Phase Transition and Anomalous Low Temperature Ferromagnetic Phase in Pr0.6Sr0.4MnO3 Single Crystals

We report on the magnetic and electrical properties of Pr_0.6Sr_0.4MnO₃ single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T _C∼301 K and a first-order structural transition at T _S∼64 K. At T _S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M–H) curves below T _S are anomalous in that the virgin curve lies outside the subsequent M–H loops. The hysteretic structural transition at T _S as well as the irreversible magnetization processes below T _S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.     

Theory of Unusual Superconducting Phase Transitions in Heavy Fermion Metals at High Magnetic Fields

We present a non-perturbative theory of paramagnetically-driven superconducting (SC) phase transitions in heavy-fermion metals, which reliably determines their stable SC phases, treats properly the corresponding finite jumps of the order parameter, and can account for unusual features reported recently for this type of materials. It is found that for quasi-2D heavy-fermion metals, such as CeCoIn₅, at high magnetic fields oriented perpendicular to the highly conducting planes, the effect of the Fulde-Ferrel (FF) modulation is too weak to prevent a direct first-order phase transition from the normal to the uniform SC state. For 3D heavy-fermion metals, such as URu₂Si₂, the FF modulation stabilizes, under a decreasing magnetic field, a non-uniform SC state via a second-order phase transition from the normal state. However, at a slightly lower field the modulated phase becomes unstable, transforming to a uniform SC state via a first-order transition.     

Slow growth of 3He-B into hypercooled 3He-A

We report observations of the first-order phase transition between the two superfluid phases of ³He. We supercool a long cylindrical sample of the higher-temperature A phase in a magnetic field in a simple dc magnetometer. We then introduce B phase at one end of the sample and observe its growth into the supercooled A phase via the magnetometer signal. For temperatures above a certain value, the phase interface travels up the tube with a nearly time-independent velocity, whose temperature dependence is in reasonable agreement with theory. We also investigate the dependence of this velocity on tube diameter and magnetic field. Our measured values of the difference between A phase and B phase magnetization agree with previous dynamic (NMR) measurements. We also derive thermodynamic data on the AB transition from the measurements of magnetization difference and the dependence of the transition temperature on magnetic field.     

Ferromagnetic and Charge-Ordered Phases in (Nd, Na)?CMn?CO Compounds

Polycrystalline Nd_1?x Na _x MnO₃ ( $x=0.05\mbox{--} 0.20$ ) compounds were prepared in single-phase form with a Pbnm space group. Paramagnetic to ferromagnetic transitions were observed up to a doping concentration of 15% with a maximum T _C of 113?K. The x=0.20 sample exhibits a charge order transition at 180?K, with a distinct behavior in magnetization versus field curve. The magnetization versus temperature plot of all these materials exhibits an anomaly at around 40?K, with a signature of spin-glass like behavior. The x=0.10 sample is found to exhibit a maximum spontaneous magnetization value of 3.1?? _B at 20?K. The magnetization data could be analyzed based on the Brillouin function model, by accounting for the ferromagnetic interaction. The effective spin contribution of 3d electrons for the double-exchange ferromagnetic interaction and the value of magnetic spin-canting angles were estimated. The measured magnetization is explained on the basis of competing magnetic interaction due to spin-canting. The charge order quenching and the first-order transition from the charge-ordered to ferromagnetic phase are observed for a threshold field of 3?T at 12?K. The electrical resistivity of the above samples exhibits insulating behavior.     

Low-temperature magnetic behavior of HoRh4B4

Detailed measurements of the magnetization of HoRh₄B₄ and GdRh₄B₄ near the magnetic transition temperature are presented. In contrast to the ferromagnetic behavior of GdRh₄B₄, no spontaneous magnetization is found to develop below T _min HoRh₄B₄. This raises doubts about the nature of the magnetic order in that material as well as in DyRh₄B₄ and TbRh₄B₄. Anisotropy and time dependence of the magnetization are found in these last three compounds. The behavior of the very small remanent magnetization and of the paramagnetic susceptibility of HoRh₄B₄ is shown. These results point to a complex magnetic order, possibly helical or sinusoidal antiferromagnetism with a long wavelength.Partially supported by a grant from the Swiss National Science Foundation.Research in La Jolla supported by National Science Foundation Grant No. DMR77-08467.     

