Prof. Dr. Felix Bloch > Research Profile

by Luisa Bonolis

Felix Bloch

Nobel Prize in Physics 1952 together with E. M. Purcell
"for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith".

Felix Bloch’s scientific work was characterised by great originality and by diversity both in subject matter and in treatment, but it was the continuing concern with magnetism that marked his entire scientific career and led him to many fundamental achievements. He was among the pioneers in the application of quantum mechanics to many physical phenomena for which there had been no previous explanation. His early studies on superconductivity and on ferromagnetic materials opened the way to modern condensed-matter theories and to new approaches to many-particle systems and related problems of collective phenomena. His pioneering work on the determination of the magnetic moment of the neutron performed with Luis Alvarez in the period 1939-1940 eventually led to the discovery of nuclear magnetic resonance in bulk matter, a technique that Bloch developed in 1945, simultaneously with Edward Purcell. A method for the determination of the nuclear magnetic moments through resonance with radio waves had long been known and was rewarded with the Nobel Prize in Physics for the year 1944 for Isidor Rabi. Rabi carried out his investigations using molecular beams. These had the advantage that one could investigate substances in a state of very high rarefaction, but at the same time the applications were limited. The method of Bloch and Purcell implied a great simplification and generalisation in this respect, enabling its application to solid, liquid and gaseous substances. Since each kind of atom and its isotopes has a sharply defined and characteristic nuclear frequency, one can, for any object placed between the poles of an electromagnet, seek out and examine with radio waves all the various kinds of atom and isotopes present in the object in question. The advantage is that this can be done even in only small quantities, and, which is the essential point, without in any perceptible way affecting the sample, its form, crystalline structure, etc. An analysis in situ of extraordinary sensitiveness is thus obtained, which can be adapted for many scientific and technical fields. For this achievement Bloch was awarded the 1952 Nobel Prize in Physics, jointly with Edward Purcell. The practical value of research in chemistry and biology, using refinements of the original method, has been immense. In particular, the magnetic resonance imaging method, or MRI, later developed based on this technique, represented the greatest advance in medical imaging since the discovery of X-rays in 1895, at the same time being complementary to X-ray studies, since it allows investigations in physiological and metabolic processes.

