Microwave Absorption and the Magnetic Hyperthermia Applications of

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Microwave absorption and the magnetic hyperthermia applications of Li Zn Co Fe O nanoparticles in multi-walled carbon nanotubes matrix 0.3

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Madhumita Dalal, Jean-Marc Greneche, Biswarup Satpati, Tayssir B. Ghzaiel, Frédric Mazaleyrat, Raghumani S. Ningthoujam, and Pabitra Kumar Chakrabarti ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12091 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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ACS Applied Materials & Interfaces

Microwave absorption and the magnetic hyperthermia applications of Li0.3Zn0.3Co0.1Fe2.3O4 nanoparticles in multi-walled carbon nanotubes matrix

Madhumita Dalal1, Jean-Marc Greneche2, Biswarup Satpati3, Tayssir B. Ghzaiel4, Frédric Mazaleyrat5, Raghumani S. Ningthoujam6 and Pabitra K. Chakrabarti1* 1

Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan – 713 104, West Bengal India

2

Institut des Molécules et Matériaux du Mans -IMMM UMR CNRS 6283, Le Mans Université, 72085, Le Mans Cedex 9, France

3

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Kolkata-700064, India

4

Université de Tunis El Manar Faculté des Sciences de Tunis, UR11ES18 Unité de Recherche de Chimie Minérale Appliquée, 2092, Tunis, Tunisie 5

ENS de Cachan, 61, Avenue du Président Wilson, 94235 Cachan CEDEX, France 6

Chemistry Division, Bhabha Atomic Research Centre, Mumbai - 400085, India *E mail: [email protected] (P. K. Chakrabarti)

1 ACS Paragon Plus Environment


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Abstract Nanoparticles of Li0.3Zn0.3Co0.1Fe2.3O4 (LZC) were prepared by sol-gel method and dried in furnace at ~ 200 ˚C. The dried sample was annealed at 500, 600, 700 and 800 ˚C for 5 h each. Rietveld analysis of X-ray diffraction (XRD) patterns confirm the cubic Fd3m phase formation with lattice parameters ranged from 8.376 up to 8.390 Å and allow the crystallite sizes (dcryst) to be estimated. To enhance the microwave (MW) absorption as well as the effectiveness for hyperthermia treatment, nanoparticles are taken in the matrix of multi-walled carbon nanotubes (MWCNTs) and the morphology of the so prepared samples (LZC@MWCNT) was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. Both static and dynamic magnetic properties were investigated on the samples of LZC nanoparticles and compared to those of the samples of LZC@MWCNT. The samples annealed at 500, 600 and 800 ˚C are excellent candidates in cancer treatment as AC magnetic heating analysis shows that the hyperthermia temperature (42 ˚C) was successfully achieved for an applied ac magnetic field of 420 Oe and 300 kHz frequency. MW absorption study also reveals that the samples of LZC@MWCNT could be used as potential MW absorbing material for which a maximum reflection loss (RL) of ~ -21 dB was achieved at a frequency of 15.27 GHz for only a 1 mm layer thickness. KEYWORDS: X-ray diffraction; Electron microscopy; Magnetic properties; Ferrites; Carbon nanotube; Microwave absorption; AC induction heating

1.

Introduction Nanocrystalline spinel ferrites have been widely investigated in different fields of research

due to their interesting applications in the low frequency region as well as high frequency region.


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One of the recent applications in the low frequency region is the use of ferrites as heat mediator in magnetic fluid hyperthermia (MFH) treatment

1-11

. Besides, in the high frequency region

ferrites are best used as microwave (MW) absorber 12-20. In MFH, a ferrofluid can be used as the heat mediator in a low-frequency ac magnetic field. The mechanism consists of a controlled local heating to destroy the cancerous tissues without affecting the healthy ones. At ~ 42 ˚C cancer cells are damaged according to a pathobiological effect in which the normal structure of the proteins, nucleic acids and phospholipids are modified causing deterioration of cell-structure integrity 1. So, the 42 ˚C temperature is believed to be the hyperthermia temperature 2. The effectiveness of a material for hyperthermia treatment depends on its magnetization and anisotropy. When single domain superparamagnetic (SPM) nanoparticles are placed in an ac magnetic field, heat is generated due to Brownian and Néel’s spin relaxations. In this direction, iron oxides (Fe3O4, α-Fe2O3 and intermediate compositions) are the most studied spinel ferrites for hyperthermia treatment hyperthermia treatment

7-8

3-6

. But, there are also instances of using other materials for

. The main parameter to declare a material as an effective heat

mediator is its specific absorption rate (SAR) which is defined as the energy amount converted into heat per unit time and per unit mass. The SAR of several spinel ferrites like CoFe2O4, MnFe2O4, LiFe2O4 etc. are already reported

9-11

. SAR can be enhanced by improving both the

magnetization of the nanoparticles and the anisotropy. Besides, the study of MW absorption potential of spinel ferrites is another established field of research. The high permeability and high resistivity of spinel ferrites lead to the superior absorption quality of these ferrites. Due to their high resistivity, magnetic losses arising from the eddy currents can be neglected 2. However, in a frequency range from 1 MHz up to few hundred MHz of incident electromagnetic (EM) wave, the magnetic loss arising due to domain-wall



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resonance becomes effective

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. Again, for the EM wave with frequency of 2 to 15 GHz,

magnetic loss arises mainly due to the natural and exchange resonances

13

. When the ferrite

nanoparticles in multi-walled carbon nanotubes (MWCNTs) matrix is placed in EM field, electrons can migrate through the interfaces between ferrite nanoparticles and MWCNTs originating an interfacial polarization. This leads to strong dielectric loss of EM wave in case of ferrite nanoparticles in MWCNTs matrix. The perfect matching of magnetic and dielectric losses in these samples causes a strong absorption of the incident EM waves. Furthermore, the lightweight and the strong absorption efficiency of ferrite nanoparticles in MWCNTs matrix 14, 15 make them potential candidate to be used as MW absorber as well as MW devices. For example, it was established that Fe3O4 in MWCNTs

16

, CoFe2O4 in MWCNTs

17

, Li-Zn-Cu-ferrite in

MWCNTs 18 and others 19, 20 cause effective absorption of EM waves. The soft magnetic Li-Zn-ferrites with high saturation magnetization and SPM behaviour were studied by many authors also considered

23, 24

21, 22

and in some cases doping by Mg or Ti in Li-Zn-ferrites are

. The structural, magnetic, initial permeability and Mössbauer studies of Co

substituted Li-Zn-ferrite with compositional formula Li0.4–0.5xZn0.2CoxFe2.4–0.5xO4 (x = 0.0 ≤ x ≤ 0.1 in steps of 0.02) have also been studied by Soibam et al.

