A review is given on the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R-Fe-C permanent magnets. First, we analyse the phase diagrams, crystal structure of hard magnetic phases and composition ranges in which these phases form solid solutions, as a result of various types of substitution. Then, the magnetic properties of and -based compounds are described. The routes used for manufacturing R-Fe-B and R-Fe-C permanent magnets are detailed. The magnets' microstructure as well as their coercivities are described in correlation with composition and manufacturing routes. The properties of nanostructure magnets are then presented. Finally, the stability of Nd-Fe-B and Nd-Fe-C magnets in various working conditions is analysed.

Permanent magnets based on R-Fe-B and R-Fe-C alloys

Microstructure, magnetic properties and exchange-coupling interactions for one-dimensional hard/soft ferrite nanofibers

The nanocomposite SrFe 12 O 19 /Ni 0.5 Zn 0.5 Fe 2 O 4 ferrite aligned hollow microfibers with the hollow diameter to the fiber diameter estimated about 3/5 have been prepared by the gel precursor transformation process. The nanocomposite binary ferrites with different mass ratios are formed after the precursor calcined at 900 °C for 2 h, fabricating from SrFe 12 O 19 nanoparticles and Ni 0.5 Zn 0.5 Fe 2 O 4 nanoparticles with a uniform phase distribution. These nanocomposite ferrite microfibers show a combination of magnetic characteristics for the hard (SrFe 12 O 19 ) and soft (Ni 0.5 Zn 0.5 Fe 2 O 4 ) phase with an enhanced remanence owing to the exchange-coupling interactions. The aligned microfibers exhibit a shape anisotropy.

Magnetic hard/soft nanocomposite ferrite aligned hollow microfibers and remanence enhancement

The coercivity mechanism of Nd‐Fe‐B sintered magnets is investigated using a continuously varying model for a single variable K. The results show that the starting field Hs (which includes the critical field H0 and the expanding field He) is greater than the nucleation field Hn and is the main element that controls the coercive force in Nd‐Fe‐B sintered magnets. The comparison between theory and experiment on the dependence of coercivity on the field strength aligning the grains Ha indicates that the starting field theory is in better agreement with the experimental result than other theories discussed.

Coercivity and its dependence on the strength of alignment magnetic field in Nd‐Fe‐B sintered magnets

Synthesis and magnetic properties of SrFe12-x -yAlxCoyO19 nanocomposites prepared via autocombustion technique

Investigations on magnetization reversal processes in oriented sintered Nd-Fe-B alloys

The theories and models of exchange-coupling interaction between grains in nanocrystalline magnetic materials are introduced here. Their insufficiencies and distinctions frcrn the experimental results are pointed out’ and some revisions are made. The effects of the phase component ratio’ the grain size and microstnjcture on the effective anistropy and magnetic properties in the nanocomposite peimanent materials and the improved models are discussed in detail. *Supported by National Natural Science Foundation of China (Grant Nos. 50371046 and 50671055) and the Research Fund for Doctoral Programme of High Education of China (Grant No. 20040422012)

Exchange-coupling interaction, effective anisotropy and magnetic property of nano-magnetic materials

The dependence of coercivity on the grain size in nanocomposite Nd2Fe14B/α–Fe magnets with different distributions of magnetically soft and hard phases is investigated by means of statistical mean. The calculations show that when there exists no soft phase, the coercivity of magnets decreases monotonically with hard grain size reducing. For a given volume fraction of hard phase, the coercivity of nanocomposite Nd2Fe14B/α–Fe magnets with a random distribution of soft and hard grains shows a peak value as a function of hard grain size. When the hard grain size is larger than an optimum value of soft grain size (15nm), the nanocomposite Nd2Fe14B/α–Fe magnets with the multilayer structure of soft and hard grains can possess a higher coercivity than that with the random distribution of soft and hard grains.

Gao Ru-Wei

Papers

Investigations on magnetization reversal processes in oriented sintered Nd-Fe-B alloys

Exchange-coupling interaction, effective anisotropy and magnetic property of nano-magnetic materials

Dependence of coercivity on phase distribution and grain size in nanocomposite Nd2Fe14B/α–Fe magnets

Research on the origin of magneto-crystalline anisotropy in R2Fe14B compounds

Exchange-coupling interaction and effective anisotropy of NdFeB nanocomposite permanent magnetic materials