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Effects of Ga doping on the electric and magnetic properties of DyMn ₁ _x Ga _x O ₃

S.Z. Li

^a^,^b^,^c^,^*

, S.S. Chen

^a

, X.Z. Wang

^d

, J.M. Liu

^c

aSchool of Physics, HuBei Polytechnic University, Huangshi 435003, China

bHuBei Key Laboratory of Mine Environmental Pollution Control and Remediation, HuBei Polytechnic University, Huangshi 435003, China

cLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

dInstitute for Advanced Materials, HuBei Normal Univerisity, Huangshi 435003, China

A R T I C L E I N F O Keywords:

Multiferroics Polycrystalline Raman spectrum Electric polarization Speciﬁc heat

Antiferromagnetic property

A B S T R A C T

Ga^3þfor Mn^3þsubstitution in multiferroic DyMn_1xGa_xO₃(x¼0, 0.02, 0.05, 0.1, 0.2, 0.3 and 0.4) has been performed to study the change on Mn^3þordering, which also inﬂuences on the ordering of Dy^3þ. The samples are pure and their dielectric constant, electric polarization and magnetic properties have been investigated. When the content of Ga^3þincreases above 0.1, multiferroic properties completely disappear. These results indicate that Ga^3þfor Mn^3þsubstitution in DyMnO₃bulk can reduce the exchange interactions ofJ_MnMn,J_DyMnand the bond angle of Mn-O-Mn, while the electric polarization is observed to decrease rapidly, even for small Ga doping, and the anomaly at low T inP(T)associated with the Dy ordering is no longer observed forx¼0.05.

1. Introduction

Multiferroic oxides have attracted more and more attention in recent years due to their possible application toward storage materials and intriguing fundamental physics. Among the naturally existing multiferroic oxides, the presence of magnetoelectric (ME) materials, which simultaneously possess at least two orders among ferroelectric (FE), ferromagnetic (FM) or antiferromagnetic (AFM), and ferroelastic order parameters, are more[1–3]. For example, LuFe2O4[4], BiFeO3[5], and other hexagonal and orthorhombic manganites (RMnO₃R¼Y, Gd to Lu) exist[6,7]. The strong ME coupling has been obtained in orthorhombic manganites with the discovery of ferroelectricity resulting from cycloidal-spin ordering for RMnO₃(R¼Gd, Tb and Dy)[8–15].

Especially, the orthorhombic manganites and their mixtures, RMnO3(R¼Tb, Dy and Gd) and EuxY_1xMnO3[16]are comprehensively studied. They crystallize in a distorted perovskite structure (Pbmn) and ferroelectricity has been shown to originate from a spiral magnetic structure that breaks both time reversal and inversion symmetry, which all shows the strong magnetic-electric coupling. Especially for TbMnxGa1xO3, it shows that Ga for Mn substitution has been performed in order to study the inﬂuence of Mn-magnetic ordering on the Tb-magnetic sublattic[17].

But for DyMnO₃, the mechanism of the magnetic-ﬁeld inducing po- larizationﬂop is more complicated. BelowT_N^Dy¼6.5 K, Dy magnetic

moments order is in a commensurate structure. Simultaneous with the Dy magnetic ordering, an incommensurate lattice modulation is evolved while the original Mn induced modulation is suppressed. Furthermore, when the magneticfield (H¼2T) was applied alongb, an incommensurate structure is induced by Mn spiral ordering and the polarization increases. At T_lockin¼18 K, an additional component of Mn moment along c axis gives rise to a spiral (cycloidal) magnetic order and breaks the inversion symmetry. Finally, it confirms that Mn-inducing Dy spins contributes to the enhancement of the polarization through the external magneticfield[18–20].

In this letter, we present the polarization resulting from doping with non-magnetic Ga ion, which results in a change of the exchange interactions between Mn- and Dy-ions,J_DyMnand between Mn- and Mn- ions,J_MnMnand then inﬂuences on the whole polarization.

