X-ray resonant magnetic scattering by Fe/Cr superlattices

We have studied the structural and magnetic properties of an antiferromagnetically (AF) coupled Fe/Cr superlattice by means of soft x-ray resonant magnetic scattering. Strong and purely magnetic Bragg peaks are observed at the half-order positions in reciprocal space parallel to the [001] growth direction and in between the structural Bragg reflections from the superlattice periodicity. The magnetic hysteresis loops measured at the first-order and at the half-order Bragg peaks clearly demonstrate the strong AF coupling of the Fe/Cr multilayer. Transverse scans and off-specular reflectivity measurements confirm an AF domain structure of the superlattice in remanence with large perpendicular correlation. In addition, the transverse scan of the half-order Bragg peak exhibits a Lorentzian line shape at zero field, which diminishes in higher fields, indicative of a remanent multidomain state approaching a single-domain state towards saturation.     

Effective-medium approach for surface-localized magnetic polaritons in magnetic (ferromagnetic and antiferromagnetic) superlattices

The general effective-medium dispersion relations are derived for surface-localized magnetic polaritons which propagate parallel to the surface between a superlattice and semi-infinite bulk material, as applied to ferromagnetic and antiferromagnetic superlattices, in the situation when a static magnetic field is applied in the plane of the layers and parallel to the magnetization. The dependence of the energy of the surface waves on the volume fraction of the ferromagnetic superlattice component and the influence of the external magnetic field on the spectrum of the surface magnetic polaritons for the antiferromagnetic superlattice are investigated. The spectrum of the surface-localized magnetic polaritons which appear at the junction of the magnetic (ferromagnetic and antiferromagnetic) superlattice with the magnetic material are more complex, in contrast to the cases of semi-infinite magnetic material or semi-infinite magnetic SL. It is essential that in all cases in the presence of the external magnetic field the spectrum of the magnetic polaritons are non-reciprocal. The properties of surface polaritons are discussed in detail for the system ferromagnetic superlattice (YIG/non magnet)/YAG and the antiferromagnetic superlattice (MnF₂/ZnF₂)/FeF₂.     

Exchange bias in spin-engineered double superlattices

Exchange bias has been observed in sputtered magnetic double superlattices which consist of a ferromagnetically coupled superlattice grown on an antiferromagnetically (AF) coupled superlattice. This system exhibits a parallel domain wall, a spin flop transition, and exchange bias when the anisotropy is large in the AF block. This work shows that neither the domain wall nor the spin flop are directly related to exchange bias but that the anisotropy is essential.     

Transport in a shunted surface superlattice with a perpendicular magnetic field

We experimentally investigate the transport through a shunted surface superlattice under the influence of a magnetic field applied perpendicular to the current direction. The current–voltage characteristics of these surface superlattices exhibit a peak which is followed by a wide region of negative differential resistance. The application of a transverse magnetic field has a profound influence on the position and height of this peak. The recorded shifts are compared to the predictions of different superlattice transport theories. Since these theories predict a different dependence on the magnetic field strength, the transport mechanism in the surface superlattice structures can be uniquely determined.     

Dispersion relation for surface-localized magnetic polaritons and magnetostatic waves in antiferromagnetic superlattice

In addition to the general dispersion equation for surface-localized magnetic polaritons and magnetostatic waves, which propagate in the system antiferromagnetic superlattice-antiferromagnet, we derive a simplified result for the long-wavelength limit λ?L (L is the period of superlattice) when the superlattice is found to behave like an anisotropic bulk medium (effective-medium approach). The dispersion curves and frequency region of the existence of the surface magnetic polaritons and magnetostatic waves are presented numerically for several values of the external magnetic field and for different antiferromagnetic materials.     

