Showing papers on "Exchange interaction published in 1988"

Spin-density-wave antiferromagnetism in chromium

Abstract: A comprehensive account is given of the macroscopic and microscopic physical properties of chromium (and where appropriate those of its dilute alloys) that relate to its antiferromagnetism. Neutron scattering is treated in great detail, first in the historical introduction, then as an experimental probe of both the magnetic structure and the excitations of the incommensurate spin-density-wave state and (with the assistance of x rays) of the concomitant charge-density wave and strain wave. Neutron scattering is considered as a tool to explore not only the disappearance of long-range order with increasing temperature through the growth of excitations as the weak first-order N\'eel transition is approached, but also the persistence of these spin fluctuations well into the paramagnetic state---processes that are still little understood. The article surveys, without mathematical details, model systems designed to reproduce the magnetic and thermodynamic properties of Cr. The energy-band structure calculations are given a more comprehensive review. Special attention is paid to calculations of the wave-vector-dependent susceptibility that reproduce the observed wave vector of the spin-density wave, and to a recent finite-temperature calculation that gives almost the right N\'eel temperature. The review of Fermi-surface studies emphasizes those designed to relate the spin-density wave vector (and its pressure dependence) to the nesting vector of the Fermi surface. An account is given of the spectroscopic determination of the energy gap(s), whose theoretical analysis is still unclear, and of experiments aimed at determining physical properties that throw light on the origin of the weak first-order N\'eel transition. The article describes the use of magnetic anomalies in the elastic moduli to determine the volume dependence of the exchange interaction responsible for antiferromagnetism in Cr. The experimental features of the spin-flip transition are reviewed, although a theory of this phenomenon is wanting. The experimental study of microscopic structure by the use of hyperfine-interaction properties is surveyed. An account is given of both experimental and theoretical studies of the surface of Cr and of Cr films and sandwiches. Finally, "technical antiferromagnetism" is discussed: the effect of severe internal strain in producing a commensurate antiferromagnetic state, wave-vector Q domains, polarization S domains (for which the experimental evidence is scanty), and ultrasonic attenuation as a tool to study them.

Exact Jastrow-Gutzwiller resonating-valence-bond ground state of the spin-(1/2 antiferromagnetic Heisenberg chain with 1/r2 exchange.

Abstract: A set of Jastrow wave functions comprises exact eigenstates of a family of S= $\frac{1}{2}$ antiferromagnetic chains with ${\mathrm{r}}^{\mathrm{\ensuremath{-}}2}$ exchange. The ground state of the isotropic model is in this set, and is identical to the U\ensuremath{\rightarrow}\ensuremath{\infty} limit of the Gutzwiller wave function, also identified as Anderson's ``resonating-valence-bond'' state. The full set of energy levels of this model is obtained; the spectrum exhibits remarkable ``supermultiplet'' degeneracies suggesting the existence of a hidden continuous symmetry.

Mechanisms of exchange anisotropy (invited)

Abstract: Exchange anisotropy refers to a group of phenomena which appear in ferromagnetic‐antiferromagnetic sandwiches, particularly to a field offset in the hysteresis loop. After a brief review of experiment and earlier theory, a new theory is described involving a random field at the interface which causes the antiferromagnet to break up into domains, whose size is inversely proportional to the exchange field offset. The theory is extended by considering the temperature dependence and also the topological properties of the domains, in particular nonzero winding numbers which increase the size of the domains and give them added stability. The metastability of such structures provides an explanation of the magnetic ‘‘training’’ effect observed in multiple cycles of the hysteresis loop.

Micromagnetic studies of thin metallic films (invited)

Abstract: A computer simulation model has been developed to conduct micromagnetic studies of thin magnetic films. Thin‐film media are modeled as a planar hexagonal array of hexagonally shaped grains. Each grain is a single domain particle whose magnetization reverses by coherent rotation. The computation utilizes coupled gyromagnetic dynamic equations with phenomenological Landau–Lifshitz damping. In particular, the effects of particle interactions are investigated. The effect of media microstructure on magnetic hysteresis is examined as well as the effect of intergranular exchange coupling. The difference between planar and completely random orientation of the crystalline anisotropy axes is discussed. Recorded transitions are simulated by allowing a pair of perfect transitions to relax. With no intergranular exchange coupling, the transitions show profound irregularity and zig‐zag structure. Intergranular exchange coupling produces more uniform transitions with increased zig‐zag structure amplitude. For a closely ...

