multiferroic features with magneto-capacitance...

Chapter 5

MULTIFERROIC FEATURES WITH

MAGNETO-CAPACITANCE AND

ELECTROCNIC STRUCTURE STUDIES

of La0.8Bi0.2Fe1-xMnxO3 SYSTEM

This chapter puts forward the contribution of Mn ions on the multiferroic

feature exhibited by La0.8Bi0.2Fe1-xMnxO3 (0.0 ≤ x ≤ 0.4) system. This had been

done by measuring the dielectric properties in the presence of magnetic field.

Electronic structure has been confirmed using XAS and charge transfer

multiplet calculations.

Ph.D Thesis Ghazala Anjum

Chapter 5 Magneto-capacitance and electronic……… Page 90

5.1 Introduction

Multiferroics offer the unique opportunity to study fundamental physical links among spins

and charges. External magnetic and electric fields can induce respectively, electric

polarization and magnetization in these materials [1, 2]. Consequently, they have potential

for technological applications. Coupling between different order parameters produce ME

effect. ME effect provides an additional degree of freedom in designing of actuators, sensors

and next generation memory devices which are unachievable separately in either FE or

magnetic materials [3]. However, a high inherent coupling between multiferroic order

parameters has not yet been found in a single phase compound which hinders their

applications. Thus, attempts to design multiferroics in the same phase have proven

unexpectedly difficult. The usual atomic level mechanisms driving the two orders are

mutually exclusive, because ferroelectricity requires empty and FM necessitates partially

filled transition metal d orbitals [3]. This identification has driven a flurry of research

activities on finding the alternative mechanisms for achieving desired condition for ME

coupling in a material for applications purpose. Higher FE and magnetic ordering temperatures

are also desired. It is expected that any type of substitution at Mn site will lead to a substantial

change in their magnetic and dielectric properties, because of the change in magnetic coupling

among Mn atoms. It is important as well as interesting to design a novel single phase

multiferroic material which is also easy to synthesis and has the essential properties. Here the

doping of FE element (Bi3+

) at La site and Mn at Fe site are used for modifying the

imbalance of d holes/electrons in antiferromagnetic (LaFeO3) host. The net magnetic moment

is achieved through uncompensated AF spin because of difference in ionic radii of Mn and

Fe ions and their valence states. Since the physical behavior of samples is closely related to

their electronic and local atomic structures, studies on the change and/or the hybridization of

the electronic states in multiferroic samples were not sufficient and not examined in detail

except for a few [4–7]. Preliminary material LaFeO3, is a high-spin charge-transfer insulator

with 6A1 symmetry [8]. From the dielectric and magnetic studies on LBFMO ( 0.0 ≤ x ≤ 0.4)

samples, it was found that both these phenomena became enhanced after the substitution of

Mn ions at Fe sites as discussed in detail in Chapter 4 of this thesis. The presence of EB in

the LBFMO system is an additional and intriguing aspect for technological application in

novel magnetic memory devices. Therefore, the fact that all the electrical and magnetic

properties are closely related to the electronic structure, makes it inevitable to study the



chemical environment of a system. Without knowledge of this, one may never be sure about

the mechanism responsible for different properties which the system exhibits. In Chapter 4,

crystallographic nature of samples, dielectric response and magnetic properties were

discussed in detail but the feature that arises due to coupling between them was not much

highlighted. For understanding the nature and origin of them this study was undertaken.

This chapter gives the detail of work performed to find the magnitude of

coupling between the electric and magnetic orders by means of MC measurement. An insight

into the changes of electronic and local structure of transition metal ions present in LBFMO

(0.0 ≤ x ≤ 0.4) multiferroic samples was perceived by varying the Mn content. For that

NEXAFS at the O K-edges is performed to gain information regarding the hybridization of O

2p orbitals with 3d orbitals of the neighbouring transition metal ions and to understand the

local electronic structure [9]. NEXAFS at L3,2-edges of 3d transition metal ions (Fe and Mn)

is performed to get an insight into the chemical surroundings, oxidation state and crystal field

parameters, which is also further confirmed by K-edge measurement. Charge transfer atomic

multiplet calculations on L3,2-edge of Fe, Mn and M4,5 edge of La ions are also performed to

confirm the valence states and symmetry aspects of the samples under study.

