AFM Exchange Research Articles

Azomethine Fe3+ coordination compounds containing carbazole units: Synthetic approach, spectral characterization, and magnetic studies

The main aim of our study is to investigate various types of hybrid systems with a paramagnetic iron (III) ion as a core and photoactive derivatives of carbazole at the periphery. We have synthesized a variety of biligand coordination compounds with the composition [FeL2]A (where L is 2‐[2‐[(E)‐[4‐[4‐(3,6‐di‐tert‐butylcarbazol‐9‐yl)benzoyl]oxy‐2‐phenolate]methyleneamino]ethylamino]ethyl 4‐(3,6‐di‐tert‐butylcarbazol‐9‐yl)benzoate, A− is NO3− (I), Cl− (II), PF6− (III)). These systems were studied by superconducting quantum interference device (SQUID) magnetometry and X‐band electron paramagnetic resonance (EPR) spectroscopy. Magnetic measurements revealed mixed spin states (high spin [HS], low spin [LS]) of Fe (III) ions at room temperatures. Estimates of the corresponding spin contributions were made. It was found that all samples exhibit AFM exchange interactions between iron (III) ions. The ground spin state at 2.0 K was established and analyzed. EPR measurements confirmed the HS states of iron (III) and revealed two types of them: with weak distorted and strong low‐symmetry octahedral crystal fields.

A first principle +U correction to the investigation of intrinsic vacancy and zinc vacancy-cluster induced magnetism in w-ZnO

Pure ZnO shows the probable origin of the magnetic nature. Microscopic correlation to point defect VO and the cluster VZn is obtained by first-principle calculation and +U interaction energy of the p-d orbital subjected to improve band and DOS. Hydrogen contribution to the magnetic moment (MM) due to the coupling of vacancy and 1 s electron correlates with the experimental outcomes. Intrinsic vacancy modulates Fermi surface electron density, leading to the stoner criterion. A higher-order magnetic moment is associated with the cluster vacancy, and the change in the MM has accounted for the +U adjustment to the relevant orbital impact. Nonlinear relations are due to cluster vacancy complexes VZn-x/Oi-x (x = H, 2H), and the FM and AFM exchange has been analyzed. The material encloses large grains; hence, it is advantageous for VZn to develop near the grain edge, and in O-rich environments, chemisorbed O2 is formed near the grain surface.

Transition from AFM Spin Canting to Spin Glass-AFM Exchange as Particle Size Decreases in LaFeO3.

In this work, we have studied structural and magnetic properties of LaFeO3 as a function of the particle size d, from bulk (d >> 1 µm) to nanoscale (d ≈ 30 nm). A large number of twins were observed for large particles that disappear for small particle sizes. This could be related to the softening of the FeO6 distortion as particle size decreases. It was observed that the bulk sample showed spin canting that disappeared for d ~ 125 nm and can be associated with the smoothening of the orthorhombic distortion. On the other hand, for d < 60 nm, the surface/volume ratio became high and, despite the high crystallinity of the nanoparticle, a notable exchange effect bias appeared, originated by two magnetic interactions: spin glass and antiferromagnetism. This exchange bias interaction was originated by the formation of a "magnetic core-shell": the broken bonds at the surface atoms give place to a spin glass behavior, whereas the inner atoms maintain the antiferromagnetic G-type order. The LaFeO3 bulk material was synthesized by the ceramic method, whereas the LaFeO3 nanoparticles were synthesized by the sol-gel method; the particle size was varied by annealing the samples at different temperatures. The physical properties of the materials have been investigated by XRD, HRTEM, TGA, and AC and DC magnetometry.

