Magnetic materials

Magnetic Material

Chapter 4

Magnetic properties

Paramagnetic >increase slightly with increasing magnetic field

Diamagnetic decreases with magnetic field

Ferromagnetic increases dramatically ….

Magnetic units

B=m_oH in vacuum

Permeability in vacuum

Magnetic flux density ,induction

Magnetic field strength

M=XH

Xm is magnetic susceptibility

Ur is relative permeability

Diamagnetic material ur<1

Paramagnetic material 1<ur<1.1

Ferromagnetic material ur>>1

Ferromagnetism

1. Alignment of magnetic moments of atoms in given direction
2. If the magnetic moments are ordered, then how do we explain the demagnetized state
3. Weiss domain theory

Ferromagnetic Domains

Within ferromagnetic domains all magnetic moments of atoms point in the same direction.

Demagnetization Field

Does the shape of the specimen have any effect on the magnetic field in the specimen? Yes

Shape anisotropy, Alnico Magnets

Easy axis of magnetization decided by the shape of the particle

Why do domains exist at all?

The energy should be minimized.

What is the self energy of a single-domain particle?

Energy per unit volume of a dipole

Demagnetizing field

Energy of a single domain

How does the domain arise from a single domain specimen as the magnetic field is reduced?

By creating domain wall, the total energy of the system is reduced even though energy is required to create domain wall.

Hysteresis Loop

Important Properties of Ferromagnetic Materials

Permeability(range 10-105) high for permalloys and super alloys(Ni-Fe alloys)

Remanence: This distinguishes between ferromagnets and paramagnets. It describes how well a material can keep its magnetization

Hysteresis: Reversible and irreversible processes

Saturation: magnetization depends only upon the magnitude of the atomic magnetic moment m and the number of atoms per unit volume

Remanence : The remanent induction and the remaining magnetization M_R.

Coercivity: the magnetic induction can be reduced to zero by areverse magnetic field of strength H_c. This field strength is called coercivity.

Curie Temperature: All ferromagnetic materials when heated to a sufficiently high temperature become paramagnetic. The transition temperature is called the Curied temperature.

Magnetic Order can be destroyed by thermal energy

Exchange Energy

Magnitude of the co-operation between adjacent atoms can be approximate by the thermal energy necessary to destroy it. K_Tc (Curie Energy)

Magnetocrystalline Energy

If different amounts of energy are required to magnetize a material along different crystallographic directions, then we have magnetocrystalline anisotropy.

Hard and Soft Magnetic Materials

Coercivity(H_c):

-The magnetic field necessary to reverse magnetization from the remanence to zero(B=0)

-Hard magnets: H_c>10kA/m

-Soft magnets: H_c>1kA/m

Uses of a Soft Magnet

1. One of the most important applications of magnetic materials is the enhancement magnetization by a current –carrying coil.
2. For this application the magnet should be:

-Easily magnetized and demagnetized

Properties of a Soft Magnet

A good soft magnet should have:

1. A large saturation magnetization (max B)

2. Magnetization should be large in small fields

i.e. soft magnets should have a high permeability (u_m)

u_m=B/H

The area under a hysteresis curve represents the energy losses

Area =Hysteresis Losses (J/m³)

Magnets that are subjected to high frequency alternating magnetic fields should have a narrow hysteresis curve, i.e. a low coercivity

Properties of a Soft Magnet

• A low coercivity corresponds to easy movement of domain walls.
• Structural defects restrict wall movement

-particles

-voids

-grain bounderies

• Soft magnets should be free of structural defects

Zero Frequency Oscillating Fields

§ Devices: Electromagnet Cores

§ Materials:

-Iron (99.95% Fe)

-Iron-cobalt (49% Fe, 49% Co, 2% Vn)

§ Ideal Properties

- high saturation magnetization

§ Properties:

-Iron

-Iron-Cobalt

Low Frequency Oscillating Fiedls

Device:

Main Transformer Cores (50-60 Hz)

Motors

Materials

-Iron-silicon Alloys

-97%Fe, 3% Si

Ideal Properties:

-high permeability

-large saturation magnetization

Low Frequency Oscillating Fields

Iron-Silicon Alloys

Benefits of adding Silicon:

- Increase resistivity (Lower eddy-current Losses)

- Improvement of punchability

- Ferrite former

Adverse Effects of adding Silicon:

- Lowers ductility – limits Si addition

- Lowers saturation magnetization

- Lowers Curie temperature

High Frequency Oscillating Fileds

At high Frequencies, metallic cores are not useful due to eddy-current losse

Devices:

