Ferrite materials evince the ferrimagnetism phenomenon. Ferrimagnetic materials maintain the opposite line-up of spin magnetic moments with different measures [1]. Generally, this classification of oxides becomes interesting materials due to the manifestation of the wide potential window referring to electric, magnetic, and dielectric features [2]. In the near past, an important structure of ferrites pronounced as spinel structure has refashioned the electronic industry because of their extensive implementation in high-density recording media, magnetic fluids, biological tagging, data storage, gas sensors, etc [3]. In respect of technological applications, soft magnetic materials like manganese ferrites have proved themselves as the best candidates in technological applications like magnetic recording heads, hyperthermia, and drug delivery, transformer cores, transducers, electromagnetic interference shielding, etc [4-7]. The MnFe2O4 is a soft ferromagnetic material with a normal spinel structure when tetrahedral sites and octahedral sites collectively create magnetic sub-lattices [8, 9]. In the normal spinel crystal structure, the tetrahedral sites are dwelled by divalent metallic cations while the octahedral sites by trivalent metallic cations [10, 11]. The antiparallel exchange interaction of spin magnetic moments of Fe3+ neutralizes the magnetic effect because of equal distribution of Fe3+ ions at both A and B sites. Only the spin magnetic moments of Fe2+ got aligned in the same orientation to produce net magnetization of ferrites [12].
Substitution of rare-earth ions causes structural modifications which result in the disturbance of net magnetic moment per unit volume (magnetization) of ferrite materials. 3d-4f electronic coupling induced by rare-earth ion substitution at B-sites in ferrite is responsible for increased electrical and magnetic properties. Rare-earth ions possess some good advantageous properties like a suitable magnetic moment, ionic stability, and site preference which make them the best choice to dope rare earth in ferrite to tune their electrical and magnetic properties [7, 13]. Magnetism is associated with the presence of unpaired electrons in the subshells of an atom. Rare-earth ions contain unpaired 4f electrons while iron contains 3d unpaired electrons. When substitution of rare-earth ions is done in ferrites, then spin-orbit coupling commonly called L-S coupling supposed to occur between 3d and 4f electrons. This coupling brings modifications in the structure which ultimately enhances the electrical and magnetic properties of the sample [14, 15]. This is the reason why researchers prefer rare-earth atoms as their best candidates to employ them in ferrites to tune their physical and chemical properties according to their demand and need.
Rezlescu et al. reported the effect of Gd3+, Dy3+, Sm3+, and Yb3+ substitution on the physical properties of Lithium-Zinc ferrite. The study disclosed the variation in growth, Curie temperature, and resistivity [16]. Murugesan et al. described the structural and magnetic properties of Gd-substituted Mn-ferrites. Sol-gel auto-combustion method was employed for the synthesis of Gd-substituted Mn-ferrite nanoparticles. He reported a decrease in values of saturation magnetization and coercivity with Gd3+ content [17]. Naik et al. have reported the impact of Nd3+ ions substitution on structural and magnetic properties of Mn-Zn ferrites via combustion method. He reported a rise in values of saturation magnetization (Ms) with the increase in the concentration of Gd3+ ions [18]. Wu et al. prepared lanthanum substituted Ni-Zn ferrite nanoparticles via solid-state reaction. They observed that the calcination temperature affected the magnetic properties of La3+ substituted Ni-Zn-ferrites and the value of coercivity achieved at 800 oC was 120 Oe [19]. Wang et al. worked on the analysis of structural and morphological results of La3+ substituted Ni-Zn ferrite nanoparticles using the sol-gel method. Results revealed the fall in saturation magnetization with the addition of lanthanum ions[19]. Among rare earth elements, Dysprosium with the highest magnetic moment and possesses high resistivity. Therefore, to meet the demand for highly resistive material we wish to explore the substitution of Dy3+ ions in ferrite which knocks out the Fe3+ ions at the octahedral site. This sort of exploration has not been studied deeply to the best of our knowledge. We plan to discuss in detail the room temperature measured magnetic and electrical properties of Dy3+ substituted soft manganese ferrite nanoparticles with different concentrations of Dy3+ ions (0.0 ≤ x ≤ 0.16). The detailed synthesis and structural parameters of these spinel ferrite nanoparticles have been discussed in our previous publication [7].