Ferromagnetic and antiferromagnetic orders in low dimensional PbBiFe1-xMxO4 (M = Mn and Co) solid solutions

A new solid solution of composition PbBiFe1-xMxO4 (M ​= ​Mn and Co, 0 ​≤ ​x ​≤ ​0.15–0.2) with low structural dimensions has been synthesized by solid-state reaction at 873–923 ​K. The crystal structures have been investigated using X-ray and neutron diffraction. The presence of Co2+/Co3+ on the Fe3+ sites and oxygen vacancies in PbBiFe1-xCoxO4 is revealed by its unusual volume increase and TGA, while the volume contraction of PbBiFe1-xMnxO4 indicates the substitution of Mn4+/Mn3+ on the Fe3+ sites. This leads to unexpected greater and smaller metal-metal distances inside the chains for the Co and Mn case, respectively, compared with the undoped materials. All the PbBiFe1-xMxO4 materials display long-range antiferromagnetic order but are suppressed by cationic substitution, as evidenced by a decrease in TN. Additional intrinsic short-range ferro- (or ferri-) magnetic transitions in the Co-doped materials are evidenced by magnetic hysteresis and frequency-dependent χ’ peak around TF. The introduction of ferromagnetic intrachain clusters in PbBiFe1-xCoxO4 is rationalized through a competitive mechanism between direct AFM exchange interaction and 90°- type FM superexchange interactions, which provided insights to tune the magnetic properties and design low dimensional functional materials.

Joint Studies of Spin Frustration Induced by Doping Small ZnSe Nanoparticles with Fe Atoms

AbstractAs a 1.8 nm ZnSe nanocrystal is progressively doped with 1%, 5%, and 10% Fe, it shows a progressive change in its magnetic properties from a superparamagnetic FM‐dominated exchange type to an onset of AFM exchange with evidence of spin frustration. Magnetization measurements allow to obtain exchange coupling constants that are compared to the results of a Broken‐Symmetry Density Functional Theory (BS‐DFT) model of a doped (ZnSe)34 cluster. DFT shows a capability to reproduce the experimental pattern of the increasing influence of AFM exchange as doping concentration increases. The material phase segregates at the edges where strained rhombic surface sites are the preferred doping sites of iron. Large concentrations of iron leads to the formation of Fe clusters and complex exchange patterns that result in spin frustration in some iron trimers but none in the others. The spin frustration of these complex systems by assuming mirror symmetry of the sites when fitting by using BS‐DFT formalism is classified and analyzed. While some individual J constants obtained have significant errors, the averaged exchange constants are generally in good agreement with our experimental data.

Dynamic spin fluctuations in the frustrated spin chain compound Li3Cu2SbO6

We report the signatures of dynamic spin fluctuations in the layered honeycomb Li$_3$Cu$_2$SbO$_6$ compound, with a 3$d$ S = 1/2 $d^9$ Cu$^{2+}$ configuration, through muon spin rotation and relaxation ($\mu$SR) and neutron scattering studies. Our zero-field (ZF) and longitudinal-field (LF)-$\mu$SR results demonstrate the slowing down of the Cu$^{2+}$ spin fluctuations below 4.0 K. The saturation of the ZF relaxation rate at low temperature, together with its weak dependence on the longitudinal field between 0 and 3.2 kG, indicates the presence of dynamic spin fluctuations persisting even at 80 mK without static order. Neutron scattering study reveals the gaped magnetic excitations with three modes at 7.7, 13.5 and 33 meV. Our DFT calculations reveal that the next nearest neighbors (NNN) AFM exchange ($J_{AFM}$ = 31 meV) is stronger than the NN FM exchange ($J_{FM}$ = -21 meV) indicating the importance of the orbital degrees of freedom. Our results suggest that the physics of Li$_3$Cu$_2$SbO$_6$ can be explained by an alternating AFM chain rather than the honeycomb lattice.

