arXiv subjects

M. Bartkowiak

Publications and source records attributed to M. Bartkowiak.

At least 37 records · Page 2Linked to original sources

arXiv2021

Magnetization process of atacamite: a case of weakly coupled $S = 1/2$ sawtooth chains

We present a combined experimental and theoretical study of the mineral atacamite Cu$_2$Cl(OH)$_3$. Density functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateau-like magnetization at about $M_{\rm sat}/2$. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

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arXiv2021

High-field spin-flop state in green dioptase

The high-field magnetic properties and magnetic order of the gem mineral green dioptase Cu$_6[$Si$_6$O$_{18}]\cdot 6$H$_2$O have been studied by means of single-crystal neutron diffraction in magnetic fields up to $21~$T and magnetization measurements up to $30~$T. In zero field, the Cu$^{2+}$-moments in the antiferromagnetic chains are oriented along the $c$-axis with a small off-axis tilt. For a field applied parallel to the $c$-axis, the magnetization shows a spin-flop-like transition at $B^*=12.2~$T at $1.5~$K. Neutron diffraction experiments show a smooth behavior in the intensities of the magnetic reflections without any change in the periodicity of the magnetic structure. Bulk and microscopic observations are well described by a model of ferromagnetically coupled antiferromagnetic $XXZ$ spin-$\frac{1}{2}$ chains, taking into account a change of the local easy-axis direction. We demonstrate that the magnetic structure evolves smoothly from a deformed Néel state at low fields to a deformed spin-flop state in a high field via a strong crossover around $B^*$. The results are generalized for different values of interchain coupling and spin anisotropy.

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arXiv2020

Using uniaxial stress to probe the relationship between competing superconducting states in a cuprate with spin-stripe order

We report muon spin rotation and magnetic susceptibility experiments on in-plane stress effects on the static spin-stripe order and superconductivity in the cuprate system La2-xBaxCuO4 with x = 0.115. An extremely low uniaxial stress of 0.1 GPa induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from 10 to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress. These results show not only that large-volume-fraction spin-stripe order is anti-correlated with 3D superconducting (SC) coherence, but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order, the spatially-modulated 2D and uniform 3D SC orders, imposing an important constraint on theoretical models.

cond-mat.supr-con↗

arXiv2020

Noncollinear magnetic structure in U2Pd2In at high magnetic fields

We report an unexpected magnetic-field-driven magnetic structure in the 5f-electron Shastry- Sutherland system U2Pd2In. This phase develops at low temperatures from a noncollinear antiferromagnetic ground state above the critical field of 25.8 T applied along the a-axis. All U moments have a net magnetic moment in the direction of the applied field, described by a ferromagnetic propagation vector qF = (0 0 0) and an antiferromagnetic component described by a propagation vector qAF = (0 0.30 1/2 ) due to a modulation in the direction perpendicular to the applied field. We conclude that this surprising noncollinear magnetic structure is due to a competition between the single-ion anisotropy trying to keep moments, similar to the ground state, along the [110]-type directions, Dzyaloshinskii-Moryia interaction forcing them to be perpendicular to each other and application of the external magnetic field attempting to align them along the field direction.

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arXiv2019

Nature of the field-induced magnetic incommensurability in multiferroic Ni$_3$TeO$_6$

Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pmδ$),$δ\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. $Q_{IC}$ is found to drift both as function of magnetic field and temperature. Preliminary inelastic neutron scattering reveals that the spin wave gap in zero field has minima exactly at $Q_{IC}$ and a gap of about 1.1 meV consisting with a cross-over around 8.6 T. Our findings excludes the possibility of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magneto-electric coupling earlier observed in this material and advocates for the symmetric exchangestriction as the origin of this effect.

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arXiv2019

Magnetic ground state of the frustrated spin-1/2 chain compound $β$-TeVO$_4$ at high magnetic fields

Frustrated spin-1/2 chains, despite the apparent simplicity, exhibit remarkably rich phase diagram comprising vector-chiral (VC), spin-density-wave (SDW) and multipolar/spin-nematic phases as a function of the magnetic field. Here we report a study of $β$-TeVO$_4$, an archetype of such compounds, based on magnetization and neutron diffraction measurements up to 25 T. We find the transition from the helical VC ground state to the SDW state at $\sim$3 T for the magnetic field along the $a$ and $c$ crystal axes, and at $\sim$9 T for the field along the $b$ axis. The high-field (HF) state, existing above $\sim$18 T, i.e., above $\sim$1/2 of the saturated magnetization, is an incommensurate magnetically ordered state and not the spin-nematic state, as theoretically predicted for the isotropic frustrated spin-1/2 chain. The HF state is likely driven by sizable interchain interactions and symmetric intrachain anisotropies uncovered in previous studies. Consequently, the potential existence of the spin nematic phase in $β$-TeVO$_4$ is limited to a narrow field range, i.e., a few tenths of a tesla bellow the saturation of the magnetization, as also found in other frustrated spin-1/2 chain compounds.

