Antiferromagnetic Coupling Strength Research Articles

Sub-terahertz excitations in a synthetic antiferromagnet with perpendicular anisotropy

In this paper, micromagnetic simulations are employed to investigate terahertz (THz) magnetic excitations in a spin torque nano-oscillator (STNO) with a perpendicularly magnetized synthetic antiferromagnetic (SAF) free layer. The magnetization precession of the free layer can be finely tuned into the sub-THz range without the necessity of external magnetic fields. The excited frequency exhibits two distinctive regions, namely region-I and region-II, depending on the applied current strength. In region-I, characterized by relatively small currents, the two ferromagnetic layers are stabilized at two separate precession orbits. The frequency in this region decreases with current strength, exhibiting similar features as the Néel vector change observed in antiferromagnets. In contrast, region-II is defined by currents where the two ferromagnetic layers synchronize into the same precession orbit. The frequency increases with current, correlating with the variation in the net magnetization of the SAF layer. An analytical model is developed through the canonical transformation of Lagrange’s equation, which can describe the frequency dependence on both the applied current and the antiferromagnetic interlayer coupling strengths. The simulations and the analytical model show good agreement, offering a more profound understanding of the magnetic excitation properties in STNOs with ultrathin SAF free layers. These insights are crucial for the design of advanced terahertz spintronic devices.

Frustrated magnetism of the spin-1 kagome antiferromagnet β-BaNi3(VO4)2(OH)2

We propose the newly synthesized β-BaNi3(VO4)2(OH)2 (space group: ) as a candidate for the spin-1 kagome Heisenberg antiferromagnet (KHA). The compound features a uniform kagome lattice of Ni2+ (S = 1) ions with a large interlayer distance. High-field measurements at low temperatures reveal a susceptibility local minimum at ∼9 T, resembling a 1/3 magnetization plateau as predicted by the pure S = 1 KHA model. Below ∼6 K, approximately 1% of the spins exhibit spin-glass order, which may be attributed to the nanocrystalline grain size of ∼50 nm. Despite the antiferromagnetic exchange coupling strength of ∼7 K, the majority of spins remain disordered down to ∼0.1 K as indicated by the observed power-law behaviors in magnetic specific heat . Our results demonstrate that the low-energy magnetic excitations in β-BaNi3(VO4)2(OH)2 are gapless, which contradicts the current theoretical expectations of the ideal model.

Noncollinear interlayer exchange coupling across IrFe spacer layers

In this paper, we show that IrFe spacer layers enable the control of noncollinear interlayer exchange coupling between ferromagnetic Co layers over a spacer layer composition and thickness range that is larger than any observed before for other spacer layer materials. Moreover, in the studied Co|IrFe|Co structures, the bilinear coupling strength, J1, is above 2mJ/m2 and the biquadratic coupling strength, J2, is above 1mJ/m2 over a wide spacer layer composition and thickness range. Notably, the largest antiferromagnetic bilinear coupling strength ever observed across spacer layers of 0.6nm or thicker is achieved, with J1 peaking at 3.4mJ/m2. After annealing, orthogonal coupling is favoured over a wide spacer layer composition and thickness range, which could be useful for magnetic sensor devices. As the temperature is reduced below room temperature, the coupling parameters of the as-deposited films vary slowly, which could be a desirable feature for applications. Measurements of the saturation magnetization of Co|IrFe|Co structures reveal that IrFe has a magnetic moment in the spacer layer composition range for which noncollinear coupling is observed. This indicates that the origin of noncollinear coupling in Co|IrFe|Co could be attributed to the competition between the antiferromagnetic coupling of magnetic atoms across Ir atoms in the spacer layer and the ferromagnetic coupling of neighbouring magnetic atoms.

Classification of hysteresis loops for exchange biased F/NM/F trilayer with antiferromagnetic interlayer coupling

Major hysteresis loops of the F/NM/F/AF structure are classified according to the magnetization process, with the assumption of coherent rotation for each F component and antiferromagnetic coupling between the two F layers. Magnetic stable states of the system as well as specific reversal modes of magnetization during each stage of the major hysteresis cycle are studied in detail based on energetics analysis, in the whole range of exchange bias field and interlayer antiferromagnetic coupling strength. The analysis shows 12 different types of major hysteresis loops for this system and is confirmed by simulation. Strategies to determine key material parameters for the F/NM/F/AF structure from major hysteresis loops are proposed and illustrated by numerical simulation. The research shows a subtle tuning of the magnetization process in the F/NM/F/AF structure, collectively by exchange biasing and interlayer antiferromagnetic coupling. Practically, this serves as a theoretical grounding for the identification of root causes of failure in the product analysis of GMR (giant magnetoresistance) sensors.

