H. ZHAO et al.: EFFECT OF MANGANESE DOPING ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES ... 891–895 EFFECT OF MANGANESE DOPING ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES OF ZINC FERRITE VPLIV DOPIRANJA Z Mn NA MAGNETNE IN MAGNETNO-KALORI^NE LASTNOSTI Zn FERITA Haitao Zhao * , Xuehan Li, Hui Zhao, Yulian Wang School of Materials Science and Engineering, Shenyang Ligong University, Shenyang, China Prejem rokopisa – received: 2019-04-26; sprejem za objavo – accepted for publication: 2019-07-23 doi:10.17222/mit.2019.089 MnxZn1-xFe2O4 ferrites with x = 0.2, 0.4, 0.6 and 0.8 were successfully prepared with the solvothermal method. The structural characteristics, morphology and magnetic properties of the composite powders were obtained with X-ray diffraction (XRD), a scanning electron microscope (SEM) and a vibrating-sample magnetometer (VSM). The results show that MnxZn1-xFe2O4 ferrite has a pure cubic spinel structure with a particle size of about 200–300 nm. Ethylene glycol plays an important role during the formation of monodisperse particles. Synthesized particles exhibit ferromagnetic behavior with a small hysteresis at room temperature. Ms reaches the maximum value of 71.99 emu/g when the amount of manganese ions is x = 0.6. At 600 s, the temperature of Mn0.8Zn0.2Fe2O4 can rise to 69.9 °C, showing an excellent magnetocaloric effect. Keywords: zinc ferrite, solvothermal synthesis, magnetic properties, magnetocaloric effect Avtorji so uspe{no sintetizirali MnxZn1-xFe2O4 ferite z x = 0,2, 0,4, 0,6 in 0,8 s solvotermalno metodo. Z rentgensko difrakcijo (XRD), vrsti~no elektronsko mikroskopijo (SEM) in magnetometrom na treso~i se vzorec (VSM), so dolo~ili strukturne lastnosti, morfologijo in magnetne lastnosti kompozitnih prahov. Rezultati analiz so pokazali, da ima MnxZn1-xFe2O4 ferit ~isto kubi~no-{pinelno strukturo z velikostjo delcev okoli 200 nm do 300 nm. Etilenglikol igra pomembno vlogo pri tvorbi enovite disperzije delcev ferita. Sintetizirani delci imajo feromagnetne lastnosti z majhno histerezo pri sobni temperaturi. Maksimalna vrednost magnetizacije nasi~enja (Ms) je bila 71,99 emu/g pri vsebnosti Mn ionov x = 0,6. Po 600 s temperatura Mn0.8Zn0.2Fe2O4 lahko naraste do 69,9 °C, kar ka`e na odli~en magnetno-kalori~en u~inek. Klju~ne besede: cink-ferit, solvotermalna sinteza, magnetne lastnosti, magnetno-kalori~no delovanje 1 INTRODUCTION MnZn ferrites attracted considerable investigations because of their potential applications in catalysis, mag- netic storage, cancer therapy, medical imaging and electronic devices. 1–5 The chemical processes currently in vogue for the synthesis of MnZn ferrite particles in- clude co-precipitation, 6–9 the sol–gel auto-combustion method, 10–13 the solid-state reaction method, 14–16 solvo- thermal technique, 17–19 microwave processing tech- nique, 20–22 hydrothermal synthesis etc. 23–27 The solvothermal technique has the advantages of functioning at low temperatures, being a simple, low- cost synthetic process, exhibiting an ease of composit- ional control and producing ultrafine particles with a narrow-sized distribution. Therefore, the solvothermal technique is an economical and simple strategy for the synthesis of monodisperse powders. G. S. Wang et al. 28 successfully prepared ZnFe 2 O 4 nanocrystal clusters with a surfactant-assisted solvothermal method and investig- ated them as a potential magnetorheological material. W. Shen et al. 29 demonstrated a simple solvothermal route for the synthesis of monodisperse Fe 3 O 4 and studied the effects of the reaction parameters on the structure. However, their investigations focused on structural, magnetic and electrical properties at lower frequencies (<1 GHz). Until now, there has been little research work on the magnetic properties and magnetocaloric effect of MnZn ferrites. In the present study, monodispersed Mn x Zn 1-x Fe 2 O 4 (x = 0.2, 0.4, 0.6 and 0.8) ferrite particles were synthesized using a solvothermal approach. The samples were characterized with various experimental techniques and the properties of ferrite particles were investigated. 