Becoming Heisenberg’s Assistant in Leipzig
Felix Bloch was born in Zürich on 23 October 1905. In the fall of 1924 Bloch entered the Federal Institute of Technology (ETH) in Zürich, planning to study engineering, but his interests really lay in theoretical directions and the next year he decided to move to physics. During his university years, the foundations of physics were being replaced with totally new concepts and Bloch learned the early contemporary developments of quantum mechanics from Peter Debye, who deeply influenced him. He also had the possibility of attending lectures and seminars at the neighbouring University of Zürich given by the great mathematician Hermann Weyl and by Erwin Schrödinger, at a time when the latter developed his wave equation.
Bloch’s period at ETH came to a sudden end in the fall of 1927. He was about to start his thesis, when at the same time Schrödinger, Debye and Weyl all left Zürich, so that it became clear that he had to join the exodus. Debye suggested Bloch to follow him and move to Leipzig, where he had been appointed new director of the Institute of Physics. Debye had also persuaded the twenty-six-year-old Werner Heisenberg, who was already famous as one of the founders of the new quantum mechanics, to accept the professorship for theoretical physics. So Bloch started to work as Heisenberg’s first doctoral student and immediately had the greatest admiration for his young professor, establishing with him a very close and excellent connection. It was from Heisenberg that he received the encouragement to make his own early contributions.
At the time, Heisenberg was enthusiastic about the application of quantum mechanics to solids, and asked Bloch to examine the problems related to the conductivity of metals. These issues had been the subject of the Solvay conference of 1924, during which it had clearly emerged that appropriate methods had not yet been developed for a comprehensive description of the properties of matter. In 1926, going beyond the earlier work of Paul Drude and Hendrik Lorentz, Wolfgang Pauli had given a first new impetus to the field. In treating the conduction electrons as an ideal gas, obeying the newly formulated Fermi-Dirac statistics, he was able to derive the temperature independence of the paramagnetism of conduction electrons. By further application of the new statistics within the framework of the classical Drude-Lorentz theory, Sommerfeld had discussed the consequences for the specific heat and the electric conductivity of metals showing how only a small fraction of the electrons contribute to the specific heat, thus explaining qualitatively the relatively low specific heat observed in experiments, which had been a dilemma for a long time. Sommerfeld's and Pauli’s work involved new concepts, such as quantum statistics and electron spin. However, it remained very unsatisfactory that they had treated the conduction electrons in metals as an ideal gas of free electrons. How could the electrons be considered as free in the presence of the obviously very strong variations of the potential energy inside the metal? The problem of explaining “how the electrons could sneak by all the ions,” became the subject of Bloch’s doctoral thesis, which was published in the Zeitschrift für Physik in 1928 with the title “Über die Quantenmechanik der Elektronen in Kristallgittern” (The quantum mechanics of electrons in crystal lattices), and which is recognised as the basis of the modern theory of solids. He took a major step forward by approximating the lattice by a three-dimensional periodic potential, in order reduce the problem to a one-body calculation. In this potential, the electrons were tightly bound with an energy much larger than the kinetic energy of their motion through the lattice. He found that the allowed energy states were not discrete, but were distributed in continuous bands. The full machinery of quantum mechanics was brought to bear on solids, opening the way to the modern quantum theory of metals. Heisenberg was immediately very enthusiastic and exclaimed: “This is the solution of the problem!” Important concepts such as Bloch’s theorem, Bloch states and Bloch wave functions, go back to this work, which laid the foundations of the quantum theory of electrons in lattices and the band theory of solids. In establishing the modern picture of the structure of metals and insulators, it also played later a key role in much research on semiconductor devices, providing again a remarkable example of the interplay between fundamental and applied science.

Superconductivity and Ferromagnetism
Bloch’s successful theory accounted for metals, semiconductors, and insulators but not for superconductors. A satisfactory theory of superconductivity, which had been discovered by Kamerlingh Onnes in 1911, was still missing. After a suggestion by Wolfgang Pauli, Bloch started working on the problem while he spent the year 1928-1929 as Pauli’s assistant in Zürich. Bloch’s approach, using single electrons to derive the resistance of metals at low temperatures, could not give rise to superconductivity. In his new interpretation, he suggested an analogy with ferromagnetism and showed that the most stable state of a conductor, in the absence of an external magnetic field, was a state with no currents. And since superconductivity was a stable state displaying persistent currents without external fields, it was difficult to see how a theory for superconductivity could be constructed. However, in 1933, the discovery of the Meissner effect, according to which a superconductor expels magnetic fields upon entering the superconducting state, led in the following year to the formulation by Fritz and Heinz London of the phenomenological theory of the electrodynamics of a superconductor.
In parallel, Heisenberg had also suggested that Bloch take up the problem of ferromagnetism, which Heisenberg himself had already recently tackled within the frame of quantum mechanics. While he was still in Zürich, Bloch made an important improvement of Heisenberg’s theory, on which he continued to work during the academic year 1929-1930, which he spent in Utrecht. There he developed the concept of spin-waves, providing the first quantitative result on the dependence of the magnetic moment on absolute temperature in the low-temperature region. Spin-waves also proved to be an important precursor of quasi-particle theories. After returning to Leipzig, Bloch wrote his habilitation paper (Zur Theorie des Ferromagnetismus), published in 1930 in Zeitschrift für Physik, where he systematically studied exchange interaction problems and residual magnetisation in ferromagnets, developing much of the formalism which ever since has been used in condensed-matter theory and problems of collective phenomena. He established the nature of the boundaries between domains, i.e., the transition layers that separate adjacent areas of a ferromagnetic material, which are magnetised in different directions. These boundarie subsequently came to be known as Bloch walls. This work became a bridge between the quantum theory of ferromagnetism in the 1930s and present theories of many-particle systems.
After spending time in Copenhagen with Niels Bohr, during which Bloch worked on his well-known contribution to the theory of the stopping of fast charged particles in matter, he returned to Leipzig as Privatdozent and continued to be interested in quantum theory and in the nascent quantum electrodynamics. In March 1933, Bloch, being Jewish, decided to leave Germany, because of Hitler’s rise to power. During the summer, he travelled to Paris, Utrecht and Copenhagen, then spent part of his Rockefeller Fellowship in Rome with Enrico Fermi, with the intention of spending the other half in Cambridge. However, in the fall of 1933, after receiving a telegram from Stanford offering him a position in the Physics Department, he decided to follow Bohr’s recommendation and accept this opportunity.

