arXiv subjects

P. J. Jensen

Publications and source records attributed to P. J. Jensen.

At least 19 recordsLinked to original sources

arXiv2008

Elementary transitions and magnetic correlations in two-dimensional disordered nanoparticle ensembles

The magnetic relaxation processes in disordered two-dimensional ensembles of dipole-coupled magnetic nanoparticles are theoretically investigated by performing numerical simulations. The energy landscape of the system is explored by determining saddle points, adjacent local minima, energy barriers, and the associated minimum energy paths (MEPs) as functions of the structural disorder and particle density. The changes in the magnetic order of the nanostructure along the MEPs connecting adjacent minima are analyzed from a local perspective. In particular, we determine the extension of the correlated region where the directions of the particle magnetic moments vary significantly. It is shown that with increasing degree of disorder the magnetic correlation range decreases, i.e., the elementary relaxation processes become more localized. The distribution of the energy barriers, and their relation to the changes in the magnetic configurations are quantified. Finally, some implications for the long-time magnetic relaxation dynamics of nanostructures are discussed.

cond-mat.mtrl-sci↗

arXiv2006

Two-dimensional Heisenberg antiferromagnet in a transverse field

We investigate the magnetic reorientation in a two-dimensional anisotropic antiferromagnet due to a transverse magnetic field. Using a many-body Green's function approach, we show that the magnetization component perpendicular to the applied field (and along the easy-axis of the antiferromagnet) initially increases with increasing field strength. We show that this unexpected result arises from the suppression of quantum and thermal fluctuations in the antiferromagnet. Above the Néel temperature, this effect leads to a reappearance of a magnetic moment along the easy-axis.

cond-mat.str-el↗

arXiv2005

Coupled ferro-antiferromagnetic Heisenberg bilayers investigated by many-body Green's function theory

A theory of coupled ferro- and antiferromagnetic Heisenberg layers is developed within the framework of many-body Green's function theory (GFT) that allows non-collinear magnetic arrangements by introducing sublattice structures. As an example, the coupled ferro- antiferromagnetic (FM-AFM) bilayer is investigated. We compare the results with those of bilayers with purely ferromagnetic or antiferromagnetic couplings. In each case we also show the corresponding results of mean field theory (MFT), in which magnon excitations are completely neglected. There are significant differences between GFT and MFT. A remarkable finding is that for the coupled FM-AFM bilayer the critical temperature decreases with increasing interlayer coupling strength for a simple cubic lattice, whereas the opposite is true for an fcc lattice as well as for MFT for both lattice types.

cond-mat.stat-mech↗

arXiv2005

Magnetic reordering in the vicinity of a ferromagnetic/antiferromagnetic interface

The magnetic arrangement in the vicinity of the interface between a ferromagnet and an antiferromagnet is investigated, in particular its dependence on the exchange couplings and the temperature. Applying a Heisenberg model, both sc(001) and fcc(001) lattices are considered and solved by a mean field approximation. Depending on the parameter values a variety of different magnetic configurations emerge. Usually the subsystem with the larger ordering temperature induces a magnetic order into the other one (magnetic proximity effect). With increasing temperature a reorientation of the magnetic sublattices is obtained. For coupled sc(001) systems both FM and AFM films are disturbed from their collinear magnetic order, hence exhibit a similar behavior. This symmetry is absent for fcc(001) films which, under certain circumstances, may exhibit two different critical temperatures. Analytical results are derived for simple bilayer systems.

cond-mat.str-el↗

arXiv2005

Comment on "Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100)"

In a recent Letter the phase diagram of Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100) trilayers was determined experimentally as well as theoretically. Whereas we do not object the presented results, we remark that published work was not commented on. Especially, much more information can be extracted out of the measurements by a quantitative comparison with an appropriate model, allowing for a determination of important parameters such as exchange coupling constants.

