Periodic Reporting for period 1 - PRESSMAG (The pressure effects on magnetic properties of an ideal quasi-2D planar antiferromagnet BaNi2V2O8 at the base and finite temperatures)
Reporting period: 2022-05-15 to 2024-05-14
Recent years remarkable progress was achieved in fundamental understanding of quantum phenomena in condensed matter physics. A wide range of exotic magnetic excitations and unconventional states of matter have been theoretically predicted and then experimentally observed in materials which are found to be physical realization for theoretical models. However, the experimental studies of many intriguing phenomena are still limited by lack of the ideal physical realizations for certain physical models. Therefore, searching for a possibility to control and change physical properties of the materials in the demanded way is an important scientific task.
Recent studies of the effects of pressure on the magnetic properties of solid-state magnets reveal that the external pressure can change the magnetic exchange interactions and induce quantum phase transitions giving experimental access to novel states of matter. This means that pressure can be used as a tool to customize the magnetic properties of solid-state compounds by changing the underlying Hamiltonian, potentially generating new and unconventional phenomena.
The PRESSMAG project is dedicated to exploring the hydrostatic pressure effects on the magnetic structure, Hamiltonian and critical behaviour of novel quasi-two-dimensional planar antiferromagnet BaNi2V2O8 at base and finite temperatures where the signatures for the rare Berezinskii-Kosterlitz-Thouless (BKT) criticality have been recently experimentally observed at ambient conditions.
In the conventional picture the long-rage magnetic order is ruled out by the Mermin-Wagner theorem in two-dimensional planar (2D XY) magnets at finite temperatures. However, Kosterlitz and Thouless, independently with Berezinskii, predicted a topological phase transition in 2D XY systems from disordered to quasi-ordered state at finite transition temperature TBKT brought by the pairing of the topological defects, spin-vortices, into spin-vortex/antivortex pairs. Although initially the BKT phenomena was expected only in 2D XY model, later theoretical studies suggest that it can also appear in quasi- two-dimensional magnets with planar anisotropy extending the class of compounds where BKT behaviour can be experimentally observed. This attracts strong attention to the search and investigation of the physical realizations of such systems.
Recent studies found BaNi2V2O8 to be an ideal physical realization for the quasi-two-dimensional magnet. Despite conventional long range magnetic order below TN, this compound exhibits magnetic properties of various 2D models over a wide temperature range at ambient pressure. In particular, BaNi2V2O8 behaves as 2D XY, 2D XXZ or 2D Haeisenberg magnet depending on temperature regime. In the planar 2D XY regime at ambient pressure BaNi2V2O8 reveals signatures of Berezinskii – Kosterlitz – Thousless criticality as seen experimentally and confirmed by theoretical computations. Because the estimated T_BKT is lower than TN, the quasi-ordered state of spin-vortex/antivortex-pairs in BaNi2V2O8 is hidden by conventional long range magnetic so that only signatures for the decomposed pairs are experimentally observed just above TN in ambient pressure.
The aim of PRESSMAG project is to study the effects of hydrostatic pressure on the overall magnetic properties such as Hamiltonian, magnetic structure and critical behaviour of model quasi-2D magnetic system BaNi2V2O8 and probing the hydrostatic pressure as a tool to customize TN, TBKT and planar 2D XY regime in BaNi2V2O8 making BKT phenomena more experimentally accessible. The results of PRESSMAG project are expected to be general and can be applied for the other quasi-2D magnets with similar Hamiltonian.
Recent studies of the effects of pressure on the magnetic properties of solid-state magnets reveal that the external pressure can change the magnetic exchange interactions and induce quantum phase transitions giving experimental access to novel states of matter. This means that pressure can be used as a tool to customize the magnetic properties of solid-state compounds by changing the underlying Hamiltonian, potentially generating new and unconventional phenomena.
The PRESSMAG project is dedicated to exploring the hydrostatic pressure effects on the magnetic structure, Hamiltonian and critical behaviour of novel quasi-two-dimensional planar antiferromagnet BaNi2V2O8 at base and finite temperatures where the signatures for the rare Berezinskii-Kosterlitz-Thouless (BKT) criticality have been recently experimentally observed at ambient conditions.
In the conventional picture the long-rage magnetic order is ruled out by the Mermin-Wagner theorem in two-dimensional planar (2D XY) magnets at finite temperatures. However, Kosterlitz and Thouless, independently with Berezinskii, predicted a topological phase transition in 2D XY systems from disordered to quasi-ordered state at finite transition temperature TBKT brought by the pairing of the topological defects, spin-vortices, into spin-vortex/antivortex pairs. Although initially the BKT phenomena was expected only in 2D XY model, later theoretical studies suggest that it can also appear in quasi- two-dimensional magnets with planar anisotropy extending the class of compounds where BKT behaviour can be experimentally observed. This attracts strong attention to the search and investigation of the physical realizations of such systems.