Magnetic ratchet effects in a two-dimensional electron gas

The effect of the magnetic field on the generation of an electric current in a two-dimensional electronic ratchet is theoretically studied. Mechanisms of the formation of magnetically induced photocurrent are proposed for a structure with a two-dimensional electron gas (quantum well, graphene, or topological insulator) with a lateral asymmetric superlattice consisting of metallic strips on the external surface of the structure. The ratchet with the spatially oscillating magnetic field generated by the ferromagnetic lattice, as well as the nonmagnetic ratchet placed in the uniform magnetic field both classically weak and strong quantizing, is considered. It is established that the ratio of the amplitude of the magnetic oscillations of photocurrent to the ratchet photocurrent in zero field can exceed two orders of magnitude.     

Superlattice, rhombus, square, and hexagonal standing waves in magnetically driven ferrofluid surface

Standing wave patterns that arise on the surface of ferrofluids by (single frequency) parametric forcing with an ac magnetic field are investigated experimentally. Depending on the frequency and amplitude of the forcing, the system exhibits various patterns including a superlattice and subharmonic rhombuses as well as conventional harmonic hexagons and subharmonic squares. The superlattice arises in a bicritical situation where harmonic and subharmonic modes collide. The rhombic pattern arises due to the nonmonotonic dispersion relation of a ferrofluid.     

Polariton dynamics of a one-dimensional magnetic photonic crystal in crossed magnetic and electric fields

By an example of a semibounded one-dimensional magnetic photonic crystal (MPC) of easy-axis antiferromagnet-nonmagnetic dielectric type placed in mutually orthogonal dc magnetic and electric fields, conditions are found under which the quadratic magneto-optic interaction leads to a number of anomalies in the reflection and propagation conditions for TM or TE waves incident from outside on the surface of the superlattice. It is assumed that the easy magnetic axis is collinear to the electric field.     

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.     

Conductivity of a semiconductor superlattice in a transverse magnetic field

The conductivity of a semiconductor superlattice in a magnetic field perpendicular to the superlattice axis is calculated. The dependence of the conductivity on the magnetic field strength is studied. It is found that there is a magnetic-field range where the resistivity is proportional to the magnetic field strength.     

Tuning magnetotransport through a magnetic kink crystal in a chiral helimagnet

We consider magnetotransport properties in a conducting chiral helimagnet, where the magnetic kink crystal (MKC) is formed under weak magnetic field applied perpendicular to the helical axis. The MKC behaves as a magnetic superlattice potential and results in Bragg scattering of conduction electrons. Tuning of the weak magnetic field enables us to control the size of the superlattice Brillouin zone and gives rise to a series of divergent resistivity anomalies originating from resonant Bragg scatterings. We discuss as well a nontrivial magnetic structure in the resonant states realized in the subsystem of the itinerant electrons.     

Superconducting spin valve effect of a v layer coupled to an antiferromagnetic [Fe/V] superlattice

We studied superconducting V layers deposited on an antiferromagnetically coupled [Fe(2)V(11)](20) superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T(S) decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe2 layer is reduced if at a distance of 1.5 nm a second Fe2 layer with antiparallel spin orientation exists.     

Collective modes of a superlattice - plasmons,LO phonon-plasmons,and magnetoplasmons

The wavevector-frequency dependent dielectric constant is investigated for a superlattice structure. Attention here is focused on the collective modes of the GaAs-GaAlAs superlattice with doped (or modulated doped) quantum wells. For wells widely separated in space, such that Bloch wave function overlap between wells is negligible, a Bloch-like plasmon can propagate along the superlattice direction mediated entirely by Coulomb interaction alone. Interaction of these plasmons with optical phonons and with a magnetic field is investigated.     

Electrodynamic response of charge carriers in doubly periodic semiconductor <Emphasis Type="Italic">n</Emphasis>-type superlattices in a permanent homogeneous magnetic field

The electromagnetic response of the two-dimensional electron gas of a surface superlattice placed in a perpendicular permanent homogeneous magnetic field is studied. The magneto–optic Kerr and Faraday effects are calculated. The conditions of transparency of model semiconductor structures in the terahertz frequency region are found and the field-induced spin density of the electronic states is calculated. The features of the frequency dependences of complex Kerr and Faraday angles are connected with the symmetry of the spinor states of the charge carriers in a superlattice.     