Theory of magnetically polarizable superconductors

Superconductors containing a magnetically polarizable medium under an applied magnetic field are discussed theoretically. It is shown that instead of the Pauli limiting field found by Chandrasekhar and Clogston for nonmagnetic superconductors, the maximum critical field is limited by the effect of the susceptibility X_f due to the polarizable medium and is greatly reduced as X_f increases, indicating the appearance of the first-order phase transition from the superconducting to the normal state. The magnetic behavior of the ferromagnet superconductor ErRh₄B₄ under applied fields is analyzed in terms of this idea.     

Phase transformations and properties of the Ni-Mn-In Heusler alloy

The phase and magnetic transformations in a ferromagnetic alloy of the Ni-Mn-In system are studied using dilatometry, magnetometry, metallography, and scanning electron microscopy. The effect of a magnetic field of varying intensity on the martensitic transformation and magnetic transitions of the Ni₄₇Mn₄₂In₁₁ alloy is found. It is shown that the linear expansion coefficient and magnetization exhibit an anomalous behavior in the region of the magnetostructural transition.     

Dependence of Magnetization on Temperature, Magnetoresistance Effect and Magnetic Phase Diagram for the Layered Gd5Ge4 Compound

The dependence of dc magnetization and electrical transport on temperature were systemically studied for the layered Gd₅Ge₄ compound. A complex coexistence behavior of ferromagnetism (FM) with antiferromagnetism (AFM) phase and significant magnetoresistance effect were found under the inducement by the applied magnetic field. The results show an unusual magnetoresistance (MR) effect with opposite behavior at the high and low temperatures, which is positive at high temperature region and negative at low temperature region for the layered Gd₅Ge₄ alloy. And its MR maximum is close to 50%, which can be a very large numerical value e.g. for giant magnetoresistance (GMR) materials. From the magnetization loop under different temperatures, also a magnetization step behavior was found below ∼6.2 K. The results proved an existence of reentrance AFM phase at low temperature. From experimental data, also the magnetic phase diagram is obtained. This kind of complex magnetization is analyzed and discussed in the frame of phase separation. The experimental results opened also the possibility of application of the layered Gd₅Ge₄ compound in the fields of information record and sensor technique.      

Magnetization studies of near-room-temperature flux-trapping phenomenon in CuO-based materials

DC magnetization studies on a variety of ceramic and crystalline YBaCuO samples indicate a superconducting-like flux-trappinig behavior occurring at similar temperatures as zero-resistance transition temperatures observed near room temperature. The flux-trapping behavior is exemplified by diamagnetic-like deviations from a positive magnetic background in the zero-field-cooled magnetization, by a divergence between the zero-field-cooled and field-cooled magnetization data, or by a positive Meissner effect in the field-cooled magnetization. These behaviors are consistent with a high-T _c (>250 K) superconducting phase in an inhomogeneous superconductor where the superconducting regions exist as defect structures or surface coatings and whose volume fraction is less than a few percent.     

Investigation of the occurrence of superconductivity below the antiferromagnetic transition in Ho(Ir0.7Rh0.3)4B4

The pseudoternary compound Ho(Ir_0.7Rh_0.3)₄B₄, which had previously been reported to exhibit the occurrence of antiferromagnetism above the superconducting transition, has been examined using heat capacity, ac magnetic susceptibility, upper critical magnetic field, thermal expansion, magnetostriction, electrical resistance, thermal conductivity, and static magnetization measurements. The data confirm the bulk nature of the antiferromagnetic phase transition and are consistent with the presence of bulk superconductivity coexisting with magnetic order.     

