Ferrimagnetic Yttrium Iron Garnet Research Articles

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.

Tm3Fe5O12/Pt Heterostructures with Perpendicular Magnetic Anisotropy for Spintronic Applications

With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub‐10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin‐current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR‐driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature‐dependent PMA of the TmIG thin film.

Bose-Einstein condensation of magnons pumped by the bulk spin Seebeck effect

We propose inducing Bose-Einstein condensation of magnons in a magnetic insulator by a heat flow oriented toward its boundary. At a critical heat flux, the oversaturated thermal gas of magnons accumulated at the boundary precipitates the condensate, which then grows gradually as the thermal bias is dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin) Seebeck effect must generally overcome both the local Gilbert damping associated with the coherent magnetic dynamics as well as the radiative spin-wave losses toward the magnetic bulk, in order to achieve the threshold of condensation. We quantitatively estimate the requisite bias in the case of the ferrimagnetic yttrium iron garnet, discuss different physical regimes of condensation, and contrast it with the competing (so-called Doppler-shift) bulk instability.

Theory of the spin Seebeck effect in antiferromagnets

The spin Seebeck effect (SSE) consists in the generation of a spin current by a temperature gradient applied in a magnetic film. The SSE is usually detected by an electric voltage generated in a metallic layer in contact with the magnetic film resulting from the conversion of the spin current into charge current by means of the inverse spin Hall effect. The SSE has been widely studied in bilayers made of the insulating ferrimagnet yttrium iron garnet (YIG) and metals with large spin-orbit coupling such as platinum. Recently the SSE has been observed in bilayers made of the antiferromagnet $\mathrm{Mn}{\mathrm{F}}_{2}$ and Pt, revealing dependences of the SSE voltage on temperature and field very different from the ones observed in YIG/Pt. Here we present a theory for the SSE in structures with an antiferromagnetic insulator (AFI) in contact with a normal metal (NM) that relies on the bulk magnon spin current created by the temperature gradient across the thickness of the AFI/NM bilayer. The theory explains quite well the measured dependences of the SSE voltage on the sample temperature and on the applied magnetic field in $\mathrm{Mn}{\mathrm{F}}_{2}/\mathrm{Pt}$.

Length Scale of the Spin Seebeck Effect.

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50 μm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found.

Nonlinear dynamics of three-magnon process driven by ferromagnetic resonance in yttrium iron garnet

We report an investigation of the dynamics of the three-magnon splitting process associated with the ferromagnetic resonance (FMR) in films of the insulating ferrimagnet yttrium iron garnet (YIG). The experiments are performed with a 6 μm thick YIG film close to a microstrip line fed by a microwave generator operating in the 2–6 GHz range. The magnetization precession is driven by the microwave rf magnetic field perpendicular to the static magnetic field, and its dynamics is observed by monitoring the amplitude of the FMR absorption peak. The time evolution of the amplitude reveals that if the frequency is lowered below a critical value of 3.3 GHz, the FMR mode pumps two magnons with opposite wave vectors that react back on the FMR, resulting in a nonlinear dynamics of the magnetization. The results are explained by a model with coupled nonlinear equations describing the time evolution of the magnon modes.

Design of a polarisation reconfigurable patch antenna using ferrimagnetic materials

In this study the authors introduce the utilisation of ferrimagnetic compounds into an antenna design and investigate the capability of controlling the antenna properties by means of an external magnetic field. After intensive study of the material properties, the ferrimagnetic compound yttrium iron garnet (YIG) was found to be the best candidate for the proposed novel antenna design. The authors provide a patch antenna design with a part of the dielectric substrate replaced by YIG compound. They have investigated the influence of this compound on the antenna's polarisation properties under the application of an external magnetic field. The authors clearly demonstrate that the type and the sense of the antenna polarisation are strongly influenced by the YIG substrate, because they change with respect to the direction and the magnitude of the external magnetic field. Moreover, the authors have proved that the presence of this material induces non-reciprocal properties in the antenna's operation, changing the antenna properties with respect to receiving or transmitting mode.

Spin Pumping by Parametrically Excited Exchange Magnons

We experimentally show that exchange magnons can be detected by using a combination of spin pumping and the inverse spin-Hall effect proving its wavelength integrating capability down to the submicrometer scale. The magnons were injected in a ferrite yttrium iron garnet film by parametric pumping and the inverse spin-Hall effect voltage was detected in an attached Pt layer. The role of the density, wavelength, and spatial localization of the magnons for the spin pumping efficiency is revealed.

