a) Polarisation profile of the MSP J0034−0534 at 150 MHz and 400 MHz. All the flux scales are in arbitrary units. b) RM spectra for PSR J0034−0534 from the LOFAR data: (i) RM spectrum before removing the instrumental response, centred at RM ≈ 0 rad m^-2; (ii) spectrum after subtracting the instrumental response from the data. The black arrow indicates the position of the expected RM for this pulsar, under the assumptions stated in Sect. 8.3.1.