Physica E 46 (2012) 213–217

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Physica E

1386-94

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Room temperature ferromagnetic property of Ag2Mo2O7 nanowires

Muhammad Hashim a, Chenguo Hu a,n, Cuiling Zhang a,b, Xiao Xie a

a Department of Applied Physics, Chongqing University, Chongqing 400044, PR Chinab Chongqing Technology and Business University, Chongqing 400067, PR China

H I G H L I G H T S

G R A P H I C A L A

c Room temperature ferromagneticproperty of the Ag2Mo2O7 nano-wires was studied first time.

c The value of coercivity was 127.7 Oeand remanent magnetization was16.9�10�3 emu/g.

c Magnetic property is caused fromdeformation of lattices and electronstrapped by oxygen vacancies.

c The photoluminescence spectrummeasured at room temperature con-firmed the oxygen vacancies.

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

x.doi.org/10.1016/j.physe.2012.09.023

esponding author. Tel.: þ86 23 65678362; fa

ail address: hucg@cqu.edu.cn (C. Hu).

B S T R A C T

Room temperature magnetic property of the anorthic phase Ag2Mo2O7 nanowires was found, whichmainly comes from the deformation of the crystal lattices and electrons trapped by the oxygenvacancies in Ag2Mo2O7 crystal.

a r t i c l e i n f o

Article history:

Received 18 February 2012

Accepted 26 September 2012Available online 4 October 2012

a b s t r a c t

Magnetic property of anorthic phase Ag2Mo2O7 nanowires with width of 2 mm and length up to 40 mm

synthesized by hydrothermal method was studied for the first time. The Ag2Mo2O7 nanowires exhibit

ferromagnetic property at room temperature with coercivity 127.7 Oe and remanent magnetization

16.9�10�3 emu/g. The magnetic mechanism has been discussed according to the calculated results of

the spin-polarized density functional theory (DFT), which was carried out by the Vienna ab initio

Simulation Package (VASP) using projector augmented wave (PAW) pseudopotential to describe the

interaction between electron and ion for Ag2Mo2O7 crystal, demonstrating that the magnetism is

caused by oxygen vacancies. The photoluminescence spectrum measured at room temperature also

indicates the oxygen vacancies in the Ag2Mo2O7 nanowires.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

There have been recent attempts at bringing about the loss ofcenter of symmetry in some specific inorganic compounds bygenerating lattice strain so that ferroelectric property could beinduced [1,2]. A nanostructure that gives rise to the formation ofsurface defects leads to a ferromagnetic behavior of the material.Sundaresan et al. studied room-temperature ferromagnetism of thenanoparticles (7–30 nm diam) [3], such as CeO2, Al2O3, ZnO, In2O3,and SnO2, and they found that the saturated magnetic moments inCeO2 and Al2O3 nanoparticles are comparable to those observed in

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transition-metal-doped wideband semiconducting oxides. The originof ferromagnetism in these materials is assumed to come from theexchange interactions between localized electron spin momentsresulting from the oxygen vacancies at the surfaces of the nanopar-ticles. They suggested that ferromagnetism may be a universalcharacteristic of nanoparticles of metal oxides. These findings haveprompted some research groups to synthesize semiconducting oxidenanostructures to induce room-temperature ferromagnetism. How-ever, ferromagnetism varies from materials due to different elemen-tal compositions and crystal structures with different symmetricgroups.

Metal molybdate is an important family of inorganic mater-ials which has wide application in various fields, such as photo-luminescence [4], microwave applications [5], optical fibers [6],scintillator materials [7,8], humidity sensors [9], and catalysts [10].

