arXiv subjects

Ivica Zivkovic

Publications and source records attributed to Ivica Zivkovic.

At least 19 recordsLinked to original sources

arXiv2025

Evidence of rotational and tilting disorder of ReO6 octahedra in single crystals of a 5d1 double perovskite Ba2CaReO6

We present results of an experimental study on single crystals of a 5d1 double perovskite Ba2CaReO6. Magnetization measurements reveal a weak splitting between zero-field-cooled and field-cooled protocols below 12 K. At magnetic fields above 1 T the splitting is absent and the magnetic susceptibility is featureless. A detailed specific heat study in a wide temperature range and comprising different heat pulses did not reveal any indication of a thermodynamic phase transition. At low temperatures we do observe specific heat deviating from a phonon background, leading to a total electronic entropy release of ~Rln2. Resonant and non-resonant x-ray diffraction of characteristic Bragg peaks indicates a significant presence of disorder, potentially related to random tilts and rotations of rigid ReO6 octahedra.

cond-mat.str-el↗

arXiv2024

Dynamic Jahn-Teller effect in the strong spin-orbit coupling regime

Exotic quantum phases, arising from a complex interplay of charge, spin, lattice and orbital degrees of freedom, are of immense interest to a wide research community. A well-known example of such an entangled behavior is the Jahn-Teller effect, where the lifting of orbital degeneracy proceeds through lattice distortions, often accompanied by ordering of spins and metal-insulator transitions. Static distortions, including cooperative behavior, have been associated with colossal magneto-resistance, multiferroicity, high-$T_\mathrm{C}$ superconductivity and other correlated phenomena. Realizations of the dynamic Jahn-Teller effect, on the other hand, are scarce since the preservation of vibronic symmetries requires subtle tuning of the local environment. Here we demonstrate that a highly-symmetrical 5d$^1$ double perovskite Ba$_2$MgReO$_6$, comprising of a 3D array of isolated ReO$_6$ octahedra, fulfils these requirements, resulting in a unique case of a dynamic Jahn-Teller system with strong spin-orbit coupling. Thermodynamic and resonant inelastic x-ray scattering experiments undoubtedly show that the Jahn-Teller instability leads to a ground-state doublet, invoking a paradigm shift for this family of compounds. The restoration of vibronic degrees of freedom arises from a quantum-mechanical zero-point motion, as revealed by detailed quantum chemistry calculations. The dynamic state of ReO$_6$ octahedra persists down to the lowest temperatures, where a multipolar order sets in, allowing for investigations of the interplay between a dynamic JT effect and strongly correlated electron behavior.

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arXiv2023

Cavity-Magnon-Polariton spectroscopy of strongly hybridized electro-nuclear spin excitations in LiHoF4

We first present a formalism that incorporates the input-output formalism and the linear response theory to employ cavity-magnon-polariton coupling as a spectroscopic tool for investigating strongly hybridized electro-nuclear spin excitations. A microscopic relation between the generalized susceptibility and the scattering parameter |S11| in strongly hybridized cavity-magnon-polariton systems has been derived without resorting to semi-classical approximations. The formalism is then applied to both analyze and simulate a specific systems comprising a model quantum Ising magnet (LiHoF4) and a high-finesse 3D re-entrant cavity resonator. Quantitative information on the electro-nuclear spin states in LiHoF4 is extracted, and the experimental observations across a broad parameter range were numerically reproduced, including an external magnetic field titraversing a quantum critical point. The method potentially opens a new avenue not only for further studies on the quantum phase transition in LiHoF4 but also for a wide range of complex magnetic systems.

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arXiv2023

Diffuse spin waves, zero modes and cluster excitations in the spin-3/2 kagomé antiferromagnet chromium jarosite, KCr$_3$(OD)$_6$(SO$_4$)$_2$

The dynamics of the S = 3=2 kagome antiferromagnet chromium jarosite, KCr$_3$(OD)$_6$(SO$_4$)$_2$ was studied using high-resolution neutron time-of-flight spectroscopy on a polycrystalline sample with a nearly stoichiometric magnetic lattice (2.8(2)% Cr vacancies). Neutron spectroscopy reveals diffuse spin wave excitations in the ordered phase with an incomplete gap and significant finite life-time broadening as well as a pronounced zero mode. Using linear spin wave theory, we estimate the exchange couplings, with the leading nearest neighbor value being $J_1$ = 0:884 meV. Above $T_N$ diffuse excitations from cooperative paramagnetism dominate. A model for two-dimensional magnetic cluster excitations is shown to capture the essential features of the data in the cooperative paramagnetic phase.

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arXiv2023

Doping Dependence of the in-Plane Transition in Co$_3$Sn$_2$S$_2$

In Co$_3$Sn$_2$S$_2$ two transitions are observed, the main one to a ferromagnetic state at $T_C = 174$ K and the second one, involving in-plane components at $T_P = 127$ K. We follow their doping dependence as Sn is replaced with In, which causes a reduction of $T_C$ and $T_P$. Importantly, both transitions follow the same doping dependence, indicating a single energy scale involved with both processes.