Large ultrafast photoinduced magnetic anisotropy in a cobalt-substituted yttrium iron garnet

We demonstrate experimentally that excitation of a Co-substituted ferrimagnetic yttrium iron garnet thin film with linearly polarized 100 fs laser pulses triggers large-angle magnetization precession with an amplitude, phase, and frequency determined by the characteristics of the laser pulse. The precession results from a light-induced anisotropy field with a characteristic lifetime of 20 ps, the direction of which is determined by the polarization of the light. Its strength for a pump intensity of $25\text{ }\text{mJ}/{\text{cm}}^{2}$ is 250 G which is comparable to the intrinsic anisotropy of the sample. By choosing the proper laser-pulse parameters, we were able to excite a precession with an amplitude as large as $20\ifmmode^\circ\else\textdegree\fi{}$ and a precession frequency modified by up to 50%.

Control of nonlinear collapse in magnetooptical Kerr media

We demonstrate a novel approach to the control of the collapse of optical beams in Kerr media. To this end, we consider a combination of two magnetooptical (MO) phenomena, the Cotton-Mouton and Faraday effects, in order to introduce a suitable and tunable interplay between the induced linear and circular birefringences. Our numerical analysis confirms that a properly applied external magnetic field may alter the dynamics of the beam propagation in magnetic self-focusing bulk Kerr media. For a chosen value of the optical power, the acceleration, the delay or even the arrest of the collapse can be achieved. We demonstrate experimentally the magnetically-induced acceleration of the collapse in a ferrimagnetic yttrium iron garnet crystal (YIG). Full Text: PDF

Element selective X-ray magnetic circular and linear dichroisms in ferrimagnetic yttrium iron garnet films

X-ray magnetic circular dichroism (XMCD) was used to probe the existence of induced magnetic moments in yttrium iron garnet (YIG) films in which yttrium is partly substituted with lanthanum, lutetium or bismuth. Spin polarization of the 4d states of yttrium and of the 5d states of lanthanum or lutetium was clearly demonstrated. Angular momentum resolved d-DOS of yttrium and lanthanun was shown to be split by the crystal field, the two resolved substructures having opposite magnetic polarization. The existence of a weak orbital moment involving the 6p states of bismuth was definitely established with the detection of a small XMCD signal at the Bi M 1 -edge. Difference spectra also enhanced the visibility of subtle changes in the Fe K-edge XMCD spectra of YIG and {Y, Bi}IG films. Weak natural X-ray linear dichroism signatures were systematically observed with all iron garnet films and with a bulk YIG single crystal cut parallel to the (1 1 1) plane: this proved that, at room temperature, the crystal cannot satisfy all requirements of perfect cubic symmetry (space group: Ia 3 ¯ d ), crystal distortions preserving at best trigonal symmetry ( R 3 ¯ or R 3 m ). For the first time, a very weak X-ray magnetic linear dichroism (XMLD) was also measured in the iron K-edge pre-peak of YIG and revealed the presence of a tiny electric quadrupole moment in the ground-state charge distribution of iron atoms. Band-structure calculations carried out with fully relativistic LMTO-LSDA methods support our interpretation that ferrimagnetically coupled spins at the iron sites induce a spin polarization of the yttrium d-DOS and reproduce the observed crystal field splitting of the XMCD signal.

Nonreciprocal Polarization Converter Consisting of Asymmetric Waveguide With Magnetooptic Cladding: Theory and Simulation

A nonreciprocal polarization converter compatible with InP-based photonic integrated circuits is proposed. The device consists of an asymmetric InGaAsP waveguide combined with a ferrimagnetic cerium-substituted yttrium iron garnet layer. It makes use of the nonreciprocal conversion of the polarization state in the waveguide. A nonreciprocal TE-TM conversion efficiency of 93% at 1.55 mum wavelength can be obtained with a device length of 0.27 mm.

Spin-Wave Ferromagnetic Film Combiner as a NOT Logic Gate

An on-chip spin-wave combiner was fabricated by structuring a ferrimagnetic yttrium iron garnet film. Interference of magnetostatic spin-wave pulses in the combiner was studied using time and space resolve Brillouin light scattering spectroscopy. This investigation demonstrates functionality of the combiner as a NOT logic gate. We believe that it is an important step towards implementation of all-spin-wave on-chip logical gates.

