Phd Defence: Antiferromagnetism in YBCO and CoO Nanoparticles

The high temperature superconductor, YBCO (YBa2Cu3O6+x) is probably the most famous member of a large family of high temperature superconductors, the cuprates, which are antiferromagnetically ordered at low doping and superconducting at high hole doping. The magnetic properties of 30 nm disc-shaped nanoparticles YBCO have been investigated using neutron scattering and muon spin rotation (μSR). The results show that the magnetic order parameter of the nanoparticles exhibits significant finite size effects. Most importantly the magnetization curves differ in shape compared to bulk and the Néel temperature is reduced compared to bulk, most pronounced at low doping. μSR measurements similarly show altered properties compared to bulk. Most significantly μSR shows the samples to exhibit reemergence of the native-like (non-frustrated) Néel state at low temperature, a phenomenon previously only observed in bulk systems. The reemergence is thought to be a freezing of dynamic holes, yielding most spins non-frustrated. This observation strongly supports the view that reemergence is intrinsic to the cuprates. Critical phenomena close to the Néel temperature in nanoparticles of the simple 3D Ising antiferromagnet CoO have been studied through neutron diffraction. In ideal/bulk systems the critical correlation length diverges at the transition temperature. However, our investigations show a distinct cut-off in the correlation length of the nanoparticle systems, strongly related to the sample particle size (see figure above). This has never been observed before and casts light onto the physics of phase transitions in finite sized systems.

Opponents: Dir. Kurt N. Clausen PSI, Villigen (CH), Prof. Bella Lake, HMI, Berlin (D)
Assoc. Prof. Jesper Nygård, NBI.
Supervisors: Kim Lefmann, Risø/NBI, Christof Niedermayer, PSI, Villigen (CH),
Luise Theil Kuhn, Risø DTU, Morten Bo Madsen, NBI.