arXiv subjects

A. K. Bera

Publications and source records attributed to A. K. Bera.

At least 19 recordsLinked to original sources

arXiv2026

Neutron scattering evidence for two-dimensionally coupled spin-dimerized antiferromagnetic lattice in α-Cu2P2O7

The microscopic magnetic model of the low-dimensional quantum magnet alpha-Cu2P2O7 has remained controversial. We present a comprehensive study of its magnetic ground state and excitation spectrum using temperature-dependent inelastic neutron scattering, neutron diffraction, magnetization measurements, and comprehensive spin-wave modeling. Our results unambiguously establish alpha-Cu2P2O7 as a two-dimensionally coupled spin-dimerized antiferromagnetic (AF) lattice within the bc plane, with a dominant AF exchange J2 = 7.73 meV (hereafter referred to as "intradimer exchange") and weaker exchange couplings J1, J3, and J4 in the two-dimensional lattice (hereafter referred to as "interdimer exchange"), in agreement with LDA-based density functional theory and in contrast to previous GGA+U predictions. The dominant intradimer AF exchange is found between seventh-nearest-neighbor Cu-Cu ion pairs [d(Cu-Cu) = 5.125(3) A] rather than nearest-neighbor Cu-Cu ion pairs [d(Cu-Cu) = 3.014(1) A] of the structural dimers. Weak interlayer coupling (J5 = 0.03 meV) stabilizes long-range antiferromagnetic order below TN = 25 K. We further identify a weak single-ion anisotropy, associated with the distorted CuO5 polyhedra, that opens a gap in the spin-excitation spectrum and drives a field-induced metamagnetic transition. Systematic spin-wave calculations elucidate the distinct roles of interlayer coupling J5 and anisotropy term D in producing two distinct energy gaps at different antiferromagnetic zone centers. Complementary neutron diffraction and magnetization measurements as a function of applied magnetic field uncover a previously overlooked metamagnetic transition near 13 kOe and allow construction of the magnetic phase diagram in the H-T plane.

cond-mat.str-el↗

arXiv2026

Frustration-induced degenerate spin state with up-up-down-down ordering in corner-connected Heisenberg square-plaquettes

We investigate frustrated magnetism of a corner-connected square plaquette Heisenberg model with exchange interactions along the edges $(J_1)$, along inter-plaquette links $(J_2)$, and along square diagonals $(J_3)$. Using Luttinger--Tisza (LT) minimization of the Fourier interaction matrix $\mathcal J(\mathbf q)$ together with large-scale Monte Carlo (MC) simulations, we obtain a classical low-temperature magnetic phase diagram in the normalized plane $(J_1/|J_2|, J_3/|J_2|)$. The two methods play complementary roles: the LT analysis provides the zero-temperature candidate ordering wave vectors, while the MC simulations elucidate the resulting ordering tendencies under the hard-spin constraint at low but finite temperatures. Three regimes emerge at low temperatures, an antiferromagnetic phase (AF), a ferromagnetic phase (FM) and a degenerate spin state with \textit{up-up-down-down (uudd)} ordering (DS). For the frustrated degenerate spinstate, LT exhibits line-like minima along $q_x=\pm q_y$ in the $hk$-plane, revealing a highly degenerate spin configuration which violates the hard-spin constraint. The MC results uncover a state with quasi-two-dimensional ordering. The DS regime is intrinsically multi-$\mathbf{q}$: the ordered texture assembles itself from symmetry-related modes on the lines, producing a ``\textit{uudd}'' spin arrangement comprising of distinctive strong antiferromagnetic correlations on diagonals, ferromagnetic correlations on corner links, and highly suppressed correlations on the edges. The field-temperature phase diagram for a representative parameter point in the DS regime, determined by MC simulations, yields field induced distinct regions of negatively and positively correlated layers. These two regions are separated by a curve corresponding to negligible inter-layer correlations.

cond-mat.str-el↗

arXiv2026

Phonon anomalies and critical scaling in the spin-$1/2$ trimer chain Na$_2$Cu$_3$Ge$_4$O$_{12}$

