Coherent Rotation Mode Research Articles

Field orientation dependent magnetization reversal and dynamics in sub-100 nm wide permalloy nanowires

We have investigated the magnetization reversal mechanism and dynamic behaviour of 90 nm wide permalloy nanowires patterned over a large area as a function of thickness (t) in the range from 20 nm to 70 nm and applied field orientation. The reversal processes and dynamic properties are probed using the magneto-optical Kerr effect and broadband ferromagnetic resonance (FMR) spectroscopy respectively. We have observed that both the static and dynamic properties are markedly sensitive to the film thickness. For t= 20 nm, the magnetization reversal process is dominated by coherent rotation mode due to the strong shape anisotropy and the onset of curling mode is observed. The FMR spectra display the existence of two well-separated modes, unlike the single mode which existed in the large width nanowires of the same thickness. The two modes at high and low frequencies correspond to the excitations at the centre and edges of the wire respectively. For t ⩾ 50 nm, the magnetization reversal process is dominated by curling mode of reversal. The centre mode frequency is found to be sensitive to the film thickness and shows two fold anisotropy. The experimental results are substantiated with micromagnetic simulations. These results have potential implications in designing reconfigurable magnonic crystals for microwave devices.

Effects of ferromagnetic branch structures on coercivity of alnico permanent magnets

The effects of various branches geometry and dimensions such as length, thickness, and width for H-, U-, O-, YH- and YU-shaped alnico structures on coercivity (Hci) using micromagnetic simulation for coherent rotation and curling modes are investigated. The simulation results suggest that the H-shaped structure needs long and short branch length for the coherent rotation and curling, respectively, regardless of branch thickness and width to realize high Hci. Short branch length with thin thickness and short width are recommended for both rotations for the U- and O-shaped structures. Lastly, both Y-shaped structures need branch with long length, thin thickness, and mid-long width for the coherent rotation, but short width for the YH-shaped and mid-long width for the YU-shaped regardless of length and thickness for curling are desired. Furthermore, among the five studied structures, H- or YH-shaped structure for coherent rotation and O-shaped structure for curling are highly recommended for fabrication to realize a high Hci.

Perforated alumina templates as a tool for engineering of CoPd film magnetic properties

Abstract The patterning of the CoPd films was performed by the deposition of the Co/Pd multilayers on the nanoperforated templates of anodic aluminum oxide (AAO). We observed that the morphology of aluminum oxide is represented by the hexagonal packed hemispherical wells with a pore in the middle. The morphology of the patterned films follows the features of the initial templates; the magnetic material is deposited mainly in the regions between the pores and on their perimeter. The Co atoms diffuse into the Pd layer already during the film deposition, and discontinuous interfaces of Co and Pd are formed leading to the creation of the CoPd solid solution. The out-of-plane hysteresis loops of the nanoporous films exhibited high values of the coercive field (HC = 2.0–3.6 kOe) and squareness ratio (Mr/MS = 0.7–0.8). This indicates that the nanoporous films retain the perpendicular magnetic anisotropy observed for continuous film. Deteriorated values of effective anisotropy constant Keff for the porous films with respect to the continuous film are explained by the complex morphology of the nanoporous films, which leads to partial deviation of magnetic moments from the direction normal to the sample plane. On the other hand the increase of the values of the coercive field HC of the nanoporous films in comparison to the continuous films is related to the pinning of the magnetic moments on the nanopores. We observed the transition of the magnetization reversal mechanism from the domain wall motion observed for the continuous film, to coherent rotation mode for the nanoporous films. We show that this approach allows us to tune the coercive field HC and the effective magnetic anisotropy constant Keff of the CoPd film by engineering the pore size of the templates.

Anhysteretic remanent magnetization: model of grain size distribution of spherical magnetite grains

A phenomenological model based on a linear relationship between the magnetic coercivity field and the reciprocal of the grain diameter is applied to explain the anhysteretic remanent magnetization (ARM) imparted to artificial samples with different concentrations of a very well characterized magnetite powder. By analyses of scanning electron microscopy images, the spherically shaped single domain synthetic magnetite is found to follow a lognormal grain size distribution with ~86 nm of mean diameter. The proposed model, fitted to ARM measurements up to a peak alternating field of 100 mT, yields a very good agreement. The coercivity behaviour predicted by micromagnetism theory disagrees with the experimental results of this work. A likely explanation for the discrepancy is that the magnetite particles, which consist of a mixture of grains in coherent rotation and curling modes, produce similar observations as domain processes.

