Magnetic properties of nanocrystalline Co–Ni ferrite

A spinel structure which is formed by a nearly close-packed fcc array of anions with holes partly filled by the cations can be represented by the formula AB₂O₄ [1], A represents metallic ions located in A interstitial (tetrahedral) sites and B metallic ions located in B (octahedral) sites. Due to the large electronegativity of oxygen, the ionic type of bonds prevails in almost all oxide spinels [2]. Magnetic properties of nanoparticles have been of great interest in recent years partly because of the development of high-density magnetic storage media with nanosized constituent particles or crystallites [3, 4]. Much attention have been attracted by the investigation of nanophase spinel ferrite particles owing to their technological importance in the application areas, such as microwave devices, high speed digital tape and disk recording, ferrofluids, catalysis, and magnetic refrigeration systems [5]. Magneto-optical properties of CoFe₂O₄ and NiFe₂O₄ have been investigated in references [6–9]. The investigation of ferromagnetic resonance properties in Co_xNi_1−xFe₂O₄ with low cobalt concentration (X ≤ 0.1) were performed many years ago [10–13]. Cobalt ferrite, CoFe₂O₄, is a well-known hard magnetic material [14]. Recently, Ni²⁺ ions were added to Co ferrite films to improve the magnetic recording properties of cobalt ferrite films, which are promising as high-density perpendicular recording media [15, 16]. To our knowledge, the systematic investigation of the magnetic properties of nanocrystalline Co–Ni ferrite Co_xNi_1−xFe₂O₄ with x variated from 0 to 1 has not been reported. In this article, the crystal structure and the magnetic properties of nanosized Co–Ni ferrite prepared by the polyvinyl alcohol (PVA) sol–gel method are reported.

Co_xNi_1−xFe₂O₄ (0.0 ≤ x ≤ 1.0) ferrite powders were produced by the PVA sol–gel method [17]. Cobalt nitrate, nickel nitrate and ferric nitrate were mixed at a Co:Ni:Fe ion ratio of x:1–x:2 and dissolved in deionized water, this mixture was then added to the aqueous PVA solution with continuous stirring for 30 min. After that, the mixed solution was dehydrated at 80 °C until the dry gel-type precursor was obtained. Portion of this precursor was calcined at 800 °C for 2 h, yielding the materials examined in this work.

The crystallographic properties of the samples were examined by powder X-ray-diffraction (XRD) measurements with CuK_α radiation (Philips x’ pert, Holland). The macromagnetism measurements were performed using a vibrating sample magnetometer (VSM) (Lakeshore 7304, USA).

Figure 1 shows X-ray diffraction patterns of Co_xNi_1−xFe₂O₄. All samples were found to be single phase spinel. The average particle sizes as determined from the X-ray diffraction line breadths are about 34 ± 2 nm. The lattice constant a ₀ of each sample listed in Table 1 is obtained by plotting the lattice constant versus the Nelson–Riley function and extrapolating the result to θ = 90°. A comparison between the lattice constant of the nanosized Co_xNi_1−xFe₂O₄ and that of the bulk material [1] is shown in Fig. 2. The lattice constant a ₀ increases almost linearly with the increasing cobalt concentration (x). This can be explained by the fact that the ionic radius of Co²⁺ (0.78 Å) is larger than that of Ni²⁺ (0.69 Å). The larger value of the lattice constant for the nanosized Co_xNi_1−xFe₂O₄ than for the bulk material with x ranging from 0 to 1.0 may be due to the lattice expansion induced by the reduced particle size and increased surface–body ratio in Co_xNi_1−xFe₂O₄ ferrite nanoparticles.

Fig. 1

X-ray diffraction patterns of Co_xNi_1−xFe₂O₄ (x = 0.0, 0.4, 0.6, 1.0)

