Layer-heterogeneous magnetic states in metallic nanosystems

The formation of layer-heterogeneous periodic magnetic states in metallic systems is explained in terms of the dependence of the energy of itinerant electrons on the magnetic ordering of localized spins. The interaction of the local magnetic moments with conduction electrons with a certain spin projection is described by a set of spin-dependent δ-function potentials. A matrix method is developed permitting one to calculate the energy spectrum and the density of states of itinerant electrons in the presence of a layered magnetic heterogeneity. This method is used to explain oscillations of the interlayer exchange interaction in metallic magnetic superlattices and the stabilization of spin-density wave structures in transition metals and alloys.     

Staggered potential and spin polarization effects on RKKY interaction in armchair graphene nanoribbon

Indirect exchange interaction between two magnetic external atoms, named by Ruderman–Kittle–Kasuya–Yosida (RKKY) interaction, has been presented in the staggered armchair graphene nanoribbon. We have studied RKKY interaction as a function of distance between localized moments. It has been shown that a magnetic ordering along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. The static spin susceptibility components of armchair graphene nanoribbon have been calculated to find exchange interaction between arbitrary components of magnetic moments. We have exploited Green’s function approach in order to calculate spin susceptibility components of electronic gas in nanoribbon structure in the context of tight binding model Hamiltonian. The effects of parameter and ribbon width on the dependence of exchange interaction on distance between moments are investigated. Our results show the spin polarization along perpendicular to the plane leads to anisotropic behavior for exchange interaction between the two magnetic moments. In other words the spatial behavior of RKKY interaction between longitudinal components of magnetic moments is different from that of transverse components.     

Influence of amorphous environment on magnetic structure

The influence of disordered medium on the magnetic structure of crystalline clusters is discussed. In a plate-like crystalline cluster with localized moments the disordered magnetic environment leads to preference of a spiral magnetic structure over the antiferromagnetic collinear ordering that would be otherwise realized. By the numerical solution of a specific model qualitative information was obtained about the influence of the exponential rate of decrease of the exchange interaction, plate thickness and degree of disorder of the medium on the magnetic structure of the cluster.     

A model for magnetic abnormalies of FeNi Invar alloys

A model based on the idea of localized magnetic moments is presented which allows to calculate the local magnetic moment expectation values of FeNi alloys. The only parameters of the model are the exchange integralsJ _FeFe,J _FeNi,J _NiNi. By assuming a “mixed” exchange interaction the concentration dependence of the exchange integralsJ _FeFe andJ _FeNi is calculated. The model allows the iron magnetic moments to orient parallel or antiparallel to the magnetization axis, depending on the local environment. It explains the magnetic abnormalies of FeNi Invar alloys as for example the concentration dependence of the mean magnetic moment and the Curie temperatures as well as the characteristic “flat” courves of the spontaneous magnetization.     

The effects of electron–phonon interaction on anisotropic RKKY interaction in graphene nanoribbon

We study the Ruderman–Kittle–Kasuya–Yosida (RKKY) interaction in doped armchair graphene nanoribbon. The effects of both external magnetic field and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments has been analyzed. It has been shown that a magnetic field along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain both transverse and longitudinal static spin susceptibilities of armchair graphene nanoribbon in the presence of electron-phonon coupling and magnetic field. The spin susceptibility components are calculated using the spin dependent Green’s function approach for Holstein model Hamiltonian. The effects of spin polarization on the dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of magnetic field on the spatial behavior of in-plane and longitudinal RKKY interactions are different in the presence of magnetic field.     

Shallow donor impurities in magnetic semiconductors

We introduce the macroscopic magnetization to treat the localized magnetic moments in a magnetic semiconductor. We obtain a set of coupled equations for the magnetization and the electronic wavefunction of a shallow donor impurity exchange coupled to the localized moments. A variational solution to the equations of motion indicates that near to the critical temperature the wavefunction contracts and the electron becomes more bound. We discuss the relevance of this model for the understanding of the activation energy for conductivity in the paramagnetic phase of EuO.     

