The composition dependence of magnetization and curie temperature of Fe-P, Co-P and Ni-P

Journal of Magnetism and Magnetic Materials 53 (1985) 103-110 North-Holland, Amsterdam

103

THE COMPOSITION DEPENDENCE OF MAGNETIZATION AND CURIE TEMPERATURE OF Fe-P, Co-P AND N i - P K. H(]LLER, G. DIETZ, R. HAUSMANN and K. KI)LPIN II. Physikalisches Institut der Universitgtt zu Krln, Z~lpieher Str. 77, D-5000 Cologne 41, Fed. Rep. Germany Received 6 May 1985

The saturation magnetization a0 of electrodeposited Fe-P, C o - P and N i - P (0-27 at% P) alloys at low temperatures has been measured with a vibrating sample magnetometer while the Curie temperatures T¢ have been derived from the breakdown of the magnetic moments induced by a small external field. The magnetic moment per atom decreases linearly with growing phosphorus content. The facts that the slopes of the lines are the same for the three alloy systems and that their values are smaller than predicted do not fit to known theoretical models, e.g. to modified rigid band models and to the valence bond model. N i - P and C o - P behave similarly, as the extrapolation of the corresponding straight lines to zero phosphorus content reveals the known o0's and Tc's of the pure crystalline metals. This is not true in the case of Fe-P. These observations allow some conclusions concerning the short range order in the three alloy systems.

1. Introduction Up to now a large number of ferromagnetic transition metal (TM)-metalloid alloys has been investigated. As the metalloids are chemically quite different from transition metals these materials are not only of technical but also of scientific interest. Different simplified models have been developed to describe the dependence of the magnetic moment per atom or per metal atom on composition. The most distant positions are occupied by the rigid band model and its modifications and the valence bond model. The first one neglects the influence of local environments on the net magnetic moment whereas the latter one claims to describe both local order and magnetic moment by chemical bonding between the TM and the metalloid atoms. The models seem to fit some systems with reasonable or even excellent accuracy but fail in others [1,2]. In the light of this situation it is necessary to work out systematical trends of experimental data and to correlate them with trends predicted by theories or elementary physical ideas. Simple systems which can be prepared as crystalline and amorphous materials over a wide concentration

range are the binary alloys of iron, cobalt and nickel with boron or phosphorus. The boron alloys have already been extensively investigated [3]. Some of our recent work has filled the data gaps concerning the phosphorus alloys. In this paper these data will be discussed in the light of the above mentioned theories. The metalloid concentration was varied as far as supersaturated solutions of phosphorus in the metals could be obtained, that was between zero and 27 at% P. The fundamental magnetic properties are mainly determined by the first coordination shell. Therefore our results also reveal some information on the short range order present in the investigated alloy series. Recent structural models describe the amorphous structure assuming units consisting of metalloid atoms surrounded by metal atoms [4]. A similar approach is proposed in ref. [2] to motivate the valence bond model. It will be shown that the consequent application of this model contradicts the just mentioned structural concept. Instead of this we shall try to relate the fundamental magnetic properties to coordination numbers and interatomic distances which both can be determined by diffraction experiments.

0304-8853/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

K. HUller et al. / Magnetization and Curie temperature of F e - , Co-, N i - P

104

2. Experimental The phosphorus alloys of iron, cobalt and nickel investigated in this paper were prepared by electrodeposition [5,6]. Material having less than about 12 at% P was microcrystalline. With more than about 13 at% P it was amorphous. The boundary concentrations varied a little for the three systems [7]. The compositions of all the samples were determined by wet chemical analysis and, if possible, additionally by a preceding magnetic analysis. The specific saturation magnetization a 0 = as(0 ), i.e. the spontaneous specific magnetization at T = 0 K, was derived from os(T)-curves measured by means of a vibrating sample magnetometer in external fields up to 17.5 kOe [7]. The Curie temperatures T~ were derived from the breakdown of the magnetic moments of the samples. The moments were induced by the residual field in the air gap of an electromagnet and were observed as functions of temperature, fig. 1. This method has the advantage that the magnetic field within the material is too small (about 30 Oe) to produce forced magnetization in the neighbourhood of Tc. The samples were about 5 mm in diameter and 0.2-0.4 mm thick. So the demagnetizing factor mainly determines the apparent susceptibility up to a temperature immediately below T~. The width of the interval where the small magnetic moment vanishes depends on the inclination of the curve as(T ) near T~. For pure crystalline nickel it amounts to about 5 K. For homogeneous material T~ results from the intersection of the linearly

