Tc = 97 K and Ts at 25.2 K.23 The CoCr2 O4 nanoparticles with y = 80% SiO2 coating concentration have Tc = 95 K and Ts at 20 K The Tc and Ts show decreasing trend with increasing SiO2 concentration The Ts and TF transitions are rather weak in coated nanoparticles which is due to the fact that SiO2 highly disturbs the surface of nanoparticles and may create distortions on the surface The shift of Tc and Ts towards lower temperature with increasing SiO2 concentration can be attributed to decreasing crystallite size with SiO2 concentration In addition to these transitions, a lock-in transition (TF ) which usually occurs in pure CoCr2 O4 at 14 K as observed by Choi et al.24 and Yamasaki et al.,25 is also observed in our samples but at TF = 12 K (for uncoated nanoparticles) The TF value slightly decreases with decreasing crystallite size which is also due to finite size effects.26 Fig (a) shows the M-H loops of CoCr2 O4 /(SiO2 )y nanoparticles with y = 0, 45 and 80 wt.% under maximum applied field of ± 5T at T = 25 K M-H loops show typical ferrimagnetic behaviour for all the samples The loops are not saturated even at ± T which is typical for nanoparticles due to disordered surface spins The non-saturation behaviour of the loops increases with decreasing crystallite size or increasing SiO2 concentration Fig (b) shows the variation of saturation magnetization (Ms ) and coercivity (Hc ) with increasing SiO2 concentration We ascribe variation of Ms to FIG (a) Temperature dependence of ZFC and FC magnetization curves under magnetic field of 50 Oe for CoCr2 O4 /(SiO2 )y nanoparticles with y = 0, 45 and 80 wt.%, (b) variation in Tc , Ts and TF value with SiO2 concentration Dashed lines just show the trends 025011-5 Kamran et al AIP Advances 7, 025011 (2017) FIG (a) M-H loops of CoCr2 O4 /(SiO2 )y nanoparticles with y = 0, 45, and 80 wt.% at T = 25 K, (b) variation of Ms and Hc with SiO2 concentration Dashed lines just show the trends the non-magnetic amorphous behaviour of SiO2 which enhances surface spins disorder by creating a surrounding layer around nanoparticle and leads to decrease in Ms value The surface to volume ratio becomes also high with the decrease in particle size.27,28 It is also observed that the Ms value of bare (y = 0) nanoparticles is greater than coated nanoparticles which is due to their large average crystallite size of uncoated nanoparticles as compared to coated nanoparticles.29 Similar phenomenon was also reported for NiFe2 O4 and CoFe2 O4 ferrite nanoparticles.30,31 The variation of coercivity (Hc ) with SiO2 concentration depends upon the anisotropy of nanoparticles The coercivity shows maximum value for nanoparticles with y = 45 % The Hc value is related with the magnetization reversal of the nanoparticles which may be affected by surrounding SiO2 coating material This anomalous behaviour was also reported by Georgea et al.32 Fig shows the in-phase part of AC susceptibility (ZFC) as a function of temperature for CoCr2 O4 /(SiO2 )y nanoparticles with y = 0, 45 and 80 wt.% under applied AC field of amplitude (HAC ) = Oe and frequency (f) = Hz A sharp peak in χ is associated with the ferrimagnetic transition temperature at 94 K for uncoated CoCr2 O4 nanoparticles (y = 0%) which is nearly the same as observed by Lawes et al.9 and Mantlikova et al.33 However, Zakutna et al.34 and Rath et al.35 reported lower T values in the vicinity of 76 K It can be seen clearly that transition peak decreases with increasing SiO2 concentration and gets broadened and these findings are consistent with the dc ZFC/FC magnetization curves The obvious reason for decreasing transition peak temperature and its broadening is the presence of amorphous SiO2 which enhances surface spins disorder and decreases crystallite size Anomalies associated with 025011-6 Kamran et al AIP Advances 7, 025011 (2017) FIG ZFC AC susceptibility (in-phase part) of CoCr2 O4 /(SiO2 ) nanoparticles with y = 0, 45 and 80 wt.% Ts and TF are also observed in ZFC AC susceptibility but very weak due to very low applied AC field IV CONCLUSIONS The SiO2 coating effects on the structural and magnetic properties of the CoCr2 O4 nanoparticles with concentration (y) = 0, 45 and 80 wt.% have been reported XRD studies confirm the formation of single phase normal spinel CoCr2 O4 nanoparticles and average crystallite size lies in the range of 19 – 28 nm for different SiO2 concentration The lattice parameter and average crystallite size show decreasing trend with increasing SiO2 concentration Uncoated CoCr2 O4 nanoparticles with y = % undergo a ferrimagnetic transition at Tc = 101 K, which decreases to 95 K with increasing SiO2 concentration up to 80% The Ts and TF transition values also shift to lower temperatures with increasing SiO2 concentration The shift in Tc , Ts and TF are attributed to finite size effects in these nanoparticles M-H loops shows decrease in Ms value with increasing SiO2 concentration, which was due to formation of smaller nanoparticles at higher SiO2 concentration The temperature dependent ZFC AC-susceptibility also indicates the decrease in the transition temperature Tc from 94 to 85 K with increasing SiO2 concentration In conclusion, SiO2 coating concentration can be very effective and useful in controlling the crystallite size and tuning the magnetic properties of the CoCr2 O4 nanoparticles ACKNOWLEDGMENTS Authors acknowledge Higher Education Commission of Pakistan for financial support M K Sharif et al., “Synthesis and characterization of Zr and Mg doped BiFeO3 nanocrystalline multiferroics via micro emulsion route,” Journal of Alloys and Compounds 667, 329–340 (2016) Y Han et al., “Effect of aliovalent Pd substitution on 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