Exchange coupled nanocomposite hard magnetic alloys

Abstract

The processing, structures and phase constitutions and the magnetic properties of nanocomposite hard magnetic alloys are reviewed. The emphasis is on rare earth (RE)–iron–boron alloys in which the hard magnetic phase RE2Fe14B is intermixed with one or more soft magnetic phases. Processing–structure–property relationships are the principal focus, in particular, the role of the hard and soft nanocrystallite dimensions in promoting intergrain ferromagnetic exchange coupling and the consequent enhancement of remanent magnetisation and the technologically important maximum energy density. The powder processing, chill block melt spinning, mechanical alloying and thin film deposition routes to develop nanocrystalline and nanocomposite structures are reviewed. The coercivity mechanism in ultrafine grained alloys and the influence of crystallite dimensions are discussed, as are the effects on intrinsic and extrinsic properties of RE substitutions, replacement of iron by other transition metals and enrichment of the boron content. Exchange enhancements in Sm–Co based nanocomposite bulk alloys and in nanoscale FePt/ α-Fe composite thin films are briefly considered, together with thin film materials involving exchange coupling between ferromagnetic and antiferromagnetic phases, in core–shell type structures of transition metal compounds surrounded by oxides and in mechanically alloyed materials. The processing and magnetic properties of bonded magnets based on nanocrystalline/nanocomposite REFeB alloys are discussed. The possibility of producing anisotropic hard/soft composites with properties approaching the theoretical maximum is considered and the extent to which this goal has been realised for fully dense alloys identified.

Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles

“…Traditionally the search for new permanent magnets has been focused mainly on the search for materials with large anisotropies, mainly based on rare-earth elements, e.g., SmCo 5 or Fe 14 Nd 2 B [42,[357][358][359][360][361][362][363][364][365]. However, the ever increasing demand for permanent magnets has triggered a shortage of rare-earth raw materials resulting in a significant increase in price.…”

“…Since the discovery of exchange bias (i.e., the loop shift in the field axis of the hysteresis loops [33,34]) in Co/CoO nanoparticles [35], FM/AFM and inverse AFM/FM core/shell nanoparticles have been extensively studied [36][37][38][39][40] Interestingly, less attention has been paid to FM or FiM "conventional" hard/soft and "inverted" soft/hard, core/shell nanoparticles (see Fig. 1) although it has been demonstrated for bulk and thin film systems that these bi-component materials can exhibit very appealing properties [41][42][43][44][45][46]. However, in recent years substantial advancement has occurred in this field, particularly in permanent magnets , magnetic recording media [64,[72][73][74][75][76][77][78][79], microwave absorption [80][81][82], ferrofluids [83] or biomedical applications [84][85][86][87][88], where it has been shown that for certain applications the use of bimagnetic core/shell nanoparticles can be advantageous over single magnetic nanoparticles.…”

Comparison of Additive Manufacturing Materials and Human Tissues in Computed Tomography Scanning

“…The magnetic coupling can be helpful for combining the magnetic hardness of the rare earth magnets and the high magnetization of the soft magnetic material and can result in a high magnetic energy density product in nanoparticle composites. [18][19][20][21] Such an exchange coupling can be realized in nanocomposites of Nd 2 Fe 14 B with either α-Fe or FeCo. [22][23][24][25][26][27][28][29][30][31][32][33] …”

The magnetic, structure and mechanical properties of rapidly solidified (Nd7Y2.5)–(Fe64.5Nb3)–B23 nanocomposite permanent magnet

“…These magnets have the advantages over sintered magnets due to lower raw materials cost, isotropic and in some cases with higher energy products, easier to magnetize in any direction, less contamination or forming oxides due to short processing cycle and higher corrosion resistance due to involvement of lower rare earth (RE) content. A maximum energy product of 400 kJ/m 3 has been predicted for the isotropic nanocomposite magnets by micromagnetic calculations [11], but so for energy product in the range of 80-160 kJ/m 3 [12][13][14][15][16] has been achieved. This discrepancy is attributed to the difficulty in obtaining optimized microstructure containing nanoscale phase grains with homogenous grains distribution.…”