D. V. Dimitrov

Magnetic properties and microstructure of Fe-O and Co-O thin films

Iron oxide and cobalt oxide thin films were prepared by reactive DC magnetron sputtering in a mixture of Ar and O/sub 2/ gases. The stoichiometry of the oxide films was studied as a function of the amount of O/sub 2/ during deposition. Sputtering conditions, including dc power and oxygen-to-argon ratio, were found which allowed pure FeO, Fe/sub 3/O/sub 4/, /spl alpha/-Fe/sub 2/O/sub 3/, CoO, and Co/sub 3/O/sub 4/ films to be fabricated. The microstructure of the films, and especially the grain shape and size distribution, were quite different for oxide films with different stoichiometry. The crystallographic axes of Fe-O films were randomly oriented in contrast to Co-O films in which a strong texture was found. A strong ferromagnetic-like behavior was observed in FeO and /spl alpha/-Fe/sub 2/O/sub 3/ films, in contrast to the well-established antiferromagnetic behavior in bulk. The reason for this anomalous behavior is attributed to clusters of defects in FeO films and uncompensated surface spins in /spl alpha/-Fe/sub 2/O/sub 3/ films. A large shift (3800 Oe) was observed in the low temperature hysteresis loop of a field cooled sample consisting of 50% Co and 50% CoO. A similar but less pronounced effect was observed in FeO and /spl alpha/-Fe/sub 2/O/sub 3/ films. This result is believed to be caused by a strong exchange coupling between the ferromagnetic and antiferromagnetic constituents in these composite films.

Defect clusters in Fe1−xO and their ferrimagnetic properties

Ferrimagnetic properties, large saturation magnetization (Ms) and low temperature coercivity (Hc) was observed in nonstoichiometric Fe1−xO films. This unusual behavior was attributed to the existence of spinel type defect clusters coherently embedded in the FeO matrix. A model, which explains the Fe1−xO properties and predicts a linear dependence between Ms and the Fe1−xO lattice parameter a with the Fe3O4 data falling on the same line, is proposed. The experimental Ms vs a curve is in excellent agreement with the theoretical prediction, which indicates the existence of a fundamental relation between FeO, Fe1−xO, and Fe3O4.

The role of the bulk spin-dependent scattering on the origin of the enhanced magnetoresistance in magnetic multilayers

Abstract A detailed analysis of the current density distribution along the perpendicular direction of a spin-valve-like structure is presented. It is shown that, even when only bulk spin-dependent scattering is considered, the carriers responsible for the enhanced magnetoresistance in these systems are flowing in a narrow strip of the ferromagnetic layer close to the ferromagnetic/nonmagnetic interface. A modification of the bulk transport properties of this region affects drastically the giant magnetoresistance (GMR) of the overall structure.

Memory effect and temperature behavior in spin valves with and without antiferromagnetic subsystems

Temperature behavior and memory effect in standard spin valves (SV) and SVs with synthetic antiferromagnetic (Co/Ru/Co) (SV-SAF) subsystems have been studied. SV-SAFs show much better temperature stability. Memory effect refers to the phenomenon that the exchange bias can be altered at temperatures (TR’s) much lower than the blocking temperature (TB), and these temperatures (TR’s) are imprinted into SVs. The memory effect greatly deteriorates the magnetoresistance behaviors in SV. Our results suggest that the memory effect is caused by a distribution of local blocking temperatures (Tb’s). The magnetization state in the pinned layer is critical in determining the temperature behavior of HE and magnetoresistance. By partially reversing the magnetization in the pinned ferromagnetic (FM) layers, we are able to separate the temperature dependencies of the local exchange bias (He) associated with regions consisting of different Tb’s. Two features have been observed: (1) the local exchange bias (He) with a narro...

Surface-induced magnetism in α-Fe2O3/Ag multilayers

Abstract Anomalous ferromagnetic-like behavior, with significant magnetization and very large coercivities at low temperatures was observed in α-Fe 2 O 3 /Ag multilayers. Mossbauer studies showed the presence of two magnetic components, bulk-like and surface-like with substantially lower hyperfine field. It was found that the observed magnetic moment per particle can be accounted for by the contribution of only about 5% of the total number of surface Fe 3+ ions. This result can be explained if the magnetic moments on the surface are predominantly randomly oriented resembling a spin glass state. The negligible quadrupole interaction, observed in the Mossbauer spectra at low temperature, strongly supports the last interpretation. Large shifts in the hysteresis loops of field-cooled samples indicate strong exchange coupling between the antiferromagnetic core and the net magnetic moments on the surface.

Stoichiometry and magnetic properties of iron oxide films

The stoichiometry, structural and magnetic properties, and Moessbauer spectra of reactively sputtered Fe-O films were studied as a function of the O{sub 2} partial pressure during the deposition. By increasing the amount of O{sub 2} films with the following crystallographic structures and stoichiometry were fabricated; amorphous Fe-O, mixture of Fe and FeO, off stoichiometric single-phase Fe{sub x}O, mixture of FeO and Fe{sub 3}O{sub 4}, single-phase Fe{sub 3}O{sub 4}, mixture of Fe{sub 3}O{sub 4} and {gamma}-Fe{sub 2}O{sub 3}, mixture of Fe{sub 3}O{sub 4} and {alpha}-Fe{sub 2}O{sub 3}, and single-phase {alpha}-Fe{sub 2}O{sub 3}. The Verwey transition in Fe{sub 3}O{sub 4} films was observed in the coercivity versus temperature curve and in the thermomagnetic data. FeO and {alpha}-Fe{sub 2}O{sub 3} films showed anomalous ferromagnetic-like behavior, in contrast to their antiferromagnetic nature in bulk. The unusual magnetic properties were attributed to the formation of clusters of tetrahedrally coordinated Fe{sup 3+} ions in FeO and to uncompensated surface spins, in {alpha}-Fe{sub 2}O{sub 3} films.

Magnetoresistance in NiCoO/Py/Cu/Py spin valves

Room temperature transport properties of two sets of CIP spin valve structures were studied. In addition to the conventional collinear four-probe technique, the dependence of the resistivity on the angle between the magnetizations of the free and pinned Py layers has been studied. We have shown that this approach allows one to separate the contributions of giant magnetoresistance, the anisotropic magnetoresistance and the nonmagnetic resistivity, which is not possible by using the conventional four-probe method.

Effect of Co impurity layers on the AMR enhancement of Ni thin films

Abstract The transport properties of Ni thin films with Co and Cu impurity layers were studied as a function of the distance between the impurity layers. The films were fabricated by DC magnetron sputtering and the nominal thickness was kept constant at 1500 A. Co and Cu impurity layers were inserted into the films using deposition times insufficient to fill one atomic layer. The distance between these layers, L , was varied from 10 to 500 A. In the case of Ni films with Co impurity layers, the anisotropic magnetoresistance (AMR) increases significantly with decreasing L , and approaches the value of the Ni mean free path. At 20 K, the longitudinal MR is 1.3% for pure Ni films and increases to 1.8% and 2.8% when L = 40 and 10 A, respectively. A much lower effect has been observed in the case of Cu, and this indicates the importance of the magnetic nature of the impurity interface layers in the enhancement of MR.

Stoichiometry and Magnetic Properties of Reactively Sputtered Fe-O Films

The stoichiometry, microstructure and magnetic properties of reactively sputtered Fe-O films were studied as a function of the amount of O2 during the deposition. Films which were comprised of mixtures of oxides resulted for most of the O2/Ar ratios. However, for some conditions pure FeO, Fe3O4 and α - Fe2O3 films were produced. The magnetic properties of Fe3O4 films were consistent with the results of other studies. In particular the Verwey transition was clearly observed in the coercivity versus temperature curve and in the thermomagnetic data. FeO and α - Fe2O3 films showed ferromagnetic-like behavior, in contrast to their well established antiferromagnetic properties in bulk. The unusual magnetic properties are attributed to the formation of clusters of tetrahedraly coordinated Fe3+ ions in FeO and to uncompensated surface spins, in α - Fe2O3 films as suggested by Mossbauer studies.

Size effects in the stoichiometry and magnetic properties of Fe-O/Ag multilayers

Abstract The stoichiometry and magnetic properties of reactively sputtered Fe-O/Ag multilayers were studied as a function of the Fe-O layer thickness. The thinnest Fe-O (50 A) layers were highly disordered. However, when the thickness was increased to more than 100 A, well-crystallized films comprised of a mixture of FeO and Fe 3 O 4 were formed. The relative amount of FeO and Fe 3 O 4 was thickness-dependent. Very thick (more than 3500 A) films were essentially single-phase Fe 3 O 4 .