Maintaining high conductivity in NiCo2O4 epitaxial thin films to 625 °C via an overlayer stabilization approach

The inverse spinel ferrimagnetic NiCo2 O4 presents a unique model system for studying the competing effects of crystalline fields, magnetic exchange, and various types of chemical and lattice disorder on the electronic and magnetic states. Here, magnetotransport anomalies in high-quality epitaxial NiCo2 O4 thin films resulting from the complex energy landscape are reported. A strong out-of-plane magnetic anisotropy, linear magnetoresistance, and robust anomalous Hall effect above 300 K are observed in 5-30 unit cell NiCo2 O4 films. The anomalous Hall resistance exhibits a nonmonotonic temperature dependence that peaks around room temperature, and reverses its sign at low temperature in films thinner than 20 unit cells. The scaling relation between the anomalous Hall conductivity and longitudinal conductivity reveals the intricate interplay between the spin-dependent impurity scattering, band intrinsic Berry phase effect, and electron correlation. This study provides important insights into the functional design of NiCo2 O4 for developing spinel-based spintronic applications.

Transition metal oxides (TMOs) with perpendicular magnetic anisotropy (PMA) and metallic behavior have promising potential for application in the development of new generation spintronic devices with high density, low power consumption, and nonvolatility. Although much progress has been made, the simultaneous coexistence of robust PMA and excellent metallicity at room temperature or higher temperatures in TMOs remains a huge challenge, limiting their practical application. Herein, high-quality NiCo2O4 (NCO) epitaxial films are reported, which have low resistivity, strong room temperature PMA with highly tunable coercive field, a sign-reversible anomalous Hall effect (AHE), as well as an exchange bias (EB)-like effect. The AHE sign reversal in higher substrate temperature (Tsub.) samples is attributed to the competition between the intrinsic mechanism contributed by the Berry curvature and the extrinsic mechanism dominated by impurity scattering. The EB-like effect in higher Tsub. samples is caused by the presence of non-stoichiometric NCO in films, which in turn leads to local antiferromagnetic coupling. Finally, a perpendicular magnetic tunnel junction based on the NCO homojunction is designed. This work reveals the relevance of cations in NCO to the crystal structure, magnetism and transport properties, which opens up new opportunities for the use of NCO films in the design of novel materials and devices.

The origin of alternating wavy dark-bright stripe-like contrast in strain contrast transmission electron microscopy images of NiCo2O4 (NCO) epitaxial thin films grown by pulsed laser deposition has been investigated. The nanoscale stripe-like pattern is determined to be associated with coexisting rock salt (RS) and inverse spinel crystal phases. The presence of two different phases, not addressed in previous reports, is experimentally confirmed by both electron diffraction and high resolution transmission electron microscopy imaging. First principles based calculations, together with compressive strain present in the films, support the formation of such coexisting crystallographic phases in NCO. Similar microstructural patterns and RS structure are not observed in epitaxial films of two other oxides of the spinel family, namely, NiFe2O4 and CoFe2O4. A correlation between the coexisting structures and the macroscopic physical properties of NCO is discussed.

High quality epitaxial bilayer films of spinel CoFe2O4 (CoF) and M-type hexagonal ferrite BaFe12O19 (BaM) were deposited onto (111) magnesium oxide (MgO) substrate by pulsed laser ablation deposition. X-ray diffraction patterns of both BaM/CoF and CoF/BaM films showed only (0001) BaM, (111) CoF, and (111) MgO peaks. The highest coercive field of Hc=1.4 kOe was obtained for a BaM/CoF film where the CoF layer was deposited at 400 °C, which was higher than typical Hc values of ∼0.4 kOe found for single layer BaM films on (111) MgO. This was less than the Hc∼3.0 kOe found for (111) CoF films deposited at 400 °C, probably due to in situ annealing effects during the growth of the overlying BaM film at 900 °C. This Hc enhancement for BaM/CoF bilayers as compared to BaM/MgO films may provide a means to combine large coercive fields with high quality hexaferrite films.

