Coercivity and squareness enhancement in ball-milled FeCo–MnO nanocomposites

FeCo–MnO nanocomposites were prepared by ball milling mixtures of Fe, Co and Mn, where MnO was obtained from the oxidation of Mn after or during the milling process. The coercivity and squareness of FeCo–MnO nanocomposites increase with increasing milling time. After annealing treatment, the coercivity and squareness increase and exhibit a maximum at 120 h milling time. The temperature dependence of the magnetic properties of FeCo–MnO was also studied and there is a distinct boundary at 120 K. The exchange-bias field and the coercivity decrease quickly with increasing temperature from 30 to 120 K. However, there are no shifted hysteresis loops and the coercivity decreases slightly with increasing temperature from 120 K to room temperature. The enhancement of the coercivity and squareness is mainly attributed to the exchange-bias effects and the reduced magnetic interactions between the FeCo particles by the efficient isolation in MnO matrix.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Temperature dependent coercivity and magnetization of nickel ferrite nanoparticles

Magnetic nanoparticles of nickel ferrite (size: 24±4 nm) have been synthesized by chemical coprecipitation method using stable ferric and nickel salts. Coercivity of nanoparticles has been found to increase with decrease in temperature of the samples. It has been observed that the coercivity follows a simple model of thermal activation of particle’s moment over the anisotropy barrier in the temperature range (10-300 K), in accordance with Kneller’s law for ferromagnetic materials. Saturation magnetization follows the modified Bloch’s law in the temperature range from 300 to 50 K. However, below 50 K, an abrupt increase in magnetization of nanoparticles was observed. This increase in magnetization at lower temperatures was explained with reference to the presence of freezed surface-spins and some paramagnetic impurities at the shell of nanoparticles that are activated at lower temperatures in core-shell nickel ferrite nanoparticles.     

Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.     

Influence of annealing temperature on morphological and magnetic properties of La0.9Sr0.1MnO3

The nanocrystalline samples of La_0.9Sr_0.1MnO₃ (LSMO) have been prepared by the combustion method. The thermo gravimetric analysis of precursor was carried out. The X-ray diffraction study confirms the rhombohedral crystal structure without any other impurity phases. The morphology and magnetic properties change with annealing temperature. The saturation magnetization increases linearly and coercivity of the nanoparticles varies significantly as annealing temperature increases. The maximum saturation magnetization and lower coercivity found for the sample heat treated at 1200 °C are 52.5 emu/g and 10.7 Oe respectively.     

Low-temperature ordering and enhanced coercivity of L10-FePt thin films with Al underlayer

FePt multilayer films with and without Al underlayer were prepared by magnetron sputtering on SiO₂ substrate and subsequently annealed in vacuum. Experimental results suggest that the existence of Al underlayer can effectively reduce the ordering temperature and increase the coercivity of FePt films. Due to the slight larger lattice constant of Al underlayer than that of FePt films, [Fe (0.66 nm)/Pt (0.84 nm)]₃₀ films begin to order at 350 °C and the coercivity of them reach to 5.7 kOe after annealing at 400 °C for half an hour.     

Magnetic properties in Fe-doped NiO synthesized by co-precipitation

The magnetic properties of 1.5 at% Fe-doped NiO bulk samples were investigated. The samples were prepared by sintering the corresponding precursor in air at temperatures between 400 and 800 °C for 6 h. The synthesis was by a chemical co-precipitation and post-thermal decomposition method. In order to allow a comparison, a NiO/0.76 at% NiFe₂O₄ mixture was also prepared. The X-ray diffraction pattern shows that the samples that were sintered at 400 and 600 °C remain single phase. As the sintering temperature increased to 800 °C, however, the sample becomes a mixture of NiO and NiFe₂O₄ ferrite phases. The samples were investigated by measuring their magnetization as a function of magnetic field. The samples sintered between 400 and 800 °C and the one mixed directly with NiFe₂O₄ nanoparticles show a coercivity value of H_c≈200, 325, 350 and 110 Oe, respectively. The magnetic properties of the samples depend strongly on the sintering temperature. Simultaneously, the field-cooling hysteresis loop shift also observed after cooling the sample sintered at 600 °C to low temperature suggests the possibility of the existence of a ferromagnetic/antiferromagnetic exchange coupling.     

