Polycrystalline Fe Films Research Articles

Room-temperature intrinsic and extrinsic damping in polycrystalline Fe thin films

We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping parameters of $\approx$ 0.0024 for Fe films with thicknesses of 4-25 nm, regardless of their microstructural properties. The remarkable invariance with film microstructure strongly suggests that intrinsic Gilbert damping in polycrystalline metals at room temperature is a local property of nanoscale crystal grains, with limited impact from grain boundaries and film roughness. By contrast, the in-plane FMR linewidths of the Fe films exhibit distinct nonlinear frequency dependences, indicating the presence of strong extrinsic damping. To fit our in-plane FMR data, we have used a grain-to-grain two-magnon scattering model with two types of correlation functions aimed at describing the spatial distribution of inhomogeneities in the film. However, neither of the two correlation functions is able to reproduce the experimental data quantitatively with physically reasonable parameters. Our findings advance the fundamental understanding of intrinsic Gilbert damping in structurally disordered films, while demonstrating the need for a deeper examination of how microstructural disorder governs extrinsic damping.

Exchange bias effect in Fe/LaAlO3: An interface induced effect

The present work reports the appearance of the exchange bias (EB) effect at low temperatures in epitaxial Fe thin film deposited on single crystalline LaAlO3(001) substrate. The observed EB effect vanishes around 30 K indicating that it could be due to the formation of antiferromagnetic FeO layer at the interface between Fe and LaAlO3 substrate. The EB in Fe/LaAlO3(001) is further substantiated by comparing the observations with polycrystalline Fe film deposited on silicon substrate, kept adjacent to LaAlO3 substrate during the deposition. Magnetic microstructure measured by Kerr microscopy corroborates the structural data of Fe film i.e., cubic four-fold anisotropy and isotropic nature for Fe film deposited on LaAlO3 and silicon substrates, respectively. Further, low temperature Kerr microscopy results reveal that the magnetization reversal process significantly gets modified due to the induced unidirectional anisotropy as a consequence of FeO layer formation at Fe/LaAlO3 interface.

Experimental and Theoretical In Situ Spectral Magneto-Ellipsometry Study of Layered Ferromagnetic Structures

A method for processing of in situ spectral magneto-ellipsometry data has been developed to analyze planar ferromagnetic nanostructures. A multilayer model containing a ferromagnetic layer with two interfaces, a nonferromagnetic buffer layer, and a nonferromagnetic substrate has been tested within a new approach to the interpretation of magnetic-field-modulated spectral ellipsometric measurements involving the magnetooptical Kerr effect in the transverse configuration. In particular, the effect of the thickness of the ferromagnetic layer on the results of magneto-ellipsometric measurements has been analyzed. The measurements have been performed with polycrystalline Fe films with different thicknesses on a nonferromagnetic SiO2/Si(100) surface. The diagonal and off-diagonal components of the complex dielectric tensor in the spectral range of 1.38–3.45 eV have been determined by processing spectral magneto-ellipsometric data. The results have been compared to the available data obtained by other authors and to the calculation of the dielectric tensor of Fe within the density functional theory.

Electrical and Magnetic Properties of Ultrathin Polycrystalline Fe Films Grown on SiO2/Si(001)

Ultrathin polycrystalline Fe films have been grown on the oxidized surface of a Si(001) substrate. The resistivity and magnetic hysteresis of Fe films have been measured in the range of thickness from 2.5 to 10 nm. Based on the analysis of the data obtained, it is suggested that there is a transition to the structurally continuous film at a thickness of ~6 nm. It is found that Fe grains in this film acquire the preferred (111) orientation during this transition.

Электрические и магнитные свойства ультратонких поликристаллических пленок Fe, выращенных на SiO-=SUB=-2-=/SUB=-/Si(001)

AbstractUltrathin polycrystalline Fe films have been grown on the oxidized surface of a Si(001) substrate. The resistivity and magnetic hysteresis of Fe films have been measured in the range of thickness from 2.5 to 10 nm. Based on the analysis of the data obtained, it is suggested that there is a transition to the structurally continuous film at a thickness of ~6 nm. It is found that Fe grains in this film acquire the preferred (111) orientation during this transition.

Structure of Ultrathin Polycrystalline Iron Films Grown on SiO2/Si(001)

The structure of polycrystalline Fe films grown on an oxidized Si(001) surface at room temperature has been studied by the technique of high-energy electron diffraction. It has been found that the grain orientation in the films depends of the amount of deposited iron. In Fe films less than 5 nm thick, grains have been found to be randomly oriented. Fe films more than 5 nm in thickness exhibit the (111) texture with an axis coinciding with the surface normal. The angular dispersion of the [111] direction in the Fe lattice from the surface normal is ±25°. It has been found that as the Fe films become thicker, the (111) texture changes to the (110) texture.

