Gergely T. Zimanyi

Extracting the intrinsic switching field distribution in perpendicular media: A comparative analysis

We introduce a method based on the first-order-reversal-curve (FORC) diagram to extract the intrinsic (microscopic) switching-field distribution (SFD) of perpendicular recording media (PRM). To demonstrate the viability of the method, we micromagnetically simulated FORCs for PRM with known SFD. The extracted SFD is compared with the SFD obtained by means of two different methods that are based on recoil loops, too, which, however, rely on mean-field approximations and assumptions on the shape of the SFD. The FORC method turns out to be the most accurate algorithm over the technologically relevant range of magnetic quality factors Q, where the other methods overestimate the width of the SFD.

Quantum Phase Slips and Transport in Ultrathin Superconducting Wires

We present a microscopic study of the quantum fluctuations of the superconducting order parameter in thin homogeneous superconducting wires at all temperatures below T{sub c}. The rate of quantum phase-slip processes determines the resistance R(T) of the wire, which is observable in very thin wires, even at low temperatures. Furthermore, we predict a new low-temperature metallic phase below a critical wire thickness in the 10-nm range, in which quantum phase slips proliferate. {copyright} {ital 1997} {ital The American Physical Society}

Anisotropy dependence of irreversible switching in Fe∕SmCo and FeNi∕FePt exchange spring magnet films

Magnetization reversal in exchange-spring magnet films has been investigated by a first-order reversal curve sFORCd technique and vector magnetometry. In Fe/epitaxial-SmCo films, the reversal proceeds by a reversible rotation of the Fe soft layer, followed by an irreversible switching of the SmCo hard layer. The switching fields are clearly manifested by separate steps in both longitudinal and transverse hysteresis loops, as well as sharp boundaries in the FORC distribution. In FeNi/ polycrystalline-FePt films, particularly with thin FeNi, the switching fields are masked by the smooth and step-free major loop. However, the FORC diagram still displays a distinct onset of irreversible switching and transverse hysteresis loops exhibit a pair of peaks, whose amplitude is larger than the maximum possible contribution from the FeNi layer alone. This suggests that the FeNi and FePt layers reverse in a continuous process via a vertical spiral. The successive versus continuous rotation of the soft/hard layer system is primarily due to the different crystal structure of the hard layer, which results in different anisotropies.

Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves

To develop a full understanding of interactions in nanomagnet arrays is a persistent challenge, critically impacting their technological acceptance. This paper reports the experimental, numerical and analytical investigation of interactions in arrays of Co nanoellipses using the first-order reversal curve (FORC) technique. A mean-field analysis has revealed the physical mechanisms giving rise to all of the observed features: a shift of the non-interacting FORC-ridge at the low-HC end off the local coercivity HC axis; a stretch of the FORC-ridge at the high-HC end without shifting it off the HC axis; and a formation of a tilted edge connected to the ridge at the low-HC end. Changing from flat to Gaussian coercivity distribution produces a negative feature, bends the ridge, and broadens the edge. Finally, nearest neighbor interactions segment the FORC-ridge. These results demonstrate that the FORC approach provides a comprehensive framework to qualitatively and quantitatively decode interactions in nanomagnet arrays.

Disorder-induced magnetic memory: Experiments and theories

Beautiful theories of magnetic hysteresis based on random microscopic disorder have been developed over the past ten years. Our goal was to directly compare these theories with precise experiments. To do so, we first developed and then applied coherent x-ray speckle metrology to a series of thin multilayer perpendicular magnetic materials. To directly observe the effects of disorder, we deliberately introduced increasing degrees of disorder into our films. We used coherent x rays, produced at the Advanced Light Source at Lawrence Berkeley National Laboratory, to generate highly speckled magnetic scattering patterns. The apparently “random” arrangement of the speckles is due to the exact configuration of the magnetic domains in the sample. In effect, each speckle pattern acts as a unique fingerprint for the magnetic domain configuration. Small changes in the domain structure change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed. Our experiments quickly answered one longstanding question: How is the magnetic domain configuration at one point on the major hysteresis loop related to the configurations at the same point on the loop during subsequent cycles? This is called microscopic return-point memory RPM. We found that the RPM is partial and imperfect in the disordered samples, and completely absent when the disorder is below a threshold level. We also introduced and answered a second important question: How are the magnetic domains at one point on the major loop related to the domains at the complementary point, the inversion symmetric point on the loop, during the same and during subsequent cycles? This is called microscopic complementary-point memory CPM. We found that the CPM is also partial and imperfect in the disordered samples and completely absent when the disorder is not present. In addition, we found that the RPM is always a little larger than the CPM. We also studied the correlations between the domains within a single ascending or descending loop. This is called microscopic half-loop memory and enabled us to measure the degree of change in the domain structure due to changes in the applied field. No existing theory was capable of reproducing our experimental results. So we developed theoretical models that do fit our experiments. Our experimental and theoretical results set benchmarks for future work.

Complex dynamical flow phases and pinning in superconductors with rectangular pinning arrays

We examine vortex pinning and dynamics in thin-film superconductors interacting with square and rectangular pinning arrays for varied vortex densities including densities significantly larger than the pinning density. For both square and rectangular pinning arrays, the critical depinning force shows maxima at only certain integer matching fields where the vortices can form highly ordered arrays. For rectangular arrays the depinning force and commensurability effects are anisotropic with a much lower depinning threshold for vortex motion in the easy flow directions. We find evidence for a crossover in pinning behavior in rectangular pinning arrays as the field is increased. We also show analytically, and confirm with simulations, that for

Vertically graded anisotropy in Co/Pd multilayers

B = 2B_{\phi}

REVISITING THE THEORY OF FINITE SIZE SCALING IN DISORDERED SYSTEMS : NU CAN BE LESS THAN 2/D

the strongest pinning can be achieved for rectangular pinning arrangements rather than square for one direction of driving force. Under an applied driving force we find a remarkable variety of distinct complex flow phases in both square and rectangular arrays. These flow phases include stable sinusoidal and intricate pinched patterns where vortices from different channels do not mix. As a function of the driving force certain flow states become unstable and transitions between different phases are observed which coincide with changes in the net vortex velocities. In the rectangular arrays the types of flow depend on the direction of drive. We also show that two general types of plastic flow occur: stable flows, where vortices always flow along the same paths, and unstable or chaotic flows.

TUNNEL JUNCTIONS OF UNCONVENTIONAL SUPERCONDUCTORS

Author(s): Kirby, Brian J.; Davies, J. E.; Liu, Kai; Watson, S. M.; Zimanyi, G. T.; Shull, R. D.; Kienzle, P. A.; Borchers, J. A. | Abstract: Depth grading of magnetic anisotropy in perpendicular magnetic media has been predicted to reduce thefield required to write data without sacrificing thermal stability. To study this prediction, we have producedCo/Pd multilayers with depth-dependent Co layer thickness. Polarized neutron reflectometry shows that thethickness grading results in a corresponding magnetic anisotropy gradient. Magnetometry reveals that theanisotropy gradient promotes domain nucleation upon magnetization reversal - a clear experimental demonstrationof the effectiveness of graded anisotropy for reducing write field.

Thermal stability of graded exchange spring media under the influence of external fields

For phase transitions in disordered systems, an exact theorem provides a bound on the finite size correlation length exponent: