.
`(cid:14)
`Thin Solid Films 334 1998 196]200
`
`Magnetic properties of fcc iron in Ferfcc metal multilayers
`
`F. PanU, M. Zhang, B.X. Liu
`Department of Materials and Science and Engineering, Tsinghua Uni¤ersity, Beijing 100084, People’s Republic of China
`
`Abstract
`
`Vapor-deposition technique was employed to grow the fcc iron in the FerCu, FerPd and FerPt multilayers. The thickness,
`periodicity, chemical composition, microstructure, and magnetic properties of the films were characterized and measured by
`various methods. The experimental results indicated that, when the Fe layers were thinner than 2]3 nm, the Fe atoms could
`grow in the fcc structure on the polycrystalline fcc non-magnetic metal layers with a fixed thickness of 6.5]7.5 nm. The fcc Fe
`in the Ferfcc metal multilayers exhibited ferromagnetic behavior, and its magnetic moment can be as high as 3.4 m for
`B
`Fe 1.2 nm rCu 7.5 nm , 3.2m for Fe 1.6 nm rPd 6.5 nm , and 2.1 m for Fe 2.3 nm rPt 7.0 nm , respectively. The
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`.
`B
`B
`modification of magnetic properties of fcc Fe was attributed to a significant change in the distance among Fe atoms in the fcc
`lattice, resulting in a considerable changing in electron-couple, compared with that in the normal Fe-bcc structure. Q 1998
`Elsevier Science S.A. All rights reserved.
`
`Keywords: Magnetic properties; Fcc iron; Multilayers
`
`1. Introduction
`
`In the last 10 years, magnetic multilayered films on
`a nanometre scale with artificial periodicity have at-
`tracted much attention because these films may fea-
`ture some anomalous magnetic properties, such as
`changes in magnetization as the magnetic layer thick-
`ness is reduced, appearance in some cases of a uniax-
`ial
`interfacial anisotropy, and giant magnetoresis-
`tance. These phenomena are probably related to the
`existence of surface and interface state, i.e. the re-
`duced coordination number and symmetry of atoms in
`the surface, transitional structure sublayer, interface
`roughness, and associated chemical disordering, etc.
`w
`x
`1]4 . In Ferfcc metal multilayers, the metastable fcc
`Fe was obtained in some systems by various deposi-
`tion methods, and these films exhibited very different
`magnetic properties. For example, recently, Himpsel
`w x
`w x
`5 and Macedo et al. 6 reported that fcc Fe grew
`epitaxially on Cu single crystals and formed a sharp
`
`U Corresponding author. Tel.: q86 10 62784546; fax: q86 10
`62771160; e-mail: panf@mail.tsinghua.edu.cn
`
`w x
`interface. In our recent study 3 , it was also found
`that Fe grew epitaxially on polycrystalline Cu by elec-
`tron-beam vapor deposition. In FerPt system, fcc Fe
`w x
`was obtained by Croft et al. 7 and the author’s group
`w x8 . In FerPd multilayers, the metastable fcc Fe phase
`was also obtained and the fcc Fe exhibited ferromag-
`w x
`netic behavior 9 . With extensive data obtained for
`many systems, research interest is therefore to study
`the magnetic properties of fcc Fe in Ferfcc metal
`multilayers, which is helpful for understanding the
`origin of the magnetic property of magnetic materials.
`We report, in this paper, the magnetic properties of
`fcc Fe observed in the Ferfcc metal multilayers pre-
`pared by electron-beam vapor deposition, the correla-
`tion between the magnetic properties and the mi-
`crostructure of the films, and discuss the possible
`mechanism responsible for the observed magnetic
`properties.
`
`2. Experimental procedure
`The FerCu, FerPd and FerPt multilayered films
`were prepared by depositing alternately the pure con-
`stituent metal 99.99% at rates of 0.01]0.2 nmrs
`(cid:14)
`.
`
`0040-6090r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved.
`(cid:14)
`.
`P I I S 0 0 4 0 - 6 0 9 0 9 8 0 1 1 4 3 - 2
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`Lambeth Magnetic Structures, LLC Exhibit 2009
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`F. Pan et al. r Thin Solid Films 334 1998 196]200
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`(
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`197
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`onto a glass substrate of 0.1 mm thickness for mag-
`.
`netic property study and a NaCl single crystal with
`(cid:14)
`.
`freshly cleaved surface for microstructure analysis in
`an e-gun evaporation system at a vacuum level of
`5 =10y5 ]2 =10y6 Pa. The thickness of
`the con-
`stituent metal, varied from 1.2 to 14 nm controlled by
`an in situ quartz oscillator. The total thickness of the
`films was controlled in the range 75]275 nm. Samples
`were analyzed by Transmission Electron Microscopy
`(cid:14)
`.
`(cid:14)
`.
`TEM , Selected Area Diffraction SAD , and X-ray
`diffraction to identify the structure. Rutherford
`(cid:14)
`.
`Backscattering RBS was also employed to measure
`the thickness, periodicity, and chemical composition
`of the samples. The magnetic properties were mea-
`(cid:14)
`.
`sured with a Vibrating-sample magnetometer VSM ,
`with a resolution of 5 =10y6 emu, in a magnetic field
`of up to 10 kOe at room temperature. The size of the
`VSM samples were 4 mm=6 mm or 5 mm=5 mm.
`First, a hysteresis loop of the substrate and holder
`(cid:14)
`.
`was measured and the saturation magnetization MS
`was found to be approx. 4 =10y4 emu, which was one
`or two orders of magnitude lower than that of the
`Ferfcc metal multilayer films. Then the hysteresis
`loops of the samples were measured, and the magne-
`tization of the substrate and holder was subtracted
`automatically by the computer. To reduce the experi-
`mental error, measurements were made on an assem-
`bly of four similar specimens. Consequently, the mag-
`netic moment from the substrate and holder had a
`negligible effect on the measured values, and the
`precision of the measured magnetic moment of the
`films was estimated to be better than 1%. After
`measuring the magnetic properties, the films were
`dissolved in 5 ml aqua regia HNO :HCls1:3 and
`(cid:14)
`.
`3
`the Inductive Coupled Plasma Atomic Emission Spec-
`(cid:14)
`.
`trum ICP was employed to determine the Fe con-
`tent in the multilayers. An average magnetic moment
`per Fe atom was then obtained using these data. The
`error involved in the ICP measurement was approx.
`5%, and therefore the total error was around 6%.
`
`3. Results and discussion
`
`3.1. Structure characterization
`
`Results of low-angle X-ray diffraction and RBS
`confirmed the artificial periodicity of all the multilay-
`ered films, and the periodicities obtained, respectively
`from low-angle X-ray diffraction, agreed well with the
`RBS. For example, Fig. 1 shows a low-angle X-ray
`w
`.x
`diffraction pattern for an Fe 3 nm rPd 6.5 nm 14
`(cid:14)
`.
`(cid:14)
`multilayers taken with Cu K a radiation. From this
`figure, the second-, third-, fourth- and fifth order
`the FerPd bilayers can be
`diffraction peaks of
`observed. According to these data, the periodicity of
`the multilayers is approx. 9.8 nm, confirming the
`
`w
`Fig. 1. Low-angle X-ray diffraction pattern of an Fe 3 nm rPd 6.5
`(cid:14)
`.
`(cid:14)
`.x
`nm
`multilayers.
`14
`
`deposited metal thickness. Fig. 2 shows a RBS spec-
`w
`.x
`trum of the Fe 8.0 nm rPt 7.0 nm 10 multilayers.
`(cid:14)
`.
`(cid:14)
`This spectrum was obtained with 2.023 MeV Heq
`ions, and the laboratory backscattering angle was
`1658. In order to resolve individual
`layers by the
`detector at this energy, which presented a resolution
`of approx. 10 nm, the sample was tilted by 608. From
`this figure, one can see that both iron and platinum
`spectra consist of ten peaks, corresponding to ten
`FerPt bilayers in the multilayers. The total thickness
`of the sample is estimated to be approx. 150 nm by
`resolving the RBS spectrum, which agrees with the
`nominal thickness.
`The microstructure of the films was examined by
`(cid:14)
`.
`means of transmission electron microscopy TEM ,
`(cid:14)
`.
