Antiferromagnetic System Research Articles

Magnetocaloric effect as a signature of quantum level-crossing for a spin-gapped system

Recent research dealing with magnetocaloric effect (MCE) study of antiferromagnetic (AFM) low dimensional spin systems have revealed a number of fascinating ground-state crossover characteristics upon application of external magnetic field. Herein, through MCE investigation we have explored field-induced quantum level-crossing characteristics of one such spin system: (NCP), an AFM spin 1/2 dimer. Experimental magnetization and specific heat data are presented and the data have been employed to evaluate entropy, magnetic energy and magnetocaloric properties. We witness a sign change in magnetic Grüneisen parameter across the level-crossing field BC. An adiabatic cooling is observed at low temperature by tracing the isentropic curves in temperature-magnetic field plane. Energy-level crossover characteristics in NCP interpreted through MCE analysis are well consistent with the observations made from magnetization and specific heat data.

Hole doping and antiferromagnetic correlations above the Néel temperature of the topological semimetal (Sr1−xKx)MnSb2

Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal {\Ksub} confirm the three dimensional (3D) C-type antiferromagnetic (AFM) ordering of the Mn$^{2+}$ moments at $T_{\rm N}=305 \pm 3$ K which is slightly higher than that of the parent SrMnSb$_2$ with $T_{\rm N}=297 \pm 3$ K. Susceptibility measurements of the K-doped and parent crystals above $T_{\rm N}$ are characteristic of 2D AFM systems. This is consistent with high temperature neutron diffraction of the parent compound that display persisting 2D AFM correlations well above $T_{\rm N}$ to at least $\sim 560$ K with no evidence of a ferromagnetic phase. Analysis of the de Haas van Alphen magnetic oscillations of the K-doped crystal is consistent with hole doping.

Simulating frustrated antiferromagnets with quadratically driven QED cavities

We propose a class of quantum simulators for antiferromagnetic spin systems, based on coupled photonic cavities in presence of two-photon driving and dissipation. By modeling the coupling between the different cavities through a hopping term with negative amplitude, we solve numerically the quantum master equation governing the dynamics of the open system and determine its non-equilibrium steady state. Under suitable conditions, the steady state can be described in terms of the degenerate ground states of an antiferromagnetic Ising model. When the geometry of the cavity array is incommensurate with the antiferromagnetic coupling, the steady state presents properties which bear full analogy with those typical of the spin liquid phases arising in frustrated magnets.

Antiferromagnetic ordering in van der Waals 2D magnetic material MnPS3 probed by Raman spectroscopy

Magnetic ordering in the two-dimensional (2D) limit has been one of the most important issues in condensed matter physics for the past few decades. The recent discovery of new magnetic van der Waals materials heralds a much-needed easy route for the studies of 2D magnetism: the thickness dependence of the magnetic ordering has been examined using Ising- and XXZ-type magnetic van der Waals materials. Here, we investigated the magnetic ordering of MnPS3, a 2D antiferromagnetic material of the Heisenberg-type, by Raman spectroscopy from bulk all the way down to the bilayer. The phonon modes that involve the vibrations of Mn ions exhibit characteristic changes as the temperature gets lowered through the Néel temperature. In bulk MnPS3, the Raman peak at ~155 cm−1 becomes considerably broadened near the Néel temperature, and upon further cooling is subsequently red-shifted. The measured peak positions and polarization dependences of the Raman spectra are in excellent agreement with our first-principles calculations. In few-layer MnPS3, the peak at ~155 cm−1 exhibits the characteristic red-shift at low temperatures down to the bilayer, indicating that the magnetic ordering is surprisingly stable at such a thin limit. Our work sheds light on the hitherto unexplored magnetic ordering in the Heisenberg-type antiferromagnetic systems in the atomic-layer limit.

