A study on fe induced structural, magnetic and transport properties in colossal magnetoresistive nd0 67sr0 33mno3 polycrystalline bulk and films 3

3 Effect of Fe Substitution for Mn on the Structural, Magnetic and Transport Properties in Polycrystalline Manganite Nd0.67Sr0.33MnO3 System 3.1 Introduction The perovskite manganite N

3 Effect of Fe Substitution for Mn on the Structural, Magnetic and

Transport Properties in Polycrystalline Manganite Nd0.67Sr0.33MnO3

System

3.1 Introduction

The perovskite manganite Nd1-xSrxMnO3 compound can be labeled as a material

having characteristics common to compounds in the “intermediate” bandwidth subset of

manganese oxides [88] due to the presence of a stable charge-ordered phase at x = 0.5,

state which can be easily destroyed by a magnetic field in a first order transition In this

chapter, a considerable emphasis will be given to the analysis of Nd0.67Sr0.33MnO3 (Nd

1-xSrxMnO3 with x = 0.33) compound At this composition, the compound is found to

exhibit the highest Curie temperature, T c and the moments are close to the expected

spin-only value The compounds with Nd3+Mn 3+O3 (x = 0) [89] and Sr2+Mn 4+O3 (x = 1) [90]

are antiferromagnetic and insulating in its ground state However when the two

compounds are mixed, ferromagnetic Nd0.67Sr0.33Mn3 + Mn O

67

33

0 3 compound is formed

The mixed valency of the manganates leads to strong ferromagnetic (FM) interactions

arising from the exchange of electrons via Mn3+ – O – Mn4+ bonds as explained by the

double-exchange (DE) mechanism [28] At this composition, the DE interaction prevails

over the other interactions (eg superexchange interaction from Mn3+ – O – Mn3+ bonds)

and one sees the occurrence of ferromagnetism, metal-to-insulator transition, and large

magnetoresistance occurring at and around the ferromagnetic Curie temperature, T c The

onset of ferromagnetism arises from the core spin S = 3/2 of both Mn3+ and Mn4+ due to

half-filled crystal field split t orbital and a strong intra-atomic Hund’s rule coupling

which spin aligns the electrons in the e g orbital and causes them to take part in the

electronic transport The metallic state in the manganites is thus caused by the

ferromagnetic interactions In addition to DE interaction, lattice distortions are believed

to play an important role through strong electron-lattice coupling which arises from the

Jahn-Teller distortion [91, 44] around the Mn3+ ions What happens to the ferromagnetic

order, metallic state and magnetoresistance when we mix a double exchange (DE) type

ferromagnet such as the manganate with an antiferromagnetic insulator like ironate? To

answer this question we have investigated the polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (x =

0 – 1) system This is done in a systematic way where Fe ion is added in steps to

Nd0.67Sr0.33MnO3 until it has completely substituted the Mn ions in the compound,

Nd0.67Sr0.33FeO3 The main reason Fe is chosen in this study is because the Fe ion has

identical ionic radius to that of the Mn ion, so we would expect the otherwise strong

lattice effects to be maintained and the resultant effects are mainly due to changes in the

electronic structure However the transition metals (TM) in the middle of the row of the

Periodic table such as Fe and Mn are expected to reflect the transitional behavior of the

early and late TM oxides [92] The magnetic and electronic nature in oxides of type Nd

1-xSrxMnO3 and Nd1-xSrxFeO3 are completely different [93, 94] For example, the end

member of the system, Nd0.67Sr0.33MnO3 (NSMO) undergoes a paramagnetic (PM) to

ferromagnetic (FM) transition at Curie temperature T c ≈ 274 K [95] arising from the

mixed valency of both Mn3+ and Mn4+ The Mn ions with valency +3 and +4 exist in the

high spin state with the electronic configurations t , and , ,

respectively Both the valence states carry spin and the conduction band arises from the

e

1 3

(NSFO) has a ground state as an antiferromagnetic (AFM) insulator arising from

bondings of Fe3+ - O2- - Fe3+, Fe4+ - O2- - Fe4+, and Fe3+ - O2- - Fe4+, which can be

explained by a superexchange (SE) model The electronic configuration of the Fe ions is

