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Scripta Materialia 67 (2012) 316–319

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Room temperature magnetoelectric coupling enhancementin Mg-substituted polycrystalline GdFeO3

Jyoti Shah and Ravinder Kumar Kotnala⇑

National Physical Laboratory, Council of Scientific and Industrial Research, Dr. K.S. Krishnan Road, New Delhi 110012, India

Received 19 January 2012; revised 26 April 2012; accepted 1 May 2012Available online 7 May 2012

The magnetic, dielectric and magnetoelectric coupling coefficients of GdFe1-xMgxO3 (x = 0.00, 0.02, 0.04 and 0.06) have beenmeasured at room temperature. An increase in lattice constants and cell volume was observed by X-ray diffraction. The Neel tem-perature of GdFeO3 was reduced to 356 �C from 401 �C by Mg substitution. The dielectric constant increased from 0.011 to 123.70with Mg substitution. The magnetoelectric coupling coefficient, a, observed for GdFeO3 was 0.76 mV cm�1 Oe�1 and increased to1.67 mV/cm-Oe with x = 0.06 Mg substitution at room temperature.� 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Antiferromagnetism; GdFeO3; Neel temperature; Multiferroics; Magnetoelectric coupling

The trend towards device miniaturization has in-creased interest in research into multifunctional materi-als. In view of this, extensive research is being carriedout in order to understand magnetoelectric coupling inmultiferroic materials [1–5]. Multiferroic materials areinteresting not only because they exhibit ferroelectricand magnetic properties but also due to their “magneto-electric effect” (ME), whereby induced electrical polari-zation and magnetization can be mutually controlledby applying a magnetic and an electric field, respectively.The ME effect in any magnetoelectric material is mea-sured in terms of the voltage induced by applying a mag-netic field [6–8]. Since the prediction of the ME effect byPierre Curie in 1894, numerous research papers on mul-tiferroic compounds have been published [9]. Most ofthe multiferroic compounds either in single or compositeform exhibit strong ME coupling due to multiple spinarrangements. It is commonly observed in antiferromag-netic compounds [10,11]. The ME coupling coefficient inmultiferroic compounds was established experimentallyby Rivera [12], and enabled the linear, a, and quadratic,b, coupling coefficients of the ME effect to be deter-mined. The first experimentally observed ME couplingin single-phase antiferromagnetic Cr2O3 showedazz = 4.13 psm�1 [13]. The ME effect in various com-pounds and composites has been widely investigated

1359-6462/$ - see front matter � 2012 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2012.05.003

⇑Corresponding author. Tel.: +91 11 45608599; fax: +91 1145609310; e-mail: rkkotnala@nplindia.org

[14]. Orthoferrites (RFeO3) exhibit complex magneticproperties due to the interaction of two interpenetratingsublattices of R3+ and Fe3+ in a molecular field treat-ment [15]. Tokunga et al. induced magnetization inGdFeO3 single crystal by the application of electric field,and vice versa [16]. They swept the electric field from+15 to �15 kV cm�1 during sample magnetization mea-surement and observed an increase in magnetic momentfrom �0.035 to 0.00395 lB f.u.�1 Similarly, by thesweeping magnetic field from +1.5 to �1.5 kOe, chargepolarization increased from ±1.5 to 0.01 lC cm�2 at2.2 K. However, this ME coupling and temperature istoo low for the practical realization of a spintronic de-vice. In view of this, we prepared GdFeO3 with Mg sub-stitution in order to observe room temperature MEcoupling by the dynamic magnetic field method (MEB).

A series of GdFe1-xMgxO3 (0.00 < x < 0.06) sampleswas synthesized by a solid-state reaction method usingmolar ratios of Gd2O3 (99.9%), Fe2O3 (99.9%) andMgO (99.9%) precursor. Oxide powders were groundin an agate mortar and pestle for 1 h. Homogeneouslymixed powder was kept in a furnace at 1100 �C for24 h in air. Presintered powders were again ground for1 h followed by pelletization under 10 ton pressure.Rectangular pellets were then kept for sintering at1300 �C for 24 h in air. X-ray diffraction measurementsof the sample series were taken with Bruker Axes equip-ment. Magnetization measurements were taken with aLakeshore Model 7304 vibrating sample magnetometer.

sevier Ltd. All rights reserved.

20 30 40 50 60 70 80

x=0.06

x=0.04

x=0.02

x=0.00

GdFe1-xMgxO3

Inte

nsity

(a.u

.)

Angle (2θ)

Figure 1. X-ray diffraction pattern of the GdFe1-xMgxO3 series.