An ab initio DFT study of the optical and magnetic properties of Mn doped GaFeO3

In this study, the effect of Mn substitution on the structural, magnetic, electronic and optical properties of GaFeO3 (GFO) were investigated with density functional theory (DFT). A detailed discussion on the magnetic exchange interaction was performed and new aspects like antiferromagnetic coupling strength and exchange coupling parameter were studied. GFO has an antiferromagnetic structure and spontaneous electric polarization in the ground state. The antiferromagnetic structure arises from the magnetic exchange interaction between Fe1 and Fe2 atoms via O atoms. Replacing the Fe1 atom with the Mn atom changes the magnetic exchange interaction in GFO and as a result alters the ground state properties of GFO. Structural investigations indicate that the lattice parameters and unit cell volume are increased with Mn substitution. Also, increasing the Mn concentration leads to an increment in the total spin magnetic moment because of the different magnetic moments of Fe and Mn atoms. This increase is accompanied by an antiferromagnetic to ferrimagnetic (AFM-FIM) transition. Furthermore, the antiferromagnetic coupling strength is decreased with the Mn substitution. Also, electronic investigations reveal the formation of new states around the Fermi surface in spin up channel after the substitution of Mn in the GFO host structure. These new states make possible optical transitions at energies less than the GFO band gap. The number of these new states is increased with increasing the Mn concentration. The imaginary component of the dielectric function, absorption coefficient, and real component of optical conductivity are increased with an increase in the Mn concentration in the energy range up to about 3 eV.

Competing magnetic interactions and emergent phase diagrams in double perovskite Y2NixCo1−xMnO6

We present a comparative study of double perovskites Y2NixCo1−xMnO6 for x = 1, 0.5, and 0. The polycrystalline samples of Y2NixCo1−xMnO6 with space group P21/n were synthesized via sol-gel technique. X-ray photoelectron spectroscopy (XPS) confirms the presence of majority Ni2+/Mn4+ in Y2NiMnO6 (YNMO) and a mixed valence state Ni2+/3+/Co2+/3+/Mn4+/3+ in co-doped Y2Ni0.5Co0.5MnO6 (YNCMO). The temperature dependent magnetization results suggest homogeneous substitution of Ni ions with Co ions with minimal contribution of 3 + cationic magnetic interactions in YNCMO. The superexchange ferromagnetic (FM) interaction has a major contribution to the magnetism of YNMO. The strength of antiferromagnetic (AFM) coupling resulting from the antiphase boundaries in Y2NixCo1−xMnO6 increases with the increase in the Co concentration. The AC susceptibility (AC-χ) study further validates the presence of higher density of antiphase boundaries, which is reflected by the dynamics of domain walls in YNCMO. The isothermal magnetic entropy change (ΔSM) as functions of temperature and magnetic field is exploited to assess the stabilization of different magnetic phases. The findings of ΔSM (T,µ0H) lead to the proposed new magnetic phase diagrams for Y2NixCo1−xMnO6 for x = 1 and 0.5, in comparison with the previously established phase diagram of Y2CoMnO6.

Interlayer coupling effect on skyrmion dynamics in synthetic antiferromagnets

Skyrmions in synthetic antiferromagnets (SAFs) could be immune to the skyrmion Hall effect and are, thus, promising in spintronics applications. We introduce breathing modes that can be realized by changing the magnetocrystalline anisotropy periodically in time to generate spin waves around a deformed SAF skyrmion. The net momentum transferred from the magnon spin currents results in a motion of the SAF skyrmion, which is two orders of magnitude faster than that of a ferromagnetic skyrmion. We also reveal that the velocity of the SAF skyrmion can be manipulated by the strength of antiferromagnetic coupling between layers, which is different from ferromagnetic and antiferromagnetic systems. This phenomenon originates from the damping-like character of the antiferromagnetic coupling and offers a dimension to optimize skyrmion dynamics in SAFs.