Magnetization reversal in epitaxial highly anisotropic CoFe2O4 hetero-structures

Magnetization reversal in epitaxial cobalt ferrite films grown on (110) MgO substrate has been investigated through angular studies of hysteresis loop and DCD remanence curve. The angular dependences of the coercivity and the switching field of these films strongly deviate from those typically known for coherent and incoherent rotation modes. However, the best fit for these angular curves suggests that domain wall nucleation is the appropriate mode for the magnetization reversal in these hetero-structures.

Magnetization Reversal and Magnetic Anisotropy in Ordered CoNiP Nanowire Arrays: Effects of Wire Diameter

Ordered CoNiP nanowires with the same length of 4 µm and varying diameters (d = 100 nm–600 nm) were fabricated by electrodeposition of CoNiP onto polycarbonate templates. X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy confirmed the quality of the fabricated nanowires. Magnetic measurements and theoretical analysis revealed that the magnetization reversal and magnetic anisotropy were significantly influenced by varying of the diameters of the nanowires. There existed a critical wire diameter (dc ≈ 276 nm), below which the magnetization reversal occurred via a coherent rotation mode, and above which the magnetization reversal occurred via a curling rotation mode. The easy axis of the magnetization tended to change in direction from parallel to perpendicular with respect to the wire axis as the wire diameter exceeded dc ≈ 276 nm. With increasing wire diameter, the coercive field (Hc) and the remanent to saturation magnetization ratio (Mr/Ms) were also found to rapidly decrease in the range d = 100–400 nm and gradually decrease for d > 400 nm.

Magnetization Dynamics and Reversal Mechanisms in Ni Nanowire and Nanotube Arrays

Highly ordered arrays of Ni nanowires (NWs) and nanotubes (NTs) were electrodeposited into porous anodic alumina templates with 200 nm pore diameter. The geometrical parameters of the NW/NT arrays were tuned by the deposition conditions. The fabricated NWs and NTs had various lengths depending upon deposition time and NTs had a wall thickness of ~40 nm. Morphological characterizations were performed using a scanning electron microscope and transmission electron microscope yields the topology of NWs and NTs, structural properties were determined using X-ray diffraction, and magnetic characterization was done using SQUID. Dynamic properties have been studied by ferromagnetic resonance technique in frequency sweep mode. A comparative study of the magnetization reversal processes was also performed by analyzing the angular variation of resonance frequency of NWs/NTs. The resonance frequency increases linearly with magnetic field for studied arrays. In NWs the magnetization reversal mode is curling mode, whereas for NTs it is coherent rotation mode.

Magnetic and structural properties of MnBi multilayered thin films

Magnetic and structural properties of MnBi films with thicknesses up to 50 nm were investigated. Thin films of the MnBi LTP (Low Temperature Phase) were fabricated onto silica-glass substrates by sputter-deposition of Bi/Mn multilayer, followed by a subsequent annealing at about 550 °C for 30 min. Coercivity of such thin films is higher than 15 kOe, even though the film thickness is about 10 nm. These thin films show the preferential growth of c-axis of the LTP along the film normal. Moreover, high resolution transmission electron microscopy indicates that the LTP regions of 30–50 nm in size are physically isolated by Bi. The magnetization reversal mechanism of such a LTP region is mainly governed by a coherent rotation mode based on the δM curve measurement.

Magnetization reversal studies in structurally tailored cobalt nanowires

Cobalt nanowires (NWs) having hcp crystal structure are structurally tailored for different preferred orientations (PO) of (0002), (101¯0), (112¯0) and (101¯1) by varying bath temperature and bath concentration in commercially available 50nm pore diameter polycarbonate (PCT) and 20nm pore diameter anodic alumina (AAO) membranes. The magnetization studies show orientation dependent competition of magneto-crystalline anisotropy with shape anisotropy. The large effective anisotropy, Keff (along longitudinal direction) of 1.42×106erg/cc is observed in (0002) PO NWs, which changes sign (−1.50×106erg/cc) in (101¯0) PO NWs. The angular dependence of coercivity [HC(θ)] in (0002) oriented Co NWs exhibits a non-monotonic behavior in both the 50nm and 20nm samples. The fitting of HC(θ) data reveals that the magnetization reversal mechanism initially takes place by curling and subsequently changes to coherent rotation mode after a certain transition angle, which is higher in case of denser NW array. This increase in transition angle can be attributed to the increased magneto-static interactions in the AAO membrane array having 103 times higher NW areal-density than that in PCT membrane array.