Ultrafast depinning of domain walls in notched antiferromagnetic nanostructures

The pinning and depinning of antiferromagnetic (AFM) domain wall is certainly the core issue of AFM spintronics. In this work, we study theoretically the N\'eel-type domain wall pinning and depinning at a notch in an antiferromagnetic (AFM) nano-ribbon. The depinning field depending on the notch dimension and intrinsic physical parameters are deduced and also numerically calculated. Contrary to conventional conception, it is revealed that the depinning field is remarkably dependent of the damping constant and the time-dependent oscillation of the domain wall position in the weakly damping regime benefits to the wall depinning, resulting in a gradual increase of the depinning field up to a saturation value with increasing damping constant. A one-dimensional model accounting of the internal dynamics of domain wall is used to explain perfectly the simulated results. It is demonstrated that the depinning mechanism of an AFM domain wall differs from ferromagnetic domain wall by exhibiting a depinning speed typically three orders of magnitude faster than the latter, suggesting the ultrafast dynamics of an AFM system.

Landau-Lifshitz-Bloch equation for domain wall motion in antiferromagnets

In this work, we derive the Landau-Lifshitz-Bloch equation accounting for the multi-domain antiferromagnetic (AFM) lattice at finite temperature, in order to investigate the domain wall (DW) motion, the core issue for AFM spintronics. The continuity equation of the staggered magnetization is obtained using the continuum approximation, allowing an analytical calculation on the domain wall dynamics. The influence of temperature on the static domain wall profile is investigated, and the analytical calculations reproduce well earlier numerical results on temperature gradient driven saturation velocity of the AFM domain wall, confirming the validity of this theory. Moreover, it is worth noting that this theory could be also applied to dynamics of various wall motions in an AFM system. The present theory represents a comprehensive approach to the domain wall dynamics in AFM materials, a crucial step toward the development of AFM spintronics.

Bloch oscillations of multimagnon excitations in a Heisenberg XXZ chain

The studies of multi-magnon excitations will extend our understandings of quantum magnetism and strongly correlated matters. Here, by using the time-evolving block decimation algorithm, we investigate the Bloch oscillations of two-magnon excitations under a gradient magnetic field. Through analyzing the symmetry of the Hamiltonian, we derive a rigorous and universal relation between ferromagnetic and anti-ferromagnetic systems. Under strong interactions, in addition to free-magnon Bloch oscillations, there appear fractional bounded-magnon Bloch oscillations which can be understood by an effective single-particle model. To extract the frequencies of Bloch oscillations and determine the gradient of magnetic field, we respectively calculate the fidelity in the time domain and the sub-standard deviation in the frequency domain. Our study not only explore the interaction-induced Bloch oscillations of multi-magnon excitations, but also provides an alternative approach to determine the gradient of magnetic field via ultracold atoms in optical lattices.

Interfacial spin Seebeck effect in noncollinear magnetic systems

The interplay between spin and heat currents at magnetic insulator|nonmagnetic metal interfaces has been a subject of much scrutiny because of both fundamental physics and the promise for technological applications. While ferrimagnetic and, more recently, antiferromagnetic systems have been extensively investigated, a theory generalizing the heat-to-spin interconversion in noncollinear magnets is still lacking. Here, we establish a general framework for thermally-driven spin transport at the interface between a noncollinear magnet and a normal metal. Modeling the interfacial coupling between localized and itinerant magnetic moments via an exchange Hamiltonian, we derive an expression for the spin current, driven by a temperature difference, for an arbitrary noncollinear magnetic order. Our theory reproduces previously obtained results for ferromagnetic and antiferromagnet systems.

Ab initio calculations on three dimensional antiferromagnet Cu3TeO6

We have carried out the ab initio density functional theory calculations to study the electronic structure and magnetic properties of three dimensional antiferromagnetic system Cu3TeO6. The hopping interactions between various Cu2+ ions are calculated here. We find there are twelve important hopping interactions present in the system. The exchange paths are visualized by Wannier function plotting. We have also studied the effect of the spin-orbit coupling (SOC) in Cu3TeO6. The results of the calculations reveal that the SOC stabilizes the magnetic order in this system. The antiferromagnetic insulating state is the lowest energy state within LSDA + U + SOC calculation. We have calculated the magnetocrystalline anisotropy and (111) is the easy axis for this system as agreed with the experiment.

