The high-quality inverse spinel CuCo2O4 thin films are epitaxially grown on (001) MgAl2O4 substrates by radio frequency magnetron sputtering. The electrical transport properties exhibit typical semiconducting characteristics, accompanying the enhancement of resistivity with the thinning of CuCo2O4 thickness. The transport properties could be well understood by the Mott variable range hopping model. The anomalous Hall effect with a clear hysteresis loop is observed below 100 K, indicating the existence of out-of-plane magnetization in the epitaxial-grown CuCo2O4 films. In addition, the negative magnetoresistance at low temperature reverses to the positive magnetoresistance (≥100 K), which is related to the changes from the decrease in spin/carrier scattering under the magnetic field at low temperature to the enhancement of carrier deflection due to the conventional Lorenz force (≥100 K). The observed physical properties are closely related to the orbital occupation of Cu ion in CuCo2O4 films, which is a significant difference compared to that of documented metallic NiCo2O4. This work is a good comprehensive study of inverse spinel oxide thin films.

Transition-metal oxides with spinel (AB2O4) structure exhibit diverse functional properties such as ferrimagnetism,1,2 multiferroics,3,4 and anomalous Hall effect,5 exhibiting enormous potential for applications in the field of spintronics.6–8 They are highly tunable due to the close interplay between the charge, spin, orbital, and lattice degrees of freedom. A series of inverse spinel structure oxides, such as Co3O4,9,10 NiCo2O4,11,12 NiFe2O4,13,14 CoFe2O4,15,16 and Fe3O4,17,18 have been widely studied. Among them, the inverse spinel ferrimagnetic NiCo2O4 (NCO) is a rising candidate for spintronics19–21 due to its versatile electrochemical properties,22,23 robust magnetic ordering,24 high conductivity,25,26 room-temperature perpendicular magnetic anisotropy,27,28 and anomalous Hall effect.29 In an ideal inverse spinel case, NCO configuration can be expressed as [Co3+]Td[Ni2+Co3+]OhO4. However, due to the multivalent states of Ni and Co ions in NCO and the complexity of ion occupation,30,31 it has been reported that the actual configuration is typically expressed as [Co1x3+Cox2+]Td[Ni1x2+Nix3+Co3+]OhO4. This brings difficulties in studying the origin of emerging phenomena such as perpendicular magnetic anisotropy and anomalous Hall effect in NCO, further hindering the potential application of NCO in spintronics.

Since the NCO emerges various intriguing physical properties, it is natural to propose whether the nearby oxide CuCo2O4 (CCO) persists similar properties or not. The Cu element is next to the Ni element in the Periodic Table, which may make NCO and CCO supposed to be with similar physical properties. The electron configuration of Cu2+ is [Ar]3d9 with one itinerant electron at 3d orbital. Generally, Ni ions in NCO exhibit multivalent states, while Cu ions usually only show bivalent state. An inverse spinel crystal structure of CCO is displayed in Fig. 1. In an ideal inverse spinel structure, Co3+ ions occupy tetrahedral (Td) sites, while Co3+ and Cu2+ ions share octahedral (Oh) sites.3,32 The orbitals of Ni2+ ions in NCO [Fig. 1(c)] and Cu2+ in CCO [Fig. 1(d)] at the octahedral sites are split into t2g and eg. Due to the crystal field, the t2g orbital exhibit the lower energy. The Td sites are occupied by the Co3+ ions, with the e states at the lower energy [Fig. 1(b)].33–35 The magnetic moments of the Cu2+ and Co3+ ions at the Oh and Td sites are antiferromagnetically coupled.36 In addition, the Co3+ ions at the Oh sites do not contribute to magnetization due to the low spine state configuration (t2g6eg0, S = 0).30 Therefore, CCO is a good prototype system to study the origin of magneto-transport properties of spinel oxide, which could be compared to the emerging NCO. Unfortunately, few studies on the magneto-transport properties of CCO have been explored so far.

FIG. 1.

(a) Crystal structure of CCO. The ideal case of electron configurations of 3d states for (b) Co3+ ions at the Td site in both the NCO and CCO, (c) Ni2+ ions at the Oh site in the NCO, and (d) Cu2+ ions at the Oh site in the CCO.

FIG. 1.

(a) Crystal structure of CCO. The ideal case of electron configurations of 3d states for (b) Co3+ ions at the Td site in both the NCO and CCO, (c) Ni2+ ions at the Oh site in the NCO, and (d) Cu2+ ions at the Oh site in the CCO.

