University Duisburg-Essen (founded 1.1.2003)

Campus Duisburg

Campus Essen

41.000 students One of the 10 biggest in Germany Less than 50 years old

www.uni-due.de/physik

www.uni-due.de/agfarle/wiedwald

Nanostructured Hard-Magnets

by Interface Design

Ulf Wiedwald

Thessaloniki, December 14, 2015

Shell

(~2 nm)

HRTEM

Core

Outline

Motivation and Introduction

FeCo nanowires

CoNi nanorods

Motivation

Electromagnetic sensors/actuators and motors/generators:

Permanent magnets are everywhere!

Magnetocrystalline Anisotropy

Experimentalphysik-Ag FarleIntroMagn

Anisotropy constants for various magnetic

materials show trends that reveal correlations

between atomic structure, crystal structure

and magnetism. Anisotropy is a fundamental

property defining the suitability of magnetic

materials for different applications whose

microscopic origin is still being unraveled.

Experimental probes that are sensitive to the

spin-resolved electronic states underlying

anisotropy in complex materials are needed

to explain these trends at a microscopic level.

[Figure courtesy of D. Weller, IBM, based on

data taken from B. D. Cullity, Introduction to

Magnetic Materials, Addison-Westley,

Reading, MA, 1972, pg. 381, and T. Klemmer

et al., Scripta Metallurgica et Materialia 33

(1995) 1793.]

Volume anisotopy of materials

Surface effects

Enhanced surface contribution

(= reduced symmetry):

30% surface atoms in spherical

particle with d = 6 nm

lattice distortions?

surface reconstruction?

electronic bandstructure?

magnetism?

-3-

0

1x104

2x104

3x104

4x104

5x104

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

surf

ace

fra

ctio

n

diameter (nm) n

o. of ato

ms

Nanomagnetism- Fundamentals II - Wiedwald , 15.12.2015

Consequences for Nanostructures

• Structural changes

Change of electronic structureModification of exchange and density of statesParamagnetic metals may become ferromagnetic nanoparticles

• Large surface/volumelower coordination (in average) = lower symmetry= larger orbital moment

• Distortion of crystal lattice / surface reconstructioninhomogeneous orbital magnetic moments„core/shell“ types of magnetic anisotropy

0 50 100 150 200 250 300 350 4000.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

1.2x10-4

1.4x10-4

TB = 278 K

FC (Bcool

=5T, Btest

=20G)

ZFC (Btest

=20G)

M(e

mu

)

T(K)Blocking temperature TB

Zero-field-cooled / field-cooled magnetization

Tmax=278K

11.4 nm Co assembly

„time window“: minutes

Blocking Temperature

C. B. Murray et al, MRS Bulletin (2001) p. 985

Magnetic anisotropy of a Nanoparticle

Shape anisotropy(dipole-dipole interaction)

Magnetocrystalline anisotropy(spin-orbit interaction)

Surface Volume Interface Step

Anisotropy

Effective magnetic

Anisotropy Keff

Exchange anisotropy(exchange interaction at FM/AFM interface)

M

8 nm

40 Atome

2 nm2 nm2 nm

HR-TEM

Goal

The figure of merit for a PM: (BH)max

Large Br

Large Hc

T. Maurer, et. al., APL 91, 172501 (2007).

In this talk:

Use magnetic shape

anisotropy of nanowires

Development of novel nanomaterials for high performance

permanent magnets (PM) whithout any rare earths or platinum.

Nanowires for permanent magnets?

for FeCo: μ0HC = μ0Heff ≈ ½ μ0Ms = 1.2 T

The largest remanent induction the strongest magnetic field

Co(hcp) Fe FeCo Ni Nd2Fe14B1

μ0Ms [T] 1.76 2.15 2.4 0.61 1.6

K [kJ/m3] 450 46 15 5 5000

Heff = HMA + (N┴ – N‖) Ms

Shape anisotropy

(N┴ – N‖)Ms

Magnetocrystalline anisotropy

HMA = HV + HS

HV – volume contribution

HS – surface-induced anisotropy(N┴ – N‖) = ½

for high aspect

ratio NW

IDEAL case: coherent reversal process in isolated

single domain FeCo NW

FeCo Nanowires

Coercive field limitations

Micromagnetic simulations

Domain wall (DW) nucleation and

depinning at the ends of the NWs

I. Panagiotopoulos et. al., JAP 144, 143902 (2013).

Magnetic dipolar coupling:

coercivity depends on

packing fraction of NWs

FeCo : μ0HC ≈ 0.3 T

Design goal

Electrochemically prepared.

