Proc. Computational Science Workshop 2014 (CSW2014)
JPS Conf. Proc. 5, 011017 (2015)
http://dx.doi.org/10.7566/JPSCP.5.011017

Magnetization reversal of permalloy film by purespin current injection: relation between reversal
time and injected surface
Syuta HONDA* and Takashi KIMURA1,2
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
1
Faculity of Sciences, Kyushu University, Fukuoka 812-8581, Japan
2
Japan Science and Technology Agency, CREST, Tokyo 102-0075, Japan
E-mail: honda@hermes.esys.tsukuba.ac.jp(Received October 20, 2014)
The dynamics of the magnetization reversal process in a nanosized permalloy film by the purespin current (PSC) injection is analyzed. We have performed micromagnetic simulation basedon the Landau-Lifshitz equation taking into account both the relaxation term and spin transfer
torque term caused by the PSC. PSC is injected into the permalloy from a side of the film. We
compare the magnetization reversal times among the PSCs injected from each of sides. The
fast magnetization reversal occurs for the PSC injected from the side which area is smallest,
i.e. highest spin current density, if we impose a condition that the intensity of the spin current
is constant irrespective to the area of the side from which the PSC is injected.
KEYWORDS: Landau-Lifshitz equation, magnetization switching1.

Introduction

In a magnetic device or spintronics device such as a magnetoresistance element and amagnetic random access memory, a ferromagnet (FM) chip is used on a memory cell. It
also need to control magnetization direction quickly in the cell with low power.
Magnetization is usually controlled by a magnetic field or spin-transfer torque (STT) of aspin-polarized current [1, 2].
Magnetization reversal by injecting a pure spin current (PSC) is one of the methods ofSTT switching [3]. A PSC is generated by using a spin valve structure [4], which is ajunction of a FM and a nonmagnetic metal. The PSC drifts into a target FM and theinjected PSC decays in the FM. STT is developed around the injection side [5, 6] and themagnetization reverses. Recently, this magnetization reversal is demonstrated with thepermalloy (Py) thin film [3].
The PSC can be injected from each of sides into the film because the generation FM
can be set apart from the target FM. However, the relation between the side from which
PSC is injected and the magnetization reversal is unclear. We study the relation by using
the micro-magnetic simulation based on the Landau-Lifshit equation with the relaxation
term and STT term of PSC in this paper.

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JPS Conf. Proc. 5, 011017 (2015)

5 nm

30 nm
zyx
2.5 nm x 2.5 x 2.5 nm

2.

Fig. 1. Image of the PSC injection into the Py filmfrom the middle area side. The large box of 5 nm  15nm  30 nm is the film and the small box of 2.5 nm  2.5nm  2.5 nm is one of the divided small cells. The smallarrows and large arrow is the spin-polarization direction
and current direction of the PSCs, respectively. The
shaded sides of the large box is the side into which thePSC is injected.

Model and Methods

We simulated the magnetization reversal process in the small Py film by using themicro-magnetic simulation. The film is cuboid as depicted in figure 1 and the size offilm was 5 nm  15 nm  30 nm, which was divided into the small cells of 2.5 nm  2.5nm  2.5 nm. Magnetization of the film was reversed to –z-direction from +z-direction
by injecting the PSC which spin-polarization is along –z-direction. The PSC wasinjected in three ways, namely injected from the top, front, and left sides of the film,
which areas are 5 nm  15 nm, 5 nm  30 nm, and 15 nm  30 nm, respectively. The
schematic of the PSC injected from the front side into the film is shown in figure 1.
The magnetic moment (MM) in each small cell was solved by using theLandau-Lifshit (LL) equation with the STT term by the PSC [7 - 9], which ism1
  |  | m  H eff   |  | (m  (m  H eff )) (m  (m js )) ,(1)tMSwhere m is a unit vector of the MM of each small cell, is the gyro-magnetic ratio of 1.76
 107 rad/(s∙Oe), Heff is a combination of static and exchange magnetic fields, is thedamping constant of 0.02, and js is the PSC density, which is the tensor withspin-current vector and spin-polarization direction, and MS is a saturated magnetization
for the Py of 850 emu/cm3. The exchange stiffness constant is 1.3×10−6 erg/cm forestimation of an exchange magnetic field.
We assumed that the PSC decays exponentially in the film, since the PSC is relaxedwithin several nanometers in Py films [10]. We introduced a quantity u = |js|/MS which
decays with a characteristic diffusion length  of 4 nm as u = u0 exp(−x/), where u andu0 have a dimension of velocity, and x is the distance from the side of the film fromwhich the PSC is injected. We call u0 the injection velocity of the PSC. The PSCs wereuniformly injected from each of sides. When the PSC is injected from the left side, thePSC does not fully decay in this model because the thickness of the film is close to thediffusion length. However, we assumed that the injected PSC did not move outside thefilm and fully decayed in the film forcibly.
The initial MMs at t = 0 were obtained by one nanosecond of relaxation from thoseoriginally oriented along +z-direction. The PSC injection began at t = 0 and then u0 is
kept constant for t > 0. We employed the injection time when the magnetization is mostly
inverted to –z-direction, i.e. <mz> = –0.95, as the magnetization reversal time (tr).

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JPS Conf. Proc. 5, 011017 (2015)

51tr (ns)

<mz>

40

3
2

1-1

0

1

2t (ns)3Fig. 2.
Average of normalized magnetic
moments during the magnetization reversal
process in the film with the PSC injection. (a)The PSCs with u0 = 100 m/s is injected into thefilm from the left side (broken curve), frontside (bold solid curve), and top side (thin solidcurve), respectively.

