Magnetic and M ossbauer studies of amorphous . Fe–Al–Er–Bribbons
H. Oukris
a,*, H. Lassri
a, E.H. Sayouty
b, J.M. Greneche
caLaboratoire de Physique des Materiaux et de Micro-! electronique, France!
bLaboratoire de Physique Nucleaire, Universit! e Hassan II, Facult! e des Sciences Ain Chock, B.P. 5366 Ma! arif, Route d’El Jadida,# Km-8, Casablanca, Morocco
cLaboratoire de Physique de L’Etat Condense, UMR CNRS 6087, Universit! e du Maine, 72085 Le Mans Cedex 9, France!
Received 25 October 2001
Abstract
We have studied the magnetization of melt spun amorphous Fe74xAlxEr6B20alloys with 0pxp15 under magnetic fields up to 8 T, and have analyzed the results at 4.2 K on the basis of the random magnetic anisotropy (RMA) model.
The local random anisotropy constant is found to be (1.570.2)107erg/cm3and is aluminum content independent.
The Mossbauer studies show that the average hyperfine field decreases linearly with the addition of Al.. r2002 Elsevier Science B.V. All rights reserved.
PACS: 74.25.Ha; 75.50.Kj; 75.50.Lk; 75.30.Gw; 76.80.+y
Keywords: Amorphous ribbons; Rare earth-transition metal alloys; Magnetization; Local random anisotropy; Mossbauer effect.
1. Introduction
Amorphous alloys based on rare-earth (R), transition metal (T) and metalloid (M) elements, such as T–R and T–R–M, show interesting magnetic properties and have been studied in the past by a number of authors [1–3]. As usually observed in the intermetallics, the magnetic mo- ment of the heavy rare-earth in amorphous alloys couples antiferromagnetically to that of the transition metal. One of the fascinating behaviors in such amorphous alloys arises from the random magnetic anisotropy (RMA) which results from the topological disorder present in these materials.
Indeed, Harris, Plischke and Zuckermann have
first introduced the random axial anisotropy model for amorphous alloys by adding to the exchange interaction term, a single ion magnetic anisotropy term with different randomly oriented directions at each site [4].
Rare-earth metal atoms with an orbital moment are thus well known to give rise to large random anisotropy in amorphous alloys [4]. Some theore- tical models have been developed to calculate the random anisotropy and related parameters from the analysis of the approach to magnetic saturation [5,6]. In this work, we describe magnetic and Mossbauer studies performed on. amorphous Fe74xAlxEr6B20 alloys prepared by conventional melt-spinning technique and the results are discussed on the basis of the RMA model.
*Corresponding author.
0304-8853/03/$ - see front matterr2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 0 5 9 - 8
2. Experimental methods
Amorphous Fe74xAlxEr6B20 ribbons with 0pxp15; were quenched in an inert atmosphere of Ar using the melt-spinning technique. The melt- spun ribbons were about 30mm thick with different widths varying from about 3 to 5 mm.
X-ray diffraction was used to check the amor- phous structure. The exact chemical composition of the samples was determined by electron probe microanalysis. The magnetization was measured in the temperature range 4.2–300 K, under applied fields up to 8 T. The Curie temperature TC was also determined using a vibration sample magnet- ometer. Mossbauer samples made of parallel. ribbons were placed perpendicular to the g-beam.
We used a triangular waveform spectrometer and a source of 57Co diffused into a rhodium matrix.
Mossbauer experiments without magnetic field. were performed at 77 and 300 K. The Mossbauer. spectra were fitted with a least-square fitting program using the histogram method, constrain- ing the linewidth of each elementary component to be the same. The broad lines and the asymmetrical shape of spectra were described by a discrete distribution of hyperfine fields linearly correlated to that of isomer shift. The isomer shift values are quoted to that of a-Fe at room temperature.
3. Results and discussion 3.1. Magnetic studies
The field dependence of magnetization shows that saturation is attained only forH of about 3 T at all temperatures. A very small high field susceptibility (whf) is seen and weakly increases with Al content, the highest value is of the order of 104emu/gOe as generally observed in such metallic glasses. To obtain spontaneous magneti- zation values, the linear part of M¼fðHÞ has been extrapolated atH¼0:Table 1 shows the Al concentration dependence of the magnetizationM at 4.2 K and TC: Both M and TC decrease with addition of Al. As the Er concentration is constant, those decreases are due to the dilution
of Fe in the alloy with the increasing Al content in the first coordination shell of Fe atoms and due to the antiferromagnetic coupling between Fe and Er moments.
