The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5% and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While this system is considered as the archetypal quantum spin glass, the detailed analysis of our magnetization data indicates the absence of a phase transition, not only in a transverse applied magnetic field, but also without field. A zero-Kelvin phase transition is also unlikely, as the magnetization seems to follow a non-critical exponential dependence on the temperature. Our analysis thus unmasks the true, short-ranged nature of the magnetic properties of the LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations suggesting the lack of phase transition in this system.
We show in this comment that the claim by Ancona-Torres et al. of an equilibrium quantum or classical phase transition in the LiHo$_x$Y$_{1-x}$F$_4$ system is not supported by a rigorous scaling analysis.
We consider the quantum magnet $\rm LiHo_xY_{1-x}F_4$ at $x = 0.167$. Experimentally the spin glass to paramagnet transition in this system was studied as a function of the transverse magnetic field and temperature, showing peculiar features: for example (i) the spin glass order is destroyed much faster by thermal fluctuations than by the transverse field; and (ii) the cusp in the nonlinear susceptibility signaling the glass state {\it decreases} in size at lower temperature. Here we show that the hyperfine interactions of the Ho atom must dominate in this system, and that along with the trans…
The physics of the spin glass (SG) state, with magnetic moments (spins) frozen in random orientations, is one of the most intriguing problems in condensed matter physics. While most theoretical studies have focused on the Edwards-Anderson model of Ising spins with only discrete `up/down' directions, such Ising systems are experimentally rare. LiHo$_{x}$Y$_{1-x}$F$_{4}$, where the Ho$^{3+}$ moments are well described by Ising spins, is an almost ideal Ising SG material. In LiHo$_{x}$Y$_{1-x}$F$_{4}$, the Ho$^{3+}$ moments interact predominantly via the inherently frustrated magnetostatic dipole…
AC susceptibility measurements are presented on the dilute, dipolar coupled, Ising magnet LiHo$_x$Y$_{1-x}$F$_4$ for a concentration $x = 0.045$. The frequency and temperature dependences of the susceptibility show characteristic glassy relaxation. The absorption spectrum is found to broaden with decreasing temperature suggesting that the material is behaving as a spin glass and not as an exotic spin liquid as was previously observed. A dynamical scaling analysis suggests a spin glass transition temperature of 42.6 mK $\pm 2$ mK with an exponent $zν= 7.8 \pm 0.23$.
We analyze recent experiments on the dilute rare-earth compound LiHo_xY_(1-x)F_4 in the context of an effective Ising dipolar model. Using a Monte Carlo method we calculate the low-temperature behavior of the specific heat and linear susceptibility, and compare our results to measurements. In our model the susceptibility follows a Curie-Weiss law at high temperature, chi ~ 1/(T-T_cw), with a Curie-Weiss temperature that scales with dilution, T_cw ~ x, consistent with early experiments. We also find that the peak in the specific heat scales linearly with dilution, C_max(T) ~ x, in disagreement …
The random field Ising model (RFIM) is central to the study of disordered systems. Yet, for a long time it eluded realization in ferromagnetic systems because of the difficulty to produce locally random magnetic fields. Recently it was shown that in anisotropic dipolar magnetic insulators, the archetypal of which is the $LiHo_xY_{1-x}F_4$ system, the RFIM can be realized in both ferromagnetic and spin glass phases. The interplay between an applied transverse field and the offdiagonal terms of the dipolar interaction produce effective longitudinal fields, which are random in sign and magnitude …
As a result of the interplay between the intrinsic off-diagonal terms of the dipolar interaction and an applied {\it transverse} field $H_t$, the diluted ${\rm LiHo_xY_{1-x}F_4}$ system at x$> 0.5$ is equivalent to a ferromagnet in a longitudinal random field (RF). At low $H_t$ the quantum fluctuations between the Ising like doublet states are negligible, while the effective induced RF is appreciable. This results in a practically exact equivalence to the classical RF Ising model. By tuning $H_t$, the applied longitudinal field, and the dilution, the Ising model can be realized in the presence…
The ${\rm LiHo_xY_{1-x}F_4}$ compound is widely considered to be the archetypal dipolar Quantum Ising system, with longitudinal dipolar interactions $V_{ij}^{zz}$ between ${\rm Ho}$ spins $\{i,j \}$ competing with transverse field-induced tunneling, to give a T=0 quantum phase transition. By varying the ${\rm Ho}$ concentration x, the typical strength $V_0$ of $V_{ij}^{zz}$ can be varied over many orders of magnitude; and so can the transverse field $H_{\perp}$. A new effective Hamiltonian is derived, starting from the electronuclear degrees of freedom, and valid at low and intermediate temper…
