An electron spin resonance magnetometer-for time-varying fields

NUCLEAR

INSTRUMENTS

AND

METHODS

17 (1962)

157--160; N O R T H - H O L L A N D

PUBLISHING

CO.

AN ELECTRON SPIN RESONANCE MAGNETOMETER-FOR TIME-VARYING FIELDSt J. L. S T A H L K E *

Physics Research Laboratory, University o/Illinois, Urbana, Illinois R e c e i v e d 27 M a y 1962 F q u i p m e n t is described utilizing electron p a r a m a g n e t i c reson a n c e for m e a s u r i n g t h e i n s t a n t a n e o u s v a l u e of t h e p u l s e d m a g n e t i c field of t h e U n i v e r s i t y of Illinois 300 MeV B e t a t r o n

in t h e r a n g e 3000 to 6400 gauss. T h e m e a s u r e m e n t of a t i m e v a r y i n g field is r e d u c e d to t h e d e t e r m i n a t i o n of a m i c r o w a v e f r e q u e n c y . An a c c u r a c y of 0 . 1 % is a c h i e v e d .

1. Introduction Previous methods of measuring time-varying raagnetic fields have usually involved complex coil geometry, elaborate calibration procedures, and othei: such encumbrances. The equipment here described, developed to measure the pulsed magnetic field of the University of Illinois 300 MeV Betatron, essentially reduces the problem to raeasuring the frequency of a microwave oscillator. Field strengths between 3000 and 6400 gauss can be determined with equipment so far constructed.

magic tee bridge and crystal detector, and displayed on an oscilloscope or chart recorder. Some modifications in the usual E S R setup are made in the present application, since the purpose

2. Equipment In the conventional electron spin resonance (ESR) test setup, repeatedly described in the literature 1, z), a klystron oscillator supplies microwave energy to a paramagnetic crystal such as diphenyl picryl hydrazyl (DPPH), which is usually enclosed in a resonant cavity. The cavity is placed in a slightly modulated magnetic field, with the E lines in the cavity parallel to the magnetic field. The precession frequency of the unpaired electrons in the paramagnetic material is in resonance with the microwave oscillator frequency when the magnetic field = hf/gB. (h = Planck's constant; f = microwave frequency in cps, g = 2.0036 for D P P H , and ~tl is the Bohr magneton.) When the changing field reaches this value, the paramagnetic crystal absorbs microwave energy. The decrease in energy in Lhe microwave system is usually detected b y a t P a r t i a l l y s u p p o r t e d b y t h e j o i n t p r o g r a m of t h e U.S. Office of N a v a l R e s e a r c h a n d t h e U.S. A t o m i c E n e r g y Commission. * N o w w i t h S y l v a n i a E l e c t r o n i c S y s t e m s , M o u n t a i n View, California.

TUNING METER

Fig. 1. Block d i a g r a m .

is to measure a magnetic field rather than the absorption characteristics of various sample crystals. Fig. 1 is a block diagram of the equipment. The microwave systems operate in the X and K= bands, 8.2 kMc/s to 12.4 kMc/s, and 12.4 kMc/s to 18.0 kMc/s, respectively. The corresponding magnetic field ranges are 3000 to 4400 gauss, and 4400 to 6400 gauss. The klystron oscillator (Varian X-13 for the X band, and Varian X-12 for the Ku band) is followed b y a variable attenuator to set the power level at an optimum value as well as to isolate the klystron from the frequency-pulling effect of the wavemeter and spurious system resonances. The magic tee microwave bridge is balanced x) R. R. U n t e r b e r g e r , E l e c t r o n i c s 32 (1959) 142. 2) D. J. E. I n g r a m , S p e c t r o s c o p y a t R a d i o a n d M i c r o w a v e F r e q u e n c i e s , ( B u t t e r w o r t h s , Lonklon, 1955) pp. 30ft. 157

158

J.L.

STAHLKE

for each frequency setting b y means of the variable

short in its E arm. At balance the energy propagated down the detector arm is a minimum, and consequently the noise produced b y the detector

i

lK

are amplified and applied positive-going to a triode biased below cutoff, usually to - 9 V. In this way the absorption signal is amplified while the noise is left below cutoff. The output is nominally ,

IK

+ 150 V

T~"0"1

:3.9K T

REG

L6K

6.8K

i470

~C4 - -

/

I'ME

: , ofTT) . . . .

I

CVARIABLE BIAS(-5 TO -15 VOLTS)

_L

Fig. 2. Amplifier-discriminator s c h e m a t i c diagram.

crystal is small. When absorption occurs, the bridge becomes unbalanced and the crystal produces a signal proportional to the difference in power level in the right and left arms of the tee. In this way high average crystal current and the associated noise are eliminated. Under usual operating conditions an absorption signal of 3 to 10 mV is obtained from the detector crystal type 1N23B or 1N23E for the X-band and type 1N78B for the K, band. Careful selection of crystals is important, since approximately 50 % of type 1N23B and 30% of type 1N23E proved unsatisfactory for this application, either producing excessive noise or exhibiting low sensitivity. The 1N78B Ku band crystals are more uniformly satisfactory. An 0-5 V, 1000 ohm/V meter (tuning meter) is con-

~f~"~~ I~"~DPPH GRYSTA~S SILVER COATED POLYSTYRENE

Fig. 3. Waveguide probe.