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arXiv2023

Weyl metallic state induced by helical magnetic order

In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro- or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that Weyl nodes can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that EuCuAs develops a planar helical structure below $T_\textrm{N}$ = 14.5 K which induces Weyl nodes along the $Γ$--A high symmetry line in the Brillouin zone.

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arXiv2022

Mixed Valence Pseudobrookite Al$_{1.75}$Ti$_{1.25}$O$_5$: High Temperature Phase Transitions, Magnetism and Resistivity

Dark blue single crystals of Al$_{1.75}^{3+}$ Ti$_{1.0}^{4+}$ Ti$_{0.25}^{3+}$O$_5$ were grown with a novel synthesis method based on the reaction of a Ti3+/Ti4+ containing langbeinite melt and Al$_2$O$_3$. The obtained needles crystallize in the pseudobrookite structure and undergo two reversible phase transitions from orthorhombic Cmcm to C2/m first and subsequently to C2 symmetry. Like the known aluminum titanate pseudobrookites, anistropic thermal expansion is observed. The temperature evolution of the crystal structure reveals some insights into the mechanism leading to the decomposition of the Al$_{1.75}$Ti$_{1.25}$O$_5$ above 725$^\circ$C. The magnetic and electrical properties are discussed and compared to other reported aluminum titanate pseudobrookites.

cond-mat.mtrl-sci↗

arXiv2022

Unravelling the origin of the peculiar transition in the magnetically ordered phase of the Weyl semimetal Co3Sn2S2

Recent discovery of topologically non-trivial behavior in Co3Sn2S2 stimulated a notable interest in this itinerant ferromagnet (Tc = 174 K). The exact magnetic state remains ambiguous, with several reports indicating the existence of a second transition in the range 125 -- 130 K, with antiferromagnetic and glassy phases proposed to coexist with the ferromagnetic phase. Using detailed angle-dependent DC and AC magnetization measurements on large, high-quality single crystals we reveal a highly anisotropic behavior of both static and dynamic response of Co3Sn2S2. It is established that many observations related to sharp magnetization changes when B || c are influenced by the demagnetization factor of a sample. On the other hand, a genuine transition has been found at Tp = 128 K, with the magnetic response being strictly perpendicular to the c-axis and several orders of magnitude smaller than for B || c. Calculations using density-functional theory indicate that the ground state magnetic structure consist of magnetic moments canted away from the c-axis by a small angle (~ 1.5deg). We argue that the second transition originates from a small additional canting of moments within the kagome plane, with two equivalent orientations for each spin.

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arXiv2022

Tuning topological spin textures in size-tailored chiral magnet insulator particles

Topological spin textures such as skyrmions hold high potential for use as magnetically active elements in diverse near-future applications. While skyrmions in metallic multilayers attract great attention in this context, unleashing the myriad potential of skyrmions for various applications requires the discovery and customization of alternative host system paradigms. Here we developed and applied a chemical method to synthesize octahedral particles of the chiral insulating skyrmion host Cu2OSeO3 with both narrow size distribution, and tailored dimensions approaching the nanoscale. Combining magnetometry and neutron scattering experiments with micromagnetic simulations, we show that the bulk phase diagram of Cu2OSeO3 changes dramatically below octahedral heights of 400 nm. Further particle size-dependent regimes are identified where various topological spin textures such as skyrmions, merons and bobbers can stabilize, prior to a lower critical octahedral height of approx. 190 nm below which no topological spin texture is found stable. These findings suggest conditions under which sparse topological spin textures confined to chiral magnet nanoparticles can be stable, and provide fresh potential for insulator-based application paradigms.

cond-mat.mtrl-sci↗

arXiv2022

The magnetic structure of the topological semimetal Co$_3$Sn$_2$S$_2$

Co$_3$Sn$_2$S$_2$ has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co$_3$Sn$_2$S$_2$ below $T_\textrm{C} = 177$ K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal $c$ axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged $(\textbf{k}=\textbf{0})$ in-plane 120$^\circ$ antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between $\sim$90 K and $T_\textrm{C}$. A discontinuous change in bulk properties and neutron polarization observed at $T = 125$ K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co$_3$Sn$_2$S$_2$ is a magnetic Weyl semimetal.

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arXiv2021

Magnetic field induced quantum spin liquid in the two coupled trillium lattices of K$_2$Ni$_2$(SO$_4$)$_3$

Quantum spin liquids are exotic states of matter which form when strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of the magnetic interactions. Three-dimensional cases are especially challenging due to the significant reduction of the influence of quantum fluctuations. Here, we report the magnetic characterization of {\kni} forming a three dimensional network of Ni$^{2+}$ spins. Using density functional theory calculations we show that this network consists of two interconnected spin-1 trillium lattices. In the absence of a magnetic field, magnetization, specific heat, neutron scattering and muon spin relaxation experiments demonstrate a highly correlated and dynamic state, coexisting with a peculiar, very small static component exhibiting a strongly renormalized moment. A magnetic field $B \gtrsim 4$ T diminishes the ordered component and drives the system in a pure quantum spin liquid state. This shows that a system of interconnected $S=1$ trillium lattices exhibit a significantly elevated level of geometrical frustration.

