Antiferromagnetic Exchange Interactions Research Articles

Copper(II) halide salts of 5-bromo-2-aminopyridine and 3,5-dibromo-2-aminopyridine: syntheses, structures and magnetic behavior

Two 5-bromo-2-aminopyridinium quantum Heisenberg antiferromagnetic (QHAF) complexes of copper(II) have been synthesized and studied structurally and magnetically. The structures of (5-BAPH)2[CuCl4] (1) and (5-BAPH)(3,5-diBAPH)[CuBr4] (2) are reported [5-BAPH = 2-amino-5-bromopyridinium; 3,5-diBAPH = 2-amino-3,5-dibromopyridinium]. Single-crystal X-ray diffraction measurements show that 1 crystallizes in the triclinic space group Pī and 2 crystallizes in the monoclinic space group P21/c. Both products crystallize with a four-coordinate Cu(II) ion and are stabilized with extensive hydrogen bonding between the halide ions and pyridinium and amino substituents on each cation. The tetrahalidocuprate ions in 1 and 2 exist in distorted geometries between square planar and tetrahedral with mean trans angles of 139.48° and 131.85°, respectively. Halogen bonding is also observed. Magnetic susceptibility data were collected on 1 and 2. Magnetic susceptibility data of 1 exhibit moderate antiferromagnetic interactions which are best fit using an alternating chain model [CC = 0.4262(3) cm3 K mol−1, J/kB = −22.75(2) K, α = 0.134(3)]. Magnetic susceptibility data of 2 exhibit weak antiferromagnetic interactions, best fit using a uniform chain model [CC = 0.410(6), J/kB = −6.1(4) K].

Electrical, optical, and magnetic properties of amorphous yttrium iron oxide thin films and consequences for non-local resistance measurements

We present magnetic characterization, charge resistivity, and optical photoluminescence measurements on amorphous yttrium iron oxide thin films (a-Y–Fe–O), with supporting comparisons to amorphous germanium (a-Ge) films. We measured magnetic properties with both SQUID magnetometry and polarized neutron reflectometry. These results not only confirm that a-Y–Fe–O is a disordered magnetic material with strong predominantly antiferromagnetic exchange interactions and a high degree of frustration, but also that it is best understood electrically as a disordered semiconductor. As with amorphous germanium, a-Y–Fe–O obeys expectations for variable-range hopping through localized electron states over a wide range of temperature. We also clarify the consequences of charge transport through such a semiconducting medium for non-local voltage measurements intended to probe spin transport in nominally insulating magnetic materials. We further compare non-local resistance measurements made with “quasi-dc” automated current reversal to ac measurements made with a lock-in amplifier. These show that the “quasi-dc” measurement has an effective ac current excitation with frequency up to approximately 22 Hz, and that this effective ac excitation can cause artifacts in these measurements including incorrect sign of the non-local resistance. This comprehensive investigation of non-local resistance measurements in a-Y–Fe–O shows no evidence of spin transport on micrometer length scales, which is contrary to our original work, and in line with more recent investigations by other groups.

Low-frequency spin dynamics of the quasi-two-dimensional S = 12 antiferromagnet BaCdVO(PO4)2

A quasi-two-dimensional quantum magnet with competing ferro- and antiferromagnetic exchange interactions, $\mathrm{BaCdVO}{({\mathrm{PO}}_{4})}_{2}$, is studied using the magnetic resonance technique in a wide frequency range. The magnetic resonance spectra of the ordered phase confirm the collinear antiferromagnetic spin structure oriented along the $a$ axis. The frequency-field diagram of antiferromagnetic resonance demonstrates two low-frequency excitation branches with different energy gaps, indicating a biaxial nature of anisotropy. We observe a critical softening of the longitudinal resonance mode in a field of 3.8 T, which is the lower boundary of a previously proposed presaturation phase positioned below the saturation field of 6.5 T. An interpretation of the presaturation phase as a state with variable magnetization of spin-vacancy defects is proposed. It may be considered an alternative to the spin-nematic phase initially supposed.

