Ferrimagnetic Insulator Research Articles

A comparative transport study of Bi2Se3 and Bi2Se3/yttrium iron garnet

Bilayers of 20 quintuple layer Bi2Se3 on 30 nm thick yttrium iron garnet (YIG) have been grown with molecular beam epitaxy in conjunction with pulsed laser deposition. The presence of the ferri-magnetic insulator YIG causes additional scattering to the surface states of the Bi2Se3 topological insulator layer, as indicated by the temperature dependence of the resistivity. From the two-channel analysis of the Hall data, we find that the surface contribution in the bilayer samples is greatly reduced. Furthermore, the weak antilocalization effect from the surface states is clearly suppressed due to the presence of the YIG layer.

Proximity induced high-temperature magnetic order in topological insulator--ferrimagnetic insulator heterostructure.

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.

High-Temperature Ferrimagnetism Driven by Lattice Distortion in Double Perovskite Ca2FeOsO6

5d and 3d hybrid solid-state oxide Ca2FeOsO6 crystallizes into an ordered double-perovskite structure with a space group of P2₁/n with high-pressures and temperatures. Ca2FeOsO6 presents a long-range ferrimagnetic transition at a temperature of ~320 K (T(c)) and is not a band insulator, but is electrically insulating like the recently discovered Sr2CrOsO6 (T(c) ~725 K). The electronic stat of Ca2FeOsO6 is adjacent to a half-metallic state as well as that of Sr2CrOsO6. In addition, the high-T(c) ferrimagnetism was driven by lattice distortion, which was observed for the first time among double-perovskite oxides and represents complex interplays between spins and orbitals. Unlike conventional ferrite and garnet, the interplays likely play a pivotal role of the ferrimagnetism. A new class of 5d-3d hybrid ferrimagnetic insulators with high-T(c) is established to develop practically and scientifically useful spintronic materials.

High Cooperativity in Coupled Microwave Resonator Ferrimagnetic Insulator Hybrids

We report the observation of strong coupling between the exchange-coupled spins in a gallium-doped yttrium iron garnet and a superconducting coplanar microwave resonator made from Nb. The measured coupling rate of 450 MHz is proportional to the square root of the number of exchange-coupled spins and well exceeds the loss rate of 50 MHz of the spin system. This demonstrates that exchange-coupled systems are suitable for cavity quantum electrodynamics experiments, while allowing high integration densities due to their spin densities of the order of one Bohr magneton per atom. Our results furthermore show, that experiments with multiple exchange-coupled spin systems interacting via a single resonator are within reach.

Thermal imaging of standing spin waves

A simple method for the spatial imaging of standing spin-wave modes is demonstrated. We observed instantaneous temperature-rise images on a polycrystalline ferrimagnetic insulator Y3Fe5O12 slab using an infrared camera immediately after exciting magnetostatic standing spin-wave modes. We found that a wavy temperature distribution appears before the temperature distribution on the sample is smoothed by heat conduction. The distribution is consistent with the result of a calculation based on the Landau-Lifshitz-Gilbert equation using the effective demagnetization coefficient of the slab. This result proves that the method can image the spatially distributed patterns of standing spin waves.

First-principles investigation of magnetism and ferroelectricity in Ni-doped BiCoO3

The electronic and magnetic properties of BiCo0.875Ni0.125O3 were systematically investigated within the framework of density functional theory. Calculations show that C-type antiferromagnetic structure is the ground state in Ni-doped BiCoO3. The hybridization between Bi–O, Co–O, and Ni–O plays an essential role in the C-AFM behavior. The macroscopic magnetization caused by the short-range antiferromagnetic coupling can be greatly increased by Ni doping. The total magnetic moment of BiCo0.875Ni0.125O3 is 1.02 per 2 × 2 × 2 supercell, which is considerably enhanced, compared with BiCoO3. With the GGA + U method, a ferrimagnetic insulator can be predicted for BiCo0.875Ni0.125O3. The electronic polarization of BiCo0.875Ni0.125O3 is predicted by the Berry-phase calculations, with the value of 158.9 µC cm−2. Our results clearly demonstrate that Ni doping may lead the BiCoO3 system to show ferrimagnetic properties, while maintaining its excellent ferroelectric properties.

