Spin-wave excitation and Mössbauer spectrometry of amorphous interface in Tb/Fe multilayer

Journal of Alloys and Compounds 473 (2009) 25–27

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Journal of Alloys and Compounds

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j a l l c o m

Spin-wave excitation and Mössbauer spectrometry of amorphous interface in Tb/Fe multilayer

S. El Khiraoui ^a,∗ , M. Sajieddine ^a , H. Lassri ^b , M. Sahlaoui ^a

aLaboratoire de Physique et Mécanique des Matériaux, Université Sultan Moulay Slimane, FST, B.P. 523, 23000 Béni-Mellal, Morocco

bLaboratoire de Physique des Matériaux, Micro-électronique, Automatique et Thermique, Faculté des Sciences Aïn-Chock, Université Hassan II, B.P. 5366 Mâarif, Casablanca, Morocco

a r t i c l e i n f o

Article history:

Received 7 February 2008

Received in revised form 6 June 2008 Accepted 11 June 2008

Available online 22 July 2008

Keywords:

Magnetic films and multilayers Evaporation

Hyperfine interactions Spin-wave excitation Mössbauer spectrometry

a b s t r a c t

Magnetic properties of Tb(48 Å)/Fe(26 Å) multilayer were studied with Mössbauer spectrometry at dif- ferent temperatures before and after isothermal annealing at 673 K. For this last case, a significant perpendicular magnetic anisotropy is induced. This phenomenon is related to the existence of an amor- phous and homogenous Tb–Fe alloy, located at the interfaces, which is produced by interdiffusion during the heat processing. The thermal evolution of hyperfine field deduced for interfacial Tb–Fe alloy is found to obey the Bloch law. The spin-wave stiffness constant, the distance between nearest magnetic atoms and the exchange parameterAwere calculated from the experimental results.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Rare earth/transition metal (RE/TM) multilayers have been extensively studied for their structural and magnetic properties.

Perpendicular magnetic anisotropy has been observed for same of them which makes them potential candidates for perpendicu- lar magnetic recording devices [1–6]. The magnetic parameters in RE/Fe multilayers are iron ferromagnetism, strong anisotropy and RKKY (Ruderman–Kittel–Kasuya–Yosida) type interactions in the rare earth, ferromagnetic (ferrimagnetic) coupling between iron and light (heavy) rare earth. In Tb/Fe multilayer, the occurrence of perpendicular magnetic anisotropy in these multilayered systems depends mainly on the extensions and the nature of the interfa- cial Fe–Tb region [1–3,7–8]. The morphology and the extension of the interfaces in relation with the substrate temperature and the deposition process are investigated by

Fe Mössbauer spectrom- etry method [9]. The study showed that perpendicular magnetic anisotropy is enhanced when the interface consists of an amor- phous Fe–Tb alloy [2,10] or where the multilayers have annealed at certain temperature and duration of annealing [3,10–11]. In this work we describe the results of our studies of Tb(48 Å)/Fe(26 Å) multilayer prepared by evaporation and annealed under a vacuum

∗ Corresponding author. Tel.: +212 23 48 51 22; fax: +212 23 48 52 01.

E-mail address:elksaliha@yahoo.fr(S. El Khiraoui).

of 10

Torr at 673 K during 6 h, we have studied their interface by exploiting the evolution of the hyperfine field distribution in func- tion of the temperature of measurement and discuss them in the context of conclusions published in the literature [1–3,7,10].

2. Experimental

Multilayered films of nominal [Tb(48 Å)/Fe(26 Å)]77thicknesses were grown by evaporation and alternate condensation of each element in a high vacuum cham- ber. The pressure during the evaporation was kept below 10⁻⁸Torr. The fabrication procedure has been reported in detail in Ref.[12]. The substrate was kept at a fixed temperature of 90 K during the deposition process. To prevent oxidation upon expo- sure to atmosphere the sample was overcoated with 140 Å amorphous silicon. After annealing carried out under a vacuum of 10⁻⁷Torr, the sample is conditioned in suitable thickness for spectrometry Mössbauer. The Mössbauer measurements were performed at different temperatures using transmission Mössbauer spectrometry.

The⁵⁷Co (Rh) source was mounted on a constant acceleration triangular motion velocity transducer. The plane of the sample was set perpendicular to the incident

␥-beam direction in a cryostat with liquid helium, of traditional invoice. This unit makes it possible to cover the temperature range 4.2–300 K. The Mössbauer spectra were fitted with a least-squares technique. The direction of the magnetic anisotropy is extracted from the line intensity ratio 3:x:1:1:x:3 of the six Mössbauer lines of the magnetically split sextets, wherexis related tobyx= 4 sin²/(1 + cos²),is the angle between the direction of the magnetization and the direction of the␥-beam.

The mean values of isomer shift are given with respect to the␣-Fe at 300 K.

3. Results and discussion

The Mössbauer spectra and hyperfine field distributions for the as-deposited and annealed multilayer measured at 4.2 K are

doi:10.1016/j.jallcom.2008.06.028

26 S. El Khiraoui et al. / Journal of Alloys and Compounds 473 (2009) 25–27

Fig. 1.Mössbauer spectra for Tb(48 Å)/Fe(26 Å) multilayer collected at 4.2 K and corresponding hyperfine field distributions: (a) as deposited and (b) annealed.

presented in Fig. 1(a and b). The spectra exhibits two components:

the usual six absorption peaks typical to b.c.c. iron, due to atoms located in the core of iron layers (solid lines) and a broad contri- bution with a depleted hyperfine field. The latter contribution has been analysed using the histogram method of combined distribu- tions of hyperfine fields

) and isomer shifts. The hyperfine fields distribution displayed in Fig. 1a, present two regions [13]:

(I) a broad low field part [0–170 kOe], is attributed to the Fe atoms with low magnetic moments near the Tb layer and (II) a high field part [170–320 kOe], where the probability increases linearly with the

is attributed to the Fe atoms little perturbed by the Tb atoms. The area of the interfacial component is equivalent to 12 Å of the thickness of iron layer. For the annealed sample, the form of this component is characteristic of an amorphous alloy and involv- ing 22 Å of the thickness of iron layer, this has been attributed to an interdiffusion between Fe and Tb atoms at the interface. Table 1 lists the fitted hyperfine parameters values deduced by fitting the Mössbauer spectra. We can note that after annealing, the angle decrease indicating a significant improvement of the perpendicular magnetic anisotropy.

