Magnetic Properties And Magnetocaloric Effects In Rare-earth Transition Metal Compounds With The NaZn₁₃-type And Ce₆Ni₂Si₃-type Structures And Spinel CdCr₂S₄ Compounds

Magnetic refrigerators working near room temperature based on the magnetocaloric effect are demonstrated to be more environment friendly and higher efficient than the systems based on a gas compression-expansion process. A key point for practical application of this technology is to acquire effective magnetocaloric materials, which has attracted much attention in recent years. In this thesis, magnetic properties and magnetocaloric effects are studied both theoretically and experimentally for the rare-earth transition metal compounds with the NaZn₁₃-type and Ce₆Ni₂Si₃-type structures and spinel CdCr₂S₄ compounds, and the main results are summarized as follows:1. Maxwell relation is used to calculate the entropy changes of La_1-xR_xFe_11.5Si_1.5 (R = Ce, Pr, Nd) compounds with a first-order magnetic transition. A detailed analysis suggests that the extremely high peak does not reflect the intrinsic nature of the entropy change at Curie temperature TC because the fraction of FM in the coexistent phases does not contribute toΔS, and only PM phase makes contributions with a magnetic field applied. The spike-shapedΔS peak appearing at TC overestimates theΔS.2. Partially replacing La with R = Ce, Pr and Nd in La_1-xR_xFe_11.5Si_1.5 leads to a reduction in TC. The substitution of R for La causes an enhancement of the field-induced itinerant electron metamagnetic transition, which leads to a remarkable increase in magnetic entropy change, and in hysteresis loss as well. It is found that the increase of TC with Si content increasing in La_0.7Pr_0.3Fe_13-xSi_x. The magnetic entropy change and the hysteresis loss at TC are reduced simultaneously because the order of phase transition changes from first-order to second-order caused by replacing Fe with Si.3. It is found that the values of TC are enhanced with the increase of Co concentration in La_1-yPr_yFe_11.5-xCo_xSi_1.5, and the substitution of Co for Fe can adjust TC to around room temperature. Substitution of Co or introduction of interstitial carbon atoms in La_1-yPr_yFe_11.5-xCo_xSi_1.5 suppresses strongly the itinerant-electron metamagnetic transition, leading to a reversible dependence of magnetization on the temperature/magnetic field cycle. A large magnetocaloric effect, a small hysteresis loss and a high refrigerant capacity have been simultaneously achieved at room temperature.4. The magnetocaloric effects of the LaFe_13-xSi_x compounds substituted by different elements including rare-earth R elements, Co, Si as well as interstitial C and H are analyzed. Based on the experimental results, a relation between magnetic entropy changeΔS and Curie temperature TC is established. It is suggested that the combined substitution of R and Co in LaFe_13-xSi_x compounds can produce a large value ofΔS at the same time by adjusting TC to around room temperature, which is an effective way to acquire efficient magnetocaloric materials at room temperature.5. It is found that the maximum values of magnetic entropy change and refrigerant capacity of Gd₆Co₂Si₃ are found to beΔSm=6.3 J/kg K and RC1 = 503 J/kg, RC2 = 430 J/kg for a magnetic field change of 0?5 T at room temperature, respectively. Although the values ofΔSm for Ce₆Ni₂Si₃-type GdCoSi compounds are relatively small, their RC values are much larger than those of some magnetocaloric materials with the first-order phase transition in a similar temperature range. The RC values are also comparable with those of Gd.6. Magnetic properties and magnetocaloric effects of spinel CdCr₂S₄-based compounds with the second-order phase transition are investigated by isothermal magnetization and specific heat measurements. A large magnetocaloric effect is observed in CdCr₂S₄ systems with a colossal magnetocapacitive effect.