Ferromagnetic Thickness Research Articles

Surface Representation of Pulsed Eddy Current Sensor Signals for Improved Ferromagnetic Material Thickness Quantification

Pulsed eddy current (PEC) sensing is a nondestructive testing and evaluation technique widely used for detecting and quantification of flaws and properties of metallic test pieces. In recent years, the use of the technique for the assessment of ferromagnetic materials, specifically, for the purpose of thickness quantification of ferromagnetic plates, or wall-like structures, has risen. The detector coil-based PEC sensor architecture has been the most commonly used sensor architecture for the purpose. Recent works have demonstrated the effectiveness of the detector coil voltage decay rate for this purpose. The decay rate signal feature has been used to construct a linear functional relationship between thickness and the feature value. This functional relationship has eventually been used for thickness estimation. However, this method has proven to come with a limitation of accuracy in thickness estimates. Therefore, we present in this article a method for surface representation of PEC sensor signals. In this method, the thickness varies as a surface-like function in 3D, relying on two signal feature values provided as independent variables. We demonstrate how the surface representation yields a 2% improvement in accuracy (with respect to correlation coefficient) in previous results (i.e., thickness estimates) through a case-study of quantifying wall thickness of a gray cast iron pipe. Such an improvement in accuracy has significant cost-saving implications in the field of structural health monitoring, especially in managing critical pipes in sectors such as water and energy. Cost savings result by enabling infrastructure managers to optimize rehabilitation and thereby provide improved services.

Higher-order ferromagnetic resonances in out-of-plane saturated Co/Au magnetic multilayers

Artificial ferromagnetic (FM)/nonmagnetic multilayers, with large enough FM thickness to prevent the dominance of interface anisotropies, offer a straightforward insight into the understanding and control of perpendicular standing spin wave (PSSW) modes. Here we present a study of the static and dynamic magnetic properties of ${[\mathrm{Co}(3.0\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Au}(0.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{1\ensuremath{\le}N\ensuremath{\le}30}$ multilayers. Magnetometry reveals that the samples exhibit magnetization reversal properties typical of an effective single layer with weak perpendicular anisotropy, with the distinctive thickness-dependent magnetization reorientation transition from uniform in-plane to out-of-plane stripe domains at remanence. However, when such multilayer systems are out-of-plane saturated, the dynamic response reveals the existence of several different ferromagnetic resonances in the form of PSSW modes that strongly depend on the material modulation characteristics along the total thickness. These modes are induced by the layer stacking itself as the effective single layer model fails to describe the observed complex dynamics. In contrast to most systems considered in the past, described by a dynamic model of a single effectively homogeneous thick layer, the specific structures investigated here provide a unique platform for a large degree of tunability of the mode frequencies and amplitude profiles. We argue that the combination of periodic magnetic properties with vertical deformation gradients, arising from heteroepitaxial strain relaxation, converts the Au interlayer regions into a vertical regular array of magnetic pinning planes for the PSSW modes, which promotes the complex dynamics observed in this system.

Exchange Interactions in NiFe/Ta/IrMn Heterostructures under Conditions of Tantalum Deficiency

In the NiFe/Ta/IrMn structures, the contributions of two types of exchange interactions at the NiFe/IrMn interface, partially separated by sections of the nonmagnetic tantalum interlayer, are distinguished. One type of exchange is “contact”—it occurs directly at the NiFe/IrMn interface. Another type is “interlayer,” which takes place through the conduction electrons of the tantalum interlayer. A comparison of the temperature dependences of the exchange bias of the hysteresis loops at fixed parameters of the ferromagnetic layers and various effective thicknesses of the nonmagnetic interlayer made it possible to separate the contributions of the above types of exchange interactions and determine their temperature dependences individually. The exchange interaction energies at the NiFe/IrMn and NiFe/Ta/IrMn interfaces were determined.

Tunneling through a dielectric/ferromagnetic/ferroelectric three-step-like composite barrier

Electron transport in ferroelectric tunnel junctions (FTJs) with a three-step-like barrier combining a dielectric, ferromagnetic, and ferroelectric was investigated theoretically. A significant enhanced tunneling electroresistance effect was observed as compared to traditional FTJs with a bilayer composite barrier. The spin polarization also could be enhanced in a certain orientation of ferroelectric polarization. A metal/dielectric/ferromagnetic/ferroelectric/metal FTJ with a large ferromagnetic thickness is preferred for the optimal performance. The ferroelectricity in the ferroelectric layer has a weak modulation effect on the spin polarization. This work provides a way to enhance the performance of FTJs and/or to control the spin-polarized electronic transport by structure engineering.

