Ferrimagnetic Insulator Research Articles

Detection of induced paramagnetic moments in Pt onY3Fe5O12via x-ray magnetic circular dichroism

Magnetic moments in an ultra-thin Pt film on a ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ (YIG) have been investigated at high magnetic fields and low temperatures by means of X-ray magnetic circular dichroism (XMCD). We observed an XMCD signal due to the magnetic moments in a Pt film at the Pt $L_{3}$- and $L_{2}$-edges. By means of the element-specific magnetometry, we found that the XMCD signal at the Pt $L_{3}$-edge gradually increases with increasing the magnetic field even when the field is much greater than the saturation field of YIG. Importantly, the observed XMCD intensity was found to be much greater than the intensity expected from the Pauli paramagnetism of Pt when the Pt film is attached to YIG. These results imply the emergence of induced paramagnetic moments in Pt on YIG and explain the characteristics of the unconventional Hall effect in Pt/YIG systems.

Tailoring magnetic insulator proximity effects in graphene: first-principles calculations

We report a systematic first-principles investigation of the influence of different magnetic insulators on the magnetic proximity effect induced in graphene. Four different magnetic insulators are considered: two ferromagnetic europium chalcogenides namely EuO and EuS and two ferrimagnetic insulators yttrium iron garnet (YIG) and cobalt ferrite (CFO). The obtained exchange-splitting in graphene varies from tens to hundreds of meV depending on substrates. We find an electron doping to graphene induced by YIG and europium chalcogenides substrates, that shift the Fermi level above the Dirac cone up to 0.78 eV and 1.3 eV respectively, whereas hole doping shifts the Fermi level down below the Dirac cone about 0.5 eV in graphene/CFO. Furthermore, we study the variation of the extracted exchange and tight-binding parameters as a function of the EuO and EuS thicknesses. We show that those parameters are robust to thickness variation such that a single monolayer of magnetic insulator can induce a strong magnetic proximity effect on graphene. Those findings pave the way towards possible engineering of graphene spin-gating by proximity effect especially in view of recent experimental advancements.

Quantitative investigation of the inverse Rashba-Edelstein effect in Bi/Ag and Ag/Bi on YIG

The inverse Rashba-Edelstein effect (IREE) is a spin conversion mechanism that recently attracts attention in spintronics and condensed matter physics. In this letter, we report an investigation of the IREE in Bi/Ag by using ferrimagnetic insulator yttrium iron garnet. We prepared two types of samples with opposite directions of the Rashba field by changing a stacking order of Bi and Ag. An electric current generated by the IREE was observed from both stacks, and an efficiency of spin conversion—characterized by the IREE length—was estimated by taking into account a number of contributions left out in previous studies. This study provides a further insight into the IREE spin conversion mechanism: important step towards achieving efficient spin-charge conversion devices.

Probing current-induced magnetic fields in Au|YIG heterostructures with low-energy muon spin spectroscopy

We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet—YIG) by using low energy muon spin spectroscopy (LE-μSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maximum shift of 0.4 G in the internal field of muons at the surface of Au film which is in close agreement with the value expected for Oersted fields. As muons are implanted closer to the Au|YIG interface, the shift is strongly suppressed, which we attribute to the dipolar fields present at the Au|YIG interface. Combining our measurements with modeling, we show that dipolar fields caused by the finite roughness of the Au|YIG interface consistently explain our observations. Our results, therefore, gauge the limits on the spatial resolution and the sensitivity of LE-μSR to the roughness of the buried magnetic interfaces, a prerequisite for future studies addressing current induced fields caused by the spin-accumulations due to the spin-Hall effect.

Symmetry breaking of the surface mediated quantum Hall Effect in Bi2Se3 nanoplates using Fe3O4 substrates

Bi2Se3 nanoplate devices are synthesized on SiO2 and ferrimagnetic insulator substrates. We experimentally demonstrate that ferromagnetism is induced into the bottom surface. The symmetry broken bottom states give rise to an additional Shubnikov–de Haas frequency and leads to a decoupling of the top and bottom quantum Hall effects. We present a three-channel model that separates the bulk, top and bottom surface contributions to the Hall resistance, indicating the presence of two symmetry shifted half-integer QHEs.

