Ferromagnetic to spin glass cross over in (La,Tb)_{2/3}Ca_{1/3}MnO_{3}
aa r X i v : . [ c ond - m a t . s t r- e l ] F e b Ferromagnetic to spin glass cross over in (La,Tb) / Ca / MnO C. Raj Sankar, S. Vijayanand, and P. A. Joy ∗ Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India
In the series La / − x Tb x Ca / MnO , it is known that the compositions are ferromagnetic forsmaller values of x and show spin glass characteristics at larger values of x . Our studies on themagnetic properties of various compositions in the La / − x Tb x Ca / MnO series show that the crossover from ferromagnetic to spin glass region takes place above x ≈ x . A mixed phase region coexists in the narrow compositional range 0.1 ≤ x ≤ I. INTRODUCTION
There has been a wide interest in the study of the sub-stituted perovskite-type manganites, La − x A x MnO , inorder to understand the different aspects of the complexmagnetic behaviour exhibited by these compounds. The double exchange interactions involving Mn andMn ions give rise to ferromagnetism in the substitutedmanganites. The changes in the Mn-O-Mn bond angle,from structural distortions, are very crucial in determin-ing the strength of the ferromagnetic interactions. High-est Curie temperature in the La − x Ca x MnO series isobtained for x ≈ / A / MnO . Many inter-esting new magnetic behaviors are observed when La ispartially substituted by other trivalent rare-earth ions inLa / Ca / MnO , though there are no changesin the Mn /Mn ratio after substitution. Hwang et al. found a direct correlation between the Curie temperatureand the average ionic radius of the La-site ions, wherethe Curie temperature decreases with decreasing aver-age ionic radius in La . − x R x Ca . MnO (R = Pr, Y),indicating the role of lattice effects in determining theferromagnetic properties. Partial substitution of La bysmaller ions leads to a decrease in the Mn-O-Mn bondangle and this affects the long range ferromagnetic ex-change interactions. For La . − x Tb x Ca . MnO (in the re-ports, the chemical composition is used as(La − x Tb x ) / Ca / MnO ), a gradual decrease inthe Curie temperature is observed with increasingconcentration of Tb. This is also associated with abroadening of the magnetic transition and ultimately aspin glass behaviour is observed at larger concentrationsof Tb.
In La . − x Tb x Ca . MnO , the evolutionof a spin glass (SG) or a cluster glass (CG) state isthought to be due to the competing interactions offerromagnetic (FM) and antiferromagnetic (AFM) typesor the random distribution of Mn-O-Mn bond anglewhich suppresses the exchange strength between theMn ions significantly. At sufficiently large valuesof x ( x = 0.22), short range ordered magnetic clustersare formed with typical magnetic coherence length ofaround 18 ˚A, at low temperatures. The reported studies on La . − x Tb x Ca . MnO are performed on ferromagnetic compositions ( x ≤ x > x between 0.1 and 0.15 inLa . − x Tb x Ca . MnO have been studied. The criti-cal concentration is found to be x ≈ II. EXPERIMENTAL
The polycrystalline La . − x Tb x Ca . MnO compo-sitions were synthesized by the conventional solid stateroute from La O , Tb O , CaCO and MnO by mix-ing these oxides in the required stoichiometry for x = 0,0.03, 0.07, 0.10, 0.11, 0.12, 0.125, 0.13, 0.15, 0.20, and0.25. The well-mixed powders were initially heated at1273 K for 48 h, and subsequently at 1473 K for 48h,1573 K for 48 h and finally at 1623 K for 24h, with in-termediate grindings at every 24 h steps to ensure thesample homogeneity. Finally the powder samples werepelletized and sintered at 1673 K for 24 h. The sampleswere characterized by powder X-ray diffraction using CuK α radiation and Ni filter. The Mn contents in thecompounds were estimated by the iodometric titrationmethod. The magnetization measurements were per-formed on a vibrating sample magnetometer. Temper-ature variation of the ac magnetic susceptibility of thesamples was measured by the mutual inductance methodin a field of 2 Oe and at a frequency of 210 Hz.