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extrapolated experimental curve with the background intensity (fig. 1). Due to the electrolytic process the material may be inhomogeneously composited on a macroscopic scale [12]. In this case the extrapolation procedure reveals the highest Tc present in the sample whereas the chemical analysis yields the average phosphorus content (fig. lc). This must be kept in mind when the relation between Tc and composition is discussed. The material had to be heated as quickly as possible to prevent crystallization at temperatures below T~. The maximum available heating rate was 20 K / m i n . The actual temperature was measured by means of a thermocouple in direct thermal contact with the specimens. Only few samples (e.g. C o - P with about 25 at% P) survived one heating period without beginning to crystallize. Repeated experimental procedure applied to these samples reproduced T¢ within 1 K. The absolute values of T~ resulted in an accuracy of about 3 K as was checked with the aid of a crystalline standard nickel sample. More precise methods fail because they demand that the material be maintained for a longer time at temperatures near T~.

3. Results and discussion

3.1. Saturation magnetization of Co-P In fig. 2 magnetization curves of some C o - P alloys are depicted. The available external field of 17.5 kOe was just sufficient to saturate also the samples with 11 and less at% P. According to their X-ray diffraction patterns they were microcrystalline having an hcp structure. Alloys with 12 to 27 at% P were totally amorphous and magnetically soft. Those with a higher phosphorus content consisted of crystalline Co 2P and an amorphous component. In the present paper these materials will not be considered. In fig. 3 the composition dependence of o0 in /~B per metal atom of Col_xP x (4~
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fairly well with those obtained by Pan and Turnbull on amorphous C o - P with 21-25 at% P [8] but they are greater than some values observed on

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crystalline samples with low phosphorus contents [9,10]. Obviously the fields applied in those experiments were insufficient to saturate the material. Thus a too a strong decrease of o0 with increasing x is assumed in the lower composition range•

3.2. Saturation magnetization of Fe-P F e - P having more than 12 at% P could be saturated in the maximum available external field. Electrodeposited samples with 1 to 4 at% P were very brittle, so we failed in preparing samples with approximately ellipsoidal shapes• The specimens had large inhomogeneous demagnetizing factors and saturation could only be approached to within some per cent. The saturation magnetizations o0 of these samples are nearly equal to those of pure iron. Additionally their o0 and their saturation magnetostriction was found to vary in a similar way to that of iron between 4 and 295 K. The reason may be that phosphorus is not dissolved in the metal but mainly segregated into the grain boundaries. According to fig. 3 a 0 varies linearly in the composition range between 13 and 27 at% P independent of structure. The experimental curve also includes the value measured on tetragonal FeaP [11]. The alloys with 13-15 at% P were at least partly microcrystalline. According to small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) [12,13] the diameters of the crystallites were about 2 nm so that it was impossible to determine the crystal structure with the available technique. Electrodeposited F e - P with 15 to 27 at% P is totally amorphous. Results obtained in this composition range by other authors [11] agree well with ours.

3.3. Saturation magnetization of N i - P

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We have investigated N i - P mainly in the composition range around the transition from crystalline to amorphous material. Corresponding magnetization curves are depicted in fig. 4. Most of them show a slight paraprocess indicating the onset of weak ferromagnetism at a phosphorus content of about 11 at% P. This will be discussed in more detail in a following paper. The linear extrapola-

K. Hfdler et aL / Magnetization and Curie temperature of Fe-, Co-, N i - P

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Fig. 5 shows the reduction of the mean magnetic moment per alloy atom and fig. 6 the change of Tc with increasing phosphorus content for the three alloy systems, both in relation to the pure metals. The Curie temperatures of the C o - P alloys with less than 21 at% P were not available because these materials crystallized below Tc. The values inserted in the figure were derived from the measured spin wave stiffnesses D assuming that the ratio D / T c --- 0.23 meV A 2 / K remains constant throughout the whole concentration range [7]. The composition dependences of the properties of C o - P and N i - P differ in a characteristic manner from those of F e - P . Beginning with C o - P and N i - P one recognizes that independent of the structure the reduction of the magnetic moment per metal atom follows the same straight line for the two systems starting at the values for the pure metals. Material with less than about 11 at% P is crystalline (hcp and fcc, respectively), N i - P with 11 to 14 at% P is partly amorphous, C o - P with more than 12 at% P is totally amorphous. The reduction of Tc with increasing phosphorus con-

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tion of the magnetization curves to H = 0 reveals the spontaneous magnetization with an error of 1 e m u / g corresponding to 0.01/za/nickel atom. The lowest curve in fig. 3 shows that our experimental values agree well with earlier work [14,15]. We shall therefore include these results in our further discussion. The smearing out of the curve above 14 at% P connected with the transition from weak ferromagnetism to paramagnetism will not be considered.

