An emerging paradigm in modern electronics is that of CMOS+ \({\mathsf{X}}\) requiring the integration of standard CMOS technology with novel materials and technologies denoted by \({\mathsf{X}}\) . In this context, a crucial challenge is to develop accurate circuit models for \({\mathsf{X}}\) that are compatible with standard models for CMOS-based circuits and systems. In this perspective, we present physics-based, experimentally benchmarked modular circuit models that can be used to evaluate a class of CMOS+ \({\mathsf{X}}\) systems, where \({\mathsf{X}}\) denotes magnetic and spintronic materials and phenomena. This class of materials is particularly challenging because they go beyond conventional charge-based phenomena and involve the spin degree of freedom which involves non-trivial quantum effects. Starting from density matrices—the central quantity in quantum transport—using well-defined approximations, it is possible to obtain spin-circuits that generalize ordinary circuit theory to 4-component currents and voltages (1 for charge and 3 for spin). With step-by-step examples that progressively become more complex, we illustrate how the spin-circuit approach can be used to start from the physics of magnetism and spintronics to enable accurate system-level evaluations. We believe the core approach can be extended to include other quantum degrees of freedom like valley and pseudospins starting from corresponding density matrices.
