Session N53: Magnetism in Correlated Electron Systems I: Materials & Neutrons

Strongly correlated ferromagnetism in NiTa4Se8

In this talk, I report on the growth and characterization of a novel weakly itinerant ferromagnet NiTa₄Se₈. Magnetometry measurements reveal the existence of complex magnetic order, potentially with multiple competing order parameters. Our transport and thermodynamic measurements, in combination with density functional theory (DFT) calculations, provide evidence for strong electronic correlations arising from a combination of spin fluctuations and flat-bands near the Fermi level. This material may serve as a useful platform for the study of weak itinerant magnetism, and the interplay of strong electronic correlations with complex magnetism more generally.   

Physical properties of the vanadium-based kagome metals YV6Sn6 and GdV6Sn6

The vanadium-based kagome metals YV6Sn6 and GdV6Sn6 are synthesized using a flux-based crystal growth technique. X-ray diffraction (XRD), magnetization, magnetotransport, and heat capacity measurements are carried out to study the physical and magnetic properties of these compounds. XRD data reveal a network of V-ions coordinated by Sn that are separated by triangular lattice planes of rare-earth ions. The onset of low-temperature, likely noncollinear, magnetic order of Gd spins is detected in GdV6Sn6, while V-ions in both compounds remain nonmagnetic. They exhibit high mobility, multiband transport and present an interesting platform for controlling the interplay between magnetic order associated with the R-site sublattice and nontrivial band topology associated with the V-based kagome network. Our results invite future exploration of other RV6Sn6 (R=rare earth) variants where this interplay can be tuned via R-site substitution.   

Magnetic properties of NdCuGa3 single crystals

The study of magnetic order in the f-electron system has received significant attention due to the finding of exotic magnetic phases of matter. The detailed low-temperature physical and magnetic characterization is critical for revealing an exotic magnetic ground state. This presentation will discuss the low-temperature magnetic properties of NdCuGa3. NdCuGa3 crystallizes in a tetragonal crystal structure with C_4v symmetry. It exhibits an antiferromagnetic phase below TN = 3.5 K, and no additional phase transition was detected down to 50 mK zero-field specific heat measurements. The nature of the antiferromagnetic phase is expected to have incommensurate magnetic ordering. The presentation includes the detailed structure, magnetic, and physical properties of NdCuGa₃.   

Effects of transition metal (TM) substitution on the collapsed tetragonal transition in SrTM2P2

The dramatic reduction of the c-lattice parameter caused by a collapsed tetragonal transition gives rise to mechanical properties of technological interest, such as shape memory and a remarkable superelasticity [1]. This transition occurs in a wide variety of pnictide intermetallic compounds due to bond formation between pnictogens across ab planes. It is frequently found in P-T phase diagrams in proximity to different magnetic and/or superconducting transitions [2], and thus could potentially serve as a compass to find novel electronic and magnetic ground states. Motivated by this, we explore the case of SrNi₂P₂ which, upon cooling at ambient pressure, undergoes a transition into a partially collapsed structure, where only a third of the P ions bond across the Sr layers. We report on solution growth, transport and powder x-ray diffraction measurements to assess the effects that Co and Rh substitution have on this transition.

[1] Sypek et al., Nat. Commun. 8, 1083 (2017).

[2] Torikachvili et al., Phys. Rev. Lett., 101, 057006 (2008).   

Magnetic ordering in Co doped SrNi2P2.

SrNi₂P₂ is an intriguing system as it exhibits fully gapped superconductivity and adopts a unique third-collapsed tetragonal (tcT) phase[1,2]. It transitions from uncollapsed tetragonal (ucT) to tcT phase at T_s =< 326 K, where only one-third of P atoms bond across the Sr layer. Substituting Ni with Co completely suppresses T_s, drives sytem to ucT phase, and induces various magnetic ground states. The magnetic phase diagram of Sr(Ni_1-xCo_x)₂P₂ obtained from the magnetization measurements shows that even though both stoichiometric SrNi₂P₂ and SrCo₂P₂ are paramagnetic, Sr(Ni_1-xCo_x)₂P₂ are antiferromangetic for 0.58<≈x<≈0.94 and ferromagnetic for 0.95<≈x<≈0.98. Single-crystal neutron diffraction measurements performed on x = 0.88 sample confirms the long-range antiferromagnetic ordering with helical arrangement of spins along c-direction. Also, feature corresponding to ferromagnetic ordering was observed for x = 0.96. [1] F. Ronning et al PRB 79,134507(2009). [2] S. Xiao et al Nano Lett. 21,7913(2021).   

Large Phonon Thermal Hall Effect in the Antiferromagnetic Insulator Cu3TeO6

        Phonons are known to generate a thermal Hall effect in various insulators, including multiferroic materials [1] and cuprate Mott insulators [2,3,4], but the underlying mechanism is still unclear. Theoretical proposals for the phonon thermal Hall effect fall into two categories: intrinsic scenarios based on the coupling of phonons to their environment (e.g. [5]) and extrinsic scenarios based on the skew scattering of phonons by impurities or defects (e.g. [6]). 

