12-Synthesis of superparamagnetic Mn3O4 nanocrystallites by ultrasonic irradiation
?Correspondingauthor.Tel.:+912225592327; fax:+91225505151.
E-mail address:ikgopal@magnum.barc.ernet.in (I.K.Gopalakrishnan).
a few nanometers,energy considerations favor the
formation of single domain particles,which could
exhibit unique properties such as superparamag-
netism[12,13].In order to obtain the desired
nanostructured materials,various methods such as
chemical bath deposition method[5],solid state
reaction[13],g-ray irradiation[14],and vapor-
phase growth[15]have been developed.Of late,
sonochemical processinghas proven to be an
useful technique for generating materials with
unusual properties[16–20].Sonochemically
synthesized materials are highly active in catalysis
due to their particle size and high surface area.The
chemical effect of ultrasound arise from acoustic
cavitation phenomena,i.e.,formation,growth,
and implosive collapse of bubbles in a liquid
medium.The cavitation collapse lead to many
extreme conditions,viz.extremely high tempera-
turee45000KT,high pressuree420MpaT,
and very high cooling ratese1010K sà1T,which
produce unusual chemical and physical environ-
ments.In this paper,we report the structural
and magnetic characterization of nanocrystalline
Mn3O4synthesized usingultrasonic irradiation of
aqueous solution of manganese acetate at the
ambient.
2.Experimental procedure
About1gof mang anese acetate tetrahydrate
(Aldrich(99+))was dissolved in100ml of distilled
water.The resultant solution was sonicated for
three hours in a conical?ask.The solution was
then centrifuged,washed with distilled water and
ethanol.The reddish brown product was then
dried at501C in a vacuum oven for few hours.
XRD patterns of the sample was recorded using
Ni?ltered Cu–K a radiation by employinga
Phillips Diffractometer(model PW1071)?tted
with graphite crystal monochromater.Structural
analysis was carried usingthe Rietveld re?nement
analysis employingcomputer prog ram FULL-
PROF[21]with X-ray intensity collected for the
range20 p2y p60 .Average size of the particles was determined from the extra broadeningof the
X-ray diffraction peaks of the sample using
Scherrer’s formula applied to the strongest peak.DC magnetization measurements as a function of temperature and?eld were carried out usingan E.G.&G P.A.R vibratingsample mag netometer (model4500).
3.Results and discussion
The chemical reactions driven by intense ultra-sonic irradiation are oxidation,reduction,dissolu-tion,and hydrolysis.The mechanism of the formation of Mn3O4nanocrystals takes into consideration the radical species generated in the water molecules by the adsorption of ultrasound [18].The reaction steps as well as the explanation for sonochemical reaction process can be summar-ized as follows:
H2OTTTTTHtOH
HtH:!H2
OHtOH!H2O2
MneCH3COOT2!Mn2tt2eCH3COOT2
Mn2ttH2O2!Mn3t.
The structure of Mn3O4is body-centered tetra-gonal containing4molecules per unit cell.The space group is I41=amd(No141)with manganese positions?xed by symmetry at4a and8d and oxygen in16h.The ionic structure is Mn2t?Mn3t 2O4[22–24].The detailed structural analysis was performed on the XRD data of Mn3O4,by Rietveld pro?le re?nement method with the computer program FullProf[21]using atomic coordinates of Mn3O4.The parameters varied were scale factor,zero point,and half-width parameters in addition to lattice parameters, positional coordinates,and overall temperature factor.These parameters were?rst varied indivi-dually then in groups.Care was taken to avoid varyinghig hly correlated parameters at the same time.
The structural parameters includingatomic coordinates and isotropic temperature factor of atoms are given in Table1.The lattice parameters obtained from the re?nement are a?5:7474e5T(A and c?9:457e1T(A.These are in good agreement
with that reported in the literature[22–24].The
I.K.Gopalakrishnan et al./Journal of Crystal Growth280(2005)436–441437
observed,the pro?le ?tted as well as the difference pattern for Mn 3O 4are presented in Fig.1.It can be seen from Fig.1and Table 1that observed data
?ts very well with reported structure indicating phase purity of the Mn 3O 4synthesized by us.The average size of the particles were calculated from broadeningof the 100%XRD peak,using the Debye Scherrer formula
t ?e0:9l T=b Cos y ,
where t ?thickness of the particles,l is the X-ray wavelength,y ?Bragg peak angle and b ????????????????????????????????????