25, 26

. But the properties like MW

absorption capability or ac magnetic heating efficacy of these ferrites were not considered by the author. In this direction, the present work aims to investigate these properties for a different composition of Li-Zn-Co-ferrite: Li0.3Zn0.3Co0.1Fe2.3O4 (LZC). As non-magnetic Zn substitution improves the magnetization of Li-ferrite

22

so in the present case a greater content of Zn is

considered. Desired phase formation is analysed and different structural and microstructure parameters are extracted by analysing the X-ray diffraction (XRD) pattern with the help of material analysis using diffraction (MAUD). The prepared nanoparticles are successfully taken


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in MWCNTs matrix, as confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. In addition, the analysis of Raman spectrum of one of the samples of nanoparticles in MWCNTs matrix is also included. Magnetic and Mössbauer studies are also performed to explain the magnetic and hyperfine behaviours of the nanoparticles and those in MWCNTs matrix. Finally, the effectiveness of the nanoparticles in MFH treatment and applicability of nanoparticles in MWCNTs matrix as MW absorbing material is investigated.

2.

Experimental details

Nanoparticles synthesis Nanoparticles of LZC were synthesised by sol-gel method where LiNO3, Zn(NO3)2·6H2O, Co(NO3)2·6H2O and Fe(NO3)3·9H2O were taken as precursor materials. A clear aqueous solution of the precursor materials in a molar ratio of 0.3:0.3:0.1:2.3 was formed by means of magnetic stirring. Besides, another clear aqueous solution of citric acid was formed taking one mole of the citric acid equal to the total moles of the nitrates. Then, the citric acid solution was added to the nitrate solution under vigorous stirring condition and stirring was continued for ~ 14 hrs at a fixed temperature of ~ 80 ˚C. As soon as gel was formed the beaker was shifted to the furnace and kept there for ~ 4 hrs at 100 ˚C. Then the furnace temperature was increased up to ~ 200 ˚C for further removal of nitrates and the product sample was designated as LZC200. To get nanoparticles of different sizes, the sample, LZC200 was annealed for 5 h each at 500, 600, 700 and 800 ˚C and the corresponding products were designated as LZC500, LZC600, LZC700 and LZC800.

Nanocomposite synthesis



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Nanoparticles of the samples, LZC200 and LZC500 were taken in the matrix of MWCNTs by means of ultrasonication. LZC nanoparticles and MWCNTs were taken in a ratio of 9:1 and dispersed into xylene medium. The uniform dispersion of LZC nanoparticles was added drop wise into the dispersion of MWCNTs under ultrasonication. After addition, ultrasonication was continued for two more hours to ensure uniform incorporation of LZC nanoparticles in MWCNTs matrix. Employing the magnetic property of the nanoparticles, samples were collected at the bottom of the beaker with the help of a permanent magnet while xylene was decanted followed by slow evaporation of the remaining xylene. The two samples were accordingly designated as LZC200@MWCNT and LZC500@MWCNT which represents the LZC nanoparticles dried at 200 ˚C in MWCNTs matrix and the LZC nanoparticles annealed at 500 ˚C in MWCNTs matrix, respectively. Characterization techniques XRD patterns of the samples were recorded by means of a powder X-ray diffractometer, Model BRUKER D8 Advance with da Vinchi, using Cu Kα radiation (λ=1.5405Å) and analyzed by Rietveld method using the MAUD program to confirm the crystallographic phase formation. SEM micrographs of the samples, LZC200@MWCNT and LZC500@MWCNT were recorded using ZEISS SUPRA40 field emission gun (FEG) SEM. TEM micrographs of the sample, LZC200@MWCNT were recorded using FEI, Tecnai G2 F30-ST microscope operated at 300 kV. The same microscope is equipped with a scanning unit, a high-angle annular dark field (HAADF) detector for scanning transmission electron microscopy (STEM-HAADF) and the energy dispersive X-ray spectroscopy (EDX), attachment to perform elemental analyses. Raman spectrum of the sample, LZC500@MWCNT was recorded in HORIBA using Nd:YAG laser of excitation wavelength 532 nm and power 4 mW. Mössbauer spectra of the samples, LZC200,


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LZC600 and LZC200@MWCNT were recorded at 300 and 77 K using a bath cryostat in a transmission mode with constant acceleration driving unit using a

57

Co/Rh γ-beam source.

Standard Fe foil was used to calibrate the spectrometer and the isomer shift (IS) values were determined with reference to that of standard Fe foil at 300 K. MOSFIT program was employed to fit the spectra where elemental magnetic sextets and quadrupolar doublets were composed of Lorentzian lines. Dynamic magnetic properties were investigated by the hysteresis loops at 300 K using a digital hysteresis loop tracer supplied by Metis Instruments and Equipments NV, Belgium at a frequency of 50 Hz. Static hysteresis loops were recorded using vibrating sample magnetometer (VSM) and superconducting quantum interference device - vibrating sample magnetometer (SQUID-VSM) for a maximum applied field of 1.2 and 5 T, respectively. AC induction heating was performed in 1.5 ml microcentrifuge tube which was kept at the centre of the coil having 5 cm diameter with 4 turns, magnetic field of 420 Oe and 300 kHz frequency. The scattering parameters, S11 and S21, of the samples, LZC200@MWCNT and LZC500@MWCNT were recorded in the frequency range of 8 – 18 GHz using an Agilent E8363B vector network analyzer.

3.

Results and discussion

3.1 Structural and microstructure analysis XRD and Rietveld analysis To investigate the structural and microstructure properties of LZC nanoparticles, the experimental XRD patterns (Fig. 1) were analyzed by Rietveld method using MAUD version 2.33. Characteristic peaks of cubic Fd3m phase are found in the XRD patterns of the nanoparticles which are accordingly matched with the JCPDS file no. 71-1268. It confirms the



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spinel ferrite phase formation in the nanoparticles. However, a very small peak attributed to rhombohedral (R3c) α-Fe2O3 (JCPDS file no. 89-0599) phase is observed in the XRD pattern of the sample, LZC600 at 2θ ≅ 33.16˚ suggests that a small fraction of α-Fe2O3 phase does occur in the samples, LZC200 and LZC500 which are annealed at temperatures below 600 ˚C but the corresponding peak is not distinguishable from the background scatterings. Such segregation of α-Fe2O3 phase is a very common fact while burning iron oxides in air

27, 28

. However, in the

samples, LZC700 and LZC800 which are annealed at higher temperatures (700 and 800 ˚C, respectively), no trace of α-Fe2O3 is noticed. In case of the samples, LZC200@MWCNT and LZC500@MWCNT a peak assigned to hexagonal P63/mmc phase appears at 2θ ≅ 26˚ which ensures the presence of MWCNTs (JCPDS file no. 25-0284) in these two samples. Thus, the XRD patterns of the samples, LZC200, LZC500, LZC700 and LZC800 are fitted considering Fd3m phase only, that of the sample, LZC600 is fitted considering both Fd3m and R3c phases while those of the samples, LZC200@MWCNT and LZC500@MWCNT are described using Fd3m and P63/mmc phases. Values refined from Rietveld analysis are summarized in Table 1. Relative weight percentages of α-Fe2O3 in the sample, LZC600 and MWCNTs in the samples, LZC200@MWCNT and LZC500@MWCNT are 28.5, 19.5 and 19.2 %, respectively. As we have considered 9:1 ratio of ferrite and MWCNTs, the weight percentage of MWCNTs in the samples, LZC200@MWCNT and LZC500@MWCNT should be ~ 10 %. But, we have obtained higher values of relative weight percentages of MWCNTs which is due to the loss of comparatively denser ferrite nanoparticles during preparation. The values of the lattice parameters of the nanoparticles are in the range of 8.370 – 8.390 Å, slightly higher than that of Li0.35Zn0.2Co0.1Fe2.35O4 (8.320 Å) as reported by Soibam et al.

