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Laser-initiated switching of magnetization direction in ferrimagnetic rare-earth-transition-metal (RE-TM) alloys - whether laser induced or photothermal via compensation point - is being vigorously pursued owing to the promise of extending operating frequencies of magnetic devices into the terahertz regime. Despite intense interest, however, the effects of repeated laser exposure on the film structure and subsequent switching behavior have yet to be investigated. In order to better understand the correlated effects of femtosecond-laser irradiation on both the magnetic response and photoinduced morphological variations of RE-TM alloys, we performed in situ Fresnel transmission electron microscopy (TEM) on Tb23Co77 thin films with Ta protecting layers. Via optical access to the specimen in a modified TEM, we irradiated the thin films in situ with both individual and series of femtosecond optical pulses, and correlated laser-induced changes in magnetic domain-wall formation and growth with photothermal crystal formation and accompanying pinned magnetic sites. We find that, for a range of applied laser fluences and numbers of individual pulses, several distinct regions are formed displaying varied magnetic behavior (switchable, nonswitchable, demagnetized) and morphological features (small-to-large crystal-grain variations). Through a series of systematic studies, we quantified these linked magnetic and morphological properties as a function of laser fluence, number of pulse-train cycles, and number of individual femtosecond-laser pulses and the duration between each. Our results show how the sensitive connection between magnetic behavior and morphological structure can emerge in magneto-optic experiments across several parameters, thus illustrating the need for rigorous characterization so that potential operating regimes may be universally identified.
Bibliographical noteFunding Information:
This work was supported partially by the National Science Foundation through the University of Minnesota MRSEC under Award No. DMR-1420013. This work was partially supported by C-SPIN, one of six STARnet Centers, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA. Part of this work was carried out at the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Awards No. DMR-0819885 and No. DMR-1420013. Part of this work was carried out at the College of Science and Engineering Institute for Rock Magnetism, University of Minnesota, which is made possible in part through the Instrumentation and Facilities program of the NSF Earth Science Division. Additional support was provided by 3M through a Nontenured Faculty Award under Award No. 13673369 and by the Arnold and Mabel Beckman Foundation through a Beckman Young Investigator Award (D.J.F.). Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research under Award No. 53116-DNI7.
© 2016 American Physical Society.
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