In-plane Exchange Interactions Research Articles

Multiferroic quantum material Ba2Cu1−xMnxGe2O7 (0 ≤ x ≤ 1) as a potential candidate for frustrated Heisenberg antiferromagnet

Multiferroic Ba2CuGe2O7 was anticipated as a potential member of the exciting group of materials hosting a skyrmion or vortex lattice because of its profound Dzyaloshinskii–Moriya interaction (DMI) and the absence of single ion anisotropy (SIA). This phase, however, could not be evidenced and instead, it exhibits a complex incommensurate antiferromagnetic (AFM) cycloidal structure. Its sister compound Ba2MnGe2O7, in contrast, is characterized by a relatively strong in-plane exchange interaction that competes with a non-vanishing SIA and the weak DMI, resulting in a quasi-two-dimensional commensurate AFM structure. Considering this versatility in the magnetic interactions, a mixed solid solution of Cu and Mn in Ba2Cu1−xMnxGe2O7 can hold an interesting playground for its interactive DMI and SIA depending on the mixed spin states of the transition metal ions towards the skyrmion physics. Here, we present a detailed study of the micro- and macroscopic spin structure of the Ba2Cu1−xMnxGe2O7 solid solution series using high-resolution neutron powder diffraction techniques. We have developed a remarkably rich magnetic phase diagram as a function of the applied magnetic field and x, which consists of two end-line phases separated by a potentially quantum-critical phase at x = 0.57. An AFM conical structure at zero magnetic field is demonstrated to persist up to x = 0.50. Our results provide crucial information on the spin structure and magnetic properties, which are necessary for the general understanding and theoretical developments on multiferroicity in the frame of skyrmion type or frustrated AFM lattice where DMI and SIA play an important role.

Coexistence of spin liquid state and magnetic correlations in 3d-5d based triangular-lattice antiferromagnet Sr3CuIr2O9

Here, we report detailed lattice structure, magnetization (dc and ac) and specific heat measurements on a 3d-5d based new triple-perovskite material Sr3CuIr2O9. The Sr/Cu forms a layered structure of triangular-lattice while the Ir forms Ir2O9 dimers which lie in chain as well as simultaneously makes layered triangular-lattice with neighboring atoms. Due to random site-sharing with Sr2+, the Cu2+ (3d 9, spin-1/2) forms a diluted magnetic lattice, thus giving a disordered in-plane exchange interaction. Opposed to conventional model, the Ir5+ (5d 4, = 0) is believed to be magnetic here which participates both in-chain and in-plane magnetic interactions. This complex lattice structure driven competing exchange interaction leads the ground state to a gapless quantum-spin-liquid state which coexists with (weak) ferromagnetic spin correlations. While underling the importance of spin state (spin-1/2), we believe that the combined effect of lattice structure, geometric frustration, spin–orbit coupling and spin state has given rise this interesting ground state in this material.

Magnetism of two-dimensional honeycomb layered Na2Ni2TeO6 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.

In-plane magnetic structure and exchange interactions in the high-temperature antiferromagnet Cr2Al

The ordered tetragonal intermetallic Cr$_2$Al forms the same structure type as Mn$_2$Au, and the latter has been heavily investigated for its potential in antiferromagnetic spintronics due to its degenerate in-plane N\'{e}el vector. We present the single crystal flux growth of Cr$_2$Al and orientation-dependent magnetic properties. Powder neutron diffraction of Cr$_2$Al and first-principles simulations reveal that the magnetic ordering is likely in-plane and therefore identical to Mn$_2$Au, providing a second material candidate in the MoSi$_2$ structure type to evaluate the fundamental interactions that govern spintronic effects. The single ordering transition seen in thermal analysis and resistivity indicates that no canting of the moments along the $c$ axis is likely. Magnetometry, resistivity, and differential scanning calorimetry measurements confirm the N\'{e}el temperature to be $634 \pm 2$ K. First-principles simulations indicate that the system has a small density of states at the Fermi energy and confirm the lowest-energy magnetic ground state ordering, while Monte Carlo simulations match the experimental N\'{e}el temperature.

