B. Bhanu Prasad

Amorphous to crystalline transformation of Fe81B13.5Si3.5C2

The crystallization of Fe81B13.5Si3.5C2 (Metglas 2605SC) has been studied by resistivity, differential thermal analysis (DTA), Mossbauer and x‐ray techniques, and compared with reported results based on other techniques. The DTA shows in addition to the usual two peaks a third peak at 850 K paralleled by a change in the resistivity measurements at the same temperature. The crystallization is found to take place gradually in the temperature region 750–810 K with ∝‐iron precipitating first. Mossbauer studies provide evidence for ∝‐Fe, Fe2B, and a species with a hyperfine magnetic field ∼314 kOe which could not be unequivocally established. X‐ray data shows evidence for the presence of Fe3C in addition to ∝‐Fe and Fe2B.The crystallization of Fe81B13.5Si3.5C2 (Metglas 2605SC) has been studied by resistivity, differential thermal analysis (DTA), Mossbauer and x‐ray techniques, and compared with reported results based on other techniques. The DTA shows in addition to the usual two peaks a third peak at 850 K paralleled by a change in the resistivity measurements at the same temperature. The crystallization is found to take place gradually in the temperature region 750–810 K with ∝‐iron precipitating first. Mossbauer studies provide evidence for ∝‐Fe, Fe2B, and a species with a hyperfine magnetic field ∼314 kOe which could not be unequivocally established. X‐ray data shows evidence for the presence of Fe3C in addition to ∝‐Fe and Fe2B.

Mössbauer study of a ferromagnetic metallic glass: Fe74Co10B16

Abstract Amorphous Fe74Co10B16 (METGLAS 2605CO) has been studied in the temperature range of 77 K – 700 K by Mossbauer spectroscopy. Its crystallization temperature is found to be 665 ± 5 K and Curie temperature is estimated to be 760±10 K. The observed rapid decrease in reduced hyperfine fields can be explained well by Handrichs model for amorphous ferromagnets if one assumes a temperature dependent δ, a measure of fluctuations in the exchange interactions in such solids.

A Mössbauer study of amorphous Fe40Ni40B20

Amorphous Fe40Ni40B20 (VITROVAC 0040) alloy has been investigated using 57Fe Mossbauer Spectroscopy. The Curie temperature Tc is found to be well defined and is 695 ± 1 K. The quadrupole splitting just above Tc is 0.64 mm sec−1. The crystallization temperature is 698 ± 2 K, close to but definitely above Tc. The average hyperfine field Heff(T) of the glassy state shows a temperature dependence of Heff(0)[1 − B32(T/Tc)32 − C52(T/Tc)52 − …] indicative of the existence of spin wave excitations. The values of B32 and C52 are found to be 0.40 and 0.06, respectively, for T/Tc ⩽ 0.72. At temperatures close to Tc, Heff(T) varies as (1 − T/Tc)β where β is one of the critical exponents and its value is found to be 0.29 ± 0.02.

Absolute thermoelectric power of magnetic glassy metals

Abstract Absolute thermoelectric powers of three glassy ferromagnetic metals Fe40Ni40P14B6, Fe29Ni49P14B6Si2 and Fe74Co10B16 have been determined between 77 K – 700 K. Absolute thermoelectric powers for these metals are found to be rather small when compared to some other crystalline magnetic alloys. It is observed that the thermoelectric power varies linearly with temperature above room temperature, while the linear relationship breaks down at lower temperatures showing Kondo type behaviour, that is, a minimum in thermoelectric power versus temperature. Some structure is observed in Fe74Co10B16 and Fe29Ni49P14B6Si2 at ∑450 K and 380 K, respectively which seems to have some correlation with the change in the slope d / dT of the samples at those temperatures. Results are interpreted in terms of Grest and Nagel theory.

A Mössbauer study of amorphous alloy Fe67Co18B14Si1

Abstract Amorphous alloy Fe 67 Co 18 B 14 Si 1 (METGLAS 2605CO) has been studied by Mossbauer Spectroscopy in the temperature range 80 K – 900 K. Non-crystalline nature of the alloy is reflected in the observed broad and overlapping Mossbauer six-line pattern from 80 K – 630 K. Amorphous sample crystallizes at 650 K before becoming paramagnetic in the amorphous state. The Curie temperature has been estimated to be 830 ± 10 K. H eff (T)/H eff (O) vs (T/T C ) data has been fitted to the Handrich model of an amorphous ferromagnet in which the fluctuation exchange parameter has been taken as δ = 0.6(1−T 2 /T C 2 ). H eff (T) has also been found proportional to ( T/T C ) 3 2 with a slope B 3 2 - 0.43±0.05 . Initial crystallization studies by Mossbauer Spectroscopy and X-rays indicate that the crystallized products do not contain α-Fe but most probably FeB and (FeCo)B alloys.