Magnetic field-controlled −π transitions and their experimental signatures in superconductor-ferromagnet-superconductor junctions

Superconductor-ferromagnet-superconductor Josephson junctions are known to exist in the 0 and π states with the transitions between them controlled by the temperature and ferromagnetic interlayer thickness. We demonstrate that these transitions can be controlled also by the external magnetic field directed perpendicular to the layers. By varying the ratio of diffusion coefficients in superconducting and ferromagnetic layers, these field-controlled transitions can be made detectable for arbitrary large values of the exchange energy in the ferromagnet. We also show that the 0−π transitions in the perpendicular field can be observed as the specific features of the flux-flow conductivity dependencies on the ferromagnetic thickness in accordance with recent experimental results.

Capturing dual behavior of the parallel coercivity in FeNi/Cu nanowire arrays by fine-tuning of segment thicknesses

The emergence of novel effects such as giant magnetoresistance and magnetothermopower in single nanowires (NWs) and multilayered NWs (MNWs) makes it necessary to comprehensively understand and distinguish between their magnetic behavior and subtle characteristics. Herein, experimental fine-tuning of both ferromagnetic and non-magnetic segment thicknesses in 50 nm diameter FeNi/Cu MNW arrays from TFeNi = 25–260 nm and TCu = 2–23 nm, respectively, enables us to find the threshold TCu, below which the coercive field behavior of rod-like MNW systems magnetically saturated parallel to their long axis shows a continuous decreasing trend, which is similar to that of single NWs with integrated structure. This is in turn accompanied with enhanced magnetizing- and demagnetizing-type interactions as evidenced by first-order reversal curve (FORC) analysis. FORC measurements of disk-shaped to rod-like FeNi segments also indicate a transition from dipolar to monopolar interaction regime which coincides with continuous increasing and decreasing trends of the parallel coercivity and magnetization reversibility, being in agreement with typical MNW magnetic characteristics. Our results on the variation behavior of interaction field distribution as a function of TCu allows for evaluating the credibility of the theoretical approach proposed in the literature on the interaction field, providing threshold conditions based on the segment aspect ratio of MNW arrays.

Quantifying the Relative Thickness of Conductive Ferromagnetic Materials Using Detector Coil-Based Pulsed Eddy Current Sensors.

Thickness quantification of conductive ferromagnetic materials by means of non-destructive evaluation (NDE) is a crucial component of structural health monitoring of infrastructure, especially for assessing the condition of large diameter conductive ferromagnetic pipes found in the energy, water, oil, and gas sectors. Pulsed eddy current (PEC) sensing, especially detector coil-based PEC sensor architecture, has established itself over the years as an effective means for serving this purpose. Approaches for designing PEC sensors as well as processing signals have been presented in previous works. In recent years, the use of the decay rate of the detector coil-based time domain PEC signal for the purpose of thickness quantification has been studied. Such works have established that the decay rate-based method holds generality to the detector coil-based sensor architecture, with a degree of immunity to factors such as sensor shape and size, number of coil turns, and excitation current. Moreover, this method has shown its effectiveness in NDE of large pipes made of grey cast iron. Following such literature, the focus of this work is explicitly PEC sensor detector coil voltage decay rate-based conductive ferromagnetic material thickness quantification. However, the challenge faced by this method is the difficulty of calibration, especially when it comes to applications such as in situ pipe condition assessment since measuring electrical and magnetic properties of certain pipe materials or obtaining calibration samples is difficult in practice. Motivated by that challenge, in contrast to estimating actual thickness as done by some previous works, this work presents a protocol for using the decay rate-based method to quantify relative thickness (i.e., thickness of a particular location with respect to a maximum thickness), without the requirement for calibration.

