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Tuning the microstructural, optical and superexchange interactions with rare earth Eu doping in nickel ferrite nanoparticles
Materials Chemistry and Physics 241 (2020) 122383
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Tuning the microstructural, optical and superexchange interactions with rare earth Eu doping in nickel ferrite nanoparticles Mritunjoy Prasad Ghosh a, Saurabh Sharma b, Harendra Kumar Satyapal b, Kamar Tanbir b, Rakesh Kumar Singh b, Samrat Mukherjee a, * a b
Department of Physics, National Institute of Technology Patna, Patna, 800005, Bihar, India Aryabhatt Centre for Nanoscience and Nanotechnology, Aryabhatt Knowledge University, Patna, 800001, Bihar, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Synthesis of rare earth doped nanoferrites. � Reduction in superexchange interaction at 300 K. � Enhancement in coercivity at 5 K. � Observation of superparamagnetism at 300 K.
A R T I C L E I N F O
A B S T R A C T
Keywords: Rare earth dopant W–H plots Raman spectra Magnetic properties Superexchange interactions
A methodical study on structural, magnetic and optical properties of Eu doped NiFe2O4 nanoparticles with generic composition NiEuxFe2-xO4 (x ¼ 0.00, 0.02, 0.04 and 0.06), synthesized via co-precipitation technique has been reported in this article. The fabrication of single phase ferrite nanoparticles was verified using X-ray powder diffractograms. The analysis of W–H plots revealed the presence tensile strain in the samples along with an average crystallite size of 20 � 4 nm. The existence lowering of cubic symmetry at octahedral sublattices was detected by vibrational Raman spectra recorded at 300 K. The values of indirect optical band gap were noted in the range of 1.76 � 0.02 eV. The hysteresis loops at room temperature supported the paramagnetic character of Eu3þ ions in spinel structure. The evidence of reduced A-O-B superexchange interaction along with reduction of saturation magnetization was also found for higher Eu content nanoparticles at 300 K. The coercivity was observed to increase at 5 K. The blocking temperature and thermo-magnetic irreversibility was reduced with increasing Eu concentration.
* Corresponding author. E-mail address: [email protected] (S. Mukherjee). https://doi.org/10.1016/j.matchemphys.2019.122383 Received 19 March 2019; Received in revised form 18 October 2019; Accepted 28 October 2019 Available online 31 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. X-ray diffractograms obtained at room temperature.
1. Introduction
various tops down and bottoms up routes but among these, the standard chemical co-precipitation method is the easiest process to attain nano particles with fewer defects and also dispenses better size homogeneity in nano order. It is familiar that the properties of ferrite nanoparticles are very sensitive to fabrication techniques and cations distribution [7]. The NiFe2O4 nanoparticles display excellent stability, soft ferrimagnetic nature, moderate Curie temperature and also semi conducting character [8,9]. The rare earth Eu3þ ions consists of unpaired 4f electrons along with a magnetic moment of 3.4 μB which is observed experimentally although calculations show zero magnetic moment. It also depicts paramagnetic nature above 90.4 K (TN) whereas magnetic behavior is found due to Van-Vleck effect at certain low temperature [10,11]. In case of isolated Eu3þ and Sm3þ rare earth ions, a significant disagreement between theoretical magnetic moment and experimental magnetic moment is observed. These two particular rare earth ions have multiplet spin in both ground and first excited states. The excited multiplet state is generally close enough to the ground multiplet state; thereby the excited state overlaps with ground state. This phenomenon results in the changing of net magnetic moment of the system as well as a notable variation from theoretical magnetic moment. In case of isolated Eu3þ ions the theoretical magnetic moment is zero whereas experimentally a magnetic moment of 3.4 μB is observed. This behavior is known as
Nanotechnology permits researchers to synthesize nano size systems where classical laws of physics fail to explain the physical phenomenon and things appear differently at nano scale. The past three decades had seen a tremendous development in synthesis method of size/shape controlled, stable and monodisperse magnetic nanoparticles. Nano materials consist of large surface area hence more sensitive with external reagents. Magnetic nanoparticles are able to respond according to magnetic field and can be controlled easily with external field. The physical and magnetic properties of such particles can be tuned by varying diameter, dopant ions and its concentration depending upon applications [1–3]. The ferrite nanoparticles have broad range of in dustrial applications such as magnetic biosensor, water purifications, magnetic particles imaging (MPI), heat source in hyperthermia, catalytic exercise, targeted drug delivery, magnetic resonance imaging (MRI) etc. For biomedical applications purpose, instead of bare particles the nanoparticles are coated with non-toxic oleic acid, PVP, PVA or suitable elements of carbon. Bulk NiFe2O4 consists an inverse spinel cubic (FCC) structure where Fe3þ ions are distributed uniformly in both tetrahedral and octahedral site. The Ni2þ ions prefer to occupy octahedral voids. The ferrimagnetism in spinel ferrites originates from A-B super exchange interactions [4–6]. Nickel ferrite nanoparticles can be fabricated using 2
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Materials Chemistry and Physics 241 (2020) 122383
Fig. 2. Williamson-Hall (W–H) plot of the samples.
Van-Vleck effect [11–13]. Although several studies on rare earth (RE) ions doped in spinel ferrites were reported but only a few articles rep resented a systematic exploration of the RE dopant impact on structural and magnetic character of ferrites nanoparticles [14–16]. The behavior of rare earth Eu3þ ions inside a cubic spinel ferrite system considering the Van-vleck effect has not been explored systematically. The Eu3þ ions are also able to interrupt the superexchange interactions above 90.4 K due to its non-magnetic nature in ferrites. In this work, we have presented the impact of Eu3þ dopant on soft ferrimagnetic nickel ferrite nanoparticles. Due to its larger size, a slight amount of Eu3þ can distorted the local structure of cubic (FCC) spinel ferrites. The Eu3þ ions can vary the magnetic properties of NiFe2O4 nanoparticles differently at well above and below N� eel temperature (TN). The microstructural, magnetic and optical behavior of nickel ferrite nanoparticles with rare earth Eu3þ ions has been explored sys tematically in this article. A connection between the structural and magnetic properties has been set up for Eu doped nickel nanoferrite systems. The doping of Eu3þ ions also have a fundamental role for controlling crystallites size growth along with development of micro stain in tiny crystal of ferrites. The optical absorption region may diverge due to Eu implantation in ferrites.
ground in a mortar. The obtained power was then sintered at 500 � C for 5 h and cooled down gently. The prepared samples were then named as Eu-0, Eu-2, Eu-4 and Eu-6 according to increase of Eu content. Further all the characterizations were done using those powdered samples [5]. 2.2. Characterizations The spinel phase identification, microstructural, optical and mag netic features of prepared Eu doped samples was characterized using Xray diffraction, High resolution transmission electron microscopy, vibrational Raman spectroscopy, UV–Visible spectroscopy and VSM measurements. The X-ray diffraction graphs of all Eu doped samples were taken using Rigaku Advance X-ray Diffractometer with a source of copper having wavelength of 1.5406 Å (Kα - line). The X-ray diffracto grams were observed with a step rate 0.020/sec over the angular range of 20� � 2θ � 80� . The structural images of the synthesized nano particles were taken using high resolution transmission electron mi croscopy (HRTEM, JEM-2100F, JEOL, Japan, Accelerating voltage ¼ 200 kV). The Raman spectra were obtained at 300 K over the range 200 cm 1 to 800 cm 1 by Renishaw Raman spectrometer with a diode laser source of 473 nm wavelength. The UV DRS spectrometer (UV-2550, Shimadzu, USA) was used to perform optical behavior mea surement. Magnetic properties of all samples were characterized by
2. Experimental details 2.1. Synthesis process
Table 1 Structural and refined parameters of all Eu doped nickel ferrite nanoparticles.
