The other word to seach around is " Magnetic saliency"
For an SPM with a uniform rotor core, peak torque occurs when the stator field is precisely aligned with the quadrature (q) axis, ie 90 deg to the direct (d) axis.
For an IPM, with high saliency, the position of peak torque is directly dependant upon that saliency, and the position of peak torque is skewed towards to d axis. How far it is skewed depends upon both fixed geometrical effects, and dynamics ones such as reluctance and saturation.