Py Layer Research Articles

Annealing effect of sputter-grown Pt/Ni80Fe20/Pt sandwich trilayer films on Gilbert damping

The annealing effect of Pt/Permalloy(Py: Ni80Fe20)/Pt sandwich trilayer films on Gilbert damping was investigated through ferromagnetic resonance, x-ray diffraction (XRD), and transmission electron microscopy (TEM) measurements. We estimated the variation of the effective Gilbert damping constant with 30-min-post-annealing temperature in the range of Ta = 600–800 K. The damping constant for the as-grown sample was estimated to be αeff = 0.021, but it linearly increased with Ta until 700 K and then drastically increased by 4.5 times up to αeff = 0.093 at Ta = 800 K. Delicate analyses of the XRD and TEM data revealed that the damping constant in such a thin-layered structure is closely related to the structural characteristics of the interfaces and atomic intermixing through them. The linear increase in αeff in the range of Ta = 650–700 K was ascribed to the alloying/intermixing effect between the Pt and Py layers through both interfaces, whose effect was found to be related to the degradation of the Pt (111) texture. Meanwhile, in the range of Ta = 750–800 K, αeff further and rapidly increased relative to the values in the range of Ta = 650–700 K. Specifically, the 800-K-post-annealed sample showed that its damping constant was 4.5 times larger than that of the as-grown sample, its saturation magnetization decreased by ∼54%, and also that new stoichiometric compounds such as NiPt, FePt, and NiFePt appeared. TEM equipped with energy-dispersive x-ray spectroscopy also revealed that Ta gives rise to atomic intermixing between the Pt and Py thin layers through both interfaces, resulting in interface shifting. This work provides the means to control the Gilbert damping constant and better to understand complex multi-layered structures in potential spintronic devices.

Fabrication of Graphene-Inserted Tunneling Device (GiTD) for Emerging Spin Devices

Objective:Spintronics, which utilizes the spin degree of freedom of elementary particles, holds the promise for many attractive applications including non-volatile logic and storage. However, the main challenge is to achieve spin manipulation (spin injection, transport and detection) with high efficiencies. Here in this work, we focus on spin injection from the nanomagnet to another layer, which is typically degraded by any conductivity mismatch and/or interfacial imperfections. Inserting a tunnel barrier between the ferromagnetic and channel layers has shown to relieve this problem.The best way to quantitatively study spin polarization of a ferromagnetic thin film that is modified by any spin flipping at the interfaces is to directly measure the tunneling spin polarization across the ferromagnet and the tunnel barrier using superconducting tunneling spectroscopy (STS) [1]. One practical challenge in using this technique to investigate a role of any specific material of interest (any insertion layer other than the ferromagnet itself) in terms of spin polarization is to fabricate such a multi-layered device structure. The difficulty mainly arises from oxidation of underlying metallic layers.Recently, graphene has been considered as a promising material for spintronic applications. Advantages of graphene include weak spin-orbit coupling and low lattice mismatch with conventional ferromagnets such as Permalloy (Ni80Fe20). This leads researchers to re-think graphene as the long-range spin transport channel or the efficient tunnel barrier material(or both) [2]. The primary objective of this work is to demonstrate the graphene-inserted tunneling device (GiTD) that enables subsequent STS measurements to study how graphene impacts tunneling spin polarization and ultimately use it to enhance the spin injection efficiency in spin devices.New results:Fig. 1 illustrates a schematic device structure (GiTD) investigated in this work. Initially, a superconducting thin layer (5 nm) of Al was deposited by sputtering. A CVD-grown monolayer graphene was then transferred on top, which was followed by deposition of Permalloy (Py). Because a thin tunnel barrier of AlOx is easily formed by exposing the sample to ambient air, we study the role of graphene in the F/I/S device stack (F: ferromagnet, I: insulator, S: superconductor) with a native thin oxide formed at the graphene/S-layer interface. We patterned the graphene layer by photolithography and O2-plasma etching to avoid an electrical short between the top and bottom layers in the cross-bar geometry. Details of the process flow are depicted in Fig. 2, and a micrograph image of the fabricated GiTD structure (mounted on the STS holder) is shown in Fig. 3. Raman spectroscopy [3] was also conducted to confirm both the presence and absence of graphene in respective regions (Fig. 4).One important piece of innovation introduced in this work is an “offset contact” method that was developed to ensure any unnecessary oxidation of contact electrodes does not adversely impact the electrical testing. Since the STS measurement requires the sample to be mounted on a chip carrier wire-bonded to separate contact pads on the substrate, deposition of any additional metallic layer (known to facilitate the wire bonding process) may result in oxide formation on the underlying contact layer unless all constituent layers are deposited in-situ. As seen in Fig. 2 (steps (2) through (7)), we deposited first a thin film of Py and then put the S-layer (Al) with a slight offset to Py. By depositing another Py layer on top, a direct link between the top Py and bottom Py was established to bypass the oxidized Al layer in between. Fig. 5 shows the current-voltage (I-V) characteristic of the fabricated control device (Py/AlOx/Al) measured at room temperature, clearly demonstrating that (1) current flows well through the multi-layered contact pads and (2) memristor-like switching behavior is observed due to formation of a thin metal oxide (AlOx).Significance:By conducting the STS measurements that compare the control device with the graphene-inserted device (GiTD), the role of graphene in terms of tunneling spin polarization will be quantitatively studied. This will enable us to develop the novel tunnel barrier material, possibly consisting of 2D graphitic nanostructures. It will be best interfaced with both the ferromagnetic contact (spin injector or detector) and the graphene-based spin transport channel. Ultimately, this study will greatly impact the development of active spin devices such as spin FET where achieving the high spin injection still remains the biggest challenge.

