Net Magnetization Research Articles

Emergent topological quasiparticle kinetics in constricted nanomagnets

The ubiquitous domain wall kinetics under magnetic field or current application describes the dynamic properties in nanostructured magnets. However, when the geometrical size of a nanomagnetic system is constricted to the limiting domain wall length scale, the competing energetics between anisotropy, exchange, and dipolar interactions can cause emergent kinetics due to quasiparticle relaxation, similar to bulk magnets of atomic origin. This paper presents a joint experimental and theoretical study to support this argument: constricted nanomagnets, made of antiferromagnetic and paramagnetic neodymium thin film with honeycomb motif, reveal fast kinetic events at picosecond timescales due to the relaxation of topological quasiparticles that persist to low temperature in the absence of any external stimuli. This discovery is especially important considering the fact that paramagnets or antiferromagnets have no net magnetization. Yet, the kinetics in neodymium nanostructures is quantitatively similar to that found in ferromagnetic counterparts and only varies with the thickness of the specimen. This suggests that a universal, topological quasiparticle-mediated dynamical behavior can be prevalent in nanoscopic magnets, irrespective of the nature of the underlying magnetic material. Published by the American Physical Society 2024

Electrical Excitation and Detection of Chiral Magnons in a Compensated Ferrimagnetic Insulator.

Magnon chirality refers to the precessional handedness of magnetization around the external magnetic field, which is fixed as right-handed in ferromagnets. Compensated ferrimagnets accommodate parallel and antiparallel configurations of net magnetization and angular momentum, and thus serve as an ideal platform for studying magnon chirality. Through performing spin-torque ferromagnetic resonance experiments, we experimentally study the reversal of low-frequency magnon chirality across the magnetization and angular momentum compensation temperatures in a Gd_{3}Fe_{5}O_{12}/Pt bilayer. In particular, we demonstrate that dampinglike spin torque could sensitively excite and detect the reversal of low-frequency magnon chirality. By solving the coupled Landau-Lifshitz-Gilbert equations, the close correlation between the reversal of low-frequency magnon chirality and the sign of net angular momenta is established. The electrical excitation and detection of low-frequency magnon chirality in compensated ferrimagnetic insulators could be useful for building chiral spintronics.

Spin-Polarized Antiferromagnetic Metals

Spin-polarized antiferromagnets have recently gained significant interest because they combine the advantages of both ferromagnets (spin polarization) and antiferromagnets (absence of net magnetization) for spintronics applications. In particular, spin-polarized antiferromagnetic metals can be useful as active spintronics materials because of their high electrical and thermal conductivities and their ability to host strong interactions between charge transport and magnetic spin textures. We review spin and charge transport phenomena in spin-polarized antiferromagnetic metals in which the interplay of metallic conductivity and spin-split bands offers novel practical applications and new fundamental insights into antiferromagnetism. We focus on three types of antiferromagnets: canted antiferromagnets, noncollinear antiferromagnets, and collinear altermagnets. We also discuss how the investigation of spin-polarized antiferromagnetic metals can open doors to future research directions.

Photocurrent Spectroscopy of Dark Magnetic Excitons in 2D Multiferroic NiI2.

Two-dimensional (2D) antiferromagnetic (AFM) semiconductors are promising components of opto-spintronic devices due to terahertz operation frequencies and minimal interactions with stray fields. However, the lack of net magnetization significantly limits the number of experimental techniques available to study the relationship between magnetic order and semiconducting properties. Here, they demonstrate conditions under which photocurrent spectroscopy can be employed to study many-body magnetic excitons in the 2D AFM semiconductor NiI2. The use of photocurrent spectroscopy enables the detection of optically dark magnetic excitons down to bilayer thickness, revealing a high degree of linear polarization that is coupled to the underlying helical AFM order of NiI2. In addition to probing the coupling between magnetic order and dark excitons, this work provides strong evidence for the multiferroicity of NiI2 down to bilayer thickness, thus demonstrating the utility of photocurrent spectroscopy for revealing subtle opto-spintronic phenomena in the atomically thin limit.

