Structural, optical, dielectric and magnetic studies of gadolinium-added Mn-Cu nanoferrites

Accepted Manuscript Structural, optical, dielectric and magnetic studies of Gadolinium-added Mn-Cu nanoferrites R. Rajesh Kanna, N. Lenin, K. Sakthipandi, A. Senthil Kumar PII: DOI: Reference:

S0304-8853(17)33019-6 https://doi.org/10.1016/j.jmmm.2018.01.019 MAGMA 63597

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

25 September 2017 21 December 2017 8 January 2018

Please cite this article as: R.R. Kanna, N. Lenin, K. Sakthipandi, A.S. Kumar, Structural, optical, dielectric and magnetic studies of Gadolinium-added Mn-Cu nanoferrites, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.01.019

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Structural, optical, dielectric and magnetic studies of Gadolinium‒added Mn‒Cu nanoferrites R. Rajesh Kanna1, N. Lenin1, K. Sakthipandi1*, A. Senthil Kumar2 1

Department of Physics, Sethu Institute of Technology, Kariapatti 626 115, Tamil Nadu, India

2

Department of Mechanical Engineering, Sethu Institute of Technology, Kariapatti 626 115,

Tamil Nadu, India Abstract Spinel ferrite with the general formula Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) was synthesized using the standard sonochemical method. The structure, optical, morphology, dielectric and magnetic properties of the prepared Mn1-xCuxFe1.85Gd0.15O4 nanoferrites were exhaustively investigated using various characterization techniques. The phase purity, secondary phase and crystallite parameters were studied from X-ray diffraction patterns. Fourier transform infrared spectra showed two absorption bands of transition metal oxides in the frequency range from 400 to 650 cm-1, which are related to asymmetric stretching modes of the spinel ferrites (AB2O4). Raman spectra have five active modes illustrating the vibration of O 2- ions at both tetrahedral (A) site and octahedral (B) site ions. The wide and narrow scan spectrum from X-ray photoelectron spectroscopy results confirmed the presence of Mn, Cu, Gd, Fe, C and O elements in the composition. The oxidation state and core level of the photo electron peaks of Mn 2p, Cu 2p, Gd 3d, Fe 2p and O 1s were analyzed. The influence of the Cu2+ concentration in Mn1xCuxFe1.85Gd0.15O4 on

the morphology, varying from nanorods, nanoflakes to spherical, was

explored on the basis of scanning electron microscopy images. Ultraviolet diffuse reflectance spectroscopy studies indicated that the optical bandgap (5.12‒5.32 eV) of the nanoferrites showed an insulating behavior. The dielectric constant, loss tangent and complex dielectric 1

constant values decreased with an increase in frequency with the addition of Gd 3+ content. A vibrating sample magnetometer showed that the prepared nanoferrites had a soft ferromagnetic nature. The magnetic parameter changed markedly with an increase in the Cu content in Mn1xCuxFe1.85Gd0.15O4

nanoferrites. The optical, dielectric and magnetic properties were

considerably enhanced with the addition of Gd3+ ions in the spinel nanoferrites. Keywords: Nanostructured materials, Optical properties, Dielectric response, Magnetic measurements *Corresponding author Tel

: +91-4566-308001-4 (4 Lines)

Fax

: +91-4566-30800/6

Email

: [email protected]

1. Introduction In the past few years, there has been an increase in the demand for microwave devices in automotive, consumer and industrial radar systems to reduce the cost of the device [1-3]. Devices such as computers, radar, wireless communication systems, medical equipment, microwave ovens and household appliances emit microwaves highly [2-4]. These microwaves affect the efficiency, protection and performance of electronic devices, which leads to a reduction in the lifetime of electronic equipment and causes environmental pollution [5-7]. Therefore, considerable attention has been focused on reducing microwave interference in effective microwave absorption materials, along with low density, strong absorption, low cost, wide frequency range and high resistivity [8-10].

2

Conventional microwave-absorbing materials occur in the form of ferrite (BaFe12O19, Ba3Co2Fe24O41), conductive polymer composites (PANI/Fe2O3) and carbon-based materials (CNTs/Polymer, RGO/MnFe2O4 nanocomposites, porous carbon) [1, 2, 8-10]. However, these materials cannot meet basic requirements such as a wide wave-absorbing band, lightness, thinness and strength [11, 12]. Spinel ferrites are considered to be the best material for microwave absorbers because of their stability, high resistivity, high magnetization and high Curie temperature [1, 13]. These spinel ferrites are widely used for different purposes such as in microwave devices, photo catalysis, magnetically guided drug delivery, high-frequency devices and hyperthermia [13-15]. The properties of spinel ferrites can be modified easily and extensively by doping rare-earth metals and different transition metals in order to optimize their use in special applications [15, 16]. The magnetic, dielectric and microwave absorption properties of magnetic nano materials can be enhanced by doping of rare-earth elements [17]. It has been reported in the literature that the microwave properties of spinel ferrites can be modified by doping of gadolinium because of the existence of super exchange interaction between iron and Gd3+ ions [18, 19]. Among spinel ferrites, manganese ferrite (MnFe2O4) has received considerable attention in the field of electronic devices and microwaves owing to their high electrical resistance and magnetic permeability [20, 21]. The application of MnFe2O4 in various fields involves magnetic recording media, biomedical, computer memory chips, rod antenna, radio frequency coil fabrication, transformer cores, etc [22, 23]. To improve the properties of MnFe2O4 material, Mn ions can be partially replaced with other transition metals such as Li, Ni, Co and Cu [24-26]. Copper-substituted manganese ferrite materials have been found to possess interesting structural and magnetic properties [27-29]. The substitution of Cu can alter the magnetic properties of 3

