Wolfgang Schmidt

Institut Laue-Langevin, Grenoble, Rhône-Alpes, France

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Publications (7)18.88 Total impact

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    ABSTRACT: We report on the first single crystal study of SrTm$_2$O$_4$. Magnetization measurements along the crystallographic axes of a nearly stoichiometric Sr$_{1.07(3)}$Tm$_{2.07(6)}$O$_{4.00(2)}$ sample show either a positive or a negative Curie-Weiss temperature indicative of a competition between antiferromagnetic and ferromagnetic couplings. The field-dependent magnetization suggests an effective Zeeman splitting of the high-level \emph{J}-multiplets above $\sim$8.3 T and that the paramagnetism may originate from only one of the two inequivalent Tm$^{3+}$ crystallographic sites. Our single-crystal polarized neutron scattering and powder unpolarized neutron diffraction data show no evidence for either long- or short-range magnetic order even down to $\sim$65 mK. We reveal two TmO$_6$ octahedral distortion modes, i.e., one distortion is stronger than the other especially at low temperatures, which is attributed to different crystal fields of the two inequivalent octahedra. Therefore, the compound SrTm$_2$O$_4$ is unique and different from its brethren in the family of frustrated Sr$RE_2$O$_4$ (\emph{RE} = rare earth) magnets. We propose that crystal field anisotropy may dominate over weak dipolar spin interactions in SrTm$_2$O$_4$, thereby leading to a virtually non-ordered magnetic state.
    06/2015; DOI:10.1039/C5TC01607J
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    ABSTRACT: Identifying the nature of magnetism, itinerant or localized, remains a major challenge in condensed-matter science. Purely localized moments appear only in magnetic insulators, whereas itinerant moments more or less co-exist with localized moments in metallic compounds such as the doped-cuprate or the iron-based superconductors, hampering a thorough understanding of the role of magnetism in phenomena like superconductivity or magnetoresistance. Here we distinguish two antiferromagnetic modulations with respective propagation wave vectors at Q± = (H ± 0.557(1), 0, L ± 0.150(1)) and QC = (H ± 0.564(1), 0, L), where (H, L) are allowed Miller indices, in an ErPd2Si2 single crystal by neutron scattering and establish their respective temperature- and field-dependent phase diagrams. The modulations can co-exist but also compete depending on temperature or applied field strength. They couple differently with the underlying lattice albeit with associated moments in a common direction. The Q± modulation may be attributed to localized 4f moments while the QC correlates well with itinerant conduction bands, supported by our transport studies. Hence, ErPd2Si2 represents a new model compound that displays clearly-separated itinerant and localized moments, substantiating early theoretical predictions and providing a unique platform allowing the study of itinerant electron behavior in a localized antiferromagnetic matrix.
    Scientific Reports 02/2015; 5:7968. DOI:10.1038/srep07968 · 5.58 Impact Factor
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    ABSTRACT: The N$\acute{\rm e}$el temperature of the new frustrated family of Sr\emph{RE}$_2$O$_4$ (\emph{RE} = rare earth) compounds is yet limited to $\sim$ 0.9 K, which more or less hampers a complete understanding of the relevant magnetic frustrations and spin interactions. Here we report on a new frustrated member to the family, SrTb$_2$O$_4$ with a record $T_{\rm N}$ = 4.28(2) K, and an experimental study of the magnetic interacting and frustrating mechanisms by polarized and unpolarized neutron scattering. The compound SrTb$_2$O$_4$ displays an incommensurate antiferromagnetic (AFM) order with a transverse wave vector \textbf{Q}$^{\rm 0.5 K}_{\rm AFM}$ = (0.5924(1), 0.0059(1), 0) albeit with partially-ordered moments, 1.92(6) $\mu_{\rm B}$ at 0.5 K, stemming from only one of the two inequivalent Tb sites mainly by virtue of their different octahedral distortions. The localized moments are confined to the \emph{bc} plane, 11.9(66)$^\circ$ away from the \emph{b} axis probably by single-ion anisotropy. We reveal that this AFM order is dominated mainly by dipole-dipole interactions and disclose that the octahedral distortion, nearest-neighbour (NN) ferromagnetic (FM) arrangement, different next NN FM and AFM configurations, and in-plane anisotropic spin correlations are vital to the magnetic structure and associated multiple frustrations. The discovery of the thus far highest AFM transition temperature renders SrTb$_2$O$_4$ a new friendly frustrated platform in the family for exploring the nature of magnetic interactions and frustrations.
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    ABSTRACT: Our single-crystal polarized neutron scattering at 65 mK and powder unpolarized neutron diffraction at 0.5 K show no evidence for a long-range magnetic order and even detect no sign of diffuse magnetic neutron scattering in single-crystal SrTm2O4. The data refinements reveal that the two TmO6 octahedral distortion modes are the same as those of the TbO6 octahedra in SrTb2O4, i.e., one distortion is stronger than the other one especially at low temperatures, which is attributed probably to different crystal electric fields for the two inequivalent octahedra. Consequently, we conclude that SrTm2O4 has no magnetic order, neither long-ranged nor short-ranged, even down to 65 mK. Therefore, SrTm2O4 is a different compound from its brethren in the new family of frustrated SrRE2O4 (RE = Gd, Tb, Dy, Ho, Er, and Yb) magnets. We propose that crystal field anisotropy may dominate over weak dipolar spin interactions in SrTm2O4, leading to a virtually nonmagnetic ordering state.
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    Frontiers in Physics 01/2014;
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    ABSTRACT: Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around T(N) the PMRV translates to negative, down to ~-10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices.
    Scientific Reports 10/2012; 2:750. DOI:10.1038/srep00750 · 5.58 Impact Factor
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    ABSTRACT: We report on the magnetic excitation spectrum in the normal state of the heavy-fermion superconductor CeCu92)Si(2) on approaching the quantum critical point (QCP). The magnetic response in the superconducting state is characterized by a transfer of spectral weight to energies above a spin excitation gap. In the normal state, a slowing-down of the quasielastic magnetic response is observed, which conforms to the scaling expected for a QCP of spin-density-wave type. This interpretation is substantiated by an analysis of specific heat data and the momentum dependence of the magnetic excitation spectrum. Our study represents the first direct observation of an almost critical slowing-down of the normal state magnetic response at a QCP when suppressing superconductivity. The results strongly imply that the coupling of Cooper pairs in CeCu(2)Si(2) is mediated by overdamped spin fluctuations.
    Physical Review Letters 06/2011; 106(24):246401. DOI:10.1103/PhysRevLett.106.246401 · 7.73 Impact Factor

Publication Stats

16 Citations
18.88 Total Impact Points

Institutions

  • 2012–2015
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 2011
    • Forschungszentrum Jülich
      • Jülich Centre for Neutron Science (JCNS)
      Düren, North Rhine-Westphalia, Germany