Journal of Magnetism and Magnetic Materials 324 (2012) 928–933

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Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Glassy behavior in the layered perovskites La2�xSrxCoO4 ð1:1rxr1:3Þ

S. Mukherjee a,n, Rajarshi Mukherjee a, S. Banerjee b, R. Ranganathan b, Uday Kumar c

a Department .of Physics, The University of Burdwan, Burdwan 713104, Indiab Saha Institute of Nuclear Physics, Kolkata 700 064, Indiac Department of Physical Sciences, Indian Institute of Science Education and Research – Kolkata, Mohonpur 741252, India

a r t i c l e i n f o

Article history:

Received 2 May 2011

Received in revised form

21 July 2011Available online 17 October 2011

Keywords:

Cobaltite

Phase separation

Magnetic property

Spin glass

53/$ - see front matter & 2011 Elsevier B.V. A

016/j.jmmm.2011.07.060

esponding author

ail address: sanseb68@yahoo.co.in (S. Mukhe

a b s t r a c t

The glassy behavior of the phase segregated state in the layered cobaltite La2�xSrxCoO4 has been

studied. The role of the inter-cluster interactions as well as the disordered spins at the paramagnetic–

ferromagnetic interface, behind the observed glassy behavior have been investigated. The disordered

spins at the interface appear to be strongly pinned, and they contribute little to the observed glassy

behavior. On the other hand, the inter-cluster interactions play the key role. Both the Co4þ and Co3þ

ions are in the intermediate spin state.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The two-dimensional confinement of the B–O–B network in thelayered perovskite oxides A2BO4, having K2NiF4 structure, reduces theone-electron band width of the eg electrons, and thereby introducesstrong electron correlation. Interestingly, charge- as well as spin-ordered state have been observed in such systems [1,2]. Studies ofsuch systems also show nontrivial coupling among different degreesof freedom such as charge, spin and orbital; and thereby promotescomplex electronic phenomena [3]. In addition, different spin statesfor the same valence state of the Co ions make the cobaltitesparticularly more interesting in comparison with the other perovskiteoxides [4]. A correlation between the structural and the transportproperties has already been proposed based on changes in theaverage oxidation and spin state of the Co ions [5]. Recently, thephase segregated state (PSS) in the manganite [6] and cobaltite [7,8]has attracted attention, with the experimental observation of glassymagnetic features. However, about the origin of glassy behavior, adefinite conclusion is yet to be reached. For the PSS of manganite(La0.25Nd0.75)0.7Ca0.3MnO3, Rivadulla et al. [6] suggest that the glassyfeatures actually originate from inter-cluster interactions. Moreover,the authors conclude that this idea should be applicable to othersystems also, close to a first order electronic transition. However, forthe PSS of Cobaltite Sr1.5Pr0.5CoO4, the authors [7] show that incontrast to the La-based manganite, the observed glassy featurescannot be explained by considering only the inter-cluster interactions.Instead, both spin glass like phase and inter-cluster interactions

ll rights reserved.

rjee).

contribute to the glassy magnetic behavior. It is well known thatthe frustration of the spin state is essential for the observation of spinglass phase in canonical systems [9,10]. According to Rivadulla et al.[6], the concentration of the clusters depending upon the compositionand magnetic field introduces the frustration effect, rather than thecompeting ferromagnetic (FM) double exchange (DE), and antiferro-magnetic (AFM) superexchange (SE) interactions. The inter-clusterinteraction plays an important role in the PSS, and the relative size ofthe FM clusters to the non-FM matrix is crucial. A systematic study ofthe PSS with different relative sizes of the FM clusters, either bytuning the composition or the magnetic field may provide essentialinformation about the magnetic state.

