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In current-perpendicular-to-plane magnetoresistance, the resistance difference between parallel and antiparallel ferromagnetic layers relies on spin-dependent transmission and retention of spin information. Here, an electron traversing the stack must pass a platinum layer between the two Co films. Strong spin-orbit coupling and associated spin relaxation in Pt, together with spin-memory loss at interfaces, can reduce the polarization reaching the second Co layer and weaken giant magnetoresistance. This is more relevant to the stated perpendicular current than an in-plane shunting explanation.

First, redesign the spacer path so a low-spin-relaxation conductor such as Cu directly separates the two ferromagnets, while retaining perpendicular anisotropy through other interfaces or a different layer sequence. Minimize unnecessary Pt traversal and improve interface sharpness. Merely thinning the Cu layer is not sufficient if it reintroduces magnetic coupling or leaves the Pt spin-relaxation bottleneck unchanged.

Second, replace the metallic spin-transport section with a suitable thin insulating tunnel barrier, for example an engineered MgO-based magnetic tunnel junction, and choose compatible magnetic electrodes. Spin-selective tunnelling can give a much larger resistance contrast, but pinholes, disorder and a poorly matched barrier defeat it. Retaining distinct switching fields and a stable antiparallel configuration remains essential in either design.

The giant magnetoresistance mechanism is spin-dependent conduction through the two ferromagnets. Exchange splitting gives majority and minority $d$ bands different density of states values at the Fermi energy. Scattering into those bands, together with interface scattering, produces different resistances for the two spin channels. The relevant quantity is the spin-dependent transport/scattering rate, not simply a count of all available states.

For parallel layer magnetizations, one global spin channel encounters relatively weak scattering in both Co and NiFe, providing a low-resistance path. For antiparallel magnetizations, each global spin channel encounters one more strongly scattering ferromagnet, so both paths are comparatively resistive. The channels conduct in parallel while each crosses the layers in series. This is current-perpendicular-to-plane magnetoresistance, because the current crosses the interfaces.^[ref]

In the simplest two-current resistor model, with equal magnetic layers having channel resistances $a,b$ and spacer resistance $r$ per channel,Real Co/NiFe layers need not have equal parameters, but the same-sign spin asymmetries normally give $R_{AP}>R_P$. Their different magnetic coercivities allow the two configurations to be selected separately.