arXiv subjects

M. Baenitz

Publications and source records attributed to M. Baenitz.

At least 73 records · Page 4Linked to original sources

arXiv2015

Alternating-spin-chain compound AgVOAsO$_4$ probed by $^{75}$As NMR

$^{75}$As NMR measurements were performed on a polycrystalline sample of spin-1/2 alternating-spin-chain Heisenberg antiferromagnet AgVOAsO$_4$. Temperature-dependent NMR shift $K(T)$, which is a direct measure of the intrinsic spin susceptibility, agrees very well with the spin-1/2 alternating-chain model, justifying the assignment of the spin lattice. From the analysis of $K(T)$, magnetic exchange parameters were estimated as follows: the leading exchange $J/k_{\rm B} \simeq 38.4$ K, alternation ratio $α= J'/J \simeq 0.68$, and spin gap $Δ/k_{\rm B} \simeq 15$ K. The transferred hyperfine coupling between the $^{75}$As nucleus and V$^{4+}$ spins obtained by comparing the NMR shift with bulk susceptibility amounts to $A_{\rm hf} \simeq 3.3$ T/$μ_{\rm B}$. Our temperature-dependent spin-lattice relaxation rate $1/T_1(T)$ also shows an activated behaviour at low temperatures, thus confirming the presence of a spin gap in AgVOAsO$_4$.

cond-mat.mtrl-sci↗

arXiv2015

Sc_{2}Ga_{2}CuO_{7}: A possible Quantum spin liquid near the percolation threshold

Sc$_{2}$Ga$_{2}$CuO$_{7}$ (SGCO) crystallizes in a hexagonal structure (space group: $P$$6$$_{3}/mmc$),\textcolor{black}{{} which can be seen as an alternating stacking of single and double triangular layers. Combining neutron, x-ray, and resonant x-ray diffraction we establish that the single triangular layers are mainly populated by non-magnetic Ga$^{3+}$ ions (85\% Ga and 15\% Cu), while the bi-layers have comparable population of Cu$^{2+}$ and Ga$^{3+}$} ions \textcolor{black}{(43\% Cu and 57\% Ga)}. Our susceptibility measurements in the temperature range 1.8 - 400 K give no indication of any spin-freezing or magnetic long-range order (LRO). We infer an effective paramagnetic moment $μ_{eff}=1.79\pm0.09$ $μ_{B}$ and a Curie-Weiss temperature $θ_{CW}$ of about $-44$ K, suggesting antiferromagnetic interactions between the Cu$^{2+}$($S=1/2$) ions. Low-temperature neutron powder diffraction data showed no evidence for LRO down to 1.5 K. In our specific heat data as well, no anomalies were found down to 0.35 K, in the field range 0-140 kOe.\textcolor{black}{{} The magnetic specific heat, $C_{m}$, exhibits a broad maximum at around 2.5 K followed by a nearly power law $C$$_{m}$$\propto$ $T$$^α$ behavior at lower temperatures, with $α$ increasing from }0.3\textcolor{black}{{} to} 1.9\textcolor{black}{{} as a function of field for fields upto 90 kOe and then remaining at 1.9 for fields upto 140 kOe. Our results point to a disordered ground state in SGCO. }

cond-mat.str-el↗

arXiv2015

S=1/2 quantum critical spin ladders produced by orbital ordering in Ba2CuTeO6

The ordered hexagonal perovskite Ba2CuTeO6 hosts weakly coupled S=1/2 spin ladders produced by an orbital ordering of Cu2+. The magnetic susceptibility chi(T) of Ba2CuTeO6 is well described by that expected for isolated spin ladders with exchange coupling of J~86 K but shows a deviation from the expected thermally activated behavior at low temperatures below T*~25 K. An anomaly in chi(T), indicative of magnetic ordering, is observed at T_mag=16 K. No clear signature of long-range ordering, however, is captured in NMR, specific heat or neutron diffraction measurements at and below T_mag. The marginal magnetic transition, indicative of strong quantum fluctuations, is evidence that Ba2CuTeO6 is in very close proximity to a quantum critical point between a magnetically ordered phase and a gapped spin liquid controlled by inter-ladder couplings.