Single-molecule Magnets (SMM) spin channels connecting FeMn antiferromagnet and NiFe ferromagnetic electrodes of a tunnel junction

The integration of single-molecule magnets (SMMs) into magnetic tunnel junctions (MTJs) offers significant potential for advancing molecular spintronics, particularly for next-generation memory devices, quantum computing, and energy storage technologies such as solar cells. In this study, we present the first demonstration of SMM-induced spin-dependent properties in an antiferromagnet-based MTJ molecular spintronic device (MTJMSD). We engineered cross-junction-shaped devices comprising FeMn/AlOx/NiFe MTJs. The AlOx barrier thickness where the exposed junction edges meet was comparable to the SMM length, facilitating the incorporation of SMM molecules as spin channels for spin-dependent transport. The SMM channels enabled long-range magnetic moment ordering around molecular junctions, which were precisely engineered via fabrication processes. The SMM, composed of a [Mn6(μ3-O)2(H2N-sao)6(6-atha)2(EtOH)6] (H2N-saoH = salicylamidoxime, 6-atha = 6-acetylthiohexanoate) complex, featured thioester groups at the ends that upon hydrolysis they form bonds with the magnetic electrodes. SMM-treated junctions demonstrated a significant current enhancement, reaching up to 7 μA at an input voltage of 60 mV. Furthermore, SMM-doped junctions exhibited current stabilization in the μA range at lower temperatures, whereas the bare electrodes showed current suppression to the picoampere range. Magnetization measurements conducted at 55 K and 300 K on pillar-shaped devices revealed a reduction in magnetic moment at low temperatures. Additionally, Kelvin probe atomic force microscopy (KPAFM) measurements confirmed that SMM integration transformed the electronic properties over long ranges.These findings are attributed to the spin channels formed between magnetic metal electrodes, which enhance spin polarization at each magnetic electrode. Our research highlights the potential of using antiferromagnetic materials, characterized by minimal stray fields and zero net magnetization, to transform MTJMSD devices.

PT Symmetry Breaking Induced Anomalous Valley Hall Effect in 2D Antiferromagnetic Semiconductor.

Realizing the anomalous valley Hall (AVH) effect in two-dimensional (2D) materials is of crucial importance for information processing and recording technology. While the research in this field mainly focuses on ferromagnetic systems, little is known about antiferromagnetic systems. Here, using k·p model analysis, we report a novel mechanism of realizing the AVH effect in 2D antiferromagnetic materials. This physics is related to the PT symmetry breaking induced by intrinsic staggered sublattice potential, which is introduced by asymmetric magnetic ions located at different sublattices. With reversal of the magnetic orientation on different sublattices, the AVH effect can be reversed. Based on first-principles calculations, we further demonstrate this mechanism in an antiferromagnetic monolayer of NiRuCl6. Intriguingly, due to the d orbital mismatch near the Fermi level, monolayer NiRuCl6 simultaneously owns zero net magnetization and large spin splitting and valley polarizations, which facilitates the observation of the AVH effect. Our findings greatly enrich the research on valley physics in antiferromagnetic systems.

Altermagnetism: Exploring New Frontiers in Magnetism and Spintronics

AbstractRecent developments have introduced a groundbreaking form of collinear magnetism known as “altermagnetism”. This emerging magnetic phase is characterized by robust time‐reversal symmetry breaking, antiparallel magnetic order, and alternating spin‐splitting band structures, yet it exhibits vanishing net magnetization constrained by symmetry. Altermagnetism uniquely integrates traits previously considered mutually exclusive to conventional collinear ferromagnetism and antiferromagnetism, thereby facilitating phenomena and functionalities previously not achievable within these traditional categories of magnetism. Initially proposed theoretically, the existence of the altermagnetic phase has since been corroborated by a range of experimental studies, which have confirmed its unique properties and potential for applications. This review explores the rapidly expanding research on altermagnets, emphasizing the novel physical phenomena they manifest, methodologies for inducing altermagnetism, and promising altermagnetic materials. The goal of this review is to furnish readers with a comprehensive overview of altermagnetism and to inspire further innovative studies on altermagnetic materials which can potentially revolutionize applications in technology and materials science.

