university duisburg-essenmater.physics.auth.gr/materials/download/wiedwald.pdf · university...
TRANSCRIPT
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