The modulation of magnetism and charge carrier in magnetic topological insulator MnBi4Te7 by Pb and Sb co-doping

The intrinsic magnetic topological insulator (MTI) MnBi2Te4 family containing magnetic order and nontrivial topology are fantastic materials for studying exotic topological states, such as quantum anomalous Hall effect, but the strong antiferromagnetic (AFM) coupling hinders potential practical applications. Here, we carry out magnetic and transport measurements on (Mn(1−x)Pbx)Bi4Te7 (0 ≤ x ≤ 0.52) single crystals. As x increases from 0 to 0.45, the AFM exchange interaction is gradually weakened by the dilution of magnetic moments. When x reaches up to 0.52, the AFM coupling is completely suppressed and it turns into a ferromagnetic -like ground state. Additionally, Sb substitutions on Bi sites in this system can not only alter the charge carriers but also further influence the magnetism and bring multiple magnetic phase transitions under low temperatures. Our results demonstrate a MTI candidate with adjustable magnetic ground state and charge carriers in Pb and Sb co-doped MnBi4Te7, which provides a promising platform to study rich topological quantum phases and benefits the potential applications in the future.

Topological Spin Textures in a Non-Collinear Antiferromagnet System.

Topologically protected magnetic "whirls" such as skyrmions in antiferromagnetic materials have recently attracted extensive interests due to their nontrivial band topology and potential application in antiferromagnetic spintronics. However, the room-temperature skyrmions in natural metallic antiferromagnetic materials with merit of probably conveniently electrical manipulation have not been reported. Here, we realize room-temperature skyrmions in a non-collinear antiferromagnet Mn3 Sn capped with a Pt overlayer. The evolution of spin textures from coplanar inverted triangular structures to Bloch-type skyrmions is achieved via tuning the magnitude of interfacial Dzyaloshinskii - Moriya interaction. Beyond that, the temperature can induce an unconventional transition from skyrmions to antiferromagnetic meron-like spin textures at approximately 220 K in our Mn3 Sn/Pt samples. Combining with the theoretical calculations, we find the transition is originated from the temperature dependence of antiferromagnetic exchange interaction between kagome sublayers within the Mn3 Sn crystalline unitcell. Our findings open the avenue for the development of topological spin-swirling-based antiferromagnetic spintronics. This article is protected by copyright. All rights reserved.

Tunable topological magnon phases in layered ferrimagnets

The introduction of effective spin-orbit couplings into magnon systems provides the opportunities to discover exotic and fascinating topological phases. In this work, we theoretically study the magnon spin-orbit coupling due to the natural dipole-dipole interaction in a layered collinear easy-axis ferrimagnet with two types of ferromagnetic sublayers coupled via antiferromagnetic interlayer exchange interaction, where we predict a nodal-loop semimetal phase and a Weyl semimetal phase in perpendicularly and laterally magnetized geometries, respectively. By driving the spin texture into a spin-canting phase with a magnetic field perpendicular to the easy axis, the nodal loop with a normal easy axis reduces to two Weyl points in the noncollinear spin configuration. Magnon arcs in the Weyl semimetal phases in both geometries are predicted in ferrimagnetic film with finite layers. Our results reveal the role of noncollinear spin texture on magnon topology, which may enrich the investigations in topological ferrimagnonics.

Thermal and magnetic properties of Cu4O(SeO3)3 composed by ferrimagnetic O2Cu6 units of edge-sharing OCu4 tetrahedra

Monoclinic Cu4O(SeO3)3 has been prepared hydrothermally with subsequent heating in evacuated silica tubes. It has been characterized in measurements of dc and ac magnetic susceptibility χ and specific heat Cp. The title compound orders magnetically at T1 = 13.2 K and evidences a pre-transitional anomaly at T2 = 17.2 K. In the range T1 < T < T2, the application of external magnetic field rapidly suppresses excessive magnetization of this intermediate phase, but the hump in specific heat appears. The magnetic system of the title compound is constituted by the layered network of isolated Cu atoms and O2Cu6 units composed by edge-sharing anion-centered OCu4 tetrahedra. First principles calculations provided values of both ferromagnetic and antiferromagnetic exchange interactions within O2Cu6 units J1 – J11 and J12 – J22, and antiferromagnetic intralayer J23 – J30 and interlayer J31 – J32 exchange interactions. Basing on signs and values of these interactions a magnetic model of monoclinic Cu4O(SeO3)3 is suggested.

