| The oxides are very ubiquitous in nature and accordingly the functional materials made up of the oxide magnetic materials are widely applied to different fields of industrial manufacture and people's daily life.In condensed matter physics,many kinds of novel physical properties can emerge for these oxide magnetic materials due to the interactions among lattice,charge,spin and orbital degrees of freedom.These magnetic compounds not only provide the fertile ground for basic scientific research such as colossal magnetoresistance materials,multiferroic materials and magnetic-thermal switch but also have some practical applications in information storage,energy conversion and so on.With the rapid development of science and technology,there will be a great chance for the booming progress of oxide magnetic materials.In recent years,it is found that the heat transport exhibits a lot of peculiar behaviors in these oxide magnetic materials.It is valuable for us to deeply understand the magnetic excitations and magnetic ground state of the oxide magnetic materials by studying their heat transport properties.In this dissertation,the low-T heat transport properties are studied for several oxide magnetic materials,such as low-dimensional spin-gap quantum magnetCu3(CO3)2(OH)2,layered perovskite magnetic material GdBaMn2O5.0,zigzag frustrated spin chain material SrEr2O4.The dissertation is composed of four chapters,the major contents of each chapter are as follows:The first chapter reviews the progress of research on the heat transport properties of the oxide magnetic materials.First,the research history of low dimensional quantum spin systems are briefly introduced,and then some low dimensional quantum magnetic materials particularly some spin gap systems are discussed in which magnetic excitations can either scatter phonons or carry heat.Second,the structure and research progress of a new zigzag frustrated spin chain system SrLn2O4(Ln=Gd,Dy,Ho,Er,Tm and Yb)are reviewed including their existing various kinds of interactions and phase diagrams.Finally,we introduce the magnon heat transport in layered perovskite magnetic materials which exhbit strong anisotropy in thermal conductivity with heat currents along different directions.In chapter Two,the thermal conductivity(k)of a distorted spin diamond-chain system,Cu3(CO3)2(OH)2,is studied at low temperatures and high magnetic fields.In zero field,the k(T)curve with heat current along the chain direction has very small magnitudes and shows a pronounced three-peak structure.The magnetic fields along and perpendicular to the chains change the k strongly in a way having good correspondence to the changes of magnetic specific heat in fields.The data analysis based on the Debye model for phononic thermal conductivity indicates that the heat transport is due to phonons and the three-peak structure is caused by two resonant scattering processes by the magnetic excitations.In particular,the spin excitations of the chain subsystem are strongly scattering phonons rather than transporting heat.In chapter Three,the thermal conductivity of double perovskite ferrimagnet GdBaMn2O5.0 is studied.It is found the kc(T)curve shows a broad hump below the Neel temperature(TN = 144 K),which indicates the magnon heat transport along the c axis.Whereas,the ka(T)shows a kink at TN,caused by a magnon-phonon scattering effect.This anisotropic behavior is caused by the anisotropy of spin interactions along different directions.At very low temperatures,magnetic-field-induced changes of Ka(T)and Kc(T),which is likely due to phonon scattering by free Gd3+ spins,is rather weak.This indicates that the spin coupling between Gd3+ and Mn2+/Mn3+ is rather strong at low temperatures.Chapter Four reports the thermal conductivity of zigzag frustrated spin chain material SrEr2O4 single crystals with magnetic field along a and c directions at very low temperatures.We find a distinct different heat transport behavior with magnetic field along these two different axis:when magnetic field was applied along c direction,a magnetic transition happened at about 0.5 T for Erl spins with a formation of intermediate state,then will be gradually polarized with increasing magnetic field and finally Er2 spins also polarized where a plateau-like feature can be observed between these two polarizing processes in k(H)curves;while when magnetic field was applied along a direction,two dips can be found in k(H)curves at low magnetic fields which can be attributed to the 1/3 plateau transition in magnetization measurements at about 0.2 and 1.3 T.In addition,a very large recovery can be observed in our thermal conductivity measurements with applying magnetic field which indicates a strong spin phonon coupling in SrEr2O4 system. |
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