Terahertz Spin Current Pulses in Antiferromagnetic Oxide: The Role of Vacancy‐Induced Ferromagnetism

Antiferromagnetic oxides have attracted increasing attention for their outstanding peculiarities in spintronics. Crystal lattice defects that are present in antiferromagnetic oxides can influence their physical properties, such as vacancy‐induced ferromagnetism. Meanwhile, the generation and manipulation of ultrafast spin currents of antiferromagnetic insulators are highly desired. Although the generation and detection of terahertz spin current pulses in antiferromagnetic oxides have been realized, the effect of vacancy‐induced ferromagnetism on spin current in antiferromagnetic oxides is not yet known. Herein, the role of vacancy‐induced ferromagnetism on the terahertz spin current in antiferromagnetic nickel oxide thin films is reported. Structural and magnetic characterizations reveal that nickel vacancies effectively break the strong antiferromagnetic exchange coupling, giving rise to the coexistence of antiferromagnetism and ferromagnetism in NiO thin films. Notably, the enhancement of terahertz radiation associated with the photo‐induced ultrafast spin current of NiO thin film with the strongest ferromagnetism is the most significant. Besides, the nonlinear susceptibility tensor parameters related to the antiferromagnetic property of NiO thin films also change distinctly. The findings indicate that the defects of antiferromagnetic materials play a decisive role in the application of antiferromagnetic spintronics in the terahertz field.


Introduction
[3][4][5] In recent years, the generation, transportation, and detection of the spin current in AFM materials have been studied extensively.The usual generation mechanism of the spin current in antiferromagnet is the spin Hall effect (SHE), [6][7][8] and spin Seebeck effect (SSE). [9,10]The inverse spin Hall effect (ISHE) in heavy metals is one of the accessible methods to detect the spin current. [11]18][19][20] The introduction of crystal lattice defects and impurities is an effective means for modulating the physical properties of magnetic materials, such as optoelectronic properties, [21,22] and magnetic properties, [23][24][25] etc.[28] As a prototype AFM insulator, the transition metal oxide NiO has a rock-salt crystal structure (point group Fm3m).The defect-induced and doping-induced ferromagnetism in antiferromagnetic NiO has also been reported. [29,30]Meanwhile, there are many researches on ultrafast magnetic dynamics in single-crystal NiO due to its high Néel temperature (T N = 523 K). [31][32][33] Especially, Qiu et al. [34] implemented the generation of the ultrafast spin current in a NiO/Pt heterostructure by terahertz emission spectroscopy.Their research considers the AFM insulating NiO thin film a defect-free single crystal.However, the effects of defect-induced ferromagnetism in NiO thin films on the generation of the ultrafast spin current have not been studied so far.
In the AFM oxide NiO, the Ni-O-Ni AFM super-exchange interaction and Néel order are crucial factors, [35] related to the optical generation of spin currents [36] and magnetic switching. [37,38]ere, we present that the generation of the terahertz-wave emission spectrum associated with the ultrafast spin current in the heterostructures consists of the defective NiO thin films deposited on the (111)-oriented MgO substrate and Pt.The main reason for the defects in the NiO thin films is the difference due to the substrate temperature and oxygen pressure during the deposition.As the substrate temperature increases from room temperature to 600 °C, the structural characterization shows that the dominant defects in NiO thin films gradually change from oxygen vacancies (V O ) to nickel vacancies (V Ni ), and the NiO thin film exhibits weak ferromagnetism.The appearance of weak FM can be attributed to the presence of V Ni breaking the balance of up spin and down spin of NiO thin films. [30]Due to the different strengths of FM between NiO thin films, the measured THz radiation associated with the ultrafast spin current differs, and its intensity is positively correlated with the strength of ferromagnetism.Besides, by studying the relationship between the THz wave intensity and samples azimuth angle θ, we find that the nonlinear optical coefficients of the V Ni -dominant NiO films are also changed significantly compared with those of the V O -dominant NiO films, and V Ni -induced FM can promote the generation of ultrafast spin current in NiO thin films.Our results show that vacancy-induced FM plays an important role in the modulation of the spin current in AFM materials and it opens an avenue of fundamental research in the field of ultrafast AFM spintronics.

