It is observed a turning point in the particle size for which the coercivity Hci of a Sm-Co alloy reaches a peak. Using a broad size range from 20 nm to 5 mm, the turning point of the flake thickness for SmCo5 nanoflakes is determined in the range of 100–180 nm with Hci peak at ∼20 kOe. A lower coercivity at a particle size well below the turning point is likely related to a more detailed nanoscale morphology that controls coercivity. The effect of particle size on crystallinity for high energy milled powder is also discussed with four observations.

Articles you may be interested inMagnetic texture and coercivity of anisotropic nanocrystalline SmCo6.1Si0.9 magnets J. Appl. Phys. 115, 17A701 (2014); 10.1063/1.4860658 Effect of milling parameters on SmCo5 nanoflakes prepared by surfactant-assisted high energy ball milling J. Appl. Phys. 113, 013913 (2013); 10.1063/1.4773323Effect of surfactant molecular weight on particle morphology of SmCo5 prepared by high energy ball milling Influence of the type of surfactant and hot compaction on the magnetic properties of SmCo5 nanoflakesEstablishing an effective process to synthesize anisotropic magnetic nanopowders that have both high energy products and high coercivity is highly desirable for many applications. A recent effort using surfactant-assisted high energy milling is proved to be capable of making SmCo 5 nanoflakes/ submicron-flakes with high performance. Using the cast alloy as a starting powder, a series of the flakes with various thicknesses were made, which had energy product values up to 22 MGOe and coercivity values up to 21 kOe. The highest coercivity corresponds to a typical flake thickness of 180 nm. The key to our technique is retaining the crystallinity, which allows a high degree of anisotropy to be produced. Results of the analyses from the micrograph of SEM, XRD patterns, and magnetic measurements demonstrate the effect of flake thickness on the magnetic properties. More importantly, a relationship of property-morphology correlation in nanoscale is established for rare earth magnetic powder/flakes.

The magnetic properties of RFe12−xNbx and R(Fe1−yCoy)11.3Nb0.7 compounds with R=Y, Tb, and Dy have been investigated in the concentration region defined by 0.6<x<0.8 and y⩽0.3, where the compounds are single phase with the ThMn12-type of structure. The Curie temperature TC of the RFe12−xNbx compounds is almost independent of the Nb content whereas the saturation magnetization Ms decreases with increasing Nb content. The spin-reorientation temperature Tsr of the TbFe12−xNbx and DyFe12−xNbx compounds decreases monotonically with increasing x. Substitution of Co for Fe in RFe11.3Nb0.7 leads to a remarkable increase of TC and the appearance of a maximum in the Co-concentration dependence of Ms. In contrast, Tsr decreases monotonically with increasing Co content for both R=Tb and Dy. The modification of T- and R-sublattice anisotropy originating from a change of the Nb content and from substitution of Co for Fe was analyzed by combining crystalline electric field theory and the individual-site model.

The structural and magnetic properties of Gd3(Fe1−xCox)25Cr4 compounds with x=0–0.6 have been investigated. The structures of all the compounds investigated are found to possess monoclinic symmetry and belong to the A2/m space group. Substitution of Co for Fe leads to a clear contraction of the unit-cell volume. The Curie temperature increases from 492 K for x=0 to 762 K for x=0.6. The composition dependence of the saturation magnetization at 5 K reaches a maximum around x=0.3. It is noteworthy that substitution of Co for Fe results in a significant change of the magnetocrystalline anisotropy of the Co sublattice, and changes the easy magnetization direction of Gd3(Fe1−xCox)25Cr4 compounds from basal plane to easy axis.

Structural and magnetic properties, especially the magnetocrystalline anisotropy of DyFe12-xNbx compounds with x = 0.55-0.85, have been investigated. The easy magnetization directions at room temperature for all the compounds are along the c-axis. With decreasing temperature the magnetocrystalline anisotropy changes from easy axis to easy cone at Tsr2, then to easy plane at Tsr1. A spin phase diagram has been constructed for DyFe12-xNbx. The change of magnetocrystalline anisotropy was further investigated by the angular dependence of magnetization with respect to the magnetic field at various temperatures. The temperature dependence of the cone angle was determined for DyFe11.3Nb0.7. It is noteworthy that with increasing Nb content both Tsr1 and Tsr2 decrease monotonically while the Curie temperature is almost independent of Nb content. The cone angle increases monotonically with decreasing temperature from Tsr2 and discontinuously comes up to 90° at Tsr1. The temperature dependence of the cone angle can be quite well defined in terms of crystal field theory. In addition, an obvious magnetohistory effect was observed for all compounds at low temperature. The critical field of magnetohistory was found to be smaller than 0.5 T for DyFe11.3Nb0.7.