Presence Of Antiferromagnetic Coupling Research Articles

Insight into ferromagnetic interactions in CuII-LnIII dimers with a compartmental ligand.

In the last two decades, efforts have been devoted to obtaining insight into the magnetic interactions between CuII and LnIII utilizing experimental and theoretical means. Experimentally, it has been observed that the exchange coupling (J) in CuII-LnIII systems is often found to be ferromagnetic for ≥4f7 metal ions. However, exchange interactions at sub-Kelvin temperatures between CuII and the anisotropic/isotropic LnIII ions are not often explored. In this report, we have synthesized a series of heterobimetallic [CuLn(HL)(μ-piv)(piv)2] complexes (LnIII = Gd (1), Tb (2), Dy (3) and Er (4)) from a new compartmental Schiff base ligand, N,N'-bis(3-methoxy-5-methylsalicylidene)-1,3-diamino-2-propanol (H3L). X-ray crystallographic analysis reveals that all four complexes are isostructural and isomorphous. Magnetic susceptibility measurements reveal a ferromagnetic coupling between the CuII ion and its respective LnIII ion for all the complexes, as often observed. Moreover, μ-SQUID studies, at sub-Kelvin temperatures, show S-shaped hysteresis loops indicating the presence of antiferromagnetic coupling in complexes 1-3. The antiferromagnetic interaction is explained by considering the shortest Cu⋯Cu distance in the crystal structure. The nearly closed loops for 1-3 highlight their fast relaxation characteristics, while the opened loops for 4 might arise from intermolecular ordering. CASSCF calculations allow the quantitative assessment of the interactions, which are further supported by BS-DFT calculations.

Defect-driven ferrimagnetism and hidden magnetization in MnBi2Te4

MnBi$_2$Te$_4$ (MBT) materials are promising antiferromagnetic topological insulators where field driven ferromagnetism is predicted to cause a transition between axion insulator and Weyl semimetallic states. However, the presence of antiferromagnetic coupling between Mn/Bi antisite defects and the main Mn layer can reduce the low-field magnetization, and it has been shown that such defects are more prevalent in the structurally identical trivial magnetic insulator MnSb$_2$Te$_4$ (MST). We use high-field magnetization measurements to show that the magnetization of MBT and MST occur in stages and full saturation requires fields of~$\sim$~60 Tesla. As a consequence, the low-field magnetization plateau state in MBT, where many determinations of quantum anomalous Hall state are studied, actually consists of ferrimagnetic septuple blocks containing both a uniform and staggered magnetization component.

Highly efficient spin-orbit torque in Pt/Co/Ir multilayers with antiferromagnetic interlayer exchange coupling

We have studied the spin orbit torque (SOT) in Pt/Co/Ir multilayers with 3 repeats of the unit structure. As the system exhibits oscillatory interlayer exchange coupling (IEC) with varying Ir layer thickness, we compare the SOT of films when the Co layers are coupled ferromagnetically and antiferromagnetically. SOT is evaluated using current induced shift of the anomalous Hall resistance hysteresis loops. A relatively thick Pt layer, serving as a seed layer to the multilayer, is used to generate spin current via the spin Hall effect. In the absence of antiferromagnetic coupling, the SOT is constant against the applied current density and the corresponding spin torque efficiency (i.e. the effective spin Hall angle) is $\sim$0.09, in agreement with previous reports. In contrast, for films with antiferromagnetic coupling, the SOT increases with the applied current density and eventually saturates. The SOT at saturation is a factor of $\sim$15 larger than that without the antiferromagnetic coupling. The spin torque efficiency is $\sim$5 times larger if we assume the net total magnetization is reduced by a factor of 3 due to the antiferromagnetic coupling. Model calculations based on the Landau Lifshitz Gilbert equation show that the presence of antiferromagnetic coupling can increase the SOT but the degree of enhancement is limited, in this case, to a factor of 1.2-1.4. We thus consider there are other sources of SOT, possibly at the interfaces, which may account for the highly efficient SOT in the uncompensated synthetic anti-ferromagnet (SAF) multilayers.

