A study on fe induced structural, magnetic and transport properties in colossal magnetoresistive nd0 67sr0 33mno3 polycrystalline bulk and films 1

Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites Introduction to Magnetoresistance Phenomenology and Theory of Colossal Magnetoresistive Manganites 1.1 Introduction In recent years, the discovery of colossal magnetoresistance (CMR) properties in mixed valence manganites [1 - 3] has demonstrated for the second time after the break through about high Tc superconducting cuprates that oxides offer a very promising field for the investigation of new materials with specific properties susceptible to be involved in device applications. Magnetic systems of great potential are those with limited ability to transport electricity in zero field, resulting from competing dissimilar ground states. Giant magnetoresistance (GMR) multilayer metallic films show a relatively large sensitivity to magnetic fields. The mechanism in these films is largely due to spin valve effect between polarized metals. If an electron in a regular metal is forced to move across spin-polarized metallic layers, it will suffer spin-dependent scattering. The observed magnetoresistance (MR) is a few percent and has the very important advantage of not being limited to low temperatures. Its giant magnetoresistive head proved to be an improvement from the inductive read head and showed a clear advantage in read signal amplitude. With the rapid development in the magnetic data storage industry spin valve head with higher sensitivity was evolved. It has been used in the magnetic storage industry for several years now. While the physical mechanism that produces the magnetoresistance is well understood, the technological challenges involved in the Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites production of small devices of high sensitivity are the bottleneck of an industry ever hungry for smaller-faster-better sensors. On the equally speculative note, it would seem that colossal magnetoresistive material has a bright potential future in the world of magnetic recording industry. Manganites are ideal compounds for magnetic sensor devices since the ground states are metallic and semiconducting, respectively. It has been known and studied as long as 50 years ago (van Santen and Jonker, 1950) [4, 5], however a more recent development involves the study of the unusually large effect an external field has on their ability to transport electricity and heat. The energy scale of the phenomenon produces the most interesting effects, such as the metal-insulator transition and the maximum sensitivity to external field, at temperatures close to room temperature [6]. Manganites are prototypical of correlated electron systems where spin, charge, and orbital degrees of freedom are at play simultaneously. The rich solid state physics make these compounds unique in terms of colossal magnetoresistance and also the potential applications as not only sensor devices but also as solid electrolytes, catalysts, and novel electronic materials. Several excellent review articles such as by Ramirez [7], Coey et al. [8], Tokura and Tomioka [9], Rao et al. [10], Salamon and Jaime [11] and edited books by Rao and Raveau [12], Tokura [13] are available in the literature now which discuss the properties of manganites in detail. We have organized this chapter into three main modules. The choice of material is intended to provide the basic concepts of the various field of physics involved in the manganites system. The first part examines the basic principle of various types of magnetoresistance, MR with emphasize on the colossal magnetoresistive manganites. A Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites simple introduction to the general description of various magnetoresistance phenomena, following the chronological sequence of discovery will be reviewed. In the second part, the basic concepts and underlying physical properties of manganites and the various magnetoresistance phenomena, observed by state of art of experiments and interpreted with modern theories are presented. We also introduce the subject with a brief historical summary of the experimental results and theoretical developments since the discovery of manganites in the early 1950s. This includes the correlation between crystal structure, magnetic and transport properties in the systems. Special attention will be paid to the “colossal magnetoresistive” material. The fundamental physical properties of the perovskite-structure, ABO3 (A = trivalent/divalent cations, B = cations) and the underlying physics involved, observed by the state of art of experiments, will be discussed. The concept of double exchange and the importance of Jahn-Teller coupling in particular are presented. More recent research is then described, treating the variety of ground states including ferromagnetic metals, orbital- and charge-ordered antiferromagnets etc. that emerge as divalent atoms are substituted for trivalent rare earth ions. Due to the scientific importance and potential applications in magnetic sensors for storage devices and nonvolatile magnetic random access memory, the last section is devoted to review the atypical magnetic, electrical transport and magnetoresistive properties of the half-metallic ferromagnetic colossal magnetoresistive compounds. We end the chapter with a summary and an outlook of the overall dissertation. Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites 1.2 Overview of the magnetoresistance (MR) phenomenology Magnetoresistance (MR) refers to the relative change in electrical resistivity ρ of a metal or semiconductor when placed in a magnetic field. It is generally defined by MR = [∆ρ ρ (0, T )] = [ρ (0, T ) − ρ (H , T )]/ ρ (0, T ) (1 – 1) where ρ (H , T ) and ρ (0, T ) are the resistances or resistivities at a given temperature in the presence and absence of a magnetic field, H, respectively. It can be positive or negative depending on its definition. Generally, the MR effect depends on both the strength of the magnetic field and the relative direction of the magnetization with respect to the current. In this section, we will briefly review the mechanism involved in the four distinct types of MR. They are the normal magnetoresistance (NMR), anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and colossal magnetoresistance (CMR), the latter being the main focus of this dissertation. 1.2.1 Normal magnetoresistance (NMR) Normal magnetoresistance (also known as ordinary magnetoresistance) refers to the change in electrical resistance/resistivity of a metal or semiconductor when placed in an applied magnetic field. The existence of this phenomenon was intuited by E. H. Hall in 1879. However, Hall’s effort to determine the extra resistance was unsuccessful. It was demonstrated that the NMR effect originated from the Lorentz force on the electron trajectories in an applied magnetic field. Let us assume a simple classical one carrier model (free electron theory). We consider the effect of field B acting on a conduction electron having an average velocity v perpendicular to the applied magnetic field in a sample. It experiences a force acting normal to both directions (v and B) and moves in Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites response to this force and the force effected by the transverse electric field. We assume that a constant current I flows along the x-axis from left to right in the presence of a zdirected magnetic field. Electrons subject to the Lorentz force initially drift away from the current line toward the negative y-axis, resulting in an excess surface electrical charge on the side of the sample. This charge results in the Hall voltage cause by the transverse electric field, a potential drop across the two sides of the sample. The accumulation of the charge continues until the resulting transverse electric field becomes large enough to cause a force that is equal and opposite to the magnetic field. Under equilibrium conditions, the motion of the charge carriers is identical in the presence or absence of a magnetic field. Therefore, no resultant magnetoresistance was observed because of this transverse electric field. However, in the two-band model and complex Fermi surface where both electrons and holes are present, then all charge carriers not have the same properties. When the current flow is disturbed by the presence of a magnetic field, some of the charge carriers are forced to travel a different path between the electrodes, which lead to more scattering than in the absence of a field. Then a larger observed resistivity is expected. The difference between the zero field resistivity and the measured resistivity under the applied magnetic field is known as normal magnetoresistance. This magnetoresistance is always positive and varies as B2. The magnitude of this type of magnetoresistance is usually negligible (eg. 0.8% for Fe and ∼ 0% for Fe80B20) [14]. Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites 1.2.2 Anisotropic magnetoresistance (AMR) In contrast to the normal magnetoresistance, anisotropic magnetoresistance (AMR) is only observed in ferromagnetic (FM) metal and alloys. AMR originates from the spin-orbit interaction between the electron trajectory (orbit) and the magnetization (spin) [15]. It depends on the relative orientations of the magnetization and the electric current. When an external field is applied to the sample, the electron cloud about each nucleus deforms slightly as the direction of the magnetization rotates. This deformation changes the amount of scattering undergone by the conduction electrons when traversing the lattice as shown in figure – 1. e I M Magnetic field e I M Magnetic field Figure – Schematic diagram demonstrating the physical origins of AMR. Shaded yellow ovals represent the scattering cross-sections of bound electronic orbits. The arrows (black) indicate the deflected conduction electrons when traversing the electronic orbits. The sample resistance is the greatest when the current, I flows parallel to the magnetization, M. If the magnetization and current are oriented parallel to the external field, then the electronic orbits are perpendicular to the current. The cross-section for scattering is increased giving a high resistance. Conversely, if the magnetization and current are oriented perpendicular to the external field, then the electronic orbits are in Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites the plane of the current. The cross-section for scattering is decreased giving a low resistance state. 