Takao Imagawa

Distribution of blocking temperature in bilayered Ni81Fe19/NiO films

Exchange paths were investigated for unidirectional exchange coupled 40 nm Ni81Fe19/50 nm NiO films by performing several field cooling experiments. Our experimental data were very consistent with the assumed existence of a variety of exchange paths. Each exchange path seemed to produce its own local unidirectional anisotropy and different local blocking temperature. The measureable exchange coupling could be described as consisting of the sum of the respective exchange paths, each with its own local blocking temperature. On the other hand, an observed blocking temperature of about 230 °C was determined from the exchange paths having the highest local blocking temperature. The local blocking temperatures were thought to be widely distributed, ranging from room temperature to about 230 °C, and the maximum existence probability was most likely at about 215 °C. This indicated that the exchange paths having the local blocking temperature of 215 °C made the largest contribution to the exchange coupling field a...

Characteristics of a thin film microtransformer with circular spiral coils

A thin-film 3.14-mm*3.88-mm microtransformer has been fabricated, using photolithography technology, to support high-frequency switching regulators. The microtransformer is composed of sputtered amorphous magnetic layers, electroplated copper windings with 20- mu m width, and insulator layers. The coupling factor between the primary and secondary windings of the microtransformer is 0.97 for f=3 MHz. An input-output voltage ratio of 0.95 at 15 MHz is obtained at no-load. Observation of the domain images under high-frequency excitation indicates that the magnetic anisotropy of the microtransformer films is induced by rotating magnetic field annealing so as to increase the output voltage. >

Magnetic exchange coupling for bilayered Ni81Fe19/NiO and trilayered Ni81Fe19/NiFeNb/NiO films

Exchange coupling between a ferromagnetic film and an antiferromagnetic NiO film was investigated. Bilayered ferromagnetic Ni81Fe19/antiferromagnetic NiO films had a large exchange coupling field and blocking temperature of about 200 °C. In trilayered Ni81Fe19/ferromagnetic (Ni81Fe19)100−xNbx/NiO films, a way could be developed to control the exchange coupling field at a small value by increasing the Nb of the intermediate film. The most important factor in the control seemed to be that the numbers of magnetic Fe and Ni atoms of (Ni81Fe19)100−xNbx, which contributed to the exchange coupling between (Ni81Fe19)100−xNbx and NiO, varied with the existence of nonmagnetic Nb at their interface. From experimental results with other trilayered Ni81Fe19/ferromagnetic (Ni100−xFex)93Nb7/NiO films, it was ascertained that the exchange coupling field seemed to be independent of the magnetic moment of the ferromagnetic film although unidirectional anisotropy constant was proportional to it. As for blocking temperature,...

Rotational hysteresis loss study on exchange coupled Ni81Fe19/NiO films

Investigations on unidirectional anisotropy and rotational hysteresis loss of exchange coupled Ni81Fe19/NiO films have been conducted to clarify the nature of the exchange coupling mechanism. The interfacial exchange coupling regions, which had been considered to be scattered among the nonexchange coupling regions matrix, were found to be composed of many local regions of two kinds: (i) blockable regions which can give the Ni81Fe19 film a unidirectional anisotropy and (ii) unblockable regions which can have exchange coupling, but cannot give the Ni81Fe19 film unidirectional anisotropy. These unblockable regions begin to change gradually to blockable regions on decreasing the temperature below around 100–110 K. This change is probably caused by the antiferromagnetic NiO anisotropy of unblockable regions being strengthened below that temperature. Moreover, the decrease in size of the exchange coupling field and lowered blocking temperature for tNiO<50 nm (tNiO:NiO film thickness) seems to originate from a d...

Thickness effect on ferro/antiferromagnetic coupling of Co/CrMnPt systems

To investigate ferro/antiferromagnetic coupling in Co/CrMnPt layers of spin valve films, spin valve films of glass/Ta/NiFe/CoFe/Cu/Co 30 A/CrMnPt D A/Ta 30 A were made with various CrMnPt thicknesses indicated D. Shift fields of the hysteresis loop indicated Hp and coercivities indicated Hc were evaluated. With the increase of D from 150 to 1000 A, the blocking temperature increases from RT to 340 °C. By increasing the temperature, the coercivity Hc increases, peaks at around the blocking temperature and then decreases. The temperature at which Hc peaks increases with an increase of D. The maximum value of Hc decreases with the increase of D. The grain size distribution of CrMnPt was measured and temperature dependencies of Hp and Hc were calculated based on a thermal fluctuation model using the grain size distribution. Quantitatively, the calculated results agree with the experimental results. The change of temperature dependence of Hp and Hc are explained by the change of the local blocking temperature ...