The nickel ferrite nanoparticles containing different weight per centage of rare earth Eu3þ ions with a generic formula NiEuxFe2-xO4 (x ¼ 0.00, 0.02, 0.04 and 0.06) were synthesized via standard coprecipitation method [7,17]. In this process, stoichiometric amount of chemicals Fe(NO3)3⋅9H2O, Ni(NO3)2⋅6H2O and Eu(NO3)⋅6H2O were dissolved in deionized water of 200 ml. The solution was kept in a beaker and stirred continuously using magnetic stirrer. The precursor NaOH solution was then put drop wise slowly to attain the final pH 10 of the entire solution. The solution was corroded at 80 � C for next 2 h so that no reagent left unreacted. Thus the precipitate was washed using distilled water and ultimately in alcohol several times to attain pH 7. After that, the precipitate was dried under ambient atmosphere and
Sample-Id
Eu-0
Eu-2
Eu-4
Eu-6
Space group
Fd3m
Fd3m
Fd3m
Fd3m
8.357 � 0.001 21 � 1 2.91 � 0.01
8.379 � 0.001 24 � 1 4.53 � 0.01
8.385 � 0.001 21 � 1 3.65 � 0.01
2.26 1.79 8.81 1.07
2.49 1.98 10.34 1.17
2.21 1.75 6.89 1.06
Cell parameters a (Å) 8.348 � 0.001 D (nm) 16 � 1 Strain (X 0.21 � 0.01 3 10 ) Refinement factors Rwp (%) 2.11 Rp (%) 1.66 R2F (%) 9.60 χ2 1.13
3
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Table 2 Contain ideal cationic distribution and various structural parameters of all samples. SampleId
Asite
B-site
rA (Å)
rB (Å)
I220/ I222
I422/ I222
Eu-0 Eu-2
Fe3þ Fe3þ
0.645 0.645
0.667 0.671
2.31 1.76
0.98 0.92
Eu-4
Fe3þ
0.645
0.673
1.38
0.78
Eu-6
3þ
Ni2þFe3þ Ni2þEu3þ0.02 Fe3þ0.98 Ni2þEu3þ0.04 Fe3þ0.96 Ni2þEu3þ0.06 Fe3þ0.94
0.645
0.676
1.47
0.68
Fe
Table 3 Contain hopping length (LA and LB), theoretical lattice constant (ath) and oxygen positional parameter (u) of all Eu doped nanoparticles. Sample-Id
LA (Å)
LB (Å)
ath (Å)
u
Eu-0 Eu-2 Eu-4 Eu-6
3.615 3.619 3.628 3.631
2.951 2.955 2.962 2.965
8.393 8.404 8.410 8.417
0.388(2) 0.388(1) 0.387(9) 0.387(8)
16 T-VSM-PPMS (Quantum design). Fig. 3 (b). TEM image of Eu-6 ferrite nanoparticles.
3. Results and discussion
temperature and are shown in Fig. 1. The diffraction of X-ray from crystallographic plans (220), (311), (222), (400), (440) and (511) satisfying Bragg’s condition are associated with cubic spinel structure [5]. All observed characteristic peaks of synthesized nanoparticles were
3.1. Structural studies The X-ray diffractograms of all Eu doped nanoparticles taken at room
Fig. 3 (a). TEM image of Eu-0 ferrite nanoparticles. 4
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Materials Chemistry and Physics 241 (2020) 122383
Fig. 4. Room temperature Raman spectra of all Eu doped samples.
Fig. 5. Tauc plots of all Eu doped nickel ferrite nanoparticles.
found to match with JCPDS card no. 10–325 along with space group Fd-3m. The Rietveld refinement of diffraction patterns were carried out using GSAS programme with EXPUGI interface and pseudo voigt
function was adopted for profile fitting. The structural and refinement parameters were collected in Table 1. The goodness of fit (χ2) near unity along with reliability factors (RP, RWP, R2F) less than 10% confirmed the 5
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For investigation on structural distortion produced by large size dopant ions, it is important to have information about the average volume of voids that can be occupied by the ions. The average ionic radius of voids at tetrahedral (A) and octahedral (B) sites is evaluated using the following expressions [22,23].
Table 4 Vibrational Raman active modes of all Eu doped nickel ferrite nanoparticles. Sample-Id Eu-0 Eu-2 Eu-4 Eu-6
Raman active modes (cm
1
)
A1g(1)
A1g(2)
B2/E
T2g(3)
T2g(2)
Eg
T2g(1)
696 702 705 706
658 675 684 685
—— 641 652 653
571 551 562 559
481 468 477 475
329 305 317 314
205 197 200 198
rA ¼ [C(Fe3þ) r(Fe3þ) þ C(Ni2þ) r(Ni2þ)] � � � � � � � � � 1 C Fe3þ r Fe3þ þ C Ni2þ r Ni2þ rB ¼ 2 � � � �� þ C Eu3þ r Eu3þ where C represents concentration of ions, r(Fe3þ), r(Ni2þ) and r(Eu3þ) corresponds to the ionic radius of Fe3þ (0.645 Å), Ni2þ (0.69 Å) and Eu3þ (0.947 Å) ions respectively. The minute increment in ionic radius (rB) at the B sites is due to the size difference between Fe3þand Eu3þ ions. The theoretical lattice parameter (ath) can be obtained using the stated relation [13,24]. � � �� �� 8 ath ¼ rA þ Ro þ √3 rB þ Ro 3√3 where Ro (1.32 Å) represents the radius of oxygen ions. The theoretical lattice parameter (ath) is found to increase with increase of Eu content, collected in Table 3. A similar kind of trend was also noticed for lattice constant estimated from the Rietveld refinement of X-ray diffraction data. The oxygen locus parameter (u) is another crucial parameter to get an idea about slight rupture present in spinel structure. The obtained value of u was found minutely larger with respect to ideal value 0.375 (considering origin at tetrahedral site). It is due to the push of ions present in B-sites which reflects in “u” value. The oxygen locus param eter (u) is commonly estimated by following equation [20,25]. � � � � 1 1 u ¼ R0 þ rA þ 4 ath √3
Fig. 6. Hysteresis loops of all the samples obtained at room temperature.
