Jens Jensen
Research Subjects
Research Interests
Jens Jensen has a total of 202 co-authors. Here are the top co-authors by number of shared publications:
- Keith A. McEwen
University College London
12 shared publications - Per Hedegård
University of Copenhagen Jagtvej 155A
7 shared publications - Henrik Moodysson Rønnow
École Polytechnique Fédérale de Lausanne (EPFL)
6 shared publications - Niels Bech Christensen
Technical University of Denmark
6 shared publications A. R. Mackintosh
Niels Bohr Institute, O/rsted Laboratory, Universitetsparken 5, DK-2100 Copenhagen, Denmark
5 shared publications- Desmond F. McMorrow
London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London, UK
5 shared publications
Latest Publications
- Open Access
- 6 February 2026
- Published by American Physical Society (APS) in Physical Review X
- Vol. 16 (1)
Inducing Ferroquadrupolar Order with Applied Magnetic Field in TbPO 4
In this work, we demonstrate a remarkable field-induced ferroquadrupolar phase emanating from the magnetoelastic coupling. Using synchrotron x-ray and neutron diffraction, heat capacity measurements, as well as mean-field modeling, we show that above the critical magnetic field, ferroquadrupolar O 2 − 2 order is stabilized as the main order parameter, in competition with the field-induced magnetic polarization. This is revealed by a severe lattice distortion and the suppression of antiferromagnetic ordering of the dipole moments. Even in zero magnetic field, the previously reported antiferromagnetic order is a mixed quadrupolar-dipolar phase, further highlighting the crucial role of spin-lattice interactions. This complete understanding of the magnetoelastic phase diagram establishes TbPO 4 as a model system for studying nematic-antiferromagnetic transitions and provides key Hamiltonian parameters for its description.
- 6 February 2026
- Published by American Physical Society (APS) in Physical Review X
- Vol. 16 (1)
Open AccessInducing Ferroquadrupolar Order with Applied Magnetic Field in TbPO 4
In this work, we demonstrate a remarkable field-induced ferroquadrupolar phase emanating from the magnetoelastic coupling. Using synchrotron x-ray and neutron diffraction, heat capacity measurements, as well as mean-field modeling, we show that above the critical magnetic field, ferroquadrupolar O 2 − 2 order is stabilized as the main order parameter, in competition with the field-induced magnetic polarization. This is revealed by a severe lattice distortion and the suppression of antiferromagnetic ordering of the dipole moments. Even in zero magnetic field, the previously reported antiferromagnetic order is a mixed quadrupolar-dipolar phase, further highlighting the crucial role of spin-lattice interactions. This complete understanding of the magnetoelastic phase diagram establishes TbPO 4 as a model system for studying nematic-antiferromagnetic transitions and provides key Hamiltonian parameters for its description.
- 15 June 2018
- Published by American Physical Society (APS) in Physical Review B
- Vol. 97 (22) , 224417
Magnetoelastic phase diagram ofThe magnetic phase diagram of the quaternary borocarbide is investigated by direct means and by studying magnetically induced modifications of the crystal structure. Detailed superconducting quantum interference device measurements reveal a complex phase diagram with five distinct magnetic phases. The phase boundaries are mapped out comprehensively. Synchrotron hard x-ray measurements in applied magnetic fields are employed to probe the magnetoelastic distortions throughout the phase diagram. The determination of the wave vectors of these field-induced lattice deformations suggests a range of commensurate spin-slip-type magnetic structures at low temperatures with wave vectors of the form with and . The proposed magnetic structures yield values of magnetization well in-line with observations. The scattering intensity due to the magnetoelastic deformations exhibits a drastic jump at the phase boundary at 1.3 T and low temperatures.- 15 June 2018
- Published by American Physical Society (APS) in Physical Review B
- Vol. 97 (22) , 224417
Magnetoelastic phase diagram ofThe magnetic phase diagram of the quaternary borocarbide is investigated by direct means and by studying magnetically induced modifications of the crystal structure. Detailed superconducting quantum interference device measurements reveal a complex phase diagram with five distinct magnetic phases. The phase boundaries are mapped out comprehensively. Synchrotron hard x-ray measurements in applied magnetic fields are employed to probe the magnetoelastic distortions throughout the phase diagram. The determination of the wave vectors of these field-induced lattice deformations suggests a range of commensurate spin-slip-type magnetic structures at low temperatures with wave vectors of the form with and . The proposed magnetic structures yield values of magnetization well in-line with observations. The scattering intensity due to the magnetoelastic deformations exhibits a drastic jump at the phase boundary at 1.3 T and low temperatures.- 22 January 2018