cond-mat.mtrl-sci↗

arXiv2004

Many-body Green's function theory of ferromagnetic Heisenberg systems with single-ion anisotropies in more than one direction

The behaviour of ferromagnetic systems with single-ion anisotropies in more than one direction is investigated with many-body Green's function theory generalizing earlier work with uniaxial anisotropies only. It turns out to be of advantage to construct Green's functions in terms of the spin operators S^x, S^y and S^z, instead of the commonly used S^+,S^- and S^z operators. The exchange energy terms are decoupled by RPA and the single-ion anisotropy terms by a generalization of the Anderson-Callen decoupling. We stress that in the derivation of the formalism none of the three spatial axes is special, so that one is always able to select a reference direction along which a magnetization component is not zero. Analytical expressions are obtained for all three components of the magnetization and the expectation values <(S^x)^2>, <(S^y)^2> and <(S^z)^2> for any spin quantum number S. The formalism considers both in-plane and out-of-plane anisotropies. Numerical calculations illustrate the behaviour of the magnetization for 3-dimensional and 2-dimensional systems for various parameters. In the 2-dimensional case, the magnetic dipole-dipole coupling is included, and a comparison is made between in-plane and out-of-plane anisotropies.

cond-mat.stat-mech↗

arXiv2003

Magnetic relaxation and dipole-coupling-induced magnetization in nanostructured thin films during growth: A cluster Monte Carlo study

For growing inhomogeneous thin films with an island nanostructure similar as observed in experiment, we determine the nonequilibrium and equilibrium remanent magnetization. The single-island magnetic anisotropy, the dipole coupling, and the exchange interaction between magnetic islands are taken into account within a micromagnetic model. A cluster Monte Carlo method is developed which includes coherent magnetization changes of connected islands. This causes a fast relaxation towards equilibrium for irregularly connected systems. We analyse the transition from dipole coupled islands at low coverages to a strongly connected ferromagnetic film at high coverages during film growth. For coverages below the percolation threshold, the dipole interaction induces a collective magnetic order with ordering temperatures of 1 - 10 K for the assumed model parameters. Anisotropy causes blocking temperatures of 10 - 100 K and thus pronounced nonequilibrium effects. The dipole coupling leads to a somewhat slower magnetic relaxation.

cond-mat.stat-mech↗

arXiv2003

Low-energy properties of two-dimensional magnetic nanostructures: interparticle interactions and disorder effects

The low-energy properties of two-dimensional ensembles of dipole-coupled magnetic nanoparticles are studied as function of structural disorder and particle coverage. Already small deviations from a square particle arrangement lift the degeneracies of the microvortex magnetic configuration, and result in a strongly noncollinear magnetic order of the particle ensemble. The energy distribution of metastable states is determined. For a low degree of disorder a strongly asymmetric shape with a pronounced peak of the ground state energy results. In contrast, for a strong disorder a Gaussian-like distribution is obtained. The average dipole coupling energy $\bar E_\mathrm{dip}$ decreases with increasing structural disorder. The role of vacancies has been studied for a square particle array by determining the angular distribution of the preferred microvortex angle as function of the vacancy concentration. Indications for a preferred angular direction along the axial as well as along the diagonal directions of the square array are revealed. A corresponding investigation for disturbed square arrays results in a different angular distribution. The effect of dipole-quadrupole corrections resulting from the finite size of the particles is quantified.

cond-mat.dis-nn↗

arXiv2003

Scaling behavior of the dipole coupling energy in two-dimensional disordered magnetic nanostructures

