EARLY HISTORY OF MAGNETIC RESONANCE

Norman F. Ramsey

Lyman Laboratory of PhysicsHarvard University

Cambridge, Massachusetts 02138

INTRODUCTION

In the title of this report, emphasis should begiven to the word early. Some readers may evenbelieve that "Pre-History" would be a better titlethan early history. The report will cover theperiod from 1921 to the first nuclear resonanceabsorption experiments of Purcell, Torrey andPound and the first nuclear induction experi-ments of Bloch, Hansen and Packard eventhough from some points of view the history ofmagnetic resonance can be said to begin with theexperiments that end this report.

My interest in the history of magnetic reso-nance began with preparations for my Ph.D.final examination in 1939. Since mine was thefirst Ph.D. thesis based on nuclear magnetic,resonance, I feared that my examining commit-tee would ask searching questions as to the ori-gins of the ideas of magnetic resonance and ofthe molecular beam technique we used to detectthe resonance transitions.

EARLIEST SEARCH FOR A DEPENDENCEOF MAGNETIC SUSCEPTIBILITY ON

FREQUENCY

The earliest reported search for a dependenceof magnetic susceptibility on frequency was car-ried out by Belz(l) in 1922 for solutions of avariety of paramagnetic salts. No frequencydependence was found. Acting on a suggestion ofLenz and Ehrenfest, G. Breit(2) searched for afrequency dependence of the magnetic suscepti-bility of various paramagnetic substances butfound no dependence on frequency. Perhaps thisdisappointment contributed to Breit's decision toconcentrate in theory, where he later had such aproductive career.

SPACE QUANTIZATION WHENDIRECTION OF MAGNETIC FIELD

CHANGES

The origins of the molecular beam magneticresonance method can be traced back to earlytheoretical speculations and experiments on the

change in the quantum mechanical space quanti-zation when the direction of a magnetic field ischanged. The problem was first posed and par-tially solved in 1927 by C. G. Darwin(2) and hisanalysis was subsequently improved by P.Gutinger(3), E. Majorana(4), and L. Mote andM. Rose(4).

In the period 1931-33 several experiments inOtto Stern's laboratory in Hamburg successfullymeasured the changes in the space quantizationwhen the direction of the magnetic field waschanged. The experiments of Phipps andStern(5) and Frisch and Segre(6) partly agreedwith the best theory and partially disagreed. I.I. Rabi(7) pointed out that the discrepancybetween theory and experiment was due to theneglect of nuclear spins in previous theories.Although the magnetic moment of the electron isabout 2000 times larger than the typical nuclearmagnetic moment, the angular momenta arecomparable in size and at the low fields used insome of the experiments the nuclear spin angu-lar momenta were tightly coupled to the electronspin making large effects on the observations. Inall of these experiments the direction of the fieldwas changed in space as the atoms went by.Since the atoms had a thermal velocity distribu-tion the frequency components were different fordifferent velocities, so on averaging over thevelocity distribution, no sharp resonances wereeither anticipated or observed. Rabi(8) andSchwinger(9) in 1937 calculated the transitionprobability for molecules that passed through aregion in which the direction of the field variedrapidly.

FIRST ATTEMPT TO OBSERVEDNUCLEAR MAGNETIC RESONANCE IN

CONDENSED MATTER

In 1936 with calorimetric techniques, C. J.Gorter(lO) successfully observed a frequencydependence of the paramagnetic relaxation of anumber of alums. He found that the observedeffects depended on the frequency, v, as vxwhere x was a number, usually between 1 and 2.No resonance effects were observed. Gorter(lO)

94 Bulletin of Magnetic Resonance

also utilized the same calorimetric method in anattempt to look at 7Li nuclear magnetic reso-nance in LiCl and for an *H resonance in A1Kalum but found no such resonance. The follow-ing year, Lasarew and Schubnikowt(21) showedat low temperature that the nuclear magneticmoments in solid hydrogen contributed signifi-cantly to the observed static magnetic suscepti-bility of solid hydrogen.

