Metamagnetic Phase Research Articles

Magnon dynamics during phase transitions in FeRh by Brillouin light scattering

FeRh has been extensively studied because of metamagnetic phase transitions between the antiferromagnetic (AFM) and the ferromagnetic (FM) phase observed in this material. In this work, Brillouin light scattering is used to detect magnons in metamagnetic FeRh. Appearance and disappearance of magnon scattering peaks with variation of temperature illustrate the transformation of FeRh into the FM and AFM phases, respectively. The dependence of frequency on temperature indicates significant change thus characterizing metamagnetic phase transitions in FeRh, since the temperature dependence of the magnon frequency shift mainly follows the saturation magnetization temperature dependence. In addition, a considerable increase of full width at half maximum linewidth of the Stokes peak for FeRh is explained by the enhancement of magnetic damping. It is further found that the Dzyaloshinskii–Moriya interaction (DMI) takes place in single-layer FeRh with the DMI energy D affected by the first-order phase transitions in FeRh. These results provide an insight into the magnon dynamics in metamagnetic materials as well as offering new ways toward a modulation of magnetic damping in a wide range, which will be useful in the design of multifunctional spintronic devices.

Determination of sub-ps lattice dynamics in FeRh thin films

Understanding the ultrashort time scale structural dynamics of the FeRh metamagnetic phase transition is a key element in developing a complete explanation of the mechanism driving the evolution from an antiferromagnetic to ferromagnetic state. Using an X-ray free electron laser we determine, with sub-ps time resolution, the time evolution of the (–101) lattice diffraction peak following excitation using a 35 fs laser pulse. The dynamics at higher laser fluence indicates the existence of a transient lattice state distinct from the high temperature ferromagnetic phase. By extracting the lattice temperature and comparing it with values obtained in a quasi-static diffraction measurement, we estimate the electron–phonon coupling in FeRh thin films as a function of laser excitation fluence. A model is presented which demonstrates that the transient state is paramagnetic and can be reached by a subset of the phonon bands. A complete description of the FeRh structural dynamics requires consideration of coupling strength variation across the phonon frequencies.

A Simplified Method Characterizing Magnetic Ordering Modulated Photo‐Thermoelectric Response in Noncentrosymmetric Semimetal Ca3Ru2O7

Recently, the spin entropy related to magnetic‐order transition, which contributes to thermoelectric power is attracting more and more attention. Here, it is demonstrated that the photo‐thermoelectric (PTE) response can be reshaped when Ca3Ru2O7 undergoes a meta‐magnetic phase (MMP) transition driven by both temperature and magnetic field. First, a sign change is observed crossing T S = 48 K and the linear polarization angle‐dependent PTE current maximizes along the a‐axis above T S, while it maximizes along the b‐axis below T S, which indicates that the antiferromagnetic spin order contributes to such spatial anisotropy. Second, in the temperature range of around 40–50 K, the PTE current is found to be sharply suppressed when the external magnetic field is applied in plane along the a‐axis but is only gradually suppressed when the applied field is along the b‐axis, which gives out two critical fields. Such suppression of PTE current under a magnetic field is attributed to the suppression of the spin entropy in the phase transition between the antiferromagnetic state and the MMP state and the H–T phase diagrams of Ca3Ru2O7 are redrawn accordingly. The work provides a convenient yet efficient method to understand the magnetic phase transition, which may also find applications in other correlated spin materials in general.

Control of metamagnetic phase transition in epitaxial FeRh films by changing atomic order degree

Near-equiatomic FeRh alloys undergo a first-order metamagnetic phase transition around room temperature. Here, we show that the phase transition parameters of epitaxial FeRh films can be adjusted to a great extent through changing the growth temperature, annealing temperature, and annealing time. The influences of the surface morphology and Fe (Rh) element distribution before and after heat treatments have been analyzed. The experimental results show that the increase of annealing temperature reduces the displacement of Fe (Rh) atoms. As a result, the resistivity is reduced, the phase transition interval is narrowed, and the phase transition is shifted to high temperature. The experimental results indicate that the first-order phase transition parameters in epitaxial FeRh films can be controlled by heat treatments and are correlated with the atomic order degree of Fe (Rh).

Dynamical tuning of the thermal conductivity via magnetophononic effects

Strategies for tuning the thermal conductivity of crystals by means of external fields are rare. Here, we predict the existence of large magnetophononic effects in materials that undergo antiferromagnetic (AFM) $\ensuremath{\leftrightarrow}$ ferromagnetic (FM) phase transitions, which allow for the modulation of the lattice heat conductivity, ${\ensuremath{\kappa}}_{L}$, via the application of magnetic fields. Specifically, by using first-principles methods we predict a large and anomalous ${\ensuremath{\kappa}}_{L}$ increase of $\ensuremath{\approx}40%$ for the metamagnetic phase transition occurring in bulk FeRh near room temperature. The disclosed magnetophononic effects are caused by large anharmonic spin-phonon couplings, namely, significant differences in the phase space of allowed phonon-phonon collision processes taking place in the respective AFM and FM phases.