Magnonic Floquet Quantum Spin Hall Insulator in Bilayer Collinear Antiferromagnets

We study irradiated two-dimensional insulating bilayer honeycomb ferromagnets and antiferromagnets coupled antiferromagnetically with a zero net magnetization. The former is realized in the recently synthesized bilayer honeycomb chromium triiodide CrI3. In both systems, we show that circularly-polarized electric field breaks time-reversal symmetry and induces a dynamical Dzyaloshinskii-Moriya interaction in each honeycomb layer. However, the resulting bilayer antiferromagnetic system still preserves a combination of time-reversal and space-inversion ({bf{P}}{bf{T}}) symmetry. We show that the magnon topology of the bilayer antiferromagnetic system is characterized by a {{bf{Z}}}_{{bf{2}}} Floquet topological invariant. Therefore, the system realizes a magnonic Floquet quantum spin Hall insulator with spin filtered magnon edge states. This leads to a non-vanishing Floquet magnon spin Nernst effect, whereas the Floquet magnon thermal Hall effect vanishes due to {bf{P}}{bf{T}} symmetry. We study the rich {{bf{Z}}}_{{bf{2}}} Floquet topological magnon phase diagram of the system as a function of the light amplitudes and polarizations. We further discuss the great impact of the results on future experimental realizations.

Thermodynamic properties of a multiferroic antiferromagnetic spin system: The influence of Dzyaloshinskii Moriya interaction

In this paper, we address the influence of Dzyaloshinskii Moriya (DM) interaction on the thermodynamic properties of a multiferroic antiferromagnetic spin system using the spin wave theory (SWT) as a diagonalization method which, associated to the statistical physics, helps to evaluate the statistical sum. The basic factors in thermodynamics, such as the Boltzmann entropy and the specific heat capacity at thermal equilibrium, are obtained. Analyzing the numerical results, it follows that the DM interaction is the best candidate that can help to maintain in long time the system in its coherent state. Furthermore, we find that the influence of the DM interaction is more accentuated at low-temperature and that it enables the system to release heat energy. On the other hand, the fact that the specific heat capacity tends to the constant clearly shows that the system obeys the Dulong–Petit law. In addition, our results also show that for high magnetic field (greater than B[Formula: see text]=[Formula: see text]30 Tesla), the magnetic field dependence of both entropy and specific heat shows a peak-like behavior and that the DM interaction raises the corresponding critical magnetic field but lowers the peak amplitude. It follows that the spin-flop transition occurs in the system and that the strong magnetic field withdraw the antiferromagnetic phase. Overall, we find that the DM interaction is a promising candidate for the control of our system.

A Novel STT-MRAM Design With Electric-Field-Assisted Synthetic Anti-Ferromagnetic Free Layer

Spin-transfer torque magnetic random-access memory (STT-MRAM) is one of the next-generation nonvolatile memories, which has the most promising industrial prospect and has generated lots of new ideas. One of the continuous challenges in STT-MRAM development is to reduce the critical write current. Recently, one published paper reported that under a small electric bias voltage, the synthetic anti-ferromagnetic (SAF) multilayer system could be switched between an anti-ferromagnetic coupling state and a ferromagnetic coupling state. Based on this phenomenon, we propose a new type of STT-MRAM of which the critical write current can be reduced by an assisting electric-field (E-field). Micromagnetic simulation has been performed to study the dynamic switching behavior of the magnetization of the SAF free layer with tunable Ruderman–Kittel–Kasuya–Yosida interaction under the impact of an E-field.

Role of phase transition in the dielectric and magnetic properties of Na containing NiO

In this study, we systematically investigated the phase transition (cubic (90°) → rhombohedral (60.01°)) and its role in the electronic structure, dielectric, and magnetic behavior of Ni1−xNaxO (0.02≤x≤0.2). X-ray photoelectron spectroscopy indicated non-local screening of the divalent Ni by oxygen and monovalent sodium ions together with giant multiplet splitting. At 310 K, the longitudinal optical phonon mode (500 cm−1) dominated over all the vibrational excitations, but for the dilute dispersion of Na, the transverse optical phonon mode (455 cm−1) exhibited the highest intensity among all the modes. Even at a very low level (x∼ 0.02) of Na substitution, the intensity of the 2-magnon mode (1530 cm−1) was significantly suppressed. The onset of the structural transition and antiferromagnetic Néel temperature (TN) shifted to a high temperature region with moderate Na substitution, which has significant implications for the dependence of the relative dielectric permittivity (εR(T)) and magnetic susceptibility (χmag(T)) on the temperature. The dependence of the ac-resistivity ρac(T, f) on the temperature and frequency followed Mott's variable range-hopping mechanism for charge carriers between the localized states, where the activation energies were highly dependent on the spin (S = 1/2) of the type-II antiferromagnetic system. The magnetization isotherms (M–H) were analyzed using a modified Langevin function M=MoL(μPHkBT)+χaH and we extracted TN for the Ni1−xNaxO system.

