F Bert

Publications (55)108.27 Total impact

• Article: Quantum Tunneling in Half-Integer-Spin Kagome-Lattice Langasites

  A. Zorko, F. Bert, P. Mendels, A. Potočnik, A. Amato, C. Baines, K. Marty, P. Bordet, P. Lejay, E. Lhotel, V. Simonet, R. Ballou
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  ABSTRACT: Employing the muon spin relaxation technique we evidence temperature independent magnetic fluctuations persisting down to the lowest temperatures in the samarium-based ($J=5/2$) kagome-lattice Langasite. A detailed bulk-magnetization characterization and comparison to the neodymium-based ($J=9/2$) compound allow us to assign the persistent spin dynamics to a quantum tunneling process. This is facilitated by pairwise anisotropic magnetic interactions, leading to a universal scaling of the muon relaxation. Our study reveals a remarkable analogy between weakly interacting half-integer-spin rare-earth magnets and molecular nanomagnets.
  10/2012;
• Article: Exchange energies of Kapellasite from high-temperature series analysis of the kagome lattice J1-J2-Jd-Heisenberg model

  B. Bernu, C. Lhuillier, E. Kermarrec, F. Bert, P. Mendels
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  ABSTRACT: We present a method to build magnetic models for insulators based on high-temperature expansions (HTE) by fitting both the magnetic susceptibility and the low temperature specific heat data. It is applied to the frustrated magnet Kapellasite (Cu3Zn(OH)6Cl2) with the J1-J2-Jd-Heisenberg model on the kagome lattice. Experimental data are reproduced with a set of competing exchange energies centered at J1 = -12K, J2= -4K and Jd=15.6K, where Jd is the third neighbor exchange energy across the hexagon. Strong constrains between these exchange energies are established. These values confirm the results of B. F\"ak et al. (Phys. Rev. Lett., 109, 037208 (2012)) regarding the location of Kapellasite in the "cuboc2" phase of the Heisenberg model. The quality and limits of this modeling are discussed.
  10/2012;
• Article: Singlet Ground State of the Quantum Antiferromagnet Ba3CuSb2O9

  J. A. Quilliam, F. Bert, E. Kermarrec, C. Payen, C. Guillot-Deudon, P. Bonville, C. Baines, H. Luetkens, P. Mendels
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  ABSTRACT: We present local probe results on the honeycomb lattice antiferromagnet Ba3CuSb2O9. Muon spin relaxation measurements in zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.
  07/2012;
• Article: Kapellasite: a kagome quantum spin liquid with competing interactions.

  B Fåk, E Kermarrec, L Messio, B Bernu, C Lhuillier, F Bert, P Mendels, B Koteswararao, F Bouquet, J Ollivier, A D Hillier, A Amato, R H Colman, A S Wills
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  ABSTRACT: Magnetic susceptibility, NMR, muon spin relaxation, and inelastic neutron scattering measurements show that kapellasite, Cu3Zn(OH)6Cl2, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphic with herbertsmithite, is a gapless spin liquid showing unusual dynamic short-range correlations of noncoplanar cuboc2 type which persist down to 20 mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to bulk susceptibility data and possesses competing exchange interactions. The magnetic specific heat calculated from these exchange couplings is in good agreement with experiment. The temperature dependence of the magnetic structure factor and the muon relaxation rate are calculated in a Schwinger-boson approach and compared to experimental results.
  Physical Review Letters 07/2012; 109(3):037208. · 7.37 Impact Factor
• Article: Magnetic behavior of Ba3Cu3Sc4O12.

  B Koteswararao, A V Mahajan, F Bert, P Mendels, J Chakraborty, V Singh, I Dasgupta, S Rayaprol, V Siruguri, A Hoser, S D Kaushik
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  ABSTRACT: The chain-like system Ba(3)Cu(3)Sc(4)O(12) has potentially interesting magnetic properties due to the presence of Cu(2+) and a structure-suggested low dimensionality. We present magnetization M versus magnetic field H and temperature T, T- and H-dependent heat-capacity C(p), (45)Sc nuclear magnetic resonance (NMR), muon spin rotation (μSR), neutron diffraction measurements and electronic structure calculations for Ba(3)Cu(3)Sc(4)O(12). The onset of magnetic long-range antiferromagnetic (AF) order at T(N) ∼ 16 K is consistently evidenced from the whole gamut of our data. A significant sensitivity of T(N) to the applied magnetic field H (T(N) ∼ 0 K for H = 70 kOe) is also reported. Coupled with a ferromagnetic Curie-Weiss temperature (θ(CW) ∼ 65 K) in the susceptibility (from a 100 to 300 K fit), it is indicative of competing ferromagnetic and antiferromagnetic interactions. These indications are corroborated by our density functional theory based electronic structure calculations, where we find the presence of significant ferromagnetic couplings between some copper ions whereas AF couplings were present between some others. Our experimental data, backed by our theoretical calculations, rule out the one-dimensional magnetic behavior suggested by the structure and the observed long-range order is due to the presence of non-negligible magnetic interactions between adjacent as well as next-nearest chains.
  Journal of Physics Condensed Matter 05/2012; 24(23):236001. · 2.55 Impact Factor
• Article: Kapellasite: a kagome quantum spin liquid

