  To reduce material and processing costs of commercial permanent magnets and
to attempt to fill the empty niche of energy products, 10 - 20 MGOe, between
low-flux (ferrites, alnico) and high-flux (Nd2Fe14B- and SmCo5-type) magnets,
we report synthesis, structure, magnetic properties and modeling of Ta, Cu and
Fe substituted CeCo5. Using a self-flux technique, we grew single crystals of I
- Ce15.1Ta1.0Co74.4Cu9.5, II - Ce16.3Ta0.6Co68.9Cu14.2, III -
Ce15.7Ta0.6Co67.8Cu15.9, IV - Ce16.3Ta0.3Co61.7Cu21.7 and V -
Ce14.3Ta1.0Co62.0Fe12.3Cu10.4. X-ray diffraction analysis (XRD) showed that
these materials retain a CaCu5 substructure and incorporate small amounts of Ta
in the form of \dumb-bells", filling the 2e crystallographic sites within the
1D hexagonal channel with the 1a Ce site, whereas Co, Cu and Fe are
statistically distributed among the 2c and 3g crystallographic sites. Scanning
electron microscopy, energy dispersive X-ray spectroscopy (SEM-EDS) and
scanning transmission electron microscopy (STEM) examinations provided strong
evidence of the single-phase nature of the as-grown crystals, even though they
readily exhibited significant magnetic coercivitie of ~1.6 - ~1.8 kOe caused by
Co-enriched, nano-sized, structural defects and faults that can serve as
pinning sites. Formation of the "composite crystal" during the heat treatment
creates a 3D array of extended defects within a primarily single grain single
crystal, which greatly improves its magnetic characteristics. Possible causes
of the formation of the "composite crystal" may be associated with Ta atoms
leaving matrix interstices at lower temperatures and/or matrix degradation
induced by decreased miscibility at lower temperatures. Fe strongly improves
both the Curie temperature and magnetization of the system resulting in
(BH)max:~13 MGOe at room temperature.