The LiHo$_x$Y$_{1-x}$F$_4$ magnetic material in a transverse magnetic field $B_{x}\hat x$ perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in a random disordered system. We show that the $B_{x}-$induced magnetization along the $\hat x$ direction, combined with the local random dilution-induced destruction of crystalline symmetries, generates, via the predominant dipolar interactions between Ho$^{3+}$ ions, {\it random fields} along the Ising $\hat z$ direction. This identifies LiHo$_x$Y$_{1-x}$F$_4$ in $B_x$ as a new random field Ising sys…
In LiHo_xY_{1-x}F4, the magnetic Holmium Ho^{3+} ions behave as effective Ising spins that can point parallel or antiparallel to the crystalline c-axis. The predominant inter-Holmium interaction is dipolar, while the Y^{3+} ions are non-magnetic. The application of a magnetic field Bx transverse to the c-axis Ising direction leads to quantum spin-flip fluctuations, making this material a rare physical realization of the celebrated transverse field Ising model. The problems of classical and transverse-field-induced quantum phase transitions in LiHo_xY_{1-x}F_4 in the dipolar ferromagnetic (x=1)…
We study the effective long-range Ising dipole model with a local exchange interaction appropriate for the dilute magnetic compound LiHo_{x}Y_{1-x}F_4. Our calculations yield a value of 0.12 K for the nearest neighbor exchange interaction. Using a Monte Carlo method we calculate the phase boundary T_c(x) between the ferromagnetic and paramagnetic phases. We demonstrate that the experimentally observed linear decrease in T_c with dilution is not the simple mean-field result, but a combination of the effects of fluctuations, the exchange interaction and the hyperfine coupling. Furthermore, we fi…
We present specific heat data on three samples of the dilute Ising magnet $\HoYLF$ with $x = 0.018$, 0.045 and 0.080. Previous measurements of the ac susceptibility of an $x = 0.045$ sample showed the Ho$^{3+}$ moments to remain dynamic down to very low temperatures and the specific heat was found to have unusually sharp features. In contrast, our measurements do not exhibit these sharp features in the specific heat and instead show a broad feature, for all three samples studied, which is qualitatively consistent with a spin glass state. Integrating $C/T$, however, reveals an increase in resid…
The rare-earth material $\mathrm{LiHoF_4}$ is believed to be an experimental realization of the celebrated (dipolar) Ising model, and upon the inclusion of a transverse field $B_x$, an archetypal quantum Ising model. Moreover, by substituting the magnetic Ho ions by non-magnetic Y ions, disorder can be introduced into the system giving rise to a dipolar disordered magnet and at high disorders to a spin-glass. Indeed, this material has been scrutinized experimentally, numerically and theoretically over many decades with the aim of understanding various collective magnetic phenomena. One of the …
The random dipolar magnet LiHo$_x$Y$_{1-x}$F$_4$ enters a strongly frustrated regime for small Ho$^{3+}$ concentrations with $x<0.05$. In this regime, the magnetic moments of the Ho$^{3+}$ ions experience small quantum corrections to the common Ising approximation of LiHo$_x$Y$_{1-x}$F$_4$, which lead to a $Z_2$-symmetry breaking and small, degeneracy breaking energy shifts between different eigenstates. Here we show that destructive interference between two almost degenerate excitation pathways burns spectral holes in the magnetic susceptibility of strongly driven magnetic moments in LiHo$_x$…
The nature of ordering in dilute dipolar interacting systems dates back to the work of Debye and is one of the most basic, oldest and as-of-yet unsettled problems in magnetism. While spin-glass order is readily observed in several RKKY-interacting systems, dipolar spin-glasses are subject of controversy and ongoing scrutiny, e.g., in ${{\rm LiHo_xY_{1-x}F_4}}$, a rare-earth randomly diluted uniaxial (Ising) dipolar system. In particular, it is unclear if the spin-glass phase in these paradigmatic materials persists in the limit of zero concentration or not. We study an effective model of ${{\r…
We use quantum Monte Carlo methods and various analytic approximations to solve the Ising spin-glass model in a transverse field in the disordered phase. We focus on the behavior of the frequency dependent susceptibility of the system above and below the critical field . We establish the existence of an exact equivalence between this problem and the single-impurity Kondo model at the quantum critical point. Our predictions for the long-time dynamics of the model are in good agreement with experimental results on $LiHo_{0.167}Y_{0.833}F_4$.
