大学物理实验报告英文版

大学物理实验报告

Ferroelectric Control of Spin Polarization

ABSTRACT

A

current

drawback

of

spintronics

is

the

large

power

that

is

usually

required

for

magnetic

writing,

in

contrast

with

nanoelectronics,

which

relies

on

“zero

-

current,”

gate

-controlled

operations.

Efforts

have

been

made

to

control

the

spin-relaxation

rate,

the

Curie

temperature,

or

the

magnetic

anisotropy with a gate voltage, but these effects are usually small and volatile. We used

ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and

nonvolatile control of carrier spin polarization by electrically switching ferroelectric

polarization. Our results represent a giant type of interfacial magnetoelectric coupling and

suggest a low-power approach for spin-based information control.

Controlling the spin degree of freedom by purely electrical means is currently an

important challenge in spintronics (

1

,

2

). Approaches based on spin-transfer torque

(

3

) have proven very successful in controlling the direction of magnetization in a

ferromagnetic

layer,

but

they

require

the

injection

of

high

current

densities.

An

ideal

solution would rely on the application of an electric field across an insulator, as in

existing nanoelectronics. Early experiments have

demonstrated

the volatile modulation

of spin-based properties with a gate voltage applied through a dielectric. Notable

examples

include the gate control

of the spin-orbit interaction in

III-V

quantum

wells

(

4

), the Curie temperature

T

C

(

5

), or the magnetic anisotropy (

6

) in magnetic

semiconductors with carrier- mediated exchange interactions;

for example, (Ga,Mn)As or

(In,Mn)As. Electric field

–

induced modifications of magnetic anisotropy at room

temperature have also been reported recently in ultrathin Fe-based layers (

7

,

8

).

A nonvolatile extension of this approach involves replacing the gate dielectric by a

ferroelectric and taking advantage of the hysteretic response of its order parameter

(polarization) with an electric field. When combined with (Ga,Mn)As channels, for

instance, a remanent control of

T

C

over a few kelvin was achieved through

polarization-driven charge depletion/accumulation (

9

,

10

), and the magnetic

anisotropy was modified by the coupling of piezoelectricity and magnetostriction

(

11

,

12

).

Indications

of

an

electrical

control

of

magnetization

have

also

been

provided

in magnetoelectric heterostructures at room temperature (

1 3

–

17

).

Recently,

several

theoretical

studies

have

predicted

that

large

variations

of

magnetic

properties

may

occur

at

interfaces

between

ferroelectrics

and

high-

T

C

ferromagnets

such

as

Fe

(

18

–

20

),

Co

2

MnSi

(

21

),

or

Fe

3

O

4

(

22

).

Changing

the

direction

of

the

ferroelectric

polarization has been predicted to influence not only the interfacial anisotropy and

magnetization, but also the spin polarization. Spin polarization [., the normalized

difference

in

the

density

of

states

(DOS)

of

majority

and

minority

spin

carriers

at

the

Fermi

level

(

E

F

)]

is

typically

the

key

parameter

controlling

the

response

of

spintronics

systems,

epitomized

by

magnetic

tunnel

junctions

in

which

the

tunnel

magnetoresistance

(

TMR

)

is

related

to

the

electrode

spin

polarization

by

the

Jullière

formula

(

23

).

These

predictions suggest that the nonvolatile character of ferroelectrics at the heart of

ferroelectric random access memory technology (

24

) may be exploited in spintronics

devices such as magnetic random access memories or spin field-effect transistors (

2

).

However, the nonvolatile electrical control of spin polarization has not yet been

demonstrated.

We address this issue experimentally by probing the spin polarization of electrons

tunneling from an Fe electrode through ultrathin ferroelectric BaTiO

3

(BTO) tunnel

barriers (

Fig. 1A

). The BTO polarization can be electrically switched to point toward

or

away

from

the

Fe

electrode.

We

used

a

half-metallic (

25

)

bottom

electrode

as

a

spin

detector

in

these

artificial

multiferroic

tunnel

junctions

(

26

,

27

).

Magnetotransport

experiments provide evidence for a large and reversible dependence of the

TMR

on

ferroelectric polarization direction.

Fig. 1

(

A

) Sketch of the nanojunction defined by electrically controlled nanoindentation. A

thin resist is spin- coated on the BTO(1 nm)/LSMO(30 nm) bilayer. The nanoindentation

is

performed

with

a

conductive-tip

atomic

force

microscope,

and

the

resulting

nano-hole

is filled by sputter-depositing Au/CoO/Co/Fe. (

B

) (Top) PFM phase image of a BTO(1

nm)/LSMO(30 nm) bilayer after poling the BTO along 1-by-4

–μm stripes with either a

negative or positive (tip-LSMO) voltage. (Bottom) CTAFM image of an unpoled area of a

BTO(1

nm)/LSMO(30

nm)

bilayer.

Ω,

ohms.

