Magnetic Viscosity Measurements Research Articles

Experimental determination of Lévy flight distributions of the energy barriers in spin glasses

Zero-field magnetic viscosity measurements at low temperatures were made on a series of Pd40Ni40−xFexP20 (x=10–20) samples in order to determine the apparent magnetic-moment-weighted energy barrier distributions for these spin-glass alloys. The distributions observed are best described by a stretched exponential in the form of a truncated Lévy flight distribution. This form suggests a hierarchical landscape of apparent energy barriers arising from interactions between randomly oriented magnetic clusters within the material. The degree of stretching of the exponential form of the energy barrier distributions is found to increase with decreasing iron concentration.

New sensors for measuring M and H in high magnetic fields

Two different small flat pick-up coil (FPC) devices, a flat pick-up coil developed at the TU-Vienna using a thin film technique, and a wire-wound flat pick-up coil commercially available, are compared for the characterization of magnetic materials in high pulsed magnetic fields. The FPC devices allow measuring the local surface magnetization. A small Hall probe to perform local magnetization measurements at high pulse field was also tested. The non-linear dependence of such probe was corrected mathematically and the results were compared with pick-up coils measurements. Magnetic viscosity measurement using the Hall probe is presented.

Thermal relaxation of the magnetic properties in hard disk recording media

The thermal relaxation of the magnetic properties of thin films for hard disk media is studied. The analyzed magnetic layers are constituted of ternary or quaternary Co-based alloy with different thickness. The relaxation is analyzed with measurements of magnetic viscosity and of coercive field variability. The magnetization switching of the magnetic films is also studied with the rotational hysteresis analysis. The different stability in the films with different compositions and thickness is discussed in relationship with the evolution of the basic processes of magnetization.

Magnetic interactions and reversal behavior ofNd2Fe14Bparticles diluted in a Nd matrix

Reversible magnetization measurements, micromagnetic modeling, the temperature dependence of coercivity, and magnetic viscosity measurements have been used to clarify the magnetic reversal mechanism of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ particles contained in a Nd matrix. The coercivity was observed to increase markedly as the dilution of the ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ phase was increased. The increase in coercivity was accompanied by a change in the reversal mechanism. In the least dilute samples, domain wall motion involving several grains governed by intergrain interactions was active. In the most dilute samples nonuniform reversal of individual grains was dominant, reversal occurring particle by particle and resembling the behavior of isolated Stoner-Wohlfarth particles. The value of the coercivity in the most dilute sample was in excellent agreement with micromagnetic modeling results for isolated particles when the effect of thermal activation of magnetization reversal was accounted for. Despite the single particle reversal mechanism of the most dilute samples, a linear dependence of coercivity on packing fraction was not observed. This is attributed to a clustering of the grains in the samples and changes in grain shape with composition. In all samples, regardless of dilution, the initial magnetic state after thermal demagnetization was found to be one in which a substantial proportion of grains were in a multidomain state. However, micromagnetic simulations for isolated particles of similar shape to those in the most dilute sample showed that the single domain state is the lowest energy state. It is concluded that thermal demagnetization can result in the system remaining in a local metastable state and not the global energy minimum. Micromagnetic calculations showed that one or more domain walls can arise in a grain during thermal demagnetization and that magnetostatic effects provide a significant energy barrier in zero field to the removal of a domain wall once it is formed.

Studies of the magnetic and reversal properties for thin CoCrTa films

The effects of magnetic layer thickness on film structural and magnetic properties were studied systematically with emphasis on the thermal effects on thin recording media films. X-ray diffraction measurements reveal structural changes as thickness decreases, and the existence of a “Cr enriched phase” associated with the interface. The saturation magnetization M s decreases with thickness and the thickness of the “dead layer” was found to be ∼23 Å. Systematic measurements of effective anisotropy, coercivity and saturation magnetization as functions of temperature have been carried out. Magnetic viscosity measurements reveal that thermal stability is affected not only by grain sizes but also by anisotropy reduction associated with nanostructure evolution, as the film thickness decreases.