ed 2nano àd 2
standard Tq ;d beingthe peak width at half maxima.The value comes out to be about 15nm.DC magnetization measurements as a function of temperature at different applied ?elds in the temperature range 5–200K are presented in Fig.2.As shown in the ?gure,at 100Oe applied ?eld,both the zero ?eld cooled (ZFC)and the ?eld cooled (FC)magnetization plots of the compound show a ferrimagnetic onset temperature eT c Tof about 39K.The observed T c is less than those reported eT c ?43K Tfor the bulk and the single crystalline compounds prepared by various
2θ(°)
Fig. 1.Room temperature X-ray diffractogram of Mn 3O 4nanocrystallites.The solid line is Rietveld ?t to the data.The bottom trace is the difference between ?t and observed pattern.The vertical lines indicate the peak positions.
Table 1
Structural parameters for as determined by Rietveld re?nement of X-ray data Wavelength (A
) 1.5406a (A ) 5.7674(5)b (A ) 5.7674(5)c (A )9.457(1)V e(A 3T314.579Space group I41=amd
Positional parameters Atom Site x y z Mn 2t4a 00.250.875Mn 3t8d 00.500.50O 2à
4f
0.484(4)0.255(9)
Overall temperature factor B e(A 2T 1.009(3)
R -factors e%T
R p 10.8R wp 14.5R exp 12.6R B
8.36Goodness of ?t 1.33
DW-statistics
d ?1:6584;Q ?1:8623
I.K.Gopalakrishnan et al./Journal of Crystal Growth 280(2005)436–441
438
synthesis routes [24–26].In general,a bulk ferromagnetic compound is multidomain in nature below its ferromagnetic transition temperatures.Although formation of domain walls adds to its energy,the mutidomain character reduces the total magnetization and hence the magneto-static en-ergy to zero,leading to the minimization of the total energy of the system.This multidomain character of a ferromagnetic system can be lost if the particle size is decreased below a critical size,at which the division of the aligned spins within a ferromagnetic system into domains no longer leads to the minimization of its total energy.The ferromagnetic particles are then termed as single domain with their magnetization aligned along the one of the easy directions of magnetizations.Like the bulk ferromagnetic systems,the single domain ferromagnetic systems also show hysteresis in their M–H behavior (?nite coercive ?eld and remanent magnetization)and thermomagnetic irreversibility (TMI)between their ZFC and FC plots.There is,however,an important difference;in the case of bulk form the hysteresis and the TMI result from the energy barrier associated with restriction in the domain wall formations and movements,whereas that in the case single domain form they result from energy barrier associated with restriction in orientation process of the single domain spins as
imposed by the anisotropy energy of the system.The anisotropy energy is de?ned as E ?KV Sin 2y ,
where V ?volume of the particle,y is the angle between the actual direction of magnetization and the easy direction of magnetizations and K is anisotropy constant.Hysteresis in the single domain ferromagnetic particles vanishes when the particle size becomes so small that the maximum anisotropy energy (KV for y ?90 )becomes close to the thermal energy,so that the process of ?ippingof the sing le domain spin becomes uninhibited.This state of the ferromag-netism is called ‘superparamagnetism’as it does not show any hysteresis in its M–H behavior and like in the case of paramagnetism,the magnetiza-tion never gets saturated even at very high applied ?eld.When superparamagnetic particles are cooled below certain temperature they transform to ferromagnetic state.The thermal energy at this temperature becomes signi?cantly less than the anisotropy energy eKV Tand thus cannot facilitate uninhibited ?ippingof the sing le domain spins.The ferromagnetic onset temperature decrease with decrease in size of the single domain particles.In the present study,the observation of lower value of the ferrimagnetic onset temperature (T c )as compared to those reported for the bulk [24–26]indicates that the particles are single domain in nature.Recently Changet al.[15]reported synthesis of nanocrystalline Mn 3O 4by vapor-phase deposition method and showed that a decrease in crystallite size from 80to 25nm results in a decrease in T c from 44to 42K.In the single domain state the Mn 3O 4particles should show superparamagnetism between bulk T c (43K)and the observed blockingtemperature eT B %37K T,as deduced from the peak in the ZFC plot.It can be seen from Fig.3(inset),they indeed show superparamagnetic behavior in their M–H plot recorded at 40K.It is known that both T B and TMI are sensitive function of applied magnetic ?eld as they decrease with increase in applied ?eld.In agreement with this we have observed a signi?cant decrease in TMI on increasing the applied ?eld from 100to 8000Oe.This also results in decrease of T B from 37to 10K.When the ?eld
0.0
0.5
1.0
1.5
T(K)
Fig. 2.DC magnetization of Mn 3O 4nanocrystallites as a function of temperature recorded at two different applied ?elds.Open symbols correspond to ZFC data while closed symbols indicate FC data.
