Intralayer Exchange Coupling Research Articles

Monte Carlo study of internal energy and specific heat of a nano-graphene bilayer in a longitudinal magnetic field

The thermodynamic properties of a nano-graphene bilayer, consisting of the upper layer A of spin-3/2 with antiferromagnetic intralayer exchange coupling and the bottom layer B of spin-5/2 with ferromagnetic intralayer exchange coupling, have been studied by the use of Monte Carlo simulation. We find a number of characteristic phenomena. The effects of the exchange coupling, the single-ion anisotropy and the longitudinal magnetic field on the internal energy, the specific heat and the blocking temperature of the mixed-spin bilayer system have been investigated in detail. The internal energy and the specific heat profiles are clarified. In particular, we have found that the specific heat curve may show two peaks phenomenon for appropriate values of the system parameters.

Fingerprints of field-induced Berezinskii–Kosterlitz–Thouless transition in quasi-two-dimensional S=1/2 Heisenberg magnets Cu(en)(H2O)2SO4 and Cu(tn)Cl2

Organo-metallic compounds Cu(en)(H2O)2SO4 (en=C2H8N2) and Cu(tn)Cl2 (tn=C3H10N2) representing S=1/2 quasi-two-dimensional Heisenberg antiferromagnets with an effective intra-layer exchange coupling J/kB≈3K, have been examined by specific heat measurements at temperatures down to nominally 50mK and magnetic fields up to 14T. A comparative analysis of magnetic specific heat in zero magnetic field revealed nearly identical contribution of short-range magnetic correlations and significant differences were observed at lowest temperatures. A phase transition to long-range order was observed in Cu(en)(H2O)2SO4 at TC=0.9K while hidden in Cu(tn)Cl2. A response of both compounds to the application of magnetic field has rather universal features characteristic for a field-induced Berezinskii–Kosterlitz–Thouless transition theoretically predicted for ideal two-dimensional magnets.

Monte Carlo Investigation of the Magnetic and Thermal Properties of Four Sublattice Ferromagnetic– Antiferromagnetic Double-Layer Superlattices

Using Monte Carlo simulation, we have studied the magnetic and thermal properties of a four sublattice ferromagnetic (FM) and anti-FM (AFM) double-layer superlattices system in a longitudinal magnetic field. The effects of the intralayer AFM exchange coupling $J_{{2}}$ , the single-ion anisotropy ${D}$ and the longitudinal magnetic field ${h}$ on the magnetization ${M}$ , the susceptibility ${x}$ , the internal energy ${U}$ , and the specific heat ${c}$ of the system are discussed in detail. Particular attention has been paid to the variations of the hysteresis loops of the system with the AFM exchange coupling $J_{ {2}}$ and the anisotropy ${D}$ . Some interesting results, such as the triple hysteresis and the exchange bias behavior, have been found under certain conditions.

Study of perpendicular anisotropy L1-FePt pseudo spin valves using a micromagnetic trilayer model

A trilayer micromagnetic model based on the Landau-Lifshitz-Bloch equation of motion is utilized to study the properties of L10-FePt/TiN/L10-FePt pseudo spin valves (PSVs) in direct comparison with experiment. Theoretical studies give an insight on the crystallographic texture, magnetic properties, reversal behavior, interlayer coupling effects, and magneto-transport properties of the PSVs, in particular, with varying thickness of the top L10-FePt and TiN spacer. We show that morphological changes in the FePt layers, induced by varying the FePt layer thickness, lead to different hysteresis behaviors of the samples, caused by changes in the interlayer and intralayer exchange couplings. Such effects are important for the optimization of the PSVs due to the relationship between the magnetic properties, domain structures, and the magnetoresistance of the device.

Quantum critical universality and singular corner entanglement entropy of bilayer Heisenberg-Ising model

We consider a bilayer quantum spin model with anisotropic intra-layer exchange couplings. By varying the anisotropy, the quantum critical phenomena changes from XY to Heisenberg to Ising universality class, with two, three and one modes respectively becoming gapless simultaneously. We use series expansion methods to calculate the second and third Renyi entanglement entropies when the system is bipartitioned into two parts. Leading area-law terms and subleading entropies associated with corners are separately calculated. We find clear evidence that the logarithmic singularity associated with the corners is universal in each class. Its coefficient along the Ising critical line is in excellent agreement with those obtained previously for the transverse-field Ising model. Our results provide strong evidence for the idea that the universal terms in the entanglement entropy provide an approximate measure of the low energy degrees of freedom in the system.