In an experiment reported in 1942 subse-quent to the successful molecular beam nuclearmagnetic resonance experiments described in thenext two sections, Gorter and Broer(lO)attempted to observe nuclear magnetic resonancein powders of LiCl and KF, but no resonancewas observed. It is still a mystery as to whyGorter did not detect a resonance. In part hesuffered from a poor choice of material since R.V. Pound much later showed that pure crystal-line LiF has an unusually long nuclear spin-lat-tice relaxation time. However, that alone doesnot explain the failure of Gorter's inspiredexperiments since at a much later date N. Blo-embergen found one of Gorter's original crystalsand was able to observe an NMR signal with iteven though the relaxation time was large. Themost likely explanation for the failure of Gorter'sexperiments was an unfavorable signal-to-noiseratio in his apparatus. It is of interest to notethat the first appearance of the phrase "nuclearmagnetic resonance" in a publication title is inGorter's 1942 paper, but he attributes the coin-ing of this phrase to I. I. Rabi.

TRANSITIONS INDUCED BY PASSAGE OFMOLECULES THROUGH

DIFFERENTLY ORIENTATED MAGNETICFIELDS

While Gorter was pursuing his unsuccessfulNMR experiments, I. I. Rabi was independentlystudying transitions induced when atoms or mol-ecules in a molecular beam traversed a region inspace of space in which the directions of themagnetic field change successively. In his bril-liant 1937 theoretical paper entitled "SpaceQuantization in a Gyrating Magnetic Field",Rabi(8) assumed for simplicity that the field wasoscillatory in time even though the initial appli-cation was to a field varying along the beamrather than oscillatory with time. As a conse-quence, all the formulae in that paper are appli-cable to the resonance case with oscillatory fieldsand the paper, without alteration, provides thefundamental theory for present molecular beammagnetic resonance experiments as well as forother experiments with magnetic resonance.

MOLECULAR BEAM MAGNETICRESONANCE

While writing his paper on the gyrating field,Rabi discussed with some of his colleagues thepossibility of using oscillatory rather than spacevarying magnetic fields, but Rabi's laboratoryhad a full .program of important experimentswhich did not require oscillatory fields, and noexperiments utilizing oscillatory fields werestarted during the first six months following thesubmission of Rabi's theoretical paper on thegyrating magnetic field. In September 1937, C.J. Gorter visited Rabi's laboratory(12) anddescribed his brilliantly conceived but experi-mentally unsuccessful efforts to observe nuclearmagnetic resonance in lithium fluoride, asdescribed in Gorter's publications of the previousyear(10). The research efforts in Rabi's labora-tory at Columbia University were soon directedprimarily toward the construction of molecularbeam magnetic resonance experiments withoscillator driven magnetic fields. Two successfulmagnetic resonance devices were soon con-structed by Rabi(13,14), Zacharias(13,14), Mill-man(13), Kusch(13), Kellogg(14), and Ram-sey(14, 15), A schematic view(13) of the methodis shown in Figure 1. In these experiments theatoms or molecules were deflected by a firstinhomogeneous magnetic field and refocused by asecond one. When the resonance transition wasinduced in the region between the two inhomoge-neous fields, the occurrence of the transitioncould easily be recognized by the reduction ofintensity associated with the accompanying fail-ure of refocusing. For transitions induced by theradiofrequency oscillatory field, the apparentfrequency was almost the same for all moleculesindependent of molecular velocity. As a result,when the oscillator freguency was equal to theLarmor angular frequency « o of a nucleus, asharp resonance was obtained where

(1)

is the angular precession frequency of a classicalmagnetized top with the same ratio Yj of mag-netic moment to angular momentum when in amagnetic field Ho. Figure 2 shows the firstreported nuclear magnetic resonance curve; thecurve was obtained with a beam of LiCl mol-ecules(13).