cond-mat.str-el↗

arXiv2015

Towards ferromagnetic quantum criticality in FeGa3-xGex: 71Ga NQR as a zero field microscopic probe

71Ga NQR, magnetization and specific heat measurements have been performed on polycrystalline Ge doped FeGa3 samples. A crossover from an insulator to a correlated local moment metal in the low doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, xC=0.15, (1/T1T) exhibits a pronounced T4/3 power law over two orders of magnitude in temperature which indicates 3D quantum critical ferromagnetic fluctuations. Furthermore, for the ordered x = 0.2 sample (TC = 6 K) 1/T1T could be fitted well in the frame of Moriya's SCR theory for weakly FM systems with 1/T1T proportional to chi. In contrast to this, the low doping regime nicely displays local moment behavior where 1/T1T proportional to chi2 is valid. For temperature tending to zero the Sommerfeld ratio = (C/T) is enhanced (70 mJ/mole-K2 for x = 0.1) which indicates the formation of heavy 3d-electrons.

cond-mat.str-el↗

arXiv2015

Frustrated three-dimensional antiferromagnet Li$_{2}$CuW$_{2}$O$_{8}$: $^7$Li NMR and the effect of non-magnetic dilution

We report a $^7$Li nuclear magnetic resonance (NMR) study of a frustrated three-dimensional \mbox{spin-$\frac12$} antiferromagnet Li$_{2}$CuW$_{2}$O$_{8}$ and also explore the effect of non-magnetic dilution. The magnetic long-range ordering in the parent compound at $T_{\rm N}\simeq$ 3.9~K was detected from the drastic line broadening and a peak in the spin-lattice relaxation rate ($1/T_1$). The NMR spectrum above $T_{\rm N}$ broadens systematically, and its full width at half maximum (FWHM) tracks the static spin susceptibility. From the analysis of FWHM vs. static susceptibility, the coupling between the Li nuclei and Cu$^{2+}$ ions was found to be purely dipolar in nature. The magnitude of the maximum exchange coupling constant is $J_{\rm max}/k_{\rm B}\simeq 13$\,K. NMR spectra below $T_{\rm N}$ broaden abruptly and transform into a double-horn pattern reflecting the commensurate nature of the spin structure in the ordered state. Below $T_{\rm N}$, 1/$T_1$ follows a $T^5$ behavior. The frustrated nature of the compound is confirmed by persistent magnetic correlations at high temperatures well above $T_{\rm N}$. The dilution of the spin lattice with non-magnetic Zn atoms has dramatic influence on $T_N$ that decreases exponentially similar to quasi-one-dimensional antiferromagnets, even though Li$_2$CuW$_2$O$_8$ has only a weak one-dimensional anisotropy. Heat capacity of doped samples follows power law ($C_{\rm p} \propto T^α$) below $T_{\rm N}$, and the exponent ($α$) decreases from 3 in the parent compound to 1 in the 25\% doped sample.

cond-mat.mtrl-sci↗

arXiv2015

Anisotropic Ru3+ 4d5 magnetism in the alpha-RuCl3 honeycomb system: susceptibility, specific heat and Zero field NMR

Hexagonal alpha-Ru trichloride single crystals exhibit a strong magnetic anisotropy and we show that upon applying fields up to 14 T in the honeycomb plane the successive magnetic order at T1 = 14 K and T2 = 8 K could be completely suppressed whereas in the perpendicular direction the magnetic order is robust. Furthermore the field dependence of chi(T) implies coexisting ferro- and antiferromagnetic exchange between in-plane components of Ru^3+-spins, whereas for out-of-plane components a strong antiferromagnetic exchange becomes evident. 101^Ru zero-field nuclear magnetic resonance in the ordered state evidence a complex (probably non coplanar chiral) long-range magnetic structure. The large orbital moment on Ru^3+ is found in density-functional calculations.

cond-mat.str-el↗

arXiv2014

Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons in YbFe2Al10

We present magnetization, specific heat, and 27Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of about 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic scaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T) simultaneously tracks the valence fluctuations from the 4f -Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature, the latter evolve out of an Yb 4f admixed conduction band.

