Low-field Colossal Magnetoresistance Research Articles

Publisher's Note: “Uncovering the path to low-field colossal magnetoresistance: A microscopic view of field driven percolative insulator-to-metal transition in manganites” [Appl. Phys. Lett. 122, 112402 (2023)

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Uncovering the path to low-field colossal magnetoresistance: A microscopic view of field driven percolative insulator-to-metal transition in manganites

For perovskite manganites, their colossal magnetoresistance (CMR) requires a large field, which limits their potential applications. In order to uncover the path to achieve low-field CMR, it is crucial to understand the microscopic process of the field driven insulator-to-metal transition (IMT) in manganites. This is particularly true considering the fact that the IMT is of a percolative type, in which the interplay between nucleation and growth of the electronic phase separation domains under magnetic field is not well investigated. In this work, we investigate the magnetic field driven percolative IMT in a model system of La1−x−yPrxCayMnO3 in real space via magnetic force microscopy (MFM). Our experimental observations show unambiguously three stages of the IMT phase transition where domain nucleation and domain growth exhibit distinctly different features in the global initial magnetization measurements. Moreover, MFM reveals that domain growth requires a much lower field than domain nucleation, which provides critical information on how to achieve low-field CMR. It is believed that the exchange field provided by ferromagnetic metallic domains at the boundary with antiferromagnetic insulating domains plays a critical role in assisting the domain growth process. Optimizing such internal exchange fields in manganites is a potential route to achieve CMR without the need of a large external field.

Current driven discontinuous insulator-metal transition and low-field colossal magnetoresistance in Sm0.6Sr0.4MnO3

We show that with increasing magnitude of current (I), insulator-metal (I-M) transition in Sm0.6Sr0.4MnO3 becomes discontinuous and shifts down in temperature, which is accompanied by an abrupt decrease in temperature of the sample while cooling. The I-M transition exhibits a hysteresis while warming. A magnetic field of μ0H=0.5 T results in −99% magnetoresistance for I=11 mA compared to −32% for I=1 mA. Our results underscore the importance of inhomogeneous Joule heating that leads to coexistence of the high temperature paramagnetic phase with the low temperature ferromagnetic phase over a wide temperature range.

Tunneling magnetoresistance in phase-separated manganite nanobridges

The manganite $(\text{La},\text{Pr},\text{Ca}){\text{MnO}}_{3}$ is well known for its micrometer-scale phase separation into coexisting ferromagnetic metallic (FMM) and insulating regions. Fabrication of bridges with widths smaller than the phase-separation length scale has allowed us to probe the magnetic properties of individual phase-separated regions. At the onset of phase separation, a magnetic field induced insulator-to-metal transition among a discrete number of domains within the narrow bridges gives rise to abrupt, low-field colossal magnetoresistance steps at well-defined switching fields. At lower temperatures when the FMM phase becomes energetically favorable, the insulating regions shrink to form thin insulating strips separating adjacent FMM regions with different coercive fields. Tunneling magnetoresistance is observed across the naturally occurring intrinsic insulating strips (tunnel barriers) spanning the width of the bridges. The presence of such intrinsic tunnel barriers introduces an alternative approach to fabricating novel nanoscale magnetic tunnel junctions.

Room temperature low-field colossal magnetoresistance in La0.7Sr0.3MnO3

It is found that La0.7Sr0.3MnO3 crystal shows hysteretic nonlinear V-I characteristics at high currents. The authors have reproduced such a behavior by using a simple thermal equation. Utilizing magnetic field dependence of the V-I curves, magnetoresistance ratio of R(0Oe)∕R(2.8kOe)=230% is realized at room temperature in the presence of bias currents.