Magnetic and Microstructure Characterization
We fabricated seven types of NiO thin film samples by pulsed laser deposition (PLD).The seven NiO thin films are labeled as Sample No. 1 to No. 7, depending on the difference in the substrate temperature (T sub ) and oxygen pressure (P oxy ) during deposition, as detailed in the Experimental Section and Table 1.For confirming the magnetic properties of the NiO thin film samples, static magnetization measurements were carried out by a superconducting quantum interference vibrating sample magnetometer (SQUID-VSM) at room temperature.The applied magnetic field is parallel to the film surface.The magnetization curves of NiO thin films after subtracting the MgO substrate signal are shown in Figure 1a.The inset is the magnetization curves without subtraction of the MgO substrate signal.However, after deducting the substrate signal, we find that all the samples show significant magnetic hysteresis loops, of which sample 1 (T sub = RT, P oxy = 0 Pa) has the weakest magnetization (0.8 emu cm À3 ), and as the T sub increased, the magnetization of NiO thin films gradually increased until the temperature reached 400 °C (3.5 emu cm À3 ).Compared with the coercivity of NiO thin film reported by Sugiyama et al., [30] the measured coercivities of NiO thin films are small.Meanwhile, the magnetic force microscope (MFM) patterns of NiO thin films shown in Figure S1, Supporting Information, indicate the magnetic response of sample 5 (T sub = 400 °C, P oxy = 1.2 Pa) is the strongest compared with sample 1 and sample 2, corresponding to the magnetic curves of sample 5.Moreover, the temperature dependences of magnetization (M-T curves) of NiO thin films (see Figure S2, Supporting Information) and exchange bias effect in the heterostructures with the Co as the FM layer, samples 1 and 2, and 5, (Figure 1b) which demonstrated that the NiO thin films exhibit AFM properties.These results reveal the coexistence of antiferromagnetism and ferromagnetism in the NiO thin films.
We measured the X-ray Diffraction (XRD) patterns of the seven samples to figure out the differences in their crystal structures as shown in Figure 1c.The XRD patterns do not show any additional diffraction peaks, except the peaks of the MgO (111) and NiO (111) diffraction, which indicate that the grown NiO thin films are single crystalline with the (111) crystalline orientation.However, the small NiO (111) diffraction peaks differ in their positions from sample to sample.To explain this variation, we estimated the out-of-plane lattice constants and strains based on the XRD theory. [39,40]As shown in Figure 1f, with the increase in T sub , the strain decreases gradually, which is consistent with the change in the out-of-plane lattice constant c.Especially, the out-of-plane lattice constant (c = 4.30 Å) of sample 1 (T sub = RT and 0 Pa) increases in comparison to the lattice constant (c = 4.18 Å) of bulk NiO. [41]The strain is positive, which indicates that the NiO thin films are subjected to tensile stress increasing the out-of-plane lattice constant c.To further investigate the evolution of the crystal structures of the NiO samples, the Raman measurements were carried out at room temperature.The Raman spectra of the NiO thin films were studied extensively, which included LO, TO, 2TO, 2LO, and 2M modes. [42]The lattice defects of the NiO thin films are reflected by the changes in the intensities of these modes.Generally, the intensity of the LO (2LO) modes is weak (strong) in the defect-free NiO thin film. [43]s shown in Figure 1d, the intensities of the LO and 2LO modes of the Raman spectra of all the samples are different.We extracted the relative intensity ratio of the 2LO and LO modes, as shown in Figure 1e.Notably, the ratio of sample 1 (T sub = RT and 0 Pa) is less than 1, which indicates that this is the most defective sample.
To confirm the single crystalline properties of the NiO thin films, the high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns were investigated for sample 2 (T sub = RT, P oxy = 1.2 Pa) (seen Figure S3, Supporting Information) and sample 5 (T sub = 400 °C, P oxy = 1.2 Pa) (shown in Figure 2a,b, respectively).The HRTEM patterns indicate that the NiO thin film is perfectly epitaxial on the MgO single-crystal substrate.The SAED patterns show that when the substrate temperature is lowered from 400 °C to RT, the crystal structure of NiO thin film changes from the cubic phase (point group Fm3m) to a tetragonal phase (point group I4/mmm) under the strain. [44]We also performed scanning TEM (STEM) observations combined with electron energy-loss spectroscopy (EELS) analysis.Figure 2c presents the STEM image of a vacancy core in the NiO thin film grown under T sub = 400 °C, P oxy = 1.2 Pa conditions.The dislocation induced by vacancies can be seen.Furthermore, we focus on the structures of the O K-edge spectra, which can reflect the deficiency from the hybridized 3d 8 bands of NiO. [45]Figure 2d shows O K-edge EELS taken at the vacancy core and in the bulk region.The spectrum exhibits the same peak positions as previous studies in the bulk region. [45]As marked by the blue arrow in Figure 2d, the pre-peak (%528 eV) is observed only in the spectrum taken at the vacancy core, which is absent in the bulk.The prepeak in the O K-edge EELS has been observed in Ni-deficient nickel oxide. [30,46]Our findings manifest that the vacancy cores should be locally nonstoichiometric in the NiO thin films.
The optical spectra of all the NiO samples were measured by using a UV-vis-NIR spectrometer.The optical absorbance spectra are shown in Figure S4a, Supporting Information.There is no obvious variation in the absorbance of samples 2-7 except in that of sample 1.Then, the optical bandgaps of the NiO films were extracted by using Tauc's equations, [47] as shown in Figure S4b, Supporting Information.As can be seen in Figure S4c, Supporting Information, at T sub = RT and P oxy = 0 Pa deposition conditions, the optical bandgap (E g = 3.18 eV) of the NiO thin film abruptly decreases in comparison to the optical bandgap (E g % 4.0 eV) of bulk NiO. [47]This result demonstrates that the band spectra of the NiO thin films are significantly affected by different growth conditions.We believe that the difference between the samples is due to the vacancies caused by different deposition conditions.Hence, it is crucial to clarify the source of the vacancies.

