Ferromagnetic Magnons Research Articles

High-field magnons in ferromagnets with random anisotropy axes.

We investigate magnons in ferromagnets with random anisotropy axes. It is assumed the applied field is large in comparison with the exchange and anisotropy fields. A coherent-anisotropy-field approximation is introduced, where the effect of the disorder is represented by a complex, energy-dependent anisotropy field, analogous to the coherent potential in the random-alloy problem. The real part of the anisotropy field represents the shift in the magnon energy, whereas the imaginary part gives the damping. The shift and damping of the uniform mode and the damping rate across the magnon band are calculated for both a uniform and a nonuniform distribution in the cosine of the angle between the anisotropy axis and the applied field.

PRA degenerate d-band complex magnons in metallic ferromagnets at zero and finite temperatures: Inclusion of system specificity and d-electron(hole) induced charge and spin polarity fluctuations

The Barnea-Horwitz (BH) RPA nondegenerate Hubbard model (NDHM) of complex magnons in metallic ferromagnets at zero and finite temperatures is generalized to include ten-fold d-electron(hole) degeneracy using the Moriya-Kemeny-Siegel (MKS) ten-fold degenerate Hubbard model (TDHM). As such, in the RPA, Hartree-Fock (HF) one-electron(hole) regime, metallic ferromagnet specificity is automatically included via the inherent d-bandwidth ( w TB d) and d-bandfilling (⨍ B) dependence of the TDHM electron(hole)-electron(hole) interaction. As such, local HF limit electron(hole) spin and charge polarity fluctuations, via the increased possibility of multiple electron(hole) site occupancy, even in the RPA-HF, are automatically included.

Lowest Fe2+spin excitations in disordered Fe1-xMnxCl2mixtures by microwave absorption at high frequencies

Far-infrared absorption measurements in Fe1-xMnxCl2 are reported in the frequency range 60 to 600 GHz. The low-energy excitations of the Fe2+ spin system have been studied at low temperature over the whole range of concentration. At low Mn2+ concentration, in both antiferromagnetic and ferromagnetic phases, the authors observe a resonance line corresponding to the excitation of antiferromagnetic and ferromagnetic magnons. For x 0.5 the absorption signal becomes very broad.

Saturation of Parallel-Pumped Magnons in the Two-Dimensional Ferromagnets K2CuF4 and (CH3NH3)2CuCl4

Parallel pumping of magnons in the two-dimensional ferromagnets K 2 CuF 4 and (CH 3 NH 3 ) 2 CuCl 4 has been studied at a pumping frequency of 8.91 GHz and T =1.4 K. Both the real and the imaginary components of nonlinear susceptibility, χ' and χ'', of the parallel-pumped magnons with wave number from 0 to the Brillouin-zone-boundary have been measured up to 25 dB in power beyond the threshold for spin-wave instability. The principal mechanism to limit the amplitude of the parallel-pumped magnons is attributed to the nonlinear damping due to an increase of the magnon interaction between the parallel-pumped magnons.

Self-consistent calculation of the surface magnetization profile and the localized magnons in a Heisenberg ferromagnet

Static and dynamic surface properties of a Heisenberg ferromagnet are studied in RPA over the whole temperature range. Magnetization at the 3rd plane and the bulk are taken equal. Self-consistent results for magnetization and surface magnon energies are reported. The bulk drives the surface critical behaviour. Dimensional cross-over appears as the surface becomes decoupled from the bulk.

Nonequilibrium states of magnons in ferromagnet under nonresonance alternating magnetic field

The kinetic equations have been obtained to describe the magnon nonequilibrium states when the ferromagnet is under the parametrically nonresonance effect of the external alternating magnetic field. The changes of magnetization and of the field energy damping are also calculated.

Enhanced Coulomb and exchange interaction crystal fields in the modified Zener model of magnons in metallic ferromagnets

The Bartel treatment of magnons in metallic ferromagnets within the modified Zener model of itinerant-localized (s-electron or hole) exchange is generalized to include ten-fold d-electron (hole) spin-orbital degeneracy. As such one can ab initio include enhanced intra-cell Coulomb repulsive crystal fields and exchange attractive crystal fields. This treatment includes itinerant-itinerant (d-electron or hole) Coulomb and exchange interactions as well as itinerant-localized (d-electron or hole) interactions; Hund's rule coupling is included automatically. Also, ab initio the ferromagnet specificity is included via the d-bandfilling and d-bandwidth inherent in the degenerate theory; the possibility of finite temperatures is also included. Acoustic and optic ferromagnon branches are recalculated and corrected and the position of the Stoner particle-hole continuum boundary is recalculated at zero and finite temperatures.

