University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Wednesday, September 5th 2018
1:25 pm:
Speaker: Nikolaos Biniskos, Jülich Centre for Neutron Science JCNS
Subject: : Inelastic neutron scattering investigations in the series Mn5-xFexSi3

The magnetocaloric cooling process is based on the magnetocaloric effect (MCE) where entropy changes of a magnetic material in an applied magnetic field are tied to adiabatic changes in temperature. An entropy transfer between crystal lattice and the magnetic spin system has to take place. A large MCE at room temperature and low magnetic field for a material with abundant and environmentally friendly elements opens the way for magnetic cooling devices. The MCE potentially occurs in any magnetic ordering process and inelastic neutron scattering (INS) that microscopically probes the magnetization dynamics is a key tool to tackle the question of the ingredients that favor large MCE.

Mn5-xFexSi3 compounds are showing a moderate MCE (2 to 4 J/kg K depending on x) at low magnetic field change from 0T to 2T, which is promising for magnetic refrigeration applications [2]. The ferromagnetic compound MnFe4Si3 has a magnetic phase transition at about 300K. The magnetic excitation spectrum of the magnetocaloric compound MnFe4Si3 has been investigated by means of polarized and unpolarized INS on single crystals. Spectra were collected in the FM phase (TC ≈ 305 K), as well as in the paramagnetic state, in order to understand the nature of the magnetism in MnFe4Si3. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the q-dependent linewidths in the paramagnetic state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of kBTC pointing to a behavior typical of many ferromagnets. In addition, the in- and out-of-plane spin fluctuations are found to be isotropic around TC and can be suppressed by a magnetic field of 2 T [3].

The parent compound Mn5Si3 undergoes two antiferromagnetic transitions at TN1=66K (AF1) and TN2=99K (AF2). Experiments with unpolarized INS in the paramagnetic (PM) state and in the AF2 and AF1 phases revealed that AF1 is characterized by sharp spin-waves, but AF2 is characterized by a diffuse signal that resembles the one of the PM state, indicating strong spin fluctuations [4]. These fluctuations may play an essential role in the MCE.

[1] O. Tegus et al., Nature 415 (2002), 150.
[2] Songlin et al., J. Alloys Compounds, 334 (2002), 249–252.
[3] N. Biniskos et al.,. Phys. Rev. B 96, 104407 (2017).
[4] N. Biniskos et al.,. Phys. Rev. Lett. 120, 257205 (2018).

Faculty Host: Martin Greven

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