Colossal Magnetoresistive Oxides Research Articles

Theoretical study of the role of charge ordering in antiferromagnetically ordered manganites

We address the interplay of charge and magnetic orderings in colossal magnetoresistive material manganese oxides. We propose here on-site double exchange spin–spin interaction in the presence of Heisenberg-type spin–spin interaction in localized t2g core electrons. We consider charge-density wave (CDW) interaction in the crystal lattice as an extra mechanism in the itinerant eg band, to take into account of colossal magnetoresistance (CMR) in the system. We calculate electron Green’s functions by Zubarev’s Green’s function technique and hence calculate the charge-ordering gap and magnetic gap in the conduction band as well as core electron states. These orders are solved self-consistently for different model parameters of the system. We observe that the induced magnetic gap in the conduction band exists near the antiferromagnetic Néel temperature, which accounts for the CMR in the system. For all values of temperature, the CDW coupling lies in the range of g = 0.04 to 0.06, where the induced magnetic gap exists. The temperature-dependent specific heat exhibits anomalous jumps near charge-ordering and magnetic-ordering temperatures. The eg electron density of states exhibits a two-gap structure which explains tunneling conductance spectra measurements.

Nonlinear transport in nanoscale phase separated colossal magnetoresistive oxide thin films

We report a study of the I-V characteristics of 2.5–5.4 nm epitaxial La1−xSrxMnO3 (x = 0.33 and 0.5) and La0.7Ca0.3MnO3 thin films. While La0.67Sr0.33MnO3 films exhibit linear conduction over the entire temperature and magnetic field ranges investigated, we observe a strong correlation between the linearity of the I-V relation and the metal-insulator transition in highly phase separated La0.5Sr0.5MnO3 and La0.7Ca0.3MnO3 films. Linear I-V behavior has been observed in the high temperature paramagnetic insulating phase, and an additional current term proportional to Vα (α = 1.5–2.8) starts to develop below the metal-insulator transition temperature TMI, with the onset temperature of the nonlinearity increasing in magnetic field as TMI increases. The exponent α increases with decreasing temperature and increasing magnetic field and is significantly enhanced in ultrathin films with thicknesses close to that of the electrically dead layer. We attribute the origin of the nonlinearity to transport through the nanoscale coexisting metallic and insulating regions. Our results suggest that phase separation is not fully quenched even at low temperatures and high magnetic fields.

Magnetic frustration effects in the new colossal magnetoresistance oxide NaCr2O4

We have investigated the magnetic properties of the new colossal magnetoresistance oxide NaCr2O4 with the calcium-ferrite-type structure by using 23Na, 53Cr nuclear magnetic resonance (NMR) and magnetization measurements. This chromate is found to show an antiferromagnetic order with the spin moments slightly canted from the b axis at temperatures below T N=125 K. The spin structure may be governed by the competition between double-exchange ferromagnetic and superexchange antiferromagnetic interactions in the double chains. The origin of the large magnetoresistance is discussed for several scenarios such as phase separation, rearrangement of the spin structure induced by the magnetic field, and domain walls.

Natural media with negative index of refraction: Perspectives of complex transition metal oxides (Review Article)

The capabilities of perovskite-like compounds with the effect of colossal magnetoresistance (CMR) and some other complex oxides to have a negative index of refraction (NIR) are considered. Physical properties of these compounds are also analyzed from the standpoint of designing tunable metamaterials on their base. Of particular interest are temperature and magnetic field driven first-order transformations in oxides with perovskite structure and in spinels. These transformations give rise to nanophase separated states, using which the properties of negative refraction can be affected. The magnetic-field controlled metamaterials with CMR oxides as a boundary NIR media for a photonic crystal are discussed.

New Correlated Model of Colossal Magnetoresistive Manganese Oxides

A new minimal model is constructed for the doped manganese oxides which exhibit colossal magnetoresistance, involving a broad spin-majority conduction band as well as nearly localized spin-minority electron states. A simple mean-field analysis yields a temperature-dependent hybridized band structure with suppressed carrier weight at the Fermi level. Spin stiffness is complex, indicating strong spin-wave damping. Further investigations are needed to verify the relevance of the proposed model.