1.2.3 Giant magnetoresistance (GMR) Giant magnetoresistance (GMR) was originally discovered by Baibich et al. in 1988 in Fe/Cr multilayers [16]. Like other magnetoresistive effects, GMR is the change in electrical resistance in response to an applied magnetic field. Contrary to NMR and AMR, GMR arises from the dependence of the resistivity in layered and granular magnetic structures on the local magnetic configuration. It was called “giant magnetoresistance” or GMR because the observed magnetoresistive effect was found to be much larger than either NMR or AMR. The underlying physics can be qualitatively understood using Mott’s model, which was introduced as early as 1936 [17] to explain the sudden increase in resistivity of FM metals as they are heated above room temperature. The GMR that is present in magnetic/nonmagnetic metal multilayers can easily be explained by Mott’s argument as follows: First we consider the magnetizations in the multilayer to be aligned collinearly as shown in figure – 2. Up Up Down spin Down spin Figure - Schematic illustration of electron transport in magnetic/nonmagnetic metal multilayers for antiparallel (left) and parallel (right) magnetization of successive ferromagnetic (dark green) and nonmagnetic (light green) metal layers. Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites The magnetic layers are antiferromagnetically coupled (or antiparallel) in the absence of external field. The electrons with spin projections parallel and antiparallel to the magnetization of the ferromagnetic layers will experience different scattering rates when they enter the multilayer [18]. This is called spin-dependent scattering. Let us assume that electrons with spin antiparallel to the magnetization are scattered strongly. We shall see that in the antiparallel-aligned multilayer, the up-spin and down-spin electrons are scattered strongly both in the first and second ferromagnetic layers. Hence the total resistance of the multilayer in its antiferromagnetic configuration is high. When a strong enough field is applied to align the magnetizations of the adjacent magnetic layers as in the parallel-aligned magnetic layers case, the up-spin electrons will pass through the structure with minimum scattering due to parallel arrangement of the spins with the magnetization of the layers. On the contrary, the down-spin electrons are scattered strongly within both ferromagnetic layers because their spins are antiparallel to the magnetization of both layers. This is represented in figure – which shows the resistance change as a function of applied magnetic field. The same argument can also be used to explain the GMR observed in granular materials. The magnetic moments of the ferromagnetic granules are randomly oriented in the absence of a magnetic field. Both the up- and down-spin electrons are scattered strongly by the granules. Thus the resistance will be large. When a saturating magnetic field is applied, the magnetic moments are aligned and the resistance will be low. Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites R RAP RP HS H Figure – Schematic representation of the GMR effect. The graph shows the change in the resistance of the magnetic multilayer as a function of applied magnetic field. The figure shows the magnetization configuration (indicated by the arrows in white) of the trilayer at various magnetic fields. The magnetizations are aligned antiparallel at zero field and parallel at an external field H larger than the saturation field Hs. Thus we can adopt the definition of ‘optimistic’ GMR ratio as the ratio of the maximum difference in resistance over the minimum resistance: GMR ratio ≡ Rap − R p Rp (1 – 2) where R p is the resistance when the magnetizations are parallel and Rap is the resistance when the magnetizations are antiparallel. The optimistic GMR ratio is unbounded but the ‘pessimistic’ GMR ratio is Rap − R p Rap , which is also in use, is never greater than 1. GMR is always the property of a device consisting of alternating layers of different materials. The discovery of GMR has created great excitement since this magnetoresistive effect has important applications, particularly in magnetic information storage technology. Magnetoresistive reading heads are commercially available and have since become the leading technology beyond the year 1990 [19]. The highest published values of GMR to Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites date are 220% in Fe/Cr multilayer [20] and 120% in Co/Cu multilayer [21]. From the results published, it is crucial that a good matching of the multilayer is done since the magnitude of GMR varies considerably depending on the chemical constituents of the multilayer. 1.2.3.1 Spin Valve Spin valve structure consists of uncoupled magnetic thin films which can be switched from the antiparallel to parallel configuration. It was developed by Dieny et al. [22] in order to overcome the large saturating fields required to rotate the magnetization to the ferromagnetic configuration in GMR structure. For example, the saturation fields in the Fe/Cr multilayer [16] are of the order 10 – 20 kG which is three orders of magnitude higher than the field required for applications. The spin valve shown in figure – has four layer structures consisting of a magnetically soft ferromagnetic (FM) layer (free layer) and a second FM layer (pinned layer), which is exchange-coupled to an antiferromagnetic (AFM) layer. The two thin magnetic films are separated by a nonmagnetic spacer. Lead Free FM Layer Spacer Hard Magnet Pinned FM Layer + NM Layer AFM Pinning Layer Lead Hard Magnet NM Layer Figure - Schematic cross section of a spin valve GMR read head with exchangepinned layer and longitudinal hard bias. The nonmagnetic (NM) layer under the hard magnet is for controlling the coercivity and thickness of the hard magnet. 