Field-induced Magnetic Phase Transitions Research Articles

Statistical theory of magnetic field induced phase transitions in negative diamagnetic anisotropy liquid crystals doped with carbon nanotubes

A statistical mean-field theory is proposed for describing the magneto-orientational ordering of a composite material, namely a liquid crystal suspension of carbon nanotubes. A nematic liquid crystal with negative anisotropy of diamagnetic susceptibility with the molecules, which have long axes tending to be orthogonal to the applied magnetic field, has been considered as a dispersion medium. Since carbon nanotubes possess positive anisotropy of the diamagnetic susceptibility, they tend to align parallelly to the field, causing competition with the ordering of the liquid crystal matrix. Phase transitions induced by temperature and magnetic field have been studied for different values of the coupling energy of carbon nanotubes with a liquid crystal matrix. It is shown that depending on the temperature, magnetic field induces the transition from a highly ordered nematic phase to a weakly ordered paranematic phase with both the positive and the negative value of the order parameter of the liquid crystal, which respectively corresponds to the orientational anisotropy of the “easy-axis” and “easy-plane” types.

Impact of Mn3+ substitution on magnetization and electric polarization behavior in geometry frustrated CuFe1−xMnxO2

The Mn3+ doped single crystal samples CuFe1−xMnxO2 with x = 0.01, 0.03 and 0.05 were synthetic by floating zone method, and their magnetization and electric polarization were measured with static and pulsed high magnetic field up to 50 T in the temperature ranging from 2 to 300 K. A series of field-induced magnetic phase transitions and electric polarization, which show obviously anisotropy, were observed in all the doped samples, and their critical fields were directly related to the Mn3+ doping level. The pulsed high magnetic dielectric polarization measurements show that, in the ferroelectric incommensurate phase, the spontaneous electric polarization region is enhanced by the Mn3+ dopant. The impact of the Mn3+ substitution on magnetic and electric polarization behaviors may associate with the partial release of spin frustration in the CuFeO2. Based on the experimental results, the impact of Mn3+ doping on the magnetization and electric polarization behavior as well as their anisotropy are investigated and a detailed magnetic diagram are assumed.

Critical phenomenon and phase diagram of Mn-intercalated layered MnNb3S6

The magnetization of Mn-intercalated layered MnNb3S6 single crystal has been systematically investigated. The angle-dependent magnetization (, , and ) displays that the magnetization of MnNb3S6 with is much stronger than that with H//c, but no magnetic anisotropy within the ab-plane. In addition, a field-induced magnetic phase transition is found when . The investigation of the critical behavior gives the critical exponents , , and with K, which belong to the universality class of three-dimensional Heisenberg model. The critical behavior and exponents indicate that the magnetic coupling in MnNb3S6 is of an isotropic short-range type. Based on universality scaling, the H − T phase diagram with is constructed. The magnetic behaviors and phase diagram of MnNb3S6 are analogous to those of CrNb3S6 which exhibits a chiral magnetic soliton lattice.

Spin dynamics and field-induced magnetic phase transition in the honeycomb Kitaev magnet α−Li2IrO3

The layered honeycomb iridate $\alpha$-Li$_2$IrO$_3$ displays an incommensurate magnetic structure with counterrotating moments on nearest-neighbor sites, proposed to be stabilized by strongly-frustrated anisotropic Kitaev interactions between spin-orbit entangled Ir$^{4+}$ magnetic moments. Here we report powder inelastic neutron scattering measurements that observe sharply dispersive low-energy magnetic excitations centered at the magnetic ordering wavevector, attributed to Goldstone excitations of the incommensurate order, as well as an additional intense mode above a gap $\Delta\simeq2.3$ meV. Zero-field muon-spin relaxation measurements show clear oscillations in the muon polarization below the N\'{e}el temperature $T_{\rm N}\simeq15$ K with a time-dependent profile consistent with bulk incommensurate long-range magnetism. Pulsed field magnetization measurements observe that only about half the saturation magnetization value is reached at the maximum field of 64 T. A clear anomaly near 25 T indicates a transition to a phase with reduced susceptibility. The transition field has a Zeeman energy comparable to the zero-field gapped mode, suggesting gap suppression as a possible mechanism for the field-induced transition.