Low-dimensional quantum magnets provide an ideal platform to explore spin-lattice coupling-mediated quantum correlations, which give rise to emergent quasiparticle excitations. The antiferromagnetically coupled spin-1/2 trimer chain of copper ions in Na$_2$Cu$_3$Ge$_4$O$_{12}$ (NCGO) hosts high-energy spin excitations of different species, whose energy scales overlap with those of lattice vibrations. Here, we report a comprehensive temperature-dependent Raman spectroscopic study performed between 80 and 400 K. The dynamic spin susceptibility, as obtained from the analysis of the broad spectral background, reveals the emergence of quasiparticle excitations below 170 K. We further identify an unusual crossover of phonon dynamics when the material transits from a normal paramagnetic state to a correlated quantum magnetic state. A power law dependence of the integrated Raman susceptibility of the phonon modes, $I_{χ^{\prime\prime}}^{i}\sim|T-T_{c}|^β$, is observed with the critical temperature $T_c$=167$\pm$1 K, and critical exponent $β= 0.24\pm 0.02$. The combined results obtained from the broad spectral background and sharp phonon peaks further indicate that the phonon renormalization observed across the crossover is driven by dynamic spin states. Additionally, statistical correlations among phonon energy eigenvalues, quantified through matrix-norm and power-test analyses of 200 spectra recorded at 80 K, reveal an unexpected linear correlation among phonon modes, also indicating that the collective lattice response is governed by spin correlations. These findings establish NCGO as a model system for investigating cooperative spin-lattice coupling and critical scaling behavior of phonon dynamics in low-dimensional magnetic materials.

cond-mat.str-el↗

arXiv2026

Fermionic Band Dispersions and an Evidence of Cooperon Excitations in a Spin-$1/2$ Trimer Chain

We obtain the solution of the Hamiltonian of an antiferromagnetically coupled spin-$1/2$ trimer chain in terms of three bands that host three different species of fermions. While the lowest two bands correspond to spin-$1/2$ fermions, the fermions in the highest band are of spin-$3/2$. Because the bands are for different species of fermions, the particle-hole excitation channel across the bands is closed. However, fractionalized excitations as spin-$1/2$ and spin-$3/2$ fermions in pairs open a cooperon channel of excitations in Raman scattering. The background spectral intensity profile obtained by Raman scattering measurements in Na$_2$Cu$_3$Ge$_4$O$_{12}$ having a trimer chain consisting of spin-$1/2$ Cu ions, has comprehensively been shown to be consistent with these excitations.

cond-mat.str-el↗

arXiv2026

Fermionic Approach to Elementary Excitations and Magnetization Plateaus in an S=1/2 XX Hybrid Trimer-Dimer Chain

We study the elementary excitations and magnetization of a one-dimensional spin-1/2 XX chain comprising trimer-dimer units (the J1-J1-J2-J3-J2 topology) under a transverse magnetic field h. Using Green's function theory and the Jordan-Wigner transformation, we map the system onto spinless fermions and focus on antiferromagnetic (AFM) interactions. At zero temperature, distinct 1/5 and 3/5 magnetization plateaus emerge, determined by the global periodicity Q=5, with the number of plateaus matching the number of excitation gaps above the Fermi level of the spinless fermions. The magnetic phase diagram in the (h-Js) plane features a Luttinger liquid (LL) state, a gapless AFM state, two magnetization plateau states, and a fully polarized gapped magnetic state. The widths of the LL and gapless AFM phases are found to be proportional to the bandwidths gamma = |E(k=0)-E(k=pi)| of the corresponding elementary excitations, whereas the widths of the magnetization plateau states are governed by the excitation gaps. Our study opens new directions for exploring interacting trimer-dimer spin chains in quantum magnetism using experimental techniques such as neutron scattering, as well as theoretical and numerical approaches including quantum Monte Carlo (QMC) and density-matrix renormalization group (DMRG) methods. Furthermore, we extend the Oshikawa-Yamanaka-Affleck (OYA) condition to generalized cluster chains, demonstrating that the allowed magnetization plateaus are governed by the global periodicity of the chain (e.g., Q=5 for a trimer-dimer chain), rather than by the local periodicity of individual units (Q=3 for a trimer or Q=2 for a dimer).

