FELIX BLOCH AND MAGNETIC RESONANCE

Erwin L. Hahn

Physics Department University of California Berkeley, California 94720 Presented at the Felix Bloch Memorial Symposium* American Physical Society April 25, 1984 Washington D.C.

Among the versatile and fundamental contri- damping in quantum , the periodicity butions of Felix Bloch to , the science of of in crystals and metals, susceptibility magnetic resonance in condensed matter, also and conductivity in metals, , introduced independently by the Purcell Harvard charged particle stopping power, quantum elec- group, was essentially the crowning achievement tro-dynamics, x-ray spectra, x-ray and Compton of his career. The background for the early scattering, Auger effect, beta decay and super- investigations of the Stanford group in what was conductivity. What was most important in then called "nuclear induction" must be viewed Bloch's future development was his decision to in the context of Bloch's early scientific career. leave Europe in 1934 and to begin his career at In his early youth it was unheard of to think of Stanford where he eventually began to do exper- making an assured living as a theoretical physi- imental physics. The seed of his magnetic reso- cist, so a proper career to choose was engineer- nance or "nuclear induction" idea stemmed from ing, and he became a student of engineering in his interest in the . I quote from Bloch's the Institute of Technology in Zurich. He quickly words in his magnetic resonance Nobel Lecture, became bored of these studies, and against the as follows: "The idea that a neutral elementary advice of his professors switched to physics. He particle should possess an intrinsic magnetic was aware that only fanatics do theoretical moment had a particular fascination for me — It physics and was told that he would probably seemed important to furnish a direct experimen- starve at it if he was mediocre. As a young tal proof for the existence of the magnetic , eager to capitalize on all the new moment of the free neutron." In 1936 Bloch physics which could be solved by the new quan- published a paper on the magnetic scattering of tum mechanics, Bloch made a grand tour of in which he suggested that a beam of studies at prominent centers of European physics slow neutrons in passing through iron would before World War II. He interacted with verita- experience a highly localized interaction with the ble Who's Who of 20th century - Bohr, iron atoms. He predicted that neutron spins Schrodinger, Pauli, Debye, Heisenberg, Kram- would become polarized as well as scattered, ers, Fokker, Ehrenfest and Fermi. Very soon leading to the present day methods of neutron afterward in the early 1930's, Bloch acquired an magnetic scattering. His thinking about mag- eminent stature of his own in the physics of sol- netic resonance began by his conception of a ids. Beginning in 1928 to the time of his death resonance depolarization experiment in which a in 1983, his papers ranged through a remarkable polarized neutron beam was passed through range of subjects. Leaving out his magnetic space with spins parallel to a strong constant resonance publications, they included radiation magnetic field H. As acknowledged in 1938 by Rabi and col- laborators, Bloch apparently, along with Gorter, had the magnetic resonance principle in mind, *The author is grateful to Carson Jeffries and with Bloch interested in carrying out neutron Martin Packard for access to source material spin resonance. Of course Rabi first accom- concerning F. Bloch. plished the experiment in 1938 with a molecular

82 Bulletin of Magnetic Resonance I a H S" •.

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Figure 1. Calculations of nuclear Boltzmann factors from original notes of F. Bloch.

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Figure 2. First estimates of the nuclear induction signal from original notes of F. Bloch.

Figure 3. F. Bloch. H. Staub, and W. Hansen viewing a homemade oscilloscope in the early 50's.

