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1.
M. Abou Ghantous A. Khater 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):335-341
We present the solution of the full magnetic problem arising from the absence of magnetic translation symmetry in two dimensions
due to an extended magnetic surface step on the surface boundary of an insulating magnetic substrate. The calculation concerns
in particular the spin fluctuation dynamics of a magnetic atomic step in the surface of a ferromagnetic simple cubic lattice,
the spin order being in the direction normal to surface boundary. Only exchange interactions are considered between the spins
in the model. The theoretical approach determines the evanescent spin fluctuation field in the two dimensional plane normal
to the direction of the step edge. This field arises owing to the absence of magnetic translation symmetry in this plane,
and is completely independent of the form of the surface defect, underlying the general character of the calculation. We show
the existence of optical localised magnon modes propagating along the step, their fields being evanescent in the plane normal
to the step direction.
Received 17 February 1999 相似文献
2.
P. Fröbrich P.J. Kuntz 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(4):445-455
The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of
thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which
require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier
work, on the magnetic properties of thin ferromagnetic films are compared.
Received 31 October 2002 Published online 7 May 2003 相似文献
3.
P. Fröbrich P.J. Jensen P.J. Kuntz A. Ecker 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):579-594
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 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.
Received 19 June 2000 and Received in final form 2 August 2000 相似文献
4.
A. Saffarzadeh 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(2):149-153
Spin-polarized tunneling in FMS/M/FMS double tunnel junctions where FMSs are ferromagnetic semiconductor layers and M is a
metal spacer is studied theoretically within the single-site coherent potential approximation (CPA). The exchange interaction
between a conduction electron and localized moment of the magnetic ion is treated in the framework of the s-f model. The spin polarization in the FMS layers is observed to oscillates as a function of the number of atomic planes in
the spacer layer. Amplitude of these oscillations decreases with increasing the exchange interaction in FMS layers.
Received 9 June 2001 and Received in final form 20 August 2001 相似文献
5.
We investigate the magnetic excitations for the magnetic problem arising from the absence of magnetic translation symmetry in one dimension due to the presence of an impurity layer embedded within a semi-infinite ferromagnet. A Heisenberg model is employed to investigate the possibility that localized modes can occur with an impurity layer implanted within a semi-infinite ferromagnet. No electronic effects are considered. The theoretical approach employs the matching procedure in the mean field approximation and determines the propagating and evanescent spin amplitude fields including the contribution due to an applied field. The results are used to calculate the energies of localized modes associated with the impurity layer and with the surface. Numerical examples of the modes are given and they are found to exhibit various effects due to the interplay between the impurity layer and surface modes. It is shown that more localized modes can occur and the modification of the spin wave spectra can be signaled by the appearance of surface and impurity modes, besides the bulk excitations. Also, the bulk spin fluctuations field, the spin waves localized on the surface as well as on impurity layer depend are shown to depend on the nature of the exchange coupling between spin sites, the values of spin sites and the position of the impurity layer from the surface. 相似文献
6.
P. Fröbrich P.J. Jensen P.J. Kuntz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):477-489
The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function
theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire
temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus
a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation:
RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation
of the magnetization is determined by the spin components (), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient.
Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant
differences between these approaches.
Received 6 April 1999 and Received in final form 9 July 1999 相似文献
7.
F. Mila Fu-Chun Zhang 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(1):7-10
One-dimensional spin 1 systems may have a rich phase diagram including Haldane gap and dimerized phases if the usually very
small biquadratic exchange becomes significant. We show that this unlikely condition may be fulfilled in electron systems
with quasi-degenerate orbitals. This mechanism may have been experimentally realized in the spin 1 chain LiVGe2O6. The implications for the exploration of the physics and quantum chemistry of spin 1 chains are discussed.
Received 4 January 2000 相似文献
8.
N. Keller J. Mistrík Š. Višňovský D.S. Schmool Y. Dumont P. Renaudin M. Guyot R. Krishnan 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(1):67-73
Orthoferrites present, as bulk materials, reorientation transitions of their magnetic moment alignment at temperatures depending
on the rare-earth (RE) ion. In particular, orthoferrites (REFeO3) with RE = Sm, Dy, present this transition at T
SRT = 443 K and 36 K, respectively. The spectra of the complex Kerr and Faraday angle have been measured on orthoferrite thin
films (RE = Sm, Dy, Y), which were prepared by pulsed laser deposition on amorphous quartz substrates. The obtained spectra
exhibit contributions of both surfaces and interfaces. Propagation effects of the polarized light in the magneto-optical medium
which is interpreted in terms of a simplified theoretical formalism, is also observed. For selected photon energies, temperature
dependent Faraday rotation measurements, , on orthoferrite thin films (RE = Sm, Dy, Y) have been performed. A quite different thermal variation compared to the bulk
magnetization has been observed. Curie temperatures are found to be close to the bulk values or slightly larger by 10 K to
20 K as in the case of DyFeO3 and YFeO3. For RE = Sm and Dy, increases with increasing temperature contrary to the saturation magnetization, passes through a maximum at about 460 K and
vanishes with a T
C of 647±18 K, 695 K for RE = Sm and Dy respectively.