Investigation of induced Pt magnetic polarization in Pt/Y3Fe5O12 bilayers

Using X-ray magnetic circular dichroism (XMCD) measurements, we explore the possible existence of induced magnetic moments in thin Pt films deposited onto the ferrimagnetic insulator yttrium iron garnet (Y3Fe5O12). Such a magnetic proximity effect is well established for Pt/ferromagnetic metal heterostructures. Indeed, we observe a clear XMCD signal at the Pt L3 edge in Pt/Fe bilayers, while no such signal can be discerned in XMCD traces of Pt/Y3Fe5O12 bilayers. Integrating the XMCD signals allows to estimate an upper limit for the induced Pt magnetic polarization in Pt/Y3Fe5O12 bilayers.

Directional control of spin-wave emission by spatially shaped light

In future spintronics it is anticipated that spin waves will function as unique information carriers that are free from Joule heating1,2,3,4,5,6,7. Directional control of spin-wave emission has been desired for the realization of switching devices. Here, we propose a promising technique that makes use of a spatially shaped light pulse with circular polarization. Focusing this light pulse on a magnet generates spin waves via the inverse Faraday effect. Moreover, the wavenumber distribution of the spin waves is determined by the spatial intensity distribution of the light spot. We demonstrate the principle of this technique both theoretically and experimentally. We successfully control the direction of the energy flow by shaping the light spot into an ellipse, with its major axis parallel or perpendicular to the magnetic field. Our findings will open up the possibility of fast and arbitrary synthesis of spin-wave patterns by using a more sophisticated light-shaping technique, for example using a computer-generated hologram8. Researchers use spatially shaped light to control the direction of spin-wave emission from the ferrimagnetic insulator Gd4/3Yb2/3BiFe5O12. They capture the essential features of the observations by employing a simple model that maps the spatial profile of the pump pulse onto the dispersion relation of the spin wave.

Kondo Metal and Ferrimagnetic Insulator on the Triangular Kagome Lattice

We obtain the rich phase diagrams in the Hubbard model on the triangular kagome lattice as a function of interaction, temperature, and asymmetry by combining the cellular dynamical mean-field theory with the continuous time quantum Monte Carlo method. The phase diagrams show the asymmetry separates the critical points in the Mott transition of two sublattices on the triangular kagome lattice and produces two novel phases called plaquette insulator with a clearly visible gap and a gapless Kondo metal. When the Coulomb interaction is stronger than the critical value U(c), a short range paramagnetic insulating state, which is a candidate for the short rang resonating valence-bond spin liquid, emerges before the ferrimagnetic order is formed independent of asymmetry. Furthermore, we discuss how to measure these phases in future experiments.

Long-Wavelength Helimagnetic Order and Skyrmion Lattice Phase inCu2OSeO3

We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe, and Fe(1-x)Co(x)Si, i.e., binary isostructural siblings of Cu2OSeO3 that order helimagnetically. The temperature dependence of the specific heat of Cu2OSeO3 is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far.

Enhanced spin pumping at yttrium iron garnet/Au interfaces

Spin injection across the ferrimagnetic insulator yttrium iron garnet (YIG)/normal metal Au interface was studied using ferromagnetic resonance. The spin mixing conductance was determined by comparing the Gilbert damping parameter α in YIG/Au and YIG/Au/Fe heterostructures. The main purpose of this study was to correlate the spin pumping efficiency with chemical modifications of the YIG film surface using in situ etching and deposition techniques. By means of Ar+ ion beam etching, one is able to increase the spin mixing conductance at the YIG/Au interface by a factor of 5 compared to the untreated YIG/Au interface.

