An experiment was carried out to investigate the effect of Nd
2O
3 substitution, molar ratio n {=Fe
3+/(Co
2++Ni
2++Nd
3+)}, concentration of alkali, inflowed time of alkali on the magnetic and physical properties of Co-Ni-Nd system spinel ferrite fine particles prepared by the chemical coprecipitation method without post-annealing. The chemical coprecipitation compositions were chosen according to the formula (CoO)
0.5(NiO)
0.5-x(Nd
2O
3)
0.5x·n/2(Fe
2O
3), where x varied between 0 and 0.2 and n varied between 2.00 and 2.75. The typical magnetic properties were saturation magnetization σ
s=47.0×10
-6 Wb·m/kg, residual magnetization σ
r=23.6×10
-6 Wb·m/kg, coercivity H
cJ =620.7 kA/m. To remove the superparamagnetic substance, the coprecipitation fine particles were etched with dilute sulfuric acid. As a result, in the (CoO)
0.5(NiO)
0.45(Nd
2O
3)
0.025·1.25(Fe
2O
3) composition, when the excess NaOH and inflowed time were 1.1 mol and 1 s respectively, the following magnetic and physical properties were obtained: σ
s=75.1×10
-6 Wb·m/kg, σ
r=56.7×10
-6 Wb·m/kg, H
cJ=861.8 kA/m, K
1=3.56×10
5 J/m
3, K
2=-13.1×10
5 J/m
3, and H
A=3.90 MA/m.
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