Coupling Energy Density Research Articles

The positive exchange bias property with hopping switching behavior in van der Waals magnet FeGeTe

The magnetic exchange bias (EB) effect is one of the representative interlayer magnetic coupling phenomena and is widely utilized in numerous technological applications. However, its mechanism is still elusive even in a simple magnetic bilayered system because of the complex interface magnetic orders. Van der Waals (vdW) layered magnetic materials may provide an essential platform for deeply understanding the detailed mechanism of the EB owing to its ideal interface structure. Here we first observed the positive exchange-biased anomalous Hall effect with a hopping switching behavior in the FeGeTe vdW nano-flakes. After systemically studying the cooling field dependence properties of the EB effect, we propose that the coexistence of stable and frustrated surface magnetization of the antiferromagnetic (AFM) phase will modify the total interface coupling energy density between the ferromagnetic and AFM phases. This model could provide a consistent description for such unusual EB effect based on microspin simulation.

Interfacial Antiferromagnetic Coupling and Dual-Exchange Bias in Tetragonal SrRuO3-PrMnO3 Superlattices.

The functional properties of oxide heterostructures depend on the interfaces accommodating ions, their spins, and structural mismatches. Here, by stabilizing tetragonal symmetry, we achieve the in-plane antiferromagnetic (AFM) ordering and dual-exchange bias in the superlattices consisting of two ferromagnets SrRuO3 (SRO) and PrMnO3 (PMO). The tetragonal symmetry of this superlattice system achieved after the octahedral rotations yield an elongation of the c-axis parameter with Ru-O-Mn bond angle close to 180°, induces an interfacial antiferromagnetic ordering, which is suppressed as the ferromagnetic (FM) ordering in the PMO layer increases. The 0.1 T in-plane cooling field (Hcool) leads to the shift (ca. -0.04 T) of minor hysteresis loop along the negative field axis due to the presence of -0.87 erg/cm2 AFM interfacial exchange coupling energy density (ERu,Mn) at 20 K. The exchange bias field (HEB) switches from negative to positive value with the increase in Hcool. For 5 T Hcool, the HEB is positive, but the ERu,Mn is -1.25 erg/cm2 for n ≤ 8 (n = number of unit cells of PMO) and 1.52 erg/cm2 for n ≥ 8. The HEB and its switching from negative to positive with the increase in Hcool are explained by the interplay of strong antiferromagnetic coupling energy and Zeeman energy at the interfaces. The results demonstrate that the SRO-PMO superlattice could be a model system for the investigation of the interfacial exchange coupling in functional oxides.

Antiferromagnetically coupled perpendicular magnetic anisotropic CoFeB/MgO films across a Mo spacer with high thermal stability

The magnetic and magneto-transport properties of the multilayered MgO/CoFeB/Mo/CoFeB/MgO films have been investigated. Perpendicular magnetic anisotropy and alternating ferromagnetic and antiferromagnetic interlayer coupling along the perpendicular direction as well as giant magnetoresistance have been observed in this Mo separated CoFeB/MgO structure. More importantly, the antiferromagnetic interlayer coupling of perpendicular layers is thermally stable against annealing temperature up to 400 °C, showing that the present perpendicular synthetic antiferromagnetic structure is promising for the magnetic engineering of nanodevices based on the perpendicular CoFeB/MgO system. The largest antiferromagnetic exchange coupling is obtained for Mo thickness of 0.8 nm, the corresponding exchange coupling energy density is 0.036 erg/cm2.

Influence of ferromagnetic layer on the exchange coupling of antiferromagnetic NiO-based films

Strong effects of ferromagnetic layer (FMCo, and Ni80Fe20) on the magnitude and blocking temperature of exchange coupling are observed in antiferromagnetic NiO-based films NiO (5nm)/FM1 (tnm)/FM2 (6-tnm). The existence of interfacial spins configuration in glass-like state and FM anisotropy are proposed to interpret a minimum shown in thermal magnetization curves for films with strong exchange coupling effect. The microstructural change of FM layer and the long-range interaction of exchange bias are taken into account to explain a strong dependence of exchange coupling energy density on the thickness tF of FM layer when tF<5nm.

