La0.7Sr0.3MnO3 - Based Spintronics

1

La0.7Sr0.3MnO3 - based SpintronicsInvestigation on fundamental issues and applications

Umberto Scotti di Uccio

DiMSAT, Università di Cassino

CNR – Coherentia Napoli

-50 0 50

0.0

0.2

0.4

0.6

MR

(%

)

µ0H (mT)

2

Research group & Institutions•where

Monte Cassino AbbeyFounded in 529 DCDestroyed in 1944, feb 15th

University of Cassino

CNR - Coherenthia labs.

3

Di.M.S.A.T. - University of Cassino

Research group & Institutions•who

CNR– Coherentia Labs.

R. Vaglio (Coherentia Labs leader)

U. Scotti di Uccio

P. Perna

F. Miletto

N. Lampis

M. Radovic

N. Russo

A. Sambri

G. Pepe

A. Ruotolo

M. Salluzzo

R. Di CapuaSTMdevicesCooperations:

4

Research groupScientific interests and background

Oxide films growth

HTc Josephson

Devices fabrication and characterization

Structural and transport characterization

RSM R(T)XRD STM

Physics of the film surface

LEEDGXRD

530 540

LSMO

O1s

I (a

. u

.)

BE (eV)

110

100

PES

The core activity of the group is the investigation of oxide films, mainly

perovskites. At present we can resort to different techniques to characterize the

structural and transport properties of samples, including x ray and electron

diffraction, scanning tunneling microscopy, and photoemission spectroscopy. As

far as devices are concerned, our experience mainly come from the fabrication

and characterization of high Tc Josephson junctions,…

5

Research groupScientific interests and background

Magnetoelectronics

Devices fabrication and characterization

Structural and transport characterization

RSM R(T)XRD STM

Physics of the film surface

LEEDGXRD

530 540

LSMO

O1s

I (a

. u

.)

BE (eV)

110

100

PES

Oxide films growth

-50 0 50

0.0

0.2

0.4

0.6

MR

(%

)

µ0H (mT)

…but since a couple of years we started to work on magnetoelectronics devices

based on LSMO, that are the topic of this talk.

6

Why LSMO?

Optimal dopingx = 0.3 – 0.4

MnOLa, SrLa1-xSrxMnO3

is a perovskiticmanganite

La1-xSrxMnO3

is a ferromagnet

La1-xSrxMnO3 has a large spin polarization at the Fermi level

Normal FM Half Metal

1NN

NNP ≈

+

−=

↓↑

↓↑

So, why LSMO. LSMO is a perovskitic material. The content x in Strontium acts

as a doping that controls the number of carriers, actually holes, at tha Fermi level.

At the optimal doping that we considered in this work LSMO is a robust

ferromagnet with Curie temperature well above room temperature. Most

importantly LSMO is an almost perfect half metal, that is, the conduction band is

mostly filled up with one orientation of spin, opening the door to application to

spin injection.

7

Outline

• LSMO films

• Overview on spintronics

• Preliminary experiments on LSMO

• Device fabrication and characterization

• Conclusions and future plans


8

Outline

• LSMO films






9

LSMO films deposition conditions

Eccimer laser KrF - λ = 248 nm Target – substrate 41 mmP(O2) = 0.1 mbar

Effective Fluency: 80 mJ /2.6 mm2

Repetition rate: 2 HzDeposition temperature: 850 °C

Laser ablation

This work: STO substrate

Coherentia Labs.Napoli

Modular system forOxideDeposition andAnalysesUHV base P < 10-11 mbar

AFMSTM

LEED

XPS-UPS PLD

This is a picture of our lab. It shows the MODA system, that is a Modular system

for Oxide Deposition and Analysis. It mainly consists of a chamber devoted to

pulsed laser deposition, that is connected to several analysis chambers, that is, X

ray photoemission Spectroscopy, Low Energy Electron Diffraction, Scanning

Tunneling Microscopy.