nected across the detector crystal to monitor level and detect the resonant dip due to the meter. The amplifier-discriminator used is in fig. 2. The positive-going 3-10 mV input

power waveshown pulses

30 V. Conventional power supplies are used. To make field measurements possible at any frequency within the range of the klystron and waveguide system, no cavity is used, at some sacrifice in absorption signal amplitude. The system is terminated b y the waveguide probe shown in fig. 3.The probe is a transition section of waveguide, converting from air filled metal guide to metalliccoated polystyrene guide. The polystyrene section is used to permit thin conducting walls which are necessary to minimize eddy currents in the rapidly varying (60 cps) field of the betatron. Dimensions of the dielectric waveguide are reduced from the corresponding air filled waveguide size by the square root of the dielectric constant to provide the same propagation characteristics as the air filled guide. The metal portions of the transition sections are formed on a steel mandrel from standard brass X and K. band waveguide, with part of the material cut away. After coating the polystyrene with silver paint (DuPont 4817) to a thickness of about 0.001 inch, it is inserted firmly into the metal taper section, and the junction painted liberally with silver. Even small discontinuities at this junction caused a reduction in signal amplitude b y a factor of 3 or 4. The D P P H is inserted into a { inch hole drilled { inch deep in the end of the polystyrene waveguide. The hole is then covered with Scotch cellophane tape, and the waveguide end painted with silver. (See fig. 3.) At present, about

AN E L E C T R O N

SPIN RESONANCE

20 feet of waveguide is used between the D P P H and the magic tee b~idge.

3. Operation The klystron freqL'~ncy is set to the desired value, and checked b y observing the dip in crystal voltage as the wavemeter is tuned through resonance. When measuring frequency, the attenuator is set for the smallest value of crystal voltage which still permits observation of the resonant dip. This minimizes the coupling between klystron and wavemeter, thus minimizing the frequency pulling effect of the wavemeter on the klystron. The wavemeter is now detuned. After adjusting the variable short for minimum crystal voltage, the attenuator is set for 0.5 V across the crystal, approximately title optimum value. This completes the tuning procedure. The pulse produced as the field passes tlhrough the resonant value is displayed on an oscilloscope synchronized with the varying field. At this laboratory, the shape of the pulsed field being measured corresponds approximately to onehalf of a 60 cycle sine wave. It recurs 6 times per ,,;econd. If the field is gradually increased while the microwave frequency is fixed, a pulse appears as tlhe field enters the region of resonance. It increases in amplitude to a maximum, and, with further increases in field strength, begins to split into two pulses. The point of maximum amplitude with one pulse is the correct point to determine the peak field strength from the microwave frequency.

4. Accuracy The nominal line width of D P P H under these conditions is 3 gauss. The effective value will be greater than this if a relatively large sample is used in an inhomogeneous field. In this equipment, a volume of 25 mm 3 has been found to provide sufficient signal amplitude without undue pulse broadening. Larger volumes of D P P H , of course, produce larger absorption signals. Fields produced b y eddy currents induced in the silver coating of the plastic waveguide above and below the D P P H crystals add or substract from the field seen b y the D P P H , and need to be minimized. The resistivity of the silver paint (ap-

MAGNETOMETER

159

plied without thinning) was found to be 1.5 x 10-* ohm-cm, some two orders of magnitude greater than that of metallic silver. In one test, conducted with the sinusoidal field reaching a peak value of 4050 gauss, the coating thickness was increased in increments b y a factor of 10. With the thickness increased 10 times, it was found necessary to increase the field 0.13% (microwave frequency held constant) to bring the field at the D P P H to its original value. Thus an error of 0.013 % would be expected with one coating. However, the rate of change of the field is small throughout the region of resonance when measuring the peak value, and larger effects would occur at points where the rate of change is larger. The resistivity of the coating can be increased by thinning the paint with a solvent such as amyl acetate. Further reductions in eddy current effects could be realized b y striating the silver coating above and below the D P P H . All the constants appearing in the expression relating frequency to field are known with considerable accuracy, the accuracy with which the frequency is measured being the limiting value. Cavity-type microwave frequency meters are available with stated accuracies exceeding 0.1%. The effective width of the absorption peak does not limit the accuracy, since the center of the peak can be determined to 1 gauss or better. Thegvalue for D P P H is taken as 2.0036 + 0.0023). Using this value, the field in gauss can be found from H = f/k, where k = 2.8044, a n d f i s in Mc/sec. A comparison between a proton resonance magnetometer*) and the E S R equipment was made in a field of 3600 gauss sinusoidally modulated over a range of 30 gauss at a rate of 60 cps. Agreement within 0.1% was observed, with the E S R equipment yielding values consistently high b y approximately 3 gauss, or 0.08 %. This was expected, since the proton resonance probe used was intended for use only in fixed fields, and some eddy currents were present. Absorption pulses from both magnetometers were displayed on a dual trace oscilloscope using sine wave sweep phased to produce superposition of the E S R pulses. The magnitude: a) See ref. 2), p. 203. 4) H. W. Knoebel, a n d E. L. H a h n , Rev. Sci. I n s t r . 22 (195D 904.

160

J. L. S T A H L K E

of the eddy current fields in the proton resonance probe could then be determined by measuring the displacement of the proton absorption pulses. The value obtained was 3.4 gauss, indicating close agreement between the two magnetometers. Acknowledgements The author is indebted to Professor A. O. Han-

son, who suggested this work, and to Professor C. S. Robinson for his very helpful advice. Mr. D. Skaperdas and others of the Coordinated Science Laboratory are thanked for lending and setting up the original microwave equipment. Mr. C. L. Rogers assisted in many ways, especially in designing the waveguide transition section. The author also thanks Dr. G. R. Miller for many illuminating discussions on ESR.