Asymmetry of the magnetization reversal mechanism probed by relaxation measurements in La-Ca-Mn-O ferromagnetic/antiferromagnetic multilayers

In exchange-coupled ferromagnetic/antiferromagnetic ${\mathrm{La}}_{0.60}{\mathrm{Ca}}_{0.40}{\mathrm{MnO}}_{3}(\mathrm{FM})/{\mathrm{La}}_{0.33}{\mathrm{Ca}}_{0.67}{\mathrm{MnO}}_{3}(\mathrm{AF})$ multilayers the magnetization reversal mechanism is probed by magnetic viscosity and remanence measurements below (5 K) and above (80 K) the blocking temperature ${T}_{B}=70 \mathrm{K}.$ The magnetic relaxation follows the $\mathrm{ln}(t)$ behavior at 5 K, which is a universal feature of a slow relaxation process, whereas at 80 K the time decay requires an additional exponential term. Below ${T}_{B},$ the observed loop asymmetries in the irreversible susceptibility $({\ensuremath{\chi}}_{\mathrm{irr}})$ and the magnetic viscosity (S) reveal two inequivalent reversal mechanisms between the field increasing and field decreasing branches of the hysteresis loop.

Crystallographic defects in CoCrPt thin film media: effect on interactions and magnetic viscosity

We report synchrotron x-ray and magnetic measurements on a set of five CoCrPt thin films, each sputter-deposited onto CrV underlayers after successively increasing the base pressure to which the system was pumped prior to charging with argon. We find that the increase of residual nitrogen gas in the chamber promotes the formation of fcc-like regions in the mainly hcp Co-alloy grains by increasing the stacking fault density. The type and probability of stacking faults were determined from the x-ray data and were found to increase with increasing base pressure, varying from 9% to 30% for samples grown at 10-8 and 10-4 Torr, respectively. These stacking faults cause an increase in regions of low coercivity thus promoting thermally activated magnetization reversal. We have examined the irreversiblemagnetization reversal processes and interactions in detail using remanence, ΔM and magnetic viscosity measurements. ΔM measurements provide a measure of the intergranular interactions. Significant differences were observed in the switching field distributions and ΔM curves. A large increase in activation volume was also observed for films with over 20% stacking faults. We propose that this is due to the reduction in the anisotropy fields caused by the crystallographic defects which then allows exchange coupling to dominate the reversal, causing cooperative reversal. We have also investigated the degree of in- and out-of-plane c-axis texture in the films and find that this is also dependent on the level of nitrogen contamination, as the c-axis texture is very poor in the film grown in the worst vacuum conditions. In the other films we see evidence of some degree of circumferential c-axis texture in the plane of the sample.

Apparent magnetic energy-barrier distribution in horse-spleen ferritin: Evidence for multiple interacting magnetic entities per ferrihydrite nanoparticle

Magnetic viscosity measurements were made on native horse-spleen ferritin in zero applied magnetic field at temperatures between 2 and 21 K. The data have been used to calculate the apparent magnetic-moment-weighted energy barrier distribution for the sample of ferritin. The distribution is composed of a log-normal distribution plus a second distribution that is well described by an exponential decay of barrier frequency with increasing barrier height. The two distributions contribute approximately equally to the overall distribution. The log-normal distribution has its peak at an energy barrier of approximately $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\mathrm{J},$ while the decay constant for the second distribution has a value of approximately $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\mathrm{J}.$ The existence of the low-energy barrier distribution with exponentially decaying shape in conjunction with the observation of shifted field-cooled magnetic hysteresis loops is interpreted as strong evidence for the existence of multiple interacting magnetic entities within each ferritin particle.

Role of thermal energy on the magnetic properties of laminated antiferromagnetically coupled recording media

The effect of thermal energy (kBT), which has been found to play some important roles in the magnetic properties of recently developed antiferromagnetically coupled media, is described. It was observed that the thermal energy helps to obtain an antiparallel configuration of moments at remanence. Therefore, a reduction in the remnant moment–thickness product (Mrδ) is observed, even for smaller values of J (interface coupling constant) than those used in simulations that do not consider thermal energy. The magnetic viscosity measurement helps to distinguish the magnetization decay behavior of the top and bottom layers. The magnetic moments of top and bottom layers show maximum decay at different fields and the decay rates approximately scale with their thickness. Viscosity results also point out that the magnetization reversal of the bottom layer should occur in the first quadrant, in order to obtain a low noise and thermally stable media. Micromagnetic simulation was performed by including thermal effects. In that case, Mrδ reduction could be obtained for smaller values of J than in the case where thermal energy is not included in the simulation.

Thermally and field driven magnetization processes in nanometer-scale particle arrays

Nanometer-scale iron particles were fabricated by Scanning Tunneling Microscopy assisted chemical vapor deposition. With this, the particles can precisely be placed at a predetermined position and their dimensions can easily be controlled. The latter enabled us to perform a systematic study of the dependence of thermal activation on the particles' diameter (9-20 nm for the different arrays). Magnetic measurements included Hall magnetometry and variable field MFM. From the temperature dependence of the switching fields of our elongated particles we inferred a thermally activated nucleation-propagation type of magnetization reversal even for the smallest (9 nm) particles. The energy barrier for magnetization reversal in elongated particles is derived, from which dynamic coercivities can be calculated. The impact of thermal activation was found to scale with the particles' cross-section, in agreement with magnetic viscosity measurements.

Magnetic viscosity measurements reveal reversal asymmetry in exchange-biased bilayers

We have used time-dependent magnetization measurements to probe the asymmetry in the magnetization-reversal mechanisms in exchange-biased ${\mathrm{MnF}}_{2}/\mathrm{F}\mathrm{e}$ bilayers. Analysis of the magnetic viscosity on the magnetizing and demagnetizing sides of the exchange-shifted hysteresis loops reveals a striking asymmetry. This is due to different mechanisms for reversal on the two sides of the loop as elucidated previously by polarized-neutron reflectometry and anisotropic-magnetoresistance measurements. The viscosity data reveal asymmetric features that are intrinsically related to the nature of the exchange anisotropy.

Characterization of Perpendicular Recording Media of CoPtCrO Granular Thin Films.

Thermal stability of CoPtCrO granular thin films with perpendicular magnetic anisotropy is investigated. Magnetization decay with time M(t) exhibits almost a linear dependence with ln(t), similar to that for longitudinal recording media. In comparison with amorphous rare earth-transition metal alloys media which shows a non-linear decay with ln(t), the differences in magnetization reversal process and energy barrier distribution are discussed. From magnetic viscosity measurements, activation volume is determined and shows a minimum of about 2x10-18 cm3 for CoPtCrO at fields near its coercivity, leading to a coupled region of 10 nm average diameter. Also recording performance is investigated using a conventional merged GMR head and magnetic force microscope. The linear recording density dependence of signal amplitude reveals that the 50% rolloff density D50 is about 300 kfci.

Thermally activated magnetization reversal in nanometer-size iron particles

Arrays of nanometer-scale iron particles were fabricated by scanning tunneling microscopy assisted chemical vapor deposition. This allowed precise control over the particles' dimensions (diameters down to 9 nm) and their arrangement. The temperature dependence of the switching fields of these elongated particles revealed a thermally activated nucleation/propagation type of magnetization reversal even for our smallest particles. Improved high sensitivity Hall magnetometry allowed magnetic viscosity measurements which confirmed an activation volume much smaller than the particle size.

Disentangling distribution effects and nature of the dynamics in relaxation measurements: the RMR method

We discuss here the nature of the low-temperature magnetic relaxation in samples of magnetic nanoparticles. In addition to usual magnetic viscosity measurement, we have used the residual memory ratio (RMR) method. This procedure enables us to overcome the uncertainties usually associated with the energy barrier distribution, thus giving a more detailed insight on the nature of the observed dynamics. A custom-made apparatus coupling dilution refrigeration and SQUID magnetometry allowed measurements of very diluted samples at temperatures ranging between 60 mK and 7 K. Two types of particles have been studied: γ-Fe 2O 3 of moderate anisotropy, and CoFe 2O 4 of higher anisotropy where quantum effects are more likely to occur. In both cases, the data cannot simply be interpreted in terms of mere thermally activated dynamics of independent particles. The deviation from thermal activation seems to go opposite of what is expected from the possible effect of particle interactions. We therefore believe that it suggests the occurrence of quantum dynamics at very low temperatures.

