Spin correlations in the extended kagome system YBaCo ₃FeO ₇

The transition metal-based oxide YBaCo ₃FeO ₇ is structurally related to the mineral Swedenborgite SbNaBe ₄O ₇, a polar noncentrosymmetric crystal system. The magnetic Co ₃Fe sublattice consists of a tetrahedral network containing kagome-like layers with trigonal interlayer sites. This geometry causes frustration effects for magnetic ordering, which were investigated by magnetization measurements, Mössbauer spectroscopy, polarized neutron diffraction, and neutron spectroscopy. Magnetization measurement and neutron diffraction do not show long range ordering even at low temperature (1 K), although a strong antiferromagnetic coupling (∼2000 K) is deduced from the magnetic susceptibility. Below 590 K we observe two features, a spontaneous weak anisotropic magnetization hysteresis along the polar crystallographic axis and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the interlayer sites remain idle. Below ∼50 K the onset of a hyperfine field shows the development of moments static on the Mössbauer time scale also for the Fe interlayer sites. Simultaneously, an increase of spin correlations is found by polarized neutron diffraction. The relaxation part of the dynamic response has been further investigated by high-resolution neutron spectroscopy, which reveals that the spin correlations start to freeze below ∼50 K. Monte Carlo simulations show that the neutron scattering results at lower temperatures are compatible with a recent proposal that the particular geometric frustration in the Swedenborgite structure promotes quasi-one-dimensional partial order.