Ferromagnetic Layer Thickness Research Articles

Micromagnetic study of exchange bias effect in sub-micron dots of Co2MnSi interfaced with uncompensated IrMn

Owing to its crucial role in spintronics devices, the exchange bias (EB) phenomenon has been extensively investigated in various ferromagnet (FM) and antiferromagnet (AFM) bilayers since its discovery in Co/CoO core–shell nanoparticles. In this study, we present the emergence of negative EB for the first time in the Co2MnSi Heusler alloy interfacing with an uncompensated AFM, exhibiting analogous anisotropy to the IrMn. Due to the high pinning and IrMn anisotropy values, EB is stronger here. Investigation into the influence of ferromagnetic layer thickness (tFM) on exchange bias reveals an inverse relationship, while coercivity displays a non-monotonic increase. The analysis of spin canting angles suggests the presence of a maximum canting angle in the Co2MnSi layer close to the interface. We thoroughly analyze the spin configurations at the interface as well as away from it in the Co2MnSi (25 nm)/IrMn (5 nm) bilayer to better understand the mechanism of magnetization reversal. Interestingly, our findings unveiled distinct spin behaviors for the first and second reversals. In cases of small AFM thicknesses (tAFM), the exchange field is proportionate to the tAFM, contrasting with large tAFM, where it scales as 1/tAFM. Notably, coercivity demonstrates an increasing behavior across all tAFM variations. The angular dependence of the Heusler alloy revealed a four-fold symmetry indicative of cubic anisotropy and a two-fold symmetry representative of uniaxial anisotropy. The angular dependency study of exchange bias indicated similar clockwise (CW) and counterclockwise (CCW) rotations, with cos (θ) unidirectional dependence. However, loop shifting revealed that the lower pinning ability at 0° was due to a low Meiklejohn-Bean parameter (R) value. Additionally, through the manipulation of the R-parameter, we can tune the magnitude of the coercive field and EB. All these results are crucial for the utilization of the Co2MnSi/IrMn heterostructures for various applications in spintronics-based devices.

Quantum well oscillations in giant magnetoresistance and conductance with ferromagnetic free layer thickness in spin-valve structures with inverted [Co/Pt]n/Co reference layer

We present a study on the conductance and giant magnetoresistance effect in Co/Cu/(Co/Pt)4/Co/IrMn spin valves, where the inverted [Co/Pt]4/Co reference layer and the Co free layer exhibit fcc (111) and hcp (100) orientations along the z-axis. The term ’inverted’ refers to sub-nanometer thickness for both Co and Pt layers, with Pt having a smaller thickness compared to Co. By varying the thickness of the Co free layer from 0.3 to 3.2 nm, we observe oscillations in both in-plane and perpendicular conductance as well as giant magnetoresistance, displaying a superposition of two periods (2.6 and 3.7 monolayers of the Co free layer). First-principle calculations suggest that these oscillations can be attributed to quantum well states occurring in minority channels between fcc Co(111) and hcp Co(100) within multilayered structures of Co/Cu(111)/Co, thereby providing an opportunity for precise manipulation of spin electrons within sub-nanometer scale textured layers.

Interfacial Magnetic Anisotropy Controlled Spin Pumping in Co60Fe20B20/Pt Stack

Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of the CoFeB thin film by tilting the magnetization from in-plane to out-of plane direction due to interfacial anisotropy from higher thickness of CoFeB thin film. The ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 1.10[Formula: see text]MA/m2 to 2.40[Formula: see text]MA/m2, with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices.

