A study on fe induced structural, magnetic and transport properties in colossal magnetoresistive nd0 67sr0 33mno3 polycrystalline bulk and films 5 7

5 Fe-induced Magnetic, Transport and Magnetoresistance Behavior in Nd0.67Sr0.33MnO3 Epitaxial Films and Thickness Dependent Magnetic, Electrical Transport and Coefficient of Resistance i

5 Fe-induced Magnetic, Transport and Magnetoresistance Behavior in Nd0.67Sr0.33MnO3 Epitaxial Films and Thickness Dependent Magnetic, Electrical Transport and Coefficient of Resistance in Nd0.67Sr0.33Mn1-xFexO3 (x = 0, 0.05) Strain-relaxed Films

This chapter is divided into two parts Both parts have their focus on the fabrication of Fe-doped Nd0.67Sr0.33MnO3 epitaxial films by pulsed-laser deposition However, the first part concentrates on the experimental studies of the Fe – induced effect on magnetic, electrical transport and magnetoresistance properties in

Nd0.67Sr0.33MnO3 epitaxial films Upon doping, no structural changes have been found However, the Curie temperature, the associated metal-to-insulator transition temperature and the magnetization decrease drastically with Fe doping The resistivity

in the paramagnetic regime for all the samples follows Emin-Holstein’s theory of

small polaron The polaron activation energy, W and resistivity coefficient, A

increase with Fe doping This effect may be ascribed to the fact that upon Fe doping, the long-range ferromagnetic order is destroyed and the polaron mobility is reduced in this system As compared to the La-based system, Fe doping has a stronger tendency

to destabilize the long-range ferromagnetic order in the Nd-based system Large MR (as high as 90%) observed in the epitaxial NSMFO film may be attributable to the good lattice-matching between the grown film and substrate

p

The second part focuses mainly on the thickness-dependent magnetic, electrical transport and temperature coefficient of resistance in Nd0.67Sr0.33Mn1-xFexO3

(x = 0, 0.05) strain-relaxed films for t = 150 and 450 nm films It is found that the

films well reproduce the properties intrinsic to the polycrystalline bulk Fe substitution at Mn sites reduces the saturation magnetization, ferromagnetic Curie

temperature, T c , metal-insulator temperature, T p and leads to an overall increase in

magnetoresistance (MR) The resistivity in the T > T p regime follows Emin-Holstein’s

theory while the resistivity in the T < T p regime follows the empirical relation of ρ(H,

T) = ρo + ρ2(H)T 2 + ρ5(H)T 5 Both show Fe-doping at Mn sites reduces the long-range ferromagnetic order in all the samples As the film thickness increases, the resistivity decreases indicating a reduction of short-range disorder in the film In contrast to Bi

substitution which raises the temperature coefficient of resistance (TCR) of the film,

TCR decreases upon Fe substitution in its Nd0.67Sr0.33MnO3 bulk

5.1 Fe-induced magnetic, transport and magnetoresistance

behavior in Nd0.67Sr0.33MnO3 epitaxial films

5.1.1 Introduction

Having known the importance of colossal magnetoresistance (CMR) perovskite-type manganites A1-xBxMnO3 (A = La, Nd, Pr; B = Ca, Sr, Pb) [75, 145, 146] to both scientific and technological field [147, 7] due to its promising potential applications as read heads for magnetic information storage [148], infrared detector [149], and low-field magnetic sensors [150] as described from the previous chapters, section 5.1 is devoted to the study of Fe-doped manganites epitaxial thin films Most

of these studies on the Fe-doped manganites that have focused mainly on the La

1-xCaxMnO3 and La1-xSrxMnO3 [151 – 153] systems are in the form of polycrystalline samples In polycrystalline thin films, their properties are very similar to the polycrystalline ceramics of the same composition whereby the transport property

show strong grain size dependence The resistivity and MR response in these

polycrystalline manganites are due to both intrinsic effect arising from within the grains and extrinsic effect from intergrain tunneling process across GB Therefore, high-quality epitaxial thin films allow us to minimize the GB effects and study the Fe-induced effect with greater reliability In this chapter, we report the Fe-induced magnetic, electrical and magnetoresistance induced behavior in Nd0.67Sr0.33Mn1-

xFexO3 films Nd0.67Sr0.33MnO3 (NSMO) has drawn much attention due to its CMR effect as explained in the earlier chapters [115, 154] As compared to other manganite

materials, NSMO polycrystalline target has a MR ratio as large as 34% near its Curie temperature of 270 K [115] Besides its CMR effect, Si et al [155] observed a large

magnetic entropy change in NSMO which makes it a potential candidate for magnetic refrigeration material, replacing the conventional gas-compression refrigerator

