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In the magnetic hard disk drive industry, a continuous increase in the recording density requires higher anisotropy media in order to maintain thermal stability. However, further advances by scaling have run into a stumbling block due to limitations on the required magnetic fields, particularly for writing, which is currently being addressed by alternative approaches such as heat-assisted magnetic recording and microwave-assisted magnetic recording technologies. In this work, we investigate and demonstrate another alternative approach which is based on the effect of the spin transfer torque (STT). The approach uses tunneling spin-polarized currents, instead of magnetic fields, between a nanoscale magnetic probe and a magnetic recording media, both with a perpendicular anisotropy. Writing is performed by spin polarized electrons injected from the probe into the media, due to the STT effect. Reading is produced by the tunneling magnetoresistance (TMR) effect between the two magnetic layers, in the probe writer and the media substrate, respectively. The energy-efficient switching, with an energy of 3.1 MA/cm2, is confirmed through the TMR and the magneto-optical Kerr effect. The demonstrated STT-based magnetic recording overcomes the magnetic field limitations to both writing and reading and thus paves the way for the next-generation energy-efficient and extremely high-density recording.

In this article, a detailed overview of the methodology to design a write transducer for recording onto perpendicular media at areal densities beyond 1 Tbit/in.2 is presented. The two basic modes of perpendicular recording, single-layer recording media in combination with a ring type head and double-layer recording media with a soft underlayer in combination with a single pole head, are compared with each other theoretically and experimentally. Moreover, perpendicular recording is compared to longitudinal recording from the perspective of the writing process. The system efficiency is redefined for perpendicular recording to take into account the critical role of the soft underlayer. The effects of using “soft” magnetic shields around the trailing pole are analyzed. It is shown that at least a factor of 2 increase in the field can be obtained at areal densities beyond 500 Gbit/in.2 if shields are used. Such an open issue as the skew angle sensitivity in perpendicular recording is analyzed. It is shown that...

The physics of the playback process in perpendicular recording is explored. It is shown that due to the existence of effectively two layers of the “magnetic charges” in a perpendicular medium, the stray field sensed by a reader rolls off with the areal density essentially differently than it does in longitudinal recording. Comparing three recording modes, longitudinal recording and perpendicular recording without and with a soft underlayer (SUL), at equivalent conditions, with a 10 nm thick recording layer at a bit aspect ratio of 1:1, the density roll-off is the fastest for the longitudinal mode, while it is the slowest for the perpendicular mode without a SUL. It is shown that although at relatively low linear and track densities the use of a SUL increases the playback signal, the signal does not depend on the use of a SUL at high densities. It is shown that for both perpendicular modes, although at sufficiently low track densities (below ∼50 ktpi) the signal disappears at relatively low linear densitie...

Magnetic force microscopy measurements of perpendicular media with a CoCr-based hard layer and a CoFe soft underlayer has been made to investigate skew angle effects. The recorded tracks were produced using a focused ion beam made single pole head with a track width of approximately 400 nm and a gap thickness of approximately 1 μm at a 25 nm flying height. Magnetic force microscopy images of tracks recorded at write currents above and below the saturation value of approximately 100 mA turn were studied at different values of the linear density, 20–100 kfci, at zero skew angle as well as −15° and +15°. Calculated perpendicular recording fields versus the distance down the track, and across the track, near the trailing pole edge was performed using three-dimensional boundary element modeling and compared to the results from the microscopy data. Drawbacks and possible solutions to the skew angle problem for perpendicular recording are discussed.

The long-term stability of a CoCrPt perpendicular storage media has been investigated by thermal aging at temperatures ranging from 95 to 280 °C during otherwise normal ambient conditions. The signal-to-noise ratio from magnetic force microscopy images has been extracted by the autocorrelation method and compared for the different aging conditions. During aging at elevated temperatures, the media stack undergoes some structural and crystallographic changes. It is found that the severity of the aging process is higher for low-density bit patterns due to the higher value of demagnetizing fields for large bits. A comparison to the thermal aging is done with a set of disks in which a deterioration of the magnetic bit pattern was induced by an external magnetic field. Two experiments, including fields with a single polarity as well as with a cycled polarity, mimic different magnetic situations for the disk. During the external field experiments, no structural or crystallographic changes occur in the media. The magnetic background noise of the disks is also analyzed. For aging in a magnetic field, it is found that below a certain threshold value, mostly low-density bit patterns are affected by the aging process. However, above the threshold value, both low and high-density bit patterns are affected.

