Session D50: Ferromagnetic Order and Instabilities

Time-stable remanence in Dzyaloshinskii-Moriya Interaction driven Weak ferromagnets

We observe that a number of Dzyaloshinskii-Moriya Interaction (DMI) driven weak ferromagnets (WFMs) including α-Fe₂O₃, MnCO₃ and NiCO₃ exhibit two distinct time scale in the magnetization relaxation process as explored through SQUID magnetometry. One of the time scales of relaxation is short and, therefore, leads to a quick decay while the other is ultraslow leading to the observation of a time-stable remanence [1]. Furthermore, the time-stable remanence also varies with the strength of the magnetic field in a very unexpected way. We propose that these unique features of remanence explored in systems of different length scales covering bulk single-crystal to nanocrystallites can be taken as the footprint of weak ferromagnetism. We also find that this time-stable remanence which is intrinsic to WFMs can considerably be tunable by nano scaling as explored in the room temperature WFM α-Fe₂O₃ synthesized in various shapes, size, and morphologies. Corroborated by the synchrotron XRD measurements the study has provided us crucial insights to optimize the magnitude of time-stable remanence in α-Fe₂O₃ which has a prominent role in antiferromagnetic spintronics [2].

[1]. N. Pattanayak et al. Phys. Rev. B, 2017, 96, 104422
[2]. N. Pattanayak et al. J. Phys. Condens. Matter, 2019, 31, 365802.   

Ferromagnetic kinetic exchange interaction

Anderson's superexchange mechanism is indispensable for the description and prediction of the magnetic interactions in localized spin systems. Within the framework, weak ferromagnetism originates from Goodenough's mechanism and potential exchange when strong antiferromagnetic kinetic exchange interaction is suppressed. However, Anderson's model and also its modified version, Zaanen-Sawatzky's model, fail to catch an important ferromagnetic contribution. Here, we report ferromagnetic kinetic exchange mechanism arising from overlapping of magnetic orbitals and their strong covalency with the bridging orbitals. Contrary to the conventional weak ferromagnetic exchange mechanisms, the present one can be larger than the antiferromagnetic kinetic exchange interaction, resulting in strong stabilization of ferromagnetic state. Based on first principles calculations, we demonstrate that the present mechanism is responsible for strong ferromagnetism in several existing materials with diamagnetic metal bridges: Fe³⁺-Co³⁺-Fe³⁺ and Cu²⁺-Cr⁶⁺-Cu²⁺ complexes, quasi-one-dimensional Cu chain, La₄Ba₂Cu₂O₁₀, and Co chain, Ca₃Co₂O₆.

  

Small Moment, Itinerant Ferromagnetism Discovered in Single Crystalline La5Co2Ge3

Single crystals of monoclinic La₅Co₂Ge₃ were grown using a self-flux method and were characterized by room-temperature powder X-ray diffraction, anisotropic temperature and field dependent magnetization, temperature dependent resistivity, specific heat, and muon spin resonance. La₅Co₂Ge₃ has a Curie temperature (T_C) of ∼3.8 K, as well as a clear loss of spin disorder scattering in resistivity data and a sharp specific heat anomaly. The magnetism associated with La₅Co₂Ge₃ has μ_eff = 0.92 μ_B per mol-Co, μ_sat = 0.11 μ_B per mol-Co, and a change in the entropy at T_C of ∼0.05 R ln2 per mol-Co making it a rare, itinerant, low T_C compound.   

Avoided ferromagnetic quantum critical point in pressurized La5Co2Ge3

A quantum phase transition is a phase transition that occurs at 0 K. For many second-order phase transitions, application of a magnetic field, doping or pressure can suppress the transition temperature and lead to a quantum critical point (QCP). However, avoided quantum criticality is often observed in intermetallic, ferromagnetic systems. The mechanism behind this phenomenon has been discussed intensively in recent years. Lately, we discovered a new ferromagnetic compound, La₅Co₂Ge₃, which has a Curie temperature T_c ~ 3.8 K. The physical properties under pressure and the pressure dependence of Tc are studied up to ~ 5 GPa. We find that a ferromagnetic QCP is avoided by the appearance of a new, most likely magnetic, phase. Details of the T-p phase diagram and the possible nature of the new phase will be discussed.   

Effect of Fe and Cr doping on electronic and magnetic properties of Inverse Heusler alloys: Mn2CoCrxGa(1-x) and Mn2CoFexGa (1-x)

Mn and Co based Heusler alloys found to exhibit interesting electronic and magnetic properties. We have investigated the effect of electronic and magnetic properties on Mn₂CoGa after doping Fe and Cr. Entire calculations were performed through plane wave pseudo-potential method using quantum espresso.

