Spin-wave Measurements Research Articles

Probing spin waves in Co3O4 nanoparticles for magnonics applications.

The magnetic properties of spinel nanoparticles can be controlled by synthesizing particles of a specific shape and size. The synthesized nanorods, nanodots and cubic nanoparticles have different crystal planes selectively exposed on the surface. The surface effects on the static magnetic properties are well documented, while their influence on spin waves dispersion is still being debated. Our ability to manipulate spin waves using surface and defect engineering in magnetic nanoparticles is the key to designing magnonic devices. We synthesized cubic and spherical nanoparticles of a classical antiferromagnetic material Co3O4 to study the shape and size effects on their static and dynamic magnetic proprieties. Using a combination of experimental methods, we probed the magnetic and crystal structures of our samples and directly measured spin wave dispersions using inelastic neutron scattering. We found a weak, but unquestionable, increase in exchange interactions for the cubic nanoparticles as compared to spherical nanoparticle and bulk powder reference samples. Interestingly, the exchange interactions in spherical nanoparticles have bulk-like properties, despite a ferromagnetic contribution from canted surface spins.

Phase diagram and spin waves in the frustrated ferro-antiferromagnet SrZnVO(PO4)2

Single crystals of the frustrated S=1/2 ferro-antiferromagnetic proximate square lattice material SrZnZnVO(PO$_4$)$_2$ are studied in magnetometric, calorimetric, neutron diffraction and inelastic neutron scattering experiments. The measured spin wave spectrum reveals a substantial degree of magnetic frustration and a large quantum renormalization of the exchange constants. The H-T magnetic phase diagram is established. It features a novel pre-saturation phase, which appears for only one particular field orientation. The results are discussed noting the similarities and differences with the previously studied and similarly structured Pb$_2$VO(PO$_4$)$_2$ compound.

Evidence for an electromagnon in GdMn2O5 : A multiferroic with a large electric polarization

We report in this paper the dynamical properties of ${\mathrm{GdMn}}_{2}{\mathrm{O}}_{5}$ studied by inelastic neutron scattering and infrared spectroscopy assisted by ab initio calculations. Our work sheds light on the electromagnon, a magnetic mode that can be excited by an electric field. Combining spin-wave measurements, simulations, and ab initio calculations of the single-ion anisotropies and the superexchange interactions, we describe in detail the magnetic contribution to this mode. An exhaustive study of the temperature and polarization dependence of its electroactivity completes this comprehensive study of the complex ${\mathrm{GdMn}}_{2}{\mathrm{O}}_{5}$ system.

Multiphase magnetism in Yb2Ti2O7

We use neutron scattering to show that ferromagnetism and antiferromagnetism coexist in the low T state of the pyrochlore quantum magnet [Formula: see text] While magnetic Bragg peaks evidence long-range static ferromagnetic order, inelastic scattering shows that short-range correlated antiferromagnetism is also present. Small-angle neutron scattering provides direct evidence for mesoscale magnetic structure that we associate with metastable antiferromagnetism. Classical Monte Carlo simulations based on exchange interactions inferred from [Formula: see text]-oriented high-field spin wave measurements confirm that antiferromagnetism is metastable within the otherwise ferromagnetic ground state. The apparent lack of coherent spin wave excitations and strong sensitivity to quenched disorder characterizing [Formula: see text] is a consequence of this multiphase magnetism.

Three-dimensional magnetism and the Dzyaloshinskii-Moriya interaction in S = 3/2 kagome staircase Co3V2O8.

Time-of-flight neutron data reveal spin waves in the ferromagnetic ground state of the kagome staircase material Co3V2O8. While previous work has treated this material as quasi-two-dimensional, we find that an inherently three-dimensional description is needed to describe the spin wave spectrum throughout reciprocal space. Moreover, spin wave branches show gaps that point to an unexpectedly large Dzyaloshinskii-Moriya interaction on the nearest-neighbor bond, with D 1 ≥ J 1/2. A better understanding of the Dzyaloshinskii-Moriya interaction in this material should shed light on the multiferroicity of the related Ni3V2O8. At a higher temperature where Co3V2O8 displays an antiferromagnetic spin density wave structure, there are no well-defined spin wave excitations, with most of the spectral weight observed in broad diffuse scattering centered at the (0, 0.5, 0) antiferromagnetic Bragg peak.

