Vicinity Of TN Research Articles

Magnetic Ordering and Interplay between the Magnetic and Charge Subsystems in New Francisite‐Analog Cu3Dy(SeO3)2O2Cl as Studied by the Spectroscopy of Kramers Doublets

The rare‐earth (RE) counterparts of francisite mineral Cu3Bi(SeO3)2O2Cl demonstrate strong interplay between d‐ and f‐magnetic subsystems and interesting magnetic properties, including multiferroicity. Herein, an optical spectroscopic study of a new member of the RE‐francisite family, dysprosium francisite, Cu3Dy(SeO3)2O2Cl, is presented. The inherent property of the Kramers degeneracy to be lifted by a magnetic field is used to study the magnetic and thermodynamic properties of the crystal. The splitting of the spectral lines corresponding to f–f transitions in the Dy3+ Kramers ion unambiguously points to the magnetic ordering of the crystal at a temperature of TN = 39 K. The value of the single‐ion anisotropy of the Dy3+ ion is estimated; it dominates the magnetic anisotropy of the copper subsystem. The specific behavior of the spectral lines in the vicinity of TN clearly indicates low‐dimensional magnetism and the interaction between the magnetic and charge degrees of freedom in Cu3Dy(SeO3)2O2Cl, i.e., the multiferroicity, previously reported in the literature for Cu3Bi(SeO3)2O2Cl.

Metamagnetism and Magnetocaloric Properties in a van der Waals Antiferromagnet CrOCl

Herein, the anisotropic magnetic properties of 2D van der Waals (vdW) insulator, chromium oxychloride (CrOCl), are presented. Temperature‐dependent bulk magnetization reveals a paramagnetic to antiferromagnetic phase transition at ≈14 K (Néel temperature, TN) while cooling. From the magnetic field‐dependent magnetization study (M–H) at low temperature, a first‐order field‐induced (at or above 40 kOe) metamagnetic transition from antiferromagnetic to ferrimagnetic state has been observed. Noticeably, switching of the easy plane (ab‐plane) of the magnetization to the c‐axis can be identified above 29 kOe. Moreover, in the vicinity of TN, under an applied magnetic field, magnetic entropy change (ΔSM) can be estimated as 4.31 J kg−1 K−1 for H//c. The results evoke further experiments on charge/spin transport properties of few‐layer CrOCl in the nanoelectronic circuit framework for understanding the connection between crystallographic deformation and metamagnetism.

Theoretical investigation on the magnetocaloric effect of BiFeO3

The magnetic entropy change (MEC) properties of BiFeO3 (BFO) were investigated theoretically by means of quantum microscopic calculations based on the Green's function, the classical thermodynamical theory and Maxwell's relation. The MEC increase as the temperature rises, while decline steeply from max to zero at the magnetic phase transition temperature TN. This anomaly of the MEC can be explained as the antiferromagnetic phase transition. The temperature dependence of MECs shows two kinks. One kink appears at the temperature T1 = 425K, which is interpreted as the result of the dependence of the spin cycloid anharmonicity on temperature. Another abnormal kink of MEC appears at nearly T2 = 550 K, which is associated with the onset of structural changes. Besides, it is found that TN shifts to higher temperature as the exchange coupling constants (J1) between one magnetic spin and its nearest neighbor, the exchange coupling constants (J2) between the two next- nearest neighbor magnetic spins, the exchange coupling constant (I) between the nearest neighbor pseudo-spin and magnetoelectric coupling constant (g) increase. What's more, it is also found that the magnetic entropy is decrease as the J1 and J2 increase. At last but not least, the I and g affect magnetic entropy only in the vicinity of TN. Those achieved conclusions are in good accordance with the experimental results and other theory findings.

Electrocaloric effect anomaly in multiferroic BiFeO3

Electrocaloric Effect of multiferroic BiFeO3 (BFO) has been studied by using the method of Green’s function. The calculations demonstrate that the isothermal entropy change (IEC) of BFO exhibits a cusp-like structure at ferromagnetic transition temperature TN. This anomaly in IEC can be explained as the effect of magnetic order disappearance above TN. Note that a maximum IEC appears at the temperature of T0, which is coincide with the ferroelectric transition temperature TC. Besides, the IEC increases slightly with the anisotropy constant KD increase in the vicinity of TN. At last but not least, the IECs decrease with the ferroelectric interaction (I) and the magnetoelectric coupling constant (g) increase, whereas the TN shift to higher temperature with the I and the g increase. Those conclusions drawn are in good accordance with the experimental results and other theory findings.

