Session W52: Cooperative Magnetic Phenomena: Spin-Lattice Effects and Dynamics

Magnetic properties near the quantum critical point in the pyrochlore iridates (Eu1-xLax)2Ir2O7

Pyrochlore iridates, R₂Ir₂O₇ (R = rare-earth and Y), have been extensively studied over the past decade. A wealth of emergent properties including metal insulator transitions (MITs) and non-trivial topological phases arise due to the combination of spin orbit coupling, geometric frustration and strong electron correlation. The ground state is highly sensitive to the ionic size of R, and evolves from an insulating All-In All-Out (AIAO) antiferromagnetic (AFM) ground state for smaller rare-earth ions (R = Gd, Tb, Dy, Ho, Er, Yb, Y), to a non-magnetic metallic state for large R = Pr. The intermediate members with R = Nd, Sm and Eu, however, show a sharp MIT concurrent with AIAO type AFM long-range ordering. This quantum critical point is attributed to the change of the iridium electronic structure with chemical pressure, although direct experimental evidence remains elusive. Previous divalent (Ca) and trivalent (Bi) cation doping studies create either Ir⁴⁺-Ir⁵⁺ charge disproportionation or large Bi 6p-Ir 5d hybridization. Here we investigate this quantum critical point between R = Nd and Pr with a new doping strategy, replacing Eu³⁺ with larger La³⁺ in Eu₂Ir₂O₇, to produce (Eu_1-xLa_x)₂Ir₂O₇ with an expanded lattice. On doping to x = 0.5, a large lattice parameter is achieved that is equivalent to the critical point. We can then systematically study the magnetism which arises solely from the Ir-sublattice and see how it evolves with increasing chemical pressure on. Magnetic susceptibility studies show a field cooled, zero field cooled splitting at T_c, and the systematics of these dependencies in the vicinity of the quantum critical point will be presented and discussed.   

Piezomagnetism in uranium dioxide probed by x-ray diffraction in pulsed magnetic fields

5f-electron spin systems exhibit strong spin-lattice coupling and electronic correlations and are predicted to host new emergent phenomena. One recent example is the observation of piezomagnetism and magneto-elastic memory effect in the antiferromagnetic Mott-Hubbard insulator, UO₂ [1]. Here, we give a short overview of piezomagnetic behavior in this material with a focus on X-ray diffraction studies of oriented UO₂ crystals under strong pulsed magnetic fields. We show how the high-resolution single-crystal diffraction allows us to study details of subtle unit-cell distortions below and above the structural and magnetic phase transition in this material. We show that direct microstructural observations of the piezomagnetic and switching effects are being a direct consequence of the non-collinear 3k magnetic order that breaks time-reversal symmetry in a non-trivial way. We also observe the presence of magnetic domains with distinct magnetic-field evolution in the magnetically ordered state of UO₂, both when the field is applied repeatedly in a single direction and when the field direction is alternated. 

[1] Jaime, M. et al. Piezomagnetism and magnetoelastic memory in uranium dioxide. Nat. Commun. 8, 99 (2017).   

Probing antiferromagnetic domains and textures at criticality using single-shot Resonant Coherent Soft X-ray Scattering

Strongly correlated oxides host a rich phase diagram due to the strong interplay between structural, electronic and spin degrees of freedom. For example, the perovskite nickelate compound PrNiO3 (PNO) exhibits a first-order phase transition from paramagnetic metal to bond-ordered antiferromagnetic insulator, in which structural, charge and spin degrees of freedom are all coupled. Here we investigate a thin film of PNO grown on a substrate of tensile-strain inducing (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT), and use coherent resonant soft x-ray scattering at the Ni L3 edge to study the emergence of the magnetic order parameter in the critical regime of the phase transition. We observe a small-q periodic modulation on the magnetic bragg peak which we argue to be the ‘fingerprint’ of a particular domain configuration. By analyzing the symmetry of these small-q modulations it was possible to directly guess the corresponding real-space domain arrangement and reproduce the observed diffraction patterns. This process is facilitated by the 2-dimensional nature of the domain arrangement, and represents a powerful new approach to single-shot imaging of 2-dimensional mesoscopic antiferromagnetic spin textures.   