5.1.2 Experiments

A standard pulsed laser deposition (PLD) system, incorporating a stainless steel deposition chamber [156], is employed as shown in chapter two The targets used have a nominal composition of Nd0.67Sr0.33Mn1-xFexO3, NSMFO (x = 0, 0.05 and

0.1) made by using standard solid state reaction procedure given in chapter two from high purity oxide powders of Nd2O3, MnO2, SrCO3 and Fe2O3 After repeated grinding and sintering at 1250 °C for 24 h in air, the targets are found to be of a single phase using X-ray diffraction (XRD) Thick films of this material around 4500 Å are grown on (001)-oriented SrTiO3 (STO), 5 × 10 × 0.5 mm3 in size, substrates using a Lambda Physik KrF excimer laser 248 nm in wavelength, 30 ns in pulse width and 5

Hz in repetition rate This is to avoid the strain effect arising from lattice mismatch at the interface of the substrate and film It is well known that such films deviations from the crystal structure may become more pronounced due to influence of the substrate, which may lead to the larger possibilities for atomic arrangements as a result of diffusion during film deposition The laser frequency was 1.8 Jcm-2 The substrate is heated to a constant temperature of 750 °C and the chamber is held at 0.5 mbar of pure oxygen ambient pressure during film growth The as-deposited films were post-

annealed in situ for 1 h at 750 °C under 500 mbar of O2 pressure

The structure and orientation of these targets and films are checked by XRD using a Phillips diffractometer with Cu Kα radiation The chemical composition is determined by means of energy dispersive X-ray spectroscopy (EDX) The film thickness is measured by an Alpha-step 500 surface profiler and confirmed by an atomic force microscope The magnetic properties are measured using an Oxford superconducting vibrating sample magnetometer (VSM) In order to correct for the

diamagnetic effects of the substrates, their magnetization curves are measured before film deposition The conventional dc four-probe method is used to measure the electrical and MR properties of the film samples The temperature ranges are 77 – 300

K for electrical and magnetic measurements

5.1.3 Experimental results and discussions

5.1.3.1 Structural Characterization

Figure 5 – 1 depicts the X-ray diffraction (XRD) patterns for NSMFO targets

and films with x = 0, 0.05 and 0.1 at room temperature, collected by step scanning

over angular range ° at a step size of 0.01° The measurements reveal that all of the sintered Nd

602

of the NSMFO films are single phase The crystal structure of the epitaxial NSMFO films deposited onto (001)-oriented STO can be indexed with its [100] direction perpendicular to the surface of the film The FWHM of its rocking curve is ~ 0.56° for

x = 0.05 sample, as shown in the inset of figure 5 – 1 The -reflections for x = 0,

0.05 and 0.1 are found to be 1.9183, 1.9198 and 1.9225 Å, respectively The X-ray linewidths can be used to estimate the average particle sizes through the classical Scherrer formulation , where is the diameter of the particle in

Å, is a constant (shape factor ~0.9) [157], B is the difference of the width of the

half-maximum of the peaks between the sample and the standard of KCl used to

θ2

−

θ2

k

Figure 5 – 1 The XRD θ – 2θ patterns for Nd0.67Sr0.33Mn1-xFexO3 for ceramic targets

and epitaxial films with x = 0, 0.05 and 0.1 grown by PLD on (001)-oriented SrTiO3

substrates The inset gives the FWHM of the selected range 10° ≤ Ω ≤ 14° rocking curve for the Nd0.67Sr0.33Mn0.95Fe0.05O3 film

calibrate the intrinsic width associated with the equipment, and λ is the wavelength of the X-rays For x = 0, 0.05 and 0.1 films, an average crystal size of 30 nm is obtained

This value is consistent when compared to the measurement taken by AFM

5.1.3.2 Magnetic and Electrical Transport Properties

After zero-field cooling (ZFC) down to 77 K, the magnetization data is collected in an applied magnetic field of 2 kOe during the warming process In figure

5 – 2, it can be seen that Fe doping drives the Curie temperature, T c, lower The values

of T c are determined as the temperatures at which the ( )