5.2 Results and discussion

5.2.1 Magneto-capacitive coupling

To confirm ME coupling between magnetic and dielectric parameters, MC measurement has

been performed in the temperature range of 80 – 300 K for a representative sample

La0.8Bi0.2Fe0.7Mn0.3O3 (LBFMO3). MC measurements for other samples (x = 0.1, 0.2, and

0.4) of the series also have also been performed but the best result is found in the sample with

x = 0.3 concentration. MC measurement is a standard method to confirm the coupling

between magnetic and FE orders [4, 10-11]. Temperature dependence of observed ε' (at 123

Hz) at applied fields of 0 and 3 tesla is shown in Fig 5.1 (a). Coupling is observed for a wide

range of temperature (180 – 280 K), rather than at a single temperature which is a good

indication from application point of view. Also dielectric loss illustrates a decrease in

magnitude with the application of magnetic field [Fig 5.2 (b)], which is the most noticeable

feature observed in these materials for their application in high magnetic field. The MC has

been calculated using the relation,

………(5.1)



Fig 5.1 (a) Temperature variation of ε' for La0.8Bi0.2Fe0.7Mn00.3O3 sample in the absence

and presence of a magnetic field (H = 3 tesla). (b) Variation of tan δ in the absence

and presence of magnetic field. Inset shows the enlarged part of dotted portion that

depicts the coupling.



Fig 5.2 Percentage magneto-capacitive coupling as a function of temperature at an

applied frequency of 123 Hz for the La0.8Bi0.2Fe0.7Mn00.3O3 sample.

where ε' (H,T) represents ε' at a certain magnetic field (H) and temperature (T) and ε' (0,T) is

ε' when the external magnetic field is zero at the same temperature.

Method used for the calculation of MC is similar to that used by Sun et al [12].

Coupling for the La0.8Bi0.2Fe0.7Mn0·3O3 composition is estimated to be about 18% at ~200 K

as shown in Fig 5.2, which confirms the presence of better MC coupling. Coupling has also

been confirmed by finding the capacitance value as a function of magnetic field at a fixed

frequency (123 Hz) and temperature (230 K) in decreasing as well as increasing magnetic

field (0 - 3 tesla) cycles (Fig 5.3).



Fig 5.3 Capacitance vs magnetic field (0 – 3 tesla) plots of La0.8Bi0.2Fe0.7Mn0.3O3 sample

measured at 230 K and 123 Hz.

Interestingly, different paths followed by capacitance values for decreasing and increasing

magnetic fields show the coupling between two order parameters. Similar, variation of tan δ

as a function of magnetic fields is also observed as shown in Fig 5.4 which is in accordance

with the ME coupling. The possibility of ME coupling due to magneto-resistance (MR)

combined with the Maxwell–Wagner effect has been the subject of debate on multiferroics

showing MC and was explained by Catalan and Scott [13, 14]. This has also been analysed

for the present system and ruled out by fitting with the UDR model in the high f region

(discussed in Chapter 4). The MR of the samples also comes out to be negligible. Apparent

MC may be due to two possible reasons. It may be owing to coupling between two the order



Fig 5.4 Variation of dielectric loss (tanδ) with applied magnetic field (0 – 3 tesla) for

La0.8Bi0.2Fe0.7Mn0.3O3 sample at 230K and 123 Hz.

parameters or because of MR of the sample. To rule out the possibility of MR effect, we

have measured it at a magnetic field of 3 tesla and found to be less than 1%. It further verifies

that the observed phenomenon is basically due to the coupling between the electric and

magnetic dipoles.