Thickness-Dependent Structural and Magnetic Study of Co/Si/Co Trilayers Grown by Ion Beam Sputtering

The magnetic properties of the metal/semiconductor structure can be modulated by the semiconductor layer thickness and therefore in the present paper, a series of trilayers of ion beam sputtered Cobalt–Silicon–Cobalt (Co/Si/Co) were grown to study the interface characteristics and their connections with magnetic properties. The thickness of Co layer, [Formula: see text], is fixed to 3[Formula: see text]nm, while varying the thickness of Si sandwich layer, [Formula: see text], from 1.5[Formula: see text]nm to 4[Formula: see text]nm, respectively. Grazing incidence X-ray diffraction (GIXRD), grazing incidence X-ray reflectivity (GIXRR), Atomic Force Microscopy (AFM) and Magneto-Optic Kerr Effect (MOKE) techniques were employed to study the crystal structure, surface and interface structure, morphology and magnetic characteristics of thin films. X-ray reflectivity measurements show substantial intermixing between the layers leading to trilayers of complicated structure at the interface during deposition. At [Formula: see text][Formula: see text]nm, whole Si layer is converted into silicide, whereas at higher spacer layer thickness ([Formula: see text]), in addition to silicide layer, an unreacted elemental layer of Si also remains in the spacer. A magneto-optical measurement reveals the presence of anti-ferromagnetic coupling in these samples and the strength of coupling between Co layers is found to be depended on [Formula: see text].

An adaptable heterometallic trinuclear coordination cluster in the synthesis of tailored one-dimensional architecture: Structural characterization, magnetic analysis and theoretical calculations

A new trinuclear complex [(CuLα-Me)2Co(bnz)2] (1) has been synthesized by using a metalloligand [CuLα-Me] (H2Lα-Me=N,N′-bis(α-methylsalicylidene)-1,3-propanediamine, bnz=benzoate) with trans-coordinated syn–syn bridging benzoate group (1κO:2κO′). The syn–syn coordinative selectivity of carboxylate towards this trinuclear unit leads exclusively to the formation of linear coordination cluster. Such coordinative adaptability is exploited for supramolecular assembly using a dicarboxylate linker, terephthalate (tph, 1,4-benzenedicarboxylate) which yielded a tailored one-dimensional quasi-linear coordination polymer [(CuLα-Me)2Co(tph)]n·2nH2O (2) having the linear trinuclear node. Isothermal magnetization measurement at 2K suggests that both 1 and 2 posses S=1/2 ground spin state indicating the presence of antiferromagnetic coupling at low temperature. The variable-temperature magnetic susceptibility measurements also reveal that both compounds are antiferromagnetically coupled with exchange coupling constants (J) of −17.3 and −9.2cm−1 for 1 and 2, respectively. The nature and magnitude of exchange interactions are further corroborated by density functional calculations.

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.

Domain wall creep in a 2D magnetic wire in the presence of antiferromagnetic coupling

We have studied the dynamics of propagation in one dimension of a magnetic domain wall (DW) in the presence of an exchange bias field in nanowires patterned in two-dimensional ultra-thin perpendicularly magnetized ferromagnetic–antiferromagnetic (F–AF) films. The DW velocity v(H) is strongly reduced when the DW propagates against the exchange bias field, and its field dependence is consistent with a creep process with a critical exponent μ = 1/4. The increase in energy barrier responsible for the reduction of v(H) cannot be explained by the introduction of a macroscopic exchange bias field. Instead it arises from the AF–F interface that provides an extra-source of random pinning.

Synthesis and characterization of a tri-nuclear Co(II) complex with a Bis(catecholate) ligand

Abstract A tri-nuclear Co(II) complex of formula [Co3(phen)3(L1)2] [L1 = 2,2′,3,3′-tetrahydroxy-benzaldazine, phen = 1,10-phenanthroline] has been prepared. The complex was characterized by X-ray crystal analysis, ESI-MS and its UV–Vis spectrum. The room temperature value of χmT is 7.50 emu K mol− 1, which is larger than the expected value of 5.58 emu K mol− 1 (for three isolated high spin Co(II) ions). When the temperature is lowered the χmT product continuously decreases down to 5.0 K, which indicates the presence of antiferromagnetic coupling between the CoII ions.

