| 1. |
- Henelius, Patrik, et al.
(author)
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Quantum Monte Carlo simulation of thin magnetic films
- 2002
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:9
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Journal article (peer-reviewed)abstract
- The stochastic series expansion quantum Monte Carlo method is used to study thin ferromagnetic films, described by a Heisenberg model including local anisotropies. The magnetization curve is calculated, and the results compared to Schwinger boson and many-body Green's function calculations. A transverse field is introduced in order to study the reorientation effect, in which the magnetization changes from out of plane to in plane. Since the approximate theoretical approaches above differ significantly from each other, and the Monte Carlo method is free of systematic errors, the calculation provides an unbiased check of the approximate treatments. By studying quantum spin models with local anisotropies, varying spin size, and a transverse field, we also demonstrate the general applicability of the recent cluster-loop formulation of the stochastic series expansion quantum Monte Carlo method.
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| 2. |
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| 3. |
- Edberg, Richard, et al.
(author)
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Dipolar spin ice under uniaxial pressure
- 2019
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:14
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Journal article (peer-reviewed)abstract
- The magnetically frustrated spin ice family of materials is host to numerous exotic phenomena such as magnetic monopole excitations and macroscopic residual entropy extending to low temperature. A finite-temperature ordering transition in the absence of applied fields has not been experimentally observed in the classical spin ice materials Dy2Ti2O7 and Ho2Ti2O7. Such a transition could be induced by the application of pressure, and in this work we consider the effects of uniaxial pressure on classical spin ice systems. Theoretically, we find that the pressure-induced ordering transition in Dy2Ti2O7 is strongly affected by the dipolar interaction. We also report measurements of the neutron structure factor of Ho2Ti2O7 under pressure and compare the experimental results to the predictions of our theoretical model.
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| 4. |
- Edberg, Richard, et al.
(author)
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Effects of uniaxial pressure on the spin ice Ho2Ti2O7
- 2020
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 102:18
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Journal article (peer-reviewed)abstract
- The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 are experimental and theoretical exemplars of highly frustrated magnetic materials. However, the effects of applied uniaxial pressure are not well studied, and here we report magnetization measurements of Ho2Ti2O7 under uniaxial pressure applied in the [001], [111], and [110] crystalline directions. The basic features are captured by an extension of the dipolar spin ice model. We find a good match between our model and measurements with pressures applied along two of the three directions, and we extend the framework to discuss the influence of crystal misalignment for the third direction. The parameters determined from the magnetization measurements reproduce neutron scattering measurements that we perform under uniaxial pressure applied along the [110] crystalline direction. In the detailed analysis, we include the recently verified susceptibility dependence of the demagnetizing factor. Our work demonstrates the application of a moderate applied pressure to modify the magnetic interaction parameters. The knowledge can be used to predict critical pressures needed to induce new phases and transitions in frustrated materials, and in the case of Ho2Ti2O7 we expect a transition to a ferromagnetic ground state for uniaxial pressures above 3.3 GPa.
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| 5. |
- Giblin, S. R., et al.
(author)
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Pauling Entropy, Metastability, and Equilibrium in Dy2Ti2O7 Spin Ice
- 2018
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 121:6
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Journal article (peer-reviewed)abstract
- Determining the fate of the Pauling entropy in the classical spin ice material Dy2Ti2O7 with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice-the dipolar spin ice model-predicts an ordering transition at T approximate to 0.15 K, but recent experiments by Pomaranski et al. suggest an entropy recovery over long timescales at temperatures as high as 0.5 K, much too high to be compatible with the theory. Using neutron scattering and specific heat measurements at low temperatures and with long timescales ( 0.35 K/10(6) s and 0.5 K/10(5) s, respectively) on several isotopically enriched samples, we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Furthermore, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez et al. are, after all, essentially correct: The short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.
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| 6. |
- Sandberg, L. o., et al.
(author)
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Design, calibration, and performance of a uniaxial pressure cell for neutron scattering studies of quantum magnetism
- 2023
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 94:10
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Journal article (peer-reviewed)abstract
- We provide an overview of a pressure cell designed to apply uniaxial pressure to single crystals for the study, by neutron scattering techniques, of strongly correlated magnetic systems and, in particular, quantum magnets. A detailed overview of the pressure cell components, their requirements, and links to the scientific and technical specifications are presented. The pressure cell is able to accommodate a 200 mm(3) single crystal that can be pressurized up to 2 GPa at cryogenic temperatures. The pressure cell is consistent with the requirements of inelastic neutron scattering and, importantly, neutron polarization analysis. A particular strength of the uniaxial pressure cell is the highly uniform and low background for a wide scattering angle of 360(degrees) horizontally and +/- 20(degrees) vertically. We show the performance of the uniaxial pressure cell using a relevant neutron scattering instrument, the polarized diffuse scattering instrument, D7. The experiments confirm that the cell complies with the scientific and technical requirements. This uniaxial pressure cell will provide a useful additional tool in the sample environment suite available for the study of quantum magnetism.