5.3 Electronic structure

5.3.1 O K- edge spectra

Figure 5.5 presents the normalized O K-edge NEXAFS spectra of the LBFMO (0.0 ≤ x ≤

0.4) samples along with the spectra of the reference compounds MnO2, MnO, Fe2O3 and

Bi2O3. It represents the spectral feature of the O 2p unoccupied density of states (DOS) in the

conduction band which are hybridized with the orbitals of La, Bi, Fe and Mn ions in the

surroundings. From O K-edge spectra, one can probe into the local unoccupied DOS above

the Fermi level (EF) by employing the selection rules (∆l = ±1, ∆s = 0). Groot et al [9] have

systematically analysed the pre-peak structure of O K-edge for a series of transition metal



Fig 5.5 Normalized O K-edge NEXAFS spectra of LBFMO (0.0 ≤ x ≤ 0.4)

samples along with those of MnO2, Bi2O3, MnO and Fe2O3 as reference compounds.


Chapter 5 Magneto-capacitance and electronic……… 7

oxides. Two major changes are observed in the spectra after the substitution of Mn at the Fe

site. The first one is an increase in the intensity of the feature marked as ‘a’ and the second

one is the spectacular behavior observed in the feature denoted by the arrow. The O K-edge

spectra of the Mn-substituted LBFMO samples may be considered as composed of different

spectral features marked a, a' at ~ 530.15 eV and 531.55 eV, b, b' at ~ 535.15 eV and 536.68

eV and c, c' at ~ 540.26 eV and 542.679 eV respectively. The peaks are assigned on the basis

of comparison with the reference spectra of MnO2, MnO, Bi2O3 and Fe2O3 (Fig 5. 5). The

features marked by a and a' are attributed to the transitions of electrons from O 1s core levels

to the 2p states which are hybridized with the 3d orbitals of Mn/Fe and split into t2g and eg

orbitals under the influence of the octahedral CFS. The energy separation between these

peaks is ~ 1.4 eV for sample with x = 0.0 and increases up to ~1.7 eV for the sample with x =

0.4. The results are consistent with those reported by Abbate et al [8] for Sr doped LaFeO3.

The oxygen 1s spectra of undoped parent compound LaFeO3 also consist of

two sharp peaks of t2g and eg orbitals which split due to the octahedral crystal field.

Consequently, it is related to the crystal field splitting 10 Dq ~1.8 eV [8]. It is noticed that

spectral feature ‘a’ gains weight as the Mn concentration is increased to (x = 0.4). This

clearly indicates the increased hybridization of the Mn 3d orbitals with the O 2p states. The

spectral feature denoted by arrows at ~533 eV is assigned to the hybridization of O 2p with

the Bi 6s/6p orbitals. Designation of this peak is based on the spectral features observed at

the O K-edge of Bi2O3 reference compound. It is not clearly visible in minimal doped

samples, which may be due to the fact that it is hidden in the low-energy tail of the La 5d

band. Enhancement of this feature is owing to the polarization of Bi 6s2 lone pair of

electrons, which is favored after Mn doping. Since the unit cell contracts with Mn doping,

periodic distortions in the rare earth polyhedron take place and dipoles are created.

Complementary to this opinion, enhanced dielectric properties are also observed with Mn

doping as demonstrated in Chapter 4. Peaks b and b' at ~535.15 eV and 536.68 eV

respectively are associated with the hybridization of O 2p orbital primarily with the La 5d

character. However, the broad feature in the higher energy region starting at ~ 540 eV

involves the mixing of higher energy metal states of Mn 4sp, Fe 4sp and La 5sp orbitals. A

shift in peak positions towards the lower energy region with the increase in Mn concentration

suggests the possibility of the presence of mixed valence states of Mn or Fe ions in the

system. To understand the valence states of the transition metal ions, namely Mn and Fe, L3,2-

and K-edge spectra have been also studied.