strong harmony between theoretical and experimental diffractions data [18,19]. The broadening of the diffraction profile peaks usually origi nate from tiny crystallites sizes, micro strain in crystals and instrumental broadening effect. The instrumental broadening contribution was minimized using proper calibration before measurement. The contri butions of crystallite size effect and micro strain in tiny crystal in line width (β) can be filtered out using Williamson Hall (W–H) plot, stated below [20]. � Kθ βcosθ¼ ε ð4sinθ þ D
The minute decrement in “u” with increasing Eu content in nickel ferrite nanoparticles is attributed to the enhancement of lattice parameter. 3.2. HRTEM images analysis Fig. 3 (a) and 3 (b) shows the HRTEM micrographs of as-synthesized Eu-0 and Eu-6 ferrite nanoparticles respectively. The HRTEM studies revealed the spherical nature of prepared nanoparticles together with size of the nanoparticles were found to have perfect match with the mean crystallite size calculated using Scherrer’s formula. The observed slight agglomeration among the nanoparticles is attributed to the Van der Waals interactions. The existence of crystalline ordering was verified by the concentric rings formation of the selective area electrons diffraction (SAED) pattern [26]. The diffraction of electrons from (311), (400) and (440) were identified and shown in inset of Fig. 3 (a). A careful examination of SAED pattern revealed that the concentric rings were not equip-placed, the first two rings were near to each-other whereas the third ring was little bit far apart [5,27]. This particular configuration of rings in SAED pattern confirmed that face centred cubic (fcc) like structure is present for these spinel ferrites although the FCC structure is different compared to conventional face centred cubic lattice.
where D is the average crystallite size, K is constant depending on ge ometry of synthesized particles (it attains a value 0.89 for spherical shape particles), λ is the wavelength of X-ray used, ε belongs to the micro strain and θ is the corresponding Bragg’s angle [17]. The W–H plots of all samples are presented in Fig. 2. The average crystallites size of pre pared nanoparticles was found in the range of 20 � 4 nm with an increasing tensile strain for high Eu content. The increment in tensile strain with Eu content in the samples is attributed to the presence of larger size Eu3þ ions in comparison to Fe3þ ions. The reflection of X-ray beam associated with (422), (222) and (220) crystallographic planes are usually sensitive to cationic distribution. The intensity of reflected beam from (220) and (422) plans can vary with minute change in cationic distribution of tetrahedral voids while reflection of X-ray beam from (222) plane depends on ions related to octahedral voids [21]. The observed reduction in intensity ratio of I220/I222 and I422/I222 confirmed that the rare earth Eu3þ ions preferred to occupy octahedral voids in
3.3. Raman spectra analysis
spinel structure, presented in Table 2. The hopping length (LA ¼ √3 4a
and LB ¼ √2 4a ) of A-sites and B-sites were noticed to increase with higher Eu concentration [5], listed in Table 3. The increment in hopping lengths reflects the requirement of more energy for charge carriers to move from one to another cationic site thereby decreasing the conduc tivity [5].
The Raman spectroscope is a sensitive probe to investigate various vibrational mode and local structure of a material. The vibrations of molecules which allow the change in polarizability are usually vibra tional Raman active mode. The Raman spectra observed at room tem perature of all Eu doped samples are shown in Fig. 4. The Raman peaks 6
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Materials Chemistry and Physics 241 (2020) 122383
Fig. 7. Hysteresis loops of all the samples obtained at 5 K.
translational movement of FeO4. The presence of all these active modes in pristine and doped nickel ferrites proved the formation of spinel structure as displayed in Table 4. The manifestation of B2/E Raman active mode for higher Eu content samples is attributed to P4122 space group [19]. The doping of rare earth Eu3þ ions having large ionic radius minutely ruptured the octahedral sites as a result the cubic symmetry of octahedral sites lowers into tetragonal symmetry. The broadening of the Raman peaks is due to the size effect of the nanoparticles [29,30].
Table 5 Magnetic parameters of all Eu doped samples. SampleId
Eu-0 Eu-2 Eu-4 Eu-6
Magnetic Studies 5K
300 K
MS (emu/ g)
HC (Oe)
MS (emu/ g)
HC (Oe)
39 38 24 44
448 464 515 527
27 26 22 16
81 23 21 15
TB
Tirr
K
K
242 � 1 198 � 1 183 � 1 187 � 1
— 268 � 1 234 � 1 229 � 1
3.4. UV–Vis studies The UV–Visible spectroscopy is a useful tool to investigate the ab sorption, transmission or reflection band of a semiconducting material. We have conducted the UV–Vis spectra measurement of all synthesized samples in absorption mode over the range of 200 nm � λ � 800 nm. The optical band gap of all Eu doped nanoparticles were analyzed using the equation [13,31].
are fitted using Lorentzian shaped function. The bulk nickel ferrites contain tetrahedral (A) and octahedral (B) sites associated with cubic spinel group with Fd-3m space group. The tetrahedral and octahedral identical sites are belonged to Td and D3d point groups respectively [18, 20]. The group theory computes five vibrational Raman active modes (A1g, Eg and 3T2g) for spinel ferrite system. The active modes associated with higher wave number (˃ 600 cm 1) belong to tetrahedral sites whereas below of it is related to the octahedral sites [28]. The peak associated with A1g mode reflects the equable stretching of Fe3þ and O2 ions present in A-sites. The relative low energy Eg, T2g(2) and T2g(3) active modes originates due to equal and unequal bending of O2 ions in respect to Fe at B-sites while the T2g(1) mode arises due to the
α(υ). hυ� C(hυ-Eo)n where α is the absorption co-efficient, hυ is the energy of incident photon, C is a constant, Eo corresponds to the optical band gap of the material and n is an index which attains a value of 2 for allowed indirect band gap [32,33]. The Tauc plots of all prepared Eu doped samples for measuring allowed indirect band gap are presented in Fig. 5. Nickel 7
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Materials Chemistry and Physics 241 (2020) 122383
Fig. 8. M-T graphs of all the samples.
300 K weakens the A-O-B superexchange interactions which results in reduction of coercivity along and saturation magnetization. It is apparent from the hysteresis loops that coercivity reduces with incre ment in Eu content in nanoparticles. The overall magnetic moment in spinel ferrite system is the difference of magnetic moment of the octa hedral (B) sites and the tetrahedral (A) sites. The Eu3þ ions preferentially occupy the octahedral sites due to its large ionic radius. Because of reduction in net magnetic moment the saturation magnetization was observed to decrease at room temperature [14]. As the Eu3þ content increased in the samples, the coercivity and remanence was reduced to a negligible amount. The samples were in superparamagnetic state. Fig. 7 shows the hysteresis plots of all Eu doped samples recorded at 5 K in 50 kOe magnetic field. The observed magnetic moment of Eu3þ ions at low temperature (below TN) is 3.4 μB while the theoretical calculated value is zero. This can be explained by Vanvleck effect. The substitution of rare earth Eu3þ(3.4μB) ions in place of Fe3þ(5.9μB) ions leads to reduction in net magnetic moment of octahedral sites which is the notable cause of observed decrease in saturation magnetization at 5 K. The effects of spins canting, spins pinning at low temperature due to nano size of particles are more pronounced and also contributed to this decrease of magnetization. The overall magnetic moment (mT) in terms of Bohr’s magnetron can be obtained by the following relationship [20, 23].