Composition dependence of the spin-anomalous Hall effect in a ferromagnetic Fe–Co alloy

We investigated the spin-anomalous Hall effect (SAHE) in ferromagnetic Fe–Co binary alloys with different compositions. The ferromagnetic resonance (FMR) measurement in Fe1−xCox/Ti/Py trilayer structures was performed to detect the SAHE in Fe1−xCox. The modulation of FMR linewidth in the Py layer due to the SAHE was observed. The effective spin-anomalous Hall angle changed from a positive value (+0.027 at x = 0.15) to a negative value (−0.012 at x = 0.65) with increasing Co composition. This study provides a deeper understanding of the SAHE.

Computational Study of Magnetic Switching Mechanism of Nanoscale MRAM Cells

Investigation of magnetic switching of nanoscale single MRAM cells of different shapes and sizes is imperative for their applications in future magnetic memory devices. To this end, we have investigated the magnetic switching mechanism of nanoscale single MRAM cells of two different shapes with varying lateral aspect ratios by computational micromagnetic simulation. We have analysed how various parameters such as the coercive field, remanence and saturation field were affected by the variation in magnetic field. We have also analysed the change in shape of the hysteresis loops of the various samples. The magnetization reversal states were simulated to justify the spatial coherence of magnetization switching. As a result, the cells with higher aspect ratio show the Py and Fe layers forming antiparallel states in the plateau similar to synthetic antiferromagnets. As we reduce the aspect ratio, more complex quasi-uniform magnetic states are observed which are even more complicated for elliptical cells. The rectangular cell with the highest aspect ratio of 2.5 shows the most coherent and predictable switching behaviour, showing its suitability for the application of MRAM cells.

Spin-orbit precession effect in a Py/Pt/Co tri-layer structure detected by ferromagnetic resonance

Spin–orbit torques (SOTs) acting in a Py/Pt/Co tri-layer, where the Py and Co layers are in-plane and perpendicularly magnetized, respectively, are investigated via the ferromagnetic resonance method. The resonance linewidth of the Py layer is clearly modulated by the DC bias current due to the damping-like SOT originated from the spin Hall effect in the Pt. In addition to this, the linewidth modulation efficiency is found to depend on the magnetization polarity of the perpendicularly magnetized Co layer. We conclude that this behavior comes from an additional damping-like SOT originating from the spin–orbit precession effect at the Pt/Co interface.

Interfacial magnetic anisotropy in Py/MoS2 bilayer

The anisotropic magnetoresistance (AMR) in permalloy (NiFe, Py)/MoS2 bilayer is studied and a strong enhancement of AMR in Py layer is observed due to the MoS2 coating on its surface. The spin Hall magnetoresistance and the induced magnetic moment by interfacial magnetic proximity effect at the Py/MoS2 interface cannot explain the observed AMR enhancement. In order to find the origin of the observed MR increase in Py/MoS2 sample, the magnitude of magnetic anisotropy constants are derived from torque curves on the basis of AMR data. The enhanced AMR in the Py/MoS2 bilayers can be attributed to the interfacial hybridization at the interface. Our findings enrich the understanding of the magnetoresistance of metallic FM/transition metal dichalcogenide (TMDC) bilayers and present AMR enhancement for application in spintronic devices.

