Magnetic History Dependence Research Articles

Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions

Ca2Co2O5 in the brownmillerite form was synthesized using a high-pressure optical-image floating zone furnace, and single crystals with dimensions up to 1.4x0.8x0.5 mm3 were obtained. At room temperature, Ca2Co2O5 crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group Pcmb (No. 57) with unit cell parameters a=5.28960(10) {\AA}, b=14.9240(2) {\AA}, and c=10.9547(2) {\AA}. With decreasing temperature, it undergoes a re-entrant sequence of first-order structural phase transitions (Pcmb to P2/c11 to P121/m1 to Pcmb) that is unprecedented among brownmillerites, broadening the family of space groups available to these materials and challenging current approaches for sorting the myriad variants of brownmillerite structures. Magnetic susceptibility data indicate antiferromagnetic ordering in Ca2Co2O5 occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, the specimen exhibits a weak ferromagnetic component directed primarily along the b axis that shows a pronounced thermal and magnetic history dependence.

Vibrating-Coil Magnetometry of the Spin Liquid Properties ofTb2Ti2O7

We have explored the spin liquid state in Tb(2)Ti(2)O(7) with vibrating-coil magnetometry down to ~0.04 K under magnetic fields up to 5 T. We observe magnetic history dependence below T*~0.2 K reminiscent of the classical spin ice systems Ho(2)Ti(2)O(7) and Dy(2)TiTi(2)O(7). The magnetic phase diagram inferred from the magnetization is essentially isotropic, without evidence of magnetization plateaus as anticipated for so-called quantum spin ice, predicted theoretically for [111] when quantum fluctuations renormalize the interactions. Instead, the magnetization for T≪T* agrees semiquantitatively with the predictions of "all-in-all-out" (AIAO) antiferromagnetism. Taken together, this suggests that the spin liquid state in Tb(2)Ti(2)O(7) is akin to an incipient AIAO antiferromagnet.

Magnetization jumps and relaxation effect in doped CeFe2

The dynamic antiferromagnetic phase present in CeFe2 gets stabilized with Ga and Si substitutions. Phenomena such as strain-induced first order jumps in the magnetization curves, asymmetry between the M-H curves during the increasing and decreasing field cycles, the envelope curve being inside the virgin curve, occur in these compounds. Temperature and time dependences of magnetization show that the compounds possess glassy behavior at low temperatures. Multi-step magnetization behavior, unusual relaxation effect, thermal and magnetic history dependence, which are signatures of the martensitic scenario due to the strong magneto-structural coupling, are found to be present in this system. We also show that one can induce the magnetization steps with the help of appropriate measurement protocol. Detailed magnetization relaxation studies have been carried out to understand the dynamics of magnetic phase transition.

High-field history dependence of magnetization and resistivity in phase-separatedY0.2Ca0.8MnO3

We studied the high-field history dependence of magnetization and resistivity of ${\mathrm{Y}}_{0.2}{\mathrm{Ca}}_{0.8}\mathrm{Mn}{\mathrm{O}}_{3}$ in the frame of magnetic and structural phase transitions. With increasing magnetic fields, the difference between field-cooled and zero-field-cooled data becomes increasingly large, and the onset temperature of the difference shifts toward high temperatures. The x-ray diffraction and electron spin resonance measurements reveal that the orthorhombic paramagnetic phase disappears, and the orthorhombic ferromagnetic and monoclinic antiferromagnetic phases coexist below $\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Based on these results, a phenomenological model was proposed to explain the high-field history dependent behavior.

Magnetotransport of the low-carrier density one-dimensional S=1/2 antiferromagnet Yb4As3

The transport properties of the semimetallic quasi-one-dimensional S=1/2 antiferromagnet Yb4As3 have been studied by performing low-temperature (T≥0.02 K) and high magneticfield (B≤60 T) measurements of the electrical resistivity ρ(T, B). For T ≿ 2 K a ‘heavy-fermion’-like behavior Δρ(T)=AT2 with huge and nearly field-independent coefficient A ≈ 3 µΩ cm/K2 is observed, whereas at lower temperatures ρ(T) deviates from this behavior and slightly increases to the lowest T. In B>0 and T ≾ 6 K the resistivity shows an anomalous magnetic-history dependence together with an unusual relaxation behavior. In the isothermal resistivity Shubnikov-de Haas (SdH) oscillations, arising from a low-density system of mobile As-4p holes, with a frequency of 25 T have been recorded. From the T- and B-dependence of the SdH oscillations an effective carrier mass of (0.275±0.005)m0 and a charge-carrier mean-free path of 215 A are determined. Furthermore, in B≥15 T, the system is near the quantum limit and spin-splitting effects are observed.

Magnetic history dependence of metastable states in thin films with dipolar interactions

We present the results of a Monte Carlo simulation of the ground state and magnetic relaxation of a model of a thin film consisting of a two-dimensional square lattice of Heisenberg spins with perpendicular anisotropy K, exchange J and long-range dipolar interactions g. We have studied the ground state configurations of this system for a wide range of the interaction parameters J/ g, K/ g by means of the simulated annealing procedure, showing that the model is able to reproduce the different magnetic configurations found in real samples. We have found the existence of a certain range of K/ g, J/ g values for which in-plane and out-of-plane configurations are quasi-degenerated in energy. We show that when a system in this region of parameters is perturbed by an external force that is subsequently removed, different kinds of ordering may be induced depending on the followed procedure. In particular, simulations of relaxations from saturation under an AC demagnetizing field or in zero field are in qualitative agreement with recent experiments on epitaxial and granular alloy thin films, which show a wide variety of magnetic patterns depending on their magnetic history.

MAGNETIC FIELD AND HISTORY DEPENDENCE OF THE HARMONIC SUSCEPTIBILITY IN A GRANULAR SUPERCONDUCTOR

We report on the generation of harmonics of the ac susceptibility in a typical Y(123) granular superconductor. The intergrain harmonic response has been studied at T=77 K as a function of the ac field, both with and without superposed dc fields. We find that our results can be adequately described by the Anderson-Kim model. The effect of superposed dc fields on the ac field dependence of the even and odd harmonics has been qualitatively explained. Magnetic history dependence has ben studied in the field cooled and remnant states. Effects of field cooling indicate that the intergrain fields in this case are slightly smaller than for the corresponding zero field cooled condition. Remnant field studies clearly show the role of trapped flux in generating even harmonics.

Strong magnetic history dependence of magnetic relaxation in high- Tc superconductors

Very detailed flux creep and relaxation measurements of intragrain magnetization in sintered high- T c superconductors at 4.2 K are presented. The M-relaxation rate is not a single valued function of the applied B-field but depends strongly on the prior magnetic field cycling procedure. The largest relaxation rate is measured on the envelope curves of the M-hysteresis area. On the transition curves (minor loops) the relaxation rate is reduced and even disappears locally. This is of great importance for applications of high- T c superconductors in electronics. The results can also be applied to low- T c superconductors like NbTi and are of interest for accelerator magnets with superconducting windings. Theoretical expressions for the relaxation process are derived for all physically relevant parts of the M-hysteresis curve on the basis of Bean's model. As the grain M influences also the integrain critical transport current and microwave absorption granular high- T c material, our results on M-relaxation behavior are a basis to explain the already observed time dependence of those phenomena.