Ferromagnetic Thin Films Research Articles

Ferromagnetic resonance in FePt thin films at elevated temperatures

Understanding of damping processes in ferromagnetic thin films at elevated temperatures has significant implications for heat-assisted magnetic recording, spin-transfer torque memory, and magnetic sensors operating at high temperatures. Through cavity-based high-temperature ferromagnetic resonance (FMR) measurements, this work examined the FMR linewidth and damping properties of continuous cubic FePt thin films at elevated temperatures. The data show that the FMR linewidth and the Gilbert damping constant both increase monotonically when temperature is increased from room temperature toward the Curie temperature. This temperature dependence is opposite to that observed previously in FePt thin films that are granular, rather than continuous, and have L10 structure, rather than cubic structure; in those films, the FMR linewidth decreases monotonically with an increase in temperature [PR Applied 10, 054046 (2018)]. These opposite results originate from the difference in the crystalline structure and microstructure of the films. In the previous work, the granular L10-order FePt films hold dense material imperfection and thereby may host strong two-magnon scattering (TMS); the TMS-produced damping decreases with an increase in temperature, giving rise to reduced FMR linewidths at high temperatures. In the current work, the continuous cubic FePt films have much less imperfection and thereby host weak TMS, and the dominant damping mechanism is spin-flip magnon-electron scattering (SF-MES). The SF-MES process becomes stronger with an increase in temperature, giving rise to larger linewidth and higher damping at high temperatures. This work and the previous work together demonstrate that for a given thin-film material, the temperature dependence of the FMR linewidth critically relies on the structural properties of the film. They also indicate that one can engineer damping in magnetic thin films through the control of the structural properties of the films.

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.

Skyrmion lattice hosted in synthetic antiferromagnets and helix modes

Thin ferromagnetic films can possess unconventional magnetic properties, opening a new road for using them in spintronic technologies. In the present work exploiting three different methods, we comprehensively analyze phason excitations of a skyrmion lattice in synthetic antiferromagnets. To analyze phason excitations of the skyrmion lattice, we have constructed an analytical model based on three coupled helices and found a linear gapless mode. Micromagnetic simulations also support this result. Moreover, a similar result has been achieved within the rigid skyrmion lattice model based on the coupled Thiele's equations, when the coupling between skyrmions in different layers of the synthetic antiferromagnetic is comparable to or larger than the intralayer coupling. In addition, we also consider the orbital angular momentum and spin pumping current associated with phason excitations. Due to the gapless excitations in the case of skyrmion lattice, the pumping current is nonzero for the arbitrary frequency of pumping microwaves. In the case of individual skyrmions, no current is pumped when microwave frequency is inside the gap of the spectrum of individual skyrmions.

Investigation of ferromagnetism and dual donor defects in Y-doped ZnO thin films

We report here the room temperature ferromagnetism in Y-doped ZnO thin films grown by RF magnetron sputtering. XRD and Raman spectra analysis revealed that pristine and Y-doped ZnO thin films are crystallised in hexagonal wurtzite structure. XPS was performed to confirm the electronic states of elements present in the films. FESEM images of the films were recorded to study their morphological properties. EDAX analysis was also performed to confirm the presence of Y in ZnO thin films. The point defects in the samples were analysed using photoluminescence spectrometer and EPR spectrometer to understand the mechanism behind the magnetic properties exhibited. It is found that zinc interstitial (Zni +) and oxygen vacancy (VO +) defects coexist in the samples and help in setting ferromagnetic properties in Y-doped ZnO thin films. VSM analysis indicated that Y-doped ZnO thin films show ferromagnetic nature while the pure ZnO thin film exhibits diamagnetic behaviour. The dual donor defect (Zni + and VO +) assisted bound magnetic polaron model has been proposed to explain the Y-doped ZnO diluted magnetic semiconductor system. It has been observed that the results are more reliable and reproducible.

Separation of domain walls with nonlocal interaction and their renormalised energy by Γ-convergence in thin ferromagnetic films

We analyse two variants of a nonconvex variational model from micromagnetics with a nonlocal energy functional, depending on a small parameter ϵ>0. The model gives rise to transition layers, called Néel walls, and we study their behaviour in the limit ϵ→0. The analysis has some similarity to the theory of Ginzburg-Landau vortices. In particular, it gives rise to a renormalised energy that determines the interaction (attraction or repulsion) between Néel walls to leading order. But while Ginzburg-Landau vortices show attraction for degrees of the same sign and repulsion for degrees of opposite signs, the pattern is reversed in this model.In a previous paper, we determined the renormalised energy for one of the models studied here under the assumption that the Néel walls stay separated from each other. In this paper, we present a deeper analysis that in particular removes this assumption. The theory gives rise to an effective variational problem for the positions of the walls, encapsulated in a Γ-convergence result. In the second part of the paper, we turn our attention to another, more physical model, including an anisotropy term. We show that it permits a similar theory, but the anisotropy changes the renormalised energy in unexpected ways and requires different methods to find it.