Magnetic transformation of Mn from anti-ferromagnetism to ferromagnetism in FeCoNiZMnx (Z = Si, Al, Sn, Ge) high entropy alloys

We design high entropy alloys (HEAs) with different induction elements (Si/Al/Sn). In order to keep the crystal structure invariant and to investigate how the increment in saturation magnetization (Ms) is caused only by the change of electron spin state, each set of HEAs contains a different amount of Mn. Synergistic effects among induction elements that induce the magnetic transformation of Mn from anti-ferromagnetism to ferromagnetism are found. Ms of added Mn reduces when a particular induction element (Si0.4/Al0.4/Sn0.4) exists, while a larger increment of Ms appears when two induction elements coexist, Si0.4Al0.4 (25.79 emu/g) and Sn0.4Al0.4 (15.43 emu/g). This is reflected in the microcosmic magnetic structure for the emergence of closed domains due to large demagnetization energy, which is confirmed by the Lorentz transmission electron microscope (LTEM) data. The calculated magnetic moments and the exchange integral constants from density functional theory based on the Exact Muffin-Tin Orbits formalism reveal that the magnetic state and the strength of ferromagnetic and anti-ferromagnetic coupling determine the variation of Ms in different chemical environments. The difference in energy levels of coexisting multiple induction elements also leads to a larger increment of Ms, Si0.4Al0.4Sn0.4 (29.78 emu/g), and Si0.4Al0.4Ge0.4Sn0.4 (31.00 emu/g).

Current-induced linear motion of antiferromagnetically coupled skyrmion-like bubble domains stabilized without Dzyaloshinskii–Moriya interaction

Magnetic domains are used to represent binary digits in magnetic memories. In domain-motion magnetic memories, the domain must move longitudinally along a ferromagnetic nanowire. Antiferromagnetically coupled magnetic skyrmions, which are domains meeting this requirement, can be stabilized via the Dzyaloshinskii–Moriya interaction (DMI) in an antiferromagnetically coupled bi-layered perpendicularly magnetized thin wire. We find that antiferromagnetically coupled skyrmion-like bubble domains (BDs) may be produced in such a wire without DMI. The stability and the dynamics of such domains are studied by micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation. The BDs in the two layers have clockwise and anti-clockwise Bloch type domain walls, so the BD cores have anti-parallel magnetic moments and are antiferromagnetically coupled. Domain size decreases with decreasing wire width and/or increasing antiferromagnetic coupling strength of the wire. The minimum value of the BD radius is about 10 nm. When a spin-polarized current flows through the wire, the BD moves with the current.

Magnetic characteristics of epitaxial NiO films studied by Raman spectroscopy

Raman spectroscopy is utilized to study the magnetic characteristics of heteroepitaxial NiO thin films grown by plasma-assisted molecular beam epitaxy on MgO(100) substrates. For the determination of the Néel temperature, we demonstrate a reliable approach by analyzing the temperature dependence of the Raman peak originating from second-order scattering by magnons. The antiferromagnetic coupling strength is found to be strongly influenced by the growth conditions. The low-temperature magnon frequency and the Néel temperature are demonstrated to depend on the biaxial lattice strain and the degree of structural disorder, which is dominated by point defects.

Physical Properties of Sr-Doped Double Perovskite La2NiMnO6

A series of double perovskite manganese oxide La2−xSrxNiMnO6 (x = 0, 0.05, 0.1) samples were prepared by solid-state synthesis. The XRD patterns show that the three samples have perovskite crystal structures. The temperature dependence of the magnetization curves for La2−xSrxNiMnO6 (x = 0, 0.05, 0.1) near the Curie temperature indicates that Sr-doping weakens the ferromagnetism while enhancing the antiferromagnetism of this system, and the field-dependent magnetization curves further confirm this conclusion. The hysteresis loops, the enlarged views of the hysteresis loops at 2 K, and the Raman spectra of the three samples indicate that Sr-doping increases the antisite disorder degree, the number of antisite defects, and the antiphase boundaries, and the antiferromagnetic coupling between the antiphase boundaries and the surrounding ferromagnetic regions is enhanced. At the same time, the temperature dependence of the inverse magnetic susceptibility changes from an upward deviation from the Curie-Weiss law to a downward trend. This phenomenon can be explained by the opposing changes in the relative strength between the antisite defects and the antiferromagnetic coupling strength. After Sr-doping, the field-cooling curve and field-warming curve of the LSNMO system do not coincide, which is typical of a first-order phase transition. This phenomenon is further confirmed by the rescaling and arrott curves. The temperature dependence of the resistivity curves shows that the La2−xSrxNiMnO6 (x = 0, 0.05, 0.1) samples are all semiconductor materials. Following Sr-doping, the metal-insulator transition temperature of the system decreases, and the difference between the resistivity values measured at 0 T and 2 T increases.