T

T M

∂

∂

curves each shows a

minimum T c for x = 0 is about 260 K This value agrees with that given in the literature for fully oxygenated NSMO crystal [158] However, T c decreases to 165 K

for NSMFO (x = 0.05) film For x = 0.1 film, T c drops below 77 K, and no apparent transition is observed within the measured temperature range for the sample Besides

lowering T c, Fe doping also weakens the ferromagnetism in the system The magnitude of the magnetization for 5% Fe-doped NSMO film decreases almost by ½ compared to non-doped NSMO film According to the double-exchange (DE) mechanism, the magnetic behavior in the manganese oxide materials is determined by the ferromagnetic interaction between Mn3+ and Mn4+ in the systems, where the e g

electrons hop between the two partially filled d orbitals of neighboring Mn3+ and

Mn4+ ions via the Mn3+ – O2- – Mn4+ couplings

Figure 5 – 2 Magnetization, M of Nd0.67Sr0.33Mn1-xFexO3 films as a function of

temperature, T at a field of H = 0.2 T for x = 0, 0.05 and 0.1 samples The arrows indicate the ferromagnetic Curie temperature, T c

x = 0

x = 0.05

x = 0.10

Figure 5 – 3 Field dependence of saturation magnetization, M(H) for

Nd0.67Sr0.33Mn1-xFexO3 films with x = 0, 0.05 and 0.1 at 77 K

It is proved from Mossbauer spectroscopy studies [159, 160] that Fe ions which exist

as Fe3+ are antiferromagnetically coupled to the ferromagnetic Mn – O network The Mossbauer shift indicates that Fe ions are in their high spin 3+ states Therefore Fe3+

with a full spin t configuration does not allow for the transfer of electrons via Fe

– O – Mn networks This is manifested by the fact that T

2 3

2g e g

c is driven to a lower temperature and magnetization exhibits a drop with Fe doping

To get a clearer picture of the magnetization behavior, we measure the field, H dependence of the magnetization, M at 77 K for NSMFO (x = 0, 0.05 and 0.1) films as presented in figure 5 – 3 The M – H curve for x = 0 shows a ferromagnetic (FM) shape and saturates at a field of 2 T For x ≥ 0.05, the magnetization increases

consistently with applied field, without saturation, rising rapidly with increasing Fe

content The resultant magnetization curve for x = 0.05 is essentially the superposition

of both FM and AFM components Further Fe doping suppresses the ferromagnetism

of NSMO and AFM state sets in for x = 0.1 film Therefore, one can conclude from

the magnetization results that the probability of ferromagnetic coupling between the

Fe and Mn sublattices can be excluded and Fe doping enhances the AFM ordering This observation is very different from Ru and Cr-doped manganites [161, 162],

where instead of lowering its T drastically, Ru and Cr doping cause a slight or only very marginal decrease in T Though the positive influence of these ions aid in the magnetic ordering and insulator-metal transition, the overall T is still too high

(above room temperature) for potential applications

transition temperatures (taken as maximum resistivity temperature), T for x = 0 and

0.05 are 258 K and 115 K, respectively It is observed that in the ferromagnetic

region, all samples show metallic behavior (a positive dρ/dT) Near the ferromagnetic

transition, spin disorders lead to a sharp increase in resistivity The application of an external magnetic field suppresses the magnetic disorder, leading to a decrease in the resistivity, hence the largest magnetoresistance occurs close to the magnetic transition

temperature For the undoped NSMO film, T is not much different from T This

agrees well with our hypothesis earlier that NSMO film is fully oxygenated and

hence, the NSMO film is in the ferromagnetic metallic (FMM) state below T ≈ T

In this case, one can say that the NSMO film exhibits inherent properties which are

intrinsic to the NSMO target material (T = 270 K and T = 268 K [115]) For 5% Fe-doped film, T is found to be far below T The Nd

said to be insulating in the high temperature region and it turns metallic at about T

~ 115 K According Krisnan and Ju [163], the difference in temperature, of about

50 K, may be ascribed to the effect of grain boundaries or loss of oxygen However,

in our case, the inhomogeneities created by the antiferromagnetic insulating (AFI) matrix results in the loss in volume of the FM phase and may be one of the reasons for the observed being apart fromT

T

∆

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Figure 5 – 4 Temperature dependence of the resistivity for Nd0.67Sr0.33Mn1-xFexO3

epitaxial films with x = 0 and 0.05 at zero field (open symbols) and 1 T (closed

symbols) applied field The resistivity of the undoped NSMO film is plotted on the

right-hand-side expanded scale

Figure 5 – 5 ln(ρ/T) is plotted against the inverse temperature for Nd0.67Sr0.33Mn

