Magnetic Thin Film Structures Research Articles

Influence of annealing temperature on the structural and magnetic properties of FeGaSiB thin films

Understanding the relationship between the structure and magnetic properties of soft magnetic thin films is important in the development of magnetic sensors and electronic devices. This work has systematically studied how the annealing temperature changes the morphology and hence the magnetic properties of 60 nm FeGaSiB thin films. The as-grown and 100 °C annealed films were amorphous/nanocrystalline with soft magnetic properties. As the annealing temperature increased, so did the coercive field of the films, which was found to be due to the formation of polycrystalline grains, with random texture. The grain size increased with increasing annealing temperature, while the Curie Temperature and magnetostriction constant decreased. For the saturation induction, there was a large decrease between the 100 °C annealed film and the 250 °C annealed film, due to the film morphology changing from amorphous to polycrystalline. Thus there is strong correlation between the crystallinity and the soft magnetic behaviour of FeGaSiB films.

Magnetic interference gonioapparent thin film based on metal-dielectric-metal structure for optical security devices

In recent years, magnetic interference thin films have gained wide attention in optical security devices field by virtue of their gonioapparent and dynamic 3D effects. Based on the color mechanism of metal-dielectric Fabry-Perot structure, a novel seven-layer magnetic thin film structure is proposed by adopting the ultrathin metal layer as a bonding layer and a pure metallic Ni layer as a magnetic layer as well as a reflective layer. Color target optimization optimac method is utilized that realizes the seven-layer metal-dielectric optically variable magnetic thin film structure with green at the normal incidence and purple-red at 60°. The structure effectively solves the delaminate problem and simplifies the multilayer structure. Through different combined magnetic field designs, the magnetic orientation experiment of the prepared magnetic optically variable thin film is carried out, and the 3D anti-counterfeiting with remarkable dynamic color change effect is obtained, which provides a new solution for optical security devices.

Finite element micromagnetic simulation of the magnetic domain structures in thin films with uniaxial anisotropy

In this work, finite element micromagnetic simulations were performed in order to study the magnetization reversal mechanisms in thin films with perpendicular magnetocrystalline anisotropy. Magnetization loops are taken both for perpendicular and planar external magnetic fields, at different anisotropy constants and film sizes. Magnetization vortices, stripe domains and magnetic bubbles are ab initio simulated and they are investigated on the basis of the film characteristics.

Effect of in-situ annealing temperature on magnetic domain structure and magnetism of Zr2Co11 thin films

Rhombohedral Zr2Co11-based materials have shown very promising magnetic properties, but this rhombohedral phase is elusive and metastable. In this study, nanocrystalline Zr2Co11 films were obtained by magnetron co-sputtering on Si (100) substrate with the in-situ annealing. Temperature dependence of magnetic domain structure and magnetic properties of nanocrystalline Zr2Co11 have been investigated. Proper in-situ annealing temperature enhances the content of hard magnetic phase and improves the magnetic properties. Compared with Zr2Co11 thin films grown at low temperature (823K) and higher temperatures, thin films with in-situ annealing temperature of 1023K have coercivity Bc (Bc=μ0Hc) of 0.15T and energy product BHmax of 16.0kJ/m3. Magnetic Force Microscopy images show the highest bright-dark magnetic domain contrast with highest root-mean-square values of the phase shift for thin film with in-situ annealing temperature of 1023K. These findings have correlated magnetic domain structure and magnetic properties of Zr2Co11 thin films, and elaborated the effect of different in-situ annealing temperatures on magnetic domain structure.

