arXiv subjects

R. S. Freitas

Publications and source records attributed to R. S. Freitas.

At least 19 recordsLinked to original sources

arXiv2025

Excitation spectrum and low-temperature magnetism in disordered defect-fluorite Ho2Zr2O7

In this work, we report on the thermomagnetic characterization and crystalline-electric field (CEF) energy scheme of the disordered defect-fluorite Ho2Zr2O7. This structural phase is distinguished by the coexistence of magnetic frustration and extensive disorder, with Ho3+ and Zr4+ sharing randomly the same 4a site with even 50% occupancy, and an average 1/8 oxygen vacancy per unit cell. AC magnetic susceptibility measurements performed on powder samples down to 0.5 K revealed signs of slowing spin dynamics without glassy behavior, including a frequency dependent peak at 1 K. Yet, no evidence for long-range magnetic order is found down to 150 mK in specific heat. Inelastic neutron scattering measurements show a weak, low-lying CEF excitation around 2 meV, accompanied by a broad level centered at 60 meV. To fit our observations, we propose an approach to account for structural disorder in the crystal-field splitting of the non-Kramers Ho3+. Our model provides an explanation to the broadening of the high-energy, single-ion excitations and suggests that the zirconate ground-state wave function has zero magnetic moment. However, structural disorder acts as guarantor of the magnetism in Ho2Zr2O7, allowing the mixing of low lying states at finite temperatures. Finally, we show that this scenario is in good agreement with the bulk properties reported in this work.

cond-mat.str-el↗

arXiv2019

Melting of Spin Ice state through structural disorder in Dy2Zr2O7

Neutron scattering, a.c. magnetic susceptibility and specific heat studies have been carried out on polycrystalline Dy2Zr2O7. Unlike the pyrochlore spin ice Dy2Ti2O7, Dy2Zr2O7 crystallizes into the fluorite structure and the magnetic Dy3+ moments randomly reside on the corner-sharing tetrahedral sublattice with non-magnetic Zr ions. Antiferromagnetic spin correlations develop below 10 K but remain dynamic down to 40 mK. These correlations extend over the length of two tetrahedra edges and grow to 6 nearest neighbors with the application of a 20 kOe magnetic field. No Pauling's residual entropy was observed and by 8 K the full entropy expected for a two level system is released. We propose that the disorder melts the spin ice state seen in the chemically ordered Dy2Ti2O7 compound, but the spins remain dynamic in a disordered, liquid-like state and do not freeze into a glass-like state that one might intuitively expect.

cond-mat.dis-nn↗

arXiv2019

Dynamic magnetism in the disordered hexagonal double perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$

Magnetic frustration and disorder are key ingredients to prevent the onset of magnetic order. In the disordered hexagonal double perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$, Mn$^{4+}$ cations, with $S=3/2$ spins, can either form highly correlated states of magnetic trimers or dimers or remain as weakly interacting orphan spins. At low temperature ($T$), the dimer response is negligible, and magnetism is dominated by the trimers and orphans. To explore the role of magnetic frustration, disorder and possibly of quantum fluctuations, the low-$T$ magnetic properties of the remaining magnetic degrees of freedom of BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$ are investigated. Heat-capacity data and magnetic susceptibility display no evidence for a phase transition to a magnetically ordered phase but indicate the formation of a correlated spin state. The low-temperature spin dynamics of this state is then explored by $μ$SR experiments. The zero field $μ^{+}$ relaxation rate data show no static magnetism down to $T=19$ mK and longitudinal field experiments support as well that dynamic magnetism persists at low $T$. Our results are interpreted in terms of a spin glass state which stems from a disordered lattice of orphans spins and trimers. A spin liquid state in BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$, however, is not excluded and is also discussed.

cond-mat.str-el↗

arXiv2017

Magnetic-field-induced ordered phase in the chloro-bridged copper(II) dimer system [Cu2(apyhist)2Cl2](ClO4)2

