Enhancement of magnetic and dielectric properties of Ni0.25Cu0.25Zn0.50Fe2O4 magnetic nanoparticles through non-thermal microwave plasma treatment for high-frequency and energy storage applications

Citation: Munir, M.A.; Naz, M.Y.; Shukrullah, S.; Ansar, M.T.; Farooq, M.U.; Irfan, M.; Mursal, S.N.F.; Legutko, S.; Petr˚u, J.; Pagáˇc, M. Enhancement of Magnetic and Dielectric Properties of Ni0.25Cu0.25Zn0.50Fe2O4Magnetic Nanoparticles through Non-Thermal Microwave Plasma Treatment for High-Frequency and Energy Storage Applications. Materials 2022,15, 6890. https://doi.org/10.3390/ ma15196890 Academic Editor: Emil Babi´c Received: 31 August 2022 Accepted: 29 September 2022 Published: 4 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Enhancement of Magnetic and Dielectric Properties of Ni0.25Cu0.25Zn0.50Fe2O4Magnetic Nanoparticles through Non-Thermal Microwave Plasma Treatment for High-Frequency and Energy Storage Applications Muhammad Adnan Munir 1, Muhammad Yasin Naz 1,*, Shazia Shukrullah 1,*, Muhammad Tamoor Ansar 2, Muhammad Umar Farooq 3, Muhammad Irfan 4, Salim Nasar Faraj Mursal 4, Stanislaw Legutko 5, Jana Petr˚u 6and Marek Pagáˇc 7 1Department of Physics, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan 2Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan 3State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China 4Electrical Engineering Department, College of Engineering, Najran University, Najran 61441, Saudi Arabia 5Faculty of Mechanical Engineering, Poznan University of Technology, 60-965 Poznan, Poland 6Department of Machining, Assembly and Engineering Metrology, Mechanical Engineering Faculty, VŠB-Technical University of Ostrava, 17, Listopadu 2172/15, 70800 Ostrava, Czech Republic 7FME, Department of Machining, Assembly and Engineering Metrology, VSB Technical University of Ostrava, 17. listopadu 2172/15, 70800 Ostrava, Czech Republic *Correspondence: [email protected] (M.Y.N.); [email protected] (S.S.) Abstract: Spinel ferrites are widely investigated for their widespread applications in high-frequency and energy storage devices. This work focuses on enhancing the magnetic and dielectric properties of Ni 0.25 Cu 0.25 Zn 0.50 ferrite series through non-thermal microwave plasma exposure under lowpressure conditions. A series of Ni 0.25 Cu 0.25 Zn 0.50 ferrites was produced using a facile sol–gel auto-ignition approach. The post-synthesis plasma treatment was given in a low-pressure chamber by sustaining oxygen plasma with a microwave source. The structural formation of control and plasmamodified ferrites was investigated through X-ray diffraction analysis, which confirmed the formation of the fcc cubical structure of all samples. The plasma treatment did not affect crystallize size but significantly altered the surface porosity. The surface porosity increased after plasma treatment and average crystallite size was measured as about ~49.13 nm. Morphological studies confirmed changes in surface morphology and reduction in particle size on plasma exposure. The saturation magnetization of plasma-exposed ferrites was roughly 65% higher than the control. The saturation magnetization, remnant magnetization, and coercivity of plasma-exposed ferrites were calculated as 74.46 emu/g, 26.35 emu/g, and 1040 Oe, respectively. Dielectric characteristics revealed a better response of plasma-exposed ferrites to electromagnetic waves than control. These findings suggest that the plasma-exposed ferrites are good candidates for constructing high-frequency devices. Keywords: ferrite nanoparticles; non-thermal plasma; dielectric properties; magnetization; conductivity; energy storage 1. Introduction Ferrites are ceramic materials containing a large amount of iron oxide (Fe2O4) mixed with metallic elements such as zinc (Zn), cobalt (Co), nickel (Ni), and manganese (Mn) in minuscule proportions. Iron oxide possesses ferrimagnetic and electrically non-conducting behavior [ 1 ]. Ferrites are categorized into two types, i.e., soft and hard ferrites. Soft ferrites have the potential to magnetize and demagnetize easily, while hard ferrites, due to greater H c and M r ,are difficult to magnetize and demagnetize [ 2 , 3 ]. Among soft ferrites, spinel Materials 2022,15, 6890. https://doi.org/10.3390/ma15196890 https://www.mdpi.com/journal/materials Materials 2022,15, 6890 2 of 16 ferrites are presented by MFe 2 O 4 where Mreveals divalent ions, generally known as Mn 2+ , Zn 2+ , Fe 2+, and Co 2+ . Excellent magnetic and electrical and microwave characteristics make spinel ferrites a strong candidate for numerous applications [ 4 ], such as high-frequency applications [ 5 ], recording