We report a field-switchable first-order antiferromagnetic-antiferromagnetic (AFM-AFM) phase transition in single-crystalline $\mathrm{H}{\mathrm{o}}_3\mathrm{C}{\mathrm{u}}_4\mathrm{S}{\mathrm{n}}_4$, which differs from the widely studied AFM-FM and FM-FM first-order magnetic transitions. Electrical resistivity and magnetic susceptibility measurements reveal sharp, discontinuous jumps and hysteresis between the two AFM states, with a large positive magnetoresistance change exceeding 55%, a value that far exceeds the typical magnitude of intrinsic magnetoresistance ($\sim 0.1\%-10\%$) in conventional antiferromagnetic materials. Notably, this AFM-AFM transition can be fully switched by a modest magnetic field of only 0.4 T, in stark contrast to conventional AFM-FM first-order transitions, which typically require $\sim 2-5$ T external magnetic fields. This work constructs the detailed magnetic phase diagram of this system, reveals the critical role of sinusoidal spin reorientation in the giant magnetoresistance effect, and provides an experimental platform for studying low-field-tunable antiferromagnetic phase transitions.

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• Magnetic phase transitions
• Antiferromagnets
• Magnetization measurements