Electrical resistivity and thermoelectric power of glassy Fe74Co10B16 and Fe67Co18B14Si1 alloys

Abstract Electrical resistivity and thermoelectric power measurements of Fe 74 Co 10 B 16 and Fe 67 Co 18 B 14 Si 1 have been performed in the temperature range 77 to 900 K. For both the samples the crystallization is taking place in a two step process. Thermoelectric power S ( T ) for both samples is found to be negative and at lower temperatures, a minima in S ( T ) is observed which shows a Kondo type behaviour.

Magnetic, electrical and thermoelectric studies on metallic glass Fe39Ni39Mo4Si6B12

Abstract Metallic glass Fe39Ni39Mo4Si6B12 has been investigated using Mossbauer Spectroscopy, electrical resistivity, and thermoelectric power techniques. Heff(O), Tc and Tx are found to be 275 ± 5 kOe and 725 ± 3 K by Mossbauer Spectroscopy. Heff(T)/Heff(O) vs T/Tc gives a good fit to the Handrich model of an amorphous ferromagnet in which fluctuation parameter δ is taken as δ = 0.65 (1−T2/Tc2). Heff(T) has also been found proportional to ( T / T c ) 3 2 with proportionality constan B 3 2 = 0.52 ± 0.05 . Electrical resistivity measurements show Tc = 560 ± 5 K and the onset of crystallization at 700 K which gets completed at 760 K (heating rate 5 K/min.). Thermoelectric power, S, is found to be negative with a maximum |S| = −3.2 μV/K. It has a temperature dependence similar to other iron-rich metallic glasses observed earlier.

ANNEALING AND CRYSTALLIZATION BEHAVIOUR OF Fe67Cc18B14Si1

Annealing and crystallization behaviour of amorphous Fe67Co18B14Si1 alloy have been investigated by Mossbauer spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mossbauer measurements on as received samples annealed upto 573 K for 8,12,24,48 and 96 hrs, separately and at 673 K for 4 and 8 hrs do not show any significant change in the hyperfine parameters indicating no change in topological and chemical short range order. Relief of internal stress on annealing changes the direction of the magnetization axis slightly which, in general, is found to be out of the ribbon plane. The sample becomes crystallized when annealed at 673 K for 12 hrs showing the presence of three sextets with hyperfine fields 363 270 and 213 kOe. XRD and TEM studies on the crystallized sample indicate precipitation of α-Fe and Fe2B phases but Mossbauer study on the sample annealed at 673 K for 12 hrs clearly indicates the presence of α-(Fe0.7Co0.3), (Fe0.3Co0.7)2B and (Fe-Co)3B phases.

Studies on glassy ferromagnet Fe67Co18B14Si1

57Fe Mossbauer spectroscopy has been used to study magnetic properties, annealing, and crystallization behavior of metallic glass Fe67Co18B14Si1 (Metglas 2605C0). Its crystallization temperature, Tx, is observed to be 650 K. The Curie temperature, estimated from temperature dependence of magnetic hyperfine field, is found to be 830±10 K. Probability distribution of hyperfine fields P(H) shows a single peak of FWHM 91 kOe which decreases with increase in temperature. Effect of annealing at temperatures from 373 to 673 K for 4 to 96 h indicates that the sample gets crystallized at 673 K after heat treating for longer periods (∼12 h), a temperature higher than Tx (650 K) obtained from temperature dependence of Mossbauer spectra indicating dependence of Tx on heat treatment. The ‘‘as received’’ sample when annealed at 673 K for 12 h or more shows the presence of α‐(Fe0.7Co0.3) (Fe0.3Co0.7)2B, and (Fe–Co)3B phases. However, the sample used for the temperature dependence of Mossbauer spectra up to 900 K shows t...

Thermoelectric behaviour of amorphous magnetic alloys

The Thermoelectric emfs of thermocouples formed by amorphous METGLAS 2826 (Fe40Ni40P14B6) and METGLAS 2826B (Fe29Ni49P14B6Si2) with standard thermocouple wires like copper, chromel, alumel, etc., were measured as a function of temperature between −196° C and 30° C to assess their suitability as thermoelectric temperature sensors. Thermoelectric emfs generated by METGLAS 2826/Cu and METGLAS 2826B/Cu thermocouples at −196° C are about an order of magnitude smaller when compared to thermal emfs of a standard copper/constantan thermocouple at the same temperature.