Skyrmion quantum numbers and quantized pumping in two-dimensional topological chiral magnets

We investigate the general conditions to achieve the adiabatic charge and spin polarizations and quantized pumping in 2D magnetic insulators possessing inhomogeneous spin structures. In particular, we focus on the chiral ferrimagnetic insulators that are generated via spontaneous symmetry breaking from correlated two-dimensional topological insulators. Adiabatic deformation of the inhomogeneous spin structure generates the spin gauge flux, which induces adiabatic charge and spin-polarization currents. The unit pumped charge and spin are determined by the product of two topological invariants, which are defined in momentum and real spaces, respectively. The same topological invariants determine the charge and spin quantum numbers of skyrmion textures. It is found that in noncentrosymmetric systems, a new topological phase, dubbed the topological chiral magnetic insulator, exists in which a skyrmion defect is a spin-1/2 fermion with electric charge $e$. Considering the adiabatic current responses of generic inhomogeneous systems, it is shown that the quantized topological response of chiral magnetic insulators is endowed with the second Chern number.

Intriguing sequence of GaFeO3structures and electronic states to 70 GPa

Structural studies of the ferrimagnetic (${T}_{\mathrm{N}}$ = 200 K) Mott insulator GaFeO${}_{3}$ (SG Pc2${}_{1}n$) to 70 GPa, complemented by ${}^{57}$Fe M\"ossbauer spectroscopy and resistance (R) measurements at compression, decompression, and recompression, reveal a fascinating sequence of structures. Starting at \ensuremath{\sim}25 GPa a new structure, an orthorhombic perovskite (Pv) (SG Pbnm), is sluggishly formed followed by a volume V(P) drop of 5.4%. The complete formation of the Pv occurs at 42 GPa. In the 0--33 GPa range ${T}_{\mathrm{N}}$ reaches 300 K and R(P) decreases by one order of magnitude. At 53 GPa an isostructural transition is detected, characterized by a discontinuous drop of V(P) by \ensuremath{\sim}3%. M\"ossbauer spectra (MS) reveal a nonmagnetic component coexisting with the magnetic one at \ensuremath{\sim}60 GPa. Its abundance increases and above 77 GPa no sign of a magnetic hyperfine interaction is detected down to 5 K. Concurrently, one observes a continuous yet precipitous decrease in R(P) taking place in the 53--68 GPa range, leading to an onset of the metallic state at P = 68 GPa. These electronic/magnetic features of the high pressure (HP) Pv are consistent with a Mott transition. With pressure decrease below 50 GPa, the insulating Pv is recovered, and at \ensuremath{\sim}24 GPa a 1st-order structural transition takes place to a LiNbO${}_{3}$-type structure with SG R3c. This structure remains stable down to ambient pressure and with recompression it is stable up to 50 GPa, afterwards it transforms back to the HP Pv structure. It is noteworthy that this transition occurs at the same pressure, regardless of the preceding structures: Pbnm or R3c. The results are compared with hematite (Fe${}_{2}$O${}_{3}$, SG $R\overline{3}c$) and other ferric oxides. The mechanisms of the transitions are discussed.

Spin Pumping at the Magnetic Insulator (YIG)/Normal Metal (Au) Interfaces

Spin injection across the ferrimagnetic insulator (YIG)/normal metal (Au) interface was studied by ferromagnetic resonance. The spin mixing conductance was determined by comparing the Gilbert damping in bare YIG films with those covered by a Au/Fe/Au structure. The Fe layer in Au/Fe/Au acted as a spin sink as displayed by an increased Gilbert damping parameter α compared to that in the bare YIG. In particular, for the 9.0 nm YIG/2.0 nm Au/4.3 nm Fe/6.1 nm Au structure, the YIG and Fe films were coupled by an interlayer exchange coupling, and the exchange coupled YIG exhibited an increased Gilbert damping compared to the bare YIG. This relationship between static and dynamic coupling provides direct evidence for spin pumping. The transfer of spin momentum across the YIG interface is surprisingly efficient with the spin mixing conductance g(↑↓) ≃ 1.2 × 10(14) cm(-2).

