Magnetic Compensation Temperature Research Articles

Strongly heat-assisted spin–orbit torque switching of a ferrimagnetic insulator

Ferrimagnetic insulators promise low-power and high-speed spintronic applications, thanks to their insulating nature and fast dynamics near compensation points. In a ferrimagnetic insulator/heavy metal heterostructure, we investigate field- and current-induced magnetization switching at various temperatures and observe distinct magnetization switching behaviors owing to spin–orbit torque (SOT) and heating effect. We have realized SOT switching across the magnetization compensation temperature and discovered that the SOT switching is strongly heat-assisted: the temperature is always above the compensation temperature while the SOT switching happens in our case. Moreover, we show that the SOT efficiency is strongly magnetization-dependent by characterizing the current dependence of SOT efficiency and explaining the anomalous SOT switching back phenomena in the presence of a titled external field. Our results reveal the critical role of Joule heating on the dynamics of magnetic insulators and pave the way for the application of spintronic devices based on magnetic insulators.

Complex anomalous Hall effect of CoGd alloy near the magnetization compensation temperature

In this article, the temperature-dependent complex Hall effect near the magnetization compensation temperature of CoGd alloy films is systematically studied. We find anomalies of Hall loops with characteristics similar to the topological Hall effect (THE), which is usually considered to indicate the occurrence of topological spin texture. However, the THE is ruled out by detailed magnetic and transport measurements on CoGd alloy films with different composition ratios. After considering the influence of the spin-flop effect and the inhomogeneity of the components naturally formed in the ferrimagnetic CoGd alloy, we constructed a phenomenological model that can explain the complex Hall effect near the magnetization compensation point without relying on the THE. The main result is helpful for explaining the microscopic magnetic properties and transport properties of rare-earth transition metal ferrimagnetic alloys near the compensation point.

Current-Induced Spin Torques on Single GdFeCo Magnetic Layers.

Spintronics exploit spin-orbit coupling (SOC) to generate spin currents, spin torques, and, in the absence of inversion symmetry, Rashba and Dzyaloshinskii-Moriya interactions. The widely used magnetic materials, based on 3d metals such as Fe and Co, possess a small SOC. To circumvent this shortcoming, the common practice has been to utilize the large SOC of nonmagnetic layers of 5d heavy metals (HMs), such as Pt, to generate spin currents and, in turn, exert spin torques on the magnetic layers. Here, a new class of material architectures is introduced, excluding nonmagnetic 5d HMs, for high-performance spintronics operations. Very strong current-induced torques exerted on single ferrimagnetic GdFeCo layers, due to the combination of large SOC of the Gd 5d states and inversion symmetry breaking mainly engineered by interfaces, are demonstrated. These "self-torques" are enhanced around the magnetization compensation temperature and can be tuned by adjusting the spin absorption outside the GdFeCo layer. In other measurements, the very large emission of spin current from GdFeCo, 80% (20%) of spin anomalous Hall effect (spin Hall effect) symmetry is determined. This material platform opens new perspectives to exert "self-torques" on single magnetic layers as well as to generate spin currents from a magnetic layer.

Unusual anomalous Hall effect in the ferrimagnetic GdFeCo alloy

We have investigated the anomalous Hall effect (AHE) of a ferrimagnetic GdFeCo film with perpendicular magnetic anisotropy. In the vicinity of magnetization compensation temperature TM, the peak structure or triple loop of the Hall resistance loops is mainly caused by the opposite magnetic moments of the two sublattices of Gd and FeCo, and that can be explained by the mechanism of the two-channel AHE. Furthermore, we demonstrated that the transport properties of the GdFeCo film can be manipulated by ionic liquid gating. When the gate voltage is −4 V, the TM of the GdFeCo film was changed up to 29 K. Our research provides experimental evidence for the design of novel electronic devices.

Magnetic Properties and Compensation Temperature in Tri-layer Graphyne-Like Structure: Monte Carlo Simulations

In this paper, we study the magnetic properties of a tri-layer graphyne-like structure using Blume-Capel Ising model. The studied system is composed by a mixed spins σ = ± 5/2, ± 3/2 and ± 1/2 (in the plane B) and S = ± 2, ± 1 and 0 (in the plane A). The study is performed through Monte Carlo simulations under the Metropolis algorithm. Besides, we examine the effects of the exchange coupling interactions on the obtained compensation temperature. Additionally, we investigate the effects of the crystal and external magnetic fields on the partial and total magnetizations. The magnetic and thermal properties of the considerate system are largely affected by the exchange coupling interactions, the crystal and external magnetic fields.

