Enhancing the magnetic moment of ferrimagnetic NiCo₂O₄ via ion irradiation driven oxygen vacancies

Heteroepitaxial strain engineering has been extensively used to modulate the structure of materials, leading to the design of new functionalities. The changes in the crystal lattice can effectively control the microscopic correlations leading to exotic and unexpected states of matter.^1–5 For instance, the epitaxial strain in oxide thin-films using different substrates is known to efficiently modulate and create a multitude of properties such as superconductivity, even in otherwise non-superconducting compounds, ferroelectricity, and ferromagnetism.^6–9 This method has proven to be very effective. However, it involves perturbation in all three lattice directions and besides the unavailability of suitable substrates; the standard biaxial strain has only shown the ability to change the physical properties in a discrete manner. These limitations constrain the ability of further tuning the crystal lattice and, hence, the resulting film properties. However, a new paradigm in this area would be a particular modification of the lattice strain along a single axis.^10–12 Recently, injection of He-ions into oxide films has emerged as a powerful tool in effectively controlling the out-of-plane lattice expansion, independent of changes in the in-plane lattice.^9,13 Prominent examples are the uniaxial control of the magnetic and electric properties of La_0.7Sr_0.3MnO₃ and the structural phase transition in the SrRuO₃ thin-films.^9,13 This approach not only allows one to gain an unprecedented control over the physical properties of oxides but also extends the possible use of oxide films toward commercial applications.

The spinel oxides exhibit rich magnetic and electronic structures, and correlations between those properties promise novel and superior spintronic functionalities, such as spin injection and/or filtering.¹⁴ In particular, NiCo₂O₄ (NCO) is a mixed-valent inverse spinel^15,16 that exhibits a ferrimagnetic transition well above room temperature and has large p-type conductivity.¹⁷ Furthermore, these properties can be controlled by means of growth temperature in epitaxially grown NCO thin-films.^18,19 The low-cost availability and the inherent correlations of magnetic ordering and conductivity of NCO along with its electrocatalytic activity and infrared transparency make it a most suitable candidate for a multitude of applications.^20–24 Owing to its viability in contemporary spintronics, we have used an ion-irradiation approach for further modifying the properties of epitaxial NCO thin-films. We present the observation of an increase in the magnetic moment along with the resistivity in the NCO films due to high energy He-ion irradiation induced effects. Besides the irradiation induced strain,⁹ our analysis also indicates the role of the induced defects especially the role of oxygen vacancies and lattice occupation.

NCO thin-films (a_p = 8.116 Å) of thickness ranging from ∼81 to 93 nm were epitaxially grown on (001) oriented MgAl₂O₄ (a = 8.083 Å) using the pulsed laser deposition technique. After deposition, these films were irradiated with He-ions at an energy of 100 keV with incident fluence ranging from 0.5 × 10¹⁶/cm² to 3 × 10¹⁶/cm². The samples are named by the numbers 0.5 × 10¹⁶–3 × 10¹⁶ in the manuscript. The high energy He-ions penetrate the NCO films and are implanted into the substrate resulting in a relatively depth-independent vacancy distribution in the films [Fig. 1(a)]. The resulting total displacements and the He-ion distribution are estimated in the supplementary material, allowing for a comparison with different ions and energies.²⁵ Figure 1(b) shows θ-2θ scans around the (004) substrate reflection. The existence of thickness fringes, even after irradiation, depicts the high quality of the films. The dashed line illustrates the shift in the NCO peak toward lower 2θ values, i.e., an implantation induced expansion of the out-of-plane lattice parameter.^9,13 However, the chosen implantation conditions lead to a different reaction of the almost defect free substrate and the much less perfect films on the increasing implantation fluence: Whereas this effect saturates for the NCO layer almost after the first implantation steps, a pronounced shoulder appears at the low angle side of the substrate reflection demonstrating nicely an increasing strain with increasing fluence. To verify that theirradiation does not lead to any (in-plane) lattice relaxation reciprocal space maps (RSMs) of all samples at the asymmetric MgAl₂O₄ (115) reflection were recorded. Figure 1(c) shows exemplarily the RSM scan of the representative 1 × 10¹⁶ film. The film peak lies exactly below the substrate peak and indicates the coherent growth of the NCO layer on the substrate. We have not found any noticeable NCO peak intensity loss and broadening of the peak in the irradiated films. Even more RSM measurements (see the supplementary material) confirm that the implantation does not lead to a detectable enhancement of the diffuse scattering indicating that the amount of defects additionally introduced by implantation is well below the sensitivity of our HR-XRD measurement.

