Enhancement of MR ratios using thin oxide layers in PtMn and α-Fe2O3-based spin valves

Spin valves having thin oxide layers in the pinned and/or free layers were prepared by sputtering. MR ratios of the spin valves were increased from 8.1 to 11.9% by inserting the oxide layer into the pinned layer in Ta/PtMn/CoFe/Cu/CoFe/Ta spin valves. MR ratio of 13.9% and considerably large sheet ΔR of 2.55 Ω were obtained in the PtMn-based spin valves having the oxide layer in the pinned and free layer. Larger MR ratio of 17.3% and the sheet ΔR of 1.3 Ω were obtained in the PtMn-based dual-type spin valves having the oxide layer in both pinned layers. α-Fe₂O₃ based spin valves having thin oxide layers were also prepared. MR ratios of the spin valves were increased from 11.9 to 14.3% by inserting the oxide layer into the free layer in α-Fe₂O₃/CoFe/Cu/CoFe/Ta spin valves. The enhancement of the MR ratios may be attributed to the specular scattering effect of the conduction electrons by the thin oxide layers.     

Synthetic Co50Pt50/Ru/CoFe hard–soft trilayer in spin valves

The influence of deposition power and seedlayer on the properties of hard magnet Co₅₀Pt₅₀ was studied. Co₅₀Pt₅₀(/Co₉₀Fe₁₀)/Ru/Co₉₀Fe₁₀ trilayer was used as pining/pinned layer in spin valves. The influences of different hard layer, soft layer and free layer on exchange bias, interlayer coupling, and magnetoresistance (MR) ratio were studied. Weak antiferromagnetic interlayer coupling was obtained by adjusting the thickness of hard and soft layers. MR of a spin valve with structure Cr2/CoFe0.5/CoPt4/CoFe0.5/Ru0.8/CoFe2.2/Cu2.05/CoFe2.6/Cu1.1/Ta1 reached 10.68% (unit in nm), which is comparable to those of IrMn-based synthetic spin valves. The increment of the coercivity of the free layer is mainly due to the static magnetic interaction between the hard layer and the free layer.     

Annealing effect of NiO/Co90Fe10 thin films: From bilayer to nanocomposite

Exchange-biased bilayers are widely used in the pinned layers of spintronic devices. While magnetic field annealing (MFA) was routinely engaged during the fabrication of these devices, the annealing effect of NiO/CoFe bilayers is not yet reported. In this paper, the transition from NiO/Co₉₀Fe₁₀ bilayer to nanocomposite single layer was observed through rapid thermal annealing at different temperatures under magnetic field. The as-deposited and low-temperature (<623 K) annealed samples had rock salt (NiO) and face center cubic (Co₉₀Fe₁₀) structures. On the other hand, annealing at 623 K and 673 K resulted in nanocomposite single layers composed of oxides (matrix) and alloys (precipitate), due to grain boundary oxidization and strong interdiffusion in the NiO/CoFe and CoFe/SiO₂ interfaces. The structural transition was accompanied by the reduction of grain sizes, re-ordering of crystallites, incensement of roughness, and reduction of Ni²⁺. When measured at room temperature, the bilayers exhibited soft magnetism with small room-temperature coercivity. The nanocomposite layers exhibited an enhanced coercivity due to the changes in the magnetization reversal mechanism by pinning from the oxides. At 10 K, the increased antiferromagnetic anisotropy in the NiO resulted in enhanced coercivity and exchange bias in the bilayers. The nanocomposites exhibited weaker exchange bias compared with the bilayers due to frustrated interfacial spins. This investigation on how the magnetic properties of exchange-biased bilayers are influenced by magnetic RTA provides insights into controlling the magnetization reversal properties of thin films.     

TiN underlayer and overlayer for TbFeCo perpendicular magnetic recording media

TiN thin film is prepared by DC reactive sputtering in Ar+N₂ atmosphere and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media as well as its effect on the magnetic properties of the latter have been studied. Only 5 nm TiN overlayer and 20 nm under layer can successfully protect the TbFeCo film from oxidation. Initially the coercivity is increased sharply from about 2 to 6 kOe for an increase of underlayer thickness to 60 nm then the increasing rate of coercivity becomes very slow. The saturation magnetization remains almost constant with the underlayer thickness. The remanent squareness ratio remains constant at 1.0 with the underlayer thickness up to 60 nm then decreases.     

