Structural origin of perpendicular magnetic anisotropy in Ni–W thin films

The magnetic properties and microstructure of electrodeposited Ni–W thin films (0–11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0–2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2–7 at% W), and an amorphous phase (>7 at% W). In the transition region, (4–5 at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni–W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni–W films with PMA is composed of isolated columnar crystalline grains (27–36 nm) with the FCC phase surrounded by the Ni–W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni–W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni–W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of 〈1 1 1〉 columnar grains and its shape magnetic anisotropy. It is concluded that Ni–W electrodeposited films (4–5 at% W) may be applicable for perpendicular magnetic recording media.     

Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.     

Magnetic properties of nanocrystalline Fe86.7Zr3.3B4Ag6 thin film

The changes of magnetic properties with annealing temperature were studied in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film. The thin films were deposited by a DC magnetron sputtering method, annealed at 300–700°C for 1 h in vacuum under a field of 1.5 kOe parallel to the film plane, and then furnace-cooled. As a result, it has been found that the Ag addition to Fe–Zr–B amorphous thin films resulted in the decrease of crystallization temperature to 400°C due to promoted crystallization ability. Also, it gave rise to formation of fine BCC α-Fe crystalline precipitates with a grain size smaller than 10 nm in the amorphous matrix near 400°C, and led to prominent enhancement in the magnetic properties of the Fe_86.7Zr_3.3B₄Ag₆ thin films. Significantly, excellent magnetic properties such as a saturation magnetization of 1.7 T, a coercive force of 1 Oe and a permeability of 7800 at 50 MHz were obtained in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film containing 7.2 nm-size BCC α-Fe, which was annealed at 400°C. Also, core loss of 1.4 W cm⁻³ (B_m=0.1 T) at 1 MHz in the thin film was obtained, and it is a much lower value than had been obtained in any existing soft magnetic materials. Such excellent properties are inferred to originate from the uniform dispersion of nano-size BCC α-Fe in the amorphous matrix.     

A thin film magnetic field sensor of sub-pT resolution and magnetocardiogram (MCG) measurement at room temperature

We developed a very sensitive high-frequency carrier-type thin film sensor with a sub-pT resolution using a transmission line. The sensor element consists of Cu conductor with a meander pattern (20 mm in length, 0.8 mm in width, and 18 μm in thickness), a ground plane, and amorphous CoNbZr film (4 μm in thickness). The amplitude modulation technique was employed to enhance the magnetic field resolution for measurement of the high-frequency field (499 kHz), a resolution of 7.10×10^?13 T/Hz^1/2 being achieved, when we applied an AC magnetic field at 499 kHz. The phase detection technique was applied for measurement of the low frequency field (around 1 Hz). A small phase change was detected using a dual mixer time difference method. A high phase change of 130°/Oe was observed. A magnetic field resolution of 1.35×10^?12 T/Hz^1/2 was obtained when a small AC field at 1 Hz was applied. We applied the sensor for magnetocardiogram (MCG) measurement using the phase detection technique. We succeeded in measuring the MCG signal including typical QRS and T waves, and compared the MCG with a simultaneously obtained conventional electrocardiogram (ECG) signal.     

The influence of the geometry of micro-patterned thin films on the effective permeability and resonance frequency by domains development

For the application in high-frequency micromagnetic devices, the permeability and resonance frequency of ferromagnetic components is of high interest. It is dominantly influenced by different factors, the external field and direction and the domain distribution, shape and orientation. By the use of micromagnetic simulation, the domain pattern in films was determined and the effective permeability was calculated. The results of the calculations were compared with the domain shape of patterned microstructures of thin FeCoTaN-films, which were deposited onto oxidised silicon substrates by reactive r.f.-magnetron sputtering by employing 6-in Fe₃₇Co₄₆Ta₁₇ targets. To achieve a high-frequency suitability, the films have to be annealed in a static magnetic field of 50 mT between 400 and 500 °C, which are typical temperatures used in CMOS processes, to induce an in-plane uniaxial anisotropy needed for the high-frequency performance. Magnetic softness was obtained by producing amorphous or nanocrystalline films, and additionally, by aspiring low magnetocrystalline anisotropies for, e.g., certain Fe/Co fractions. The unpatterned films with a lateral dimension of 5×5 mm² were measured in a strip line permeameter in a frequency range up to 5 GHz and exhibited ferromagnetic resonance frequencies between 2 and 2.5 GHz within a low-loss permeability spectrum (low width of imaginary part of permeability). For possible integrations in passive microelectronic components the films were patterned to a few tenths of micrometers by near ultra-violet lithography and plasma beam etching, and then consequently annealed to obtain the static and dynamic magnetic properties. To influence the amount of closure domains, designs were conceived to influence the domain formation by creating additional internal boundaries. As a result, the ferromagnetic resonance frequency and the effective permeability are strongly driven by internal and external boundaries.     