From the calculation it is observed that the band gap of Mn₂CoGa shifted (increases) on 25% doping of Fe (or Cr) to its pristine form due to the effect of which shifting the band gap on the right side and shows perfect half-metallicity character. The band gap is found to be 0.307eV eV and 0.352eV for Fe and Cr doping respectively which is also greater than the minority band gap in the pristine (0.113 eV). The decreasing rate of bandgap is more in case of Fe doping than Cr doping. This maybe due to exchange splitting and coulomb repulsion competition. At this case system follows the Slater Pauling’s rule and hence gives rise to integer magnetic moments. If percentage of doping increases non-integer value of total spin magnetic moment is observed indicating that half-metallicity were destroyed. Doping in higher degree drastically increases the Magnetic moments.   

Small angle neutron scattering study of magnetic length scales in disordered ferromagnetic Ni-V alloys.

We used small angle neutron scattering (SANS) to reveal magnetic microstructures in a disordered ferromagnet (FM) close to quantum critical point (QCP) where long range order is suppressed with critical temperatures Tc approaching zero. First results of the alloy Ni-V with low Tc are presented on well characterized polycrystalline samples that demonstrated the quantum Griffiths phase signatures for “random” magnetic inhomogeneities [1]. SANS data were collected at NG7SANS, NIST. Full polarization analysis was effective to resolve weak magnetic scattering from dominating nuclear scattering. Angular dependent spin-flip scattering identified magnetic short-range clusters. Angular dependent non-spin-flip interference term recognized long-range magnetic domain contributions. The disordered FM Ni-V presents short-range correlations coexisting with long-range correlations at low temperatures well below Tc in the quantum Griffiths regime close to the QCP.
[1] R. Wang et al, Phys. Rev. Lett. 118, 267202 (2017)   

Unveiling electronic correlation and ferromagnetic superexchange mechanism in van der Waals crystal CrSiTe3

The recent discovery of intrinsic ferromagnetic order in atomically thin van der Waals crystal CrXTe₃ (X=Si,Ge) stimulates intensive studies on the nature of low-dimensional magnetism. By combining advanced many-body calculations with angle-resolved photoemission spectroscopy we investigate CrSiTe₃ single crystals and unveil the pivotal role played by the strong electronic correlations at both high- and low-temperature regime. Above the Curie temperature (Tc), Coulomb repulsion (U) drives the system into a charge transfer insulating phase. In contrast, below Tc the crystal field arranges the Cr-3d orbitals such that the ferromagnetic super-exchange profits, giving rise to the bulk ferromagnetic ground state with which the electronic correlations compete. The excellent agreement between theory and experiment establishes CrSiTe₃ as a prototype low dimensional crystal with the cooperation and interplay of electronic correlation and ferromagnetism. Ref. [1] J. X. Zhang et al., Phys. Rev. Lett. 123, 047203 (2019)   

Existence of ferromagnetism in a two-dimensional two species bosonic Hubbard model with the presence of confinement

With recent experimental advances on cold atoms in atomic traps, the realization of exotic magnetic phases in the presence of confinement is an area of renewed interest. We investigate a two-dimensional bosonic Hubbard
model with two species in the presence of a harmonic trapping potential. In particular, we focus on the appearance of a novel high entropy ferromagnetic phase at finite doping and polarization. We find that in the presence of confinement, this ferromagnetic phase appears in both hard-core and soft-core bosonic models. In the hard-core limit, we observe phase separation where the heavy species is localized in the center
of the trap, and it is surrounded by the light species. In the soft-core limit, we observe that the light species gets trapped in the center of the lattice, and it is surrounded by the heavy species. We link our results with experimental observables relevant for magnetic phases of trapped cold atoms.   

Z3 ferromagnet to Valence Bond Solid transition in 1d

We continue recent efforts to discover examples of deconfined quantum criticality in one-dimensional models. In this work we focus on the transition between Z3 ferromagnet and Valence Bond Solid (VBS) in a 1d chain with a Z3xZ3 global symmetry. We use a model with alternating projective representations on the sites of the two sublattices. This allows the model to connect to an exactly solvable point which has VBS character as SU(3)-invariant singlets. We find a direct transition from this phase to a ferromagnetically ordered phase breaking Z3 symmetry. Numerical evidence suggests a second-order or extremely weak first-order transition. We characterize the apparent criticality using entanglement scaling and the critical scaling of lattice operators.   

Physical properties of the van der Waals material Fe5GeTe2

van der Waals bonded materials that possess magnetic order at high temperatures are of great interest for integrating magnetic effects into engineered heterostructures. Of the cleavable materials studied so far, Fe_5-xGeTe₂ possesses the highest reported Curie temperature T_C in the bulk and maintains a high T_C in exfoliated flakes. This presentation will discuss the physical properties of Fe_5-xGeTe₂ single crystals, with an emphasis on the metastable nature of the compound and how magnetism on one Fe sublattice dominates the transport properties. The ability to tune the magnetism through cobalt doping will also be briefly discussed.
  

