We report results on highly oriented, face-centered tetragonal ordered CoPt and FePt thin films grown nonepitaxially by directly depositing films on thermally oxidized Si substrates and subsequent annealing. By controlling the thickness, composition, and annealing conditions, and/or depositing a proper underlayer, nearly perfect (001)-oriented CoPt and FePt films can be obtained. Magnetic measurements reveal large perpendicular anisotropy for such films

Powers, Nathan D.

Sellmyer, David J.

Yan, M.L.

Zeng, H.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy 4-1-2002 Orientation-controlled nonepitaxial L10 CoPt and FePt films H. Zeng University of Nebraska - Lincoln M.L. Yan University of Nebraska - Lincoln Nathan D. Powers University of Nebraska-Lincoln, ndp5@byu.edu David J. Sellmyer University of Nebraska-Lincoln, dsellmyer@unl.edu Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer  Part of the Physics Commons Zeng, H.; Yan, M.L.; Powers, Nathan D.; and Sellmyer, David J., "Orientation-controlled nonepitaxial L10 CoPt and FePt films" (2002). David Sellmyer Publications. 47. https://digitalcommons.unl.edu/physicssellmyer/47 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Orientation-controlled nonepitaxial L10 CoPt and FePt filmsH. Zeng, M. L. Yan, N. Powers, and D. J. Sellmyera)Center for Materials Research and Analysis and Department of Physics and Astronomy,University of Nebraska, Lincoln, Nebraska 68588~Received 29 October 2001; accepted for publication 4 February 2002!We report results on highly oriented, face-centered tetragonal ordered CoPt and FePt thin filmsgrown nonepitaxially by directly depositing films on thermally oxidized Si substrates andsubsequent annealing. By controlling the thickness, composition, and annealing conditions, and/ordepositing a proper underlayer, nearly perfect ~001!-oriented CoPt and FePt films can be obtained.Magnetic measurements reveal large perpendicular anisotropy for such films. © 2002 AmericanInstitute of Physics. @DOI: 10.1063/1.1464663#Recently, L10 structured CoPt and FePt films,1–4 CoPtand FePt based nanocomposite films5,6 and FePt self-assembled nanoparticles7 have attracted much attention.These high anisotropy materials show great potential forhigh-density magnetic recording because of their ability toachieve extremely small grain sizes.7,8 Under normal growthconditions, the films often possess ~111! preferred or randomorientations.1,4 For data-storage applications, it is desirablefor the magnetic easy axis to orient either in the plane orperpendicular to the plane.9 Hence, CoPt and FePt basedfilms with ~001! texture have an advantage over randomly or~111!-oriented films. Previously, ~001! texture were obtainedby epitaxial growth on heated MgO~001! substrates.10 Pt un-derlayers also have been used for obtaining ~001!textured.11,12 Only recently have ~001!-oriented FePt:B2O3~Refs. 6 and 13! and CoPt/Ag ~Ref. 14! nanocomposite filmsbeen reported. However, the mechanism of texture formationremains mostly unclear.In this work, we have prepared nearly perfect ~001!-textured CoPt and FePt films by directly sputtering films onthermally oxidized Si substrates and subsequent annealing.Such films are interesting because the orientation mechanismis totally different from that of epitaxial films. The textureevolution during annealing takes place in three distinctivestages: initial nucleation of nanocrystallites, disorder–ordertransition, and grain growth and coalescence. Each stageshows different orientation, with different characteristic timescales particular to the material. By controlling these pro-cesses, film thicknesses, and compositions, we are able tocontrol the orientation of films.CoPt and FePt films were prepared by sputtering(Co/Pt)n and (Fe/Pt)n multilayers, followed by conventionalor rapid thermal annealing. The thickness of Co ~Fe! layerwas varied while keeping Pt layer-thickness constant, so thatthe composition