III. RESULTS AND DISCUSSION
The samples were characterized for their phase purityby X-ray diffraction. All samples showed single phase na-ture and the diffraction patterns were indexed on the dis-torted orthorhombic structure with space group
Pbnm . The lattice parameters were obtained by least squaresrefinement of the diffraction patterns and found to be
30 K / m a x T (K)
FIG. 1: Temperature variation of the ac susceptibility of somecompositions in La . − x Tb x Ca . MnO . The numbers in-dicate the values of x . comparable for those of the corresponding compositionsreported previously. The Mn content was found tobe matching with the expected value of 33% for all thecompositions.The ac susceptibility curves of the unsubstituted andsome Tb-substituted compositions are shown in Fig. 1.With increasing concentration of Tb, the magnetic tran-sition temperature is decreased, and the susceptibilitycurves show a cusp-like feature when the concentrationis increased beyond x = 0.1. The shapes and features ofthe ac susceptibility curves of the Tb-substituted com-positions (for x ≤ x > Although well-defined mag-netic transitions are observed for the samples with lowconcentrations of Tb, there is a distinct anomalous fea-ture in the susceptibility curves of these compositions atlow-temperatures. A decrease in the susceptibility is ob-served below ∼
30 K, as soon as a small amount of Tb isincorporated ( x = 0.03).As shown in Fig. 2, for x = 0.1, a small step in thesusceptibility curve is observed below the main magnetictransition. For x = 0.11, there is a cusp like feature ob-served at a higher temperature which is succeeded by thenormal flat curve, and finally a drop in the susceptibilitybelow 30 K. This is a complex feature and is not com-monly reported for the manganite compositions. As theconcentration of Tb ions is further increased, it can beseen that the cusp-like feature becomes more prominent,the flat region gradually vanishes, but a dip in suscep-tibility is still observed at 30 K for the compositions upto x = 0.125. For x > x = 0.33 is reportedto show spin glass behavior. Neutron diffraction studieshave indicated ferromagnetically ordered state for x =0.067 at low temperatures, whereas short range orderedclusters are found for x = 0.2 (the authors have used the / m a x T (K)
FIG. 2: Temperature dependence of the ac susceptibility ofthe compositions in La . − x Tb x Ca . MnO , for 0.1 ≤ x ≤ x . chemical formula as (La − x Tb x ) / Ca / MnO , so that x = 0.1 and 0.3, respectively, correspond to x = 0.067 and0.2 in La . − x Tb x Ca . MnO ). These clusters showspin glass-type of behavior due to competition betweenFM and AFM types of exchanges.
For the compositions with x > x is increased. Also, it was found that the relativesusceptibility value also decreased with the increasing Tbconcentration. These observations point out the evolu-tion of short range ordering due to the decreasing Mn-O-Mn bond angle and consequent formation of magneticclusters as the concentration of Tb is increased. Thisclustering is likely to be due to the breaking or weaken-ing of the double exchange closer to the Tb-sites causedby the local structural distortion and the spin glass likefeature originates from the formation of such clusters.The decreasing temperature corresponding to the cusp,identified as T g henceforth, suggests the confinement ofmagnetic clusters to shorter length scales as the numberof Tb centers is increased (increasing values of x ).As shown in Fig. 3, the ferromagnetic transition tem-perature decreases almost linearly with increasing x , upto x = 0.125. For the compositions showing both fer-romagnetic and spin glass characteristics (0.1 ≤ x ≤ T c and T g are determined as illustrated in theinset of Fig. 3 for x = 0.11. Here, the χ -T data for x =0.15 (spin glass composition) is shifted towards the rightside and that of x = 0.07 (ferromagnetic composition) isshifted towards the left side along the x -axis, to matchwith the observed data of the mixed phase compositionand the curves are normalized with respect to the maxi-mum values. T c is taken as the mid point of the magnetictransition. T g changes abruptly around x = 0.125 andvaries from 66 K for x = 0.13 to 43 K for x = 0.25. T g has been found to be almost independent (40-50 K) of x for higher Tb concentrations. Thus, the lowest value of
50 100 150 T C T g T ( K ) x in La Tb x Ca MnO x = 0.11 x = 0.07 x = 0.15T g T c T (K)
FIG. 3: Variation of T c and T g as a function of x inLa . − x Tb x Ca . MnO . Inset: illustration of the methodof extracting T c and T g , for x = 0.11, using the data for x =0.15 (for T g ) and x = 0.07 (for T c ). possible T g is larger than the temperature (30 K) wherea decrease in the ac susceptibility is observed for 0 < x ≤ / Ca / MnO has been ascribed to spin glassbehavior. However, neutron diffraction studies showedferromagnetic ordering for x = 0.067 down to 7 K. Thus, the feature at 30 K for 0.03 ≤ x ≤ ions are randomly dis-tributed in the lattice, the double exchange is disturbedaround the Tb centers and therefore tiny magnetic clus-ters are formed with reduced Mn-O-Mn angle. These tinymagnetic clusters remain isolated until x = 0.125 (1/8)above which larger magnetic clusters are formed due tothe breaking or considerable weakening of the three di-mensional long range ordering. Thus, the temperatureat which a decrease in the susceptibility is observed, dueto these small clusters, remains the same until x = 1/8.The three dimensional ordering is affected when x > like local envi-ronment in the lattice of La . − x Tb x Ca . MnO . ForTbMnO , an incommensurate-commensurate phase tran-sition, which is accompanied by a ferroelectric transition,associated with a lattice modulation, is observed close to ∼