Stripe Discommensuration and Spin Dynamics of Half-Doped Pr_{3/2}Sr_{1/2}NiO_{4}.

We present inelastic neutron scattering measurements of magnetic excitations in stripe ordered Pr_{3/2}Sr_{1/2}NiO_{4} at T∼10 K. For the observed magnetic incommensurability ε=0.4, we have incorporated a stripe discommensuration model in our linear spin wave calculation and obtained best agreement with the measured spin wave dispersion, especially to explain the symmetrical outward shift of the magnetic peaks from Néel ordered zone center in energy range 35 to 45meV. Our study indicates the prerequisite to consider a discommensurated spin stripe unit with proper out-of-plane and in-plane exchange interactions in between Ni^{2+} spins to describe the observed spin wave characteristics in Pr_{3/2}Sr_{1/2}NiO_{4}.

Magnetic structure and spin waves in the frustrated ferro-antiferromagnet Pb2VO(PO4)2.

Single crystal neutron diffraction, inelastic neutron scattering, and electron spin resonance experiments are used to study the magnetic structure and spin waves in Pb2VO(PO4)2, a prototypical layered S = 1/2 ferromagnet with frustrating next-nearest neighbor antiferromagnetic interactions. The observed excitation spectrum is found to be inconsistent with a simple square lattice model previously proposed for this material. At least four distinct exchange coupling constants are required to reproduce the measured spin wave dispersion. The degree of magnetic frustration is correspondingly revised and found to be substantially smaller than in all previous estimates.

Spin dynamics of the magnetocaloric compound MnFe4Si3

The magnetic excitation spectrum of the magnetocaloric compound ${\mathrm{MnFe}}_{4}{\mathrm{Si}}_{3}$ has been investigated by means of polarized and unpolarized inelastic neutron scattering on single crystals. Spectra were collected in the ferromagnetic phase (${T}_{C}\ensuremath{\approx}305\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), as well as in the paramagnetic state, in order to understand the nature of the magnetism in ${\mathrm{MnFe}}_{4}{\mathrm{Si}}_{3}$. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the $q$-dependent linewidths in the paramagnetic state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of ${k}_{B}{T}_{C}$ pointing to a behavior typical of many ferromagnets. In addition, the in- and out-of-plane spin fluctuations are found to be isotropic around ${T}_{C}$ and can be suppressed by a magnetic field of 2 T.

Spin waves in the fcc lattice antiferromagnet: competing interactions, frustration, and instabilities in the Hubbard model

Spin waves in the type-III ordered antiferromagnetic state of the frustrated t-t′ Hubbard model on the face-centred-cubic (fcc) lattice are calculated to investigate finite-U-induced competing interaction and frustration effects on magnetic excitations and instabilities. Particularly strong competing interactions generated due to the interplay of fcc lattice geometry and magnetic order result in significant spin wave softening. The calculated spin wave dispersion is found to be in qualitative agreement with the measured spin wave dispersion in the pyrite mineral MnS2 obtained from inelastic neutron scattering experiments. Instabilities to other magnetic orders (type I, type II, spiral, non-collinear), as signalled by spin wave energies turning negative, are also discussed.

Optical Emission Spectroscopy Study of Competing Phases of Electrons in the Second Landau Level.

Quantum phases of electrons in the filling factor range 2≤ν≤3 are probed by the weak optical emission from the partially populated second Landau level and spin wave measurements. Observations of optical emission include a multiplet of sharp peaks that exhibit a strong filling factor dependence. Spin wave measurements by resonant inelastic light scattering probe breaking of spin rotational invariance and are used to link this optical emission with collective phases of electrons. A remarkably rapid interplay between emission peak intensities manifests phase competition in the second Landau level.

