Nanocomposite Permanent Magnet Materials
Nanocomposite permanent magnet materials, combining magnetically hard and soft phases in nanoscale, is a new permanent magnetic material. Compared with traditional permanent magnetic material, nanocomposite lowers rare earth content and cost, exhibits high coercivity of hard magnetic phase and high saturation magnetization of soft magnetics phase, upgrades in temperature stability, heat resistance and antioxidant ability. Till now, the main nanocomposite permanent magnets are Nd2Fe14B/Fe3B, Nd2Fe14B/α-Fe, Pr2Fe14B/α-Fe, Pr2Fe14C/α-Fe, Sm2Fe17Nx/α-Fe, and Sm2Fe17Cx/α-Fe. It is reported that Sumitomo Special Metals Co. Ltd(MMSC) succeeded in practical application and mass production for Nd2Fe14B/Fe3B material, other materials are still at the laboratory research stage.
After years of domestic and foreign scholars’ efforts, great progress has been made in theoretical and experimental researches on nanocomposite permanent magnet material. And researches show that, although nanocomposite permanent magnet greatly increases in remanence, slightly decreases in coercivity, limits the increase of maximum energy product. Besides, bonding process mostly used in nanocomposite magnet forming, to further reduce its magnetic properties. Therefore, how to improve coercivity under the premise of ensuring high remanence has become a hotspot for research. 
Nanocomposite permanent magnet material has excellent theoretical magnetic properties and broad application prospects. However, experimental and industrial magnetic properties are far from theoretical values, which limits its application and development. To achieve large-scale production of nanocomposite permanent magnet, in addition to strengthen the research on exchange coupling mechanism, reverse magnetization direction and coercive force itself, it is extremely important to study material composition, adding elements and preparation process.
It is always a goal of the people engaged, relying on the correct theoretical guidance, to seek a process route workable for different materials. It can be predicted, with the perfection of theoretical and experimental methods, nanocomposite permanent magnet material will have a broader space for development.
The influence on the magnetic properties of nanocristalline ribbons and powders has character of microstructure, between others – the grain size, volume of hard and soft magnetic phases and their distribution. Magnetic properties of ribbons and powders depend mainly on their chemical composition and parameters of their heat treatment. Technology of magnets from nanocristalline ribbon consists of the following process: preparing the Nd-FeB alloy, preparing the ribbon, powdering of the ribbon, heat treatment of the powder and finally preparing the magnets. Nanocomposite permanent magnet materials based on Nd-FeB alloy with Nd low content are a new type of permanent magnetic material. The microstructure of this nanocomposite permanent magnet is composed of a mixture of magnetically soft and hard phases which provide so called exchange coupling effect.
Keywords: melt-spun Nd-Fe-B, heat treatment, nanocomposite, exchange coupling, magnetic properties
Nanocomposite permanent magnet materials based on Nd-Fe-B alloy with Nd low content are a new type of permanent magnetic material. The microstructure of this nanocomposite permanent magnet is composed of a mixture of magnetically hard and soft phases. Principle of exchange coupling between soft and hard magnetic grains. Depending on the alloy composition, soft magnetic phases are one or two of α-Fe, Fe3B and the hard magnetic phase is Nd2Fe14B. Essential conditions for microstructure of such material is uniform distribution of phases on a scale of the order of 5 – 20 nm, and exchange coupling between hard and soft phases. The nanocomposite magnet produced by this route exhibitet very high remanence, relatively high energy product (BH)max ~ 95 – 100 kJ/m3, in spite of the fact that the fraction of the hard magnetic Nd2Fe14B phase was only 15% of the alloy. Nd content in nanocomposite magnetic materials is decreased for 50at% in relation to Nd2Fe14B with stehiometric Nd content. Among the advantages of these magnets is low material cost due to the reduction of the content of the expensive hard magnetic phase. These materials have a high potential to be developed into high – performance permanent magnets with very high energy product.
During the research of high coercive magnetic materials based on Nd-FeB alloy, special activities in research are focused on nanocomposite Nd-Fe-B materials with Nd low content. The rapid quenching technology (melt-spun) for obtaining high-coercive magnets of this type gives the possibility to influence on the grain size and microstructure through the cooling rate. The cooling rate range in which optimal magnetic properties are achieved is rather narrow so that the heat treatment is needed in order to achieve the optimal magnetic microstructure. Optimization of microstructure is the key to improve the hard magnetic properties of these nanocomposite magnets.
Magnetic Nanoparticles and Nanocomposites
Melt-spun (Nd11.4Fe82.9B5.7)0.99M1 ribbons (M = Zr, Nb, Ga, Zr + Ga, Nb + Ga) were prepared by melt-spinning technique. Ga addition is found to be effective for the orientation of c-axis of Nd2Fe14B grains perpendicular to the ribbon plane. Better magnetic properties can be achieved by adding both the two kinds of elements Zr + Ca, Nb + Ca, and it is found that the preferred orientation is further improved. The alignment degree changes with ribbon thickness and is highest when ribbon thickness is 120 μm. Heat treatment can improve the texture degree, but lead to coarser grains. Cryogenic treatment is first applied for the treatment of nanocomposite Nd2Fe14B/α-Fe melt-spun ribbons. The effects on magnetic properties and texture degree of nanocomposite magnets after cryogenic treatment were studied. The result shows that cryogenic treatment is beneficial to the enhancement of texture degree of melt-spun ribbon and the grain size has no obvious change.
Investigated Nd-Fe-B alloy with Nd low content was prepared by rapid quenching (R/Q) under Ar atmosphere. The selected cooling rate was 20 m/s.
In presented research different methods of analysis were used. The temperature behavior aimed at selection of a favorable regime of heat treatment of the investigated melt-spun alloy was examined by the DTA. The phase transformation in the function of cooling rate and heat treatment regime for defined initial chemical composition of melt-spun Nd-Fe-B alloy were investigated by determining the phase composition, by application of XRD and with Mössbauer spectroscopic phase analysis. For determining the critical temperature of phase and magnetic transformations, thermomagnetic measurements were carried out. The thermomagnetic curves were completed with hysteresis loops in stages before and after the TM treatment. Magnetic properties in the function of the investigated parameters were measured on the VSM (vibratory sample magnetometer) with magnetic field strength of 50kOe. Comparison of the experimental results obtained by different investigation techniques enabled more complete comprehension of the crystallization process and phase composition during the heat treatment.
Mössbauer spectra were taken in the standard transmission geometry with the Co57 (Rh) source at room temperature. The calibration was done against the a-iron foil data. For the spectra fitting and decomposition the CONFIT package was used. For thermomagnetic measurements weakly compacted material of the cylindrical shape with a diameter of 2 mm and thickness of about 1.5 mm was used. The thermomagnetic measurement was carried out in the field of 50 Oe with the temperature sweep of 4 K/min.
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