Multiferroic phase stability in non-stoichiometric MnWO4H. W. Yu, X. Li, L. Li, M. F. Liu, Z. B. Yan, and J.-M. Liu Citation: Journal of Applied Physics 115, 17D722 (2014); doi: 10.1063/1.4866087 View online: http://dx.doi.org/10.1063/1.4866087 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Modulated multiferroic phases and electric polarization in Mn1−xRuxWO4+δ J. Appl. Phys. 117, 17D912 (2015); 10.1063/1.4916503 Ferroelectricity and competing interactions in Ho-deficient non-stoichiometric orthorhombic HoMnO3 J. Appl. Phys. 117, 17D903 (2015); 10.1063/1.4906529 Ion doping effects in multiferroic MnWO4 J. Appl. Phys. 111, 083906 (2012); 10.1063/1.4703913 Field-induced continuous rotation of the polarization in multiferroic Mn0.95Co0.05WO4 J. Appl. Phys. 111, 07D903 (2012); 10.1063/1.3671419 Re-entrant spiral magnetic order and ferroelectricity in Mn 1 − x Fe x WO 4 ( x = 0.035 ) J. Appl. Phys. 105, 07D913 (2009); 10.1063/1.3079865

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Multiferroic phase stability in non-stoichiometric MnWO4

H. W. Yu, X. Li, L. Li, M. F. Liu, Z. B. Yan, and J.-M. Liua)

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

(Presented 7 November 2013; received 14 September 2013; accepted 14 November 2013; published

online 26 February 2014)

We investigate the multiferroic phase stability of MnWO4 in response to the non-stoichiometry of Mn

and W, given the Mn:W ratio g. It is observed that the non-stoichiometry does not affect remarkably

the ferroelectric transition point (the AF3-AF2 transition point) and the AF2-AF1 transition point, but

the non-polar AF1 phase is partially replaced by the ferroelectric AF2 phase. The measured electric

polarization is slightly enhanced with increasing stoichiometric deviation jg� 1j. The possible

underlying mechanism for these effects is discussed. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4866087]

Multiferroic materials have inspired much attention in

recent years. The coexistence of spontaneous polarization

and magnetization in the same phase and the magnetoelectric

coupling between them makes the mutual control of magne-

tism and ferroelectricity possible.1–3 These potentially useful

phenomena are observed in a wide class of materials, includ-

ing orthorhombic rare-earth manganites,4–6 triangular

cuprates,7–9 Ni3V2O8,10 Ca3CoMnO6,11 and so on. While

most of them show more than one magnetic phase, the magne-

toelectric coupling is available only in certain phases that fea-

ture noncollinear spin orders. The Dzyaloshinskii-Moriya

(DM) interaction is believed to induce such couplings based

on a magnetic structure that results in a macroscopic electric

polarization,12–14 mainly in 3d magnetic transition metal

oxides of noncollinear spin order.15,16

MnWO4 (MWO) has been well studied in recent years.

MWO undergoes successive magnetic phase transitions upon

decreasing temperature T. The first antiferromagnetic transi-

tion occurs at T¼ TAF3¼ 13.5 K, where a sinusoidal incom-

mensurate spin structure is developed (AF3 phase). The AF3

phase is replaced by the AF2 phase with a tilted elliptical

spiral structure at T¼TAF2¼ 12.6 K. The collinear commen-

surate AF1 phase with characteristic ""## spin order appears

at T¼TAF1¼ 7.8 K and below. It is noted that only the heli-

cal AF2 phase is ferroelectric with polarization P aligned

along the b-axis. Plenty of works have been done to modu-

late the stability of these phases, based on the fact that the

spin structure is sensitive to any small perturbations such as

external fields17,18 and chemical substitutions.19–22

The purpose of this work is to investigate the multiferroic

phase stability of MWO against the Mn:W non-stoichiometry.

The spin structure is a compromise between multifold

interactions’ competition and the effective interactions can be

over the 11th-neighbor.23 The spin-lattice interaction is

believed to play a substantial role. It was reported that substi-

tution of Mn2þ by Co, Zn, or Mg can seriously suppress the

AF1 phase and slightly destabilize the AF2 phase.20,21,24,25

Since these substitutions all result in chemical disorder and

lattice contraction, it is argued that the spin-lattice interactions

in addition to the multifold spin interactions are major ingre-

dients for the magnetic transitions.

