165 research outputs found

    Magnetic pyroxenes LiCrGe2O6 and LiCrSi2O6: dimensionality crossover in a non-frustrated S=3/2 Heisenberg model

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    The magnetism of magnetoelectric SS = 3/2 pyroxenes LiCrSi2_2O6_6 and LiCrGe2_2O6_6 is studied by density functional theory (DFT) calculations, quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as low-field and high-field magnetization measurements. In contrast with earlier reports, we find that the two compounds feature remarkably different, albeit non-frustrated magnetic models. In LiCrSi2_2O6_6, two relevant exchange integrals, J1J_1 \simeq 9 K along the structural chains and Jic1J_{\text{ic1}} \simeq 2 K between the chains, form a 2D anisotropic honeycomb lattice. In contrast, the spin model of LiCrGe2_2O6_6 is constituted of three different exchange couplings. Surprisingly, the leading exchange Jic1J_{\text{ic1}} \simeq 2.3 K operates between the chains, while J1J_1 \simeq 1.2 K is about two times smaller. The additional interlayer coupling Jic2J_{\text{ic2}} \simeq J1J_1 renders this model 3D. QMC simulations reveal excellent agreement between our magnetic models and the available experimental data. Underlying mechanisms of the exchange couplings, magnetostructural correlations, as well as implications for other pyroxene systems are discussed.Comment: 11 pages, 8 figures, 3 tables + Supplementary informatio

    High-magnetic field phase diagram and failure of magnetic Gr\"uneisen scaling in LiFePO4_4

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    We report the magnetic phase diagram of single-crystalline LiFePO4_4 in magnetic fields up to 58~T and present a detailed study of magneto-elastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at \tn\ in the thermal expansion coefficient α\alpha imply pronounced magneto-elastic coupling. Quantitative analysis yields the magnetic Gr\"uneisen parameter γmag=6.7(5)107\gamma_{\rm mag}=6.7(5)\cdot 10^{-7}~mol/J. The positive hydrostatic pressure dependence dTN/dp=1.46(11)dT_{\rm N}/dp = 1.46(11)~K/GPa is dominated by uniaxial effects along the aa-axis. Failure of Gr\"uneisen scaling below 40\approx 40~K, i.e., below the peak temperature in the magneto-electric coupling coefficient [\onlinecite{toft2015anomalous}], implies several competing degrees of freedom and indicates relevance of recently observed hybrid excitations~[\onlinecite{yiu2017hybrid}]. A broad and strongly magnetic-field-dependent anomaly in α\alpha in this temperature regime highlight the relevance of structure changes. Upon application of magnetic fields BbB||b-axis, a pronounced jump in the magnetisation implies spin-reorientation at BSF=32B_{\rm SF} = 32~T as well as a precursing phase at 29~T and T=1.5T=1.5~K. In a two-sublattice mean-field model, the saturation field Bsat,b=64(2)B_{\rm sat,b} = 64(2)~T enables the determination of the effective antiferromagnetic exchange interaction Jaf=2.68(5)J_{\rm af} = 2.68(5)~meV as well as the anisotropies Db=0.53(4)D_{\rm b} = -0.53(4)~meV and Dc=0.44(8)D_{\rm c} = 0.44(8)~meV

    Direct observation of band-gap closure for a semiconducting carbon nanotube in a large parallel magnetic field

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    We have investigated the magnetoconductance of semiconducting carbon nanotubes (CNTs) in pulsed, parallel magnetic fields up to 60 T, and report the direct observation of the predicted band-gap closure and the reopening of the gap under variation of the applied magnetic field. We also highlight the important influence of mechanical strain on the magnetoconductance of the CNTs.Comment: 4 pages, 4 figure

    Large zero-field cooled exchange-bias in bulk Mn2PtGa

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    We report a large exchange-bias (EB) effect after zero-field cooling the new tetragonal Heusler compound Mn2PtGa from the paramagnetic state. The first-principle calculation and the magnetic measurements reveal that Mn2PtGa orders ferrimagnetically with some ferromagnetic (FM) inclusions. We show that ferrimagnetic (FI) ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled (ZFC) EB during the virgin magnetization process. The complex magnetic behavior at low temperatures is characterized by the coexistence of a field induced irreversible magnetic behavior and a spin-glass-like phase. The field induced irreversibility originates from an unusual first-order FI to antiferromagnetic transition, whereas, the spin-glass like state forms due to the existence of anti-site disorder intrinsic to the material.Comment: 5 pages, 4 figures, supplementary material included in a separate file; accepted for publication in PR

    Collinear order in a frustrated three-dimensional spin-12\frac12 antiferromagnet Li2_2CuW2_2O8_8

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    Magnetic frustration in three dimensions (3D) manifests itself in the spin-12\frac12 insulator Li2_2CuW2_2O8_8. Density-functional band-structure calculations reveal a peculiar spin lattice built of triangular planes with frustrated interplane couplings. The saturation field of 29 T contrasts with the susceptibility maximum at 8.5 K and a relatively low N\'eel temperature TN3.9T_N\simeq 3.9 K. Magnetic order below TNT_N is collinear with the propagation vector (0,12,0)(0,\frac12,0) and an ordered moment of 0.65(4) μB\mu_B according to neutron diffraction data. This reduced ordered moment together with the low maximum of the magnetic specific heat (Cmax/R0.35C^{\max}/R\simeq 0.35) pinpoint strong magnetic frustration in 3D. Collinear magnetic order suggests that quantum fluctuations play crucial role in this system, where a non-collinear spiral state would be stabilized classically.Comment: published version with supplemental material merged into the tex