Dynamic magnetic properties of Fe70Co30(100) single-crystal thin films deposited at various substrate temperatures

Crystallographic orientation dependence of Gilbert damping constant, α , has been measured under different static magnetic fields for Fe70 Co30 (100) single-crystal thin film samples prepared at various substrate temperatures. Lorentz type resonance peaks are observed for the samples prepared below 200 oC. For a sample deposited at 400 oC, broadening of the resonance peak is observed, which is related with an increased surface roughness. α value decreases monotonically with increasing the static field when the field is applied parallel to the easy magnetization axis, whereas it increases when the field is applied along the hard magnetization axis up to around the anisotropy field and then decreases. The minimum value of α is determined to be 0.012 for the hard axis under static fields greater than 650 Oe. The value is twice larger compared with that of Fe(100) single-crystal film. The static magnetization curves measured for Fe70 Co30 films show large deviations from the curves expected by coherent rotation mode. Such deviation seems to be a possible reason for a larger α value of Fe70 Co30 thin film.

Improvement of magnetic intergranular isolation and evaluation of read/write characteristics on SmCo 5 perpendicular magnetic thin films

SmCo 5 alloy is a promising candidate for ultra-high-density perpendicular magnetic recording (PMR) media because of its high uniaxial magnetocrystalline anisotropy K u of more than 1.1×10 8 erg/cm 3. Previously, we successfully achieved high K u in a sputter-deposited SmCo 5 thin film by introducing a Cu/Ti dual underlayer. However, in order to apply the SmCo 5 films to practical PMR media, it is necessary to decrease medium noise. A granulated magnetic film comprising of small and magnetically decoupled grains is effective in reducing the medium noise. In this paper, we have proposed a new granular film that is fabricated by partial thermodiffusion of Cu between the Sm–Co continuous layer and the Cu underlayer, which is granulated using compositional segregation caused by the addition of Ta 2O 5. We have analyzed the magnetic properties, magnetic domain size, and magnetization reversal process of the proposed SmCo 5 film. The magnetic domain size decreased and the magnetization reversal process changed from the magnetic-wall-motion mode to a coherent rotation mode to some extent on isolation of magnetic grains. The read/write characteristics of granulated SmCo 5 double-layered media were also evaluated. The medium noise decreased and the signal-to-noise ratio increased for the granulated double-layered (PMR) medium.

Direct-current effects on magnetization reversal properties of submicron-size Permalloy patterns for radio-frequency devices

Ferromagnetic resonance (FMR) spectroscopy is used to measure direct-current (dc) effects on the magnetization reversal properties of submicron-sized lateral patterned magnetic material. The observed FMR frequency-field relationship shows that for both 240 and 550 nm wide Permalloy (Py) nanowires the coercivity is reduced by ∼33% when a 50 mA dc passes through the transmission line where the nanowires are incorporated. The temperature dependence of the coercivity has a T relationship which suggests the coherent rotation mode tendency in such 100 nm thick Py nanowires.

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.

Switching field and mechanism of cubic and flat square particles

Switching fields and mechanisms of cubic and flat square particles were investigated using computer simulation. The switching fields can be estimated from the nucleation fields obtained from the analytical equation for the coherent rotation or the curling modes of a sphere with the same diameter as the cubic particle or of an oblate spheroid with the same length of the principal axes as the flat square particle. When the magnetic moments rotate coherently in the initial stage (Mzav/Ms∼0.9, Mzav is the average magnetization in the easy direction and Ms is the saturation magnetization), the nucleation field of the coherent rotation mode accounts for the switching field. The switching fields of the flat square particle with chips were also investigated when the exchange stiffness constant was not small (=1.0×10−6 erg/cm). It was found that the chips increase the switching field and that the sum of the anisotropy field and the field due to shape anisotropy calculated from the average demagnetizing fields in the three directions (x,y,z) accounts for the switching field.

Nucleation-type magnetization reversal by spin-polarized current in perpendicularly magnetized FePt layers

The magnetic field dependence of current-induced magnetization reversal was investigated for current-perpendicular-to-plane giant magnetoresistance pillars with perpendicularly magnetized FePt layers. Current-induced magnetization reversal under the tilted external magnetic field showed that a reversed domain is nucleated by spin-transfer torque and then propagates to complete magnetization reversal, although the formation of a multiple-domain state is not observed by static measurement. This nucleation-type current-induced magnetization reversal is characteristic of a system having large magnetic anisotropy and a small number of pinning sites for the domain wall, and explains the large magnetic field dependence of the critical current density for magnetization reversal $({J}_{c})$. As the magnetic anisotropy of the FePt free layer is reduced, the magnetic field dependence of ${J}_{c}$ becomes close to the values theoretically calculated for the coherent rotation mode, implying that the mode of the current-induced magnetization reversal changes from the nucleation of a reversed domain to the rotation of magnetization.