The deposition condition of our films are quite standard. In this work we only

employed Srontiun Titanate substrates.

10

22.5 23.0 23.5 24.0 24.50.0

0.5

1.0 (002) LSMO

(002) STO

FWHM = 0.07°

Cou

nts

(a

.u.)

ωωωω (deg)

Films structure and morphology

Epitaxial, high structural quality

Smooth surfaces

Cube-on-cubeepitaxy(001) STO substrate 3 µm

45 46 47 48

1

10

log(I

/Io)

2θ

STO (002)

LSMO (002)

These are some data for LSMO grown on (001) STO. The films are smooth, and

show a high crystal quality. They also are fully strained at least up to 100 nm.

11


(110) STO substrate Cube-on-cubeepitaxy

Epitaxial, high structural quality

Smooth surfaces

In device fabrication we mostly employed films grown on (110) STO. The

LSMO grows in the usual “cube on cube” fashion also in this case, and also in

this case it shows a very high crystal quality and smooth surfaces…

12


(110) STO substrate

High Curie Temperature

Nice transport properties

0 100 200 300 4000.0

0.5

1.0

R (Ω

)M/M

(0)

T (K)

0

300

600

900

1200

TC ≈≈≈≈ 350 K

FM

PM

TC

I

M

..and nice transport and magnetic properties, with Curie Temperature well above

room temperature. By the way, the comparison between magnetization and

resistivity that I’m showing here demonstrate the well known fact that in the

ferromagnetic phase LSMO is a good metal, and in the paramagnetic phase it is a

bad conductor.

13

Magnetic properties

(110) STO substrate

easy

hard(001)

(110)_

-0.10 -0.05 0.00 0.05 0.10

-5x10-4

0

5x10-4

M(e

mu

)

µo H (T)

T = 100 K

(001)

(1-10)

in plane

In this viewgraph I show the magnetic properties of LSMO with (110)

orientation. As known, the LSMO has a strong magnetostriction, so that the easy

axis in films depend on the strain. In the case of STO we have tensile strain, and

the easy axis lies parallel to the substrate. The standard characterization of the

magnetic hysteresis cycles was performed in a vibrating sample magnetometer.

We see that there is a strong anisothropy between the two in-plane directions, and

that the easy axis is aligned to the (001) direction.

14

Outline

• LSMO films






So it is time to introduce the research on devices with a brief overview on

spintronics devices

15

The GMR effect

Pictures from

Gary A. Prinz, SCIENCE 282, 1660 (1998)

Hard disk read heads

Review data

Normal FM Half Metal

Spin-polarized transport occurs in ferromagnetic half metals. The orientation of the majority spins determines the magnetization of the

sample and it is controlled by the external field. When two half metals are

put in contact through a spacer, there is a state with low resistance when

the magnetic moments are parallel, and a state with high resistance when the magnetic moments are antiparallel. This is the magnetoresistance effect

that is exploited in the read heads of our hard disks.

16

TMR devices

Stuart S. P. Parkin, et al., nature materials 862, (2004)

300 K

M. Coey, et al., nature materials, 9 (2005)

( ) ( )( )HR

HR0RTMR

−= Review data

In the TMR devices the spacer between the two magnetized layers is a tunnel

barrier. The quality factor for the device is the TMR ratio, defined as the percent

variation of resistivity in presence of an external magnetic field. Tunnel devices

demonstrate a very high TMR ratio and are serious candidates for many

commercial devices. At present the best results are based on MgO barriers

separating Cobalth-Iron alloys electrodes.