Spectral Function of Holes in a Quantum Antiferromagnet

Abstract: By use of an effective Hamiltonian which takes into account the constraints on the motion of a hole in a quantum antiferromagnet, the spectral function of the hole is calculated. For small exchange and away from the antiferromagnetic zone boundary, it is found to be dominated by incoherent multiplespin-wave processes. The dispersion of the quasiparticle part and the possible implications for disordering of the quantum antiferromagnet are discussed.

Quantum energy gap in two quasi‐one‐dimensional S=1 Heisenberg antiferromagnets (invited)

Abstract: Following the Haldane conjecture, the antiferromagnetic (AF) Heisenberg chain of integer spins has a singlet ground state separated from the excited states by an energy gap. Recent numerical calculations on finite AF chains with S=1 supported this conjecture and provided an approximate value for the energy gap: EG≂0.4‖ J‖, where J is the intrachain exchange interaction. We report experimental studies on two Ni (II) quasi‐one‐dimensional (1D) AF with large intrachain interaction, J/k≂−50 K, Ni(C2H8N2)2NO2(ClO4) (NENP) and Ni(C3H10N2)2NO2(ClO4), (NINO). In both compounds, the magnetic susceptibility along the three crystal axes steeply decreases below T≂15 K and no 3D long‐range magnetic order could be detected down to 1.2 K. These features are consistent with the Haldane conjecture. Inelastic neutron scattering experiments performed on NENP show two energy gaps, with an average value of about 0.4‖ J‖, which are explained by a splitting of the Haldane gap by single‐ion anisotropy.

Exchange interactions in quantum well subbands

Abstract: It is shown that exchange interactions in the two‐dimensional electron gas in quantum wells could cause observable effects on subband energies and intersubband transition energies. In the case of doped quantum wells, the intrasubband exchange interaction can produce an energy shift which is substantially larger than the direct Coulomb energy shift. Theoretical estimates of such shifts are compared with experimental measurements of the infrared photoconductivity of multiple quantum well AlGaAs/GaAs structures with wells doped at about 1018 cm−3.

Exchange forces between domain wall and electric current in permalloy films of variable thickness

Abstract: Wall displacements are induced by large current pulses crossing a wall, in Ni81Fe19 films. In films of thickness w<35 nm containing Neel walls, the sense of wall motion is found to be independent of the magnetization sense in the two domains adjacent to the wall, and is identical to the sense of motion of the electronlike charge carriers. This shows that the wall motion is not caused by stray magnetic fields, but rather by s‐d exchange forces generated by conduction electrons. The value ≂5×107 A cm−2 of the needed current density agrees with predictions of a theory based on s‐d exchange. In the case of cross‐tie walls in films with 35 nm

Ferromagnetic resonance studies of exchange-biased Permalloy thin films.

Abstract: Ferromagnetic resonance (FMR) spectra of Permalloy thin films exchange-coupled to iron-manganese films are analyzed. Studies were made on bilayer, ferromagnetic-antiferromagnetic (FA) and trilayer (AFA) structures, as a function of both F and A layer thicknesses in the range 20--800 A\r{}. Data are presented at a frequency of 9.3 GHz for both in-plane and perpendicular directions of the applied field, and at 34.1 GHz, in-plane. Analysis of these data enables extraction of the magnetization, gyromagnetic ratio, and an exchange shift due to spin-wave stiffness and perpendicular-surface anisotropy, as a function of layer thickness. The azimuthal dependence of the in-plane resonance is used to determine the magnitude of the exchange anisotropy (bias field). The magnetization and gyromagnetic ratio show little dependence on the thickness of either the F or A layer down to 50 A\r{}, implying that the interfaces are sharp on a scale of a few lattice constants. Within this interfacial region the magnetization is reduced as a result of interaction with the antiferromagnet. We suggest that the perpendicular-surface anisotropy is created by exchange coupling to the antiferromagnet whose easy axes are not in the plane of the interface. Finally, we suggest a model for exchange anisotropy in which the antiferromagnetic domain pattern is not totally locked, but adjusts in response to the ferromagnetization. Such a model qualitatively explains the bias field exerted by the antiferromagnetic layer deposited before the ferromagnet, the field-training effect, the FMR linewidth, and the magnitude of the bias field.