Confirmation of Vacancies in NiO Thin Film
Figure 3a,c shows the X-Ray photoelectron spectroscopy (XPS) spectra of the Ni 2p and O 1s states of the NiO thin film deposited at T sub = RT and 0 Pa, respectively.According to the binding energies of Ni 2p and O 1s for the NiO thin film, [48] we fitted the spectra to obtain the relative contents of Ni and O elements.In the Ni 2p spectrum, the content of Ni 3þ is negligible compared to that of Ni 2þ .Of course, the same is true in other samples also (see Figure S5, Supporting Information).So, we will not consider the influence of Ni 3þ in the following discussion.In the O 1s spectrum, the three subspectra are separated, which attribute to the lattice oxygen (O l ), oxygen vacancy (O v ), and chemisorbed oxygen (O c ). [48,49] As illustrated in Figure 3b,d, the relative content ratios of Ni 2þ /O 2À l and O v /O l are the maximum for sample 1 and the minimum for sample 5, and both trends are consistent with the variation trend of the defects of the Raman spectra analysis shown in Figure 1e.Taken together, this trend indicates the formation of the lattice of the Ni-deficiency NiO thin film at T sub = 400 °C and P oxy = 1.2 Pa, and the largest number of oxygen vacancies at T sub = RT and 0 Pa deposition conditions.Under the growth conditions of T sub = 500 and 600 °C, the Ni vacancies appreciably decrease, which can be attributed to the defect-free lattice formation under high-temperature conditions. [50]To determine the vacancy types more accurately in NiO thin films, we measure the positron annihilation spectroscopy (PAS), which can characterize the microstructural defects in NiO thin films.Using the Lifetime 9 program, [51] the lifetime spectra are evaluated to two-lifetime components, named τ 1 and τ 2 in sample 1 and sample 5, shown in Figure 3e,f, respectively.Generally, the positrons are only trapped by negative vacancy (V Ni ) or neutral-charged vacancy clusters.The τ 1 and τ 2 represent the lifetime of the monovacancy and vacancy cluster, respectively. [52]The τ 1 , τ 2 , and their corresponding relative intensities (I 1 , I 2 ) in sample 1 (T sub = RT and P oxy = 0 Pa) are the balance between up and down spins along each atomic column, and the presence of vacancy clusters also breaks the strong coupling between local ferromagnetic order and antiferromagnetic order, which results in ferromagnetic behavior and small coercivities in the defective NiO thin film. [30]

