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Magnetic anisotropy in nanoscaled materials probed by ferromagnetic resonance
Abstract and Figures
Ferromagnetic resonance measurements probe the dynamical response of magnetic systems due to an excitation within the microwave regime. Offering high sensitivity and energy resolution in the μeV range of ferromagnetic resonance this technique is well suited for the investigation of magnetic anisotropy in nanoscale systems. Ferromagnetic Resonance experiments give direct and quantitative access to magnetic anisotropy based on an analysis that uses the Landau-Lifshitz equation of motion. This will be demonstrated for the case of ultrathin magnetic 5-20ML thick Fe films on {4×6}GaAs(001) (2D system) which have been grown and measured in situ in ultra high vacuum, magnetic MnAs stripes (1D system) grown on GaAs(001) as well as for arrays of highly monodisperse FePt nanoparticles (quasi 0D system).
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... The in-plane dependences reveal a strong uniaxiality that is superposed on the fourfold anisotropy favoring the in-plane Fig. 3). Our Fe results are consistent with the data of Zakeri et al. 28 In the case of Fe on GaAs(001), the precise microscopic origin of this thickness dependence is still controversial. Generally, magnetoelastic coupling or the unidirectional Fe-As bonds of the topmost Fe monolayer on c(4×4) and 2×4 reconstructions 29 are considered to cause the uniaxiality. ...
... 30 In order to experimentally clarify the effect of strain, we measured the magnetoelastic coupling constant B 2 (see Table II) of the Fe 1−x Si x films with the cantilever-beam magnetometer. B 2 is related to the magnetoelastic anisotropy constant by K MEL 2 = −B 2 33 , as derived in Ref. 28. Using the strain 33 = 0.01213, as determined using XRD in Ref. 12, we obtain K MEL 2 = −26.7 kJ/m 3 . ...
... III, μ 0 M eff includes the additive shape anisotropy Table II. K 2⊥ is reported to be strain induced and thus connected to the magnetoelastic coupling constant B 1 , as discussed in Ref. 28. We assume the alloy composition, the film thickness, and the magnitude of the strain affecting B 1 , both in sign and magnitude. ...
Ferromagnetic resonance studies and magnetoelastic coupling of a set of epitaxial Fe1−xSix films on GaAs(001)c(4×4) are reported. Fe1−xSix alloys form a solid solution for low Si concentrations and atomic ordering at the composition of the Heusler compound Fe3Si. The provided magnetic anisotropy constants are discussed with respect to the growth parameters. The high uniaxial in-plane anisotropy is related to the interface as evidenced by its thickness dependence. The contribution of the magnetoelastic coupling to this uniaxiality is low. For layers grown at 250 ∘C, the formation of a two-phase system is indicated by the dependence of the cubic fourfold anisotropy on the Si concentration. The resonance linewidths are less than 2 mT, thus corroborating high magnetic and crystallographic quality. The angular out-of-plane dependence of the linewidth is explained by a contribution from two-magnon scattering; the unusual symmetry observed in plane is discussed in the framework of the diverse relaxation mechanisms.
... In the past, several theoretical calculations and phenomenological models have been implemented to calculate the effect of magnetic anisotropy field (H K ) or constant (K) on magnetic properties. 29,[52][53][54][55] Such models often incorporate various approximations to extract K, and hence H K , from M(H) loops. Although in most cases the approximations remain valid, an ad hoc incorporation of a model to quantify H K is non-trivial and may lead to erroneous interpretation. ...
Nanostructured magnetic materials with well-defined magnetic anisotropy are very promising as building blocks in spintronic devices that operate at room temperature. Here we demonstrate the epitaxial growth of highly oriented Fe3O4 nanorods on a SrTiO3 substrate by hydrothermal synthesis without the use of a seed layer. The epitaxial nanorods showed biaxial magnetic anisotropy with an order of magnitude difference between the anisotropy field values of the easy and hard axes. Using a combination of conventional magnetometry, transverse susceptibility, magnetic force microscopy (MFM) and magneto-optic Kerr effect (MOKE) measurements, we investigate magnetic behavior such as temperature dependent magnetization and anisotropy, along with room temperature magnetic domain formation and its switching. The interplay of epitaxy and enhanced magnetic anisotropy at room temperature, with respect to randomly oriented powder Fe3O4 nanorods, is discussed. The results obtained identify epitaxial nanorods as useful materials for magnetic data storage and spintronic devices that necessitate tunable anisotropic properties with sharp magnetic switching phenomena.
