| RefNo | EC/1983/41 |
| Previous numbers | Cert XXI, 129 |
| Level | Item |
| Title | Abragam, Anatole: certificate of election to the Royal Society |
| Date | 1982 |
| Description | Certificate of Candidate for Election to Foreign Membership. Citation typed |
| Citation | In 1958 Abragam proposed the method of polarizing nuclear spins dynamically using the "solid effect", and demonstrated its success experimentally. A nuclear paramagnet at about 1 K, containing a small quantity of an electronic paramagnetic impurity, is irradiated in a high magnetic field by a microwave frequency corresponding to the electronic precession frequency plus or minus the nuclear precession frequency. The nuclear spin system acquires a polarization close to that of the electronic impurity, over 95% of the saturation value. This method of "dynamic nuclear polarization" has become the standard process for producing polarized targets, and is used in nuclear physics laboratories around the world.
In later experiments with polarized nuclear spin systems, Abragam has achieved a number of outstanding feats. Using the technique of "adiabatic demagnetisation in the rotating frame", in which the nuclear spins are brought into resonance with a radio-frequency field whose strength is then reduced to zero, he has attained an effective temperature of the nuclear spin system of less than 10 -6 K. The system then enters a magnetically ordered state under the influence of the dipole-dipole interactions between the nuclear magnets. The nature of the ordered state depends on the initial conditions, such as the orientation of the steady magnetic field with respect to the crystal axes. For 19F nuclear spins in CaF2, states of antiferromagnetic order and ferromagnetic order (with domains) have been predicted and confirmed, using nuclear magnetic resonance of the weakly abundant (0.13%) isotope 43Ca as a probe. Furthermore, the nuclear moments may be polarized parallel to the applied field, or antiparallel. Abragam has shown that in the latter case, which corresponds to a thermodynamic state with a "negative temperature", different states of nuclear order can occur.
In a later tour de force (1978), Abragam has carried out neutron scattering experiments from a nuclear spin system at a temperature of about 10 -7 K. These demonstrated directly the presence of an ordered antiferromagenetic state in the nuclear spins in lithium hydride through Bragg scattering of neutrons from the magnetic superlattice. The mechanism which makes this possible is the large spin dependent nuclear reactions between the neutrons and the nuclear spins in the crystal, which are comparable in size with the magnetic interactions between neutrons and electron magnetic moments.
Using the formal analogy with magnetic coupling, Abragam showed how the nuclear coupling to the nuclear spins could be ascribed to a "pseudo-magnetic nuclear moment". This novel and important concept led to his experiments in which the shift in the precession frequency of a neutron beam in traversing a system of polarized nuclei has been used to determine the magnitude of the "pseudo-magnetic nuclear moments" for a wide range of nuclei.
"Dynamic nuclear polarization", "nuclear antiferromagnetism" and "nuclear pseudo-magnetism" form new and significant fields of physics which have flowed from the insight and genius of Professor Abragam. His three books "Nuclear Magnetism" (1961), "Electron Paramagnetic Resonance" (1970, with B. Bleaney), and "Nuclear Magentism: Order and Disorder" (1982, with M. Goldman) are the authoritative texts, all written in English and published by O.U.P. |
| AccessStatus | Closed |
Fellows associated with this archive
| Code | PersonName | Dates |
| NA3892 | Abragam; Anatole (1914 - 2011) | 1914 - 2011 |