magnetic anisotropy and chemical long-range order in epitaxial ferrimagnetic crpt3 films

Journal of Magnetism and Magnetic Materials 218 (2000) 151}164

Magnetic anisotropy and chemical long-range order in epitaxialferrimagnetic CrPt

3"lms

M. Maret!,",*, M. Albrecht!, J. KoK hler!, R. Poinsot", C. Ulhaq-Bouillet",J.M. Tonnerre#, J.F. Berar#, E. Bucher!

!Fakulta( t fu( r Physik, Universita( t Konstanz, P.O. Box M 621, D-78457 Konstanz, Germany"Institut de Physique et Chimie des Mate& riaux de Strasbourg, UMR46 CNRS-ULP, 23 rue du Loess, F-67037 Strasbourg, France

#Laboratoire de Cristallographie, CNRS, B.P. 166, F-38042 Grenoble, France

Received 20 December 1999; received in revised form 7 April 2000; accepted 7 April 2000

Abstract

Thin "lms of CrPt3

were prepared by molecular beam epitaxy on both Al2O

3(0 0 0 1) and MgO(0 0 1) substrates, either

directly by co-deposition of Cr and Pt at high temperatures or after in situ annealing of superlattices [Cr(2As )/Pt(7As )]. Insitu RHEED observations and X-ray di!raction measurements have allowed us to check the single-crystal quality ofCrPt

3"lms and to determine the degree of L1

2-type long-range order (LRO). In "lms co-deposited between 8503C and

9503C a nearly perfect LRO has been observed. As in bulk alloys, such ordering yields a ferrimagnetic order, while thedisordered "lms are non-magnetic. In contrast with the ferromagnetic L1

2-type ordered CoPt

3(1 1 1) "lms, the ferrimag-

netic CrPt3(1 1 1) "lms exhibit perpendicular magnetic anisotropy with quality factors, K

u/K

d, as large as 5 and large

coercivities around 450kA/m. Such anisotropy could be related to the arrangement of Cr atoms, which owing to theirlarge magnetic moment, oppositely directed to the small Pt moment, drive the magnetization. Since the six Cr nearestneighbours around a Cr atom are along the S00 1T directions, making an angle of 54.743 with the [1 1 1] growthdirection, the overlap of their electron distribution favors an easy axis of magnetization normal to the "lm plane. Thisidea is supported by the absence of magnetic anisotropy in ferrimagnetic CrPt

3(0 0 1) "lms. ( 2000 Elsevier Science

B.V. All rights reserved.

PACS: 6110F; 6155H; 7530G; 7560; 7570

Keywords: CrPt3

alloy "lms; Chemical long-range order; Magnetic anisotropy

1. Introduction

The discovery of the improved short-wavelengthmagneto-optical properties of the Co-Pt alloy "lms

*Correspondence address. FakultaK t fuK r Physik, UniversitaK tKonstanz, J.-Bruckhard-str. 29, P.O. Box M 621, D-78457 Kon-stanz, Germany. Tel.: #49-7531-88-3858; fax: #49-7531-88-3090.

E-mail address: [email protected] (M. Maret).

compared with the multilayers Co/Pt [1] has givenrise to a renewal of interest in the study of the T}Pt(T"Cr, Mn, Fe and Co) transition-metal magneticalloys. Around the equiatomic alloy composition,the growth of the L1

0chemically ordered phase

with the tetragonal c-axis along the "lm normalusing MgO(0 0 1) substrate, leads to alloy "lmswhich exhibit very strong perpendicular aniso-tropy and consequently are very promising

0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 3 6 7 - X

magneto-optical media, such as FePt(0 0 1)(K

u"10 MJ/m3) [2], CoPt(0 0 1) (K

u"3MJ/m3)

[3] and FePd(0 0 1) (Ku"1.5MJ/m3) [4]. More-

over, as it was demonstrated for the FePt alloy [5],when using MgO(1 1 0) substrate the tetragonal c-axis lies in the "lm plane and therefore FePt(1 1 0)"lms are very attractive for longitudinal magneticrecording. In the L1

0phase, the stacking sequence

along the tetragonal easy direction consists ofalternate pure 3d metal and pure Pt planes. Theselective hybridisation along the c-axis between 3datoms with large magnetic moment and Pt atomswith large spin}orbit (SO) coupling is the sourceof the large magnetocristalline anisotropy in theL1

0compounds. As calculated by Oppeneer et al.

