Effect of Tm substitution on the magnetic properties and local magnetic anisotropy of amorphous Fe80-xTmxB20 ribbons

Effect of Tm substitution on the magnetic properties and local magnetic anisotropy of amorphous Fe _{80 − x} Tm _x B ₂₀ ribbons

E.H. Sayouty

, F. Annouar

, H. Lassri

, N. Randrianantoandro

, J.M. Greneche

aLaboratoire de Physique Nucl´eaire, Universit´e Hassan II, Facult´e des Sciences Ain Chock, B.P. 5366 Maˆarif, Route d’El Jadida, km-8 Casablanca, Morocco

bLaboratoire de Physique des Mat´eriaux et de Micro-´electronique, Universit´e Hassan II, Facult´e des Sciences Ain Chock, B.P. 5366 Maˆarif, Route d’El Jadida, km-8 Casablanca, Morocco

cLaboratoire de Physique de l’Etat Condens´e, UMR CNRS 6087, Universit´e du Maine, Le Mans, France

Received 4 January 2005; received in revised form 24 January 2005; accepted 24 January 2005 Available online 9 March 2005

Abstract

We have carried out magnetic and M¨ossbauer studies of amorphous Fe80−xTmxB20alloys (0≤x≤16). With an increasing Tm content, both the Curie temperature TCand the magnetic moment of Fe atomµFedecrease. We have extracted the value of exchange constant A from TC

and that of the local magnetic anisotropy constant KLfrom the coercivity. M¨ossbauer studies were performed in a transmission geometry and also using the conversion electron spectroscopy. Both M¨ossbauer spectrometry techniques show that the average hyperfine field decreases linearly with the addition of rare-earth.

© 2005 Elsevier B.V. All rights reserved.

Keywords: Amorphous ribbons; Local magnetic anisotropy; M¨ossbauer spectrometry

1. Introduction

Metallic glasses based on transition metals and metalloids have been studied extensively. The influence of addition of various magnetic and/or non-magnetic atoms on the magnetic and transport properties has been well documented[1–5]. In particular Fe-based alloys produced by melt spinning tech- niques hold promise for several applications such as mag- netic shields, power and electronic transformers and record- ing heads. Many efforts have been made to study the influence of the rare-earth elements R substitution in Fe-based amor- phous alloys[6–11]. One of the fascinating behaviors in such alloys arises from the random magnetic anisotropy which is the result of the topological disorder present in these materi- als. Some theoretical models have been developed to calcu- late the random anisotropy and related parameters from the analysis of the approach to magnetic saturation[12,13]. We have briefly reported our first magnetic studies of amorphous

∗Corresponding author. Tel.: +33 2 4383 3301; fax: +33 2 4383 3518.

E-mail address: greneche@univ-lemans.fr (J.M. Greneche).

Fe80−xTmxB20 alloys where the Tm spin structure is found to be non-collinear [14] while several important magnetic parameters, such as local magnetic anisotropy constant, ex- change constant, ferromagnetic correlation length have been derived[11]. The magnetic properties and the structural re- laxation of these magnetic amorphous alloys have been stud- ied mainly by magnetic measurements and⁵⁷Fe M¨ossbauer spectrometry. In this work, we describe magnetic and par- ticularly M¨ossbauer studies on Fe_80−xTm_xB₂₀, performed in both transmission and reflexion geometries.

2. Experimental methods

Amorphous Fe_80−xTm_xB₂₀alloys with 0≤x≤16 all ex- pressed in at.%, were quenched by melt spinning in pure argon atmosphere. The starting materials were of purity bet- ter than 4N. The ribbon samples were about 30␮m thick with different widths varying from about 3 to 5 mm. X-ray diffraction was used to verify the amorphous structure. The chemical composition of the samples was determined by elec-

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doi:10.1016/j.jallcom.2005.01.047

tron probe microanalysis. Vibration sample magnetometer was used to measure for fields up to 1.8 T. The M¨ossbauer measurements were performed at 300 K using the conversion electron M¨ossbauer spectrometry (CEMS) and transmission M¨ossbauer spectrometry (TMS) at 300 and 77 K. The CEM spectra were determined with the use of a flowing He-5% CH4

gas proportional counter in which the sample was placed in backscattering geometry. The⁵⁷Co(Rh) source was mounted on a constant acceleration triangular motion velocity trans- ducer. The values of isomer shift are referred to that of␣-Fe absorber at 300 K. The M¨ossbauer spectra were fitted with a least-square fitting program using the histogram method, constraining the linewidth of each elementary component to be the same[15]. The broad lines and the asymmetrical shape of spectra were described by a discrete distribution of hyper- fine fields linearly correlated to that of isomer shift.