cond-mat.str-el↗

arXiv2014

Magnetic properties and heat capacity of the three-dimensional frustrated S=1/2 antiferromagnet PbCuTe2O6

We report magnetic susceptibility (chi) and heat capacity Cp measurements along with ab-initio electronic structure calculations on PbCuTe2O6, a compound made up of a three dimensional 3D network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature (theta_CW) of about -22 K from the chi(T) data. The magnetic heat capacity (Cm) data show a broad maximum at T^max ~ 1.15 K (i.e. T^max/theta_CW ~ 0.05), which is analogous to the the observed broad maximum in the Cm/T data of a hyper-Kagome system, Na4Ir3O8. In addition, Cm data exhibit a weak kink at T^* ~ 0.87 K. While the T^max is nearly unchanged, the T^* is systematically suppressed in an increasing magnetic field (H) up to 80 kOe. For H > 80 kOe, the Cm data at low temperatures exhibit a characteristic power-law (T^α) behavior with an exponent α slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that PbCuTe2O6 is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields.

cond-mat.str-el↗

arXiv2014

Hindered magnetic order from mixed dimensionalities in CuP$_2$O$_6$

We present a combined experimental and theoretical study of the spin-1/2 compound CuP$_2$O$_6$ that features a network of two-dimensional (2D) antiferromagnetic (AFM) square planes, interconnected via one-dimensional (1D) AFM spin chains. Magnetic susceptibility, high-field magnetization, and electron spin resonance (ESR) data, as well as microscopic density-functional band-structure calculations and subsequent quantum Monte-Carlo simulations, show that the coupling $J_{2D}\sim$ 40 K in the layers is an order of magnitude larger than $J_{1D}\sim$ 4 K in the chains. Below $T_N\sim$ 8 K, CuP$_2$O$_6$ develops long-range order (LRO), as evidenced by a weak net moment on the 2D planes induced by anisotropic magnetic interactions of Dzyaloshinsky-Moriya type. A striking feature of this 3D ordering transition is that the 1D moments grow significantly slower than the ones on the 2D layers, which is evidenced by the persistent paramagnetic ESR signal below $T_N$. Compared to typical quasi-2D magnets, the ordering temperature of CuP$_2$O$_6$ $T_N/J_{2D}\sim$ 0.2 is unusually low, showing that weakly coupled spins sandwiched between 2D magnetic units effectively decouple these units and impede the long-range ordering.

cond-mat.mtrl-sci↗

arXiv2013

Interplay between localized and itinerant magnetism in Co substituted FeGa$_3$

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe$_{1-x}$Co$_x$Ga$_3$ was studied by means of electrical resistivity, magnetization, ab-initio band structure calculations, and nuclear spin-lattice relaxation $1/T_1$ of the $^{69,71}$Ga nuclei. Temperature dependencies of the electrical resistivity reveal that the evolution from the semiconducting to the metallic state in the Fe$_{1-x}$Co$_x$Ga$_3$ system occurs at $0.025<x<0.075$. The $^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of $2\!-\!300$ K for the concentrations $x = 0.0,$ $0.5,$ and $1.0$. In the parent semiconducting compound FeGa$_3$, the temperature dependence of the $^{69}(1/T_1)$ exhibits a huge maximum at about $T\!\sim\!6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa$_3$, demonstrates a metallic Korringa behavior with $1/T_1$ $\propto T$. In Fe$_{0.5}$Co$_{0.5}$Ga$_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1\propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with both magnetization measurements, showing localized magnetism with a relatively low effective moment of about 0.7 $μ_B$/f.u., and ab initio band structure calculations, where a ferromagnetic state with an ordered moment of 0.5 $μ_B$/f.u. is predicted. The results are discussed in terms of the interplay betwen the localized and itinerant magnetizm including in-gap states and spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2013

Unconventional magnetism in multivalent charge-ordered YbPtGe$_2$ probed by $^{195}$Pt- and $^{171}$Yb-NMR

