Synthesis and magnetic properties of novel Ln[subscript 2-x]MxCuO[subscript 4+y] compositions (Ln = La,Pr,Nd,Sm,Eu,Gd; M = Ca,Sr,Ba; 0<=x<=0.2; -0.5<=y<=0.5)

Thumbnail Image
Cho, Jin Hyung
Major Professor
D. C. Johnston
Committee Member
Journal Title
Journal ISSN
Volume Title
Research Projects
Organizational Units
Organizational Unit
Journal Issue
Is Version Of
Physics and Astronomy

Ln[subscript]2CuO[subscript]4 (Ln = La, Pr, Nd, Sm, Eu, Gd) undergo structural transitions to oxygen-reduced structures Ln[subscript]2CuO[subscript]4-d with Ln = La, d = 1/3 and Ln = Pr, Nd, Sm, Eu, Gd, d = 1/2. The Nd[subscript]2CuO[subscript]4-type (T[superscript]'-phase) La[subscript]2CuO[subscript]4+d is observed after reoxygenation of oxygen-reduced La[subscript]2CuO[subscript]3.67 structure below 500°C. The magnetic properties of T/O (K[subscript]2NiF[subscript]4-type structure)-phase La[subscript]2CuO[subscript]4 systems show quite different behaviors compared with those of conventional solid state reacted La[subscript]2CuO[subscript]4 systems. The implications of these nonsuperconducting behaviors within the superconducting carrier concentrations are discussed from the structural point of view;La[subscript]2-xM[subscript]xCuO[subscript]4+d (M = Ca, Sr; x = 0.05, 0.15), synthesized by the low temperature route, also contain excess oxygen and show different physical properties. [superscript]139La NQR measurements of nonsuperconducting La[subscript]1.85Sr[subscript]0.15CuO[subscript]4.04 show metallic properties. The tolerance factor, which is based on ionic radii, is quite successful to describe structural properties of K[subscript]2NiF[subscript]4 related cuprate compounds. With anisotropic thermal motions of apical oxygens in La[subscript]2CuO[subscript]4, we discuss the crucial role of out-of-plane oxygens for stabilization of cuprate structures;The magnetic phase diagram and phase separation of La[subscript]2-xSr[subscript]xCuO[subscript]4+[delta] system (0.000≤ x≤ 0.030, 0.0≤[delta]) are reported. From the decrease of T[subscript]c by doping level, we infer the localization and pair-breaking mechanism induced by doped holes. Phase separation is realized within the appearance of three-dimensional antiferromagnetic order. We also observe scaling properties of susceptibilities versus renormalized temperatures and infer the finite size effects by doped holes from the scaling;[superscript]139La NQR spin-lattice relaxation rates vs temperature are presented for La[subscript]2-xSr[subscript]xCuO[subscript]4 with x = 0.02 to 0.08. The spin-lattice relaxation rate below ~2T[subscript]N(x) shows a power-law critical behavior while above ~2T[subscript]N(x) it follows an exponential law with a small x-dependent spin-stiffness constant, where T[subscript]N is the magnetic ordering temperature. It is argued that the spin-lattice relaxation rate arises from fluctuations of the staggered magnetization in locally ordered mesoscopic domains. We infer that the magnetically ordered state is not a conventional spin glass.

Subject Categories
Wed Jan 01 00:00:00 UTC 1992