Collaboration with M. Vázquez

Increase the coercive field in FeCo NWs

suppress vortex formation

by pinning the tips to an antiferromagnet

(a) native oxide

(b) AFM layer

AAO membrane

Diana C. Pinto Leitão, PhD Thesis, Universidade do Porto (2010)

Josefin Nissa, Master Thesis, Lund University (2013)

Poinern et al., Materials, 4, 487-526 (2011)

Li et al., Nanotechnology, 225604 (2008)

Two step anodization process:

(99.999%)

Masuda and Fukuda, Science, Vol. 268 (1995)

Al + acid + voltage = nano-pores

Top view of the AAO membrane:

dip - interpore distance

dc-c - distance center to center

D - diameter of pore

FeCo NWs: Morphology and composition

SampleD

[nm]dc-c

[nm]Length L [μm]

Aspect ratio L/ D

Porosity factor P

FeCo20 20 ± 1.5 nm 53 ± 2 nm 6 300 0.126

FeCo40 41 ± 2 nm 105 ± 2 nm 7.5 188 0.130

FeCo40 FeCo20Characteristics of the samples:

EDS Quantitative Results:

Composition of NWs:

Fe : 30.6 ± 0.5

Co : 69.4 ± 0.6

Fe30Co70

FeCo NWs: magnetic characterization

Sample name μ0Ms [T] Mr/Ms μ0Hc [mT] μ0Heff [mT]

FeCo20 1.88± 0.02 0.84 ± 0.02 240 ± 2 585 ± 10

FeCo40 1.95± 0.02 0.92 ± 0.02 220 ± 2 645 ± 10

SQUID of samples as-prepared at T = 300 K:

Perpendicular

Parallel

B

B

Sara Liébana-Viñas et al.. Nanotechnology 26, 415704 (2015)

FeCo NWs: Magnetic hardening

Glue on Si substrate Remove Au layer Partial removal AAO

membraneDeposition NiMn

Sara Liébana-Viñas et al.. Nanotechnology 26, 415704 (2015)

FeCo NWs: steps of magnetic hardening

402.85 nm

0.00 nm

200nm

AFM measurement

AAO etched ~ 50-70nm

Sara Liébana-Viñas et al.. Nanotechnology 26, 415704 (2015)

FeCo NWs: magnetic characterization (FeCo 40 nm)

Δ=12mT

Δ=52mT

Δ=52mT

Δ=60mT

Δ=60mT

Δ=15mT1 After removing Au

2 After removing membrane

3 After depositing NiMn

NO INCREASE IN COERCIVITY AFTER COVER

WITH NiMn ALLOY!

II NW long axis 0 Initial state

2

c cexch

H HH

Hexch(10K) = 6 mT

FeCo NWs: magnetic characterization

Δ=52mT

1 After removing Au 2 After removing membrane 3 After depositing NiMn

No increase of Hc (NiMn)

0 Initial state

Au

Δ=52mTΔ=12mT

NiMn only at one tip!

Fe30Co70

FexCoyOz

Ni50Mn50

Fe30Co70

FexCoyOz

FexCoyOz

Fe30Co70

FexCoyOz

FexCoyOz

Fe30Co70

FexCoyOz

Conclusions

• Two sources of magn. anisotropy: dipole and LS

• Permanent magnets with 3d alloys

• Surface and interface design

• Align different contributions to anisotropy field

• Preparation of CoNi rods and FeCo nanowires

• Natural oxidation increases

coercive field and remanent magnetization

• AFM NiMn: both tips must be capped!

Collaborations

Sara Liébana-Viñas

Marina Spasova

Ruslan Salikhov

Anna Elsukova

Juliane Perl

Benjamin Zingsem

Anna S. Semisalova

Behnaz Arvan

Xiang Yao

Michael Farle

Verónica Salgueiriño

Cristina Bran

Ester M. Palmero

Manuel Vazquez

Peter Toson

Josef Fidler

Coordinated by D. Niarchos

Madrid