DSpace at VNU: Optical spectra of the colloidal Fe-doped manganate CaMn1-xFexO3 (x = 0, 0.01, 0.03, 0.05)

Optical Spectra of the Colloidal Fe-doped ManganateCaMn1−xFexO3 x = 0, 0.01, 0.03, 0.05 Duc Huyen Yen Pham, Duc Tho Nguyen, Duc Thang Pham and Nam Nhat Hoang∗ Faculty of Technical Physi

Optical Spectra of the Colloidal Fe-doped Manganate

CaMn1−xFexO3 ( x = 0, 0.01, 0.03, 0.05)

Duc Huyen Yen Pham, Duc Tho Nguyen, Duc Thang Pham and Nam Nhat Hoang∗

Faculty of Technical Physics and Nanotechnology, Vietnam National University, University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Ha Noi, Viet Nam

The Tan Pham

Faculty of Basic Science, Hung Yen University of Technology and Education, Hung Yen, Viet Nam

(Received 31 May 2012, in final form 19 December 2012)

We report the optical behaviors of the Fe-doped CaMnO3 family of compounds at low doping

concentrations x ≤ 5% The study aims at assisting the evaluation of the competition between

ferro-and antiferromagnetic orderings, which is believed to be a cause of many interesting properties of this

class of compounds, including the magnetization reversal effect recently discovered The structural

characterization showed a predominant orthorhombic phase with slightly increased cell constants

due to doping The Raman spectra revealed changes associated with the Mn sites, and the IR

absorption spectrum showed a characteristic Fe band at 1.2 eV, which should be accompanied by

a change of spin The analysis of the magnetization data allowed us to predict that while the

doping reduced the ferromagnetic coupling strength, and therefore the TC, the maximal doping

concentration for the effective exchange to be zero was around 14%

PACS numbers: 75.47.Lx; 75.50.Ee; 74.25.Fy; 75.30.Kz

Keywords: Perovskite, Manganate, Structure, Optical

DOI: 10.3938/jkps.62.2133

I INTRODUCTION

The multiferroics based on doped CaMnO3 have

at-tracted much attention from scientists because of their

many potential applications in modern spintronics [1]

Recently, Fe-doped CaMnO3[2] was reported to show a

magnetization reversal effect (MRE) in response to

tem-perature at low fields and low doping contents This

effect may be of extreme importance for

temperature-control devices Earlier reports indicated that this effect

in ferrimagnets was found in YVO3 [3]; (the

tempera-ture at which the magnetization reversal response

hap-pened was around 80K, which is twice as large as that (40

K) in CaMn1−xFexO3, LaVO3 [4,5] and spinel Co2VO4

[6] In doped CaMnO3, the effect may also be observed

in Gd1−xCaxMnO3 [7] and Dy1−xCaxMnO3 [8]. The

chrome-based perovskite compound La1−xPrxCrO3 [9]

also revealed a magnetization reversal in response to

tem-perature Unlike YVO3 whose MRE is associated with

a first-order phase transition and is believed to originate

from the different alignments of the spins of the

sub-lattices according to temperature (from the presence of

a so-called anti-symmetric Dzyaloshinsky-Moriya (DM)

∗E-mail: namnhat@gmail.com

interaction between two nearest-neighbor V ions), the MRE in Fe-doped CaMnO3 was not accompanied by

structural changes and seemed to follow from a compe-tition between antiferromagnetic (AFM) and ferromag-netic (FM) orderings within a unified structural frame Because the development of the FM interaction is important for understanding the physics of Fe-doped CaMnO3 multiferroics, we present here an investigation

of the Curie temperature T C and the optical behaviors

of this class of compound at low doping contents (x ≤

0.05) The maximal doping content is below the critical

value of x = 0.08 where the MRE has been reported [2],

so the compounds should fall into a region where FM develops within the dominance of the AFM interaction Thus, clarifying the first stage of the FM formulation is important It is worthwhile noting that pure CaMnO3

possesses a G-type AFM ground state with a net Mn4+

magnetic moment of 2.46 µB [10] (close to the experi-mental value of 2.65 µB [11]).