Fig. 2

Dependence of the lattice parameter on cobalt concentration for Co_xNi_1−xFe₂O₄

Some typical room temperature hysteresis loops of Co_xNi_1−xFe₂O₄ are shown in Fig. 3 with field up to 12 kOe. The maximum magnetization (M _s) in field of 12 kOe, residual magnetization ratio (M _r/M _s) and coercivities (H _c) for all samples are summarized in Table 1. Figure 4 shows the dependence of M _s on cobalt concentration (x). The increase of cobalt concentration yields the monotonic increase of M _s, which may be caused by the substitution of Ni²⁺ ions by Co²⁺ ions on octahedral sites. The magnetic moment μ per ion for Co²⁺ ions (3 μ_B) is larger than that for Ni²⁺ ions (2 μ_B). Therefore, the increasing Co²⁺concentration (x) on octahedral sites may result in an increasing magnetic moment per formula of Co_xNi_1−xFe₂O₄, and equivalently, an enhancement of magnetization. As shown in Fig. 5, with the increasing cobalt concentration, the residual magnetization ratio (M _r/M _s) and coercivities (H _c) increase in the range of low concentration and then decrease in the range of high concentration. At high cobalt concentrations, a decreasing coercivity and simultaneously, an increasing saturation magnetization have been observed, which can be easily controlled by increasing the content of cobalt in Co_xNi_1−xFe₂O₄. Similar results have been reported in modified Ba-ferrite and Sr-ferrite [18, 19]. Our observations may have some significance. Magnetic recording medium, for instance, requires a high saturation magnetization and a moderately high coercivity [20]. Sometimes, a too high coercivity is not suitable for recording medium application and has to be modified, and it is important to keep a high saturation magnetization when the coercivity is reduced [18]. The coercivity of an array of single domain particles interacted with each other is given by [21]

$$ H_{\text{c}} (p) = H_{\text{c}} (0)(1 - p). $$

Fig. 3

Hysteresis loops of Co_xNi_1−xFe₂O₄ with (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, (e) x = 0.8, and (f) x = 1.0

Table 1 Lattice parameter (a ₀), maximum magnetization (M _s), coercivity (H _c) and residual magnetization ratio (M _r/M _s) for Co_xNi_{1− x}Fe₂O₄

Fig. 4

Dependence of maximum magnetization on Co²⁺ concentration in Co_xNi_1−xFe₂O₄

Fig. 5

Dependence of coercivity and residual magnetization ratio on Co²⁺ concentration in Co_xNi_1−xFe₂O₄

Here p is the volume fraction, which can be described as v _p/v _s, v _s is the total volume of the sample, and v _p the total volume of single domain particles in the sample. H _c(0) is the coercivity of an array of non-interacting particles with uniaxial anisotropy, given by [22]

$$ H_{\text{c}} (0) = 0.48H_{\text{A}} $$

$H_{\text{A}} = 2K/\mu _0 M_{\text{s}} ,$ so we obtain

$$ H_{\text{c}} (p) = 0.96K(1 - p)/\mu _0 M_{\text{s}} . $$

It may be roughly assumed that the value of p is same for all the samples considering the fact that all the samples were prepared and examined almost under the same conditions. If this model (array of single domain particles interacted with each other) is qualitatively applicable to our samples. There will be a dependence of anisotropy constant K on the cobalt concentration x, which can be evaluated from the given values of M _s and H _c for our samples (Fig. 6). As the anisotropic effect of Co²⁺, especially Co²⁺ in B sites, is strong it usually outweighs other possible contributions to the induced anisotropy [23]. In the range of low concentrations Co²⁺ ions may be considered as isolated. This may explain the approximate linear dependence of K on x in the low x range (Fig. 6). In the range of high concentrations Co²⁺ ions cannot be considered as isolated and contributions of more complicated local configurations including pairs become important. For example, isolated Co²⁺–Co²⁺ pairs might induce a quadratic dependence of K on x [24]. This might cause the complicated variation of K in the range of high cobalt concentration as shown in Fig. 6. Similar phenomenon have been demonstrated in Co_xNi_1−xFe₂O₄ systems [25, 26] but not been reported in Co_xNi_1−xFe₂O₄ systems yet.

Fig. 6

Dependence of anisotropy constant K on Co²⁺ concentration in Co_xNi_1−xFe₂O₄

In conclusion, the PVA sol–gel method has been used to produce nanosized Co–Ni ferrite Co_xNi_1−xFe₂O₄ (0.0 ≤ x ≤ 1.0). The lattice constants of Co–Ni ferrite nanoparticles are larger than that of bulk materials and increases with the increasing cobalt concentration (x). The increase of cobalt concentration yields the monotonic increase of M _s. The residual magnetization ratio and coercivities increase in the range of low cobalt concentration and then decrease in the range of high concentration, which may originate from the variation of anisotropy induced by Co²⁺ ions in octahedral sites.