[6] using density-functional band-structure theory,the large 3d moment and Pt SO coupling arealso the main factors responsible for the large mag-neto-optical Kerr rotation observed in the TPt

3compounds crystallizing in the cubic L1

2-type

(AuCu3) structure. However, due to the isotropic

character of the L12

structure, no perpendicularanisotropy was observed in the L1

2ordered

CoPt3(1 1 1) "lms grown on Al

2O

3(0 0 0 1) or

mica(0 0 1) substrates at growth temperatures (¹')

higher than 5003C [7,8]. In contrast, for lower¹

'close to 4003C, the CoPt

3"lms develop a

strong perpendicular anisotropy with Ku

values aslarge as 1 MJ/m3. The reason for this drasticchange is that these "lms exhibit no L1

2chemical

long-range order but an anisotropic local ordercharacterized by preferential CoCo pairs in the"lm plane balanced by preferential CoPt pairs outof the plane [9]. Such anisotropic e!ects are pro-moted by the Pt segregation e!ect at the advancingsurface, allowed by dominant surface di!usionduring the codeposition process, and are thenfrozen-in as the free surface advances due tonegligible bulk di!usion. Recently, from angle-dependent X-ray magnetic circular dichroismmeasurements in the CoPt

3"lms, such structural

e!ects were correlated to the existence of Co-3dand Pt-5d orbital moment anisotropies, which canbe considered as the microscopic origin of themagnetocrystalline anisotropy [10].

Among the class of 3d-Pt alloy "lms, interestingfor both basic and applied research, we have pre-pared CrPt

3"lms by molecular-beam epitaxy

(MBE). In contrast with the CoPt3

alloy whichis ferromagnetic in the FCC disordered and L1

2ordered states, the CrPt

3alloy is ferrimagnetic in

the L12

ordered state and non-magnetic in thedisordered state. Polarized neutron di!ractionmeasurements on bulk CrPt

3alloys have led to

values of opposited magnetic moments equal to3.37 and !0.26l

Bfor Cr and Pt atoms, respective-

ly [11]. In this paper, we present an extensive studyof the chemical long-range order and magneticanisotropy measured in CrPt

3"lms grown on

Al2O

3(0 0 0 1) and MgO(00 1) substrates, which

were obtained either directly by codeposition ofCr and Pt atoms at high temperatures or afterin situ annealing of superlattices. In particular,we show that surprisingly, the ferrimagneticorder in the ordered CrPt

3(1 1 1) "lms induces

a marked perpendicular anisotropy. With respectto the "rst study of CrPt

3"lms, which were

obtained after annealing of sputtered multi-layers [12], here the preparation of monocrystal-line "lms on two di!erent surface symmetriesallows us to understand the structural origin ofsuch anisotropy.

2. Experimental procedures

Epitaxial "lms were grown on Al2O

3(0 0 0 1) or

MgO(00 1) substrates on which a Pt bu!er layer of50As was grown at 6503C with a rate of 0.1As /s, ina (Varian) MBE system under ultra-high-vacuumconditions by evaporation of Cr and Pt from twoelectron-gun sources. CrPt

3alloy "lms were pre-

pared either directly by codeposition of Pt and Crat temperatures ranging from 4003C to 9503C, orby in situ high-temperature annealing of superlatti-ces [Cr(&2 As )/Pt(&7As ]

n(n&20}35) grown pre-

viously at room temperature. For co-deposition,the growth rates of Pt and Cr were equal to 0.2 and0.04As /s, respectively and alloy thicknesses weretypically around 470As . In order to obtain the rightCrPt

3composition after annealing the superlatti-

ces, the thicknesses of the Cr and Pt layers werechosen equal to i/2 and 7As on Al

2O

3(0 0 0 1),

values deduced from the interatomic distancesd1 1 0

for BCC Cr and d1 1 1

for FCC Pt, ii/1.5 and6As on MgO(0 0 1), the Cr layer thickness was

152 M. Maret et al. / Journal of Magnetism and Magnetic Materials 218 (2000) 151}164

chosen such that the atomic volume of Cr atomsis equal to that in the BCC structure when theCr}Cr nearest distance "ts the Pt}Pt distance in the(1 0 0) planes. All the alloy "lms and annealedsuperlattices were covered by a protective layer of20As thick Ru grown at room temperature. Thegrowth of the bu!er layer, alloy "lm or superlatticewas characterized in situ using re#ection high-en-ergy electron di!raction (RHEED) with a grazingincidence of about 33 and a beam energy of 10 keV.The recording of RHEED intensities has allowedus to follow the L1

2chemical ordering during

co-deposition and also during annealing of super-lattices.

Accurate X-ray di!raction measurements in "lmsgrown on Al

2O

3(0 0 0 1) were carried out at the

European Synchrotron Radiation Facilities inGrenoble on the seven-circle di!ractometer of theCRG BM02 beamline, using a photon wavelengthequal to 1.5517As . All fundamental and superstruc-ture peaks were measured in re#exion, using eithera symmetric geometry (h}2h scans) or asymmetricgeometry (u}2h scans) with h the Bragg angle andx the angle between the incident beam and the "lmplane (as decribed in Ref. [13]); they provide in-formation on both chemical ordering and the per-fection of the FCC stacking.

The structure of the "lms grown on MgO(0 0 1)was controlled on a high-resolution (HR) X-rayPhilips di!ractometer using a four-Ge(2 2 0) crystalmonochromator providing a pure Cu K

a1parallel

beam. Low-angle re#ectivity measurements wereperformed on the HR di!ractometer by setting slitsof 40 and 100lm at the exit of the monochromatorand before the detector, respectively. Simulations ofthe specular scans using the code of Fischer [14]led to the determination of the bu!er and alloythicknesses, and the interfacial roughness includinginterdi!usion.