30 K and large magnetic field controlled polarizationeffects are reported at this temperature.
A local phase separation exists in the x = 0.1 sample,as evidenced by a small step- like magnetic transition, in-dicating that ferromagnetic clusters are started formingat this value of x . It is possible that, above this value of x , some magnetic clusters with short range ordering areseparated whose size decreases with increasing x . Thus,at intermediate values of x , the lattice is consisting of -15 -10 -5 0 5 10 15-90-60-300306090 At 12 K M ( e m u / g ) H (kOe) x = 0.0 x =0.10 x =0.13 x = 0.15 x = 0.20 At 15 kOe M ( e m u / g ) x in La Tb x Ca MnO FIG. 4: The M-H curves of La . − x Tb x Ca . MnO for dif-ferent values of x . Inset: variation of magnetization at 15 kOeas a function of x . larger ferromagnetic clusters with sufficiently long rangeordering and smaller short range ordered clusters. Also,for the larger long range ordered part, the magnetic tran-sition temperature decreases due to the decrease in theMn-O-Mn bond angle. Thus, the clustering may be seento start when x = 0.11, where a cusp is also observedalong with the normal magnetic features.Fig. 4 shows the dc magnetization curves of differentcompositions, measured at 12 K, up to a maximum fieldof 15 kOe. The magnetization is saturated above 10kOe for x ≤ x in La / − x Tb x Ca / MnO is shown in the insetof Fig. 4. The magnetization remains almost the sameup to x = 0.1 and then decreases above this value of x . Also, the magnetization is not saturated for x > . Ca . Mn − x Ga x O . QCP is defined asa second order transition accompanied by the change ofa non-thermal parameter. The observation of QCP inLa . Ca . Mn − x Ga x O system is as predicted by thetheoretical calculations and the QCP in this system wasexpected for a value of 10-20% of Ga substitution. Thesubstitution of a nonmagnetic ion like Ga at the Mn-sublattice of the perovskite-type oxide causes the local-ization of the electronic states suppressing the double ex-change mechanism. For La . Ca . Mn − x Ga x O , thespontaneous magnetic moment calculated from the ex-perimental neutron diffraction patterns recorded at 1.5K decreased for x > x > / − x Tb x Ca / MnO shows almost a similartrend, except for the finite value of the magnetization athigher values of x , suggesting the possible existence ofQCP in this system also. However, this needs to be ver- x = 0.125 M ( e m u / g ) x = 0.13 x = 0.15 M ( e m u / g ) H (kOe) 0 5 10 1502040 x = 0.20 H (kOe)
FIG. 5: The virgin magnetization (red curves) and part of thehysteresis loop (blue curves) of x = 0.125, 0.13, 0.15, and 0.2in La . − x Tb x Ca . MnO . Note that the H scale is differentfor x = 0.125. ified with the help of neutron diffraction measurements,as made in the case of the Ga substituted system.Another interesting behavior observed in the low tem-perature M − H measurements for the compositions im-mediately above x = 1/8 is an irreversible jump to a fer-romagnetic state at higher magnetic fields, as shown inFig. 5. This is observed only for the virgin magnetizationmeasurements. Up to x = 0.125, a normal feature is ob-served, where the virgin magnetization curve lies insidethe hysteresis loop. For x = 0.2, the behaviour is similarto that observed for some typical spin glass systems, where the virgin magnetization curve initially lies out-side the loop and then merges with the loop at higherfields. On the other hand, for x = 0.13 and 0.15, the en-tire virgin magnetization curve lies outside the hysteresisloop above a certain small field (this small crossing fieldis observed for x = 0.2 and 0.25 also, and increases with x ), and an anomalous step-like feature is observed in thevirgin magnetization curve, similar to that of a metamag-netic transition. However, this transition is completely irreversible. The field above which a broad step is ob-served is larger for x = 0.15 compared to that for x =0.13. After the magnetic field is increased in the negativedirection to -15 kOe and when brought back to +15 kOethrough H = 0, the transition is not observed. This isan irreversible ferromagnetic transition in the sense thatthe step-like feature is never obtained when the measure-ments were repeated immediately or even after a time gapof 30 minutes. In the subsequent measurements, the firstpart of the curve always lies inside the hysteresis loop,like that for the compositions for x ≤ whereas Woodward et al. explained the observations interms of an avalanche behavior. It may be noted thatthe first part of the virgin curve is similar to that of thespin glass composition x = 0.2, in the present case, andtherefore, it is possible that the second jump is due to afield induced growth of the larger ferromagnetic clusters.Once the clusters are grown, it is not possible to revertback to the original state due to the unavailability of suf-ficient thermal energy. The original state is found onlywhen the temperature is raised above the peak tempera-ture and then cooled back in zero field. IV. CONCLUSIONS
The present studies made on a series of close compo-sitions in La . − x Tb x Ca . MnO indicate that singlephase ferromagnetic compositions are possible for x < ≤ x ≤ x . The ferromagnetic clusters present in the com-positions immediately above the cross over region showmagnetic field induced growth and give larger magnetiza-tion at higher fields. Further detailed studies are requiredto understand the complex magnetic behavior shown bythese Tb substituted manganite compositions at the in-termediate and the cross over regions. ∗ Electronic address: [email protected] Y. Tokura,
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