Transverse Spin Density Research Articles

Oscillatory exchange bias due to an antiferromagnet with incommensurate spin-density waves.

Oscillatory exchange bias in both magnitude and in sign has been observed in epitaxial (100)Cr/Ni(81)Fe19 bilayers due to the incommensurate spin-density waves in antiferromagnetic (100)Cr layers. Salient effects due to the spin-flip transition between longitudinal and transverse spin-density waves as well as that of expanding wavelength have been observed.

Fincher-Burke excitations in single-Q chromium

The low-energy excitations of incommensurate antiferromagnetic Cr have been investigated by means of high-resolution, inelastic neutron scattering with unpolarized, cold neutrons within an energy range E<9 meV. In agreement with previous measurements we observe Fincher–Burke excitations in the transverse spin density wave phase that appear between the unresolved spin-wave peaks at the incommensurate positions Q±=(1±δ,0,0). In contrast to the previous measurements, our high-resolution data shows that the Fincher–Burke modes do not follow a linear dispersion. Therefore, they have nothing in common with the acoustic phonon branch. The major part of the scattering is concentrated in the range 4 meV<E<8 meV.

Influence of strain on the anti-ferromagnetic ordering in epitaxial Cr(001) films on MgO

The influence of epitaxial strain on the anti-ferromagnetic ordering was studied in a 450-nm-thick Cr film grown on a MgO(001) substrate by molecular beam epitaxy. The anti-ferromagnetic nature of the films was probed by neutron diffraction. A detailed structural characterization of the film was carried out by atomic force microscopy, Rutherford backscattering/channeling spectrometry, and X-ray diffraction. At low temperatures a transverse spin density wave with propagation vector in the film plane is detected, and is attributed to the presence of epitaxial strain. Crystal defects in the sample lead to the appearance of the commensurate anti-ferromagnetic state at higher temperatures.

Phase transitions in a magnetic field inV2−yO3(y=0and0.04)

The effect of a magnetic field on the two long-range-ordered magnetic phases, the collinear insulating antiferromagnetic (AFI) and the incommensurate metallic transverse spin density wave (SDW), is investigated for the vanadium sesquioxide system. A field of 18 T has little effect on the AFI phase of a nominal ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ sample. The transverse SDW phase of ${\mathrm{V}}_{1.96}{\mathrm{O}}_{3}$ can be suppressed by a 4.6(3)-T magnetic field applied in the plane of spiraling spins, while the same magnetic field applied along the spiral axis has little effect on the SDW phase.

Optimising imaging parameters for post mortem MR imaging of the human brain.

MR imaging of post mortem brains has the potential to yield volumetric information and define the extent of structural changes prior to pathological examination. Although standard T2-weighted clinical imaging sequences have been used for the examination of formalin-fixed brains, these protocols may not yield optimum contrast. We examined the effect of varying durations of formalin fixation on the transverse relaxation time (T2) and the tissue spin density. Three post mortem brains were examined weekly during formalin fixation from the unfixed state to 35 days fixation. Standard MR spin-echo imaging was used at 5 echo times (20-100 ms) to calculate transverse relaxation time (T2) and spin density. T2 decreased significantly (ANOVA, p<0.001) in both grey and white matter by 7 days fixation and there was a further (but non-significant) trend towards lower values between 7 and 35 days. Grey and white matter T2 times converged with fixation. Conversely, the grey to white matter spin density ratio increased from 1.19+/-0.01 to 1.54+/-0.06 over five weeks of fixation. Our results suggest that spin density-weighted imaging sequences would provide improved grey to white matter contrast over T2-weighted sequences.

