Ferromagnetic Resonance Line Research Articles

Angle dependent inverse spin Hall effect in NiFe/Pt thin film

In NiFe/Pt bilayer, when spin current originating from the magnetization procession of NiFe is inject into the adjacent Pt layer under ferromagnetic resonance (FMR), the direct current (DC) voltage VISHE generated by inverse spin Hall effect (ISHE) will be added to the voltage VSRE generated by spin rectification effect (SRE), therefore the measured voltage in experiment is the sum of VISHE and VSRE. It is crucial to separate these contributions, which has been often overlooked before, in order to make a reasonable comparison of the ISHE among different materials. The voltages having symmetric (Lorentz type) and anti-symmetric (dispersive type) components both vary with the static magnetic field strength. However, they have different static magnetic field angle dependences according to our theoretical analysis. In order to distinguish the contribution of ISHE from that of SRE, in this paper, we employ a method, in which the voltage across the sample is measured when the static magnetic field is applied to different directions, to analyze the voltage by varying magnetic field angle in a range from 0° to 360° in steps of 10°, thereby separating the VISHE. The separation is carried out by fitting the angle dependent symmetric and anti-symmetric curves to different theoretical formulas of ISHE and SRE. The voltages of the two different contributions together with the phase angle of the microwave are obtained. At the same time, the FMR line width and the resonant field can be read out. The results show that the ferromagnetic resonance line width in NiFe(20 nm)/Pt(10 nm) sample is larger than that in NiFe(20 nm) sample due to the injection of spin current from NiFe to Pt in the bi-layer sample. We notice that in the curves of voltage vs. static magnetic field, the Lorentz symmetry components of the voltage from the bi-layer sample weight more than those from the single-layer sample. This is explained as a result of the existence of the ISHE in the bi-layer sample, where the spins are pumped from the magnetic layer to the adjacent nonmagnetic layer. The spin pumping effect does not show up in the single-layer sample. There are a large portion of symmetric components in the double layer sample, which is attributed to the ISHE. Although the voltage caused by the SRE is smaller than that by the ISHE, the SRE voltage cannot be ignored. Our work is crucial to understanding the spin-related effects in ferromagnetic/nonmagnetic metal material and provides an improved analysis method to study the spin pumping and the ISHE.

Ferromagnetic Resonance Study of Magnetic Anisotropy in Epitaxial Fe<sub>3</sub>O<sub>4</sub> Thin Film

Ferromagnetic resonance (FMR) is used to investigate the magnetic anisotropy of a 25.5 nm thin Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> film above and below its Verwey transition temperature (TV). Epitaxial smooth film of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> is deposited by oxygen-assisted molecular beam epitaxy. FMR spectra are recorded using a 9.49 GHz Brucker EMX system from 80 K to room temperature with a magnetic field applied along [100] direction. The resonance field (Hr) and linewidth (AH) are extracted by fitting the FMR line with a Lorentzian function. Temperature dependence of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> and ΔH shows a transition at T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</sub> . Furthermore, the data of in-plane angular dependence of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> at 300 and 83 K indicated a change in symmetry from fourfold to twofold below TV, consistent with a transformation of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> from cubic (fourfold) to monoclinic (twofold) structure with its easy axis switching from [110] to [100]. Complex symmetry pattern is found in angular dependence of ΔH below TV, which is attributed to the in-plane twinning of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> lattice.

Crystal Structure and Magnetic Properties of Mn-Doped Zn-Ferrite Nanoparticles

The Mn x Zn 1-x Fe 2 O 4 nanoparticles with (x = 0, 0.25) , and 0.5 were synthesized by a chemical coprecipitation method. The X-ray diffraction (XRD) patterns were well fitted with single-phase spinel ferrite structure using Rietveld analysis as Fd -3 m space group. The average crystallite size of the nanoparticles estimated from XRD data analysis varies between 4 and 7 nm. The Mossbauer spectra of the nanoparticles at room temperature were fitted with a well-defined single quadrupole splitting for doublet pattern. The magnetization versus magnetic field data at 5 K show hysteresis loops indicating ferromagnetic (FM) cluster behavior. The superparamagnetic nature of the samples is evident. The temperature-dependent magnetization plots in zero field cooled and field cooled mode show increase in blocking temperature from 31 to 115 K with increase in Mn content from 0 to 0.5. The FM resonance (FMR) spectra indicate slight decrease in FM interactions as x increases from 0 to 0.25 followed by a significant increase with further increase in x value to 0.5. The decrease in resonance field ((H r ) and increase in peak-to-peak linewidth ΔH PP of the FMR line with decreasing temperature indicates weak antiferromagnetic coupling between the octahedral and tetrahedral sublattices of the spinel ferrite. The replacement of Zn by 50% Mn in Zn-ferrite results in an increase in FM interactions with no significant change in the crystallite size.

