Continuous Magnetic Phase Transition Research Articles

Theoretical analysis of magnetic properties and the magnetocaloric effect using the Blume-Capel model

This work investigates the magnetic properties and the magnetocaloric effect in the spin-1 Blume-Capel model. The study was carried out using the mean-field theory from the Bogoliubov inequality to obtain the expressions of free energy, magnetization and entropy. The magnetocaloric effect was calculated from the variation of the entropy obtained by the mean-field theory. Due to the dependence on the external magnetic field and the anisotropy included in the model, the results for the magnetocaloric effect provided the system with first-order and continuous phase transitions. To ensure the results, the Maxwell relations were used in the intervals where the model presents continuous variations in magnetization and the Clausius-Clapeyron equation in the intervals where the model presents discontinuity in the magnetization. The methods and models for the analysis of a magnetic entropy change and first-order and continuous magnetic phase transitions, such as mean-field theory and the Blume-Capel model, are useful tools in understanding the nature of the magnetocaloric effect and its physical relevance.

Magnetic properties and phase diagram of quasi-two-dimensional Na2Co2TeO6 single crystal under high magnetic field

We investigated the magnetic characteristics of Na2Co2TeO6 at different temperatures and magnetic field. The experimental results indicated that the magnetic field can disturb the antiferromagnetic interaction and lead to the disorder. Magnetization curves measured with different angles θ (θ is between the magnetic field direction and c axis) express the magnetocrystalline anisotropy in this system. When the angle θ = 0 (magnetic field parallel to c axis), two continuous magnetic phase transitions at critical temperature T N1 and T N3 were observed. As θ changes, T N1 is almost independent on θ, indicating the magnetic ordering at T N1 was a spontaneous behavior with a robust AFM characteristic. On the other hand, as θ increases from 0 to 180, T N3 presents extreme value at θ = 90 (magnetic field perpendicular to c axis). It indicates that T N3 were sensitive to temperature and magnetic fields. At some angles closing to ab plane, an additional phase transition was observed at T N2. This phase transition at T N2 may mainly result from the long range antiferromagnetic ordering within ab-plane. Furthermore, the magnetization measurement up to 50 T revealed the strong antiferromagnetic coupling in the system, and in which the magnetic coupling within the honeycomb layers is strong and the magnetic coupling interaction between honeycomb layers is weaker. Based on the experimental results, we have obtained the complete magnetic phase diagram.

Limits to magnetic quantum criticality from nuclear spins

The phenomenology of quantum phase transitions concerns physics at low temperatures and energies, and corresponding solid-state experiments often reach millikelvin temperatures. However, this is a scale where in many solids the influence of nuclear spins and their hyperfine interaction is no longer negligible. This may limit the observability of electronic quantum critical phenomena. Here we discuss how continuous magnetic quantum phase transitions get influenced, modified, or destroyed by the coupling to nuclear spins. We use simple yet paradigmatic spin models for magnetic quantum criticality and determine modifications to the phase diagram, the excitation spectrum, and thermodynamics due to the presence of nuclear spins. We estimate crossover scales below which purely electronic quantum criticality is no longer observable, and discuss the novel physics emerging at low temperatures. Our results are relevant for a variety of compounds displaying magnetic quantum phase transitions and, more generally, highlight the sensitivity of quantum critical systems to small perturbations.

Structural stability, magneto-electronic properties, and tuning effects for transition metal-doped net-Y nanoribbons