5.3.2 Mn L3,2 edge spectra

The normalized NEXAFS spectra of the Mn L3,2-edge for the LBFMO (0.1 ≤ x ≤ 0.4)

samples taken in TEY mode along with the spectra of the reference compounds (MnO and

Mn2O3) are shown in Fig 5.6. The main features in the L3,2-edge spectra are dominated by the

dipole transition from the core 2p level to the unoccupied 3d (2p6 3d

n → 2p

5 3d

n+1) states and

depend on the local electronic structure because of a large Coulomb interaction between the

Mn 3d orbital and the surrounding oxygen 2p orbitals. Thus the analysis of the L3,2-edge

spectra presents the information about the oxidation states and crystal field symmetry of the

3d TM ions [15]. The Mn 2p core states split into two broad multiplets, namely the L3 (2p3/2)

at ~ 643 eV and L2 (2p1/2) at ~ 654 eV due to the spin–orbit interaction. The separation

between these two peaks is found to be ~ 11 eV. Each of these two regions further splits into

t2g and eg subbands owing to the CF effect. It is a well known fact that if there is any change

in the valence state of Mn, say from Mn2+

to Mn3+

/Mn4+

states, the NEXAFS spectra at L3-

edge show a shift towards higher energy and the spectral shape also modifies significantly

with the number of 3d electrons [16]. It is clear from the spectra of the reference compound

MnO that Mn has a +2 (3d5) valence state with orbitals filled by majority spin ↑ electrons.

Therefore, the intense feature at lower energy has been attributed to t2g and the higher energy

feature to eg subbands. The larger intensity of the peak is due to the fact that, for Mn2+

there

are three vacant states in the t2g orbitals whereas eg has only two. This assignment of peak is

done qualitatively.

To further understand the origin of the multiplet marked from a to c as shown

in Fig 5.6 for the LBFMO samples in the L3,2-edge spectra, we have also performed atomic

multiplet calculations in various symmetries [see Fig 5.7 (a – h)] using different 10 Dq

values for MnO, MnO2 and Mn2O3 respectively. Figures 5.7 (a – h) present the simulated

spectra corresponding to the three valence states (+2, +3 and +4) of Mn ions present in

LBFMO multiferroic system amid the weighted percentage spectra for the sample with x =

0.1 shown in the inset of Fig 5.7 (a). The simulated spectrum in support of Mn4+

state of Mn

ions is illustrated in the inset of Fig 5.7 (e). It is found that the best reproduced fit for MnO is

obtained corresponding to 10 Dq = 0.6 eV in octahedral symmetry as displayed in Fig 5.7 (b)

which is in accordance with the results of Lee et al [17] for (Fe, Mn)3O4 spinel oxides.

Similarly, the optimal fits for MnO2 and Mn2O3 to experimental data appear corresponding to

the crystal field splitting value, 10 Dq = 1.8 eV and 2.4 eV as shown in Fig 5.7 (g) and the



Fig 5.6 Normalized Mn L2,3-edge NEXAFS spectra of LBFMO (0.1 ≤ x ≤ 0.4)

samples with the spectra of MnO and Mn2O3 for comparison.



inset of Fig 5.7 (e), respectively. Mn ions in MnO2 are in +4 oxidation state with 3d3

configuration having half filled t2g and empty eg orbitals. Subsequently, the unoccupied part

of eg will contribute to a higher energy peak at L3-edge. Correspondingly, in the case of

Mn2O3, Mn is in +3 (3d4) valence state with the main contribution to L3-edge from eg

orbitals. Atomic multiplet calculations are carried out by reducing the Slater integrals to 80%

of their atomic values [15]. Theoretical spectra are broadened with a Lorentzian and a

Gaussian to simulate lifetime effects and instrumental resolution respectively. Lorentzian

FWHM values of 0.20 eV and 0.10 eV are used for multiplets in the L3- and L2-peaks

respectively, whereas FWHM is taken as 0.3 eV for Gaussian.

After finding these simulated spectra for Mn ions matching to different

valence states, we have tried to establish that Mn ions possess the mixed valence states.

Closer inspections of the spectral features of the Mn-substituted LBFMO samples (see Fig

5.6) illustrate their complex multiplet features. Using the theoretically simulated spectra for

three valence states, weighted sum (48% Mn2+

+ 29% Mn3+

+ 23% Mn4+

) is calculated for

the sample with x = 0.1 and we are capable of producing the observed spectra to some extent,

as shown in the inset of Fig 5.7 (a). This calculation is also performed with a number of

other possible combinations of the weighted percentage for three valence states (+2, +3 and

+4) but the best fit is obtained corresponding to this value only. However, the calculated

spectra do not fully mimic the observed one but give us an indication, which is further

confirmed by comparing with the spectra of the reference compounds as shown in Fig 5.6.