Close modal

In this work, the high-quality CCO films were epitaxially grown by radio frequency magnetron sputtering on (001) MgAl2O4 (MAO) substrates. The electrical transport properties exhibited typical semiconducting properties for various thicknesses of CCO films. The resistivity is enhanced by thinning the CCO film thickness. The transport properties could be well understood by the Mott variable range hopping model. The anomalous Hall effect with a clear hysteresis loop is observed below 100 K while a conventional linear Hall resistance is disclosed at 100 K. The negative magnetoresistance at low temperatures appears at low temperatures related to the decrease in spin/carrier scattering and the increase in conductivity. The negative magnetoresistance changes to positive magnetoresistance at 100 K, indicating the enhancement of carrier deflection due to the conventional Lorenz force. The X-ray photoelectron spectroscopy reveals that the observed physical properties are linked to the electron occupation in CCO films. The observed properties are significantly different from those of documented metallic NiCo2O4.

The high-quality CCO films were epitaxially grown on MAO (001) substrates (5 × 5 mm2) by radio frequency (RF) magnetron sputtering with a pressure of 7.5 mTorr (Ar/O2 = 1:1) at different growth temperatures (300 and 400 °C). The applied RF power was 30 W. The thickness of the film was controlled by the deposition time. The surface morphology of the films was studied by atomic force microscopy (AFM, MFP-3D Infinity). The crystal structure was characterized by the X-ray diffraction (XRD, Rigaku Smartlab) with a Cu Kα wavelength of 1.5406 Å, while the thickness of the films was evaluated by the X-ray reflectivity. The magnetotransport properties of the CCO films were measured in a physical properties measurement system (PPMS, Quantum Design) with the van der Pauw geometry under a low current excitation (10–30 μA).37 The valence states of Cu, Co, and O ions were measured by X-ray photoelectron spectroscopy (XPS, Axis supra+).

The topographies of as-grown CCO films are revealed by the AFM images [as shown in Figs. 2(a) and 2(b)]. It indicates that the as-grown CCO films display a uniformly clean surface with a flat surface roughness of 0.7 nm (300 °C) and 0.9 nm (400 °C), corresponding to the high quality of sputtered CCO films.31,38 Figure 2(c) displays the X-ray reflectivity(XRR) of a typical CCO film grown at 400 °C. The clear oscillation is observed with the calculated thickness of 21.0 nm. It indicates that the CCO film has a high interface quality, which is consistent with the flat surface revealed by AFM. Figure 2(d) shows the XRD patterns of CCO films deposited at 300 and 400 °C. Only (00l) diffraction peaks from the CCO films and MAO substrate are observed. No impurity phases are identified. Obviously, a clear (004) CCO peak near the MAO substrate is displayed for the CCO film grown at 300 °C, while a weak (004) CCO peak is collected for the film grown at 400 °C, corresponding to a higher quality of CCO film grown at 300 °C.31 The calculated out-of-plane lattice constants are 8.31 and 8.27 Å for CCO films grown at 300 and 400 °C, respectively. The XRD pattern also reveals that the film grown at 400 °C exhibits weak crystallinity, which may be from the decomposition at the elevated temperature due to the small growth window similar to that of NiCo2O4 films.3 

FIG. 2.

The AFM images of CCO films grown at (a) 300 °C and (b) 400 °C. (c) The typical collected XRR data of CCO film grown at 400 °C. (d) The XRD θ–2θ scans of the CCO films.

FIG. 2.

The AFM images of CCO films grown at (a) 300 °C and (b) 400 °C. (c) The typical collected XRR data of CCO film grown at 400 °C. (d) The XRD θ–2θ scans of the CCO films.

Close modal

To study the transport properties of CCO films, the temperature-dependent resistivity (ρxx) has been collected in the temperature range of 10–380 K (Fig. 3). Figures 3(a) and 3(b) display the temperature dependence of ρxx for the CCO films with various thicknesses grown at 300 and 400 °C, respectively. Generally, the ρxx exponentially increases with the decrease in temperature over the whole temperature range, exhibiting a typical semiconducting behavior for all samples,39,40 irrespective of the growth temperature. As for the CCO films grown at 300 °C, the room temperature ρxx increases from 4.27 × 10−3 to 1.45 × 10−2 Ω cm as the reduction of film thickness from 23.7 to 4.0 nm. Similar temperature-dependent and thickness-dependent ρxx for CCO films grown at 400 °C is revealed as compared to that of films grown at 300 °C. The room-temperature ρxx varies from 8.02 × 10−3 to 4.87 × 10−2 Ω cm with a decrease in film thickness from 21.0 to 3.5 nm [Fig. 3(e)]. The enhanced room-temperature ρxx (400 °C) is closely related to the degraded quality of CCO film as revealed by the XRD data.28 Strikingly, the semiconducting CCO film shows a significant difference as compared to the metallic NCO film, considering the close element position in the Periodic Table. In the NCO film, the metallicity originates from the mixed valence states of Ni2+ and Ni3+ at the Oh sites and the double exchange interaction.32 However, the electrons in the CCO film reveal a localization property with a bivalent valence of Cu ion, resulting in the semiconductor nature of CCO.