Preference orientation and magnetic properties of CoxFe3−xO4 nanocrystalline thin films on quartz substrate

Nanocrystalline spinel ferrite thin films of Co_xFe_3−xO₄ (x=0.3$x=0.3$, 0.5, 0.8, and 1.0) have been prepared by RF sputtering on quartz substrate without a buffer layer at room temperature and annealed at the temperature range from 200 to 600 °C in air. The as-sputtered films exhibit the preferred orientation and the high magnetization and coercivity. After annealing, the preferred orientations become poor, but the magnetization and coercivity increase. The sample with a magnetization of 455 emu/cm³, a coercivity of 2.8 kOe, a remanence ratio of 0.72, and a maximum energy product of 2.4 MGOe has been obtained. The influence of Co ions and annealing temperature on the magnetic properties has been discussed.     

Phase segregation and exchange biasing in TbFeCo films with perpendicular anisotropy

A TbFeCo film was deposited by DC magnetron sputtering and studied by transmission electron microscopy, polar and longitudinal magneto-optical Kerr effect, and magnetometry measurements. Transmission electron microscopy has shown the existence of lateral compositional inhomogeneity. Magneto-optical measurements have shown that the initial layer at the bottom consists of only magnetic perpendicular component and the top surface layer has a compositional inhomogeneity and consists of in-plane components and perpendicular one. The perpendicular components in the bottom and the surface layers have identical composition. Two in-plane components have been shown by magnetometry measurements. It is shown that phase segregation exists in the TbFeCo film and possible form of compositional inhomogeneity has been discussed. The two in-plane components are exchange coupled with a magnetization off-alignment of 35°. For the soft in-plane component, the in-plane and out-of-plane angular dependence of the exchange biasing is similar to those of the conventional one. Within temperatures from 100 to 300 K, the exchange field and the coercivity are both linear functions of temperature.     

Magnetoresistance study and interlayer coupling of CoFe/Os/CoFe thin films

The CoFe/Os/CoFe thin films were deposited on natural oxidized Si(1 0 0) substrates at room temperature by an ultra-high vacuum DC-magnetron sputtering system with a base pressure less than 1×10⁻⁸ Torr. The thickness of the ferromagnetic layers was 100 Å in all cases and a series of trilayers with Os spacer ranging from 3 to 20 Å was made. Effects of the Os layer thickness on the magnetoresistance (MR) and magnetic properties were investigated. The results showed that the magnetism switched from ferromagnetic (Os thickness=3, 5 Å) to antiferromagnetic (Os thickness=7–13 Å) and then ferromagnetic (Os thickness=20 Å) again. From the MR study, we see that the AMR ratio decreased from 4.64% to the minimum value 0.69% at 9 Å and then increased; GMR ratio increased from 0.01% to the maximum value 0.43% at 9 Å and then decreased. From the hysteresis loops, the results exhibited that coercivity increased from 16 Oe to the maximum value 92 Oe at 9 Å and then decreased, and squareness value decreased from 0.97 to the minimum value 0.17 at 9 Å and then increased. Dependence of saturation field on Os spacer-layer thickness for CoFe trilayers showed a maximum value 216 Oe at 9 Å. This suggests that the small GMR effect may be related to the small exchange coupling strength in CoFe/Os/CoFe thin films.     

Magnetic,structural and electrical properties of ordered and disordered Co50Fe50 films

Co₅₀Fe₅₀ films with thickness varying from 100 to 500 Å were deposited on a glass substrate by sputtering process, respectively. Two kinds of CoFe films were studied: one was the as-deposited film, and the other the annealed film. The annealing procedure was to keep the films at 400 °C for 5 h in a vacuum of 5×10⁻⁶ mbar. From the X-ray study, we find that the as-deposited film prefers the CoFe(1 1 0) orientation. Moreover, the body-centered cubic (bcc) CoFe(1 1 0) line is split into two peaks: one corresponding to the ordered body-centered tetragonal (bct) phase, and the other, the disordered bcc phase. After annealing, the peak intensity of the ordered bct phase becomes much stronger, while that of the disordered bcc phase disappears. The annealing has also caused the ordered CoFe(2 0 0) line to appear. When the amount of the ordered bct phase in Co₅₀Fe₅₀ is increased, the saturation magnetization (M_s) and coercivity (H_c) become larger, but the electrical resistivity (ρ) decreases. From the temperature coefficient of resistance (TCR) measurement, we learn that the bct grains in the CoFe film start to grow at temperature 82 °C.     

Temperature dependence of current-induced magnetization switching in spin valves with a ferrimagnetic CoGd free layer

The temperature dependence of current-induced magnetization switching of ferrimagnetic CoGd free layers in spin valves is explored. At temperatures well above and well below the magnetization compensation temperature (T(MC)) of CoGd, a current flowing from the free layer to the CoFe fixed layer aligns the moments of the two layers parallel, and a current flowing in the opposite direction aligns them antiparallel. However, for intermediate temperatures just above T(MC), the current-induced alignment of the moments is reversed. We attribute this effect to the different compensation temperatures of the net magnetization and angular momentum of CoGd.     

