Antiferromagnetic Metal Research Articles

One-loop effective potential for two-dimensional competing scalar order parameters

Using the method of the effective potential of quantum field theory, we compute the quantum corrections to the phase diagram of systems with competing order parameters. This is specially useful to study metallic systems with competing antiferromagnetic and superconducting ground states. We focus on the two-dimensional case that is relevant for high Tc superconductors and heavy fermion systems. We consider two different types of couplings between the order parameters and obtain the modifications in the phase diagrams due to critical quantum fluctuations in these systems with conflicting orders. We consider z=1, as well as a dissipative z=2 dynamics, typical of antiferromagnetic metals close to the magnetic quantum critical point. Our results show that these depend strongly on both dimensionality and dynamics of the propagators describing the excitations of the possible ordered states. We find stable unconventional coexisting phases, as well as the enhancement of the region of coexistence by fluctuations.

Magnetic Behavior in TiS3 Nanoribbon.

The electronic structure, magnetic properties and strain response of N-a-TiS3 nanoribbons are investigated by first-principles calculations. We find that the magnetic ground state is strongly dependent on width of a-TiS3. When N equals an odd number the ground state is a ferromagnetic (FM) metal, meanwhile, when N equals an even number the ground state is an anti-ferromagnetic (AFM) metal. More interestingly, a tensile strain as large as 6% can tune the 9-a-TiS3 nanoribbon from a FM metal to a half metal. A 4% tensile strain also causes a phase transition from AFM to FM ground state for 10-a-TiS3 nanoribbon. Our findings show that N-a-TiS3 is a promising candidate for spintronic and electronic applications.

Integration of the noncollinear antiferromagnetic metal Mn3Sn onto ferroelectric oxides for electric-field control

Noncollinear antiferromagnetic materials have received dramatically increasing attention in the field of spintronics as their exotic topological features such as the Berry-curvature-induced anomalous Hall effect and possible magnetic Weyl states could be utilized in future topological antiferromagnetic spintronic devices. In this work, we report the successful integration of the antiferromagnetic metal Mn3Sn thin films onto ferroelectric oxide PMN-PT. By optimizing growth, we realized the large anomalous Hall effect with small switching magnetic fields of several tens mT fully comparable to those of bulk Mn3Sn single crystals, anisotropic magnetoresistance and negative parallel magnetoresistance in Mn3Sn thin films with antiferromagnetic order, which are similar to the signatures of the Weyl state in bulk Mn3Sn single crystals. More importantly, we found that the anomalous Hall effect in antiferromagnetic Mn3Sn thin films can be manipulated by electric fields applied onto the ferroelectric materials, thus demonstrating the feasibility of Mn3Sn-based topological spintronic devices operated in an ultralow power manner.

Synthesis and characterization of a new quasi-one-dimensional antiferromagnet CoF2(H2O)2(pyrazine)

A new one-dimensional (1D) antiferromagnetic transition metal hybrid fluoride CoF2(H2O)2(pyz) (pyz = pyrazine) together with two new isostructural MF2(H2O)2(pyz) (M = Ni2+, Zn2+) have been successfully synthesized by the hydrothermal method. Their structures were determined by single crystal X-ray diffraction and they all crystallize in the same space group of C2/m. The magnetic property of CoF2(H2O)2(pyz) 1 was measured and discussed. The Weiss constant of -20.6 K indicates dominant antiferromagnetic interaction and the effective magnetic moment of about 5.66 μB suggests large unquenched orbital contribution and anisotropy in 1. The quantum fluctuations in the quasi-1D antiferromagnet of 1 prevent the appearance of long range magnetic order until 2.9 K, suggesting good 1D magnetism.

Magnetic and Structural Properties of Exchange Coupled Heusler Alloy NiO/Co2FeAl Interfaces with n-and p-Type Silicon Substrates

Exchange coupled structures of half metallic ferromagnets, Heusler Alloy (FM), Co2FeAl (CFA) and antiferromagnet (AF), NiO were fabricated by electron beam evaporation technique on n- and p-type Si substrates. These fabricated bilayer structures were further characterized for structural, morphological and magnetic point of views using x-ray diffraction (XRD), atomic/magnetic force microscopy (AFM/MFM) and vibrating sample magnetometer (VSM) techniques. Structural study shows the presence of CFA alloy and NiO phase. Surface morphology shows the granular feature of the top surface with grain size of 200–300 nm due to clustering of smaller grains. Smaller sized magnetic domains have been observed for the structures. Hysteresis behaviour of NiO/CFA/Si structures shows the ferromagnetic behaviour for in-plane orientation with negligible exchange bias whereas a significant exchange bias for out of plane orientation has been observed. The observed result could be understood due to surface roughness and arrangements of spins near the interface of ferromagnetic and anti-ferromagnetic layers.

