# Competing Magnetism in Layered Mixed Transition Metal Chalcogenides KCo2–x Ni x Se2, KCo2–x Ni x S2, and CsCo2–x Ni x Se2

## TL;DR

This paper studies how substituting nickel in layered transition metal chalcogenides affects their magnetic properties, revealing transitions from ferromagnetism to antiferromagnetism and paramagnetism.

## Contribution

The study reveals how nickel substitution alters magnetic ordering in KCo2–xNi xSe2 and related compounds, showing a transition from ferromagnetic to antiferromagnetic and paramagnetic states.

## Key findings

- KCo2Se2 is ferromagnetic below 90 K with localized moments of 0.6 μB per cobalt ion.
- At x = 0.5, the system shows antiferromagnetic order with localized moments of around 1 μB per transition metal.
- Higher nickel content suppresses magnetic ordering, with KCo0.5Ni1.5Se2 showing no long-range magnetic order.

## Abstract

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 KCo2–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 μB 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 μB 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 KCo0.5Ni1.5Se2 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 KCo2–x
Ni
x
Se2 series has a dramatic effect on the
physical properties. The analogous sulfide series - KCo2–x
Ni
x
S2shows
similar behavior, and the series CsCo2–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.