    Frustration of square cupola in Sr(TiO)Cu4_{4}(PO4_{4})4_{4}

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    The structural and magnetic properties of the square-cupola antiferromagnet Sr(TiO)Cu4_{4}(PO4_{4})4_{4} are investigated via x-ray diffraction, magnetization, heat capacity, and 31^{31}P nuclear magnetic resonance experiments on polycrystalline samples, as well as density-functional band-structure calculations. The temperature-dependent unit cell volume could be described well using the Debye approximation with the Debye temperature of θD\theta_{\rm D} \simeq 550~K. Magnetic response reveals a pronounced two-dimensionality with a magnetic long-range-order below TN6.2T_{\rm N} \simeq 6.2~K. High-field magnetization exhibits a kink at 1/31/3 of the saturation magnetization. Asymmetric 31^{31}P NMR spectra clearly suggest strong in-plane anisotropy in the magnetic susceptibility, as anticipated from the crystal structure. From the 31^{31}P NMR shift vs bulk susceptibility plot, the isotropic and axial parts of the hyperfine coupling between 31^{31}P nuclei and the Cu2+^{2+} spins are calculated to be Ahfiso6539A_{\rm hf}^{\rm iso} \simeq 6539 and Ahfax952A_{\rm hf}^{\rm ax} \simeq 952~Oe/μB\mu_{\rm B}, respectively. The low-temperature and low-field 31^{31}P NMR spectra indicate a commensurate antiferromagnetic ordering. Frustrated nature of the compound is inferred from the temperature-dependent 31^{31}P NMR spin-lattice relaxation rate and confirmed by our microscopic analysis that reveals strong frustration of the square cupola by next-nearest-neighbor exchange couplings.Comment: 11 pages, 13 figures, 1 table, Phys. Rev. B (Accepted, 2018

    Direct measurements of the magnetocaloric effect in pulsed magnetic fields: The example of the Heusler alloy Ni50_{50}Mn35_{35}In15_{15}

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    We have studied the magnetocaloric effect (MCE) in the shape-memory Heusler alloy Ni50_{50}Mn35_{35}In15_{15} by direct measurements in pulsed magnetic fields up to 6 and 20 T. The results in 6 T are compared with data obtained from heat-capacity experiments. We find a saturation of the inverse MCE, related to the first-order martensitic transition, with a maximum adiabatic temperature change of ΔTad=7\Delta T_{ad} = -7 K at 250 K and a conventional field-dependent MCE near the second-order ferromagnetic transition in the austenitic phase. The pulsed magnetic field data allow for an analysis of the temperature response of the sample to the magnetic field on a time scale of 10\sim 10 to 100 ms which is on the order of typical operation frequencies (10 to 100 Hz) of magnetocaloric cooling devices. Our results disclose that in shape-memory alloys the different contributions to the MCE and hysteresis effects around the martensitic transition have to be carefully considered for future cooling applications.Comment: 5 pages, 4 figure

    Singlet ground state in the alternating spin-1/21/2 chain compound NaVOAsO4_4

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    We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NaVOAsO4_4 by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and 75^{75}As nuclear magnetic resonance (NMR) measurements as well as density-functional band structure calculations. Temperature-dependent magnetic susceptibility, ESR intensity, and NMR line shift could be described well using an alternating spin-1/21/2 chain model with the exchange coupling J/kB52J/k_{\rm B}\simeq 52 K and an alternation parameter α0.65\alpha \simeq 0.65. From the high-field magnetic isotherm measured at T=1.5T=1.5 K, the critical field of the gap closing is found to be Hc16 H_{\rm c}\simeq 16 T, which corresponds to the zero-field spin gap of Δ0/kB21.4\Delta_0/k_{\rm B}\simeq 21.4 K. Both NMR shift and spin-lattice relaxation rate show an activated behavior at low temperatures, further confirming the singlet ground state. The spin chains do not coincide with the structural chains, whereas the couplings between the spin chains are frustrated. Because of a relatively small spin gap, NaVOAsO4_4 is a promising compound for further experimental studies under high magnetic fields.Comment: 14 pages, 10 figures, 2 table

    Huge First-Order Metamagnetic Transition in the Paramagnetic Heavy-Fermion System CeTiGe

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    We report on the observation of large, step-like anomalies in the magnetization (ΔM=0.74\Delta M = 0.74\,μB\mu_{\rm B}/Ce), in the magnetostriction (Δl/l0=2.0103\Delta l/l_{0} = 2.0 \cdot 10^{-3}), and in the magnetoresistance in polycrystals of the paramagnetic heavy-fermion system CeTiGe at a critical magnetic field μ0Hc\mu_0 H_c \approx 12.5\,T at low temperatures. The size of these anomalies is much larger than those reported for the prototypical heavy-fermion metamagnet CeRu2_2Si2_2. Furthermore, hysteresis between increasing and decreasing field data indicate a real thermodynamic, first-order type of phase transition, in contrast to the crossover reported for CeRu2_2Si2_2. Analysis of the resistivity data shows a pronounced decrease of the electronic quasiparticle mass across HcH_c. These results establish CeTiGe as a new metamagnetic Kondo-lattice system, with an exceptionally large, metamagnetic transition of first-order type at a moderate field.Comment: 5 pages, 4 figure
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