more complicated due to the possibility of the existence of the Fe ions in the low spin and

high spin states The high spin Fe3+ ion has a configuration of , and the low

spin (LS) Fe

2 3

=

S

2

3+ ion has a configuration of , The high spin (HS) state is

energetically more favorable The magnetic susceptibility measurements for the

paramagnetic state, Mossbauer and powder neutron diffraction studies for the related Fe

perovskites have proved Fe

0 5

3 2

3+ to be in the HS state [96, 97] The tetravalent Fe ion,

according to ligand field theory, can assume either a HS t , configuration or a

LS , configuration in an octahedral crystal field, with the latter being stabilized

when the crystal field splitting is large However, based on the measured magnitude of

the magnetic moment with by neutron scattering, Takeda, Komura, and

Watanabe [98] have suggested that the Fe

4+ ions in SrFeO3 are to be in the HS state with

three electrons filling the t 2g band, with the remaining e g electron itinerant Apart from

that, Chul et al [99] have pointed out convincingly in the Mossbauer spectra (MS) of the

Nd1-xCaxFeO3-y that Fe4+ exists as the HS configuration in the octahedral environment

This finding was further supported by studies related to MS [100] as well as X-ray and

UV photoemission spectroscopy [101] on La1-xSrxFeO3 and La1-xSrxMnO3 compounds

Hence, in our system we will consider Fe3+ and Fe4+ ions to be predominantly in the high

spin state However, we do not rule out the possibility of Fe ion to being in a different

spin state given its proximity to Mn ions

Thus, the correlation between the valence state and spin state of the TM ions is

believed to influence strongly the competing interactions such as the magnetic and

electrical transport properties of the rare earth TM oxides Therefore, it is of interest to

trace out the changes in their physical properties as the concentration of the oxides is

shifted from pure manganate to pure ironate The objectives of this work are to find the

changes in structural composition in relation to the valence state of the Fe ions and at the

same time the influence of Fe doping on the magnetic and electrical transport properties

in the polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (NSMFO) oxides The FM behavior was

observed for x < 0.1 compounds, while AFM nature was observed for x > 0.3 compounds

Intermediate composition with x = 0.1, 0.15 and 0.3 showed the presence of both FM and

AFM behavior arising from the complex electronic structure in the system

3.2 Experiments

Polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (NSMFO) ceramics in this work have been

prepared by solid state reaction method as described before in Chapter 2 with x = 0, 0.05,

0.1, 0.3, 0.4, 0.6, 0.8 and 1 The final products were characterized by a fine-step-mode

x-ray diffraction (XRD), model Phillips Diffractometer with Cu Kα source at room

temperature An Oxford superconducting vibrating sample magnetometer (VSM) was

used to measure the magnetic property of the samples The magneto- and electrotransport

properties were measured via the standard four-point probe on gold-coated surfaces

X-ray photoelectron (XPS) measurements were carried out using a spectrometer equipped

with Mg Kα source with hν = 1253.6 eV

3.3 Experimental results and discussions

3.3.1 Structural Characterization

Figure 3 – 1 presents the XRD patterns of polycrystalline samples of

Nd0.67Sr0.33Mn1-xFexO3 (x = 0 – 1) taken at room temperature The patterns show clean

single-phase patterns without any detectable impurity for all samples All the samples

were found to be crystalline in an orthorhombic structure obtained from the program

DICVO91 [102] No apparent macroscopic structural changes can be identified by small

amount of Fe doping However, the peaks shift slightly to lower two theta degree as Fe

doping increases Upon increasing the amount of Fe ions substituting Mn ions of different

ionic radii, the observed shift in the XRD position in figure 3 – 1(b) is expected (ionic

radii of Mn3+ = 0.785, Mn4+ = 0.670, Fe3+ = 0.785 and Fe4+ = 0.725 [103]) Table 3 – 1

shows the room-temperature unit cell volumes and axis a, b and c, respectively The

lattice parameters for lower Fe-doped samples (x < 0.3) remain almost unaltered The unit

cell volume increases linearly as the iron concentration increases as depicted in figure 3 –

2 The observations indicate that the MnO6 octahedra are strongly distorted by the

introduction of Fe ions The increase in the unit cell volume is hypothesized to be due to

increasing amount of Mn ions being replaced by the Fe ions of variable valence state,

such as Fe3+ and Fe4+ in the samples

Figure 3 – 1 (a) The XRD θ − 2θ patterns of polycrystalline Nd0.67Sr0.33Mn1-xFexO3

samples with x = 0, 0.05, 0.08, 0.1, 0.3, 0.4, 0.6, 0.8 and 1 (b) A section of the XRD

patterns of polycrystalline Nd0.67Sr0.33Mn1−xFe Ox 3 samples from 32o < 2θ < 33.5o Note

the systematic decrease in 2θ with increasing Fe doping as indicated by the vertical

dotted line

Table 3 – 1 Unit cell parameters of the Nd0.67Sr0.33Mn1-xFexO3 (x = 0 – 1) manganites

c

V

Figure 3 – 2 Dependence of the lattice parameters (left axis) and the volume (right axis)

on Fe at % x in Nd0.67Sr0.33Mn1-xFexO3 (x = 0 – 1) manganites

3.3.2 X-ray Photoelectron Spectroscopy (XPS)

For the perovskite manganites, the Mn3+ content and the mole ratio of Fe4+ ions to

the total Fe ions have important effects on the magnetic and transport properties Ahn et

population of the electron hopping electrons, and the number of available hopping sites in

the La1-xCaxMn1-yFeyO3 system These results originate from the suppression of double

exchange due to depopulation of hopping electrons by Fe doping, resulting in the

reduction of ferromagnetism and metallic conduction Early study by Jonker [105] has

shown that the presence of Fe3+, Mn3+ and Mn4+ for x < 0.85 and Fe3+, Fe4+ and Mn4+ for

0.85 < x < 1.00 plays a crucial role on the conductivity of La0.85Ba0.15Mn1-xFexO3 as the

widths and energies of their e g bands dictate the electron distribution of the Fe and Mn

ions From these, we can infer that the presence of the amount of Fe4+ and Mn3+ have

close relation with the structural, magnetic and conductivity properties in the samples

Noting the importance of valence state of Mn and Fe ions, XPS measurements

have been employed to further analyze the valency of Mn and Fe in the system Fe 2p and

Mn 2p XPS spectra of NSMFO are shown in figure 3 – 3(a) and (b), respectively In

figure 3 – 3(b), Mn 2p peaks can be resolved into two spin-orbit doublets, 2p 3/2 and 2p 1/2

of the Mn 2p core hole They are Mn2O3 and MnO2 at binding energies of 641.5 and

643.4 eV, respectively The valence state of Mn ions in NSMFO system can be attributed

to a mixed valence state of +3 and +4 depending on Fe concentration The average Mn

valency of NSMO is ∼ +3.3

Figure 3 – 3 XPS spectra of (a) Fe 2p and (b) Mn 2p in NSMFO system for x = 0, 0.05,

0.08, 0.1, 0.3, 0.4, 0.6, 0.8 and 1 The shift is shown by the vertical line drawn

As iron content increases, the Mn peaks decrease in intensity This implies an

overall gradual decrease in the average Mn valency in the system At x = 1, no peaks are

observed Thus the FM ground state is weakened by Fe substitution and this is followed

by the onset of AFM ground state in x = 1 sample End member Nd Sr FeO3

(NSFO) is taken as a reference An appreciable shift towards lower energy value (as

shown by slanted dotted line) is observed for Fe 2p spectra in figure 3 – 3(a) The shift in

binding energy as Fe concentration increases is expected to be due to the sensitivity of

the TM spectra to the symmetry of the ground state [106] According to Abbate et al

[107], such chemical shift is due to a change in the electrostatic energy at the TM site

Thus, when an increasing amount of Fe ions replaces Mn sites, the electrostatic energy at

the Mn sites changes due to a change in the 3d count It is worth noting that up to x ≈ 0.1,

the overall shapes of the multiplets hardly change, resembling the spin orbit doublet, 2p

+ 3 67 0

2

g e

+ 2 33 0

3/2

of the Fe 2p core hole The binding energy of the Fe 2p peaks is very close to that in

