185

MAGNETIZATION IN AMORPHOUS Ni-Fe-BASED ALLOYS

J. SCHNEIDER, K. ZAVETA*, A. H A N D S T E I N , R. H E S S K E and W. HAUBENREISSER* ZFW, 8027 Dresden, Postfach, DDR

We report on magnetic measu remen t s on amorphous (FexNi, x)a0P,0B~o alloys with 0 ~ x ~< 0.5. The amorphous Ni-rich alloys are characterized as weak itinerant ferromagnets . The curvature of the Arrott plots are used for d iscuss ing the appearance of an inhomogeneous magnetizat ion. Deviat ions from the linearity in the Arrott plots have been observed in

the critical concentra t ion range as well as for higher Fe-concentrat ion. Chemical inhomogenei t ies on a fine spatial scale, which may bring about local regions with different magnetic structure, seem mainly to cause the he terogeneous magnetization.

1. Introduction

At the present time there exists much ex- perimental data on the occurrence of fer- romagnetism in amorphous solids. Most of the experimental work has been done on transition metal (TM) alloys with glass formers (GF). The samples are prepared using various techniques like quenching, deposition, etc. All these amor- phous alloys exhibit a high degree of com- positional as well as topological disorder [1]. Therefore , questions arise as to the effect of the various kinds of disorder on the intrinsic magnetic properties.

Amorphous solids are of particular interest in magnetism to obtain fundamental information on the effect of structure on the formation of the magnetic moments as well as the nature of the exchange interaction. The observed nonin- tegral number of magnetic moment per atom, the values of qJqs higher than unity as well as the low critical concentrat ion for the ap- pearance of ferromagnetism in these TM-GF alloys, may give evidence for the itinerant character of the electrons carrying the magnetic moments [2]. The saturation magnetization and the Curie temperatures are considerably re- duced compared to the corresponding crystal- line alloys, this being usually attributed to to- pological disorder. For studying its influence the properties of amorphous and crystalline single phases of the same chemical composition should be compared. The stable crystalline al- loys are usually heterogeneous. Therefore it is difficult to separate the effect of chemical com- position, and chemical and topological disorder on the magnetic properties [3]. However , there

* Institute of Solid State Phys ics , Prague, CSSR. ' ZFW, 69 Jena, Helmhol tzweg 4, DDR.

is also the possibility that supersaturated solid solutions and new metastable crystalline phases are formed by quenching as appears during an- nealing of the amorphous samples [4]. Com- paring the magnetic quantities of single phase amorphous and metastable crystalline alloys of the same chemical composition it has been found that the difference in these quantities is quite small. The magnetic properties appear to be determined mainly by the chemical com- position and the chemical disorder rather than by topological disorder [3].

There arises also the question of the influence of the different kinds of disorder on the mag- netic structure in the amorphous alloys. Is there only a simple overall arrangement of the mo- ments in these amorphous ferromagnets, or does a much more complex magnetic structure appear?

In this paper we report on bulk magnetic measurements on amorphous (FexNil-x)80Pj0B~0 alloys with 0~<x ~<0.5. The curvature of the Arrott plots are used for discussing the ap- pearance of inhomogeneities in the spatial dis- tribution of the magnetization.

2. Experimental facts

All the samples were prepared by rapid quenching from the melt using a technique for producing microwires [5]. The structure of all samples was considered to be amorphous, since the X-ray diffraction pattern showed only broad haloes.

The magnetization measurements were made using a ballistic magnetometer in the tem- perature range 4.2-77 K and in fields up to 40 kOe. The measurements were carried out on cylindrical samples of these amorphous alloys.

The low temperature magnetic isotherms of

Physica 91B (1977) 185-189 © North-Holland

186

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187

all the amorphous alloys with x ~<0.3 show a remarkable nonlinear increase of the mag- netization with the field well above technical saturation. The high field susceptibility de- creases with increasing field but is nonzero also at the highest field used. Near ly vanishing high- field susceptibilities are obtained for x I>0.5. The technical saturation was reached in any case in fields of 600Oe. The saturation field itself decreases with increasing Fe-content. This behaviour suggests an incomplete alignment of the magnetic moments for the Ni-rich alloys which may give also a spatial nonuniform magnetization.