2 EXPERIMENTAL PART Mn x Zn 1-x Fe 2 O 4 (x = 0.2, 0.4, 0.6 and 0.8) ferrite particles were synthesized using a solvothermal approach. Analytical-grade FeCl 3 ·6H 2 O, Zn(NO 3 ) 2 ·6H 2 O and MnSO 4 ·H 2 O were used to prepare ferrite powders. The metal salt, weighed according to the stoichiometric proportion, was dissolved evenly in ethylene glycol to obtain a homogeneous solution. After that, the resulting mixture was transferred into a 100-mL Teflon-lined stainless-steel autoclave, and heated to 200 °C for 12 h. The system was then cooled down to room temperature. The products were obtained with centrifugation, sequentially washed with deionized water and ethanol Materiali in tehnologije / Materials and technology 53 (2019) 6, 891–895 891 UDK 620.1:544.227:543.316/.318:67.017 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(6)891(2019) *Corresponding author's e-mail: zht95711@163.com (Haitao Zhao) for several times and then dried in a vacuum oven at 60 °C for 12 h. The phase formation of the synthesized product was identified with X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) using Cu-K radiation ( = 0.15418 nm) ina2 range of 20–70°. The morphology of the products was observed with a scanning electron microscope (SEM) on an S-3400N microscope operating at 25 kV. Their magnetic properties were measured with a Lake Shore 7410 vibrating-sample magnetometer (VSM) at room temperature. 3 RESULTS H. ZHAO et al.: EFFECT OF MANGANESE DOPING ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES ... 892 Materiali in tehnologije / Materials and technology 53 (2019) 6, 891–895 Figure 5: Curves of Mn amounts in Ms and Hc Figure 2: SEM images of Mn x Zn 1-x Fe 2 O 4 particles: a) x = 0.2, b) x = 0.4, c) x = 0.6, d) x = 0.8 Figure 4: Hysteresis loops of Mn x Zn 1-x Fe 2 O 4 nanoparticles at room temperature (300 K) Figure 1: XRD patterns of Mn x Zn 1-x Fe 2 O 4 ferrites Figure 3: Particle-size distributions of Mn x Zn 1-x Fe 2 O 4 ferrites: a) x = 0.2, b) x = 0.4, c) x = 0.6, d) x = 0.8 4 DISCUSSION To identify the phase change, the XRD patterns of Mn x Zn 1-x Fe 2 O 4 with x = 0.2, 0.4, 0.6 and 0.8 are shown in Figure 1. The positions and relative intensities of all diffraction peaks are assigned to the (111), (220), (311), (400), (511), (440) and (533) reflections of those from the JCPDS card (No.73-1963) for Mn x Zn 1-x Fe 2 O 4 . Sharp and strong peaks confirm the obtained powders have a single-phase cubic spinel structure. The average crystal size (D) for each sample is estimated from the diffraction data for the (311) plane from the XRD patterns, in accordance with the Debye-Scherrer formula in Equation (1): D = 089 . cos (1) where is the X-ray wavelength, is the value of the full width at half maximum (FWHM) of the (311) diff- raction peak, and is the Bragg angle of the (311) peak. The average crystal size of Mn x Zn 1-x Fe 2 O 4 ferrites, as deduced from the X-ray data, is in a range of 205 nm to 287 nm for all the samples. There is a systematic increase in the crystal size with the increase in the Mn concentration. This phenomenon suggests that the Mn 2+ ion doping influences the particle crystallization during the solvothermal process and promotes the grain growth of crystallite Zn 2+ owing to crystal-lattice inflations. SEM images of the Mn 1-x Zn x Fe 2 O 4 particles are shown in Figure 2. It reveals that the obtained particles demonstrate the high quality and uniformity of the particle size. The particles are monodisperse, exhibiting a spherical shape with a narrow-sized distribution. These particles have an average size of about 200–300 nm. Ethylene glycol plays an important role during the formation of monodisperse particles, which can slow down the aggregation growth of crystals due to fewer surface hydroxyls and a higher viscosity of the system, providing enough time for the powders to rotate adequately, finding a suitable configuration interface. Particle-size distribution of the Mn 1-x Zn x Fe 2 O 4 particles is shown in Figure 3. It reveals that