High resolution measurement and modelling of magnetic domain structures in epitaxial FePd (0 0 1) L10 films with perpendicular magnetisation

Magnetic domain structures in two 50 nm thick chemically-ordered FePd (0 0 1) epitaxial films with different perpendicular anisotropies have been studied using Lorentz microscopy. Domain and domain wall structures vary significantly according to the magnitude of the anisotropy. For lower anisotropy films, a stripe domain structure with a period of ≈100 nm is formed in which there is a near-continuous variation in orientation of the magnetisation vector. By contrast, in the film with higher anisotropy, a maze-like domain structure is supported. The magnetisation within domains is perpendicular to the film plane and adjacent domains are separated by narrow walls, less than 20 nm wide. Micromagnetic modelling is generally in good quantitative agreement with experimental observations and provides additional information on the domain wall structure.     

Field electron emission from amorphous carbon thin films grown by RF magnetron sputtering

Using a RF magnetron sputtering, amorphous carbon (a-C) and N-doped a-C (a-C:N) thin films were fabricated as field electron emitter. These thin films were deposited on Si(0 0 1) substrate at several temperatures. The field emission property was improved for a-C thin films grown at higher substrate temperatures. Furthermore, a-C:N film exhibits field emission property better than that of undoped a-C film. These results are explained in terms of the change in surface morphology and structural properties of a-C film.     

Structural and magnetic properties of single-step electrochemically deposited nanocrystalline cobalt ferrite thin films

In this paper, we study the structural, surface morphological and magnetic properties of single-step electrochemically deposited cobalt ferrite thin films. The prepared films were nanocrystalline with cubic crystal structure. Scanning electron micrograph image showed that the cobalt ferrite thin film was uniformly distributed over the substrate in addition to some random overgrowth of porous particles. The saturation magnetization of 298 emu/c was confirmed when films were used in magnetic studies.     

A comparative study of the magnetocaloric effect in Gd3Co and Gd3Ni

Magnetic and magnetocaloric properties of polycrystalline samples of Gd₃Co and Gd₃Ni have been studied. Both these compounds are antiferromagnets and undergo metamagnetic transitions in the antiferromagnetic phase. The Neel temperatures are found to be 128 and 99 K, respectively for Gd₃Co and Gd₃Ni. Though both these compounds have the same crystal structure, their magnetic structures seem to be different. It is found that Gd₃Ni possesses larger magnetic anisotropy compared to Gd₃Co. The maximum values of isothermal magnetic entropy change are 11 and 18.5 J Kg⁻¹ K⁻¹ for Gd₃Co and Gd₃Ni respectively, while their refrigerant capacities are 2.6 J cm⁻³ for both of them. Magnetic entropy change in the paramagnetic region shows a quadratic dependence on the magnetic field in the case of Gd₃Ni, indicating the presence of spin fluctuations above the Neel temperature.     

Irreversible nanoscale morphology transformation of an Fe film on Mo(1 1 0) induced by a magnetic STM tip

STM tip-induced surface roughening is reported for a 6 Å Fe film grown on vicinal Mo(1 1 0) at 760 ± 15 K. Using a STM tip of the antiferromagnetic alloy MnNi and tunneling parameters of 0.1 nA and 70 mV, the film morphology was completely transformed over a period of 1 h at room temperature. The results indicate that there is a strong promotion of surface diffusion and interlayer mass transport by the local electric field between the tip and sample and/or a magnetic interaction between the tip and the film. The strain state of the film plays a part in the propagation of this transformation across the scanned area.     