Formation of helical spin alignment in the AFM/FM/AFM trilayers by spin–orbit torque controlled exchange bias

Non-collinear spin structures can exhibit unusual magnetic properties that cannot be expected in an ordinary collinear ferromagnet (FM) due to the chiral alignment of magnetic moments, offering new opportunities for applications in the field of spintronics. In the present study, we demonstrate that exchange bias pinning can be applied to a single FM layer in two different directions simultaneously, resulting in modified magnetic behaviors due to the formation of non-collinear helical spin structures in the multilayers of Co0.7Ni0.3O (antiferromagnet, AFM)/Co0.7Ni0.3 (FM)/Co0.7Ni0.3O (AFM)/Pt (heavy metal, HM). The pinning of spins at one interface between FM and AFM/HM was controlled by spin Hall current originating from the electrical current through the HM layer at room temperature, while the spins at the other interface between FM and AFM were pinned in a fixed direction, hence allowing for the formation of a helical spin structure along the FM layer thickness with controllable chirality at room temperature. Modified magnetic behaviors of a helical spin structure were confirmed from measurements of magnetic hysteresis and magnetoresistance, as well as direct observation of magnetic domains.

Planar tunneling Hall effect in two-layer system

We investigate planar Hall effect and related phenomena in the two-layer system consisting of ferroelectric barrier with spin–orbit coupling and thin ferromagnetic layers. We demonstrate that planar Hall effect in this system is a size effect which originates from finite thickness of ferromagnetic layer and spin–orbit coupling linear in wave number in the barrier. Our calculations reveal that Hall current, spin Hall current and spin accumulation in the barrier layer as well as in the ferromagnetic layer can be manipulated by the reversal of electrical polarization of the barrier and by changing the direction of magnetization of ferromagnetic layer.

The Role of Ferromagnetic Layer Thickness and Substrate Material in Spintronic Emitters.

In this article, we investigate optically induced terahertz radiation in ferromagnetic FeCo layers of varying thickness on Si and SiO2 substrates. Efforts have been made to account for the influence of the substrate on the parameters of the THz radiation generated by the ferromagnetic FeCo film. The study reveals that the thickness of the ferromagnetic layer and the material of the substrate significantly affect the generation efficiency and spectral characteristics of the THz radiation. Our results also emphasize the importance of accounting for the reflection and transmission coefficients of the THz radiation when analyzing the generation process. The observed radiation features correlate with the magneto-dipole mechanism, triggered by the ultrafast demagnetization of the ferromagnetic material. This research contributes to a better understanding of THz radiation generation mechanisms in ferromagnetic films and may be useful for the further development of THz technology applications in the field of spintronics and other related areas. A key discovery of our study is the identification of a nonmonotonic relationship between the radiation amplitude and pump intensity for thin films on semiconductor substrates. This finding is particularly significant considering that thin films are predominantly used in spintronic emitters due to the characteristic absorption of THz radiation in metals.

Exchange bias effect in polycrystalline Bi0.5Sr0.5Fe0.5Cr0.5O3 bulk

Bulk Bi0.5Sr0.5Fe0.5Cr0.5O3 (BSFCO) is a new compound comprising the R3c structure. The structural, magnetic property and exchange bias (EB) details are investigated. The material was in the super-paramagnetic (SP) state at room temperature. Exchange bias usually occurs at the boundary between different magnetic states after field cooling (HFC) acts on the sample. Here the result shows that changing HFC from 1 to 6 T reduces the HEB value by 16% at 2 K at the same time. Meanwhile, HEB diminishes as the ferromagnetic layer thickness increases. The variation of (the thickness of ferromagnetic layer) tFM with the change of HFC leads to the tuning of HEB by HFC in BSFCO bulk. These effects are obviously different from the phenomenon seen in other oxide types.

Magnetization Dynamics in Proximity-Coupled Superconductor-Ferromagnet-Superconductor Multilayers. II. Thickness Dependence of the Superconducting Torque

In this work, we study magnetization dynamics in superconductor-ferromagnet-superconductor thin-film structures. Results of the broad-band ferromagnetic resonance spectroscopy are reported for a large set of samples with varied thickness of both superconducting and ferromagnetic layers in a wide frequency, field, and temperature ranges. Experimentally the one-dimensional anisotropic action of superconducting torque on magnetization dynamics is established; its dependence on thickness of layers is revealed. It is demonstrated that experimental findings support the recently-proposed mechanism of the superconducting torque formation via the interplay between the superconducting kinetic inductance and magnetization precession at superconductor-ferromagnet interfaces.