The magnetic moment of the neutron
Bloch arrived in Stanford in early April 1934, bringing there a new kind of physics. He was acting as associate professor and soon set up a joint seminar on theoretical physics with Robert Oppenheimer, who was teaching at Berkeley. In 1935, during a trip to Europe, he visited Copenhagen, and had some inspiring discussions with Bohr, who directed his attention to the neutron, which was becoming an important focus of physical research. Since 1932, when James Chadwick had demonstrated its existence, the neutron had deeply changed the perspectives on the nature of matter. The neutron itself offered a new tool to explore the structure of nuclei and the physics of neutron interactions was actually gaining momentum after the discovery made by Fermi and his group in Rome in 1934 that slow neutrons were incredibly effective in producing artificial radioactivity. Being now recognised as the elementary constituents of nuclear matter, the intrinsic properties and particularly the magnetic moments of protons and neutrons had become of considerable interest. The study of hyperfine structure in atomic spectra showed that nuclei possess an intrinsic angular momentum, spin, and, parallel to its orientation, a magnetic moment, as first suggested in 1924 by Pauli. The energy of interaction of this magnetic moment with the magnetic field, produced by the atomic electrons at the position of the nucleus, leads to the observed small splitting of the energy levels. However, the optical determination of nuclear moments has several disadvantages, which limited the accuracy to a few percent at best. Moreover, hyperfine splittings tend to decrease with decreasing atomic number with the result that it is not possible, by optical means, to observe them in the case of the greatest fundamental importance, that of hydrogen.