10 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites splitting of the degenerate levels to be given by the ferromagnetic transition temperature k B Tc and using classical argument, predicts the electrical conductivity to be as follows:  xe  Tc    σ =   ah  T  (1 – 3) where x is the doping concentration and a is the Mn-Mn distance. Equation (1 – 3) agrees with the Jonker and van Santen’s available experimental results in the limited region of 0.2 < x < 0.4. An alternative way to visualize DE process was presented in detail by Anderson and Hasegawa (1955) and it involves a second-order process in which the two states described above go from one to the other using an intermediate state Mn13↑+ O3↓ Mn23↑+ . The effective hopping for the electron to move from one Mn site to the next is proportional to the square of the hopping involving the p-oxygen and d-manganese orbitals (tpd). In addition, if the localized spins are considered classical and with an angle θij between nearest-neighbor, then the effective hopping becomes proportional to cos(θ/2). The effective hopping expression can be given as t pd → t eff = t cos θ ij (1 – 4) where θij is the angle between spins of the sites i and j while t is the hopping matrix element of the extra electron. For purely antiferromagnetic ordering θij = π and teff = 0, then the hopping cancels. On the other hand, if the system is made ferromagnetic, θij = teff = t, then hopping is the largest and the electrons can move freely. The quantum version of this process has been described by Kubo and Ohata (1972) [32]. 24 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites 1.2.4.4 Electrical transport properties van Santen and Jonker [4, 5] have first reported the resistivity measurements on ceramic samples of (La1-xAx)MnO3 (A = Ba, Ca or Sr) as a function of temperature, T and compositions. They found striking correlation between the magnitude of the resistivity and the magnetic state of the compounds. Most of these manganese perovskites are paramagnetic insulators at ordinary temperatures and exhibit an increase in electrical resistivity with a decrease in temperature. Compositions that are ferromagnetic show an insulator-like behaviour above Tc and a decrease in electrical resistivity as they are cooled below Tc. The insulator-to-metal (I-M) transition is identified as a maximum in the resistivity and different compositions induces different insulator-to-metal transition temperature, Tim. The sharpness of the transition in polycrystalline samples and films is often dependent on the purity of the samples. A) Paramagnetic state with T > Tc The temperature dependence of the resistivity, ρ, of the manganites in the paramagnetic state generally show two types of behavior as seen in figure – 8. The manganites that are doped with large A-site cations radii such as Sr, Ba and Pb have larger bandwidth. As a result the paramagnetic state can be metallic (PMM) or insulating (PMI) with a small gap. However, for manganites with relatively smaller cations like Ca, the bandwidth would be small and the paramagnetic phase is generally insulating (PMI). Thus, whether the T > Tc phase is PMM or PMI depend entirely on the bandwidth. In order to investigate what gives rise to the different physical behavior above Tc, three 25 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites models are used to fit the resistivity data of the manganites above Tc [33]. They are a simple Arrhenius law [34], a polaron model [35], and a variable-range hopping model [36]. We discuss these briefly here. (1) An Arrhenius law  Eg  ρ = ρ ∞ exp    2kT  (1 – 5) is generally used to model activated behavior due to a band gap E g or a mobility edge. Here ρ ∞ is the resistivity at T = ∞ and k is the Boltzman constant. It should be kept in mind that a pure Arrhenius law might be a very crude approximation. The resistivity, ρ = 1/(n e µ) usually displays a temperature-dependent prefactor due to the carrier mobility µ or the carrier density n. Though the carrier concentration is dominated by the thermally activated form, it might nevertheless display a temperature-dependent prefactor determined by the particular density of states. (2) Nearest-neighbor hopping of small polarons (SPH) model in the adiabatic regime. In the adiabatic regime, the charge-carrier motion is faster than the lattice vibrations. Thus the resistivity is given by ρ= W p  πkT exp   ne 3ea ω  kT  (1 – 6) with W p = E p / − t . E p denotes the polaron formation energy, t the electronic transfer integral, ω0 the longitudinal optical-phonon frequency, a the hopping distance, and e the 26 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites electronic charge. Equation (1 – 6) is valid for temperatures larger than half the Debye temperature ΘD. It is fulfilled for samples which have a Curie temperature Tc > ΘD/2 ∼ 150 – 200 K [37]. In the nonadiabatic limit, the charge-carrier’s motion is slow compared to the lattice vibrations and the small polaron model gives inconsistent results. (3) Mott’s variable range hopping (VRH) [38] type of expression  T  14  ρ ≈ ρ ∞ exp     T   (1 – 7) holds good [36, 38] above Tc for the carriers which are localized by random potential fluctuations. Here the temperature scale T0 is related to the localization length ξ [38] by kT0 = 24 πN (E F )ξ (1 – 8) where N (EF ) denotes the density of states at the Fermi level. B) Ferromagnetic metallic state with T < Tc In the region just below Tc, ρ varies rapidly with temperature. Experimental evidence for metallic behavior comes from power-law dependences of R [39 – 41] have 27 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites Figure – 13 The magnetization, resistivity, and magnetoresistance of La1-xCaxMnO3 (x = 0.25), as a function of temperature at various fields. The inset is ρ at low temperatures. Reproduced from Schiffer et al. (1995) [27]. 