Changing film thickness to manipulate microstructural properties has been considered as a potential method in practical application. Here, we report that atomic-scale structural properties are regulated by film thickness in an NiCO2O4(NCO)/CuFe2O4(CFO) bilayer heterostructure prepared on (001)-MgAl2O4 (MAO) substrate by means of aberration-corrected scanning transmission electron microscopy (STEM). The misfit dislocations at the NCO/CFO interface and antiphase boundaries (APBs) bound to dislocations within the films are both found in NCO (40 nm)/CFO (40 nm)/MAO heterostructures, contributing to the relaxation of mismatch lattice strain. In addition, the non-overlapping a/4[101]-APB is found and the structural transformation of this kind of APB is resolved at the atomic scale. In contrast, only the interfacial dislocations form at the interface without the formation of APBs within the films in NCO (10 nm)/CFO (40 nm)/MAO heterostructures. Our results provide evidence that the formation of microstructural defects can be regulated by changing film thickness to tune the magnetic properties of epitaxial bilayer spinel oxide films.

The present study probes the effect of cation distributions in the structural, optical, electronic, and magnetic properties of mixed-valent inverse-spinel NiCo2O4 (NCO) nanoparticles (NPs). NCO NPs were prepared using the sol–gel combustion method and the grain size was obtained in the magnetic exchange length range assumed to be from single-ion anisotropy. The Raman and photoluminescence spectroscopies confirm the presence of an inverse-spinel structure with different oxidation states, and vibrating sample magnetometry clarifies the existence of ferromagnetism with the presence of magnetic anisotropy among the cations. These NPs annealed at a higher grain-growth temperature accumulate ferrimagnetic properties and produce magneto-crystalline anisotropy making NCO an assuring material for spintronic applications. A detailed x-ray absorption spectroscopy and x-ray magnetic circular dichroism studies reveal an indestructible correlation between the distribution of the present cations and the element-specific origin of ferrimagnetic behavior. Ni is found to be accountable for the magnetic moment and electronic conduction, whereas Co is associated mainly with the generation of the magnetic anisotropy even in the polycrystalline NP form. This describes the anti-ferromagnetic coupling between Co and Ni ions that is pivotal in demonstrating the exchange interaction between these cations. The above result signifies the site-dependent cation valence states for the magnetic properties, and the extent of growing conditions are related to such cation-site dysfunction. This depicts further tunability in NCO as a functional oxide material.

Epitaxial CoFe2O4(CFO)/CoO bilayers were fabricated by pulsed laser deposition on flexible muscovite mica substrate. Samples with different CFO thicknesses were employed to study the phenomenon of exchange bias involving strongly anisotropic ferromagnet. Magnetic measurements exhibited great enhancement in the features of exchange bias. Raman and X-ray absorption spectroscopies indicated that a new phase emerged within the CFO layer because of the cation charge redistribution in CFO layer under bending, which in turn gave rise to anomalous hysteresis loops exhibited in the bent bilayers. These results provide a fundamental understanding about the mechanisms of exchange bias prevailing in these bilayers and call attention to the implementation of spintronic devices using flexible heterostructures such as the present CFO/CoO bilayers.

Magnetoresistance of epitaxial SrRuO3 thin films on a flexible CoFe2O4-buffered mica substrate

Heterogeneous bilayer of Aurivillius compound Bi6Fe2Ti3O18 (BFTO) and Cubic spinel structure CoFe2O4 (CFO), a class of highly effective multiferroic composites, have been synthesized on fused quartz substrate by sol–gel method. Both BFTO and CFO layers are synthesized separately without any chemical reaction which can be confirmed by XRD pattern and Raman spectroscopy for two individual sets of maps. SEM and AFM images also verify two separate layers by showing the boundary between two layers clearly. Furthermore, linear increased ferromagnetism with increasing CFO component has been detected, further proving the independence of BFTO and CFO phases. Non-linear changed electron transition energy has been observed which may be derived from the crystal field distortion at the interface of the two layers. Therefore, an appropriate component can be found which have both excellent magnetic and optical properties, showing great potential on solar-energy conversion devices and multiferroic applications. In addition, variable temperature transmission spectra have been detected and the results are worth to be analyzed deeply.

Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science. The present work investigates the effect of cation redistribution on the structural, electrical, optical and magnetic properties of mixed-valent inverse spinel NiCo2O4(NCO) thin films. It is observed that the films grown at low temperatures (T < 400 °C) exhibit metallic behavior while that grown at higher temperatures (T > 400 °C) are insulators with lower ferrimagnetic-paramagnetic phase transition temperature. So far, n-type Fe3O4 has been used as a conducting layer for the spinel thin films based devices and the search for a p-type counterpart still remains elusive. The inherent coexistence and coupling of ferrimagnetic order and the metallic nature in p-type NCO makes it a promising candidate for spintronic devices. Detailed X-ray Absorption and X–ray Magnetic Circular Dichroism studies revealed a strong correlation between the mixed-valent cation distribution and the resulting ferrimagnetic-metallic/insulating behavior. Our study clearly demonstrates that it is the concentration of Ni3+ions and the Ni3+–O2−Ni2+ double exchange interaction that is crucial in dictating the metallic behavior in NCO ferrimagnet. The metal-insulator and the associated magnetic order-disorder transitions can be tuned by the degree of cation site disorder via growth conditions.