Dielectric and polarization studies of magnetoelectric coupling in non-relaxor Pb(Fe1/2Ta1/2)O3 multiferroic ceramics

ABSTRACTDielectric permittivity ϵ and polarization / electric field hysteresis loops of highly-resistive Li-doped non-relaxor Pb(Fe1/2Ta1/2)O3 ceramics obtained by one-step sintering of the mechanically activated mixture of starting oxides and Li2CO3 have been studied. Antiferromagnetic phase transition temperature TN ≈ 150 K was determined using Mossbauer spectroscopy studies at different temperatures. Though no anomalies in ϵ(T) curve at TN were observed, 1/ϵ(T) dependence exhibits change of the slope in the vicinity of TN. Anomalies in the temperature dependences of coercive field EC and derivative of the spontaneous polarization PS are observed near TN as well. These anomalies are attributed to magnetoelectric coupling.

Lattice and magnetic dynamics of a quasi-one-dimensional chain antiferromagnet PbFeBO4

A group of recently synthesized orthorhombic Pnma crystals PbMBO4 (M = Cr, Mn, Fe) demonstrates a number of unusual structural and magnetic properties. We report on polarized Raman scattering study of the lattice and magnetic dynamics in single crystals of an antiferromagnet PbFeBO4 below and above K. Polarization properties of the observed magnetic excitations below TN as well as intense quasi-elastic scattering support the quasi-one-dimensional character of the magnetic structure of PbFeBO4. Frequency overlapping of magnetic excitations and low-frequency phonons in the range of 90–200 cm−1 leads to pronounced asymmetric anomalies thus confirming intrinsic coupling of magnetic and lattice subsystems. This conclusion is also supported by observation of anomalous temperature behaviour of higher frequency phonons in the vicinity of TN. Experimental investigations are supported by relevant magnetic symmetry analysis which allows us to explain previously observed anomalous results.

The crystal field effects in hexagonal 4H-SrMnO3

ABSTRACTWe have provided an explanation for the EPR signal in 4H-SrMnO3, revealed at the vicinity of TN and in the paramagnetic region, as related with the doublet originating from the 4A2g subterm of the Mn4+ ion. Our calculations, taking into account the spin–orbit coupling and the crystal–field interactions with a trigonal distortion, reproduce the doublet ground state which is characterised by magnetic moment of ±0.975μB in very good agreement with experimental value of ±0.983μB. It confirms fundamental importance of the spin–orbit coupling and the orbital magnetism for description of 3d oxides. We claim that the EPR result provides a strong argument for the existence in the 4H-SrMnO3 crystal of the discrete atomic-like states with the energy separation of part of meV.

Role of iso-electronic Ru5+ in thermal transport behavior of YMn1-xRuxO3 compounds

The perovskite manganites compounds with nominal composition YMn1-xRuxO3 (YMRO) (x = 0, 0.05, 0.1 and 0.2) were investigated systematically through structural, morphology and thermal transport measurements. Our thermopower and specific heat measurements indicate that the ground state of rare earth manganite compounds is highly sensitive to the ruthenium (Ru) concentration at Mn-site and henceforth to its cationic radius. These properties were analyzed by considering various models and empirical relations. The high value and complicated temperature dependence of thermopower (S) is an indication of electron–magnon scattering in these systems. The observation of giant suppression in the S with increasing Ruthenium content confirms the mixed-valence subsistence of Mn - Ru network. Specific heat measurements depict an anomaly in the vicinity of TN and associated entropy change, was then obtained by subtracting the lattice part from the experimental data, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples.

