arXiv subjects

A. A. Tsirlin

Publications and source records attributed to A. A. Tsirlin.

At least 91 records · Page 5Linked to original sources

arXiv2015

Microscopic analysis of the magnetic form factor in low-dimensional cuprates

We analyze the magnetic form factor of Cu$^{2+}$ in low-dimensional quantum magnets by taking the metal-ligand hybridization into account explicitly. In this analysis we use the form of magnetic Wannier orbitals, derived from the first-principles calculations, and identify the contributions of different atomic sites. Having performed local density approximation calculations for cuprates with different types of ligand atoms, we discuss the influence of the on-site Coulomb correlations on the structure of the magnetic orbital. The typical composition of Wannier functions for copper oxides, chlorides and bromides is defined and related to features of the magnetic form factor. We propose easy-to-use approximations of the partial orbital contributions to the magnetic form factor in order to give a microscopic explanation for the results obtained in previous first-principles studies.

cond-mat.str-el↗

arXiv2014

Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn2RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn2RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

cond-mat.mtrl-sci↗

arXiv2014

Importance of tetrahedral coordination for high-valent transition metal oxides: YCrO$_4$ as a model system

We have investigated the electronic structure of the high oxidation state material YCrO$_4$ within the framework of the Zaanen-Sawatzky-Allen phase diagram. While Cr$^{4+}$-based compounds like SrCrO$_3$/CaCrO$_3$ and CrO$_2$ can be classified as small-gap or metallic negative-charge-transfer systems, we find using photoelectron spectroscopy that YCrO$_4$ is a robust insulator despite the fact that its Cr ions have an even higher formal valence state of 5+. We reveal using band structure calculations that the tetrahedral coordination of the Cr$^{5+}$ ions in YCrO$_4$ plays a decisive role, namely to diminish the bonding of the Cr $3d$ states with the top of the O $2p$ valence band. This finding not only explains why the charge-transfer energy remains effectively positive and the material stable, but also opens up a new route to create doped carriers with symmetries different from those of other transition-metal ions.

cond-mat.str-el↗

arXiv2014

Magnetic transitions in the spin S=5/2 frustrated magnet BiMn2PO6 and strong lattice softening in BiMn2PO6 and BiZn2PO6 below 200 K

The crystallographic, magnetic and thermal properties of BiMn2PO6 and BiZn2PO6 were investigated by x-ray diffraction, magnetic susceptibility χ, heat capacity Cp, and 31P NMR measurements as well as by density-functional band-theory calculations. Both compounds show a strong monotonic lattice softening on cooling, where the Debye temperature decreases by a factor of two from Θ_D=650K at T=300K to Θ_D=300K at T=2K. The χ(T) data for BiMn2PO6 above 150K follow a Curie-Weiss law with a Curie constant consistent with a S=5/2 with g=2 and an antiferromagnetic (AFM) Weiss temperature θ_CW= -78K. The χdata indicate long-range AFM ordering below TN=30K, confirmed by a sharp lambda-type peak in Cp(T) at 28.8K. The band-theory calculations indicate that BiMn2PO6 is an AFM compound with dominant interactions J1/kB= 6.7K and J3/kB= 5.6K along the legs and rungs of a two-leg spin-ladder, respectively. In addition to the long-range AFM ordering at TN=30K, a second transition at 10 K is observed from the χand NMR measurements but is not evident in the Cp data. The Cp data at low T suggest a significant contribution from AFM spin waves moving in three dimensions and the absence of a spin-wave gap. A detailed analysis of the NMR spectra indicates commensurate magnetic order between 10K and 30K and a possible incommensurate modulation and/or spin canting below 10K. The commensurate order is consistent with microscopic simulations that yield a collinear Neel-type AFM spin arrangement both within and between the ladders, despite the presence of multiple weak interactions frustrating this magnetic structure. Frustration for AFM ordering and 1D spatial anisotropy of the 3D spin interactions are manifested in the frustration ratio f=|θCW|/TN=2.6, indicating a suppression of TN from 68K in the absence of these effects to 30K in BiMn2PO6.

