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OBSERVATIONS ON THE MICROSTRUCTURE AND MAGNETIZATION OF ALNICO PERMANENT
MAGNETS
M. Hetherington, A. Cerezo, J. Jakubovics, G. Smith
To cite this version:
M. Hetherington, A. Cerezo, J. Jakubovics, G. Smith. OBSERVATIONS ON THE MICROSTRUC- TURE AND MAGNETIZATION OF ALNICO PERMANENT MAGNETS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-429-C9-434. �10.1051/jphyscol:1984971�. �jpa-00224459�
JOURNAL DE PHYSIQUE
Colloque C9, supplément au n012, Tome 45, décembre 1984 page C9-429
OBSERVATIONS ON THE MICROSTRUCTURE AND MAGNETIZATION OF ALNICO PERMANENT MAGNETS
M.G. Hetherington, A. Cerezo, J.P. Jakubovics and G.D.W. Smith Department o f MetaZZurgy and Science of Matemals, Parks Road, Oxford 0x1 3PH, U.K.
Résumé - Des spécimens d'Alnico5 et 7 ont été étudiés avec l'aide de la microscopie ionique dechamp, de la microanalyse parsonde atomique et de la microscopie électronique haute tension. La composition et la morphologie des phases ont été déterminées avec précision. Ces résultats ont montré que les changements d'aimantation sont produits par le mouvement "Bloch wall".
Abstract - Alnico5 and 7 specimens have been studied by field ion microscopy, atom probe microanalysis and high voltage electron microscopy.
The composition and morphology of the phases was accurately determined. From the results, it was shown that magnetization changes occurred by Bloch wall motion.
1. INTRODUCTION
Alnico has been one of the most important permanent magnet materials for the past forty years and despite recent developments remains of great technological significance. However, the explahation of its magnetic properties is open to criticism. The magnetization process is currently interpreted in terms of single-domain behaviour of a system of isolated, single-domain particles. The simplest process by which the magnetization could change in such a system is by coherent rotation /1/. However, calculations using this model give incorrect values for the coercivity and for the angular dependence of al1 important parameters. A more sophisticated version of this theory allows magnetization to change by fanning, buckling or curling /2-6/. This model gives more accurate numerical values of the important properties and their angular variation. However, it cannot prov'ide any explanation for the 'interaction' domains observed by many workers /7-Il/.
Two further shortcomings are worth noting. First, the assumption of no interactions between particles is completely unjustified because of their interconnectivity (discussed further in this paper), and because most measurements of saturation magnetization as a function of temperature show two Curie temperatures /12-14/, indicating that both phases are magnetic. Second, the calculations which previous workers have made assume a demagnetizing factor of the form
Ha 1 - p,
where p is the volume fraction of the magnetic phase. Néel /15/ assumed a priori a relation of this form. Compaan and Zijlstra /16/ provided a proof of this formula.
However, their assumption that the system is isotropie is not applicable to Alnico.
In this paper, measurements made on the Field Ion Microscope (F.I.M.), Atom Probe (A.P.) and High Voltage Electron Microscope (H.V.E.M.) are shown to lead to a new interpretation of the magnetic properties of Alnico. Both the main types of anisotropic Alnico were studied (Alnico5 and the higher coercivity Alnico7) in order to explain the differences between their properties.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984971
JOURNAL DE PHYSIQUE
2. FIELD ION MICROSCOPY
Specimens were prepared for F.I.M. by electrochemical polishing in a solution of 75% acetic acid and 25% perchloric acid. The new V.G. FIMIOO in Oxford was used to obtain microanalysis measurements using a time of flight detector. Images were obtained using a pressure of 1ow5mb of neon, voltages applied to the tip were in the range 3-20kV.
