Isolated Dc Dc Converter Matlab

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Room-temperature perpendicular magnetization switching through giant spin-orbit torque from sputtered BixSe(1-x)

Topological Insulator Material

Mahendra DC1, Mahdi Jamali2, Jun-Yang Chen2, Danielle Reifsnyder Hickey3, Delin Zhang2, Zhengyang Zhao2, Hongshi Li3, P. Quarterman2, Yang Lv2, Mo Li2, K. Andre Mkhoyan3 and

Jian-Ping Wang2,1,3,*

The spin-orbit torque (SOT) arising from materials with large spin-orbit coupling promises a path for ultra-low power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1-x) thin films in BixSe(1-x)/CoFeB heterostructures by using a dc planar Hall method. Remarkably, the spin Hall angle (SHA) was found to be as large as 18.83, which is the largest ever reported at room temperature (RT). Moreover, switching of a perpendicular CoFeB multilayer using SOT from the BixSe(1-x) has been observed with the lowest-ever switching current density reported in a bilayer system: 2.3 × 105 A/cm2 at RT. The giant SHA, smooth surface, ease of growth of the films on silicon substrate, successful growth and switching of a perpendicular CoFeB multilayer on BixSe(1-x) film opens a path for use of BixSe(1-x) topological insulator (TI) material as a spin-current generator in SOT-based memory and logic devices.

Summary Sentence: We demonstrated growth of smooth BixSe(1-x) films on a large silicon wafer with the largest spin Hall angle ever reported using a semiconductor industry compatible sputtering process; furthermore, we developed and switched a perpendicular CoFeB multilayer on BixSe(1-x) films for the first time at RT by BixSe(1-x), a TI material, which enables a path for reliable and efficient beyond-CMOS devices.

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Currently, SOT in spin Hall material (SHM)/ferromagnet (FM) heterostructures is of great interest due to its efficient switching of magnetization in proposed spin-based memory and logic devices (1–3). SOT has been calculated theoretically (4, 5) and observed experimentally in heavy metals (HMs)(6–10) and TIs (11–16). The in-plane charge current injected into an SHM/FM/oxide heterostructure is scattered in the vertical direction, either up or down, depending upon the spin orientation of electrons due to the spin Hall effect (SHE) (6, 7, 10, 17) or Rashba effect (9, 18). Thus, accumulated spin-polarized current in the interface between the SHM and FM transfers spin angular momentum to the FM (6, 7, 19), which can rotate the magnetization of the FM and is known as SOT.

Conventionally, the spin-polarized current is generated by using a ferromagnetic polarizer that transfers its spin angular momentum to a FM layer. Spin-transfer torque (STT) based devices suffer from large power consumption and reliability issues due to the low efficiency of the ferromagnetic polarizer (6, 20).

SOT-based memory and logic devices are superior compared to the STT-based devices because they do not require a separate polarizer for the generation of spin-polarized current and could potentially have a much more efficient spin-current source (1, 6, 10). This results in a lower writing current density and, thus, much better device reliability.

The most commonly studied spin-current generators in SHM/FM heterostructures are HMs, such as Ta (6, 8, 21), W (10, 17), Pt (7, 9), and TIs such as Bi2Se3(11, 12, 14), and (Bi0.5 Sb0.5)2Te3 (13). It has been reported by several groups that a large current density on the order of 106 -108 A/cm2 is required to switch the magnetization using SOT from HMs due to their small SHA (6, 7, 9). In the case of a TI as the spin-current generator, switching of the magnetic doped TI (Cr0.08Bi0.54Sb0.38)2Te3 layer at 1.9 K has been observed with a current density 8.9 × 104 A/cm2 (13). It is also expected that TIs will be able to demonstrate a low switching current density for the switching of magnetization at RT as a result of their large spin-orbit coupling (SOC) (11–14). However, the switching of magnetization has not yet been observed on any TI/FM bilayer system at RT. Furthermore, the practical application of single crystalline TI grown by molecular beam epitaxy (MBE) is limited due to its strict demand on the single crystal

