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Disorder-Induced Microscopic Magnetic Memory Print

The magnetic-recording industry deliberately introduces carefully controlled disorder into its materials to obtain the desired magnetic properties. But as the density of magnetic disks climbs, the size of the magnetic domains responsible for storage must decrease, posing new challenges. Beautiful theories based on random microscopic disorder have been developed over the past ten years. To directly compare these theories with precise experiments, an American–European team, led by researchers from the University of Washington, Seattle, first developed and then applied coherent x-ray speckle metrology to microscopic magnetic domains in a series of thin multilayer perpendicular magnetic materials of varying disorder. Their results, at odds with all previous theories, have set a new reference point for future theories.

The Persistence of Memory

Magnets are not just for refrigerator doors—they are of paramount importance in today’s digital information age. In the face of a rapidly growing appetite for data storage, shrinking the size of magnetic devices has resulted in astounding increases in magnetic storage capacity. But the march of magnetic miniaturization poses challenges. For example, as the storage density of magnetic disks climbs, the size of the magnetic regions (domains) responsible for storage must decrease. How can we reliably make these domains and control their behavior? Given their importance, it is surprising that related fundamental questions have not been answered: Do the magnetic domains return to precisely the same pattern after the configuration is changed by an external magnetic field, to completely different patterns, or to something in between? In other words, is their memory perfect, nonexistent, or partial?

By studying magnetic domains 100 nanometers wide, Pierce et al. have gleaned new insight into how they evolve in changing magnetic fields by means of a new x-ray technique that directly probes the spatial structure of the magnetic domain configuration as it changes in the field. In particular, they have shown that disorder within the domains influences their evolution and that it enables them to partially remember their previous arrangement. These results are at odds with all previous theories and have set a new benchmark for future theories.

To investigate how microscopic disorder influences the evolution of the magnetic domains—and perhaps enables them to remember their previous arrangement as these systems are cycled around their major hysteresis loops—the collaborators deliberately introduced disorder in the form of interfacial roughness into a series of thin cobalt–platinum multilayer films representative of the materials in the newest ultrahigh-density disk drives. The films had domain widths of about 100 nm and their magnetization was perpendicular to the plane of the film.

Diagram of the experiment. Soft x rays from the undulator pass through a small pinhole and then scatter through a sample. A soft x-ray CCD collects the radiation. A uniform magnetic field applied to the sample can manipulate the magnetic domains.

The coherent magnetic speckle metrology technique at ALS Beamline 9.0.1 was used to directly probe the effect of disorder on the spatial structure of the magnetic domain configuration. In this technique, linearly polarized, soft x rays pass through the films, and a CCD camera collects the scattered light. To be sensitive to the magnetic domains, the photon energy was tuned to the cobalt 2p→3d resonant transition. Dependent on the polarization of the incident light relative to the orientation of the electron spin, this resonant scattering process is the quantum analogue of the classical Faraday effect.

Coherent x rays, produced by passing them through a pinhole, generate highly speckled scattering patterns, with the random arrangement of speckles being 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 will change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed.

Three-square-micron magnetic force microscopy images of the magnetic domain structure at remanence for samples with (from left to right) increasing disorder. Note the apparent disappearance of the labyrinthine structure as the disorder grows.

The experiments immediately 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). For the smooth samples with magnetically soft loops, the researchers found little or no RPM. In sharp contrast, they always found strong RPM for rougher, hard-loop samples. In short, the RPM was partial and imperfect.

The researchers also posed for the first time and then 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 complementary-point memory (CPM). As with RPM, they found the CPM was also partial and imperfect. They found no CPM for the lowest-disorder samples, but for the more disordered samples, they found significant, nonzero CPM, and the CPM was always a little smaller than the RPM.

Major hysteresis loop and evolution of the magnetic domains in the sample with the lowest disorder and their corresponding speckle patterns. Starting from positive saturation (all the magnetic spins are pointed up), small areas of the sample begin to reverse their magnetization, then a labyrinth of magnetic domains is produced, and finally the remaining up domains grow smaller and eventually disappear completely when the spins are uniformly pointed down.

No existing theory was capable of reproducing these experimental results. With the asymmetry between RPM and CPM suggesting a theory that incorporates a small component to break the spin inversion symmetry, the researchers developed two new theories fit their data. Their future work will focus on determining whether or not one or both of the new theories is correct and on determining whether RPM and CPM exist in other magnetic systems.

Research conducted by M.S. Pierce, C.R. Buechler, and L.B. Sorenson (University of Washington, Seattle); J.J. Turner and S.D. Kevan (University of Oregon); E.A. Jagla (Abdus Salam International Centre for Theoretical Physics, Italy); J.M. Deutsch, T. Mai, and O. Narayan (University of California, Santa Cruz); J.E. Davies and K. Liu (University of California, Davis); J.H. Dunn (MAX Laboratory, Sweden); K.M. Chesnel and J.B. Kortright (Berkeley Lab); and O. Hellwig and E.E. Fullerton (Hitachi Global Storage Technologies).

Research Funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); Lawrence Livermore National Laboratory; and Deutsche Forschungsgemeinshaft. Operation of the ALS is supported by BES.

Publications about this research: M.S. Pierce, C.R. Buechler, L.B. Sorenson, J.J. Turner, S.D. Kevan, E.A. Jagla, J.M. Deutsch, T. Mai, O. Narayan, J.E. Davies, K. Liu, J.H. Dunn, K.M. Chesnel, J.B. Kortright, O. Hellwig, and E.E. Fullerton, "Disorder-induced microscopic magnetic memory," Phys. Rev. Lett. 94, 017202 (2005); E.A. Jagla, "Hysteresis loops of magnetic thin films with perpendicular anisotropy," Phys. Rev. B 72, 094406 (2005); and J.M. Deutsch and T. Mai, "Mechanism for nonequilibrium symmetry breaking and pattern formation in magnetic films," Phys. Rev. E 72, 016115 (2005).