Interlayer coupling between out-of-plane magnetized multilayers across a thin antiferromagnetic spacer

The interlayer exchange coupling between Co/Pt perpendicular-to-plane magnetized layers across a thin IrMn spacer layer was experimentally studied. In contrast to earlier studies on interlayer coupling through antiferromagnetic NiO, which revealed an oscillatory coupling behavior as a function of NiO thickness, a ferromagnetic coupling was observed here in the range of IrMn thickness between 0.6 and 1.5 nm and antiferromagnetic between 1.5 and 2.5 nm. The antiferromagnetic coupling is attributed to an orange peel magnetostatic mechanism whereas the ferromagnetic coupling is attributed to an out-of-plane polarization of the antiferromagnetic IrMn layer induced by the interfacial exchange interaction with the adjacent out-of-plane ferromagnetic layers. Measurements of hysteresis loops versus temperature show that the coupling vanishes at 510 K for t_IrMn=1 nm. This critical temperature is far below the Néel temperature of bulk IrMn, but above the blocking temperature of IrMn/Co bilayers at such thickness. Using a one-dimensional model describing a partial domain wall in the antiferromagnet, we explain the coupling in terms of an out-of-plane tilt of the Mn moments at the IrMn/(Co/Pt) interfaces yielding a weak net polarization of the IrMn. Finally, the non-oscillatory decay of the coupling was attributed to the compensated spin structure of the IrMn in the parallel to the interfaces.     

Deposition of superconducting CeCoIn5 thin films by co-sputtering and evaporation

Thin superconducting films of CeCoIn₅ were prepared in situ by simultaneous thermal evaporation of indium and dc magnetic field assisted sputtering of planar metallic Ce and Co targets. To achieve an effective sputtering of the magnetic Co target a special geometry with two facing planar targets (Ce and Co) and magnetic field perpendicular to the targets was used. The stoichiometric (0 0 1)-oriented CeCoIn₅ films were grown on r-cut sapphire substrates with a high-rate of 100 nm/min. The temperature dependence of the electrical resistivity revealed the characteristic heavy-fermion behavior and a superconducting transition at about 2 K in agreement with the literature data for CeCoIn₅ bulk material and thin films.     

Vortex lattice matching effects in a washboard pinning potential induced by Co nanostripe arrays

An advanced mask-less nanofabrication technique, focused electron beam-induced deposition (FEBID), has been employed on epitaxial Nb thin films to prepare ferromagnetic decorations in the form of an array of Co stripes. These substantially modify the non-patterned films’ superconducting properties, providing a washboard-like pinning potential landscape for the vortex motion. At small magnetic fields B ? 0.1 T, vortex lattice matching effects have been investigated by magneto-transport measurements. Step-like drops in the field dependencies of the films resistivity ρ(B) have been observed in particular for the vortex motion perpendicular to the Co stripes. The field values, corresponding to the middle points of these drops in ρ(B), meet the vortex lattice parameter matching the pinning structure’s period. These disagree with the results of Jaque et al. (2002) [11], who observed matching effects corresponding to the stripe width in Nb films grown on periodically distributed submicrometric lines of Ni.     

10.6 μm Infrared light photoinduced insulator-to-metal transition in vanadium dioxide

Vanadium dioxide has excellent phase transition characteristic. Before or after phase transition, its optical, electrical, magnetic characteristic hangs hugely. It has a wide application prospect in many areas. Now, the light which can make vanadium dioxide come to pass photoinduced phase transition range from soft X-ray to medium infrared light (6.9 μm, 180 meV). However, whether 10.6 μm (117 meV) long wave infrared light can make vanadium dioxide generate photoinduced phase transition has been not studied. In this paper, we researched the response characteristic of vanadium dioxide excited by 10.6 μm infrared light. We prepared the vanadium dioxide and test the changes of vanadium dioxide thin film’s transmittance to 632.8 nm infrared light when the thin film is irradiate by CO₂ laser. We also test the resistivity of vanadium dioxide. Excluding the effect of thermal induced phase transition, we find that the transmittance of vanadium dioxide thin film to 632.8 nm light and resistivity both changes when irradiating by 10.6 μm laser. This indicates that 10.6 μm infrared light can make the vanadium dioxide come to pass photoinduced phase transition. The finding makes vanadium has a potential application in recording the long-wave infrared hologram and making infrared detector with high resolution.     

Chemical solution processing and properties of Sr2FeMoO6 thin films

Crack-free and oriented Sr₂FeMoO₆ (SFMO) thin film with double perovskite structure has been fabricated by the chemical solution deposition (CSD) method. A homogeneous and stable SFMO precursor solution was successfully prepared by controlling the reaction of starting metal-organic compounds in a mixture solvent of 1-propanol and 2-methoxyethanol. SFMO thin films with c-axis preferred orientation could successfully be synthesized on MgO (0 0 1) and SrTiO₃ (0 0 1) substrates by optimizing the several processing conditions. SFMO thin film prepared on SrTiO₃ (0 0 1) showed a magnetoresistance effect at a low magnetic field.     