Unsubstituted and Dy3+ substituted Mn ferrite nanoparticles (DyxMnFe2-xO4 (x=0.0, 0.01, 0.02, 0.04, 0.08, 0.16) were prepared via optimized wet chemical reverse microemulsion preparation approach as reported in our previous publications [7, 20]. One of the special features of this optimized method is its cost-effectiveness using a low-cost surfactant (Triton X-100) and a commonly available oil phase (Paraffin oil). The detailed synthesis and structural characterization by XRD and FTIR have been discussed in our previous article [7]. In this short communication, we aim to discuss in detail the magnetic and electrical parameters of rare earth Dy3+ cations substituted MnFe2O4. The samples were given the names Dy-0, Dy-1, Dy-2, Dy-3, Dy-4, and Dy-5 respectively with the increase in the concentration of substituent. Pellets of all these solid spine ferrite nanoparticles were prepared to study electric resistivity by the same procedure, as already reported in our previous publication [21].
Figure 1 (a) shows a scanning electron microscopic image of Dy3+ substituted MnFe2O4 (Dy-5) nanoparticles. The average particle size was calculated by Image J software and found ~ 60 nm. The synthesized product was in spherical morphology with a lower order of aggregation. Moreover, the elemental composition was also examined and EDX spectra of Dy3+ substituted Mn ferrites nanoparticle are depicted in Figure 1 (b). The EDX spectra of sample Dy-5 confirmed the existence of all the constituent elements in the final product and thus justified the synthesis of Dy3+ substituted Mn ferrites nanoparticles.
To explore the impact of Dy3+ substitution on magnetic properties, the magnetic study was carried out on the prepared samples at room temperature using a vibrating sample magnetometer (VSM). Figure 2 illustrated the magnetization curves for Dy3+ substituted Mn ferrites. The hysteresis loops were used to calculated saturation magnetization (Ms), Residual magnetization (Mr), Coercivity (Hc), and are given in table 1. Other parameters magnetic moment (nB), anisotropy constant (K1) and the remanence ratio R was also calculated and given in table 1. The magnetization curves described the superparamagnetic behavior that confirmed the single domain nature of particles. The superparamagnetic behavior of prepared samples could be associated with their nano size which overcomes the magnetic anisotropy [22]. According to Neel's two sub-lattice models, the magnetic moment is given by the following equation 1.
In this equation, the MB stands for the magnetic moment of site B and MA for site A [23, 24][Yousuf, 2020 #783]. The magneton numbers can be calculated by equation 2.
![](https://myfiles.space/user_files/87796_83ef146087a3bf32/87796_custom_files/img1626276036.PNG)
In equation 2, the Mw is molecular weight, Ms is saturation magnetization. Neel’s mode l[25] was also used to explain the magnetic behavior of samples. The decreased saturation magnetization can be associated with structural modifications as a result of spin-exchange interaction with Dy3+ substitution. The bond lengths and bond angles influence the entire magnetic strength caused by exchange interactions [26]. Overall magnetic strength is directly related to bond angles while it is inversely related to bond lengths. An increase in bond lengths and a decrease in bond angles reveal the weakening mechanism of A-A and A-B exchange interactions along with the strengthening mechanism of B-B interactions. These weakening and strengthening scenarios lead to a decrease in superexchange interactions between A and B sites which eventually results in a fall in magnetization. The drop in saturation magnetization values may be associated with the spin magnetic moments relating to rare-earth ions usually trigger from unpaired electrons localized in 4f subshells [27]. The entering and adjustment of Dy3+ ions in the octahedral sites are responsible for weak Dy3+-Fe3+ interactions and strong Fe-Fe interactions. This leads to a fall in saturation magnetization [28]. Lattice defects, surface magnetic moment disorder, and presence of an immobile surface layer of nanoparticles are the accepted reasons for smaller saturation magnetization values. The presence of the secondary phase as explained in our previous communication [7] may also affect on saturation magnetization. The production of secondary phase DyFeO3 may produce distortion in the structure. It was observed that as substituent concentration increase, Ms increase in Dy-2. This was due to the replacement of Dy3+ ion with Fe3+ ions at B-site and increases the Dy3+-Fe3+ interactions. However in the remaining samples, the Ms value decreased when compared with Dy-0 [21]. The variation of Ms and Mr with an increase in the concentration of Dy3+ ions was shown in Figure 3 (a).
