Change search
Link to record
Permanent link

Direct link
BETA
Publications (10 of 20) Show all publications
Giblin, S. R., Twengström, M., Bovo, L., Ruminy, M., Bartkowiak, M., Manuel, P., . . . Fennell, T. (2018). Pauling Entropy, Metastability, and Equilibrium in Dy2Ti2O7 Spin Ice. Physical Review Letters, 121(6), Article ID 067202.
Open this publication in new window or tab >>Pauling Entropy, Metastability, and Equilibrium in Dy2Ti2O7 Spin Ice
Show others...
2018 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 6, article id 067202Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-235135 (URN)10.1103/PhysRevLett.121.067202 (DOI)000441021400016 ()30141658 (PubMedID)
Funder
Swedish Research Council, 2013-03968Stiftelsen Olle Engkvist Byggmästare, 2014/807
Note

QC 20180919

Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2018-09-19Bibliographically approved
Bovo, L., Twengström, M., Petrenko, O. A., Fennell, T., Gingras, M. J., Bramwell, S. T. & Henelius, P. (2018). Special temperatures in frustrated ferromagnets. Nature Communications, 9(1), Article ID 1999.
Open this publication in new window or tab >>Special temperatures in frustrated ferromagnets
Show others...
2018 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, no 1, article id 1999Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Nature Publishing Group, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-229285 (URN)10.1038/s41467-018-04297-3 (DOI)000432535300003 ()29784922 (PubMedID)2-s2.0-85047443782 (Scopus ID)
Funder
Swedish Research CouncilStiftelsen Olle Engkvist Byggmästare
Note

QC 20180601

Available from: 2018-06-01 Created: 2018-06-01 Last updated: 2018-06-01Bibliographically approved
Twengström, M., Bovo, L., Gingras, M. J., Bramwell, S. T. & Henelius, P. (2017). Microscopic aspects of magnetic lattice demagnetizing factors. PHYSICAL REVIEW MATERIALS, 1(4), Article ID 044406.
Open this publication in new window or tab >>Microscopic aspects of magnetic lattice demagnetizing factors
Show others...
2017 (English)In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 4, article id 044406Article in journal (Refereed) Published
Abstract [en]

The demagnetizing factor N is of both conceptual interest and practical importance. Considering localized magnetic moments on a lattice, we show that for nonellipsoidal samples, N depends on the spin dimensionality (Ising, XY, or Heisenberg) and orientation, as well as the sample shape and susceptibility. The generality of this result is demonstrated by means of a recursive analytic calculation as well as detailed Monte Carlo simulations of realistic model spin Hamiltonians. As an important check and application, we also make an accurate experimental determination of N for a representative collective paramagnet (i.e., the Dy2Ti2O7 spin ice compound) and show that the temperature dependence of the experimentally determined N agrees closely with our theoretical calculations. Our conclusion is that the well-established practice of approximating the true sample shape with "corresponding ellipsoids" for systems with long-range interactions will in many cases overlook important effects stemming from the microscopic aspects of the system under consideration.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-220295 (URN)10.1103/PhysRevMaterials.1.044406 (DOI)000416582000002 ()
Note

QC 20171222

Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2018-05-02Bibliographically approved
Henelius, P., Lin, T., Enjalran, M., Hao, Z., Rau, J. G., Altosaar, J., . . . Gingras, M. J. (2016). Refrustration and competing orders in the prototypical Dy2Ti2O7 spin ice material. Physical Review B. Condensed Matter and Materials Physics, 93(2), Article ID 024402.
Open this publication in new window or tab >>Refrustration and competing orders in the prototypical Dy2Ti2O7 spin ice material
Show others...
2016 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 2, article id 024402Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Physical Society, 2016
Keywords
ISING PYROCHLORE MAGNETS, HIGHLY FRUSTRATED PYROCHLORES, LOW-TEMPERATURE PROPERTIES, MONOPOLE DYNAMICS, COULOMB PHASE, MONTE-CARLO, MODEL, STATE, HO2TI2O7, ENTROPY
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-181359 (URN)10.1103/PhysRevB.93.024402 (DOI)000367662400004 ()2-s2.0-84955241780 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20160205

Available from: 2016-02-05 Created: 2016-02-01 Last updated: 2017-11-30Bibliographically approved
Revell, H. M., Yaraskavitch, L. R., Mason, J. D., Ross, K. A., Noad, H. M., Dabkowska, H. A., . . . Kycia, J. B. (2013). Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice. Nature Physics, 9(1), 34-37
Open this publication in new window or tab >>Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice
Show others...
2013 (English)In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 9, no 1, p. 34-37Article in journal (Refereed) Published
Abstract [en]