  B. Fåk, E. Kermarrec, L. Messio, B. Bernu, C. Lhuillier, F. Bert, P. Mendels, B. Koteswararao, F. Bouquet, J. Ollivier, A. D. Hillier, A. Amato, R. H. Colman, A. S. Wills
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  ABSTRACT: Magnetic susceptibility, NMR, muSR, and inelastic neutron scattering measurements show that kapellasite, Cu3Zn(OH)6Cl2, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphous with the herbertsmithite mineral, is a gapless spin liquid with frustrated interactions showing unusual dynamic short-range correlations of non-coplanar cuboc2 type which persist down to 20 mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to thermodynamical data. The experimental data are compared to theoretical calculations using the Schwinger-boson approach.
  Physical Review Letters 03/2012; 109:037208. · 7.37 Impact Factor
• Article: Magnetic behavior of Ba_{3}Cu_{3}Sc_{4}O_{12}

  B. Koteswararao, A. V. Mahajan, F. Bert, P. Mendels, J. Chakraborty, V. Singh, I. Dasgupta, S. Rayaprol, V. Siruguri, A. Hoser, S. D. Kaushik
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  ABSTRACT: The chain-like system Ba_{3}Cu_{3}Sc_4O_{12} has potentially interesting magnetic properties due to the presence of Cu^{2+} and a structure-suggested low-dimensionality. We present magnetization M versus magnetic field H and temperature T, T- and H-dependent heat-capacity C_{p}, ^{45}Sc nuclear magnetic resonance (NMR), muon spin rotation ({\mu}SR), neutron diffraction measurements and electronic structure calculations for Ba_{3}Cu_{3}Sc_{4}O_{12}. The onset of magnetic long-range antiferromagnetic order at T_{N} ~16 K is consistently evidenced from the whole gamut of our data. A significant sensitivity of T_{N} to the applied magnetic field H (T_{N}~0 K for H=70 kOe) is also reported. Coupled with a ferromagnetic Curie-Weiss temperature ({\theta}_{CW}~65 K) in the susceptibility (from a 100 K-300 K fit), it is indicative of competing ferromagnetic and antiferromagnetic interactions. These indications are corroborated by our density functional theory based electronic structure calculations where we find the presence of significant ferromagnetic couplings between some copper ions whereas AF couplings were present between some others. Our experimental data, backed by our theoretical calculations, rule out one-dimensional magnetic behaviour suggested by the structure and the observed long-range order is due to the presence of non-negligible magnetic interactions between adjacent as well as next-nearest chains.
  03/2012;
• Article: Magnetic behavior of Ba 3Cu 3Sc 4O 12

  B Koteswararao, A V Mahajan, F Bert, P Mendels, J Chakraborty, V Singh, I Dasgupta, S Rayaprol, V Siruguri, A Hoser, S D Kaushik
  Journal of Physics Condensed Matter. 01/2012; 24(23).
• Source

  Available from: ArXiv

  Article: Field-induced freezing of a quantum spin liquid on the kagome lattice.

  M Jeong, F Bert, P Mendels, F Duc, J C Trombe, M A de Vries, A Harrison
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  ABSTRACT: We report (17)O NMR measurements in the S=1/2 (Cu(2+)) kagome antiferromagnet Herbertsmithite ZnCu(3)(OH)(6)Cl(2) down to 45 mK in magnetic fields ranging from 2 to 12 T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-Kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments ≲0.1 μ(B) and gapped excitations. The H-T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field μ(0)H(c)=1.55(25) T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagome Heisenberg antiferromagnet model.
  Physical Review Letters 12/2011; 107(23):237201. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Quantum kagome antiferromagnet: ZnCu3(OH)6Cl2

  P. Mendels, F Bert
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  ABSTRACT: Herbertsmithite, ZnCu3(OH)6Cl2, is the prototype candidate for a spin liquid behavior on a geometrically perfect kagome lattice. Its discovery and the absence of any evidence for spin-freezing down to the lowest probed temperature to-date enable one to explore the properties of kagome-related physics in an unprecedented temperature range. We review its properties and discuss some open issues. A tentative comparison to models is also performed.
  07/2011;
• Source

