Neutron Scattering Studies on Magnetic Structure of the Double-Layered Manganite La_{2-2x}Sr_{1+2x}Mn_2O_7 ($0.30 \leq x \leq 0.50$)

3 Pages·0.1 MB·English

by  M. Kubota

Neutron Scattering Studies on Magnetic Structure of the Double-Layered
Manganite La Sr Mn O (0.30 ≤ x ≤ 0.50)
2−2x 1+2x 2 7
M. Kubota,† H. Fujioka,‡ K. Ohoyama3 K. Hirota,‡ Y. Moritomo,4 H. Yoshizawa,† and Y. Endoh‡
†Neutron Scattering Laboratory, I.S.S.P., University of Tokyo, Tokai, Ibaraki, 319-1106, Japan
‡CREST, Department of Physics, Tohoku University, Aoba-ku, Sendai, 980-8578, Japan
3Institute for Materials Research Tohoku University, Aoba-ku, Sendai 980-8577, Japan
9 4Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya, 464-01, Japan
9
9 Systematicpowderdiffractionstudieshavebeencarriedouttoestablishthemagneticphasediagram
1 of La2−2xSr1+2xMn2O7 (LSMO327) in awidehallconcentration range(0.30≤x≤0.50), usingthe
n HERMES diffractometer. LSMO327 exhibits a planar ferromagnetic structure for 0.32 ≤x≤0.38
a at low temperatures. A finite canting angle between planar magnetic moments on neighboring
J planesstartsappearingaroundx∼0.40andreaches180◦ (A-typeantiferromagnet)atx=0.48. At
1 x=0.30, on the otherhand, themagnetic moments are aligned parallel to thec-axis.
2

Keywords: A: magnetic materials, B: crystal growth, C: neutron scattering, D: magnetic structure
]
l
e
- PerovskiteMnoxideR1−xAxMnO3(R:Rare-earthion, The prescribed amount of dried La2O3, SrCO3, and
r
t A:Alkaline-earthelement)providesanidealstagetosys- Mn3O4 are thoroughly mixed and calcined in the air at
s
. tematically study complex physics resulting from spin- 1200–1450◦C for 4 days with frequent grinding. Sample
t
a charge-lattice degrees of freedom. [1,2] Recently, Morit- rodsweremelt-growninafloatingzoneopticalimagefur-
m omo et al. [3,4] have discovered that the layered Per- nace, then powderizedagain. All the samples were char-
- ovskite Mn oxide La2−2xSr1+2xMn2O7 (LSMO327) with acterizedby x-raydiffractions,whichshownodetectable
d
x=0.40showscolossalmagnetoresistance(CMR),which impurities.
n
is much more enhanced than that of similarly doped We have taken powder diffraction patterns at room
o
c La1−xSrxMnO3. LSMO327 (I/4mmm, a=3.871 ˚A and temperature, intermediate temperature (100− 120 K),
[ c = 20.126 ˚A. [5,6]) has MnO2 double layers separated and low temperature (∼ 10 K) using HERMES, [11]