Here, for non-stoichiometric MWO, either Mn- or

W-deficiency can be viewed as a kind of chemical disorder

too. In this case, the relative stability of one magnetic phase

over the other will be modulated. By synthesizing a series of

MWO samples with different g, we investigate the stability

of the AF1 and AF2 phases.

We define an atomic ratio g¼Mn/W and quantity jg� 1jmeasures the non-stoichiometry. Our experiment is on poly-

crystalline samples prepared using standard solid-state reac-

tions. The high-purity WO3 and MnO powder was chosen as

reagents and thoroughly mixed for 24 h. The dried mixture was

ground for 1 h each and then annealed in air for 12 h at 600 �C.

After another intermediate grindings, the mixture was com-

pressed into pellets and annealed at 950 �C for 20 h in air.

The sample crystallinity was checked using X-ray dif-

fraction (XRD) with Cu Ka radiation at room temperature.

The valence states of Mn and W ions and chemical composi-

tion were examined by X-ray photoelectron spectroscopy

(XPS; ULVAC-PHI PHI5000 VersaProbe using Al Ka radia-

tion). The magnetization M and dc magnetic susceptibility vwere measured using the Quantum Design Superconducting

Quantum Interference Device (SQUID) in the zero-field

cooled (ZFC) mode and field-cooling (FC) mode, respec-

tively. The cooling and measuring fields were both 1000 Oe.

The electric polarization as a function of T was measured

using the pyroelectric current method by Keithley 6514 elec-

trometer connected to the Physical Properties Measurement

System.26 The samples were first poled in a static electric

field of �10 kV/cm, and then the pyroelectric currents meas-

ured through warming the samples were integrated to obtain

polarization P as a function of T.

The high-precision XRD data of several samples are

plotted in Fig. 1. All the spectra fit the standard database sat-

isfactorily. Here, it is critical to exclude any manganese

oxides or tungsten oxides. By focusing on the local (030)

and (022) reflections in Fig. 1(b), one sees gradual shifting

of the two peaks towards the high-angle with increasing

jg� 1j, indicating clearly the lattice contraction. For clarify-

ing the details, we perform the Rietveld refining of the XRD

data using the GSAS program. For a reliable refining of the

a)Author to whom correspondence should be addressed. Electronic mail:

liujm@nju.edu.cn.

0021-8979/2014/115(17)/17D722/3/$30.00 VC 2014 AIP Publishing LLC115, 17D722-1

JOURNAL OF APPLIED PHYSICS 115, 17D722 (2014)

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data, one needs to know the variations of lattice occupation

in these non-stoichiometric samples. First, we employ the

XPS to probe the g value and Mn/W valence states. For

examples, the XPS spectra for samples g¼ 1.04, g¼ 1, and

g¼ 0.94 are shown in Fig. 1(d) for W and (e) for Mn. No

identifiable shift of the peaks corresponding to the Mn-

2p1/2,3/2 and W-4f5/2,7/2 is observed, implying that the va-

lence states of Mn and W ions remain to be Mn2þ and W6þ,

although the existence of tiny Mn and/or W ions with differ-

ent valences cannot be excluded. Second, generation of ionic

defects in the g 6¼ 1 samples is needed to meet the electric

neutrality condition. It is noted that Mn2þ ion is bigger than

W6þ ion and there is big valence difference between them.

For g> 1, no possibility for occupation of the W-vacant site

by Mn ion is expected and thus, the W-vacancies are

assumed with oxygen vacancies surrounding the W vacan-

cies. On the other hand, for g< 1, two possible situations

may appear. One is that the excess W6þ ions occupy the

Mn2þ-vacant sites. The electrostatic energy due to the big

valence difference between Mn2þ and W6þ can be high. The

other is the generation of Mn vacancies and surrounding ox-

ygen vacancies, which are favored from the energy

consideration.

Based on this site occupation model, high precision

Rietveld refining is obtained. The evaluated lattice constants

(a, b, and c) and lattice unit volume V as a function of g are

plotted in Figs. 1(e)–1(h). These parameters decrease with

increasing jg� 1j. This feature is similar to those cases with

the Co, Zn, or Mg-substituted MWO.20,21,24,25 It is then sug-

gested that the AF1 phase may be destabilized, and the AF2

phase may be or not. The AF1 phase will most likely be par-

tially replaced by the AF2 phase, to be confirmed below.

The v-T data in both the ZFC and FC modes for five

samples are presented in Fig. 2. Rather than any quantitative

discussion, we list several qualitative features. First, the

magnetization over the whole T range is suppressed with

increasing jg� 1j. This phenomenon may be understandable

for g< 1 since the magnetic Mn species is deficient.