Hubbard model on an anisotropic checkerboard lattice at finite temperatures: Magnetic and metal–insulator transitions

We study magnetic and Mott transitions of the Hubbard model on the geometrically frustrated anisotropic checkerboard lattice at half filling using cellular dynamical mean-field theory. Phase diagrams over a wide area of the parameter space are obtained by varying the interparticle interaction strength, geometric frustration strength, and temperature. Our results show that frustration and thermal fluctuations play a competing role against the interactions and in general favor a metallic phase without antiferromagnetic order. Due to their interplay, the system exhibits competition between antiferromagnetic insulator, antiferromagnetic metal, paramagnetic insulator, and paramagnetic metal phases in the intermediate-interaction regime. In the strong-interaction limit, which reduces to the Heisenberg model, our result is consistent with previous studies.

Machine-learning-assisted prediction of magnetic double perovskites

Magnetism is an important property of materials that plays a key role in many different applications. In the present paper, we use a combination of computational tools: a machine-learning technique for screening of stable candidates, an evolutionary algorithm for crystal structure determination, and first-principles calculations for characterization of electronic and magnetic properties to make predictions on magnetic double perovskites, which are yet to be synthesized. Out of 412 scanned candidates of ${A}_{2}{BB}^{\ensuremath{'}}{O}_{6}$ composition with $3d$ and $4d$ or $5d$ transition metals at B and ${B}^{\ensuremath{'}}$ sites, we found 33 compounds to form stable double-perovskite structures, 25 of which were further considered for characterization of their structure and properties. Our exercise predicted 21 double perovskites of varying magnetic and electronic properties, ranging from ferromagnetic half metals to ferri- and antiferromagnetic insulators to ferromagnetic metals and a rare example of antiferromagnetic metals. Our computational study is expected to help in discovering new magnetic double perovskites.

Tuning of Al doping on martensitic-like transition in phase separated manganites La0.6Ca0.4Mn1-xAlxO3-δ

The tuning effects of Al3+ doping on the magnetic and magnetotransport properties of La0·6Ca0·4MnO3-δ have been studied by preparing series La0·6Ca0·4Mn1-xAlxO3-δ (x = 0, 0.1, 0.125 and 0.15). Al3+ doping suppresses the ferromagnetic metal phase of La0·6Ca0·4MnO3-δ by weakening the double-exchange interaction of Mn3+-O2--Mn4+, leading to a phase separation state. Al3+ doping dependent irreversible metamagnetic transitions are observed at lower temperatures, especially a martensitic-like transition appears in x = 0.125 and 0.15 system below 4.2 K. Phase diagrams of La0·6Ca0·4Mn1-xAlxO3-δ (0.1 ≤ x ≤ 0.15) in the B-T plane are presented, in which the antiferromagnetic insulator and ferromagnetic metal phases boundary is figured out. The antiferromagnetic matrix and magnetic disorders caused by the random distribution of Al3+ ions and the spontaneous oxygen vacancies grow into a pinning center to pin the Mn moments. As magnetic field is increased to the critical field, the localized magnetic moments rotate to parallel to the magnetic field direction, leading to a martensitic-like transition.

Coupling between the heavy-fermion superconductor CeCoIn5 and the antiferromagnetic metal CeIn3 through the atomic interface