The magnetic properties were studied using aSQUID magnetometer with applied "eld up to4]103kA/m (i.e. 50 kOe in cgs). Measurementswere done at 5 and 295K with the "eld appliednormal and parallel to the "lm plane. The e!ectivemagnetic anisotropies (K

%&&"K

u!0.5l

0M2

4) were

calculated from the area enclosed between the per-pendicular and parallel magnetization hysteresiscurves.

3. Film growth

Fig. 1a shows the RHEED patterns recordedafter the growth of a Pt(1 1 1) bu!er onAl

2O

3(0 0 0 1) along the Pt[1 0 11 ] and [1 1 21 ] azi-

muths. The well-de"ned streaks reveal the highlymonocrystalline quality and the #atness of the Ptlayer. The epitaxial relationships with the substrateare [[1 1 21 0]Al

2O

3(0 0 0 1) E [2 11 11 ] Pt(1 1 1),

[1 0 11 0]Al2O

3(0 0 0 1) E [1 0 11 ] Pt(1 1 1).The other

RHEED patterns shown in Fig. 1 were recorded,after the co-deposition of an alloy "lm at 8503C(b), after the growth of a superlattice [Cr(2As )/Pt(7As )]19.5 at room temperature (c) and after insitu annealing up to 10503C (d) (just after reaching10503C the sample was rapidly cooled). The super-lattice consists of an half-integer of bilayers, i.e. thelast deposited layer is a Cr layer. For the alloy "lm,in addition to the fundamental streaks which con-"rms its epitaxial growth on Pt(1 1 1), intermediatestreaks are clearly visible and are characteristic ofL1

2chemical ordering. As it can be seen from the

schematic view of a CrPt3(1 1 1) plane in Fig. 2, the

atomic rows parallel to the two directions [1 1 21 ]and [1 0 11 ] show alternate Pt pure and mixed rows.The RHEED pattern of the as-deposited superla-ttice shows a single set of broader streaks for eachazimuth, slightly spotty, indicating that the Crlayers adopt the FCC stacking of the Pt layers.After annealing the appearance of superstructurestreaks con"rms the formation of the L1

2ordered

phase and the narrowing of all streaks indicatesa marked smoothing of the surface.

During the increase of the annealing temper-ature, we have also recorded the RHEED inten-sities of the fundamental and superstructurestreaks, I

&and I

4, measured along the [1 1 21 ] azi-

muth. Fig. 3 shows the change of I4

corrected forthe background as a function of the annealing time;the corresponding graduated temperatures are alsoindicated. Between 4503C and 8503C the temper-ature was regularly increased and the temperature-dependent variation of I

Sshown in the insert indi-

cates that L12-type long-range ordering increases

weakly up to 7003C, then rapidly above 8003C.LRO is thus maximal around 8503C, decreasesfrom 9003C and is almost vanishing after a long-time annealing at 10503C. Around 4003C, a small

M. Maret et al. / Journal of Magnetism and Magnetic Materials 218 (2000) 151}164 153

Fig. 1. RHEED patterns along the two azimuths [1 0 11 ] and [1 1 21 ] recorded after the growth of (a) 50 thick Pt bu!er grown at 6503C onAl

2O

3(0 0 0 1); (b) a 470 co-deposited alloy of CrPt

3grown at 8503C; a superlattice [Cr(2As )/Pt(7As )]

19.5grown at room temperature;

and (c) and after in situ annealing up to 10503C (d).


Fig. 2. Schematic view of a (1 1 1) plane for the L12

orderedCrPt

3"lm; alternate pure Pt and mixed Cr}Pt rows parallel to

the directions [1 0 11 ] and [1 121 ] (white and dark circles repres-ent Pt and Cr atoms, respectively).

Fig. 3. Change of the superstructure RHEED intensity cor-rected for background recorded during annealing of the[Cr(2As )/Pt(7As )]

19.5superlattice, as a function of the annealing

time (in insert as a function of the annealing temperature duringthe linear increase from 4503C to 8503C).

increase of the RHEED intensity around the super-structure streak position is clearly visible. Usinga line pro"le mode this intensity is spread out in thereciprocal space and corresponds to a large bumpbetween two strong fundamental peaks; such e!ectdisappears rapidly above 4003C. This e!ect inducedby the tendency of Pt atoms to segregate at the

surface through the Cr top layer of the superlatticecould be explained by the formation of a mediumrange ordering between Cr and Pt atoms at thesurface, vanishing when the surface is completelycovered by Pt atoms. This segregation processwas also seen from the in-plane lattice parametermeasurement recorded during the annealing of thesuperlattice Cr/Pt.

