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1.
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
2.
The tight-binding linear muffin tin orbital (TB-LMTO) method within the local density approximation is used to calculate structural, electronic and magnetic properties of GdN under pressure. Both nonmagnetic (NM) and magnetic calculations are performed. The structural and magnetic stabilities are determined from the total energy calculations. The magnetic to ferromagnetic (FM) transition is not calculated. Magnetically, GdN is stable in the FM state, while its ambient structure is found to be stable in the NaCl-type (B1) structure. We predict NaCl-type to CsCl-type structure phase transition in GdN at a pressure of 30.4 GPa. In a complete spin of FM GdN the electronic band picture of one spin shows metallic, while the other spin shows its semiconducting behavior, resulting in half-metallic behavior at both ambient and high pressures. We have, therefore, calculated electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli and magnetic moments for GdN in the B1 and B2 phases. The magnetic moment, equilibrium lattice parameter and bulk modulus is calculated to be 6.99 μB, 4.935 Å and 192.13 GPa, respectively, which are in good agreement with the experimental results. 相似文献
3.
本文计算了简单立方铁磁体中由于磁性杂质的存在而形成的局域自旋波。其空间对称有s型模、p型模和dr型模三种。求出了各类局域模的波函数和自旋偏离的相角分布,其相应的能级与参量J′S′/JS的关系均由曲线表出。其次分析了在体心立方和面心立方铁磁体中可能出现的局域模的对称类型。我们指出,s型局域自旋波是最有可能实际观测到的。作为观测局域自旋波的样品的铁磁体,其居里点应随渗杂浓度的增加而上升。按照这一判据,我们举出一些可以试用的实验样品。最后,通过总自旋量子数的分析,一般地断定了辐射场不可能激
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4.
Hongxia Wang Y. Sh. Mohammed Xiaobo Du Kai Li Hanmin Jin 《Journal of magnetism and magnetic materials》2009,321(19):3114-3119
The electronic structure and ferromagnetic stability of Co-doped SnO2 are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective UCo transforms the ground state of Co-doped SnO2 to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+UCo calculations show that the pure substitutional Co defects in SnO2 cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+UCo show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of UCo enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb UO on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+UCo calculations are not changed by the correction of band gap. 相似文献
5.
A. Birsan 《Current Applied Physics》2014,14(11):1434-1436
The electronic structure and magnetic properties of Zr2CoAl bulk material were investigated within the Density Functional Theory (DFT) framework. The material, basically a complete spin polarized half-metallic ferromagnet in the ground state, crystallizes in the ordered full-Heusler inverse structure (Hg2CuTi-type structure). The energy band gap, localized in minority spin channel is 0.48 eV at equilibrium lattice parameter, 6.54 Å. The total magnetic moment calculated, equal to 2 μB/f.u., is an integral, in agreement with the Slater-Pauling curve for full-Heusler alloys. 相似文献
6.
A theoretical study is made for the role of an impurity layer embedded within a semi-infinite ferromagnet in determining the spectra of (0 0 1) surface spin waves and the layer magnetization for the surface and impurity layer. The calculations are described using simultaneously a closed form of the spin-wave Green's function and the matching procedure in the random-phase approximation. Analytic expressions for the Green's functions are also derived in a low-temperature spin-wave approximation. The theoretical approach determines the bulk and evanescent spin fluctuation fields in the two-dimensional plane normal to the surface. The results are used to calculate the energies of localized modes associated with the impurity layer as well as 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. The results derived from the dynamic correlation functions between a pair of spin operators at any two sites are employed to evaluate the spin deviation in the ferromagnet due to the localized modes associated with the surface and with the impurity layer obtained by means of the matching procedure. The correlation functions and the layer magnetization are then illustrated as function of the impurity layer distance from the surface for a given temperature. 相似文献
7.
A superconductor with 4-fermion attraction, considered by Maćkowiak and Tarasewicz is modified by adding to the Hamiltonian
a long-range magnetic interaction V between conduction fermions and localized distinguishable spin 1/2 magnetic impurities. V has the form of a reduced s-d interaction. An upper and lower bound to the system’s free energy density f(H, β) is derived and the two bounds are shown to coalesce in the thermodynamic limit. The resulting mean-field equations for the
gap Δ and a parameter y, characterizing the impurity subsystem are solved and the solution minimizing f is found for various values of magnetic coupling constant g and impurity concentration. The phase diagrams of the system are depicted with five distinct phases: the normal phase, unperturbed
superconducting phase, perturbed superconducting phase with nonzero gap in the excitation spectrum, perturbed gapless superconducting
phase and impurity phase with completely suppressed superconductivity. 相似文献
8.