00100

200
u0 (m/s)300

Fig. 3. Magnetization reversal time (tr) as afunction of the injection velocity (u0). The PSC
is injected from the left (broken curve), front(bold solid curve), and top small side (thin solidcurve). The open and closed circles are
obtained by the same intensity of PSCs injectedfrom the front side with u0 = 100 m/s and 150
m/s of the injection, respectively.

3. Results
First, we simulated the magnetization reversal process with injecting the PSC of u0 =100 m/s into the film from each of the sides. Figure 2 shows the z-component of averagedm (<mz>) as a function of the injection time of the PSC (t). At the beginning, <mz>changes little because the MMs hardly take STT from the PSC since the spin-polarization
direction of the PSC is nearly anti-parallel to the MMs. STT of m  (m js ) increases
with decreasing |<mz>|, i.e. m is tilted from +z-direction, and magnetization dramaticallydecreases. Finally magnetization reversal occurred even though the PSC decayed aroundthe injection edge. Magnetization reversed at 0.7 ns, 2.0 ns, and 3.4 ns for the PSC
injected from the left, front, and top side of the film, respectively. The size of a film inwhich magnetization reversal can occurs was discussed in Ref [7]. [If we use a model ofthe PSC going through the film without decay, the magnetization reversal time is 0.9 ns
for the PSC injected from the left side.]
We compare the magnetization reversal times for PSCs injected in three ways with thesame injection velocity. Figure 3 shows the reversal time as a function of the injection
velocity. The reversal time was decreased in approximately inversely proportion to theinjection velocity. Magnetization reversal for the PSC injected from the left side is thefastest because the intensity of the PSC is the largest.
We compare the magnetization reversal times among three types of the injection forthe same intensity of the PSC, i.e. the product of the injection velocity and injection
area (u0  S) is kept constant irrespective of the side. The ratio of the injection velocity
estimated for the PSCs injected form the left (top) side to the front side is 1/3(2.0). The
magnetization reversal times for the PSC injected from front side with u0 = 100 m/s(150 m/s) are plotted as the open (closed) circles in figure 3. Magnetization is reversed

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JPS Conf. Proc. 5, 011017 (2015)z<mz> = 1

0.5

0−0.5

−1yFig. 4. (Color online) Snapshots of the magnetic moments in the film at <mz> ~ 1, 0.5, 0, –0.5, and–1 during the magnetization reversal process. The PSC with u0 = 200 m/s is injected from the top sideof the film (the top side in this figure). Each arrow represents the direction of the magnetic moment oneach of small cells.
(a)

(b)

zyFig. 5. (Color online) Snapshots of the magnetic moments in the film at <mz> ~ 0 during themagnetization reversal process by injecting the PSC with (a) u0 = 100 m/s from the front side of thefilm (the left side in this figure) and (b) u0 = 33.3 m/s from the left side of the film (the front side inthis figure).

at tr = 2.0 ns (1.2 ns). The reversal times for the PSC injected from the other side arealso plotted with the same marks in figure 3. We found that the magnetization reversal
for the PSC injected from the top side is the fastest of tr = 1.3 ns (0.84 ns), and that fromthe left side is the slowest of tr = 2.2 ns (1.3 ns) even though the PSC is fully relaxed inthe film.
Next, we introduce the snapshots of the MMs in the film during the magnetization
reversal process which correspond to the results marked by the open circles in Figure 3.
Figure 4 shows snapshots for the PSC injected from the top side with u0 = 200 m/s. The
PSC was injected from top side of the figure. At first, the MMs near the injection siderotated with STT of the PSC. The rotating MMs change the effective static fields andexchange fields for the around MMs. Especially, the exchange fields tilt the MMs same
direction. Finally, magnetization is reversed.
We also show the snapshots of the MMs at <mz> ~ 0 for the PSC injected from thefront side with u0 = 100 m/s and the left side with u0 = 33.3 m/s in figure 5. During theinjection from the front side, a small magnetic vortex was produced. However it
depends whether a vortex or a magnetic wall was produced or the magnetizationsmoothly reverse. When the PSC was injected from the top side, the magnetization
reverses with roughly uniform rotation of MMs. When magnetization is reversed withsmall spin injection velocity of u0 < 25 m/s, all MMs rotate more uniformly during themagnetization reversal process for the PSCs injected from each of sides.
When the PSC with a larger spin injection velocity is injected, the rotation of MMsJPS Conf. Proc. 5, 011017 (2015)

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due to the STT of PSC is faster than these due to the external magnetic fields. And theMMs experience the larger magnetic fields produced from differences in the direction
between the MMs. Hence, the rotations of the MMs accelerate. As a result, the fastermagnetization reversal occurs.

3. Conclusion
The dynamics of the magnetization reversal process in the nanosized permalloy filmby the pure spin current injection has been analyzed. The micro-magnetic simulation baseon the on the Landau-Lifshit equation with the spin transfer torque term has been used.
We have compared the magnetization reversal time among the PSCs injected from eachof sides of the film. The fast magnetization reversal occurs for the PSC injected from theside which area is smallest, i.e. highest spin current density, if we impose a condition thatthe intensity of the spin current is constant irrespective to the area of the side from which
the PSC is injected.
Acknowledgment
This work was supported by JSPS KAKENHI Grant Number 25820135 and theCREST Program on “Research of Innovative Material and Process for Creation of
Next-generation Electronics Devices” by JST.
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