When performed in the proper low field, low frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in transverse field, $\text{LiHo}_x\text{Y}_{1-x}\text{F}_4$, prove the existence of a spin glass transition for $x$ = 0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.
We present ac susceptibility and specific heat measurements taken on samples of LiHo$_x$Y$_{1-x}$F$_4$ in the dilute limit: x = 0.018, 0.045, 0.080 and 0.12. Susceptibility measurements show glassy behavior including wide absorption spectra that continually broaden with decreasing temperature. Dynamical scaling analyses show evidence of finite-temperature spin-glass transitions, the temperatures of which match those of recent theoretical work. A surprisingly long intrinsic time constant is observed in these samples and is found to be inversely correlated with the concentration of magnetic mome…
We perform Metropolis Monte Carlo simulations of the behaviour of a film of insulating material containing a distribution of magnetic nanoparticles. We assume that these particles only interact through dipolar forces and we find that their behaviour at low T shows characteristics of a spin-glass with a freezing T_f at which the linear susceptibility and the specific heat show a maximum. We obtain the spin glass order parameter as a function of temperature and we also calculate the time auto-correlation of the spin at the center of the system. We find that these results are consistent with the …
Systems in which the dipolar energy dominates the magnetic interaction, and the crystal field generates strong anisotropy favoring the longitudinal interaction terms, are considered. Such systems in external magnetic field are expected to be a good experimental realization of the transverse field Ising model. With random interactions this model yields a spin glass to paramagnet phase transition as function of the transverse field. Here we show that the off-diagonal dipolar interaction, although effectively reduced, destroys the spin glass order at any finite transverse field. Moreover, the res…
By means of parallel tempering Monte Carlo simulations we find strong evidence for a finite-temperature spin-glass transition in a system of diluted classical Heisenberg dipoles randomly placed on the sites of a simple cubic lattice. We perform a finite-size scaling analysis of the spin-glass susceptibility, $χ_{\rm SG}$, and spin-glass correlation length, $ξ_{L}$. For the available system sizes that can be successfully equilibrated, the crossing of $ξ_{L}/L$ versus temperature is strongly affected by corrections to scaling and possibly by a short-length scale ferromagnetic spin blocking. Simi…
In this paper we investigate the relationship between glassy and ferromagnetic phases in disordered Ising ferromagnets in the presence of transverse magnetic fields, $Γ$. Iterative mean field simulations probe the free energy landscape and suggest the existence of a glass transition as a function of $Γ$ which is distinct from the Curie temperature. New experimental field-cooled and zero-field-cooled data on LiHo$_x$Y$_{1-x}$F$_4$ provide support for our theoretical picture.