First order spin flop transition in yttrium iron garnet (YIG)

We have studied the field dependence of the sublattice magnetization of ferrimagnetic yttrium iron garnet (YIG) using neutron scattering. In contrast to the macroscopic spontaneous magnetization that shows the normal field dependence of a soft ferromagnet (sudden saturation at the demagnetization field and no hysteresis) in neutron scattering a field induced first order spin flop transition with considerable hysteresis is observed at a critical field of H c ∼ 580 G (external field). Considering that with neutron scattering the antiferromagnetic component of ∼4/5 of the total moment is detected preferentially while in the macroscopic magnetization samples the ferromagnetic component of ∼1/5 exclusively it becomes clear that ferromagnetic and antiferromagnetic component have a completely independent field (and temperature) dependence. This indicates that the two magnetic structures have to be viewed as two weakly coupled order parameters. In the zero field ground state the moment orientations of the two ordering structures are orthogonal. Only for fields H 0 > H c a nearly collinear ferrimagnetic order is established by the field.

Modifications of the magnetic properties of ferrites by swift heavy ion irradiations

Single crystal plates of ferrimagnetic yttrium iron garnet (111)-YIG:Si (Y3Fe4.94Si0.06O12) and barium hexaferrite (00.1)-BaM (BaFe12O19) or (00.1)-BaM:Co,Ti (BaFe9.1Co1.4Ti1.5O19) are irradiated with swift heavy ions (3.8 GeV Xe129 or 6.0 GeV Pb208) in the electronic slowing down regime, above the threshold (≈20 keV nm−1) of formation of continuous and homogeneous cylindrical amorphous tracks. The modifications of the magnetic properties are studied by Fe57 Mössbauer spectroscopy and ac magnetic permeability measurements versus ion fluence. In the doped crystals having a planar magnetic anisotropy (YIG:Si and BaM:Co,Ti), the room-temperature Mössbauer spectra show that the magnetization is flipped perpendicularly to the sample plane at a critical amorphous fraction around 30% in both compounds. This corresponds to a 90% drop of the measured in-plane magnetic permeability. No such effect is seen in the undoped BaM samples with the axial [00.1] anisotropy. These data are interpreted by a magnetomechanical effect generated by the stress field induced by the amorphous tracks in the sample plane which flips the magnetization along the track-axis direction when the stress-induced anisotropy constant surpasses the pristine crystal anisotropy constant at the critical amorphous fraction. In the case of YIG:Si single crystal, a track-induced anisotropy field around 0.1 T is deduced from the Mössbauer spectra under a magnetic field applied in the sample (111) plane which rotates the magnetization back to the easy 〈111〉 magnetization axis lying near the sample (111) plane in a reversible manner. The magnetic ordering of amorphous YIG:Si below 70 K is also studied by Mössbauer spectroscopy under high magnetic field (5 T). A two-dimensional Bruggeman model used for the calculation of the permeability of the crystal+amorphous track composites yields track-core radii larger by about 40% than the values previously obtained for both compounds by Mössbauer spectroscopy. We think that such discrepancies show that the samples cannot be treated like standard two-phase composites since the undamaged crystal undergoes a variable stress field depending on the damaged fraction.

Microwave compatible YBa2Cu3O7−x films on ferrimagnetic garnet substrates

High quality epitaxial YBa2Cu3O7−x (YBCO) thin films have been deposited onto ferrimagnetic yttrium iron garnet (100) substrates using a double buffer layer lattice engineering scheme. The YBCO films are c-axis oriented and show superconductive transition temperatures of 88–89 K with transition widths ≤0.5 K. The films also exhibited Jc’s of 2.7×106 A/cm2 in zero field and 1.1 ×106 A/cm2 in 2000 G at 77 K. Dielectric resonator measurements indicated unloaded Q values of 29 000 at 77 K and 24 GHz for unpatterned films. These values correspond to a surface resistance (RS) of 500 μΩ at 77 K, 10 GHz, which is the lowest obtained to date for a YBCO film on a ferrimagnetic substrate. Such films could be used for fabricating miniature-high-temperature superconducting microwave elements such as nonreciprocal and passive microwave devices, making possible, their integration on a single ferrimagnetic wafer.