Magnetic structure, excitations and short-range order in honeycomb Na2Ni2TeO6

Na2Ni2TeO6 has a layered hexagonal structure with a honeycomb lattice constituted by Ni2+ and a chiral charge distribution of Na+ that resides between the Ni layers. In the present work, the antiferromagnetic (AFM) transition temperature of Na2Ni2TeO6 is confirmed at T N ≈ 27 K, and further, it is found to be robust up to 8 T magnetic field and 1.2 GPa external pressure; and, without any frequency-dependence. Slight deviations from nominal Na-content (up to 5%) does not seem to influence the magnetic transition temperature, T N. Isothermal magnetization curves remain almost linear up to 13 T. Our analysis of neutron diffraction data shows that the magnetic structure of Na2Ni2TeO6 is faithfully described by a model consisting of two phases described by the commensurate wave vectors , and , with an additional short-range order component incorporated in to the latter phase. Consequently, a zig-zag long-range ordered magnetic phase of Ni2+ results in the compound, mixed with a short-range ordered phase, which is supported by our specific heat data. Theoretical computations based on density functional theory predict predominantly in-plane magnetic exchange interactions that conform to a J 1–J 2–J 3 model with a strong J 3 term. The computationally predicted parameters lead to a reliable estimate for T N and the experimentally observed zig-zag magnetic structure. A spin wave excitation in Na2Ni2TeO6 at E ≈ 5 meV at T = 5 K is mapped out through inelastic neutron scattering experiments, which is reproduced by linear spin wave theory calculations using the J values from our computations. Our specific heat data and inelastic neutron scattering data strongly indicate the presence of short-range spin correlations, at T > T N, stemming from incipient AFM clusters.

Magnetic and electronic properties of layered Sr2NiO2Cl2 with square planar coordination

Local coordination geometry is crucial in determining the physical properties of transition metal compounds. In this work, we report a detailed density functional theory study on the magnetic and electronic properties of layered Sr2NiO2Cl2, which possesses an unusual square planar coordination. Due to the crystal field and spin splitting, Ni 3d electrons fully occupy the dxz, dyz, and dxy orbitals, while partially occupy the dz2 and dx2-y2 states, leading to the high spin state of Ni2+ ions and strong antiferromagnetic in-plane nearest-neighbor exchange interaction J1. Thus, Sr2NiO2Cl2 is predicted to exhibit a G-type magnetic ground state, which all nearest-neighbor magnetic moments are antiparallel to each other in the NiO2 plane. A band gap is opened irrespective of the specific magnetic state since the unique square planar crystal field splitting. Finally, large orbital moments and giant easy-plane magnetocrystalline anisotropy energy are obtained, which is again related to the unique coordination environment.

Large easy-plane magnetic anisotropy in square planar coordinate Sr2CoO2X2 (X = Cl, Br)

Large magnetocrystalline anisotropy energy (MAE) is an important requirement for applications in spintronic devices. Here, we focus on layered cobalt oxyhalides Sr2CoO2X2 (X = Cl, Br) with unusual square planar coordination, which are naturally expected to possess a strong MAE. Based on density functional calculations, we find Co ions adopt a high spin state due to the large spin-exchange splitting and exhibit a sizable orbital moment of about 0.6 μB for both compounds. The in-plane nearest-neighbor exchange interaction J1 is antiferromagnetic coupling, and at least one order of magnitude stronger than other exchange interactions, which results in the observed G-type magnetic ground state. As expected, Sr2CoO2X2 have a giant MAE of about 13 meV per Co atom, and the magnetic moments prefer to lie in the ab plane, which is largely originated from the unique crystal field environment and could be explained by perturbation theory.

Stripe Discommensuration and Spin Dynamics of Half-Doped Pr_{3/2}Sr_{1/2}NiO_{4}.