Fig. 4 shows the RHEED patterns of aMgO(00 1) substrate (a), a Pt(0 0 1) bu!er layer (b)and a superlattice [Cr(1.5As )/Pt(6 As )]33.5 before (c)and after high-temperature anneal (d). After thegrowth of Pt(0 0 1), the RHEED pattern along theazimuth [1 1 0] is characteristic of a &(1]5)' surfacereconstruction, already observed in several (1 0 0)crystal faces, such as Ir(1 0 0) and Au(1 0 0) andexplained by a hexagonal rearrangement of the toplayer; from LEED studies more complicate recon-structions, like

A14 1

!1 5Bwere reported for Pt(1 0 0) (see, for example, Ref.[15]). The RHEED pattern of the as-depositedsuperlattice con"rms the epitaxial growth of theCr layer, with the following in-plane relationship:Pt[1 1 0]DDCr[1 0 0]. Along the [1 1 0] azimuth, anintermediate streak, corresponding to a period of1.5d

1 1 0is clearly visible suggesting some recon-

struction of the Cr top layer, involving likely the"rst underlayer of Pt atoms, in order to "t the largemis"t between the d

1 0 0Cr}Cr (2.885As ) and

d1 1 0

Pt}Pt (2.775As ) distances. After annealing,very intense intermediate RHEED streaks appearalong the [1 0 0] azimuth. Along the [0 0 1] growthdirection the stacking sequence of a perfectly or-dered CrPt

3"lm consists of alternate mixed and

pure Pt planes. From the schematic view of a (0 0 1)mixed plane shown in Fig. 5a, it appears clearlyalong the [1 0 0] azimuth alternate rows of pure Ptand pure Cr atoms, causing the superstructureRHEED streaks, while all the atomic rows parallelto the [1 1 0] direction consist of mixed atoms.Therefore, the weak intermediate streaks seen inFig. 4 along the [1 1 0] azimuth could be ascribedeither to some reconstruction of the surface, highlyimproved after annealing, or to the existence


Fig. 4. RHEED patterns along the two azimuths [1 1 0] and [1 0 0] of (a) a MgO(0 0 1) substrate and after the growth of (b) 50As thick Ptbu!er grown at 6503C, (c) a superlattice [Cr(1.5As )/Pt(6As )]

33.5grown at room temperature and (d) after its annealing (1 h at 8503C).


Fig. 5. Schematic views of (a) a perfect (0 0 1) mixed plane for theL1

2ordered "lm of CrPt

3, showing alternate pure Pt and mixed

Cr}Pt rows parallel to the direction [1 0 0] and only mixedCr}Pt rows parallel to the direction [1 1 0]; and (b) a (0 0 1) planecontaining an antiphase boundary with a displacement vector[0 1

212] leading to Pt rows parallel to [1 1 0].

1The L12

(A3B) structure can be represented by four inter-

penetrating cubic sublattices: three are equivalent and construc-ted from the centers of two opposite faces in the FCC lattice(sublattices 1, 2, 3) and the fourth consists of the corners of theFCC lattice (sublattice 4). The chemical LRO parameter isde"ned as (P1, 2, 3

P5!x

P5)/(1!l1, 2, 3) where P1, 2, 3

P5is the occu-

pancy rate of the (1, 2, 3) sublattices by Pt atom, xP5

the Pt alloycomposition, and l1, 2, 3 the ratio of the number of sites belong-ing to the three sublattices 1, 2, 3 to the total number of sites inthe FCC lattice (equal to 3

4), i.e x

P5"P1, 2, 3

P5l1, 2, 3#P4

P5l4.

of periodic antiphase domains. As illustrated inFig. 5b, the existence of an antiphase boundarylocated in the (1 11 0) plane with a displacementvector of [0 1

212] leads to the presence of pure Pt

rows.

4. X-ray di4raction

In Fig. 6, a typical h}2h X-ray scan measuredon BM02 is shown for a CrPt

3alloy "lm grown

at 8503C on Pt(1 1 1). From the full-width athalf-maximum (FWHM) of the 111 peak, we havedetermined a normal coherence length, ¸

M"

j/2*u1 1 1

cos h1 1 1

, very close to the alloy thick-ness, indicating the high crystalline perfectionof the "lm. The higher intensity level on the left-sidecomes from the Pt(1 1 1) bu!er. The rocking curveacross the CrPt

3(1 1 1) re#ection drawn in the in-

sert can be decomposed into two Gaussian compo-nents. We can assume that the narrow peak(FWHM"0.0283) is characteristic of the mosaicspread of the alloy "lm, such a small value resultsfrom the high quality of our sapphire substrates(FWHM"0.00353 across the 006 peak), while thebroad peak (FWHM"0.173) yields a lateral co-herence length, ¸

,"j/2*u

1 1 1sin h

1 1 1about

750As . The numerous well-de"ned Kiessig fringeswith a period corresponding to the sum of the Ptbu!er and alloy thicknesses, indicates that the in-terface Pt/CrPt

