CoNbZr Layers Research Articles

Magnetoresistance and performance of amorphous-CoNbZr∕Co∕Cu∕Co magnetic multilayers

A pseudo spin-valve (PSV) using amorphous CoNbZr alloy as a soft magnetic layer and buffer layer was proposed. The giant magnetoresistance (GMR), thermal annealing effect, and the performance were investigated. Antiparallel magnetization alignments were observed in the samples with 2–4nm CoNbZr layer and a maximum GMR of 6.5% was obtained. Furthermore, sandwich with relatively thick CoNbZr layer has a superior thermal stability to 400°C and a GMR enhancement to about 9%. After patterning to 6μm×1μm elliptic stripe, a single domain forms and the dynamic MR behavior indicates the stripe has a linear and stable GMR response. Therefore, it is believed that the PSVs with amorphous CoNbZr layers have good potentials for spin-electronic devices.

Surface roughness effect on bias voltage characteristics in CoNbZr-based magnetic tunnel junctions

Abstract CoNbZr layers have been employed in magnetic tunnel junctions (MTJs) to substitute traditionally used Ta layers. The CoNbZr-based junctions exhibited better V h (the voltage where MR ratio becomes half) characteristics than Ta-based ones in as-deposited state (164 mV vs. 151 mV at 10 K). Moreover, V h gradually increased at higher temperatures for CoNbZr-based junctions while almost no change was observed for Ta-based ones (345 mV vs. 144 mV at RT, respectively). This is because CoNbZr-based junctions offer smoother interface structure than Ta-based one.

Characteristics of magnetic tunnel junctions consisting of amorphous CoNbZr layers

Magnetic tunnel junctions comprising amorphous CoNbZr layers have been investigated. Co85.5Nb8Zr6.5 (in at. %) layers were employed to substitute traditionally used Ta layers with an emphasis given on understanding underlayer effect. The typical junction structure was SiO2/CoNbZr or Ta 2/CoFe 8/IrMn 7.5/CoFe 3/Al 1.6+oxidation/CoFe 3/CoNbZr or Ta 2 (nm). For both as-deposited state and after annealing, the CoNbZr-underlayered structure showed superior surface smoothness up to the tunnel barrier than Ta-underlayered one (0.16 vs 0.34 nm). Ultrasmooth layer structure was proven beneficial for reducing temperature-dependent magnetoresistance properties. A longer annealing degraded the properties most likely due to Mn interdiffusion toward the bottom ferromagnetic electrode as confirmed by Auger electron spectroscopy. By a slight change in the bottom electrode thickness (CoNbZr 4/CoFe 10), the tunneling magnetoresistance ratio can be increased up to 32% after 10 min annealing at 300 °C.

Thermal stability of spin-valves incorporating amorphous CoNbZr under and capping layers

We introduce amorphous CoNbZr-based spin-valves (SVs) where CoNbZr substituted Ta layers that have been traditionally used for under and capping layers. The purpose of this study was to investigate thermal stability of the new structure at elevated temperatures. A comparison was also made with a similarly structured Ta-based SV. Magnetoresistance (MR) ratio and exchange bias field (Hex) of CoNbZr-based SV increased up to about 50% (from 4.5% to 6.8%) and 52% (from 294 Oe to 448 Oe), respectively, after annealing at 300 °C for 240 min. The root-mean-square (rms) surface roughness values for CoNbZr and Ta single layers were 0.07 nm and 0.43 nm, respectively, indicating that an amorphous CoNbZr offers much smoother surface than the Ta layer. Because CoNbZr-based SVs possess fine and dense microstructures, they were able to resist interdiffusion at elevated temperature, resulting in an improvement in thermal stability. In addition, Mn diffusion into the pinned CoFe layer might be reduced probably due to the fact that CoNbZr capping layer provided almost equal chemical potential for Mn diffusion into the pinned CoFe layer.

Soft magnetic properties of composite-anisotropy CoNbZr/ceramics multilayers (invited)

CoNbZr/ceramics multilayered films, the metal layers of which have sequentially changed anisotropy directions, were investigated. Such newly processed film is called ‘‘composite-anisotropy multilayer film.’’ The effects of this new processing on soft magnetic properties were studied. In the case where the shift angle in the easy magnetization direction between neighboring CoNbZr layers is 45°, a smooth B-H loop with low coercivity was obtained. Furthermore, annealing from 200 to 400 °C resulted in high permeability (μ′) of 1000–1500 and excellent frequency responses of μ′ above 100 MHz in all directions in the plane of the film. These results could be explained in terms of magnetic interactions of the CoNbZr magnetic layers which are in close contact with one another across the intervening ceramics layers. These findings indicate that such composite film has many advantages over conventional uniaxial multilayers in applications to magnetic devices. Since these characteristics are thought to be very significant for microdevices composed of disc-shaped magnetic cores, we fabricated a planar microtransformer composed of circular spiral coils and disk-shaped magnetic cores, 2.4 mm in size, using photolithographic technology. The input–output voltage ratio, VOUT/VIN, was measured in the frequency range of 2–40 MHz and showed a high value of 0.83 at 40 MHz.

