Helsham’s Lodestone and Magnetism in Magnetite

Lodestone: A lodestone is a rock that is rich in the mineral magnetite and has become magnetised naturally.  Lodestones have been known to have the property that they attract iron since antiquity. The lodestone in Trinity’s School of Physics instrument collection was donated to Richard Helsham by the Lord Chancellor, Thomas Wyndham, Baron of Finglas.  In its pure form, magnetite exists as shiny black crystals and its chemical formula is Fe₃O₄.  It is unusual in that it retains its magnetised state well above room temperature.  The gritty, black crystals which accumulate in a magnetic trap in domestic heating systems are largely composed of magnetite. 

The word ‘lode’ comes from old English and means way or journey. In the ancient world, compasses made with lodestones were used for navigation. This was done by marking the lodestone with the North direction, the lodestone was then placed on top of a piece of cork in a water basin. Under these conditions, the lodestone rotates to face magnetic North. Humans have used Lodestones for centuries, dating back even as far as the 6^th century BC. The Lodestone was humanity’s first introduction to the world of magnetism. The earliest reference from Europe is in the 6^th century BC by Greek philosopher Thales of Miletus, who is often credited with discovering lodestone’s attraction to iron. Thales’s explanation for the magnetic abilities of the lodestone was the belief that they possessed souls. In the Lushi Chunqi, a classical Chinese text from the 2^nd century BC, the attraction of iron to a lodestone is stated. In the Lunheng, a Chinese text published in 80 AD, the first mention of a needle’s attraction to a lodestone is mentioned.

The magnetism of lodestones was investigated by William Gilbert in the 16^th century. He devised the rule that like poles repel, unlike poles attract and defined the term ‘pole’. Gilbert noted that an iron bar brought into contact with a lodestone could be magnetised. Practically shaped, less expensive magnets were produced this way. Stacking these new magnets increased their magnetic intensity and rendered lodestones obsolete. Gilbert observed that a magnet exerts its force of attraction on iron without surface contact, as well as through thin sheets of other metals. This is expressed later in Faraday’s idea of a ‘field of force’ around a magnet, the magnetic field. Lodestones are mentioned in Helsham’s Lectures on Natural Philosophy, Lecture III on the topic of Repulsion and Central Forces, where he says, ‘For if the disagreeing pole of a lodestone be moved towards a magnetical needle floating on water, the needle will recede; and the nearer the stone is brought to it, with the greater violence and precipitation will it fly off; the repelling power, like the attractive, exerting itself with greater vigor at smaller distances’.

The inscription on Helsham’s Lodestone reads: ‘The Gift of his Excel (Excellency) Thomas Lord Wyndham, Baron of Finlas (Finglas) Lord Chancelour (Chancellor) and one of the Lord Justices of Ireland to Trinity College near Dublin’

Magnetite: Magnetism in magnetite retains its fascination for physicists to this day.  The atomic structure, ferrimagnetism and electric conductivity of magnetite are complex.  In 1939, the Dutch chemist, Evert Verwey, discovered an abrupt change in the atomic structure and electric conductivity of magnetite at 120 degrees Kelvin (-150 C). The microscopic origin of magnetism is the ‘spin’ on magnetic ions in the material. In the case of magnetite, these are several kinds of iron ion.  Essentially, each magnetic ion behaves as a microscopic bar magnet with its poles oriented in some direction. A magnetised material has a majority of its spins oriented in a specific direction.  Magnetisation of the various ion types in magnetic oppose each other. However, they have different magnitudes resulting a net magnetisation. This is known as ferrimagnetism.

The fundamental unit which is repeated in a magnetite crystal below 120 Kelvin contains 120 atoms and its electrical resistance increases 100 fold below this temperature.  The large number of atoms and tendency to form microscopic domains, which are microscopic crystalline regions with different spatial orientations, made it difficult to determine the atomic structure. In 2012, Paul Attfield at the University of Edinburgh used a micron sized crystal containing just one of these domains to determine the crystal structure for the first time. It reveals a structure named ‘trimerons’ by Attfield in which zig-zag chains of three iron ions interact strongly.  In 2000, magnetic resonance spectroscopy was used by the Japanese physicist, Moriji Mizoguchi, to observe spins of individual magnetic ions in magnetite and in 2014, following the work of Attfield, Charles Patterson in the Trinity School of Physics performed calculations which analysed the magnetic resonance data of Mizoguchi to reveal the connection between the crystal structure and electron and spin distribution in magnetite.

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