Magneto-transport properties of [Co/Bi]n wire structures

The present study examines the artificial control of grain-boundary resistance and its contribution to the magneto-transport properties of [Co(1 nm)/Bi(2.5 nm)]_n (n=10 or 20) line structures on the Si(0 0 1)/SiN_x substrate. Conventional patterning and deposition processes are applied for the fabrication of a device that consists of five-line structures with a line width of 2 μm. A ΔR/R=80% ratio was observed in the five-line structure of [Co(1 nm)/Bi(2.5 nm)]₁₀ multilayers at 10 K. Our measurements indicate that grain-boundary effects can be associated with the large ΔR/R ratio of transverse magnetoresistance.     

Soft magnetic properties of FeCo films with Co underlayer

Bilayered Fe₆₅Co₃₅ (=FeCo)/Co films were prepared by facing targets sputtering with 4πM_s∼24 kg. The soft magnetic properties of FeCo films were induced by a Co underlayer. H_c decreased rapidly when the Co underlayer was 2 nm or more. The films showed well-defined in-plane uniaxial anisotropy with the typical values of H_ce=10 Oe and H_ch=3 Oe, respectively. High frequency characteristics of the films show the films can work at 0.8 GHz with real permeability as high as 250.     

Effect of substrate temperature on structure and electrical resistivity of laser-ablated IrO2 thin films

IrO₂ thin films were prepared on Si(1 0 0) substrates by laser ablation. The effect of substrate temperature (T_sub) on the structure (crystal orientation and surface morphology) and property (electrical resistivity) of the laser-ablated IrO₂ thin films was investigated. Well crystallized and single-phase IrO₂ thin films were obtained at T_sub = 573-773 K in an oxygen partial pressure of 20 Pa. The preferred orientation of the laser-ablated IrO₂ thin films changed from (2 0 0) to (1 1 0) and (1 0 1) depending on T_sub. With the increasing of T_sub, both the surface roughness and crystallite size increased. The room-temperature electrical resistivity of IrO₂ thin films decreased with increasing T_sub, showing a low value of (42 ± 6) × 10⁻⁸ Ω m at T_sub = 773 K.     

Effect of Co layer thickness on the magnetic properties of Tb30Co70/SiNx/Co thin films

The SiN_x (20 nm)/Tb₃₀Co₇₀ (90 nm)/SiN_x (5 nm)/Co (3–37 nm)/SiN_x (10 nm)/Si multilayer films are deposited on naturally oxidized Si wafer by magnetron sputtering. The saturation magnetization (M_s) of the multilayer films is increased with the thickness of high M_s ferromagnetic Co layer. The perpendicular coercivity (_Hc⊥$H_{{\mathrm{c}}^{\perp }}$) value is increased with Co layer thickness as the thickness of the Co layer is lower than 15 nm and then decreases drastically when the thickness of the Co layer further increased. The increase of the _Hc⊥$H_{{\mathrm{c}}^{\perp }}$ value is owing to the interlayer exchange effect [Li Zhang, Physica B 390 (2007) 373] between TbCo and Co layers. Co under-layer with in-plane magnetic anisotropy would pin the magnetic moment of the TbCo layer near by the Co layer and cause the value of _Hc⊥$H_{{\mathrm{c}}^{\perp }}$ to increase. However, as the Co layer is thicker than a critical thickness, the _Hc⊥$H_{{\mathrm{c}}^{\perp }}$ value of the multilayer film would decrease. Therefore, the Co layer with in-plane magnetic anisotropy and soft magnetic properties is expected to dominate the magnetic properties of the multilayer films.     

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO₃ (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42μ_B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21μ_B/Co and 0.65μ_B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.     

Electronic transport properties and Microstructures of TiO2:Co magnetic semiconductors

TiO₂:Co thin films of high Co concentration were investigated by the high resolution transmission electron microscopy, physical property measurement system and energy dispersive X-ray. The as-deposited films are amorphous magnetic semiconductors and we did not find any Co metal particles in them. The electronic transport process in the low-temperature range below 80 K could be described by the spin-dependent variable-range-hopping process. However, after the samples were annealed at 300 °C, large amounts of Co metal particles were observed to connect each other and the films show a metallic behavior. The origin of ferromagnetism of the thin films is also discussed.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Giant Faraday rotation in Fe–ZnSe granular films

