Chapter-Ill Theoretical Background
4.7 Magnetization Measurement Techniques
4.7.2 Principle of VSM
If a sample is placed in a uniform magnetic field, created between the poles of a electromagnet, a dipole moment will be induced. If the sample vibrates with sinusoidal motion a sinusoidal electrical signal can be induced in suitable placed pick- up coils. The signal has the same frequency of vibration and its amplitude will be
76
Driving Coil
Oscillating capacitor plate
Fixed capacitor plate
Saniple Holder
Pole Piece
'F~
Sample /
Pick-up
proportional to the magnetic moment. amplitude, and relative position with respect to the pick-up coils system. Fig 4.8 shows the block diagram of vibrating sample magnetometer. The sample is fixed to a sample holder located at the end of a sample rod mounted in a electromechanical transducer. The transducer is driven by a power amplifier which itself is driven by an oscillator at a frequency of 90 Hz. So. the sample vibrates along the Z axis perpendicular to the magnetizing field. The latter induced a signal in the pick-up coil system that is fed to a differential amplifier. The output of the differential amplifier is subsequently fed into a tuned amplifier and an internal lock-in amplifier that receives a reference signal supplied by the oscillator.
The output of this lock-in amplifier, or the output of the magnetometer itself.
is a DC signal proportional to the magnetic moment of the sample being studied. The electromechanical transducer can move along X. Y and Z directions in order to find the saddle point. Calibration of the vibrating sample magnetometer is done by measuring the signal of a pure Ni standard of known saturation magnetic moment placed in the saddle point. The basic instrument includes the electromechanical system and the electronic system (including a personal computer). Laboratory electromagnets or superconducting coils of various maximum field strengths may he used.
Power Amplifier
llV Amplifier
Range Attenuator
~ 1 1
'tuned Differential Amplifier Amplifier
ipliftcr
I .ock-in Amplifier
Digital Readout
Integrated Amplifier
Fig. 4.8 Block diagram of vibrating sample magnetometer
77
Fig. 4.9 Vibrating sample magnetometer
78
Chapter
-V
Result and Discussions
Chapter-V Result and Discussions
5.1 Crystallization Behavior of (Fe0.95C00.05)73.5CuiNb3Si13.5B9 Alloy
The understanding of the crystallization kinetics of magnetic amorphous I nanocrystalline alloys has various scientific and technical interest because of' the enhancement as well as deterioration of magnetic properties extremely depends on the degree of crystallization. Calorimetric studies of amorphous alloys provide substantial fundamental information concerning the kinetics of crystallization and structural relaxation effects. The kinetics of the onset of crystallization has been studied calorimetrically by Clernents and Cantor [5.11 and both calorimetrically and magnetically by Loborsky [5.2] in a variety of amorphous magnetic alloys. Crystallization kinetics is otien determined from Differential Scanning Calorimetry (l)SC), Differential Thermal Analysis (DTA) and in magnetic materials from the Thermomagnetic Analysis (TMA) [5.3-5.5]. Crystallization kinetics have also been studied by a variety of techniques including high resolution transmission electron microscope (14RTEM). in-situ XRD (X- ray Diffraction), Extended X-ray Absorption Fine Structure (EXAFS) measurements and resistance measurements.
- If the amorphous alloy is to be used as a precursor for the production of nanocrystalline FINEMET of composition Fe-CU-Nb-B-Si then the primary and secondary crystallization temperatures are of importance. Because the structure of the beneficial ferromagnetic nanocrystalline phase is composed of Fe(Si), which is the product of primary crystallization. The secondary crystallization product is the Fe-B phase. The phase is detrimental for the soft magnetic properties because of its high anisotropy energy. For the present research on (Feo 9 Coo.05)73 5Cu1 Nb3Si135B9 alloy. DTA has been performed to identify the crystallization temperatures as well as activation energy required for crystallization and the XRD experiment was undergone to identify the evolution of phases with heat treatment.
5.1 .1 DTA Results of (Feo 95Co005)735Cu Nb3Si 13cB9 Alloy
Crystallization is non-reversible, exothermic process [5.6-5.7]. Calorimetric studies of amorphous alloy provide substantial and fundamental information concerning the kinetics of crystallization. DTA is a direct and effective way to analyze the kinetics of nanocrystalline materials in respect of phase transition.
The change of composition affects the growth kinetics in a complicated way, which can only be determined experimentally. The composition of the alloy affects both the primary and secondary crystallization phases, because the time needed for the constituent atom to have long range order depends on their bond energies [5.8-5.9]. Good soft magnetic properties of the materials require not only small grain size but at the same time the absence of boron compound. The separation between the primary crystallization of bcc Fc(Si) and the secondary product of Fe-B compounds not only is determined by the Cu and Nb additions but also on boron content. With the increase of boron content the separation between the two products decreases [5.101. We kept at a moderate level of boron content in the nominal composition in order to obtain an optimum nanoscaled structure.
Regarding the study of FINEMET as an amorphous alloy, the identification of primary and secondary crystallization temperature carries much significance. Because the nanocrystalline phase comprising of FeCo(Si), is the product of primary crystallization.
Again identification of the secondary crystalline product is FeCoB is also important because evolution of this phase is detrimental to soft magnetic properties. The separation between these two phases is required to be verified because the ultra soft magnetic behavior expected from the FeCoSi phase can be suitably achieved when boride phases do not overlap with this phase.
The crystallization process is affected by heating rate as well as by composition. To keep the stability of the nanocrystalline alloy to achieve the expected soft magnetic behavior, both the nucleation and growth rate of nanocrystallites must be controlled. To study the effect of composition on the stability of the amorphous and nanocrystalline alloy, heating
rate must be kept constant at any single DTA treatment. DTA is a direct and effective technique for analyzing the kinetics of crystallization of amorphous materials.