Spectral features denoted by arrow (~ 639.3 eV) and a (~ 640.64 eV) can be directly

compared with the MnO reference compound and are attributed to the +2 valence state.

These features are diminished in the sample with x = 0.4 putting forward the decreased

contribution from the +2 state. The multiplet features marked a' (~ 641.27 eV), b (~ 641.97

eV), b' (~ 642.50 eV) and c (~ 643.34 eV) of Fig 5.6 are attributed to mix valence states,

Mn3+

/ Mn4+

. The increase in spectral weight of feature c (Fig 5.6) specifies that contribution

from Mn3+

becomes dominant with a small input from Mn4+

. These findings are coherent

with the results of the O K-edge spectra and also support the LBFMO system’s magnetization

data [18]. Primary compound, LaFeO3 as mentioned earlier is AF in nature and Bi being a

non-magnetic ion has only a marginal effect on primary AF exchange interactions in these

samples. The Mn substituted samples exhibit the occurrence of a weak FM behaviour owing

to the presence of mixed valence Mn states, which in turn favours the multichannel double



Fig 5.7 Calculated spectra of Mn L3,2- edge with various values of CF

splitting (10 Dq) using CTM4XAS calculations, (a - d) show the simulated

spectra for Mn2+ state. Inset: (a) spectra obtained from weighted sum for 10% doped

samples. (e - h) correspond to simulated spectra for Mn3+ state. Inset: (e) displays the

simulated spectra for Mn4+ state.



exchange mechanism (Mn2+

– O – Mn3+

and Mn3+

– O – Mn4+

). Apart from the presence of

mixed valence states of Mn ions being primarily responsible for modifying all the electrical

and magnetic properties, the function of oxygen stoichiometry can never be ruled out. Hence,

it needs a study for the utility of oxygen variation with doping.

In order to confirm, we have tried to determine the oxygen content using the

iodometric titration technique for the LBFMO (0.1 ≤ x ≤ 0.4) samples [19]. It is found that

the variation of oxygen is from 2.97 to 3.05 which is also consistent with the results of

Kundu et al [20]. This slight variation of oxygen is responsible for the mixed valence states

of Mn as Fe is found to be always in +3 state throughout the series of present samples. The

XAS data also support this argument.

5.3.3 Mn K- edge spectra

To further understand and support the presence of mixed valence states of the Mn ions in the

LBFMO (0.1 ≤ x ≤ 0.4) samples, measurements at the Mn K-edge is also carried out. It is

well known that pre-edge features exist in the K-absorption spectra of most transition metal

ion compounds. They are usually associated with electronic transitions from the 1s core

orbital to the localized molecular orbitals of primarily 3d character [21]. Since electronic

quadrupole transitions (1s, 3d) are much weaker than electronic dipole transitions, the

strength of these pre-edge transitions depends strongly on the local symmetry of transition

metal ions, which affects the degree of mixing of p states of oxygen with the 3d states.

Consequently, the pre-edge features are very sensitive analytical tools in the electronic

structure studies of transition metal ion compounds.

The normalized NEXAFS spectra of the Mn K-edge for the LBFMO (0.1 ≤ x

≤ 0.4) samples along with those of the reference compounds (MnO, MnO2) for comparison

are displayed in Fig 5.8. The characteristic spectral features are marked from a to d. The pre-

edge peak marked a at ~ 6541.37 eV is attributed to the quadruple transition from the 1s core

level to unoccupied bounded Mn 3d orbitals. For the sample with x = 0.1, the pre-edge peak

is found to have a small intensity with a broad feature. There is a slight increase in its

intensity with the enhancement of Mn ion concentration at the Fe site. This relatively broad

feature further signifies the presence of mixed valence states of Mn ion. Inspection of the

spectra reveals that the feature marked b at ~ 6547.7 eV resembles well that of MnO and

indicates that Mn ions are also in the +2 valence state. The main absorption edge (marked c)

at ~ 6556.97 eV is assigned to purely electric dipole allowed transitions from the 1s core