cond-mat.str-el↗

arXiv2025

Probing the magnetic ground state and magnetoelastic coupling in double perovskite ruthenate: Ca2ScRuO6

Ruthenates, materials with a single magnetic Ruthenium (Ru) atom, often display an exotic array of ground states ranging from superconductivity to altermagnetism. In this work, we investigated the magnetic ground state of a least explored member of the 4d3 double perovskite ruthenate series A2ScRuO6 (A = Ca, Sr, Ba): Ca2ScRuO6. Interestingly, temperature-dependent bulk susceptibility curve shows ferrimagnetic-like behaviour above the magnetic ordering at around 40 K, which were corroborated by the identification of the mixed valence states, Ru5+ and Ru4+ via X-ray absorption spectroscopy. Structural analysis further revealed atomic-site exchange between the Ru and Sc sites, which results in the Ru mixed valence states. Neutron powder diffraction measurements detected the presence of magnetic Bragg peaks at a low temperature near 4 K and a moderate magnetoelastic coupling near the ordering temperature of 40 K. However, the corresponding symmetry analysis shows a weak Type I antiferromagnetic ground state with a reduced magnetic moment of 1.1μB/Ru atom. Our findings establish an unusual magnetic ground state in the Mott insulating Ca2ScRuO6, where a long range ordered antiferromagnet coexists with small magnetic clusters, which manifests a ferrimagnetic-like high temperature inverse magnetic susceptibility. This system presents a unique platform to study long-range magnetic order in the presence of antisite disorder.

cond-mat.str-el↗

arXiv2025

Multiscale Magnetic Correlations in La2Mn2-xNixO6: Role of Crystal Structure in Double Perovskites

The magnetic correlations in double perovskites La2Mn2-xNixO6 (x = 0.5, 0.75, 1.0, 1.25 and 1.5) have been systematically investigated across macroscopic, mesoscopic, and microscopic length scales using temperature-dependent bulk DC magnetization, neutron depolarization, and neutron powder diffraction measurements, respectivitly. The magnetic properties evolve from a long-range ferromagnetic (FM) order to a cluster ferromagnetic or spin-glass (FM or SG) behavior as the Ni concentration increases. This evolution is directly linked to changes in the crystal structure, transitioning from pure orthorhombic (x=0.5) to mixed orthorhombic and monoclinic (x=0.75-1.0), and eventually to mixed trigonal and monoclinic symmetries (x=1.25-1.5). Ni substitution enhances the magnetic ordering temperature from 170 K (x=0.5) to 280 K (x=1.0), but this is accompanied by a reduction in both magnetization and ordered magnetic moment. Beyond x=1.0, any long-range magnetic ordering is absent. Additionally, all compositions exhibit a reentrant spin-glass-like phase at low temperatures (below about 50 K). Neutron diffraction analysis confirms that long-range FM order occurs only in the orthorhombic phase, while the monoclinic and trigonal phases lack such magnetic ordering. The temperature-dependent magnetic correlations are closely connected to variations in crystal structural parameters, including lattice constants and unit cell volume. The electrical conductivity behavior, following the variable range hopping (VRH) model, highlights the role of multivalence Mn and Ni ions on the electrical properties. This study elucidates the microscopic mechanisms behind the tunable magnetic and electrical properties of La2Mn2-xNixO6, offering valuable insights for the design of advanced materials for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Evidence for magnetoelastic coupling and chiral magnetic ground state in quasi-van der Waals tr-Cr$_{1.22}$Te$_{2}$

