Antiferromagnetic Exchange Coupling Research Articles

Coupled skyrmion breathing modes in synthetic ferri- and antiferromagnets

We present micromagnetic simulations of the dynamic GHz-range resonance modes of skyrmions excited by either out-of-plane AC magnetic fields or spin torques in prototypical synthetic ferri- and antiferromagnetic trilayer structures. The observed features in the calculated power spectra exhibit a systematic dependence on the coupling strength between the individual magnetic layers and are related to pure in-phase and antiphase breathing modes as well as to hybridizations of breathing and spin-wave modes that are characteristic for the considered circular-shaped geometry. The experimental detection of these resonant oscillation modes may provide a means for skyrmion-sensing applications and for the general characterization of skyrmion states in multilayer stacks with antiferromagnetic interlayer exchange coupling.

Two new copper(II) binuclear complexes with 2-[(E)-(pyridine-2yl-hydrazono)methyl]phenol: Molecular structures, quantum chemical calculations, cryomagnetic properties and catalytic activity

The Schiff base is synthesized by the reaction of 2-hydrazinopyridine and salicylaldehyde. The copper(II) binuclear complexes [Cu2(µ-ClO4)(L)2(H2O)2](ClO4)·H2O(1) and [Cu2(µ-pyrazine)(L)2](ClO4)2(2)with 2-[(E)-(pyridine-2yl-hydrazono)methyl]phenol (HL), were synthesized and characterized using various physicochemical techniques. The molecular structures of both binuclear complexes were evaluated by single-crystal X-ray analysis. Analysis of the supramolecular synthons and their effect on crystal packing is conferred in the context of crystal engineering. The electron paramagnetic spectra are reported as well. The electrochemical behavior of both complexes was explored using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The electronic and spectral properties are described by quantum chemical calculations (TD and DFT).The cryomagnetic investigation (2–300 K) reveal antiferromagnetic exchange coupling between two copper(II) centres of different strength: JCuCu = −21.6 and JCuCu = -6.8 cm−1 for 1 and 2, respectively. The strong antiferromagnetic coupling between the copper centers found in 1 could be explained not only by magnetic exchange through hydrogen bonds but also through the π-π stacking of the Schiff base ligands. Both binuclear complexes exhibit catalytic activity toward the dismutation of superoxide anion at physiological pH. Although the activity in both complexes is lower than the native enzyme, they have potential as antioxidant SOD model for pharmaceutical applications.

Entanglement entropy in the spin-1/2 Heisenberg chain with hexamer modulation of exchange

The spin-1/2 isotropic alternating Heisenberg chains with hexamer modulation of exchange are considered. Recently, the ground state phase diagram of the model was obtained for different patterns of modulations in the limit of strong antiferromagnetic exchange coupling on odd links. In addition to the zero and saturated-magnetization plateaus, two gapped plateau states were recognized in the presence of a uniform magnetic field. Here, using the numerical Lanczos method, the ground state of finite chain systems is calculated and the behavior of the entanglement entropy (EE) and the concurrence is studied. First, we divided the chain system into two subsystems A and B and focus on EE which shows how much subsystems A and B are entangled. Results show that a pair of spins located on a strong antiferromagnetic link is entangled in the gapped mid plateau states. On the other hand, a pair of spins located on a weak link is completely entangled with the rest of the chain in the zero-magnetization plateau state. Second, we focused on the entanglement between nearest neighbor pair spins and the concurrence is calculated numerically. Pair spins on non-modulated links will be entangled only in the first or the second gapped plateau phases.

Synthetic domain walls in [TbFeGa/TbFe]2 multilayers

Here we report the possibility of creating synthetic domain walls in nominal [Fe72Ga28/Tb33Fe67]2 multilayers. The magnetization as a function of the temperature reveals the absence of Compensation temperature in the samples which can be understood considering an interdiffusion process that results in the formation of TbFeGa alloys at the nominal FeGa layers. Therefore, samples actually comprise TbFeGa and TbFe layers. The hysteresis loops exhibit magnetization steps related to the nucleation of domain walls formed because of the competition between different interactions: (i) the antiferromagnetic exchange coupling between the heavy rare earth (Tb) and the transition magnetic metal (Fe) inside each layer, (ii) the antiferromagnetic coupling between Tb and Fe at the interfaces, and (iii) the Zeeman energy. In good agreement with the experimental values, the nucleation field of the domain walls has been theoretically calculated taking into account the tilt of the magnetization with respect to the sample plane. Our experimental results indicate that by a proper thickness adjustment, it can be tuned, both the value of the nucleation field and the layer in which this firstly occurs. Experimental values for the exchange bias field have also been calculated.

