Magnetic Properties And Magnetocaloric Effect Of NaZn₁₃-type La(Fe, Co, Si)₁₃B_y Compounds

NaZn13-type La(Fe,Si)₁₃ compounds are one of the promising candidates for magnetic refrigerant due to their large magnetocaloric effect, low cost and friendly environment. However, the disadvantages of the compounds are lower Curie temperature and the unstable phase structure. In order to improve these advantages, we have systematically investigated the magnetic properties and magnetcaloric effect in La(Fe,Si)₁₃ compounds by introducing interstitial atom B and substituting Fe atoms by Co atoms. LaFe_13-xSi_xB_y(x=1.5 and 1.6; y=0, 0.1, 0.2, 0.3, 0.4 and 0.5), LaFe_11.9-xCoxSi1.1B0.2(x=0.7, 0.8 and 0.9), LaFe10.1Co0.9Si2Bx(x=0, 0.1, 0.2) and La1.2Fe_13-xSi_x(x=1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.8 and 2.0) were successfully prepared by arc melting the raw material with low purity. The structure, magnetic properties and magnetocaloric effect have been investigated by means of x-ray diffraction, Scanning electron microscopy (SEM), magnetic measurements and the direct measurement of the adiabatic temperature change. The main results are as follows:The annealing time of LaFe_13-xSi_xB_y(x=1.5 and 1.6; y=0, 0.1, 0.2, 0.3, 0.4 and 0.5) compounds can be reduced obviously by introducing interstitial atom B. The lattice parameter slightly increases with increasing B content. By changing the boron concentration from 0 to 0.5, the Curie temperature (TC) increases from 183 to 186 K for x=1.5 and from 186 to 192 K for x=1.6, respectively. It can be seen that the small thermal hysteresis between heating and cooling curves for these compounds, but the thermal hysteresis is not affected by B addition. The maximal magnetic entropy change decreases from 20.0 to 10.7 J/kgK by changing y from 0 to 0.5 for x=1.5 and from 19.1 to 7.1 J/kgK for x=1.6, respectively. It is clear that the itinerant electron metamagnetic (IEM) transition for low B content above the Curie temperature. The IEM transition disappears for y=0.5.The large negative lattice expansions are observed in LaFe11Co0.9Si1.1B0.2 compound near TC. The TC can be tuned to room temperature by changing the Co concentration. The maximal magnetic entropy changes are 17.3,14.9 and 14.8 J/kgK for a magnetic field change from 0 to 5 T and 9.7, 8.1 and 7.5 J/kgK for a magnetic field change from 0 to 2 T for x=0.7, 0.8 and 0.9, respectively. The maximum magnetic entropy change of the LaFe_11.9-xCo_xSi_1.1B0.2 (x=0.7, 0.8 and 0.9) compounds markedly exceed that of Gd (–ΔSM=9.8 J/kgK for a magnetic field change from 0 to 5 T and–ΔSM=4.5 J/kgK for a magnetic field change from 0 to 2 T). The relative cooling power of LaFe_11.9-xCoxSi1.1B0.2 (x=0.7, 0.8 and 0.9) compounds are as large as that of Gd.The structural and magnetic properties were studied by means of x-ray diffraction and magnetic measurement for LaFe10.1Co0.9Si2Bx (x=0, 0.1 and 0.2) compounds. The lattice parameters and Curie temperature have nothing change before and after annealing for x = 0; but when x=0.1 and 0.2, the lattice parameters and Curie temperatures slightly increase after annealed. The lattice parameters and Curie temperatures of the annealed and non-annealed samples increase with increasing x. The maximum values of the magnetic entropy change of LaFe10.1Co0.9Si2Bx samples are 1.8, 1.9 and 2.4 J/kgK for x=0, 0.1 and 0.2 for a field change from 0 to 1.5 T.By over addition of La to the LaFe_13-xSi_x (x=1.2, 1.3, 1.4, 1.5, 1.6, 1.8 and 2.0) compounds, we first found that the stabilized NaZn13 cubic structure single-phase (space group is Fm-3c) can be more easily obtained by annealing at 1100°C for 50 hours. Simultaneously, the LaFeSi (tetragonal structure with space group of P4/nmm) and a small amount of the a-Fe phase are found for low Si content compounds. The Curie temperature increases linearly with increasing Si concentration. Large magnetic entropy change and a typical IEM transition have been observed in this series compounds. The peak values of–ΔSM notably exceed that of Gd.