Effect of AFM and FM exchange interaction on magnetic anisotropy properties of single domain SmFeO3 at nanoscale

Multifunctional Samarium orthoferrites (SmFeO3) nano-particles have been studying extensively due to their applications in digital memory, ultrafast switching, temperature dependent spin valves and sensor applications. Magnetic properties of SFO were investigated by varying particle size, initial magnetic state of material and applied field under Zero Field Cooling (ZFC) and Field Cooling (FC) protocol. It was found that the remnant magnetization, M−H behavior and coercive field are extremely dependent on particle size. It was also found that the magnetic behavior of single domain SFO nanoparticle is completely different to that of multi domain bulk due to uniaxial magneto crystalline anisotropy. The magnetization vs temperature (M−T) behavior of nano-particle depends on unidirectional anisotropy along with uniaxial magneto-crystalline anisotropy of SFO. The Sm3+ spin ordering in nano-SFO compensates to that of Fe3+ spin ordering at higher temperature (~13 K) compare to that of bulk (~4K). The Sm3+ spin unidirectional magnetic anisotropy is more effective in between ~13 K to ~65 K and Fe3+ spin unidirectional anisotropy is effective at higher temperature beyond ~65 K. Magnetically poled nano-particles show completely different behavior to that of non-poled nano-SFO particles at low temperature. In nutshell, the manuscript shows why one can’t predict the magnetic behavior of the SFO without knowing its particle or grain size, initial magnetic state of the particle, procedure of measurement, the magnitude of applied field and temperature etc., since all these affects the magneto crystalline anisotropy as well as unidirectional anisotropy of SFO.

Magnetism of 3d and 4d doped Mn0.7T0.3NiGe (T = Fe, Co, Ru and Rh): bulk magnetization and ab initio calculations

We compare the magnetic properties of 3d (Fe and Co) and 4d (Ru and Rh) transition metals doped MnNiGe using the combined results of magnetization and ab initio calculations. The alloys crystallize in austenite Ni2In-type hexagonal phase (space group: P63/mmc) with insignificant difference in the lattice parameters. Mn0.7Fe0.3NiGe and Mn0.7Co0.3NiGe exhibit spin-glass behavior, resulting from the competing ferro- and antiferromagnetic interactions. These alloys exhibit spontaneous exchange bias field of about Oe and 323 Oe, respectively. From the 4d-metal doped alloys, Mn0.7Ru0.3NiGe shows glassy behavior while long-range ferromagnetic order is confirmed in Mn0.7Rh0.3NiGe. In Mn0.7Rh0.3NiGe, in agreement with experiment and the theoretical calculations, the ground state is confirmed to be ferromagnetic because of the FM exchange interactions of the Mn magnetic moments. But in Mn1−x(Fe,Co,Ru)xNiGe alloys the calculations revealed the competing and comparable FM and AFM exchange interaction parameters, resulting in the formation of spin-glassy characteristics.

Phosphonate-assisted tetranuclear lanthanide assemblies: observation of the toroidic ground state in the TbIII analogue.

The reaction of LnCl3·6H2O with a multidentate flexible Schiff base ligand (LH4), H2O3PtBu and trifluoroacetic acid (tfaH) afforded a series of homometallic tetranuclear complexes, [Ln4(LH2)2(O3PtBu)2(μ2-η1η1tfa)2][2Cl] (Ln = DyIII (1), TbIII (2) and GdIII (3)). The tetranuclear lanthanide core contains two structurally different lanthanide centres, one being in a distorted trigonal dodecahedron geometry and the other in a distorted trigonal prism. Complexes 1-3 were investigated via direct and alternating current (DC and AC) magnetic susceptibility measurements. Only 1 revealed a weak single-molecule magnet (SMM) behaviour. Alternating current (ac) magnetic susceptibility measurements on 1 reveal a frequency-dependent out-of-phase signal. However, the absence of distinct maxima in the χ'' peak (within the temperature/frequency range of our experiments) prevented deduction of the experimental energy barrier for magnetization reversal (Ueff) and the relaxation time. We have carried out extensive ab initio (CASSCF + RASSI-SO + SINGLE_ANISO + POLY_ANISO) calculations on complexes 1-2 to gain deeper insights into the nature of magnetic anisotropy. Our calculations yielded only one exchange coupling parameter between the two LnIII centres bridged by the ligand (neglecting the exchange between the LnIII centres that are not proximal wrt each other). All the extracted J values indicate a weakly antiferromagnetic coupling between the metal centres (J = -0.025 cm-1 for 1 and J = -0.015 cm-1 for 2). Calculated exchange coupled Ucal values of ∼5 and ∼1 cm-1 in 1 and 2 respectively nicely corroborated the experimental observations regarding weak and no SMM characteristics. Our calculations indicated the presence of a net single-molecule toroidal (SMT) behaviour in complex 2. On the other hand, fitting the magnetic data (susceptibility and magnetization) in the isotropic cluster 3 revealed weak AFM exchange couplings of J1 = 0.025 cm-1 and J2 = -0.020 cm-1 which are consistent with those for GdIII ions.

Controlled growth of Ni/NiO composite nanoparticles and its influence on exchange anisotropy and spin glass features

The current study focus on the exchange bias properties of Ni/NiO ferromagnetic-antiferromagnetic nanocomposite by fabricating Ni (FM) using a one-step solution combustion method. The prepared Ni nanoparticles were oxidized at 400 °C for different time durations (τ = 3, 6, 12, 24, 48, 72, 96, 120, 144 and 168 h) in the ambient atmosphere to grow NiO (AFM). The X-ray diffraction pattern of the Ni/NiO nanocomposite indicate good crystallinity of the samples. Raman spectroscopy exhibited a short-range magnon excitation around 1400 cm−1 that indicate the contribution of AFM exchange energy between the Ni2+-O2--Ni2+ chain in the composites. The presence of Ni and NiO phases were confirmed by room temperature magnetization measurement. For the better understanding of the exchange bias of the Ni/NiO nanocomposite subjected to heat treatment for several hours (0–168 h), zero-field cooled and field cooled M-H and M-T measurements were made. The hysteresis loop shift along the field axis and enhanced coercivity values shows the exchange bias effect in the Ni/NiO structure in the controlled oxidation process. The Ni and NiO interfacial effect and phase fraction have significant effect on the observed exchange bias. Moreover, finite size effects contribution on the antiferromagnetic exchange and spin glass features observed at a temperature of 24 K of the Ni/NiO system contribute to the large exchange bias and enhanced coercivity.

Effect of cobalt doping on martensitic transformations and the magnetic properties of Ni50−xCoxMn37Sn13 (x = 1, 2, 3) Heusler ribbons

The effect of cobalt doping on martensitic transformations and the magnetic properties of Ni50−xCox Mn37Sn13 (x = 1, 2, 3) magnetic shape memory alloys obtained by melt spinning in the form of ribbons is studied. The crystallographic structure of all of the ribbons at room temperature is austenite cubic L21. SEM micrographs indicate the formation of textured ribbons with columnar grains growing up perpendicular to the ribbon plane. For all of the ribbons, DSC cyclic scans reveal the martensitic transformation on cooling and the reverse austenitic transformation upon heating below room temperature. In addition, they indicate that the martensitic transformation temperatures decrease with an increasing cobalt content.Thermomagnetic curves reveal the coexistence of AFM and FM exchange interactions at low temperatures. Likewise, the magnetization change (ΔM) between the martensite and austenite phase, as well as the austenite Curie temperature (TCA), increase with increasing cobalt content in the ribbons. Structural transformations are also sensitive to the external applied magnetic field. This fact suggests that the structural transformation temperatures of the ribbons could be tuned to the desired functional temperature by controlling the cobalt amount through the replacement of Ni as well as by changing the applied magnetic field, which could lead to an enhancement of the magnetic properties.