could be adjusted. The nominal total thick-ness was varied from 20 to 900 Å. The structural propertieswere characterized by x-ray diffraction ~XRD!. The magneticproperties were measured by alternating-gradient-force andsuperconducting-quantum-interference-device magnetome-ters.Figure 1 shows the XRD pattern for a 112 Å Co50Pt50sample annealed at 750 °C for different durations. The an-nealing time in Fig. 1~a! is 2 s. Only one peak at 48.4° isclearly visible, which corresponds to neither ~200! (47.6°)nor ~002! (49.3°) of the face-centered tetragonal ~fct! or-dered CoPt, but rather the position is in between. The hys-teresis loop shows very small coercivity, indicating that thesample structure is disordered face-centered-cubic ~fcc! with~100! texture. Upon annealing from 30 to 300 s, the ~001!superlattice peak emerges and becomes stronger, while thefcc ~200! peak first broadens and eventually splits into twopeaks corresponding to ~200! and ~002! of the ordered fctphase @Figs. 1~b! and 1~c!#. Upon further annealing for 600 s,the ~111! peak intensity increases and becomes predominant,indicating the development of ~111! texture @Fig. 1~d!#.It is expected that the possible texture evolution processas a function of annealing time takes on three distinctivestages, with different time scales. The first stage, initialnucleation of disordered fcc ~100! nanocrystallites, occurs ina few seconds. The ~100! texture indicates that the nucleiorientations are not evenly represented, and the ~100! orien-tation prevails over other orientations. This is presumablydue to the interaction between the film and the substrate, e.g.,strain. The second stage, the disorder–order transition, takesseveral minutes. Atomic diffusion likely involves only neigh-boring cells, so that some Co and Pt atoms interchange theirsites without significantly rearranging the whole lattice. Theresulting orientation thus has two components, with the su-a!Electronic mail: dsellmyer@unl.eduFIG. 1. XRD pattern of 112 Å Co50Pt50 films annealed at 750 °C for ~a! 2,~b! 30, ~c! 300, and ~d! 600 s.APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 13 1 APRIL 200223500003-6951/2002/80(13)/2350/3/$19.00 © 2002 American Institute of PhysicsDownloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspperlattice plane parallel @~001! component# and perpendicu-lar @~100! component# to the film plane, both originatingfrom the disordered ~100! structure. During this stage, somegrain growth and coalescence occurs.15 The final stage, pro-longed thermal treatment, leads to significant grain growthand coalescence. To minimize the surface energy, the filmgradually develops ~111! texture, since ~111! is the closest-packed plane. The grain sizes estimated from the Scherrer’sformula confirm the grain growth process during annealing.Based on this analysis, suppressing grain growth is es-sential in promoting ~001! and ~100! orientations. This canbe done by reducing film thicknesses. For thinner films afterannealing, the films are no longer continuous but rather con-stitute separate island-like grains. Once the material betweengrains is depleted, high diffusion barriers prohibit furthergrain growth and coalescence. Liou et al.16 found that long-time annealed 10 nm CoPt films form well-separated nano-particles. The ~001! and ~100! textures of the nuclei can thusbe preserved for thinner films even after long annealingtimes. Figure 2 shows XRD for Co50Pt50 films annealed at750 °C for 300 s, with the thickness varying from 28 to 896Å. It can be seen that the thinnest sample shows both ~001!and ~100! orientations, with only a very weak ~111! peak;while the thickest one shows only ~111! texture. For interme-diate thicknesses, the texture is a mixture of ~001!, ~100!, and~111!.We have systematically varied the Co atomic ratio from40% to 60%, in an attempt to achieve highly oriented ~001!CoPt films. It can be seen from Fig. 3 that for films annealedat 750 °C for 300 s, the 40% Co film @Fig. 3~a!