Effect of defects, magnetocrystalline anisotropy, and shape anisotropy on magnetic structure of iron thin films by magnetic force microscopy

Microstructures of magnetic materials, including defects and crystallographic orientations, are known to strongly influence magnetic domain structures. Measurement techniques such as magnetic force microscopy (MFM) thus allow study of correlations between microstructural and magnetic properties. The present work probes effects of anisotropy and artificial defects on the evolution of domain structure with applied field. Single crystal iron thin films on MgO substrates were milled by Focused Ion Beam (FIB) to create different magnetically isolated squares and rectangles in [110] crystallographic orientations, having their easy axis 45° from the sample edge. To investigate domain wall response on encountering non-magnetic defects, a 150 nm diameter hole was created in the center of some samples. By simultaneously varying crystal orientation and shape, both magnetocrystalline anisotropy and shape anisotropy, as well as their interaction, could be studied. Shape anisotropy was found to be important primarily for the longer edge of rectangular samples, which exaggerated the FIB edge effects and provided nucleation sites for spike domains in non-easy axis oriented samples. Center holes acted as pinning sites for domain walls until large applied magnetic fields. The present studies are aimed at deepening the understanding of the propagation of different types of domain walls in the presence of defects and different crystal orientations.

Interface-driven noncollinear magnetic structure and phase transition of Fe thin films

We investigate the magnetic structure and its temperature dependence of Fe thin films grown on MgO(001) and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(0001) surfaces by means of nuclear resonant x-ray scattering. By fabricating Fe films $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{doped}$ with $^{57}\mathrm{Fe}$ at the interface and middle of the film, depth-resolved analysis of the magnetic structure is performed. On MgO(001), the magnetization is dominantly out-of-plane at the interface, whereas it is mainly in-plane at the middle of the film, indicating that the Fe film has a noncollinear magnetic structure. On ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(0001), on the other hand, the magnetization is mainly in-plane in the entire film. The noncollinear magnetic structure on MgO(001) is confirmed to be energetically stable with the aid of first-principles calculations. We also investigate the temperature dependence of the internal magnetic field in a depth-resolved way. The experimental data suggest the internal magnetic field at the interface is smaller than that of the middle of the film on both substrates, suggesting that the magnetic phase transition starts at the interface at a lower temperature than the entire film.

A magnetometer of weak quasi-stationary and high-frequency fields on resonator microstrip transducers with thin magnetic fields

A high-sensitivity magnetometer for simultaneous measurements of three components of a weak quasi-stationary or high-frequency magnetic-field vector was developed and investigated. Microstrip structures that are based on irregular resonators serve as the magnetometer transducers. An anisotropic thin-film magnetic structure is used as the sensing element. This structure consists of two thin magnetic films that are prepared by magnetron sputtering of a Ni75Fe25 permalloy target and separated by a silicon monoxide layer. It is demonstrated that the transducer exhibits the maximum sensitivity, when the easy magnetization axis of the film structure is orthogonal to the polarization direction of the pumping microwave magnetic field in the microstrip resonator and at an optimal value of a constant magnetic bias field and its optimal deflection from the pumping-field polarization direction which is parallel to it. The magnetometer is characterized by a wide dynamic range of measured magnetic fields, 10–10–10–4 Т, and a wide frequency range, 10–1–105 Hz.

Critical thickness for stripe domain formation in FePt thin films: Dependence on residual stress

Magnetically soft FePt thin films of varying thickness (20 nm ≤ d ≤ 100 nm) were sputter-deposited at different Ar pressures in order to systematically modify the residual stress and hence the magnetic anisotropy. The magnetic domain structure of FePt thin films showed a transition from planar to nearly parallel stripes above a critical thickness, dcr, which was found to depend on an anisotropy contribution perpendicular to the film plane, originated essentially in magnetoelastic effects. A careful structural characterization was made in order to obtain the strain and the stress induced magnetic anisotropy in the samples. Vibrating sample magnetometry and magnetic force microscopy were used to investigate the changes occurring in the magnetic domain structure and the critical thickness of each set of films. Joining together structural and magnetic results, we have been able to construct a phase diagram that divided regions of different domain structures, either by changing the film thickness or the perpendicular magnetic anisotropy. The experimental results could be satisfactorily explained by using a model developed by Murayama. The observed dependence of the magnetic properties of soft FePt thin films on the fabrication conditions opens the possibility to tune the magnetic domain configuration from planar to stripe-like domains by changing the argon sputtering pressure used during film deposition.

