Gd 055Sr MnO Ceramics - Hindawi

Hindawi Publishing CorporationPhysics Research InternationalVolume 2012, Article ID 632016, 7 pagesdoi:10.1155/2012/632016

Research Article

Metamagnetic Phase Transitions in(Sm0.5Gd0.5)0.55Sr0.45MnO3 Ceramics

Fedor N. Bukhanko

Department of Electronic Properties of Metals, Donetsk Physical and Technical Institute NASU, Donetsk 83114, Ukraine

Correspondence should be addressed to Fedor N. Bukhanko, [email protected]

Received 4 August 2011; Revised 18 January 2012; Accepted 18 January 2012

Academic Editor: Manh-Huong Phan

Copyright © 2012 Fedor N. Bukhanko. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The temperature dependences of ac magnetic susceptibility of the (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics provide evidence of theformation of a mixed insulating state with a specific quantum-disordered phase, in which the domains with a long-rangeantiferromagnetic order dominate at temperatures below TN

∼= 48, 5 K. Irreversible metamagnetic phase transition in theferromagnetically ordered state is carried out at the critical field Hc1 ∼ 25 kOe which remains constant in the temperature rangefrom 4.2 K to 60 K. From the analysis of the magnetization isotherms, the conclusion can be made about a spontaneous phasetransition into the state with charge and orbital ordering at temperatures below the critical value TCO ∼ 60 K. It is supposed thatshort-range charge correlations exist in unusually large interval of temperatures∼90 K at temperatures aboveTCO that considerablyexceeds the area of existence of local orbital correlations ∼50 K.

1. Introduction

Change of the average radius 〈rA〉 of a rare-earth ion replac-ing lanthanum in A-positions of perofskite-like matrixes inR0.55Sr0.45MnO3 (R = Sm, Eu, Gd) manganites leads to thebicritical phase diagram where the metallic ferromagneticphase is separated from the dielectric antiferromagneticphase with charge/orbital (CO/OO) ordering by the firstorder phase transition [1, 2]. Prominent features whichshould be observed in experiment close to bicritical pointhave been predicted: (1) phase transition from a paramag-netic state in the ferromagnetic should be the of first order;(2) the critical temperatures of phase transitions TCO and Tc

close to bicrytical point should decrease sharply; (3) highsensibility of the sample to external magnetic field near tointerphase boundary. These effects arise because of hugefluctuation of several order parameters related to bicriticalbehavior, which sharply amplifies near to boundary of twophases, where more than two order parameters meet. As aresult of the local lattice distortion in R0.55Sr0.45MnO3, largestructural disorder enhances the phase fluctuation betweenferromagnetic metal and charge/orbital-ordered insulatorand suppresses their long-range orders [3–5].

The transition between a ferromagnetic (FM) metal andan insulator with short-range charge/orbital correlation in-duced by the change of average radius 〈rA〉 has been investi-gated for the single crystals (Sm1-y Gdy)0.55Sr0.45MnO3 (0 ≤y ≤ 1) [6]. A systematic study indicates that the long-rangeferromagnetic order is kept up to y ∼0.5 (corresponding toR = Eu) with Tc reduced from ∼130 K to ∼50 K, while itchanges to a spin-glass-like insulator for y≥ 0.6. Strong com-petition was found between long-range ferromagnetic orderand paramagnetic disorder or a spin-glass-like insulator withshort-range charge/orbital correlation, being controlled bychanges of the average radius 〈rA〉 or the external magneticfield. No macroscopic phase separation is discerned whenhigh-quality single-crystal specimens are used.

The results of measurements of magnetic susceptibilityχ(T) and magnetization isotherms M(H) in the samplesof ceramics of (Sm0.5Gd0.5)0.55Sr0.45MnO3 in strong staticand pulse magnetic fields essentially differ from the samemeasurements made earlier on the single crystals. It is sup-posed that the reason of a significant disparity of magneticproperties of these systems is the various nature of theground state in single crystals and the ceramics, related todifferent values of the structural disorder.