Characterisations of CoCu films electrodeposited at different cathode potentials

Structural and magnetic properties of CoCu films electrodeposited on polycrystalline Cu substrates were investigated as a function of cathode potential used for their deposition. The compositional analysis, performed by energy dispersive X-ray spectroscopy, demonstrated that an increase in the deposition potential results in an increase in Co content of CoCu films. The crystal structure of the films was studied using the X-ray diffraction (XRD) technique. It was observed that they have a face centred cubic (fcc) structure, but also contain partly hexagonal close-packed phase. XRD results revealed that the (1 1 1) peak of fcc structure splits into two as Co (1 1 1) and Cu (1 1 1) peaks and the peak intensities change depending on the deposition potential and hence the film composition. The magnetic measurements were carried out at room temperature using a vibrating sample magnetometer. The magnetic findings indicated that coercivity decreases and saturation magnetisation increases with the increase of Co:Cu ratio caused by the deposition potential and also all films have planar magnetisation.     

Visible luminescence of nanocrystalline AlN:Er thin film by co-deposition of AlN,Er, and SiO2

We report a visible luminescence of Er³⁺ ions in an amorphous-nanocrystalline AlN:Er thin film prepared by co-deposition using AlN, Er, and SiO₂ targets. A PL emission spectrum of Er³⁺ in the AlN:Er film annealed at 750 °C showed a strong bluish green emission of Er³⁺ in the amorphous-nanocrystalline AlN:Er thin film, which is attributed to the intra-4fEr³⁺ transitions of ²H_11/2 → ⁴I_15/2 and ⁴F_7/2 → ⁴I_15/2. It was found that crystallite diameters were between 3 and 5 nm by high-resolution transmission electron microscopy. The occurrence of the strong Er³⁺ emission in the annealed AlN:Er thin film with a mixture of amorphous and nanocrystalline phases may be contributed to an increase in the number of excitation Er³⁺ centers and a presence of oxygen related to Er³⁺ excitation and recombination process in the AlN:Er thin film.     

Microstructures and soft magnetic properties of as-deposited Fe–Sm–O thin films

The influences of O₂ partial pressure on saturation magnetization, coercivity and effective permeability of the as-deposited Fe–Sm–O thin films, which were fabricated by RF magnetron reactive sputtering method, were investigated. The nanocrystalline Fe_83.4Sm_3.4O_13.2 thin film fabricated at O₂ partial pressure of 5% exhibited the best magnetic softness with a saturation magnetization of 1.43 MA/m, coercivity of 65.2 A/m and effective permeability of about 2600 in the frequency range from 0.5 to 100 MHz. The electrical resistivity of Fe_83.4Sm_3.4O_13.2 was 130 μΩ cm. The microstructures and electrical resistivity were investigated in this work.     

Exchange bias effects in ferromagnetic wires

We have investigated the exchange bias effect in micron-sized ferromagnetic wires made from Co and Ni₈₀Fe₂₀ films. The wires were fabricated using optical lithography, metallization by sputtering and lift-off technique. Magnetotransport measurements were performed at temperatures ranging from 3 to 300 K. We observed marked changes in the magnetoresistance (MR) properties as the temperature is varied. At 300 K, the field at which the sharp peak occurs corresponding to the magnetization reversal of the Co wires is 167 Oe and is symmetrical about the origin. As the temperature was decreased to 3 K, we observed a shift in the peak positions of the MR characteristics for both the forward and reverse field sweeps corresponding to a loop shift of 582 Oe in the field axis. The asymmetric shift in the MR loops at low temperatures clearly indicates the exchange bias between ferromagnetic (Co) and antiferromagnetic parts (Co-oxide at the surfaces) from natural oxidation. Ni₈₀Fe₂₀ wires of the same geometry showed similar effect with a low exchange bias field. The onset of exchange biasing effect is found to be 70 and 15 K for the Co and Ni₈₀Fe₂₀ wires, respectively. A striking effect is the existence of exchange biasing effect from the sidewalls of the wires even when the wires were capped with Au film.     

Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca,Sr)

The magnetic and electrical properties of polycrystalline Pr_1?xA_xCoO₃ cobaltites with A=Ca, Sr and 0≤x≤0.5 were studied in the temperature range 4 K≤T≤1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO₃ is a paramagnetic insulator in the range from 4 to 1000 K. The Sr-doped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions.