Solid solutions of Tl1-xKxCo2Se2 (0 ≤ x ≤ 1.0) were prepared by high temperature solid state reactions. All the samples are highly metallic (ρ ≈ 10-4-10-5 Ω cm). The magnetic phase diagram was determined by magnetic susceptibility measurements: the thallium-rich phases (0 ≤ x ≤ 0.45) order antiferromagnetically, the potassium-rich phases (0.55 ≤ x ≤ 1.0) order ferromagnetically, and the intermediate phases (x = 0.50) show competition between antiferromagnetic and ferromagnetic behavior. The possible mechanism of the transition from antiferromagnetic to ferromagnetic behavior is discussed.
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