Strong Magnetoelastic Coupling Research Articles

Competition between magnetic interactions and structural instabilities leading to itinerant frustration in the triangular lattice antiferromagnet LiCrSe2

LiCrSe2 constitutes a recent valuable addition to the ensemble of two-dimensional triangular lattice antiferromagnets. In this work, we present a comprehensive study of the low temperature nuclear and magnetic structure established in this material. Being subject to a strong magnetoelastic coupling, LiCrSe2 was found to undergo a first order structural transition from a trigonal crystal system (Pbar{3}m1) to a monoclinic one (C2/m) at Ts = 30 K. Such restructuring of the lattice is accompanied by a magnetic transition at TN = 30 K. Refinement of the magnetic structure with neutron diffraction data and complementary muon spin rotation analysis reveal the presence of a complex incommensurate magnetic structure with a up-up-down-down arrangement of the chromium moments with ferromagnetic double chains coupled antiferromagnetically. The spin axial vector is also modulated both in direction and modulus, resulting in a spin density wave-like order with periodic suppression of the chromium moment along the chains. This behavior is believed to appear as a result of strong competition between direct exchange antiferromagnetic and superexchange ferromagnetic couplings established between both nearest neighbor and next nearest neighbor Cr3+ ions. We finally conjecture that the resulting magnetic order is stabilized via subtle vacancy/charge order within the lithium layers, potentially causing a mix of two co-existing magnetic phases within the sample.

Long-range, non-local switching of spin textures in a frustrated antiferromagnet

Antiferromagnetic spintronics is an emerging area of quantum technologies that leverage the coupling between spin and orbital degrees of freedom in exotic materials. Spin-orbit interactions allow spin or angular momentum to be injected via electrical stimuli to manipulate the spin texture of a material, enabling the storage of information and energy. In general, the physical process is intrinsically local: spin is carried by an electrical current, imparted into the magnetic system, and the spin texture will then rotate in the region of current flow. In this study, we show that spin information can be transported and stored “non-locally" in the material FexNbS2. We propose that collective modes can manipulate the spin texture away from the flowing current, an effect amplified by strong magnetoelastic coupling of the ordered state. This suggests a novel way to store and transport spin information in strongly spin-orbit coupled magnetic systems.

Strong magnetoelastic coupling in MnCoSi compounds studied in pulsed magnetic fields

The orthorhombic MnCoSi compounds have been found to present a large magnetoelastic coupling, which is regarded as the source for the magnetocaloric effect (MCE) and the magnetostrictive effect. As a result, these compounds are potential materials for caloric applications such as solid-state refrigeration. In the present study, we offer fundamental insights in the magnetoelastic coupling in these compounds based on their structural, metamagnetic, and MCE behavior. The directly measured adiabatic temperature change ($\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}$) in different initial temperatures (down to 18 K) and pulsed magnetic fields (up to 40 T) presents a moderate MCE performance (the maximum $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}=--3.1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for a field change of 13 T), which results from the metamagnetic behavior of these compounds. Furthermore, the magnetization measurements in pulsed (and static) magnetic fields indicate that the magnetoelastic coupling is significantly enhanced for increasing fields resulting in an improved saturation magnetization. The metamagnetic transition is continuously pushed to lower temperatures in higher fields. The phase diagram constructed from the experimental transition temperatures ${T}_{t}$ and the critical magnetic fields ${\ensuremath{\mu}}_{0}{H}_{\mathrm{cr}}$ indicate that the transition is terminated below 18 K and that ferromagnetism is stabilized for fields above 22.3 T. Our results provide unique insights into the strong magnetoelastic coupling under high pulsed magnetic fields, providing guidelines for the design of giant magnetocaloric materials for future caloric applications.

Vibronic collapse of ordered quadrupolar ice in the pyrochlore magnet Tb2+xTi2−xO7+y

While the spin-liquid state in the frustrated pyrochlore ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2\ensuremath{-}x}{\mathrm{O}}_{7+y}$ has been studied both experimentally and theoretically for more than two decades, a definite description of this unconventional state still needs to be achieved. Using synchrotron-based THz spectroscopy in combination with quantum numerical simulations, we highlight a significant link between two features: the existence of a quadrupolar order following an ice rule and the presence of strong magnetoelastic coupling in the form of hybridized ${\mathrm{Tb}}^{3+}$ crystal-field and phonon modes. The magnitude of this so-called vibronic process, which involves quadrupolar degrees of freedom, is significantly dependent on small off-stoichiometry $x$ and favors all-in--all-out-like correlations between quadrupoles. This mechanism competes with the long-range ordered quadrupolar ice, and for slightly different stoichiometry, is able to destabilize it.

Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn2O4.

Spinel compounds AB2X4 consist of both tetrahedral (AX4) and octahedral (BX6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn2O4 has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn2O4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be (Mn0.92+Fe0.13+)A(Mn3+Fe0.93+Mn0.12+)BO4. While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at TS ≈ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn3+ cation. Strong magnetoelastic coupling is unveiled at TN1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to TN2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn3+/Fe3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ∼25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O-B-O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn2O4. Our study indicates that FeMn2O4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.

Site-specific atomic substitution in a giant magnetocaloric Fe2P -type system

Giant magnetocaloric (GMC) materials constitute a requirement for near room temperature magnetic refrigeration. (Fe,Mn)$_2$(P,Si) is a GMC compound with strong magnetoelastic coupling. The main hindrance towards application of this material is a comparably large temperature hysteresis, which can be reduced by metal site substitution with a nonmagnetic element. However, the (Fe,Mn)$_2$(P,Si) compound has two equally populated metal sites, the tetrahedrally coordinated $3f$ and the pyramidally coordinated $3g$ sites. The magnetic and magnetocaloric properties of such compounds are highly sensitive to the site specific occupancy of the magnetic atoms. Here we have attempted to study separately the effect of $3f$ and $3g$ site substitution with equal amounts of vanadium. Using formation energy calculations, the site preference of vanadium and its influence on the magnetic phase formation are described. A large difference in the isothermal entropy change (as high as 44\%) with substitution in the $3f$ and $3g$ sites is observed. The role of the lattice parameter change with temperature and the strength of the magnetoelastic coupling on the magnetic properties are highlighted.

On the magnetic-structure origin of giant magnetostrictive effect in MnCoSi-based metallic helimagnets

Giant magnetostrictive effect, the huge volume and/or shape change of a material under an external magnetic field, is recently discovered in helimagnets with large magnetoelastic interactions. Here we revisit this effect in MnCoSi, a helical metamagnet with a unique Lifshitz tricritical point. By introducing minimal amount of Ni, the modified competing exchange interaction simultaneously lowers the tricritical temperature and even critical field of metamagnetism, leading to a giant, reversible, low-field magnetostrictive effect at ambient temperature. When applying a cyclic low field of 0.5 T, a giant reversible magnetostriction of 550 ppm can be achieved and maintained at 300 K. Interestingly, by advanced 3D-vector magnetic field measurement, a strong relation is found between magnetocrystalline anisotropy and the critical field of magnetostrictive effect. In-situ neutron powder diffraction results further reveal that the incommensurate helical antiferromagnetic structure with moment lying in ab plane transits into a ferromagnetic structure along b axis with increasing magnetic fields. During this metamagnetic transition, dramatical spin rotation triggers sharp anisotropic lattice change via strong magnetoelastic coupling, resulting in giant magnetostrictive effect. The saturated magnetostriction value is closely related with the amplitude of spin rotations. These findings not only design of a potential MnCoSi-based magnetostrictive material but also uncover the specific evolution of magnetic structures in the metamagnetic transition and the possible mechanism behind the macroscopic size change.

Combined experimental and theoretical study of hydrostatic He-gas pressure effects in α−RuCl3

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on the structural and magnetic aspects of the layered honeycomb antiferromagent $\alpha$-RuCl$_{3}$. Magnetic susceptibility measurements performed under almost ideal hydrostatic-pressure conditions yield that the phase transition to zigzag-type antiferromagnetic order at $T_N$ = 7.3 K can be rapidly suppressed to about 6.1 K. A further suppression with increasing pressure is impeded due to the occurrence of a pressure-induced structural transition at $p \geq$ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, as reflected by large discontinuous changes in $\chi$ and pronounced hysteresis effects, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of $\chi$ under varying pressure in the paramagnetic regime ($T > T_N$) and before dimerization ($p <$ 100 MPa) reveal a considerable increase of $\chi$ with pressure. These experimental observations are consistent with the results of ab-initio Density Functional Theory (DFT) calculations on the pressure-dependent structure and the corresponding pressure-dependent magnetic model. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults. Using different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome, transforming the magnetic state from a multiple-$T_N$ into a single-$T_N$ state.