Enhanced Anomalous Hall Effect of Pt on an Antiferromagnetic Insulator with Fully Compensated Surface

We report a significantly enhanced anomalous Hall effect (AHE) of Pt on antiferromagnetic insulator thin film (3-unit-cell La0.7Sr0.3MnO3, abbreviated as LSMO), which is one order of magnitude larger than that of Pt on other ferromagnetic (e.g. Y3Fe5O12) and antiferromagnetic (e.g. Cr2O3) insulator thin films. Our experiments demonstrate that the antiferromagnetic La0.7Sr0.3MnO3 with fully compensated surface suppresses the positive anomalous Hall resistivity induced by the magnetic proximity effect and facilitates the negative anomalous Hall resistivity induced by the spin Hall effect. By changing the substrate’s temperature during Pt deposition, we observed that the diffusion of Mn atoms into Pt layer can further enhance the AHE. The anomalous Hall resistivity increases with increasing temperature and persists even well above the Neel temperature (T N) of LSMO. The Monte Carlo simulations manifest that the unusual rise of anomalous Hall resistivity above T N originates from the thermal induced magnetization in the antiferromagnetic insulator.

Tailoring optical excitation to control magnetic skyrmion nucleation

A single laser pulse can create nanometer-scale magnetic skyrmions in a thin film ferromagnet. However, the skyrmions appear randomly distributed within the excited film area hampering application of the process. Here, the authors present a reliable approach to gain nanometer spatial control over the skyrmions' positions: reflective masks fabricated on the material's backside locally affect the optical excitation amplitudes reached. The backside masks precisely define preferred and inhibited regions for skyrmion nucleation while maintaining frontside access to the magnetic film.

Excitation and detection of coherent sub-terahertz magnons in ferromagnetic and antiferromagnetic heterostructures

Excitation of coherent high-frequency magnons (quanta of spin waves) is critical to the development of high-speed magnonic devices. Here we computationally demonstrate the excitation of coherent sub-terahertz (THz) magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a photoinduced picosecond acoustic pulse. Analytical calculations are also performed to reveal the magnon excitation mechanism. Through spin pumping and spin-charge conversion, these magnons can inject sub-THz charge current into an adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using a dynamical phase-field model that considers the coupled dynamics of acoustic waves, spin waves, and EM waves, we show that the emitted EM wave retains the spectral information of all the sub-THz magnon modes and has a sufficiently large amplitude for near-field detection. These predictions indicate that the excitation and detection of sub-THz magnons can be realized in rationally designed FM or AFM thin-film heterostructures via ultrafast optical-pump THz-emission-probe spectroscopy.

A short-circuited coplanar waveguide for low-temperature single-port ferromagnetic resonance spectroscopy setup to probe the magnetic properties of ferromagnetic thin films

A coplanar waveguide shorted in one end is proposed, designed, and implemented successfully to measure the properties of magnetic thin films as a part of the vector network analyzer ferromagnetic resonance (FMR) spectroscopy setup. Its simple structure, potential applications, and easy installation inside the cryostat chamber made it advantageous especially for low-temperature measurements. It provides a wide band of frequencies in the gigahertz range essential for FMR measurements. Our spectroscopy setup with a short-circuited coplanar waveguide has been used to extract the Gilbert damping coefficient and effective magnetization values for standard ferromagnetic thin films, such as Permalloy (Py) and Cobalt (Co). The thickness and temperature-dependent studies of those magnetic parameters have also been done here for the afore-mentioned magnetic samples.

A large unidirectional magnetoresistance in Fe–Sn heterostructure devices

Unidirectional magnetoresistance (UMR) is an attractive magnetic-field sensing technique as it enables us to detect the in-plane direction of the external magnetic field with a single element. However, the UMR amplitude is typically several orders of magnitude smaller than those of other directional magnetoresistances exhibited in ferromagnetic thin films, hindering sensing applications using the UMR. For a directional magnetic sensor using the UMR, an in-depth understanding of the mechanisms of the UMR and its enhancement are highly desirable. In this study, the structural dependence of the UMR in Fe–Sn heterostructure devices is investigated. We find a weak dependence of the UMR on interface configuration in the oxide cap/Fe–Sn heterostructures on various oxide substrates. In contrast, the normalized UMR amplitude is enhanced by a factor of 27 with increasing Fe–Sn layer thickness from 4.0 to 100 nm. These results suggest that the magnetothermal effect dominates the large UMR in the Fe–Sn heterostructure devices.

