arXiv subjects

P. Schiffer

Publications and source records attributed to P. Schiffer.

At least 19 recordsLinked to original sources

arXiv2023

Deconstructing Magnetization Noise: Degeneracies, Phases, and Mobile Fractionalized Excitations in Tetris Artificial Spin Ice

Direct detection of spontaneous spin fluctuations, or "magnetization noise", is emerging as a powerful means of revealing and studying magnetic excitations in both natural and artificial frustrated magnets. Depending on the lattice and nature of the frustration, these excitations can often be described as fractionalized quasiparticles possessing an effective magnetic charge. Here, by combining ultrasensitive optical detection of thermodynamic magnetization noise with Monte Carlo simulations, we reveal emergent regimes of magnetic excitations in artificial "tetris ice". A marked increase of the intrinsic noise at certain applied magnetic fields heralds the spontaneous proliferation of fractionalized excitations, which can diffuse independently, without cost in energy, along specific quasi-1D spin chains in the tetris ice lattice.

cond-mat.mes-hall↗

arXiv2022

Collective ferromagnetism of artificial square spin ice

We have studied the temperature and magnetic field dependence of the total magnetic moment of large-area permalloy artificial square spin ice arrays. The temperature dependence and hysteresis behavior are consistent with the coherent magnetization reversal expected in the Stoner-Wohlfarth model, with clear deviations due to inter-island interactions at small lattice spacing. Through micromagnetic simulations, we explore this behavior and demonstrate that the deviations result from increasingly complex magnetization reversal at small lattice spacing, induced by inter-island interactions, and depending critically on details of the island shapes. These results establish new means to tune the physical properties of artificial spin ice structures and other interacting nanomagnet systems, such as patterned magnetic media.

cond-mat.mes-hall↗

arXiv2022

Magnetic-Field-Dependent Thermodynamic Properties of Square and Quadrupolar Artificial Spin Ice

Applied magnetic fields are an important tuning parameter for artificial spin ice (ASI) systems, as they can drive phase transitions between different magnetic ground states, or tune through regimes with high populations of emergent magnetic excitations (e.g., monopole-like quasiparticles). Here, using simulations supported by experiments, we investigate the thermodynamic properties and magnetic phases of square and quadrupolar ASI as a function of applied in-plane magnetic fields. Monte Carlo simulations are used to generate field-dependent maps of the magnetization, the magnetic specific heat, the thermodynamic magnetization fluctuations, and the magnetic order parameters, all under equilibrium conditions. These maps reveal the diversity of magnetic orderings and the phase transitions that occur in different regions of the phase diagrams of these ASIs, and are experimentally supported by magneto-optical measurements of the equilibrium "magnetization noise" in thermally-active ASIs.

cond-mat.mes-hall↗

arXiv2021

Experimental Realization of the 1D Random Field Ising Model

We have measured magnetic-field-induced avalanches in a square artificial spin ice array of interacting nanomagnets. Starting from the ground state ordered configuration, we imaged the individual nanomagnet moments after each successive application of an incrementally increasing field. The statistics of the evolution of the moment configuration show good agreement with the canonical one-dimensional random field Ising model. We extract information about the microscopic structure of the arrays from our macroscopic measurements of their collective behavior, demonstrating a process that could be applied to other systems exhibiting avalanches.

cond-mat.mes-hall↗

arXiv2020

Field-Induced Magnetic Monopole Plasma in Artificial Spin Ice

Artificial spin ices (ASIs) are interacting arrays of lithographically-defined nanomagnets in which novel frustrated magnetic phases can be intentionally designed. A key emergent description of fundamental excitations in ASIs is that of magnetic monopoles -- mobile quasiparticles that carry an effective magnetic charge. Here we demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, high-density plasma-like regimes of mobile magnetic monopoles. By passively "listening" to spontaneous monopole noise in thermal equilibrium, we reveal their intrinsic dynamics and show that monopole kinetics are minimally correlated (that is, most diffusive) in the plasma phase. These results open the door to on-demand monopole regimes having field-tunable densities and dynamic properties, thereby providing a new paradigm for probing the physics of effective magnetic charges in synthetic matter.

cond-mat.mes-hall↗

arXiv2015

Quenched crystal field disorder and magnetic liquid ground states in Tb2Sn2-xTixO7

Solid-solutions of the "soft" quantum spin ice pyrochlore magnets Tb2B2O7 with B=Ti and Sn display a novel magnetic ground state in the presence of strong B-site disorder, characterized by a low susceptibility and strong spin fluctuations to temperatures below 0.1 K. These materials have been studied using ac-susceptibility and muSR techniques to very low temperatures, and time-of-flight inelastic neutron scattering techniques to 1.5 K. Remarkably, neutron spectroscopy of the Tb3+ crystal field levels appropriate to at high B-site mixing (0.5 < x < 1.5 in Tb2Sn2-xTixO7) reveal that the doublet ground and first excited states present as continua in energy, while transitions to singlet excited states at higher energies simply interpolate between those of the end members of the solid solution. The resulting ground state suggests an extreme version of a random-anisotropy magnet, with many local moments and anisotropies, depending on the precise local configuration of the six B sites neighboring each magnetic Tb3+ ion.

