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

National Institute of Standards and Technology における超伝導研究及び生活

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

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

We present a detailed study of static and dynamic magnetic behavior of Fe3O4 nanoparticles with average particle sizes 〈d〉 ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, TV, shifts down to ∼20 K for 〈d〉=50 nm and is no longer observable for smaller particles. The smallest particles (〈d〉=5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at TB∼45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization MS at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with 〈d〉=5 nm, dynamic ac susceptibility measurements show...

Static and dynamic magnetic properties of spherical magnetite nanoparticles

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.

Antiphase boundaries (APBs) were observed in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal films grown on MgO. The APBs are an intrinsic consequence of the nucleation and growth mechanism in films. Across an APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened, reversing the dominant interaction from that found in the bulk. Thus the APB separates oppositely magnetized regions, consistent with Lorentz microscopy measurements. The APBs induce very large saturation fields and nearly random magnetization distribution in zero field.

Origin of the Anomalous Magnetic Behavior in Single Crystal Fe 3 O 4 Films

${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ films were grown on Si, $〈100〉$ and $〈110〉$ MgO, $〈111〉$ Mg${\mathrm{Al}}_{2}$${\mathrm{O}}_{4}$, and $〈0001〉{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ by reactive sputter deposition X-ray diffraction and TEM studies of the films grown on MgO show they are uniformly strained epitaxial single crystal specimens Conversion electron M\"ossbauer spectroscopy (CEMS) spectra for films on all substrates show the presence only of the stoichiometric ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ phase, and values for the hyperfine fields and isomer shifts of the $A$ and $B$ sites consistent with bulk ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ However, the CEMS spectra exhibit an anomalous out-of-plane moment distribution, although the moments are expected to be in the plane of the film due to the large shape anisotropy Furthermore, the magnetization remains unsaturated in fields as large as 70 kOe, and torque measurements of films grown on MgO remain unsaturated at 21 kOe The extrapolated values for the anisotropy, derived from torque curves taken both in and out of the film plane, are much smaller than that required to cause the lack of saturation in high fields and the anomalous CEMS spectra, and are fairly well explained as a combination of crystalline, magnetoelastic, and shape anisotropy of bulk single crystal ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ subjected to in-plane tensile stress The magnetoelastic anisotropy derived for ideal epitaxial ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ films grown on $〈100〉$ and $〈110〉$ MgO, using bulk values for the magnetostriction and elastic constants, agrees well with values determined experimentally A comparison of films with thicknesses ranging from 001 to 6 \ensuremath{\mu}m indicates the anomalous behavior to be a volume, as opposed to a surface, effect The anomalous behavior is exhibited in films grown on other substrates and by other techniques (evaporation and molecular beam epitaxy), and is independent of thickness and deposition conditions It appears to be an intrinsic property of all ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ films Possible origins of this behavior are discussed

Anomalous moment and anisotropy behavior in Fe3O4 films.

As first noted by Neel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Neel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 A diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may ...

Magnetic anomalies in NiO nanoparticles

Reliably erasing data from storage media (sanitizing the media) is a critical component of secure data management. While sanitizing entire disks and individual files is well-understood for hard drives, flash-based solid state disks have a very different internal architecture, so it is unclear whether hard drive techniques will work for SSDs as well.We empirically evaluate the effectiveness of hard drive-oriented techniques and of the SSDs' built-in sanitization commands by extracting raw data from the SSD's flash chips after applying these techniques and commands. Our results lead to three conclusions: First, built-in commands are effective, but manufacturers sometimes implement them incorrectly. Second, overwriting the entire visible address space of an SSD twice is usually, but not always, sufficient to sanitize the drive. Third, none of the existing hard drive-oriented techniques for individual file sanitization are effective on SSDs.This third conclusion leads us to develop flash translation layer extensions that exploit the details of flash memory's behavior to efficiently support file sanitization. Overall, we find that reliable SSD sanitization requires built-in, verifiable sanitize operations.

/pdf/reliably-erasing-data-from-flash-based-solid-state-drives-1jf7f0r51i.pdf

Frederick E. Spada

Papers

Giant magnetoresistance in heterogeneous Cu-Co alloys.

Origin of the Anomalous Magnetic Behavior in Single Crystal Fe 3 O 4 Films

Anomalous moment and anisotropy behavior in Fe3O4 films.

Magnetic anomalies in NiO nanoparticles

Reliably erasing data from flash-based solid state drives