Nanomagnetometry W. Wernsdorfer, E. Bonet Orozco and B. Barbara Lab - - PowerPoint PPT Presentation

Nanomagnetometry

• W. Wernsdorfer, E. Bonet Orozco and B. Barbara

Lab L. Néel - CNRS, Grenoble, France

• A. Benoit

CRTBT - CNRS , Grenoble, France

• D. Mailly

L2M, Bagneux, Paris, France and a lot of collaborators

Different techniques

• Torque balance [Morrish 1956]
• "Rotation method" [Knowles 1978]
• Vibrating sample magnetometer 107 µB [Richter 1989]
• Lorentz microscopy 107 µB [Salling 1991]
• MFM 107 µ B [Chang 1993, Ledermann 1994]
• Hall sensor 106 µB [Kent 1994]
• Micro - SQUID 104 µ B [Wernsdorfer 1995]
• Transport measurements 104 µB [Giordano 1995]

...

2D Hall probes

H I V

Principle: Deviation of electrons induced by a magnetic field (Lorentz force) Semi-conductor heterostructure : GaAs - GaAlAs (à 4K) electron density : n = 3 1011 cm-2 mobility : 800 000 cm2V-1s-1 Hall resistant : 2000

• /T

resistance : 20

at 4K and 2000

at 300K

2D Hall bridge

I I V sample H

A.D. Kent, D.D. Awschalom et al., JAP, 76, 6656 (1994) sensitivity of 106 µB A.K. Geim et al. APL, 71 (16), (1997) Luise Theil Hansen, <theil@meyer.fys.ku.dk> sensitivity of 104 µB

Electric transport measurements

Magnetoresistance

• K. Hong, N. Giordano, JMMM, 151, 396 (1995)

depinning of a domain wall in an isolated Ni wires

• F. Coppinger et al., PRL 75, 3513 (1995)

Single domain switching of small ErAs clusters investigated using telegraph noise spectroscopy

Giant magnetoresistance

• V. Gros, A. Fert et al.

Co/Cu/Co structures

Spin-dependent tunneling with Coulomb blockade

L.F. Schelp, A. Fert et al., PRB, 56. R5747 (1997) Co/Al2O3/Co tunnel junctions with cobalt clusters in the Al2O3 layer

Superconducting Quantum Interference Device (SQUID)

Different types of Josephson junctions :

• point junctions
• tunnel junctions
• micro bridge junctions

Theoretical limit : 1 µ B !!!

with a coupling factor of 4*107 µB/Φo

Roadmap of the micro-SQUID technique

1 100 10 4 10 6 10 8 10 10 1993 1995 1997 1999 2001 2003 Quantum limit of a SQUID

information storage

0 3 nm

Years

S (µB)

Studied nanostructures

S = 10

20 10 10 10 8 10 6 10 5 10 4 10 3 10 2 10 1

clusters individual spins molecular clusters nanoparticles wires submicron particles

Ho

Micro-SQUID magnetometry

• sensitivity :

10 -4 Φ Φ Φ Φo

• fabricated by electron beam lithography

(D. Mailly, LPM, Paris)

particle Josephson junctions

stray field

≈ 1 µm B

• 102 – 103 µB, i.e. (2 nm)3 of Co

10-18 – 10-17 emu

SQUID details

• fabricated by electron beam lithography
• D. Mailly, L2M - CNRS, Bagneux
• dimension :

1 - 2 µm

• material :

Nb

• temperature :

< 7K

• direct coupling with the SQUID
• sensitivity :

10-4 Φo

• ✁

104 µB i.e. (6nm)3 of Co

• ✁

10-16 emu conventional SQUID : 10-7 emu

m ic ro -SQUID s e t -up

2 4 conne ct e d SQUIDs

x

1 0 A/ 1 0 V

y

1 0 A/ 1 0 V

z

1 0 A/ 1 0 V

Macint os h

PLD

Curre nt s ource s Ele ct roniq ue SQUID Scann e r 2 PCI cart e s

1 ...2 4 1 µ A - 3 mA

t I

0 .0 3 - 6 K

Naïve theory

I1 I2

n0

? 300 ns

Ic I t

• 15
• 10
• 5

5 10 15

• 150
• 100
• 50

50 100 150 U(mV) I(µA)

A E F C B D

Ic min Ic

Critical current measurements

40 50 60 70 80 90 100 110 120 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 I c(µA) µ µ µ µο ο ο ο Η(µΤ) Η(µΤ) Η(µΤ) Η(µΤ)

