magnetic ordering in fe2−xznxmoo4(x=0.1–1) spinel

*Corresponding author. fax:#091-033-3374637.E-mail address: [email protected] (R. Ranganathan).

Journal of Magnetism and Magnetic Materials 223 (2001) 39}49

Magnetic ordering in Fe2~x

ZnxMoO

4(X"0.1}1) spinel

Anindita Ray!, R.N. Bhowmik!, R. Ranganathan!,*, Abhijit Roy",J. Ghose", Sujeet Chaudhury#

!Low Temperature Physics Section, Saha Institute of Nuclear Physics, Sector 1, Block 1/AF Bidhan Nagar, Calcutta 700 064, India"Chemistry Department, Indian Institute of Technology, Kharagpur 721302, India

#Low temperature Physics Laboratory, Centre For Advanced Technology, Indore 452009, India

Received 20 July 2000; received in revised form 3 October 2000

Abstract

We have studied the diluted Fe2~x

ZnxMoO

4spinel ferrite which shows a frozen disordered magnetic state at low

temperature. Magnetic properties are examined by DC magnetisation measurements as a function of temperature, "eldand time and AC susceptibility experiment. Our measurements show that this disordered magnetic system at low "elds,shares many common features with spin glass or cluster glass like phases. Results suggest that the interaction graduallychanges as the magnetic ion concentration decreases by the substitution of non-magnetic Zn on A site and causinga perturbation to the magnetically ordered spins and magnetic order decreases. ( 2001 Elsevier Science B.V. All rightsreserved.

PACS: 75.50.Lk; 75.50.Gg

Keywords: Spinel; Ferrimagnetic; Magnetisation; Spin-glass; Disorder

1. Introduction

Among the disordered materials which displaynon-conventional magnetic properties, those withthe spinel structure have received a great deal ofinterest over the last 15 years and a variety ofperturbed magnetic orders have been reported.A large but non-exhaustive list of the investigatedcompounds is given in Refs. [1}3]. Systems withspinel structure are found to be more interestingbecause they bring in the possibility of introducinga variety of magnetic disorder and frustration in the

system. The spinel structure consists of a closedlattice of oxygen ions with two types of interstitialsites for cations, the A and B sites with tetrahedraland octahedral symmetry, respectively. The highlystable spinel structure allows di!erent cations to belocated on the same type of site and owing to thesite preference of the cations many selective mag-netic substitutions and also various degrees ofmagnetic dilutions in the two sublattices may beobtained. Thus the strength of the exchange inter-actions may be controlled not only by the choice ofcations but also by the choice of the anion.

Spinel oxides of general formulaA

xD

1~x[B

yD

1~y]2O

4where A and B are para-

magnetic cations on the A and B sites and D isa diamagnetic ion display a NeH el ferrimagnetic

0304-8853/01/$ - see front matter ( 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 5 9 0 - 4

structure for x"y"1. Generally in the absence ofany dilution the three exchange integrals J

AB, J

AA,

JBB

are negative, the inter sublattice interactionJAB

being much stronger than either of the twointra sublattice interactions J

AAand J

BB. Thus all

the A site moments are antiparallel to the B sitemoments in the undiluted spinels where as AA andBB bonds may remain frustrated because they areunsatis"ed. As a result of this unsatis"ed bondsincreasing magnetic dilution accentuates the com-petition between various exchange forces givingrise to a variety of magnetic structures [1}4]. Inorder to classify di!erent substituted ferrite systemsVillain [4] has developed a model for spin-glassand semi-spin-glass phase. A semi-spin-glass phaseis one where two transitions are expected: the "rstat a high temperature for the longitudinal spincomponent which orders ferrimagnetically and thesecond at a low temperature for the transverse spincomponent which has a spin-glass-type order.A reentrant state predicted from mean "eld theoryis phenomenologically equivalent to the semi-spin-glass state. Dormann and Nogues [1}3] havereviewed the magnetic structures of various sub-stituted ferrites and have proposed a phase diagramto classify them especially the systems with local-ised canted states (LCS). They have shown thatthis LCS result from both the dilution and thefrustration of the exchange interaction. At low tem-peratures a localised canted state (LCS) is phenom-enologically equivalent to that of reentrant state orsemi-spin-glass phase. The only di!erence lies in theappearance of a collinear structure at high temper-ature which is not expected in LCS model.