We present inelastic neutron scattering measurements of magnetic excitations in stripe ordered Pr_{3/2}Sr_{1/2}NiO_{4} at T∼10 K. For the observed magnetic incommensurability ε=0.4, we have incorporated a stripe discommensuration model in our linear spin wave calculation and obtained best agreement with the measured spin wave dispersion, especially to explain the symmetrical outward shift of the magnetic peaks from Néel ordered zone center in energy range 35 to 45meV. Our study indicates the prerequisite to consider a discommensurated spin stripe unit with proper out-of-plane and in-plane exchange interactions in between Ni^{2+} spins to describe the observed spin wave characteristics in Pr_{3/2}Sr_{1/2}NiO_{4}.

Spin dynamics of a magnetic Weyl semimetal Sr1−xMn1−ySb2

Dirac matters provide a platform for exploring the interplay of their carriers with other quantum phenomena. Sr$_{1-x}$Mn$_{1-y}$Sb$_2$ has been proposed to be a magnetic Weyl semimetal and provides an excellent platform to study the coupling between Weyl fermions and magnons. Here, we report comprehensive inelastic neutron scattering (INS) measurements on single crystals of Sr$_{1-x}$Mn$_{1-y}$Sb$_2$, which have been well characterized by magnetization and magnetotransport measurements, both of which demonstrate that the material is a topologically nontrivial semimetal. The INS spectra clearly show a spin gap of $\sim6$ meV. The dispersion in the magnetic Mn layer extends up to about 76 meV, while that between the layers has a narrow band width of 6 meV. We find that the linear spin-wave theory using a Heisenberg spin Hamiltonian can reproduce the experimental spectra with the following parameters: a nearest-neighbor ($SJ_1\sim28.0$ meV) and next-nearest-neighbor in-plane exchange interaction ($SJ_2\sim9.3$ meV) , interlayer exchange coupling ($SJ_c\sim-0.1$ meV), and spin anisotropy constant ($SD\sim-0.07$ meV). Despite the coexistence of Weyl fermions and magnons, we find no clear evidence that the magnetic dynamics are influenced by the Weyl fermions in Sr$_{1-x}$Mn$_{1-y}$Sb$_2$, possibly because that the Weyl fermions and magnons reside in the Sb and Mn layers separately, and the interlayer coupling is weak due to the quasi-two-dimensional nature of the material, as also evident from the small $SJ_c$ of -0.1 meV.

Interface assisted high magnetoresistance in BiFeO3/Fe97Si3 thin film at room temperature

Bilayer thin films of multiferroic oxide BiFeO3 and metallic alloy Fe97Si3 deposited by RF-magnetron sputtering have been investigated to understand spin polarized electron transport across bilayer interface. Effect of BiFeO3 thickness variation on magnetic and magnetotransport properties of bilayer thin films has been analyzed. An induced unidirectional magnetic anisotropy in bilayer thin films has been observed by interface coupling between antiferromagnetic BiFeO3 and ferromagnetic Fe97Si3 spin magnetic moments. Increasing interface roughness with increase in BiFeO3 film thickness has been found to be a regulatory factor for inducing in-plane unidirectional anisotropy and exchange interaction at bilayer interface. A high magnetic moment ∼315 emu/cc originated at room temperature by uncompensated interface spin density has been measured in 30 nm thick BiFeO3 film. Magnetic inhomogeneities at interface give rise to a spin glass like phase responsible for spin flip scattering at bilayer interface. Enhanced low field magnetoresistance ∼30% in 60 nm thick BiFeO3 film is ascribed to intensified scattering from disordered interface states with increasing BiFeO3 thickness. Magnetodielectric measurements (applied magnetic field 1 kOe) have revealed five times increase in relaxation time of charge carriers inside grains confirming intergrain charge transport dominated by alignment of magnetic moments of nearby grains via exchange coupling. High value of magnetoresistance ∼46% in 60 nm thick film of increased grain size is due to enhanced critical length for charge transport. Observed room temperature magnetoresistive properties of BiFeO3/Fe97Si3 thin films are critical for developing multiferroic antiferromagnetic based spintronic devices.