heads [ 6 ], antimicrobial agents [ 7 ], microwave absorbers [ 8 ], and catalysts [ 9 ]. With the advancement of electronic devices regarding compact design, multifunctionality, and miniaturization, high-power inductors are attracting the attention of researchers for new developments in electronics. Multilayer chip inductors (MLCIs) are extensively utilized in the miniaturization of energy storage appliances. NiCuZn ferrite has been widely tested as a substrate magnetic material for MLCIs due to its high resistance and exceptional magnetic characteristics in high-frequency ranges [ 10 ]. MLCIs are key components in video cameras, smartphones, and notebook computers, where improved magnetic attributes of nanoferrites are very important for efficient MLCIs to minimize ferrite volume in the chip. So, with the advancement of critical component downsizing, the formulation of NiCuZn ferrite with excellent characteristics has sparked broad interest [ 11 ]. To achieve the technological merits of miniaturizing dense cores for high frequencies, more research on NiCuZn ferrites has been conducted to investigate the physical phenomenon responsible for magnetic modifications. Chu et al. [ 12 ] investigated the role of Mn 2+ doping in improving magnetic characteristics of spinel NiCuZn ferrites. Experimental results revealed that saturation magnetization reduced from 67 to 62 emu/g when Mn 2+ content increased up to 0.03%. With a rise in sintering temperature, both remanence and saturation magnetization increased while coercivity was decreased. Mn 2+ addition cause an increase in permeability with a drop in resonance frequency. Peng et al. [ 13 ] synthesized NiCuZn ferrite via a co-precipitation method with ultrasonic assistance and analyzed the influence of temperature and ultrasonic radiation on magnetic characteristics and microstructures of nanoferrites. The best magnetic properties were obtained at room temperature (RT) with assistance of 60W ultrasonic power. M-H investigation revealed that with the same ultrasonic power NiCuZn ferrite exhibited maximum saturation magnetization and coercivity, but at a 50 ◦ C reaction temperature ferrites exhibited ferromagnetic behavior. It was concluded that ultrasound has a progressive effect on the microstructure and magnetic attributes of ferrite NPs and this technique also has the potential for the controlled preparation of ferrites at an industrial scale. Sujatha and colleagues investigated how the sintering temperature affected the electromagnetic properties of NiCuZn ferrite. Experimental findings revealed that temperature directly affects material features and excellent magnetic and dielectric properties were exhibited with increased sintering temperature [ 14 ]. Researchers have adopted numerous routes such as the sol–gel auto-combustion technique [ 15 ], solid-state route [ 16 ], hydrothermal method [ 17 ], co-precipitation [ 18 ], and egg white technique [ 19 ] for the synthesis of NiCuZn ferrite. Magneto-electric characteristics can be tuned using different additives such as Bi 2 O 3 , V 2 O 5 , and Cu [ 20 – 22 ]. Microwave heating has also been utilized to synthesize NiCuZn ferrite due to several benefits over other techniques such as minimum reaction time, rapid and selective heating, and direct interactions of dipoles and charges in pristine specimens [ 23 ]. Post-synthesis plasma processing of nanoparticles is another approach adopted to improve the opto-magnetic characteristics of materials [ 24 ]. Microwave plasma is non-thermal plasma generated by an electrode system under low pressure and a high-power microwave source. Microwave radiation generates electrons, ions, and high-energy neutral species which directly react with the target substance and alter the texture and characteristics of that material [ 25 ]. In fact, the plasma-assisted process generates free radicals, which produce crystalline vacancies on inorganic substances, altering the electronic distribution and supplying appropriate active sites [26,27]. To date, no detailed research has been reported on improving the magnetic and dielectric characteristics of NiCuZn ferrite using post-synthesis plasma treatment. Therefore, in the current research, our prime motivation is to probe the impact of microwave plasma treatment on NiCuZn ferrites for high-frequency devices. In this research, NiCuZn ferrite is successfully fabricated by the sol–gel auto-ignition method. The post-synthesis impact Materials 2022,15, 6890 3 of 16 of plasma exposure is tested for ferroelectric, dielectric, and magnetic attributes of spinel ferrite, which reveals substantial enrichment in magnetic and dielectric characteristics. 