Fig 5.8 Normalized Mn K-edge NEXAFS spectra of LBFMO (0.1 ≤ x ≤ 0.4)

samples together with those of MnO2 and MnO as reference compounds.



level to the 4p continuum states of Mn which are not localized. It resembles well with that of

Mn2O3 reference spectra depicting the fact that Mn ions are mainly present in the +3 valence

state along with contribution from +2 and +4 states which is in good agreement with the Mn

L3,2-edge spectra of the LBFMO system (Fig 5.6) [22]. Secondary peaks (marked d) that

occur at ~ 12 eV above the main absorption peak is attributed to the multiple scattering of the

electron by neighbouring atoms (Fe and Mn) in the LBFMO samples. These observations

further substantiate the fact that Mn in the LBFMO samples is in mixed valence states.

5.3.4 Fe L3,2-edge spectra

To investigate the valence state of Fe ions in the LBFMO (0.0 ≤ x ≤ 0.4) system, we have

also evaluated the Fe L3,2-edge spectra of the samples. Figure 5.9 displays the normalized

NEXAFS spectrum of the Fe L3,2-edge for the LBFMO (0.0 ≤ x ≤ 0.4) samples taken in TEY

mode along with the spectra of the reference compound Fe2O3. These spectra primarily probe

the Fe 2p – 3d transition and are strongly influenced by the core–hole potentials [23]. The

intensity of these spectral lines can be regarded as a measure of the total unoccupied Fe 3d

states. Spectra of all LBFMO samples illustrate two separate structures comprising the L3 (2

p3/2) and L2 (2 p1/2) under the influence of spin orbit splitting. L3- and L2-edge separation is ~

12 eV. The Fe 3d orbitals degenerate in spherical symmetry which further split into two

subsets of t2g and eg orbitals as a consequence of the crystal field effect.

For the L3 structure, the peaks marked a and a' at ~ 708.51 eV and ~710.14

eV respectively, are approximately 1.6 eV apart. A comparison of the observed features

marked a and a' with the reference spectra implies that Fe ions are in +3 states in all doped

samples consistent with the report by Kundu et al [20]. Also, the orbitals in Fe 3d5 state have

electronic configurations as follows:

t 2g (dxy, dyz, dxy) and eg (dx2-y

2, dz2 )

Fe3+

↑ ↑ ↑ ↑ ↑

which is a stable state as the energy of crystal field splitting is smaller than the Hund energy

of coupling between two electrons. Therefore, Fe ions remain in the high-spin state. It has

been also mentioned in the introduction of this chapter, that LaFeO3 is a high-spin charge-

transfer insulator. According to Chang et al [24], while Mn3+

and Mn4+

enhance the double



Fig 5.9 Normalized Fe L2,3-edge spectra of LBFMO (0.0 ≤ x ≤ 0.4) along with

the Fe2O3 samples.



exchange, Fe3+

and Fe4+

enhance the SE interaction in the system, although Fe4+

increases the

electron transfer via the singly unoccupied eg orbital. The presence of a larger amount of Fe3+

in the system as compared to Fe4+

shifts the entire system from the FM to AF state. Hence,

the possibility of another valence state is also not in accordance with the results of the

LBFMO multiferroic system.

In order to further analyse the spectrum, atomic multiplet calculations were

also performed [see Fig 5.10 (a-d)] on Fe 2p edge in octahedral (Oh) symmetry which was

also demonstrated by de Groot and Fuggle [25], Droubay and Chambers [26]. Best match

between experimental and simulated results was obtained corresponding to the Oh symmetry

[Fig 5.10 (d)]. In our calculation, crystal field splitting (10 Dq) was varied from 0.9 to 1.8 eV

in order to obtain the best possible fit to the experimentally obtained spectra. The multiplet

was broadened with Lorentzian value of FWHM ~ 0.2 eV and Gaussian value of 0.3 eV to

simulate the instrumental broadening. The best fit was obtained corresponding to the value of

10 Dq ~ 1.8 eV [Fig 5.10 (d)]. It is worth noting that the best fit value of 10 Dq obtained after

simulation is in close agreement with the reported values for α-Fe2O3 and the spectra are also

similar to the Fe2O3 compound showing that Fe in these samples is in the +3 valence state in

octahedral symmetry [8, 16].