Inhomogeneous magnetic behaviour is ex- pected quite generally due to fluctuations in the exchange field and the local anisotropy fields which depend sensitively on the short range chemical and geometrical ar rangement of the atoms. Typical M 2 against H / M curves (Arrott plots) for x = 0.02, 0.07, 0.1 and 0.16 are given in figs. 1 (a)-(d). Arrott plots for various Fe-con- centration at 4.2 K are drawn in fig. 2. A crude est imation of the critical field for domain ro- tation shows that for all these curves the

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measuring field was much larger than the critical one. The spatially nonuniform uniaxial aniso- t ropy is of the order of 104erg/cm 3 and the values of the saturation magnetization are in the range up to 500 G.

The critical concentrat ion in these alloys is about x - 0.02. The onset of ferromagnet ic or- dering has been deduced both f rom the manifestat ion of the transition tempera ture T~ and the appearence of finite hysteresis. But finite hysteresis can be also measurable above To. This may be caused by some degree of magnetic short range ordering [6].

Because of the curvature in the Arrott plots the usual method of Belov does not provide a unique way of determining To. Nevertheless , using this method we found T~ = 4 0 K for x = 0.07, T ~ = 7 1 K for x = 0 . 1 and T c - 1 4 3 K for x = 0.13, An estimation using a method given by Murani et al. [7] where ( H l M ) n ~ o is plotted vs. T, gives T c - 1.5 K for x = 0.02.

3. D i scuss ion

The onset of magnetic ordering in these amorphous metallic solids does not seem to be different f rom that observed for dilute mag- netic crystalline metallic alloys. The latter also have a random distribution of the magnetic moments and may therefore be regarded as an amorphous magnetic material. This conclusion is also supported by recent measurements on amorphous FeGe alloys in the critical concen- tration range by Endoh et al. [8]. The onset of fer romagnet ism is attributed to interactions between superparamagnet ic clusters appearing due to chemical inhomogeneit ies or magnetic polarization clouds [9, 10]. In the critical concentrat ion range mostly a highly in- homogeneous spatial distribution of magnetiza- tion results [9, 11].

For magnetic homogeneous sys tems the Ar- rott plots give straight lines in the case of weak ferromagnet ic materials except at the lowest fields where domain rotations occur [6]. An inhomogeneous character of the magnetization for the amorphous Fe -Ni alloys near the critical concentrat ion is therefore associated with de- viations f rom the linearity in the Arrott plots. There is a significant amount of curvature at low fields for x = 0.02 [fig. l(a)], which changes f rom being convex to a more linear curve as we pass

188

to lower temperatures. There is also a change in the curvature from convex to concave as we go from below to above the critical concentration [see figs. I (a) and (b)]. This supports the s t a t e -

m e n t that there exists a close analogy between what happens for a fixed concentration when the temperature is changed and the case where the temperature is fixed and the concen- tration is w~ried (see [121). These plots show qualitatively the shape given by Shtrikman and Wohlfarth [13] for heterogeneous weak itinerant magnetic alloys, where such deviations from the linearity of the Arrott plots tire attributed to spatial inhomogeneities of the magnetization due to fluctuations in the concentration. The observed M 2 versus HIM curves, the reasonable constancy of M(O,O)/T~ [141 lind the values qJq~>l [2] corroborate to charac- terize the amorphous Ni-rich (Fe,Nil ,)s0PI~BI~ alloys as weak itinerant ferromagnets.

Whereas nonlinear Arrott plots in weak fer- romagnetic crystalline disordered alloys have been observed above the critical concentration up to twice the critical concentration [11], we found in the amorphous Fe-Ni-based alloys high values of the susceptibility as well as de- viations from the linearity of Arrott plots also for much higher Fe-concentration (figs. l(b)-(d) and fig. 2). The slope of the Arrott plots in- creases with increasing Fe-content (see fig. 2). But in the case of higher Fe-concentration the form of the Arrott plots differs more from the calculated one given in [13, 15]. The theoretical predictions apply only for weak ferromagnetic materials with very small fluctuations in concentration. But for our rapidly quenched Fe- Ni-based alloys large chemical inhomogeneities on a fine spatial scale, which may bring about local regions with different magnetic structure, seem mainly to cause the heterogeneous mag- netization. These chemical inhomogeneities may appear as spatial compositional fluctuations or as phase precipitations. Both may bring about a certain range of finite fluctuation lengths of the magnetization. In connection with this effect the neglect of the VZM(v) term in a theoretical analysis [13] seems to be incorrect.

The high-field dependence of the magnetiza- tion in these amorphous Fe-Ni-based alloys may be compared with those found for crystal- line Fe-AI alloys [16]. The anomalous magnetic properties of Fe-AI alloys have also been at-

tribuled to chemical and structural inhomo- geneities (different phases with different crystal- line struclure).