1 by (La1−xSrx)2O2, stacking along the c-axis. which is a powder neutron diffractometer with multi-
v The magnetic structure of LSMO327 has been estab- detectors with the Ge (3 3 1) monochromater (λ =
1 lished between x = 0.40 and 0.48 by neutron-diffraction 1.819 ˚A). Temperature dependences of magnetic reflec-
1 studyusingsinglecrystals.[6]Thelow-temperaturemag- tions were measured using the triple-axis spectrometers
2
neticphaseconsistsofplanarferromagnetic(FM)andA- GPTAS and TOPAN, where the (0 0 2) reflection of
1
type antiferromagnetic (AFM) components, indicating a pyrolytic graphite (PG) was used to monochromate, to-
0
9 canted AFM ordering. With increasing x, the canting getherwithPGfilterstoeliminatehigherordercontami-
9 angle between planes changes from 6.3◦ at x = 0.40 to nations. ThesespectrometersarelocatedintheJRR-3M
/ 180◦ at x = 0.48, while the FM ordering temperature reactor in JAERI.
t
a T decreases from 120 K to 0 K. It was also discovered Figure 1 shows five possible models of magnetic struc-
C
m
that the A-type AFM ordering remains above T up to tureandcorrespondingscatteringpatternsforLSMO327.
C
- T ∼ 200 K. At x = 0.30, on the other hand, it is re- Hereweassumethatspins areferromagneticallycoupled
d N
n ported that the low-temperature magnetic structure is withinaplane,i.e.,FMorA-typeAFMphase. Provided
o AFMwiththeeasyaxisparalleltothec-axisandthatthe that spins are antiferromagnetically modulated within
c easy-axis shows a canting at higher temperature, giving theabplane,thereshouldbemagneticsuperlatticepeaks
:
v the in-plane components. [7,8]As for x=0.50,superlat- at(hk l)withhalfintegerofhand/ork,whichisclearly
i tice peaks are observedby electrondiffraction [9] as well not the case for the present study. Models a (FM-I) and
X
as neutron diffraction, [10]which are ascribed to charge- b (FM-II) correspondto FM structures with spins align-
r
a ordering. The A-type AFM structure is realized at low ingwithintheabplaneandalongthec-axis,respectively.
temperatures, while additional magnetic reflections are Models c (AMF-I), d (AFM-II) and e (AFM-III) repre-
seen at intermediate temperatures, indicating the coex- sent A-type AFM structures with different alternating
istence of CE-type and A-type AFM structures. pattern along the c-axis. Note that a canted AFM mag-
LSMO327 exhibits a variety of magnetic structures netic structure is represented by a combination of FM
with changing temperature and the hole concentration and AFM components, which thus gives a combination
x. Therehavebeen, however,no systematic study ofthe of FM and AFM reflection patters shown in Fig. 1.
magnetism of LSMO327 in a wide temperature and hole
concentration range so far. To establish the magnetic
phase diagram, we have carried out extensive neutron-
diffraction studies on LSMO327 powder samples pre-
pared in consistent manner.
1
La Sr Mn O La Sr Mn O
2-2x 1+2x 2 7 2-2x 1+2x 2 7
1.0 2000
0.5 (a)FM-I (002) (004) (101)(103)(006) (105)(110) 1000 FM-II (101) x(103)=0.30(105)(110)
1.0 2000
s )0.5 (b)FM-II (101)(103) (105)(110) 5 min )1000 FM-I (002) (004) (101) (103) x(006)=0.35(105)(110)
t 1
ni1.0 2000
/
y ( arb. u0.5 (c) (001) (003) (102) (104)AFM(007)-I ( counts 1000 FM-I+(001)AFM-I(002) (003) (004) (100)(101) x(103)=(006)0.39(105)(110)
t1.0 y2000
nsi (d) 1) AFM-II sit FM-I+AFM-I x=0.45
e 00 n
Int0.5 ( (003) (005)(100) (102) (104) (007) Inte1000 (001) (002) (003) (004) (100)(101)(102)(103)(006)(104) (105)(110)
1.0 2000
0.5 (e) (002) (004) (101) (103)(006) AFM-III 1000 AFM-I (001) (003) (102) x=0(104).50
0.0 0
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40
scattering angle ( degree ) scattering angle ( degree )
FIG.1. Fivepossiblemodelsandcorrespondingreflections FIG. 2. Powder patterns in the low temperature phase
for magnetic structures which would explain the present re- (∼10K)ofLa2−2xSr1+2xMn2O7(0.30≤x≤0.50). Thedata
sults of La2−2xSr1+2xMn2O7 (0.30≤x≤0.50). takenat room temperatureissubtractedfrom each profileas
background. Hatched peaks correspond to AFM reflections,
We have carried out comprehensive powder neutron- and theothers correspond toFM reflections.
diffraction measurements at x = 0.30, 0.32, 0.335, 0.35,
0.38, 0.39, 0.40, 0.45, 0.48 and 0.50. Figure 2 summa-
La Sr Mn O
rizes the results for x = 0.30, 0.35, 0.39, 0.45 and 0.50 2-2x 1+2x 2 7
at 10 K. We have noticed that the powder diffraction 250
patterns for x = 0.32 ≤ x ≤ 0.50 at low temperatures
T
can be explained by a combination of FM-I and AFM-I TC
phases: the FM-I phase is dominant at low doping re- 200 TN
) CO
T
gion, while the AFM-I phase (hatched peaks) gradually K CE
I
takesovertheFM-Iphasewithincreasingx. Thepresent (

