Spin-related transport in a polycrystalline NiCo2O4 film: Drastic current-induced change in resistivity-temperature characteristics via spin injection

Technical Summary: Spin-Related Transport in Polycrystalline NiCo₂O₄ Films

Problem Statement
Silicon-based spin-transport devices are promising for low-power next-generation electronics due to Si's long spin lifetime and CMOS compatibility. Realizing high-performance devices requires efficient spin injection, often achieved using ferromagnetic (FM) metal electrodes with insulating tunnel barriers to overcome conductivity mismatch. Conductive, half-metallic, and ferrimagnetic NiCo₂O₄ (NCO) is a candidate for such electrodes. However, while fully epitaxial NCO films exhibit metallic resistivity-temperature ($\rho-T$) characteristics and high saturation magnetization ($M_s$), NCO films grown on Si substrates are typically polycrystalline due to lattice mismatch. Polycrystalline NCO generally contains numerous defects, particularly near grain boundaries, leading to weaker double-exchange interactions, reduced $M_s$, and semiconducting $\rho-T$ behavior ($d\rho/dT < 0$). Conventional transport models, such as variable range hopping (VRH) and nearest neighbor hopping (NNH), treat the sample as homogeneous and fail to capture the spatially inhomogeneous defect distributions intrinsic to polycrystalline films. Furthermore, the mechanism behind current-induced changes in transport properties in these materials remains unclear.

Methodology
The authors investigated a 36-nm-thick polycrystalline NCO film deposited on a 1-nm amorphous MgAl₂O₄ (MAO) buffer layer on a Si(001) substrate. The film was characterized as having a single spinel phase with an $M_s$ of $1.5\mu_B$/f.u. (approximately 75% of the theoretical value), indicating the presence of defects. A Hall-bar device was fabricated to measure electrical transport properties.

The study systematically analyzed the temperature dependence of resistivity ($\rho-T$) from 4 K to 290 K under various in-plane currents ranging from 100 nA to 1 mA (current densities of 3.1 A/cm² to 31 kA/cm²). Additionally, current-voltage ($I-V$) characteristics and magnetoresistance (MR) measurements under in-plane magnetic fields ($H_{\parallel}$) were performed to support the transport model.

Key Contributions and Proposed Model
The primary contribution of this work is the proposal of a spin-related transport model that extends conventional hopping models by incorporating the spatial, current, and temperature dependencies of the degree of spin alignment.

The model posits that polycrystalline NCO consists of two distinct regions:

1. Grain bodies: Nearly defect-free regions with a higher Curie temperature ($T_{C,b}$) and well-aligned spins at low temperatures.
2. Interfacial regions (grain boundaries): Defect-rich regions with a lower Curie temperature ($T_{C,int}$) and a lower degree of spin alignment.

The model suggests that spin-polarized electrons are injected from the grain bodies into the interfacial regions. This spin injection enhances the degree of spin alignment in the interfacial regions, effectively increasing their local Curie temperature ($T_{C,int}$) and reducing the energy barriers for electron transfer (strengthening double-exchange interactions). The transport mechanism transitions between VRH, NNH, and metallic conduction depending on the relative magnitudes of temperature, current, and the local Curie temperatures.

Results
The experimental results demonstrate a drastic current-induced evolution in the $\rho-T$ characteristics:

• At 100 nA: The film exhibits entirely semiconducting behavior ($d\rho/dT < 0$) across the 4–290 K range, well-described by VRH and NNH models.
• At 10 µA: The curve becomes non-monotonic, showing a peak at 15 K, with a downward branch at lower temperatures.
• At 100 µA and 1 mA: The resistivity decreases significantly across the entire temperature range. The peak shifts to higher temperatures (98 K and 164 K, respectively), and the curve approaches metallic behavior ($d\rho/dT > 0$) over a wide temperature range. At 1 mA, the room-temperature resistivity drops to 0.7 mΩ cm.

The $I-V$ characteristics reveal a multi-step structure, with a critical current ($I_c$) marking the onset of a drastic resistivity decrease. This behavior is attributed to the reduction of the effective width of the interfacial regions ($w_{int}$) as the spin alignment is enhanced by the injected spin current. Magnetoresistance measurements show a sign reversal from negative to positive as the current increases, supporting the transition from a hopping-dominated regime to one where metallic transport and specific spin-alignment configurations in the interfacial regions dominate.

Significance
The authors claim that these findings provide direct evidence of spin-polarized electrons existing within the grain bodies of polycrystalline NCO, highlighting the material's potential as an efficient spin source for Si-based devices. The study establishes a physical basis for understanding spin-related transport in polycrystalline double-exchange systems by demonstrating that the interplay between spatially inhomogeneous defect distributions and current-induced spin alignment can fundamentally alter transport mechanisms. The proposed model comprehensively explains the transition from semiconducting to metallic behavior without requiring a change in the material's intrinsic composition, attributing the evolution instead to the enhancement of spin alignment in defect-rich regions via spin injection.