25

. It is due to the fractional

substitution of Li1+ and Fe3+ ions of smaller ionic radii (0.68 and 0.64 Å, respectively) by Zn2+


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ion of higher ionic radius (0.74 Å). Crystallite sizes (dcryst) of the nanoparticles are calculated from Rietveld analysis using a lognormal type of distribution and those lie in the range of 5 – 69 nm (Table 1). These values are similar to those (Fig. 2a) calculated using Debye-Scherrer equation

29

. Very small amount of lattice strain (~ 10-3 – 10-4) is present in the samples.

Occupancy of Zn2+ ions in B-site is higher in smaller nanoparticles which support the behaviour of Zn2+ ions when approaching nano dimensions 30. But Li1+ and Co2+ ions are found to occupy B-site. The atomic structure of the sample, LZC500 (Fig. 2b) is derived using the lattice parameters obtained from Rietveld analysis by means of visualization for electronic and structural analysis (VESTA) (ver. 3.3.1). SEM analysis To study the structural morphology of the nanoparticles in MWCNTs matrix, SEM micrographs (Fig. 3) of the two samples, LZC200@MWCNT and LZC500@MWCNT, are analyzed. Micrographs clearly show the presence of MWCNTs and LZC nanoparticles. LZC nanoparticles are spherical in shape as obvious from the micrographs but those are mostly agglomerated due to strong magnetic interaction among those. Diameters of nanoparticles of the sample, LZC200@MWCNT lie in the range of 13 to 22 nm which agrees well with dcryst of the sample, LZC200@MWCNT (~ 8 nm) calculated using Debye-Scherrer equation. However, the average diameter (~ 19.2 nm) calculated by lognormal fitting of the distribution of diameters of the nanoparticles is much higher than that calculated using Debye-Scherrer equation. This difference mainly comes from the strong agglomeration tendency of the nanoparticles as well as the very small size of those. Similarly, the average diameter of the nanoparticles of the sample, LZC500@MWCNT obtained by lognormal fitting of the distribution of diameters of the



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nanoparticles is ~ 25.1 nm which is close to dcryst of the sample, LZC500@MWCNT (~22.5 nm) calculated using Debye-Scherrer equation. TEM analysis TEM is a very important tool to transfer the information from materials to images. In order to get information about the particle size, shape, crystallinity and incorporation of nanoparticles in MWCNTs matrix, the TEM images (Figs. 4a and b) of the sample, LZC200@MWCNT are taken. Spherical shape of the nanoparticles is confirmed having a lognormal size distribution with an average diameter of ~ 12 nm which is close to dcryst (~ 8 nm) of the sample, LZC200@MWCNT as calculated by Debye-Scherrer. Thus, here the average diameter of the nanoparticles is determined more accurately compared to SEM analysis. High-resolution TEM (HRTEM) images (Figs. 4c and d) indicate that the particles are mostly nanocrystallites. Presence of MWCNTs is further confirmed from the micrographs though encapsulation is not ensured everywhere. Different crystallographic planes are marked in the selected area diffraction (SAED) pattern (Fig. 4e) by calculating the inter planer spacing (d-spacing) from the concentric rings and those include the crystallographic planes of ferrite nanoparticles as well as that of MWCNTs. The (113) plane of α-Fe2O3 with d-spacing of 0.22 nm is also marked in the SAED pattern which confirms the presence of α-Fe2O3 in the sample, LZC200@MWCNT though the fraction might be small. Crystallographic planes of ferrite nanoparticles and MWCNTs are also confirmed from the HRTEM (Figs. 4c and d). The lattice fringes with separations of 0.48, 0.25 and 0.35 nm correspond to the (111), (311) planes of ferrite and (002) plane of MWCNTs, respectively. Here, ferrite nanocrystallites appear to be surrounded by the lattice fringes of MWCNTs. To investigate the chemical compositions of the sample, STEM-HAADF-EDX analysis is performed and the presence of Fe, Zn, O and C is confirmed (Fig. 4f). Fe, Zn and O


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signals are coming from the ferrite nanocrystallites and C signal is due to the MWCNTs. However, C signal also includes the C of the C-coated grid and the trace of Cu is appeared as we have used a Cu grid. No trace of Li is recorded as it is a light element which cannot be easily evidenced from EDX analysis. Also, no trace of Co is found in the EDX spectrum (Fig. 4f). Thus, to confirm the brighter contrast in the sample (Fig 4f), we carried out EDX line scan (Fig. 4h) along the line displayed in Fig. 4f and it confirms the presence of C, Fe, Zn and also Co.

Raman analysis Raman spectrum of the sample, LZC500@MWCNT is recorded in the wave number range of 200 – 1800 cm-1 at 300 K (Fig. 5). The bands appeared in the low wave number range (200 – 800 cm-1) correspond to the Raman active modes of ferrites. To get a detailed analysis of the obtained spectrum in the low wave number range, it is de-convoluted into components (inset of Fig. 5) where each component is fitted with Lorentzian line shape. As shown in Fig. 2b the unit cell of the sample, LZC500 contains 56 atoms while the smallest Bravais cell contains 14 atoms only. Hence there could be 42 vibrational modes among which only five modes are Raman active. These five modes represent the vibrations of O ions and both the O and A-site ions

31

.

Though the modes appeared here (Fig. 5) are very much similar to the Raman active modes of other ferrites

32, 33

but their number is more in our case. We have obtained nine modes in the

wave number range 200 – 800 cm-1 which are at 254, 288, 334, 466, 520, 576, 608, 649 and 686 cm-1. Among these, the three major components at 334, 466 and 686 cm-1 are assigned to Eg, T2g(2) and A1g, respectively all of which represent the normal mode motion of AO4 tetrahedron 31

. Eg represents the symmetric bend of O with respect to A-site ions, T2g(2) represents the

asymmetric stretch of O and A-site ions and A1g represents the symmetric stretch of O along A ˗



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O bonds. At the low frequency side of A1g mode there is another component at 608 cm-1 with comparable intensity which also represents A1g mode. This doublet like structure has also been observed in earlier reports on Zn0.25M0.75Fe2O4 Li0.35Zn0.30Fe2.35O4

22

31

, CoFe2O4 and substituted CoFe2O4

33

and

. The degeneracy in A1g mode comes from the locally distorted AO4

tetrahedron. In Fe3O4, A- and B-sites are completely occupied by Fe ions whereas in case of the sample, LZC500 it is seen from the Rietveld analysis that A-site is occupied by Zn and Fe ions while B-site is occupied by Li, Co, Zn and Fe ions. Due to this redistribution of cations having different ionic radii the bond distances are also redistributed which produces distortion in the AO4 tetrahedron. Two other components at 576 and 649 cm-1 also appear due to this locally distorted tetrahedron. The component at 520 cm-1 is assigned to the T2g(3) Raman active mode and represents the asymmetric bend of O with respect to A-site ions. The extreme left component at 254 cm-1 is assigned to the T2g(1) mode and represents the translatory motion of AO4 tetrahedron as a whole. As reported by Misra et al. the Raman modes of Li0.35Zn0.30Fe2.35O4 annealed at 400 ˚C are appeared at 355, 495, 670 and 715 cm-1