A theoretical study using density functional theory (DFT) showed that 2D and finite systems (nanoclusters) may exhibit FM ordering at surfaces that penetrates into the bulk with a penetration depth of 2.7 nm Thus, al-though FM is absent in bulk CaMnO3, due to surface

re-laxation, it may appear and prevail in nanoclusters [10]

In undoped CaMnO3 the FM interaction is a result of

-2133-an exch-2133-ange between Mn4+and Mn3+ions, which occurs

due to impurities or oxygen vacancies The occurrence of

Mn3+ ions may also be induced by substitution, which

in the case of Fe-doping brings a complicated picture

of microscopic interactions because Fe is a multivalent

ion The available studies of the CaFeO3 family (and

derived compounds) show that Fe4+ ions may exhibit

at low temperatures a so-called charge

disproportiona-tion (CD) phenomenon [12–14]; that is, the unpaired e1

g

electron of the high spin Fe4+ ions (t3

2g e1g) is dispropor-tionally located so that two Fe4+ ions are transformed

to Fe3+and Fe5+: 2Fe4+(t3

2g e1g)→ Fe3+(t3

2g e2g) + Fe5+

(t3

2g e0g) An exact knowledge of the amount of the Fe4+

species is important in many cases because Fe-based

per-ovskites are highly efficient catalysts in oxidation

reac-tions of propane and ethanol [15] Therefore the possible

interaction involves, except Mn3+ and Mn4+ pairs, the

coupling pairs of Fe ions themselves (Fe4+, Fe3+, Fe5+)

and of Fe ions and Mn ions (Mn3+ or Mn4+) At low

doping concentration, if we were suggesting a

double-exchange mechanism (DE, according to Refs 16 and 17),

and at high temperatures, where the CD phenomenon is

absent, the two Fe ions may be considered as being far

enough away from each other, for the possible FM

in-teraction pairs to be reduced to Mn3+ and Mn4+, Fe4+

(substituted for Mn4+) and Mn3+, and Fe3+(substituted

for Mn3+) and Mn4+ Therefore, we will show here that

if this scenario holds, then Fe-doping introduces a

de-crease in the overall FM strength and, together with

this, a decrease in T C The conclusion may not hold

for higher doping concentration because one cannot rule

out the possible occurrence of Fe2+, which may exhibit

a high spin state (d xy , d yz)3(d xy)1(d1

z2)(d x2−y2)1(S = 2)

with magnetic moments of 3.1 and 3.6 µB at 293 and 10

K, respectively, as in SrFeO2 [18].

For CaMn1−xFexO3one may expect a behavior similar

to those of SrMn1−xFexO3[19] and of the oxygen-deficit

SrMn1−xFexO3−δ [20] These compounds possess

anti-ferromagnetic ordering at low and heavy doping contents

whereas intermediate substitution leads to a spin-glass

behavior Near 50% doping, two types of ordering, AFM

and FM, were found to co-exist The authors argued

that the spin-glass behavior was a result of competing

AFM and FM interactions between Mn4+ and observed

Fe3+ and Fe5+ ions Note that SrMn

1−xFexO3 also

ex-hibits a G-type AFM ground state as do CaMnO3 and

SrMnO3 At the other end, SrFeO3−σ also possesses an

AFM structure (T N = 134 K) [21] However, a fully

oxygenated SrFeO3shows a metallic character (no static

Jahn-Teller distortion inspite of the one unpaired

elec-tron in the Fe4+ e1

g orbital) For the purpose of this work, which is to show that the overall FM strength

de-creases according to the doping, the correct

determina-tion of T C and the evolution of T C upon doping are of

major importance

II EXPERIMENTS

The CaMn1−xFexO3 (CMFO) bulk samples were

prepared by using solid-state reaction technique with CaCO3, MnO2 and Fe2O3 powders as the precursors

(Merck, 99.9%) As common for ceramic materials, the technological factors usually have strong influences on the properties of the materials prepared We utilized the following route: First, the parent oxides were dried

to minimize the possible dampness; these were then weighted in the required molar proportions, mixed to-gether, and ground for 4 hours The mixture was ground again for 4 more hours in ethanol and were then pressed into pellet (at the pressure of 5 tons/cm2, and without

bonding colloid) with heights of 2 mm and diameters of

10 mm The pellets were sintered at 700◦C for 4 hours.