Low-dimensional carbon allotropes have attracted much attention due to their great potential for applications in future electronic and magnetic devices. Recently, new graphene-like nanoribbons consisting of four-, six-, and eight-membered carbon rings have been synthesized successfully, and a related two-dimensional structure net-Y was subsequently proposed. Here, we report theoretical investigations on the structural stability and magneto-electronic properties for the net-Y armchair nanoribbons doped with low-concentration transition metal (TM) atoms (Mn, Ni, Fe, Co, and V), especially focusing on their electric-magnetic and mechano-magnetic coupling effects. The calculated binding energy and molecular dynamics simulation confirm that all these ribbons hold a high stability. Particularly, by TM-doping, the intrinsic non-magnetic semiconducting ribbon can be converted to a bipolar magnetic semiconductor (BMS), a half metal (HM), or a spin-degenerate magnetic semiconductor (MSC), presenting rich magnetic features. Meanwhile, this magnetism is flexibly tunable by the transverse electric field or stretched strain. For example, a low electric field can transform the ribbon into the HM phase, and the mechanic strain can realize a continuous magnetic phase transition among BMS, magnetic metal (MS), MSC, HM, and half semiconductor. These findings indicate that the TM-doped net-Y ribbon possesses favorable magnetism, which might have promising applications in magnetic nanodevices.

Spin dynamics and a nearly continuous magnetic phase transition in an entropy-stabilized oxide antiferromagnet

Studying magnetism in entropy-stabilized oxides (ESOs) is an exciting new research avenue following the recent discovery of antiferromagnetic order in (Mg${}_{0.2}$Co${}_{0.2}$Ni${}_{0.2}$Cu${}_{0.2}$Zn${}_{0.2}$)O (MgO-ESO). Muon spin relaxation and neutron scattering experiments reveal a continuous transition that occurs across a broad distribution of antiferromagnetic ordering temperatures throughout the MgO-ESO sample. The ground state exhibits a magnon gap and significant quasielastic neutron scattering, suggesting unusual intrinsic magnetic fluctuations. These results resolve several open questions about magnetic order in MgO-ESO and pave the way for future investigations of magnetic ESOs.

Imaging and control of critical fluctuations in two-dimensional magnets.

Strong magnetization fluctuations are expected near the thermodynamic critical point of a continuous magnetic phase transition. Such critical fluctuations are highly correlated and in principle can occur at any time and length scales1; they govern critical phenomena and potentially can drive new phases2,3. Although critical phenomena in magnetic materials have been studied using neutron scattering, magnetic a.c. susceptibility and other techniques4-6, direct real-time imaging of critical magnetization fluctuations remains elusive. Here we develop a fast and sensitive magneto-optical imaging microscope to achieve wide-field, real-time monitoring of critical magnetization fluctuations in single-layer ferromagnetic insulator CrBr3. We track the critical phenomena directly from the fluctuation correlations and observe both slowing-down dynamics and enhanced correlation length. Through real-time feedback control of the critical fluctuations, we further achieve switching of magnetic states solely by electrostatic gating. The ability to directly image and control critical fluctuations in 2D magnets opens up exciting opportunities to explore critical phenomena and develop applications in nanoscale engines and information science.

Caloric effects around phase transitions in magnetic materials described by ab initio theory: The electronic glue and fluctuating local moments

We describe magneto-, baro-, and elastocaloric effects (MCEs, BCEs, and eCEs) in materials, which possess both discontinuous (first-order) and continuous (second-order) magnetic phase transitions. Our ab initio theory of the interacting electrons of materials in terms of disordered local moments has produced explicit mechanisms for the drivers of these transitions, and here, we study associated caloric effects in three case studies where both types of transition are evident. Our earlier work had described FeRh’s magnetic phase diagram and large MCE. Here, we present calculations of its substantial BCE and eCE. We describe the MCE of dysprosium and find very good agreement with experimental values for isothermal entropy (ΔSiso) and adiabatic temperature (ΔTad) changes over a large temperature span and different applied magnetic field values. We examine the conditions for optimal values of both ΔSiso and ΔTad that comply with a Clausius–Clapeyron analysis, which we use to propose a promising elastocaloric cooling cycle arising from the unusual dependence of the entropy on temperature and biaxial strain found in our third case study—the Mn3GaN antiperovskite. We explain how both ΔSiso and ΔTad can be kept large by exploiting the complex tensile strain–temperature magnetic phase diagram, which we had earlier predicted for this material and also propose that hysteresis effects will be absent from half of the caloric cycle. This rich and complex behavior stems from the frustrated nature of the interactions among the Mn local moments.

