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MAGNETIC PROPERTIES OF RAPIDLY QUENCHED Fe-Ni BASED ALLOYS

J. SCHNEIDER, A. HANDSTEIN, R. HEBKE and K. ZAVETAt ZFW, 8027 Dresden, Postfach, DDR

We report for these alloys on the ratios qJq~, of the moment measured above T¢ from the magnetic susceptibility and below T, from the saturation magnetization. They are in general higher than unity, which may be taken as indication of band ferromagnetism in these amorphous metallic alloys. Possible explanations are discussed of the fact that most of these ratios lie above the Rhodes-Wohlfarth plot.

I. Introduction

Amorphous solids are of particular interest in magnetism to get fundamental information about the effect of structure on the formation of the magnetic moments as well as the nature of the exchange interaction. Most of the experimental work has been done on transition metal (TM) alloys with glass formers. The amorphous alloys are in general ferromagnetic if the corresponding alloys without the glass former atoms are fer- romagnetic. The saturation magnetization and the Curie temperatures are considerably reduced compared to the corresponding crystalline al- loys, this being usually attributed to topological disorder. For studying its influence the proper- ties of amorphous and crystalline single phases of the same chemical composition should be compared. The stable crystalline alloys are usu- ally heterogeneous. Therefore it is difficult to separate the effect of chemical composition, chemical, and topological disorder on the magne- tic properties [1, 2].

Chemical and structural inhomogeneities may also cause a more complicated magnetic struc- ture than a simple ferromagnetic one. The change of the saturation magnetization may be therefore attributed to changes in the density of states as well as tomixed magnetism.

Theoretical work has mainly concentrated on Heisenberg systems. The Heisenberg model, however, seems not to be appropriated to TM and their alloys, where in general the magnetic moment per atom is nonintegral. All the amorph- ous alloys containing a high concentration of ferromagnetic TM exhibit the characteristics of a metal. The expected considerable degree of itin- erancy of the d-electrons leads us to use the model of itinerant electrons, where the change in the

f Institute of Solid State Physics, Prague, CSSR

electronic structure by charge transfer can also be described. Thereby all kinds of disorder as well as different cluster effects should be in- cluded to describe the magnetic properties. Ac- cording to [3] the ratio qc/q.~ may serve as a criterion for the differentiation of localized mo- ments behaviour or itinerant description of fer- romagnets. Here qc equals the magnetic moment per atom deduced from the Curie-Weiss con- stant, and qs is the corresponding moment from the saturation magnetization.

In this paper we report on observed values of qJqs of different amorphous solids.

2. Experimental procedure and results

Amorphous alloys having the composition (Fex Ni~_x)s0PloB1o with x = 0.0 to 0.5, Fes~P~7As2 and (Fe05Mnos)s3P17 were prepared by rapid quenching from the melt. All the samples used were examined by X-ray diffraction using Debey-Scherrer method. The diffraction pat- terns show only broad haloes indicating an amorphous structure. The magnetic measure- ments were carried out using a ballistic mag- netometer in fields up to 39 kOe in the tempera- ture range 4.2K to 77K and on a magnetic balance in fields up to 7 kOe. Experimental val- ues of qJqs are summarized in table I, where for comparison the earlier observed qc/qs on fer- romagnetic amorphous alloys and intermediate phases have been included.

3. Discussion

It can be seen that q¢/qs for all these amorph- ous TM alloys is higher than unity. This fact may be taken as evidence for a model of itinerant electrons [3]. This conclusion, however, is some- what ambiguous. Most of the given qc/qs fall above the Rhodes-Wohlfarth plot.

Physica 86-88B (1977) 301-302 O North-Holland

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Table I Data of measured qJq~ values of ferromagnetic amorphous alloys and crystalline intermediate phases

Substance q~/q~ T~

Amorphous : (Fe,, ,Ni,, ~)~oP,,B., 3.2 ~570 K (Fe,, ,~Nio,7),oP,,B m 11.5 -+ 0.5 ~ 143 K (Feo ,Nio ~)~oP,oB,, 5.2 71 K (Feo oTNi,, ~3).,,P,oB,,, 8.1 40 K Fe,,P,~As: 2.5 +_ 0.3 ~570 K (Feo ~Mn,~,),,P,v 2.3 _+ 0.3 ~375 K Ni,~,,,P ..... 2.4 + 0.5 190 K [5] Ni,3 ~P,~,., 12 125 K [5] (Feo~Mn,,3)v~P,,C,. 3.67 210 K [4] (Feo ~Mn.,)~,P,,C ,. 7 140 K [4]

Crystalline : Fe~.P 2.78 306 K [6] Fe.~C ~2.71 520 K [6] Fe~C ~ 1.73 485 K [6l

Two questions now arise: What is (I) the nature of the magnetic moments and (II) the magnetic structure in these amorphous alloys. Concerning the first point both the nonintegral number of q~ and the low critical concentration of Fe in the amorphous (Fex Nil_~ )80P 10B m system for the onset of ferromagnetism support the model of itinerant electrons. Whereas qs of Ni80PIoB10 is zero the initial susceptibility vs. temperature curve show Curie-Weiss behaviour with a value of about 0.45 for qc. This value is smaller than that for crystalline Ni (0.89 [3]), which may indicate the extent of the electron transfer. Despite the fact that the Curie-Weiss law is usually taken as indication of localized magnetic moments, it may be also obtained within a model of itinerant electrons [7]. The existence of localized spin fluctuations or magne- tic polarization clouds caused by various kinds of disorder cannot be excluded. Nevertheless the observable spin density distribution may be in any case a collective property of the whole electron system.

Notwithstanding the nature of the magnetic moments the large values of qc/q~ and their

deviation from the Rhodes-Wohlfarth plot (see table 1) may indicate a complex magnetic con- figuration. This is also supported by measure- ments of the high field magnetization at low temperatures in amorphous Fe-Ni alloys.

One finds a high value of the susceptibility as well as deviations from the linearity of Arott plots not only near the critical concentration but also for higher Fe-concentration, which indicate an inhomogeneous magnetic behaviour [8]. Spa- tially non-uniform magnetization is expected quite generally due to fluctuations in the ex- change field and the local anisotropy fields which depend sensitively on short range chemical and geometrical arrangement of the atoms. In the critical concentration range magnetic polariza- tion clouds may be the origin of inhomogeneous magnetization. But for larger Fe concentration chemical inhomogeneities due to crystalline or amorphous phase precipitates on a fine spatial scale, which may bring about local regions with different magnetic structure, seem to cause the most important effect. Then any analysis of the Xo ~ curve will be a complicated task. The large differences Tp- Tc, which are an unique feature of amorphous ferromagnets, may also indicate strong magnetic short range order as well as competition between opposing magnetic interac- tions.

References

[1] J. Schneider and H. Wiesner, phys. stat. sol. 29(a) (1975) 151,503.

[2l G.S. Cargill, in: AlP Conference Proc. Nr. 24 (New York, 1975) p. 138.

[3] P. Rhodes and E.P. Wolfarth, Proc. Roy. Soc. 273 (1963) 247.

[4] A.K. Sinha, J. Appl. Phys. 42 (1971) 338. [5] D. Pan and D. Turnbull, in: AlP Conference Proc. Nr. 18

(New York, 1974) p. 646. [6] E. Vogt, in: Magnetism and Metallurgy, Vol. 1 (Academic

press, N.Y., 1969) p. 247. [7] E.P. Wolfarth, Comments Sol. State Phys. 6 (1975) 123. [8] J. Schneider and J. Konetzky phys. stat. sol. (a) 36 (1976)

L10. J. Schneider et al., to be published.