Properties Of Ferromagnetic Thin Films Research Articles

Study of the Effect of Inverse Magnetostriction in Ferromagnet/Ferroelectric Heterostructures Using Ab Initio Calculations

Fe/BaTiO3, Fe/SrTiO3, Co/BaTiO3, and Co/SrTiO3 heterostructures, which exhibit magnetoelectric effect, have been investigated. It is shown that the magnetic properties of thin ferromagnetic films can be controlled using an external electric field. The structural, electronic, and magnetic properties of the heterostructures have been investigated applying ab initio calculation methods. It is shown that, using the inverse piezoelectric effect, one can reduce the absolute value of the ferromagnet magnetization vector. This approach may be a basis for controlling the properties of one of the ferromagnetic layers of a superconducting spin valve and, as a consequence, the superconducting properties of the valve.

Magnetic Properties of Amorphous Ta/CoFeB/MgO/Ta Thin Films on Deformable Substrates with Magnetic Field Angle and Tensile Strain.

Recently, the application of cobalt iron boron (CoFeB) thin films in magnetic sensors has been widely studied owing to their high magnetic moment, anisotropy, and stability. However, most of these studies were conducted on rigid silicon substrates. For diverse applications of magnetic and angle sensors, it is important to explore the properties of ferromagnetic thin films grown on nonrigid deformable substrates. In this study, representative deformable substrates (polyimide (PI), polyethylene naphthalate (PEN), and polydimethylsiloxane (PDMS)), which can be bent or stretched, were used to assess the in-plane magnetic field angle-dependent properties of amorphous Ta/CoFeB/MgO/Ta thin films grown on deformable substrates. The effects of substrate roughness, tensile stress, deformable substrate characteristics, and sputtering on magnetic properties, such as the coercive field (Hc), remanence over saturation magnetization (Mr/Ms), and biaxial characteristics, were investigated. This study presents an unconventional foundation for exploring deformable magnetic sensors capable of detecting magnetic field angles.

Investigation of magnetic properties of ferromagnetic thin films with three spin layers using fourth order perturbed Heisenberg Hamiltonian

The Ceylon Journal of Science is a peer-reviewed journal published quarterly by the University of Peradeniya, Sri Lanka in March, June, September and December. It is aimed at publishing high quality research articles on topics related to different disciplines in Science. The journal accepts original research articles, book reviews, reviews and mini-reviews, short communications, opinions, research notes, and commentaries and notes. The Journal has its own website https://site.pdn.ac.lk/cjs/. The journal strictly adheres to publication ethics as emphasized by the Committee on Publication Ethics (COPE). It is indexed in Sri Lanka Journals Online (SLJOL), Directory of Open Access Journals (DOAJ), Google Scholar and Zoological Records. According to the Google Scholar;H5-Index: 12H5-Median: 15According to the Exaly (1970 – 2021);Impact Factor: 0.6 (top 19%)Extended IF: 0.6 (top 19%)H-Index: 8 (top 28%)Citations/paper: 1.42

Effect of external stress anisotropy on the ferromagnetic resonance properties in thin film

Effects of in-plane and out-of-plane external stress anisotropy of ferromagnetic thin film with in-plane and out-of-plane uniaxial anisotropy on the ferromagnetic resonance frequencies have been theoretically investigated by using ferromagnetic resonance method. It is shown that resonance frequency can be enhanced by increasing in-plane external stress anisotropy field and uniaxial anisotropy field, decreasing out-of-plane uniaxial anisotropy field. For out-of-plane external stress case, however, the resonance frequency and two critical fields have no significant change. Also, compared with the case of the in-plane external stress anisotropy, the FMR frequencies are smaller. Additionally, interesting features are obtained for the stress case and compared with the case of without stress. The obtained results provide further insights on the effect of external stress on the properties of ferromagnetic thin film.