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substrate. It is also constrained due to the presence of valleys or voids with different size and height as a result of triangular spirals (22). In this paper, we report BixSe(1-x) films with giant spin Hall angles at RT grown on silicon substrate by magnetron sputtering, which is a semiconductor industry compatible process. The dc planar Hall method is used for the characterization of SOT in a BixSe(1-x)/CoFeB heterostructure with in-plane magnetic easy-axis (23–25). At RT, the SHA (the ratio of spin-polarized current density to charge current density) of the sputtered BixSe(1-x) film is up to two orders of magnitude larger than that of HMs and approximately one order of magnitude greater than the crystalline TI, Bi2Se3. In particular, we developed a perpendicular CoFeB multilayer on BixSe(1-x) films and demonstrated switching of the magnetization using SOT arising from the BixSe(1-x) with the lowest switching current density in bilayers at RT.

Moreover, the spin Hall conductivity (SHC), which is the product of SHA and electrical conductivity of the SHM, is determined to be comparable to or larger than the previously reported values of other spin- current generators. Furthermore, the sputtered BixSe(1-x) layer on silicon substrate shows an excellent surface smoothness on a wafer scale for a better practical application in future.

In order to characterize the SOT arising from the BixSe(1-x) films, thin films with the multilayer structure Si/SiO2/MgO (2 nm)/BixSe(1-x) (tBiSe nm)/CoFeB (5 nm)/MgO (2 nm)/Ta (5 nm) were prepared, with tBiSe= 4, 8,16, and 40 nm as shown in the schematic drawing in figure 2A. Unless otherwise stated, we will use the labeling BS4, BS8, BS16, BS40 for the samples with tBiSe= 4, 8, 16, 40 nm, respectively.

Figure 1A,B shows the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of samples BS4 and BS8, respectively. The HAADF-STEM images show that BixSe(1-x) has a polycrystalline structure and that the atomic layers of Bi and Se are continuous in both the BS4 and BS8 samples. Additionally, the average grain orientation in sample BS4 is 2° with a standard deviation of 9° (from vertical c-orientation), which is almost identical to the average grain orientation in the BS8 sample (2° with a standard deviation of 8°). The energy dispersive X-ray spectroscopy (EDS) line scan shows that at the top of the BixSe(1-x) films there is stoichiometric Bi2Se3; however, there is a

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gradient of the Bi concentration from top to the bottom of the films (figure S2). The average ratio x in BixSe(1-x) film is 0.47 with ± 3% uncertainty determined by the Rutherford back scattering (RBS). Figure 1C shows the atomic force microscopy (AFM) images of Si/SiO2/MgO (2 nm)/BixSe(1-x) 4 nm film. The root mean square (RMS) value of the surface roughness of the 4 nm BixSe(1-x) film is 0.51nm.

Furthermore, we probed surface roughness propagation in full stack as shown in figure 1D. The RMS value of surface roughness 0.38 nm in BS4 sample confirms the smoothness of the full stack for future device fabrication on a wafer level.

The multilayer films are patterned into Hall cross bars 5-30 μm wide and 70 μm long, and dc planar Hall measurement is performed on the Hall cross bar with dimensions 10 μm × 70 μm, as shown in figure 2B. The bipolar input current of magnitude up to 8.5 mA is injected along the x-direction, and the

R

I α ) is measured under the application of a constant 0.5 T in-plane magnetic field while rotating the sample in the xy plane from -7 to 365° . The in-plane SOT exerted by the accumulated spin current on the interface of the BixSe(1-x)/CoFeB (figure 2A) is obtained by using

‖

, where  is the reduced Planck’s constant,   sJ is the spin-polarized current density, e is an electronic charge,

M Is The Saturation Magnetization, T Is The

thickness of the ferromagnetic layer, ˆm is the magnetization unit vector, and ˆσ is the spin-polarized current accumulation unit vector. The associated out-of-plane magnetic field with the in-plane torque (τ‖)

Ħ

. In addition to the in-plane torque, there is also an out-of-plane component of SOT due to the spin accumulation at the interface, which is given by

T

α is a coefficient that determines the efficiency of current-induced effects. The associated in- plane magnetic field with the out-of-plane torque (τ ⊥) is given by

