High-Field Optical Spectroscopy of the Spin-Crossover Complex [MnIII(taa)]

In this study, we have observed the optical absorption spectra of the manganese(III) complex [Mn^III(taa)] (H₃taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine), in high magnetic fields. From the measurements at zero field, we have found a variation of the spectrum between 100 K and 4.2 K due to the thermally-induced spin-crossover transition, which occurs around T _c ≈47 K. Furthermore, from the high-field experiment, a magnetochromism, accompanying with the field-induced spin-crossover, has been observed just below T _c .     

Magnetic,magnetocaloric properties,and critical behavior in a layered perovskite La<Subscript>1.4</Subscript>(Sr<Subscript>0.95</Subscript>Ca<Subscript>0.05</Subscript>)<Subscript>1.6</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript>

We report the results of magnetic, magnetocaloric properties, and critical behavior investigation of the double-layered perovskite manganite La_1.4(Sr_0.95Ca_0.05)_1.6Mn₂O₇. The compounds exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature T _C = 248 K, a Neel transition at T _N = 180 K, and a spin glass behavior below 150 K. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition range. Magnetic entropy change (??S _M) was estimated from isothermal magnetization data. The critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T _C = 248 K, β = 0.594, γ = 1.048, and δ = 2.764. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M _S(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (??S _M) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change [(??S _M) vs. M ²] and the classical extrapolation from the Arrott curves (µ₀H/M vs. M ²), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well.     

Spin Wave Excitation in a Mechanically Alloyed Co2MnSi Solid Solution

The spin wave excitation has been analyzed in a Co₂MnSi solid solution compound. A Co₂MnSi sample was prepared by utilizing a mechanical alloying technique in Ar atmosphere. After 72 hours of milling time, the Co₂MnSi solid solution was transformed from a multiphase to a single broadening phase, which is essential in understanding the spin wave excitation in a nanocrystalline material. The magnetization was measured from 7 K to room temperature by using a superconducting quantum interference device (SQUID) magnetometer. The thermal-magnetization curve was found to obey the Bloch law, M _S(T)=M _S(0)(1?BT ^3/2?CT ^5/2). Based on this formula, the spin wave stiffness constant was calculated from the magnetization data at low temperatures. The values were 0.264 eV?Ų and 0.325 eV?Ų for 72 and 96 hours of milling time, respectively. The spectroscopic splitting g-factor was obtained via electron spin resonance (ESR) measurements at X-band (9.45 GHz).     

Above room temperature magnetocaloric effect in perovskite Pr0.6Sr0.4MnO3

The dependence of magnetization M on the applied magnetic field H and temperature T was measured carefully, near the Curie temperature T_C for the perovskite manganite sample Pr_0.6Sr_0.4MnO₃. The experimental results indicate that this specimen exhibit ferromagnetic (FM) to paramagnetic (PM) transition at T_C ~ 320 K. In the 200 K–45 K temperature range the spontaneous magnetization was decreasing, probably due to spin canted state between manganese and praseodymium spin systems. At 46 K the magnetization presents a second little transition, which can be ascribed to very weak traces of secondary Mn₃O₄ phase, and remains constant between 10 K and 46 K. The maximum value of the magnetic entropy change obtained from the M(H) plot data is |ΔS_M^max| = 2.3 Jkg^? 1 K^? 1 for applied magnetic field of 2.5 T. At this value of magnetic field the relative cooling power (RCP) is 34.5 Jkg^? 1. The temperature corresponding to ΔS_M maximum value is almost equal to T_C. The large entropy change can be attributed to the fact that the ferromagnetic transition enhances the effect of the applied magnetic field greatly. It is suggested by the results that this compound can be used as the working material in an active magnetic regenerative refrigerator above room temperature.     

High-Field ESR and Magnetization Study of a Novel Macrocyclic Chelate Trinuclear Ni(II) Complex

We have investigated magnetic properties of a novel macrocyclic chelate Ni(II) complex [Ni₃(L)(OAc)₂], by means of the static magnetization M and high field frequency tunable electron spin resonance (HF-ESR). Magnetic field B dependencies of M reveal the magnetic ground state with a total spin S $^{\mathit{tot}}_{0}=1$ and a strong antiferromagnetic coupling between three Ni(II) ions. HF-ESR measurements at frequencies ν=80–350 GHz and B up to 15 T yield a magnetic anisotropy gap of the order of 60 GHz (2.9 K) and a g-factor of 2.2. In addition, the modelling reveals a positive single ion anisotropy (D>0) corresponding to an easy plane situation for the Ni₃ complex.     