Geometry, induced magnetism and modified electronic behaviors for magnetic atom adsorption on antimonene nanotubes.

Antimonene nanotubes, a class of important derivatives of the 2D counterpart (Sb monolayer), with transition metal (TM) atom adsorption were investigated systematically based on the first-principles calculations. For a stable geometry, the lengths of TM-Sb bonds on the tube surface strongly depend on their relative electronegativity. In particular, we find that the intrinsic magnetic moment magnitude of the TM atom plays a decisive role in inducing tube magnetism, and only TM atoms with a larger intrinsic magnetic moment (≥3.0 μB) can induce the magnetism for tubes. The strong interaction and coupling between the TM d-orbital and Sb p-orbital lead to variously favorable magnetic phases, such as the spin bipolar semiconductor and half-semiconductor, which is predicted to be stable beyond room temperature. In addition, the weakening or quenching of the magnetism for the adsorbed TM atom is intimately related to the expansion of the TM atom valence electron configuration and the charge transfer. Furthermore, the TM adsorption can also effectively regulate the tube carrier mobility to the difference of several orders of magnitude, and results in significant carrier polarity and spin polarity of mobility. A sensitive electric-magnetic coupling effect was also shown to cause continuous magnetic phase transition, providing more opportunity for obtaining magneto-electric materials.

Continuous magnetic phase transition in artificial square ice

Critical behavior is very common in many fields of science and a wide variety of many-body systems exhibit emergent critical phenomena. The beauty of critical phase transitions lies in their scale-free properties, such that the temperature dependence of physical parameters of systems differing at the microscopic scale can be described by the same generic power laws. In this work we establish the critical properties of the antiferromagnetic phase transition in artificial square ice, showing that it belongs to the two-dimensional Ising universality class, which extends the applicability of such concepts from atomistic to mesoscopic magnets. Combining soft x-ray resonant magnetic scattering experiments and Monte Carlo simulations, we characterize the transition to the low temperature long range order expected for the artificial square ice system. By measuring the critical scattering, we provide direct quantitative evidence of a continuous magnetic phase transition, obtaining critical exponents which are compatible with those of the two-dimensional Ising universality class. In addition, by varying the blocking temperature relative to the phase transition temperature, we demonstrate its influence on the out-of-equilibrium dynamics due to critical slowing down at the phase transition.

Adsorption of flexible polymer chains on a surface: Effects of different solvent conditions.

Polymer chains undergoing a continuous adsorption-desorption transition are studied through extensive computer simulations. A three-dimensional self-avoiding walk lattice model of a polymer chain grafted onto a surface has been treated for different solvent conditions. We have used an advanced contact-density chain-growth algorithm, in which the density of contacts can be directly obtained. From this quantity, the order parameter and its fourth-order Binder cumulant are computed, as well as the corresponding critical exponents and the adsorption-desorption transition temperature. As the number of configurations with a given number of surface contacts and monomer-monomer contacts is independent of the temperature and solvent conditions, it can be easily applied to get results for different solvent parameter values without the need of any extra simulations. In analogy to continuous magnetic phase transitions, finite-size-scaling methods have been employed. Quite good results for the critical properties and phase diagram of very long single polymer chains have been obtained by properly taking into account the effects of corrections to scaling. The study covers all solvent effects, going from the limit of super-self-avoiding walks, characterized by effective monomer-monomer repulsion, to poor solvent conditions that enable the formation of compact polymer structures.

Kibble-Zurek scalings of continuous magnetic phase transitions in spin-1 spin-orbit-coupled Bose-Einstein condensates

We investigate the universal dynamics of a continuous magnetic phase transition from a unmagnetized phase to a ferromagnetic phase in a spin-1 spin-orbit-coupled Bose-Einstein condensate. When the system approaches the critical point, the Landau critical velocity gradually decreases to zero and so the correlation length diverges. Therefore, during the slow passage through the critical point, ferromagnetic domains are spontaneously created according to the Kibble-Zurek mechanism. Through calculating the Bogoliubov excitations, we give the Landau critical velocity and the correlation length, from which we derive the Kibble-Zurek scalings. Furthermore, we numerically simulate the critical spatial-temporal dynamics of the bifurcation delay and spontaneous domain formation, and extract the universal scaling exponents. The numerical scalings extracted from the critical spatial-temporal dynamics are very consistent with the scalings derived from the correlation length.

