High field magnetic and Mössbauer studies in amorphous Fe-Er-B-Si alloys

Journal of Magnetism and Magnetic Materials 98 (1991) 155-161 North-Holland

155

High field magnetic and Miissbauer studies in amorphous Fe-Er-B-Si alloys *

R. Krishnan, H. Lassri and J. Teillet ’ Laboratoire de MagnPtisme, CNRS, 92195 Meudon, France

Received 8 November 1990

We describe the results of magnetization and Mossbauer studies under high magnetic field on amorphous Fe,,_,Er,B,,Si, alloys with 0 I x I 15. The Er moment is 8~~ at 4.2 K which indicates a conical spin structure with an average cone angle of 54“. The Fe moment decreases only slightly with increasing x. For x = 15, the field dependence of magnetization shows a discontinuity at H = 3 T and it begins to increase remarkably with H. This is attributed, in the light of Mossbauer experiments under fields, to the opening of the canted Fe spin structure. The results are discussed.

1. Introduction

Rare-earth metal atoms with spin-orbit mo- ment are known to give rise to large random anisotropy in amorphous alloys [l]. The literature abounds in studies on amorphous Rare Earth (RE)-transition metals films [2,3]. On the other hand, the melt spun metallic glasses based on TM-metalloids have also been studied extensively in the past decade and are well documented in the literature [4]. However, investigations of metallic glasses containing RE metals are relatively very few [5]. The discovery of permanent magnets based on Nd-Fe-B has also led to some studies of such alloys prepared by melt spinning [6]. We have reported recently on metallic glasses of the type TM-RE-B-Si where TM = Co and RE= Er, Gd, . . . [7,8]. We have shown that under intense magnetic fields the anti-parallel arrangement of Co and Er spins breaks down leading to an in- crease in the net alloy magnetization, and that the field needed to reach this situation also depended strongly on the Er content [7,9]. The above results

* Part of the PhD thesis by H. Lassri, Universite de Rouen (January 1990).

’ INSA URA CNRS 808, 7631 Mt-St-Aignan, France.

have been interpreted in terms of the relative strengths of the interactions, namely, the ferro- magnetic J,,_ co, JEr_ Er and antiferromagnetic J ,--_ Er, and the random anisotropy energy of the Er atoms. In the present work we study the high field behaviour of Fe-Er-B-Si alloys which is found to be somewhat different from that of Co based alloys. We have also carried out Miissbauer studies under magnetic field to get more informa- tion about the spin structure of Fe which enable us to understand better the magnetization be- haviour.

2. Experimental details

The amorphous Fe,,_,Er,B,,Si, alloys with 0 I x I 15, in the form of ribbons were prepared by melt spinning technique in an inert atmo- sphere. The exact composition was determined by Electron Probe Micro Analysis (EPMA). The amorphous structure was verified by X-ray dif- fraction. The magnetic moment M was measured with a precision better than fO.l%, under mag- netic fields up to 15 T and in the temperature range from 4 to 295 K.

Mijssbauer samples made of parallel ribbons

0304-8853/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

156 R. Krishnan et al. / Amorphous Fe-D-B-S alloys and high fields were set perpendicular to the y-beam. Mijssbauer

spectra were obtained with a triangular waveform spectrometer using a source of ‘%o in a rhodium matrix. Experiments under an external field of 4.4 and 8 T applied parallel to the y-beam were performed at the Francis Bitter National Magnet Laboratory at MIT and also at the Universite du Mans (France). Amorphous spectra were fitted with a least square technique [lo] using the histo- gram method relative to discrete distributions for single crystal [ll], constraining the width of each elementary spectrum to be the same. The isomer shifts are relative to a-Fe at room temperature.

3. Results and discussions

The ribbons were about 2 mm wide and about 30 pm thick and were X-ray amorphous.

3.1. Magnetization at H = 3 T

Saturation could be obtained for about 3 T at all temperatures. Let us first discuss the results at 4.2 K. The alloy magnetic moment (expressed in Bohr magnetons (pa)) decreases with the addition of Er which indicates the antiparallel coupling between the Fe and Er moments. In order to calculate the moment of Er (pEr) we followed the following procedure [7]. It is known that the Fe moment (pw) diminishes when it is alloyed with a rare-earth metal due to the hybridization of the 3d and the 5d orbitals, but this effect is negligible for small concentrations. So we took pre = 1.96~~

obtained for the alloy with x = 0, and assumed this to be the same in the alloy with x = 4.8.

