H. T. Kim

Nanocrystalline NdFeB magnets fabricated by a modified hot-working process

Nanocrystalline and textured NdFeB magnets were prepared from isotropic melt-spun Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 powder by a modified hot-working process using direct joule heating. The isotropic magnet obtained by current applied (CA) press was composed of randomly oriented Nd2Fe14B grains of 50–100 nm in diameter, and the optimized magnetic properties were Br=0.81 T (8.1 kG), iHc=2022 kA/m (25.4 kOe), and (BH)max=117 kJ/m3 (14.7 MG Oe). After CA deformation, the grains grew to platelets with 50–100 nm in thickness and 200–400 nm in length, and the magnet showed excellent properties of Br=1.31 T (13.1 kG), iHc=1457 kA/m (18.3 kOe), and (BH)max=333 kJ/m3 (41.8 MG Oe).

A sensitive 10 T pulsed magnetometer

An inductive magnetometer is described for measurements using the pulsed magnetic fields up to 10 T at room temperature. The sensing element of the magnetometer is a modification of a three-axis-compensated design. A distinctive feature of the magnetometer described is the independent compensation of signals induced by each of the spatial components of the pulsed magnetic field. Sensitivity of the magnetometer is 5×10−4 emu and is limited mainly by irreproducibility of the magnetometer compensation (10−6 of the signal from the working coil with an inner diameter of 4 mm). The magnetometer demonstrates high sensitivity for volume magnetization and magnetic susceptibility measurements (10−2 emu/cm3 and 10−7, accordingly, for samples with a volume of 60 mm3). As examples, the magnetization curves of some hard magnetics (bulk and thin films) and common diamagnetic materials are measured.

Pulsed field magnetometery for high coercivity permanent magnets

A high sensitivity pulsed field magnetometer for the measurement of thin films or small volume of high coercive rare earth permanent magnets has been constructed. The pickup coil designed in principle of 3-axis compensation has the sensitivity of 5×10−4 emu at tested field of 76 kOe. The compensation has been repeatable within 4×10−4–10×10−4 emu, which is enough for measurement within 2% accuracy for the materials with magnetic moment as small as 5×10−2 emu. Measurements on NdFeB magnet show excellent repeatability within 0.3% discrepancy for remanence and coercivity. The magnetic hysteresis loop of a NdFeB thin film (1050 nm) was measured satisfactorily. Estimation of the eddy current effect, by measuring the similar dimensioned but lower resistive nonmagnetic metals, has also been investigated.

Magnetic Properties and Microstructure of Nanocrystalline NdFeB Magnets Fabricated by a Modified Hot Working Process

Magnetic properties, microstructure and texture of NdFeB magnets fabricated by a modified hot working process from commercial melt-spun powders (Magnequench; MQPA, MQPB and MQPB+) have been investigated. The hot-pressed isotropic magnet made from MQPA powder, which contains higher Nd content than that of MQPB or MQPB+, shows higher coercivity. The magnet also shows homogenous and fine grains with higher coercivity for higher consolidation pressure. The hot-deformed MQPA magnet shows a strong anisotropy along the press direction with homogeneous platelet Nd₂Fe₁₄B grains of 50~100 ㎚ in thickness and 200~500 ㎚ in length. The hot-deformed MQPB+ magnet, however, shows low remanence and low coercivity. The microstructure of the magnet consists of two areas; undeformed Nd₂Fe₁₄B grains and well-aligned but large grains with 3~4 ㎛ in length. Low Nd content attributes to the formation of the two different area.

Thermomagnetic analysis of NdFeB melt‐spun alloys with low Nd content

The phase transformation of NdFeB melt‐spun alloys with low Nd content of 4–8 at. % was investigated by thermomagnetic analysis and x‐ray diffractometry. Experimental results have shown that the metastable Nd2Fe23B3 compound formed in the alloys is considered to be transformed to Nd2Fe23B3+α‐Fe+Fe3B in the temperature range of 550–690 °C, α‐Fe+Fe3B+Nd1+eFe4B4 in the temperature range of 690–730 °C and finally α‐Fe+Nd1+eFe4B4 above 840 °C. From the results, it has been concluded that Nd2Fe14B is not formed from metastable Nd2Fe23B3. On the other hand, the melt‐spun alloy of Nd2Fe23B3 (∼Nd7.1Fe82.1B10.7) annealed under optimum conditions has been found to be composed of α‐Fe, Fe3B, and Nd2Fe14B phases. The alloy has a coercivity comparable to Fe3B‐based Nd4Fe77B19 and relatively high‐energy product of about 71.6 kJ/m3 (∼9 MG Oe).

Effect of additives on the magnetic properties of Nd–(Fe, Co)–B melt–spun ribbons

Effect of additives and heat treatment on the magnetic properties of Nd–(Fe, Co)–B melt–spun ribbons have been investigated. The optimum annealing temperatures were in the range of 600 °C – 700 °C. Among the additives studied, Ga, Cu, Ti, and Nb were found to be effective for the increase of coercivity. Particularly, Cu– and Ga–containing alloys show high coercivity exceeding 20 kOe.