Ferromagnetic Phase Boundary Research Articles

Domain engineering in ferromagnetic morphotropic phase boundary with enhanced and non-hysteretic magnetostriction: A phase-field simulation

Giant and non-hysteretic magnetostriction is critical for magneto-mechanical applications, such as transducers, sensors, and actuators. In this work, the introduction of domain engineering into the single crystals near the ferromagnetic morphotropic phase boundary (MPB) is achieved through phase-field simulation, with the aim of exerting control over domain configuration and magnetostriction. It is found that intermediate external magnetic fields applied along non-easy magnetic axes during the nucleation stage can significantly affect the subsequent domain growth and evolution kinetics. The simulations reveal a domain wall broadening mechanism to explain the significantly improved non-hysteretic magnetoelastic responses observed in engineered tetragonal crystals. Our simulation results additionally demonstrate a discernible correlation between the domain wall densities and external fields in the engineered tetragonal domain. This research provides a potential pathway for the advancement of giant and non-hysteretic magnetostrictive material designs.

A Comparison of the magnetic characteristics and exchange spring effect of Y-substituted Co2Mn1-xYxO4 composite spinel oxides

Magnetic spinel oxides are materials that exhibit unique magnetic properties due to their crystal structure and composition. Herein, we report the sol-gel synthesis of Y-substituted Co2Mn1-xYxO4 (0 ≤ x ≤ 0.1) inverse spinel oxides. X-ray diffraction pattern confirms the single-phase cubic structure with space group Fd3m. Magnetic susceptibility measurement results exhibited the para-ferrimagnetic phase transition near TC = 174 K. Also, another phase transition near a specific temperature T* = 76 K, below TC of Y-doped samples. Furthermore, the magnetic entropy change analysis also confirmed the magnetic phase transition near the temperature of T*. Moreover, with the phase transition near T*, Y-doped samples show magnetic anomalies at low temperatures. Similarly, the analysis of magnetic entropy change and the magnetic hysteresis loops show the coexistence of “soft " and “hard” ferromagnetic phases at low temperatures in Y-substituent Co2Mn1-xYxO4 samples. Using the coupling of the “soft/hard” ferromagnetic phase boundary, a noticeable exchange spring effect can be observed at low temperatures. Due to the exchange spring effect, the magnetic energy product (BH)max up to 3.1 MJ m−3 can be obtained at 2 K. Such insights can help to optimize the behavior of magnetic materials and structures, leading to improved performance and functionality in applications such as magnetic data storage, magnetic sensors, spintronics, and magnetic nanoparticles.

Ultrasensitive magnetostrictive responses at the pre-transitional rhombohedral side of ferromagnetic morphotropic phase boundary

The morphotropic phase boundary (MPB) has been utilized extensively in ferroelectrics and recently been extended to ferromagnets, especially for the magnetostrictive materials. Here, guided by phenomenological theories and phase-field simulations, we proposed a design strategy for obtaining the ultrasensitive magnetoelastic response at the pre-transitional rhombohedral side of ferromagnetic MPB, by further flattening the energy landscape while maintaining large intrinsic magnetostriction. To validate this, we judiciously introduced the light-rare-earth-based $$\hbox {Tb}_{0.1}\hbox {Pr}_{0.9}$$ system to the Co-doped $$\hbox {Tb}_{0.27}\hbox {Dy}_{0.73}\hbox {Fe}_{2}$$ alloy, as $$\hbox {Tb}_{0.1}\hbox {Pr}_{0.9}$$ is an anisotropy compensation system with large intrinsic strains and the transition metal dopant of Co tends to optimize the magnetostriction. Phase-field modeling was used to determine the detailed magnetic domain evolution of the investigated multi-component Laves phase compounds, the results of which were compared with experimental results. At room temperature, an ultrahigh magnetoelastic response $${d}_{33}$$ was found in $$\hbox {Tb}_{0.253}\hbox {Dy}_{0.657}\hbox {Pr}_{0.09}(\hbox {Fe}_{0.9}\hbox {Co}_{0.1})_{2}$$ recompensation system especially at low fields, which is superior to that of the commercial $$\hbox {Tb}_{0.27}\hbox {Dy}_{0.73}\hbox {Fe}_{2}$$ (Terfenol-D) polycrystal. The ultrahigh magnetostrictive sensitivity, together with low raw material cost makes it one of the strongest candidates for magnetostriction applications.

