Basics of Magnetic Fields
5. Magnetic materials
Learning outcomes
- I can distinguish between ferromagnetic, paramagnetic, and diamagnetic materials.
- I can explain how magnetic domains affect magnetism.
- I can describe how materials become magnetized.
- I can compare soft magnetic and hard magnetic materials.
- I can identify applications of magnetic materials in technology.
Magnetic Materials
How Materials Respond to Magnetic Fields
Different materials respond differently when placed in a magnetic field. Some are strongly attracted to magnets, some are only weakly attracted, and others are weakly repelled.
Three important categories are:
- ferromagnetic materials
- paramagnetic materials
- diamagnetic materials
Their different behaviours arise from the magnetic properties of atoms and electrons within the material.
Where Magnetism Comes From
Magnetism in materials ultimately comes from electrons.
Electrons have magnetic moments associated with their quantum properties. In an atom, the magnetic effects of different electrons may:
- cancel
- partially cancel
- reinforce one another
The way these atomic magnetic moments interact determines how a material responds to an external magnetic field.
This microscopic behaviour produces the different types of magnetism we observe on a larger scale.
Ferromagnetic Materials
Ferromagnetic materials respond very strongly to magnetic fields and can become strongly magnetized.
Important examples include:
- iron
- nickel
- cobalt
- many steels
- some rare-earth alloys
A ferromagnetic material can be strongly attracted to a magnet.
Some ferromagnetic materials can also retain their magnetization after the external magnetic field is removed.
Magnetic Domains
A major feature of ferromagnetic materials is the formation of magnetic domains.
A magnetic domain is a region in which many atomic magnetic moments are aligned in approximately the same direction.
Within one domain, the magnetic effects of many atoms reinforce one another.
A piece of ferromagnetic material can contain a very large number of domains.
Domains in an Unmagnetized Material
An unmagnetized piece of iron still contains magnetic domains.
However, the domains may be oriented in many different directions.
For example:
↑ ↘ ← ↗ ↓ →
The magnetic effects of the domains largely cancel.
Therefore, the object has little overall magnetization.
It is incorrect to say:
"An unmagnetized material has no magnetic domains."
The domains are present; their overall magnetic effects simply cancel.
Domains in a Magnetized Material
When a ferromagnetic material becomes magnetized, its domain structure changes.
Domains aligned favourably with the applied magnetic field can grow, and magnetic moments can become more strongly aligned.
The material develops a net magnetization.
A simplified representation might look like:
→ → → → → →
The magnetic effects now reinforce one another instead of largely cancelling.
The object can therefore produce a strong external magnetic field.
How a Material Becomes Magnetized
A ferromagnetic material can be magnetized by exposing it to a sufficiently strong magnetic field.
Possible methods include:
- placing it inside a current-carrying coil
- stroking it repeatedly with a permanent magnet
- placing it close to a strong magnet
The applied field encourages the magnetic domains to become more strongly aligned.
This process is called:
magnetization
Magnetizing with a Solenoid
A common method of magnetization uses a solenoid.
A solenoid is a coil of wire.
When current flows through it, the coil produces a magnetic field.
If a ferromagnetic object is placed inside the solenoid, this magnetic field can cause its domains to become more aligned.
A stronger field generally produces greater magnetization until the material approaches magnetic saturation.
Magnetic Saturation
As the applied magnetic field becomes stronger, more of the material's magnetic structure becomes aligned.
Eventually, increasing the applied field produces relatively little additional magnetization.
The material has approached:
magnetic saturation
At saturation, the material is close to its maximum possible magnetization under those conditions.
Magnetic Induction
A ferromagnetic material can become temporarily magnetized when placed near a permanent magnet.
This is called:
magnetic induction
For example, an iron nail placed near a magnet can become temporarily magnetic and attract another nail.
The external magnetic field changes the domain arrangement within the iron.
When the external field is removed, the iron may lose much of its magnetization.
Paramagnetic Materials
Paramagnetic materials are weakly attracted to an external magnetic field.
Examples include:
- aluminium
- magnesium
- platinum
- oxygen
Their magnetic response is much weaker than that of ferromagnetic materials.
In a paramagnetic material, atomic magnetic moments tend to align slightly with an applied magnetic field.
However, thermal motion prevents strong long-range alignment.
When the external field is removed, the material generally does not remain strongly magnetized.
Paramagnetic Behaviour
Without an external field, the atomic magnetic moments of a paramagnetic material are largely randomly oriented.
When a magnetic field is applied:
some moments tend to align with the field
This produces a weak net magnetization in the same direction as the applied field.
Therefore:
paramagnetic materials are weakly attracted toward stronger magnetic fields.