3is di!used. Fig. 7 shows the experi-

mental and simulated low-angle re#ectivity curvesobtained for the same "lm grown at 8503C. A singleseries of fringes is observable, proving some inter-di!usion between the Pt bu!er and the alloy "lm.The simulation yields thicknesses of Pt bu!er andalloy "lms equal to 44 and 469As , respectively, andan interfacial roughness around 8 As . The mainstructural parameters of several alloy "lms grownat various temperatures on Al

2O

3(0 0 0 1) and an

annealed superlattice are summarized in Table 1.The degree of L1

2chemical ordering is character-

ized by a LRO parameter, S, related to the occu-pancy rate of Pt atoms in the four sublatticesdescribing the L1

2structure (see Ref. [16]).1 S is


Fig. 6. h}2h X-ray di!raction scan around the 1 1 1 re#ection with in insert the rocking curve across 1 1 1 decomposed into two Gaussiancomponents (upper curves) and X-ray patterns of the 1 1 3 and 1 12 re#ections measured in asymmetric re#ection geometry (lowercurves) for a codeposited CrPt

3"lm grown at 8503C (the measurements were performed on the BM02 beamline at the ESRF, using

a photon wavelength of 1.5517As ).

determined from the integrated intensities of there#ections 1 1 3 and 1 1 2 measured in asymmetricre#exion geometry (shown in Fig. 6), including thewidths of their rocking curves. The re#ections 1 1 3and 1 1 2 can be decomposed into two Gaussiancomponents corresponding to di!erent coherentlengths. For 1 1 3, an improvement of the "t can beachieved by adding a supplementary low-anglecomponent coming from the Pt bu!er which is nottaken into account for the calculation of S.

Using a linearly polarized photon beam onBM02, S is deduced from the integrated intensitiesI1 1 3

and I1 1 2

, corrected for sample absorptionand Lorentz factor (¸

hkl"1/sin 2h

hkl) as follows:

S"F1 1 3

F1 1 2

SI1 1 2

A1 1 3

¸1 1 3

I1 1 3

A1 1 2

¸1 1 2

. (1)

Fhkl

is the structure factor for the re#ection hkl, in-cluding a Debye}Waller factor (exp!B sin2h

hkl/j2).


Fig. 7. X-ray re#ectivity curve of a 470As thick CrPt3"lm grown

at 8503C on a 50As thick Pt bu!er: experimental data (crosses)and simulated curve (solid line) (the measurements were carriedout on a HR Philips di!ractometer, using the Cu K

a1wavelength of 1.5406As ).

Table 1Structural parameters of CrPt

3co-deposited "lms and an annealed superlattice [Cr(2As )/Pt(7As )]19.5 , grown on Pt(1 1 1)/Al

2O

3(0 0 0 1)!

Method d1 1 1

t *u1 1 1

S$0.1 *2h1 1 3

*u1 1 3

*2h1 1 2

*u1 1 2

a3)0 a3)0

¹'

(3C) As (As ) (deg) (deg) (deg) (deg) (deg) (As ) (deg)

Co-deposited (650) 2.232 483 0.25 0.195 0.406 0.26 2.683 3.76 3.882 90.307Co-deposited (800) 2.237 469 0.247 0.55 0.27 0.105 0.925 0.713 " *

Co-deposited (850) 2.236 469 0.173 0.95 0.405 0.24 0.404 0.738 3.889 90.2536Co-deposited (900) 2.237 464 0.134 0.9 0.39 0.29 0.43 0.47 * *

Co-deposited (950) 2.237 471 0.14 0.5 0.39 0.184 0.40 0.425 3.891 90.249Superlattice annealed 2.241 169 0.12 0.65 0.66 0.33 0.96 0.682 3.8895 90.161

!d1 1 1

, spacing of (1 1 1) planes (very close to that of bulk L12

CrPt3

equal to 2.238 [17], t, alloy thickness; *u1 1 1

, full-width athalf-maximum of the broad component of the rocking curve across 1 1 1(shown in Fig. 5); S, chemical order parameter; *2h

1 1 3and

*2h1 1 2

, full-widths at half-maximum of the main Gaussian component of the 1 1 3 and 1 1 2 peaks in a u/2h scan; *u1 1 3

and *u1 1 2

,full-widths at half-maximum of the rocking curves across 1 1 3 and 11 2; a3)0 and a3)0, re"ned lattice parameters of the rhombohedral cell,reproducing the deformation of the FCC stacking along the [1 11] growth direction."No re"nement due to an insu$cient number of re#ections.

If we assume that both species have the sameB displacement factor, it can be determined fromthe ratio of the integrated intensities (I

1 1 1/I

2 2 2).

The B values are ranging from 0.7 to 1 As 2. Theabsorption factors A

hklare given by

Ahkl

"

1

k(1#(sin h!/)/(sin h#/))

]A1!exp (!kt)A1

sin h#/#

1

sinh!/BB,(2)

where k is the linear absorption factor of the alloy"lm, t the "lm thickness, h the Bragg angle and/ the angle between the re#ecting plane and the"lm surface.

