We present the experiment observation of a giant topological Hall effect (THE) in a frustrated kagome bilayer magnet Fe₃Sn₂. The negative topologically Hall resistivity appears when the field is below 1.3 T and it increases with increasing temperature up to 300 K. Its maximum absolute value reaches ~2.01 μΩ·cm at 300 K and 0.76 T. The origins of the observed giant THE can be attributed to the coexistence of the field-induced skyrmion state and the non-collinear spin configuration, possibly related to the magnetic frustration interaction in Fe₃Sn₂.

We present an overview in the understanding of spin-transfer torque (STT) induced magnetization dynamics in spin-torque nano-oscillator (STNO) devices. The STNO contains an in-plane (IP) magnetized free layer and an out-of-plane (OP) magnetized spin polarizing layer. After a brief introduction, we first use mesoscopic micromagnetic simulations, which are based on the Landau-Lifshitz-Gilbert equation including the STT effect, to specify how a spin-torque term may tune the magnetization precession orbits of the free layer, showing that the oscillator frequency is proportional to the current density and the z-component of the free layer magnetization. Next, we propose a pendulum-like model within the macrospin approximation to describe the dynamic properties in such type of STNOs. After that, we further show the procession dynamics of the STNOs excited by IP and OP dual spin-polarizers. Both the numerical simulations and analytical theory indicate that the precession frequency is linearly proportional to the spin-torque of the OP polarizer only and is irrelevant to the spin-torque of the IP polarizer. Finally, a promising approach of coordinate transformation from the laboratory frame to the rotation frame is introduced, by which the nonstationary OP magnetization precession process is therefore transformed into the stationary process in the rotation frame. Through this method, a promising digital frequency shift-key modulation technique is presented, in which the magnetization precession can be well controlled at a given orbit as well as its precession frequency can be tuned with the co-action of spin polarized current and magnetic field (or electric field) pulses.

We present a study of magnetocaloric effect of the quasi-two-dimensional (2D) ferromagnet (CH₃NH₃)₂CuCl₄ in ab plane (easy-plane). From the measurements of magnetic field dependence of magnetization at various temperatures, we have discovered a large magnetic entropy change associated with the ferromagnetic-paramagnetic transition. The heat capacity measurements reveal an abnormal adiabatic change below the Curie temperature T_c~8.9 K, which is caused by the nature of quasi-2D layered crystal structure. These results suggest that perovskite organic-inorganic hybrids with a layered structure are suitable candidates as working substances in magnetic refrigeration technology.

Single-grain models with different cerium contents or structural parameters have been introduced to investigate the reversal magnetization behaviors in cerium-containing magnets. All the micromagnetic simulations are carried out via the object oriented micromagnetic framework (OOMMF). As for single (Nd,Ce)₂Fe₁₄B type grain, the coercivity decreases monotonously with the increase of the cerium content. Four types of grain structure have been compared:single (Nd,Ce)₂Fe₁₄B type, core ((Nd,Ce)₂Fe₁₄B)-shell (Nd₂Fe₁₄B) type with 2 nm thick shell, core (Ce₂Fe₁₄B)-shell (Nd₂Fe₁₄B) type, and core (Nd₂Fe₁₄B)-shell (Ce₂Fe₁₄B) type. It is found that core ((Nd,Ce)₂Fe₁₄B)-shell (Nd₂Fe₁₄B) type grain with 2 nm thick shell always presents the largest coercivity under the same total cerium content. Furthermore, the relationship between the coercivity and the shell thickness t in core ((Nd,Ce)₂Fe₁₄B)-shell (Nd₂Fe₁₄B) type grain has been studied. When the total cerium content is kept at 20.51 at.%, the analyzed results show that as t varies from 1 nm to 7 nm, the coercivity gradually ascends at the beginning, then quickly descends after reaching the maximum value when t=5 nm. From the perspective of the positions of nucleation points, the reasons why t affects the coercivity are discussed in detail.