3.2. Random magnetic anisotropy modeling
The approach to saturation in the magnetization can be described in the following two ways according to Chudnovsky et al. [6–8]. For applied fields higher than the exchange field H>Hex;the field dependence is expected to follow anH2law, whereas when HoHex; which incidentally is appropriate to our study, the dependence is best described by anH1=2 law. Therefore, in the latter case if one plotsMas a function ofH1=2;a linear dependence will be obtained and one can then write;
ðM0MÞ=M0¼ ðHS=HÞ1=2=15; ð1Þ where
HS¼Hr4=Hex3: ð2Þ The exchange fieldHexcan also be expressed as Hex¼2A=M0ðRaÞ2; ð3Þ where Ra is the length over which the local anisotropy axes show a correlation (short-range structural order). We assume R¼10A, as deter-( mined experimentally on similar alloys [9–13]. The exchange constant follows from the relation [14]:
A¼CSFekBTC=4ð1þSFeÞrFeFe; ð4Þ where C is the iron concentration, SFe is the Fe spin, andrFeFe;the interatomic Fe–Fe distance, is taken as 2.5A.(
Table 1
Spontaneous magnetization at 4.2 K and Curie temperature of amorphous Fe74xAlxEr6B20alloys
x M(emu/g) (72 emu/g) TC(K) (710 K)
0 94 540
4 89 527
6 87 512
10 74 488
15 71 452
M0 is the saturation magnetization andHr the random local anisotropy field. They are related to the local anisotropy energy KL by the relation [7,12]
Hr¼2KL=M0: ð5Þ
Fig. 1 shows the H1=2 dependence of M for 0pxp15 at 4.2 K. The data points align well in the intermediate regime HoHex; one can obtain HSfrom the slopeM0ðHS1=2Þ=15:But one observes a deviation from the linear dependence in the high field regimeH>Hex:This field region is known as the crossover field. Table 2 shows the various
parameters obtained from the analysis of the data using the models described above.
Following Chudnovsky’s work [6–8], the dimen- sionless parameterlis expressed as
l¼ ð2=15Þ1=2Hr=Hex¼ ð2=15Þ1=2KLR2a=A: ð6Þ
It is found that the l is less than unity in our alloys (Table 2), that suggest a ferrimagnetic system with weak anisotropy. In addition, the exchange constant decreases when the Al content increases and the local energy anisotropy is practically independent of Al content.
The values ofKLdeduced from the Chudnovsky’s model are in the range 1.4–1.6107erg/cm3, which are about ten times higher than normal 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 Er which is a rare earth possessing an important magnetic anisotropy, and on the other hand the Fe for which the mean magnetic moment is lower than that of the metallic counter- part. This situation shows that the Fe orbital momentum is incompletely quenched in the alloy, then we will find a spin-orbit interaction which will give rise to a local magnetic aniso- tropy in the Fe sub-network [15]. Finally the magnetic random local anisotropy constant evaluated by Sarkis [16] is a function of the inter-sublattice exchange interactions (nErFe¼ 30T=mB evaluated by fitting of M–T curves [17]) and the sub-networks local anisotropies. The Sarkis’s model gives us an effective anisotropy
Fig. 1. TheH1=2dependence of the magnetizationMat 4.2 K.