Spin-transfer-assisted parametric pumping of magnons in yttrium iron garnet

The combination of parametric pumping and spin-transfer torque is a powerful approach that enables high-level control over magnetic excitations in thin-film ferromagnets. The excitation parameters, such as pumping power and external field strength, affect the instabilities of individual magnon modes. We theoretically explore how the simultaneous effects of parametric pumping and spin transfer torque influence these magnetic instabilities in a thin-film ferromagnet. Within the Landau-Lifshitz-Gilbert framework, we perform micromagnetic simulations of magnon excitations in yttrium iron garnet by pumping, spin transfer torque, and a combination of the two. We find that consistent with experimental results, the magnitude and direction of the spin-transfer torque tune the parametric instability thresholds.

Surface roughness effects on magnetic properties of thin films: A computational Monte Carlo study

By means of Monte Carlo simulation, a computational approach is proposed to investigate the surface roughness effects on the critical and hysteresis behaviors of Heisenberg ferromagnetic thin films. Here, the atomic sites of the film’s surface are changed systematically from their regular positions using a disorder parameter, thereby allowing us to smoothly interpolate between smooth and rough surfaces. Threshold values of the structural disorder level at which the system critical temperature increases are identified. Furthermore, the surface roughness increases is observed to have a profound impact on the system’s magnetic hysteresis loops. In particular, a significant increase in the loop’s area is observed as the roughness increases, thereby increasing the coercivity of the thin magnetic film.

Applications of a novel electron energy filter combined with a hybrid-pixel direct electron detector for the analysis of functional oxides by STEM/EELS and energy-filtered imaging

The performance and suitability of a new electron energy filter in combination with a hybrid pixel, direct electron detector for analytical (scanning) transmission electron microscopy are demonstrated using four examples. The STEM-EELS capabilities of the CEOS Energy Filtering and Imaging Device (CEFID) were tested with focus on weak signals and high spatio-temporal resolution. A multiferroic, multilayer structure of REMnO3 (RE = Yb, Er, Tb, Y), grown on yttria-stabilized zirconia (YSZ), is used to exemplify that this new instrumental setup produces valuable electron energy-loss spectroscopy (EELS) data at high energy losses even when using short acquisition times, providing detailed chemical information about the interfaces in this complex multilayer sample. Another functional oxide, namely a ferromagnetic La2NiMnO6 thin film grown on SrTiO3, demonstrates that atomically resolved spectrum images can be recorded, using short dwell times and moderate beam currents in order to warrant the integrity of the sample. In a third example, inhomogeneously Er-doped YSZ shows by EELS spectrum imaging that elements at low concentrations can be detected semi-quantitatively, uncovering the expected layered Er distribution but revealing substantial interdiffusion. In a final example, we simply demonstrate that the hybrid pixel detector in combination with the energy filter can also be used for energy-filtered imaging and thus for elemental mapping complementary to EELS in scanning transmission mode.

Optimization of the Growth Process of Double Perovskite Pr2-δNi1-xMn1+xO6-y Epitaxial Thin Films by RF Sputtering.

Epitaxial thin films of Pr2−δNi1−xMn1+xO6−y (PNMO) double perovskite were grown on (001)-oriented SrTiO3 substrates by RF magnetron sputtering. The influence of the growth parameters (oxygen pressure, substrate temperature, and annealing treatments) on the structural, magnetic and transport properties, and stoichiometry of the films was thoroughly investigated. It is found that high-quality epitaxial, insulating, and ferromagnetic PNMO thin films can only be obtained in a narrow deposition parameter window. It is shown that a careful selection of the growth conditions allows for obtaining a high degree of Ni/Mn cation ordering, which is reflected in the values of the Curie temperature, TC, and saturation magnetization, MS, which are very close to those of bulk material.

