Public lecture - How magnets work

Q&A Session

Q: Does the shape of the ring array affect the emergent behaviour?

A: Yes, very much. We're learning more and more about this and it is an active area of our research. This could open up ways of tuning the array size in order to control the emergent behaviour, to adapt to different problems being tackled. 

Q: How do we read and write data onto solid state drives?

A: Solid-state drives (SSDs) use a very different technology to the magnetic hard disk drives (HDDs) I described. SSDs mostly use semiconductors based on a type of transistor (a 'MOSFET'). These are known as 'Flash' type memories and work by storing electrical charge. Data writing and readout is, therefore, electrical. These devices are quicker than HDDs but are more expensive and drives have lower capacity. Most laptops now have SSDs instead of HDDs. A desktop computer might have both to combine speed (SSD) and large memory (HDD). 

Q: What was the sample used in the optical microscope demonstration

This was a 'garnet' type sample obtained from this site: https://www.telatomic.com/all-produts/magnetic-bubble-apparatus. It is wonderful for demonstrations and teaching students about the behaviour of magnetic materials. 

Q: Can magnetic field be manipulated to such accuracy as to arrange metal powder particles into a specific shape?

The control of magnetic particles is of huge interest to researchers developing biomedical applications. The magnetic particles tend to be at the nanoscale, typically 20-100 nm (nanometres) in diameter, although some particles in the micrometer scale are also studied. The particles can be used in various ways, e.g. as carriers to deliver drugs to a specific location in the body, or heated or physically rotated to kill problem cells in the body. They can be guided using field coils from magnetic resonance imaging (MRI) instruments or simply captured by permanent magnets placed close to the body. Otherwise, antibodies attached to the magnetic particles can help to locate them to particular regions of the body, particularly if the antibodies target a specific problem, such as a cancer cell.

I should mention that I have a project just about to start that aims to use various methods to move particles into shapes and then using another stimulus to make the particles grab each other to hold the shape when all stimuli are removed. The idea is to be able to have lots of small particles that can be arranged into various useful shapes but turned back into the individual particles at the flick of a switch, and ready to make a new object. This could be the ultimate in reconfigurable materials! The project is called 'What happens when you cross LEGO and a Star Trek Replicator?' and you can see the official summary at https://gow.epsrc.ukri.org/NGBOViewGrant.aspx?GrantRef=EP/T028661/1

Q: Is increase in storage capacity in HDD is on reduced possible grain structure or obtained with use of complex advance doped material use.

A: Yes it can but we're coming to the limit of what can be achieved using this approach. The heat-assisted magnetic recording (HAMR) technology has extended the usefulness of this approach and will allow the technology roadmaps to be followed for a few more years. Beyond this, a new technology is needed, and Seagate (and others) are proposing a change in approach to large single 'grains' (in which atoms are perfectly ordered) that each hold a single bit of information. These large grains will still be smaller than the total bit size that is used currently, but their greater size than individual grains used today means they will have an advantage in greater stability.  

Q: Is the polarisation of a magnetic field similar to that of vector addition where the polarisation is the average direction of all the moments?

A: Yes, that's right. Magnetic fields are vector properties and so multiple magnetic fields at the same point in space add vectorially. It is, therefore, possible to cancel out one magnetic field with another. 

Q: How big are the rings in your system?

A: The rings are 1 - 4 micrometers (1 - 4 mm) in diameter (1 mm = 0.001 mm). We have made arrays that are 2 cm x 2 cm in total, so made up of over 1 million rings. The useful arrays can be as small as 25 x 25 rings though, so this would be only 100 mm x 100 mm (0.1 mm x 0.1 mm) for the largest rings. 

Q: Why does hitting magnets together cause them to lose their magnetisation?

A: Hitting the magnets together (or striking with another object, such as a hammer) creates mechanical shock waves that pass through the materials. This can change the position of atoms in the material and change the direction of atoms' magnetic moments. Once the shock wave has passed, it can leave some magnetic domains (remember these? They are the regions in which atomic magnetic moments are all aligne) pointing in different directions, which reduces the overall magnetisation of the object. In extreme cases, the new domains are in completely random directions and the magnetisation goes to zero. 

Q: Is there any glass material that is magnetic

A: There are a few answers to this:

  • Metals can have a 'glass-like' structure, with no ordering to the position of atoms. These are known as 'metallic glasses' and have 'amorphous' atomic ordering. These have some interesting mechanical properties (e.g. metallic glass golf clubs make balls go much further!). There are many magnetic materials that have this structure.
  • Glasses around us are often made of oxide materials. There are many magnetic materials that are oxides too. These include particles used in the seals on your refrigerator to the magnetic domain viewer used in the webinar.
  • 'Magnetic glasses' (usually called 'spin glasses') are also an area of study. These generally have weakly-interacting magnetic elements that have magnetic moments that are only partially ordered or not ordered at all (this is in contrast to the 'ferromagnetic order' I discussed in the webinar, where atomic magnetic moments lined up with those of their neighbours). Spin glasses are mostly used to explore fundamental physical principles but need to be taken to cryogenic (very cold!) temperatures, typically 1 - 77 Kelvin!

Key dates:

Registration deadline:

10 July 2020