28 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites been reported. Typically, the resistivity for T < Tc can be fitted using the expression below ρ (T ) = ρ + ρn T n (1 – 9) where ρ and ρ n are fitting parameters. In general for T < 0.2 Tc, a T2 dependence with n = seems to make the major contribution. Here the T2 dependence might be correlated to the general electron-electron scattering within a Fermi liquid model.The resistivity can also be well fitted up to T < 0.5 Tc with n ≈ 2.5. Schiffer et al. [27] found that the resistivity data for high-quality La1-xCaxMnO3 (x = 0.2, 0.33 and 0.45) films below Tc can be fitted well with n ≈ 2.5. However, at somewhat high temperature, contribution from an extra term faster than T2 is needed. For example, ρ (T ) = ρ + ρ1T + ρ 2T 4.5 (1 – 10) where ρ is the resistivity due to domain boundaries and other temperature-independent scattering mechanisms while the ρ1T and ρ2 T 4.5 terms are empirical fits to the data which represent a combination of electron-electron and electron-magnon scatterings. Here, different physical states and preparation conditions of the samples will result in different ρ and ρ n , indicating that perturbation in the solid solution manganites also contributes to the electron-scattering processes. 29 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites 1.2.4.5 Magnetoresistance properties The magnetoresistance, MR in RE1-xAExMnO3 manganites as defined in equation (1 – 1) arises from both intrinsic and extrinsic effects. Effects intrinsic to a material are distinguished from extrinsic effects which depend on the direction of the magnetization in adjacent ferromagnetic regions. The latter arises from contacts between particles and grain boundary, exhibiting high MR value under a low magnetic field at low temperatures. The intrinsic magnetoresistance of nonstoichiometric EuO [8, 43] and mixed-valence manganites [3, 8] is always huge and it occurs near the vicinity of Curie temperature, Tc. We will discuss these two kinds of MR briefly here. (1) The intrinsic MR effect The gigantic MR in the perovskite manganites have already been known at the very early stage of the study on transition metal oxides. The early work of Zener [28, 29] was to explain the simultaneous occurrence of ferromagnetism and metallicity in transition metals using the ‘double exchange’ (DE) mechanism which has been described in the earlier section. By creating electron-vacancy sites in the lattice, the eg electron can hop depending on the relative configuration of the local spins. The ferromagnetic metallic state is stabilized by maximizing the kinetic energy of the conduction electrons (θij = 0). When the temperature is raised near or above Tc, the configuration of the spin is dynamically disordered and the effective hopping interaction is subject to disorder and reduced on average. This would lead to enhancement of the resistivity near and above Tc and therefore, large MR can be expected around Tc. 30 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites Anderson and Hasegawa [30] further showed that the transfer integral varies as the cosine of the angle between neighboring spins as in equation (1 – 4). As temperature is lowered and spins fluctuation decrease, the combined itinerant/local-moment system lowers its total energy by aligning the spins ferromagnetically. However, Millis et al. [44] have shown that a Hamiltonian incorporating only the DE interaction cannot explain the magnitude of the change in resistivity at the ferromagnetic transition. With these, they, as well as Roder et al. [45] proposed, in addition to the DE, an electron-phonon coupling term. The paper by Searle and Wang [46, 47] reported thoroughly the magnetic field dependence of the resistivity for a flux grown crystal of La1-xPbxMnO3, in particular the large MR near Tc. Soon after, Kubo and Ohata [32] gave a theoretical account for this phenomenon using the findings of those mentioned above. The physics of CMR is obviously more complex. We have seen examples of the failure of the simplest DE model [44], as manifested for example by insulating behavior above Tc relevant to the CMR effect, anomalously diffusive charge dynamics even in the ferromagnetic ground state, versatile charge- and orbital-ordered states adjacent to or competitive with the DE ferromagnetic state, etc. There are certainly other important factors than the above simplest DE scenario, which are necessary to interpret important features observed experimentally. Those features are, for example, electron-lattice interaction, ferromagnetic/antiferromagnetic superexchange interaction between local spins, intrasite and intersite Coulomb repulsion interactions among eg electrons, etc. These interactions occasionally compete with the ferromagnetic DE interaction, producing complex but intriguing electronic phases as well as the gigantic response of the system to an external field, such as CMR effect. Figure – 13 shows the 31 