Knowing the alloy moment (Pi) and using the relation,

PA = { (80 - x)(P,G) - X(P,,))/loo

we calculated pnr to be 8~~. This moment which is a projection along the applied field is smaller than the theoretical value (gJ) of 91_la. This reduc- tion could be attributed to the non-collinear and conical spin structure of Er with an average apex angle of 54”. This phenomenon is the resultant of the strong random anisotropy of Er and the anti-

n

-a 2 1 i Ii

0 5 10 15

XcatXEr)

Fig. 1. The alloy and the iron moments as a function of Er concentration at 4K.

ferromagnetic JFe_ Er interactions which normally lead to a “ sperimagnetic” structure [12]. It is interesting to note that the projection of the Er moment for Fe based alloys is higher than 6.8~~

found in the Co based alloys indicating a higher cone angle and a stronger random anisotropy in the latter [7]. Now pre for other alloys could be calculated based on the reasonable assumption that pEr is independent of x. Fig. 1 shows that at 4 K, on the one hand, pa decreases with Er and the compensation of the moments occur for about x = 14. On the other hand, it is seen that pre decreases only very slightly with increasing x un- like in the case of Co [7].

3.2. Magnetization at 4.2 k for 3 T < H I 20 T Fig. 2 shows the field dependence of M of the samples studied. For x = 4.8, 6.3 and 11 satura- tion is attained only for H close to 3 T. This would indicate that the Fe spin structure is non- collinear. This is a well known phenomenon for the alloys rich in Fe as has been reported by us in a-Fe-Zr alloys. This is so because in such cases the local structure of Fe becomes fee like [13]

leading to competing interactions. Furthermore, in

the present case, the conical spin structure of Er

could also impose some canting of Fe spins in

order to satisfy JFe_Er < 0.

R. Krishnan et al. / Amorphous Fe-Er-B-Si alloys and high fields 157

. . . * . ^x=4

. . . .

loo- *

. . . ..a * 63

. . . . a’ *

.

75 -

50-

. . 1 . -

. *

. * * . 1

. * * .

. ’ .

.

25 - .

. . . . . . . * . *

I I I

0 5 10 15

H(T)

Fig. 2. The high magnetic field dependence of the magnetiza- tion in a-Fe-Er-B-Si alloys at 4K.

When the field is increased above 3 T, for samples with x < 11, only a very small high-field susceptibility is observed, which is normally found in the case of rare-earth based alloys. This result is quite different from that recently reported by us [9] in a-Co-Er-B alloys where even in this range of Er concentration, a steep increase in the magne- tization was observed for fields above a certain critical value. However, a similar result is found for x = 15, where there is a remarkable increase in M with the field. This composition is to be dis- tinguished from the others because at 4.2 K the Er sub-network magnetization dominates. Therefore the applied field in this case is along the direction of Er spins and opposite to those of Fe. It is seen in fig. 2 that for x = 15 there is a discontinuity in the M dependence of H at H 2: 3 T and M starts increasing linearly from 10 emu g-’ to 35 emu g -’ as H increases from 3 to 15 T. This increase could be due to two reasons, namely, either i) the

conical spin structure of Er closes under the action of the high field since they are now parallel, or ii) the small canting of the Fe spin structure which we have mentioned earlier now begins to open up under the action of the external field because they are now antiparallel to each other. So it is seen that the high field behaviour of Fe based alloys is different from that of Co based ones. This leads us to propose the hypothesis that in the latter, due to the fact that the random anisotropy of Er is so high, that it dominates the antiferromagnetic J co_Er interaction, which therefore breaks down more easily at high fields. At this point it is necessary to discuss results from Mossbauer stud- ies in order to get some more information about the local Fe spin structure.

3.3. h4iissbauer studies with H = 0

The magnetic spectra recorded with H = 0, at 77 and 4 K are similar and fig. 3 shows the results at 4 K. They exhibit broadened lines, increasing from the inner to the outer lines, typical of the amorphous state, due to distributions of the hy- perfine field parameters. Because of the structural disorder, firstly the quadrupolar shift is assumed to be zero and secondly, each magnetic domain is assumed to have the same hyperfine field distribu- tion which therefore, does not depend on the magnetic texture. The overlapping of the external (numbered 1 and 6) and the intermediate (num- bered 2 and 5) lines does not allow us to measure very accurately the relative intensity of these dif- ferent lines, nor therefore, the angle 8 between the hyperfine field and the y-beam calculated from the ratio of the intensities of lines [Z&Z,,, = 4 sin2t9/3(1 + cos’e)], especially for low values of 13. The spectra, practically symmetrical about the

