magnetic thermometer

Magnetic thermometer

A thermometer whose operation is based on Curie's law, which states that the magnetic susceptibility of noninteracting (that is, paramagnetic) dipole moments is inversely proportional to absolute temperature. Magnetic thermometers are typically used at temperatures below 1 K (-458°F). The magnetic moments in the thermometric material may be of either electronic or nuclear origin. Generally the magnetic thermometer must be calibrated at one or more reference temperatures. See Electron, Nuclear moments, Paramagnetism

At temperatures from a few millikelvins upward, the thermometric material is preferably an electronic paramagnet, typically a nonconducting hydrous rare-earth salt. For higher temperatures, an ion is selected with a large magnetic moment in a crystalline environment with a high density of magnetic ions. In contrast, for low temperature use the magnetic exchange interactions between the magnetic ions should be small, which is accomplished by selecting an ion with a well-localized moment and by maintaining a large separation between the magnetic ions by means of diamagnetic atoms. This is the case in cerium magnesium nitrate (CMN) [2Ce(NO3)3 · 3Mg(NO3)3 · 24H2O]. Here, the Ce3+ ion is responsible for the magnetic moment, which is well localized within the incompletely filled 4f shell relatively deep below the outer valence electrons. To reduce the magnetic interactions between the Ce3+ ions further, Ce3+ may be partly substituted with diamagnetic La3+ ions. Lanthanum-diluted CMN has been used for thermometry to below 1 mK. See Exchange interaction

A mutual-inductance bridge, originally known as the Hartshorn bridge, has been the most widely employed measuring circuit for precision thermometry. The bridge is driven by a low-frequency alternating-current source. The inductance at low temperatures consists of two coils, which are as identical as possible. The voltages induced across them by the drive current are compared by means of a high-input-impedance ratio transformer. The output level of this voltage divider is adjusted to equal that of the midpoint between the two coils, using as null indicator a narrow-band preamplifier and a phase-sensitive (lock-in) detector. Thus, without a paramagnetic specimen, the bridge is balanced with the decade divider adjusted at its midpoint, while with the specimen inside one of the coils the change in the divider reading at bridge balance is proportional to the sample magnetization. For high-resolution thermometry it has become standard practice to replace the room-temperature zero detector with a SQUID magnetometer circuit. This also allows the mass of the sample to be reduced from several grams to the 1-mg level. See Inductance measurement, SQUID

Nuclear magnetic moments are smaller by a factor of 103 and are used for thermometry only in the ultralow-temperature region. For this the Curie-law behavior is generally sufficient down to the lowest temperatures. The nuclear paramagnetic thermometer loses adequate sensitivity for calibration purposes above 50–100 millikelvins, unless it is operated in a high polarizing field (H greater than 0.1 tesla). It can be utilized as a self-calibrating primary thermometer if the spin-lattice relaxation time is measured in parallel with the nuclear Curie susceptibility. Pulsed NMR measurement on the 195Pt isotope in natural platinum metal provides presently the most widely used thermometry at temperatures below 1 mK. In the Curie-susceptibility measuring mode, it has been extended down to 10 μK. See Low-temperature thermometry, Magnetic relaxation

magnetic thermometer

[mag′ned·ik thər′mäm·əd·ər]
(solid-state physics)
A sample of a paramagnetic salt whose magnetic susceptibility is measured and whose temperature is then calculated from the inverse relationship between the two quantities; useful at temperatures below about 1 K.
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