Φ Φ Φ Φ ο

ο ο ο

Ic statistics

100 200 300 400 500 600 700 count Ic -180µA 0.5 1 1.5 2

Histogram of 60000 Ic measurements

• Magnetization measurement : average of N

measurements of Ic precision increases with

• limitation of the cycling frequency of Ic measurement :

length of the current ramp

• 100 µs

cooling of SQUID

• 1 µs
• sensitivity :

10000 measurements per second :

• Ex. : our sensitivity : 104 µB

cluster of Co of 5 nm in diameter

Feedback mode

Ic Ic0

• stable zone

measure Ic continuously

if Ic > Ic0, apply positive external flux

if Ic < Ic0, apply negative external flux

external flux compensates sample's fux

Jump detection: “cold mode”

• 0.3
• 0.2
• 0.1

0.1 0.2 0.3

• 150
• 100
• 50

50 100 150 Flux( ) Φ/Φο Φ/Φο Φ/Φο Φ/Φο

ο ο ο ο

µ µ µ µ Η(µΤ) Η(µΤ) Η(µΤ) Η(µΤ)

40 50 60 70 80 90 100 110 120 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Α Α Α Α Β Β Β Β I c (µA)

µ0 H –

• +
• ✁

SQUID polarized below the critical current

magnetization jump

• SQUID transition

the SQUID heats only after the magnetization jump

H M T t t + 1

• 1

0.04 K 0.04 K < T measure < 30 K Hsat

• H sat

( i) t est

H

² t t

Blind mode

apply a test field, we may (or may not) have reversal

measure after the fact with a second field

field out of plane, high T, microwaves...

Ex: “large” particles

• 0.3
• 0.2
• 0.1

0.1 0.2 0.3

• 150
• 100
• 50

50 100 150 Flux( ) Φ/Φο Φ/Φο Φ/Φο Φ/Φο

ο ο ο ο

µ µ µ µ Η(µΤ) Η(µΤ) Η(µΤ) Η(µΤ)

Co particle: 70 nm x 50 nm x 25nm

0.5 1 0¡ 30¡ 60¡ 90¡ 120¡

• 150¡
• 120¡
• 90¡
• 60¡
• 30¡

hsw

S < 1 S = 1.4 S = 2.4

Ni wires: (40-100) nm x (4-5) µm

Smaller systems

FeS particle: length 200 nm, diameter 20 nm

50 100 150 200 250 0¡ 30¡ 60¡ 90¡ 120¡ 210¡ 240¡ 270¡ 300¡ 330¡ µ µ µ µ

ο ο ο ο

Η Η Η Η

σω σω σω σω

(µΤ) (µΤ) (µΤ) (µΤ)

Co nanoparticles: diameter 20 nm

0.1 0.2 0.3 0.4 0¡ 30¡ 60¡ 90¡ 120¡ 210¡ 240¡ 270¡ 300¡ 330¡ µ µ µ µο

ο ο ο

Η Η Η Η

σω σω σω σω

(Τ) (Τ) (Τ) (Τ) 0

Coupling between nanoparticles

0.1 0.2 0.3 0¡ 30¡ 60¡ 90¡ 120¡ 210¡ 240¡ 270¡ 300¡ 330¡

µ0 H(T)

0.14 0.145 0.15 0.155 0.16 0.165

• 0.125
• 0.12
• 0.115
• 0.11

µ oHy(T) µ oHx(T)

dH/dt

3 nm cobalt cluster

DPM - Villeurbanne: LASER vaporization and inert gas condensation source Low Energy Cluster Beam Deposition regime

HRTEL along a [110] direction fcc - structure, faceting Ideal case: truncated octagedron with 1289 or 2406 atoms for diameters of 3.1 or 3.8 nm blue: 1289-atoms truncated octahedron grey: added atomes, total of 1388 atomes

Low energy cluster beam deposition

Nb

embedded clusters

clusters

Micro-SQUID magnetometry

SQUID is fabricated by electron beam lithography

• D. Mailly, LPN-CNRS

sensitivity :

• 102 - 103 µB i.e. (2 nm)3 of Co

i.e.

• 10-18 - 10-17 emu

clusters in Nb - matrix

• M. Jamet, V. Dupuis, A. Perez, DPM-CNRS, Lyon

Acknowledgment: B. Pannetier, F. Balestro, J.-P. Nozières

embedded clusters

Josephson junctions 1 µm embedded clusters

3 nm