Several disordered systems likeZnCr

2xGa

2~2xO

4with certain ranges of dilution

(0.4)x(0.85) have shown to exhibit spin-glass-like behaviour [5}7]. Local canted state (LCS)behaviour was observed in Zn

xMg

1~xFe

2O

4(x'0.5) [8]. A perturbed magnetic ordering inthe Zn

xCo

1~xFeCrO

4(x* 0.5) [9] and also in

ZnCr2x

Ga2~2x

O4

(0.1)x)1) [10] have alsobeen reported. In all the studies the observationswere based on AC susceptibility, DC magnetisa-tion, Hysteresis loop, Mossbauer spectroscopy andneutron di!raction measurements. Neutron di!rac-tion can give an unambiguous microscopic evid-ence of long- or short-range magnetic order and

may provide a distinction between spin-glass andsemi-spin-glass states. Although numerous com-pound have been investigated, the existing experi-mental results and theoretical interpretationsindicate that the complete understanding of theproblem of complex ferrite structures arising fromthe change of magnetic composition and dilution ofsublattices by various non-magnetic ions has notyet been achieved.

We report the magnetic measurements carriedout on the spinel ferrite Fe

2~xZn

xMoO

4,

(x"0.1, 0.2, 0.4, 0.6, 0.8, 1). Fe2MoO

4is an inter-

esting spinel oxide and has been studied by variousworkers [11}14]. This system shows a ferrimag-netic Curie temperature (¹

C) 348K and a spin com-

pensation temperature (¹K)&160K, where the two

sublattice magnetisations are equal so the net mag-netisation is zero. The formal valence distributionof Fe

2MoO

4spinel oxide may be represented as

Fe2`B

#Fe3À

#Mo3` H Fe3`B

#Fe2À

#Mo3`

HFe2`B

#Fe2À

#Mo4` so that Mo4` and Mo3`

along with Fe2` and Fe3` ions are present on theoctahedral sites (B) and Fe2` ion and Fe3` ions onthe tetrahedral site (A) of the spinel lattice [15].Recently, Roy et al. [16] have studied the low-"eldmagnetisation of titanium-substituted Fe

2MoO

4,

i.e. Fe2Mo

1~yTi

yO

4samples. For y)0.4 they ob-

served a strong negative remanence and a spin com-pensation temperature below Curie temperature. Itis observed that as Ti replaces Mo, the Curie temper-ature and the compensation temperature decreases(¹

C&345}120K and ¹

K&160}110K for y"

0}0.8). In Fe2MoO

4, Fe is present in both octa-

hedral and tetrahedral sites as in Fe2TiO

4. If the

AA and BB interactions in the former ferrite areonly due to Fe then the magnetic interactionsshould be similar to those in latter ferrite. M vs.¹ plots predicted by Neel of the two ferrites are notsimilar. This implies that the B}B interaction in thisferrite is not comparable or greater than the A}Binteraction. However studies on this ferrite haveindicated that the A}A and B}B interaction arecomparable [15]. Thus it appears that the magneticinteraction in Fe

2MoO

4is not due to Fe alone and

a stronger B}B interaction giving rise to compensa-tion temperature suggests that the Mo present onthe octahedral sites are also taking part in themagnetic interactions of the ferrite. On substituting

40 A. Ray et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 39}49

Fig. 1. Temperature dependence of the "eld-cooled and zero"eld-cooled magnetisations of Fe

1.9Zn

0.1MoO

4at 30Oe.

Mo by Ti, however, both Fe3` and Mo3` ions areremoved thereby the strength of the B}B interac-tions in Fe

2Mo

1~yTi

yO

4is decreased. Thus

a lowering of compensation temperature with y isexpected and implying that the B}B interaction issmall and likely to be less than the A}B interaction.

Roy et al. [17] in their preliminary work haveshown how magnetic interactions of Fe

2MoO

4change as the tetrahedral site magnetic ions Fe2` isprogressively replaced by the diamagnetic ionZn2` (Fe

2~xZn

xMoO

4). They have concluded that

with the substitution of Zn2`, A}A interaction be-comes weak and thereby the M vs. ¹ plots do notshow any compensation temperature. A decrease inthe magnetic ions (Fe2`) on the A site also leads toweaker A}B interactions, and a complex order isintroduced. In view of the above we have decided toperform low-"eld magnetisation, thermo-remanentmagnetisation (TRM) relaxation, high-"eld mag-netisation measurements to understand the natureof ordering in these spinels Fe

2~xZn

xMoO

4.

