study of alnico 8 by mössbauer spectroscopy

6
E. F. MAKAROV e t al.: Study of Alnico 8 by Miissbauer Spectroscopj- 45 phys. stat. sol. 24, 45 (1967) Subject classification: 6.1; 18.2; 21.1.1 Institute of Chemical Physics, Academy of Sciences of the USSR, Moscozc (a), and V N I I Elektromekhaniki, Moscow (b) Study of Alnico 8 by Mossbauer Spectroscopy BY E. F. MAKAROV (a), V. A. POVITSKII (a), E. B. GRANOVSKII (b), and A. A. FRIDMAN (b) The hard magnetic alloy "alnico 8" is investigated by Mossbauer spectroscopy. Two non-equivalent positions are found for the Fe atoms in the hardened alloy. After tempering and a magnetic-field heat treatment cycle (MFT) some of the Fe atoms are found to have a paramagnetic environment. The MFT has the effect of increasing the fraction of the Fe atoms which have such an environment and a maximum value of 7% is obtained for this fraction. The magnetic fields at the Fe67 nuclei are determined for various states of the alloy. The results are discussed together with the results of Mossbauer investigations of "alnico 5". Die magnetisch harte Legierung ,,alnico 8" wird mit MoBbauerspektroskopie untersucht. Zwei nichtaquivalente Platze fur die Fe-Atome werden in der abgeschreckten Legierung gefunden. Nach dem Anlassen und einem thermomagnetischen Behandlungszyklus (MFT) wird gefunden, da13 ein Teil der Fe-Atome sich in einer paramagnetischen Umgebung be- findet. Der MFT-Zyklus bewirkt ein Anwachsen des Anteils der Fe-Atome, die eine solche Umgebung besitzen. Ein Maximalwert von 7% wird fur diesen Anteil erhalten. Die inneren magnetischen Felder am Fe5'-Kern werden fur verschiedene Zustande der Legierung be- stimmt. Die Ergebnisse werden zusammen mit Ergebnissen von MoBbauer-Untersuchungen a n ,,alnico 5" diskutiert. 1. Introduction Hard magnetic alnico 8 alloys get the optimum magnetic properties after a magnetic field heat treatment cycle (MFT), i.e. isothermal quenching in the magnetic field followed by two-stage tempering. As was shown by electron microscopy and X-ray analyses, the alloy in this state has u + u' duplex structure [l], in which particles of one of the phases, elongated in the magnetic field direction, are uniformly distributed within the second phase. The tetra- gonal body-centered lattices of both phases are so connected that the common for both lattices C-axes have equal periods and coincide with the long axes of the particles [2, 31. The particle size in alnico 8 alloy is 300 x 300 x 4000 A on the average [3j. The high coercitivity of the alnico alloys is explained theoretically by the fact that isolated precipitates of one of the phases have high magnetization, the second phase has low magnetization, and magnetization reversal processes in an external field proceed according to the Stoner-Wohlfarth model 141. But because of the structure dispersity the experimental investigations of the magnetic nature and chemical composition of the phases were unsuccessful up to the present. By means of magnetic measurements on alnico 5, whose composition is near to that of alnico 8, it was found that it possesses two phases in the high coerci-

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Page 1: Study of Alnico 8 by Mössbauer Spectroscopy

E. F. MAKAROV e t al.: Study of Alnico 8 by Miissbauer Spectroscopj- 45

phys. stat. sol. 24, 45 (1967)

Subject classification: 6.1; 18.2; 21.1.1

Institute of Chemical Physics, Academy of Sciences of the USSR, Moscozc (a) , and V N I I Elektromekhaniki, Moscow ( b )

Study of Alnico 8 by Mossbauer Spectroscopy BY

E. F. MAKAROV (a), V. A. POVITSKII (a), E. B. GRANOVSKII (b), and A. A. FRIDMAN (b)

The hard magnetic alloy "alnico 8" is investigated by Mossbauer spectroscopy. Two non-equivalent positions are found for the Fe atoms in the hardened alloy. After tempering and a magnetic-field heat treatment cycle (MFT) some of the Fe atoms are found t o have a paramagnetic environment. The MFT has the effect of increasing the fraction of the Fe atoms which have such an environment and a maximum value of 7% is obtained for this fraction. The magnetic fields a t the Fe67 nuclei are determined for various states of the alloy. The results are discussed together with the results of Mossbauer investigations of "alnico 5".