Phase field simulation of grain size effects on the phase coexistence and magnetostrictive behavior near the ferromagnetic morphotropic phase boundary

The grain size effects on the phase coexistence and magnetostrictive response of Tb1−xDyxFe2 polycrystals near the ferromagnetic morphotropic phase boundary (MPB) are revealed through phase-field modeling. It shows that phase coexistence is a universal phenomenon in polycrystals for both ferromagnetic and ferroelectric MPB and that the range of compositions for phase coexistence increases with decreasing grain sizes. A large, reversible, and anhysteretic magnetostrictive response at low external fields is also found in the fine-grained polycrystals around the ferromagnetic MPB, which offers us a route to developing nanocrystalline magnetostrictive materials.

Effects of Bi3+ doping on charge ordering and high magnetic field phase diagram of Nd0.6-xBixCa0.4MnO3

We report the doping effects and pulsed high magnetic field-induced charge ordering to ferromagnetic phase transitions in manganites Nd0.6-xBixCa0.4MnO3 (0 ≤ x ≤ 0.4). The experimental results reveal that the charge ordering is significantly enhanced upon Bi3+ doping, and the charge ordering temperature increases with increasing Bi3+ content. With applying pulsed high magnetic field, the charge ordering phase collapses into ferromagnetic phase through a metamagnetic transition. The phase diagram of Nd0.6-xBixCa0.4MnO3 has been determined by the high magnetic field studies, in which the charge ordering and ferromagnetic phase boundary is clearly figured out. The critical fields for melting the charge ordering phase of Nd0.6-xBixCa0.4MnO3 increase significantly with increasing Bi3+ content, which is revealed to reach up to 33.5 T for the sample with x = 0.3. Based on the hybridization between Bi-6s orbitals and O-2p orbitals, we assume that the 6s2 lone-pair character of Bi3+ plays a crucial role in favoring the charge ordering in Nd0.6-xBixCa0.4MnO3 (0 ≤ x ≤ 0.4).

Polar-antipolar transition and weak ferromagnetism in Mn-doped Bi0.86La0.14FeO3

Having been considered as a prime example of a room-temperature magnetoelectric multiferroic, BiFeO3 continues to attract much interest. Since functional properties of this material can be effectively influenced by chemical, electrical, magnetic, mechanical and thermal stimuli, it can serve as a model for the investigation of cross-coupling phenomena in solids. Special attention is currently paid to the study of chemical pressure-driven magneto-structural transformations. In this paper, we report on the effect of the Mn doping on the crystal structure and magnetic behavior of the Bi1−xLaxFeO3 multiferroics near their polar-antipolar (antiferromagnetic-weak ferromagnetic) phase boundary. Synchrotron x-ray and neutron powder diffraction measurements of the Bi0.86La0.14Fe1−xMnxO3 (x = 0.05, 0.1, 0.15) compounds have been performed. The diffraction data suggest that the Mn substitution results in the suppression of the ferroelectric polarization and gives rise to the appearance of the antiferroelectric (generally, PbZrO3-related) phase characteristic of the phase diagrams of the Bi1−xRExFeO3 (RE = rare-earth) systems. Depending on the Mn concentration (determining phase composition of the Bi0.86La0.14Fe1−xMnxO3 samples at room temperature), either complete or partial revival of the polar phase can be observed with increasing temperature. Magnetic measurements of the samples indicate that the Mn doping affects the stability of the cycloidal antiferromagnetic order specific to the polar phase, thus resulting in the formation of a ferroelectric and weak ferromagnetic state.

Simulation study on exchange interaction and unique magnetization near ferromagnetic morphotropic phase boundary

Extensive efforts have been made in searching enhanced functionalities near the so-called morphotropic phase boundaries (MPBs) in both ferroelectric and ferromagnetic materials. Due to the exchange anti-symmetry of the wave function of fermions, it is widely recognized that the exchange interaction plays a critical role in ferromagnetism. As a quantum effect, the exchange interaction is magnitudes larger than electric interaction, leading to a fundamental difference between ferroelectricity and ferromagnetism. In this paper, we establish an energetic model capturing the interplay among the anisotropy energy, magnetostatic energy and the exchange energy to investigate systematically the effects of the exchange energy on the behavior of the ferromagnetic MPB. For the first time, it is found that the exchange energy can narrow the width of MPB region in the composition temperature phase diagram for ferromagnetic MPB systems. As temperature increases, MPB region becomes wider because of the weakening of the exchange interaction. Our simulation results suggest that the exchange energy play a critical role on the unique behavior of ferromagnetic MPB, which is in contrast different from that of ferroelectric MPB.