Magnetic field induced antiferromagnetic tricritical points in Ce2Sb and Ce2Bi

We present a detailed investigation of the physical properties of Ce$_2 $Sb and Ce$_2 $Bi single crystals, which undergo antiferromagnetic transitions at around 8.2 and 10 K respectively. When magnetic fields are applied parallel to the $c$ axis, metamagnetic transitions are observed at low temperatures, corresponding to a magnetic field-induced phase transition. It is found that the field-induced transition changes from second-order at higher temperatures, to first-order at low temperatures, suggesting the existence of tricritical points (TCPs) in both compounds. Since replacing Bi with Sb suppresses the TCP to lower temperatures and corresponds to a positive chemical pressure, these results suggest that applying pressure to Ce$_2$Sb may suppress the TCP to lower temperatures, potentially to zero temperature at a quantum tricritical point.

Field-induced phases in a heavy-fermion U(Ru0.92Rh0.08)2Si2 single crystal

We report the high-field induced magnetic phases and phase diagram of a high quality \urxrs~single crystal prepared using a modified Czochralski method. Our study, that combines high-field magnetization and electrical resistivity measurements, shows for fields applied along the $c$-axis direction three field-induced magnetic phase transitions at $\mu_{0} H_{c1}$ = 21.60 T, $\mu_{0} H_{c2}$ = 37.90 T and $\mu_{0} H_{c3}$ = 38.25 T, respectively. In agreement with a microscopic up-up-down arrangement of the U magnetic moments the phase above $H_{c1}$ has a magnetization of about one third of the saturated value. In contrast the phase between $H_{c2}$ and $H_{c3}$ has a magnetization that is a factor of two lower than above the $H_{c3}$, where a polarized Fermi-liquid state with a saturated moment $M_{s}$ $\approx$ 2.1 $\mu_{B}$/U is realized. Most of the respective transitions are reflected in the electrical resistivity as sudden drastic changes. Most notably, the phase between $H_{c1}$ and $H_{c2}$ exhibits substantially larger values. As the temperature increases, transitions smear out and disappear above $\approx$ 15 K. However, a substantial magnetoresistance is observed even at temperatures as high as 80 K. Due to a strong uniaxial magnetocrystalline anisotropy a very small field effect is observed for fields apllied perpendicular to the $c$-axis direction.

Фазовые переходы в жесткой доменной структуре феррит-гранатовой пленки

Spontaneous and magnetic field-induced phase transitions in a rigid domain structure of a uniaxial ferrite-garnet film are studied. It is shown that the temperature and field stability intervals of the lattice of cylindrical magnetic domains depend on the structure of the domain boundaries.

Magnetic field-induced electronic phase transition in the Dirac semimetal state of black phosphorus under pressure

Different instabilities have been confirmed to exist in the three-dimensional (3D) electron gas when it is confined to the lowest Landau level in the extreme quantum limit. The recently discovered 3D topological semimetals offer a good platform to explore these phenomena due to the small sizes of their Fermi pockets, which means the quantum limit can be achieved at relatively low magnetic fields. In this work, we report the high-magnetic-field transport properties of the Dirac semimetal state in pressurized black phosphorus. Under applied hydrostatic pressure, the band structure of black phosphorus goes through an insulator-semimetal transition. In the high pressure topological semimetal phase, anomalous behaviors are observed on both magnetoresistance and Hall resistivity beyond the relatively low quantum limit field, which is demonstrated to indicate the emergence of an exotic electronic state hosting a density wave ordering. Our findings bring the first insight into the electronic interactions in black phosphorus under intense field.

Multiple magnetic transitions in single crystals of Ce12Fe57.5As41 and La12Fe57.5As41

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce12Fe57.5As41 and La12Fe57.5As41. The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds. Both compounds exhibited multiple magnetic orders within 2-300 K and metamagnetic transitions at various fields. Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce12Fe57.5As41 and La12Fe57.5As41, respectively, followed by antiferromagnetic type spin reorientations near Curie temperatures. The magnetic properties underwent complex evolution in the magnetic field for both compounds. An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce12Fe57.5As41. The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure. A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce12Fe57.5As41. A temperature-field phase diagram was present for these two rare earth systems. In addition, a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150-300 K, which is rarely found in 3D-based compounds. It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.