5.3.5 La M4,5-edge spectra

The normalized NEXAFS spectra of the La M4,5-edge for the LBFMO (0.1 ≤ x ≤ 0.4)

samples are presented in Fig 5.11 (a). The M4,5-edge absorption spectra are helpful in finding

the exact valence information because of the localized nature of 4f electrons. The 3d x-ray

absorption process excites a 3d core electron into the empty 4f shell and the transition can be

described as 3d10

4f n → 3d

9 4f

n+1. In the case of rare earths, M4,5-edge spectra are dominated

by the spin–orbit splitting of 3d holes which provides the information about the unoccupied

4f valence states [27]. In the spectra of this series of samples, M5- ( related to the 3D1 state)

and M4- edges ( related to the 1P1 state) occur at ~ 834.2 eV and ~ 848.5 eV respectively with

the difference in peak position ( ~ 16.3 eV), which matches well with earlier reports [28].

Atomic multiplet calculations in intermediate coupling, including all possible states of

configuration, were performed with 4f 0

ground state and 3d94f

1 excited states and were found

to be in accordance with those reported by Thole et al [28]. For the calculated spectra, the

lifetime broadening, i.e. Lorentzian parameter and the experimental resolution, i.e.



Fig 5.10 Simulated XAS spectra of Fe 2p L-edge using atomic multiplet calculation

with various crystal field strength 10 Dq (0.9 -1.8 eV) from (a) to (d).



Fig 5.11 (a) Normalized La M4,5- edge spectra of LBFMO (0.1 ≤ x ≤ 0.4) samples.

(b) Simulated spectra of La M4,5- edge using atomic multiplet calculations.



Gaussian parameters were taken to be 0.2 for the M5-edge and 0.4 for the M4-edge, plus a

Gaussian of 0.25, respectively. The calculated spectra shown in Fig 5.11 (b) provides the

clear evidence of La being in the +3 valence state with no change in the spectral feature due

to Mn doping even up to x = 0.4. Thus, all the above NEXAFS results suggest that

substitution of Mn ions in the LBFMO multiferroic system modifies the electronic structure

of these materials to a large extent. The existence of mixed valence states of Mn ions as

complemented by magnetization studies (Chapter 4) is also confirmed by the Mn L3,2- and K-

edge spectra.

5.4 Conclusions

In summary, simultaneous existence of ferroelectric and ferromagnetic phases in the present

system demonstrates that these ceramic oxides are multiferroic materials. MC properties

further confirm the presence of magneto-capacitive coupling. Electronic structure of Mn

substituted LBFMO (0.0 ≤ x ≤ 0.4) samples is obtained by means of NEXAFS spectroscopy

at O K-, Mn L3,2-, Mn K-, Fe L3,2- and La M4,5-edges along with reference compounds.

Studies reveal that the Mn ions of the samples are present in mixed valence states. Mn ions

are mainly in +3 state along with a small admixture of +2 and +4 states. However, Fe remains

in the +3 state for all doping values. M4,5-edges spectra of La confirm the +3 state of La ions

in the system. Atomic multiplet calculations on Fe L3,2-, Mn L3,2- and LaM4,5-edges further

substantiate these valence specific arrangements of the respective ions in the LBFMO (0.0 ≤

x ≤ 0.4) samples.



References [1] N. A. Spaldin and M. Fiebig Science 309 391 (2005).

[2] M. Fiebig, J. Phys. D: Appl Phys. 38 R123 (2005).

[3] N. A. Hill, J. Phys. Chem. B 104 6694 (2000).

[4] N. E. Rajeevan, R. Kumar, D. K. Shukla, P. Thakur, N. B. Brookes, K. H. Chae, W. K.

Choi, S. Gautam, S. K. Arora, I. V. Shvets and P. P. Pradyumnan, J. Phys.: Condens.

