Spin Fluctuations Drive the Inverse Magnetocaloric Effect in ${\mathrm{Mn}}_5{\mathrm{Si}}_3$

Inelastic neutron scattering measurements are performed on single crystals of the antiferromagnetic compound ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ in order to investigate the relation between the spin dynamics and the magnetothermodynamics properties. It is shown that, among the two stable antiferromagnetic phases of this compound, the high temperature one has an unusual magnetic excitation spectrum where propagative spin waves and diffuse spin fluctuations coexist. Moreover, it is evidenced that the inverse magnetocaloric effect of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$, the cooling by adiabatic magnetization, is associated with field induced spin fluctuations.

Figure 1

(a) Projection of the structure of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ in the $(a,b)$ plane of the orthorhombic cell according to single crystal neutron diffraction data [16, 17]. The triangles with continuous and dashed lines are located in different planes separated by $c/2$. The red symbols ⊗ and ⊙ in the AFM1 phase indicate magnetic moments pointing in and out of the page, respectively. (b) Temperature dependence of the purely magnetic (1, 2, 0) and (0, 3, 0) Bragg peak intensities. (c) Color-coded intensity plot of the INS data collected at a constant energy transfer of 5 meV as a function of $\mathbf{Q}=(Q_h,2,0)$ and temperature. Data were collected with an unpolarized neutron beam using the three-axis spectrometer 2T1. In the inelastic spectra, the background is subtracted and the measured intensity is corrected by the detailed balance factor.

Figure 2

(a)–(c) FlatCone measurements in the scattering plane $(b,c)$ taken with the three-axis spectrometer ThALES in the AFM1 (10 K) and AFM2 (80 K) phases and in the PM (120 K) state at a constant energy transfer of 3 meV. The “holes” in the spectra correspond to spurious scattering that has been masked during the data evaluation. (d) Raw data obtained with the three-axis spectrometer IN12 with an unpolarized neutron beam around $\mathbf{Q}=(Q_h,2,0)$ at a constant energy transfer of 3 meV. Lines in the AFM1 phase (50 K) and in the PM state (120 K) indicate fits with Gaussian and Lorentzian functions, respectively. The line in the AFM2 phase (80 K) is a guide for the eyes.

Figure 3

Inelastic spectra at a constant energy transfer of 3 meV around $\mathbf{Q}=(Q_h,2,0)$ obtained with polarized neutrons in the PM state (120 K) and in the AFM2 phase (80 K) with IN12. (a) In the PM state (120 K), the solid line corresponds to a Lorentzian function fit. (b) In the AFM2 phase (80 K), the solid lines represent overall fits and the dashed lines the individual signal from spin waves (SW) and spin fluctuations (Fluc.) (see the details in the Supplemental Material [23]). The horizontal dashed line indicates the background level.

Figure 4

(a) ($T\text{−}H$) phase diagram of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ for $\overrightarrow{H}\parallel c$ after Ref. [20]. The vertical dashed line indicates the temperature (50 K) where neutron scattering measurements were performed under a magnetic field. (b),(c) Field dependence of magnetic Bragg peaks (1, 2, 0) and (0, 3, 0) at 50 K. For (1, 2, 0), a 9 mm plexiglass attenuator was in place. (d) Inelastic spectra obtained at 50 K around $\mathbf{Q}=(Q_h,2,0)$ under a 0 and 10 T magnetic field for an energy transfer of 3 meV. The red line is a guide for the eyes, and the black line corresponds to fits with Gaussian functions. Data were obtained with an unpolarized neutron beam using IN12.