This temperature was chosen to assure the burn out of organic substances and the dehydration of coordinated water The sintered pellets were ground again for 4 hours and pressed; then, they were calcined at 1200◦C for 24

hours in ambient conditions and at a constant ramping rate 2◦C/min.

The structures of the samples were characterized by using X-ray diffraction with a Bruker D5005 diffratome-ter The Raman scattering measurements were carried out by using a Renishaw Invia Microscope and using

He-Ne excited radiation (λ = 632.8 nm) The Curie

temper-atures of the samples were determined from the M(T) curves measured by using a vibrating sample magne-tometer (DMS 880) having a sensitivity of about 10−6

emu/g

III RESULTS AND DISCUSSION

1 Structure Characterization

Figure 1 shows the XRD patterns of CaMn1−xFexO3

(x = 0, 0.01, 0.03, 0.05) The data show no secondary

phase except the one with the orthorhombic structure Pnma with (hkl) indices As seen, with increasing Fe concentration, the peaks shift to lower angles, signifying increases in the lattice constants (Table 1) Because Fe was substituted into the Mn sites, the increases in the lattice parameters and the volume may be interpreted

in terms of the larger ionic radius of Fe ions in com-parison with that of Mn ions (particularly, 0.585 ˚A for 6-coordinated Fe4+ and 0.530 ˚A for Mn4+) Thus, the

observed lattice expansion caused by doping may serve

as a signature of site occupation of Fe ions Indeed, if

Fe ions are substituted in Ca sites (12-coordinated oxy-gen atoms) or as impurities in the grain boundaries, the developments of the lattice constants may be quite dif-ferent In the orthorhombic symmetry with slight

defor-mation as for our cases, the bond angles <Mn-O-Mn>

are not changed so much, so the expected values of the

Table 1 Lattice parameters, Mn-O bond length and expected exchange integral.

0.00 5.2934, 5.279, 5.264 [22] 7.4860, 7.448 [22] 209.76, 207.0 [22] 1.8715, 1.895 [22] 0.30

Fig 1 (Color online) XRD patterns of CaMn1−xFexO3

bulk samples

exchange integrals (both super-exchange or double

ex-change) completely depend on the values of the bond

lengths (Mn-O) As the lattice parameters are increased

due to doping, one may expect a corresponding decrease

in the strength of the exchange interaction Table 1

gives the expected values of the double exchange

inte-gral JMnMn evaluated using first-principles calculation

with the local density approximation (LDA) functional

To compare our results with the results previously

published, we list the structural parameters reported

for undoped polycrystalline CaMnO3, which were

deter-mined from the X-Ray and the Neutron scattering data

given in Ref 22 The structure of the undoped CaMnO3

has also been studied at temperature from room

tem-perature to 800◦C by using high-resolution synchrotron

X-ray powder diffraction The CaMnO3 structure was

found to remain orthorhombic in the Pbnm space group

over the entire temperature range [23]

2 Raman Scattering Measurement

To confirm the correct substitution of Fe into the Mn

sites, we performed Raman scattering measurements for

undoped CaMnO3 and doped CaFexMn1−xO3, and the

results are shown in Fig 2 According to the group

Fig 2 (Color online) Raman scattering spectra of (a) CaMnO3 [24] and (b) CaFexMn1−xO3

theory, the vibrations of atoms in the lattice of CaMnO3

(Pnma) consist of 24 Raman active modes:

ΓCaMnO 3 = 7A g + 5B 1g + 7B2g + 5B 3g

+10B 1u + 8B 2u + 10B 3u + 8A u (1)

In comparison with the theoretical values from Bhat-tacharjee et al [24], the vibration modes may be

as-signed as follows: B 2g(258 cm−1), Ag(280, 322, and 467

cm−1), B

3g (433 cm−1), and B1g (489 cm−1) For

vari-ous doping concentrations, the vibration modes may be extinguished (for example 489 cm−1) or enhanced (i.e.,

280, 298, 376, 393, 466, 613, 632 and 736 cm−1)

Accord-ing to Ref 25, the vibration mode at 301 cm−1 belongs

to a rotation around the x axis of the BO6 octahedron.

