Table Of ContentDegradation of LaMnO3−y surface layer in LaMnO3−y/ metal interface
A.Plecenik and K. Fr¨ohlich
Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 84239 Bratislava, Slovak Republic
J.P. Espin´os and J.P.Holgado
2 Instituto de Ciencia de Materiales de Sevilla, Centro de Investigaciones Cientficas ”Isla de la Cartuja”, C/ Americo
0 Vespucio, s/n, 41092 Isla de la Cartuja, Sevilla, Spain
0
2 A. Halabica and M. Pripko
n Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 84239 Bratislava, Slovak Republic
a
J A. Gilabert
5 University of Nice Sophia Antipolis, UMR CNRS 6622, Parc Valrose, 06108 Nice Cedex 2, France
2
]
i surfaceboundaryaresignificantlydifferentfromthoseof
c
the bulk. In addition, several experiments indicated the
s
Wereportelectricalmeasurementsshowingthedegradation
- creation of an oxygen depleted layer of LaAMnO inter-
rl processes of LaMnO3−y (LaMnO) in LaMnO/normal metal faces. Mievilleetal.2 studiedtheinterfaceresistanceand
t interfaceinbothpointcontactandplanar-typejunctions. Im-
m transport across conducting ferromagnetic oxide/metal
mediatelyafterthepreparationoftheinterface,thedegrada-
interfaces. Theymeasureda veryhighresistanceofLaS-
. tion process was followed by measuring the evolution of the
t rMnO/Al(Nb) junctions, which could be explained by
a junction resistanceversustime. Thisprocessischaracterized
m bytheappearanceofasecondmaximumintheresistancevs. the existence of a degraded surface layer of ferromag-
neticoxideduetoalossofoxygen. AlsodeTeresaetal.3
- temperature (R-T) dependence at temperatures lower than
d theCurietemperatureTc,atwhichthemetal-insulatortran- observeda dependence of TMR onthe type of tunneling
n sitionoccursinthebulk. Theseeffectsareexplainedinterms barrier. The present work was stimulated by the above
o oftheformationofadepletedinterfacelayerinLaMnOcaused mentionedresults,butmainlybytheresultspublishedin
c by an out-diffusion of oxygen from the manganite surface to our previous paper4. LaMnO/Al2O3/Nb junctions were
[
thenormalmetal. ThisassumptionisconfirmedbyXPSmea- fabricated for the study of the influence of illumination
1 surement. Similar resultsonLaSrMnO3−y interfaces arealso on the electrical properties of MTJ. The Al2O3 insulat-
v obtained. ing barrier was prepared using well-knownthin films Nb
0 pacs: 73.40.Cg, 68.35.Fx,79.60.Jv technology5: An aluminium thin film was sputtered on
6
The magnetoresistive rare earth per- LaMnO,andthenoxidizedinanoxygenatmosphere. Fi-
4
1 ovskites La1−xAxMnO3−y (LaAMnO, A=Ca, Sr, Pb), nally a semi-transparent Nb upper electrode was sput-
0 which include LaMnO, have been in the research spot- tered on the top. Using the same technique with the
2 lightfor a decade. Optimally doped LaAMnO exhibits a same preparation parameters, the width of the barrier
0 paramagnetic-insulatingbehaviorabovetheCurietem- createdon the LaMnO was about two times higher than
t/ perature Tc and a ferromagnetic - metallic behavior be- that for Nb/Al2O3/Nb junctions.
a low Tc. In the recent years,the giant magnetoresistance In this paper we study the LaMnO surface properties
m
phenomenon (MR) has been intensively studied in these in contact with Al, In, Au and Pb metals in order to
- materials. However till low-field, MR effects in granular explainthe physicalprocessesinthe interfacewhichgive
d
films and tunneling of spin-polarized electrons through risetoanunusualbehaviorofMTJ.TheLaMnO/normal
n
an insulating barrier in LaAMnO/Insulator/LaAMnO metal point contact exhibits a change of resistance im-
o
c magnetic tunnel junctions (MTJ) gave promise to their mediately after its preparation. In the planar junctions
: technological applications like magnetic field sensor and (morestablethanpointcontact)theappearanceofasec-
v
nonvolatile magnetic random access memory. One can ond maximum in the R-T dependence was observedsev-
i
X expect a near 100% value of tunneling magnetoresis- eral hours after junction preparation. These effects are
r tance (TMR) in tunnel junctions based on half metal- explained by an out-diffusion of oxygen from LaMnO,
a
lic LaAMnO ferromagnet. TMR depends on the quality and they were confirmed by XPS spectroscopy.