Diamagnetic Materials
Diamagnetic materials are weakly repelled by magnetic fields.
Examples include:
- bismuth
- copper
- silver
- gold
- water
- graphite
When an external magnetic field is applied, it induces a very small magnetic response in the material that opposes the applied field.
Therefore:
diamagnetic materials are weakly repelled from stronger magnetic fields.
Diamagnetism
Diamagnetism occurs because an applied magnetic field changes the motion of electrons within atoms.
The resulting induced magnetic effect opposes the applied field.
This is related to a general physical tendency for induced magnetic effects to oppose the change that produces them.
The effect is usually extremely weak.
However, in strong magnetic fields it can become observable.
All Materials Have Some Diamagnetic Response
Diamagnetism is a fundamental response found in matter.
However, in materials that are strongly ferromagnetic or paramagnetic, those stronger magnetic effects usually dominate.
Therefore, we normally classify a material according to the dominant magnetic behaviour that is experimentally observed.
Comparing the Three Types
| Property | Ferromagnetic | Paramagnetic | Diamagnetic |
|---|---|---|---|
| Response to field | Strong attraction | Weak attraction | Weak repulsion |
| Net response | With applied field | With applied field | Opposes applied field |
| Can retain strong magnetization? | Often possible | Generally no | No |
| Domain behaviour important? | Yes | Not in the same ferromagnetic-domain sense | Not in the same ferromagnetic-domain sense |
| Example | Iron | Aluminium | Copper |
| Relative effect | Very strong | Weak | Very weak |
A Common Misconception About Aluminium
Aluminium is often casually described as "non-magnetic."
For everyday situations, this is usually reasonable because ordinary magnets do not strongly attract aluminium.
More precisely, however:
aluminium is paramagnetic
Its attraction to a magnetic field is simply much weaker than the attraction experienced by ferromagnetic iron.
Similarly, copper is diamagnetic rather than completely unaffected by magnetic fields.
Magnetic Susceptibility
A material's response to an applied magnetic field can be described using magnetic susceptibility.
It is commonly represented by:
χ
Very broadly:
ferromagnetic → very large positive response
paramagnetic → small positive χ
diamagnetic → small negative χ
A positive susceptibility means the induced magnetization tends to align with the applied field.
A negative susceptibility means the induced response tends to oppose it.
Soft Magnetic Materials
A soft magnetic material is easy to:
- magnetize
- demagnetize
Soft magnetic materials respond readily when an external magnetic field changes.
They are useful when magnetism must be repeatedly switched or reversed.
Examples include:
- soft iron
- silicon steel
- some ferrites
Applications of Soft Magnetic Materials
Soft magnetic materials are useful in:
- electromagnets
- transformers
- electric motors
- generators
- relays
- inductors
For example, an electromagnet should become strongly magnetized when current flows but lose much of that magnetization when the current is switched off.
A soft magnetic material is ideal for this purpose.
Hard Magnetic Materials
A hard magnetic material is more difficult to magnetize, but once strongly magnetized it resists demagnetization.
Hard magnetic materials retain a large amount of their magnetization.
They are therefore useful for making:
permanent magnets
Examples include suitable:
- hard steels
- neodymium-iron-boron alloys
- samarium-cobalt alloys
- some ferrites
- alnico alloys
Applications of Hard Magnetic Materials
Hard magnetic materials are used when a persistent magnetic field is required.
Applications include:
- permanent magnets
- speakers
- headphones
- electric motors
- generators
- magnetic sensors
- magnetic fasteners
- data and position-sensing systems
Soft vs Hard Magnetic Materials
| Soft Magnetic Materials | Hard Magnetic Materials |
|---|---|
| Easy to magnetize | Generally harder to magnetize strongly |
| Easy to demagnetize | Difficult to demagnetize |
| Low magnetic coercivity | High magnetic coercivity |
| Useful with changing fields | Useful for permanent fields |
| Electromagnets | Permanent magnets |
| Transformer cores | Speakers and permanent-magnet motors |
The words soft and hard refer to magnetic behaviour, not necessarily mechanical softness or hardness.
Retentivity
Retentivity, or remanence, describes a material's ability to retain magnetization after the external magnetizing field is removed.
A permanent-magnet material should have substantial remanent magnetization.
Soft magnetic materials are generally designed to retain relatively little magnetization after the applied field is removed.
Coercivity
Coercivity describes how strongly a material resists being demagnetized.
A hard magnetic material has:
high coercivity
A soft magnetic material has:
low coercivity
Therefore:
high coercivity → good for permanent magnets
low coercivity → good for transformer and electromagnet cores
Hysteresis
When a ferromagnetic material is repeatedly magnetized in opposite directions, its magnetization does not respond instantaneously to the changing applied field.