17

LSMO based TMR devices

F. Pailloux, PRB 66, 014417 (2002)

4.2 K

M. Bowen, et al., APL 82, 233 (2003)…LSMO is attractive because of its high spin polarization leading to high expected MR

Review data

Actual materials are not ideal half metals, but…

21

21

PP

PP

R

R

−≈

∆

1

↓↑

↓↑

+

−=

NN

NNP

M. Julliere, Phys. Lett. 54A, 225 (1975)

The actual materials are not ideal half metals but they instead have some

population of minirity spins at the Fermi level, that results in a reduction of the

TMR. In this context LSMO is attractive because of its high degree of spin

polarization leading to high magnetoresistance. This was observed for instance in

TMR junctions with STO barrier.

18

M. Bowen, et al., APL 82, 233 (2003)

LSMO based TMR devices

The problem is that the TMR drops with temperature

Review dataUsual interpretation: degraded properties at LSMO interface

21

21

PP

PP

R

R

−≈

∆

1

…LSMO is attractive because of its high spin polarization leading to high expected MR

↓↑

↓↑

+

−=

NN

NNP

M. Julliere, Phys. Lett. 54A, 225 (1975)

Actual materials are not ideal half metals, but…

The problem with LSMO however is that the TMR drops fastly with temperature

and it is small at room temperature. This effect is generally interpreted in terms

of some degradation of the magnetic properties at the surface of LSMO, or in the

case of trilayers at the interface with barrier.

19

Outline

• LSMO films





Our experience on:•Degradation of surface• Intrinsic properties


Now I would like to show what is our experience on the properties of LSMO in

connection to the decrease of magnetoresistance. I will try to demonstrate that

this effect may be connected both to degradation of the surface and to more

intrinsic properties of LSMO.

20

The dead layer

0 5 10 15 20 25 30 350

5

10

15

20

M x

t (

em

u m

-2)

t (nm)

(110) LSMO

MODA lab

t ≈ 5 nmtop

bottom

?

First of all, I would like to comment on the so called “dead layer”. The existence

of the dead layer was demonstrated both for LSMO and for common

ferromagnetic metals and alloys by comparing the magnetic moment or also the

electrical conductance of samples with different thickness. For instance, in our

measurements on (110) LSMO grown on STO we find that the extrapolated

magnetization of a layer as thin as about 5 nm is zero, so that we infer a non

magnetic layer of about 5 nm in the samples. Reported values are similar and are

slightly dependent on the substrate and on the growth technique. However bulk

measurements don’t distinguish between sample surface and interface with the

substrate.

21

The metallic state at the surface

La0.7Sr0.3MnO3 on (001) STO

LEED on as-deposited samplesdemonstrate metallicity at the surface

Not probing for magnetic properties:LSMO is not a real insulator in the PM state

1 × 1

So let’s consider surface sensitive analyses. In this picture I’m showing the

electron diffraction pattern taken on an as-deposited LSMO sample. Since the

electron diffraction is only observed on conducting surfaces, the measurements

demonstrate that the LSMO surface of as-deposited samples is conducting. This

is important, but it is not probing, because LSMO is not a real insulator even

when in the paramagnetic state.

22

x ray source e- detector

x ray source e- detector

θcosll

Experimental technique: XPS at different emerging angles

Degradation of the surface propertiesof LSMO: evidence by XPS

Normal emission

Shallow angle emission

PS sampling depth ≈ 1-2 nmShallow angle: still lower

We can get some more information by photoemission. The photoemission has a

sampling depth of a few nanometers, because the mean free path of

photoelectrons is short. Photoemission at shallow angle has a still lower sampling

depth. Thus, in order to get information on the very last layer I will compare

between measurements taken at different emission angles.

23


Mn 3d

t2g

eg

O 2p

pπ

pσ

CB

VB

La0.7Sr0.3MnO3 : 2/3 filling eg

First let me remind that in LSMO the conduction band is the lower Mn3d eg

band, that is partially filled, while the valence band is made by overlapping Mn

t2g and O 2p states.

24

0 2 4 6 8 10

0.0

0.5

1.0

normal emission

shallow emission

Co

unts

(a.u

.)