- Published by American Physical Society (APS) in Physical Review B
- Vol. 97 (2) , 024423
Comprehensive cluster-theory analysis of the magnetic structures and excitations inThe magnetic properties of are analyzed in the mean-field/random-phase approximation using a basis of clusters with four spins along the -axis chains of Co ions. The model gives a unifying account of the bulk properties, the spin waves, and the higher-order cluster-spin excitations. The theory describes accurately the neutron scattering measurements of the excited doublet of the ions in both the antiferromagnetic and the paramagnetic phases. The theory has been applied by Larsen et al. [ Phys. Rev. B 96, 174424 (2017)] for analyzing the quantum phase transition at a transverse field of 160 kOe and is found to agree closely with their observations.- 22 January 2018
- Published by American Physical Society (APS) in Physical Review B
- Vol. 97 (2) , 024423
Comprehensive cluster-theory analysis of the magnetic structures and excitations inThe magnetic properties of are analyzed in the mean-field/random-phase approximation using a basis of clusters with four spins along the -axis chains of Co ions. The model gives a unifying account of the bulk properties, the spin waves, and the higher-order cluster-spin excitations. The theory describes accurately the neutron scattering measurements of the excited doublet of the ions in both the antiferromagnetic and the paramagnetic phases. The theory has been applied by Larsen et al. [ Phys. Rev. B 96, 174424 (2017)] for analyzing the quantum phase transition at a transverse field of 160 kOe and is found to agree closely with their observations.- Open Access
- 20 November 2017
- Published by American Physical Society (APS) in Physical Review B
- Vol. 96 (17) , 174424
Spin excitations and quantum criticality in the quasi-one-dimensional Ising-like ferromagnetin a transverse fieldWe present experimental evidence for a quantum phase transition in the easy-axis anisotropic quasi-one-dimensional ferromagnet in a transverse field. Elastic neutron scattering shows that the magnetic order parameter vanishes at a transverse critical field (4) T, while inelastic neutron scattering shows that the gap in the magnetic excitation spectrum vanishes at the same field value, and reopens for . The field dependence of the order parameter and the gap are well described by critical exponents and close to , implying that the quantum phase transition in differs significantly from the textbook version of a Ising chain in a transverse field. We attribute the difference to weak but finite three-dimensionality of the magnetic interactions.- 20 November 2017
- Published by American Physical Society (APS) in Physical Review B
- Vol. 96 (17) , 174424
Open AccessSpin excitations and quantum criticality in the quasi-one-dimensional Ising-like ferromagnetin a transverse fieldWe present experimental evidence for a quantum phase transition in the easy-axis anisotropic quasi-one-dimensional ferromagnet in a transverse field. Elastic neutron scattering shows that the magnetic order parameter vanishes at a transverse critical field (4) T, while inelastic neutron scattering shows that the gap in the magnetic excitation spectrum vanishes at the same field value, and reopens for . The field dependence of the order parameter and the gap are well described by critical exponents and close to , implying that the quantum phase transition in differs significantly from the textbook version of a Ising chain in a transverse field. We attribute the difference to weak but finite three-dimensionality of the magnetic interactions. - Open Access
- 21 February 2017
- Published by American Physical Society (APS) in Physical Review B
- Vol. 95 (6) , 064421
Field-induced reentrant magnetoelectric phase inUsing pulsed magnetic fields up to we have measured the bulk magnetization and electrical polarization of and have studied its magnetic structure by time-of-flight neutron Laue diffraction. Our data establish the existence of a reentrant magnetoelectric phase between and . We show that a magnetized version of the zero field commensurate structure explains the magnetoelectric response quantitatively. The stability of this structure suggests a field-dependent spin anisotropy. Above , a magnetoelectrically inactive, short-wavelength incommensurate structure is identified. Our results demonstrate the combination of pulsed fields with epithermal neutron Laue diffraction as a powerful method to probe even complex phase diagrams in strong magnetic fields.- 21 February 2017
- Published by American Physical Society (APS) in Physical Review B
- Vol. 95 (6) , 064421