22

. Compared to that, here the

Raman modes are appeared at comparatively lower wave numbers viz., 334, 466, 608 and 686 cm-1. This red shift occurs due to the substitution of Li and Fe by Co having higher atomic mass 33

. Again, the component at 288 cm-1 with very low intensity matches the Eg mode of α-Fe2O3 34,

35

which confirms the presence of a small fraction of α-Fe2O3 in the sample,

LZC500@MWCNT. The other two major bands at 1347 and 1576 cm-1 (Fig. 5) are related to the stretching of SP2 hybridized carbon in MWCNTs. Thus, the analysis of spectral bands confirms the phase formation of the samples of LZC nanoparticles in MWCNTs matrix.

3.2 Mössbauer analysis


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Mössbauer spectra of the samples, LZC200, LZC600 and LZC200@MWCNT are recorded at 300 and 77 K (Fig. 6a). At 300 K the spectrum of the sample, LZC200 appears as a doublet merged with a small trace of sextet. Thus the spectrum of the sample, LZC200 is fitted with a quadrupolar doublet and a hyperfine field distribution. These two patterns together indicate the presence of a mixed state of SPM and blocked ferrimagnetic nanoparticles of the sample, LZC200. Similar type of behaviour is found in the spectrum of the sample, LZC200@MWCNT which indicates that the behaviour of the nanoparticles of the sample, LZC200 is not changed due to incorporation in MWCNTs matrix. Though, in the later case the relative area (RA) percentage of SPM nanoparticles is slightly increased viz., the RA percentage of doublet patterns in the samples, LZC200 and LZC200@MWCNT are 40 and 42 %, respectively. The IS values of the doublet and the hyperfine distribution of the two samples vary from 0.3 to 0.4 mm/s which is in agreement with the earlier results on Li-Zn- and Li-Zn-Co-ferrites

21, 26

. However at 77 K,

doublet patterns are hardly visible in any of the spectra rather sextets are appeared in the spectra of both samples. In the spectra of the sample, LZC200, the difference between IS values at 300 K (IS = 0.3 mm/s) and 77K (IS = 0.43 mm/s) is ~ 0.13 mm/s and this agrees well with the difference in IS values (~ 0.15 mm/s) assuming the change in IS due to second order Doppler (SOD) as - 0.0007 mm/s

36

. At 77 K, the mean internal hyperfine field Bhf of the samples,

LZC200 and LZC200@MWCNT are 45.3 and 45.9 T, respectively which is similar to the other reported values

21, 26

. Besides, the spectra of the sample, LZC600 at 300 and 77 K appear to be

clear sextets suggesting that the nanoparticles of the sample, LZC600 are mostly in ferrimagnetic state. Moreover, the spectrum of the sample, LZC600 at 77 K is fitted with two distinct sextets which mean for the Fe3+ ions in A- and B-sites of the spinel structure. Usually, the sextet with larger Hint and larger IS is assigned to the Fe3+ ions at B-site 37. Thus, here the sextet with IS ~



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0.46 mm/s and Bhf ~ 53.1 T is assigned to the Fe3+ ions at B-site and the other one with IS ~ 0.42 mm/s and Bhf ~ 50.9 T is assigned to the Fe3+ ions at A-site. The relative absorption areas of the two sextets are 25 and 75 %, respectively, but they are strongly dependent on the fitting procedure: indeed the lack of resolution prevents an accurate estimation of the octahedral and tetrahedral Fe content. It is necessary to apply an external magnetic field of about 6-8 T to resolve the hyperfine structure according the ferromagnetic structure. To confirm the ferrimagnetic state of the nanoparticles, Mössbauer spectra of other three samples (LZC500, LZC700 and LZC800) are also recorded at 77 K (Fig. 6b). The spectra of these three samples are individually fitted with three sextets and the average IS values are obtained as 0.43, 0.43 and 0.42 mm/s, respectively. These values of IS confirm the presence of Fe3+ ions only. The average values of Bhf of these three samples are 50.8, 51.2 and 51.0 T, respectively, i.e. completely equivalent.

3.3 Magnetic analysis Dynamic magnetic properties To inspect SPM relaxation in the samples, 300 K dynamic hysteresis loops are recorded at a frequency of 50 kHz for a maximum applied field of ~ 33 kA/m (415 Oe). The loop of the samples, LZC200 and LZC200@MWCNT (Fig. 7) are very narrow and linear in nature which confirms the SPM nature of these two samples as predicted by Mössbauer analysis. Gradual widening of the loops with the increase of annealing temperature indicates an increase in the ferrimagnetic fraction of nanoparticles of the samples annealed at higher temperatures. Maximum magnetizations (Bm) of the samples, LZC200, LZC500, LZC600, LZC700, LZC800, LZC200@MWCNT and LZC500@MWCNT are 16.9, 23.1, 29.7, 35.6, 40.4, 16.6 and 21.4


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emu/g, respectively. Earlier, Sutradhar et al. reported the dynamic magnetic properties of Li0.32Zn0.26Cu0.1Fe2.32O4 annealed at 600 ˚C

18

and a comparison to this report reveals that LZC

nanoparticles have slightly lower values of Bm and coercivity (Hc) though annealed at a same temperature. This difference arises mainly due to the different stoichiometry of the samples and the inclusion of Co instead of Cu, as Cu and Co species are known to increase magnetization and anisotropy, respectively. Also, the values of Bm of the two samples, LZC200@MWCNT and LZC500@MWCNT have lower values compared to those of the corresponding bare ferrite nanoparticles (LZC200 and LZC500, respectively). Again, Bm is increased with the increase of dcryst though Hc exhibits a different behaviour (shown in the inset of Fig. 7). With the variation of dcryst, Hc first increases up to ~ 90 Oe at dcryst ~ 23 nm but then decreases with further increase of dcryst. This type of variation of Hc can be explained by the domain size effect 38. In single domain nanoparticles magnetization is built up by the spin rotation whereas in multi-domain particles magnetization is built up by the domain-wall movement. The energy required for magnetization reversal in single-domain particle is higher compared to that required for multi-domain particles. This leads to higher value of Hc in single-domain particles than multi-domain counterpart. Additionally, from the observed variation of Hc, single domain critical size could be estimated at ~ 23 nm. The two samples, LZC200@MWCNT and LZC500@MWCNT, have Hc of 30.8 and 84.1 Oe, respectively. Remnant magnetizations (Br) of the samples, LZC200, LZC500, LZC600, LZC700, LZC800, LZC200@MWCNT and LZC500@MWCNT are 1.1, 5.5, 7.1, 8.6, 7.8, 1.3, 4.6 emu/g, respectively. An increase in Hc and Br of the sample, LZC200@MWCNT is observed compared to those of the sample, LZC200 which causes an increase in the loop area and directs towards an enhancement of the effective anisotropy while nanoparticles are taken in MWCNTs matrix. Again, SAR could be directly derived from the loop area using the relation: SAR = Area



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× frequency

39

and the knowledge of SAR is essential to know the heating efficiency for

hyperthermia. The SAR values of the samples, LZC200, LZC500, LZC600, LZC700, LZC800, LZC200@MWCNT and LZC500@MWCNT as derived from the dynamic hysteresis loops are 57, 274, 328, 338, 286, 75 and 218 W/g, respectively at a frequency of 50 Hz of the applied ac magnetic field. The SAR of the sample, LZC200@MWCNT is increased compared to that of the sample, LZC200.