Numerical calculations of the average dipole-coupling energy $\bar E_\mathrm{dip}$ in two-dimensional disordered magnetic nanostructures are performed as function of the particle coverage $C$. We observe that $\bar E_\mathrm{dip}$ scales as $\bar E_\mathrm{dip}\propto C^{α^*}$ with an unusually small exponent $α^*\simeq 0.8$--1.0 for coverages $C\lesssim20%$. This behavior is shown to be primarly given by the contributions of particle pairs at short distances, which is intrinsically related to the presence of an appreciable degree of disorder. The value of $α^*$ is found to be sensitive to the magnetic arrangement within the nanostructure and to the degree of disorder. For large coverages $C\gtrsim20%$ we obtain $\bar E_\mathrm{dip}\propto C^α$ with $α=3/2$, in agreement with the straighforward scaling of the dipole coupling as in a periodic particle setup. Taking into account the effect of single-particle anisotropies, we show that the scaling exponent can be used as a criterion to distinguish between weakly interacting ($α^* \simeq 1.0$) and strongly interacting ($α^* \simeq 0.8$) particle ensembles as function of coverage.

cond-mat.dis-nn↗

arXiv2003

Anisotropic susceptibility of ferromagnetic ultrathin Co films on vicinal Cu

We measure the magnetic susceptibility of ultrathin Co films with an in-plane uniaxial magnetic anisotropy grown on a vicinal Cu substrate. Above the Curie temperature the influence of the magnetic anisotropy can be investigated by means of the parallel and transverse susceptibilities along the easy and hard axes. By comparison with a theoretical analysis of the susceptibilities we determine the isotropic exchange interaction and the magnetic anisotropy. These calculations are performed in the framework of a Heisenberg model by means of a many-body Green's function method, since collective magnetic excitations are very important in two-dimensional magnets.

cond-mat.str-el↗

arXiv2003

In-plane magnetic reorientation in coupled ferro- and antiferromagnetic thin films

By studying coupled ferro- (FM) and antiferromagnetic (AFM) thin film systems, we obtain an in-plane magnetic reorientation as a function of temperature and FM film thickness. The interlayer exchange coupling causes a uniaxial anisotropy, which may compete with the intrinsic anisotropy of the FM film. Depending on the latter the total in-plane anisotropy of the FM film is either enhanced or reduced. Eventually a change of sign occurs, resulting in an in-plane magnetic reorientation between a collinear and an orthogonal magnetic arrangement of the two subsystems. A canted magnetic arrangement may occur, mediating between these two extremes. By measuring the anisotropy below and above the Néel temperature the interlayer exchange coupling can be determined. The calculations have been performed with a Heisenberg-like Hamiltonian by application of a two-spin mean-field theory.

cond-mat.stat-mech↗

arXiv2002

In-plane dipole coupling anisotropy of a square ferromagnetic Heisenberg monolayer

In this study we calculate the dipole-coupling-induced quartic in-plane anisotropy of a square ferromagnetic Heisenberg monolayer. This anisotropy increases with an increasing temperature, reaching its maximum value close to the Curie temperature of the system. At T=0 the system is isotropic, besides a small remaining anisotropy due to the zero-point motion of quantum mechanical spins. The reason for the dipole-coupling-induced anisotropy is the disturbance of the square spin lattice due to thermal fluctuations ('order-by-disorder' effect). For usual ferromagnets its strength is small as compared to other anisotropic contributions, and decreases by application of an external magnetic field. The results are obtained from a Heisenberg Hamiltonian by application of a mean field approach for a spin cluster, as well as from a many-body Green's function theory within the Tyablikov-decoupling (RPA).

cond-mat.str-el↗

arXiv2002

Quantum Monte Carlo simulation of thin magnetic films

The stochastic series expansion quantum Monte Carlo method is used to study thin ferromagnetic films, described by a Heisenberg model including local anisotropies. The magnetization curve is calculated, and the results compared to Schwinger boson and many-body Green's function calculations. A transverse field is introduced in order to study the reorientation effect, in which the magnetization changes from out-of-plane to in-plane. Since the approximate theoretical approaches above differ significantly from each other, and the Monte Carlo method is free of systematic errors, the calculation provides an unbiased check of the approximate treatments. By studying quantum spin models with local anisotropies, varying spin size, and a transverse field, we also demonstrate the general applicability of the recent cluster-loop formulation of the stochastic series expansion quantum Monte Carlo method.

