De-Hua Han

NIFE/NIO BILAYERS WITH HIGH EXCHANGE COUPLING AND LOW COERCIVE FIELDS

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...

TEXTURE AND SURFACE/INTERFACE TOPOLOGICAL EFFECTS ON THE EXCHANGE AND COERCIVE FIELDS OF NIFE/NIO BILAYERS

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...

Effect of stress on exchange coupling field, coercivity, and uniaxial anisotropy field of NiFe/NiO bilayer thin films

The effects of stress on the exchange coupling field, Hex, and coercivity, Hc, in NiFe/NiO bilayers deposited onto a Si wafer using rf-reactive sputtering were experimentally studied. Samples of the bilayers were externally and constantly uniaxial strained (either tensile or compressive) using a specially designed sample holder. The hysteresis loops of the stressed NiFe/NiO bilayer samples were measured in situ along the easy and hard axis of the NiFe films using a vibrating sample magnetometer. The composition of the NiFe film was characterized to be Ni80.2Fe19.8 using x-ray photoelectron spectroscopy. The Hc of the bilayers were significantly affected by the stress, while the Hex was not apparently changed by the same stress. The large changes in the coercivity in the stressed NiFe/NiO bilayer were produced by the large change of the effective uniaxial anisotropy field, HK, of the bilayer. The control and reduction of both intrinsic and external stresses are important in the fabrication of spin-valve gi...

Stress effects on exchange coupling field, coercivity, and uniaxial anisotropy field of NiO/NiFe bilayer thin film for spin valves

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...

Time decay of magnetization in longitudinal CoCrTa/Cr high density thin film media

A systematic experimental study of the time decay of the magnetization (M-decay) in longitudinal CoCrTa/Cr high density thin film media with different thicknesses at various levels of in situ reverse magnetic field which simulates the demagnetization field in recorded bits is presented. Two series of thin film media with the thickness of magnetic layer ranging from 90 /spl Aring/ to 1500 /spl Aring/ were deposited by RF/DC magnetron sputtering. The magnetic properties and M-decay were measured at room temperature using a vibrating sample magnetometer and an alternating gradient force magnetometer. The emphasis was placed on the M-decay at various film thickness. It is found that the coefficient of magnetic viscosity is not a constant over 5 decades decay time (sec.), the M-decay percentage exhibits a well pronounced peak at the reverse field value-of the remanent coercivity, and the mechanism of M-decay is apparently different in thin and thick film recording media. The mechanisms of affecting the magnetic viscosities of thermally-induced time decay are explored.

Effects of surface/interface morphology on giant magnetoresistance and magnetic field sensitivity of NiO-based spin-valves

The effects of surface/interface morphology on the giant magnetoresistance (GMR) and magnetic field sensitivity (MFS) of NiFe/Co/Cu/Co/NiFe/NiO/Si and Co/Cu/Co/NiO/Si bottom spin-valve films were studied. The NiO-based spin valves were fabricated on NiO films with different surface roughnesses. A GMR of 4.02% and a MFS of 0.83%/Oe were obtained from the spin-valves with a smoother surface/interface. A GMR of 5.46% and a MFS of 0.22%/Oe in low field and a GMR of 6.26% and a MFS of 0.16%/Oe in high field were obtained from the spin-valve films with a rougher surface/interface. It was found that the smoother the surface of the spin-valve films, the smaller the GMR but the larger the MFS. On the other hand, the rougher the surface of the spin-valve films, the larger the GMR but the smaller the MFS. The mechanism for surface/interface morphology affecting the GMR and MFS is also explored.

Magnetic anisotropy of the Fe-SiO/sub 2/ and Fe/sub 20/Ni/sub 80/-SiO/sub 2/ granular solid measured using the magnetic relaxation method

A method was reported by which the effective anisotropy of the Fe-SiO/sub 2/ and Fe/sub 20/Ni/sub 80/-SiO/sub 2/ granular solid can be measured using the magnetic relaxation equations. The pure Fe-SiO/sub 2/ and Fe/sub 20/Ni/sub 80/-SiO/sub 2/ granular solid samples were fabricated using a sol-gel method in which the nanoscale bcc structure Fe and fcc structure Fe/sub 20/Ni/sub 80/ particles are dispersed in the silica matrix. The average size of the metal particles in the two discussed Fe-SiO/sub 2/ and Fe/sub 20/Ni/sub 80/-SiO/sub 2/ granular solid samples are 10 nm and 8 nm, respectively. The obtained values of the effective magnetic anisotropy are in the order of the 10/sup 6/ erg/cm/sup 3/ and almost independent with temperature from 80 K to 300 K for these granular solid samples. These values are far larger than the magnetocrystalline anisotropy for the bulk bcc structure Fe and fcc structure Fe/sub 20/Ni/sub 80/. The larger stress magnetic anisotropy may result in these larger magnetic anisotropy for these granular solid samples. It was found that the effective magnetic anisotropy of the Fe-SiO/sub 2/ granular solid is larger than that of the Fe/sub 20/Ni/sub 80/-SiO/sub 2/ granular solid, which may result from the larger stress anisotropy in Fe-SiO/sub 2/ granular solid than in Fe/sub 20/Ni/sub 80/-SiO/sub 2/ granular solid and the reason is discussed.

Steep magnetoresistance change with low saturation fields in Co/Ni multilayer thin films

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 ...

Magnetization Decay In Cr/CoCrTa Thin Film Recording Media

Area recording density in hard disk drives has been rapidly increasing.[l] To maintain sufficient level of signal-to-noise ratio (SNR) requires a reduction of grain size in recording media, at least in the same rate as the reduction of recording track width. Recent computer simulations on thermal decay of recorded transitions in thin film media have shown that in the presence of demagnetization field for closely recorded transitions, significant thermal decay could occur at area recording density lower than what is expected.[2] The key understanding yet to be achieved is the superparamagnetic behavior in a strong interactive environment, such as closely packed grains in a longitudinal film disk.

Structural, magnetic properties and steep magnetoresistance change with low saturation magnetic fields in Co/Ni multilayer thin films

We have fabricated Co/Ni multilayer thin films with a constant Co thickness of 2.2 nm, Ni thickness changing from 0.5 to 5.0 nm, and a total of 20 bilayers using dc magnetron sputtering. The magnetic measurements show that these multilayers have an in-plane uniaxial anisotropic properties. These multilayers show a steep magnetoresistance change with low saturation magnetic field, 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/spl deg/ to the current I, a magnetoresistance ratio of 15.0% with a FWHM of 13 Oe was obtained. While for H perpendicular to the multilayer easy axis and H at 45/spl deg/ to the current I, a magnetoresistance ratio of 19.1% with a FWHM of 34 Oe was obtained. The former implies a magnetoresistance sensitivity of 1.15%/Oe, while the sensitivity of the latter is just 0.56 %/Oe. All the peaks of the magnetoresistance curve appeared at the coercive field H/sub c/ of the multilayers. The most important characteristic of the magnetoresistance change were a sharp increase in resistance followed by a very steep decrease with increasing H.