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.

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.

Entropic topography associated with field-induced quantum criticality in a magnetic insulator DyVO4

Exploration of low temperature phase transitions associated with quantum critical point is one of the most mystifying fields of research which is under intensive focus in recent times. In this work, through comprehensive experimental evidences, we report the possibility of achieving quantum criticality in the neighborhood of a magnetic field-tuned tricritical point separating paramagnetic, antiferromagnetic and metamagnetic phases in a magnetic insulator, DyVO4. Magnetic susceptibility and heat capacity indicate to the presence of a long-range second order antiferromagnetic transition at TN ~ 3.2 K. Field variation of Magnetic susceptibility and heat capacity, along with differential magnetic susceptibility and DC field dependent AC susceptibility gives evidence of the modification of the antiferromagnetic structure below the tricritical point; implying the presence of a field-induced first order metamagnetic transition which persists down to 1.8 K. Further, the magnetic field dependence of the thermodynamic quantity − dM/dT, which is related to magnetic Gruneisen parameter, approaches a minimum, followed by a crossover near 5 kOe to a maximum; along with a hyperbolic divergence in temperature response of dM/dT in the critical field regime. Temperature response of heat capacity at 5 kOe also shows a deviation from the conventional behavior. Entropic topography phase diagram allows tracking of the variation of the entropy, which indicates towards the emergence of the peak at quantum critical point into a V-shaped region at high temperatures. Our studies yield an inimitable phase diagram describing a tricritical point at which the second-order antiferromagnetic phase line terminates followed by a first order line of metamagnetic transition, as the temperature is lowered, leading to metamagnetic quantum critical end point.

Controlling the Metamagnetic Phase Transition in FeRh/MnRh Superlattices and Thin-Film Fe50-xMnxRh50 Alloys.

Equiatomic and chemically ordered FeRh and MnRh compounds feature a first-order metamagnetic phase transition between antiferromagnetic and ferromagnetic order in the vicinity of room temperature, exhibiting interconnected structural, magnetic, and electronic order parameters. We show that these two alloys can be combined to form hybrid metamagnets in the form of sputter-deposited superlattices and alloys on single-crystalline MgO substrates. Despite being structurally different, the magnetic behavior of the alloys with substantial Mn content resembles that of the FeRh/MnRh superlattices in the ultrathin individual layer limit. For FeRh/MnRh superlattices, dissimilar lattice distortions of the constituent FeRh and MnRh layers at the antiferromagnetic-ferromagnetic transition cause double-step transitions during cooling, while the magnetization during the heating branch shows a smooth, continuous trend. For Fe50-xMnxRh50 alloy films, the substitution of Mn at the Fe sites introduces an effective tensile in-plane strain and magnetic frustration in the highly ordered epitaxial films, largely influencing the phase transition temperature TM (by more than 150 K). In addition, Mn acts as a surfactant, enabling the growth of continuous thin films at higher temperatures. Thus, the introduction of hybrid FeRh-MnRh systems with adjustable parameters provides a pathway for the realization of tunable spintronic devices based on magnetic phase transitions.

Correlation between magnetic and crystal structural sublattices in palladium-doped FeRh alloys: Analysis of the metamagnetic phase transition driving forces

FeRh alloys doped with the third element exhibit a change in the lattice and magnetic subsystems, which are manifested in antiferromagnetic- ferromagnetic (AFM-FM) first-order phase transition temperature, the shrinkage of the temperate hysteresis under transition, and the reduction of the saturation magnetization. All aforementioned parameters are crucial for practical applications. To control them it is quite important to determine the driving forces of the metamagnetic transition and its origins. In this manuscript ab initio calculations and experimental studies results are presented, which demonstrate the correlation between the structural and magnetic properties of the Fe50Rh50−xPdx alloys. The qualitative analysis of the metamagnetic phase transition driving forces in palladium-doped FeRh alloys was performed to determine their contribution to the evolution of magnetic and lattice subsystems. In addition, the impact of the impurities phases together with its magnetic behavior on the AFM-FM phase transition was considered.

Magnetoentropic Signatures of the Textured Metamagnetic Phase of an Antiferromagnetic Polar Metal: Ca3Ru2O7

We report the magnetocaloric effect of a bilayered perovskite ruthenate Ca3Ru2O7 that has recently been recognized as an antiferromagnetic polar metal. The magnetic entropy change obtained from tem...

