M. Valldor

Publications (5)0 Total impact

• Article: Anisotropic Monopole Heat Transport in the Spin-Ice Compound Dy$_2$Ti$_2$O$_7$

  G. Kolland, M. Valldor, M. Hiertz, J. Frielingsdorf, T. Lorenz
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  ABSTRACT: We report a study of the thermal conductivity $\kappa$ of the spin-ice material Dy$_2$Ti$_2$O$_7$. From the anisotropic magnetic-field dependence of \kappa$ and by additional measurements on the phononic reference compounds Y$_2$Ti$_2$O$_7$ and DyYTi$_2$O$_7$, we are able to separate the phononic and the magnetic contributions to the total heat transport, i.e. $\kappa_{ph}$ and $\kappa_{mag}$, respectively, which both depend on the magnetic field. The field dependent $\kappa_{ph}$ arises from lattice distortions due to magnetic-field induced torques on the non-collinear magnetic moments of the Dy ions. For $\kappa_{mag}$, we observe a highly anisotropic magnetic-field dependence, which correlates with the corresponding magnetization data reflecting the different magnetic-field induced spin-ice ground states. The magnitude of $\kappa_{mag}$ increases with the degree of the ground-state degeneracy. This anisotropic field dependence as well as various hysteresis effects suggest that $\kappa_{mag}$ is essentially determined by the mobility of the magnetic monopole excitations in spin ice.
  02/2013;
• Article: Thermal Conductivity and Specific Heat of the Spin-Ice Compound Dy$_2$Ti$_2$O$_7$: Experimental Evidence for Monopole Heat Transport

  G. Kolland, O. Breunig, M. Valldor, M. Hiertz, J. Frielingsdorf, T. Lorenz
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  ABSTRACT: Elementary excitations in the spin-ice compound Dy$_2$Ti$_2$O$_7$ can be described as magnetic monopoles propagating independently within the pyrochlore lattice formed by magnetic Dy ions. We studied the magnetic-field dependence of the thermal conductivity {\kappa}(B) for B || [001] and observe clear evidence for magnetic heat transport originating from the monopole excitations. The magnetic contribution {\kappa}_{mag} is strongly field-dependent and correlates with the magnetization M(B). The diffusion coefficient obtained from the ratio of {\kappa}_{mag} and the magnetic specific heat is strongly enhanced below 1 K indicating a high mobility of the monopole excitations in the spin-ice state.
  05/2012;
• Source

  Available from: ArXiv

  Article: Magnetostrictive Neel ordering of the spin-5/2 ladder compound BaMn2O3: distortion-induced lifting of geometrical frustration

  M. Valldor, O. Heyer, A. C. Komarek, A. Senyshyn, M. Braden, T. Lorenz
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  ABSTRACT: The crystal structure and the magnetism of BaMn$_2$O$_3$ have been studied by thermodynamic and by diffraction techniques using large single crystals and powders. BaMn$_2$O$_3$ is a realization of a $S = 5/2$ spin ladder as the magnetic interaction is dominant along 180$^\circ$ Mn-O-Mn bonds forming the legs and the rungs of a ladder. The temperature dependence of the magnetic susceptibility exhibits well-defined maxima for all directions proving the low-dimensional magnetic character in BaMn$_2$O$_3$. The susceptibility and powder neutron diffraction data, however, show that BaMn$_2$O$_3$ exhibits a transition to antiferromagnetic order at 184 K, in spite of a full frustration of the nearest-neighbor inter-ladder coupling in the orthorhombic high-temperature phase. This frustration is lifted by a remarkably strong monoclinic distortion which accompanies the magnetic transition.
  10/2010;
• Article: Crystal structure and properties of the manganese containing Swedenborgite YBaMn3AlO7

  M. Valldor, O. Breunig
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  ABSTRACT: Small single crystals of the new Swedenborgite YBaMn3AlO7 were synthesized in a floating-zone image furnace. The crystal structure is well described using the hexagonal symmetry P63mc (a = 6.37081(4), c = 10.3873(1) Å) and contains a geometrically frustrated metal sublattice of (Mn3Al) including kagome layers and trigonal interlayer sites. Thermodynamic and magnetic measurements indicate that YBaMn3AlO7 undergoes a magnetic freezing close to 16 K, below which a spin-glass like state is observed. The glassy state is a result of geometric frustration and Mn–Al disorder.Graphical abstract
  Solid State Sciences. 13(5):831-836.
• Article: Magnetostrictive Néel ordering of the spin-5/2 ladder compound BaMn_ {2} O_ {3}: Distortion-induced lifting of geometrical frustration

  M. Valldor, O. Heyer, A. C. Komarek, A. Senyshyn, M. Braden, T. Lorenz
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  ABSTRACT: The crystal structure and the magnetism of BaMn2O3 have been studied by thermodynamic and diffraction techniques using large single crystals and powders. BaMn2O3 is a realization of an S= 5/2 spin ladder as the magnetic interaction is dominant along 180° Mn–O–Mn bonds forming the legs and the rungs of a ladder. The temperature dependence of the magnetic susceptibility exhibits well-defined maxima for all directions, proving the low-dimensional magnetic character in BaMn2O3. The susceptibility and neutron powder diffraction data, however, show that BaMn2O3 exhibits a transition to antiferromagnetic order at 184 K, in spite of a full frustration of the nearest-neighbor interladder coupling in the orthorhombic high-temperature phase. This frustration is lifted by a remarkably strong monoclinic distortion, which accompanies the magnetic transition.
  Phys. Rev. B. 83(2).