BE (eV)

0.0

0.2

0.4

0.6 diffe

rence

O 2p

Mn t2gMn eg

difference

EF


EFCB

VB

bulk

surface

Spectral weight transfer:There is a reduction in the DOS close to the Fermi edge

At the surface:Higher BE of eg

Low conducting state

These are the results. We see that the density of states at the LSMO at the surface

is different. We interpret the weight transfer as a sign of bad metallicity because

the eg states are pushed far from the Fermi level. (In this case there is also an

enhanced occupation, that is possibly due to the presence of oxygen vacancies,

because oxygen vacancies are electron donors.).

25

0 100 200 300 4000

500

1000

R (

Ω)

T (K) 250 300 350

0.4

0.8

ρ (a

. u.)

dρ/

dT

(a.

u.)

T (K)

0.4

0.8

LT

HT

La0.7Sr0.3MnO3 on STO(110)

coexistence

TM

Transport properties in manganite thin films Phys. Rev. B 71, 064415 (2005)

The inhomogeneous phase transition

( ) HTLT f1f ρ−+ρ=ρ• R changes smoothly• The magnetic properties are not well established and not uniform

The behaviour of ρ(T) has been described in terms of an inhomogeneous transition with phase separation

LT : conducting FM phase

HT : insulating PM phase

HTLT

HT

HT

Let me present a different point of view. This is again the ρ(T) plot of a sample

deposited on (110) STO, and on the right I show a magnification of the region of

the metal-insulator transition. The red curve is the derivative dr/dT and TM is

Curie temperature. We see that there is a region of about 100 K where the

resistivity changes dramatically. This is the crossover between the conducting

ferromagnetic phase at low temperature, to the insulating paramagnetic phase at

high temperature. In a recent paper, we demonstrated that this crossover can be

described in terms of a phase separation as shown in this formula where f is the

fraction of conducting phase. So we have a large region where the magnetic

properties are not well established and not uniform.

26


250 300 350

0.4

0.8

ρ (a

. u.)

dρ/

dT

(a.

u.)

T (K)

0.4

0.8


STM in conductance map mode

Vo = 2 V , feedback disconnected

0 100 200 300 4000

500

1000

R (

Ω)

T (K)


77 K

LT

J. Phys.: Condens. Matter 18 11595 (2006)500 × 500 nm2

This mechanism influences surface properties. Here I’m showing a conductance

map taken by low temperature STM, that is highly sensitive to surface. In this

mode of operation, the false colors indicate the conductance of the junction

between tip and sample, that in a first approximation is the density of states of the

sample. We see that the map is homogeneous in spite of the details of sample

morphology.

27


250 300 350

0.4

0.8

ρ (a

. u.)

dρ/

dT

(a.

u.)

T (K)

0.4

0.8

0 100 200 300 4000

500

1000

R (

Ω)

T (K)


LT

coexistence

77 K 300 KLa0.7Sr0.3MnO3 on STO(110)


Dark regions : low conductivity

Light regions : high conductivity500 × 500 nm2 500 × 500 nm2

J. Phys.: Condens. Matter 18 11595 (2006)

At room temperature the situation changes. The phase separation separation

mechanism is at play…

28


0 100 200 300 4000

500

1000

R (

Ω)

T (K)


LT

77 K

300 K

coexistence

500 × 500 nm2 500 × 500 nm2



Dark regions : low conductivity

Light regions : high conductivity

…and we see the coexistence of insulating and conducting regions that are

spatially separated. Now, this is an intrinsic effect and there is no reason to

suspect that it should not be at play not only at surfaces but also at interfaces,

reducing the performance of TMR junctions. Concluding this section, I would

like to comment that non intrinsic mechanisms such as oxygen loss or

contamination can be perhaps controlled by a suitable technology, but the phase

separation mechanism seems a more fundamental problem with LSMO.

29


77 K

300 K500 × 500 nm2 500 × 500 nm2

Concluding this section…

• Non intrinsic mechanisms such as oxygen loss or contamination can beperhaps controlled by a suitable technology

• PS is an intrinsic, thermodynamical effect

• PS can be at play also at interfaces (but how?)