cond-mat.str-el↗

arXiv2014

The quantum origins of skyrmions and half-skyrmions in Cu2OSeO3

The Skyrme-particle, the $skyrmion$, was introduced over half a century ago and used to construct field theories for dense nuclear matter. But with skyrmions being mathematical objects - special types of topological solitons - they can emerge in much broader contexts. Recently skyrmions were observed in helimagnets, forming nanoscale spin-textures that hold promise as information carriers. Extending over length-scales much larger than the inter-atomic spacing, these skyrmions behave as large, classical objects, yet deep inside they are of quantum origin. Penetrating into their microscopic roots requires a multi-scale approach, spanning the full quantum to classical domain. By exploiting a natural separation of exchange energy scales, we achieve this for the first time in the skyrmionic Mott insulator Cu$_2$OSeO$_3$. Atomistic ab initio calculations reveal that its magnetic building blocks are strongly fluctuating Cu$_4$ tetrahedra. These spawn a continuum theory with a skyrmionic texture that agrees well with reported experiments. It also brings to light a decay of skyrmions into half-skyrmions in a specific temperature and magnetic field range. The theoretical multiscale approach explains the strong renormalization of the local moments and predicts further fingerprints of the quantum origin of magnetic skyrmions that can be observed in Cu$_2$OSeO$_3$, like weakly dispersive high-energy excitations associated with the Cu$_4$ tetrahedra, a weak antiferromagnetic modulation of the primary ferrimagnetic order, and a fractionalized skyrmion phase.

cond-mat.str-el↗

arXiv2014

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

The magnetism of magnetoelectric $S$ = 3/2 pyroxenes LiCrSi$_2$O$_6$ and LiCrGe$_2$O$_6$ 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 LiCrSi$_2$O$_6$, two relevant exchange integrals, $J_1 \simeq$ 9 K along the structural chains and $J_{\text{ic1}}$ $\simeq$ 2 K between the chains, form a 2D anisotropic honeycomb lattice. In contrast, the spin model of LiCrGe$_2$O$_6$ is constituted of three different exchange couplings. Surprisingly, the leading exchange $J_{\text{ic1}}$ $\simeq$ 2.3 K operates between the chains, while $J_1$ $\simeq$ 1.2 K is about two times smaller. The additional interlayer coupling $J_{\text{ic2}}$ $\simeq$ $J_1$ 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.

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

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

Magnetism of CuX2 frustrated chains (X = F, Cl, Br): the role of covalency

Periodic and cluster density-functional theory (DFT) calculations, including DFT+U and hybrid functionals, are applied to study magnetostructural correlations in spin-1/2 frustrated chain compounds CuX2: CuCl2, CuBr2, and a fictitious chain structure of CuF2. The nearest-neighbor and second-neighbor exchange integrals, J1 and J2, are evaluated as a function of the Cu-X-Cu bridging angle, theta, in the physically relevant range 80-110deg. In the ionic CuF2, J1 is ferromagnetic for theta smaller 100deg. For larger angles, the antiferromagnetic superexchange contribution becomes dominant, in accord with the Goodenough-Kanamori-Anderson rules. However, both CuCl2 and CuBr2 feature ferromagnetic J1 in the whole angular range studied. This surprising behavior is ascribed to the increased covalency in the Cl and Br compounds, which amplifies the contribution from Hund's exchange on the ligand atoms and renders J1 ferromagnetic. At the same time, the larger spatial extent of X orbitals enhances the antiferromagnetic J2, which is realized via the long-range Cu-X-X-Cu paths. Both, periodic and cluster approaches supply a consistent description of the magnetic behavior which is in good agreement with the experimental data for CuCl2 and CuBr2. Thus, owing to their simplicity, cluster calculations have excellent potential to study magnetic correlations in more involved spin lattices and facilitate application of quantum-chemical methods.

cond-mat.str-el↗

arXiv2013

Structure and magnetism of Cr2BP3O12: Towards the quantum-classical crossover in a spin-3/2 alternating chain

Magnetic properties of the spin-3/2 Heisenberg system Cr2BP3O12 are investigated by magnetic susceptibility chi(T) measurements, electron spin resonance, neutron diffraction, and density functional theory (DFT) calculations, as well as classical and quantum Monte Carlo (MC) simulations. The broad maximum of chi(T) at 85K and the antiferromagnetic Weiss temperature of 139 K indicate low-dimensional magnetic behavior. Below TN = 28 K, Cr2BP3O12 is antiferromagnetically ordered with the k = 0 propagation vector and an ordered moment of 2.5 muB/Cr. DFT calculations, including DFT+U and hybrid functionals, yield a microscopic model of spin chains with alternating nearest-neighbor couplings J1 and J1' . The chains are coupled by two inequivalent interchain exchanges of similar strength (~1-2 K), but different sign (antiferromagnetic and ferromagnetic). The resulting spin lattice is quasi-one-dimensional and not frustrated. Quantum MC simulations show excellent agreement with the experimental data for the parameters J1 ~= 50 K and J1'/J1 ~= 0.5. Therefore, Cr2BP3O12 is close to the gapless critical point (J1'/J1 = 0.41) of the spin-3/2 bond-alternating Heisenberg chain. The applicability limits of the classical approximation are addressed by quantum and classical MC simulations. Implications for a wide range of low-dimensional S = 3/2 materials are discussed.

