Magnetic Properties And Magnetocaloric Effects In Rare-earth Transition Metal Compounds With The NaZn₁₃-Ttype And Ce₆NiI₂SiI₃-type Structures

The structure, magnetism, magnetic transition, magnetic entropy change, magnetic hysteresis, and refrigeration capacity of La(Fe,M)₁₃ (M=Si,Al) and (Tb_1-xDy_x6Co_1.67Si₃ compounds, were systematically studied in present paper. The main results are as follows:1. The structure, magnetism, and magnetic entropy change of La_0.6Pr_0.4Fe_11.5Si_1.5 compound were studied utilizing magnetic measurement and neutron diffraction method. It is found that the coexistent phase area about 1.1K for Pr_0.4La_0.6Fe_11.5Si_1.5, measured with temperature cooling or heating, results in the false peak of magnetic entropy. The magnetic field induced metamagnetic transition takes place above Curie temperature. And the critical field grows up with increasing temperature.2. The substitution of Co for Fe strengthens the exchange interaction among T-T in La_0.5Pr_0.5Fe_11.5-xCo_xSi_1.5C_0.2 compounds, and leads to the Curie temperature increasing obviously. Large magnetic entropy and efficient refrigeration capacity are retained. A little bit of Co can eliminate the magnetic hysteresis and drive the magnetic phase transition from first-order to second-order.3. Magnetic properties and magnetic entropy changes have been investigated in La_0.5Pr_0.5Fe_11.4Si_1.6H_x (x=0, 0.9, 1.6) hydrides. It is found that the Curie temperature can be tuned to room temperature by adjusting hydrogen content. It is attractive that both thermal and magnetic hysteresis are remarkably reduced because of the weakness of the itinerant-electron metamagnetic transition after hydrogenation, while the large magnetic entropy change and considerable RC are retained.4. La_0.7Pr_0.3Fe_11.5Si_1.5C_0.2H_x (x=0, 0.6, 1.2) compounds were successfully prepared by solid-solid and gas-solid phase reactions. The structure, magnetism, magnetic entropy change, magnetic hysteresis, and refrigeration capacity of La_0.7Pr_0.3Fe₍11.5_Si_1.5C_0.2H_x were studied. It is found that small content carbon introducing can not sacrifice the large magnetic entropy, while the maximal hysteresis loss at TC decreases remarkably. The Curie temperature TC can be tuned to ³20 K by the following introduction of interstitial hydrogen. It is attractive that both thermal and magnetic hysteresis are eliminated after hydrogenation,while the large magnetic entropy change is retained. Moreover, the introduction of a small content of carbon is benefit to the following preparation of hydride and improved the mechanical performance and the thermostability.5. The LaFe_11.5Al_1.5H_x and LaFe_11.5Al_1.5C_0.2H_x compounds were prepared by gas-solid phase reaction, and the magnetism and magnetic entropy change were well studied. The Curie temperature TC can be tuned to 295 K for LaFe_11.5Al_1.5H_x compounds by the introduction of interstitial hydrogen (x=1.3). It is obvious that LaFe_11.5Al_1.5H_1.3 compound shows the second-order transition feature. The maximal values of -ΔS for LaFe_11.5Al_1.5H_1.3 compound is 12.3 J·kg^-1 K^-1 at TC for a field change of 0-5 T. The maximal values of -ΔS for LaFe_11.5Al_1.5C_0.2H_1.0 compound is ₁₃.8 J·kg^-1 K^-1 at TC for a field change of 0-5 T, which is 40% higher than that of Gd. Moreover, the LaFe_11.5Al_1.5C_0.2H_1.0 compound is stable even at 623 K.6. The magnetic and magnetocaloric properties of (Tb_1-xDy_x)₆Co_1.67Si₃ (x=0, 0.2, 0.4, 0.6, and 0.8) have been experimentally investigated. The compounds undergo multiple second-order magnetic transitions. The Curie temperature decreases linearly as the content of Dy grows from 0 to 0.8. The maximal magnetic entropy change slightly decreases when Dy is introduced. Large refrigeration capacity (RC) value are achieved for (Tb_1-xDy_x)₆Co_1.67Si₃ (x=0, 0.2, 0.4, 0.6, 0.8) compounds under a field change of 0-5 T, which is benefit for application.