Electrical resistance is a crucial and well-understood property of systems ranging from computer microchips to nerve impulse propagation in the human body. Here we study the motion of magnetic charges in spin ice and find that extra spins inserted in Dy2Ti2O7 trap magnetic monopole excitations and provide the first example of how defects in a spin-ice material obstruct the flow of monopoles-a magnetic version of residual resistance. We measure the time-dependent magnetic relaxation in Dy2Ti2O7 and show that it decays with a stretched exponential followed by a very slow long-time tail. In a Monte Carlo simulation governed by Metropolis dynamics we show that surface effects and a very low level of stuffed spins (0.30%)-magnetic Dy ions substituted for non-magnetic Ti ions-cause these signatures in the relaxation. In addition, we find evidence that the rapidly diverging experimental timescale is due to a temperature-dependent attempt rate proportional to the monopole density.

Keywords
Ising Pyrochlore Magnets, Zero-Point Entropy
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-116419 (URN)10.1038/NPHYS2466 (DOI)000312635200013 ()2-s2.0-84871620509 (Scopus ID)
Note

QC 20130121

Available from: 2013-01-21 Created: 2013-01-18 Last updated: 2017-12-06Bibliographically approved
Biltmo, A. & Henelius, P. (2012). Unreachable glass transition in dilute dipolar magnet. Nature Communications, 3, 857
Open this publication in new window or tab >>Unreachable glass transition in dilute dipolar magnet
2012 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 3, p. 857-Article in journal (Refereed) Published
Abstract [en]

In magnetic systems the combined effects of disorder and frustration may cause the moments to freeze into a disordered state at a spin-glass transition. Recent experiments have shown that the rare earth compound LiHo0.045Y0.955F4 freezes, but that the transition is unreachable because of dynamics that are 10(7) times slower than in ordinary spin-glass materials. This conclusion refutes earlier investigations reporting a speed-up of the dynamics into an exotic anti-glass phase caused by entanglement of quantum dipoles. Here we present a theory, backed by numerical simulations, which describes the material in terms of classical dipoles governed by Glauber dynamics. The dipoles freeze and we find that the ultra-slow dynamics are caused by rare, strongly ordered clusters, which give rise to a previously predicted, but hitherto unobserved, Griffths phase between the paramagnetic and spin-glass phases. In addition, the hyperfine interaction creates a high energy barrier to flipping the electronic spin, resulting in a clear signature in the dynamic correlation function.

Keywords
Ising Spin-Glasses, Disordered Magnet, Quantum, Relaxation, Phase, Dynamics, Lihof4
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-98336 (URN)10.1038/ncomms1857 (DOI)000304611400052 ()2-s2.0-84864449821 (Scopus ID)
Note
QC 20120625Available from: 2012-06-25 Created: 2012-06-25 Last updated: 2017-12-07Bibliographically approved
Gingras, M. J. & Henelius, P. (2011). Collective phenomena in the LiHo xY 1-xF 4 quantum ising magnet: Recent progress and open questions. Paper presented at International Conference on Frustration in Condensed Matter, ICFCM; Sendai; Japan; 11-14 January 2011. Journal of Physics: Conference Series, 320, 012001
Open this publication in new window or tab >>Collective phenomena in the LiHo xY 1-xF 4 quantum ising magnet: Recent progress and open questions
2011 (English)In: Journal of Physics: Conference Series, ISSN 1742-6588, Vol. 320, p. 012001-Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
Collective phenomena, Ising spins, Nonmagnetics, Physical realization, Quantum phase transitions, Recent progress, Spin-flip, Transverse-field Ising model, Ferromagnetic materials, Ferromagnetism, Glass, Holmium, Ising model, Magnetic fields, Solids, Spin fluctuations, Spin glass, Quantum theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-150675 (URN)10.1088/1742-6596/320/1/012001 (DOI)000299350600001 ()2-s2.0-81055125404 (Scopus ID)
Conference
International Conference on Frustration in Condensed Matter, ICFCM; Sendai; Japan; 11-14 January 2011
Note

QC 20140909

Available from: 2014-09-09 Created: 2014-09-08 Last updated: 2015-10-09Bibliographically approved
Biltmo, A. & Henelius, P. (2009). The ferromagnetic transition and domain structure in LiHoF4. Europhysics letters, 87(2)
Open this publication in new window or tab >>The ferromagnetic transition and domain structure in LiHoF4
2009 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 87, no 2Article in journal (Refereed) Published
Abstract [en]