# shows astrong ~200! peak, besides ~001! and ~002! peaks, while 60%Co film exhibits predominantly ~001! orientation @Fig. 3~d!#.At the same time, with increasing Co composition, all peaksshift slightly toward higher angles. This is to be expectedsince Co has a smaller atomic radius, which decreases latticespacing. For fct CoPt or FePt, the lattice parameter c isslightly smaller than a , which may suggest that ~001! orien-tation has lower surface energy than ~100! orientation. Thereason why Co rich films favor ~001! texture to ~100! textureis presumably related to the change of surface energy bymutual substitution of Co and Pt atoms.As can be seen next, by depositing a B2O3 underlayer,and utilizing the interdiffusion between the underlayer andthe magnetic film, it is possible to further control the grainsize and separations. Figure 4 shows 200 Å Co55Pt45 films,with and without a 20 Å B2O3 underlayer annealed at 750 °Cfor 600 s. For CoPt films thicker than 60 Å without an un-derlayer, long-time annealing leads to significant ~111! orien-tation. However, with only a 20 Å B2O3 underlayer, the tex-ture after long-time annealing is still ~001!, and the grain sizeis significantly smaller than without the underlayer. This sug-gests that during annealing, B2O3 diffuses into the grainboundaries and builds up diffusion barriers, which sup-presses the grain growth and coalescence. Grain boundarysegregation of nonmagnetic materials also helps to reducethe intergrain exchange coupling.8So far, we have seen that obtaining highly oriented ~001!texture requires a balance of the thermal treatment time toFIG. 3. XRD pattern of 120 Å CoxPt1002x films annealed at 750 °C for 300s, with x5 ~a! 40, ~b! 50, ~c! 55, and ~d! 60.FIG. 4. XRD pattern of 200 Å Co55Pt45 films annealed at 750 °C for 600 s.~a! without an underlayer, and ~b! with a 20 Å B2O3 underlayer.FIG. 5. XRD pattern of Fe55Pt45 films annealed at 550 °C for 2 s, withvarying thickness: ~a! 60, ~b! 120, and ~c! 240 Å.FIG. 2. XRD pattern of Co50Pt50 films annealed at 750 °C for 300 s, withvarying thicknesses: ~a! 28, ~b! 56, ~c! 224, and ~d! 896 Å.2351Appl. Phys. Lett., Vol. 80, No. 13, 1 April 2002 Zeng et al.Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspensure sufficient ordering while at the same time minimizingthe grain growth. Thinner films with slightly Co rich compo-sitions are also essential. A suitable underlayer can help toachieve nearly perfect ~001! texture.FePt films show similar behavior. However, the orderingtakes place at significantly lower annealing temperature andshorter time, suggesting a large difference in diffusion effi-ciency from CoPt. Figure 5 shows XRD data for Fe55Pt45films annealed at 550 °C for 2 s, with nominal thicknessesfrom 60 to 240 Å. All the samples show only ~001! and ~002!superlattice peaks, aside from the ~200! peak of Si. UnlikeCoPt, the ordering occurs within a few seconds of annealing,so that there is neither a significant disordered fcc phase norsignificant grain growth and coalescence leading to ~111!texture during this stage. These characteristics ensure thatnearly perfect ~001! texture can be achieved for FePt films atmoderate annealing temperatures and short times, with verysmall grain sizes, as opposed to CoPt films. The grain sizeestimated from Scherrer’s formula for the 240 Å FePt sampleis only 6.7 nm.Typical hysteresis loops for a 100 Å CoPt film annealedat 750 °C for 300 s. and FePt film at 550 °C for 5 s, indirections both perpendicular and parallel to the film, areshown in Fig. 6. It is seen that for both films, the easy axis isin the perpendicular direction, which is consistent with XRDresults. The perpendicular loops show large coercivity(.7 kOe) and high remanence ratio.In summary, we have produced nearly perfect ~001! ori-ented CoPt and FePt films. These films are grown nonepi-taxially on thermally oxidized Si substrates. In the case