- Cores for recording heads

- Cores for receiving antennas

Material: Cubic Ferrites (Ferrite Spinels)

- AKA: ferroxcube

- Made by sintering

Properties

- High resistivity (insulator)

High Frequency Oscillating Fields

Cubic Ferrites

- High porosity, domain walls do not move easily

- Magnetization rotation plays a big fole , therefore it is important to minimize anisotropy

At lower frequencies (<>

- Mn-Zn Ferrite – has largest magnetization

At high frequencies(>10 MHz)

- Ni-Zn ferrite- has a higher resistivity

Electrical Steel: Core materials of electrical machinery and equipment

-Grain-oriented steel

-Non-oriented steel

Electrical energy loss depends on magnetic properties.

-Core loss = hysteresis loss + eddy current loss

-Permeability (magnetic induction):

Dependent on crystallographic texture

Power losses depend on direction along which the steel is magnetized

Why is Si added?

- Reduction of core loss

- Improvement punchability

- Ferrite former

Hoq Fe-Si Grain oriented steel processed?

Heat from BOF>Ferrosilicon Addition to Ladle>Casting>Sealing>Hot Rolling>Pickle Line>First Cold Reduction>First Anneal (Normallized)>Second Cold Reduction>Second Anneal (Decarburization)>MgO coating>High Temp. Anneal>Coating>Thermal Flattening>Slitting>>>>

Non-Oriented Electrical Steels ( applications)

- Used for core materials for rotating electric machinery

- Magnetic materials

-low core loss

-high permeability

-Si content 0.1%-3.0%

How is Fe-Si Non-Oriented steel proceeded?

In non-oriented steels most of the research has been focused on grain size control

- Chemical composition: Si, Al, Mn, P, C, O, N

- Processing conditions : hot rolling condition, Hot-band annealing, cold roling method, intermediate annealing, final annealing

- Suitable testure can reduce the power losses

Full-processed non-oriented electrical steels without stress relief annealing

Continuous casting>Hot rolling>Hot band annealing>Acid pickling>1^st cold rolling>Intermediate annealing>2^nd cold rolling>Final annealing>Insulating coating>Punching>motor

Effect of Grain Size non-oriented electrical steels

There exists an optimum grain size

Grains increase, eddy-current loss increases and hysteresis loss decrease

Amorphous Metal for Electric Power Distribution

• Absence of grain boundaries,easier domain wall movement
• High Resistivity (eddy-current)
• The absence of crystal anisotropy)
• Core losses reduced by 75%
• The amorphous metal tends to be thinner, harder and more fragile than silicon steel (engineering design different)
• Processing 10⁵ C/s quench rate (planar flow castings)
• The molten alloy is forced through the slotted nozzle in close proximity (about 0.5 mm) to the surface of a moving substrate

Application of soft magnetic materials

• Answering machine
• Cordless phone
• Cell phone
• Digital camera
• Battery recharger
• Video camera battery replacer/recharger
• Digital clock
• Electric toothbrush
• Electric razor
• Electric screwdriver
• Electric drill
• Laptop computer
• Office phone
• Ink jet printer
• Speaker system on the computer

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Hard Magnets

How are Hard magnets rated

Magnets are characterized by three main characteristics

• Residual inducton(Br). This is an indication of how strong the magnet is
• Coercive Force (Hc)

This is an indication of how difficult is to demagnetize the magnet

-Maximum Energy Product (BHmax)

The energy required to demagnetize the permanent magnet.

High Energy Product

- The maximum energy of BH can be found in the second quadrant of the hystersis loop. It is related to the hystersis loop. Measured in KJ/m^3

Alnico Magnets

- Discovery of a group alloys called Alnico alloy I early 1930s

- Typical composition weight%: Fe-35, Co-35, Ni-15, Al-7, Cu-4, Ti-4

- Still used today as they have a high Curie temperature(~850 C)operates at high higher temperatures)

- More stable properties around room temperature

- The magnets are either sintered or directionally cast, and then annealed in a magnetic field

- This process develops an oriented micro-structure of strongly magnetic Fe-Co in a weak magnetic Ni-Al. The coercivity derives from the rod shaped the phase, which generates shape anisotropy. The a phase is pinning the domain walls movement.