2 Physics Research International

2. Experimental Procedures

In this work, the ceramic samples of(Sm1-y Gdy)0.55Sr0.45MnO3 (0 ≤ y ≤ 1) were preparedusing the standard solid-state reaction technique. In briefdescription, homogeneous powders were heat treatedduring three stages at temperatures of 1000, 1100◦C for10 h, and 1150◦C for 24 h with intermediate grinding ofas-obtained products. Compacted under pressure of 10kbar, pellets were sintered in air at 1150◦C for 10 h withsucceeding temperature drop to the room temperature at therate of 70◦C/hour. Temperature dependences of magneticsusceptibility χ(T) were measured by induction method atfrequencies of 1, 5, 7, and 10 kHz in the temperature rangeof 4,2–100 K with the help of PPMS-10. Field dependencesof magnetization M(H) have been measured in an intervalof temperatures of 4,2–200 K in the strong pulse and staticmagnetic fields. Measurements in pulse fields to 300 kOeat temperatures varied from 20 to 200 K were made bynonindustrial pulse magnetometer, and measurements instatic fields to 80 kOe at 4,2 K have been carried out byapplication of vibrating magnetometer VSM EGG, PrincetonAppplied Reserch.

3. Experimental Results and Discussion

According to [6], in the (Sm1-y Gdy)0.55Sr0.45MnO3 singlecrystals, the metal ferromagnetic ground state is saved withgrowth of Gd content only in the range of concentrationof 0 ≤ y ≤ 0.5. The further increase of y results in thefirst-order phase transition in dielectric low-temperaturestate with characteristic signs of spin-glass phase. Unlikethe results of article [2], in the given work, the metalferromagnetic phase was destroyed completely already fory = 0.5, that is, associated apparently with larger structuraldisorder of ceramic samples. Results of measurement ofthe temperature dependence of ac magnetic susceptibilityof (Sm0.5Gd0.5)0.55Sr0.45MnO3 gave evidences of formationof long-range antiferromagnetic structure in the sampleswith the critical temperature TN

∼= 48, 5 K. The sharp peakof ac magnetic susceptibility close to 48,5 K is observedindependently of the frequency of measurement (Figure 1)and with a small temperature hysteresis∼2 K, which stronglydiffers from a wide peak χ(T) in the vicinity of TG ≈ 45 Kfor single crystals [6]. The same sharp peak of magneticsusceptibility had been observed earlier in various antiferro-magnets with Heisenberg, XY and Ising type of interaction,on magnetic lattices of dimensionality 1, 2, and 3 [7]. Atdecrease of the temperature, there is fast growth of χ(T) forT ≥ TN and then a sharp drop of the susceptibility wasfound for T ≤ TN. As shown in Figure 1, sharp peak ofmagnetic susceptibility is imposed on continuous paramag-netic increase of a susceptibility with the temperature dropin the wide temperature interval. The magnetic susceptibilityintensively rises with fall of temperature of the sampleand diverges near to absolute zero of temperatures, thatis, characteristic for quantum phase transitions in low-dimensional quantum Heisenberg antiferromagnets with thestructural disorder [8–10]. In this phase, low-temperature

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χ(e

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Figure 1: Initial ac magnetic susceptibility curves χ(T) in the(Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics in zero external magnetic field.

uniform susceptibility diverges algebraically with nonuni-versal exponents. This is a signature that the quantum-disordered phase is a quantum Griffiths phase. Superpositionof two various contributions to a low-temperature magneticsusceptibility testifies to the inhomogeneity nature of theground state of the (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics ina zero external magnetic field. Apparently, in these ceramicsamples, long-range antiferromagnetism coexists with short-range magnetic correlations in the large temperature intervalnear absolute zero. It is supposed that the mixed insulatingstate with a special feature of a quantum-disordered phasewith dominating domains of long-range antiferromagnetismis found in the (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics at thetemperatures below TN

∼= 48, 5 K. Similar mixed state closeto quantum critical point had been discovered recently in theLa2/3Ca1/3Mn1-x GaxO3 manganites in the vicinity of a metal-insulator transition [11, 12].