Epitaxial thin films of spinel NiCo2O4 (NCO) grown on MgAl2O4 (001) substrates are reported to exhibit dramatic changes in the magnetic and transport properties with deposition temperature. While films grown at lower temperatures (&amp;lt;450°C) are ferrimagnetic with metallic characteristics, those grown at higher temperatures are non-magnetic and insulating. Detailed polarized Raman spectroscopy studies indicate that the higher temperature films have close to the ideal inverse spinel cation distribution, Co3+[Ni2+Co3+]O42−, whereas those deposited at lower temperature are characterized by mixed cation/charge distribution at both the tetragonal (A) and octahedral (B) sites. Additionally, temperature-dependent Raman studies demonstrate that, unlike bulk polycrystalline samples, all the NCO films are robust against thermal treatment with full reversibility after annealing at 600°C in oxygen and air. However, partial decomposition is observed after annealing in vacuum.

We investigated the influence of oxygen vacancies on the magnetic and transport properties of ferrimagnetic NiCo2O4 (NCO) epitaxial films. Oxygen vacancies were introduced by annealing under reducing atmospheres NCO films whose cation composition was close to the stoichiometric one. We find that annealing NCO films under the vacuum reduces their magnetizations and increases their electrical resistivities. The perpendicular magnetic anisotropy, on the other hand, is almost unaffected by annealing treatments. X-ray absorption spectroscopy shows that oxygen vacancies introduced in NCO films preferentially lower the Ni valence state while leaving the Co valence state unchanged. The lowering in the Ni valence state explains the reduced magnetizations and the increased resistivities for the films annealed under the vacuum. On the other hand, the Co valence state, which dominantly determines the orbital magnetic moments responsible for magnetic anisotropy, is insensitive to oxygen vacancies. Therefore, the perpendicular magnetic anisotropy is maintained even when the oxygen vacancies are introduced.

Compression-tolerant supercapacitor based on NiCo2O4/Ti3C2Tx MXene/reduced graphene oxide composite aerogel with insights from density functional theory simulations

Controlling nanostructures and crystallographic orientations in epitaxial nanocomposite thin films are important for tuning their physical properties. Here, we present epitaxial nanocomposite thin films of (110) oriented CoFe2O4 (CFO) and (1110) oriented Bi5Ti3FeO15 (BTFO15) grown on SrTiO3 (110) substrates with a vertically aligned pillar–matrix type structure. The size and density of CFO pillars embedded in the BTFO matrix were controlled by the growth temperature and CFO concentration. Moreover, BTFO takes intergrowth phases with the general formula of Bi4Ti3O12 • nBiFeO3 (n = 1–∼1.5) depending on the growth temperature. Scanning probe analysis on the ferroelectric properties of BTFO15–CFO nanocomposite thin films suggested that the BTFO has a switchable out-of-plane polarization component originating from the tilted orientation of its a–b plane polarization. For high CFO pillar density, however, a non-negligible number of conductive paths which might be formed at the pillar–matrix interface are likely to prevent the polarization reversal.

High resolution electron energy loss spectroscopy (HREELS) is utilized to probe the optical band gaps at the nanoscale in epitaxial NiCo2O4 (NCO) thin films with different structural order (cation/charge). The structure of NCO deviates from the ideal inverse spinel (non-magnetic and insulating) for films grown at higher temperatures (>500 °C) towards a mixed cation structure (magnetic with metallic conductivity) at lower deposition temperatures (<450 °C). This significantly modifies the electronic structure as well as the nature of the band gap of the material. Nanoscale regions with unoccupied tetrahedral A site cations are additionally observed in all the samples and direct measurement from such areas reveals considerably lower band gap values as compared to the ideal inverse spinel and mixed cation configurations. Experimental values of band gaps have been found to be in good agreement with the theoretical mBJLDA exchange potential based calculated band gaps for various cation ordering and consideration of A site cation vacancy defects. The origin of rich variation in cation ordering observed in this system is discussed.