Low-temperature ordering of L10 FePt phase in FePt thin film with AgCu underlayer

FePt (20 nm) films with AgCu (20 nm) underlayer were prepared on thermally oxidized Si (0 0 1) substrates at room temperature by using dc magnetron sputtering, and the films annealed at different temperature to examine the disorder–order transformation of the FePt films. It is found that the ordered L1₀ FePt phase can form at low annealing temperature. Even after annealing at 300 °C, the in-plane coercivity of 5.2 kOe can be obtained in the film. With increase in annealing temperature, both the ordering degree and coercivity of the films increase. The low-temperature ordering of the films may result from the dynamic stress produced by phase separation in AgCu underlayer and Cu diffusion into FePt phase during annealing.     

Particle size effect on phase and magnetic properties of polymer-coated magnetic nanoparticles

Polymer-coated magnetic nanoparticles are hi-tech materials with ample applications in the field of biomedicine for the treatment of cancer and targeted drug delivery. In this study, magnetic nanoparticles were synthesized by chemical reduction of FeCl₂ solution with sodium borohydride and coated with amine-terminated polyethylene glycol (aPEG). By varying the concentration of the reactants, the particle size and the crystallinity of the particles were varied. The particle size was found to increase from 6 to 20 nm and the structure becomes amorphous-like with increase in the molar concentration of the reactant. The magnetization at 1 T field (M_1T) for all samples is > 45 emu/g while the coercivity is in the range of 100-350 Oe. When the ethanol-suspended particles are subjected to an alternating magnetic field of 4 Oe at 500 kHz, the temperature is increased to a maximum normalized temperature (3.8 °C/mg) with decreasing particle size.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

Magnetic properties of Nd–Fe–B film magnets prepared by RF sputtering

Highly oriented films of ∼6 μm in thickness consisting of the Nd₂Fe₁₄B compound phase were obtained by a three-dimensional sputtering method at room temperature and the subsequent crystallization by annealing. The c-axis orientation and coercivity of film samples were sensitive to the sputtering parameters and annealing conditions. The optimum temperature and time for annealing were 650 °C and 30 min to show the highest coercivity without any deterioration for the orientation of Nd₂Fe₁₄B grains, and furthermore the degree of c-axis orientation was increased by decreasing the Ar gas pressure or input power for sputtering. The resultant film magnets with good magnetic properties of B_r=∼1.06 T, H_C=∼371 kA/m, and (BH)_max=∼160 kJ/m³ were obtained under the optimized parameters for sputtering.     

Thermally assisted writing for perpendicular MRAM

Spin valves composed of TbCo/CoFe/Cu/CoFe/TbFeCo were fabricated with perpendicular magnetization and GMR ratios of 4.5%. The (TbCo/CoFe) layers and (CoFe/TbFeCo) layers are referred to the free and the pinned layers, respectively. The compositions of two layers were chosen to have a lower Curie temperature (130 °C) but higher coercivity (13.2 kOe) of the free layer at room temperature than those of the pinned layer; therefore, the free layer is quite stable at room temperature but its magnetization can be easily switched at a relatively low temperature. Spin valves were patterned into 100-μm-wide cells and their coercivity was reduced with increasing writing current due to the temperature rise by current-heating. When the current density of the writing current was increased to 2.1×10⁶ A/cm², the required switching field for the free layer was only 10 Oe.     

Evolution of magnetic and microstructural properties of thick sputtered NdFeB films with processing temperature

Ta (100 nm)/NdFeB (5 μm)/Ta (100 nm) films have been deposited onto Si substrates using triode sputtering (deposition rate ∼18 μm/h). A 2-step procedure was used: deposition at temperatures up to 400 °C followed by ex-situ annealing at higher temperatures. Post-deposition annealing temperatures above 650 °C are needed to develop high values of coercivity. The duration of the annealing time is more critical in anisotropic samples deposited onto heated substrates than in isotropic samples deposited at lower temperatures. For a given set of annealing conditions (750 °C/10′), high heating rates (?2000 °C/h) favour high coercivity in both isotropic and anisotropic films. The shape and size of Nd₂Fe₁₄B grains depend strongly on the heating rate.     

