Table Of ContentEpitaxial growth and magnetic properties of Sr CrReO thin films
2 6
S. Gepr¨ags,1,∗ F.D. Czeschka,1 M. Opel,1 S.T.B. Goennenwein,1 W. Yu,2 W. Mader,2 and R. Gross1,3,†
1Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, D-85748 Garching, Germany
2Institut fu¨r Anorganische Chemie, Universita¨t Bonn, D-53117 Bonn, Germany
3Physik-Department, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany
(Dated: January 28, 2009)
The double perovskite Sr CrReO is an interesting material for spintronics, showing ferrimag-
2 6
netism up to 635K with a predicted high spin polarization of ≈ 86%. We fabricated Sr CrReO
2 6
epitaxial films by pulsed laser deposition on (001)-oriented SrTiO substrates. Phase-pure films
3
with optimum crystallographic and magnetic properties were obtained by growing at a substrate
9 temperature of 700◦C in pure O2 of 6.6×10−4mbar. The films are c-axis oriented, coherently
0 strained,andshowlessthan20%anti-sitedefects. Themagnetizationcurvesrevealhighsaturation
0 magnetization of 0.8µB per formula unit and high coercivity of 1.1T, as well as a strong magnetic
2 anisotropy.
n
PACSnumbers: 75.70.-i81.15.Fg,85.75.-d
a
J
8 INTRODUCTION filmswithexcellentcrystallinequalityaswellasmagnetic
2
properties comparable to bulk SCRO.
] The ferrimagnetic double perovskites A BB(cid:48)O , with
i 2 6
c BamagneticandB(cid:48)anon-magnetictransitionmetalion
s
[1, 2, 3], are attractive magnetic materials. Of particu- GROWTH AND CRYSTALLOGRAPHIC
-
l lar interest is Sr CrReO (SCRO) [4, 5], as it exhibits a CHARACTERIZATION
r 2 6
t ferrimagnetic transition temperature of T ≈635K well
m C
above room temperature, in combination with a quasi We have used pulsed laser deposition (PLD) [21] to
.
t half-metallic band structure according to theoretical cal- growtheSCROfilmsfromastoichiometrictarget,which
a
m culations [6]. This high TC is strongly correlated to a was fabricated using an oxygen getter control technique
largeinducedmagneticspinmomentatthenon-magnetic to ensure stoichiometry [22]. To identify the optimal
-
d Re site [7, 8]. TC can be further increased by electron growth conditions, we have deposited films (i) in three
n doping, as demonstrated e.g. in Sr2FeMoO6, Sr2CrWO6, different atmospheres: O2, Ar, and a mixture of Ar/O2
o or Sr2CrOsO6 by 5d band filling [9, 10, 11]. The strong, (99/1 by volume), (ii) at different pressures, and (iii) at
c
[ long-range magnetic interaction is attributed to a ki- different substrate temperatures 450◦C ≤ TG ≤ 900◦C.
netic energy driven exchange process, in which the hy- The structural and magnetic properties of the SCRO
1 bridization between itinerant electrons on the B(cid:48) sites thin films were analyzed using in-situ reflection high en-
v
and the localized moments on the B sites leads to an ergyelectrondiffraction(RHEED),highresolutionx-ray
9
9 induced magnetic moment of the non-magnetic B(cid:48) ions diffraction (HRXRD), SQUID magnetometry, and high
4 [12, 13]. Regarding device applications, the strong in- resolution transmission electron microscopy (HRTEM).
4 fluence of the growth conditions on the structural, mag- The film thickness was determined by x-ray reflectome-
1. netic, and electronic properties of double perovskites is try.
0 demanding. In particular, it became evident that a key Comparing samples grown in O , Ar, and Ar/O , we
2 2
9 parameter to understand the properties of double per- find that high-quality, phase-pure SCRO films are ob-
0 ovskites is the amount of anti-site defects (B ions on B(cid:48) tained only in a growth atmosphere containing oxygen.