Lattice and magnetic dynamics in perovskite Y1−xLaxTiO3

Inelastic neutron scattering combined with the dynamic pair density function (DPDF) analysis were used to investigate the magnetic and lattice dynamics in the orbitally active ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ as it crosses the antiferromagnetic (AFM) to ferromagnetic (FM) phase boundary. Upon doping, the FM state present in ${\mathrm{YTiO}}_{3}$ is suppressed on approaching a critical concentration of ${x}_{c}\ensuremath{\sim}0.3$ in which ${T}_{C}\ensuremath{\simeq}0$, and is replaced by the AFM phase of ${\mathrm{LaTiO}}_{3}$. Below ${x}_{c}$, magnetic scattering from spin waves is dominant at low energies. At ${x}_{c}$ with a ${T}_{C}\ensuremath{\simeq}0$, magnetic scattering is also observed and is most likely due to AFM fluctuations. At the same time, local atomic fluctuations extending to 50 meV are observed above and below the magnetic transitions from $0\ensuremath{\le}x\ensuremath{\le}1$ that show distinct characteristics with $x$. From Y to La, a clear difference is observed in the phonon density of states as a function of doping as well. At $x=0.15$ and 0.3, low-energy modes involving predominantly the rare-earth ion become suppressed with increasing temperature, while in $x=1$, strong suppression of phonon modes across a wide range in energy is observed above ${T}_{N}$. It is likely that in the Y heavy samples, phonon modes below 20 meV have a stronger influence on the orbital excitations, while in ${\mathrm{LaTiO}}_{3}$, a strong phonon dependence is observed upon cooling up to ${T}_{N}$.

Phase-field simulation of domain structures and magnetostrictive response in Tb1−xDyxFe2 alloys near morphotropic phase boundary

Phase-field method micromagnetic microelastic modeling is employed to simulate the thermal domain stability and enhanced magnetostrictive responses around the ferromagnetic morphotropic phase boundary (MPB) in giant magnetostrictive Tb1−xDyxFe2 (x≈0.27) single crystal. The simulation shows that the rhombohedral and tetragonal phases coexist in equilibrium in the vicinity of MPB region due to the balance of weak magnetocrystalline anisotropy and strong exchange, magnetostatic and ferroelastic interaction. Enhanced magnetostrictive response is found in the vicinity of MPB, which could be attributed to the low-energy rotating pathways of local magnetization vectors in the phase coexisting region.

High magnetic field phase diagram in electron-doped manganites La(0.4)Ca(0.6)Mn(1-y)Cr(y)O3.

We report the charge-order to ferromagnetic phase transition induced by pulsed high magnetic field and impurity doping effects in manganites La0.4Ca0.6(Mn1−yCry)O3 (0 ≤ y ≤ 0.2). Significant charge-order suppression and ferromagnetic tendency upon the Cr3+-doping are evidenced, and three different ground states are identified, namely the charge-order state, the phase separated state, and the spin-glass like state. Phase diagram in the H-y plane at 4.2 K is determined by the high magnetic field study, in which the charge-order and ferromagnetic phase boundary is clearly figured out. The critical magnetic field for melting the charge-order phase of La0.4Ca0.6MnO3 is revealed to reach up to 46 T at 4.2 K. Interestingly, distinct responses of the three states to the high magnetic field are observed, indicating the special physics regarding the charge order melting process in each state. The mechanism of the doping induced charge-order suppression and ferromagnetism promotion can be understood by the competition between the antiferromagnetic interaction of Cr-Mn and local enhancement of electron hopping by Cr3+.

Highq-state clock spin glasses in three dimensions and the Lyapunov exponents of chaotic phases and chaotic phase boundaries

