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When a current loop is placed in a magnetic field it experiences a torque. It is given by

\(\vec{\tau}=\vec{M} \times \vec{B}\)

where, \(\vec{M}\) is the magnetic moment of the loop and \(\vec{B}\) is the magnetic field. 

or \(\quad \tau=M B \sin \theta\) where \(\theta\) is angle between \(M\) and \(B\) When \(\vec{M}\) and \(\vec{B}\) are parallel (i.e. \(\theta=0^{\circ}\) ) the equilibrium is stable and when they are antiparallel (i.e. \(\theta=\pi\) ) the equilibrium is unstable.