Ferrimagnetic structure in the high-pressure phase of α−Mn

The α−Mn phase exhibits a large anomalous Hall effect (AHE) in its pressure-induced weak ferromagnetic (WFM) state, despite its relatively small spontaneous magnetization of ∼0.02μB/Mn. To understand the underlying mechanism behind this AHE, we performed single crystal neutron diffraction measurements at 2.0 GPa to determine the magnetic structure of the WFM phase. Our investigation reveals a ferrimagnetic structure characterized by nearly collinear magnetic moments aligned along the [001] direction at sites I, II, III-1, and IV-1. In contrast, the small moments at sites III-2 and IV-2 lie within the (001) plane. The calculated net magnetization of this magnetic structure, (−0.08±0.10)μB/Mn atom, is in agreement with the experimentally determined spontaneous magnetization. The observation of a magnetic reflection at q=(0,0,0) satisfies a key condition for the emergence of the AHE. Published by the American Physical Society 2024

Altermagnetism Induced by Sliding Ferroelectricity via Lattice Symmetry-Mediated Magnetoelectric Coupling.

Altermagnets, distinct from conventional ferromagnets or antiferromagnets, have recently attracted attention as the third category of collinear magnets, which exhibit the coexistence of zero net magnetization and spin polarization due to their unique lattice symmetries. Meanwhile, the additional layer degrees of freedom in multilayer sliding ferroelectrics offer opportunities for coupling with lattice symmetries, paving the way for an innovative approach to constructing multiferroic lattices. In this study, altermagnetic tuning in SnS2/MnPSe3/SnS2 heterostructures is achieved by breaking and restoration of lattice inversion symmetry through sliding ferroelectric switching. First-principles calculations reveal that the spin density and corresponding time-reversal symmetry of MnPSe3 can be manipulated by lattice symmetry, triggering phase transitions between antiferromagnetism and altermagnetism. This research establishes a novel form of magnetoelectric coupling mediated by lattice symmetry and provides a theoretical basis for the design of miniature information processing and memory devices based on altermagnetism.

Crossover from Relativistic to Non-Relativistic Net Magnetization for MnTe Altermagnet Candidate

We experimentally study magnetization reversal curves for MnTe single crystals, which is the altermagnetic candidate. Above 85 K temperature, we confirm the antiferromagnetic behavior of magnetization M, which is known for α-MnTe. Below 85 K, we observe anomalous low-field magnetization behavior, which is accompanied by the sophisticated \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M(\\alpha )$$\\end{document} angle dependence with beating pattern as the interplay between \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M(\\alpha )$$\\end{document} maxima and minima: in low fields, \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M(\\alpha )$$\\end{document} shows ferromagnetic-like 180° periodicity, while at high magnetic fields, the periodicity is changed to the 90° one. This angle dependence is the most striking result of our experiment, while it can not be expected for standard magnetic systems. In contrast, in altermagnets, symmetry allows ferromagnetic behavior only due to the spin–orbit coupling. Thus, we claim that our experiment shows the effect of weak spin–orbit coupling in MnTe, with crossover from relativistic to non-relativistic net magnetization, and, therefore, we experimentally confirm altermagnetism in MnTe.

Valley-Related Multipiezo Effect and Noncollinear Spin Current in an Altermagnet Fe2Se2O Monolayer.

An altermagnet exhibits many novel physical phenomena because of its intrinsic antiferromagnetic coupling and natural band spin splitting, which are expected to give rise to new types of magnetic electronic components. In this study, an Fe2Se2O monolayer is proven to be an altermagnet with out-of-plane magnetic anisotropy, and its Néel temperature is determined to be 319 K. The spin splitting of the Fe2Se2O monolayer reaches 860 meV. Moreover, an Fe2Se2O monolayer presents a pair of energy valleys, which can be polarized and reversed by applying uniaxial strains along different directions, resulting in a piezovalley effect. Under the strain, the net magnetization can be induced in the Fe2Se2O monolayer by doping with holes, thereby realizing a piezomagnetic property. Interestingly, noncollinear spin current can be generated by applying an in-plane electric field on an unstrained Fe2Se2O monolayer doped with 0.2 hole/formula unit. These excellent physical properties make the Fe2Se2O monolayer a promising candidate for multifunctional spintronic and valleytronic devices.