Die magnetisch harte Legierung ,,alnico 8" wird mit MoBbauerspektroskopie untersucht. Zwei nichtaquivalente Platze fur die Fe-Atome werden in der abgeschreckten Legierung gefunden. Nach dem Anlassen und einem thermomagnetischen Behandlungszyklus (MFT) wird gefunden, da13 ein Teil der Fe-Atome sich in einer paramagnetischen Umgebung be- findet. Der MFT-Zyklus bewirkt ein Anwachsen des Anteils der Fe-Atome, die eine solche Umgebung besitzen. Ein Maximalwert von 7% wird fur diesen Anteil erhalten. Die inneren magnetischen Felder am Fe5'-Kern werden fur verschiedene Zustande der Legierung be- stimmt. Die Ergebnisse werden zusammen mit Ergebnissen von MoBbauer-Untersuchungen a n ,,alnico 5" diskutiert.

1. Introduction Hard magnetic alnico 8 alloys get the optimum magnetic properties after

a magnetic field heat treatment cycle (MFT), i.e. isothermal quenching in the magnetic field followed by two-stage tempering. As was shown by electron microscopy and X-ray analyses, the alloy in this state has u + u' duplex structure [l], in which particles of one of the phases, elongated in the magnetic field direction, are uniformly distributed within the second phase. The tetra- gonal body-centered lattices of both phases are so connected that the common for both lattices C-axes have equal periods and coincide with the long axes of the particles [2, 31. The particle size in alnico 8 alloy is 300 x 300 x 4000 A on the average [3j.

The high coercitivity of the alnico alloys is explained theoretically by the fact that isolated precipitates of one of the phases have high magnetization, the second phase has low magnetization, and magnetization reversal processes in an external field proceed according to the Stoner-Wohlfarth model 141. But because of the structure dispersity the experimental investigations of the magnetic nature and chemical composition of the phases were unsuccessful up to the present.

By means of magnetic measurements on alnico 5, whose composition is near to that of alnico 8, it was found that it possesses two phases in the high coerci-

Page 2: Study of Alnico 8 by Mössbauer Spectroscopy

46 E. F. MAKAROV, V. A. POVITSKII, E. B. GRANOWKII, and A. A. FRIDMAN

tivity state which are ferromagnetic with magnetizations of 1600 and 100 G a t room temperature [5]. Mossbauer measurements on alnico 5 gave more precise results. It was found that part of the Fe atoms is located in paramagnetic surroundings [6, 71. The discovered broadening of the lines of a quenched specimen in comparison with the heat-treated one is connected with changes of the phase structure. These and other results of [6, 71 have shown the per- spectives of Mossbauer measurements for a study of this type of alloys.

The purpose of this paper is to study alnico 8 by the Mossbauer technique.

2. Samples and Experimental Procedure An alnicoalloycontaining 33.2 a t %Fe, 31.5aty0Co, 12.6aty0Ni, 13.8atyoA1,

3.3 a t% Cu, and 5.6 atyo Ti was prepared from materials of high purity by high frequency melting in vacuum. The samples were subjected to different heat treatments: No. 1 - for 1 h a t 1200 "C in hydrogen followed by quenching in water, No. 2 and No. 3 - the same quenching followed by tempering for 30 min a t 800 and 820 "C respectively, No. 4 - MFT, i.e. quenching from 1200 to 800 "C, followed by an exposure to a magnetic field a t that bemperature for 12 min, tempering for 5 h at 650 "C and for 20 h a t 550 "C. The samples were ground in a hard metal ball mill for 24 h. The particle size was smaller than 0.025 mm. X-ray diffraction analyses demonstrated no oxidation of the powder, additional Debye-Scherrer lines belong to WC. The absorbers were prepared from powders, their Fe density being 0.01 glcm2. The measurement of the Mossbauer spectra were done with an electrodynamical transducer in the con- stant acceleration regime. The scintillation y-spectrometer with a multichannel analyser served as registrating system. The source (c05~ in Cr matrix) held a t room temperature.

X-ray diffraction data were obtained from powders and single crystals using the radiation of a Co anode.