Quantum critical behaviour in the (Cr97.8Si2.2)100−yMoy alloy system

The Cr-Si system offers an ideal reference alloy as a case study to follow the evolution of the well-known first-order magnetic transition in dilute Cr-Si by doping with Mo, which is iso-electronic with Cr. This alloy system, in addition, then also offers the opportunity to search for effects of quantum criticality (QC) associated with the process of driving the Néel transition temperature (TN) to zero. Previous investigations on the ternary (Cr97.8Si2.2)100−yMoy alloy system (0 ≤ y ≤ 9) indicated possible quantum critical behaviour at a critical concentration yc ≈ 3.8. The present paper extends these studies to additional concentrations in the alloy series, as well as to explore key indicators of QC, such as Hall coefficient (RH) and magnetic susceptibility (χ) measurements. Recent results indicate the possible existence of two critical points in the (Cr97.8Si2.2)100−yMoy alloy system. The first critical point at yc1 is postulated to be related to the concentration where the first-order transition gives way to a second-order-like continuous magnetic phase transition. Behaviour normally associated with QC is observed at the second critical concentration yc2, accompanying a transition to paramagnetism.

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

We present the specific heat, magnetization, optical spectroscopy measurements and the firstprinciple calculations on the Weberite structure Ca2Os2O7 single crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A \lambda-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, small amount of itinerant carriers with short life time \tau are observed, which is gapped at 20 K with a direct gap of 0:24 eV . Our first principle calculations indicate that the anti-ferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t2g bands and push them away from Fermi level(EF). On the other hand, a non-collinear magnetic interaction is needed to push the Os(4c) bands away from EF, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and non-collinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

Magnetic phase transition in confined MnO nanoparticles studied by polarized neutron scattering

We have investigated the magnetic ordering and the phase transition in MnO nanoparticles confined in a porous glass using polarized neutron scattering. These MnO nanoparticles are best described as extended wormlike structures with a mean diameter of $70\text{ }\text{\AA{}}$. We observe an apparent continuous magnetic phase transition in MnO nanoparticles, in contrast to the well-known discontinuous phase transition in bulk MnO. By polarization analysis, separating the magnetic scattering, it is found that within the individual MnO nanoparticles about 60% of atoms remain disordered in the low-temperature limit, presumably due to interactions between nanoparticles and glass walls. The continuous character of the phase transition and the unusual temperature dependence suggests a surface-induced disorder phenomenon.

Stoner ferromagnetic phase of graphene in the presence of an in-plane magnetic field

We study the effects of an in-plane magnetic field on the ground-state properties of both gapless and gapped graphene sheets within random-phase approximation. The critical magnetic field which leads to a fully spin polarized phase increases by decreasing the carrier density at zero gap indicating that no spontaneous magnetic phase transition occurs. However, at large energy-gap values it decreases by decreasing the density. We find a continuous quantum magnetic phase transition (Stoner phase) for Dirac fermions in a doped graphene sheet. Novel in-plane magnetic field dependence of the charge and spin susceptibilities are uncovered.

Evolution of the magnetic phase transition in MnO confined to channel type matrices: Neutron diffraction study

Neutron diffraction studies of antiferromagnetic MnO confined to MCM-41 type matrices with channel diameters 24-87 A demonstrate a continuous magnetic phase transition in contrast to a discontinuous first order transition in the bulk. The character of the magnetic transition transforms with decreasing channel diameter, showing the decreasing critical exponent and transition temperature, however the latter turns out to be above the N\'eel temperature for the bulk. This enhancement is explained within the framework of Landau theory taking into consideration the ternary interaction of the magnetic and associated structural order parameters.