Efficient spin transport through polyaniline

By using the spin pumping process, we show that it is possible to transport a pure spin current across layers of conducting polyaniline (PANI) with several hundred nanometers sandwiched between a film of the ferrimagnetic insulator yttrium iron garnet (YIG) and a thin layer of platinum. The spin current generated by microwave-driven ferromagnetic resonance of the YIG film, injected through the YIG/PANI interface, crosses the whole PANI layer and then is injected into the Pt layer. By means of the inverse spin Hall effect in the Pt, the spin current is converted into charge current and electrically detected as a dc voltage. We measured a spin diffusion length in PANI of 590 \ifmmode\pm\else\textpm\fi{} 40 nm, which is very large compared with normal metals, demonstrating that PANI can be used as an efficient spin current conductor and poor charge current conductor, opening the path towards spintronics applications based in this very attractive material.

Control of Magnon-Photon Coupling Strength in a Planar Resonator/Yttrium-Iron-Garnet Thin-Film Configuration

A systematic study of the coupling at room temperature between ferromagnetic resonance and a planar resonator is presented. The chosen magnetic material is a ferrimagnetic insulator (yttrium iron garnet, YIG) which is positioned on top of a stop band (notch) filter based on a stub line capacitively coupled to a 50 Ω microstrip line resonating at 4.731 GHz. Control of the magnon-photon coupling strength is discussed in terms of the microwave excitation configuration and the YIG thickness from 0.2 to 41 μm. From the latter dependence, we extract a single spin-photon coupling of g 0 /2π = 162 ± 6 mHz and a maximum of an effective coupling of 290 MHz.

Ba2NiOsO6 : A Dirac-Mott insulator with ferromagnetism near 100 K

The ferromagnetic semiconductor $\mathrm{B}{\mathrm{a}}_{2}\mathrm{NiOs}{\mathrm{O}}_{6}$ (${T}_{\mathrm{mag}}\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) was synthesized at 6 GPa and 1500 \ifmmode^\circ\else\textdegree\fi{}C. It crystallizes into a double perovskite structure [$Fm\ensuremath{-}3m$; $a=8.0428(1)\phantom{\rule{0.16em}{0ex}}\AA{}$], where the $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{6+}$ ions are perfectly ordered at the perovskite $B$ site. We show that the spin-orbit coupling of $\mathrm{O}{\mathrm{s}}^{6+}$ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a $>21\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$ magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te (${T}_{\mathrm{mag}}<180\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), the spin-gapless semiconductor $\mathrm{M}{\mathrm{n}}_{2}\mathrm{CoAl}$ (${T}_{\mathrm{mag}}\ensuremath{\sim}720\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), and the ferromagnetic insulators EuO (${T}_{\mathrm{mag}}\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and $\mathrm{B}{\mathrm{i}}_{3}\mathrm{C}{\mathrm{r}}_{3}{\mathrm{O}}_{11}$ (${T}_{\mathrm{mag}}\ensuremath{\sim}220\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of $\mathrm{B}{\mathrm{a}}_{2}\mathrm{NiOs}{\mathrm{O}}_{6}$ should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.

Interface electronic structure at the topological insulator–ferrimagnetic insulator junction

An interface electron state at the junction between a three-dimensional topological insulator film, Bi2Se3, and a ferrimagnetic insulator film, Y3Fe5O12 (YIG), was investigated by measurements of angle-resolved photoelectron spectroscopy and x-ray absorption magnetic circular dichroism. The surface state of the Bi2Se3 film was directly observed and localized 3d spin states of the Fe3+ in the YIG film were confirmed. The proximity effect is likely described in terms of the exchange interaction between the localized Fe 3d electrons in the YIG film and delocalized electrons of the surface and bulk states in the Bi2Se3 film.

Gd-doping-induced insulator-metal transition in SrTiO3

Recently, insulator-metal transition was found experimentally in Gd-doped SrTiO3 films. Here, we present first-principle investigation on the structural, electronic and magnetic properties of Sr1−xGdxTiO3 within density-function theory. The spin-polarized calculations give a diamagnetic insulator at x=0, a ferrimagnetic metal 0.125≤ x≤0.5 and a ferrimagnetic insulator x=1 and all Ti ions moments are antiparallel to Gd ions moments. Magnetic Gd-doping distorts the structures of Sr1−xGdxTiO3 films and results in ferrimagnetism. Doped electrons occupy the bottom of conduction bands so that insulator-metal transition occurs. These calculated results are in agreement with available experiments.