Detailed $^{195}$Pt- and $^{171}$Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe$_2$ are reported. The temperature dependence of the $^{195}$Pt-NMR shift $^{195}K(T)$ indicates the opening of an unusual magnetic gap below 200\,K. $^{195}K(T)$ was analyzed by a thermal activation model which yields an isotropic gap $Δ/k_B \approx 200$\,K. In contrast, the spin-lattice relaxation rate $^{195}$($1/T_1$) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreover, $^{195}$($1/T_1$) follows a simple metallic behavior, similar to the reference compound YPtGe$_2$. A well resolved NMR line with small shift is assigned to divalent $^{171}$Yb. This finding supports the proposed model with two sub-sets of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the YbPtGe$_2$ lattice.

cond-mat.str-el↗

arXiv2013

Magnetization and spin dynamics of the spin S=1/2 hourglass nanomagnet Cu5(OH)2(NIPA)4*10H2O

We report a combined experimental and theoretical study of the spin S=1/2 nanomagnet Cu5(OH)2(NIPA)4*10H2O (Cu5-NIPA). Using thermodynamic, electron spin resonance and 1H nuclear magnetic resonance measurements on one hand, and ab initio density-functional band-structure calculations, exact diagonalizations and a strong coupling theory on the other, we derive a microscopic magnetic model of Cu5-NIPA and characterize the spin dynamics of this system. The elementary five-fold Cu2+ unit features an hourglass structure of two corner-sharing scalene triangles related by inversion symmetry. Our microscopic Heisenberg model comprises one ferromagnetic and two antiferromagnetic exchange couplings in each triangle, stabilizing a single spin S=1/2 doublet ground state (GS), with an exactly vanishing zero-field splitting (by Kramer's theorem), and a very large excitation gap of Δ~68 K. Thus, Cu5-NIPA is a good candidate for achieving long electronic spin relaxation (T1) and coherence (T2) times at low temperatures, in analogy to other nanomagnets with low-spin GS's. Of particular interest is the strongly inhomogeneous distribution of the GS magnetic moment over the five Cu2+ spins. This is a purely quantum-mechanical effect since, despite the non-frustrated nature of the magnetic couplings, the GS is far from the classical collinear ferrimagnetic configuration. Finally, Cu5-NIPA is a rare example of a S=1/2 nanomagnet showing an enhancement in the nuclear spin-lattice relaxation rate 1/T1 at intermediate temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Complex Chiral Modulations in FeGe close to Magnetic Ordering

We report on detailed polarized small-angle neutron scattering on cubic FeGe in magnetic fields applied either along (transverse) the scattering vector or parallel (longitudinal)to the neutron beam. The ($H,T$) phase diagrams for all principal axes contain a segmented $A$-phase region just below the onset of magnetic order. Hexagonal Bragg-spot patterns were observed across the entire $A$-phase region for the longitudinal geometry. Scattering intensity was observed in parts of the A phase for both scattering configurations. Only in a distinct pocket ($A_1$) vanishing scattering intensity was found in the transverse geometry.

cond-mat.mtrl-sci↗

arXiv2012

Spin liquid behaviour in Jeff=1/2 triangular lattice Ba3IrTi2O9

Ba3IrTi2O9 crystallizes in a hexagonal structure consisting of a layered triangular arrangement of Ir4+ (Jeff=1/2). Magnetic susceptibility and heat capacity data show no magnetic ordering down to 0.35K inspite of a strong magnetic coupling as evidenced by a large Curie-Weiss temperature=-130K. The magnetic heat capacity follows a power law at low temperature. Our measurements suggest that Ba3IrTi2O9 is a 5d, Ir-based (Jeff=1/2), quantum spin liquid on a 2D triangular lattice.

cond-mat.str-el↗

arXiv2012

Field tuned critical fluctuations in YFe2Al10: Evidence from magnetization, 27Al (NMR, NQR) investigations

We report magnetization, specific heat, and NMR investigations on YFe2Al10 over a wide range in temperature and magnetic field and zero field (NQR) measurements. Magnetic susceptibility, specific heat and spin-lattice relaxation rate divided by T (1/T1T) follow a weak power law (T^-0.4) temperature dependence, which is a signature of critical fluctuations of Fe moments. The value of the Sommerfeld-Wilson ratio and linear relation between 1/T1T and chi(T) suggest the existence of ferromagnetic correlations in this system. No magnetic ordering down to 50 mK in Cp(T) and the unusual temperature and field scaling of the bulk and NMR data are associated with a magnetic instability which drives the system to quantum criticality. The magnetic properties of the system are tuned by field wherein ferromagnetic fluctuations are suppressed and a crossover from quantum critical to FL behavior is observed with increasing magnetic field.