Tunable spin wave properties in [Co/Ni80Fe20]r multilayers with the number of bilayer repetition

Tunability of spin waves in [Co/Ni80Fe20]r multilayer films by varying the number of bilayer repetition (r) is investigated using Brillouin light scattering (BLS) spectroscopy. Conventional magnetometry shows a monotonic increase in the in-plane anisotropy with r. BLS spectra show a number of distinct modes, which were found to be Damon–Eshbach and perpendicular standing spin wave modes confined in several bilayers with different mode numbers by macrospin modelling. The extracted values of the magnetic parameters from the spin wave measurements agree reasonably well with the magnetometry results. A qualitative analysis of the dependence of BLS intensity of PSSW modes on the corresponding thickness has also been performed using a light-scattering theory for confined geometries. The results of the calculations are in agreement with experimental data.

Influence of tantalum underlayer on magnetization dynamics in Ni81Fe19 films

The effect of tantalum (Ta) underlayer is investigated in Ni81Fe19 thin films for magnetization dynamics. The damping parameters extracted from spin wave measurements increase systematically with increasing Ta thickness, whereas the damping parameters from ferromagnetic resonance measurements are found to be weakly dependent on the Ta thickness. The difference is attributed to propagating properties of spin wave and short spin diffusion length in Ta. The group velocity of spin waves is found to be constant for different Ta thicknesses, and nonreciprocity of spin waves is not affected by the Ta thickness. The experimental observations are supported by micromagnetic simulations.

Spin waves and metallic state of magnetoresistive manganites

La1−x(B = Sr,Ca,Ba)xMnO3 manganites are known to undergo upon doping a transition from an insulating ferromagnetic state to a puzzling metallic ferromagnetic phase showing colossal magnetoresistance properties. Revisiting spin wave measurements in this metallic phase, we observe strong similarities with the insulating x ~ 1/8 regime, suggesting that the same physics of small ferromagnetic domains is at play [1, 2]. We argue that all the anomalies in this metallic state could be explained in this inhomogeneous picture, in continuity with the x ~ 1/8 range.

Quantized Spin Waves in the Metallic State of Magnetoresistive Manganites

Spin wave measurements have been carried out in ferromagnetic (F) La(1-x)(Sr,Ca)(x)MnO(3) with x(Sr) = 0.15, 0.175, 0.2, 0.3, and x(Ca) = 0.3. In all q directions, close to the zone boundary, the spin wave spectra consist of several energy levels, with the same values in the metallic and the x approximately 1/8 doping ranges. For x(Sr) = 0.15, the data are quantitatively interpreted in terms of quantized spin waves within 2D ordered F domains or clusters of 4a size. The same picture holds in the metallic state with, however, disordered clusters embedded in a 3D F matrix.

Renormalization of antiferromagnetic spin waves in heavy fermion compounds with [formula omitted

Recent inelastic neutron scattering experiments in CeIn 3 and CePd 2 Si 2 single crystals measured spin wave excitations at low temperatures. These two heavy fermion compounds exhibit antiferromagnetic (AF) long-range order, but a strong competition between the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and Kondo effect is evidenced by their nearly equal Néel and Kondo temperatures. Our aim is to show how magnons such as measured in the AF phase of these Ce compounds, can be described within a microscopic Heisenberg–Kondo model introduced by Iglesias, Lacroix and Coqblin, previously applied to study the non-magnetic phase. The model includes the Ce-4f local moments, assumed in AF order, and Kondo-coupled with the electrons in the conduction band, where we also allow for Hubbard correlation possibly inducing an AF state. We consider competing RKKY (Heisenberg-like J H ) and Kondo ( J K ) AF couplings. Carrying on a series of unitary transformations, we perturbatively derive a second-order effective Hamiltonian which, averaged onto the AF electron ground state, describes the AF spin wave excitations, renormalized by their interaction with correlated itinerant electrons. We numerically study how the different parameters of the model influence the renormalization of the magnons, yielding useful information for the analysis of inelastic neutron scattering (INS) experiments in AF heavy fermion compounds. We also compare our results with the available experimental data, finding good agreement with the spin wave measurements in cubic CeIn 3 .

Fermionic representation for the ferromagnetic Kondo lattice model: Diagrammatic study of spin-charge coupling effects on magnon excitations

A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zone-boundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.