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arXiv2019

Evolution of Magnetic Order from the Localized to the Itinerant Limit

Quantum materials that feature magnetic long-range order often reveal complex phase diagrams when localized electrons become mobile. In many materials magnetism is rapidly suppressed as electronic charges dissolve into the conduction band. In materials where magnetism persists, it is unclear how the magnetic properties are affected. Here we study the evolution of the magnetic structure in Nd(1-x)Ce(x)CoIn(5) from the localized to the highly itinerant limit. We observe two magnetic ground states inside a heavy-fermion phase that are detached from unconventional superconductivity. The presence of two different magnetic phases provides evidence that increasing charge delocalization affects the magnetic interactions via anisotropic band hybridization.

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arXiv2018

Deformation of the Magnetic Skyrmion Lattice in MnSi under Electric Current Flow

Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense observed for opposite sample edges aligned with the direction of current flow. The peak broadening (and the corresponding skyrmion lattice rotations) remain finite even after switching off the electric current. These results indicate that skyrmion lattices under current flow experience significant friction near the sample edges, and plastic deformation due to pinning effects, these being important factors that must be considered for the anticipated skyrmion-based applications in chiral magnets at the nanoscale.

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arXiv2018

Electric field-driven topological phase switching and skyrmion lattice metastability in magnetoelectric Cu$_{2}$OSeO$_{3}$

Due to their topological protection and nanometric size, magnetic skyrmions are anticipated to form components of new high-density memory technologies. In metallic systems skyrmion manipulation is achieved easily under a low density electric current flow, although the inevitable thermal dissipation ultimately limits the energy efficacy of potential applications. On the other hand, a near dissipation-free skyrmion and skyrmion phase manipulation is expected by using electric \emph{fields}, thus meeting better the demands of an energy-conscious society. In this work on an insulating chiral magnet Cu$_{2}$OSeO$_{3}$ with magnetoelectric coupling, we use neutron scattering to demonstrate directly i) the creation of metastable skyrmion states over an extended range in magnetic field and temperature, and ii) the in-situ electric field-driven switching between topologically distinct phases; the skyrmion phase and a competing non-topological cone phase. For our accessible electric field range, the phase switching is achieved in a high temperature regime, and the remnant (E=0) metastable skyrmion state is thermally volatile with an exponential lifetime on hour timescales. Nevertheless, by taking advantage of the demonstrably longer-lived metastable skyrmion states at lower temperatures, a truly non-volatile and near dissipation-free topological phase change memory function is promised in magnetoelectric chiral magnets.

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arXiv2018

Pauling entropy, metastability and equilibrium in Dy$_2$Ti$_2$O$_7$ spin ice

Determining the fate of the Pauling entropy in the classical spin ice material Dy$_2$Ti$_2$O$_7$ with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice - the dipolar spin ice model - predicts an ordering transition at $T\approx 0.15$ K, but recent experiments by Pomaranski $et\ al.$ suggest an entropy recovery over long time scales at temperatures as high as $0.5$ K, much too high to be compatible with theory. Using neutron scattering and specific heat measurements at low temperatures and with long time scales ($0.35$ K$/10^6$ s and $0.5$ K$/10^5$ s respectively) on several isotopically enriched samples we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Further, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez $et\ al.$ are, after all, essentially correct: the short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.

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arXiv2018

Distinct domain switching in Nd0.05Ce0.95CoIn5 at low and high fields

Nd0.05Ce0.95CoIn5 features a magnetic field-driven quantum phase transition that separates two antiferromagnetic phases with an identical magnetic structure inside the superconducting condensate. Using neutron diffraction we demonstrate that the population of the two magnetic domains in the two phases is affected differently by the rotation of the magnetic field in the tetragonal basal plane. In the low-field SDW-phase the domain population is only weakly affected while in the high-field Q-phase they undergo a sharp switch for fields around the a-axis. Our results provide evidence that the anisotropic spin susceptibility in both phases arises ultimately from spin-orbit interactions but are qualitatively different in the two phases. This provides evidence that the electronic structure is changed at the quantum phase transition, which yields a modified coupling between magnetism and superconductivity in the Q-phase.

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arXiv2017

Skyrmion formation in a bulk chiral magnet at zero magnetic field and above room temperature

We report that in a $β$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field-cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature $T_\mathrm{c}$ $\sim$ 400 K. The once-created metastable skyrmions survive at zero magnetic field both at and above room temperature. Such robust skyrmions in a wide temperature and magnetic field region demonstrate the key role of topology, and provide a significant step toward technological applications of skyrmions in bulk chiral magnets.