Theoretical study on the magnetic properties of cathode materials in the lithium-ion battery.

The layered LiMO2 (M = Co, Ni, and Mn) materials are commonly used as the cathode materials in the lithium-ion battery due to the distinctive layer structure for lithium extraction and insertion. Although their electrochemical properties have been extensively studied, the structural and magnetic properties of LiNiO2 are still under considerable debate, and the magnetic properties of monoclinic LiMnO2 are seldom reported. In this work, a detailed study of LiNiO2, LiMnO2, and a half-doped material LiNi0.5Mn0.5O2 is performed via both first-principles calculations and Monte Carlo simulations based on the effective spin Hamiltonian model. Through considering different structures, it is verified that a structure with a zigzag-type pattern is the most stable one of LiNiO2. Moreover, in order to figure out the magnetic properties, the spin exchange interactions are calculated, and then magnetic ground states are predicted in these three systems. The results show that LiNiO2 forms a spiral order that is caused by the competition from both the short-range and long-range spin exchange interactions, whereas the magnetic ground state of LiMnO2 is collinearly antiferromagnetic due to its nearest and next-nearest neighbor antiferromagnetic spin exchange interactions. However, LiNi0.5Mn0.5O2 is collinearly ferrimagnetic because of the ferromagnetic nearest neighbor Ni-Ni and Mn-Mn exchange interactions. Our work demonstrates the competition between the different exchange interactions in these cathode materials, which may be relevant to the performance of the lithium-ion battery.

The exact solution of the edge-modified graphene nanoribbon-like structure Ising-Heisenberg spin (1,1/2) system

A two-fold rotational symmetry ferromagnetic Ising-Heisenberg model modified by edge spin ( S = 1 / 2 ) with antiferromagnetic Heisenberg exchange interaction, whose shape is graphene nanoribbon-like has been constructed, and the host Ising spin ( S = 1 ). By use of the star-triangle transformation and the transfer matrix method, the edge states, thermodynamics and magnetization behavior of the system display various properties. The results perform that the unique edge-spin states and the host-spin states of the system are influenced with each other. And there are eleven different ground state phases, which are presented as the values of magnetic susceptibility multiplied by temperature at ultra-low temperature in the parameter space of single-ion anisotropy and antiferromagnetic Heisenberg interaction. Because of a large number of excited states close to the ground state, the specific heat curve shows multi-peak phenomena, which are analyzed by corresponding to the entropy of the system. The magnetization process of the system is complex, due to the direct correlation of the edge-spin states and the host-spin states of system, in which many different magnetization plateaus appear. The rigorous results of the number and width of magnetization plateaus and the saturation of the magnetization are written. This study offers unique opportunities for future technological applications such as proposed designs of graphene-based quantum spintronic devices as well as magnetic ordering.

Tunable magnetic interactions in mono-transition metal spinels MgCr2X4 (X=O,S,Se) and ACr2S4 (A=Ca,Sr,Ba)

We investigate the electronic and magnetic properties of the spinel materials $\mathrm{Mg}{\mathrm{Cr}}_{2}{X}_{4}$ ($X$ = O, S, Se) and $A{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ ($A$ = Ca, Sr, Ba), a special type of mono-transition metal spinel material. The magnetic properties of these materials are understood by inspecting the competition between the ${90}^{\ensuremath{\circ}}$ ferromagnetic superexchange interactions and the antiferromagnetic direct exchange interactions. We found the direct exchange interaction is extremely sensitive to the distance between the Cr ions, ${d}_{\text{Cr-Cr}}$. The overall magnetic interaction can be transformed from antiferromagnetic at short ${d}_{\text{Cr-Cr}}$ to ferromagnetic at larger ${d}_{\text{Cr-Cr}}$. Therefore, we can realize different magnetic states in the same material by controlling the ${d}_{\text{Cr-Cr}}$ with external strain. Furthermore, the ferromagnetism is strengthened as tensile strain is employed, and a strategy to improve the Curie temperature, which is essential for spintronics devices, is proposed.

