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Hartmann, Ola
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Publications (10 of 25) Show all publications
Huang, W., Mooij, L. P. A., Droulias, S. A., Palonen, H., Hartmann, O., Pálsson, G. K., . . . Hjörvarsson, B. (2017). Concentration dependence of hydrogen diffusion in clamped vanadium (001) films. Journal of Physics: Condensed Matter, 29(4), Article ID 045402.
Open this publication in new window or tab >>Concentration dependence of hydrogen diffusion in clamped vanadium (001) films
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 4, article id 045402Article in journal (Refereed) Published
Abstract [en]

The chemical diffusion coefficient of hydrogen in a 50 nm thin film of vanadium (0 0 1) is measured as a function of concentration and temperature, well above the known phase boundaries. Arrhenius analysis of the tracer diffusion constants reveal large changes in the activation energy with concentration: from 0.10 at 0.05 in II V-1 to 0.5 eV at 0.2 in II V-1. The results are consistent with a change from tetrahedral to octahedral site occupancy, in that concentration range. The change in site occupancy is argued to be caused by the uniaxial expansion of the film originating from the combined hydrogen induced expansion and the clamping of the film to the substrate.

Keywords
diffusion, hydrogen, single crystal
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-312024 (URN)10.1088/1361-648X/29/4/045402 (DOI)000389233200001 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2017-01-05 Created: 2017-01-04 Last updated: 2018-09-14Bibliographically approved
Huang, W., Palonen, H., Droulias, S. A., Hartmann, O., Wolff, M. & Hjörvarsson, B. (2017). Diffusion of hydrogen in ultra-thin V(001) layers. Journal of Alloys and Compounds, 723, 484-487
Open this publication in new window or tab >>Diffusion of hydrogen in ultra-thin V(001) layers
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2017 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 723, p. 484-487Article in journal (Refereed) Published
Abstract [en]

We report on investigations of the influence of one dimensional confinement on the diffusion of hydrogen, in the low concentration limit (alpha-phase). The confinement is obtained by utilising single crystal Fe/V(001) superlattices, in which hydrogen preferably resides in the V layers. The diffusion along the [110] direction in the V(001) layers can thereby be determined. Activation energy and attempt jump rates are extracted from an Arrhenius analysis. No effects are observed from the confinement on the hydrogen diffusion in the thickness range 7-28 monolayers (approximate to 1.1-4.2 nm) of V(001).

Keywords
Confinement, Optical transmission, Diffusion, Interstitials
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-333734 (URN)10.1016/j.jallcom.2017.06.194 (DOI)000407009400062 ()
Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2018-09-14Bibliographically approved
Huang, W., Pálsson, G. K., Brischetto, M., Droulias, S. A., Hartmann, O., Wolff, M. & Hjörvarsson, B. (2017). Experimental observation of hysteresis in a coherent metal-hydride phase transition. Journal of Physics: Condensed Matter, 29(49), Article ID 495701.
Open this publication in new window or tab >>Experimental observation of hysteresis in a coherent metal-hydride phase transition
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 49, article id 495701Article in journal (Refereed) Published
Abstract [en]

We investigate the hysteresis obtained in the hydrogen absorption and desorption cycle for a single crystal Pd/V-28 [Fe-4/V-28](11) superlattice. Below the critical temperature, a small but clear hysteresis is observed in the pressure-composition isotherms, while it is absent above. The experimental results thereby prove the relevance of macroscopic energy barriers for obtaining hysteresis in coherent structural transformations. The textured Pd layer exhibits substantially larger hysteresis effects, which can be related to an irreversible energy loss caused by defect generation in Pd.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keywords
hysteresis, coherent, incoherent, phase transition
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-342651 (URN)10.1088/1361-648X/aa9696 (DOI)000415854900001 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2018-02-28 Created: 2018-02-28 Last updated: 2018-05-14Bibliographically approved
Huang, W., Pálsson, G. K., Brischetto, M., Palonen, H., Droulias, S. A., Hartmann, O., . . . Hjörvarsson, B. (2017). Finite size effects: deuterium diffusion in nm thick vanadium layers. New Journal of Physics, 19, Article ID 123004.
Open this publication in new window or tab >>Finite size effects: deuterium diffusion in nm thick vanadium layers
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2017 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 19, article id 123004Article in journal (Refereed) Published
Abstract [en]