`selected area electron diffraction SAD , and X-ray
`diffraction. Table 1 shows the crystal structure of the
`constituent metals in the multilayers. From this table,
`one can see that for the FerCu multilayers, when
`t F1.5 nm and t s7.5 nm, the metastable fcc Fe
`Fe
`Cu
`was obtained, which grew epitaxially on polycrys-
`talline fcc Cu, as the difference of the atom radius
`w x
`between iron and copper is only approx. 3% 3 . The
`lattice parameter of fcc Fe was approx. 0.360"0.005
`
`.x
`w
`Fig. 2. The RBS spectrum of the Fe 8.0 nm rPt 7.0 nm
`(cid:14)
`.
`(cid:14)
`10
`layers.
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`
`F. Pan et al. r Thin Solid Films 334 1998 196]200
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`
`Table 1
`The crystal structure of the constituent metals in the multilayers
`
`Specimen
`
`FerCu d s7.5 nm, d F1.5 nm
`.
`(cid:14)
`Cu
`Fe
`FerCu d s7.5 nm, d )2.5 nm
`(cid:14)
`.
`Cu
`Fe
`FerPd d s6.5 nm, d F6.5 nm
`(cid:14)
`.
`Pd
`Fe
`FerPd d s6.5 nm, d )7.0 nm
`(cid:14)
`.
`Pd
`Fe
`FerPt d s7.0 nm, d F3.4 nm
`(cid:14)
`.
`Cu
`Fe
`FerPt d s7.0 nm, d )5.6 nm
`(cid:14)
`.
`Cu
`Fe
`
`Crystal structure
`
`fcc Feqfcc Cu
`bcc Feqfcc Cu
`fcc Feqfcc Pd
`bcc Feqfcc Pd
`fcc Feqfcc Pt
`bcc Feqfcc Pt
`
`Lattice parameter
`(cid:14)
`.
`of fcc Fe nm
`
`0.360"0.05
`]
`0.360"0.05
`]
`0.389"0.02
`]
`
`Growth model
`
`Epitaxial growth
`
`Metastable phase
`
`Epitaxial growth
`
`Fe
`
`atoms in an fcc structure on thick Pt layers is respon-
`sible for observing only one fcc structure. While t )
`Fe
`5.6 nm, the Fe can not grow in an fcc structure on the
`Pt layer because of the internal stress caused by the
`(cid:14)
`.
`large mismatch between Fe and Pt approx. 8% , and
`the films consist of bcc Fe and fcc Pt.
`
`3.2. Magnetic properties
`
`The VSM results indicated that all the fcc phases in
`FerCu, FerPd and FerPt multilayers exhibit ferro-
`magnetic behavior and show an in-plane easy axis of
`magnetization. For example, Fig. 3 shows three typical
`hysteresis loops of Fe 1.5 nm rCu 7.5 nm , Fe 1.6
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`nm rPd 6.5 nm and Fe 3.4 nm rPt 7.0 nm in a
`.
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`.
`magnetic field of 5 kOe, respectively. Table 2 shows
`the magnetic properties of the fcc Fe phase in various
`systems. From the table, one can see that with the
`exception of the FerPt multilayers, the magnetic mo-
`ment per Fe atom in the FerCu and FerPd films was
`(cid:14)
`obviously higher than that of the bulk bcc Fe 2.15
`.m , and that the enhancement of the magnetic mo-
`B
`ment increased with decreasing Fe layer thickness,
`(cid:14)
`reaching a maximum value of 3.27 m for Fe 1.6
`B
`nm rPd 6.5 nm films and 3.44 m for Fe 1.5
`(cid:14)
`.
`(cid:14)
`.
`B
`
`nm. While t )2.5 nm, the films consist of polycrys-
`Fe
`talline bcc Fe and fcc Cu, and the grain size of the
`films is approx. 1]5 nm.
`For FerPd multilayers, the experimental condition
`to obtain the metastable fcc Fe phase is t F6.5 nm
`and t s6.5 nm. Under such condition, the SAD
`Pd
`patterns of the films consist of two sets of sharp
`diffraction rings from two fcc phases, i.e one is the fcc
`Pd phase and the other is a metastable Fe phase also
`w x
`with an fcc structure 9 . Their lattice parameters
`were approx. 0.389"0.005 nm and 0.360"0.005 nm,
`respectively. While t )7.0 nm, the diffraction lines
`Fe
`from metastable fcc Fe disappeared, and the films
`consist of bcc Fe and fcc Pd phases. The lattice
`parameter of the bcc phase was approx. 0.286"0.005
`nm, which was the same as that of the bulk bcc Fe.