Superparamagnetic clusters at room temperature, anomalous magnetic coercivity and spin-flop transition in novel dilute antiferromagnetic nanoparticles of La2Fe0.875Cr0.125GaO6

Abstract Magnetism at the nanoscale is intriguing, but that on the magnetic properties of a dilute antiferromagnetic (AFM) system is rarely studied. Presence of uncompensated spins at the lattice as well as on the nanoparticle surfaces could lead to interesting magnetic behaviour. We present the combined effect of reduction in particle size and dilution of exchange interactions on the magnetic properties of an AFM perovskite oxide. A dilute AFM compound, La2Fe0.875Cr0.125GaO6 nanoparticles (average particle size ∼ 44.5 (7) nm), has been synthesized where non-magnetic Ga-ions are introduced to interrupt the superexchange pathways. The compound crystallizes in orthorhombic structure with Pbnm space group, where Fe, Cr and Ga ions are distributed randomly in the B-site. DC magnetic studies show superparamagnetic behaviour at room temperature, spin-flop transition and a prominent hysteresis loop with sufficiently high coercivity around 6.69 kOe at ∼138 K. The analysis of thermoremanent and isothermoremanent magnetization curves reveal SPM behaviour at 300 K and 2-dimensional dilute antiferromagnetic in field like nature at 5 K. AC magnetic susceptibility analysis confirms the presence of non-interacting clusters having SPM behaviour with blocking temperature ∼280 K. Furthermore, temperature variation of coercivity shows a Gaussian nature, mainly associated with competing Dzyaloshinskii-Moriya interactions between disordered B-site ions along with strong uncompensated surface spin effects.

Topological thermal Hall effect driven by spin-chirality fluctuations in frustrated antiferromagnets

By revealing an underlying relation between the Dzyaloshinskii-Moriya interaction (DMI) and the scalar spin chirality, we develop the theory of magnon thermal Hall effects in antiferromagnetic systems. The dynamic fluctuation of the scalar chirality is shown to directly respond to the nontrivial topology of magnon bands. In materials such as the jarosites compounds KFe$_3$(OH)$_6$(SO$_4$)$_2$ and veseignite BaCu$_3$V$_2$O$_8$(OH)$_2$ in the presence of in-plane DMI, the time-reversal symmetry can be broken by the fluctuations of scalar chirality even in the case of coplanar $\mathbf{q}=0$ magnetic configuration. The spin-wave Hamiltonian is influenced by a fictitious magnetic flux determined by the in-plane DMI. Topological magnon bands and corresponding nonzero Chern numbers are presented without the need of a canted non-coplanar magnetic ordering. The canting angle dependence of thermal Hall conductivity is discussed in detail as well. These results offer a clear principle of chirality-driven topological effects in antiferromagnetically coupled systems.

Probing hole-doping of the weak antiferromagnet TiAu with first principles methods

We investigate hole-doping by Sc substitution for Ti in the weak antiferromagnet (wAFM) system TiScAu. Behavior reported so far fits a weak itinerant AFM picture, and experimentally leads to a quantum critical point at . Here we study supercells with several rational fractions of Ti replaced by Sc. We find, unexpectedly, definite local moment-like behavior, e.g. magnetic moments of the Ti atom comparable to or even larger than in the bulk persist even into the Ti-poor regime of this alloy system. Itinerant signatures persist, however, as the Ti projected density of states displays van Hove singularity peaks near the Fermi level in most cases, revealing a striking similarity to nonmagnetic bulk TiAu. The current picture of this system, midway between itinerant and local moment, will be provided and discussed in light of experimental observations.