To study the mutual interaction between unconventional superconductivity and magnetic order through an interface, we fabricate Kondo superlattices consisting of alternating layers of heavy-fermion superconductor CeCoIn$_5$ and antiferromagnetic (AFM) heavy-fermion metal CeIn$_3$. The strength of the AFM fluctuations is tuned by applying hydrostatic pressure to CeCoIn$_5(m)$/CeIn$_3(n)$ superlattices with $m$ and $n$ unit-cell-thick layers of CeCoIn$_5$ and CeIn$_3$, respectively. Superconductivity in CeCoIn$_5$ and AFM order in CeIn$_3$ coexist in spatially separated layers. At ambient pressure, N\'{e}el temperature $T_N$ of the CeIn$_3$ block layers (BLs) of CeCoIn$_5$(7)/CeIn$_3(n)$ shows little dependence on $n$, in contrast to CeIn$_3(n)$/LaIn$_3$(4) superlattices where $T_N$ is strongly suppressed with decreasing $n$. This suggests that each CeIn$_3$ BL is magnetically coupled by the RKKY interaction through the adjacent CeCoIn$_5$ BL and a 3D magnetic state is formed. With applying pressure to CeCoIn$_5$(7)/CeIn$_3$(13), $T_N$ of the CeIn$_3$ BLs is suppressed up to 2.4 GPa, showing a similar pressure dependence as CeIn$_3$ single crystals. An analysis of upper critical field reveals that the superconductivity in the CeCoIn$_5$ BLs is barely influenced by the AFM fluctuations in the CeIn$_3$ BLs, even when the CeIn$_3$ BLs are in the vicinity of the AFM quantum critical point. This is in stark contrast to CeCoIn$_5$/CeRhIn$_5$ superlattice where the superconductivity in the CeCoIn$_5$ BLs is profoundly affected by AFM fluctuations in the CeRhIn$_5$ BLs. The present results show that although AFM fluctuations are injected into the CeCoIn$_5$ BLs from the CeIn$_3$ BLs through the interface, they barely affect the force which binds superconducting electron pairs. These results demonstrate that 2D AFM fluctuations are essentially important for the pairing interactions in CeCoIn$_5$.

Magnetic Phase Transitions and the Anisotropy of Charge Carrier Scattering in Antiferromagnetic Metal Ho0.5Lu0.5B12 with Dynamic Charge Stripes

The transverse magnetoresistance, heat capacity, and magnetization of monodomain single crystals of dodecaboride Ho0.5Lu0.5B12 with a cage-glass structure are investigated at helium temperatures. It is shown that the anisotropy of charge carrier scattering in the antiferromagnetic phase is due to dynamic charge stripes oriented along directions 〈110〉 in the fcc lattice.

Diversity of Coordination Modes in a Flexible Ditopic Ligand Containing 2-Pyridyl, Carbonyl and Hydrazone Functionalities: Mononuclear and Dinuclear Cobalt(III) Complexes, and Tetranuclear Copper(II) and Nickel(II) Clusters

Syntheses, crystal structures and characterization are reported for four new complexes [Cu4Br2(L)4]Br2 (1), [Ni4(NO3)2(L)4(H2O)](NO3)2 (2), [Co2(L)3](ClO4)3 (3) and [Co(L)2](ClO4) (4), where L− is the monoanion of the ditopic ligand N′-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide (LH) built on a picolinoyl hydrazone core fragment, and possessing a bidentate and a tridentate coordination pocket. The tetranuclear cation of 1·0.8H2O·MeOH is a strictly planar, rectangular [2 × 2] grid. Two 2.21011 L− ligands bridge adjacent CuII atoms on the short sides of the rectangle through their alkoxide oxygen atoms, and two 2.11111 ligands bridge adjacent CuII atoms on the long sides of the rectangle through their diazine groups; two metal ions are 5-coordinate and two are 6-coordinate. The tetranuclear cation of 2·0.2H2O·3EtOH is a square [2 × 2] grid. The two 6-coordinate NiII atoms of each side of the square are bridged by the alkoxide O atom of one 2.21011 L− ligand. The dinuclear cation of 3·0.8H2O·1.3MeOH contains two low-spin octahedral CoIII ions bridged by three 2.01111 L− ligands forming a pseudo triple helicate. In the mononuclear cation [Co(L)2]+ of complex 4, the low-spin octahedral CoIII center is coordinated by two tridentate chelating, meridional 1.10011 ligands. The crystal structures of the complexes are stabilized by a variety of π–π stacking and/or H-bonding interactions. Compounds 2, 3 and 4 are the first structurally characterized nickel and cobalt complexes of any form (neutral or anionic) of LH. The 2.01111 and 1.10011 coordination modes of L−, observed in the structures of complexes 3 and 4, have been crystallographically established for the first time in coordination complexes containing this anionic ligand. Variable-temperature, solid-state dc magnetic susceptibility and variable-field magnetization studies at 1.8 K were carried out on complexes 1 and 2. Antiferromagnetic metal ion···metal ion exchange interactions are present in both complexes. The study reveals that the cation of 1 can be considered as a practically isolated pair of strongly antiferromagnetically coupled (through the diazine group of L−) dinulear units. The susceptibility data for 2 were fit to a single-J model for an S = 1 cyclic tetramer. The values of the J parameters have been rationalized in terms of known magnetostructural correlations. Spectral data (infrared (IR), ultraviolet/visible (UV/VIS), 1H nuclear magnetic resonance (NMR) for the diamagnetic complexes) are also discussed in the light of the structural features of 1–4 and the coordination modes of the organic and inorganic ligands that are present in the complexes. The combined work demonstrates the ligating flexibility of L−, and its usefulness in the synthesis of complexes with interesting structures and properties.