2. Experiments

Polycrystalline DyMn_1xGaxO3(x¼0, 0.02, 0.05, 0.1, 0.2, 0.3 and 0.4) samples were prepared by thoroughly mixing high-purity Dy2O3(99.99%), Ga2O3(99.99%) and Mn2O3(99%) in a stoichiometric ratio. The sample crystallinity was conﬁrmed by powder XRD. The magnetization was measured with a SQUID magnetometer (SQUID, Quantum Design Inc.). The ferroelectricity was probed using the pyro- electric current method (using Keithley 6514), combined with the

* Corresponding author. School of Physics, HuBei Polytechnic University, Huangshi 435003, China.

E-mail address:[email protected](S.Z. Li).

Contents lists available atScienceDirect

Physica B: Physics of Condensed Matter

journal home page:www.el sevier. com/l oca te/ physb

https://doi.org/10.1016/j.physb.2017.10.053

Received 21 June 2017; Received in revised form 11 September 2017; Accepted 12 October 2017 Available online 16 October 2017

Physica B: Physics of Condensed Matter 530 (2018) 49–52

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Quantum Design Physical Properties Measurement System (PPMS, Quantum Design Inc.), whereas the dielectric data were collected using an impedance analyzer (HP 4294A). For probing the polarizationP, each sample under a poling electricfieldE¼440 kV/m and various magnetic fieldH(0–4 T) wasfirst cooled down toT¼2 K, followed by a sufficient long time short-circuit procedure. As Dy^3þordering causes abnomalies in the dielectric constant, a magneticfield intensity (H¼0-4T) was applied during the whole measurement.

3. Results and discussions

The prepared samples for different Mn/Ga ratios all exhibit the orthorhombic phase (Pbmn) of the unsubstituted phase, without any secondary phase observed, as shown inFig. 1(a). The diffraction peaks shift to low angle with increasing Ga content as shown in the insert graph ofFig. 1. It conﬁrms that Ga^3þdoping expands the volume, which is in accordance to larger ionic radius of Ga^3þ(Ga^3þ~0.69Å, Mn^3þ~0645Å).

In order to investigate the effect of partial substitution of Mn^3þby doping

Ga^3þ, the Raman spectroscopies for DyMn1xGaxO3(x¼0, 0.2, 0.3 and 0.4) were studied. Ag(1) and B2g(1) denote, respectively, antistretching and stretching vibrations of oxygen atoms in the xz planes, Ag(3) and B2g(3) correspond to bending of MnO6octahedra, Ag(4) denotes MnO6

rotations, and“B2g(2) scissor-like”oxygen rotations. With increasing the Ga^3þcontent to 0.2, Ag(1), Ag(3) and Ag(4) disappear inFig. 2, which is related with the multiferroic properties of DyMn_1xGa_xO₃(x¼0, 0.02, 0.05, 0.1). So we can infer that the multiferroic properties of DyMn_1xGaxO3(x¼0.2, 0.3 and 0.4) disappear, too. The peak for B2g(1) is becoming weaker with increasing the Ga^3þcontent, which is related with the bond distance of Mn-O-Mn and the bond angle.

To further study the multiferroic property by doping Ga^3þ, wefirst show temperature profiles of the electric polarizationP for DyMnO₃ ceramic at different magneticfields (H) (0-4T), which was applied in the whole temperature range. Due to the random orientation of the grains in the polycrystalline sample, the measured polarization has to be lower than the intrinsicPfor a single grain or FE domain under the same condition. And below 10 K, the change of the polarization has resulted from the different magneticfields at the same electric field and the antiferromagentic(AFM) transition temperature of Dy is about 9 K for polycrystalline. A sudden increase ofPcomes up atH¼2T. This might indicate that the independent Dy ordering has disappeared and the propagation vector is induced to that of Mn spiral ordering. Prokhnenko et al. observed the evolution of ICM ordering of Dy moments with the same periodicity as the Mn spiral ordering and the coupling between Dy and Mn is stronger than that of other RMnO₃[20]. So we can argue that magnetic field makes a difference for Dy ordering below T_N^Dy and Fig. 1.X Ray diffraction spectrometer for DyMn1xGaxO3(x¼0, 0.02, 0.05, 0.1, 0.2, 0.3

and 0.4).

