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Equipment for Continuous and Pulsed RF-Mössbauer Experiments
I. Bibicu, D. Barb
To cite this version:
I. Bibicu, D. Barb. Equipment for Continuous and Pulsed RF-Mössbauer Experiments. Journal de Physique III, EDP Sciences, 1995, 5 (11), pp.1865-1869. �10.1051/jp3:1995230�. �jpa-00249421�
Classification
Physics Abstracts
76.80 78.70G 75.50 K
Equipment for Continuous and Pulsed RF-M6ssbauer
Experiments
1. Bibicu and D. Barb
Institute of Atomic Physics, P-O-Box MG-06, Bucharest, Romania
(Received 5 )lay 1995, accepted 25 July1995)
Abstract. The present paper reports on a versatile equipment for continuous and switched RF-M6ssbauer measurements with an increased capacity to minimize the RF-heating effects.
1. Introduction
An outline of the background of experimental and theoretical aspects of the effects of Radio
Frequency (RF) magnetic fields on the M6ssbauer spectra of ferromagnetic materials has been presented by Julian and Daniels iii. A proper understanding of RF collapse effects requires that the RF induced effects be unambiguously separated from spectral narrowing which might result from RF heating [2-6]- Equipment that allows the RF-heating effects in an adjustable way to be minimized is desirable. The present paper reports on versatile equipment for switched and continuous RF-M6ssbauer experiments with an increased capacity to minimize the RF-heating
effects.
2. The Equipment Description
The equipment, performed as additional part to AME-50 M0ssbauer spectrometer and a
Promeda-01 programmable data acquisition and processing system, is schematically repre-
sented in Figure 1- The Interface Circuit (IC) receives the start signals from the Function Generator (FG) and produces the control signals for the Gate circuit (G), the new start signals
for the data acquisition system (MCA) and the control signals for the Coincidence Circuit
(CC). The coincidence circuit routes the signals from a Single Channel Analyzer (SCA)- By
means of the gate circuit, the RF power provided by the Radiofrequency Oscillator (RFD and
amplified by the RF Power Amplifier (PA), is applied continuously or switched to the Absorber
IA), placed inside the coil IL of a resonant LC-tank circuit. This final output passes through
a Bird directional Power Meter (PM), which permits the monitoring of both forward and re- flected power. The LC-tank circuit is used to enhance the field strength applied to the sample.
This type of circuit is commonly used in Nuclear-Magnetic Resonance (NMR) studies [7j.
@ Les Editions de Physique 1995
1866 JOURNAL DE PHYSIQUE III N°11
nAV ADDR
ASTART FG ic
~
PA M
A/2
S L
PC SA
v
D
Fig- I- Schematic view of the apparatus for continuous and pulsed RF M6ssbauer experiments.
A: Absorber, L: Coil, MA: Matching Assembly, S: radioactive Source, MVT: M6ssbauer Velocity Transducer, MD: M6ssbauer Drive, FG: Function Generator, IC: Interface Circuit, G: Gate circuit.
CC: Coincidence Circuit, RFO: Radiofrequency Oscillator, PA: Power Amplifier, PM: Power Meter~
PC:,Proportional Counter, SA: Spectroscopy Amplifier, SCA: Single Channel Analyser, MCA: pro-
grammable data acquisition and processing system, D: nitrogen Dewar, V: Valve.
The resonant circuit is divided into two parts in order to separate physically the sample
from the rest of the electronics. The first part, the Matching Assembly (MA) consists of two capacitors C and C'. The capacitor C together with the inductance L forms the resonant
tank, the capacitor C' serves to match the impedance to the 50 ohms input cable. The second part of the resonant circuit consists of the coil in which the sample is placed. The coil is a
simple helix of eight turns of copper wire wrapped about a polyvinyl chloride (PVC) tube for extra stability. The copper is covered with silver, its diameter being lmm. Like the matching assembly the coil and the sample are contained in a grounded metal chassis. The metal chassis and the PVC tube are provided with windows so that the attenuation of the M6ssbauer gamma radiation is minimized. The matching assembly and the coil are linked together by a coaxial
cable, one half wave length long, designated in Figure I by 1/2- The purpose of the half-wave line is to isolate physically the coil from the tuning and matching capacitors, allowing adequate cooling of the sample.
