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.

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.

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