Static magnetic properties To get an insight into the magnetic properties of the nanoparticles, static hysteresis loops of the nanoparticles are recorded at 300 K for a maximum applied field of 1.2 T using VSM. The loops (Fig. 8a) are very similar to that obtained using dynamic magnetic measurement with similar type of Hc variation with dcryst (shown in the inset of Fig. 8a). However, the Hc values obtained here are higher compared to that obtained from dynamic hysteresis loops. It is due to the better precision of the measuring instrument. Very low value of Hc (~ 10.7 Oe) of the sample, LZC200 is the manifestation of SPM nature of the nanoparticles of this sample. Observed Hc variation with dcryst (shown in the inset of Fig. 8a) confirms the single domain critical size to be ~ 23 nm. For applied field of 1.2 T, saturation magnetization (Ms) of the samples, LZC200, LZC500, LZC600, LZC700 and LZC800 are 37.6, 62.0, 70.9, 82.2 and 86.5 emu/g, respectively while the corresponding remnant magnetizations (Mr) are 0.7, 12.1, 13.5, 12.8 and 9.0 emu/g, respectively. Ms gradually increases with the increase of dcryst which confirms the built up of ferrimagnetic fraction with the increase of annealing temperature. Interestingly, Hc of the sample, LZC500 (127.5 Oe) is increased by almost ten times compared to Hc of Li0.32Zn0.36Fe2.32O4 (10.4 Oe) annealed at same temperature (500 ˚C) and having dcryst ~ 21.4 nm 21. Such increase in Hc is


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attributed to the 10 % Co present in LZC nanoparticles and indicates towards the improvement of anisotropy which is very significant for the application of LZC nanoparticles as heat mediator in hyperthermia treatment. However, in this process Ms of LZC nanoparticles (62.0 emu/g) is diminished from that of Li0.32Zn0.36Fe2.32O4 (68 emu/g)

21

. Again, an improvement in the

magnetization of the sample, LZC800 at 300 K (86.5 emu/g) is noticed compared to Li0.35Zn0.2Co0.1Fe2.35O4 nanoparticles (55 emu/g) as reported by Soibam et al.

26

. This

improvement is probably achieved by the better crystallinity of the nanoparticles of the sample, LZC800 (dcryst ~ 58 nm as obtained from Debye-Scherrer) though annealed at a lower temperature

(800

˚C)

compared

to

the

annealing

temperature

(1050

˚C)

of

Li0.35Zn0.2Co0.1Fe2.35O4 nanoparticles (dcryst ~ 26 nm). Additionally, an increased Zn content may also be another reason for increasing magnetization. For more detailed investigation, magnetic properties of the sample, LZC200 are measured using SQUID-VSM at low temperature (Fig. 8b). For a maximum applied field of 5 T, Ms of the sample, LZC200 at 300 and 150 K are 44.4 and 61.7 emu/g, respectively and the corresponding Hc values are 28.4 and 68.3 Oe, respectively. Both the values of Ms and Hc are increased with the increase of maximum applied field. These high values of Ms and Hc of the sample, LZC200 cause high magnetic loss which helps to perfectly match the magnetic and dielectric losses coming from the ferrite and MWCNTs, respectively. Thus reliable MW absorption in the samples, LZC@MWCNT is assured. Again, the loop area is a major determining factor in case of hyperthermia since it is directly related to the SAR value as mentioned earlier. The loop area of the samples, LZC200, LZC500, LZC600, LZC700 and LZC800 as calculated from the 300 K hysteresis loops are 0.21, 1.98, 2.19, 1.94 and 1.44 J/kg, respectively. Those results suggest that the nanoparticles of the sample, LZC600 with maximum loop area may be more favourable for hyperthermia. Using the value of Ms (91.7



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emu/g) of LZC800 at 10 K, degree of inversion (1-x) can be calculated assuming the cation distribution as Zn Fe Li. Zn. Co. Fe . O

40

. The value of x (= 0.18) of the

sample, LZC800 calculated in this way comes out to be exactly equal to that calculated from the Rietveld analysis (Table 1). The values Hc and Mr of the sample, LZC800 at 10 K are 224.8 Oe and 35.4 emu/g, respectively. At 10 K, the squareness ratio (Mr/Ms) is reached a value of ~ 0.4 ( ~ 0.1 at 300 K). This increase in Mr/Ms ratio points towards the freezing of magnetic spins along the applied field at low temperatures. The low value of Hc (53.5 Oe) and Mr/Ms ratio (0.1) with a high value of Ms (86.5 emu/g) at 300 K suggests that LZC nanoparticles could also be used in soft magnetic devices. To determine the magnetocrystalline anisotropy parameters, FC and ZFC magnetizations of the samples, LZC200 and LZC800 are recorded in the temperature range of 300 – 5 K (Figs. 8c and d). From the behaviour of the curve, it is confirmed that TB is far above 300 K. It interprets that the thermal energy at 300 K is not sufficient to flip the magnetic moments of all the nanoparticles of the samples, LZC200 or LZC800. The magnetic moments of a fraction of nanoparticles having comparatively larger size may remain aligned along the direction of applied magnetic field. However, TB is not a single temperature but rather a distribution which arises due to the distribution of particle size as resulting from TEM analysis. The distribution function of TB can be written as fT =

√ ! "!

exp %−

'()* !,

!+

"/!

Here, the symbols have their usual meanings

-.

/

41

0

(1)

. The difference between FC and ZFC

magnetizations can be estimated from fT and is given by 42, 43 ∆M =

34/5 6 789::

;1 − erf '

() ⁄ ! √"!

−

"!

.?

√


(2)

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Here we have considered T = T exp@√2λ xC and x = F

() ! ⁄ !+ . √"!