In the switched mode, the radio frequency is applied during a time period equal to the
sweeping time of the full velocity range. For every 2~ (n
= 0,1, 2,3,4) start signals received,
the IC circuit generates one start signal for MCA and by means of a CC circuit the M6ssbauer spectrum is recorded only in the presence of the RF field. Significantly, the choice of different values for n allows the variation of the time between successive RF pulses, adjusting the proper
duration for cooling the sample.
Additional cooling of the absorber is provided by the continuous flux of liquid nitrogen from the Dewar (D), which is controlled by a Valve (V). The electronics are protected from the
influence of the RF radiation.
The temperature of the sample during RF exposure is measured with an infrared pyrometer.
The AME-50 M6ssbauer spectrometer (FG, MD, MVT) is operated in the constant acceler- ation mode, using a sawtooth velocity wave form and ~~Co (Pd) Source (S) of tens mci. The
~J
W( 400
C
~(
o 1
Fig- 2- Temperatures of the sample obtained for different n values. The error bars are included in the data symbols.
M0ssbauer gamma radiation emitted by the source is detected by the Proportional Counter
(PC), amplified by the Spectroscopy Amplifier (SA) and selected by the Single Channel Ana-
lyzer (SCA)-
The power amplifier is able to amplify RF signals over a range of frequencies from I to 100 MHz up to a power level of100 W- The quality factor (Q) of the resonant LC-tank circuit at 55 lIHz frequency is Q
" 60. Along its axis the RF field generated in the helical coil is
parallel to the absorber plane and the gamma radiation is perpendicular to the absorber plane.
In the absence of any ferromagnetic material, the peak amplitude of the induced oscillating RF
magnetic field inside the coil is given by HRF t 3 (PQ/Vu)~/~ [8], where P is the RF power in watts supplied by the final amplifier, Q is the quality factor. V is the volume of the coil in
cubic centimeters and u is the resonant frequency in MHz- In the case of a coil loaded with a
ferromagnetic foil, the field density is increased [9]. For the values of Q = 60, u
= 54.8 MHz
and P
= 50 W, corresponds an HRF m 15 Oe. The flux of liquid nitrogen is controlled from 0 to I liter per hour.
We have applied the radiofrequency field (54.8 MHz, amplitudes up to 15 Oe) to the soft
ferromagnetic amorphous alloy FesSi3.5B13 5C2 (Metglas 2605 SC) in order to study the RF
collapse [10] and the effects of the pulsed RF annealing [10-12]-
3. Results
The temperatures of the sample during different RF exposures, obtained for n
= 0,1, 2 are
plotted in Figure 2. Temperatures are recorded for the same RF power (50 W) and the same
flux ofliquid nitrogen. There is a significant decrease of the sample temperature as n increases.
For n
= 3,4 the temperature values fall outside the measuring range ofour infrared pyrometer.
By a proper choice of both n value and the liquid nitrogen flux, it is possible to obtain any desired temperatures of the sample.
1868 JOURNAL DE PHYSIQUE III N°11
P(QS)
C O
~ :
u1 . .
k
u1 . .
Cfl0.98
~ . ~
g ~
jj
. o~
~ -2 -1 o 2 o os lo is 2.o
imm/s as imm/s
a) b>
Fig- 3- RF collapsed spectrum of amorphous Fe81B13.5S13 5C2 (54.8 Mhz, 15 Oe) a), and distribu- tion of QS b), resulting from the fit in a)-
The RF collapsed spectrum recorded over four hours in a continuous RF field of15 Oe at 54.8 MHz is shown in Figure 3. The ribbon exhibits a Quadrupole Splitting (QS) distribution with
an average value of QS
= 0.52 mm Is. The solid line in Figure 2a represents the fit of the QS distribution to the experimental points. The QS distribution P (QS), extracted directly from the spectrum using the constrained Hesse-Rubartsch method [13,14] is shown in Figure 3b.