Recent studies found BaNi2V2O8 to be an ideal physical realization for the quasi-two-dimensional magnet. Despite conventional long range magnetic order below TN, this compound exhibits magnetic properties of various 2D models over a wide temperature range at ambient pressure. In particular, BaNi2V2O8 behaves as 2D XY, 2D XXZ or 2D Haeisenberg magnet depending on temperature regime. In the planar 2D XY regime at ambient pressure BaNi2V2O8 reveals signatures of Berezinskii – Kosterlitz – Thousless criticality as seen experimentally and confirmed by theoretical computations. Because the estimated T_BKT is lower than TN, the quasi-ordered state of spin-vortex/antivortex-pairs in BaNi2V2O8 is hidden by conventional long range magnetic so that only signatures for the decomposed pairs are experimentally observed just above TN in ambient pressure.
The aim of PRESSMAG project is to study the effects of hydrostatic pressure on the overall magnetic properties such as Hamiltonian, magnetic structure and critical behaviour of model quasi-2D magnetic system BaNi2V2O8 and probing the hydrostatic pressure as a tool to customize TN, TBKT and planar 2D XY regime in BaNi2V2O8 making BKT phenomena more experimentally accessible. The results of PRESSMAG project are expected to be general and can be applied for the other quasi-2D magnets with similar Hamiltonian.
To explore the effects of hydrostatic pressure on the magnetic properties of the quasi-two-dimensional planar magnet BaNi2V2O8 the series of the high-pressure experiments have been performed on the powdered and single-crystalline samples of BaNi2V2O8. The powdered sample of BaNi2V2O8 was synthesized by the researcher in the beginning of the project while the single crystalline samples of BaNi2V2O8 were already available to the start of the project.
First, the thermodynamic measurements of the single crystalline sample of BaNi2V2O8 have been done under several applied hydrostatic pressures to get overview of the pressure effects on the magnetic properties of compound. The single crystal magnetic susceptibility data of BaNi2V2O8 were collected on BaNi2V2O8 under applied pressures of 3, 5.6 ,5.8 and 8 kbar and at the magnetic field of 1 T applied parallel to the c - crystallographic axis over the temperature range 2K -200 K. The results reveal that (1) pressure above 5.6 kbar shifts characteristics minimum Txy - temperature of the crossover to the 2D XY planar regime, towards to higher temperatures. The shift ΔTxy is found to be 4 K which is independent on pressure within the pressure range from 5.6 – 8 kbar. The shift of the characteristic minimum Txy to higher temperatures indicates that pressure above 5.6 kbar is increasing the planar anisotropy term in the Hamiltonian of BaNi2V2O8.
To explore the effects of pressure on the Hamiltonian of BaNi2V2O8 the series of the single crystal high-pressure inelastic neutron scattering experiments have been done at neutron large scale facilities where time for the measurements were obtained on the competitive ground. In particular, the measurements of the magnetic excitation spectra of BaNi2V2O8 under applied pressure have been done using the TASP and EIGER spectrometers at Paul Scherrer Institute (PSI), LET spectrometer at the ISIS Neutron and Muon Source (ISIS), IN20 spectrometer at Institute Laue–Langevin (ILL) where both clamp and gas pressure cell were used.
The results reveal that hydrostatic pressure noticeable effects the Hamiltonian of BaNi2V2O8 leading to (1) increase of both easy-axis and planar anisotropy terms which was observed experimentally via the increase of the energy gaps in the magnetic excitation spectra of BaNi2V2O8 under applied pressure. In particular, the applied hydrostatic pressure of 9 kbar shifts the low energy gap on 1.5 meV towards higher energies and high-energy gap on 0.3-0.4 meV towards higher energies. The raising of these energy gaps indicates the increase of the easy-axis and planar (XY) anisotropies terms in the Hamiltonian of BaNi2V2O8. (2) The pressure consistently affects the magnetic exchange couplings so that the pressure of 6.2 kbar leads to an increase of the second neighbor interaction and further increasing pressure to 9 kbar results in a 0.5-0.6 meV increase of the dominant first-neighbor magnetic exchange interaction. These changes were observed experimentally via the shifts of top of the energy dispersion. It is important to note that all inelastic neutron scattering measurements under applied pressures faced challenge of long counting time due to the low transmission and high background of the pressure cell. The counting time varied depending on the particular experiment, but in some experiments were 8 times longer than the counting time for similar data points at the ambient conditions. These conditions limited the amount of inelastic neutron scattering data which were collected during the experiments.
To explore the pressure effects on the magnetic structure and critical behaviour of BaNi2V2O8 the elastic neutron scattering measurement and muon-spin rotation measurements of BaNi2V2O8 have been done under applied pressure using WISH diffractometer at ISIS, D10 diffractometer at ILL, TASP spectrometer at PSI and GPD muon instrument at PSI. The results reveal that the magnetic structure of BaNi2V2O8 under applied pressure of 3 GPa remains the same with one at the ambient conditions. This result shows that pressure does not induce structural or magnetic phase transition in BaNi2V2O8. The transition temperature TN to the long range magneticaly ordered state was found to be TN=49 K at 7.3 kbar and TN= 50K at 9 kbar, revealing only slightly shift with respect to the TN=47.75 K at ambient conditions.