In this work, we employ the Landau-Lifshitz-Bloch equation to study the ferromagnetic resonance of a magnetic nanoparticle assumed in a mono-domain with uniaxial anisotropy. We consider a constant magnetic field applied perpendicularly to the easy axis of anisotropy. We investigated the temperature effect on the resonance spectrum through the complex magnetic susceptibility tensor. We found a decrease in the resonance frequency as the temperature is incremented and a decrement in the resonance field range when the driven frequency increases.
This Topical Review presents an overview of the recent experimental and theoretical attempts on designing magnonic crystals for operation at different frequencies. The focus is put on the microscopic physical mechanisms involved in the formation of the magnonic band structure, allowed as well as forbidden magnon states in various systems, including ultrathin films, multilayers and artificial magnetic structures. The essential criteria for the formation of magnonic bandgaps in different frequency regimes are explained in connection with the magnon dynamics in such structures. The possibility of designing small-size magnonic crystals for operation at ultrahigh frequencies (terahertz and sub-terahertz regime) is discussed. Recently discovered magnonic crystals based on topological defects and using periodic Dzyaloshinskii--Moriya interaction, are outlined. Different types of magnonics crystals, capable of operation at different frequency regimes, are put within a rather unified picture.
Ferromagnetic resonance (FMR) uses the same equipment as EPR to study strong interactions between spins characterising crystallised ferromagnetic materials. The parameters describing these very anisotropic interactions are deduced from the variation in the position of the resonance line as a function of the direction of the applied magnetic field. This technique is illustrated by experiments performed on several types of nano structured materials: Fe films deposited by epitaxy on GaAs, thin layers of semiconductors doped with Mn²⁺ and ferrofluids. Its applications relate to magnetic recording, spintronics, and biomedical fields.
A detailed lineshape analysis of the ferromagnetic resonance (FMR) spectra taken on pulse electrodeposited nanocrystalline (nc-) Ni sheets (with the average crystallite size, d, varying from 10 nm to 40 nm) at temperatures ranging from 113 K to 325 K yield accurate values for saturation magnetization, Ms(T), Landé splitting factor, g, anisotropy field, Hk(T), resonance field, Hres, and FMR linewidth, ΔHpp(T). Thermally-excited spin-wave (SW) excitations completely account for Ms(T) and the SW description of Ms(T) gives the values for the saturation magnetization and spin-wave stiffness at absolute zero of temperature, i.e., Ms(0) and D0, for nc-Ni samples of different d that are in excellent agreement with the corresponding values deduced previously from an elaborate SW analysis of the bulk magnetization data. While Ms(0) varies with d as Ms(0)∼d-3/2,D0 follows the power law D0∼d4/3. The angular variations of Hres in the ‘in-plane’ as well as ‘out-of-plane’ sample configurations, demonstrate that the main contribution to Hk(T) comes from the cubic magnetocrystalline anisotropy. The exchange-conductivity mechanism describes the observed thermal decline of ΔHpp reasonably well but fails to explain the very large magnitude of ΔHpp at any given temperature. By comparison, the Landau-Lifshitz-Gilbert (LLG) damping gives a much greater contribution to ΔHpp but the LLG contribution is relatively insensitive to temperature.