S. Krompiewski 《Physics letters. A》1979,71(1):133-136
The Wolff-Clogston model for a dilute alloy with interactions between electrons in the host metal is considered. A localized magnon mode is suggested to appear above the magnon band for a fcc structure, provided the impurity is “less magnetic” (δU<0) than a strongly ferromagnetic host. 相似文献
9.
V. V. Val’kov S. V. Aksenov E. A. Ulanov 《Journal of Experimental and Theoretical Physics》2014,119(1):124-137
Quantum transport of electrons through a magnetic impurity located in an external magnetic field and affected by a substrate is considered using the Keldysh diagram technique for the Fermi and Hubbard operators. It is shown that in a strongly nonequilibrium state induced by multiple reflections of electrons from the impurity, the current-voltage (I–V) characteristic of the system contains segments with a negative conductivity. This effect can be controlled by varying the anisotropy parameter of the impurity center as well as the parameters of coupling between the magnetic impurity and metal contacts. The application of the magnetic field is accompanied by an increase in the number of Coulomb steps in the I–V curve of the impurity. The effect of appreciable magnetoresistance appears in this case. We demonstrate the possibility of switching between magnetic impurity states with different total spin projection values in the regime of asymmetric coupling of this impurity with the contacts. 相似文献
10.
Electronic structure and magnetic properties of perovskite EuZrO3 have been investigated using the ab initio density-functional calculations with local spin density approximation (LSDA) and LSDA+U methods. The results that are obtained reveal that the antiferromagnetic G-type arrangement is more stable than other possible configurations. The ground G-AFM state shows the insulator property with an energy gap of about 0.27 eV at U=0 eV. It is found that the energy gap strongly depends on the correction potential parameter of U due to the strong interaction of the f electrons of Eu in EuZrO3. The spin magnetic moment of Eu ions is predited to be 6.82μB, which is in well agreement with the experimental result of 6.87μB. 相似文献
11.
The magnetic hyperfine fields for 119Sn impurity atoms, localized in Ga sites of ferromagnetic intermetallic compounds RGa (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), were measured by the Mössbauer spectroscopy technique. At T=5 K, the hyperfine field value (Bhf) varies from 3.3 T in TmGa to 28.0 T in GdGa. Huge deviation from the proportionality between Bhf and the projection of the R3+ ion spin (Sz=(g−1)J) was found. As the atomic number of the R element increases, the Bhf/Sz ratio drastically decreases from 12.6 T for PrGa to 3.3 T for TmGa. This unexpected result can be explained by the strong dependency of Bhf value on the relationship between the Sn-R atomic separation (Rnn) and the radius of the magnetic 4f shell (R4f). In the framework of this concept, the available experimental data for Sn atom in the rare-earth compounds with non-magnetic sp elements were considered. The data may be described by the universal dependency on the single parameter, λ=Rnn/R4f. 相似文献
12.
S. V. Sazonov 《Physics of the Solid State》2006,48(4):670-677
Two modes of nonlinear propagation of two-frequency acoustic pulses in a low-temperature crystal containing paramagnetic resonance impurities with an effective spin S = 1 in an external magnetic field and a field of the static strain are considered. It is shown that the spin-phonon transitions occurring within spin triplets according to the V scheme are responsible for two-frequency self-induced acoustic transparency. When the spin-phonon transitions follow the Λ scheme, there can arise an acoustic effect similar to electromagnetically induced transparency in a pulsed mode, which is accompanied by trapping of the population of the spin sublevels. 相似文献
13.
The spin wave excitation and its size effect has been studied in Al-capped Fe films grown on low-symmetry GaAs(1 1 3)A substrates. The temperature dependence of saturation magnetization follows an effective Bloch's law as long as magnetization remains larger than about 70% of its saturation value. A significant increase of the spin wave parameter B is found in Al-capped ultrathin Fe films grown on GaAs(1 1 3)A compared to bulk Fe, Fe films on GaAs(0 0 1) and other systems. This is explained as a result of the reduction in uniaxial magnetic anisotropy observed in this orientation for the same thickness range. However, this observed uniaxial magnetic anisotropy is found to be a likely reason for stabilizing the ferromagnetism. 相似文献
14.