the average particle sizes of x = 0.2, 0.4, 0.6 and 0.8 are (200, 220, 293 and 269) nm, respectively. The statistical results are basically consistent with the XRD analysis. In Fig- ure 3a, less than5%ofthesamples is taken up by the total particle sizes of 198 nm and 204 nm, while (20.05, 22.13 and 18.72) % of the samples are taken up by the sizes of (199, 200 and 201) nm, respectively. The combined proportion of the three is 60.90 %. In Figure 3b, the proportions of the particles of 219 nm and 220 nm are 25.12 % and 23.34 %, respectively; both values together amount to 48.46 %, while the other-size particles amount to 50 %. In Figure 3c, the particles of 291 nm, and 298 nm account for less than5%ofthe samples, while the particles of (293, 294 and 295) nm account for about 60 % of the samples. The particle size of the prepared samples is basically between 293 nm and 295 nm. It can be concluded that the particle-size distribution of the prepared samples is relatively wide. In Figure 3d, about 73 % of the particles were (269, 270 and 271) nm. It can be seen that the particle size of the synthesized powders is uniform and the distribution is concentrated when x = 0.8. As can be seen from Fig- ure 3, the proportion of the particulate matter can reach about 25 % in some samples, while in other samples, it is less than 5 %. The particle spacing is quite different, and the zinc ion content has a certain influence on the part- icle size of the product. In summary, different stoichio- metric ratios are closely related to the particle size and particle distribution. In order to obtain products with a smaller particle-size difference, it is necessary to adjust the ratio of manganese and zinc ions in the reaction system. The magnetic properties of Mn x Zn 1-x Fe 2 O 4 ferrites were studied using a vibrating-sample magnetometer at room temperature. In Figure 4, it can be observed that Mn-Zn ferrites synthesized at different molar ratios show ferromagnetic properties because the residual saturation magnetization and coercivity are small at room temperature. The corresponding magnetic properties of Mn x Zn 1-x Fe 2 O 4 are shown in Table 1. Figure 5 shows the variation trend of the saturation magnetization (Ms) and coercivity (Hc) with different amounts of manganese ions. It can be observed that Ms reaches the maximum value of 71.99 emu/g when the amount of manganese ions is x = 0.6, and the coercivity curve first shows a decreasing trend and then an increase with the increase in the amount of manganese ions. When x = 0.8, the H. ZHAO et al.: EFFECT OF MANGANESE DOPING ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 891–895 893 Figure 6: Time-temperature curves of Mn x Zn 1-x Fe 2 O 4 particles Table 1: Main magnetic properties of Mn x Zn 1-x Fe 2 O 4 particles Formula Ms (emu/g) Mr (emu/g) Hc (Oe) μ B Mn 0.2 Zn 0.8 Fe2O4 66.87 10.35 119.42 2.88 Mn0.4 Zn 0.6 Fe 2 O 4 68.73 10.90 121.85 2.94 Mn0.6 Zn 0.4 Fe 2 O 4 71.99 12.36 131.32 3.03 Mn0.8 Zn 0.2 Fe 2 O 4 69.53 10.30 132.84 2.91 coercivity is 131.84 Oe. This phenomenon shows that the saturation magnetization and coercivity of manganese- zinc ferrite are closely related to the substitution degree of zinc ions for a constant external magnetic field. With the increase in the Mn 2+ addition, the degree of substi- tution for Zn 2+ increases, and the saturation magneti- zation of the product continues to increase. If x > 0.6, the degree of substitution for Zn 2+ gradually decreases. With the increasing concentration of manganese ions, manganese ions preferentially occupy the octahedral B site, which results in an equal number of trivalent iron ions entering the tetrahedral A site. In the Néel model, the magnetic moment μ B ( ) can be expressed with the following Equation (2): BBA () () () =− MM (2) In this formula, M A and M B represent the magnetic moments of the tetrahedral structure position (A) and octahedral structure position (B) in the Mn-Zn ferrite, respectively. The magnetic moment μ B is closely related to the structure position, the distribution of metal cations and