Holmium (Ho)

Holmium holds a record that sounds modest and is not: no element has a larger magnetic moment per atom. A holmium ion carries about 10.6 Bohr magnetons, against roughly 2.2 for an atom of iron. Iron makes better magnets because its atoms cooperate at room temperature and holmium's do not — but atom for atom, holmium is the most magnetic thing there is.

That single quantity turns up behind most of what the element is used for, sometimes in places where magnetism is not obviously the point.

Pole pieces and helices

The immediate use of an enormous atomic moment is flux concentration. Insert holmium pole pieces into the bore of a superconducting magnet and the field at the sample rises, because the holmium's own magnetisation adds to the applied field. Research magnets chasing the highest attainable static fields have used holmium for exactly this — a gain of a tesla or two that nothing else can supply as compactly.

Holmium's own magnetic ordering is peculiar and historically important. Below about 19 K it is a straightforward ferromagnet. Between 19 K and 132 K the moments arrange themselves into a helix — each atomic layer's magnetisation rotated by a fixed angle from the layer below, spiralling up the crystal. Helical magnetic order of this kind was one of the first exotic structures resolved by neutron diffraction, and holmium remains the textbook specimen.

Soret's "element X" and Cleve's holmia

Two groups arrived at holmium in the same year by different routes, and the credit has never been cleanly assigned.

In 1878 in Geneva, Jacques-Louis Soret and Marc Delafontaine examined erbium fractions spectroscopically and found absorption bands belonging to nothing known. Not knowing what it was, they called the responsible substance element X. That is the spectroscopic discovery, and it is the one the data card above credits.

Independently, Per Teodor Cleve in Uppsala was breaking erbia down chemically, and in 1878-79 he obtained two new substances from it. One was brown, one was green. He named the brown one holmia after Holmia, the Latin name for Stockholm, and the green one thulia. Cleve did the chemistry, produced material, and supplied the name that stuck.

Neither group had a pure element. Boisbaudran demonstrated in 1886 that Cleve's holmia was itself a mixture, and pulled dysprosium out of it. The pattern was standard for the period: every rare earth discovered turned out on closer inspection to contain at least one more.

Two point one microns

Holmium's most widely encountered application is a laser wavelength. A holmium-doped yttrium aluminum garnet crystal emits at about 2100 nanometres, in the mid-infrared, and that number matters because it sits on a strong absorption band of water.

Living tissue is mostly water, so holmium laser light is absorbed within a few tenths of a millimetre of where it lands. Energy does not travel; it deposits. The result is a beam that cuts and vaporises precisely, seals small vessels as it goes, and does not damage structures just behind the target. It also travels down ordinary silica optical fibre, which means it can be delivered through a flexible endoscope.

Two procedures dominate:

• Stone fragmentation. A holmium laser fibre passed up the ureter breaks kidney stones of any composition — including the cystine and calcium oxalate monohydrate stones that resist shockwave treatment — and has been the standard endoscopic method since the 1990s.
• Prostate enucleation. Holmium laser enucleation of the prostate removes obstructing tissue for benign enlargement with much less bleeding than electrosurgical resection, and unlike older techniques its results do not deteriorate with gland size.

Holmium's near-monopoly here is now under pressure from thulium fibre lasers, which operate slightly shorter and offer finer control, and the two are being compared head-to-head in urology trials.

A microsphere that can be seen while it works

Radioembolisation treats liver tumours by lodging radioactive microspheres in the vessels feeding them. The standard material is yttrium-90, which works well and has one blind spot: it emits almost nothing that can be imaged, so where the dose actually went has to be inferred.

Holmium-166 microspheres, approved for use in Europe, solve that by exploiting the element's magnetism. Holmium-166 emits therapeutic beta particles with a 27-hour half-life — and because holmium is so strongly paramagnetic, the spheres themselves are directly visible on MRI. The same particles that deliver the treatment show up as a quantifiable signal on a scan, so the delivered dose distribution can be mapped in the patient rather than modelled. A small scout dose can even be given first to check where the spheres will go before the full treatment is committed.

A much longer-lived form, holmium-166m1 with a half-life of around 1,200 years, serves a quieter purpose: it emits gamma rays at many well-separated energies and is used as a reference source for calibrating gamma spectrometers.

The filter that tells a spectrometer it is lying

Every ultraviolet-visible spectrophotometer needs its wavelength scale checked, and holmium is what checks it.

Holmium oxide in glass, and holmium perchlorate in solution, produce a series of very narrow absorption bands at precisely known wavelengths spread across the ultraviolet and visible range. The bands are narrow because they are 4f transitions, shielded from the surrounding material, and they are consequently almost unaffected by temperature or by the host medium. That combination makes holmium an almost ideal transfer standard, and national metrology institutes issue certified holmium filters and solutions for the purpose. There is a holmium reference somewhere in most analytical laboratories in the world.

The same sharp bands give holmium compounds a striking optical trick. Holmium-doped glass and cubic zirconia look yellow under daylight and pink under fluorescent light, because the narrow absorption lines interact very differently with a continuous spectrum and a line spectrum.

One atom, one bit

In 2017 a team at IBM's Almaden laboratory placed single holmium atoms on a magnesium oxide surface, cooled the assembly to a little above one kelvin, and used a scanning tunnelling microscope to write and read the magnetic orientation of individual atoms — which held their state for hours.

Holmium was chosen because of its large moment and because the symmetry of its electronic states on that particular surface protects them against the relaxation processes that flip smaller magnets almost instantly. The demonstration built a two-bit memory from two atoms. It is not a storage technology and was never presented as one; what it establishes is the physical floor — that the smallest possible magnetic bit is one atom, and that the atom for the job is holmium.