of the magnetic state of the LaAMnO surface, interface 200nmthickLaMnOepitaxialthinfilmswithT =270
c
quality,surfaceroughnessofthe electrodes,etc. Anoma- K weredepositedonsingle crystallineSrTiO3 substrates
lous tunneling behavior observed in MTJ could be ex- by low pressure liquid source metal-organic chemical
plainedbyextrinsicfactorsplayingadominantroleinthe vapour deposition (MOCVD)6. LaMnO/metal point
MTJ interfaces. This assumptionhas been made by sev- contacts were realized using a holder which exerted a
eralauthors. Parket al.1 showedthat evenin a fully ox- constant pressure of the tip onto the sample. Bulk Au,
idized LaSrMnO sample, the magnetic properties at the Al, or Pb were used for the upper electrode. The shape
1
of the sharp point was prepared by mechanical sharpen- In the next measurements the planar junctions were
ing. After the electrical measurements, a 200x200 µm2 used because they were more stable than the point con-
geometric area was evaluated for the Pb tip. tactones. BecauseAlisaveryreactivemetalwithavery
Forthepreparationofplanarjunctions,a40µm-wide short time of interaction, indium as an upper electrode
and 260 µm - long LaMnO3 base electrode was formed was used for the following measurement. The R-T char-
by wet etching through resist mask. After stripping the acteristic measured on a LaMnO/In planar junction at
photoresist,the areafor deposition ofnext layersby lift- different times ( 3, 5 and 23 hours) after preparation of
offtechniquewasdefinedinpositivephotoresist. Shortly the contactsis showninFig.2. We observedthe classical
before the deposition of the barrier and the upper elec- maximum correlated with the I-M transition of the bulk
trodes, the surface of LaMnO3 was etched to eliminate material at Tc around 275 K of the bulk material. This
a contaminated and degraded upper layer. A 100 nm maximumdidnotchangeverymuchwithtime. Itmeans
thick Al or In layer was deposited by thermal evapora- that the oxygen content in the bulk material does not
tion. Junctionswithanareaof40×40µm2 werefinalized change. After a longtime (23 hours)a secondmaximum
after the removal of the photoresist. appears around T’ = 185 K in the R-T characteristics.
c
The R-T and resistance vs. time (R-t) characteris- WeknowthatinLaAMnOsystemstheTcdecreaseswith
tics were measured by a computer controlled four point a decrease of the oxygen content, and it disappears for
method . deoxygenatedsamplesthatexhibitsemiconductingprop-
XPSspectrawereobtainedinanESCALAB210spec- erties. This new maximum is related to the oxygen de-
trometer that consisted of two separate independently- pletedinterface,anditisbroaderduetoadistributionof
pumped chambers. Pressures in the range of 6x10−11 T’ . ThisvalueofT’ =185Kcorrespondstoanaverage
c c
mbar and 10−8 mbar were obtained in the analysis and oxygen content at the interface of y around 0.1.8
preparation chambers, respectively. Al has been evapo- To confirm the occurrence of out-diffusion processes
ratedintheanalysischamberfromaresistivelyheatedfil- in the LaMnO surface layer, XPS measurements of
amentmadeofAlwirewrappedaroundathicktungsten LaMnO/Alinterfaceswerestudied. Fig.3shownsspectra
wire. The power was maintained to give a reproducible of the O1s, Mn2p, Al2p and La3d regions measured on
evaporation rate of 1 monolayer of Al per minute. Con- clean (original)LaMnO surface (curves a) at the normal
trol evaporation carried out onto a clean Au foil showed acquisitionangle,afterthedepositionofaround1mono-
thatunder ourexperimentalconditions,onlymetallic Al layer of Al (curves b), at the normal adquisition angle,
is evaporated, maintaining this oxidation state through- around2monolayersofAl,atthenormaladquisitionan-
outalltheacquisitiontime. Ahemisphericalelectronen- gle (curves c), and curves d were measured as curves c
ergy analyser working in the pass energy constant mode atagrazingadquisitionangle. Onecanseethe following
atavalueof50eVwasused. UnmonochromatizedAlKa features in the XPS spectra after the deposition of Al:
radiationwasusedastheexcitationsource. Spectrawere - two peaks correlated with Al and Al2O3,
energy-calibrated by taking the La3d5/2 peak at 834.6 - two peaks in the O1s spectra correlated with oxygen
eV (BE). The spectra were acquired at 90 (normal) and bonding in LaMnO3 at529.9eV andAl2O3 at 531.6eV,
20 (grazing) degrees with respect to sample surface. - shift of the peaks in the Mn2p spectra due to the de-
In Fig 1, the LaMnO/metal point contact resistance creasing of Mn valency.