Ba8MnNb6O24 : A model two-dimensional spin- 52 triangular lattice antiferromagnet

We successfully synthesized and characterized the triangular lattice anitferromagnet Ba$_8$MnNb$_6$O$_{24}$, which comprises equilateral spin-5/2 Mn$^{2+}$ triangular layers separated by six non-magnetic Nb$^{5+}$ layers. The detailed susceptibility, specific heat, elastic and inelastic neutron scattering measurements, and spin wave theory simulation on this system reveal that it has a 120 degree ordering ground state below T$_N$ = 1.45 K with in-plane nearest-neighbor exchange interaction ~0.11 meV. While the large separation 18.9 A between magnetic layers makes the inter-layer exchange interaction virtually zero, our results suggest that a weak easy-plane anisotropy is the driving force for the k$_m$ = (1/3 1/3 0) magnetic ordering. The magnetic properties of Ba$_8$MnNb$_6$O$_{24}$, along with its classical excitation spectra, contrast with the related triple perovskite Ba$_3$MnNb$_2$O$_9$, which shows easy-axis anisotropy, and the iso-structural compound Ba$_8$CoNb$_6$O$_{24}$, in which the effective spin-1/2 Co$^{2+}$ spins do not order down to 60 mK and in which the spin dynamics shows sign of strong quantum effects.

Stabilization of E -type magnetic order caused by epitaxial strain in perovskite manganites

The magnetism and spin-induced ferroelectricity of perovskite $R{\mathrm{MnO}}_{3}$ ($R=\text{La}$-Lu) epitaxial films grown on ${\mathrm{YAlO}}_{3}$ substrate have been investigated by means of first-principles density-functional-theory calculations. We show that the epitaxial strain leads to an $E$-type magnetic ground state for all of the rare-earth manganites, which explains the large ferroelectric polarization observed in ${R\mathrm{MnO}}_{3}$ ($R=\text{Gd}$-Lu) epitaxial films. The transition of the ground state from the $A$-type or spiral antiferromagnetic phase to the $E$-type phase can be attributed mainly to the significant weakening of ferromagnetism in the in-plane nearest-neighbor exchange interaction, which changes from ferromagnetic to antiferromagnetic with the increase of $R$ ionic radius. The enhancement of Jahn-Teller distortion caused by the epitaxial strain plays an important role in the change of exchange interactions. The predicted $E$-type phase in strained films for larger $R$ ions ($R=\text{La}$-Dy) can greatly improve the ferroelectric polarization. However, for the smaller $R$ ions ($R=\text{Ho}$-Lu) whose ground state of the bulk is the $E$-type phase, the ferroelectric polarization of strained films is slightly reduced compared to the bulks, resulting from the competition between the opposite trends of electronic and ion polarization. The ferroelectric distortion caused by the magnetostriction effect is enhanced by the epitaxial strain, while the polarization induced by the pure electronic mechanism is significantly suppressed. The introduction of Coulomb repulsion $U$ in the calculations overestimates the tendency of exchange interactions toward ferromagnetism, leading to the incorrect ground states.

Influence of apical oxygen on the extent of in-plane exchange interaction in cuprate superconductors

In high Tc superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, for three antiferromagnetic parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical atoms, we compare the magnetic spectra measured by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature.

Time-of-Flight Elastic and Inelastic Neutron Scattering Studies on the Localized 4d Electron Layered Perovskite La5Mo4O16

The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La$_5$Mo$_4$O$_{16}$ were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the $ab$ plane with antiferromagnetic stacking along the $c$ axis where the magnetic propagation vector is $\mathbf{k}=\left(0,0,\frac{1}{2}\right)$. The ordered magnetic moments are estimated to be $0.54(2)\mu_\text{B}$ for Mo$^{5+}$ ($4d^1$) ions and $1.07(3)\mu_\text{B}$ for Mo$^{4+}$ ($4d^2$) ions at 4 K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the $c$ axis due to the spin-orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La$_5$Mo$_4$O$_{16}$.