Table 6 Contain atomic mass (M), effective magnetic moment (mT) and anisotropy constant (K) of all Eu doped samples. SampleId
Atomic mass (per formula unit)
mT (μB) at 5K
mT (μB) at 300 K
K (X 105) erg/cm3
Eu-0 Eu-2 Eu-4 Eu-6
234.39 236.31 238.23 240.16
1.64 � 0.01 1.61 � 0.01 1.02 � 0.01 1.89 � 0.01
1.13 � 0.01 1.10 � 0.01 0.94 � 0.01 0.69 � 0.01
3.89 1.41 0.87 1.33
ferrite is known to be indirect band gap soft ferrimagnetic semi conducting material. It shows an indirect band gap of 1.64 eV as re ported in research articles. The estimated indirect optical band gaps were 1.765 eV, 1.778 eV, 1.782 eV and 1.747 eV respectively with gradually increase of Eu content in nickel ferrite nanoparticles. The prepared nanoparticles was displayed an opaque character near IR re gion of EM spectrum. 3.5. Magnetic studies The room temperature applied field (20 kOe) dependent magneti zation M(H) plots of all Eu doped samples are displayed in Fig. 6. The coercive field (HC), saturation magnetization (MS) and other magnetic parameters obtained from magnetic measurements are given in Table 5. The Eu ions have a paramagnetic character at 300 K while the magnetic ordering starts below N� eel temperature (TN) of 90.4 K [11]. The para magnetic nature of Eu3þ dopant ions in nickel ferrite nanoparticles at
mT ¼
MS M 5585
where MS and M corresponds to the saturation magnetization and atomic mass per formula unit respectively. The net magnetic moment 8
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was observed to decrease in both 5 K and room temperature as per Table 6. The coercivity (HC) is also affected by saturation magnetization (MS) of the nanoparticles system below TB and follows the stated relation [34]. HC
magnetic properties can be achieved by substitution of proper amount of rare earth ions in cubic spinel phase. Declaration of competing interest
2K ¼ MS
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
where K corresponds to the effective anisotropy constant which have contributions from size, shape, volume, surface and stress anisotropies [6,8]. The MS and K were observed to reduce with increasing Eu con centration in nickel ferrites nanoparticles which results increment in coercive field in blocked magnetic state [35]. Zubair et al. also observed similar trend in Eu doped cobalt ferrite nanoparticles [16]. The M-T measurements in both zero field cooled (ZFC) and field cooled (FC) protocol were performed at 500 Oe static magnetic field for all the Eu doped samples, shown in Fig. 8. All the ZFC curves displayed a broaden peak with a maxima. The corresponding temperature in ZFC protocol is designated as blocking temperature (TB) which is found to shift towards low temperature with increasing Eu content. This repre sents the superparamagnetic (SPM) characteristics of synthesized nanoparticles [5]. The magnetic spins are blocked randomly along easy axis below TB while spins are free to rotate above TB. The thermal energy (kBTB) at blocking temperature is the threshold for activation of mag netic spins. The weakening of A-O-B superexchange interaction due to Eu substitution and tiny size of particles are responsible for the decrease in blocking temperature. The almost horizontal nature of FC curves of all Eu doped samples below TB indicated the presence of interparticle in teractions. The observed blocking temperature is just a mean value because of the nanoparticles ensemble contains a size distribution of particles [36]. The values of the effective anisotropy constant (K) are found to decrease with increase of Eu concentration and are given in Table 6. The temperature in M-T plots where ZFC and FC curves begin to separate is termed as the thermo-magnetic irreversibility temperature (Tirr). It also reflects the existence of non equilibrium magnetic state along with blocking point of largest size particles in the ensemble [36]. The observed reduction in Tirr with increasing Eu content is attributed to weak A-O-B superexchange interactions gradually. The separation be tween magnetization in ZFC-FC protocol of M-T plots near 5 K can be expressed by the stated relation [37]. � � HA MZFC ¼ MFC HA < 2HC 2HC
Acknowledgment The authors are grateful to the UGC-DAE CSR Indore centre for providing room temperature Raman spectroscopy and magnetization measurement facilities. References [1] B. Isso, I.M. Obaidat, B.A. Albiss, Y. Haik, Int. J. Mol. Sci. 14 (2013) 21266–21305. [2] K. Bhattacharjee, S.P. Pati, G.C. Das, D. Das, K.K. Chattopadhyay, J. Appl. Phys. 116 (2014) 233907. [3] V.R. Reddy, D. Kothari, A. Gupta, S.M. Gupta, Appl. Phys. Lett. 94 (2009), 082505. [4] G.S. Shahane, A. Kumar, M. Arora, R.P. Pant, K. Lal, J. Magn. Magn. Mater. 322 (2010) 1015–1019. [5] R. Mohan, M.P. Ghosh, S. Mukherjee, J. Magn. Magn. Mater. 458 (2018) 193–199. [6] S. Kumar, V. Singh, S. Aggarwal, U.K. Mandal, R.K. Kotnala, Mater. Sci. Eng. B 166 (2010) 76–82. [7] S.E. Shirsath, D. Wang, S.S. Jadhav, M.L. Mane, S. Li, Ferrites obtained by sol-gel method, in: L. Klein, M. Aparicio, A. Jitianu (Eds.), Handbook of Sol-Gel Science and Technology, Springer, Cham, 2018, pp. 695–735. [8] C. Caizar, Mater. Sci. Eng. B 100 (2003) 63. [9] M. Zhang, Z. Zhenfa, Q. Liu, P. Zhang, X. Tang, X. Zhu, Y. Sun, J. Dai, Adv. Mater. Sci. Eng. (2013) 1–10, 609819. [10] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, USA, 1974. [11] J.M.D. Coey, Magnetism and Magnetic Materials, Cambridge Univ. Press, 2009. [12] S. Blundell, Magnetism in Condensed Matter, Oxford Univ. Press, 2001. [13] M.P. Ghosh, S. Mukherjee, J. Magn. Magn. Mater. 489 (2019) 165320. [14] S.E. Jacobo, S. Duhalde, H.R. Bertorello, J. Magn. Magn. Mater. 272–276 (2004) 2253–2254. [15] R. Hoschschild, H. Fuess, J. Mater. Chem. 10 (2000) 539–542. [16] A. Zubair, Z. Ahmad, A. Mahmood, W.C. Cheong, L. Ali, M.A. Khan, A.H. Chughtai, M.N. Ashique, Results Phys. 7 (2017) 3203–3208. [17] P. Nordblad Aakash, R. Mohan, S. Mukherjee, J. Magn. Magn. Mater. 441 (2017) 710. [18] A. Roychowdhury Aakash, D. Das, S. Mukherjee, J. Alloy. Comp. 668 (2016) 33. [19] A. Roychowdhury Aakash, D. Das, S. Mukherjee, Ceram. Inter. 42 (2016) 7742. [20] M.P. Ghosh, S. Mukherjee, J. Am. Ceram. Soc. (2019), https://doi.org/10.1111/ jace.16687 . [21] A.M.M. Farea, S. Kumar, K.M. Batoo, A. Yousef, C.G. Lee, Alimuddin, J. Alloy. Comp. 464 (2008) 361. [22] V.K. Lakhani, T.K. Pathak, N.H. Vasoya, K.B. Modi, Solid State Sci. 13 (2011) 539–547. [23] R. Sharma, P. Thakur, M. Kumar, P.B. Barman, P. Sharma, V. Sharma, Ceram. Inter. 43 (2017) 13661–13669. [24] G. Kumar, J. Shah, R.K. Kotnala, V.P. Singh, Sarveena, G. Garg, S.E. Shirsath, K. M. Batoo, M. Singh, Mater. Res. Bull. 63 (2015) 216–225. [25] U. Kodam, K. Bharathi, V.R. Reddy, S. Rayaprol, V. Siruguri, M. Garimalle, J. Appl. Phys. 121 (2017), 055101. [26] K.M. Batoo, D. Salah, G. Kumar, A. Kumar, M. Singh, M. Abd El-Sadek, F.A. Mir, A. Imran, D.A. Jameel, J. Magn. Magn. Mater. 411 (2016) 91. [27] G. Rana G, U.C. Johri, J. Alloy. Comp. 577 (2013) 376–381. [28] J.D. Baraliya, H.H. Joshi, Vib. Spectrosc. 74 (2014) 75. [29] A. Samavati, M.K. Mustafa, A.F. Ismail, M.D.H. Othman, M.A. Rahman, Mater. Express 6 (2016) 473. [30] A. Kumar, M.A. Dar, P. Sharma, D. Varshney, AIP Conf. Proc. 1591 (2014) 1148. [31] S. Singhal, S. Bhukal, J. Singh, K. Chandra, S. Bansal, J. Nanotechnol. 930243 (2011) 1–6. [32] B.S. Holinsworth, D. Mazumder, H. Sims, Q.C. Sun, M.K. Yurtisigi, S.K. Sarker, A. Gupta, W.H. Buther, J.L. Musfeldt, Appl. Phys. Lett. 103 (2013), 082406. [33] Q.C. Sun, H. Sims, D. Mazumder, J.X. Ma, B.S. Holinsworth, K.R.O. Neal, G. Kim G, W.H. Butler, A. Gupta, J.L. Musfeldt, Phys. Rev. B. 86 (2012), 205106. [34] I. Ali, M. Lslam, M. Ishaque, H.M. Khan, M.N. Ashiq, M. Rana, J. Magn. Magn. Mater. 324 (2012) 3773–3777. [35] J. Chand, G. Kumar, P. Kumar, S.K. Sharma, M. Knobel, M. Singh, J. Alloy. Comp. 509 (2011) 9638. [36] D. Carta, M.F. Casula, A. Falqui, D. Loche, G. Mountjoy, C. Sangregorio, A. Corrias, J. Phys. Chem. C 113 (2009) 8606–8615. [37] K.S. Ramakrishna, C. Srinivas, C.L. Prajapat, S.S. Meena, M.V.K. Mehar, D. M. Potukuchi, D.L. Sastry, Ceram. Inter. 44 (2018) 1193.
