= Manganese monoxide neutron diffraction experiment
In 1949, <Clifford Shull> and <J. Samuel Smart> compared powdered <manganese(II) oxide> at room <temperature> and $80\,\mathrm K$. Powder preparation supplies many <crystal> orientations. Their Fig. 1 records scattered-neutron <intensity (physics)> versus counter angle. Nuclear reflections remained, while the cold pattern gained strong reflections forbidden by the chemical <unit cell>. They indexed these with a magnetic <unit cell> having twice the chemical cell edge. The warm pattern already contained diffuse magnetic scattering: loss of sharp magnetic peaks does not mean loss of individual <magnetic moments>.<Detection of Antiferromagnetism by Neutron Diffraction by Clifford Glenwood Shull and J. Samuel Smart>[Shull and Smart, Detection of Antiferromagnetism by Neutron Diffraction (1949), Fig. 1 and accompanying paragraphs; original reprint inspected at Carnegie Mellon University]{ref}
The source was Oak Ridge's graphite-moderated reactor. In Early Development of Neutron Scattering (1994), <Clifford Shull> describes how the early two-axis instruments used a monochromating <crystal> to select the incident-neutron <wavelength>; a shielded neutron counter rotated around the specimen to record angular <intensity (physics)>. This specifies the source–<monochromator>–sample–counter path, without identifying which instrument revision produced the MnO trace.<Early Development of Neutron Scattering by Clifford Glenwood Shull>[Early Development of Neutron Scattering (1994), pp. 99–103, Figs. 2–4, and p. 106, Fig. 8; inspected CMU reprint pp. 99–106]{ref}
The control was the same compound above and below magnetic ordering, with retained nuclear peaks identifying the underlying chemical periodicity. The original paper labels an aluminium impurity peak and defers a complete magnetic-structure account. It supplies neither a chemical synthesis recipe nor detailed cold-stage construction. The inference is periodic magnetic order, rather than a claim that every possible structural change was excluded at arbitrary resolution.<Detection of Antiferromagnetism by Neutron Diffraction by Clifford Glenwood Shull and J. Samuel Smart>[Fig. 1: aluminium impurity label, nuclear/magnetic indexing and room-temperature/80 K comparison; final paragraph before conclusion]{ref}
The connection to the measured angle is the <elastic Bragg scattering condition>. If the incident and outgoing <wave vectors> have equal magnitude $2\pi/\lambda$ and make an angle $2\theta$, their difference has magnitude
$$
|\mathbf Q|=\frac{4\pi}{\lambda}\sin\theta.
$$
A longer real-space period supplies additional, smaller <reciprocal lattice> spacings, which can put magnetic reflections at new counter angles. In the one-dimensional toy model above, doubling the period changes the allowed spacing from $2\pi/a$ to $\pi/a$. The actual MnO indexing is three-dimensional; the toy row does not assert its reflection positions or relative peak strengths.
<Louis Néel> supplied the ordering theory; the 1949 diffraction work was a later experimental test. His 1970 <Nobel Prize in Physics> recognized <antiferromagnetism> and <ferrimagnetism>. <Clifford Shull>'s neutron-diffraction award followed in 1994.<The Nobel Prizes in Physics 1901–2000 by NobelPrize.org>[sections on neutron diffraction and magnetic ordering: Néel 1970 and Shull 1994]{ref}
Back to article page