Standing Spin Waves in Ultrathin Magnetic Films: A Method to Test for Layer-Dependent Exchange Coupling

We introduce a method to test theoretical models for the layer dependence of exchange coupling constants in ultrathin magnetic films. The method is based on the observation of high-energy and high-momentum standing spin wave modes using high-resolution electron energy loss spectroscopy. Experimental data are presented for 5-8 layers of fcc cobalt deposited on Cu(100). The power of the method is illustrated by comparison to two theoretical studies predicting rather different results concerning the ratio of the interlayer and intralayer exchange coupling constants near the surface. Only the theory with a large interlayer coupling shows sufficient energy spreading in the layer dependence of the dispersion curves to match the experimental data. We furthermore discuss the reason for the surprising success of the simple nearest-neighbor Heisenberg model with a single exchange constant matched to experiment.

Extremely Long Cu···O Contact as a Possible Pathway for Magnetic Interactions in Na2Cu(CO3)2

Chemical binding in a mixed copper sodium carbonate Na2Cu(CO3)2, a layered material showing ferromagnetic intralayer exchange and weak antiferromagnetic interlayer coupling, was examined within the topological analysis of experimental (from high-resolution X-ray diffraction) and theoretical (from periodic quantum chemical calculations) electron density functions in its crystal. Together with modeling of a superexchange pathway within the LSDA and DFT+U approach, the results obtained reveal a very weak Cu···O interaction (0.5 kcal/mol worth) between the copper-carbonate layers that is nevertheless stabilizing (bonding) and may serve as a possible pathway for antiferromagnetic interactions.

Interfacial coupling in multiferroic/ferromagnet heterostructures

We report local probe investigations of the magnetic interaction between BiFeO3 films and a ferromagnetic Co0.9Fe0.1 layer. Within the constraints of intralayer exchange coupling in the Co0.9Fe0.1, the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO3 ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO3 films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO3 film based on our X-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO3.

Short-time dynamics of Fe2/V13 magnetic superlattice models

Critical relaxation from a low-temperature fully ordered state of Fe2/V13 iron-vanadium magnetic superlattice models has been studied using the method of short-time dynamics. Systems with three variants of the ratio R of inter- to intralayer exchange coupling have been considered. Particles with N = 262144 spins have been simulated with periodic boundary conditions. Calculations have been performed using the standard Metropolis algorithm of the Monte Carlo method. The static critical exponents of magnetization and correlation radius, as well as the dynamic critical exponent, have been calculated for three R values. It is established that a small decrease in the exchange ratio (from R = 1.0 to 0.8) does not significantly influence the character of the short-time dynamics in the models studied. A further significant decrease in this ratio (to R = 0.01), for which a transition from three-dimensional to quasi-twodimensional magnetism is possible, leads to significant changes in the dynamic behavior of iron-vanadium magnetic superlattice models.

Magnon energy gap and quantum fluctuation in a ferro-anti-ferromagnetic multisublattice system

Within the framework of the linear spin wave approximation and the retarded Green's function method, the magnetic properties of a ferro-anti-ferromagnetic Heisenberg system are studied. Under certain cases, five magnon energy gaps exist in the energy band. The effects of upper surface anisotropy, spin numbers, exchange coupling, and antiferromagnetic intralayer exchange coupling on magnon energy gaps, as well as the effects of the interlayer exchange couplings on magnetization and quantum fluctuation, are discussed. Results show that antiferromagnetic (ferrimagnetic) interlayer exchange coupling plays a significant role in the magnetic properties of the system.

Experimental Study of the Magnetocaloric Effect in the Two-Dimensional Quantum System Cu(en)(H2O)2SO4

Magnetocaloric studies have been performed on a two-dimensional (2D) quantum system Cu(en)(H2O)2SO4 (en=ethylendiamine=C2N2H8) in the temperature range from 0.6 K to 4 K in magnetic fields up to 2 T by adiabatic magnetization and demagnetization measurements. The title compound has been previously identified as a potential realization of the quasi-two dimensional spatially anisotropic triangular Heisenberg antiferromagnet with spin 1/2 and effective intralayer exchange coupling, J/kB= −1.4 K. A phase transition to the magnetically ordered state has been observed in zero magnetic field at TN = 0.91 K. The normal magnetocaloric effect (MCE) was observed in the temperature range from 0.6 K to 0.98 K. In contrast, at temperatures near the phase transition the character of MCE changes to inverse and above the temperature 2 K the normal MCE was observed again. This change can be ascribed to the onset of short-range magnetic correlations previously observed in specific heat and susceptibility studies.