Magnetic States and Metal—Insulator in Strongly Correlated Systems (Scientific Summary)

Various slave particle approaches to the consideration of electron correlations in many-electron models, in particular, in application to layered copper oxide systems, have been analyzed. The magnetic phase diagrams of the ground state have been considered within the t—t’ Hubbard and s—d exchange models for the square and cubic lattices depending on the degree of band filling and the interaction parameter with allowance for collinear ferromagnetic, antiferromagnetic, and incommensurate (helical) magnetic phases, as well as magnetic phase separation. The generalized Hartree—Fock and slave boson approaches including correlations have been used. The latter approach makes it possible to correctly include the contribution of doubly occupied sites to the energy and to adequately describe the paramagnetic phase. A criterion of a metal—insulator transition has been obtained by numerical calculations and the analytical expansion in the transfer integral between next-nearest neighbors t’ and in the direct antiferromagnetic gap Δ. The order of the transition in the square lattice changes from the second to the first with increasing t’ at a small value of t’ ∼ 0.05t, which is due to the presence of a van Hove singularity near the band center. For the simple and body-centered cubic lattices, the transition from the antiferromagnetic insulator state occurs to an antiferromagnetic metal phase, is a second-order transition, and is followed by a transition to a paramagnetic metal. The inclusion of the intersite Heisenberg interaction changes these results and the transition can become a first-order transition.

Mn2C Monolayer: Hydrogenation/Oxygenation-Induced Strong Ferromagnetism and Potential Applications

Two-dimensional (2D) ferromagnetic materials with strong ferromagnetism and high Curie temperature are significantly desired for the applications of nanoscale devices. Here, based on first-principles computations, we report that hydrogenated/oxygenated Mn2C monolayer is a such 2D material. The bare Mn2C monolayer is an antiferromagnetic metal with the local magnetic moment of Mn ∼3 μB. However, the antiferromagnetic coupling of Mn atoms can transform into the ferromagnetic order under hydrogenation/oxygenation. Especially, the intrinsic magnetic moments in the hydrogenated/oxygenated Mn2C monolayer can be as large as 6 μB per unit cell, and the Curie temperatures are above 290 K. Beside the potential applications in spintronic devices, our work suggests that the Mn2C monolayer is also promising to be used in hydrogen/oxygen detection and removal devices.

Layer-controlled laws of electron transport in two-dimensional ferromagnets

Recent discoveries of intrinsic two-dimensional ferromagnetism in layered compounds drive intense research on their fundamental properties and spintronic applications in atomically thin materials. In these materials, ferromagnetism engenders remarkable transport and optical phenomena and can be controlled electrically. However, coupling of carriers with magnetic moments in lateral charge transport is yet to be explored. Here, we report layer-dependent electron transport in two-dimensional rare-earth ferromagnets MSi 2 , formed by silicene, evolving from an antiferromagnetic metal in the bulk to a ferromagnetic semiconductor in one monolayer. Small integer numbers of monolayers (1, 2, etc. ) result in qualitatively different transport regimes following simple analytical laws. Most remarkably, a gap opening in the monolayer limit gives rise to a colossal negative magnetoresistance, depending exponentially on the magnetic field. The results call for applications in nanoelectronics and stimulate studies of fundamental spin behaviors.

From Fieldlike Torque to Antidamping Torque in Antiferromagnetic Mn2Au

Efficient electrical switching of antiferromagnets (AFMs) is key to their use in high-density, ultrafast, nonvolatile spintronic memory. Mn${}_{2}$Au, an AFM with opposite spin sublattices, is a unique metallic material, in that fieldlike spin torque can switch its AFM moments. However, switching induced by antidamping torque remains to be verified in metallic AFMs. Here the authors demonstrate current-induced switching of AFM moment in both a (103)-oriented Mn${}_{2}$Au single layer and a Mn${}_{2}$Au/Pt heterojunction by fieldlike torque and antidamping torque respectively. The simultaneous realization of both torque types in metallic Mn2Au makes it a promising candidate for AFM spintronics.

Large spin-orbit torque efficiency enhanced by magnetic structure of collinear antiferromagnet IrMn.