84 Bulletin of Magnetic Resonance beam. Bloch thereafter acknowledged Rabi's magnetic resonance principle to flip the neutron spin, and in 1940 in collaboration with Luis Alvarez at Berkeley, the neutron moment was measured to an accuracy of 1%. This accuracy was determined by flip coil measurements, which could not be better than 0.4%. Bloch's first notion, which he quickly abandoned as too awk- ward, was to improve the accuracy of this meas- urement by calibrating the magnetic field by a molecular beam technique. Bloch began to won- der whether it might be possible to measure the magnetic resonance of nuclei in ordinary con- densed matter? He was unaware that Gorter had failed in 1936 and in 1942 to carry out such an experiment in a crystal. This is not surpris- ing since Bloch was not a conscientious surveyor of the literature. He preferred to rediscover and work things out ior himself - a marked charac- teristic of his independent personality. Nor was he aware of the experiments contemplated or initially carried out on nuclear adsorption at Harvard. Figure 4. M. Packard, R. Sands and F. Bloch. Bloch began from first simple principles to calculate the voltage that would be induced in an inductance by the precessing macroscopic magnetization due to protons in 1 mL of water at room temperature, subjected to a strong field H apparatus -- and that was about it For the and a perpendicular r.f. field H,. (See Figures 1 nuclear induction experiment they used a rather and 2). He stated to me personally and to oth- primitive lecture demonstration magnet (Figure ers, on querying him about his train of thought 5) with current supplied by the small Stanford then, that he was amazed how such a simple cyclotron. calculation, taking into account a small Boltz- They first failed repeatedly in their search mann factor (of the order 10" *), could give such for resonance signals at the field supposedly large voltage signals of the order a millivolt from adjusted to the correct value of 1826 gauss for nuclear induction, well above amplifier noise. He proton resonance at 7.76 MC. In fact, returning recalled, having confirmed ample nuclear signal to the worry about thermal equilibrium, the size, that the worry of long thermal equilibrium sample of water was allowed to sit in the mag- times might cause failure to see any netic field all day to make certain that enough signal whatsoever. As I proceed further on, this macroscopic magnetization would build up to concern about relaxation will be brought out. provide a signal before making a search. Little The first attempt to measure nuclear induc- did they know that they had to wait only about 3 tion took place in the fall of 1945, after Bloch seconds for protons to relax in water. Finally returned to Stanford from wartime service at the something happened after they switched off the Radio Research Laboratory in Cambridge, Mas- magnet one day (sometime after Christmas, sachusetts, where he worked on radar scattering 1945) to see what was wrong. A signal blip and theory of the magnetron. With his student appeared on the scope unexpectedly. They saw Martin Packard and colleague W. W. Hansen, an their first adiabatic fast passage signal by drop- apparatus was assembled, with Bloch working on ping the magnetic field through the resonance the magnet and Packard and Hansen assembling condition w = YH. It was quickly confirmed the radio transmitter and receiver. In those that the signal could be improved by using a days the physics apparatus at Stanford was in a paramagnetic iron nitrate solution to shorten the sorrowfully primitive state with none of the ele- relaxation time. Now nuclear induction was gant equipment Bloch and Hansen were accus- established as a reality. After this success Bloch tomed to during their research sojourns else- gave a colloquium lecture while Martin Packard where during the war. The Stanford physics valiantly tried to reproduce the experiment on basement was cluttered with antique x-ray the lecture table in front of the audience, but he

Vol. 7, No. 2/3 85 quantum mechanical susceptibility and absorp- tion. Many new findings developed in the years to follow. The concept of motional narrowing in liquids developed explicitly from the Harvard group. Their style focused on microscopic effects of local fields, line widths and shapes, and effects of fluctuations of local fields upon the magnetic resonance signal and its relaxation. When nuclear induction signals in liquids were first investigated at Stanford, their observed line widths, which were predominantly caused by magnetic field inhomogeneity, were interpreted in the beginning to be due to dipolar broadening by neighboring spins in the sample. The germ of motional narrowing was of course finally evident to the Stanford group, since the effect of long relaxation times was included in the Bloch equa- tions. The Stanford group interest, however, was more in the measurement of spins and moments. The Bloch method of crossed-coils was adapted quite efficiently to the search for new Figure 5. Lecture demonstration magnet used to resonances which W. W. Hansen had in mind as obtain the first nuclear induction signal. a master project, interrupted, however, by his untimely death. With Nicodemus and Staub, Bloch returned to his old interest in the neutron, and made a precision measurement of its mag- netic moment by observing in the same field two couldn't find the signal — the spins were still shy precession frequencies, that of a beam of neu- trons in vacuum, and that of protons in water. about exposing themselves to the public, having 3 been left alone in their incoherent privacy for an Later tritium ( H) was measured by Levinthal, and Packard worked on the precise moment of eternity. 2 These preliminary results, reported in 1946, H. Warren Proctor developed a search NMR were followed by Bloch's famous paper on spectrometer, and he began to measure the spins Nuclear Induction in which the phenomenological and moments of a large number of nuclei. In the theory of macroscopic spin dynamics was given. course of these searches it was inevitable for It is of historical interest again to reproduce a Proctor and Yu that they should discover the certain page (Figure 6) of Bloch's original note- chemical shift - the same nucleus in different book. This shows clearly his reasoning in arriv- compounds has a different resonance frequency because contributions of the bonded electrons to ing at what are now the standard dynamical the effective field at the nucleus. This shift was equations of nuclear magnetic resonance, and the also discovered independently by Dickenson at famous transverse T and longitudinal T, relax- 2 MIT in the laboratory of Francis Bitter. Proctor ation times. and Yu observed the NMR shift of nitrogen-14 in A few weeks after the first successful exper- NH4NO3 dissolved in water: one nitrogen reso- iments at Stanford, Bloch received the news that nance came from the NH group, shifted from a a closely related discovery had been made simul- 4 second nitrogen resonance in the N0 3 group. taneously at Harvard. Their discoveries were This effect excited Bloch very much at first, announced in the same issue of , thinking that perhaps there existed stable January 1946. For a brief period of time it was nuclear isomers that had slightly different prop- thought that possibly these two investigations erties. The other possibility of course was that it were dealing with different things. Finally it could be just a nasty diamagnetic chemical shift was realized the two experiments were observing which would not and did not excite him. After it the same phenomena from different points of was confirmed as chemical in origin the shift did view. Bloch's approach was in terms of dynamic not command Bloch's interest as fundamental in macroscopic voltage signals induced by preces- his list of priority problems in physics. To con- sion and the Faraday effect, whereas the Purcell firm whether or not the two nitrogen resonances group description was in optical terms of