Three-magnon bound states in the two-dimensional isotropic and anisotropic Heisenberg ferromagnet

The existence is shown of bound complexes of three magnons in a two-dimensional simple cubic Heisenberg ferromagnet. The Heisenberg spin hamiltonian is replaced by Dyson's boson hamiltonian. The spurious features of this hamiltonian are treated explicitly. Using Faddeev's formalism, detailed calculations of the three-magnon bound state energy curves have been performed both for the isotropic and the longitudinally anisotropic ferromagnet. Attention is paid to the possibility of observing these three-magnon bound states experimentally.

Ferromagnetic resonance in FeCl2 and FeBr2

We have used far-i.r. radiations from a HCN laser (336.55 and 310.88 μm) in conjunction with a pulsed magnetic field (0–350 kOe) to observe the ferromagnetic resonance absorption in FeCl2 and FeBr2 monocrystals. The resonant lines were obtained for high magnetic field (⋍ 100 kOe) well above the metamagnetic transition field and correspond to the excitation of ferromagnetic magnons at k = 0. From experimental results we found that the gz spectroscopic tensor component of both FeCl2 and FeBr2 is gz = 3.7 ± 0.05 and that the gap energies of antiferromagnetic magnons in zero field are respectively 16 and 17.3 cm-1 for FeCl2 and FeBr2.

Nonlinear effects involving localized magnon modes in impure ferromagnetic insulators

A simple theoretical treatment for the study of nonlinear processes involving impurities in ferromagnetic insulators is presented. In particular, the possibility of exciting ferromagnetic magnons with very large wave vectors is analyzed theoretically. The proposed excitation mechanism is analogous to the first-order Suhl instability and takes place via nonlinear processes involving localized modes. A rough estimate of the critical power of a far-infrared laser source necessary to attain the instability threshold is given. It is noted that the threshold value is within the power capabilities of present pulsed far-infrared lasers, suggesting that the process is experimentally feasible.

The relation between one-magnon light scattering and the complex magneto-optic effects in YIG

A theory is presented for light scattering from acoustic magnons in a cubic ferromagnet and expressed in terms of the complex magneto-optic effects. The treatment gives a description of the scattered intensity extending into the region of optical absorption. The analysis when applied to YIG shows that the contribution to the scattered intensity from the anti-Hermitian (dissipative) terms in the magneto-optic coupling dominates towards the short-wavelength end of the visible spectrum. The theory is found to relate accurately the magneto-optic data to the scattered intensity measured at four laser wavelengths spanning the visible region.

Parallel Pumping of the Brillouin-Zone-Boundary Magnons in a Two-Dimensional Ferromagnet K2CuF4

There is a very flat branch in the spin-wave spectrum of two-dimensional ferromagnets. Since the energy of the lowest Brillouin-zone-boundary magnon is very small in this case, those magnons can be excited by a conventional parallel pumping experiment. Instability of the zone-boundary magnons in a two-dimensional ferromagnet K 2 CuF 4 is observed and a magnon relaxation time is measured at a pumping frequency of 8.9 GHz. Another feature of this case is the existence of measurable steady state absorption below the instability threshold (subthreshold absorption), because the magnon density of states of the lowest branch is very large. It is usually difficult to observe this absorption in three-dimensional ferromagnets. From the shape of the subthreshold absorption observed at pumping frequencies of 8.9, 35 and 73 GHz, the interlayer exchange field in K 2 CuF 4 is determined to be 126 Oe (or equivalent to 0.017 K). The ratio of the interlayer to the intralayer exchange fields is estimated to be 0.00042.

Parametric excitation of spin waves in ferromagnets via localized magnon modes

The possibility to excite ferromagnetic magnons with very large k is analyzed theoretically. The proposed excitation mechanism is analogous to the first-order Suhl instability and takes place via non-linear processes involving localized modes associated with imputities in ferromagnetic insulators. A rough estimate of the critical power of a far-infrared laser source necessary to attain the instability threshold shows that the processes are experimentally feasible.