dependence versus time shows a significant increase at The deposition of the first monolayer of Al on the
severalhundredsecondsafterthepreparationofthecon- LaMnO surface provokes the oxidation of Al to Al3+,
tact. These changes were observed even if noble metal whose peak arises at about 75 eV (BE). This behaviour
(Au) was used as the upper electrode material. Similar was tested by a simultaneous deposition of Al on Au.
resultsonhigh-T superconductors(HTS)weredescribed ThedetectionofmetallicAlduringthewholeexperiment
c
in our previous work7. The change of the point contact (morethan1hour)excludetheoxidationofAlduetothe
resistance was explained within an oxygen out-diffusion residual O2 partial preasure in the analytic chamber. In
model. additional, when a second amount of Al was deposited
The total point contact resistance with a tip made of (curve c in Fig.3a), most of it remained metallic (peak
nonreactive metal (e.g. Au) can be expressed as R = at 72.6 eV BE). Finally, when this situation is examined
R +R +R , where R is the tunneling resis- at the grazing adquisition angle (curve d in Fig.3a), the
M LaMnO T T
tance andR andR the contactresistanceswith relative intensity of the peak from Al(0), in comparison
M LaMnO
metal and LaMnO, respectively. We suppose that R with that from Al+3, increases substantially, indicating
M
and RT are constant in time for stable nonreactive met- that metallic Al is located on the top of the Al2O3.
als, and the change of resistance R is then given by the The oxidation of Al in the LaMnO3/Al interface ac-
change of R . The creation of additional barriers companyingtheappearenceoftwopeaksintheO1sspec-
LaMnO
from a LaMnO oxygen depleted layer as well as from tra is correlated with a change of binding energy due to
the oxide of the upper metallic electrode (created from thecreationofAl2O3. Thesmoothsecondpeakwasmea-
reactive metal like Al, Pb) creates a complex interface. suredalsooncleanLaMnOsurface(curveainFig.3c)as
Butinallcases,theCurietemperatureT intheLaMnO wellasaftercleaningtreatment. Theoriginofthesecond
c
oxygen depleted region must be changed also. peak can be explained by the contamination of LaMnO
2
by CO= species within the preparation procedure of the 7M.Grajcar, A.Plecenik, Sˇ.Benˇaˇcka, Ju.Revenko,
3
LaMnO thin film. Anyway, the intensity of the peak at V.M.Svistunov,Physica C 218, 82 (1993).
the energy of 520.6 eV increase after the deposition of 8A.M.De Leon-Guevara; P.Berthet; J.Berthon;
Al (curves b and c in Fig.3c) due to the arising of O= F.Millot; R.Revcolevschi; A.Anane; C.Dupas; K.Le Dang;
ions in Al2O3. The measurement at the grazing angle J.P.Renard; P.Veillet Phys.Rev.B 56, 6031(1997)
shows that Al2O3 is created on the LaMnO surface (the
globalintensity of Al2p signalgrowsand the Mn and La
signals decrease). These facts aslo indicate the existence
1.2
of metallic Al on the top of Al2O3.
eVAbseifgonreifiAcalnetvcahpeomraitciaolnshtoift64o1f.M4 enV2papfteearkAflroemvap64o2ra.5- u.] LaMnO 3-x/Pb
tsitoanteswtriothngalysuibnsdeiqcauteenstachdaencgreeaosfethoef MMnn3+va/lMennc4y+troat+io3, [a.0 LaMnO 3/Al
R 1.1
typical for oxygen-deficient LaAMnO. R/
From XPS measurements described above, we can
confirm the loss of oxygen from the LaMnO in LaMnO3-x/Au
the LaMnO/Al interface and the creation of a
LaMnO/Al2O3/Al junction with a subsequentchange in
the Mn3+/Mn4+ ratio. 1.0
0 500 1000 1500 2000
Inconclusion,wepresentedR-TandR-tmeasurements
on LaMnO/metal point contact and planar junctions. Time [sec.]