The values determined for the co-deposited "lmsin Table 1 show that LRO is maximal in a growthtemperature range between 8503C and 9003C, inagreement with our RHEED observations. Let usnote that no 1 1 2 peak was observed in a "lmgrown at 4003C, indicating that the volume di!u-sion is too small to promote LRO across the "lmthickness. Therefore, the bump observed byRHEED around 4003C is likely to be a surfacealloying e!ect. A highly ordered state was alsofound in a superlattice annealed for a relativelyshort time between 8003C and 10503C (30min).

The measurements of supplementary re#ections,namely 1 3 1, 3 1 1, 4 0 0, 2 2 2, have allowed us todetermine a small rhombohedral deformation ofthe FCC stacking corresponding to a slight com-pression along the [1 1 1] growth direction, as in-dicated by a value of the a3)0 angle larger than 903.For the co-deposited "lms, the re"ned a3)0 valueslisted in Table 1 decrease with increasing growthtemperature. This change can be attributed to a re-laxation of the epitaxial stress induced by the Ptbu!er.

Fig. 8 shows specular scans for a co-depositedalloy "lm grown on MgO(0 0 1) at 8503C, measuredon the HR di!ractometer. The intensity and the


Fig. 8. h}2h X-ray di!raction scan for a co-deposited CrPt3"lm

grown at 8503C on a MgO(00 1) substrate (solid line), using theCu K

a1wavelength; the rocking curves across the 0 01 and 0 0 2

re#ections are represented in dashed line.

narrowing of the superstructure 0 0 1 Bragg peaks,compared with those of the fundamental 0 0 2 peak,re#ect a large degree of L1

2ordering. The centers

of the 0 0 1 and 0 0 2 peaks lead to a FCC latticeparameter of 3.88As $0.005 in agreement with thatof bulk CrPt

3. The normal coherence lengths de-

duced from the 0 0 1 and 0 0 2 peaks are about 240and 275As (i.e. 60% of the alloy thickness), respec-tively. In comparison with the CrPt

3(1 1 1) "lms,

this shorter coherence length can be in part at-tributed to the presence of a small volume fraction(about 2%) of 111-oriented grains evidenced by thepeak at 20.23. Let us note that in the "lms grown onAl

2O

3(0 0 0 1), the 0 0 2 re#ection was never ob-

served. The rocking curves across the 0 0 1 and 0 0 2re#ections represented in dashed lines are charac-terized by a single broad component, due to thelarger mosaic spread of the MgO(0 0 1) substrate(FWHM"0.0173) compared to that of the sap-phire substrate. Therefore, we cannot separate thee!ects of mosaic spread and lateral coherence in thealloy "lm; the FWHM value across the main peak0 0 2 is equal to 0.63, i.e. about 4 times larger thanthat of the broad component for the "lms depositedon sapphire (*u

1 1 1in Table 1), suggesting an

in-plane coherence length about 200As . The LROparameter was deduced from the ratio of the integ-rated intensities I

463/I

&6/$, corrected for sample ab-

sorption, polarization and Lorentz factor andtaking account for the widths of their rocking

curves. Since the 0 0 l re#ections refer to planesparallel to the surface, the absorption factors arecalculated from Eq. (2) with /"0. The averageDebye}Waller displacement deduced from the ra-tio I

0 0 2/I

0 0 4is about 2As 2. The LRO parameter is

equal to 0.6, which is signi"cantly smaller than theone of the alloy "lm grown on Al

2O

3(0 0 0 1) at the

same temperature (S"0.95). Such lower orderingcompared to the 1 1 1-oriented "lms, has also beenobserved in a superlattice grown on MgO(0 0 1) andannealed at 8503C for 1 h with a LRO parameter of0.4. These smaller values of S could arise from thelateral coherence length about 4 times smaller inCrPt

3(0 0 1) "lms than in CrPt

3(1 1 1) "lms. With

regard to the spatial coherence length of the X-raybeam (about 0.1lm), the in#uence of antiphasedomains which tends to decrease the intensities ofthe superstructure re#ections should be more im-portant in the CrPt(0 0 1) "lms. However, a depend-ence of ordering kinetics with the surface symmetry,especially for the co-deposited "lms, cannot bediscarded.

5. Magnetic characterization

Magnetization hysteresis loops measured at 5 Kare shown in Fig. 9 for three co-depositedCrPt

3(1 1 1) "lms grown at 8003C, 8503C and

9503C and an annealed superlattice. For all sam-ples, the easy axis is along the "lm normal. Thecorrection for the large diamagnetic contribution ofthe substrates in the in-plane hysteresis loops canbe somewhat over-estimated when the magnetiz-ation is not completely saturated in the maximumapplied "eld of 4000kA/m, as seen for the almostperfectly ordered "lm grown at 8503C. As expected,the magnetization increases with LRO, the largestvalue found in the "lm grown at 8503C equal to300$30 kA/m at 5K (250 kA/m at 295K) is higherthan the values reported for the annealed sputteredCrPt