Trigonal tr-Cr$_{1+δ}$Te$_{2}$ is a well-known ferromagnetic material that has recently drawn much attention due to the discovery of zero-field skyrmion state, unusual anomalous Hall effect, topological Hall effect, and topological Nernst effect. This quasi-van der Waals (vdW) layered material with intercalated Cr atoms possesses many peculiar features that depend on the amount of Cr intercalation, although the microscopic magnetic ground state is still elusive. We reveal the structural and magnetic properties of tr-Cr$_{1.22}$Te$_{2}$ by low-temperature x-ray diffraction, magnetization, temperature-dependent Raman spectroscopy, and single-crystal neutron diffraction studies. Magnetization measurements under small applied magnetic field indicate two successive magnetic transitions, one from a ferromagnetic (FM) state to an antiferromagnetic (AFM) state (T$_\mathrm{C}=197$ K), and second from AFM to a paramagnetic state (T$_\mathrm{N}=211$ K). The FM transition is sharp with a strong presence of magnetoelastic coupling, but is not accompanied by any structural phase transition. The magnetic structure obtained from zero-field single crystal neutron diffraction reveals that the Cr1 and Cr2 moments are ferromagnetically aligned along the c-axis, while the Cr3 and intercalated Cr4 atoms induce an AFM component in the ab-plane leading to an umbrella-like spin structure which possesses a finite spin chirality. The presence of a finite spin chirality is responsible for the observation of the topological Hall effect (THE).

cond-mat.str-el↗

arXiv2025

Magnetic Ground State and Spin Excitations in the 2D Trimerized Collinear-II Lattice Antiferromagnet Li2Ni3P4O14

We report the magnetic ground state, spin excitations, and spin Hamiltonian of the 2D spin-1 trimerized Heisenberg antiferromagnet Li2Ni3P4O14. Below the magnetic ordering temperature TN = 14.5 K, the compound exhibits a canted long-range antiferromagnetic order with a propagation vector k = (0 0 0), consistent with the magnetic space group P21/c.1 (No. 14.75). The ground state magnetic structure consists of ferromagnetic spin-trimers of Ni2+ ions. The spin-trimers are coupled antiferromagnetically along the c-axis and ferromagnetically along the a-axis. Inelastic neutron scattering (INS) reveals gapped and dispersive magnon excitations below the TN, and gapless quasi-elastic scatterings at higher temperature. The linear spin-wave theory simulations reveal the essential features of the excitation spectrum; by a spin Hamiltonian composed of ferromagnetic intra-trimer exchange interaction J1 and inter-trimer exchange interactions J2 (FM) and J3(AFM) within the bc plane. The J2 and J3 along the b-axis and c-axis, respectively, with strengths of J2/J1=0.79 and J3/J1=-0.91. In addition, a weak inter planer ferromagnetic exchange interaction J4 (|J4/J1|~0.12) is found along the a-axis. The determined exchange constants reveal a 2D trimerized Collinear-II spin lattice within the bc-plane. The analysis of INS spectra by linear spin-wave theory also yields a moderate single-ion anisotropy (D/J1=0.48) which accounts for the observed spin gap below TN as well as the metamagnetic transition near 44 kOe in dc magnetization (M vs H) curves. These findings identify Li2Ni3P4O14 as a rare realization of a two-dimensional trimerized spin system and offer the direct experimental confirmation of theoretically predicted magnon excitations, unveiling the fundamental characteristics of the expected excitation spectrum.

cond-mat.str-el↗

arXiv2025

Magnetic excitations and exchange parameters of a nickel chain compound PbMn$_2$Ni$_6$Te$_3$O$_{18}$: Neutron scattering and density functional theory studies