Unconventional magnetoresistance induced by sperimagnetism in GdFeCo

We investigate the magnetoresistance of ferrimagnetic GdFeCo across the magnetization compensation temperature ${T}_{\mathrm{M}}$. The magnetic field dependence of longitudinal resistivity (${\ensuremath{\rho}}_{\mathrm{xx}}$) shows opposite trends below and above ${T}_{\mathrm{M}}$, and the variation of ${\ensuremath{\rho}}_{\mathrm{xx}}$ with $B$ becomes more significant as the temperature decreases. The observed unconventional magnetoresistance is attributed to the sperimagnetism of GdFeCo. Further investigations on the transverse resistivity (${\ensuremath{\rho}}_{\mathrm{xy}}$) of GdFeCo unveils that, contrary to the recent reports that the transition metal dominates transport of rare-earth transition-metal ferrimagnets, the Gd contribution to magnetoresistance is comparable to the FeCo contribution, showing that the transport of GdFeCo is antiferromagnetic. Our results therefore show that ferrimagnets are a convenient platform for studying antiferromagnetic spin transport and also are potential materials that can enable antiferromagnetic spin devices.

Nonvolatile electric-field-controlled anomalous Hall effect in ferrimagnetic GdFeCo film

We report on the electric-field-modulated anomalous Hall effect (AHE) under various temperatures in ferrimagnetic GdFeCo/Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMN-PT) structures with perpendicular magnetic anisotropy. The coercive field H c and the magnetic compensation temperature T M are modulated by applying an electric field to the multilayer. The resistance ratio (ΔR xy/R xy) plotted as a function of the symmetric bipolar electric field takes the form of a nonvolatile butterfly-like bipolar curve. The resonance field H r of the ferromagnetic resonance spectra as a function of the applied electric field shows a typical butterfly-like shape, indicating that strain is the dominant effect in the electric-field-controlled AHE in the ferrimagnetic heterostructure. In addition, a four-state nonvolatile memory is observed upon simultaneous application of pulsed magnetic field and electric field. The results for the electric-field-controlled AHE should prove useful for research into the development of ultra-energy-efficient ferrimagnets.

Spin Polarization Compensation in Ferrimagnetic Co1−xTbx / Pt Bilayers Revealed by Spin Hall Magnetoresistance

Rare-earth-transition-metal (RETM) ferrimagnets have received great attention for their small magnetic moments and fast dynamics in the development of next-generation spintronic devices. Here, we study spin Hall magnetoresistance (SMR) in the RETM ferrimagnet and heavy-metal bilayers of ${\mathrm{Co}}_{1\text{\ensuremath{-}}x}{\mathrm{Tb}}_{x}/\mathrm{Pt}$. Anomalous temperature dependence of the SMR is observed, where the SMR changes nonmonotonically and has a valley next to the magnetization compensation temperature. Additionally, we find that the SMR varies significantly for different chemical compositions of ${\mathrm{Co}}_{1\text{\ensuremath{-}}x}{\mathrm{Tb}}_{x}$ alloys. These anomalous behaviors are interpreted as the dramatically altered spin polarization of the ${\mathrm{Co}}_{1\text{\ensuremath{-}}x}{\mathrm{Tb}}_{x}$ layer near the compensation point. By deducing spin polarization from the abnormal temperature dependence of SMR, we provide a convenient way to access spin polarization of the RETM ferrimagnets. These experimental results show implications for tunable spin-current sources of RETM ferrimagnets.

Field-driven spin reorientation in SmMnO3 polycrystalline powders

The SmMnO3 perovskite system shows interesting magnetic properties and making them an attractive material for specific technological application. However, these materials on nanosized dimension display properties that can be significantly different from their bulk dimension. In this regard, using the sol-gel method, the successful synthesis of SmMnO3 polycrystalline powders is herein reported. The structure, morphology, vibrational and magnetic properties of the as-produced sample have been investigated. X-ray diffraction pattern shows a single-perovskite SmMnO3 crystalline phase structure, with a crystalline size of ∼70 nm. The room temperature Raman spectrum confirms the formation of the perovskite SmMnO3 crystalline structure observed by XRD. DC magnetic measurements show an antiferromagnetic-like transition at 60 K (TN ≈ 60 K), attributed to the antiferromagnetic ordering of the Mn3+ spins, magnetic measurements also reveal a temperature-induced magnetization reversal phenomenon characterized by the magnetic compensation temperature (Tcomp ≈ 6 K), attributed to antiparallel orientation of weak ferromagnetic Mn3+- and Sm3+-ion magnetic moments induced by Sm3+-Mn3+ exchange interaction. Magnetization loops, recorded from the as-synthesized SmMnO3 sample contain some singularity compared to previous studies of SmMnO3, which are assigned to small particle size and single domain character of the particles, which are characterized by two well-defined anomalies, both connected with the spin inversion phenomena: one at low fields showing increasing and decreasing branches intersecting each other; another at high fields revealing sudden jumps. The study of a Schottky anomaly in the specific heat measurements recorded at low temperatures (below 10 K) under DC magnetic fields (in the range 5–14 T), is consistent with the presence of a field-driven spin reorientation, from antiferromagnetic to a net ferromagnetic order.