where HA is the applied field (500 Oe) and HC corresponds to coercivity. 4. Conclusion A series of rare earth Eu doped NiFe2O4 nanoparticles were prepared. The room temperature XRD patterns verified the existence of cubic spinel structure. The W–H plots revealed the presence of tensile strain in the system together with average diameter of nanoparticles 20 � 4 nm. Raman spectra also confirmed the transformation of octahedral cubic symmetry to octahedral tetragonal symmetry for higher Eu concentra tion. This was attributed to the difference in the cationic sizes of Ni and Fe compared to Eu. The Tauc plots revealed an indirect allowed band gap in the range of 1.76 � 0.02 eV. The signature of paramagnetic behavior of Eu3þ ions at room temperature was confirmed by M-H curves. The blocking temperature (TB) and thermo-magnetic irrevers ibility (Tirr) decreased with increment in Eu ions in nickel ferrite nanoparticles. The weakening of superexchange interactions at the paramagnetic region of Eu3þ ions was also found. The coercive field was enhanced while saturation magnetization was reduced at low tempera ture (below TB). We can conclude that tailoring of structural, optical and
9
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Tuning the microstructural, optical and superexchange interactions with rare earth Eu doping in nickel ferrite nanoparticles Mritunjoy Prasad Ghosh a, Saurabh Sharma b, Harendra Kumar Satyapal b, Kamar Tanbir b, Rakesh Kumar Singh b, Samrat Mukherjee a, * a b
Department of Physics, National Institute of Technology Patna, Patna, 800005, Bihar, India Aryabhatt Centre for Nanoscience and Nanotechnology, Aryabhatt Knowledge University, Patna, 800001, Bihar, India
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Synthesis of rare earth doped nanoferrites. � Reduction in superexchange interaction at 300 K. � Enhancement in coercivity at 5 K. � Observation of superparamagnetism at 300 K.
A R T I C L E I N F O
A B S T R A C T
Keywords: Rare earth dopant W–H plots Raman spectra Magnetic properties Superexchange interactions
A methodical study on structural, magnetic and optical properties of Eu doped NiFe2O4 nanoparticles with generic composition NiEuxFe2-xO4 (x ¼ 0.00, 0.02, 0.04 and 0.06), synthesized via co-precipitation technique has been reported in this article. The fabrication of single phase ferrite nanoparticles was verified using X-ray powder diffractograms. The analysis of W–H plots revealed the presence tensile strain in the samples along with an average crystallite size of 20 � 4 nm. The existence lowering of cubic symmetry at octahedral sublattices was detected by vibrational Raman spectra recorded at 300 K. The values of indirect optical band gap were noted in the range of 1.76 � 0.02 eV. The hysteresis loops at room temperature supported the paramagnetic character of Eu3þ ions in spinel structure. The evidence of reduced A-O-B superexchange interaction along with reduction of saturation magnetization was also found for higher Eu content nanoparticles at 300 K. The coercivity was observed to increase at 5 K. The blocking temperature and thermo-magnetic irreversibility was reduced with increasing Eu concentration.
* Corresponding author. E-mail address: [email protected] (S. Mukherjee). https://doi.org/10.1016/j.matchemphys.2019.122383 Received 19 March 2019; Received in revised form 18 October 2019; Accepted 28 October 2019 Available online 31 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. X-ray diffractograms obtained at room temperature.
1. Introduction
various tops down and bottoms up routes but among these, the standard chemical co-precipitation method is the easiest process to attain nano particles with fewer defects and also dispenses better size homogeneity in nano order. It is familiar that the properties of ferrite nanoparticles are very sensitive to fabrication techniques and cations distribution [7]. The NiFe2O4 nanoparticles display excellent stability, soft ferrimagnetic nature, moderate Curie temperature and also semi conducting character [8,9]. The rare earth Eu3þ ions consists of unpaired 4f electrons along with a magnetic moment of 3.4 μB which is observed experimentally although calculations show zero magnetic moment. It also depicts paramagnetic nature above 90.4 K (TN) whereas magnetic behavior is found due to Van-Vleck effect at certain low temperature [10,11]. In case of isolated Eu3þ and Sm3þ rare earth ions, a significant disagreement between theoretical magnetic moment and experimental magnetic moment is observed. These two particular rare earth ions have multiplet spin in both ground and first excited states. The excited multiplet state is generally close enough to the ground multiplet state; thereby the excited state overlaps with ground state. This phenomenon results in the changing of net magnetic moment of the system as well as a notable variation from theoretical magnetic moment. In case of isolated Eu3þ ions the theoretical magnetic moment is zero whereas experimentally a magnetic moment of 3.4 μB is observed. This behavior is known as
Nanotechnology permits researchers to synthesize nano size systems where classical laws of physics fail to explain the physical phenomenon and things appear differently at nano scale. The past three decades had seen a tremendous development in synthesis method of size/shape controlled, stable and monodisperse magnetic nanoparticles. Nano materials consist of large surface area hence more sensitive with external reagents. Magnetic nanoparticles are able to respond according to magnetic field and can be controlled easily with external field. The physical and magnetic properties of such particles can be tuned by varying diameter, dopant ions and its concentration depending upon applications [1–3]. The ferrite nanoparticles have broad range of in dustrial applications such as magnetic biosensor, water purifications, magnetic particles imaging (MPI), heat source in hyperthermia, catalytic exercise, targeted drug delivery, magnetic resonance imaging (MRI) etc. For biomedical applications purpose, instead of bare particles the nanoparticles are coated with non-toxic oleic acid, PVP, PVA or suitable elements of carbon. Bulk NiFe2O4 consists an inverse spinel cubic (FCC) structure where Fe3þ ions are distributed uniformly in both tetrahedral and octahedral site. The Ni2þ ions prefer to occupy octahedral voids. The ferrimagnetism in spinel ferrites originates from A-B super exchange interactions [4–6]. Nickel ferrite nanoparticles can be fabricated using 2
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Fig. 2. Williamson-Hall (W–H) plot of the samples.