Formation of gapless Z 2 spin liquid phase manganites in the (Sm1–y Gd y )0.55Sr0.45MnO3 system in zero magnetic field: Topological phase transitions to states with low and high density of 2D-vortex pairs induced by the magnetic field

The evolution of the ground state of the manganese spin ensemble in the (Sm1–y Gd y )0.55Sr0.45MnO3 in the case of isovalent substitution of rare-earth samarium ions with large radii with gadolinium ions with significantly smaller radii is studied. The measured temperature dependences of the ac magnetic susceptibility and the field dependences of the dc magnetizations are analyzed using the Heisenberg–Kitaev model describing the transition from the ordered spin state with classical isotropic AFM exchange to the frustrated spin state with quantum highly anisotropic FM exchange. A continuous transition from the 3D ferromagnetic state of manganese spins in the initial sample with y = 0 to zigzag AFM ordering of CE-type spins in ab planes for y = 0.5, coexisting in samples with y = 0.5, 0.6, and 0.7 at temperatures below T N ≅ 48.5 K with a disordered phase such as a quantum Griffiths phase is identified. As the gadolinium concentration further increases, the CE-type zigzag AFM structure is molten, which leads to the appearance of an unusual phase in Gd0.55Sr0.45MnO3 in the temperature range close to the absolute zero. This phase has characteristic features of a gapless Z 2 quantum spin liquid in zero external magnetic field. The step changes in the magnetization isotherms measured at 4.2 K in the field range of ±75 kOe are explained by quantum phase transitions of the Z 2 spin liquid to a phase with topological order in weak magnetic fields and a polarized phase in strong fields. The significant difference between critical fields and magnetization jumps in isotherms indicates the existence of hysteretic phenomena in quantum spin liquid magnetization–demagnetization processes caused by the difference between localization–delocalization of 2D vortex pairs induced by a magnetic field in a quantum spin liquid with disorder.

Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe2-xCrxO4 Nanoparticles Synthesized by Chemical Combustion.

A series of nickel-chromium-ferrite NiFe2-xCrxO4 (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mössbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (TN is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mössbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (Fe0.75Ni0.25)[Ni0.75Cr1.25]O4. The saturation magnetization MS in the largest NPs is about (0.98-0.95) μB, which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O4. At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about Tcom = 360-365 K. This value significantly exceeds the point Tcom in bulk ferrites NiFexCr2-xO4 (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFe0.75Cr1.25O4 with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure.

Geometrically frustrated Fe2P-like systems: beyond the Fe-trimer approximation

FeP-like structures can be strongly frustrated magnets due to their Kagome/triangular intercalated-layer structure. A complete magnetic solution of the complex spin architecture, and hence the full potential of the magnetic phenomena in FeP-like material prototypes, is yet to be found. A previous magnetic model for a representative FeCrAs-like system used a mean-field effective-spin to describe the 3g-Wyckoff located Fe-triangles. Such an approach demonstrated the outstanding magnetocaloric properties of the material but left the question of whether the intra-trimer interaction could lead to new physical phenomena and therefore more potentially useful properties. In this work Monte Carlo simulations are employed in order to understand both the influence of the additional degrees of freedom introduced by the Fe-trimers and the changes caused by all the possible exchange couplings between them. Complex scenarios arise, in which FM coupling in the trimers gives rise to both in-plane and out-of-plane inter-layer AFM states; whereas AFM exchange in the trimers gives rise to three distinct states, i.e. AFM-canted layers, a non-collinear superposition of ferromagnetic Kagome/triangular orderings, and tilted inter-planar AFM order. These last three configurations generate a double bifurcated magnetic phase diagram while the first one mimics the behavior seen in a model that treats the trimer as an effective-spin under an applied magnetic field.