the spin/tilt effect. As the concentration of Mn 2+ gradually increases, the corresponding magnetic moment μ B first increases and then decreases. This is because when the amount of Mn 2+ is relatively small, Mn 2+ can preferentially occupy position B of the octahedron structure, which forces the same amount of Fe 3+ in the reaction system to enter position A of the tetrahedron structure. This results in a gradual increase in the super- exchange force between A–B sites, net magnetic moment and saturation magnetization Ms. When the amount of Mn 2+ exceeds 0.4, the A–B exchange force begins to weaken, while the B–B exchange force in- creases. As the content of manganese ions increases con- tinuously and occupies the B position, the redundant part of Mn 2+ enters the tetrahedron-to-tetrahedron structure position so that the same amount of Fe 3+ in the tetra- hedron gap returns to the octahedron structure position, resulting in an increase in the magnetic moment of the B position. This rearrangement rapidly increases the exchange force of the B–B position, forcing the number of reverse parallel-spin coupling pairs in the octahedron gap to increase continuously so that the saturation-mag- netization intensity is decreased. μ B is the experimental value of the magnetic moment of the ferrite which can be calculated with the following formula in Equation (3): B s = × MM 5585 (3) μ B – experimental value of the magnetic moment; M – molar molecular mass; M s – saturation magnetization. According to Equation (3), the calculation results for μ B are listed in Table 1. From Table 1, it can be con- cluded that the values of the magnetic moment and saturation magnetization are closely related, and μ B is proportional to Ms. Figure 6 is a time-temperature curve of the Mn x Zn 1-x Fe 2 O 4 particles under a 50-kHz external alternating magnetic field. It can be seen from Figure 6 that Mn x Zn 1-x Fe 2 O 4 can convert part of the electromag- netic energy into internal energy under the action of an external alternating magnetic field, which can increase the temperature of the product. When x = 0.2, 0.4, 0.6 and 0.8, the final temperatures can reach (42.5, 50.4, 60.8 and 69.9) °C, respectively. In the period of 0–200 s, the rate of the temperature rise is fast, but during the period of 200–400 s, it is slower. The temperature of the product during the period of 400–600 s is no longer on the increase, gradually reaching a stable value. This is because the heat production and environmental-heat dissipation of the sample are in a relative equilibrium state and the heat is no longer diffused. The increase in the product temperature is closely related to its magnetic loss, which consists of the eddy current loss, hysteresis loss and residual loss. As the Mn-Zn ferrite synthesized with the solvothermal method has a high resistivity, the effects of the eddy current loss and residual loss on the energy conversion can be neglected, so the final heat of the prepared product is closely related to the hysteresis loss. When the power of the applied magnetic field is fixed to a certain value, the heat generated by the hysteresis loss can be indirectly expressed by the product of the saturation magnetization, M s , and the coercivity, Hc. When x = 0.8, the product of the saturated magnetization and coercivity reaches the maximum value, also indicating the fastest heating rate under the alternating magnetic field, and the temperature of the sample reaches the highest value. At 600 s, the temperature can rise to 69.9 °C, showing an excellent magnetocaloric effect. 