Table 2
Some magnetic parameters of Fe74xAlxEr6B20alloys
x M0(emu/g)
(70.5 emu/g) at 4.2 K
A(108erg/cm) at 4.2 K
KL(107erg/cm3) at 4.2 K
lat 4.2 K /BS(T) (70.5 T) at 77 K
/BS(T) (70.5 T) at 300 K
0 95.0 26.6 1.40 0.19 24 20.3
4 91.6 26.1 1.37 0.19 22 18.6
6 91.3 24.7 1.37 0.20 21.3 18
10 78.0 23.3 1.44 0.24 19.3 15.3
15 77.6 19.2 1.66 0.32 16.8 12.9
constant by
Keff¼KL¼ ½KFeþKEr
þ ð2KErKFe=nErFeMErMFeÞ=½1þ2ðKFe MFe2 þKErMEr2Þ=nErFeMErMFeM2: ð7Þ To apply this model to the amorphous ribbons we consider that the sublattice anisotropy con- stants found in formula (7) are as per the contents in the alloy. Thus for the transition metal we can write the anisotropy constant as KFe¼
½ð74xÞ=100KFeatom where KFeatom is the local anisotropy constant per atom for the Fe, we follow the same way with the rare earth constant.
We take the rare earth anisotropy per ion to be constant, equal to 2.5108erg/cm3 which is the value evaluated for the Co35Er65 amorphous ribbon [18], and the local anisotropies per atom are found to be 5107for Fe and are independent of Al content.
The values for KFeatom deduced by us are in agreement with the values of the local magnetic anisotropy calculated by F.ahnle using a semi- empirical Hartree–Fock perturbation approach for the local spin-orbit coupling operator [19].
3.3. Mossbauer studies.
Fig. 2 shows the Mossbauer spectra of amor-. phous Fe59Al15Er6B20 alloy measured at 77 and 300 K. The spectra are characteristic of magnetic amorphous alloys. The sextets exhibit broadened lines due to the atomic structural disorder, and a small asymmetry. These two features can be described by means of an hyperfine field distribu- tion linearly correlated to that of an isomer shift.
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 PðBÞ of the amorphous Fe74xAlxEr6B20 alloys for 0pxp15 at 77 K are shown in Fig. 3. They can be well described by two Gaussian compo- nents, the center positions of which are decreasing when the Al content increases. Indeed, it is seen that as Al concentration increases, the
bimodal shape of PðBÞ is rather unchanged, but the PðBÞ shifts to lower B values. Nevertheless, a low field component tends to grow in the PðBÞ distribution when the Al content increases as observed in the case of Er. Let us emphasize that the position of this component cannot be attributed to a texture effect, i.e. a misfit proce- dure. Consequently, this contribution might be assigned to an increasing number of Al within the first Fe coordination shell, that is in agreement with the decrease of M and Tc; as previously mentioned.
Fig. 2. Mossbauer spectra for amorphous Fe. 59Al15Er6B20
alloys at 77 and 300 K.
Fig. 3. Hyperfine field distributions PðBÞ for amorphous Fe74xAlxEr6B20alloys with 0pxp15 at 77 K.
The angleybetween the hyperfine field and the g-beam is given by the ratio of the intensities of lines
A2¼I2;5=I1;6¼4=3 sin2y=ð1þcos2yÞ: ð8Þ As y changes from 01 to 901, the ratio A2 increases from 0 to 4/3. For a completely random distribution of magnetic moments, A2¼2=3:The values ofywhich range from 601to 651, show that the iron moments are weakly preferentially or- iented within the ribbon plane.
Many similarities exist between the results obtained with our Fe74xAlxEr6B20 samples and the Fe80xErxB12Si8alloys studied by us [20,21]. It is also noted that the amorphous Fe74Er6B20alloy exhibits a smaller isomer shift (0.13 mm/s) than 0.20 mm/s for amorphous Fe80B20 alloy [22,23].
The shift, produced by the chemical environment, results from the change in shielding the s electron experienced when the 3d electron density is changed.
The variations of the average hyperfine field /BS with increasing Al concentration at 77 and 300 K is shown in Table 2. It can be seen that B diminishes with increasing Al content, and the rate of decrease of the hyperfine field in T can be represented by the relation B¼240:48x at 77 K, wherexis the Al concentration.
4. Conclusion
In conclusion, we have prepared amorphous Fe74xAlxEr6B20 alloys and carried out magneti- zation and transmission Mossbauer studies. It was. found that when the Al content increases both the magnetization and the Curie temperature decrease.
The random local anisotropy is close to 1.5107erg/cm3. The exchange field is higher than that of the random anisotropy field, which corresponds to a ferrimagnetic system with weak
anisotropy. A study by M.ossbauer spectrometry shows that the average hyperfine field decreases linearly with increasing Al content.
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