Session M51: Impurities, Disorder & Spin Textures

Electronic structure of the frustrated diamond lattice magnet NiRh2O4

NiRh₂O₄ is the first known realization of a S=1 diamond lattice magnet and is predicted to host various exotic phenomena such as topological paramagnetism, spiral spin liquid, and excitonic magnetism, caused by frustrated nearest and next-nearest neighbor exchange, as well as orbital degeneracy. Thermodynamic measurements found no sign of magnetic order and inelastic neutron scattering found excitations suggestive of a valence bond solid. Theoretical works using both ab-initio and effective models have explained these results by proposing a spin-orbital singlet ground state for NiRh₂O₄. Our recent RIXS measurements are mostly consistent with these predictions, but contain a feature that cannot be explained by a single-ion model with Coulomb interaction, crystal field, and spin-orbit coupling. Based on ab-initio calculations, we find that this feature is consistent with an interatomic orbital excitation between Ni and Rh sites. In addition, the spectral lineshapes suggests significant electron-phonon coupling, common to orbitally degenerate A-site spinels. These results provide insight into frustrated magnetism beyond fully localized moments in systems with coupled spin, orbital, and lattice degrees of freedom.   

Hidden energy scale in geometrically frustrated magnets

We study the effect of quenched disorder on geometrically frustrated (GF) magnets, a class of systems in which spin liquids are widely sought. We analyse the available data on the spin-glass freezing transition in GF system and show that its phenomenology is qualitatively different from that of conventional (non-GF) glasses. Whereas conventional systems have a glass-transition temperature that increases with increasing disorder, GF systems have a glass temperature that increases with decreasing disorder, approaching, in the clean limit, a finite value. This behaviour implies the existence of a hidden energy scale (far smaller than the Weiss constant) which is independent of disorder and drives the glass transition in the presence of disorder. Motivated by these observations, we propose a scenario in which the interplay of interactions and entropy in the disorder-free system yields a temperature-dependent magnetic permeability with a crossover temperature that determines the hidden energy scale. The relevance of this scale for quantum spin liquids is discussed.   

Strong limit spin and exchange disorder effects on magnetism

Exchange and spin disorder provide access to quantum criticality, frustration, and spin dynamics, but broad tunability and a deeper understanding of strong limit disorder is lacking, as traditional enthalpy-driven synthesis leads to unintended secondary crystal phases or defects when more than one or two elements are combined on a sublattice. To overcome this, a range of single crystal high entropy oxide La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ films are synthesized to probe the role of site-to-site spin and exchange interaction variances in stabilizing magnetic responses. The complexity of the system provides tunability and functionality not present in any of the ternary or half-doped quaternary parents or as a sum of parent properties. Neutron diffraction and magnetometry show that the compositionally disordered systems can paradoxically host long-range magnetic order, while manipulation of the S and J parameters through cation ratio permits continuous control of magnetic phase from antiferromagnetism (AFM), to degenerate, to ferromagnetism (FM). Tuning of the coexisting magnetic phase composition allows design of exchange bias behaviors in monolithic single crystal films, which have, until now, only been observable in AFM-FM bilayer heterojunctions or 2D layered bulk systems.  Despite the extraordinary levels of microstate complexity, a classical Heisenberg model can predict astonishingly accurate magnetic phase diagrams when highest probability local microstate populations are considered.   

Stripe order, impurities, and symmetry breaking in a diluted frustrated magnet

We investigate the behavior of the frustrated J1-J2 Ising model on a square lattice under the influence of random dilution and spatial anisotropies. Spinless impurities generate a random-field type disorder for the spin-density wave (stripe) order parameter. These random fields destroy the long-range stripe order in the case of spatially isotropic interactions. Combining symmetry arguments, percolation theory and large-scale Monte Carlo simulations, we demonstrate that arbitrarily weak spatial interaction anisotropies restore the stripe phase. More specifically, the transition temperature Tc into the stripe phase depends on the interaction anisotropy ∆J via Tc ∼ 1/| ln(∆J)| for small ∆J. This logarithmic dependence implies that very weak anisotropies are sufficient to restore the transition temperature to values comparable to that of the undiluted system. We analyze the critical behavior of the emerging transition and find it to belong to the disordered two-dimensional Ising universality class, which features the clean Ising critical exponents and universal logarithmic corrections. We also discuss the generality of our results and their consequences for experiments.   