ofFePt, optimum magnetic properties can be achieved at mod-erate annealing temperature and short annealing time, withvery small grain sizes. The nanostructures can be furthercontrolled by the proper underlayer for better texture andsmaller grain sizes and intergrain exchange coupling. Theprocesses and substrates used are much more compatiblewith practical applications than those used to grow epitaxialfilms.Research supported by NRI, NSIC, IBM, and CMRA.1 K. R. Coffey, M. A. Parker, and J. K. Howard, IEEE Trans. Magn. 31,2737 ~1995!.2 C. P. Luo and D. J. Sellmyer, IEEE Trans. Magn. 31, 2764 ~1995!.3 C. P. Luo, Z. S. Shan, and D. J. Sellmyer, J. Appl. Phys. 79, 4899 ~1996!.4 R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, A. Cebollada, and G.R. Harp, J. Appl. Phys. 79, 5967 ~1996!.5 M. Yu, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett. 75, 3992 ~1999!.6 C. P. Luo, S. H. Liou, L. Gao, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett.77, 2225 ~2000!.7 S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287,1989 ~2000!.8 D. J. Sellmyer, C. P. Luo, and H. Zeng, in Magnetic Storage SystemsBeyond 2000, edited by G. C. Hadjipanayis ~Kluwer, Dordrecht, 2002!,p. 163.9 C. D. Mee, Magnetic Recording Technology ~McGraw–Hill, New York,1996!.10 B. M. Lairson, M. R. Visokay, R. Sinclair, and B. M. Clemens, Appl.Phys. Lett. 62, 639 ~1993!.11 M. R. Visokay and R. Sinclair, Appl. Phys. Lett. 66, 1692 ~1995!.12 M. Watanabe and M. Homma, Jpn. J. Appl. Phys., Part 2 35, L1264~1996!.13 C. P. Luo, Ph.D Dissertation, University of Nebraska, 2001.14 V. Karanasos, I. Panagiotopoulos, D. Niarchos, H. Okumura, and G. C.Hadjipanayis, Appl. Phys. Lett. 79, 1255 ~2001!.15 L. Eckertova´, Physics of Thin Films ~Plenum, New York, 1986!.16 S. Liou, Y. Liu, S. S. Malhotra, M. Yu, and D. J. Sellmyer, J. Appl. Phys.79, 5060 ~1996!.FIG. 6. Typical hysteresis loops for ~a! 100 Å CoPt annealed at 750 °C for300 s and ~b! 100 Å FePt annealed at 550 °C for 5 s.2352 Appl. Phys. Lett., Vol. 80, No. 13, 1 April 2002 Zeng et al.Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Orientation-controlled nonepitaxial L1\u3csub\u3e0\u3c/sub\u3e CoPt and FePt films

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
David Sellmyer Publications Research Papers in Physics and Astronomy 
4-1-2002 
Orientation-controlled nonepitaxial L10 CoPt and FePt films 
H. Zeng 
University of Nebraska - Lincoln 
M.L. Yan 
University of Nebraska - Lincoln 
Nathan D. Powers 
University of Nebraska-Lincoln, ndp5@byu.edu 
David J. Sellmyer 
University of Nebraska-Lincoln, dsellmyer@unl.edu 
Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer 
 Part of the Physics Commons 
Zeng, H.; Yan, M.L.; Powers, Nathan D.; and Sellmyer, David J., "Orientation-controlled nonepitaxial L10 
CoPt and FePt films" (2002). David Sellmyer Publications. 47. 
https://digitalcommons.unl.edu/physicssellmyer/47 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Orientation-controlled nonepitaxial L10 CoPt and FePt films
H. Zeng, M. L. Yan, N. Powers, and D. J. Sellmyera)
Center for Materials Research and Analysis and Department of Physics and Astronomy,
University of Nebraska, Lincoln, Nebraska 68588
~Received 29 October 2001; accepted for publication 4 February 2002!
We report results on highly oriented, face-centered tetragonal ordered CoPt and FePt thin films
grown nonepitaxially by directly depositing films on thermally oxidized Si substrates and
subsequent annealing. By controlling the thickness, composition, and annealing conditions, and/or
depositing a proper underlayer, nearly perfect ~001!-oriented CoPt and FePt films can be obtained.
Magnetic measurements reveal large perpendicular anisotropy for such films. © 2002 American
Institute of Physics. @DOI: 10.1063/1.1464663#
Recently, L10 structured CoPt and FePt films,1–4 CoPt
and FePt based nanocomposite films5,6 and FePt self-
assembled nanoparticles7 have attracted much attention.
