Ferromagnetic Order of Reduced Magnetic Moments in a Frustrated Sawtooth Chain of the Magnetic Semiconductor ZnYb${}_2$S${}_4$

Abstract

In a sawtooth spin chain, competing nearest- and next-nearest-neighbor interactions suppress long-range order, yielding novel quantum states such as a spin-dimer singlet, 1/2 magnetization plateau, and spin contraction. Here, we investigate the magnetic properties of the orthorhombic semiconductor ZnYb${}_2$S${}_4$, in which Yb${}^{3+}$ ions with an effective spin-1/2 form a sawtooth chain along the $b$-axis. The specific heat exhibits a sharp peak at $T_{\mathrm{m}}$ $=$ 1.4 K, at which the magnetic entropy $S_{\mathrm{m}}$ reaches only 27% of $R$ln2. This reduced $S_{\mathrm{m}}$ at $T_{\mathrm{m}}$ indicates the entropy release of the ground state doublet of Yb${}^{3+}$ even for $T$ $>$ $T_{\mathrm{m}}$. The isothermal magnetization $M(B)$ at 0.28 K exhibits hysteresis for $\left|B\right|\le 0.2$ T and increases monotonically for $B>0.2$ T. The spontaneous magnetization is only 0.1 ${\hat{I}¼}_{\mathrm{B}}$$/$Yb, an order of magnitude smaller than that expected for the ground state doublet of Yb${}^{3+}$. Moreover, in powder neutron diffraction measurements, no superlattice reflections due to antiferromagnetic order are observed for $T$ $<$ $T_{\mathrm{m}}$. Therefore, in the ground state, the Yb moments are ferromagnetically aligned, but their amplitude is reduced by magnetic frustration in the sawtooth Yb chain.

6 pages, 6 figures

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