Structural distortions and magnetism in transition-metal cluster compound GaNb4Se8

Lacunar spinels AB₄Q₈ (A=Al, Ga, Ge; B=V, Nb, Mo, Ta; Q=S, Se) display a variety of novel phenomena including skyrmion lattices and pressure-induced insulator-metal transitions and superconductivity. The high temperature structure features unique tetrahedral transition-metal clusters with S=1/2 moments on an FCC lattice. Low temperature distortions of these clusters are crucial to the properties of lacunar spinels. In GaNb₄Se₈, antiferromagnetic interactions lead to geometric frustration and a magnetic transition associated with a tetragonal distortion, proposed to be a singlet state at low temperatures. We present results of structural characterization on lacunar spinel GaNb₄Se₈ under temperature and pressure in connection to its magnetic properties.   

X-ray Photon Fluctuation Spectroscopy on amorphous FeGe near magnetic phase transition at LCLS-II.

Non-trivial magnetic spin textures, e.g. Skyrmions, have attracted attention lately due to their potential application in data storage and unique topological protection. Despite this, much is still unknown about the stabilization of these textures and how the symmetric (Hesienberg) and antisymmetric (Dzyaloshinskii-Moriya interaction) exchange couplings compete. In this talk, I present results from a recent experiment at LCLS-II where we probed sub-nanosecond time scales near the magnetic phase transition of amorphous FeGe. We investigated the transition between a helical and paramagnetic state, mediated by temperature. This work not only provides further insight into the competition between symmetric and antisymmetric exchange in chiral materials but also provides a blueprint for future sub-nanosecond experiments at X-ray Free Electron Lasers.   

Well-defined paramagnons in YCo2

Cubic YCo₂ is an exchanged-enhanced metallic paramagnet on the verge of ferromagnetic order. In such itinerant magnets, competition between magnetic states can be seen in the magnetic excitation spectrum.  Here we present results from inelastic neutron scattering, diffraction, and thermodynamic measurements on YCo₂.  We find steep well-defined over-damped magnetic excitations centered at the Γ point of the Brillouin zone which extend past 100 meV.  The excitations weaken with increasing temperature but exist up to at least 300 K. We will discuss the excitations in terms of the compound's itinerant ferromagnetism and proximity to ferromagnetic order.   

Scalar susceptibility of a diluted classical XY model

The Higgs (amplitude) mode at the disordered superfluid-Mott glass quantum phase transition was recently shown to feature unusual localization properties that violate naive scaling [1,2,3]. To test whether analogous behavior also occurs in the classical case, we analyze the Higgs mode in a diluted 3D classical XY-model near the magnetic phase transition. We calculate the amplitude correlation function and the corresponding scalar susceptibility by means of Monte Carlo simulations. In contrast to the quantum case, we find that the scalar susceptibility fulfills naive scaling (employing the clean critical exponents, as expected from the Harris criterion) as the temperature is varied across the phase transition for several dilutions.

[1] M. Puschmann, J. Crewse, J. A. Hoyos, and T. Vojta, Phys. Rev. Lett. 125, 027002 (2020).

[2] J. Crewse and T. Vojta, Phys. Rev. B 104, 014511 (2021).

[3] M. Puschmann, J.C. Getelina, J.A. Hoyos, and T. Vojta, Ann. Phys. in press , arXiv:2101.11065   

Local structure of giant magnetocaloric system (Mn,Fe)2(P,Si)

(Mn,Fe)₂(P,Si) is a promising system for magnetocaloric applications, with a tunable magnetostructural transition that ranges from about 320 K to 200 K, depending on the precise composition. As with other giant magnetocaloric systems with first-order magnetostructural transitions, the local interactions between neighboring atoms and magnetic moments are of crucial importance for the behavior of the system. To probe the local structure of (Mn,Fe)2(P,Si), we have conducted pair distribution function (PDF) analysis of x-ray and neutron total scattering data collected from powder samples with a range of Mn:Fe ratios, including measurements with an in situ magnetic field. A significant rearrangement of the local atomic structure is observed as a function of temperature and applied magnetic field. We present an analysis of the local atomic structure of these compounds and discuss the relevance to the observed magnetocaloric properties.   