Possible origins of the magnetoresistance gain in colossal magnetoresistive oxide La0.69Ca0.31MnO3: Structure fluctuation and pinning effect on magnetic domain walls

The spatial fluctuation of the magnetic domain (MD) and charge/orbital ordering (CO/OO) structure at around the Curie temperature (TC) was directly observed in a colossal magnetoresistance (CMR) compound, La0.69Ca0.31MnO3, in which extraordinary anisotropic magnetoresistance (AMR) has also been observed. It was found that the long range MD structure collapses upon the emergence of short range CO/OO in a narrow temperature regime, which provides abundant evidence in support of a gain in magnetoresistance at around TC. Moreover, the pinning effect on the MD wall was discerned and it may contribute to the CMR as well as to the extraordinary AMR effect.

Magnetoresistance of ferromagneticCrxTi1−xNsolid solution nitride

${\text{Cr}}_{x}{\text{Ti}}_{1\ensuremath{-}x}\text{N}$ solid solution nitrides were prepared by nitridation of Cr-Ti alloys to measure their magnetic and transport properties. The experiment was motivated by calculations on magnetism pointing out similarities of CrN and ${\text{LaMnO}}_{3}$ (the parent compound of colossal magnetoresistance oxides). The bulk ${\text{Cr}}_{0.5}{\text{Ti}}_{0.5}\text{N}$ showed strong ferromagnetism $({T}_{c}\ensuremath{\sim}130\text{ }\text{K})$ while ferromagnetism was very weak for ${\text{Cr}}_{0.67}{\text{Ti}}_{0.33}\text{N}$, being consistent with our previous experimental results for epitaxial thin-film samples [Inumaru et al., Appl. Phys. Lett. 91, 152501 (2007)]. These results confirmed that the ferromagnetism is an intrinsic characteristic of the bulk alloy and cannot be ascribed to lattice strains due to the epitaxy of the films. ${\text{Cr}}_{0.5}{\text{Ti}}_{0.5}\text{N}$ showed magnetoresistance of approximately 5% at 5 T. The magnetoresistance was expressed well by a scaling function $\ensuremath{-}(\ensuremath{\rho}\ensuremath{-}{\ensuremath{\rho}}_{0})/{\ensuremath{\rho}}_{0}=C{(M/{M}_{s})}^{2}$, where $M$ and ${M}_{s}$ are the magnetization for a given magnetic field and saturated magnetization, respectively. The constant $C$ for the nitride was 0.14, which was much smaller than that for manganese oxides $(C\ensuremath{\sim}4)$.

Local Crystal Structure Modifications in Pulsed Laser Deposited Colossal Magnetoresistive Oxide Thin Films

ABSTRACTThe local structure around the manganese atom is probed by extended x-ray absorption spectroscopy (EXAFS) measurements in pulsed laser deposited thin films of La11xCaxMnO3 (x=0.1210.53). The thin films were deposited on various single crystal oxide substrates. The effect of the lattice parameter of the substrate on the local structural modifications around Mn atom is investigated. All the x-ray absorption experiments were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. By detailed EXAFS theoretical modeling for the possible local structures and the least square fitting to the EXAFS data using these models, the overall substrate and the Ca concentration effects are probed. The EXAFS results indicate a rigid Mn-O bonding, but response of the Mn-O-Mn bond angle upon variations of the substrate lattice constant and Ca substitution.

Giant magnetoimpedance and high frequency electrical detection of magnetic transition in La0.75Sr0.25MnO3

We show that high frequency electrical transport is an efficient technique for detecting the magnetic transition hidden by the scattering of charges at grain boundaries in colossal magnetoresistive oxides, even in the absence of any external magnetic field. The dc resistivity which shows only a weak anomaly at the Curie temperature in La0.75Sr0.25 MnO3 transforms into abrupt jumps in both resistive (Z′) and reactive (Z″) parts of the ac impedance, Z(f, T, H) = Z′(f, T, H) + jZ″(f, T, H) at higher frequencies (f = 0.1–5 MHz). The anomaly in Z′ and Z″ at TC decreases as much as 19% and 15%, respectively, in a dc magnetic field of H = 65 mT compared with 1% dc magnetoresistance, suggesting a possible giant low-field magnetoimpedance effect which could be exploited for room temperature practical applications. We interpret our observations due to changes in the magnetic penetration depth induced by the spontaneous ordering of spins and by the applied field.

