VOLUME 80, NUMBER 9
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`P H Y S I C A L R E V I E W L E T T E R S
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`2 MARCH 1998
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`Magnetic Moment of fcc Fe(111) Ultrathin Films by Ultrafast Deposition on Cu(111)
`
`J. Shen, P. Ohresser, Ch. V. Mohan, M. Klaua, J. Barthel, and J. Kirschner
`Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale, Germany
`(Received 19 September 1997)
`Thermally deposited ultrathin Fe films on Cu(111) at room temperature or below have been known
`to grow in a multilayer mode with a low net magnetic moment 共⬃(cid:48)(cid:46)(cid:53)m(cid:66)兲. In the present work we have
`used pulsed laser deposition to produce isotropic fcc Fe兾Cu共(cid:49)(cid:49)(cid:49)
`films which grow layer by layer with
`a magnetic moment of more than (cid:50)m(cid:66). We attribute the larger magnetic moment of the pulsed laser
`deposited films to their structural perfection as seen by low energy electron diffraction and scanning
`tunneling microscopy.
`[S0031-9007(98)05386-1]
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`¢
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`PACS numbers: 75.70.Ak, 68.55.–a, 75.30.Cr, 81.15.Fg
`
`fcc g(cid:45)Fe is known to have various magnetic structures.
`These include paramagnetic, antiferromagnetic, ferrimag-
`netic, low-moment ferromagnetic, or high-moment ferro-
`magnetic states depending on the lattice constant [1,2].
`Generally for g(cid:45)Fe a larger lattice constant favors a larger
`moment as well as the ferromagnetic alignment. How-
`ever, the most interesting high-moment 共.(cid:50)m(cid:66)兲 ferromag-
`netic phase has never been experimentally observed in
`any isotropic fcc Fe system despite theoretical predictions
`[3,4]. In bulk, fcc g(cid:45)Fe has zero net moment: It can only
`be stabilized either above 1200 K in a paramagnetic state,
`or quenched as antiferromagnetic inclusions in a Cu matrix
`[5].
`In the case of ultrathin films, a paradoxical situation
`arises: On the one hand, one has to epitaxially grow Fe
`on an fcc substrate with relatively large lattice constant to
`achieve large atomic volume and thus the high-moment
`phase of Fe; on the other hand, growing Fe on such sub-
`strates will unavoidably lead to a vertical expansion of the
`Fe lattice constant, which makes the films no longer fcc-
`like. A possible substrate for growing isotropic fcc Fe is
`copper due to the small lattice misfit. On the (100) sur-
`face, the high-moment phase was observed to exist up to
`(cid:52) ML [6] but in this thickness region the films are tetrag-
`onally distorted [7]. The net moment of Fe films falls
`immediately [8] after they become fcc-like (5 to 10 ML)
`[7]. On the (111) surface, the Fe films appear to have near
`isotropic fcc structure [9] but magnetic measurements on
`films capped by copper [10] as well as on films supported
`by a stepped substrate [11] indicate that Fe possesses a sig-
`nificantly smaller net moment of about (cid:48)(cid:46)(cid:53)m(cid:66). Therefore,
`the very fundamental question, i.e., whether an isotropic
`fcc Fe system can be high-moment ferromagnetic or not,
`has yet to be answered experimentally.
`In this Letter we demonstrate the first experimental
`finding of an isotropic fcc Fe system which has a high-
`moment ferromagnetic phase. The system is ultrathin
`Fe film on Cu(111) prepared by pulsed laser deposition
`(PLD). The PLD technique greatly increases the instan-
`taneous deposition rate by 5 or 6 orders relative to that of
`thermal deposition (TD). The PLD films have an isotropic
`fcc structure and a layer-by-layer morphology up to 6 ML
`
`as compared to the multilayer-island morphology of the
`TD Fe films. Most importantly, we have observed the
`high-moment ferromagnetic phase in the PLD Fe films up
`to 3 ML. We will discuss the mechanisms supporting the
`high-moment phase later in the paper.