Magnetization reversal mechanisms in highly corrugated thin films

Nanopatterned ferromagnetic (FM) thin films have specific characteristics that make them a workhorse for sensors based on magnonic, magnetoplasmonic, or anisotropic magneto-resistive effects. Undulated FM thin films have been studied because of their tunable uniaxial anisotropy. They have been traditionally understood by means of Schlömann's model taking account of shape-induced magnetic anisotropies in softly corrugated systems. Here, we show how it cannot describe accurately the magnetic behavior of highly corrugated FM systems within a thickness region of less than the ripple amplitude. We report on the magnetization reversal processes detected in Permalloy films deposited onto highly corrugated patterns (250 nm in periodicity, 180 nm in amplitude) in a wide thickness range (15–150 nm), finding both that the anisotropy of the system does not correspond to a uniaxial type for FM thicknesses larger than 40 nm and that the anisotropy of the system increases with the FM thickness. Based on the results, we hypothesize that whereas Schlömann's model is valid for softly corrugated thin films, it fails to explain magnetization reversal processes of highly corrugated thin films, especially when the ripple amplitude is much greater than the deposited FM layer thickness. By means of micromagnetic simulations, we find an increment of anisotropy with thickness, just as in the experimental, as well as determine the arise of magnetic domains at the ridges of high thickness corrugated FM thin films. This approach will help to get a better understanding of operating mechanisms in magnetic field sensors based on undulated ferromagnetic materials.

Nonvolatile memory cell using a superconducting-ferromagnetic π Josephson junction

Storage of a single magnetic flux quantum in a superconducting loop containing a Josephson junction represents a promising unit cell configuration for construction of a cryogenic memory of superconducting digital circuits. However, application of a DC bias current is required for operation of such a memory cell to maintain trapping of the flux quantum in the storage loop. In this work, we present a superconducting memory cell that uses a superconducting-magnetic π junction. The cell characteristics show flux quantum hysteresis centering at the zero-bias current. We develop a fabrication process that combines superconductor–ferromagnet–superconductor (SFS) junctions with superconductor–normal metal–superconductor (SNS) junctions. The critical current density of the SFS junctions shows a 0–π oscillation as a function of the ferromagnetic layer thickness. The formation of the π junction is confirmed further by the flux modulation curves of a superconducting quantum interference device made from SNS junctions with an additional SFS junction.

Interference phenomena in Josephson junctions with ferromagnetic bilayers: Spin-triplet correlations and resonances

We study the Josephson effect in planar $\mathrm{S}{\mathrm{F}}_{1}{\mathrm{F}}_{2}\mathrm{S}$ junctions that consist of conventional $s$-wave superconductors (S) connected by two metallic monodomain ferromagnets (${\mathrm{F}}_{1}$ and ${\mathrm{F}}_{2}$) with an arbitrary transparency of interfaces. We solve the scattering problem in the clean limit based on the Bogoliubov--de Gennes equation for both spin-singlet and odd in frequency spin-triplet pairing correlations. We calculate numerically the Josephson current-phase relation $I(\ensuremath{\phi})$. While the first harmonic of $I(\ensuremath{\phi})$ is completely generated by spin-singlet and short-range spin-triplet superconducting correlations, for noncollinear magnetizations of ferromagnetic layers the second harmonic has an additional long-range spin-triplet component. Therefore, for a strong ferromagnetic influence, the long-range spin-triplet contribution to the second harmonic dominates. We find an exception due to the geometric resonance for equal ferromagnetic layers when the first harmonic is strongly enhanced. Both first and second harmonic amplitudes oscillate with ferromagnetic layer thicknesses due to $0\text{\ensuremath{-}}\ensuremath{\pi}$ transitions. We study the influence of interface transparencies and find additional resonances for finite transparency of the interface between ferromagnetic layers.