FIG. 3.

The temperature-dependent resistivity ρxx for the CCO films with various thicknesses grown at (a) 300 °C and (b) 400 °C. The fittings of RT curves with variable range hopping model for the semiconducting CCO films grown at (c) 300 °C and (d) 400 °C. (e) ρxx vs t at room temperature (300 K). (f) The extracted fitting parameter T0 vs t at room temperature (300 K).

FIG. 3.

The temperature-dependent resistivity ρxx for the CCO films with various thicknesses grown at (a) 300 °C and (b) 400 °C. The fittings of RT curves with variable range hopping model for the semiconducting CCO films grown at (c) 300 °C and (d) 400 °C. (e) ρxx vs t at room temperature (300 K). (f) The extracted fitting parameter T0 vs t at room temperature (300 K).

Close modal

Since the temperature-dependent ρxx follows the exponential variation, the Mott variable range hopping (Mott-VRH) model is employed to fit the data.41,42 The Mott-VRH model is widely used to describe the conduction mechanism in semiconductor/insulator, which has a high carrier localization as well as narrow energy bandwidths.31,43 The Mott-VRH model can be expressed as R(T)=R0e(T0/T)1/4, where R0 and T0 are the fitting parameters. The parameter T0 is used to represent the degree of electron localization, namely, a large T0 corresponds to a strong electron localization. The extracted T0 values are 1.11 × 106, 7.28 × 104, 2.85 × 104, and 1.03 × 104 K with the increase in CCO thickness (300 °C) [Figs. 3(c) and 3(f)], indicating the enhanced electron localization in the thinner film. As for the CCO films grown at 400 °C, the T0 values are 3.69 × 106, 5.66 × 105, 1.39 × 105, and 1.31 × 105 K for various thicknesses. The observed variation of T0 is consistent with the trend of resistance modulation as shown in Fig. 3(e).44 

Figures 4(a) and 4(b) show the Hall resistivity Rxy as a function of H at various temperatures for the 23.7 and 21.0 nm CCO films grown at 300 and 400 °C, respectively. The Hall measurements were performed by the van der Pauw geometries, where the longitudinal resistance Rxx is included in the raw data of Rxy(H). To eliminate the influence of Rxx, the magnetic field reversal is employed, and the pure Hall signal is extracted through the relation Rxy(H) = [Rxy(+H) − Rxy(−H)]/2. Clear hysteresis anomalous Hall resistance is observed below 100 K for CCO films, disclosing that the films may hold the out-of-plane magnetic ordering.28,45 Linear field-dependent Rxy is displayed at 100 K, corresponding to the normal anomalous Hall effect related to the field-driven carrier deflection. It seems the Curie temperature of CCO films is near 100 K. It should be noted that the signal-to-noise ratio of the Hall resistance of the CCO films at 10 K is low because the Hall resistance is hard to measure considering the insulating nature of CCO films at low temperatures. Correspondingly, an obvious negative magnetoresistance with a two-peak feature is revealed below 100 K as shown in Figs. 4(c) and 4(d). The peak position is the coercivity as shown in the Hall resistance. Both the CCO films exhibit similar behavior in the whole temperature range. The magnetoresistance ratio is defined as MRR = [Rxx(H) − Rxx(0)]/Rxx(0).33,46 The MMR for CCO films reaches 6.4% at 50 kOe (300 °C) and 13.9% at 50 kOe (400 °C). The negative magnetoresistance is linked to the alignment of out-of-plane magnetic moment, which could reduce the scattering of charge carriers, namely, enhancing the conductivity.47 Meanwhile, the negative magnetoresistance is strongly influenced by the presence of grain boundaries in the epitaxial films. The MRR decreases with the increase in temperature, accompanying the disappearance of the double-peak feature. It indicates the weakening of the magnetic ordering when the temperature is elevated. Interestingly, the negative MRR changes to the positive MRR at 100 K. The positive MRR may originate from the classical deviation of electrons due to the Lorenz force under the magnetic field, resulting in the increase in electron scattering as well as the increase in resistivity. Although the anomalous Hall effect and magnetoresistance of CCO film are significantly different compared to that of NCO film, the small MRR feature is preserved in both films. The main difference between the NCO and CCO is the orbital occupation as shown in Fig. 1, which results in a vast of variation in the physical properties.