3. Results and discussion 3.1. Magnetic studies

At 10 K, the magnetic saturation occurs for applied fields smaller than 1 T, although a very small high field suscep- tibility (χhf) is evidenced, the highest value is of the order of 4×10⁻⁴emu/g Oe, as usually observed in such metal- lic glasses. The spontaneous magnetization µa (expressed in Bohr magnetons␮B) decreases with the addition of Tm which indicates the antiparallel coupling between Fe and Tm moments. It is well established that the Fe momentµFedi- minishes when it is alloyed with a rare-earth metal due to 3d–5d hybridization, but this effect is negligible for small concentrations. So we considerµFe= 2.08␮B obtained for the alloy with x = 0, and assume this to be the same in the alloy with x≤6. Knowing the alloy moment (µa) and using the relation,

µa= |MFe−MTm| = |(80−x)µFe−xµTm|/100, (1) where M_Fe and M_Tm denote the magnetization of the two sublattices, Tm momentµTm is estimated at 5.7±0.5␮B. This moment which is a projection along the applied field is smaller than the theoretical value (g J␮B) of 7␮B. This re- duction could be attributed to the non-collinear and conical spin structure of Tm. This phenomenon is due to the strong random anisotropy of Tm. NowµFe for other alloys could be calculated based on the reasonable assumption thatµTm

is independent of x. The values of the Fe moment are shown inTable 1. The decrease of the Fe moment with the Tm con- tent can be understood as due to an increasing filling of the 3d spin-up band of the Fe atom by the 6s²/5d electrons of Tm and by the sp electrons from B atoms since the relative concentration of B with respect to Fe increases.

Curie temperatures T_Cof amorphous Fe_80−xTm_xB₂₀ al- loys were determined from the thermomagnetic data. The values of T_Cthus obtained are reported inTable 1. The de- crease in T_C could be caused by the weakening of Fe–Fe interaction and the results are characteristic of antiferromag- netic interaction between Tm and Fe atoms which is well known.

As regards the anisotropy, Alben and Becker[16] have developed a model which relates the coercivity (H_C) to the local anisotropy constant (K_L) and one can write

HC=K_L⁴d⁶/20A³M0, (2) where d is the characteristic correlation length of anisotropy directions (short range structural order), and we take d = 10 ˚A, which was determined experimentally on similar alloys [17,18]. M0 is the saturation magnetization. The exchange constant follows from the relation[9]:

A=CSFekBTC/4(1+SFe)rFeFe, (3) where C = (80−x)/100 is the iron concentration, SFe is the spin of Fe, and rFeFe, the interatomic distance, is taken as 2.5 ˚A. We found that exchange constant A decreases from 38×10⁻⁸to 19×10⁻⁸erg/cm when x is increased from 0 to 15 (Table 1).

From the experimental H_Cvalues we calculated the ran- dom local anisotropy constant and the results at 10 K are shown inTable 1. H_C gradually increases with increasing Tm content and the local magnetic anisotropy constant (K_L) shows a small increase.

The values of K_L are in the range 0.8×10⁷– 1.3×10⁷erg/cm³, which are about 10 times higher than usual values for anisotropy in crystalline transition metals. In our case, we have the contribution of two sub-networks at the magnetic anisotropy in one hand the Tm which is a rare- earth possessing an important magnetic anisotropy (KTm), and on the other hand the Fe for which the mean magnetic moment is lower than that of the metallic counterpart. This situation shows that the Fe orbital momentum is incompletely quenched in the alloy, then we will find a spin–orbit interac- tion, giving rise to a local magnetic anisotropy in the Fe sub-

Table 1

Some magnetic parameters of amorphous Fe80−xTmxB20alloys. Saturation magnetization, Fe moment, exchange constant, coercivity, local magnetic anisotropy constant, at 10 K and Curie temperature of amorphous Fe80−xTmxB20alloys

x M0(emu/g)±2 emu/g µFe(␮B)±0.02␮B TC(K)±10 K A (10⁻⁸erg/cm) HC(Oe)±1 Oe KL(10⁷erg/cm³)

0 190 2.08 660 38 2.6 0.8

4 140 2.00 590 32 6.7 0.8

8 99 1.92 518 27 30 0.9

12 58 1.82 433 22 260 1.2

15 37 1.80 370 19 900 1.3

network (K_Fe). Finally the magnetic random local anisotropy constant evaluated by Sarkis and Callen[19]and by Itri et al.[20]is a function of the inter-sublattice exchange inter- actions (nTmFe= 40 T/␮Bevaluated by fitting ofµa–T curve [21]) and the sub-networks local anisotropies.

This model gives us a local magnetic anisotropy constant by

KL≈(KFe+KTm)× {1+[2(KTmMFe

−KFeMTm)²/nTmFeMFeMTmµ²a(KFe+KTm)]}⁻¹. (4) To apply this model to the amorphous ribbons, we consider that the sublattice anisotropy constants found in formula (4) are taken per their contents in the alloy. Thus for the transition metal, the anisotropy constant is expressed asKFe=(80− x)KFe^atom/100 whereK^atomFe is the local anisotropy constant per atom for the Fe. The same manner can be used to determine the rare-earth anisotropy constant. The local anisotropies per atom are found to be 10⁷and 5×10⁷erg/cm³for Fe and Tm, respectively, and are independent of x. The present value for K^atom_Fe is in agreement with that calculated by F¨ahnle using a semi-empirical Hartree–Fock perturbation approach for the local spin–orbit coupling operator[22].

