In a sawtooth spin chain, competing nearest- and next-nearest-neighbor exchange interactions are expected to yield novel quantum ground states, as theoretical studies predicted, including spin-dimer singlet, noncollinear order, 1/2 magnetization plateau, and spin contraction. In this study, the magnetic properties of the Yb-based semiconductor ZnYb₂S₄, which hosts an antiferromagnetic (AFM) effective spin-1/2 sawtooth chain of Yb³⁺ ions with 4f¹³ configuration along the b-axis in the orthorhombic crystal structure, were investigated by measuring the specific heat, magnetic susceptibility, isothermal magnetization, and powder neutron diffraction.

The magnetic susceptibility obeys the Curie–Weiss law for an Yb³⁺ ion with a total angular momentum J = 7/2. The negative paramagnetic Curie temperature indicates AFM interactions between the Yb moments. Owing to the isolated Kramers-doublet ground state of the Yb³⁺ ion produced by the strong crystalline electric field (CEF), this compound can be regarded as a frustrated effective spin-1/2 sawtooth chain system.

These results reveal that the ground state below T_m = 1.4 K is a ferromagnetic (FM) order, with Yb moments contracted by frustration. The specific heat exhibits a peak at T_m. The magnetization at B = 0.03 T increases on cooling below T_m. This behavior is a characteristic of FM order. Notably, the magnetic entropy at T_m is only 27% of Rln2, smaller than Rln2 expected for a phase transition of the Kramers-doublet. In addition, the magnetization of 0.08 m_B at 0.28 K is considerably smaller than 1.3 m_B /Yb³⁺ expected for the Kramers doublet. These results indicate that the ordered Yb moments are contracted. Powder neutron diffraction measurements show no superlattice reflections due to AFM order. Notably, the amplitude of the ordered Yb moments is restored by applying magnetic fields, suggesting a release of the magnetic frustration.  

This uniform alignment of the reduced moments differs markedly from the theoretical predictions for an AFM sawtooth chain. This discrepancy may be attributed to additional exchange interactions, such as interchain exchange interactions. Otherwise, theoretical analyses using Hubbard models for the quarter-filled insulating case indicate that the ground state could be a FM order over a broad range of parameters, close to the paramagnetic regions with AFM correlations. Thereby, ZnYb₂S₄ is a 4f-electron system exhibiting a characteristic ground state arising from magnetic frustration in a sawtooth effective spin-1/2 chain.

(Written by Shinji Okada and Takahiro Onimaru on behalf of all authors.)

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