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tent in the two systems also agrees. The values for N i - P entered in fig. 6 are taken mainly from ref. [141. The straight line fitted to the /~B/atom measured on F e - P has nearly the same slope between 13 and 25 at% P as that for N i - P and C o - P , but it is shifted so that its linear extrapolation does not touch the experimental values observed on the alloys with less than 4 at% P and does not intersect the ordinate at A/~B = 0. Otherwise than in N i - P and in C o - P the Curie temperature remains nearly constant. Extrapolation of the experimental curve reveals neither the value for bcc iron nor that for the crystalline F%P. The observations on N i - P and C o - P suggest similar short range orders in the amorphous and the crystalline alloys, possibly gradually changing with phosphorus content. This comes from the similarity of the dense packings in the two structural states having nearly the same co-ordination numbers [16], and as a consequence having similar first co-ordination shells which mainly determine the fundamental magnetic properties. In F e - P the situation is different. The bcc iron is not densely packed. The co-ordination number is 8. As has been shown in a previous paper [17] the amorphous F e - P alloys have a bcc-like mechanical density but the number of nearest neighbours proved to be about 13 [18]. Therefore, the topological configuration in glassy F e - P resembles more that in N i - P and C o - P than that in bcc-iron. This is probably the reason for the identical slopes

107

of the experimental lines in fig. 3. According to this concept the line measured on F e - P should extrapolate to a value corresponding to a hypothetical iron with a not densely packed structure characterized by 14 nearest neighbours. The Curie temperatures of phosphorus rich amorphous F e - P and crystalline Fe3P are different from similar structural grounds. The structure of the compound Fe3P is complicated, having 8 formula units per unit cell. It is improbable that a corresponding short range order is established during rapid solidification even in amorphous alloys with compositions at or near Fe75P2s. The same may be true for C o - P alloys with more than 20 at% P. Although in this region their composition is nearer to that of the stable compound Co2P than to pure cobalt the diminution of the magnetic moment per atom (fig. 5) follows the straight line determined by the material with low phosphorus content. Probably the deviation of the experimental points at the upper end of the concentration range is due to a change of the local environment tending to the short range order of Co 2P. These conclusions correlate to observations obtained by other authors on crystalline (Fel_yNy)3Ge [19]. For y < 0.1 it has hcp structure, for 0.1 < y < 0.55 bcc structure and for y > 0.55 fcc structure. At the boundary compositions the values of Tc and the slopes of T~(y) and the slopes of a0(y) change but not the values of o0. Obviously the latter property depends only on composition but not on structure. The same conclusion may be drawn from our measurements as T~ of crystalline F%P does not agree with the T~ of amorphous Fe7sP25 whereas the Ms of the two materials are equal. From these observations it appears that T~ is mainly determined by the atomic configuration around the metal atoms and the mutual interactions between them, i.e. the short range order, whereas the magnetic moment is mainly affected by the metalloid content.

5. The experimental results in the light of rigid band models Malozemoff et al. [1] have argued that Friedel's modification of the rigid band model [20] should

