Temperature: from doublet to sextet

Magnetic ordering

Every magnetic material has an ordering temperature, called the Curie temperature for ferro- and ferrimagnets and the Néel temperature for antiferromagnets. Above it, the magnetic moments of the iron atoms point in random, constantly changing directions, so the nucleus feels no steady field and you see a doublet. Below it the moments line up, the nucleus feels a hyperfine field and you see a sextet. The field grows as the sample is cooled further.

Ordering temperatures vary widely. Hematite and magnetite order far above room temperature, goethite at around 400 K, and lepidocrocite only below about 77 K, which is why lepidocrocite is a doublet at room temperature.

Superparamagnetism in small particles

In very small particles, a few nanometres across, thermal energy can flip the magnetisation of a whole particle from one direction to another. A Mössbauer measurement takes about 10⁻⁸ seconds to “see” a field. If the flipping is faster than that, the field averages out to zero and the particle gives a doublet, even though it is magnetically ordered. This is called superparamagnetism.

Cooling slows the flipping down. Below the blocking temperature the magnetisation stays put for long enough, and the sextet appears. Larger particles have higher blocking temperatures. Real samples contain a range of particle sizes, so over a range of temperatures you see a sextet and a doublet together.

Try lowering the temperature below. The spectrum keeps the same scale throughout, so you can watch the doublet shrink as the sextet grows. The plot underneath shows how the share of iron in the sextet and the doublet changes with temperature.

About this model

The plot is an illustration, not a particular mineral. It assumes the particles have a range of sizes, so they block at different temperatures, and that the field drops a little as each particle nears its blocking temperature. Real spectra in this range often show broad, uneven lines from magnetic relaxation, which this simple model leaves out.

Why measure at 77 K and 4.2 K?

Many Fe(III) minerals give almost the same doublet at room temperature. Cooling with liquid nitrogen (77 K) or liquid helium (4.2 K) makes them order or block one after another, so they can be told apart. A 2-line ferrihydrite, for example, is still a doublet at 77 K but a sextet at 4.2 K. The guide on iron oxides shows this mineral by mineral.

Magnetite is best measured at about 140 K

The ratio of the octahedral (B) to tetrahedral (A) site areas in magnetite shows how far it has oxidised towards maghemite. At room temperature the two sites absorb with different efficiency (their recoil-free fractions differ), which skews the ratio. Cooling brings them closer together, but below the Verwey transition (about 120 K) the spectrum becomes too complex to fit with two sites. Gorski and Scherer (2010) therefore recommend measuring at about 140 K, cold enough to improve the recoil-free fractions but still above the Verwey transition.

Other effects of temperature

Isomer shift. The second-order Doppler shift makes δ rise on cooling, by roughly 0.1 mm/s between room temperature and 4.2 K. Compare values measured at the same temperature.
Absorption strength. The recoil-free fraction increases on cooling, so lines get deeper and relative areas become more reliable.
Phase transitions. Some minerals change their magnetic structure on cooling. Hematite passes through the Morin transition at about 260 K, where its quadrupole shift changes sign, and magnetite passes through the Verwey transition at about 120 K.