Thus, the He-ion irradiation with such a high energy, 100 keV, creates an additional perturbation in the films. But due to the fact that there is a significant amount of defects already in the as-grown films and that the lattice expansion is approximately proportional to the total defects including the intrinsic ones; the strain saturates at rather low fluence reaching an upper limit of the defect concentration, as it was similarly observed for ion irradiated Si.²⁶ 

Temperature (T) dependent magnetization (M) data were collected in an external field of 100 Oe [Fig. 2(a)]. The M-T graph of the as-grown film exhibits the ferrimagnetic-paramagnetic phase transition (T_C) ∼350 K. However, irradiation has increased the moment of films by an order in magnitude with a decrease in the transition temperature. Figure 2(b) represents the magnetization versus magnetic field isotherms collected for all films at 5 K. The saturation moment in the 2.5 × 10¹⁶ film is ∼2.5 μ_B/f.u., which is more than twice larger than in the as-grown film as well as the reported bulk value.²⁷ In earlier studies, an epitaxy-induced reordering of cations along with a comparable enhancement of the magnetization has been observed for the ferromagnetic epitaxial NiFe₂O₄ film on SrTiO₃.²⁸ However, the present study reports the tuning and enhancement of the magnetic property, almost two-times, in oxide films upon the application of He-ion irradiation. The predicted magnetization of NCO is only 2 μ_B/f.u., therefore the observed moment in the irradiated film (∼2.5 μ_B/f.u.) can be rationalized as the redistribution of the cation in the spinel structure.²⁹ Further increase in fluence to 3 × 10¹⁶/cm² results in a slight decrease of the moment and T_C compared with the film irradiated with a fluence of 2.5 × 10¹⁶/cm², suggesting the latter to be an optimal He-ion fluence value. The advantage of using high energy He-ions lies in the fact that it will only induce a perturbation in the NCO layer without any implantation of and doping by foreign atoms.^9,30 It is noteworthy that the magnetization of films remarkably scales with the fluence of He-ions and the magnetic moment almost doubles as the fluence increases from 0.5 × 10¹⁶/cm² to 2.5 × 10¹⁶/cm² [Fig. 2(c)].

To explore the effect of irradiation on the electronic properties of NCO and its correlation with the magnetic properties, temperature dependent resistivity (ρ) data were collected [Fig. 2(d)]. The as-grown film exhibits a metal-insulator transition (T_MI) at ∼350 K [the inset of Fig. 2(c)].^18,19 Below T_MI, the as-grown film shows minima in the vicinity of ∼50 K, indicating a disorder-induced quantum interference effect,¹⁹ and re-enters into the insulating state. As the films were irradiated, the high-temperature phase begins to lose conductivity systematically. The salient features observed in the ρ-T data of irradiated films are as follows: (i) the 0.5 × 10¹⁶ film shows a resistive behavior and T_MI similar to the as-grown film, (ii) as fluence increased up to 2.5 × 10¹⁶/cm², the overall resistivity is almost doubled compared with the as-grown film, but with a reduction in T_MI, and (iii) upon further increasing the fluence to 3 × 10¹⁶/cm², the resistivity decreases relative to the 1 × 10¹⁶ and 2.5 × 10¹⁶ films. The increased resistivity below T_MI in irradiated films compared with the as-grown film can be associated with the disorder induced by He-ions. This induced disorder causes expansion in the uniaxial strain which acts as the main factor in influencing the resistivity upturn.³¹ Therefore the resistivity upturn will significantly increase with the fluence with a shift in the resistivity minima toward a higher temperature side.

