Metamagnetic Transition Research Articles

Nonunitary triplet superconductivity tuned by field-controlled magnetization: URhGe, UCoGe, and UTe2

We report on theoretical studies on ferromagnetic superconductors, URhGe, and UCoGe and identifies the pairing state as a non-unitary spin-triplet one, analogous to superfluid $^3$He-A phase. A recently found superconductor UTe$_2$ with almost ferromagnet is analyzed by the same manner. Through investigating their peculiar upper critical field $H_{\rm c2}$ shapes, it is shown that the pairing symmetry realized in all three compounds can be tuned by their magnetization curves under applied fields. This leads to the reentrant $H_{\rm c2}$ in URhGe, an S-shaped in UCoGe and an L-shaped $H_{\rm c2}$ in UTe$_2$ observed under the field direction parallel to the magnetic hard axis in orthorhombic crystals in common. The identification with double chiral form: ${\bf d}(k)=(\hat{b}+i\hat{c})(k_b+ik_c)$ in UTe$_2$ naturally enables us to understand (1) multiple phases with A$_1$, A$_2$, and A$_0$ phases observed under pressure, (2) the enhanced reentrant $H_{\rm c2}$ for the off-axis direction fields associated with first order meta-magnetic transition, and (3) Weyl point nodes oriented along the $a$-axis. All three compounds are found to be topologically rich solid-state materials worth further investigation.

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.

Enhancing ferromagnetism in the kinetically arrested LaFe12B6 by partial La/Nd substitution

The intrinsic magnetic properties of the La0.9Nd0.1Fe12B6 compound have been studied by magnetization measurements. The compound orders ferromagnetically (FM) at the Curie temperature, TC, of ~58 K, which shifts toward higher temperatures with increasing magnetic field. At 2 K isothermal magnetization data indicate a mixed magnetic state with ~2/3 of the material remaining FM and ~1/3 becoming antiferromagnetic (AFM) after cooling in a zero magnetic field. The AFM state fully transforms into the FM state via a field-induced metamagnetic transition, and the critical field is ~HC = 34 kOe at 2 K. Another kind of metamagnetic transition is observed above the low-field TC, where a field-induced metamagnetic transition is between the paramagnetic (PM) and FM states. Both metamagnetic transitions are accompanied by large thermal and magnetic hystereses, and the Arrott plots confirm that the transitions are first order in nature. A large magnetocaloric effect in the La0.9Nd0.1Fe12B6 compound is observed in the vicinity of the field-induced PM-FM transition.

Emergence of Nontrivial Spin Textures in Frustrated Van Der Waals Ferromagnets.

In this work, first principles ground state calculations are combined with the dynamic evolution of a classical spin Hamiltonian to study the metamagnetic transitions associated with the field dependence of magnetic properties in frustrated van der Waals ferromagnets. Dynamically stabilized spin textures are obtained relative to the direction of spin quantization as stochastic solutions of the Landau–Lifshitz–Gilbert–Slonczewski equation under the flow of the spin current. By explicitly considering the spin signatures that arise from geometrical frustrations at interfaces, we may observe the emergence of a magnetic skyrmion spin texture and characterize the formation under competing internal fields. The analysis of coercivity and magnetic hysteresis reveals a dynamic switch from a soft to hard magnetic configuration when considering the spin Hall effect on the skyrmion. It is found that heavy metals in capped multilayer heterostructure stacks host field-tunable spiral skyrmions that could serve as unique channels for carrier transport. The results are discussed to show the possibility of using dynamically switchable magnetic bits to read and write data without the need for a spin transfer torque. These results offer insight to the spin transport signatures that dynamically arise from metamagnetic transitions in spintronic devices.

Giant volume magnetostriction in La0.85Ce0.15Fe12B6

We report the discovery of giant magnetostriction and huge negative thermal expansion (NTE) in the itinerant-electron metamagnetic compound La0.85Ce0.15Fe12B6. This system presents multiple magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), driven by changes in both external field and temperature. The magnetic-field–dependent magnetostriction exhibits irreversible abrupt jumps at T ≤ 5 K, while it evolves smoothly with applied field above 5 K. A giant positive volume magnetostriction of ΔV/V (20 K, 6 T) = 0.80% is observed across the field-induced first-order AFM–FM metamagnetic transition. It is further revealed that the FM–PM transition is accompanied by a remarkably large NTE with a coefficient of linear thermal expansion αL = −34 × 10−6 K−1 over a temperature window of ΔT ∼ 63 K.