Kellogg, Rabi, Ramsey, and Zacharias(14,15) soon extended the method to the moleculesH2, D2 and HD for which the resonance fre-quencies depended not only on eqn. 1 but also on

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ffl

Figure 1. Schematic diagram(13) showing the principle of the first molecular beam magnetic resonanceapparatus. The two solid curves indicate two paths of molecules having different orientations that arenot changed during passage through the apparatus. The two dashed curves in the region of the B mag-net indicate two paths of molecules whose orientation has been changed in the C region so the refocusingis lost due to the change in the component along the direction of the magnetic field.

IOO

75

IlIS II2OMAGNET CURRENT IN AMPERES

Figure 2. Curve showing refocused beam inten-sity at various values of the homogeneous field.One ampere corresponds to about 18.4 Gauss.The frequency of the oscillating field was heldconstant at 3.518 X 10* cycles per second.

internal interactions within the molecule. Thetransitions in this case occurred whenever theoscillatory field was at a Bohr angular frequency

for an allowed transition

rw = Ej - Ef (2)

For the first time the authors described theirresults as "radiofrequency spectroscopy". Theradiofrequency spectrum for H2 is shown in Fig-ure 3.

The first molecular beam magnetic resonanceexperiments were with *2 molecules for whichthe primary interactions were those of thenuclear magnetic moments in external magneticfields, but in 1940 Kusch, Millman and Rabi(16,17) first extended the method to paramagneticatoms and in particular to AF = ± 1 transitionsof atoms where the relative orientation of thenuclear and electronic magnetic moments werechanged, in which case the resonance frequencieswere determined dominantly by fixed internalproperties of the atom rather than by interac-tions with an externally applied magnetic field.

In 1949, N. F. Ramsey(18,20) invented theseparated oscillatory field method for magneticresonance experiments. In this new method, theoscillatory field, instead of being distributedthroughout the transition region, was

96 Bulletin of Magnetic Resonance

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«

H in H2FREQUENCY6 9 8 7 MCI, : O 5 AMP

I6SOM'AGNETIC FIELD IN GAUSS

Figure 3. Radiofrequency spectrum of H2 in the vicinity of the proton resonance frequency(14). Theresonance frequencies are primarily determined by the interaction of the proton magnetic moment withthe external magnetic but the state of different mj and mj are displaced relative to each other by thedifferent values of the nuclear spin-nuclear spin interaction energies and of the spin-rotational interac-tion.

concentrated in two coherently driven oscillatoryfields in short regions at the beginning and endof the resonance region. In an alternative ver-sion of the same method, the coherent oscillatoryfields are applied in two short pulses - at thebeginning and end of the observation time. Themethod has the following advantages(20): (1)the resonances are 40% narrower than even themost favorable Rabi resonances with the samelength of apparatus; (2) the resonance are notbroadened by field inhomogeneities: (3) thelength of the transition region can be muchlonger than the wavelength of the radiation, pro-vided that the two oscillatory field regions areshort, whereas there are difficulties with theRabi method due to phase shifts when the lengthof the oscillatory region is comparable to thewave length; (4) the first-order Doppler shift canmostly be eliminated when sufficiently shortoscillatory field regions are used; (5) the sensi-tivity of the resonance can be increased by thedeliberate use of appropriate relative phaseshifts between the two oscillatory fields; and (6)with short lived states the resonance width canbe narrowed below that expected from the life-time of the state and the Heisenberg uncertaintyprinciple if the separation of the oscillatory fieldsis sufficiently great that only molecules living

longer than average in the excited state canreach the second oscillatory field before decaying.