Tailoring of magnetic anisotropy by ion irradiation for magnetic tunnel junction sensors

Magnetic Tunnel Junctions (MTJ) are employed in a range of technologies such as data storage, sensing, and so on due to their compact nature and high field sensitivity. In magnetic films and structures, ion irradiation is capable of modifying the crystal structure, phases and magnetic properties including magnetic anisotropy, which enables tunability of the sensing axis of devices such as MTJ sensors. Irradiation can also adjust the magnetic properties of specific layers without affecting the others, making it one of the most effective methods to post-process multi-layered devices. In this article we review the literature of light ion irradiation effects on MTJs and their component materials, focusing on the effects most relevant to MTJ sensors, such as magnetic anisotropy and magnetization. We first briefly review the effects of the ion-solid interaction, then discuss the causes of magnetic anisotropy, and lastly, we summarize the state of the field of ion-induced modification of magnetic properties in ferromagnetic thin films, antiferromagnetic structures, and MTJs.

Microstructure control for magnetic thin films with high functionality

In most magnetic devices with thin films, the magnetic properties are governed by not only the intrinsic properties but also the microstructure. To realize high performance in magnetic devices, understanding the correlation between magnetic properties and microstructure is important. In this study, the development of ferromagnetic thin films for high-density recording media and large output current perpendicular to plane giant magnetoresistance devices are reviewed. Based on the experimental data, the research direction for next-generation high-areal-density magnetic recording is discussed.

Theory of Spin Waves in a Thin Ferromagnetic Film with a Periodic System of Circular Antidots. Solutions that Correspond to the Crystal Band Theory

Theory of Spin Waves in a Thin Ferromagnetic Film with a Periodic System of Circular Antidots. Solutions that Correspond to the Crystal Band Theory

Thickness-dependent structural phase transition and self-intercalation of two-dimensional ferromagnetic chromium telluride thin films

The quest for next generation spintronic devices has promoted the exploration of ferromagnetism in a two-dimensional (2D) limit which enriches the family of 2D materials. Here, we realized the molecular beam epitaxial growth of atomically flat chromium telluride (CrTex) films on Si(111) substrates in the 2D limit and discovered a thickness-dependent structural phase transition with self-intercalation during the growth. Combining the in situ reflection high-energy electron diffraction, scanning tunneling microscopy, x-ray photoemission spectroscopy, and ex situ x-ray diffraction, we found that the first layer of CrTex films formed in a CrTe2 crystalline phase as a buffer layer for further growth. Afterward, the chromium atoms began to intercalate into the layers of CrTe2, and the Cr3Te4 phase dominated the following growth over the second layer. Subsequent superconducting quantum interference device measurements verified the ferromagnetism in the chromium telluride film down to one layer limit. Our results provide important information on the structural phase transition during the growth of CrTex films, which would be an ideal platform for studying ferromagnetism in 2D systems, and the growth of high-quality CrTex films on Si substrates would benefit the further applications of 2D ferromagnetic films.

Magnetization recoil-curve overshoot and interaction plots in Co thin films

Recoil magnetization curves measured on Fe, Co and Ni films have recently been found to lie entirely and way outside the major hysteresis loop, a phenomenon referred to as a recoil-curve overshoot, RCO. A model of pairs of exchange-coupled grains with slightly-misaligned anisotropy axes has been able to reproduce key features of these films such as the collapse of the hard-magnetization axis with greatly enhanced remanent magnetization and coercivity, the RCO as well as kinks some major loops present near saturation. In the present work we employ an interaction plot technique in order to probe the nature of the interactions influencing the peculiar magnetization reversal. Our study on a 25-nm-thick magnetron-sputtered Co film reveals another unforeseen, in thin ferromagnetic films, features, disclosed in the vicinity of the collapsed hard axis where scissors-like magnetization reversal takes place—wholly negative interaction plots and hysteretic loops with positive recoil fields. Through model simulations we show that, contrary to the widespread notion, ferromagnetic intergrain coupling might be solely responsible for these puzzling features of ferromagnetic hysteresis.

Structural, magnetic and Magneto-transport properties of Mn-doped SiGe thin films

Mn-doped polycrystalline SiGe thin films are fabricated by radio frequency magnetron sputtering and subsequent rapid thermal annealing. Structural characterizations show that the Mn atoms are distributed uniformly in the SiGe matrix and mainly occupy the substitutional sites with 2+ state. X-ray magnetic circular dichroism signals reveal the spin and orbital magnetic moment information of the samples. Magnetic measurements show that the Curie temperature is up to ∼ 290 K and the effective magnetic moment is about 1.57 μB/Mn. Magneto-transport measurements reveal anomalous Hall effect, another signal of ferromagnetism in the samples. Our results provide insights into the structure and the ferromagnetism of the Mn-doped SiGe thin films and may be useful for related spintronic applications.