Frustration-mediated crossover from long-range to short-range magnetic ordering in Y1–xLuxBaCo4

The $R\mathrm{BaC}{\mathrm{o}}_{4}{\mathrm{O}}_{7}$ system is a prototype geometrically frustrated magnet in which kagome planes and triangular layers of Co-O tetrahedra interleave. For $R=\mathrm{Y}$, an antiferromagnetic ground state is realized due to a frustration-breaking trigonal-orthorhombic phase transition. For $R=\mathrm{Lu}$, however, a long-range ordered state has rarely, if ever, been reported despite a similar symmetry-breaking transition, albeit at a significantly lower temperature. To explore this dichotomy, we present a comprehensive magnetic and structural phase diagram for ${\mathrm{Y}}_{1--x}\mathrm{L}{\mathrm{u}}_{x}\mathrm{BaC}{\mathrm{o}}_{4}{\mathrm{O}}_{7}$, established through complementary neutron diffraction and magnetization measurements. Our results outline the phase evolution of the nuclear structures in response to changes in composition and temperature. The temperature of the trigonal ($P31c$) to orthorhombic ($Pbn{2}_{1}$) transition, ${T}_{\mathrm{s}1}$, decreases monotonically with increasing Lu content from 310 K for $x=0.0$ to 110 K for $x=1.0$. In Lu-rich compositions ($0.7\ensuremath{\le}x\ensuremath{\le}1.0$), first-order structural transitions are observed with coexisting and competing orthorhombic $Pbn{2}_{1}$ and metastable monoclinic $\mathit{Cc}$ phases. For the magnetically ordered Y-rich compositions, $T$- and $x$-dependent refinements of the magnetic structure reveal an antiferromagnetic ``ribbonlike'' arrangement of Co spin pairs in both the triangular and the kagome layers. A gradual suppression of long-range magnetic order is observed with increasing the Lu content, accompanied by the development of short-range magnetic correlations present in all the samples.