Optimization of Pt composition with magnetic thin films for magnetic field sensor application

Heavy metals like Pt play an important role in the field of spintronics due to their high spin-orbit coupling. Yet optimization condition for using it in proximity to magnetic materials is still challenging. In this letter, using post-annealing of permalloy (Py)/Pt multilayers, different composition structures of Pt with Py have been fabricated. Structural analysis showed that at temperatures higher than 200 °C the Pt diffuses into the crystal structure of Py while at lower temperatures Pt is in proximity to the Py layer. At annealing temperatures below 200 °C the magnetoresistance (MR) ratio and sensitivity increases and at higher annealing temperatures the MR ratio and sensitivity decreases. Magnetic hysteresis loop measurement showed that at lower annealing temperature, magnetic moment and therefore MR ratio increases. Results can be considered for optimization of heavy metal mixing with magnetic materials for magnetic field sensor enhancement.

Spin pump induced inverse spin Hall effect observed in Bi-doped n -type Si

An inverse spin Hall effect (ISHE) in n-type silicon was observed experimentally when conduction electrons were scattered on the spin-orbit potential of bismuth. The spin current in the silicon layer was generated by excitation of the magnetization precession during ferromagnetic resonance in a thin permalloy (Py) layer deposited on a Si layer doped by phosphor and bismuth. From the angular dependences of the dc voltage for different Py/n-Si:Bi structures aligned along the [011] or [100] crystal axes, we were able to distinguish the planar Hall effect (PHE) and ISHE contributions. The ISHE dc voltage signal was proportional to sin{\theta}*sin2{\theta} product for the structure aligned to the [011] crystal axis and to sin{\theta}*cos2{\theta} for the [100] direction. In addition, the PHE dc voltage was observed for the angles corresponded to the sin2{\theta} dependence. It means that for silicon as a many-valley semiconductor, the scattering of spins due to the spin-orbit potential induced by shallow donor in n-type material is dependent on the orientation of the valley axes relative to the direction of the magnetic field.

Spinwave detection by nitrogen-vacancy centers in diamond as a function of probe–sample separation

Magnetic field noise from magnons can reduce the lifetimes of proximate spins and degrade the performance of spin based technologies. However, spatial and temporal averaging over the area of typical field sensors makes measuring magnetic field noise challenging. Here, we use an ensemble of nitrogen-vacancy (NV) point-defects in diamond to measure the spectral profile of thermally excited spinwave noise at room temperature as a function of the distance away from a 20 nm thick Permalloy (Py) thin film. We systematically vary the separation between the NV and Py layers using a silicon-dioxide wedge and measure the longitudinal relaxation rate of the NV center ms = 0 state as a function of the separation. The measured spinwave-induced relaxation of an ensemble of NV centers is well described by a magnetostatic model of dipole fields from the spinwaves. We furthermore find that our all-optical, nonperturbative measurements of the spinwave noise can be used to extract information about the ferromagnetic source, such as magnetization, damping, and fluctuating amplitude. This technique is amenable to application with stand-off from ferromagnetic elements and from buried structures.

The influence of Cu spacer morphology in Cu/Py/Cu/Co/IrMn spin valves with induced non-collinear spin structures

Non-collinear exchange couplings between Py and Co layers (0°⩽φ⩽90°) were achieved in Py(10)/Cu(5)/Co(10)/IrMn(8) (thickness values in nm) spin valves by modifying their layer morphologies using oblique deposition in a confocal sputtering setup. Two series of spin valves were prepared by keeping the sample holder fix during the depositions, except for the Cu spacers that were deposited using either fixing (Series-I) or spinning mode (Series-II). Series-I (Series-II) has Cu spacer layers composed by oriented (non-oriented) grains and Cu/Co interfacial roughnesses relatively rougher (smoother), favoring lower (higher) interlayer exchange couplings and exchange bias fields. The magnetization easy axis of Py and the unidirectional anisotropy axis followed the expected directions, while the Co uniaxial axis is surprisingly dephased by (15±5)° for 0°⩽φ<90°. The angular offset between the Co uniaxial axis and the Py uniaxial (unidirecional anisotropy) axis was more pronounced in Series-I, indicating that the Cu spacer roughness played an important role in the interlayer exchange couplings and also in the Co spin reorientation effect. The magnetoresistance magnitudes were similar to those found in the literature for ordinary Py/Cu/Co multilayers. However, it reduces as the φ-angle increased, an effect associated with the enhancement of the interface roughness due to the misalignment of the oriented grains of adjacent layers.