Signal-to-noise ratio (SNR) from a limited bit sequence has been extracted from magnetic force microscopy (MFM) images by two different methods and compared. The autocorrelation SNR properties extracted from the MFM image data are in better agreement with spinstand data than “normal” Fourier extracted SNR values. Transition noise data were also extracted and analyzed, “Squeezing” of the noise was measured as the periodicity of the recorded bit pattern became smaller than 250 nm and subsequently a relaxation at ∼200 nm occurred. MFM analysis based SNR data for two media types (CoCrPtTa alloy and CoB/Pd multilayer based) are compared. Ni45Fe55 alloy was used for the soft underlayer material.

In regard to cancer therapy, magnetoelectric nanoparticles (MENs) have proven to be in a class of its own when compared to any other nanoparticle type. Like conventional magnetic nanoparticles, they can be used for externally controlled drug delivery via application of a magnetic field gradient and image-guided delivery. However, unlike conventional nanoparticles, due to the presence of a non-zero magnetoelectric effect, MENs provide a unique mix of important properties to address key challenges in modern cancer therapy: (i) a targeting mechanism driven by a physical force rather than antibody matching, (ii) a high-specificity delivery to enhance the cellular uptake of therapeutic drugs across the cancer cell membranes only, while sparing normal cells, (iii) an externally controlled mechanism to release drugs on demand, and (iv) a capability for image guided precision medicine. These properties separate MEN-based targeted delivery from traditional biotechnology approaches and lay a foundation for the comp...

A focused ion beam process was utilized to physically define a “point dipole” magnetic force microscopy (MFM) tip with cylindrical dimensions as small as 50 nm in diameter and 10 nm in height. By controlling the preferred orientation of the magnetization, it is possible to define directional sensitivity. The preferred orientation is controlled either by choosing a proper magnetic material with preferred crystalline anisotropy or by applying a sufficiently strong external magnetic field. The signal selectivity with respect to the imaged field orientation allows more direct information to be obtained from the imaged object, as compared to the information obtained from the same object when using a conventional MFM tip.

This work addresses the issue of cubic phase formation in boron nitride thin-film systems. The presented data suggest strain-induced formation of cubic phase. The compliant nature of the turbostratic boron nitride buffer layer is shown to be an essential factor for cubic phase nucleation. Two distinct regimes of cubic phase formation are observed. First, a cubic boron nitride seed layer is formed on top of the turbostratic boron nitride buffer layer under nitrogen ion irradiation of the growth surface. This is followed by the growth mode of the cubic phase, which requires a different set of growth conditions. The role of nitrogen ion irradiation in two deposition regimes is discussed.

We have investigated Co/Pd multilayers deposited on either Ta or indium tin oxide (ITO) seed layers as a potential perpendicular recording media. We have examined the microstructural evolution of the films deposited on the two different types of seed layers and related it to the magnetic properties of the films. Ta underlayer produces a strong 〈111〉 fiber texture in the multilayer while ITO produces randomly oriented grains. Transmission electron microscopy reveals a microstructure of columnar grains separated by less dense material at the boundaries for the multilayers with an ITO underlayer. However, the less dense material is absent when using a Ta underlayer. The films exhibited strong perpendicular anisotropy and a higher coercivity of ∼6800 Oe and squareness of ∼0.99 are obtained for the films deposited on an ITO seed layer. The differences in the value of coercivity and squareness in the films can be correlated with the differences in the evolution of microstructures for different seed layers.

Application of reflection high-energy electron diffraction (RHEED) to texture determination is demonstrated for magnetic thin film applications. Diffraction pattern analysis based on kinematic theory of RHEED is outlined. Application of the technique is demonstrated for texture characterization of hexagonal-close-packed CoCr alloys that are used as recording layers in perpendicular recording media. It is shown that both texture orientation and texture angular dispersion extracted from RHEED patterns are in agreement with the data obtained using x-ray diffraction.