Tunable room temperature ferromagnetism in fullerene thin film induced by 1 MeV proton microbeam irradiation

We report tunable ferromagnetic properties of thin films of fullerene upon 1 MeV proton microbeam ion irradiation, by varying the ion fluence. Focused microbeam scanning of 1 MeV proton ions have been performed on the fullerene thin films. Consequently, a stable and significant ferromagnetic ordering at room temperature has been observed in the fullerene thin films. The proton microbeam irradiation induces a maximum magnetic ordering in fullerene with optimum dosage and subsequently higher fluence irradiations yield, diminishing effect upon the observed ferromagnetic ordering due to the higher degree of damages. The X-ray diffraction and Raman analysis confirm the damage due to proton microbeam irradiation. The reduction in the saturation magnetic moment induced, is very sharp as a result of exposure to the larger ion fluence. The approximate distance between defects has been simulated computationally, and its relation with observed ferromagnetism has been established. The irradiation has been performed at moderate ion flux (low microbeam current) to avoid the possible annealing effects of the ion irradiation.

Magnetic properties of ferromagnetic thin films with three spin layers as described by fourth-order perturbed Heisenberg Hamiltonian

The magnetic properties of ferromagnetic thin films with three spin layers were investigated for the first time using fourth-order perturbed Heisenberg Hamiltonian. Total magnetic energy was determined for ferromagnetic materials with simple cubic lattice. The simulation was carried out for Heisenberg Hamiltonian with different second-order magnetic anisotropy constants in different spin layers. The total magnetic energy periodically varies with spin-exchange interaction, azimuthal angle of spin, and the second-order magnetic anisotropy constant in each spin layer. The peaks along the axis of the angle are closely packed except in the 3-D plot of energy versus angle and second-order magnetic anisotropy constant of the middle spin layer. 2-D plots were perceived by rotating 3-D plots in MATLAB. According to the 3-D plot, the angle between magnetic easy and hard directions is 90 degrees. The minimum energy range was found in the 3-D plot of energy versus angle and the second-order anisotropy constant of the middle spin layer. The samples with the minimum magnetic anisotropy energies have potential application in transformer cores, magnetic switching, and magnetic amplifiers.

Theoretical investigation of ferromagnetic resonance in a ferromagnetic thin film with external stress anisotropy

We use the ferromagnetic resonance (FMR) method to study the properties of ferromagnetic thin film, in which external stress anisotropy, fourfold anisotropy and uniaxial anisotropy are considered. The analytical expressions of FMR frequency, linewidth and the imaginary part of magnetic susceptibility are obtained. Our results reveal that the FMR frequency and the imaginary part of magnetic susceptibility are distinctly enhanced, and the frequency linewidth or field linewidth are broadened due to a strong external stress anisotropy field. The hard-axis and easy-axis components of magnetization can be tuned significantly by controlling the intensity and direction of stress and the in-plane uniaxial anisotropy field.

Unidirectional planar Hall voltages induced by surface acoustic waves in ferromagnetic thin films

The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on LiNbO$_3$ substrates are patterned into Hall-bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.

Uniaxial strain modulation of the skyrmion phase transition in ferromagnetic thin films

Mechanical control of magnetic order through strain is an important and promising approach in the development of advanced spintronics devices. Here, using a real-space phase-field model based on Ginzburg-Landau theory, we explore the uniaxial strain modulation of topological phase transition in ferromagnetic thin films. In the phase-field model, a magnetoelastic coupling between magnetization and strain, rather than the strain-induced anisotropic Dzyaloshinskii-Moriya interaction, is introduced to describe the strain-modulated anisotropic deformation and topological phase transition of skyrmions. The phase-field simulations show that a uniaxial tensile strain in the ferromagnetic thin films can not only change the wavelength and propagation direction of the spin spiral in helical phase but also induce topological phase transitions of skyrmions. Under specific magnetic fields, the ferromagnetic thin film exhibits the ferromagnetic-to-skyrmion and skyrmion-to-helical phase transitions serially when the uniaxial tensile strain increases, which are different from those induced by uniaxial stress in bulk ferromagnets. The present work not only provides a magnetoelastic coupling mechanism on the uniaxial strain control of topological magnetic structures but also suggests an effective way to tune the electromagnetic properties of ferromagnetic thin films for a new generation of spintronic devices based on skyrmions.

