Competing Magnetism in Layered Mixed Transition Metal Chalcogenides KCo2–xNixSe2, KCo2–xNixS2, and CsCo2–xNixSe2

Layered transition metal chalcogenides are a versatile class of compounds that exhibit exotic physical phenomena, including superconductivity, thermoelectric properties and magnetic properties. The magnetic properties of ThCr₂Si₂-type solid solutions KCo_2-x Ni _x Ch ₂ (Ch = S, Se; 0 ≤ x ≤ 2) with metallic properties were probed using magnetometry and powder neutron diffraction (PND). KCo₂Se₂ is ferromagnetic below ∼90 K and powder neutron diffraction (PND) showed evidence for long-range ferromagnetic order with localized moments of 0.6 μ_B per cobalt ion. With increasing nickel substitution, the system starts to order antiferromagnetically at x = 0.5. In these cases, PND experiments showed long-range A-type antiferromagnetic order with localized moments of around 1 μ_B per transition metal at 5 K. The Néel temperature (T _N) for three-dimensional long-range ordering exhibits a maximum at x = 1, suggesting that nickel substitution enhances the antiferromagnetic interactions along the stacking direction. Higher nickel content suppresses the magnetic ordering temperature, and KCo_0.5Ni_1.5Se₂ shows no magnetic long-range order with a lack of measurable Bragg peaks by PND (although a magnetic transition is evident by magnetometry), and further increasing the nickel content causes the system to become paramagnetic in the region 1.6 ≤ x ≤ 2. Our results show that increasing the electron count in the KCo_2-x Ni _x Se₂ series has a dramatic effect on the physical properties. The analogous sulfide series - KCo_2-x Ni _x S₂shows similar behavior, and the series CsCo_2-x Ni _x Se₂, containing a larger alkali metal ion, is comparable apart from the lack of a ferromagnetic region at high Co contents in the absence of an applied magnetic field.