Probing nanoscale inhomogeneities in transition metal oxides with ultrafast mid-infrared spectroscopy

Transition metal oxides are a rich area of investigation for experimentalists and theorists alike due to their coupling of multiple degrees of freedom with similar interaction strengths. This complexity leads to intrinsic phase inhomogeneities that are believed to play a significant role in the fascinating phenomena observed in these systems. We use ultrafast mid-infrared spectroscopy to probe quasiparticle dynamics in the colossal magnetoresistive oxides, Nd 0.5Sr 0.5MnO 3 and Tl 2Mn 2O 7. Our results demonstrate for the first time that ultrafast spectroscopy is sensitive to the presence of nanoscale phase inhomogeneities, strongly indicating the universality of phase coexistence in complex transition metal oxides.

Ferromagnetic CrxTi1−xN solid solution nitride thin films grown by pulsed laser deposition and their magnetoresistance

Epitaxial nitride films of CrxTi1−xN were grown on MgO (001) by pulsed laser deposition under nitrogen radical irradiation, and their magnetic and transport properties were investigated, which was motivated by calculations on magnetism pointing out similarities of CrN to LaMnO3, the parent compound of colossal magnetoresistance oxides. The films showed ferromagnetism for 0.28⩽x⩽0.5 and the TC showed a maximum (140K) at x=0.5. When x exceeded 0.5(x=0.58) ferromagnetism disappeared completely. At x=0.5, the nitride film showed a large magnetoresistance (6%–7% at 5T), which had a maximum at 130K.

Origin of ferromagnetic response in diluted magnetic semiconductors and oxides

This paper reviews the present understanding of the origin of ferromagnetic response thathas been detected in a number of diluted magnetic semiconductors (DMSs) and dilutedmagnetic oxides (DMOs) as well as in some nominally magnetically undoped materials. Itis argued that these systems can be grouped into four classes. To the first belongcomposite materials in which precipitations of a known ferromagnetic, ferrimagnetic orantiferromagnetic compound account for magnetic characteristics at high temperatures.The second class forms alloys showing chemical nanoscale phase separation into the regionswith small and large concentrations of the magnetic constituent. Here, high-temperaturemagnetic properties are determined by the regions with high magnetic ion concentrations,whose crystal structure is imposed by the host. Novel methods enabling a controlof this spinodal decomposition and possible functionalities of these systems aredescribed. To the third class belong (Ga, Mn)As, heavily doped p-(Zn, Mn)Te,and related semiconductors. In these solid solutions the theory built on the p–dZener model of hole-mediated ferromagnetism and on either the Kohn–Luttingerkp theory or the multi-orbital tight-binding approach describes qualitatively, and oftenquantitatively, thermodynamic, micromagnetic, optical, and transport properties.Moreover, the understanding of these materials has provided a basis for the development ofnovel methods, enabling magnetization manipulation and switching. Finally, in anumber of carrier-doped DMSs and DMOs a competition between long-rangeferromagnetic and short-range antiferromagnetic interactions and/or the proximity of thelocalization boundary lead to an electronic nanoscale phase separation. Thesematerials exhibit characteristics similar to colossal magnetoresistance oxides.

Origin of ferromagnetism and nano-scale phase separations in diluted magnetic semiconductors

This paper reviews various origins of ferromagnetic response that has been detected in diluted magnetic semiconductors (DMS). Particular attention is paid to those ferromagnetic DMS in which no precipitation of other crystallographic phases has been observed. It is argued that these materials can be divided into three categories. The first consists of (Ga,Mn)As and related compounds. In these solid solutions the theory built on p–d Zener's model of hole-mediated ferromagnetism and the Kohn–Luttinger kp theory of semiconductors describes quantitatively thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods enabling magnetisation manipulation and switching. To the second group belong compounds, in which a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation that results in characteristics similar to colossal magnetoresistance oxides. Finally, in a number of compounds a chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent is present. It has recently been suggested that this spinodal decomposition can be controlled by the charge state of relevant magnetic impurities. This constitutes a new perspective method for 3D self-organised growth of coherent magnetic nanocrystals embedded by the semiconductor matrix.

Investigation of activated transport in hole doped rare earth manganites in the high temperature paramagnetic regime

Electronic transport in the high temperature paramagnetic regime of the colossal magnetoresistive oxides, La 1− x A x MnO 3, A=Ca, Sr, Ba, x≃0.1–0.3, has been investigated using resistivity measurements. The main motivation for this work is to relook into the actual magnitude of the activation energy for transport in a number of manganites and study its variation as a function of hole doping ( x), average A-site cation radius (〈 r A〉), cationic disorder ( σ 2) and strain ( ε zz ). We show that contrary to current practice, the description of a single activation energy in this phase is not entirely accurate. Our results clearly reveal a strong dependence of the activation energy on the hole doping as well as disorder. Comparing the results across different substituent species with different 〈 r A〉 reveals the importance of σ 2 as a metric to qualify any analysis based on 〈 r A〉.