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites typical magnetization, resistivity and magnetoresistance of La1-xCaxMnO3 (x = 0.25) as a function of temperature, reproduced from Schiffer et al. (1995) [27]. As seen, it shows the existence of a robust MR, larger than 80% which is expected to be favorable for potential applications such as magnetic storage devices. The CMR transition is correlated with the ferromagnetic transition in the magnetization. However, this large MR only appears within a narrow range of temperature and a large field is needed to produce such effect. In order to overcome these drawbacks, the understanding of the fundamental electronic, structural and magnetic properties of the perovskite manganites has been initiated. (2) The Extrinsic MR effect Recent experiments on epitaxial manganite films with a single grain boundary [48] have allowed the high-field, intrinsic CMR to be separated from the low-field extrinsic MR effect due mainly to heterogeneous magnetization distribution in adjacent grains [49]. The effect can also be most directly seen in pressed powder compacts of manganites [50] and other half-metallic ferromagnets [51]. Figure – 14 shows the temperature (T) dependence of MR at H = 1500 Oe and the temperature (T) dependence of zero-field resistivity for polycrystalline LCMO and LSMO films. Unlike the high-field CMR which is an intrinsic property, usually greatest near Tc, the low-field effect exists at all temperatures below Tc in polycrystalline material and increases with decreasing grain size. A characteristic but unexplained feature of the lowfield extrinsic MR in manganite ceramics [52], polycrystalline films [53, 54], and tunnel 32 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites junctions [55, 56] is its rapid decay with increasing temperature. Thus, it appears that a fundamentally different process is dominating the MR of the polycrystalline samples. Hwang et al. [52] have pointed out that the MR in the polycrystalline sample is dominated by transport across grain boundaries that are extremely sensitive to an applied field. They suggested that the conduction is carried out by a process called the intergrain spin-polarized tunneling. Since the electron spin is conserved in the tunneling process, there is an additional magnetic coupling energy when the magnetic moments of the neighboring grains are not parallel. Figure – 14 Resistivity and MR as a function of temperature for polyscrystalline (average grain size of 14 µm) LCMO and LSMO films. Reproduced from Li et al. [53]. 33 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites By considering the magnetic field dependence of the intergrain coupling energy, the extrinsic MR is given by ∆ρ / ρ o = − [ ] JP m (H , T ) − m (0, T ) 4k B T (1 – 11) where J is intergrain exchange constant, P is the electron polarization, and m is the magnetization normalized to the saturation value. However, an alternative explanation for the low-field MR has been proposed by Li et al. [53]. According to their observations on the polycrystalline and epitaxial LSMO and LCMO thin films, they suggested that the conductivity at low temperatures is dominated by the intergrain spin-polarized scattering. By using wide-field Kerr microscope, the magnetic domains with different coercivities are observed to be weakly coupled in a polycrystalline LSMO film and they orient successively in an increasing magnetic field. The magnetic inhomogeneity at grain boundaries serves as strong scattering centers whose strength is field dependent. A large field reduces the scattering because of the alignment of the domains associated with the grains in a parallel configuration. The MR results [57, 58] can be approximately described as ∆ρ / ρ ≈ cos θ ij ≈ − A(M / M s ) (1 – 12) where M is the global magnetization, Ms the saturation magnetization, A the prefactor that determines the size of the MR, and θ ij is the angle between the axes of the ferromagnetic grains. In both the tunneling and scattering models, ρ is expected to be a maximum at the coercive field and decreases as the relative orientation of the magnetization between grains changes with the application of a field as shown in figure 34 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites – 15. The MR data in figure – 15 is fitted quite well by a (M / M s ) dependent. The MR hysteresis loops mirror the global magnetization, M, with the MR being proportional to (M / M s )2 [59]. Figure – 15 Magnetoresistance (MR) as a function of magnetic field, H measured at 10 kOe for a µm averaged grain size LCMO film for both field-parallel and fieldperpendicular alignments. The magnetic hysteresis loop for the sample obtained at the same temperature using SQUID magnetometer is also plotted. After [53]. In addition to the above two proposed models, the drop in MR as the temperature is raised can also be explained by other contributions such as spin-flip scattering process. The presence of an oxygen-deficient layer at SrTiO3/La0.7Sr0.3MnO3 interface may lead to enhanced electron-magnon scattering, which induces spin flipping [60]. Whatever the explanation, the low-field MR (LFMR) effect could be more significant for practical applications in the next generation of digital recording and sensing devices [61] than the intrinsic CMR. Studies are needed to have a better understanding of the resistivity 35 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites response to an applied field and to improve the magnetic field required for significant MR effects. Experiments such as nonferromagnetic/metal mixed composite sintering [62, 63], artificial CMR/insulator/CMR multilayer structure fabrication [64], interface-related disorder [65], and on-spin clustering [66] have been designed to enhance and improve the MR effect. High-spin polarization materials such as manganites and Sr2FeMoO6 [67] with higher Tc have also been extensively studied and considered. However, the enhancement of low-field MR, especially at room temperature, remains as a challenge. 