From Ferromagnetic Resonance Research Articles

Growth temperature-controlled Gilbert damping and anisotropies in PLD grown epitaxial Co2FeSi Heusler alloy thin film

The utilization of spin rather than charge as a functional aspect in the electronic devices will result in an energy-efficient, thermally stable, and fast operation. Cobalt-based Heusler alloy Co2FeSi (CFS) offers excellent possibilities due to its high curie temperature, large spin polarization, and low Gilbert damping (α). Small damping makes the alloy fascinating for spin transfer torque magnetoresistive random-access memory (STT-MRAM) devices where a low α causes a low value of switching current. The present investigation explored the effect of deposition temperature on the chemical, structural and magnetic properties of the pulsed laser deposition grown single-crystal CFS film of thickness 50 nm on a lattice-matched (001) MgO substrate. The XRD data suggest that Co site atomic ordering and overall film crystallinity improved with temperature, resulting in a minimal value of α obtained from ferromagnetic resonance (FMR) experiment in the film grown at 600°C. The azimuthal angular variation (φ) of FMR field data gives a fourfold anisotropy pattern that confirms the single-crystal CFS film epitaxially grown on the MgO substrate. Two in-plane anisotropies, namely cubic (four-fold) and uniaxial, were obtained in all the samples. This investigation is crucial in the fabrication of fast spintronics based memory and processing devices.

Control of site occupancy by variation of the Zn and Al content in NiZnAl ferrite epitaxial films with low magnetic damping

The structural and magnetic properties of Zn/Al doped nickel ferrite thin films can be adjusted by changing the Zn and Al content. The films are epitaxially grown by reactive magnetron sputtering using a triple cluster system to sputter simultaneously from three different targets. Upon the variation of the Zn content the films remain fully strained with similar structural properties, while the magnetic properties are strongly affected. The saturation magnetization and coercivity as well as resonance position and linewidth from ferromagnetic resonance (FMR) measurements are altered depending on the Zn content in the material. The reason for these changes can be elucidated by investigation of the x-ray magnetic circular dichroism spectra to gain site and valence specific information with elemental specificity. Additionally, from a detailed investigation by broadband FMR a minimum in g-factor and linewidth could be found as a function of film thickness. Furthermore, the results from a variation of the Al content using the same set of measurement techniques is given. Other than for Zn, the variation of Al affects the strain and even more pronounced changes to the magnetic properties are apparent.

Asymmetric Interfaces in Epitaxial Off-Stoichiometric Fe3+xSi1−x/Ge/Fe3+xSi1−x Hybrid Structures: Effect on Magnetic and Electric Transport Properties

Three-layer iron-rich Fe3+xSi1−x/Ge/Fe3+xSi1−x (0.2 < x < 0.64) heterostructures on a Si(111) surface with Ge thicknesses of 4 nm and 7 nm were grown by molecular beam epitaxy. Systematic studies of the structural and morphological properties of the synthesized samples have shown that an increase in the Ge thickness causes a prolonged atomic diffusion through the interfaces, which significantly increases the lattice misfits in the Ge/Fe3+xSi1−x heterosystem due to the incorporation of Ge atoms into the Fe3+xSi1−x bottom layer. The resultant lowering of the total free energy caused by the development of the surface roughness results in a transition from an epitaxial to a polycrystalline growth of the upper Fe3+xSi1−x. The average lattice distortion and residual stress of the upper Fe3+xSi1−x were determined by electron diffraction and theoretical calculations to be equivalent to 0.2 GPa for the upper epitaxial layer with a volume misfit of −0.63% compared with a undistorted counterpart. The volume misfit follows the resultant interatomic misfit of |0.42|% with the bottom Ge layer, independently determined by atomic force microscopy. The variation in structural order and morphology significantly changes the magnetic properties of the upper Fe3+xSi1−x layer and leads to a subtle effect on the transport properties of the Ge layer. Both hysteresis loops and FMR spectra differ for the structures with 4 nm and 7 nm Ge layers. The FMR spectra exhibit two distinct absorption lines corresponding to two layers of ferromagnetic Fe3+xSi1−x films. At the same time, a third FMR line appears in the sample with the thicker Ge. The angular dependences of the resonance field of the FMR spectra measured in the plane of the film have a pronounced easy-axis type anisotropy, as well as an anisotropy corresponding to the cubic crystal symmetry of Fe3+xSi1−x, which implies the epitaxial orientation relationship of Fe3+xSi1−x (111)[0−11] || Ge(111)[1−10] || Fe3+xSi1−x (111)[0−11] || Si(111)[1−10]. Calculated from ferromagnetic resonance (FMR) data saturation magnetization exceeds 1000 kA/m. The temperature dependence of the electrical resistivity of a Ge layer with thicknesses of 4 nm and 7 nm is of semiconducting type, which is, however, determined by different transport mechanisms.