These high anisotropy materials show great potential for
high-density magnetic recording because of their ability to
achieve extremely small grain sizes.7,8 Under normal growth
conditions, the films often possess ~111! preferred or random
orientations.1,4 For data-storage applications, it is desirable
for the magnetic easy axis to orient either in the plane or
perpendicular to the plane.9 Hence, CoPt and FePt based
films with ~001! texture have an advantage over randomly or
~111!-oriented films. Previously, ~001! texture were obtained
by epitaxial growth on heated MgO~001! substrates.10 Pt un-
derlayers also have been used for obtaining ~001!
textured.11,12 Only recently have ~001!-oriented FePt:B2O3
~Refs. 6 and 13! and CoPt/Ag ~Ref. 14! nanocomposite films
been reported. However, the mechanism of texture formation
remains mostly unclear.
In this work, we have prepared nearly perfect ~001!-
textured CoPt and FePt films by directly sputtering films on
thermally oxidized Si substrates and subsequent annealing.
Such films are interesting because the orientation mechanism
is totally different from that of epitaxial films. The texture
evolution during annealing takes place in three distinctive
stages: initial nucleation of nanocrystallites, disorder–order
transition, and grain growth and coalescence. Each stage
shows different orientation, with different characteristic time
scales particular to the material. By controlling these pro-
cesses, film thicknesses, and compositions, we are able to
control the orientation of films.
CoPt and FePt films were prepared by sputtering
(Co/Pt)n and (Fe/Pt)n multilayers, followed by conventional
or rapid thermal annealing. The thickness of Co ~Fe! layer
was varied while keeping Pt layer-thickness constant, so that
the composition could be adjusted. The nominal total thick-
ness was varied from 20 to 900 Å. The structural properties
were characterized by x-ray diffraction ~XRD!. The magnetic
properties were measured by alternating-gradient-force and
superconducting-quantum-interference-device magnetome-
ters.
Figure 1 shows the XRD pattern for a 112 Å Co50Pt50
sample annealed at 750 °C for different durations. The an-
nealing time in Fig. 1~a! is 2 s. Only one peak at 48.4° is
clearly visible, which corresponds to neither ~200! (47.6°)
nor ~002! (49.3°) of the face-centered tetragonal ~fct! or-
dered CoPt, but rather the position is in between. The hys-
teresis loop shows very small coercivity, indicating that the
sample structure is disordered face-centered-cubic ~fcc! with
~100! texture. Upon annealing from 30 to 300 s, the ~001!
superlattice peak emerges and becomes stronger, while the
fcc ~200! peak first broadens and eventually splits into two
peaks corresponding to ~200! and ~002! of the ordered fct
phase @Figs. 1~b! and 1~c!#. Upon further annealing for 600 s,
the ~111! peak intensity increases and becomes predominant,
indicating the development of ~111! texture @Fig. 1~d!#.
It is expected that the possible texture evolution process
as a function of annealing time takes on three distinctive
stages, with different time scales. The first stage, initial
nucleation of disordered fcc ~100! nanocrystallites, occurs in
a few seconds. The ~100! texture indicates that the nuclei
orientations are not evenly represented, and the ~100! orien-
tation prevails over other orientations. This is presumably
due to the interaction between the film and the substrate, e.g.,
strain. The second stage, the disorder–order transition, takes
several minutes. Atomic diffusion likely involves only neigh-
boring cells, so that some Co and Pt atoms interchange their
sites without significantly rearranging the whole lattice. The
resulting orientation thus has two components, with the su-
a!Electronic mail: dsellmyer@unl.edu
FIG. 1. XRD pattern of 112 Å Co50Pt50 films annealed at 750 °C for ~a! 2,
~b! 30, ~c! 300, and ~d! 600 s.
APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 13 1 APRIL 2002
23500003-6951/2002/80(13)/2350/3/$19.00 © 2002 American Institute of Physics
Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
perlattice plane parallel @~001! component# and perpendicu-
lar @~100! component# to the film plane, both originating
from the disordered ~100! structure. During this stage, some
grain growth and coalescence occurs.15 The final stage, pro-
longed thermal treatment, leads to significant grain growth
and coalescence. To minimize the surface energy, the film
gradually develops ~111! texture, since ~111! is the closest-
packed plane. The grain sizes estimated from the Scherrer’s
formula confirm the grain growth process during annealing.