`The experiments were performed in an ultrahigh vacuum
`(UHV) multichamber system including facilities for scan-
`ning tunneling microscopy (STM), Auger electron spec-
`troscopy (AES), low energy electron diffraction (LEED),
`and magneto-optical Kerr effect (MOKE). The base pres-
`sure is better than (cid:53) 3 (cid:49)(cid:48)2(cid:49)(cid:49) mbar and never exceeds
`(cid:50) 3 (cid:49)(cid:48)2(cid:49)(cid:48) mbar during deposition. Prior to film deposi-
`tion the copper(111) substrate 共miscut ,(cid:48)(cid:46)(cid:49)–兲 was cleaned
`by cycles of Ar1 sputtering followed by 700 K anneal-
`ing until a contamination free Auger spectrum and a sharp
`LEED 共(cid:49) 3 (cid:49)兲 pattern have been achieved. The substrate
`was then kept at 220 K and placed 100 mm away from an
`Fe target (99.99% purity). The output of an excimer laser
`with KrF (248 nm wavelength, 34 ns pulse length, typical
`pulse energy (cid:50)(cid:55)(cid:48) (cid:51)(cid:48)(cid:48) mJ, and repetition rate 5 Hz) was
`focused onto the Fe target, resulting in an instantaneous de-
`position rate, i.e., the deposition rate during the duration of
`the plasma plume (on the order of microsecond) created by
`each laser pulse, of Fe on Cu(111) of about (cid:49)(cid:48)(cid:54) ML兾min.
`The film thickness was controlled by reflection high energy
`electron diffraction and cross-examined by STM and AES
`afterwards. For comparison, films on the same substrate at
`the same temperature were also prepared by thermal evap-
`oration from an iron wire (5N purity) heated by e-beam
`bombardment. The magnetic properties of both types of
`the films were recorded by MOKE.
`By pulsed laser deposition,
`the morphology of the
`Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films has been improved remarkably towards
`layer by layer. Figure 1 shows a side-by-side comparison
`of the morphology of 1.0, 2.0, and (cid:51)(cid:46)(cid:53) ML Fe兾Cu共(cid:49)(cid:49)(cid:49)兲
`films prepared by thermal (left column) and pulsed laser
`(right column) deposition. The thermally deposited Fe
`films have a typical multilayer morphology: The substrate
`has not been wetted after 2 ML of Fe deposition. All the
`islands are bilayer (marked as 2) or trilayer (marked as
`3) high at nominal thickness of 1 ML, and contain five
`
`1980
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`0031-9007兾98兾80(9)兾1980(4)$15.00
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`© 1998 The American Physical Society
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`Lambeth Magnetic Structures, LLC Exhibit 2001
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`VOLUME 80, NUMBER 9
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`P H Y S I C A L R E V I E W L E T T E R S
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`2 MARCH 1998
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`pulsed laser deposited films have a significantly more
`stable fcc structure with respect to the fcc ! bcc phase
`transformation. Figure 2 shows the interlayer spacing
`of the PLD films as a function of the thickness as
`calculated from LEED intensity vs energy curves based
`on a kinematic model. Below 6 ML the interlayer spacing
`is close to that of the substrate and above 6 ML it
`decreases to a distinctly smaller value. The inset LEED
`patterns indicate that the films are in fcc(111) structure
`below 6 ML and transform into the bcc(110) structure
`with Kurdjumov-Sachs orientation at higher thickness.
`Therefore, both our LEED and STM studies suggest that
`the pulsed laser deposited films remain isotropic fcc below
`6 ML. Figures 1 and 2 unambiguously prove that by
`means of ultrafast deposition, i.e., pulsed laser deposition,
`the morphology of the Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films is indeed largely
`improved towards layer by layer, and the fcc ! bcc phase
`transformation has been considerably delayed.