3"lms exhibiting the highest remanence

(M4"198 kA/m for M

3/M

4"0.97) [12] but lower

than the value for the bulk alloy (380 kA/m), cal-culated from the magnetic moments of Cr and Ptdetermined by polarized neutrons. For orderedbulk alloys, it was shown that the magnetization ismaximum for a Pt atomic fraction of 0.75 and


Fig. 9. Magnetization hysteresis loops measured at 5K for three co-deposited CrPt3(1 1 1) "lms grown at (a) 8003C, (b) 8503C and (c)

9503C, respectively, and (d) a superlattice [Cr(2As )/Pt(7As )]19.5

annealed up to 10503C (all "lms were grown on Al2O

3(0 0 0 1)). The

applied "eld was perpendicular (*) and parallel to the "lm plane (- - -). (Let us recall the conversion from SI to cgs units: for (volume)magnetizations 1 kA/m gives 1 emu/cm3, and for magnetic "elds 1 kA/m gives 4p Oe.).

decreases rapidly when the composition deviatesfrom this value [18]. Therefore, the smaller valuefound in the "lm could be explained by some com-positional deviation. Since the lattice parameter ofbulk FCC alloys for atomic fractions of Pt rangingfrom 0.7 to 0.8 is almost constant, the check of the"lm composition from the value of the spacing of(1 1 1) planes is within $0.05. For the co-deposited"lms, the e!ective magnetic anisotropy, K

%&&, is

shown in Fig. 10, together with the LRO parameterand the perpendicular remanence as a function ofthe growth temperature. The quality factors K

u/K

d,

deduced from K%&&

and M4, range from 4 to 6.7. It

appears clearly that perpendicular anisotropy andremanence are enhanced with LRO. Besides, all theordered "lms exhibit large coercivities rangingfrom 600 to 1100kA/m. The S and K

%&&values of

the annealed "lm equal to 0.65 and 0.17MJ/m3 arealso in agreement with this behaviour.

Fig. 11 shows the magnetization loops measuredat 295 K, for two di!erently oriented "lms preparedat 8503C by co-deposition on Al

2O

3(0 0 1) and

MgO(00 1) surfaces. The perpendicular anisotropy,still well pronounced in the CrPt

3(1 1 1) "lm (with

K%&&

"0.23MJ/m3 and remanence of 66%), hascompletely vanished in the CrPt

3(0 0 1) "lm. In

CrPt3(0 0 1), the easy axis lies in the "lm plane

due to the demagnetizing energy (0.5l0M2

4"

!0.015MJ/m3), but the two equivalent directions[0 0 1] and [1 0 0] are easy axes. These results con-"rm well the existence of a signi"cant magnetocrys-talline anisotropy in the L1

2ordered CrPt

3"lms

whose origin is discussed further. Magnetic forcemicroscopy experiments were performed usinga OMICRON UHV STM/AFM system, in non-contact mode, using single-crystal Si cantileverwith integrated tips covered by 40 nm thick Coalloy. The MFM images map the resonance


Fig. 11. Magnetization hysteresis loops measured at 295K for two co-deposited CrPt3(1 1 1) "lms grown at 8503C on (a) Pt(1 1 1) and (b)

Pt(0 0 1), respectively. The applied "eld was perpendicular (*) and parallel to the "lm plane (- - -). For the CrPt3(0 0 1) "lm the "eld

applied in the plane was parallel to the [1 0 0] direction.

Fig. 10. Change of the chemical LRO parameter, S, e!ectiveanisotropy energy, K

%&&, and perpendicular remanence, M

3/M

4as a function of the growth temperature for co-depositedCrPt

3(1 1 1) "lms grown on Al

2O

3(0 0 0 1).

Fig. 12. MFM image of a CrPt3"lm as-grown at 8503C on

Al2O

3(0 0 0 1): 2]2lm2.

frequency shifts of the cantilever. The magneticdomain pattern of the CrPt

3(1 1 1) "lm grown at

8503C without preliminary magnetic treatment isshown in Fig. 12 and consists in a network ofbubbles slightly elongated of width 70 nm. Forcomparison, we have measured the MFM patternof an 470 thick epitaxial CoPt

3(1 1 1) "lm grown at

4003C on Al2O

3(0 0 0 1). Such a "lm develops

a strong perpendicular anisotropy with remanenceclose to 100% and its MFM pattern consists ofsegmented stripes of width 110 nm, with highlycontrasting up and down perpendicularly magnet-ized domains. Based on the stripe domain modeldeveloped by Kooy and Enz [19], it was predicted

that when the "lm thickness, t, is larger than a char-acteristic length, D

0"p

8/2pM2

4(p

8is the Bloch

wall energy), the domain size increases with t asJtD

0for systems exhibiting high quality factors

[20]. From Ref. [21], the p8

value for CoPt3

isapproximately 0.004 J/m2, yielding a characteristiclength of about 200As . Since the two measuredCoPt

3and CrPt

3"lms have the same thickness,


the ratio of their domain sizes leads to a character-istic length of 80As for CrPt

3, indicating that as a

consequence of the smaller demagnetizing energy,the wall energy in the CrPt

3"lms is roughly an

order of magnitude smaller than for CoPt3. This

value should be con"rmed by complementaryMFM measurements in CrPt

3"lms with di!erent

thicknesses (all grown at 8503C).