Note on temperature dependence of exchange constant in magnetite

Abstract: We calculated the temperature dependence of the exchange constant between 20°C and 573°C using three previously published data sets from inelastic neutron scattering by spin wave excitations in magnetite. The exchange constant in magnetite varies as a function of temperature approximately as the saturation magnetization to the power 1.7. Our synthesis of temperature dependent spin wave dispersion data provides an experimental foundation for the temperature variation of the exchange energy which is critical for micromagnetic domain structure calculations and for an understanding of the acquisition of thermoremanent magnetization in rocks.

Solid‐state ‘‘magic‐angle’’ sample‐spinning nuclear magnetic resonance spectroscopic study of group III–V (13–15) semiconductors

Abstract: We have obtained 27 Al, 69 Ga, 71 Ga, 113 In, and 115 In static and ‘‘magic‐angle’’ sample‐spinning (MASS) solid‐state nuclear magnetic resonance (NMR) spectra of a series of polycrystalline III–V semiconductors (AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb) at magnetic field strengths of 8.45 and 11.7 T. Line‐broadening mechanisms have been identified by comparing static and MASS NMR results. By applying the MASS technique, dipolar, first‐order quadrupolar, and pseudodipolar interactions are removed. Thus, the MASS spectral linewidth (of the central, 1/2↔−1/2, transition) is dominated by exchange and/or second‐order quadrupolar interactions. For compounds having the cubic zinc blende structure, the exchange interaction dominates, and exchange interaction constants can be determined. For AlP, AlAs, AlSb, GaP, GaAs, InP, and InAs, first‐order quadrupole effects are evident as spinning sidebands (SSBs), due to the satellite transitions. These effects are due to a small distribution of electric field gradients caused by lattice defects, and result in an overall Lorentzian profile for the SSB envelope. For compounds having the hexagonal wurtzite structure (GaN and AlN), the second‐order quadrupolar interaction is the main line‐broadening mechanism, and we find isotropic chemical shifts δi of 333 ppm [from 1 M Ga(NO3 )3 ] for GaN, and of 115 ppm [from 1M Al(H2 O)6 Cl3 ] for AlN, and nuclear quadrupole coupling constant e2 qQ/h of 2.8 MHz for 69 GaN, 1.7 MHz for 71 GaN, and of 2.2 MHz for AlN.

Observation of the exchange interaction at the surface of a ferromagnet.

Abstract: Etude de la dependance en temperature de l'aimantation de surface dans le regime onde de spin

Fermi liquid description of La2−xSrxCuO4

Abstract: We treat a model with orbitals on the copper and oxygen sites and hopping between them. In the limit of infinite U on the Cu site, the problem is treated using the slave boson method. In the presence of doping, we find a Fermi liquid solution with mass enhancement proportional to x −1 . Low frequency optical absorption shows deviation from a simple Drude behavior, with oscillator strength proportional to x. Interactions between quasi-particles via the exchange of slave bosons is calculated. The Heisenberg exchange interaction J arises as a second order boson exchange and may give rise to singlet pairing between quasi-particles. Our treatment is essentially a perturbation theory valid when J is less than the renormalized bandwidth. Consequently the Fermi liquid description may break down for small doping.

Three-body-exchange interaction in dense rare gases.

Abstract: Self-consistent phonon calculations are employed to demonstrate the importance of the three-body-exchange interaction in dense rare-gas solids. A Slater-Kirkwood form is adopted, the parameters of which are adjusted on ab initio calculations for helium and argon and calculated by a scaling law for krypton and xenon. When used in conjunction with realistic (Aziz) pair potentials and the Axilrod-Teller terms, the exchange interaction brings the calculated equation of state for each rare gas into excellent accord with experiment. This type of interaction should stabilize the hcp structure for all rare gases below 90 GPa.