Based on this analysis, suppressing grain growth is es-
sential in promoting ~001! and ~100! orientations. This can
be done by reducing film thicknesses. For thinner films after
annealing, the films are no longer continuous but rather con-
stitute separate island-like grains. Once the material between
grains is depleted, high diffusion barriers prohibit further
grain growth and coalescence. Liou et al.16 found that long-
time annealed 10 nm CoPt films form well-separated nano-
particles. The ~001! and ~100! textures of the nuclei can thus
be preserved for thinner films even after long annealing
times. Figure 2 shows XRD for Co50Pt50 films annealed at
750 °C for 300 s, with the thickness varying from 28 to 896
Å. It can be seen that the thinnest sample shows both ~001!
and ~100! orientations, with only a very weak ~111! peak;
while the thickest one shows only ~111! texture. For interme-
diate thicknesses, the texture is a mixture of ~001!, ~100!, and
~111!.
We have systematically varied the Co atomic ratio from
40% to 60%, in an attempt to achieve highly oriented ~001!
CoPt films. It can be seen from Fig. 3 that for films annealed
at 750 °C for 300 s, the 40% Co film @Fig. 3~a!# shows a
strong ~200! peak, besides ~001! and ~002! peaks, while 60%
Co film exhibits predominantly ~001! orientation @Fig. 3~d!#.
At the same time, with increasing Co composition, all peaks
shift slightly toward higher angles. This is to be expected
since Co has a smaller atomic radius, which decreases lattice
spacing. For fct CoPt or FePt, the lattice parameter c is
slightly smaller than a , which may suggest that ~001! orien-
tation has lower surface energy than ~100! orientation. The
reason why Co rich films favor ~001! texture to ~100! texture
is presumably related to the change of surface energy by
mutual substitution of Co and Pt atoms.
As can be seen next, by depositing a B2O3 underlayer,
and utilizing the interdiffusion between the underlayer and
the magnetic film, it is possible to further control the grain
size and separations. Figure 4 shows 200 Å Co55Pt45 films,
with and without a 20 Å B2O3 underlayer annealed at 750 °C
for 600 s. For CoPt films thicker than 60 Å without an un-
derlayer, long-time annealing leads to significant ~111! orien-
tation. However, with only a 20 Å B2O3 underlayer, the tex-
ture after long-time annealing is still ~001!, and the grain size
is significantly smaller than without the underlayer. This sug-
gests that during annealing, B2O3 diffuses into the grain
boundaries and builds up diffusion barriers, which sup-
presses the grain growth and coalescence. Grain boundary
segregation of nonmagnetic materials also helps to reduce
the intergrain exchange coupling.8
So far, we have seen that obtaining highly oriented ~001!
texture requires a balance of the thermal treatment time to
FIG. 3. XRD pattern of 120 Å CoxPt1002x films annealed at 750 °C for 300
s, with x5 ~a! 40, ~b! 50, ~c! 55, and ~d! 60.
FIG. 4. XRD pattern of 200 Å Co55Pt45 films annealed at 750 °C for 600 s.
~a! without an underlayer, and ~b! with a 20 Å B2O3 underlayer.
FIG. 5. XRD pattern of Fe55Pt45 films annealed at 550 °C for 2 s, with
varying thickness: ~a! 60, ~b! 120, and ~c! 240 Å.
FIG. 2. XRD pattern of Co50Pt50 films annealed at 750 °C for 300 s, with
varying thicknesses: ~a! 28, ~b! 56, ~c! 224, and ~d! 896 Å.
2351Appl. Phys. Lett., Vol. 80, No. 13, 1 April 2002 Zeng et al.
Downloaded 16 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
ensure sufficient ordering while at the same time minimizing
the grain growth. Thinner films with slightly Co rich compo-
sitions are also essential. A suitable underlayer can help to
achieve nearly perfect ~001! texture.