FIG. 4.

The typical magnetic field-dependent Hall resistance Rxy for (a) 23.7 nm CCO films grown at 300 °C and (b) 21.0 nm CCO films grown at 400 °C. The magnetoresistance ratio measured at various temperatures for (c) 23.7 nm CCO films (300 °C) and (d) 21.0 nm CCO (400 °C).

FIG. 4.

The typical magnetic field-dependent Hall resistance Rxy for (a) 23.7 nm CCO films grown at 300 °C and (b) 21.0 nm CCO films grown at 400 °C. The magnetoresistance ratio measured at various temperatures for (c) 23.7 nm CCO films (300 °C) and (d) 21.0 nm CCO (400 °C).

Close modal

To further resolve the difference in physical properties between the NCO film and CCO film, the XPS was carried out to analyze the valence state and chemical composition of CCO films. Figure 5 shows the typical XPS spectra of the Cu 2p, Co 2p, and O 1s core levels of CCO films grown at 400 °C. The XPS spectrums were fitted using the Gaussian–Lorentzian function. The black dot line is the experimental collected data and the red line is the sum of the fitting results. The XPS curve of the Cu 2p3/2 core level in the CCO film consists of a dominant peak sitting at the binding energy of ∼933.82 eV, revealing the copper is Cu2+. Moreover, a satellite peak is located at a binding energy of ∼941.99 eV, which indicates that the electrons of Cu2+ ions are localized at the 3d orbital (Fig. 1), resulting in semiconducting properties.48 The Co 2p3/2 spectra were fitted by two main peaks at a binding energy of ∼780.90 and ∼779.48 eV, attributing to the coexistence of Co2+ and Co3+, respectively. The ratio of Co2+/Co3+ is 2.1.49 The fitting O 1s spectra reveal the coexistence of three peaks at ∼529.56, ∼530.95, and ∼532.13 eV, which could be assigned to lattice O2−, oxygen defects in the crystal lattice, and the hydroxyl/carbonate groups, respectively.50–52 The fitting results indicate that the presence of Cu2+ ions leads to different magnetotransport properties observed in CCO films compared to that of NCO films.30 The temperature-dependent resistivity of CCO films exhibits semiconductor properties and follows the Mott-VRH model. The Mott-VRH model is widely used to describe the conduction mechanism in semiconductors/insulators with high carrier localization and narrow energy bandwidths, indicating the highly localized nature of electrons of Cu2+ in CCO. In contrast, the Ni2+/Ni3+ and Co3+ ions exhibit metallic properties through the double exchange mechanism at the Oh sites in NCO films. The above-mentioned factors contribute to the differences in the magnetic and electrical transport properties between CCO films and NCO films.

FIG. 5.

The typical XPS spectra of the CCO films grown at 400 °C for (a) Cu 2p, (b) Co 2p, and (c) O 1s.

FIG. 5.

The typical XPS spectra of the CCO films grown at 400 °C for (a) Cu 2p, (b) Co 2p, and (c) O 1s.

Close modal

In summary, the epitaxial CCO (001) thin films were grown on MAO substrates at different temperatures. The electrical transport properties exhibited typical semiconducting properties for various thicknesses of CCO films. The resistivity could be modulated by the CCO film thickness as well as the growth temperature. The clear anomalous Hall effect with hysteresis loop is observed below 100 K, irrespective of the sample growth temperature. The Hall resistance changes to a linear relation at 100 K due to the conventional Lorenz force. The negative magnetoresistance at low temperature changes to the positive magnetoresistance at 100 K, which originated from the changes from the decrease of spin/carrier scattering under a magnetic field at low temperature to the enhancement of carrier deflection due to the conventional Lorenz force (≥100 K). The X-ray photoelectron spectroscopy reveals that the observed properties are related to the electron occupation in CCO films. The observed properties are significantly different from those of documented metallic NiCo2O4. The present results advance our fundamental understanding and provide significant guidance for the application of spinel oxides in spintronics.

