Magnetization Reversal Phenomena Research Articles

Pressure-Induced Sequential Magnetic Pole Inversion and Antiferromagnetic−Ferromagnetic Crossover in a Trimetallic Prussian Blue Analogue

The flexibility of the structure of Prussian blue analogues and its ability to incorporate a variety of competing magnetic interactions have allowed the design of mixed ferro-ferrimagnets, which span the whole spectrum of magnetic behavior, including the rare phenomenon of magnetization reversal in response to a change in temperature. Hydrostatic pressure is used here to induce multiple reversals of the direction of the spontaneous magnetization in the trimetallic Prussian blue analogue, Rb0.64Ni0.31Mn0.87[Fe(CN)6].2.8H2O. Remarkably, the magnetic response is extremely sensitive to pressure, and the magnetization flips from positive to negative and back to positive in a very narrow pressure range (0 < P < 0.6 kbar). A further increase in pressure to 4.0 kbar induces an internal redox reaction, and the magnetic order switches from ferrimagnetic to bulk ferromagnetism.

Magnetization reversal phenomena in (Cr0.70Ti0.30)5S6

Magnetization reversal phenomena (MRP) along magnetic order-order transitions have recently been reported on impurity-doped magnetic systems. Because imperfect long-range magnetic order exists in these systems, it is expected that a systematic investigation of MRP will give physical information on thermomagnetic processes of magnetic systems in the range from the micro- to nanoscales. As a typical order-order transition (a state doubly modulated by helical and canting orders to a collinear ferrimagnetic state) has been known to occur on Cr5S6 at a transition temperature Tt, we investigate the magnetizations of (Cr0.70Ti0.30)5S6 on heating and cooling runs in various magnetic fields. At 20Oe, the field-cooled magnetization just below the Curie temperature has a positive sign; the sign turns negative below the compensation temperature TCM (first step) and finally returns to positive below Tt (second step). The first-step MRP observed in this system is explained by the potential barriers resulting from anisotropy energy when the preferred direction of collinear ferrimagnetic moment reverses. The proposed mechanism for second-step MRP is the pinning effect caused by the impurity atoms (Ti) in the helical long-range-order chains. Comparing other examples of MRPs, we discuss the roles of local impurity centers in the thermomagnetic process in magnetic order-order transitions.

Magnetic microstructures and their dynamics studied by X-ray microscopy

Full-field soft X-ray microscopy in combination with X-ray magnetic circular dichroism as contrast mechanism is a powerful technique to image with elemental specificity magnetic nanostructures and multilayered thin films at high lateral resolution down to 15 nm by using Fresnel zone plates as X-ray optical elements. Magnetization reversal phenomena on a microscopic level are studied by recording the images in varying external magnetic fields. Local spin dynamics at a time resolution below 100 ps can be addressed by engaging a stroboscopic pump-and-probe scheme taking into account the time pattern of synchrotron storage rings. Characteristic features of magnetic soft X-ray microscopy are reviewed and an outlook into future perspectives with regard to increased lateral and temporal resolution is given.

Study of magnetization reversal phenomena in CoCrTaPt perpendicular recording thin film media in a tilted magnetic field

Magnetization reversal process in a tilted magnetic field was investigated for CoCrTaPt thin films with perpendicular magnetic anisotropy. Angular-dependent magnetic properties studied by the Hall effect analysis and VSM measurement suggested strong influence of the Pt content in the magnetization process. Specimen with Pt content of 6 at% indicated magnetization rotation type reversal dominant, while the Pt-rich (12at%) specimen was considered to reverse by a combination of magnetization rotation and domain wall motion.

Competing field pulse induced dynamic transition in Ising Models

We discuss dynamic magnetization-reversal phenomena in the Ising model under a finite-duration external magnetic field, which competes with the existing order for . The nature of the phase boundary is estimated from the mean-field (MF) equation of motion. The susceptibility and relaxation time diverge at the MF phase boundary. A Monte Carlo study also shows divergence of the relaxation time and order-parameter fluctuations at the phase boundary. The fourth-order cumulant shows two distinct behaviors. For low temperatures and pulse durations, the value of the cumulant at the crossing point for different system sizes is far less than that for the static transition in the same dimension. This suggests a new universality class for the dynamic transition. For higher temperatures and pulse durations, the transition falls in a MF-like weak-singularity universality class.