A decisive step forward was made in 1933, with the fundamental experiments of Isaac Stern and his collaborators Otto Frisch and Immanuel Estermann, in which they determined the magnetic moments of the proton and the deuteron by deflections of molecular beams in strong inhomogeneous fields. In 1922, using a similar method, Stern and Walther Gerlach had successfully demonstrated the existence of the spatial quantisation in silver atoms evaporated in an oven and flying through a magnetic field. Instead of on the emitted light, the molecular beam method was focused on measuring the deflection of the beam particles in an inhomogeneous field. Stern and Gerlach found that the parent beam split into two distinct parts, with no silver atoms in the central region. This two-component splitting suggested that there exist only two allowed orientations for the silver atoms in the magnetic field. The splitting depends upon the magnetic moment of the silver atom, which arises from its electronic structure. Nuclear moments are three orders of magnitude smaller than their atomic counterparts, so that the application of molecular beam methods to their quantitative measurement required both modifying and refining the methods employed in the Stern-Gerlach experiment. In Stern’s improved experiments of the early 1930s, an analysis of the deflection pattern of magnetic moments yielded the proton moment to within ten percent. A most important result was that instead of having the expected value of one nuclear magneton it was 2.5 times larger. Of similar importance was the result that the magnetic moment of the deuteron was between 0.5 and 1 nuclear magneton. From the simplest plausible considerations of the structure of this nucleus, containing one proton and one neutron, such quantitative data also suggested a first approximate experimental value for the neutron magnetic moment, to which one should ascribe a value of about 2 nuclear magnetons. In 1943, “for his contribution to the development of molecular ray method and his discovery of the magnetic moment of the proton,” Stern was awarded the Nobel Prize in Physics.
The Stern-Gerlach experiment, an early triumph of the molecular beam method, offering other-than-spectroscopic evidence that quantum objects exhibit behaviour incompatible with classical physics, had stunned and intrigued the American physicist Isidor Rabi ever since when he was a student. Now, the fundamental character of Stern and his collaborators’ measurements prompted him to set up his own experiment at Columbia University to measure the proton’s - as well as the deuteron’s - magnetic moment. Starting from 1934, Rabi greatly developed the technique of molecular beams and carried out a series of brilliant investigations with his collaborators. If it was assumed that the deuteron was a compound nucleus made up of the proton and the neutron, and if, within the confines of the deuteron, the magnetic moments of the proton and deuteron were assumed to be additive, then the magnetic moment of the neutron could be inferred from a knowledge of the magnitudes and the signs of the proton’s and deuteron’s magnetic moments. Using a new arrangement, they determined for the first time that the magnetic moments of the proton and deuteron are positive, i.e., oriented like the spin, and were able to reduce the uncertainty in the measured value of the proton’s magnetic moment from 10 percent to 5 percent. Measurements for the deuteron gave 4 percent instead of 26 percent. From these results, published in 1936, the anomalous value of -2.0 nuclear magnetons was estimated for the moment of the neutron, but a direct measurement of that quantity remained an open problem.
During this period Bloch had become particularly fascinated by the idea that a neutral elementary particle should possess an intrinsic magnetic moment, as he said in his Nobel lecture, “... since it was in such striking contrast to the then only existing theory of an intrinsic moment which had been given by P.A.M. Dirac for the electron. Combining relativistic and quantum effects, he had shown that the magnetic moment of the electron was a direct consequence of its charge and it was clear that the magnetic moment of the neutron would have to have an entirely different origin.” It seemed thus important to furnish a direct experimental proof for the existence of a magnetic moment of the free neutron, but the great collimation required for this type of experiment, which might easily be obtained with the remarkable improvements introduced by Rabi’s group in the molecular beam method, would be almost impossible with the neutron sources available at the time. Bloch was definitely aware of these experimental difficulties, so that he proposed observing the scattering of slow neutrons in iron as a simpler and more suitable way of obtaining direct information about the magnetic moment of the neutron. In early summer 1936, Bloch pointed out that slow neutrons moving through magnetised matter are scattered by atomic nuclei, not only due to the direct interaction but also due to the magnetic coupling between the magnetic field arising from the magnetic moment of the atom and the neutron’s magnetic moment. HIs earlier work on ferromagnetism then suggested that if a beam of neutrons is incident on an iron plate magnetised by an external magnetic field, those neutrons with moments parallel to the magnetising field are scattered more strongly, with the result that the emerging beam of neutrons is partially polarised. A second magnetised iron plate could be used as an analyser, allowing only a select subset of beam particles to enter the detector.

