Magnetotransport Properties And Magnetocaloric Effect In Magnetic Oxides

In recent years, spintronics is a new branch of condensed matter physics based on the development of semiconductor electronics and magnetoelectronics. Spintronics lies in the area of current scientific interest in physics and material sciences due to abundant physical conceptions and applied potentials. An important result is the discovery of giant magnetoresistance with the application in spintronics devices. Non-volatile magnetic computer memory (MRAM) is expected to have a large economic impact, and has been significantly focused on its experimental research. The key of giant magnetoresistance (GMR) is how to obtain a large low-field magnetoresistance (MR) at room temperature. Half-metal materials with high spin polarization, such as CrO₂, Fe₃O₄ and doped manganites, have been paid much attention in enhanced extrinsic MR and intrinsic MR. Theoretical and experimental results indicate that enhanced low-field MR originates from the grain boundary or particle boundary effect in half-metal granular composite. Improving the properties of boundary barrier is an effective method to obtain enhanced MR. Furthermore, 3d/4d/5d transition metal perovskite oxides are of lasting interests due to unique physical phenomena (such as colossal magnetoresistance, percolation, phase separation etc.) that result from highly correlated d-band electrons and strong electron-lattice coupling. Compared to 3d transition metal oxides, metallic conductivity is rather more common among 4d and 5d transition metal oxides. In strongly correlated electron systems different degrees of freedom, such as the spin, orbitals, and lattice deformations, are inextricably coupled, which lead to large electronic, magnetic responses and giant magnetoresistance etc.On the other hand, rapid development of a new magnetic refrigeration technology, based upon the magnetocaloric effect (MCE), has attracted an immense increase in interest in magnetic materials. Magnetic refrigeration exhibits more considerable advantages than conventional vapor-cycle refrigeration, such as high energy efficiency, small volume, ecological cleanliness, etc. Until recently, a gadolinium (Gd) rare-earth metal and Gd₅(Ge_1-xSi_x)₄ series with large MCE have been considered as the most active magnetic refrigerant in room-temperature magnetic refrigerators, but its usage is somehow limited because the expensive cost, chemical metastable, and large thermal and field hysteresis. Therefore, research in the magnetic cooling field has been focused on the search for new materials that are cheaper but displaying large MCEs.Based on above discussed background, the thesis is composed of two parts. Firstly, magnetotransport properties in half-metallic CrO₂ and Fe₃O₄ particles are studied. At the same time, magnetic phase transition and magnetoresistance are widely discussed in 3d/4d transition metal oxides SrRu_1-xMn_xO₃. Secondly, the magnetocaloric effect is investigated in half-metal CrO₂ nano-rods. Also, we have addressed the effect of Mn doping on magnetic phase transition and magnetocaloric effect in polycrystalline SrRu_1-xMn_xO₃.The main results of our study are listed as follows:1. The study of magnetotransport properties in magnetic oxides.(1) Enhanced Magnetoresistance (MR) and magnetoimpedance (MI) effect are obtained in ball milled Fe₃O₄ particles by improving particle boundary properties. The resistivity, magnetic properties, magnetoresistance, magnetoimpedance effect and properties of boundary barriers are studied systematically. MR and MI exhibit the maximum values when t=350 hour, which are strongly related to the barrier properties of grain boundaries. The MI value increases from -7.0% to-12.3% at room temperature and under 5000 Oe. The complex impedance analysis is effectively used to evaluate the contribution of grain and grain boundary to conduction process in Fe₃O₄ powders. Based on Simmons model, the changes of barrier thickness and barrier height with respect to ball milling time are obtained from a series of non-linear current-voltage (I-V) curves. We have a detailed discussion on magnetotransport with barrier properties, which maybe develop new high frequency spintronics devices.