Negative Tunnel Magnetoresistance Research Articles

Tunneling-MR in Fe–Cr–O metal-nonmetal granular systems

Tunneling magnetoresistance (TMR) has been achieved for Fe–Cr–O granular films prepared by RF cosputtering methods. These films are composed of nanoscaled Fe granules with an average diameter of 18–28 nm embedded in the crystalline insulating Cr 2O 3 matrix, and Fe volume fraction f v ranges from 15% to 55%. Ferromagnetic properties exist in all samples. With the decrease of f v, TMR increases from −0.24% for f v=55% to −1.9% for f v=39%, the maximum, then decreases to −0.23% at f v=28%. It is interesting that TMR becomes positive 2% at f v=15%. The relationship between ln ρ(0) and 1/ T 1/2 shows a clear linear relation in the temperature range from 200 to 290 K for the sample with the maximum negative TMR, indicating a tunneling conductive mechanism. Beyond this temperature range (5–200 K) the relationship is obviously nonlinear, implying that some other effects may exist, which needs to be further studied. Our results imply that superparamagnetic state of the metal granules is not the necessary condition for TMR, and it may not be the main factor affecting the TMR, while the microstructure of films, such as the interfaces between the metal and oxide may be important to TMR as the electronic structure of metal-oxide interface can influence the spin polarization.

Junction-like magnetoresistance of intergranular tunneling in field-aligned chromium dioxide powders

The magnetic and magnetotransport properties of field-aligned half-metallic ${\mathrm{CrO}}_{2}$ powders have been studied. Needle-shaped nanoparticles of ${\mathrm{CrO}}_{2}$ have been aligned in a strong magnetic field. The aligned powder sample shows a strong anisotropy along the alignment direction. The conduction mechanism of the aligned ${\mathrm{CrO}}_{2}$ powder sample has been examined and is consistent with the intergranular spin-dependent tunneling. Negative tunneling magnetoresistance of about 41% is achieved in a small field in the vicinity of the coercive field at 5 K. The magnetoresistance (MR) versus field curve shows two well-separated narrow peaks at the coercive fields and resembles that of a magnetic tunnel junction. This junctionlike MR results from the narrow switching field distribution of the aligned powders. Our results suggest that the aligned magnetic ${\mathrm{CrO}}_{2}$ particles may find novel applications in spin-transport structures and devices.