The existence of this effect was clearly demonstrated in 1937 by several investigations prompted by Bloch’s 1936 paper. In these experiments the polarisation of neutrons was used as a tool to determine the neutron moment by a change of the polarisation produced by a magnetic field between the polariser and the analyser. These early experiments provided direct proof of the neutron’s magnetic moment and were consistent with the value inferred indirectly by Rabi. Even if they provided only an order of magnitude for the neutron moment, they opened up the possibility of further work with polarised neutron beams and in fact later experiments of the same kind, published in 1938, were able to determine the sense of the neutron’s procession in the field and thus successfully verified the sign of the neutron’s moment. However, they were less successful regarding its magnitude, so that, as Bloch recalled in his Nobel Lecture “the most desirable goal” still remained that of “accurately measuring the magnetic moment of the neutron.”
In the meantime, Rabi’s group had designed a new experimental apparatus very similar to that used to determine the signs of the proton and deuteron moments. As in the previous setup, each beam particle was deflected in a first strong inhomogeneous magnetic field, and then in a second one with the opposite orientation, whose field strength was set to exactly undo what the first magnet did. In this way, both fast and slow atoms would be refocused into the detector by the second inhomogeneous field, avoiding complications associated with the distributed velocities of the beam particles. A most important innovation was the introduction of an oscillating field between the two deflecting magnets, which was superimposed at right angles to a strong constant homogeneous field. A static magnetic field exerts a torque on the component of each magnetic moment that is not aligned along the field, continuously changing its direction, but leaving its magnitude constant. The result is a precession around the field with a frequency, called the Larmor precession frequency, f = γH, proportional to the strength of the field through the gyromagnetic ratio, a constant characteristic of each nucleus or nucleon. The gyromagnetic ratio is also the constant of proportionality between the magnetic moment and the spin angular momentum: γ= µ/I. The strength of the constant magnetic field is slowly varied, thereby varying the Larmor precession frequency and bringing it into coincidence with the frequency of the oscillating field, which can be controlled precisely by a radio-frequency oscillator. Once the resonance condition is achieved, a tilt of the nuclear moments is induced, inducing transitions. Such transition occurs from one Zeeman level to another, when the alternating field satisfies Bohr’s frequency condition for the energy difference between two states, analogous to the resonance absorption of visible light. However, instead of optical frequencies, one normally deals here with frequencies in the radio range, so that this application of the magnetic resonance method is properly labelled as belonging to the new field of radiofrequency spectroscopy. Many atoms flip to another orientation and are thus no longer focused on to the detector by the second inhomogeneous field; the observed intensity diminishes registering a marked resonance minimum. If the spin is known, the simultaneous knowledge of the strength of the resonance field and the Larmor frequency directly yields the gyromagnetic ratio and makes it possible to determine the nuclear magnetic moment with extraordinary precision.
The magnetic resonance method led to a determination of the magnetic moments of the proton and the deuteron with an accuracy of about one part in a thousand. It was a great advance over previous molecular-beam methods for measuring magnetic moments and it was soon extended to the molecules of hydrogen, deuterium and tritium. These measurements provided further information on the neutron, as well. In 1944 the Nobel Prize for Physics was awarded to Rabi “for his resonance method for recording the magnetic properties of atomic nuclei”.