cond-mat.str-el↗

arXiv2012

Pressure-induced phase transitions and high-pressure tetragonal phase of Fe1.08Te

We report the effects of hydrostatic pressure on the temperature-induced phase transitions in Fe1.08Te in the pressure range 0-3 GPa using synchrotron powder x-ray diffraction (XRD). The results reveal a plethora of phase transitions. At ambient pressure, Fe1.08Te undergoes simultaneous first-order structural symmetry-breaking and magnetic phase transitions, namely from the paramagnetic tetragonal (P4/nmm) to the antiferromagnetic monoclinic (P2_1/m) phase. We show that, at a pressure of 1.33 GPa, the low temperature structure adopts an orthorhombic symmetry. More importantly, for pressures of 2.29 GPa and higher, a symmetry-conserving tetragonal-tetragonal phase transition has been identified from a change in the c/a ratio of the lattice parameters. The succession of different pressure and temperature-induced structural and magnetic phases indicates the presence of strong magneto-elastic coupling effects in this material.

cond-mat.supr-con↗

arXiv2012

Decorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2

We report the microscopic magnetic model for the spin-1/2 Heisenberg system CdCu2(BO3)2, one of the few quantum magnets showing the 1/2-magnetization plateau. Recent neutron diffraction experiments on this compound [M. Hase et al., Phys. Rev. B 80, 104405 (2009)] evidenced long-range magnetic order, inconsistent with the previously suggested phenomenological magnetic model of isolated dimers and spin chains. Based on extensive density-functional theory band structure calculations, exact diagonalizations, quantum Monte Carlo simulations, third-order perturbation theory, as well as high-field magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2 are accounted for by a frustrated quasi-2D magnetic model featuring four inequivalent exchange couplings: the leading antiferromagnetic coupling J_d within the structural Cu2O6 dimers, two interdimer couplings J_t1 and J_t2, forming magnetic tetramers, and a ferromagnetic coupling J_it between the tetramers. Based on comparison to the experimental data, we evaluate the ratios of the leading couplings J_d : J_t1 : J_t2 : J_it = 1 : 0.20 : 0.45 : -0.30, with J_d of about 178 K. The inequivalence of J_t1 and J_t2 largely lifts the frustration and triggers long-range antiferromagnetic ordering. The proposed model accounts correctly for the different magnetic moments localized on structurally inequivalent Cu atoms in the ground-state magnetic configuration. We extensively analyze the magnetic properties of this model, including a detailed description of the magnetically ordered ground state and its evolution in magnetic field with particular emphasis on the 1/2-magnetization plateau. Our results establish remarkable analogies to the Shastry-Sutherland model of SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice.

cond-mat.str-el↗

arXiv2011

Magnetic model for A2CuP2O7 (A = Na, Li) revisited: 1D versus 2D behavior

We report magnetization measurements, full-potential band structure calculations, and microscopic modeling for the spin-1/2 Heisenberg magnets A2CuP2O7 (A = Na, Li). Based on a quantitative evaluation of the leading exchange integrals and the subsequent quantum Monte-Carlo simulations, we propose a quasi-one-dimensional magnetic model for both compounds, in contrast to earlier studies that conjectured on the two-dimensional scenario. The one-dimensional nature of A2CuP2O7 is unambiguously verified by magnetization isotherms measured in fields up to 50 T. The saturation fields of about 40 T for both Li and Na compounds are in excellent agreement with the intrachain exchange J1 ~ 27 K extracted from the magnetic susceptibility data. The proposed magnetic structure entails spin chains with the dominating antiferromagnetic nearest-neighbor interaction J1 and two inequivalent, nonfrustrated antiferromagnetic interchain couplings of about 0.01*J1 each. A possible long-range magnetic ordering is discussed in comparison with the available experimental information.

