Series of Bulk Magnetic Phase Transitions in NaxCoO2: a muSR study

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by  P. Mendels

Series of Bulk Magnetic Phase Transitions in Na CoO : a µSR study
x 2
P. Mendels,1 D. Bono,1 J. Bobroff,1 G. Collin,2 D. Colson,3 N. Blanchard,1
H. Alloul,1 I. Mukhamedshin,1 F. Bert,1 A. Amato,4 and A.D. Hillier5
1Laboratoire de Physique des Solides, UMR 8502, Universit´e Paris-Sud, 91405 Orsay, France
2 Laboratoire L´eon Brillouin, CE Saclay, CEA-CNRS, 91191 Gif-sur-Yvette, France
3 SPEC, CE Saclay, CEA-CNRS, 91191 Gif-sur-Yvette, France
5 4Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
0 5ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11, OQX, United Kingdom
0 (Dated: February 2, 2008)
2
Using muon spin rotation, well-defined bulk ∼ 100% magnetic phases in NaxCoO2 are revealed.
n
A novel magnetic phase is detected for x = 0.85 with the highest transition temperature ever
a
observedfor x≥0.75. Thisstresses thediversityofx≥0.75magnetic phasesandthelink between
J
magneticandstructuraldegreesoffreedom. Forthecharge-orderedx=0.50compound,acascadeof
0
transitionsisobservedbelow85K.Fromadetailedanalysisofourdata,weconcludethattheordered
1
momentvariescontinuouslywithtemperatureandsuggestthatthetwosecondarytransitionsat48K

and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.
]
l
e PACSnumbers: 75.30.-m,76.75.+i,75.25.+z,(71.27.+a,71.30.+h)
-
r
t
s
. Beyond their long-known ionic mobility which opened pound. Furthermore, on the basis of a second nominally
t
a the route to industrial applications, cobaltates recently x = 0.90 sample where an incommensurate (IC)SDW
m receivedconsiderableattentionafterthediscoveryofhigh 25-50%fractionordersetsinbelow20K,adome-shaped
- thermoelectric power in metallic Na0.7CoO2 [1] and su- continuous phase diagram was proposed, in a Hubbard
d
perconductivity, maybe unconventional, in the hydrated approach which links magnetism to doping [8]. Bulk C-
n
o Na0.35CoO2compound[2]. Inaddition,averyrichphase SDW was reported to occur below 20 K [9], only in one
c diagram [3], still to be explored in detail, seems to in- x=0.82(2)singlecrystal,withaµSRsignaturedifferent
[ volve many and possibly competing parameters such as fromtheprevioussamples. Sortingouttheactualimpact

1 doping, charge order, magnetism, frustration and strong of doping on magnetism of the CoO2 planes clearly calls
v electronic correlations. This spans over most of topical for a more refined study.
3 problems in condensed matter, more specifically in the The importance of the structure might be well illus-
0 field of correlated systems. Whether one parameter has tratedinthe peculiar x=0.50case. Awelldefinedcom-
2
a leading role over others is a central issue for under- mensurate superstructure has been observed [4] where
1
0 standing the fundamentals of physics in Na-cobaltates. Na+/vacanciesand Co3+/Co4+ arrangein orderto min-
5 Cobaltates are layered compounds, like high-T imize both Na-Na and Na-Co Coulomb repulsion. A
c
0 cuprates. Magnetic and conducting properties set in magnetic state was recently reported below 50 K [10]
/
t CoO2 layers, issued from edge sharing CoO6 octahedra and a metal-insulator transition sets in around 30 K [3].
a stacked along the c-axis and separated by Na+ partially In addition a susceptibilty anomaly is observed around
m
filled layers. In a naive model, the increase of Na+ con- 90 K [3], which origin is still not known.
-
d tentleadstoaconversionoftheformalchargeofCofrom In this Letter, we present a study of 100% mag-
n 4+ (low-spin, S = 1/2) to 3+ (S = 0) -hence a depleted netic phases, detected through µSR. The∼structures of
o antiferromagnetic frustrated quantum triangular lattice- all investigated samples were checked by room-T X-ray
c
: and/or to electron doping occuring in a S = 1/2 back- Rietveld refinements. For x 0.75, we isolate two pure
v ground, which per se are interesting problems. magnetic phases, including ≥a novel one for x = 0.85.
i
X Experimentally,bothmetallicandmagneticcharacters We clearly demonstrate the absence of phase separation,
r can be found for various x but, at variance with high- stress the underlying role of the structure , hence the
a T ’s, Na+ and Co3+/4+ charge-orderingsmightset in for complexity of the magnetic, likely non-continuous phase
c
well defined compositions as shown by recent electronic diagram. For x =0.50, we clearly establish, for the first
diffraction [4], cristallographic [5] and NMR studies [6]. time, that 3 magnetic transitions occur below 85 K.