Metamagnetic Phase Transitions in La1−x Nd x Mn2Si2

Experimental measurements for the magnetization at various temperatures are analyzed using a mean field model for the purely ferromagnetic spin configuration near the metamagnetic phase transitions in La_1?x Nd _x Mn₂Si₂ (x=0.3) at a constant magnetic field (50 mT). By fitting the temperature dependence of the magnetization from the free energy in the mean field model to the experimental data for this compound, the coefficients in the free energy expansion are determined. Our analysis of the magnetization describes a first-order character of the metamagnetic transition in La_1?x Nd _x Mn₂Si₂ (x=0) on the basis of the mean field model studied here.     

Evidence for supercooling across a vortex-matter phase transition: studies on CeRU2 and Bi2Sr2CaCu2O8

We present DC magnetization data indicating a first-order phase transition in the vortex state of CeRu₂, with the higher entropy phase exhibiting enhanced pinning. Minor hysteresis loops show evidence of supercooling of the higher entropy phase as the phase boundary is crossed both isothermally as well as at constant field. These features are shown to be absent across the Bragg-glass to vortex-glass transition in Bi₂Sr₂CaCu₂O₈. The supercooling is more persistent in the constant field case.     

Radio-frequency acoustic powder echoes in high-Tc Bi2Sr2CaCu2O8 around the H-T phase boundary

Temperature dependence of intensity of rf acoustic powder echoes is studied for powdered samples of Bi₂Sr₂CaCu₂O₈ ceramics around the H-T phase boundary and compared with combined magnetization data in field-cooled and zero-field cooled conditions for the same powder. Rf acoustic powder echoes are found to vanish at H_C2 rather than H_irr determined from the magnetization data and we conclude that the intrinsic pinning is important for the observation of rf acoustic powder echoes in the high-T_C Bi₂Sr₂CaCu₂O₈ ceramics. Hysteretic behavior observed around H_C2, suggests an existence of the first-order phase transition at the H_C2 line.     

The nature of the transition atH c2 in low-κ type II lead-indium superconducting alloys

The anomalous magnetic and calorimetric properties atH _c2, which we reported for a lead-indium sample, are in agreement with the prediction of a bulk first-order phase transition from recent theories for critical κ superconductors \(\kappa \approx 1/\sqrt 2 \) . Other experimentalists, however, explain this behavior as an effect of the superconducting surface sheath. In an attempt to clarify the issue, magnetization and specific heat measurements are reported here on two samples with nominal concentrations of 1.9 and 1.4 at % indium in lead. The latter is type I above 6 K (T _c =7.12 K). It is shown that surface effects are relevant for the effect reported previously. However, after empirical surface treatments have been applied to eliminate or at least minimize such effects, most of the results for the sample, which fulfills the theoretical requirements, are in agreement with theoretical predictions of Fischer, Usadel, and Jacobs. A bulk first-order transition atH _c2 is thus, nevertheless, likely to occur for strong-coupling type II superconductors with critical κ values.     

Annealing Influence on the Microstructure and Magnetic Properties of Ni–Mn–In Alloys Ribbons

We report on the structure, microstructure and inverse magnetocaloric effect associated with the first-order martensitic phase transition, in Heusler Ni_50.0Mn_35.5In_14.5 alloy ribbons. We have studied the short-time vacuum annealing influence at 1048?K, 1073?K, 1098?K, and 1123?K in these properties. At room temperature, an increase in the degree of structural order for ribbons annealed up to 1098 K was observed, corresponding to cubic L2₁ austenite phase. Meanwhile, for the sample annealed at 1123?K a monoclinic 10M martensitic phase was detected. A comparison of magnetic entropy change as a function of the applied field, after using zero-field-cooling thermomagnetic and isothermal magnetization measurements, has been made for the sample annealed at 1073?K.     