Epitaxial Fe Thin Films on {100} Y2Ti2O7: Model Interfaces for Nano-Oxide Dispersion Strengthened Steels

Nanostructured Ferritic Alloys, a variant of oxide dispersion strengthened steels, contain a high density of ≈2.5 nm Y-Ti-O nano-oxides (NOs) that provide remarkable irradiation tolerance, enhance recombination of vacancies and self-interstitial irradiation defects, and trap He in fine-scale bubbles at their interfaces. To complement studies of embedded NOs, mesoscopic-scale metal-oxide interfaces were fabricated by electron beam deposition of Fe films on {100} Y2Ti2O7 (YTO) bulk single-crystal substrates. We report, for the first time, the dominant epitaxial orientation relationship (OR) for the polycrystalline Fe film: {110}Fe\\{100}YTO and 〈111〉Fe\\〈110〉YTO. Further, one large grain region had an OR that is also found in embedded NOs: {100}Fe\\{100}YTO and 〈100〉Fe\\〈110〉YTO. HRTEM studies show clean, semicoherent interfaces with misfit dislocation spacings of 0.7 and 1.4 nm, respectively. These observations are important for the development of first principles models of metal-oxide interfaces, and the bilayers themselves will be used to observe the He partitioning between the Fe, YTO, and the corresponding interface.

In situ investigations of magneto-optical properties of thin Fe layers

A thin polycrystalline Fe film on a single-crystal Si substrate with natural SiO2 oxide is obtained by thermal evaporation in ultrahigh vacuum. The magneto-optical properties of the resultant structure are investigated in situ by the methods of spectral ellipsometry. The values of the coercive force for the Fe film are obtained, and the magnetization reversal loop and the energy dependence of the equatorial Kerr effect are constructed. The effectiveness of magnetoellipsometry for in situ analysis of the geometrical and magnetooptical properties of thin ferromagnetic layers is demonstrated.

Scaling of the coercivity with the geometrical parameters in epitaxial Fe antidot arrays

We studied a series of square lattice antidot arrays, with diameter and lattice parameter from hundreds of nanometers to some microns, fabricated using two lithography techniques in epitaxial Fe(001) films. The coercivity increase of each array with respect to its base film can be scaled to a simple geometric parameter, irrespective of the lithography technique employed. Magnetic transmission x-ray microscopy studies, in arrays fabricated on polycrystalline Fe films deposited on silicon nitride membranes, evidenced the propagation of reversed domains from the edges of the arrays, in agreement with the coercivity analysis of the epitaxial arrays and with micromagnetic models.

Magneto-Optical Stokes Polarimetry and Nanostructured Magnetic Materials

Stokes parameters fully characterize the polarization state of light in an experimentally accessible manner. Photoelastic modulator (PEM) based Stokes polarimetry offers a very high sensitivity which is particularly suitable for the investigation of the magneto-optical properties of nanostructured magnetic materials. In this paper, we shall describe a robust methodology recently developed by us that utilizes a dual PEM setup. As an example of its application, we report on the magneto-optical characteristics of focused Ga ion beam patterned Fe films. We have investigated Ga ion irradiation of single-layer polycrystalline Fe films deposited on Si3N4 substrates, which allows us to study the effects of ion implantation with minimum added complications. Complemented by structural and other characterization techniques, the absolute measurement of magneto-optical effects through the determination of Stokes parameters has enabled us to effectively separate the various contributions from film thinning due to sputtering, structural modifications and compositional changes caused by Ga incorporation. A comparison is also made between the magneto-optical behavior of patterned thin films and that of anodic aluminum oxide embedded magnetic nanowire arrays.

Effect of growth parameters on the structure and magnetic properties of thin polycrystalline Fe films fabricated on Si〈1 0 0〉 substrates

This paper deals with the experimental investigation of the structure and magnetic properties of thin polycrystalline Fe films. Two sets of 50±2nm thick Fe films were fabricated on Si〈100〉 substrates with native oxides in place by varying (i) the sputter pressure pAr and (ii) the Fe sputter power PFe. X-ray diffraction (XRD) study revealed that all films grew with strong 〈110〉 texture normal to the film plane. No higher order peaks were observed in any of the films studied. For both film sets, the lattice constant (a) was less than the bulk Fe lattice constant (a0=2.866Å), which suggested the existence of compressive strain in all films. Two regions of homogeneous strain were observed over the range of pAr studied. Magneto-optical Kerr effect (MOKE) measurements showed that all films exhibited magnetically isotropic behaviour. The magnetic properties were observed to be influenced strongly by pAr. The film grown at pAr=4μbar was the most softest (Hs=100±8 kAm−1, Mr/Ms=0.87±0.02) film among all the films studied. The magnetic properties were found to be independent of PFe. The effective saturation magnetostriction constant λeff determined (using the Villari method) was positive (4±1ppm) and observed to vary within the calculated error.