Alnico Magnets

Advantages:

- Mechanical stability

- Small reversible changing of magnetic values with temperature

- High application temperature (about 500C)

- Easy to magnetize

Disadvantages

- Contains cobalt (raw material price)

- Limited design due to low coercivity

- Cannot be exposed to significant demagnetizing fields

- Magnetic stability at reverse fields

Hard Ferrite

- Second advance in the development of permanent magnets, in the 1950s

- Composition

- Low remanence(~400 mT), the maximum energy product is only ~40KJ/m^3

- High coercivity, (~250 kA/m) could be exposed to moderate demagnetizing fields, applications in permanent magnet motors

- The magnets are made by a powder metallurgy comprise by very small grains(<1nm)

Advantages

- Raw material base (lowest price per energy unit)

- Thermal stability at elevated temperatures

- Easy to magnetize

- Stable magnetization

Disadvantages

- Low values (Br, (BH) max)

- Brittleness (ceramic)

Rare Earth

Rare earth magnets include Sm-Co (Samarium-Cobalt) magnets and Nd-Fe-B (Neodymium-iron-Boron) magnets.

Ytrium and Samarium cobalt Magnets

- YCo₅ magnets were discovered in 1966

- The first rare-earth(RE) and a transition metal (TM) magnet

- Re provides the anisotropy to the phase

- TM provides the high magnetization and high Curie temperature

- SmCo₅ was discovered in 1967, sintered magnets can have energy product as high as ~160KJ/m^3

- High Coercivity is achieved by prevention of the nucleation of reverse domains

- Maximum energy product was increases to 240kJ/m63, with a Sm₂Co₁₇ based alloy, in 1976.

Samarium Cobalt Magnets

- General Cobalt Magnets Sm₂(Co, Fe, Cu, Zr)₁₇

- The magnets are produce by powder metallurgy

- Solution treated at ~1100C, are single phases

- Sm₂ Sm₂Co₁₇ type phase, is enriched in Fe

- Cell boundaries comprise of a layer of Sm₂Co₅ type phase, is enriched in Cu

- The Magnetic domain wall energy is greatly reduced within the cell boundary , pinning the domain walls leads to permanent magnetic properties

- Samarium is much less abundant than other light rare-earth elements

Advantages

- high thermal stability (reversible and irreversible)

- The potential is that it can be operated at high temp.

- In general no corrosion protection required

Disadvantages

- High raw material prices

- Mechanical manufacture ability/handling(brittle)

- High magnetic field strength for saturation needed

- Cobalt is a strategically important metal, sale are restricted

NdFeB Magnet

- NdFeB magnets were discovered simultaneously by General Motors in the USA and Sumitomo Special Metals in Japan in 1984. Different processing routes employed

- Based on the magnetic phase Nd₂Fe₁₄B

- High coercivity, high remanence, Tc around 300-500 C

- Maximum energy products of ~450kJ/m^3, by improved heat treatment and the use of high percentage of iron

- Sintered NdFeB achieve their coercivity by creating a Nd-rich phase at the grain boundaries to produce liquid phase during sintering. This makes nucleation of reverse magnetic domains more difficult

Advantages:

- Highest remanence and energy product

- Raw material price compared to SmCo

(No cobalt and Nd less expensive than Sm)

- Well controlled manufacture and handling

Disadvantages:

- For high temperature applications special grades required

- In many cases corrosion protection required

Methods used to make Nd FeB

Powdering and Sintering

- The material is melted in an inert atmosphere to prevent oxidation

- Milled into a powder with an average diameter of 3 um

- The particles are then aligned in 800 kA/m., magnetic field

- Compacted at 200 MPa pressure

- Sintered at 1050-1150 C in an inert atmosphere

The Rapid Quench

- The individual components (Nd, Fe, B) are “melt spun” in an argon atmosphere ( particle size 20-80 nm)

- They are bounded with epoxy with an intermediate maximum energy product 72 kJ/m^3

- They can be hot pressed to form aligned textured magnets with a high maximum energy product around 450 kJ/m^3

The Surface Treatment

- The corrosion resistance of NdFeB is poor

- Painting, coating, or plating required

- Nickel, Zinc, or Tin plating provided good corrosion resistance for NdFeB magnets

- Cadmium chromate or aluminum chromate plate NdFeB using vacuum deposition techniques

- A variety of organic coating have also been successfully developed for NdFeB, exhibiting good corrosion resistance characteristics