Systematic Raman study of optical phonons in RBa2Cu3O6+δ ( R=Y , Dy, Gd, Sm, Nd): Antiferromagnetic coupling strength versus lattice parameters

We present a systematic study of the interplay between lattice parameters and the energy of the optical phonons as well as the antiferromagnetic coupling strength, $J$, in the high-$T_{\text{c}}$ superconducting cuprate $R$Ba$_2$Cu$_3$O$_{6+\delta}$ (\mbox{R-123}, $R =$ Y, Dy, Gd, Sm, Nd) with hole-doping $p$ ($0.00<p \lesssim 0.04$). The energy of the $B_{1g}$-mode at $\nu_{B1g}\approx~335$ cm$^{-1}$ has been found to relate systematically to the inverse of the lattice parameter $a$. Our results confirm the temperature dependent phonon splitting for Nd-123 at low doping, which has been reported for optimally-doped Nd-123. Surprisingly, $J$ is independent of $a$ for the first four $R$ families, and a general consistency between $T_{\text{c}}^{\text{max}}$ and $J$, as suggested in a previous investigation, could not be confirmed.

Study on adjacent radicals short contact interaction, spin density, overlap integral and strong anti-ferromagnetic coupling strength

Through theoretical calculations and magnetostructural study of nine radical compounds that exhibit strong anti-ferromagnetic coupling strength, it is revealed preliminarily that the overlap integral between adjacent radicals short contact atoms is relevant to associated strong anti-ferromagnetic coupling strength. It is also confirmed that spin density and overlap integral between adjacent radical short contact atoms are the two major factors that dominate intermolecular strong anti-ferromagnetic coupling strength. The study also reveals that the factor of the overlap integral can replace the factors of both interatomic short contact distance and slippage degrees of π-π stacking. The study also reveals for the first time that the overlap integral changes with different short contact atom and different central coordinated atom.

Static and dynamic behavior of interlayer exchange coupled Ni80Fe20/Ru continuous films and nanowires

We present the static and dynamic results of engineered interlayer exchange coupled Ni80Fe20 (Py)/Ru continuous films and arrays of nanowires. We demonstrate a robust control of the coupling mechanism by varying the number of ferromagnetic layers and the thickness of Ru. We found that the antiferromagnetic coupling strength affects both the static and dynamic behaviors of the ferromagnetic layers (a strong antiferromagnetic coupling when the Ru thickness is 1 nm and negligible antiferromagnetic coupling when it is 1.4 nm). Most interestingly, multiple resonant modes were found for five-layer nanowires at remanence due to the interlayer exchange coupling. There is a good qualitative agreement between the experiment and the micromagnetic simulation. The results may suggest a novel way in the design of spintronic devices.

First principles study of magnetism induced by topological frustration of bowtie-shaped graphene nanoflake

Fine-tuning magnetic states by understanding topological frustration inducing magnetic mechanism should allow greater flexibility for the design of graphene-based spintronics. Based on first-principles calculations, it is predicted that bowtie-shaped graphene nanoflake (GNF) is of spin-polarized ground state exhibiting antiferromagnetic (AFM) ordering between two individual triangular GNFs. It is demonstrated that strength of antiferromagnetic coupling of both symmetric and asymmetric bowtie-shaped GNF displays strong zero-energy-state-orientated behavior due to non-trivial nature of topological frustration, with implications for designing graphene nanostructures with predefined magnetic states. It also proposes a specific example of structures that can serve as nanoscale molecular logic gates composed by asymmetric bowtie-shaped GNFs units, which augment the special antiferromagnetic function through structural configuration of multi-bowtie-shaped GNFs.

Thin Co/Ni-based bottom pinned spin-transfer torque magnetic random access memory stacks with high annealing tolerance

Spin-transfer torque magnetic random access memory (STT-MRAM) is considered as a replacement for next generation embedded and stand-alone memory applications. One of the main challenges in the STT-MRAM stack development is the compatibility of the stack with CMOS process flows in which thermal budgets up to 400 °C are applied. In this letter, we report on a perpendicularly magnetized MgO-based tunnel junction (p-MTJ) on a thin Co/Ni perpendicular synthetic antiferromagnetic layer with high annealing tolerance. Tunnel magneto resistance (TMR) loss after annealing occurs when the reference layer loses its perpendicular magnetic anisotropy due to reduction of the CoFeB/MgO interfacial anisotropy. A stable Co/Ni based p-MTJ stack with TMR values of 130% at resistance-area products of 9 Ω μm2 after 400 °C anneal is achieved via moment control of the Co/Ta/CoFeB reference layer. Thinning of the CoFeB polarizing layer down to 0.8 nm is the key enabler to achieve 400 °C compatibility with limited TMR loss. Thinning the Co below 0.6 nm leads to a loss of the antiferromagnetic interlayer exchange coupling strength through Ru. Insight into the thickness and moment engineering of the reference layer is displayed to obtain the best magnetic properties and high thermal stability for thin Co/Ni SAF-based STT-MRAM stacks.