Magnetic properties of polycrystalline [formula omitted] alloys

We have investigated the magnetic properties of the Heusler phase Co 2 Cr 1 - x Fe x Al in the composition regime ( x = 0.3 - 0.5 ) in the disordered B2 phase. Both bulk and surface static and dynamic magnetic aspects were addressed by employing alternating gradient magnetometry (AGM), magneto-optical Kerr effect (MOKE) and Brillouin light scattering (BLS). All samples show ferromagnetic hysteresis loops and a tendency of increasing saturation magnetization M s with the iron content. With BLS the behavior of bulk spin waves and the Damon Eshbach (DE) surface spin wave mode have been studied. The spectra are typical for opaque bulk ferromagnetic samples with strong exchange. The measured spin wave frequencies as a function of magnetic field are in good agreement with the calculated values. Saturation magnetization and gyromagnetic ratio g have been determined from the field-dependent peak positions of the bulk and the DE modes. The g -factor extracted from the DE mode shows a clear tendency of increase with increasing Fe-content. However, we could not find any peculiarities of the alloy with x = 0.4 , which had been proposed as a Heusler phase on the basis of electronic structure calculations [T. Block, C. Felser, G. Jakob, J. Ensling, B. Mühling, P. Gütlich, R.J. Cava, J. Solid State Chem. 176 (2003) 646].

High-wave-vector spin waves in ultrathin Co films on W(110)

We present an experimental study of high-wave-vector spin waves in 8 monolayer (ML) thick hexagonal closed-packed (hcp) Co films performed by spin-polarized electron energy loss spectroscopy (SPEELS). Using inelastic electron scattering, we were able to follow the spin wave dispersion up to the surface Brillouin zone boundary $(\overline{K})$, i.e., up to a wave-vector transfer of $1.64\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$. The spin wave dispersion was found to agree surprisingly well with the dispersion relation of a surface spin wave calculated by a nearest-neighbor Heisenberg model. From this description, we obtain a value for the product of the exchange coupling constant $(J)$ and the magnetic moment $(S)$ of $JS=14.8\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. This value, within the error bar, is identical to our results obtained on thin fcc Co films on Cu(001). We also find that the spin wave features measured by SPEELS at high-wave-vectors are strongly broadened. This is in agreement with expectations from nonadiabatic theoretical descriptions in which the broadening is ascribed to a strong damping of these high-wave-vectors spin waves by Stoner excitations. Similar to the observations in previously studied systems, we also observe a strong dependence of the measured spin wave intensities on the kinetic energy of the incident electrons $({E}_{\mathrm{kin}})$. Highest spin wave intensities were found for low kinetic energies $({E}_{\mathrm{kin}}<10\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$.

Magnetic ordering and spin-liquid state ofYMnO3

We have measured the bulk properties as well as the elastic and inelastic neutron scattering of ${\mathrm{YMnO}}_{3}$ in order to understand the static and dynamic properties of the Mn moments. Our measurements clearly show that above ${T}_{N}$ there are short range correlations between spins at the nearest neighbor and next nearest neighbor Mn sites, which also fluctuate in time. This, together with other bulk properties, demonstrates the presence of a spin liquid phase above ${T}_{N}$ arising from the geometrically frustrated Mn moments. Below ${T}_{N},$ a well-defined spin wave develops and we can understand the experimentally measured spin wave spectrum in terms of a Heisenberg Hamiltonian with a small easy plane anisotropy. However, even in the ordered phase we have found evidence of short range spin correlations.

Spin-wave measurements on MBE-grown zinc-blende structure MnTe by inelastic neutron scattering

Spin-wave excitations of a MBE-grown single crystal of MnTe with a zinc-blende structure have been measured by inelastic neutron scattering in the type-III antiferromagnetic low-temperature phase of this compound. The experimental data have been modeled with a Heisenberg Hamiltonian including isotropic exchanges and magnetic anisotropy. Exchange constants and anisotropy terms have been estimated, which allowed a discussion on the relative importance of distant neighbors. The relevance of anisotropic exchanges is also discussed. The assumption of a collinear spin arrangement provides a consistent description of the spin-wave spectrum, including inelastic-neutron-scattering cross section. The temperature dependence of the spin waves revealed an anomalously strong damping, which starts well below the transition temperature.