Matter. 21 406006 (2009).

[5] T. Higuchi, Jpn. J. Appl. Phys. 47 7570 (2008).

[6] J. A. McLeod, Z. V. Pchelkina, L. D. Finkelstein, E. Z. Kurmaev, R. G. Wilks, A.

Moewes, I. V. Solovyev, A. A. Belik and E. T. Muromachi, Phys. Rev. B 81 144103

(2010).

[7] C. C. Hsieh, T. H. Lin, H. C. Shih, J. Y. Lin, C. H. Hsu, C. W. Luo, K. H. Wu, T. M. Uen

and J. Y. Juang J. Phys.: Conf. Ser. 150 042062 (2009).

[8] M. Abbate, F. M. F. de Groot and J. C. Fuggle, Phys. Rev. B 46 4511 (1992).

[9] F. M. F. de Groot, M. Grioni, J. C. Fuggle, J. Ghizsen, G. A. Sawatzky and H. Petersen,

Phys. Rev. B 40 5715 (1989).

[10] G. Catalan, Appl. Phys. Lett. 88 102902 (2006).

[11] A. K. Singh, S. D. Kaushik, B. Kumar, P. K. Mishra, A. Venimadhav, V. Siruguri and S.

Patnaik, Appl. Phys. Lett. 92 132910 (2008).

[12] Z. H. Sun, B. L. Cheng, S. Dai, L. Z. Cao, Y. L. Zhou, K. J. Jin, Z. H. Chen and G. Z.

Yang, J. Phys. D: Appl Phys. 39 2481 (2006).

[13] J. F. Scott, J. Mater. Res. 22 2053 (2007).

[14] G Catalan and J. F. Scott, Nature 448 E4 (2007).

[15] G. Van der Laan and I. W. Kirkman, J. Phys.: Condens. Matter. 4 4189 (1992).

[16] F. M. F. de Groot, J. C. Fuggle, B. T. Thole and G A Sawatzky, Phys. Rev. B 42 5459

(1990).

[17] H. J. Lee, G. Kim, D. H. Kim, J. S.Kang, C. L. Zhang, S. W. Cheong, J. H. Shim, S. Lee,

H. Lee, J. Y.Kim,B. H. Kim and B. I. Min, J. Phys.: Condens. Matter. 20 295203 (2008).

[18] G. Anjum, R. Kumar, S. Mollah, D. K. Shukla, S. Kumar and C. G. Lee, J. Appl.

Phys. 107 103916 (2010).

[19] P. Barahona, V. Bodenez, T. Guizouarn, O. Pena, J. Ortiz and E. Rios, R. Pastene and J.

L. Gautier, J. Chil. Chem. Soc. 50 495 (2005).

[20] A. K. Kundu, R. Ranjith, B. Kundys, N. Nguyen, V. Caignaert, V. Pralong, W. Prellier

and B. Raveau, Appl. Phys. Lett. 93 052906 (2008).

[21] G. Subias, J. Garcia, M. G. Proietti and J. Blasco, Phys. Rev. B 56 8183 (1997).

[22] M. Croft, D. Sills, M. Greenblatt, C. Lee, S.W. Cheong, K. V. Ramanijachary and D.

Tran, Phys. Rev. B 55 14 (1997).

[23] F. J. Mele and J. J. Ritsko, Phys. Rev. Lett. 43 68 (1979).

[24] Y. L. Chang and C. K. Ong, Appl. Phys. Lett. 82 1721 (2003).

[25] F. M. F. de Groot and J. C. Fuggle, Phys. Rev. B 42 9 (1992).

[26] T. Droubay and S. A. Chambers, Phys. Rev. B 64 205414 (2001)

[27] G. Kaindl, W. D. Brewer, G. Kalkowski and F. Holtzberg, Phys. Rev. Lett. 51 2056

(1983).

[28] B. T. Thole, G. V. Laan, J. C. Fuggle, G. A. Sawatzky, R. C. Karnatak and J. M. Esteva,

Phys. Rev. B 32 5107 (1985).

multiferroic features with magneto-capacitance...

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