We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

https://iopscience.iop.org/article/10.1088/0034-4885/58/11/003/pdf

The flux-line lattice in superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.

The Flux-Line Lattice in Superconductors

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

CMR manganites: physics, thin films and devices

Colossal-magnetoresistance materials: manganites and conventional ferromagnetic semiconductors

The amplitude $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$ and temperature ${T}_{M}$, where the colossal magnetoresistance (CMR) response of ${L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ manganites are maximum, are found to be controlled by the radius of the lanthanide $({L}^{3+})$ which modifies the bending of the Mn-O-Mn bond. Increasing the bond distortion lowers ${T}_{M}$ and enhances $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$. Enhanced CMR arises from (1) a shift to lower temperatures of ${T}_{M}$, (2) a reduced mobility of the doping holes, and (3) an increase of the coupling between itinerant and localized electrons. The resistivity $\ensuremath{\rho}\left(H\right)$ follows an $\ensuremath{\approx}{\mathrm{BM}}^{2}\left(H\right)$ law and the parameter $B$ is also tuned by the Mn-O-Mn bond angle. The narrowing of the electronic bandwidth is the fundamental parameter controlling the observed CMR.

Colossal magnetoresistance of ferromagnetic manganites: Structural tuning and mechanisms.

The complex microstructure of directionally solidified ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}}$${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ composites having up to 30 vol % 2:1:1 precipitates with sizes down to 0.1 \ensuremath{\mu}m and critical currents above ${10}^{5}$ A/${\mathrm{cm}}^{2}$ at 77 K and zero magnetic field has been investigated through detailed transmission electron microscopy observations. It is shown that samples with a high ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ concentration display polygonization as a mechanism to relieve stress while only small microcracks occur which are very efficiently stopped by small ${\mathrm{Y}}_{2}$${\mathrm{BaCuO}}_{5}$ precipitates. This crystal polygonization leads to the formation of either sharp or diffuse interfaces within the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ matrix. In the first case low angle grain boundaries occur which have a minor perturbation of the critical currents, while strongly disordered smooth interfaces are observed in other cases which because of their irregular distribution can only have a minor relevance on the flux-pinning mechanism of these superconductors. Stacking faults are found to be a common defect in the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ matrix but they are very inhomogeneously distributed thus preventing to establish a clear correlation between their density and critical currents.

Microstructure of directionally solidified high-critical-current YBa 2 Cu 3 O 7 - Y 2 BaCuO 5 composites

We have studied the field, frequency, and amplitude dependence of the ac susceptibility on a single crystal of the electron-doped superconductor Pr[sub 1.85]Ce[sub 0.15]CuO[sub 4[minus][ital y]]. We show that a thermally activated flux motion over effective energy barriers [ital U]([ital T],[ital H])=[ital U][sub 0](1[minus][ital T]/[ital T][sub [ital c]])[ital H][sup [minus]0.6], with [ital U][sub 0][approx]40 meV, can account for the dc-field and frequency effects. The amplitude dependence of the ac response in this superconducting system is discussed and it is argued that it is a signature of the critical state.

ac response of the vortex system in a Pr1.85Ce0.15CuO4-y single crystal.

We have measured the magnetoresistivity of some ${\mathit{L}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ (L=Pr,Nd,Sm) superconducting single crystals along the ab plane and the c axis. For most crystals the resistivity along both directions displays a very sharp rise just below ${\mathit{T}}_{\mathit{c}}$ and before the zero-resistance state is reached. This resistance anomaly is quenched by a magnetic field. The effect of the field is more pronounced for H\ensuremath{\parallel}c than for H\ensuremath{\parallel}(ab). Several possible scenarios to account for the resistance anomaly are discussed. We propose that the resistance peak is a manifestation of a quasireentrant behavior resulting from an intrinsic granularity of these n-type high-temperature superconductors. The relevance of these results and ac susceptibility measurements for the anomalous behavior reported for the upper critical field is discussed.

S. Piñol

Papers

Colossal magnetoresistance of ferromagnetic manganites: Structural tuning and mechanisms.

Microstructure of directionally solidified high-critical-current YBa 2 Cu 3 O 7 - Y 2 BaCuO 5 composites

ac response of the vortex system in a Pr1.85Ce0.15CuO4-y single crystal.

Giant resistive peak close to the superconducting transition in L2-xCexCuO4 single crystals.

Bridgman growth and enhanced critical currents in textured YBa2Cu3O7 — Y2BaCuO5 composites