We have investigated the quasi-one dimensional Ni-chain compound PbMn$_2$Ni$_6$Te$_2$O$_{18}$ using theoretical DFT calculations, inelastic neutron scattering and optical spectroscopy in order to understand the nature of magnetic exchange interactions. Our inelastic neutron scattering study at 5 K on a powder sample reveals two bands of magnetic excitations, the first near 8 meV and the second near 18 meV originating from the antiferromagnetic zone center near $Q$ = 1~Å. On the other hand at 100 K (which is above T$_N$ = 86 K) a broad diffuse scattering signal is observed indicating the presence of short range magnetic correlations. We have analyzed the magnetic excitations based on the Linear Spin Wave Theory (LSWT) and compared the experimentally estimated exchange parameters with the DFT calculations. Our analysis reveals that the value of the exchange parameter at the larger distance (d=3.654 $Å$) $J_3$=4.21(8) meV between Ni-Ni (from inter-chain) is the strongest amongst the allowed six exchange parameters, which suggests that this system is not really a quasi-one-dimensional and confirmed by the absence of a Haldane gap. We have also presented the electronic structure calculations. The spin-polarized partial density of states (DOS) projected onto the Mn-d and Ni-d orbitals reveals that the Ni-d$_{x^2-y^2}$ contribution is dominant below the Fermi level in the spin-up and spin-down channel, while a minimal contribution from spin-up Mn states in the occupied region, suggesting a nearly high-spin state. The estimated Néel temperature, based on experimental exchange parameters is found to be in close agreement with the experimental value.

cond-mat.str-el↗

arXiv2024

Effect of antisite disorder on the magnetic and transport properties of a quaternary Heusler alloy

Spin gapless semiconductors based Heusler alloys are the special class of materials due to their unique band structure, high spin polarization and high Curie temperature. These materials exhibit a distinct electronic structure: a nonzero band gap in one spin channel while the other spin channel remains gapless, making them highly suitable for tunable spintronics. In this study, a comprehensive analysis of structural, magnetic, thermoelectric, and transport properties of the quaternary Heusler alloy CoFeMnSn is conducted. X-ray diffraction and Neutron diffraction analyses confirm a well ordered structure with partial antisite disorder between Co, Fe and Mn, Sn atoms. Magnetic studies show that the material exhibits room-temperature ferromagnetism, with a Curie temperature of around 660 K. Notably, we observe an anomalous Hall effect linked to intrinsic mechanisms driven by Berry curvature, underscoring the intricate relationship between structural disorder and electronic behavior. Transport measurements also highlight the impact of antisite disorder on the systems, with resistivity decreasing as temperature increases. These insights position CoFeMnSn as a promising material for future spintronic devices and advanced technological applications.

cond-mat.mtrl-sci↗

arXiv2024

Structural modulation driven Curie temperature enhancement in Cr-doped SrRuO3

Strongly correlated system with competing ground states are often poised close to the quantum critical point. External perturbations such as pressure, strain, electric field, and chemical doping can stabilise its ground state with exotic physical properties. Cr-doping is the lone exception which enhances the Curie-temperature in one of such correlated system SrRuO$_3$. To find the origin of $T_C$ enhancement, we investigate temperature-dependent structure, spectroscopic, magnetic and magnetotransport properties in SrRu$_{1-x}$Cr$_x$O$_3$. Cr-doping squeezes the unit cell volume which effectively enhances the stretching octahedral distortion by nearly five times than pure SrRuO$_3$. The Curie temperature increment by $\sim$ 22 K for x = 0.15 is found to be intertwined with the structural-modulation. Temperature-dependent Neutron diffraction analysis indicate that the unit cell volume minima coincide exactly with the enhanced ferromagnetic ordering ($\sim$ 190 K). Further analysis reveals that the effect of Cr-doping not only freezes the octahedral tilt below 100 K but also suppresses the complex magnetism responsible for exchange bias and topological hall effect in SrRuO$_3$. The spectroscopic measurements find a reduction of itinerancy of d-electrons with Cr-doping. The magnetotransport measurements portray an evolution from itinerant to localised ferromagnetism.

cond-mat.str-el↗

arXiv2024

Antiferromagnetic ordering and glassy nature in NASICON type NaFe$_2$PO$_4$(SO$_4$)$_2$