Magnetic-field-induced structural phase transition and giant magnetoresistance in La0.85Ce0.15Fe12B6

Magnetoelastic coupling, structural, magnetic, electronic transport, and magnetotransport properties of ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ have been studied by a combination of macroscopic [magnetization, electrical resistivity, and magnetoresistance (MR)] and microscopic temperature- and magnetic-field-dependent x-ray powder diffraction measurements. The itinerant-electron system ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ exhibits an antiferromagnetic (AFM) ground state and multiple magnetic transitions, AFM-ferromagnetic (FM) and FM-paramagnetic (PM), triggered by changes in both temperature and magnetic fields. At low temperatures, the field-induced first-order AFM-FM metamagnetic phase transition is discontinuous, manifesting itself by extremely sharp steps in magnetization as well as in MR and is accompanied by large magnetic hysteresis. A remarkably large negative MR of \ensuremath{-}73% was discovered. In addition, the time evolution of the electrical resistivity displays a colossal spontaneous jump when both the applied magnetic field and temperature are constant. Diffraction data reveal a magnetic-field-induced structural phase transition associated with the AFM-FM and PM-FM transformations. The lattice distortion is driven by magnetoelastic coupling and converts the crystal structure from rhombohedral ($R\overline{3}m$) to monoclinic $(C2/m)$. The AFM and PM states are related to the rhombohedral structure, whereas the FM order develops in the monoclinic symmetry. A huge volume magnetostriction of \ensuremath{\sim}0.9% accompanies this symmetry-lowering lattice distortion. Meanwhile, a highly anisotropic thermal expansion involving giant negative thermal expansion with an average volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}195\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ was observed. The consistency seen in these different experimental data constitutes direct evidence of the strong correlations between charge, magnetic, and crystallographic degrees of freedom in this material.

Subpicosecond metamagnetic phase transition in FeRh driven by non-equilibrium electron dynamics

Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350 ± 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.

Experimental evidence for field-induced metamagnetic transition of EuCd2As2

Recent studies have shown that layered compound EuCd2As2 could exhibit diverse topological states depending on the different magnetic structures, such as Weyl semimetal, Dirac semimetal and topological insulator. In order to further study the interplay between magnetism and topology of EuCd2As2, it is necessary to figure out its magnetic structure. Here, by magnetization (M) measurements and negative magnetostriction (λ) along the [001] direction measured by scanning tunneling microscopy on EuCd2As2 single crystals, we observe field-induced metamagnetic phase transition from A-type antiferromagnetic (AFM) ground state to field-polarized state, with canted AFM (CAFM) state in between. Magnetization and magnetostriction are more sensitive to the in-plane field than the out-of-plane field, indicating the magnetic moments lying in the ab plane. The absence of abrupt jump on M-H and λ-H curves demonstrates that the phase transition is a second-order type. In CAFM state, M increases linearly with the field and λ is proportional to M2. Tunneling conductance spectra show the field-induced evolution of the electronic density of states. Our results provide experimental evidence for understanding the magnetic structure of EuCd2As2.

Probing exchange bias at the surface of a doped ferrimagnetic insulator

With the realization of stress-induced perpendicular magnetic anisotropy, efficient spin-orbit torque switching, and room temperature topological Hall effect, interest in rare earth iron garnets has been revived in recent years for their potential in spintronic applications. In this study, we investigate the magnetic properties of micrometer-thick Bi and Ga substituted thulium iron garnets (BiGa:TmIG) grown by the liquid-phase epitaxy method. Above the magnetization compensation (MC) temperature, anomalous triple hysteresis is observed in BiGa:TmIG/Pt heterostructures by anomalous Hall effect measurements. X-ray magnetic circular dichroism and energy dispersive spectroscopy measurements reveal its origin as an internal exchange bias (EB) effect arising from inhomogeneities localized at the surface of the film. Possibly depending on the difference in thickness and defect realization of the EB layer, two types of magnetization reversal mechanisms, namely, the Stoner-Wohlfarth type and the reversible domain-wall motion type, are observed. Our results show that rich meta-magnetic phases exist in garnets close to MC, which can be robustly tuned by chemical composition engineering and conveniently probed by electrical transport measurements.

Nonhysteretic metamagnetic phase transition in Ho2In1-xAlx (0 ≤ x ≤ 0.4) by hetero-structural alloying

By hetero-structural alloying ferromagnetic hexagonal Ho2In with antiferromagnetic orthorhombic Ho2Al, the magnetic and structural changes have been observed in Ho2In1-xAlx (0 ≤ x ≤ 0.4) alloys. At the critical composition x = 0.1, the highest maxima of magnetic entropy change of 16.3 Jkg-1K−1 and the adiabatic temperature change of 6.5 K have been obtained via an unconventional hysteresis-free metamagnetic phase transitionm, which is higher than 9 Jkg-1K−1 and 5.4 K in Ho2In benchmark alloy undergoing second-order phase transition. Such increment is associated with the intensified magnetoelastic coupling. The deeper origin is proposed to be from the delocalized 5d electrons of rare-earth atoms by external field since the magnetoresistence is also maximized at the critical composition. The hetero-structural alloying strategy is demonstrated to be effective to intensify the MCE effect without causing the undesirable hysteresis.