1-xFexO3 (x = 0 and 0.05) films The solid lines are fits of the adiabatic small-polaron

model

Therefore it is obvious that Fe doping creates some forms of AFI matrix which

separates the FMM background into isolated domains in the region between 115 < T <

165 K, as proposed by Hong et al [164], in Nd0.67Sr0.33Mn0.95Fe0.05O3 film Hence this result further supports the hypothesis made earlier that FM and AFM phases coexistence in the Fe-doped manganite

At low temperatures, the temperature-independent residual value

(resistivity at 77 K) varies by several orders of magnitude from x = 0 to x = 0.05

samples Therefore, the carriers seem to undergo a loss of mobility, or else very few

of them participate in the conduction process in the Fe-doped NSMFO film In order

to examine the conduction process above the ferromagnetic transition, the resistivity

data in the paramagnetic phase of the NSMFO (x = 0, 0.05) films were fitted using the

nearest-neighbour small-polaron in the adiabatic regime and Mott’s variable-range hopping (VRH) models It is found that resistivity of epitaxial NSMFO films is best

described by the small polaron hopping model,

model by the

Emin-Holstein theorem [165] in the adiabatic regime Here W is the polaron activation energy, A the resistivity coefficient and k the Boltzmann constant From figure 5 – 5,

we obtained W from the gradient of the slope and A from the y-intercept These data are also summarized in Table 5 – 1 W and A rise with increasing x Our findings are

in qualitative agreement with the reported results of La

p

p

0.7Sr0.3Mn1-xFexO3 [166] films and La0.67Ca0.33Mn1-xGaxO3 [167] samples The standard error of the estimate involved

for the polaron fit is 0.11 which is insignificant small and R 2 = 0.995 giving a highly

correlated observed and fitted values for x = 0.05 sample deTeresa et al [168] proposed that the increase in W was due to Mn – O lattice distortion upon doping p

However, Sun et al [167] suggested that Fe doping has its main influence on the local

magnetic structure in the system As the local DE ferromagnetism is weakened upon increasing Fe concentration, the magnetic characteristics due to lattice polaron decays,

causing the increase of W Thus with the doping of Fe ion, the increase of activation

energy reflects the increase in polaron binding energy as Fe ions bind the polaron more strongly than Mn ions in the lattice

p

We now turn to the resistivity coefficient A Our findings present a different view from Huang et al [166] Instead of decreasing in low doped samples (x < 0.1), A

increases substantially even in the 5% Fe-doped NSMFO sample From this view, we

ascribe the rise of A in the x = 0.05 sample to a combination of polaron

nearest-neighbor and non-nearest-nearest-neighbor hopping due to the strong on-site Coulomb

repulsion as suggested by Sun et al [167] Therefore the magnetic Fe3+ substitution seems to have a stronger tendency than the other nonmagnetic ions to destabilize the long-range ferromagnetic order in the Nd-based than the La-based manganite system

in the high temperature regime

5.1.3.3 Magnetoresistance

Figure 5 – 6 shows the temperature, T dependence of the magnetoresistance,

MR at H = 1 T for the polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (x = 0, 0.05 and 0.1) targets and (x = 0, 0.05) epitaxial films The calculated values of MR, defined in

equation 3 – 1 as MR = [ρ(H = 0) - ρ(H = 10 kOe)]/ ρ(H = 0), where ρ(H = 0)

and ρ(H = 10 kOe) are the films and targets resistivities in zero and 10 kOe magnetic field, respectively, are 45% at maximum MR temperature (refers to the temperature at

which MR is maximum), T = 255 K for x = 0 and 86% at T = 110 K for x = 0.05 MR MR

Table 5 – 1: Magnetic and transport parameters for Nd0.67Sr0.33Mn1-xFexO3 (x = 0, 0.05 and 0.1) films T c is the magnetic transition temperature, T IM is the insulator-to-

metal transition temperature, MR max is the maximum MR ratio, T MR the maximum MR

temperature, W is the activation energy and A the resistivity coefficient p

Sample W p (meV) A (mΩcm/K) T c (K) T IM (K) T MR (K) MR max (%)

