PHYSICAL REVIEW B 85, 174414 (2012)

Lattice distortion accompanied by magnetization reversal in A-type antiferromagnetic manganites

Jong-Suck Jung, Ayato Iyama, Hiroyuki Nakamura, Yusuke Wakabayashi, and Tsuyoshi KimuraDivision of Materials Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

(Received 23 February 2012; published 11 May 2012)

Magnetostriction was investigated for layered A-type antiferromagnetic SmMnO3 showing large magnetoca-pacitive effects around a temperature (TTP) where ferrimagnetically coupled Mn 3d and Sm 4f moments werereversed simultaneously. Upon sweeping temperature or a magnetic field, a significant lattice distortion wasobserved at TTP or the coercive field, respectively. This indicates that the lattice is strongly coupled with themagnetic configuration. We discuss the lattice distortion accompanied by the magnetization reversal in terms ofa partial change in the orbital state of Mn eg electrons.

DOI: 10.1103/PhysRevB.85.174414 PACS number(s): 75.80.+q, 75.85.+t, 75.60.−d

I. INTRODUCTION

Studies of a new type of multiferroics in which ferroelectric-ity is driven by a magnetic order have received considerableinterest in terms of both fundamental science and potentialapplication to novel memory devices.1,2 A typical exampleof the multiferroics is the perovskite manganites RMnO3

(R = rare-earth ions) with the Pbnm orthorhombic structure. Inmultiferroic manganites, ferroelectricity is induced by a mag-netic order into a noncollinear spiral3,4 or collinear E-type5,6

antiferromagnetic (AFM) structure, breaking the inversionsymmetry through the inverse Dzyaloshinskii-Moriya (DM)mechanism7–9 or the exchange striction.10,11 These two mag-netic structures become stable in RMnO3 (R = Gd,Tb, . . . ,Tm) with relatively small ionic radii of R ions rR , whereas,the ground state of RMnO3 (R = La,Pr, . . . ,Eu) with large rR

is the layered A-type AFM one which preserves the inversionsymmetry. Thus, in terms of the multiferroic research, theA-type AFM RMnO3’s had never attracted much attentiondue to their nonferroelectric nature.

Recently, however, it has been reported that the Neel N -typeferrimagnetic SmMnO3, one of the A-type AFM RMnO3’s,shows large magnetocapacitive effects12 despite a lack ofspin-driven ferroelectricity. In SmMnO3, Mn moments showthe A-type AFM order at TN ≈ 60 K as illustrated in theinset of Fig. 1.13,14 Because of the DM interaction,15,16 theA-type AFM structure becomes canted with finite cantingangle �CA, and then the system shows weak ferromag-netism along the c axis below TN.17,18 Furthermore, in alow magnetic field applied along c (Bc < ∼1 T), SmMnO3

shows temperature (T )-induced magnetization reversal at thecompensation temperature (Tcomp ≈ 9.4 K), below whichmagnetization along c (Mc) becomes negative13 [see Fig. 1(a)].This T -induced magnetization reversal can be attributed to thedevelopment of the Sm moments’ polarization being alignedantiparallel to the direction of net Mn moments (and of Bc)due to the antiferromagnetic Sm-Mn exchange interaction.13

In higher Bc (> ∼1 T), by contrast, T profiles of Mc show anabrupt change with thermal hysteresis at a temperature (TTP)around Tcomp, suggesting that a first-order-like phase transitiontakes place at TTP [see the 4 and 8T data in Fig. 1(a)].12