The intrinsic domain wall resistance of Fe films with a periodic domain pattern

The intrinsic domain wall resistance (DWR) of 180° Néel walls in a polycrystalline Fe film is determined by creating a periodic domain pattern, obtained by locally inducing exchange bias. After field cooling, the coercivity is spatially modulated, resulting in periodic 180° domain walls. To determine the intrinsic DWR, a rotating magnetic field is used to reversibly create and annihilate the domain walls. After correcting for the anisotropic magnetoresistance, the extracted DWR is positive.

유리 위에 증착된 다결정 Fe 자성박막의 이차조화파 회전 이방성

유리판 위에 증착된 다결정 Fe 자성박막의 두께에 따른 표면 결정 구조가 Ti : Sapphire 레이저를 사용하여 조사되었다. 비선형 광학 유전율의 결정구조에 따른 대칭성에 대한 계산으로부터 박막 표면은 <TEX>$C_{2v}$</TEX>에 매우 가까운 <TEX>$C_s$</TEX> 구조를 갖음을 보였다. 이러한 광학적 회전 이방성은 임의 방향의 그레인으로 구성된 다결정 구조의 박막에서는 매우 이례적인 일이다. 이차조화파로부터 얻은 이 결과는 (110) bcc 형태로 자란 X선 회절 결과와도 일치하는 것으로 매우 얇은 상태일 때 가지고 있던 박막 표면의 등방 구조가 두께가 증가함에 따라 바뀌면서 bulk 상태의 구조를 점차적으로 갖게 됨을 보여주고 있다. The surface structure of polycrystalline Fe films of various thicknesses on glass substrates have been studied using a Ti: Sapphire laser at 780 nm. We found that the surface structure possesses C<TEX>$_s$</TEX> crystal structure close to <TEX>$C_{2v}$</TEX> through symmetry consideration. We present one-fold intensity profile with one mirror plane in second harmonic (SH) intensity as a proof of <TEX>$C_s$</TEX> symmetry. <TEX>$C_s$</TEX> and <TEX>$C_{2v}$</TEX> surface symmetries usually appear at the (110) surface of a cubic diamond single crystal [1]. Therefore this surface symmetry would be related to bcc (110) growth orientation coinciding with XRD measurement. Further we treated surface normalized SH asymmetry with various thicknesses. The SH asymmetry increases with increasing of film thickness. From these results, it is observed that the surface structure of thin polycrystalline Fe film below 5 nm is almost isotropic, while in the case of the thicker Fe films, surface structure have <TEX>$C_s$</TEX> symmetry structure close to <TEX>$C_{2v}$</TEX>.

Weak-Localization Correction to the Anomalous Hall Effect in Polycrystalline Fe Films

In situ transport measurements have been made on ultrathin (<100 A thick) polycrystalline Fe films as a function of temperature and magnetic field for a wide range of disorder strengths. For sheet resistances Rxx less than approximately 3kOmega, we find a logarithmic temperature dependence of the anomalous Hall conductivity sigmaxy, which is shown for the first time to be due to a universal scale dependent weak-localization correction within the skew-scattering model. For higher sheet resistance, granularity becomes important and the break down of universal behavior becomes manifest as the prefactors of the lnT correction term to sigmaxx and sigmaxy decrease at different rates with increasing disorder.

Study of the water gas shift reaction on Fe in a high temperature proton conducting cell

The water gas shift (WGS) reaction was studied in a double-chamber high temperature proton conducting cell (HTPC). The proton conductor was a strontia–ceria–ytterbia (SCY) disk of the form: SrCe 0.95Yb 0.05O 3−α and the working electrode was a polycrystalline Fe film. The reaction temperature and the inlet partial pressure of CO varied between 823 and 973 K, and between 1.0 and 10.6 kPa, respectively. The inlet partial pressure of steam ( P H 2 O ) was kept constant at 2.3 kPa. An increase in the production of H 2 was observed upon “pumping” protons away from the catalyst surface. The Faradaic efficiency ( Λ) was lower than unity, indicating a sub-Faradaic effect. The highest value of rate enhancement ratio ( ρ) was approximately 3.2, at T = 823 K. The proton transport number (PTN) varied between 0.45 and 1.0. An up to 99% of the produced H 2 was electrochemically separated from the reaction mixture.