Magnetic states have been reported for long for x All samples were prepared by solid state reaction of
≥
0.75. A commensurate spin density wave (C-SDW) was Co3O4 and Na2CO3. They were further annealed in air
detected below 22 K in minority volume fraction in a for twice 12 hrs and quenched with intermediate grind-
◦
nominal x = 0.75 sample [7]. Given the reported c-axis ing,thenannealedinflowingoxygenfor24hrsat600 C.
parameteris rathertypicalofthe non-magneticx=0.70 The x = 0.50 composition was reached by immerging
phase [6], the invoked magnetic inhomogeneity might and stirring for 4 days the starting x = 0.70 material in
ratherrelatetothemultiphasingoftheinvestigatedcom- a sodium hypochlorite solution. Hexagonal parameters
2

0.3 mp 4 100
A
x = 0.75 urier ) TF 50 G 75
0.2 Fo Hz 2
y 0.1 F2req. (MH4z) cy (M x = 0.75 2550 (%)a
r n 0 ar
et ue 100 Ap
m q
sym 0.0 Amp Fre 2 TF 50 G75
A 0.2 x = 0.85 Fourier x = 0.85 2550

0
F2req. (MH4z) 5 10 15 20 25 30
0.1 T (K)
FIG. 2: Left, closed symbols: T- variation of the 4 ZF fre-
quencies used in the fits. ∼ (T −T )0.28 lines emphasize the
o
0 1 2 3 scalingofalltheirvariations. Thewidthsaresmallerorofthe
orderofthepointsizes. Right,opencircles: wTFasymmetry
Time (ms)
.
FIG.1: ZFasymmetries(linesareforfits)andFTat5Kfor
x=0.75and0.85(PSI).CirclesonFTareforfittedfrequen-
cies The asymmetry of the high frequency line is accounted Fourier transforms (FT) which give the distribution of
for by 2 frequencies (i.e. 2 µ+ sites) for x=0.75. H over all µ+ sites. The FTs are quite well peaked
int
clearly indicating a simple magnetic order in view of the
number of µ+ sites expected (see x = 0.5 discussion).
a = 2.81511(3) ˚A and c = 11.1314(2) ˚A were measured Accordingly, µSR signals and their T-evolution are best
with a Pnmm orthorhombicsuperstructure (a√3,2a,c). described by damped cosine functions (see ref. [7, 9] for
Nasites,attheverticaleitherofaCosite(Na1)orofthe details),theweightofwhichcanbefixedatlowT values.
centerofaCotriangle(Na2)haveanequaloccupancyof In Fig. 2, we plot the weak transverse field (wTF)
0.25, in agreement with the nominal composition. The asymmetry which monitors the non-magnetic fraction,
x = 0.75 sample displays an hexagonal P63/mmc lat- found to be less than 10% in both samples. The sharp-
tice with a = 2.84175(4) ˚A , c = 10.8087(2) ˚A , typical ness of the decrease of the wTF signal allows us to ex-
of the H2 phase of ref. [5] and exhibits only a few very tracttransitiontemperatures,T =20.8(5)and27(1)K,
o
weak (incommensurate) additional diffraction peaks for for x = 0.75 and 0.85. For each sample, the frequencies
◦
25<2θ <40 . Structural refinements lead to site occu- (Fig. 2) scale with each other on the whole T-range and
pancies 0.22(1)for Na1 and 0.52(1)for Na2, close to the decreasesmoothlywhenT increasestovanishatT ,sim-
o
nominalcontent0.75. Thismaterialdecomposessponta- ply reflecting the variation of the order parameter of a
neously in air leading to the x = 0.70 typical diffraction unique magnetic phase probed at 4 different µ+ sites.