During the summer 1937, without being aware of Rabi’s contemporary advancement of his research with molecular beams through the resonance method, Bloch had the idea of using a polarised beam of neutrons that would pass through a region of constant magnetic field which could be varied to the point where the Larmor precession of the neutrons was in resonance with the frequency of an oscillating magnetic field. The ratio of the resonance value of the constant field to the known frequency of the oscillating field of course could immediately give the value of the magnetic moment, according to the resonance condition.
A first experiment carried out in the summer 1938, when Rabi was visiting Stanford, was unsuccessful due to instrumental effects which were hard to discover with the small intensity of the natural neutron sources used and unfortunately postponed the actual realisation of Bloch’s version of the magnetic resonance method. By that time Rabi and his collaborators had already published their remarkable results on the proton and deuteron’s moments. Bloch went on with his plans and in the fall of 1938 he spoke to Ernest Lawrence, proposing the use of the 37-inch cyclotron, in operation at Berkeley since 1936, which was an indirect source of neutrons, providing a sufficient flux for the experiment. In Berkeley, the young experimental physicist Luis Alvarez joined Bloch and they begun their experimental work in the spring and summer of 1939, first spending several months developing their polariser-analyser equipment, then moving on to the cyclotron, which unfortunately worked only sporadically at the time, so that the experiment took a long time to be completed.
The beam of neutrons obtained by passing the unpolarised neutron beam from the cyclotron through the polariser, a strongly saturated plate of magnetised iron, emerged with a great number of neutrons oriented antiparallel to the magnetising field. Off resonance, the degree of polarisation remained intact. At resonance, the polarisation of the incident beam was changed, and the scattering of the beam in the second plate could be detected. Since the intensity of the neutron beam emerging from the analyser depended on the polarisation, the intensity was a function of the oscillating field frequency.
In an article submitted to the Physical Review in October 1939, Alvarez and Bloch reported for the magnetic moment of the neutron a value of 1.935±0.02 nuclear magnetons, with a negative sign with respect to the proton’s moment. This result was consistent with the magnetic moment of the deuteron measured by Rabi and collaborators, assuming one could simply add the proton and neutron moments. However, in that same period, Rabi’s group announced that the deuteron has an electrical quadrupole moment, indicating that the charge of the nucleus is not distributed evenly over a sphere. It was shown that, as a consequence, there should be small departures from additivity of the magnetic moments within the deuteron. Bloch wanted to reduce the uncertainty of the neutron moment result from 1% to at least one part in a thousand, in order to test the small deviations from the additivity of the moments of the proton and the neutron. The success of his method, which could be applied to any nucleus, led Bloch to decide that a small cyclotron should be built at Stanford, which would provide the opportunity of making further measurements in house.
In the meantime, however, dramatic events were changing the world panorama. On September 1, 1939, Hitler’s invasion of Poland marked the beginning of World War II. In 1942, Bloch was involved in war activities and later joined the Radio Research Laboratory at Harvard University where he worked in radar research, in particular with microwaves. In early 1945, he began to think about what one could do after the war. He discussed a lot with Rabi, who was the head of the Research Division, and together they wrote the review article “Atoms in variable magnetic fields”, but Bloch was eager to go back to the neutron work: “That was my obsession,” he said to Rabi. Then he realised that instead of using molecular beams to study the nuclear magnetic resonances, one should be able to do it in condensed matter, not in vacuum. He began to do his calculations in the evening, convincing himself “that this should at least be possible.”

Nuclear Magnetic Resonance in Bulk Matter
Bloch returned to Stanford from the Harvard Radio Research Laboratory with clear ideas about the experiments he wanted to perform. The essential fact of the magnetic resonance consists in the change of orientation of nuclear moments, and the methods to be employed in molecular and atomic beams as well as in neutron beams were primarily dictated by different ways to detect this change. The acquaintance with radio techniques during the war suggested Bloch still another and much simpler way, that of detecting the electromagnetic induction caused by the reorientation of nuclear moments through the normal methods of radio reception. In order to understand the basic principle one can consider the magnetic moments of protons contained in a small sample of water, which are oriented in a completely random manner in the absence of an external magnetic field, with no energetic difference for any particular orientation. When the sample is placed in a magnetic field, the moments immediately begin to precess with the Larmor frequency, regardless of their initial spatial orientation and at the same time an ordered system begins to appear, moments aligning in a parallel or anti-parallel orientation. There is a high-energy state and a low-energy state depending on the relative orientations of the moments to the external field. These two orientations form a two-level system: i.e., a system that has two states and a definite energy difference, or splitting, between them. Photons with energy E=hν, exactly matching the energy difference between the two states are at resonance with the system and can thus cause transitions between the two energy levels. The resonance frequency, in turn, depends on the strength of the static magnetic field and occurs in the radio-frequency region of the spectrum for a relatively strong magnetic field (about 7000 Gauss). As in the nuclear resonance method, if a weak oscillating magnetic field is superposed on the constant field, at resonance the orientation of magnetic moments suddenly reverses and Bloch thought that, therefore, one could expect a measurable effect in form of an oscillating induced voltage in a pick-up coil. It was for this reason that he considered the term nuclear induction most suitable and descriptive of the phenomenon as a whole. The exact value of the frequency that gives the maximum signal can then be used to calculate the magnetic moment.
Later this technique provided a new and simple method for measuring magnetism: once the magnetic moment of a given nucleus is known, it can be used to determine the strength of any unknown magnetic field, less easily measurable otherwise. One remarkable application is the possibility of measuring the strength of the earth's magnetic field to an incredible accuracy – and thus to measure very small variations due, e.g., to the presence of ferrous objects - simply by tuning in to the precession rate of protons with a special instrument, the proton magnetometer. Bloch’s collaborators in the experiments were William Hansen and a graduate student, Martin Packard. They established this new effect, using water at room temperature and observing the signal induced in a coil by the rotation of the proton moments. Within their experimental error, the proton moment was found to be in close agreement with the value that had been already determined by Rabi in his experiments with molecular beams.
In December of 1945, Bloch and Edward Purcell of Harvard met at the annual meeting of the American Physical Society and realised that they were working on similar problems. Simultaneously and independently, Purcell and his collaborators Henry Torrey and Robert Pound had developed a technique that was nearly identical to Bloch’s. They decided that Bloch would continue his researches and investigate liquids, whereas Purcell would concentrate on crystals. Their results appeared at the same time and both methods came to be known as nuclear magnetic resonance (NMR), soon to become the foundation of new fields in physics, chemistry, biology, physiology and medicine. The development of resonance methods led unexpectedly to the development of a set of very powerful and sensitive analytical tools, useful for a multitude of purposes. Moreover NMR is so non-destructive that it can be used to study living organisms without damaging them. The successes of magnetic resonance imaging in diagnostic medicine are now rivalling - and at the same time complementing - the tremendous advance brought about by Wilhelm Roentgen’s discovery of X rays at the end of the 19th century.