Influence of modified surface effects on the magnetocaloric properties of ferromagnetic thin films

We have studied the influence of surface effects on the magnetocaloric properties of ferromagnetic thin films. Using the effective field theory, we have computed the refrigerant capacity, isothermal entropy change, and the magnetic phase transition temperature as functions of the film thickness, and surface exchange coupling. We have shown that reduced and enhanced surfaces due to the presence of modified surface exchange couplings may drastically affect the magnetocaloric properties of the system. According to our results, in contrast to the conventional bulk ferromagnetic materials, depending on the film thickness and surface exchange coupling strength, maximal entropy change in thin film systems can be observed at temperatures well below the magnetic phase transition temperature. This observation can be beneficial from view point of technological applications.

Development of in situ magneto-ellipsometry for studying correlation between the optical and magneto-optical properties of ferromagnetic thin films

In this work we present the way of nanostructured films study by means of magneto-ellipsometry. The method of interpretation of in situ magneto-optical ellipsometry spectra from the in situ molecular beam epitaxy setup with an integrated magneto-ellipsometric real time synthesis control is described. The method has been successfully tested on Fe/SiO2/Si nanostructures within the model of a homogeneous semi-infinite medium. As a result, the dielectric tensor components for Fe layer were calculated using a developed approach.

Hidden peculiar magnetic anisotropy at the interface in a ferromagnetic perovskite-oxide heterostructure

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the eg and t2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.

Monte Carlo simulation of magnetic multilayered structures with giant magnetoresistance effects

Description of giant magnetoresistance effects in magnetic multilayered structures with the use of the anisotropic Heisenberg model for determination of magnetic properties of thin ferromagnetic films forming these structures is given. Monte Carlo simulations of magnetic properties for structures, which are constructed from two ferromagnetic films divided by nonmagnetic film, are carried out. The temperature and magnetic field dependencies are considered for ferromagnetic and antiferromagnetic configurations of these structures. The calculation of the magnetoresistance is carried out for different thicknesses of the ferromagnetic films. It was shown, that the obtained temperature dependence for the magnetoresistance is agreed very well with experimental results, measured for the magnetic multilayered structures similar to structures, which are considered in our investigations.

The effects of post-deposition ion-beam bombardment with oxygen on the Co surface in modifying the magnetic properties of Co thin films

Ion beam bombardment is a useful surface modification method in tailoring the microstructure and magnetic properties of ferromagnetic thin films. In this paper, Co thin films were bombarded by Ar+ ion beam with oxygen and then capped with Al (to prevent oxidation). The oxygen content was altered from 0 to 41% to investigate the ion-beam bombardment effect. An oxide layer containing Al2O3, Co3O4 and CoO was formed after the ion-beam bombardment with oxygen, as characterized by x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, and x-ray diffraction. Enhanced coercivities (Hc's) (from 4.14kA/m to 7.96kA/m) and loop squareness (Mr/Ms. from 0.5 to 0.8) was induced by pure Ar+ ion-beam bombardment that resulted in domain wall pinning due to bombardment-induced defects and Co spin reordering. Hc exhibited a strong dependence on the oxygen content in the ion beam, due to the nature of the cobalt oxide formation. Field cooling to 180K resulted in increased squareness (Mr/Ms.=0.9) and reduced Hc. The low-field magnetization measured during zero field and field cooling to further investigate the exchange bias magnetism identified an irreversibility temperature at 320K in a film treated by ion-beam bombardment with an O2/Ar ratio of 21%.

Monte Carlo simulations of dynamic phase transitions in ferromagnetic thin-films

By means of detailed Monte Carlo (MC) simulations, we have presented dynamic phase transition (DPT) properties of ferromagnetic thin-films. Thermal variations of surface, bulk and total dynamical order parameters (DOP) for a film and total order parameter for the films with different thicknesses have been examined. Opposite regimes of reduced exchange interaction (surface to bulk ratio values in R<Rc and R>Rc regimes where Rc is the critical value at which the critical temperature becomes independent of film thickness L) have also been taken into consideration. The average magnetizations with varying temperature of each layer are reversed in these two regimes. Based on the results, we have confirmed that the system represents a crossover behavior between ordinary and extraordinary transition in the presence of surface exchange enhancement.