However, it is true for g> 1 too. A reasonable argument is

that the non-stoichiometry releases the spin frustration and

stabilizes the spin structures. Second, a comparison of the

ZFC and FC data shows that the separation between the two

modes becomes weaker with increasing jg� 1j, and the two

modes even merge together at g¼ 0.96. This also evidences

that the spin phase stability against, e.g., thermal activations,

is enhanced with increasing jg� 1j. Third, the v-T curves at

the ZFC mode show quite similar shapes, indicating that the

magnetic transition sequence does not change much for dif-

ferent g. Unfortunately, the AF3-AF2 and AF2-AF1 transi-

tion points cannot be clearly identified from the v-T data

although several weak anomalies can be roughly seen, which

are marked as TAF1, TAF2, and TAF3 shown in Fig. 2.

Finally, we consult to the ferroelectricity for illustrating

the effect of Mn/W non-stoichiometry. The measured I-Tcurves and evaluated P-T curves for several samples are shown

in Figs. 3(a)–3(f). The P-T data for the g¼ 1 sample is similar

to earlier results.17 Several clear features are shown. First, if

defining the initiating point of the pyroelectric current I from

the high-T side as TAF2, and the initiating point from the low-Tside as TAF1, one sees that both TAF2 and TAF1 do not shift with

increasing jg� 1j, noting that the two points for the g¼ 1 sam-

ple coincide well with earlier reports.17,21,22,24,25 This is differ-

ent from those Co, Mg, or Zn-substituted MWO where TAF1

disappears immediately and TAF2 is slightly down-shifted. For

the Fe-substituted case, however, TAF1 is up-shifted and TAF2 is

down-shifted. Second, for both g> 1 and g< 1, not only the Pwithin TAF1<T<TAF2 is gradually enhanced but also non-

zero polarization is identified below TAF1. Meanwhile, the

polarization at T¼ 2 K, P(2 K), increases too. The P(max) and

P(2 K) data as functions of g are plotted in Figs. 3(g) and 3(h),

respectively. In those substituted MWO, however, the P of the

AF2 phase is gradually damaged upon the increasing substitu-

tion. Third, the above data show the incomplete AF2-AF1

phase transitions initiating at TAF1 in these g 6¼ 1 samples, lead-

ing to the AF1 and AF2 phase coexistence below TAF1. This is

also different from those substituted MWO.20–22,24

From the measured results, we can highlight the multi-

ferroic phase diagram in the (g, T) plane at H¼ 0, as shown

FIG. 1. (a) Measured h-2h XRD spectra of several samples, and (b) the

amplified (030) and (022) reflections. The XPS spectra of Mn(2p1/2,3/2) and

W(4f5/2,7/2) core levels for g¼ 1.04, 1.00, and 0.94 are shown in (c) and (d).

The evaluated lattice constants (a, b, c) and lattice unit volume V are plotted

in (e)-(h).

FIG. 2. Measured v-T curves under the ZFC and FC modes for five samples

g¼ 1.04, 1.02, 1.00, 0.98, and 0.96. The measuring field is 1000 Oe.

17D722-2 Yu et al. J. Appl. Phys. 115, 17D722 (2014)

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113.198.210.198 On: Thu, 14 May 2015 02:49:32

in Fig. 4. In the low-T range, the phase diagram is divided

into three regions: the AF1 phase close to g¼ 1 and the

AF1þAF2 coexisting regions on the two sides.

Furthermore, from the ferroelectric data, the TAF1 at which

the AF2-AF1 phase transition begins does not change. Here,

we acknowledge that the proposed phase diagram is still

more or less qualitative, and more data on the spin structure

are needed for a quantitative phase diagram.

The obtained structural and ferroelectric data allows us

to have a qualitative discussion on the underlying mecha-

nism for the effects of the Mn/W non-stoichiometry.

First, the similarity between the present materials and

those Co, Zn, and Mg-substituted MWO lies in the fact that

chemical disorder is induced and the lattice contracts. The

chemical disorder is believed to destabilize the magnetic

phases via the spin-lattice interactions.24,25 This effect seems

more significant in suppressing the AF1 phase rather than the

AF2 phase. The induced lattice contraction is equivalent to

the effect of a chemical pressure, believed to destabilize the

non-polar AF1 phase.27 The chemical disorder induced here

seems weak, and the lattice contraction at the maximal value

here is only one-tenth of that for those Co, Zn, and Mg-

substituted MWO. Therefore, it is reasonable to observe an

incomplete taking-over of the AF1 phase by the AF2 phase,

leading to the AF1þAF2 phase coexistence.