NIO STRUCTURE-EXCHANGE ANISOTROPY RELATION IN THE NI81FE19/NIO FILMS AND THERMAL STABILITY OF ITS NIO FILM

We studied an antiferromagnetic (AF) NiO film for an exchange‐biased layer, focusing especially on the relationships between the exchange coupling properties of the Ni81Fe19(top)/NiO(bottom) films and the character of its NiO film. Among the variable sputtering conditions, our experimental data showed that the dominant factor determining the exchange coupling properties was the Ar pressure during the NiO film preparation. Better exchange coupling properties resulted when the NiO film was deposited at low Ar pressure which was attributed to: (i) the smooth surface of the NiO film and (ii) the presence of relatively large particle sizes within it. The former was thought to bring about not only an increase in the number of unidirectional exchange coupled Ni81Fe19/NiO spins, but also the appearance of exchange paths having large local exchange anisotropies. The latter was thought to produce an increase in the AF clusters with a particle volume larger than KeiA/KAFi, where Kei, A, and KAFi are local unidirecti...

Thermal stability of magnetic properties of electroplated Ni -Fe-In ternary alloy films

Magnetic properties and internal stresses of Ni-Fe-In thin films prepared by electroplating were investigated along with Ni-Fe films. In both films magnetostriction constants and internal stresses increased through annealing. Also coercive forces increased with increasing magnetostriction constants. Ni-Fe-In ternary permalloy films were found to have lower coercive forces after annealing than Ni-Fe binary permalloy films at same magnetostriction constants. This is because indium prevents grain growth during annealing.

Magnetic properties of sputtered Co-Ni-Fe-Pd films for thin-film heads

Magnetic properties of Co-Ni-Fe-M (M=Rh, Ir, Pd, Pt) films prepared by sputtering are investigated. It is found that addition of Pd decreases the magnetostriction constant of the films from 1*10/sup -5/ to around zero. On the other hand, addition of other elements, such as Rh, Ir, and Pt, increases it. However, coercive forces of Co-Ni-Fe-Pd films become more than 10 Oe when the magnetostriction is less than 2*10/sup -6/. Multilayered films are investigated to obtain films with low coercive force. 43Co-27Ni-15Fe-15Pd films of 0.17 mu m and Al/sub 2/O/sub 3/ films of 0.01 mu m in thickness are layered time-sequentially. This multilayered film has saturation induction of 1.4 T, approximately=0 magnetostriction, and a low coercive force of 1.5 Oe. Furthermore, Co-Ni-Fe-Pd films are ascertained to be as resistant to corrosion as Permalloy films. Recording heads with multilayered Co-Ni-Fe-Pd films with Al/sub 2/O/sub 3/ interlayers as magnetic cores have been fabricated. Recording characteristics were evaluated. These laminated Co-Ni-Fe-Pd/Al/sub 2/O/sub 3/ heads exhibit about 6 dB better overwrite than Permalloy heads. >

Surface Morphology of the Grain of Permalloy Films

Surface morphologies of sputtered permalloy films were investigated by SEM. Grain sizes decreased with slanting of the substrate surface or the presence of N2 in the sputtering gas. The preferred orientations of permalloy films were changed from ≪111≫ to ≪100≫ by the N2 presence. SEM observations of crystal orientations suggested that the changes in the preferred orientations of permalloy films were caused by reduction of the surface energy, which was, in turn, thought to be caused by nitrogen adsorption.

Preparation of Magnetic Oxide Films by Sputtering Method

Expe_rimental Procedure The substr ates we used wer-e Si02, GGG and sapphire with different ot-ientations. The targets were composed of BiYDyFe4 . 4 Inco.r)012 and BiSmErFe4.4Ino.EC01,2. The sputtering device was of the high-frequency magnetron type, and the bias potential was 0 -250V. Table 1 shows the sputtering and heat treatment conditions. The film structure was studied with X-ray and electron beam diffraction. The magnetic properties were measured with a VSM.