Magnetism in BiFe<Subscript>1−x</Subscript>Ni<Subscript>x</Subscript>O<Subscript>3</Subscript>: studied through electron spin resonance spectroscopy

The effect of Ni doping in BiFe_1?xNi_xO₃ (BFNO) multiferroics are studied by X-ray diffraction (XRD), Fourier transmission infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), hysteresis loop (M–H), temperature dependent magnetization (FC-ZFC) measurements and electron spin resonance (ESR) techniques. The XRD and FTIR studies indicate that the BFNO compounds remain in rhombohedral (R3c) phase without appearance of any structural transformation due to Ni doping. The XPS studies show the oxidation states of Fe ions as 3⁺, whereas Bi is found to be in a mixed valence state of 2⁺ and 3⁺ in BFNO. The Ni ion doping enhances the saturation magnetization from 0.179 emu/g (x?=?0.025) to 2.38 emu/g (x?=?0.20), which is higher than the reported values found in literature. The FC-ZFC magnetization studies suggest the presence of a magnetic phase transition from a weak ferromagnetic to a spin glass state at low temperature. The ESR studies confirm the ferromagnetic state of BFNO samples.     

Superconductivity of Bi Confined in an Opal Host

Superconductivity is observed in a composite of rhombohedral crystalline bismuth nanoparticles imbedded in an insulating porous opal host via electrical transport and AC magnetic susceptibility. The onset of superconductivity in this system occurs in two steps, with upper transition temperature T _c,U =4.1 K and lower transition temperature of T _c,L =0.7 K, which we attribute to the granular nature of the composite. The transition at T _c,U is observed to split into two transitions with the application of a magnetic field, and these have upper critical fields extrapolated to T=0 K of H _c2,1(0)=0.7 T and H _c2,2(0)=1.0 T, corresponding to coherence lengths of ξ ₁(0)=21 nm and ξ ₂(0)=18 nm, respectively. We suggest that because of the lack of bulk-like states in the Bi nanoparticles due to confinement effects, superconductivity originates from surface states arising from Rashba spin-orbit scattering at the interface.     

Observation of Field-induced Anomaly in High-field Magnetization on a Complex Spin-Driven Multiferroic Compound, LiCu2−z Zn z O2

We performed high-field magnetization measurements in the pulse magnetic fields up to 55 T at low temperatures on a doped LiCu_2?z Zn _z O₂ (z=0.07). When H // c-axis, one anomaly was clearly observed in $\frac{dm}{dH}$ curves at ～9 T. The anomaly broadens as temperature increases and disappears at ～15 K, which is close to the critical temperature of spiral spin phase and the formation of isolated spin-dimer. Interestingly, an additional peak shows up at 4.2 K, indicating a possible boundary at ～5 K. On the other hand, when H // a-axis, one broad anomaly can be distinguish which is centered around 11.8 T. However, this anomaly only appears at T≤5.5 K, which is much lower than 15 K, indicating the origin of this anomaly could be different with those in H // c-axis. It is probably comes from the broken one-dimensional chain and the complex inter-chain interactions.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

High-Field and Multi-Frequency ESR in the Quasi Two-Dimensional Triangular-Lattice Antiferromagnet CuCrO2

We have performed high-field and multi-frequency electron spin resonance (ESR) and magnetization measurements in magnetic fields up to about 53 T on single crystals of a delafossite triangular-lattice antiferromagnet CuCrO₂, which has been proposed to show an out-of-plane 120° spiral spin structure below about 24 K. We calculated ESR resonance modes and magnetization curves assuming an Ising-like Heisenberg Hamiltonian with a spiral spin structure by a mean-field theory. The experimental results can be well reproduced using the following parameters, the intralayer exchange constant J/k _B=26.9 K, the easy-axis anisotropy D/k _B=?0.47 K, and the in-plane g-factor g _ab =2.0.     

Magnetic State and Magnetocaloric Effect of SmMnO3

We present a study of magnetization and magnetocaloric effect for the SmMnO₃ compound. This compound was synthesized by combustion reaction and its magnetic and structural properties studied by X-ray powder diffraction (XRD) and magnetization (M) measurements as a function of temperature and under magnetic fields. The XRD pattern at room temperature confirmed the presence of a single phase with orthorhombic structure. From magnetization versus temperature, we observe two magnetic orderings, the first one at 6 K due to Sm³⁺, and the other one at T _N =57(2) K is the anti-ferromagnetic long-range ordering. The magnetic entropy change, ΔS _M , was obtained from magnetization isotherms close to T _N where it reaches a maximum value of about 8.0 J/kg K for an applied field of 7 T.     

Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We report synthesis and superconductivity at 3.7 K in PrO_0.5F_0.5BiS₂. The newly discovered material belongs to the layered sulfide based REO_0.5F_0.5BiS₂ compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ^°C in a single step. Detailed structural analysis has shown that the as synthesized PrO_0.5F_0.5BiS₂ is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) Å, c=13.362(4) Å. Bulk superconductivity is observed in PrO_0.5F_0.5BiS₂ below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (T _c ) onset at 3.7 K and T _c (ρ=0) at 3.1 K. The hump at T _c related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ～6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (H _c1) of around 8 Oe.     

A New Method for the Estimation of H c2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the γ(T ) = H ^ab _c2/H^c2 with γ(T _c) ≈ 3.     

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB₂Cu₃O₇ (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T _dp very close to the critical temperature T _c (T _dp～0.95T _c), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T _dp (～0.5T _c). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO₃ (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T _c does not support a low T _dp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T _dp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T _dp close to T _c. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.     

Anisotropic Magnetocaloric Effect in Single-crystalline Pr0.52Sr0.48MnO3

We report the magnetocaloric effect (MCE) in a Pr_0.52Sr_0.48MnO₃ single crystal estimated from the isothermal magnetization curve using the Maxwell relation. Isothermal magnetization curves are measured over the range 20 K to 320 K where the field was applied parallel (??) and perpendicular (??) to the [110] direction of the perovskite structure with Pbnm space group. A peak in the temperature (T) dependence of magnetic entropy change (??S _M) with a fairly large negative value (???3.3 J/kg?K) is observed at 275 K close to the Curie temperature (T _C) for a change in field of ??H=40 kOe. The ?? and ?? components of ??S _M deviate from each other below ??260 K and an inverse MCE is observed below ??150 K. We note that the Landau theory of phase transitions satisfactorily explains the ??S _M vs T plot around the second-order transition at T _C.     

Magnetic and magnetotransport properties of single-crystalline R_2PdGe_6(R = Pr,Gd and Tb)

Single crystals of R_2PdGe_6(R = Pr, Gd and Tb) compounds were grown by the Bi-flux method. Pr_2PdGe_6 is an antiferromagnetic compound with Néel temperature T_N= 15 K, in which a field-induced magnetic transition(spin flip) occurs when a magnetic field is applied along either a or b axis; a small magnetization and hysteresis loop were observed when a field is applied along c axis. Gd_2PdGe_6 is a collinear antiferromagnetic compound with T_N= 37 K along b axis. Tb_2PdGe_6 is an antiferromagnetic compound with T_N= 48 K and its hard magnetization direction is along b axis. The temperature dependences of the resistance of the entire three compounds present inflection points at the respective T_N. A large resistance(as well as magnetoresistance) change can be found at the spin flip transition of Pr_2PdGe_6, but the change is not obvious at the spin flop transition of Gd_2PdGe_6.     

