Giant and Oscillatory Junction Magnetoresistance via RKKY-like Spin Coupling in Spin-Gapless Mn$_2$CoAl/SiO$_2$/p-Si Heterostructures

Here, we report spin-selective transport and exceptionally large positive junction magnetoresistance (JMR) in sputter-deposited Mn$_2$CoAl/native-SiO$_2$/p-Si heterostructures. Highly ordered inverse-Heusler Mn$_2$CoAl thin films with near-ideal XA chemical ordering (S$\approx$0.97) and a Curie temperature of $\sim$590 K are realized using magnetron sputtering process. The spin-gapless semiconducting nature of Mn$_2$CoAl is experimentally supported by a weakly temperature-dependent resistivity with a very small negative temperature coefficient of resistance ($\mathrm{TCR} \approx -4.2 \times 10^{-9}\,Ω\cdot \mathrm{m} \cdot \mathrm{K}^{-1}$) and a nonsaturating linear magnetoresistance over a wide range of magnetic fields and temperatures. A giant positive JMR of $\sim$825% at 10 K and $\sim$134% at room temperature is observed despite the presence of only a single ferromagnetic electrode. Systematic variation of the SiO$_2$ tunnel barrier thickness reveals a reproducible oscillatory sign reversal of the JMR accompanied by a monotonic decay in magnitude. This behavior reflects thickness-dependent modulation of spin-selective tunneling mediated by phase-coherent interfacial carriers. It can be described phenomenologically by an RKKY-like functional form without invoking conventional metallic exchange interactions. These results identify Mn$_2$CoAl/native-SiO$_2$/p-Si heterostructures as robust and scalable platforms for room-temperature spin-selective transport, with potential applications in semiconductor-compatible spin filters, magnetic field sensors, and reconfigurable spintronic logic elements.