強磁性ホール効果によるＣｏ／Ｐｄ多層膜の磁化機構評価

The magnetization process in Co/Pd multilayers under a tilted magnetic field was studied using ferromagnetic Hall effect analysis. The loci of the tip of an effective magnetization vector were determined from the simultaneously detected anomalous and planar Hall voltage signals contributed by the perpendicular and in-plane magnetization components, respectively, inside a multilayered specimen. The evaluated magnetization reversal process exhibited a clear dependence on the multilayer structure due to the variation of the Co or Pd layer thickness. For different types of multilayers, Hall effect study under a tilted magnetic field revealed the difference in the magnetization reversal phenomenon, while no significant difference was detected in the characteristics measured in a perpendicular applied field.

Magnetic anisotropy and switching process in diluted Ga1−xMnxAs magnetic semiconductor films

Magnetic anisotropy and magnetization reversal phenomena are studied in Ga1−xMnxAs films grown on GaAs(001). From the azimuthal dependence of the magneto-optical Kerr rotation and magnetic linear dichroism, the in-plane 〈100〉 directions are unambiguously assigned as axes of easy magnetization. The values of the cubic and uniaxial anisotropies are determined. Magnetization reversal proceeds by noncoherent spin rotation and irreversible jumps by formation of 90° domains.

Magnetization reversal through soliton flip in a biquadratic ferromagnet with varying exchange interactions

We study the phenomenon of magnetization reversal in the form of a soliton flip in a biquadratic ferromagnetic spin chain induced by varying bilinear and biquadratic exchange interactions. This is carried out by analyzing the evolution of the velocity and amplitude of the soliton using a perturbation analysis.

Multiscale nature of hysteretic phenomena: Application to CoPt-type magnets

We suggest a workable approach for the description of multiscale magnetization reversal phenomena in nanoscale magnets and apply it to CoPt-type alloys. We show that their hysteretic properties are governed by two effects originating at different length scales: a peculiar splitting of domain walls and their strong pinning at antiphase boundaries. We emphasize that such multiscale nature of hysteretic phenomena is a generic feature of nanoscale magnetic materials.

Magnetization Reversal Phenomena and Domain Wall Behaviors in Nanostructured Magnetic Systems

Several recent experiments on micro- (or nano-) structured samples of ferromagnetic materials are introduced. Magnetization reversal phenomena are investigated on submicron wire samples of trilayer structure using the giant magnetoresistance effect. Domain wall movements are sensitively monitored by resistivity measurements and the velocity of propagation is determined. The contribution of domain wall to the resistivity is argued from the results on artificially designed samples of a spring-magnet system. In circular dots of permalloy, the existence of vortex magnetization is confirmed and the reversal of the vortex core magnetization is studied from magnetic force microscopy measurements.

Reversal mechanism in permanent magnetic materials

The phenomenon of magnetization reversal in a hard magnet is studied analytically within a framework of a simplified energy minimization model for a two grain system. The transition region (a domain wall-like moment structure) is found to play a crucial role in nucleating a reversed domain and in the subsequent formation of a domain wall near the grain boundary. This is shown to result in a considerable reduction in coercivity. Temperature effects are taken into account by considering thermally activated jumps over energy barriers that hinder the motion of the transition region. We found that thermal fluctuations at room temperature can substantially reduce coercivity provided the grains are sufficiently small.

正方形パッドに連結したサブミクロン磁性細線の反転磁場分布

The distributions of the switching field (HSW) for single magnetic wires (150 nm width) were investigated as a function of temperature between 5 and 300 K. One end of each wire was connected to a square pad (large area), and magnetization reversal phenomena were very sensitively detected by using the giant magnetoresistance effect. While the distribution width of the HSW for the Co layer in NiFe/Cu/Co drastically broadened below 100 K, that of the HSW for the NiFe layer in NiFe/Cu/NiFe had three narrow peaks at each temperaure down to 5 K. The origin of these three peaks can be attributed to the existence of three different kind of magnetic domain structures in the pad area, which was suggested by MFM observation.