Theoretical investigation of electronic, magnetic, transport and optical properties of the pure and doped cuprate superconductor HgBa2CuO4+δ

We report detailed DFT calculations and Monte Carlo simulations on the pure and doped cuprate superconductor HgBa2CuO4+δ. For the pure compound (δ = 0), we have obtained an insulating behavior with strong antiferromagnetic copper spin correlations in the CuO2 plane. The high value of the calculated Néel temperature TN = 333 K reflects the large in-plane exchange interaction J = -145 meV. The obtained optical properties and critical exponents demonstrate the anisotropic quasi-2D character of this type of materials. As for the doped compound the electronic structure and the transport properties have been investigated for various values of doping. Based on these data, we suggest a prediction of the value of optimum doping for HgBa2CuO4+δ(δoptimal = 0.125).

Periodically driven Kondo impurity in nonequilibrium steady states

We study the nonequilibrium dynamics of a periodically-driven anisotropic Kondo impurity model. The periodic time dependence is introduced for a local magnetic field which couples to the impurity spin and also for an in-plane exchange interaction. We obtain the exact results on the time evolution for arbitrary periodic time dependence at the special point in the parameter space known as the Toulouse limit. We first consider a specific case where the local magnetic field is periodically switched on and off. When the driving period is much shorter than the inverse of the Kondo temperature, an intriguing oscillating behavior (resonance phenomenon) emerges in the time average of the impurity spin polarization with increasing the local magnetic field intensity. By taking the high frequency limit of the external driving, we elucidate that the system recovers the translational invariance in time and can be described by a mixture of the zero-temperature and infinite-temperature properties. In certain cases, the system is governed by either the zero-temperature or infinite-temperature properties, and therefore can be properly described by the corresponding equilibrium state.

Magnetic structure and magnon dynamics of the quasi-two-dimensional antiferromagnetFePS3

Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of FePS3, an S = 2 Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of k(M) = [01 1/2] and the moments are collinear with the normal to the ab planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were J(1) = 1.46, J(2) = -0.04, and J(3) = -0.96 meV. The single-ion anisotropy is large, Delta = 2.66 meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, J' = -0.0073 meV, proving that FePS3 is a very good approximation to a two-dimensional magnet.

Co—Fe Prussian Blue Analogue Intercalated into Diamagnetic Mg—Al Layered Double Hydroxides

A heterostructure of diamagnetic magnesium–aluminium layered double hydroxides (Mg–Al LDHs) and photomagnetic cobalt–iron Prussian Blue analogue (Co–Fe PBA) was designed, synthesized and then designated as LDH–PB. The cyanide-bridged Co–Fe PBA was two-dimensionally intercalated into the Mg–Al LDH template by the stepwise anion exchange method. LDH–PB showed ferrimagnetic properties with in-plane antiferromagnetic exchange interactions, as well as small photo-induced magnetization by visible light illumination due to the low dimensional structures and the characteristic photo-induced electronic states of the mixed valence of FeIII(low spin, S = 1/2)–CN–CoII(high spin, S = 3/2)–NC–FeII (low spin, S = 0).

Spin waves and spatially anisotropic exchange interactions in theS=2stripe antiferromagnetRb0.8Fe1.5S2

An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb$_{0.8}$Fe$_{1.5}$S$_2$ throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of $27.6\pm4.2\mu_B^2$/Fe, consistent with $\mathrm{S \approx 2}$. Unlike $X$Fe$_2$As$_2$ ($X=$ Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb$_{0.8}$Fe$_{1.5}$S$_2$ is a Mott-like insulator with fully localized $3d$ electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.

Charge transfer and orbital reconstruction in the (La2/3Sr1/3MnO3)m/(SrRuO3)n superlattices

The structural, electronic, and magnetic properties of (La2/3Sr1/3MnO3)m/(SrRuO3)n superlattices have been investigated based on the first principles calculations. An obvious Jahn-Teller distortion, which depends on m, n, appears in MnO6 octahedron in the superlattices. The stretch along c-axis of MnO6 octahedron at the interface lifts the Mn eg orbital degeneracy, with electrons preferring the lower energy 3z2−r2 to the higher energy x2−y2. Benefitting from the charge transfer at the interface, the still occupied x2−y2 orbital can mediate a robust in-plane double exchange interaction. La2/3Sr1/3MnO3 block is ferromagnetic and metallic, even for the superlattice with m = n = 1.