Insight into the magnetism of a distorted Kagome lattice, Dy3Ru4Al12, based on polycrystalline studies

The layered compound with distorted Kagome nets, Dy3Ru4Al12, was previously reported to undergo antiferromagnetic ordering below (TN=) 7 K, based on investigations on single crystals. Here, we report the results of our investigation of AC and DC magnetic susceptibility (χ), isothermal remnant magnetization (MIRM), heat-capacity, magnetocaloric effect and magnetoresistance measurements on polycrystals. The present results reveal that there is an additional magnetic anomaly around 20 K, as though the Néel order is preceded by the formation of ferromagnetic clusters. We attribute this feature to geometric frustration of magnetism. In view of the existence of this phase, the interpretation of the linear-term in the heat-capacity in terms of spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in the vicinity of TN, AC χ shows a prominent frequency dependence and, below TN, MIRM exhibits a slow decay with time. This raises a question whether the antiferromagnetic structure in this compound is characterized by spin-glass-like dynamics. In contrast to what was reported earlier, there is a change in the sign of the magnetoresistance (MR) at the metamagnetic transition. A butter-fly-shaped (isothermal) MR loop (interestingly spanning over all the four quadrants) is observed at 2 K with distinct evidence for the magnetic phase co-existence phenomenon in zero field after travelling through metamagnetic transition field. The results on polycrystals thus provide additional information about the magnetism of this compound, revealing that the magnetism of this compound is more complex than what is believed, due to geometric frustration intrinsic to Kagome net.

Magnetocaloric and phase coexistence in La0.5Ca0.5–xSrxMnO3 manganites

Structural, magnetic, and magnetocaloric properties of the phase separated La0.5Ca0.5–xSrxMnO3 (x = 0, 0.2, 0.3, 0.4, and 0.5) manganites have been studied. The results show that the phase coexistence state can be investigated by magnetocaloric studies (especially the field dependence of magnetic entropy change at constant temperature). Magnetic entropy change (△SM) shows positive (negative) peak at the vicinity of TN (TC). However, in the intermediate temperatures between TN and TC, both the sign and magnitude of △SM are strongly dependent on temperature and magnetic field, manifesting the competition of ferromagnetic (FM) (negative △SM) and non-FM (positive △SM) phases. This behavior is more pronounced in the parent compound, La0.5Ca0.5MnO3, in which the intermediate phase separation ranges between 160 and 225 K. The substitution of Ca by Sr enhances the ferromagnetic state, weakens the phase separation, and thus narrows the temperature range in which the field related effects (such as the sign change of △SM) are observed.

Studies of Ferroelectric and Magnetic Phase Transitions in Multiferroic PbFe0.5Ta0.5O3

X-ray, magnetic, and Mossbauer studies of a PbFe0.5Ta0.5O3 powder have been carried out. For the first time temperature dependence of the unit cell parameters was obtained. Antiferromagnetic Neel temperature TN ≈180 K was determined using the results of Mossbauer studies. In the vicinity of TN an anomaly in the temperature dependence of the monoclinic angle β was revealed in PFT. This anomaly seems to be due to the magnetoelectric coupling.

Field induced changes in cycloidal spin ordering and coincidence between magnetic and electric anomalies in BiFeO3 multiferroic

The temperature dependences of ZFC and FC magnetization was measured for BiFeO3 ceramics in magnetic field up to μ0H=10T and in temperatures from the range 2–1000K. The antiferromagnetic order was detected from the hysteresis loops below the Néel temperature TN=646K. The anomaly in M(H), which occurred in the low magnetic field range, was ascribed to the field-induced transition from circular cycloid to the anharmonic cycloid. At high field limit, we observed the field-induced transition to the homogeneous spin order. From the M(H) dependence, we deduced that the spin cycloid becomes anharmonic above the field Ha that caused nonlinear magnetization. The cycloid vanished above the field Hc and the system again exhibited linear magnetization M(H). The anomalies in the electric properties, ε′(T), tanδ(T), and σ(T), which are manifested within the 635–670K range, coincide with the anomaly in the temperature dependence of magnetization M(T), which occurs in the vicinity of TN=646K. We propose that this coincidence can be explained by the critical behavior of chemical potential µS, related to the magnetic phase transition.