Fig. 2.Raman spectrum for DyMn_1xGaxO3(x¼0, 0.1, 0.2, 0.3 and 0.4). Ag(1) and B2g(1) denote, respectively, antistretching and stretching vibrations of oxygen atoms in the xz planes, Ag(3) and B2g(3) correspond to bendings of MnO6octahedra, Ag(4) denotes MnO6

rotations, and“B2g(2) scissor-like”oxygen rotations.

Fig. 3.(a)Temperatures profiles of electric polarization for DyMnO3at different magnetic field (0-6T); (b) Temperatures profiled of electric polarization for DyMn1xGax(x¼0, 0.02, 0.05 and 0.1).

S.Z. Li et al. Physica B: Physics of Condensed Matter 530 (2018) 49–52

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contributes to the existing of ICM of Dy ordering. InFig. 3(b), we give the temperature (T) dependence of polarization (P) for DyMn_1xGaxO3(x¼0, 0.02, 0.05 and 0.1) at zero magneticﬁeld. Sub- stantial changes inPexist with increasing the doping content forP. The kink clearly observed atx¼0 shows a sudden decrease for DyMn0.98-

Ga0.02O3ceramic contrast to that for DyMnO3sample and it vanished for DyMn0.95Ga0.05O3sample. This is an indirect evidence of the effective reduction of the strength of Mn-Mn interactions,J_MnMn. Furthermore, we deduce thatJ_DyMnmust also be reduced while decreasing the content of Mn ions.

As well known, the ionic radiusR_Mn^3þis 0.645 nm andR_Ga^3þis 0.69 nm and it is nonmagnetic. When Ga^3þion was doped into DyMnO3, it oc- cupies the Mn^3þsite and it makes a difference for the interaction of Mn-O- Mn, thus Ga^3þdoped DyMnO₃could result in the change of Mn-O-Mn angle that influences the spiral order of Mn that impacts onP. On the other hand, we believe that Ga-doped DyMnO3first influences on the action of Dy^3þordering, inducing the change of Mn spiral ordering. Ga substitution for Mn has actually reduced the exchange reactions of Dy- and Mn- ions. When the kink becameflat for DyMn0.98Ga0.02O3ceramic, it may indicate that the exchange reactions of Dy- and Mn-ions were completely destroyed. The partial substitution of Mn by Ga has an effect onPsimilar to that of applying and external magneticfield. As the doped content increases up to 0.1, it completely breaks the Mn spiral ordering and reducesJMnMn, so thatPcompletely disappears.

To further understand the changes of Ga doped DyMnO3, the temperature dependent dielectric constant and speciﬁc heat measurements are given at zero magneticﬁled as shown inFig. 4for DyMnO₃ and DyMn0.95Ga0.05O3samples. The ferroelectricPand dielectric constant at 500 kHz for polycrystalline DyMnO3and DyMn0.95Ga0.05O3as shown in Fig. 4(a) and (b). They are different from previous DyMnO₃single crystalline, respectively. The dielectric constant of DMO single crystalline shows a clear peak. But for our samples, the dielectric constants become

much broader. The broad dielectric constant starts increasing at about 11 K and decreasing above 20 K associated with theT_lockininFig. 4(a).

And for DyMn0.95Ga0.05O3, the dielectric constant also turns broad and there is aλ-shaped peak at aboutT_lockin, 18 K. This phenomenon is similar with the dielectric constant of the potassium dihydrogen phos- phate KH2PO4 [21]. Just this is corresponding to the point that the rare-earth manganites are governed by relaxation effect long before[22].