:
v sites and vice versa). It was shown that the saturation In particular, the presence of oxygen during the growth
Xi magnetization MS decreases with increasing amount of seems to be essential to achieve Cr/Re sub-lattice order-
anti-site defects [14]. Therefore, a thorough understand- ing [23]. In the following, we therefore focus on films
r
a ingofthegrowthprocessisakeyprerequisiteforobtain- grown in pure O atmosphere of p =6.6×10−4mbar.
ing thin films with optimum physical properties. While Higher pressure2was found to leadO2to parasitic phases,
the growth of Sr2FeMoO6 and Sr2CrWO6 thin films has lower pressure to a reduced saturation magnetization.
been investigated in detail[15, 16, 17, 18], little is known Another important deposition parameter is the laser flu-
about Sr2CrReO6 so far [5, 19, 20]. ence PL on the target, determining the energy of the
Here, we report on the epitaxial growth of c-axis ori- ablated particles. We found no significant change of the
ented SCRO thin films on (001) SrTiO (STO) sub- magnetic and crystalline quality upon varying P from
3 L
strates, and discuss the effects of growth temperature, 1.0 to 2.5J/cm2. Moreover, the laser pulse repetition
growth atmosphere, and pressure on the film quality. rate f also had no noticeable effect on the anti-site
L
WeidentifyoptimalgrowthparametersleadingtoSCRO disorder in our SCRO films, in contrast to what was
2
m(cid:1)(cid:2)(cid:5)(cid:11)(cid:4)(cid:13)(cid:4)(cid:3)(cid:1)(cid:12)(cid:6)000 ...468 (cid:18)(cid:14)(cid:8)(cid:11)(cid:1)(cid:10)(cid:5)(cid:10)(cid:1)(cid:20)(cid:1)(cid:7)(cid:6)(cid:4)(cid:9)(cid:1)(cid:17)(cid:16)(cid:15)(cid:19) 000 ...001 680 (cid:7)(cid:10)(cid:8)(cid:9)(cid:1)(cid:2)(cid:14)(cid:3) Intensity (cps)111111 000000 123456 (cid:2)(cid:22)(cid:3)SCRO 002STO 001 Intensity 4 5(cid:15)(cid:17)SCRO006(cid:19)(cid:5)(cid:12)STO(cid:5)(cid:18)4(cid:9)0036(cid:17) (cid:19)(cid:5)(cid:20)(cid:5)4SCRO(cid:7)(cid:17)7008STO 004
M
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0
0 .2 0 .0 4 (cid:1) (cid:7)(cid:1)(cid:1)(cid:2)(cid:31)(cid:3)
4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0
T (cid:1)(cid:2)(cid:21)(cid:12)(cid:3) s)8 0 0 (cid:2)(cid:23)(cid:3) (cid:19)(cid:12)(cid:18)(cid:17)(cid:1)(cid:5)(cid:5)(cid:7) 1 0 4 (cid:2)(cid:24)(cid:3) (cid:19)(cid:20)(cid:17)(cid:7)(cid:7)(cid:7)
(cid:13) (cp6 0 0 1 0 3 (cid:19)(cid:12)(cid:18)(cid:17)
Fµof0IGHth.e=1:S7CSTRatO(ufur(lal0t0cioi4rn)clmreesafl)geancstewitoienzlal(taoiospnFenWMsHSqMumaeroaefsstu)hroeefdrosaicntkgi2nl5egKpchuaarnvsdee Intensity24 00 000 (cid:13)(cid:30)(cid:1)(cid:21)(cid:5)(cid:4)(cid:14)(cid:5)(cid:10)(cid:16)(cid:31)(cid:1) 111 000 012 (cid:19)(cid:1)(cid:1)(cid:6)(cid:12)(cid:5)(cid:18)(cid:6)(cid:17) (cid:1)(cid:1)(cid:9)(cid:5)(cid:9)
SCRO thin films grown at pO2 = 6.6×10−4mbar plotted 1 1 .1 1 1(cid:2).2(cid:1)(cid:2)(cid:31)1(cid:3)1 .3 1 1 .4 1 9 2(cid:7)0(cid:1)(cid:1)(cid:2)8(cid:31)5(cid:3) 8 6
versus the substrate temperature TG. 0 .3 9 6 (cid:2)(cid:25)(cid:3) (cid:2)(cid:26)(cid:3)
(cid:19)(cid:20)(cid:17) 0 .5 9 2
found for Sr2FeMoO6 films [24]. We therefore used a q (rlu)^00 ..33 89 82 (cid:19)(cid:12)(cid:5)(cid:5)(cid:18)(cid:7)(cid:17) 00 .. 55 88 48 (cid:19)(cid:6)(cid:5)(cid:20)(cid:8)(cid:17)
PL = 2.0J/cm2 and fL = 2Hz in all subsequent growth 0 .3 8 4 (cid:5)(cid:5)(cid:9) 0 .5 8 0 (cid:1)(cid:19)(cid:1)(cid:1)(cid:6)(cid:12)(cid:6)(cid:18)(cid:11)(cid:17)(cid:1)
runs. -0 .0 0 3 0 .0 0 0 0 .0 0 3 0 .1 9 5 0 .1 9 8 0 .2 0 1
q (cid:1)(cid:2)(cid:28)(cid:27)(cid:29)(cid:3) q (cid:1)(cid:2)(cid:28)(cid:27)(cid:29)(cid:3)
The influence of the substrate temperature T on the || ||
G
structuralandmagneticpropertiesofSCROthinfilmsis
FIG. 2: HRXRD of SCRO thin films grown at 700◦C in
summarized in Fig. 1. It is evident that the full width
p =6.6×10−4mbar. Panels(a)-(c)correspondtoaSCRO
at half maximum (FWHM) of the rocking curve of the O2
film with a thickness of d = 31nm, panels (d) and (e) to a
SCRO (004) reflection stays almost constant on a low film with d=65nm. (a) ω-2θ scan showing Laue oscillations
value of about 0.05◦ for 550◦C ≤ T ≤ 750◦C. Only at (LO), (b) rocking curve of the (002) reflection, (c) ω-2θ scan
G
the lowest and highest T , the FWHM increases up to of the superstructure (101) reflection, (d,e) reciprocal space
G
0.08◦. ThisindicatesthatSCROthinfilmscanbegrown maps around the (004) and (116) reflections.
with low mosaic spread in a large temperature window
from550◦Cto750◦C.Wenotethatthemosaicspreadof
theSCROthinfilmsisclosetothatoftheSTOsubstrate and (404) reflections. Comparing the measured inten-
(typically0.01◦−0.02◦). Incontrast,thesaturationmag- sitytosimulations,performedusingthesoftwarepackage
netization M peaks around T = 700◦C. As a strong TOPAS from Bruker company, the amount of anti-site
S G
correlationbetweenM andthesub-latticeorderisoften defects is estimated to less than 20%. More detailed in-
S
observed in ferrimagnetic double perovskites [14], Fig. 1 formation about the mosaicity and the strain state can
suggests that SCRO films grown at T ≈ 700◦C show a be obtained from the reciprocal space maps around the
G
highdegreeofsub-latticeorderandthusasmallfraction (004) and (116) reflections shown in Figs. 2(d) and (e).
of anti-site defects. That is, SCRO thin films with opti- Clearlynoq -shiftofthesymmetric(004)andasymmet-
(cid:107)
mal structural and magnetic properties are obtained for ric (116) SCRO film reflections with respect to the cor-
T =700◦CinapureO atmosphereof6.6×10−4mbar. responding STO substrate reflections is observed. This
G 2
Fig.2showsthestructuralpropertiesachievedforepi- demonstratespseudomorphicgrowthuptothemaximum
taxial SCRO films which were deposited at the optimum film thickness of 65nm. The reciprocal space maps and
growth parameters described above. The ω-2θ scan re- the in-plane cube-on-cube growth derived from an in-
veals no crystalline parasitic phases. As shown in the plane φ-scan (not shown) reveal a tetragonal symme-
inset, the observation of Laue oscillations provides evi- try of the√SCRO thin films, with lattice parameters of
denceforacoherentfilmgrowth. Furthermore,thesmall aSCRO = 2aSTO = 5.523˚A and cSCRO = 7.90˚A. Com-
FWHM of the rocking curve of the SCRO (002) reflec- paringthemeasuredin-planelatticeconstanttothevalue
tion of only 0.05◦ demonstrates high crystalline qual- abulk =5.5206˚A of bulk SCRO [2] shows that the tensile
ity with low mosaic spread. The Cr/Re sub-lattice or- epitaxial coherency strain of the SCRO films grown on
der of our SCRO films can be inferred from the in- STO substrates is negligibly small and the films can be
tensity of the superstructure (101) reflection shown in regarded as strain-free.