Structural and magnetic properties of NiFe2O4 thin films grown on isostructural lattice-matched substrates

Nickel ferrite (NiFe2O4) thin films are grown epitaxially on two different isostructural substrates, (100)-oriented MgGa2O4 and ZnGa2O4, using pulsed laser deposition. These spinel substrates have a lattice mismatch of 0.62% and 0.04%, respectively, with NiFe2O4 crystal. While the films grown on MgGa2O4 substrates exhibit significant strain resulting in a tetragonal distortion of the crystal structure, the films on ZnGa2O4 substrate are essentially strain-free and retain their cubic structure because of the near-perfect lattice match. Magnetometry data suggest that film strain is the principal factor determining the anisotropy of these NiFe2O4 films. This is also confirmed by the effective magnetization values obtained from ferromagnetic resonance (FMR) measurements. While there is only a modest decrease in effective Gilbert damping constant with strain reduction, an enhancement in the spin voltage is observed in the spin Seebeck effect (SSE) measurements for NiFe2O4 films grown on ZnGa2O4 substrate with thickness ≤200 nm.

Model of Magnetic Damping and Anisotropy at Elevated Temperatures: Application to Granular FePt Films

Understanding the damping mechanism in finite size systems and its dependence on temperature is a critical step in the development of magnetic nanotechnologies. In this work, nano-sized materials are modeled via atomistic spin dynamics, the damping parameter being extracted from Ferromagnetic Resonance (FMR) simulations applied for FePt systems, generally used for heat-assisted magnetic recording media (HAMR). We find that the damping increases rapidly close to Tc and the effect is enhanced with decreasing system size, which is ascribed to scattering at the grain boundaries. Additionally, FMR methods provide the temperature dependence of both damping and the anisotropy, important for the development of HAMR. Semi-analytical calculations show that, in the presence of a grain size distribution, the FMR linewidth can decrease close to the Curie temperature due to a loss of inhomogeneous line broadening. Although FePt has been used in this study, the results presented in the current work are general and valid for any ferromagnetic material.

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation. In this study, three Fe film samples with thicknesses of 50 monolayers (ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used. The magnetic anisotropy constant obtained from ferromagnetic resonance (FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy (MCA) at the normal deposition angle with larger Fe thickness. However, the in-plane uniaxial magnetic anisotropy (UMA) is induced by a higher oblique deposition angle and a smaller thickness. Its hard axis lies between the [100] and [010] directions. The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA. Due to MCA, when the magnetic field is weaker than the saturated field, the magnetization direction does not always align with the external field. The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07. Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) systems. Thus, a vector network analyzer ferromagnetic resonance (VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.

Stress-induced magnetic properties of PLD-grown high-quality ultrathin YIG films

Yttrium iron garnet (YIG:Y3Fe5O12) thin films were grown on (111) gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates using pulsed-laser deposition under several different deposition and annealing conditions. X-ray diffraction measurements revealed that the crystallographical orientation of the YIG films is pseudomorphic to and the same as that of the GGG substrate, with a slight rhombohedral distortion along the surface normal. Furthermore, X-ray reciprocal space mapping evidenced that in-situ annealed YIG films during film growth are under compressive strain, whereas ex-situ annealed films have two different regions under compressive and tensile strain. The saturation magnetization (4πMS) of the films was found to vary, according to the deposition conditions, within the range of 1350 to 1740 G, with a very low coercivity of HC &amp;lt; 5 Oe. From ferromagnetic resonance (FMR) measurements, we estimated the effective saturation magnetization (4πMeff) to be 1810 to 2530 G, which are larger than that of single crystalline bulk YIG (∼1750 G). Such high values of 4πMeff are attributable to the negative anisotropy field (HU) that increases in size with increasing compressive in-plane strain induced in YIG films. The damping constant (αG) of the grown YIG films was found to be quite sensitive to the strain employed. The lowest value of αG obtained was 2.8 × 10−4 for the case of negligible strain. These results suggest a means of tailoring HU and αG in the grown YIG films by the engineering of strain for applications in spintronics and magneto-optical devices.

Ferromagnetic Relaxation and Magnetic Properties of Co40Fe40B20 Thin Films

The Co40Fe40B20 thin films with varying thickness using silicon dioxide (SiO2) as substrate were fabricated by magnetron sputtering system (ULVAC, Japan) with a base pressure P = 10−8 Torr to understand magnetodynamic interactions and structural properties. The samples were characterized using X-band ferromagnetic resonance experiments at room temperature. The XRD pattern shows that the thin film is amorphous and we got a strong peak (400) of substrate silicon dioxide (SiO2). The SEM result of Co40Fe40B20 thin film demonstrates that it has uniform and homogeneous morphology. It has been observed from ferromagnetic resonance (FMR) results that the thin film depicts anisotropic behavior and easy axis lies out of the plane. The dependence of the resonance fields on the angle between the normal of the film and out of plane dc magnetic field indicates the presence of uniaxial magnetic anisotropy associated with thickness of Co40Fe40B20 thin films. The inplane and out-of-plane angular dependences of the resonance field (HR) and line width (ΔHpp) of FMR spectra were measured and explained using the Landau–Lifshitz–Gilbert equation. The origin of magnetic damping has been discussed by considering spin–orbit, s–d interactions, and two magnonscattering mechanism. Some background molecular vibrations at wave number 2000 and 3800 cm−1 in three samples are also identified from FTIR, which further show that background molecular vibrations can be reduced by decreasing the size. This study will be helpful to understand spin transfer torque (STT) and the timescale for magnetization reversal in the spintronic devices

Directional change of magnetic easy axis of arrays of cobalt nanowires: Role of non-dipolar magnetostatic interaction

Abstract Room temperature magnetization of two dimensional (2D) arrays of cobalt nanowires (NWs) having diameter 50 and 150 nm prepared by electrodeposition are studied in details. Diffraction patterns of the NWs reveal that the crystallites of the NWs become more textured on decreasing their diameter. Magnetic hysteresis loop measurements show the magnetic easy axis changes its direction from axial to perpendicular direction of NWs on increasing the length of the NWs. The magnetostatic interaction among the NWs, known as the key factor in defining the easy direction is found not to be dipolar at all the circumstances. An aspect ratio (length/diameter of NWs) dependence of the non-dipolar interaction in 150 nm NWs is evident from the static magnetization as well as from ferromagnetic resonance (FMR) measurements.

Characterization of a Y-type hexagonal ferrite-based frequency tunable microwave absorber

A Y-type hexaferrite rod with the composition of Ba2Co1.8Cu0.2Fe12O22 was presented as an absorbing material with high absorbance. Its high absorbance and wide absorption band result from ferromagnetic resonance (FMR) that is self-biased by strong shape and magnetocrystalline anisotropy fields. Around the FMR frequency the specimen of the ferrite rods exhibits very high absorbance and the FMR frequency can be tuned by the rod dimension. In addition to the high absorbance and the wide tunable absorption band, the microwave absorber has another advantage of light weight due to the use of the ferrite rods instead of ferrite slabs.

Low‐field non‐resonant microwave absorption in glass‐coated Co‐rich microwires

AbstractA study of low‐field non‐resonant microwave absorption (LFA) at 9.8 GHz, on as‐cast amorphous Co‐rich CoFeBSi microwires under different measuring geometries is presented. Results confirm that LFA is associated with the magnetization processes from the unmagnetized state (HDC = 0) to the saturated condition, in many aspects similar to Giant Magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA signal showed large variations in its maximum–minimum separation as a function of the measuring geometry, which is interpreted in terms of the total anisotropy in the process. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of magnetic materials by low-field microwave absorption techniques

A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe ⩽ H DC⩽+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA strongly depends on the anisotropy field of the sample. In contrast with FMR (which can be described as the homogeneous precession of spins in the saturated state), LFA can be thought as a spin rotation process occurring during the magnetic saturation. In this work, we present a detailed study of the basic features of LFA in several types of materials: ferrites and amorphous microwires and ribbons; in particular the effects sample shape, temperature up to the Curie transition, the influence of easy axis and the effects of annealings. These examples show that once LFA is fully understood, it can become a powerful characterization tool.

Magnetic properties of nanostructrured (Co/Cu/Co)/Fe/Si multi-layer

Magnetic multi-layer (Cu/Co) 60/Fe film on Si(0 0 1) substrate have been investigated. The magnetic parameters such as saturation magnetization, coercive field, and magnetic anisotropies have been determined from ferromagnetic resonance (FMR) and DC magnetization data.

Magnetic damping in Fe-based films deposited by laser ablation in magnetic field

Magnetic films were deposited on oxidized Si by KrF laser ablation of amorphous ribbons (composition Fe 60 Ni 20 B 20 , Fe 64− x Nb x Co 21 B 15 with x = 0–5 and Fe 73.5− x Ni x Cu 1 Nb 3 Si 13.5 B 9 with x = 0–40). The deposited films are amorphous. Deviation from stoichiometry is low. The magnetization motion damping parameter λ and magnetization of the films were obtained from ferromagnetic resonance (FMR) spectra at 18, 25, 49 and 70 GHz. The lowest λ value obtained is 5.1 × 10 7 rad/s for a Fe 33.5 Ni 40 Nb 3 Cu 1 Si 13.5 B 9 film. Here, the magnetization was only 7.99 kG because of low content of Fe. But high magnetization around 16 kG, combined with acceptable damping parameter ( λ = 9.1–9.8 × 10 7 rad/s), was measured in Fe 60 Ni 20 B 20 films. For the samples deposited in the external magnetic field of 30 mT parallel to the substrate surface an in plane uniaxial anisotropy is observed, which is 5 × 10 −3 erg/cm 3 in the film with the lowest damping parameter. The Fe-rich film Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 shows a much higher anisotropy of 17.9 × 10 −3 erg/cm 3 . Such combinations of properties are appropriate for the application of this kind of films in fast magnetic devices.

Rotatable anisotropy and mixed interfaces: Exchange bias inFe/KNiF3

Results from ferromagnetic resonance experiments made on epitaxially grown $\mathrm{Fe}/{\mathrm{KNiF}}_{3}$ bilayers are presented. The bilayers exhibit exchange bias and coercivity previously studied using low-temperature magnetometry. In this paper, results from ferromagnetic resonance (FMR) experiments on the ${\mathrm{FeKNiF}}_{3}$ bilayers are reported. A rotatable anisotropy is identified, and temperature dependence of all anisotropy fields are examined. A possible correlation between the bias field and the rotatable anisotropy is proposed. Linewidths are also studied and possible correlations between coercive fields and FMR linewidths are discussed. A model for the rotatable anisotropy is proposed which ascribes the existence of exchange bias to small uncompensated regions at the interface and rotatable anisotropy to exchange coupling combined with long-range disorder within the antiferromagnet. A possible realization of this mechanism through exchange-coupled grains is discussed in relation to other models for exchange bias in disordered structures.

