Magnetic torsion springs are capable of producing unique and useful torque-displacement responses that are not possible with elastic springs. Millimeter-scale magnetically-actuated robots, which are gaining increasing interest in biomedical applications, would benefit from the use of magnetic torsion springs. However, existing magnetic torsion springs are difficult to fabricate at that scale and can only produce sinusoid-like responses. Here we show that the magnets embedded in the links of a robot for actuation purposes can also be leveraged to produce torsion spring-like behavior. This Simultaneous Magnetic Actuation and Restoring Torque (SMART) spring design can enable switching or pop-up behaviour in millimeter-scale magnetically-actuated mechanisms. A novel analytical model, validated both numerically and experimentally, is used to design constant-stiffness and nonlinear bistable SMART springs. These springs are integrated into a novel 3.5 mm diameter magnetic robot manipulator.