Figure 5 – 6 Temperature dependence of the magnetoresistance, MR curves for

Nd0.67Sr0.33Mn1-xFexO3 with x = 0, 0.05 and 0.1 (open symbols) polycrystalline targets and x = 0 and 0.05 (close symbols) epitaxial films

films, respectively The MR for x = 0.1 film increases below the measured temperature range Note that T ≈ T suggests that the intrinsic MR is dominant in the system For the epitaxial films, MR occurs within a narrow temperature range and

decreases in the low-temperature phase This is in contrast to the polycrystalline

targets where MR increases gradually with decreasing temperature Spin-polarized

intergrain tunneling and spin-dependent scattering at the grain boundaries have been proposed to explain the observed behavior in the polycrystalline targets [52] As compared to the polycrystalline targets, the enhanced MR observed in these films may

be attributable to the growth of a good epitaxial film on lattice-matched substrates These films have been grown thick enough to accommodate the epitaxial strain arising from the lattice mismatch between the substrate and film

5.1.4 Conclusion

The microstructure, magnetic and electrical transport properties of epitaxial Fe doping in Nd0.67Sr0.33Mn1-xFexO3 (x = 0, 0.05 and 0.1) films have been systematically

studied No structural changes were observed as Mn is being substituted by Fe

However, with increasing doping level, the magnetization M is suppressed and both and T are lowered After careful analysis of the transport properties with the

Emin-Holstein’s theory, we suggest that the transport process of Nd

5.2 Thickness-dependent magnetic, electrical transport and temperature coefficient of resistance in Nd0.67Sr0.33Mn1-xFexO3 (x

= 0, 0.05) strain-relaxed films

5.2.1 Introduction

It is known that Mn3+/Mn4+ ratio in the perovskite-type manganites

A1-xBxMnO3 (A = La, Nd, Pr; B = Ca, Sr, Pb) and the microstructural of Mn-O

network are key parameters controlling the DE interaction, magnetism and transport

properties in the manganite systems From the previous chapters, we learned that

doping at its respective A and/or Mn sites changes the physical properties of the

perovskite manganites By doing so the mean ionic radii will be varied and this will

lead to structural modification due to the adjustment of Mn–O–Mn bond angle/length

[28, 169] Many researchers have directly replaced parts of the Mn ions with other

elements such as Fe, Al, Cr and Ru [115, 161, 170] It is found that the substitution of

Mn sites by Fe and Al ions suppresses the DE interaction and thus the FM metallic

state, promoting the AFM insulating behavior This is in contrast to Ru doping which

weakens the charge ordering and induces ferromagnetism and metallicity in the

manganite system [161] The possibility of using thin films epitaxial growth paves

another way for controlling the band distance and bond angle of the Mn-O-Mn local

arrangement through tailoring of the biaxial epitaxial strain Average strain which

arises from lattice mismatch between the substrate and manganite thin films is

expected to be dependent on film thickness Consequently, the magnetization and

conductivity in these manganite materials follow the overall strain state In the past,

the magnetic and the electrical transport properties of these CMR films were

discussed in terms of the strain state [171], oxygen content [172] and annealing

conditions [173] In addition, Jin et al [3] have investigated the thickness dependence

of the CMR for La-Ca-Mn-O films The different MR behavior reported is

hypothesized to be closely related to the optimization of the perovskite lattice parameter Recently, most of the work related to the study of the thickness dependent

magnetism, resistivity and MR ratio has been carried out for ultrathin films [174, 175]

Their studies claimed the existence of a structural disorder layer (dead layer) between the lattice and substrate interface Our present work aims to report the CMR,

temperature coefficient of resistance (TCR defined as

dT

dR R

1), magnetic and electrical properties of Nd0.67Sr0.33Mn1-xFexO3, NSMFO (x = 0 and 0.05) epitaxial strain – relaxed films with thicknesses, t = 150 and 450 nm The magnetic and electrical

properties of the polycrystalline bulk have also been included A detailed study to display the intrinsic properties of perovskite manganites is accomplished by comparing the behavior of polycrystalline bulk with that of the strain-relaxed films

5.2.2 Experiments

The experimental and characterization techniques by growing epitaxial

Nd0.67Sr0.33Mn1-xFexO3, NSMFO (x = 0 and 0.05) films on (001)-SrTiO3 (STO) perovskite substrates are exactly the same as in section 5.1.2 The deposition is performed with a KrF excimer laser (wavelength of 248 nm, a pulse width of 30 ns) from a rotating stoichiometric NSMFO target onto the substrate All films are deposited at a substrate temperature of 750 °C under an oxygen ambient pressure of

approximately 0.5 mbar and a laser fluency of 5 Hz Annealing is carried out in-situ at

750 °C in an oxygen pressure of 500 mbar for one hour before cooling down to room temperature The thickness is calibrated by growing the film on partially covered STO

substrate and measuring the height of the resulting step using an Alpha – step 500 surface-profiler method A triangle – wave ac magnetic field of 10 kOe and 0.005 Hz

is applied parallel to the sample surface The diamagnetic contribution of the substrate

is subtracted from the data The temperature ranges are 77 – 300 K for electrical and magnetic measurements

5.2.3 Experiment results and discussions

the epitaxial films are often characterized under some degrees of ab-plane biaxial tensile stress and c-axis compressive stress on the film This leads to the growth of tetragonal unit cell with large ab-plane lattice constant and subsequently a shorter c-

axis Figure 5 – 7 exemplifies the XRD patterns collected at room temperature for

NSMFO (x = 0 and 0.05) bulks and t = 150 nm films The NSMFO targets are of

single-phase without any secondary or impurity phase It is observed that the film

exhibits strong preferences in the [l00] direction perpendicular to the surface of the film The fact that most of the non c-axis oriented diffraction peaks are not recorded

proves that the films are epitaxially grown with high purity The full width at half maximum (FWHM) of the rocking angles on (200) planes for

Nd0.67Sr0.33Mn0.95Fe0.05O3 (t = 150 and 450 nm) films are in the range of 0.5° - 0.8° The inset to figure 5 – 7 shows a θ − 2θ scan of the (200) reflections for NSMFO (x =

0 and 0.05) bulk and t = 150 nm films From the XRD measurements, the out-of-plane lattice constant calculated for t = 450 nm NSMO film is a c = 3.848 Å It is comparable

to the bulk value of 3.849 Å, which indicates strain relaxation in the c-axis direction The lattice constants for t = 150 nm NSMFO (x = 0 and 0.05) films range between 3.845 < a c < 3.848 Å This also shows that the NSMFO films are in almost complete

strain- relaxed state along the c – axis Therefore, as the thickness of the film increases, the c – axis lattice constant relaxes towards its bulk lattice constant as

observed from our XRD analysis Our result is consistent with other authors [176] who claimed that the manganite films relax the lattice mismatch through point defects, dislocations or different grain structures in a layer near the substrate/film interface, leaving an almost non-strained film near the surface

5.2.3.2 Magnetic Properties

Figure 5 – 8(a) and (b) show the zero-field-cooled magnetization as a function

of temperature for NSMFO (x = 0 and 0.05) bulk and films in a magnetic field of 2 kOe The Curie temperature, T c is defined as the temperature at which the slope of the

magnetization is maximum The T c for the NSMFO bulks are estimated to be 270 K (x

= 0) and 188 K (x = 0.05) while that for the NSMFO films are 258 K (x = 0, t = 150 nm), and 260 K (x = 0, t = 450 nm) and 165 K (x = 0.05, t = 450 nm) It is well documented that samples without sufficient oxygen content exhibit lower T c, resistance peak temperature, saturation magnetization and a higher resistance [177,

178] In situ annealing of the films increases the oxygen stoichiometry and eliminates

part of the static defects in the samples such as vacancies and interstitials in the film

Hence, from the magnetization data with T cbulk ≈ T cfilm, our films are expected to be

Figure 5 – 7: XRD spectra for Nd0.67Sr0.33Mn1-xFexO3, NSMFO (x = 0 and 0.05) bulks and t = 150 nm films The inset gives the θ scan of the (200) reflections of NSMFO bulks and films

θ2

−

fully stoichiometric It is observed that the saturation magnetization is almost

thickness independent As seen in figure 5 – 8(b), NSMO (t = 150 and 450 nm) films

are seen to display very similar saturation magnetization due to continuous growth over the film surface This result is also demonstrated in the strain-relaxed

La0.7Ca0.3MnO3 films [179] All the NSMO films exhibit inherent properties which are intrinsic to the NSMO polycrystalline bulk

As the film thickness decreases from 300 down to 4 nm, Ziese et al [179] have observed a drop in T c of about 200 K This observation, as claimed by Zhang and his co-workers [180], is thickness dependent With the proposed finite size

scaling theory, Fisher et al [181] predicted that T c in thin film will shift to lower temperatures than that of the bulk when the spin-spin correlation exceeds the film

thickness This explains the slight difference in T c observed when t varies from 150 to