Magnetic and electronic properties of KMn1-xMxBi (M = Cu, Mg, Zn) solid solutions

Materials displaying antiferromagnetic exchange interactions are promising for spintronic applications. The ability to control exchange interactions and electronic properties of antiferromagnetic materials is key in this context. Here, we investigate by means of first-principles calculations to what extent the structural, electronic and magnetic properties of theKMnBi compound can be tailored through chemical doping. It is shown that the KMnBi compound features antiferromagnetic ordering and a direct gap of 0.287 eV. It is found that all solid solutions of KMn0.5M0.5Bi (M = Cu, Mg, Zn) present antiferromagnetic behavior in the ground state. In contrast, the compositions of KMn0.75M0.25Bi (M = Cu, Mg, Zn) are ferrimagnetic with total magnetization of 3.89 μB/cell, 4.45 μB/cell, and 4.41 μB/cell for 25%Cu, 25%Mg and 25%Zn doped KMnBi, respectively. This study shows that the magnetic response of KMnBi can be manipulated to its advantage by appropriately substituting transition metals in the Mn cell site. As a result, transition metal doped KMnBi could have potential applications based on their magnetic response, in particular, in magnetic spintronics.

New Dinuclear Macrocyclic Copper(II) Complexes as Potentially Fluorescent and Magnetic Materials.

Two dinuclear copper(II) complexes with macrocyclic Schiff bases K1 and K2 were prepared by the template reaction of (R)-(+)-1,1'-binaphthalene-2,2'-diamine and 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde K1, or 4-tert-butyl-2,6-diformylphenol K2 with copper(II) chloride dihydrate. The compounds were characterized by spectroscopic methods. X-ray crystal structure determination and DFT calculations confirmed their geometry in solution and in the solid phase. Moreover, intermolecular interactions in the crystal structure of K2 were analyzed using 3D Hirshfeld surfaces and the related 2D fingerprint plots. The magnetic study revealed very strong antiferromagnetic CuII-CuII exchange interactions, which were supported by magneto-structural correlation and DFT calculations conducted within a broken symmetry (BS) framework. Complexes K1 and K2 exhibited luminescent properties that may be of great importance in the search for new OLEDs. Both K1 and K2 complexes showed emissions in the range of 392-424 nm in solutions at various polarities. Thin materials of the studied compounds were deposited on Si(111) by the spin-coating method or by thermal vapor deposition and studied by scanning electron microscopy (SEM/EDS), atomic force microscopy (AFM), and fluorescence spectroscopy. The thermally deposited K1 and K2 materials showed high fluorescence intensity in the range of 318-531 nm for K1/Si and 326-472 nm for the K2/Si material, indicating that they could be used in optical devices.

СИНТЕЗ, СТРОЕНИЕ И МАГНИТНЫЕ СВОЙСТВА КОМПЛЕКСОВ Cu(II) НА ОСНОВЕ 4,6-ДИМЕТИЛПИРИМИДИЛГИДРАЗОНА ДИАЦЕТИЛМОНОКСИМА: ЭКСПЕРИМЕНТАЛЬНОЕ И ТЕОРЕТИЧЕСКОЕ ИССЛЕДОВАНИЕ

In order to study the effect of the dimethylpyrimine fragment nature on the structure and magnetic properties of coordination compounds, new binuclear Cu(II) metallochelates were synthesized. The coordination compounds are [Cu2(HL)2(CH3OH)2](ClO4)2 (1) and [Cu2(HL)2(NO3)2] (2), where HL – the monodeprotonated form of 2-hydrazino-4,6-dimethylpyrimidine and diacetylmonoxime condensation product. The structure and properties of complexes were established using elemental analysis, 1H NMR, IR-spectroscopy, magnetochemistry. It was shown that the coordination compounds are diamagnetic at room temperature due to the strong antiferromagnetic exchange interaction between paramagnetic Cu(II) ions: −2J &gt; 1000 cm−1. The data obtained are in a good agreement with the results of quantum chemical calculations (theory level B3LYP/6 311G(d)).