Monte Carlo simulation of multilayer magnetic structures and calculation of the magnetoresistance coefficient

The Monte Carlo study of three-layer and spin-valve magnetic structures with giant magnetoresistance effects has been performed with the application of the Heisenberg anisotropic model to the description of the magnetic properties of thin ferromagnetic films. The dependences of the magnetic characteristics on the temperature and external magnetic field have been obtained for the ferromagnetic and antiferromagnetic configurations of these structures. A Monte Carlo method for determining the magnetoresistance coefficient has been developed. The magnetoresistance coefficient has been calculated for three-layer and spin-valve magnetic structures at various thicknesses of ferromagnetic films. It has been shown that the calculated temperature dependence of the magnetoresistance coefficient is in good agreement with experimental data obtained for the Fe(001)/Cr(001) multilayer structure and the CFAS/Ag/CFAS/IrMn spin valve based on the Co2FeAl0.5Si0.5 (CFAS) Heusler alloy.

Comparison of the effect of the exchange and single-ion anisotropy on the magnetic properties of thin ferromagnetic films

Using the many-body Green function theory, we caculate the magnetic reorientation temperature and the anisotropic parallel susceptibility of ferromagnetic Heisenberg thin films with the exchange and single-ion anisotropy. Particularly, we compare the effect of these anisotropies on the above mentioned observables. On the basis of our results, one cannot generally claim that these anisotropies are equivalent in the whole rangle of their parameters.

Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

We report on the controlled modification of relaxation parameters and magnetic moments of epitaxial Fe thin films through Dy doping. Ferromagnetic resonance measurements show that an increase of Dy doping from 0.1% to 5% gives a tripling in Gilbert damping, and more importantly a strongly enhanced anisotropic damping that can be qualitatively understood through the slow-relaxing impurity model. X-ray magnetic circular dichroism measurements show a pronounced suppression of the orbital moment of the Fe with Dy doping, leading to an almost threefold drop in the orbital to spin moment ratio, ml/ms. Doping with Dy can therefore be used to control both dynamic and static properties of thin ferromagnetic films for improved performance in spintronics device applications, mediated through the antiferromagnetic interaction of the 4f and 3d states.

Scaling of the thickness dependent anomalous Hall effect in amorphous ferromagnetic thin films

We testified the scaling theory of room temperature anomalous Hall effect in amorphous Co90Zr10 ferromagnetic thin films with the thickness between 5 nm to 600 nm. The experiment results did not meet the theory expectation if we simply put σxy and σxx into a log-log plot, because a negative exponent in σxy = σxxβ appeared for the films thicker than 100 nm. We removed the thickness independent bulk contribution to the scattering, and rebuilt the scaling relation by using only the surface/interface contribution. A crossover from the intrinsic moderately dirty regime, where β = 0.82 to the dirty regime, where β = 1.76, had been observed, which matched previous results and theory expectations. Therefore, it is necessary to remove the bulk contribution in establishing the AHE scaling relation when using the thickness to tune the transportation properties of ferromagnetic thin films.

Modulation of the properties of thin ferromagnetic films with an externally applied electric field in ferromagnetic/piezoelectric/ferromagnetic hybrids

In many cases, technological advances are based on artificial low-dimensional structures of heterogeneous constituents, thus called hybrids, that when come together they provide stand-alone entities that exhibit entirely different properties. Such hybrids are nowadays intensively studied since they are attractive for both basic research and oncoming practical applications. Here, we studied hybrids constituted of piezoelectric (PE) and ferromagnetic (FM) components in the form FM/PE/FM, ultimately aiming to provide means for the controlled modulation of the properties of the FM electrodes, originating from the strain imposed to them by the PE mediator when an electric field is applied. The PE component is in single crystal form, 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT), while the FM outer layers are Cobalt (Co) in thin film form. Detailed magnetization measurements performed under variation of the electric field applied to PMN-PT demonstrated the efficient modulation of the properties of the Co electrodes at low temperature (coercive field modulation up to 27% and saturation magnetization absolute modulation up to 4% at T = 10 K for electric field not exceeding 6 kV/cm). The modulation degree faints upon increase of the temperature, evidencing that the thermal energy eventually dominates all other relevant energy scales. Candidate mechanisms are discussed for the explanation of these experimental observations. The results presented here demonstrate that commercially available materials can result in quantitatively noticeable effects. Thus, such elemental Co/PMN-PT/Co units can be used as a solid basis for the development of devices.