THz Emission from NiO/Pt Heterostructures
To study the ultrafast spin current in NiO thin films under different deposition conditions, we fabricated heterostructures of the NiO/Pt, which are detected by THz time-domain spectroscopy (THz-TDS).The THz emission setup is schematically shown in Figure 4a.In a pump laser, the fs pulses propagate along the z-axis, i.e., the (111) orientation of the NiO thin film.Additional experimental details can be found in the Experimental Section and Figure S6, Supporting Information.The THz emission time-domain and corresponding Fourier-transformed spectra of the NiO/Pt heterostructure, which deposited under T sub = RT and P oxy = 0 Pa (see Figure S7, Supporting Information), respectively.The broadband frequency range (0-3 THz) is restricted by the ZnTe crystal.According to previous research, [34,53] the generation of the longitudinal spin current J s of the defect-free NiO thin film can be attributed to the photo-induced impulsive magnetic moment ΔM.J s transport into the Pt layer and convert into the in-plane charge current J c due to inverse spin Hall effect (ISHE), which expressed as J c = θ SH σ Â J s , where θ SH and σ are the spin Hall angle and the unit vector in the spin polarization direction, respectively.The ISHE conspires to yield efficient emission of electric pulses in the THz band. Figure S7c, Supporting Information, shows the linear dependence between the THz amplitude and pump fluence of the laser, which indicates the linear dependence of photo-induced spin current density on the optical intensity in the NiO thin film. [54]igure 4b,c shows the THz emission signals and corresponding frequency-domain spectra of all seven samples.The intensities of the THz emission are different.Moreover, we measure the THz emission signal of the bare NiO samples (see Figure S8, Supporting Information).The THz signal of the NiO/Pt samples is stronger than that of the bare NiO thin films.A weak THz signal from the bare NiO thin films is observed, which indicates that the heavy metal Pt layer with strong SOC is essential for the generation of a strong THz signal, and the influence of the quadrupoles or electric dipoles induced by the nonlinear optical effects in NiO single layer in the THz signal in the NiO/Pt heterostructure can be excluded. [18]The peak-to-peak values of the samples are extracted as shown in Figure 4d.The THz amplitude of sample 5 (T sub = 400 °C, P oxy = 1.2 Pa) is the most strong compared with those of other samples.Meanwhile, we find that the variation trend of the THz amplitude is consistent with that of the vacancy among different samples, except for sample 1 (T sub = RT, P oxy = 0 Pa).The phenomenon manifests the amplitude of the generated ultrafast spin currents, which can be manipulated by the introduction of vacancies and vacancyinduced FM.At a AE0.1 T in-plane external magnetic field, the polarization of the THz wave is not changed as shown in Figure 4e, which is attributed to the AFM property of NiO thin films.

Dependences of THz Amplitudes on Sample Azimuth
To further investigate the effect of the vacancies on the nonlinear optical effect in the NiO thin films, we studied the dependences of the THz amplitudes of different samples on the sample azimuth angle θ.When the samples are rotated through θ around the z-axis, the amplitude of the THz emission becomes the minimum at θ % 120°and then the polarity is inversed at θ % 180°for sample 1 (T sub = RT and P oxy = 0 Pa).However, the amplitudes of the THz emission are the minimum at θ % 180,°and then the polarity is inversed at θ % 60°for sample 5 (T sub = 400 °C and P oxy = 1.2 Pa) (seen Figure S9, Supporting Information).As shown in Figure 5a-g, the relationships between the peak-topeak values of the THz pulses and sample azimuth angle θ are summarized for the seven samples, respectively.The figures clearly show that as the substrate temperature increases, the symmetry changes from onefold to threefold.Generally, in a special crystal structure, the NiO single crystal has four T-domain states, each of which has three S-domain states, resulting in a total of 12 domain states. [53]The spin current generation from the NiO layer can be described by the coherent second-order nonlinear optical response as follows [34] M where A and B are coefficients related to χ is the nonlinear susceptibility tensor related to the AFM exchange coupling constant and domain distribution, [15,55] and η is connected with the ratio of contribution from the spin domain in the AFM oxide NiO.It is worth noting that Equation (1) only considers the nonlinear optical response in defect-free AFM NiO thin film.However, in this study, the contribution of ferromagnetism in the defective NiO thin films to spin current generation needs to be taken into account.In ferromagnetic materials, the generation of ultrafast spin current can be attributed to the spin Seebeck effect and spin Hall effect in the presence of an external magnetic field. [18,56]In zero external magnetic field, the correlation between sample azimuth angle and the excitation of ultrafast spin currents composed of ferromagnetic thin films can be expressed as Csinð2θÞ, which is closely related to the nonlinear optical effect in ferromagnetic thin films.The coefficient C is also concerned with χ ð2ÞMEE xyz . [57,58]ence, the spin current generation from the defective NiO thin films with weak ferromagnetic behaviors can be described by The THz amplitude can be expressed as x .We fit the THz amplitudes as a function of θ by using Equation (2) (solid lines in Figure 5a-g).Different fitting values of A, B and C for every sample are shown in Figure 5h, indicating that the vacancies of the NiO films have not only an effect on the AFM exchange coupling constant and 12 domains distribution of the NiO crystal, and also the vacancies-induced ferromagnetism in NiO thin film significantly contributes to the generation of spin current.

