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As the Cr concentration increases, the ferromagnetic fraction, magnetization, and magnitude of the hyperfine field decrease, whereas the magnitude of the isomer shift increases.. The Fe

Structural and magnetic studies of sputtered Fe 1−x Cr x thin films

N H Duc, A Fnidiki, J Teillet, J Ben Youssef, and H Le Gall

Citation: Journal of Applied Physics 88, 4778 (2000); doi: 10.1063/1.1289780

View online: http://dx.doi.org/10.1063/1.1289780

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/88/8?ver=pdfcov

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Structural and magnetic studies of sputtered Fe1 À xCrx thin films

N H Duca)

Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334 Nguyen Trai,

Thanh Xuan, Hanoi, Vietnam

A Fnidiki and J Teillet

Laboratoire de Magne´tisme et Applications, GPM-UMR 6634, Universite´ de Rouen,

76821 Mont-Saint-Aignan, France

J Ben Youssef and H Le Gall

Groupe des Laboratoires de Bellevue, CNRS, 92195 Meudon Cedex, France

共Received 1 March 2000; accepted for publication 28 June 2000兲

X-ray diffraction, magnetization, and Mo¨ssbauer effect investigations have been performed for

sputtered Fe1⫺xCrx (0⭐x⭐0.54) thin films The body-centered-cubic 共bcc兲 phase appears for x

⬍0.32, while the␴phase is formed for 0.38⭐x⭐0.44 For 0.32⭐x⬍0.38 and 0.44⬍x⭐0.54, the

samples are composed of both bcc and ␴ phases As the Cr concentration increases, the

ferromagnetic fraction, magnetization, and magnitude of the hyperfine field decrease, whereas the

magnitude of the isomer shift increases The Fe and Cr magnetic moments, isomer shift, and

hyperfine field of the bcc Fe–Cr phase have been deduced and are discussed consistently in terms

of charge and spin distributions, as well as magnetic valence © 2000 American Institute of

Physics. 关S0021-8979共00兲08419-X兴

I INTRODUCTION

Electronic and magnetic investigations of bulk

body-centered-cubic 共bcc兲 Fe–Cr alloys have been performed

ex-tensively for many years.1–9 This is one of only two

Fe-based alloy series共the other is Fe–V兲 in which a wide

solid-solution range occurs In thin films, the magnetism of Cr

atoms has recently became the focus of interest because of its

mediating role in exchange coupled superlattice and giant

magnetoresistance materials.10–12 Substitution of Fe by Cr

atoms strongly changes not only the magnetic properties but

also the structural and mechanical properties.13–15This is an

interesting aspect in magnetism as well as in technical

appli-cations

Magnetism of bcc Fe–Cr can be described in terms of

the Cr-nearest neighbor environment.4,6 This is directly

re-lated to the charge and spin distributions In this context,

information on hyperfine parameters deduced from

Mo¨ss-bauer spectrometry studies is worth considering Indeed, a

linear correlation between the hyperfine field (Bhf) and the

isomer shift 共IS兲 was found for bulk bcc Fe–Cr alloys by

Dubiel and Zukrowski.6It has successfully been used to

con-firm the charge and spin transfers in this system These

ex-perimental results were well reproduced by electronic

struc-ture calculations.9 Structure, magnetization, and the

Mo¨ssbauer effect have been studied for nonequilibrium

Fe–Cr thin films.13In Ref 13, the difference in local atomic

configurations of the␴phase and A15 phase in the

paramag-netic state was shown, however, the Fe and Cr magparamag-netic

moments, charge, and spin transfer were not mentioned

In this article, we present our experimental

investiga-tions of the crystal structure, magnetization, and Mo¨ssbauer

effect for sputtered Fe1⫺xCrx thin films Charge and spin transfer in the bcc Fe–Cr phase are discussed by considering the configuration of the Fe and Cr magnetic moments, the relationship between the hyperfine field and magnetization, the relationship between the hyperfine field and the isomer shift, as well as the magnetic valence