Colossal magneto-resistive relaxation effects in La0.9Ce0.1Fe12B6

The study of the magnetic, electronic transport, and magnetotransport properties of La0.9Ce0.1Fe12B6 itinerant-electron system has been performed by combining magnetization, electrical resistivity, and magnetoresistance experiments. Along with the antiferromagnetic (AFM) ordering at TN = 35 K, two consecutive magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), occur upon heating under certain magnetic field values. At fixed temperatures, it is revealed that both AFM and PM phases can be converted into the FM phase irreversibly and reversibly via a first-order metamagnetic transition associated with a large hysteresis. Below 8 K, the metamagnetic transition is discontinuous, manifesting itself by multiple sudden jumps in magnetoresistance and magnetization. A giant negative magnetoresistance effect of about −78% is found. We further demonstrate that the time dependencies of the electrical resistivity and the magnetization exhibit colossal spontaneous steps after an incubation time in conditions where both the applied magnetic field and temperature are constant. Another intriguing observation in the phase diagram is the presence of a critical point at the crossover of the three distinct PM, FM, and AFM magnetic states.

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.

Magnetocaloric properties and specifics of the hysteresis at the first-order metamagnetic transition in Ni-doped FeRh

Measurements of the magnetization in quasistatic and pulsed magnetic fields with different sweep rates, measurements of the specific heat in various magnetic fields, and direct measurements of the adiabatic temperature change have been employed to study the metamagnetic phase transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in an ${({\mathrm{Fe}}_{0.98}{\mathrm{Ni}}_{0.02})}_{49}{\mathrm{Rh}}_{51}$ alloy with a critical AF-FM transition temperature, ${T}_{\mathrm{tr}}$, reduced to 266 K. Based on the obtained results, a magnetic phase diagram for this alloy has been constructed. The AF-FM transition induced by the magnetic field below 10 K is found to occur in a steplike fashion in contrast to smooth behavior at $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}<T<{T}_{\mathrm{tr}}$. The adiabatic temperature change $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ in the magnetic field of 2 T exceeds 6.5 K in pulsed fields (\ensuremath{\sim}100 T/s) and in the Halbach setup (\ensuremath{\sim}0.5 T/s), which is in agreement with the estimation from the S-T diagram constructed based on the specific heat measurements. The reversible $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ reaches \ensuremath{-}4.6 K under cyclic conditions in the Halbach setup (2 T). A complete transformation to the FM state in the whole temperature range requires a magnetic field of 14 T. Direct measurements of $\mathrm{\ensuremath{\Delta}}{T}_{ad}$ in pulsed fields of 14 T revealed an irreversible part of the magnetocaloric effect associated with the presence of magnetic hysteresis and respective losses during the magnetization process.

Topological Hall effect in the antiferromagnetic Dirac semimetal EuAgAs

The nontrivial magnetic texture in real space gives rise to the intriguing phenomenon of the topological Hall effect (THE), which is relatively less explored in topological semimetals. Here, we report a large THE in the antiferromagnetic (AFM) state in single crystals of EuAgAs, an AFM Dirac semimetal. EuAgAs hosts an AFM ground state below ${T}_{N}=12$ K with a weak ferromagnetic component. The in-plane isothermal magnetization below ${T}_{N}$ exhibits a weak metamagnetic transition. We also observe chiral anomaly induced positive longitudinal magnetoconductivity, which indicates a Weyl fermion state under an applied magnetic field. The first-principles calculations reveal that EuAgAs is an AFM Dirac semimetal with a pair of Dirac cones, and therefore a Weyl semimetallic state can be realized under time-reversal symmetry breaking via an applied magnetic field. Our study establishes that EuAgAs is a system for exploiting the interplay of band topology and the topology of the magnetic texture.

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.

Local origin of the strong field-space anisotropy in the magnetic phase diagrams of Ce1−xLaxB6 measured in a rotating magnetic field

Cubic f-electron compounds commonly exhibit highly anisotropic magnetic phase diagrams consisting of multiple long-range ordered phases. Field-driven metamagnetic transitions between them may depend not only on the magnitude, but also on the direction of the applied magnetic field. Examples of such behavior are plentiful among rare-earth borides, such as RB$_6$ or RB$_{12}$ ($R$ = rare earth). In this work, for example, we use torque magnetometry to measure anisotropic field-angular phase diagrams of La-doped cerium hexaborides, Ce$_{1-x}$La$_x$B$_6$ ($x$ = 0, 0.18, 0.28, 0.5). One expects that field-directional anisotropy of phase transitions must be impossible to understand without knowing the magnetic structures of the corresponding competing phases and being able to evaluate their precise thermodynamic energy balance. However, this task is usually beyond the reach of available theoretical approaches, because the ordered phases can be noncollinear, possess large magnetic unit cells, involve higher-order multipoles of 4f ions rather than simple dipoles, or just lack sufficient microscopic characterization. Here we demonstrate that the anisotropy under field rotation can be qualitatively understood on a much more basic level of theory, just by considering the crystal-electric-field scheme of a pair of rare-earth ions in the lattice, coupled by a single nearest-neighbor exchange interaction. Transitions between different crystal-field ground states, calculated using this minimal model for the parent compound CeB6, possess field-directional anisotropy that strikingly resembles the experimental phase diagrams. This implies that the anisotropy of phase transitions is of local origin and is easier to describe than the ordered phases themselves.