The error function is

introduced as D e E dt = @√π⁄2CerfcT, where erfcT = 1 − erf T. The difference of FC /

and ZFC magnetizations (∆M) is also plotted as a function of temperature (shown in the inset of Figs. 8c and d). The observed ∆M is fitted using Eq. 2 where different parameters viz., TB, Keff and λB are varied systematically. For a fixed applied field of 100 Oe, Keff of the samples, LZC200 and LZC800 are 4.8 × 106 and 6.7 × 106 erg/cm3, respectively. Keff of the sample, LZC200 (dcryst ~ 8 nm) is greater than that of Li0.32Zn0.36Fe2.32O4 (~2.9 × 106 erg/cm3) with dcryst ~ 21.4 nm and obtained for an applied field of 500 Oe 21. Also, Keff of the sample, LZC800 (dcryst ~ 58 nm) is greater than that of Li0.35Zn0.30Fe2.35O4 (~ 1.1 × 106 erg/cm3) with dcryst as high as 110 nm for an applied field of 5 kOe 22. This increase of Keff is due to the presence of Co in the LZC nanoparticles. Apart from this, shape anisotropy, grain boundary and lattice defects also contribute to magnetocrystalline anisotropy. Finally, the high value of Keff of LZC nanoparticles causes large magnetic loss

44

and makes them suitable for hyperthermia treatment as well as

good MW absorbing medium.

3.4 Magnetic hyperthermia The heating efficiency of the samples, LZC200, LZC500, LZC600, LZC800 and LZC200@MWCNT are studied by recording the time dependent temperature curves (Figs. 9a and b) at two different concentrations (2 and 10 mg/ml) in an applied ac magnetic field of 420 Oe and 300 kHz frequency. With a concentration of 2 mg/ml of the samples, LZC500, LZC600 and LZC800, the hyperthermia temperature i.e., 42 ˚C

45, 46

is achieved within 3 minutes.

However, in the case of the sample, LZC200 with the same concentration, a maximum temperature of 41 ˚C is reached after 10 minutes. Thus, the heating behaviour of the sample, 19 ACS Paragon Plus Environment


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LZC200 is recorded with a higher concentration of 10 mg/ml and the hyperthermia temperature is attained within 2 minutes. The heating efficiency of the sample, LZC200@MWCNT is also studied with a concentration of 10 mg/ml and 42 ˚C is reached within 5 minutes. In the other three samples (LZC500, LZC600 and LZC800), a sharp rise in the temperature is noted for a concentration of 10 mg/ml and after reaching 50 ˚C the instrument is stopped. Heat generation in the samples, LZC200, and LZC500 and LZC200@MWCNT having dcryst less than or comparable to the single domain critical size is due to Brownian and Néel’s spin relaxations while that in other two samples (LZC600 and LZC800) having multidomains is mainly due to hysteresis losses

45

. Since Li-Zn-Co ferrite has high resistivity, the loss due to

eddy currents may be ignored. In single domain particles, relaxation losses are caused either by the rotation of the magnetic particles related to Brownian motion or by the reversal of magnetic spins originating from SPM relaxation phenomena. In the first case it is known as Brownian relaxation and the second one is known as Néel’s relaxation. The power loss due to Brownian or Néel’s relaxation can be calculated by the following formula: P = mHωτ⁄2τkTρV1 + ω τ

(3)

Here, m, H and ω denote the particle magnetic moment, the amplitude and angular frequency of the applied magnetic field, respectively, while τ, k, ρ and V represent the relaxation time, the Boltzmann constant, the density of LZC nanoparticles and the particle volume, respectively. According to Debye, P is maximum when ωτ = 1 47. Considering Keff = 4.8 × 106 erg/cm3, ω = 18.85 × 105 s-1 (f = 300 kHz) and T = 300 K the estimated value of particle size for maximum power loss due to Néel’s relaxation (dNéel) is ~ 4 nm. Also, with the frequency of the applied magnetic field the estimated value of hydrodynamic particle size for maximum power loss due to Brownian relaxation (dBrown) is ~ 11 nm

45

. In case of the samples, LZC200 and


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LZC200@MWCNT dcryst (~ 8 nm) is much greater than dNéel but it is below dBrown. So in these two cases Brownian relaxation may be dominating over Néel’s relaxation. Again, from the HRTEM micrographs (Figs. 4a and b) it is seen that the nanoparticles are mostly agglomerated due to strong magnetic dipolar interaction among those. Hence, power loss due to the Brownian or Néel’s relaxation remains relatively smaller than the possible peak values at the applied frequency of the magnetic field. In case of the sample, LZC500 dcryst (~ 23 nm) is much above dNéel and dBrown. Again, the hysteresis loop area of the sample, LZC500 has considerable value (~ 1.98 J/kg). Thus, in case of the sample, LZC500 both spin relaxation and hysteresis loss causes heat generation. In the other two samples (LZC600 and LZC800) main contribution to heat generation comes from the hysteresis loss. The applicability of a material as a heat mediator in MFH treatment is best measured by the SAR of the material in an ac magnetic field. The SAR can be calculated using the relation: ∆

SAR = C ∆E X

(4)

YZ[\

Here the symbols have usual meanings

45

. The SARs calculated from the ac magnetic heating

differ from those estimated from the loop area of the dynamic hysteresis loops which is due to the difference of frequency in the two measurements (300 kHz and 50 Hz, respectively) and also due to that of maximum applied magnetic field for measurements (420 and 415 Oe, respectively). A maximum SAR of ~ 379 W/gFerrite is obtained for 2 mg/ml of the sample, LZC600 with dcryst ~ 36 nm (as obtained from Debye-Scherrer) for an applied ac magnetic field of 420 Oe and 300 kHz. This result supports the prediction based on the hysteresis loop area, as mentioned earlier. For the same concentration (2 mg/ml), SAR of the sample, LZC800 decreases down to 167 W/gFerrite. Indeed, the sample, LZC800 with larger dcryst (~ 58 nm) possess less grain boundaries and thus less pinning sites whereas the sample, LZC600 with smaller dcryst (~ 36 nm) 21 ACS Paragon Plus Environment


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have more pinning sites. These pinning sites prevent domain wall motion and thus require additional energy to keep domain walls moving across the grain boundaries resulting more heat generation. As well, SAR of the samples, LZC200 and LZC500 at 2 mg/ml concentration are 205 and 199 W/gFerrite, respectively. Furthermore, the SAR of the sample, LZC200@MWCNT for a concentration of 10 mg/ml is ~ 31 W/gFerrite which remains much below the SAR of the sample, LZC200 (83 W/gFerrite) for the same concentration. However, at 10 mg/ml concentration of the samples, LZC200, LZC600 and LZC800, it is difficult to get approximate value of SAR since temperature rises in a short span of time (a few seconds) and time-temperature plot is not curve type. SAR of the sample, LZC500 at 10 mg/ml concentration is ~ 38 W/gFerrite. There are several reports on the heating efficiency of iron oxide in various forms; for example, nanoparticles 48, nanoplates 49, nanocubes 50 and among these various forms of Fe3O4, maximum SAR is obtained for nanocubes ( 2452 W/gFe) for an applied ac magnetic field of 29 kA/m (or 364 Oe) and 520 kHz

50

. Kim et al.

11

have reported the SAR of various ferrites viz., Fe-, Li-,

Ni/Zn/Cu-, Co-, Co/Ni, Ba- and Sr-ferrites. Li0.5Fe2.5O4 and CoFe2O4 have specific loss power of 13.52 and 30.36 W/gFerrite for an applied field of 110 A/m and 6.77 MHz 11. Compared to these earlier reports

48-51

, the obtained SAR of LZC nanoparticles at this magnitude and frequency of

applied ac magnetic field establishes that these nanoparticles do act as excellent candidates in hyperthermia cancer treatment.