FMR evidence of ferromagnetic correlations at zigzag edge states in single-layer graphene

Magnetic properties of zigzag edges of single-layer graphene were investigated by ferromagnetic resonance (FMR)/electron spin resonance (ESR) technique. We observed the FMR lines in single-layer graphene, which provide the most direct experimental proof of the theoretical prediction of ferromagnetic correlations at zigzag edges. To explain the observed temperature dependence of the FMR lines we analyzed the magnetic spin correlation length, which is responsible for magnetic ordering in these quasi-1D ferromagnetic nanodomains formed by zigzag edges. ESR spectra of conduction electrons were also studied.

Study of the structural and magnetic characteristics of epitaxial Fe3Si/Si(111) films

The results of the structural and magnetic studies of the epitaxial structure prepared during the simultaneous evaporation from two iron and silicon sources on an atomically pure Si(111)7 × 7 surface at a substrate temperature of 150°C have been presented. The epitaxial structure has been identified as a single-crystal Fe3Si silicide film with the orientation Si[111]‖Fe3Si[111] using methods of the X-ray structural analysis, transmission electron microscopy, and reflection high-energy electron diffraction. It has been established that the epitaxial Fe3Si film at room temperature has magnetic uniaxial anisotropy (Ha = 26 Oe) and a relatively narrow uniform ferromagnetic resonance line (ΔH = 11.57 Oe) measured at a pump frequency of 2.274 GHz.

Microwave resonant and zero-field absorption study of doped magnetite prepared by a co-precipitation method.

Fe3O4 and ZnxFe3−xO4 pure and doped magnetite magnetic nanoparticles (NPs) were prepared in aqueous solution (Series A) or in a water-ethyl alcohol mixture (Series B) by the co-precipitation method. Only one ferromagnetic resonance line was observed in all cases under consideration indicating that the materials are magnetically uniform. The shortfall in the resonance fields from 3.27 kOe (for the frequency of 9.5 GHz) expected for spheres can be understood taking into account the dipolar forces, magnetoelasticity, or magnetocrystalline anisotropy. All samples show non-zero low field absorption. For Series A samples the grain size decreases with an increase of the Zn content. In this case zero field absorption does not correlate with the changes of the grain size. For Series B samples the grain size and zero field absorption behavior correlate with each other. The highest zero-field absorption corresponded to 0.2 zinc concentration in both A and B series. High zero-field absorption of Fe3O4 ferrite magnetic NPs can be interesting for biomedical applications.

Temperature induced ferromagnetic resonance frequency change and resonance line broadening of a Fe–Co–Hf–N film with in-plane uniaxial anisotropy – a theoretical and experimental study

A soft ferromagnetic Fe–Co–Hf–N film was produced by reactive r.f. magnetron sputtering, in order to study its high-frequency behaviour by means of frequency domain permeability measurements up to the GHz range. It resulted in the composition Fe33Co43Hf10N14 and exhibits a saturation polarisation Js of around 1.35T. The film is consequently considered as being uniformly magnetised due to an in-plane uniaxial anisotropy of approximately μ0.Hu≈4.5mT after annealing it at 400°C in a static magnetic field for 1h. While heating the film from room temperature to 300°C during the high-frequency measurement procedure a marked ferromagnetic resonance peak shift (maximum of the imaginary part of the frequency-dependent permeability) from 2.35GHz down to 1.84GHz is conspicuous. This is in a very good agreement with the theory established by taking the “real” ferromagnetic resonance formula for ferromagnetic films into account. Simultaneously, the full width at half maximum (FWHM) ΔfFMR of the resonance line, which is a consequence of precession damping of the magnetic moments, clearly increases. This behaviour does not correlate with the ferromagnetic resonance value decrease, and is qualitatively discussed in terms of exchange interaction with the intrinsic spin–lattice relaxation process due to not totally supressed orbital momenta (〈L〉≠0) of Fe2+ and Co2+ or the occupation change of their spectral levels within the induced uniaxial anisotropy field.

