Electronic structure and local magnetic moments for the epitaxially grown fcc Fe films included between the Cu(001) substrate and Cu(001) covering layers are calculated. The lattice constant of the Fe film is assumed to be equal to that of Cu homogeneously in the whole film. Interface parts of the Fe film are found to be ordered ferromagnetically, while inner part of the Fe film is obtained as ordered layer-by-layer antiferromagnetically for odd number of layers. For even number of Fe layers the most favourable configuration includes antiferromagnetism of inner layers with two middle layers coupled ferromagnetically.
Within a tight-binding LMTO approach in the atomic sphere approximation we calculate the exchange interaction, J, between magnetic Fe slabs in a periodic multilayer arrangement m Fe/n Cu/ .... We concentrate on cases, where Cu adapts to the bcc structure of Fe. For the "open" (100)-interface our results suggest that a minimal number of m = 13 Fe monolayers per Fe slab is necessary to get antiferromagnetic coupling. For m = 16 we find antiferromagnetic coupling for 8 ≤ n ≤ 14, i.e. a long period of ca. 11 and a short period of 2 Cu monolayers. In contrast, for the (110)-interfaces, only a long period of 14 monolayers is found, i.e. the coupling is antiferromagnetic for 7 ≤ n ≤ 14 already for small values of m. We then model the roughness by mutual interchange of 50% of the Fe and Cu atoms, respectively, in the interface layers and find that, due to the interchange, the coupling constant J, as a function of the Cu-spacer thickness n, roughly shifts by half a mono-layer to the right. In the (100)-case, the short period gets washed out by the roughness.
Very peculiar electronic structure and local magnetic moment distribution for fcc Fe ultrathin films of 1, 2, 3, and 4 monolayers of thickness, epitaxially grown on Cu(001) substrates and covered with some layers of Cu, are calculated. The lattice constant of the Fe film is assumed to be equal to that of Cu, homogeneously in the whole film. The competition between the surface ferromagnetism and bulk antiferromagnetism results in ferromagnetism of 2-monolayer film, asymmetric solution for 3-monolayer film and antisymmetric solution for 4-monolayer film.
Possibilities of the modeling of the flux density distribution in thin films are significantly limited using the finite elements method due to the fast increase of the number of tetrahedral elementary cells with reduction of the thickness. For this reason, method of the moments is very important alternative for finite elements method in the case of thin layers, where layer's thickness should be considered. Method of the moments overcomes this barrier, due to the possibility of operation on uniform grids with limited number of cells. Moreover, in opposite to the finite elements method, the method of the moments requires solving of the well defined linear equations, instead of the set of ill-posed differential equations. Paper presents the generalization of the method of the moments for thin layers with given thickness. Layers are defined as the 2D rectangular grids. Within the generalization, four key equations describing the influence of rectangular cell's border on the magnetization of cells are stated. On the base of these dependences, the set of 2NM linear equations was determined, where N and M are the numbers of rectangular cells in the rows and columns of regular grid. Finally, the set of linear equations is solved and magnetic flux density distribution in the thin layer is calculated.
The magnetooptical anisotropy in strained quantum well structures is studied theoretically. As the source of this anisotropy the magnetic field induced mixing of light and heavy hole states is found. The anisotropy is also found in the frame of the multiple-band envelope-function approximation studying the influence of the barrier material on electronic states in the quantum well.
Quasi-one-dimensional spin systems described by an Ising-like Hamiltonian with a strong space anisotropy (s=1/2) are investigated. Magnetic properties of this model are examined in the approximation including Gaussian fluctuations of molecular field. This paper reports an attempt at obtaining more accurate results for Gaussian fluctuation of molecular field by an exact formula for mean fluctuations of a spin.
Grain boundaries play an important role in low-field magnetoresistance of La_{0.7}Ca_{0.3}MnO_{3} and La_{0.7}Sr_{0.3}MnO_{3} thin films deposited by magnetron sputtering and pulsed laser deposition on YSZ(100) and silicon substrates buffered by YSZ. Well-pronounced low-field magnetoresistance hysteresis was observed in magnetic fields applied in in-plane and out-of-plane directions. High values of local magnetoresistance sensitivity d(MR)/dH in the vicinity of the coercive field were obtained reaching up to 0.2%/Oe for La_{0.7}Ca_{0.3}MnO_{3} samples at 5 K.
