Transition Metal Manganese, Nickel Oxide And Sdw Cr Alloy Electric, Magnetic And Thermal Properties Of Study

Study on the transition-metal oxides has become one of the most active research fields in the present condensed matter physics. Since the spin, charge, orbit and lattice degree of freedom are strongly corrected, the competition between and the coexistence of all kinds of interactions (e.g. double-exchange interaction, super-exchange interaction, electron-phonon interaction, spin-phonon interaction etc.) lead to various exotic physical phenomenon in the system, including the colossal magnetoresistance(CMR) effect and the charge-ordered(CO) state. The phenomenon of giant magnetoresistance (GMR) is observed in the spin-density-wave (SDW) antiferromagnetic Cr alloy. The Cr alloy will show several kinds of electrical and magnetic transitions, controlling the species and the concentration of the doped metal element. As we known, most studies are focused on the electiacl and/or magnetic measurements, but little work on thermal physical properties is reported.In this dissertation, the perovskite manganites with the CMR effect, the nickelates with the charge-ordered stripe and the SDW antiferromagnetic Cr alloy are choosen. We study the complex interaction and the underlying mechanism in different magnetic states and phase transitions by the thermal conductivity and thermal expansion combining with the electrical, magnetic and infrared spectra measurements. The dissertation consists five chapters. The arrangement of the chapters is presented as fellows:In chapter 1, we make an overview on the physical properties of the perovskite manganites and the the SDW antiferromagnetic Cr alloy. We give a review on thermal physical properties in solids, including thermal conductivity and thermal expansion. We present our motive of this dissertation.In chapter 2, the magnetic, electrical and thermal transport properties as well as infrared spectra for the perovskites manganites Nd_0.7Sr_0.3Mn_1-xCr_xO₃(0.01≤x≤0.15) and La_0.7Ca_0.3Mn_1-xCr_xO₃(0.01≤x≤0.6) have been studied in different temperature. Two systems have similar behaviors. When the sample enters metal state from the insulating state, thermal conductivity k shows an observius increase. We suggest that such an increase is contributed to not only the phonon thermal conductivity but also the magnon thermal conductivity. The local lattice distortion and spin disorder introduced by Cr-doping depressed this increase. Double peaks are observed in the resistivity-temperature curve when the Cr content reaches 0.1. The higher temperature peak is related to the metal-insulator transiton. The second peak at the lower temperature comes out and k is depressed, because the Cr doping destroys the double exchange and the ferromagnetic super-exchange of Cr³⁺-O-Mn³⁺ appears, k is depressed and shows a similar behavior of disordered and uncrystal samples in the temperature region where the anti- and ferromagnetic interactions are competitive and coexisted. The phenomenon is contributed to the spin-dependent scattering on phonons.In chapter 3, the temperature dependence of thermal conductivity, resistivity and low field ac. susceptibility of nickelates Nd_2-xSr_xNiO₄ (x=0.33, 1.35) have been measured. The thermal conductivity anomalies associated with charge order and spin order are observed simultaneously in Nd_1.67Sr_0.33NiO₄ for the first time. The thermal conductivity is enhanced below charge-ordered transition, but is suppressed after the spins are antiferromagnetic ordered. Only the charge order phenomenon is observed in the resistivity-temperature curve. The anomalous phonon thermal conductivity in Nd_1.67Sr_0.33NiO₄ indicates that there are strong charge-phonon interaction and the spin-phonon interaction in this compound. Thermal conductivity measurement should be an effective way to study charge and spinIn chapter 4, Thermal expansion coefficients as a function of temperatureα(T) are studied in three typical samples La_0.7Ca_0.3Mn_0.9Cr_0.1O₃,La_0.4Ca_0.6Mn_0.99Cr_0.01O₃ and Cr_97.02Al_2.98. It is found thatα(T) shows different behavior at the metal-insulator (M-I)transition, the charge-ordered (CO) transition and the spin-density-wave (SDW) antiferromagnetic transition.α(T)show an obviouscontraction near the M-I transition temperature T_mi in La_0.7Ca(0.3)Mn_0.9Cr_0.1O₃ and a sharp increase around the CO transition temperature T_CO in La_0.4Ca_0.6Mn_0.99Cr_0.01O₃. These findings are attributed to the strong charge-lattice coupling associated with the different electron states in manganites. In Cr_97.02Al_2.98,α(T) shows a jump-likedecrease near the SDW antiferromagnetic transition temperature T_n, because the magnetovolume effect related to the magnetic transition gives a negative thermal expansion coefficient.In chapter 5, we study the temperature dependence of the magnetic, electrical and thermal physical properties in Cr-Fe-Mn, Cr-Fe-V and Cr-Al-V alloys. We explore how the magnetic properties affact the electrical transport and the thermal physical properties with the different doping elements and/or the diffenent concentration. In Cr-Fe-Mn, the ferromagnetic(FM) clusters can grow more easily at the antiferromagnetic(AFM) network. The GMR effect has been observed in the studied alloy. The strength of the magnetoresistance depends on not only the FM interactions but also the AFM background. Thermal conductivity shows a similar behavior of disordered and uncrystal samples in the temperature region where the FM clusters are embedded in the AFM network. The effect of the spin-dependent scattering on phonons is also observed in Cr alloy. In the Cr_97.02Al_2.98 alloy, the increase of the resistivity is associated with the progressive opening up of the energy gap, and the jump-like decrease of the thermal expansion coefficient is attributed to the magnetovolume effect. SDW is depressed by doping V in Cr-Al alloy. In the Cr-Fe-V alloy, the Neel temperature T_n is depressed obviously, but the Curie temperature T_c is not affected. The increase of resistivity below T_n can be welldescribed by the function 2Δ∝(T_N -T)^1/2 .