An electron spin resonance study of Eu doping effect in La4/3Sr5/3Mn2O7 single crystal

The effect of Eu3+ ion doping in the La sites of single-crystal La4/3Sr5/3Mn2O7 was investigated. Electron spin resonance (ESR) was applied to La4/3Sr5/3Mn2O7 and (La0.8Eu0.2)4/3Sr5/3Mn2O7 single crystals. A phase separation and phase transitions were observed from the ESR spectra data. Between 350 K and 300 K, both paramagnetic resonance (PMR) and anisotropic ferromagnetic resonance (FMR) lines were observed in the ab plane and the c axis direction, suggesting a coexistence of the paramagnetic (PM) phase and the ferromagnetic (FM) phase. The magnetization measurement reveals a spin-glass-like behavior in single-crystal (La0.8Eu0.2)4/3Sr5/3Mn2O7 below the temperature of spin freezing Tf (∼ 29.5 K).

Thin Magnetic Films with Artificial Texture on Substrate: Microwave Properties

Using the scanning spectrometer of ferromagnetic resonance (FMR) the experimental dependences of the resonance field and FMR line width of thin permalloy magnetic films, which were deposited in vacuum on the substrate with an artificial texture, were obtained. The texture was produced by putting parallel grooves using a diamond cutter on glass substrates with period from 5 to 100 μm. It was found that the presence of the texture led to a considerable increase of the resonance field and FMR line width, when the external field was directed orthogonal to the grooves. On the base of a numerical micromagnetic simulation the explanation of the nature of observable in thin magnetic films effects was given.

Magnetic and structural properties of Co2FeAl thin films grown on Si substrate

The correlation between magnetic and structural properties of Co_{2} FeAl (CFA) thin films of different thickness (10 nm<d< 100 nm) grown at room temperature on MgO-buffered Si/SiO2 substrates and annealed at 600\lyxmathsym{\textdegree}C has been studied. XRD measurements revealed an (011) out-of-plane texture growth of the films. The deduced lattice parameter increases with the film thickness. Moreover, pole figures showed no in-plane preferential growth orientation. The magneto-optical Kerr effect hysteresis loops showed the presence of a weak in-plane uniaxial anisotropy with a random easy axis direction. The coercive field measured with an applied field along the easy axis direction and the uniaxial anisotropy field increase linearly with the inverse of the CFA thickness. The microstrip line ferromagnetic resonance measurements for in-plane and perpendicular applied magnetic fields revealed that the effective magnetization and the uniaxial in-palne anisotropy field follow a linear variation versus the inverse CFA thickness. This allows deriving a perpendicular surface anisotropy coefficient of -1.86 erg/cm2

Grain growth, densification, and gyromagnetic properties of LiZnTi ferrites with H3BO3-Bi2O3-SiO2-ZnO glass addition

LiZnTi (Li0.43Zn0.27Ti0.13Fe2.17O4) ferrites doped with 0.35 wt. %–1.5 wt. % H3BO3-Bi2O3-SiO2-ZnO (BBSZ) were synthesized through a low temperature ceramic sintering process. The grain growth of LiZnTi ferrites was discussed by using the liquid phase sintering mechanism. BBSZ promoted grain growth via liquid phase sintering, and the optimum addition of BBSZ could reduce porosity of the sample. Meanwhile, selected parameters including saturation induction (BS), coercivity (HC) and ferromagnetic resonance line width (ΔH) were measured as functions of doping content, and their relationships with ferrite porosity and microstructure were also discussed. The LiZnTi ferrite samples containing x = 0.5, 0.65, and 0.8 sintered at 920 °C, 900 °C, and 880 °C, respectively, exhibited high BS and low ΔH values at 9.3 GHz. The addition of proper content of BBSZ can not only improve BS but also reduce HC and ferromagnetic resonance line width (ΔH) by low temperature (∼900 °C) liquid phase sintering.