Study of the fields scattered by a periodic strip structure of thin magnetic films

Components of the fields scattered by a periodic planar strip structure of thin magnetic films possessing a uniaxial magnetic anisotropy in the plane have been calculated using the phenomenological model. Regularities in the dependence of these fields on the design parameters of the structure have been studied. The results obtained agree with the numerical analysis of the micromagnetic model of this structure. It has been shown that, near the edges of strips magnetized orthogonally to the major axis, the components of the scattered field can exceed the external magnetizing field by a few orders of magnitude. This fact makes it possible to design highly efficient magnetoresistive elements on the basis of a strip structure of magnetic films and thin semiconductor films.

Hard Magnetic Property Improvement of Sputter-Prepared FePd Films on Glass Substrates by Underlayering With Refractory Nb, Mo, and W Elements

Magnetic properties and structure of sputter-prepared FePd thin films on the glass substrates by underlayering with refractory elements M (M = Nb, Mo, and W) have been studied. The structural analysis shows that FePd films have a (111) preferred orientation. All studied FePd films exhibit in-plane magnetic anisotropy. For single-layer FePd films, the coercivity ( $H_{c})$ and magnetic energy product [(BH) $_{\mathrm {max}}]$ are increased with annealing temperature ( $T_{a})$ and reached the maximum value of 1.3 kOe and 3.8 MGOe in the sample annealing at 600 °C. With the further increase of $T_{a}$ , magnetic properties drop rapidly. Dependence of magnetic hardening on $T_{a}$ can be explained by dominant solid reactions in different region of $T_{a}$ including ordering and grain growth. Interestingly, magnetic properties of L10-FePd are largely improved by underlayering with 5 nm-thick Nb, Mo, and W layers. Both the coercivity and energy product are significantly increased to 1.6 kOe and 4.9 MGOe for M = Nb, 2.5 kOe and 6.4 MGOe for M = Mo, and 3.3 kOe and 8.7 MGOe for M = W, respectively. Structural analysis suggests that this magnetic enhancement is related to both higher ordering degree and refined microstructure.

Cell culture arrays using micron-sized ferromagnetic ring-shaped thin films

Cell patterning has become an important technology for tissue engineering. In this research, domain walls are formed at the two ends of a ferromagnetic ring thin film after applying a strong external magnetic field, which can effectively attract magnetically labeled cells and control the position for biological cell. Magnetophoresis experiment was conducted to quantify the magnetic nanoparticle inside the cells. A ring-shaped magnetic thin films array was fabricated through photolithography. It is observed that magnetically labeled cells can be successfully attracted to the two ends of the ring-shaped magnetic thin film structure and more cells were attracted and further attached to the structures. The cells are co-cultured with the structure and kept proliferating; therefore, such ring thin film can be an important candidate for in-vitro biomedical chips or tissue engineering.

Magnetic property improvement of sputter-prepared FePd films on glass substrates with W underlayer

Magnetic properties and structure of sputter-prepared FePd thin films on the glass substrates with W underlayer have been studied. The structural analysis shows that FePd films have a (111) preferred orientation. All studied FePd films exhibit in-plane magnetic anisotropy. In the single layer samples, the coercivity (Hc) and magnetic energy product ((BH)max) are increased with Ta and reached the maximum value of 2.3kOe and 4.3MGOe in the sample annealing at 650°C. With further increase of Ta, magnetic properties drop rapidly. Dependence of magnetic hardening on Ta can be explained by dominant solid reactions in different region of Ta including ordering and grain growth. Interestingly, magnetic properties of L10-FePd are largely improved with the insertion of a 5-nm-thick W underlayer. Hc and (BH)max are significantly increased to 3.9kOe and 8.5MGOe, respectively. Structural results suggest that this magnetic enhancement is related to both higher ordering degree and refined microstructure.

