Metastable Structure And Magnetic Properties Of Melt Spun Smfe Based Compounds

The permanent magnetic materials with metastable structure like SmFe based compounds have attracted much attention due to the high magnetic properties and rich physical phenomenon. The research of the formation and transformation between these metastable and equilibrium state has vital significance on the understanding of crystal structures relationship and thermodynamic stability.So, SmFe based compounds are prepared by melt spun and subsequent heat treating. The influences of composition and preparing method to the phase transition, microstructure and magnetic properties are tested by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and Mossbauer Spectroscopy.Firstly the SmFex(3≤x≤12) compounds were prepared by melt spinning (40m/s) and subsequent heat treatment (600-1000℃×1h). The formation and transition of the metastable phases are mainly indexed by XRD. The relationship diagram of Phase-Composition-Temperature (PCT diagram) is ploted. The glass forming ability can be enhanced by the increase of Sm content to x<5. Metastable Sm5Fe17-type structure exists when3<x<5and temperature lower than800℃. SmFe2-type structure is stable when x<4or temperature lower than900℃when4<x<8. The SmFeg compounds with TbCu7-type structure are very sensitive to composition and preparation process. The stability of all the phase in this experiment can be listed as:Sm2Fe17-type, SmFe3-type> SmFe2-type> Sm5Fe17-type, SmFe9-type> amorphous.The Sm5Fe17and SmFeg based compounds according to the PCT diagram are studied in detail. The structure and magnetic properties of Sm5Fe17under different quenching velocity (10-50m/s) and different heat treatment (600-900℃) process are analysed. The highest coercivity of33.8kOe are prepared under30m/s and700℃×1h. The coervicity is increased by the "domain wall pinning" mechanism due to SmFe2phase, which also caused the "kink" in the demagnetization curve. Saturation magnetization and coercivity are dropping obviously after the adding of Terbium (decreased by50%, from30kOe to17kOe after adding10%Tb). The main reason is due to the increase of crystal size from20-50nm to100nm and emerging of soft magnetic phase Tb6Fe23in the700℃×1h heat treated samples, which caused the coercivity mechanism changed from domain wall pinning to nucleation gradually. The exchange coupling effect in the (Sm,Tb)5Fe17compounds smoothed the demagnetization curve.It’s different to elevate the final magnetic properties of SmFe9Nx compounds due to the unstable TbCu7type structure under40m/s. The similar atomic radius of Zr to Sm leads to the occupation of1α crystal site to avoid increasing of crystal elastic strain energy. The TbCu7type structure is stabilized as the c/a value increase to higher than0.86.The coercivity of Sm1-xZrxFe9.5+xNxmagnetic powders after crystalized and nitrided is increasing with the heat treating temperature. As to saturation magnetization, no obvious change can be observed from750to850℃while falling rapidly after900℃. Considering about composition, better properties are those Sm1-xZrxFe9.5+xNy samples with x=0.1and0.2. The coercivity and saturation magnetization dropped steeply when the content of Zr and Fe increase, especially when x>0.4and the magnetic properties fall to about zero. The best properties can be obtained when x=0.2and the procedure of heat treated at850℃×1h and nitrided at440℃×16h, which can be (BH)m=6.63MGOe, Hcj=6.00kOe, Br=7.17kGs. The nitride powders have obvious domain wall pinning mechnism to explain high coercivity while more reverse magnetization nucleation center formed in the higher a-Fe content samples which cause the decrease of coercivity.Besides of Zr, the combined adding of Si and B can stabilized the metastable TbCu7-type structure because of low mixing enthalpy with Sm and Fe. The samples’ saturation magnetization decreases with the added non-magnetic elements Si. As to SmFe93+xSi0.2B0.1(x=0,0.5,1.0), the extra Fe atoms prefer to take2e site, which bring the shrink of atom distance between Fe2e-Fe2e sites and increase of the value of c/a. The highest hyperfine field of29T and Curie temperature of477.68K can be obtained when x=0.5. After nitridation, the Curie temperature increase about265K. The best magnetic properties of Hcj=4.31kOe,(BH)m=3.5MGOe can be found when x=0.5.B in the melt spun Sm0.7Zr0.1Fe9.6Bx samples can elevated the glass forming ability and inhibited the precipitation of a-Fe. No a-Fe can be found in the730℃×90min treated samples. The high c/a value of0.87means the stability of this SmZrFeB compounds. After crystallization, the Smo.7Zr0.1Fe9.6Bx samples have obvious permanent magnetic properties. The coercivity increase with the Boron content to highest of2.8kOe at x=0.8. The boron rich phase emerged as the Boron content continue to increase. The preferential occupation of2e or3g sites may the main reason to have these permanent magnetic properties.