Localized Magnons in Ferromagnet with Impurity Spin. III

The energies of the localized magnons around an impurity spin in a linear chain of ferromagnet in the case of S = S '=1/2 were calculated, where S and S ' are the magnitude of the host and impurity spin, respectively. Two types of the localized magnons were obtained; one is the s type which is the energy of the ground state when the impurity spin couples antiferromagnetically with its nearest neighboring host spins, and it is already obtained by Wang and Callen, and the other is the p type. An approximate method to get the ground state for S '≥1 is proposed. The energies of the two localized magnons were calculated by solving approximately the Dyson equation for Green functions. The spin wave method by Ishii, Kanamori and Nakamura is discussed.

Temperature dependence of nuclear resonance frequencies in the two-dimensional ferromagnet K 2CuF 4

The resonance frequencies of 19F, 63Cu and 65Cu nuclei in zero external field show approximately a linear temperature dependence between 1.4 and 4.2°K. This variation is supported by a simple model of magnons in a two-dimensional ferromagnet in the presence of magnetic anisotropy.

Green's-Function Theory of a Damped Boson System (Application to an Interacting Magnon System)

The many-time Green's-function theory for a damped boson system subjected to an external time-dependent excitation is presented. The damping is properly taken into account by means of a generalized form of the Senitzky-Haken theory of a boson system interacting with a heat bath. The theory leads to a hierarchy of equations for the Green's functions of the system, which are averaged over the heat-bath variables and which take into account the damping of the various boson modes. Two examples for a ferromagnetic magnon system coupled by an anharmonic interaction are given.

Theory of Macroscopic Excitations of Magnons

The excitation of ferromagnetic magnons by a macroscopic time-varying magnetic field is analyzed quantum mechanically. The quantization of the spin excitations is made by the method of Holstein and Primakoff, generalized for the case of a general nonuniform static field. Both linear and nonlinear excitation mechanisms are considered, with the driving field being either perpendicular or parallel to the static field. Particular attention is given to the coherence properties of the magnon states generated.

Cross Section for "Ultracold" Neutrons I. Theory of Magnetic Scattering

Abstract Very cold neutrons (typical wavelengths are about 102 Å) are available by recently developed techniques 2. The features of the scattering of these “ultracold” neutrons are investigated in the present paper discussing a few examples of magnetic scattering. The diffraction of neutrons by the vortex lattice in superconductors is treated as an example of elastic scattering, while in most other cases the scattering is inelastic. We discuss the scattering by ferromagnetic magnons at low temperatures and the critical scattering from an isotropic ferromagnet above the critical tempera­ture. Simple models are used to derive the total cross sections. It is shown that relevant information about the inealsticity can be gained by the measurement of total cross sections in these cases.

Tunneling Spectroscopy in Ferromagnetic Semiconductors

The observation of zero-bias anomalies in tunneling has received much attention but their interpretation has suffered from a lack of definition of the character of the tunneling barrier. Junctions formed from a metal with Eu-chalcogenide ferromagnetic semiconductors offer a potential means of overcoming this difficulty. We have made rectifying junctions of EuS:Eu on In. The results indicate: 1) a large resistivity peak at zero-bias voltage which, in the ferromagnetic region, has a strong magnetic-field dependence, and 2) a resistance maximum near 30 mV which has been tentatively assigned to excitation of collective modes in the bulk of the semiconductor. We interpret the present results by considering excitation of ferromagnetic magnons in the barrier region.

Renormalization of the EffectivegFactor of Magnons

Within the framework of the Heisenberg model we have calculated the renormalization of the effective $g$ factor of magnons in cubic ferromagnets and antiferromagnets. We find a $f(k){T}^{\frac{3}{2}}$ enhancement of $g$ in ferromagnets, where $f(k)\ensuremath{\sim}{k}^{2}$ for small $k$, and a $\ensuremath{-}\frac{1}{2}\frac{{\ensuremath{\chi}}_{\ensuremath{\parallel}}}{{\ensuremath{\chi}}_{\ensuremath{\perp}}}$ depression of $g$ for all antiferromagnetic magnons. We estimate that this effect will have experimental consequences of 10% or more at reasonable temperatures.