The variation of an external static magnetic field in aninterval ±6 T at 4,2 K had led to irreversible destructionof mixed phase accompanied by formation of the steadyferromagnetic phase (Figure 2). Field-induced stable FMphase was formed as a result of continuous metamagneticphase transition induced by growth of an external magneticfield to extremely small critical value Hc1 = 25 kOe with asharp increase in magnetization M(H) in the form of a stepin height ΔM. The further increase of the field to 60 kOeresulted only in insignificant growth of magnetization. At themagnetic field reduction, the induced ferromagnetic phase issaved up to zero field. The subsequent variation field in aninterval ±15 kOe has allowed receiving of a wide hysteresisloop corresponding apparently to an anisotropic ferromag-netic state of the sample at 4,2 K. A distinction of the receivedmagnetization curve M(H) at 4.2 K is large width of thehysteresis loop (∼4,4 kOe), which corresponds to coercivefield Hc

∼= 2, 2 kOe, and rather small size of magnetization ofsaturation in the field of 60 kOe∼60 emu/g, related evidentlywith the canted state of the manganese spins.

Physics Research International 3

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mu

/g)

Figure 2: Evolution of a magnetization isotherm M(H) of the(Sm0.5Gd0.5)0.55Sr0.45MnO3 sample at 4.2 K induced by metamag-netic phase transition.

Similar behavior of the field dependence of the mag-netization was observed in Pr0.7Ca0.3MnO3 at 5.0 K [13].According to neutron-diffraction experiments, without amagnetic field, Pr0.7Ca0.3MnO3 demonstrates three phasetransitions: change of the lattice symmetry at TB = 200 K,antiferromagnetic ordering at TN = 140 K, and canted anti-ferromagnetism at TCA = 110 K. It shows a canted ferromag-netic structure accompanied with antiferromagnetic (AF)components of so-called pseudo-CE-type antiferromagneticstructure at the temperatures below TCA. Furthermore, thecharge ordering of Mn3+ and Mn4+ ions takes place simul-taneously with a lattice distortion described as buckling,where MnO6 octahedra show alternating tilting and cause adoubling of the periodicity of the the lattice along the [010]direction. Although the resistivity of Pr0.7Ca0.3MnO3 showsinsulating behavior at zero field, it exhibits an insulator-metal transition at around 4.0 T. An applied field enforces aalignment of the AF components towards the field directionand drives Pr0.7Ca0.3MnO3 into the metallic state by actu-ating the double-exchange mechanism and destroying thecharge ordering. This an insulator-metal transition inducedby external magnetic field is accompanied also by a largemagnetic hysteresis, that indicates a strong coupling betweenspins and charges. It has been suggested that irreversiblemetamagnetic phase transition in Pr0.7Ca0.3MnO3 can beviewed as a melting of the charge ordering.

It is well known that the field-induced insulator-metalphase transition in half-doped manganites occurs at rel-atively small magnetic fields ∼10–40 T. Rather little valueof the critical magnetic field necessary for generation of aferromagnetic metal state is explained within the frameworkof double-fluid model [14, 15] by the competition betweencanting induced metallicity and inhomogeneity arising fromthe trapping of carriers by the Jahn-Teller defects. It has been

found that with growth of intensity of an external magneticfield basic state sharply leaves from the JT-distorted cantedinsulator state and is seized in a metal phase with the optimal(high) canting angle. In the given work, metamagneticphase transitions in the (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramicslocated in immediate vicinity of the insulator-metal transi-tion take place at extremely small value of the critical externalmagnetic field. This suggests that the phase transitionsin a ferromagnetic state in (Sm0.5Gd0.5)0.55Sr0.45MnO3 areinduced by a delocalization of self-trapped eg-electrons ofMn accompanied by disappearance of the local JT distortionsand the locking in a metal ferromagnetic phase.