This behaviour is called:
magnetic hysteresis
It can be represented using a hysteresis loop.
The shape of the loop tells us about important properties such as:
- remanence
- coercivity
- energy losses
Soft Materials and Hysteresis
Soft magnetic materials generally have a relatively:
narrow hysteresis loop
This means relatively little energy is lost when the material is repeatedly magnetized and demagnetized.
That is important in transformers, where the magnetic field reverses many times every second.
Reducing hysteresis losses improves efficiency.
Hard Materials and Hysteresis
Hard magnetic materials generally have a wider hysteresis loop and high coercivity.
This means they resist changes to their magnetization.
That is useful for permanent magnets because the material should maintain its magnetic state despite:
- external magnetic fields
- vibration
- ordinary environmental changes
Transformer Cores
Transformers require a magnetic field that changes continuously with alternating current.
The core should:
- magnetize easily
- demagnetize easily
- reverse its magnetization repeatedly
- lose as little energy as practical
Therefore, transformer cores use suitable:
soft magnetic materials
Electromagnets
An electromagnet usually contains a coil surrounding a soft ferromagnetic core.
When current flows:
current → magnetic field → core magnetizes → stronger magnetic field
When the current stops, the core should lose most of its magnetization.
Applications include:
- scrapyard lifting magnets
- relays
- magnetic locks
- bells
- solenoid actuators
Permanent-Magnet Motors
Permanent-magnet motors use hard magnetic materials to provide a persistent magnetic field.
The permanent magnet interacts with magnetic fields produced by electric currents.
This interaction produces forces that can generate rotation.
Strong rare-earth magnets allow powerful motors to be made relatively small.
Applications include:
- electric vehicles
- drones
- computer equipment
- power tools
- robotics
Speakers and Headphones
Speakers often contain a permanent magnet made from a hard magnetic material.
A current-carrying coil sits within the magnetic field.
The interaction between the current and magnetic field produces a force.
The coil moves a diaphragm or cone, creating sound waves.
Permanent magnets are useful because they provide the required magnetic field without continuous electrical power.
Magnetic Data Storage
Magnetic materials can also represent information.
In magnetic storage systems, small regions of material can have different magnetization states.
These states can represent digital information.
Magnetic storage has been used in technologies such as:
- hard disk drives
- magnetic tape
- magnetic stripes
The material must maintain its magnetic state reliably until deliberately changed.
Magnetic Shielding
Some magnetic materials are useful for controlling the path of magnetic fields.
Materials with suitable magnetic properties can provide a preferred path for magnetic flux.
This can reduce magnetic-field strength in a protected region.
Magnetic shielding is useful around:
- sensitive instruments
- sensors
- electronic equipment
- scientific experiments
Magnetic Sensors
Magnetic materials and magnetic-field effects are used in many sensors.
Applications include:
- vehicle wheel-speed sensing
- position detection
- door sensors
- current measurement
- compasses
- industrial automation
Magnetic sensing is useful because it can often work without physical contact between moving parts.
Heating and Ferromagnetism
Heating can affect magnetic ordering.
As temperature increases, thermal motion makes magnetic alignment more difficult.
Above a characteristic temperature called the:
Curie temperature
a ferromagnetic material loses its long-range ferromagnetic ordering.
It then behaves differently magnetically.
The Curie temperature depends on the material.
Demagnetizing a Material
A magnetized material can be demagnetized by disrupting its magnetic alignment.
Methods can include:
- strong heating
- mechanical shock
- applying an opposing magnetic field
- applying an alternating magnetic field that gradually decreases
The goal is to reduce the material's overall magnetic alignment.
Magnetizing by Stroking
A simple classroom method involves repeatedly moving one pole of a permanent magnet along a ferromagnetic object in the same direction.
The repeated applied magnetic field encourages greater domain alignment.
The object can then develop an overall north and south pole.
This works much better with suitable ferromagnetic materials than with paramagnetic or diamagnetic materials.
Magnetizing by Electric Current
A stronger and more controllable method is to place a ferromagnetic object inside a solenoid.
When current flows:
electric current → magnetic field → domain alignment
Increasing the current can increase the applied magnetic field.
The resulting magnetization depends on:
- material
- applied field strength
- previous magnetic history
- temperature
Choosing a Magnetic Material
Engineers select magnetic materials according to the job they need to perform.
For an electromagnet:
soft magnetic material
is generally preferred.
For a permanent magnet:
hard magnetic material
is preferred.
For a transformer:
soft material with low hysteresis loss
is desirable.
For a compact powerful permanent-magnet motor:
strong hard magnetic material
may be desirable.
Material selection is therefore an important part of engineering design.