The cubic pyrochlore Dy₂Pt₂O₇ was synthesized under 4 GPa and 1000℃ and its magnetic and thermodynamic properties were characterized by DC and AC magnetic susceptibility and specific heat down to 0.1 K. We found that Dy₂Pt₂O₇ does not form long-range magnetic order, but displays characteristics of canonical spin ice such as Dy₂Ti₂O₇, including (1) a large effective moment 9.64 μ_B close to the theoretical value and a small positive Curie-Weiss temperature θ_CW=+0.77 K signaling a dominant ferromagnetic interaction among the Ising spins; (2) a saturation moment ～4.5 μ_B being half of the total moment due to the local <111> Ising anisotropy; (3) thermally activated spin relaxation behaviors in the low (～1 K) and high (～20 K) temperature regions with different energy barriers and characteristic relaxation time; and most importantly, (4) the presence of a residual entropy close to Pauling's estimation for water ice.

We use quantum Monte Carlo simulations to study an S=1/2 spin model with competing multi-spin interactions. We find a quantum phase transition between a columnar valence-bond solid (cVBS) and a Néel antiferromagnet (AFM), as in the scenario of deconfined quantum-critical points, as well as a transition between the AFM and a staggered valence-bond solid (sVBS). By continuously varying a parameter, the sVBS-AFM and AFM-cVBS boundaries merge into a direct sVBS-cVBS transition. Unlike previous models with putative deconfined AFM-cVBS transitions, e.g., the standard J-Q model, in our extended J-Q model with competing cVBS and sVBS inducing terms the transition can be tuned from continuous to first-order. We find the expected emergent U(1) symmetry of the microscopically Z₄ symmetric cVBS order parameter when the transition is continuous. In contrast, when the transition changes to first-order, the clock-like Z₄ fluctuations are absent and there is no emergent higher symmetry. We argue that the confined spinons in the sVBS phase are fracton-like. We also present results for an SU(3) symmetric model with a similar phase diagram. The new family of models can serve as a useful tool for further investigating open questions related to deconfined quantum criticality and its associated emergent symmetries.

The multiferroicity in the RMn₂O₅ family remains unclear, and less attention has been paid to its dependence on high-temperature (high-T) polarized configuration. Moreover, no consensus on the high-T space group symmetry has been reached so far. In view of this consideration, one may argue that the multiferroicity of RMn₂O₅ in the low-T range depends on the poling sequence starting far above the multiferroic ordering temperature. In this work, we investigate in detail the variation of magnetically induced electric polarization in GdMn₂O₅ and its dependence on electric field poling routine in the high-T range. It is revealed that the multiferroicity does exhibit qualitatively different behaviors if the high-T poling routine changes, indicating the close correlation with the possible high-T polarized state. These emergent phenomena may be qualitatively explained by the co-existence of two low-T polarization components, a scenario that was proposed earlier. One is the component associated with the Mn³⁺-Mn⁴⁺-Mn³⁺ exchange striction that seems to be tightly clamped by the high-T polarized state, and the other is the component associated with the Gd³⁺-Mn⁴⁺-Gd³⁺ exchange striction that is free of the clamping. The present findings may offer a different scheme for the electric control of the multiferroicity in RMn₂O₅.

We report a comprehensive Raman scattering study on layered MPS₃ (M=Mn, Fe, Ni), a two-dimensional magnetic compound with weak van der Waals interlayer coupling. The observed Raman phonon modes have been well assigned by the combination of first-principles calculations and the polarization-resolved spectra. Careful symmetry analysis on the angle-dependent spectra demonstrates that the crystal symmetry is strictly described by C_2h but can be simplified to D_3d with good accuracy. Interestingly, the three compounds share exactly the same lattice structure but show distinct magnetic structures. This provides us with a unique opportunity to study the effect of different magnetic orders on lattice dynamics in MPS₃. Our results reveal that the in-plane Néel antiferromagnetic (AF) order in MnPS₃ favors a spin-phonon coupling compared to the in-plane zig-zag AF in NiPS₃ and FePS₃. We have discussed the mechanism in terms of the folding of magnetic Brillouin zones. Our results provide insights into the relation between lattice dynamics and magnetism in the layered MPX₃ (M=transition metal, X=S, Se) family and shed light on the magnetism of monolayer MPX₃ materials.

We reported a study of tunnel magnetoresistance (TMR) effect in single manganite nanowire via the combination of magnetotransport and magnetic force microscopy imaging. TMR value up to 290% has been observed in single (La_1-yPr_y)_1-xCa_xMnO₃ nanowires with varying width. We find that the TMR effect can be explained in the scenario of opening and blockade of conducting channels from inherent magnetic domain evolutions. Our findings provide a new route to fabricate TMR junctions and point towards future improvements in complex oxide-based TMR spintronics.