1.3 Summary and Outlook Fundamental features of the colossal magnetoresistive (CMR) manganites with perovskite-related structures have been described. First the combined metal-insulator and ferro-paramagnetic transition and its description in terms of a double-exchange (DE) mechanism provide us with the basic starting ground for description of the various ground states observed. Besides the proposed models, examples of deviation from DE model have been provided, which was ascribed partly to Jahn-Teller-type electron-lattice coupling effect. In fact, more complex but intriguing features such as antiferromagnetic superexchange, orbital-ordering, charge-ordering, and orbital-lattice interaction emerge due to the competition between the DE and other instabilities in the systems. An important motivation for the study of CMR is the future potential of this rapidly expanding field in the magnetic storage and sensors industry. In this dissertation, the studies of the iron-valency induced effect on the magnetic, electrical and magnetotransport properties of Nd0.67Sr0.33MnO3 polycrystalline bulk and 36 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites epitaxial thin films, temperature sintering effects on the magnetic, electrical and magnetotransport properties of La0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 composites and bilayers are reported in detail. This dissertation is organized as follows: In chapter two, a summary of the different types of experimental and characterization techniques used were described in detail. A brief account of the fundamental process for preparing the polycrystalline manganite ceramics and thin films and the basics of the various conventional techniques for characterizing the microstructure, magnetic and electrical transport properties of the sample were discussed. Chapter three reviewed the Fe valencyinduced effect on the magnetic, electric and magnetotransport properties in Nd0.67Sr0.33Mn1-xFexO3 polycrystalline system studied by means of x-ray diffraction, x-ray photoelectron spectroscopy, vibrating sample magnetometer and standard four-point probe techniques. The system was found to display mixed properties of Nd0.67Sr0.33MnO3 and Nd0.67Sr0.33FeO3 depending on the Fe concentration. It was shown that orthorhombic solid solutions existed over the entire range of iron concentrations ≤ x ≤ 1. The magnetic and transport properties were altered when Fe was substituted for Mn, demonstrating a rapid decrease in Curie temperature and a remarkable enhancement in magnetoresistance. This chapter concluded that Mn ions enhance the ferromagnetic double exchange in the system and Fe ions enhance the antiferromagnetic superexchange interaction in the system. In chapter four, the effect of temperature sintering on the magnetic, electrical and transport properties of La0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 composites was presented. The results suggested that sintering temperature had a prominent effect on the properties of grain boundary, which played an important role in determining the electrical transport 37 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites behavior of the composites. At 900 °C, the intergrain exchange couplings between the adjacent particles gave rise to an increase in resistance. Raising the sintering temperature induced the formation of an interfacial phase near the La0.67Sr0.33MnO3 and Nd0.67Sr0.33MnO3 grain boundaries. Broad magnetoresistance across room temperature was observed in a composite sintered at 1300 °C. This observation may be ascribed to the coexistence of multiphase in the composite that led to a decrease in resistance of the composites. Chapter five had been divided into two parts. Both parts had their focus on the fabrication of Fe-doped Nd0.67Sr0.33MnO3 epitaxial films by pulsed-laser deposition. However, the first part concentrated on the experimental studies of the Fe – induced effect on magnetic, electrical transport and magnetoresistance properties in Nd0.67Sr0.33MnO3 epitaxial films in detail. The second part focused 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 was found that the films well reproduce the properties intrinsic to the polycrystalline bulk. Fe substitution at Mn sites reduced the saturation magnetization, ferromagnetic Curie temperature, Tc, metal-insulator temperature, Tp and led to an overall increase in magnetoresistance (MR). The resistivity in the T > Tp regime followed EminHolstein’s theory while the resistivity in the T < Tp regime followed the empirical relation of ρ(H, T) = ρo + ρ2(H)T2 + ρ5(H)T5. Both showed Fe-doping at Mn sites reduced the long-range ferromagnetic order in all the samples. As the film thickness increased, the resistivity decreased indicating a reduction of short-range disorder in the film. 38 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites Chapter six was based on the study of