Study on magnetocaloric effect and phase transition in La0.7(La–Ce)0.3FexAl11.5−xSi1.5 alloys

Magnetocaloric materials with high performance are crucial to the achievement of magnetic refrigeration which is a promising cooling technology to replace traditional gaseous compression refrigeration. In this work, the magnetocaloric effect and phase transition of La0.7(La–Ce)0.3FexAl11.5−xSi1.5 alloys have been investigated. The structural and magnetic properties were characterized by means of X-ray diffraction and magnetization measurements. Belov-Arrott plots and the exponent n from field dependence of isothermal entropy change were applied to determine the nature of the phase transition. The XRD patterns indicate that the alloys crystallize mainly in NaZn13-type phase with small amounts of α-Fe and LaFeSi as secondary phases. With an increase of x from 11.0 to 11.4, the Curie temperature decreases from 211.2 K to 185.4 K while the magnetocaloric effect increases from 4.9 J kg−1 ⋅K−1 to 23 J kg−1 ⋅K−1 for a field change of 2 T. The alloys show refrigeration capacity between 111 J ⋅kg−1 and 152 J ⋅kg−1 for a magnetic field of 2 T. Analyses of the Belov-Arrott plot and the exponent of n reveal that the first-order itinerant-electron metamagnetic phase transition in the alloys is weakened by decreasing x, and the alloys with x = 11.2, 11.3 and 11.4 undergo a second-order phase transition.

High-magnetic-field solidification on magnetostrictive effect of Ni-doped MnCoSi alloy

Ni-doped MnCoSi alloy was prepared by high-magnetic-field solidification with a slow cooling rate. At a 3 at% Ni substitution for Mn, the room-temperature critical field was decreased to 0.4 T and the tricritical temperature between first-order and second-order antiferromagnetic-ferromagnetic metamagnetic phase transformation was tuned to 280 K. As a result, we obtained oriented and dense Mn0.97Ni0.03CoSi alloy and observed anisotropic reversible magnetostrictive effects under the magnetic field of 3.1 T near room temperature.

Magnetic imaging and statistical analysis of the metamagnetic phase transition of FeRh with electron spins in diamond

Magnetic imaging based on nitrogen-vacancy (NV) centers in diamond has emerged as a powerful tool for probing magnetic phenomena in fields ranging from biology to physics. A key strength of NV sensing is its local-probe nature, enabling high-resolution spatial images of magnetic stray fields emanating from a sample. However, this local character can also form a drawback for analyzing the global properties of a system, such as a phase transition temperature. Here, we address this challenge by using statistical analyses of magnetic-field maps to characterize the first-order temperature-driven metamagnetic phase transition from the antiferromagnetic to the ferromagnetic state in FeRh. After imaging the phase transition and identifying the regimes of nucleation, growth, and coalescence of ferromagnetic domains, we statistically characterize the spatial magnetic-field maps to extract the transition temperature and thermal hysteresis width. By analyzing the spatial correlations of the maps in relation to the magnetocrystalline anisotropy and external magnetic field, we detect a reorientation of domain walls across the phase transition. The employed statistical approach can be extended to the study of other magnetic phenomena with NV magnetometry or other sensing techniques.

N+ Irradiation and Substrate-Induced Variability in the Metamagnetic Phase Transition of FeRh Films

Metamagnetic FeRh has been the focus of numerous studies for its highly unique antiferromagnetic (AF) to ferromagnetic (FM) metamagnetic transition. While this phase transition usually occurs above room temperature (often Tc > 400 K), both ion irradiation and strained epitaxial growth have been used to bring it to applicable temperatures. Nevertheless, cross sample variability is pervasive in these studies. Here we explore the optical and magnetic properties of 35 nm thick FeRh grown by magnetron sputter deposition simultaneously on two different single crystal substrates: epitaxially on MgO (001) and highly strained with large lattice mismatch on Al2O3 (1000). We then irradiate the epitaxial film with 5 keV N+ ions to introduce disorder (and to a lesser extent, modify chemical composition) without effecting the surface morphology. We find that the phase-transitional properties of both films are strikingly different due to the large lattice mismatch, despite being grown in tandem with nominally identical growth conditions including Fe/Rh stoichiometry, pressure, and temperature. We observe that N+ implantation lowers Tc by ~60 K, yielding a sample with nominally the same transition temperature as the non-epitaxial film on sapphire, yet with a significantly increased magnetic moment, a larger magnetization change and a more abrupt transition profile. We attribute these differences to the Volmer-Weber type growth mode induced by the sapphire substrate and the resulting rougher surface morphology.