Fe2O3 (around 711.4 eV), which suggests the Fe ions are in the trivalent state This

indicates that at least in the first x = 0.1 series, the Fe ions essentially remain in a 3d5

(Fe3+) configurations This is consistent with results from Mossbauer spectroscopy (MS)

studies described earlier [108] with the detection of trivalent Fe ions at a low doping level

(x < 0.1) For x > 0.1, we observe that the Fe 2p spectra grow and dominate the region

resemble that of 2p3 peaks of NSFO At the intermediate range, the ground state has

become a combination of both NSMO and NSFO depending on the doping concentration

For x = 1, the Fe 2p XPS results for NSFO correspond well to that of SrFe

2 /

4+O3 [107, 109]

and LaFe3+O3 [107] MS studies, based on the behavior of doped Ln 1-x A xFeO3 (Ln = La,

Pr, and Nd; A = Sr and Ca) oxides These results have proved that Fe3+ ( 3 ,

2g t

2

5

=

S ) and

Fe4+ ( t , 2 ) are in the high spin states [94, 110, 111] As compared to NSFO, the

multiplets for the higher doped Fe content samples hardly shift in position resembling

that of NSFO However the only difference is in their intensity It is observable that both

figure 3 – 3(a) and (b) complement each other such that when Fe substitution increases,

the Fe 2p peak intensity in figure 3 – 3(a) increases while the Mn 2p peak intensity in

figure 3 – 3(b) decreases The results of Mn

from 31.8% for x = 0 to 15.1% for x = 0.4 as the Fe doping level increases in the nominal

composition of the samples The decrement of Mn4+ contents correspond well with other

related Al, Fe and In-doped La-based compounds measured by redox titration method

[112 - 114] However, we do not exclude the possibility of the formation of oxygen

vacancies by means of preserving the charge equilibrium in the sample The above result

also helps to explain our earlier experimental observation that the unit cell undergoes an

increase in volume as a consequence of more Mn4+ ions of smaller ionic size being

replaced by Fe ions of bigger ionic radii in higher Fe-doped compounds

3.3.3 Magnetic Properties

After zero-field cooling (ZFC) down to 77 K, the magnetization data were

collected in a 100 Oe magnetic field during the warming process Figure 3 – 4(a) and (b)

depict the temperature dependence of magnetization, M(T) for polycrystalline

Nd0.67Sr0.33Mn1 Fe Ox 3 samples with x = 0, 0.05, 0.08, 0.1, 0.15 and x = 0.4, 0.6, 0.8 and

1, respectively It is clearly shown that only samples from x = 0, 0.05, 0.08 and 0.1 in

figure 3 – 4(a) undergo a ferromagnetic (FM) to paramagnetic (PM) phase transition

within the narrow temperature range For Fe concentration x ≤ 0.05, the compositions are

characterized by sharp transition from magnetically ordered states to paramagnetic states

In compositions with 0.08 ≤ x ≤ 0.15, the transition gradually disappears with increasing

x because of the higher degree of structural disorder in the Mn lattice For x ≥ 0.4, the FM

to PM transition completely disappears This is witnessed by the onset of the Neel

temperature, T N at 140 K in the x = 0.4 sample For 0.6 ≤ x ≤ 1, the magnetization values

greatly reduce to less than 0.3 emu/g and continue to decrease with increasing

temperature For x = 1, the magnetization value is so insignificant that it is hardly

noticeable over a wide temperature range Based on MS studies, Chul et al [94] reported

the antiferromagnetic insulating behavior in Nd1-xSrxFeO3 (x = 0 – 1) samples The Curie

temperature T is defined as the temperature corresponding to the minimum of dM(T)/dT

For x = 0, T is 270 K which is in agreement with the previous report [115] In the range

of x = 0 – 0.1, T shifts to a lower temperature progressively with increasing amount of

Fe doping The values of T

to drop by approximately 18 K It is interesting to compare this value with that of ∼12 K

for Fe-doped La0.7Sr0.3Mn1-xFexO3 (LSMFO) system in Xianyu et al [116], which leads

us to conclude that Fe doping has a stronger effect on weakening the ferromagnetism in

Nd- based systems than that in the La-based systems