`Knowing the morphology and structure of the films,
`we now compare the magnetic properties of thermally
`and pulsed laser deposited films. As the present Let-
`ter is focused on the magnetic moment, we will concen-
`trate here only on the saturation magnetization while we
`leave most of the magnetic properties such as Curie tem-
`perature, magnetic anisotropy, and the spin reorientation
`to a forthcoming paper [12]. The saturation magnetiza-
`tion was obtained from saturated MOKE hysteresis loops,
`which were measured always in the same polar geometry
`
`¢
`
`the pulsed laser deposited
`spacing of
`Interlayer
`FIG. 2.
`Fe兾Cu共(cid:49)(cid:49)(cid:49)
`films as a function of thickness. The inset pictures
`are typical LEED patterns corresponding to films below and
`above 6 ML, respectively. The decrease of the interlayer spac-
`ing at 6 ML is consistent with the LEED patterns indicating a
`structural transformation from fcc(111) to bcc(110).
`
`1981
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`FIG. 1. STM topography images of Fe兾Cu共(cid:49)(cid:49)(cid:49)
`films pre-
`pared by thermal deposition (left column) and pulsed laser
`deposition (right column). The pulsed laser deposited films
`show typical morphology of a good layer-by-layer growth. The
`thermally deposited films grow in a multilayer mode. At thick-
`ness as low as 1 ML, the thermal film is formed by bilayer
`islands (marked as 2), trilayer islands (marked as 3), and bcc
`precipitates. At 2 ML, the islands typically contain five layers
`in height. Note at 3.5 ML the pulsed laser deposited film stays
`fcc-like, while the thermal film has become mainly bcc-like.
`
`In addition, some
`layers at nominal thickness of 2 ML.
`elongated ridgelike structures have been observed in the
`films. These structures are recognized as bcc(110) precipi-
`tates because they have the same elongated morphology
`(along 具(cid:48)(cid:49)(cid:49)典 directions) as that of the thick films 共.(cid:52) ML兲
`whose structure has been determined to be bcc(110) by
`LEED. When the thickness increases to 3.5 ML, the
`bcc precipitates become dominant and the LEED pattern
`(not shown here) accordingly shows structure of bcc(110)
`with Kurdjumov-Sachs orientation. The fcc ! bcc phase
`transformation quickly proceeds between 2 and 4 ML,
`above which the films have been fully transformed into
`bcc morphology according to our STM studies.
`For the pulsed laser deposited films, the morphology
`is close to a layer-by-layer one. At 1 ML, more than
`90% of the substrate surface has been covered by the
`Fe atoms, while in the 2 ML film the second layer
`contributes more than 85% of the total surface area.
`Moreover, we have obtained clear evidence that
`the
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`VOLUME 80, NUMBER 9
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`P H Y S I C A L R E V I E W L E T T E R S
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`2 MARCH 1998
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`irrespective whether the easy magnetization axis is paral-
`lel or perpendicular to the film surface. Typical hysteresis
`loops of both thermally and pulsed laser deposited films
`at 1.6 ML are shown in Fig. 3. The two loops, measured
`at 40 K, differ strongly. The PLD loop has a well-defined
`rectangular shape and a very small coercivity of 135 Oe.
`The TD loop is significantly tilted with a large coercivity
`of 1150 Oe, reflecting the imperfection of the TD films as
`shown in Fig. 1. In this respect, the very small coercivity
`and rectangular shape of the hysteresis of the PLD film is
`yet another proof of its structural perfection.
`The most remarkable difference, however, is the satu-
`ration magnetization. At the same nominal thickness of
`1.6 ML, the saturation magnetization of the PLD film is
`almost 4 times larger than that of the TD film.
`In Fig. 4
`the saturation magnetization is displayed as a function of
`thickness. Here all the data points, except two points for
`the PLD films below 1.5 ML, were acquired at 150 K by
`liquid (cid:78)(cid:50) cooling instead of 40 K by liquid He cooling.
