Structural And Magnetic Disorder In Crystalline Materials PDF Download

Abstract: In this entire work, we characterize the structural, electrical, and magnetic properties of many novel magnetic polycrystalline materials. Our focus is to investigate these properties to enhance the scientific community and possibly pave the road to explore into other directions, such as growing thin films. we investigate the effect of charge carrier substitution with chemical disorder for Ca1-xLaxMn0.5Ru0.5O3 (x = 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, and 0.0) double perovskites. X-ray diffraction measurements showed these polycrystalline powders to be phase pure and to exhibit Pnma symmetry with a b+a- octahedral tilting. Neutron powder diffraction (NPD) showed that the crystal structure and magnetic moments do not change significantly across the substitution range. Ca0.5La0.5Mn0.5Ru0.5O3 exhibits ferrimagnetism. NPD further concluded the lack of cooperative Jahn Teller distortion. Density functional theory (DFT) calculations confirmed that the extra 1/2 electron is responsible for opening the band gap, in that 0.295e and 0.17e is being transferred to the Ru and Mn bands, respectively. Thus, the Fermi level is being tuned when La3+ is substituted into the system. DFT calculations also showed ferrimagnetism to be the most energetically stable magnetic phase. Overall we conclude that despite the chemical disorder, Ca0.5La0.5Mn0.5Ru0.5O3 is an insulating ferrimagnet with a Curie temperature (TC) of 233 K, which is a rare finding to have chemical disorder with magnetic order. The next chapter, we examine the effects of isovalent substitution in Ba1-xSrxMn0.5Ru0.5O3 system. We explore how the larger cation, Ba2+, impacts the structural, electrical, and magnetic properties. We find that BaMn0.5Ru0.5O3 crystallizes in the 9R polymorph with R-3m symmetry. BaMn0.5Ru0.5O3 is an insulating canted antiferromagnet with a TN of 36 K. From neutron diffraction, Mn1 and Mn2 have magnetic moment values of 1.60(4) uB/f.u. and 2.15(2) uB/f.u., respectively. For simplicity, the Ru moments were not refined. While the refined moments are low for Mn3+/4+, it is assumed that Ru is contributing to the magnetic moments. Ba0.5Sr0.5Mn0.5Ru0.5O3 crystallizes with the 6H crystal structure which has P63/mmc symmetry. From low temperature neutron diffraction, Ba0.5Sr0.5Mn0.5Ru0.5O3 exhibits glassy behavior since it lacks long range magnetic order. Resistivity measurements show this compound to be an insulator. In the following chapter, we continue studying Ba1-xAxMn0.5Ru0.5O3 perovskites by substituting in charge carriers. We explore the effects of aliovalent substitution, La3+, by investigating the structural, magnetic, and electrical properties for the phase diagram of Ba1 xLaxMn0.5Ru0.5O3 system. The end members were found to be phase pure solid solutions while the in between compositions were heterogeneous mixtures. While BaMn0.5Ru0.5O3 exhibits the 9R as noted above, Ba0.5La0.5Mn0.5Ru0.5O3 manifests the 3C (I4/mcm space group symmetry) crystal structure and is an insulating ferrimagnet with a TC of 205 K. From neutron diffraction, both the ferromagnetic and ferrimagnetic structures have similar refinement values. However, the moments obtained from a ferrimagnetic structure (Mn = 3.7 uB/f.u. and Ru = -0.88(2) uB/f.u.) are more reasonable than for a ferromagnetic structure (Mn = 1.707(7) uB/f.u. and Ru = 1.707(7) uB/f.u.). Plus the magnetization shows Ba0.5La0.5Mn0.5Ru0.5O3 to have 1.54 uB/f.u., which is closer in value to the expected spin only ferrimagnetic moment for Mn3+ + Ru4+ oxidation states. Thus we conclude that Ba0.5La0.5Mn0.5Ru0.5O3 is a ferrimagnet based on the refined magnetic moments gathered from neutron diffraction and the magnetization results from SQUID. Thin films of Sr2CrReO6 have been reported to be metallic (possibly half-metallic) with a high Curie temperature (635K), which makes this material a feasible candidate for spintronic applications. To understand the role of doping, investigations of the structural, electrical, and magnetic properties of on and off stoichiometric Sr2+xCrReO6 (x = -0.07, 0.0, 0.05, 0.075, and 0.10), Sr2Cr1+yRe1 yO6 (y = 0.1, 0.05, 0.05, and 0.1), and A-site substituted Sr2-zAzCrReO6 (A = K, Ca, and La; z = 0.10 and 0.20; z = 1.0 only for Ca) samples have been achieved. Varying the Cr/Re ratio and aliovalent substitutions on the A-site lead to systematic variations in the magnetization and Curie temperatures. As the Re oxidation state increases, the saturation magnetization increases and the Curie temperature decreases. In the last chapter, we investigate the structural, magnetic, and electrical properties of phase pure polycrystalline Sr2CrOsO6. From X-ray powder diffraction, Sr2CrOsO6 adopts the symmetry and is 73.6(3) % ordered. Our magnetic results show Sr2CrOsO6 to be a ferrimagnet with a small net moment of 0.224 uB/f.u. and the coercitivity to be 7.84 T at 4.5 K in a field strength of 35 T, and the Curie temperature is 660 K. Sr2CrOsO6 is strongly insulating with resistivity of 0.0861 [omega]*cm at 300 K and shows activated electron transport.