108

K. H~ller et al. / Magnetization and Curie temperature of F e - , C o - , Ni - P

also apply to transition metal-metalloid alloys. N o charge transfer from the metalloid to the metal atoms is assumed in accordance with photoemission experiments performed on C o - P and N i - P [21,22]. Like other rigid band models this concep tion predicts a dependence of the magnetic moment on the valence of the alloying constituents and neglects the effect of the local environment. The average number n av of Bohr magnetons per a t o m should linearly decrease according to /'/av = n 0 - x [ 1 0 - ( Z T M - g x ) ] , (1) n o is the value for the pure metal, x is the concentration of the metalloid, z x and ZTM are the valences of the metalloid and the metal, respectively. Eq. (1) predicts different slopes of the straight lines for materials with different Z.rM - z x , namely - 7 for F e - P , - 6 for C o - P and - 5 for N i - P provided z x = 5. (The dashed lines in fig. 5.) But, independent of the transition metal, the experimental curves in fig. 5 reveal slopes of 4.2. This means - the effect of phosphorus on the magnetic moment per atom is weaker than predicted, - the change of the magnetic moments per atom of T M - P alloys does not depend on the valence of the transition metal. These two statements can be generalized taking into consideration results known from the literature. The data collected in [1] show that the influence of Si, Ga, C, Ge and B ( = M) on the atom-averaged atomic magnetic moments of F e - M alloys is smaller than predicted by the model. Additionally K a z a m a et al. [23] have found that the change of the magnetic moment per atom with x of FesoP20_xMx and Co75B25_xM x does not correlate with the valence but with the size of M ( = C , B, P, Si, Ge, As). As a result of these considerations we state that the interaction between the electronic systems of the transition metal and the metalloid depends less on the choice of the alloy components than assumed in band models and stressed in ref. [1]. 6. The experimental results in the light of the valence bond model

The valence bond model as proposed by Corb et al. [2] assumes that transition metal and metal-

loid atoms interact by chemical bonding. The number of the chemical bonds per atom determines the magnetization. This concept predicts the number n of Bohr magnetons per T M - a t o m to be for dilute bcc iron alloys

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n o is the value for the pure metal, Z the number of bonds formed by each metalloid atom, % the p-electron valence of the metalloid, x the concentration of the metalloid. Z is the number of T M neighbors of a metalloid atom, e.g. Z = 12 for dilute fcc N i - P and hcp C o - P and Z = 9 for CozP and for phosphorus rich amorphous C o - P alloys. According to eqs. (2) the change of the magnetic moment per T M - a t o m is plotted versus x / ( 1 - x) in fig. 7. As expected by Corb et al. the bond model has no success with F e - P alloys, except for the case of bcc samples with low phosphorus content. The dashed lines calculated from eqs. (2b) and (2c) with the just mentioned Z ' s take their courses below the experimental curves. Within the framework of the model one has to conclude that the phosphorus atoms do not bond to all their T M neighbours or that the reduction of the magnetic moment per chemical bond is overestimated in the model equations. This may happen if the phosphorus atoms are not able to build up a compound-like coordination shell during deposition but have to arrange themselves with an already present metal structure. This diminishes the number of formed bonds because not all TM-neighbours of a phosphorus atom have the right position. As a consequence we have to regard Z as a fitting parameter. If we do so we find Z = 7.6 for C o - P with less than 23 at% P. To account for the nonmagnetic behaviour of the compound Co 2P we have to assume Z = 10. Data on C o - B can be fitted by Z = 6. This suggests the following conclusion: Z = 6, 7.6 or 10 for amorphous and partly crystalline C o - B , for amorphous and hcp C o - P and for Co2P, respectively, indicate that the bonding (or in other words

K. Hfdler et al. / Magnetization and Curie temperature of F e - , Co-, N i - P

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the short range order) of C o - P alloys is more similar to that of C o - B alloys than to that of crystalline Co2P. The behaviour of the different Co-metalloid alloys resembles each other more than that predicted from the different structures of the corresponding stable compounds (Co2P and Co3B ). The interpretation of the experimental results obtained on N i - P is even less straightforward because eq. (2c) includes Vp as an additional parameter. Assuming op = 3 as the p-valence of phosphorus the experimental line yields Z = 3.5. This value is obviously too small. We have to regard vp also as a fit parameter and under this point of view eq. (2c) loses any evidence. In spite of the failure of the valence model in the present form it is clear that chemical interactions play a prominent role in stabilizing the structure of metal-metalloid alloys. This must be taken into consideration in an improved model.

7. Conclusions

The decrease of the saturation magnetization and the Curie temperature with increasing metal-

loid content observed on C o - P and N i - P shows no direct effect of structural order. Either there is none or it is hidden behind the composition dependence in the sense that the closed packed arrangement changes gradually with composition. The alloys have distorted metal structures up to 20-25 at% P. The metal atoms are replaced by phosphorus atoms. So a 0 and Tc are reduced. In F e - P on the other hand the alloying effect on Tc is opposed by an increasing iron-iron coordination number which leads to a slight increase of the Curie temperature with increasing phosphorus content over a wide concentration range. Observations on F e - B can be interpreted in the same way. The differences in the behaviours of C o - P and N i - P on the one hand and iron alloys on the other arise from the fact that C o - P and N i - P are as closely packed as the pure metals are whereas the iron alloy series starts with a bcc order which is different from the dense packing already in the first coordination shell. We have to assume that the short range order changes with increasing metalloid content. This is the reason why the extrapolation of the straight line fitted to the experimental points does not intersect the ordinate

