Reduction of critical current density for out-of-plane mode oscillation in a mag-flip spin torque oscillator using highly spin-polarized Co2Fe(Ga0.5Ge0.5) spin injection layer

Bosu, S.; Sepehri-Amin, H.; Sakuraba, Yuya; Hayashi, Masamitsu; Abert, Claas; Suess, Dieter; Schrefl, T.; Hono, K.

doi:10.1063/1.4942373

Cited by 26 publications

(19 citation statements)

References 26 publications

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“…The values of the parameters used in the above calculations are derived from typical experiments 8,11,14,15 . As shown in Fig.…”

Section: Pacs Numbersmentioning

confidence: 99%

Crossover between fast and slow excitation of magnetization by spin torque

Taniguchi¹

2016

Appl. Phys. Express

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A crossover between two mechanisms destabilizing the magnetization in equilibrium by the spin transfer effect is found in a ferromagnetic multilayer consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer, where an in-plane magnetic field is applied, and electric current flows from the pinned to the free layer. A fast transition from the in-plane to the out-of-plane state occurs in the low-field region, whereas a slow transition with small-amplitude oscillation becomes dominant in the high-field region. On the other hand, only the fast transition mechanism appears for the opposite current direction. PACS numbers:Excitation of magnetization dynamics such as switching and self-oscillation in a ferromagnetic/nonmagnetic multilayer by the spin transfer effect 1,2 has been studied extensively for application in practical devices such as magnetic memory and microwave generators 3-15 . It has been recognized that there is a threshold value of the electric current necessary to destabilize the magnetization in equilibrium and excite any type of magnetization dynamics by the spin torque effect. Evaluation of the threshold current is important because it determines the performance of spin torque devices, for example, power consumption. To this end, a deep understanding of the physical mechanism of magnetization instability due to the spin torque is necessary.In this letter, the physical mechanism destabilizing the magnetization in a ferromagnetic multilayer consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer is studied theoretically. We find that this system shows a crossover between two mechanisms destabilizing the magnetization in equilibrium, depending on the magnitude of an in-plane applied magnetic field. A fast transition, on the order of nanoseconds, from an in-plane stable state to an out-ofplane state is dominant in the low-field region, whereas a slow transition in a time range exceeding 100 ns with a small-amplitude oscillation principally determines the instability threshold in the high-field region. This crossover appears only when the electric current flows perpendicular to the plane from the pinned layer to the free layer. When the current direction is reversed, the instability threshold is determined solely by the fast transition. Figure 1 schematically shows the system under consideration. The z-axis is perpendicular to the film-plane, and an in-plane magnetic field is applied along the x-axis. The unit vectors pointing in the magnetization directions of the free and pinned layers are denoted by m and p, respectively. The magnetization of the pinned layer points in the positive z-direction, p = +e z . The magnetization dynamics in the free layer is described by the LLG equation, where γ and α are the gyromagnetic ratio and Gilbert damping constant, respectively. The magnetic field consists of an applied field H appl and demagnetization field along the z-direction. Throughout this letter, the magnetic field is considered to poin...

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“…The values of the parameters used in the above calculations are derived from typical experiments 8,11,14,15 . As shown in Fig.…”

Section: Pacs Numbersmentioning

confidence: 99%

Crossover between fast and slow excitation of magnetization by spin torque

Taniguchi¹

2016

Appl. Phys. Express

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“…Since, the used model uses the spin-diffusion model, it is strictly valid only for metallic structures, using conducting layers. Interesting structures which could be directly modeled are for example Heusler alloys 12 which show a strong degree of spin polarization and could be a potential candidate for MRAM cells. Higher currents can be applied at which the tunnel barrier degeneration for large currents has not to be considered.…”

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confidence: 99%

Significant reduction of critical currents in MRAM designs using dual free layer with perpendicular and in-plane anisotropy