β-Tantalum, a better candidate for spin-to-charge conversion

Abstract Development of thermoelectric energy harvesting devices has hit a stumbling block due to the intrinsically linked electrical and thermal conductivities of materials. However, this field can still be improved by employing devices that take advantage of spin-based effects. A temperature gradient can be converted to a spin-polarized current in a ferrimagnetic insulator by the spin Seebeck effect (SSE), and that spin current can be converted to an electrical voltage in a heavy metal by the Inverse spin Hall effect (ISHE). Thus, the thermal energy capture and charge production steps can be separated into two distinct regions of the thermoelectric device, allowing separate tuning of electrical and thermal conductivities. The second step of this process, spin current to electrical voltage conversion, is controlled by the strength and sign of ISHE in the metal, and platinum has become the standard for this purpose. However, here we report a better candidate, β-Tantalum, which shows a spin Seebeck voltage approximately 4 times higher than that of Pt at room temperature. The temperature dependence of spin Seebeck in YIG/β-Ta also closely follows that of YIG/Pt, consistent with magnon spin current theory. The sign of the spin Seebeck voltage in Ta found to be opposite to that of Pt, making the two materials highly complementary for fabricating spintronics-based thermoelectric modules for practical applications.

Anomalous Hall hysteresis inTm3Fe5O12/Ptwith strain-induced perpendicular magnetic anisotropy

We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically flat ferrimagnetic insulator Tm3Fe5O12 films grown with pulsed laser deposition on substituted-Gd3Ga5O12 substrate which maximizes the tensile strain at the interface. In bilayers consisting of Pt and TIG, we observe large squared Hall hysteresis loops over a wide range of thicknesses of Pt at room temperature. When a thin Cu layer is inserted between Pt and TIG, the Hall hysteresis magnitude decays but stays finite as the thickness of Cu increases up to 5 nm. However, if the Cu layer is placed atop Pt instead, the Hall hysteresis magnitude is consistently larger than when the Cu layer with the same thickness is inserted in between for all Cu thicknesses. These results suggest that both the proximity-induced ferromagnetism and spin current contribute to the anomalous Hall effect.

Optomagnonic Whispering Gallery Microresonators.

Magnons in ferrimagnetic insulators such as yttrium iron garnet (YIG) have recently emerged as promising candidates for coherent information processing in microwave circuits. Here we demonstrate optical whispering gallery modes of a YIG sphere interrogated by a silicon nitride photonic waveguide, with quality factors approaching 10^{6} in the telecom c band after surface treatments. Moreover, in contrast to conventional Faraday setups, this implement allows an input photon polarized colinearly to the magnetization to be scattered to a sideband mode of orthogonal polarization. This Brillouin scattering process is enhanced through triply resonant magnon, pump, and signal photon modes within an "optomagnonic cavity." Our results show the potential use of magnons for mediating microwave-to-optical carrier conversion.

Epitaxial synthesis and physical properties of double-perovskite oxide Sr2CoRuO6 thin films

We report epitaxial structures and physical properties of double-perovskite Sr2CoRuO6 films grown using pulsed-laser deposition. Samples with a degree of Co/Ru order of 2–73% were obtained by changing growth temperature. X-ray absorption spectroscopy (XAS) on the highest ordered sample revealed that Co ions were trivalent with a high-spin configuration and Ru ions were pentavalent. We found large differences in magnetization and resistivity between the highest and lowest ordered samples as well as the absence of strong magnetism and metallicity, which are common characteristics of SrCoO3 and SrRuO3. Using resonant photoemission spectroscopy and XAS, dominant d-orbital components at the top of the occupied state (the bottom of the unoccupied state) were identified to be Ru 4d t2g (Co 3d and Ru 4d t2g). These results suggest that the ground state of double-perovskite Sr2CoRuO6 is a ferrimagnetic insulator.