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arXiv2021

Magnetic and electronic structure of the topological semimetal YbMnSb$_2$

The antiferromagnetic (AFM) semimetal YbMnSb$_2$ has recently been identified as a candidate topological material, driven by time-reversal symmetry breaking. Depending on the ordered arrangement of Mn spins below the Néel temperature, $T_\mathrm{N}$ = 345 K, the electronic bands near the Fermi energy can ether have a Dirac node, a Weyl node or a nodal line. We have investigated the ground state magnetic structure of YbMnSb$_2$ using unpolarized and polarized single crystal neutron diffraction. We find that the Mn moments lie along the $c$ axis of the $P4/nmm$ space group and are arranged in a C-type AFM structure, which implies the existence of gapped Dirac nodes near the Fermi level. The results highlight how different magnetic structures can critically affect the topological nature of fermions in semimetals.

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arXiv2021

Triplons, Magnons, and Spinons in a Single Quantum Spin System: SeCuO3

Quantum spin systems exhibit an enormous range of collective excitations, but their spin waves, gapped triplons, fractional spinons, or yet other modes are generally held to be mutually exclusive. Here we show by neutron spectroscopy on SeCuO$_3$ that magnons, triplons, and spinons are present simultaneously. We demonstrate that this is a consequence of a structure consisting of two coupled subsystems and identify all the interactions of a minimal magnetic model. Our results serve qualitatively to open the field of multi-excitation spin systems and quantitatively to constrain the complete theoretical description of one member of this class of materials.

cond-mat.str-el↗

arXiv2020

Magnetic and structural properties of Ni-substituted magnetoelectric Co$_4$Nb$_2$O$_9$

The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig structure with magnetic moments in the ab-plane. The compound with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly non-collinear ferrimagnetic structure with two possible configurations in ab plane was derived from ND study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provide further information on the magnetic structure of the compounds.

cond-mat.mtrl-sci↗

arXiv2020

Ferrimagnetic 120$^\circ$ magnetic structure in Cu2OSO4

We report magnetic properties of a 3d$^9$ (Cu$^{2+}$) magnetic insulator Cu2OSO4 measured on both powder and single crystal. The magnetic atoms of this compound form layers, whose geometry can be described either as a system of chains coupled through dimers or as a Kagomé lattice where every 3rd spin is replaced by a dimer. Specific heat and DC-susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with a $S=1/2$ degree of freedom per Cu$^{2+}$, and so is the effective moment extracted from DC-susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to a $\sim120$ degree spin structure in which ferromagnetic intra-dimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that \sample \ represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

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arXiv2020

Peculiar magnetic dynamics across the in-field transition in Ca3Co2O6

The discovery of multiple coexisting magnetic phases in a crystallographically homogeneous compound Ca$_3$Co$_2$O$_6$ has stimulated an ongoing research activity. In recent years the main focus has been on the zero field state properties, where exceedingly long time scales have been established. In this study we report a detailed investigation of static and dynamic properties of Ca$_3$Co$_2$O$_6$ across the magnetic field induced transition around 3.5 T. This region has so far been practically neglected while we argue that in some aspects it represents a simpler version of the transition across the $B = 0$ state. Investigating the frequency dependence of the ac susceptibility we reveal that on the high field side ($B > 3.5$ T) the response corresponds to a relatively narrow distribution of magnetic clusters. The distribution appears very weakly dependent on magnetic field, with an associated energy barrier of around 200 K. Below 3.5 T a second contribution arises, with much smaller characteristic frequencies and a strong temperature and magnetic field dependence. We discuss these findings in the context of intra-chain and inter-chain clustering of magnetic moments.

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arXiv2019

Persistent antiferromagnetic order in heavily overdoped Ca$_{1-x}$La$_x$FeAs$_2$

In the Ca$_{1-x}$La$_x$FeAs$_2$ (112) family of pnictide superconductors, we have investigated a highly overdoped composition (x = 0.56), prepared by high-pressure, high-temperature synthesis. Magnetic measurements show an antiferromagnetic transition at TN = 120K, well above the one at lower doping (0.15 < x < 0.27). Below the onset of long-range magnetic order at TN, the electrical resistivity is strongly reduced and is dominated by electron-electron interactions, as evident from its temperature dependence. The Seebeck coefficient shows a clear metallic behavior as in narrow band conductors. The temperature dependence of the Hall coefficient and the violation of Kohler's rule agree with the multiband character of the material. No superconductivity was observed down to 1.8 K. The success of the high-pressure synthesis encourages further investigations of the so far only partially explored phase diagram in this family of Iron-based high temperature superconductors.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic nano-fluctuations in a frustrated magnet

Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures.

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