(2) The enhanced magnetoresistance is obtained in CrO₂ powder compact by an oxidization reaction process. An aqueous potassium permanganate (KMnO₄) is used to react with the CrO₂ particles coated naturally with Cr₂O₃ layer. The experiment indicates that the strong oxidant can effectively adjust thickness of the natural Cr₂O₃ layer, and thereby change the surface state of the CrO₂ particles. An optimal reaction process yields an obvious increase up to -33% in magnetoresistance at a temperature of 5 K for the chemical treated CrO₂ powder, compared to MR=-27% for the original CrO₂ powder. The magnetic and magnetotransport properties are systematically investigated with the variation of barrier properties. The simple chemical approach has a potential to achieve an enhanced magnetoresistance in a metallic particle system by adjusting the surface state of the magnetic nanoparticle.(3) The present work is originally intended to investigate the magnetoresistance in the binary half-metal CrO₂/Fe₃O₄ composites with opposite spin polarization. A detailed investigation on percolation, magnetoresistance and microstructure has been studied. Experimental results indicate that the conductivity exhibits an abrupt increase when CrO₂ volume fraction is close to 0.15, where a percolation behavior happens. Magnetoresistance exhibits a minimum value in the vicinity of percolation threshold at a temperature of 77 K. Moreover, we have addressed a detailed discussion on the mechanism of magnetoresistance near the percolation region in the binary half-metal granular CrO₂/Fe₃O₄ composite with opposite sign of spin polarizations.(4) The magnetic phase transition and large magnetoresistance are firstly studied in 3d/4d transition metal oxides SrRu_1-xMn_xO₃ (0≤x≤1). A large low temperature magnetoresistance (MR) of -41% at 10 K, which is the largest MR reported in Mn-doped SrRuO₃. The large MR=-35% at T_c is also observed in MR-T curves for sample x=0.55. Importantly, we firstly report the magnetic phase diagram in polycrystalline SrRu_1-xMn_xO₃. Substitution of Mn ions for Ru drives the system from ferromagnetic state SrRuO₃, to an antiferromagnetic state SrMnO₃. The measurement of x-ray photoelectron spectroscopy supports that small amount of Mn³⁺ and Ru⁵⁺ exists in present samples. The presence of combined ions with different valence states may result in a variety of magnetic interactions including Mn³⁺-Mn⁴⁺ DE ferromagnetic interaction, Mn³⁺-Ru⁴⁺(Ru⁵⁺) FM superexchange interaction, Mn⁴⁺-Mn⁴⁺ and Ru-Ru AFM interaction. Therefore, the magnetic phase transitions with x are more complicated and pronounced than those in single crystals. Furthermore, the systematical studies in magnetic and magnetotransport properties at different temperature have been discussed. The mechanisms of large magnetoresistance at different magnetic states are addressed.2. The study of magnetocaloric effect in magnetic oxides.(1) The magnetic entropy changeâ–³S_M and adiabatic temperature changeâ–³T_ad of half-metallic CrO₂ particles is firstly investigated by superconducting quantum interference device (SQUID) measurements. The experimental results indicate that the acicular CrO₂ particles yield a largeâ–³S_M=-5.1 J/kg-K andâ–³T_ad=2.0 K at an applied field of 15 kOe near Curie temperature. The observations are one of the best results measured ever among those magnetic oxides, and also can be comparable to that for a pure metal Gd. At the same time, the magnetocaloric effect in different CrO₂ granular has been studied. In order to make a further study of MCE in half-metal CrO₂ nano-rods, molecular field model, Landau theory and Debye approximation are used to unveil origins of the magnetic entropy change associated with a second phase transition in the system including the contribution of spin, magnetostriction and electronic entropy change.(2) The magnetocaloric effect (MCE) in 4d itinerant ferromagnet SrRuO₃ has been firstly investigated by superconducting quantum interference device (SQUID). The experiment indicates that SrRuO₃ exhibits magnetic entropy changâ–³S_M=-2.5 J/kg-K and adiabatic temperature changeâ–³T_ad=3.1 K at an applied field of 6.5 T and near Curie temperature (-160 K). The second order phase transition for SrRuO₃ not only makes the large magnetic entropy change retain over a broad temperature range of 28 K, but also leads to a large relative cooling power (RCP) of 70 J/kg. Based on the analysis of Debye approximation, the present work demonstrates the importance of the lattice entropy change in SrRuO₃ material due to a remarkable change in lattice deformation at T_c.With the Mn doping, magnetic state exhibits a phase transition from ferromagnetic state through spin glass/cluster glass to antiferromagnetic state. The magnetic entropy changeâ–³S_M presents a negative value, peaking at both Curie temperature T_c and spin freezing temperature T_f. Significantly, a positive magnetic entropy changeâ–³S_p is observed at N(?)el temperature T_N. Therefore, we have addressed a detailed investigation on the mechanism of magnetic entropy changes with the variation of magnetic properties.