The giant magneto-optical Faraday effect (giant Faraday rotation) of ferromagnetic metal–semiconductor matrix Fe–ZnSe granular films prepared by radio frequency sputtering is studied. The result shows that the Faraday rotation angle θ_F value of the granular films sample with Fe volume fraction x=35% is of the order of 10⁵ °/cm at room temperature. Temperature dependence of the θ_F of Fe_0.35(ZnSe)_0.65 granular films shows that θ_F value, below 150 K, increases rapidly with the decrease of the temperature, and when T=10 K θ_F value is 6×10⁵ °/cm. Through the study of the structure and dependence of magnetic properties on temperature, it has been found that the remarkable increase of the θ_F value of Fe_0.35(ZnSe)_0.65 granular films below 150 K seems to arise from the sp–d exchange interaction inside the granular films.     

Magnetic properties, microstructures and magnetoresistance effect in Co/Alq3 granular films

A series of Co_x(Alq₃)_1−x granular films were prepared using the co-evaporating technique. HRTEM images show typical characteristics of granular films with the average size of 2-5 nm. It shows a gradual change from superparamagnetism to ferromagnetism with the rise of x. Negative magnetoresistance (MR) was observed, reaching −5.31% in x=0.44 sample at 30 K under the field of 10 kOe. The concentration dependence of MR and resistivity is also investigated. The inter-particle tunneling is believed to account for the negative MR effect.     

Exchange bias in Co nanoparticles embedded in an Mn matrix

Magnetic properties of Co nanoparticles of 1.8 nm diameter embedded in Mn and Ag matrices have been studied as a function of the volume fraction (VFF). While the Co nanoparticles in the Ag matrix show superparamagnetic behavior with T_B=9.5 K (1.5% VFF) and T_B=18.5 K (8.9% VFF), the Co nanoparticles in the antiferromagnetic Mn matrix show a transition peak at ∼65 K in the ZFC/FC susceptibility measurements, and an increase of the coercive fields at low temperature with respect to the Ag matrix. Exchange bias due to the interface exchange coupling between Co particles and the antiferromagnetic Mn matrix has also been studied. The exchange bias field (H_eb), observed for all Co/Mn samples below 40 K, decreases with decreasing volume fraction and with increasing temperature and depends on the field of cooling (H_fc). Exchange bias is accompanied by an increase of coercivity.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.     

Coexistence of positive and negative exchange bias in CrMn/Co bilayers

Exchange-biased CrMn/Co bilayers with various thicknesses of Co sputtered onto Si(1 0 0) substrates by the RF sputtering system have been studied. Double-shifted loops have been observed with the thickness of Co layer in a narrow range and become single-shifted loops after some cycles of measurement. Those results are interpreted as the association of positive and negative exchange bias.     

Influence of spin-glass-like phase on magnetic and electrical transport properties of reactive sputtered Al1−xFexN films

Nanocrystalline Al_1−xFe_xN films were fabricated using the reactive sputtering method. A large amount of spin-glass-like moments are in the films. With the decrease of temperature, the films turn from the spin-glass-like behavior to ferromagnetism. At low temperatures, the saturation magnetization increases and the coercivity decreases with the increase of x. The coercivity increases significantly below 50 K due to the pinning effect of the frozen disordered spin-glass-like moments. All of the films are semiconducting. The low-temperature transport mechanism turns from tunneling to hopping as x increases. Magnetoresistance (MR) shows weak saturation trend with the applied field because of the hard alignment of the frozen moments. Meanwhile, MR follows the relation of log |MR|=a+bT−1, and the spin polarization satisfies P(T)=P₀e−βTα, related with the disordered spin-glass-like moments.     

Planar Hall effect and magnetoresistance in Ni81Fe19 and Co square shaped thin films

Ni₈₁Fe₁₉ and Co thin films have been fabricated and their transport properties have been investigated for potential applications in ultra sensitive magnetic field sensors. The Ni₈₁Fe₁₉ films exhibit an anisotropic magnetoresistance (AMR) of 2.5% with a coercivity 2.5 Oe and the Co films exhibit an AMR of 0.7% with coercivity 11 Oe. Large planar Hall effect magnetoresistance values at room temperature are reported for both cases. An unbalanced Wheatstone bridge model is proposed to describe quantitatively the observed experimental Planar Hall Effect data.     