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arXiv2017

Magnetic structure in U(Ru0.92Rh0.08)2Si2 single crystal studied by neutron diffraction in static magnetic fields up to 24 T

We report the high-field induced magnetic phase in single crystal of U(Ru0.92Rh0.08)2Si2. Our neutron study combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at Hc1 = 21.6 T has an uncompensated commensurate antiferromagnetic structure with propagation vector Q2 = ( 2/3 0 0) possessing two single-Q domains. U moments of 1.45 (9) muB directed along the c axis are arranged in an up-up-down sequence propagating along the a axis, in agreement with bulk measurements. The U magnetic form factor at high fields is consistent with both the U3+ and U4+ type. The low field short-range order that emerges from the pure URu2Si2 due to Rh-doping is initially strengthened by the field but disappears in the field-induced phase. The tetragonal symmetry is preserved across the transition but the a axis lattice parameter increases already at low fields. Our results are in agreement with itinerant electron model with 5f states forming bands pinned in the vicinity of the Fermi surface that is significantly reconstructed by the applied magnetic field.

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arXiv2017

Complete decoupling of magnetic order and superconductivity in a conventional superconductor

Superconductivity and magnetic order strongly compete in many conventional superconductors, at least partly because both tend to gap the Fermi surface. In magnetically-ordered conventional superconductors, the competition between these cooperative phenomena leads to anomalies at magnetic and superconducting phase boundaries. Here we reveal that in Pr2Pt3Ge5 superconducting and multiple magnetic order are intertwined within the same HT-phase space, but remain completely decoupled. Our thermal conductivity measurements provide evidence for normal electrons in the superconducting phase from which magnetic order emerges with negligible coupling to electron bands that contribute to superconductivity.

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arXiv2015

Dynamic reorganization of vortex matter into partially disordered lattices

We report structural evidence of dynamic reorganization in vortex matter in clean NbSe$_2$ by joint small angle neutron scattering and ac-susceptibility measurements. The application of oscillatory forces in a transitional region near the order-disorder transition results in robust bulk vortex lattice configurations with an intermediate degree of disorder. These dynamically-originated configurations correlate with intermediate pinning responses previously observed, resolving a long standing debate regarding the origin of such responses.

cond-mat.supr-con↗

arXiv2015

Incommensurate magnetic structure, Fe/Cu chemical disorder and magnetic interactions in the high-temperature multiferroic YBaCuFeO5

Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO5 up to temperatures TN2 as high as 230K [1,2] we report here for the first time a model for the incommensurate magnetic structure of this material that we complement with ab-initio calculations of the magnetic exchange parameters. Using neutron powder diffraction we show that the appearance of polarization below TN2 is accompanied by the replacement of the high temperature collinear magnetic order by a circular inclined spiral with propagation vector ki=(1/2, 1/2, 1/2+-q). Moreover, we find that the polarization approximately scales with the modulus of the magnetic modulation vector q down to the lowest temperature investigated (T=3K). Further, we observe occupational Fe/Cu disorder in the FeO5-CuO5 bipyramids, although a preferential occupation of such units by Fe-Cu pairs is supported by the observed magnetic order and by density functional calculations. We calculate exchange coupling constants for different Fe/Cu distributions and show that, for those containing Fe-Cu dimers, the resulting magnetic order is compatible with the experimentally observed collinear magnetic structure (kc=(1/2, 1/2, 1/2), TN2 > T > TN1 = 440K). Based on these results we discuss possible origins for the incommensurate modulation and its coupling with ferroelectricity.

cond-mat.str-el↗

arXiv2014

Electric field-induced Skyrmion distortion and giant lattice rotation in the magnetoelectric insulator Cu2OSeO3

Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected magnetic spin vortex-like objects, display a magnetoelectric coupling and can be manipulated by externally applied electric (E) fields. Here, we explore the E-field coupling to the magnetoelectric Skyrmion lattice phase, and study the response using neutron scattering. Giant E-field induced rotations of the Skyrmion lattice are achieved that span a range of $\sim$25$^{\circ}$. Supporting calculations show that an E-field-induced Skyrmion distortion lies behind the lattice rotation. Overall, we present a new approach to Skyrmion control that makes no use of spin-transfer torques due to currents of either electrons or magnons.

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arXiv2014

Fermi-surface topology of the iron pnictide LaFe$_2$P$_2$

We report on a comprehensive de Haas--van Alphen (dHvA) study of the iron pnictide LaFe$_2$P$_2$. Our extensive density-functional band-structure calculations can well explain the measured angular-dependent dHvA frequencies. As salient feature, we observe only one quasi-two-dimensional Fermi-surface sheet, i.e., a hole-like Fermi-surface cylinder around $Γ$, essential for $s_\pm$ pairing, is missing. In spite of considerable mass enhancements due to many-body effects, LaFe$_2$P$_2$ shows no superconductivity. This is likely caused by the absence of any nesting between electron and hole bands.

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