Change in the magnetic structure of (Bi,Sm)FeO3 thin films at the morphotropic phase boundary probed by neutron diffraction

We report on the evolution of the magnetic structure of BiFeO3 thin films grown on SrTiO3 substrates as a function of Sm doping. We determined the magnetic structure using neutron diffraction. We found that as Sm increases, the magnetic structure evolves from a cycloid to a G-type antiferromagnet at the morphotropic phase boundary, where there is a large piezoelectric response due to an electric-field induced structural transition. The occurrence of the magnetic structural transition at the morphotropic phase boundary offers another route towards room temperature multiferroic devices.

Magnetic domain structure in nanocrystalline Ni-Zn-Co spinel ferrite thin films using off-axis electron holography

We report a study of the magnetic domain structure of nanocrystalline thin films of nickel-zinc ferrite. The ferrite films were synthesized using aqueous spin-spray coating at low temperature (∼90 °C) and showed high complex permeability in the GHz range. Electron microscopy and microanalysis revealed that the films consisted of columnar grains with uniform chemical composition. Off-axis electron holography combined with magnetic force microscopy indicated a multi-grain domain structure with in-plane magnetization. The correlation between the magnetic domain morphology and crystal structure is briefly discussed.

Evolution of microwave ferromagnetic resonance with magnetic domain structure in FeCoBSi antidot arrays

Magnetic domain structure of FeCoBSi antidot array thin films of varying thickness were characterized using surface magneto-optic Kerr effect. Vibrating sample magnetometry and microstrip transmission line measurements helped to associate the microwave magnetic analysis of the antidot arrays with hysteresis studies. The domain structure evolution from quasi-continuous domains to strip domains induced by the competing exchange and dipolar interaction resulted in the change of ferromagnetic resonance from multi-band to single-band. Hence, the mechanisms of multi-resonance are proposed to be related to domain wall motion, natural resonance and spin wave modes. This phenomenon can be used to control the magnetization dynamics in spin wave devices.

Optimization of magneto-resistive response of ion-irradiated exchange biased films through zigzag arrangement of magnetization

Exchange coupled ferromagnetic-antiferromagnetic Ni81Fe19/Ir23Mn77 films with a zigzag alignment of magnetization are prepared by local ion irradiation. The anisotropic magneto-resistive behavior of the magnetic thin film structures is correlated to the magnetic structure and modeled. A unique uniaxial field sensitivity along the net magnetization alignment is obtained through the orthogonally modulated and magnetic domain wall stabilized magnetic ground state. Controlling local thin film magnetization distributions and, thus, the overall magnetization response opens unique ways to tailor the magneto-resistive sensitivity of functional magnetic thin film devices.

Combinatorial Fe–Co thin film magnetic structures obtained by thermionic vacuum arc method

Magnetoresistive Fe–Co based thin film structures were produced using thermionic vacuum arc method. The purpose of this work was to obtain significant magnetic response on different granular combinatorial structures. The Giant Magnetoresistive (GMR) and combination between GMR and Tunneling Magnetoresistive (TMR) properties of the thin film structures were the aim of the work. The proposed method in order to obtain the desired granular structures is based on electron beam emitted by an externally heated cathode, accelerated by a high anodic potential.The work consisted in preparing two sets of samples, first being a combination between Fe–Co as magnetic materials embedded in a Cu matrix, with a total thickness of 200nm. The second structure was a combination between Fe–Co (50%–50%) alloy, embedded in a matrix of Cu combined with MgO with a thickness of 200nm. Both sets of samples were obtained by three simultaneous types of discharges. Because of the substrate positioning in respect with the three anode–cathode systems, different material concentrations were obtained, confirmed by Energy Dispersive Spectroscopy (EDS) measurement results. Structural and morphological properties were investigated using Scanning Electron Microscopy; Atomic Force Microscopy and EDS. Electrical properties of the obtained samples were studied using the 4 point measurement method. The magnetic properties were first studied using a non-destructive optical method called MOKE (Magneto-Optical Kerr Effect). Electrical resistance behavior of the granular type structures was studied for different values of the magnetic field, up to 0.3T, at different values of the sample temperature. The magnetoresistive effect obtained for the two sets of samples varied from 1.5% to 81% in respect with the substrate positioning and sample temperature for a constant magnetic field.

