Magnetic Domains in Soft Ferromagnets

A ferromagnet

consisting of a single domain creates a large external field, which costs energy ( B2). This stray field

is reduced

by forming several domains. It is like cutting up the magnet and letting the pieces

orient them-

selves optimally (i.e., with North and South poles next to each other).

This occurs in pure and defect-free

materials (soft magnets). An external field is needed to magnetize them by aligning the domains (electromagnet).

Imaging Magnetic Domains with Iron Powder

Small iron particles are attracted to the high magnetic stray field at the transition between domains. They scatter light in a dark field microscope. The orientation of the domains is determined by adding an external field and watching them grow or shrink.

Magnetic Domain Walls

0.1 m

M

Saturation Magnetization MsRemanence Mr

H

Coercivity Hc

Magnetic Hysteresis Curve: M(H)

Small Coercivity

: Soft Ferromagnets

Simple Magnetic Hysteresis in nm

-

Sized Particles and Layers

(Smaller than a Domain Wall

Single Domain)

Permanent MagnetsThe strongest permanent magnets (hard magnets) are alloys

of 3d transition metals

(Fe, Co) with rare earths. They exhibit the highest energy product B·H.

Examples are neodymium-iron-boron

(Nd2

Fe14

B) and samarium-cobalt (SmCo5) .

Such magnets are used for electric motors and for generators in wind turbines, both important ingredients in becoming more energy-efficient. A Prius

contains 1 kg of neodymium. That has caused increasing demand for rare earths.

To prevent the formation of domains in a permanent magnet, one uses materials with a high defect density. Defects

prevent domains from forming and thereby “pin”

the magnetization.

Two neodymium-iron-boron magnets that crunched a fingertip.

B, H, and M

in a Bar Magnet

The magnetization M

is constant in a permanent magnet (by definition). The two other fields need to satisfy:B

= H

+ 4M

insideB

= H

outsideH|| is continuous at a boundaryB

is continuous at a boundary

The H-field inside the magnet is the demagnetizing field Hd

. It is related to the stray field outside the magnet by the continuity of H|| at the side of the magnet. Since the outside field lines bend 180o

from the side to the end and since B

is continuous at the end

face, Hd

opposes B

inside.

To

con-

struct

the field lines, start with M, then jump to B, and do H last.

M

H

B

Magnetostriction

and Piezoelectricity

Magnetostriction

has a large influence on magnetic nanostructures, such as hard disk reading heads. Small structures tend to be strained by the surrounding material, and the strain magnetizes them permanently. Their magnetization cannot be switched anymore. In permalloy

(Ni0.8

Fe0.2

) the magnetostriction

vanishes, which makes it a key material for spintronics.

Piezoelectricity

makes it possible to position a STM tip with sub-atomic precision. A piezoelectric ceramic changes its length proportional to an applied voltage. A common design is a tube-scanner which can move in all three directions (x,y,z).

Change in B, E

Length Change