Abstract
The quasi-one-dimensional (1D) spin chain system α–CoV2O6 is well known for exhibiting
field-induced magnetic transitions that take the system from an antiferromagnetic (AFM)
ground state to a ferrimagnetic (FI) state, and then to a ferromagnetic (FM) state. The
occurrence of these transitions in α–CoV2O6 is caused by the strong Ising-like anisotropy of
the high-spin (HS) state Co2+ ions. This enables the compound to have a wide range of practical
applications in various fields including magnetic refrigeration, spintronics, and
supercapacitors.
The present study investigates the structural, chemical, morphological, and magnetic
properties of α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05. A wet chemical synthesis method
was followed in the synthesis of powder α–Co(V1−xCrx)2O6 samples for the first time. The
structural properties of α–Co(V1−xCrx)2O6 were probed using room temperature X-ray
diffraction (XRD). Structural analysis was followed by chemical composition and elemental
composition analysis using X-ray photoelectron spectroscopy (XPS) and energy dispersive Xray
electron spectroscopy (EDS). The sizes and morphology of powder particles were probed
using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Field (μ0H) and temperature (T) dependence of magnetization (M) were used to probe the
magnetic properties of α–Co(V1−xCrx)2O6 samples. In addition to these investigations, the
optical properties of the α–Co(V1−xCrx)2O6 samples were investigated using ultraviolet visible
near infrared (UV-Vis-NIR).
Single phase monoclinic α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05 were formed by
calcining the raw synthesized sample at 450 ℃ and at 500 ℃. Binary phases of α–CoV2O6 and
γ−CoV2O6 are formed when calcination temperature is set below 450 ℃ and above 500 ℃ and
when x > 0.05. The presence of Cr in single phase α–CoV2O6 causes a compression distortion
v
of the α–CoV2O6 crystal structure along the crystallographic c axis, as well as distortions of
the CoO6 octahedra.
XPS analysis showed that the Co, and V ions in α–CoV2O6 take on oxidation states Co2+
and V5+, respectively. The dopant Cr ions exist in a Cr3+ oxidation state and prefer to reside in
the Co2+ ionic sites when doped into α–CoV2O6. Considerable amounts of oxygen vacancies
are formed during crystallization of the α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05 samples.
The presence of Co, V, O, and Cr in the samples were confirmed using EDS. SEM and TEM
analysis revealed that the average size of particles in the powder decreased with a reduction of
calcination temperature, as well as with increasing dopant concentration. Size reduction is
accompanied by an agglomeration of the particles.
Magnetization as a function of applied field, M(μ0H), isotherms showed metamagnetism in
the α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05, characterized by three magnetization steps.
The presence of M(μ0H) hysteresis at fields below 5 T at 2 K, demonstrate that magnetization
is metastable and suggest magnetic irreversibility. Zero-field-cooled (ZFC) and field-cooled
(FC) temperature dependence of magnetization, M(T), curves revealed an AFM ordering of the
α–Co(V1−xCrx)2O6 samples at fields, M(μ0H) ≤ 1 T. Increasing field strength to 2.5 T causes a
transition from the AFM state to a FI state, indicated by a shift of TN to lower temperatures.
Further increase of field to 5 T causes a transition from the FI state to a FM state. The FM state
is characterized by a monotonic increase of magnetization with decreasing temperature. Cr
doping was shown to increase TN when the system is in the AFM and FM states.
Spin-glass behaviour was observed in the form of ZFC and FC bifurcation at fields between
1.0 T and 2.5 T, with spin-glass freezing occurring at TSG. The presence of spin-glass behaviour
in powder α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05, is described using the Gabey-Toulouse
(G-T) model for systems with weak anisotropy. Remanent magnetization relaxation
vi
measurements, M(t), of α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05, are described using a
stretched exponential decay function used for spin-glass systems.
Analysis of optical properties of α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤ 0.05, revealed that
Cr doping and particle size reduction has very small effects on the size of optical band gap in
α–CoV2O6. However, the presence of Cr results in the formation of electronic states within the
optical band gap, allowing for electronic transitions within the band gap.
The successful synthesis and characterization of α–Co(V1−xCrx)2O6 samples with 0 ≤ x ≤
0.05 serves as a foundation and offers a novel approach in investigating the physical properties
of doped α–CoV2O6.