The Role of Vacancy-Induced Ferromagnetism
Given all the above results, we can conclude that the vacanciesinduced ferromagnetism can manipulate the spin current generated from defective NiO thin films.The opposite spins of Ni 2þ ions align antiferromagnetically in two adjacent {111} planes below T N , as shown in Figure 6, the defect-free NiO crystal has zero magnetic moments.Moreover, the NiO crystal has two exchange interaction constants, namely, the nearest neighbor (nn) exchange coupling constant J 1 and the next nearest neighbor (nnn) coupling constant J 2 . [44]Due to the existence of nickel vacancies in the NiO lattice, as shown in Figure 6a, the balances between up spin and down spin are broken, which make the NiO thin film exhibits ferromagnetism.
According to the XPS and PAS analysis, nickel vacancy increases from sample 2 (T sub = RT, P oxy = 1.2 Pa) to sample 5 (T sub = 400 °C, P oxy = 1.2 Pa), which results in strong ferromagnetism.J s increase with ferromagnetism of NiO thin film, except for the NiO thin film under the deposition condition of T sub = RT and 0 Pa.J s of sample 1 with the highest vacancy cluster increased abnormally.For samples 6 (T sub = 500 °C, P oxy = 1.2 Pa) and sample 7 (T sub = 600 °C, P oxy = 1.2 Pa), the ferromagnetism weak due to the reduction of nickel vacancies of the NiO thin films at high temperature, which decreases J s .Moreover, as the growth temperature decreases, the oxygen (nickel) vacancy increases (decreases), and the cubic structure of NiO is transformed into a tetragonal structure under the strain, as shown in Figure 6b.For the relationship between the THz amplitudes and sample azimuth angle θ, coefficients A and B mainly focus on the coherent effect of the 12 domains and the change of AFM exchange coupling constant of NiO, [55] and coefficient C represents the contribution of ferromagnetism.As seen in Figure 5h, coefficient A and C is the largest in sample 5 (T sub = 400 °C, P oxy = 1.2 Pa), which indicates that the 12 domains in the NiO thin film are coherently superimposed on χ ð2ÞMEE xyz and ferromagnetism-induced spin current make the most significant contribution, consistent with the static magnetic characterization.As the substrate temperature decreases, the oxygen vacancies gradually increase.When T sub = 300 °C and P oxy = 1.2 Pa (sample 4), coefficient B increases greatly, and coefficient A and C decrease, which illustrates that the single spin domain starts to play a dominant role in NiO thin film, which prompt the symmetry conversion from threefold to onefold.At lower substrate temperatures, the increase of oxygen vacancies leads to the formation of more vacancy clusters, and the ferromagnetism of NiO thin films is weak, which causes the coefficient C to tend to 0. Furthermore, in sample 1 (T sub = RT, P oxy = 0 Pa), the coefficient A is slightly larger than that of sample 2, which illustrates that in addition to the effect of single domains, the multidomain structure in sample 1 also acts on the χ ð2ÞMEE xyz .This demonstrates the formation of more vacancy clusters may influence the domain distribution and AFM exchange coupling constant in NiO, which leads to the generation of a larger spin current in sample 1. [55] At T sub = 500 and 600 °C, the decreasing nickel vacancies in NiO thin film have an effect on the distribution of domains, which results in the decreasing spin current J s .The above discussion illustrates that vacancy-induced ferromagnetism plays a crucial role in the ultrafast spin current generation of the NiO thin film.