cond-mat.str-el↗

arXiv2011

Magnetic properties of the low-dimensional spin-1/2 magnet α-Cu_2As_2O_7

In this work we study the interplay between the crystal structure and magnetism of the pyroarsenate α-Cu_2As_2O_7 by means of magnetization, heat capacity, electron spin resonance and nuclear magnetic resonance measurements as well as density functional theory (DFT) calculations and quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O chains in the crystal structure represent a realization of a quasi-one dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with relevant pathways through non-magnetic AsO_4 tetrahedra. Owing to residual 3D interactions antiferromagnetic long range ordering at T_N\simeq10K takes place. Application of external magnetic field B along the magnetically easy axis induces the transition to a spin-flop phase at B_{SF}~1.7T (2K). The experimental data suggest that substantial quantum spin fluctuations take place at low magnetic fields in the ordered state. DFT calculations confirm the quasi-one-dimensional nature of the spin lattice, with the leading coupling J_1 within the structural dimers. QMC fits to the magnetic susceptibility evaluate J_1=164K, the weaker intrachain coupling J'_1/J_1 = 0.55, and the effective interchain coupling J_{ic1}/J_1 = 0.20.

cond-mat.str-el↗

arXiv2011

Phase separation and frustrated square lattice magnetism of Na1.5VOPO4F0.5

Crystal structure, electronic structure, and magnetic behavior of the spin-1/2 quantum magnet Na1.5VOPO4F0.5 are reported. The disorder of Na atoms leads to a sequence of structural phase transitions revealed by synchrotron x-ray powder diffraction and electron diffraction. The high-temperature second-order alpha <--> beta transition at 500 K is of the order-disorder type, whereas the low-temperature beta <--> gamma+gamma' transition around 250 K is of the first order and leads to a phase separation toward the polymorphs with long-range (gamma) and short-range (gamma') order of Na. Despite the complex structural changes, the magnetic behavior of Na1.5VOPO4F0.5 probed by magnetic susceptibility, heat capacity, and electron spin resonance measurements is well described by the regular frustrated square lattice model of the high-temperature alpha-polymorph. The averaged nearest-neighbor and next-nearest-neighbor couplings are J1 ~ -3.7 K and J2 ~ 6.6 K, respectively. Nuclear magnetic resonance further reveals the long-range ordering below TN = 2.6 K in low magnetic fields. Although the experimental data are consistent with the simplified square-lattice description, band structure calculations suggest that the ordering of Na atoms introduces a large number of inequivalent exchange couplings that split the square lattice into plaquettes. Additionally, the direct connection between the vanadium polyhedra induces an unusually strong interlayer coupling having effect on the transition entropy and the transition anomaly in the specific heat. Peculiar features of the low-temperature crystal structure and the relation to isostructural materials suggest Na1.5VOPO4F0.5 as a parent compound for the experimental study of tetramerized square lattices as well as frustrated square lattices with different values of spin.

cond-mat.mtrl-sci↗

arXiv2011

Peculiar long-range superexchange in Cu2A2O7 (A = P, As, V) as a key element of the microscopic magnetic model

A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined theoretical and experimental study. Despite a dominant intradimer coupling J1, sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K. The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in the coupling regime of these isostructural compounds is governed by the nature of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the superexchange is realized via O-O edges of the tetrahedron. Implications for a broad range of systems are discussed.

cond-mat.str-el↗

arXiv2010

Antiferromagnetic spin-1/2 chains in (NO)Cu(NO3)3: a microscopic study

We report on the microscopic model of the recently synthesized one-dimensional quantum magnet (NO)Cu(NO3)3. Applying density functional theory band structure calculations, we obtain a leading antiferromagnetic exchange coupling J ~ 200 K, which runs via NO3 groups forming spin chains along the b direction. Much weaker couplings J' ~ 2 K link the chains into layers in a non-frustrated manner. Our calculations do not support the earlier conjecture on an anisotropic frustrated square lattice physics in (NO)Cu(NO3)3. In contrast, the model of uniform spin chains leads to a remarkably good fit of the experimental magnetic susceptibility data, although the low-temperature features of the intrinsic magnetic susceptibility measured by electron spin resonance might call for extension of the model. We outline possible experiments to observe the suggested long-range magnetic ordering in (NO)Cu(NO3)3 and briefly compare this compound to other spin-1/2 uniform-chain systems.

cond-mat.str-el↗