Broadband ferromagnetic resonance system and methods for ultrathin magnetic films

Spintronics requires the development of magnetic thin film structures having a wide range of magnetic properties. Ferromagnetic resonance (FMR) is a well understood experimental technique that has proven to be an invaluable tool to probe the static and dynamic magnetic properties of ultrathin films, multilayer nanostructures, and superlattices. In order to achieve a full characterization of thin film materials, one needs to carry out FMR measurements at a wide range of microwave frequencies. In this paper, we show that one does not have to use a broadband vector network analyzer; similar performance can be achieved by using a broadband microwave signal generator, a coplanar waveguide, and a broadband microwave detector. To obtain a good signal to noise ratio, one needs to employ a modulation technique in order to use lock-in detection; in this paper, we use low frequency external field modulation (105Hz) and microwave power amplitude pulse modulation (10kHz). The sensitivity and the performance of this broadband microwave system is demonstrated on two types of samples: molecular beam epitaxy grown single crystal GaAs(001)/Fe/Au and sputter deposited textured Si(111)/Ta/Ru/Co/Ru superlattice structures. The samples were mounted on a coplanar waveguide, allowing one a broadband measurement, ~0.1–50GHz, of DC field swept FMR signals. The results are compared to traditional field swept, field modulated measurements in microwave cavity resonators. Despite the fact that the FMR signal can be very different from that obtained by standard microwave cavities, we show that the analysis of the FMR signal is fairly simple using an admixture of the in-phase and out-of-phase components of rf susceptibility and that the resulting fitted magnetic parameters are in excellent agreement. Additionally, we demonstrate that microwave power amplitude pulse modulation can be used to greatly speed up data collection times, especially for very weak and broad FMR signals.

Polarization enhancement and ferroelectric switching enabled by interacting magnetic structures in DyMnO3 thin films

The mutual controls of ferroelectricity and magnetism are stepping towards practical applications proposed for quite a few promising devices in which multiferroic thin films are involved. Although ferroelectricity stemming from specific spiral spin ordering has been reported in highly distorted bulk perovskite manganites, the existence of magnetically induced ferroelectricity in the corresponding thin films remains an unresolved issue, which unfortunately halts this step. In this work, we report magnetically induced electric polarization and its remarkable response to magnetic field (an enhancement of ~800% upon a field of 2 Tesla at 2 K) in DyMnO3 thin films grown on Nb-SrTiO3 substrates. Accompanying with the large polarization enhancement, the ferroelectric coercivity corresponding to the magnetic chirality switching field is significantly increased. A picture based on coupled multicomponent magnetic structures is proposed to understand these features. Moreover, different magnetic anisotropy related to strain-suppressed GdFeO3-type distortion and Jahn-Teller effect is identified in the films.

An investigation of the magneto-optical properties of thin-film magnetic structures

This study reports on the performance of multilayer film structures, which are a very prospective material for thin-film magnetic sensors. The magnetic and magneto-optical properties of iron and cobalt thin films and also ferromagnetic (FM)/non-magnetic layer (NML)/FM trilayers, prepared using a magnetron sputtering system, are presented. The FM layer thickness of tFe and tCo in trilayers varied from 25 to 100 Å and the NML thickness of tNML varied from 5 to 2000 Å. In the NML/FM samples, the NML thickness varied from 0 to 400 Å. The dependences of the hysteresis characteristics of Fe films on the NML thickness were found. The dependence of the transverse Kerr effect (TKE) magnitude on tFe was established. It was shown experimentally that TKE is sensitive to the magnetization up to a certain depth range below the surface of ferromagnetic—the information depth. It was discovered that the in-plane hysteresis characteristic of the trilayers is strongly dependent on tnml. So existence of the exchange coupling between FM layers through NML and its oscillatory behaviour (from antiferromagnetic (AF) to ferromagnetic (F) order) were experimentally established. It was found that the period AF–F–AF oscillations of exchange coupling is equal to 5–10 Å.