In this paper, we discuss the concept and prototype fabrication of reconfigurable and non-volatile vertical magnetic logic gates. These gates consist of two input layers and a RESET layer. The RESET layer allows the structure to be used as either an AND or an OR gate, depending on its magnetization state. To prove this concept, the gates were fabricated using a multi-layered patterned magnetic media, in which three magnetic layers are stacked and exchange-decoupled via non-magnetic interlayers. We demonstrate the functionality of these logic gates by conducting atomic force microscopy and magnetic force microscopy (MFM) analysis of the multi-layered patterned magnetic media. The logic gates operation mechanism and fabrication feasibility are both validated by the MFM imaging results.

We compute the switching field in a disk-shaped polycrystalline exchange-coupled bit (similar material to those found in bit-patterned media (BPM)) with micromagnetics, by varying physical parameters of a test grain. It was found that the size and the anisotropy of the test grain have substantial effects on the switching field, while its location has only minor influence. Scaling of the bit and the test grain dimensions result in similar switching properties. Switching field was reduced when the number of the low anisotropy test grains increased. Additionally, it was established that the intergranular exchange coupling needs to be at least 10% of the intragrain exchange for the bit to behave as one exchange-coupled entity. This investigation provides some insights for optimizing the material microstructure for the BPM application.

Switching field variations due to low anisotropy inclusions in disk-shaped magnetic nanostructures with vertical magnetic anisotropy for applications in perpendicular recording bit-patterned medium are analyzed micromagnetically. The influence of various material parameters and geometry on switching field is analyzed. It is found that the size of the low anisotropy inclusion strongly affects the switching field. However, the dependence significantly weakens when inclusions’ size becomes comparable to the exchange lengths, also known as domain wall thickness. The location of the inclusion within a bit has only a weak influence on the switching field. Scaling of the bit and the inclusion dimensions result in a system with very similar switching properties. The observed deviations from scalability are attributed to the presence of a non-scalable parameter of micromagnetic models, namely, the domain wall thickness, also known as exchange length. The switching field strongly depends on the bit diameter when th...

In this letter, a direct measurement of “easy” magnetization switching indicating zero-magnetization remanence in a magnetic probe with a cross section as narrow as 60×60 nm2, and as tall as 750 nm, is presented. Magnetic force microscopy was utilized to test focused-ion-beam-fabricated nanomagnetic probes. The data directly indicate that unlike a regular solid probe, a probe with a tubelike ending (nanotube) provides substantially “easier” switching.

A micromagnetic study employing OOMMF1 simulator of magnetic switching wave propagation along magnetic data channels is presented. The optimization of the wave propagation as a function of geometry and physical parameters is investigated, which resulted in an order of magnitude increase in the depth of magnetic signal propagation. Unlike unoptimized rapidly decaying waves, the resulting wave stays practically the same throughout the channel, improving wave detection characteristics. When the in-plan dimensions of the channel are scaled proportionally, the optimized propagation length remains relatively constant. This indicates a possibility of the scalability in the data channel, which has significant technological applications.

Magneto-optical materials have widespread applications in communication and optical devices. Besides existing applications such as optical diodes, untapped potential applications could be accessed should magneto-optical properties be improved such that smaller magnetic fields can be employed. Here we present an efficient method for fabricating oxide materials that possess excellent optical and magnetic properties—they are transparent to visible light yet have high magnetic susceptibility. Combined, these properties produce large Faraday rotations; the measured Verdet constant is >−300 rad T−1 m−1 at 632.8 nm, a high value for a thick, optically transparent material. Because this Verdet constant is more than twice that of the state of the art material, these nanocrystalline oxides produce polarized light rotations with less than half the applied magnetic field necessary. They are made by densifying rare earth nanocrystalline powder into dense, large-sized bodies using an electric current activated technique (sometimes known as spark plasma sintering). The processing temperature is optimized in order to achieve sufficient density without causing excessive phase changes that would destroy light transparency. This process produces materials quickly (

The magnetic properties, structural properties, and thermal conductivity of FePt films deposited on Ag and Cu heat sink layers designed for use in heat-assisted magnetic recording (HAMR) were investigated. It has been found that FePt films grown on Cu have a well-defined L10-FePt (001) texture while the FePt films grown on Ag appear to be more isotropic. As the thickness of the heat sink layer increases from 15 to 120 nm the coercivity of the FePt films decreases from 1.7 to 1.5 T for Cu and from 1.3 to 1.0 T for Ag. The thermal conductivity measurements, carried out with the “laser-flash” technique, revealed that the overall thermal resistance of the examined structures is dominated by the thermal boundary resistance and the interface effects. The increase in the thickness of Ag and Cu heat sink layers does not lead to the higher effective thermal conductivity of the layered structure in the cross-plane direction. The obtained results are important for optimization of the FePt-based structures for HAMR.