Understanding Multiferroic Hexagonal Manganites by Static and Ultrafast Optical Spectroscopy

Multiferroic hexagonal manganites ReMnO3studied by optics are reviewed. Their electronic structures were revealed by static linear and nonlinear spectra. Two transitions located at~1.7 eV and~2.3 eV have been observed and attributed to the interband transitions from the lower-lying Mn3+dxy/dx2-y2anddxz/dyzstates to the Mn3+d3z2-r2state, respectively. These so-called d-d transitions exhibit a blueshift as decreasing temperatures and an extra blueshift nearTN. This dramatic change indicates that the magnetic ordering seriously influences the electronic structure. On the other hand, the ultrafast optical pump-probe spectroscopy has provided the important information on spin-charge coupling and spin-lattice coupling. Because of the strongly correlation between electronic structure and magnetic ordering, the amplitude of the initial rising component inΔR/Rshows striking changes at the vicinity ofTN. Moreover, the coherent optical and acoustic phonons were observed on optical pump-probe spectroscopy. Both the amplitude and dephasing time of coherent phonons also exhibit significant changes atTN, which provide the evidence for spin-lattice interaction in these intriguing materials.

Interaction Between Troponin and Myosin Enhances Contractile Activity of Myosin in Cardiac Muscle

Ca(2+) signaling in striated muscle cells is critically dependent upon thin filament proteins tropomyosin (Tm) and troponin (Tn) to regulate mechanical output. Using in vitro measurements of contractility, we demonstrate that even in the absence of actin and Tm, human cardiac Tn (cTn) enhances heavy meromyosin MgATPase activity by up to 2.5-fold in solution. In addition, cTn without Tm significantly increases, or superactivates sliding speed of filamentous actin (F-actin) in skeletal motility assays by at least 12%, depending upon [cTn]. cTn alone enhances skeletal heavy meromyosin's MgATPase in a concentration-dependent manner and with sub-micromolar affinity. cTn-mediated increases in myosin ATPase may be the cause of superactivation of maximum Ca(2+)-activated regulated thin filament sliding speed in motility assays relative to unregulated skeletal F-actin. To specifically relate this classical superactivation to cardiac muscle, we demonstrate the same response using motility assays where only cardiac proteins were used, where regulated cardiac thin filament sliding speeds with cardiac myosin are >50% faster than unregulated cardiac F-actin. We additionally demonstrate that the COOH-terminal mobile domain of cTnI is not required for this interaction or functional enhancement of myosin activity. Our results provide strong evidence that the interaction between cTn and myosin is responsible for enhancement of cross-bridge kinetics when myosin binds in the vicinity of Tn on thin filaments. These data imply a novel and functionally significant molecular interaction that may provide new insights into Ca(2+) activation in cardiac muscle cells.

Influence of Hydrogen Bonding on Device Parameters and Field Response in N* and SmC* Phases of a Ferroelectric Liquid Crystal, HBFLC: 12 bpa

Isotropic to chiral nematic (IN*) and chiral nematic to chiral SmC(N*C*) transitions are studied by LF dielectric method in a hydrogen bonded ferroelectric liquid crystal, viz., 12 bpa. They agree with TM and DSC reports. Tilt angle, electro-clinic effect, spontaneous polarization, and dielectric relaxations are investigated and compared with that of an odd homologue 11 bpa. Growth of tilt and spontaneous polarization as order parameters in SmC* phase agree with mean-field prediction. Effect of H-bonding on the growth of tilt and PS order parameters is discussed. Observed large EC-effect is explained by the inclined configuration of soft-covalent interaction. Dielectric dispersion revealed a collective response, viz., Goldstone mode at ∼100 Hz in SmC* and an LF-mode in N* phases. The two HF-relaxations, i.e., Types I and II in N* and SmC* phases are explained through a refined dipole model. Shift of GM with field in SmC* is explained. Off-centred dispersion is explained through the Cole-Davidson relation. HF relaxations are related to the individual dipole reorientation. Shift of fR with T infers higher activation energy for Type I and lower activation energy for Type II relaxations. Soft mode permittivity follows the Curie–Weiss law in the vicinity of TN*C*. Device parameters, viz., thermal stability, tilt angle, and spontaneous polarization in SmC* phase are found to be influenced by H-bonding in FLCs. Results are is discussed and compared with the data reported on 11 bpa and other FE materials.