2. Experimental Work 2.1. Synthesis of NiCuZn Ferrite Ni 0.25 Cu 0.25 Zn 0.50 ferrite samples were fabricated via the sol–gel auto-combustion method. Estimated quantities of metal nitrates, i.e., zinc nitrate (Zn (NO 3 ) 2≥ 98.5%], nickel nitrate (Ni (NO 3 ) 2≥ 99.98%], iron nitrate (Fe (NO 3 ) 3≥ 98.5%], and copper nitrate (Cu (NO 3 ) 2 99.95%], were used as precursors. Citric acid C 6 H 8 O 7 was utilized as fuel for combustion and liquid ammonia NH 3 was used to maintain the pH of the solution at 7. In the first step, metal nitrates were dissolved individually in DI water. Then, an aqueous solution of citric acid was introduced to the metal nitrate solution, keeping the nitrate to citrate proportion at 1:1. Liquid ammonia was added to this solution dropwise under continuous stirring. After adjusting the pH of the nitrate citrate solution, constant heat at 90 ◦ C was supplied during the hydrolysis of the solution. Continuous heating and stirring resulted in the formation of xerogel, which further burned due to the exothermic process generating flames in the beaker. The final product was black ash-like powder. The obtained fluffy powder was dried at 100 ◦ C and then calcined at 800 ◦ C for 4 h. Three samples were fabricated using 30, 40, and 45 mL of DI water and named as UT30, UT40, and UT45, respectively. Finally, one half was taken from all prepared NiCuZn ferrite samples and was exposed to microwave plasma. Plasma-treated samples with different DI water concentrations were named as PT30, PT40, and PT45, respectively. All synthesized untreated and plasma-treated samples were converted into a toroid using a hydraulic press. Schematic of synthesis and plasma treatment process is provided in Figure 1. Materials 2022, 15, x FOR PEER REVIEW 3 of 17 radicals, which produce crystalline vacancies on inorganic substances, altering the electronic distribution and supplying appropriate active sites [26,27]. To date, no detailed research has been reported on improving the magnetic and dielectric characteristics of NiCuZn ferrite using post-synthesis plasma treatment. Therefore, in the current research, our prime motivation is to probe the impact of microwave plasma treatment on NiCuZn ferrites for high-frequency devices. In this research, NiCuZn ferrite is successfully fabricated by the sol–gel auto-ignition method. The post-synthesis impact of plasma exposure is tested for ferroelectric, dielectric, and magnetic attributes of spinel ferrite, which reveals substantial enrichment in magnetic and dielectric characteristics. 2. Experimental Work 2.1. synthesis of NiCuZn ferrite Ni0.25Cu0.25Zn0.50 ferrite samples were fabricated via the sol–gel auto-combustion method. Estimated quantities of metal nitrates, i.e., zinc nitrate (Zn (NO3)2 ≥ 98.5%], nickel nitrate (Ni (NO3)2 ≥ 99.98%], iron nitrate (Fe (NO3)3 ≥ 98.5%], and copper nitrate (Cu (NO3)2 99.95%], were used as precursors. Citric acid C6H8O7 was utilized as fuel for combustion and liquid ammonia NH3 was used to maintain the pH of the solution at 7. In the first step, metal nitrates were dissolved individually in DI water. Then, an aqueous solution of citric acid was introduced to the metal nitrate solution, keeping the nitrate to citrate proportion at 1:1. Liquid ammonia was added to this solution dropwise under continuous stirring. After adjusting the pH of the nitrate citrate solution, constant heat at 90 °C was supplied during the hydrolysis of the solution. Continuous heating and stirring resulted in the formation of xerogel, which further burned due to the exothermic process generating flames in the beaker. The final product was black ash-like powder. The obtained fluffy powder was dried at 100 °C and then calcined at 800 °C for 4 h. Three samples were fabricated using 30, 40, and 45 mL of DI water and named as UT30, UT40, and UT45, respectively. Finally, one half was taken from all prepared NiCuZn ferrite samples and was exposed to microwave plasma. Plasma-treated samples with different DI water concentrations were named as PT30, PT40, and PT45, respectively. All synthesized untreated and plasma-treated samples were converted into a toroid using a hydraulic press. Schematic of synthesis and plasma treatment process is provided in Figure 1. Figure 1. Schematic illustration of synthesis and plasma treatment of ferrites. Figure 1. Schematic illustration of synthesis and plasma treatment of ferrites. 2.2. Non-Thermal Plasma Treatment Non-thermal plasma generated by microwaves with a 900 W power source was used to treat ferrites. Figure 2displays the plasma treatment setup comprise of different components. First of all, specimens were placed in a sample holder and the sample holder was further placed in a shielding box to prevent radiation leakage. The chamber was fully evacuated using a vacuum pump which created low pressure in the chamber. After creating the required pressure, oxygen gas was supplied as a plasma precursor. The precursor gas Materials 2022,15, 6890 4 of 16 was provided to the fully evacuated chamber. The plasma was made by using a microwave source that breaks the gas atoms into free electrons and highly energetic ions, collectively called plasma. A gradual increase in microwave power resulted in plasma production at a certain power level and then continuous production of plasma started in the chamber at a power of 1200 W. Each pristine specimen was exposed to plasma for half an hour, which directly interacted with specimens and caused surface modifications and changes in magnetic and dielectric characteristics of ferrites. Both pristine and processed ferrite samples were pressed into pellets with a hydraulic press at an external force of 40 k N. Materials 2022, 15, x FOR PEER REVIEW 4 of 17 2.2. Non-Thermal Plasma Treatment Non-thermal plasma generated by microwaves with a 900 W power source was used to treat ferrites. Figure 2 displays the plasma treatment setup comprise of different components. First of all, specimens were placed in a sample holder and the sample holder was further placed in a shielding box to prevent radiation leakage. The chamber was fully evacuated using a vacuum pump which created low pressure in the chamber. After creating the required pressure, oxygen gas was supplied as a plasma precursor. The precursor gas was provided to the fully evacuated chamber. The plasma was made by using a microwave source that breaks the gas atoms into free electrons and highly energetic ions, collectively called plasma. A gradual increase in microwave power resulted in plasma production at a certain power level and then continuous production of plasma started in the chamber at a power of 1200 W. Each pristine specimen was exposed to plasma for half an hour, which directly interacted with specimens and caused surface modifications and changes in magnetic and dielectric characteristics of ferrites. Both pristine and processed ferrite samples were pressed into pellets with a hydraulic press at an external force of 40 k N. Figure 2. Illustration of plasma processing setup. 2.3. Characterization A Bruker D8 X-ray diffractometer was used to determine the phases of the fabricated ferrite specimens. A radiation source with a wavelength of 1.54 Å was used in this characterization. The morphology was probed using a NovaNano SEM 450 FESEM instrument. Magnetic characteristics of pristine and plasma-treated ferrites were measured using a Lakeshore-7410 VSM instrument (vibrating sample magnetometer). The dielectric behavior of specimens was analyzed by employing a Wayne-Kerr 6500B precision analyzer. The ferroelectric characteristics of plasma-treated and pristine samples were examined using a multiferroic tester from Radiant Technology Inc (Albuquerque, NM, USA). 3. Results and Discussion 3.1. Phase Analysis X-ray diffraction was employed to probe crystal structure, phase purity, and other structural parameters of ferrite specimens. The synthesis process and annealing temperature have a significant role in determining the crystal structures of nanoparticles [28]. XRD analysis of Ni0.25Cu0.25Zn0.50 was conducted within the 2θ range of 20°–80° to probe the structural and phase compositions of both pristine and plasma-processed NiCuZn ferrite Figure 2. Illustration of plasma processing setup. 2.3. Characterization A Bruker D8 X-ray diffractometer was used to determine the phases of the fabricated ferrite specimens. A radiation source with a wavelength of 1.54 Å was used in this characterization. The morphology was probed using a NovaNano SEM 450 FESEM instrument. Magnetic characteristics of pristine and plasma-treated ferrites were measured using a Lakeshore-7410 VSM instrument (vibrating sample magnetometer). The dielectric behavior of specimens was analyzed by employing a Wayne-Kerr 6500B precision analyzer. The ferroelectric characteristics of plasma-treated and pristine samples were examined using a multiferroic tester from Radiant Technology Inc (Albuquerque, NM, USA). 