Journal of Magnetism and Magnetic Materials 328 (2013) 66–71

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Journal of Magnetism and Magnetic Materials

0304-88

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E-m

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Magnetic and dielectric properties of quasi-two-dimensionalmanganese oxide Sr7Mn4O15

S. Gao a, Y.D. Lv a, X.X. Wang a, H.S. Yang a, K.Q. Ruan a,n, X.G. Li a,b

a Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of Chinab Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 27 September 2012Available online 11 October 2012

Keywords:

Quasi-two-dimensional

Manganese oxide

Electron spin resonance

Dielectric permittivity

Spin–lattice coupling

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.10.007

esponding author. Tel.: þ86 551 3600652; fa

ail address: kqruan@ustc.edu.cn (K.Q. Ruan).

a b s t r a c t

Using dc magnetic susceptibility, electron spin resonance (ESR) and dielectric spectroscopy techniques,

we investigate the magnetic and dielectric properties of Sr7Mn4O15, which has a layered structure with

pairs of face-sharing octahedra Mn2O9. Magnetic susceptibility reveals the Neel temperature

TN�70.5 K, above which Sr7Mn4O15 manifests intense short range order. ESR spectra indicate the

existence of a transition at about 370 K, confirming the suggested spin pairing in the Mn2O9 units. In

the dielectric spectra, a sudden drop of permittivity is observed at TN, revealing the existence of strong

spin–lattice coupling in Sr7Mn4O15.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The relationship between a material’s property and its structurehas always been the central issue in condensed matter physics.A well-known example is the colossal magnetoresistance (CMR) inmanganese oxides [1]. In CMR manganites, coupling of electronicstates with the lattice vibronic modes, or the so-called Jahn–Tellerdistortion, assist the electronic localization, constructing a neces-sary ingredient for the nanoscale phase separation. Another inter-esting example can be found in the recent pursuit of multiferroics[2], where magnetism and ferroelectricity coexist and interact witheach other. In materials like hexagonal RMnO3 (R is Y or rare-earthion with small radius), atomic displacement induced by magneticexchange interaction plays the key role in their magneto-electricproperty [3]; And in materials such as CaMn7O12, this exchangeinduced displacement is even deemed as the direct cause for theirsubstantial ferroelectric polarization [4].

With regard to the magnetic properties, a crucial factor thatneeds particular attention is the structural dimension [5]. Whencompared with its three-dimensional counterparts, low dimen-sional system usually manifests stronger short-range-order (SRO)effects, which can be reflected in the measurement of thermo-dynamic quantities like specific heat and magnetization. For thelayered antiferromagnetic (AF) Sr7Mn4O15 (structure shown inFig. 1) [6–10], whose building blocks are face-sharing octahedraMn2O9 units, a cusp of the magnetic susceptibility has beenobserved at about 100 K, which can be ascribed to this SROeffect [9]. Besides this low dimensional character, it is suggested

ll rights reserved.

x: þ86 551 3601073.

that the Mn2O9 units also play a vital part in the magnetic propertyof Sr7Mn4O15. According to the conjecture in previous report [9], thetwo Mn4þ spins in the Mn2O9 may have already aligned oppositelywith each other at about 350 K, resulting in anomalous Curie–Weissfitting parameters below room temperature (meff¼4.47 mB per Mn,Y¼�490 K under 100 G). In fact, the existence of such a Curie–Weiss behavior is anomalous in itself if we compare Sr7Mn4O15

with other manganites having similar Mn2O9 units [11–14]:4H–SrMnO3 transits into AF order at temperature as high as 280 K[11]; and 4H–BaMnO3, with TN�50 K due to magnetic frustration,already shows an obvious deviation from Curie–Weiss law abovethe room temperature [14], implying strong interaction between theMn4þ spins in the Mn2O9 pair. However, apart from these phenom-enal comparisons, none direct evidence has been provided for thestrong spin interaction in Mn2O9 units of Sr7Mn4O15. The onlyknown magnetic report on this material is performed below 300 K[9], indicating the necessity for further investigation at highertemperature. Moreover, the dielectric property of Sr7Mn4O15 hasremained unexplored, demanding investigation on the possiblecoupling between magnetic and dielectric properties in this system.

In this paper, we investigate the magnetic and dielectricproperties of Sr7Mn4O15. Above room temperature, direct evi-dence for the proposed scenario of spin pairing in Mn2O9 units isclearly observed from ESR spectra. Additionally, dielectric mea-surement shows a distinct anomaly at TN, illustrating Sr7Mn4O15

as a system with a strong coupling between lattice and spin.

