Characterization of magnetoresistance hysteresis of Permalloy thin-film using near-field microwave microscope

A near-field scanning microwave microprobe (NSMM) technique has been used to investigate the magnetic properties of the Permalloy (Ni₈₁Fe₁₉) thin film. To characterize the hysteresis behavior of the magnetoresistance (MR) of Permalloy (Py) thin films, the microwave reflection coefficient, S₁₁ was measured. The change of the estimated MR was observed under in-plane external magnetic fields, and was confirmed with variation of MR measured by the 4-probe method. The magnetic properties of the Py thin film were examined by a vibrating sample magnetometer. The observed MR was correlated with the changes of the relative magnetic permeability, Δμ of the Py. We also directly imaged the Py thin film changes by NSMM. MR of Py was determined from the visualized microwave reflection coefficient changes ΔS₁₁ at the thin film interface with high sensitivity. The present methodology can be extended to investigations of other magnetic thin films or magnetic materials using the NSMM system.     

Demagnetizing field computation for dynamic simulation of themagnetization reversal process

The magnetic-field distribution for a thin magnetic film is computed using the fast Fourier transform technique. The method is quite general and accommodates any two-dimensional magnetization distribution. It allows the computation of fields both inside the film (demagnetizing fields) and outside (stray fields and leakage)     

Nanoscale Magnetic Characterization of Tunneling Magnetoresistance Spin Valve Head by Electron Holography

Nanostructured magnetic materials play an important role in increasing miniaturized devices. For the studies of their magnetic properties and behaviors, nanoscale imaging of magnetic field is indispensible. Here, using electron holography, the magnetization distribution of a TMR spin valve head of commercial design is investigated without and with a magnetic field applied. Characterized is the magnetic flux distribution in complex hetero‐nanostructures by averaging the phase images and separating their component magnetic vectors and electric potentials. The magnetic flux densities of the NiFe (shield and 5 nm‐free layers) and the CoPt (20 nm‐bias layer) are estimated to be 1.0 T and 0.9 T, respectively. The changes in the magnetization distribution of the shield, bias, and free layers are visualized in situ for an applied field of 14 kOe. This study demonstrates the promise of electron holography for characterizing the magnetic properties of hetero‐interfaces, nanostructures, and catalysts.     

Magnetic properties of thin ferromagnetic films in relation to their structure

The influence of film structure on magnetic properties is primarily due to the creation of spatially fluctuating local anisotropies which make the magnetization direction inhomogeneous. The deviation from homogeneous magnetization of domains, the so-called ripple, influences a great number of static, quasistatic and dynamic effects via the intrinsic demagnetizing field. The ripple theory is expressed in terms of only one phenomenological constant, the structure constant S, which covers completely the influence of the film structure on the deviations of real thin film behaviour from that of ideal single domains.     

Off-Axis Electron Holography of Single Ferromagnetic Nanowires

Off-axis electron holography has been applied to study the remanent magnetization state of single ferromagnetic Co₉₃Cu₇ nanowires a few micrometer in length and a typical radius of 40nm. Because the objective lens of the electron microscope has to be switched off, the spatial resolution of the reconstructed phase images is presently limited to approximately 70nm. The magnetization reversal of an individual nanowire has been followed by observing a series of remanent states, obtained ex situ by applying different external magnetic field sweeps parallel to the nanowire axis. The relation between misoriented crystal grains and nonuniform magnetization states has been studied by the combination of electron holography and conventional transmission electron microscopy.     

Study of Spin-Valve Operation in Permalloy–SiO2–Silicon Nanostructures

Magnetotransport studies are performed on nanoscale Permalloy(Py)–(Mg)–SiO₂-degenerate Si(100) tunneling devices in spin-valve geometry with and without Mg interlayer. Highly remanent, single domain Py electrodes (15–100-μm length, 100–1000-nm width) are realized by electron-beam lithography, electron-beam evaporation, and subsequent lift-off. Different widths ensure subsequent switching of the Py nanoelectrodes in increasing magnetic fields. A suppression of spin-polarized current is expected for antiparallel magnetization configuration of source and drain contacts (i.e. positive magnetoresistance) if spin injection and detection have been successfully implemented. Magnetic hysteresis curves of Py nanowire arrays measured at temperatures from 5 K up to 300 K reveal increasing coercive fields (up to 40 mT) for decreased nanowire widths as required for device operation. Small positive magnetoresistance is observed for the spin-valve geometry with Mg interlayer at 4.2 K, contrary to the negative anisotropic magnetoresistance measured of single wires.     

SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> ferrites and hard magnetic PVA nanocomposite: investigation of magnetization,coecivity and remanence

In this work, various morphologies of SrFe₁₂O₁₉ (SrFe) nanostructures were synthesized via a simple sol–gel method. The effect of concentration, temperature and various surfactants on the morphology and particle size of the magnetic seeds was investigated. The prepared magnetic products were characterized by X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy techniques. Alternating gradient force magnetometry reveals that the samples exhibit hard magnetic property with the coercivity up to 5300 Oe. Strontium ferrite was added to poly vinyl alcohol to prepare the magnetic polymeric matrix thin film nanocomposites. The saturation magnetization and coercivity decreased due to agglomeration of magnetic nanoparticles in polymer matrix.     

Efficient Spin-Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr2Ge2Te6

Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr₂Ge₂Te₆, is studied using the anomalous Hall effect in Cr₂Ge₂Te₆/tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out-of-plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in-plane magnetic field and a charge current flowing through Ta—a heavy metal exhibiting giant spin Hall effect—is used. In the presence of in-plane fields of 20 mT, charge current densities as low as 5 × 10⁵ A cm^–2 are sufficient to switch the out-of-plane magnetization of Cr₂Ge₂Te₆. This finding highlights that current densities required for spin-orbit torque switching of Cr₂Ge₂Te₆ are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low-power memory and logic applications.     

Investigation of Structural, Magnetic and Magnetotransport Properties of Electrodeposited Co–TiO2 Nanocomposite Films

Nanocomposite Co?CTiO₂ thin films were prepared by simultaneous electrodeposition of Co and TiO₂ on a Cu substrate from a solution based on Co sulfate in which TiO₂ nanoparticles were suspended by stirring. We investigated the influence of the TiO₂ nanoparticles concentration in the bath on the morphology, composition, magnetic and magnetotransport properties of the films. The Co?CTiO₂ thin films were characterized by using scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction analyses, and their magnetic properties were evaluated by using an induction type device with data acquisition system and a torque magnetometer. The current in-plane transport properties of the films have been investigated. The results showed that the films were composed of a Co metal matrix containing embedded TiO₂ nanoparticles and cobalt hydroxide which is formed simultaneously with cobalt metal deposition. The amount of TiO₂ in the film increases with the rising concentration of TiO₂ nanoparticles in the plating bath. This complex structure favored the increase of the magnetoresistance. The Co?CTiO₂ nanocomposite films (containing about 1.3 at.% Ti) exhibit a giant magnetoresistance contribution of 47.6 %. From the magnetic measurements, we have found that the saturation magnetization, the magnetic susceptibility, and the effective magnetic anisotropy constant decrease with the increasing content of TiO₂ in the thin layer. The easy magnetization axis direction changes from in-plane to almost perpendicular-to-plane, with increasing TiO₂ nanoparticles content in the film. The existence of a giant magnetoresistance effect in Co?CTiO₂ is very promising for potential applications in spintronics.     

Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot‐Electron Pulses

Current‐induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic, and nondestructive in order to function in device applications. Therefore, single‐ electronic‐pulse‐driven deterministic switching of the magnetization on the picosecond timescale represents a major step toward future developments of ultrafast spintronic systems. Here, the ultrafast magnetization dynamics in engineered Gd_x [FeCo]_1?x ‐based structures are studied to compare the effect of femtosecond laser and hot‐electron pulses. It is demonstrated that a single femtosecond hot‐electron pulse causes deterministic magnetization reversal in either Gd‐rich and FeCo‐rich alloys similarly to a femtosecond laser pulse. In addition, it is shown that the limiting factor of such manipulation for perpendicular magnetized films arises from the formation of a multidomain state due to dipolar interactions. By performing time‐resolved measurements under various magnetic fields, it is demonstrated that the same magnetization dynamics are observed for both light and hot‐electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current‐induced magnetization manipulation is enhanced due to the ballistic transport of hot electrons before reaching the GdFeCo magnetic layer.     