The form of the distribution shows that a homogenous and amorphous interfacial alloy is formed after annealing. In such amor- phous alloy, we can apply the theory of spin waves to calculate the exchange stiffness constant. The thermal evolution of the average hyperfine field relating to the interfacial component made it pos- sible to estimate the Curie temperature

= 450 K of formed alloy at the interface. This temperature corresponds to a concentration

out of iron for the amorphous alloys Fe

Tb

ranging between 0.6 and 0.63 [14].

According to [15], each Fe atom is surrounded by 12 neighbours atoms, the number of

first neighbours Tb of Fe atom placed at

Table 1

Mössbauer parameters of⁵⁷Fe nuclei for as deposited and annealed multilayer mea- sured at 4.2 K

Sample Hhyp(kOe) Area (%) of interface component

Thickness of interface (Å)

(^◦)

As deposited 282 46 12 48

Annealed 250 84 22 27

the origin for a concentration

of iron is deduced from a binomial distribution. The probabilities deduced from the curves of hyperfine field distribution correspond to

= 7 at 4.2 K.

Extensive reports are available in literature to prove the exis- tence of long wavelength magnetic excitation in amorphous ferromagnets [16,17]. In amorphous materials, the mean field the- ories do not always explain the local magnetic excitations. For an accurate description of the low-temperature behavior of magnetic properties, the spin-wave theory can be used. The spin-wave energy can be expressed by

hω(q)

2

(1) where

is the effective energy arising from dipole–dipole interactions,

is the wave vector of the spin wave,

is the spin- wave stiffness constant, and the higher-order dispersion terms

have generally been found to be negligible in amorphous alloys.

Thus, it would be of interest to examine the low-temperature behavior of systems with this type of magnetic ordering. The inter- diffusion at the interface between Tb and Fe layers change the ferromagnetic order, and the effect of Tb seems to be specific.

An interpretation of the temperature dependence of the hyper- fine magnetic field in terms of the magnetization behavior is plausible if the reduction of the data to 4.2 K is possible. Thus, the effects of the surface modification of the magnetic ground state can be eliminated. Even for thin films we can assume the propor- tionality between the reduced

and the reduced magnetization using arguments holding for bulk iron [18], because the character of the hyperfine coupling remains unchanged. The relative change of magnetization

(T) and the average hyperfine field

(T) at low temperatures in ferromagnetic materials obeys the expression [19–21]:

Hhyp

(T )

(4.2 K)

H_hyp

(4.2 K)

(2)

Equation (2) is a good approximation of low-temperature magne-

tization in both crystalline and amorphous ferromagnets [22,23],

where

is related to the spin-wave stiffness constant

as given by

S. El Khiraoui et al. / Journal of Alloys and Compounds 473 (2009) 25–27 27

Fig. 2.T^3/2dependence of the hyperfine magnetic field relating to the interfacial component in annealed multilayer.

the relation [24], namely:

B=

2.612

(0)

B

4D

(3) where

is the g-factor (g

= 2),

is the Boltzmann constant,

is the Bohr magneton and there is a proportionality between

and the magnetization

which is of 158 kOe/

for amorphous rare earth–Fe alloys [25–26]. The temperature dependence

of

(T)/H

(4.2 K) is shown in Fig. 2. It is seen that Bloch’s law is verified about

/3, this result is similar to that reported in Ref. [27].

In Fig. 2, we have adjusted experimental data by using Eq. (2) which allowed us to find

value of about 3.03

10

K

.

Using the value of the

coefficient

from the Mössbauer hyperfine field data [16], it is possible to calculate the spin stiffness constant using (3) which is found to be equal to 72.7 meV Å

.

, which measures the range of the exchange interaction, is about 0.16 meV Å

K

which is less than 0.27 meV Å

K

in crystalline iron [20].

According to the Heisenberg model,

fulfill the equation:

D= kBr_ij²TC

2(S

1) (4)

where

is the distance between nearest magnetic atoms (Fe) and

is the spin moment of the Fe atom. Using Eq. (4) we found

to be about 2.58 Å which is in agreement with previous work [28].

The anomalously small values of

imply that the bulk mag- netization is being renormalized with increasing temperature in large degree by demagnetization processes other than the long- wavelength spin waves bound to the character antiferromagnetic of the Tb atoms. Babic et al. [29], on the basis of magnetization data,

have suggested a possible low-lying Stoner band with a gap in the region 4–6 meV to explain the demagnetization.

From the coefficient

mentioned above, it is possible to calcu- late the exchange constant

is related to

by the following relation:

A= MS

(0)D 2g

(5) The value obtained for

is about 28.8 meV.

4. Conclusion

Perpendicular magnetic anisotropy for a Tb(48 Å)/Fe(26 Å) mul- tilayer is reinforced by the presence of an amorphous alloy formed at the interface between Fe and Tb layers [2]. We have studied the magnetization of the interface of Tb(48 Å)/Fe(26 Å) annealed multilayer in terms of the spin-wave theory which allowed us to determine the Fe–Fe distance, the nature of the interactions in the magnetic interface of sublattice and the exchange parameter

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