I Α Shows Two-Fold Symmetry With Extrema At

approximately 45° in increments of 90º. In principle, the

R

is due to the combined effects of the external field, current-induced effective fields, and the anisotropy field acting on the magnetization (

, Where Β Is The Angle

between the magnetization and current flow direction). The

Is Due To The Pulling Of The

magnetization in the out-of-plane direction by a current-induced effective field (

M Is

the z-component of magnetization). The current-induced effective fields HT and HOOP can be extracted by characterizing the

I Α

for positive and negative input currents. The difference of the Hall resistance is given by (23),

(1),

where C is the resistance offset that accounts for the Hall bar imbalance,

Is The Gradient Of The

change in the anomalous Hall signal versus the externally applied out-of-plane magnetic field, and α is the angle between field and current flow direction. Figure 2D shows

I Α Versus An Externally

applied field angle for the sample BS4 at different input currents. The

I Α Increases With Increase

in input current and has a maximum located at about 180° . The

Is Obtained By Sweeping The Out-

of-plane field at a small constant input current. After considering current shunting and short circuit effects

Dh

is determined to be - 6.75, 7.33, -7.37, -7.55, -6.51 for BS4-BS40 samples, respectively.

H

versus the current density is presented in figure 2E. The

J

determined by the linear fit is as large as (99.94 ± 0.17) Oe (10-6)/ (A/cm2) for the BS4 sample, where

J

is the current density in the BixSe(1-x) layer (the uncertainty is the standard error from the linear fit).



) of the BS4 sample is determined to be 18.83 ± 0.03. Sample BS4 has the largest SHA value at RT that has been reported to date, which means that it is the most efficient spin-

J

for BS6-BS40 samples is (23.17 ± 0.48), (14.19 ± 0.24), (9.27 ± 0.03), and (2.14 ± 0.11) Oe (10-6)/(A/cm2), respectively. The SHAs for BS6-BS40 samples are determined to be 4.36 ± 0.09, 2.67 ± 0.04, 1.74 ± 0.05, 0.4 ± 0.02, respectively. The SHC is defined as

, Where Σ

is electrical conductivity of the SHM. The SHC is determined to be as large as 1.47 ± 0.02 × 105

 Ω-

1m-1 for the BS4 sample. The BixSe(1-x) films have both σ and SHC values comparable to previous reports on TI (11–14). A summary of σ , SHA, SHC for our samples and the best previously reported SHMs is presented in Table 1.

In our measurement technique, after we use a large in-plane magnetic field to make the CoFeB film single domain, we did not observe any significant HT and Oersted field which can be clearly seen in

R

I α signal that has perfect cosα nature (figure 2D). This indicates that the large SOT in BixSe(1- x) films is due to the SHE. The origin of SHE in our BixSe(1-x) films is investigated by studying variations

Σ

with σ . Analogous to the scaling law of AHE in the FM films (27, 28), the SHC can be written

Σ

is the intrinsic SHC and δ is a constant which determines the

Σ

versus the square of electrical conductivity is presented in figure 2E. By fitting the above equation over the experimental data,

Confirms

that the SHE in the BixSe(1-x) films is mainly due to the intrinsic SHE.

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The SOT arising from BixSe(1-x) can be directly observed by switching a FM in close proximity to the spin channel(6–9). We prepared a Si/SiO2/MgO (2 nm)/BixSe(1-x) (4 nm)/Ta (0.5 nm)/CoFeB (0.6 nm)/Gd (1.2 nm/CoFeB (1.1 nm)/MgO (2 nm)/Ta (2 nm) sample and a control sample Si/SiO2/MgO (2 nm)/Ta (5 nm)/CoFeB (0.6 nm)/Gd (1.2 nm)/CoFeB (1.1 nm)/MgO (2 nm)/Ta (2 nm) for the switching experiment. The films were patterned into Hall cross bars using optical lithography. Figure 3B shows the

R

loop of the BixSe(1-x) switching sample obtained by sweeping the out-of-plane field at a constant

R

at zero magnetic field confirms the easy axis of the magnetization along the out-of-plane direction. The perpendicular magnetic anisotropy (PMA) originated from the exchange interaction between the CoFeB and Gd layers. The