Magnetic properties of the one-dimensional S=32 Heisenberg antiferromagnetic spin-chain compound Na2Mn3O7

We have performed magnetic susceptibility $(\ensuremath{\chi})$, heat capacity $({C}_{p})$, and nuclear magnetic resonance (NMR) measurements on the layered compound ${\mathrm{Na}}_{2}{\mathrm{Mn}}_{3}{\mathrm{O}}_{7}$ in which the magnetic ${\mathrm{Mn}}^{4+}$ ions constitute a maple-leaf lattice. At high temperature, $\ensuremath{\chi}$ follows the Curie-Weiss (CW) law with CW temperature as high as 152 K and ${\mathrm{Mn}}^{4+}$ ions are found to be in the high-spin state $(S=\frac{3}{2})$. With decreasing temperature, $\ensuremath{\chi}$ passes through a broad maximum at 115 K and decreases rapidly at low temperature. A similar broad peak appears at around 62 K in the magnetic contribution to heat capacity $({C}_{m})$. The temperature dependence of $\ensuremath{\chi}$ and ${C}_{m}$ are the characteristics of an one-dimensional Heisenberg antiferromagnetic spin chain with a nonmagnetic ground state. The obtained values of spin-gap energy $({\mathrm{\ensuremath{\Delta}}}_{s})$ and nearest-neighbor antiferromagnetic exchange coupling $(J)$ are 28 and 27 K, respectively. There is no evidence of a long-range magnetic ordering down to 2 K in heat capacity data, in agreement with spin-singlet ground state. These observations are further corroborated by $^{23}\mathrm{Na}$ NMR studies through measurements of both the NMR shift and the spin-lattice relaxation rate.

Temperature-induced antiferromagnetic interlayer exchange coupling in (Ga,Mn)(As,P)-based trilayer structure

We present the observation of temperature-induced transition between ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange coupling (IEC) in trilayer structures consisting of two (Ga,Mn)(As,P) ferromagnetic layers and a GaAs:Be spacer. Hall resistance measurements were performed on trilayers prepared in as-grown and annealed states. Anomalous Hall resistance (AHR) measurements show two-step hysteresis loops, indicating that coercive fields of the two (Ga,Mn)(As,P) layers in the trilayer are different. Minor loops of AHR show characteristic shifts that correspond to FM IEC at low temperatures and AFM IEC at high temperatures in both as-grown and annealed samples. This temperature-induced transition from FM to AFM IEC occurs at a lower temperature in the annealed sample than in the as-grown sample. We attribute such an enhanced IEC transition effect in the annealed sample to the increase of carrier concentrations obtained by annealing, a characteristic feature of ferromagnetic semiconductors. The observation of AFM IEC in the (Ga,Mn)(As,P)-based trilayer suggests the possibility of realizing synthetic antiferromagnetic systems in such structures, which can be used for antiferromagnetic spintronic device applications.

Laser-induced terahertz emission from layered synthetic magnets

We investigated laser-pulse-induced terahertz (THz) emission in Ta/CoFeB/Ir/CoFeB/Ta layered synthetic magnets with different Ir spacer thicknesses. THz emissions were observed under no applied magnetic fields for samples exhibiting interlayer antiferromagnetic exchange couplings, and those THz emissions also showed oscillatory behavior against the Ir spacer thicknesses. These results are qualitatively explained with the THz emission mechanism due to the inverse spin Hall effect for the laser-induced spin current in the layered structures. We also discuss the effect of a non-collinear configuration of magnetizations on THz emission with macrospin simulation.

Lattice dynamics in the spin- 12 frustrated kagome compound herbertsmithite

We investigate the lattice dynamics in the spin-1/2 frustrated kagome compound herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$ by a combination of infrared spectroscopy measurements and ab initio density functional theory calculations, and provide an unambiguous assignment of infrared-active lattice vibrations involving in-plane and out-of-plane atom displacements in the kagome layers. Upon cooling, non-thermal red-shifts and broadening appear specifically for modes that deform the kagome layer or affect the Cu-O-Cu bond angles, thus creating pronounced modifications of the antiferromagnetic exchange coupling. Our results indicate the presence of a strong magnetoelastic coupling to the spin system. We discuss the effects of this coupling and its relation to recent experiments reporting a global symmetry reduction of the kagome lattice symmetry.

Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

Vanadium dichalcogenides have attracted increasing interests for the charge density wave phenomena and possible ferromagnetism. Here, we report on the multiphase behavior and gap opening in monolayer VTe2 grown by molecular beam epitaxy. Scanning tunneling microscopy (STM) and spectroscopy study revealed the (4×4) metallic and gapped (2√3×2√3) charge-density wave (CDW) phases with an energy gap of ≈ 40 meV. Through the in-plane condensation of vanadium atoms, the typical star-of-David clusters and truncated triangle-shaped clusters are formed in the (4×4) and (2√3×2√3) phases respectively, resulting in different surface morphologies and electronic structures as confirmed by density functional theory (DFT) calculations with on-site Coulomb repulsion. The CDW-driven reorganization of the atomic structure weakens the ferromagnetic superexchange coupling and strengthens the antiferromagnetic exchange coupling on the contrary, suppressing the long-range magnetic order in monolayer VTe2. The electron correlation is found to be important to explain the gap opening in the (2√3×2√3) phase.

(FeCoB/Ru/FeCoB)/ZnO]n superlattice multilayer: A real optical mode ferromagnetic resonance thick-film

A pure optical mode (OM) resonance with ultrahigh resonance frequency (frO) and permeability μ at zero bias field was observed in 50-nm FeCoB/Ru/FeCoB sandwich trilayers due to the presence of strong antiferromagnetic interlayer exchange coupling. However, the necessary thickness for creating practical microwave devices cannot be readily achieved by increasing the FeCoB layer thickness or stacking FeCoB/Ru units due to dramatically deteriorated high-frequency performance. In this study, several 50-nm FeCoB/Ru/FeCoB OM units were stacked to form a superlattice structure, with 10-nm ZnO insulator spacers separating the units. It is interesting that the superlattice multilayers not only retain excellent high-frequency OM resonance performance comparable to the single OM unit but also increase the effective magnetic film thickness by a factor of 5. This can be attributed to the positive superposition of the almost identical OM units as well as the effective decoupling between them by thick ZnO spacers. This study provides a promising way to fabricate thicker films while still maintaining excellent OM resonance performance.

Crystalline salts of the ring-reduced tin(IV) dichloride hexadecachlorophthalocyanine and octachloro- and octacyanotetrapyrazinoporphyrazine macrocycles with strong electron-withdrawing ability

Substituted tin(IV) dichloride phthalocyanine and tetrapyrazinoporphyrazines with acceptor chloro- or cyano-substituents: SnIVCl2(PcCl16), SnIVCl2(TPyzPzCl8) and SnIVCl2{TPyzPz(CN)8}, where PcCl16 = hexadecachlorophthalocyanine; TPyzPz(CN)8 = octacyanotetrapyrazinoporphyrazine, TPyzPzCl8 = octachlorotetrapyrazinoporphyrazine were synthesized. The reduction of these metallomacrocycles by fullerene {cryptand(Na+)}(C60•−) salt (1, 3 and 4) or potassim graphite (KC8) in the presence of cryptand (2) yields crystalline anionic salts: {cryptand(Na+)}2[SnIVCl2(PcCl164−)]2−⋅C6H4Cl2 (1), {cryptand(K+)}2[{SnII(PcCl164−)}0.926{SnIVCl2(PcCl164−)}0.074]2−⋅2C6H4Cl2 (2), {cryptand(Na+)}[SnIVCl2{TPyzPz(CN)8•3−}]•−⋅C6H4Cl2 (3) and {cryptand(Na+)}2[SnIVCl2(TPyzPzCl84−)]2−⋅2C6H4Cl2 (4). Reduction of SnIVCl2(macrocycle2−) with dianion macrocycles is accompanied by the formation of radical trianion or tetraanion macrocycles in 1–4. As stronger reductant, KC8 reduces the macrocycle and in addition nearly complete metal centered reduction from tin(IV) to tin(II) took place. Strong blue shift of the Q-band and new absorption bands are observed in the NIR range in the spectra of 1–4. Partial disruption of macrocycle aromaticity and alternation of the C – Nmeso bonds were found for the reduced macrocycles. Macrocycles are planar, and SnIV atoms are positioned exactly in the macrocycle plane. The only exception is strong distortion of the PcCl16 macrocycle in SnII(PcCl164−) (2). Structures of 1–4 contain layers from the macrocycles having the side-by-side contacts in 1, 2, 4 and separated by cationic layers. Salt 3 presents the first structure of the TPyzPz(CN)8 macrocycle with strongest acceptor properties amomg known porphyrins and porphyrazines. It was shown that tetraanion macrocycles are diamagnetic in 1, 2 and 4. The [SnIVCl2{TPyzPz(CN)8)•3−}]•− radical anions are paramagnetic in 3 with effective magnetic moment of 1.64 μB, and salt 3 shows moderate antiferromagnetic coupling of spins with Weiss temperature of −14 K.