cond-mat.str-el↗

arXiv2014

Spin Order by Quantum Frustration in Triangular Lattice Mott Insulator NaCrO2 : A Neutron Scattering Study

We report high resolution neutron scattering measurements on the triangular lattice Mott insulator Na$_x$CrO$_2$ ($x$=1) which has recently been shown to exhibit an unusually broad fluctuating crossover regime extending far below the onset of spin freezing ($T_c\sim$41K). Our results show that below some crossover temperature ($T\sim0.75T_c$) a small incommensuration develops which helps resolve the spin frustration and drives three-dimensional magnetic order supporting coherent spin wave modes. This incommensuration assisted dimensional crossover suggests that inter-layer frustration is responsible for stabilizing the rare 2D correlated phase above 0.75$T_c$. In contrast to the host compound of 2D cobaltate superconductor such as Na$_x$CoO$_2$ ($ξ_c>50$Å), no magnetic long-range order is observed down to 1.5K ($ξ_c<16$Å).

cond-mat.str-el↗

arXiv2014

Epitaxial Growth of VO$_{2}$ by Periodic Annealing

We report the growth of ultrathin VO$_{2}$ films on rutile TiO$_{2}$ (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO$_{2}$ via solid-phase epitaxy. Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change ΔR/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm.

cond-mat.mtrl-sci↗

arXiv2013

Non-monotonic residual entropy in diluted spin ice: a comparison between Monte Carlo simulations of diluted dipolar spin ice models and experimental results

Spin ice materials, such as Dy2Ti2O7 and Ho2Ti2O7, have been the subject of much interest for over the past fifteen years. Their low temperature strongly correlated state can be mapped onto the proton disordered state of common water ice and, consequently, spin ices display the same low temperature residual Pauling entropy as water ice. Interestingly, it was found in a previous study [X. Ke {\it et. al.} Phys. Rev. Lett. {\bf 99}, 137203 (2007)] that, upon dilution of the magnetic rare-earth ions (Dy^{3+} and Ho^{3+}) by non-magnetic Yttrium (Y^{3+}) ions, the residual entropy depends {\it non-monotonically} on the concentration of Y^{3+} ions. In the present work, we report results from Monte Carlo simulations of site-diluted microscopic dipolar spin ice models (DSIM) that account quantitatively for the experimental specific heat measurements, and thus also for the residual entropy, as a function of dilution, for both Dy2Ti2O7 and Ho2Ti2O7. The main features of the dilution physics displayed by the magnetic specific heat data are quantitatively captured by the diluted DSIM up to, and including, 85% of the magnetic ions diluted (x=1.7). The previously reported departures in the residual entropy between Dy2Ti2O7 versus Ho2Ti2O7, as well as with a site-dilution variant of Pauling's approximation, are thus rationalized through the site-diluted DSIM. For 90% (x=1.8) and 95% (x=1.9) of the magnetic ions diluted, we find a significant discrepancy between the experimental and Monte Carlo specific heat results. We discuss some possible reasons for this disagreement.

cond-mat.stat-mech↗

arXiv2012

Low temperature dynamic freezing and the fragility of ordering in Tb2Sn2O7

We have probed the low temperature magnetic behavior of the ordered spin ice material Tb2Sn2O7 through ac magnetic susceptibility measurements of both the pure material and samples with small percentages of Ti substituted on the Sn sublattice. We observe a clear signature for the previously reported ordering transition at TC = 850 mK, as well as evidence for dynamic freezing at temperatures well below TC, confirming the persistence of significant magnetic fluctuations deep in the spin-ordered regime. The long range ordering transition was completely suppressed with as little as 5% Ti for Sn substitution, and 10% Ti substitution resulted in a spin-glass-like spin freezing transition near 250 mK. The results demonstrate that the long range magnetic ordering is surprisingly fragile in this system.

cond-mat.str-el↗

arXiv2012

High temperature onset of field-induced transitions in the spin-ice compound Dy2Ti2O7

We have studied the field-dependent ac magnetic susceptibility of single crystals of Dy2Ti2O7 spin ice along the [111] direction in the temperature range 1.8 K - 7 K. Our data reflect the onset of local spin ice order in the appearance of different field regimes. In particular, we observe a prominent feature at approximately 1.0 T that is a precursor of the low-temperature metamagnetic transition out of field-induced kagome ice, below which the kinetic constraints imposed by the ice rules manifest themselves in a substantial frequency-dependence of the susceptibility. Despite the relatively high temperatures, our results are consistent with a monopole picture, and they demonstrate that such a picture can give physical insight to the spin ice systems even outside the low-temperature, low-density limit where monopole excitations are well-defined quasiparticles.

cond-mat.str-el↗

arXiv2012

Reversible Control of Magnetic Interactions by Electric Field in a Single Phase Material