3. Results and Discussion 3.1. Phase Analysis X-ray diffraction was employed to probe crystal structure, phase purity, and other structural parameters of ferrite specimens. The synthesis process and annealing temperature have a significant role in determining the crystal structures of nanoparticles [ 28 ]. XRD analysis of Ni 0.25 Cu 0.25 Zn 0.50 was conducted within the 2 θ range of 20 ◦ –80 ◦ to probe the structural and phase compositions of both pristine and plasma-processed NiCuZn ferrite samples calcined at 800 ◦ C. The XRD peak indexing was executed using a technique elucidated by Cullity [ 29 ]. XRD intensity peaks were reported at 2 θ of 30.03 ◦ , 35.35 ◦ , 36.96 ◦ , 42.94 ◦ , 53.29 ◦ , 56.79 ◦ , 62.31 ◦ , and 73.76 ◦ , which are attributed to (220), (311), (222), (400), (422), (511), and (440) hkl planes, respectively, mentioned in Figure 3. These hkl planes indicated the growth of pure fcc spinel structures of NiCuZn ferrite and obtained peaks agreed with the previously reported literature [ 30 ]. Obtained XRD patterns also endorsed the statement that the sol–gel auto-ignition technique is favorable for synthesizing spinel ferrites using described conditions [31]. Materials 2022,15, 6890 5 of 16 Materials 2022, 15, x FOR PEER REVIEW 5 of 17 samples calcined at 800 °C. The XRD peak indexing was executed using a technique elucidated by Cullity [29]. XRD intensity peaks were reported at 2θ of 30.03°, 35.35°, 36.96°, 42.94°, 53.29°, 56.79°, 62.31°, and 73.76°, which are attributed to (220), (311), (222), (400), (422), (511), and (440) hkl planes, respectively, mentioned in Figure 3. These hkl planes indicated the growth of pure fcc spinel structures of NiCuZn ferrite and obtained peaks agreed with the previously reported literature [30]. Obtained XRD patterns also endorsed the statement that the sol–gel auto-ignition technique is favorable for synthesizing spinel ferrites using described conditions [31]. Figure 3. Indexed XRD spectra of pristine and plasma-processed NiCuZn ferrite. The magnificent crystallinity of specimens was palpable, as shown by the sharp peak associated with the (311) plane and FWHM related to the development of crystallites [32]. Table 1 shows different parameters, including crystallite size, lattice parameters, X-ray density, bulk density, and porosity. The lattice parameters of all specimens were estimated by applying Equation (1) [33]: 𝑎 = 𝜆 𝑠𝑖𝑛𝜃  ( ℎ  + 𝑘  + 𝑙  ) (1) A simple relation (V = a3) was used to find out the unit cell volume [33], while grain size was obtained from Scherrer’s formula in Equation (2) by using the (311) characteristic peak for all specimens [29]. 𝐷   = 𝐾𝜆 𝛽𝐶𝑂𝑆𝜃 (2) Here, 𝜃 is the diffraction angle, β denotes FWHM, and K is the shape factor. The Xray is denoted by 𝜆, which is typically 1.54 Å. Furthermore, X-ray density 𝜌𝑥 was evaluated by applying Equation (3) [33]. 𝜌𝑥 = 𝑍𝑀 𝑉 𝑁  (3) Here, NA is Avogadro’s no., V is unit cell volume, M defines molecular weight, and Z reveals the no. of formula units existing in the unit cell. The bulk density of pressed samples was estimated using Equation (4) [34]. Figure 3. Indexed XRD spectra of pristine and plasma-processed NiCuZn ferrite. The magnificent crystallinity of specimens was palpable, as shown by the sharp peak associated with the (311) plane and FWHM related to the development of crystallites [ 32 ]. Table 1shows different parameters, including crystallite size, lattice parameters, X-ray density, bulk density, and porosity. The lattice parameters of all specimens were estimated by applying Equation (1) [33]: a=λ sinθq(h2+k2+l2)(1) A simple relation (V = a 3 ) was used to find out the unit cell volume [ 33 ], while grain size was obtained from Scherrer’s formula in Equation (2) by using the (311) characteristic peak for all specimens [29]. Dhkl =Kλ βCOSθ(2) Here, θ is the diffraction angle, β denotes FWHM, and Kis the shape factor. The X-ray is denoted by λ , which is typically 1.54 Å. Furthermore, X-ray density ρx was evaluated by applying Equation (3) [33]. ρx=ZM VNA (3) Here, N A is Avogadro’s no., Vis unit cell volume, Mdefines molecular weight, and Zreveals the no. of formula units existing in the unit cell. The bulk density of pressed samples was estimated using Equation (4) [34]. ρB=m v(4) Here, m is the mass of pellets prepared using a hydraulic press while v is the volume of pellets calculated by using the relation given in Equation (5). V=πr2h(5) Materials 2022,15, 6890 6 of 16 In this relation, ris the pellet’s radius, while hpresents the pellet’s height. The porosity of the synthesized specimens was also calculated using the relation between X-ray density and bulk density presented in Equation (6) [31]. Porosity =1−ρB ρx×100% (6) All calculated parameters mentioned in Table 1revealed that plasma exposure did not alter the phase and structure formation of spinel ferrites. Table 1. The data extracted from XRD spectra of pristine and plasma-processed NiCuZn ferrites. Sample Crystallite Size (nm) Lattice Constant (Å) Unit Cell Volume X-ray Density (g/cm3) Bulk Density (g/cm3) Porosity (%) UT40 49.1212 ±0.0091 8.3983 ±0.0081 593.44 ±1.9242 5.3757 ±0.0173 5.1732 ±0.0015 5.881 ±0.9878 PT40 49.1324 8.3800 593.12 5.3787 5.1723 7.934 UT45 49.1434 8.4001 597.05 5.3433 5.1701 6.321 PT45 49.1201 8.3971 595.99 5.3528 5.1708 7.541 3.2. Microstructural Analysis The morphological representation and statistical grain distribution of pristine and processed ferrite nanoparticles are given in Figures 4and 5. It is conspicuous from Figure 4a,c that untreated NiCuZn ferrites exhibited dense and homogeneous morphologies with non-uniform grain distribution. Nanoparticles exhibited an almost spherical shape with a cluster-like appearance. Micrographs also show that pristine NiCuZn ferrites demonstrated fuzzy grain boundaries with agglomeration among nanoparticles. Generally, agglomeration appeared because of high annealing temperature magnetic dipole–dipole interactions among the nanoparticles [ 35 ]. When the agglomeration phenomena dominated, the grains clumped together, reducing porous behavior. Figure 4b,d shows SEM micrographs of plasma-treated specimens. It is conspicuous that plasma exposure significantly affected the morphology of ferrite specimens. Plasma treatment alters not only the shape but also the size of grains. A decrease in grain size caused a decline in agglomeration due to plasma treatment which was also reported before [ 36 ]. Some grains exhibited a cubical shape, while most grains exhibited a spherical shape with well-defined grain boundaries. Changes in morphology and grain shape indicate that plasma treatment is a very useful technique in surface modifications. These changes in surface morphology correspond to the plasma etching effect [ 37 ]. A noticeable transformation in shape and size was reported, which are influential factors for determining the magnetic and dielectric features of spinel ferrite [ 38 ]. The majority of grains showed sizes ranging from 0.6 to 1 µ m in the case of pristine ferrite specimens, as shown by histograms in Figure 5. 3.3. Magnetic Features VSM investigation was accomplished to examine the magnetic attributes such as M s , H c , M r , etc. The magnetic characteristics of spinel ferrites are affected by various factors, including synthesis route, cation distribution on tetrahedral and octahedral sites, and crystallite size, as presented in Figure 5[ 30 ]. Magnetic characteristics of ferrite nanoparticles can be modified using different doping ions or changing the synthesis technique and calcination temperature [ 39 , 40 ]. Hysteresis loops of both untreated and plasma-treated ferrites were plotted using applied field and corresponding magnetization. Values of magnetic parameters were calculated using hysteresis loop and are presented in Table 2. It is evident from Figure 6that plasma treatment effectually increased M s ,H c , and M r . A more than 60% increase in M s was observed for plasma-treated ferrite specimens and the highest value was 74.46 emu/g, which agreed with that previously reported for La-doped NiCuZn ferrites [ 41 ]. Saturation magnetization reported in this research is the highest magnetization Materials 2022,15, 6890 7 of 16 observed for pure NiCuZn ferrite and also greater than for ferrite mixed with additives, i.e., Cu, Tb, Co, and Bi 2 O 3 -Nb 2 O 5 [ 22 ]. That escalation in saturation magnetization is associated with direct contact of ferrites specimens with highly energetic microwave plasma [42]. Materials 2022, 15, x FOR PEER REVIEW 7 of 17 Figure 4. SEM micrograph of pristine UT40, UT45 4 (a, c), and plasma-treated PT40, PT45 4 (b, d) specimens. Figure 5. A statistical representation of grain distribution for pristine and plasma−treated ferrite. 3.3. Magnetic Features VSM investigation was accomplished to examine the magnetic attributes such as Ms, Hc, Mr, etc. The magnetic characteristics of spinel ferrites are affected by various factors, including synthesis route, cation distribution on tetrahedral and octahedral sites, and crystallite size, as presented in Figure 5 [30]. Magnetic characteristics of ferrite nanoparticles can be modified using different doping ions or changing the synthesis technique and Figure 4. SEM micrograph of pristine UT40, UT45 4 ( a , c ), and plasma-treated PT40, PT45 4 (b,d) specimens. Materials 2022, 15, x FOR PEER REVIEW 7 of 17 Figure 4. SEM micrograph of pristine UT40, UT45 4 (a, c), and plasma-treated PT40, PT45 4 (b, d) specimens. Figure 5. A statistical representation of grain distribution for pristine and plasma−treated ferrite. 