`For FerPt multilayers, the metastable fcc Fe was
`w x
`also obtained when t F3.4 nm and t s7.0 nm 8 .
`However, the fcc Fe in FerPt films was different from
`in the FerCu and FerPt pairs. The lattice
`that
`parameters of the new metastable fcc Fe phase in the
`FerPt system was approx. 0.389"0.002 nm, which is
`greater than that observed in FerCu and FerPd
`films. From the SAD patterns of FerPt films, it was
`found that when t F3.4 nm, there was only one fcc
`Fe
`phase. According to the RBS and low-angle X-ray
`diffraction results, the films had a good periodic struc-
`ture, and the lattice parameter of the observed fcc
`phase was about the same as that of pure platinum,
`i.e. 0.392 nm. It could therefore be thought that,
`under our experimental conditions, the growth of Fe
`
`Fe
`
`Pt
`
`Table 2
`The magnetic properties of the fcc Fe phase in various systems
`
`Specimen
`Fe 1.5 nm rCu 7.5 nm
`.
`(cid:14)
`.
`(cid:14)
`Fe 1.2 nm rPd 6.5 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 1.6 nm rPd 6.5 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 3.0 nm rPd 6.5 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 4.3 nm rPd 6.5 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 6.5 nm rPd 6.5 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 1.2 nm rPt 7.0 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 2.3 nm rPt 7.0 nm
`(cid:14)
`.
`(cid:14)
`.
`Fe 3.4 nm rPt 7.0 nm
`(cid:14)
`.
`(cid:14)
`.
`
`(cid:14)
`Magnetic moment m
`B
`
`.
`
`(cid:14)
`Hc Oe
`
`.
`
`3.44"0.28
`2.77"0.17
`3.27"0.20
`3.04"0.19
`2.77"0.17
`2.85"0.18
`1.85"0.12
`2.12"0.13
`2.08"0.13
`
`19
`21
`18
`19
`20
`18
`14
`12
`9
`
`(cid:14) . w
`Fig. 3. Three typical hysteresis loops of a Fe 1.5 nm rCu 7.5
`(cid:14)
`.
`(cid:14)
`(cid:14) . w
`.x
`(cid:14) . w
`.x
`nm ; b Fe 1.6 nm rPd 6.5 nm ; and c Fe 3.4 nm rPt 7.0
`.
`(cid:14)
`(cid:14)
`.
`(cid:14)
`(cid:14)
`11
`15
`.x
`nm
`multilayers in a magnetic field of 5 kOe, respectively. To
`14
`reduce the measuring error, four identical specimens were put
`together in one measurement to obtain the hysteresis loops.
`
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`199
`
`in the
`the magnetic moment
`enhancement of
`Pdrmagnetic metal Fe, Co multilayers is due to the
`(cid:14)
`.
`w
`x
`polarization of the Pd atoms 16]18 , because Pd is a
`strong paramagnetic and a small addition of magnetic
`elements will induce a magnetic moment at the Pd
`x
`. w
`(cid:14)
`sites approx. 0.36 m 19 . Therefore the magnetic
`B
`moment of the Pd atom was probably partly responsi-
`ble, besides the enhanced magnetic moment of the fcc
`Fe phase, for the observed magnetic enhancement in
`the FerPd multilayers.
`For FerPt multilayers, the fcc Fe had a lattice
`parameter of 0.389"0.002 nm, which was larger than
`that in the FerCu and FerPd films. Moruzzi et al.
`
`w x20 calculated the total energy and magnetization of
`the bulk fcc iron, and pointed out that the magnetism
`is related to the Wigner]Seitz radius, which con-
`tained the same volume as that of an atom in the
`actual lattice. The Wigner]Seitz cell volume of the
`fcc Fe in the FerPt films is 20]30% greater than that
`in the FerCu and FerPd films, and this results in a
`smaller magnetic moment per Fe atom in the FerPt
`films. Besides, the atomic exchange force decreases
`with increasing atomic distance, hence, a large lattice
`parameter will also result in a reduction of electron
`spin density and lower the magnetic moment. This
`may give a possible explanation for the observed
`magnetic properties of fcc Fe in the FerPt multilay-
`ers.
