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JOURNAL OF RESEARCH of the Notional Bureau of Standa rds-C. Engineering and Instrumentation

Vol. 73C, Nos. 3 & 4, July- December 1969

The Nuclear Quadrupole Resonance Magnetometer: A New Method for the Precision Measurement of the Magnitude and Direction of Magnetic Fields

Melvin Linzer

Institute for Materials Research , National Bureau of Standards, Washington, D.C. 20234

(September 11, 1969)

A theore ti ca l investigation of the feasibility of a magne tometer b ased on the observation of the Zeeman s plitting of a nu c lea r quadrupole resonance line is present ed. With a s ingle c rys tal of potas' s ium chlo rate as the se nsor, an nqr magnetome ter with ce nte r freque ncy of 28 MHz and bandwidth of 10 MHz could cover a ran ge of 10 nT to greate r than 1 T. Prec is ion a t s pec ifi c c rysta l o ri entations sho uld approach 10 nT in the magnitude of the field, at flux densities exceeding 100 ",,1' , and (65/8) microradians in the direction of the fi e ld where 8 is the flux density in mT. If circ ul a rl y po larized rf e xc itation is e mployed, a precis ion of 10 nT could be achieved at flux dens ities less than 100 ""T. Accuracy of the flu x density measure ments is limit ed b y the uncertaint.y in the gyromagnetic ratio of 3sC I, whi ch at the present tim e is approximately 1 X 10- ·. The effert of the s mall asymmetry parame ter whjch charac te rizes the ""C! resonance in KCIOa is analyzed. Various experimental arrangements are proposed to record the magn itude and d irection of the fi e ld , to obtain high re solution at low field s and good linearity at high fields, to overcome the difficulties due to the presence of a small asymmetry parameter, and to make an ins tantaneous measurement of the Zeeman splitting frequency.

Key words: Magne tome ter; nuclear £Iuadru po le resonan ce.

1. Basic Theory

This paper presents a th eoreti cal investigati on of the feasibility of a magn etometer based on the observa· tion of the Zeeman splitting of a nuclear quadrupole resonance (nqr) lin e. A detailed treatment of the Zee· man effect can be found in the monograph of Das and Hahn [lJ,1 in the papers quoted therein, and in several subsequent publications [2-8]. We shall confine our· selves in this section to a discussion of the theory relevant to Zeeman splittings in single crystals and its application to the measurement of the magnitude and direction of magnetic fields. The nuclei will be assumed to be characterized by an asymmetry parameter, YJ, equal to zero. Moreover, the quadrupole energy will be co nsidered sufficiently greater than the Zeeman energy such that the Zeeman splitting is essentially linear with respect to the magnetic fi eld.

Special emphasis will be placed on the 11/21 ~ 13/21 transitions; these are respon sible for the 35Cl nqr spectrum in potassium chlorate (KCI03), which, as will be shown later, is very suitable for magnetic field measurements. The effect of the small asymmetry parameter in KCIO:) is discussed in the next section.

I Figu res in brackets indicate the li terat ure references a t the e nd of thi s pape r.

67

Transitions between the ± m and ±(m+ 1) levels, with m ;:;': 1, give rise to two frequencies spaced symmetrically about the pure quadrupole frequency, VQ, with separation

Ilv = 2v I. cos (J

where Vt is the Larmor frequency, h'l is the absolute value of the gyromagnetic ratio, Bois the magnetic flux density, and (J is the angle between the magnetic field and the symmetry axis of the electric field gradient [1]. The intensities of the two lines are one· half the intensity of the original line and are inde- pendent of the orientation of the magnetic field.

For transitions between ± 1/2 and ± 3/2 levels, two pairs of splittings are in general observed because of the zeroth-order mixing of the ± 1/2 states in the presence of a magnetic field .2 The energy level diagram is shown in figure 1 for the case applicable to the 35Cl spectrum in KCl0 3 _ When YJ = 0 and VL ~ 0,

~ For co nvenience, we have labeled the four quan tum states as ± 1/2, ±3/2. The con- ven tional des ignation of the ± 1/2 levels in the presence of a small magnetic field is "±"; ,a t flux densities suffIciently great to cause an appreciable admixture of a ll the levels. a dif- fere nt notation is usualIy employed [9],

/ /

/ /

/

/

U I f3

/ ,

, ,

z) I a Va Vf3

1 - 2

+~ 2

3 2

3 T 2"

FIG URE 1. Low·field energy level diagram for the 1//21- 13/21 transitions of a nucleus with hal/integral spin, negative quad· rupole coupling and positive gyro magnetic ratio.

