1896 1920 1987 2006

Properties of Materials

Chapter Two Magnetic Properties of Materials

Key Magnetic Parameters

How does M respond to H?

Fe, Co, Ni

Ferromagnetic

Fe3O4

Ferrimagnetic

Paramagnetic

Antiferromagnetic

Diamagnetic

• Diamagnetic

– Linear ，χ＜0，~ 10-5 ~ 10-6

• Paramagnetic

– Linear ， χ＞0，~10-3 ~ 10-6

• Ferromagnetic

– nonlinearity， χ＞0 ，

χ is the largest

• Ferrimagnetic

– nonlinearity， χ＞0 ，

χ is relatively large

• Antiferromagnetic

– Linear， χ＞0， χ is relatively small

All materials have response to external magnetic field to some extent.

Ferromagnetism Possess a permanent magnetic

moment in the absence of an

external field and manifest very

large and permanent

magnetizations;

Occurs in transition metals iron

(as BCC -ferrite), cobalt, nickel,

and some rare earth metals such

as gadolinium (Gd);

χ can be 106

M = χ H

Origin of Ferromagnetism • Permanent magnetic moments

results from atomic magnetic

moments due to uncancelled

electron spins

• Small contribution from

orbital magnetic moment

• Quantum mechanical exchange

interactions favour parallel

alignment of moments

• Alignment within a domain

Ferrimagnetism

• Macroscopically similar to ferromagntism

• Antiferromagnetic exchange interactions

• Different sized moments on each sublattice results in net magnetization (incomplete cancellation of spin moments)

• Example: MFe2O4, magnetite, maghemite

Anti-ferromagnetism • In some materials, exchange

interactions favor antiparallel alignment of atomic magnetic moments

• Materials are magnetically ordered but have zero remnant magnetization and very low

• Many metal oxides are antiferromagnetic

Anti-ferromagnetic Materials

Above the Neel temperature they become paramagnetic

Transition metal compounds, especially oxides: hematite, metals such as chromium, alloys such as iron manganese (FeMn), and oxides such as nickel oxide (NiO).

• Thermal energy can be used to overcome exchange interactions

• Magnetic order is broken down at the Néel temperature (c.f. Curie temp)

Ferro/Ferri/Antiferro-magnetism

Spontaneous Magnetization

Anti- ferromagnetic

Iron group Rare earth

Ferromagnetic

A－Exchange interaction constant

• when A＞0，spontaneous

magnetization;

•when A＜0，no spontaneous

magnetization;

cos22 221 ASSSAEex

When the two atoms are close to each other, their electrons in 3d layer

and 4s layer can exchange positions, and such interaction forces the spin

magnetic moments ( ) in the adjacent atoms to align in order.

The exchange interaction energy:

S

Magnetic Domain Theory

Magnetic Domain Theory

total internal wall externalU U U U

Magnetic Domain Theory

Magnetic domain structure in a single crystals of Fe - 3%Si alloy photomicrographs Magnetic domains change shape as a magnetic field (H) is applied. Domains favorably oriented with the field grow at the expense of the unfavorably oriented domains.

Magnetic Hysteresis Loop

Ms, Bs

Magnetic Anisotropy Single crystal

Soft/Hard Magnetic Materials

Size and shape of hysteresis loop for ferromagnetic and ferrimagnetic materials are of considerable practical importance.

Loop area: energy loss per unit volume per mag-demag cycle (heat)

Soft Magnetic Materials

Large Mr and small Hc desirable for transformer and motor cores to minimize energy dissipation with AC fields.

Features: easy to be magnetized and demagnetized.

Properties: low Bs, Br, Hs, Hc; high μ

Application: magnetic conductors, such as: transformer, relay, induction coil, iron core; motor rotor, stator; magnetic circuit connection, magnetic screen, switch, storage element.

Material:

– Industrial pure iron, silicon steel

– Fe-Ni alloy

– Soft magnetic ferrite

Used in devices subject to alternating magnetic fields and energy loss must be low, such as transformer

Transformer

Industrial pure iron/silicon steel

Hard Magnetic Material

Features: Maintain high

magnetization intensity with no

external magnetic field.

Properties: high Bs, Br, Hs, Hc.

Applications:electrical meter,

motor, telephone, radio, tape

recorder...

Materials: Maraging steel; Cast

aluminum nickel; Oxide ferrite;

Rare earth cobalt, neodymium

iron boron

Large Mr and Hc desirable for permanent magnets and magnetic recording and memory devices.

Application of Hard Magnetic Materials

Electrical Motor

Magnetic Data Storage

easy to be magnetized (low Hs)

• high remanence (high Br)

• easy to be demagnetized (low Hc)

Magnetic Tape

Classification of Magnetic Materials

All are technologically important

External field H = 0

Classification of Magnetic Materials

Magnetostrction Magnetostriction -- When a ferromagnet is magnetized, the

length changes along the magnetic field direction (elongation

or shortening).

0

0

l

ll

• when λ＞ 0 ，Positive magnetostriction（Elongation）

• when λ＜ 0 ，Negative magnetostriction（Shorten）

Magnetostrictive coefficient:

Magnetostrictive Materials Saturation magnetostriction

With the strength of the external magnetic field (H) increasing,

the magnetization intensity of ferromagnet (M) also increases, and the

absolute value of λ increases. When the magnetization reaches the

saturation value Ms , λ→λs (saturation magnetostriction coefficient)

Magnetoresistance Ordinary Magnetoresistance (OMR) discovered in 1856 by Lord Kelvin Magnetoresistance (MR) is the change of resistance of a conductor in an external magnetic field. In typical metal, at room temperature, OMR effects are very small, at most of the order of a few per cent.