3.3. Magnetic Features VSM investigation was accomplished to examine the magnetic attributes such as Ms, Hc, Mr, etc. The magnetic characteristics of spinel ferrites are affected by various factors, including synthesis route, cation distribution on tetrahedral and octahedral sites, and crystallite size, as presented in Figure 5 [30]. Magnetic characteristics of ferrite nanoparticles can be modified using different doping ions or changing the synthesis technique and Figure 5. A statistical representation of grain distribution for pristine and plasma−treated ferrite. Materials 2022,15, 6890 8 of 16 Materials 2022, 15, x FOR PEER REVIEW 8 of 17 calcination temperature [39,40]. Hysteresis loops of both untreated and plasma-treated ferrites were plotted using applied field and corresponding magnetization. Values of magnetic parameters were calculated using hysteresis loop and are presented in Table 2. It is evident from Figure 6 that plasma treatment effectually increased Ms, Hc, and Mr . A more than 60% increase in Ms was observed for plasma-treated ferrite specimens and the highest value was 74.46 emu/g, which agreed with that previously reported for La-doped NiCuZn ferrites [41]. Saturation magnetization reported in this research is the highest magnetization observed for pure NiCuZn ferrite and also greater than for ferrite mixed with additives, i.e., Cu, Tb, Co, and Bi2O3-Nb2O5 [22]. That escalation in saturation magnetization is associated with direct contact of ferrites specimens with highly energetic microwave plasma [42]. Figure 6. Variation in saturation magnetization, coercivity and remanence magnetization with plasma treatment. When ferrite specimens are directly exposed to microwave plasma, the movement of magnetic dipoles significantly improved due to rising temperature, which boosts saturation magnetization [43]. An increase in coercivity and remanence can be associated with surface effects generated during plasma treatment [44]. Analysis of magnetic characteristics endorsed the statement that microwave plasma treatment is an efficient technique used for the modification of ferrites and it can be valuable in the latest applications. Table 2. Measurements of magnetic properties of NiCuZn ferrite samples. Sample Saturation Magnetization (Ms) Remnant Magnetization (Mr) Squareness Ratio (Mr/Ms) Coercivity (Hc) UT-30 31.85 11.32 0.355 437 PT-30 53.38 18.35 0.343 518 UT-40 42.52 15.31 0.360 443 PT-40 64.48 22.24 0.344 551 UT-45 48.01 20.42 0.425 1033 PT-45 74.46 26.35 0.353 1040 Figure 6. Variation in saturation magnetization, coercivity and remanence magnetization with plasma treatment. When ferrite specimens are directly exposed to microwave plasma, the movement of magnetic dipoles significantly improved due to rising temperature, which boosts saturation magnetization [ 43 ]. An increase in coercivity and remanence can be associated with surface effects generated during plasma treatment [ 44 ]. Analysis of magnetic characteristics endorsed the statement that microwave plasma treatment is an efficient technique used for the modification of ferrites and it can be valuable in the latest applications. Table 2. Measurements of magnetic properties of NiCuZn ferrite samples. Sample Saturation Magnetization (Ms) Remnant Magnetization (Mr) Squareness Ratio (Mr/Ms) Coercivity (Hc) UT-30 31.85 11.32 0.355 437 PT-30 53.38 18.35 0.343 518 UT-40 42.52 15.31 0.360 443 PT-40 64.48 22.24 0.344 551 UT-45 48.01 20.42 0.425 1033 PT-45 74.46 26.35 0.353 1040 3.4. Ferroelectric Properties The ferroelectric characteristics of pristine and plasma-treated NiCuZn ferrites were analyzed via P-E loops by plotting curves using polarization and electric field at a 50 Hz frequency and RT and are reflected in Figure 7[ 45 ]. From the formation of P-E loops, it is evident that both pristine and plasma-treated specimens exhibited weak ferroelectric characteristics [ 46 ]. The remnant polarization (P R ), maximum polarization (P m ), and coercive field (E c ) were significantly affected by plasma exposure. Untreated ferrite specimens exhibited narrow P-E loops, while plasma-treated specimens revealed oval-like P-E loops indicating deterioration in ferroelectric attributes for plasma-treated ferrites [ 47 ]. The development of wider P-E loops revealed substantial leakage current, which is anticipated due to the existence of conducting ferrite phase [ 48 ]. The excellent ferroelectric characteristics of untreated ferrite specimens can be related to the restricted mobility of ferroelectric phase Materials 2022,15, 6890 9 of 16 domains [ 49 ]. Overall analysis validated that plasma treatment significantly reduced the ferroelectric properties of ferrite