The use of the q uantum numbers ±m. to de signat e the s lates in the case 8 0 #- 0 is fo r cO ll venience onl y,

the splittings are symmetrical about the pure quad- rupole frequency, with the inner pair of lines, denoted by aa' , always equal to or stronger in intensity than the outer pair of lines, denoted by f3f3'. The splittings and relative inte nsities are give n by the equations [lJ:

~Vaa' = 13 - IlvD cos 0 , Iaa' = (f+ 1)/21 ~V!3!3'= (3+/)vD cos e, I !3!3' = (f-1)/21

1 = [l + (I + 1/2)2 tan 2 e] 1/2

and are shown in figures 2 and 3, respectively. The a components correspond to the transitions ± 1/2 ~±3/2 , whereas the f3 components arise from the transitions =+ 1/2~ ±3/2.

Simplified spectra are obtained at three special orientations of the magnetic field. At e = 0°, only the splitting of the a components is observed with

90 /

80 /

/

/ 70 /

/ Vi 60 / w /

~ 50 / 1f3 w / 0 CD 4 0 /

I

30 I I a

I 20 r

r r

10 r I I

~Vaa' = 2VD. The f3 compone nts give rise to twice thi s splitting but have zero intensity at this orientation. At e=90°, the a and f3 components have identical intensiti es and s plittings, with ~Vaa' = ~V!3!3' =(/+1/2)vI .. At e = tan - I [2V2/(/+1/2)], the splittings are given by

For 1=3/2, e and ~V!3!3' are equal to 54°44' (one-half of the tetrahedral angle) and 2 \1'3 VL , respectively. The separation between a and f3 components is therefore \1'3 v L at thi s orientation.

Because of their simplified spectral patterns, these special orientations are easily obtained experimentally. All three alinements give comparable line separations, with maximum splittings betw~en adjacent lines appearing at e= oo and 90°.

Deviations from linearity with respect to the magni- tude of the magnetic field depend very strongly on the orientation .of the symmetry axis. These deviations may be expressed in terms of a power series [9] in ~, where ~ = VI./VQ is the ratio of the Larmor frequency to the pure quadrupole frequency and is considered to be much less than one. At e = 0°, both the a and (vanishing) f3 component splittings are exactly linear with the field. For the ()= 54°44' configuratjon, the f3 splitting is approximately equal to 2\1'3 (l-e/3) but the composite a line is merely broadened by ~ e· At ()=90°, the fractional deviation is -3/2 e for the a splitting and zero for the f3 splittin g.

The Zee man splittings of the lines are least sensitive to angle at () = 0° and most sensitive at e = 90°. Inspec- tion of fi gure 2 reveals several large regions of almost uniform slope for the s plittings. Values of I d~v/d()1 as well as other pertinent characteristi cs of the a and f3 components are given in table 1 for the three special orientations.

I a

" " , " " " '\

"- '\

"-, /3 ' \

\ \ \ \ \ \ \ \ \ I I \ I I

OL_~2~----_~1~.5-------_~1 ------_-.~5-------0~----~.5~----~~----~.5~----~~

ZEEMAN SPL\ TTl NG (iJ'B,/2rr)

FIGURE 2. Low·field Z eeman splittingsfor 11/21 - 13/21 transitions in the case 1) = 0. The aa'{3{3 ' not ation corresponds to the s ituation depicted in figure 1.

68

.75

w . 5 > ;::

lines. Thus, the necessity of determining the width or shape of the resonance line, and the corresponding proble ms associated with the depende nce of these parameters on te mperature gradients (see below) are avoided. For arbitrary crys tal orientation, however, accurate field measure ments will still be difficult be- cause the composite lines are in general asym metri cal due to the different intensities of the 0' and f3 compo- nents. This proble m does not exist if 8 is equal to 0° or if the transitions ± m ~ ± (m + 1), with m ~ 1, are studied. Under these circumstances, only one line will be observed for each direction of polarization , thereb y eliminating the overlap and improving the resolution at very low magnetic field s.

2. The Potassium Chlorate Sensor

The foregoing discussion de monstrates that Zeeman splitlin gs in nqr can yield information regarding the magnitude and direction of a magnetic field. Thus, in principle, an nqr spectrometer with a properly chose n sample can function as a magnetometer. To examine the usefulness of this techniqu e, we shall consider the case of a single crystal of potassium chlorate. This co mpound has several desirable features which make it a suitable sensor for this application. The CI0 3" ions in the unit cell are completely equivale nt with regard to their resonant behavior in an external magnetic field. The unsplit ~5C l resonance line occurs at an experi- mentally convenient freque ncy of 28.1 MHz at room temperature and is characterized by an asymmetry parameter close to zero (YJ ~ 0.005) ,3 an

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