35 research outputs found
Crystal Symmetry Lowering in Chiral Multiferroic BaTaFeSiO observed by X-Ray Magnetic Scattering
Chiral multiferroic langasites have attracted attention due to their
doubly-chiral magnetic ground state within an enantiomorphic crystal. We report
on a detailed resonant soft X-ray diffraction study of the multiferroic
BaTaFeSiO at the Fe and oxygen edges. Below
() we observe the satellite reflections ,
, and where . The dependence of the scattering intensity on X-ray polarization and
azimuthal angle indicate that the odd harmonics are dominated by the
out-of-plane (-axis) magnetic dipole while the
originates from the electron density distortions accompanying magnetic order.
We observe dissimilar energy dependences of the diffraction intensity of the
purely magnetic odd-harmonic satellites at the Fe edge. Utilizing
first-principles calculations, we show that this is a consequence of the loss
of threefold crystal symmetry in the multiferroic phase
Ultrafast relaxation dynamics of the antiferrodistortive phase in Ca doped SrTiO3
The ultrafast dynamics of the octahedral rotation in Ca:SrTiO3 is studied by
time resolved x-ray diffraction after photo excitation over the band gap. By
monitoring the diffraction intensity of a superlattice reflection that is
directly related to the structural order parameter of the soft-mode driven
antiferrodistortive phase in Ca:SrTiO3, we observe a ultrafast relaxation on a
0.2 ps timescale of the rotation of the oxygen octahedron, which is found to be
independent of the initial temperaure despite large changes in the
corresponding soft-mode frequency. A further, much smaller reduction on a
slower picosecond timescale is attributed to thermal effects. Time-dependent
density-functional-theory calculations show that the fast response can be
ascribed to an ultrafast displacive modification of the soft-mode potential
towards the normal state, induced by holes created in the oxygen 2p states
Crystal symmetry lowering in chiral multiferroic observed by x-ray magnetic scattering
Chiral multiferroic langasites have attracted attention due to their doubly chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft x-ray diffraction study of the multiferroic Ba3TaFe3Si2O14 at the Fe L2,3 and oxygen K edges. Below TN (≈27K) we observe the satellite reflections (0,0,τ), (0,0,2τ), (0,0,3τ), and (0,0,1−3τ) where τ≈0.140±0.001. The dependence of the scattering intensity on x-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane (ˆc axis) magnetic dipole while the (0,0,2τ) originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependencies of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe L3 edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase
The Spin-Reorientation Transition in TmFeO3
X-ray magnetic circular and linear dichroism (XMCD and XMLD) have been used
to investigate the Fe magnetic response during the spin reorientation
transition (SRT) in TmFeO3. These experiments are complemented with resonant
magnetic diffraction experiments at the Tm M5 edge to study simultaneously the
induced magnetic order in the Tm 4f shell and the behavior of the Tm orbitals
through the SRT. Comparing the Fe XMLD results with neutron diffraction and
magnetization measurements on the same sample indicate that the SRT has an
enhanced temperature range in the near surface region. This view is supported
by the resonant soft x-ray diffraction results at the Tm M5 edge. These find an
induced magnetic moment on the Tm sites, which is well-described by a dipolar
mean field model originating from the Fe moments. Even though such a model can
describe the 4f response in the experiments, it is insufficient to describe the
SRT even when considering a change in the 4f anisotropy. Moreover, the results
of the Fe XMCD are indicative of a decoupling of spin canting and
antiferromagnetic spin rotation in the near surface regime close to the SRT,
which remains to be understood.Comment: 28 pages, 12 figure
Ultrafast Laser-Induced Melting of Long-Range Magnetic Order in Multiferroic TbMnO3
We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray
diffraction probe measurements to investigate the coupling between the
photoexcited electronic system and the spin cycloid magnetic order in
multiferroic TbMnO3 at low temperatures. We observe melting of the long range
antiferromagnetic order at low excitation fluences with a decay time constant
of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously
observed in other systems. To explain the data we propose a simple model of the
melting process where the pump laser pulse directly excites the electronic
system, which then leads to an increase in the effective temperature of the
spin system via a slower relaxation mechanism. Despite this apparent increase
in the effective spin temperature, we do not observe changes in the wavevector
q of the antiferromagnetic spin order that would typically correlate with an
increase in temperature under equilibrium conditions. We suggest that this
behavior results from the extremely low magnon group velocity that hinders a
change in the spin-spiral wavevector on these time scales.Comment: 9 pages, 4 figure
Magnetic properties of strained multiferroic CoCr2O4: a soft X-ray study
Using resonant soft X-ray techniques we follow the magnetic behavior of a
strained epitaxial film of CoCr2O4, a type-II multiferroic. The film is
[110]-oriented, such that both the ferroelectric and ferromagnetic moments can
coexist in plane. X-ray magnetic circular dichroism (XMCD) is used in
scattering and in transmission modes to probe the magnetization of Co and Cr
separately. The transmission measurements utilized X-ray excited optical
luminescence from the substrate. Resonant soft X-ray diffraction (RSXD) was
used to study the magnetic order of the low temperature phase. The XMCD signals
of Co and Cr appear at the same ordering temperature Tc~90K, and are always
opposite in sign. The coercive field of the Co and of Cr moments is the same,
and is approximately two orders of magnitude higher than in bulk. Through sum
rules analysis an enlarged Co2+ orbital moment (m_L) is found, which can
explain this hardening. The RSXD signal of the (q q 0) reflection appears below
Ts, the same ordering temperature as the conical magnetic structure in bulk,
indicating that this phase remains multiferroic under strain. To describe the
azimuthal dependence of this reflection, a slight modification is required to
the spin model proposed by the conventional Lyons-Kaplan-Dwight-Menyuk theory
for magnetic spinels. Lastly, a slight increase in reflected intensity is
observed below Ts=27K when measuring at the Cr edge (but not at the Co edge).Comment: 28 pages, 15 figure
The ultrafast Einstein–de Haas effect
The Einstein-de Haas effect was originally observed in a landmark experiment1 demonstrating that the angular momentum associated with aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetization using an external magnetic field. A related problem concerns the timescale of this angular momentum transfer. Experiments have established that intense photoexcitation in several metallic ferromagnets leads to a drop in magnetization on a timescale shorter than 100 femtoseconds—a phenomenon called ultrafast demagnetization2,3,4. Although the microscopic mechanism for this process has been hotly debated, the key question of where the angular momentum goes on these femtosecond timescales remains unanswered. Here we use femtosecond time-resolved X-ray diffraction to show that most of the angular momentum lost from the spin system upon laser-induced demagnetization of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, launching a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the X-ray data to simulations and optical data, we estimate that the angular momentum transfer occurs on a timescale of 200 femtoseconds and corresponds to 80 per cent of the angular momentum that is lost from the spin system. Our results show that interaction with the lattice has an essential role in the process of ultrafast demagnetization in this system