Phase diagram of the helical structure of a two-subsystem frustrated antiferromagnet

The expansion of the thermodynamic potential for the two-subsystem antiferromagnet with frustrated intersubsystem isotropic exchange is obtained. It is demonstrated that this expansion contains the first derivatives with respect to the antiferromagnetic vectors of the subsystems, i.e., the Lifshitz invariant. The equation for the temperature-field boundary of the helical phase for the two-subsystem frustrated antiferromagnet is derived by linearizing the variational equations for the minimum free energy within the mean-field approximation. Relationships are obtained for the critical field at T = 0, the angle of canting of moments of the antiferromagnetic sublattices, and the temperature of spontaneous appearance of helical ordering in the absence of an external field. It is revealed that there is a second higher temperature of formation of the helical magnetic structure induced by the magnetic field with the wave vector of the helix nonmonotonically depending on the external field. The phase boundary of the helical phase and the temperature dependence of the orientation of moments of the magnetic subsystem with weak exchange interaction are determined using numerical minimization of the free energy. It is shown that the transition to the commensurate phase is a first-order transition with a small magnetization jump. A comparative analysis of models with different spatial displacements of ions in the subsystems along the direction of the vector of the helical structure is performed. A criterion is proposed for the choice of the direction of the vector of the incommensurate magnetic structure.     

A theory of double exchange in infinite dimensions

This paper gives a simplified model of the double exchange which is a kind of indirect exchange interaction between localized magnetic moments. The presented model is solved exactly in the case of infinite - dimensional space. Equations for single-particle Green's function and magnetization of the localized spins subsystem are obtained. It is shown that our simple double exchange model reveals an instability to the ferromagnetic ordering of localized moments. Magnetic and electric properties of this system on Bethe lattice with are investigated in detail. Received: 24 January 1997 / Revised: 14 February 1997 / Received in final form: 18 August 1997 / Accepted: 25 August 1997     

Noncollinear magnetic ordering in a magnetic dimer supported on a metallic substrate

Self-consistent electronic structure calculations for a magnetic dimer supported on a metal surface are performed on the basis of the microscopic Hamiltonian for itinerant electrons. The possibility of a noncollinear ground state in the absence of spin-orbit interaction and magnetic frustration is shown. The effective exchange interaction of moments is determined by the number of quasi-localized d-electrons per atom and, in general, is not described by the Heisenberg model.     

Spin fluctuation and ultrasonic attenuation in magnetic superconductors

The staggered susceptibility in the superconducting state of magnetic superconductors is calculated by taking account of the electromagnetic interaction between the localized magnetic moments and the persistent current. With the use of the susceptibility, the ultrasonic attenuation caused by spin fluctuations is obtained. The attenuation shows the characteristic dependence on temperature, magnetic field, and the direction of sound propagation.     

Effect of impurities on the properties of superconductor with helical order of localized spins

The Cooper pairing of electrons in the crystal with regularly distributed localized spins is considered. We study the case when the exchange interaction of electrons and localized spins leads to the helical magnetic order in superconductivity while in the absence of the superconductivity the ferromagnetic order occurs. We show that the scattering of electrons on the nonmagnetic impurities narrows the region of existence of the superconducting phase with helical order of spins (HS-phase). This narrowing is not considerable and condition on the purity of the crystals under which the HS-phase may be observable is not severe.     

一维铁磁链中量子孤波的能级和磁矩

使用Hartree近似和一种简化的准离散多标度方法,研究了具有交换作用和经典磁矩相互作用的一维铁磁链中的量子孤波解. 这种一维铁磁链中不仅存在着运动的量子孤波,也存在着静态的量子孤波(即量子内禀局域模).利用所获得的量子孤波解,进一步研究了量子孤波的能级和由量子孤波所携带的磁矩. 研究表明,量子孤波的能量和磁矩都是量子化的,这些结果为正确理解磁性材料中像磁滞回线的量子台阶等宏观量子特性提供了一条可能的途径.     

Dependence of the metastability of the magnetic states of two-phase nanoparticles on mechanical stress

The influence of mechanical stress on the magnetic states of multiaxial heterophase magnetic materials is investigated using a model of two-phase nanoparticles. The range of critical fields for the reversal of the magnetic moments of phases is calculated and phase diagrams are built to assess the effect of interface exchange interaction and mechanical stress on the metastability of the magnetic states of two-phase nanoparticles.     

Theory of magnetic susceptibility of Bloch electrons: Effect of localized magnetic moments

We derive for the first time a first-principles theory of the magnetic suceptibility (χ) of Bloch electrons including the effects of periodic potential, spin-orbit interaction and localized magnetic moments, and believe it to be the most general and thorough treatment which has yet been made of this problem. The importance of the theory in possible applications is discussed.     