        To shed new light on this question, we have studied an entirely different class of materials, the antiferromagnetic insulator Cu₃TeO₆. In this simple cubic material, we find the largest thermal Hall conductivity κ_xy of any insulator so far. The fact that κ_xy is ~ 50 times larger than in the cuprate Sr₂CuO₂Cl₂, for example, is strong evidence that the phonons are responsible for the Hall signal, since the phonon-dominated longitudinal thermal conductivity, κ_xx, is also ~ 50 times larger. So κ_xy seems to grow in proportion with the conductivity of phonons.

        We discuss the implications of the finding that the ratio κ_xy / κ_xx is roughly the same in these two very different materials, as well as in other insulating materials.

[1] Ideue et al., Nature Materials 16, 797 (2017). 

[2] Grissonnanche et al., Nature 571, 376 (2019). 

[3] Grissonnanche et al., Nature Physics 16, 1108 (2020).

[4] Boulanger et al., Nature Communications 11, 5325 (2020).

[5] Ye, Savary & Balents, arXiv:2103.04223 (2021).

[6] Sun, Chen & Kivelson, arXiv:2109.12117 (2021).   

Mixed magnetic phases in AlxCoCrNiFe high entropy alloys

The large distribution in atomic sizes and masses in high-entropy alloys results in extreme local environments, which manifests strongly in the thermal and magnetic properties. In this work, high-entropy alloys of (Fe,Co,Cr,Ni)Al_x, 0 < x < 2, are prepared and their temperature-dependent magnetic and electronic properties determined. Magnetometry results show that all the samples are ferromagnetic, with a high-temperature phase, T_C >200 K, and a second low-temperature phase with T_C≈20 K. However, the high-temperature phase is not associated with an open hysteresis loop, suggesting superparamagnetic behavior. The closed hysteresis loop suggests the ferromagnetism appears as small clusters, a theory explored with small-angle neutron scattering (SANS). The samples also show downturn in the magnetization at T<10 K which can be associated with an antiferromagnetic phase. Recent works have suggested that extreme strain distributions can induce antiferromagnetic ordering. This research used resources at the High Flux Isotope Reactor and the Spallation Neutron Source, as appropriate, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory, and was funded by the DOE DE-SC0021344.   

Long-range magnetic order in the magnetodielectric regime of Ce2O3

The metastable sesquioxide, Ce₂O₃, has been a material of intense interest in recent years due to reports of an anomalous giant magnetodielectric effect and the emergence of vibronic modes below a putative antiferromagnetic transition at T_N = 6.2K. The claim of long-range magnetic order in this material is based on heat capacity and temperature-dependent susceptibility measurements. Curiously, three previous neutron diffraction studies were unable to distinguish any magnetic Bragg peaks, casting doubt on the nature of the low temperature state. To address this point, we undertook a comprehensive study of polycrystalline Ce₂O₃ using neutron diffraction, triple-axis and time-of-flight inelastic neutron scattering, and muon spin rotation. I will report evidence of magnetic order in Ce₂O₃ and discuss these results in light of our current understanding of the underlying physics.   

Neutron scattering investigation of the spin density wave ground state in CeAuSb2

CeAuSb₂ is a tetragonal heavy-fermion compound (s.g. P4/nmm) displaying exotic spin density wave (SDW) order below its Néel temperature T_N = 6 K. Magnetic field suppresses T_N toward absolute zero, but instead of reaching a quantum critical point, its SDW ground state evolves into a unique double-q magnetic order above B_c1 = 2.8 T. The magnetism may be coupled to nematic charge order induced by a structural transition just above T_N, and the coupling remains intact into the multi-q state [1]. Furthermore, earlier neutron diffraction [2] revealed a second harmonic Bragg peak below B_c1 suggesting the SDW is not a simple sinusoid. Even-order Bragg peaks have been associated with a SDW-induced lattice distortion, which can further be accompanied by a charge density wave. Its connection, if any, to a nematic state is an intriguing possibility. In this talk we will present recent zero-field magnetic neutron scattering measurements on a single crystal of CeAuSb₂. To address possible coupling of magnetic and charge orders, we performed polarized neutron diffraction in the SDW ground state. Additionally, we will present our results on magnetic critical scattering across the phase transition.

1. S. Seo et al, Phys Rev X 10, 011035 (2020)

2. G. Marcus et al, Phys Rev Lett 120, 097201 (2018)   

Complex ordering of Spin Density Waves in Ruddlesden-Popper oxides

Pr4Ni3O10 is a metallic, trilayer, Ruddlesden-Popper oxide that possesses an anomalous metal-metal transition (MMT) at a temperature of ~ 158 K. Below the MMT, Pr4Ni3O10 develops a three-dimensionally correlated charge density wave similar to its cousin La4Ni3O10, along with a spin-density wave (SDW) that is weakly correlated along the c-axis. In addition, diffusive rods along [0,0,l] are observed in the magnetic signal from Neutron diffraction. We could successfully reproduce the diffuse scattering patterns by a numerical model of short-range SDW stacking. However, unlike La4Ni3O10, as the temperature is further lowered below ~ 30 K, the local magnetic fields created by the spin density wave induce magnetic moments on the Pr3+ cations. Exchange interactions couple the Pr and Ni moments which drives the spin density wave to cross over into a three-dimensionally correlated magnetic order that is commensurate along the c-axis but incommensurate and locked to the charge density wave-vector in the basal plane.   