Magnetic Properties of Composite Anisotropy CoNbZr/Ceramics Multi-Layers

The authors investigated CoNbZr/ceramic multilayer films, in which the anisotropy direction of the CoNbZr layers, separated by only thin ceramic layers, was changed sequentially (hereafter "composite anisotropy multilayer films"). The effect of this new lamination method on the B-H hysteresis loop and magnetic permeability was studied. When there are twelve CoNbZr layers and the angle between the easy-axis direction of successive CoNbZr layers is 45°, a smooth hysteresis loop with low coercivity was obtained. This same film also had a large permeability (μ') and an excellent frequency response. In addition, the μ' values remained unchanged when bias fields of 2 to 3 Oe were applied. This result is significant for applications to magnetic devices, as it contrasts with conventional uniaxial magnetic multilayer films the permeability of which is changed by an applied bias field.

ＣｏＮｂＺｒ／Ｃｅｒａｍｉｃｓ複合異方性多層膜の磁気特性

The CoNbZr/ceramics multi-layers having sequentially changed anisotropy directions for CoNbZr layers in close contact with one another beyond ceramics layers were investigated. We call such a thin film “Composite Anisotropy Multi-Layers”, hereafter. The effect of this new multilaying method on B-H hysteresis loop and magnetic permeability was studied. When the number of CoNbZr layers is twelve and the value of the shift angle between the easy magnetization directions for the nearest neighbor CoNbZr layers is 45°, the smooth hysteresis loop with a low coercivity was obtained. Furthermore, this film showed a large permeability (μ') and an excellent frequency response. In addition, the μ' values remained unchanged by the bias fields of 2∼3 Oe. This finding is noteworthy for applications of magnetic devices, compared with the conventional uni-axial magnetic multi-layers in which permeabilities are changed by the applied bias field.

Geometrically induced domain pattern variations in laminated CoNbZr structures

Domain patterns of the upper magnetic layers of CoNbZr structures laminated with SiO2 or Al2O3 spacers were observed with a Kerr-effect microscope. In these relatively long laminated structures the width was varied in the range 20 μm ≤ w ≤ 150 μm, the spacer thickness in the range 8 nm ≤ b ≤ 100 nm and the magntic layer thickness in the range 0.35 mum ≤ D ≤ 2 μm. For the larger spacer thicknesses and/or the smaller pattern widths, an in-plane closure domain pattern similar to that of an unlaminated layer was observed, which suggests that the CoNbZr layers are magnetically uncoupled. For the other geometries, domain patterns featuring large easy-axis oriented domains were found, indicating that the CoNbZr layers are magnetically coupled through flux closure around the spacer. The transition from uncoupled to coupled magnetic layers takes place through an intermediate domain pattern incorporating enlarged easy-axis orientated domains which are only partially closed in the film plane by closure domains. The region in which this intermediate domain pattern occurs was found to coincide with the boundary between an in-plane closure domain pattern and a single easy-axis domain pattern as calculated according to Slonczewski et al. (IEEE Trans. Magn. MAG-24 (1988) 2045). Magnetic coupling between upper and lower CoNbZr layers is also shown to modify the behaviour of thin-film transformers having laminated CoNbZr cores.

Enhanced magnetic behaviour of narrow track thin-film heads determined from thin-film transformers

Measurements are presented in which the mutual magnetic inductance of a thin-film transformer has been measured as a function of the track width in the frequency range from 1 to 8 MHz. The thin-film transformer was made using a 2 μm structured amorphous CoNbZr layers as flux guide. It was found that the relative permeability at 1 MHz, determined from the mutual inductance, increased by a factor of 2.5 when the track width decreased from 250 to 10 μm. At 7 MHz, however, this effect was found to have almost disappeared. Domain observations using a Kerr-effect microscope were made of CoNbZr patterns which were similar in lay-out to those applied in the transformers. For all track widths, a closure domain pattern was observed, the periodicity of which changed with the track width. No relationship between both the relative area of the rotationally sensitive domains or of the closure domains and the measured mutual inductance was found for these layers. Instead, a relationship was found between the length of the 90° walls of the closure domains per unit area and the increased mutual inductance observed at 1 MHz.

Soft-magnetic properties and structure of Fe/CoNbZr multilayers

The magnetic and structural properties of ion beam sputtered Fe/CoNbZr multilayers were studied as a function of the CoNbZr and Fe layer thickness. Good soft-magnetic properties (coercivity <40 A/m and permeability >1000) are obtained only when the Fe grains are small, which is the case for multilayers with Fe layer thicknesses below 10 nm and CoNbZr layer thicknesses above 4 nm. Thick CoNbZr layers (>4 nm) are amorphous and repeated nucleation of Fe grains is observed. When the CoNbZr layers are too thin (<3 nm) they are crystalline and do not interrupt the Fe grain growth. The resulting columnar growth of Fe throughout the total film thickness leads then to a drastic decrease of permeability and increase of coercivity.

Induced anisotropy of amorphous CoFeSiB and CoNbZr magnetic materials

Uniaxial anisotropy induced by magnetic annealing was studied on amorphous CoFeSiB and CoNbZr layers. The kinetics of inducing the anisotropy is slowed down by a preanneal at 400°C. This effect is much stronger for CoNbZr than for CoFeSiB. An addition of 2% transition metal (TM) atoms does not influence the kinetics after preannealing appreciably. Simultaneously with inducing anisotropy magnetic annealing changes the magnetostriction. Two effects determine the anisotropy, orientation of pairs of different magnetic atoms and a contribution which also occurs in alloys with a single magnetic atom. The latter contribution is independent of type and size of the non-magnetic atom(s) and is attributed to Co - clusters.