Static and dynamic magnetic properties of Fe‐Cr‐layered structures with antiferromagnetic interlayer exchange

Abstract: Thin films of Fe when separated by approximately 8 A of Cr couple antiferromagnetically. Evidence for this comes from spin‐wave‐mode spectra as measured by means of light scattering and hysteresis curves, here obtained by means of the magneto‐optic Kerr effect. Based on new and more‐detailed data we would like to discuss the behavior of a particular spin‐wave mode due to the spin canting induced by the antiferromagnetic coupling. It is related to a Goldstone mode. The experiments also show that antiferromagnetic coupling occurs not only for films grown along [100] but also for films grown along [110]. We discuss the implications of this observation.

Effect of hydrogen on the curie temperature of Nd2(Fe15M2); M ≡ Al, Si, Co

Abstract: The Curie temperature of Nd 2 Fe 17 can be increased by hydrogenation or by substitution of aluminium, silicon or cobalt for iron, but these increases are not simply additive. They can be understood in terms of the distance dependence of exchange interactions and band-structure effects.

Exchange effects on excitons in quantum wells.

Abstract: Exchange splitting of quasi-two-dimensional excitons confined in quantum wells is studied theoretically by performing a simplified calculation with a scaling argument. Both the exchange energy and the longitudinal-transverse splitting are found to be enhanced drastically when the confinement of the exciton increases. Taking into account the difference of the enchange interaction between the heavy-hole and the light-hole exciton, the intrinsic splittings of the exciton ground states are calculated for GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As quantum wells within the envelope-function approximation.

Interlayer exchange in magnetic superlattices (invited)

Abstract: A simple model for calculating the Ruderman–Kittel–Kasuya–Yosida exchange interaction between layers of a magnetic rare‐earth (RE) metal separated by yttrium is presented. The calculation is semiphenomenological and makes use only of previously measured or calculated quantities. It is valid when the thickness of the magnetic RE layers is much smaller than that of the Y layers. The coupling is found to be of fairly long range and to be consistent in sign with the ordering observed in Gd‐Y superlattices. For the actual case of comparable layer thicknesses, a discussion of the superlattice wave functions is given. We show that our simple model is very plausible for constituent metals with simple band structures. For the actual band structure of Gd, eigenstates that are evanescent in the Gd region may provide a justification of our model and account for the agreement with experiment. Further theoretical and experimental work is necessary for understanding these systems.

Dependence of the R-R (R = Rare Earth) Exchange Interactions on the Nature of the R Atom in Intermetallics

Abstract: In rare-earth (R) intermetallics, the molecular field coefficient nRR describing exchange interactions between R spin moments decreases, in a given series, by almost an order of magnitude from compounds with light R to those with heavy R elements. Simultaneously a decrease is observed in the transferred hyperfine field at nonmagnetic sites. These results are compared to the recent observation in R-transition metal (M) compounds of a decrease of the molecular field coefficient nRM across a given series of compounds (M fixed, R varied). These behaviours are coherently understood by considering that both R-R and R-M indirect exchange interactions in R intermetallics occur via 5d conduction electrons and that the 4f-5d exchange at R sites decreases from Pr to Tm.

Theory of magnetic surface anisotropy and exchange effects in the Brillouin scattering of light by magnetostatic spin waves (invited)

Abstract: A new theory of the Brillouin shift in the inelastic scattering of light by magnetostatic spin waves is presented. Contrary to previous work, the present calculations do include exchange effects and treat the magnetic surface anisotropy constants Ks and Kss directly rather than via the stratagem of effective volume anisotropies. The experimental data for {110} Fe on W are explained about as well by the present theory as by previous work. A detailed analysis reveals the previously unnoticed fact that the signs of Ks and Kss for (110) Fe on W are opposite to those for (110) Fe on GaAs. Some new spin‐wave modes arising from exchange are predicted and shown to occur outside the frequency range which has been investigated experimentally. A quantitative explanation is proposed for the occasional applicability of a theory based on effective volume anisotropies and zero exchange.