Phase-separated Manganites Research Articles

Electrical current effect in phase-separated La5∕8−yPryCa3∕8MnO3: Charge order melting versus Joule heating

We have studied the effect of electric field on transport properties of the prototypical phase-separated manganite La5∕8−yPryCa3∕8MnO3 with y=0.34. Our results show that the suggested image, in which the charge ordered state is melted by the appliance of an electric current and∕or voltage, has to be revised. We explain the observed resistivity drop in terms of an artifact related to Joule heating and the particular hysteresis that the system under study displays, common to many other phase-separated manganites.

Action of strong coupling on steplike magnetization and transport properties in phase-separated manganite

For phase-separated La0.275Pr0.35Ca0.375MnO3 manganite, a steplike charge-ordered antiferromagnetic-ferromagnetic transition was observed by field induction and accompanied by a sudden drop of resistivity. The results can be explained in terms of the spin reorientation in the antiferromagnetic (AFM) phase and simultaneous destruction of orbital ordering by a magnetic field. Combined with specific heat and magnetization measurements, the electronic state density at Fermi energy was greatly enhanced at 5T, which shows the increase of carrier number due to delocalization effect, by analogy with the spin reorientation due to destruction of orbital ordering by magnetic field. The present results prove that the strong coupling among spin, charge, and orbital ordering would be the main cause of existing complex physical behavior at low temperature, which depends sensitively on the spin orientation of adjacent charge-ordered AFM domains and can be controlled by applied field.

Intrinsic interface exchange coupling of ferromagnetic nanodomains in a charge ordered manganite

We present a detailed magnetic study of the Pr1/3Ca2/3MnO3 manganite, where we observe the presence of small ferromagnetic (FM) domains (diameter ~ 10A) immersed within the charge-ordered antiferromagnetic (AFM) host. Due to the interaction of the FM nanodroplets with a disordered AFM shell, the low-temperature magnetization loops present exchange bias (EB) under cooling in an applied magnetic field. Our analysis of the cooling field dependence of the EB yields an antiferromagnetic interface exchange coupling comparable to the bulk exchange constant of the AFM phase. We also observe training effect of the EB, which is successfully described in terms of a preexisting relaxation model developed for other classical EB systems. This work provides the first evidence of intrinsic interface exchange coupling in phase separated manganites.

Magnetodielectric consequences of phase separation in the colossal magnetoresistance manganitePr0.7Ca0.3MnO3

We have studied the low-frequency dielectric properties of the phase-separated manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ as a function of applied magnetic field in the low temperature phase-separated state. The dielectric constant is strongly field dependent and also depends on the magnetic field history of the sample. The dielectric behavior appears to be associated with the hopping of polaronic charge carriers, and we can derive the field dependent hopping energy barrier from the frequency dependence of the dielectric constant. This analysis allows us to associate the metal-insulator transition observed in this material with the field-induced suppression of the polaron activation energy.

Current localization and enhanced percolative low-field magnetoresistance in disordered half metals

By modeling the magnetotransport in disordered half metals onto a random resistor network, we have performed numerical simulations on the low-field magnetoresistance (LFMR) and the current distribution in the network. It is found that there is a close relationship between the magnitude of LFMR and the current morphology. LFMR increases with the increase of current localization. In the limit of strong disorder, a quasi-one-dimensional transport channel forms and LFMR reaches its maximum. Compared with the previous two-component (metal/insulator) percolative scenario for colossal magnetoresistance in phase-separated manganites, such a current percolation transition gives further understanding of the magnetotransport in disordered half metals.

Heating and non-ohmic transport in phase-separated manganites

We investigate the features of non-ohmic transport in phase-separated manganites by Monte Carlo simulations in a random network with clusters of ferromagnetic metal and antiferromagnetic insulator. It is found that, by taking into account the heating process and the temperature dependence of resistivity, the current-voltage characteristics are highly non-ohmic and exhibit hysteresis in sweeping the voltage. We calculate the temperature dependence of this behavior, and investigate the changes of the percolative paths at different currents. The breakdown of most percolative paths by the heating effect of the current results in an interrupt transition from the metallic behavior to the insulator behavior of the system. The obtained results are in good agreement with related experiments.

Spatially inhomogeneous transport in phase-separated polycrystalline manganites

Based on intrinsic phase separation between ferromagnetic metallic domains and paramagnetic insulating domains, and extrinsic band-bending effects at the grain boundaries, a tunneling percolation model is established for the magnetotransport in phase-separated polycrystalline manganites. Within this model, the competition between intrinsic colossal magnetoresistance and extrinsic tunneling magnetoresistance in these materials is mapped onto a multicomponent bond-disordered random resistor network, where the fraction of each kind of bond is determined self-consistently. For the existence of a distribution of intergranular distance and the exponential dependence of tunneling resistance on the intergranular distance, the network exhibits strong disorder dependence of low-field magnetoresistance and is expected to yield a nonuniversal behavior of electrical transport in some particular cases, which are also magnetic field dependent. Further numerical simulations on the temperature dependence of resistance and magnetoresistance give a reasonable explanation of the dependence of magnetotransport on the intergranular connectivity and the grain size. All of these results agree will with experimental observation.