As shown in Figure 3, at temperatures smaller 60 K, iso-therms of M(H) in pulse fields practically coincide in formwith the curves of the magnetization obtained in staticfields at 4,2 K. Size of magnetization step ΔM produced bytransition in the metal ferromagnetic state practically didnot change with temperature growth. Metamagnetic phasetransitions occur at the critical field Hc1

∼= 25 kOe whichremains constant up to the temperature of 60 K. Stabilityof the induced ferromagnetic phase in the range of tem-peratures of 4,2–60 K is confirmed also by the temperaturedependence of ac magnetic susceptibility χ(T) (Figure 4)received at heating of the sample in zero dc field where theferromagnetic phase has been induced by strong magneticfield at 18 K. The temperature dependence of susceptibilitycorresponds to phase transition from the induced orderedferromagnetic state to the disordered paramagnetic statewith critical temperature of Tc ≈ 48 K. The temperatureof 60 K in this figure corresponds to full destruction oflong-range ferromagnetic ordering of Mn spins and canbe considered as a boundary of stability of the inducedferromagnetic phase, which considerably exceeds the areaof existence of the mixed state in zero magnetic field.Unusual feature of metamagnetic phase transitions in therange of temperatures of 4.2–60 K is the constancy ofcritical field Hc1, that is, an evidence of absence of effect oftemperature changes on generation of ferromagnetic phasein the (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics. It is necessaryalso to notice that irreversible metamagnetic phase transi-tions investigated in the given paper do not contain strongdiscontinuities of isotherms of M(H) that are characteris-tic of earlier investigated irreversible metamagnetic phasetransitions in the manganites [16, 17] both in static, andin pulse magnetic fields, and exist at sufficiently elevatedtemperatures. Therefore, the mechanisms of the irreversibletransitions offered in these papers cannot fully explain thegiven experimental results.

At temperatures above 60 K, behaviour of magnetizationin a strong pulse magnetic field essentially varies. As shownin Figure 3, the isotherms of magnetization in high-temper-ature paramagnetic phase agrees with well-known behaviourof M(T) curves in a Griffiths-like phase and strongly differsfrom the isotherms received in low-temperature phase withthe mixed insulating ground state with dominating domainsof long-range antiferromagnetism. At growth of intensity ofexternal magnetic field to the critical value Hc1, there is areversible metamagnetic-like transition from paramagneticto a ferromagnetic state in the form of a narrow step on


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Figure 3: Evolution of magnetization isotherms M(H) of the (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics in the pulse magnetic fields for tempera-tures of 20–110 K.

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Figure 4: Temperature dependence of the ac magnetic susceptibility χ(T) of the ceramics (Sm0.5Gd0.5)0.55Sr0.45MnO3 obtained after meta-magnetic phase transition at 18 K.


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Tco = 60 K

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(a.u

.)

Figure 5: Temperature dependence of magnetization step height ΔM(T) at the metamagnetic phase transitions in the(Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics induced in the temperature interval of 18–165 K.

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T (K)

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ΔH

c(k

Oe)

Figure 6: Temperature dependence of field hysteresiswidth ΔHc of the metamagnetic phase transitions in the(Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics induced in the temperatureinterval of 18–165 K.

M(H) curve accompanied by field hysteresis at reductionof magnetic field intensity with critical field Hc2 /= 0 belowwhich the sample comes back to the initial paramagneticstate. The values of critical fields Hc1 and Hc2 practicallylinearly increase at temperatures above 80 K, whereas adifference of critical fields ΔHc = Hc1 − Hc2 decreaseswith the temperature rising and reaches zero near 110 K.At the further rise in temperature, the field-induced phasetransition looked like a narrow step of magnetization ΔM(T)without field hysteresis, which was displaced linearly ontemperature towards stronger fields and disappeared attemperatures above 150 K (Figure 5). At temperatures above150 K corresponding to top boundary T∗ of Griffiths-likephase, we observed only linear increase in magnetizationwith field growth, characteristic for a usual paramagneticphase. Unusual behaviour of magnetization isotherms attemperatures above 60 K apparently is caused by growth oflocal charge/orbital correlations with the temperature drop

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T(K

)

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−Hc2

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FM

Figure 7: Magnetic H-T phase diagram of the(Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics, received in strong pulsemagnetic fields.