We study magnetic properties of spin glass SG systems under a random field (RF), beased on the suggestion that RFs can be induced by a weak transverse field in the compound LiHo$_x$Y$_{1-x}$F$_4$. We consider a cluster spin model that allows long-range disordered interactions among clusters and short-range interactions inside the clusters, besides a local RF for each spin following a Gaussian distribution with standard deviation $Δ$. We adopt the one-step replica symmetry breaking (RSB) approach to get an exactly solvable single-cluster problem. We discuss the behavior of order parameters, spe…
We simulate the dc magnetic response of the diluted dipolar-coupled Ising magnet LiHo\(_{0.045}\)Y\(_{0.955}\)F\(_4\) in a transverse field, using exact diagonalization of a two-spin Hamiltonian averaged over nearest-neighbour configurations. The pairwise model, incorporating hyperfine interactions, accounts for the observed drop-off in the longitudinal (c-axis) susceptibility with increasing transverse field; with the inclusion of a small tilt in the transverse field, it also accounts for the behavior of the off-diagonal magnetic susceptibility. The hyperfine interactions do not appear to lea…
We present analytical and numerical evidence for the validity of an effective S=1/2 approach to the description of random field generation in S>=1, and especially in an S=1, dipolar spin glass models with strong uniaxial Ising anisotropy and subject to weak external magnetic field Bx transverse to the Ising direction. Explicitely Bx-dependent random fields are shown to naturally emerge in the effective low-energy description of a microscopic S=1 toy model. We discuss our results in relation to recent theoretical studies pertaining to the topic of Bx-induced random fields in the LiHo$_x$Y$_{1-x…
The spin-glass phase in the $\LHx$ compound is considered. At zero transverse field this system is well described by the classical Ising model. At finite transverse field deviations from the transverse field quantum Ising model are significant, and one must take properly into account the hyperfine interactions, the off-diagonal terms in the dipolar interactions, and details of the full J=8 spin Hamiltonian to obtain the correct physical picture. In particular, the system is not a spin glass at finite transverse fields and does not show quantum criticality.
The interactions between the dielectric polarization and the fluctuations of the strain (stress) tensor in relaxor ferroelectrics are shown to give rise to the anisotropy of the anharmonic P4-term in the Landau-type free energy, however, the harmonic P2-term is still properly described by the rigid spherical random bond--random field model. These are the essential features of the compressible spherical dipolar glass model, which is used to calculate the singularities of the specific heat near field-induced critical points. The results agree with recent high-resolution calorimetric experiments …
We derive the equilibrium phase diagram of the classical dipolar Ising antiferromagnet at the mean-field level on a geometry that mimics the two dimensional Kagome lattice. Our mean-field treatment is based on the combination of the cluster variational Bethe-Peierls formalism and the cavity method, developed in the context of the glass transition, and is complementary to the Monte Carlo simulations realized in [Phys. Rev. B 98, 144439 (2018)]. Our results confirm the nature of the low temperature crystalline phase which is reached through a weakly first-order phase transition. Moreover, they a…
We study the glass transition of binary mixtures of dipolar particles in two dimensions within the framework of mode-coupling theory, focusing in particular on the influence of composition changes. In a first step, we demonstrate that the experimental system of König et al. [Eur. Phys. J. E 18, 287 (2005)] is well described by point dipoles through a comparison between the experimental partial structure factors and those from our Monte Carlo simulation. For such a mixture of point particles we show that there is always a plasticization effect, i.e. a stabilization of the liquid state due to mi…
The interplay between quantum fluctuations and disorder is investigated in a spin-glass model, in the presence of a uniform transverse field $Γ$, and a longitudinal random field following a Gaussian distribution with width $Δ$. The model is studied through the replica formalism. This study is motivated by experimental investigations on the LiHo$_x$Y$_{1-x}$F$_4$ compound, where the application of a transverse magnetic field yields rather intriguing effects, particularly related to the behavior of the nonlinear magnetic susceptibility $χ_3$, which have led to a considerable experimental and the…
Using Monte Carlo simulations, we study the character of the spin-glass (SG) state of a site-diluted dipolar Ising model. We consider systems of dipoles randomly placed on a fraction x of all L^3 sites of a simple cubic lattice that point up or down along a given crystalline axis. For x < 0.65 these systems are known to exhibit an equilibrium spin-glass phase below a temperature T_sg proportional to x. At high dilution and very low temperatures, well deep in the SG phase, we find spiky distributions of the overlap parameter q that are strongly sample-dependent. We focus on spikes associated wi…