Zero-field finite-momentum and field-induced superconductivity in altermagnets

We explore the possibilities for spin-singlet superconductivity in newly discovered altermagnets. Investigating d-wave altermagnets, we show that finite-momentum superconductivity can easily emerge in altermagnets even though they have no net magnetization, when the superconducting order parameter also has d-wave symmetry with nodes coinciding with the altermagnet nodes. Additionally, we find a rich phase diagram when both altermagnetism and an external magnetic field are considered, including superconductivity appearing at high magnetic fields from a parent zero-field normal state. Published by the American Physical Society 2024

(Invited) Magnetism of Tb3N@C80 Endofullerenes

Tb3N@C80 was one of the first trimetallic endofullerenes to be studied in view of its magnetization where the microscopic picture of 3 Tb magnetic moments non-collinearly aligned by the ligand field of the central nitrogen ion was proposed [1]. The triangular geometry results in a net molecular magnetization of two Tb3+ ions or 18 μB. Today, this is confirmed with 23 possible pseudospin arrangements of the 3 Tb magnetic moments, resulting in a frustrated magnetic ground state with fluctuating pseudospins [2]. This picture does not include coherent tunneling of the magnetization, which is predicted within an 8-dimensional Hamiltonian and can be tested with low temperature magnetization measurements. Experimentally, we find a low temperature hysteresis that identifies Tb3N@C80 as a single molecule magnet. More surprisingly, we observe room temperature ferromagnetism in cubic single crystallites of the molecules using SQUID magnetometry. Since the magnetic interaction between adjacent carbon cages is expected to be dipolar it is neglected in the explanation of magnetic order above 3 K [3] and it is challenging to explain this finding with endofullerene magnetism. We present a series of analytical chamistry results of samples with and without room temperature ferromagnetism.[1] M. Wolf et al. Angew. Chem. Int. Ed. 44, 3306 (2005).[2] R. Westerström et al. Phys. Rev. B, 89, 060406(R) (2014).[3] A. Kostanyan et al. Phys. Rev. B, 101, 134429 (2020).

Ferrimagnetic structures with rare-earth induced spin-reorientation in the Mn self-doped perovskite (Er0.7Mn0.3)MnO3

The search for spin-reorientation (SR) phase transitions, a spontaneous rotation of ordered magnetic moments, in ferrimagnetic (FiM) materials, which carry a net magnetization, is of fundamental and practical interest for applications in the field of spintronics. In this work, we have investigated Mn self-doped (Er0.7Mn0.3)MnO3 solid solution with GdFeO3-type Pnma perovskite structure by combining specific heat, magnetic susceptibility and neutron powder diffraction measurements. We provide experimental evidence for FiM order below TC = 104 K, and a spontaneous SR transition at TSR = 11 K. A FiM structure appears in all (R1-xMnx)MnO3 compounds with R = Dy–Lu for x ≥ 0.2. But in this structural family, R = Er is the only material with a SR transition. FiM order in (Er0.7Mn0.3)MnO3 appears with ferromagnetic (FM) ordering of Mn3+ and Mn4+ cations at the B site along the a-direction, which are antiferromagnetically (AFM) coupled with Er3+ and Mn2+ cations at the A site. At TSR, ordered Er3+ change direction from the magnetically harder a-axis to the magnetically easy b-axis and induce a change of the whole FiM structure, including the direction of all Mn spins from the irreducible representation mGM3+ to mGM4+. We calculated the crystal-field (CF) anisotropy for Ho3+ in (Ho0.8Mn0.2)MnO3 and Er3+ in (Er0.7Mn0.3)MnO3, based on the point charge model; the results revealed large magnetic anisotropies. For the FiM structure of (Ho0.8Mn0.2)MnO3, the magnetically easy a-axis of Ho3+ keeps the high-temperature magnetic directions along the a-axis and gives rise to a pronounced magnetization reversal effect at low temperature because of a significant rise of Ho3+ ordered moments. On the other hand, for the FiM structure of (Er0.7Mn0.3)MnO3, the magnetically easy b-axis of Er3+ gives rise to a SR phase transition at TSR, which does not lead to magnetization reversal even though Er3+ ordered moments rise significantly at low temperatures.