6. Discussion and conclusions

In comparison with the ferromagneticCoPt

3(1 1 1), the "nding of strong perpendicular

anisotropy in well-ordered L12-type CrPt

3(1 1 1)

"lms was unexpected. In CoPt3(1 1 1) "lms it was

shown that the development of such isotropic LROorder destroys the perpendicular anisotropy [8].Therefore, the di!erent magnetic behaviours ofthe L1

2ordered CoPt

3and CrPt

3"lms should be

attributed to their di!erent magnetic ordering, fer-romagnetic and ferrimagnetic, respectively. X-raymagnetic dichroism measurements in a CrPt

3(1 1 1)

"lm grown at 8503C at the ¸2,3

egdes of Pt havecon"rmed that the magnetic moment of Pt atomsis oppositely directed to the moment of Cratoms, since the XMCD signal is positive atboth ¸

2and ¸

3edges as already found in a bulk

CrPt3

alloy [22]. Therefore, the magnetizationof the CrPt

3(1 1 1) "lms is driven by the Cr atoms

of large magnetic moment of 3.37lB

(comparedto!0.26l

Bfor Pt atoms). In the L1

2-type struc-

ture, the six nearest Cr neighbors around a Cr atomare arranged as second neighbors at a distance ofaFCC

, while the 12 "rst neighbors are Pt atoms. Ina stacking of (1 1 1) planes, these six Cr neighbors liealong the S0 0 1T directions, tilted of 54.743 fromthe [1 1 1] growth direction and are thus locatedout of the [1 1 1] plane containing the Cr centralatom, (i.e. 3 atoms in the upper (1 1 1) plane and3 atoms in the lower (1 1 1) plane). It is believed thatthe overlap of their electron distribution yielda perpendicular anisotropy as observed in theCrPt

3(1 1 1) "lms. In contrast, in a stacking of

(0 0 1) planes, these six Cr neighbors are locatedalong the S1 0 0T directions, (i.e. 2 atoms along thegrowth direction [0 0 1], and two atoms each alongthe two other directions [1 0 0] and [0 1 0] lying in

the CrPt3(0 0 1) "lm plane). Consequently, since for

the CrPt3(0 0 1) "lm we have taken care to apply

the magnetic "eld in the plane along the direction[0 0 1], the small di!erence between the two hyster-esis loops (Fig. 11) is due to the shape anisotropyonly and all S1 0 0T directions are the easy axes ofmagnetization. In the ferromagnetic L1

2ordered

CoPt3(1 1 1) "lms, the magnetization is driven by

both Co and Pt atoms. Since the arrangement ofthe 12 Pt "rst nearest neighbors around a Co atom(6 atoms in the plane and 3 atoms in each adjacentplane) is isotropic, these "lms exhibit no magneticanisotropy. The absence of perpendicular aniso-tropy was also found in the L1

2ordered ferromag-

netic MnPt3(1 1 1) "lms [23].

In conclusion, thin epitaxial CrPt3(1 1 1) "lms

with high degree of L12-type ordering were pre-

pared by molecular-beam epitaxy. As in bulk alloysthis chemical ordering leads to a ferrimagnetic or-dering. Magnetization and perpendicular magneticanisotropy increase with the degree of ordering; thehighest perpendicular anisotropy was found ina co-deposited "lm grown at 8503C with an e!ec-tive anisotropy energy of 0.25MJ/m3. The existenceof strong perpendicular anisotropy observed in theferrimagnetic oriented (1 1 1) "lms could be relatedto the nearest CrCr pairs aligned along the S0 0 1Tdirections tilted of 54.743 from the [1 1 1] growthdirection, which owing to their large magnetic mo-ment drive the magnetization. Due to their smallshape anisotropy energy, the quality factors ofthese "lms deduced from SQUID measurementsrange from 4 to 7. Moreover, the magneto-opticKerr rotation measured for the 470As thick "lmgrown at 8503C at the He}Ne laser wavelength(633 nm) is 0.153, which is quite similar to thatmeasured in a CoPt

3"lm grown at 4003C on

Al2O

3(0 0 0 1) substrate and covered also by a 20

thick Ru layer. Finally, the CrPt3"lms could also

be attractive materials for magneto-optic record-ing.

Acknowledgements

We thank E. Beaurepaire of IPCM Strasbourgfor the MOKE experiments and Henry Fischer forproviding us the code for the simulation of the


low-angle re#ectivity curves. In Konstanz, this pro-ject was funded by the DFG, Sonderforschun-gbereich 513, Nanostrukturen, which is gratefullyacknowledged.

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magnetic anisotropy and chemical long-range order in epitaxial ferrimagnetic crpt3 films

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