• PS can reduce TMR performances because PM regions are present

…and we see the coexistence of insulating and conducting regions that are

spatially separated. Now, this is an intrinsic effect and there is no reason to

suspect that it should not be at play not only at surfaces but also at interfaces,

reducing the performance of TMR junctions. Concluding this section, I would

like to comment that non intrinsic mechanisms such as oxygen loss or

contamination can be perhaps controlled by a suitable technology, but the phase

separation mechanism seems a more fundamental problem with LSMO.

30

Outline

• LSMO films






Now, let us come to our activity on devices

31

What about interfaces between LSMO and conventional FM?

Can the degraded interface act as a tunnel barrier?Will it lead to tunneling magnetoresistance?

Metal

substrate

LSMOSurfacelayer

O migration?

A. Plecenick, et al., Appl. Phys. Lett. 81, 859 (2002)


Our first work was devoted to this issue: can a degradedinterface between LSMO and a magnetic alloy act as a tunnel barrier? Will it lead to tunnelingmagnetoresistance? There weresome indication in literature that a metallic contact on LCMO can show an insulating interlayer due to oxygen migration from LCMO, butlittle about the magneticproperties.

32

What about interfaces between LSMO and metals?

Ni80 – Fe20 depositionDC SputteringO.5 Pa Ar @ room temperatureFirst experiment: stencil mask

(110) STO

LSMOSurfacelayer

Permalloy (Ni0.80-Fe0.20)


In a first experiment we realized a simple bilayer LSMO/Permalloy

33

-50 0 50

-5x10-7

0

5x10-7

30 nm LSMO + 10 nm PY

30 nm LSMO

m (

A m

2)

µoH (mT)

-50 0 50-2x10

-7

-8x10-8

0

8x10-8

2x10-7

m (

A m

2)

µoH (mT)

Magnetic properties of LSMO – Py bilayers

La0.7Sr0.3MnO3/Py on STO(110)

difference

LSMO+Py

• HC = 15 mT @ 100 K

• MS = 620 emu/cm3 @ 100 K

LSMO

• Hc = 17 mT @ 100 K

• MS = 515 emu/cm3 @ 100 K

Py

• Hc = 4 mT @ 100 K• MS = 830 emu/cm3

No evidence of exchange couplingRelatively large saturation of Py

The hysteresis loops indicate that the two layers are magnetically decoupled, and

that the permalloy has a relatively high saturation field.

34

I+ V+ V- I-V+ V-

I+ I-

Low temperature MR effect

-100 -80 -60 -40 -20 0 20 40 60 80 100

-0.15

-0.10

-0.05

0.00

∆R

/RH

c (

%)

µ0H(mT)

10 times smaller

@ 4.2 K

Patterned La0.7Sr0.3MnO3 on STO(110)

-80 -60 -40 -20 0 20 40 60 80-2.0

-1.5

-1.0

-0.5

0.0

∆R

/RH

c (

%)

µo H (mT)

@ 4.2 K

30 nm LSMO – 10 nm Py on STO(110)

Hr

Jr

CPP

U. Scotti di Uccio, et al.et al., APL 88, 252504 (2006)

Here I show the MR of the bilayer at 4.2 K. The MR of the bilayer is quite small.

However it is ten times higher than a single LSMO film, and it cannot either be

ascribed to Py...

35

Low temperature MR effect

-80 -60 -40 -20 0 20 40 60 80-2.0

-1.5

-1.0

-0.5

0.0

∆R

/RH

c (

%)

µo H (mT)

-5x10-7

0

5x10-7

M (A

m2)

@ 4.2 K

30 nm LSMO – 10 nm Py on STO(110)

HC LSMO

V+ V-

I+ I-

-100 -80 -60 -40 -20 0 20 40 60 80 100

-0.15

-0.10

-0.05

0.00

∆R

/RH

c (

%)

µ0H(mT)

10 times smaller

@ 4.2 K

Patterned La0.7Sr0.3MnO3 on STO(110)

Hr

Jr

CPPI+ V+ V- I-

U. Scotti di Uccio, et al.et al., APL 88, 252504 (2006)

…because the peaks in the MR plot correspond to the coercive field of LSMO.