This work was supported by the National Key R&D Program of China (Grant No. 2022YFB3506000). This work was also partially supported by the National Natural Science Foundation of China (Grant Nos. 12104005 and 52301230), the Scientific Research Foundation of the High Education Institutions for Distinguished Young Scholars in Anhui Province (Grant No. 2022AH020012), and the Innovation Project for Overseas Researcher in Anhui Province (Grant No. 2022LCX004). This work was also partially supported by the facilities at the Center of Free Electron Laser and High Magnetic Field (FEL&HMF) in Anhui University.

The authors have no conflicts to disclose.

X.J. and B.Z. contributed equally to this work.

Xianghao Ji: Investigation (equal); Visualization (equal); Writing – original draft (equal). Biao Zheng: Investigation (equal); Visualization (equal); Writing – original draft (equal). Mingzhu Xue: Formal analysis (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Xue Liu: Formal analysis (equal); Investigation (equal). Wenshuai Gao: Formal analysis (equal); Investigation (equal). Mingliang Tian: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal). Xuegang Chen: Conceptualization (lead); Formal analysis (lead); Funding acquisition (lead); Project administration (lead); Supervision (lead); Writing – original draft (equal); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

1.
Y.
Shen
,
D.
Kan
,
Z.
Tan
,
Y.
Wakabayashi
, and
Y.
Shimakawa
, “
Tuning of ferrimagnetism and perpendicular magnetic anisotropy in NiCo2O4 epitaxial films by the cation distribution
,”
Phys. Rev. B
101
,
094412
(
2020
).
2.
Z.
Wang
,
P.
Wu
,
X.
Zou
,
S.
Wang
,
L.
Du
,
T.
Ouyang
, and
Z. Q.
Liu
, “
Optimizing the oxygen-catalytic performance of Zn–Mn–Co spinel by regulating the bond competition at octahedral sites
,”
Adv. Funct. Mater.
33
,
2214275
(
2023
).
3.
Y.
Bitla
,
Y.-Y.
Chin
,
J.-C.
Lin
,
C. N.
Van
,
R.
Liu
,
Y.
Zhu
,
H.-J.
Liu
,
Q.
Zhan
,
H.-J.
Lin
,
C.-T.
Chen
et al, “
Origin of metallic behavior in NiCo2O4 ferrimagnet
,”
Sci. Rep.
5
,
15201
(
2015
).
4.
A.
Sundaresan
and
N. V.
Ter-Oganessian
, “
Magnetoelectric and multiferroic properties of spinels
,”
J. Appl. Phys.
129
,
060901
(
2021
).
5.
D.
Kan
,
L.
Xie
, and
Y.
Shimakawa
, “
Scaling of the anomalous Hall effect in perpendicularly magnetized epitaxial films of the ferrimagnet NiCo2O4
,”
Phys. Rev. B
104
,
134407
(
2021
).
6.
X.
Xu
,
C.
Mellinger
,
Z. G.
Cheng
,
X.
Chen
, and
X.
Hong
, “
Epitaxial NiCo2O4 film as an emergent spintronic material: Magnetism and transport properties
,”
J. Appl. Phys.
132
,
020901
(
2022
).
7.
J.
Zhang
,
W.
Zhang
,
X.
Zhu
,
J.
Yang
,
J.
Xu
, and
D.
Yu
, “
Resonant slot nanoantennas for surface plasmon radiation in optical frequency range
,”
Appl. Phys. Lett.
100
,
241115
(
2012
).
8.
Y.
Hao
,
X.
Chen
,
L.
Zhang
,
M.-G.
Han
,
W.
Wang
,
Y.-W.
Fang
,
H.
Chen
,
Y.
Zhu
, and
X.
Hong
, “
Record high room temperature resistance switching in ferroelectric-gated Mott transistors unlocked by interfacial charge engineering
,”
Nat. Commun.
14
,
8247
(
2023
).
9.
C.-S.
Cheng