A theoretical investigation on the magneto-optical spectra of Ce-substituted yttrium aluminium garnet

Calculations of the Faraday rotation and ellipticity spectra at photon energies between 1.6 and 4.0 eV (wavelengths between 775 and 310 nm) resulting from Ce3+ ions in Ce-substituted yttrium aluminium garnets (YAG:Ce) based on the quantum theory are presented. The Faraday effect contributed by the Ce3+ ions is caused mainly by the intra-ionic electrical dipole transitions between the 4f and 5d configurations. The Faraday effect of the Ce3+ ions in both the diamagnetic YAG and the ferrimagnetic yttrium iron garnet (YIG) crystals has a paramagnetic spectral shape but the resonance frequencies of YAG:Ce are at 2.76 and 3.7 eV, which are obviously higher than those of YIG:Ce. This difference can be explained by the fact that the crystal field acting upon the rare-earth ions in garnets depends strongly on the nature of the next-nearest neighbours. The differences between the transition intensities for the right- and left-handed circularly polarized light from both the lowest and the second crystal-field (CF)-split 5d levels to the ground state are negative. Therefore the Faraday rotation spectrum contributed by the Ce3+ ions in YAG has a wave-like shape in the energy region considered. However, for YIG:Ce, the difference between the transition intensities for the right- and left-handed circularly polarized light from the second CF-split 5d level to the ground state is positive. So the corresponding spectrum for YIG:Ce has a different shape. The calculated results are in good agreement with experiment.

Growth and characterization of (Y3Fe5O12–Bi3Fe5O12) heterostructures by pulsed laser deposition

Superlattice heterostructures consisting of alternating single crystalline ferrimagnetic yttrium–iron–garnet (YIG) and bismuth–iron–garnet (BIG) thin film layers on gadolinium–gallium–garnet substrates show an increased saturation magnetization with respect to that of the monolayered structures grown under the same conditions. The observed effect is attributed to the distortions introduced in the YIG layers by the adjacent BIG layers. In this letter, we report our growth approach, by pulsed laser deposition, of these unusually performing thin film heterostructures.

The ultrasonic behaviour of ferrimagnetic yttrium iron garnet (YIG) crystal under pressure

This paper describes the ultrasonic behaviour of an yttrium iron garnet (YIG) crystal as function of pressure and magnetic field at room temperature, and reveals that there is magnetic-acoustic interaction in YIG. Similar ultrasonic behaviour under pressure has been revealed by measurements of sound velocity and attenuation of yttrium aluminium garnet (YAG) and YIG. Obviously, this could be attributed to the same garnet crystal structure. However, the ultrasonic behaviour of ferrimagnetic YIG and non-magnetic YAG is different when an external magnetic field is applied.

Swift heavy ions in magnetic insulators: A damage-cross-section velocity effect.

The damage in ferrimagnetic yttrium iron garnet, ${\mathrm{Y}}_{3}$${\mathrm{Fe}}_{5}$${\mathrm{O}}_{12}$ or YIG, induced by energetic heavy-ion bombardment in the electronic stopping-power regime has been studied in the low-velocity range (for a beam energy E\ensuremath{\le}3.6 MeV/amu). Epitaxial thin films of YIG on [111]-${\mathrm{Gd}}_{3}$${\mathrm{Ga}}_{5}$${\mathrm{O}}_{12}$ substrates were thus irradiated at room temperature with 15-MeV $^{19}\mathrm{F}$, 50-MeV $^{32}\mathrm{S}$, 650-MeV $^{181}\mathrm{Ta}$, 750-MeV $^{208}\mathrm{Pb}$, and 666-MeV $^{238}\mathrm{U}$. The damage-cross-section A is extracted from channeling-Rutherford-backscattering spectroscopy and compared to previous works. All the experimental results show that at one given value of dE/dx, the damage cross section is higher for low-velocity ions than for high-velocity ions over a large range of dE/dx. At constant dE/dx, the larger the difference between the ion velocities is, the larger the difference between the damage cross sections. Such a deviation might be explained by the effect of the energy deposition being more localized for the low-velocity ions than for the high-velocity ions. This work clearly indicates that the electronic stopping power is not the only key parameter in the creation of ion tracks, and that the damage cross section depends on the lateral distribution of the energy deposition.