Thermomagnetic transitions and coercivity mechanism in bulk composite Nd60Fe30Al10 alloys

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd₆₀Fe₃₀Al₁₀ alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ_B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10⁵ J/m³, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.     

Micromagnetic study of domain wall depinning driven by nanosecond current pulse in notched Permalloy nanowires

The complete understanding of domain wall (DW) dynamics is important in the design of future spintronic devices. The characteristics of faster time-scale and lower current amplitude to move DW along nanowire are crucial in fabrication upgrade. In this study, we have investigated depinning behavior of magnetic domain wall triggered by nanosecond current pulse in notched Permalloy nanowires by means of micromagnetic simulation. We introduced double-triangular notch as the constrictions in the nanowire. The non-adiabaticity of the spin-transfer-torque is considered in simulation by varying the non-adiabatic constant (β) value. We observed that the depinning current density (J_d) was not significantly affected by β for notch size (s) < 50 nm. Interestingly, we found that the depinning time (t_d) for β ≥ 0.04 was slightly constant for all the cases with s > 70 nm, where the DW structure was kept to be a transverse structure during the depinning process. The broadly applicable depinning behavior is considered to contribute to the development of high-speed memory storage devices based on magnetic domain wall.     

Theoretic 3-D study of the high-frequency magnetic moment dynamics in thin ferromagnetic films with in-plane uniaxial anisotropy by considering eddy-current generation

In the present paper, theoretic investigations of polarisation vector precession trajectories represented by a macro spin in ferromagnetic films with in-plane uniaxial anisotropy were realised. For this purpose, the Landau–Lifschitz–Gilbert differential equation (LLG) in combination with the Maxwell equations were solved for three dimensions by considering a linear progression of the magnetisation or polarisation with an external field. The frequency and time dependent polarisation trajectories illustrate how a magnetic moment precesses if effective damping and eddy-currents impacts its motion. For computation, typical parameter values like the saturation polarisation J_s=μ₀·M_s=1.4 T and in-plane uniaxial anisotropy μ₀·H_u=4.5 mT were employed. The main focus of simulation was on the variation of the effective damping parameter α_eff between 0.01 and 0.05 and ferromagnetic film thickness t_m between 200 nm and 1200 nm. The frequency-dependent calculations were carried out between 50 MHz and 6 GHz. The time-dependent simulations were done for a duration between 5 and 30 ns.     

Magnetic domain structure in Fe78.8−xCoxCu0.6Nb2.6Si9B9 nanocrystalline alloys studied by Lorentz microscopy

The magnetic domain structures of Fe_78.8−xCo_xCu_0.6Nb_2.6Si₉B₉ (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe_38.8Co₄₀Cu_0.6Nb_2.6Si₉B₉ and Fe_18.8Co₆₀Cu_0.6Nb_2.6Si₉B₉ alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy K_u, that is, the domain width d is proportional to the induced magnetic anisotropy (K_u)^−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.     

Magnetic properties of CdSb doped with Ni

Magnetic properties of the group II–V semiconductor CdSb single crystals doped with Ni (2 at%) are investigated. Deviation of the zero-field-cooled susceptibility, χ_ZFC, from the field-cooled susceptibility is observed below 300 K, along with a broad maximum of χ_ZFC (T) at T_b in fields below the anisotropy field B_K∼4 kG. T_b(B) obeys the law [T_b(B)/T_b(0)]^1/2=1–B/B_K with T_b(0)∼100 K. The magnetization exhibits saturation above ∼20–30 kG, a weak temperature dependence and anisotropy of the saturation value M_s. The coercive field is much smaller then B_K and displays anisotropy inverted with respect to that of M_s. Such magnetic behavior is expected for spheroidal Ni-rich Ni_1−xSb_x nanoparticles with high aspect ratio, broad distribution of the sizes and with orientations of the major axis distributed around a preferred direction.     

Magnetization reversal dynamics in Au/Co/Au(111) ultrathin films: Effect of roughness of the buffer layer

We present a study of the magnetization reversal dynamics in ultrathin Au/Co/Au films with perpendicular magnetic anisotropy, for a Co thickness of 0.5, 0.7 and 1 nm. In these films, the magnetization reversal is dominated by domain nucleation for t_Co=0.5, 0.7 nm and by domain wall propagation for t_Co=1 nm. The prevalence of domain nucleation for the thickness range 0.5-0.7 nm is different from results reported in the literature, for the same system and for the same thickness range, where the magnetization reversal took place mainly by domain wall motion. We attribute this difference to the effect of roughness of the Au buffer layer on the morphology of the magnetic layer.     