450 nm As Fe doping increases, the saturation magnetization deteriorates and T c

lowers accordingly as seen in figure 5 – 8 These results are manifested in both bulks and films The ferromagnetism in these materials has been suppressed upon Fe doping [115] Thus, weakening of ferromagnetism may be interpreted as the formation of AFM Fe3+ - O2- - Mn3+ and Fe3+- O2- - Fe3+ couplings as reported in Mossbauer spectroscopy (MS) studies [182]

(a)

(b)

Figure 5 – 8: Zero-field-cooled magnetization, M at H = 0.2 T as a function of temperature, T for (a) NSMFO (x = 0 and 0.05) bulks and (b) NSMO (t = 150 and

450 nm) films and Nd0.67Sr0.33Mn0.95Fe0.05O3 (t = 150 nm) film Inset in (a) shows

temperature dependence of magnetization and reciprocal magnetization of NSMO

bulk The arrows in (b) indicate the ferromagnetic Curie temperatures, T c for the respective films

5.2.3.2 Electrotransport Properties

Figure 5 – 9 presents the resistivity as a function of temperature for the

NSMFO (x = 0 and 0.05) polycrystalline bulks, t = 150 and 450 nm films at zero and

10 kOe applied field The insulator-metal transition temperature, T p (defined by

maximum slope resistivity) are 268 K, 258 K and 255 K for the NSMO target, t = 450 and 150 nm films, respectively The corresponding T p for N0.67S0.33M0.95F0.05O3 target,

t = 450 and 150 nm films are 183 K, 115 K, and 110 K respectively For the undoped

NSMO films, T p follows T c closely NSMO films turn ferromagnetic metallic at

around 260 K A decoupling between T p and T c of about 60 K as observed in

N0.67S0.33M0.95F0.05O3 films becomes noticeable [183] The resistivities with H = 0 and

10 kOe for the NSMFO bulks are larger in magnitude than those in films This indicates a greater grain boundary resistance and a more restricted conduction path in

the polycrystalline bulk than in the film As the film thickness varies, a change in T c is

also accompanied by a slight decrease in T p Although the films display almost similar saturation magnetization, the reduction of film thickness leads to an increase in

resistivity Films with t = 450 nm display a lower resistivity than that of 150 nm

although the films are earlier claimed to be free from strain effects caused by the substrate-lattice mismatch In order to explain the above observed results, several authors [184, 185] claimed the existence of interface related dead layer between the

substrate and film Sun et al [175] reported the existence of an electrically dead layer

near LCMO/substrate or LCMO/vacuum interface for films on LaAlO3 and NdGaO3

Additional work produced by Borges et al [185] further suggested that the strain

which arises from the film/substrate lattice mismatch was released through a depth

Figure 5 – 9: Temperature dependence of resistivity for Nd0.67Sr0.33Mn1-xFexO3 (x

= 0 and 0.05) targets and (t = 150 and 450 nm) films at zero (dotted lines) and 10

kOe (solid lines) applied field

which was less than the dead layer thickness Therefore we can conclude that the increase in resistivity as observed in figure 5 – 9 may best be associated with the decrease in thickness of more relaxed films

In order to study the conductive behavior of the manganites in detail,

low-temperature resistivity data for T < T p is analyzed using a polynomial expansion in

temperature We find that the experimental resistivity curves for NSMFO (x = 0, 0.05) bulks, t = 150 and 450 nm NSMO films in figure 5 – 10 are well fitted by the simple

empirical relation of ρ(H, T) = ρo + ρ2(H)T2 + ρ5(H)T5 Here, ρo, ρ2(H) and ρ5(H) are

the fitting parameters evaluated at both zero and 10 kOe magnetic field The fitted solid and dotted lines are shown in figure 5 – 10 and fitted parameters are listed in

Table 5 – 2 The range of validity can be extended to about 250 K below T p for all the fits The residual resistivity, ρo, is temperature independent and is attributed to scattering by grain boundaries and static imperfections in the system The resistivity

of the NSMO bulk is higher than the corresponding NSMO films as anticipated The values of ρo for the NSMO (t = 150 nm) film and bulk are 0.2 mΩ-cm and 1.0 mΩ-

cm, respectively The higher ρo for the bulk than the film is primarily due to the enhanced electron scattering off the polycrystalline grain boundaries In fact, the ρo

value of 0.2 mΩ-cm at 77 K for NSMO film is identical to the reported LCMO and

LSMO MOCVD films (t = 150 nm) at 5 K [186] The decrease of ρo value in NSMO films as film thickness increases indicates a reduction of short-range disorder in the

film The T2-term is often associated to electron-electron scattering and the coefficient intrinsic to the manganite compound is usually about 10-8 ΩcmK-2 [186]

