  An emerging paradigm in modern electronics is that of CMOS + $\sf X$
requiring the integration of standard CMOS technology with novel materials and
technologies denoted by $\sf X$. In this context, a crucial challenge is to
develop accurate circuit models for $\sf X$ 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 + $\sf X$ systems, where $\sf X$ 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 go higher in the computing stack, 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.