Temperature and Magnetic Field-Induced Spin Reorientation in Rare-Earth Perovskite ErFe<Subscript>0.75</Subscript>Cr<Subscript>0.25</Subscript>O<Subscript>3</Subscript>

We report the synthesis of a single-phase rare-earth perovskite ErFe_0.75Cr_0.25O₃ polycrystalline and its magnetic properties. A transition occurs at temperature T _N = 120 K below which we observe a weak magnetic moment from the canted antiferromagnetism. Interestingly, ErFe_0.75Cr_0.25O₃ reveals the compensation-like behavior at T _comp?like = 27 K, where the net magnetic moments of transition-metal ions are antiparallel and equal to the induced net moment of Er³⁺ ions, and the paramagnetic contribution of Er³⁺ moment presenting a nonzero magnetization. The temperature-dependent magnetization measurement shows a spin reorientation transition from Γ₄ to Γ₁ at 6 K. Furthermore, it is also observed that there is a spin-flop transition at low temperature induced by external magnetic field in Γ₁ state (antiferromagnetic state). The interaction between (Fe/Cr)-3d and Er-4f electrons drives an extremely interesting spin reorientation transition which is highly sensitive to magnetic field and temperature.     

Low-temperature magnetic properties of HoRh4B4

The low-temperature magnetic properties of HoRh₄B₄ have been studied by means of measurements of the magnetic susceptibility, magnetization, specific heat, thermal expansion, and magnetostriction. The ferromagnetic phase transition at T _M= 6.7 K shows almost ideal S = 1/2 mean field behavior in the specific heat. Crystal field effects due to the splitting of the J = 8 Hund's rule ground state of the Ho³⁺ ions result in Schottky anomalies in the specific heat and the thermal expansion and are also revealed in the low-field magnetic susceptibility and the magnetostriction. Information on the ground state doublet of the 4f electrons has been obtained from the nuclear contribution to the specific heat below 1 K and the high-field magnetization below T_M.This research was supported by the Schweizerische Nationalfonds zur Forderung der wissenschaftlichen Forschung (HRO), by the Department of Energy under Contract No. EY-76-S-03-0034-PA227-3 (LDW, MBM), and by the National Science Foundation under Grant No. NSF/DMR77-08469 (DCJ)     

Detection of a metastable magnetic phase in the La1.85Ba0.15CuO4−y system

We have made samples of La_1.85Ba_0.15CuO_4–y with varying heat treatments. We have measured their dc magnetization and electron paramagnetic resonance, and have examined them by electron microscopy and electron diffraction. For samples annealed only a few hours, cooling the material below 5 K causes a magnetic transition and a thermal hysteresis. These observations, taken with the EPR data, suggest a possible spin-glass or ferromagnetic phase. Annealing samples in an oxygen atmosphere appears to remove that magnetic phase. For the phase with the K₂NiF₄ structure, no superlattice spots are seen in electron diffraction at room temperature or at 77 K.     

Magnetocaloric effect in La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> for <Emphasis Type="Italic">x</Emphasis> = 0.3, 0.35, and 0.4

We present systematic studies of the magnetocaloric properties of the polycrystalline La_1−x Ca _x MnO₃ system for x = 0.3, 0.35, and 0.4 near a second-order phase transition from a ferromagnetic to a paramagnetic state. The crystal structure of the studied manganites was shown to be in good agreement with previous reports. The value of the magnetocaloric effect has been determined from the measurement of magnetization as a function of temperature and external magnetic field. The maximum entropy change detected in La_0.7Ca_0.3MnO₃ at a field of 2 T reaches 8 J/kg K which exceeds that of gadolinium. In all studied samples, the paramagnetic–ferromagnetic transition is very narrow but no hysteresis is observed and the transitions are identified as second-order ones. The phase transition in La_0.7Ca_0.3MnO₃ appears to be almost of first-order.     

Signature of room temperature ferromagnetism in Mn doped CeO2 nanoparticles

We report structural and magnetic properties of Mn doped CeO₂ nanoparticles using X-ray diffraction (XRD), field emission transmission electron microscopy (FE-TEM) and dc magnetization measurements. XRD results infer that all the samples have single phase nature and lattice parameters decrease with Mn doping. The particle size calculated using XRD and TEM analysis was found to decrease with Mn doping. Field cooled magnetization measurement shows that the transition temperature is above room temperature. Magnetic hysteresis loop studies indicate that undoped and Mn doped CeO₂ nanoparticles show weak ferromagnetic behavior at room temperature.