pattern(c 10.892(4)˚A).Finally,x=0.85materialex- Forx=0.75,the frequencies andT values aresimilar
∼ o
hibitsamonoclinicdistortionwitha10.766(2)˚Ac-lattice to that of minority 20%phases reported in [7]. On the
∼
constant (projected on the hexagonal cell). contrary,to our knowledge, for x=0.85,the frequencies
The µSR experiments were performed at the ISIS are different from existing ones and the transition tem-
(EMU)andPSI(GPS)facilities[11]. Inthecaseofmag- perature is much higher than any reported to date for
netic order, spontaneous oscillations of the muon polar- x 0.75. This points at a novel magnetic phase, com-
≥
izationaredetectedwithoutanyappliedfield(ZFsetup) mensurate, as suggested above, which is worth noticing
through the asymmetry of the positron emission due to for such non-peculiar value of x as 0.85.
theµ+ desintegration. Thefrequency(ies)correspondto We now focus on the x=0.50 composition. In Fig. 3,
the internal field(s)(Hint) at the muon(s) site(s) and, in we display for various T both asymmetries versus time
thecaseofsimplemagneticphases,trackthevariationof andtheir FT. Magnetic orderis evidentthroughsponta-
the order parameter(i.e. the localstatic moment). High neous oscillations of the ZF µSR signals below 85(1) K,
statistics counts allowedto revealclearlyall the oscillat- whichcorrespondstothehighT kinkinSQUIDdata(in-
ing frequencies for each detected magnetic phase. set,Fig.4). Unlikepreviouscaseswheretheshapeofthe
We first give a brief account for the x 0.75 samples asymmetryremainssimilarforallT inthefrozenregime,
≥
whichdisplayatextbookµSRsignatureofaphasetransi- wefind,forthefirsttime,thatthree distinctmagneticre-
tiontoanorderedmagneticstate. InFig.1,forx=0.75 gions exist for x=0.50 [12]. Depending on the T range,
and 0.85 samples, we report both ZF asymmetry and 2 or 3 frequencies were used to fit the µSR data, lead-
3

0.2 2.5
5 K z) 4.5x10-6 c(emu/g)
0.1 H
M 2.0
-6
0.2 5 K s ( 4.0x10 H = 10kG
metry0.1 27.6 K 10 K ncie 1.5 50 100
Asym00..12 47.8 K 27.6 K eque 1.0
r
35.2 K F 0.5
0.2
47.8 K
65.5 K 0.0
0.1
65.5 K 0 20 40 60 80
0 1 2 3 4 5 1 2 3 T (K)
Time (ms) Frequency (MHz)
FIG. 4: T-plot of the x = 0.5 ZF frequencies (the line is a
FIG. 3: Left panel: ZF asymmetries for x = 0.5 at various guide to the eye). Bars stand for the FWHM of the various
typical T (PSI). Lines are fits, see text. The evident change spectrallines. Theerrorsinthefitsaresmallerthanthedata
oftheoscillations shapeisreflectedintheFouriertransforms points. Closed (open) symbols are for PSI (ISIS) data taken
(right panel). There, circles mark fitted frequencies. on different batches. Vertical arrows indicate the kinks of
the magnetic susceptibility (see inset). Vertical dotted lines
separate the various regimes.
ing to a more complex T-dependence of the frequencies
(Fig.4) thaninFig.2. Twoextra-transitionsaresingled
out at lower T, namely at 29(1)and 48.0(5) K. no phase separation occurs.
At base temperature (5 K), three frequencies can be In order to further discuss our data, we now address
distinguished on the FT in the range 1.3-1.9 MHz. Sur- the issue of the µ+ location. Possible sites, minimizing
prisingly,uponincreasingthetemperature,onlythehigh the µ+ electrostatic energy are either a Na+ vacancy or
frequency line is kept, whereas the remaining spectrum a site located 1 ˚A away from an O2−, forming a O-µ−
progressively spreads downwards continuously to even bond. Thefirstonecanbediscardedsince(i)nearbyNa+
reach zero frequency around 29 K. Correlatively, the would repel the µ+ from this position (ii) the Gaussian
widths of the two lowest frequency signals noticeably in- damping in the paramagnetic state, originating from Na
crease from T = 5 K to finally overlap for T . 28 K. andCo nucleardipoles, is muchtoo smallto match with
Between30and48K,theFTspectrumismainlypeaked this site location only (∆ = 0.28(1) instead of 0.17
calc
attwofrequencies,onefairlylow(<0.5MHz)andonein µs−1) and (iii) in the case of partial occupancy, a much
thecontinuityofthehighfrequencycutoff. TheFTspec- lower frequency than observed at 5 K should be found
trumchangesabruptly around48 K andthree frequency at a Na vacant site in comparison with that at O2− site
peaks are again evident between 50 and 85 K. (ratio 1:5),as alreadyadvocatedin[7, 9]for x 0.75.
∼ ≥
Ifthehighest-frequencystillroughlymimicsthe varia- Forx=0.50,Coareknowntoformalternatingchains
tionexpectedforanorderparameter,thevariationofthe in a given plane [4]. Every 2 chains, Na alternate above
other frequencies do not scale with this one and have an andbeloweachCo,hence the Co valencein sucha chain
unconventionalnon-monotonicvariationintheT >48K is expected to be uniform and weaker than for the next
phase. The former certainly corresponds to a µ+ domi- Co, also uniform, chain. In the following, we make the
nantly coupled to a single magnetic site while the latter simple assumption that cobalt is either in the Co3+ or
are issued from several moments and are likely sensitive Co4+ state [13] and consider an ideal undistorted or-
to the relative orientation of moments. In this respect, thorhombicstructure. We arbitrarilyconsiderµ+ bound
the continuity of the highest frequency indicates that
thereisnosuddenchangeofthemomentat48and29K.
Onthe contrary,the markedchange at48 K for the oth-
3+
ersrevealsalock-inofthemagneticstructurebelow48K Co
-and down to 29 K- which is confirmed by the kink ob- 2a 2b
4+
servedin SQUID data at 48 K. wTF measurements (not Co
displayed)indicatethatthemagneticfractioninoursam-
3+ 1
ples is 90% for all T <85 K, and therefore underlines Co
∼
thebulkcharacteratallT. Finally,thechangesobserved FIG. 5: Charge order suggested in ref.4. Big circles are for
for all µ+ frequencies at these specific T either through Co4+/3+ andsquaresareforNaabovetheCoplane. Oxygens
inflexionsordiscontinuitiesclearlyestablishthatwedeal, (small circles) form atriangular network shifted from theCo
one aboveand below (not represented) theCo planes.
in each domain, with a single cristallographic phase and
4
to oxygens located above the Co plane. Inequivalent and magnetic properties have a common origin, likely
charge/magnetic configuration surrounding oxygens can structural.
be sortedbyconsideringCo trianglesnextto theµ+ site The origin of these multiple transitions is not obvious
altogether with the occupancy of nn Na+ sites (fig. 5). at the present stage. We can clearly rule out a scenario
Three inequivalent O2− are found, consistent with the where an IC-SDW would switch to a C-SDW since the
number of frequencies observed at 5 K and above 49 K. frequencies are already quite well peaked at the upper
Twohave2nn magneticCo4+ andhaveeitherann Na1 transition. Whether the problemcanbe tackledthrough
orann Na2,respectivelylabeled(2a)and(2b)[14]. The a purely ionic model where anisotropy and charge local-
third site (1) has a single nn Co4+ and 2 nn Co3+ with izationcouldinducethesecondarytransitionsorwhether
one Na1 and one Na2 filled nearby sites. aC-SDWordermightbe influencedbythese parameters
Asalreadyarguedabove,µ+ with1magneticnn Co4+ open new avenues to the debate on this peculiar x=0.5
(site 1) can be safely assigned to the highest frequency composition.
signal. A µ+ coupled to two nn Co4+ will sense pro-
In summary, we clearly show the existence of bulk
nounced changes of internal fields and may also experi-
phase transitions in high quality powdered samples with
ence quite weak fields. Our data constrains severely the
well identified structures. This truly underlines the im-
orientation of the moments, since, first, H is observed
int portance of magnetism in cobaltates, in a large range
to be weaker for sites (2) than for sites (1) for all T
of Na doping. For x = 0.5, we give a convincing evi-
and second, the ratio varies sizeably with T to reach
∼ dence that three magnetic transitions occur with an AF
1/5 value between 29 and 48 K. To illustrate this point,
order building up at 85 K and moments rearrangements
one can notice that H (2) cancels only for two antifer-
int at lower T (48 and 29 K). For 0.75 x 0.85, one can
romagnetic Co4+ moments, lying perpendicular to the ≤ ≤
now isolate three close magnetic phases, with a C-SDW
Co4+-Co4+-µ+ plane(henceperpendiculartothechain).