Spin–orbit torque generated by spin–orbit precession effect in Py/Pt/Co tri-layer structure

In this report, current-induced spin–orbit torques (SOTs) in a permalloy (Py)/Pt/Co tri-layer system are investigated. In addition to the conventional SOTs generated from the spin Hall effect in the Pt layer, we observed a distinct existence of unconventional SOT acting on the in-plane magnetized Py layer in which the polarity of SOT reverses with the magnetization of perpendicularly magnetized Co. From its symmetry and experimental verifications, we conclude that this unconventional SOT, especially the damping-like torque term, originates from the additional spin current generated by the spin–orbit precession effect at the Pt/Co interface. Moreover, we reveal that the polarity of this SOT strongly depends on the interface structure of the ferromagnetic material/non-magnetic metal (NM), which can be controlled by the choice of NM material. The interface structure dependence shows a consistent result with first principle calculations of the spin–orbit precession effect that were previously reported. Our findings not only broaden the origin of the current-induced SOT but also pave the way for the design of high-efficient SOT-based devices.

Anomalous Spin-Orbit Field via the Rashba-Edelstein Effect at theW/PtInterface

We have studied spin-orbit (SO) field in Ni$_{80}$Fe$_{20}$(Py)/W/Pt trilayer by means of spin-torque ferromagnetic resonance, and demonstrated that the W/Pt interface generates an extra SO field acting on the Py layer. This unprecedented field originates from the following three processes, 1) spin accumulation at W/Pt interface via the Rashba-Edelstein effect, 2) diffusive spin transport in the W layer, and 3) spin absorption into the Py layer through accumulation at the Py/W interface. Our result means that we can create extra SO field away from the ferromagnet/ metal interface and control its strength by a combination of two different metals.

Electrical and magneto-optical characterization of Py/MoS2 bilayer: A facile growth of magnetic-metal/semiconductor heterostructure

Ferromagnet/two-dimensional transition-metal dichalcogenides (FM/2D-TMDCs) have drawn great attention due to their potential impact in electronics, optoelectronics, and spintronics. However, their fabrication technology is very limited especially when manufacturing of a FM-metal/TMDC-semiconductor element is a purpose. Here, we report a successful demonstration of a NiFe (Py)/2D-TMDC (MoS2) metal/semiconductor bilayer device with a systematic study of its electronic transport and magneto-optical response. The MoS2 with a thickness of 50 nm is grown through electrodeposition technique on a sputter deposited 40 nm Py layer. I-V characterization ultimately demonstrates a Schottky response due to presence of a barrier formed at the interface. Magneto-optical Kerr effect (MOKE) represents a significant amplification as the MoS2 cap-layer formed an optical cavity. In addition, the coercive field of Py/MoS2 bilayer shows a decrease w.r.t. that of the Py layer because of proximity coupling formed at the interface. This study demonstrates not only a successful fabrication of FM-metal/TMDC-semiconductor contact, but also an optical path enhancement due to presence of the TMDC layers for optical and magneto-optical applications.

Spin-reorientation transition and temperature dependence of high-frequency properties in [Co/Pd]6/Permalloy multilayer thin films

High-frequency studies, of exchange-coupled multilayer films, are the key to advance the development of fast switching spintronic devices. In this paper, we utilize the first-order reversal curve (FORC) to improve the understanding of the spin-reorientation transition process when the permalloy (Py) thickness increases. We also investigate the temperature dependence of the high-frequency properties of exchange-coupled [Co/Pd]6/Py (t nm) multilayer films during the spin-reorientation transition (SRT). The results show that the phase state of the magnetic domain changes, from the perpendicular phase to the multi-domain phase as the Py layer increases during the SRT. The temperature-dependent high frequency results indicate that the resonance linewidth ΔHres increases gradually when the temperature decreases from 300 K to 50 K. Our experimental study offers new insight into the SRT process in exchange-coupled multilayer films and can help to promote the development of fast switching spintronic devices.