Corbino disc

In response to the Lorentz force, the carrier velocity v:

Magnetic field pointing into

page (screen)

Current-Carrying Wire

Direction of velocity v of

electrons

Direction of qv of (negative)

electrons

Direction of force on conduction electrons

Giant Magnetoresistance In 1988 two research groups independently discovered materials showing a very large MR, now known as giant magnetoresistance (GMR).

Superlattice

Magneto-Optic Effect

Zeeman Effect

by Dutch physicist Pieter Zeeman, is the effect of splitting a spectral line into several components in the presence of a static magnetic field.

Since the distance between the Zeeman sub-levels is a function of the magnetic field, this effect can be used to measure the magnetic field, e.g. that of the Sun and other stars or in laboratory plasmas. The Zeeman effect is very important in applications such as nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, magnetic resonance imaging (MRI) and Mössbauer spectroscopy.

Applications

Magneto-Optic Effect

Caused by left and right circularly polarized waves propagating at slightly different speeds

Farady Effect

Causes a rotation of the plane of polarization which is linearly proportional to the component of the magnetic field in the direction of propagation

θF = F L (M/Ms)

Needs high optical transmission Applications applications in measuring instruments: measure optical rotatory power and for remote sensing of magnetic fields; amplitude modulation of light, and are the basis of optical isolators and optical circulators (Faraday Rotator).

Typical materials: Y3Fe3O12 (YIG); (Yb0.3Tb1.7Bi1)Fe5O12

Magneto-Optic Effect

When the applied magnetic field is perpendicular to incident light

Cotton-Mouton Effect: birefringence induced by magnetic field

Magneto-Optic Effect

Kerr Effect Light that is reflected from a magnetized surface can change in both polarization and reflected intensity

Magneto-Optic Effect

Magneto-optical memory

Applications

Magnetocaloric Effect

where T is the temperature, H is the applied magnetic field, C is the heat capacity of the working magnet (refrigerant) and M is the magnetization of the refrigerant

using a magnet with a small heat capacity

applying a large field

using a magnet with a large change in magnetization vs. temperature, at a constant magnetic field

How to enhancemagnetocaloric effect ?

the heating or cooling (i.e., the temperature change) of a magnetic material due to the application of a magnetic field

Thermodynamic Cycle

Adiabatic demagnetization

Adiabatic magnetization

Isomagnetic enthalpic transfer

Isomagnetic entropic transfer

Heat removed by liquid or gas

In thermal contact with environment to be refrigerated

Magnetic Refrigeration

Conventional Carnot refrigeration cycle

Magnetic refrigeration VS

Magnetic refrigeration

Magnetic refrigeration advantages:

Safer & Environmental friendly

No harmful, ozone-depleting

coolant gases

Compact, quieter

Lower power consumption

Higher cooling efficiency

Can reach extremely low T

adiabatic demagnetization refrigerator

Magnetocaloric refrigeration system prototype developed by GE

Nanomagnets A nanomagnet is a submicrometric system that prevents spontaneous magnetic order (magnetization) at zero applied magnetic field (remanence).

Like the paramagnet, the superparamagnet returns to zero magnetization when the field is removed. It does so for a different reason: small size, not intrinsically weak exchange between the individual moments.

Nanomagnets

Superparamagnetic Ferromagnetic

Size Effects – Superparamagnetism

Nanomagnets Nano scale has a big impact on the magnetic properties! In a normally ferromagnetic material, nano scale reduces the moment, but it can be restored by applying a magnetic field. The good news: switchable interactions! (medical application) The bad news: There would seem to be a lower limit to the size of a magnetic particle that can hold an alignment for data storage. “Beating the superparamagnetic limit’’ by developing synthesis routes for NPs with high anisotropy constants is one way to try to compensate for thermal fluctuations that become dominant at small particle volumes.

Ferrofluid: Flowable Magnets • In the 1960’s Stephen Pappell at NASA first

developed ferrofluids as a method for controlling fluids in space;

• Colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid;

• Each tiny particle is thoroughly coated with a surfactant to inhibit clumping;

• The magnetic attraction of nanoparticles is weak enough that the surfactant's Van der Waals force is sufficient to prevent magnetic clumping or agglomeration

Ferrofluid • Dispersion stability of ferrofluid

• Magnetorheological fluid

Surfactant-stabilized nanoparticles

Micro-particles

Properties of Ferrofluid

• Nanomagents

• Viscosity

• Magnetic pressure

• Interfacial control

• Induced heat convection

• Normal-field instability

• …

Cutting-edge Research Example-1 Switchable Static and Dynamic Self-Assembly of Magnetic Droplets on Superhydrophobic Surfaces

Science, 2013, 341, 253-257.

Cutting-edge Research Example-1 Switchable Static and Dynamic Self-Assembly of Magnetic Droplets on Superhydrophobic Surfaces

Science, 2013, 341, 253-257.

The key concepts in this chapter

Magnetic moment Magnetic torque

Magnetic field strength Magnetic induction

Nanomagnet Francis Bitter

pattern Magnetization Magnetic medium

Permeability

Magnetic susceptibility Ferromagnetism Ferrimagnetism

Antiferro-magnetism Paramagnetism Diamagnetism Magnetization curve

Hysteresis loop Magnetoresistance Curie’s law Curie Weiss law

Coercivity Hard magnets Soft magnets Magnetic domain

Magnetocrystalline anisotropy Magnetostriction

Spontaneous magnetization Ferrofluid

Homework

Read one paper (related to application of magnetic

materials) that’s published in Science, Nature, Advanced

Materials, Nano Letters or other top English journals.

Write one or two paragraphs of your learning after

reading the paper.