MnFe2O4 markedly because of its smaller ionic radius and the tenure of Cu 2+ ions into the spinel lattice induces lattice deformation, which leads to alterations in the magnetic properties [19, 30]. In addition, Gd doping with MnCuFe2O4 nanoferrites can improve their properties. Generally, various methods are used to prepare nanoferrites including co‒precipitation [31, 32], thermal decomposition [33], hydrothermal [34, 35], sol–gel [2, 3, 36] and sonochemical [37‒40]. Among these, the sonochemical method is the most commonly used process as a result of its good grain size control, simplicity and low cost. In this article, a novel material was prepared with the chemical formula Mn1xCuxFe1.85Gd0.15O4 (x=

0.2, 0.4, 0.6 and 0.8) using the sonochemical method. X-rays diffraction

(XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), ultra violet-diffuse reflectance spectroscopy (UV-DRS), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) and a vibrating sample magnetometer (VSM) and dielectric spectroscopy were carried out for microwave frequency applications. 2. Experimental 2.1. Preparation of Nanoferrites Gadolinium nitrate (Gd (NO3)3 .6H2O, 99%), manganese carbonate (MnCO3.H2O, 99%), cupric nitrate hexahydrate (CuNO2O6.3H2O, 99%), ferric nitrate nonahydrate (Fe (NO3)3 .9H2O, 99%), citric acid (C6H8O7.H2O, 99%), ammonia hydroxide (NH4OH) and deionized water (all from Merck) were used without additional purification. These nanoferrites were synthesized using a sonochemical reactor (Ultrasonic cell crusher, Lark, India). The metal carbonate, metal nitrate, citric acids and ammonia hydroxide materials were used as source materials to prepare Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Initially, metal nitrates and citric 4

acid were dissolved in deionized water at a 1:1 ratio of nitrates to citric acid. The mixed solution was sonicated for 2 h. The temperature of the sonication process was maintained constant at 80 °C. After sonication, the pH value of the solution was adjusted to ~7 by adding NH4OH dropwise in the prepared solution while stirring at ambient temperature. The pH-adjusted solution was allowed to dry in a hot-air oven at a constant temperature of 80 °C for 24h. The residue formed was collected and then calcined in air at 400 °C for 3h. The calcined powder was ground to obtain a homogenous mixture, which was sintered at 1000 °C for 24h in atmospheric air. A flow chart of the detailed synthesis process of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites is shown in Fig.1. The sintered ferrites were mixed with 2 wt% polyvinyl acetate as a binder and pressed into pellets by applying a uniaxial load of 5 kN using a hydraulic press. These pellets were gradually heated to about 300 °C to remove the binder. 2.2. Characterization of nanoferrites The structural analysis of the nanoferrites was carried out using an X-ray diffractometer (Smart Lab; Rigaku, USA) using CuKα as a radiation source with a constant voltage (40 kV) and a current rating (30 mA) in the scan range from 10° to 80°. Fourier transform infrared spectra of the nanoferrites were recorded (Spectrum 100; Perkin Elmer, USA) in the spectral range of 4000–400 cm−1 using the KBr pellet method with a consistent mixture of sample and KBr at a ratio of 1:100. The Raman spectra were recorded using a LABRAM-HR (Horiba, Japan) spectrometer equipped with a charge-coupled device detector and cooled liquid nitrogen. The Raman measurements were carried out using a laser source of 514 nm and the optical intensity at the sample surface was maintained at 0.1 mW to avoid damage. A spectral range of 100-1000 cm-1 was examined in the Raman measurement at 1800 grooves/mm grating. X-ray photoelectron spectroscopy measurements were performed using an AXIS ULTRA (Axis 165, 5

UK) spectrometer with monochromatic Al Ka (1486.6 eV) radiation. The surface morphology and elemental compositional analyses of the nanoferrites were carried out using a scanning electron microscope coupled with energy-dispersive X-ray spectroscopy (ULTRA 55; Zeiss, Germany). The diffuse reflectance spectrum of the nanoferrites was obtained using a UV-2450 spectrophotometer (Shimadzu, USA) in the range from 200 to 800 nm. The frequency-dependent dielectric constant and dielectric loss tangent were measured using an LCR (Wayne Kerr 6500B, UK) precision meter bridge in the parallel mode in the frequency range 20 Hz– 5 MHz. A silver paste was applied on both sides of the pellet for better ohmic contact. Magnetic measurements were performed on the prepared nanoferrites at room temperature using a vibrating sample magnetometer (BHV 50, Riken Denshi Co. Ltd. Japan). 3. Results and discussion 3.1. XRD analyses The XRD patterns of the prepared Mn1-xCuxFe1.85 Gd0.15O4 nanoferrites for the compositions of 0.2, 0.4, 0.6 and 0.8 are shown in Fig.2. The XRD patterns are consistent in the MnCuFe2O4 (JCPDS 74-2072) with (111), (220), (311), (222), (400), (422), (511), (440), (533) and (444) diffraction planes, the Gd2O3 (JCPDS 12-0797) with (222), (411), (404), (433), (620), (543) and (642) diffraction planes and the GdFeO3 peak at a 34.66° diffraction angle with (210) diffraction planes [41]. The MnCuFe2O4 and Gd2O3 peaks are corresponding to cubic phase (main phase) of the crystal structure which presents all the prepared Mn1-xCuxFe1.85Gd0.15O4 nanoferrites. The GdFeO3 peak is related to orthorhombic phase (secondary phase) of crystal structure, it present only in the highest concentration of Cu 2+. The percentage of phase fraction was calculated for the composition of x=0.8 [42] and the percentage of phase fraction values of

6

the cubic and orthorhombic phase is 92.17 % and 7.83 %, respectively. The crystallite sizes of the prepared nanoferrites were calculated using Scherer formula as follows [27, 41]: D=

(1)

where λ is the wavelength, β is the full-width half-maximum and θ is Bragg's angle. The crystal sizes obtained are listed in Table 1 and it shows that the crystalline size of the prepared nanoferrites gradually decreased with an increase in the copper concentration. The copper ions are substituted with manganese ions and the added gadolinium ions are enter into the octahedral site of the spinel crystal which replace the Fe3+ ions. The ionic radius of copper ion (0.73 Å) is smaller than the manganese ion (0.80 Å), which leads decreases in crystal size [30]. The Gd3+ ions have comparably large ionic radii in the element present in the precursor. This resulted in limited solvability in the spinel lattice and prevented grain growth [43]. The lattice constants (a) of the prepared nanoferrites were calculated using the following equation [43]: a=

(2)

where d is the d-spacing of the diffraction planes and h, k and l are the Miller indices of the diffraction planes in the XRD pattern. The lattice constant of spinel nanoferrites decreases from 8.439(x= 0.2) to 8.410(x= 0.8) Å. This decrease in the lattice constant can basically be explained as a result of exchange of large ionic radii of gadolinium in the arrangement of smaller iron ions [44]. This was also observed in the unit cell volume parameter. The X-ray density (ρXRD) of the prepared nanoferrites was obtained using the following equation [43, 44]:

ρXRD=

(3)

where Z is the number of molecules per unit volume, M is the molecular weight of the sample, N is Avogadro's number and a is the lattice constant. The structural parameters such as crystal size, 7

lattice constant, unit cell volume and X-ray density are listed in Table 1. The density of the crystal structure varied linearly with the amount of dopant, showing a slight increase with an increase in the copper content due to an increase in the molecular weight and a decrease in the lattice constant of the spinel nanoferrites. 3.2. FTIR spectra Fig.3 shows the FTIR spectra of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Normally, spinel ferrites mainly have two absorption bands: the first absorption band at v1 (650-550 cm-1), corresponding to the tetrahedral site and the second absorption band at v2 (525-390 cm-1), corresponding to the octahedral site [44]. Further, it is clear from Fig. 3 that the wavenumbers appeared at 578, 617 and 645 cm-1, which can be ascribed to the stretching vibration of the Fe‒O bond in the tetrahedral site. The dip found at wavenumber 467 cm -1 occurred due to the Fe‒O stretching bond in the octahedral site. Moreover, two absorption bands at 1429 and 1569 cm-1 appeared due to the symmetric and antisymmetric stretching bonds of C– C, respectively. Therefore, the presence of metal–oxygen bands in the tetrahedral and octahedral sites was confirmed in the prepared nanoferrites. 3.3. Raman spectra Raman spectroscopy was used to determine the molecular structure of the prepared Mn1xCuxFe1.85Gd0.15O4 (x=

0.2, 0.4, 0.6 and 0.8) nanoferrites. The obtained pattern is shown in Fig. 4.

These spinel structured nanoferrites with the space group Fd3m had five Raman active modes, namely, A1g, Eg, T2g (1), T2g (2) and T2g (3) [45]. All the Raman active modes were present in the observed spectra of the prepared nanoferrites. The Raman active mode near 600 cm-1 was due to the vibrations of metal oxygen in the tetrahedral site. The frequency mode near 400 cm-1 was due

8

to the vibrations of metal oxygen in the octahedral site [46]. The A1g active mode was present owing to the symmetric stretching of oxygen atoms on Fe–O bonds almost at 600 cm-1 in the tetrahedral sites. The Eg active mode was due to the symmetric bending of an oxygen atom with respect to the metal ion around 216-220 cm-1. The T2g (3) mode that appeared around 385-393 cm-1 occurred because of an asymmetric bending of an oxygen atom with respect to a metal ion and the T2g (2) mode observed around 270-283 cm-1 was an asymmetric stretch of Fe and O. The T2g (1) mode occurred because of a translational shift of Fe‒O (153-157 cm-1). The active mode positions of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites are listed in Table 2 along with the occurrence of peaks. It is evident from Table 2 that there was a shift in the frequency of the peaks in the Raman spectra of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites i.e. the frequency of A1g, Eg, T2g (1) and T2g (3) modes decreased and increased, respectively, with increasing Cu2+ content, where as the T2g (2) mode frequency increased up to x= 0.6 composition and then decreased due to the addition of Gd 3+. The Gd3+ ion affected the lattice structure in the place of cation distribution of metal ions which led to a change in the frequency range and the intensity of the Raman active modes [47]. 3.4. XPS Spectra The investigation of surface properties and the oxidation state of the prepared Mn1xCuxFe1.85Gd0.15O4 nanoferrites

was performed using X-ray photoelectron spectroscopy; it was

used to determine the physical and surface properties of the nanoferrites. The XPS spectrum is the most sensitive and powerful technique for surface analysis. The surface elements of the nanoferrites were determined from the intensities of peaks with the respective ratios of the composition. The energy of the photo electrons from the nanoferrites was determined from the survey spectrum of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites 9

and is shown in Fig. 5. The elements Mn, Cu, Gd, Fe and O in the prepared nanoferrites were visualized clearly within the binding energy of 0‒1300 eV in the wide scan spectrum (Fig. 5a). Moreover, the oxidation state and the presence of individual elements of the nanoferrites were shown by the narrow scan XPS spectrum. The narrow scan XPS spectrum of the O 1s, Mn 2p, Fe 2p, Cu 2p and Gd 3d is shown in Fig. 5b‒5f. The narrow scan O 1s spectrum of the prepared nanoferrites is shown in Fig. 5b. The O 1s spectra yielded more information on the structure of the nanoferrites than the other spectra because of an electron located in the oxygen region. It yielded two peaks with binding energy peaks at 529.57±0.32 eV due to ionic bonding or double bonding to an oxygen atom with a cation. The binding energies at 531.10±0.21 eV corresponded to covalent bonding to two atoms (Fe-O-Fe/Cu-O-Cu/Fe-O-Cu) that appeared as a spinel ferrite structure [48‒51]. The XPS spectra (Fig. 5c) of the Mn2+ surface element showed binding energies at 710.66±0.31 (2p3/2) and 724.65±0.42 (2p1/2) eV across the intense peaks, respectively. It fact, high-spin Mn2+ was evident in the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites [48, 51]. The Fe 2p narrow scan spectra showed Fe 2p3/2 and Fe 2p1/2 peaks, respectively, at binding energies of 710.66±0.52 and 724.65± 0.41 eV based on the spin–orbit coupling exchange interaction involving the unpaired outer 3d electrons and the left over 3s electron of the atom. Fig. 5d shows that the peaks of Fe 2p3/2 and Fe 2p1/2 are separated by 13.83 eV, which indicates that iron had a Fe3+ electron state [49, 51]. The distribution of the Cu content between tetrahedral and octahedral sides has a major influence on the spinel ferrite structure. The Cu 2p narrow scan spectrum (Fig. 5e) contains doublet peaks such as Cu 2p3/2 and Cu 2p1/2 peaks owing to spin–orbit coupling. Major peaks were present at 933.38±0.32 and 953.67±0.31 eV at binding energies of the Cu 2p3/2 and Cu 2p1/2 intense peaks, respectively. These peaks occurred due to the presence of Cu2+ on octahedral sites. The minor peaks at binding energies of

10

937.53±0.21 and 958.31±0.42 eV peaks occurred due to the presence of Cu 2+ on the tetrahedral site. Further, the peaks at 943.18±0.11 and 961.53±0.12 eV represented the satellite peaks of Cu 2p spectra. The presence of the main peaks and the satellite peaks confirmed the presence of Cu2+ in spinel nanoferrites [49]. The surface analysis of Gd 3d is shown in Fig. 5f, where Gd 3d5/2 and Gd 3d3/2 peaks were located at, respectively, 1186.83±0.23 and 1226.58±0.12 eV. The spin–orbit splitting occurred at 39.75 eV, which indicates that the oxidation state of Gd is 3+ [52, 53]. 3.5. UV-DRS spectra The influence of gadolinium on manganese copper nanoferrites was studied by UV-DRS. The UV- DRS spectra of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites are shown in Fig. 6. The absorption values were observed to be in the range of 200300 nm for the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. The absorption values were 242 (x= 0.2), 239(x= 0.4), 234 (x= 0.6) and 231 (x= 0.8) nm. The direct and indirect optical bandgap energy values of nanoferrites were determined using the following equation, which is related to the photon energy and absorbance as [54]: E(gap) =

(4),

and

hvα

(

- Egap)n

(5)

where h is the Planck constant, v is the frequency, α is the absorbance,  is the wavelength of absorption and n is the different types of electronic transitions (n= ½ and 2 for, respectively, direct and indirect transition). A direct transition graph (Tauc plotted between (Ephot) 2and Ephot) 11

was used to determine the intercept values of direct bandgap energy (Fig. 6b). The optical direct bandgap of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites were 5.27, 5.33, 5.41 and 5.48 eV, respectively. Fig. 6c shows an indirect transition (Tauc plots of (1/2vs.Ephot) for all the prepared nanoferrites. The indirect bandgap values of the prepared nanoferrites were 5.12, 5.21, 5.28 and 5.32 eV, which were estimated from the extrapolation of the Tauc plot at the intersection with the energy axis. Generally, the absorption is indirectly proportional to the bandgap energy value of nanoferrites; specifically, if the bandgap energy value increases, the wavelength of absorption decreases and vice versa. This was clear from the observed results, which are presented in Table 4. The optical bandgap increases with a resultant decrease in the crystal size with an increase in the dopant concentration of Cu 2+. This indicates an inverse correlation of crystal size with optical bandgap energy due to the defects within the spinel structure and results in micro strain. This causes a split in the ionic radii of Gd 3+ and Fe3+ ions, which leads to an increase in the optical bandgap energy of the spinel nanoferrites [55, 56]. From the literature, the optical bandgap energy values of the pure Mn–Cu nanoferrites occur in the range of 1.73‒2.81 eV [27, 28]. However, the addition of the rare-earth gadolinium ion to Mn– Cu nanoferrites leads to a blue shift (5.12‒5.32 eV) in the absorption band of the nanoferrites due to the high resistivity of Gd3+. This implies that the prepared nanoferrites could have potential applications in microwave devices [20]. 3.6. SEM and EDX analysis SEM images of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites are shown in Fig. 7. It shows that the materials have nano structured morphology. Interestingly, the morphology of the nanoferrites varies with an increase in the Cu 2+ content. The morphology of the composition x= 0.2 has a nanorods-like structure as shown in Fig. 7a. Fig. 7b 12

shows a nanoflakes-like morphology for the composition of x= 0.4. Further, an increase in copper content beyond x= 0.4 leads to a spherical morphology (Fig. 7c‒7d). The morphology shape of the nanoferrites mainly depends on their interaction with stabilizers and preparation method [57]. Additionally, change in morphology occurs when the nucleation and growth process are affected by dangling bonds and the charge of the surface states in the starting materials [58]. In the present investigation, sonochemical method cause high temperature and pressure in the solution due to the bubbles formation and collapse phenomena. These phenomena create cavitations around the particles, cause different shapes of the prepared nanoferrites [59]. Moreover, the nucleation growth occurred when the increase of copper content because of the behavior of electrostatic and Vander Waals forces interactions leads to changes in morphology [30]. Similar reports are available for the changing morphology when the copper content increases [30, 60]. The EDX spectra of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites are shown in Fig. 8. It shows the peaks for O, Mn, Fe, Cu and Gd elements in the prepared nanoferrites. The element compositions of Mn: Cu: Fe: Gd: O estimated from the EDX spectrum are 0.8: 0.19: 1.85: 0.15: 4 (x= 0.2), 0.61: 0.40: 1.85: 0.15: 4 (x= 0.4), 0.41: 0.59: 1.84: 0.14: 4 (x= 0.6), 0.21: 0.81: 1.85: 0.15: 4 (x= 0.8), respectively. No other material was found and the prepared nanoferrites were uniform, with a fixed stoichiometric ratio. Hence, the sonochemical method promotes the formation of spinel nanoferrites and the particle size of the prepared nanoferrites is on average 30-50 nm. 3.7. Dielectric property The dielectric behavior of ferrites is caused by the electric dipole moments developed as a result of the charge transfer caused by ion exchange (divalent and trivalent metal cations) with

13

in the spinel structure. The dielectric constant (ε) of the Mn1-xCuxFe1.85Gd0.15O4 (x=0.2, 0.4, 0.6 and 0.8) nanoferrites were measured using the following equation [61]: ε =

(6)

where C is the capacitance of the pellet, d is the thickness, A is the cross sectional area of the flat surface pellet and ε is the dielectric permittivity of the free space. Dielectric constants of the prepared nanoferrites are shown in Fig. 9. The dielectric constants gradually decrease with increasing frequency at lower frequencies for all compositions of the prepared Mn1xCuxFe1.85Gd0.15O4 (x=

0.2, 0.4, 0.6 and 0.8) nanoferrites. At higher frequencies, the dielectric

constant values were identical for all compositions of the prepared nanoferrites. The loss tangent (tanδ) and complex dielectric constant (ε") values of the prepared nanoferrites were measured using the following equations [61]:



tanδ =

(7),

and ε" = ε tanδ

(8)

where ρ is the resistivity and f is the frequency of the applied field. The variation in loss tangent and the complex dielectric constant as a function of frequency are shown, respectively, in Fig. 10 and Fig. 11. The values of tan δ and ε" decreased with an increase in the applied frequency and remained constant at higher frequencies and were similar to the dielectric constant (ε). The dielectric constant, loss tangent and complex dielectric constant values of the prepared Mn1xCuxFe1.85Gd0.15O4 (x=

0.2, 0.4, 0.6 and 0.8) nanoferrites are presented in Table 5 for ease of

comparison. The variation in the dielectric constant can be explained on the basis of polarization. 14

At this point, a decrease in the dielectric constant at lower frequencies is mostly caused by spacecharge polarization [62]. At higher frequencies, the electric dipoles are inability to follow the rapid variation with the applied electric field. Thus, the dielectric constant (ε) is independent of frequency and constant in the high-frequency region [61]. The loss tangent leads to the occurrence of structural inhomogeneities and also polarization lags behind the altering field applied [63]. The loss tangent is found to decrease lower than the dielectric constant and the complex dielectric constant at low frequencies and the variation is the same at higher frequencies. The variations in the dielectric constant and loss tangent with the prepared nanoferrites are shown in Fig. 12. It is clear that the values of ε, ε" and tanδ decreased with an increase in the Cu concentration owing to the addition of Gd3+ ions in the Mn‒Cu nanoferrites. The Gd3+ ions are good electrical insulators with high electrical resistivity. They may increase the resistivity, which decreases the polarization and dielectric constant of the nanoferrites [64]. Interestingly, dielectric study of the prepared nanoferrites indicated that the dielectric properties improved more with the addition of Gd3+ compared with Mn‒Cu nanoferrites [27, 28] and this very low loss tangent material is used for high-frequency applications to absorb unwanted microwaves from electronic devices [20, 28, 41]. 3.8. Magnetic property The magnetic property of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites was analyzed using a vibrating sample magnetometer at room temperature. The magnetic hysteresis loop of the prepared nanoferrites is shown in Fig. 13. It is clear from the results that the prepared nanoferrites show a soft and ferromagnetic behavior. It is clear from the magnetic hysteresis loop that the saturation magnetization decreases from 29.38 (x= 0.2) to 18.25 (x= 0.8) emug-1 with an increase in the copper content in the spinel nanoferrite when the crystallite size of 15

the prepared nanoferrite decreases from 66.52 to 33.40 nm. This can be ascribed to the surface effect of the magnetic nanoferrites caused by their small crystallite size; also, it was found that the gadolinium ions added generally occupied octahedral (B) sites because of their larger ionic radii [13]. Consequently, the addition of Gd3+ ions to manganese copper ferrite lattices is similar to the addition of non-magnetic atoms at the octahedral (B) sites and, for this reason, the saturation magnetization of octahedral (B) sites is reduced [41]. The saturation magnetization of spinel nanoferrites is the difference between the saturation magnetization of octahedral (B) sites and tetrahedral (A) sites, and as a result, the saturation magnetization is decreased [65]. From Fig.14, it is clear that the coercivity increases with an increase in the Cu 2+ concentration up to x = 0.6 in Mn1-xCuxFe1.85Gd0.15O4 and decreases when x= 0.8. The remnant magnetization values also showed a trend similar to that of the coercivity values. Generally, coercivity depends on factors such as strain, magneto‒crystallinity, grain size, morphology, crystal phase and anisotropy [56]. In XRD measurements, the secondary phase is observed at a higher concentration of copper substitution, which decreases the coercivity values [41, 66, 67]. In addition, coercivity increases with the Cu2+ content, which is a result of an increase in effective anisotropy due to strong lattice deformation [13]. The anisotropy constant (K) and the squareness ratio were calculated using the following equations [50, 56]: K=

(9),

and Squareness ratio=

(10)

where Mr is the remnant magnetization, Ms is the saturation magnetization and Hc is the coercivity. The anisotropy constant and squareness ratiovalues of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 16