M. Hashim et al. / Physica E 46 (2012) 213–217214

The magnetic properties of metal molybdate have also been inves-tigated. The magnetic susceptibility measurements of scheeliterelated compounds AgLnMo2O8 (Ln¼ lanthanide) showed that thecompounds are paramagnetic down to 2 K [11]. The paramagneticwas also found for the complex oxide Na3Fe2Mo5O16 down to 10 Kand then turns to antiferromagnetic ordering [12]. Praseodymiummolybdate Pr2(MoO4)3 reveals Curie–Weiss paramagnetism withpredominant antiferromagnetic interactions between Pr3t-magneticmoments and no evidence of magnetic transitions down to T¼5 K[13]. Molybdate hydrates MMoO4ynH2O (M¼Co, Mn, n¼0, 3/4, 1)nano/microcrystals showed the antiferromagnetic ordering ofMnMoO4.H2O (o12 K), MnMoO4 (o12 K), NiMoO4 �H2O (o18 K)and CoMoO4.3/4H2O (o9 K) [14]. Therefore, all these molybdate areparamagnetic and they only show antiferromagnetic property atextremely low temperature. Very recently, we found weak ferro-magnetic property of the FeMoO4 nanorods at room temperature asa result of oxygen vacancies in the nanorods [15].

In this paper we report for the first time room temperatureferromagnetic property of the anorthic phase Ag2Mo2O7 nanowires.Both coercivity and remanent magnetization are much larger thanFeMoO4 nanorods. The magnetism mechanism has been discussedaccording to the calculated results of spin-polarized density

10 µm

Fig. 1. SEM image (a) and XRD pattern (b) of Ag2Mo2O7 nanowires synthesized by

hydrothermal method at 150 1C for 48 h.

functional theory (DFT) for Ag2Mo2O7 crystal with consideration ofoxygen vacancies by using the Vienna ab initio simulation package.

2. Experimental

2.1. Synthesis of Ag2Mo2O7 nanowires

Ag2Mo2O7 nanowires were synthesized by the hydrothermalmethod without any surfactant or template [16]. Typically0.25 mmol of H2MoO4 and 0.5 mmol of AgNO3 was dissolved in12 mL distilled water respectively. AgNO3 solution was added toH2MoO4 solution under magnetic stirring at room temperature.The resultant solution at pH 2 by HNO3 was transferred to Teflon-lined autoclave of 30 mL capacity and then put into stove at150 1C. After reacting for 48 h the Teflon-lined autoclave wastaken out and allowed to cool down to room temperature. Thenafter washing several times with deionized water and ethanol, thelight green product of Ag2Mo2O7 nanowires was obtained forinvestigations.

2.2. Characterization

The morphology and size of the synthesized nanowires werecharacterized by scanning electron microscopy (Nova 400 NanoSEM) at 20 kV. To investigate the crystal phase of the synthesized

-6000 -3000 0 3000 6000

-0.1

0.0

0.1

M (e

mu/

g)

H (Oe)

B

-200 -100 0 100 200-0.050

-0.025

0.000

0.025

0.050

M (e

mu/

g)

H (Oe)

coerecivity = 127.7 Oeremnant magnetization = 0.0169 emu/g

Fig. 2. Hysteresis loop (a) of the Ag2Mo2O7 nanowires with field sweeping from

�7000 to þ7000 Oe and the enlarged M–H curve and (b) in the center part.

M. Hashim et al. / Physica E 46 (2012) 213–217 215

sample, an X-ray diffractometer (XRD) with Cu K alpha radiationat 4@/min scanning speed were employed. Magnetic measure-ments of the synthesized Ag2Mo2O7 nanowires were carried outby ADE magnetics Model Ev 11 Vibrating Sample Magnetometerat room temperature. The photoluminescence (PL) spectrum weremeasured on the Ag2Mo2O7 nanowires on a glass slice under theirradiation of 30 mW HeCd laser at wavelength of 325 nm.

3. Results and discussion

Fig. 1a gives a SEM image of the synthesized sample showingwire-like morphology with length up to 40 mm and width 100–200 nm. XRD peaks of the synthesized product can be indexed withanorthic phase Ag2Mo2O7 with card no. 75-1505 as shown in Fig. 1b.The magnetic property of the Ag2Mo2O7 nanowires has been

Fig. 3. Crystal structure (a) of 2�1�1 supercell of Ag2Mo2O7 containing 22 atoms (pu

energy band structures of the Ag2Mo2O7 without (b) and with (c) oxygen vacancies re

reader is referred to the web version of this article.)