Now, it is imperative to understand the origin of the enhancement of the magnetic moment of irradiated films and their correlation with the electronic properties. The experimental results suggest a strong coupling of the electronic and magnetic properties to the structural deviations of the film from the relaxed, pristine bulk phase. As the data do not indicate the formation of extended 1D, 2D, or volume defects, three potential types of structural defects remain to be considered: First, there is the epitaxy-induced biaxial strain state of all studied films, which is compressive in-plane and becomes increasingly non-Poissonian tensile in the out-of-plane direction upon ion-irradiation. Second, spinel-type oxides are prone to disorder and exchange processes on the cation sublattices with the normal and inverse spinel as the two limiting cases with ordered sublattice occupation.^14,27 Third, disorder on the cation sublattice perturbs the ideal local coordination environments in the normal spinel structure, which contains formally three-fold positive cations in six-fold coordinated octahedral sites (O_h) and two-fold positive cations in four-fold-coordinated tetrahedral sites (T_d).³² Disorder may lead to a local charge imbalance, which can be compensated by the presence of oxygen vacancies.

To separately study the influence of epitaxial strain, cation disorder, and oxygen vacancies introduced by ion-irradiation on NCO from first principles, we performed spin-polarized electronic structure calculations with density functional theory (DFT). By considering the strain as an additional parameter, this extends a recent theoretical study,³³ whereas earlier studies including electronic modeling on this system are rather limited.^14,34 We employ supercells of eight spinel formula units (Fig. 3), which allow accommodating different spin structures and are either fully geometry-optimized or clamped to the smaller in-plane lattice parameter of the MgAl₂O₄ substrate according to experimental data. The structures comprise the ground-state structure, the β-type inverse spinel (β_i-NCO) with Co in T_d, and c-ordered planes of Ni and Co in O_h, as well as the high symmetric Ni(T_d)-Co(O_h) normal spinel (n-NCO) [Figs. 3(a)–3(c)]. In addition, we took into account oxygen deficiency according to the symmetry-inequivalent site: V_O1, V_O2 for β_i-NCO, and V_O for n-NCO [Figs. 3(a) and 3(b)]. Here, we only relaxed internal coordinates fixing supercells to the relaxed c parameter of the stoichiometric clamped cell. The calculated total energies and magnetization are summarized in Table I.

The defect-free crystal β_i-NCO is marginally more stable than n-NCO by 2 meV/f.u. in the relaxed and 69 meV/f.u. in the clamped case. This may lead to cation disorder in the bulk and give rise to thermally activated cation exchange for epitaxially strained films.^14,27 In the clamped case, elongation of the out-of-plane lattice parameter takes place for all structures, most significantly for the magnetic n-NCO. A zero net magnetization state exists for n-NCO, which is considerably less favorable than the ferrimagnetic state for the relaxed (+159 meV/f.u.) and clamped cases (+115 meV/f.u.). This picture changes in the presence of oxygen vacancies. Again, the most stable structure is β_i-NCO with vacancies on site V_O1. Vacancy formation in the n-NCO, on the other hand, is significantly less favorable with about +30 meV/f.u. per vacancy. In addition, vacancies in β_i-NCO on site V_O2 are with +44 meV/f.u. per vacancy comparably stable, which further adds to a possible degeneracy of the electronic structure in the case of a high vacancy concentration. Thus, from a stability point of view, the as-grown film may either consist of significant patches of both rather perfect normal and inverse spinel structures or it already hosts a significant amount of oxygen vacancies in a predominantly inverse spinel structure on site V_O1. Ion irradiation may enhance both cation disorder and oxygen vacancy content.

