Colossal Magnetoresistance Behavior Research Articles

Novel Insights of Enhanced Magnetization and Magnetoelectric Coupling in the Nanocomposite of BiFeO3 with Lanthanum Strontium Manganite

AbstractNovel insights on the magnetic, ferroelectric and magnetoelectric coupling of the nano composites of BiFeO3 (BFO) and La0.7Sr0.3Mn0.95Fe0.05O3 (LSMFO) with the latter exhibiting colossal magnetoresistance (CMR) behavior are provided in this study for the first time. Appreciable magnetoelectric coupling is observed in the composite and is seen to be significantly varying at temperatures below the blocking temperature of LSMFO. The local structure and magnetic properties of both the BFO and LSMFO particles in the composite are investigated in a detailed manner using Mössbauer spectroscopy studies specially devised to address the problem concerned. Enhanced values of the effective magnetic fields are observed at the Fe sites associated with BFO particles which are in contact with LSMFO particles. Interestingly, an appreciable increase in the magnetic hyperfine fields at Fe sites corresponding to LSMFO is also observed in the nanocomposites. New insights are derived based on the results of Mössbauer studies toward comprehending the magnetic and magnetoelectric coupling effects as deduced in these composites.

Thermal treatment method: A novel approach to prepare Nd0.7Sr0·3MnO3 manganite

The discovery of colossal magnetoresistance (CMR) in perovskite manganites has attracted considerable interest due to their utilisation in spintronic devices. Various techniques were adopted to investigate the influence of their particle size on magnetic and transport properties. Unfortunately, the complexity of conventional methods underlines the need for a simple preparation technique. Here, single phase Nd0.7Sr0·3MnO3 (NSMO) was successfully synthesised by thermal treatment method with polyvinylpyrrolidone (PVP) acting as capping agent. FTIR spectra confirmed the decomposition of PVP after a high-temperature sintering process. The atomic composition of the sample was found very close to the nominal ratio. NSMO exhibited paramagnetism at room temperature, and its TC was recorded at 246 K. Typical CMR behaviour was perceived and accompanied by metal-insulator transition (TMI) at 182 K. Thus, this study has provided a novel approach to prepare high-purity Nd-based manganite with an easier operation.

Reversibility of magnetic field driven transition from electronic phase separation state to single-phase state in manganites: A microscopic view

Electronic phase separation (EPS) is a common phenomenon in strongly correlated oxides. For colossal magnetoresistive (CMR) manganites, the EPS is so pronounced that not only does it govern the CMR behavior, but also raises a question whether EPS exists as a ground state for systems or a metastable state. While it has been well known that a magnetic field can drive the transition of the EPS state into a single-phase state in manganites, the reversibility of this transition is not well studied. In this work we use magnetic force microscopy (MFM) to directly visualize the reversibility of the field driven transition between the EPS state and the single-phase state at different temperatures. The MFM images correspond well with the global magnetic and transport property measurements, uncovering the underlying mechanism of the field driven transition between the EPS state and the single-phase state. We argue that EPS state is a consequence of system quenching whose response to an external magnetic field is governed by a local energy landscape.

Magnetotransport properties of La1−xCaxMnO3 (0.52 ≤ x ≤ 0.75): Signature of phase coexistence

We report the temperature and magnetic field dependence of transport properties in epitaxial films of the manganite La1−xCaxMnO3 in the overdoped region of the phase diagram for x > 0.5, where a charge–ordered (CO) and an antiferromagnetic (AF) phase are present. Resistivity, magnetoresistance and angular dependence of magnetoresistance were measured in the temperature interval 4.2K <T < 300K, for three concentrations x=0.52,0.58 and 0.75 and in magnetic fields up to 5T. The semiconductor/insulator–like behavior in zero field was observed in the entire temperature range for all three concentrations x and the electric conduction, at lower temperatures, in the CO state obeys 3D Mott's variable-range hopping model. A huge negative magnetoresistance for x=0.52 and x=0.58, a metal–insulator transition for B > 3 T for x=0.52 and the presence of anisotropy in magnetoresistance for x=0.52 and x=0.58 show the fingerprints of colossal magnetoresistance (CMR) behavior implying the existence of ferromagnetic (FM) clusters. The declining influence of the FM clusters in the CO/AF part of the phase diagram with increasing x contributes to a possible explanation that a phase coexistence is the origin of the CMR phenomenon.

