Investigation of Physical Properties of a Low Dimensional Quantum Magnet

Abstract

The aim of the present study is to investigate the low temperature bulk mag- netic properties and transport properties of a low dimensional quantum magnet SrCuO2. Along with another member from the cuprate family Sr2CuO3 , this compound o ffers an excellent realization of 1D spin chains in a bulk crystal. However, unlike Sr2CuO3 which has well separated linear chains of Cu-O-Cu, the basic structural units of the spin chains in SrCuO2 comprises of two linear chains of Cu-O-Cu coupled together to form a ribbon. The nearest neighbour (with a localized spin) of each Cu ion in such an arrangement is another Cu ion in the neighbouring chain of the ribbon. Thus, the Cu-Cu chain is a zig-zag chain which explains why this compound is also popularly known as a zig-zag chain compound. This unique structure leads to an inherent frustration in the system due to the fact that the next nearest neighbour Cu spins along the lin- ear Cu-O-Cu chain couple antiferromagnetically, while the nearest neighbour Cu spins couple ferromagnetically. This inherent frustration is understood to play a key role at determining the behaviour of the system at very low temperatures. Previous studies on the magnetic ground state of this compound have re- vealed that there is no long range order (LRO) down to temperatures as low as 2K. Frustration is believed to play a key role in suppressing the LRO in the system. In order to better understand the role played by frustration we have successfully grown single crystals of SrCuO2, which are lightly doped with vary- ing concentrations (0.25%, 0.5% and 1%) of non-magnetic (Zn) and magnetic (Co) impurities. The technique used to grow these crystals is the Traveling Solvent Floating Zone (TSFZ) method using the four-mirror optical furnace at IISER (Pune). DC Magnetization/Susceptibility measurements were performed by Koushik Karmakar (PhD Student, Physics, IISER Pune) at IISER, where it was found that the non-magnetic impurities (Zn(2+)) break the chains into segments, where at the free ends of these segments e ffective paramagnetic moments are induced. However, no LRO is observed down to 2K. For magnetic impurities (Co(2+)) the behaviour is more complex. The susceptibility shows a highly anisotropic behaviour which is also temperature dependent. There is no transition peak ob- served in susceptibility measured along the crystallographic a and b directions, whereas a prominent peak in the susceptibility at around 5K is measured along the c direction, which indicates a highly anisotropic ordering behaviour. The transition temperature scales with the concentration of the Co(2+) ions which can be taken as an evidence to assert that the Co ions are present in the spin chains. Below the transition peak, ZFC and FC measurements reveal a split- ting/hysteresis which points towards a spin glass/freezing behaviour. Thermal transport studies were performed on two concentrations of Co doped samples at the IFW Dresden under the supervision of Dr. Christian Hess to gain an improved understanding of the eff ect of doping of magnetic impurities on the elementary excitations (spinons) in the spin chains and their coupling with phonons. In the near future it is planned to perform µSR and neutron scattering studies at the Paul Scherrer Institute (PSI), Switzerland in the temperature range below 5K to understand the local spin behaviour in the case of Co doped single crystals of SrCuO2