Received 28 July 2000 相似文献
9.
T. Kamppeter S.A. Leonel F.G. Mertens M.E. Gouvêa A.S.T. Pires A.S. Kovalev 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(1):93-102
The properties of dynamical solitons (magnon droplets) in the classical, two-dimensional anisotropic Heisenberg model with
easy-axis exchange anisotropy are studied. The solution of the Landau-Lifshitz equation in the continuum limit for the soliton
with topological charge q = 1 is obtained numerically using a shooting method. We analized a wide range of the anisotropy parameter and our results
are in good agreement with results obtained from spin dynamics simulations. The dependence of an internal precession frequency
of the soliton on both the anisotropy parameter and the radius of the soliton is also investigated. Finally, the limits of
applicability of the continuum approach are discussed.
Received 22 August 2000 相似文献
10.
T. Domański 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(1):49-56
The method of continuous canonical transformation is applied to the double exchange model with a purpose to eliminate the
interaction term responsible for non conservation of magnon number. Set of differential equations for the effective Hamiltonian
parameters is derived. Within the lowest order (approximate) solution we reproduce results of the standard (single step) canonical
transformation. Results of the selfconsistent numerical treatment are compared with the other known studies for this model. 相似文献
11.
C.J. Calzado J.-P. Malrieu 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):375-381
Accurate ab initio calculations on embedded Cu4O12 square clusters, fragments of the La2CuO4 lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J
= 124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic
second-neighbor interaction (J'
= 6.5 meV) and four-spin cyclic exchange (K
= 14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic
coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect
seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic
coupling with the Cu-O distance is Δ
J
/Δ
d
Cu - O
∼ 1700 cm-1?-1.
Received 20 November 2000 相似文献
12.
V.P. Dyakonov M.T. Borowiec A. Jedrzejczak M. Górska N.A. Doroshenko E. Zubov Yu. Bukhantsev T. Zayarnyuk H. Szymczak 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):9-14
Magnetic phase transition in the CsDyW2O8 magnet has been studied by means of low temperature specific heat C
(
T
) measurements. The magnetic ordering temperature of the Dy3+ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions
between Dy3+ ions. The behavior of the C
(
T
) dependencies above and below T
N
is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization
are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure
of the ground state.
Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002 相似文献
13.
N. Guskos V. Likodimos S. Glenis J. Typek H. Fuks M. Wabia D.G. Paschalidis D. Tossidis C.L. Lin 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(3):277-282
The magnetic properties of the rare earth molecular compounds with hydrazone ligands containing Nd3+, Gd3+, and Yb3+ have been investigated by electron paramagnetic resonance (EPR) and magnetization measurements. For the Gd-compound, partially
resolved fine structure due to Gd3+ and exchange narrowing effects at low temperatures are observed in the EPR spectra, suggesting, consistent with the EPR and
dc magnetic susceptibility, weak antiferromagnetic exchange interactions. Paramagnetic behavior sustained down to low temperatures
is derived for Yb3+ ions, whereas substantial ferromagnetic exchange coupling is inferred for the lighter Nd3+ ions, indicating significant variations of the exchange integrals along the lanthanide series.
Received 29 April 2002 Published online 31 July 2002 相似文献
14.
P. Bonville J.A. Hodges M. Shirakawa M. Kasaya D. Schmitt 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):349-355
Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T
N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic
moments along the crystal a axis. From a 151Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that
in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure
sine-wave just below T
N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics
of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show
that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that
the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop
transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor.
Received 17 January 2001 and Received in final form 20 March 2001 相似文献
15.
M.M. Savosta A.N. Ulyanov N.Yu. Starostyuk M. Maryško P. Novák 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):393-396
The results of 55Mn NMR, dc magnetization, and ac susceptibility studies are presented for La0.7Ca0.15Ba0.15MnO3, La0.7Sr0.15Ba0.15MnO3, and La0.7Ba0.3MnO3 ferromagnetic manganites. While is a function of the mean radius of the La and alkaline-earth ions and the cation disorder, the form of the temperature dependence
of the magnetic moment may be expressed as function of only. The phase transition is continuous for all three compounds.
Received 5 March 1999 相似文献
16.