`Since the Curie temperature of both PLD and TD films is
`above 200 K [12], 150 K turns out to be sufficiently be-
`low the Curie temperature where the magnetization falls
`rapidly. Below 1.5 ML, the Curie temperature of the PLD
`films is lower than 150 K; therefore, the two data points
`were measured at 40 K. It is also important to note here
`that we consider the measured Kerr intensity, no matter
`from the fcc or transformed bcc films, to be proportional
`to the magnetization [11].
`For the thermally deposited films in Fig. 4, their mag-
`netization is strongly correlated with the fcc ! bcc phase
`transformation. In the thickness range where the fcc phase
`is dominant 共,(cid:50) ML兲, the magnetization of the films has
`low values increasing near linearly with film thickness.
`Between 2 and 4 ML, the magnetization increases steeply,
`in accordance with the rapid progress of the fcc ! bcc
`phase transformation in this region.
`In the bcc region
`
`¢
`
`FIG. 3. Comparison of Kerr hysteresis loops of a 1.6 ML
`Fe兾Cu共(cid:49)(cid:49)(cid:49)
`film prepared by thermal deposition and pulsed
`laser deposition. The loops were measured at 40 K. The
`saturation signal of the pulsed laser deposited film is about
`4 times larger than that of the thermal film. The former has
`also a square shape with a much smaller coercivity reflecting
`the improved morphology shown in Fig. 1.
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`1982
`
`共.(cid:52) ML兲, the magnetization of the films again rises pro-
`portionally with thickness but is nearly 4 to 5 times larger
`than the expected values based on the linear extrapolation
`from the fcc films 共,(cid:50) ML兲. This means that by thermal
`growth, the net magnetic moment of the fcc films is about
`4 to 5 times smaller than that of the bcc films. Assum-
`ing the typical moment of (cid:50)(cid:46)(cid:50)m(cid:66) for the transformed bcc
`films, we estimate the net moment of the TD films in the
`fcc regime to be about (cid:48)(cid:46)(cid:53)m(cid:66), which is consistent with
`previous results from the copper capped films [10] and the
`films supported by a stepped substrate [11].
`two different
`For the pulsed laser deposited films,
`regions can be distinguished in Fig. 4. Below 3 ML
`(region I),
`the magnetization increases linearly with
`increasing thickness, reaching its maximum at about
`3 ML. The magnetic moment in this region appears to
`be close to that of the transformed TD bcc films, i.e.,
`(cid:50)(cid:46)(cid:50)m(cid:66) per atom. This moment value corresponds to the
`high-moment ferromagnetic phase since it is clearly larger
`than the net moment of any other magnetic structure
`predicted by the theories [1,2]. Above (cid:51) ML (region II),
`the magnetization falls abruptly to 30% to 40% of that
`of the 3 ML film. Upon further thickness increasing, the
`
`¢
`
`FIG. 4. Saturation magnetization as a function of the thick-
`ness of the thermally (open circles) and pulsed laser deposited
`(full circles) Fe兾Cu共(cid:49)(cid:49)(cid:49)
`films. The thermal films have low
`magnetization below 2 ML ((cid:48)(cid:46)(cid:53)m(cid:66) per atom per monolayer).
`Between 2 and 4 ML, their magnetization increases quickly,
`in accordance with the rapid progress of the fcc ! bcc phase
`In the bcc region 共.(cid:52) ML兲, the
`transformation in this region.
`magnetization of the films is nearly 4 times larger than the ex-
`pected values based on the linear extrapolation from the fcc
`films 共,(cid:50) ML兲. For the pulsed laser deposited films, the mag-
`netization initially increases linearly at a rate of about (cid:50)m(cid:66) per
`atom per monolayer reaching the maximum at about 3 ML.
`Above 3 ML, the magnetization first falls down to 30% to 40%
`of that of the 3 ML film, and then linearly increases with a
`slope of about (cid:48)(cid:46)(cid:55)m(cid:66) per atom per monolayer. Note that at a
`given nominal thickness below 3 ML, the magnetization of the
`pulsed laser deposited films is strongly enhanced, up to a factor
`of 4 to 5, as compared to that of the thermal films.
