Irreversible Magnetization Switching Research Articles

Active magnetic force microscopy of Sr-ferrite magnet by stimulating magnetization under an AC magnetic field: Direct observation of reversible and irreversible magnetization processes

Understanding the dynamic magnetization process of magnetic materials is crucial to improving their fundamental properties and technological applications. Here, we propose active magnetic force microscopy for observing reversible and irreversible magnetization processes by stimulating magnetization with an AC magnetic field based on alternating magnetic force microscopy with a sensitive superparamagnetic tip. This approach simultaneously measures sample's DC and AC magnetic fields. We used this microscopy approach to an anisotropic Sr-ferrite (SrF) sintered magnet. This is a single domain type magnet where magnetization mainly changes via magnetic rotation. The proposed method can directly observe the reversible and irreversible magnetization processes of SrF and clearly reveal magnetic domain evolution of SrF (without stimulating magnetization—stimulating reversible magnetization—stimulating irreversible magnetization switching) by slowly increasing the amplitude of the external AC magnetic field. This microscopy approach can evaluate magnetic inhomogeneity and explain the local magnetic process within the permanent magnet.

Irreversible magnetization switching at the onset of superconductivity in a superconductor ferromagnet hybrid

We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case, switching is driven by changes in the magnetostatic energy due to spontaneous Meissner screening currents forming in the superconductor below the critical temperature. Our scanning Hall probe measurements also reveal vortex-mediated pinning of the ferromagnetic domain structure due to the pinning of quantized stray fields in the adjacent superconductor. The ability to use temperature as well as magnetic field to control the local magnetisation structure raises the prospect of potential applications in magnetic memory devices.

Magnetic Properties of Cobalt Nanowires and Nanotubes

Development of novel magnetic materials has played an important role in modern day science and technology. Nanomagnetism research not only sparks interests in fundamental physics, but also renders promising technological applications. Ferromagnetic Co nanowires, synthesized via low voltage electrodeposition method. High resolution transmission electron microscopy and x-ray diffraction results show that the nanowires are uniform in size, and consist predominantly hcp structure with the magnetocrystalline easy axis (c-axis) perpendicular to the wire axis. SQUID measurement illustrates the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire axis. Furthermore, the magnetic domain structures of individual, segmented or multiple nanowires are studied via magnetic force microscopy. It shows a strong dipole at the ends of the wire, together with a spatial magnetization modulation along the wire with a period around 700 nm. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis. Low temperature magnetoresistance measurement shows irreversible magnetization switching in addition to the anisotropic behavior due to magnetization orientation dependent electron scattering.On the other hand, Cobalt nanotube arrays embedded in hexagonally ordered anodic aluminum oxide templates are fabricated by direct electrodeposition with low current density under several mA/cm2. The wall thickness is about 15 nm and outer diameters are in accordance with pore diameter around 80 nm. Nearly 100% filling rate is achieved which benefits from the through pores which enable the efficient penetration of electrolyte. Transmission electron microscopy and selected-area electron diffraction are performed to show that the nanotubes are predominantly hcp single crystals with c-axis (easy axis) perpendicular to the tube axis. SQUID and MFM results indicate that the magnetization follows a circumferential direction around the tube, which is manifested in the weak magnetic signal in the MFM imaging as well as a sheared hysteresis dependence for field applied along the tube axis. This phenomenon has been confirmed by theoretical modeling taking into account the magnetocrystalline, shape demagnetization and magnetic exchange energies.

Imaging the Magnetization Processes in Epitaxial Exchange Coupled SmCo$_{5}$/Fe/SmCo$_{5}$ Trilayers

A detailed understanding of the magnetization processes in an exchange coupled SmCo5/Fe/SmCo5 trilayer is derived from imaging the domain configuration of the sample in a succession of remanent states by magnetic force microscopy. Domain nucleation and domain wall movement processes are identified and are interpreted in terms of the dominating coercivity mechanism. As in single hard magnetic SmCo5 layers, strong pinning of domain walls governs the magnetization reversal and determines coercivity. The coupling to the Fe layer, however, leads to an increased domain size in the thermally demagnetized state and to a larger nucleation density prior to the irreversible magnetization switching. Due to slightly different switching fields of the two SmCo5 layers, a compensated state can be adjusted, in which the moments of the Fe layer form 180° spin spirals with laterally varying chirality. The magnetic contrast arising from these spin spirals is imaged and the irreversible changes in a small in-plane field perpendicular to the easy axis are studied.

Strain-induced reversible and irreversible magnetization switching in Fe/BaTiO3 heterostructures

Magnetization switching of an Fe film in Fe/BaTiO3 heterostructures is demonstrated due to the interface lattice distortion caused by the structural phase transition of BaTiO3. The temperature dependence of in-plane magnetization of the Fe film in both zero and a small negative applied magnetic field clearly reveals that the reversible and irreversible magnetization switching processes occur in the Fe/BaTiO3 heterostructures. The variation in the magnetization orientation is corroborated by the fact that the symmetry of the magnetic anisotropy changes as the BaTiO3 undergoes the structural phase transition from the tetragonal to orthorhombic phases.

Experimental determination of the magnetization dependent part of the demagnetizing field in hard magnetic materials

A method for extracting the magnetization dependent part of the demagnetizing field from minor hysteresis loops is described. It applies to hard magnetic materials with irreversible magnetization switching. The method’s validity is tested on the simulated magnetization curves of an assembly of hard magnetic grains, as well as on a thin NdFeB film with out of plane magnetization. Effective demagnetization factors are extracted from the analysis. These factors are smaller than the usually applied sample shape dependent demagnetizing factors.

