Search

The time dependences of the magnetic moment m(t) of thin GaMnSb films containing MnSb clusters are measured. It is found that the m(t) curves are straight in semilogarithmic coordinates m(lnt). The slope of the m(lnt) lines correspond to the magnetic viscosity S. It is found that the field dependences of the magnetic viscosity S(H) and magnetic moment m(H) at low temperatures are determined by a log-normal distribution of the magnetic anisotropy energy of the MnSb clusters.

Louis Neel (1904–2000) was probably the most influential figure in applied magnetism in the 20th century. He pioneered or developed concepts such as antiferromagnetism, ferrimagnetism, superparamagnetism, magnetic viscosity, surface and induced anisotropy, domain walls, orange-peel coupling, and exchange anisotropy, many of which are of great importance in modern thin-film devices. A brief summary is given of his career and scientific contributions.

Time decay and remanence measurements have been made on a series of very thin Fe–SiO2 granular thin films of varying composition. The susceptibility and the magnetic viscosity decreases drastically when the film thickness is thin enough to reach a two-dimensional arrangement of the granules. The measured values of the magnetic viscosity are very low (2% loss per decade) in the thinnest films so that further investigations in granular magnetic systems with higher anisotropy promise greater advantages for recording media.

In this work, we report on measurements of magnetization with time [M(t)] around the pinned layer loop in spin-valve materials. These time-dependent effects are manifest via changes in M(t) in constant field (H) and via variations in the apparent coercivity with the sweep-rate of the applied field. The changes in the apparent coercivity on the pinned layer loop differ for the magnetizing and demagnetizing branches as does the form of M(t) data.

Starting from an exact quantum-statistical description, the influence of the shape of the energy landscape on the magnetic viscosity is investigated. Magnetic phase-space analysis based on Kramers’ escape-rate theory of chemical reaction kinetics theory shows that the activation entropy associated with thermally activated hopping modifies the magnetic viscosity by reducing the attempt-frequency prefactor compared to an earlier prediction by Brown [W. F. Brown, Phys. Rev. 130, 1677 (1963)]. Energetic contributions are analyzed in terms of a model applicable to a range of coherent and noncoherent magnetization processes, and in the long-time limit deviations from the linear logarithmic magnetic-viscosity law are found.

This article presents experimental evidence for scaling and data collapse in magnetic viscosity. It is demonstrated that, by using appropriate scaling, the “bell-shaped” viscosity coefficient curves observed for different temperatures collapse onto one universal curve. This data collapse reveals a self-similar structure of the viscosity coefficient as a function of two variables: magnetic field and temperature.

Measurements of reversible and irreversible magnetization have been used to examine the reversal process on the initial magnetization curve in sintered NdFeB. It is found that the reversal process involves equal and opposite changes of irreversible and reversible magnetization, described here as isomagnetic reversal, resulting in no net change in magnetization during reversal. This result is consistent with the observation that magnetic viscosity is negligible on the initial magnetization curve.

The magnetization reversal mechanism of a permanent magnet has long been a controversial issue, which is closely related to the so-called coercivity problem. It is well known that the energy barrier for magnetization reversal contains essential information on reversal process. In this study, we propose a method to analyze the energy barrier function for the magnetization reversal. Preferentially (001) oriented Nd-Fe-B films with and without a Nd overlayer are used as model magnets. By combining the magnetic viscosity and time dependent coercivity measurements, the barrier function has been successfully evaluated. As a result, although the Nd-Fe-B films with and without Nd overlayer exhibit different magnetic behaviors, the power indices for their energy barrier are almost the same, suggesting that the magnetization reversal proceeds in a similar mode.

We systematically vary the strength of the vertical interlayer exchange coupling (VIEC) between the magnetic layers (MLs) of an advanced exchange coupled composite media and experimentally investigate, through magnetic viscosity and dynamic coercivity measurements, the impact on the thermal stability. The strength of the VIEC is controlled through exchange coupling layers of varied thicknesses, whereas MLs are unchanged. We estimate the thermal energy barrier ΔEo as a function of the VIEC within the standard Arrhenius-Neel relaxation and Sharrock formalisms and characterize the effect of the VIEC on the degree of incoherency in the grain reversal mechanism. The analysis shows that while ΔEo drops for weaker VIEC, the ratio of ΔEo to MsHo energy per activation volume is maintained near 2 and weakly optimizes for intermediate VIEC.

Measurement of first order reversal curves (FORC) is emerging as a common characterization method to understand the exchange and magnetostatic interactions in magnetic films. In this paper, the FORC of the top and bottom layers of an antiferromagnetically coupled (AFC) perpendicular recording media is studied. The AFC media shows four peaks in the FORC contours, as against a single peak observed in the FORC of typical magnetic samples. Moreover, it is also pointed out that the FORC contours may be affected by time-dependent magnetization effects.