Among the vacancy-free structures, both magnetic spinels exhibit a significant magnetic moment of 2 μ_B/f.u. However, the existence of a significant amount of non-magnetic n-NCO domains might give rise to the observed low magnetization in the as-grown state. For vacancy-containing supercells the magnetization is significantly reduced, and accordingly the saturation flux density B_S of originally 0.36 T decreases by up to a factor of two. Considering the total energies of the different structures, an increasing oxygen content is likely to promote clamped β_i-NCO in the vicinity of a vacancy, assuming that the necessary energy is provided to overcome the barrier of the cation reordering due to the released energy of He-irradiation. This finding suggests that the initial state of the material is partly n-NCO with a low vacancy concentration. The tendency toward increasing magnetization upon ion-irradiation may then be indicative of an increasing amount of β_i-NCO. Decreasing magnetic moment even after higher ion-fluence may then be correlated with a high vacancy concentration of V_O1 and an additional vacancy formation of V_O2, or alternatively, with cation reordering to the n-NCO phase. An analysis of the electronic density of states (DOS) [Figs. 3(d)–3(i)] supports the first alternative. Non-magnetic n-NCO is half-metallic, whereas β_i-NCO exhibits small bandgaps. With increasing amount of vacancies on site V_O1 rather than V_O, the insulating domains of the film increase concomitantly with the measured resistance.

From theory, both scenarios with and without vacancies in the as-grown film are likely, thus a differentiation must rely on spectroscopy capable of distinguishing the oxidation states of both metal cations.

We have performed the Co-L_2,3 and Ni-L_2,3 edge XAS experiments of NCO in order to understand the contrasting magnetic and electronic behavior of irradiated films (Fig. 4). The spectra show shifts in the L₂ and L₃ edges compared to the as-grown film which indicates a reduced valence state in Ni and Co ions resulting from the oxygen vacancy induced by the irradiation [Figs. 4(a) and 4(b)].³⁵ Two spectral features A and B in the Co-L₃ edge of irradiated films relative to the as-grown film in the vicinity of ∼777 eV and ∼779 eV are the characteristic features for Co²⁺ in O_h [Fig. 4(c)].^19,36 This strongly hints toward an increase in the content of Co²⁺ at O_h, along with Co³⁺, and consequently explains the moment in the irradiated films. A lower energy shift of the average peak weight in the irradiated films than in the as-grown film, clearly evident at the Ni-L₃ edge [Fig. 4(d)], suggests a decrease of the average oxidation state of Ni ions, due to the vacancy creation.³⁷ This suggests that the valence state of Ni at the lattice site changes formally from Ni³⁺ to Ni²⁺, which gives rise to the increased resistivity of irradiated films.

The ground state structure of NCO is an inverse spinel, where Ni-ions are located at O_h.^14,19,27,29 However, inset 1 of Fig. 4(b) does highlight a small decrease in the intensity of the near-edge spectral feature of the irradiated films. The near-edge spectral feature is generally used to distinguish between the T_d and O_h cations as this transition is only allowed for T_d. The decrease in the intensity of this spectral feature clearly indicates the presence of Ni-ions in T_d for the as-grown film, while less Ni-ions occur at T_d for irradiated films.^38,39 This indeed supports our theoretical model of the existence of significant patches of both normal and inverse spinel structures in the as-grown film. Upon ion irradiation, the decrease in the intensity is indicative of an increased amount of the inverse spinel structure. Irradiation has provided the necessary energy to the films to change the cation site occupancies in conjunction with vacancies, thereby driving the irradiated films close to the inverse spinel structure [inset 2 of Fig. 4(b)]. Therefore, the observed tendency toward an increasing net magnetization upon ion-irradiation, 0.5 × 10¹⁶/cm²–2.5 × 10¹⁶/cm², may then be indicative of an increasing amount of inverse spinel.²² At a fluence of 3 × 10¹⁶/cm², the intensity of the spectral feature increases in the vicinity of 836–839 eV compared with the 2.5 × 10¹⁶ film [inset 2 of Fig. 4(b)]. This suggests again an increase of the normal spinel structure in the 3 × 10¹⁶ film which consequently decreases the magnetization and the resistivity [Figs. 2(b)–2(d)]. Therefore, the variation in fluence alters the oxygen vacancy concentration and their mutual arrangement in the irradiated films leading to the shuffling of Ni and Co oxidation states and hence the fine tuning of electronic and magnetic properties.