A bis-calix[4]arene-supported [CuII16] cage.

Reaction of 2,2'-bis-p-tBu-calix[4]arene (H8L) with Cu(NO3)2·3H2O and N-methyldiethanolamine (Me-deaH2) in a basic dmf/MeOH mixture affords [CuII16(L)2(Me-dea)4(μ4-NO3)2(μ-OH)4(dmf)3.5(MeOH)0.5(H2O)2](H6L)·16dmf·4H2O (4), following slow evaporation of the mother liquor. The central core of the metallic skeleton describes a tetracapped square prism, [Cu12], in which the four capping metal ions are the CuII ions housed in the calix[4]arene polyphenolic pockets. The [CuII8] square prism is held together "internally" by a combination of hydroxide and nitrate anions, with the N-methyldiethanolamine co-ligands forming dimeric [CuII2] units which edge-cap above and below the upper and lower square faces of the prism. Charge balance is maintained through the presence of one doubly deprotonated H6L2- ligand per [Cu16] cluster. Magnetic susceptibility measurements reveal the predominance of strong antiferromagnetic exchange interactions and an S = 1 ground state, while EPR is consistent with a large zero-field splitting.

Antiskyrmions and their electrical footprint in crystalline mesoscale structures of Mn1.4PtSn

Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effect. However, the electrical detection of antiskyrmions, in both thin films and bulk samples has been challenging to date. Here, we apply magneto-optical microscopy combined with electrical transport to explore the antiskyrmion phase as it emerges in crystalline mesoscale structures of the Heusler magnet Mn1.4PtSn. We reveal the Hall signature of antiskyrmions in line with our theoretical model, comprising anomalous and topological components. We examine its dependence on the vertical device thickness, field orientation, and temperature. Our atomistic simulations and experimental anisotropy studies demonstrate the link between antiskyrmions and a complex magnetism that consists of competing ferromagnetic, antiferromagnetic, and chiral exchange interactions, not captured by micromagnetic simulations.

Strain-driven antiferromagnetic exchange interaction in SrMnO3 probed by phase-shifted spin Hall magnetoresistance

Multiferroics have found renewed interest in topological magnetism and for logic-in-memory applications. Among them, ${\mathrm{SrMnO}}_{3}$, possessing strong magnetoelectric coupling, is gaining attention for the design of coexisting magnetic and polar orders upon straining. Here we demonstrate antiferromagnetic exchange interactions in strained SMO thin films extracted from a new feature in the phase response of spin Hall magnetoresistance, which has not been explored in earlier works, such as in magnetic insulators. We explain our findings with a model that incorporates magnetic anisotropy along the [110] direction, corroborates with density functional theory studies, and is consistent with the direction of ferroelectric polarization in ${\mathrm{SrMnO}}_{3}$. The fundamental insights obtained from our studies establishes the potential of this material in magnetoelectrically coupled devices for different logic and memory applications.

Structural, Spectroscopic Characterization and Application of a Co(II) Coordination Polymer [Co(suc)(bpm)] ⋅ 2H2O

AbstractA Co(II) coordination polymer [Co(suc)(bpm)] ⋅ 2H2O (1) built from succinate (suc) and bipyrimidine (bpm) is synthesized hydrothermally and characterized by different methods. It crystallizes in a monoclinic space group P21/n with an intriguing 3D polymeric architecture. The weak antiferromagnetic (AF) exchange interactions were evinced by Curie Constant (C)=1.803 cm3mol−1K and a negative Weiss constant (φ)=−30.163 K. Type VI adsorption isotherm behavior of 1 at 77 K revealed its non‐porous property. Compound 1 was investigated and successfully utilized as an additive in lowering the decomposition temperature of ammonium nitrate (AN) based fuels as demonstrated from TG and DSC experiments performed in nitrogen atmosphere.