Low-lying magnetic excitations inNi3Aland their suppression by a magnetic field

Results of high-resolution magnetization (M) measurements performed on well-characterized polycrystalline ${\mathrm{Ni}}_{3}\mathrm{Al}$ sample over wide ranges of temperature and external magnetic field are presented and discussed in the light of existing theoretical models. Contrary to the earlier claims that either Stoner single-particle excitations or nonpropagating spin fluctuations solely determine the temperature dependence of spontaneous magnetization $M(T,0),$ at low temperatures, we find that propagating transverse spin-density fluctuations (spin waves) almost entirely account for the thermal demagnetization of both $M(T,0)$ and ``in-field'' magnetization $M(T,H),$ at temperatures $T\ensuremath{\lesssim}{0.28T}_{C}$ ${(T}_{C}=\mathrm{Curie}\mathrm{point}).$ The spin-wave stiffness possesses a field-independent value of $69.6(14) \mathrm{meV} {\mathrm{\AA{}}\mathrm{}}^{2}$ which conforms well with those determined earlier from small-angle and inelastic neutron-scattering experiments. In the temperature range ${0.32T}_{C}\ensuremath{\lesssim}T\ensuremath{\lesssim}{0.92T}_{C},$ enhanced nonpropagating spin-density fluctuations (SF) give a contribution to $M(T,0)$ and $M(T,H)$ that completely overshadows the one arising from spin waves. In accordance with the predictions of a modified spin-fluctuation theory, proposed by the authors recently, the thermally excited SF's get strongly suppressed by magnetic field H while the zero-point SF's are relatively insensitive to H.

Spin energetics in a GaAs quantum well: Asymmetric spin-flip Raman scattering

We demonstrate an asymmetric dependence of the spin-flip electronic Raman spectrum from a two-dimensional electron gas on the direction of circular polarization of the photons, resulting from an interference of light scattered from longitudinal and transverse spin-density fluctuations. By exploiting these selection rules, we are able to determine experimentally that the sign of the band-structure parameter ${a}_{42},$ which describes the bulk ${k}^{3}$ conduction-band spin splitting in zinc-blende semiconductors, is negative for GaAs.

Depth-dependent investigation of the distribution of the spin density waves in thin chromium films with surface X-ray and neutron scattering

For thin epitaxial Cr(0 0 1) films capped with a ferromagnetic Fe layer a transverse spin density wave (SDW) is expected which propagates in the out-of-plane direction with the Cr spins aligned parallel to the film plane in the direction of the Fe magnetization vector. Synchrotron and neutron scattering experiments show, however, that the SDW wave propagates parallel to the film plane with spins oriented out-of-plane. In addition, a commensurate antiferromagnetic phase is found. The re-orientation of the SDW is caused by a frustrated Fe–Cr exchange coupling introduced by monoatomic steps at the Fe–Cr interface. Complete re-orientation takes place over some distance close to the interface reducing severely the coherence length of the SDW structure. With the surface scattering method we have measured the coherence length of the SDW as a function of depth. Furthermore, we have investigated the role of the commensurate antiferromagnetic phase near the Fe–Cr interface. We find no scattering from a commensurate order, implying a layering of the two phases with the incommensurate phase on top.

Instabilities of the Hubbard chain in a magnetic field

We find and characterize the instabilities of the repulsive Hubbard chain in a magnetic field by studing all response functions at low frequency \omega and arbitrary momentum. The instabilities occur at momenta which are simple combinations of the (U=0) \sigma =\uparrow ,\downarrow Fermi points, \pm k_{F\sigma}. For finite values of the on-site repulsion U the instabilities occur for single \sigma electron adding or removing at momenta \pm k_{F\sigma}, for transverse spin-density wave (SDW) at momenta \pm 2k_F (where 2k_F=k_{F\uparrow}+k_{F\downarrow}), and for charge-density wave (CDW) and SDW at momenta \pm 2k_{F\uparrow} and \pm 2k_{F\downarrow}. While at zero magnetic field removing or adding single electrons is dominant, the presence of that field brings about a dominance for the transverse \pm 2k_F SDW over all the remaining instabilities for a large domain of $U$ and density n values. We go beyond conformal-field theory and study divergences which occur at finite frequency in the one-electron Green function at half filling and in the transverse-spin response function in the fully-polarized ferromagnetic phase.