Electrical spin, which is the key element of spintronics, has been regarded as a powerful substitute for the electrical charge in the next generation of information technology, in which spin plays the role of the carrier of information and/or energy in a similar way to the electrical charge in electronics. Spin-transport phenomena in different materials are central topics of spintronics. Unlike electrical charge, spin transport does not depend on electron motion, particularly spin can be transported in insulators without accompanying Joule heating. Therefore, insulators are considered to be ideal materials for spin conductors, in which magnetic insulators are the most compelling systems. Recently, we experimentally studied and theoretically discussed spin transport in various antiferromagnetic systems and identified spin susceptibility and the Néel vector as the most important factors for spin transport in antiferromagnetic systems. Herein, we summarize our experimental results, physical nature, and puzzles unknown. Further challenges and potential applications are also discussed.

We report the detailed physical properties of quaternary compound Ba₂BiFeS₅ with the key structural ingredient of isolated FeS₄ tetrahedra. Magnetization and heat capacity measurements clearly indicate that Ba₂BiFeS₅ has a paramagnetic to antiferromagnetic transition at about 30 K. The calculated magnetic entropy above ordering temperature is much smaller than theoretical value for high-spin Fe³⁺ ion with S=5/2, implying the possible short-range antiferromagnetic fluctuation in Ba₂BiFeS₅.

The spin transparency at the normal/ferromagnetic metal (NM/FM) interface was studied in Pt/YIG/Cu/FM multilayers. The spin current generated by the spin Hall effect (SHE) in Pt flows into Cu/FM due to magnetic insulator YIG blocking charge current and transmitting spin current via the magnon current. Therefore, the nonlocal voltage induced by an inverse spin Hall effect (ISHE) in FM can be detected. With the magnetization of FM parallel or antiparallel to the spin polarization of pure spin currents (σ_sc), the spin-independent nonlocal voltage is induced. This indicates that the spin transparency at the Cu/FM interface is spin-independent, which demonstrates that the influence of spin-dependent electrochemical potential due to spin accumulation on the interfacial spin transparency is negligible. Furthermore, a larger spin Hall angle of Fe₂₀Ni₈₀ (Py) than that of Ni is obtained from the nonlocal voltage measurements.

CrI₃ in two-dimensional (2D) forms has been attracting much attention lately due to its novel magnetic properties at atomic large scale. The size and edge tuning of electronic and magnetic properties for 2D materials has been a promising way to broaden or even enhance their utility, as the case with nanoribbons/nanotubes in graphene, black phosphorus, and transition metal dichalcogenides. Here we studied the CrI₃ nanoribbon (NR) and nanotube (NT) systematically to seek the possible size and edge control of the electronic and magnetic properties. We find that ferromagnetic ordering is stable in all the NR and NT structures of interest. An enhancement of the Curie temperature T_C can be expected when the structure goes to NR or NT from its 2D counterpart. The energy difference between the FM and AFM states can be even improved by up to 3-4 times in a zigzag nanoribbon (ZZNR), largely because of the electronic instability arising from a large density of states of iodine-5p orbitals at E_F. In NT structures, shrinking the tube size harvests an enhancement of spin moment by up to 4%, due to the reduced crystal-field gap and the re-balance between the spin majority and minority populations.

Heavily Mn-doped SiGe thin films were grown by radio frequency magnetron sputtering and then treated by post-growth thermal annealing. Structural characterizations reveal the coexistence of Mn-diluted SiGe crystals and Mn-rich nanoclusters in the annealed films. Magnetic measurements indicate the ferromagnetic ordering of the annealed samples above room temperature . The data suggest that the ferromagnetism is probably mainly contributed by the Ge-rich nanoclusters and partially contributed by the tensile-strained Mn-diluted SiGe crystals. The results may be useful for room temperature spintronic applications based on group IV semiconductors.