Time dependent behavior of the magnetization, i.e., magnetic viscosity, in ferromagnetic materials is well known. Less well known is the phenomenon of spontaneous remagnetization, where following a magnetic history that results in the dc demagnetized state, a ferromagnetic material spontaneously develops a magnetic moment as a function of time. Spontaneous remagnetization behavior of the bulk amorphous ferromagnets Nd60Fe30Al10 and Nd60Fe20Co10Al10 is investigated as a function of temperature from 50 to 400 K. At all temperatures the spontaneous remagnetization, Mspon, follows the relationship Mspon=Sspon ln(t+t0), where Sspon is a measure of the spontaneous remagnetization processes, t is the time, and t0 is a reference time. Sspon is strongly temperature dependent, increasing approximately linearly with temperature from 50 to ≈300 K, where it reaches a peak, before decreasing rapidly. This behavior is similar to that observed for the magnetic viscosity coefficient, S, in ferromagnetic materials.

The magnetization processes in nanocomposite PrCo4.5C0.5 ribbons that are composed of two hard magnetic phases PrCo5Cδ and Pr5Co19Ce have been systematically investigated. The magnetic reversal of the samples is found to be mainly determined by inhomogeneous domain wall pinning. Different from the nanocomposite magnets with hard and soft magnetic phases, the reverse field, where the reversible susceptibility χrev peaks, is found to be 5.8kOe, which is much lower than the field corresponding to the χtotal maximum. The study of magnetic viscosity indicates that the time dependence of magnetization is primarily determined by the reversal of PrCo5Cδ phases, and a larger activation volume, v=3.2×10−18cm3, is deduced for the PrCo4.5C0.5 ribbons.

In this paper we report on the low‐temperature magnetic relaxation of CrO2 particles of digital compact cassette tapes. The observed temperature dependence of the magnetic viscosity, S≡∂M/∂ ln t, shows three different regimes below room temperature. The most remarkable is that for T≤5 K, S(T) remains constant, suggesting the occurrence of macroscopic quantum tunneling of magnetization.

The intrinsic energy barriers 31rb2, as well as the lgHc–lgHf plot, were calculated from a localized weak domain wall pinning model which fit the experimental data well. These results support the localized weak domain wall pinning model in NdFeB sintered magnets.

Magnetic viscosity and the phenomenon of the sweep‐rate dependence of the coercivity are important for the characterization of magnetic recording media. The activation volume of magnetization reversal or switching volume is directly related to medium noise. The magnetic viscosity behavior of annealed Fe/Pt multilayers was studied and the switching volumes were obtained by both measurements of the sweep‐rate dependence of coercivity and time dependence of magnetization. It is found that samples with larger coercivities have smaller switching volumes, and an estimate of the magnetic grain diameter is about 9 nm. The coercivity mechanisms are also discussed.

The theory of magnetic viscosity due to solute atom pairs has been verified on several bcc alloys. The results on Fe‐Si alloys with Si content up to 6% wt show that solute atom pairs give magnetic viscosity. If the strong effect of the temperature is taken into account, the induced anisotropy energy deduced from the viscosity field can be considered in agreement with the value measured directly from the magnetization curves. The results on a series of Fe‐Al alloys up to 25 at. % Al, where order occurs, further confirm the theory. The viscosity field is, with reasonable agreement, proportional to the number of solute atom pairs, as deduced from studies on ordering of the alloy and from the internal friction resulting from corresponding stress‐induced ordering.

A specimen of iron containing 0.01% of C in solid solution has been cold worked by 7% and aged at room temperature. After this treatment it shows a new peak of magnetic viscosity at 180°C. The diffusion process giving rise to the peak is controlled by an activation energy of about 32 000 cal/g atom and a time constant at infinite temperature of about 10−14 sec. These figures agree with those of a peak of internal friction observed in similar specimens by Ke and by Koster, and interpreted as due to diffusion of C atoms in the surroundings of the dislocations. Hence the peak of magnetic viscosity can be related to the same mechanism.

Magnetic viscosity has been measured in aged MnAlC alloys having an intrinsic coercive force of 1.44 kOe at 295 K rising to 1.97 kOe at 77 K. The measurements are used to estimate the derivative (∂E/∂H) of the activation energy E which must be supplied thermally to move segments of domain wall through coherent jumps. This derivative is found to vary linearly with the reciprocal of the effective applied field. These results are consistent with a domain wall pinning model which allows a range parameter 4b to be deduced, from the viscosity results, as 44 A. This result is close to the estimated wall width of 49 A.

A previously described suspension employing permanent magnets is shown to be a sensitive device for measuring magnetic viscosity. A large increase in viscosity concomitant to the loading of the suspended mass with nonmagnetic weights suggests that some of the observed viscosity originates in the mechanism of magnetic suspension per se.