Violation of Pauli-Clogston limit in the heavy-fermion superconductor CeRh2As2 : Duality of itinerant and localized 4f electrons

We theoretically propose a mechanism to understand the violation of the Pauli-Clogston limit for the upper critical field H$_{\rm c2}$ observed in the Ce bearing heavy Fermion material CeRh$_2$As$_2$ from the view point of spin singlet pairing. It is based on a duality concept, the dual simultaneous aspects of an electron: the itinerant part and localized part of quasi-particles (QPs) originated from the 4f electrons of the Ce atoms. While the itinerant QPs directly participate in forming the Cooper pairs, the localized QPs exert the internal field so as to oppose the applied field through the antiferromagnetic exchange interaction between them. This is inherent in the dense Kondo lattice system in general. We argue that this mechanism can be applied not only to the locally noncentrosymmetric material CeRh$_2$As$_2$, but also to globally inversion symmetry broken Ce-based materials such as CePt$_3$Si. Moreover, we point out that it also works for strongly Pauli limit violated spin triplet pairing systems, such as UTe$_2$.

Structural and electrical properties of Mg–Cu- and Mg–Cu–Li-doped bismuth niobate semiconductors with the pyrochlore structure

This work studies a series of synthesized Bi1.6Mg0.8-xCuxNb1.6O7-δ (x = 0.2, 0.4) semiconductors and their Li-doped compositions. A detailed structure investigation combining high-resolution neutron-, synchrotron-, and X-ray diffraction methods, as well as DFT calculations, revealed the preferential location of Cu and Li atoms at the Bi sites and Mg atoms at the Nb ones. According to high-temperature X-ray diffraction data, a structural modification caused by the activation of oxygen transport occurs at 200 °C. The linear thermal expansion coefficient was found to be 3.6–4.6·10−6 K−1 (50–400 °C). Magnetic susceptibility measurements allowed us to determine weak antiferromagnetic exchange interactions. The direct band gap was predicted using the DFT-HSE03 hybrid functional calculation, and the optical direct band gap was estimated at 2.3–2.4 eV. Impedance spectroscopy and a dc four-probe technique were also employed to examine the samples' electrical properties. The high mixed electronic-ionic conductivity of the pyrochlores was detected, while the vacancies created by Li-doping in Bi1.5-yLiyMg0.375Cu0.375Nb1.5O7-δ have been found not to affect the conductivity. Besides, the pyrochlores are chemically compatible with the La0.7Sr0.3MnO3 perovskite (up to 800 °C). These make us believe that the studied Mg–Cu- and Mg–Cu–Li-doped bismuth niobate semiconductors can become the basis for composite electrodes to boost their oxygen conductivity.

Solvent-dependent formation of 1D coordination polymers based on polynuclear copper(II)-carboxylate fragments and 4-(pyridine-3-yl)methyleneamino-1,2,4-triazole linkers

The solvent used for the synthesis and crystallization was shown to affect the composition and structure of compounds formed in the reaction of copper(II) trimethylacetate with a Schiff base, 4-(pyridin-3-yl)methyleneamino-1,2,4-triazole (L). The reaction of [Cu2(piv)4(Hpiv)2] (Hpiv – trimethylacetic acid) in MeCN was found to give the crystals of 1D coordination polymer {[Cu2(piv)4(L)]·MeCN}n (1), the structure of which contains binuclear {Cu2(piv)4} units linked by the bridging organic ligand L. A similar reaction in MeOH or EtOH gave 1D coordination polymers {[Cu6(OH)2(piv)10(L)2(ROH)2]∙4ROH}n (2: R = Me, 3: R = Et) whose structure is composed of alternating binuclear {Cu2(piv)4} and tetranuclear {Cu4(OH)2(piv)6(L)2(ROH)2} units bound by N-donor ligand L. Storage of the mother liquor obtained after the isolation of the crystals of 3 leads to the crystallization of chained polymer [Cu(piv)2L]n (4). The structures of new complexes 1–4 were determined by single crystal X-ray diffraction. According to magnetochemical measurements for 1 and 3, antiferromagnetic exchange interactions are observed in these compounds between CuII ions (S = ½) in binuclear {Cu2(piv)4} units (J = −190.5 and −215 cm−1, respectively), as well as dominate in tetranuclear {Cu4(OH)2(piv)6(L)2(EtOH)2} units of 3 (J1 = −202 cm−1, J2 = –23 cm−1, J3 = 18 cm−1).