Photon Factory Activity Report 2022 #40 (2023)
BL-16A/2021G501

Magnetic Evidence of NiCo2O4 using Soft X-Ray Magnetic Circular
Dichroism
R. DAWN1, S. JENA1, K. AMEMIYA2 and V. R. SINGH1*
1
Department of Physics, Central University of South Bihar, Gaya 824236.
2

Photon Factory, IMSS, High Energy Accelerator Research Organization,Tsukuba, Ibaraki 305-0801, Japan.

1. INTRODUCTION: The d-band metal oxides arerenowned among materials due to the complexity of
their phase diagram that exhibits a formidable array ofclose and almost overlapping transitions between
metallic, insulating, magnetic, and even forsuperconducting phases [1]. Spintronics is one of themost important emerging areas of research inrecent years. Its potential lies in providing highspeed memory devices with less power
consumption and high storage density. To date,
only the ferromagnetic materials have been studied
widely for their applications in spintronics devices.
But ferrimagnetic spinel oxides, in some cases, cangive much better performance than its
ferromagnetic counterpart. The ferrimagnetic
inverse spinel Nickel Cobaltite (NiCo2O4) has
attracted extensive research interest for its versatileelectrochemical properties, robust magnetic order,
high conductivity, as well as its highly tunable
nature due to the closely coupled charge, spin,
orbital, lattice, and defect effects. The cation
distribution in mixed-valent inverse spinel NCO
can be expressed in terms of a general formula as
Cox2+ Co1-x3+[Co3+ Ni1-x2+ Nix3+] O42- where
tetrahedral site is occupied by high spin Co2+ andCo3+ while Ni2+, low spin Ni3+ and Co3+ occupy theoctahedral sites. Out of all cations, Ni3+ ionsexhibit a strong Jahn-Teller Distortion. Besides,
further investigation will be performed tocharacterize the electronic and magnetic structure
through spectroscopic measurements. The
fundamental understanding of these phenomena
will facilitate the functional design of NiCo2O4
nanoparticles
fornanoscalespintronic
applications. The intrinsic coupling offerrimagnetic order and the metallic nature in
NiCo2O4 makes it a unique candidate for spintronic
applications. X-ray Absorption Spectroscopy(XAS) results not only provide a direct and solid
evidence to the scenario of mixed-valent cation
distribution of NCO but also establishes a
distinction between the metallic and insulatingphases in terms of their relative concentrations.
Thus, enabling us to understand the mechanism

responsible for such contrasting behaviors. Pairs ofXAS spectra (μ+/μ−) are measured with circularly
polarized x-rays with ±0.1T magnetic fields
applying to the samples. The main objective of thepresent work is to evolve a mechanism that leads
to metallic behavior in NiCo2O4 ferrimagnet andhence is related to the charge localization at Ni
ions, due to possible charge/orbital ordering. YBitla et al [2] reported that NiCo2O4, in form of thinfilms grown at low temperatures (T<400°C) conserves
metallic behaviour while that grown at higher
temperatures (T>400°C) are insulators with low
ferrimagnetic-paramagnetic phase transition. Suchinherent coupling coexistence of ferrimagnetic orderand the metallic nature makes NCO a promising
candidate for spintronic devices.

We propose a facile coprecipitation method and subsequently postannealing approach where Cetrimonium Bromide(CTAB) acts as a chelating ligand and Iso-propanol(IPA) as solvent for synthesizing NiCo2O4 samples.
The sample was placed a pre-heated oven of 300oC
for calcine for 2 hrs and after that the sample isannealed at 700oC in air-atmosphere for a highly
polycrystalline pure inverse spinel phase formationthat has been verified through X-ray Powder
Diffraction. The effects of reaction parameters suchas initial molar concentration of reactants, reaction
time and species of solvent on product size andshape were investigated along with the outcomes of
2. EXPERIMENT:

imaging experiments were performed at the variablepolarization soft x-ray beam-line BL-16A of the Photon
Factory (KEK, Japan). Experimental geometry of soft xray absorption (XAS) and x-ray magnetic circulardichroism (XMCD) experiments are shown in Fig. 1 and2. The sample was placed in the vacuum chamber with apressure of 10−9 Torr equipped with a 5T
superconducting magnet. XAS and XMCD signals were
measured at ±0T, ±1T and ±2T varying magnetic-fields(only the data of ±1T has been produced here) with rightand left circularly polarized (RCP and LCP) x-rayshaving an energy resolution of 0.1 eV using the Surfacesensitive total electron yield (TEY) method near Ni L2,3

Photon Factory Activity Report 2022 #40 (2023)
and Co L2,3 absorption edges with right and left circularly
polarized (RCP and LCP) x-rays.
3. RESULTS AND DISCUSSIONS: The XAS spectra
obtained with applied magnetic fields of +1.0 and -1.0 T
are denoted by + and - which represent left and rightcircularly polarized light, respectively as shown in Fig

Fig 1(a) Normalised XAS spectra and Fig 1(b)Normalised XMCD spectra of Ni L2,3 site. Fig 1(c)
XMCD/XAS Intensity ratio w.r.t. applied magneticfield 0 T, ±1.0 T and ±2.0 T .
1(a) and 2(a). The XMCD spectrum was recorded bytaking a difference between the XAS spectra withnegative and positive helicity of the circular polarized
light. Ni and Co, both shows a tendency of multiplet
structure at their L3 and L2 absorption edges, respectively.

Well-resolved peaks at the absorption maxima at E =853.77 eV and 871.55 eV at the L3 and L2 edges,
respectively are clearly observable at Ni L edge. Thisverifies that the fine structures of Ni 2p → 3d transition
should result from the localization of Ni 3d electrons
rather than the oxidation of the material. Co, on the other
hand, have multiplet absorption peaks at L3 edges. The L3

Fig 2(a) Normalised XAS spectra and Fig 2(b)Normalised XMCD spectra of Co L2,3 site. Fig 2(c)
XMCD/XAS Intensity ratio w.r.t. applied magneticfield 0 T, ±1.0T and ±2.0T.
peak with absorption energy, E = 780.84 eV arrives witha satellite peak at 779.42 eV; whereas, L2 shows a singlet
peak at 795.16 eV. Such results implies that unlike Nisite, Co 2p → 3d transition is non-localised and infusesPhoton Factory Activity Report 2022 #40 (2023)
more oxidation of the material. XMCD signal measured
in a magnetic field of B =1.0 T (as produced here) for Ni
L2,3 and Co L2,3 is shown in Fig.1(b) and Fig.2(b),respectively. The XMCD feature of Ni and Co L2,3 isanti-parallel to each other. The XMCD/XAS intensity
ratio w.r.t. the variable magnetic field with 0 T, ±1.0T
and ±2.0T are shown in Fig 1(c) for Ni-edges and in Fig
2(c) for Co-edges. This result is quite relatable with theXMCD graphs. Fig 1(c) exhibits that the intensity ratioof XMCD/XAS is increasing with increasing magneticfield. Fig 2(c) produces a steeper increasing ratio withincreasing magnetic-field. This suggests that Co involves
more in magnetic anisotropy which indicates an increasein ferromagnetism of the NPs. If anyone wants for aquantitative spin and orbital magnetic moment
calculation, through sum-rule [3-7] it can be derived.

ACKNOWLEDGEMENTS: The experiment at thePhoton Factory was approved by the Program AdvisoryCommittee (Proposal Nos. 2021G501. Authors
acknowledges support from UGC-BSR Start-up
Research Grant F.30-395/2017(BSR).
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Zhu, H-J Liu, Q. Zhan, H-J Lin, C-Te Chen, Y-H Chu& Q. He, Scientific Reports, 5:15201. DOI:10.1038/srep15201.

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