Open AccessField-induced reentrant magnetoelectric phase inUsing pulsed magnetic fields up to we have measured the bulk magnetization and electrical polarization of and have studied its magnetic structure by time-of-flight neutron Laue diffraction. Our data establish the existence of a reentrant magnetoelectric phase between and . We show that a magnetized version of the zero field commensurate structure explains the magnetoelectric response quantitatively. The stability of this structure suggests a field-dependent spin anisotropy. Above , a magnetoelectrically inactive, short-wavelength incommensurate structure is identified. Our results demonstrate the combination of pulsed fields with epithermal neutron Laue diffraction as a powerful method to probe even complex phase diagrams in strong magnetic fields. - 28 November 2016
- Published by American Physical Society (APS) in Physical Review B
- Vol. 94 (17) , 174443
Phase diagram of diluted Ising ferromagnetWe present a systematic study of the phase diagram of () Ising ferromagnets obtained from neutron scattering measurements and mean-field calculations. We show that while the thermal phase transition decreases linearly with dilution, as predicted by mean-field theory, the critical transverse field at the quantum critical point is suppressed much faster. This behavior is related to competition between off-diagonal dipolar coupling and quantum fluctuations that are tuned by doping and applied field, respectively. In this paper, we quantify the deviation of the experimental results from mean-field predictions, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description.- 28 November 2016
- Published by American Physical Society (APS) in Physical Review B
- Vol. 94 (17) , 174443
Phase diagram of diluted Ising ferromagnetWe present a systematic study of the phase diagram of () Ising ferromagnets obtained from neutron scattering measurements and mean-field calculations. We show that while the thermal phase transition decreases linearly with dilution, as predicted by mean-field theory, the critical transverse field at the quantum critical point is suppressed much faster. This behavior is related to competition between off-diagonal dipolar coupling and quantum fluctuations that are tuned by doping and applied field, respectively. In this paper, we quantify the deviation of the experimental results from mean-field predictions, with the aim that this analysis can be used in future theoretical efforts towards a quantitative description.- Open Access
- 27 November 2015
- Published by IOP Publishing in Journal of Physics: Conference Series
- Vol. 663 (1) , 012010
Low-energy spin-wave excitations in amplitude-modulated magnetic structure of PrNi2Si2
Inelastic neutron scattering (INS) experiments and random phase approximation calculations have been used to investigate the low-energy spin-wave excitations in PrNi2Si2. The modulated magnitude of the ordered magnetic moments of Pr 3+ ions implies that the associate, longitudinally polarized magnetic excitations are more intense and dispersive than the usual transverse spin waves. Within the random phase approximation the results are in good overall agreement with the predictions made by the model determined previously from the paramagnetic excitations. The most unusual observation is the well-defined amplitude mode detected close to the magnetic Bragg point existing simultaneously with the phason mode. At low energies, an extra mode is observed to hybridize with the magnetic phasons in the neighborhood of the magnetic Brillouin zone center. A magnetoelastic interaction between the magnetic excitations and the longitudinal phonons is able to explain part of the disturbances, but it is concluded that the extra mode must be of some other, unknown origin.
- 27 November 2015
- Published by IOP Publishing in Journal of Physics: Conference Series
- Vol. 663 (1) , 012010
Open AccessLow-energy spin-wave excitations in amplitude-modulated magnetic structure of PrNi2Si2
Inelastic neutron scattering (INS) experiments and random phase approximation calculations have been used to investigate the low-energy spin-wave excitations in PrNi2Si2. The modulated magnitude of the ordered magnetic moments of Pr 3+ ions implies that the associate, longitudinally polarized magnetic excitations are more intense and dispersive than the usual transverse spin waves. Within the random phase approximation the results are in good overall agreement with the predictions made by the model determined previously from the paramagnetic excitations. The most unusual observation is the well-defined amplitude mode detected close to the magnetic Bragg point existing simultaneously with the phason mode. At low energies, an extra mode is observed to hybridize with the magnetic phasons in the neighborhood of the magnetic Brillouin zone center. A magnetoelastic interaction between the magnetic excitations and the longitudinal phonons is able to explain part of the disturbances, but it is concluded that the extra mode must be of some other, unknown origin.