Specific heat and magnetization measurements of the compound [Cu2(apyhist)2Cl2](ClO4)2, where apyhist = (4-imidazolyl)ethylene-2-amino-1-ethylpyridine), were used to identify a magnetic-field-induced long-range antiferromagnetic ordered phase at low temperatures (T < 0.36 K) and magnetic fields (1.6 T < H < 5.3 T). This system consists of a Schiff base copper(II) complex, containing chloro-bridges between adjacent copper ions in a dinuclear arrangement, with an antiferromagnetic intradimer interaction |Jintra|/kB = 3.65 K linked by an antiferromagnetic coupling |Jinter|z/kB = 2.7 K. The magnetic-field-induced ordering behavior was analyzed using the mean field approximation and Monte Carlo simulation results. The obtained physical properties of the system are consistent with the description of the ordered phase as a Bose-Einstein Condensation (BEC) of magnetic excitations. We present the phase diagram of this compound, which shows one of the lowest critical magnetic field among all known members of the family of BEC quantum magnets.

cond-mat.str-el↗

arXiv2016

Crossover from one- to three-dimensional behavior in the S = 1/2 Heisenberg antiferromagnet Cu(N2H5)2(SO4)2

From experimental and theoretical analyses of magnetic and specific-heat properties, we present the complete magnetic phase diagram of the quasi-one-dimensional antiferromagnet Cu(N$_2$H$_5$)$_2$(SO$_4$)$_2$. On cooling and at zero magnetic field this compound enters a one-dimensional regime with short-range magnetic correlations, marked by a broad maximum in the specific heat and magnetic susceptibility at $T_\mathrm{max}\sim 2\,\mathrm{K}$, followed by an ordered antiferromagnetic phase below $T_\mathrm{N}\sim 1\,\mathrm{K}$ induced by small interchain couplings. The intermediate-temperate one-dimensional regime can be modeled using exact quantum-transfer-matrix calculations, which perfectly describe the nonmonotonic behavior of T_max as a function of the magnetic field, giving $J = 4.25\,\mathrm{K}$ for the intrachain exchange parameter. The analysis of magnetic specific-heat and susceptibility data at low temperature indicates that the interchain exchange couplings are an order of magnitude smaller than the coupling inside the chains.

cond-mat.str-el↗

arXiv2016

Universal Dynamic Magnetism in Yb-Pyrochlores with Disparate Ground States

The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra and interacting via anisotropic exchange. Structurally, only the non-magnetic B-site cation, and hence, primarily the lattice parameter, is changing across the series. Nonetheless, a range of magnetic behaviors are observed: the low temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character, while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low temperatures. While the static properties of the ytterbium pyrochlores are distinct, inelastic neutron scattering measurements reveal a common character to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless continuum of spin excitations, resembling over-damped ferromagnetic spin waves at low Q. Furthermore, the specific heat of the series also follows a common form, with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly at T_C or T_N. The novel spin dynamics we report correlate strongly with the broad specific heat anomaly only, remaining unchanged across the sharp anomaly. This result suggests that the primary order parameter in the ytterbium pyrochlores associated with the sharp anomaly is "hidden" and not simple magnetic dipole order.

cond-mat.str-el↗

arXiv2016

Avalanche-like metamagnetic transition in (LaNd)CaMnO manganites

We investigate the behavior of ultrasharp metamagnetic transitions in La(5/8-y)Nd(y)Ca(3/8)MnO(3) manganites. These compounds change from a low temperature ferromagnetic metallic state at low Nd doping to a charge-ordered antiferromagnetic insulator for high Nd content. At an intermediate doping a phase-separated state is established. At low temperatures (2 K), we observe an avalanche-like field-induced metamagnetic transition, when the entire compound changes abruptly from one phase to the other. We investigate the signatures of this ultrasharp transition using magnetization and specific heat measurements. We observe a first order transition in the specific heat associated with discontinuous jumps in the magnetization. A strong increase of the sample temperature is simultaneously observed. The results are interpret in terms of latent heat release from the field induced enhancement of the ferromagnetic fraction, triggering the avalanche process.