Electric detection of the spin-Seebeck effect in magnetic insulator in the presence of interface barrier

The spin-Seebeck effect (SSE), the spin-voltage generation as a result of a temperature gradient, has recently been observed in ferrimagnetic insulator LaY2Fe5O12 films by means of the inverse spin-Hall effect in Pt films. Here we investigate the SSE using LaY2Fe5O12/SiO2(Cu)/Pt systems, where the LaY2Fe5O12 and Pt layers are separated by SiO2 (Cu) thin-film barriers. The experimental results show that the SSE signal disappears in the LaY2Fe5O12/SiO2/Pt system, but the finite signal appears in the LaY2Fe5O12/Cu/Pt system, indicating that the direct contacts between the LaY2Fe5O12 and normal metals is necessary for generating the SSE signal.

Observation of longitudinal spin-Seebeck effect in magnetic insulators

We propose a longitudinal spin-Seebeck effect (SSE), in which a magnon-induced spin current is injected parallel to a temperature gradient from a ferromagnet into an attached paramagnetic metal. The longitudinal SSE is measured in a simple and versatile system composed of a ferrimagnetic insulator Y3Fe5O12 slab and a Pt film by means of the inverse spin-Hall effect. The experimental results highlight the intriguing character of the longitudinal SSE due to its own geometric configuration.

Electronic structures of quasi-one-dimensional ferrimagnetic insulator Ca 3Co 2O 6

The electronic structures of quasi-one-dimensional ferrimagnetic Ca 3Co 2O 6 are investigated using the generalized gradient approximation (GGA) as well as the GGA plus on-site Coulomb interaction ( GGA + U ) scheme. GGA + U calculations reveal that the interchain ferrimagnetic Ca 3Co 2O 6 is a Mott–Hubbard insulator rather than a metal given from GGA. In addition, we found an on-site U induced 3 z 2 − r 2 orbital ordering on Co pri sublattice which drives the intrachain ferromagnetic coupling along c-axis. Our findings suggest that strong electron-electron correlation plays an important role in Ca 3Co 2O 6.

Tracks of 30-MeV C 60 clusters in yttrium iron garnet studied by scanning force microscopy

Yttrium iron garnet (Y 3Fe 5O 12 or YIG), an amorphizable ferrimagnetic insulator, is probably the best studied material with respect to track formation and damage morphology. This paper presents first scanning force microscopy (SFM) of surface damage induced by energetic C 60 clusters. YIG single crystals were irradiated at normal incidence with 30-MeV C 60 cluster ions (kinetic energy ∼0.04 MeV/u) provided by the tandem accelerator of the Institute of Nuclear Physics in Orsay (IPNO). The SFM topographic images show nano-protrusions on the YIG surface; where each hillock is generated by one C 60 cluster. The role of stopping power and deposited energy density is discussed in terms of dimensional analysis of the nanostructures. Hillocks created by C 60 clusters are compared with those produced by monatomic ions.

Enhanced magnetization of CuCr2O4 thin films by substrate-induced strain

The first synthesis of epitaxial spinel CuCr2O4 thin films is reported, which exhibit enhanced magnetization in excess of 200% of the accepted bulk value when grown on single-crystal (110) MgAl2O4 substrates. Bulk CuCr2O4 is a ferrimagnetic insulator with a low net magnetic moment of 0.5μB due to its tetragonal unit cell with a c∕a ratio of 1.29 and frustrated moment configuration. It is shown that through epitaxial growth and substrate-induced strain, it is possible to enhance the magnetization of these films by reducing the tetragonality of its unit cell which, in turn, effectively decreases magnetic moment frustration allowing for a greater net moment.

Y 3Fe 5O 12 yttrium iron garnet and lost magnetic moment (computing of spin density)

The yttrium iron garnet (YIG) has been intensively studied in the past as a convenient ferrimagnetic insulator for resonance devices and magnetic memories. The YIG was considered as the model of ionic compound with the ion Fe 3+ and 5 μ B on each of two iron sites with a resulting magnetization of 5 μ B. Contrarily, all neutron diffraction studies lead to a localized moment close to 4 μ B. The present work is a tentative to understand these noticeable discrepancies by using the electronic structure computing.