146 research outputs found
Fast strain wave induced magnetization changes in long cobalt bars: Domain motion versus coherent rotation
A high frequency (88 MHz) traveling strain wave on a piezoelectric substrate is shown to change the magnetization direction in 40 lm wide Co bars with an aspect ratio of 103. The rapidly alternating strain wave rotates the magnetization away from the long axis into the short axis direction, via magnetoelastic coupling. Strain-induced magnetization changes have previously been demonstrated in ferroelectric/ferromagnetic heterostructures, with excellent fidelity between the ferromagnet and the ferroelectric domains, but these experiments were limited to essentially dc frequencies. Both magneto-optical Kerr effect and polarized neutron reflectivity confirm that the traveling strain wave does rotate the magnetization away from the long axis direction and both yield quantitatively similar values for the rotated magnetization. An investigation of the behavior of short axis magnetization with increasing strain wave amplitude on a series of samples with variable edge roughness suggests that the magnetization reorientation that is seen proceeds solely via coherent rotation. Polarized neutron reflectivity data provide direct experimental evidence for this model. This is consistent with expectations that domain wall motion cannot track the rapidly varying strain
Beyond the Interface Limit: Structural and Magnetic Depth Profiles of Voltage-Controlled Magneto-Ionic Heterostructures
Electric-field control of magnetism provides a promising route towards
ultralow power information storage and sensor technologies. The effects of
magneto-ionic motion have so far been prominently featured in the direct
modification of interface chemical and physical characteristics. Here we
demonstrate magnetoelectric coupling moderated by voltage-driven oxygen
migration beyond the interface limit in relatively thick AlOx/GdOx/Co (15 nm)
films. Oxygen migration and its ramifications on the Co magnetization are
quantitatively mapped with polarized neutron reflectometry under thermal and
electro-thermal conditionings. The depth-resolved profiles uniquely identify
interfacial and bulk behaviors and a semi-reversible suppression and recovery
of the magnetization. Magnetometry measurements show that the conditioning
changes the microstructure so as to disrupt long-range ferromagnetic ordering,
resulting in an additional magnetically soft phase. X-ray spectroscopy confirms
electric field induced changes in the Co oxidation state but not in the Gd,
suggesting that the GdOx transmits oxygen but does not source or sink it. These
results together provide crucial insight into controlling magnetic
heterostructures via magneto-ionic motion, not only at the interface, but also
throughout the bulk of the films
Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices
We demonstrate that delta-doping can be used to create a dimensionally
confined region of metallic ferromagnetism in an antiferromagnetic (AF)
manganite host, without introducing any explicit disorder due to dopants or
frustration of spins. Delta-doped carriers are inserted into a manganite
superlattice (SL) by a digital-synthesis technique. Theoretical consideration
of these additional carriers show that they cause a local enhancement of
ferromagnetic (F) double-exchange with respect to AF superexchange, resulting
in local canting of the AF spins. This leads to a highly modulated
magnetization, as measured by polarized neutron reflectometry. The spatial
modulation of the canting is related to the spreading of charge from the doped
layer, and establishes a fundamental length scale for charge transfer,
transformation of orbital occupancy and magnetic order in these manganites.
Furthermore, we confirm the existence of the canted, AF state as was predicted
by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained
elusive
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