`
`B
`
`F. Pan et al. r Thin Solid Films 334 1998 196]200
`)
`(
`nm rCu 7.5 nm films, respectively, i.e. approx. 1.5
`.
`(cid:14)
`.
`times that of the bulk Fe.
`Based on an all-electron total energy local spin
`w
`x
`density approach, Freeman et al. 10]12 predicted
`that there would be a significant enhancement in
`two-dimensional magnetism at the surfaces and inter-
`faces in the transition metals grown on noble metals,
`e.g. Fe in a thin-film form with fcc structure can
`exhibit ferromagnetic behavior, in contrast to its bulk
`fcc phase, which is non-magnetic. The magnetic mo-
`ment of the Fe atom, in comparison to its value of
`2.15 m in the bcc bulk, could be up to 2.98 m for
`B
`B
`(cid:14)
`.
`the topmost Fe overlayer and the clean Fe 001 sur-
`face. The first-principles all-electron linearized aug-
`mented plane wave method investigations
`in
`w
`x
`FerCu 001 superlattices by Zhou et al. 13 , also
`(cid:14)
`.
`predicted that the magnetic moment of the interface
`Fe layer is stabilized at the high spin state of the fcc
`crystals. Tight-binding calculations of the magnetic
`surface, interface and multilayers by Krompiewski et
`w
`x
`al. 2,14,15 gave a similar prediction. These calcula-
`tions can therefore explain the observed magnetic
`properties of fcc Fe, i.e exhibiting ferromagnetic be-
`havior.
`From the experimental results, it can also be found,
`even though Fe can grow with an fcc structure in
`various systems, the magnetic properties of fcc Fe
`changes with the lattice parameter of the fcc Fe
`phase, which depends on the other metals in the
`multilayers. The maximum magnetic moment per Fe
`(cid:14)
`atom with fcc structure was 3.44 m for Fe 1.5
`B
`nm rCu 7.5 nm films, 3.27 m for Fe 1.6 nm rPd 6.5
`(cid:14)
`.
`(cid:14)
`.
`(cid:14)
`.
`B
`nm films and 2.12 m for Fe 2.3 nm rPt 7.0 nm
`.
`(cid:14)
`(cid:14)
`.
`.
`films. The magnetic moment of fcc Fe in the FerCu
`and FerPd multilayers is significantly higher than
`that in the FerPt multilayers. The main reason for
`the observed difference in magnetism of fcc Fe phases
`in the FerCu, FerPd and FerPt multilayers may be
`explained in the following way. For the FerCu sys-
`tem, the interface between the magnetic layer and
`non-magnetic layer could be considered to be an ideal
`situation which was used in theoretical investigation,
`as evidenced by the epitaxial growth of the metastable
`fcc Fe on a thick Cu layer, and the lattice parameter
`of fcc Fe was around 0.360 nm. Consequently, the Fe
`thin film with fcc structure in the FerCu system have
`a higher magnetic moment than the bcc Fe.
`As for the FerPd multilayers, even though an fcc
`Fe with a lattice parameter of 0.360"0.005 nm was
`also formed at the FerPd interface, Fe could not
`grow epitaxially on Pd because of the radius differ-
`ence. An ideal interface situation used in theoretical
`investigation could not be formed, because Fe and Pd
`have a large solid solubility, which resulted in a slightly
`lower magnetic moment of fcc Fe than that in the
`FerCu films. It is generally assumed that a partial
`
`4. Conclusion
`
`In summary, we have shown that fcc iron in the
`FerCu, FerPd and FerPt multilayers exhibits ferro-
`magnetic behavior, and its magnetic moment was
`enhanced considerably in the FerCu and FerPd mul-
`tilayers because of the epitaxial growth of the thin Fe
`layer on Cu and an fcc metastable phase as0.360"
`(cid:14)
`.
`0.005 nm on Pd. The maximum magnetic moments
`per Fe atom in an fcc structure in the FerCu FerPd
`and FerPt films were 3.44, 3.27 and 2.12 m respec-
`B
`tively, which is probably correlated with the lattice
`parameter of metastable fcc phase iron, i.e. a large
`lattice parameter results in a reduction of the mag-
`netic moments in thin films.
`
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