The independent random walk method (which is equivalent to the noninteracting spin wave method) with no further approximations was used to calculate the low temperature expansion of: (i) the local mean spin values of an isotropic Heisenberg ferromagnet for semi-infinite crystal and (ii) the spontaneous magnetization of an anisotropic Heisenberg ferromagnet for monolayer and double layer. The full low temperature expansion starting with T^{3/2} term was obtained for the semi-infinite Heisenberg ferromagnet and it was shown that the spontaneous magnetization for thin films of anisotropic Heisenberg ferromagnets can exhibit a quasi-linear behaviour in certain temperature region related to the magnitude of the anisotropy.
The spin-wave resonance in the thin FeBr_{2} field-induced metamagnet in the paramagnetic phase with the (001) surfaces and at low temperatures is examined theoretically. It is found that the absorption spectrum is strongly affected by modifications of the surface exchange parameters. Also, the conditions for the appearance of various surface and bulk spin-wave features are discussed.
Magnetization directions of ferromagnetic ultra-thin films can be altered by increasing the thickness. The transition between in-plane and out-of-plane axes is induced by the competition among the magnetostatic, magnetocrystalline and magnetoelastic anisotropy energies. Such an effect has attracted more interest recently, due to the applications in magneto-optical recording technologies. In this study, we have investigated by magneto-optical Kerr effect the magnetization properties of magnetron sputtered nickel thin films.
La_{1-x}MnO_3 films grown by metal organic chemical vapor deposition technique on r-plane cut Al_2O_3 substrates were investigated. The change of the optical response over the La_{1-x}MnO_3/Al_2O_3 sample surface was investigated along with the temperature dependence of magnetization. The mostly pronounced difference in the spectra of dielectric function occurred in the region of the d-d transitions of Mn-ions. The changes in the optical spectra and magnetic properties were correlated to the structural features of thin film.
Soft magnetic multicomponent Co-Fe-Ni-Si-B thin films have been grown by pulsed laser deposition onto single-crystal sapphire and silicon substrates. The static hysteresis measurements for different substrate temperatures are presented. Thin films with a coercive force smaller than 1 Oe were grown at substrate temperatures from 250°C to 350°C. X-ray diffraction measurements proved that the structure of films is amorphous. The surface morphology of grown thin films was observed by scanning electron microscopy. The chemical composition of deposited films corresponds to the composition of bulk alloy.
The influence of temperature and annealing on giant magnetoresistance of Si(100)/Cu(20 nm)/Py(2 nm)/(Cu(2 nm)/Py(2 nm))_{100} multilayer (Py = Ni_{83}Fe_{17}) sputtered at room temperature in double face-to-face configuration is reported. It was found that giant magnetoresistance value, ΔR_{GMR}/R_{sat} (where R_{sat} is the resistance in saturation), monotonically decreases with increasing temperature (4.5% at 173 K to about 1% at 373 K). This results from the decrease in magnetic change of resistance, ΔR_{GMR}, and to the lesser extent from an increase in R_{sat}, though both of them are caused by the shortening of electrons mean free path. The observed almost linear decrease in giant magnetoresistance saturation field with increasing temperature is explained by temperature changes of magnetization profile. Vibrating sample magnetometer measurements revealed that the increase in temperature results in pronounced decrease in remnant to saturation magnetization ratio (M_{r}/M_{s}) suggesting that at low temperatures magnetic bridges between Py layers play an important role in magnetization process. It is shown that proper annealing, by an annihilation of bridges and/or lateral decoupling, leads to an increase in giant magnetoresistance ratio from 3.4% in as deposited state to 4.7%.