Using Monte Carlo simulations we verify that the rare-earth compound LiHoF4 is a very good realization of a dipolar Ising model. With only one free parameter our calculations for the magnetization, specific heat and inverse susceptibility match experimental data at a quantitative level in the 0.5-3 kelvin range, including the ferromagnetic transition at 1.53 K. Using parallel tempering methods and reaching system sizes up to 32000 dipoles with periodic boundary conditions, we are able to give evidence of the logarithmic corrections predicted in renormalization group theory. Due to the long range and angular dependence of the dipolar model, sample shape and domains play a crucial role in the ordered state. We consider surface corrections to Griffiths's theorem, which arise infinite macroscopic samples and lead to a theory of magnetic domains. We find a domain wall energy of 0.059 erg/cm(2) and predict that the ground-state domain structure for cylinders with a demagnetization factor N > 0 consists of thin parallel sheets of opposite magnetization, with a width depending on the demagnetization factor.

Keywords
LITHIUM HOLMIUM FLUORIDE, CRITICAL-BEHAVIOR, LOGARITHMIC CORRECTIONS, COMPUTER-SIMULATION, MONTE-CARLO, MAGNET, PHASE, MODEL, CRYSTAL, DIPOLES
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-13943 (URN)10.1209/0295-5075/87/27007 (DOI)000269665400027 ()2-s2.0-79051468890 (Scopus ID)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2017-12-12Bibliographically approved
Henelius, P. & Fishman, R. S. (2008). Hybrid quantum-classical Monte Carlo study of a molecule-based magnet. Physical Review B. Condensed Matter and Materials Physics, 78(21)
Open this publication in new window or tab >>Hybrid quantum-classical Monte Carlo study of a molecule-based magnet
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 21Article in journal (Refereed) Published
Abstract [en]

Using a Monte Carlo (MC) method, we study an effective model for the Fe(II)Fe(III) bimetallic oxalates. Within a hybrid quantum-classical MC algorithm, the Heisenberg S=2 and S-'=5/2 spins on the Fe(II) and Fe(III) sites are updated using a quantum MC loop while the Ising-type orbital angular momenta on the Fe(II) sites are updated using a single-spin classical MC flip. The effective field acting on the orbital angular momenta depends on the quantum state of the system. We find that the mean-field phase diagram for the model is surprisingly robust with respect to fluctuations. In particular, the region displaying two compensation points shifts and shrinks but remains finite.

Keywords
Heisenberg model, iron compounds, magnetic transitions, molecular, magnetism, Monte Carlo methods, quantum theory, honeycomb lattice, mixed-valency, m-iii, oxalate, ferromagnetism, magnetization, ferrimagnets, cations, spin-2, system
Identifiers
urn:nbn:se:kth:diva-18084 (URN)10.1103/PhysRevB.78.214405 (DOI)000262244400050 ()2-s2.0-57649203001 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
Biltmo, A. & Henelius, P. (2008). Low-temperature properties of the dilute dipolar magnet LiHoxY1-xF4. Physical Review B. Condensed Matter and Materials Physics, 78(5)
Open this publication in new window or tab >>Low-temperature properties of the dilute dipolar magnet LiHoxY1-xF4
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 5Article in journal (Refereed) Published
Abstract [en]

We analyze recent experiments on the dilute rare-earth compound LiHoxY1-xF4 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, X similar to 1 / (T- T-cw), with a Curie-Weiss temperature that scales with dilution, T-cw similar to x, consistent with early experiments. We also find that the peak in the specific heat scales linearly with dilution, C-max(T)similar to x, in disagreement with recent experiments. This difference could be caused by the hyperfine interaction which is not included in our calculation. Experimental studies do not reach a consensus on the functional form of the susceptibility and specific heat, and in particular, we do not see reported scalings of the form X similar to T-0.75 and X similar to exp(-T/T-0). Furthermore, we calculate the ground-state magnetization as a function of dilution and re-examine the phase diagram around the critical dilution x, = 0.24 +/- 0.03. We find that the spin-glass susceptibility for the Ising model does not diverge below x, while some recent experiments give strong evidence for a stable spin-glass phase in LiHo0.167Y0.833F4.

Keywords
SPIN-GLASSES
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-13942 (URN)10.1103/PhysRevB.78.054437 (DOI)000259368200093 ()2-s2.0-50449103708 (Scopus ID)
Note
QC 20100705Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2017-12-12Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-3228-2826

Search in DiVA

Show all publications