We study a quantum extension of the spherical $p$-spin-glass model using the imaginary-time replica formalism. We solve the model numerically and we discuss two analytical approximation schemes that capture most of the features of the solution. The phase diagram and the physical properties of the system are determined in two ways: by imposing the usual conditions of thermodynamic equilibrium and by using the condition of marginal stability. In both cases, the phase diagram consists of two qualitatively different regions. If the transition temperature is higher than a critical value $T^{\star}$…
In a recent letter [Phys. Rev. Lett. {\bf 75}, 2360 (1996)] we briefly discussed the existence and nature of ferroelectric order in positionally disordered dipolar materials. Here we report further results and give a complete description of our work. Simulations of randomly frozen and dynamically disordered dipolar soft spheres are used to study ferroelectric ordering in non-crystalline systems. We also give a physical interpretation of the simulation results in terms of short- and long-range interactions. Cases where the dipole moment has 1, 2, and 3 components (Ising, XY and XYZ models, resp…
Ferromagnetic Ising systems with competing interactions are considered in the presence of a random field. We find that in three space dimensions the ferromagnetic phase is disordered by a random field which is considerably smaller than the typical interaction strength between the spins. This is the result of a novel disordering mechanism triggered by an underlying spin-glass phase. Calculations for the specific case of the long-range dipolar LiHo_xY_{1-x}F_4 compound suggest that the above mechanism is responsible for the peculiar dependence of the critical temperature on the strength of the r…
We study equilibration and ordering in the classical dipolar kagome Ising antiferromagnet, which we show behaves as a disorder-free fragile spin glass. By identifying an appropriate order parameter, we demonstrate a transition to the ordered state proposed by Chioar et al. [Phys. Rev. B 93, 214410 (2016)] with a 12-site unit cell that breaks time-reversal and sublattice symmetries, and further provide evidence that the nature of the transition is first order. Upon approaching the transition, the spin dynamics slow dramatically. The system readily falls out of equilibrium, overshooting the tran…
The physical properties of magnetic nanoparticles have been investigated with focus on the influence of dipolar interparticle interaction. For weakly coupled nanoparticles, thermodynamic perturbation theory is employed to derive analytical expressions for the linear equilibrium susceptibility, the zero-field specific heat and averages of the local dipolar fields. By introducing the averages of the dipolar fields in an expression for the relaxation rate of a single particle, a nontrivial dependence of the superparamagnetic blocking on the damping coefficient is evidenced. This damping dependenc…
Using Monte Carlo simulation we investigated time of flight current transients predicted by the dipolar glass model for a random spatial distribution of hopping centers. Behavior of the carrier drift mobility was studied at room temperature over a broad range of electric field and sample thickness. A flat plateau followed by $j\propto t^{-2}$ current decay is the most common feature of the simulated transients. Poole-Frenkel mobility field dependence was confirmed over 5 to 200 V/$μ$m as well as its independence of the sample thickness. Universality of transients with respect to both field and…
Both orientational glasses and dipolar glasses possess an intrinsic random field, coming from the volume difference between impurity and host ions. We show this suppresses the glass transition, causing instead a crossover to the low $T$ phase. Moreover the random field is correlated with the inter-impurity interactions, and has a broad distribution. This leads to a peculiar variant of the Imry-Ma mechanism, with 'domains' of impurities oriented by a few frozen pairs. These domains are small: predictions of domain size are given for specific systems, and their possible experimental verification…
We present the results of Monte-Carlo simulations of the charge carrier transport in a disordered molecular system containing spatial and energetic disorders using the dipolar glass model. Model parameters of the material were chosen to fit a typical polar organic photoconductor polycarbonate doped with 30% of aromatic hydrazone, whose transport properties are well documented in literature. Simulated carrier mobility demonstrates a usual Poole-Frenkel field dependence and its slope is very close to the experimental value without using any adjustable parameter. At room temperature transients ar…