3.5 Microwave absorption An interesting property of the carbon based ferrite nanocomposites is their enhanced MW absorption capability. Exceptional absorption efficiency in these nanocomposites is ensured by the perfect matching between the dielectric and magnetic losses. Dielectric loss arises from the


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carbon based material i.e., either carbon nanotube or graphene and magnetic loss arises from the ferrite nanoparticles. The dielectric and magnetic losses are figured by the imaginary parts (ε″ and µ″) of the complex permittivity (ε∗^ ) and complex permeability (μ∗^ ), respectively. Using a non-iterative method

52

the EM parameters (ε∗^ and μ∗^ ) can be estimated from the scattering

parameters, namely S11 and S21, the reflection and transmission coefficients, respectively. The real parts of ε∗^ and μ∗^ represent the ability of the material to store the external dielectric and magnetic energies, respectively when kept in an external EM field. The frequency variations of real (ε′ and µ′) and imaginary parts (ε″ and µ″) of ε∗^ and μ∗^ of the two samples, LZC200@MWCNT and LZC500@MWCNT in the X- and Ku-bands of frequency (8 – 12 and 12 – 18 GHz, respectively) are displayed in Fig. 10. In the X-band of frequency µ″ decreases with the increase of frequency (Fig 10c) and this is in fair agreement with the earlier result of Soibam et al.

25

. In Fig. 11, the frequency dependence of the dielectric and magnetic loss

tangents are displayed. The magnetic loss could be due to magnetic hysteresis, domain wall resonance, eddy current loss, exchange resonance, etc. However in this frequency range (8 – 18 GHz) magnetic loss is mainly due to natural and exchange resonances

12, 13

. From the earlier

discussions it is clear that the nanoparticles of the sample, LZC200 having dcryst ~ 8 nm are mainly SPM in nature and thus magnetic loss in case of the sample, LZC200 mainly comes from SPM relaxation phenomena. In addition, hysteresis loss arising from the processes like domain wall motion, movement of dislocations and grain boundaries also contribute to the magnetic loss though the contribution may be small in comparison to those due to SPM relaxation. Besides, the hysteresis loss of the sample, LZC500 significantly contributes to the magnetic loss. As these small nanoparticles of the samples, LZC200 (~ 8 nm) and LZC500 (~22.5 nm) have high resistivity, micro-eddy currents could not flow through those which is also clear from the low



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values of eddy current coefficient, μaa μa f in the frequency range under consideration (Fig. 11)

53

. Furthermore, if magnetic loss arises only from eddy current loss then, the eddy current

coefficient must remain constant with the change of frequency. But the unsteady nature of eddy current coefficient in the considered frequency range, confirms that magnetic loss is not only arising from eddy current loss 53. However, when these nanoparticles incorporated in conducting MWCNTs matrix are kept in an external EM field, the change in external field direction forces the flux density of the nanoparticles to follow the external field. The change in flux density through the conducting MWCNTs induces an emf and causes a current to flow through the MWCNTs

54

. According to Lenz’s law, this current opposes the cause, i.e. the change in flux

density and consequently magnetic losses arise. Thus, the sample becomes highly conductive at the frequency of the maximum reflection loss (RL), and the broad peak in ε′ occurs around this particular frequency (Fig. 10f). The dielectric loss mainly depends on the free electrons present in the materials

12, 53

and it is enhanced while the LZC nanoparticles are taken in MWCNTs

matrix due to the dipole and interfacial polarizations 13. The dipole polarization is caused by the surface functional groups and several types of defects in MWCNTs whereas interfacial polarization is caused by the interfaces among the ferrite nanoparticles and MWCNTs 53. However, for a material in which loss is coming from both dielectric and magnetic losses, loss tangent in general is defined by 55 tan δ =

d″

(5)

d′

Where δ′ = μ′ε′ − μ″ε″ and δ″ = μ′ε″ + μ″ε′. With these values, the attenuation constant in nepers per unit length can be determined by the following equation 55 α=

√d′ λ+

f√1 + tan δ − 1

(6)


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Here, λ0 is the free space wavelength. In Figs. 12a and b, the attenuation constant is plotted as a function of frequency. One does emphasize that the transmission line theory establishes that RL is dependent on ε∗^ and μ∗^ , according to the following equation 12, 56 R g dB = 20 log|Zm) − 1⁄Zm) + 1|

(7)

where Zin which corresponds to the normalised input impedance, is expressed as Zm) = μ∗^ ⁄ε^∗ ⁄ tanhoj2πfd⁄cμ∗^ ε∗^ ⁄ q

(8)

Here, f, d and c denote the frequency of the MW, the thickness of the absorber and the velocity of EM waves in free space, respectively. In the X-band of frequency (Fig. 12c), no significant absorption is observed except at some parts where RL of the sample, LZC500@MWCNT with 2 mm layer thickness is slightly below -10 dB. But RL of -10 dB is very significant while considering EM absorption as it defines ~ 90 % absorption of the incident EM wave. Interestingly, in the Ku-band of frequency (Fig. 12d), the two samples, LZC200@MWCNT and LZC500@MWCNT with thicknesses 1 and 2 mm exhibit significant reflection losses (Table 2). The sample, LZC500@MWCNT exhibits maximum RL of ~ -21 dB at a frequency of 15.27 GHz for 1 mm thick layer and the total frequency bandwidth in the Ku-band where RL is less than -10 dB is ~ 4.4 GHz. However, for a relatively thick layer (2 mm) of the same sample, maximum RL is reduced to ~ -20 dB and shifted to at a slightly lower frequency of 15.24 GHz while the bandwidth is increased up to ~ 4.8 GHz. The sample, LZC200@MWCNT also exhibits strong absorption deeps in the Ku-band. For a fixed layer thickness, RL of the sample, LZC200@MWCNT is lower than that of the sample, LZC500@MWCNT. Such a difference is due to that of their magnetizations. MW absorption is all about the matching of magnetic and dielectric losses. The increase in the magnetization of the sample, LZC500 (Ms ~ 62 emu/g) compared to that of the sample, LZC200 (Ms ~ 37.6 emu/g) provides better matching between



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Page 26 of 48

the two losses in the former and causes an increased RL. Hence, LZC nanoparticles in MWCNTs matrix could be considered as significant MW absorbing materials.

4.