First, the thermodynamic measurements of the single crystalline sample of BaNi2V2O8 have been done under several applied hydrostatic pressures to get overview of the pressure effects on the magnetic properties of compound. The single crystal magnetic susceptibility data of BaNi2V2O8 were collected on BaNi2V2O8 under applied pressures of 3, 5.6 ,5.8 and 8 kbar and at the magnetic field of 1 T applied parallel to the c - crystallographic axis over the temperature range 2K -200 K. The results reveal that (1) pressure above 5.6 kbar shifts characteristics minimum Txy - temperature of the crossover to the 2D XY planar regime, towards to higher temperatures. The shift ΔTxy is found to be 4 K which is independent on pressure within the pressure range from 5.6 – 8 kbar. The shift of the characteristic minimum Txy to higher temperatures indicates that pressure above 5.6 kbar is increasing the planar anisotropy term in the Hamiltonian of BaNi2V2O8.
To explore the effects of pressure on the Hamiltonian of BaNi2V2O8 the series of the single crystal high-pressure inelastic neutron scattering experiments have been done at neutron large scale facilities where time for the measurements were obtained on the competitive ground. In particular, the measurements of the magnetic excitation spectra of BaNi2V2O8 under applied pressure have been done using the TASP and EIGER spectrometers at Paul Scherrer Institute (PSI), LET spectrometer at the ISIS Neutron and Muon Source (ISIS), IN20 spectrometer at Institute Laue–Langevin (ILL) where both clamp and gas pressure cell were used.
The results reveal that hydrostatic pressure noticeable effects the Hamiltonian of BaNi2V2O8 leading to (1) increase of both easy-axis and planar anisotropy terms which was observed experimentally via the increase of the energy gaps in the magnetic excitation spectra of BaNi2V2O8 under applied pressure. In particular, the applied hydrostatic pressure of 9 kbar shifts the low energy gap on 1.5 meV towards higher energies and high-energy gap on 0.3-0.4 meV towards higher energies. The raising of these energy gaps indicates the increase of the easy-axis and planar (XY) anisotropies terms in the Hamiltonian of BaNi2V2O8. (2) The pressure consistently affects the magnetic exchange couplings so that the pressure of 6.2 kbar leads to an increase of the second neighbor interaction and further increasing pressure to 9 kbar results in a 0.5-0.6 meV increase of the dominant first-neighbor magnetic exchange interaction. These changes were observed experimentally via the shifts of top of the energy dispersion. It is important to note that all inelastic neutron scattering measurements under applied pressures faced challenge of long counting time due to the low transmission and high background of the pressure cell. The counting time varied depending on the particular experiment, but in some experiments were 8 times longer than the counting time for similar data points at the ambient conditions. These conditions limited the amount of inelastic neutron scattering data which were collected during the experiments.
To explore the pressure effects on the magnetic structure and critical behaviour of BaNi2V2O8 the elastic neutron scattering measurement and muon-spin rotation measurements of BaNi2V2O8 have been done under applied pressure using WISH diffractometer at ISIS, D10 diffractometer at ILL, TASP spectrometer at PSI and GPD muon instrument at PSI. The results reveal that the magnetic structure of BaNi2V2O8 under applied pressure of 3 GPa remains the same with one at the ambient conditions. This result shows that pressure does not induce structural or magnetic phase transition in BaNi2V2O8. The transition temperature TN to the long range magneticaly ordered state was found to be TN=49 K at 7.3 kbar and TN= 50K at 9 kbar, revealing only slightly shift with respect to the TN=47.75 K at ambient conditions.
The PRESSMAG project revealed that relatively low hydrostatic pressure (below 1 GPa) can efficiently affects the Hamiltonian of the quasi -2D magnetic system without inducing structural or magnetic phase transitions. This encourages further investigations of pressure as a tool to access and modify the Hamiltonian of model magnetic compounds.
Moreover, PRESSMAG project proves the feasibility of high- pressure inelastic neutrons scattering experiments using various instruments, including instruments on the spallation neutron sources, which are characterized by lower flux with comparison to the reactor sources and performing measurements on the relatively small samples. This extends the range of projects and systems which can be studied under applied hydrostatic pressure using neutron scattering since the feasibility is a key issue in the high-pressure experiments. In addition, the PRESSMAG project is an unique example where the series of neutron scattering measurements have been done at almost the same pressure conditions using different instruments including combination inelastic and elastic neutron scattering measurements. These factors make results of PRESSMAG project valuable from both scientific and technical/methodological sides.
Moreover, PRESSMAG project proves the feasibility of high- pressure inelastic neutrons scattering experiments using various instruments, including instruments on the spallation neutron sources, which are characterized by lower flux with comparison to the reactor sources and performing measurements on the relatively small samples. This extends the range of projects and systems which can be studied under applied hydrostatic pressure using neutron scattering since the feasibility is a key issue in the high-pressure experiments. In addition, the PRESSMAG project is an unique example where the series of neutron scattering measurements have been done at almost the same pressure conditions using different instruments including combination inelastic and elastic neutron scattering measurements. These factors make results of PRESSMAG project valuable from both scientific and technical/methodological sides.