Asymmetries of spin-flip electronic Raman scatteringin a III-V semiconductor quantum well

We have calculated the angular and the polarization dependence of spin-flip electronic Raman scattering from a III-V semiconductor quantum well with its conduction band spin split due to broken inversion symmetry. We found that the interference of light scattered from the longitudinal and transverse spin-density fluctuations leads to a dependence of the Raman spectrum on the direction of circular polarization of photons. This phenomenon at zero magnetic field is entirely due to the intrinsic electron spin dynamics in the spin-split band. The predicted asymmetric polarization dependence is preserved when the sample becomes dirty, such that the elastic-electron scattering rate exceeds the characteristic frequency of electron spin precession in the spin-split band.

Relativistic calculation of the electronic structure of antiferromagnetic chromium with a sinusoidal spin-density wave

A simple method for calculating the electronic structure of antiferromagnetic chromium with a sinusoidal spin-density wave, based on the variational Ritz procedure, is suggested. The four-component Dirac wavefunctions, defined in a crystal without allowance for magnetic order, are chosen as basis functions. The calculations of the electronic spectrum of chromium performed for different types of spin-density wave polarization show that taking into account the relativistic effects results in a difference between the electronic structures of the longitudinal and transverse spin-density waves.

Neutron diffraction and reflectivity studies of the Cr Néel transition in Fe/Cr (001) superlattices

The effects on the interlayer coupling of the Cr Néel transition is studied in Fe/Cr(001) superlattices. The Néel transition is suppressed for Cr layer thickness < 42 Å. For > 42 Å of Cr, the Néel temperature T N initially increases rapidly and then asymptotically approaches its bulk value with a three-dimensional transition-temperature shift exponent value of λ = 1.4 ± 0.3. Neutron diffraction confirms both the Cr antiferromagnetic order and the existence of the incommensurate, transverse spin density wave whose nesting wave vector is the same as that of bulk Cr. The ordering of the Cr dramatically alters the coupling of the Fe layers. The biquadratic Fe interlayer coupling observed for T > T N vanishes below T N as confirmed by polarized neutron reflectivity. The behavior can be understood in terms of finite-size and spin frustration effects at rough Fe-Cr interfaces.

Short-range order in spin density wave antiferromagnets

The model of the short-range order state for itinerant antiferromagnets with SDW is developed. The thermodynamic transverse spin density fluctuation are shown to influence essentially on the formation of the long-range magnetic order. In the wide temperature range these occurs the shortprange, magnetic order regim. The properties of the low-frequency transverse excitations of the spin density are analized. The existence of a fully diffusive mode at small wave vectors is predicted.

Skyrme and meron crystals in quantum Hall ferromagnets

The ground state of a two-dimensional electron gas at Landau level filling factor ν near 1 is studied in the Hartree-Fock approximation. For Zeeman coupling different from zero, the ground state is a Skyrme crystal with both spontaneous long range order in the charge density and spontaneous long range order in the transverse spin density. The energy of the Skyrme crystal is lowest for a square lattice structure with opposing postures for topological excitations on opposite sublattices. We discuss interpretations of our results in terms of non-linear σ and generalized spin models for quantum Hall ferromagnets. For zero Zeeman coupling, we find that the ground state can be interpreted as a meron crystal with two interpenetrating sublattices, each supporting quasiparticles with charge e/2.

Stability and symmetry of the spin-density-wave state in chromium.

The group-theoretical nonadiabatic Heisenberg model proposed recently by the author for better understanding of the material properties of superconductors is applied to the band structure of chromium. Starting from the symmetry of the Bloch functions of a distinct narrow, roughly half-filled energy band of Cr, it is shown that, within this new model, the electrons may lower their Coulomb correlation energy by forming a state that possesses a spin structure with the space group ${D}_{4h}^{6}$ of perfect antiferromagnetic Cr. We give a general condition for the stability of itinerant antiferromagnetism. The commensurate-incommensurate transition is ascribed, as usual, to the peculiar geometric features of the Fermi surface of Cr and to band degeneracies at the Fermi level that split at this transition. We have determined the magnetic groups of the transversely and longitudinally modulated spin structure and give the corepresentations to which the commensurate, the transverse, and the longitudinal spin-density-wave states belong. It turns out that, in contrast to the longitudinal, the transverse spin-density wave is not quite incommensurate because its periodicity is an even multiple of the lattice constant. We speculate that this behavior might be responsible for the spin-flip transition at ${T}_{\mathrm{sf}=123}$ K. The special form of the resulting corepresentations shows that the two incommensurate states are accompanied by lattice distortions and that all three magnetic states are N-electron Bloch states with nonvanishing wave vector. While the commensurate and the transverse spin-density waves, because of their symmetry, are exactly antiferromagnetic, we cannot exclude the longitudinally modulated structure's being slightly ferromagnetic.