2. Experimental details

Polycrystalline Sr7Mn4O15 was synthesized through the organicpolymeric gel route [15]. Dried SrCO3 was dissolved in aqueous

Fig. 1. (a) The crystal structure of Sr7Mn4O15 viewed along the c axis, showing the

layers formed along the bc plane. (b) A unit cell of Sr7Mn4O15 viewed along the

a axis, showing the face-sharing octahedra Mn2O9 units.

Fig. 2. Observed (crosses) and calculated (solid line) X-ray diffraction intensities

for Sr7Mn4O15 at 300 K. The vertical marks show the Bragg peak positions and the

bottom curves show the difference between observed and calculated intensities.

Agreement factors are Rwp¼0.0769 and Rp¼0.0593.

Fig. 3. The dc magnetic susceptibility w(T) measured in 1 kOe under ZFC and FC

conditions. The solid line shows the Curie–Weiss fit for ZFC in the temperature

range of 200–300 K.

S. Gao et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 66–71 67

solution of citrate acid, where stoichiometric Mn(NO3)2 and ethy-lene glycol were added. During this process, several drops of nitriteacid were added to adjust the PH in order to prevent precipitation.The resulted solution was stirred for 4 h before heated at 130 1C for30 h to form a gel, which was then transferred to furnace andheated to 600 1C to decompose the organic component. Theobtained ash was ground, pelletized, and fired at 1300 1C for 2 daysand subsequently quenched to room temperature at 1000 1C.Powder x-ray diffraction (XRD) with CuKa radiation was performedon Rigaku D/Max-rA to characterize the purity and crystal structureof the sample. Dc magnetization was obtained using a QuantumDesign SQUID over the temperature range 10–380 K in an appliedfield of 1000 G. Bruker ER200D spectrometer at 9.07 GHz (X band)was employed to record the ESR spectra for loose-packed crushedcrystals through 100–470 K. For the dielectric measurements, thepellets of Sr7Mn4O15 were polished and deposited with Au electro-des on both sides. Dielectric properties were recorded with anAgilent 4294A precision impedance analyzer in the frequencywindow of 1 k–1 MHz.

3. Results and discussion

Fig. 2 displays the powder XRD pattern of Sr7Mn4O15 at roomtemperature. Results of Rietveld refinement [16,17] carried out inspace group P21/c are also shown. The obtained lattice parametersare a¼6.8187 A, b¼9.6218 A, c¼10.3791 A, b¼91.88951, whichagree well with previous reports [6,9]. As mentioned earlier,Sr7Mn4O15 forms layers in the bc plane, whose building blocks

are the face-sharing MnO6 octahedra (Mn2O9 units). At roomtemperature, the Mn–Mn distance in the Mn2O9 units is 2.57 A,whereas it is about 3.73 A between different Mn2O9 units.Besides, the shortest Mn–Mn distance between adjacent layersis equal to 5.71 A, making inter-layer exchange interaction muchweaker than that of intra-layer. Due to these distinctive structuralcharacters, we expect unique magnetic and dielectric behavior inSr7Mn4O15.

3.1. Magnetic susceptibility

The dc magnetic susceptibility w(T) is shown in Fig. 3 as afunction of temperature. The measurements were taken duringwarming after cooling the sample at zero field (ZFC) or at 1000 G(FC). With decreasing temperature, w gradually increases, forming acusp around 100 K, and then drops sharply, indicating an antiferro-magnetic (AF) transition. In a previous study [9], the broad max-imum around 100 K was claimed to cover the AF transitiontemperature. However, when taking the low-dimension structuralcharacter into consideration, an exact transition point can still beobtained from w(T) measurements. In low dimensional system, thestatic pair correlation functions GrðTÞ ¼ 3/Sz

0SzrS=S Sþ1ð Þ (r¼0, 1,

2, y, N) are effective tools to depict the evolution of short-rangeorder [5]. For a finite distance of r, Gr will exhibit an inflexion at thetransition temperature, resulting in a negative maximum in itsderivative. Using such correlation functions, zero field magneticsusceptibility for AF system can be expressed as

wT=C ¼ 1þXra0

GrðTÞ � 1�f ðTÞ9G1ðTÞ9 ð1Þ

where f(T) is a slow varying function of the order unity. It can beseen from Eq. (1) that at TN, a maximum for the derivative of wT willemerge, and the corresponding result for Sr7Mn4O15 is presented inFig. 4. The expected transition peak can be observed at T¼70.5 K,which agrees well with the Neel temperature of 75 K obtained frompowder neutron diffraction [9].