Synthesis, electrochemical, magnetic, catalytic and antimicrobial studies of N-functionalized cyclam based trinuclear copper(II) and nickel(II) complexes

Trinuclear copper(II) and nickel(II) complexes have been prepared by using Schiff base ligands derived from 1,8-[bis(3-formyl-2-hydroxy-5-methyl) benzyl]-4,11-dimethyl-l,4,8,11-tetraazacyclotetradecane, and 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-4,11-dimethyl-l,4,8,11-tetraazacyclotetradecane with aliphatic and aromatic diamines. All the complexes were characterized by elemental and spectroscopic analysis. Electrochemical studies of the copper(II) complexes in DMF solution show three irreversible one electron reduction process around E pc 1 = −0.59 to −0.80 V, E pc 2 = −0.89 to −1.14 V and E pc 3 = −1.17 to −1.29 V, and for nickel(II) complexes it is around E pc 1 = −0.63 to −0.77 V, E pc 2 = −1.20 to −1.35 V and E pc 3 = −1.60 to −1.74 V. ESR spectra and magnetic moments of the trinuclear Cu(II) complexes show the presence of antiferromagnetic coupling. Cryomagnetic investigation of the trinuclear copper(II) complexes show that the observed −2J values are in the range of 116–178 cm−1. The rate constants for hydrolysis of 4-nitrophenylphosphate by the complexes are in the range of 2.68 × 10−2 to 9.81 × 10−2 min−1. The rate constants values for the catecholase activity of the copper(II) complexes fall in the range of 3.03 × 10−2 to 9.32 × 10−2 min−1. All the complexes.

Characteristics of the magnetic ordering of Fe∕Au∕Tb multilayer films

Temperature and field investigations of the magnetic properties of multilayer films, consisting of a sequence of alternating magnetic layers of iron and the rare-earth terbium separated by layers of nonmagnetic gold, are performed. The samples were obtained by electron-beam deposition. The field dependence of the magnetic moment of a film with H∥n are analyzed to determine the characteristics of the uniaxial anisotropy. It is shown that a nonuniform uniaxial anisotropy results in nonlinearity in the magnetization of the film in H∥n. The process of magnetization of a film in H⊥n, when the transition to a single-domain state occurs only as a result of the motion of domain walls, is described. It is shown on the basis of the data on the field dependences of the magnetic moment at low temperatures that the magnetic moments of the Fe and Tb layers are oriented oppositely with respect to one another. A compensation point is observed in the temperature dependence of the magnetization; this indicates the presence of antiferromagnetic coupling between the Fe and Tb layers.

Synthesis and structural, spectroscopic and magnetic studies of two new polymorphs of Mn(SeO3)·H2O

Two new manganese(II) selenite polymorphs with formula Mn(SeO3)·H2O have been synthesized by slow evaporation from an aqueous solution. The crystal structure of both compounds (1) and (2) have been solved from X-ray diffraction data. The structure of (1) was determined from single-crystal X-ray diffraction techniques. The compound crystallizes in the Ama2 space group, with a=5.817(1), b=13.449(3), c=4.8765(9)Å and Z=4. The structure of (2) has been solved from X-ray powder diffraction data. This phase crystallizes in the P21/n space group with unit-cell parameters of a=4.921(3), b=13.121(7), c=5.816(1)Å, β=90.03(2)° and Z=4. Both polymorphs exhibit a layered structure formed by isolated sheets of MnO6 octahedra and (SeO3)2− trigonal pyramids in the (010) plane. These layers, which contain one manganese and selenium atom crystallographically independent, are formed by octahedra linked between them through the selenite oxoanions. The difference of both compounds consists in the stacking of the layers along the b-axis. The IR spectra show the characteristic bands of the selenite anion. Studies of luminescence performed at 6K and diffuse reflectance spectroscopy have been carried out for both phases. The Dq and Racah (B and C) parameters, from luminescence and diffuse reflectance spectroscopy, are Dq=705, B=750, C=3325cm-1 for (1) and Dq=720, B=745, C=3350cm-1 for (2). The ESR spectra of both compounds are isotropic with g-values of 1.99(1). Magnetic measurements indicate the presence of antiferromagnetic couplings in both phases. The J-exchange parameters have been estimated by fitting the experimental magnetic data to a model for square-planar lattice. The values obtained are J/k=-0.83, −0.91K and J′/k=-0.97, −1.20K, for polymorphs (1) and (2), respectively.

