Search

The texture and surface/interface topological effects on the exchange field Hex and coercivity Hc in rf-sputtered NiO/NiFe bilayers were studied. The NiO/NiFe bilayers with NiO crystalline textures of (111), (200), and (220), and different surface/interface roughness, were fabricated by changing the sputtering conditions. The Hex of the NiO/NiFe bilayers was inversely proportional to the thickness of NiFe. The NiO/NiFe bilayers with different NiO textures of (111) dominant, (200) dominant, or combinations of (111), (200), and (220), respectively, showed almost the same dependence of Hex on NiFe thickness. Thus, Hex was insensitive to the texture of NiO films. The surface/interface roughness has a significant effect on Hc of the NiO/NiFe bilayers. By controlling the sputtering process, NiO (45 nm)/NiFe (10 nm) bilayers with a Hex of 39 Oe, a Hc of 4.1 Oe, and an Hex/Hc ratio of 9.51 could be obtained. A new parameter of Hex/Hc was introduced to describe the exchange coupling and coercive properties in ferr...

Mn‐ and Co‐doped acicular γ‐Fe2O3 particles with a coercivity of 923 Oe and Co content of 3.5 wt % have been prepared by a special process. This involves high‐temperature ion diffusion, then step annealing. These particles exhibit a high magnetic thermal stability and low‐temperature coefficient of coercivity. The latter demonstrates a striking contrast to the ordinary cobalt‐body‐doped γ‐Fe2O3 particles. The induced uniaxial anisotropy constant Ku and the magnetocrystalline anisotropy constant K1 of the particles were determined by torque measurements. The novel properties of these particles are mainly due to the high uniaxial anisotropy, resulting from the anisotropic arrangements of cobalt ions. With the above characteristics and low Co content, these acicular particles can be one of the promising candidates for particulate high‐density magnetic recording materials.

NiO/Ni81Fe19 bilayers with high exchange coupling, Hex, and low coercivity, Hc, were fabricated. These bilayers with NiO crystalline textures of (111), (200), and (220), and different surface/interface roughness were deposited onto a Si(100) wafer by changing the rf reactive sputtering conditions. A new parameter of exchange coupling to coercivity ratio, Hex/Hc, was used to quantify the effectiveness of the exchange bias for the bilayer film. An Hex=39 Oe and Hc=4.1 Oe were obtained from a NiO (45 nm)/NiFe (10 nm) bilayer, yielding a ratio of Hex/Hc=9.51, which was significantly higher than any previously reported results. An atomic force microscopy study showed that the key to the low coercivity was the reduction of surface roughness of the NiO/NiFe bilayer. It is proposed that the roughness-induced magnetostatic coupling in the NiFe film could be one of the main mechanisms for the relating high coercivity in NiO exchange-biased NiFe bilayer. NiO/NiFe/Co/Cu/Co/NiFe spin-valve films were deposited and a g...

The effects of uniaxial stress on the exchange coupling field, Hex, coercivity, Hc, and uniaxial anisotropy field, HK, in NiO/NiFe bilayers were experimentally studied. The NiO/NiFe bilayers were deposited onto a Si(100) wafer using radio frequency reactive sputtering. Samples of the bilayers were externally and constantly uniaxial stressed (either tensile or compressed) using a specially designed sample holder with a fixed radius of curvature. The hysteresis loops of the stressed NiO/NiFe bilayer samples were measured in situ along the easy and hard axes of the NiFe films using a vibrating sample magnetometer. The composition of the NiFe film in the NiO/NiFe bilayer was characterized as Ni80.2Fe19.8 using x-ray photoelectron spectroscopy. The Hc and HK of the bilayers were significantly affected by the stress, while the Hex was apparently not changed by the same stress. The large changes in the coercivity in the stressed NiO/NiFe bilayer were produced by the change of the effective uniaxial anisotropy fi...

The causes of the coercivity enhancement and the coercivity instability of acicular cobalt ferrite particles containing 0–9.2 wt % Fe2+ ions under the influence of magnetic field and temperature were investigated by means of transverse magnetic field annealing, torque, and Mossbauer experiments. The coercivity enhancement and the instability of Co modified iron oxide particles are closely related to the uniaxial magnetic anisotropy. A Fe2+‐CO2+ pair model was proposed to explain the uniaxial magnetic anisotropy of Co modified iron particles. To verify the nature of the Fe2+‐Co2+ pair, the probability of one or more Co ions appearing in the neighborhood of a B site Fe2+ ion was calculated using the chemical composition, the Mossbauer parameters, Pi(Fe+2) and Pi(Fe+3) and discussed conditions for forming the pair between the nearest‐neighbor Co2+ and Fe2+ ion.

Fine equiaxed γ‐CoxFe2−xO3 (x=0, 0.06) particles with a diameter ranging from 200 to 1000 A were prepared by chemical precipitation. The average crystallite sizes were determined from x‐ray line broadening measurements. The saturation magnetization and magnetocrystalline anisotropy of the particles were determined by using the approach to saturation. An empirical linear dependence of the specific saturation magnetization σs on the specific surface area Sa of the fine crystallites was obtained in the form of σs(S)=σs(∞)(1−ASa). The slope A which reflects the surface spin canting anomaly is different for γ‐Fe2O3 and γ‐Co0.06Fe1.94O3 particles. Under the supposition of the fine crystallite consisting of two parts, i.e., the surface layer, whose magnetic moment cannot be turned entirely along the direction of the applied field, but makes an average canting angle with the field, and the inner part, whose magnetic moment can be aligned along the direction of the applied field, the above formula can be interpret...

Co/Ni multilayer thin films with a constant Co thickness of 2.2 nm, Ni thickness ranging from 0.5 to 5.0 nm, and a total of 20 bilayers were fabricated using dc magnetron sputtering. These multilayers show a steep magnetoresistance (MR) change with low saturation magnetic field, in‐plane uniaxial anisotropic properties, and an especially large magnetoresistance change between magnetic field H applied parallel and perpendicular to the in‐plane easy axis. For H parallel to the multilayer easy axis and H at 45° to the current I, a MR ratio of 15% with a FWHM of 13 Oe was obtained. While for H perpendicular to the easy axis and at 45° to I, a MR ratio of 19.1% with a FWHM of 34 Oe was obtained. The former implies a MR sensitivity of 1.15%/Oe, while the sensitivity of the latter is 0.56%/Oe. All the peaks of the MR curve appeared at the coercive field Hc of the multilayers. The most important characteristics of the MR change were a sharp increase in resistance followed by a very steep decrease with increasing ...