Accompanied by the Mc anomalies, the dielectric constantalong the c axis (ε′

c) displays a remarkable jump at TTP [seeFig. 1(b)].12 Furthermore, as seen in isothermal Mc [Fig. 1(c)]and magnetocapacitance [Fig. 1(d)] curves, the magnetization

reversal at the coercive field (Bcoer) accompanies a suddenchange in ε′

c. Here, a distinct behavior is that the butterflyshaped ε′

c-Bc curves below and above TTP exhibit signsopposite each other, which indicates the opposed magneticconfigurations in the two T ranges for the SmMnO3 system. Ithas been discussed that the anomalies in Mc and ε′

c at TTP

(or Bcoer) are ascribed to the simultaneous reversal of theferrimagnetically coupled Mn 3d and Sm 4f moments and theresultant sudden change in the canting angle of Mn moments.12

More recently, Cheng and co-workers reported that theirmeasurements of in-field x-ray diffraction and specific heat forSmMnO3 showed no evidence that the transition at TTP is firstorder (e.g., volume change and latent heat) despite its abruptnature.19 To further examine the transition nature at TTP, westudy magnetostrictive effects for single crystals of SmMnO3

by measurements of the striction L with high resolution (�L/L < ∼10−5). Our striction data demonstrate the presence ofa discontinuous lattice distortion at TTP. We discuss the originof the lattice distortion at TTP in terms of the change in orbitaloccupancy of Mn eg electrons.

II. EXPERIMENTAL PROCEDURES

A single crystal of SmMnO3 was grown by the floating-zonemethod. The growth was carried out with the use of a halogen-lamp image furnace at a growth rate of 8 mm/h in a flow ofN2 gas. The obtained crystals were oriented by using Lauex-ray diffraction patterns and were cut into rectangular plates(≈4 × 4 × 2 mm3) with the widest faces along crystallographicthree directions, the a, b, and c axes, in the Pbnm setting,respectively. For the striction (�L/L) measurements, uniaxialstrain gauges with a length of 1 mm were used, which wereattached to the widest face of the samples. Magnetic fields (Bc)were applied only along the c axis. To minimize the unwantedmagnetoresistive effect of the gauges, the conventional activeand dummy (a copper plate) method was carried out. Theobtained striction data were corrected by subtracting thethermal expansion of copper.20 Magnetization (Mc) and thereal part of the dielectric constant (ε′

c) were also measuredas reported previously.12 For comparison with the SmMnO3

results, we also grew a single crystal of EuMnO3, whichwas unaffected by rare-earth Eu moments to its magneticproperties, and measured the magnetostriction along the c axis.

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JUNG, IYAMA, NAKAMURA, WAKABAYASHI, AND KIMURA PHYSICAL REVIEW B 85, 174414 (2012)

FIG. 1. (Color online) T dependence of (a) Mc and (b) ε′c (10 kHz) measured at selected B, and B dependence of (c) Mc and (d)

magnetocapacitance (10 kHz) measured at 5 and 12 K, below and above Tcomp (≈9.4 K) for single crystals of SmMnO3, respectively. For allthe measurements, magnetic fields were applied along the c axis (B||c). The inset illustrates crystal and magnetic structures of A-type AFMRMnO3. Red arrows denote Mn moments, and �CA corresponds to the canting angle of the Mn moments.

III. RESULTS

Figures 2(a)–2(c) show T dependence of the strictionparallel to the a, b, and c axes [�La(T )/La (70 K),�Lb(T )/Lb (70 K), and �Lc(T )/Lc (70 K)], respectively, inthe absence and presence of applied Bc. All the striction datawere measured upon the heating process. First, let us focus onthe data at Bc = 0 T. With decreasing T , an anomaly is evidentat TN in the data of �La(T )/La (70 K) and �Lb(T )/Lb (70 K),whereas, no anomaly is observed in �Lc(T )/Lc (70 K). Withfurther lowering of T , �La(T )/La (70 K) steadily elongates,whereas, �Lb(T )/Lb (70 K) and �Lc(T )/Lc (70 K) graduallycontract. A similar magnetostrictive effect, i.e., the elongationalong a and the contraction along b below TN, has beenobserved in a typical A-type AFM manganite, LaMnO3, by aneutron-diffraction measurment.21 Namely, the A-type AFMorder in the manganites gives rise to a lattice distortion,which is the most pronounced within the ab plane. Thislattice distortion can be understood in terms of the exchangestriction.22,23 The development of the A-type AFM orderwith the ferromagnetic coupling within the ab plane probablyexpands the Mn-O-Mn bond angle ϕ in the ab plane throughthe exchange striction. In fact, the a and b axes in RMnO3

