Continuous Ferromagnetic Film Research Articles

Controlling the three dimensional propagation of spin waves in continuous ferromagnetic films with an increasing out of plane undulation

The role of three-dimensionality in a ferromagnetic medium in ruling the propagation properties of spin-waves (SW) has been one of the main focuses of the research activity in recent years. In this context, we investigate the evolution of the SW dispersion (frequency vs wave vector) induced by a progressive vertical undulation of a ferromagnetic film. The geometric undulation is taken along a single direction and is periodic with constant period, while the amplitude (differential maximum height with respect to the film thickness) is gradually increased from 0 to 60 nm. We study the characteristic modification of the internal effective field and link it to the resulting SW dispersions and spatial profile. These systems display at once features both of a planar film and a discretized medium, and the dispersion curves change not only when SWs propagate along the undulation direction, but also perpendicular to it. We discuss the geometric and magnetic conditions for having either the invariance of the SW group velocity with respect to even major changes in the undulation, or a large group velocity for some edge modes. We address a potential dual-band activity, namely the simultaneous propagation of two independent SW-signals, with separated frequency bands and disjoint oscillation regions.

Skyrmion Brownian circuit implemented in continuous ferromagnetic thin film

The fabrication of a circuit capable of stabilizing skyrmions is important for the realization of micro- to nano-sized skyrmion devices. Ultralow power Brownian computers have been theoretically proposed and are a promising example of a skyrmion-based device. However, such devices have not been realized as it would require skyrmions to be stabilized and easily movable within a circuit. Skyrmion circuits fabricated by the etching of ferromagnetic films often decrease the dipolar magnetic field stabilizing the skyrmions, thus preventing their formation. In this study, a skyrmion Brownian circuit has been implemented in a continuous ferromagnetic film with patterned SiO2 capping to stabilize the skyrmion formation. The patterned SiO2 capping controls the saturation field of the ferromagnetic layer and forms a wire-shaped skyrmion potential well, which stabilizes skyrmion formation in the circuit. Moreover, using this patterned SiO2 capping, we have implemented a Y-junction hub circuit exhibiting no pinning site at the junction, contrary to conventional etched hubs. Thus, this technique enables the efficient control of skyrmion-based memory and logic devices to move closer toward the realization of Brownian computers.

Creation and annihilation of topological meron pairs in in-plane magnetized films

Merons which are topologically equivalent to one-half of skyrmions can exist only in pairs or groups in two-dimensional (2D) ferromagnetic (FM) systems. The recent discovery of meron lattice in chiral magnet Co8Zn9Mn3 raises the immediate challenging question that whether a single meron pair, which is the most fundamental topological structure in any 2D meron systems, can be created and stabilized in a continuous FM film? Utilizing winding number conservation, we develop a new method to create and stabilize a single pair of merons in a continuous Py film by local vortex imprinting from a Co disk. By observing the created meron pair directly within a magnetic field, we determine its topological structure unambiguously and explore the topological effect in its creation and annihilation processes. Our work opens a pathway towards developing and controlling topological structures in general magnetic systems without the restriction of perpendicular anisotropy and Dzyaloshinskii–Moriya interaction.

Anomalous birefringence and enhanced magneto-optical effects in epsilon-near-zero metamaterials based on nanorods' arrays.

Hyperbolic metamaterials based on the ordered arrays of metal nanorods in a dielectric media are of great interest owing to new optical effects appearing in such artificial media. Here we study the effects in the polarization state of light passing through a nanocomposite material consisting of Au nanorods in porous alumina and a similar structure supplemented by a nanolayer of ferromagnetic nickel. We demonstrate that close to the epsilon-near-zero dispersion point, under the transition to the hyperbolic dispersion region, the nanocomposites reveal anomalously high modulation of the polarization state of light, which appears as polarization plane rotation and ellipticity changes of probing radiation with a zero ellipticity. This effect is applied for the giant enhancement of the Faraday effect in a continuous ferromagnetic film staying in contact with hyperbolic material. These findings open a path for the design of polarization state control by using hyperbolic metamaterials.

Superferromagnetism and Domain-Wall Topologies in Artificial "Pinwheel" Spin Ice.

For over ten years, arrays of interacting single-domain nanomagnets, referred to as artificial spin ices, have been engineered with the aim to study frustration in model spin systems. Here, we use Fresnel imaging to study the reversal process in "pinwheel" artificial spin ice, a modified square ASI structure obtained by rotating each island by some angle about its midpoint. Our results demonstrate that a simple 45° rotation changes the magnetic ordering from antiferromagnetic to ferromagnetic, creating a superferromagnet which exhibits mesoscopic domain growth mediated by domain wall nucleation and coherent domain propagation. We observe several domain-wall configurations, most of which are direct analogues to those seen in continuous ferromagnetic films. However, charged walls also appear due to the geometric constraints of the system. Changing the orientation of the external magnetic field allows control of the nature of the spin reversal with the emergence of either one- or two-dimensional avalanches. This property of pinwheel ASI could be employed to tune devices based on magnetotransport phenomena such as Hall circuits.