2. Experimental

Fe2~x

ZnxMoO

4(x "0.0, 0.2, 0.4, 0.6, 0.8, 1.0) is

an inverse spinel form solid solutions throughoutthe entire concentration range as shown by X-raydi!raction. The detailed sample preparations aregiven in Ref. [17]. FC, ZFC magnetisation, hyster-esis, AC susceptibility measurements were carriedout on the powder samples in the form of pelletswith typical dimensions, length &10 mm, breadth&6 mm, and thickness &3 mm. Low-"eld mag-netisation measurements were carried out witha home made DC magnetometer [18] over thetemperature range 15}300K in the magnetic "eldsupto 60Oe. High-"eld magnetisation measure-ments were carried out with SQUID magnetometerbetween 5 and 300K in the magnetic "elds upto30 kOe. In ZFC measurements the sample wascooled from room temperature to the lowestmeasurable temperature in the absence of any ap-plied magnetic "eld, then magnetisation measure-ments were made in the warming cycle by applyingthe desired magnetic "eld. No e!orts were made tocompensate earth's magnetic "eld. In the FC case,the sample was cooled from room temperature to

the set temperature in presence of an applied mag-netic "eld and then magnetisation measurementswere carried out while heating the sample. Thedecay of thermoremanent magnetisation (TRM)with time and temperature were obtained under30Oe FC condition.

3. Results and discussions

Roy et al. [17] shows that the Zn-substitutedFe

2MoO

4(x"0.2 to 1) samples do not have any

compensation temperature ¹K. Even for Zn"0.1,

i.e. Fe1.9

Zn0.1

MoO4

sample there is no spin com-pensation temperature, shown in Fig. 1. This im-plies that with the substitution of Fe

Aby Zn

Athe

anomalous magnetic behaviour of Fe2MoO

4is no

longer observed probably due to changes in themagnetic interactions in the ferrite. PreliminaryDC magnetisation measurements at 30Oe by Royet al. show that the magnetisation increases as Fe isprogressively substituted by Zn but only uptox"0.6, at higher doping level of Zn (x'0.6) mag-netisation decreases. The reason for this may be thefollowing: there are more B sites than A sites so thenet magnetisation is simply the di!erence betweenthe B and A sublattice magnetisations. Low dopingconcentrations lead to a decrease in the number of

A. Ray et al. / Journal of Magnetism and Magnetic Materials 223 (2001) 39}49 41

Fig. 2. Temperature and "eld dependence of "eld-cooled andzero "eld-cooled magnetisations of Fe

1.8Zn

0.2MoO

4sample.


1.6Zn

0.4MoO

4sample.

spins occupying the A sublattice and this leads toan increased net magnetisation in the direction ofthe B sublattice. As Zn doping is increased theB spins are no longer held rigidly in place byantiferro-magnetic coupling to the few remainingA spins and the antiferromagnetic interaction be-tween B spins may lead to canting on the B sublat-tice. This in turn may lead to a decrease in the netmagnetisation.

Since low-"eld measurements are often used toidentify the nature of spin or cluster glass likephases, we start with the low-"eld results of x"0.2sample. Fig. 2 shows M

FC(¹) and M

ZFC(¹) at di!er-

ent low "elds for x"0.2 sample. ZFC maximum(¹

.!9) and irreversibility temperatures (¹

*33) are

weakly "eld dependent and shifts to lower temper-ature with the increase of "eld. Here ZFC magnet-isation does not reach zero at ¹)15 K. Fig. 3shows FC, ZFC data at di!erent "elds for x"0.4.For this concentration strong irreversibilities and"eld dependence of ZFC maximum temperaturehave been observed. However the nature of the FCmagnetisation curves for x"0.2 and 0.4 at lowtemperature are di!erent. FC magnetisation ofx"0.4 sample continuously increases until thetemperature&20K is reached but for x"0.2sample a maximum in FC magnetisation (at

¹&200K at 60 Oe) has been observed. Sucha sharp rise in FC magnetisation or the large re-manence (i.e. the di!erence between M

FCand

MZFC

) may be associated with the appearance ofthe "nite-range ferromagnetic coupling, formingthe clusters around ¹

C&300 K [19]. Absence of

true long-range order at low temperature is appar-ent from the large di!erence between M

FC(¹) and

MZFC

(¹) curves.As the A site is more and more diluted, J

ABde-

creases, thus increasing frustration and disorder onB site. Fig. 4 shows the ZFC and FC magnetisationdata for x"0.6 sample for DC "elds 2}60Oe. Inall "elds M