cond-mat.str-el↗

arXiv2016

XY Antiferromagnetic Ground State in the Effective S=1/2 Pyrochlore Yb2Ge2O7

We report neutron scattering and muon spin relaxation measurements (muSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent with an S_eff = 1/2 description for the Yb moments. muSR measurements reveal the presence of an ordering transition at T_N = 0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k = (000) Gamma_5 ground state with an ordered magnetic moment of 0.3(1) mu_B per Yb ion. We compare and contrast this phenomenology with the low temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical S_eff = 1/2 XY pyrochlore magnets.

cond-mat.str-el↗

arXiv2015

Ground state selection in XY pyrochlore antiferromagnets R$_{2}$Ge$_{2}$O$_{7}$ (R = Er, Yb)

Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $Γ_5$ manifold. The ground state is a $ψ_{3}$ phase for the Er sample and a $ψ_{2}$ or $ψ_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].

cond-mat.str-el↗

arXiv2011

Increase in the magnitude of the energy barrier distribution in Ni nanoparticles due to dipolar interactions

The energy barrier distribution Eb of five samples with different concentrations x of Ni nanoparticles using scaling plots from ac magnetic susceptibility data has been determined. The scaling of the imaginary part of the susceptibility Chi"(nu, T) vs. Tln(t/tau_0) remains valid for all samples, which display Ni nanoparticles with similar shape and size. The mean value increases appreciably with increasing x, or more appropriately with increasing dipolar interactions between Ni nanoparticles. We argue that such an increase in constitutes a powerful tool for quality control in magnetic recording media technology where the dipolar interaction plays an important role.

cond-mat.str-el↗

arXiv2008

Magnetic structures of quaternary intermetallic borocarbides RCo2B2C (R=Dy, Ho, Er)

The magnetic structures of the title compounds have been studied by neutron diffraction. In contrast to the isomorphous RNi2B2C compounds wherein a variety of exotic incommensurate modulated structures has been observed, the magnetic structure of ErCo2B2C is found to be collinear antiferromagnet with k=((1/2),0,(1/2)) while that of HoCo2B2C and DyCo2B2C are observed to be simple ferromagnets. For all studied compounds, the moments are found to be confined within the basal plane and their magnitudes are in good agreement with the values obtained from the low-temperature isothermal magnetization measurements. The absence of modulated magnetic structures in the RCo2B2C series (for ErCo2B2C, verified down to 50 mK) is attributed to the quenching of the Fermi surface nesting features.

cond-mat.str-el↗

arXiv2008

Synthesis and magnetic characterization of TmCo2B2C

A new quaternary intermetallic borocarbide TmCo2B2C has been synthesized via a rapid-quench of an arc-melted ingot. Elemental and powder-diffraction analyses established its correct stoichiometry and single-phase character. The crystal structure is isomorphous to that of TmNi2B2C(I4/mmm) and is stable over the studied temperature range. Above 7 K, the paramagnetic state follows the modified Curie-Weiss behavior (X=C/(T-theta)+X0 wherein X0=0.008(1) emu/mole and the temperature-dependent term reflecting the paramagnetism of the Tm subsystem: ueff=7.6(2) uB [in agreement with the expected value for a free Tm3+ ion] and theta = -4.5(3) K. Long range ferromagnetic order of the Tm sublattice is observed to develop around ~1 K. No superconductivity is detected in TmCo2B2C down to 20 mK, a feature which is consistent with the general trend in the RCo2B2C series. Finally, the influence of the rapid-quench process on the magnetism (and superconductivity) of TmNi2B2C will be discussed and compared to that of TmCo2B2C.