Josephson current through a ferromagnetic bilayer: Beyond the quasiclassical approximation

Based on the Bogoliubov-de Gennes equations, we provide an exact numerical solution for the critical current of Josephson junctions with a composite ferromagnetic bilayer. We demonstrate that for the antiparallel orientation of the magnetic moments of the bilayer, the presence of a potential barrier at the bilayer interface results in large oscillations of the critical current as a function of ferromagnet thickness and/or exchange field. Because of this, and remarkably, in the range of small exchange field and thicknesses, the magnetism leads to the increase of the critical current. This effect is well pronounced at low temperature but disappears near $T_c$. If the potential barrier is replaced by a spin-active barrier at the bilayer interface the conventional 0-$\pi$ transition, similar to the case of an uniform ferromagnetic Josephson junction, is observed. Strikingly, for a parallel orientation of the magnetic moments of the bilayer, the presence of the spin-active barrier restores the anomalous behavior---potential barrier in the antiparallel case. These behaviors result from the resonant tunneling of Cooper pairs across the composite barrier---an effect related to the spin-dependent Fermi vector in the presence of the ferromagnets' exchange field.

Interfacial Perpendicular Magnetic Anisotropy in Magnetic Tunnel Junctions Comprising CoFeB with FeNiSiB Layers

Controlling ferromagnetic thickness (t) and properties such as saturation magnetization (Ms) and effective magnetic anisotropy constant (Keff) has been regarded as critical for the performance of magnetic tunnel junctions (MTJs) with interfacial perpendicular magnetic anisotropy. Here, we report the effects of hybridizing a CoFeB layer with a FeNiSiB layer as part of a magnetic free layer structure. We deposited thin film stacks by magnetron sputtering on Si wafers with thermal oxides and carried out post-deposition heat treatment at 300 °C for 1 h in a vacuum under a magnetic field. We found that Ms and Keff could be tuned by adding a layer of amorphous FeNiSiB. While the Ms and Keff values were modified, the tunneling magnetoresistance (TMR) ratios of the MTJs were maintained, even though the CoFeB thickness was decreased by half. Moreover, an asymmetric bias voltage dependence of TMR was suppressed in the MTJs with FeNiSiB/CoFeB hybrid free layers due to improvements in the interface quality between the CoFeB/MgO interfaces.

Tuning spinterface properties in iron/fullerene thin films

In ferromagnetic (FM) metal/organic semiconductor (OSC) heterostructures charge transfer can occur which leads to induction of magnetism in the non-magnetic OSC. This phenomenon has been described by the change in the density of states in the OSC which leads to a finite magnetic moment at the OSC interface and it is called the ‘spinterface’. One of the main motivations in this field of organic spintronics is how to control the magnetic moment in the spinterface. In this regard, there are several open questions such as (i) which combination of FM and OSC can lead to more moment at the spinterface? (ii) Is the thickness of OSC also important? (iii) How does the spinterface moment vary with the FM thickness? (iv) Does the crystalline quality of the FM matter? (v) What is the effect of spinterface on magnetization reversal, domain structure and anisotropy? In this context, we have tried to answer the last four issues in this paper by studying Fe/C60 bilayers of variable Fe thickness deposited on Si substrates. We find that both the induced moment and thickness of the spinterface vary proportionally with the Fe thickness. Such behavior is explained in terms of the growth quality of the Fe layer on the native oxide of the Si (100) substrate. The magnetization reversal, domain structure and anisotropy of these bilayer samples were studied and compared with their respective reference samples without the C60 layer. It is observed that the formation of spinterface leads to a reduction in uniaxial anisotropy in Fe/C60 on Si (100) in comparison to their reference samples.

Thickness effect on the easy axis distribution in exchange biased Co/IrMn bilayers

It is commonly assumed that in uncompensated FM/AF interfaces, the easy axes of antiferromagnetic grains are aligned toward the magnetic field direction applied during the film growth. However, due to several intrinsic and extrinsic factors, these easy axes can be aligned away from the unidirectional axis. In this context, the aim of this work is to study the relationship between the easy axes distribution (EAD) and thickness of both the ferromagnetic (FM) and antiferromagnetic (AF) layers in Co/IrMn bilayer systems. A phenomenological model for the free energy of the system that takes into account a domain wall formation in the AF layer and a Gaussian-type distribution of their easy axes is used. Thus, we simulate and fit the magnetization curves and get several magnetic parameters like rotatable anisotropy field, exchange bias field, coercivity and magnetic anisotropy. Our results show that when the AF thickness is increased, the width of the easy-axis distribution also increases following a power-law relationship. However, no correlation is observed when the FM thickness is varied. Furthermore, considering an easy-axis distribution we observe a significant reduction in coercivity as well as a slight decrease of the exchange bias field.

