Structural, electrical and magnetic properties of cobalt ferrite with Nd³⁺ doping

Abstract

A systematic study on the influence of Nd³⁺ substitution on structural, magnetic and electrical properties of cobalt ferrite nanopowders obtained by sol–gel auto-combustion route was reported. The formation of spinel phase was confirmed by X-ray diffraction (XRD) data, and percolation limit of Nd³⁺ into the spinel lattice was also observed. Fourier transform infrared spectroscopy (FTIR) bands observed ≈ 580 and ≈ 390 cm⁻¹ support the presence of Fe³⁺ at A and B sites in the spinel lattice. The variation in microstructure was investigated by scanning electron microscopy (SEM), and the average grain size varies from 5.3 to 3.3 µm. The substitution of Nd³⁺ significantly affects the formation of pores and grain size of cobalt ferrite. Room-temperature saturation magnetization and coercivity decrease from 60 to 30 mA·m²·g⁻¹ and 19.9–17.8 mT, respectively, with Nd³⁺ substitution increasing. These decreases in magnetic properties are explained based on the presence of non-magnetic nature of Nd³⁺ concentration and the dilution of super-exchange interaction in the spinel lattice. The room-temperature direct-current electrical resistivity increases with Nd³⁺ concentration increasing, which is due to the unavailability of Fe²⁺ at octahedral B sites.

1 Introduction

In recent years, among the family of spinel ferrites, cobalt ferrite has been rigorously investigated due to its tremendous applications in high-density magnetic recording media, microwave devices, high-sensitivity sensor and biomedical industries [1,2,3,4]. Apart from the promising electronic applications, they are also suitable and widely used in environmental remediation applications due to their excellent physical and chemical properties like high saturation magnetization, low cost, size- and shape-dependent and catalytic properties [5, 6].

The structural, electrical, magnetic and dielectric properties of cobalt ferrite are governed by the factors like method of preparation, sintering time and temperature, chemical composition, type and concentration of dopant. The spinel unit cell consists of cubic closed-pack arrangement of oxygen ions with 64 tetrahedral (A) and 32 octahedral interstitial sites (B). Out of these 96 interstitial sites, only 8 and 16 cations are occupied in A and B sites, respectively, which are aligned in a mutually opposite direction. Thus, there is a great probability to tune the properties of the cobalt ferrite by rearranging the cations present in A and B sites. This is because magnetic and electrical properties are predominantly depending on cationic and charge distribution. Introducing a fraction of magnetic or non-magnetic ions with different valency states in the spinel ferrite affects significantly its structure, which further plays a crucial role in tuning its physical properties for various industrial applications. The rare-earth ions have unpaired 4f electrons, and they have a strong spin–orbit coupling. The substitution of rare-earth ions with Fe³⁺ site will cause 4f–3d coupling which is helpful to determine magneto-crystalline anisotropy of the material; therefore, it is possible to develop a magnetic core useful for low- and high-frequency applications. Several authors reported that substitution of rare-earth ions enhances the electromagnetic characteristics of the cobalt ferrite. Nikumbh et al. [7] reported decrease in magnetic parameters due to the substitution of Nd³⁺, Sm³⁺ and Gd³⁺ in cobalt ferrite. On the other hand, Tahmineh et al. [8] and Dascalu et al. [9] observed that Tb³⁺ could enhance the saturation magnetization of cobalt ferrite and make it a suitable candidate for recording head applications. Thus, the objective of the present study is to investigate the influence of Nd³⁺ doping on structural, electrical and magnetic properties of cobalt ferrite system. Moreover, due to the larger ionic radii of Nd³⁺, it requires higher energy to enter into the spinel lattice. Therefore, standard ceramic route has been chosen in the present work and it is expected that they must occupy the larger B sites, which produce an unusual magnetic behavior upon Nd³⁺ substitution.

2 Experimental

Neodymium (Nd³⁺)-substituted cobalt ferrite with chemical composition CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) was synthesized by standard ceramic method. High-purity analytical-grade (analytical reagent (AR)) precursors of CoO (99.9%), Fe₂O₃ (99.9%) and NdO(99.9%) were used as starting materials in desired stoichiometric proportions. The starting precursors were weighed and mixed thoroughly in agate motor and calcined at 1173 K for 4 h. These powders were compacted in the form of pellets by adding 5% polyvinyl alcohol (PVA) as a binder. Finally, the powder materials and pellets were sintered again at 1473 K for 4 h followed by natural cooling to room temperature. The sintered samples are subjected to various characterization techniques to understand the modifications in structural, electrical and magnetic properties as a function of Nd³⁺ concentration.

Cobalt ferrite powders were analyzed to identify the phase formation by using Panalytical X’Pert Pro MPD X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154056 nm) in the range of 10^°–80^°. The infrared spectra were recorded in the range of 4000–400 cm⁻¹ with Shimadzu Fourier transform infrared spectroscopy (FTIR) Prestige-21. Microstructural changes were studied using LEO 435 VP scanning electron microscope (SEM). Prior to scanning, surfaces of the pellets were coated with a thin layer of platinum. The magnetization measurement was carried out by LAKESHORE VSM-7410 vibrating sample magnetometer (VSM) with maximum applied field of 2 T at room temperature. The room-temperature direct-current (DC) electrical resistivity measurements were done on the disk-shaped pellets using standard two-probe method at a small electric field of 1 V·cm⁻¹ for all samples.