Multifunctional epitaxial systems on silicon substrates

Multifunctional heterostructures can exhibit a wide range of functional properties, including colossal magneto-resistance, magnetocaloric, and multiferroic behavior, and can display interesting physical phenomena including spin and charge ordering and strong spin-orbit coupling. However, putting this functionality to work remains a challenge. To date, most of the work reported in the literature has dealt with heterostructures deposited onto closely lattice matched insulating substrates such as DyScO3, SrTiO3 (STO), or STO buffered Si(100) using concepts of lattice matching epitaxy (LME). However, strain in heterostructures grown by LME is typically not fully relaxed and the layers contain detrimental defects such as threading dislocations that can significantly degrade the physical properties of the films and adversely affect the device characteristics. In addition, most of the substrates are incompatible with existing CMOS-based technology, where Si (100) substrates dominate. This review discusses recent advances in the integration of multifunctional oxide and non-oxide materials onto silicon substrates. An alternative thin film growth approach, called “domain matching epitaxy,” is presented which identifies approaches for minimizing lattice strain and unwanted defects in large misfit systems (7%–25% and higher). This approach broadly allows for the integration of multifunctional materials onto silicon substrates, such that sensing, computation, and response functions can be combined to produce next generation “smart” devices. In general, pulsed laser deposition has been used to epitaxially grow these materials, although the concepts developed here can be extended to other deposition techniques, as well. It will be shown that TiN and yttria-stabilized zirconia template layers provide promising platforms for the integration of new functionality into silicon-based computer chips. This review paper reports on a number of thin-film heterostructure systems that span a variety of ferroelectric, multiferroic, magnetic, photocatalytic, and smart materials. Their properties have been extensively investigated and their functionality found to be comparable to films grown on single-crystal oxide substrates previously reported by researchers in this field. In addition, this review explores the utility of using laser processing to introduce stable defects in a controlled way and induce magnetism and engineer the optical and electrical properties of nonmagnetic oxides such as BaTiO3, VO2, NiO, and TiO2 as an alternative for incorporating additional magnetic and conducting layers into the structure. These significant materials advancements herald a flurry of exciting new advances in CMOS-compatible multifunctional devices.

Multifunctional heterostructures integrated on Si (100)

Complex oxide films and heterostructures exhibit a wide range of functional properties, including colossal magneto-resistance, magnetocaloric effects, coupled magnetic and polarisation (multiferroic) behaviour, and some interesting physical phenomena including spin, charge and orbital ordering. However, putting this functionality to work remains a challenge. To date, most of the work reported in the literature has dealt with heterostructures deposited on closely lattice-matched insulating substrates such as DyScO3 (DSO), NdGaO3 (NGO), MgO and SrTiO3 (STO). Unfortunately, these substrates are incompatible with existing complementary metal oxide semiconductor (CMOS)-based technology, where silicon (Si) (100) substrates dominate. This review covers the major advances in the integration of multifunctional (oxide and non-oxide) materials onto Si (100) substrates reported in the recent past by the authors’ group using pulsed laser deposition in conjunction with a novel layer growth approach called ‘domain-matching epitaxy’ (DME). This paper focuses on the growth of several important oxide systems, including BiFeO3 (BFO), La0·7Sr0·3MnO3 (LSMO), BaTiO3 (BTO), and non-oxide films including permalloy/MgO and Ni/MgO. It is shown that one can achieve thin-film epitaxy over an extended misfit scale by using the paradigm of DME. This will be critical for the future integration of multifunctional heterostructures onto CMOS-based chips in order to create smart structures for the next-generation solid-state devices. In addition, this paper explores the utility of using laser processing to introduce defect populations that induce magnetism in non-magnetic oxides such as STO and BTO as an alternative to incorporating more traditional magnetic layers into the structure.