Review of Pulsed-Eddy-Current Signal Feature-Extraction Methods for Conductive Ferromagnetic Material-Thickness Quantification

Thickness quantification of conductive ferromagnetic materials has become a common necessity in present-day structural health monitoring and infrastructure maintenance. Recent research has found Pulsed Eddy Current (PEC) sensing, especially the detector-coil-based PEC sensor architecture, to effectively serve as a nondestructive sensing technique for this purpose. As a result, several methods of varying complexity have been proposed in recent years to extract PEC signal features, against which conductive ferromagnetic material thickness behaves as a function, in return enabling thickness quantification owing to functional behaviours. It can be seen that almost all features specifically proposed in the literature for the purpose of conductive ferromagnetic material-thickness quantification are in some way related to the diffusion time constant of eddy currents. This paper examines the relevant feature-extraction methods through a controlled experiment in which the methods are applied to a single set of experimentally captured PEC signals, and provides a review by discussing the quality of the extractable features, and their functional behaviours for thickness quantification, along with computational time taken for feature extraction. Along with this paper, the set of PEC signals and some MATLAB codes for feature extraction are provided as supplementary materials for interested readers.

Observation of magnetic skyrmions in unpatterned symmetric multilayers at room temperature and zero magnetic field

Magnetic skyrmions are promising candidates for the next generation of spintronic devices due to their small size and topologically protected structure. One challenge for using these magnetic states in applications lies on controlling the nucleation process and stabilization that usually requires an external force. Here, we report on the evidence of skyrmions in unpatterned symmetric Pd/Co/Pd multilayers at room temperature without prior application of neither electric current nor magnetic field. Decreasing the ferromagnetic interlayer thickness, the tuning of the physical properties across the ferromagnetic/non-magnetic interface gives rise to a transition from worm like domains patterns to isolated skyrmions as demonstrated by magnetic force microscopy. On the direct comparison of the measured and simulated skyrmions size, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) was estimated, reveling that isolated skyrmions are just stabilized at zero magnetic field taking into account non-null values of iDMI. Our findings provide new insights towards the use of stabilized skyrmions for room temperature devices in nominally symmetric multilayers.

Reversal of domain wall chirality with ferromagnet thickness in W/(Co)FeB/MgO systems

We investigate the effect of the Dzyaloshinskii-Moriya interaction (DMI) on domain wall (DW) configurations in W/(Co)FeB/MgO structures with varying ferromagnet (FM) thicknesses. The DW chirality and effective DMI field are evaluated from field-induced DW motion. The results indicate a reversal of DW chirality with the FM thickness irrespective of the FM material (CoFeB or FeB) and the crystallographic phase of W (α or β phase). The observed change in the magnitude of the DMI field is supported by an additional measurement of spin-orbit torque assisted magnetization switching under in-plane magnetic fields. The present findings offer previously unknown insights into the origin of interfacial DMI and indicate the co-existence of multiple factors governing DW chirality in systems with broken inversion symmetry.

Thickness-Dependent Perpendicular Magnetic Anisotropy and Gilbert Damping in Hf/Co20Fe60B20/MgO Heterostructures

Command over fundamental magnetic properties, such as magnetic anisotropy and Gilbert damping, could lead to a universal magnetic memory with long data retention and ultralow power consumption. The authors study these properties in film heterostructures fabricated on industry-friendly platforms, with subnanometer deposition control. By tweaking the ferromagnet's thickness by one monolayer, a 25% reduction in power consumption is projected for memory devices with the proposed film stack, by virtue of a 50% reduction in the damping parameter. These discoveries have immediate implications for memory applications, and will accelerate their development.

Aging Effects in the Nonequilibrium Behavior of Magnetic Superstructures and Their Manifestation in Magnetoresistance

The nonequilibrium behavior of the magnetic superstructures consisting of alternating magnetic and nonmagnetic nanolayers is numerically simulated by Monte Carlo methods. An analysis of the calculated two-time dependence of an autocorrelation function during the evolution from a high-temperature initial state has revealed aging effects, which are characterized by slowing down of the correlation effects in the system when the waiting time increases. In contrast to bulk magnetic systems, the aging effects are shown to appear in magnetic superstructures both near the critical ferromagnetic ordering temperature Tc in films and in a low-temperature phase at T ≤ Tc. The aging effects in the correlation processes in a magnetic multilayer structure weaken when ferromagnetic film thickness N increases at T = Tc(N), and these effects increase with film thickness N at temperatures T = Tc(N)/2. When simulating the transport properties of the Co/Cu(001)/Co structure, we calculated the temperature dependence of equilibrium magnetoresistance and were the first to reveal the influence of nonequilibrium behavior of the structure on the magnetoresistance and the manifestation of the aging effects in it.

Non-destructive evaluation of ferromagnetic material thickness using Pulsed Eddy Current sensor detector coil voltage decay rate

A ferromagnetic material thickness quantification method based on the decay rate of the Pulsed Eddy Current sensor detector coil voltage is proposed. An expression for the decay rate is derived and the relationship between the decay rate and material thickness is established. Pipe wall thickness estimation is done with a developed circular sensor incorporating the proposed method, and results are evaluated through destructive testing. The decay rate feature has a unique attribute of being lowly dependent on properties such as sensor shape and size, and lift-off, enabling the method to be usable with any detector coil-based sensor. A case study on using the proposed method with a commercial sensor is also presented to demonstrate its versatility.

MODELING OF CLAMPING MAGNETS INTERACTION WITH FERROMAGNETIC SURFACE FOR WHEEL MOBILE ROBOTS

The mathematical models of clamping permanent and electro- magnets for clamping force determination by means of field theory (method of secondary sources and features of integrated forces calculation) are offered for magnetically operated movers of mobile robots (MR) for moving on inclined or vertical ferromagnetic surfaces. The results of experimental investigations of magnetic induction and clamping force measurements in different spatial positions of clamping magnet relative to the ferromagnetic surface are given. The developed simulation models based on field theory and finite element method allow to validate the obtained functional dependencies with respect to gaps between magnet and working surface, caused by excrescences of shells, and ferromagnetic surface thickness changes due to its technological features, as well as help at the stage of MR’s design. Authors propose control and measuring system of clamping force that can be successfully used for new and existing MRs’ constructions for their reliable operating on complex surfaces taking into account the nature of their uncertainties (ferromagnetic or non-ferromagnetic).

First principles calculation for Gilbert damping constants in ferromagnetic/non-magnetic junctions

We evaluated an intrinsic α in ferromagnetic (FM)/non-magnetic (NM) junctions from first principles (FM = Co, Fe, and Ni and NM = Cu, Pd, and Pt) to investigate the effects of the inserted NM layer. α is calculated by liner muffin-tin orbital methods based on the torque-correlation model. We confirmed that Gilbert damping is enhanced and saturated as NM thickness increases, and that the enhancement is greater in NM materials having a stronger spin-orbital interaction. By contrast, the calculated FM thickness dependences of α show that Gilbert damping tends to decrease and be saturated as the FM thickness increases. Under the torque-correlation model, the dependences of α on FM and NM thickness can be explained by considering the electronic structure of the total system, including junction interfaces, which exhibit similar behaviors derived by spin pumping theory.

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

Josephson junctions containing ferromagnetic layers are of considerable interest for the development of practical cryogenic memory and superconducting qubits. Such junctions exhibit a ground-state phase shift of π for certain ranges of ferromagnetic layer thicknesses. We present studies of Nb based micron-scale elliptically shaped Josephson junctions containing ferromagnetic barriers of Ni81Fe19 or Ni65Fe15Co20. By applying an external magnetic field, the critical current of the junctions is found to follow characteristic Fraunhofer patterns and display sharp switching behavior suggestive of single-domain magnets. The high quality of the Fraunhofer patterns enables us to extract the maximum value of the critical current even when the peak is shifted significantly outside the range of the data due to the magnetic moment of the ferromagnetic layer. The maximum value of the critical current oscillates as a function of the ferromagnetic barrier thickness, indicating transitions in the phase difference across the junction between values of zero and π. We compare the data to previous work and to models of the 0-π transitions based on existing theories.