Dipolar‐Distribution Cavity γ‐Fe2O3@C@α‐MnO2 Nanospindle with Broadened Microwave Absorption Bandwidth by Chemically Etching

Developing microwave absorption materials with ultrawide bandwidth and low density still remains a challenge, which restricts their actual application in electromagnetic signal anticontamination and defense stealth technology. Here a series of olive‐like γ‐Fe₂O₃@C core–shell spindles with different shell thickness and γ‐Fe₂O₃@C@α‐MnO₂ spindles with different volumes of dipolar‐distribution cavities were successfully prepared. Both series of absorbers exhibit excellent absorption properties. The γ‐Fe₂O₃@C@α‐MnO₂ spindle with controllable cavity volume exhibits an effective absorption (2O₃@C spindle reaches as high as ?45 dB because of the optimized electromagnetic impedance balance between polymer shell and γ‐Fe₂O₃ core. Intrinsic ferromagnetism of the anisotropy spindle is confirmed by electron holography. Strong coupling of magnetic flux stray lines between spindles is directly imaged. This unique morphology and facile etching technique might facilitate the study of core–shell type microwave absorbers.     

Magnetic domains in thin-film recording heads as observed in the SEM by a lock-in technique

In a thin-film magnetic recording head, a magnetic circuit made from two Permalloy thin films, one above the other, is magnetized by a spixally plated thin-film copper coil which is located (together with insulating layers) between those two films. The magnetic domains in a thin Permalloy film can be studied by type-2 magnetic constrast using backscattered electrons (BSE) in the scanning electron microscope (SEM) provided that the film has a smooth surface and is deposited on a flat substrate. In practice, the upper Permalloy film follows the contours of the underlying layers. This gives a topographic signal which is large enough to mask the type-2 magnetic contrast signal in its simple form. The domain walls can, however, be seen if the magnetic recording head is excited with a sinusoidal current and if the video waveform is processed with a lock-in amplifier referenced either to the fundamental or to the second harmonic of the excitation frequency. This lock-in image processing technique has now been applied to obtain images of the magnetic domains both from the upper Permalloy film of an incompletely fabricated head and also from the exposed cross sections of the Permalloy films in an operational thin-film head.     

Multidimension‐Controllable Synthesis of MOF‐Derived Co@N‐Doped Carbon Composite with Magnetic‐Dielectric Synergy toward Strong Microwave Absorption

Metal–organic framework (MOF) is highly desirable as a functional material owing to its low density, tunable pore size, and diversity of coordination formation, but limited by the poor dielectric properties. Herein, by controlling the solvent and mole ratio of cobalt/linker, multidimension‐controllable MOF‐derived nitrogen‐doped carbon materials exhibit tunable morphology from sheet‐, flower‐, cube‐, dodecahedron‐ to octahedron‐like. Tunable electromagnetic parameters of Co@N‐doped carbon composites (Co@NC) can be obtained and the initial MOF precursor determines the distribution of carbon framework and magnetic cobalt nanoparticles. Carbonized Co@NC composites possess the following advantages: i) controllable dimension and morphology to balance the electromagnetic properties with evenly charged density distribution; ii) magnetic‐carbon composites offer plenty of interfacial polarization and strong magnetic coupling network; iii) a MOF‐derived dielectric carbon skeleton provides electronic transportation paths and enhances conductive dissipation. Surface‐mediated magnetic coupling reflects the stray magnetic flux field, which is corroborated by the off‐axis electron holography and micro‐magnetic simulation. Optimized octadecahedral Co@NC sample exhibits the best microwave absorption (MA) of ?53.0 dB at the thickness of 1.8 mm and broad effective frequency from 11.4 to 17.6 GHz (Ku‐band). These results pave the way to fabricate high‐performance MA materials with balanced electromagnetic distribution and controlled morphology.     

The micromagnetic propertis of high-coercivity metallic thin films and their effects on the limit of packing density in digital recording

The micromagnetic structures of the high-coercivity, isotropic, and high-squareness thin films of sputtered Co-Re have been investigated using transmission electron microscope (TEM) Lorentz imaging and electron deflection methods. From the behavior of the magnetic ripple structure under applied field and the configuration of the local surface fields observed in these experiments, the existence of magnetic clusters in these films was verified. Based on the interpretation of the field dependence of the ripple formation and the hysteretic properties of the film, it is concluded that the formation of the magnetic clusters is a spontaneous process resulting from intercrystalline interactions and local inhomogeneities in the anisotropy. The effects of such cluster formation on longitudinal magnetic recording were investigated. The results show that the reduction of dipole energy at the transition region between two oppositely magnetized regions can be achieved by a stepwise rotation of the magnetization vector of an individual cluster in the form of a vortex. This type of rotation creates a finite transition length which is limited by the size of the magnetic cluster of the film. Consequently, it is concluded that the maximum packing density for saturation recording in these types of films would be less than that predicted by the phenomenological equation, which was derived based solely on considerations of the demagnetization field and the coercivity of the film.     