ZFC(¹) shows a broad maximum while

MFC

continuously increases with the decrease intemperature for ¹(20K. It is apparent that theFC}ZFC branching (¹

*33) and ZFC maximum

(¹.!9

) [194K (2Oe), 190K (10Oe), 174K (30Oe)and 160 K (60Oe)] are strongly "eld dependent, i.e.shifted to low temperature with higher "elds. In allcases ZFC magnetisation at about 20K is zero.These features are also observable in x"0.4sample. It may be recalled that strong irreversibili-ties between FC and ZFC magnetisation is one ofthe characteristic feature of spin-glass, cluster glass,semi-spin-glass type of systems. Villain [4] has pre-dicted two transitions in a semi-spin glass. The NeH el



1.4Zn

0.6MoO

4sample.

Inset shows the "eld dependence of the reduced temperatures¹

*33/¹

4and ¹

.!9/¹

4. Dotted curves are "t to Eq. (1), described

in the text.

Fig. 5. Temperature dependence of the "eld-cooled and zero"eld-cooled magnetisations (a) for Fe

1.4Zn

0.6MoO

4at 20 kOe.

(b) for FeZnMoO4

at 30kOe.

temperature corresponds to collapse of longitudi-nal spin component and transverse spin componentfreezes at some lower temperature ¹

G. Results of

x"0.6 sample do not show such behaviour.In an attempt to relate the FC}ZFC branching

with the mean "eld d'Almeida-Thouless line [20]i.e. a spin-glass-like freezing, we have plotted thereduced temperatures ¹

.!9/¹

4(¹

.!9is the temper-

ature where MZFC

(¹) attains a maximum, and ¹4is

the temperature at which magnetisation starts toincrease &238K) and ¹

*33/¹

4(¹

*33is the temper-

ature where MZFC

(¹) and MFC

(¹) irreversibilitystarts) vs. applied "eld, are shown in the inset ofFig. 4. It has been found that these temperaturescould scale with [21] as follows:

¹.!9

/¹4,¹

*33/¹

4a1!

A

¹4

Hn. (1)

The best "ts are obtained with AK0.58 andnK1.07 for the case of ¹

*33/¹

4and AK1.76,

nK0.79 for the case of ¹.!9

/¹4. For real spin glass

¹4

is replaced by the spin-glass transition temper-

ature. Mean "eld theory predicts n"23. However,

this is only a necessary feature of spin glass but nota su$cient one.

Fig. 5(a) shows that there is no di!erence be-tween the FC, ZFC curves in a "eld of 20 kOe. This


Fig. 6. Magnetisation vs. applied "eld for Fe1.4

Zn0.6

MoO4

atdi!erent temperatures.

Fig. 7. Temperature and frequency dependence real s@ andimaginary sA parts of AC susceptibility of Fe

1.4Zn

0.6MoO

4.

Inset shows the frequency dependence of sA peaks "tted withVogel}Fulcher law.

implies that the irreversibility decreases as the mag-netic "eld is increased from low "eld ((100 Oe) toa high "eld. M(H) data for three di!erent temper-atures 5, 125, 200 K are shown in Fig. 6. The sampleexhibits a hysteresis loop at 5 K and M(H) curve at5K is nearly saturated at 10 kOe "eld but 125,200K curve is nearly saturated at 2.5 kOe. Thissaturation of magnetisation suggests the ferrimag-netic behaviour.

AC susceptibility and thermoremanent magnet-isation measurements are useful to understand thedynamics of freezing of the magnetic glassy system.For example in spin-glass systems the temperatureassociated with the maxima of s@, corresponding toa PM}SG transition varies slowly with the measur-ing frequency. The strong frequency dependencehas been suggested to be signature of clusterfreezing at ¹)¹

#. Moreover the susceptibility

itself is frequency dependent even above ¹&

forsuperparamagnetic clusters, whereas it is slowlyfrequency dependent below ¹

&only for spin glass.

So study of frequency dependence is useful to com-pare number of systems. Fig. 7 shows s@, sA fordi!erent frequencies in the range 37}7 kHz at H

AC

K1 Oe. The maximum of s@(¹) and sA(¹) are fre-quency dependent. s@(¹) maximum shifts to highertemperature and decreases in magnitude with theincrease in frequency. The shift in sA(¹) maximumwith frequency is also consistent. An attempt hasbeen made to "t the frequency dependence of sA(¹)maxima using the Vogel}Fulcher law

u"u0

exp[!E!/k

B(¹

&!¹

0)] (2)


Fig. 8. ZFC magnetisation relaxation for Fe1.4

Zn0.6

MoO4.