We investigate crystal structure and magnetic properties including spin relaxation and magnetocaloric effect in NASICON type NaFe$_2$PO$_4$(SO$_4$)$_2$ sample. The Rietveld refinement of x-ray and neutron diffraction patterns show a rhombohedral crystal structure with the R$\bar{3}$c space group. The core-level spectra confirm the desired oxidation state of constituent elements. The {\it dc}--magnetic susceptibility ($χ$) behavior in zero field-cooled (ZFC) and field-cooled (FC) modes show the ordering temperature $\approx$50~K. Interestingly, the analysis of temperature dependent neutron diffraction patterns reveal an A-type antiferromagnetic (AFM) structure with the ordered moment of 3.8 $μ_{B}$/Fe$^{3+}$ at 5~K, and a magnetostriction below $T_{\rm N}=$ 50~K. Further, the peak position in the {\it ac}--$χ$ is found to be invariant with the excitation frequency supporting the notion of dominating AFM transition. Also, the unsaturated isothermal magnetization curve supports the AFM ordering of the moments; however, the observed coercivity suggests the presence of weak ferromagnetic (FM) correlations at 5~K. On the other hand, a clear bifurcation between ZFC and FC curves of {\it dc}--$χ$ and the observed decrease in peak height of {\it ac}--$χ$ with frequency suggest for the complex magnetic interactions. The spin relaxation behavior in thermo-remanent magnetization and aging measurements indicate the glassy states at 5~K. Moreover, the Arrott plots and magnetocaloric analysis reveal the AFM--FM interactions in the sample at lower temperatures.

cond-mat.str-el↗

arXiv2023

Field-induced phase transitions and anisotropic magnetic properties of the Kiteav-Heisenberg compound Na$_2$Co$_2$TeO$_6$

Spin systems with honeycomb structures have recently attracted a great deal of attention in connection with the Kitaev quantum spin liquid state (QSL) predicted theoretically. One possible Kitaev QSL candidate is Na$_2$Co$_2$TeO$_6$ realizing a honeycomb lattice of pseudo-spin-1/2. Field-dependent single-crystal neutron diffraction technique allows us to determine the microscopic spin-spin correlations across the field induced phase transitions for H//a and H//a* in plane field directions. Our results reveal phase transitions, initially to a canted zigzag antiferromagnetic state at approximately 60 kOe, followed by a possible transition to a partially polarized state over the range of 90-120 kOe, and finally to a field-induced fully polarized state above 120 kOe. We observe distinct field dependencies of the magnetic peak intensities for H//a and H//a*. In addition, low-temperature electron spin resonance in magnetic fields H//c yields a complete softening for one of the antiferromagnetic resonances at ~ 40 kOe, revealing a field-induced phase transition. The present work, thus, provides new insights into the field evolution of the important Kitaev-Heisenberg spin system Na$_2$Co$_2$TeO$_6$.

cond-mat.str-el↗

arXiv2023

Two-dimensional short-range spin-spin correlations in the layered spin-3/2 maple leaf lattice antiferromagnet Na2Mn3O7 with crystal stacking disorder

We report the nature of magnetic structure, microscopic spin-spin correlations and their dependence on the underlying crystal structure of the geometrically frustrated layered spin-3/2 maple-leaf-lattice (MLL) antiferromagnet Na2Mn3O7 by a comprehensive neutron diffraction study. Crystal structural studies by x-ray and neutron diffractions reveal that the MLL layers (constituted by Mn3O72- units) are well separated by non-magnetic Na layers. The studies also conclude the presence of stacking faults (in-plane sliding of magnetic MLL layers) as well as a distortion in the MLL of Mn4+. Temperature dependent magnetic susceptibility, heat capacity, and neutron diffraction data yield a short-range antiferromagnetic (AFM) ordering below ~ 100 K without a long-range magnetic ordering down to 1.5 K. The analysis of the diffuse magnetic neutron scattering patterns by reverse Monte Carlo method reveals 2D spin-spin correlations within the MLL layers. Additionally, we establish a relation between the correlation length of the short-range magnetic ordering with the stacking faults through a varying synthesis condition. The present study, therefore, explores a microscopic picture of the crystal- and spin-structures, as well as their correlation, hence, provides an experimental insight of the magnetic ordering in a MLL AFM. Further, we have outlined the formation of several 2D frustrated lattice geometry having triangular plaquettes, including the MLL, by crystal engineering of the triangular lattice and their role on the stabilization of multiple novel chiral spin states which opens up a door for study of novel chiral spin states.