Structure driven magnetic correlations and magnetodielectric coupling in 6H-perovskite Ba3DyRu2O9

The 6H-perovskites Ba3(R/M)Ru2O9 (R = rare Earth, M = transition metal) exhibit complex magnetism and have been extensively studied recently for their magnetodielectric (MD) properties. Here, we present a detailed study of structural, magnetic, thermodynamic and MD properties of a 6H-perovskite Ba3DyRu2O9. This compound is found to undergo long range antiferromagnetic ordering below ∼5.8 K (T N), along with the presence of metamagnetic transition at low temperatures. The heat capacity shows two additional anomalies at ∼28 K (T 1) and ∼33 K (T 2), besides the anomaly at T N. Signature of these anomalies is also visible in the derivative of magnetization curve. The dielectric response also shows weak anomalies around T 1 and T 2 at zero field whereas anomaly at T 2 gets suppressed at 80 kOe. The observed MD coupling of ∼2%–4% at 80 kOe field below ∼30 K temperature range, is among the highest values observed for the compounds of this family. Low temperature crystal structures of the compound show sharp distortion of Ru2O9 octahedra near T 2. Our study points toward the emergence of structurally driven spin correlations of Ru moments resulting in the observed MD coupling in this compound.

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.

Emergence of metamagnetic transition, re-entrant cluster glass and spin phonon coupling in Tb2CoMnO6

The double perovskite compound Tb2CoMnO6 has been investigated using x-ray absorption spectroscopy (XAS), Raman spectroscopy, magnetic measurements and ab initio band structure calculations. It is observed that both anti-ferromagnetic (AFM) and ferromagnetic (FM) phase coexist in this material. The presence of anti-site disorder (ASD) has been established from the analysis of neutron diffraction data. Moreover, a prominent metamagnetic transition is observed in the M(H) behavior that has been explained with the drastic reorientation of the pinned domain which are aligned antiparallel by the antiphase boundaries (APBs) at zero field. The ASD further gives rise to spin frustration at low temperature which leads to the re-entrant cluster glass ∼33 K. The coupling between phononic degree of freedom and spin in the system has also been demonstrated. It is observed that the theoretical calculation is consistent with that of the experimentally observed behavior.

Strain-modulated helimagnetism and emergent magnetic phase diagrams in highly crystalline MnP nanorod films

We explore strain-modulated helimagnetism in highly crystalline MnP nanorod films grown on Si(100) substrates using molecular beam epitaxy. The strained MnP film exhibits a paramagnetic to ferromagnetic (FM) phase transition at ${T}_{C}\ensuremath{\sim}279\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the FM to helical phase transition at ${T}_{N}\ensuremath{\sim}110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The value of ${T}_{N}$ is greater than ${T}_{N}\ensuremath{\sim}47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for the MnP single crystal, indicating strong strain-modulated helimagnetic states in the MnP nanorod film. The presence of significant thermal hysteresis in the helical phase indicates the coexistence of competing magnetic interactions, leading to the first-order metamagnetic transition. Similar to its single-crystal counterpart, an anisotropic magnetic effect is observed in the MnP film, which is independently confirmed by magnetic hysteresis loop and radio-frequency transverse susceptibility (TS) measurements. The evolution of the screw to the cone and fan phases is precisely tracked from magnetization vs magnetic field/temperature measurements. The temperature dependence of the anisotropy fields, extracted from the TS spectra, yields further insight into the competing nature of the magnetic phases. Unfolding of the different helical phases at $T<120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ $(\ensuremath{\sim}{T}_{\mathrm{N}})$ is analyzed by the temperature- and field-dependent magnetic entropy change. Based on these findings, the comprehensive magnetic phase diagrams of the MnP nanorod film are constructed for both the in-plane and out of plane magnetic field directions, revealing emergent strain/dimensionality-driven helical magnetic features that are absent in the magnetic phase diagram of the MnP single crystal.