Suess

Vogler

Bruckner

et al. 2017

Applied Physics Letters

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Abstract:One essential feature in MRAM cells is the spin torque efficiency, which describes the ratio of the critical switching current to the energy barrier. Within this paper it is reported that the spin torque efficiency can be improved by a factor of 3.2 by the use a of dual free layer device, which consists of one layer with perpendicular crystalline anisotropy and a second layer with in-plane crystalline anisotropy. Detailed simulations solving the spin transport equations simultaneously with the micromagnetics equation were performed in order to understand the origin of the switching current reduction by a factor of 4 for the dual layer structure compared to a single layer structure.The main reason could be attributed to an increased spin accumulation within the free layer due to the dynamical tilting of the magnetization within the in-plane region of the dual free layer.Magnetic random access memory (MRAM) has the potential to become a major player in embedded magnetoresitive non-volatile memory technology due to the fast access and write times. An important design guideline in MRAM development is to reduce the critical switching current while maintaining a high thermal stability. Different designs of spin-transfer torque MRAM (STT-MRAM) cells have been proposed in order to reduce the critical current density. Dual pinned layer were proposed in order to reduce the critical current. Here, the second pinned layer has to point in opposite direction to the first pinned layer and it is separated by a second MgO barrier from the free layer. The critical current was reported to be a factor of 2-3 times smaller than that of a single MgO barrier 1 reaching values of the critical current density of j c = 0.02 TA/m² as reported in Ref[ 2 ]. In this reported structure both pinned layers consist of synthetic antiferromagnetically (SAF) coupled and exchange biased layers leading to layer thicknesses of about 40 nm, which reduces the scalability.

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“…(16). The current magnitude above which the difference between the theoretical and numerical results appears is on the order of 10 8 A/cm 2 , which is one to two orders of magnitude larger than the current magnitude used in typical experiments [14,18,21,24]. Therefore, we consider that the present formula works well to analyze experiments for wide range of the applied field angle and magnitude.…”

Section: Applicability Of the Present Theorymentioning

confidence: 88%

“…The condition that our formula of the threshold current density works better than the linear analysis to instability threshold is Eq. (21). We also derived the theoretical condition, Eq.…”

Section: Discussionmentioning

confidence: 99%

“…In particular, a spin-torque oscillator consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer has been an attractive research subject in the field of magnetism. [11][12][13][14][15][16][17][18][19][20][21]. In this type of spin-torque oscillator, the spin torque forces the magnetization of the free layer into out of plane, and excites a large amplitude oscillation around the perpendicular axis.…”

Section: Introductionmentioning

confidence: 99%

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Instability analysis of spin-torque oscillator with an in-plane magnetized free layer and a perpendicularly magnetized pinned layer

Taniguchi

2016

Phys. Rev. B

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We study the theoretical conditions to excite a stable self-oscillation in a spin-torque oscillator with an in-plane magnetized free layer and a perpendicularly magnetized pinned layer in the presence of magnetic field pointing in an arbitrary direction. The linearized Landau-Lifshitz-Gilbert (LLG) equation is found to be inapplicable to evaluate the threshold between the stable and self-oscillation states because the critical current density estimated from the linearized equation is considerably larger than that found in the numerical simulation. We derive a theoretical formula of the threshold current density by focusing on the energy gain of the magnetization from the spin torque during a time shorter than a precession period. A good agreement between the derived formula and the numerical simulation is obtained. The condition to stabilize the out-of-plane self-oscillation above the threshold is also discussed.

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Reduction of critical current density for out-of-plane mode oscillation in a mag-flip spin torque oscillator using highly spin-polarized Co2Fe(Ga0.5Ge0.5) spin injection layer

Cited by 26 publications

References 26 publications

Crossover between fast and slow excitation of magnetization by spin torque

Crossover between fast and slow excitation of magnetization by spin torque

Significant reduction of critical currents in MRAM designs using dual free layer with perpendicular and in-plane anisotropy

Instability analysis of spin-torque oscillator with an in-plane magnetized free layer and a perpendicularly magnetized pinned layer

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