Presence Of Antiferromagnetic Exchange Coupling Research Articles

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters.

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

Magnetic excitations of skyrmions in antiferromagnetic-exchange coupled disks

Magnetic skyrmions are topologically protected spin textures. They can exist in bilayer synthetic antiferromagnets and have some exotic behaviors in terms of dynamics. In this work, we theoretically studied the dynamics of magnetic skyrmions in antiferromagnetic-exchange coupled disks in terms of microwave response using micromagnetic simulations. It is well known that clockwise (CW)/counterclockwise (CCW) circular resonance modes of skyrmions can be excited when an in-plane microwave is applied to the system. We found three coupled excitation modes (CW-CW, CCW-CCW, and CW-CCW) in the presence of antiferromagnetic-exchange coupling and the type of excitation mode depends on the frequency of the in-plane microwave. Furthermore, the topological trajectories of excitation modes are elliptical. For the low frequency (less than 10 GHz) excitation, the trajectory is close to linear mode since the major axis of the ellipse is much larger than its minor axis.

A polynuclear and two dinuclear copper(II) Schiff base complexes: Synthesis, characterization, self-assembly, magnetic property and DFT study

Three elongated (4+1) square pyramidal copper(II) complexes, [Cu2(L)2(μ1,1-N3)2]·H2O (1), [Cu2(L)2(μ1,1-NCO)2] (2) and [Cu(L)(μ1,5-dca)]n (3) [HL=1((2-(methylamino)ethylimino)methyl)naphthalen-2-ol and dca=dicyanamide], have been synthesized using a tridentate N2O donor Schiff base ligand (HL) and characterized. Complexes 1 and 2 are centrosymmetric dimers in which copper(II) centres are connected by asymmetric double end on pseudohalide bridges. Complex 3 features a 1D zigzag chain in which copper(II) centres are connected by end to end dca bridges. Variable temperature (2–300K) magnetic susceptibility measurements indicate the presence of antiferromagnetic exchange coupling between copper(II) centres in complexes 1 (J=−2.313cm−1) and 3 (J=−0.344cm−1), whereas complex 2 shows ferromagnetic exchange coupling between copper(II) centres (J=0.513cm−1). DFT calculations also corroborate the data. The fluid solution EPR spectra recorded at 293K are typical of copper(II) species. Significant supramolecular interactions are explored using high level DFT calculations (BP86-D3/def2-TZVP) and characterized by Bader’s theory of “atoms-in-molecules”.

Features of the magnetic properties of Co/Si/Co thin-film systems

The magnetic properties of Co/Si/Co thin-film structures grown by magnetron sputtering have been studied using magnetooptical techniques. It is established that the saturation field (H S) of trilayers exhibits oscillations as a function of the thickness of the semiconductor (silicon) interlayer. This behavior is explained by structural features of the Co/Si/Co system and the presence of antiferromagnetic exchange coupling between magnetic layers via the silicon interlayer.

Write Field Dynamics in the Presence of Antiferromagnetic Coupling of Writer Pole

A micromagnetic simulation is conducted to study the write field dynamics in the presence of antiferromagnetic exchange coupling (AFC) of writer pole for a perpendicular magnetic recording (PMR) writer head. It is found that, depending on the actual configuration of the AFC coupling in the yoke and the pole tip, there could be an intermediate transient state near zero write field during the magnetization reversal, compared to the normal writer design without AFC coupling. This transient state may cause recording performance degradation. We further investigated the AFC coupling effect on the remanent field in the degauss process. For different performance aspects, i.e., field dynamics, maximum field, and remanent field, the AFC in the yoke and in the pole tip plays different roles.

Changes in magnetic properties from solid state to solution in a trinuclear linear copper(ii) complex

A linear trinuclear copper(II) complex containing phenoxido- and alkoxido-bridges between the metal centers has been isolated and structurally characterized. The complex cation consists of a linear array of three copper ions, assembled by means of two doubly deprotonated ligands. The octahedral coordination sphere of the two peripheral copper(II) ions is completed by weakly bound methanol molecules, and the square-planar central metal ion is located on an exact, crystallographic inversion center. Temperature-dependent magnetic susceptibility studies reveal the presence of antiferromagnetic exchange coupling between the copper(II) ions in the trinuclear unit along with small intermolecular antiferromagnetic interactions in the low temperature range. The results were fitted in two different ways, (i) taking into account solely the exchange interaction between the adjacent metal centers or, (ii) regarding exchange interactions between both adjacent and non-adjacent copper(II) ions. Solid-state temperature-dependent X-band EPR studies in the range 4.2–250 K indicate a doublet ground spin state |½, 1〉. In solution, the ground spin state of the complex is found to be a quartet (S = ), suggesting a modification of the exchange coupling interactions between the copper(II) ions. The simulation of the 4.2 K solution spectrum gives rise to the best parameters D > 0.8 cm−1, g⊥ = 2.04 and g∥ = 2.21.

Thermodynamic properties of [Cu II (en) 2 (H 2 O)][Cu II (en) 2 Ni 2 Cu 2 I (CN) 10]·2H 2 O−S=1/2 magnet with a layered crystal structure

The specific heat of [CuII(en)2(H2O)][CuII(en)2Ni2Cu2I(CN)10]·2H2O was measured in the temperature range from 1.5 K to 10 K. The studies did not reveal any phase transition into an ordered state. Magnetization measurements, at 4.2 K and 13.7 K and in fields up to 60 kG, confirmed the assumption of weak exchange interactions, resulting from the knowledge of the crystal structure. Magnetic susceptibility data show deviations from Curie-Weiss law near 5.5 K. The analysis of the inverse susceptibility data indicates the presence of antiferromagnetic exchange coupling J/kB≈−0.65 K.

Giant Magnetoresistance in As-Sputter-Deposited Au/NiFe Multilayer Thin Films

ABSTRACTGiant magnetoresistance with low saturation fields (Hs’s) is reported in Au and permalloy (Ni0.82Fe0.18) or Co-doped permalloy multilayer thin films as-deposited on Ta-overcoated Si and glass substrates. A ΔR/R as high as 4.0% with ≈25 Oe Hs was observed at 295 K for the film consisting of 10 layers of 24 Å Au/13 Å Ni0.82Fe0.18 deposited on a 3 Å Ta-overcoated glass at 50 °C. A Hs value as low as ≈20 Oe with a 15% smaller ΔR/R has been observed for the films with a thicker (e.g., 50 Å) Ta underlayer. Magnetic hysteresis loops of these films indicate the presence of antiferromagnetic exchange coupling between the Ni0.82Fe0.18 layers. This exchange coupling is much smaller for the multilayer films without the Ta underlayer, resulting in a 6x smaller ΔR/R and lOx larger Hs observed for these films. Results of x-ray diffraction analysis indicate stronger (111) texturing for the multilayer films with a Ta underlayer, consistent with the stronger antiferromagnetic coupling between the the Ni0.82Fe0.18 layers in the film. The addition of 2–10 % Co moderately increases the ΔR/R value, but also increases substantially the Hs (up to ≈200 Oe).

Magnetic Susceptibilities and Exchange Effects in Four Organic Free Radicals

Studies of EPR intensity and linewidth in four free radicals at temperatures between 300° and 1.5°K have shown the presence of antiferromagnetic exchange coupling. In two cases a low-temperature paramagnetism occurs following a Curie-Weiss law with positive Weiss constant. A model involving isolated spins in a magnetically dilute system is proposed to explain these results.