Ubiquitous first-order transitions and site-selective vanishing of the magnetic moment in giant magnetocaloric MnFeSiP alloys detected by 55Mn NMR

We report on a study on a representative set of FeP-based MnFePSi samples by means of Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the 3f sites than at the 3g sites on approaching TC, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the 3f spin density instead of enhanced spin fluctuations. The temperature-driven 3f spin extinction is demonstrated to evolve into a truly nonmagnetic state of the 3f Mn ions well above TC, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides “normal” 3f Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at 3f sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average 3f moment previously reported at large Mn concentrations.