Post on 17-Mar-2020
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.