with large ϕ are longer and shorter than those in RMnO3 withsmall ϕ, respectively [e.g., a = 5.5367(1) A, b = 5.7473(1) A,ϕ = 155.11(5)◦ in LaMnO3 (Ref. 24); a = 5.3548(6) A, b =5.8131(7) A, ϕ = 147.53(5)◦ in SmMnO3 (Ref. 25)]. In light ofthese relationships, the present observation, i.e., the elongationof a and the contraction of b at TN, can be reasonably explained.

Next, we focus on the striction data in Bc = 4 and 8 T.At TTP < T < TN, though the effect of Bc on �La(T )/La

(70 K) is negligibly small, �Lb(T )/Lb (70 K) and �Lc(T )/Lc

(70 K) gradually contract and elongate with increasing Bc,

respectively. Another noteworthy feature in the in-field stric-tion data is that sudden jumps appear in all the directions at TTP

where Mc-T and ε′c-T curves show discontinuous anomalies.

As seen in the insets of Fig. 2, �La(T )/La (70 K) and�Lb(T )/Lb (70 K) show a discontinuous jump at TTP towardlower temperatures, whereas, �Lc(T )/Lc (70 K) suddenlydrops at TTP. With increasing Bc, TTP approaches Tcomp inthe manner of Mc and ε′

cin Figs. 1(a) and 1(b), and thechange in �L(TTP)/L increases in all directions. The largestmagnetostrictive effect is observed in �Lc(TTP)/Lc. Then, incontrast with the data at T > TTP, �La(T )/La (70 K) and�Lb(T )/Lb (70 K) elongate, whereas, �Lc(T )/Lc (70 K)contracts with increasing Bc at T < TTP. Though no latticeanomaly at TTP had been observed in the previous in-fieldx-ray diffraction measurement at B = 5 T,8 our strictionmeasurement with higher resolution clearly demonstrates thepresence of the lattice anomaly at TTP.

To further examine the B effects on the striction behaviorfor SmMnO3, we measured the isothermal magnetostrictionalong the a, b, and c axes [�La(B)/La , �Lb(B)/Lb,and �Lc(B)/Lc]. Figure 3 shows (a) �La(B)/La , (b)�Lb(B)/Lb, and (c) �Lc(B)/Lc as functions of Bc at 5 and 12K, i.e., below and above TTP. One of the remarkable featuresin Fig. 3 is that all the striction data except for �La(B)/La

at 12 K display drastic changes at around Bcoer, ±1.2 T for5 K and ±2.7 T for 12 K [compare Fig. 3 and Fig. 1(c)],respectively. Though no anomaly was observed in �La(B)/La

at 12 K (above TTP), the result was consistent with that in the T

dependence of �La(T )/La (70 K) shown in Fig. 2(a) in whichthe negligibly small B effect was observed at TTP < T < TN.The drastic changes in �L(B)/L at Bcoer mean that the magne-tization reversal also affects the lattice. The magnitude of the

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FIG. 2. (Color online) T profiles of the striction parallel to the(a) a, (b) b, and (c) c axes [�La(T )/La (70 K), �Lb(T )/Lb (70 K),and �Lc(T )/Lc (70 K)] at selected Bc, respectively. All the datawere measured during the heating process. The insets show themagnification around Tcomp. Magnetic fields are applied along thec axis (B||c).