Field Angle and Thickness Dependence of Coercivity in Electrodeposited CoNi–Cu Multilayer Nanowires

Variation in the coercivity of CoNi-Cu multilayer nanowires grown by electrodeposition using a hole pattern of alumina templates has been systematically studied as a function of the CoNi layer thickness and the angle between an applied magnetic field and the nanowire axis. We have found that the magnetization reversal for the nanowires with thin disk-shaped CoNi layers is in the coherent rotation mode, while that for the nanowires with long rod-shaped CoNi layers can be interpreted by a combination of the coherent rotation and curling modes. With decreasing CoNi layer thickness t(CoNi) from 6.8plusmn0.8 to 1.7plusmn0.2 nm, the coercivity rapidly decreases from 305 to 10 Oe, indicating that for t(CoNi)<1.7plusmn0.2 nm, the CoNi layers of the nanowires become superparamagnetic at room temperature. The room temperature superparamagnetism is further confirmed by magnetization measurements at low temperatures using a superconducting quantum interference device. The estimated blocking temperatures for t(CoNi)=1.7plusmn0.2 and 1.0plusmn0.1 nm are 181 and 136 K, respectively

The influence of Ni-rich nanoclusters on the anisotropic magnetic properties of CdSb doped with Ni

The magnetic properties of oriented CdSb single crystals doped with 2 at% of Ni are investigated. From measurements of magnetic irreversibility defined by deviation of the zero-field-cooled (ZFC) susceptibility from the field-cooled (FC) susceptibility, the value of the mean anisotropy field BK ∼ 4 kG is obtained. The ZFC susceptibility displays a broad maximum at a blocking temperature, Tb, depending on B according to the law [Tb(B)/Tb(0)]1/2 = 1 − B/BK with Tb(0) ∼ 100 K. The field dependence of the magnetization exhibits saturation above ∼20–30 kG with values of Ms different for B along the [1 0 0], [0 1 0] and [0 0 1] axes. The temperature dependence of Ms is weak, increasing slightly upon cooling the sample below ∼100 K. The temperature dependence of the coercive field, Bc(T), is weak above Tb but is enhanced strongly with decreasing temperature below Tb. The anisotropy of Bc is inverted with respect to the anisotropy of Ms. Such behaviour can be attributed to spheroidal Ni-rich Ni1−xSbx nanoparticles with a high aspect ratio, broad size distribution and distribution of the orientation of the major axis around a preferred direction. The relation Bc ≪ BK and the anisotropies of Ms and Bc are consistent with reversal of the magnetization by the curling mode, whereas the Tb(B) dependence is typical of the coherent rotation mode. This difference is connected to the proximity of the average transversal cluster radius to a critical value for transition between the two magnetization reversal modes within a wide crossover interval, due to broad distribution of the cluster sizes.

Magnetization reversal process in FePt L10 nanoparticles

We have studied the magnetization reversals of FePt L10 particles. Lithographically fabricated single FePt particles and well isolated epitaxial FePt L10 nanoparticles have been investigated for a wide range of particle diameters D. Careful analyses of the experiments have revealed that for particles larger than 20 nm the magnetization reversal behaviors are governed by either nucleation or expansion of a reversed domain depending on the temperature, and the coherent rotation mode becomes dominant for D<20 nm.

Magnetization reversal of iron nanoparticles studied by submicron Hall magnetometry

Magnetization of an array of 16 elongated iron nanoparticles with diameter of ∼7 nm and height of ∼100 nm has been studied with a submicron semiconductor Hall magnetometer. Details of the magnetization hysteresis curves and the angular dependence of switching fields are examined. The results indicate that the magnetization reversal in these cylindrical nanoparticles cannot be described with a single coherent rotation mode.

The magnetic reversal in dot arrays recognized by the self-organized adaptive neural network

The remagnetization dynamics of monolayer dot array superlattice XY 2-D spin model with dipole–dipole interactions is simulated. Within the proposed model of array, the square dots are described by the spatially modulated exchange-couplings. The dipole–dipole interactions are approximated by the hierarchical sums and spin dynamics is considered in regime of the Landau–Lifshitz equation. The simulation of reversal for 40 000 spins exhibits formation of nonuniform intra-dot configurations with nonlinear wave/anti-wave pairs developed at intra-dot and inter-dot scales. Several geometric and parametric dependences are calculated and compared with oversimplified four-spin model of reversal. The role of initial conditions and the occurrence of coherent rotation mode is also investigated. The emphasis is on the classification of intra-dot or inter-dot (interfacial) magnetic configurations done by adaptive neural network with varying number of neurons.