Observing the Great Spin and Orbital Swap

The interplay between electronic charge, spin, and orbital degrees of freedom in solids is at the heart of the complex and exotic magnetic behavior. The electrons, whose spins play a central role in these magnetic effects, primarily interact through the Coulomb force, which is spin-independent. However, the Pauli exclusion principle imposes constraints that give rise to so-called exchange interactions, which couple the spin and orbital degrees of freedom of the electrons. The exchange coupling is not a normal force but is instead the outcome of a symmetry that a quantum system must obey when two of its electrons swap their spins and (orbital) positions. Exchange interactions are in fact the essential ingredient behind “orbital magnetism,” a phase in which electrons confined in localized orbitals interact with one another and with other valence electrons populating different orbitals. Orbital magnetism includes ferromagnetism, as well as more exotic effects like colossal magnetoresistance (an enormous decrease of resistance in certain manganese oxides exposed to an external magnetic field [1]) and heavy fermion behavior (a thousandfold increase in the effective mass of electrons in rare-earth or actinide compounds [2]). But this type of magnetism isn’t well understood: Simplified models can’t accurately describe these nontrivial magnetic behaviors and, in spite of intensive efforts, many open questions remain in our understanding of orbital magnetism and its consequences.

Investigation of orbital disorder in Ca-doped LaMNO3

Studies of materials that exhibit colossal magnetoresistance (CMR) have been well documented over the past decade [1]. La1-xCaxMnO3 is one such CMR material that has attracted significant attention as it is relatively cheap and straightforward to synthesize. CMR materials have potential use among the next generation of magnetic memory devices, magnetic-field sensors, and transitors. For that to be possible a clear understanding has to be determined for the origin of the CMR phenomenon. Recent studies of CMR manganites have shown that a `polaron glass' phase precedes CMR behaviour at high temperatures (figure 1) [2]. This glasslike state sits midway between the high-temperature polaron liquid and the fully orbital-ordered solid with polarons. The polarons are localized charge density fluctuations associated with lattice deformation which gives rise to static, short-range correlations. At low temperature these polaron correlations are thought to evolve into the nanoscale polarons responsible for CMR itself. This poster describes our reasoning for the anticipated connection between the orbital disorder and CMR in La1-xCaxMnO3 revealed via reverse Monte Carlo modelling of X-ray and neutron total scattering data.

Intrinsic antiferromagnetic coupling underlies colossal magnetoresistance effect: Role of correlated polarons

A commonly believed picture of colossal magnetoresistance (CMR) effect is related to a first-order phase transition and electronic phase separation with coexisting ferromagnetic metallic and antiferromagnetic insulating phases. However, the underlying mechanism, i.e., the characteristic energy scale of the interacting phases and their spatial extent, is still under debate. Here we present experimental evidence on the existence of an effective antiferromagnetic coupling between the ferromagnetic nanodomains in epitaxial thin films of a classical CMR material $({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y}){}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ with Pr doping, $y$ = 0.375 and 0.4. This coupling yields to peculiar low-field CMR behavior with magnetic hysteresis and slow resistance relaxation, both induced by the magnetization reversal. The coercive field obeys a square-root temperature dependence for $T\ensuremath{\ll}{T}_{\mathrm{C}}$ and increases anomalously close to the phase transition. We modeled the magnetic structure within the phase-separation scenario as an assembly of single-domain ferromagnetic nanoparticles, antiferromagnetically coupled (pinned) by correlated Jahn-Teller polarons. The concentration of polarons increases drastically close to phase transition as indicated by the third harmonic of the electrical conductivity as well as Raman spectroscopy.

Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3

We demonstrate that magnetic phase separation and competing spin order in the colossal magnetoresistive (CMR) manganites can be directly explored via tuning strain in bulk samples of nanocrystalline La${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$. Our results show that strain can be reversibly frozen into the lattice in order to stabilize coexisting antiferromagnetic domains within the nominally ferromagnetic metallic state of La${}_{5/8}$Ca${}_{3/8}$MnO${}_{3}$. The measurement of tunable phase separation via magnetic neutron powder diffraction presents a direct route of exploring the correlated spin properties of phase separated charge/magnetic order in highly strained CMR materials and opens a potential avenue for realizing intergrain spin tunnel junction networks with enhanced CMR behavior in a chemically homogeneous material.