Magnetization, high pressure, and magnetocaloric studies of MnRuxRh1-xAs (x = 0.05, 0.1): Experimental and theoretical approaches

The magnetic properties of the MnRuxRh1-xAs system with x = 0.05, 0.10 were studied under hydrostatic pressure up to 1.2 GPa in the 80–420 K temperature range. The (p,T) magnetic phase diagram was determined. The ferromagnetic component in an intermediate (AF2+f) domain was stabilized with pressure for both samples, and the critical field variation versus pressure was determined. In addition, dc magnetization measurements were performed in magnetic fields up to 10 T, and the magnetic field–induced phase transitions were pointed out. The reverse and conventional magnetocaloric effect for both compounds was analyzed. Electronic band structure computations for the MnRuAs and MnRhAs parent compounds were performed by using the Korringa–Kohn-Rostoker method. The results indicate that significant changes of electronic states in the vicinity of EF on the application of external pressure tend to favor a ferromagnetic-type ordering of the Mn magnetic moments.

Combined electric and magnetic field-induced anisotropic tunable electronic phase transition in AB-stacked bilayer phosphorene

With the aid of the two-dimensional tight-binding effective model in combination with effective models for perpendicular electric and magnetic fields, we describe the dynamics of carriers in Bernal bilayer phosphorene. Both electric and magnetic fields are in reliable ranges. We study the electronic phase transition beyond the continuum approximation focusing on the electronic band structure and density of states. In the present work, it is shown that the system subjected to the perpendicular electric field takes a semiconductor-semimetal phase transition with linear- and quadra-like bands along the Γ − X and Γ − Y direction, respectively when the magnetic field is absent. However, turning on the magnetic field shows again the semiconductor-semimetal phase transition for electric field-induced bilayer phosphorene but with only a linear-like band along the Γ − Y direction. Furthermore, we found that by varying the magnetic field, the Landau levels exhibit different treatments in the absence and presence of the electric field. Concisely, we argue that these tunable phase transitions with new rich features give proper motivations to experimentalists to tune the electro-optical properties of bulk phosphorus.

X-ray diffractometer for the investigation of temperature- and magnetic field-induced structural phase transitions

This article reports the development and characterization of a laboratory-based high-resolution X-ray powder diffractometer equipped with a 5.5 T magnet and closed-cycle helium cryostat that is primarily designed for the investigation of magneto-structural phase transitions. Unique features of the diffractometer include the position-sensitive detector, allowing the collection of an entire diffraction pattern at once, and the high energy resolution with Mo Kα1radiation. The ability to utilize a lower energy resolution but higher photon flux by switching to an X-ray mirror monochromator makes it a versatile setup for a variety of compounds. In this contribution, details of the design and performance of the instrument are presented along with its specifications.

Magnetostructural transformation and magnetocaloric effect in Ni42Mn47.5Sn10.5 and Ni41.5Mn47.5Sn10.5Zn0.5 ferromagnetic shape memory alloys

A comparative investigation on magnetostructural transformation and coupled magnetocaloric properties of Ni42Mn47.5Sn10.5 and Ni41.5Mn47.5Sn10.5Zn0.5 ferromagnetic shape memory alloys prepared by arc-melting method is carried out through structural and magnetic measurements. As Zn is doped in NiMnSn alloy, the cell volume decreases and the martensitic transformation temperature increases by 21 K. Obvious metamagnetic behavior and magnetic hysteresis is observed in isothermal magnetization curves, indicating the magnetic field induced magnetostructural transformation. Magnetic field induced phase transition is also confirmed under applied field of 2 T and martensitic transformation temperature is decreased by 4.1 and 5.29 K for Ni42Mn47.5Sn10.5 and Ni41.5Mn47.5Sn10.5Zn0.5 alloys, respectively. The entropy change in Ni41.5Mn47.5Sn10.5Zn0.5 alloy with change in magnetic field from 0 to 2 T is 12.3 J · kg−1 · K−1, which is larger than that in Ni42Mn47.5Sn10.5 alloy. The achieved low field entropy change in Zn doped alloy suggests its potential application as magnetic cooling refrigerant.