Magnetic properties of polycrystalline thin films of chromium chalcogenide spinels (CdCr_{2}Se_{4} lightly doped with indium and CdCr_{2x}In_{2-2x}Se_{4}) were studied. The ferromagnetic (FMR) and spin-wave resonance (SWR) techniques were used to investigate the temperature dependences of both the spin-wave stiffness constant D and the saturation magnetization M_{s}. The resonance spectra were recorded in the temperature range extending from 4.2 K to 300 K. The influence of indium concentration on M_{s}(T) and D(T) was studied. It was shown that lightly doped samples (In/Cd < 1% at.) exhibited the ferromagnetic ordering with M_{s}(T) and D(T) being the linear functions of T^{3/2} and T^{5/2}, respectively. Higher concentration of indium produced the reentrant transition and spin-glass state of magnetic ordering in CdCr_{2x}In_{2-2x}Se_{4}. The temperature dependence of M_{s} was also found from the FMR data for these two magnetic phases.
Temperature dependence of the local magnetization in the spin-wave regime is calculated within the framework of the multiband model for ultrathin films consisting of 5, 7 and 9 monolayers. The temperature range in which the calculated results can be fitted to the Bloch T^{3/2} law is found in all cases. The Bloch coefficient Bay corresponding to temperature dependence of the average film magnetization is found to be proportional to 1/D, where D is the thickness of the film. The spatial distribution of the local magnetization is obtained. The Bloch coefficient corresponding to the surface layer appears to be greater than the one corresponding to the central layer, namely B_{s} > B_{c}. The ratio B_{s}/B_{c} is increasing with an increase of the film thickness. The calculated results are well consistent with experimental ones obtained for ultrathin films of various thicknesses.
The magnetic interactions in superlattices formed by alternating layers of magnetic and nonmagnetic semiconductors with common anions were studied theoretically. Within a one-dimensional tight-binding model by minimising the total electronic energy we show the existence of an efficient long range mechanism of magnetic correlations between the neighbouring magnetic layers in such superlattices. The cases of magnetic ions in the barriers (e.g., EuTe/PbTe) and in the wells, with the order within the magnetic layers being either ferromagnetic or antiferromagnetic, have been considered and compared with the paramagnetic case. In the case of antiferromagnetic ordering within magnetic layers we have found that for even and odd numbers of magnetic monolayers different magnetic superlattices are energetically favourable.
In this contribution, we will discuss the effect of interface misfit strain on magnetic coupling of ferromagnetic layers, across intervening, metallic spacer in the epitaxial superlattice system. We calculate the interface lateral lattice deformation, due to ionic radii misfit within the Frenkel-Kontorova model and derive its influence on surface magnetic anisotropy and electronic properties of the nonmagnetic spacer. We will study also the origin of the interface magnetoelastic asymmetry in superlattices and its effect on magnetic coupling between magnetic layers. Also the magnetoelastic contribution to the in-plane exchange coupling is widely discussed.
The interlayer coupling between ferromagnetic EuS layers separated by spacer layers of diamagnetic insulators, YbSe and SrS, is studied within a 3D tight-binding model. The dependencies of the coupling strength on the energy structure of the spacer, on strains resulting from the lattice mismatch between the superlattice constituents, as well as on an applied hydrostatic pressure and lattice deformations, are presented. The sign and the range of the obtained coupling agree with the behavior of magnetic correlations observed recently in neutron reflectivity spectra of EuS/YbSe superlattices.
We report a relatively large exchange bias effect observed for the first time in Ni-Mn-Sn thin films with different microstructure and composition: a Ni_{50}Mn_{36}Sn_{14} epitaxial film (A), a Ni_{50}Mn_{43}Sn_7 film which is phase decomposed (B), and a NiMn/Ni_{50}Mn_{25}Sn_{25} bilayer (C). Despite the samples differ markedly in both microstructure and composition H_{EB} does not substantially differs at 5 K. Exchange bias decreases with increasing T approximately as H_{EB} (T) ∝ H_{EB} (5K)/T with H_{EB} (5K) of 180 Oe and 60 Oe for sample B and C, respectively and almost linearly for sample A with H_{EB} (5K) = 65 Oe. Blocking temperature where the exchange bias vanishes is 40, 50 and 80 K for sample A, C and B, respectively. The results suggest that the role of AFM/FM interfaces is not substantial in formation of exchange bias in Ni-Mn-Sn Heusler alloy films and exchange bias is rather related to AFM/FM interactions in nanoscale.
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