Investigation of phonons and magnons in [Ni80Fe20/Au/Co/Au]10 multilayers

The dispersion relations of surface acoustic waves and spin waves propagating in magnetic [Ni80Fe20/Au/Co/Au]10 multilayers deposited on a silicon substrate have been investigated by high-resolution Brillouin spectroscopy. We measured the spectra of spin waves for two canonical geometries where the spin waves propagate with the wave vector oriented parallel and perpendicular to the direction of static magnetization in-plane magnetized Py layer. We investigated experimentally the crossing of phononic and magnonic dispersion relations and discussed the mechanism of magnetoelastic interaction in this system.

Metal/metal-oxide thin layer heterostructure by laser treatment for memristor application

Metal-oxide memristors are one of the most important sets of memristors for use as the future memory and computing elements. In this context, developing novel methods for producing metal/metal-oxide thin film heterostructures is demanded. Here, we use a 354 nm pulsed laser for treatment of a thin (40 nm) Ni80Fe20 (Permalloy, Py) layer and observe its memristive characteristics. The laser treatment (LT) resulted in oxidation of the surface of the Py layer and therefore forming a uniform Py/Py-oxide heterostructure. The oxygen vacancy formation and rupture for memristivity mechanism is evidenced by I-V measurement of two samples with different oxide thicknesses. The sample with lower thickness of oxidized Py shows lower voltage threshold and higher ON/OFF ratio. Results show that LT method can be used for producing functional thin memristive layers from metallic materials.

All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe2/ferromagnet heterostructures.

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals1-3 or topological insulators4,5 provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure6,7; the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation8. The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling6,9 and non-trivial band structures10 with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states9-11. Atomically flat surfaces, which can be produced with high quality12, facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of ~2.96 × 105 A cm-2 suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 ± 0.06 mJ m-2 induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.

Magnetic and magnetocaloric properties of Py/Gd/CoFe/IrMn stacks

Abstract We explore whether it is possible to enhance the magnetocaloric effect (MCE) in layered systems, where a refrigerant (weak ferromagnet) is sandwiched by strong ferromagnets, by magnetizing the spacer with the effective field that results from its exchange coupling to the surroundings. Temperature variation around the Curie point of the spacer (TC) leads to a change in interlayer coupling between strongly ferromagnetic surroundings, which manifests in increasing the switching field and/or the coercivity of the softer from the surroundings. In order to verify the hypothesis above, we have prepared layered stacks of Py(10 nm)/Gd(x nm)/Co90Fe10(7 nm)/Ir20Mn80(15 nm) (Py = Ni80Fe20 is permalloy) with the Gd spacer in thickness varied from x = 20 nm to x = 5 nm. In this system we clarified how the magnetization switching in the Py layer was sensitive to temperature varied in the range between 300 K and 50 K. We have found that strong enough exchange between the Py and Gd layers emerges only at low temperatures (T

Proposal for a NbPy-based superconducting spin-valve

Superconducting critical temperature, Tc, in proximity coupled Nb/Py/Nb trilayers with Py thickness dPy = 20–432 nm is investigated as a function of the magnetic history of the samples. An enhancement of Tc is observed when an ordered pattern of magnetic stripe domains is induced in the Py layer, namely for dPy > 300 nm. This result is interpreted as a manifestation of the domain-wall superconductivity effect. The possibility to exploit these results for the design of a NbPy-based superconducting spin-valve is discussed.

Misaligned anisotropies in spin-valve films studied through magnetoresistance and magnetization measurements

Magnetization and magnetoresistance properties of Py/Cu/Py/IrMn spin valve (SV) films are studied in the framework of the modified antiferromagnetic domain-wall model applied to a granular multidomain system. In the simulations, a misalignment between the in-plane easy magnetization axes of the Py and IrMn is considered. Magnetization and magnetoresistance data are simulated for a number of field orientations and a fairly good agreement with the experiment is found. The same holds for the respective distributions of the coercive and exchange-bias field values determined from magnetoresistance first-order reversal curves (MR-FORC) obtained for magnetic field parallel to the direction of that applied during the sample deposition. Both experimental and theoretical data of the angular variations of the magnetoresistance at constant fields are successfully used to obtain the misalignment angles. For some samples, mostly those with thinner Py layer coupled to IrMn one, our results indicate that the misalignment is due to interfacial magnetic frustration. Moreover, it is shown that MR-FORC diagrams are useful to extract information about the alignment of the grains along the field-cooling directions in SVs, as well as that these can be used to determine the threshold of the continuity of the pinned magnetic layer.