Application of the Stoner-Wohlfarth model with interaction for the determination of the saturation magnetisation, anisotropy field, and mean field interaction in bulk amorphous ferromagnets

Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd₆₀Fe₃₀Al₁₀, Nd₆₀Fe₂₀Co₁₀Al₁₀ and Pr₅₈Fe₂₄Al₁₈ have been measured in applied magnetic fields up to 9 T at temperatures in the range 10-350 K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner-Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation M_s, anisotropy field H_a, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of M_s, H_a, d and K over the range 20-350 K are found to be qualitatively similar, though there are quantitative differences. In all cases M_s increases with decreasing temperature, both H_a and K increase with decreasing temperature, reaching a peak in the range 75-120 K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls.     

Order-disorder transformation in FePt nanoparticles studied by ferromagnetic resonance

We have investigated the ferromagnetic resonance (FMR) response of as-made and temperature annealed FePt magnetic nanoparticles. The as-made nanoparticles, which have been fabricated by a chemical route, crystallize in the low magnetic anisotropy fcc phase and have a diameter in the range of 2-4 nm. The annealing of the particles at high temperatures (T_A=550, 650 and C) in an inert Ar atmosphere produces a partial transformation to the high magnetocrystalline anisotropy L1₀ phase, with a significant increase in particle size and size distribution. FMR measurements at X-band (9.5 GHz) and Q-band (34 GHz) show a single relatively narrow line for the as-synthesized particles and a structure of two superimposed lines for the three annealed samples. The origin of this line shape has been attributed to the presence of the disordered fcc phase. Assuming that the system consists of a collection of identical particles with a random distribution of easy axes, we have been able to estimate a mean value for the magnetic anisotropy constant of the particles in the fcc phase, K∼2×10⁶ erg/cm³. The measured line shape in the annealed samples can be explained if we consider that the magnetic anisotropy of the particles has a gaussian distribution with a relatively broad width.     

In-plane magnetic anisotropy and coercive field dependence upon thickness of CoFeB

The structural and magnetic properties of as-grown 5–50 nm thin ion-beam sputter deposited transition metal–metalloid Co₂₀Fe₆₀B₂₀ (CFB) films are reported in this communication. A broad peak observed at 2θ∼45° in the glancing angle X-ray diffraction pattern revealed the formation of very fine nano-sized grains embedded in majority amorphous CFB matrix. Although no magnetic field is applied during deposition, the longitudinal magneto-optic Kerr effect measurements performed at 300 K in these as-grown films clearly established the presence of in-plane uniaxial magnetic anisotropy (K_u). It is argued that this observed anisotropy is strain-induced. This is supported by the observed dependence of direction of K_u on the angle between applied magnetic field and crystallographic orientation of the underlying Si(100) substrate, and increase in the coercivity with the increase of the film thickness.     

Magnetic reversal mechanism in strong perpendicular magnetic anisotropic CoPt/AlN multilayer film

Magnetic reversal mechanism of the Sub/AlN5 nm/[CoPt2 nm/AlN5 nm]₅ nano multilayer film, which shows strong perpendicular magnetic anisotropy (K_u=6.7×10⁶ erg/cm³), has been studied. The angle-dependent magnetic hysteresis loops of this highly perpendicular anisotropic CoPt/AlN multilayer film were measured in the present work, applying a magnetic field along different angles φ with respect to the film normal. It demonstrates that the magnetic reversal of the CoPt ultrathin layers in the CoPt/AlN multilayer film is occurred by the reversible magnetization rotation and the irreversible displacement of domain walls. The φ-dependent part of coercive field is resulted from the internal stress according to the Kondorsky and Kersten model. The φ-independent part of coercive field implies some random and isotropy pinning centers (e.g., vacancies, dislocations, grain boundaries) in the ultrathin CoPt layers. Our work is useful for coercivity control of metal/ceramics layered structures, in particular the perpendicular magnetic tunneling junctions.     