Monte Carlo study of the surface effect on the compensation and critical behaviour in a molecular-based magnetic film AFeIIFeIII(C2O4)3

Monte Carlo simulation is used to study the magnetic properties and critical behaviour of a molecular-based magnetic film AFeIIFeIII(C2O4)3(A = N(n − CnH2n+1)4, n = 3–5). The effects of surface exchange coupling, surface single-ion anisotropy and layer thickness on the compensation temperature and critical temperature of a ferrimagnetic mixed spin-2 and spin-5/2 Ising model on a honeycomb lattice are discussed in detail. The results show that the surface intralayer exchange coupling R1 = J1S/|J1| and surface interlayer exchange coupling R2 = J2S/|J1| play important roles in the compensation and critical behaviour of the system. In particular, we have found two special points R1c at which the compensation and critical temperatures are independent of the layer thickness L and the surface single-ion anisotropy D1S/|J1|. A number of characteristic phenomena, such as the possibility of two and three compensation points, are found. The effects of the layer thickness L on the susceptibility, internal energy and specific heat with temperature are numerically investigated.

Mean Field Analysis of the Exchange Coupling (J) for Two- and Three-Dimensional Structured Tetracyanoethenide (TCNE•–)-Based Magnets

Mean field expressions based on the simple Heisenberg model were derived to correlate the inter- and intralayer exchange couplings to the critical temperatures, Tc, for several tetracyanoethylene (TCNE)-based magnets with extended two- and three-dimensional (2-D and 3-D) structure types. These expressions were used to estimate the exchange coupling, J, for 2-D ferrimagnetic [MII(TCNE)(NCMe)2]+ (M = Mn, Fe), 3-D antiferromagnetic MnII(TCNE)[C4(CN)8]1/2, and 3-D ferrimagnetic MnII(TCNE)3/2(I3)1/2. The type and magnitude of the exchange coupling are in accord with the previously reported magnetic data.

Monte Carlo Simulation of Layer Thickness Influence on a Mixed Spin-2 and Spin-5/2 Ising Ferrimagnetic System

In this paper, the influence of the layer thickness and the surface intralayer exchange coupling on the magnetic and thermodynamic properties of the mixed spin-2 and spin-5/2 ferrimagnetic systems are studied in detail by the Monte Carlo simulation. A number of interesting phenomena have been found. The results show that there exist the significant dependences of the compensation temperature, phase transition temperature, and the hysteresis loop of the system on both layer thickness and surface intralayer exchange coupling.

Specific Heat of a Two-Layer Magnetic Superlattice

The specific heats of both a two-layer ferromagnetic superlattice and a two-layer ferrimagnetic one are studied. It is found that the spin quantum numbers, the interlayer and intralayer exchange couplings, the anisotropy, the applied magnetic field, and the temperature all affect the specific heat of these superlattices. For both the ferromagnetic and ferrimagnetic superlattices, the specific heat decreases with increasing the spin quantum number, the absolute value of interlayer exchange coupling, intralayer exchange coupling, and anisotropy, while it increases with increasing temperature at low temperatures. When an applied magnetic field is enhanced, the specific heat decreases in the two-layer ferromagnetic superlattice, while it is almost unchanged in the two-layer ferrimagnetic superlattice at low field range at low temperatures.

