# Mixed magnetism, nanoscale electronic segregation and ubiquitous first order transitions in giant magnetocaloric MnFeSiP alloys detected by $^{55}$Mn NMR

## TL;DR

This study uses $^{55}$Mn NMR to investigate magnetic transitions, phase separation, and mixed magnetism in MnFePSi alloys, revealing the first-order nature of transitions and the origin of magnetism variations.

## Contribution

It provides detailed NMR evidence for the first-order magnetic transition and mixed magnetism in MnFePSi alloys, clarifying the role of spin density and phase separation.

## Key findings

- First-order magnetic transition confirmed by NMR measurements.
- Phase separation of ferromagnetic and fluctuating Mn regions observed.
- Mixed magnetism driven by vanishing spin density at specific sites.

## Abstract

We report on a study on a representative set of Fe$_{2}$P-based MnFePSi samples by means of $^{55}$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 on approaching $T_C$, characteristic of the mixed magnetism of these materials, is demonstrated to be driven by a vanishing spin density rather than enhanced spin fluctuations at the $3f$ site. An upper limit of 0.03~$\mu_B$ is set for the fluctuating Mn moment at the $3f$ site by the direct detection of a $^{55}$Mn NMR resonance peak in the Mn-rich samples above $T_C$, showing nearly temperature-independent frequency shifts. A sharper secondary peak observed at the same compositions reveals however the disproportionation of a significant $3f$-Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character across the transition, down to the lowest available temperatures. Such a diamagnetic fraction qualitatively accounts for the reduced average $3f$ moment previously reported at large Mn concentrations.