II EXPERIMENT

The Fe1⫺xCrx (0⭐x⭐0.54) thin films were deposited

onto a glass substrate at 300 K using a triode rf-sputtering system To avoid corrosion and oxidation, the film stacks were covered with a 10 nm thick Nb layer on top The film thickness ranged from 0.5 to 1.5 ␮m The composition was analyzed using energy dispersive x-ray共EDX兲 analysis The structure of the samples was investigated by high-angle x-ray diffraction 共XRD兲 a using a cobalt anticathode (␭CoK␣

⫽0.1790 nm)

The magnetization was measured with a vibrating sample magnetometer共VSM兲 in magnetic fields up to 1.4 T applied in the film–plane directions

Conversion-electron Mo¨ssbauer spectra共CEMS兲 at room temperature were recorded using a conventional spectrom-eter equipped with a homemade helium–methane propor-tional counter The source was 57Co in a rhodium matrix The film was set perpendicular to the incident␥ beam The spectra were fitted with a least-squares technique using a histogram method with respect to discrete distributions, thereby constraining the linewidths of each elementary spec-trum to be the same Isomer shifts are given relative to␣-Fe

at 300 K The average ‘‘cone angle’’␤between the incident

␥-ray direction 共the film–normal direction兲 and that of the

hyperfine field Bhf共or the Fe magnetic moment direction兲 is

a 兲Corresponding author; Electronic mail: duc@cryolab.edu.vn

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estimated from the line-intensity ratios 3:x:1:1:x:3 of the

six Mo¨ssbauer lines, where x is related to ␤ by sin2␤

⫽2x/(4⫹x).

III EXPERIMENTAL RESULTS AND DISCUSSION

The crystal structure of the Fe1⫺xCrx(0⭐x⭐0.54) thin

films is clearly detected by the x-ray diffraction

investiga-tion The bcc phase appears for x⬍0.32 关Fig 1共a兲兴 The ␴

phase, which is classified as the topologically closed packed

one, is formed for 0.38⭐x⭐0.44 关Fig 1共c兲兴 For 0.32⭐x

⭐0.38 and 0.44⭐x⭐0.54, samples are composed of both the

bcc and␴phase关Fig 1共b兲兴 The phase diagram is presented

in Fig 2共a兲 It is usually obtained for Fe1⫺xCrx thin films

prepared by the sputter-deposition method.13 In Ref 13, however, the composition range for the formation of the ␴

phase is 0.45⬍x⬍0.55 For comparison, these data are also

included in Fig 2共b兲 For the Fe–Cr films prepared by ther-mal coevaporation, an amorphous phase can be formed in the composition range of 0.4⭓x⭓0.75.15 These distinctions re-flect a strong effect of the deposition process on the forma-tion of Fe–Cr alloys

The spontaneous magnetization at 300 K is determined

by linearly extrapolating the magnetization curve from high

field to zero field The spontaneous magnetization ( M S) re-ferring to the volume of the bcc phase is shown in Fig 3 Note that, as Fe is substituted by Cr, the room temperature

M S value of the bcc alloys decreases and seems to tend to

zero at x⬎0.6 with a slope of approximately ⫺3.7␮B/at This result is comparable with that reported in Ref 13 At low temperature, however, a slope of ⫺2.4␮B/at, which is rather close to that given from a simple Slater–Pauling analysis, was found.4,13This difference between the low and

room temperature M S variations may be related to the change in film ordering temperature with Cr concentration

However, we do not know the T C values for this system so far The spontaneous magnetization of the ␴alloys does not follow the tendency found for the bcc alloys, but is almost zero This is due to the fact that the␴phase is ferromagnetic with low ordering temperature and very small magnetic moment.13