Systematic study on the spacer-dependent magnetic properties of Mn δ-doped GaAs/(Ga, Mn)As ferromagnetic heterostructures: from first principles

First-principles calculations based on density-functional theory have been performed on the spacer-dependent magnetic properties of δ-doped GaAs/(Ga, Mn)As ferromagnetic heterostructures (DFH). It is found that all the structures show ferromagnetic (FM) alignment as the favoured configuration. Their electronic structures are half-metallic independent of the spacer thickness d. The interlayer exchange coupling (IEC) between two magnetic layers decreases sharply at first with increasing d, then reaches a stationary value at d = 4a0 (a0 is the lattice constant of GaAs), where the FM and antiferromagnetic states become energetically degenerate. The charge density and the strong spin-polarized holes are concentrated mostly in the vicinity of MnAs magnetic layers for all the structures. After the injection of free holes into the system, we confirm the free-hole mediated mechanism for the intralayer exchange coupling and IEC. Furthermore, the injection of holes enhances the intralayer exchange coupling much more than the IEC. From these results, it is definitely shown that the magnetic coupling of DFH can get across a nonmagnetic spacer. It is closely related to the spacer thickness d, and the redistribution of injection holes plays a key role when d is small.

Magnetic properties of [formula omitted] two-dimensional quantum antiferromagnet Cu(D 2O) 2(C 2H 6D 2N 2)SO 4

Specific heat and susceptibility measurements of Cu(D 2O) 2(C 2H 6D 2N 2)SO 4 have been performed to investigate the influence of the partial deuteration on the magnetic properties of Cu(H 2O) 2(C 2H 8N 2)SO 4 — a representative of an S = 1 / 2 spatially anisotropic triangular antiferromagnet with intralayer exchange coupling J e f f / k B ≈ 1.5 K . The analysis of experimental data at helium temperatures revealed a slight, less than 10% reduction of magnetic coupling as well as ordering temperature. The origin of the observed behavior is discussed.

Spin‐wave spectra and magnetization of ferro–ferrimagnetic double layers

AbstractThe spin‐wave spectra and magnetization of the ferro–ferrimagnetic double layers are studied by using a linear spin‐wave approximation and retarded Green's‐function method. We obtain the four branches of the spin‐wave spectra. Two energy gaps are found to exist in the energy band. The effects of the interlayer exchange coupling, the intralayer exchange coupling and the spin quantum numbers on the spin‐wave spectra and the energy gaps are discussed. The minimum (maximum) value point on the spin‐wave spectra and energy gaps correspond to a system that has a high symmetrical magnetic structure and the balance of quantum competitions among the exchange couplings and the spin quantum numbers of the system. There is a crossover between sublattice magnetizations in ferromagnetic layer that is affected by quantum fluctuations, thermal fluctuations and frustration of spins. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magneto-Structural Correlations in Cu(tn)Cl2 (tn = 1,3-Diaminopropane): Two-Dimensional Spatially Anisotropic Triangular Magnet Formed by Hydrogen Bonds

A novel polymeric one-dimensional compound Cu(tn)Cl2 (tn = 1,3-diaminopropane) was prepared and structurally characterized, and its spectral, magnetic, thermodynamic, and thermal properties were studied. The unique structure shows ladderlike chains composed of Cu(II) atoms and chloro bridging ligands [Cu(-mu(3)-Cl-)Cu2] running along the crystallographic c axis. The coordination geometry about copper (4 + 2) approximates that of a strongly elongated octahedron. The equatorial plane of the coordination octahedron is formed by a chelate N-bonded tn ligand and two chloro ligands. One of the chloro ligands is terminal, and the other one, mu3-Cl-, forms two additional longer bonds to the neighboring copper atoms and thus occupies the axial octahedral positions. The electronic ground state of the Cu(II) ion is of d(z)2 symmetry and suggests the activation of intraladder and interladder Cl...H-N hydrogen bonds as exchange paths that form a two-dimensional pattern of a triangular symmetry. The interaction due to the hydrogen bonds seems to play an important role in molecular packing and magnetic coupling. The studies of magneto-structural correlations including electron paramagnetic resonance measurements and thermodynamic and magnetic properties revealed a two-dimensional character of magnetic correlations with the effective intralayer exchange coupling J/k(B) approximately -3 K. No phase transition to the ordered state has been observed down to 60 mK. Cu(tn)Cl2 with the interlayer coupling J' approximately 10(-3)J and moderate intralayer interaction represents an excellent example of a two-dimensional magnetic system.

The magnetizations and the phase transitions in the spin-1/2 Ising superlattice with impurities

The critical temperature of the spin-1/2 Ising superlattice with impurities is examined using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions. We discuss the effects of the interlayer and the intralayer exchange couplings, the concentration of magnetic atoms.