Second, the ferroelectric polarization in the AF2 phase

is enhanced in the g 6¼ 1 MWO, while it is suppressed for

those Co, Zn, and Mg-substituted MWO. This P-enhance-

ment may be related to the overall high stability of the mag-

netic phases (both AF2 and AF1) in the g 6¼ 1 MWO with

respect to the g¼ 1 MWO (Fig. 2). This higher stability and

thus the ferroelectric domains in the AF2 phase against inter-

nal fluctuations reasonably allows larger polarization.

Third and surely, both magnetic and non-magnetic

substitutions in the g 6¼ 1 samples and those Co, Zn, and

Mg-substituted MWO definitely impose impact on the spin

interactions. It was claimed that the effects of non-magnetic

substitutions seem independent of the nature of substituting

species, and the three-dimensional nature of magnetic inter-

actions and the spin frustration remain less affected.25 The

Mn-deficient MWO is equivalent to a kind of non-magnetic

substitution, while the W-deficient MWO does not include

additional magnetic species either since the substituted sites

are W-vacancies. Therefore, a reasonable argument is that, sim-

ilar to the Zn/Mg-substituted cases, the non-stoichiometry here at

low jg� 1j imposes less influence on the magnetic interactions,

and the major contributions are from the chemical disorder and

modulated spin-lattice interactions.

This work was supported by the National 973 Projects

of China (Grant No. 2011CB922101), the Natural Science

Foundation of China (Grant Nos. 11234005 and 51332006),

and the Priority Academic Program Development of Jiangsu

Higher Education Institutions, China.

1Y. Tokura, Science 312, 1481 (2006).2S. W. Cheong and M. Mostovoy, Nat. Mater 6, 13 (2007).3K. F. Wang, J.-M. Liu, and Z. F. Ren, Adv. Phys. 58, 321 (2009).4T. Kimura et al., Nature (London) 426, 55 (2003).5T. Goto et al., Phys. Rev. Lett. 92, 257201 (2004).6N. Hur et al., Nature (London) 429, 392 (2004).7S. Park et al., Phys. Rev. Lett. 98, 057601 (2007).8T. Kimura et al., Phys. Rev. B 73, 220401(R) (2006).9K. Kimura et al., Phys. Rev. Lett. 103, 107201 (2009).

10G. Lawes et al., Phys. Rev. Lett. 95, 087205 (2005).11L. Lin et al., J. Appl. Phys. 111, 07D901 (2012).12H. Katsura et al., Phys. Rev. Lett. 95, 057205 (2005).13M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006).14I. A. Sergienko and E. D. Dagotto, Phys. Rev. B 73, 094434 (2006).15M. Kenzelmann et al., Phys. Rev. Lett. 95, 087206 (2005).16S. G. Condran et al., J. Phys.: Condens. Matter 22, 162201 (2010).17K. Taniguchi et al., Phys. Rev. Lett. 97, 097203 (2006).18K. Taniguchi et al., Phys. Rev. Lett. 101, 207205 (2008).19F. Ye et al., Phys. Rev. B 78, 193101 (2008).20Y. S. Song et al., Phys. Rev. B 79, 224415 (2009).21R. P. Chaudhury et al., Phys. Rev. B 83, 014401 (2011).22H. W. Yu et al., Phys. Rev. B 87, 104404 (2013).23H. Ehrenberg et al., J. Phys.: Condens. Matter 11, 2649 (1999).24F. Ye et al., Phys. Rev. B 86, 094429 (2012).25L. Meddar et al., Chem. Mater. 21, 5203 (2009).26G. Q. Zhang et al., Phys. Rev. B 84, 174413 (2011).27R. P. Chaudhury et al., Physica B 403, 1428 (2008).

FIG. 3. (a)-(f) Measured pyroelectric current I (red solid curves) and eval-

uated electric polarization P (blue dashed curves) as a function of T. The

arrows indicate TAF2 and TAF1, respectively. (g) and (h) The evaluated

P(2 K) and P(max) as a function of g.

FIG. 4. Evaluated multiferroic phase diagram in the g�T plane at H¼ 0.

17D722-3 Yu et al. J. Appl. Phys. 115, 17D722 (2014)

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