So there is some indication that the device works based on some spin scattering

mechanism, but it is still unclear the role of the interface, that on this basis could

well be something as a metallic spacer. Moreover, the MR of the device doesn’t

show a flat baseline, probably due to the MR of LSMO itself.

36

Flat baselineno MR from LSMO

Broad peakshigh HS of Py

-60 -40 -20 0 20 40 600

1

2

µo H (mT)

% M

R

LSMO – Py TMR devices

Ion milling etching30 × 30 100 × 70 µm2

100 × 70 µm2

Ohmic behavior

R ≈ 580 Ω tunnel barrier

Reduced role of film resistanceU. Scotti di Uccio, et al.et al., APL 88, 252504 (2006)

To get more information we patterned devices with a small contact area. Here I

show the MR of a 70 x 100 micron square junction. The I vs V plot is linear, with

a resistance that is much higher than the resistance of the electrodes. This is an

indication of a tunneling mechanism. Moreover we have the suppression of the

MR due to the LSMO film resulting in a flat baseline. The peaks are broadened

due to the not well established antiparallel state between LSMO and Py, due to

the large saturation field of Py.

37

LSMO – Py TMR devices

Simulation

based on the Julliere model:• Measured M(H) on a similar bilayer

• Computation of

• ~10% spin polarization

21

21

PP

PP

R

R

−≈

∆

1

-60 -40 -20 0 20 40 600

1

2

µo H (mT)

% M

R

-60 -40 -20 0 20 40 600

1

2

% M

R

µo H (mT)

simulation

A computation, that is not actually a fit, based on the Jullier model reproduces

quite well the general behavior and indicates a very small spin polarization at the

interface. Of coarse the device is much worse than all-LSMO TMR devices.

Nevertheless, the interesting thing is that the degraded layer at the surface of

LSMO can act as a tunnel barrier.

38

A completely different concept

Devices based on the DW resistance

LSMO nanoconstrictions

LSMO

Top view

nanoconstrictions

substrate

P. Bruno Phys. Rev. Lett. 83, 2425 (1999)

DW thickness ≈≈≈≈ Costriction width

The DW are trappedat the costrictions

Picture adapted from A. M. Haghiri, “Half metallic devices for spintronics”Thinner DW higher MR

The principle of operation is the following. The domain walls between regions

with different orientation of magnetization scatter the electrons. So we can make

a device if we can control the formation of domain walls. This is achieved by

realizing nanocostrictions, because it was demonstrated that nanocostrictions pin

the domain walls, and that when located at nanocostrictions the walls become

thinner and scatter electrons more efficiently.

39

C. Rüster, et al.,PRL 91, 216602 (2003)

Nanoconstrictions in (Ga,Mn)As

J. Appl. Phys., Vol. 89, (2001)

APPL. PHYS. LETT. 87, 083102 2005

Review dataarXiv:cond-mat/0610338 v1 12 Oct 2006

LSMOnanoconstrictions

This is not a new concept, and it was already exploited in several studies, among

which I remind those made at the CRISMAT by prof. Raveau team and recently

by prof. Mercey team.

40

MM

Low MR

H

MR

0 Hc1 Hc2



Picture adapted from A. M. Haghiri, “Half metallic devices for spintronics”


So this is a cartoon of device operation. At the beginning the resistance il low

41

MM

High MR

H

MR

0 Hc1 Hc2

The central part has higher coercivity





When the field is increased, the side arms flip earlier, because the central region

has higher coercivity for geometrical reasons.