Instability of dipole magnetoexcitons in quantum wells' and graphene superlattices

We propose the Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field. For this system the instability of the ground state of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is found. The density of superfluid component ns(T) and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two-dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both QW and graphene realizations.     

Phase transitions of a spin-one alternating magnetic superlattice

We examine the critical behavior of a magnetic superlattice which made up of two magnetic materials, A and B. Using the effective field theory with a probability distribution technique that accounts for the single-site spin correlation, we derive the analytical equation for the Curie temperature of the superlattice which alternates as ABAB...AB. The dependence of the Curie temperature on the interface coupling strength J_ab and the layer number of the finite superlattice was calculated. The effects of the surface modification are also studied. Received 2 March 2001     

2D chaotic quantum states in superlattices

The semiclassical motion of an electron along the axis of a superlattice may be calculated from the miniband dispersion curve. Under a weak electric field the electron executes Bloch oscillations which confines the motion along the superlattice axis. When a magnetic field, tilted with respect to the superlattice axis, is applied the electron orbits become chaotic. The onset of chaos is characterised by a complex mixed stable-chaotic phase space and an extension of the orbital trajectories along the superlattice axis. This delocalisation of the orbits is also reflected in the quantum eigenstates of the system some of which show well-defined patterns of high probability density whose shapes resemble certain semiclassical orbits. This suggests that the onset of chaos will be manifest in electron transport through a finite superlattice. We also propose that these phenomena may be observable in the motion of trapped ultra-cold atoms in an optically induced superlattice potential and magnetic quadrupole potential whose axis is tilted relative to the superlattice axis.     

Current oscillation and chaotic dynamics in superlattices driven by crossed electric and magnetic fields

We have theoretically studied current oscillation and chaotic dynamics in doped GaAsAlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.     

Mössbauer studies of the morin transition in bulk and microcrystalline α-Fe2O32

Mössbauer spectra of bulk and microcrystalline hematite have been analyzed at the Morin transition for both the weak-ferromagnetic (WF) and anti-ferromagnetic (AF) phases simultaneously, increasing precision in the extracted Mössbauer parameters. The Morin transition in bulk took place in less than 0.4 K, and in the microcrystals decreased linearily with expanding lattice in agreement with its increase under external pressure. Temperature hysteresis measurements in the microcrystals gave values of the fourth-order magnetic anisotropy energy which indicated possible surface spin-pinning effects. In bulk the magnetic field decreased by 7 ± 0.5 kOe in going from the WF to the AF phase on decreasing temperature, and the isomer shift decreased by $\text{0.014 ± 0.003}\text{mm}\text{sec.}$ With lattice dilation the AF quadrupolar interaction appeared to decrease, while the isomer shift change across the transition goes positive. In contrast the change in magnetic field is not simply related to this lattice dilation, indicating surface spin-pinning effects.     

Specific Heat of a Two-Layer Magnetic Superlattice

The specific heats of both a two-layer ferromagnetic superlattice and atwo-layer ferrimagnetic one are studied. It is found that the spin quantumnumbers, the interlayer and intralayer exchange couplings, the anisotropy,the applied magnetic field, and the temperature all affect the specificheat of these superlattices. For both the ferromagnetic and ferrimagneticsuperlattices, the specific heat decreases with increasing the spin quantumnumber, the absolute value of interlayer exchange coupling, intralayerexchange coupling, and anisotropy, while it increases with increasingtemperature at low temperatures. When an applied magnetic field is enhanced, the specific heat decreases in the two-layer ferromagnetic superlattice, while it is almost unchanged in the two-layer ferrimagnetic superlattice at low field range at low temperatures.