Single-Crystal Permanent Magnets: Extraordinary Magnetic Behavior in theTa-,Cu-, andFe-Substituted

Tej N. Lamichhane

¹

,

Michael Onyszczak

²

,

Olena Palasyuk

³

et al.

Abstract

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 highflux (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 coercivities of ∼1.6-∼1.8 kOe caused by Co-enriched, nano-sized, structural defects and faults that can serve as pinning sites. Heat treatments at 1040 • C for 10 h and a hardening at 400 • C for 4 h lead to the formation of a so-called "composite crystal" with a bimodal microstructure that consists of a Tapoor matrix and Ta-rich laminal precipitates. 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.

Computational screening of Fe-Ta hard magnetic phases

“…An improved coercivity has been reported in magnets such as (Fe,Co) 2 B and Ce(Co,Fe,Cu) 5 after the incorporation of small amount of Ta 6,7 . While the Co-Ta phase diagram shows up to seven stable binary phases 8,9 , only two structures have been found in the phase diagram of Fe-Ta: Laves phase (C14) Fe 2 Ta space group (SG) 194, P6 3 /mmc and µ-phase FeTa SG 166, R-3m.…”

Temperature dependence of semi-hard iron silicide rare-earth free magnet

“…This CeCo 5 PM shows an energy product of 103.45 kJ m −3 at room temperature. Although it shows better PM performance than the ferrites, it still contains a very expensive Ce element [7]. Consequently, it is highly desirable to find rare-earth free and low-cost gap magnets.…”