Nickel acetate (Ni(CH₃COO)₂) and Dihydrate zinc acetate (Zn(CH₃COO)₂·2H₂O) were acquired from Shanghai Zhanyun Chemical Co., Ltd. Anhydrous ethanol was provided by Sinopharm Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP) came from Sigma Aldrich Co., Ltd.

First, Zn(CH₃COO)₂ and Ni(CH₃COO)₂, with a molar ratio of 1:1 for Zn/Ni atoms, were separately added into glass bottles. Subsequently, 9 g of anhydrous ethanol and 1 g of PVP were added to prepare the electrospinning precursor solution. During the electrospinning process, a rotating drum served as the collector. Electrospinning was conducted using a voltage of 10 kV and a feed rate of 1 mL h^-1. The resulting nanofiber membrane was then subjected to calcination in a muffle furnace. The temperature was increased at a rate of 3 °C min^-1 until reaching 700°C, where it was held for 2 h. This process yielded a micro-nanostructure.

Finally, a low-temperature conductive silver glue was employed to attach conductive leads to an ITO-coated glass. The obtained micro/nanostructures was fixed onto the conductive glass to create the NiO/ZnO optoelectronic device. Additionally, single NiO and ZnO micro/nanostructures were also prepared as controls.

X-ray diffraction (XRD; Rigaku SmartLab) was used to analyze the type and phase composition of the samples. Scanning electron microscopy (SEM, Phenom Pro G6, Thermo Fisher Scientific) was employed to study the crystal morphology of the samples. The comprehensive physical property measurement system (PPMS-9; Quantum Design) with Electronic Transport Option (ETO) was used to investigate the variation of resistance with the magnetic field in the samples. Lasers with wavelengths of 450 nm (blue light) and 635 nm (red light) were used for illumination. The direction of the applied magnetic field was parallel to the direction of lighting and the direction of the current, as shown in Figure 1.

The morphology of the samples was analyzed by scanning electron microscopy (SEM) (Supplementary Figures S1, S2 and Figure 2A). In particular, the NiO/ZnO heterostructures exhibit an obvious network structure, indicating that the specific surface area is large, the defect density is high, and the ring current can be induced in the changing magnetic field (Figure 2A). X-ray diffraction (XRD) analysis (Figure 2B) shows diffraction peaks of ZnO at 31.66°, 34.36°, 36.16°, 47.46°, and 56.5°, corresponding to the (100), (002), (101), (102), and (110) crystal planes, respectively. Similarly, the XRD peaks of pure NiO are located at 36.82° and 42.9°, corresponding to the (111) and (200) crystal planes, confirming the cubic phase of NiO. No impurity peaks were observed in the XRD data of the sample, indicating successful preparation of NiO/ZnO heterostructures.

Doping can introduce defects that affect the magnetic properties of the materials. After doping antiferromagnetic NiO [Supplementary Figures S3A, B (Supporting Information)] with non-magnetic ZnO (Supplementary Figures S3A), the magnetization-temperature (M-T) curve (Figure 2C) shows a significant difference between the ZFC and FC curves, with a clear bifurcation between them. As the temperature decreases before the bifurcation point, the difference between the two curves becomes more pronounced, indicating that the composite micro-nanostructures exhibit a spin glass state. The interaction between NiO and ZnO alters the material’s magnetism (Supplementary Figure S4). Additionally, hysteresis loops of the material at different temperatures were measured (Figure 2D), revealing enhanced ferromagnetic properties at lower temperatures, accompanied by increased magnetization in zero magnetic field.

The resistance of NiO/ZnO heterostructures in varying magnetic fields was measured (Figure 3). The green dashed line represents the applied magnetic field. It was observed that applying or removing an external magnetic field resulted in a peak in the resistance. The magnetoresistance (MR), defined as the change in resistance relative to the resistance under zero magnetic field (R₀), is expressed as Equation 1: M R = R H − R 0 R 0 × 100 % (1)

This study investigated the maximum resistance variation in a magnetic field (R_H), with respect to the resistance under zero magnetic field (R₀). The variation of MR with time is shown in Figure 3, where the y-axis represents MR. The change in sample conductivity continued to decrease even after the magnetic field stopped increasing, possibly due to the long relaxation time of the spin glass state. The magnetic relaxation time for paramagnetic materials (the time from the start to the completion of magnetization) is almost negligible, whereas for spin glasses, it can range from minutes to hours and increases with decreasing temperature, consistent with the relationship between temperature and magnetism [38].

Resistance spikes tests in varying magnetic fields were conducted at room temperature for NiO, ZnO, and NiO/ZnO micro/nanostructures. Significant spikes were observed only in the NiO/ZnO heterostructures at room temperature (Figure 3A), indicating the formation of a heterostructure rather than a simple composite. The heterojunction structure likely extends the observable effect to higher temperatures due to longer relaxation times and easier excitation of electron-hole pairs brought about by the heterojunction structure [39]. Extended testing showed that the composite micro-nanostructures maintained a stable response over a longer period (Figure 3B), while ZnO and NiO exhibited no response under the corresponding magnetic field (Supplementary Figure S5).