Fig. 2(c). This superstructure peak has finite intensity ThequalityoftheinterfacebetweentheSTOsubstrate
only for a finite amount of ordering on the Cr/Re sub- and the SCRO film was investigated by HRTEM. The
lattice. The amount of anti-site defects can be deter- bright-fieldHRTEMmicrographalongthe[110]-direction
mined from the measured intensity ratio of the (101) oftheSTOsubstrateshowninFig.3(a)demonstratesthe
3
M = 0.89µ /f.u. of our polycrystalline target mate-
S B
rial and the literature values ranging between 0.8 and
0.9µ /f.u. at low temperature [4, 19]. Recent model
B
calculations including spin-orbit coupling [6] predicted
M = 1.28µ /f.u. for perfect sub-lattice order. As-
S B
suming that M decreases about linearly with increas-
S
ing sub-lattice disorder, the measured amount of anti-
site defects of less than 20% translates into an expected
saturation magnetization larger than 0.77µ /f.u., again
B
in good agreement with the measured value. The mea-
FIG. 3: (a) HRTEM micrograph of a 65nm thick, c-axis
sured M(H) curves also reveal a very large coercive field
oriented SCRO film grown on (001) STO. The arrow marks
µ H = 1.1T. This value again agrees well with litera-
the interface. (b) Electron diffraction pattern of SCRO in 0 c
[100] orientation with odd-indexed super-lattice reflections. turedatarangingfrom1to1.5Tat4.2K[4,19],demon-
stratingthattheSCROcompoundisahardferromagnet.
However, in contrast to a previous study [19] we observe
perfectepitaxialgrowthofSCROonSTO.Neitherdislo- a strong magnetic anisotropy in our SCRO films. This is
cations nor structural defects could be detected. To ob- evident from the M(H) hysteresis loop with H oriented
tain information on the Cr/Re sub-lattice ordering, elec- perpendicular to the film plane. For this field orienta-
tron diffraction was performed, as shown in Fig. 3(b). tion, the magnetization loop is rectangular-shaped, with
After foil thickness determination from the intensity ra- a much lower coercive field µ0Hc = 62mT. Further-
tioI(020)/I(000),theanti-sitedefectsoftheCr/Resub- more, the field at which the hysteresis loop closes is also
lattice can be estimated to be less than 17%, e.g. from reducedtoabout0.7T,ascomparedtomorethan3Tfor
the intensity ratio I(011)/I(020). Note that the (011) themagneticfieldappliedinthefilmplane. Wenotethat
peak is a superstructure peak with vanishing intensity the apparent dependence of MS on the field orientation
for complete disorder on the Cr/Re sub-lattice. This can be attributed to the large saturation field of SCRO,
provides additional evidence for a highly ordered Cr/Re making the unambiguous determination of the diamag-
sub-lattice in our SCRO films. neticcontributionofthesubstratedifficult. Additionally,
the large spin-orbit coupling in SCRO is expected to re-
sult in strong magnetic anisotropy.
MAGNETIC PROPERTIES
CONCLUSIONS
We finally address the magnetic properties of the
SCRO films. Fig. 4 exemplarily shows the M(H)
In conclusion, we have identified the optimal param-
curves of a 31nm thick SCRO film recorded at 25K.
eters for the epitaxial growth of c-axis oriented SCRO
For the external magnetic field applied in-plane along
filmson(001)STObypulsedlaserdeposition. Thefilms
[100], the measured saturation magnetization M =
S have excellent crystalline quality with a high degree of
0.8µ /formula unit (f.u.) is in good agreement with
B Cr/Re sub-lattice order. The FWHM of the rocking
curves is as small as 0.05◦, M reaches 0.8µ /f.u., and
S B
the coercivity is 1.1T. These parameters make SCRO
1 .0
thinfilmsanoutstandingferrimagneticdoubleperovskite
H || [1 0 0 ] for spintronic applications.
0 .5
) Financial support by the DFG via the priority pro-
.
.u grams 1157 and 1285 (project nos. GR 1132/13 and
f
/B 0 .0 1132/14), GO 944/3-1, and the Excellence Cluster
m
( H || [0 0 1 ] ”Nanosystems Initiative Munich (NIM)” are gratefully
M - 0 .5 T = 2 5 K acknowledged.
- 1 .0
- 6 - 4 - 2 0 2 4 6
(cid:1) H ( T )
0 ∗ Electronic address: [email protected]
† Electronic address: [email protected]
FIG.4: Magnetizationversusappliedmagneticfieldcurvesof
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