-4000 -2000 0 2000 4000

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

-800 -400 0 400 800-0.2

-0.1

0.0

0.1

0.2

M (e

mu/

g)

H (Oe)

x=0.00 x=0.02 x=0.04 x=0.06

GdFe1-xMgxO3

Magnetic Field H (Oe)

Mag

netic

Mom

ent M

(em

u/g)

0 100 200 300 400 5004.0x103

6.0x103

8.0x103

1.0x104

1.2x104

1.4x104

1.6x104

(280oC)(356oC)

(401oC)

T'N

Temperature (oC)

1/Su

scep

tibilit

y (χ

-1dc

)

TN

Figure 2. Magnetization curve for the GdFe1-xMgxO3 series (inset) andInverse DC susceptibility vs. temperature curve for the GdFe1-xMgxO3

series.

101 102 103 104 105 106

0

20

40

60

80

100

120

140x=0.00x=0.02x=0.04x=0.06

GdFe1-xMgxO3

Die

lect

ric C

onst

ant (

K)Frequency (Hz)

Figure 3. Dielectric constant vs. frequency curve at room temperaturefor the GdFe1-xMgxO3 series.

0 5 10 15 20 25 30 350.00

0.01

0.02

0.03

0.04

0.05R=0.9917

R=0.9929

R=0.9941

R=0.9957

DC Magnetic Field=0 Oe

GdFe1-xMgxO3

x=0.00 x=0.02 x=0.04 x=0.06

Line

ar M

E O

utpu

t Vol

tage

(V/c

m)

AC Magnetic Field (Oe)

Figure 4. Linear ME coupling voltage plot at room temperature forthe GdFe1-xMgxO3 series.

J. Shah, R. K. Kotnala / Scripta Materialia 67 (2012) 316–319 317

Dielectric measurements were carried out with a WayneKerr 6540A impedance analyzer, in the frequency range20 Hz–1 MHz. The magnetoelectric coupling coefficientwas measured by the dynamic magnetic field (MEB)method by a system built in-house .

X-ray diffraction of the sample series was carried outfrom 20� to 80� angle sweep as shown in Figure 1. AllXRD peaks obtained coincide well with the orthorhom-bic space group D16

2h. These space groups belong to fourdistorted perovskite units in a crystallographic unit cell.The XRD peaks were refined using the CHEKCELLprogram for the compositions as shown in Table 1. Thisreveals that Mg substitution increases the unit cell vol-ume of GdFeO3. Using the CHEKCELL lattice constantrefinement program, an increase in lattice constant wasobserved with Mg substitution. The increase in unit cellvolume with standard deviation error is also given inTable 1. Since the ionic radius of Mg2+ = 0.78 A is largerthan Fe3+ = 0.67 A, Mg substitution would have re-sulted in lattice increment/strain in order to attain elec-tronic stabilization of the structure.

The magnetic characteristics of the samples weredetermined by measuring the M–H loop at room tem-perature (Fig. 2 inset) and the Neel temperature(Fig. 2). The magnetization loop confirms a weak ferro-magnetic behavior of GdFeO3. The weak ferromagne-tism in orthoferrites measured by various researcherssuggests that it is due to an intrinsic property of thecompound [17]. GdFeO3 belongs to the distorted perov-skite group [18]. According to Neel, magnetization inrare earth ferrates is due to differences in the molecularmagnetic fields of the sublattices [19]. At 280 �C a smalldip is observed for GdFeO3 which exhibits sublatticemagnetization as shown in Figure 2. Dzyaloshinskii-Moriya found the antisymmetric exchange interactionDij.SiXSj to be responsible for the weak ferromagnetism[20]. GdFeO3 exhibits G-type antiferromagnetism: Fe3+

ions are situated at the corner of the cube surrounded

Table 1. Refined lattice constants and unit cell volume of GdFe1-xMgxO3 (x

GdFe1-xMgxO3 Lattice Constant

a b c

x = 0. 00 5.3479 5.6090 7.6589x = 0.02 5.3489 5.6084 7.6606x = 0.04 5.3461 5.6122 7.6669x = 0.06 5.3535 5.6119 7.6764

octahedrally by oxygen ions with a Gd3+ ion at centerof the cube. Each Fe ion is coupled by superexchangeto six nearest-neighbor Fe ions. In the case of antiferro-magnetic crystals, the antisymmetric exchange interac-tion is compatible with tilting of spins causing weakferromagnetism. Anderson has measured the increase

= 0.00–0.06).

Cell volume (A3) Standard error in cell volume

229.740 .2482229.808 .2255230.033 .2694230.623 .2118

Table 2. Magnetic, dielectric and linear coupling coefficients observed for GdFe1-xMgxO3 (x = 0.00–0.06) at room temperature.