Manipulation of Magnetic Properties by Tunable Magnetic Dipoles in a Ferromagnetic Thin Film

We demonstrate how a unique nanomodulation within a continuous ferromagnetic film can induce magnetic dipoles at predefined, submicrometer scale locations, which can tune the global magnetic properties of the film due to dipole–dipole interactions. Arrays of tunable magnetic dipoles are generated with in-plane and out-of-plane directions, which can be rotated in-plane within the three-dimensional (3-D) modulated structure of a continuous film. In-plane magnetic dipole rotation enables a methodology to control overall magnetic properties of a ferromagnetic thin film. Formation of magnetic dipoles and their tunability were studied in detail by magnetic force microscopy, high-resolution magnetic measurements, and micromagnetic simulation of a nanomodulated Ni45Fe55 alloy film. A pattern larger than a single magnetic domain would normally form a vortex in the remanent state. However, here the unique 3-D nanostructure prevents vortex formation due to the competition between in-plane and out-of-plane dipole–dipole interaction giving rise to a metastable state. Experimentally, at zero remanence, the magnetization goes through a transformation from a metastable to a stable state, where the dipole–dipole interaction depends on their geometrical arrangement. Thus, the magnetic properties of the continuous film can be varied by the proposed pattern geometry. A detail analytical study of the dipolar energy for the system agrees well with the experimental and simulated results.

Domain wall engineering through exchange bias

The control of the structure and position of magnetic domain walls is at the basis of the development of different magnetic devices and architectures. Several nanofabrication techniques have been proposed to geometrically confine and shape domain wall structures; however, a fine tuning of the position and micromagnetic configuration is hardly achieved, especially in continuous films. This work shows that, by controlling the unidirectional anisotropy of a continuous ferromagnetic film through exchange bias, domain walls whose spin arrangement is generally not favored by dipolar and exchange interactions can be created. Micromagnetic simulations reveal that the domain wall width, position and profile can be tuned by establishing an abrupt change in the direction and magnitude of the exchange bias field set in the system.

Ordered magnetic dipoles: Controlling anisotropy in nanomodulated continuous ferromagnetic films

In this paper, the research focus is how to entangle magnetic dipoles to control/engineer magnetic properties of different devices at a submicron/nano scale. Here, we report the generation of synthetic arrays of tunable magnetic dipoles in a nanomodulated continuous ferromagnetic film. In-plane magnetic field rotations in modulated Ni${}_{45}$Fe${}_{55}$ revealed various rotational symmetries of magnetic anisotropy due to dipolar interaction with a crossover from lower to higher fold as a function of modulation geometry. Additionally, the effect of aspect ratio on symmetry shows a novel phase shift of anisotropy, which could be critical to manipulate the overall magnetic properties of the patterned film. The tendency to form vortex is in fact found to be very small, which highlights that the strong coupling between metastable dipoles is more favorable than vortex formation to minimize energy in this nanomodulated structure. This has further been corroborated by the observation of step hysteresis, magnetic force microscopy images of tunable magnetic dipoles, and quantitative micromagnetic simulations. An analytical expression has been derived to estimate the overall anisotropy accurately for nanomodulated film having low magnetocrystaline anisotropy. Derived mathematical expressions based on magnetic dipolar interaction are found to be in good agreement with our results.

Bias-Current and Optically Driven Transport Properties of the Hybrid Fe/SiO<sub>2</sub>/p-Si Structures

Pronounced optical-and bias-current-sensitive features of the transport properties of a Fe/SiO2/p-Si hybrid structure in planar geometry at temperature variation are investigated. Comparative analysis of two Fe/SiO2/p-Si samples, one with a continuous Fe film and the other with two electrodes formed from a Fe layer and separated by a micron gap, shows that these features are due to the MIS transition with a Schottky barrier near the interface between SiO2 and p-Si. Resistance of such a MIS transition depends exponentially on temperature and bias. In the structure with a continuous ferromagnetic film, the competition between conductivities of the MIS transition and the Fe layer results in the effect of current channel switching between the Fe layer and a semiconductor substrate. Within certain limits, this process can be controlled by a bias current and optical radiation. The mechanism of the optical effect is photogeneration of electron-hole pairs in the semiconductor substrate near its boundary with SiO2 layer.

Magnetic-field- and bias-sensitive conductivity of a hybrid Fe/SiO2/p-Si structure in planar geometry

Pronounced magnetic-field- and bias-sensitive features of the transport properties of a Fe/SiO2/p-Si hybrid structure in planar geometry at temperature variation are investigated. Comparative analysis of two Fe/SiO2/p-Si samples, one with a continuous Fe film and the other with two electrodes formed from a Fe layer and separated by a micron gap, shows that these features are due to the metal-insulator-semiconductor (MIS) transition with a Schottky barrier near the interface between SiO2 and p-Si. Resistance of such a MIS transition depends exponentially on temperature and bias. In the structure with a continuous ferromagnetic film, the competition between conductivities of the MIS transition and the Fe layer results in the effect of current channel switching between the Fe layer and a semiconductor substrate. Within certain limits, this process can be controlled by a bias current and a magnetic field. Positive magnetoresistance of the structures at high temperatures is determined, most likely, by disorder-induced weak localization. In the structure with the gap, negative magnetoresistance is observed at certain temperature and bias. Its occurrence should be attributed to an inversion layer formed in the semiconductor near the SiO2/p-Si interface when MIS transition is in the inversion regime.