We investigate the effect of finite size on the chemical diffusion of deuterium in extremely thin V(001) layers. A five fold increase in the diffusion coefficient is observed at concentrations around 0.2 [D/V], when the thickness of the V is decreased from 28 to 14 atomic layers (approximate to 2.1-4.2 nm). The size dependent deuterium-deuterium interaction energy is argued to be the root of the observed changes as the diffusion rates are similar at low concentrations. The results demonstrate the feasibility of using finite-size effects to enhance the chemical diffusion of light interstitials in solids. We discuss the general applicability of these effects to other systems.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
Keywords
superlattices, optical transmission, deuterium diffusion, octahedral sites
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-347665 (URN)10.1088/1367-2630/aa94df (DOI)000424885300004 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2018-04-05 Created: 2018-04-05 Last updated: 2018-05-14Bibliographically approved
Mooij, L., Huang, W., Droulias, S. A., Johansson, R., Hartmann, O., Xin, X., . . . Hjörvarsson, B. (2017). The influence of site occupancy on diffusion of hydrogen in vanadium. Physical Review B, 95(6), Article ID 064310.
Open this publication in new window or tab >>The influence of site occupancy on diffusion of hydrogen in vanadium
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064310Article, review/survey (Refereed) Published
Abstract [en]

We investigate the effect of site occupancy on the chemical diffusion of hydrogen in strained vanadium. The diffusion rate is found to decrease substantially, when hydrogen is occupying octahedral sites as compared to tetrahedral sites. Profound isotope effects are observed when comparing the diffusion rate of H and D. The changes in the diffusion rate are found to be strongly influenced by the changes in the potential energy landscape, as deduced from first-principles molecular dynamics calculations.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-275058 (URN)10.1103/PhysRevB.95.064310 (DOI)000395988800002 ()
Note

The manuscript version of this article is part of two PhD theses: http://uu.diva-portal.org/smash/record.jsf?pid=diva2:900624

http://uu.diva-portal.org/smash/record.jsf?pid=diva2:950756

Available from: 2016-01-28 Created: 2016-01-28 Last updated: 2018-05-14Bibliographically approved
Karlsson, E. B., Hartmann, O., Chatzidimitriou-Dreismann, C. A. & Abdul-Redah, T. (2016). The hydrogen anomaly in neutron Compton scattering: new experiments and a quantitative theoretical explanation. Measurement science and technology, 27(8), Article ID 085501.
Open this publication in new window or tab >>The hydrogen anomaly in neutron Compton scattering: new experiments and a quantitative theoretical explanation
2016 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 27, no 8, article id 085501Article in journal (Refereed) Published
Abstract [en]

No consensus has been reached so far about the hydrogen anomaly problem in Compton scattering of neutrons, although strongly reduced H cross-sections were first reported almost 20 years ago. Over the years, this phenomenon has been observed in many different hydrogen-containing materials. Here, we use yttrium hydrides as test objects, YH2, YH3, YD2 and YD3, Y(HxD1-x)(2) and Y(HxD1-x)(3), for which we observe H anomalies increasing with transferred momentum q. We also observe reduced deuteron cross-sections in YD2 and YD3 and have followed those up to scattering angles of 140 degrees corresponding to high momentum transfers. In addition to data taken using the standard Au-197 foils for neutron energy selection, the present work includes experiments with Rh-103 foils and comparisons were also made with data from different detector setups. The H and D anomalies are discussed in terms of the different models proposed for their interpretation. The 'electron loss model' (which assumes energy transfer to excited electrons) is contradicted by the present data, but it is shown here that exchange effects in scattering from two or more protons (or deuterons) in the presence of large zero-point vibrations, can explain quantitatively the reduction of the cross-sections as well as their q-dependence. Decoherence processes also play an essential role. In a scattering time representation, shake-up processes can be followed on the attosecond scale. The theory also shows that large anomalies can appear only when the neutron coherence lengths (determined by energy selection and detector geometry) are about the same size as the distance between the scatterers.