Strontium ferromolybdate (Sr2FeMoO6–δ
, SFMO) is a material exhibiting promising magnetoresistive properties. We have synthesized SFMO samples out of simple oxides (SrCO3, Fe2O3, and MoO3 or partially
reduced SrFeO3–x
(SFO) and SrMoO4–y
(SMO) precursors. The samples have been experimentally investigated using X-ray diffraction, temperature-dependent magnetization, Mössbauer effect and ferromagnetic resonance measurements. Samples of the
first type contain a high density of defects, especially [FeMo], [MoFe] antisites, and do not exhibit any superstructural ordering of the iron and molybdenum ions (P = 0%). These samples comprise iron cations in a mixed valence state, Fe2+
/
3+,
and are characterized by a higher magnetic inhomogeneity than those synthesized out of precursors. The use of the latter increases the sample density and brings about a growth acceleration, synthesis temperature reduction, as well as the appearance of a superstructural ordering of the Fe3+
and Mo5+ cations with P = 64%. The samples exhibit magnetic anisotropy and consist of nanosize grains. Zero-field-cooling measurements of the temperature dependences of the magnetization reveal a sudden leap of the magnetization at low temperatures (below 23 K) that witnesses
the existence of magnetic regions with a low coercivity, in which a superparamagnetic state exists. The obtained results are important for the optimization of the synthesis technology of SFMO for device applications.
FeNi alloy thin films with different thickness deposited on Indium Tin Oxide (ITO) conductive glass substrates from the electrolytes by electrodeposition method have been studied by magnetic force microscopy (MFM), scanning electron microscopy (SEM) and ferromagnetic resonance (FMR) technique. For these films possessing an in-plane isotropy, the remanence decreases with the increasing of film thickness and the critical thickness that a stripe domain structure emerges is about 116 nm. Characteristic differences of the FMR spectra of different thickness are also observed. The results show that the resonance field at high measured angle increases firstly then decreases with increasing thickness, which may be related to the striped domain structure.
The spin and orbital magnetism of 8nm thick Fe2.8Si1.2 , Fe3Si , and Fe3.2Si0.8 films epitaxially grown on MgO(001) was determined experimentally by ferromagnetic resonance and superconducting quantum interference device magnetometry and theoretically by fully relativistic density functional theory calculations. The experimental average spin (orbital) moment of the stoichiometric Fe3Si [muS(L)av=1.38(0.051)muB] is in reasonable agreement with the theoretical one [muS(L)av=1.75(0.029)muB] . Slight increases (reductions) of the Fe content are experimentally found to increase (decrease) the spin and orbital moments as predicted by theory. The results reveal an important step toward tailoring spin and orbital magnetism in the binary Heusler alloys.
Using ferromagnetic resonance (FMR) technique, we have investigated the temperature dependence and angular dependence of line width and resonance magnetic field of Co nanoparticles capped with novel alkane carboxylic acids of varying chain lengths. The magnetic properties such as blocking temperature and anisotropy sensitively depend on the chain length as evidenced by the temperature dependence of line width. These results indicate that the magnetic properties of these samples are critically governed by the interparticle interactions which are decided by the chain length. The presence of anisotropy even up to very high temperature above the blocking temperature observed in these studies confirms the presence of inter-particle magnetic interactions as well as intra-particle exchange interaction between the core and shell regions as evidenced by our earlier ac susceptibility and transverse susceptibility measurements on similar system.
A full-thickness dependent, from two monolayers to bulk, absolute
determination of the total magnetic moments of Co/Cu(001) films is
carried out in situ with the help of UHV high-Tc SQUID
magnetometer. The total moment is enhanced by 11% compared to the bulk
value of 1.73μB/atom measured in the thick limit. These
values agree perfectly with polarized neutron diffraction measurements
for the bulk and confirm theoretical predictions for the ultrathin
limit.
The dynamical magnetic properties of ultrathin Fe/GaAs(001) films in the thickness range 5–100 Å have been studied by in situ Brillouin light scattering. Measurement of the spin-wave frequency as a function of both the intensity and the in-plane direction of the applied magnetic field, as well as of the incidence angle of light, enabled us to determine the magnetic parameters of the films. A continuous evolution from uniaxial to biaxial in-plane magnetic anisotropy has been observed with increasing thickness, together with a marked increase of the effective magnetization. Remarkably, we also found that the presence of a Cu capping layer, even 1 ML thick, completely suppresses the uniaxial anisotropy in the thinnest samples. This effect is discussed in terms of the possible underlying physical mechanisms.