Fig. 9. Time dependence of the thermoremanent magnetisationat di!erent temperatures after cooling in 30Oe "eld andt8"100 s for Fe

1.4Zn

0.6MoO

4sample. Solid lines are logarith-

mic "ts, Eq. (3), as described in the text.

[shown as inset of Fig. 7] with ¹0(¹

&,

u0"1.6]105Hz and E

!/k

B"2.29. These values

are smaller than those for canonical spin glass (e.g.for CuMin u

0K108 Hz) and for systems described

in terms of progressive freezing of clusters [19].Fig. 8 represents the relaxation of ZFC magnet-

isation measured at ¹"100K with waiting timet8"100, 1000, 2100 s. Age-dependent behaviour is

observed in spin glass, reentrant spin-glass, clusterglass systems, etc. with di!erent relaxation rates[22}24]. Various functional forms have been pro-posed to describe the magnetisation as function oftime and wait time. The commonly used stretchedexponential function does not "t well the TRM(t)data measured with t

8"100 s. Linearity of M vs.

log(t) curves as shown in Fig. 9 indicate that mag-netic relaxation can be described by the logarithmiclaw

M(t)"M0!S log(t), (3)

where &S' is the magnetic viscosity. The logarithmiclaw can be obtained on the basis of a NeH el clustermodel taking into account of the existence of a widedistribution of relaxation times [25,26]. In the en-tire temperature range (75}190K) &S' varies be-tween 0.011 and 0.14.

The experimental results namely the "eld-depen-dent ¹

.!9, ¹

*33at low "elds, frequency-dependent

AC s data, ageing e!ect as discussed above forx"0.6 sample suggest that there exists short-rangeferrimagnetic order with frustration. At low tem-perature and at low "eld the system exhibitsnonequilibrium glassy features which indicatesthe presence of frustration and short-range order(cluster-glass or spin-glass-like freezing as shownin Figs. 4 and 7). But this system stabilises to aferrimagnetic behaviour at higher "elds (Figs. 5(a)and 6).

As the A sites are more and more dilutedJAB

decreases, thus one would expect increasingfrustration and disorder. FC, ZFC magnetisationmeasurements with temperature for x"0.8 sampleis shown in Fig. 10. The ZFC maximum and irre-versibility temperature are "eld dependent for this



1.2Zn

0.8MoO

4sample.

Inset shows the "eld dependence of the reduced temperature¹

.!9/¹

4"tted with Eq. (1).

Fig. 11. Time dependence of the thermoremanent magnetisa-tion at di!erent temperatures after cooling at 30 Oe "eld andt8"100 s for Fe

1.2Zn

0.8MoO

4sample. Solid lines are logar-

ithmic "ts, Eq. (3), as described in the text.

concentration also like x"0.6 sample. ZFC maxi-ma (¹

.!9) occur at 73 K (2Oe), 67K (10Oe), 62 K

(30Oe) and 56K (60Oe). Inset of Fig. 10 showsthe "eld dependence of the reduced temperature¹

.!9/¹

4(where ¹

4&120K), "tted with Eq. (1) and

the &n' value is 0.366 which is less than the mean"eld value n"2

3. The logarithmic law (described by

the Eq. (3)) "ts well with the TRM(t) data measurewith t

8"100 s as shown in Fig. 11. It is apparent

that for x'0.6 the concentration of Fe2` in A siteprobably become very small and B}B interactionbecoming comparable or greater than A}B interac-tion. This in turn may give rise to a complex mag-netic order.

In Fe2~x

ZnxMoO

4, by replacing Fe with Zn

in tetrahedral site the chemical formula may berewritten as Zn

(1~xA )Fe2`

xA[Fe2`

xBMo4`

0.5]2O

4. For

Zn"0.2 concentration xA"0.8, x

B"0.5. On

comparing these values with the magnetic rangeorder diagram for the diluted spinels [27] it is clearthat the long-range ferrimagnetic order (LRFO)and short-range ferrimagnetic order (SRFO) bothare present but the M(¹) data at low "elds for

Zn"0.2 sample [Fig. 2] suggest SRFO not theLRFO. When Zn concentration is increased to 0.6,i.e. x

A"0.4, x

B"0.5. Ref. [27] suggests that the

SRFO is large compared to LRFO. For Zn"0.8,i.e. x

A"0.2, x

B"0.5, short-range antiferromag-

netic order (SRAO) is prominent one. We believethat the Zn"0.6, and 0.8 samples are in thecrossover region of SRFO to SRAO. This maybe the reason for the "eld dependence of irreversi-bility temperature at low "eld. Frequency-depen-dent AC susceptibility data supports the glassyfeatures.