At this point, it is essential to elaborate on the systematic modification of the physical properties by the controlled and optimized He-ion fluence. The impact of an increasing fluence on the magnetization and resistivity depends on the difference in the amount of oxygen vacancies. The effect of vacancies drives the lattice expansion and consequently the unpreserved unit-cell volume, not within the limit of Poisson’s effect.^36,40 The magnetization of NCO scales with the He-fluence. Our XAS data and theoretical model suggest that the initial state in the as-grown film is an admixture of the normal and inverse spinel structures. The induced vacancy content and the subsequent cation redistribution are indicative of the increased magnetization upon ion-irradiation and hence the formation of an increased fraction of inverse spinel structure in irradiated films. Previously, Torres et al. have also demonstrated the critical role of octahedral cation vacancies and the oxygen vacancy in enhancing the magnetic moment of zinc ferrite thin films.⁴¹ With increasing fluence, the amount of vacancies is enhanced in the clamped inverse structure and the magnetization is increased with a decrease of T_C. However, scaling of the magnetization does no longer pertain for the film subjected to a fluence of 3 × 10¹⁶/cm², which may be rationalized by an ion-energy-induced partial cation reordering to the oxygen-deficient normal spinel. In this film, the moment marginally decreases compared with the 2.5 × 10¹⁶ film, which may also be attributed to the fact that ferromagnetic ordering decreases at large fluence and the competing antiferromagnetic ordering increases, as is also discernible by the first-order phase transition (see the supplementary material). At the same time, we observed a systematic increase of the overall resistivity with fluence up to 2.5 × 10¹⁶/cm². Thus, the conduction mechanism in the film correlates with the ferromagnetic double-exchange interactions. In this respect, variation of the fluence will alter the oxygen vacancy concentration, resulting in the reshuffling of the Ni and Co oxidation state and hence changes the intrinsic interactions between the ions. The assertion of a redistribution on the cation sublattices further supported by XAS data suggesting an increased Ni²⁺ (insulating) fraction in the films with increasing fluence. Our studies suggest that the change in the vacancy concentration mediated by ion-irradiation doping dictates a competition between ferromagnetic and antiferromagnetic order which governs the underlying intrinsic mechanism of magnetism and conduction in the system.

To conclude, we have effectively employed the He-ion irradiation technique for tailoring the oxygen deficiency in NCO thin-films. The magnetic moment of NCO has been increased by more than two-times by an optimized and controlled increase in the fluence of He-ions. This observed increase of magnetization is correlated with a uniaxial lattice expansion. Spin-polarized electronic structure calculations with DFT indicate that the observed behavior is related to disorder of magnetic and non-magnetic phases in the as-grown NCO films and a redistribution of phase partitioning due to an enhanced oxygen deficiency induced by the irradiation. Our XAS results, in particular peak-height changes and energy shifts of the Ni and Co L_2,3-edge, confirm the model of oxygen deficiency and the cation reordering. Remarkably, this study not only demonstrates an approach to induce, enhance, and, in fact, tune the magnetism and conductivity by controlling the fluence and energy of ion-irradiation but also introduces a new pathway to engineer the strain in oxide films for the exploration of inaccessible phases which may be suitable for spintronic applications.

See supplementary material for the details of film synthesis, experimental and theoretical techniques, He-ion irradiation procedure, symmetric RSM scans, magnetic properties before and after annealing, thermoremanence, and resistivity.

P. Pandey appreciates Deutsche Forschungsgemeinschaft (ZH 225/6-1), Germany, for the financial support, Dr. Rakesh Rana for fruitful discussion, and Dr. R. Böttger for ion-irradiation. Support by the Ion Beam Center (IBC) at HZDR is gratefully acknowledged.