Aging, Rejuvenation, and Memory Effects in Single Crystal Spin Glass

Spin glasses are a prototype of complex systems for which dynamics can be studied experimentally only out of equilibrium. We are systematically studying the phenomena of aging, rejuvenation, and memory, common to many glassy systems.  We use ac susceptibility measurements made on a single crystal of CuMn with a 7.91% concentration of manganese and a T_g of 41 K. Aging occurs when the temperature is held fixed, leading to a relaxation or decrease of the susceptibility [1]. Rejuvenation occurs when the sample “forgets” that it aged and returns to a reference cooling curve (hence it gets “younger”) [1]. In the memory effect, after temperature cycling, the heating curve seems to “remember” its previous cooling history [1]. In this talk, we discuss these effects with a focus on rejuvenation and our attempts to understand its underlying mechanisms both through a droplet model (in the form of Bray-Moore temperature chaos [2]) and a hierarchical model. The ac frequency was 1 Hz and the ac field was 5 Oe.  The temperatures investigated ranged from 37 K to 8.5 K. Our data show that both the rejuvenation and memory results appear to favor a temperature dependent hierarchical model.

 

[1] Jonason, K., et al. "Memory and chaos effects in spin glasses." Physical Review Letters 81.15 (1998): 3243.

[2] Bray, Alan J., and Michael A. Moore. "Chaotic nature of the spin-glass phase." Physical review letters 58.1 (1987): 57.   

The Fine Structure of the Relaxation Rates of the Spin Glass Aging Curves

Contrary to the traditional interpretation that there is only one extremum in the relaxation rates of spin-glass aging curves, we find that there are three local extrema: two minima and one maximum for the relaxation rates of the thermoremanent magnetization. We studied the properties of the extrema under variation of magnetic field at 0.9 T_g of a CuMn sample. The global minimum of the relaxation rates gradually switches from the minimum at longer time scale to the minimum at shorter time scale as the magnetic field strength increases. The time scale corresponding to the local maximum increases with the field, and the variation of the second minimum is related to the nonlinear Zeeman energy term. The underlying physical process will be presented.   

Probing AgMn spin glass thin film with 1/f noise

Analysis of electronic 1/f noise data can provide the energy barrier distribution of the spin glass state in thin films.  In overly simple terms, the electronic noise spectrum of a spin glass is related to the differences in the spin configurations that occur by transitions over the energy barriers. The power spectral density is proportional to 1/f^α and the exponent, α, of the noise spectrum can be used to determine the energy barrier distribution. In general, the noise measurements are limited to a frequency bandwidth and this limitation requires the temperature dependence of the exponent to provide a more complete barrier distribution; the resulting distribution is a zero temperature distribution. We have used this technique to determine the energy barrier distribution for thin films of the spin glass alloy AgMn (12 at.%) for which the solid solubility for Mn is larger than CuMn. In our measurements, different thicknesses were investigated but here we focus on the results for a 40nm thick film. In addition to the AgMn results, we make a comparison with the results from similar measurements on CuMn.   

Control of Mn3GaN Magnetism and Anomalous Hall Conductivity with N content

Antiperovskite Mn₃GaN is a metallic antiferromagnet whose frustrated non collinear spin configurations can generate spin Hall and anomalous Hall transport phenomena with potential spintronic applications. These properties can be tuned with N stoichiometry, which we pursue in epitaxial thin films with substate clamping to constrain the in-plane lattice constant. We use electronic transport combined with SQuID magnetometry to probe the magnetic properties. These data indicate that reducing the N gas fraction in the reactive deposition environment increases the Neel transition temperature, and induces an anomalous Hall signature and a low-temperature magnetic moment. Using this anomalous Hall signature, we generate a phase diagram summarizing the electronic properties as a function of N growth fraction. We discuss these results in the context of the microscopic magnetic structures.   

Low energy structure of classical spiral spin liquids

Spiral spin liquids are a class of spin liquids that have been experimentally discovered and theoretically studied in literature.  However, a concrete physical picture of its spin liquid nature has not been established so far, considering that its zero-temperature ground states do not contain any local degeneracy. In this work we illustrate the low-energy structure of 2D spiral spin liquids, and reveal its connection to fracton and elasticity physics. We find that the local momentum vector can form new types of vortices in the system, which have very different properties from the commonly known spin vortices. The proliferation of such vortices leads the system into a liquid phase at low temperature. Furthermore, the effective theories of such vortices show that they are equivalent to quadrupoles of fractons/disclinations in rank-2 U(1) theory/elasticity. At very low temperature, the system freezes with these vortices forming a rigid network. Our work sheds light on the nature of classical spiral spin liquids, and also paves the way toward understanding the quantum limit of them.   