RF magnetron sputtering ferroelectric PbZr0.52Ti0.48O3 thin films with (001) preferred orientation on colossal magneto-resistive layers

Deposition by RF magnetron sputtering of PbZr0.52Ti0.48O3 (PZT) ferroelectric thin films on two types of colossal magneto-resistive (CMR) oxide electrodes, La0.67Sr0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO), is demonstrated in this work. The multiferroic heterostructure is grown on a STO substrate, which causes a 〈001〉 preferred orientation to develop. Ferroelectric, retention of polarisation and local piezoelectric properties were measured for assessing the success of the integration from the ferroelectric point of view. Remnant polarisation Pr and coercive field Ec were found to be ∼40 μC/cm2 and ∼100 kV/cm, respectively. Films presented good retention of polarisation and piezoresponse loops. These results show that ferroelectric layers with good functionality can be grown on CMR oxide films, and open the possibility of designing a piezoelectrically driven spin valve memory cell device based on this heterostructure.

Monoclinic La1‐xBaxMnO3 (x = 0.185) at 160 K.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Monoclinic La1−xBaxMnO3(x= 0.185) at 160 K

Single-crystal X-ray diffraction has shown that lanthanum barium manganese trioxide, La0.815Ba0.185MnO3, is monoclinic (I2/c) below a first-order phase transition at 187.1 (3) K. This result differs from the Pbnm symmetry usually assigned to colossal magnetoresistance oxides, A1-xA'xMnO3 with x approximately 0.2, which adopt a distorted perovskite-type crystal structure. The Mn atom lies on an inversion center, the disordered Li/Ba site is on a twofold axis and one of the two independent O atoms also lies on a twofold axis.

Resonant Mn L Emission Spectra of Layered Manganite La1.2Sr1.8Mn2O7

Resonant Mn L α,β soft X-ray emission spectra of a colossal magnetoresistive manganese oxide La 1.2 Sr 1.8 Mn 2 O 7 have been measured at the Mn L 2,3 -edges excited by monochromatized undulator radiation. The excitation energy dependence of the spectra has been analyzed in terms of d – d and charge-transfer excitations. The experimental spectra are reproduced by the multiplet calculation for the Mn 3+ configuration, which is consistent with the dominant occupancy of the 3 d x 2 - y 2 orbital and probably a phase separation.

Highly dense and compositionally inhomogeneous nano-agglomerates in an epitaxial La0.8Sr0.2MnO3 thin film grown on (100)SrTiO3

Microstructures in the thin film of La0.8Sr0.2MnO3 grown on (100) SrTiO3 by laser molecular beam epitaxy were characterized by transmission electron microscopy. Highly dense and dimensionally uniform nano-agglomerates were found embedded in thin film of La0.8Sr0.2MnO3. High-angle angular dark-field imaging, elemental mapping, and compositional analysis revealed that the nano-agglomerates are rich in manganese and poor in lanthanum. The ratio of Mn/La in the nano-agglomerates fluctuates. A salient feature of this compositional fluctuation within the nanoscale isthe formation of cubic MnO phase, which appears as the core of the nano-agglomerates.The La0.8Sr0.2MnO3 film is domain-oriented and two domains were identified on the basis of orthorhombic lattice. The orientation relationships between La0.8Sr0.2MnO3 domains and MnO were determined as [010]LSMO,1//[001]MnO and (100)LSMO,1//(110)MnO; [101]LSMO,2//[001]MnO and (010)LSMO,2//(100)MnO. The domain structuresand compositional inhomogeneities within nanoscale result in a textured microstructure, which is one of the most important parameters for tuning electronic properties in colossal magnetoresistance oxides.

Textured nano-agglomerates embedded in the thin film of La0.8Sr0.2MnO3

We have observed nano-scale chemical composition separation in the thin film of La0.8Sr0.2MnO3 by means of high-angle-angular-dark-field imaging, elemental mapping, and composition line-scanning in a transmission electron microscope. It is found that highly dense and dimensionally uniform nano-agglomerates are embedded in the film. The ratio of Mn∕La in the nano-agglomerates is fluctuated and generally much higher than that in their surrounding medium. Such compositional inhomogeneities result in a textured microstructure, which is one of the most important parameters for tunning electronic properties in the colossal magnetoresistance oxides.