(

C

)

X-ray

absorption

spectra

collected

at

room

temperature close to the Fe L

3,2

(top), Ba M

5,4

(middle), and Ti L

3,2

(bottom) edges on

an AlO

x

nm)/Al nm)/Fe(2 nm)/BTO(1 nm)/LSMO(30 nm)(

D

) HRTEM and (

E

) HAADF images of the

Fe/BTO

interface

in

a

Ta(5

nm)/Fe(18

nm)/BTO(50

nm)/LSMO(30

nm)The

white

arrowheads

in

(D) indicate the lattice fringes of {011} planes in the iron layer. [110] and [001]

indicate pseudotetragonal crystallographic axes of the BTO perovskite.

The tunnel junctions that we used in this study are based on BTO(1 nm)/LSMO(30 nm)

bilayers

grown

epitaxially

onto

(001)-oriented

NdGaO

3

(NGO)

single-crystal

substrates

(

28

).

The

large

(~180°)

and

stable

piezoresponse

force

microscopy

(PFM)

phase

contrast

(

28

) between negatively and positively poled areas (

Fig. 1B

, top) indicates that the

ultrathin BTO films are ferroelectric at room temperature (

29

). The persistence of

ferroelectricity

for

such

ultrathin

films

of

BTO

arises

from

the

large

lattice mismatch

with the NGO substrate (

–

%), which is expected to dramatically enhance ferroelectric

properties in this highly strained BTO (

30

). The local topographical and transport

properties of the BTO(1 nm)/LSMO(30 nm) bilayers were characterized by conductive-tip

atomic

force

microscopy

(CTAFM)

(

28

).

The

surface

is

very

smooth

with

terraces

separated

by one-unit- cell

–

high steps, visible in both the topography (

29

) and resistance

mappings (

Fig. 1B

, bottom). No anomalies in the CTAFM data were observed over lateral

distances on the micrometer scale.

We defined tunnel junctions from these bilayers by a lithographic technique based on

CTAFM (

28

,

31

). Top electrical contacts of diameter ~10 to 30 nm can be patterned by

this

nanofabrication

process.

The

subsequent

sputter

deposition

of

a

5-nm-thick

Fe

layer,

capped by a Au(100 nm)/CoO nm)/Co nm) stack to increase coercivity, defined a set of

nanojunctions (

Fig. 1A

). The same Au/CoO/Co/Fe stack was deposited on another BTO(1

nm)/LSMO(30 nm) sample for magnetic measurements. Additionally, a Ta(5 nm)/Fe(18

nm)/BTO(50 nm)/LSMO(30 nm) sample and a AlO

x

nm)/Al nm)/Fe(2 nm)/BTO(1 nm)/LSMO(30 nm)

sample were realized for structural and spectroscopic characterizations.

We used both a conventional high-resolution transmission electron microscope (HRTEM)

and the NION UltraSTEM 100 scanning transmission electron microscope (STEM) to

investigate

the

Fe/BTO

interface

properties

of

the

Ta/Fe/BTO/LSMO

sample.

The

epitaxial

growth of the BTO/LSMO bilayer on the NGO substrate was confirmed by HRTEM and

high-resolution

STEM

images.

The

low- resolution,

high-angle

annular

dark

field

(HAADF)

image

of

the entire

heterostructure shows the sharpness

of

the LSMO/BTO interface over

the studied area (

Fig. 1E

, top).

Figure 1D

reveals a smooth interface between the

BTO and the Fe layers. Whereas the BTO film is epitaxially grown on top of LSMO, the

Fe

layer

consists

of

textured

nanocrystallites.

From

the

in-plane

(

a

)

and

out-of-plane

(

c

)

lattice

parameters

in

the

tetragonal

BTO

layer,

we

infer

that

c

/

a

= ±

,

in

good

agreement with the value of found with the use of x-ray diffraction (

29

). The

interplanar

distances

for

selected

crystallites

in

the

Fe

layer

[.,

~

?

(

Fig.

1D

,

white

arrowheads)] are consistent with the {011} planes of body-centered cubic (bcc) Fe.

We investigated the BTO/Fe interface region more closely in the HAADF mode of the STEM

(

Fig. 1E

, bottom). On the BTO side, the atomically resolved HAADF image allows the

distinction

of

atomic

columns

where

the

perovskite

A-site

atoms

(Ba)

appear

as

brighter

spots. Lattice fringes with the characteristic {100} interplanar distances of bcc Fe

(~

?

) can be distinguished on the opposite side. Subtle structural, chemical, and/or

electronic modifications may be expected to occur at the interfacial boundary between

the BTO perovskite-type structure and the Fe layer. These effects may lead to

interdiffusion of Fe, Ba, and O atoms over less than 1 nm, or the local modification

of

the

Fe

DOS

close

to

E

F

,

consistent

with

ab

initio

calculations

of

the

BTO/Fe

interface

(

18