cond-mat.str-el↗

arXiv2022

Emergent many-body composite excitations of interacting spin-1/2 trimers

Understanding exotic forms of magnetism in quantum spin systems is an emergent topic of modern condensed matter physics. Quantum dynamics can be described by particle-like carriers of information, known-as quasiparticles that appear from the collective behaviour of the underlying system. Spinon excitations, governing the excitations of quantum spin-systems, have been accurately calculated and precisely verified experimentally for the antiferromagnetic chain model. However, identification and characterization of novel quasiparticles emerging from the topological excitations of the spin system having periodic exchange interactions are yet to be obtained. Here, we report the identification of emergent composite excitations of the novel quasiparticles doublons and quartons in spin-1/2 trimer-chain antiferromagnet Na2Cu3Ge4O12 (having periodic intrachain exchange interactions J1-J1-J2) and its topologically protected quantum 1/3 magnetization-plateau state. The characteristic energies, dispersion relations, and dynamical structure factor of neutron scattering as well as macroscopic quantum 1/3 magnetization-plateau state are in good agreement with the state-of-the-art dynamical density matrix renormalization group calculations.

cond-mat.str-el↗

arXiv2022

Magnetic structure and properties of a vanthoffite mineral Na6Mn(SO4)4

A detailed analysis of the magnetic properties of a vanthoffite type mineral Na6Mn(SO4)4 basedon dc magnetization, low temperature neutron powder diffraction and theoretical calculations is reported. The mineral crystallizes in a monoclinic system with space group P21/c, where MnO6 octahedra are linked via SO4 tetrahedra. This gives rise to super-exchange interaction between two Mn2+ ions mediated by two nonmagnetic bridging anions and leads to an antiferromagnetic ordering below 3 K. The magnetic structure derived from neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin arrangement in the bc plane of the crystal. The magnetic properties are modelled by numerical calculations using exact diagonalization technique, which fits the experimental results and provides antiferromagnetic ground state of Na6Mn(SO4)4.

cond-mat.mtrl-sci↗

arXiv2022

Magnetism of the 2D honeycomb layered Na2Ni2TeO6 compound driven by intermediate Na-layer crystal-structure

The microscopic spin-spin correlations in the 2D layered spin-1 honeycomb lattice compound Na2Ni2TeO6 have been investigated by neutron diffraction and inelastic neutron scattering. The honeycomb lattice of spin-1 Ni2+ ions, within the crystallographic ab planes, are well separated along the c axis by an intermediate Na layer whose crystal structure contains chiral nuclear density distributions of Na ions. The chirality of the alternating Na layers is opposite. Such alternating chirality of the Na layer dictates the magnetic periodicity along the c axis where an up-up-down-down spin arrangement of the in-plane zigzag AFM structure is found. Besides, the above described commensurate (CM) zigzag AFM order state is found to coexist with an incommensurate (ICM) AFM state below the TN ~ 27.5 K. The ICM state is found to appear at much higher temperature ~ 50 K and persists down to lowest measured temperature of 1.7 K. Our reverse Monte Carlo (RMC) analysis divulges a two dimensional (2D) magnetic correlations (within the ab plane) of the ICM AFM state over the entire temperature range 1.7-50 K. Further, the spin-Hamiltonian has been determined by carrying out inelastic neutron scattering experiments and subsequent linear spin-wave theory analysis which reveals the presence of competing inplane exchange interactions up to 3 rd nearest neighbours consistent with the zigzag AFM ground state, and weak interplanar interaction as well as a weak single-ion-anisotropy. The values of the exchange constants yield that Na2Ni2TeO6 is situated well inside the zigzag AFM phase (spans over a wide ranges of J2/J1 and J3/J1 values) in the theoretical phase diagram. The present study, thus, provides a detailed microscopic understanding of the magnetic correlations and divulges the intertwining magneto-structural correlations.

cond-mat.str-el↗