T. Shinjo

Effect of Joule heating in current-driven domain wall motion

It was found that high current density needed for the current-driven domain wall motion results in the Joule heating of the sample. The sample temperature, when the current-driven domain wall motion occurred, was estimated by measuring the sample resistance during the application of a pulsed current. The sample temperature was 750 K for the threshold current density of 6.7×1011A∕m2 in a 10-nm-thick Ni81Fe19 wire with a width of 240 nm on thermally oxidized silicon substrate. The temperature was raised to 830 K for the current density of 7.5×1011A∕m2, which is very close to the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5×1011A∕m2, an appearance of a multidomain structure in the wire was observed by magnetic force microscopy, suggesting that the sample temperature exceeded the Curie temperature.

Magnetization reversal in submicron magnetic wire studied by using giant magnetoresistance effect

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(200 A)/Cu(100 A)/NiFe(50 A) was investigated by measuring the electric resistance in an external magnetic field. A giant magnetoresistance (GMR) effect of about 0.8% was observed when the magnetizations in two NiFe layers are oriented antiparallel. It is demonstrated that magnetization reversal phenomena can be very sensitively investigated by utilizing the GMR effect.

Dynamics of a magnetic domain wall in magnetic wires with an artificial neck

Magnetization reversal in submicron magnetic wires consisting of a NiFe/Cu/NiFe trilayer with an artificial neck was investigated by utilizing the giant magnetoresistance effect. A magnetic domain wall was injected into the wire by a local magnetic field applied at the end of the wire. Pinning and depinning of the magnetic domain wall were detected as sharp changes in resistance. It was found that the neck works as a pinning site of a domain wall and that the depinning field increases with a decrease of the neck width.

Magnetoresistance of Multilayers Prepared on Microstructured Substrates

A new class of multilayers was prepared by using substrates with V-groove micro-structures. The magnetoresistance ratio of a sample [Cu(58 A)/Co(12 A)/Cu(58 A)/NiFe(12 A)]× 180 fabricated on a substrate with a V-groove period of 2 µ m shows a large anisotropy; 12% for the current perpendicular to the grooves and 8.1% for the current parallel to the grooves at room temperature.

Magnetoresistance of multilayers with two magnetic components

Abstract Multilayers consisting of Cu, Co, Cu and Ni(Fe) alloy layers were prepared and magnetoresistance (MR) properties were studied. In the magnetization process, a giant ferrimagnetic state is realized at moderate external fields because of the difference of the coercive fields in the two magnetic layers and then the resistance is greatly enhanced. MR properties were examined by varying the total layer number and the Cu layer thickness. The largest MR ratio was observed in [Cu(55 A)/Co(25 A)/Cu(55 A)/Ni 80 Fe 20 (25 A)]×15; 10% at 300 K and 28% at 80 K.

Propagation of a magnetic domain wall in magnetic wires with asymmetric notches

The propagation of a magnetic domain wall (DW) in a submicron magnetic wire consisting of a magnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated by utilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled by a pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. It was found that the depinning field of the DW from the notch depends on the propagation direction of the DW.

Evidence for antiferromagnetic coupling between Fe layers through Cr from neutron diffraction

Small-angle neutron diffraction measurements have been made for a multilayer, [Fe(27 A)/Cr(12 A)]×30, at room temperature. It was found that the magnetizations of adjacent Fe layers are coupled in antiparallel. The decrease of antiferromagnetic peak intensity as a function of external field was observed. It is confirmed that the giant magnetoresistance in Fe/Cr multilayers is due to the antiferromagnetic ordering of Fe layers caused by the interlayer coupling across an intervening Cr layer.

The magnetization process and magnetoresistance of exchange-spring bilayer systems

A perfectly reversible magnetization process was observed in NiFe/CoSm bilayers. During this process, the magnetic moments in the soft magnetic layer (NiFe) are pinned at the interface with the hard magnetic layer (CoSm), so that the direction of the magnetic moment distributes successively like a Bloch wall. The characteristic reversible magnetization process is explained by an atomic layer model. The magnetoresistance also exhibits a reversible change reflecting the magnetization process. The basic feature of the reversible magnetoresistance curve is understood to be anisotropic magnetoresistance.

Reversible magnetization process and magnetoresistance of soft-magnetic (NiFe) /hard-magnetic (CoSm) bilayers

Abstract The magnetization process and magnetoresistance were studied for soft-magnetic (NiFe)/hard-magnetic (CoSm) bilayers. In the course of the magnetization reversal, the magnetic moments in the soft magnetic layer rotate reversibly, while they are pinned by the hard magnetic layer at the interface; consequently, the direction of the magnetic moment distributes successively as in a Bloch wall. The magnetoresistance also shows a reversible change, reflecting the magnetization process. The change is explained by means of the anisotropic magnetoresistance due to angle distributed magnetic moments.

Two types of magnetic vortex cores in elliptical permalloy dots

Elliptical (track-shaped) permalloy (Ni19Fe81) dots, in which magnetic circular vortex and antivortex structures are stabilized, were prepared and the magnetic properties of perpendicular magnetization spots (turned-up magnetizations) at the cores of both types of vortices were studied. Using magnetic force microscopy, the direction of the turned-up magnetization was detected and the switching field was measured. It was found that the value of the switching field of the turned-up magnetization at the antivortex core is smaller by about 1000 Oe than that at the circular vortex core. It was confirmed that the switching of the turned-up magnetization in the antivortex is not influenced by the directions of the turned-up magnetizations in the neighboring circular vortices. Vanishing and regenerating processes of turned-up magnetizations were observed by increasing and decreasing the magnetic field applied to the in-plane direction.