The effective magnetic anisotropy Keff of chemically disordered Fe70Pt30 particles with a mean diameter of 2.3 nm is shown to be temperature dependent between 50 K and 350 K. From the determination of the blocking temperatures by field-cooled and zero-field-cooled magnetisation measurements and ferromagnetic resonance experiments, that is in two different time windows, we find Keff = (8.4 ± 0.9)×105 J/m3 at 23 K. This is found to be one order of magnitude larger than the bulk material value for the disordered phase. This value is confirmed by quantitative simulations of the experimentally determined zero-field-cooled magnetisation and can be explained by the large contribution of surface anisotropy, small deviations from a spherical shape and the presence of an approximately one monolayer thick iron oxide shell.
We present magnetization isotherms and 9GHz ferromagnetic resonance
(FMR) spectra recorded on powders containing 6nm-diameter nanocrystals,
prepared by a high-temperature organometallic route. Upon lowering the
temperature from 400K towards the blocking temperature,
Tb~50K, deviations from Langevin isotherms and shifts of the
FMR field occur, which for the first time are consistently explained by
introducing a uniaxial anisotropy which, for both CoPt3 and
FePt2, strongly decay with the 6th power of the particle
moments μp(T).
Recent widespread interest in the research and application potential of ferromagnetic films and layers owes much to the fact that many of their magnetic properties differ from those of the bulk materials. One reason for this difference is the reduced dimensionality of the interaction space which gives rise to different anisotropic fields. Furthermore, modern processing techniques enable new materials to be produced, such as sandwiches of layers with alternating magnetic properties.
Thin films of MnAs grown on GaAs(0 0 1) show a self-organized structure of coexisting ferromagnetic α- and paramagnetic β-MnAs stripes in the temperature interval from 10 to 40°C. We quantify the magnetic anisotropies of the α-stripes via ferromagnetic resonance and superconducting quantum interference device magnetometry for samples with thicknesses of 57 and 165 nm. The easy axis of magnetization is found to be located perpendicular to the stripe direction, whereas the direction parallel to the stripes is a hard one. While the intrinsic anisotropies show a bulk-like behavior and explain the direction of the hard axis, the key to understanding the direction of the easy axis is given by the demagnetizing fields due to the stripe formation.
On different As-rich GaAs(001) templates, well characterized by reflectance difference spectroscopy, nucleation and growth of NiAs-type MnAs is investigated in real time by reflection high-energy electron diffraction. Using very high As4/Mn flux ratios and low growth rates, one of the two occurring azimuthal alignments of the (1¯100) orientation can be nearly suppressed even in the nucleation stage, and it vanishes completely with further growth. Annealing is found to be very effective in surface smoothing. In dependence on the As/Mn ratio the MnAs(1¯100) surface develops different reconstructions. This finding is important for further investigations in the growth of double heterostructures. High-resolution transmission electron microscopy of as-grown MnAs/GaAs samples reveals an abrupt interface. The lattice mismatch accommodation is anisotropic with regularly arranged misfit dislocations along the [1¯10] direction and less localized coherency strain in the [110] direction, consistent with a near-coincidence-site lattice model. © 1999 American Vacuum Society.
The magnetic anisotropy of nanometer thin films and of nanosize structures is
discussed. Experimental methods for the quantitative determination of magnetic
anisotropy are described. Magnetocrystalline, shape, and magnetoelastic anisotropy
contributions are reviewed, and recent examples for the non-bulk-like magnetic
anisotropy and of the temperature dependence of both the magnetization and
magnetic anisotropy of nanoscale materials are presented. It is shown that film strain
and its relaxation give rise to film thickness dependent anisotropy, which can be
misinterpreted as a surface anisotropy. The decisive role of the surface anisotropy
for adsorbate-induced spin-reorientation transitions (SRT) is elucidated. The
application of x-ray magnetic circular dichroism (XMCD) for the determination of
magnetic anisotropy of nanosize islands down to the single atom size is presented.