Tuning the Coexistence Regime of Incomplete and Tubular Skyrmions in Ferromagnetic/Ferrimagnetic/Ferromagnetic Trilayers.

The development of skyrmionic devices requires a suitable tuning of material parameters to stabilize skyrmions and control their density. It has been demonstrated recently that different skyrmion types can be simultaneously stabilized at room temperature in heterostructures involving ferromagnets, ferrimagnets, and heavy metals, offering a new platform of coding binary information in the type of skyrmion instead of the presence/absence of skyrmions. Here, we tune the energy landscape of the two skyrmion types in such heterostructures by engineering the geometrical and material parameters of the individual layers. We find that a fine adjustment of the ferromagnetic layer thickness, and thus its magnetic anisotropy, allows the trilayer system to support either one of the skyrmion types or the coexistence of both and with varying densities.

Setting of the magnetic structure of chiral kagome antiferromagnets by a seeded spin-orbit torque.

The current-induced spin-orbit torque switching of ferromagnets has had huge impact in spintronics. However, short spin-diffusion lengths limit the thickness of switchable ferromagnetic layers, thereby limiting their thermal stability. Here, we report a previously unobserved seeded spin-orbit torque (SSOT) by which current can set the magnetic states of even thick layers of the chiral kagome antiferromagnet Mn3Sn. The mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface with spin currents arising from an adjacent heavy metal while also heating the layer above its magnetic ordering temperature. This interface region seeds the resulting spin texture of the entire layer as it cools down and, thereby, overcomes the thickness limitation of conventional spin-orbit torques. SSOT switching in Mn3Sn can be extended beyond chiral antiferromagnets to diverse magnetic systems and provides a path toward the development of highly efficient, high-speed, and thermally stable spintronic devices.

Annealing Temperature and Thickness Dependencies of Perpendicular Magnetic Anisotropy and Dzyaloshinskii– Moriya Interaction of Pt/Co/MgO Thin Films

Effects of the ferromagnetic layer thickness and annealing temperature were studied in Pt-/Co-/MgO-based systems grown on thermally oxidized Si substrates using magnetron sputtering system. The Co thicknesses were varied in the range of 1–6 nm and the samples were annealed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex situ</i> at 200 °C, 300 °C, and 400 °C. Vibrating sample magnetometry was used to determine the magnetization at saturation and the magnetic dead layer thickness that were found to be insignificantly affected by the annealing. Brillouin light scattering, in Damon–Eshbach configuration, was used to measure the thermally excited spin wave (SW) frequencies versus the in-plane applied magnetic field and the SW vector which were used to investigate the perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii–Moriya interaction (iDMI), respectively. The Co effective thickness dependence of the effective magnetization ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M_{\mathrm {eff}}$ </tex-math></inline-formula> ), which provides information on the PMA constants, shows the existence of two linear regimes with different slopes due to misfit strain-induced magnetoelastic anisotropy contributions to volume and surface terms. The volume magnetocrystalline anisotropy constant increases monotonously with annealing temperature, whereas the pure surface (Néel type) constant increases with annealing up to 300 °C and degrades at 400 °C. The surface iDMI constant decreases with increasing annealing temperature and no obvious correlation with interface PMA constant has been found, suggesting different interfaces contribution to PMA and iDMI.

Photon spin angular momentum driven magnetization dynamics in ferromagnet/heavy metal bilayers

Thin-film magnetization controlled by optical helicity has been recently reported. Although circularly polarized light has spin angular momentum, helicity-dependent all-optical magnetization switching is mediated by the stochastic thermal process, such as magnetic circular dichroism, and the effect of photon spin angular momentum is considered to be a secondary role. Conversely, the inverse Faraday effect in ferromagnetic thin films and photon spin angular momentum injection into heavy metal thin films have been observed, which can induce torque on metallic thin-film magnets. In this study, we show photon spin angular momentum driven magnetization dynamics in bilayers of Co/(Pt, Au) thin films with various thicknesses. The heavy metal Pt, Au, and ferromagnetic Co layer thickness dependencies of photon spin angular momentum driven torques are discussed in terms of field-like torque owing to the inverse Faraday effect and spin-transfer torque caused by photon spin angular momentum injection into the heavy metal layer with details of optical and magnetic properties. This study provides a better understanding of photon spin angular momentum induced magnetization dynamics in metallic thin-film heterostructures for efficient photon-driven magnetization manipulation.

Effects of underlayer on structures and magnetic properties of Fe&lt;sub&gt;65&lt;/sub&gt;Co&lt;sub&gt;35&lt;/sub&gt; alloy films

Fe&lt;sub&gt;100-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Co&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 30–40) alloys have the highest saturation magnetizations, 4π&lt;i&gt;M&lt;/i&gt;&lt;sub&gt;s&lt;/sub&gt; ≥ 24 kG (1 G = 10&lt;sup&gt;–4&lt;/sup&gt; T). Therefore, FeCo thin flms have been widely used in microwave magnetic devices. However, the as-deposited FeCo film has a large coercivity, which is attributed to the large saturation magnetostriction and high magneto-crystalline anisotropy&lt;i&gt;.&lt;/i&gt; On the basis of maintaining high saturation magnetization, adding an appropriate underlayer is a simple and effective method to reduce the coercivity of the film and facilitate the magnetic field-induced in-plane uniaxial magnetic anisotropy. Since these kinds of films are used in a high-frequency environment, the eddy current loss in GHz band must be considered. For a certain film material, the thinner the film, the lower the eddy current loss is. However, at present, the thickness of ferromagnetic layer is generally tens of nanometers or even hundreds of nanometers, which will not help to suppress the eddy current loss at high frequency. In the present study, to obtain FeCo films with good soft magnetic properties and excellent high-frequency characteristics, Fe&lt;sub&gt;65&lt;/sub&gt;Co&lt;sub&gt;35&lt;/sub&gt; alloy films with a thickness of 13 nm and different underlayers (Cu, Co and Ni&lt;sub&gt;80&lt;/sub&gt;Fe&lt;sub&gt;20&lt;/sub&gt;) are prepared by magnetron sputtering. The effects of different underlayer materials and different NiFe underlayer thickness values on the structures and magnetic properties of FeCo films are studied. The results show that the introduction of underlayers can increase the in-plane uniaxial magnetic anisotropies of films, and the soft magnetic properties of films are significantly improved. The reason why the good soft magnetic properties can be achieved is attributed to the grain refinement, the dipolar interaction between layers, and the reduction of surface roughness. For different underlayer materials with the same thickness, NiFe underlayer can obviously improve the soft magnetic properties of FeCo films: the covercivity of easy axis is 23 Oe. By changing the thickness of NiFe underlayer, the dynamic magnetic properties of films can be adjusted. The resonance frequency changes from 3.13 GHz for NiFe(1 nm)/FeCo(13 nm) film to 2.78 GHz for NiFe(9.3 nm)/ FeCo(13 nm) film. For all NiFe/FeCo bilayer films, the real part of the permeability &lt;i&gt;μ′&lt;/i&gt; at low frequency has a large value of 350–450, and the damping coefficient &lt;i&gt;α&lt;/i&gt; shows a small value of 0.01–0.02. In addition, the smaller film thickness can reduce eddy current loss, which contributes to its application in high-frequency microwave magnetic devices.

Role of disorder in the superconducting proximity effect in a−NdNi5/Nb bilayers

The search for optimal materials to be used in superconductor/ferromagnet hybrids is fundamental to the development of superconducting spintronic devices. One of the main issues which should be tackled is the optimum ferromagnetic material that should be used in these systems, i.e., a ballistic ferromagnet with a large value of the exchange energy or a weak ferromagnet with diffusive transport. Here, we investigate the proximity effect between Nb and amorphous ${\mathrm{NdNi}}_{5}$. We observe that the electrical transport is influenced by the strong disorder in the ferromagnetic layers. For small values of the ferromagnetic layer thickness, the depression of the superconducting critical temperature is stronger than as observed in metallic ferromagnetic layers with comparable values of the exchange energy. On the contrary, for a large thickness of the ferromagnet the interface transparency is very small and the effect of the ferromagnet on the superconductor becomes negligible. The results are in line with recent observations and confirm the prominent role played by the structural and morphological properties of the ferromagnet on the physics of superconductor/ferromagnet bilayers.

Readout of an antiferromagnetic spintronics system by strong exchange coupling of Mn2Au and Permalloy

In antiferromagnetic spintronics, the read-out of the staggered magnetization or Néel vector is the key obstacle to harnessing the ultra-fast dynamics and stability of antiferromagnets for novel devices. Here, we demonstrate strong exchange coupling of Mn2Au, a unique metallic antiferromagnet that exhibits Néel spin-orbit torques, with thin ferromagnetic Permalloy layers. This allows us to benefit from the well-established read-out methods of ferromagnets, while the essential advantages of antiferromagnetic spintronics are only slightly diminished. We show one-to-one imprinting of the antiferromagnetic on the ferromagnetic domain pattern. Conversely, alignment of the Permalloy magnetization reorients the Mn2Au Néel vector, an effect, which can be restricted to large magnetic fields by tuning the ferromagnetic layer thickness. To understand the origin of the strong coupling, we carry out high resolution electron microscopy imaging and we find that our growth yields an interface with a well-defined morphology that leads to the strong exchange coupling.

Size and heat treatment effects in the electrical conduction of the [Fe/SiO]n discontinuous multilayers

The present work shows the comprehensive investigations of crystal structural and electrical properties of [Fe/SiO]n discontinuous multilayers with various effective thickness of ferromagnetic (dFe) and insulator (dSiO) layers. Results of the study performed by the transmission electron microscopy have demonstrated that the formation of discontinuous magnetic layers occurs at the decrease of dFe from 10 nm to 4 nm. Namely, the discontinuous multilayers at dF = 4 nm are granular nanostructures that consist of an array of Fe particles separated by the SiO insulator channels. It was shown that depending on the heat treatment conditions and an effective thickness of ferromagnetic and insulator layers, three modes of conductivity (metallic, transitional and insulation) were realized. The temperature coefficient of resistance takes a positive, negative, or close to zero value depending on the conductivity mode. The correlation between the crystal structure and electrical properties were discussed.

Effects of ferromagnetic-material thickness on magnetoelectric voltage transformation in a multiferroic heterostructure

A magnetoelectric (ME) voltage transformer is fabricated on the basis of a ferromagnetic (FM)–piezoelectric (PE) heterostructure comprising two equally thick laminated layers of an amorphous FM alloy and a piezoceramic lead zirconate-titanate layer sandwiched between them. The structure, placed inside an excitation coil, is electrically poled and magnetized in the direction of the long axis. The primary voltage is applied to the coil and the secondary voltage is measured between the electrodes of the PE material. It is shown for the first time that the change in the total thickness of magnetic layers significantly influences the transformer´s characteristics. At the largest total thickness of FM layers of 138 µm, the open-circuit voltage transformation ratio K has a maximum value of about 20, and the power transfer efficiency η at a matched resistive load of about 20 kΩ reaches 45%. The variation of the control magnetic field in the range of 0–21.6 kA m−1 makes it possible to change the voltage transformation ratio K from zero to the maximum value. A simple model allows one to calculate the dependence of the characteristics of the ME transformer on the frequency of the primary voltage, thickness of the FM layers, control magnetic field, and the load.