- FET based marginal R.F. Oscillator
- Digital diaplay of frequency
- Excellent peaks display
- Digital display of Helmoltz Coil Current
- Compatible with general pupose CRO in X-Y mode
In recent years Magnetic Resonance has developed
into a very useful and powerful tool in solid state research.
In this method, use is made of the Zeeman interaction of the
magnetic dipoles associated with the nucleus or electron,
when placed in an external magnetic field. Accordingly, they
are identified as NMR (Nuclear Magnetic Resonance) or ESR
(Electron Spin Resonance). This form of spectroscopy finds
many applications in the investigation of crystal structures,
environmental effects, dynamic effects, defects in solids
and in many diverse branches of Physics, Chemistry and Biology.
Elementary Magnetic Resonance
We know that the intrinsic angular momentum (spin) of the
electron S couples with the orbital angular momentum of the
to give a resultant
and this coupling gives rise to the ‘fine structure’
of the spectra. Further, under the influence of an external
magnetic field (H) each of the level will split into (2j+1)
sublevels (Zeeman effect) and the splitting of a level will
DE = (gµ0H)mj
where µ0 is the Bhor magneton,
g is the Lande’ g-factor and mj is the magnetic quantum
number. As can be seen, the splitting is not same for all
levels; it depends on the
and of the
level (s=½ always for one electron). However, the sublevels
will split equally by an amount
DE = gµ0H
0r = hn0
is the frequency of the system. Now if the electron is subjected
to a perturbation by an oscillating magnetic field with its
direction perpendicular to the static magnetic field and its
frequency n1such that
the quantum n1 is equal to E=hn0,
we say that there is a resonance between n1 and n0.
This willinduce transition between neighbouring sublevels
(mj=±1) and in turn will absorb energy from oscillating
field. Thus, at resonance, we get a peak due to the absorption
of energy by the system
If we consider a free electron and substitute
the proper value of constants in the equation: g=2.00, µ0=0.927X10-20
erg/gauss & h=6.625 X l0-27 erg sec, we get
That is ESR can be observed at radio frequencies
in a magnetic field of a few gauss or in the microwave region
in a magnetic field of a few kilogauss. The latter alternate
has many advantages: improved signal-to-noise ratio, high
resolution etc. and is always preferred for accurate work,
though it is very sophisticated and expensive. However, if
the basic understanding of the subject is the main criteria
as is usually the requirement of class room experiments, the
observation of ESR in low magnetic field and in a radio frequency
region makes it a lot simple, inexpensive and within the reach
of every post-graduate laboratory.
Description of the ESR Spectrometer
A block diagram of the ESR Spectrometer is given below in
Fig. 1, and a brief description follows.
The first stage of the ESR circuit consists of a
critically adjusted (marginal) radio frequency oscillator with 4-digit frequency display.
This type of oscillator is required here, so that the slightest
increase in its load decreases the amplitude of oscillation
to an appreciable extent. The sample is kept inside the tank
coil of the oscillator, which in turn, is placed in the 50Hz
magnetic field, generated by the Helmholtz coils. At resonance,
i.e. when the frequency of oscillation equal to the Larmour’s
frequency of the sample, the oscillator amplitude registeres
a dip due to the absorption of power by the sample. This obviously,
occurs periodically four times in each complete cycle of the
supply voltage. The result is an amplitude modulated carrier
which is then detected using a FET demodulator and amplified by an op-amp circuit.
Highly stabilised and almost ripple free
power supply for the above circuit is obtained using an integrated
This can compensate the undermined phase difference
which may be introduced in the amplification stages of the
spectrometer and oscilloscope.
50Hz Sweep Unit
A 50Hz current flows through Helmholtz coils which
provides a low frequency magnetic field to the sample. As
the resonance is observed at few gauss only, no static magnetic
field is applied.
Oscilloscope (not supplied with the Spectrometer)
Any inexpensive oscilloscope normally available
in the laboratory would be quite suitable.
Advantages and Limitations
of our Spectrometer
- The instrument is basically designed
for postgraduate laboratories keeping in view their requirements
- The observation of ESR at low magnetic
fields and consequently in radio-frequency region makes
its instrumentation and working a lot simple and within
the reach of a postgraduate students. Good resonance peaks
can be obtained as a class room exercise.
- The spectrometer is complete in all
respects including a sample DPPH (except a CRO).