Mean-field theory of the spin-1/2 transverse field Ising model with a negative thermal expansion

A generalized spin-1/2 transverse field Ising model with a negative thermal expansion of the lattice is introduced and investigated using standard methods of statistical mechanics. Besides the volume-dependent magnetic energy, the static lattice energy, and anharmonic Einstein phonon energy are also considered in calculations. Analytic relations for the Gibbs free energy, magnetic moments, and equations of state are obtained, taking into account a simple volume dependence of all energy contributions. The ground-state and finite-temperature phase diagrams are discussed in detail for the strong and weak magneto-elastic coupling. It is clearly demonstrated that the generalized spin-1/2 transverse field Ising model exhibits a novel critical behavior, due to the strong negative expansion of the lattice, which is controlled by the strength of magneto-elastic coupling. The presented approach can be easily modified to study also other magnetic and non-magnetic crystalline models of solids.

Noncollinear magnetism in two-dimensional CrTe2

The discovery of two-dimensional (2D) van der Waals magnets opened unprecedented opportunities for the fundamental exploration of magnetism in quantum materials and the realization of next generation spintronic devices. Here, based on a multiscale modelling approach that combines first-principles calculations and a Heisenberg model supplied with ab-initio parameters, we report a strong magnetoelastic coupling in a free-standing monolayer of CrTe2. We demonstrate that different crystal structures of a single CrTe2 give rise to non-collinear magnetism through magnetic frustration and emergence of the Dzyaloshinskii–Moriya interaction. Utilizing atomistic spin dynamics, we perform a detailed investigation of the complex magnetic properties pertaining to this 2D material impacted by the presence of various types of structural distortions akin to charge density waves.

Magneto-Elastic Coupling in a Sinusoidal Modulated Magnet Cr2GaN

We use neutron powder diffraction to investigate the magnetic and crystalline structure of Cr2GaN. A magnetic phase transition is identified at T ≈ 170 K, whereas no trace of structural transition is observed down to 6 K. Combining Rietveld refinement with irreducible representations, the spin configuration of Cr ions in Cr2GaN is depicted as an incommensurate sinusoidal modulated structure characterized by a propagating vector k = (0.365, 0.365, 0). Upon warming up to the paramagnetic state, the magnetic order parameter closely resembles to the temperature dependence of c-axis lattice parameter, suggesting strong magneto-elastic coupling in this compound. Therefore, Cr2GaN provides a potential platform for exploration of magnetically tuned properties such as magnetoelectric, magnetostrictive and magnetocaloric effects, as well as their applications.

Correlating Crystallography, Magnetism, and Electronic Structure Across Anhysteretic First-Order Phase Transition in Pr2In

Temperature-dependent powder X-ray diffraction and magnetization measurements of Pr2In conclusively prove that the unusual anhysteretic first-order paramagnetic-ferromagnetic phase transition in the compound is related to concurrent changes in both the magnetic and crystallographic lattices. At the same time, the hexagonal Ni2In-type structure is stable at least between 6 and 298 K, including at T C = ∼57 K. From the density functional theory calculations, the electronic structure of the compound is extraordinarily sensitive to minor changes in lattice parameters that occur across the phase transition, revealing the origin of strong magnetoelastic coupling. In the vicinity of T C, the maximum entropy change, ΔS Max = −16 J Kg−1 K−1 induced by a moderate magnetic field change of 20 kOe (ΔS Max = −20 J Kg−1 K−1 for 50 kOe magnetic field change) is comparable to other known potentially functional materials demonstrating large cryogenic magnetocaloric effect.

Magnetostriction and heat-capacity study on the metamagnetic phase transition of Dy2In1-xAlx alloys

In Dy2In1-xAlx (x = 0, 0.2, 0.3 and 0.4) alloys, the hexagonal structure is gradually transformed into orthorhombic one with x = 0.3 as the critical composition. In Dy2In0.7Al0.3 alloy, the highest magnetic entropy change is accompanied with highest saturation magnetostriction ∼995 ppm under 7 T near phase transition temperature, indicating the typical strong magnetoelastic coupling. From heat-capacity (Cp) measurement, a sharp peak is observed on the Cp - T curves for Dy2In1-xAlx (x = 0, 0.2 and 0.3) alloys. Meanwhile, more Al substitution for In leads into the reduced density of state near Fermi level of each alloy judged from Cp/T - T2 curves. The maximized ΔTad for a field of 7 T is slightly increased from 6.5 K in Dy2In to 7 K in Dy2In0.7Al0.3.

Role of spin-phonon and electron-phonon interactions in the phonon renormalization of (Eu1−xBix)2Ir2O7 across the metal-insulator phase transition: Temperature-dependent Raman and x-ray studies

We report temperature-dependent Raman scattering and X-ray diffraction studies of pyrochlore iridates, (Eu$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$, for x=0, 0.02, 0.035, 0.05 and 0.1. The temperature variation in Raman experiments spans from 4 K to 300 K, covering the metal-insulator phase transition accompanied by paramagnetic to all-in/all-out (AIAO) spin ordering (T$_N$). These systems also show a Weyl semi-metal (WSM) phase at low temperatures (below ~50 K). We show that the Ir-O-Ir bond bending mode, A$_{1g}$ (510 cm$^{-1}$), shows anomalous softening in the magnetically ordered AIAO state, arising primarily from the spin-phonon interaction due to the phonon-modulation of the Dzyaloshinskii-Moriya (DM) spin-exchange interaction. The two stretching modes, T$_{2g}^1$ (307 cm$^{-1}$) and T$_{2g}^2$ (382 cm$^{-1}$) harden significantly in the magnetic insulating phase. The T$_{2g}$ phonons also show anomalous temperature dependence of their mode frequencies, hitherto unreported, due to strong electron-phonon coupling. The signatures of the WSM state are observed in phonon renormalization below 50 K due to strong electron-phonon interaction. Our experimental results establish strong magneto-elastic coupling below T$_N$ and significant electron-phonon interactions in the metallic phase above T$_N$ as well as in the low-temperature WSM state.

Structural correlation to ferroelectric order, non-Griffiths like phase and magnetocaloric effect in YbCrO4

We report a ferroelectric order around ∼99 K (T FE), which is considerably above the long range ferrimagnetic order at 25 K (T N ). The value of saturation electric polarization is considerable as ∼570 μC m−2 for a poling field of 5 kV cm−1. The ferroelectric order is associated with a significant magnetoelectric coupling below ∼90 K. A weak signature of T FE is observed in the dielectric constant, which is associated with a linear magnetodielectric response at 18 K . A transition to a polar structure has been correlated with the occurrence of ferroelectric order. A non-Griffiths like phase is observed around ∼48 K, which is also linked with the structural distortion. A reasonable value of the magnetic entropy change of 5.07 J Kg−1 K−1 is noted at T N for a change in field of 50 kOe, which involves a strong magnetoelastic coupling. The results indicate considerable structural instabilities, which is linked with the observed multifunctional properties of YbCrO4.

Unconventional metamagnetic phase transition in R2In (R=Nd, Pr) with lambda-like specific heat and nonhysteresis

• The field-triggered metamagnetism with lambda-like peak on the specific heat is found in Nd 2 In and Pr 2 In alloys. • Neither thermomagnetic hysteresis nor lattice changes are observable during phase transition. • Giant magnetoentropy changes of 13.2 J kg −1 K −1 and 19.5 J kg −1 K −1 are achieved with enhanced magnetoelastic coupling. • The max adiabatic temperature changes of 6.5 K and 7.4 K are obtained for Nd 2 In and Pr 2 In alloys under 7 T. • It is proposed to be from the delocalized 5 d electrons evidenced by the negative magnetoresistance. The phase transition of Nd 2 In and Pr 2 In has been studied and found to be between the first-order and second-order nature. Hardly any hysteresis nor obvious cell volume changes are presented. Concurrently the field-triggered metamagnetism is demonstrated with lambda-like peak on specific heat curve. For the field change of 7 T, giant magnetic entropy changes of 13.2 J kg −1 K −1 and 19.5 J kg −1 K −1 and adiabatic temperature changes of 6.5 K and 7.4 K have been achieved in Nd 2 In and Pr 2 In alloys respectively, which is partially contributed by strong magnetoelastic coupling represented by high saturation magnetostriction of ∼450 ppm. Such unique feature is proposed to be originated from the delocalized 5 d electrons, as evidenced by the negative magnetoresistance caused by the phase transition.

Carrier doping-induced strong magnetoelastic coupling in 2D lattice.