Dilute dipolar Ising magnets remain a notoriously hard problem to tackle both analytically and numerically because of long-ranged interactions between spins as well as rare region effects. We study a new type of anisotropic dilute dipolar Ising system in three dimensions [Phys. Rev. Lett. {\bf 114}, 247207 (2015)] that arises as an effective description of randomly diluted classical spin ice, a prototypical spin liquid in the disorder-free limit, with a small fraction $x$ of non-magnetic impurities. Metropolis algorithm within a parallel thermal tempering scheme fails to achieve equilibration …
We investigate the dielectric response in the glass-electret state of two dipolar glass-forming materials. This unusual polar glassy state of matter is produced when a dipolar liquid is supercooled under the influence of a high electric dc field, which leads to partial orientational order of the molecules carrying a dipole moment. Investigation of the prepared glass-electrets by using low-field dielectric spectroscopy reveals a clear modification of their dielectric response in the regime of the Johari-Goldstein beta-relaxation, pointing to a small but significant increase of its relaxation st…
Ultracold gases offer an unprecedented opportunity to engineer disorder and interactions in a controlled manner. In an effort to understand the interplay between disorder, dipolar interaction and quantum degeneracy, we study two-dimensional hard-core dipolar lattice bosons in the presence of on-site bound disorder. Our results are based on large-scale path-integral quantum Monte Carlo simulations by the Worm algorithm. We study the ground state phase diagram at fixed half-integer filling factor for which the clean system is either a superfluid at lower dipolar interaction strength or a checker…
By tempered Monte Carlo simulations, we study site-diluted Ising systems of magnetic dipoles. All dipoles are randomly placed on a fraction x of all L^3 sites of a simple cubic lattice, and point along a given crystalline axis. For x_c< x<=1, where x_c = 0.65, we find an antiferromagnetic phase below a temperature which vanishes as x tends to x_c from above. At lower values of x, we find an equilibrium spin-glass (SG) phase below a temperature given by k_B T_{sg} = x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy. We study (a) the relative mean square deviation D_q^2 of …
The LiHo$_x$Y$_{1-x}$F$_4$ Ising magnetic material subject to a magnetic field, $B_x$, perpendicular to the Ho$^{3+}$ Ising direction has shown over the past twenty years to be a host of very interesting thermodynamic and magnetic phenomena. Unfortunately, the availability of other magnetic materials other than LiHo$_x$Y$_{1-x}$F$_4$ that may be described by a transverse field Ising model remains very much limited. It is in this context that we use here mean-field theory to investigate the suitability of the Ho(OH)$_{3}$, Dy(OH)$_{3}$ and Tb(OH)$_3$ insulating hexagonal dipolar Ising-like ferr…
Heat bath Monte Carlo simulations have been used to study a 12-state discretized Heisenberg model with a new type of random field on simple cubic lattices of size $128 \times 128 \times 128$. The 12 states correspond to the [110] directions of a cube. The model has the standard nonrandom two-spin exchange term with coupling energy $J$ and a random field which consists of adding an energy $h_R$ to two of the 12 spin states, chosen randomly and independently at each site. We report on the case $h_R / J = 3$, which has a sharp phase transition at about $T_c / J = 1.40625$. Below $T_c$, the model …
Far tails of the density of state (DOS) are calculated for the simple models of organic amorphous material, the model of dipolar glass and model of quadrupolar glass. It was found that in both models far tails are non-Gaussian. In the dipolar glass model the DOS is symmetric around zero energy, while for the model of quadrupolar glass the DOS is generally asymmetric and its asymmetry is directly related to the particular geometry of quadrupoles. Far tails of the DOS are relevant for the quasi-equilibrium transport of the charge carriers at low temperature. Asymmetry of DOS in quadrupolar glass…
We search for a Gardner transition in glassy glycerol, a standard molecular glass, measuring the third harmonics cubic susceptibility $χ_3^{(3)}$ from slightly below the usual glass transition temperature down to $10K$. According to the mean field picture, if local motion within the glass were becoming highly correlated due to the emergence of a Gardner phase then $χ_3^{(3)}$, which is analogous to the dynamical spin-glass susceptibility, should increase and diverge at the Gardner transition temperature $T_G$. We find instead that upon cooling $| χ_3^{(3)} |$ decreases by several orders of mag…
Dynamic nonlinear (cubic) susceptibility in quantum d-dimensional Ising spin glass with short-range interactions is investigated on the basis of quantum droplet model and quantum-mechanical nonlinear response theory. Nonlinear response depends on the tunneling rate for a droplet which regulates the strength of quantum fluctuations. It shows a strong dependence on the distribution of droplet free energies and on the droplet length scale average. Comparison with recent experiments on quantum spin glasses like disordered dipolar quantum Ising magnet is discussed.