I.K.Gopalakrishnan et al./Journal of Crystal Growth 280(2005)436–441
439
is applied at low temperatures for recordingthe ZFC plot,an incomplete alignment of super-paramagnetic particles along the ?eld direction results in a low value of observed magnetization at temperatures much below than the ferrimagnetic onset temperature.The reason for this is that at low temperatures a weak magnetic ?eld is unable to force the single domain particles to overcome the anisotropy energy barrier and align them along the ?eld direction.The peak in ZFC magnetization plot,which is taken as average blocking tempera-ture (T B ),results from two opposingeffects,viz.an increase in thermal energy with increase in temperature,which helps more and more particles overcomingthe anisotropy energ y barrier;and a decrease in magnetization on approaching the ferromagnetic onset temperature eT c T.In the FC mode the magnetic ?eld is applied above the ferromagnetic onset temperature thus resulting in a higher value of low-temperature magnetization than in the correspondingZFC plots.An increase in the applied magnetic ?eld leads to a decrease in this difference between the ZFC and the FC magnetizations (TMI).This is attributed to an increase in low temperature ZFC magnetization as a stronger magnetic ?eld can force more and more single domain particles to align along the ?eld direction even at low thermal energy.Fig.2shows an apparent increase in the ferromagnetic onset temperature with an increase in the applied ?eld from 100to 8000Oe.This is due to an increase in the magnetization of superparamag-netic particles with increase in the magnetic ?eld as shown in Fig.3.
It can be seen from isothermal magnetization plots for of Mn 3O 4nanoparticles recorded at different temperatures presented in Fig.3that the superparamagnetic character at 40K is no longer maintained as the temperature lowered to 10and 5K.At these two temperatures blockingof the single domain spins by the anisotropy forces gives rise to hysteresis in the M–H plots.At 5K the observed coercive ?eld is about 2500Oe,much less than that observed (4600Oe)for the multidomain single crystalline Mn 3O 4[25].In the single domain magnetic particles the coercive ?eld is proportional to the anisotropy density (KV )and it decreases with decrease in the particle size (V ).The
-10000-7500-5000-2500025005000750010000
-0.08
-0.06-0.04-0.020.00
0.020.040.060.08
H(Oe)
Fig.3.Isothermal magnetization vs.?eld behaviors of Mn 3O 4nanocrystallites at three different temperatures (5,10and 40K).Inset shows an enlarged view of the magnetization plot at 40K showing superparamagnetic behavior.
I.K.Gopalakrishnan et al./Journal of Crystal Growth 280(2005)436–441
440
difference in the coercive?eld observed by us to that reported for the single crystalline Mn3O4can thus be attributed to the difference in the blocking mechanism of the spins and the small size of the nanoparticles prepared by use%15nmT.A de-crease in the coercive?eld with an increase of temperature from5to10K is consequence of easingof blockingin the sing le domain spins in presence of higher thermal energy.The effect of size on the magnetic properties is also re?ected in the observation of a low value of saturation magnetizationeM STof the nanoparticles at5K. The value is as low as0:07m B=molecule of Mn3O4, much lower than the reported value of1:85m B for the single crystalline Mn3O4at an applied?eld of 10,000Oe.Such large reduction in the saturation magnetizationeM STvalues on reducingthe size of the particles from bulk to the nanometer scale has been observed in the case of other magnetic spinels like g-Fe2O3[27].A disorderingof the mag neti-cally ordered spins due to their cantingat the surface as well as at the core of the nanoparticles results in a net decrease in saturation magnetiza-tion[28,29].Mn3O4nanoparticles can be visua-lized as havinga ferrimag netic core surrounded by a surface layer of disordered spins.
In conclusion,in this paper we report room temperature synthesis of nanocrystalline Mn3O4 by ultrasonic irradiation of Mn-acetate solution in water.Rietveld re?nement analysis of the XRD data suggests the formation of a phase pure Mn3O4with average particle size of about15nm. DC magnetization studies show that the particles are single domain in nature with the observation of superparamagnetic behavior at40K. References
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