Coatings

- Zinc-coated Magnets

- Nickel-coated Magnets

- Epoxide-coated Magnets

Examples of Magnet Applications

- Mechanical to mechanical- such as attraction and repulsion

- Mechanical to electrical- such as generators and microphones

- Electrical to mechanical-such as motors, loud speakers

- Mechanical to heat- such as eddy-current and magnetic torque devices

- Special effects-such as magneto-resistance, Hall effect devices, and magnetic resonance

Magnetic Recording Technology

- The storage of analog or digital signals on a magnetic medium for subsequent retrieval and use

- Low-frequency analog audio-recording

- High speed digital recording of data

- The recording of video information (the highest density of recording, 2660 flux reversal per millimeter, 4000 reversals in possible)

- Storage of data for computers is achieved by magnetic mathods (magnetic tapes, floopy disks and magneto-optic disks)

- A large industry (US $ 170 billion a year in 2001)

Areal Density Progress

- Area density is doubling each year

- We expect progress to continue at roughly this pace to 100Gb/in^2 using current technology, and using new ones beyond 100 Gb/in^2

Conventional Magnetic Recording; Write Head

- Longitudinal media/vertical recording media

- The medium is magnetize using a small electromagnet called the “write” head

- The write head is made of a soft ferrite ore permalloy and had a small gap of typically 0.8 um

How is “information” written into the medium?

- What happens when the magnetic tape passes the recording head gap?

- A change in the state of magnetization may happen, depending on the strength and direction of the magnetic field in the head gap

- The magnetic imprint of the tape is then recorded(the remanence of one direction B_R could represent the state “1” and remanence of the other B_R could represent the state “o”.

How is information “read” from the magnetic medium?

- A tape moves past the “read head” recorder

- Changes of the leakage flux from the surface of the tape

- The flux passes through the body of the read head and is detected by a coil wrapped around the head

- The read head must be magnetically soft

- The voltage in the coil is proportional to df/dt

Criteria for Magnetic Recording Media

- A relatively high coercivity (so the information cannot be erased by small magnetic field)

- The coercivity small enough (erasure of information and subsequent re-use of the medium when desired)

- The medium should have a relative high saturation magnetization and remanence so that the leakage field from the surface can be easily measured by the read head.

- Coercivity of the recording media: 20-150 KA/m (250-1875 Oe) saturation magnetization: 0.3-2.0 MA/m

Magnetic Recording Materials

Which materials are used?

- Gramma iron oxide

- Chromium dioxide

- Cobalt surface-modified

- Hexagonal ferrites

- Ferromagnetic powders

- Cobalt alloys

Analog Recording

- The tape has a permanent magnetic record of the sound or data signal

- The number of the flux lines is proportional to the recorded signal strength

- Their duration is inversely proportional to the recorded frequency

- The duration represents a certain wavelength on the tape and be be expressed as:

- Wavelength = Tape speed/frequency

Analog Recording: Example

If a high frequency response of 15 KHz is desired

- As in reproduction of music, the wavelength should be as short as passible to save tape

- If we want to record and reproduce 15kHz at a tape speed of 9 cm/sec

- Wavelength = 9/15000 = 6 um

- This is a very short wavelength; ¼ of the tape thickness

Perpendicular recording in Operational System

Advantages should be:

- higher storage density ( bits per cm)

- higher and more sharply-desired leakage field

Problems:

- poor signal-to-nose ration(high noise on playback)

- need for a very small head-to-medium distance(flying height)

Materials used

- textured Co-Cr 18% film

- barium ferrite

Perpendicular Media

- Avoids (for sometime) superparamagnetism

- Thicker magnetic media

- Magnetic stability

- Larger signal to head

Prospect of Perpendicular Recording

- Perpendicular recording has an expected density of 10^8 bits/in^2

- This density is comparable with the storage density expected from magneto-optic recording

- Continual improvements in conventional longitudinal recording have been made

- The use of magnetoresistive heads for detecting the signals

Writing

- Data is written on a magneto-optic disk using a laser beam

- The disk consists of a material with low ordering temperature and a relatively high angle of Kerr rotation

- The material is heated by incident laser light (as temperature rises the coercivity is reduced).

- The magnetization direction of the region is altered

- The region is cooled, the coercivity rises

- Data is stored

The reading Process

- Madneto-optic Kerr effect: a heavy polarized beam of laser light, has a direction of polarization rotated by direction of magnetization of the reflecting medium

- The direction of magnetization at various points on the magneto-optic disk can be determined

- The laser beam used for reading has a lower intensity than that used for writing(so the data are not corrupted by overheating).