Colossal electroresistance and low-field colossal magnetoresistance in the single-crystal bilayered manganiteLa1.2Sr1.8Mn2O7

We report the observation of current-induced abrupt transition from a metallic to an insulating state in a bilayered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ single crystal. A colossal electroresistance effect $[\ensuremath{\rho}(50\phantom{\rule{0.3em}{0ex}}\mathrm{mA})\ensuremath{-}\ensuremath{\rho}(1\phantom{\rule{0.3em}{0ex}}\mathrm{mA})]∕\ensuremath{\rho}(50\phantom{\rule{0.3em}{0ex}}\mathrm{mA})=93%$ is found at $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Moreover, the current-induced transition is very sensitive to external magnetic fields. At the measuring current of $50\phantom{\rule{0.3em}{0ex}}\mathrm{mA}$ a low-field colossal magnetoresistance effect, $[\ensuremath{\rho}(H)\ensuremath{-}\ensuremath{\rho}(0)]∕\ensuremath{\rho}(0)=\ensuremath{-}86%$, occurs at $H=500\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$. This low-field magnetoresistance effect can be repeatedly modulated by external magnetic field. We interpret these phenomena as the consequence of the percolative conduction and local Joule thermal effect.

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.

Effect of sintering temperature on electronic transport and low-field colossal magnetoresistance in sol-gel prepared polycrystals of nominalLa2/3Ca1/3Mn0.96Cu0.04O3

It is shown that transport and magnetoresistive properties of sol-gel prepared polycrystalline samples of nominal ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{3}$ $(x=4%)$ strongly depend on sintering temperature ${(T}_{s}).$ Lowering ${T}_{s}$ causes a large decrease of the insulator-metal transition temperature ${(T}_{\mathrm{IM}})$ and a substantial increase in resistivity at $T<~{T}_{\mathrm{IM}}.$ Above ${T}_{\mathrm{IM}},$ resistivity data can be fitted to the variable-range hopping model, $\ensuremath{\rho}\ensuremath{\propto}{e}^{{(T}_{0}{/T)}^{1/4}}$, with a strong dependence of ${T}_{0}$ on ${T}_{s}.$ For ${T}_{s}<~1200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ upon application of a low magnetic field (0.3 T), the samples exhibit colossal magnetoresistance (CMR) effect near ${T}_{\mathrm{IM}}.$ The largest CMR with a value $\ensuremath{\sim}74%$ is obtained at ${T}_{s}\ensuremath{\sim}1100\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ and decreases on varying ${T}_{s}$ from $\ensuremath{\sim}1100\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ We also show that the sample at ${T}_{s}=1000\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ exhibits an almost constant magnetoresistance with a value $\ensuremath{\sim}65%$ for the 0.3 T field over the whole low-temperature range of $T<77\mathrm{K}.$

Annealing induced grain growth and grain connectivity in an epitaxial film of La 0.67Ca 0.33MnO 3 and its effect on low field colossal magnetoresistance

We have studied the effect of annealing on a strain-relaxed film of La 0.67Ca 0.33MnO 3 (thickness 300 nm) grown epitaxially on SrTiO 3, by using local probes like scanning tunneling microscopy and scanning tunneling potentiometry (STP) and bulk probes like resistivity and magnetoresistance. Annealing enhances the film resistivity near the peak and suppresses the peak temperature. We find that annealing leads to growth of large sized grains, but the larger grains have relatively high resistance grain boundary as seen by STP. This in turn enhances the low field MR of the annealed films.

Enhancement of magnetoresistance in the intermediate state of Pr0.5Sr0.3Ca0.2MnO3

Current–voltage (I–V) characteristics of Pr0.5Sr0.3Ca0.2MnO3 are measured at different temperatures and different fields. Our data of I–V curves show a crossover from linear to nonlinear behavior at the temperature lower than the charge ordering temperature. New findings in this work include (i) abrupt jumps/drops of voltage in each I–V curve below 100 K and (ii) an enhancement of magnetoresistance (MR) ratio from 15% to 20% which is associated with the unusual voltage jumps/drops in I–V curves. The enhancement of MR ratio evidences the existence of an intermediate state between the charge ordering state and the charge ordering breakdown state. Most importantly, the existence of such intermediate state provides a new route to achieve a low field colossal magnetoresistance for future applications of spintronics.

Strain effect and the phase diagram ofLa1−xBaxMnO3thin films

We report magnetic and transport properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{MnO}}_{3}$ $(x=0.05--0.33)$ epitaxial thin films. Compared with the corresponding bulk materials, the ferromagnetic transition temperature is reduced in the compressive strained ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{MnO}}_{3}$ thin films with $x=0.3$ and 0.33, but enhanced significantly in the tensile strained thin films with $x<~0.2.$ Especially, ferromagnetism and low field colossal magnetoresistance effect were observed around room temperature in $x=0.1$ thin film, and as $x=0.05,$ a spin-canting insulating state in bulk shifts to ferromagnetic metallic state in thin film. The phase diagram of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{MnO}}_{3}$ thin films was obtained, and strain effect on these novel properties was discussed.

Highly sensitive low-temperature low-field colossal magnetoresistance in screen printed La 0.63Y 0.07 Ca 0.30MnO 3 thick films

Thick films of La 0.63Y 0.07Ca o.30MnO 3 were fabricated on (100) oriented single crystal LaAlO 3 (LAO) and on polycrystalline Al 2O 3 substrates by a screen printing technique. The films were sintered at 1200°C in air and oxygen atmosphere. All these thick films show a metal-insulator (M-I) transition at T p1∼220 K. For both substrates, the films sintered in oxygen flow have slightly higher T p1. The film on polycrystalline Al 2O 3 sintered in oxygen flow shows 71% magnetoresistance (MR) at 220 K in the presence of an 8 T applied field. The salient feature of the colossal magnetoresistance observed in these thick films is that low-temperature low-field MR is highly sensitive than only Ca doped manganites.

A large low field colossal magnetoresistance in the La0.7Sr0.3MnO3 and CoFe2O4 combined system

La 0.7 Sr 0.3 MnO 3 and CoFe2O4 samples have been prepared by a sol-gel process. A mixture of the two samples was annealed at 800 °C and the scanning electron micrograph shows large CoFe2O4 grains well surrounded by La0.7Sr0.3MnO3 grains. The powder x-ray diffraction patterns indicate no evidence of reaction between the La0.7Sr0.3MnO3 and CoFe2O4. The resistivity of the combined sample (with 20 wt% CoFe2O4) is about an order of magnitude larger than that of the same-grain-sized La0.7Sr0.3MnO3 sample, while a large low field magnetoresistance (MR) has been obtained in the combined samples. Under an applied field of 5 kOe, the MR of the combined sample is 10% even at 280 K and 5% at 290 K, whereas the same nanosized La0.7Sr0.3MnO3 sample is only 2% and 1%, respectively.

Low-field colossal magnetoresistance in bandwidth-controlled manganites

We have investigated magnetoresistance (MR) phenomena relevant to the charge ordering (CO), namely, the real space ordering of both Mn3+ and Mn4+, in single crystals of (Nd1−ySmy)1/2Sr1/2MnO3, in which the one-electron bandwidth (W) is systematically controlled by varying the average ionic radius of the A site. The low-field colossal MR is observed for the small-W region of y⩾0.5; e.g., ρ(0)/ρ(H)>103 in a field of 0.4 T at 115 K for the y=0.94 crystal. This is viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a lattice-structural change. In the small-W region, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature Tc. The competition between the ferromagnetic double-exchange and antiferromagnetic CO interactions gives rise to such a lattice-coupled first-order phase transition induced by a relatively low magnetic field.

Low-Field Colossal Magnetoresistance in Manganite Tunnel Junctions

ABSTRACTLarge magnetoresistance values are obtained on tunnel junctions epitaxially deposited by pulsed-laser deposition and consisting of ferromagnetic manganite La0.67Sr0.33MnO3 electrodes separated by various tunnel barriers: SrTiO3, PrBaCu2.8Ga0.2O7 and CeO2. The magnetoresistance can be decomposed into a low-field and a high-field contribution. The latter is attributed to the presence of canted interfacial manganite phases, as confirmed by the temperature behaviour of the resistance. A low-field magnetoresistance ratio of 450% below 100 Oe is obtained on a sample with a SrTiO3 barrier, indicating a spin polarization value in excess of 0.83 for the manganite.