Figure 4 shows typical CEM spectra at 300 K for several investigated Fe1⫺xCrx thin films A ferromagnetic sextet is

observed for the bcc phase with x⭐0.32 关Figs 4共a兲 and

4共b兲兴 With increasing x, however, the Mo¨ssbauer lines

broaden due to a random distribution of the hyperfine field Moreover, a center contribution superimposed on these fer-romagnetic sextets is enhanced and becomes prominent for

the bcc Fe–Cr films with x⭓0.32 关Figs 3共c兲–3共e兲兴 This paramagnetic contribution is attributed to the ␴ phase For the pure ␴ alloys共i.e., for 0.38⭐x⬍0.44兲, only quadrupole

splitting characterized for the paramagnetic contribution is observed This is, however, not presented here

The CEM spectra of the Fe1⫺xCrx thin films were fitted

with a wide distribution of hyperfine field P(Bhf) to take into account all the environments experienced by the57Fe nucleus

FIG 1 Typical x-ray diffraction patterns of several Fe 1⫺xCrx thin films: x

⫽共a兲 0.28, 共b兲 0.32, and 共c兲 0.44.

FIG 2 Phase diagram of sputtered Fe 1⫺xCrxthin films: 共a兲 present study

and 共b兲 from Ref 13.

FIG 3 Magnetic moment of the Fe 1⫺xCrxthin films: 共䊉兲 bcc, and 共䊊兲 ␴ phase.

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关Figs 4共a兲–4共e兲兴 This provides average values of the

hyper-fine field (具Bhf典), the isomer shift共具IS典兲, the cone angle

be-tween the Fe moment direction and the film–normal

direc-tion 共具␤典兲, and the relative ferromagnetic fraction (Aferro)

The results of Aferro 共in the Fe1⫺xCrx alloys兲 and of 具Bhf典,

具IS典, 具␤典 共referred to the bcc Fe–Cr phase兲 obtained are

plot-ted in Figs 5共a兲–5共d兲, respectively Here,具Bhf典 and具IS典 are

negative Note that the accuracy of the isomer shift values of

the two compounds with x⫽0.52 and 0.54 is low Their

re-ported 具IS典 values correspond to a minimum of ␹2 test in

fitting It can be seen from Fig 5 that, although Aferro,

兩具Bhf典兩 and 具␤典 decrease with increasing the Cr

concentra-tion,兩具IS典兩 increases In accordance with the XRD results, the

decrease of Aferrois associated with the appearance of the␴

phase The change of hyperfine field is related to a change of

spin density and the change of the isomer shift indicates a

change of charge density The increase of the兩具IS典兩 value can

be described in terms of the increase in the density of s and

p electrons at Fe nuclei due to the substituted Cr atoms,

whose electronegativity is lower than that of Fe atoms With

regard to the charge and spin transfer, Dubiel and

Zukrowski6 considered the relationship between the

hyper-fine field and the isomer shift in bulk Fe–Cr alloys They

found a linear correlation between具Bhf典and具IS典 This plot is

presented in Fig 6 for the bcc Fe1⫺xCrx phase Here, the

data are also consistent with a linear relation, although they

do not prove it With regard to the Dubiel and Zukrowski

approach, a linear dependence can be expressed as 具Bhf典

⫽32⫹165具IS典 The slope of this line 关d具Bhf典/d/具IS典

⫽165 T/共mm/s)] is close to that of 172.5 T/共mm/s兲 reported

for bulk Fe–Cr alloys.6Such a strong correlation means that

the charge transfer is directly related to the spin transfer We

will come back to details of the charge and spin distribution later The variation of具␤典 reflects a random orientation of the

Fe magnetic moments in high Cr concentration alloys

The hyperfine magnetic field is not proportional the

3d(Fe) magnetic moment, but can be empirically given by4,6

FIG 4 Mo¨ssbauer spectra and hyperfine field distributions in bcc Fe 1⫺xCrx

thin films: x⫽共a兲 0.08, 共b兲 0.28, 共c兲 0.32, 共d兲 0.52, and 共e兲 0.54.