Comparing all-optical switching in synthetic-ferrimagnetic multilayers and alloys

We present an experimental and theoretical investigation of all-optical switching by single femtosecond laser pulses. Our experimental results demonstrate that, unlike rare-earth transition-metal ferrimagnetic alloys, $\mathrm{Pt}/\mathrm{Co}/{[\mathrm{Ni}/\mathrm{Co}]}_{N}/\mathrm{Gd}$ can be switched in the absence of a magnetization compensation temperature, indicative for strikingly different switching conditions. In order to understand the underlying mechanism, we model the laser-induced magnetization dynamics in Co/Gd bilayers and GdCo alloys on an equal footing, using an extension of the microscopic three-temperature model to multiple magnetic sublattices and including exchange scattering. In agreement with our experimental observations, the model shows that Co/Gd bilayers can be switched for a thickness of the Co layer far away from compensating the total Co and Gd magnetic moment. We identify the switching mechanism in Co/Gd bilayers as a front of reversed Co magnetization that nucleates near the Co/Gd interface and propagates through the Co layer driven by exchange scattering.

Tuning magnetization compensation and Curie temperatures in epitaxial rare earth iron garnet films

An ab initio effective Hamiltonian is developed and used to reveal the effect of misfit strain on magnetic properties of epitaxial (001) films made of a prototypical rare earth iron garnet: Gadolinium iron garnet. It is found that both the magnetization compensation temperature (${T}_{\mathrm{M}}$) and the Curie temperature (${T}_{\mathrm{C}}$) can be significantly and linearly enhanced when varying the strain from tensile to compressive values. Consequently, ${T}_{\mathrm{M}}$ can be designed to coincide with room temperature, and the magnitude of the magnetization can be optimized at around 300 K when choosing the appropriate substrate. The microscopic origins of these technologically promising effects are further revealed.

Investigation of negative magnetization and impedance spectroscopy of Sm-substituted gadolinium iron garnets

Samarium substituted gadolinium iron garnet samples were synthesized in pure-phase form by solid-state chemical reaction method. The lattice constant increases (12.4624 Å to 12.5211 Å) with Sm-concentration. Magnetization measurements show an increase in ferrimagnetic (FIM) transition temperature (TC = 567 K to 575 K) and a decrease in magnetic compensation temperature (Tcomp = 286 K to 70 K) upon Sm-substitution. In addition to magnetic compensation, an interesting negative magnetization under field-cooled condition is achieved for a sample with x = 2.0, and it has been explained in terms of enhanced magnetic anisotropy due to Sm-substitution. A significant increase in saturation magnetization is attained at room temperature by Sm-substitution. Frequency dependence of complex impedance shows relaxation behavior and the relaxation frequency follows the Arrhenius power-law, along with an anomaly around FIM TC. Such anomaly is ascribed to magneto-electric coupling.

High-field anomalies of equilibrium and ultrafast magnetism in rare-earth–transition-metal ferrimagnets

Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in the equilibrium and ultrafast magnetic properties of the ferrimagnetic rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the magnetization compensation temperature, each of the magnetizations of the antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis loops. Contrary to state-of-the-art theory, which explains such loops by sample inhomogeneities, here we show that they are an intrinsic property of the rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and have a non-zero orbital momentum in the ground state and, therefore, a large magnetic anisotropy, we are able to reproduce the experimentally observed behavior in equilibrium. The same theory is also able to describe the experimentally observed critical slowdown of the spin dynamics in the vicinity of the magnetization compensation temperature, emphasizing the role played by the orbital momentum in static and ultrafast magnetism of ferrimagnets.

Role of compensation points in all-optical magnetization switching

We study theoretically the role of compensation points in all-optical magnetization switching (AOS) in rare-earth transition-metal ferrimagnets. We use the s-d model in which localized spins are coupled with spins of itinerant electrons via the s-d exchange interaction. The main advantage of the model is that it considers the dynamics of both localized and itinerant spin population. We show that the compensation temperature, which is relevant to AOS, is determined by the net spin polarization, i.e., by the sum of the spin polarizations of localized and itinerant spins. This temperature does not coincide exactly with either the magnetization compensation temperature or the angular momentum compensation temperature.