Crossover from relativistic to non-relativistic net magnetization for MnTe altermagnet candidate

Abstract We experimentally study magnetization reversal curves for MnTe single crystals, which is the altermagnetic candidate. Above 85 K temperature, we confirm the antiferromagnetic behavior of magnetization M, which is known for α–MnTe. Below 85 K, we observe anomalous low-field magnetization behavior, which is accompanied by the sophisticated M(α) angle dependence with beating pattern as the interplay between M(α) maxima and minima: in low fields, M(α) shows ferromagnetic-like 180° periodicity, while at high magnetic fields, the periodicity is changed to the 90° one. This angle dependence is the most striking result of our experiment, while it can not be expected for standard magnetic systems. In contrast, in altermagnets, symmetry allows ferromagnetic behavior only due to the spin-orbit coupling. Thus, we claim that our experiment shows the effect of weak spin-orbit coupling in MnTe, with crossover from relativistic to non-relativistic net magnetization, and, therefore, we experimentally confirm altermagnetism in MnTe.

Long-lived enhanced magnetization-A practical metabolic MRI contrast material.

Noninvasive tracking of biochemical processes in the body is paramount in diagnostic medicine. Among the leading techniques is spectroscopic magnetic resonance imaging (MRI), which tracks metabolites with an amplified (hyperpolarized) magnetization signal injected into the subject just before scanning. Traditionally, the brief enhanced magnetization period of these agents limited clinical imaging. We propose a solution based on amalgamating two materials-one having diagnostic-metabolic activity and the other characterized by robust magnetization retention. This combination slows the magnetization decay in the diagnostic metabolic probe, which receives continuously replenished magnetization from the companion material. Thus, it extends the magnetization lifetime in some of our measurements to beyond 4 min, with net magnetization enhanced by more than four orders of magnitude. This could allow the metabolic probes to remain magnetized from injection until they reach the targeted organ, improving tissue signatures in clinical imaging. Upon validation, this metabolic MRI technique promises wide-ranging clinical applications, including diagnostic imaging, therapeutic monitoring, and posttreatment surveillance.

Tailoring Bi1−x−yBaxFeyFeO3 for desired properties: insights into structural, dielectric, and magnetic responses

In present study, samples of Bi1−x−yBaxFeyFeO3 have been synthesized, where x = 0.15, y = 0; x = 0.10, y = 0.10; and x = 0.15, y = 0.10 utilizing a swift two-stage solid-state reaction method. X-ray diffraction (XRD) data has been Rietveld refined to evaluate the structural parameters. Micro-strain is also calculated using Williamson Hall method. Temperature (300 K to 660 K) dependent measurements of the dielectric constant have been conducted at various frequencies (100 kHz, 500 kHz, and 1000 kHz). The dielectric constant (ε′) rises as the temperature increases. Two dielectric anomalies around 450 K and 613 K have been noticed in ε′ versus T curves for all the samples which might be related with defect dipoles and the magnetic transition respectively. Further, an insignificant value of loss tangent (0.2) specifically at around 300 K is a signal of small leakage current in the samples. The source of high dielectric constant is discussed by considering the structural distortions in the ceramics. A clear hysteresis loop has been observed for all the samples which is a sign of collapse of antiferromagnetic nature of BiFeO3. Further, in case of co-doped samples, almost a saturation in magnetization with magnetization value 5.9718 emu g−1 has been noticed in hysteresis curve indicating a major contribution of ferromagnetic interaction. Enhancement in the net magnetization is briefly discussed by considering the ferromagnetic type direct interaction among Fe3+ ions and suppressing the anti-ferromagnetic type super exchange interaction.