3.2. M¨ossbauer studies

Fig. 1shows some transmission M¨ossbauer spectra of the amorphous Fe_80−xTm_xB₂₀ alloys for 0≤x≤16 at 77 and 300 K. The sextets exhibit broadened lines due to the atomic structural disorder, and a small asymmetry. These two fea- tures can be described by means of a hyperfine field distri- bution correlated to that of an isomer shift. The correlation

is assumed to be linear. In agreement with the asymmetrical shape of sextets, the isomer shift decreases together with the hyperfine field, i.e. when the Tm content increases. The de- crease when Tm is substituted for Fe can be explained as the total s density at the⁵⁷Fe nucleus increases with Fe replaced by Tm, i.e. decrease of the 3d electron density. 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 distribution which, therefore, does not depend on the magnetic texture. The hyperfine field distributions P(B_hf) of the amorphous Fe_80−xTm_xB₂₀alloys for 0≤x≤15 at 300 and 77 K are shown inFig. 2. One ob- serves thus that P(B_hf) shifts to lower B_hf values when Tm concentration increases in agreement with magnetic data, i.e.

decrease of T_C, as previously discussed. They can be well described by two Gaussian components, one distinguishes a high field component attributed to Fe preferentially sur- rounded by B atoms and a low field component which tends to grow when the Tm content increases, as observed in the case of Fe74−xAlxEr6B20amorphous alloys when the Al con- tent increases[23]. Let us first emphasize that the position of this component cannot be attributed to a misfit procedure, i.e.

the presence of a texture effect which remains rather low in all samples. Then this suggests that the low-field peak arises from non-magnetic Fe atoms (i.e. atoms with no intrinsic mo- ment) or paramagnetic atoms (i.e. atoms with a moment but subject to such a weak exchange interaction that they do not order magnetically).

As shown inFig. 3, CEM spectra show quite similar hyper- fine structure as those of TM spectra but the mean hyperfine field values obtained from CEM spectra are found slightly larger. In addition, the CEMS measurement showed that in the surface layer (about 1000 ˚A thick) the iron spins are pref- erentially oriented in the plane of the sample of the ribbons

Fig. 1. Transmission M¨ossbauer spectra at 300 K (left) and 77 K (right).

Fig. 2. P(Bhf) at 300 K (left) and 77 K (right) for amorphous Fe80−xTmxB20alloys with x = 4, 8, 12.

studied while they are close to be randomly oriented in the total thickness according to TMS measurement.

The variations of the average hyperfine field B_hf with Tm content are shown inFig. 4. The values extrapolated to x = 0 are in good agreement with those published for Fe₈₀B₂₀ [24]. It is clear that the increase in the Tm content leads to a decrease of the hyperfine field values. Many similarities exist between the results obtained on different amorphous alloys:

Fe_80−xTm_xB₂₀, Fe_80−xV_xB₁₂Si₈ and Fe_80−xR_xB₂₀ (R = Er, Ho; 0≤x≤16)[5,25,26], as previously reported.

B_hfshows a linear decrease with Tm (Fig. 4) and the rate of decrease of the hyperfine field in T can be represented by the relationBhf= 25.2−0.85x, where x is the Tm concen- tration. This decrease is attributed to the decrease in the Fe sub-network magnetization that arises from the hybridization

Fig. 3. Conversion electron M¨ossbauer spectra and corresponding P(Bhf) at 300 K for amorphous Fe80-−xTmxB20alloys with x = 4, 8, 12, and 15.

Fig. 4. Composition dependences of average hyperfine field at 300 K for Fe_80−xTmxB20ribbons.

of 3d orbitals of Fe with the 5d ones of Tm, as pointed out earlier. However, the metalloid atoms have also some influ- ence on the hyperfine magnetic field: both by their electrons and by the perturbation of the Fe–Fe distances of near-by Fe atoms. The widths of the hyperfine field distribution do not increase very much with the increase in Tm content. How- ever, the Tm atoms reduce the average hyperfine magnetic field at 300 K by as much as about 0.85 T per one substituted Tm atom.

4. Conclusions

In conclusion we have prepared amorphous Fe80−x

Tm_xB₂₀alloys and carried out magnetization and M¨ossbauer studies. The substitution of iron atoms in Fe₈₀B₂₀amorphous alloys by Tm atoms causes changes in the magnetic moment of Fe, Curie temperature, exchange constant and coercitive force. It was found that when the Tm content increases the coercitive field increases whereas both the Curie temperature

and the iron magnetic moment decrease. At 10 K, the local magnetic anisotropy of the ribbons is close to 10⁷erg/cm³. At 300 and 77 K, a study by transmission M¨ossbauer spectrom- etry shows that the hyperfine field decreases with increasing Tm content in accordance with the decrease in the Fe mo- ment due to hybridization effects. On the other hand, the 300 K CEMS measurement showed that in the surface layer (about 1000 ˚A thick) the iron spins are preferentially oriented in the plane of the sample in all the ribbons studied.

Acknowledgements

The authors are grateful to Prof G. Marest (Lyon, France) for performing CEM spectra. The last part of this work was supported by A.I. no. MA/04/95.

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