3 Results and discussion

3.1 XRD results

XRD patterns of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples are shown in Fig. 1. It is clear from that all the samples exhibit cubic spinel phase, and prominent peaks corresponding to (220), (311), (222), (400), (422), (511) and (440) planes are in accordance with standard JCPDS card No. 22-1086. The lattice constant a was calculated according to the following equation:

$$a_{\exp } = d\sqrt {h^{2} + k^{2} + l^{2} }$$

(1)

where d is the inter-planar distance of each plane and (hkl) are Miller indices. Nelson–Riley extrapolation method was used to find the accurate lattice constant, as listed in Table 1 [10]. It is clear from Table 1 that lattice constant increases linearly with Nd³⁺ and then decreases when x > 0.02. The observed increase is due to the replacement of Fe³⁺ (0.065 nm) at B site by larger ionic radii Nd³⁺ (0.0995 nm). Similar variation in lattice constant with Nd³⁺ substitution in different ferrite systems has been reported in studies [11, 12]. However, the decrease in lattice constant for the composition x ≥ 0.020 is ascribed to the possible diffusion of Nd³⁺ to the grain boundaries instead of entering into the lattice site. Therefore, the percolation limit of Nd³⁺ concentration to accommodate into the octahedral spinel lattice is 0.02 mol %. Hameda et al. [13] and Farid et al. [14] observed similar variation in lattice constant in their investigation with Nd³⁺ substitution. The average crystallite size for all the samples was calculated using the following Debye–Scherer’s equation, as listed in Table 1:

$$D_{311} = \frac{0.9\lambda }{\beta \cos \theta }$$

(2)

where D₃₁₁, λ, β and θ are volume-averaged crystallite size, wavelength of X-ray (0.15406 nm), full width at half maximum of (311) peak and diffraction angle, respectively. The average crystallite size varies randomly with Nd³⁺ content increasing between 46 and 49 nm. The calculated X-ray density (d_x) of all the samples was calculated by the following formula:

$$d_{x} = \frac{{ZM_{\text w} }}{{N_{\text A} V_{\text C} }}$$

(3)

where Z is the number of formula units in a unit cell, M_w is the molecular weight of the sample, N_A is the Avogadro’s number and V_C is volume of the cell. From Table 1, a linear dependence of X-ray density on Nd³⁺ concentration is observed. This variation in density is a direct consequence of higher molecular weight of Nd³⁺ than Fe³⁺ concentration. It is worthwhile to mention the influence of large ionic radii on hoping lengths of spinel lattice. The following equations were used to calculate the tetrahedral hopping length (L_A) and octahedral hopping length (L_B):

$$L_{\text{A}} = \frac{1}{4}a\sqrt 3 \;{\text{and}}\;L_{\text{B}} = \frac{1}{4}a\sqrt 2$$

(4)

Figure 2 shows the composition dependence of hoping lengths at A and B sites. It can be observed that hopping lengths follow the similar trend with that of lattice constant, suggesting the increase in the distance between the ions in the respective sublattice. The separation between the ions due to the substitution of larger Nd³⁺ modifies the magnetic and electrical properties.

Fig. 1

XRD patterns of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples

Table 1 Average crystallite size, X-ray density, FTIR frequency band positions (ν₁ and ν₂) and grain size of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples

Fig. 2

Hopping lengths (L) of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples

3.2 FTIR and microstructure study

Infrared spectra of Nd³⁺-substituted cobalt ferrite are shown in Fig. 3. Two prominent absorption bands in the range of 350–400 and 550–600 cm⁻¹ are noticed. The high-frequency absorption band ν₁ corresponds to the stretching vibration of the octahedral metal oxygen bond, and low-frequency band ν₂ is due to the metal oxygen vibrations at tetrahedral sites. The positions of the bands are listed in Table 1. Both tetrahedral and octahedral band positions are shifted to higher frequency side. The shift in band positions is due to the variation in Fe³⁺↔ O²⁻ bond length and cation redistribution. In the present work, occupation of Nd³⁺ in octahedral B site with larger ionic radii is responsible for the observed shift in absorption bands. It is also observed from Fig. 3 that broadening of ν₂ band increases with Nd³⁺ concentration increasing, which suggests the occupancy of Nd³⁺ on octahedral B sites [15].

Fig. 3

FTIR spectra of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples

The typical SEM images of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) are presented in Fig. 4. It is well known that grain size and structure influence the physical properties of ferrites. Grain growth is closely related to the grain boundary mobility, because there is a competition between the driving force for grain boundary movement and the retarding force exerted by the pores during the grain growth [16]. The grain size is calculated using linear intercept method and presented in Table 1. It is to be noted that the addition of Nd³⁺ significantly impedes the grain growth. The observed variations in grain size and formation of secondary phase are consistent with reported results in the literature by various authors, but in different ferrite systems [17, 18].