Supercritical hydrothermal synthesis, thermal, spectroscopic and magnetic studies of two new polymorphs of Mn(SeO3)

Two new manganese(II) selenite polymorphs with formula Mn(SeO3) have been synthesised using supercritical hydrothermal conditions. The crystal structure of both compounds (1) and (2) has been solved from single-crystal X-ray diffraction data. The structures consist of a three-dimensional framework formed by MnO6 octahedra and (SeO3)2- selenite anions with trigonal pyramidal geometry. Compound (1) shows chains of elongated, corner-sharing MnO6 octahedra. These chains are linked alternately by Mn2O10 dimers of edge-sharing octahedra. Conversely, compound (2) exhibits MnO6 octahedra sharing edges with three further octahedra, giving rise to a complex three-dimensional framework. The IR spectra show the characteristic bands of the selenite anion. Studies of luminescence and diffuse reflectance spectroscopy, performed at 6 K and at room temperature, respectively, have been carried out for both compounds. The Dq and Racah parameters are Dq= 830, B= 500 and C= 3790 cm(-1) for (1) and Dq= 795, B= 520 and C= 3785 cm(-1) for (2). The EPR spectra of both compounds are isotropic with a g-value of 1.99(1), which remains unchanged with variation in temperature. Magnetic measurements indicate the presence of antiferromagnetic couplings as the major interactions in both phases, but with compound (2) exhibiting at low temperature a canting of antiferromagnetically aligned spins. The estimated J-exchange parameters are J/k=-2.2 and -1.93 for (1) and (2), respectively, with J'= -0.87 and -0.55 K.

Coordination Behaviour of Novel Macrobicyclic Tricompartmental Ligands: Synthesis, Magnetic, Electrochemical and Kinetic Studies of Unsymmetrical Mono- and Binuclear Copper(II) Complexes

Several mono- and binuclear copper(II) complexes of general formula [CuL](ClO4) and [Cu2L(ClO4)](ClO4) have been synthesized from macrobicyclic ligands having different compartments originating from their corresponding precursor compound (PC: 3,4:9,10-dibenzo-1,12[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,8-dioxacyclohexadecane). Electrochemical studies show that one quasi-reversible reduction wave (E pc=-0.75 to -0.83 V) for the mononuclear complexes and two quasi-reversible one-electron transfer reduction waves (=-0.78 to -0.86 V, =-1.01 to -1.28 V) for the binuclear complexes are obtained in the cathodic region. Room temperature magnetic moment studies indicate the presence of antiferromagnetic coupling in the binuclear complexes (μ eff=1.44–1.56 BM), which is confirmed by the broad ESR spectra with g=2.10–2.11, whereas the mononuclear complexes show hyperfine splitting in the ESR spectra and have magnetic moment values close to the spin-only value (μ eff=1.69–1.72 BM). Variable temperature magnetic susceptibility study of the complex finds that the observed -2J values for the binuclear complexes [Cu2L1(ClO4)](ClO4) and [Cu2L4(ClO4)]-(ClO4) are 216 cm-1 and 236 cm-1, respectively. The observed initial rate constant values for catechol oxidation using the complexes as catalysts range from 6.57×10-3 to 4.93×10-2 min-1 and the values are found to be higher for the binuclear complexes than the corresponding mononuclear complexes. It is proposed that a distortion of the copper ion geometry arises as the macrocyclic ring size increases. This is supported by spectral, electrochemical and catalytic studies.

First report of singly phenoxo-bridged copper(ii) dimeric complexes: synthesis, crystal structure and low-temperature magnetic behaviour studyElectronic supplementary information (ESI) available: structural data for compounds 1, 3, 2. See http://www.rsc.org/suppdata/nj/b3/b300217a/

Three singly phenoxo-bridged dinuclear Cu(II) complexes [Cu2(L)2(SCN)(H2O)](ClO4) 1, [Cu2(L)2(N3)(H2O)](ClO4) 2, [Cu2(L)2(NCO)(H2O)](ClO4) 3 (HL is the Schiff base ligand derived from O-vanillin and 2-aminomethyl ethylenediamine) have been synthesised and characterised by infrared, UV-Vis spectra, electrochemical study and single crystal X-ray diffraction study. In all the complexes (1–3), there are two geometrically different square pyramidal copper(II) centres, with N3O2 and N2O3 donor sets for the two Cu(II) centres. In the dimeric unit, the copper atoms are held together by only one distinct rare μ2-phenolate oxygen of one Schiff base. The Cu–Cu distances are 3.291(2), 3.244(1) and 3.244(1) Å for complexes 1, 2 and 3, respectively, and the Cu–O–Cu angles are found to be 111.5(1), 109.3(2), 109.6(1)° for complexes 1, 2 and 3, respectively. Variable temperature magnetic susceptibility study on the complexes 1,2 and 3 indicate the presence of antiferromagnetic coupling between the Cu(II) centres with 2J values −109.8, −103.9 and −95.9 cm−1, respectively.