The magnetization-direction-dependent inverse spin Hall effect (ISHE) has been observed in NiFe film during spin Seebeck measurement in IrMn/NiFe/Cu/yttrium iron garnet (YIG) multilayer structure, where the YIG and NiFe layers act as the spin injector and spin current detector, respectively. By using the NiFe/IrMn exchange bias structure, the magnetization direction of YIG (M_YIG) can be rotated with respect to that of NiFe (M_NiFe) with a small magnetic field, thus allowing us to observe the magnetization-direction-dependent inverse spin Hall effect voltage in NiFe layer. Compared with the situation that polarization direction of spin current (σ_s) is perpendicular to M_NiFe, the spin Seebeck voltage is about 30% larger than that when σ_s and M_NiFe are parallel to each other. This phenomenon may originate from either or both of stronger interface or bulk scattering to spin current when σ_s and M_NiFe are perpendicular to each other. Our work provides a way to control the voltage induced by ISHE in ferromagnets.

The critical properties and the nature of the ferromagnetic-paramagnetic phase transition in the 2D organic-inorganic hybrid (CH₃NH₃)₂CuCl₄ single crystal have been investigated by dc magnetization in the vicinity of the magnetic transition. Different techniques were used to estimate the critical exponents near the ferromagnetic-paramagnetic phase transition such as modified Arrott plots, the Kouvel-Fisher method, and the scaling hypothesis. Values of β=0.22, γ=0.82, and δ=4.4 were obtained. These critical exponents are in line with their corresponding values confirmed through the scaling hypothesis as well as the Widom scaling relation, supporting their reliability. It is concluded that this 2D hybrid compound possesses strong ferromagnetic intra-layer exchange interaction as well as weak interlayer ferromagnetic coupling that causes a crossover from 2D to 3D long-range interaction.

In quasi-one-dimensional (q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic (AFM) compound YbAlO₃, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations, and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave (LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.

We report a strong antiferromagnetic (AFM) interlayer coupling in ferromagnetic La_0.67Sr_0.33MnO₃/SrRuO₃ (LSMO/SRO) superlattices grown on (111)-oriented SrTiO₃ substrate. Unlike the (001) superlattices for which the spin alignment between LSMO and SRO is antiparallel in the in-plane direction and parallel in the out-of-plane direction, the antiparallel alignment is observed along both the in-plane and out-of-plane directions in the present sample. The low temperature hysteresis loop demonstrates two-step magnetic processes, indicating the coexistence of magnetically soft and hard components. Moreover, an inverted hysteresis loop was observed. Exchange bias tuned by the temperature and cooling field was also investigated, and positive as well as negative exchange bias was observed at the same temperature with the variation of the cooling field. A very large exchange field (H_EB) was observed and both magnitude and sign of the H_EB depend on the cooling field, which can be attributed to an interplay of Zeeman energy and AFM coupling energy at the interfaces. The present work shows the great potential of tuning a spin texture through interfacial engineering for the complex oxides whose spin state is jointly determined by strongly competing mechanisms.

The structural, magnetic properties, and electronic structures of hexagonal FeCoSn compounds with as-annealed bulk and ribbon states were investigated by x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscope (TEM), scanning electron microscope (SEM), magnetic measurements, and first-principles calculations. Results indicate that both states of FeCoSn show an Ni₂In-type hexagonal structure with a small amount of FeCo-rich secondary phase. The Curie temperatures are located at 257 K and 229 K, respectively. The corresponding magnetizations are 2.57 μ_B/f.u. and 2.94 μ_B/f.u. at 5 K with a field of 50 kOe (1 Oe=79.5775 A·m^-1). The orbital hybridizations between 3d elements are analyzed from the distribution of density of states (DOS), showing that Fe atoms carry the main magnetic moments and determine the electronic structure around Fermi level. A peak of DOS at Fermi level accounts for the presence of the FeCo-rich secondary phase. The Ni₂In-type hexagonal FeCoSn compound can be used during the isostructural alloying for tuning phase transitions.

We investigate the critical exponents and magnetocaloric effect of Co_2-xZrSn polycrystal. The Co_2-xZrSn undergoes a second-order ferromagnetism phase transition around the Curie temperature of T_c~280 K. The critical behavior in the vicinity of the magnetic phase transition has been investigated by using modified Arrott plot and Kouvel-Fisher methods. The obtained critical exponents, β, γ, and δ can be well described by the scaling theory. The determined exponents of Co_2-xZrSn obey the mean-field model with a long-range magnetic interaction. In addition, the maximum magnetic entropy change -ΔS_M^max of Co_2-xZrSn is about 0.57 J·kg^-1·K^-1 and the relative cooling power (RCP) is 14.68 J·kg^-1 at 50 kOe (1 Oe=79.5775 A·m^-1).