TOPICAL REVIEW: Supermagnetism

TOPICAL REVIEW: Nanomagnetics

A review is given of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets. These compounds include R2Fe14B, R2Fe14C and R2Co14B and the large class of ternary rare earth compounds having the tetragonal ThMn12 structure. Special emphasis is given to the changes in magnetic properties of R2Fe17 compounds observed after interstitial solution of C or N atoms. The magnetic properties of all these compounds are discussed in terms of current models based on intersublattice and intrasublattice exchange and the interplay between the rare earth sublattice anisotropy and 3D sublattice anisotropy. A substantial portion of the review is devoted to manufacturing routes of permanent magnets and a description of the coercivity mechanisms operative in the magnets. A comparison is made of the performance and economic costs of various types of magnets and novel applications are briefly discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/54/9/001/pdf

New developments in hard magnetic materials

Recent work on magnetic properties of transition-metal nanowire arrays produced by electro-deposition is reviewed. The wires, which are electro-deposited into self-assembled porous anodic alumina, form nearly hexagonal arrays characterized by wire diameters down to less than 10 nm, wire lengths up to about 1 µm, and variable centre-to-centre spacings of the order of 50 nm. The fabrication and structural characterization of the arrays is summarized, magnetic data are presented and theoretical explanations of the behaviour of the wires are given. Emphasis is on extrinsic phenomena such as coercivity, magnetization reversal and interactions of the magnetic nanowires. In particular, we analyse how wire imperfections give rise to magnetic localization and dominate the hysteresis behaviour of the wires. Potential applications are outlined in the last section.

https://iopscience.iop.org/article/10.1088/0953-8984/13/25/201/pdf

Magnetism of FE, CO and NI nanowires in self-assembled arrays

Amorphous rare‐earth transition‐metal alloys of composition REl−xTMx with RE=Dy, Ho; TM=Fe,Co and 0<x<1 and Dyl−x(Fe,Co)x were prepared by evaporation. The saturation magnetization, uniaxial anisotropy, coercivity, and Faraday rotation were investigated as a function of composition and temperature. Also, the spectral variation of the Kerr rotation was measured. The magnetization data indicate a strong dispersion of the RE moments due to randomly oriented local crystal field axes. The strong turndown of the Curie temperature for the Fe‐rich alloys suggests that an additional dispersion is present in the Fe subnetwork. The mean field theory was used to analyze the temperature variation of the magnetization yielding smaller TM spin values and exchange coupling constants as compared with those of the Gd and Tb analogs. The uniaxial anisotropy constant Ku for Dy‐Co based alloys was found to vary with the square of the Dy subnetwork magnetization as predicted by the random single‐ion theory. The anisotropy of D...

Magnetic and magneto‐optical properties of rare‐earth transition‐metal alloys containing Dy, Ho, Fe, Co

Experimental approach to coercivity analysis in hard magnetic materials

Cobalt magnetism in RCo5-intermetallics: Onset of 3d magnetism and magnetocrystalline anisotropy (r=rare earth or Th)

Coercivity analysis in R17Fe83−χBχ magnets

Large magnetization anisotropy in uniaxial YCo5 intermetallic

Coercivity is the essential property characterizing hard magnetic materials, in particular with respect to normal ferromagnets. It exists when low field domain wall motion does not take place, either the magnetization being already saturated in zero field, or domain walls being pinned at structural heterogeneities. In these two cases, the magnetic configuration of the system is an energy minimum in the field, although it may not be the lowest energy minimum. This energy minimum vanishes at H = He, the coercive field. Then magnetization reversal takes place and the system reaches the lowest energy state.

Q. Lu

Papers

Experimental approach to coercivity analysis in hard magnetic materials

Cobalt magnetism in RCo5-intermetallics: Onset of 3d magnetism and magnetocrystalline anisotropy (r=rare earth or Th)

Coercivity analysis in R17Fe83−χBχ magnets

Large magnetization anisotropy in uniaxial YCo5 intermetallic

Coercivity in Hard Magnetic Materials