Characterization of magnetic materials by

Lorentz microscopy and

electron holography

JEOL JEM-2200FS transmission electron microscope @HU-Berlin

Structure and chemical composition (CTEM,STEM,EDX,EELS)

Magnetic properties(Lorentz microscopy, electron holography)

Electron biprism

Objective mini lens

NANOPERM

Fe88Zr7B4Cu1

FINEMETFe73.53Si16.07B6.43Cu1.09Nb2.88

HITPERM: (Fe,Co)-M-B-Ge-CuM:Nb, Zr, Hf

Carnegie Mellon University (USA)

FeCo based alloys: (Fe0.5Co0.5)80Nb4B13Ge2Cu1

Image : Hitachi metal company

HITPERM

Saturation flux density Bs(T)

Rel

ativ

e pe

rmea

bilit

y µ

r

Introduction to softmagneticmaterials

Low coercivity

Easily Magnetized and demagnetized. (Easily moving domain walls).

Applications

•Power Transformers

• Data communication

•Sensors

•Magnetic heads

Soft

Hard

Why we use nanocrystalline materials?

Herzer G. In: Buschow KHJ, editor. Handbook of magnetic materials, vol. 10. Amsterdam:Elsevier Science, 1997. p. 415 [chapter 3].

Relationship between structure and magnetic properties

HAADF images

nanocrystalline

nanocrystalline

Annealed at 550ºC for 1 hour Annealed at 610ºC for 1 hour

Electron diffraction

Halo ring

[111]

110

SAED Nano beam diffraction (NBD)

(Fe0.5Co0.5)80Nb4B13Ge2Cu1Annealed at 550ºC for 1 hour

110200211220222310 bcc structure

Electron diffraction

[111]

SAED NBD

[111]

SAED NBD

550ºC1 hour

610ºC1 hour

HRTEM images

15.3nm

Moiré fringes

Nanocrystalline

2.0Å

2.0Å

(110)

(110)

Amorphous phase

Annealed at 550ºC for 1 hour

HRTEM images

Amorphous phase

Nanocrystalline

Annealed at 610ºC for 1 hour

Lorentz microscopy

N N

S S

2T SpecimenN N

S S

Obj mini lens

CTEM mode Lorentz mode

Objective lens

ON OFF

x x x x x x x x· · · ·x x x x · · · · x x x x

Over focused

Under focused

Lorentz microscopy

In focus Under focusedOver focusedAnnealed at 550ºC for 1 hour

Lorentz microscopy

Annealed at 610ºC for 1 hour

In focus Under focusedOver focused

Gun

Biprism

Condenser lens

Objective lens

Specimen

Reference wave

Hologram

Object wave

Electron holography

Electron hologram and it’s Fourier transform

Side band

FFT

550ºC for 1 hour

Fourier transform of a reference hologram

Over focusedIn focus Under focused

PhaseAmplitude

Reconstructed phase and amplitude compared to Lorentz mode image

550ºC for 1 hour

Surface plot of phase

Reconstructed phase compared to Lorentz mode image

Annealed at 610ºC for 1 hour

Under focusedIn focus Phase

Objective lens

N N

S S

H

θ

N N

S S

H//

Specimen In-plane magnetic field

H//=H·Sinθ

N N

S S

Objective lens off Objective lens slightly active

Tilt specimen

Specimen

Objective lens

Change of applied field by tilting sample

App

lied

field

(Fe0.5Co0.5)80Nb4B13Ge2Cu1Annealed at 550ºC for 1 hour

Domain walls’ motion in Lorentz mode

550ºC for 1 hour

Conclusion

Large magnetic domain and few pinning sites in sample annealed at 550℃ for 1 hour were observed by Lorentz microscopy and electron holography. This kind of domain configuration is easy to move in small magnetic field.

Domain configuration becomes more complicated and more pinning sites exist when the annealing temperature was elevated to 610 ℃. This means domain walls in sample need more energy to overcome these barriers. In other words, the coercivity of sample becomes large.