Essentially the same magnetic resonancetechnique as developed by Rabi for measuringnuclear magnetic moments with a molecularbeam was used by Alvarez and Bloch(21) tomeasure the magnetic moment of the neutronwith a neutron beam. Since the first publicationon the neutron magnetic resonance studies waspublished about two years after the first molecu-lar beam magnetic resonance papers appeared, itis often considered that the neutron studies ofAlvarez and Bloch were merely adaptations ofthe resonance methods developed by Rabi and hisassociates. However, Alvarez recently has toldme that Bloch had thought of doing the neutronbeam magnetic resonance experiment beforeeither Alvarez or Bloch had heard of the molecu-lar beam magnetic resonance experiments ofRabi and his associates. It must have been abitter disappointment to Bloch and Alvarez tolearn that their clever idea for magnetic reso-nance had been anticipated by Rabi and hisassociates. It is to their credit that they did notlet this disappointment blight their researchcareers; instead each went on to win separateNobel Prizes for subsequent research.

Work on both molecular beam and neutron

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beam magnetic resonance experiments wereinterrupted by World War n . In 1944 Rabi andRamsey spent one evening together in Cam-bridge, Massachusetts, planning possible post-war research experiments. Two ideas emergedas leading candidates. One was to use themolecular beam magnetic resonance method tomeasure the hyperfine separation in atomichydrogen since a presumably exact theoreticalcalculation of this separation existed. Thisexperiment was eventually carried out and led tothe first indication of an anomalous magneticmoment of the electron. The other idea was todetect the existence of nuclear magnetic reso-nance transitions by their effect on the oscillator.To our pleasant surprise, the signal-to-noise cal-culations were favorable and we became quiteenthusiastic about the possibility. We then real-ized that we were merely reinventing Gorter'snuclear magnetic resonance experiments andthat those experiments had failed for unknownreasons. We, therefore, decided that efforts inthat direction should be given a low prioritycompared to the various molecular and atomicbeam experiments, including the one on theatomic hydrogen hyperfine separation.

ELECTRON PARAMAGNETICRESONANCE EXPERIMENTS IN

CONDENSED MATTER

In addition to his unsuccessful efforts toobserve nuclear magnetic resonance, Gorter(lO)successfully observed paramagnetic relaxation incondensed matter. However, his attempts toobserve an electron paramagnetic resonancefailed. The first successful paramagnetic reso-nance experiments in condensed matter werethose of Zavoisky(23). His observed paramag-netic resonance with CrCl3 is shown in Figure 4,was first reported in a 1944 Ph.D. thesis, andseveral years elapsed before there was wide-spread recognition of his accomplishment.Shortly after Zavoisky's pioneering work, obser-vations of electron paramagnetic resonanceswere made by Cummerow and Halliday (24) andothers.

NUCLEAR MAGNETIC RESONANCEEXPERIMENTS IN CONDENSED MATTER

Following World War II, two groups in theUnited States sought to develop nuclear mag-netic resonance experiments with condensedmatter. One was E. M. Purcell, N. G. Torreyand R.V. Pound(25) at Harvard University andthe other was F. Bloch, W. Hansen and M. E.

0

by the Physical Review announcing the success-ful observation of nuclear magnetic resonanceabsorption of the protons in a paraffin filled 30MHz resonant cavity whose output was balancedagainst a portion of the signal generator output.When the magnetic field passed through reso-nance, an unbalanced signal 20 times noise wasobserved.

When Bloch, Hansen and Packard (26)started their experiments, they were fully awareof Gorter's experiments but they were encour-aged to proceed because they thought they knewthe source of the previous failure and a meansfor overcoming it. They believed that Gorter'sexperiment had failed because the thermalrelaxation time T, was much longer than Goiterhad allowed for. To overcome this difficulty theyproposed to put their water sample in a strongmagnetic field for several days to allow thenuclear spin system to reach thermal equilib-rium. They in fact did so: when their apparatuswas all ready for a first test they inserted thewater in the high field and before attempting acareful search for a resonance Bloch went off ona ski trip to allow the system to come to equilib-rium. When he returned he and his associatesfound the desired resonance after some initialsearching, but they also found that the relaxa-tion time was short and not long. Instead ofwaiting several days to begin their observations,a few seconds would have sufficed. The detec-tion method of Bloch, Hansen and Packard(26)was rather different from that of Purcell, Torreyand Pound(25). Instead of observing the absorp-tion signal with a single coil, they used twoorthogonal coils and picked up the signal inducedin the second coil by the coherently precessingnuclei driven by the first coil. For this reasonthey called their experiments nuclear induction.A letter(2 6) announcing their successful experi-ment was received by the Physical Review onJanuary 29, 1946.