Van-Vleck effect [11–13]. Although several studies on rare earth (RE) ions doped in spinel ferrites were reported but only a few articles rep resented a systematic exploration of the RE dopant impact on structural and magnetic character of ferrites nanoparticles [14–16]. The behavior of rare earth Eu3þ ions inside a cubic spinel ferrite system considering the Van-vleck effect has not been explored systematically. The Eu3þ ions are also able to interrupt the superexchange interactions above 90.4 K due to its non-magnetic nature in ferrites. In this work, we have presented the impact of Eu3þ dopant on soft ferrimagnetic nickel ferrite nanoparticles. Due to its larger size, a slight amount of Eu3þ can distorted the local structure of cubic (FCC) spinel ferrites. The Eu3þ ions can vary the magnetic properties of NiFe2O4 nanoparticles differently at well above and below N� eel temperature (TN). The microstructural, magnetic and optical behavior of nickel ferrite nanoparticles with rare earth Eu3þ ions has been explored sys tematically in this article. A connection between the structural and magnetic properties has been set up for Eu doped nickel nanoferrite systems. The doping of Eu3þ ions also have a fundamental role for controlling crystallites size growth along with development of micro stain in tiny crystal of ferrites. The optical absorption region may diverge due to Eu implantation in ferrites.
ground in a mortar. The obtained power was then sintered at 500 � C for 5 h and cooled down gently. The prepared samples were then named as Eu-0, Eu-2, Eu-4 and Eu-6 according to increase of Eu content. Further all the characterizations were done using those powdered samples [5]. 2.2. Characterizations The spinel phase identification, microstructural, optical and mag netic features of prepared Eu doped samples was characterized using Xray diffraction, High resolution transmission electron microscopy, vibrational Raman spectroscopy, UV–Visible spectroscopy and VSM measurements. The X-ray diffraction graphs of all Eu doped samples were taken using Rigaku Advance X-ray Diffractometer with a source of copper having wavelength of 1.5406 Å (Kα - line). The X-ray diffracto grams were observed with a step rate 0.020/sec over the angular range of 20� � 2θ � 80� . The structural images of the synthesized nano particles were taken using high resolution transmission electron mi croscopy (HRTEM, JEM-2100F, JEOL, Japan, Accelerating voltage ¼ 200 kV). The Raman spectra were obtained at 300 K over the range 200 cm 1 to 800 cm 1 by Renishaw Raman spectrometer with a diode laser source of 473 nm wavelength. The UV DRS spectrometer (UV-2550, Shimadzu, USA) was used to perform optical behavior mea surement. Magnetic properties of all samples were characterized by
2. Experimental details 2.1. Synthesis process
Table 1 Structural and refined parameters of all Eu doped nickel ferrite nanoparticles.
The nickel ferrite nanoparticles containing different weight per centage of rare earth Eu3þ ions with a generic formula NiEuxFe2-xO4 (x ¼ 0.00, 0.02, 0.04 and 0.06) were synthesized via standard coprecipitation method [7,17]. In this process, stoichiometric amount of chemicals Fe(NO3)3⋅9H2O, Ni(NO3)2⋅6H2O and Eu(NO3)⋅6H2O were dissolved in deionized water of 200 ml. The solution was kept in a beaker and stirred continuously using magnetic stirrer. The precursor NaOH solution was then put drop wise slowly to attain the final pH 10 of the entire solution. The solution was corroded at 80 � C for next 2 h so that no reagent left unreacted. Thus the precipitate was washed using distilled water and ultimately in alcohol several times to attain pH 7. After that, the precipitate was dried under ambient atmosphere and
Sample-Id
Eu-0
Eu-2
Eu-4
Eu-6
Space group
Fd3m
Fd3m
Fd3m
Fd3m
8.357 � 0.001 21 � 1 2.91 � 0.01
8.379 � 0.001 24 � 1 4.53 � 0.01
8.385 � 0.001 21 � 1 3.65 � 0.01
2.26 1.79 8.81 1.07
2.49 1.98 10.34 1.17
2.21 1.75 6.89 1.06
Cell parameters a (Å) 8.348 � 0.001 D (nm) 16 � 1 Strain (X 0.21 � 0.01 3 10 ) Refinement factors Rwp (%) 2.11 Rp (%) 1.66 R2F (%) 9.60 χ2 1.13
3
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Table 2 Contain ideal cationic distribution and various structural parameters of all samples. SampleId
Asite
B-site
rA (Å)
rB (Å)
I220/ I222
I422/ I222
Eu-0 Eu-2
Fe3þ Fe3þ
0.645 0.645
0.667 0.671
2.31 1.76
0.98 0.92
Eu-4
Fe3þ
0.645
0.673
1.38
0.78
Eu-6
3þ
Ni2þFe3þ Ni2þEu3þ0.02 Fe3þ0.98 Ni2þEu3þ0.04 Fe3þ0.96 Ni2þEu3þ0.06 Fe3þ0.94
0.645
0.676
1.47
0.68
Fe
Table 3 Contain hopping length (LA and LB), theoretical lattice constant (ath) and oxygen positional parameter (u) of all Eu doped nanoparticles. Sample-Id
LA (Å)
LB (Å)
ath (Å)
u
Eu-0 Eu-2 Eu-4 Eu-6
3.615 3.619 3.628 3.631
2.951 2.955 2.962 2.965
8.393 8.404 8.410 8.417
0.388(2) 0.388(1) 0.387(9) 0.387(8)
16 T-VSM-PPMS (Quantum design). Fig. 3 (b). TEM image of Eu-6 ferrite nanoparticles.
3. Results and discussion
temperature and are shown in Fig. 1. The diffraction of X-ray from crystallographic plans (220), (311), (222), (400), (440) and (511) satisfying Bragg’s condition are associated with cubic spinel structure [5]. All observed characteristic peaks of synthesized nanoparticles were
3.1. Structural studies The X-ray diffractograms of all Eu doped nanoparticles taken at room
Fig. 3 (a). TEM image of Eu-0 ferrite nanoparticles. 4
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Fig. 4. Room temperature Raman spectra of all Eu doped samples.
Fig. 5. Tauc plots of all Eu doped nickel ferrite nanoparticles.
found to match with JCPDS card no. 10–325 along with space group Fd-3m. The Rietveld refinement of diffraction patterns were carried out using GSAS programme with EXPUGI interface and pseudo voigt
function was adopted for profile fitting. The structural and refinement parameters were collected in Table 1. The goodness of fit (χ2) near unity along with reliability factors (RP, RWP, R2F) less than 10% confirmed the 5
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For investigation on structural distortion produced by large size dopant ions, it is important to have information about the average volume of voids that can be occupied by the ions. The average ionic radius of voids at tetrahedral (A) and octahedral (B) sites is evaluated using the following expressions [22,23].