Spin-orbit torque (SOT) offers promising approaches to developing energy-efficient memory devices by electric switching of magnetization. Compared to other SOT materials, metallic antiferromagnet (AFM) potentially allows the control of SOT through its magnetic structure. Here, combining the results from neutron diffraction and spin-torque ferromagnetic resonance experiments, we show that the magnetic structure of epitaxially grown L10-IrMn (a collinear AFM) is distinct from the widely presumed bulk one. It consists of twin domains, with the spin axes orienting toward [111] and [-111], respectively. This unconventional magnetic structure is responsible for much larger SOT efficiencies up to 0.60 ± 0.04, compared to 0.083 ± 0.002 for the polycrystalline IrMn. Furthermore, we reveal that this magnetic structure induces a large isotropic bulk contribution and a comparable anisotropic interfacial contribution to the SOT efficiency. Our findings shed light on the critical roles of bulk and interfacial antiferromagnetism to SOT generated by metallic AFM.

Anomalous Hall conductivity of noncollinear magnetic antiperovskites

The anomalous Hall effect (AHE) is a well-known fundamental property of ferromagnetic metals, commonly associated with the presence of a net magnetization. Recently, an AHE has been discovered in non-collinear antiferromagnetic (AFM) metals. Driven by non-vanishing Berry curvature of AFM materials with certain magnetic space group symmetry, anomalous Hall conductivity (AHC) is very sensitive to the specific type of magnetic ordering. Here, we investigate the appearance of AHC in antiperovskite GaNMn$_{3}$ as a representative of broader materials family ANMn$_{3}$ (A is a main group element), where different types of non-collinear magnetic ordering can emerge. Using symmetry analyses and first-principles density-functional theory calculations, we show that with almost identical band structure, the nearly degenerate non-collinear AFM $\Gamma_{5g}$ and $\Gamma_{4g}$ phases of GaNMn$_{3}$ have zero and finite AHC, respectively. In a non-collinear ferrimagnetic $M$-1 phase, GaNMn$_{3}$ exhibits a large AHC due to the presence of a sizable net magnetic moment. In the non-collinear antiperovskite magnets, transitions between different magnetic phases, exhibiting different AHC states, can be produced by doping, strain, or spin transfer torque, which makes these materials promising for novel spintronic applications.

Linear Trimer Formation with Antiferromagnetic Ordering in 1T-CrSe2 Originating from Peierls-like Instabilities and Interlayer Se-Se Interactions.

Anomalous successive structural transitions in layered 1T-CrSe2 with an unusual Cr4+ valency were investigated by synchrotron X-ray diffraction. 1T-CrSe2 exhibits dramatic structural changes in in-plane Cr-Cr and interlayer Se-Se distances, which originate from two interactions: (i) in-plane Cr-Cr interactions derived from Peierls-like trimerization instabilities on the orbitally assisted one-dimensional chains and (ii) interlayer Se-Se interactions through p-p hybridization. As a result, 1T-CrSe2 has the unexpected ground state of an antiferromagnetic metal with multiple Cr linear trimers with three-center-two-electron σ bonds. Interestingly, partial substitution of Se for S atoms in 1T-CrSe2 changes the ground state from an antiferromagnetic metal to an insulator without long-range magnetic ordering, which is due to the weakening of interlayer interactions between anions. The unique low-temperature structures and electronic states of this system are determined by the competition and cooperation of in-plane Cr-Cr and interlayer Se-Se interactions.

Probing the quantum phase transition in Mott insulator BaCoS2 tuned by pressure and Ni substitution

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.

Mott transition in ruby lattice Hubbard model**Project supported by Inner Mongolia Natural Science Foundation, China (Grant No. 06021601).

Mott transition in a ruby lattice with fermions described by the Hubbard model including on-site repulsive interaction is investigated by combining the cellular dynamical mean-field theory and the continuous-time quantum Monte Carlo algorithm. The effect of temperature and on-site repulsive interaction on the metallic–insulating phase transition in ruby lattice with fermions is discussed based on the density of states and double occupancy. In addition, the magnetic property of each phase is discussed by defining certain magnetic order parameters. Our results show that the antiferromagnetic metal is found at the low temperature and weak interaction region and the antiferromagnetic insulating phase is found at the low temperature and strong interaction region. The paramagnetic metal appears in whole on-site repulsive interaction region when the temperature is higher than a certain value and the paramagnetic insulator appears at the middle scale of temperature and on-site repulsive interaction.