Temperature dependence of switching field distribution in a NiFe wire with a pad

The distribution of switching fields ( H sw) in a NiFe wire was investigated as a function of temperature between 5 and 300 K. The sample structure under investigation is Ta/NiFe/Cu/NiFe wire (150 nm width) connecting to a square pad (large area) at an end. Magnetization reversal phenomena are very sensitively detected using the giant magnetoresistance effect. With repeating magnetoresistance measurements, we obtained a histogram of H sw with three narrow peaks at each temperature. The origin of three peaks can be attributed to the existence of three different kinds of magnetic domain structures at the pad area, which was confirmed by magnetic force microscopy observation.

Nonlinear magnetization reversal in copper-permalloy composite wires induced by a high-frequency current

The phenomenon of magnetization reversal in copper-permalloy composite wires induced by a high-frequency ac current field was studied. The frequency spectrum of a voltage measured in the probing coil wound on the wire contains only even harmonics in a broad range of amplitudes of the ac excitation current and the bias magnetic field strength. The amplitude of the even harmonics measured is highly sensitive with respect to the bias field. The results can be used to develop weak magnetic field sensors.

Study of magnetization reversal in submicron wires using the giant magnetoresistance effect

Magnetization reversal phenomena in submicron magnetic wire systems were investigated from magnetoresistance measurements. Wire arrays and also single wire samples consisting of two magnetic layers with different coercivities were prepared and the resistivity was measured by applying external magnetic fields. Owing to the non-coupled type of giant magnetoresistance effect, the resistivity of wires is enhanced in proportion to the relative area of antiparallel magnetization. It is demonstrated that the resistivity measurements can sensitively detect the magnetization reversal phenomena in submicron wires. The dynamic properties of domain walls in wire samples are studied and the velocity of domain wall propagation is estimated.

GMR effect in a single trilayer wire with submicron width

Magnetization reversal phenomena in a submicron magnetic wire with a trilayer structure were investigated by measuring the electric resistance in external magnetic fields. The critical field for domain wall nucleation was observed to be slightly different for each field sweep. The magnetoresistance measurements were repeated 200 times with a sweep rate of 6 Oe/s. The obtained distribution of critical fields had two peaks and both moved to higher fields as temperature was decreased. This result suggests that domain wall nucleation occurs through a thermally activated process down to 5 K.

Propagation of the magnetic domain wall in submicron magnetic wire investigated by using giant magnetoresistance effect

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(400 Å)/Cu(200 Å)/NiFe(50 Å) was investigated by measuring the electric resistance in external magnetic fields. It is shown that the magnetization reversal can be very sensitively investigated by utilizing the giant magnetoresistance effect. The time variation of resistance during the magnetization reversal was also measured and the velocity of the magnetic domain wall propagating in the wire was determined at 77 K.

Temperature-induced magnetization reversal in a YVO3 single crystal

The total energy of a magnet in a magnetic field is lowest when the magnetic moment is aligned parallel to the magnetic field. Once aligned, the magnetic moment can be reversed by applying a sufficiently large field in the opposite direction. These properties form the basis of most magnetic recording and storage devices. But the phenomenon of magnetization reversal in response to a change in temperature (in a small magnetic field) is rarer. This effect occurs in some ferrimagnetic materials consisting of two or more types of antiferromagnetically ordered magnetic ions1, and forms the operational basis of ferrimagnetic insulators. Here we report the observation of multiple temperature-induced magnetization reversals in YVO3. The net magnetic moment is caused by a tilting of the antiferromagnetically aligned moments of (crystallographically identical) V3+ ions, due to orthorhombic distortion in the crystal structure. We observe an abrupt switching at 77 K associated with a first-order structural phase transition, and a gradual reversal at ∼95 K without an accompanying structural change. The magnetization always reverses if the crystal is cooled or warmed through these two temperatures in modest fields. We propose a possible mechanism involving a change in orbital ordering which may be generic to a broad class of transition metal oxides.

Magnetization reversal in submicron magnetic wire studied by using giant magnetoresistance effect

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(200 Å)/Cu(100 Å)/NiFe(50 Å) was investigated by measuring the electric resistance in an external magnetic field. A giant magnetoresistance (GMR) effect of about 0.8% was observed when the magnetizations in two NiFe layers are oriented antiparallel. It is demonstrated that magnetization reversal phenomena can be very sensitively investigated by utilizing the GMR effect.