Magnetization dynamics of the compensated ferrimagnet Mn2RuxGa

Here we study both static and time-resolved dynamic magnetic properties of the compensated ferrimagnet from room temperature down to 10K, thus crossing the magnetic compensation temperature $T_{M}$. The behaviour is analysed with a model of a simple collinear ferrimagnet with uniaxial anisotropy and site-specific gyromagnetic ratios. We find a maximum zero-applied-field resonance frequency of $\sim$160GHz and a low intrinsic Gilbert damping $\alpha$$\sim$0.02, making it a very attractive candidate for various spintronic applications.

Investigation of magnetic and relaxor dielectric properties of polycrystalline gadolinium iron garnet by Bi substitution

We have synthesized Bi-substituted gadolinium iron garnet [(Gd3−xBix)Fe5O12] samples with x = 0, 0.5, and 1.0 and studied their structural, magnetic, and dielectric properties. All samples are found to be in single-phase form with cubic structure as per the Rietveld analysis based on $$Ia\bar{3}d$$ space group. Bi substitution leads to increase in lattice constant, Fe(a)–Fe(d) bond length and Fe(a)–O–Fe(d) bond angles, and they have direct influence on magnetic properties. All samples undergo ferrimagnetic transition followed by magnetic compensation behavior at low temperature. The ferrimagnetic transition temperature (Tc) is found to increase from 576 K for x = 0 to 596 K for x = 1.0. However, the magnetic compensation temperature (Tcomp) decreases from 296 to 176 K. They are explained in terms of increase in Fe(a)–O–Fe(d) bond angle and the dilution of magnetic moment at Gd site. The analysis of impedance spectra at room temperature and at higher temperature shows that Bi substitution gives rise to larger impedance due to the localization as well as reduction in the concentration of charge carriers. The activation energy of charge-carrier relaxation is found to be in the range of 0.9–1.0 eV. The temperature variation of dielectric constant data and their analysis based on modified Curie–Weiss law show the relaxor ferroelectric transition with a typical diffuseness exponent, γ close to 2.0.

Magnetic properties of ferrimagnetic Tb/CoFeB/MgO films

We studied perpendicular magnetic anisotropy (PMA) and magnetization compensation temperature (TM) in Tb/CoFeB/MgO films. Interlayer antiferromagnetic coupling between Tb and CoFeB layers enables useful antiferromagnetic properties containing a CoFeB/MgO junctions, which are important for the practical applications to obtain giant tunnel magnetoresistance and voltage-control of magnetic anisotropy. While the PMA is mainly attributed to the interface at CoFeB/MgO, TM can be controlled by the thickness ratio of Tb and CoFeB layers as the magnetic moments of them are antiferromagnetically coupled. Our results provide many possibilities to add exciting new functions to ferrimagnetic spintronics.

Exploring interfacial exchange coupling and sublattice effect in heavy metal/ferrimagnetic insulator heterostructures using Hall measurements, x-ray magnetic circular dichroism, and neutron reflectometry

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

Метод анализа доменной структуры в микрочастицах магнитных порошков

It is offer the method analysis of the domain structure in the microparticles of the magnetic powders on the basis of the Mӧssbauer spectroscopy. The method has been experimentally verified on the particles from gadolinium iron garnet ferrite Gd3Fe5O12 in diameter 40 ± 5 μm in the region of the magnetic compensation temperature Tcm (Tcm =286 К). It is shown that near the Tcm the particles of the ferrite is monodomains and leave from Tcm the particles turn into much domain state.

Exchange coupling torque in ferrimagnetic Co/Gd bilayer maximized near angular momentum compensation temperature

Highly efficient current-induced motion of chiral domain walls was recently demonstrated in synthetic antiferromagnetic (SAF) structures due to an exchange coupling torque (ECT). The ECT derives from the antiferromagnetic exchange coupling through a ruthenium spacer layer between the two perpendicularly magnetized layers that comprise the SAF. Here we report that the same ECT mechanism applies to ferrimagnetic bi-layers formed from adjacent Co and Gd layers. In particular, we show that the ECT is maximized at the temperature TA where the Co and Gd angular momenta balance each other, rather than at their magnetization compensation temperature TM. The current induced velocity of the domain walls is highly sensitive to longitudinal magnetic fields but we show that this not the case near TA. Our studies provide new insight into the ECT mechanism for ferrimagnetic systems. The high efficiency of the ECT makes it important for advanced domain wall based spintronic devices.