Single crystal Fe films grown on Ge (001) substrates by magnetron sputtering

Single crystal Fe films were grown on Ge (001) substrates by using dc magnetron sputtering. It was found that the microstructures and magnetic properties of Fe films on Ge substrates were strongly dependent upon the substrate temperature during the deposition process. There existed a narrow substrate temperature window of 125±25°C for achieving single crystal Fe film on Ge. Lower substrate temperature led to polycrystalline Fe films due to limited mobility of Fe atoms, while higher substrate temperatures resulted in amorphous Fe–Ge alloy due to severe interdiffusion.

Swift heavy ion irradiation of a-Si∕Fe∕c-Si trilayers

Si ∕ Fe ∕ Si trilayers, with 12nm amorphous Si and 45nm polycrystalline Fe films deposited on Si(100) wafers, were irradiated with 350MeV Au26+ ions at fluences of (0.6–11.3)×1014ions∕cm2. The ion-induced modifications of their structural and magnetic properties were characterized by means of Rutherford backscattering, glancing angle x-ray diffractometry, and the magneto-optical Kerr effect. The mixing rate at the upper a-Si∕Fe interface was three times as high as that at the lower Fe∕c-Si interface. A simple formula is proposed, which on the basis of (nuclear) thermal-spike mixing reproduces the observed (electronic) mixing rates. Ion irradiation at a moderate fluence (6.7×1014∕cm2) induced a magnetic anisotropy in the sample, which was magnetically isotropic after deposition. At the highest fluence, full interface mixing occurred and the magnetic anisotropy almost disappeared. The results are compared with those obtained in Fe∕Si and Ni∕Si bilayers ion irradiated in the regimes of nuclear and electronic stopping.

Overlapping XAFS L spectra of 3d metals: a new application of the regularization method

L XAFS spectra of polycrystalline Fe and Cr films are investigated by EXAFS and TEY (Total Electron Yield) techniques. A new method of obtaining local structure information from overlapping L XAFS spectra for 3d metals is proposed. Tikhonov regularization method of solving ill-posed problem is used. In contrast to the conventional methods (Fourier transformation and fitting procedures) this method does not demand any model assumption and any special procedure of deconvolution of L1 –, L2 –, L3 – contributions. The parameters obtained from L XAFS spectra of polycrystalline Fe and Cr films (interatomic distance and coordination number for the first and the second coordination shells) agree with the expected crystallographic values. Up to now the regularization method was applied only to nonoverlapping XAFS spectra.

Brillouin light scattering from collective spin waves in Fe–Al–O granular films

Spin waves (SWs) propagating along the inplane easy and hard directions of highly resistive and soft magnetic Fe 67Al 13O 20 granular films of ∼1 μm in thickness have been studied at room temperature by Brillouin light scattering in magnetic fields of up to 4.0 kOe. The granular films possess an isotropic inplane resistivity of ρ∼150 μΩ cm and exhibit an inplane magnetic anisotropy of H K ∼150 Oe. We also examined SWs of a polycrystalline Fe film of ∼1 μm in thickness. SW spectra observed from the granular films consist of a bulk SW peak on both frequency sides and a surface SW peak only on the positive frequency side in this study, and are very similar to the SW spectra of the Fe film. Also, the SW peak widths of the granular films are found to be comparable to the SW peak widths of the Fe film. Magnetostatic analyses for a uniform medium with an anisotropic exchange coupling and an inplane uniaxial magnetic anisotropy successfully reproduce the SW frequencies as a function of the magnetic field. We found that the SW stiffness constants in the Fe–Al–O granular films are highly anisotropic; the easy-axis constant can be more than 10 times larger than the hard-axis constant.

Increase of magnetic damping in thin polycrystalline Fe films induced by Cu/Fe overlayers

The ferromagnetic resonance properties of thin polycrystalline layers in the sequence Cu/Fe/Cu/Fe/Cu, grown on Si wafers, were studied. Fe grown on Cu can have a very narrow ferromagnetic resonance (FMR) linewidth. Similar structures are of interest for spin transport studies and for giant magnetoresonance applications. The thinner Fe underlayer ranged from 2 to 5 nm, the intermediate Cu spacer thickness from 2 to 10 nm, and the Fe outer layer was 20 nm thick. The increased damping of the thinner Fe layer in this structure, as reflected in the FMR linewidths observed at 9.46 and 33.5 GHz, is similar to that predicted by Berger and recently observed experimentally using epitaxial single crystal Fe films. In addition to confirming the 1/(Fe layer thickness) dependence of the additional damping, the present measurements show a decrease of damping with increasing Cu spacer thickness, implying a short spin transport decay length in our Cu of about 3.5 nm. The linewidth in isolated Cu/Fe/Cu layers also increased with decreasing Fe thickness, as predicted and observed in some other cases. Magnetization data indicate negligible magnetic coupling of the Fe layers except for the smallest interlayer Cu thicknesses used. The linewidths studied increased moderately with cooling to 77 K.