,
M.
Serizawa
,
H.
Sakata
, and
T.
Hirayama
, “
Electrical conductivity of Co3O4 films prepared by chemical vapour deposition
,”
Mater. Chem. Phys.
53
,
225
(
1998
).
10.
Z. N.
Kayani
,
S.
Arshad
,
S.
Riaz
, and
S.
Naseem
, “
Investigation of structural, optical and magnetic characteristics of Co3O4 thin films
,”
Appl. Phys. A
125
,
196
(
2019
).
11.
X.
Chen
,
X.
Zhang
,
M. G.
Han
,
L.
Zhang
,
Y.
Zhu
,
X.
Xu
, and
X.
Hong
, “
Magnetotransport anomaly in room-temperature ferrimagnetic NiCo2O4 thin films
,”
Adv. Mater.
31
,
1805260
(
2019
).
12.
A.
Shawky
,
M.
Alhaddad
,
R.
Mohamed
,
N. S.
Awwad
, and
H. A.
Ibrahium
, “
Magnetically separable and visible light-active Ag/NiCo2O4 nanorods prepared by a simple route for superior photodegradation of atrazine in water
,”
Prog. Nat. Sci.: Mater. Int.
30
,
160
(
2020
).
13.
K.
Chand Verma
,
V.
Pratap Singh
,
M.
Ram
,
J.
Shah
, and
R.
Kotnala
, “
Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films
,”
J. Magn. Magn. Mater.
323
,
3271
(
2011
).
14.
S.
Regmi
,
Z.
Li
,
A.
Srivastava
,
R.
Mahat
,
S.
Kc
,
A.
Rastogi
,
Z.
Galazka
,
R.
Datta
,
T.
Mewes
, and
A.
Gupta
, “
Structural and magnetic properties of NiFe2O4 thin films grown on isostructural lattice-matched substrates
,”
Appl. Phys. Lett.
118
,
152402
(
2021
).
15.
P.
Dorsey
,
P.
Lubitz
,
D.
Chrisey
, and
J.
Horwitz
, “
CoFe2O4 thin films grown on (100) MgO substrates using pulsed laser deposition
,”
J. Appl. Phys.
79
,
6338
(
1996
).
16.
N.
Labchir
,
A.
Hannour
,
A. A.
Hssi
,
D.
Vincent
,
K.
Abouabassi
,
A.
Ihlal
, and
M.
Sajieddine
, “
Synthesis and characterization of CoFe2O4 thin films for solar absorber application
,”
Mater. Sci. Semicond. Process.
111
,
104992
(
2020
).
17.
X.
Wang
,
Y.
Liao
,
D.
Zhang
,
T.
Wen
, and
Z.
Zhong
, “
A review of Fe3O4 thin films: Synthesis, modification and applications
,”
J. Mater. Sci. Technol.
34
,
1259
(
2018
).
18.
E.
Liu
,
Y.
Yin
,
L.
Sun
,
Y.
Zhai
,
J.
Du
,
F.
Xu
, and
H.
Zhai
, “
Increasing spin polarization in Fe3O4 films by engineering antiphase boundary densities
,”
Appl. Phys. Lett.
110
,
142402
(
2017
).
19.
H.
Sukegawa
,
Y.
Miura
,
S.
Muramoto
,
S.
Mitani
,
T.
Niizeki
,
T.
Ohkubo
,
K.
Abe
,
M.
Shirai
,
K.
Inomata
, and
K.
Hono
, “
Enhanced tunnel magnetoresistance in a spinel oxide barrier with cation-site disorder
,”
Phys. Rev. B
86
,
184401
(
2012
).
20.
Y.
Shen
,
D.
Kan
,
I.
Lin
,
M.-W.
Chu
,
I.
Suzuki
, and
Y.
Shimakawa
, “
Perpendicular magnetic tunnel junctions based on half-metallic NiCo2O4
,”
Appl. Phys. Lett.
117
,
042408
(
2020
).
21.
B.
Zheng
,
X.
Ji
,
M.
Xue
,
C.
Jia
,
C.
Kang
,
W.
Zhang
,
J.
Yang
,
M.
Tian
, and
X.
Chen
, “
Robust room temperature perpendicular magnetic anisotropy and anomalous Hall effect of sputtered NiCo2O4 film
,”
J. Phys.: Condens. Matter
36
,
275701
(
2024
).
22.
Y.
Huang
,
J.
Tao
,
W.
Meng
,
M.
Zhu
,
Y.
Huang
,
Y.
Fu
,
Y.
Gao
, and
C.
Zhi
, “
Super-high rate stretchable polypyrrole-based supercapacitors with excellent cycling stability
,”
Nano Energy
11
,
518
(
2015
).
23.
J.
Wu
,
W.-M.
Lau
, and
D.-S.
Geng
, “