Studies of magnetic entropy change and phase transitions in SrRu1−xMnxO3 perovskite

This paper reports a systematical investigation on the role of magnetic phase transitions on magnetocaloric effect in polycrystalline SrRu1−xMnxO3 (0⩽x⩽0.72), combining with the temperature dependent magnetization measurement by a superconducting quantum interference device. Mn substitution drives the magnetic phase transition from ferromagnetic state to antiferromagnetic state via a short-range magnetic order. The magnetic entropy change (ΔSM) presents negative values and a maximum at Curie temperature TC and/or spin freezing temperature Tf for 0⩽x⩽0.27, which is due to the contribution of the spin ordering induced by magnetic field. In addition, we also observe positive magnetic entropy changes (ΔSP) at Néel temperature TN for high Mn concentration (x⩾0.41) as the charge ordering is negligible. Since the magnetization is insensitive to external field in the vicinity of TN, the positive magnetic entropy change is considered to stem from the dramatic temperature dependence of the magnetization.

Magnetic heterogeneity in electron dopedLa1−xCaxMnO3 manganites studied by means of electron spin resonance

Electron spin resonance (ESR) has been applied to investigate the magnetic heterogeneity in electrondoped La1−xCaxMnO3 (0.80≤x≤0.95). A low field ferromagnetic resonance (FMR) mode is observed for lightly doped compounds(x = 0.90,0.95), signifying the formation of ferromagnetic (FM) spin clusters within the antiferromagneticG-type AFM phase. The anomalous temperature variations of the resonance field,linewidth and FMR intensity, as well as the observation of thermal cycling effects belowTC, emphasize the non-trivial dynamics of the FM phase, which is attributedto the temperature dependent size evolution of the underlying spin clusterstowards canted AFM and FM domains. For heavier electron doping(x = 0.80,0.85), distinct AFM behaviour is evinced in the vicinity ofTN in the monoclinic C-type AFM phase, characterized by the absenceof critical relaxation. Additional weak FMR lines are observed forx = 0.80 and 0.85, whereas a narrow superparamagnetic-like signal is detected forx = 0.95.

Elastic and Anelastic Properties of the Ceramics PbFe0.5Nb0.5O3: Anomalies Near T C and T N

Low-frequency elastic and anelastic properties of the ceramic PbFe0.5Nb0.5O3 were investigated by the inverted torsion pendulum in the temperature ranges including the Curie and Neel temperatures. Correlation between elastic and dielectric properties was found to take place within the diffuse ferroelectric phase transition. The shear modulus and the internal friction anomalies observed in the vicinity of TN are due to magnetostriction connection between a strain and magnetic spin ordering.

Surface and volume phase states of Fe1−x GaxBO3 crystals in the vicinity of the Néel point

The behavior of the magnetic system of a surface layer of macroscopic Fe1−xGaxBO3 crystal (x=0, 0.3) in the vicinity of the Neel temperature TN was studied. The studies were made by a method involving simultaneous gamma, x-ray, and electron Mossbauer spectroscopy that made it possible to obtain information simultaneously from surface layers and from the bulk of a macroscopic crystal. It was found that the temperature TN(L) at which a thin layer at a depth L from the surface switches to a disordered state is lower than TN for the bulk and is lower the closer this layer is to the surface. In the vicinity of TN, a nonuniform state is observed in which the bulk of the crystal is magnetically ordered and the surface layer is disordered. The transition temperature TN(L) decreases from TN to its surface value within a surface layer of a “critical” thickness.

Magnetocaloric effect in Gd2PdSi3

The influence of the application of a magnetic field (H) on the temperature (T) dependence of heat capacity and isothermal magnetization has been investigated in Gd2PdSi3, a compound ordering antiferromagnetically below (TN=)21 K in zero H. Among other findings, the one to be emphasized is the observation of significant magnetocaloric effect (MCE) over about a 30 K range with a peak in the vicinity of TN. This finding suggests that this material could be useful for magnetic refrigeration below 50 K. The results also establish that either of the two experimental methods—heat capacity or magnetization—can be applied to identify materials with large MCE.