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| 7. |
- Bovo, L., et al.
(author)
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Special temperatures in frustrated ferromagnets
- 2018
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In: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9:1
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Journal article (peer-reviewed)abstract
- The description and detection of unconventional magnetic states, such as spin liquids, is a recurring topic in condensed matter physics. While much of the efforts have traditionally been directed at geometrically frustrated antiferromagnets, recent studies reveal that systems featuring competing antiferromagnetic and ferromagnetic interactions are also promising candidate materials. We find that this competition leads to the notion of special temperatures, analogous to those of gases, at which the competing interactions balance, and the system is quasi-ideal. Although induced by weak perturbing interactions, these special temperatures are surprisingly high and constitute an accessible experimental diagnostic of eventual order or spin-liquid properties. The well characterised Hamiltonian and extended low-temperature susceptibility measurement of the canonical frustrated ferromagnet Dy2Ti2O7 enables us to formulate both a phenomenological and microscopic theory of special temperatures for magnets. Other members of this class of magnets include kapellasite Cu3Zn(OH)6Cl2 and the spinel GeCo2O4.
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| 8. |
- Chakraborty, P. B., et al.
(author)
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Theory of the magnetic phase diagram of LiHoF4
- 2004
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:14
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Journal article (peer-reviewed)abstract
- The properties of LiHoF4 are believed to be well described by a long-range dipolar Ising model. We go beyond mean-field theory and calculate the phase diagram of the Ising model in a transverse field using a quantum Monte Carlo method. The relevant Ising degrees of freedom are obtained using a nonperturbative projection onto the low-lying crystal-field eigenstates. We explicitly take the domain structure into account, and the strength of the near-neighbor exchange interaction is obtained as a fitting parameter. The on-site hyperfine interaction is approximately taken into account through a renormalization of the transverse applied magnetic field. Finally, we propose a spectroscopy experiment to precisely measure the most important parameter controlling the location of the phase boundary.
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| 9. |
- Gingras, M. J. P., et al.
(author)
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Collective phenomena in the LiHo xY 1-xF 4 quantum ising magnet : Recent progress and open questions
- 2011
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In: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 320, s. 012001-
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Journal article (peer-reviewed)abstract
- In LiHo xY 1-xF 4, 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-Ho 3+ interaction is dipolar, while the Y 3+ ions are non-magnetic. The application of a magnetic field B x 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-xF 4 in the dipolar ferromagnetic (x = 1), diluted ferromagnetic (0.25 ≤∼ x < 1) and highly diluted x ≤∼ 0.25 dipolar spin glass regimes have attracted much experimental and theoretical interest over the past twenty-five years. Two questions have received particular attention: (i) is there an antiglass (quantum disordered) phase at low Ho 3+ concentration and (ii) what is the mechanism responsible for the fast B x-induced destruction of the ferromagnetic (0.25 ≤∼ x < 1) and spin glass (x ≤∼ 0.25) phases? This paper reviews some of the recent theoretical and experimental progress in our understanding of the collective phenomena at play in LiHo xY 1-xF 4, in both zero and nonzero B x.
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| 10. |
- Henelius, Patrik, et al.
(author)
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Refrustration and competing orders in the prototypical Dy2Ti2O7 spin ice material
- 2016
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In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 93:2
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Journal article (peer-reviewed)abstract
- Spin ices, frustratedmagnetic materials analogous to common water ice, have emerged over the past 15 years as exemplars of high frustration in three dimensions. Recent experimental developments aimed at interrogating anew the low-temperature properties of these systems, in particular whether the predicted transition to long-range order occurs, behoove researchers to scrutinize our current dipolar spin ice model description of these materials. In this work, we do so by combining extensive Monte Carlo simulations and mean-field theory calculations to analyze data from previous magnetization, diffuse neutron scattering, and specific-heat measurements on the paradigmatic Dy2Ti2O7 spin ice material. In this work, we also reconsider the possible importance of the nuclear specific heat C-nuc in Dy2Ti2O7. We find that C-nuc is not entirely negligible below a temperature similar to 0.5 K and must therefore be taken into account in a quantitative analysis of the calorimetric data of this compound below that temperature. We find that in this material, small effective spin-spin exchange interactions compete with the magnetostatic dipolar interaction responsible for the main spin ice phenomenology. This causes an unexpected " refrustration" of the long-range order that would be expected from the incompletely self-screened dipolar interaction and which positions the material at the boundary between two competing classical long-range-ordered ground states. This allows for the manifestation of new physical low-temperature phenomena in Dy2Ti2O7, as exposed by recent specific-heat measurements. We show that among the four most likely causes for the observed upturn of the specific heat at low temperature [an exchange-induced transition to long-range order, quantum non-Ising (transverse) terms in the effective spin Hamiltonian, the nuclear hyperfine contribution, and random disorder], only the last appears to be reasonably able to explain the calorimetric data.
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