Now we check the magnetic property in the range of 150–380 K. It is interesting to find that Sr7Mn4O15 obeys the Curie–Weiss (CW) law in this temperature range and the fit is shown inFig. 3 as the solid line. The best fit gives the CW parameters ofeffective magnetic moment meff¼4.40 mB per Mn and Weisstemperature Y¼�401 K. Compared with the spin-only magneticmoment of 3.84 mB for Mn4þ , the fitted meff is abnormally large insuch a non-frustrated system [9]. To ensure the intrinsic nature ofthis unusual Curie–Weiss behavior, the temperature derivativeof 1/w is plotted in Fig. 4. If impurities like the diamagnetic one

Fig. 4. Temperature dependence of the temperature derivative of wT (left axis) and

1/w (right axis).

Table 1Comparison for manganese oxides with similar face-sharing octahedral Mn2O9 units.

Listed terms are the Mn–Mn distance in the Mn2O9 unit (Intra Mn–Mn distance),

existence of frustration (Frustration), the occurrence of Curie–Weiss behavior under

room temperature (CW behavior) and the transition temperature (TN).

Manganites Intra Mn–Mn

distance (A)

Frustration CW

behavior

TN (K) Reference

4H–SrMnO3 2.500 � � �278 [11]

6H–SrMnO3 2.511 Ja� 235 [12]

2H–BaMnO3 2.407 J � 59 [13]

4H–BaMnO3 2.532 J � 50 [14]

Sr7Mn4O15 2.569 � J 70.5 —

a The ratio 9Y9=TN � 3:2 in cited literature indicates that frustration in

6H–SrMnO3 should be very weak.

Fig. 5. Typical ESR spectra of Sr7Mn4O15 measured at 110 K, 200 K and 450 K.

S. Gao et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 66–7168

exist [18], an artificial CW fit would lead to unrealistic fittingparameters, and dð1=wÞ=dT should exhibit an obvious temperaturedependence. Whereas for the present system shown in Fig. 4,dð1=wÞ=dT stays invariant above 150 K, excluding the extrinsicimpact on the CW fitting parameters.

In order to understand this anomalous fitting parameter, we referto the special character of face-sharing octahedral in Sr7Mn4O15. Asdescribed earlier, the Mn–Mn distance in the Mn2O9 units is muchshorter than that of neighboring units, leading to a stronger exchangeinteraction between the inner spin pairs. Under such a stronginteraction, two Mn4þ spins in the Mn2O9 units might haveoppositely aligned with each other at a temperature much higherthan TN, which greatly suppressed the magnetic susceptibility. There-fore, the anomalous meff can be attributed to this w(T) suppressingeffect and might be viewed as an indirect evidence for the Mn4þ spinpairing in Mn2O9 units [9].

Indirect support for this spin-pairing scenario can also beobtained through comparison with other manganese oxides thatpossess similar face-sharing MnO6 octahedra units. Table 1 listsfour typical systems that contain Mn2O9 units, where their mag-netic properties and the average Mn–Mn distance in Mn2O9 areshown. In contrast to Sr7Mn4O15, none of these manganites exhibitCurie–Weiss behavior below room temperature, indicating thestrong interaction of Mn spins in the Mn2O9 dimer. Additionally,as pointed out in the introduction part, systems free from frustra-tion all have a much higher transition temperature than that ofSr7Mn4O15. Thus it can be concluded that the exchange interactionin Mn2O9 units is able to align the Mn4þ spin pairs at a temperaturemuch higher than 70 K, making it reasonable to attribute thesuppressed w(T) in Sr7Mn4O15 to the Mn4þ spin pairing effect.