Effects of cation size disorder and lattice distortion on metamagnetism in phase-separated manganites

The effects of A-site cation size disorder in ABO 3 type charge-ordered and antiferromagnetic Pr 0.5Ca 0.5MnO 3 system have been studied by substituting La 3+, Sr 2+ or Ba 2+, while keeping the valency of Mn ions and the tolerance factor ( t=0.921) constant in the substituted compounds. We find that the substitutions by these larger cations induce successive sharp step-like metamagnetic transitions at 2.5 K. The critical field for metamagnetism is the lowest for 3% Ba substituted compound, which has the largest A-site cation size disorder and the least distorted MnO 6 octahedra, among the compounds reported here. These cation substitutions give rise to ferromagnetic clusters within antiferromagnetic matrix, indicating phase-separation at low temperatures. The growth of the clusters is found to vary with the substitution amount. The local lattice distortion of MnO 6 octahedra enhances the charge ordering temperature and reduces the magnetization at high fields (>1 T) in these manganites.

Current Oscillation and Low-Field Colossal Magnetoresistance Effect in Phase-Separated Manganites

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.

Power-law distribution of avalanche sizes in the field-driven transformation of a phase-separated oxide

At low temperature $(T\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the isothermal magnetization curves of a phase-separated manganese oxide $({\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}{\mathrm{Mn}}_{0.96}{\mathrm{Ga}}_{0.04}{\mathrm{O}}_{3})$ are found to exhibit a large number of small steps. Analysis of this phenomenon shows several features which are typical of avalanchelike dynamics. These findings lend further support to the martensitic origin of the magnetization steps encountered in phase-separated manganites.

Abrupt field-induced transition triggered by magnetocaloric effect in phase-separated manganites

The occurrence at low temperatures of an ultrasharp field-induced transition in phase-separated manganites is analyzed. Experimental results show that magnetization and specific-heat steplike transitions below $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are correlated with an abrupt change of the sample temperature, which happens at a certain critical field. This temperature rise, a magnetocaloric effect, is interpreted as produced by the released energy at the transition point and is the key to understand the existence of the abrupt field-induced transition. A qualitative analysis of the results suggests the existence of a critical growing rate of the ferromagnetic phase, beyond which an avalanche effect is triggered.

Magnetic-field-induced structural homogeneity of a phase-separated manganite

Magnetic field (MF) dependence of the phase separation (PS) in the manganite Pr0.65(Ca0.7Sr0.3)0.35MnO3 was studied using high-energy x-ray powder diffraction. The compound shows intrinsic inhomogeneities in the form of coexisting competing phases below a temperature Tc. Application of MFs not only eliminates the multiple phases below Tc but also significantly affects the structure above Tc. The MF-induced structural phase transition occurs abruptly at 2 K but is smooth at higher temperatures. Moreover, the MF dependence of some reflection intensities clearly indicates a complicated PS. This MF-induced homogeneity should play a key role in the colossal magnetoresistance effect.

Magnetization Steps in Phase Separated La0.5Ca0.5Mn1-yFeyO3

We report a low temperature magnetic study on the phase separated polycrystalline manganites La0.5Ca0.5Mn1−y Fe y O3 (0.02≤y≤0.06). Several abrupt and sizeable magnetization steps are observed in the M(H) curves of all samples at 2 K, which are associated to avalanche-like growth of the ferromagnetic phase. This work contributes to confirm that this feature is inherent to phase separated manganites, and is not restricted to particular compounds or kinds of doping.

Characteristics of the phase-separated state in manganites: Relationship with transport and magnetic properties

The temperature and magnetic field dependence of the resistivity, magnetoresistance, and magnetic susceptibility of phase-separated manganites in the temperature range corresponding to nonmetallic behavior are considered within the framework of a model of inhomogeneous state with allowance for the existence of ferromagnetically correlated regions even in the absence of long-range magnetic order. The main attention is given to the interval of high temperatures and weak fields. The main characteristics of the phase-separated state of manganites are evaluated from a comparison of the theoretical results with available experimental data.

Local magnetization and transport properties in phase-separated (La,Pr)1−Ca MnO3 in the vicinity of the phase transition

Abstract Local magnetization has been studied in a phase-separated manganite (La 1− y Pr y ) 0.7 Ca 0.3 MnO 3 ( y =0.7) by a magneto-optical imaging technique simultaneously with resistivity measurements. The observed image demonstrates a region in which magnetization changes corresponding to a telegraph noise in resistive fluctuation. This observation is the first direct evidence for a switching domain in phase-separated manganites.

Neutron scattering evidence for magnetic-field-driven abrupt magnetic and structural transitions in a phase-separated manganite

Substitutions at the Mn site of the charge-ordered ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ manganite is an effective way to induce abrupt jumps on the magnetic-field-driven magnetization curve. In order to get new insights into the origin of this remarkable feature, the ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Ga}}_{0.03}{\mathrm{O}}_{3}$ perovskite manganite has been studied by neutron diffraction versus temperature and at 2.5 K in an applied magnetic field up to 6 T. A weak and complex antiferromagnetic order is found for the low-temperature ground state. Magnetic transitions, associated with structural ones, are evidenced for certain strengths of magnetic field, which gives rise to the steplike behavior corresponding to the magnetization curve. Small-angle neutron scattering provides evidence for a nucleation process of micron-size ferromagnetic domains which follows the magnetization behavior.

Staircase-like metamagnetic transitions in phase-separated manganites: Influence of thermal and mechanical treatments

Substitutions in the Mn sublattice of antiferromagnetic, charge, and orbitally ordered manganites was recently found to produce intriguing metamagnetic transitions consisting of a succession of sharp magnetization steps separated by plateaus. The compounds exhibiting such features can be divided into two categories, depending on whether they are sensitive to thermal cycling effects or not. One compound in each category has been considered in the present study. This article reports on the influence of two treatments: high-temperature annealing and grinding. It is shown that both of these treatments can dramatically affect the phenomenon of magnetization steps. The results provide us with information about the origin of these jumps in magnetization.

Small-scale phase separation and electron transport in manganites

The magnetoresistance and the noise power of non-metallic phase-separated manganites are studied. The material is modelled by a system of small ferromagnetic metallic droplets (magnetic polarons or ferrons) in an insulating matrix. The concentration of metallic phase is assumed to be far from the percolation threshold. The electron tunnelling between ferrons causes the charge transfer in such a system. The magnetoresistance is determined both by the increase in the volume of the metallic phase and by the change in the electron hopping probability. In the framework of such a model, the low-field magnetoresistance is proportional to H 2 and decreases with temperature as T −n , where n can vary from 1 to 5, depending on the parameters of the system. In the high-field limit, the tunnelling magnetoresistance grows exponentially. Different mechanisms of the voltage fluctuations in the system are analysed. The noise spectrum generated by the fluctuations of the number of droplets with extra electrons has a 1/ f form over a wide frequency range. In the case of strong magnetic anisotropy, the 1/ f noise can also arise due to fluctuations of the magnetic moments of ferrons. The 1/ f noise power depends only slightly on the magnetic field in the low field range whereas it can increase as H 6 in the high-field limit.

Oxygen vacancies effect on phase separation in Pr0.5Ca0.2Sr0.3MnO3−δ

We study the effect of the introduction of a controlled amount of oxygen vacancies on the magnetic and transport properties of the phase separated manganite Pr0.5Ca0.2Sr0.3MnO3−δ, for δ=0 and 0.01. This compound presents a paramagnetic to ferromagnetic (FM) transition below TC∼240 K, and then to a charge-ordered (CO) and antiferromagnetic (AFM) phase at TCO∼175 K. However, below TCO an appreciable FM component (∼20%) still survives, related to a FM volume immersed within the CO/AFM matrix. At low temperatures, in the mainly CO phase, the introduction of oxygen vacancies enhances the FM ordering, with the consequent decrease of resistivity and magnetoresistance (MR). On the contrary, in the FM phase (in the range TCO<T<TC) the magnetization is reduced and the resistivity and MR increase. These results are discussed in the frame of percolative transport in metal–insulator mixtures, and frustrated double exchange interaction due to the interruption of some Mn–O–Mn chains.

Abrupt jumps in the metamagnetic transitions of orbitally ordered manganites

An experimental study of the magnetic properties of the Pr0.5Ca0.5Mn0.97Ga0.03O3 perovskite manganite is presented. The ac magnetic susceptibility measurements show that the low T orbitally and charge ordered state has been destabilized by the nonmagnetic impurity substituted for Mn. In this compound, at 2.5 K, well-defined, sharp steps on the field-dependent magnetization curves are observed. This unusual metamagnetic transition is found to be sensitive to thermal cycling. The latter induces a training effect for the transition, which strongly supports the proposed martensitic-like physics of some phase-separated manganites.