to the critical value TCO∼= 60 K associated with transition

of the sample to a state with long-range charge-ordering.The strong magnetic field destroys both long-range charge-ordering state (T < TCO) and the state with short-rangecharge/orbital correlation (T > TCO) that results in field-induced phase transitions found in this work. It is possible toassume that sharp falling to zero of width of a hysteresis ΔHc

of the critical fields of metamagnetic phase transition withgrowth of temperature (Figure 6) is related to destructionof local correlations of orbital order ξOO(T) at temperaturesabove 110 K, whereas field-induced magnetization step ΔM,falling to zero in the range of temperatures of 60–150 K,implies destruction of local charge-order and reduction tozero of the correlation length ξCO(T) at the temperatureT∗ ∼= 150 K. Magnetic H-T phase diagramme of the(Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics had been received instrong pulse magnetic fields (Figure 7).


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/dH

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/dH

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/dH

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0.006

Figure 8: Field dependences of derivative of magnetization dM/dH received near to critical field Hc1 of transition in a ferromagnetic stateat temperatures above TCO

∼= 60 K.

Special interest is represented by additional step structureof magnetization isotherms M(H) close to critical field Hc1

where charge/orbital correlations are broken, received onlyin the rapid pulse fields with sweep rate of magnetic field≥2 T/μs. In experiment, an existence of one or two smallnarrow steps of the magnetization was shown, which arisewith growth of magnetic field near to critical value Hc1 andare absent in hysteresis curves at field reduction close to Hc2.More clearly, this feature is shown in field dependences ofmagnetization derivative dM/dH close to Hc1 in the form ofadditional narrow peaks of the derivative, having by the basispeak in width ∼20 kOe (Figure 8). These steps of isothermsof magnetization exist only at temperatures lower 110 K thatallows us to assume that the reason of their occurrence isdestruction of short-range orbital correlations forming thedomain structure of JT distortions of a crystal lattice forT < 110 K. In vicinity of TCO

∼= 60 K, there is a splittingof dM/dH single peak evidently connected with existence ofclose TCO incommensurate lattice modulation of an elasticfield JT distortions with wave number incommensurate withthe lattice constant.

4. Conclusion

The results of measurement of the temperature dependencesof ac magnetic susceptibility χ(T) of the(Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics illustrate formationof the mixed insulating state with a special feature of aquantum-disordered phase with dominating domains witha long-range antiferromagnetism at the temperatures belowTN

∼= 48, 5 K. In extremely weak magnetic field Hc1∼=

25 kOe, field-induced irreversible metamagnetic phasetransition from a mixed insulating state to metallic ferromag-netic one was found out, being related to destroy of long-range charge/orbital correlation which amplifies the localantiferromagnetic correlations of Mn spins. From theanalysis of isotherms of magnetization, the conclusion wasmade about spontaneous phase transition with charge/orbital ordering at temperatures below the critical valueTCO

∼= 60 K. At temperatures above 60 K, there arereversible field-induced metamagnetic-like transitions inthe ferromagnetic state, related to destroy of short-rangecharge/orbital correlations. It is supposed that nanoscalecorrelations of a charge exist in unusually large interval of


temperatures ∼90 K at temperatures above TCO that con-siderably surpasses area of existence of short-range orbitalcorrelations ∼50 K, creating an elastic field Jahn-Teller de-formations of the crystal lattice. An additional steps structureof magnetization isotherms close to Hc1 was found, whichexist only at magnetization in very fast pulse fields withsweep rate of magnetic field ≥2 T/μs. It is supposed theexistence of close to TCO incommensurate lattice modulationof elastic field JT distortions with wave number incommen-surate with the lattice constant.

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[13] H. Yoshizawa, H. Kawano, Y. Tomioka, and Y. Tokura, “Neu-tron-diffraction study of the magnetic-field-induced metal-insulator transition in Pr0.7Ca0.3MnO3,” Physical Review B, vol.52, no. 18, pp. R13145–R13148, 1995.

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Gd 055Sr MnO Ceramics - Hindawi