Spin-glass phases and phase transitions for $q$-state clock models and their $q\ensuremath{\rightarrow}\ensuremath{\infty}$ limit the $XY$ model, in spatial dimension $d=3$, are studied by a detailed renormalization-group study that is exact for the $d=3$ hierarchical lattice and approximate for the cubic lattice. In addition to the now well-established chaotic rescaling behavior of the spin-glass phase, each of the two types of spin-glass phase boundaries displays, under renormalization-group trajectories, their own distinctive chaotic behavior. These chaotic renormalization-group trajectories subdivide into two categories, namely as strong-coupling chaos (in the spin-glass phase and, distinctly, on the spin-glass--ferromagnetic phase boundary) and as intermediate-coupling chaos (on the spin-glass--paramagnetic phase boundary). We thus characterize each different phase and phase boundary exhibiting chaos by its distinct Lyapunov exponent, which we calculate. We show that, under renormalization group, chaotic trajectories and fixed distributions are mechanistically and quantitatively equivalent. The phase diagrams of arbitrary even $q$-state clock spin-glass models in $d=3$ are calculated. These models, for all non-infinite $q$, have a finite-temperature spin-glass phase. Furthermore, the spin-glass phases exhibit a universal ordering behavior, independent of $q$. The spin-glass phases and the spin-glass--paramagnetic phase boundaries exhibit universal fixed distributions, chaotic trajectories and Lyapunov exponents. In the $XY$ model limit, our calculations indicate a zero-temperature spin-glass phase.

Magnetic properties of the spinel-type [formula omitted

The spinel sulphide CuCr 2 S 4 is a metallic ferromagnet with a Curie temperature T c ≃ 380 K , while CuHf 2 S 4 has no magnetic anomaly. Magnetic properties of the quaternary spinel-type Cu ( Cr 1 - x Hf x ) 2 S 4 system have been studied. With increasing x the ferromagnetic properties are weakened gradually from a predominant ferromagnetic, a spin-glass, finally to a simple paramagnetic behavior. For the composition of x ≃ 0.50 , a re-entrant spin-glass phase could emerge, even though the Curie temperature is ill-defined as a ferromagnetic phase boundary. Specimens with x ≥ 0.90 remain paramagnetic down to 4.2 K. A spin crossover phenomenon is found around 160 K in the specimens of x = 0.50 – 0.70 . A step-like anomaly is manifestly detected in the magnetization, which corresponds with the change of the spin state. This crossover indicates that the spin state converts from high temperature S = 2 into low temperature S = 3 2 states. In the ordered states in T < 160 K , the magnetic moment originates from only Cr 3 + ions.

Magnetic anisotropy in the ferromagnetic insulating state ofLa1−xCaxMnO3

Ferromagnetic resonance (FMR) has been applied to study the variation of magnetic anisotropy and phase separation tendencies in the ferromagnetic insulating state of stoichiometric hole-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}(0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}0.19)$. A strongly anisotropic ferromagnetic phase is identified in the ferromagnetic insulating regime. The magnetic anisotropy evolves from positive uniaxial at $x=0.125$ and 0.15 to negative cubiclike at $x=0.175$ with increasing magnitude, whereas it decreases appreciably at $x=0.19$, which is close to the ferromagnetic metallic phase boundary. Minor contributions from ferromagnetic inhomogeneities characterized by weaker magnetic anisotropy are traced at low temperatures for $x=0.125$ and 0.15, while their temperature dependence implies coupling with the major anisotropic phase. A growing tendency towards the formation of spatially separate ferromagnetic regions in the paramagnetic regime is observed at $x\ensuremath{\geqslant}0.175$, as the ferromagnetic metallic phase is approached. Persistent anomalies in the temperature dependence of the FMR parameters are traced concurrently with the structural transformation at $60--70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Three-body correlation effects on the spin dynamics of double-exchange ferromagnets

We present a variational calculation of the spin wave excitation spectrum of double--exchange ferromagnets in different dimensions. Our theory recovers the Random Phase approximation and 1/S expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three--body correlations between a Fermi sea electron--hole pair and a magnon excitation and show that they strongly affect the spin dynamics in the parameter range relevant to experiments in the manganites. The manifestations of these correlations are many-fold. We demonstrate that they significantly change the ferromagnetic phase boundary. In addition to a decrease in the magnon stiffness, we obtain an instability of the ferromagnetic state against spin wave excitations close to the Brillouin zone boundary.Within a range of intermediate concentrations, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness, which changes into hardening for other concentrations. We discuss the relevance of these results to experiments in colossal magnetoresistance ferromagnets.

Ferromagnetic phase boundary in the bond frustrated Heisenberg model

We determine the ferromagnetic phase boundary for the short range ±J bond frustrated Heisenberg model in three dimensions using a very efficient Monte Carlo algorithm which eliminates the critical slowing down usually experienced at a second order phase transition. The phase boundary is identified by measuring the correlation length directly, a method which we show to be superior to more conventional methods such as the crossing of the Binder cumulant. The critical concentration of antiferromagnetic bonds beyond which ferromagnetism is lost is xc=0.208(2).