FMR study of carbon nanotubes filled with Fe3O4 nanoparticles

Ordered arrays of carbon nanotubes were produced using a non-catalytic chemical vapour deposition. The multiwall nanotubes with an inner diameter of 120nm were grown inside porous alumina templates and then filled with a magnetic colloid of 7.5nm Fe3O4 particles. X-ray diffraction, electron microscopy and ferromagnetic resonance (FMR) were used to characterize structural and magnetic properties of the grown samples. To estimate the filling factor from the angular dependence of the FMR resonance field, we have derived an effective demagnetization factor for our system using the formalism proposed by Skomsky and Dubowik. Angular dependence of the FMR line width allows one to conclude about the non-uniform filling, while temperature dependent measurements reflect typical features of a superparamagnetic resonance.

Ferromagnetic Resonance Studies of Exchange Biased CoO/Fe Bilayer Grown on MgO Substrate

Dynamic and static magnetizations of an exchange biased bilayer system which is constructed as a proximity of a CoO layer on an Fe-layer grown on the (100) oriented MgO substrate by ion beam sputtering technique have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The room-temperature FMR measurements reveal that the Fe layer is epitaxially grown on MgO substrate with four-fold magnetocrystalline anisotropy and the hard magnetization axis of the sample is the [100] crystallographic directions of MgO substrate. We have determined the g-value, effective magnetization, magnetocrystalline anisotropy constants and contributions to FMR linewidth due to the intrinsic Gilbert damping and inhomogeneity of magnetization by using Landau–Lifshitz–Gilbert (LLG) equation. We observed an unusual FMR line shape attributed to impedance switching of resonance cavity and complex component of conductivity of sample system. The low-temperature FMR measurement shows asymmetric hysteretic behavior of resonance field related to magnetic coupling of ferromagnetic and antiferromagnetic layers. From both FMR and VSM measurements between 10–300 K, the magnetocrystalline anisotropy is observed to dominate above blocking temperature, while unidirectional anisotropy is observed to dominate below blocking temperature over internal magnetic anisotropy. FMR spectra have a comparatively small linewidth between 40–100 Oe, which indicates to a high crystallinity of the Fe film. Gilbert constant was calculated as 0.007 from the linewidth fitting of FMR spectra. This small value is a suitable for reducing the critical switching current used in magnetic tunneling junction. Detailed exchange bias studies were carried out for hard and easy axis of the sample in the temperature range of 10–300 K. From both low-temperature FMR and VSM measurements, the blocking temperature of the system was determined as ∼60 K.

Universal Method for Separating Spin Pumping from Spin Rectification Voltage of Ferromagnetic Resonance

We develop a method for universally resolving the important issue of separating spin pumping from spin rectification signals in bilayer spintronics devices. This method is based on the characteristic distinction of spin pumping and spin rectification, as revealed in their different angular and field symmetries. It applies generally for analyzing charge voltages in bilayers induced by the ferromagnetic resonance (FMR), independent of FMR line shape. Hence, it solves the outstanding problem that device-specific microwave properties restrict the universal quantification of the spin Hall angle in bilayer devices via FMR experiments. Furthermore, it paves the way for directly measuring the nonlinear evolution of spin current generated by spin pumping. The spin Hall angle in a Py/Pt bilayer is thereby directly measured as 0.021±0.015 up to a large precession cone angle of about 20°.

Manufacturing garnet ferrite film resonators using the chemical etching technique

A procedure of manufacturing microwave resonators on the basis of epitaxial Y3Fe5O12 and (Y, Sm, Lu, Ca)3(Fe, Ge)5O12 garnet ferrites with thicknesses of ≤5 μm, which are grown on gallium-gadolinium garnet substrates, is described. To obtain circular garnet ferrites—resonators with diameters of 0.5–3.0 mm-via chemical etching in orthophosphoric acid at T = 60–175°C, protecting SiO2, silicon adhesive, and photoresist coatings are used. It is established that the resonators that were manufactured using a protective FP-383 photoresist coating have the narrowest ferromagnetic-resonance line.

