R. Kamal

A Mössbauer spectroscopic study of Nd2(Fe1−xCox)14B at x = 0.13 between 100 K and 700 K

Abstract 57Fe Mossbauer spectra of Nd2(Fe1−xCox)14B at x = 0.13 were recorded in the temperature range 100 K–700 K. The Curie temperature was found to be 675(5) K and the temperature coefficient of the internal magnetic field B hf averaged over various iron unit cell sites was found to be −0.05% K−1 near 300 K. Both of these results were in agreement with vibrating sample magnetometer (VSM) measurements of the magnetic moment μ for this sample. It was observed that between 150 K and 450 K, B hf and μ decreased almost linearly with increasing temperature. It was noticed that the iron atoms with respect to the cobalt atom had a preference for the j2 site as has been found in an earlier study of Nd2Co14B(57Fe). The Bhf temperature dependence at different iron sites was also measured and is discussed. The temperature dependence was found to be smallest for j2 when compared with the other sites. It appeared that at j2 sites there were no atoms of cobalt. The spin reorientation temperature did not appear to change with the substitution of 13% iron by cobalt in Nd2Fe14B.

Interpretation of Mössbauer effect measurements near the spin-reorientation temperature and the electric field gradient tensor of Nd2Fe14B

Abstract Electric field gradient (efg) tensor axes and signs at various crystallographic sites of Nd2Fe14B have been used for the interpretation of Mossbauer effect measurements near the spin-reorientation temperature Ts, i.e. at 150 and 100 K. It is observed that not only the variation of quadrupole splitting values DQm following spin reorientation but also the relative magnitude of DQm at different iron atom sites could be predicted by efg tensor analysis based on a point-charge model. It is also observed that the magnetic hyperfine field at different iron atom sites exhibits variations near Ts similar to those in Tm2Fe14B.

Magnetic hyperfine fields of (Nd2Fe1-xCox)14B AT x = 0.25 between 100 and 780 K

Abstract The 57 Fe Mossbauer spectra are recorded in Nd 2 (Fe 1- x Co x ) 14 B at x = 0.25 in the temperature range 100 to 780 K. T c the Curie temperature, B hf , the magnetic hyperfine field average over various Fe nuclei of the unit cell and its temperature coefficient α( B hf ) in the vicinity of 300 K are found to be 760(5) K, 34.0(3) T and -0.08(1)% K -1 , respectively. The magnetic moment at Fe atoms is estimated to increase up to 12% as a result of the partial substitution by Co atoms. The dependence of the fields upon temperature is observed to be least at the j 2 and k 2 sites as compared to the other sites of Fe. The results for the variation of B hf at all of the six sites of Fe with respect to temperature are given. A site preference of Fe atoms for the j 2 sites is observed.

Study of the influence of titanium on the NdFeCoB magnetic phase by Mössbauer spectroscopic measurements between 100 and 650 K

Abstract Elements in atomic ratios corresponding to Nd2(Fe0.8Co0.1Ti0.1)14B were melted and annealed. A Mossbauer spectroscopic study was then carried out between 100 and 650 K. The Curie temperature Tc measured from vibrational sample magnetometric measurements of the magnetic moment \ gm with temperature is found to be 650(10) K. The effect of titanium on Tc appears to be insignificant. However, the Mossbauer spectra reveal two transitions, one at low temperature (about 275 K) and the other at high temperature (above 650 K). The spectra could be analysed in terms of Nd2(Fe, Co)14B as the main phase and TiFe2 and Nd1.1Fe4B4 as the sister phases. The fraction of iron distributed among the various phases is also estimated from the spectra. It appears that titanium also inhibits the formation of Nd2Fe17 and forms a Laves phase TiFe2 with iron atoms. Titanium could, therefore, be helpful in preventing the formation of the low coercivity iron-rich structures in NdFeCoB magnets. The technique adopted here for the identification of various phases could be extended to study the influence of elements other than titanium in these magnets.

Magnetic hyperfine field changes at different Fe-sites below spin reorientation temperature in polycrystalline untextured Nd2Fe14B

Abstract In order to estimate the magnetic hyperfine fields at different sites, an analysis of the Mossbauer spectra of polycrystalline untextured Nd 2 Fe 14 B, recorded at temperatures above and below the spin reorientation temperature T s (≈150 K), has been made. Here the signs of the electric field gradients (efg) and the principal axes of efg components have been constrained according to the available crystal structure information below and above T s . It has been observed that the magnetic hyperfine field changes are similar to the recently reported magnetic moment changes at different sites in an isostructural alloy Tm 2 Fe 14 B.