Skyrmions and merons in two-dimensional antiferromagnetic systems

We have investigated (numerically) skyrmions constituted by two merons in two-dimensional Heisenberg antiferromagnets. The behavior and stability of these topological pseudo-particles are analyzed in discrete and finite lattices. The influence of lattice defects and external magnetic fields is also studied. The topological stability dictates that the two merons must be kept apart but processes of annihilation and fusion of merons are observed for finite lattices.

Long-Range Order, “Tower” of States, and Symmetry Breaking in Lattice Quantum Systems

In a quantum many-body system where the Hamiltonian and the order operator do not commute, it often happens that the unique ground state of a finite system exhibits long-range order (LRO) but does not show spontaneous symmetry breaking (SSB). Typical examples include antiferromagnetic quantum spin systems with Neel order, and lattice boson systems which exhibit Bose-Einstein condensation. By extending and improving previous results by Horsch and von der Linden and by Koma and Tasaki, we here develop a fully rigorous and almost complete theory about the relation between LRO and SSB in the ground state of a finite system with continuous symmetry. We show that a ground state with LRO but without SSB is inevitably accompanied by a series of energy eigenstates, known as the "tower" of states, which have extremely low excitation energies. More importantly, we also prove that one gets a physically realistic "ground state" by taking a superposition of these low energy excited states. The present paper is written in a self-contained manner, and does not require any knowledge about the previous works on the subject.

Wide Magnetic Thermal Memory Effect (∼55 K) Above Room Temperature Coupled to a Structure Phase Transition of Lattice Symmetry Reduction in High-Temperature Phase in an S = 1/2 Spin Chain Molecule Crystal.

One-dimensional (1D) S = 1/2 Heisenberg antiferromagnetic (AFM) chain system shows frequently a spin-Peierls-type transition owing to strong spin-lattice coupling. From high-temperature phase (HTP) to low-temperature phase (LTP), the spin chain distortion leads to the reduction in lattice symmetry in LTP, called the symmetry breaking (SB) phase transition. Herein, we report the first example of 1D S = 1/2 AFM molecular crystal, [Et3( n-Pr)N][Ni(dmit)2] (Et3( n-Pr)N+ = triethylpropylammonium, dmit2- = 2-thioxo-1,3-dithiole-4,5-dithiolate), which shows a structural phase transition with lattice symmetry increase in LTP, which is contrary to the SB phase transition. Particularly, the structure phase transition leads to magnetically bistable state with TC↑ ≈ 375 K, TC↓ ≈ 320 K, and surprisingly large thermal hysteresis (∼55 K). Additionally, LTP and HTP coexist in a temperature region near TC but not at TC in this 1D spin system. The large hysteresis is related to the huge deformation of anion stack, which needs high activation energy for the structure transformation and magnetic transition between LTP and HTP. This study would not only provide new insight into the relationship of spin-Peierls-type transition and structure phase transition but also offer a roadmap for searching molecular-scale magnetic bistable materials, which are in huge demand in future electronic, magnetic, and photonic technologies.

Magnetic Properties of XXZ Heisenberg Antiferromagnetic and Ferrimagnetic Nanotubes**Supported by the National Nature Science Foundation of China under Grant No. 11772090

The spin-1/2 antiferromagnetic and spin-(1/2, 1) ferrimagnetic single-walled nanotubes are described by XXZ Heisenberg model. The sublattice magnetization and the critical temperature of the system are calculated by using the double-time spin Greenʼs function method. At zero temperature, with the increase of the exchange interaction in the circumferential direction, a maximum value appears in the sublattice magnetization curves of antiferromagnetic and ferrimagnetic systems. As the diameter of the tube increases, the spin quantum fluctuations and thermal fluctuations are suppressed. In addition, the spin quantum fluctuation of the spin-1/2 antiferromagnetic system is greater than that of the spin-(1/2, 1) ferrimagnetic system. The critical temperature of the system increases firstly and then tends to a constant with the increase of the diameter of tube, and it decreases to zero as the exchange anisotropy of the system disappears.