Small value residual magnetization Mr indicated that homogenous distribution of nanoparticles in the structure. Coercivity values of samples described the magnetic field strength (magnetic force) which is essential to exceed or excel the magnetic anisotropy hurdle and allow magnetic moments of nanoparticles to pursue the magnetizing force orientation. The magnetic properties of crystalline material are not the same in all crystallographic directions. That material is called magnetically anisotropic material and the phenomenon is called magnetic anisotropy [29].
Dy3+ substitution in ferrite reduces the net magnetic moments of ions present at octahedral B sites. As a result, the net resultant magnetic moments of the whole material get reduced. Tahira et al. explained this decreased value that RE3+ ions have empty 6s valance shell that converts Dy3+ ions nonmagnetic and decrease the Ms value. The higher concentration of Dy3+ ions cannot enter into the lattice structure and produce stain in the structure. Although the magnetic moment of Dy3+ ions is higher than Fe3+ ions.
This variation of Hc values may be attributed to the union of size effect and its anisotropy as shown in figure 3 (b). The rare earth Dy+3 ions have larger atomic radius as compared to Fe3+ ions causes magnetocrystalline anisotropy. This anisotropy due to the larger size of rare earth is the reason for lowering the net magnetization of samples [30]. The commencement of anisotropy due to structural disorder caused by the introduction of a larger size of Dy+3 could drastically influence the coercive force needed to counterbalance the magnetic spins on the grain surface. Anisotropic constant value of the sample can be calculated by;
![](https://myfiles.space/user_files/87796_83ef146087a3bf32/87796_custom_files/img1626276045.PNG)
It is clear from Figure 3 (b) that Dy+3 ion substitution enhanced the magnetic anisotropy which eventually increased the Hc values of prepared samples it has been found that the coercivity values increased because magnetocrystalline anisotropy enhanced with anisotropic Fe3+ ions located at 2a sublattice site which normally found in rare-earth ions substitution in ferrites. The increased coercivity might also be associated with the existence of the second phase on or close to grain walls that limited the movement of domain walls [31]. The value of remanence ratio R less than 1 showed that structure belong to a single domain structure [32].
Table 1: Various parameter calculations (Ms, Mr, Hc, nB, K1, R, and resistivity) of MnFe2-xDyxO4 nanoparticles.
#
|
Ms
(KOe/g)
|
Mr
(emu/g)
|
Hc
(Oe)
|
nb
|
K1
|
R=
(Mr/Ms)
|
(ohm-m)
|
Dy-0
|
50.42
|
5.4
|
146.03
|
2.082
|
3681.29
|
0.107
|
6.31×10-11
|
Dy-1
|
43.56
|
3.1
|
84.89
|
1.807
|
1848.80
|
0.071
|
8.47×10-8
|
Dy-2
|
51.49
|
6.9
|
142.70
|
2.146
|
3673.30
|
0.134
|
2.33×10-8
|
Dy-3
|
39
|
2.6
|
56.89
|
1.640
|
1109.26
|
0.066
|
0.52×10-8
|
Dy-4
|
44.9
|
4.8
|
133.78
|
1.923
|
3003.25
|
0.106
|
0.01×10-8
|
Dy-5
|
28.7
|
1.5
|
36.91
|
0.943
|
529.587
|
0.052
|
1.28×10-8
|
The electrical resistivity values of the synthesized samples were determined by the 2 probe current-voltage technique and the obtained I-V curves were presented in Figure 4. Finally, the results from I-V measurements were further explored by using the formula [33].