cond-mat.str-el↗

arXiv2002

Dipole coupling induced magnetic ordering in an ensemble of nanostructured islands

The magnetic ordering due to the long range dipole coupling in an ensemble of magnetic islands is investigated. If the islands are large enough and closely separated, the average dipole energy per island can explain the magnitude of the observed ordering temperature of such an ensemble. The energetical degeneracy with respect to a continuous in-plane rotation of the magnetic moments in a periodic ensemble of islands is lifted in presence of an island size dispersion and an irregular island array. Many different (metastable) magnetic states are obtained, reminiscent of a spin-glass behavior. We obtain that the average magnetic binding energy per island due to the dipole coupling increases with increasing positional disorder. The island ensembles exhibit non-collinear magnetic structures, resulting in non-saturated ensemble magnetizations. The calculations are performed with a classical spin model for ensembles of islands in unit cells with periodic boundary conditions. The point dipole sums are augmented by an island areal correction.

cond-mat.dis-nn↗

arXiv2001

Model study for the nonequlibrium magnetic domain structure during the growth of nanostructured ultrathin films

The nonequilibrium magnetic domain structure of growing ultrathin ferromagnetic films with a realistic atomic structure is studied as a function of coverage and temperature. We apply a kinetic Monte Carlo method to a micromagnetic model describing the transition from superparamagnetic islands at low coverages to a closed ferromagnetic film. The magnetic relaxation and the island growth happen simultaneously. Near the percolation threshold a metastable magnetic domain structure is obtained with an average domain area ranging between the area of individual magnetic islands and the area of the large domains observed for thicker ferromagnetic films. We conclude that this micro-domain structure is controlled and stabilized by the nonuniform atomic nanostructure of the ultrathin film, causing a random interaction between magnetic islands with varying sizes and shapes. The average domain area and domain roughness are determined. A maximum of the domain area and a minimum of the domain roughness are obtained as a function of the temperature.

cond-mat.dis-nn↗

arXiv2000

Magnetic storage device with improved temporal stability

The current efforts to fabricate non-volatile magnetic recording media with a high areal density is deteriorated by the increasing temporal instability of the stored information. If the stored energy per magnetic particle competes with the thermal energy, spontaneous magnetic reversal processes may occur. Deposition of the magnetic particles on top of an antiferromagnetic substrate will increase its energy barrier due to the exchange coupling between the two subsystems. For this the magnetic moments of the antiferromagnet in the vicinity of the magnetic particle have to deviate from their undisturbed arrangement. This disturbation vanishes within a few lattice constants. In the framework of a classical spin model we calculate the spin arrangements and the resulting energy barriers for typical systems.

cond-mat.mtrl-sci↗

arXiv2000

Schwinger boson theory of anisotropic ferromagnetic ultrathin films

Ferromagnetic thin films with magnetic single-ion anisotropies are studied within the framework of Schwinger bosonization of a quantum Heisenberg model. Two alternative bosonizations are discussed. We show that qualitatively correct results are obtained even at the mean-field level of the theory, similar to Schwinger boson results for other magnetic systems. In particular, the Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite temperatures is not found if the ground state of the anisotropic system exhibits a continuous degeneracy. We calculate the magnetization and effective anisotropies as functions of exchange interaction, magnetic anisotropies, external magnetic field, and temperature for arbitrary values of the spin quantum number. Magnetic reorientation transitions and effective anisotropies are discussed. The results obtained by Schwinger boson mean-field theory are compared with the many-body Green's function technique.

cond-mat↗

arXiv2000

Many-body Green's function theory for the magnetic reorientation of thin ferromagnetic films

The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term, an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms. The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good approximation for realistic coupling strengths. We apply the new method to monolayers with spin $S\geq 1$ and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some significant differences.

cond-mat.stat-mech↗