Thetimeevolutionofpointcontactjunctionresistanceas
FIG.1. Time evolution of the LaMnO/metal (Au - open
wellanarisingofasecondpeakontheR-Tcharacteristics
circles,Al-solid squaresandPb-solidcircles) pointcontact
were observed. These effects were explained in terms of
resistance. The measurement was started immediately after
the formationofa depleted interfacelayerinLaMnO3−x thepoint contact preparation.
causedbytheout-diffusionofoxygenfromthemanganite
surface to the normal metal. This assumption was con-
firmedbyXPSmeasurements. Thesimilarresults(XPS
as well as time evolution of point contact resistance) on
23 hours after junction preparation
LaSrMnOmagnetoresistivethinfilmswerealsoobtained. 160 5 hours after junction preparation
3 hours after junction preparation
base LaMnO electrode
ACKNOWLEDGEMENT m] 120 LaMnO /In junc3tion
h 3
O
This work was supportedby the Slovak Grant Agency k 80
R[
for Science (Grants No.2/7199/20 and 1/7072/20) and
in part by the EuropeanCommission (project GRT-CT-
40
2000-05001MULTIMETOX)
0
100 150 200 250 300
Temperature [K]
FIG. 2. R-T dependence of the LaMnO electrode (solid
line) and RT-T dependence of LaMnO/In planar junction
1J.-H.Park, E.Vescovo. H.-J.Kim, C.Kwon, R.Ramesh and measured 3 hours (solid sguares), 5 hours (open circles) and
T.Venkatesan 81, Phys.Rev.Lett.81, 1953 (1998). 23 hours (solid circles) after the deposition of the In upper
2L.Mieville, D.Worledge, T.H.Geballe, R.Contreras and electrode.
K.Char, Appl.Phys.Lett. 73, 1736 (1998).
3J.M. de Teresa, A.Barth´el´emy, J.P.Contour and A.Fert,
J.Magn.Magn.Mater. 211, 160 (2000).
4A.Gilabert, A.Plecenik, K.Fr¨ohlich, Sˇ.Gaˇzi, M.Pripko,
Zˇ.Mozolov´a, D.Machajd´ık, Sˇ.Benˇaˇcka and M.G.Medici,
Appl.Phys.Lett.78, 1712 (2001).
5M.Gurwitch, M.A.Washington and H.A.Huggins,
Appl.Phys.Lett.42, 472 (1983).
6K.Fr¨ohlich,J.Sˇouc,D.Machajd´ık, A.P.Kobzev,F.Weiss,
J. P. Senateur, K.H. Dahmen, Journ. de Physique IV C5,
533 (1995).
3
Al2p La3d
$O(cid:19)
x 10
$O(cid:22)(cid:14) d)
)
a.u. c) x 2
(
y d)
sit
n b)
e c) x 2
nt
I
b)
a)
a)
85 80 75 70 65 845 840 835 830
2LQ (cid:3)$(cid:3)O2 O1s Mn2p 0Q(cid:22)(cid:14)
(cid:21) (cid:22) 2 (cid:3)
LQ(cid:3)/D(cid:3)0Q2 d)
(cid:22) x 10
u.) d)
a. x 5
sity ( c) c) x 2
n x 2
e b)
nt
I b) x 2
a)
&2 a) 0Q(cid:23)(cid:14)
(cid:22)
540 535 530 525 520 660 650 640
Binding Energy (eV)
FIG.3.
XPS spectra of the O1s, Mn2p, Al2p and La3d regions mea-
sured on the clean surface of LaMnO (curves a) after the
depositionofonemonolayerandtwomonolayersofAl,atthe
normal adquisition angle (curves b and c, respectively) and
thesame that curvesc, at the grazing adquisition angle.
4