Suppression of the magnetic phase transition in manganites close to the metal-insulator crossover

We report the suppression of the magnetic phase transition in La1-xCaxMnO3 close to the localized-to-itinerant electronic transition, i.e. at x = 0.2 and x = 0.5. A new crossover temperature Tf can be defined for these compositions instead of TC. Unlike in common continuous magnetic phase transition the susceptibility does not diverge at Tf and a spontaneous magnetization cannot be defined below it. We propose that the proximity to the doping-induced metal-insulator transition introduces a random field which breaks up the electronic/magnetic homogeneity of the system and explains these effects.

The Ising model on a dynamically triangulated disk with a boundary magnetic field

We use Monte Carlo simulations to study a dynamically triangulated disk with Ising spins on the vertices and a boundary magnetic field. For the case of zero magnetic field we show that the model possesses three phases. For one of these the boundary length grows linearly with disk area, while the other two phases are characterized by a boundary whose size is on the order of the cut-off. A line of continuous magnetic transitions separates the two small boundary phases. We determine the critical exponents of the continuous magnetic phase transition and relate them to predictions from continuum 2-d quantum gravity. This line of continuous transitions appears to terminate on a line of discontinuous phase transitions dividing the small boundary phases from the large boundary phase. We examine the scaling of bulk magnetization and boundary magnetization as a function of boundary magnetic field in the vicinity of this tricritical point.

X-ray-scattering study of the two magnetic correlation lengths in uranium antimonide.

In common with some other recent critical scattering studies we have observed the presence of a second component in the critical fluctuations close to the continuous magnetic phase transition in USb. It is found that the magnetic anisotropy of this second component does not exhibit the same directional dependence as the {open_quote}{open_quote}normal{close_quote}{close_quote} critical scattering and that the critical behavior of this feature is not consistent with an Ornstein-Zernike treatment of the scaling. {copyright} {ital 1996 The American Physical Society.}

Quantum-liquid regimes for spin chains coupled to phonons: Phonon density wave versus magnetic order.

We consider a chain of localized spins, coupled to phonons. Recently this problem has been solved exactly for a ``basic model,'' a family of spin-phonon Hamiltonians ${\mathit{H}}_{\mathrm{BM}}$ characterized by one parameter (coupling constant K), and a zero-T first-order phase transition from the magnetic (ferro or antiferro) state at low couplings to the nonmagnetic state with a phonon density wave at high couplings was found. Here we probe the general case, constructing an effective Hamiltonian H for low-energy degrees of freedom by means of regular expansion in deviations \ensuremath{\delta}H=H-${\mathit{H}}_{\mathrm{BM}}$ of the general Hamiltonian H from that of the basic model. In linear approximation in \ensuremath{\delta}H the problem appears to be exactly solvable as well, due to an infinite number of conservation laws. If K is far enough from the critical value ${\mathit{K}}_{\mathit{c}}$, then the character of the basic model solution is not altered. In the vicinity of ${\mathit{K}}_{\mathit{c}}$ the magnetic state is dramatically reconstructed: Here the ground state is a gapless magnetic quantum liquid, consisting of mobile singlet spin-phonon complexes and unbound spins. The fraction of singlets increases gradually upon approaching ${\mathit{K}}_{\mathit{c}}$, and the magnetic order parameter gradually vanishes. Thus we have here a partial screening of spins by phonons without formation of a phonon density wave. The latter appears only at K=${\mathit{K}}_{\mathit{c}}$ in the first-order phase transition. Corrections, quadratic in \ensuremath{\delta}H, destroy the integrability of the system, but outside a narrow critical region around ${\mathit{K}}_{\mathit{c}}$ they only lead to an opening of a small gap in the spectrum of the quantum liquid. The behavior of the system within the critical region is an open question. Most likely the continuous magnetic phase transition at K=${\mathit{K}}_{\mathit{c}}$ becomes a first-order one, but close to second order. The relevance of our results for three-dimensional systems and possible applications to compounds with anomalously weak magnetism are briefly discussed. \textcopyright{} 1996 The American Physical Society.