The parent compound La2CoO4 shows commensurate AFMorder of Co2þ ions (S¼3/2: high spin (HS) state) below TN¼275 K[11,12]. On the other hand, Sr2CoO4 with the same K2NiF4

structure is a metallic ferromagnet with a relatively high Curietemperature (Tc¼255 K) [13]. Hole-doping into La2CoO4 sup-presses the commensurate AFM order of the parent compound.For La2�xSrxCoO4 compounds, the commensurate AFM orderpersists up to a Sr content of x¼0.3 [3]. La1.7Sr0.3CoO4 showsshort range AFM ordering below 130 K [3]. This short rangeordering is manifested in the spin glass like behavior and theirreversibility effect observed in the zero field cooled (ZFC)susceptibility data [14]. The incommensurate spin ordering, withthe modulation closely following the amount of doping has beenobserved for 0:3oxr0:6 [3]. In La1.5Sr0.5CoO4, checkerboardcharge ordering occurs at high temperature, and the incommen-surate AFM ordering appears below Tmag¼40 K. Tmag decreaseswith further increment in Sr-doping (e.g., Tmag � 80 K for x¼0.4,Tmag � 60 K for x¼0.45, Tmag � 50 K for x¼0.5, Tmag � 15 K forx¼0.6) [3]. The study performed on single crystals of this system

2000

20

40

60

225 K

200 K

0

0

100

200

300

400

500

La0.9Sr1.1CoO4

ZFC ( ), FC ( )

T (K)

M (

emu

/ mol

)

M (

emu/

mol

)

La0.7Sr1.3CoO4

ZFC ( ) ,

FC ( )

T (K)50 100 150 200 250 300

250

Fig. 1. Magnetization M as a function of temperature for La2�xSrxCoO4, under zero

field cooled (ZFC) and field cooled (FC) conditions, measured at a dc field of 1 kOe;

inset shows the high temperature part.

S. Mukherjee et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 928–933 929

shows a significant reduction in the effective magnetic moment(meff ) beyond x� 0:7 [15]. The pure Co3þ (x¼1) compoundLaSrCoO4 is paramagnetic down to the lowest temperature [15].Further Sr doping introduces Co4þ ions. Introduction of Co4þ ionscauses the onset of ferromagnetic order at around 150 K in 3Dperovskite La1�xSrxCoO3 [16,17]. In La2�xSrxCoO4 for xZ1:1,where Co4þ ions coexist with Co3þ ions, magnetization M(H, T)data show the formation of ferromagnetic regions [16]. Magneti-zation M (H¼5 T) increases rapidly with the concentration ofCo4þ ions, suggesting the increase of ferromagnetic fraction withx [15]. For x41, the increase of M (H¼5 T), and the decrease ofresistivity with increasing x, suggest that the ferromagnetism ismediated by charge carriers, and the double-exchange interactionmay be the possible origin [16,18]. This gives rise initially to ashort range FM order, and finally stabilizes to true long range FMorder for higher x values. The glassy behavior of ferromagneticclusters depicted by a maximum in MZFC at TSG, and difference ofMFC and MZFC below TSG have been observed at low temperaturesfor 1rxr1:4 [19]. The electron doping by partial substitution ofY in Sr2CoO4 also reduces Tc to 150 K in Sr1.5Y0.5CoO4, and theferromagnetism disappears at xZ0:67 for Sr2�xYxCoO4 [20]. Inthe intermediate concentrations, it is highly plausible to get phasesegregation.

From our discussion on La2�xSrxCoO4, it is clear that in theregion 1:1rxr1:3, the system exhibits phase segregation, i.e.,the presence of ferromagnetic clusters in paramagnetic matrix.The system also shows characteristics of spin glass systems at lowtemperatures in this region of concentration. In this paper anattempt has been made to study the PSS cobaltite La2�xSrxCoO4

for 1:1rxr1:3 to understand the role of inter-cluster interac-tions as well as the disordered spins at the paramagnetic–ferromagnetic interface behind the glassy behavior.