FePt films show similar behavior. However, the ordering
takes place at significantly lower annealing temperature and
shorter time, suggesting a large difference in diffusion effi-
ciency from CoPt. Figure 5 shows XRD data for Fe55Pt45
films annealed at 550 °C for 2 s, with nominal thicknesses
from 60 to 240 Å. All the samples show only ~001! and ~002!
superlattice peaks, aside from the ~200! peak of Si. Unlike
CoPt, the ordering occurs within a few seconds of annealing,
so that there is neither a significant disordered fcc phase nor
significant grain growth and coalescence leading to ~111!
texture during this stage. These characteristics ensure that
nearly perfect ~001! texture can be achieved for FePt films at
moderate annealing temperatures and short times, with very
small grain sizes, as opposed to CoPt films. The grain size
estimated from Scherrer’s formula for the 240 Å FePt sample
is only 6.7 nm.
Typical hysteresis loops for a 100 Å CoPt film annealed
at 750 °C for 300 s. and FePt film at 550 °C for 5 s, in
directions both perpendicular and parallel to the film, are
shown in Fig. 6. It is seen that for both films, the easy axis is
in the perpendicular direction, which is consistent with XRD
results. The perpendicular loops show large coercivity
(.7 kOe) and high remanence ratio.
In summary, we have produced nearly perfect ~001! ori-
ented CoPt and FePt films. These films are grown nonepi-
taxially on thermally oxidized Si substrates. In the case of
FePt, optimum magnetic properties can be achieved at mod-
erate annealing temperature and short annealing time, with
very small grain sizes. The nanostructures can be further
controlled by the proper underlayer for better texture and
smaller grain sizes and intergrain exchange coupling. The
processes and substrates used are much more compatible
with practical applications than those used to grow epitaxial
films.
Research supported by NRI, NSIC, IBM, and CMRA.
1 K. R. Coffey, M. A. Parker, and J. K. Howard, IEEE Trans. Magn. 31,
2737 ~1995!.
2 C. P. Luo and D. J. Sellmyer, IEEE Trans. Magn. 31, 2764 ~1995!.
3 C. P. Luo, Z. S. Shan, and D. J. Sellmyer, J. Appl. Phys. 79, 4899 ~1996!.
4 R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, A. Cebollada, and G.
R. Harp, J. Appl. Phys. 79, 5967 ~1996!.
5 M. Yu, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett. 75, 3992 ~1999!.
6 C. P. Luo, S. H. Liou, L. Gao, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett.
77, 2225 ~2000!.
7 S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287,
1989 ~2000!.
8 D. J. Sellmyer, C. P. Luo, and H. Zeng, in Magnetic Storage Systems
Beyond 2000, edited by G. C. Hadjipanayis ~Kluwer, Dordrecht, 2002!,
p. 163.
9 C. D. Mee, Magnetic Recording Technology ~McGraw–Hill, New York,
1996!.
10 B. M. Lairson, M. R. Visokay, R. Sinclair, and B. M. Clemens, Appl.
Phys. Lett. 62, 639 ~1993!.
11 M. R. Visokay and R. Sinclair, Appl. Phys. Lett. 66, 1692 ~1995!.
12 M. Watanabe and M. Homma, Jpn. J. Appl. Phys., Part 2 35, L1264
~1996!.
13 C. P. Luo, Ph.D Dissertation, University of Nebraska, 2001.
14 V. Karanasos, I. Panagiotopoulos, D. Niarchos, H. Okumura, and G. C.
Hadjipanayis, Appl. Phys. Lett. 79, 1255 ~2001!.
15 L. Eckertova´, Physics of Thin Films ~Plenum, New York, 1986!.
16 S. Liou, Y. Liu, S. S. Malhotra, M. Yu, and D. J. Sellmyer, J. Appl. Phys.
79, 5060 ~1996!.
FIG. 6. Typical hysteresis loops for ~a! 100 Å CoPt annealed at 750 °C for
300 s and ~b! 100 Å FePt annealed at 550 °C for 5 s.
2352 Appl. Phys. Lett., Vol. 80, No. 13, 1 April 2002 Zeng et al.
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https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1046&amp;context=physicssellmyer

Orientation-controlled nonepitaxial L1\u3csub\u3e0\u3c/sub\u3e CoPt and FePt films

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