Low-temperature deposition of L10 FePt films for ultra-high density magnetic recording

A method based on strain-induced phase transformation was used to lower the ordering temperature of FePt films. The strain resulted from the lattice mismatch between the FePt film and the substrate or underlayer favored the ordering. The relationships between the lattice mismatch, the ordering of FePt film, and the corresponding magnetic anisotropic constant were investigated. A critical lattice mismatch near 6.33% was believed to be most suitable for improving the chemical ordering of the FePt films. CrX (X=Ru, Mo, W, Ti) alloys with (2 0 0) texture was used to control the easy axis and ordering temperature of FePt films on glass substrate. Large uniaxial anisotropy constant K_u?1×10⁷ erg/cm³, good magnetic squareness (∼1) and FePt(0 0 1) texture (rocking curve −5°) were obtained at the temperature T_s?250 °C when using CrRu underlayer. The diffusion from overlying layers of Ag and Cu and an inserted Ag pinning layer were effective in reducing the exchange decoupling and changing the magnetization reversal. The media noise was effectively reduced and the SNR was remarkably enhanced when a 2 nm Ag was inserted.     

Intrinsic coercivities of molecular beam epitaxy grown single-crystal Ni films on Ag buffer layer

Single-crystal Ni films were made by the molecular beam epitaxy (MBE) method on Si(1 0 0) and Si(1 1 0) substrates, respectively, with an 100 Å thick Ag buffer layer. The growth temperature T_S was 270 °C, and the film thickness t was 500 Å. From reflection high-energy electron diffraction (RHEED) patterns, the crystalline symmetries of the two films are clear and as expected. Intrinsic coercivities, H_C(1 0 0) and H_C(1 1 0), are plotted as a function of the angle of rotation ? around the crystal axes [1 0 0] and [1 1 0], respectively. The results show that both H_C(1 0 0) and H_C(1 1 0) exhibit mixed features of the crystalline (K_C) and the induced uniaxial magnetic (K_u) anisotropies. K_u is the magneto-elastic energy, due to lattice mismatch at the Ni/Ag interface. Moreover, the crystalline anisotropy fields, H_K(1 0 0) and H_K(1 1 0), and the induced anisotropy filed, H_u, can be calculated as a function of ?, respectively. Then, each H_C curve is fitted by the equation: H_C = H_o + H_K + H_u, where H_o is the isotropic pinning field. Meanwhile, domain structures were examined by the Bitter method, using Ferrofluid 707. On the Ni(1 0 0) film, we observed the charged cross-tie walls, and on the Ni(1 1 0) film, the un-charged Bloch walls.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

Effect of thermal process on magnetic anisotropy in FeCoB soft underlayer

Relationship between magnetic anisotropy field H_k and thermal processes during the preparation has been studied for FeCoB thin films. The FeCoB films deposited on the glass substrates by facing targets sputtering successfully showed strong magnetic anisotropy when the substrate was heated at the substrate temperature T_s above 100 °C. Additionally, the lattice spacing of FeCo(1 1 0) in the perpendicular direction was found to decrease depending on the substrate temperature T_s. Among various temperature histories, the heating processes with a phase of increasing T_s revealed the further improvement of H_k. Meanwhile, high H_k in the films disappears after the post-deposition annealing at the temperature above 400 °C.     

Anisotropy at nano-grain boundary and effective anisotropy between magnetically soft and hard nano-grains

An expression of anisotropy at grain boundary suitable for different coupling conditions has been given, based on which the effective anisotropy between soft and hard grains 〈K_sh〉 has been calculated in nanoscaled magnetic materials. The results showed that 〈K_sh〉 increases rapidly first, then the increase becomes slow, and 〈K_sh〉 decreases with increasing D_s for the same value of D_h. 〈K_sh〉 decreases with increasing D_s for all given D_h, and the decrease rate becomes slow with increasing D_h. In order to obtain higher effective anisotropy between magnetically soft and hard grains, D_h should not be less than 25 nm, and D_s should be about 10 nm.     