magnetic, electrical and magntotransport properties of La0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 bilayered film. A combination of LSMO and NSMO layers into a bilayer produced a smooth resistance curve, although the NSMO film exhibited considerable temperature dependence in resistance. The monolayered LSMO film had an MR of about 1% near room temperature while the monolayered NSMO had an MR of about 13% at TMR ≈ 246 K. The MR behavior of NSMO/LSMO bilayer resembled that of the NSMO monolayer whereas the LSMO/NSMO bilayer showed a broader MR transition in the range from 200 K < T < 300 K. Although a smooth resistance curve over a wide temperature was observed, the MR was not seriously suppressed in the bilayered system. In chapter seven, a brief conclusion of the dissertation and further suggestions for future work was presented. 39 [...]... panel (b) La1-xSrxMnO3 (left), (c) Nd1-xSrxMnO3 (middle), and (d) Pr1xCaxMnO3 (right) The PI, PM and CI denote the paramagnetic insulating, paramagnetic metallic and spin-canted insulating states, respectively The FI, FM and AFM denote the ferromagnetic insulating, ferromagnetic metallic and antiferromagnetic metallic states, respectively The COI and CAFI denote the charge-ordered insulating and canted... the familiar antiferromagnetic arrangement in the three directions, while B is the familiar ferromagnetic arrangement Figure 1 – 7 Ferromagnetic and antiferromagnetic moments as a function of sample ion composition Taken from [26] 17 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites Figure 1 – 8 The magnetic as well as electronic phase diagrams of upper panel (a) La1xCaxMnO3,... states and preparation conditions of the samples will result in different ρ 0 and ρ n , indicating that perturbation in the solid solution manganites also contributes to the electron-scattering processes 29 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites 1. 2.4.5 Magnetoresistance properties The magnetoresistance, MR in RE1-xAExMnO3 manganites as defined in equation (1 – 1) arises... samples A) Paramagnetic state with T > Tc The temperature dependence of the resistivity, ρ, of the manganites in the paramagnetic state generally show two types of behavior as seen in figure 1 – 8 The manganites that are doped with large A- site cations radii such as Sr, Ba and Pb have larger bandwidth As a result the paramagnetic state can be metallic (PMM) or insulating (PMI) with a small gap... showed that the transfer integral varies as the cosine of the angle between neighboring spins as in equation (1 – 4) As temperature is lowered and spins fluctuation decrease, the combined itinerant/local-moment system lowers its total energy by aligning the spins ferromagnetically However, Millis et al [44] have shown that a Hamiltonian incorporating only the DE interaction cannot explain the magnitude... LaMnO3-SrMnO3, and LaMnO3BaMnO3 The end members of La3+Mn3+O32-, Ca2+Mn4+O32-, Sr2+Mn4+O32-, and Ba2+Mn4+O32- are in the AFM and insulating state CaMnO3 forms a cubic perovskite 12 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites ABO3 structure with a lattice constant of 3.73 Å Mn ions are six fold coordinated with oxygen ions along the Cartesian axes: , < 010 > and directions,... DE ferromagnetic state, etc There are certainly other important factors than the above simplest DE scenario, which are necessary to interpret important features observed experimentally Those features are, for example, electron-lattice interaction, ferromagnetic/antiferromagnetic superexchange interaction between local spins, intrasite and intersite Coulomb repulsion interactions among eg electrons,... conclude this chapter with a summary and an organization of this dissertation 1. 2.4 .1 Crystalline Structure Nearly half a century ago, Jonker and van Santen [5] discovered a striking correlation between magnetic order and conductivity in a number of mixed oxides containing manganese or cobalt They have also successfully reported the existence of ferromagnetism in mixed crystals of LaMnO3-CaMnO3, LaMnO3-SrMnO3,... both intrinsic and extrinsic effects Effects intrinsic to a material are distinguished from extrinsic effects which depend on the direction of the magnetization in adjacent ferromagnetic regions The latter arises from contacts between particles and grain boundary, exhibiting high MR value under a low magnetic field at low temperatures The intrinsic magnetoresistance of nonstoichiometric EuO [8, 43] and. .. usually greatest near Tc, the low-field effect exists at all temperatures below Tc in polycrystalline material and increases with decreasing grain size A characteristic but unexplained feature of the lowfield extrinsic MR in manganite ceramics [52], polycrystalline films [53, 54], and tunnel 32 Chapter One: An Overview and Theory of Colossal Magnetoresistive Manganites junctions [55, 56] is its rapid . Tomioka [9], Rao et al. [10 ], Salamon and Jaime [11 ] and edited books by Rao and Raveau [12 ], Tokura [13 ] are available in the literature now which discuss the properties of manganites in detail important applications, particularly in magnetic information storage technology. Magnetoresistive reading heads are commercially available and have since become the leading technology beyond. spin-canted insulating states, respectively. The FI, FM and AFM denote the ferromagnetic insulating, ferromagnetic metallic and antiferromagnetic metallic states, respectively. The COI and CAFI