Lowering the temperature revealed resistance spikes in the NiO sample at 200 K, 190 K, and 180 K, with MR values of 0.28%, 0.34%, and 1.8%, respectively (Figure 3C). In contrast, ZnO micro-nanostructures did not exhibit resistance spikes in a magnetic field at any temperature. However, the NiO/ZnO composite material still showed magnetoresistance spikes at 290 K and 280 K, with MR values of 5.2% and 10.7%, respectively (Figure 3D). These results align with those of NiO micro/nanostructures, indicating that resistance spikes increase as temperature decreases, albeit with reduced stability. The incorporation of ZnO extends the operational temperature range and enhances the magnetic response of the composite material due to its temperature-dependent magnetic properties. Notably, the composite exhibits room-temperature ferromagnetism with significant magnetic response, which becomes more pronounced at lower temperatures.

The impact of magnetic field orientation and strength was investigated to understand their effects on NiO/ZnO heterostructures. Tests were conducted in different magnetic field directions, revealing that applying a magnetic field in the opposite direction slightly increased resistance spikes (Figure 4A). Furthermore, varying magnetic field magnitudes were tested, showing that higher magnetic field strengths led to increased peak magnetoresistance spikes and broader peaks, indicating enhanced resistance spikes (Figure 4B).

Next, a 450 nm blue laser was applied to induce photoexcitation in the samples, particularly affecting NiO. Figure 4C shows that exposure to light increased resistance spikes from 0.28% without light to 0.53% with light. Interestingly, under light-induced spikes, the presence of a magnetic field exacerbated this effect.

Additionally, Figure 4D demonstrates sustained photoelectric spikes in the sample, with a conductivity change of approximately 10%. Even after continuous exposure to light for one hundred seconds, the sample’s photoconductivity remained stable and unsaturated, indicating a long lifetime of photo-generated charge carriers. This stability suggests that the composite material, likely due to its defects and oxygen vacancies, is affected by light-induced changes in internal spin polarization, a phenomenon widely reported in literature [40–42].

Control experiments conducted under red light (RL), blue light (BL) and in the absence of light (no light, NL) confirmed significant changes in NiO/ZnO heterostructures even without light, suggesting a competitive effect between the heterojunction structure and light exposure (Figure 4D). The influence of magnetic field on oxygen vacancies could also contribute to observed resistance spikes [43].

However, we performed Electron Paramagnetic Resonance (EPR) tests (Supplementary Figure S6) on the samples to detect oxygen vacancies. The results showed that only ZnO and NiO/ZnO micro/nanostructures exhibited characteristic signals of oxygen vacancies, while NiO sample did not have oxygen vacancies. Therefore, the intensified resistance spike under illumination is not attributed to the effect of oxygen vacancies. Compared to NiO, the photoelectric spike of NiO/ZnO heterostructures increased by approximately 34.7%. We attribute this enhanced spike to the coupling effect between the heterojunction and the magnetic field.

NiO/ZnO heterostructures exhibit a network structure that generates an induced current in a varying magnetic field (Supplementary Figure S7). Furthermore, the NiO/ZnO heterostructures form a built-in electric field, causing orderly movement of charges (Figure 5A). The induced current appears in the built-in electric field (Figure 5B), causing disturbances that lead to disordered movement of the previously orderly charges, increasing the probability of collision scattering among the charges, thereby causing resistance spikes.

At lower temperatures, resistance typically increases, whereas illumination tends to decrease resistance. Interestingly, both factors enhance resistance spikes, suggesting a dissociation between resistance spikes and absolute resistance values. From the perspective of the electron spin, the rearrangement of electron spin and magnetization directions within NiO/ZnO heterostructures occurs during changes in magnetic fields (Figure 5C). One-dimensional semiconductor nanowires and a spin glass state can prolong spin lifetimes [38, 44]. Lower temperatures enhance material magnetization, while monochromatic laser excitation polarizes electrons, increasing scattering rates. Electron scattering rates depend on the alignment between electron spins and material magnetization directions: when aligned, electron passage through the magnetic layer is facilitated (low impedance); when opposite, electron passage is hindered (high impedance). As electron spins realign with material magnetization post-magnetic field change, resistance returns to pre-magnetic field levels, marking the end of resistance spikes.

The synergistic effect of electron spins and heterojunctions in the magnetic field collectively causes the resistance spikes of the NiO/ZnO heterostructures. The above mechanisms challenge the conventional theory linking resistance spikes to absolute resistance values and emphasize the role of electron spin dynamics, particularly under spin glass states with response times potentially extending to hundreds of seconds.