Loops Of The Bixse(1-X)

switching sample as a result of dc current sweep under the application of a constant in-plane bias field along the current channel are shown in figure 3C,D. In the presence of a + 80 Oe bias field (figure 3C), the dc current sweep from positive to negative favors the magnetization in the downward direction (Mz<0), and the magnetization switching occurs at -7 mA (~2.3 × 105 A/cm2). The subsequent reverse sweep from negative to positive current at identical magnitude and direction of the field favors the magnetization in the upward direction (Mz>0), and the magnetization switching occurs at 7 mA. Upon changing the polarity of the external field, the chirality of the

Loop Changes, Which Is Consistent With

the results of previous reports (6–8). The steps in switching confirm that the switching is mainly due to the domain wall nucleation and domain wall motion. The orientation of the

Loop In The Ta Switching

sample is opposite to the BixSe(1-x) switching sample, which suggests that the two samples do have opposite SHA polarity (figures 3C,D, and S5C,S5D). We also calculate the switching efficiency (29),

J Is The Switching

current density. The switching efficiency of the BixSe(1-x) is determined to be 2.57 kOe per 105 A/cm2. The Ta has switching efficiency of 4.7 kOe per 107 A/cm2, which is comparable to the previous reports (29). The two orders of magnitude larger switching efficiency and opposite

Deployment In Out-Of-Position Situations

D. Bendjaballah1, A. Bouchoucha1, M. L. Sahli1,2* and J-C. Gelin2

Abstract

Side-impact collisions represent the second greatest cause of fatality in motor vehicle accidents. Side-impact airbags have been installed in recent model year vehicle due to its effectiveness in reducing passengers’ injuries and fatality rates. In meeting these requirements, simulations of folding and deploying airbags are very useful and are widely used. The paper presents a simulation method for the deploying airbags using three materials in different working conditions. Finite element analysis is primarily used to evaluate this concept. In these simulations, the gas flow is described by the conservation laws of mass, momentum, and energy. The numerical results indicate that the FE method in this paper is capable of capturing airbag deploying process accurately.

Keywords: Airbag simulations, Out-of-position, Crash, Modeling, Out-of-position

Background

The passive safety of cars has become a very high prior- ity issue for the automotive industry. Today, there are not only one or two airbags in a car; certain models have ten times more than that. With the increasing usage of airbags, the number of accidents where the airbag itself can cause an injury to the occupant also increases

(Augenstein Et Al. 2003; Gabauer And Gabler 2010;

Audrey et al. 2011). As is well known, safety belts are also now devices designed to provide protection to the users of vehicles during crash events, minimizing the loads necessary to adapt their movement to the move- ment of the car (Freesmeier and Butler 1999; Schmitt et al. 1997). In general, the seat belt is designed to restrain the occupant in the vehicle and prevent the

Occupant From Having Harsh Contacts With Interior

surfaces of the vehicles. The airbag acts to cushion any impact with vehicle structure and has positive internal pressure, which can exert distributed restraining forces over the head and face. As a safety component of auto- mobile, an airbag decreases occupants’ injury likelihood effectively in case of an accident (Ruff et al. 2007). These safety elements can reduce the death rates on the roads, and its protection effects have been widely approved (Crandall et al. 2001; Teru and Ishikawa 2003). With computational tools such as finite element methods designed for dynamic contact problems, crashworthiness simulations can now be used with reliable accuracy to evaluate occupant protection in various collision condi- tions with safety metric/parameters such as acceleration, head injury criteria, intrusion distance, intrusion vel- ocity, and neck forces (neck injury risk or whiplash).

Thus, new types of airbag products are being developed to handle different collision scenarios.

Become Standard Equipment On Most New Passenger

vehicles (Braver and Kyrychenko 2004; Teng et al. 2007; Yoganandan et al. 2007). The airbag cushion is com- posed of a woven fabric which is rapidly inflated during a car crash. The airbag dissipates the passenger’s kinetic energy thereby reducing injury through biaxial stretching of the fabric bag and escaping gas through vents. There- fore, the performance of the airbag is greatly influenced by the mechanical properties of the fabric. Generally, air bags are designed to deploy in a crash that is equivalent to a vehicle crashing into a solid wall at 8 to 14 mph.

