Dai Dao-Sheng

Low temperature magnetic properties of NdxCo1−x amorphous thin films

Abstract Amorphous thin films of Nd x Co 1− x alloys were prepared over the composition range 0.07 ⩽ x ⩽ 0.96 by the vacuum evaporation method with two heating sources. The field dependence of magnetization σ( H ) in 0–7 T and at 1.5 K for all samples were investigated. The magnetic structure of these films were studied by magnetic measurement. It was found that the effective magnetic moment of the Co atom μ Co = 1.87 ƒ(x)μ B , ƒ(x) being the probability for bearing magnetic moment of cobalt atoms, the effective magnetic moment of the Nd atom μ Nd = 2.28μ B for 0.07 ⩽ x ⩽ 0.50, and μ Nd = 2.28 - 1.76 (x − 0.50) for 0.50 ⩽ x ⩽ 0.96, and the maximum distribution angle ψ O of magnetic moment of neodymium atoms is about 66° at x ψ O = 66° + 63.3° ( x − 0.50) at x ⩾ 0.50. The temperature dependence of the magnetization σ( T ) at 1.5−200 K with H = 7.0 T has been measured. When the neodymium concentration is larger than 24 at %, there is a sudden drop of σ( T ) at about 20 K, and this temperature does not change with increase of Nd component. The results show that the demagnetized state of the amorphous Nd-Co films may be composed of asperomagnetic and speromagnetic structures. The sudden drop of σ( T ) near 20 K for samples with x ⩾ 0.24 may be explained by the phase transition between speromagnetism and paramagnetism.

Behavior of the amorphous ferromagnetic ribbons in the very low fields

The D.C. and A.C. magnetic properties and loss factors of five Co‐based amorphous ribbons with different compositions in the low magnetic fields were measured. The results show that there is a distinct turning field Hp for B(H), μ(Hm) and 2πtan δ/μ−Bm. The existence of Hp can be explained by means of the domain wall pinning effect in the low fields. The measured Hp corresponds to the minimum pinning field (∼10 mOe) in samples. It is suggested that the large gradient of intrinsic stresses in the ribbons is main cause of the low field pinning effect on domain walls; then we obtained a formula for Hp which was estimated to be 2–20 milli‐Oersteds.

Magnetic properties of FexCu1−x granular alloy films

The magnetic properties of the FexCu1−x granular system, as functions of iron particle size, were studied. Using high resolution transmission electron microscopy, iron particle size was determined to be ranged from 1–4 nm in as‐deposited samples as x=0.06–0.34. Magnetic measurements revealed that samples exhibit superparamagnetic relaxation below 300 K as x≤0.22. At 1.5 K, the average magnetic moment per each iron atom is reduced as the average number N of iron atoms in a particle is less than 450, and approaches the value of bulk iron as N≥600. Moreover, the temperature dependence of magnetization for Fe–Cu granular alloys was found to obey the Bloch’s T3/2 law below 300 K as x≳0.22. We suggest that spin wave excitations of long wavelength occur due to the weak exchange coupling among iron particles as x≳0.22. This behavior was confirmed by the ferromagnetic resonance study.

Interlayer exchange coupling study of Ni80Fe20/Cr multilayered structures

Abstract The evidences of antiferromagnetic interlayer exchange coupling in Ni80Fe20/Cr multilayered structures were reported. In Ni80Fe20/Cr multilayered structures, both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance have been found to oscillate with the Cr spacer layer thickness with a period of about 13 A. Ferromagnetic resonance (FMR) spectra have revealed that for antiferromagnetically (AF) coupled films ( t Cr = 12, 25 A ) there is a weak absorption mode of higher field than that of the regular in-phase mode, and that there are only some lower field resonance modes for ferromagnetically coupled films. Moreover, the magnetization vs temperature curves for the AF films have been found to have sizable thermal magnetic hysteresis. From the FMR spectrum for t Cr = 12 A and the temperature dependence of the magnetization in different magnetic fields for t Cr = 12, 25 A , the maximum strength of the AF coupling has been estimated to be about 0.017 erg cm-2.

Oscillatory interlayer exchange coupling in Ni80Fe20/Cr multilayers

Abstract Evidence of antiferromagnetic interlayer exchange coupling in Ni 80 Fe 20 /Cr multilayers is reported. In Ni 80 Fe 20 /Cr multilayers, both the magnitude of the interlayer magnetic exchange coupling and the saturation magnetoresistance have been found to oscillate with the Cr space layer thickness with a period of about 13 A. FMR spectra revealed a weak resonance mode above the regular in-phase mode for the antiferromagnetically coupled films, and only some lower-field resonance modes for ferromagnetically coupled films.

The magnetic anisotropy and interlayer magnetic coupling of evaporated Ag/Ni multilayers

Abstract A series of Ag/Ni multilayers has been prepared by e-beam deposition and studied by ferromagnetic resonance technique as a function of the static magnetic fied orientation in a plane perpendicular to the film plane. Interface mode has been observed in nearly perpendicular geometry for multilayers with Ni layer thicknesses less than 30 A. Interface anisotropy, which is parallel to the film plane, has been obtained and understood. Moreover, the effect of interlayer magnetic coupling on the ferromagnetic spectrum has been discussed.

Magnetic excitations in amorphous Sm-Ni thin films☆

Abstract The temperature dependence of magnetization has been studied for amorphous Sm x Ni 1 -x thin film alloys. It was found that both spin wave excitations and Stoner excitations occur in the temperature range 70–200 K, and a marked deviation from the T 3/2 law was noted below 70 K. Kaneyoshis theory was used to account for the deviation.

A theory of internal stress field and configuration of magnetic domain in amorphous ribbons

Abstract The distribution of internal stress field of different regions in the amorphous ribbon are found, and the magnetic anisotropies in these regions are also calculated. On the basis of these results we could easily understand the various configurations of domain structure and some experimental phenomena of domain observation on the amorphous ribbons.

New permanent magnetic MnBiDy alloy films

We have studied the magnetic properties of MnBiDy films fabricated by the thermal coevaporation method and annealed subsequently in a vacuum oven at temperatures of 350 °C–425 °C. The thicknesses of these films are in the range from 60 to 1200 nm. X‐ray diffraction analyses have confirmed that those samples have an hcp structure. Magnetic measurements revealed that the saturation magnetization σs=46–78 emu/g, remanence ratio R=0.67–0.98, coercive force μHc=2.2–7.3 kOe, and magnetic energy product (BH)m=3.3–14.3 MGOe. These values are sensitive to the film thicknesses. It was found that μHc reaches a maximum at d≊400 nm.

Bias dependence in spin-polarized tunneling of ferromagnet ferromagnetic insulator (semiconductor)/ferromagnet junctions

Based on the nearly-free-electron approximation, the bias dependencies of electron transport properties of ferromagnet/ferromagnetic insulator (semiconductor)/ferromagnet junctions have been studied. Resonances appear in electron transmission probability. These resonances cause oscillations in the zero-temperature tunnel current and the resonances occur in tunnel conductance. Tunnel magnetoresistance (TMR) is an oscillatory function of bias. The TMR can reach a value as high as 100%. The bins dependencies of electron transport properties relate to the magnetic configurations of the junctions.