  Available from: ArXiv

  Article: Spin Liquid Ground State in the Frustrated Kagome Antiferromagnet MgCu3(OH)6Cl2

  E. Kermarrec, P. Mendels, F Bert, R. H. Colman, A. S. Wills, P. Strobel, P. Bonville, A. Hillier, A. Amato
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  ABSTRACT: We report muSR experiments on Mg{x}Cu{4-x}(OH)6Cl2 with x \sim 1, a new material isostructural to Herbertsmithite exhibiting regular kagome planes of spin 1/2 (Cu^{2+}), and therefore a candidate for a spin liquid ground state. We evidence the absence of any magnetic ordering down to 20 mK (\sim J/10^4). We investigate in detail the spin dynamics on well characterized samples in zero and applied longitudinal fields and propose a low T defect based interpretation to explain the unconventional dynamics observed in the quantum spin liquid phase.
  06/2011;
• Source

  Available from: ArXiv

  Article: Ground State and Intrinsic Susceptibility of the Kagome Antiferromagnet Vesignieite as seen by 51V NMR

  J. A. Quilliam, F Bert, R. H. Colman, D. Boldrin, A. S. Wills, P. Mendels
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  ABSTRACT: The intrinsic magnetic susceptibility and local magnetization of the near-kagome quantum magnet vesignieite, Cu3BaV2O8(OH)2, are presented as measured using 51V NMR. The NMR line shift gives an accurate measurement of the intrinsic susceptibility of the kagome sites which closely resembles that of the quantum spin liquid herbertsmithite [A. Olariu et al. Phys. Rev. Lett. 100, 087202 (2008)]. It is therefore surprising that, at Tc ~ 9 K, a transition to a heterogeneous ground state is observed. A gradual wipeout of half the NMR intensity indicates a slowly fluctuating spin liquid component and a detailed analysis of the linewidth reveals the onset of static magnetism at the remaining half of the sites. It is proposed that this transition and unusual ground state originate from a nearby quantum critical point induced by the Dzyaloshinskii-Moriya interaction.
  05/2011;
• Source

  Available from: arxiv.org

  Article: Ground state of the easy-axis rare-earth kagome langasite Pr3Ga5SiO14.

  A Zorko, F Bert, P Mendels, K Marty, P Bordet
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  ABSTRACT: We report muon spin relaxation and {69,71}Ga nuclear quadrupolar resonance local-probe investigations of the kagome compound Pr3Ga5SiO14. Small quasistatic random internal fields develop below 40 K and persist down to our base temperature of 21 mK. They originate from hyperfine-enhanced 141Pr nuclear magnetism which requires a nonmagnetic Pr3+ crystal-field (CF) ground state. In addition, we observe a broad maximum of the relaxation rate at approximately 10 K which we attribute to the population of the first excited magnetic CF level. Our results yield a Van Vleck paramagnet picture, at variance with the formerly proposed spin-liquid ground state.
  Physical Review Letters 02/2010; 104(5):057202. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Ground State of the Easy-Axis Rare-Earth Kagom\'e Langasite Pr$_3$Ga$_5$SiO$_{14}$

  A. Zorko, F Bert, P. Mendels, K Marty, P. Bordet
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  ABSTRACT: We report muon spin relaxation ($\mu$SR) and $^{69,71}$Ga nuclear quadrupolar resonance (NQR) local-probe investigations of the kagom\'e compound Pr$_3$Ga$_5$SiO$_{14}$. Small quasi-static random internal fields develop below 40 K and persist down to our base temperature of 21 mK. They originate from hyperfine-enhanced $^{141}$Pr nuclear magnetism which requires a non-magnetic Pr$^{3+}$ crystal-field (CF) ground state. Besides, we observe a broad maximum of the relaxation rate at $\simeq 10$ K which we attribute to the population of the first excited magnetic CF level. Our results yield a Van-Vleck paramagnet picture, at variance with the formerly proposed spin-liquid ground state. Comment: minor changes
  12/2009;
• Source

  Available from: Libu K Alexander

  Article: Spin dynamics in Heisenberg triangular antiferromagnets: A muSR study of LiCrO2

  A Olariu, P. Mendels, F Bert, L. K. Alexander, A. V. Mahajan, A D Hillier, A. Amato
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  ABSTRACT: We report a muSR study of LiCrO2, which has a magnetic lattice made up of a stacking of triangular Heisenberg antiferromagnetic (Cr3+, S = 3/2) layers. A static magnetically ordered state is observed below the transition temperature T_N = 62 K, while the expected peak of the relaxation rate is slightly shifted downward by a few kelvins below T_N. We draw a comparison with the isostructural compound NaCrO2, where an exotic broad fluctuating regime has been observed [A. Olariu, P. Mendels, F. Bert, B. G. Ueland, P. Schiffer, R. F. Berger, and R. J. Cava, Phys. Rev. Lett. 97, 167203 (2006)] and was suggested to originate from topological excitations of the triangular lattice. Replacing Na by Li strongly narrows the exotic fluctuating regime formerly observed in NaCrO2, which we attribute to a more pronounced inter-plane coupling in LiCrO2. Comment: 6 pages, 5 figures
  06/2009;
• Article: Electron spin resonance investigation of the spin-1/2 kagomé antiferromagnet ZnCu3(OH)6Cl2