Comparison of the irreversible thermomagnetic behaviour of some ferro- and ferrimagnetic systems

The magnetic behaviour of two ferromagnetic oxides and two ferrimagnetic oxides (ferrites) are compared to study the effect of magnetocrystalline anisotropy on thermal-history-dependence of magnetization of these ordered magnetic systems. All four compounds show thermomagnetic irreversibility (M _FC >M _ZFC) below a certain temperature,T _irr. The highly anisotropic ferromagnetic oxide, SrRuO₃ and the hard ferrite, SrFe₁₂O₁₉ show sharp peaks belowT _c in theirM _ZFC(T) curves, whereas for the soft ferrite Ni_0.8Zn_0.2Fe₂O₄ and the low anisotropic ferromagnetic oxide La_0.7Ca_0.3MnO₃ only a broad maximum is observed inM _ZFC, when measured in small magnetic fields. The shapes of theM _ZFC(T) curves are inversely-related to the magnitude of the coercivities (H _c) of the compounds in relation to the applied field, and the temperature dependence of H_c.M _FC andM _ZFC are related through the coercivity for all four magnetic systems.     

Superconductivity at 14 K in SmFe0.9Co0.1AsO

We report superconductivity in the SmFe_0.9Co_0.1AsO system being prepared by most easy and versatile single-step solid-state reaction route. The parent compound SmFeAsO is non-superconducting but shows the spin density wave (SDW) like antiferromagnetic ordering at around 140 K. To destroy the antiferromagnetic ordering and to induce the superconductivity in the parent system, the Fe²⁺ is partially substituted by Co³⁺. Superconductivity appears in SmFe_0.9Co_0.1AsO system at around 14 K. The Co doping suppresses the SDW anomaly in the parent compound and induces the superconductivity. Magnetization measurements show clearly the onset of superconductivity with T _c^dia at 14 K. The isothermal magnetization measurements exhibit the lower critical fields (H _c1) to be around 200 Oe at 2 K. The bulk superconductivity of the studied SmFe_0.9Co_0.1AsO sample is further established by open diamagnetic M(H) loops at 2 and 5 K. Normal-state (above T _c) linear isothermal magnetization M(H) plots excluded the presence of any ordered magnetic impurity in the studied compound.      

Observations and Analysis of the Thermal Disclosure of Hidden Magnetic Moments in High T c Bi:2223/Ag Tape During Warming from Low Temperatures (20, 30, and 50 K) to T c:H ⊥ c Axis

We report on measurements of the evolution of the magnetization 〈M〉 upon warming from T ₀ (20, 30, 50 K) to T _c, for Bi:2223/Ag tape prepared by the continuous filling and forming (CTFF) with the magnetization 〈M〉 initially situated along bridges between the envelopes of the major hysteresis curves. We focused on the cases where 〈M〉=0, the constituent moments cancel each other exactly. We have measured the release of magnetic moments as a function of temperature. 〈M〉 versus T traces S shaped curves where after tracing a valley the paramagnetic magnetization is followed by the appearance of a diamagnetic peak, which is then extinguished as T attains T _c. Our observations, evidence for hidden magnetization, have been explained on the basis of the critical state model assuming certain dependence of the critical current density and Meissner surface current on temperature.     

Superconducting Properties of In Situ Mg1.05(B1−xCx)2 Doped with Melanin (C16H2O3N2) via Ultrasonication in Ethanol

In this paper, we have reported melanin (C₁₆H₂O₃N₂) as a dopant of MgB₂ for the first time. Here, the effects of melanin doping to the microstructures and superconducting properties of bulk MgB₂ are thoroughly studied from XRD, SEM, TEM, magnetization, and resistivity data. We have analyzed the critical current density (J _c), irreversibility field (H _irr), flux pinning, resistivity, lattice parameters, grain sizes, critical temperatures (T _c), and other microstructures of all the samples. We have varied the doping percentage according to the nominal atomic ratio of Mg_1.05(B_1?x C _x )₂, x=0,0.02,0.06,0.08,0.1. The J _c of all the melanin-doped samples are improved as compared to that of the undoped sample in high-field region (above 6 Tesla) at low temperature. The 8 and 10 % doped samples give the best results. The 8 % doped sample registers an enhancement of J _c by a factor of 3.6 at 7 T and 5 K as compared to that of the undoped one. But, in the low-field region, melanin doping reduces J _c. The H _irr shows remarkable enhancement at low temperatures below 20 K. The best value of H _irr was found for the 8 % doped sample. However, H _irr reduces at high temperatures above 20 K in all the melanin-doped samples. The volume pinning strength of all the doped samples is enhanced over the entire field range. Further improvement in superconducting properties can be achieved by further reducing the size of the melanin particles, increasing density, and improving the homogeneity of doping.