Spin–Spin Artificial DNA Intercalated with Silver Cations: Theoretical Prediction

The indirect nuclear spin-spin coupling constants of Ag(+) cation intercalated between imidazole rings in DNA chains are calculated by means of DFT with relativistic effects taken into account by the use of the zeroth-order regular approximation Hamiltonian (DFT-ZORA). The calculations model how the (1)J((15)N,(109)Ag) coupling constant is affected by different types of geometry deformations and by the presence of water, which is simulated by means of the polarizable continuum model and explicitly present water molecules. Calculations for systems containing two and three imidazole pairs are also carried out to model the influence of stacking interactions. The computed (1)J((15)N,(109)Ag) spin-spin coupling constant is in the range of 85-105 Hz (depending on the computational model) and is in good agreement with the experimental value (ca. 92 Hz). This coupling constant is very little affected by the presence of solvent, stacking interactions, and geometry deformations. Such behavior is explained by visualization of the coupling path by means of coupling energy density (CED). Bigger models allow the coupling constant between two adjacent silver ions to be computed, and give a value of approximately 1 Hz, which is probably too small to be of practical interest. The (2)J((15)N,(15)N) value is calculated to be about 2.5 Hz, and is therefore of measurable magnitude.

Theoretical Prediction of the Spin–Spin Coupling Constants between an Axis and Macrocycle of a Rotaxane

The indirect nuclear spin-spin coupling constants between nuclei belonging to the axis and to the macrocycle of three structurally related rotaxanes have been calculated by means of density functional theory. It has been shown that the through-space axis-macrocycle proton-proton coupling constants can be as large as 0.4-0.5 Hz and therefore of measurable values. The largest through-space axis-macrocycle carbon-proton and nitrogen-proton coupling constants are 0.2-0.3 Hz. Visualization of coupling pathways by means of the coupling energy density method indicates that the larger proton-proton couplings are indeed transmitted through the space between the coupled nuclei. Thus, it seems that measurement of such couplings should be possible and that indirect spin-spin couplings can be actually transmitted through-space, with no covalent or hydrogen bonds between the coupled nuclei.

Measurement of the coupling strength distribution in ferromagnetic (F)/antiferromagnetic (AF) exchange bias systems

We propose a method for determination of the distribution function P( j) of the coupling energy density j in polycrystalline textured ferromagnetic (F)/antiferromagnetic (AF) film systems. P( j) governs the entire film coupling J and the exchange bias field H e and was not measurable until now. The method is verified by torquemetry in a high magnetic field and by reversing its rotation sense. The transition to a new magnetic steady state after rotation reversal is analyzed within a Stoner–Wohlfarth model including thermal relaxation. This transition is completed earlier for strongly coupled grains than for grains with smaller j, which is reflected in the torque curves. We determined P( j) for a sputtered NiFe(16 nm)/IrMn(0.8 nm) film at T=50 K in the hysteretic range of coupling energies and found that P strongly decreases for increasing j.

Understanding the Conformational Dependence of Spin-Spin Coupling Constants: Through-Bond and Through-SpaceJ(31P,31P) Coupling in Tetraphosphane-1,4-diides [M(L)x]2[P4R4

The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF).

Antiferromagnetic energy loss and exchange coupling of IrMn/CoFe films: experiments and simulations

The energy loss density E 2 π and the coupling energy density J of antiferromagnetic (1 1 1) textured IrMn films of different thickness t sputtered onto ferromagnetic CoFe films have been measured by torque magnetometry for high in-plane rotating magnetic fields at a low temperature ( T=10 K). The results are compared with model calculations which assume independent grains of equal size with a statistical distribution of individual grain coupling energies. Analytical expressions for E 2 π ( t) and J( t) are used to explain the measurements for small thicknesses t<3.5 nm. The simulations fit the experiments well if a thickness dependent triaxial anisotropy energy constant K( t) is introduced. At t=3 nm, K≈1.3×10 5 J/m 3.

Magnetic order and energetics of dipolar coupling in magnetic superconductors

The exact (not just nearest-neighbor) dipolar coupling energy densities for the observed structures of the rare earth Chevrel compounds are calculated by the method of Luttinger and Tisza. The dipolar coupling energy density for the most probable spin configuration is comparable to the observed magnetic transition temperature, T(M), but the most probable ground state is not found experimentally. The discrepancy between the observed magnetic ground state and that predicted from dipole coupling may arise from conduction electron effects or possibly from some strong crystal field effect and should be included in any electronic theory of the superconductive state below T(M).