specimens. Materials 2022, 15, x FOR PEER REVIEW 9 of 17 3.4. Ferroelectric Properties The ferroelectric characteristics of pristine and plasma-treated NiCuZn ferrites were analyzed via P-E loops by plotting curves using polarization and electric field at a 50 Hz frequency and RT and are reflected in Figure 7 [45]. From the formation of P-E loops, it is evident that both pristine and plasma-treated specimens exhibited weak ferroelectric characteristics [46]. The remnant polarization (PR), maximum polarization (Pm), and coercive field (Ec) were significantly affected by plasma exposure. Untreated ferrite specimens exhibited narrow P-E loops, while plasma-treated specimens revealed oval-like P-E loops indicating deterioration in ferroelectric attributes for plasma-treated ferrites [47]. The development of wider P-E loops revealed substantial leakage current, which is anticipated due to the existence of conducting ferrite phase [48]. The excellent ferroelectric characteristics of untreated ferrite specimens can be related to the restricted mobility of ferroelectric phase domains [49]. Overall analysis validated that plasma treatment significantly reduced the ferroelectric properties of ferrite specimens. Figure 7. Polarization−electric field profiles of UT30, PT30 (a) UT40, PT40 (b), and UT45, PT45 (c). 3.5. Dielectric Characteristics and Complex Impedance Parameters Currently, the world is facing environmental and energy challenges. Researchers are working diligently to design efficient energy storage devices with lower environmental hazards [50]. Dielectric substances can be utilized for numerous applications, such as enhancing the performance of capacitors, semiconductors, and other energy storage systems. The dielectric attributes can be probed by using real and imaginary components of permeability (ε′ and ε”), loss tangent (tanδ), and ac-conductivity (σac) [51]. The dielectric analysis of ferrites provides evidence about the nature of charge carriers and the conduction process accruing in ferrites. The dielectric behavior of magnetic materials is generally influenced by factors such as cation distribution, synthesis technique, temperature, applied field, grain size, and composition of the material [52]. Temperature and frequency dependency has a significant impact on the dielectric characteristics. To examine the dielectric response, the specimens in a pellet shape were placed between a couple of copper electrodes. Resistances and capacitances of ferritebased specimens were measured using an ac signal in the frequency range of 20Hz to Figure 7. Polarization−electric field profiles of UT30, PT30 (a) UT40, PT40 (b), and UT45, PT45 (c). 3.5. Dielectric Characteristics and Complex Impedance Parameters Currently, the world is facing environmental and energy challenges. Researchers are working diligently to design efficient energy storage devices with lower environmental hazards [ 50 ]. Dielectric substances can be utilized for numerous applications, such as enhancing the performance of capacitors, semiconductors, and other energy storage systems. The dielectric attributes can be probed by using real and imaginary components of permeability ( ε0 and ε00 ), loss tangent (tan δ ), and ac-conductivity ( σac ) [ 51 ]. The dielectric analysis of ferrites provides evidence about the nature of charge carriers and the conduction process accruing in ferrites. The dielectric behavior of magnetic materials is generally influenced by factors such as cation distribution, synthesis technique, temperature, applied field, grain size, and composition of the material [52]. Temperature and frequency dependency has a significant impact on the dielectric characteristics. To examine the dielectric response, the specimens in a pellet shape were placed between a couple of copper electrodes. Resistances and capacitances of ferrite-based specimens were measured using an ac signal in the frequency range of 20Hz to 20MHz. The magnitude of ε0 of a substance is considered the amount of energy that can be stored in any substance. This energy can be estimated by using the following relation [53]: ε0= Cpd/Aεo. (7) In this equation ε0 represents the dielectric permittivity, εo denotes the permittivity of free space (8.85 × 10 −12 F/m), C p represents the parallel capacitance of the specimens, d is the width of the sample pellet, and Aindicates the area of the pellet. Figure 8a–f reveal the typical behavior of ε0 and ε00 while input frequency is supplied at RT. Both pristine and plasma-processed ferrite specimens showed a steady decrease in ε0 with increasing frequency. 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