Effects of processing parameters on the properties of lanthanum manganite

A systematic study of the effects of processing parameters on the structure, microstructure, magnetic, and magnetotransport properties of lanthanum manganite (LMO) ceramics are presented. LMO ceramics are prepared using solid-state reaction technique by applying wide range of sintering temperatures 1100–1500 °C with varying holding times. It is observed that phase pure LMO can be formed at a sintering temperature of 1300 °C for a short duration of 2 h with measured density 4.8 g cm −3. Density does not improve with lower sintering temperature and longer duration. The grains appeared are quite homogeneous and uniform and do not differ much with further increase of temperature, however, less voids and higher leakage currents are observed for the LMO samples sintered at higher temperature. As the physical properties of manganites depend of structure, composition and oxidation states of Mn ions, XPS analysis are done to study the valence states. XPS analysis shows that LMO ceramics sintered at higher temperature have lower Mn valence and values approaches towards +2 as well as the loss of oxygen stoichiometry. All the samples show colossal magnetoresistance (CMR) behavior, however, the LMO prepared by sintering at 1300 °C for 2 h shows the highest MR% ∼60%.

Colossal magnetoresistance accompanied with magnetorelaxor behavior in phase-separated Ca1−xCexMnO3 thin films and CaMnO3/Ca0.92Ce0.08MnO3 superlattices

We report on the transport properties of electron-doped manganite Ca1−xCexMnO3 (CCMO, 0≤x≤0.08) films and superlattices composed of insulating layers CaMnO3 (CMO) and Ca0.92Ce0.08MnO3 (CCMO8), deposited on nearly lattice-matched NdAlO3 substrates. The CCMO (x=0.06 and 0.07) films show colossal magnetoresistance (CMR) accompanied with magnetorelaxor behavior, which can be ascribed to the phase separation of canted G-type antiferromagnetic metal and C-type antiferromagnetic insulator. The (CMO)m/(CCMO8)n superlattices with 4≤m, n≤8 (unit cells) resemble the solid-solution CCMO (x=0.06 and 0.07) films in CMR and magnetorelaxor behavior, suggesting that the phase separation takes place in the superlattices. The CMR and magnetorelaxor behavior of the (CMO)m/(CCMO8)n superlattices strongly depend on the thicknesses of constituent CMO and CCMO8 layers. The origin of the phase separation in the superlattices is discussed in terms of the charge transfer and the phase competition at the interfaces.

Magnetoelectric behavior of sodium doped lanthanum manganites

Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 (0.025⩽x⩽0.25) were prepared by polyvinyl alcohol assisted precursor method. After characterizing the samples by x-ray diffraction and transmission electron microscopy a systematic investigation of electrical, magnetic, and thermopower properties has been undertaken. The resistivity data were analyzed using effective medium approximation. From the analysis it has been found that the metallic fraction is increasing up to x=0.10 and remains constant with further doping. A close examination of the resistivity data clearly indicates that the sodium doped samples are slowly transformed from colossal magnetoresistance behavior to charge ordering behavior. Thermoelectric power data at low temperatures were analyzed by considering the magnon drag concept, while the high temperature data were explained by small polaron conduction mechanism.

Magnetic transport behavior of nano-crystalline Pr 0.67A 0.33MnO 3 (A = Ca, Sr, Pb and Ba) manganites

With a view to understand the magneto transport behavior of Pr 0.67A 0.33MnO 3 (A = Ca, Sr, Pb and Ba) exhibiting colossal magnetoresistance behavior, a systematic investigation has been undertaken. For this purpose, samples were prepared by poly vinyl alcohol (PVA)-gel route and after structural characterization of the samples by X-ray diffraction (XRD) and SEM, the electrical and magnetic transition temperatures ( T C and T P) were determined. The variation of both the transition temperatures is explained on the basis of size variance parameter. A systematic investigation of magnetization as a function of both magnetic field and temperature has been undertaken. It has been found from the magnetization versus temperature plot of PCMO that it exhibits reentrant type of spin glass behavior at 44 K. It has been concluded from the M– H plots that these materials are having soft ferromagnetic nature. Finally, of all the four samples under investigation, Pr 0.67Pb 0.33MnO 3 is found to exhibit highest magnetoresistance (MR) of 97%.