FIG 5 Ferromagnetic fraction (Aferro ), hyperfine field (具Bhf典), isomer shift 共具IS典兲, and cone angle 具 ␤ 典 in Fe 1⫺xCrxthin films: 共䊉兲 bcc, 共䊊兲 ␴ , and 共⽧兲 mixed bcc ⫹ ␴ phase.

FIG 6 Correlation between具Bhf典and 具IS典 in the bcc Fe 1⫺xCrxphase.

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where the magnetic moments of Fe atoms ( MFe) and of the

alloy ( M S) could themselves be concentration dependent,

but a and b are assumed to be constant The correlation

be-tween 具Bhf典 and M S is presented in Fig 7 Clearly, good

linear variation 具Bhf典( M S) is observed, except for a few

points at high Cr concentrations 共i.e., up to x⫽0.3 only兲.

Similar behavior with almost the same values of a and b was

found for Fe–Cr and Fe–Al alloys.4,16 At low temperature

(T⫽5 K), Shiga and Nakamura4 found that the Bhf vs M S

curve shows good linearity for the Fe1⫺xCrx up to x⫽0.7

They proposed that in this concentration range the Fe

mag-netic moment remains almost constant and the decrease in

具Bhf典is mainly due to a reduction in M S The magnitude of

the Fe magnetic moment is simply determined from the

value of the a MFeterm in Eq 共1兲 The procedure is as

fol-lows As a guide to the eye, the variation of experimental

points can be described by the equation 具Bhf典⫽22⫹5M S

共Fig 7兲 In comparison with Eq 共1兲, it turns out that aMFe

⫽22 T and b⫽5T/␮B Taking MFe⫽2.2␮B/at for bcc Fe,

one can derive that a ⫽10T/␮B This result is comparable

with that reported for the bulk Fe–Cr alloys.4These deduced

a and b values are assumed to be constant for all the

inves-tigated Fe–Cr alloys For the detail of each composition,

from Eq 共1兲, MFe(x) is estimated as MFe(x) ⫽关Bhf(x)

⫺bM S (x) 兴/a The results of MFe(x) obtained are plotted in

Fig 8 This result is supported by a neutron diffraction

study.3From these values of MFe and the measured

sponta-neous magnetization data, the Cr magnetic moment ( MCr), which is oriented antiparallel with respect to the Fe moment,

is estimated by applying the expression 兩MCr兩⫽兩关MS⫺(1

⫺x)MFe兴兩/x It results in with increasing Cr concentration,

兩MCr兩 decreasing from the value of 1.9␮B/Cr at and being

annulled at x⬎0.32 共Fig 8兲 This finding is comparable with those deduced from the magnetization,2,4 neutron scattering diffraction,3 and electronic structure calculations9 and those reported for the Cr magnetic moment in Fe/Cr interfaces.11,12 The magnetic and hyperfine properties can globally be

described as a result of the hybridization between the 3d(Fe) states and 3d 共early transition metal兲 states.9–17In this case,

it is worth recalling here the main arguments given by Li and Luo.9 In the Fe–Cr alloys, the Cr moment is negative with respect to the Fe moment This is due to the fact that the majority 共spin-up兲 3d(Cr) states are located near the Fermi level (E F), while the minority共spin-down兲 states are below

E F.9,17The minority 3d(Cr) band thus strongly overlaps the minority 3d(Fe) band This leads to enhancement of spin-down 3d(Cr) – 3d(Fe) interactions Consequently, the poten-tial energies of the minority 3d band may be lowered relative

to those of the corresponding majority 3d band Finally, spin-up 3d(Fe) electrons transfer to the spin-down 3d(Cr) subband, resulting in the reduction of the 3d(Fe) magnetic moment This decrease of the number of the 3d(Fe)

elec-trons leads to less shielding of the Fe nucleus, hence to a

larger s, p electron density at the Fe origin and then to the

observed increase of具IS典 The total magnetic moment of the

Fe atoms, however, also comes from polarization of the 4s and 4 p magnetic moments Indeed, in pure iron metal, due to 3d(Fe)-(s, p) hybridization, a negative s, p magnetic mo-ment relative to the 3d(Fe) momo-ment is usually formed.9,17