The realization of intertwined ferroelasticity and ferromagnetism in two-dimensional (2D) lattices is of great interest for broad nanoscale applications but still remains a remarkable challenge. Here, we propose an alternative approach to realize the strongly coupled ferromagnetism and ferroelasticity by carrier doping. We demonstrate that prototypical 2D β-PbO is dynamically, thermally and mechanically stable. Under hole doping, 2D β-PbO possesses ferromagnetism and ferroelasticity simultaneously. Moreover, the robustness of ferromagnetic and ferroelastic orders is doping tunable. In particular, 2D β-PbO features an in-plane easy magnetization axis that is coupled with the lattice direction, enabling the ferroelastic manipulation of the spin direction. Furthermore, the efficient ferroelastic control of the anisotropic optical property and spin splitting in 2D β-PbO are also clarified. Our study highlights a new direction for 2D magnetoelastic research and enables the possibility for multifunctional devices.

Magnetoelastic standing waves induced in UO2 by microsecond magnetic field pulses

Magnetoelastic dilatometry of the piezomagnetic antiferromagnet UO2 was performed via the fiber Bragg grating method in magnetic fields up to 150 T generated by a single-turn coil setup. We show that in microsecond timescales, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from 10 to 300 K, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100-µs observation window, are attributed to the strong magnetoelastic coupling in UO2 combined with the high crystalline quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known nonmonotonic magnetostriction background. A clear phase shift of π in the lattice oscillations is observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch field H[Formula: see text] T. We present a theoretical argument that explains this unexpected behavior as a result of the reversal of the antiferromagnetic order parameter at Hc.

Multiple field-induced phases in the frustrated triangular magnet Cs3Fe2Br9

The recently discovered material Cs$_3$Fe$_2$Br$_9$ contains Fe$_2$Br$_9$ bi-octahedra forming triangular layers with hexagonal stacking along the $c$ axis. In contrast to isostructural Cr-based compounds, the zero-field ground state is not a nonmagnetic $S=0$ singlet-dimer state. Instead, the Fe$_2$Br$_9$ bi-octahedra host semiclassical $S=5/2$ Fe$^{3+}$ spins with a pronounced easy-axis anisotropy along $c$ and interestingly, the intra-dimer spins are ordered ferromagnetically. The high degree of magnetic frustration due to (various) competing intra- and inter-dimer couplings leads to a surprisingly rich magnetic phase diagram. Already the zero-field ground state is reached via an intermediate phase, and the high-field magnetization and thermal expansion data for $H\parallel c$ identify ten different ordered phases. Among them are phases with constant magnetization of 1/3, respectively 1/2 of the saturation value, and several transitions are strongly hysteretic with pronounced length changes reflecting strong magnetoelastic coupling.

Magnetic phase diagram, magnetoelastic coupling, and Grüneisen scaling in CoTiO3

High-quality single crystals of ${\mathrm{CoTiO}}_{3}$ are grown and used to elucidate in detail structural and magnetostructural effects by means of high-resolution capacitance dilatometry studies in fields up to 15 T which are complemented by specific heat and magnetization measurements. In addition, we refine the single-crystal structure of the ilmenite ($R\overline{3}$) phase. At the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}$ pronounced $\ensuremath{\lambda}$-shaped anomaly in the thermal expansion coefficients signals shrinking of both the $c$ and $b$ axes, indicating strong magnetoelastic coupling with uniaxial pressure along $c$ yielding six times larger effect on ${T}_{\mathrm{N}}$ than pressure applied in-plane. The hydrostatic pressure dependency derived by means of Gr\"uneisen analysis amounts to $\ensuremath{\partial}{T}_{\mathrm{N}}/\ensuremath{\partial}p\ensuremath{\approx}2.7(4)\phantom{\rule{4pt}{0ex}}\mathrm{K}/\mathrm{GPa}$. The high-field magnetization studies in static and pulsed magnetic fields up to 60 T along with high-field thermal expansion measurements facilitate in constructing the complete anisotropic magnetic phase diagram of ${\mathrm{CoTiO}}_{3}$. While the results confirm the presence of significant magnetodielectric coupling, our data show that magnetism drives the observed structural, dielectric, and magnetic changes both in the short-range ordered regime well above ${T}_{\mathrm{N}}$ as well as in the long-range magnetically ordered phase.