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@PHDTHESIS{Biniskos:851282,
      author       = {Biniskos, Nikolaos},
      title        = {{I}nelastic neutron scattering on magnetocaloric compounds},
      volume       = {186},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2018-04977},
      isbn         = {978-3-95806-362-4},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {III, 92 S.},
      year         = {2018},
      note         = {RWTH Aachen, Diss., 2018},
      abstract     = {The search for more efficient use of energy has been
                      leading to a growing interest in the research field of
                      magnetocaloric materials. The magnetocaloric effect (MCE)
                      refers to a change of temperature or entropy of a magnetic
                      material exposed to a change of magnetic field. The MCE
                      requires the exchange of magnetic, lattice and/or electronic
                      entropy during an adiabatic (de-)magnetization process. 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. From the
                      Mn$_{5-x}$Fe$_{x}$Si$_{3}$ system, that exhibits a moderate
                      MCE at low magnetic fields, two materials in single crystal
                      form are under investigation: the ferromagnetic (FM)
                      compound MnFe$_{4}$Si$_{3}$ and the parent compound
                      Mn$_{5}$Si$_{3}$. The aim of this thesis is to investigate
                      the spin and lattice dynamics and their couplings in these
                      compounds that are up to nowadays unexplored with inelastic
                      neutron scattering (INS)and inelastic X-ray scattering (IXS)
                      measurements. Such studies might help to point out
                      ingredients that may favour large MCE, such as phonon-magnon
                      interaction, effect of spin fluctuations etc. The FM
                      compound MnFe$_{4}$Si$_{3}$ is a promising candidate for
                      applications since it exhibits a moderate MCE near room
                      temperature. Its magnetic excitation spectrum has been
                      investigated by means of polarized and unpolarized INS.
                      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 $\textit{q}$-dependent
                      linewidths in the paramagnetic (PM) state. The correlation
                      lengths indicate a short-range order, while the average
                      linewidth is of the order of k$_{B}$T$_{C}$ pointing to a
                      behavior typical of many ferromagnets. In addition, the in-
                      and out-of-plane spin-fluctuations are found to be isotropic
                      around T$_{C}$ and can be suppressed by a magnetic field of
                      2 T. In order to study the spin and lattice dynamics and
                      their interactions in MnFe$_{4}$Si$_{3}$, a combination of
                      IXS and INS (polarized and unpolarized) measurements was
                      performed. A remarkable feature evidenced by this
                      combination of measurements is that along the (h00)
                      direction the magnon branch close to the zone boundary falls
                      exactly on the two transverse acoustic (TA) phonons.
                      Furthermore, a large difference of intensities in the two
                      non-spin-flip (NSF) channels was observed for one TA phonon
                      mode. This difference of intensity between the two NSF
                      channels can be attributed to the nuclear-magnetic
                      interference term. The parent compound Mn$_{5}$Si$_{3}$ has
                      been extensively characterized as a model system by many
                      groups in the past decades by magnetometry, X-ray and
                      neutron diffraction on powder and single crystal samples.
                      Previous studies indicate the existence of two stable
                      antiferromagnetic (AF) phases at about 100K (AF2)and 66K
                      (AF1), respectively. AF2 and AF1 transitions are of
                      first-order and the inverse MCE (the sample heats up when an
                      external magnetic field is applied adiabatically) is
                      associated with the AF1-AF2 phase transition. INS
                      experiments revealed that AF1 is characterized by sharp
                      spin-waves, but AF2 is characterized by a mixed signal that
                      resembles the one of the AF1 and PM state, indicating strong
                      spin-fluctuations coexisting with spin-waves. Moreover, the
                      application of a magnetic field in the AF1 phase induces
                      spin-fluctuations, which points to their importance for the
                      inverse MCE in Mn$_{5}$Si$_{3}$.},
      cin          = {JCNS-2 / PGI-4 / JARA-FIT / JCNS-ILL},
      cid          = {I:(DE-Juel1)JCNS-2-20110106 / I:(DE-Juel1)PGI-4-20110106 /
                      $I:(DE-82)080009_20140620$ / I:(DE-Juel1)JCNS-ILL-20110128},
      pnm          = {144 - Controlling Collective States (POF3-144) / 524 -
                      Controlling Collective States (POF3-524) / 6212 - Quantum
                      Condensed Matter: Magnetism, Superconductivity (POF3-621) /
                      6213 - Materials and Processes for Energy and Transport
                      Technologies (POF3-621) / 6G4 - Jülich Centre for Neutron
                      Research (JCNS) (POF3-623)},
      pid          = {G:(DE-HGF)POF3-144 / G:(DE-HGF)POF3-524 /
                      G:(DE-HGF)POF3-6212 / G:(DE-HGF)POF3-6213 /
                      G:(DE-HGF)POF3-6G4},
      experiment   = {EXP:(DE-MLZ)PUMA-20140101 /
                      EXP:(DE-Juel1)ILL-IN12-20150421},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/851282},
}