Microwave absorption in Ni–Zn ferrites through the Curie transition

Microwave absorption measurements were carried out on Ni 0.35Zn 0.65Fe 2O 4 polycrystalline ferrites, at a constant frequency of 9.4 GHz (X-Band) and dc magnetic fields ( H dc) in the 0–0.8 T range. The measuring temperature was varied from 300 to 500 K. A clear evolution from ferromagnetic resonance (FMR) to paramagnetic resonance (EPR) was observed as a function of temperature which is related with the passage through the Curie point (∼430 K), as observed by thermal variations of magnetic permeability. In addition, a low field ( H dc<0.06 T) absorption signal was observed with the same phase as the FMR absorption. However, in contrast with FMR, this signal exhibited hysteresis by cycling the dc field and disappears with temperature (∼350 K) before the Curie point is reached. The assignment of this signal as due to non-resonant microwave absorption processes by the partially magnetized sample is discussed.

Thermomagnetic properties and spin-wave excitation of the nanocrystalline Fe 68.5Co 5Mo 3Cu 1Si 13.5B 9 alloy

The effect of annealing temperatures on the spin-wave excitations for the nanocrystalline Fe 68.5Co 5Mo 3Cu 1Si 13.5B 9 alloy have been investigated by observation of their low temperature magnetization curves. The temperature dependence of magnetization was measured with a SQUID magnetometer and spectroscopic splitting values g were estimated from ferromagnetic resonance (FMR) experiments. The thermomagnetization curve is found to obey Bloch's law. From these experimental results, spin-wave stiffness constants under annealing conditions were calculated. Moreover, coercive force, initial permeability, ac power loss and magnetostriction of the annealed alloys have been studied systematically.

Determination of surface magnetic profiles of thin Permalloy films using ferromagnetic resonance (abstract)

The motivation for this study arose from experimental evidence of oxidation phenomena of Permalloy films, which causes large variation in their magnetic properties as a function of film thicknesses.1 We report a numerical determination of surface magnetic profiles for films of different thicknesses and explain the modification of these profiles in terms of their oxidation. Magnetic profiles of Permalloy films can be obtained from ferromagnetic resonance (FMR) experiments. Approximate calculated profiles of saturation magnetization, exchange constant, and induced anisotropies can be determined using perpendicular resonance alone. For maximum information and accuracy, both perpendicular and parallel resonance spectra are necessary, giving the most probable magnetic profile of the films. To quantify the magnetic profiles of 50–1000-Å-thick NiFe films, we calculated the ferromagnetic resonance of a model structure using a graded saturation magnetization and exchange constant at the interface. This resonance calculation involves, in its simplest form, one second-order differential equation for perpendicular and two coupled differential equations for parallel polarization.2 The results of these calculations are matched to experimental spin-wave resonance spectra obtained at 9.5 and 34.1 GHz in perpendicular and parallel polarization, respectively. Finally, this FMR numerical solution allows us to solve the ambiguity in the source of the pinning of the magnetic moment in these films. It is due to the graded saturation magnetization at the interface and not to any significant surface anisotropy.

TEM investigation of Co-Cr film microstructure

Transmission electron microscopy (TEM) was used to reconstruct the microstructure of Co-Cr thin films. It was found that the initial layer consists of randomly oriented small crystal grains, that further growth favors well-aligned grains and that microstructural details may be correlated with inferences from ferromagnetic resonance (FMR).

Ferromagnetic resonance in the study of surface demagnetization due to chemisorption

Reduced surface magnetization due to chemisorption is determined from ferromagnetic resonance (FMR) absorption of nickel and iron thin films. Magnetically “dead” or “live” layers, i.e. layers of decreased or enhanced magnetization in comparison with the bulk, can be excluded at free Ni (111) surfaces. Approximately one hydrogen atom eliminates the contribution of one nickel atom to the surface magnetization. Twice the effect is found for CO at low coverages θ < 0.3.