Transition Metal-Nonmetal in Conductivity of Ceramic Hole-Doped Cobaltites

In bulk granulated cobaltite La1–xSrxCoO3 with the size of granules of order of 1 micron at strontium hole doping with replacement factor x = 0.35, a transition “metal-nonmetal” in the conductivity was revealed, presumably connected with AFM ordering of the moments of granules. The assumption is proved by the agreement between the experiment and results of calculation within the limits of a model offered for electron transport based on the account of in-granule double exchange Zener mechanism and intergranule mechanism of spin-polarized tunneling on the nearest neighbours with AFM exchange interaction. The calculation differs in that conductivities within granules are summarized, while total resistance of the system is represented as a sum of resistances of the granules. In addition, the existence of AFM interaction between granules is supported by the observed insensitivity of conductivity to a low external magnetic field (up to 5 kOe).

Molecular Spin Ladders Constructed from [Ni(dmit)2]− Building Blocks: Synthesis, Structure and Physical Properties

A new ion-pair complex [1-(4-bromobenzyl)-3-methylpyridinium][Ni(dmit)2] (1), in which dmit = 4,5-dimercapto-1,3-dithiole-2-thione, has been synthesized and structurally characterized. The anions of [Ni(dmit)2]− stack into dimers, which further construct into a two-leg ladder through lateral S···S interactions. The weak H-bonding interactions of C–H···S and van de Waals interactions between anion and cations were observed. The magnetic susceptibilities measured from 2–300 K indicate an AFM exchange interaction domination and an AFM ordering below ~8 K. The best fit to magnetic susceptibility above 40 K, using a dimer model with s = ½, gives rise to Δ/kB = 29.8 K, zJ′ = −0.72 K, C = 3.40 × 10−3 emu K mol−1 and χ 0 = −5.8 × 10−6 emu mol−1 with a fixed g = 2.0. Cyclovoltammetry revealed two quasi-reversible one-electron steps, which are attributed to Ni(IV/III) and Ni(III/II) redox couples. [A novel complex [1-(4-bromobenzyl)-3-methylpyridinium][Ni(dmit)2] has been synthesized and structurally characterized. The anions of [Ni(dmit)2]− stack into dimers, which further construct into two-leg ladder through lateral S···S interactions. The weak H–bonding interactions of C–H···S and van de Waals interactions between anion and cations were observed. Moreover, its magnetic property and electrochemical property have been investigated] .

Isostructural M(RL) 2(hfac) 2 complexes with RL = 5-(4-[ N- tert-butyl- N-aminoxyl]phenyl)pyrimidine

5-(4-( N- tert-Butyl- N-aminoxylphenyl))pyrimidine (RL, 4PPN) forms crystallographically isostructural and isomorphic pseudo-octahedral M(RL) 2(hfac) 2 complexes with M(hfac) 2, M = Zn, Cu, Ni, Co, and Mn. Multiple close contacts occur between sites of significant spin density of the organic radical units. Magnetic behavior of the Zn, Cu, Ni, Co complexes appears to involve multiple exchange pathways, with multiple close crystallographic contacts between sites that EPR (of 4PPN) indicates to have observable spin density. Powder EPR spectra at room temperature and low temperature are reported for each complex. Near room temperature, the magnetic moments of the complexes are roughly equal to those expected by a sum of non-interacting moments (two radicals plus ion). As temperature decreases, AFM exchange interactions become evident in all of the complexes. The closest fits to the magnetic data were found for a 1-D Heisenberg AFM chain model in the Zn(II) complex ( J/ k = (−)7 K), and for three-spin RL—M—RL exchange in the other complexes ( J/ k = (−)26 K, (−)3 K, (−)6 K, for Cu(II), Ni(II), and Co(II) complexes, respectively).