- 23 October 2015
- Published by American Physical Society (APS) in Physical Review B
- Vol. 92 (14) , 144422
Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism in(, Ho, Er, Tm, and Yb)We present a systematic study of the crystal-field interactions in the (, Ho, Er, Tm, and Yb) family of rare-earth magnets. Using detailed inelastic neutron scattering measurements, we have been able to quantify the transition energies and wave functions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to line-shape broadening in and level splitting in . Furthermore, using high-resolution neutron scattering from , we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.- 23 October 2015
- Published by American Physical Society (APS) in Physical Review B
- Vol. 92 (14) , 144422
Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism in(, Ho, Er, Tm, and Yb)We present a systematic study of the crystal-field interactions in the (, Ho, Er, Tm, and Yb) family of rare-earth magnets. Using detailed inelastic neutron scattering measurements, we have been able to quantify the transition energies and wave functions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to line-shape broadening in and level splitting in . Furthermore, using high-resolution neutron scattering from , we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.- 11 April 2014
- Published by American Physical Society (APS) in Physical Review B
- Vol. 89 (13) , 134407
Lineshape of the singlet-triplet excitations in the dimer system SrCrOto first order in the high-densityexpansionThe Cr ions in SrCrO constitute a strongly correlated spin-1/2 dimer system. Experiments show that the collective singlet-triplet excitations in this system are well defined in the zero-temperature limit, but, when heated, the inelastic neutron scattering peaks decrease rapidly in intensity and acquire a nonzero line width. When including the fluctuations to leading order in , where is the coordination number, the diagrammatic high-density expansion is found to offer an accurate description of the singlet-triplet excitations. The theory explains not only the temperature dependencies of the intensities and line widths, but also that strong correlation causes the lineshapes to become asymmetric at temperatures comparable to the excitation energies.- 11 April 2014
- Published by American Physical Society (APS) in Physical Review B
- Vol. 89 (13) , 134407
Lineshape of the singlet-triplet excitations in the dimer system SrCrOto first order in the high-densityexpansionThe Cr ions in SrCrO constitute a strongly correlated spin-1/2 dimer system. Experiments show that the collective singlet-triplet excitations in this system are well defined in the zero-temperature limit, but, when heated, the inelastic neutron scattering peaks decrease rapidly in intensity and acquire a nonzero line width. When including the fluctuations to leading order in , where is the coordination number, the diagrammatic high-density expansion is found to offer an accurate description of the singlet-triplet excitations. The theory explains not only the temperature dependencies of the intensities and line widths, but also that strong correlation causes the lineshapes to become asymmetric at temperatures comparable to the excitation energies.- 14 August 2013
- Published by American Physical Society (APS) in Physical Review B
- Vol. 88 (5) , 054413
Magnetic ordering in GdNiBC revisited by resonant x-ray scattering: Evidence for the double-modelRecent theoretical efforts aimed at understanding the nature of antiferromagnetic ordering in GdNiBC predicted double- ordering. Here we employ resonant elastic x-ray scattering to test this theory against the formerly proposed, single- ordering scenario. Our study reveals a satellite reflection associated with a mixed-order component propagation wave vector, viz., () with reciprocal lattice units, the presence of which is incompatible with single- ordering but is expected from the double- model. A () wave vector (i.e., third-order) satellite is also observed, again in line with the double- model. The temperature dependencies of these along with that of a first-order satellite are compared with calculations based on the double- model and reasonable qualitative agreement is found. By examining the azimuthal dependence of first-order satellite scattering, we show the magnetic order to be, as predicted, elliptically polarized at base temperature and find the temperature dependence of the “out of - plane” moment component to be in fairly good agreement with calculation. Our results provide qualitative support for the double- model and thus in turn corroborate the explanation for the “magnetoelastic paradox” offered by this model.- 14 August 2013
- Published by American Physical Society (APS) in Physical Review B
- Vol. 88 (5) , 054413
Magnetic ordering in GdNiBC revisited by resonant x-ray scattering: Evidence for the double-modelRecent theoretical efforts aimed at understanding the nature of antiferromagnetic ordering in GdNiBC predicted double- ordering. Here we employ resonant elastic x-ray scattering to test this theory against the formerly proposed, single- ordering scenario. Our study reveals a satellite reflection associated with a mixed-order component propagation wave vector, viz., () with reciprocal lattice units, the presence of which is incompatible with single- ordering but is expected from the double- model. A () wave vector (i.e., third-order) satellite is also observed, again in line with the double- model. The temperature dependencies of these along with that of a first-order satellite are compared with calculations based on the double- model and reasonable qualitative agreement is found. By examining the azimuthal dependence of first-order satellite scattering, we show the magnetic order to be, as predicted, elliptically polarized at base temperature and find the temperature dependence of the “out of - plane” moment component to be in fairly good agreement with calculation. Our results provide qualitative support for the double- model and thus in turn corroborate the explanation for the “magnetoelastic paradox” offered by this model.