The effect of Ge substitution on optical properties of FeGa3 single crystals

Studied initially for its large thermopower effect, the intermetallic semiconductor FeGa₃ has recently been shown to provide a suitable platform for studying electron correlations, such as metal-insulator transition, non Fermi liquid behavior and ferromagnetism. In particular, with Ge-doping at Ga site the ground state is tuned from a Kondo insulator to a paramagnetic and further to a ferromagnetic metal. Here we present optical reflectance measurements on single crystals of FeGa_3−xGe_x, for different values of x. The temperature was varied between 300 K and 5 K, and the frequency ranged from 80 cm⁻¹ to 50 000 cm⁻¹. From Kramers-Kronig transformation we obtained various optical functions and dis- cuss here the effect of Ge substitution on optical conductivity across all three proposed ground states.   

Intrinsically weak magnetic anisotropy of cerium in potential hard-magnetic intermetallics

We study the magnetic properties of the "1-12" family of prospective hard-magnetic Ce-Fe intermetallics using an ab initio dynamical mean-field theory approach to treat the complex many-electron physics of the Ce 4f¹ shell. By considering the hypothetical binary compound CeFe₁₂, as well as the realistic compositions CeFe₁₁Ti and CeFe₁₁TiN, we show that the localization of the Ce 4f electron is strongly sensitive to alloying and interstitials. The onset of heavy-fermion behavior accompanied by a significant reduction of the Ce-4f moment—and concomitantly its contribution to the magnetic anisotropy—is found to occur in CeFe₁₂ for a wide temperature range up to about 400K. This 4f delocalization is moderately reduced by Ti substitution on the Fe site and, unexpectedly, markedly increased by nitrogen interstitials. However, even for localized Ce-4f electrons their contribution to the magnetic anisotropy is rapidly suppressed with increasing temperature.
  

Thickness-dependent magnetic order in CrI3 single crystals

Two-dimensional (2D) materials with intrinsic ferromagnetism provide unique opportunity to engineer new functionalities in nano-spintronics. One such material is CrI₃, showing long-range magnetic order in monolayer with the Curie temperature (T_c) of 45 K. Here we study detailed evolution of magnetic transition and magnetic critical properties in response to systematic reduction in crystal thickness down to 50 nm. Bulk T_c of 61 K is gradually suppressed to 57 K, however, the satellite transition at T^* = 45 K is observed layer-independent at fixed magnetic field of 1 kOe. The reduction of thickness facilitates a field-driven metamagnetic transition around 20 kOe with out-of-plane field, in contrast to the continuous changes with in-plane field. The critical analysis around T_c elucidates a three-dimensional (3D) long-range magnetic coupling in bulk CrI₃, then mean-field type interactions in microscale-thick flake, and evolves into 2D Ising-like ferromagnetism in monolayer.
  

Magnetic-Competition-Induced Colossal Magnetoresistance in n-Type HgCr2Se4 under High Pressure

HgCr₂Se₄ is a well-known ferromagnetic(FM) semiconductor with T_C = 106 ~ 120 K¹ and it has received renewed interest recently. Here, we performed the first comprehensive high-pressure study on n-type HgCr₂Se₄ single crystals and surprisingly found that the FM metallic ground state is destabilized and gradually replaced by an antiferromagnetic (AF) insulating ground state under high pressure. Our combined magnetic susceptibility and neutron measurements under high pressure indicated that the AF order is most likely a spiral-type. On the other hand, the application of external magnetic fields can restore the FM metallic state again at high pressures, resulting in a colossal magnetoresistance as high as ~ 3 ^ 10¹¹ % under 5 T and 2 K at 4 GPa. Our results demonstrate that HgCr₂Se₄ situates at a critical point where the competition between FM and AF exchange interactions can be easily tuned by pressure and magnetic field. Thus, our work provides a means for realizing novel state where the extremely large magnetoresistance can be obtained via switching between two distinct electronic ground states in a single-valent system².
1. P. K. Baltzer et al., Phys. Rev. Lett. 1965, 15, 493
2. J. P. Sun et al., Phys. Rev. Lett. 2019, 123, 047201   

Electronic band structures of FeGe driven by thermal-fluctuation

Fluctuation-induced first-order transitions have been discussed in superconductors, liquid crystals, and magnetic materials. Recently, chiral magnet, such as FeGe, were found thermal-fluctuation first-order transition above T_C, which forms fluctuation disordered state (FD) of magnetic structure. Here, we unveil the Fermi surface symmetry and the evolution of electronic structures across the transition temperature of helimagnetic-paramagnetic phase at T_C~274K by means of angle-resolved photoemission spectroscopy (ARPES) on high-quality FeGe(111) films epitaxially grown on Si(111) substrate. The evolution of electronic structures with temperature suggest that FeGe is a Stoner itinerant magnetism. In contrast to ordinary ferromagnetism, a small kink in the integral energy distribution curve (IEDC) at binding energy of 0.5eV in the regime of 274K≤T_C≤276K can be identified. This temperature range is the same with the range of FD measured by AC magnetic susceptibility. Therefore, the small kinks on IEDCs imply the contribution of Dzyaloshinskii-Moriya interaction (DMI) on electronic band structure. Our finding paves the path for the theory of the relationship between freedom of spin and electron in itinerant DM helimagnet.