From the time of these experiments onward,developments in magnetic resonance occurred ata rapid pace. For this reason, I have chosen thattime to bring to an end this account of the earlyhistory of magnetic resonance.

REFERENCES1M. H. Belz, Phil Mag. 44, 479 (1922) and

G. Beit, Comm. K. Lab. 168c (1926).2C. G. Darwin, Proc Roy. Soc. 117, 258

(1927).

3G. Gutinger, Zeits. f. Physik 73, 169 (1931).4E. Maiorana, Nuovo Cimento 9, 43 (1932)

and L. Motz and M. E. Rose, Phys. Rev. 50, 348(1936).

5T. E. Phipps and O. Stern, Zeits f. Physik 73,185 (1931).

•O. Frisch and E. Segre, Zeits f. Physik 80,610 (1933).

7 1 .1 . Rabi, Phys. Rev. 49, 324 (1936).•I . I. Rabi, Phys. Rev. 51, 652 (1937).' J . Schwinger, Phys. Rev. 51, 645 (1937).1 c C. J. Gorter, Physica 3, 503, 995 and 1006

(1936) and C. J. Gorter and I. J. F. Broer,Physica 9, 591 (1942).

1XR. V. Pound, Phys. Rev. 81, 156 (1951)..12Rabi(13) refers to the visit of Gorter in a

footnote to the first paper experimentally dem-onstrating a successful molecular beam magneticresonance and Gorter 29 years later publishedan article giving his own somewhat differentrecollections of the same visit [Physics Today 20,76 (Jan. 1967)].

1 3 1 . I. Rabi, J. R. Zacharias, S. Millman andP. Kusch, Phys. Rev. 53, 318 (1938) and 55,526 (1939).

1 4 J . M. B. Kellogg, I. I. Rabi, N. F. Ramseyand J. R. Zacharias, Phys. Rev. 55, 729 (1939);57, 728 (1939); and 57, 677 (1940).

l s N . F. Ramsey, Phys. Rev. 58, 226 (1940).1«;P. Kusch, S. Millman and I. I. Rabi, Phys.

Rev. 57, 765 (1940).17 S. Millman and P. Kusch, Phys. Rev. 57,

438 (1940).1 4 N. F. Ramsey, Phys. Rev. 76, 966 (1949).1»N. F. Ramsey and H. B. Silsbee, Phys. Rev.

84, 506 (1951).2 ° N. F. Ramsey, Physics Today 33, 25 (July

1980).2 1 L . Alvarez and F. Bloch, Phys. Rev. 51, 11

(1940).2 2 J . E. Nafe. E. B. Nelson and I. I. Rabi,

Phys. Rev. 71, 914, (1947) and 73, 718 (1948).2 3 E . K. Zavoisky, Ph.D. Thesis 1944 and J.

Phys. USSR 9, 211 and 245 (1945) and 10, 197(1946).

2 * R. L. Cummerow and D. H. Halliday, Phys.Rev. 70, 483 (1946).

2 S E. M. Purcell, H. G. Torrey and R. V.Pound, Phys. Rev. 69, 37 (1946).

2*F. Bloch, W. Hansen and M. E. Packard,Phys. Rev. 69, 127 (1946).

2 7 B . G. Lasarew and L. W. Schubnikow,Phys. Zeits. Sowjet 11, 445 (1937).

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