Thermal Stability of CoFe/Cu/CoFe/IrMn Top Spin Valve

We present a study of thermal stability of the top spin valve with a structure of seed Ta （Snm）/Co75Fe25 （5 nm ） /Cu （2.5 nm） /Co75Fe25 （5 nm ） /Ir20Mn80（12 nm） /cap Ta （8 nm） deposited at room temperature by magnetron sputtering. A vibrating sample magnetometer fixed with a heater was used to record the magnetic hysteresis loops at variational temperatures and x-ray diffraction was performed to characterize the structure of the multilayer. The exchange field Hex and the coercivity of the pinned CoFe layer Hop decrease monotonically with increasing temperature. The coercivity of the free CoFe layer Hcf in the spin valve shows a maximum at 498K. The temperature dependences of Hex, Hop and Hcf have also been discussed.     

High temperature magnetoresistance in (Co,CoFe and NiFe)/Cu/CoFe/IrMn spin-valves

The MR characteristics of temperature variations ranging from room temperature to 570 K for IrMn spin-valves which consist of free layer Co, CoFe and NiFe were studied. Co-SV had the highest MR value in all temperature ranges among them and even at 510 K the MR ratio held more than half of room temperature value. Whereas CoFe-SV had a slightly higher MR than NiFe-SV below 450 K, above 450 K CoFe-SV showed a lower MR ratio than NiFe-SV. MR loops of large coercivity such as CoFe free magnetic layer collapsed in the lower elevated temperature compared to NiFe and Co. Small coercivity of the free magnetic layer would be preferable to spin-valves.     

Magnetization switching of CoFeSiB free-layered magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprising amorphous Co_70.5Fe_4.5Si₁₅B₁₀, possessing low saturation magnetization of 560 emu/cm³, as a free layer have been investigated. The switching behaviours were confirmed for the micrometer-sized elements experimentally, using the scanning magneto-optical Kerr effect (scanning MOKE). A micromagnetic modelling study was also carried out for the submicrometer-sized elements. By using either a CoFeSiB single or a synthetic antiferromagnetic free-layer structure, the magnetization switching field became much lower than conventionally used CoFe free layered MTJs.     

Exchange interaction effect of TbCo/FePt bilayers

Interlayer exchange coupling in dc-magnetron sputtered Tb_29.6Co_70.4/FePt bilayers with different annealing temperatures of the FePt film have been investigated. The dependence of ordering degree on perpendicular magnetic properties of the FePt film was studied. The Tb_29.6Co_70.4/FePt film has high perpendicular coercivity and high saturated magnetization about 7.5 kOe, and 302 emu/cm³, respectively as the substrate temperature is 500 °C and annealing at 500 °C for 30 min. It also shows a strong exchange coupling between this FePt layer and Tb_29.6Co_70.4 layer. We also examined the interface wall energy in the exchange coupled Tb_29.6Co_70.4/FePt double layers.     

Spin valve with a composite dysprosium-based pinned layer as a tool for determining Dy nanolayer helimagnetism

Spin valves with nanostructure CoFe/Dy/CoFe and three-layer structures metal/Dy/metal were prepared by magnetron sputtering. The measurements of field dependences of magnetoresistance and magnetization were held at different temperatures. The changes of magnetotransport properties of spin valve containing CoFe/Dy/CoFe structure were used for getting information on the magnetic ordering in the dysprosium layer. The characteristic changes of magnetotransport properties caused by the formation of helical ordering in dysprosium layer were detected. Special attention was paid to the estimation of Neel temperature and to the investigation of microstructure of dysprosium nanolayer.     

Studies of magnetization reversals about two kinds of MR curves observed in the spin valve using the (110) magnetite pinning layer

In the spin valves composed of Co/Cu/Co on the epitaxial (110) Fe₃O₄ as the pinning layer, we found out that shapes of magnetoresistance (MR) curves depended on thickness of the cobalt pinned layer (PL) with the field applied in the 〈110〉 direction of Fe₃O₄: (1) the flat-shaped MR curve showed low MR ratio under 2 nm thickness of cobalt pinned layer (PL): (2) the unusually shaped MR curve showed high MR ratio over 5 nm thickness of PL in spite of the hard direction of Co layers. We assumed that the synchronous magnetization reversal (SR) of PL and Fe₃O₄ would occur at the MR switching field due to 90° coupling between PL and Fe₃O₄ layers. Then, only occurrence of SR of PL cause the drastic change of the magnetization relative angle between FL and PL, indicating the observation of the unusually shaped MR curve having high MR ratio. On the other hand, the SR of cobalt free layer (FL) together with the PL flip also occur due to the large contribution of Néel-type ferromagnetic coupling between FL and PL, which lead to less changing the relative angle of FL and PL during magnetization processes, indicating the observation of a flat-shaped MR curve having low MR ratio. This dependence of PL thickness on MR curves might come from the balance of Néel (ferromagnetic) and stray field (antiferromagnetic) coupling due to magnetic free pole at edge of PL.     

