Lars Borjesson

The magnetism of double perovskites is a complex phenomenon, determined from intra- or interatomic magnetic moment interactions, and strongly influenced by geometry. We take advantage of the complementary length and timescales of the muon spin rotation, relaxation, and resonance (${\mu }^{+}\mathrm{SR}$) microscopic technique and bulk ac/dc magnetic susceptibility measurements to study the magnetic phases of the ${\mathrm{LaCaNiReO}}_6$ double perovskite. As a result, we are able to discern and report ferrimagnetic ordering below $T_{\mathrm{C}}=102\mathrm{K}$ and the formation of different magnetic domains above $T_{\mathrm{C}}$. Between $T_{\mathrm{C}}<T<270\mathrm{K}$, the following two magnetic environments appear, a dense spin region and a static-dilute spin region. The paramagnetic state is obtained only above $T>270\mathrm{K}$. An evolution of the interaction between Ni and Re magnetic sublattices, in this geometrically frustrated fcc perovskite structure, is revealed as a function of temperature through the critical behavior and thermal evolution of microscopic and macroscopic physical quantities.

We present results on the temperature dependence of ultrafast electron and lattice dynamics, measured with pump-probe transient reflectivity experiments, of an epitaxially grown LaCoO${}_{3}$ thin film under tensile strain. Probing spin-polarized transitions into the antibonding ${e}_{g}$ band provides a measure of the low-spin fraction, both as a function of temperature and time after photoexcitation. It is observed that femtosecond laser pulses destabilize the constant low-spin fraction ($$\sim${}$63%\char21{}64%) in equilibrium into a thermally activated state, driven by a subpicosecond change in spin gap $$\Delta${}$. From the time evolution of the low-spin fraction, it is possible to disentangle the thermal and lattice contributions to the spin state. A lattice mediated spin repulsion, identified as the governing factor determining the equilibrium spin state in thin-film ${\mathrm{LaCoO}}_{3}$, is observed. These results suggests that time-resolved spectroscopy is a sensitive probe of the spin state in ${\mathrm{LaCoO}}_{3}$ thin films, with the potential to bring forward quantitative insight into the complicated interplay between structure and spin state in LaCoO${}_{3}$.