Study Of The Magnetic Phase Transition And Anisotropic Magnetocaloric Effect On TbFe_1-xMn_xO₃

The rare-earth orthoferrites RFeO3 and manganites RMnO3 (R=rare-earth elements) are classified as an important family of functional materials with strongly correlated electrons. The magnetism of R(Fe,Mn)O3 are mainly contributed by two kinds of ions, i.e., R3+ and Fe3+/Mn3+. The R(Fe,Mn)O3 compounds have a perovskite structure with Fe/MnO6 octahedral, in which the R3+ ions have large magnetic moments and strong anisotropy while the transition metal ions and oxygen combine to stabilize the octahedral. The mutual interactions between R3+and Fe3+/Mn3+ ions can give rise to some interesting magnetic behaviors, such as spin reorientation, spin-flop and abundant magnetic ordered states. These novel magnetic phenomena have been attracting much attention and investigation in academic community and even in industry due to their potential application in spintronic devices. This dissertation focuses on the magnetic structure and magnetic properties of R(Fe,Mn)O3. It is found that these materials not only preserves the physical properties of parent orthoferrites RFeO3 and manganites RMnO3 but also shows some abnormal magnetic properties not observed in parent ones. This dissertation is organized as followingIn chapter 1, we briefly review the research status on the crystalline structure,electron structure and magnetic properties of rare-earth orthoferrites and manganites. Then we present an introduction to the colossal magnetoresistance effect, Jahn-Teller distortion effect and magnetoelectric effect in rare-earth manganites. The progress on the spin-reorientation phenomenon and magnetocaloric effect in rare-earth orthoferrites are also introduced.In chapter 2, we introduce the main experimental methods and principles of the utilized techniques. This includes the synthesis routes of poly crystalline and single crystal samples grown by floating zone technique, the characterization methods of crystalline structure analysed by powder x-ray diffraction and the crystalline axis determined by Laue diffraction,the magnetic structure analysed by neutron powder diffraction, surface morphology and magnetic induction lines observed by Lorentz transmission electron microscopy, and the magnetic properties measured by vibrating sample magnetometer attached to a physical property measurement system.In chapter 3, we present the results of the surface morphology and spontaneous magnetization for TbFe0.75Mn0.25O3 single crystals, including the observation of surface morphology and magnetic induction lines, the electron holograms image under magnetic field and selected area electron diffraction (SAED) patterns. We found that the fractal-like pattern appears on the edge of the samplein bright field image due to the strain. The x=0.25 sample shows no magnetic induction line in ab plane but plenty of magnetic induction lines in bc plane, signifying that there is a spontaneous magnetization exis existing in be plane. By SAED, we further demonstrate that the c axis is the spontaneous magnetization exis. Moreover, the magnetic domain walls were not observed with dimension up to about 3 ?m×3?m , which means the sample probably has a large single domain structure , as an indicative of a large magnetocrystalline anisotropy in this system.In chapter 4, we will study how the spin reorientation phenonmenon correlates the magnetic structure of TbFe1-xMnxO3 system. By performing both magnetization and neutron diffraction measurements, it is found a spin reorientation from?4(Gx,Ay,Fz) to ?1(Ax, Gy, Cz) magnetic configuration near room temperature and a re-entrant transition from ?1(Ax, Gy, Cz) to ?4(Gx, Ay, Fz)at low temperature emerges in Mn doped TbFe1-xMnxO3 system. These new transitions are distinguished from the well-known ?4 - ?2 transition observed in TbFeO3, and the sinusoidal antiferromagnetism to complex spiral magnetism transition observed in multiferroic TbMnO3. The ?4-?1 spin reorientation temperature can be improved to room temperature when x is around 0.5?0.6. We further study the evolution of magnetic entropy change (-?SM) versus Mn concentration to reveal the mechanism of the re-entrant spin reorientation behavior and the complex magnetic phase at low temperature. The observation of large magnetocaloric effect as well as the near room temperature spin reorientation reveals that the studied material could be a potential magnetic refrigerant and applied in the construction of a temperature-sensitive spintronic device. Finally, we propose an alternative route for making a temperature-sensitive spin sensor.In chapter 5, we report a comparative study of rotating magnetocaloric effect in TbMn1-xFexO3 (x=0, 0.75) single crystals. We demonstrate a large magnetic entropy change exists between a and c axes with strong magnetocrystalline anisotropy in TbMnO3 as well as TbMn0.25Fe0.75O3. The maximum of magnetic entropy change and refrigeration capacity of TbMnO3 (TbMn0.25Fe0.75O3) reach 19.20 J/Kg K(14.84 J/Kg K),411.97 J/Kg (260.80 J/Kg) under 7 T, respectively. For TbMn0.25Fe0.75O3, the large magnetocaloric effect appears close to a rare spin reorientation from ?1 to ?4 magnetic configuration at?16 K, which is distinguished from that of TbMnO3 near the antiferromagnetic ordering of Tb3+ions at ?9 K. Based on nonextensive thermodynamics,we further employ the q?Fermi Dirac statistics to demonstrate the good consistency between computed and experimental results of magnetic entropy change versus rotating angles. Our results clearly indicate the magnetocrystalline anisotropy energy plays a decisive role in the large differences of the magnetic properties and magnetocaloric properties in TbMn1-xFexO3 system.In chapter 6, we performed the study of the spin glass transition and magnetocaloric effect of Ba1-xSrxMnO3-?in order to making a contrast with RMO3 system and exploring the reason of the high magnetocaloric effect of RMO3. Series of perovskites material BaMnO3-? and Ba1-xSrxMnO3-? were synthesized by solid state reaction process. The content of oxygen vacancy can be controlled by verifying the sintering temperature and atmosphere. It is found the different anion vacancy content in BaMnO3-? causes the crystalline structure and variation of the divarication temperature point at high temperature region. The values of refrigeration coefficient of alkaline-earth manganites Bai-xSrxMnO3 are much smaller than that of rare-earth manganites due to the weak magnetocrystalline anisotropy in Ba1-xSrxMnO3. The alkaline ions cannot offer single-ion anisotropy like rare-earth ions.In chapter 7, we summarize the present results and discuss some open questions on the novel properties and application prospect for rare-earth orthoferrites and manganites.