measured at room temperature. Fig. 2a shows the hysteresis loopof the Ag2Mo2O7 nanowires with field sweeping from �7000toþ7000 Oe. The anorthic phase Ag2Mo2O7 nanowires shows ferro-magnetic behavior. The magnetization decreases under an appliedmagnetic field after reaching the saturated magnetization. In Fig. 2b,the enlarged M–H curve in the center part displays a coercive field of127.7 Oe and a remanent magnetization of 16.9�10�3 emu/g. Thecoercivity and remanent magnetization observed here for anorthicphase Ag2Mo2O7 nanowires is more than four times greater than thatof the monoclinic FeMoO4 nanorods [15]. With the best of ourknowledge we have not found any report about the magneticproperty of the anorthic phase Ag2Mo2O7.

In order to explain the room-temperature ferromagneticproperty, the spin-polarized density functional theory (DFT)calculations were carried out by the Vienna ab initio SimulationPackage (VASP) using projector augmented wave (PAW)

rple, olive and blue color represents Ag, O and Mo atom). The calculation results of

spectively. (For interpretation of the references to color in this figure legend, the

-100

10 crystal total

-2

0

2

DO

S &

PD

OS

(sta

tes/

eV)

Mo total Mo s Mo p Mo d

-4

0

4 Ag total Ag s Ag p Ag d

8 6 4 2 0 -2 -4 -6 -8-1

0

1

Energy (eV)

O total O s O p

-100

10 crystal total

-202 Mo tot

Mo-d

-202 Ag-tot

Ag-d

DO

S &

PD

OS

(Sta

tes/

eV)

6 4 2 0 -2 -4 -6-0.6-0.30.00.30.6

O tot O-p

Energy (eV)

Fig. 4. Calculated results of electron density of states (DOSs) and the electron

partial density of states (PDOSs) of the Ag2Mo2O7 without (a) and with (b) oxygen

vacancies.

300 400 500 600 700 800

Inte

nsity

(a.u

.)

Wavelength (nm)

λex = 325 nm, 300K

Fig. 5. PL spectrum of the Ag2Mo2O7 nanowires measured at room temperature.

M. Hashim et al. / Physica E 46 (2012) 213–217216

pseudopotential to describe the interaction between electron andion [17–22]. Generalized gradient approximation (GGA) in thescheme of Perdew–Bueke–Ernzerh of (PBE) was employed todescribe the exchange and correlation function. Fig. 3a presentsthe three dimensional crystal structure of 2�1�1 supercell ofAg2Mo2O7 containing 22 atoms (purple, olive and blue representsAg, O and Mo atom). Two types of the supercells, one is of perfectlattice and the other lacks one oxygen atom (form an oxygenvacancy), are considered. The pseudopotential for O, Mo and Agatoms represents O 2s22p4, Mo 4s24p64d55s1, and Ag 4s24p6

4d105s1 electron configurations, respectively. An octahedron isformed by a Mo atom surrounded by 6 oxygen atoms and Ag atomlocates at center of the tetrahedron built by 4 oxygen atoms, asshown in Fig. 3a. The calculation results of energy band structuresof the Ag2Mo2O7 without and with oxygen vacancies are shown inFig. 3b and c. From Fig. 3b, we can see that both the top of valenceband and the bottom of conduction band locate at R point and theband gap is 1.9 eV, which is underestimated in comparison withthe experimental value of 2.94 eV [20] due to the limitations ofGGA. Though the band gap calculated by the GGA is not accurate,the electronic states near the top of valence and bottom ofconduction band can be well evaluated [21]. Ag2Mo2O7 withoxygen vacancies has two new electronic levels within the bandgap compared with that of perfect Ag2Mo2O7 due to the deforma-tion of the crystal lattices and electrons trapped by the oxygenvacancies, as shown in Fig. 3c.