cond-mat.str-el↗

arXiv2008

Magnetothermal study of Dy Stuffed Spin Ice: Dy2(DyxTi2-x)O7-x/2

We have studied the thermodynamics of the stuffed spin ice material, Dy2(DyxTi2-x)O7-x/2, in which additional Dy3+ replace Ti4+ in the pyrochlore Dy2Ti2O7. Heat capacity measurements indicate that these materials lose the spin ice zero point entropy for x >= 0.3, sharply contrasting with results on the analogous Ho materials. A finite ac susceptibility is observed as T ~ 0 in both the Ho and Dy materials with x = 0.67, which suggests that spin fluctuations persist down to T ~ 0. We propose that both the entropy and susceptibility data may be explained as a result of domains of local pyrochlore type structural order in these materials.

cond-mat.str-el↗

arXiv2007

Non-monotonic zero point entropy in diluted spin ice

Water ice and spin ice are important model systems in which theory can directly account for zero point entropy associated with quenched configurational disorder. Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with non-magnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90%. Investigations of both Ho and Dy spin ices reveal that the zero point entropy depends non-monotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice.

cond-mat.str-el↗

arXiv2007

Structural Disorder and Properties of the Stuffed Pyrochlore Ho2TiO5

We report a structural and thermodynamic study of the "stuffed spin ice" material Ho2TiO5 (i.e., Ho2(Ti1.33Ho0.67)O6.67), comparing samples synthesized through two different routes. Neutron powder diffraction and electron diffraction reveal that the previously reported defect fluorite phase has short-range pyrochlore ordering, in that there are domains in which the Ho and Ho/Ti sublattices are distinct. By contrast, a sample prepared through a floating zone method has long range ordering of these sublattices. Despite the differences in crystal structures, the two versions of Ho2TiO5 display similar magnetic susceptibilities. Field dependent magnetization and measured recovered entropies, however, show a difference between the two forms, suggesting that the magnetic properties of the stuffed pyrochlores depend on the local structure.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic transition and magnetic structure of Sr4Ru3O10

We have investigated the magnetic transition and magnetic structure of triple-layered ruthenate Sr4Ru3O10 directly using neutron scattering techniques. Only one ferromagnetic phase is observed, and previously proposed antiferromagnetic phase transitions are ruled out. The complex anisotropic magnetotransport, magnetization and in-plane metamagnetic behaviors of this quasi two-dimensional (2D) material are most likely due to magnetic domain processes with strong magnetocrystalline anisotropy and a strongly anisotropic demagnetization factor.

cond-mat.mtrl-sci↗

arXiv2006

Zero Point Entropy in Stuffed Spin Ice

The third law of thermodynamics dictates that the entropy of a system in thermal equilibrium goes to zero as its temperature approaches absolute zero. In ice, however, a "zero point" or residual entropy can be measured - attributable to a high degeneracy in the energetically preferred positions of the hydrogen ions associated with the so-called "ice rules".1,2 Remarkably, the spins in certain magnetic materials with the pyrochlore structure of corner-sharing tetrahedra, called "spin ice", have an equivalent degeneracy of energetically preferred states and also have been shown to display a zero point entropy.3,4,5,6,7 Here we report that we have chemically altered Ho2Ti2O7 spin ice by stuffing extra Ho magnetic moments into normally non-magnetic Ti sites surrounding the Ho tetrahedra. The resulting series, Ho2(Ti2-xHox)O7-x/2, provides a unique opportunity to study the effects of increased connectivity between spins on a frustrated lattice. Surprisingly, the measured zero point entropy per spin appears unchanged by these excess spins, and the dynamic freezing of the spins is suppressed to lower temperatures. The results challenge our understanding of the spin ice state, and suggest a new avenue for using chemistry to study both ice-like frustration and the properties of the broad family of geometrically frustrated magnets based on the pyrochlore structure.

cond-mat.str-el↗

arXiv2006

Artificial "spin ice" in a geometrically frustrated lattice of nanoscale ferromagnetic islands

We report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the lowtemperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.

cond-mat.dis-nn↗