5 CONCLUSIONS Mn x Zn 1-x Fe 2 O 4 particles constitute a single-phase cubic spinel structure. Synthesized Mn x Zn 1-x Fe 2 O 4 part- icles are spherical and have good crystallinity. With an increase in the manganese ion amount, the particle size of the product first increased and then decreased. Mn x Zn 1-x Fe 2 O 4 powders show ferromagnetism. As the x-value gradually increased, the Ms of the product first increased and then decreased. The maximum value was 71.99 emu/g at x = 0.6. Manganese-zinc ferrites with different molar ratios lasted 600 seconds under a 50-kHz alternating magnetic field, and the temperature of the mixed dispersion solution of the samples could reach 42.5–69.9 °C. With an increase in the x-value, the mag- netocaloric properties of the Mn-Zn ferrite improved significantly, which was closely related to the hys- teresis-loop area and the hysteresis power loss of the product. H. ZHAO et al.: EFFECT OF MANGANESE DOPING ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES ... 894 Materiali in tehnologije / Materials and technology 53 (2019) 6, 891–895 Acknowledgment This work was financially supported by National Natural Science Foundation of China (51303108) and National Equipment Pre-research Project of China (61409230605) . 6 REFERENCES 1 R. P. Araújo-Neto, E. L. Silva-Freitas, J. F. Carvalho, T. R. F. Pontes, K. L. Silva, I. H. M. Damasceno, E. S. T. Egito, A. L. Dantas, M. A. Morales, A. S. Carriço, , Monodisperse sodium oleate coated magnetite high susceptibility nanoparticles for hyperthermia applications, Journal of Magnetism and Magnetic Materials, 364 (2014), 72–79, doi:10.1016/j.jmmm.2014.04.001 2 V. Tsakaloudi, D. Holz, V. Zaspalis, The effect of externally applied uniaxial compressive stress on the magnetic properties of power MnZn-ferrites, J. Mater. Sci., 48 (2013), 3825–3833, doi:10.1007/ s10853-013-7183-2 3 D. G. Li, C. Chen, W. Rao, W. H. Lu, Y. H. Xiong, Preparation and microwave absorption properties of polyaniline/Mn0.8Zn0.2Fe2O4 nanocomposite in 2-18 GHz, J. Mater. Sci.: Mater. Electron., 25 (2014), 76–81, doi:10.1007/s10854-013-1551-1 4 Q. Zhang, P. Zheng, L. Zheng, J. Zhou, H. Qin, Effect of Co-sub- stitution on the structure and magnetic properties of MnZn power ferrite, J. Electroceram., 32 (2014), 230–233, doi:10.1007/s10832- 013-9878-9 5 W. J. Wang, C. G. Zang, Q. J. Jiao, Synthesis, structure and electromagnetic properties of Mn–Zn ferrite by sol-gel combustion technique, Journal of Magnetism and Magnetic Materials, 349 (2014), 116–120, doi:10.1016/j.jmmm.2013.08.057 6 C. H. Dong, G. X. Wang, L. Shi, D. W. Guo, C. J. Jiang, D. S. Xue, Investigation of the thermal stability of Mn ferrite particles synthesized by a modified co-precipitation method, Science China: Physics, Mechanics & Astronomy, 56 (2013) 3, 568–572, doi:10.1007/s11433-012-4885-4 7 K. Rana, P. Thakur, M. Tomar, V. Gupta, A. Thakur, Investigation of cobalt substituted M-type barium ferrite synthesized via co-precipitation method for radar absorbing material in Ku-band (12–18 GHz), Ceramics International, 44 (2018) 6, 6370–6375, doi:10.1016/j.ceramint.2018.01.028 8 T. M. Hammad, J. K. Salem, A. A. Amsha, N. K. Hejazy , Optical and magnetic characterizations of zinc substituted copper ferrite synthesized by a co-precipitation chemical method, Journal of Alloys and Compounds, 741 (2018), 123–130, doi:10.1016/j.jallcom.2018. 01.123 9 V. Manikandan, X. Li, R. S. Mane, J. Chandrasekaran, Room temperature gas sensing properties of sn-substituted nickel ferrite (NiFe2O4) thin film sensors prepared by chemical co-precipitation method, Journal of Electronic Materials, 47 (2018) 7, 3403–3408, doi:10.1007/s11664-018-6295-5 10 M. R. Barati, Characterization and preparation of nanocrystalline MgCuZn ferrite powders synthesized by sol–gel auto-combustion method, J. Sol-Gel Sci. Technol., 52 (2009), 171–178, doi:10.1007/ s10971-009-2023-1 11 J. L. Liu, M. Yang, S. Y. Wang, J. Q. Lv, Y.Li, M. Zhang, Sol-gel auto-combustion synthesis and properties of Co2 Z-type hexagonal ferrite ultrafine powders, Journal of Magnetism and Magnetic Materials, 454 (2018), 1–5, doi:10.1016/j.jmmm.2018.01.049 12 N. Yasmin, I. Inam, M. Iftikhar, M. Zahid, M. N. Ashiq, S. Abdul- satar, M. Safdar, M. Mirza, Structural and magnetic properties of Cr doped strontium spinel ferrite SrFe2O4 by sol-gel auto-combustion method, Physica B: Condensed Matter, 550 (2018), 90–95, doi:10.1016/j.physb.2018.08.039 13 B. H. Guan, M. H. Zahari, K. C. Lee, Effect of La 3+ substitution on the structural and magnetic properties of Mn-Zn ferrite prepared