Magnetic interaction in Mg, Ti, Nb doped manganites 总被引:4,自引:0,他引:4
I.O. Troyanchuk M.V. Bushinsky H. Szymczak K. Bärner A. Maignan 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(1):75-80
An effect of Mn substitution with Me=Mg2+, Ti4+, Nb5+ in manganites has been investigated by preparing La0.7Sr0.3(Mn1-xMex)O3 and La1-xSrx(Mn1 - x/2Nbx/2)O3 series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La0.7Sr0.3(Mn 3 +
0.85Nb 5 +
0.15)O3 is ferromagnet with T
C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical
magnetic phase diagrams are constructed for La0.7Sr0.3(Mn1-xMex)O3 and La1-xSrx(Mn1 - x/2Nbx/2)O3. Our results show that Mn3+-O-Mn3+ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn4+ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between
antiferromagnetic Mn4+-O-Mn4+ and ferromagnetic Mn3+-O-Mn4+(Mn3+) superexchange interactions.
Received 24 January 2002 Published online 9 July 2002 相似文献
17.
P. Orgiani C. Aruta G. Balestrino S. Lavanga P.G. Medaglia A. Tebano 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(1):23-28
Transport properties of SrRuO3 thin films were studied as a function of the epitaxial strain. SrRuO3 films were grown on (100) SrTiO3 substrates by the Pulsed Laser Deposition technique equipped with Reflection High Energy Electrons Diffraction (RHEED). Samples
thickness has been varied from a few unit cells to above 1000 ? while monitoring RHEED intensity oscillations. In thicker
films epitaxial strain was found to be progressively relaxed. SrRuO3 relaxed films (thickness 1000 ?) show metallic behavior for the whole temperature range with a ferromagnetic ordering at about 150 K. For thinner
films, ferromagnetic ordering occurs at progressively lower temperatures, until in films thinner than 400 ? it disappears.
Films thinner than 80 ? show a semiconducting behavior at low temperatures. Our results provide direct evidence of the crucial
role of the strain effect for conducting and magnetic properties of SrRuO3.
Received 16 July 2001 and Received in final form 22 October 2001 相似文献
18.
A. Bernert T. Chatterji P. Thalmeier P. Fulde 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(4):535-546
We report results of a new analysis for the low-temperature structure of α
′
-NaV2O5 from synchrotron X-ray diffraction experiments. We confirm the existence of two inequivalent ladder structures in each vanadium
layer. Based on our structural data we perform a bond-valence calculation for the vanadium sites in the low temperature state.
Due to an asymmetric charge ordering we obtain only two different vanadium valences despite the three inequivalent sites.
This explains the 51V-NMR observation of only two resonant peaks in the charge ordered phase. By use of a Slater-Koster method to obtain hopping
matrix elements and cluster calculations we obtain effective vanadium-vanadium hoppings which compare well to LDA results.
Using these in a cluster calculation we obtain a superexchange of 0.047 eV between electrons on neighbouring rungs of the same ladder for the undistorted phase. For the distorted phase we find
a significant alternation in the shifts of the oxygen atoms along the legs of one of the two ladder types which leads to a
significant exchange dimerisation δ
J
≈ 0.25.
Received 24 November 2000 相似文献
19.
W. Selke M. Pleimling D. Catrein 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(3):321-327
The axial next-nearest-neighbour Ising (ANNNI) model of finite thickness is studied. Using mean-field theory, Monte Carlo
simulations, and low-temperature analyses, phase diagrams are determined, with a distinct phase diagram for each film thickness.
The robustness of the phase diagrams against varying the couplings in the surface layers is analysed.
Received 19 March 2002 and Received in final form 2 April 2002 Published online 6 June 2002 相似文献
20.
E. Müller-Hartmann A. Reischl 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(2):173-183
The derivation of effective spin models describing the low energy magnetic properties of undoped CuO2-planes is reinvestigated. Our study aims at a quantitative determination of the parameters of effective spin models from those of a multi-band model and is supposed to be relevant
to the analysis of recent improved experimental data on the spin wave spectrum of La2CuO4. Starting from a conventional three-band model we determine the exchange couplings for the nearest and next-nearest neighbor
Heisenberg exchange as well as for 4- and 6-spin exchange terms via a direct perturbation expansion up to 12th (14th for the 4-spin term) order with respect to the copper-oxygen hopping tpd. Our results demonstrate that this perturbation expansion does not converge for hopping parameters of the relevant size.
Well behaved extrapolations of the couplings are derived, however, in terms of Padé approximants. In order to check the significance
of these results from the direct perturbation expansion we employ the Zhang-Rice reformulation of the three band model in
terms of hybridizing oxygen Wannier orbitals centered at copper ion sites. In the Wannier notation the perturbation expansion
is reorganized by an exact treatment of the strong site-diagonal hybridization. The perturbation expansion with respect to
the weak intersite hybridizations is calculated up to 4th order for the Heisenberg coupling and up to 6th order for the 4-spin
coupling. It shows excellent convergence and the results are in agreement with the Padé approximants of the direct expansion.
The relevance of the 4-spin coupling as the leading correction to the nearest neighbor Heisenberg model is emphasized.
Received 8 June 2001 / Received in final form 28 May 2002 Published online 19 July 2002 相似文献