110

K. Hfdler et al. / Magnetization and Curie temperature of Fe-, Co-, N i - P

at o0 = 2.21/~ B, the value for bcc iron (fig. 3). The rigid band model and the valence bond model both overestimate the effect of the chemical nature of the alloy components and their concentrations on the saturation magnetization. It may be that a modified valence bond model describes the observations. It is accepted that chemical bonding prevents the decomposition of the supersaturated disordered (amorphous) alloys but no evidence has been found that the metalloid enforces a compound-like short range order around the metalloid sites. The interpretation of the experimental results should be based on structural models which start from the metal instead of the metalloid environment. This is suggested by comparison of the curves in fig. 3. The phosphorus reduces the magnetic moment of the metal atoms by nearly the same amount in the three alloy series (0.03/~a per at% in F e - P and in C o - P and 0.037/t a per at% in Ni-P). The experimental line representing C o - P is shifted against that of N i - P by about 1.1/xB, that of F e - P against C o - P by about 0.8/~ a. In an improved theory it must be considered that the magnetic properties of the TM-metalloid alloys are mainly determined by the metal component and are diluted by the metalloid.

Acknowledgements The support of this investigation by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 125 Aachen-Ji~lich-KOln is gratefully acknowledged.

References [1] A.P. Malozemoff, A.R. Williams and V.L. Moruzzi, Phys. Rev. B29 (1984) 1620.

[2] B.W. Corb, R.C. O'Handley and N.J. Grant, Phys. Rev. B27 (1983) 636. [3] R. Hasegawa and R. Ray, J. Appl. Phys. 50 (1979) 1586, 49 (1978) 4174. [4] P.H. Gaskell, Topics in Appl. Phys. 53 (Springer Verlag, Berlin, 1983). [5] J. Logan and M. Yung, J. Non-Crystalline Solids 21 (1976) 151. [6] A. Brenner, Electrodeposition of Alloys (Academic Press, New York-London, 1963). [7] K. Hialler and G. Dietz, J. Magn. Magn. Mat. 50 (1985) 250. [8] D. Pan and C.D. Turnbull, J. Appl. Phys. 45 (1974) 1406. [9] T. Kanbe and K. Kanematsu, J. Phys. Soc. Japan 24 (1968) 1396. [10] A.W. Simpson and D.R. Brambley, Phys. Stat. Sol. (b) 43 (1971) 291. [11] M. Mitera, M. Naka, T. Masumoto, N. Kazama and K. Watanabe, Phys. Stat. Sol. (a)49 (1978) K163. [12] R. Sonnberger, H. Bestgen and G. Dietz, Z. Phys. B 56 (1984) 289. [13] H. Bestgen, in: Rapidly Quenched Metals, vol. 1, eds. S. Steeb and H. Warlimont (North-Holland, Amsterdam, Oxford, New York, Tokyo, 1985) p. 443. [14] P.A. Albert, Z. Kovac, H.R. Lilienthal, T.R. McGuire and Y. Nakamura, J. Appl. Phys. 38 (1967) 1258. [15] A. Berrada, M.F. Lapierre, B. Loegel, P. Panissod and C. Robert, J. Phys. F 8 (1978) 845. [16] G.S. Cargill and R.W. Cochrane, J. de Phys. 35 (1974) C4-269. [17] G. Dietz and L. B6rngen, J. Non-Crystalline Solids 58 (1983) 275. [18] J. Logan, Phys. Stat. Sol. (a)32 (1975) 361. [19] K. Kanematsu and H. Takahashi, J. Phys. Soc. Japan 53 (1984) 2376. [20] J. Friedel, Nuovo Cim. Suppl. 7 (1958) 287. [21] E. Belin, D. Farques, C. Bonnelle, J. Flechon, F. Machizaud and J. Rivory, J. de Phys. C8 (1980) 427. [22] E. Belin, C. Bonnelle, S. Zuckermann and G. Machizaud, J. Phys. F 14 (1984) 625. [23] N.S. Kazama, T. Masumoto and M. Mitera, J. Magn Magn. Mat. 15-18 (1980) 1331.