Enhanced field-free current-induced magnetization switching by interlayer exchange coupling with insulating spacer layer

We investigated field-free current-induced perpendicular magnetization switching in Pt/Co/AlOx/Co/Ta structures by varying the thickness of an insulating spacer layer. A field-free spin–orbit torque switching is realized through the antiferromagnetic interlayer exchange coupling (IEC) between the bottom and top Co layers or by premagnetizing the top Co layer. Significant variations in magnetic and electrical properties are ascribed to thickness dependent IEC by changing the insulating spacer layer from 1.0 to 1.9 nm. When the thickness of the spacer layer is 1.6 nm, we found the strongest IEC field of about 300 Oe and optimal field-free current-induced magnetization switching. Micromagnetic simulation validates the existence of the Dzyaloshinskii–Moriya interaction (DMI) effect and the chirality of the domain wall configuration in the stack structures, and the field-free deterministic magnetization switching is mainly induced from DMI and IEC fields.

On the origin of positive exchange bias and coercivity enhancement in proximity to the blocking temperature

Magnetic properties of two oxidised Co thin films prepared by different sputtering techniques are analysed. Exchange bias (EB) blocking temperatures (TB) of the samples were found to be 50 K different due to difference in grain sizes of antiferromagnetic CoO. In proximity to TB the EB changes its sign from negative to positive. This positive EB behaviour is strikingly similar for the two samples with the exception of its position on the temperature scale. A computational model, taking into account temperature dependence of the antiferromagnet’s magnetocrystalline anisotropy and its sublattices’ magnetisation, reproduces positive EB in proximity to TB. This happens when a significant fraction of antiferromagnetic entities becomes involved in magnetisation reversal, and there is antiferromagnetic exchange coupling between those entities. The coercivity enhancement, accompanying positive EB, is due to anisotropy energies of the reversible antiferromagnetic entities contributing to the ferromagnetic layer’s energy barrier. The model suggests that in order for the two samples to have identical positive EB behaviour, the reversible antiferromagnetic entities should represent individual spins.

A Neutral Fe4tBuL4 All Ferric Grid Complex: Structural and Variable Temperature Magnetic Properties

A neutral all ferric [2×2] grid complex Fe4tBuL4 1 was synthesized and characterized in the solid‐state by X‐ray crystallography, variable temperature magnetic susceptibility and Mössbauer spectroscopy. The absence of any coordinate bond or other structural changes between 90(2) and 320(2) K indicated no changes to the electronic structure of 1 occurred in the solid‐state over this temperature range. Variable temperature magnetic susceptibility data were fitted to an isotropic spin Hamiltonian model and suggested weak antiferromagnetic exchange coupling between the high‐spin ferric ions (J = –16.1 cm‐1). Mössbauer spectroscopy parameters (δ = 0.416(9) mm s–1 & ΔEQ = 0.87(7) mm s‐1), powder EPR spectra, and broken symmetry density functional theory calculations supported the high‐spin ferric oxidation state assignment.

Three lanthanide–nitronyl nitroxide complexes: Syntheses, crystal structures, magnetic properties and fluorescence of selective sensing of Fe (III) ions

Three new lanthanide-nitronyl nitroxide radical complexes [Ln(hfac)3(NIT-biPh)2](Ln = Gd 1, Tb 2, Dy 3) (hfac = hexafluoroacetylacetonate; NIT-biPh = 2-([1,1′-biphenyl]-4-yl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been successfully synthesized and characterized. The X-ray crystal structure analysis revealed that three complexes are isomorphous, which exhibiting a mononuclear tri-spin structure. The central Ln (III) ion is eight-coordinated by six oxygen atoms from three hfac ligands and two oxygen atoms from the NO groups of two NIT-biPh ligands. The variable-temperature magnetic susceptibility indicated that ferromagnetic exchange interactions between the coordinated NO groups of radicals and Gd(III) ions, while antiferromagnetic exchange couplings between radicals within complex. Ac magnetic susceptibility exhibited no magnetic relaxation behavior at low temperature in compound 2. In addition, the luminescence properties of complex 2 show the characteristic emission peaks of Tb(III) ions and indicate an excellent fluorescent probe of recognizing Fe3+ ion.