Asymmetry of Critical Exponents of O(N) Model due to Spontaneous Symmetry Breaking

It has widely been believed that the critical exponents are equivalent above and below the second-order phase transition point. On the contrary, we show that asymmetry develops in the critical exponents of the O(N) symmetric model due to spontaneous symmetry breaking. Whereas only the four-point vertices are relevant in the symmetric phase, three-point vertices also become relevant in the ordered phase. One of the authors recently formulated a functional renormalization group that can reduce the number of the renormalization parameters to analyze the ordered phase[1]. Using this method to the O(N) symmetric model, we show numerically that the three-point vertices are finite at the fixed point in the ordered phase. Around the fixed point, we also find that the critical exponent of the correlation length has a value different from that of the isotropic fixed point in the symmetric phase. Our mechanism for the emergence of asymmetry in critical exponents is different from the one proposed recently[2].

[1]T. Kita, J. Phys. Soc. Jpn. 88, 054003 (2019).

[2]F. Léonard and B. Delamotte, Phys. Rev. Lett. 115, 200601 (2015).   

The extraordinary boundary transition in the 3d O(N) model via conformal bootstrap

We study the critical behavior of the 3d classical O(N) model with a boundary. Recently, it was established that upon treating N as a continuous variable, there exists a critical value N_c > 2 such that for 2 ≤ N < N_c the model exhibits a new extraordinary-log boundary universality class, if the symmetry preserving interactions on the boundary are enhanced. N_c is determined by a ratio of universal amplitudes in the normal universality class, where instead a symmetry breaking field is applied on the boundary. We investigate the normal universality class using the numerical conformal bootstrap. We found truncated solutions to the crossing equation that indicate N_c ≈ 5. Additionally, we used semi-definite programming to place rigorous bounds on the boundary CFT data of interest to conclude that N_c > 3, under a certain positivity assumption which we checked in various perturbative limits.    

Search for a spin glass phase of the edge-cubic spin model on 3D lattice

Discreteness is important for the existence of finite temperature 

phase transition, not only for regular magnetic system but also 

for random system called spin glass (SG). This is exemplified by 

the presence of three dimensional (3D) SG phase for the Ising model 

and not for the Heisenberg model.  The SG  phase  is characterized by

the low-temperature frozen random spin orientation; considered

as a temporally ordered phase rather than spatially ordered. The present 

study probes one of discrete counterparts of the Heisenberg model, i.e.,

the edge-cubic  model where spins are allowed to have 12 possible

orientations, namely to the middle point of the edges of the cubic.

We search for the existence of SG phase on the square and cubic lattices 

with random mixture of ferromagnetic and anti-ferromagnetic interactions.

In previous study, the 2D ferromagnetic version of this model was reported to indicate two consecutive phase transitions, respectively associated 

with the broken of $O_h$ and $C_{3h}$ symmetries.  In searching for the spin glass phase, we use Replica Exchange algorithm of Monte Carlo method. We estimate the critical temperature and exponents of the existing SG phase transition.   

Exploring potential energy surface of magnetic materials with non-collinear self-consistent constrain method and deep learning model

In magnetic materials, the energy surface is a function of coordinates R and the magnetization vectors M of each atom. Therefore, exploring the energy surface concerning R and M using ab-initial calculation methods has been a meaningful way to search magnetic candidate materials. However, this task is hindered by the difficulty of precisely anchoring the magnetization among the parameter space of arbitrarily varying direction or magnitude.  In this talk, we would like to present our newly developed non-collinear self-consistent magnetization constrain method and deep learning-based predicting model that explore the full M and R space, with the tolerance of magnetization up to 1e-6 Bohr magneton. Moreover, using our deep learning model, the energy, atomic force and magnetic torque (or effective field) can be predicted with quantum accuracy with respective to variables R and M. We are confident that our method can provide the possibility for applications, for example the exploring the magenetic phase space and analyzing spin interaction hamiltonian, etc.