Synthesis, crystal structures, and magnetism of the binuclear radical complex [N-methylpyridinium]2[Ni(tdas)2

The novel binuclear radical complex [N-methylpyridinium]2[Ni(tdas)2]2 (tdas = 1,2,5-thiadiazole-3,4-dithiolate) has been prepared and its crystal structures determined by X-ray crystallography at three different temperatures in order to investigate the changes in coordination structure and magnetic properties with temperature. In the binuclear radical complex the two nickel ions assume a distorted pyramidal geometry and are bridged by two S atoms of different tdas anionic ligands. At room temperature, the ESR spectrum of the binuclear radical complex in polycrystalline powder and the theoretical calculations reveal a very strong antiferromagnetic interaction, leading to diamagnetic crystals. In contrast, a peak with g = 2.05 appears in the ESR spectrum of the title complex in acetonitrile solution, which indicates that a fraction of the binuclear radical complexes dissociate. The theoretical calculations also reveal that as temperature increases the antiferromagnetic coupling strength gradually decreases, yet at 150 °C the antiferromagnetic coupling strength remains as strong as 2J = −734.26 cm−1. The strong antiferromagnetic coupling strengths should be attributed to the large spin densities on the nickel atoms and the relevant bridging sulfur atoms. This study is the first to report the correlations between the structure, magnetism, and temperature of a binuclear radical nickel complex with tdas as ligand and this study is also the first report the magnetic coupling strength of radical binuclear nickel complex with tdas as bridging ligand and with N-methylpyridinium as counter cation.

Measuring acoustic mode resonance alone as a sensitive technique to extract antiferromagnetic coupling strength

We have studied static and dynamic magnetic properties of a general asymmetric trilayer system using numerical simulations. For ferromagnetic, ${90}^{\ensuremath{\circ}}$, and antiferromagnetic coupling, the magnetizations of the two magnetic layers exhibit one, two, and three phases with increasing external field, respectively. The total magnetization and ferromagnetic resonance accordingly follow these phases of the magnetization vectors. The resonance condition is related to the interlayer coupling strength in such a way that a larger coupling constant yields a higher value of ${f}_{\mathrm{res}}/H$, where ${f}_{\mathrm{res}}$ is the resonance frequency at the external magnetic field $H$. Based on the simulation results, it is proposed that measurements of the acoustic mode resonance alone at unsaturated conditions provide a sensitive and accurate technique to extract the antiferromagnetic coupling strength. The technique is demonstrated experimentally with the broadband ferromagnetic resonance measurements of two trilayer films with weak and strong coupling strengths. The technique offers an efficient and sensitive method for antiferromagnetic coupling strength extraction, yielding coupling constant values with a precision of better than 0.03 erg/${\mathrm{cm}}^{2}$. Also, separation of the bilinear and biquadratic coupling contributions is possible with the technique.

Interlayer exchange coupling in [Pt/Co]n/MgO/[Co/Pt]2 perpendicular magnetic tunnel junctions

In this paper, we present further study on the interlayer exchange coupling of [Pt/Co]n/MgO/[Co/Pt]2 perpendicular magnetic tunnel junctions. Antiferromagnetic interlayer couplings in [Pt/Co]n/MgO/[Co/Pt]2 are observed. The strength of antiferromagnetic coupling oscillates irregularly with the repetition number n, that may be related to the Ruderman-Kittel-Kasuya-Yosida (RKKY)-type ferromagnetic interlayer coupling existing in the [Pt/Co]n hard layer. The interlayer coupling of [Pt/Co]9/MgO(22 Å)/[Co/Pt]2 magnetic tunnel junction reaches a maximum at 200 K, and decreases gradually with increasing temperature. This thermal behavior of interlayer coupling may be related to the enhanced perpendicular magnetic anisotropy of hard layer with decreasing temperature.

Charge transfer in strongly correlated systems: An exact diagonalization approach to model Hamiltonians

We study charge transfer in bridged di- and triruthenium complexes from a theoretical and computational point of view. Ab initio computations are interpreted from the perspective of a simple empirical Hamiltonian, a chemically specific Mott-Hubbard model of the complexes' π electron systems. This Hamiltonian is coupled to classical harmonic oscillators mimicking a polarizable dielectric environment. The model can be solved without further approximations in a valence bond picture using the method of exact diagonalization and permits the computation of charge transfer reaction rates in the framework of Marcus' theory. In comparison to the exact solution, the Hartree-Fock mean field theory overestimates both the activation barrier and the magnitude of charge-transfer excitations significantly. For triruthenium complexes, we are able to directly access the interruthenium antiferromagnetic coupling strengths.