Effect of annealing temperature and precipitation agent on the structure, optical and magnetic characteristics of dysprosium orthoferrite nanoparticles

Powders consisting of DyFeO3 nanoparticles were obtained by the co-precipitation method using two different precipitating agents (NaOH (SH) and (NH4)2CO3 (AC) solutions) and followed by annealing for 1 h at 850, and 950 °C. Properties of the synthesized samples were characterized by FT-IR, PXRD, BET, TEM, UV-Vis DRS, and VSM analyses. The obtained data revealed that both calcination temperature and precipitation agent had notable influence on the structural parameters and properties of the synthesized DyFeO3 nanoparticles. Increasing the calcination temperature led to the increase of structural characteristics such as average crystallite size (DScher, DW-H, DRietveld, nm), lattice cell volume (V, Å), and crystallite phase content as well as in properties such as band gap energy value (Eg, eV), coercivity (Hc, Oe), remanent magnetization (Mr, emu∙g−1), and net magnetization (Mn, emu∙g−1). Additionally, when comparing samples calcined at the same temperature, DyFeO3-AC exhibited higher structural, optical, and magnetic properties compared to DyFeO3-SH. Moreover, the DyFeO3 nanoparticles synthesized in this study exhibited lower band gap energy (1.87–1.93 eV), coercivity (0.018–0.023 Oe), and remanent magnetization (2.82∙10−6–3.87∙10−6 emu∙g−1) values, but larger net magnetization (2.43–2.62 emu∙g−1) when compared to RFeO3 perovskites synthesized by different methods (R = La, Y, Ho, Dy, Eu, Tb).

Orientation-dependent Andreev reflection in an altermagnet/altermagnet/superconductor junction

Altermagnets, an emerging class of magnetic materials distinguished by a significant non-relativistic spin splitting band structure but zero net macroscopic magnetization, have recently received considerable attention. Here, we present a theoretical study focusing on the orientation-dependent conductance induced by Andreev reflection (AR) at an altermagnet/altermagnet/superconductor junction. Conventional and equal-spin AR processes occur when the Néel vector directions of the AMs are non-collinear and controllable by the altermagnet’s orientation θ and Néel vector direction α. The conductance spectra resemble ferromagnet/ferromagnet/superconductor junctions with parallel or antiparallel magnetization configurations, depending on θ and α. Both the anisotropic altermagnetic state and the equal-spin AR signature can be probed by studying conductance spectra. These findings suggest that altermagnet/superconductor junctions are promising platforms for future superconducting spintronics applications.

Effects of inserted ultrathin Pt buffer on perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures

We investigated effects of inserted ultrathin Pt buffer on the perpendicular magnetic properties of Ta/TbFeCo/Ta layered structures. For Tb-rich TbFeCo films buffered with Ta and Ta/Pt a compensation thickness is observed at ∼ 6 nm, where the net saturation magnetization vanishes. The perpendicular magnetic anisotropy properties of two structures are found to be governed by a competition of a perpendicular bulk anisotropy and an in-plane interface anisotropy. Compared with Ta-buffered TbFeCo films, the counterparts with insertion of Pt buffer exhibits a larger negative interface anisotropy. A large variation of saturation magnetization and perpendicular magnetic anisotropy with increasing inserted thickness of Pt buffer is observed in the Ta/Pt/TbFeCo/Ta structure. This can be ascribed to the interface-driven variation of exchange interaction between Tb and FeCo at the interface, confirmed by thermomagnetic measurements. A clear correlation between the interface-driven variation of exchange interaction and perpendicular magnetic properties of the TbFeCo films is identified. Our results show the interface plays an important role in the perpendicular magnetic properties of TbFeCo films, which is critical for the application in spintronic devices.