Table 1

Hyperfine parameters of Fq,Er,,B,,Si,

T H (6) WF) e

W m (mm/s) ‘. 0 (“1

4.2 0 0.13 18.9 57

77 0 0.12 18.6 63

4.2 4.4 0.12 20.2 60

4.2 8 0.12 20.4 66

158 R. Krishnan et al. / Amorphous Fe-Er-B-Si alloys and high fields

I I !-I IT)

Fig. 3. MGssbauer spectra and fitted effective field distributions for F+,Er,,B,,Sis at 4 K for HaPP = 0, 4.4 and 8 T.

centroid, were fitted with a distribution of hyper- fine fields only (table 1 and fig. 3). The 8 values indicate that Fe moments are randomly distrib- uted. This confirms what we have mentioned earlier in connection with magnetization measure- ments. Compared to the previous results on rib- bons with lower Er content [14,15], it is seen that increasing the Er content leads to a decrease in both the average isomer shift (S) and the average hyperfine field (H) values.

3.4. Miissbauer studies under applied fields

Let us consider the results of Mossbauer ex- periments under external fields of 4.4 and 8 T applied parallel to the y-beam. Fig. 3 and table 1 show the spectra and the results of fitting at 4.2 K.

Both the average effective field H,,, = H,, + HaPP and values of 8 [f? = (Herr, y)] provide evidence for the fact that the Fe spins remain randomly oriented. This behaviour is quite different from what was obtained for alloys with a lower Er content, where the Fe subnet-work dominate at all

temperatures and where the Fe structure align- ment in any applied field was attained down to 4 K [13,14]. The slight increase of the effective field relative to the hyperfine field at 0 T is in accor- dance with the dominance of the Er subnet-work at 4 K. It is recalled that the hyperfine field is antiparallel to the Fe moment.

3.5. Temperature dependence studies

The temperature dependence of the magnetiza-

tion of the samples is typical of two sub-lattice

antiferromagnetic systems. In other words, as the

temperature is decreased the magnetization first

increases and then starts decreasing. This be-

haviour in quantitative terms, indeed is to be

expected, and it is also found to depend on the

external field. Two typical cases are shown to

illustrate this. Figs. 4 and 5 show the temperature

dependences of M at various external fields for

x = 4.8 and 15, respectively. For x = 4.8 it is seen

that at a given field, as the temperature is de-

creased, the magnetization first increases showing

R. Krishnon et al. / Amorphow Fe-Er-B-S alloys and high fields 159

t

*

1 0’

. . _IT

1 1 I .

100 LOO 500

T( K)

Fig. 4. The temperature dependence of the magnetization for the alloy with x = 4.8.

a broad maximum and it starts decreasing strongly as the temperature is lowered further. This indi- cates that the negative contribution from Er is increasing with decreasing temperature. When the external field is increased the curves are translated upwards because the net alloy moment increases

-5O- T . .*

. . . 15T 9 -

. . .

..12T l *. .*

l ^.gT .

E 0. _.

..ST

M.. . . . . .

;30;... . . l _{. .} ^=:

^.

^.,y

. . . . . . . .

-6 l ^\. l ^*

1oy .

. . I I I*

0 100 200 300

T(K)

Fig. 5. The temperature dependence of the magnetization for the alloy with x = 15.

as we have discussed earlier. However, for x = 15, the situation is more complex. For H = 1 T, a compensation point can be seen around 50 K but for H > 1 T, a clear plateau is formed below a certain temperature Tf and Tf increases with the external field. A qualitative explanation would be that the Er sub-lattice moment freezes below T,, due to a large increase in the random anisotropy and hence the net alloy magnetization remains constant. This also shows that even with a field of 15 T it is not possible to align the Er moments.

This point brings further support to our explana- tion of the increase observed in the alloy magneti- zation for H > 3 T (fig. 2), namely, that this increase cannot arise from the closing of the Er cone but on the contrary from the further opening of the Fe canted spin structure. These aspects become clearer from the Mijssbauer studies as a function of temperature which are discussed be- low.

The evolution of the Mijssbauer spectra with the temperature is shown in fig. 6 for Hap,, = 8 T.

As the temperature increases the intensity of the intermediate lines remains constant until 60 K.

For higher temperatures the intensity decreases, indicating a progressive alignment of the Fe spins along the applied field in a range of temperature of about 25 K due to the decrease of the random anisotropy of Er which seems to decrease, in the presence of the external field, the canting angle of Fe spins. Fig. 7 shows the temperature depen- dence of the average effective hyperfine field for H app = 0, 4.4 and 8 T. For T > 80 K the Fe subnet-work moment dominates and the curves are all parallel and are translated by a field equal approximately to that of the applied field, indicat- ing that the Fe moments are aligned along the external field and that there is no noticeable in- duced field. The curves with applied field intersect that without the field at a temperature T * in the range 35-40 K close to the compensation temper- ature obtained from magnetization measurements.