Table 4 Vibrational Raman active modes of all Eu doped nickel ferrite nanoparticles. Sample-Id Eu-0 Eu-2 Eu-4 Eu-6
Raman active modes (cm
1
)
A1g(1)
A1g(2)
B2/E
T2g(3)
T2g(2)
Eg
T2g(1)
696 702 705 706
658 675 684 685
—— 641 652 653
571 551 562 559
481 468 477 475
329 305 317 314
205 197 200 198
rA ¼ [C(Fe3þ) r(Fe3þ) þ C(Ni2þ) r(Ni2þ)] � � � � � � � � � 1 C Fe3þ r Fe3þ þ C Ni2þ r Ni2þ rB ¼ 2 � � � �� þ C Eu3þ r Eu3þ where C represents concentration of ions, r(Fe3þ), r(Ni2þ) and r(Eu3þ) corresponds to the ionic radius of Fe3þ (0.645 Å), Ni2þ (0.69 Å) and Eu3þ (0.947 Å) ions respectively. The minute increment in ionic radius (rB) at the B sites is due to the size difference between Fe3þand Eu3þ ions. The theoretical lattice parameter (ath) can be obtained using the stated relation [13,24]. � � �� �� 8 ath ¼ rA þ Ro þ √3 rB þ Ro 3√3 where Ro (1.32 Å) represents the radius of oxygen ions. The theoretical lattice parameter (ath) is found to increase with increase of Eu content, collected in Table 3. A similar kind of trend was also noticed for lattice constant estimated from the Rietveld refinement of X-ray diffraction data. The oxygen locus parameter (u) is another crucial parameter to get an idea about slight rupture present in spinel structure. The obtained value of u was found minutely larger with respect to ideal value 0.375 (considering origin at tetrahedral site). It is due to the push of ions present in B-sites which reflects in “u” value. The oxygen locus param eter (u) is commonly estimated by following equation [20,25]. � � � � 1 1 u ¼ R0 þ rA þ 4 ath √3
Fig. 6. Hysteresis loops of all the samples obtained at room temperature.
strong harmony between theoretical and experimental diffractions data [18,19]. The broadening of the diffraction profile peaks usually origi nate from tiny crystallites sizes, micro strain in crystals and instrumental broadening effect. The instrumental broadening contribution was minimized using proper calibration before measurement. The contri butions of crystallite size effect and micro strain in tiny crystal in line width (β) can be filtered out using Williamson Hall (W–H) plot, stated below [20]. � Kθ βcosθ¼ ε ð4sinθ þ D
The minute decrement in “u” with increasing Eu content in nickel ferrite nanoparticles is attributed to the enhancement of lattice parameter. 3.2. HRTEM images analysis Fig. 3 (a) and 3 (b) shows the HRTEM micrographs of as-synthesized Eu-0 and Eu-6 ferrite nanoparticles respectively. The HRTEM studies revealed the spherical nature of prepared nanoparticles together with size of the nanoparticles were found to have perfect match with the mean crystallite size calculated using Scherrer’s formula. The observed slight agglomeration among the nanoparticles is attributed to the Van der Waals interactions. The existence of crystalline ordering was verified by the concentric rings formation of the selective area electrons diffraction (SAED) pattern [26]. The diffraction of electrons from (311), (400) and (440) were identified and shown in inset of Fig. 3 (a). A careful examination of SAED pattern revealed that the concentric rings were not equip-placed, the first two rings were near to each-other whereas the third ring was little bit far apart [5,27]. This particular configuration of rings in SAED pattern confirmed that face centred cubic (fcc) like structure is present for these spinel ferrites although the FCC structure is different compared to conventional face centred cubic lattice.
where D is the average crystallite size, K is constant depending on ge ometry of synthesized particles (it attains a value 0.89 for spherical shape particles), λ is the wavelength of X-ray used, ε belongs to the micro strain and θ is the corresponding Bragg’s angle [17]. The W–H plots of all samples are presented in Fig. 2. The average crystallites size of pre pared nanoparticles was found in the range of 20 � 4 nm with an increasing tensile strain for high Eu content. The increment in tensile strain with Eu content in the samples is attributed to the presence of larger size Eu3þ ions in comparison to Fe3þ ions. The reflection of X-ray beam associated with (422), (222) and (220) crystallographic planes are usually sensitive to cationic distribution. The intensity of reflected beam from (220) and (422) plans can vary with minute change in cationic distribution of tetrahedral voids while reflection of X-ray beam from (222) plane depends on ions related to octahedral voids [21]. The observed reduction in intensity ratio of I220/I222 and I422/I222 confirmed that the rare earth Eu3þ ions preferred to occupy octahedral voids in
3.3. Raman spectra analysis
spinel structure, presented in Table 2. The hopping length (LA ¼ √3 4a
and LB ¼ √2 4a ) of A-sites and B-sites were noticed to increase with higher Eu concentration [5], listed in Table 3. The increment in hopping lengths reflects the requirement of more energy for charge carriers to move from one to another cationic site thereby decreasing the conduc tivity [5].
The Raman spectroscope is a sensitive probe to investigate various vibrational mode and local structure of a material. The vibrations of molecules which allow the change in polarizability are usually vibra tional Raman active mode. The Raman spectra observed at room tem perature of all Eu doped samples are shown in Fig. 4. The Raman peaks 6
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Fig. 7. Hysteresis loops of all the samples obtained at 5 K.
translational movement of FeO4. The presence of all these active modes in pristine and doped nickel ferrites proved the formation of spinel structure as displayed in Table 4. The manifestation of B2/E Raman active mode for higher Eu content samples is attributed to P4122 space group [19]. The doping of rare earth Eu3þ ions having large ionic radius minutely ruptured the octahedral sites as a result the cubic symmetry of octahedral sites lowers into tetragonal symmetry. The broadening of the Raman peaks is due to the size effect of the nanoparticles [29,30].
Table 5 Magnetic parameters of all Eu doped samples. SampleId
Eu-0 Eu-2 Eu-4 Eu-6
Magnetic Studies 5K
300 K
MS (emu/ g)
HC (Oe)
MS (emu/ g)
HC (Oe)
39 38 24 44
448 464 515 527
27 26 22 16
81 23 21 15
TB
Tirr
K
K
242 � 1 198 � 1 183 � 1 187 � 1
— 268 � 1 234 � 1 229 � 1
3.4. UV–Vis studies The UV–Visible spectroscopy is a useful tool to investigate the ab sorption, transmission or reflection band of a semiconducting material. We have conducted the UV–Vis spectra measurement of all synthesized samples in absorption mode over the range of 200 nm � λ � 800 nm. The optical band gap of all Eu doped nanoparticles were analyzed using the equation [13,31].
are fitted using Lorentzian shaped function. The bulk nickel ferrites contain tetrahedral (A) and octahedral (B) sites associated with cubic spinel group with Fd-3m space group. The tetrahedral and octahedral identical sites are belonged to Td and D3d point groups respectively [18, 20]. The group theory computes five vibrational Raman active modes (A1g, Eg and 3T2g) for spinel ferrite system. The active modes associated with higher wave number (˃ 600 cm 1) belong to tetrahedral sites whereas below of it is related to the octahedral sites [28]. The peak associated with A1g mode reflects the equable stretching of Fe3þ and O2 ions present in A-sites. The relative low energy Eg, T2g(2) and T2g(3) active modes originates due to equal and unequal bending of O2 ions in respect to Fe at B-sites while the T2g(1) mode arises due to the
α(υ). hυ� C(hυ-Eo)n where α is the absorption co-efficient, hυ is the energy of incident photon, C is a constant, Eo corresponds to the optical band gap of the material and n is an index which attains a value of 2 for allowed indirect band gap [32,33]. The Tauc plots of all prepared Eu doped samples for measuring allowed indirect band gap are presented in Fig. 5. Nickel 7
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Materials Chemistry and Physics 241 (2020) 122383
Fig. 8. M-T graphs of all the samples.