We report a combinatorial synthesis study on the magnetic properties of sputter-deposited Co/Pd multilayers with high perpendicular anisotropy and high remnant squareness for magnetic media applications such as magnetic logic systems, bit patterned media, magneto-optical recording, and multilevel three-dimensional (3D) magnetic media. The perpendicular magnetic anisotropy in the multilayers originates from the interfacial anisotropy of the alloylike structure. The deposition conditions and subsequent microstructures of the multilayers are critical factors to determine the magnetic properties of the media. We investigated the dependence of the magnetic properties on the thickness of Co and Pd layers the number of Co/Pd bilayers. For instance, we found that a 0.26-nm-thick layer of Co would produce the highest coercivity value if paired with a 0.55-nm-thick Pd layer. Our results revealed that an Ar+ milling could significantly increase the coercivity of the multilayer media. Further, we discovered that we c...

A cylindrical probe with almost perfectly flat plateaulike surface was focused ion beam (FIB) milled from an atomic force microscopy probe in order to create the required surface conditions for thin film deposition with finely controlled deposition/growth parameters. A composition of Pd(5 nm)/MgO(8 nm)/FePt(10 nm)/MgO(8 nm) was sputter deposited on the plateau probe, followed by deposition of a Pd (5 nm) protective layer. The plateau probe was then FIB-milled to produce a tip with a curvature radius of ∼25 nm. After annealing the probe at 650 °C for ∼15 min to generate an ultrahigh anisotropy L10 phase, magnetic force microscopy (MFM) imaging was performed with the probe on magnetic tracks with linear densities ranging from 200 to 1200 KFCI. The results show sub-20-nm lateral resolution in ambient conditions and magnetic tracks, which are otherwise invisible to standard MFM probes, are clearly evident with the FIB-fabricated FePt probe. With relatively high spatial resolution and coercivity values higher than 1 T, among other applications, this type of probe may be ideal for high-quality MFM study of next-generation recording media.

The physics of magnetic signal propagation in one-dimensional antiferromagnetically coupled nanomagnetic arrays is studied using micromagnetic modeling. The results are used to develop the design guidelines such as the criteria for the interelement spacing for efficient operation and the error suppression due to the magnetization misalignment in individual elements in the array. The propagation speed is found to decay significantly as the damping is increased. The external “clocking” field is applied to improve the data channel characteristics. However, premature relaxation of the end elements inhibits the proper operation of longer channels. A proposed solution is a zone-by-zone propagation scheme, which is compatible with the pipelining approach. Simulation results demonstrate a possibility of successful signal propagation at 2 GHz clocking field frequency with no limitation on the length of the channel.

A magnetic force microscopy based study on the formation of stripe domains in Permalloy (Ni80Fe20) thin films is presented. Our results show that the critical thickness for stripe domain initiation depended on the sputtering rate, the substrate temperature, and the film thickness. Beyond the stripe domain formation, an increase of the period of a highly ordered array of stripe domains was evident with increasing film thickness. Thin films sputtered at room temperature with thickness variation between ∼80 and ∼350nm exhibited square-root growth dependency on stripe domains periodicity from ∼150to∼380nm, respectively. Above a certain thickness, the domain period decreased and the periodicity deteriorated with the array becoming more random, which is a strong indicator of relatively high structural perpendicular anisotropy. To illustrate, Permalloy sputtered at 100°C initially showed linear dependence in stripe domain periodicity growth up until ∼650nm thick films. The magnetic stripe domain structure began ...

The correlation between the magnetic properties and the microstructural and chemical composition modifications of Co∕Pd magnetic multilayers upon annealing in ultrahigh vacuum at 250°C is presented. Magnetic characterization using magnetic sample magnetometer shows the vertical magnetic anisotropy increase and the switching field distribution decrease in the annealed samples. The larger values of magnetic anisotropy in the annealed samples are further shown using the magnetic force microscopy of the ac demagnetized states in the Co∕Pd multilayer films. X-ray diffraction rocking curves show an improvement in the texture and the initial magnetization curve slopes indicate the decreases in defect densities. Overall, vacuum annealing under optimal conditions improves the magnetic properties of Co∕Pd multilayers for applications in ultrahigh density magnetic recording.