Lithium-salt-based deep eutectic solvents, where the only cation is Li+, are promising candidates as electrolytes in electrochemical energy-storage devices like batteries. We have performed broadband dielectric spectroscopy on three such systems, covering a broad temperature and dynamic range that extends from the low-viscosity liquid around room temperature down to the glassy state approaching the glass-transition temperature. We detect a relaxational process that can be ascribed to dipolar reorientational dynamics and exhibits the clear signatures of glassy freezing. We find that the tempera…
We study fully occupied lattice systems of classical magnetic dipoles which point along random axes. Only dipolar interactions are considered. From tempered Monte Carlo simulations, we obtain numerical evidence that supports the following conclusions: in three dimensions, (a) there is an equilibrium spin glass phase at temperatures below $T_c$, where $k_BT_c= (0.86\pm 0.07)\varepsilon_d $ and $\varepsilon_d$ is a nearest neighbor dipole-dipole interaction energy, (b) in the spin glass phase the overlap parameter is approximately given by $\sqrt{1-T/T_c}$, and (c) the correlation length $ξ$ div…
We study the temperature-dilution phase diagram of a site-diluted Heisenberg antiferromagnet on a fcc lattice, with and without the Dzyaloshinskii-Moriya anisotropic term, fixed to realistic microscopic parameters for $IIB_{1-x} Mn_x Te$ (IIB=Cd, Hg, Zn). We show that the dipolar Dzyaloshinskii-Moriya anisotropy induces a finite-temperature phase transition to a spin glass phase, at dilutions larger than 80%. The resulting probability distribution of the order parameter P(q) is similar to the one found in the cubic lattice Edwards-Anderson Ising model. The critical exponents undergo large fini…
By tempered Monte Carlo simulations, we study 2D site-diluted dipolar Ising systems. Dipoles are randomly placed on a fraction x of all L^2 sites in a square lattice, and point along a common crystalline axis. For x_c< x<=1, where x_c = 0.79(5), we find an antiferromagnetic phase below a temperature which vanishes as x approaches x_c from above. At lower values of x, we study (i) distributions of the spin--glass (SG) overlap q, (ii) their relative mean square deviation Delta_q^2 and kurtosis and (iii) xi_L/L, where xi_L is a SG correlation length. From their variation with temperature and syst…
Strongly interacting quantum systems subject to quenched disorder exhibit intriguing phenomena such as glassiness and many-body localization. Theoretical studies have mainly focused on disorder in the form of random potentials, while many experimental realizations naturally feature disorder in the interparticle interactions. Inspired by cold Rydberg gases, where such disorder can be engineered using the dipole blockade effect,we study a Heisenberg XXZ spin model where the disorder is exclusively due to random spin-spin couplings, arising from power-law interactions between randomly positioned …
Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co80Fe20(t)/Al2O3(3nm)]x10, t = 0.9 and 1.0nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al2O3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 < T < 100K at different dc fields and as a function of frequency 0.01 < f < 1000Hz. The spectral data were successf…
The degree of contact between a system and the external environment can alter dramatically its proclivity to quantum mechanical modes of relaxation. We show that controlling the thermal coupling of cubic centimeter-sized crystals of the Ising magnet $LiHo_xY_{1-x}F_4$ to a heat bath can be used to tune the system between a glassy state dominated by thermal excitations over energy barriers and a state with the hallmarks of a quantum spin liquid. Application of a magnetic field transverse to the Ising axis introduces both random magnetic fields and quantum fluctuations, which can retard and spee…
We study the effect in geometrically frustrated antiferromagnets of weak, random variations in the strength of exchange interactions. Without disorder the simplest classical models for these systems have macroscopically degenerate ground states, and this degeneracy may prevent ordering at any temperature. Weak exchange randomness favours a small subset of these ground states and induces a spin-glass transition at an ordering temperature determined by the amplitude of modulations in interaction strength. We use the replica approach to formulate a theory for this transition, showing that it fall…