Field-induced magnetic phase transitions and memory effect in bilayer ruthenate Ca3Ru2O7 with Fe substitution

Bilayer ruthenate Ca3(Ru1−xFex)2O7 (x = 0.05) exhibits an incommensurate magnetic soliton lattice driven by the Dzyaloshinskii–Moriya interaction. Here we report complex field-induced magnetic phase transitions and memory effect in this system via single-crystal neutron diffraction and magnetotransport measurements. We observe first-order incommensurate-to-commensurate magnetic transitions upon applying the magnetic field both along and perpendicular to the propagation axis of the incommensurate spin structure. Furthermore, we find that the metastable states formed upon decreasing the magnetic field depend on the temperature and the applied field orientation. We suggest that the observed field-induced metastability may be ascribable to the quenched kinetics at low temperature.

Hall detection of anisotropic domain walls during magnetic phase transition

We investigate the electrical detection of anisotropic antiferromagnetic/ferromagnetic (AFM/FM) domain walls in (2 at%) Pd-doped FeRh films. The Hall signals keep almost at zero at both AFM and FM states, while a sizeable Hall conductivity up to 300 (Ω cm)−1 emerges during the AFM–FM phase transition of Pd-doped FeRh films around room temperature. Such Hall signals could be ascribed to the transverse current scattered by the anisotropic AFM/FM domain walls, which are mainly aligned around the [1 0 0] axis of the Pd-doped FeRh films. The highly reproducible Hall signals in both the temperature and magnetic field-induced magnetic phase transition are supported by the observation of a nonrandom formation of AFM/FM domain walls. Our findings not only advance the understanding of the magnetic phase transitions, but also propose a considerable way for the detection of AFM/FM domain walls by an electrical means.

Magnetic field induced phase transitions and phase diagrams of multiferroic Mn0.95Co0.05WO4 with cycloidal spin structure

Slightly Co-doped ${\mathrm{MnWO}}_{4}$ at the lowest 5% Co concentration, for which the multiferroic cycloidal phase becomes a ground state, has been studied in magnetic fields up to 60 T by bulk magnetic and electric polarization measurements along different crystallographic directions. The field induced magnetic transitions up to 12 T and the ways they proceed were tracked also by single-crystal neutron diffraction, and the relevant field induced magnetic structures were identified and refined. The complete magnetoelectric phase diagrams for magnetic fields along distinct directions in relation to the cycloidal spin structure have been constructed for magnetic field values exceeding those necessary to induce a spin-flip transition into the paramagnetic state. Their common feature is the existence of nonpolar sinusoidal phases identified by the disappearance of the electric polarization in a field regime slightly below the spin-flip transition. At lower magnetic fields either continuous or abrupt field induced reorientations of the cycloidal magnetic structures were observed, respectively, for a field direction along the crystallographic $b$ axis or along the easy magnetic axis, and the different character of those transitions has been attributed to specific features in the magnetic anisotropy.

Magnetostrictive hypersound generation by spiral magnets in the vicinity of magnetic field induced phase transition

In present work we have investigated magnetostrictive ultrasound generation by spiral magnets in the vicinity of magnetic field induced phase transition from spiral to collinear state. We found that such magnets may generate transverse sound waves with the wavelength equal to the spiral period. We have examined two types of spiral magnetic structures: with inhomogeneous exchange and Dzyaloshinskii–Moriya interactions. Frequency of the waves from exchange-caused spiral magnetic structure may reach some THz, while in case of Dzyaloshinskii–Moriya interaction-caused spiral it may reach some GHz. These waves will be emitted like a sound pulses. Amplitude of the waves is strictly depends on the phase transition speed. Some aspects of microwaves to hypersound transformation by spiral magnets in the vicinity of phase transition have been investigated as well. Results of the work may be interesting for investigation of phase transition kinetics as well, as for various hypersound applications.