Recent progress in cobalt-based compounds as high-performance anode materials for lithium ion batteries
,”
Rare Met.
36
,
307
(
2017
).
24.
X.
Chen
,
Q.
Wu
,
L.
Zhang
,
Y.
Hao
,
M.-G.
Han
,
Y.
Zhu
, and
X.
Hong
, “
Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo2O4 films
,”
Appl. Phys. Lett.
120
,
242401
(
2022
).
25.
K.
Zhang
,
Z.
Cen
,
F.
Yang
, and
K.
Xu
, “
Rational construction of NiCo2O4@Fe2O3 core-shell nanowire arrays for high-performance supercapacitors
,”
Prog. Nat. Sci.: Mater. Int.
31
,
19
(
2021
).
26.
M.
Xu
,
R.
Xu
,
Y.
Zhao
,
L.
Chen
,
B.
Huang
, and
W.
Wei
, “
Hierarchically porous Ni monolith@branch-structured NiCo2O4 for high energy density supercapacitors
,”
Prog. Nat. Sci.: Mater. Int.
26
,
276
(
2016
).
27.
D. P.
Dubal
,
P.
Gomez-Romero
,
B. R.
Sankapal
, and
R.
Holze
, “
Nickel cobaltite as an emerging material for supercapacitors: An overview
,”
Nano Energy
11
,
377
(
2015
).
28.
P.
Kang
,
G.
Zhou
,
H.
Ji
,
Z.
Li
,
Z.
Li
, and
X.
Xu
, “
Emergence of room-temperature perpendicular magnetic anisotropy in metallic NiCo2O4 thin film
,”
J. Magn. Magn. Mater.
553
,
169293
(
2022
).
29.
H.
Lv
,
X. C.
Huang
,
K. H. L.
Zhang
,
O.
Bierwagen
, and
M.
Ramsteiner
, “
Underlying mechanisms and tunability of the anomalous Hall effect in NiCo2O4 films with robust perpendicular magnetic anisotropy
,”
Adv. Sci.
10
,
2302956
(
2023
).
30.
M.
Xue
,
X.
Chen
,
S.
Ding
,
Z.
Liang
,
Y.
Peng
,
X.
Li
,
L.
Zha
,
W.
Yang
,
J.
Han
,
S.
Liu
et al, “
Transport anomaly in perpendicular magnetic anisotropic NiCo2O4 thin films with column-like phase separation
,”
ACS Appl. Electron. Mater.
2
,
3964
(
2020
).
31.
W.
Guo
,
C.
Zhen
,
C.
Wu
,
X.
Wu
,
G.
Li
,
L.
Ma
, and
D.
Hou
, “
Influence of growth temperature on the microstructure and electrical transport properties of epitaxial NiCo2O4 thin films
,”
Ceram. Int.
44
,
12539
(
2018
).
32.
X.
Huang
,
W.-W.
Li
,
S.
Zhang
,
F.
Oropeza
,
G.
Gorni
,
V.
de la Peña-O’Shea
,
T.-L.
Lee
,
M.
Wu
,
L.-S.
Wang
,
D.-C.
Qi
et al, “
Ni3+-induced semiconductor-to-metal transition in spinel nickel cobaltite thin films
,”
Phys. Rev. B
104
,
125136
(
2021
).
33.
C.
Zhen
,
X.
Zhang
,
W.
Wei
,
W.
Guo
,
A.
Pant
,
X.
Xu
,
J.
Shen
,
L.
Ma
, and
D.
Hou
, “
Nanostructural origin of semiconductivity and large magnetoresistance in epitaxial NiCo2O4/Al2O3 thin films
,”
J. Phys. D: Appl. Phys.
51
,
145308
(
2018
).
34.
C.
Xiao
,
Y.
Li
,
X.
Lu
, and
C.
Zhao
, “
Bifunctional porous NiFe/NiCo2O4/Ni foam electrodes with triple hierarchy and double synergies for efficient whole cell water splitting
,”
Adv. Funct. Mater.
26
,
3515
(
2016
).
35.
L.
Liu
,
C.
Zhen
,
L.
Xu
,
Z.
Shui
,
L.
Ma
,
D.
Zhao
, and
D.
Hou
, “
Semiconductor-metal transition in vulcanized NiCo2O4 film
,”
J. Phys. Chem. Solids
174
,
111189
(
2023
).
36.
R.
Nakhowong
and
R.
Chueachot
, “
Synthesis and magnetic properties of copper cobaltite (CuCo2O4) fibers by electrospinning
,”
J. Alloys Compd.
715
,
390
(
2017
).
37.
W.
Wang
,
Q.
Du
,
B.
Wang
,
Y.
Li
,
Z.
Hu
,
Y.
Wang
,
Z.
Wang
, and
M.
Liu
, “
Manipulated magnetic coercivity and spin reorientation transition in NiCo2O4 films
,”
