Permanent Magnetic Behavior Of Nanocrystalline NdFeB Based Alloys With La Or/and Ce Substitution Prepared By Rapid Quenching

Being essential components of various magnetic devices, permanent magnets have a wide application ranging from magnetic-field providers to electric motors to minimize the size of loudspeaker, disk drive, wind turbine generator, etc. Up to now, neodymium iron boron（NdFe B） based alloys are still the strongest permanent magnet with excellent room-temperature magnetic properties. However, they show reduced performance at high temperature. The use of the expensive rare earth（RE） elements like Nd, Pr and Dy raises the cost of permanent magnets. Aiming at high-performance low-cost NdFeB magnets, the effects of individual substitution of La, Ce and LaCe for Nd on room temperature magnetic properties, microstructure, characteristic temperatures, coercivity mechanism and exchange coupling have been investigated in nanocomposite and single phase(Nd_1-xMx)yFe94-yB6（M = La, Ce, or La₀.5Ce₀.5; x= 0- 0.7 and y=10, 12） alloys and [Dy_1-x（La₀.5Ce₀.5）_x]10Fe84B6 prepared by melt-spinning technique. The effects of Co substitution for Fe on room temperature properties and the thermal stability of selected [（NdDy）（La/Ce）]FeB alloys were also studied in this thesis work. The underlying physics of properties variations has been discussed.The magnetic properties of NdFeB alloys with various amounts of La substitutions have been evaluated. It is found that 10% La substitution for Nd in nanocomposites and 5% La substitution for Nd in single phase alloys are beneficial for improving the magnetic properties. The measured values of Mr,（BH）max and Hc are 106 emu/g（0.99 T）, 138 kJ/m³ and 465 kA/m, respectively for 10% La substituted nanocomposite alloy, whereas values of Mr,（BH）max and Hc are 104 emu/g（0.97 T）, 151 kJ/m³ and 721 kA/m for 5% La substituted single phase alloy, respectively. It is noted that both Curie temperature Tc and spin reorientation temperature TSR are decreased by La substitution in nanocomposite alloys.For Ce substituted NdFeB alloys, a gentle decrease in coercivity Hc, remanence Mr and the maximum energy product（BH）max were found for single phase and nanocomposite alloys. However, 20% Ce substitution for Nd leads to an anomalous increase in coercivity, thus slightly improving the room temperature magnetic properties of nanocomposite alloy. The measured values of Mr,（BH）max and Hc are 99 emu/g（0.93 T）, 103 kJ/m³ and 392 kA/m, respectively for 20% Ce substitution. The decrease of magnetic properties in single phase alloy is due to the reduced value of anisotropy field for CeFeB compared to Nd FeB and no unusual result was obtained with Ce substitution for Nd.To study the combined effect of La and Ce substitution for Nd on room temperature magnetic properties in nanocomposite alloys, La₀.5Ce₀.5 substitution for Nd was also investigated. Relatively, LaCe substituted alloys show high values of the remanent magnetization Mr, the maximum energy product（BH）max and the coercivity Hc, up to 114 emu/g（1.07 T）, 147 kJ/m³ and 471 kA/m, at x= 0.1. The unusual increase in coercivity for the alloys with 10% La or 10% La₀.5Ce₀.5 substitution is possibly attributed to the phase segregation in alloys with certain La or LaCe content at x = 0.1. Compared with La₀.75Ce₀.25 and La₀.25Ce₀.75 substituted NdFeB alloys, Ce substitution leads to more serious reduction in the magnetic properties.For a comparison, La₀.5Ce₀.5 substitution for Dy was also studied in nanocomposite [Dy_1-x（La₀.5Ce₀.5）_x]10Fe84B6 alloys. The largest values of intrinsic coercivity Hc and maximum energy product（BH）max for the alloy with 30% La₀.5Ce₀.5 substitution for Dy are 714 kA/m and 41 kJ/m³, respectively. The behavior of different elevated temperatures was also found with La₀.5Ce₀.5 substitution for Nd and Dy alloys. Interestingly, the composition dependent coercivity mechanisms with La₀.5Ce₀.5 substitution for Nd and Dy alloys are evident based on the temperature dependence of coercivity described by domain wall pinning and reverse domain nucleation models. It is found that the domain wall pinning mechanism is possibly responsible for [（LaCe）Nd]FeB alloys while reverse domain nucleation is more likely responsible for[（LaCe）Dy]FeB alloys.Composition based characteristic temperatures including Curie temperature Tc and spin reorientation temperature TSR were also studied in nanocomposite alloys with La, Ce or LaCe substitutes. The results show that these substitutions lead to a decrease in both temperatures. The nanocrystalline alloys show relatively low TSR than conventional microcrystalline alloys. TEM analysis results have revealed that a fine and uniform distributed grain structure leads to remanence enhancement for La₀.5Ce₀.5 substituted alloys.The exchange coupling based on Henkel plots, recoil loops and susceptibility curves revealed that, 10% La substitution for Nd in nanocomposite alloys and 5% La substitution for Nd in single phase alloys not only improve the room temperature magnetic properties but also enhance the exchange coupling in nanocrystalline alloys.The effect of Co substitution for Fe on room temperature and elevated temperature magnetic properties was studied in single phase and RE-rich alloys. Although remanence Jr, coercivity Hc and the maximum energy product（BH）max decrease with Co substitution for Fe, Curie temperature increases substantially in the single phase and rare earth rich alloys. As a result, Co substituted alloys show better thermal stability than other alloys. Also, nanocomposite alloys with La₀.5Ce₀.5 substitution for Nd and Dy have lower values of temperature coefficients of Jr and Hc than single phase and RE-rich alloys in the temperature range of 300-400 K.This thesis work thus demonstrated that the partial substitution of Nd by La or/and Ce can not only effectively reduce the cost of the materials but also maintain a relatively good combination of magnetic properties for melt spun nanocrystalline NdFeB based magnets. The elevated temperature behavior of La or/and Ce substituted alloys can be improved by Co substitution.