EFFECT OF GADOLINIUM CONTENT ON MAGNETIC AND STRUCTURAL CHARACTERISTICS OF NFGO NANO-PARTICLES †

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INTRODUCTION
The applications of spinel ferrite nano-particles in current context include drug delivery, photocatalysis, telecommunications, electronics, and electrical devices like transformer cores, permanent magnets, magnetic refrigeration, magnetic recording media, gas sensors, and microwave absorbers, among others [1][2][3][4][5].Spinel ferrite is generally represented by the formula AB 2 O 4 , where the B-site is referred to as octahedral at the center of the octahedron and the A-site is referred to as tetrahedral since it is positioned at the center of the tetrahedron and contains oxygen-ions at each corner.The big oxygen-ions are arranged in a face-cantered cubic configuration with space between them occupied by metal-ions [6][7].The inverse spinel structure of nickel ferrite, one of the ferrites that has been the subject of extensive research as a magnetic nanomaterial, shows that trivalent Fe +3 ions occupy both [A] and [B] sites whereas divalent Ni +2 ions occupy the octahedral [B] sites [8][9].Researchers have employed a variety of methods to analyze the structural; electrical; optical; and magnetic properties; of their ferrite nano-particles in order to determine the impact of doping elements and manufacturing methods [10][11][12].Their characteristics can be changed by doping them with rare earth elements in a cubic spinel ferrite structure [13][14].Due to its half-filled 4f 7 electronic configuration, gadolinium is a magnetically active high spin rare earth metal.The properties of the ferrites can be affected by a small amount of Gd doping.Since rare earth ions have high spinorbit coupling and unpaired (4f) electrons, it is known that Fe-Fe exchanges in ferrites are caused by the spin coupling of (3d) electrons, which affects the structural, electrical, optical, and magnetic properties.Better magnetic and electrical properties result from rare earth ion interaction with Fe, or 3d-4f coupling, when these ions occupy ferrite lattice positions [15][16].Furthermore, studies on the doping of ions in CoFe 2 O 4 nano-particles have been published, and the results indicate that the characteristics of ferrites vary significantly [13].There are few studies on Gd-doped nickel ferrites nano-particles in the literature that has been released [17][18], with the exception of those that concentrate on the composite materials' magnetic characteristics.Consequently, the composition of Gd +3 affects structural characteristics and magnetic properties in the current work, which is synthesized using the Sol gel (auto-combustion) process.The precursor materials included ammonia, citric acid, ferric and gadolinium nitrates were AR graded.De-ionized water was used to dissolve the precursor materials in a stoichiometric ratio to create a transparent solution.The solution containing Ni +2 , Gd +3 , and Fe +3 ions was chelated by adding citric acid to the organized aqueous solution.Citric acid and total moles of nitrate ions were combined in 1:3 molar ratio.The attained solution was neutralized with ammonia in an appropriate amount to maintain a p H of 7. A hot plate was then used to heat the neutralized solution to approximately 100℃ while stirring continuously.A thick gel was seen a few hours later.A loose powder [22][23] remained after the temperature was raised to 200℃, which started the ignition process and caused the generated gel to burn completely through auto-combustion.It was annealed for eight hours at 700 ℃ and was known as "as-prepared powder."

II. EXPERIMENTAL
Using a Phillips expert X-ray diffractometer, the XRD patterns for NiFe 2-x Gd x O 4 (x = 0.00, 0.010, 0.15, 0.20 & 0.25) ferrite nano-particles were obtained.The NiFe 2-x Gd x O 4 (x=0.00,0.010, 0.15, 0.20 & 0.25) ferrite nano-particles' microstructural morphology was examined using a ZEISS EVO-18 SEM.VSM were used to measure the magnetic properties at 300 K and a maximum applied magnetic field (15 kOe).Crystallite size and lattice parameter were observed to increase with Gd +3 composition and were obtained in the range of 21.0288 to 27.04125 nm and 8.3325 to respectively are tabulated in Table 1 and shown their variation with Gd composition in Figure 3.The improved values of lattice parameter confirmed the entrance of Gd +3 ions into the structure [27][28].Three different kinds of magnetic-interactions between the cations could be possible via intermediate oxygen-ions through the super-exchange mechanism because the metal-ions occupy at two different lattice sites in the spinel structure: A-A interaction, B-B interaction, and A-B interaction.The angles between cations dispersed over two sites and the distances between cations and oxygen determine an amount of interaction energy between relating cations.In order to investigate how the composition of Gd +3 affected the structural characteristics of the ferrite nanoparticles, as illustrated in Figure 4 [29].The bond-lengths between the cations at the tetrahedral and octahedral sites designated as b, c, d, e, and f were estimated.The subsequent formulas (3-7) [30] can be used to assess these:

RESULTS AND DISCUSSIONS
e = √3 ( ) The bond lengths of the ferrite nano-particles between metal-ions and oxygen at various sites are denoted by the following equations (8-11) [30] and are represented as p, q, r, and s (bond-lengths between cation-anion) in Figure-4 [29]: p = a( − ) Equations (12-16) [31] can be used to determine the bond-angles between cations (oxygen-ions) and cation-anion (metalions) of the ferrite nano-particles, which are represented as θ1, θ2, θ3, θ4 and θ5.The same was depicted in Figure-4 [28]: Tables 2, 3 & 4 contains a tabulation of all bond-length values and bond-angle values between cations and cationanions.It was discovered that the Gd +3 composition increased the interatomic distance between the cations at two distinct sites (A & B) and the cation-anion.The greater ionic radius of Gd +3 ions is the cause of this variation.A-B and B-B interactions were found strengthened, as indicated by the increased bond-angles θ 1 , θ 2 , θ 3 , θ 4 and θ 5 [32][33].The morphological study of all the compositions was done by SEM.Micrographs of all Gd doped nickel ferrite compositions are shown in Figure 5, Numerous voids and pores are visible in non-uniform agglomerated fragments [15].It was observed that every grain was dispersed randomly and had non-uniform sizes [16].The average grain size was found increased with Gd+3 composition and it was observed in the range of 140.5-176.2nm, tabulated in Table 1.At room temperature, the magnetic properties of NiFe 2−x Gd x O 4 (x = 0.00, 0.010, 0.15, 0.20 & 0.25) ferrite nanoparticles were examined using a VSM.The synthesis method, grain size, cation doping, cation re-distribution, and other factors are major determinants of the magnetic properties of ferrite nano-particles [34].Ferrite nano-particles' magnetic properties are mostly influenced by the Fe +3 -Fe +3 interaction and the spin coupling of their third-dimensional electrons.When Gd +3 enters the Fe lattice, according to the cation distribution, the Gd +3 -Fe +3 interaction happens with 3d-4f electrons spin coupling, and hence Gd +3 ions will replace Fe +3 ions in octahedral [B] site.Figure 6 illustrates the hysteresis of NiFe 2-x Gd x O 4 (x = 0.00, 0.05, 0.10, 0.15, 0.20 & 0.25) ferrite nano-particles annealed at 700℃.The derived parameters such as saturation magnetization, remanent magnetization and coercivity are tabulated in Table 5.In general, Gd +3 ions have a magnetic moment of 7.9 BM whereas Fe +3 ions have magnetic moment 5 BM.Observing the Gd +3 and Fe +3 ions magnetic moments, the magnetization for all the compositions was supposed to increase Gd +3 composition in nickel ferrites.But it is clearly observed from Table-5 that M s values and M r values were decreased from 30.28 to 15.35 emu/g and 5.07 to 3.65 emu/g respectively.The coercivity was increased from 154 Oe to 261Oe and the magnetization change may be due to Gd +3 -Fe +3 interactions.The strength of the spin-orbital coupling in ferrite materials limits their magnetic anisotropy.Presence of larger magnetic anisotropy in the ferrites results in the larger coercivity.Doping of Gd +3 ions in the nickel ferrites has resulted in the increase of coercivity which makes these ferrite compositions magnetically hard.Similar kind of results were observed for Tb +3 doped cobalt ferrites [35][36].Presence of Gd +3 ions weaken the super-exchange interactions in nickel ferrites [37][38].

CONCLUSION
Sol gel auto-combustion was used to create NiFe 2-x Gd x O 4 (x = 0.00, 0.010, 0.15, 0.20 & 0.25), ferrite nano-particles that were then annealed for eight hours at 700°C.The increased ionic radii of rare earth ions (Gd +3 ) are responsible for the structural parameters, such as crystallite size and lattice parameter, which increased from 21.0288 to 27.04125 nm and 8.3325 to 8.3367Å, respectively, with Gd +3 composition.Estimates of the tetrahedral, octahedral, and bond-angles also demonstrated significant variation with the Gd +3 composition.The average grain size, which ranged from 140.5 to 176.2 nm, was found to increase with Gd composition.The composition of Gd resulted in a decrease in both saturation and residual magnetization.Gd content increases the coercivity and gives ferrite nano-particles their magnetic hardness.

Figure 2
displays XRD patterns of NiFe 2-x Gd x O 4 (x = 0.00, 0.10, 0.15, 0.20, 0.25) ferrite nano-particles that were annealed for eight hours at 700℃ [25].Single-phase; cubic spinel structure; and Fd3m space-group; are indicated by XRD patterns.Because of nano-crystalline pattern of the prepared compositions, XRD data showed that XRD peaks grew wider with increasing Gd doping content.The variance in ionic-radius of Ni (0.74 Ả) and Gd (0.94 Ả) could be cause of slight shift in peak position observed with increase in Gd +3 composition.Various structural parameters were computed from XRD patterns using standard relations listed below.