3.2. Electron spin resonance

Electron spin resonance has been proved to be a powerfultechnique in the study of manganites as it provides a local probeof spin dynamics, revealing the coupling of spins with theirsurrounding environments [19–21]. Fig. 5 displays some typicalESR spectra obtained on powdered Sr7Mn4O15 from the tempera-ture range of 100–470 K. In ESR, the power P absorbed from thetransverse magnetic microwave field is measured as a function ofthe static magnetic field H, and a lock-in technique detecting thederivative dP/dH is used to improve the signal-to-noise ratio.Within the whole temperature range, the spectrum consists of abroad symmetric resonance line which can be fitted by

dP

dH¼ A�

d

dH

DHþa H�H0ð Þ

H�H0ð Þ2þDH2

þDHþa HþH0ð Þ

HþH0ð Þ2þDH2

!ð2Þ

where H0 is the resonance field, DH the linewidth, A the intensityfactor, and a the dispersion-to-absorption ratio. This ratio a isincluded to check the symmetry of the spectrum, and fittingresults show that it is below 10�7 in the whole temperaturerange, leading to Lorentzian lineshape and reflecting the insulat-ing property of Sr7Mn4O15 (rE2�107 O cm at 20 1C) [7]. In thefit, resonance at the reversed magnetic field �H0 is included dueto the broad linewidth [20]. g factors calculated from resonancefield H0 remain at a constant value of 1.97, as expected forantiferromagnetic compounds.

The fitting results of intensity factor A and linewidth DH areplotted in Fig. 6 as a function of temperature. Since ESR is ameasure of the imaginary part of the dynamic susceptibility, itsintensity usually displays similar temperature dependence withdc magnetic susceptibility [22]. Surprisingly, although the ESRintensity conform with the Curie–Weiss law in the range of 200–350 K, a deviation is clearly observed at T4370 K. Following theproposal of spin–pairing transition in Mn2O9 units at about 350 K[9], we ascribe this observed deviation to the spin–pairing effect.As shown in Fig. 6(a), with decreasing temperature, the increasingrate of ESR intensity is obviously reduced across this transitionpoint, corroborating the Mn2O9 spin–pairing as the origin for asuppressed w(T) and its anomalous CW fitting parameter. How-ever, it should be noted that the CW fit for ESR intensity results ina huge Weiss temperature of �949 K, which is nearly double ofthat derived from magnetic susceptibility. Such a disparity mayarise from grain boundary defect [23] but it does not affect ourconclusion in the present work.

Evidence for the Mn2O9 spin–pairing scenario can also bededuced from analysis of ESR linewidth, which is a measure of

Fig. 6. (a) Temperature variance of the lineshape parameters: (a) the intensity

factor IESR and (b) the linewidth DH. The solid line in panel (a) shows the Curie–

Weiss fit for the range of 200–300 K. Two ESR experiments using different sample

mass are performed above room temperature to confirm the variance of DH, which

result in more data points for panel (b).

Fig. 7. Temperature dependence of (a) DH � IESR � T and (b)DH � ICW � T. ICW is

the ESR intensity extrapolated from the lower temperature CW behavior. The solid

lines are linear fits in the range of 200–300 K. Gray vertical line indicates the

deviation temperature of 370 K.

S. Gao et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 66–71 69

relaxation rate for the total spin. Fig. 6(b) shows that as thetemperature is lowered from the higher side, the linewidthdecreases, reaching a minimum value of 180 G at around 120 K,and then tends to turn upwards in the lower temperature due tocritical fluctuation [24]. For the higher temperature part, we plotthe temperature dependence of DHðTÞIESRT in Fig. 7(a), where thefactor IESRT is included to remove the temperature dependence ofmagnetic susceptibility and the ESR intensity is used instead ofw(T) to account for any possible defect factor [25,26]. According torelated ESR linewidth theory in magnetic insulators [27], for theparamagnetic region TbTN that is free from critical fluctuation,DHðTÞIESRT should exhibit a linear temperature dependence if thephonon-modulation of crystalline field dominates the relaxationof total spin. However, as shown in Fig. 7(a), although lineardependence can be observed for the temperature range fromabout 2 TN to 350 K, above this a deviation sets in, making thelinear dependence quite dubious for conventional explanation inparamagnetic regime. Additionally, even when the anomaly inIESR at 370 K is excluded, meaning that the same CW dependenceof IESR as that of 200 to 330 K is assumed for the higher temperature,the deviation is still quite obvious in Fig. 7(b). Thus the paramag-netic nature is negated for the region below 370 K, evidencing thespin pairing for the face-sharing MnO6 in this temperature range.Finally, it is interesting to note that in Fig. 7(b) the deviation inlinewidth occurs nearly at the same temperature as in Fig. 7(a) evenwhen a whole range CW behavior is assumed, indicating a directinfluence of spin–pairing on the relaxation of Mn spins. Furtherexperimental and theoretical work is needed to understand therelaxation mechanism in Sr7Mn4O15 around this deviation, espe-cially the effect of spin–pairing below 370 K.