Zero-field cooling exchange bias induced by spin metastable state in MnNiGa system

<p indent="0mm">The existence of the spin metastable state in magnetic materials often causes novel physics including magnetization jumps, metamagnetic transition, and skyrmions. In this paper, we design the occupancy of Mn atoms in the MnNiGa system, and based on first-principles calculations, we anticipate the existence of spin metastable state in Mn₂Ni_1.5Ga_0.5. Therefore, Mn₂Ni₂₋_{<italic>x</italic>}Ga_{<italic>x</italic>} (<italic>x</italic>=0.36–0.60) is selected as the research platform to carry out related research. The experimental results show that near the composition of Mn₂Ni_1.5Ga_0.5, the lattice constant, crystal axis ratio, DC and AC magnetic test all have abnormal changes, and the zero-field cooling exchange bias is observed, which indicates the dual effects of the spin metastable state on structure and magnetic structure, and proves the dominant effect of spin metastable state on zero-field cooling exchange bias. This work provides a new idea for the design of spin metastable states in magnetic materials, and a new method for the realization of a zero-field cold exchange bias system.

Quantum oscillations in the field-induced ferromagnetic state of MnBi2−xSbxTe4

The intrinsic antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$ undergoes a metamagnetic transition in a c-axis magnetic field. It has been predicted that ferromagnetic MnBi$_{2}$Te$_{4}$ is an ideal Weyl semimetal with a single pair of Weyl nodes. Here we report measurements of quantum oscillations detected in the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$, where Sb substitution tunes the majority carriers from electrons to holes. Single frequency Shubnikov-de Haas oscillations were observed in a wide range of Sb concentrations (0.54 $\leq$ x $\leq$ 1.21). The evolution of the oscillation frequency and the effective mass shows reasonable agreement with the Weyl semimetal band-structure of ferromagnetic MnBi$_{2}$Te$_{4}$ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure sensitively depends on magnetism.

Metamagnetic transition and magnetocaloric properties of Ni45Mn42In13 Heusler alloy

ABSTRACT We report a study of the metamagnetic transition and magnetocaloric effect in the ternary Ni45Mn42In13 Heusler alloy system. We also investigate the structural and magnetic properties of the alloy. The crystal structure exhibited a mixture phase of martensite and austenite at room temperature which are typical for Heusler alloys. The thermomagnetization curves have martensitic and magnetic transition near room temperature and with a quite large magnetization value under 5 T applied external magnetic field. The obvious reversible metamagnetic behavior was seen between 285 and 295 K temperatures. The isothermal M–H curves also confirmed the metamagnetic transition and provided the magnetocaloric effects by Maxwell relations. This system underwent an austenite–martensite phase transition and the change in magnetic entropy was found to be quite large across this martensitic transition, so this led to an inverse magnetocaloric effect. Significant maximum magnetic entropy was obtained under 5 T applied magnetic field.

Metamagnetic Behavior in a Quadruple Perovskite Oxide.

A cubic quadruple perovskite oxide CeMn3Cr4O12 has been synthesized under high-pressure and high-temperature conditions of 8 GPa and 1273 K. The X-ray absorption spectroscopy reveals that the Ce ions are in a trivalent state, as represented by the ionic model of Ce3+Mn3+3Cr3+4O12. The magnetic study demonstrates three independent antiferromagnetic transitions attributed to Ce (∼10 K), Mn (46 K), and Cr (133 K) ions. Furthermore, a magnetic field-induced antiferromagnetic-to-ferromagnetic (metamagnetic) transition of Ce3+ 4f moments is observed at low temperatures below 20 K, exhibiting a rare example of metamagnetism in the Ce3+-oxides. This finding represents that the 3d-electron magnetic sublattices play a role in the metamagnetism of 4f-electron magnetic moments, demonstrating a new aspect of the 3d-4f complex electron systems.

Magnetoelastic transition and giant anisotropic magnetostriction effect in textured Mn&amp;lt;sub&amp;gt;1.9&amp;lt;/sub&amp;gt;Co&amp;lt;sub&amp;gt;0.1&amp;lt;/sub&amp;gt;Sb polycrystal

Mn2Sb-based alloys have extremely promising practical applications in many fields owing to their abundant and controllable magnetofuntional properties around the first-order magnetoelastic transition, as well as their low costs. However, only a few studies on the magnetostrain of these alloys have been reported. Herein, a highly textured Mn1.9Co0.1Sb polycrystal is fabricated by using the arc-melting technique, which produces a strong temperature gradient induced by rapid cooling during the arc-melting process, resulting in the highly selective orientation of crystal growth. X-ray diffraction and thermomagnetic measurements show that the grains grow in certain crystallographic directions. Metamagnetic transition is revealed by the isothermal magnetization curve measurements, which is observed to be more obvious at lower temperatures. The standard resistive strain-gauge method is adopted to measure the magnetostriction effect. Giant anisotropic magnetostrain values of up to 0.246% and −0.11% are obtained parallel and vertical to the texture of the alloy, respectively. This magnetostriction effect mainly originates from the anisotropic lattice distortion across the first-order magnetoelastic phase transition.