Magnon-based logic in a multi-terminal YIG/Pt nanostructure

Boolean logic is the foundation of modern digital information processing. Recently, there has been a growing interest in phenomena based on pure spin currents, which allows to move from charge to spin based logic gates. We study a proof-of-principle logic device based on the ferrimagnetic insulator Yttrium Iron Garnet, with Pt strips acting as injectors and detectors for non-equilibrium magnons. We experimentally observe incoherent superposition of magnons generated by different injectors. This allows to implement a fully functional majority gate, enabling multiple logic operations (AND and OR) in one and the same device. Clocking frequencies of the order of several GHz and straightforward down-scaling make our device promising for applications.

Transport and spin conversion of multicarriers in semimetal bismuth

In this paper, we report on the investigation of (1) the transport properties of multi-carriers in semi-metal Bi and (2) the spin conversion physics in this semimetal system in a ferrimagnetic insulator, yttrium-iron-garnet. Hall measurements reveal that electrons and holes co-exist in the Bi, with electrons being the dominant carrier. The results of a spin conversion experiment corroborate the results of the Hall measurement; in addition, the inverse spin Hall effect governs the spin conversion in the semimetal/insulator system. This study provides further insights into spin conversion physics in semimetal systems.

Various types of photoactive behavior of the Y3Fe5O12 ferrimagnetic insulator

The photomagnetic behavior of single-crystal yttrium iron garnet Y3Fe5O12 doped with iridium, substituting the cation of iron in the octahedron, is investigated upon illumination at room temperature. It is shown that the photomagnetic properties of Y3Fe4.97Ir0.03O12 samples are to a large degree related to the impurity distortion of the sublattice of iron atoms in octahedral coordination, rather than solely to the possible presence of Fe4+ cations, which are inactive at room temperature and may even be lacking in single crystals doped with iridium. It is concluded that the photoinduced change in the magnetic parameters of this material is determined by the location of impurity cations and increased surface imperfection of the material. The reasons for the different photoactive behavior of this promising material for spintronics, that is, a singlecrystal yttrium iron garnet, are summarized.

Gate-Tunable Spin-Charge Conversion and the Role of Spin-Orbit Interaction in Graphene.

The small spin-orbit interaction of carbon atoms in graphene promises a long spin diffusion length and the potential to create a spin field-effect transistor. However, for this reason, graphene was largely overlooked as a possible spin-charge conversion material. We report electric gate tuning of the spin-charge conversion voltage signal in single-layer graphene. Using spin pumping from an yttrium iron garnet ferrimagnetic insulator and ionic liquid top gate, we determined that the inverse spin Hall effect is the dominant spin-charge conversion mechanism in single-layer graphene. From the gate dependence of the electromotive force we showed the dominance of the intrinsic over Rashba spin-orbit interaction, a long-standing question in graphene research.

Potential multiferroic materials of Fe-substituted BiCoO3: An ab initio study

The crystal structure, ferroelectric polarization, magnetism, and electronic structure in Fe-substituted tetragonal BiCoO3 at concentrations of 25% and 50% are investigated within the framework of density functional theory. The C-type antiferromagnetic (AFM) spin configurations with intra-layer AFM coupling interactions are energetically favorable among all the considered magnetic ordering states. Fe substitutions produce 1μB net magnetic moment in Fe-substituted BiCoO3 system. Using the point charge model, the ferroelectric polarizations are predicted to be as high as 165 and 163μC/cm2 for 25% and 50% Fe substitution. Electronic band structures reveal that the Fe-substituted BiCoO3 systems are ferrimagnetic insulator. Present first-principles calculation results demonstrate that Fe substitutional doping may produce multiferroic materials simultaneously showing ferrimagnetic and excellent ferroelectric properties.

Exquisite growth control and magnetic properties of yttrium iron garnet thin films

A layer-by-layer epitaxial growth up to 227 atomic layers of ferrimagnetic insulator yttrium iron garnet (YIG) thin films is achieved on (110)-oriented gadolinium gallium garnet substrates using pulsed laser deposition. Atomically smooth terraces are observed on YIG films up to 100 nm in thickness. The root-mean-square roughness is as low as 0.067 nm. The easy-axis lies in the film plane, indicating the dominance of shape anisotropy. For (110)-YIG films, there is well-defined two-fold in-plane anisotropy, with the easiest axis directed along [001]. The Gilbert damping constant is determined to be 1.0 × 10−4 for 100 nm thick films.