In this work, the physics of magnetization reversal in patterned high anisotropy (Co∕Pd)n magnetic multilayer arrays is investigated where the magnetic island size, pitch, recording layer thickness, and the underlying multilayer magnetic properties are varied. Magnetization reversal was studied using magneto-optical Kerr effect magnetometry and magnetic force microscopy and supported by micromagnetic modeling. It is found that magnetic island dimension and/or pitch cannot alone explain the variations in the switching behavior of the patterned arrays and the observed values of switching field distribution (SFD). It is found that the ratio of switched magnetic islands to the total number of islands for a giving reversing field depends strongly on the magnetic island geometry. Stray fields from neighboring magnetic islands result in relatively minor influence on the switching characteristics. Micromagnetic modeling was used to further understand the magnetization reversal in patterned arrays. It is found tha...

A focused ion beam (FIB) fabricated nanolaser is demonstrated to be able to focus light with power of over 250nW into a 30nm spot. To fabricate a nanolaser, a 100nm thick aluminum film was deposited on the emitting edge of a diode laser. FIB was used to etch various apertures into the film. The power was measured by a scanning near-field optical microscope in the near-field regime with a 10nm separation between the probe and the air bearing surface of the nanolaser. Out of four different shapes under study, “C”-shape aperture was found to have the highest throughput.

We present evidence that low temperature atmosphere annealing of Co∕Pd magnetic multilayers reduces both the dipolar and exchange interactions, but the dipolar interaction decreases very rapidly at the lowest temperatures. We also infer cobalt oxide formation at grain boundaries. This results in a reduction of the exchange between grains, reduces the length scale of ordering in the ac demagnetized state of the films. We demonstrate that this is the result of the decoupling the grains.

Ion irradiation of continuous and patterned (Co∕Pd)n magnetic multilayer films has been studied as a mean to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. The coercivity of patterned medium was found to decrease from 11kOe for as patterned samples to 0.3kOe for samples with 800μC∕cm2 ion irradiation. Remnant squareness of the patterned samples remained essentially unchanged. As the number of bilayers increases in the sample, the effects vary, suggesting that several mechanisms of damage occur. Significantly, for typical irradiation doses used in ion-beam proximity lithography, no measurable alteration of magnetic properties was observed.

The purpose of this paper is to explore three-dimensional magnetic recording as a next generation recording technology. To defer the superparamagnetic limit in magnetic recording substantially beyond the 1Tbit∕in.2 mark, it is proposed to stack magnetic bits in a third (vertical) dimension. The vertical stacking underlies the concept of three-dimensional (3D) magnetic memory and recording—the primary subject of this paper. A clear distinction between absolute 3D memory and its trivial multilevel implementation is drawn. The paper focuses on the study of the media design and write and read processes. To minimize the intersymbol interference and improve stability, it is proposed to pattern the recording media in all three dimensions. Basic Co∕Pd-based 3D recording media necessary for this study are fabricated using cosputter deposition. Focused-ion-beam-based fabrication is used to pattern the recording media into nanoscale bit cells. The physics of 3D magnetic recording is also investigated theoretically w...

A combinatorial approach to the synthesis of high anisotropy Co∕Pd magnetic multilayer thin films is explored. Combinatorial libraries of Co∕Pd multilayer thin films were prepared using magnetron sputtering where the thicknesses of Co and Pd layers in the bilayer stack are varied. Application of multivariate regression and neural network models to the analysis of Co∕Pd multilayer combinatorial libraries is studied and the predictive capabilities of the two models are compared. The influences of the thicknesses of Co and Pd layers on magnetic properties are investigated. The models are used to design Co∕Pd multilayers with the desired magnetic properties. It is found that the neural network model enables higher accuracy than the multivariate analysis model and that the inverse design problem produces results within a good degree of precisions.