Effects of substrates on the structure and dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films

Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films were grown on Pt(1 1 1)/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt(1 1 1)/Ti/SiO₂/Si substrates by a sol-gel processing technique, respectively. The BTS thin films deposited on annealed Pt(1 1 1)/Ti/SiO₂/Si and annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates exhibited strong (1 1 1) and perfect (1 0 0) orientations, respectively. The BTS thin films grown on un-annealed Pt(1 1 1)/Ti/SiO₂/Si substrates showed random orientation with intense (1 1 0) peak, while the films deposited on un-annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrate exhibited random orientation with intense (1 0 0) peak, respectively. The dielectric constant of the BTS films deposited on annealed Pt(1 1 1)/Ti/SiO₂/Si, annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si, un-annealed Pt(1 1 1)/Ti/SiO₂/Si and un-annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates was 512, 565, 386 and 437, respectively, measured at a frequency of 100 kHz. A high tunability of 49.7% was obtained for the films deposited on annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrate, measured at the frequency of 100 kHz with an applied electric field of 200 kV/cm. The high tunability has been attributed to the (1 0 0) texture of the films and larger grain sizes.     

Influence of microstructures on exchange bias behaviors of La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 bilayers

Ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and antiferromagnetic La_0.33Ca_0.67MnO₃ (LCMO) layers were grown on SrTiO₃ (STO) substrates by the pulsed laser deposition technique. LSMO films had rougher surfaces and larger grain sizes than LCMO films. Fully strained bilayers, in which each layer was as thin as 10 nm, were prepared by changing their stacking sequences, i.e. LSMO/LCMO/STO and LCMO/LSMO/STO. The former had higher T_C (350 K) than the latter (300 K), and exchange bias effects were only observed in the former bilayers. This revealed that microstructures could play an important role in the transport and magnetic properties of manganese oxide thin films.     

Microstructures and coercivities of SmCox and Sm(Co,Cu)5 films prepared by magnetron sputtering

We have investigated the influence of composition and annealing conditions on the magnetic properties and microstructural features of SmCo_x films that were prepared by sputtering and subsequent annealing. A huge in-plane coercivity of 5.6 T was obtained from an optimally annealed Sm–Co film, which was attributed to the nanometer sized polycrystalline microstructure of the highly anisotropic SmCo₅ phase. Although a high density of planar defects were observed in the films that were annealed at high temperatures, they did not act as strong pinning sites for domain wall motion. The effect of Cu on [SmCo_4.5(9 nm)/Cu(xnm)]₁₀ multilayer thin films was also studied. An appropriate Cu content increased the coercivity.     

Out-of-plane exchange bias and magnetic anisotropy in MnPd/Co multilayers

Magnetic and structural properties in [MnPd/Co]₁₀ multilayers deposited onto Si(1 1 1) substrates have been investigated. The dependences of anisotropy and exchange bias on the thicknesses of both MnPd and Co layers have been studied. In most of the samples, the out-of-plane magnetic anisotropy and both large out-of-plane and in-plane exchange biases have been observed at cryogenic temperature below the blocking temperature T_B≈240 K. With appropriate MnPd and Co thicknesses, we have obtained samples with a large out-of-plane exchange bias along with a large out-of-plane magnetic anisotropy. The origin of the out-of-plane magnetic anisotropy in the samples has been suggested to be due to the formation of CoPd interfacial alloys which have tensile in-plane strains, while the spin structure of the antiferromagnetic layer at the interface which is believed to be responsible for exchange bias may be the same as that of the bulk material. Also, the present study shows that the interplay between the out-of-plane magnetic anisotropy and exchange bias is evident in our multilayers and plays an important role in the out-of-plane exchange-bias mechanism.     

Third-order optical nonlinearities of lead-free (Na1−xKx)0.5Bi0.5TiO3 thin films

(Na_1−xK_x)_0.5Bi_0.5TiO₃ (NKBT) (x = 0.1, 0.2, and 0.3) thin films with good surface morphology and rhombohedral perovskite structure were fabricated on quartz substrates by a sol-gel process. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties were investigated by Z-scan technique performed at 532 nm with a picosecond laser. A two-photon absorption effect closely related with potassium-doping content was found in thin films, and the nonlinear refractive index n₂ increases evidently with potassium-doping. The real part of the third-order nonlinear susceptibility χ⁽³⁾ is much larger than its imaginary part, indicating that the third-order optical nonlinear response of the NKBT films is dominated by the optical nonlinear refractive behavior. These results show that NKBT thin films have potential applications in nonlinear optics.