R.H. Yu

Magnetic properties and giant magnetoresistance in melt‐spun Co‐Cu alloys

Magnetic, structural, and transport properties of as‐quenched and annealed Co10Cu90 samples have been investigated using x‐ray diffraction and a SQUID magnetometer. The largest value of MR change was observed for the as‐quenched sample annealed at 450 °C for 30 min. The magnetic and transport properties closely correlate with the microstructure, mainly with Co magnetic particle size and its distribution. For thermal annealing the as‐quenched samples below 600 °C, the Co particle diameters increase from 4.0 to 6.0 nm with a magnetoresistance (MR) drop from 33.0% to 5.0% at 10 K. Comparison with the theory indicates that the interfacial electron spin‐dependent scattering mechanism correlates with GMR for Co particle diameters up to about 6.0 nm.

Structure, magnetic properties, and giant magnetoresistance in melt‐spun metallic copper–cobalt ribbons

We report a comprehensive investigation of structural, magnetic, and transport properties of as‐quenched and annealed CoxCu1−x (0≤x≤0.20) granular alloys prepared by melt spinning. Using x‐ray diffraction, differential scanning calorimetry measurements, and magnetic characterization, we have uncovered a Co phase separation process which results in the variation of magnetic and transport properties of Co–Cu heterogeneous alloys. In the Co composition range (0≤x≤0.15), the maximum giant magnetoresistance (GMR) was observed for CoCu samples annealed at 450 °C for 30 min, where Co particle diameters are in the range of 3.5–4.5 nm. The variation of magnetic and transport properties with the concentration and size of precipitated Co clusters is discussed, and is consistent with the prediction of the two‐channel model, in which spin‐dependent scattering is dominated by the cluster‐matrix interfaces. The reduction of GMR in high Co concentration is attributed to the appearance of magnetic coupling among magnetic ...

Magnetic properties and giant magnetoresistance of magnetic granular Co10Cu90 alloys obtained by direct-current joule heating

The direct‐current (dc) joule heating technique was exploited to fabricate giant magnetoresistance (GMR) Co10Cu90 granular alloys. The Co cluster precipitation process was investigated by calorimetric and x‐ray diffraction measurements. At T=10 K, the largest MR change of 25.0% has been observed for the melt‐spun Co10Cu90 ribbon annealed at I=5 A. The magnetoresistance scales approximately as the inverse Co particle size. At room temperature, it was found that the dc joule‐heated samples show relatively high GMR in comparison with furnace‐annealed samples. Based on the phenomenological GMR model, we assumed that it is a consequence of smaller Co particles formed in dc joule‐heated samples.

Magnetic properties and giant magnetoresistance in melt-spun Co15Cu85 alloys

The structure, magnetic properties and giant magnetoresistance (GMR) of a metallic granular Co15Cu85 alloy have been investigated using an X-ray diffractometer and a SQUID magnetometer. The granular samples were fabricated by melt spinning and subsequently thermal annealing. Structural characterization confirmed that the samples consist of ultrafine Co magnetic particles embedded in the non-magnetic Cu matrix. Above the blocking temperature TB, the magnetic behaviour of Co particles can be understood on the basis of paramagnetic theory. The largest change in magnetoresistance of 43.0% was observed for the as-quenched ribbon annealed at 450 degrees C. It was confirmed that the GMR is closely correlated with the Co particle size and density, which can be optimized by controlling the annealing conditions.

Magnetic properties and giant magnetoresistance in magnetic granular CoxCu100-x alloys

Magnetic granular CoxCu100-x alloys (x=5-20) have been prepared by melt-spinning and subsequently annealing at 450 degrees C for 30 min, which is most appropriate for obtaining the largest MR change with magnetic field in each sample. The highest magnetoresistance (MR) change of 42.5% was observed in annealed Co15Cu85 ribbons. Based on the superparamagnetic assumption, the average size of Cox particles embedded in a Cu-rich matrix for different samples, ranging from 3.5 to 5.5 nm, has been estimated by simulating the magnetization curves at room temperature which is higher than the blocking temperature TB for each sample. The value of MR change was found to strongly depend on the Co composition and annealing temperature.

Giant magnetoresistance in magnetic granular Co15Cu85 alloys annealed by direct-current Joule heating

Abstract We have studied the structure and giant magnetoresistance (GMR) in the Co 15 Cu 85 granular alloys prepared by melt-spinning and annealed by the direct-current (dc) Joule heating. At T = 10 K, GMR as large as 50.0% (in a magnetic field up to 30 kOe) has been observed in a sample annealed with I = 6 A. The Joule-heated samples show higher GMR than those annealed by the conventional method. It is attributed to less content of residual Co in the Cu matrix for the dc Joule-heated samples than that for the samples annealed in a furnace.

The magnetic behaviour in heterogeneous f.c.c. Co10Cu90 melt-spun ribbons

Abstract The metastable magnetic alloy Co10Cu90 has been prepared by the conventional melt-spinning technique. The rapid solidification process results in an extended solubility of Co in Cu although some Co particles already have precipitated during quenching. In comparison with conventional ageing methods, Joule heating could also be adopted to fabricate granular Co-Cu alloys with even finer Co particles. The finest size of 4.0nm has been achieved for Co10Cu90 alloy after annealing with a current I = 4 A for 1 rnin. Single-domain behaviour results in a value of coercivity in excess of 40.0 kAm−1 for the sample annealed with I = 9.5 A. Below the size of 6.0 nm, Co particles exhibit superparamagnetic behaviour at room temperature, which has been investigated using a superconducting quantum interference device magnetometer. A unique change in M s with annealing current or temperature was discovered.

Phase separation and magnetic properties of metastable Cu90Co10 alloy

The authors have prepared Cu{sub 90}Co{sub 10} granular alloy by the melt-spinning technique and subsequently annealing the rapidly quenched ribbons by joule heating which results in fine Co magnetic particles embedded in nonmagnetic Cu matrix. The texture and lattice constant of fcc Cu-Co matrix were analyzed based on the x-ray diffraction. The preferred orientation of fcc matrix was gradually evolved from (200) to (111) with increasing annealing current. The saturation magnetization (Ms) is closely related to formation process of magnetic Co particles during Joule heating.

Improved giant magnetoresistance in magnetic granular Co5 Cu95 alloys by direct-current joule heating

The dc joule heating technique has been used to produce giant magnetoresistance (GMR) Co5Cu95 granular alloys. At T=10 K, GMR as large as 28.5% has been observed in the as-quenched sample annealed with I=6A in a magnetic field up to 30 kOe. At room temperature, the joule-heated samples show higher GMR in comparison with that annealed by the conventional method.

Magnetic and Magnetotransport Properties In Co5cu95 Melt-spun Alloys

Giant magnetoresistance (GMR) has been observed in Co5Cu95 alloys fabricated by melt-spinning. The highest MR change of 28.0% occurs for Co5Cu95 after annealing at 450°C for 30 min. Based on the super-paramagnetic assumption, the average size of Co particles embedded in Cu matrix, ranging from 3.0 to 6.0 nm, has been determined by simulating the magnetization curves at 295 K which is higher than the blocking temperatures for the samples. Comparison with phenomenological theory for GMR indicates that the interfacial spin-dependent scattering is the dominant scattering mechanism underlying GMR origin in granular systems. Additionally, for the samples in as-quenched state or annealed at temperatureTA=350°C, the electron hybridization and super-paramagnetic behaviors of fine Co particles may be responsible for the low value of MR change.