Nickelate superconductors: Multiorbital nature and spin freezing

Superconductivity with a remarkably high $T_c$ has recently been found in Sr-doped NdNiO$_2$ thin films. While this system bears strong similarities to the cuprates, some differences, such as a weaker antiferromagnetic exchange coupling and possible high-spin moments on the doped Ni sites have been pointed out. Here, we investigate the effect of Hund coupling and crystal field splitting in a simple model system and argue that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. We then look at this system from the viewpoint of the spin-freezing theory of unconventional superconductivity, which provides a unified understanding of unconventional superconductivity in a broad range of compounds. Sr$_{0.2}$Nd$_{0.8}$NiO$_2$ falls into a parameter regime influenced by two spin-freezing crossovers, one related to the emergent multi-orbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.

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.

Exchange bias in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3/La0.7Sr0.3CrO3 heterostructures

In the recent past, heterostructures of magnetic oxide thin films have attracted a great deal of research excitement due to very interesting physical properties such as antiferromagnetic interlayer coupling, tunable exchange-bias, interfacial driven magnetic properties and high mobility electron gas across the interfaces. In this work, we report on the comprehensive magnetic properties observed from the heterostructures of (2 unit cells) La0.7Sr0.3CrO3/(8 unit cells) La0.7Sr0.3MnO3/(2 unit cells) La0.7Sr0.3CrO3, which are epitaxially deposited on SrTiO3 substrate by plasma-assisted oxide molecular beam epitaxy. Using SQUID magnetometer, the magnetic properties are studied when the magnetic field was applied both in plane and out of plane. The Curie temperature of this structure is found to be at 290 K. Most significantly, at 2 K, we observed a complete up/down shift (along magnetization axis) of hysteresis loop when the sample was cooled under a magnetic field of ± 5000 Oe in the in-plane configuration. We believe that the strong antiferromagnetic (super) exchange coupling of Mn-Cr across the two interfaces is responsible for the observed exchange bias. We will present and discuss our in-detailed experimental findings collected on this heterostructure as a function of temperature and magnetic field.

Two New Copper(Ii) Binuclear Complexes with Azo Functionalized 2-[(E)-(Pyridine-2yl-Hydrazono)Methyl]Phenol: Characterization, Molecular Structures, Quantum Chemical Calculations, Cryomagnetic Properties and Catalytic Activity

The Schiff base is synthesized by the reaction of 2-hydrazinopyridine and salicylaldehyde. The copper(II) binuclear complexes [Cu 2 (µ 2 -ClO 4 )(L) 2 (H 2 O) 2 ](ClO 4 )·H 2 O (1) and [Cu 2 (µ 2 -pyrazine)(L) 2 ](ClO 4 ) 2 (2) with 2-[(E)-(pyridine-2yl-hydrazono)methyl]phenol (HL), were synthesized and characterized using various physicochemical techniques. The molecular structures of both binuclear complexes were evaluated by single-crystal X-ray analysis. Analysis of the supramolecular synthons and their effect on crystal packing is conferred in the context of crystal engineering. The electron paramagnetic spectra are reported as well. The electrochemical behavior of both complexes was explored using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The electronic and spectral properties are described by quantum chemical calculations (TD and DFT).The cryomagnetic investigation (2-300 K) reveal antiferromagnetic exchange coupling between two copper(II) centres of different strength: J CuCu = -21.6 and J CuCu = -6.8 cm -1 for 1 and 2 , respectively. The strong antiferromagnetic coupling between the copper centers found in 1 could be explained not only by magnetic exchange through hydrogen bonds but also through the π-π stacking of the Schiff base ligands. Both binuclear complexes exhibit catalytic activity toward the dismutation of superoxide anion at physiological pH. Although the activity in both complexes is lower than the native enzyme, they have potential as antioxidant SOD model for pharmaceutical applications.

Strong antiferromagnetic coupling of the cobalt(ii)–semiquinone radical in a dinuclear complex with 2,2′-bipyrimidine ligands

A new dinuclear cobalt(ii)–semiquinone radical complex was synthesised and displayed strong antiferromagnetic exchange coupling, having −90.25 cm−1of aJvalue, between the cobalt(ii) centres and semiquinone radicals.