J. Appl. Phys.
132
,
073901
(
2022
).
38.
K.
Zhang
,
C.
Zhen
,
W.
Wei
,
W.
Guo
,
G.
Tang
,
L.
Ma
,
D.
Hou
, and
X.
Wu
, “
Insight into metallic behavior in epitaxial half-metallic NiCo2O4 films
,”
RSC Adv.
7
,
36026
(
2017
).
39.
C.
Wu
,
C.
Zhen
,
X.
Zhang
,
X.
Xu
,
J.
Xie
,
L.
Ma
,
D.
Zhao
, and
D.
Hou
, “
Sensitive metallic behavior in epitaxial NiCo2O4 films regulated by the film thickness
,”
J. Phys. Chem. Solids
160
,
110321
(
2022
).
40.
J.
Kim
and
J.
Dho
, “
Metallic ferrimagnetism and magnetic domain structure in (001) NiCo2O4 films grown at various temperatures
,”
Thin Solid Films
781
,
139978
(
2023
).
41.
N. F.
Mott
, “
Conduction in non-crystalline materials: III. Localized states in a pseudogap and near extremities of conduction and valence bands
,”
Philos. Mag.
19
,
835
(
1969
).
42.
H.
Wang
,
C.
Zhen
,
J.
Xie
,
L.
Liu
,
L.
Ma
,
D.
Zhao
, and
D.
Hou
, “
Electronic structure and magnetic configuration of Ni-substituted MnCo2O4 spinel
,”
J. Phys. Chem. C
124
,
24090
(
2020
).
43.
J.
Xie
,
C.
Zhen
,
L.
Xu
,
M.
Su
,
C.
Pan
,
L.
Ma
,
D.
Zhao
, and
D.
Hou
, “
Regulation of growth temperature on structure, magnetism of epitaxial FeCo2O4 films
,”
CrystEngComm
24
,
83
(
2022
).
44.
L.
Zhang
,
H.
Gardner
,
X.
Chen
,
V.
Singh
, and
X.
Hong
, “
Strain induced modulation of the correlated transport in epitaxial Sm0.5Nd0.5NiO3 thin films
,”
J. Phys.: Condens. Matter
27
,
132201
(
2015
).
45.
Z.
Fang
,
N.
Nagaosa
,
K. S.
Takahashi
,
A.
Asamitsu
,
R.
Mathieu
,
T.
Ogasawara
,
H.
Yamada
,
M.
Kawasaki
,
Y.
Tokura
, and
K.
Terakura
, “
The anomalous Hall effect and magnetic monopoles in momentum space
,”
Science
302
,
92
(
2003
).
46.
P.
Silwal
,
L.
Miao
,
I.
Stern
,
X.
Zhou
,
J.
Hu
, and
D.
Ho Kim
, “
Metal insulator transition with ferrimagnetic order in epitaxial thin films of spinel NiCo2O4
,”
Appl. Phys. Lett.
100
,
032102
(
2012
).
47.
S.
Jin
,
T. H.
Tiefel
,
M.
McCormack
,
R.
Fastnacht
,
R.
Ramesh
, and
L.
Chen
, “
Thousandfold change in resistivity in magnetoresistive La-Ca-Mn-O films
,”
Science
264
,
413
(
1994
).
48.
H. S.
Jadhav
,
S. M.
Pawar
,
A. H.
Jadhav
,
G. M.
Thorat
, and
J. G.
Seo
, “
Hierarchical mesoporous 3D flower-like CuCo2O4/NF for high-performance electrochemical energy storage
,”
Sci. Rep.
6
,
31120
(
2016
).
49.
J. F.
Marco
,
J. R.
Gancedo
,
M.
Gracia
,
J. L.
Gautier
,
E. I.
Ríos
,
H. M.
Palmer
,
C.
Greaves
, and
F. J.
Berry
, “
Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4
,”
J. Mater. Chem.
11
,
3087
(
2001
).
50.
K.
Xu
,
J.
Yang
, and
J.
Hu
, “
Synthesis of hollow NiCo2O4 nanospheres with large specific surface area for asymmetric supercapacitors
,”
J. Colloid Interface Sci.
511
,
456
(
2018
).
51.
R.
Zou
,
K.
Xu
,
T.
Wang
,
G.
He
,
Q.
Liu
,
X.
Liu
,
Z.
Zhang
, and
J.
Hu
, “
Chain-like NiCo2O4 nanowires with different exposed reactive planes for high-performance supercapacitors
,”
J. Mater. Chem. A
1
,
8560
(
2013
).
52.
R.
Waghmode
and
A.
Torane
, “
Hierarchical 3D NiCo2O4 nanoflowers as electrode materials for high performance supercapacitors
,”
J. Mater. Sci.: Mater. Electron.
27
,
6133
(
2016
).