cond-mat.str-el↗

arXiv2012

Ferromagnetic Ordering in CeIr2B2: Transport, magnetization, specific heat and NMR studies

We present a complete characterization of ferromagnetic system CeIr2B2 using powder x-ray diffraction XRD, magnetic susceptibility chi(T), isothermal magnetization M(H), specific heat C(T), electrical resistivity rho(T,H), and thermoelectric power S(T) measurements. Furthermore 11B NMR study was performed to probe the magnetism on a microscopic scale. The chi(T), C(T) and rho(T) data confirm bulk ferromagnetic ordering with Tc = 5.1 K. Ce ions in CeIr2B2 are in stable trivalent state. Our low-temperature C(T) data measured down to 0.4 K yield Sommerfeld coefficient gamma = 73(4) mJ/molK2 which is much smaller than the previously reported value of gamma = 180 mJ/molK2 deduced from the specific heat measurement down to 2.5 K. For LaIr2B2 gamma = 6(1) mJ/molK2 which implies the density of states at the Fermi level D(EF) = 2.54 states/(eV f.u.) for both spin directions. The renormalization factor for quasi-particle density of states and hence for quasi-particle mass due to 4f correlations in CeIr2B2 is 12. The Kondo temperature TK ~ 4 K is estimated from the jump in specific heat of CeIr2B2 at Tc. Both C(T) and rho(T) data exhibit gapped-magnon behavior in magnetically ordered state with an energy gap Eg ~ 3.5 K. The rho data as a function of magnetic field H indicate a large negative magnetoresistance (MR) which is highest for T = 5 K.While at 5 K the negative MR keeps on increasing up to 10 T, at 2 K an upturn is observed near H = 3.5 T. On the other hand, the thermoelectric power data have small absolute values (S ~ 7 μV/K) indicating a weak Kondo interaction. A shoulder in S(T) at about 30 K followed by a minimum at ~ 10 K is attributed to crystal electric field (CEF) effects and the onset of magnetic ordering. 11B NMR line broadening provides strong evidence of ferromagnetic correlations below 40 K.

cond-mat.str-el↗

arXiv2012

119Sn solid state NMR and Mössbauer spectroscopic studies of the intermediate-valent stannide CeRuSn

The ternary stannide CeRuSn is a static mixed-valent cerium compound with an or-dering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4.2 K) two tin sites can clearly be distinguished. 119Sn solid state NMR experiments are performed to probe the local hyperfine fields at the two different Sn sites. 119Sn NMR powder spectra are nicely fitted with two Sn sites with nearly the same magnetic anisotropy, but with different absolute shift values. Both Sn sites are strongly affected by crossover-like transitions between 100 and 280 K. This local-site study confirms the superstructure modulations found in previous investiga-tions. Towards lower temperatures the powder spectra are broadened giving strong evidence for the antiferromagnetically ordered ground state.

cond-mat.str-el↗

arXiv2012

Low field extension for magnetometers (TinyBee) used for investigations on low-dimensional superconductors with Bc1 < 5G

In this article a simple and easy to install low magnetic field extension of the SQUID magnetometer Quantum Design MPMS-7 is described. This has been accomplished by complementing the MPMS-7 magnet control system with a laboratory current supply for the low magnetic field region (B \leq 200G). This hard- and software upgrade provides a significant gain in the magnetic field accuracy up to an order of magnitude compared with the standard instrument's setup and is improving the resolution to better than 0.01G below 40G. The field control has been integrated into the Quantum Design MultiVu software for a transparent and user-friendly operation of this extension. The improvements achieved are especially useful, when low magnetic field strengths (B < 1G) are required at high precision. The specific advantages of this application are illustrated by sophisticated magnetic characterisation of lowdimensional superconductors like Sc3CoC4 and SnSe2{Co(η-C5H5)2}x.

cond-mat.supr-con↗