magnetostriction along the c axis around Bcoer is on the orderof 10−4, which is 1 order of magnitude larger than that alongthe a and b axes. Owing to the drastic change in �L(B)/L atBcoer, the isothermal magnetostriction curves form butterflyshapes. In all the configurations, �L(B)/L monotonicallyincreases or decreases in all B regions except for Bcoer. Anothernoteworthy feature in Fig. 3 is the sign of the magnetostriction.At 12 K (above TTP), �Lb(B)/Lb shows a sudden decreaseat Bcoer toward higher B, whereas, �Lc(B)/Lc exhibits anabrupt increase. At 5 K (below TTP), by contrast, �Lb(B)/Lb

FIG. 3. (Color online) Isothermal magnetostriction curves paral-lel to the (a) a and (b) b axes [�La(B)/La and �Lb(B)/Lb] measuredat temperatures 5 and 12 K (below and above TTP), respectively,for SmMnO3. (c) Corresponding the curves parallel to the c axis[�Lc(B)/Lc] at 5 and 12 K for SmMnO3 (thick red and blue lines)and EuMnO3 (thin black lines), respectively.

increases, whereas, �Lc(B)/Lc decreases. Related to theopposite feature, the signs of the butterfly-shaped magne-tostriction curves are reversed above and below TTP. Thiscontrasting butterfly shape in the magnetostriction remindsus of the magnetocapacitance result in Fig. 1(d).

Additionally, we also measured the magnetostriction ofanother A-type AFM manganite EuMnO3 in which the rare-earth moments do not contribute to its magnetic properties.The thin black lines in Fig. 3(c) show the �Lc(B)/Lc dataof EuMnO3 measured under the same conditions as those ofSmMnO3. The �Lc(B)/Lc at 12 K in EuMnO3 is similar tothat in SmMnO3 in terms of the butterfly-shaped hysteresis andits sign. At 5 K in EuMnO3, however, the sign of the butterflyshaped �Lc(B)/Lc curve is the same as that at 12 K, which isin sharp contrast with the result of SmMnO3. The magnitude

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of the magnetostriction in EuMnO3 is much smaller than thatin SmMnO3 as well. These results suggest that, whereas, the4f moments may enhance the magnetostrictive effect, the Mnmoments alone can give rise to the effect.

IV. DISCUSSION

By considering the magnetization and magnetostrictionresults, we infer a close connection between the magnetostric-tion and the canting angle of the weakly ferromagnetic Mnmoments �CA. The observed monotonic change in �L(B)/Lmay be ascribed to the monotonic change in �CA by applyingBc. In this case, the existence of the sudden change inL(B)/L at Bcoer suggests that the magnetization reversalaccompanies the change in �CA. Furthermore, the sign changein the magnetostriction between the data above and below TTP

supports a former proposal8 that the moment configurationscomposed of the ferrimagnetically coupled Sm and Mnmoments are opposite at temperatures below and above TTP.

Let us discuss the microscopic mechanism of the observedlattice distortion. A possible driving force to induce thelattice distortion is the DM interaction. In the A-type AFMmanganites, the spin canting toward the c axis originates fromthe DM interaction; the change in the canting angle �CA

may shift the position of oxygen ions intervening betweenadjacent Mn moments along the c axis through the inverseDM effect. At TTP < T < TN, the net magnetization due tothe canted Mn moments is aligned along Bc, and then theapplication of Bc should increase �CA and should enhancethe DM interaction. The enhancement of the DM interactionmay displace the oxygen ions further away from the midpointof adjacent Mn moments. As a result, the buckling of MnO6

octahedra should be enhanced by applying Bc. Then, the c

axis should be shortened. However, the changes in �L(T )/Lobtained by the present experimental results contradict the DMscenario. Thus, the DM scenario does not explain the observedlattice distortion.