Fig. 1 shows a number of images of the material. The bright and dark regions consist of an Fe-Co rich phase and an Ni-A1 rich phase respectively. The analysis of each of these phases is shown in the Table. The Oxford FIMIOO has several advantages over other instruments used up to now. It has a precision machined Poschenrieder-type energy compensator enabling extremely accurate measurements to
/
A1 Fe1
Co1
Ni1
CuFe-Co rich 1 at.% 1 7.3 1 59.6 1 22.8 1 10.2 1 0.4
be made. Furthermore the vacuum in the microscope is 10-llmb, al1 but eliminating the hydride problem. Care was taken to ensure that the pulse fraction was sufficiently high to eliminate inaccuracies due to preferential evaporation of any element in this complicated system. The spectra for each phase are reproduced in Fig. 2 showing the extremely high resolution. Calculations of the relative isotope fraction indicate that hydrides form less than 1% of the total ions caught. It is likely therefore that the present analysis is more reliable than'that of Zhu et al /17/, from which it differs in several respects (1). The Fe-Co phase images more brightly because it is more prominent, which also results in a higher magnification. The probe hole therefore covers a larger area of the Ni-A1 phase and a higher ion rate is observed for a given pulse fraction and d.c. voltage. However,
this means that care must be taken in estimating the relative volume fraction of each phase. This can only be obtained by calculations using the results for the relative proportions of each element in the dark and light phases. These calculations indicate a volume fraction of 81% for the bright phase, slightly higher than estimates obtained by electron microscopy /12,18/. The present results are likely to be more accurate /19/.
By 'probing down a superlattice pole, it has been possible to examine the ordering of this material. Fig. 3 shows an image with a [100] pole in the Fe-Co phase in the centre. Alternate layers are seen to image differently. Iron atoms appear brighter and the layer consisting of cobalt (plus aluminium, nickel and iron) shows up as a much fainter ring. This shows clear evidence of ordering in the Fe-Co phase. When d.c. evaporation is performed the evidence for ordering becomes even clearer as each layer has a different evaporation rate. In order to confirm these observations the atom probe was used. The channel plate was set to the maximum distance from the tip so that only atoms £rom one layer were caught in the time of flight analyser.
Fig. 4. shows an autocorrelation between alternate layers of iron, then cobalt (plus aluminium etc.). Both autocorrelation curves give a result for the wavelength of =60 atoms which is also the average number of atoms caught as two terraces collapse past the probe hole. A small size of sample (two ions) was taken in order to eliminate 'beats'. A ladder diagram also shows the ordering very elegantly. It is not clear whether the ordering is of a B2-type (FeCo) or of a DOgTtype (Fe3Co).
Some of Our results suggest that it may be a modulated structure conslsting of both phases, the average composition of the phase being between these two. Work is continuing to establish this.
This result resolves a 15-year disagreement: Bronner et al 1201 and Pfeiffer 1211 suggested that the Fe-Co phase was disordered, whereas Arbuzov and Pavlyukov /22/
suggested that it was ordered. Having independently confirmed the ordering by the F.I.M. and A.P., the electron microscopy of the material has been repeated.
Electron microscopy led Pfeiffer and Bronner to assume that the material was disordered. We obtained similar results to these workers, but they do not prove that the Fe-Co phase is disordered, but only that the Ni-A1 phase is ordered.
Observations of dark field images using a superlattice spot give images in which the contrast between the phases changes. Because of the similiarity of the scattering factors of iron and cobalt, this reversa1 in contrast would be expected even in a fully ordered material.
Considerable effort has been made to search for precipitation at the interphase boundaries in the material. It has been reported that Fe-A1 forms there 1211. No evidence has been found for this in Alnico5.
The series of micrographs in Fig. 1 show the interconnectivity of the particleS.The number under each micrograph represents the thickness of material evaporated since the previous exposure. Electron microscopy performed by ourselves and other workers show particles which appear to be lOOnm long by 30-40nm wide. The structure has a sponge-like texture, showing particles to be strongly interconnected. The depth scale indicated for these micrographs was obtained by counting the number of rings that have collapsed on a pole of known orientation. As is obvious £rom these micrographs the interweaving of particles occurs on a finer scale than is indicated by electron micrographs.