Temperature dependence of magnetic anisotropy constant in cobalt ferrite nanoparticles

The temperature dependence of the effective magnetic anisotropy constant K(T) of CoFe₂O₄ nanoparticles is obtained based on the SQUID magnetometry measurements and Mössbauer spectroscopy. The variation of the blocking temperature T_B as a function of particle radius r is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and the magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and T_B. The resultant effective magnetic anisotropy constant K(T) decreases markedly with increasing temperature from 1.1×10⁷ J/m³ at 5 K to 0.6×10⁵ J/m³ at 280 K. The attempt time τ₀ is also determined to be 6.1×10⁻¹² s which together with the K(T) best explains the temperature dependence of superparamagnetic fraction in Mössbauer spectra.     

Magnetic properties of strontium hexaferrite films prepared by pulsed laser deposition

The magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films fabricated by pulsed laser deposition on the Si(100) substrate with Pt(111) underlayer have been studied as a function of film thickness (50–700 nm). X-ray diffraction patterns confirm that the films have c-axis perpendicular orientation. The coercivities in perpendicular direction are higher than those for in-plane direction which indicates the films have perpendicular magnetic anisotropy. The coercivity was found to decrease with increasing of thickness, due to the increasing of the grain size and relaxation in lattice strain. The 200 nm thick film exhibits hexagonal shape grains of 150 nm and optimum magnetic properties of M_s=298 emu/cm³ and H_c=2540 Oe.     

Fabrication and magnetic response probed by RF transverse susceptibility in La0.67Ca0.33MnO3 nanowires

The temperature and magnetic field dependence of the radio-frequency (RF) transverse susceptibility (χ_T) of La_0.67Ca_0.33MnO₃ crystalline nanowires has been studied using a very sensitive self-resonant tunnel-diode oscillator (TDO) technique. The nanowires were synthesized using porous templates of anodized alumina by chemical solution deposition technique, and the crystalline nature of the nanowires with the average diameter of 70 nm was confirmed by TEM, SAED, and HREM. RF transverse susceptibility experiments reveal the presence of a double-peak structure at T≤245 K (the Curie temperature) but a single peak at T>245 K. This distinguishes the low temperature ferromagnetic state from the high temperature paramagnetic state. The effective magnetic anisotropy field (H_K), which corresponds to the peak location of χ_T, has been found to increase with decrease in temperature from the Curie temperature.     

Micromagnetic studies for bit patterned media above 2 Tbit/in.

Bit patterned media (BPM) recording is a candidate for extremely high density magnetic recording. A micromagnetic model is built up to analyze the phase diagram of the correct-write-in condition in BPM above 2 Tb/in.² fabricated by lithography or ion irradiation methods. The target of the study is to acquire the relationship between the recording performance and the magnetic properties of the media. The medium includes the polycrystalline grains and grain boundary. In BPM fabricated by lithography with FCT structure, two phase diagrams of the correct-write-in condition are found for the anisotropy angular distribution Δθ, the ratio of tetragonal anisotropy K₂₂ to uniaxial anisotropy K₁ and the uniaxial anisotropy distribution ΔK₁. In BPM fabricated by ion irradiation methods, two phase diagrams of the correct-write-in condition are analyzed for the ratio of saturation magnetization M′_s/M_s, anisotropy field H′_k/H_k and the exchange field H′_ex/H_ex in the ion irradiated region and the bit islands.     

Shape- and field-dependent Morin transitions in structured α-Fe2O3

We report shape- and field-dependent magnetic properties of ellipsoid-, spindle-, flattened- and rhombohedra-shaped α-Fe₂O₃ samples prepared by solvothermal technique. We observed that a magnetic spin-flip mechanism, mostly known as Morin transition (T_M), depends on the shape of α-Fe₂O₃ as well as on the applied magnetic field. In each of these structures the obtained value of T_M was less than its bulk value of 263 K. We observed that T_M shifted from highest 251.4 K for ellipsoid to lowest 220.8 K for rhombohedra structure, with intermediate values of T_M for the other two structures. However, for rhombohedra structure T_M shifted from 220.8 to 177.5 K under the external magnetic field of 100 Oe-30 kOe, respectively. The observed lowering of T_M in the structured sample was analyzed in terms of elementary size, shape of the nanocrystallites, lattice parameters and occupancy of Fe⁺³ ions as well. These parameters were determined from the Rietveld refinement process using MAUD software.