M. Nawate

Giant magnetoresistance and superparamagnetic grains in Co–Ag granular films

Co–Ag granular films having various Co grain sizes are prepared by rf sputtering under various sputtering conditions. The Co grain sizes are derived from the magnetization curves by dividing them into ferromagnetic and superparamagnetic components. As the Co content decreases, the radii of the superparamagnetic Co grains, rg, decrease and the distances between the Co grains, tg, increase. The magnetoresistance is proportional to the volume density of superparamagnetic grains or related to krg3 exp(−tg/l) with k=3.8×102 μΩ/cm2 and l=20 A, implying that the giant magnetoresistance is caused by the spin-dependent scattering in the inner part of the superparamagnetic Co grains as well as at the grain surfaces. Furthermore, it is found that the extraordinary Hall effect arises mainly from the side jump mechanism, and that significant perpendicular magnetic anisotropy appears in the Co composition range between 40 and 70 at %.

Tunneling giant magnetoresistance in Fe-SiO2 multilayered and alloyed films

Abstract Magnetic properties and magnetoresistance (MR) are examined for Fe-SiO 2 multilayers and granular films prepared on glass substrates by rf sputtering. In both the multilayers with thin Fe layers, which are discontinuous forming Fe granules, and the granular films of small Fe composition, the magnetization curve follows the Langevin function indicating superparamagnetic nature. These films exhibit the giant MR arising from spin-dependent tunneling between neighboring superparamagnetic Fe granules for both systems, and show a maximum value of 3.6% at room temperature.

Magnetization characteristics and Curie temperature of Y/Fe and Tb/Fe multilayers

Abstract In Y/Fe and Tb/Fe multilayers sputter-deposited on glass substrates, the crystallographic structure and the magnetic properties have been investigated as a function of Fe layer thickness, dFe. The saturation magnetization, Ms, becomes smaller with decreasing dFe until the critical thickness, dcFe, of 20 A for Y/Fe or 17 A for Tb/Fe where the crystallographic structure of Fe layer changes from bcc to amorphous. At dcFe, the magnetization curve at room temperature shows the paramagnetic characteristics, because the Curie temperature, TC, of the amorphous Fe is lower than room temperature. The Y/Fe multilayers of d Fe = 15−20 A show the two-dimensional magnetism, while the films of dFe

Giant magnetoresistance in Co/Cu multilayers sputter-deposited on glass substrates

Abstract Co/Cu multilayers have been prepared on glass substrates without buffer layers by rf magnetron sputtering under various conditions. The film structure for establishing the large giant magnetoresistance is suggested to have smooth, flat and sharp interfaces, and close-packed homogeneous Cu layers with low resistivity. The role of the (200) texture is also discussed.

Saturation magnetization and Curie temperature of Tb/Fe multilayers

Abstract For the amorphous region of Fe layer thickness (dFe) smaller than the critical value (dcFe) in Tb/Fe multilayers, the Curie temperature (Tc) and the saturation magnetization (Ms) have been calculated using the mean field theory. The results are consistent with the experimental data: Tc and Ms (at 300 K) increase with decreasing dFe.

GIANT MAGNETORESISTANCE OF PERPENDICULAR MAGNETIC CO/AU MULTILAYERS

Co/Au multilayers have been prepared on glass substrates by electron‐beam evaporation techniques. The perpendicular anisotropy is induced when the Co layer thickness is less than 12 A and its origin is discussed phenomenologically. In the perpendicular magnetic multilayers, large giant magnetoresistance appears and it consists of hysteretic and nonhysteretic components. The occurrence mechanism of these components has been discussed and it is suggested that the former arises from antiparallel spin alignment between adjacent Co layers due to the random wall position layer by layer, and the later is related to the granular‐type giant magnetoresistance.

Magnetic structure and perpendicular magnetic anisotropy of rare‐earth (Nd,Pr,Gd)/Fe multilayers

The structural phase and the magnetic anisotropy of Nd/Fe, Pr/Fe, and Gd/Fe multilayers have been examined using magnetization measurements and conversion electron Mossbauer spectrometry. With decreasing Fe layer thickness dFe, the structural phase of the Fe layer changes from bcc crystalline to amorphous at the critical thickness dFec of 17–20 A via an intermediate phase, which contains a paramagnetic phase in the Nd/Fe and Pr/Fe systems, but not in Gd/Fe. At just above dFec, Nd/Fe and Pr/Fe multilayers exhibit perpendicular anisotropy, which originates from the interface anisotropy due to the one ion anisotropy of Nd and Pr. However, Gd/Fe multilayers show in‐plane magnetization because of the S‐ionic Gd. Below dFec, the magnetization increases from nearly zero with decreasing dFe because of the exchange interaction between Fe and the rare earth (Nd,Pr,Gd).

Spin structures of Gd/Fe multilayers evaporated at oblique incidence

Abstract Obliquely evaporated Gd/Fe multilayers were investigated. Spin flopping was observed at a low field of about 1 kOe between 5 and 300 K. Mean field analysis of spin structure revealed that Gd and Fe spins rotate abruptly from the field direction of the spin flopping field while keeping their antiparallel coupling. Then, they rotate gradually with increasing field toward the field direction.

Magnetic properties and giant magnetoresistance of Fe/Au multilayers

Magnetic properties and magnetoresistance (MR) are examined for the Fe/Au multilayers (MLs) prepared on glass substrates by electron-beam evaporation techniques as a function of the thicknesses of Fe and Au layers; dFe and dAu. For small dFe, the perpendicular magnetic anisotropy is induced, and in the films of dFe≈5 A, the stripe magnetic domain patterns are observed. In dFe⩽3 A, however, a superparamagnetic nature and a granular-type giant magnetoresistance (GMR) appear at room temperature. In a series of [Fe(3 A)/Au(dAu)]40 MLs, the MR ratio increases with dAu and reaches the first peak at around 10 A. On the other hand, at 14 K, the ferromagnetic component having a perpendicular anisotropy becomes dominant, and the GMR arises from the random wall alignment due to the antiferromagnetic coupling between the adjacent Fe layers, causing the first MR peak at dAu≈10 A. The layered structure is obscured and the superparamagnetic Fe grains are created by annealing at 300–350 °C, followed by the crystalline gr...

Magnetization properties of bias modulated TbCo films with short periodic modulation

TbCo films modulated compositionally between 14.4 at% Tb and 20.1 at% Tb were prepared with the period of 3000 - 8 A by the bias-modulated sputtering method. Two compensation temperatures of both layers in these multi-layered films approach and merge with each other with reduction of the modulation period. The magnetic hysteresis loops change tremendously depending on the period due to exchange interaction between layers. The short periodic modulation film with layer thickness of 4 A showed the highest perpendicular anisotropy energy of 2.0 - 2.5 × 106erg/cm3, and the highest Kerr effect of 0.31 deg.