Here R stands for resistance, L stands for thickness, and A is used for the area of the made pellet. The values of electrical resistivity of samples are calculated at room temperature and depend upon the composition and structure of the crystal [34]. It varies when the concentration of Dy3+ rises. Value of electrical resistivity increases for DyxMnFe2-xO4 (x = 0.00, 0.01, 0.02, 0.04, 0.08, 0.16) in the range of 6.31×10-11 ohm-m to 1.29×10-8 ohm-m and tabulated in table 1. High resistive materials are the utmost need for modern electronic appliances in this modern world. Verwey has thrown considerable light on the conduction mechanism that occurs in ferrite nanoparticles [35]. According to his point of view, hopping of valence shell electrons at octahedral site B between positive ions of different valences does take part in the conduction process. The number of divalent iron ions becomes fewer at the octahedral B site after the conversion of Fe+2 to Fe+3. Conduction may be done by electron hopping between Mn+2 and Mn+3 but higher energy is needed to do so. It was observed that resistivity is a very sensitive and small amount of ion can change the concentration of iron ion to 10-7 [36].
A small portion of conduction may be executed by electron hopping between Mn+3 and Fe+2 at the B site [37]. The substitution of Dy+3 ions increased the sample resistivity. Actually what happens is that site preference of Dy3+ ions towards octahedral position tends to shift some of the Fe+3 ions to the tetrahedral position which reduced the quantity of Fe+3 ions at the octahedral site [38]. This forceful migration of Fe+3 ions by Dy+3 ions hindered the electron hopping between Fe+2 and Fe+3 ions. This increased the resistivity of prepared samples when compared with Dy-0. It has been found in the literature that sometimes resistivity may be reduced by introducing the rare-earth ions [39]. In such case, the Dy3+ and Fe2+ ions synergistically facilitate the conduction process and eventually, the resistivity decreased. This was due to noncorporation of empty 6s valance shell of Dy3+ ions that may not contribute to hopping process. The electron transfer model can easily explain the unusual behavior. Pure MnFe2O4 has a small resistivity value as compared with magnetite Fe3O4. This small value may be due to the production of the hematite phase along with the usual phase that was unidentified in the structure due to pressure of O2- ions in the structure [40, 41]. When substitution of Dy3+ ion increases, it increases the resistivity. However, this value is also small that may be due to the presence of a secondary phase which increases the presence of Fe2+ ions. This secondary phase insulates the grain boundaries and hinder the electron transfer between Fe2+ and Fe3+ [36, 42].
An increase in electrical resistivity in ferrites occurs due to the hindrance of electron hopping and charge transport through excited states [43]. Conduction in ferrites occurs because of electron hopping and charge transport through excited states [44]. A small concentration of Dy3+ ions at octahedral B-sites expands the inter-atomic displacement and perturbs the lattice symmetry due to the larger ionic radius of Dy3+ in comparison with Fe3+.
In conclusion, the electrical and magnetic properties of rare-earth substituted DyxMnFe2-xO4 ferrite (0≤x≤0.16) were evaluated by VSM and I-V measurements. The noticeable variation in saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and DC resistivity was noted as a result of Dy+3 substitution in MnFe2O4. These variations can be related to the modifications in the spin-exchange interactions produced as a result of structural changes due to the substitution of rare-earth ion (Dy3+). The effect of secondary phase was important in variation in saturation magnetization. The DC electrical resistivity of all prepared compositions increased with increased Dy3+ concentration. Generally, the Dy+3 substituted manganese ferrites nanoparticles showed amazing soft magnetic activities. The soft magnetic nature of the product highly recommended its creative application in a broad discipline.