Buffer effects of Ag layers on magneto-optical Co/Ge(1 0 0) ultrathin films

Magnetic properties of the Co/Ag/Ge(1 0 0) films grown at room temperature and 200 K were studied by the surface magneto-optical Kerr effect (SMOKE). More than 1.5 monolayer Ag buffer layers not only effectively block the interdiffusion between the capped Co layers and the Ge(1 0 0) substrate but also stabilize the magnetic phase. The temperature and thickness dependence on coercivity measurements show that interactions upon the interfaces are strongly correlated to the microstructures.     

Growth of crystalline/amorphous biphase Sm-Fe-Ta-N magnetic nanodroplets

Thin films of biphase (amorphous/crystalline) magnetic Sm-Fe-Ta-N nanodroplets were fabricated at room temperature with 157 nm pulse laser deposition in nitrogen from a Sm_13.8Fe_82.2Ta_4.0 target. The 50-100 nm biphase spherical nanodroplets consist of a 5-10 nm internal crystal portion surrounded by the external amorphous phase. Nitrogen fixation in the nanodroplets occurred in the plume. The films exhibit a ferromagnetic response of 2.5 kOe coercivity at room temperature. With further annealing and thermal treatment in nitrogen, the coercivity was increased to 5.0 kOe. The surrounding amorphous layer prevents post-ablation oxidization of the crystalline magnetic nucleus of the nanodroplet.     

Thermally activated magnetization reversal process of self-assembled Fe55Co45 nanowire arrays

We have investigated the temperature dependence of the magnetic properties and the magnetic relaxation of the Fe₅₅Co₄₅ nanowire arrays electrodeposited into self-assembled porous alumina templates with the diameter about 10 nm. X-ray diffraction (XRD) pattern indicates that the nanowire arrays are BCC structure with [1 1 0] orientation along the nanowire axes. Owing to the strong shape anisotropy, the nanowire arrays exhibit uniaxial magnetic anisotropy with the easy magnetization direction along the nanowire axes. The coercivity at 5 K can be explained by the sphere chains of the symmetric fanning mechanism. The temperature dependence of coercivity can be interpreted by thermally activated reversal mechanism as being the localized nucleation reversal mechanism with the activation volume much smaller than the wire volume. Strong field and temperature-dependent magnetic viscosity effects were also observed.     

Property changes of electroplated Cu/Co alloys and multilayers by organic additives

We investigated the change of magnetic properties of the electroplated Cu/Co alloys and multilayers caused by organic additives and high temperature annealing. When plated with a pure Cu/Co electrolyte, the alloy contained ∼25% of Cu and ∼75% of Co. The alloy was made of hcp-Co, fcc-Co and Cu(1 1 1) and was super-paramagnetic at room temperature. As we add a few organic additives in the plating electrolyte, the hcp-Co of the films disappeared. The organic additives contained in the electrolytes changed paramagnetic Cu/Co multilayers to ferromagnets. High-temperature thermal annealing increased coercivity due to the growth of the Co grains.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.     

Control of crystallographic orientation in L10 FePt films by using Cr and CrW underlayers

The microstructure and magnetic properties of FePt films grown on Cr and CrW underlayers were investigated. The FePt films that deposited on Cr underlayer show (2 0 0) orientation and low coercivity because of the diffusion between FePt and Cr underlayer. The misfit between FePt magnetic layer and underlayer increases by small addition of W element in Cr underlayer or using a thin Mo intermediate layer, which is favorable for the formation of (0 0 1) orientation and the transformation of FePt from fcc to fct phase. A good FePt (0 0 1) texture was obtained in the films with Cr₈₅W₁₅ underlayer with substrate temperature of 400 °C. The FePt films deposited on Mo/Cr underlayer exhibit larger coercivity than that of the films grown on Pt/Cr₈₅W₁₅ because 5 nm Mo intermediate layer depressed the diffusion of Cr into magnetic layer.     

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.