We also want to focus on the spin state of the Co3þ ions, aboutwhich different groups have different opinions. In La2�xSrxCoO4

the valence state of Coþ ion varies from Co2þ for x¼0 to Co4þ forx¼2. In the intermediate concentration Co3þ ions coexist witheither Co2þ or Co4þ . Co2þ ions are always in the high-spin (HS)state with S¼1.5 (t5

2ge2g ), whereas Co3þ ions can exist in three spin

states: the nonmagnetic low-spin (LS) state with S¼0 (t62ge0

g), theintermediate-spin (IS) state with S¼1(t5

2ge1g ), or the HS state with

S¼2 (t42ge2

g ). On the other hand, Co4þ ion has two possible spinstates: LS state (S¼0.5, t5

2ge0g ) and IS state (S¼1.5, t4

2ge1g ). Regarding

the spin state of the Co3þ ions in La1.7Sr0.3CoO4, LS state for Co3þ isconsidered favorable for efficient trapping [3]. It has been sug-gested [2] that the strong planar anisotropy leads to a quenching ofthe spin angular momentum on the Co3þ sites. In contrast, ananalysis of the superstructure is interpreted by considering Co3þ inIS state [2]. An analysis based on the anisotropy of this systemconcludes, that the spin state of Co3þ for x40:4 is the LS state, andis confirmed by a full-multiplet calculation [14]. Moritomo et al.[15] predicts HS state as the possible spin state of Co3þ ions forlower values of x, and a spin-state transition of the Co3þ ions fromthe HS state to the IS state, with an abrupt change in the effectivemagnetic moment (meff ) beyond x� 0:7. Curie–Weiss analysis forLaSrCoO4 by another group [16] suggests that Co3þ ions are in theIS state. The optical conductivity sðoÞ spectrum for LaSrCoO4 isvery similar to that of LaCoO3, in which Co3þ ions are in the IS state[15]. On the other hand, unrestricted Hartree–Fock calculations byWang et al. [21] conclude that the HS–LS ordered state of Co3þ

ions is the most probable ground state for LaSrCoO4. The partialreplacement of HS Co3þ by LS Co3þ has been identified as the mostprobable cause of the sharp decrease of magnetic moment inLa2�xSrxCoO4 near x¼0.7 [15]. Curie–Weiss analysis [15] for Co4þ

doped samples with x41 shows that meff first starts from2:722:8mB and increases to 3:1mB for x¼1.4, pointing to a stabili-zation of IS Co4þ state.

2. Experimental

Polycrystalline samples of La2�xSrxCoO4 (1:1rxr1:3) wereprepared by the high temperature ceramic method. Appropriatemolar ratios of La2O3 preheated at 1000 1C, SrCO3 and CoOpowders were thoroughly mixed in an agate mortar and initiallyfired at 1050 1C for 24 h. After regrinding, the powder was heatedfor the second time at 700 1C for 10 h. Then the mixture waspressed into a pellet after regrinding. The pellet was then sinteredat 1050 1C for 48 h in a flow of O2. The analysis of X-ray diffractionpattern obtained from Philips PW 1730 X-ray diffractometerconfirmed that all the samples were in single-phase with tetra-gonal crystal structure. The relevant lattice parameters (a, c) arerespectively (3.80675, 12.49493), and (3.80793, 12.49774) forx¼1.1, 1.3. The parameters are consistent with the reportedvalues [22]. The oxygen content of each sample has beendetermined by EDX analysis. The instrument used for EDXanalysis is Quanta 200 MK2 made by FEI Company, with an EDXdetector made by AMETEK, USA. EDX analysis suggests that thechemical formula appropriate to our samples is La2�xSrxCoO4þdinstead of La2�xSrxCoO4, where d lies between �0.1 and þ0.5 forx¼1.1 and the same is in between �0.1 and 0.2 for x¼1.3. The dcand ac magnetic measurements were carried out using a super-conducting Quantum interface device (SQUID) magnetometer.

3. Results and discussion

Fig. 1 shows the temperature dependence of the magnetiza-tion M for La2�xSrxCoO4 (x¼1.1, 1.3), measured in the tempera-ture range 5–300 K, with a dc field of H¼1 kOe, under ZFC and FCconditions. The high temperature part has been shown separately

S. Mukherjee et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 928–933930

in the inset to identify the temperature Tir, where MZFC becomesseparated from MFC. On increasing the temperature from thelowest temperature of measurement, MZFC increases and showsa peak around Tpeak¼78 K in each case. The broad maximum inMZFC and a monotonic increase of MFC below Tpeak are similar tothe cluster glass phase in La1�xSrxCoO3 for 0:18rxr0:5 [23,24].In the present study, the increase in M(T) with the lowering oftemperature below 250 K is due to the appearance of ferromag-netic ordering. This is in contrast to the sharp rise in M(T) at Tc asobserved in ferromagnets with true long range order. MFC starts toincrease below Tc¼140–150 K for both the studied samples, andreaches a plateau around 70 K. This observation is similar to thatof Chichev et al. [16] for x¼1.3 and 1.4. According to them, thismay be considered as a signature for a gradual change from ashort range or two-dimensional (2D) ferromagnetic order in theperovskite layers to a three dimensional (3D) ordered ferromag-netic regions below 70 K. We also propose that the ferromagneticvolume fraction increases with the lowering of temperature. Ourrelaxation experiments discussed in the subsequent section alsosupport this. Large separation between MFC and MZFC below Tpeak

is also typical for cluster glass phase. However, these so-calledcluster-glass-like features have also been observed insystems which exhibit true long-range ferromagnetic ordering,e.g., SrRuO3, La0.7Ca0.3MnO3 [25]; and there the glassy featureshave been suggested to originate from magnetocrystalline aniso-tropy. The present study shows an additional feature: the separa-tion between MFc and MZFC occurs at a temperature Tir higher thanTpeak for xZ1:1. Similar feature has been observed in the PSS ofcobaltite Sr1.5Pr0.5CoO4 [6]. It may be noted that in La0.95Sr1.05

CoO4 [26], MZFC starts bifurcating from MFC at T¼Tpeak. Theseparation between MZFC and MFC below Tir suggests a non-equilibrium magnetic state for the ZFC case. Our aim is to studythis non-equilibrium dynamics, and to decide whether thereexists any spin glass phase, or the non-equilibrium state arisesdue to interactions between FM clusters. Other possible origin ofthis non-equilibrium state may be domain wall relaxation due tomagnetocrystalline anisotropy. It may be noted that La2�xSrxCoO4

is highly magnetically anisotropic (planar), and it persists even inthe paramagnetic regime [14]. This magnetic anisotropy has beensuggested to appear as a result of band structure and spin–orbitcoupling. Due to this crystalline anisotropy the orbital moment isanisotropic, and the spin moment is coupled to this anisotropy viaspin–orbit coupling. The spin is finally aligned in the direction ofmaximum orbital momentum. We observe that Tpeak (� 78 K)does not change with Sr concentration x, whereas Tir is differentfor different x (Tir¼205 K for x¼1.1, Tir¼225 K for x¼1.3).

We estimate the effective magnetic moment per Co site (meff )from the Curie–Weiss analysis of the dc susceptibility data (Fig. 1) inthe temperature range 245–300 K. For x¼1.1 sample, meff is 2:14mB

with C¼0.57 emu mol�1 K and y¼ 160 K. This indicates that Co3þ

ions are in IS state (S¼1, m¼ 2:83). For x¼1.3, meff is 2:41mB withC¼0.72 emu mol�1 K, y¼ 110 K. meff increases with x. Increment inx means an increase in the number of Co4þ ions. So if meff increases,then Co4þ ions must be in IS state (S¼1.5). As pointed out earlier,Curie–Weiss analysis for Co4þ doped samples with x41 by anothergroup [16], also suggests IS Co4þ state. For all our samples y ispositive, indicating ferromagnetic exchange interaction. However,for such a highly anisotropic system with strong spin–orbit coupling,the validity of the Curie–Weiss law might be questionable.

The measurement of ac susceptibility has an advantage over thedc measurement, as it uses a very small ac field so that theinformation about the magnetic state can be obtained withoutsignificant perturbation. In addition, the magnetization dynamicsof non-equilibrium states, involving low or intermediate (10�1–10�5 s) relaxation times, can also be probed by varying thefrequency of the applied ac field. A nonzero w00 is associated with

relaxation and irreversibility effects in systems like spin glass,cluster glass, whereas the same can indicate irreversible domainwall motion in ferromagnets. In spin glass, individual magneticmoments experience random interactions with other moments; as aresult of which a metastable frozen state appears with strongirreversibility below a freezing temperature Tf [9]. The spin glassstate is characterized by a cusp in w0 at Tf. Tf is frequency dependent,and such system has its unique magnetization dynamics. Forcanonical spin glasses the frequency-shift of Tf is small, andArrhenius law (t¼ t0 expðEa=kBTÞ, Ea is the anisotropy energybarrier, t is the measurement time, i.e., the inverse of the measure-ment frequency, t0 is called the attempt frequency, and kB is theBoltzmann constant) fitting gives unphysical values of t0 and Ea

[10]. The cusp in w0ðTÞ corresponds to maximum slope in w00ðTÞ [10].For FM clusters subjected to anisotropy, w00ðTÞ shows a peak also atthe blocking temperature TB. According to Neel–Brown theory, theblocking temperature for an assembly of such noninteractingparticles can be obtained from Arrhenius law. The interactionsbetween superparamagnetic spin clusters have been taken intoaccount in the Vogel–Fulcher law t¼ t0 exp½Ea=kBðTB�T0Þ� [10].The same law has also been successfully applied to explain thefreezing dynamics of different cluster glass and spin glass systems[10]. Alternatively, the power law which assumes the existence oftrue equilibrium phase transition with a divergence of relaxationtime near the transition temperature, has also been used to explainthe relaxation behavior in cluster glass and spin glass systems [10].Although the small frequency shift of Tf in spin glass distinguishes itfrom cluster glass or superparamagnets, it is difficult to distinguishbetween cluster glass and a superparamagnet experimentally [9]. Ingeneral, larger value of t0 usually suggests the presence of clustersin the system [9]. The ac susceptibility data for La0.7Sr1.3CoO4 show acusp in both w0 and w00 around Tf¼110 K, and a modulation in slopeof w0 around 200 K (Figs. 2 and 3). The cusp is frequency indepen-dent in the range 3 Hz to 1 kHz (time scale 10�3–0.3 s). The cusp inw0 and w00 at an ac field of hac¼3 Oe around Tf¼110 K can becorrelated to the peak in MZFC at Tpeak¼78 K . The shift to the highertemperature for the former is due to the smaller value of the ac field.The effect of the dc field on Tf can be observed in Fig. 2: inset, wherethe application of a small dc field of 100 Oe shifts Tf to 105 K. For aclassical spin glass system, an increment in applied dc field shifts Tf

to lower temperature [10]. On the other hand, for assemblies ofinteracting FM clusters, increasing dc field causes further growth ofthe clusters, and as a result shifts the peak in ac susceptibility tohigher temperatures [7]. However, we feel that this is true, onlywhen the cluster sizes depend on the applied dc magnetic field. Fora system where short-range correlated ferromagnetic particles areembedded in the paramagnetic matrix, the spin structure of theinterface/surface layer has been suggested to be disordered [7].In the case of strong pinning at the interface, the system of suchinteracting ferromagnetic clusters will behave in the same way asspin glass systems with reference to the effect of dc field in themeasurement of ac susceptibility. The applied dc field will reducethe height of the potential barrier experienced by individual clusterdue to interaction with other clusters. The strong pinning is likely toappear due to anisotropy in the system. In our case, we do notobserve strong frequency dependence as observed in the case ofprogressive freezing of clusters or superparamagnetic particles [10].In fact, it does not show even very small change of Tf with frequencyas observed in metallic spin glasses. Therefore, any spin glass phasearising out of the competition between the ferromagnetic doubleexchange and the antiferromagnetic superexchange appears to beunlikely. In our case the frozen state is reached due to randominteractions of individual FM cluster with other clusters, and thefrequency independence of Tf indicates strong inter-cluster interac-tions. With the lowering of Sr concentration, the cusp turns into abroad maximum. This indicates a distribution cluster sizes, and the

0.0

0.2

0.4

0.6

50

0.0

0.1

0.2

0.3

0.4

T (K)

χ' (

emu

mol

-1 O

e-1)

hac = 3 Oe,f = 33 Hz

-Hdc = 0 Oe

-Hdc = 100 Oe

110 K

105 K

La0.7Sr1.3CoO4

χ' (

emu

mol

-1O

e-1)

χ' (

emu

mol

-1 O

e-1)

hac = 3 Oe

200 K

50

0.0

0.2

0.4

0.6

T (K)

-3.3 Hz

-33 Hz

-90 Hz

-333 Hz

-1 kHz

La0.9Sr1.1CoO4

100 150 200 250

100 150 200 250

Fig. 2. w0 , the in-phase part of the ac susceptibility as a function of temperature for

La2�xSrxCoO4 for 1:1rxr1:3. Inset shows the effect of dc field on w0 for

La0.7Sr1.3CoO4.

0.00

0.02

0.04

hac = 3 Oe

La0.7Sr1.3CoO4

χ'' (

emu

mol

-1 O

e-1)

χ'' (

emu

mol

-1 O

e-1)

50

0.00

0.02

0.04 -3.3 Hz

-33 Hz

-90 Hz

-333 Hz

-1 kHz

La0.9Sr1.1CoO4

T (K)100 150 200 250

Fig. 3. w00 , the out of-phase part of the ac susceptibility as a function of

temperature for La2�xSrxCoO4 for 1:1rxr1:3.

S. Mukherjee et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 928–933 931

clusters of different sizes have different Tf values. This is quite likelyto happen due to decrease in ferromagnetic order parameter withthe lowering of Sr concentration.

We have observed that for both the samples, the irreversibilityeffect starts below Tir bTf or Tpeak, and Tir is different for different x

(1.1, 1.3). We observe a modulation in slope in w0ðTÞ around 200 Kclose to Tir of dc measurement for La0.7Sr1.3CoO4. Such a signaturein ac susceptibility might be due to any one of the followingreasons. Firstly, it may correspond to the temperature at which thedisordered spins at the interface get pinned. In that case, theconsiderably high value of Tir suggests strong pinning effect, andthe state below Tir is a metastable one. Secondly, the system maybe an inhomogeneous one, with clusters having two peaks in thedistribution of sizes. For x¼1.1, the higher temperature anomaly inac susceptibility is not prominent. To conclude about the possibleorigin of the ac susceptibility behavior, we performed magneticrelaxation experiments for La0.7Sr1.3CoO4 at two temperatures 40 Kand 175 K, one below Tf and the other above Tf. We measured theisothermal remanent magnetization (IRM) as a function of time,after cooling the sample in zero field (ZFC), followed by cycling anexternal field 0-500 Oe-0 (Fig. 4). In our case the waiting time tw

is negligibly small in both cases. Various functional forms havebeen proposed to describe the magnetization as a function ofobservation time t. One of the most popular relations is thestretched exponential MðtÞ ¼M0 exp½�ðt=tÞ1�n] [9,10]. For glassysystems, M0, t and n depend on T for a fixed waiting time tw. For

n¼0, we have Debye type exponential relaxation with single t.On the other hand, for n¼1, M is time independent [10].A fit MðtÞ ¼M0þM1þM2 ¼M0þM10 expð�t=t1ÞþM20 expð�t=t2Þ

