In Electron Spin Resonance, what provides the energy for the transition? I recently performed an ESR experiment at M.Sc. level. The experiment manual says that the energy for the transition is provided by magnetic field oscillating at radio frequency. I am little confused by this. Shouldn't this be electromagnetic field (radio frequency em waves)? But then the sample is placed in an induction coil which is component of the tank circuit of a RF oscillator, so the filed inside the coil is magnetic field.
Can someone explain how this works and how is the energy provided for the transition?
 A: In this description of the experiment the radiofrequency is supplied externally from the B field that splits the states.

The application of the magnetic field then provides a magnetic potential energy which splits the spin states by an amount proportional to the magnetic field (Zeeman effect), and then radio frequency radiation of the appropriate frequency can cause a transition from one spin state to the other.

So the radio frequency supplies the transition energy.
A varying magnetic field, as in the induction coil you state,  generates a varying electric field and that is what an electromagnetic wave is, but one would need more details on the experimental setup, to see where the  magnet that provides the splitting field is.
A: An oscillating magnetic field always induces an oscillating electric field.  So, the "magnetic field oscillating at radio frequency" is just shorthand for a radiofrequency electromagnetic field: both electric and magnetic fields are present.
I believe the manual chooses to emphasize the magnetic field because it is the magnetic field that couples directly to the electron's (magnetic) spin.