Our conclusion of the significance of chemical inhomogeneities in the amorphous Fe-Ni alloys for the appearence of spatial inhomogeneities in the magnetization is supported by the following facts. The equilibrium solubility limits of P and B in Fe and Ni is very low. The short range order in the amorphous T M - G F alloys is similar to that in intermetallic compounds [17]. There is always a certain probability of forming different h~cal atomic configurations in such rapidly quenched systems which are heterogeneous in the equilibrium state. The amorphous-crystal- line phase transition occurs as a result of such configurational fluctuations. The homogeneity of the chemical composition may be much less complete for rapid quenching from the melt than for any other technique. As for con- ventional oxide glasses, for metallic amorphous systems a separation into different phases which are still amorphous were also observed upon annealing [4, 19]. The transformation sequences on annealing the amorphous alloys to the heterogeneous equilibrium states also give evidence for the appearance of metastable and stable intermediate phases on a fine spatial scale in the quenched states [18]. Due to these con- siderations spatial compositional fluctuations or phase precipitations may appear in such amor- phous T M - G F alloys. The local regions with different magnetic structure are based on these chemical inhomogeneities.

It seems to us that in general the intrinsic magnetic properties of these amorphous metal- lic T M - G F alloys may be caused more by chemical effects than by structural disorder. This is supported also by our measurements for some of the Fe-Ni alloys with different struc- ture, amorphous and metastable crystalline,

M 2 showing nearly the same vs. HIM curves with a large curvature in the Arrott plots in any case. Hence one has to continue the search for magnetic properties which are a direct con- sequence of the non-crystalline structure.

New experimental work for the chemical, structural, and magnetic characterization in- cluding M6ssbauer measurements and neutron scattering experiments as well as developments of theoretical models which better reflect the chemical and structural features of such

systems will be of great interest. It is especially necessary to extend the present theory of the magnetic isotherms in heterogeneous weak itinerant magnetic alloys to larger concentration fluctuations with higher than the second mo- ment and finite range. The different kinds of heterogeneity of the magnetic structure and their contributions to the nonlinearity of the Arrott plots have to be calculated and analyzed in more detail.

References

[1] See, for instance, in: H.O. Hooper and A.M. de Graft, eds., Amorphous Magnetism (Plenum, New York, 1973).

[21 J. Schneider, A. Handstein, R. Hesske and K. Zaveta, Physica B (1977), to be published.

[3] J. Schneider and H. Wiesner, Phys. Status Solidi 29 a (1975) 151, 503.

[4] H. Jones, Rep. Progr. Phys. 36 (1973) 1125. [5] H. Wiesner, Thesis, TU Dresden (1975). [6] D.M. Edwards and E. P. Wohlfarth, Proc. Roy. Soc. A

303 (!%8) 127.

189

[7] A.P. Murani, A. Tari and S.R. Coles, J. Phys. F 4 (1974) 1769.

[8] Y. Endoh et al., Solid State Commun. 18 (1976) 735. [91 W.C. Muellner and J.S. Kouvel, Phys. Rev. B 11 (1975)

4552. [10] J.S. Kouvel, in: Magnetism in Alloys, P.A. Beck and

J.T. Waber, eds. (AIME, New York, 1972) p. 244. [11] See, for instance, J. Beille, D. Bloch and M.J. Besnus,

J. Phys. F 4 (1974) 1275. [12] H.L. Alberts et al., Phys. Rev. B 9 (1974) 2233. [13] S. Shtrikman and E.P. Wohlfarth, Physica 60 (1972)

427. [14] T. K6hler and J. Schneider, to be published. [151 H. Yamada and E.P. Wohlfarth, Phys. Status Solidi (b)

58 (1973) K 151. [16] G.P. Huffmann, in: Amorphous Magnetism, H.O.

Hooper and A.M. de Graft, eds. (Plenum, New York, 1973) p. 167. T. Wakiyama, J. Phys. Soc. Jap. 32 (1972) 1222.

[17] C.C. Tsuei, in: Amorphous Magnetism, H.O. Hooper and A.M. de Graft, eds. (Plenum, New York, 1973) p. 229.

[18] P.K. Rastogi and P. Duwez, J. Non-Cryst. Solids 5 (1970) 1.

[19] C.P. Chou and D. Turnbull, J. Non-Cryst. Solids 17 (1975) 169.