GdFe1-xMgxO3 Coercivity Hc (Oe) Retentivity Mr (memu) Dielectric constant K at 20 Hz Linear magnetoelectriccoupling coefficient, a (mV cm�1 Oe�1)

x = 0.00 130 28 0.011 0.76x = 0.02 56 7 49.41 0.97x = 0.04 24 3 80.94 1.41x = 0.06 13 1 123.70 1.67

318 J. Shah, R. K. Kotnala / Scripta Materialia 67 (2012) 316–319

in exchange interaction strength due to the decrease inFe3+–O2� distance [21]. Figure 2 shows the plot of theinverse of DC susceptibility with increasing tempera-ture. The Neel temperature of GdFeO3 of 401 �C ob-tained in our sample is close to the 405 �C observed byPauthenet and Blum [22]. A further decrease in Neeltemperature has been observed with Mg substitution[23]. This suggests that the decrease in Neel temperaturemay be due to a reduction in the numbers of magneticFe3+–Fe3+ ion neighbors in the unit cell, leading to a de-crease in the antisymmetric exchange interaction. Thusthe weak magnetization of GdFeO3 is decreased dueto replacement of tilted Fe3+ neighbors by Mg2+. A de-crease in the Neel temperature of GdFe1-xMgxO3 con-firms the substitution of Mg in the Fe sites. Thesubstitution of Mg in the GdFeO3 samples causes a sig-nificant increase in conductivity from 5 � 10�8 S cm�1

(x = 0.00) to 1 � 10�4 S cm�1 (x = 0.06) at roomtemperature.

The dielectric constant vs. frequency (20 Hz–1 MHz)was measured at room temperature as shown in Figure 3.The dielectric constant (K = 0.011, GdFeO3) increaseswith Mg2+ (K = 123, Mg = 0.06) substitution. An in-crease in polarization/dielectric constant has been re-ported with Mg2+ substitution in perovskite systems[14,24,25]. The enhancement in dielectric constant byMg2+ substitution may be due to a distortion in the crys-tal structure observed by XRD peak refinement. The ob-served decrease in dielectric constant with frequency canbe explained on the basis of Koop’s phenomenologicaltheory of dielectrics [26].

The magnetoelectric coupling coefficient of sampleshas been determined by the dynamic magnetic field(MEB) method [27]. The linear coupling coefficient, a,has been measured at zero DC magnetic field whilesweeping an AC magnetic field. A function generatorhas been used to generate a 35 Oe AC magnetic fieldat a fixed frequency of 850 Hz. A buffer and high-passfilter circuit to reduce the background voltage noiseare employed in the measurement set-up to detect the in-duced MEB voltage across the sample thickness.

The output voltage Vout is given by:

Vout / ðah0 þ 2bh0H0Þwhere H = H0 + h0Sinxt, is the superimposition of theAC field, h = h0Sinxt, on the DC field, H0.

The ME coefficient in terms of voltage equation wasgiven by Rivera [12]:

MEBcofficientðaÞ ¼ dEdH¼ 1

ddVdH¼ V out

h0d

where Vout, H0, h0 and d are the induced output voltage,the DC biased magnetic field, the AC magnetic field

magnitude and the sample thickness, respectively. Beforetaking measurements, samples were electrically poled at120 �C at 20 kV cm�1 for 60 min. Figure 4 shows theME output voltage with respect to varying AC magneticfield while keeping the DC magnetic field at 0 Oe; itshows a change in slope of voltage output with AC mag-netic field due to Mg substitution. A linear behavior be-tween voltage and applied field with change in slope ispredicted based on the linear magnetoelectric couplingcoefficient, a. The value of a determined for GdFeO3 is0.76 mV cm�1 Oe�1, which increases to 1.67 mV cm�1

Oe�1 for x = 0.06 Mg substitution at room temperatureas shown in Table 2. However, we have found no litera-ture on the dynamic MEB coupling coefficient of poly-crystalline GdFeO3 at room temperature. A roomtemperature value of a of 7 mV cm�1 Oe�1 was observedfor polycrystalline single compound BiFeO3, and2.38 mV cm�1 Oe�1 at 40 K for La0.5Ca0.5MnO3

[28,29]. Tokunaga et. al. observed ME coupling in singlecrystal GdFeO3 due to Fe and Gd spinstriction at 2.2K[16]. Pyatakov et al. also studied Neel-type domain wallswith controlled chirality-driven magnetoelectric interac-tion [30–32]. A generalized symmetry theory of flexo-magnetoelectric effect for non-collinear magneticordering of weak ferromagnetic and antiferromagneticdomain walls has also been proposed [33]. Here we sug-gest that a reduction in the number of neighboring Fe3+

ions due to Mg2+ substitution somehow affects the chi-rality of antiferromagnetic domain wall, thus changingspin polarity-driven ferroelectricity.

The room temperature magnetoelectric coefficients ofthe GdFe1-xMgxO3 series have been found to increasewith nonmagnetic Mg2+ ion substitution. Mg2+ substi-tution has weakened the antisymmetric exchange inter-action and distorted the GdFeO3 structure. This effectinduces a change in magnetic property as well as inthe dielectric constant with Mg substitution, which hasbeen confirmed by measuring the linear magnetoelectriccoupling coefficient at room temperature. This impliesthat antiferromagnetic (Neel-type) domain walls play akey role in ME coupling. Furthermore, measurementsof MEB coupling are required to investigate the physicsbehind such coupling.

The authors are grateful to the Director of theNational Physical Laboratory, New Delhi for his sup-port and encouragement.

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