Air bags most often deploy when a vehicle collides with another vehicle or with a solid object like a tree. There are various types of airbags: frontal, side-impact, and curtain airbags. In general, the passenger side airbags are usually larger than the driver airbags (see Fig. 1).

Besançon, France

© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Bendjaballah et al. International Journal of Mechanical

Doi 10.1186/S40712-016-0070-2

Extensive studies have shown that the airbag deploy- ment in load cases consists of two occupant loading phases: a punch-out effect where the airbag bursts out of its container with the airbag and airbag module cover accelerating towards the occupant and a second loading phase during which the airbag is taking on its deployed shape and volume (membrane-loading effect). Bankdak et al. (2002) developed an experimental airbag test system to study airbag-occupant interactions during close proximity deployment. The results provided insight for simulating the effect of inflation energy and mass flow on target response. Bedard et al. (2002) found that while left-side (driver-side) impacts accounted for only 13.5% of all crashes, the fatality rate among these

Crashes Was 68.3% In Comparison To Front Impact

(48.3%), right-side impact (31.3%), and rear impact (38.4%). These studies underscore the importance of oc- cupant safety during side-impact collisions. In the last years, the current market requested to reduce the time and cost airbag development. In order to achieve this result, virtual simulations play an important role since they allow to minimize the number of experimental tests (Pei et al. 2013; Cao et al. 2014). Several simulation models of airbag were established (Wang et al. 2007). It is feasible to optimize the parameters of airbag deploy- ment using simulation technology. Experimental and numerical studies have quantified injury risks to close- proximity occupants from deploying side airbags. These studies have focused on the prevention of the most ad- verse effects of airbag deployment (Duma et al. 2003).

Other studies have proposed airbag characteristics to minimize particular biomechanical responses (Haland and Pipkorn 1996). In a more recent study, Marklund and Nilsson (2003) compared deformation patterns with experimental data as well as the computational costs associated with three different airbag deployment simu- lation methods; they concluded that the SPH method is relatively inexpensive and produces incremental deform- ation patterns that compare most closely to the experi- mental results. The process of inflation of an airbag is one of the determining factors in saving lives. The duration from the initial impact of the crash to the full inflation of an airbag is about 40 ms, and during this time, the airbag goes from being in a folded state to a fully inflated state, with a high internal pressure. After achieving this state, the airbag begins to deflate, thus providing a nice cushion for the body impacting it.

Ideally, the person in the crash should come into contact with the airbag at this time. In the present study, a large volume passenger side airbag model is developed to handle different collision scenarios. The main aim is evaluate the performance of deploying of passenger side airbag using finite element methods (FEM).

Materials

The tensile specimens were made in different airbags (P: Peugeot, R: Renault, and VW: Volkswagen) with a length of 200 mm long and a width of 40 mm. Table 1 shows the mechanical properties of the airbag.

Tensile Tests

To determine the mechanical properties of the material of airbag used in the test pieces, tensile tests were performed on Lloyd EZ20 universal testing machine in Constantine. These tests were conducted using rect- angular samples. The axial force and axial displacement acquired during a test are converted into stress and the strain in order to be used for the fabric material model.

The continuous recording of the stress-strain data was performed during both the load and unload phases. A minimum of five samples were made in order to check the repeatability of the measurements. All the data was collected by using a PC-based data acquisition system and analyzed by commercial software. The picture frame test device that is made for this study is shown in Fig. 2.

Fig. 1 a Frontal and side airbags. b Oblique view of facet occupant model in sitting posture following airbag deployment (Lim et al. 2014)

0.150

Bendjaballah et al. International Journal of Mechanical and Materials Engineering (2017) 12:12

Page 2 Of 9

Figure 3 shows the stress-strain relationship of the airbag sample under axial tensile loads. The results are showing a linear increase in extension with the increas- ing stresses. This is an expected output and it confirms with the theoretical behavior of a sample subjected to tensile stress. The rupture strain values for different airbags (R/P/VW) were 0.322, 0.441, and 0.472, respect- ively. The measured elastic parameters (i.e., Young’s modulus E and initial yield strength) and Poisson’s ratio are summarized in Table 2. The tensile tests of the woven fabrics can show differences on mechanical prop- erties because woven fabrics can resist in-plane shear loads once the yarn lock-up angle has been reached. The differences of material property on material direction can affect the shape of fully deployed bag (see Fig. 3b).