A more plausible driving force to induce the latticedistortion is the change in the orbital occupancy of eg electronsin Mn sites. In RMnO3, the occupied Mn eg orbital wavefunction on the two inequivalent sites can be expressed by

|±〉1 = cosθ

2|3z2 − r2〉 ± sin

θ

2|x2 − y2〉, (1)

where θ is the respective orbital component (see the lowerpanels of Fig. 4). When θ = 120◦, d3x2−r2 and d3y2−r2

orbitals are staggeredly ordered within the ab plane. ForSmMnO3, θ ≈ 114◦, which is obtained by the semiempirical

FIG. 4. (Color online) Proposed orbital-occupancy change accompanied by the lattice distortion at TTP in Bc > 1 T. (Upper panels)Schematics of the Mn moments and the Mn d3x2−r2/d3y2−r2 and d3z2−r2 orbital arrangements in the distorted MnO6 octahedra at TTP < T < TN

(left) and T < TTP (right). The lattice distortion observed at TTP is also illustrated (middle). The grayscale intensity in the orbitals representsthe occupancy of Mn eg electrons. (Lower panels) Proposed θ corresponding to the orbital state of the Mn electrons at TTP < T < TN (left) andT < TTP (right).

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LATTICE DISTORTION ACCOMPANIED BY . . . PHYSICAL REVIEW B 85, 174414 (2012)

estimate from the structural data,20 infers [cf. θ ≈ 108◦for LaMnO3 (Ref. 21)]. Apart from the DM interaction,it has been discussed that the competition between theAFM exchange and the local anisotropy can account forthe alternating tilt of the magnetization direction, whichgives rise to a small resulting moment along the c axis.13

The anisotropy may be reflected by the orbital occupancythrough the spin-orbit coupling. Hence, it is possible toconsider that the application of Bc at TTP < T < TN increasesthe canting angle �CA, enhances the c-axis component inthe magnetization, and gives rise to the partial transfer ofthe eg electrons from the dx2−y2 to the d3z2−r2 state (i.e., thedecrease in θ ). Then, by applying Bc, the elongation ofLc is induced by the increase in d3z2−r2 orbital occupancy,whereas, the contraction of the b axis arises from thedecrease in d3x2−r2/d3y2−r2 orbital occupancy at TTP < T <

TN.This orbital-occupancy scenario also well explains

the discontinuous change in L at TTP (or Bcoer) where thesimultaneous reversal of Sm and Mn moments occurs. Thenet magnetization owing to the canted Mn moments is parallelto Bc above TTP, whereas, it is antiparallel to Bc below TTP

(Ref. 12) so that the canting angle above TTP should be largerthan that below TTP. This consideration reasonably explainsthe experimental results, that is, the sudden jumps in La andLb and the abrupt drop in Lc at TTP upon cooling temperature.Figure 4 displays schematics, which summarize the latticedistortions explained by the orbital-occupancy scenario.

V. SUMMARY

We investigated the magnetostrictive effect of SmMnO3

with the A-type antiferromagnetic ground state by means ofthe strain gauge measurement. When magnetic fields above 1T were applied along the c axis, a remarkable lattice distortionwas observed at the temperature where the simultaneousreversal of the ferrimagnetically coupled Sm 4f and Mn3d moments occurred, and at the coercive field where themagnetization reversal arose. A similar but smaller latticedistortion at the coercive field was also observed in A-type antiferromagnetic EuMnO3 in which only Mn momentscontributed to its magnetic properties. This indicates thatthe Mn moments alone can induce the magnetostriction andthe rare-earth moments may enhance the effect. We explainthe origin of the magnetostriction in terms of the relativeorbital-occupancy change in Mn eg electrons.

ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for SpeciallyPromoted Research (Grant No. 20001004) and by the GlobalCOE Program (Core Research and Engineering of AdvancedMaterials-Interdisciplinary Education Center for MaterialsScience) from the Ministry of Education, Culture, Sports,Science and Technology (MEXT), Japan. It was also supportedby a Grant-in-Aid for Young Scientists (S) (Grant No.20674005) from the Japan Society for the Promotion ofScience (JSPS).

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