Magnetic resonance study of γ-Fe2O3 nanoparticles dressed in oxygen based free radicals

Two composites consisting of γ-Fe2O3 (maghemite) nanoparticles covered by two different oxygen-based free radicals derived from a 4-(methylamino)phenol sulphate and 8-hydroxy-1,3,6-trisulfonic trisodium salt acid were prepared and investigated by the magnetic resonance method in the 4–300 K range. Both composites displayed broad and very intense ferromagnetic resonance (FMR) lines originating from γ-Fe2O3 agglomerated nanoparticles. The FMR spectrum was fitted satisfactorily at each temperature by two Landau-Lifshitz functions reflecting the existence of magnetic anisotropy in the investigated system. The temperature dependence of the obtained FMR parameters (resonance field, linewidth, integrated intensity) was studied and the results were interpreted in terms of magnetic interactions between free radicals and nanoparticle agglomerates. A comparison with previously studied similar systems containing maghemite nanoparticles was made and conclusions about the role of free radicals were drawn.

Common features of low-temperature spin–charge separation and superlattice formation in multiferroic manganites and antiferromagnetic cuprates

The paper reports on new results obtained for tetragonal quasi-2D antiferromagnetic Eu2CuO4 at temperatures below 30–40 K. A set of ferromagnetic resonance lines similar to ones obtained earlier for a number of multiferroic manganites (Sanina et al 2012 J. Phys.: Condens. Matter 24 346002), with similar electric polarization and local conductivity, were measured, i.e., a multiferroic behavior was revealed in Eu2CuO4 at low temperatures. We attribute these observations to the 1D superlattices that are formed due to phase separation and self-organization of charge carriers and occupy a small crystal volume, as in the multiferroic manganites studied earlier. Characteristic features of these superlattices include a spin–charge separation and conductivity in their ferromagnetic layers. It is supposed that superlattices are domain walls separating the volume antiferromagnetic domains. A similarity between the superlattices in a number of crystals having different symmetries and bulk properties suggests that a topological order similar to that of the topological insulator exists in them at low temperatures.

Magnetic relaxation due to spin pumping in thick ferromagnetic films in contact with normal metals

Spin pumping is the most important magnetic relaxation channel in ultrathin ferromagnetic layers in contact with normal metals (NMs). Recent experiments indicate that in thick films of insulating yttrium iron garnet (YIG) there is a large broadening of the ferromagnetic resonance (FMR) lines with deposition of a thin Pt layer which cannot be explained by the known damping processes. Here we present a detailed study of the magnetic relaxation due to spin pumping in bilayers made of a ferromagnetic material (FM) and a NM. Two alternative approaches are used to calculate the transverse and longitudinal relaxation rates used in the Bloch-Bloembergen formulation of damping. In one we consider that the dynamic exchange coupling at the interface transfers magnetic relaxation from the heavily damped conduction electron spins in the NM layer to the magnetization of the FM layer while the other utilizes spin currents and the concept of the spin-mixing conductance at the interface. While in thin FM films, the relaxation rates vary with the inverse of the FM layer thickness; in thick films, they become independent of the thickness because in the FM/NM structure the FMR excitation has a surface mode character. Regardless of the thickness range the longitudinal relaxation rate is twice the transverse rate resulting in damping of the magnetization with constant amplitude characterizing a Gilbert process. The enhanced spin-pumping damping explains the experimental observations in YIG/Pt bilayers.

Determination of the concentration of conduction electrons in Y3Fe5O12 garnet crystals

Ferrites (garnets) are a model object of study and a promising material to be used in magneto-optical devices for data recording and processing and IR modulators. Due to the narrow ferromagnetic resonance line, optical transparency, and high elastic Q factor of ferrite single crystals, they are promising for solid-state microwave, optoelectronic, and computation devices. To ensure the optimal application of ferromagnetic materials, it is necessary to complete the following important task: develop a certification for samples with allowance for the degree of their imperfection caused by the deviation of crystals with garnet structure from stoichiometry, the competitive incorporation of process and dominant impurities, oxygen vacancies, etc.

Distinguishing spin pumping from spin rectification in a Pt/Py bilayer through angle dependent line shape analysis

A pure spin current driven by spin pumping is converted to a DC voltage and detected electrically in a Py/Pt bilayer sample. This DC voltage mixes with a DC voltage produced through spin rectification. The ferromagnetic resonance line shape strongly depends on the microwave magnetic h field distribution. We have systematically studied the line shapes by changing the external magnetic field orientation in plane of a Pt/Py bilayer. A method is demonstrated which allows us to calculate the microwave h field vector distribution, and distinguish spin pumping from spin rectification.