Elastic constants of antiferromagnetic chromium

The temperature dependence of the elastic constants of antiferromagnetic chromium in the poly-Q, single-Q and single-S single-Q states has been measured between temperatures in a little above the Neel temperature TN and below the spin-flip temperature. The discontinuities in the elastic constants at TN agree with the predictions of mean-field theory, and their temperature dependence in the transverse spin density wave phase agree with the predictions of a thermodynamic model, if TN is assumed to depend mainly on volume strain omega and to be linear in omega .

Ultrasonic attenuation in the transverse spin density wave phase of chromium

A strong hysteretic attenuation peak is seen when a polarizing field itH p is applied perpendicular to both the longitudinal ultrasonic wavevector itq and the wavevector itQ of the transverse spin density wave (TSDW) in chromium, i.e., itH p⊥ itq⊥ itQ, but not when ( itH p∥ itq)⊥ itQ. This effect is discussed in relation to the various models for the configuration of the polarization that have been proposed to explain the anomalous attenuation seen in the TSDW phase for itq⊥ itQ.

Magnetic excitations in chromium

We have investigated the magnetic excitations of the spin-density wave in pure chromium using inelastic neutron scattering. We have observed spin-wave modes originating from the incommensurate magnetic Bragg satellite reflections. A new additional excitation is observed with a commensurate wave vector and is interpreted as a feature specific to the transverse spin-density wave. We also present scattering data near and above the N\'eel point ${T}_{N}$. While we do not observe a divergence of the susceptibility at the incommensurate satellite positions for $T$ approaching ${T}_{N}$, there is a large increase in the diffuse scattering throughout a broad region of the reciprocal lattice near (0,0,1). We suggest connections between these data and the theory of itinerant magnetism developed by Moriya and Kawabata.

InclusiveK *+(892) andK *0(892) production inK + p interactions at 32 GeV/c

Total and semi-inclusive cross sections, longitudinal and transverse momentum distributions and spin density matrix elements of theK*+(892) andK*0(892) produced in the inclusive reactionsK+p→K*+(892)+X andK+p→K*0(892)+X at 32 GeV/c are studied in detail. The inclusive spectra of theK*(892) and their decay products are compared with pion and neutral kaon production. TheK*+(892) andK*+(892) are dominantly produced by kaon fragmentation processes. The dependence of average transverse momentum vs.x for resonances has been investigated for the first time.

Effect of random anisotropy on magnetic structure, the phase transition and the spectrum of planar antiferromagnets

The effect of random anisotropy on the magnetic structure (the pattern of directions of the antiferromagnetic vector) is studied in planar antiferromagnets. Large uniform domains can appear only due to random anisotropy fields with a correlation radius exceeding a domain wall dimension. The fields with a smaller correlation radius can generate only an amorphous structure with the antiferromagnetic vector varying continuously everywhere in the crystal volume. It is shown how parameters of magnetic structures may be extracted from experimental data of magnetic torque and broadening of AFMR linewidths. The data for AuMn, CsMnF3 and Cr are discussed. Interpretation of magnetic torque experiments in the transverse spin density wave phase of the antiferromagnetic Cr increases the scale (the polarisation domain size) of the magnetic structure 30 times compared to the scale obtained from thermal activation theory. Because of random anisotropy of random fields, an intermediate 'phase' appears in the process of the phase transition near the critical temperature of the perfect crystal.