Keywords
neutron scattering, quantum entanglement, decoherence processes
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-301015 (URN)10.1088/0957-0233/27/8/085501 (DOI)000380124800033 ()
Note

Correction in: Measurement Science and Technology, Volume: 28, Issue: 7, Article Number: 079501. DOI: 10.1088/1361-6501/aa6d91

Available from: 2016-08-17 Created: 2016-08-17 Last updated: 2017-11-24Bibliographically approved
Kalvius, G. M., Hartmann, O., Wäppling, R., Guenther, A., Krimmel, A., Loidl, A., . . . Baines, C. (2014). Magnetism of Pd1-xNix alloys near the critical concentration for ferromagnetism. Physical Review B. Condensed Matter and Materials Physics, 89(6), 064418
Open this publication in new window or tab >>Magnetism of Pd1-xNix alloys near the critical concentration for ferromagnetism
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 6, p. 064418-Article in journal (Refereed) Published
Abstract [en]

We report results of a muon spin rotation and relaxation (mu SR) study of dilute Pd1-xNix alloys, with emphasis on Ni concentrations x = 0.0243 and 0.025. These are close to the critical value x(cr) for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. Additional control data were taken for pure nonmagnetic Pd, and for an alloy where ferromagnetism is well established (x = 0.05). The 2.43 and 2.5 at.% Ni alloys exhibit similar mu SR properties. Both samples are fully magnetic, with average zero-temperature muon local fields < B-loc(T = 0)> = 2.0 and 3.8 mT and Curie temperatures T-C = 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively. The temperature dependence of < B-loc > suggests ordering of Ni spin clusters rather than isolated spins. Just above T-C, the temperature where LRO vanishes, a two-phase region is found with coexisting separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above T-C. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of T-C extrapolate to x(cr) = 0.0236 +/- 0.0027. The dynamic muon spin relaxation in the vicinity of T-C differs for the two samples: a relaxation-rate maximum at T-C is observed for x = 0.0243, reminiscent of critical slowing down, whereas for x = 0.025 no dynamic relaxation is observed within the mu SR time window. The data suggest a mean-field-like transition in this alloy.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-222164 (URN)10.1103/PhysRevB.89.064418 (DOI)000332398500001 ()
Available from: 2014-04-10 Created: 2014-04-08 Last updated: 2017-12-05Bibliographically approved
Hartmann, O., Kalvius, G. M. & Wäppling, R. (2014). Muon spin precession in ferromagnetic DyAl2. In: 13TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE: . Paper presented at 13th International Conference on Muon Spin Rotation, Relaxation and Resonance (MuSR), JUN 01-06, 2014, Grindelwald, SWITZERLAND. , 551, Article ID 012018.
Open this publication in new window or tab >>Muon spin precession in ferromagnetic DyAl2
2014 (English)In: 13TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE, 2014, Vol. 551, article id 012018Conference paper, Published paper (Refereed)
Abstract [en]

The Laves phase intermetallics REAl2 were studied by mu,SR over several decades. Results had mainly been obtained for the temperature and field dependencies of relaxation rates in the paramagnetic state. However, it turned out that spontaneous precession signals in the ordered magnetic state were very difficult to observe. We report here the observation a weak precession signal in a single crystalline sample of DyAl2. The precession signal seen has low asymmetry and is heavily damped. This is understood with the notion that the magnetic structure of DyAl2 splits the internal field at the interstitial muon stopping site into several components, and that of those only one of the field components leads to the visible precession signal. The observed temperature dependence of the precession frequency is compatible with the change of easy axis of magnetization at T = 40K.

Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 551
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-246870 (URN)10.1088/1742-6596/551/1/012018 (DOI)000347715800018 ()
Conference
13th International Conference on Muon Spin Rotation, Relaxation and Resonance (MuSR), JUN 01-06, 2014, Grindelwald, SWITZERLAND
Available from: 2015-03-11 Created: 2015-03-11 Last updated: 2015-03-11Bibliographically approved
Kalvius, G. M., Hartmann, O., Wäppling, R., Noakes, D. R. & Schaefer, W. (2013). A mu SR study of the magnetic properties of Tb2CuIn3. Physica Scripta, 87(5), 055702
Open this publication in new window or tab >>A mu SR study of the magnetic properties of Tb2CuIn3
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2013 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 87, no 5, p. 055702-Article in journal (Refereed) Published
Abstract [en]

Tb2CuIn3 shows exceptional magnetic behavior within the series of R2CuIn3 intermetallics (R denoting one of the rare earth elements), where long-range magnetic order is absent in favor of spin liquid magnetic ground states as a consequence of frustration by competing exchange. In contrast, neutron studies suggested for the magnetic ground state of Tb2CuIn3 the coexistence of long-range antiferromagnetic and short-range order. The present mu SR (muon spin rotation/relaxation) data show that at similar to 45K all Tb magnetic moments freeze into a spin-glass state. At T-N = 33K partial long-range order sets in, its volume fraction being similar to 30%. On cooling to 10 K, the antiferromagnetic fraction reaches the saturation value of 70% and hence remains coexistent with the spin-glass state for T -> 0. The antiferromagnetic state is characterized by considerable short-range spin disorder. At low temperatures the Tb moments in the two coexisting states approach the static limit. This absence of persistent moment fluctuations together with partial formation of long-range order indicates considerably lower frustration in Tb2CuIn3 compared to the rest of the R2CuIn3 compounds.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-200677 (URN)10.1088/0031-8949/87/05/055702 (DOI)000318274400025 ()
Available from: 2013-06-03 Created: 2013-06-03 Last updated: 2017-12-06Bibliographically approved
Hartmann, O., Kalvius, G. M., Wäppling, R., Guenther, A., Tsurkan, V., Krimmel, A. & Loidl, A. (2013). Magnetic properties of the multiferroic chromium thio-spinels CdCr2S4 and HgCr2S4. European Physical Journal B: Condensed Matter Physics, 86(4), 148
Open this publication in new window or tab >>Magnetic properties of the multiferroic chromium thio-spinels CdCr2S4 and HgCr2S4
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2013 (English)In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 86, no 4, p. 148-Article in journal (Refereed) Published
Abstract [en]

Powder samples of CdCr2S4 and HgCr2S4 were investigated with respect to their magnetic behavior using muon spin rotation/relaxation spectroscopy (mu SR). In CdCr2S4, a second order magnetic transition at 83 +/- 1 K is confirmed. No influence on the mu SR magnetic parameters due to the simultaneous ferroelectric transition was detected. In the limit T -> 0, persistent spin fluctuation exists as a consequence of magnetic frustration. The proposed structural transition around 150 K is barely visible in the mu SR spectra, indicating that the change in local symmetry is subtle. In HgCr2S4, we see in zero field at 27 +/- 2 K a second order transition from the paramagnetic into a long-range ordered (LRO) magnetic state. This LRO state is characterized in the mu SR spectra by a spontaneous spin precession signal. Applications of moderate magnetic fields (up to 450 mT) destroy the precession signal in favor of a rapidly decaying exponential signal. It reflects short-range order (SRO) of strongly correlated dynamic spins. The magnitude of the field required to induce this change is dependent on temperature. The observation of an increase of magnetic spin fluctuations with applied field suggests that spin dynamics play a key role in the field induced changes of the magnetic properties.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-201263 (URN)10.1140/epjb/e2013-31094-4 (DOI)000318408100033 ()
Available from: 2013-06-10 Created: 2013-06-10 Last updated: 2017-12-06Bibliographically approved
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