M. Pavkov-Hrvojevi? M. Panti? S. Radoševi? M. Rutonjski M. Škrinjar D. Kapor 《Solid State Communications》2011,151(18):1205-1209
Bulk and surface magnetic excitations of the semi-infinite ferromagnetic semiconductor (FMS) superlattices and thin films described by Heisenberg and s-d model are analyzed using the transfer matrix method, developed in our previous work. Results are discussed in the narrow-band limit. The spin-wave frequencies for the semi-infinite narrow-band semiconductors are analyzed in both low- and high-frequency regions. Energies of localized excitations are compared to the bulk and the results of Green function formalism. Depending on the parameters of the system, the surface spin waves appear as “acoustical” and “optical”, and there are only some quantitative difference in the high-frequency region, comparing our method and the Green function method. In the framework of the same methodology, bulk and surface magnetic excitations of more complicated superlattices and thin films made of the FMS superlattices are analyzed in terms of dependence of the system parameters. It is shown that the s-d interaction governs the behavior of the systems. Dependence on bulk and surface parameters is discussed. 相似文献
15.
《Solid State Communications》2007,144(12):521-523
The thermodynamic compressibility of a two-dimensional electron system in the presence of an in-plane magnetic field is calculated. We use accurate correlation energy results from quantum Monte Carlo simulations to construct the ground state energy and obtain the critical magnetic field Bc required to fully spin polarize the system. Inverse compressibility as a function of density shows a kink-like behavior in the presence of an applied magnetic field, which can be identified as Bc. Our calculations suggest an alternative approach to transport measurements of determining full spin polarization. 相似文献
16.
The electronic structures and magnetic properties of double perovskites Sr2Fe1−xCrxReO6 (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied within the local spin density approximation (LSDA) and LSDA+U schemes. The calculated results reveal that with increasing Cr content the cell volume shrinks 2.61%; the Fe/Cr site magnetic moment decreases while the Re-site moment increases. The total spin magnetic moment linearly decreases with the Cr doping from 3.00μB for x=0.00 down to 1.00μB for x=1.00 per formula unit. The magnetic coupling constants increase with increasing x. The electronic structure calculations indicate that the electronic concentration in the Re spin-down subband slightly increases resulting from the increase of bonding-antibonding interaction between the localised and the delocalised states in spin-down band; the coupling of O-2p↑ and transition-metal-3d↑ is substantially enhanced with the Cr doping. We discuss the origin of the anomalously high TC of Cr-doped Sr2FeReO6 compounds in terms of band hybridization effects. 相似文献
17.
Mario I. Molina 《Physics letters. A》2008,372(42):6388-6391
We study localized modes on a single Ablowitz-Ladik impurity embedded in the bulk or at the surface of a one-dimensional linear lattice. Exact expressions are obtained for the bound state profile and energy. Dynamical excitation of the localized mode reveals exponentially-high amplitude oscillations of the spatial profile at the impurity location. The presence of a surface increases the minimum nonlinearity to effect a dynamical selftrapping. 相似文献
18.
The temperature and angular dependence of the X-band electron spin resonance (ESR) and51V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi2-xMgxV2O8 (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the51V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT N≈ 3.5K) in doped compositions. 相似文献
19.
A physical picture of electron spin alignments in organic molecule-based ferrimagnets is given from numerical calculations of magnetic specific heat (C) and magnetic susceptibility (χ) as functions of temperature and static magnetic field (B) in terms of an Ising Hamiltonian for an alternating spin chain. The double-peak structure of specific heat appears for different parameter ratios and different magnetic field B, indicating that one peak originates from the ferromagnetic nature and the other from the antiferromagnetic nature. Meanwhile, we study successively the influence of intermolecular and intramolecular interaction on the magnetic susceptibility, showing that the ferromagnetic spin alignment in the alternating molecular chains of biradicals and monoradicals is equivalent to the ferromagnetic alignment of the effective S=1/2 spins. Our results are consistent with those of the Quantum Monte Carlo simulations and the exact diagonalization method and in qualitative agreement with the experimental ones. 相似文献
20.
We review the methods of calculating the effective activation energy Ueff(T,B,J) for both transport measurements and magnetic decay, together with some theoretical models. Then, we apply these methods to our Hg-1223 single-phase superconductor to obtain the activation energy. Transport results give that the magnetic field and temperature dependence of the Ueff can be well described as U0B−α(1−T/Tc)m. Magnetic relaxation shows that the current density dependence of U(J) can be scaled onto a single curve, which can be considered as the activation energy at some temperature T0. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J, are responsible for the magnetic decay data scaling onto a single curve at various temperatures. As temperatures close to zero and near Tc, thermally assisted flux motion model is no longer valid since other processes dominate. 相似文献