Conclusion
To summarize, we introduce vacancies in NiO thin films by deposition under different T sub and P oxy .The presence of V O affects the crystal structure and optical energy gap of NiO thin films, while the introduction of V Ni induces weak FM in the antiferromagnetic oxide NiO.Furthermore, we measure THz emission spectra from heterostructures consisting of different NiO thin films and Pt layers at zero external magnetic fields.
The THz emission differs markedly between samples.It can be attributed to the modulation role of vacancies in the generation of spin current in the NiO thin films at different growth conditions.This effect is mainly reflected in vacancy-induced FM and spin domain distributions in defective NiO thin films.Our results open a new pathway for controlling the spin current in the AFM materials.

Experimental Section
Sample Fabrication: A series of nickel oxide NiO thin films of 60 nm thickness were fabricated on double-polished (111)-oriented MgO substrates by pulsed laser deposition (PLD) at different substrate temperatures and oxygen pressure (see Table 1 for detail).The base pressure of the deposition chamber was 5 Â 10 À6 Pa.The deposition energy fluency and repetition of the KrF excimer laser pulses (248 nm) were kept at 2.4 J cm À2 and 5 Hz, respectively.Thereafter, metal Pt thin films of 3 nm thickness were grown on the NiO thin films by dc magnetron sputtering at room temperature.The sputtering power and Ar pressure during the deposition were 10 W and 0.3 Pa, respectively.During the fabrication process, the NiO/Pt heterostructures for the THz emission experiment were kept as clean as possible.
For studying the antiferromagnetic nature of samples 1, 2, and 5, we fabricated the Co/NiO heterostructures to measure magnetic hysteresis loops by VSM.A Co ferromagnetic layer of 5 nm thickness was grown on NiO thin film by dc magnetron sputtering.The sputtering power and the Ar pressure during deposition were 10 W and 0.3 Pa, respectively.Then, NiO thin films of 20 nm thickness were grown on the Co layer using the growth conditions of samples 1, 2, and 5 as shown in Table 1.
Sample Characterization: The magnetic properties were measured by a superconducting quantum interference vibrating sample magnetometer (SQUID-VSM, Quantum Design, MPMS-3) with a field of up to 3 T at room temperature.X-ray diffractions (XRD, Bruker D8) of NiO thin films were measured using Cu Kα1 radiation with λ = 1.5406Å.The Raman spectra were measured by using a microspectroscopic Raman setup (wavelength of 532 nm, InVia REFLEX, Renishaw).The surface roughness and surface magnetic response were characterized by atomic force microscopy (AFM, Bruker Multimode 8).The chemical composition and oxide state of NiO thin films were scrutinized by X-ray photoelectron spectroscopy (XPS, Thermo Kalpha).The nanostructure characterization was evaluated by a spherical aberration-corrected transmission electron microscope (STEM, FEI Theims Z).The distribution of elements was assessed through an electron energy loss spectroscopy (EELS) device, which was attached to STEM.The positron annihilation spectroscopy (PAS) measurements were performed in the air using an ORTEC "fast-fast" spectrometer (DPLS 3000) at room temperature.The resolution of the spectrometer was found to be 187ps.
THz Emission Spectroscopy: Under the conditions of room temperature and humidity of 5%, a Ti: sapphire laser oscillator with a central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1 kHz was employed for the THz emission experiments.The fs laser beam was split into two parts for the generation and detection of the THz wave.The emitted THz wave from the samples was focused on a ZnTe crystal using four parabolic mirrors for electro-optical sampling.The details of the measurement system are shown in Figure S6, Supporting Information.Unless otherwise stated, the laser pump fluence is fixed at 1 mJ cm 2 , and the THz emission spectra were measured at zero external magnetic fields and room temperature.