Exchange bias of patterned systems: Model and numerical simulation

The magnitude of the exchange bias field of patterned systems exhibits a notable increase in relation to the usual bilayer systems, where a continuous ferromagnetic film is deposited on an antiferromagnet insulator. Here we develop a model, and implement a Monte Carlo calculation, to interpret the experimental observations which is consistent with experimental results, on the basis of assuming a small fraction of spins pinned ferromagnetically in the antiferromagnetic interface layer.

Magnetic properties of Fe∕MgO granular multilayers prepared by pulsed laser deposition

Granular multilayers [Fe(tnm)∕MgO(3nm)]N with 0.4nm⩽t⩽1.5nm were prepared by sequential pulsed laser deposition. Transmission electron microscopy (TEM) images show that increasing t causes the growth of the sizes of Fe nanoparticles and broadening of the particle size distribution. For t>0.81nm, continuous Fe layers are formed. The evolution of the shapes and sizes of the particles is reflected in the magnetic properties of the investigated films. A crossover from superparamagnetic to ferromagnetic behavior upon formation of a continuous Fe layer is observed. The fit of zero field cooled and field cooled susceptibility measurements and magnetization curves using Curie–Weiss law and a weighted sum of Langevin functions, respectively, allows the estimation of the average granule size for the films with t<0.61nm. The results of the estimations correlate with the data obtained from TEM images. Reduction of saturation magnetization for Fe nanoparticles and an increase of the coercivity up to 1200Oe at low temperatures were found. It is attributed to the formation of Fe-core∕FeOx-shell structured nanocrystals. The oxide shell gives rise to a strong contribution of surface anisotropy. Isotropic tunneling magnetoresistance up to ∼3% at room temperature and in magnetic field up to 18kOe was found for the film with t=0.61nm. For higher t, an anisotropic magnetoresistance typical for continuous ferromagnetic films was observed.

Ferromagnetic coupled modes in continuous/granular multilayers: Model and experiments

We have developed an adapted model in order to describe the ferromagnetic resonance (FMR) spectra in a trilayer system in which two continuous ferromagnetic films are coupled by a granular magnetic spacer. The model allowed us to study the influence that different parameters (e.g., the Fe volume concentration and the thickness of the granular spacer, the exchange coupling field between layers, the microwave frequency, etc.) have on the overall line shape of the spectra. We present the general results predicted by the model and compare them with FMR experimental measurements made on a particular trilayer ${\mathrm{Fe}∕[\mathrm{Fe}(x)\text{\ensuremath{-}}{\mathrm{SiO}}_{2}(1\ensuremath{-}x)](t)∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}}$ formed by two continuous ferromagnetic layers, Fe and Permalloy $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$, separated by a granular film of $\mathrm{Fe}\text{\ensuremath{-}}{\mathrm{SiO}}_{2}$, in which we changed the Fe volume concentration $x$ $(0.45lxl0.85)$, and the thickness $t$ ($t=1,2,4,9$, and $18\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) of the granular spacer. Room-temperature FMR measurements were made at the $Q$ $(\ensuremath{\nu}=34\phantom{\rule{0.3em}{0ex}}\mathrm{GHz})$ and $X$ bands $(\ensuremath{\nu}=9.5\phantom{\rule{0.3em}{0ex}}\mathrm{GHz})$ with the external field applied parallel to the film plane. Two well-resolved absorption modes, one at low fields and another at higher fields, were generally observed. From the dependence of the resonance field and the relative intensity of these modes on $x$ and $t$ it was possible to deduce that the granular layer strongly interacts with the Fe layer, whereas the Permalloy layer is only weakly coupled with the rest of the layers.

Localized Ferromagnetic Resonance Force Microscopy of a Continuous Permalloy-Cobalt Film

AbstractWe report on the Magnetic Resonance Force Microscopy (MRFM) experiments performed on a 50 nm thick permalloy and a combined 20 nm thick permalloy – cobalt film. We studied the evolution of the MRFM spectra as a function of the vertical probe-sample distance and the lateral position as probe was scanned across the permalloy/cobalt interface. Our numerical simulations of the ferromagnetic resonance (FMR) modes excited in the presence of a non-uniform tip field of the cantilever compare well with experimental findings. This work demonstrates the capability of MRFM to perform local FMR spectroscopy of different materials in continuous ferromagnetic films.

Zero field resistance dip in magnetic tunnel junctions employing a granular electrode

We present magnetoresistance (MR) measurements performed on magnetic tunnel junctions in which one of the electrodes is a granular ferromagnetic film. These junctions exhibit a zero field resistance dip. The dip magnitude depends on the size of the grains. We interpret these results as a consequence of the orange peel effect between the continuous ferromagnetic film and the magnetic grains. The coupling is found to be much stronger than that between continuous ferromagnetic layers.