Fig. 5 shows Alnico5 heat treated for 48 hours at 680°C. Heat treatments at this temperature are generally regarded as reversible 123-251. The apparent change in the volume fraction is therefore due in part to the change in the relative local magnification of the dark and bright regions and underlines the care which must be taken in interpreting F.I. micrographs. It is significant that the superlattice pole shows that the Fe-Co phase is still ordered even after prolonged treatment at this elevated temperature. A new phase has been observed in the H.V.E.M. in the heat treated Alnico5 which forms long needles lying along [100] directions. It is hoped that the A.P. will establish the composition of this phase.
Measurements have been made on the older A. P. in Oxford which has poorer mass resolution than the FIM100, giving similar results to Zhu et al. However, it is difficult to resolve the cobalt peak in this instrument. However £rom a qualitive point of view it is worth noting an increase in the amount of ferromagnetic material in the dark phase. Although ordering is an important factor, it seems
likely that the Ni-A1 phase is more ferromagnetic than in Alnico5.
3. HIGH VOLTAGE ELECTRON MICROSCOPY
Lorentz microscopy (for a review see /26/) has been performed on Alnico in the A.E.I. EM7 H.V.E.M. at Oxford. Thin foils were placed outside the magnetic field of the objective lens. Fig. 6 shows the magnetic domains in Alnico5 and Fig. 7 shows, for the first time, the domains in Alnico7. The domains in Alnico7 contain only of the order of 100 particles, and therefore can only be observed by Lorentz microscopy on a high voltage electron microscope. The 1 MeV electrons are necessary since the domains only exist in specimens with a thickness greater than one 'particle' diameter thick. Using the magnetizing coils available in the Oxford H.V.E.M. /27/ domain wall motion has been observed on heat treated specimens.
4. CONCLUSIONS
The results and discussion above demonstrate the inadequacies of the present theoretical model for the change in magnetization. We are proposing a new model in which the magnetization changes through the movement of Bloch walls through the
Cg-432 JOURNAL DE PHYSIQUE
material. If we consider the change of magnetization £rom saturation in one direction to saturation in the opposite direction, initially reverse domains nucleate in the most isolated particles. Because of the exchange interaction with neighbouring particles these will extend into larger domains. When the reverse field is large enough Bloch walls are released £rom the pinning by the Ni-A1 phase and large catastrophic changes in the magnetization occur. Domains are long in the easy direction of magnetization, and end in points. If magnetization changes were occurring on a minor loop of the main hysteresis cycle, initially the points of these domains in the preferred direction will extend until once again the reverse field will liberate domain walls and large changes in magnetization will occur. The demagnetizing factor has been estimated by considering the Fourier components of the system, and was found to be of a form similar to that obtained by Stoner and Wohlfarth. This mode1 gives a reasonable value for the coercivity and correctly predicts the change in the coercivity with the direction of the applied field.
As discussed above, the Ni-A1 rich phase is more ferromagnetic in Alnico7 than in Alnico5. The increase in coercivity is due to the greater isolation and more regular arrangement of the particles.
Acknowledgments
We are grateful to Preformations Ltd and Swift-Levick Ltd for the provision of specimens.
References
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1'>
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SPECTRA FROM ALNICO 5
DARK PHASE
20nm 30nm
IMAGES OF ALNICO OBTAINED USlNG F.I.M. (B.I.V.
15 KV) DIRECTION OF ANISOTROPY IS PERPENDICULAR TO PLANE OF IMAGE
BRIGHT PHASE
l
Fig. 1
Fig. 2
[lOO] POLE IN BRIGHT PHASE
1
AUTOCORRELATION FUNCTIONS
Fig. 3 Fig. 4
R I k 1
COBALT
'-4-
--
A-,-,-- . i6alRON
C9-434 JOURNAL DE PHYSIQUE
a) F.I.M. IMAGE b)T.E.M. MICROGRAPH ALNICO 5 TREATED FOR 48 HOURS
AT 680%
ALNICO 5 MAGNETIC DOMAIN IMAGED IN FOUCAULT MODE
ALNICO 7
MODE
Fig. 7