FMR studies in the TbFeCo/GdFeCoSi bilayers

Bilayers, TbFeCo/GdFeCoSi, made by sputtering on glass substrate with buffer and capping layers were studied by measuring the hysteresis loop and by ferromagnetic resonance (FMR). When the field H was applied along the film normal, a double H_C hysteresis loop related to the two sublayers was observed. In ferromagnetic resonance measurements, a peculiar out-of-plane angular dependence of FMR spectrum was obtained. When scanning field H was 0-637 kA/m less than the anisotropy field of TbFeCo sublayer, two FMR peaks were observed. One peak was characteristic of uniaxial and unidirectional anisotropy. The anisotropy constants were obtained by fitting the data with the theory of FMR, and this peak was considered to be related to the low anisotropy GdFeCoSi layer. The second peak appeared only when the dc field H was orientated in a limited angular range around 180°. This peak was considered to be related to an uncoupled interfacial GdFeCoSi sublayer near Al capping layer. However, when H was scanned between 0-1114 kA/m, only one peak is observed due to magnetization reversal of TbFeCo layer with uniaxial anisotropy.     

Enhanced antiferromagnetic coupling in dual-synthetic antiferromagnet with Co2FeAl electrodes

We study dual-synthetic antiferromagnets (DSyAFs) using Co₂FeAl (CFA) Heusler electrodes with a stack structure of Ta/CFA/Ru/CFA/Ru/CFA/Ta. When the thicknesses of the two Ru layers are 0.45 nm, 0.65 nm or 0.45 nm, 1.00 nm, the CFA-based DSyAF has a strong antiferromagnetic coupling between adjacent CFA layers at room temperature with a saturation magnetic field of ∼11,000 Oe, a saturation magnetization of ∼710 emu/cm³ and a coercivity of ∼2.0 Oe. Moreover, the DSyAF has a good thermal stability up to 400 °C, at which CFA films show B2-ordered structure. Therefore, the CFA-based DSyAFs are favorable for applications in future spintronic devices.     

Enhancement of L10 ordered FePt by Pt buffer layer

L₁₀-ordered FePt thin films prepared by molecular-beam epitaxy on MgO (0 0 1) substrate at 320 °C with different thickness of Pt buffer layer have been investigated. The out-of-plane coercivity increases with increasing thickness of Pt buffer. The maximum values of the long-range order parameter and uniaxial magnetic anisotropy energy are 0.72 and 1.78×10⁷ erg/cm³, respectively, for films with 12 nm thick Pt buffer layer, where the c/a ratio (0.976) shows the minimum value. The reason for the enhancement in ordering is due to the proper lattice strains Pt buffer bestows on FePt layer, these strains are equal to the contraction in lattice parameter c and the expansion in a. Studies of angular-dependent coercivity revealed that the magnetization reversal behaviour shifts from a domain-wall motion dominated case towards a near rotational mode with increasing thickness of Pt buffer layer.     

Structure and property changes and femtosecond laser-induced magnetization dynamics of two-year-old high coercive TbFeCo films

An uncoated TbFeCo film on glass substrate exposed to dry air for 2 years is studied using magneto-optical polar Kerr spectroscopy and vibrating sample magnetometry (VSM). The out-of-plane hysteresis loop by VSM is found to become slanted and meanwhile, an in-plane loop is also observable. The Kerr loop measured from the exposed surface is also slanted compared to that measured from the substrate-contacted surface, which indicates that the exposed surface is oxidized. So a bilayer structure of the film is proposed. When a pump laser is switched on, the Kerr loops measured from both surfaces at the delay time of −5 ps become anomalous, showing the occurrence of magnetization reversal across magnetization compensation temperature. Unlike that measured from the substrate-contacted surface, femtosecond laser-induced magnetization dynamics measured from the exposed surface does not show magnetization reversal crossing zero magnetization. This can be explained by the bilayer structure as the compensation effect of demagnetization recovery of the oxidized layer on magnetization reversal of perpendicularly anisotropic TbFeCo layer across magnetization compensation temperature. Above experimental results show that the uncoated TbFeCo film cannot resist oxidation in dry air at room temperature for 2 years while the SiO₂-coated surface can do so for over 2 years.     

Bias voltage dependence of inverted magnetoresistance on the annealing temperature in MgO-based magnetic tunnel junctions

MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir₂₂Mn₇₈ or Fe₅₀Mn₅₀ (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5?t<2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (V_C) when an unbalanced synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. V_Cs change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different V_Cs by artificial control. The asymmetric V_C values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface.