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Simon Foner and the Vibrating Sample Magnetometer

August 23, 2013

Not all scientific information is transmitted via journal articles. Even today, when the latest discoveries can be transmitted instantaneously over the Internet , scientists feel a need for personal contact, so they attend topical conferences . It's a great way, especially for younger scientists , to meet people, and it's an excellent way to build collaborative research teams which often transcend national boundaries .

Early in my career I worked with magnetic and superconducting materials , and one topical conference in these areas was the Annual Conference on Magnetism and Magnetic Materials . As for other conferences, the most important room isn't any of the parallel session rooms, but the coffee room. Not only does the coffee keep you awake during some of the less notable presentations, but it gives you a valid excuse to sit and talk with colleagues . The coffee room is so popular, that most poster session s are held in the coffee room.

During one trip to the coffee room, I noticed an acquaintance seated with an older scientist whom I didn't recognize, so I sat to talk with them. The older scientist was Simon Foner, who was well known among magneticians (yes, that's the word we use) for inventing the vibrating sample magnetometer .[1-3]

Si, as he was called, was a Pittsburgh native. I had a fellowship at the University of Pittsburgh at that time, so we had a few talking points. Si, who had a very direct manner, then proceeded to dissect a presentation made one of my colleagues at the conference. I felt some loyalty to my colleague, and I thought that the criticism was a little too harsh. As I later found, Si was just being Si. Later, the colleague admitted that the criticism was warranted, since his model had so many parameters it could probably fit any experimental data .

Sketch of Simon Foner

Simon Foner (August 13, 1925 - October 2, 2007)

As reported in his Physics Today obituary by Robert P. Guertin and Lawrence G. Rubin, Foner was "...a tireless tinkerer with experimental apparatus, a restless gadfly, and an uncontrollable and incorrigible punster..."[4]

(Sketch derived from Internet photographs.)

Simon Foner was born on August 13, 1925, in Pittsburgh, Pennsylvania . Towards the end of World War II , Foner served in the US Navy as a sonar technician in the Pacific .[4] His hands-on electronics experience was to serve him well in later years. After the war, Foner attended what's now Carnegie-Mellon University for his undergraduate education through his 1952 Ph.D. under Emerson M. Pugh.[4,6]

After his Ph.D., Foner became a staff physicist at MIT Lincoln Laboratory , where he invented and patented the vibrating sample magnetometer (VSM).[3] One way to infer the magnetic moment of a material is by Faraday's law of induction , the simplest demonstration of which is dropping a bar magnet through a coil of wire and observing the generated voltage . Faraday's original experiment is shown below.

Faraday induction experiment

A drawing of Faraday's original induction experiment of 1831. In this case, the magnet is an electromagnet , which causes a current flow trough the larger coil when it's moved. Movement is essential. A stationary magnet causes no current flow.

(Via Wikimedia Commons.)

Generally, you aren't interested in taking data on just permanent magnets, but on the magnetic response (magnetic induction , commonly symbolized as B) of materials in response to an applied magnetic field (H). The electric field generated in a coil through induction by small samples is very small, and it would likely be masked by those generated by a ramping applied field, or by magnetic noise .

When Foner started his magnetics research, most laboratory magnets were of the pole-type in which the magnetic field was localized in a small gap between pole pieces , and the magnetic field was along the axis of these pole pieces. It's hard to generate motion along the pole axis. The vibrating sample magnetometer solved this problem, and also the problem of nulling the voltage of a changing applied field. In a VSM, the sample motion is perpendicular to the applied field, and there are pickup coils whose axes are likewise perpendicular to the applied field (see figure). Sensitivity was enhanced through the use of a phase-sensitive detector , commonly called a "lock-in amplifier."

Schematic diagram of a vibrating sample magnetometer.

A simplified diagram of a vibrating sample magnetometer

In this case, four coils with axes perpendicular to the magnetic field lines are used to attain greater sensitivity and reduced sample positioning errors. Foner gives many possible coil arrangements in his paper.[1]

(Modified Wikimedia Commons image.)

The Francis Bitter Magnet Laboratory , named after American physicist , Francis Bitter , was founded at MIT in 1961. Bitter was a pioneer in the generation of high magnetic fields. Foner was a founding member of the laboratory and became its chief scientist, and then its associate director from 1988-1989.[4]

Foner was very active in the physics community, serving in many positions in the American Physical Society , including that of consulting editor of the Review of Scientific Instruments , in which his VSM paper was published. He served, also, as chairman of the IEEE Magnetics Society , and he was an advisory editor of the Journal of Magnetism and Magnetic Materials for more than a decade.[4]

Scientists are known outside their local circle by their publications . Foner was an author of more than 400 scientific papers, and he edited four books on superconductivity, magnetism, and their applications. He was a fellow of three organizations in which I'm a member, the Institute of Electrical and Electronics Engineers , the American Physical Society, and the American Association for the Advancement of Science .[4]

Foner was a recipient of the IEEE Millennium Medal , awarded by the IEEE Magnetics Society; and the 1999 Joseph F. Keithley Award for Advances in Measurement Science by the American Physical Society for his invention of the vibrating sample magnetometer.[5] Simon Foner died on October 2, 2007.

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