To understand clearly the change of the energy band, theelectron density of states (DOSs) and the electron partial densityof states (PDOSs) of the Ag2Mo2O7 without and with oxygenvacancies crystal were calculated and, the results are shown inFig. 4. In Fig. 4a, the top of valence band is mainly composed ofthe hybridization of Ag 4d and O 2p states, while the bottom ofthe conduction band is mainly formed by the hybridization of Mo4d and O 2p states. The electron spin-up and spin-down states aresymmetric, demonstrating that no magnetism would exist in theAg2Mo2O7 without oxygen vacancies. In a perfect Ag2Mo2O7

crystal, Ag, Mo and O ion is þ1, þ6 and �2 respectively, withelectron configurations of Ag 4s2 4p6 4d10 5s0, Mo 4s2 4p6 4d0 5s0,O 2s2 2p6, respectively. Since the electron shells are fully occupiedby electrons, no magnetic properties can be observed in theperfect Ag2Mo2O7 crystal, and diamagnetism appears under theapplied magnetic field. However, with the consideration of oxy-gen vacancies in Ag2Mo2O7, near the bottom of the conductionband, the splitting of electron spin-up and spin-down states isobserved, and two impurity levels appear, which mainly comefrom the hybridization of Mo 4d, Ag 4d and O 2p states as shownin Fig. 4b. The magnetic properties appear due to the asymmetryof the spin-up and spin-down states. The lattice deformationproduces when the oxygen vacancy VO is formed with an oxygenatom removed from the supercell. VO might be positively chargedas 0, þ1 and þ2, due to ionization energies of oxygen vacancies.The electron configurations of Ag and Mo around the oxygenvacancy are changed as a result of the electrons bonded byoxygen vacancy acting on nearby Mo 4d and Ag 4d electrons,and the largest change of Mo 4s2 4p6 4d1 5s0 or 4s2 4p6 4d2 5s0

can be seen from the PDOS in Fig. 4b.According to the crystal field theory, the crystal-field is formed

by the electrostatic reaction between the central ion and thesurrounding ligands, in which atomic obits would be hybridizedor overlapped to build molecular orbits. The d orbits can bedivided into dxy,dyz,dxz(t2g) and dz2,dx2�z2(e2g) based on thesymmetry. The bond length of the octahedron formed by a Moion with 6 oxygen atoms is stretched by the deformation as aresult of the oxygen vacancies and the symmetry is also reduced.The spin-up and spin-down states below the conduction band areobviously splitting and thus forming two impurity bands because

the descent of dxy, dyz bands in Mo ion due to the Jahn–Tellereffect, where one or tow electrons of Mo 4d occupy the twocorresponding configurations of t2g1m or t2g2m. From this wecould clearly understand the results in Fig. 2. Therefore, the totalmagnetic moment is not zero and the crystal exhibits the

M. Hashim et al. / Physica E 46 (2012) 213–217 217

location- and concentration-dependant magnetism. The ferro-magnetism of the Ag2Mo2O7 nanowires showed in Fig. 2 agreeswell with the above analysis. The magnetization decreases underan applied magnetic field after reaching the saturated magnetiza-tion, revealing the obvious diamagnetism.

PL spectrum of the Ag2Mo2O7 nanowires measured at roomtemperature is shown in Fig. 5, from which we can see two peaks,one centered at 409 nm corresponding to the band edge emissionand the other broad emission ranged from 550 nm to 700 nmowing to the oxygen vacancies, providing a good evidence of theexistence of oxygen vacancies.

4. Conclusions

The anorthic phase Ag2Mo2O7 nanowires synthesized by thehydrothermal method exhibit ferromagnetic property at room tem-perature with coercivity 127.7 Oe and remanent magnetization16.9�10�3 emu/g. The theoretical calculation demonstrates twonew electronic levels within the band gap formed due to thedeformation of the crystal lattices and electrons trapped by theoxygen vacancies in Ag2Mo2O7 crystal. The spin-up and spin-downstates below the conduction band obviously split and form twoimpurity bands because the descent of t2g bands in Mo ion due to theJahn–Teller effect, where one or two electrons of Mo 4d occupy thetwo bands corresponding configurations of t2g1m or t2g2m. Therefore,the total magnetic moment is not zero and Ag2Mo2O7 crystal presentsthe location- and concentration-dependant magnetism. The oxygenvacancies in the Ag2Mo2O7 nanowires are also demonstrated by theroom temperature PL spectrum.

Acknowledgments

This work has been funded by The NSFC (60976055), SRFDP(20110191110034), and sharing fund of large-scale equipment ofChongqing University.

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