Intrinsic magnetic inhomogeneity of Eu substituted La 0.7Pb 0.3Mno 3 single crystals

The study of magnetic and magnetotransport properties of the crystals of (La 1− y Eu y ) 0.7Pb 0.3MnO 3 system has been carried out. Eu ions enter the crystals being in trivalent nonmagnetic state. Europium ions possessing of smaller ionic radius in comparison with La ions, induce local distortions of Mn–O–Mn bonds in the system that cause random distribution of magnetic exchange interactions in magnitude and, probably, in sign. The competition of magnetic interactions leads to the appearance of the inhomogeneous magnetic state in the crystals. The enhancement of concentration of Eu ions results in decrease of the Curie temperature and broadening of the inhomogeneous magnetic state area. At y=0–0.4 the coexistence of the paramagnetic phase with conductivity of the polaronic type and the ferromagnetic metallic phase is observed in a bounded temperature interval both above and below T C. Below T C the increasing of y up to 0.6 induces the magnetic state representing the coexistence of two different FM phases. These phases are spatially separated due to frustration of FM and AFM exchange interactions on phase boundaries. Above T C, up to 1.6 T C ferromagnetic clusters exist in a paramagnetic matrix similar to the case of samples with y=0–0.4. Concerning electric properties, the samples with y=0–0.4 reveal the metal–insulator transition at temperature that practically coincides with T C. The sample with y=0.6 has conductivity of insulator character up to the lowest temperatures. For all investigated compositions y=0–0.6 the CMR effect is observed in the area where the inhomogeneous magnetic state exists. The effect is determined by different conductivity of the coexisting phases, as well as by sensitivity of the inhomogeneous state to external magnetic field.

Molecular spin ladders self-assembly from [Ni(dmit)2]− building blocks: Syntheses, structures and magnetic properties

Two ion-pair compounds, consisting of 1-(4′-R-benzyl)pyridinium ([RBzPy]+, R = NO2 (1) and Br (2)) and [Ni(dmit)2]− (dmit2− = 2-thioxo-1,3-dithion-4,5-dithiolato), have been synthesized and structurally characterized. The anions of [Ni(dmit)2]− stack into dimers, which further construct into two-leg ladder through terminal S⋯S interactions in 1, lateral S⋯S interactions in 2. The weak H-bonding interactions of C–H⋯S were observed in 2, while only weak van de Waals interactions between anion and cations in 1. The magnetic susceptibilities measured in 2–300 K indicate AFM exchange interaction domination both two compounds. A peculiar magnetic transition at ∼100 K was observed in 1. An AFM ordering below ∼11 K was found in 2, and the best fit to magnetic susceptibility above 45 K in this compound, using a dimer model with s = 1/2, give rise to Δ/kB = 36.1 K, zJ = −0.91 K, C = 3.2 × 10−3 emu K mol−1 and χ0 = −4.0 × 10−6 emu mol−1 with g of 2.0 fixed.

Ferrimagnetic resonance in films of vanadium[tetracyanoethanide]x, grown by chemical vapor deposition

Ferrimagnetic resonance (FMR) measurements in thin films of $\mathrm{V}{[\mathrm{TCNE}]}_{x}$ grown by chemical vapor deposition exhibit a series of sharp lines at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The orientational dependence of these lines is a result of the sample geometry provided the magnetization tracks the applied magnetic field. The FMR intensities scale with the temperature dependence of the magnetization as measured by SQUID magnetometry. The temperature dependence of the FMR at various orientations yields an estimate of the local, negative anisotropy field, which is approximately $200\phantom{\rule{0.3em}{0ex}}\text{Oe}$ at zero temperature and decreases linearly with increasing temperature and is well fit by a model developed for spin glasses. The widths of the FMR lines track the temperature dependence of the magnetization, which suggests that they are determined by density fluctuations. The spacings between the individual FMR lines and their temperature dependence are consistent with the presence of nonlinear spin waves, whose critical microwave field is approximately ${10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\text{Oe}$. A rough estimate of the exchange stiffness constant is $A={10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}\mathrm{erg}∕\mathrm{cm}$. The small value of $A$ may be due to the magnitude of the antiferromagnetic exchange between V and adjacent TCNE and the frustration of AFM exchange between adjacent TCNE radicals.