to the present data suggest that M(t) is a sum of three terms M0, M1

and M2. M0 does not change with time within our period ofobservation. The other two parts M1 and M2 relax with timeconstants t1 and t2 respectively. Even at T¼175 K, which is wellabove Tf (� 110 K), M(t) relaxes in the same way as that at T¼40 K,well below the said temperature. t1, being in the vicinity of 90 sand t2, being in the vicinity 1400 s do not change with tempera-ture. M0 is higher at T¼175 K than that at T¼40 K, whereas M1 andM2 both are higher at T¼40 K than those at T¼175 K. The fact thatM(T) relaxes in the same way at two temperatures, one well aboveTf and the other well below Tf, indicate that passing through Tf

introduces no additional relaxation mechanism. Only we observerelative changes in the magnitude of M0, M1 and M2. This may beassociated with the relative changes in the magnitude of differentmagnetic volume fractions. M0, the time independent part isassociated with paramagnetic matrix, and the related volumefraction decreases with temperature. On the other hand, theferromagnetic volume fraction M1 and M2 increases with thelowering of temperature. So in our system we have two kinds offerromagnetic volume fraction, one relaxing with characteristictime t1 and the other with t2. Therefore, our relaxation experi-ments confirm the system to be a collection of clusters with twopreferred sizes. Moreover, t1 and t2 do not change with tempera-ture. This is in contrast to SG/CG systems, for which the relaxation

1

2

3

4

0

5.2

5.6

6.0

T = 40 K

M (

emu/

mol

)

M = M0 + M10 exp (-t/τ1) + M20 exp (-t/τ2)

M0 ~ 0.70 emu/mol

M10 ~ 1.67 emu/mol, τ1 = 85.5 s

M20 ~ 1.32 emu/mol, τ2 = 1414 s

M (

emu/

mol

)

t (s)

T = 175 K

M = M0 + M10 exp (-t/τ1) + M20 exp (-t/τ2)

M0 ~ 5.16 emu/mol

M10 ~ 0.33 emu/mol, τ1 = 91 s

M20 ~ 0.34 emu/mol, τ2 = 1405 s

1000 2000 3000 4000

Fig. 4. Isothermal remanent magnetization M as a function of time t for

La0.7Sr1.3CoO4 under ZFC condition. The solid line is a fit to M¼M0þM1

expð�t=t1ÞþM2 expð�t=t2Þ.

-12

-750

-500

-250

0

250

500

750

-12

-750

-500

-250

0

250

500

750

La0.7Sr1.3CoO4M

(em

u/m

ol)

T = 65 K -

T = 100 K

T = 150 K

-75-15

-10

-5

0

5

10

15-75

-15

-10

-5

0

5

10

15

H (k Oe)

M (

102 em

u/m

ol)

H (kOe)

-8 -4 0 4 8 12

-8 -4 0 4 8 12

-50 -25 0 25 50 75

-50 -25 0 25 50 75

Fig. 5. M versus H under ZFC condition for La0.7Sr1.3CoO4 at T¼65 K, T¼100 K and

T¼150 K. Inset shows the full hysteresis loop with field.

-12

-600

-300

0

300

600

-12

-600

-300

0

300

600

T = 65 K-

T = 90 K

T = 150 K

M (

emu/

mol

)

H (kOe)

-75

-15

-10

-5

0

5

10

15

-75

-15

-10

-5

0

5

10

15

H (kOe)

M (

102 em

u/m

ol)

-8 -4 0 4 8 12

-8 -4 0 4 8 12

La0.9Sr1.1CoO4

-50 -25 0 25 50 75

-50 -25 0 25 50 75

Fig. 6. M versus H under ZFC condition for La0.9Sr1.1CoO4 at T¼65 K, T¼125 K and

T¼160 K. Inset shows the full hysteresis loop with field.

S. Mukherjee et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 928–933932

time increases with the lowering of temperature [9,10]. Thissuggests that there is no spin glass phase.

For La0.7Sr1.3CoO4 hysteresis loop opens at temperatures below225 K (Fig. 5). The hysteresis is associated with ferromagneticvolume fractions, which appear at Tc, and increase with thelowering of temperature. We have drawn loops at three differenttemperatures, 150 K, 100 K and 65 K under ZFC condition. In anyof the three cases, saturation has not been observed even in a fieldof 7 T. It may be recalled that our system actually consists ofshort-range correlated ferromagnetic particles with disorderedboundary. In addition the clusters are strongly interacting. Thestrongly pinned disordered boundary as well as the inter-clusterinteraction prevents the sample to reach saturation. As a conse-quence, we observe a small value of MHA, i.e., M at the highestattainable field. MHA as well as the loop area increases with thelowering of temperature. This is obvious as the ferromagneticvolume fraction increases with such lowering of temperature.Identical features have been observed for x¼1.1 (Fig. 6).

The idea that the coupling between the ferromagnetic coreand the disordered interface might cause the exchange biaseffect [27], encourages us inevitably to study the hysteresis loopsunder field cooled conditions. Displaced hysteresis loop hasbeen observed under field cooled conditions for Sr1.5Pr0.5CoO4 [7].The shift is characterized by two parameters namely, HE ¼�

ðHc�þHcþ Þ=2, where Hc� and Hcþ are negative and positivecoercive fields respectively; and exchange bias magnetizationME ¼ ðMrþ þMr�Þ=2, Mrþ and Mr� being the positive and negativeremanent magnetizations respectively. One of the importantcharacteristics of the exchange bias system is the training effect[7], i.e., the reduction of exchange bias field HE with continuousfield cycling. We performed field cooled hysteresis experimentsfor La0.7Sr1.3CoO4 at T¼60 K with cooling field H¼1 k Oe as well

-20

-9

-6

-3

0

3

6

9-20

-9

-6

-3

0

3

6

9

-75

-2

-1

0

1

2

-75

-2

-1

0

1

2

H (kOe)

M (

103 em

u/m

ol)

La0.7Sr1.3CoO4

T = 60 K

M (

102 em

u/m

ol)

H (kOe)

FC, H = 1 k Oe

FC, H = 1 T

T = 10 K

La0.7Sr1.3CoO4

-15 -10 -5 0 5 10 15 20

-50 -25 0 25 50 75

-15 -10 -5 0 5 10 15 20

-50 -25 0 25 50 75

Fig. 7. Field cooled hysteresis loops at T¼60 K (with cooling field 1 kOe) and at

T¼10 K (with cooling field 1 T) for La0.7Sr1.3CoO4.

S. Mukherjee et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 928–933 933

as at T¼10 K with cooling field H¼1 T. However, we did notobserve any appreciable shift along any axis (Fig. 7), i.e., in ourcase HE as well as ME are both zero in both cases. Therefore, itappears that unlike the cobaltite system Sr1.5Pr0.5CoO4, thecoupling between the ferromagnetic core and the disorderedinterface does not play any major role.

4. Conclusion

Our study for phase-separated La2�xSrxCoO4 (1:1rxr1:3)suggests that the system may be considered as an assembly ofstrongly interacting clusters, as observed in manganite system [6].Magnetic relaxation experiments along with ac susceptibility datasuggest the existence of two preferred sizes in the distribution ofclusters. However, unlike the mentioned manganite the size ofthe magnetic clusters does not change with application of dc fieldeven of the order of 1000 Oe, although the ferromagnetic volumefraction increases with lowering of temperature. The boundaryspins at the PM–FM interface appear to be disordered andstrongly pinned. The frequency independence of the low-

temperature cusp in ac susceptibility unlike Sr1.5Pr0.5CoO4 [7]suggests strong intercluster interactions. The glassy behaviororiginates mainly from strong inter-cluster interactions, and theboundary spins do not play any significant role. Therefore, we donot observe any exchange bias effect also.

Acknowledgement

We thank Dr. Anjan Barman of S.N. Bose National Centre forBasic Sciences, India for EDAX measurements.

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