The temperature T, from which the structure de-

parts from collinearity as determined from the

change of slope in the curves of fig. 7 increases

with the field. It is noteworthy that the depen-

dence of Tf on the Happ (fig. 8) is in agreement

with the temperatures where the alloy magnetiza-

160 R. Krishnan et al. / Amorphous Fe-Er-B-S alloys and high fields (MM/S)

-9 0 $9

IO 20 30

H(T)

Fig. 6. Miissbauer spectra and fitted effective field distributions for the alloy with x = 15 and for HaPP = 8 T at 50, 70 and 80 K.

tion curves also exhibit the beginning of the might be concluded that the anisotropy of the rare plateau as shown in fig. 5. This result shows that earth is enhanced by the applied field, leading to the higher the applied field, the higher is T,. _It an earlier freezing of the spin structure.

I I I I I L I I

20 40 60 100 120 140 160

Fig. 7. Temperature dependence of the effective field for the Fig. 8. The field dependence of T, (see the text for the

alloy with x =15. explanation of T,).

0 Ii- ₀ ^II.’ _{5.0 10.0} ^’ _15.0 ^{1 ’}

Happ (T’

R. Krishnan et al. / Amorphous Fe-Er-B-S alloys and high fields 161

4. Conclusion

In conclusion we have prepared amorphous Fe-Er-B-Si alloys and carried out magnetization and MSssbauer studies under high magnetic fields.

The Er moment at 4 K is found to be 8~~ indicat- ing a conical spin structure. This has a conse- quence on the Fe spins which also tend to depart from collinearity. For example for the alloy with 15% Er, the Er subnet-work moment dominates and the alloy moment increases suddenly when the applied field is higher than 3 T. This has been interpreted in terms of the further opening of the Fe spins which are already canted since the exter- nal field is antiparallel to them. For this alloy Mbssbauer studies also confirm the random orien- tation of Fe spins. In this alloy it is also seen that the magnetization becomes independent of tem- perature below a certain temperature. This has been attributed to the large increase in the random anisotropy of Er which leads to the freezing of the spins. Finally it is seen that the stability of antifer- romagnetic coupling between Fe-Er is stronger that between Co-Er.

Acknowledgements

The authors thank Monique Rommeleure for the chemical analysis. The high field measure- ments carried out at the Service National des Champs Intenses, Grenoble are gratefully ac- knowledged. One of us (J-T.) is grateful to Dr.

Greneche of The University of Le Mans and Dr.

Papaefthymoiu, Francis Bitter National Magnet

Laboratory, MIT, for high field Miissbauer experi- ments. The Francis Bitter National Magnet Laboratory is supported by the National Science Foundation.

References

111 121 131

141 151 WI 171 181 [91

WI 1111 WI t131 1141 (151

R. Harris, M. Plischke and M.J. Zuckermann, Phys. Rev.

Lett. 31 (1973) 160.

R.W. Cochrane, R. Harris and M.J. Zuckermann, Phys.

Rep. 48 (1978) 1.

J.J. Rhyne, J.H. Schelleng and N.C. Koon, Phys. Rev. B 10 (1974) 4672.

F. Luborsky, in: Ferromagnetic Materials, vol. 1, ed. E.P.

Wohlfarth (North-Holland, Amsterdam, 1980) p. 451.

S.G. Comelison and D.J. Sellmyer, Phys. Rev. B 18 (1978) 1377.

J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Yelon, Phys. Rev. B 29 (1984) 4176.

R. Krishnan and H. Lassri, Solid State Commun. 69 (1989) 803.

R. Krishnan and H. Lassri, Solid State Commun. 73 (1990) 467.

R. Krishnan and H. Lassri, Advances in Ferrites, Proc.

Fifth Intern. Conf. on Ferrites, eds. C.M. Srivastava and J.M. Patni (Oxford & IBH, New Delhi, 1989) p. 781.

J. Teillet and F. Varret, unpublished MOSFT program.

F. Varret, Proc. Intern. Conf. Appl. Mossbauer Effect, Jaipur (1981) 129.

J.M.D. Coey, J. Appl. Phys. 49 (1978) 1646.

R. Krishnan, K.V. Rao and H.H. Libermann, J. Appl.

Phys. 55 (1984) 1823.

A. Laggoun, J. Teillet, H. Lassri, R. Krishnan and G.C.

Papaefthymoiu, Solid State Commun. 71 (1989) 79.

J. Teillet, H. Lassri, R. Krishnan and A. Laggoun, Hyper-

fine Interactions 55 (1990) 1083.