So Ga-doped DyMnO3might result in domain freezing.Fig. 4(c) and (d) give the magnetization and speciﬁc heat data for DMO and DyMn0.95-

Ga0.05O3, respectively. Magnetizations (zerofield cooling at 100 Oe) for the two samples have no obvious change except that magnetization of DyMn0.95Ga0.05O3 reduces a little, resulting from the nonmagnetic Ga-ions. For specific heat data, three sequential transitions are observed inC/TatT¼6 K,T¼18 K andT¼38 K for DMO ceramic. These features correspond to Dy AFM ordering, ferroelectric transition and the Mn sublattice AFM transition, respectively. But for DyMn0.95Ga0.05O3, the ferroelectric transition and Dy AFM transition basically vanishes. Thus, we can conclude that the sample with 5% Ga has weak ferrolelctricity but it completely breaksJ_DyMn. This corresponds to the observed behavior of P(T) for those samples. From the above mentioned results, we can get that Ga-doped DMO influences both ofJ_DyMnandJ_MnMn.

Isothermal magnetization curves at 2 K, 15 K and 50 K were measured at different magneticﬁelds for DMO and DyMn0.95Ga0.05O3inFig. 5. For DMO bulk, atT¼2 K, below the Dy ordering temperature, a two-step metamagnetic behavior is observed with aﬁrst transition around 1.3 T.

Above about 6 T Dy moment appears to be ferromagnetically aligned.

The magnetization of DMO tends to align linearly. For DyMn_0.95Ga_0.05O₃, no-step associated that might be associated with a magnetic behavior is observed with magneticﬁled. Magnetization increases linearly. So we can infer that Ga doping into DyMnO₃inﬂuences on the properties of Mn inducing Dy spin.

In order to understand the decreasing of P with increasing the content Fig. 4.(a) and (b) Temperatures proﬁles of electric polarization (left scale)and dielectric constant (right scale)for DyMnO3and DyMn0.95Ga0.05O3, respectively; (c) and (d) The speciﬁc heat (left scale)and magnetization (right scale) DyMnO3and DyMn0.95Ga0.05O3, respectively.

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of Ga^3þ, a qualitative and tentative sketch of the original and rearranged Mn spiral spin order upon the doped is shown inFig. 6. The underlying mechanism isP~eij(SiSj), withSiandSjthe two neighboring spins

andeijthe spatial vector connecting them. From XRD and our Raman results, we believe that the long range Mn spiral spin order plus the Mn- induced Dy spin order seems to be seriously disturbed when the doping exceedsx¼0.02, leading to the decay of P.

4. Conclusions

Ga for Mn substitution in DyMnO₃has been performed to study the influence of the substitution on the Mn-magnetic ordering on Dy ordering belowT_N^Mn. Polycrystalline DyMn_1xGaxO3(x¼0, 0.02, 0.05, 0.1, 0.2, 0.3 and 0.4) compounds were synthesized and characterized by electric polarization, dielectric constant, magnetization and the specific heat capacity. These results show that the light Ga substitution for Mn does not change qualitatively the magnetization of Mn but it obviously influences on the electric polarization, and Dy spin ordering. All these indicate that there exists a strong influence of Mn magnetization ordering on Dy spin ordering belowT_N^Dy.

Acknowledgements

This work was supported by the National Natureal Science Founda- tion of China (No. 51302074 and 11374147) and the Project of Hubei Polytechnic University (No. 16xjz03A).

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Fig. 5.(a) and (b) Magnetization isotherms of DyMnGaO3(x¼0 and 0.05), respectively.

Fig. 6.A qualitative sketch of the Mn spin spiral order forx¼0(a) andx¼0.02(b); The open circle represents the contrail of the Mn spin and the double head arrow indicates the period of the Mn spin spiral.

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