Ga1−xMnxAs/piezoelectric actuator hybrids: A model system for magnetoelastic magnetization manipulation

We have investigated the magnetic properties of a piezoelectric actuator/ferromagnetic semiconductor hybrid structure. Using a GaMnAs epilayer as the ferromagnetic semiconductor and applying the piezo stress along its [110] direction, we quantify the magnetic anisotropy as a function of the voltage ${V}_{p}$ applied to the piezoelectric actuator using anisotropic magnetoresistance techniques. As the magnetic anisotropy in GaMnAs substantially changes as a function of temperature $T$, the ratio of the magnetoelastic and the magnetocrystalline anistropies can be tuned from approximately 1/4 to 4. Thus, GaMnAs/piezoelectric actuator hybrids are an ideal model system for the investigation of different piezoelastic magnetization control regimes. At $T=5\text{ }\text{K}$ the magnetoelastic term is a minor contribution to the magnetic anisotropy. Nevertheless, we show that the switching fields of $\ensuremath{\rho}({\ensuremath{\mu}}_{0}H)$ loops are shifted as a function of ${V}_{p}$ at this temperature. At 50 K---where the magnetoelastic term dominates the magnetic anisotropy---we are able to tune the magnetization orientation by about $70\ifmmode^\circ\else\textdegree\fi{}$ solely by means of the electrical voltage ${V}_{p}$ applied. Furthermore, we derive the magnetostrictive constant ${\ensuremath{\lambda}}_{111}$ as a function of temperature and find values consistent with earlier results. We argue that the piezo voltage control of magnetization orientation is directly transferable to other ferromagnetic/piezoelectric hybrid structures, paving the way to innovative multifunctional device concepts. As an example, we demonstrate piezo voltage-induced irreversible magnetization switching at $T=40\text{ }\text{K}$, which constitutes the basic principle of a nonvolatile memory element.

The irreversible magnetization switching and bistability effects in ferromagnetic junctions

It is shown that, in ferromagnetic nanojunctions, i.e., conducting layered structures containing no less than two ferromagnetic layers, a high level of spin injection by a perpendicular current can be achieved. In this case, the structures exhibit fundamentally new interesting properties, specifically, an irreversible magnetization switching and a bistability.

Magnetic domains in epitaxial Fe/GaAs micro-patterned wires

The magnetic domain evolution during the magnetization reversal in in-plane magnetized epitaxial Fe/GaAs(001) wire elements with dimensions of 15 μm width × 500 μm length × 300 Å thickness has been studied using a scanning Kerr microscope. The two jump switching processes, characteristic of the magnetization reversal in continuous epitaxial Fe(001) films with four-fold in-plane anisotropy, are observed. However, the domain nucleation and growth processes which mediate the discontinuous and irreversible magnetization switching at the two critical reverse fields Hc1 and Hc2 are different from those in the continuous film. They are found to be determined by the orientation of the applied field with respect to the short and the long wire axis and hence are “shape”-anisotropic. This shape-anisotropy is a combined local dipolar field and anisotropic finite size effect, allowing the growth of domains only from the long/short wire axis and the propagation of domains only in direction of the short/long wire axis respectively according to the applied field direction.

Computer simulation of ultrahigh-density perpendicular magnetic recording

The possibility of ultrahigh-density recording higher than 10 Gb/in.2 with perpendicular magnetic recording is investigated by computer simulation for a ring-type head and single-layer medium combination. A nucleation model is used as a media model because it incorporates a nucleation site, which causes irreversible magnetization switching. Fundamental read–write characteristics are found to be entirely different from those of longitudinal recording. Recorded magnetization strongly depends on head field strength; the maximum magnetization appears around the head field strength of media coercivity, and beyond this the recorded magnetization decreases abruptly. Spacing loss in the recording process also depends on head field strength and recording density. However, saturation recording can be attained even at an ultrahigh recording density of 600 kFCI with a spacing of 30 nm. Also, a higher signal-to-noise ratio than in longitudinal recording can be obtained by introducing weak intergrain exchange interaction with a relatively large grain size in the media film. This suggests that perpendicular magnetic recording is stable in thermal fluctuation. © 1996 American Institute of Physics.

Erasure in particulate and thin-film disk media

Magnetic recording signals have been written, and subjected to erasure by external magnetic fields, on a variety of longitudinally oriented rigid particulate and thin-film disks. In general, plated thin-film disks are easier to erase than particulate disks, and particulate disks are easier to erase than sputtered thin-film disks. Decaying alternating (ac) erase fields induce greater erasure for a given peak field than do single-direction (dc) erase fields. Successive ac erasures, using the same erase field, induce greater erasure than single ac erasures; while successive dc erasures applied in the same direction do not induce greater erasure than a single dc field erasure. Successive dc erasures where the applied field direction is reversed after each erase application induce additional erasure, but are not as effective as the same number of ac erasures. Erase fields in excess of the disk coercivity induce reductions in signal amplitude, which are attributed to a region of irreversible magnetization switching, and to a region of irreversible approach to saturation as the effects of magnetic inhomogeneities are overcome.

Self-consistent calculation of stress-induced demagnetization process

The change in a magnetization transition zone in a magnetic recording medium is analyzed under stress application by the self-consistent iterative calculation method. The medium is assumed to be composed of uniaxial single domain magnetic particles with two-dimensional angular distribution of easy axes. Owing to the applied stress, the demagnetizing field temporarily increases at the localized area, because the magnetization distribution changes due to stress-induced easy axes rotation. The applied stress also changes the critical field for irreversible magnetization switching; hence, additional magnetization reversal takes place for particles which suffer critically large demagnetizing field. Consequently, the present simulation can explain the stress-induced demagnetization phenomenon, in which reproduced signal level attenuates as a result of magnetization transition broadening caused by stress.