0.4, 0.6 and 0.8) nanoferrites are in the range of 1.674‒2.583 KOe and 0.068‒0.204, respectively. The squareness ratio value of the prepared composition is less than 0.5, which points to the uniaxial anisotropy contribution of internal strains in standard magnetic materials. This uniaxial anisotropy in magnetic nanoferrites occurred as a result of surface effects [50, 56]. The Bohr magneton (ηB) and Yafet–Kittel (Y–K) angles were calculated from the following equations: [66]

ηB =

(11),

ηB = (6+x) Cos αY-K ‒ 5 (1‒x)

(12)

and

where M is the molecular weight, Ms is the saturation magnetization and x is the concentration of Cu2+ substitution. The Y‒K angle increases with an increase in the Cu 2+ content. It is implied that the existence of canted spin model of magnetization in prepared spinel nanoferrites and also the spin arrangement is appropriate on the octahedral (B) sites leading to a reduction in the A–B exchange interaction [66, 68]. Magnetic parameters such as Ms, Hc, Mr, squareness ratio, anisotropy constant, ηB and Y‒K angles are listed in Table 6. The magnetic behavior of Gdadded Cu‒Mn nanoferrites showed soft magnetic characteristics. Moreover, the low values of coercivity make them good candidates for application in microwave devices [20]. 4. Conclusion Spinel Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites were successfully prepared using a simple sonochemical reactor method. X-ray diffraction study confirmed the presence of a cubic spinel structure and the secondary phase (GdFeO3) occurred at a higher 17

concentration of Cu2+ ions. FTIR absorption spectra showed Mn‒Gd‒Cu nanoferrites based on tetrahedral (578, 617 and 645 cm−1) and octahedral (467 cm−1) group complexes. Raman active modes shifted with the frequency because of the effects of rare-earth ions. Detailed surface and core-level spectra of Mn 2p, Cu 2p, Gd 3d, Fe 2p and O 1s peaks were analyzed from the XPS spectrum. The UV-DRS showed that optical bandgap energy values red shifted (5.12‒5.32 eV) in the absorption band of the nanoferrites due to the high resistivity of Gd3+. The SEM surface morphology of the nanoferrites changes from nanorods, nanoflakes structure to a spherical-like structure. The dielectric constant, loss tangent and complex dielectric constant had low values and showed a tendency to decrease at lower frequencies. At higher frequencies, the values remained constant. Magnetic parameters such as Ms, Hc, Mr, squareness ratio, anisotropy constant, ηB and Y‒K angles were obtained from VSM. Saturation magnetization increased with an increase in the Cu2+ content and other values increased with an increase in the Cu 2+ content up to x= 0.6 and then decreased. Improved optical, dielectric and magnetic properties are useful to gain an understanding of the effects of Gd3+ on Mn‒Cu nanoferrites and they could potentially be useful in high-frequency microwave device applications. Acknowledgements K. S is grateful to the Science and Engineering Research Board (SERB), New Delhi, India, for providing the financial support to carry out this research work (Sanction no.: SR/FTP/PS-068/2014). R. R acknowledges that a part of the work (characterization only) was carried out at the CeNSE, under INUP, at IISc, which was sponsored by DeitY, MCIT, Government of India.

18

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Figure Captions Fig. 1 The flow chart of detailed synthesis process of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 2 XRD pattern of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites

28

Fig. 3 FTIR spectra of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 4 Raman spectra of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 5 XPS spectra of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 6 UV‒DRS spectra of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites Fig. 7 SEM image of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 8 EDS pattern of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 9 Dielectric constant of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 10 Dielectric loss of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 11 Complex dielectric constant of prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 12 Variation of dielectric constant and dielectric loss of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 13 Hysteresis loop of the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites. Fig. 14 Variation of Ms, Mr and Hc with the prepared Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites

29

MnCO3.H2O

Deionized water

CuNO2O6.3H2O

Fe (NO3)3 .9H2O

Mixing under the sonication at 80 °C for 2h

Gd (NO3)3.6H2O

MN: CA: 1: 1

C6H8O7.H2O

Added drop wise under stirring NH4OH Solution at pH-7 (pH ~7) Heating at 80 °C Xero-gel

Calcinated at 400 °C Burnt Ash Grinding and Sintered at1000 °C Nanoferrites powder Mn1-xCuxFe1.85Gd0.15O4