A study of a perpendicular magnetic recording transducer with the potential to be utilized at areal densities above 1Tbit∕in.2 is presented. Focused ion beam etching is used to fabricate the transducer via trimming of a regular nanoslider ring head from an air bearing surface. The transducer is compared with a regular ring-type head. With spinstand experiments, it is demonstrated that the overlapping width determines the effective trackwidth in the case of the transducer. With other conditions equal, the transducer indicates substantially higher efficiency. The saturation current values for the transducer and the ring head are measured to be above 350 and 800mAturn, respectively. The performance of the transducer is tested with two types of perpendicular recording media, with and without a soft underlayer, respectively. The experiments indicate that, contrary to the common belief, the medium without a soft underlayer demonstrates the potential for achieving substantially higher areal densities.

Selected aspects of the recording physics of magnetic data storage systems based on patterned medium are discussed. Considerations for the choice of a recording layer material are outlined. A micromagnetic study of magnetization reversal in patterned magnetic recording medium is presented. The effects of bit geometry, medium thickness, head/medium magnetic spacing, air-bearing surface geometry, write pole material, and write misregistration on magnetization reversal are explored. The influence of a recording layer design on playback resolution is evaluated. The results offer the guidelines for the design and optimization of patterned magnetic recording medium.

For the past several years, perpendicular magnetic recording has been under intense scrutiny as the primary alternative to magnetic data storage technologies in place today. Major system components, write heads and media in particular, have been the subject of extensive studies. Less attention, however, has been devoted to the playback processes in perpendicular recording systems. The playback heads used in technology demonstrations remain largely unchanged from their longitudinal recording counterparts. It is an open question whether the longitudinal playback-head design is optimal for perpendicular recording. For example, application of longitudinal playback heads in perpendicular recording leads to undesirable phenomena associated with modified playback response, increased flying height sensitivity, adjacent track interference, and calls for major modifications of the existing read channels. The subject of this work is a detailed discussion of the playback physics, in perpendicular recording systems; the focus being to establish the design guidelines for optimized perpendicular playback heads, which are equivalent or superior in their performance characteristics to conventional shielded readers used in longitudinal recording. Conformal mapping is applied to demonstrate the playback wave form equivalency between a shielded and dual-pole readers when applied in longitudinal and perpendicular recording, respectively. Utilizing extensive three-dimensional modeling and reciprocity principle to evaluate the performance of various playback-head configurations, it is demonstrated that differential reader configurations possess advantageous playback characteristics, such as higher playback amplitude, improved spatial resolution, and reduced dependence on flight-height variations as compared to conventional shielded readers. Modified design of differential readers with a single magnetoresistive sensor is proposed to overcome the manufacturability issues associated with a conventional dual-sensor differential reader.

In this letter, authors demonstrate magnetoresistance of the order of 18% at room temperature for a focused-ion-beam-fabricated nanoconstriction with critical dimensions of the order of 35nm. The main purpose of this work is to show that focused ion beam (FIB)-fabricated nanoconstrictions are relatively reproducible and thus could be further developed to obtain substantially larger magnetoresistance. Magnetoresistance is expected to increase if critical dimensions of nanoconstrictions are further reduced. The proposed focused-ion-beam-fabricated nanoconstrictions could be also used as devices to study the electron “ballistic” regime in the emerging fields of Spintronics and magnetoresistive random access memory (MRAM).

Time-resolved Kerr imaging was used to examine the magnetization dynamics of soft underlayers (SULs) within perpendicular magnetic recording media. Two media configurations were examined with permalloy SULs of 20 and 200 nm thickness. The media in both of these samples were 14 nm thick Co/Pt multilayer films separated from the soft underlayer by a thin exchange decoupling layer of Ta. For comparison, two equivalent SUL samples were examined with no media layer. The media were examined while driven at 80 MHz by one of two types of magnetic recording heads: a 2.5 μm track width longitudinal head, and a 1 μm track width perpendicular recording head. The rise time (10%–90%) of the fields from the heads was ∼1.4 ns. It was observed in the SUL films without media, that the film switching speeds were similar to that of the driving head, for both longitudinal and perpendicular heads. The switching speed of the SUL films with a recording layer was slower due to influence of the recording layer on the magnetic prop...

Magnetization studies on organic single‐crystal superconductors of κ‐(BEDT‐TTF)2Cu[N(CN)2]Br with the field H parallel to the b axis (perpendicular to the conducting plane) show anomalous field and temperature dependence of vortex pinning in the mixed state. At high temperatures, the magnetization M decays with increasing field with a power‐law dependence. The normalized relaxation rate S=d(ln M)/d(ln t) decreases monotonically with H. At low temperatures (T