Although Bloch is known among physicists for his many other achievements, it was for the development of this technique that he was awarded the Nobel Prize in Physics for 1952, jointly with Edward Purcell. The techniques and computational approaches used for computer-assisted tomography scanners developed by Allan Cormack and Godfrey Hounsfield, for which they were jointly awarded the Nobel Prize in Physiology or Medicine 1979, were applied to NMR observation techniques in the 1970s, resulting in the development of NMR scanners that can image specific chemical reactions within the human body. These scanners were found to be powerful instruments for medical diagnostics, and equipment became commercially available in the mid-1980s. So explosive were the developments in NMR spectroscopy that further Nobel Prizes were awarded in the field. In 1991, the Nobel Prize in Chemistry was assigned to Richard Ernst “for his contributions to the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy.” Then Kurt Wüthrich was awarded the Nobel Prize in Chemistry 2002 “for his development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of biological macromolecules in solution.” And the following year, Paul Lauterbur and Peter Mansfield were jointly awarded the 2003 Nobel Prize in Physiology or Medicine for their discoveries concerning magnetic resonance imaging.

Bibliography
Bloch F. (1952) The Principle of Nuclear Induction, Nobel Lecture, December 11, 1952, http: //www.nobelprize.org/nobel_prizes/physics/laureates/1952/bloch-lecture.pdf
Bloch F. (1976) Heisenberg and the early days of quantum mechanics. Physics Today 29 (12): 23-27
Bloch F. interviewed by T.S. Kuhn, May14, 1964, http://www.aip.org/history/ohilist/4509.html; interviewed by C. Weiner, August 15, 1968, http://www.aip.org/history/ohilist/4510.html; interviewed by L. Hoddeson, December 15, 1981, http://www.aip.org/history/ohilist/5004.html.
Kostas Gavroglu (2008) Bloch, Felix. In Complete Dictionary of Scientific Biography Vol. 19. Detroit: Charles Scribner’s Sons, pp. 303-308
Hofstadter R. (1994). Felix Bloch. October 23, 1905 - September 10, 1983. Biographical Memoirs 64: 35-71, http://www.nap.edu/catalog/4547.html
Rigden J. S. (1986) Quantum states and precession: The two discoveries of NMR. Reviews of Modern Physics 58(2): 433-448
Wasson, T. (ed.) (1987) Bloch, Felix. In Nobel Prize Winners, H. W. Wilson Company, New York, pp. 102-104