Thickness Dependent Magnetic Anisotropy of Ultrathin LCMO Epitaxial Thin Films

The magnetic properties of La0.7Ca0.3MnO3 (LCMO) manganite thin films were studied with magnetometry and ferromagnetic resonance as a function of film thickness. They maintain the colossal magnetoresistance behavior with a pronounced metal-insulator transition around 150-200 K, except for the very thinnest films studied (3 nm). Nevertheless, LCMO films as thin as 3 nm remain ferromagnetic, without a decrease in saturation magnetization, indicating an absence of dead-layers, although below approx. 6 nm the films remain insulating at low temperature. Magnetization hysteresis loops reveal that the magnetic easy axes lie in the plane of the film for thicknesses in the range of 4-15 nm. Ferromagnetic resonance studies confirm that the easy axes are in-plane, and find a biaxial symmetry in-plane with two, perpendicular easy axes. The directions of the easy axes with respect to the crystallographic directions of the cubic SrTiO3 substrate differ by 45 degrees in 4 nm and 15 nm thick LCMO films.

Extensions to the Kondo lattice model to achieve realistic Curie temperatures and appropriate behavior of the resistivity for manganites

We investigate the influence of the Jahn-Teller distortion and a direct antiferromagnetic moment coupling as extensions to a two-band Kondo lattice model for the magnetic and electronic properties of manganites. Those are calculated self-consistently via an interpolating self-energy model and a modified Ruderman-Kittel-Kasuya-Yosida technique using finite Hund coupling and quantum spins. We found that both effects are essential to achieve realistic Curie temperatures if we regard intraband Coulomb repulsion. Using reliable model parameters we got results which are in very good agreement with experimental data in the whole ferromagnetic doping range. In the calculated phase diagram there are ferromagnetic metal to paramagnetic insulator transitions, accompanied by a colossal magnetoresistance behavior. To improve the comparability of the measured behavior of the resistivity with the calculated one, we have to switch on interband Coulomb correlations.

Theoretical study of dopedTl2Mn2O7andTl2Mn2O7under pressure

Using first-principles density-functional-based calculations, we study the effect of doping and pressure on the manganese-based pyrochlore compound, ${\mathrm{Tl}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, which exhibits colossal magnetoresistive behavior. The theoretical study is motivated by the counterintuitive experimental observation of suppression of the ferromagnetic transition temperature upon application of pressure and its enhancement upon substitution of Mn by a moderate amount of nonmagnetic Sb ion. We also attempt to resolve the issue related to crystal structure changes that may occur upon application of pressure.

Spin dynamics of the quasi-two-dimensional ferromagnetic bilayer manganite [formula omitted

We have investigated the spin dynamics of the quasi-two-dimensional bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 , which shows colossal magnetoresistance (CMR) behaviour, by inelastic neutron scattering. We have measured the spin wave dispersion at T = 16 K along [ 1 0 0 ] , followed the temperature evolution of the spin wave dispersion close to T C ≈ 128 K and also measured the scattered neutron intensities in the ( Q , ω ) space above T C up to T = 200 K . The magnetic inelastic intensity above T C looks qualitatively very similar to those below T C and have a structure in the ( Q , ω ) space suggesting persistent spin waves.

Self-assembled nanocrystalline epitaxial manganite films on SrTiO3∕Si heterostructures

We report the growth and magnetic characterizations of La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 films on SrTiO3-buffered Si (100) and Si (111) substrates by pulsed-laser deposition. The structural studies show the epitaxial nature of the films. The microscopic studies show that the films consist of nanocrystalline particles. All films display sharp magnetic and electrical transitions associated with the colossal magnetoresistance behavior at and above room temperature, illustrating the superior quality of the films.

Buffer-Enhanced Electrical-Pulse-Induced Resistive Memory Effect in Thin Film Perovskites

A multilayer perovskite thin-film resistive memory device composed of a Pr0.7Ca0.3MnO3 (PCMO) perovskite oxide epitaxial layer on a YBa2Cu3O7-δ (YBCO) thin-film bottom electrode, a thin yttria-stabilized zirconia (YSZ) buffer layer grown on the PCMO layer, and a gold thin-film top electrode has been developed. With the addition of the YSZ buffer layer, the pulse voltage needed to switch the device is significantly reduced and the resistance-switching ratio is increased compared to a non buffered resistive memory device, which is very important for device fabrication. The magnetic field effect on the multilayer structure resistance at various temperatures shows colossal magnetoresistance (CMR) behavior for both high and low resistance states, implying a bulk material component in the switch behavior. The multilayer thin-film lattice structure has been further characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses, which indicate a high-quality heterostructure. Current imaging atomic force microscopy (I-AFM) analysis indicated nanogranular conductivity distributed uniformly throughout the PCMO film surface.