Towards a more analytic understanding of the pinning of domain walls in high-temperature permanent magnets

We discuss analytic results for the depinning of domain walls based on different models for wall pinning in permanent magnets. We propose an explanation for the different temperature dependences of the coercive field for repulsive and attractive pinning potential when the pinning boundary forms an array of pyramids and suggest that the switching dynamics are different in the two regimes. This may be tested experimentally by magnetic viscosity measurements on samples with different Cu contents. We also found analytically that when the pinning boundary forms an array of cubes, the coercive field always decreases no matter whether the pinning potential is repulsive or attractive. This is confirmed by Monte Carlo simulations.

Magnetic Viscosity of NdFe11TiNx Thin Films

NdFe11TiNx thin films were prepared by dc magnetron sputtering the NdFe11Ti films on Si (100) at 45°°C followed by subsequent nitriding. Magnetic activation volume was evaluated by measuring the coercivity as a function of the sweep rate of the applied field and also through magnetic viscosity and remanence measurement. The estimated activation volumes are 1.32×10-17 cm3 and 4.76×10-17 cm3, respectively. At room temperature, the maximum values of magnetic viscosity coefficient and irreversible susceptibility are 23.07 G/sec and 34.74 G/Oe, respectively, both occur at an applied field equal to coercivity. These parameters compare well other rare earth magnets and films.

Analysis of magnetic aftereffects in nanocrystalline SmFeCo by the Preisach hysteresis model

Magnetic viscosity measurements performed on nanocrystalline SmFeCo spring magnets with different Fe content are analyzed by the Preisach hysteresis model. The model gives a unified description of hysteresis and relaxation properties. Comparison between calculated and measured viscosity permits one to estimate the aftereffect activation volume /spl upsi/. Calculated /spl upsi//sup 1/3/ values are approximately equal to the average grain size of the hard phase.

Magnetisation reversal in mechanically milled exchange-coupled Pr 4.25Tb 1Fe 71Co 2.5Cr 3B 18 nanocomposites

Mechanically milled Pr 4.25Tb 1Fe 71Co 2.5Cr 3B 18 has been annealed to produce a material comprising a (Pr,Tb) 2(Fe,Co) 14B hard phase intimately mixed with (Fe,Cr) 2B and α-Fe soft phases. Susceptibility and magnetic viscosity measurements have been used to characterise the reversal process in the temperature range 20–300 K.

Sm(Co, Cu, Ni) thin films with giant coercivity

Nanostructured Sm(Co, Ni, Cu) thin films have been obtained by heat treating as-deposited sputtered amorphous films. The room-temperature coercivity increases from less than 100 Oe in the amorphous state to 42 kOe in a SmCo2Cu3 sample annealed 30 min at 550 °C. Structural data in the optimum samples suggest a particle size around 10 mn. Magnetic viscosity measurements indicated that the switching volume is of the same order as the crystallite size. Remanence measurements showed that interparticulate interactions are magnetizing and rather independent of the crystallization stage. These data indicate that the huge coercivity enhancement is due to domain-wall pinning at high anisotropy Sm–Co precipitates.

Nanocrystalline Sm-Co-Cu(Ni) thin films with giant coercivity

Nanostructured Srn-Co, Sm-Co-C;u, Sm-Co-Ni thin films have been obtained by crystallizing the as-deposited amorphous films which were sputtered onto Cr buffer layers. Room temperature coercivity increases from less than 100 Oe in the amorphous state up to 42 kOe in a SmCo 2Cu 3 Sample annealed 30 min. at 550 °C. Initial magnetization curves showed a behavior characteristic of uniform domain wall pining. Magnetic viscosity measurements indicated that the switching volume is of the same order as the Sm(Co,Cu) 5 crystallites size. Remanence Measuremcnts showed that interparticulate interactions were magnetizing and rather independent of the crystallization static. These data pointed to domain wall pinning at the high anisotropy Sm-C′.o precipitatcs as the origin of the huge coercivity in the annealed films.