Theoretical Background

Numerical simulations of airbags use very complex and techniques such as an orthotropic model to identify the mechanical behaviors during the airbag inflation and the fluid mechanics (gas flow) to describe the inflator gas flow (pressure gradient) and improve the representation of the pressures within the airbag. To model the airbag as an orthotropic model, three material constants have to be provided. Assuming a plane stress condition, the

Ð1Þ

where σ is the normal stress and τ is the shear stress, the subscript refers to the principal material directions, i.e., the fill and warp directions. Also, ε and γ are the strain components. The material elastic constants Qij are

Ð2Þ

where E1 and E2 are the Young’s modulus in the fill and wrap directions and G12 is the shear modulus of the fabric material. νij is the Poisson ratio of the material.

The gas exerts a pressure load on the airbag causing it to expand. This expansion puts the airbag under tensile stress lowering the expansion rate. In this study, heat conduction and heat transfer is not taken into account.

Fig. 2 A photograph of Lloyd EZ20 universal testing Fig. 3 Stress versus strain using Lloyd EZ20 machine for a three different airbags at 0° and 90° and b VW airbag test specimens at

Different Angles

Table 2 Physical and mechanical properties of the airbag

Page 3 Of 9

In the deployment of an airbag, an inflator supplies high velocity gas into an airbag causing it to expand rapidly. The gas inside the airbag is assumed to be ideal, to be of constant entropy, and to satisfy the equation of state:

Ð3Þ

Here p, ρ, and e are respectively the pressure, density, and specific internal energy, and γ is the ratio of the heat capacities of the gas. The gas flow is described by the conservation laws for mass, momentum, and energy that

Ð4Þ

here, V is a volume, A is the boundary of this volume,

N Is The Normal Vector Along The Surface A, And U

denotes the velocity vector in the volume. Applying Bernoulli’s equation in the case of an ideal gas with

Ð5Þ

Here, the subscript ex denotes quantities at the throat of the tube. Furthermore u, p, and ρ denote the quan- tities inside that part of the tube that is supplying mass.

Materials And Boundary Conditions

The airbag system mainly consists of three parts: the airbag itself, the inflator unit, and the crash sensor or diagnostic unit. Thus, to study the behavior of the airbag using FE simulations, we need to have an FE model of the airbag in the folded position. A FE model of the airbag was used to simulate the test condition as shown in Fig. 5. LS-DYNA® material model FABRIC (MAT_34) is used to simulate the airbag material. It is a variation of the layered orthotropic material model. Additionally, in the LS-DYNA® material model, fabric leakage can be accounted for. However, for this CAB material, the leak- age is almost negligible and therefore no leakage is specified. The mechanical properties can be determined from the physical test. Typical material properties for airbag fabrics are taken as given in Chawla et al. (2004a) (Table 3). These properties are used to simulate inflation process of airbag (see Table 1). The car dashboard is modeled as the rectangular thin plate using a MAT_RI-

Gid Material, And The Degrees Of Freedom Are Con-

strained in all the directions. The similar properties of thermoplastic polymer are assigned for contact purposes. The porosity of the fabric is assumed zero. The nitro- gen gas is taken for inflating the airbag. Properties of nitrogen gas and initial bag conditions are shown in Table 4. The example on which we perform the study is a typical passenger side airbag. The geometric de- tails have been measured from a commercially avail- able airbag. The initial state of the airbag is a closed rectangular whose sides are to be finished to 482 × 635 mm2 and is shown in Fig. 4.

Table 3 Material properties of airbag and rigid plate used in FE

–

Table 4 Initial values used for FE simulation of the swelling of

3.33 × 10−4

Fig. 4 The initial airbag geometry in the form of a rectangular Bendjaballah et al. International Journal of Mechanical and Materials Engineering (2017) 12:12

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