The specific heat of the two-dimensional JIsing model

The specific heat of the two-dimensional ±J Ising model has been investigated by the numerical transfer matrix method and Monte Carlo simulations from a new point of view. The region where a part of the specific heat takes a negative value has been investigated, which is characteristic of frustrated systems and reflects the non-trivial degeneracy of the ground state. The region mentioned above is found to be fairly large in the p–T plane (p is the concentration of the ferromagnetic bond and T is the temperature). Moreover, it includes the Nishimori line. Namely, it includes a part of the paramagnetic–ferromagnetic phase boundary, on which the specific heat cannot diverge. The present result indicates that the specific heat does not diverge at least on a part of the paramagnetic–ferromagnetic phase boundary above the multicritical point, which is in conflict with previous results.

Structural and magnetic ordering inPr0.65(CaySr1−y)0.35MnO3: Quantum critical point versus phase segregation scenarios

The phase diagram of Pr0.65(CaySr1-y)0.35MnO3, 0.6 <= y <= 0.8, has been determined by neutron diffraction, magnetization and electrical conductivity measurements in order to investigate the nature of the transition between ferromagnetic metallic and charge-ordered insulating states near y=0.75. Two possible scenarios for this transition have been proposed: a "quantum critical point"-like feature, near which an associated charge-disordered paramagnetic phase is present, or a phase coexistence region. We demonstrate that the latter case is true, phase segregation occurring on a mesoscopic/macroscopic length-scale (several hundred Angstroms to several microns). Our results show that no significant amount of the charge-disordered paramagnetic phase is present at low temperatures. Our data also indicate that the charge-ordered insulator to ferromagnetic metal phase boundary is temperature as well as composition-dependent.

Numerical Evidence of Infinite Number of Pure States Near Ferromagnetic Phase Boundary for Short-Range Spin-Glass Systems

Numerical evidence that the number of pure states in the spin glass phase is infinite near the ferromagnetic phase boundary for the three dimensional (3D) ± J Ising model and 3D gauge glass models ...

Local structure, transport, and rare-earth magnetismin the ferrimagnetic perovskiteGd0.67Ca0.33MnO3s

Bulk, single crystal, and metal-organic chemical-vapor deposition thin-film samples of ${\mathrm{Gd}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_{3}$ were prepared and examined for their electrical, magnetic, and structural properties. ${\mathrm{Gd}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_{3}$ is ferrimagnetic with a transition temperature between 50 and 80 K and a compensation temperature of about 15 K. A molecular field model with a ferromagnetic manganese sublattice antiparallel to the gadolinium sublattice qualitatively explains the magnetism data. A large high-field susceptibility is observed at 5 K, suggesting a sublattice rotation. The resistivity and the magnetoresistance show no anomaly near the ferrimagnetic transition. There is no noticeable change in the structure, as seen from the x-ray-absorption fine structure between 40 and 69 K, indicating that there is no structural discontinuity across the paramagnetic insulator to ferromagnetic insulator phase boundary. The resistivity of ${\mathrm{Gd}}_{0.67}$ ${\mathrm{Ca}}_{0.33}$ ${\mathrm{MnO}}_{3}$ is consistent with small polaron hopping at high temperatures (up to 1100 K), and possibly by a different mechanism at low temperatures.

Strong magnetic scattering in weak localization

Theoretical analysis of weak localization for the three dimensional system presently does not include the case with strong scattering by local magnetic impurities. For paramagnetic Fe/sub 0.33/Zr/sub 0.67/ alloy, a composition just below the ferromagnetic phase boundary, the magnetic impurity scattering time (/spl tau//sub s/) is estimated to be comparable to the spin-orbit scattering time (/spl tau//sub so/), but much less than the magnetic field dephasing time (/spl tau//sub H/) or inelastic scattering time (/spl tau//sub i/), so that the magnetic scattering occurs before the magnetic field dephasing does. When the local moments of magnetic impurities (in this case some of the Fe ions) do not interact with each other and their magnetization follows the Brillouin function, the magneto-resistance MR is proportional to n/sub s//sup 1/2/(H/T)/sup 2/ for small (H/T), where n/sub S/ is the density of local impurities of moment gS/spl mu//sub B/, and /spl Delta/p(H,T)/p/sup 2/ should be a unique function of (H/T), for a given alloy so long as the impurities are in the Curie regime. Experimental values of the MR at several temperatures for the Fe/sub 0.33/Zr/sub 0.67/ alloy lie on a single curve of /spl Delta/p(H,T)/p/sup 2/ vs. H/T.