Magnetic quantum phase transitions and entropy in Van Vleck magnet

Field-induced magnetic quantum phase transitions in the Van Vleck paramagnet with easy-plane single-ion anisotropy and competing Ising exchange between ions with the spin S=1 have been studied theoretically. The description was made by minimizing the Lagrange function at zero temperature (Т=0) and the free energy at T≠0. Stable and unstable solutions of equations corresponding to the case ψ0=|0〉 asymptotically transform into those following from the Lagrange function at Т=0. First-order phase transitions from the Van Vleck paramagnet state into the ferromagnet one were found to take place at a sufficiently high single-ion anisotropy. The entropy of such a magnet was shown to grow with its magnetization, as it occurs for antiferromagnets. At the point of quantum phase transition, the entropy has a jump, which magnitude depends on the ratio between the Ising exchange and anisotropy constants, as well as on the temperature. The described magnetic phase transition was supposed to be accompanied by the magnetocaloric effect. In the case when the Ising exchange dominates over the single-ion anisotropy, the magnetization reversal of ferromagnetic state by an external field was shown to be a phase transition of the first kind, which does not belong to orientational ones and which should be regarded as a quantum order-order phase transition.

Chiral heterobimetallic chains from a dicyanideferrite building block including a π-conjugated TTF annulated ligand.

The π-conjugated tetrathiafulvalene (TTF) annulated ligand was introduced into a dicyanometallate for the first time, leading to the synthesis of the versatile redox-active dicyanideferrite building block [(n-Bu)4N][Fe(TTFbp)(CN)2] (H2TTFbp = N-(2-(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)-5-(picolinamido) benzo[d][1,3]dithiol-6-yl) picol inamide). The incorporation of the new precursor with chiral MnIII Schiff-base complexes resulted in two enantiopure one-dimensional complexes, [Mn((R,R)-salphen)Fe(TTFbp)(CN)2]n (2-(RR)) and [Mn((S,S)-salphen)Fe(TTFbp)(CN)2]n (2-(SS)) (Salphen = N,N'-1,2-diphenylethylene-bis(salicylideneiminato) dianion), which were synthesized and structurally characterized. Circular dichroism (CD) and vibrational circular dichroism (VCD) spectra confirmed the enantiomeric nature of the optically active complexes, and structural analyses revealed the formation of neutral cyanide-bridged double chains in 2-(RR) and 2-(SS). Solution and solid state CV studies revealed the redox-active characteristics of the complexes. Antiferromagnetic couplings were detected between FeIII and MnIII centers within a chain, and a field-induced magnetic phase transition was observed (TN = 4.8 K). The introduction of electroactivity and chirality into cyanide-bridged complexes with interesting magnetic properties leads the way towards new multifunctional materials.

The magnetic properties of NdMnxCr2−xSi2C (0 < x < 2)

Carbon was successfully introduced into NdMnxCr2−xSi2 serial compounds as an interstitial atom. The compounds with x ⩽ 0.5 keep a CeMg2Si2-type structure while the structure changes slowly with increasing manganese content. Finally, a new superstructure was found and determined for x around 1.7. All the compounds for x = 1.1, 1.3, 1.5 and 1.7 have two magnetic phase transition temperatures around 43 and 314 K. What is more, a giant magnetocaloric effect has been observed around the lower phase transition temperature, TR ~ 51 K, in NdMn1.7Cr0.3Si2C. The maximum values of the magnetic entropy change −ΔSM for the field change of 2 T and 5 T are 14.9 J kg−1 K−1 and 26.0 J kg−1 K−1, which means the interstitial atom effect enhanced the MCE values for 85% and 63% compared to NdMn1.7Cr0.3Si2, respectively. NdMn1.7Cr0.3Si2C with first order magnetic transition but low hysteresis losses (thermal < ~0.58 K; magnetic field < ~0.12 T) can be a candidate for magnetic refrigerator applications in the temperature region near 50 K. Besides, two field-induced magnetic phase transitions were found in NdMn1.7Cr0.3Si2C among the temperature range from 53 to 60 K, and a linear relation between the phase transition field and temperature was found and analyzed.