Figure 1 .
Figure 1.Magnetic properties and microstructure characterizations of the NiO thin films grown under different conditions.a) Magnetization curves of NiO thin films grown by different deposition conditions.The inset is the magnetization curves without subtraction of the MgO substrate signal; b) Magnetization curves of bare Co thin film and Co/NiO bilayers.The inset is the coercivities extracted from magnetization curves.The NiO thin film is grown at T sub = RT and 400 °C, the pressure conditions are P oxy = 0 and 1.2 Pa, respectively; c, d) The XRD patterns and Raman spectrums of the NiO thin films, respectively; e) The ratio of I 2LO to I LO extracted from the Raman spectra; and f ) The out-of-plane lattice constants and strains of NiO thin films extracted from the XRD patterns.The inset black five-pointed star indicates the lattice constant of bulk NiO crystal.

τ 1 =
0.100 AE 0.003 ns (I 1 = 6.5%) and τ 2 = 0.205 AE 0.000 ns (I 2 = 93.5%).The fitting lifetimes indicate that there are few nickel vacancies (V Ni ), and its lattice defects are mainly vacancy clusters caused by oxygen vacancies (V o ) in sample 1.However, in sample 5, the lifetimes and their corresponding intensities are τ 1 = 0.137 AE 0.006 ns (I 1 = 24.7%)and τ 2 = 0.228 AE 0.001 ns (I 2 = 75.3%).The monovacancies V Ni increase evidently, and the vacancy clusters decrease in NiO thin film grown by T sub = 400 °C and P oxy = 1.2 Pa condition.The results of the XPS and PAS analysis illustrate that vacancy clusters caused by the oxygen vacancies V o in NiO thin films are the main defect types under RT-grown conditions and low oxygen pressure, and these vacancy clusters have a significant impact on the lattice constant and optical energy gap of NiO thin films.As T sub increases, V o gradually decreases and is replaced by V Ni .Up to T sub = 400 °C, V Ni dominates and vacancy clusters are also reduced in the NiO thin film.The introduction of the V Ni breaks

Figure 2 .
Figure 2. TEM patterns of NiO thin film (T sub = 400 °C, P oxy = 1.2 Pa).a) HRTEM and b) SAED patterns of sample 5, respectively.The red circle indicates the SAED region; c) Typical STEM image of a vacancy core in NiO thin film.The vacancy core is indicated by the label ⊥ in red; and d) O K-edge EELS at vacancy core and in the bulk region.

Figure 3 .
Figure 3. Vacancies in NiO thin films a,c) XPS spectrums of Ni 2p state and O 1s state of NiO thin film grown under T sub = RT and P oxy = 0 Pa, respectively.The black dotted lines represent experimental data.The solid lines of various colors are the fit to experimental data.The shaded area represents the relative contents of each element.b,d) Relative content ratios of Ni/O l and O v /O l in NiO samples, respectively.e,f ) PAS of sample 1 (T sub = RT and P oxy = 0 Pa) and sample 5 (T sub = 400 °C and P oxy = 1.2 Pa).

Figure 4 .
Figure 4. Experimental setup and THz wave emission spectrums from NiO/Pt heterostructures.a) Schematic of the THz emission spectroscopy setup.The rotation azimuth angle of the sample is denoted by θ.The coordinate (x, y, z) is adapted to the laboratory frame; b) THz emission spectra of NiO/Pt bilayers.Among them, the NiO thin films are grown by different deposition processes; c,d) Fourier-transformed spectra and THz amplitudes extracted from (a); and (e) Dependence of the THz emission on the external magnetic field.The NiO thin films are grown at T sub = 400 °C and P oxy = 1.2 Pa, and also at T sub = RT and P oxy = 0 Pa.

Figure 5 .
Figure 5. Impact of sample azimuth angle on THz amplitudes.a-g) THz amplitudes of NiO/Pt samples as a function of sample azimuth angle θ.The solid lines are the fittings of experimental data in (a-g) by using Equation (2); and h) Fitting coefficients extracted from different samples.

Figure 6 .
Figure 6.Crystal structures of NiO thin film.(a,b) Crystallographic and magnetic structures of V Ni -dominant and vacancy cluster-dominant NiO thin film, respectively.The different sizes of (a) and (b) represent changes in the lattice constant in the presence of vacancies.The blue triangle box represents the {111} crystallographic planes of NiO crystal.The arrow represents the spin direction of Ni 2þ ions.

Table 1 .
Growth conditions of NiO thin films.