30

Fig. 1

31

Fig. 2

32

Fig. 3

33

Fig. 4

34

a) Wide scan spectrum of nanoferrites

b) Narrow scan spectrum of O 1s

c) Narrow scan spectrum of Mn 2p

d) Narrow scan spectrum of Fe 2p 35

a) Narrow scan spectrum of Cu 2p

b) Narrow scan spectrum of Gd 3d

Fig. 5

36

a) UV Spectra

b) Direct bandgap

c) Indirect bandgap

Fig. 6

37

a) SEM image of Mn0.8Cu0.2Fe1.85Gd0.15O4

b) SEM image of Mn0.6Cu0.4Fe1.85Gd0.15O4

c) SEM image of Mn0.4Cu0.6Fe1.85Gd0.15O4

d) SEM image of Mn0.2Cu0.8Fe1.85Gd0.15O4

Fig. 7

38

a) EDS pattern of Mn0.8Cu0.2Fe1.85Gd0.15O4

b) EDS pattern of Mn0.6Cu0.4Fe1.85Gd0.15O4

c) EDS pattern of Mn0.4Cu0.6Fe1.85Gd0.15O4

d) EDS pattern of Mn0.2Cu0.8Fe1.85Gd0.15O4

Fig. 8 39

Fig. 9

Fig. 10 40

Fig. 11

Fig. 12

41

Fig. 13

Fig. 14

42

Table 1 Average Crystalline size, lattice constant, X-ray density and surface area of Mn1xCu xFe1.85Gd0.15O4 (x=

0.2, 0.4, 0.6 and 0.8) nanoferrites

Parameter

x= 0.2

x= 0.4

x= 0.6

x= 0.8

Crystal size (nm)

66.52

44.63

40.49

33.40

Lattice constant (a) (Å)

8.439

8.413

8.415

8.410

Unit cell volume (Å)3

600.99

595.51

594.88

594.82

5.098

5.340

5.390

X-ray density (ρXRD) (g/cm3) 5.039

Table 2 Position of various modes in Raman spectra of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites Raman peaks

Due to

(cm-1) A1g Eg T2g (3) T2g (2) T2g (1)

Asymmetric stretch of O symmetric bending of O Asymmetric bending of O Asymmetric stretch of Fe-O Translational shift of Fe-O

Composition x= 0.2

x= 0.4

x= 0.6

x= 0.8

606.1

593.9

601.3

595.7

219.4

216.5

217.2

216.5

386.5

387.2

392.4

385.8

270.2

275.6

283.2

279.1

153.5

157.1

154.9

156.4

43

Table 3 Binding energy of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites

Composition O 1s (eV)

Mn 2p

Fe 2p

Cu 2p

Gd 3d

(eV)

(eV)

(eV)

(eV)

2p3/2

2p1/2

2p3/2

2p1/2

2p3/2

2p1/2

3d5/2

3d3/2

x= 0.2

529.46

641.57 653.28

711.20 724.26

933.31 954.38

1187.42 1226.58

x= 0.4

529.69

641.16 653.42

710.42 724.36

933.46 953.62

1186.28 1226.47

x= 0.6

529.91

641.53 653.59

710.99 725.12

933.67 953.34

1186.66 1226.73

x= 0.8

529.25

640.74 653.37

710.04 724.86

933.08 953.09

1186.97 1226.56

Table 4 Absorbance and bandgap of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites Composition

Absorbance

Direct bandgap

Indirect bandgap

(nm)

(eV)

(eV)

x= 0.2

242

5.27

5.12

x= 0.4

239

5.33

5.21

x= 0.6

234

5.41

5.28

x= 0.8

231

5.48

5.32

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Table 5 Dielectric constant, complex dielectric constant and dielectric loss of Mn1xCu xFe1.85Gd0.15O4 (x=

Composition

0.2, 0.4, 0.6 and 0.8) nanoferrites Dielectric

Complex dielectric

Dielectric loss

constant (ε’)

constant (ε’’)

x= 0.2

125.52

430.53

3.43

x= 0.4

43.79

99.40

2.27

x= 0.6

10.66

11.30

1.06

x= 0.8

9.12

7.66

0.84

Table 6 Magnetic parameters of Mn1-xCuxFe1.85Gd0.15O4 (x= 0.2, 0.4, 0.6 and 0.8) nanoferrites Parameter

x= 0.2

x= 0.4

x= 0.6

x= 0.8

Magnetization (Ms) (emug-1)

29.38

28.05

25.22

18.25

Remanent magnetization (Mr) (emug-1)

2.01

2.18

5.74

3.72

Coercivity (Hc) (Oe)

55.84

58.04

142.32

138.75

Squareness ratio (Mr/Ms)

0.068

0.077

0.227

0.204

Anisotropy constant (K) (Oe)

1.674

1.661

3.662

2.583

Bohr magneton(ηB) (μB)

0.955

1.171

1.061

0.773

Yafet-Kittel (Y-K) angle (θ)

36.96

49.32

62.36

74.88

45

Highlights  Enhance the optical bandgap energy with facilitate of Gd3+ in Mn-Cu nanoferrites.  Tuning of dielectric behavior of nanoferrites for microwave frequency application  Nanoferrites show ferromagnetism with low saturation and remanence magnetisation

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