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`VOLUME 80, NUMBER 9
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`P H Y S I C A L R E V I E W L E T T E R S
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`2 MARCH 1998
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`magnetization again rises linearly with a 3 times smaller
`slope than the initial slope below 3 ML. We estimate the
`net moment of the films in region II to be about (cid:48)(cid:46)(cid:55)m(cid:66) per
`atom averaged over the film.
`As mentioned, the transition from low-moment to high-
`moment phase in the TD Fe films is the direct result of the
`fcc ! bcc phase transformation in this system. For the
`PLD films, the structural origin of the abrupt decrease of
`the net magnetic moment around 3 ML, however, appears
`to be less clear. Figure 2 indicates that neither a structural
`transformation nor a distinct change of the lattice constant
`occurs around 3 ML thickness.
`In this respect,
`the
`PLD Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 system has to be distinguished from
`the well-studied TD Fe兾Cu共(cid:49)(cid:48)(cid:48)兲 system, where an abrupt
`drop of the magnetization has also been observed around
`4 ML thickness [8]. The latter is generally considered as
`a result of the structural relaxation from fct to fcc which
`apparently does not occur in the PLD Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 case.
`An obvious question is as follows: Why do the ther-
`mally grown Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films have a low net moment in
`the first few monolayers? Two possible mechanisms may
`be responsible for the small moment. First, there exists
`two additional paths for the strain relaxation in the TD Fe
`films: (1) via the edge atoms [13] of the multilayer island
`and (2) via the bcc precipitates [14]. Therefore strain re-
`laxation is more likely to occur in the TD Fe films than
`in the PLD films, which could result in a smaller atomic
`volume, thus a smaller moment, for the TD films. Sec-
`ond, the thermally grown Fe films may have mixed signif-
`icantly with copper due to the high diffusivity of the (111)
`surface. The net moment of Fe will be reduced by copper
`diffusion: In an extreme case, the films become nonmag-
`netic when deposited at 370 K where a significant inter-
`diffusion occurs [15]. The pulsed laser deposited films,
`however, are less likely affected by copper diffusion be-
`cause of the fast deposition rate.
`The finding of the high-moment phase for the pulsed
`laser deposited Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films 共,(cid:51) ML兲 has also clearly
`demonstrated that it is not essential for the fcc Fe films to
`expand tetragonally to stabilize the high-moment phase.
`Both the tetragonally expanded Fe兾Cu共(cid:49)(cid:48)(cid:48)兲 films and the
`isotropic Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films have high-moment ferromag-
`netic phase at low thicknesses, but the transition from the
`high-moment to a low-moment phase has different origins
`in the two systems. The instability of the moment of fcc Fe
`may be understood in the following way: If changes of the
`lattice constant do occur, as in the Fe兾Cu共(cid:49)(cid:48)(cid:48)兲 system, the
`magnetic moment of the fcc Fe films will change accord-
`ingly; but even if the lattice constant remains unchanged,
`the magnetic moment of the film as a whole will still be
`
`changed if the thickness is high enough and the influence
`of the Fe兾Cu interface is low enough. Our experimental
`data (Fig. 4) suggest that in the fcc Fe(111) films the trans-
`formed phase is a low-moment ferromagnetic or ferrimag-
`netic phase in contrast to the antiferromagnetic phase in the
`fcc Fe(100) films. The determination of the detailed mag-
`netic structure of the Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films, however, needs
`theoretical studies using ab initio calculations.
`In summary, we have successfully modified the mor-
`phology of the Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 films by increasing the de-
`position rate by about 5 to 6 orders of magnitude using
`pulsed laser deposition. The pulsed laser deposited films
`共,(cid:51) ML兲 have a significantly enlarged magnetic moment
`indicating that the isotropic fcc Fe兾Cu共(cid:49)(cid:49)(cid:49)兲 ultrathin films
`are high-moment ferromagnetic.
`The authors thank G. Kroder and F. Pabisch for their
`technical support.
`
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`1983
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