Fig 3 (Color online) Infrared absorption spectra of

CaMn1−xFexO3 samples at room temperature

The enhancement of this peak due to doping may be

caused by the co-existence of FeO6and MnO6 in a unit

cell The peak at 319 cm−1 should belong to a Ca2+

os-cillation, and the peak at 375 cm−1to oscillations of both

Ca2+and O2− In the case of 1% doping, the separation

of the peaks at 467 cm−1 and 489 cm−1 was relatively

clear, but when the doping content was increased, the

peak at 467 cm−1 was extinguished whereas the one at

489 cm−1 was enhanced It should be noticed here that

strong enhancements due to doping were observed for the

peaks at 613 cm−1 and 632 cm−1 The 613-cm−1 peak

may belong to an impurity Another peak at 728 cm−1,

which corresponds to an asymmetric deformation of the

BO6 octahedron, was also enhanced when the doping

content of Fe was increased The changes in the

spec-troscopic data for Mn sites provide good experimental

evidence for the correct substitution of Fe into the Mn

sites Theoretically, such substitution may also lead to

spectral shifts and widenings of spectral lines due to a

change in (or weakening of) the metal-to-oxygen

bond-ing force constants; unfortunately the accuracy obtained

here (at a low doping concentration) was not enough to

demonstrate these effects

3 Infrared Absorption Measurement

To reveal the change of allowed optical transitions due

to Fe-doping and to determine the reduction of optical

gaps, we performed IR absorption measurements

Fig-ure 3 illustrates the result obtained for CaMn1−xFexO3

at room temperature As seen, there are three major

absorption peaks, 2.5, 4.6 and 5.7 eV For the doped

samples, a new absorption band appears at 1.2 eV We

found that the band gaps as calculated from first

princi-ples (using the LDA functional) were quite similar to the

ones extrapolated from the absorption edges (see Table

2) A red-shift can be clearly observed with increasing

Table 2 Optical band-gaps and theoretical values

∗Extrapolated from the tangent of the absorption edge

∗∗ Calculated from first principles by using the LDA

functional

Fig 4 (Color online) Partial density of states (s, p, d) for

the sample CaMn0.95Fe0.05O3

doping concentration The estimated band-gaps tended

to decrease when the doping was increased

To interpret the absorption spectrum, we calcu-lated the density of states (DOS) for 5%-doped CaMn1−xFexO3 (Fig 4) The time-dependent density

functional theory should be required to evaluate all pos-sible exited states (CI-Singles), but the DOS is adequate for qualitative interpretation In Fig 4, we found a clear correspondence between the absorption lines at 2.5

eV and 4.6 eV and the excitations of 3d electrons seen

at about 2.5 eV and 4.6 eV The appearance of a peak

at 1.2 eV is characteristic of Fe doping and should corre-spond to an antiferromagnetic exchange (with a change

of spin)

4 Evolution of T C According to Doping

We have measured the M(T) curves in an applied field

H = 500 G (field cooling (FC) mode) The obtained data are shown in Fig 5 where one may easily estimate the

values of T C by extrapolation; these values fall roughly

around 140 K Table 3 gives the estimates of T C from

the minima of the dM/dT versus T curves For the per-ovskite manganates, the values of T C are directly linked

to the coupling strength of the double-exchange (DE)

Fig 5 (Color online) Magnetization versus temperature

curves in a field of 500 Gauss for CaMn1−xFexO3samples

Table 3 Estimated TC, its decrease due to doping ∆TC=

[T C (x) − T C (x = 0)]/T C (x = 0), and the expected decrease

in the effective coupling strength ∆J ef f ≈ 1.5 ∆J.

