Taras Pokhil

“All-Heusler alloy” current-perpendicular-to-plane giant magnetoresistance

A materials system of ternary full Heusler alloys exhibiting substantial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) has been theoretically proposed and experimentally realized. Observed trends in magnetoresistance are broadly consistent with the modeling results. A CPP-GMR of 6.7% and ΔRA of 4 mΩ μm2 have been demonstrated in the bottom spin-valve configuration. The spin-stand testing of narrow-track recording heads confirmed compatibility of these materials with hard disk drive reader technology.

Spin vortex states and hysteretic properties of submicron size NiFe elements

Micromagnetic structures in submicron size circular permalloy elements of various thickness (5–50 nm) have been experimentally studied using a magnetic force microscope (MFM). One and two vortex micromagnetic states were observed in the elements. It was found that one vortex state is more favorable under external field of low values, while two vortex state forms under high fields. Switching between one and two vortex states results in a hysteresis in the micromagnetic response of the elements. The transition field from one to two vortex state increases with the increase of element thickness and, therefore, with the increase of the demagnetizing effect of the element edges. The two vortex state was not observed in the elements with a thickness of 50 nm or greater. The micromagnetic behavior correlates with hysteresis loops of a free layer in a magnetic tunnel junction of size, shape, and free layer thickness similar to elements examined with the MFM.

Heusler alloy based current-perpendicular-to-the-plane giant magnetoresistance heads for high density magnetic recording

Narrow-track current-perpendicular-to-the-plane giant magnetoresistive heads containing Heusler alloy layer have been fabricated utilizing an abutted junction hard bias design. The head performance has been tested quasistatically and dynamically under high density recording conditions using a perpendicular magnetic recording media.

Exchange anisotropy and micromagnetic properties of PtMn/NiFe bilayers

Magnetic microstructure, exchange induced uniaxial and unidirectional anisotropy and structural transformation have been studied in PtMn/NiFe bilayer films and small elements as a function of annealing time. The relationship between the fcc-fct ordering phase transformation in PtMn and the development of exchange induced magnetic properties in PtMn/NiFe bilayers is complicated by the fact that the transformation occurs throughout the entire volume of the PtMn film, while the exchange between the layers is predominantly an interface effect. Consequently, the development of the exchange anisotropy should depend primarily on the character of the structural transformation at the interface between PtMn and NiFe. The purpose of this article is to correlate the volume phase transformation in PtMn to the development of exchange anisotropy and micromagnetic behavior in PtMn/NiFe bilayers. The interface structure can be inferred from the anisotropy and micromagnetic measurements, leading to a model that explains th...

Domain wall motion in RE-TM films with different thickness

A thermally activated domain wall motion is studied in amorphous TbFe films with different thicknesses. The activation energy of the domain wall displacement process is determined for the films with various composition and thicknesses. It is about 2.5 eV. The activation volume of this process nonlinearly increases from 2*10/sup -18/ cm/sup 3/ to 7*10/sup -18/ cm/sup 3/ as the film thickness increases from 25 nm to 400 nm. The change of the domain shape with the film thickness growth and domain wall coercive force dependence on film thickness are discussed in terms of thermoactivated domain wall motion. >

STUDY OF EXCHANGE ANISOTROPY IN NIFE/NIMN AND NIFE/IRMN EXCHANGE COUPLED FILMS

Exchange anisotropy in NiFe/NiMn and NiFe/IrMn exchange coupled films was studied as a function of temperature using vibrating sample magnetometry. The exchange field was measured using three different methods: (1) as a shift of the hysteresis loop measured in an external field applied parallel to the exchange field direction; (2) calculated from the initial susceptibility in the field applied perpendicular to the exchange field; and (3) calculated from the shift of minor reversible hysteresis loops measured in external fields applied in a few different directions close to the perpendicular to the exchange field. The values of the exchange field in NiFe/NiMn samples measured using methods 2 and 3 were similar and approximately twice as high as the values measured using method 1. For the NiFe/IrMn samples methods 2 and 3 gave exchange field values slightly exceeding the values obtained using method 1. The results are explained using a model in which it is assumed that the interfacial interactions between a...

Dispersion of the pinning field direction of a ferromagnet/antiferromagnet coupled system

The exchange bias field of a ferromagnet/antiferromagnet bilayer is usually measured by the unidirectional shift of the hysteresis loop of the ferromagnetic layer. The exchange bias field results from interfacial exchange coupling between the spins in the ferromagnetic and antiferromagnetic layers. In general, the spins of the antiferromagnetic layer can vary locally in their anisotropy directions resulting in a distribution in local pinning directions. This letter reports a quantitative study on this distribution or dispersion using the anisotropic magnetoresistive (AMR) effect in a small rotating magnetic field. The AMR data as a function of the rotating angle of the field measured at a low field reveal the dispersion inside the ferromagnetic layer and at the interface of a ferromagnet/antiferromagnet system. The interaction between the domains within the ferromagnetic layer also plays a significant role in the dispersion inside the ferromagnetic layer.

Magnetic force microscope study of antiferromagnet–ferromagnet exchange coupled films

Magnetic microstructure in micron and submicron size elements made of bilayer antiferromagnet–ferromagnet (AFM/FM) (AFM: NiMn, PtMn, and IrMn; FM: NiFe and CoFe) exchange coupled polycrystalline films have been studied using a magnetic force microscope. AFM/FM elements with various thickness of FM layer (50–500 A) have been examined and compared with nonexchange biased FM elements of the same size, shape, and thickness. Micromagnetic structures observed in AFM/FM elements with thick (>200 A) FM layer indicated that, in addition to unidirectional anisotropy, the AFM layer induces uniaxial anisotropy in a FM layer. Bilayers with a NiMn or PtMn AFM layer exhibited higher induced uniaxial anisotropy than ones with IrMn. In the elements with a thin (<100 A) FM layer and NiMn or PtMn as an AFM layer, a local switching of the magnetization direction under an external applied field has been observed. The size of the “switched” areas depends on the material and thickness of the FM and AFM layers. No local switchin...

Transmission electron microscopy study of thermal effects on the free-layer reversal of a crossed-anisotropy spin valve

Transmission electron microscopy has been used to study the reversal of the free layer of a NiMn-pinned crossed-anisotropy spin valve as a function of applied field orientation and specimen temperature. By choosing the orientation of the applied field correctly it was possible to avoid the formation of domains in the reversal process. As the temperature was raised above room temperature, the mechanism remained qualitatively unchanged until temperatures of ≈200 °C were reached, beyond which irreversible behavioral change began to take place. From analysis of image sequences magnetoresistance characteristics have been constructed.

Time dependent magnetics of exchange biased spin valves

Different sweep field rates result in different pinning field and coercivity in the pinned layer of a spin valve. Annealed samples showed more profound time dependent magnetic properties as compared to the as-made films. The pinning field decreases with the sweep rate if the AFM layer is thinner than a minimum thickness. The coercivity increases with the sweep rate and is also FM layer thickness dependent. The thin FM layer showed less effect than the thicker FM layer.