Jari í Hjøllum 

Title: Antiferromagnetism in YBCO and CoO Nanoparticles

A thesis submitted for the degree of Doctor of Philosophy (PhD) on August, 2008.

Dark Cosmology Centre, Niels Bohr Institute
Planet and Geophysics, University of Copenhagen

Supervisors:

Morten Bo Madsen, Earth & Planetary Physics, Niels Bohr Institute.

Christof Niedermayer, Paul Scherrer Institute & ETH Zürich.

Kim Lefmann, AFM, Risø National Laboratory, DTU

Abstract

Antiferromagnetism in YBCO and CoO Nanoparticles 

By means of a variety of techniques, including neutron scattering and muon spin rotation, the antiferro-magnetism in two very different nanoparticle systems, the high-temperature superconductor YBCO (YBa2Cu3O6+x) and the 3D Ising antiferromagnet CoO, have been studied. With its complex chemical structure and magnetic properties YBCO is far from well-understood and the magnetic behavior of the system under different conditions is investigated. In contrast, the Ising magnetism in CoO makes it a simple system to study and the critical behavior of the system in a magnetic phase transition is investigated.

Utilizing the advantages of both neutron diffraction and muon spin rotation techniques we have for the first time mapped out the staggered magnetization of disc-shaped 30nm YBCO nanoparticles as a function of temperature as well as oxygen-doping. A significant reduction of the Néel temperature is found at low doping.
The reduction is strongest at low doping, and is diminished with increasing hole doping. Neutron scattering measurements also show a more linearly-shaped magnetic order parameter behavior compared to bulk. These observations are attributed to the confined dimensionality of the disc-shaped system. Our findings agree with a similar study of NiO nanoparticles for which the reported reduction of the Néel temperature was found to stem from finite-size effects [86].

Furthermore, a possible spin canting or spin flop mechanism is observed in neutron scattering measurements as a weak response to an applied magnetic field.
Muon spin rotation measurements reveal a distribution of Néel temperatures, seen as an increasing number of non-ordered spins, coexisting with a bulk-like behavior of the magnetic order parameter versus temperature. This exotic finite-size effect could be due to super-paramagnetic relaxation, surface spin melting or a combination of these, all of which are phenomena exclusively observed in nanoparticles.

Even more exciting, our muon spin rotation data show that reemergence of the native-like Néel state [33] at low temperature also exists in YBCO nanoparticles. The reemergence is seen as a distinct rise of the staggered magnetization at low temperature in a doped sample and is caused by changes in the internal field distribution due to the localization of holes. This removes the frustration on most spins, allowing these to form a Néel-like state with a staggered magnetization approaching that of the undoped system [20]. This observation along with previous observations in bulk YBCO [165], Y1−xCaxBa2Cu3O6 [130] and LSCO [33], strongly supports the view that the reemergence of the native-like Néel state is an intrinsic property of cuprate systems.

To study nanoscale critical phenomena, we have performed neutron scattering measurements on the simple 3D Ising antiferromagnet CoO in a novel type of experimental setup. By running a triple-axis spectrometer in two-axis mode, we have utilized a large 2D Position Sensitive Detector, to collect diffraction data with a markedly enhanced intensity around the antiferromagnetic (1/2  1/2 1/2) reflection.

For bulk- and nano- sized powders, we determine the Néel temperature (TN) and measure the critical magnetic scattering below and above this temperature. From the integrated intensity and shape of the magnetic di_raction peak we extract the magnetic order parameter, M / (TN −T)¯^v. The observed bulk value of ¯=0.28(5) agrees relatively well with the numerical prediction ' 0.33 for a 3D Ising system [64], whereas the values for nanoparticles are strongly size-dependent and found to be as large as ¯=0.46(3).  

Furthermore, comparison between X-Ray di_raction and neutron scattering results on the nanoparticles indicates that the structural size is larger than the statically ordered magnetic particle size (e.g. 19.3(8)nm and 12.87(11)nm respectively), likely because the spins at the surface are canted or disordered. Intriguingly, we observe a distinct cut-o_ in the critical correlation length » / (T − TN)−º for nanoparticles over a large temperature interval around the critical temperature. In this temperature region the critical intensity and correlation length of bulk are extremely temperature sensitive, whereas the nanoparticle system shows little or no change of either. Furthermore, the cut-off level of the correlation length clearly scales with the structural particle size.  

Presented are the first-ever observations of exotic magnetic behavior of two nanosized systems and it is shown how the observed phenomena are strongly affected by the finite size of these systems.