3. Experimental Results Fig. 1 shows Mossbauer spectra of alnico 8 a t room temperature. The spectrum

of the quenched specimen (curve (a)) represent,s overlapping of a t least two systems of hyperfine magnetic spectra, which is clearly seen from the first

Yig. 1. JIGssbarier sprvtra of the alnico 8 alloy after different heat treatments.

(a) Temperiiig at 1240 "C for 30 min and quenching in water.

(h) tempering at 800 "C for 30 mill. - I 0 7 (c) temperilia at 820 "C for 30 min,

(d) and ( e >magnetic field heat treatnient cycle veocity (mm s-1 rehtive to sfaidess steef) -

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Study of Alnico 8 by Mossbauer Spectroscopy 47

and sixth lines of the whole spectrum. These systems show the existence of two magnetically non-equivalent positions of iron within the specimen. The effective magnetic field H,ff acting on the Fe5' nuclei is (306 & 7) kOe in the first position and (260 10) kOe in the second one, which makes 0.93 and 0.77 of the value of H,ff in armco-Fe (H:;). The broadening of the spectral lines shows that apart from the two principal positions of Fe atoms in the alloy there are other positions in which less quantity of atoms is situated; therefore corresponding lines are not resolved.

The Mossbauer spectrum of specimen 2, tempered at 800 "C (curve (b)), is interpreted analogously; Heff increases up to (324 & 7) and (296 & 10) kOe. Notice that the central part of the spectrum changed : in comparison with curve (a) the resolution of the central lines became worse and the intensity of the fourth line somewhat increased. I n specimen 3, tempered a t 820 "C, both systems of lines superimpose that results in an essential broadening of the lines (curve (c)). The average value of Heff is (320 & 10) kOe, the fourth line is much wider than the third one.

The Mossbauer spectrum of specimen 4 after MFT [curve (d)) has-apart from a widened six-line spectrum with H,ff = (330 & 10) kOe-an additional narrow line in the central part of the spectrum, the appearance of which testi- fies that part of the iron atoms has paramagnetic surroundings. The fourth line of the main system is more widened than the third one, just as in specimen 3. Specimen 4 was also investigated a t -190 "C. It was found that a t liquid nitrogen temperature the intensity of the paramagnetic line slightly decreases.

X-rays data showed that the quenched specimen 1 has a Fe-Al-type lattice of a modulated (periodic and crystallographically oriented) structure ; there are satellites around the reflections of the matrix in the X-ray patterns due to the periodic change of the matrix composition in (100) direction, the modulation period being 70 to 100 A. The small intensity of satellites indicates that the composition difference is small. The period of the solid solution is 2.873 A . After having tempered a t 800 "C there are reflections from two body- centered cubic phases with parameters of 2.885 and 2.859 A in the X-ray patterns. The phase with the smaller parameter (u) has an Fe,Al type of or- dering and is enriched with Co and Fe [S], and the phase with the greater para- meter (01' ) has an FeAl type of ordering and is enriched with Ni and Al. Tem- pering at 820 "C slightly changes the quantitative phase ration.

The structure of specimen 4 was described in the introduction. The lattice parameters of the tetragonal phases are: C, = C, = 2.873 A, a, = 2.856 A,. a2 = 2.910 A.

4. Discussion After quenching in water the alloy has the FeAl-type lattice. After quenching

in oil with a less rate of cooling the Fe,Al-lattice reflections are observed. Thus one can suppose that after quenching in water there is a short-range ordering of the Fe,Al type. In the FeAl-type lattice with the degree of order near unity almost all A1 atoms are positioned in the cube centres (D-sites). 50% of atoms are to be in D-sites, but there are only 14 atyo A1 in our alloy. Analysing the X-ray diffraction intensity the authors of [9] concluded that the other D-sites are occupied by Fe and Ti atoms and the remaining atoms-Co, Ni, and Cu- occupy the vertices of the cubes (A-sites). However, we observe in the spectrum two different positions of Fe atoms in the quenched specimen. One can con-