86 Bulletin of Magnetic Resonance -

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Figure 6. Formulation of Bloch equations from original notes of F. Bloch. were not isomers, Bloch telephoned Segre at Berkeley and asked to borrow a little 1 5 N. If analyzing the numerous side effects brought in the two resonances were not present in ammo- by the many body effects of local fields in con- nium nitrate made up with x s N, then the expla- densed matter that could not be comprehended nation would be nuclear instead of chemical. without an involved series of empirical measure- Finally the sample arrived and proved again to ments. I remember distinctly the time when give two resonances. Thus the matter of chemi- Dick Norberg and I were in Felix's office, pour- cal shift was a new complicating correction factor ing out our resonance research results to him, which had to be taken into account in measuring Norberg on metals, and I with chemical echo magnetic moments. modulation effects. Bloch said: "Norberg, you The above episode was an example of how should be a metallurgist, and Hahn, you should Bloch was really not enthusiastic about be a chemist!"

Vol. 7, No. 2/3 87 Figure 8. F. Bloch in a debating mood, with E. Ginzton.

Varian in those early days which gave the com- pany the research thrust that enabled it to pro- Figure 7. M. Packard and R. Varian. duce the first efficient instrumentation for ana- lytic NMR chemistry. At this point I would like to interject some particular remarks about the Bloch equations. The Bloch equations have had wide application to In 1950 Bloch and Jeffries measured a number of physical effects which do not neces- precisely the proton magnetic moment in units of sarily involve gyromagnetic spins. They apply the nuclear magnetic by observing in the same directly to any two quantum level or equally field the NMR and cyclotron resonance of protons spaced quantum level system, and are particu- using a small anti or inverted cyclotron techni- larly useful in predicting effects involving electric que. The body of data obtained by all these dipole laser resonance phenomena. measurements was ultimately applied to the Numerous effects in quantum optics are nuclear shell model of Mayer and Jensen. analogs of spin resonance phenomena discovered Arnold, Dharmati and Packard observed in a in former days and are often predicted by Bloch very uniform magnetic field, resolution of hyper- equations. Although considerations of propaga- fine proton resonances in C2H5OH, an effect tion and fluorescence are not present in NMR observed by Gutowsky and Slichter in the steady cavity resonance experiments, there remain a state, and independently by myself in terms of host of similar effects in the time and frequency modulation beats of spin echoes. The existence domain that also appear in quantum optics. of these fine structure effects together with the Although the original phenomenological Bloch chemical shift serve as the basis of high resolu- equations work very well for fluids, in many tion NMR as it exists today. The enterprise of cases they are not rigorous for all systems, par- high resolution NMR instrumentation was pio- ticularly in . Nevertheless, the Bloch for- neered by the Varian Company, and now has mulation has stimulated new statistical investi- spread as an analytic technique throughout the gations with the that are more world. Very good scientists, a number of whom rigorous for the particular system under investi- came from Bloch's NMR group, worked for gation. Exceedingly useful is the property that

88 Bulletin of Magnetic Resonance the Bloch equations enable predictions of nonli- positive incentive and enjoyment in learning near quantum macroscopic phenomena that no physics from Felix. amount of fastidious quantum mechanical per- The legacy of contributions to science by turbation theory could predict as handily. Felix Bloch was already a monument to him Personally it was my good fortune and privi- while he was alive. Among his versatile contri- lege to study under Felix Bloch as a postdoctoral butions to physics I have emphasized his work student during the two years 1950 to 1952. It on magnetic resonance. The application and was in his to have a profound influence impact in particular of the science of NMR now on his .students. His love for physics took a .high involves the activities of thousands of research priority in his life, which induced him continually people in state physics, analytical chemis- to avoid the impediments of formal rules of try, and most recently of clinicians in the medical bureaucratic restraints that prevented him from world who apply the technique for imaging the doing things for himself. He always invited oth- human body for medical diagnosis. Not long ers to share in his search for answers, and did before his death Felix indicated to me as well as not distance himself from anyone who would join to others that the extension of magnetic reso- him in the search, regardless of his or her status. nance to humanitarian practical use was of great What many of his students gained from him satisfaction to him, intellectually was often merged with his advice Felix Bloch is among the Greats in the his- and counsel. Felix was a devoted family man, tory of science, affecting many who never met and not incidentally, he was also an accom- him, a living influence for them and a personal plished pianist. With his good wife Lore and the one for those of us who were fortunate to have family, the invitations to participate in activities known him, and who shall remember him with of family life, with musicals, hikes and parties, affection and gratitude. were all occasions indeed memorable, giving

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