A study on additional magnetic modulation vectors observed in the element neodymium

The element neodymium hosts a complex but fascinating cascade of magnetic transitions. Here we present the results of small angle neutron scattering investigations of magnetic phases of neodymium.  Our measurements demonstrate the appearance of an additional set of magnetic modulation peaks for 5.9(1) ≤ T ≤ 7.6(1) K and µ₀H ≤ 1.0(1) T for fields applied along the c-axis.  This set of magnetic peaks is described by a modulation vector of 0.06 a* and is smaller than previously reported observations from the studies performed in higher Brillouin zones. The SANS measurements facilitate a comparison between modulation vectors originating from each of the two distinct sites, typically termed cubic and hexagonal sites, in the double hexagonal closed packed structure of Nd.  From these observations we conclude that additional modulation vectors originate primarily from order on the cubic sites.      

Collective excitation continuum in the antiferromagnetic heavy-fermion system U2Zn17

We report the observation of a continuum of collective excitation in heavy-fermion antiferromagnet U₂Zn₁₇ from inelastic neutron scattering experiments, with no signatures of spin waves. The 13 Å near isotropic correlation length of these fluctuations implies they account for just 1% of the low-T Sommerfeld constant γ₀ = 200mJ/mol/ K² associated with heavy fermions. The magnetic excitations persist in the paramagnetic state up to 2T_N and the spectrum extends beyond an energy scale of 10T_N. These collective quantum fluctuations both within and beyond the ordered state of U2Zn17 are indicative of a strongly frustrated spin system ostensibly mediated by heavy fermions.   

Multi-spin excitations in an antiferromagnetic S=5/2 system

Hematite (a-Fe₂O₃), an S=5/2 antiferromagnetic system, is a promising candidate in spintronic and magnonics due to its functional spin-related properties [1, 2], such as a weak in-plane magnetic anisotropy and a low spin dissipation.

A key element for the microscopic understanding of spin-wave-based transport phenomena in insulators is the knowledge of the intrinsic spin dynamics of the system, involved in both the transport mechanism as well as in the dissipation processes. The spin dynamics of bulk hematite has been studied in the early days by inelastic neutron scattering (INS) [3], revealing the presence of an acoustic and an optical single-magnon branch up to 100 meV. The evolution of the spin dynamics in thin films - as the ones used for devices - remains still unknown due to the lack of suitable probes.

Recently, resonant inelastic x-ray scattering (RIXS) has emerged as a highly performing technique for the study of the spin dynamics in magnetic thin films [4]. Additionally, it has shown great sensitivity in unraveling the higher ranked spin excitations such as ∆s=0 and 2 [5,6], elusive to conventional techniques like INS. Here, we extend this approach to hematite thin films by using Fe L₃-edge RIXS. Below 100 meV, we resolve two modes fully consistent with the ∆s=1 magnon branches detected by INS in the bulk whereas at higher energy (up to 200 meV), multi-spin excitation modes are identified. With the support of LDA+DMFT calculations built around the Anderson impurity model [7], we interpret these to be ∆s=2 and ∆s=3 spin excitations at single magnetic site, respectively. These observations shed new light on the high-energy eigenstates of the hematite magnetic Hamiltonian, providing novel information on the spin dynamics and suggesting new pathways for the transport dissipation processes.   

Site dependent localized-delocalized magnetic moment in the magnetization-reversal in an inverse-spinel vanadate

Neutron diffraction, magnetization, and muon spin relaxation measurements, complemented by density functional theory (DFT) calculations are employed to unravel the various magnetic phases of the inverse spinel Co₂VO₄.  All measurements show a second-order magnetic phase transition at T_C = 168 K to a  colinear ferrimagnetic phase. DFT and the experimental results indicate the moments in the ferrimagnetic phase are delocalized and undergo gradual localization as the temperature is lowered below T_C . The delocalized-localized crossover gives rise to a maximum magnetization at T_NC = 138 K with a continuous decrease in the magnetization and sign-change at  T_MR= 65 K. The magnetization reversal determined at zero fields is found to be highly sensitive to an applied magnetic field, such that above B= 0.03 T, instead of a reversal, a minimum in the magnetization is apparent at T_MR. Analysis of the neutron diffraction measurements shows that the magnetization reversal is due to a gradual change in the-nearly ferromagnetic A and B sub-lattices that grow differently as the temperatures are lowered below T_C.   Muon measurements are consistent with the picture that the moments gradually become more localized as the temperature is lowered and at that at zero applied magnetic fields give rise to magnetization reversal or a minimum (at applied magnetic fields)  at T_MR.