Figure 5 – 10: Low-temperature resistivity, ρ(T) in H = 0 (open symbols) and 10 kOe (closed symbols) applied field fits to ρ(H, T) = ρo + ρ2(H)T2 + ρ5(H)T5 for the

NSMO (circle) bulk, t = 150 (triangle) and 450 nm (diamond) films The solid and dotted lines are guides to the eye for T < T p Inset shows the ρ(T) curves for

Nd0.67Sr0.33Mn0.95Fe0.05O3 bulk

Table 5 – 2: Magnetic and transport parameters for Nd0.67Sr0.33Mn1-xFexO3 (x = 0 and 0.05) bulks and films at H = 0 and 10 kOe applied field T c is the ferromagnetic Curie

temperature, T p is the insulator-metal transition temperature, MR is the

magnetoresistance ratio, TCR is the temperature coefficient of resistance, fitting parameters ρo, ρ2(H), and ρ5(H) determined using the simple empirical relation ρ(H,

Therefore, from our results, ρ2 ~10-8 ΩcmK-2 implies that the T2-term has its origin in

the electron-electron scattering Instead of using the T4.5-term which has been predicted for electron-magnon scattering in the DE theory by Kubo and Ohata [32],

the T5-term which is associated with acoustic phonon scattering is used to improve the fit in NSMFO systems The ρo, ρ2 and ρ5 values increase for the Fe-doped NSMO bulk However, no conclusion can be drawn for the N0.67S0.33M0.95F0.05O3 films as ρo

is too close to T p The main contribution of Fe substitution at Mn sites is to increase the electron-electron, electron-magnon scattering effects, thus enhancing the magnetic disorder in the system The applied magnetic field suppresses the electron scattering effects, leading to the reduction of the resistivity associated with magnetic disorder as observed in Table 5 – 2 for the lower values of ρ2 and ρ5 at H = 10 kOe Films with t

= 150 nm have higher ρo, ρ2 and ρ5 values than t = 450 nm This agrees well with our

hypothesis made earlier that the presence of a thinner relaxed region in the film above the structurally altered film-substrate region increases the short-range disorder in the film

5.2.3.4 Magnetoresistance and temperature coefficient of resistance

Figure 5 – 11 shows the dependence of MR with temperature T for NSMFO (x

= 0 and 0.05) polycrystalline bulks, t = 150 and 450 nm films at 10 kOe The MR ratio

is defined as [ρ(H = 0) – ρ(H = 10 kOe)]/ ρ(H = 0) ρ(H = 0) and ρ(H = 10 kOe) are

the resistances without and with an applied field of 10 kOe, respectively The

corresponding MR ratios for NSMFO bulks are 34% (x = 0) at the temperature which corresponds to the maximum MR, T MR = 270 K and 43% (x = 0.05) at T = 185 K The MR ratios for t = 450 nm NSMFO films are 42.6% (x = 0) at T = 255 K and

MR

MR

86.6% (x = 0.05) at T = 110 K while that for t = 150 nm NSMFO films are 45.2% (x = 0) at T = 250 K and 89.3% (x = 0.05) at T = 100 K A 5% Fe-doping in NSMO bulk gives rise to an increase of the MR ratio from ~34% to ~43% As Fe concentration increases, all the films also exhibit an increase in the MR ratio, which

can be explained by the fact that Fe ions which exist as Fe

MR

+ 3

-Application of an external magnetic field of 10 kOe realigns the depolarized Mn

spins, leading to an increase in the intrinsic MR in the Fe-doped sample This is in

accordance with the result obtained earlier that the application of magnetic field suppresses ρ2 and ρ5, giving rise to higher MR NSMFO films have a larger MR ratio

as compared to the corresponding bulk materials [183] Films with t = 150 nm show

an increment of less than 5% in its MR compared to t = 450 nm films According to Jin et al [3], the MR ratio for La0.67Ca0.33MnO3 films is found to be strongly

dependent on film thickness, with MR value reaching a maximum at t ~1000Å, and

thereafter it gradually decreases even if the thickness is further increased Thus, our present findings further demonstrate the presence of structurally altered region in the films Therefore the strain – relaxed layer mainly affects the conductivity, having less

influence on the magnetism and MR ratios in the samples This is in contrast to the

proposed strain-induced effect which has a direct influence on the magnetic and electrotransport properties in ultrathin films [185, 187]