x T C (K)∗ ∆TC [%] ∆Jef f [%] TC (K)∗∗ ∆TC [%]∗∗∗

∗ Determined from the minimum of the dM/dT versus T

curve

∗∗Determined from Eq (2)

∗∗∗ Calculated for the TC obtained from Eq (2) by using

the optical gaps given in Table 2

interaction [16] Because DE is a cause of

ferromag-netism, its competition with the super-exchange (SE)

antiferromagnetic ordering is a key factor in the

devel-opment of T C As the final effective exchange strength

J eff is a sum of the double-exchange and super-exchange

strengths J DE and J SE[26], estimating the interplay

be-tween J eff and T C is desirable

For the Fe-doped manganates, several important

experimental results should be addressed For

La0.67Ca0.33Mn0.097Fe0.03O3, Przewo´znik et al. [27]

found that J eff = 1.18 meV (for which J Mn−Mn = 1.24

and J Fe−Mn = 0.06 meV) The Fe doping in this

com-pound until 12% reduced the T C and the magnetization

but increased the magnetoresistance [28] Furthermore,

the57Fe NMR data revealed that the Fe ions exhibited

the oxidation state 3+ with anti-parallel spin in

compari-son with the spins of the neighboring Mn ions; that is, the

Fe-Mn couplings were purely of SE character [29] Hence,

the ferromagnetic domains should only be due to Mn4+

Mn3+ double-exchanged pairs [30] For this situation, if

a one-unit increase in the Fe-doping concentration

intro-duces a unit increase in−∆J on the SE side, then it has

to induce a comparable decrease in k∆J on the DE side (k is a linear scaling constant), and the new effective cou-pling J 

eff should be equal to (J SE −∆J)+(JDE −k∆J)

= J eff −(1k)∆J The estimate for k may be taken from

Refs 25-27 where−J Fe−Mn ≈ 0.5J Mn−Mn, thus implying

k = 0.5 As a result, overall decrease in the effective

cou-pling strength due to a unit increase in the doping

con-tent is ∆J eff = J eff − J 

eff ≈ 1.5∆J As the 3% doping

of Fe in La0.67Ca0.33MnO3 (T C = 344 K and J Mn−Mn

= 1.43 meV) leads to a 14.2% weakening of the J Mn−Mn

strength, a 1% increase in Fe content should weaken J eff

by about 7.1% The values of ∆J eff calculated for our

cases on the basis of k = 0.5 are given in Table 3 These

values predict that at 14% doping, the compound should

be paramagnetic because at that concentration, ∆J eff

= 100% and J eff = 0; that is, J Mn−Mn = J Fe−Mn For

comparison, Table 3 also gives the values of T Cpredicted according to Chong Der Hu’s relation [31]

where x is the substitution concentration and B is the

band gap The relation is based on previous results ob-tained by de Gennes [17] and by Kubo and Ohata [32] for La1−xAxMnO3(which gave a one order larger T C of

≈ 4×B/15) By extrapolating the theoretical TC to x = 14%, we arrived at 60 K for the T C of the paramagnetic state

IV CONCLUSION

A small doping of Fe in CaMnO3 (≤ 5%) led to

in-creases in the lattice constants and an observable

de-crease in Curie temperatures T C of all samples The analysis of the Raman scattering data showed an en-hancement of BO6resonance modes due to doping, and

the IR absorption spectrum revealed the characteristic

Fe band at 1.2 eV, which was due to the transition of electrons with a change of spin (SE interaction) The Fe doping induced an increase in the antiferromagnetic

ex-change, J Fe−Mn, between Fe and Mn sites and a strong

reduction of the ferromagnetic coupling, J Mn−Mn,

be-tween Mn sites The analysis showed that the

param-agnetic state, for which the effective exchange J eff is zero because the ferro- and the antiferro- couplings can-cel each other, should be present at a doping

concentra-tion of 14% and at T C of about 60 K

ACKNOWLEDGMENTS

The authors would like to thank the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam, project code #103.02.19.09

“Nanofluid and Application” (2009-2013), for its finan-cial supports

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