300 K weakens the A-O-B superexchange interactions which results in reduction of coercivity along and saturation magnetization. It is apparent from the hysteresis loops that coercivity reduces with incre ment in Eu content in nanoparticles. The overall magnetic moment in spinel ferrite system is the difference of magnetic moment of the octa hedral (B) sites and the tetrahedral (A) sites. The Eu3þ ions preferentially occupy the octahedral sites due to its large ionic radius. Because of reduction in net magnetic moment the saturation magnetization was observed to decrease at room temperature [14]. As the Eu3þ content increased in the samples, the coercivity and remanence was reduced to a negligible amount. The samples were in superparamagnetic state. Fig. 7 shows the hysteresis plots of all Eu doped samples recorded at 5 K in 50 kOe magnetic field. The observed magnetic moment of Eu3þ ions at low temperature (below TN) is 3.4 μB while the theoretical calculated value is zero. This can be explained by Vanvleck effect. The substitution of rare earth Eu3þ(3.4μB) ions in place of Fe3þ(5.9μB) ions leads to reduction in net magnetic moment of octahedral sites which is the notable cause of observed decrease in saturation magnetization at 5 K. The effects of spins canting, spins pinning at low temperature due to nano size of particles are more pronounced and also contributed to this decrease of magnetization. The overall magnetic moment (mT) in terms of Bohr’s magnetron can be obtained by the following relationship [20, 23].
Table 6 Contain atomic mass (M), effective magnetic moment (mT) and anisotropy constant (K) of all Eu doped samples. SampleId
Atomic mass (per formula unit)
mT (μB) at 5K
mT (μB) at 300 K
K (X 105) erg/cm3
Eu-0 Eu-2 Eu-4 Eu-6
234.39 236.31 238.23 240.16
1.64 � 0.01 1.61 � 0.01 1.02 � 0.01 1.89 � 0.01
1.13 � 0.01 1.10 � 0.01 0.94 � 0.01 0.69 � 0.01
3.89 1.41 0.87 1.33
ferrite is known to be indirect band gap soft ferrimagnetic semi conducting material. It shows an indirect band gap of 1.64 eV as re ported in research articles. The estimated indirect optical band gaps were 1.765 eV, 1.778 eV, 1.782 eV and 1.747 eV respectively with gradually increase of Eu content in nickel ferrite nanoparticles. The prepared nanoparticles was displayed an opaque character near IR re gion of EM spectrum. 3.5. Magnetic studies The room temperature applied field (20 kOe) dependent magneti zation M(H) plots of all Eu doped samples are displayed in Fig. 6. The coercive field (HC), saturation magnetization (MS) and other magnetic parameters obtained from magnetic measurements are given in Table 5. The Eu ions have a paramagnetic character at 300 K while the magnetic ordering starts below N� eel temperature (TN) of 90.4 K [11]. The para magnetic nature of Eu3þ dopant ions in nickel ferrite nanoparticles at
mT ¼
MS M 5585
where MS and M corresponds to the saturation magnetization and atomic mass per formula unit respectively. The net magnetic moment 8
M.P. Ghosh et al.
Materials Chemistry and Physics 241 (2020) 122383
was observed to decrease in both 5 K and room temperature as per Table 6. The coercivity (HC) is also affected by saturation magnetization (MS) of the nanoparticles system below TB and follows the stated relation [34]. HC
magnetic properties can be achieved by substitution of proper amount of rare earth ions in cubic spinel phase. Declaration of competing interest
2K ¼ MS
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
where K corresponds to the effective anisotropy constant which have contributions from size, shape, volume, surface and stress anisotropies [6,8]. The MS and K were observed to reduce with increasing Eu con centration in nickel ferrites nanoparticles which results increment in coercive field in blocked magnetic state [35]. Zubair et al. also observed similar trend in Eu doped cobalt ferrite nanoparticles [16]. The M-T measurements in both zero field cooled (ZFC) and field cooled (FC) protocol were performed at 500 Oe static magnetic field for all the Eu doped samples, shown in Fig. 8. All the ZFC curves displayed a broaden peak with a maxima. The corresponding temperature in ZFC protocol is designated as blocking temperature (TB) which is found to shift towards low temperature with increasing Eu content. This repre sents the superparamagnetic (SPM) characteristics of synthesized nanoparticles [5]. The magnetic spins are blocked randomly along easy axis below TB while spins are free to rotate above TB. The thermal energy (kBTB) at blocking temperature is the threshold for activation of mag netic spins. The weakening of A-O-B superexchange interaction due to Eu substitution and tiny size of particles are responsible for the decrease in blocking temperature. The almost horizontal nature of FC curves of all Eu doped samples below TB indicated the presence of interparticle in teractions. The observed blocking temperature is just a mean value because of the nanoparticles ensemble contains a size distribution of particles [36]. The values of the effective anisotropy constant (K) are found to decrease with increase of Eu concentration and are given in Table 6. The temperature in M-T plots where ZFC and FC curves begin to separate is termed as the thermo-magnetic irreversibility temperature (Tirr). It also reflects the existence of non equilibrium magnetic state along with blocking point of largest size particles in the ensemble [36]. The observed reduction in Tirr with increasing Eu content is attributed to weak A-O-B superexchange interactions gradually. The separation be tween magnetization in ZFC-FC protocol of M-T plots near 5 K can be expressed by the stated relation [37]. � � HA MZFC ¼ MFC HA < 2HC 2HC
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where HA is the applied field (500 Oe) and HC corresponds to coercivity. 4. Conclusion A series of rare earth Eu doped NiFe2O4 nanoparticles were prepared. The room temperature XRD patterns verified the existence of cubic spinel structure. The W–H plots revealed the presence of tensile strain in the system together with average diameter of nanoparticles 20 � 4 nm. Raman spectra also confirmed the transformation of octahedral cubic symmetry to octahedral tetragonal symmetry for higher Eu concentra tion. This was attributed to the difference in the cationic sizes of Ni and Fe compared to Eu. The Tauc plots revealed an indirect allowed band gap in the range of 1.76 � 0.02 eV. The signature of paramagnetic behavior of Eu3þ ions at room temperature was confirmed by M-H curves. The blocking temperature (TB) and thermo-magnetic irrevers ibility (Tirr) decreased with increment in Eu ions in nickel ferrite nanoparticles. The weakening of superexchange interactions at the paramagnetic region of Eu3þ ions was also found. The coercive field was enhanced while saturation magnetization was reduced at low tempera ture (below TB). We can conclude that tailoring of structural, optical and
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