The technique of pulsed RF annealing may complete the information regarding ordering
processes occurring in amorphous systems at high heating rate since it offers access to the short-time high temperature region of the phase transformation diagram of amorphous metals.
Pulsed 54.8 MHz RF annealing of Metglas 2605 SC induces, before any crystallisation can
be detected by X-ray diffraction, different short-range ordering effects, reflected in M6ssbauer spectra [10-12]. A quadrupole interaction analysis reveals an almost identical threshold energy for the bulk stress relief and for the chemical short-range order growth [10,11] The relative
area intensity of the QS distribution, linearly dependent on the RF field energy is proposed as
an appropriate order parameter for the onset of Compositional Short-Range Order (CSRO).
An activation energy of1.7 eV [12] was estimated for the onset of surface CSRO in the RF annealed FeBSiC system
4. Conclusion
A versatile equipment for continuous and switched RF ~I6ssbauer measurements with an in- creased capacity to minimize the RF-heating effects is reported. We have hpplied the continuous and switched radiofrequency field to the soft ferromagnetic alloy Metglass 2605 SC in order to
study the RF collapse and the effects of the pulsed RF annealing.
References
iii Julian S-R- and Daniels J-M-, Collapse of M6ssbauer spectra in strong applied radio-frequency fields, Phys- Rev. B 38 (1989) 4394.
[2] Kopcewicz M-, M6ssbauer study of the influence of thermal effects on the RF collapse effect, Phys- Stat. Sol. (a) 60 (1980) 43.
[3] Kotlicki A., M6ssbauer study of narrowing of the magnetic hyperfine splitting spectra of permalloy and invar due to the radio-frequency field, Hyp- Int. lo (1981) l167-
[4] Indurkar V-S-, Srivastava J-K- and Vijayaragharan R-, Separation of RF-collapse effect from heat-
narrowing in permalloy foil samples, Hyp- Int. 35 (1987) 1053-
[5] Kopcewicz M-, El Zayat M. and Gonser U., On the RF collapse effects in Fe-Ni alloys, Hyp. Int.
42 (1988) l123-
[6] Stenger S-, Gonser U-. Camplbell S-J- and Smirnov G-V-, Hyp. Int. 54 (1990) lo53-
iii Fukushima R-F- and Roeder S-B-W-, Experimental pulse NMR, cap-V: A nuts and bolt approach, Addison Wesley Inc. Reading England~ London (1981)-
[8] Clark W-G-, Pulsed nuclear resonance apparatus, Rev. So. Instrum. 35 (1964) 316-
[9] DePaola B-D- and Collins C-B-, Tunability of radiation generated at wavelength below I A by
anti-stokes scattering from nuclear levels, J- Opt- Soc- Amer. B1(1984) 812.
[10] Bibicu I., Rogalski M-S- and Nicolescu G-, Transmission and conversion electron M6ssbauer in- vestigation of Fe81B13 5S13-5C2 glass, Phys- Stat. Sol- (b)178 (1993) 459.
[Iii Rogalski M- and Bibicu I-, Surface short-range order induced by RF annealing of Fe81B13 5S13 5C2 glass, Mater. Lea. 13 (1992) 32.
[12] Rogalski M-S-. Bibicu I- and Sorescu M-, CEMS investigation of surface hyperfine interactions in Fe81B13 5S13 5C2 glass, Hyp- Int. 92 (1994) 1317-
[l3] Hesse J- and Rubartsch A-J-, Model independent evaluation of overlapped M6ssbauer spectra, J- Phys. E 7 (1994) 526.
[14] Lecaer G- and Dubois J-M-, Evaluation of hyperfine parameters distributions from overlapped
M6ssbauer spectra of amorphous alloys, J- Phys. E12 (1979) 526.