Molecular materials based upon organic π-donors and magnetic anions

Electrochemical combination of the magnetic dinuclear anion [MM'(C 2 O 4 )(NCS) 8 ] 4- (MM' = CrCr, CrFe) or paramagnetic mononuclear anion [Cr(NCS) 6 ] 3- with the ET organic π-donor (ET = BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) gives rise to two new series of molecular hybrid salts formulated (ET) 5 [MM'(C 2 O 4 )(NCS) 8 (MM' = CrCr (1), CrFe (2)), (ET) 8 [MM'(C 2 O 4 )(NCS) 8 ] (3), (ET) 5.5 f Cr(NCS) 6 ] (4) and (ET) 5 [(C 2 H 5 ) 4 N] [Cr(NCS) 6 ](THF) (5) (THF = tetrahydrofuran). The crystal structures consist of alternating organic and inorganic layers. The organic part does not produce any detectable effect on the magnetic properties coming from the inorganic sublattice. They indicate the presence of antiferromagnetic coupling in the homometallic dimers (1 and 3), while the coupling is ferromagnetic in the heterometallic one (2). Conductivity measurements reveal a metallic state for compounds 3 and 5.

Structural and magnetic properties of Fe/Cr and Fe/Ag multilayers

The structural and magnetic properties of Fe/Cr and Fe/Ag magnetic superlattices are discussed. The presence of antiferromagnetic coupling in Fe/Cr superlattices was clearly established, whereas Fe/Ag superlattices do not show antiferromagnetic coupling in their as-prepared state. Thermal annealing and ion irradiation lead to an increase of the magnetoresistance due to the formation of a granular FeAg alloy and a Kondo system, respectively.

Giant magnetoresistance in Fe/Mo multilayers formed by magnetron sputtering

Fe/Mo multilayers have been prepared by magnetron sputtering. Giant magnetoresistance (GMR) has been found in the samples with an antiferromagnetic interlayer coupling. The magnetoresistance (MR) ratio exceeds 12% at 4.2 K, and it oscillates as a function of Mo spacer thickness. The oscillation period is about 10–12 Å, which is consistent with the case of the reported Kerr effect. The results suggest that the presence of antiferromagnetic coupling and the absence of GMR are the result of the sharp interfaces, and that relatively rough interfaces and moderately thin Fe layer thickness are the key factors for enhancing MR in the sputtered films.

Magnetization of (111)-oriented MBE-grown Co/Cu magnetic multilayers.

We present results for the magnetization of a series of Co/Cu magnetic multilayers that were grown epitaxially in the (111) orientation. The Co layer thickness was fixed at 15 \AA{} and the Cu layer thickness varied from 5 to 20 \AA{}. The magnetization was found to increase linearly with magnetic field for fields up to about 10 kOe, which indicates the presence of antiferromagnetic coupling between the Co layers. The strength of the exchange coupling between the layers was found to oscillate as a function of the Cu thickness, with an oscillation period of about 12 \AA{}, although no corresponding oscillation was observed in the magnitude of the magnetoresistance. However, the observed approach to saturation of the magnetization corresponds closely to that of the magnetoresistance.

Giant magnetothermopower and giant magnetoresistance in molecular beam epitaxy grown Co/Cu(111) superlattices

We report the observation of a giant magnetothermopower (GMT), giant magnetoresistance (GMR), and on magnetization measurements in Co/Cu[111] superlattices grown by molecular beam epitaxy. The maximum value of the GMT (at room temperature) was 14% for a Cu thickness of 9 Å and the maximum GMR (at 4.2 K) was −26% at 7 Å of Cu. Oscillations in the remnant magnetization and the saturation field as a function of Cu thickness with a period of about 10 Å were observed. However, there were no oscillations in the GMT or the GMR. The maximum values of both the GMT and GMR are associated with saturation fields in excess of 40 kOe and with small remnant magnetizations. These results are consistent with the presence of antiferromagnetic coupling.