ground state, including the x = 0.82 of [9] [15]. The in-
We first performed calculations of the dipolar field in-
crease of T from 21 K for the hexagonal x = 0.75 to
duced on the 2 types of sites by a chain of Co4+, assum- o
27 K for the monoclinic x = 0.85 phase is quite surpris-
ing an AF coupling. The effect of moments beyond 2nd
ing since the number of magnetic Co is expected to de-
nn was found negligible. We further checked that the
crease. WhetherCo-Cocouplingsdifferbecauseofstruc-
drawnconclusions still hold without sensible changes for
turalchanges,orsomechargeorderinglocks-inadifferent
a 3D magnetic stacking either ferro or antiferromagnetic
magnetic order definitely calls for further investigations
of such AF chains. Our major findings are (i) the mo-
ofstablestructuresin the rangex=0.75 1as wellas a
ments lie close to AF order (ii) their direction is within −
clear identification of magnetic Co sites with respect to
20◦ perpendiculartotheplaneO(2)-nn Co4+chain;(iii)
the Na order. Rather than a continuous phase diagram
sites (2) frequencies are fairly sensitive to a small mis-
induced by charge doping only, the scattering of the T s
o
alignment of neighbor moments or to a change of the di-
rather point at a tight link between structures and mag-
rectionoftheAForderedmoments,e.g. 10-20◦isenough
netic order. Disentangling how charge order/disorder
to explain the variations observed in Fig. 4. Note that
and doping impact on the physical properties versus Na
the moment direction should alternate along c-axis since
content is now a crucial issue for cobaltates which ap-
oxygen triangles are inverted for two next Co planes.
parentlycombinethe physicsofmanganatesandhigh-T
c
For this structure,since site (1) is coupleddominantly
cuprates.
to a single Co, the highest frequency at 5 K can be used
to extract a value for the Co4+ moment of the order of
0.3(1) µ . Such a weak value could be explained by a
B
strong zero point quantum reduction due to the possible
1D character of the Co4+ chain. Alternatively, a com- [1] I. Terasaki, Y.Sasago and K.Uchinokura,Phys.Rev.B
mensurate spin density wave description in the frame- 56, R12685 (1997).
[2] K. Takada et al.,Nature 422, 53 (2003).
work of band magnetism -the system is metallic above
[3] M.L. Foo et al,Phys. Rev.Lett., 92, 247001 (2004).
29 K- could be a natural explanation of this value.
[4] H.W.Zandbergenetal.,Phys.Rev.B70,024101(2004).
The values of the frequencies are found to be only lit-
[5] Q. Huanget al,cond mat./0406570.
tle affected by a change in the µ+ position as compared [6] I. R. Mukhamedshin, H. Alloul, G. Collin and N. Blan-
to the effect of moments misalignement. Therefore, the chard,tobepublishedinPhys.Rev.Lettandref.therein.
changes around 29 K cannot be merely explained by a [7] J. Sugiyama et al.,Phys. Rev.B 67, 214420 (2003).
slight structural change or a charge re-ordering which [8] J. Sugiyama et al.,Phys. Rev.Lett. 92, 017602 (2004).
would affect the µ+ site. We infer that a magnetic com- [9] S.P.Bayrackietal.,Phys.Rev.B69,100410(R)(2004).
[10] Y. J. Uemura et al, cond mat./0403031.
ponent is also associated with this transition, smoother
[11] Lose powders or pressed disks were used. Dueto a more
than the 48 K one. The increase of the resistivity ob-
or less pronounced platelet shape of the grains, some c-
served in [3], rather modest in comparison with usual axispreferentialorientationcouldbeobserved∼parallel
metal-insulator transitions, clearly shows that electronic to the µ+ polarization
5
[12] ForT >48 K,no magnetism was found in [10] [14] Due to the symmetry of the charge distribution, the µ+
[13] Whatever the refinements about the charge and mag- locates in the mid-planeof the two nn Co4+ segment.
netic state of Co, we expect our discussion to hold pro- [15] Duringtherefereeing rounds,wealso isolated thisphase
vided only two dominant Co species exist for x = 0.5,
onestrongly magnetic and the otherweakly magnetic.