A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

http://science.sciencemag.org/content/sci/264/5157/413.full.pdf

Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films

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

Some of the most relevant finite-size and surface effects in the magnetic and transport properties of magnetic fine particles and granular solids are reviewed. The stability of the particle magnetization, superparamagnetic regime and the magnetic relaxation are discussed. New phenomena appearing due to interparticle interactions, such as the collective state and non-equilibrium dynamics, are presented. Surface anisotropy and disorder, spin-wave excitations, as well as the enhancements of the coercive field and particle magnetization are also reviewed. The competition of surface and finite-size effects to settle the magnetic behaviour is addressed. Finally, two of the most relevant phenomena in the transport properties of granular solids are summarized namely, giant magnetoresistance in granular heterogeneous alloys and Coulomb gap in insulating granular solids.

https://iopscience.iop.org/article/10.1088/0022-3727/35/6/201/pdf

Finite-size effects in fine particles: magnetic and transport properties

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

Magnetoresistance in magnetic manganese oxide with intrinsic antiferromagnetic spin structure

High temperature, solution phase reduction of cobalt chloride in the presence of stabilizing agents was employed to produce magnetic colloids (ferrofluids) of cobalt nanocrystals. We systematically synthesized and isolated nearly monodisperse nanocrystal samples ranging in size from 2 to 11 nm while maintaining better than a 7% std. dev. in diameter. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (e-Co). Annealing the nanocrystals at 300 °C converts them quantitatively to the more common hexagonal-close-packed crystal form. Deposition of these uniform cobalt particles on solid substrates by evaporation of the carrier solvent results in the spontaneous assembly of two-dimensional and three-dimensional magnetic superlattices (colloidal crystals). A combination of x-ray powder diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry were used to characterize both the d...

Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices (invited)

We have observed giant magnetoresistance in heterogeneous thin film Cu-Co alloys consisting of ultrafine Co-rich precipitate particles in a Cu-rich matrix. The magnetoresistance scales inversely with the average particle diameter. This behavior is modeled by including spin-dependent scattering at the interfaces between the particles and the matrix, as well as the spin-dependent scattering in the Co-rich particles.

Giant magnetoresistance in heterogeneous Cu-Co alloys.

Giant magnetoresistance in sputtered single films of Cu–Co and Ag–Co heterogeneous alloys is discussed. The films consist of Co‐rich precipitates in a nonferromagnetic matrix. The Ag–Co films have higher Δρ/ρ and Δρ values than the Cu–Co films, possibly due to less Co dissolved in the Ag matrix. Δρ scales inversely with precipitate particle size, implying that Co‐rich clusters ≤20 A diameter may be most effective for spin dependent scattering. This trend of the data and a phenomenological model suggest that interfacial spin dependent scattering is significantly stronger than the scattering within the Co‐rich particles.

Giant magnetoresistance in heterogeneous Cu–Co and Ag–Co alloy films (invited)

The dependence of magnetic properties on film thickness in as‐deposited Co‐Ag alloys is discussed. Films ranging in Co concentration from 0 to 100 vol % Co and thicknesses between 100 A and 1 μm were deposited by sputtering onto Si(100) substrates. It was found that both magnetoresistance (MR, Δρ) and MR ratio (Δρ/ρ) increase with thickness. Resistivity (ρ) decreases with increasing thickness. The dependence of MR ratio on concentration of Co is not markedly different for 200 and 2000‐A‐thick samples. Saturation magnetization is roughly constant for thicknesses greater than ∼200 A, decreasing rapidly below this cutoff. However, most of both the thin (200 A) and thick (2000 A) films begin showing magnetic hysteresis between 10 and 100 K. The differences in transport and magnetization properties may be ascribed to differences in film growth for thin versus thick films.

Dependence of giant magnetoresistance on film thickness in heterogeneous Co‐Ag alloys

Giant magnetoresistance (GMR) in heterogeneous alloy films offers an alternative to GMR in multilayers and spin valves in a potentially simpler structure. In this review, the general features or GMR in these heterogeneous films are discussed. These include film preparation, structure, magnetic properties and magnetoresistive behavior and the possible correlations among these features. It is emphasized that we are at a very early stage in the development of this subject. In particular, much more detailed microstructural data are required in order to formulate a reliable model. >

Giant magnetoresistance in heterogeneous alloy films

The observation of giant magnetoresistance (GMR) in single heterogeneous alloy films’-4 provides opportunities for developing the theory of GMR5 as well as offering new possibilities for using GMR in read heads. In this paper, we shall try to summarize the salient structural aspects of these materials, their GMR performance, their potential advantages for applications, and the device problems that must be solved.

https://ieeexplore.ieee.org/iel4/5300/14358/00662403.pdf

J. R. Mitchell

Papers

Giant magnetoresistance in heterogeneous Cu-Co alloys.

Giant magnetoresistance in heterogeneous Cu–Co and Ag–Co alloy films (invited)

Dependence of giant magnetoresistance on film thickness in heterogeneous Co‐Ag alloys

Giant magnetoresistance in heterogeneous alloy films

Giant Magnetoresistance Materials and their Potential as Read Head Sensors