| The rare earth orthochromites with ABO3 structure are attracting great attention in the past few years because of their rich magnetic properties and potential applications. In this thesis, the crystal structure, surface morphology, low temperature spin reorientation transition, glass transition behaviors and negative magnetization effects of Sm(X)Cr O3 were systematically investigated. The main contents are summarized as follow:In chapter one, we reviewed the relevant research progress of rare earth chromium oxide and glassy state behavior, we specifically introduced the low temperature magnetic structure and magnetic properties in this system and the research progress on multiferroics and magnetoelectric effects.In chapter two, we introduced the main experimental methods, including the preparation of samples, the structure analysis, the basic principles of physical property measurements and the calculation method of first principles study.In chapter three, by the method of combination of experiment and calculation, the magnetic phase transition process of Sm Cr O3 under low temperature range were studied. The t-e orbital hybridization in Cr-O-Cr ions lattices is highly correlated with the generation of weak ferromagnetism, along with the lattice distortion, which lastly leads to the nature character of the spin reorientation transitions in Sm Cr O3. The studies on the density of state show that by reasons of the exchange splitting and t-e orbital hybridization in Cr-O-Cr ions lattice, a continues change of super exchange angle affects the coupling between O2-(px, py, pz) and Cr3+(dxy, dyz, dz2, dxz, dx2), the generation and extinction of the different orientation of t-e orbital hybridization resulting in the weak ferromagnetism. The intrinsic mechanism is the influence from the t-e orbital hybridization which ultimately induced continues spin reorientation transitions, we proposed a new mechanism of spin reorientation in Sm Cr O3.In chapter four, we studied magnetic glassy dynamics behavior in Sm Cr O3, based on of magnetic cycling measurement and fitting computation between the magnetic phase transition temperature ranges, the results show the thermomagnetic irreversibility are highly related with the FM-AFM phase transitions in the system. Once the FM â†' AFM phase transitions finished, there exist some frozen AFM fractions in the magnetic domains that could not attend the AFM â†'FM phase transition, and the system would try to reach its magnetic ground state in the warming cycling. Thus the constant composition of antiferromagnetic frozen state is responsibele for the magnetic glassy state behavior, and the highest frozen fraction could reach 53.2% in the system under the external field of 50 Oe. While this glass state could be tuned by the change of the magnetic field. The magnetization as a function of time can be fitted with Kohlrausch-Williams-Watt(KWW) stretched exponential function, and the activation energy can be determined by the modified Kissinger equation.In chapter five, the influence by different dope-ion at Sm site on the glassy state properties were further studied to explore the physical mechanism. After doping nonmagnetic ion Ba2+, the frozen components reduced and we found the antiferromagnetic frozen component is not only determined by Cr-Cr interactions, Sm ions also play an important role. And after doping magnetic ion Bi3+, the magnetic phase transition characteristics change significantly accompanied with the negative magnetization and exchange bias effect, the results further verify the d-f interactions in this system.In chapter six, we studied the influence of the Sm ion replacement for new magnetoelectric materials Nd Cr Ti O5 on the electrical and magnetic properties. The internal antiferromagnetic ordered mechanism and the orgin of the magnetoelectric effect were discussed. XRD patterns imply there is no structural phase transition emerged in the doping samples, but the original magnetic phase and ferroelectric phase are broken. The DC magnetization study reveals the variation of the anti-ferromagnetic transition temperature after substitution, and the order of Cr moment plays a more important role in determination of TN in Nd Cr Ti O5. The electric polarization results further verify the d-f interactions in this system, while this interaction are remarkably interfered by Sm3+ since no polarization is detected in Nd0.5Sm0.5Cr Ti O5 at an external magnetic field.In chapter seven, the main content of our work were summarized. The study of glassy state behavior and magnetic phase transition would provide important reference on understanding the physical mechanism in rare earth chromium oxide system. At the same time, several valuable research directions related to our works in the future were illustrated. |
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