  A Zorko, S Nellutla, J van Tol, L C Brunel, F Bert, F Duc, J-C Trombe, P Mendels
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  ABSTRACT: We report a temperature and frequency dependent electron spin resonance (ESR) study of the kagomé spin-1/2 compound ZnCu3(OH)6Cl2. Our results demonstrate that two spin species are simultaneously detected; copper spins on the intra-plane Cu2+ kagomé sites and those on the inter-plane Zn2+ sites, the latter resulting from the Cu2+/Zn2+ antisite disorder. We examine all the possible magnetic anisotropy terms, which could lead to the observed extremely broad ESR lines in this system. We argue that the line broadening is due to Dzyaloshinsky-Moriya interaction.
  Journal of Physics Conference Series 02/2009; 145(1):012014.
• Source

  Available from: u-psud.fr

  Article: 29Si NMR and 69,71Ga NMR/NQR study of the kagomé compound Nd3Ga5SiO14

  A Zorko, F Bert, P Bordet, P Lejay, P Mendels
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  ABSTRACT: We report a comprehensive 29Si NMR and 69,71Ga NMR/NQR study of the large-spin magnetically anisotropic kagomé compound Nd3Ga5SiO14. We find a sizable transferred hyperfine coupling between the nuclear and electron moments, proving that these nuclei can be utilized as local probes which provide useful information about the magnetic Nd3+ ions. Our results give evidence of unexpectedly broad distributions of local environments on all nuclear sites under investigation, implying also exchange disorder.
  Journal of Physics Conference Series 02/2009; 145(1):012006.
• Article: Frustrated magnetism in the quantum Kagome Herbertsmithite ZnCu3(OH)6Cl2 antiferromagnet

  F Bert, A Olariu, A Zorko, P Mendels, J C Trombe, F Duc, M A de Vries, A Harrison, A D Hillier, J Lord, A Amato, C Baines
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  ABSTRACT: The frustration of antiferromagnetic interactions on the kagome lattice of corner sharing triangles, associated to the enhancement of quantum fluctuations for S=1/2 spins, was acknowledged long ago as a keypoint to stabilize novel ground states of magnetic matter. Only very recently, the model compound Herbersmithite, ZnCu3(OH)6Cl2, a structurally perfect kagome antiferromagnet could be synthesized and allows close comparison to theories. Muon spin resonance investigation has demonstrated the absence of any spin freezing at least down to 50 mK, an energy scale 4000 times smaller than the main antiferromagnetic interaction (J ≊ 180 K). The kagome plane susceptibility could be accurately measured from the 17O NMR lineshift. It goes through a broad maximum at T ≊ J/2 at variance with the diverging SQUID susceptibility which can be consistently interpreted as being dominated by a paramagnetic-like defect contribution arising from Zn/Cu intersite mixing. At lower temperatures (T ≊ J/100), the kagome susceptibility is found to saturate to a non zero value in agreement with the absence of a gap in relaxation measurements.
  Journal of Physics Conference Series 02/2009; 145(1):012004.
• Article: NMR and μSR study of magnetic dilution in the triangular Heisenberg antiferromagnet NaCrO2

  A Olariu, P Mendels, F Bert, B G Ueland, P Schiffer, R F Berger, R J Cava, A D Hillier
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  ABSTRACT: NaCrO2 has been shown to be a very good model compound for the study of triangular Heisenberg antiferromagnets with isotropic interactions [1]. We examine by NMR and μSR the effect of substitution of non magnetic Ga3+ ions at the Cr3+ sites (S = 3/2). The presence of non-magnetic defects is found to impact both the dynamics and the static properties of the triangular network.
  Journal of Physics Conference Series 02/2009; 145(1):012042.
• Article: Dzyaloshinsky-Moriya anisotropy in the spin-1/2 kagome compound ZnCu3(OH)6Cl2.

  A Zorko, S Nellutla, J van Tol, L C Brunel, F Bert, F Duc, J-C Trombe, M A de Vries, A Harrison, P Mendels
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  ABSTRACT: We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.
  Physical Review Letters 08/2008; 101(2):026405. · 7.37 Impact Factor