by sol-gel auto-combustion method, Materials Science Forum, 916 (2018), 91–95, doi:10.4028/www.scientific.net/MSF.916.91 14 E. M. Ibrahim, The effect of sintering time and temperature on the electrical properties of MnZn ferrites, Appl. Phys. A, 89 (2007), 203–208, doi:10.1007/s00339-007-4088-4 15 M. Mashadi, Y. Yunasfi, A. Mulyawan, Microwave absorption study of manganese ferrite in X-band range prepared by solid state reaction method, Jurnal Teknologi, 80 (2018) 2, 147–151, doi:10.11113/jt. v80.10773 16 G. P. Kumar, V. Raghavendra, C. P. Babu, Synthesis, structural studies on Ni-Zn ferrite by solid state reaction method, Chemical Science Transactions, 5 (2016) 4, 1096–1099, doi:10.7598/cst2016. 1311 17 S. Surinwong, A. Rujiwatra, Ultrasonic cavitation assisted solvother- mal synthesis of superparamagnetic zinc ferrite nanoparticles, Particuology, 11 (2013), 588–593, doi:10.1016/j.partic.2012.06.008 18 A. Kalam, A. G. Al-Sehemi, M. Assiri, G. Du, T. Ahmad, I. Ahmad, M. Pannipara, Modified solvothermal synthesis of cobalt ferrite (CoFe2O4) magnetic nanoparticles photocatalysts for degradation of methylene blue with H2O2/visible light, Results in Physics, 8 (2018), 1046–1053, doi:10.1016/j.rinp.2018.01.045 19 K. Muthukumar, D. S. Lakshmi, S. D. Acharya, S. Natarajan, A. Mukherjee, H. C. Bajaj, Solvothermal synthesis of magnetic copper ferrite nano sheet and its antimicrobial studies, Materials Chemistry and Physics, 209 (2018), 172–179, doi:10.1016/j.matchemphys. 2018.02.004 20 S. Thota, S. C. Kashyap, H. C. Gupta, T. K. Nath, Improved Magnetic Properties of Microwave-Processed Mn0.5Zn0.5Fe2O4 Nanoparticles, J. Supercond. Nov. Magn., 28 (2015), 131–136, doi: 10.1007/s10948-014-2820-9 21 V. K. Sankaranarayanan, C. Sreekumar, Precursor synthesis and microwave processing of nickel ferrite nanoparticles, Current Applied Physics, 3 (2003), 205–208, doi:10.1016/S1567- 1739(02)00202-X 22 C.l E. Patton, M. Z. Wu, K. R. Smith, V. I. Vasyuchka, Nonlinear ferrite film microwave signal processing for advanced communi- cations-physics and devices, Ferroelectrics, 342 (2006) 1, 101–106, doi:10.1080/00150190600946260 23 K. V. Sankar, S. Shanmugapriya, S. Surendran, S. C. Jun, R. K.Selvan, Facile hydrothermal synthesis of carbon-coated cobalt ferrite spherical nanoparticles as a potential negative electrode for flexible supercapattery, Journal of Colloid and Interface Science, 513 (2018), 480–488, doi:10.1016/j.jcis.2017.11.054 24 M. Q. Hua, L. Xu, F. Cui, J. Lian, Y. Huang, J. Bao, J. Qiu, Y. Xu, H. Xu, Y. Zhao, H.Li, Hexamethylenetetramine-assisted hydrothermal synthesis of octahedral nickel ferrite oxide nanocrystallines with excellent supercapacitive performance, Journal of Materials Science, 53 (2018) 10, 7621–7636, doi:10.1007/s10853-018-2052-7 25 S. Y. Liu, L. Y. Wang, K. C. Chou, Synthesis of metal-doped Mn-Zn ferrite from the leaching solutions of vanadium slag using hydrothermal method, Journal of Magnetism and Magnetic Materials, 449 (2018), 49–54, doi:10.1016/j.jmmm.2017.10.001 26 R. S. Melo, P. Banerjee, A. Franco, Hydrothermal synthesis of nickel doped cobalt ferrite nanoparticles: optical and magnetic properties, Journal of Materials Science: Materials in Electronics, 29 (2018), 14657–14667, doi:10.1007/s10854-018-9602-2 27 T. Q. Luong, V. Nguyen, Hydrothermal synthesis of superparamag- netic zinc–nickel ferrite nanoparticles, International Journal of Materials Research, 109 (2018) 6, 555–560, doi:10.3139/146.111629 28 G. S Wang, Y. Y Ma, Y. Tong, X. Dong, M. Li, Solvothermal synthesis, characterization, and magnetorheological study of zinc ferrite nanocrystal clusters, Journal of Intelligent Material Systems and Structures, 28 (2017) 17, 2331–2338, doi:10.1177/ 1045389X16685449 29 W. Shen, X. Q Chen, Y. Shi, M. Shi, H. ChenSynthesis of mono- disperse and single-crystal Fe3O4 hollow spheres by a solvothermal approach, Materials Chemistry and Physics, 132 (2012), 987–992, doi:10.1016/j.matchemphys.2011.12.046 H. ZHAO et al.: EFFECT OF MANGANESE DOPING ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 891–895 895