Common Mistakes
Mistake: Calling every material attracted to a magnet ferromagnetic.
Paramagnetic materials are also attracted, but much more weakly.
Mistake: Saying diamagnetic means completely non-magnetic.
Diamagnetic materials respond weakly by opposing an applied magnetic field.
Mistake: Saying an unmagnetized ferromagnetic material has no magnetic domains.
It has domains, but their overall magnetic effects largely cancel.
Mistake: Saying magnetization creates domains.
Magnetization mainly changes domain structure and alignment.
Mistake: Assuming aluminium is ferromagnetic because it is a metal.
Aluminium is paramagnetic.
Mistake: Assuming copper has no magnetic response.
Copper is diamagnetic.
Mistake: Thinking "soft magnetic" means physically soft.
It refers to how easily the material can be magnetized and demagnetized.
Mistake: Thinking "hard magnetic" simply means mechanically hard.
It refers primarily to resistance to changes in magnetization.
Mistake: Using hard magnetic material for every electromagnet.
Electromagnets generally require materials that can magnetize and demagnetize readily.
Key Terms
Magnetic material — material that responds to an applied magnetic field.
Ferromagnetic — strongly magnetic behaviour involving cooperative alignment of magnetic moments.
Paramagnetic — weak attraction to a magnetic field caused by partial alignment of atomic magnetic moments.
Diamagnetic — weak magnetic response that opposes an applied magnetic field.
Magnetic domain — region of a ferromagnetic material containing many similarly aligned magnetic moments.
Magnetization — overall magnetic moment per unit volume, or more generally the process/state of producing magnetic alignment in a material.
Magnetic induction — magnetization caused by exposure to an external magnetic field.
Magnetic saturation — condition in which increasing the applied field produces relatively little further magnetization.
Soft magnetic material — material that can be magnetized and demagnetized relatively easily.
Hard magnetic material — material that strongly resists demagnetization and is suitable for permanent magnets.
Remanence — magnetization remaining after an external magnetizing field is removed.
Coercivity — measure of a material's resistance to demagnetization.
Hysteresis — lagging behaviour of magnetization as an applied magnetic field changes.
Hysteresis loop — graph showing magnetic response during a complete cycle of magnetization.
Curie temperature — temperature above which long-range ferromagnetic ordering disappears.
Key Takeaways
- Materials respond differently to magnetic fields.
- Ferromagnetic materials are strongly attracted to magnetic fields.
- Iron, nickel and cobalt are important ferromagnetic materials.
- Paramagnetic materials are weakly attracted to magnetic fields.
- Aluminium is an example of a paramagnetic material.
- Diamagnetic materials are weakly repelled by magnetic fields.
- Copper and bismuth are examples of diamagnetic materials.
- Ferromagnetic materials contain magnetic domains.
- Domains contain many similarly aligned atomic magnetic moments.
- In an unmagnetized ferromagnetic material, domain contributions largely cancel.
- Magnetization changes the domain structure so the material develops a net magnetic effect.
- Strong fields can eventually drive a material toward magnetic saturation.
- Soft magnetic materials magnetize and demagnetize easily.
- Soft magnetic materials are useful in transformers and electromagnets.
- Hard magnetic materials resist demagnetization.
- Hard magnetic materials are useful for permanent magnets.
- High coercivity is important for permanent-magnet materials.
- Low coercivity is useful when magnetization must change repeatedly.
- Hysteresis can cause energy losses in magnetic materials.
- Magnetic materials are essential in motors, generators, transformers, speakers, sensors, data storage and many other technologies.
Check Your Understanding
- Name the three major categories of magnetic materials discussed in these notes.
- Define a ferromagnetic material.
- Give three examples of ferromagnetic materials.
- Define a paramagnetic material.
- Give one example of a paramagnetic material.
- Define a diamagnetic material.
- Give two examples of diamagnetic materials.
- Explain the difference between the response of paramagnetic and diamagnetic materials to an external magnetic field.
- What is a magnetic domain?
- Describe the domain arrangement in an unmagnetized ferromagnetic material.
- Explain what happens to the domain structure when a ferromagnetic material becomes magnetized.
- Describe one method of magnetizing a ferromagnetic material.
- What is magnetic saturation?
- Explain the difference between soft and hard magnetic materials.
- Why is soft iron useful in an electromagnet?
- Why are hard magnetic materials suitable for permanent magnets?
- Define coercivity.
- Explain why a transformer requires a soft magnetic material with relatively low hysteresis loss.
- Give three technological applications of magnetic materials and explain why the chosen magnetic properties are useful in each.
- Explain why the statement "copper and aluminium are completely non-magnetic" is scientifically inaccurate.