Magnetic resonance spectroscopy on the spin-frustrated magnets YBaCo3MO7 (M = Al, Fe)

We present experimental results of combined electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements on single crystals of the Swedenborgite-type compounds ${\mathrm{YBaCo}}_{3}M{\mathrm{O}}_{7}$ ($M$=Al, Fe). The magnetic lattice of these materials can be described as stacks of strongly geometrically frustrated kagome layers built up of ${\mathrm{Co}}^{2+}$ ($S=3/2$) ions. Due to the Co-$M$ site intermixing, there are $M$-type defects in the kagome planes as well as magnetic Co ions at the interplanar positions. Previous investigations revealed large antiferromagnetic (AFM) Curie-Weiss temperatures in both compounds. Yet no AFM long-range order but a spin glass behavior at low temperatures has been observed. Using $^{27}\mathrm{Al}$ NMR as well as ${\mathrm{Co}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ESR spin probes that are sensitive to local magnetic properties at different crystallographic sites, we have identified two magnetic subsystems in both compounds and have studied their distinct properties. In particular in the case of ${\mathrm{YBaCo}}_{3}{\mathrm{AlO}}_{7}$, we have observed a gradual development of the spin correlations in the two-dimensional (2D) kagome planes containing nonmagnetic Al defects and the establishment of a 3D frozen glasslike spin state of the interplanar Co ions at lower temperatures. We have found that despite a strong dependence of the ordering temperature in the kagome layers on the type of the $M$ ion (magnetic or nonmagnetic), the final 3D static ground state sets in at rather similar temperatures. We argue that the peculiar spin dynamics and a disordered magnetic ground state of the studied compounds result from the interplay of strong magnetic frustration and intrinsic structural disorder arising due to the intersite mixing of Co and $M$ atoms.