With substitution of the Cr atom, the hybridization between the 共spin-up兲 3d(Cr) and 共s, p兲 states is stronger than that

between the共spin-down兲 3d(Fe) and 共s, p兲 states This leads

to a change in the direction of the 4s, 4 p polarization from

negative 共for pure Fe兲 to positive 共for Fe–Cr alloys兲 This

increase of the s, p spin-up density was confirmed by the

electronic structure calculations.9It was the assumed mecha-nism for the maintenance of the high magnetic moment and the reduction of the magnitude of the hyperfine field at Fe sites in the Fe–Cr alloys

Magnetic properties of the transition metal alloys are described by the well-known Slater–Pauling curve.18 How-ever, a better description of the role of the elements when they appear as solutes in Fe-, Co-, and Ni-based alloys is a magnetic valence, but not a chemical one.19 Within the

simple concept of the magnetic valence Z m, not only the magnetic atoms but also the nonmagnetic ones are consid-ered In this case, the average magnetic moment per atom is written as19

where 2N s p ↑ is the number of s, p electrons in the spin band;

in the late transition metals, N s p ↑ ⫽0.3.19

For the Fe1⫺xCrx thin films, Z m is determined by the

chemical values ZFe(⫽8) and ZCr(⫽6) and by the number

N d ↑ of d electrons in the spin-up subband 共N d ↑⫽5 in the strong ferromagnets兲:

FIG 7 Correlation between具Bhf典and M sin the bcc Fe 1⫺xCrxphase.

FIG 8 Fe and Cr magnetic moments in the bcc Fe 1⫺xCrxphase.

4781

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Z m ⫽共2N d ↑ ⫺ZFe兲共1⫺x兲⫺xZCr 共3兲

The experimental and calculated magnetic moments are

presented in Fig 9 as a function of Z m Starting from pure

Fe, the experimental magnetization falls below the M ⫽Z m

⫹0.6 line This was assumed to reveal the weak

ferromag-netic character of Fe.19,20 As the Cr content increases, the

experimental magnetization data initially approach the

calcu-lated line and finally excess this plot Indeed, the

magnetiza-tion data of the bcc Fe–Cr with low Cr concentramagnetiza-tion can be

described by the equation of M ⫽Z m ⫹0.9 共i.e., with N s p ↑

⫽0.45兲 This increase of N s p ↑ is usually observed in the

al-loys of Fe and Co with early-transition metals and with

rare-earth elements.17,19,20It agrees with the above discussion that

the number of the spin-up s, p electrons increases in the

Fe–Cr alloys Moreover, this departure of the experimental

data from the M ⫽Z m⫹0.6 line may also relate to the effect

of removing electrons from the d band of the host, i.e., the

3d electrons transfer from Fe to the neighboring Cr atoms.

This results in a reduction of the number of the spin-up 3d

electrons N d ↑.

In conclusion, we have studied the crystal structure,

magnetization, hyperfine field, and isomer shift for sputtered

Fe1⫺xCrxthin films The main results have been considered consistently in terms of the Fe and Cr magnetic moment configuration, the correlation between the hyperfine field and the magnetization, the correlation between the hyperfine field and the isomer shift, as well as the magnetic valence

ACKNOWLEDGMENTS

The stay of one of the authors共N.H.D.兲 at the Groupe de Physique des Mate´riaux, University of Rouen, was supported

by the Ministe`re Franc¸ais de l’Education Nationale, de la Recherche et de la Technologie

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FIG 9 Magnetic moment as a function of the magnetic valence for bcc

Fe 1⫺xCrxthin films.

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