Magnetic properties of permalloy/permalloy-oxide multilayer thin films

Magnetic properties of permalloy/permalloy-oxide multilayer thin films are investigated. These thin films are prepared by a repeat of sputter deposition of permalloy thin film, followed by oxidation of the film surface. The total thickness of the permalloy thin films before oxidation is about 100 nm. The number of layers is one to twenty. The oxide layers are formed by oxidation in dry air. The estimated oxide layer thickness is about 2 nm. The oxide NiFe₂O₄ is identified by RHEED. The film coercivity decreases linearly with increasing layer numbers. The saturation magnetization and magnetoresistivity decrease as the number of layers increase. The coercivity decrease is due to grain growth suppression and magnetic separation by oxide film of permalloy layer, and magnetoresistivity decrease is due to electrical resistivity increase originating into electron scattering by the oxide layer.     

Magnetic properties and noise in cylindrical Permalloy thin-film magnetometers

The magnetic properties and low-frequency noise measurements of thin films of Permalloy are described. The films are obtained by electrodeposition. They are polycrystalline and single domain. The noise due to the film is shown to be an increasing function of the dispersion angle α90. A model for the noise mechanism taking into account the magnetostatic stray field arising in the grain boundaries and the thermal agitation energy is given. It is in qualitative agreement with the experimental results.     

A Living‐Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO3 Thin Films

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half‐metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare‐earth titanates RTiO₃, despite their appeal to design correlated 2D electron gases. Here, the magnetic properties of epitaxial DyTiO₃ thin films are reported. While films thicker than about 50 nm show a bulk‐like response, at low thickness a surprising increase of the saturation magnetization is observed. This behavior is described using a classical model of a “dead layer” but assuming that this layer is actually “living,” that is, it responds to the magnetic field with a strong paramagnetic susceptibility. Through depth‐dependent X‐ray absorption and photoemission spectroscopy, it is shown that the “living‐dead layer” corresponds to surface regions where magnetic (S = 1/2) Ti³⁺ ions are replaced by nonmagnetic Ti⁴⁺ ions. Hysteresis cycles at the Dy M₅ and Ti L₃ edges indicate that the surface Ti⁴⁺ ions decouple the Dy³⁺ ions, thus unleashing their strong paramagnetic response. Finally, it is shown how capping the DyTiO₃ film can help increase the Ti³⁺ content near the surface and thus recover a better ferrimagnetic behavior.     

Epitaxial Orientation and Planar Hall Effect for MnAs Films Grown on GaAs(001)

We study the planar Hall effect in high quality thin ferromagnetic films of MnAs grown on GaAs(001) exhibiting hysteresis due to the hindered rotation of the magnetic moment in the plane. The saturation magnetic field H _s, which is necessary to align the magnetization along the hard axis, depends sensitively on the epitaxial orientation of the film. By using out-of-plane magnetic fields directions, we show that H _s is strongest along the direction of the c-axis of the MnAs crystal, thus demonstrating the importance of the crystal field anisotropy for the planar Hall effect.     

Decoding of digital magnetic recording with longitudinal magnetization of a tape from a magneto-optical image of stray fields

For digital magnetic recording of encoded information with longitudinal magnetization of the tape, the connection between the domain structure of a storage medium and magneto-optical image of its stray fields obtained using a magnetic film with a perpendicular anisotropy and a large Faraday rotation has been studied. For two-frequency binary code without returning to zero, an algorithm is developed, that allows uniquely decoding of the information recorded on the tape based on analysis of an image of stray fields.     

Study of Spin-Valve Operation in Permalloy–SiO2–Silicon Nanostructures

Magnetotransport studies are performed on nanoscale Permalloy(Py)–(Mg)–SiO₂-degenerate Si(100) tunneling devices in spin-valve geometry with and without Mg interlayer. Highly remanent, single domain Py electrodes (15–100-m length, 100–1000-nm width) are realized by electron-beam lithography, electron-beam evaporation, and subsequent lift-off. Different widths ensure subsequent switching of the Py nanoelectrodes in increasing magnetic fields. A suppression of spin-polarized current is expected for antiparallel magnetization configuration of source and drain contacts (i.e. positive magnetoresistance) if spin injection and detection have been successfully implemented. Magnetic hysteresis curves of Py nanowire arrays measured at temperatures from 5 K up to 300 K reveal increasing coercive fields (up to 40 mT) for decreased nanowire widths as required for device operation. Small positive magnetoresistance is observed for the spin-valve geometry with Mg interlayer at 4.2 K, contrary to the negative anisotropic magnetoresistance measured of single wires.