Fig. 4

SEM images of CoNd_xFe_2−xO₄: a x = 0, b x = 0.010, c x = 0.015, d x = 0.020, e x = 0.025 and f x = 0.030

3.3 Electrical resistivity

The electrical properties of spinel ferrite are strongly influenced by the microstructure, availability of cations (Fe³⁺ and Fe²⁺) and their distribution among A and B sites, which in turn depend on the synthesis processes and conditions. It can be seen from Fig. 5 that room-temperature (303 K) DC electrical resistivity increases with Nd³⁺ content increasing. It is known that conduction in spinals is due to the charge transfer of electrons between cations on B sites of different valences, because A–A hopping does not exist as there are only Fe³⁺ on this sublattice and any Fe²⁺ formed during processing preferentially occupy the B sites, and B–B hopping is more dominant than A–B hopping [19]. In the present work, substitution of Nd³⁺ at the expense of Fe³⁺ reduces the availability of ferric ions at B site. This in turn enhances the resistivity of the ferrite. However, hopping takes place among Fe²⁺ and Fe³⁺ due to the presence of secondary phase at grain boundaries. This may be responsible for the slight increment in the resistivity.

Fig. 5

Room-temperature DC electrical resistivity of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples

3.4 Magnetic properties

The magnetism of the spinel ferrite is due to the super-exchange coupling of uncompensated electron spins of the individual magnetic ions through oxygen ions. Owing to this super-exchange interaction, the spins are aligned antiparallel in the two (A and B) sublattices of spinel structure [20]. Therefore, the net magnetic moment is the difference between individual magnetic moments of A and B sublattice, i.e., M = |M_B− M_A|, where M_A and M_B are magnetic moment of ions residing at A and B sites, respectively. Figure 6 represents the hysteresis loops of Nd³⁺-substituted cobalt ferrite, which clearly shows the ferromagnetic behavior. From these plots (Fig. 6), the saturation magnetization (M_s), coercivity (H_c) and ratio of remanence to saturation magnetization (M_r/M_s) were calculated, as listed in Table 2. It is well known that the 4f electrons are responsible for the magnetic moment of rare-earth ions and their magnetic ordering temperature is effective at 40 K [21]. Therefore, the effect of Nd³⁺ on net magnetic moment of spinel is almost negligible. However, due to their presence at octahedral (B) site, some of Fe³⁺ may shift to tetrahedral (A) site, which in turn alters the magnetic properties.

Fig. 6

Magnetic hysteresis loops for CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples and inset being variation of magnetization with Nd concentration

Table 2 Saturation magnetization (M_s), coercivity (H_c), remanence magnetization (M_r) and magnetic moment (n_B) of CoNd_xFe_2−xO₄ (x = 0, 0.010, 0.015, 0.020, 0.025, 0.030) samples

It is observed that as the concentration of Nd³⁺ increases, saturation magnetization (M_s) decreases. The value of M_s is seen to decrease from 60 mA·m²·g⁻¹ for undoped cobalt ferrite to 35 mA·m²·g⁻¹ for CoNd_0.03Fe_1.97O₄. The effective magnetic moment of Nd³⁺ is 3.2 µ_B, which is smaller than that of Fe³⁺ (5.0 µ_B). Therefore, substitution of Fe³⁺ by Nd³⁺ at B site causes M_s to decrease. In the present investigation, saturation magnetization values are higher than the reported values [22,23,24]. The coercivity decreases from 19.9 mT (x = 0) to 17.4 mT (x = 0.030) with Nd³⁺ content. This is due to the decrease in magnetic anisotropy of the system. The behavior of coercivity can be understood from Brown’s relation given by:

$$H_{\text{c}} = \frac{{2K_{1} }}{{\mu_{0} M_{\text{s}} }}$$

(5)

where K₁ is the magnetic anisotropy, $\mu_{\text{o}}$ is the permeability of free space and M_s is the saturation magnetization. According to the above relation, H_c is inversely proportional to M_s and directly related to K₁. It is reported that substitution of Nd³⁺ reduces the anisotropy constant [5, 18, 25]. The coercivity of the pure cobalt ferrite predominantly originated from the single-ion anisotropy of the octahedral Co²⁺. Similar to Co²⁺, Nd³⁺ also shows a strong spin–orbit coupling and contributes to the anisotropy, when they are located in the B sites of spinel ferrites. However, remarkable decrease in coercivity may be ascribed to the larger lattice distortion and smaller reduction in crystalline size. Therefore, for all samples, contribution of the anisotropy leads to the decrease in coercivity.

4 Conclusion

Nd³⁺-substituted cobalt ferrite was synthesized using standard ceramic method, and its effects on structural, electrical and magnetic properties were studied. XRD study shows the formation of single phase with cubic spinel structure. Crystallite size and grain size are affected by the substitution of Nd³⁺, suggesting that growth of crystallite size is obstructed by the substitution of Nd³⁺. The room-temperature DC electrical resistivity increases and saturation magnetization decreases with Nd³⁺ substitution increasing. Finally, it is concluded that the properties of cobalt ferrite get affected by changing parameters such as amount of substitution, method of processing, sintering temperature as well as cationic distribution and play a major role.