A Electron Paramagnetic Resonance (epr) Study of Magnetic Anisotropies in - and Two-Dimensional Copper(ii) Magnetic Insulators.
Abstract
The electron paramagnetic resonance (EPR) has been used as an experimental technique to investigate the magnetic anisotropies in some low-dimensional copper(II) magnetic insulators. The compounds studied are the two -dimensional layer perovskites, (C_6H _5(CH_2)_{ rm n}NH_3] _2CuX_4 where n = 1, 2, 3 and X is Cl or Br, and bis(cyclohexylammonium)tetrabromocuprate(II); the one-dimensional alternating linear dimer chains, piperidiniumtrichlocuprate(II) and piperaziniumhexachlorodicuprate(II); and a monomer system, bis(piperidinium)tetrabromocuprate(II). The two-dimensional layer perovskites, (C _6H_5(CH_2) _{rm n}NH_3 ]_2CuCl_4, with n = 2, 3, turned out to be the first novel copper(II) compounds whose EPR linewidths showed simultaneous contributions from the anisotropic symmetric exchange, D, the anisotropic antisymmetric exchange, d, spin diffusion and from magnetically inequivalent sites in the unit cell. The main focus of this study is to calculate all these contributions from an analysis of the EPR data. The theories describing the effects of various contributions to EPR linewidths have been put together and used for analyzing and interpreting the data. The value of the exchange interaction, J ^', between the magnetically inequivalent sites has been determined. The corresponding bromide compounds, (C _6H_5(CH_2) _{rm n}NH_3 ]_2CuBr_4, do not show either the spin diffusion effects or the effects of magnetically inequivalent sites. Increased ligand spin -orbit coupling in the compounds causes an increase in the spin anisotropies which dominate any other contributions to the linewidth. The anisotropies have been calculated for these compounds. The one-dimensional alternating linear chain dimer compounds bis(piperidinium)hexachlorodicuprate(II) and piperaziniumhexachlorodicuprate(II) show the usual behavior of an exchange coupled Ising system. No spin diffusion is observed. The EPR linewidths for the corresponding bromide compound, bis(piperidinium)hexabromodicuprate(II) turned out to be exceptionally large even at liquid nitrogen temperature (>5000 Gauss). We believe an anomalously large antisymmetric exchange mid dmid, which may exist between the dimers within the chain, may be causing the anomalously large linewidths. The structurally monomer compound bis(piperidinium)tetrabromocuprate(II) has been characterized as a linear chain, by our EPR studies. This is the first copper(II) bromide compound, to our knowledge, which showed spin diffusion behavior in EPR studies. The EPR spectra in one crystallographic plane, show two partially resolved lines corresponding to two magnetically inequivalent sites. The linewidths and g-factor values corresponding to each site are calculated and the value of the exchange interaction between the magnetically inequivalent sites, J^', is determined.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1988
- Bibcode:
- 1988PhDT.......120P
- Keywords:
-
- Physics: Condensed Matter; Chemistry: Physical