Conclusion Nanoparticles of LZC are successfully prepared by sol-gel method. XRD analysis

confirmed the cubic Fd3m phase in most of the nanoparticles while the small peak of hexagonal P63/mmc phase ascertains the presence of MWCNTs in the samples, LZC200@MWCNT and LZC500@MWCNT. In case of the annealed samples higher occupancies of Zn2+ ions in A-site are obtained from the Rietveld analysis which supports the normal spinel structure of bulk ZnFe2O4. dcryst of the nanoparticles derived from Rietveld analysis lie in the range of 5 – 69 nm while the lattice parameters are in the range 8.376 – 8.390 Å. As observed in SEM and TEM micrographs, nanoparticles are mainly spherical while two different sizes (~ 8 and 23 nm) of them have been successfully incorporated in MWCNTs matrix whose average diameter is ~ 30 – 50 nm. In addition, the two major bands at 1347 and 1576 cm-1 in the Raman spectrum of the sample, LZC500@MWCNT confirm the presence of MWCNTs. Dynamic magnetic loops revealed that the annealed samples behave as ferrimagnets whereas the sample dried at 200 ˚C and the same in MWCNTs matrix are SPM. This fact is further confirmed by the Mössbauer spectra recorded at 300 and 77 K. At 300 K, very high value of Ms ~ 86.5 emu/g is achieved for the sample, LZC800 with dcryst ~ 58 nm and Keff for the same is ~ 6.7 × 106 erg/cm3. The degree of inversion (0.18) of the sample, LZC800 calculated from the Rietveld analysis satisfactorily matches that calculated using the value of Ms at 10 K. AC magnetic heating analysis exhibits that with a small concentration (2 mg/ml) of the samples, LZC500, LZC600 and LZC800, the hyperthermia temperature (42 ˚C) is satisfactorily attained within 3 minutes for an applied field


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of 420 Oe and 300 kHz frequency. Also, SAR as high as ~ 379 W/gFerrite is obtained for nanoparticles with dcryst ~ 36 nm, suggesting that LZC nanoparticles could be an effective heat mediator for MFH treatment. Additionally, MW absorption study exhibits that a maximum RL of ~ -21 dB at a frequency of 15.27 GHz is well achieved for a 1 mm thick layer of the sample, LZC500@MWCNT. Hence, the LZC nanoparticles in MWCNTs matrix could also be used as potential MW absorbing material.

Acknowledgements The authors acknowledge the financial support provided in the DST SERD project (File no. EMR/2017/000832). The authors also acknowledge the UGC-DAE CSR, Kalpakkam Centre for providing static magnetic measurement facility. The authors sincerely thank Prof. T. K. Chini and Debraj Dey, SINP, Kolkata, for their help in SEM measurements. The authors also acknowledge the XRD facility of Department of Physics, BU procured in FIST programme (File No. SR/FST/PS-II-001/2011).

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Table 1. Results of Rietveld analysis Sample

LZC200 LZC500 LZC600 LZC700 LZC800 LZC200@

LZC500@

MWCNT

MWCNT

GOF

1.01

1.01

1.01

1.04

1.06

1.06

1.14

Rwp

3.23

3.18

3.23

10.16

10.32

10.25

11.10

Rexp

3.19

3.13

3.20

9.73

9.77

9.70

9.77

RBragg

2.57

2.53

2.57

8.06

8.25

8.05

8.90

Lattice

8.390

8.370

8.378

8.376

8.377

8.382

8.371

(0.001)

(0.001)

(0.001)

(0.001)

(0.001)

(0.001)

parameter (Å) (0.002) (error) r.m.s.

lattice 4.55×10-3 1.58×10-4 5.09×10-3 1.49×10-3 8.68×10-3 8.41×10-3

strain

Crystallite size 5 ± 1

7.86×10-4

(Fd3m)

(Fd3m)

22.28×10-3

19.19×10-3

(P63/mmc)

(P63/mmc)

19 ± 1

36 ± 2

50 ± 5

69 ± 5

4±1

23 ± 1

0.18

0.09

0.15

0.18

0.10

0.18

(nm) Zn occupancy 0.10 in A-site


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Table 2. Maximum reflection losses at different frequencies Sample

Sample

Maximum reflection At frequency Band

thickness (mm)

losses (dB)

(GHz)

width

(GHz)

X-band 1

LZC200@MWCNT

-10.7

9.83

0.27

2

LZC200@MWCNT

-10.7

9.82

0.05

LZC500@MWCNT

-11.6

8.59

0.22

-11.6

9.82

0.29

-11.0

10.90

0.28

-15.2

12.72

1.11

-20.1

15.45

3.6

-15.7

16.95

-15.1

12.72

1.11

-20.5

15.27

2.37

-13.27

17.46

0.90

-14.8

12.72

1.08

-19.6

15.24

Ku-band 1

LZC200@MWCNT

LZC500@MWCNT

2

LZC200@MWCNT

3.63 LZC500@MWCNT

-15.5

17.37

-15.5

12.72

-20.0

15.24

1.08 3.75

-15.2


17.22


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Figure captions Fig. 1. Experimental XRD patterns (dots) and MAUD fitted spectra (solid lines) of (a) LZC200, (b) LZC500, (c) LZC600, (d) LZC700, (e) LZC800, (f) LZC200@MWCNT and (g) LZC500@MWCNT. Fig. 2. (a) Variation of crystallite size with annealing temperature as obtained from DebyeScherrer equation; (b) Atomic structure of LZC500. Fig. 3. SEM micrograph of (a) LZC200@MWCNT and (b) LZC500@MWCNT. Fig. 4. (a) and (b) Low magnification TEM images of LZC200@MWCNT; (c) and (d) HRTEM images; (e) SAED pattern; (f) EDX spectrum from area 1 in (g). (g) STEM-HAADF image. (h) EDX line profile along the line 2 in (g). Fig. 5. Raman spectra of LZC500@MWCNT recorded at 300 K. Fig. 6. (a) Mössbauer spectra of LZC200, LZC600 and LZC200@MWCNT recorded at 300 and 77 K. Filled circles represent the experimental data; (b) Mössbauer spectra of LZC500, LZC700 and LZC800 recorded at 77 K. Fig. 7. Dynamic hysteresis loops. Inset: Variation of coercivity with crystallite size. Fig. 8. (a) Static hysteresis loops recorded using VSM. Inset: Variation of coercive field with crystallite size; (b) M-H loops of LZC200 recorded using SQUID-VSM; (c) and (d) Temperature variation of FC and ZFC of LZC200 and LZC500, respectively. Insets: Plot of FC - ZFC as a function of temperature. Fig. 9. (a) and (b) Time-dependent temperature curves at two different concentrations. Fig. 10. Frequency variations of (a) and (b) real part of permeability, (c) and (d) imaginary part of permeability, (e) and (f) and real part of permittivity, (g) and (h) imaginary part of permittivity.


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Fig. 11. (a) and (b) frequency variation of μaa μa f , (c) and (d) dielectric loss tangent, (e) and (f) magnetic loss tangent in X- and Ku-bands, respectively. Fig. 12. (a) and (b) Attenuation constant, (c) and (d) reflection loss profile in X- and Ku-bands. Fig. 1



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Fig. 2

Fig. 3


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Fig. 4

Fig. 5



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Fig. 6a

Fig. 6b


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Fig. 7



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Fig. 8

Fig. 9


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Fig. 10



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Fig. 11


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Fig. 12



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Table of Contents/Abstract Graphic Nanoparticles

of

Li0.3Zn0.3Co0.1Fe2.3O4

exhibits

high

saturation

magnetization

and

magnetocrystalline anisotropy which ensures their applicability in hyperthermia treatment and these nanoparticles in MWCNTs matrix have superior microwave absorption efficiency.


Microwave Absorption and the Magnetic Hyperthermia Applications of

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