Effect of substitution of vanadium in Nd2Fe14B studied by Mössbauer spectroscopy

Abstract The effect of an atomic replacement of iron by vanadium in Nd2Fe14B is studied. The temperature dependence of the hyperfine field averaged over all the 57Fe sites, B hf , was determined from Mossbauer spectroscopic measurements between 150 and 650 K in Nd2(Fe0.9V0.1)14B. It is observed that both B hf and the Curie temperature, Tc, increase upon substitution by vanadium atoms. This behaviour of vanadium in Nd2Fe14B is in contrast to that in α-Fe and other iron based alloys, where the presence of vanadium neighbours serves to decrease B hf . The variation of other hyperfine parameters at 57Fe nuclei is also studied as a function of temperature. A comparison is made between the substitution by vanadium in Nd2Fe14B and substitution by other elements.

Reduction of magnetic hyperfine fields and Curie temperature on substitution of silicon and aluminium in compounds of the type Nd2Fe14B

Etude par spectroscopie Mossbauer de composes de composition proche de Nd 2 (Fe 0,67 Si 0,33 ) 14 B et Nd 2 (Fe 0,67 Al 0,33 ) 14 B entre 100 et 295 K

Mössbauer spectroscopy investigation of the magnetic characteristics of Nd2(Fe0.8Co0.1Mn0.1)14B

Abstract The 57 Mossbauer spectra of Nd 2 (Fe 0.8 Co 0.1 Mn 0.1 ) 14 B in the temperature range of 100 to 700 K reveal several interesting features. T c , the Curie temperature, measured by the collapse of internal magnetic-hyperfine field ( B hf ) as well as by the method of thermal scanning of count rates in the central region of the spectra is found to be 625(5) K. This is in agreement with the value obtained from vibrational sample magnetometer measurements on the same sample. No evidence for spin reorientation like one in Nd 2 Fe 14 B could be obtained from our measurements. Similarly, no site preference of Mn atoms for particular Fe-sites could be observed from the present study. B hf , the average field, on 57 Fe nuclei at 300 K is 28.5(3) T. The magnetic moment of Fe atoms is estimated to decrease by ∼6% as a result of substitution of Mn and Co atoms by 10% each.

A Mössbauer spectroscopic study of the effect of molybdenum during preparation of Nd-Fe-B magnets

Abstract Elements in atomic ratios Nd2(Fe0.9Mo0.1)14B were melted in an induction furnace and annealed in order to examine the effect of molybdenum during the preparation of Nd-Fe-B magnets. A phase analysis has been made from 57 Fe Mossbauer spectroscopic measurements in the temperature range of 100 to 700 K. It is found that two iron containing phases are formed, one Mo rich Fe alloy and the other Nd-Fe(Mo)-B isostructural to Nd2Fe14B. The Nd1.1Fe4B4 phase usually found in Nd-Fe-B magnets is not observed in these samples. The Curie temperature, Tc, is found to be 605(5) K from the Mossbauer as well as vibrational sample magnetometer measurements on the same sample. At low temperatures, the average hyperfine field at Fe nuclei is found to show a decrease with respect to the value for Nd2Fe14B.

A Mössbauer spectroscopic study of the effect of molybdenum on the Nd-Fe-Co-B alloy

Abstract Elements in the atomic ratios Nd2(Fe0.8Co0.1Mo0.1)14B were melted and annealed. The 57Fe Mossbauer spectra were then recorded in the temperature range of 100 to 780 K. The molybdenum is found to enter into the main Nd2Fe14B tetragonal phase by replacing some of the iron atoms. A fraction of molybdenum is found to be present as a molybdenum-rich iron alloy. The Curie temperature is found to be increased to over 500°C, and the temperature coefficient of the average internal hyperfine field on the iron nuclei is reduced significantly. The spin-reorientation temperature is found to be unaffected by the presence of molybdenum atoms in the Nd2Fe14B unit cell. The site preference of iron atoms for the j2 site is the same as found in Nd-Fe-Co-B alloys in the previous work.