e150
result is consistent with Ref. [6] and makes it clear that r
u AFM-I
the planar FM phase is stable below x ∼ 0.40. We also at
found that the diffraction pattern at x = 0.30 is com- er100
p
pletely different from the other concentrations, and cor- m AFM
-II
responds to the FM-II phase, in which spins are aligned e II
T
ferromagneticallyalongthec-axis. Thedrasticchangein 50 FM-I Canted AFM
the magneticstructureindicatesthatthere existsacom- FM (FM-I + AFM-I)
-II
positional phase boundary around x = 0.30−0.32. To
study the intermediate temperature phase, we have also 0
0.30 0.35 0.40 0.45 0.50
meausuredpowderdiffractionsaround100−120K.Asis
hole concentration x
consistent with Ref. [6], only the AFM-I phase remains
for x=0.39−0.50,while no magnetic reflections areob-
served for x = 0.32−0.38. As x = 0.30, however, the
low-temperature FM-II phase disappears and the AFM-
II phase appears at intermediate temperatures.
2
FIG. 3. Magnetic phase diagram of La2−2xSr1+2xMn2O7 [5] J. F. Mitchell, D. N. Argyriou, J. D. Jorgensen, D. G.
(0.30 ≤ x ≤ 0.50). AFM-I (AFM-II) indicates the planar Hinks, C. D. Potter and S. D. Bader, Phys. Rev. B 51 63
A-typeAFMstructurewithFM(AFM)intra-bilayercoupling
(1997).
andFM(AFM)inter-bilayercoupling. FM-IandFM-IIstand
[6] K. Hirota, Y. Moritomo, H. Fujioka, M. Kubota, H.
fortheFMstructureswithspinwithintheabplaneandalong Yoshizawa and Y. Endoh, J. Phys. Soc. Jpn. 67 3380
the c-axis, respectively. (See Fig. 1) As for x = 0.50, only
(1998).
AFM-IexistsinthephaseI,whileAFM-IandCE-typeAFM
[7] T.Kimura,A.Asamitsu,Y.TomiokaandY.Tokura,Phys.
coexist in thephase II. Rev.Lett. 79 3720 (1997).
[8] T. G. Perring, G. Aeppli, T. Kimura, Y. Tokura and M.
We alsomeasuredthe temperaturedependence oftyp- A.Adams (unpublished).
ical magnetic reflections to complete the magnetic phase [9] J.Q.Li,Y.Matsui,T.KimuraandY.Tokura,Phys.Rev.
diagram of LSMO327, as shown in Fig. 3. As for B 57 R3205 (1998).
0.32≤x≤0.48,thereis essentiallyonelow-temperature [10] M.Kubota,H.Yoshizawa,H.Fujioka,K.Hirota,Y.Morit-
magnetic phase, i.e., a planar canted AFM, in which the omo and Y.Endoh (unpublished).
canting angle is zero below x = 0.38, becomes finite at [11] K. Ohoyama, T. Kanouchi, K. Nemoto, M. Ohashi, T.
Kajitani and Y. Yamaguchi, Jpn. J. Appl. Phys. 37 3319
x=0.39 then gradually increases with increasing x, and
finally reaches 180◦ at x = 0.48. Note that the inter- (1998).
[12] I.Solovyev,N.HamadaandK.Terakura,Phys.Rev.Lett.
mediated AFM-I phase appears at x = 0.39, where the
76 4825 (1996).
canting angle in the low-temperature phase becomes fi- [13] S.Ishihara,J.InoueandS.Maekawa,PhysicaC,263,130
nite. The magnetic phase becomes more complicated at
(1996).
x=0.50 because of the charge ordering. [9,10] Since su- [14] D.N.Argyriou,J.F.Mitchell,C.D.Potter,S.D.Bader,R.
perlattice peaks due to the chargeordering andCE-type KlebandJ.D.Jorgensen,Phys.Rev.B55R11965(1997).
AFM ordering are not confirmed in the present powder [15] Y. Moritomo, Y. Maruyama, T. Akimoto and A. Naka-
diffraction study because those peaks are very weak, we mura, J. Phys. Soc. Jpn. 67 405 (1998).
refer to the results of Ref. [10] in our phase diagram.
Themagneticphasesforx=0.30arecompletelydiffer-
entfromthe otherconcentrations. Withdecreasingtem-
perature,itbecomestheplanarAFM-IIphaseat∼100K
and then changes to the uniaxial FM-II phase at 70 K.
Perring et al. [8] also conclude that the low-temperature
phaseisAFMwithspinsaligningalongthec-axis,which
is inconsistent with the FM-II structure in the present
study, though the change of the easy-axis direction from
planartouniaxialatintermediatetemperatureissimilar.
Note that there exists a compositional phase boundary
atx=0.30−0.32,whichis consistentwith difficulties in
growingacrystal. Moredetailedstudy,particularlywith
agoodsinglecrystal,isrequiredtocompletelyclarifythe
magnetic structure around x=0.30.
Recent studies indicate that there is a close relation
betweenthe magnetismandstructure ofLSMO327,par-
ticularly through the Mn e orbital degree of freedom.
g
[6,12,13,14,15]It is thus necessaryto study the hole con-
centrationdependenceofstructureindetailandcompare
with the magnetic phase diagramwe have established in
the present study.
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[3] Y. Moritomo, Y. Tomioka, A. Asamitsu and Y. Tokura,
Phys. Rev.B 51 3297 (1995).
[4] Y.Moritomo,Y.Tomioka,A.Asamitsu,Y.TokuraandY.
Matsui, Nature(London) 380 141 (1996).
3