Intrinsic magnetoelectric coupling describes the interaction between magnetic and electric polarization through an inherent microscopic mechanism in a single phase material. This phenomenon has the potential to control the magnetic state of a material with an electric field, an enticing prospect for device engineering. We demonstrate 'giant' magnetoelectric cross-field control in a single phase rare earth titanate film. In bulk form, EuTiO3 is antiferromagnetic. However, both anti and ferromagnetic interactions coexist between different nearest neighbor europium ions. In thin epitaxial films, strain can be used to alter the relative strength of the magnetic exchange constants. Here, we not only show that moderate biaxial compression precipitates local magnetic competition, but also demonstrate that the application of an electric field at this strain state, switches the magnetic ground state. Using first principles density functional theory, we resolve the underlying microscopic mechanism resulting in the EuTiO3 G-type magnetic structure and illustrate how it is responsible for the 'giant' cross-field magnetoelectric effect.

cond-mat.mtrl-sci↗

arXiv2011

Structural and Magnetic Characteristics of MnAs Nanoclusters Embedded in Be-doped GaAs

We describe a systematic study of the synthesis, microstructure and magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by molecular beam epitaxy. Transmission electron microscopy and magnetometry studies reveal two distinct classes of nanoclustered samples whose structural and magnetic properties depend on the Mn content of the initial (Ga,Mn)As layer. For Mn content in the range 5% - 7.5%, annealing creates a superparamagnetic material with a uniform distribution of small clusters (diameter around 6 nm) and with a low blocking temperature (T_B approximately 10 K). While transmission electron microscopy cannot definitively identify the composition and crystalline phase of these small clusters, our experimental data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing results in an inhomogeneous distribution of both small clusters as well as much larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also exhibit supermagnetism, albeit with substantially larger magnetic moments and coercive fields, and blocking temperatures well above room temperature.

cond-mat.mtrl-sci↗

arXiv2011

Partial magnetization plateau emerging from a quantum spin ice state in Tb2Ti2O7

The quantum spin ice model applied to Tb2Ti2O7 predicts that magnetic fields applied along the [111] axis will induce a partial magnetization plateau [H. R. Molavian and M. J. P. Gingras, J. Phys.: Condens. Matter 21, 172201 (2009)]. We test this hypothesis using ac magnetic susceptibility and muon-spin relaxation measurements, finding features at 15 and 65 mT agreeing with the predicted boundaries of the magnetization plateau. This suggests that Tb2Ti2O7 is well described by a quantum spin ice model with an effective exchange constant of Jeff = 0.17(1) K.

cond-mat.str-el↗

arXiv2011

Measurements of Nanoscale Domain Wall Flexing in a Ferromagnetic Thin Film

We use the high spatial sensitivity of the anomalous Hall effect in the ferromagnetic semiconductor Ga1-xMnxAs, combined with the magneto-optical Kerr effect, to probe the nanoscale elastic flexing behavior of a single magnetic domain wall in a ferromagnetic thin film. Our technique allows position sensitive characterization of the pinning site density, which we estimate to be around 10^14 cm^{-3}. Analysis of single site depinning events and their temperature dependence yields estimates of pinning site forces (10 pN range) as well as the thermal deactivation energy. Finally, our data hints at a much higher intrinsic domain wall mobility for flexing than previously observed in optically-probed micron scale measurements.

cond-mat.mes-hall↗

arXiv2011

Low Velocity Granular Drag in Reduced Gravity

We probe the dependence of the low velocity drag force in granular materials on the effective gravitational acceleration (geff) through studies of spherical granular materials saturated within fluids of varying density. We vary geff by a factor of 20, and we find that the granular drag is proportional to geff, i.e., that the granular drag follows the expected relation Fprobe = η ρgrain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical cylinder with depth of insertion, hprobe, diameter dprobe, moving through grains of density ρgrain, and where η is a dimensionless constant. This dimensionless constant shows no systematic variation over four orders of magnitude in effective grain weight, demonstrating that the relation holds over that entire range to within the precision of our data.

cond-mat.soft↗

arXiv2010

Starting to move through a granular medium

We explore the process of initiating motion through a granular medium by measuring the force required to push a flat circular plate upward from underneath the medium. In contrast to previous measurements of the drag and penetration forces, which were conducted during steady state motion, the initiation force has a robust dependence on the diameter of the grains in the medium. We attribute this dependence to the requirement for local dilation of the grains around the circumference of the plate, as evidenced by an observed linear dependence of the initiation force on the plate diameter.

cond-mat.soft↗

arXiv2010

Comparing artificial frustrated magnets: tuning symmetry in nanomagnet arrays

We study the impact of geometry on magnetostatically frustrated single-domain nanomagnet arrays. We examine square and hexagonal lattice arrays, as well as a brickwork geometry that combines the anisotropy of the square lattice and the topology of the hexagonal lattice. We find that the more highly frustrated hexagonal lattice allows for the most thorough minimization of the magnetostatic energy, and that the pair-wise correlations between moments differ qualitatively between hexagonal and brickwork lattices, although they share the same lattice topology. The results indicate that the symmetry of local interaction is more important than overall lattice topology in the accommodation of frustrated interactions.

cond-mat.mes-hall↗