Improvement of magnetic properties and microstructure of Sm(Fe0.8Co0.2)12 thin films with light elements

Abstract

RFe12 (R: rare-earth elements) compounds with a tetragonal ThMn12-type crystal structure are expected to surpass the magnetic properties of Nd-Fe-B sintered magnets, since RFe12 compounds possess high saturation magnetization Ms and high anisotropy field HA by the smallest R/Fe ratio which have high molar fraction among Rm-nFe5m+2 systems. Sm(Fe0.8Co0.2)12 thin films deposited on a MgO (100) single crystal substrate with V underlayer grow epitaxially, and their representative magnetic properties of µ0Ms of 1.78 T, anisotropy field µ0HA of 12 T and Curie temperature TC of 586 ºC, which surpasses that of Nd2Fe14B compound. Since then, a lot of studies have been performed on Sm(Fe0.8Co0.2)12 compound with changing the fabrication conditions to be able to transform these excellent intrinsic magnetic properties to the extrinsic ones, particularly remanent magnetization and coercivity. We have recently realized a large coercivity of 1.2 T in an anisotropic Sm(Fe0.8Co0.2)12 thin film by the addition of B. It is considered that additive elements play an important role for improving the magnetic properties of the Sm(Fe0.8Co0.2)12 compounds. The effect of light elements on this compound has not been fully studied yet, although the effect of N addition for R-Fe compounds have been widely investigated. In this study, in order to see the effect of light elements on the structure and magnetic properties for Sm(Fe0.8Co0.2)12 thin films, Sm(Fe0.8Co0.2)12-X (X = B, C and N) have been fabricated and their structure and magnetic properties have also been investigated. The samples were prepared by using ultra-high vacuum magnetron sputtering system with base pressure of less than 1.0 × 10-8 Pa. First of all, a V underlayer of 20 nm was deposited onto the MgO (100) single crystal substrate at substrate temperature Ts of 350 ºC. Then, the Sm(Fe0.8Co0.2)12-X (X = B, C and N) layer with film thickness of 100 nm was deposited. The amount of B was changed from 0 to 11.3 at.%, the addition of C was designed by the deposition rate of each target and it was changed from 0 to 4.0 %, while for the addition of N, the flow rate ratio of N2 gas to Ar gas was changed from 0 to 1.0 %. Finally, the V layer of 10 nm was deposited as a cap layer for the prevention of oxidation. The structural analysis was performed by the XRD with Cu-Kα radiation from the out-of-plane configuration. The microstructure was examined by the blight-field transmission electron microscopy (BF-TEM), STEM with an energy dispersive X-ray spectroscopy (STEM-EDX) and 3-dimensional atom probe (3DAP). The magnetization curves were measured by using a superconducting quantum interference device (SUQID) magnetometer, the film composition was determined by EDX, and in some cases, an ICP spectroscopy analysis was performed. All measurements were performed at room temperature. From the magnetization curves, high µ0Hc of 1.11 T, high µ0Ms of 1.58 T and moderate high Ku of 3.22 MJ/ m3 were obtained for the film with B content of 11.2 at.% and tSFCB of 100 nm. It was confirmed that the addition of B was very effective to improve the magnetic properties of Sm(Fe0.8Co0.2)12 compounds and it promotes the formation of 1:12 grains surrounded by an amorphous intergranular structure. Consequently, the B rich shell can be observed close to the grain boundary phase. However, with the addition of C and N to the Sm(Fe0.8Co0.2)12 thin films, no significant improvement in magnetic properties was obtained due to the absence of the remarkable formation of the grain boundary phase. In this talk, we will discuss the correction of addition of light elements to the magnetic properties and obtained microstructure.