Earlier studies on Fe2Mo

1~yTi

yO

4[16] have

shown that Mo plays an important role in themagnetic interaction of the compounds and thecomplex magnetic order in Fe

2~xZn

xMoO

4may

be due to strong Mo}Mo interactions. The sampleFeZnMoO

4(x"1.0) is similar to that of ZnFe

2O

4(being antiferromagnetic, ¹

N&10K) if Mo takes

part in the interaction. We have made magnetisa-tion measurements in order to see the in#uence onmagnetic ordering due to the non-magnetic Znconcentration x"1. Temperature dependence of


Fig. 12. Temperature and "eld dependence of "eld-cooled andzero "eld-cooled magnetisations of FeZnMoO

4sample.

Fig. 13. Thermoremanent magnetisation MR

vs. temperatureand M

FC}M

ZFCvs. temperature at 30 Oe for FeZnMoO

4.

Fig. 14. Magnetisation vs. applied "eld for FeZnMoO4

at twodi!erent temperatures.

FC, ZFC magnetisation for three di!erent mag-netic "elds are shown in Fig. 12. FC and ZFCmagnetisation both show maximum at the sametemperature (&32 K) and there is no "eld depend-ence. The irreversibility temperatures are also "eldindependent. Remanence (M

FC}M

ZFC) is not so

large in comparison with x"0.6 sample. This low-"eld FC, ZFC behaviour (irreversibility, ZFC max-imum) is disappeared in 30 kOe "eld as shown inFig. 5(b). Temperature dependence of thermorema-nent magnetisation and M

FC}M

ZFCare shown in

Fig. 13. There is a change in slope in both thecurves at &30K where the maximum in FC, ZFCdata (&32K) occurs. M(H) curves at 10 and 50 Kis shown in Fig. 14. Hysteresis loop exists at 10 Kand magnetisation is far from saturation even ata "eld of 30 kOe. The lack of saturation of magnet-isation in such a high "eld suggests the cantedstructure. This may happen because A site isstrongly diluted by non-magnetic Zn and this inturn reducing the strength of A}B interaction andB spins are no longer held rigidly in place by thatweak AF A}B interaction. As a result magnetisa-tion value is reduced considerably for x'0.6sample. The relaxation of the low-"eld (30Oe) ther-

moremanent magnetisation for FeZnMoO4

hasbeen measured with waiting time t

8"100 s which

is shown in Fig. 15. Relaxation of TRM is charac-terized by the logarithmic law as in the case of


Fig. 15. Time dependence of the thermoremanent magnetisa-tion at di!erent temperatures after cooling in 30Oe "eld andtw"100 s for FeZnMoO

4. Solid lines are logarithmic "ts, Eq. (3),

as described in the text.

Zn"0.6 (Eq. (3)). According to Ref. [27] Zn"1system should be antiferromagnetic but experi-mental results suggest canted like state. However,we expect that the presence of impurity phase likeFe

2Mo

3O

8/Zn

2Mo

3O

8does not in#uence our re-

sult like in Fe2MoO

4[17]. Our system is magneti-

cally inhomogeneous due to Zn substitution andfrustrated causing a perturbation to the mag-netically ordered spins and hence magnetic orderdecreases. The intersublattice interaction J

ABweakens with increasing dilution in the A site andJAB

becomes comparable to JBB

giving rise to com-petition between the B}B bonds. As a consequencethere is a possibility of a strong noncollinearity inthe B site moment.

4. Conclusions

This work gives information about the nature ofmagnetic ordering. Low- and high-"eld magnetisa-tion measurements on Fe

2~xZn

xMoO

4spinel

show that on dilution of Fe at A site by non-magnetic Zn ion, increases frustration and disorder

on B site. As Zn concentration increases (x"0.6)a freezing process is established at low "elds whichevolves at higher "elds towards ferrimagnetic state.Then the system goes towards canted-like state(x"1.0). A clear description of magnetic orderingmay be obtained from MoK ssbauer spectroscopyand neutron measurements.

Acknowledgements

One of the authors RNB thank the Council ofScienti"c and Industrial research (CSIR) for pro-viding fellowship [F.NO.9/489(30)/98-EMR-I].

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magnetic ordering in fe2−xznxmoo4(x=0.1–1) spinel

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