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48 E. F. MAKAROV, V. A. POVITSKII, E. B. GRANOWKII, and A. A. FRIDMAN

clude that Fe atoms occupy D- and A-sites. Fe atoms in A-sites have eight nearest neighbours: Al, Ti, Fe and partly Co and Ni atoms substituting some Fe atoms, in the second coordination sphere we have six neighbours: Fe, Co and Ni atoms. Fe atoms in D-sites have also eight nearest neighbours: Fe, Co and Ni, and in the second sphere the neighbours are Al, Ti, and Fe (Co, Ni) atoms. Thus the two possible sites of Fe atoms differ in principle because all nearest neighbours of atoms in D-sites are ferromagnetic and those of atoms in A-sites are both ferromagnetic and non-magnetic (A1 and Ti). It is known that the nearest neighbours have the greatest influence on the value of He.. acting on Fe5' [lo]. One A1 or Ti atom in the first coordination sphere diminishes H,ff by about 7 % [lo], Ni atoms (less than 27 atyo) have no influence, and H,R increases linearly with Co addition up to 20 atyo and than decreases to the initial value 1111. These results were obtained on binary solid solutions. Sup- posing the influence of different atoms on Heff to be additive the state with a large value of H,ff (0.93 HF;) corresponds to Fe atoms in D-sites, the other with a small value (0.77 H g ) corresponds to Fe atoms in A-sites.

Comparing the areas of the extreme lines of the spectrum (curve (a)) one can estimate the quantity of Fe atoms in A- and D-sites. It turns out that Fe atoms occupy preferably D-sites (70%).

Thus the quenched alnico 8 alloy can be identified with the binary alloy, where Fe, Co, and Ni atoms occupy Fe sites and Al, Ti, and Cu atoms occupy A1 sites. I n that case alnico 8 corresponds to the alloy Fe-23y0 Al. The in- vestigation of this alloy [12] demonstrated its spectrum to be a superposition of some systems of lines with H,ff equal to 0.92 Hrg for one of them and 0.78 H Z and 0.67 Hz$ for the other which corresponds to the spectrum observed in this work. The first system of lines corresponds to D-sites, the second one to A-sites with a different number of A1 atoms in the first coordination sphere. The fact that only one possible site of Fe atoms is found in [9] is evidently accounted for by a greater concentration inhomogeneity of this specimen as the authors subjected the tempered specimen to ageing a t 650 "C for 1 h in order to increase the intensity of superstructural reflections. Such a treatment results in a clearly seen modulated structure that explains the appearance of high intensity satel- lites in the X-ray spectra.

The change of the spectrum a t solid solution disintegration during quenching can be explained by the fact that the u'-phase enriches with Ni and A1 and the a-phase enriches with Fe and Co. Evidently, because of the concentration inhomogeneity, some of the Fe atoms in the u'-phase are in paramagnetic regions at room temperature that results in the observed changes in the central part of the spectra (curves (b) and (c)). As shown in [12] for this transition it is neces- sary for the Fe atoms in A-sites to have more than five A1 atoms in the first coordination sphere. As the a'-phase enriches in A1 the fraction of these Fe atoms increases and a single line (curves (d) and (e)) appears in the spectrum of specimen 4 subjected to an entire MFT. It should be emphasized that this line corresponds not to all Fe atoms in the a'-phase but only to those having five or more A1 atoms in the first coordination sphere. Such atoms in specimen 4 make 7% of the total number of Fe atoms in the alloy. Taking into account that the exposure time a t 800 "C during heat treatment is only 12 min in com- parison with 30 min for specimen 2 i t is evident that low-temperature quenching during MFT is of considerable significance for building up paramagnetic regions. The small decrease of the intensity of a paramagnetic line from room to liquid

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Study of Alnico 8 by Mossbauer Spectroscopy 49

nitrogen temperature shows that the majority of paramagnetic regions a t the latter temperature does not yet become magnetically ordered. A paramagnetic line was found also in alnico 5 after MPT [6, 71, and the line intensity also diminished slightly when cooled. The authors of [6] separated the para- magnetic phase of alnico 5 by sedimentation. The spectrum of this phase shows two quadrupole peaks which do not vary between - 190 and +20 "C. The quadrupole doublet also exists evidently in alnico 8. One can see it from a widen- ing of the fourth spectral line of samples 2 to 4. Assuming this widening to be caused by a superposition of the second doublet line on the fourth spectral line one obtains a doublet splitting of approximately 0.8 mm/s and an isomeric shift of +0.3 mm/s with respect to stainless steel. These values coincide with the corresponding parameters of alnico 5 [el, the isomeric shift corresponds to that of FeAl which is equal to +0.36 mm/s [13].