42

Low MR

H

MR

0 Hc1 Hc2

MM





…and only at higher field the magnetic moments are again parallel.

43

Fabrication process

a) Standard lithography+ ion milling

b) FIB with Ga+ ions10 pA, 30 KeV

c) FIB with Ga+ ions1 pA, 30 KeV

d) SEM photograph

10

0 n

m

Easy axis (001) LSMO

44

10

0 n

m

Easy axis (001) LSMO

Fabrication process

a) Standard lithography+ ion milling

b) FIB with Ga+ ions10 pA, 30 KeV

c) FIB with Ga+ ions1 pA, 30 KeV

d) SEM photograph

30 – 50 nm constrictions

45

Legend:

ψ = barrier heightw = barrier thickness

2'

VV =-2 -1 0 1 2

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

I (µ

A)

V (V)

Strong non-linearity in a wide range of T

Fowler-Nordheim eq

30 nm constrictions

These are the IV curves for a 30 nm constriction at different temperatures. The

behavior is well fitted by the Fowler Nordheim equation, that is a model for

electron tunneling.

46

-2 -1 0 1 2-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

I (µ

A)

V (V)

The MR decreases with V(as in TMR)

4.2 K

30 nm constrictions

Moreover, the MR decreases with voltage, that is typical of TMR devices.

47

30 nm constrictions

Why tunneling?

a) FIB damage

b) Artificial AF DW

Lateral straggling ≈ 10 nm

Sharp Twist of magnetization

AF

Stabilization of AF state in LSMO

Not to scale

So the question is, why are the constrictions behaving like tunnel devices. One

possibility is that the lateral straggling of FIB damages the whole nanocostriction

area. The second, more intriguing effect that may play a role in the antiparallel

state is that at the constriction the twist of magnetization is so sharp that it results

in a thin antiferromagnetic insulating region. This is an open issue.

48

-60 -40 -20 0 20 40 60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

MR

(%

)

µ0H (mT)

Ibias

= 2µA

The depinning is not simultaneous

50 nm constrictions

IV characteristics almost linear

No evidenec of tunneling0 20 40 60

0.0

0.5

µ0H (mT)

MR

(%

)

H-

H+

The hysteresis loop is not symmetric

The behavior of the wider constrictions is different because the IV characteristics

are linear and the MR is lower. The plot of magnetoresistance shows that for this

device the depinning of the domain walls at the two nanocostrictions is not

simultaneous. Moreover the hysteresis loop is not symmetric. This means that the

direction of the current has influence on the pinning and depinning

mechanisms…

49

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.426

28

30

32

34

36

38

40

42

44

46

48

Hf (

mT

)

bias current (µA)

The depinning is not simultaneous

Spin transfer torque is at play?

0 20 40 60

0.0

0.5

µ0H (mT)

MR

(%

)

H-

H+

Jdx

Mdc

dx

MdMbM

+

××=τ

r

50 nm constrictions

…as also the current intensity does. This is not a classical behavior, and it is a

plausible manifestation of a spin torque acting on the domain walls because of

spin injection.

50

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0159.8

160.0

160.2

160.4

160.6

160.8

161.0

161.2

161.4

H = 0

dI/dV

(1/M

Ω)

I (mA)

The current is able to switch the state of the device without the application of an external magnetic field.

current density (H = 0)J = 1.6 × 1011 A/m

switch induced by the current

50 nm constrictions

Finally, in these constriction we can have a switch completely determined by the

current, at zero field.

51

Conclusions & perspectives

1. Both intrinsic and non-intrinsic mechanisms can lead to reduction of MR in devices based on LSMO

2. A damaged interface between LSMO and Py acts as a tunnel barrier

3. The devices based on domain wall resistance at nanoconstrictions are promising.

Physical nature of the constrained region?

Different materials?

• Next step: electron beam lithography

Date post:	27-May-2017
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La0.7Sr0.3MnO3 - Based Spintronics

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