3.3. Dielectric spectroscopy

To study the possible magnetodielectric effect, the dielectricpermittivity (e) around the Neel temperature for Sr7Mn4O15 is

measured in magnetic field up to 8 T and the results are shown inFigs. 8 and 9. For the measured temperature range, the dissipationtgd stays near zero at about 10�4 (not shown) and no relaxationprocess can be observed, as expected for systems with largeresistance [28]. As shown in Fig. 9(b), magnetic fields have littleeffect on the dielectric permittivity. However, at the magnetictransition temperature, a sudden drop in e (T) can be observed.Similar with the anomaly discussed in other systems [29–31], themagnitude of De/e is very large, reaching up to 0.09% below TN,which is nearly two orders higher than that of normal magneto-striction effect. Thus the pure geometric origin for this drop canbe excluded and the coupling of dielectric properties to themagnetic order should be considered.

Sr7Mn4O15 belongs to crystal point group 2/m, and its mag-netic point group is also 2/m since the Mn spins are alignedantiferromagnetically along the b axis [9]. According to thesymmetry analysis [32], gE2L and dE2L2 are the lowest couplingterms allowed in its free energy expression F, where E is theelectric field and L the Neel order parameter for antiferromagnet[33], g and d the coupling constants. Thus the dielectric suscept-ibility is given by

we ¼�@2F

@E2

�����E ¼ 0

¼ wð0Þe �gL�dL2ð3Þ

where the first term describes the bare dielectric susceptibilitywithout magnetic influence and the temperature dependence forits corresponding dielectric permittivity can be fitted with amodified Barrett equation

e Tð Þ ¼ e 0ð ÞþA

exp _$0=kBT� �

�1ð4Þ

here e(T) represents the bare dielectric permittivity, A is aconstant and o0 an appropriate transverse optical phonon-modefrequency. Using the data above TN, the best fit can be obtained

Fig. 8. Temperature dependence of dielectric permittivity. The solid line is the fit

to the modified Barrett equation for T480 K. The inset shows the temperature

variance of the ratio De/e74 K.

Fig. 9. Temperature variance of dielectric permittivity under. (a) different

frequency and. (b) different magnetic field.

S. Gao et al. / Journal of Magnetism and Magnetic Materials 328 (2013) 66–7170

with e (0)¼10.06, A¼0.1499, o0¼113.6 cm�1 and the result isshown in Fig. 8 as the solid curve. By subtracting this baredielectric permittivity, temperature dependence of De/e wasobtained and shown in the inset. Due to perhaps the grainboundary defect and dielectric anisotropy [31,34], temperaturedependence of De/e is not regular enough for quantified analysis.However the increasing trend below TN is obvious, correspondingto the increasing of the ordered magnetic moment on the Mn siteswith lowering temperature [9].

With reference to the temperature variance of crystallographicdata refined from neutron diffraction [9], a microscopic mechan-ism for this sudden drop in e (T) can be proposed. Due toexchange–striction effect, changes as large as several percentsare observed for Mn–Mn and Mn–O distances above and belowTN. According to Lyddane–Sachs–Teller relation [34], the dielectric

permittivity in insulators can be expressed as

ee1¼Y

i

n2li

n2ti

ð5Þ

where nli and nti are frequencies of longitudinal and transverseoptical modes, and eN is the optical dielectric permittivity.Therefore, it is possible that exchange–striction induced atomicdisplacements shift the frequencies of optical modes and finallyresult in a drop in dielectric permittivity. Recently, such amechanism has been directly confirmed in TbFe3(BO3)4 withRaman spectroscopy [31]. Further investigation should be takento verify the existence of a similar mechanism in Sr7Mn4O15.

4. Conclusions

In summary, we have investigated the magnetic and dielectricproperties of Sr7Mn4O15. The evolution of ESR spectra corrobo-rates the proposed scenario of AF spin pairing for the Mn spin inits face-sharing octahedra Mn2O9 units, whose occurrence attemperature as high as 370 K arises from the very short Mn–Mndistance in Mn2O9. At lower temperature, the quasi-two-dimensional structure of Sr7Mn4O15 results in a characteristicbroad w(T) maximum just above its long-range AF transitiontemperature of 70.5 K, which transition is also reflected in thedielectric spectra as a sudden drop. The origin of this correlationbetween magnetic and dielectric properties is attributed to theexchange–striction effect, revealing a strong spin–lattice couplingin Sr7Mn4O15.

Acknowledgments

This work is supported by the National Basic Research Programof China (No.2012CB922003).

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