= Barnett rotation magnetometry experiment
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The 2015 experiment used an air-driven nonmagnetic rotor containing polycrystalline <gadolinium>, with <magnetic shielding> and an external <fluxgate magnetometer>. Air drive reduced motor backgrounds. Measurements at 297–305 K were above the <Curie temperature>, at rotation <frequencies> up to 1.5 kHz. Reversing airflow reversed rotation and the magnetic signal; an empty capsule showed no speed-dependent signal. The readout measured stray <magnetic field>; geometry converted it to <magnetization>. Sample <temperature> was checked immediately after stopping. Heating and the conversion model limit inference.<Barnett effect in paramagnetic states by Masao Ono and others>[§§II–III, Figs. 1–2, note 22]{ref}
\Image[/barnett-rotation-magnetometry-apparatus.png]
{title=<Barnett rotation magnetometry experiment>: controlled rotation and stray-field readout}
{description=Original functional layout, with simplified geometry. The arrows independently explain the sign expected for $\gamma<0$: positive axial <angular velocity>, negative axial <magnetization>, and positive axial stray <magnetic field> at an equatorial sensor. The air line indicates drive function. Dimensions and components follow the inspected experiment; this is neither its original figure nor measured data.}
An additional analysis check follows from rotation <symmetry>. For signed sensor output $S$, separate
$$
S_{\rm odd}(\Omega)=\frac{S(+\Omega)-S(-\Omega)}2,\qquad
S_{\rm even}(\Omega)=\frac{S(+\Omega)+S(-\Omega)}2.
$$
An ideal linear <Barnett effect> appears in the odd component; a fixed offset appears in the even component. This is a proposed diagnostic, not a claim about the paper's processing. A background correlated with direction can also be odd: reversal alone is insufficient. Empty-rotor checks, zero-speed readings, calibration of sensor polarity, and controlled <temperature> changes test different errors. Equatorial stray-field sign must also be converted to <magnetization> sign: for an axial <magnetic dipole moment>, the <magnetic dipole field> at its equator points opposite to that <magnetic moment>. The apparatus diagram makes this otherwise easy-to-miss sign change visible.
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