The quadrupole effects in both alnico 5 and 8 must have identical origin. Evidently it cannot be explained by tetragonal distortions of the a'-phase lattice because the degree of tetragonality for both alloys is very different [3]. Another possible cause of this doublet could be the existence of FeO in the specimens, but as it was stated above a special X-ray analyses didn't reveal FeO. A recently found quadrupole splitting in the compound FeSi having a cubic lattice [14] can serve as some analogy. The nature of the splitting in FeSi is explained in [14] by redistribution of Fe valence electrons.

The A1 content descreases in the a-phase during quenching. This will effect Fe atoms in A-sites, first of all. The number of Fe atoms with three and two non-magnetic atoms in the first coordination sphere will increase (that causes the appearance of additional line systems), and, as a result, the average value of H,ff for A-sites will increase. H,ff for Fe atoms in D-sites will increase less than is connected with the decrease of the number of A1 atoms in the second coordination sphere. So both line systems superimpose which results in a broa- dening of the spectral lines.

I n the same manner one can explain the fact that the Mossbauer investi- gations did not permit, to separate the two types of Fe atoms [6, 71 for the tempered alnico 5 having in FeAl type of ordering. However, a considerable broadening of the spectral lines was noted. As there is no Ti in alnico 5 the total number of non-magnetic atoms decreases up to 17% and both systems super- impose.

The average value of H,,f in alnico 8 after MFT was found to be less than in alnico 5 (300 and 350 kOe respectively). Consequently, the magnetic moments of Fe atoms in alnico 8 are less which corresponds to the decrease of magneti- zation of alnico 8 by 10 t o 20% in comparison with alnico 5.

5. Conclusions

Fe atoms in the hardened alnico 8 alloy have two non-equivalent positions: one third of the atoms are in bhe cube vertices and two thirds are in the centres. The effective magnetic fields acting on Fe5' nuclei in these positions are 306 and 260 kOe respectively. After quenching one can see only a broadening of the spectra lines, and H,f f is 320 kOe after quenching from 820 "C. This is connected with the decrease in the A1 content in the a-phase. It increases in the a'-phase which causes the appearance of paramagnetic regions in the a'- 4 physica 24/1

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50 E. F. MAKAROV et al. : Study of Alnico 8 by Mossbauer Spectroscopy

phase. After the magnetic field heat treatment about 7 % of the Fe atoms are in paramagnetic surroundings. The other Fe atoms are in ferromagnetic regions which corresponds to H,ff = 330 kOe in comparison with 350 kOe in alnico 5.

Acknowledgemetit

We wish to thank Prof. V. I. Goldanskii for his interest in this work and 'Yu. V. Baldohin and Yu. M. Rabinovich for their extensive assistance in the experiments.

References [l] A. KOCH, M. V. DE STEEG, and K. DE Vos, Ber. Brbeitsgem. Ferromagnetismus 130

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i Metallovedenie 23, 444 (1967). [4] E. C. STONER and E. P. WOHLFART, Phil. Trans. Roy. SOC. (London) 0.10, 599 (1948). [5] A. S. ERMOLENKO and I. S. SHUR, Fiz. Metallov i Metallovedenie 17 , 31 (1964). [6] S. SHTRIKMAN and D. TREVES, J. appl. Phys. 37, 1103 (1966). [7] J. S. WIERENCEN and J. G. RENSEN, Z. angew. Phys. 21,69 (1966). [8] Yu. D. TYAPKIN and J. G . EROSHENKOVA-LUKaNINA, Dokl. Akad. Nauk SssR 160,325

[9] M. P. ARBUZOV, A. A. PAVLUKOV, and B. V. HAENKO, Fiz. Metallov i Metallovedenie

[lo] G. K. WERTHEIM, V. JACCARINO, J. H. WERNICK, and D. N. E. BUCHANAN, Phys. Rev.

[ll] C. E. JOHNSON, M. S. RIDOUT, and T. E. CRANSHAW, Proc. Phys. SOC. 81, 1079 (1963). [12] M. B. STEARNS, J. appl. Phys. 35, 1095 (1964). [13] K. ONO, Y. ISHIKAWA. and A. ITO, J. Phys. SOC. Japan 17 . 1747 (1962). [14] G. K. WERTHEIM, V. JACCARINO, J. H. WERNICK, J. A. SEITCHIC, H. J. WILLIAMS, and

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(Received June 19, 1967)