Chirality waves in two-dimensional magnets

We theoretically show that moderate interaction between electrons confined to move in a plane and localized magnetic moments leads to formation of a noncoplanar magnetic state. The state is similar to the Skyrmion crystal recently observed in cubic systems with the Dzyaloshinskii-Moriya interaction; however, it does not require spin-orbit interaction. The noncoplanar magnetism is accompanied by the ground-state electrical and spin currents, generated via the real-space Berry phase mechanism. We examine the stability of the state with respect to lattice discreteness effects and the magnitude of magnetic exchange interaction. The state can be realized in a number of transition metal and magnetic semiconductor systems.     

Weak antiferromagnetic ordering induced by Dzyaloshinskii-Moriya interaction and pure magnetic reflections in MnSi-type crystals

A symmetry analysis of the Dzyaloshinskii-Moriya (DM) interaction in MnSi-type crystals reveals a nontrivial antiferromagnetic pattern of tilted Mn moments remaining even after an unwinding of the ground-state helix by a strong magnetic field. The remaining tilts are caused by that component of the DM vector which is perpendicular to the component responsible for helical spiraling; both components are evaluated and related to the atomic structure using a simple model. It is shown that the tilting should induce pure magnetic reflections 00?(?=2n+1) in neutron or x-ray magnetic scattering. In addition, the DM-induced antiferromagnetic ordering is important for the core structure of intrinsic defects, for the spectra of magnetic resonances, and generally for a better understanding of spin-orbit interaction in this type of magnetics.     

Effects of finite temperatures,valence bands and energy gaps on the indirect exchange interaction in intrinsic semiconductors

The indirect exchange interaction between localized magnetic moments via virtual excitations from the valence band in intrinsic semiconductors is calculated taking into account the temperature, energy gaps, finite valence bands and effective masses. The inclusion of finite temperature effects changes significantly the phase and magnitude of the oscillations. For small gap semiconductors the oscillatory character and the possibility of ferro-magnetic as well as anti-ferromagnetic coupling are obtained. The exchange interaction oscillates with temperature suggesting interesting applications of this model.     

Enhancement of the upper critical magnetic field in La1.2−xEuxMo6S8 compounds

Measurements of the upper critical magnetic field (H_c2) of the pseudoternary system La_1.2?xEu_xMo₆S₈ were made at temperatures above 1.5 K. An enhancement of the value of H_c2 was found for 0 < x ? 0.6. The results are attributed to an increase of the orbital critical field with increasing x, compensation of the applied magnetic field by a negative exchange field due to an antiferromagnetic exchange interaction between the conduction electron spins and the Eu magnetic moments (Jaccarino-Peter effect), and exchange scattering of conduction electrons by the rare earth magnetic moments.     

Diffraction studies of the crystalline and magnetic structures of γ-Fe2O3 iron oxide nanostructured in porous glass

The crystalline and magnetic structures of γ-Fe₂O₃ maghemite synthesized in porous glass in the form of nanoparticles with a mean diameter of 106(2) Å have been determined by the neutrons and synchrotron radiation diffraction methods. Nanostructured maghemite with the spinel structure has vacancies in the octahedral and tetrahedral positions. The magnetic structure corresponds to the usual ferrimagnetic type. The measured magnetic moments are much lower than the moments in a usual sample. Moreover, the moments are strongly different in the octahedral and tetrahedral positions; this difference is explained by the difference in the exchange interaction for moments in different positins.     

Phase diagram of the disordered RKKY model in dilute magnetic semiconductors

We consider ferromagnetism in spatially randomly located magnetic moments, as in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect exchange RKKY interaction. We obtain, via Monte Carlo calculations, the magnetic phase diagram as a function of the impurity moment density n(i) and the relative carrier concentration n(c)/n(i). As evidenced by the diverging correlation length and magnetic susceptibility, the boundary between ferromagnetic and nonferromagnetic phases constitutes a line of zero temperature critical points which can be viewed as a magnetic percolation transition. In the dilute limit, we find that bulk ferromagnetism vanishes for n(c)/n(i) >0.1. We also incorporate the local antiferromagnetic direct superexchange interaction between nearest neighbor impurities and examine the impact of a damping factor in the RKKY range function.