Spin texture induced by nonmagnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3

In many geometrically frustrated magnets, a weak perturbation can potentially induce new competing ground states. In particular, novel effects induced by defects in frustrated magnets represent a highly nontrivial and interesting problem. Here, we demonstrate that a non-magnetic impurity can produce an extended spin structure (spin texture) in h-YMnO₃, a triangular lattice antiferromagnet with non-collinear magnetic order. Using inelastic neutron scattering (INS), we measured the full spin-wave spectra of single-crystalline h-Y(Mn,Al)O₃, which revealed the presence of magnon damping with clear momentum dependence. Our model calculation incorporating the spin texture well describes the INS data, which supports the formation of spin textures. Our study provides the first experimental confirmation of the impurity-induced spin textures. It provides new insights and understanding of the impurity effects in a wide variety of non-collinear magnetic systems.   

Transverse and orthogonal magnetization in antiferromagnets with higher-rank cluster multipoles

Antiferromagnets under a magnetic field develop the magnetization perpendicular to the field as well as a more conventional one parallel to the field. Up to now, two different sources of perpendicular magnetization (PM) are known. One is the transverse magnetization (TM) induced by the spin canting, and the other is the orthogonal magnetization (OM) arising from the higher-rank cluster multipoles. We firstly point out that TM and OM are indistinguishable by symmetry since PM emerges only when every crystalline symmetry other than order-two antiunitary and inversion symmetries is broken. Nevertheless, we find some reasons that TM and OM can be distinguished. First, OM depends only on cluster multipoles while TM depends also on the dominant spin interaction. We develop a microscopic theory of TM and show that the additional effective field from anisotropic spin interactions gives rise to TM by mixing the transverse to longitudinal spin components. Second, TM is from spin canting and OM is from orbital magnetization. Lastly, we show that both TM and OM contributes to the anomalous planar Hall Effect but distinct angular dependence. Because we explain all previous experimental results by our arguments, we believe that our theory provides a useful guideline for understanding the anomalous magnetic responses of the antiferromagnetic with complex magnetic structures.   

Understanding the magnetic interactions of Sr2-xLaxCoNbO6 in a pure ionic picture

The crystal field induced change in the energy levels/ bands of Sr_2-xLa_xCoNbO₆ (x=0-1) samples have been systematically understood by studying their magnetic properties, where substitution of La³⁺ at Sr²⁺ site is expected to completely transform  the Co³⁺  into Co²⁺ across the series.Temperature dependent magnetization measurements indicate the conversion from the weak ferromagnetic to the antiferromagnetic ordering for x≥0.6 due to evolution of the Co-O-Nb-O-Co exchange path resulting from the enhanced B-site ordering with the La substitution. Further, the effective magnetic moment extracted from the temperature dependent magnetization measurements show its significantly higher value (5.62 µ_B/f.u.) than the spin only moment for x=1 sample (S=3/2; 3.87 µ_B/Co²⁺), which indicate the persistence of the triply degenerate (orbital) free ion like ⁴T₁ ground state and hence significant contribution of the orbital magnetic moment. More interestingly, we observe the low temperature Schottky anomaly in the specific heat curves in case of the x≤0.4 samples due to the transition between the low lying energy levels of Co³⁺, resulting from the spin-orbit coupling and octahedral distortion, and presence of the ground state Karmer's doublet in case of Co²⁺ for x=1 sample.   

Magnetic complexity of mixed triple perovskites

Chemical doping is generally a hindrance to the formation of the spin-liquid phase. Ba₃NiIr₂O₉ with triple perovskite structure consists of interconnected corner-shared NiO₆ octahedra and face-shared Ir₂O₉ dimers, both having triangular arrangements in a-b plane and is a three-dimensional spin liquid.  We have investigated the doping effect at both Ir and Ni sublattices. Partial replacement of Ir by a 4d transition metal modifies both intra- and inter sublattice magnetic interactions. On the other hand, partial replacement of Ni by a non-magnetic 3d transition metal ion alters inter sublattice magnetic exchange.  We have found that structural symmetry remains the same in both cases. Surprisingly, the spin liquid phase is found to be preserved for doping at the Ni site. However, the doping at the Ir site destroys the spin liquid phase, resulting in long-range magnetic ordering.