Low Temperatures In High Magnetic Fields Research Articles

Structural, magnetic, dielectric and $$^{57}\text {Fe}$$ Mössbauer spectroscopic studies on $$\text {Fe}_{1-x}\text {Ce}_x\text {VO}_4$$: a type-II multiferroic material

Here, we report detailed studies on magnetic phase transitions and magneto-electric (ME) coupling in $$\text {Fe}_{1-x}\text {Ce}_x\text {VO}_4$$ . X-ray diffraction (XRD) and Raman spectroscopic measurements confirm triclinic crystal structure (P-1) with small variation in lattice parameters and Ce incorporation into the $$\text {FeVO}_4$$ lattice up to 10% of Ce. The presence of local lattice distortions and electronic inductive effect in $$\text {FeVO}_4$$ between $$\text {Fe}^{3+}$$ and $$\text {V}^{5+}$$ ions were found in X-ray absorption near edge structure (XANES) studies due to the presence of $$\text {Fe}^{3+}$$ –O– $$\text {V}^{5+}$$ linkages. Two antiferromagnetic (AFM) transitions similar to $$\text {FeVO}_4$$ appear at 21.86 K and 16.03 K, and Ce doping has little effect on the magnetic transitions. $$^{57}\text {Fe}$$ Mossbauer spectroscopic results show the invariance of Fe valance. Low temperature high magnetic field Mossbauer data depicts the presence of spiral AFM order in all the samples. A sharp peak at 16.0 K in dielectric permittivity with minimal suppression due to applied magnetic field is observed indicating the presence of ferroelectricity and magneto-dielectric coupling. Magnetic and ME transitions are almost robust against Ce doping.

Low-temperature Raman, high magnetic field Fe57 Mössbauer, and x-ray diffraction study of magnetodielectric coupling in polycrystalline GdFeO3

The present work reports the magnetodielectric (MD), ferroelectric, magnetoelectric (ME), low-temperature (LT) Raman, low-temperature high magnetic field (LTHM) $^{57}\mathrm{Fe}$ M\ossbauer and x-ray diffraction (XRD) studies of polycrystalline multiferroic ${\mathrm{GdFeO}}_{3}$. Ferroelectric ordering as a result of magnetic ordering of ${\mathrm{Gd}}^{3+}$ ions (${T}_{N,\mathrm{Gd}}$) is confirmed from the pyrocurrent measurements carried out in conventional and bias-electric field protocols. Coupling between magnetic and electric orderings, viz., magneto-electric (ME) coupling, is confirmed by measuring the dielectric constant and pyrocurrent in the presence of magnetic field. A huge magnetodielectric (MD) coefficient ($\ensuremath{\approx}\ensuremath{-}12%$) is observed below the ${\mathrm{Gd}}^{3+}$ ordering temperature, reaching a saturation value with a field of about 3 Tesla. From LT Raman data across ${T}_{N,\mathrm{Gd}}$, the presence of spin-phonon coupling and the development of new modes indicating the symmetry reduction are observed. The average displacement of cations obtained from LTHM XRD measurement corroborates the ferroelectric polarization measurements. Signatures of a transferred hyperfine field are observed from LTHM $^{57}\mathrm{Fe}$ M\ossbauer data. The variation of the effective internal magnetic field obtained from in-field M\ossbauer data gives evidence for the predicted spin structures of ${\mathrm{GdFeO}}_{3}$ with magnetic fields below ${T}_{N,\mathrm{Gd}}$. The observed anomalous variation of the center shift and resonance area from $^{57}\mathrm{Fe}$ M\ossbauer data across ${T}_{N,\mathrm{Gd}}$ ordering is explained in terms of phonon mode softening. Observed MD and ME effects are explained in terms of spin-phonon coupling as evidenced from LTHM $^{57}\mathrm{Fe}$ M\ossbauer and LT Raman data.

In-field X-ray and neutron diffraction studies of re-entrant charge-ordering and field induced metastability in La0.175Pr0.45Ca0.375MnO3-δ

Low-temperature high-magnetic field (2 K, 8 T) (LTHM) powder X-ray diffraction (XRD) and time of flight powder neutron diffraction (NPD), low-temperature transmission electron microscopic (TEM), and resistivity and magnetization measurements have been carried out to investigate the re-entrant charge ordering (CO), field induced structural phase transitions, and metastability in phase-separated La0.175Pr0.45Ca0.375MnO3-δ (LPCMO). Low-temperature TEM and XRD studies reveal that on cooling under zero-field, paramagnetic Pnma phase transforms to P21/m CO antiferromagnetic (AFM) insulating phase below ∼233 K. Unlike reported literature, no structural signature of CO AFM P21/m to ferromagnetic (FM) Pnma phase-transition during cooling down to 2 K under zero-field was observed. However, the CO phase was found to undergo a re-entrant transition at ∼40 K. Neutron diffraction studies revealed a pseudo CE type spin arrangement of the observed CO phase. The low-temperature resistance, while cooled under zero-field, shows insulator to metal like transition below ∼105 K with minima at ∼25 K. On application of field, the CO P21/m phase was found to undergo field-induced transition to FM Pnma phase, which shows irreversibility on field removal below ∼40 K. Zero-field warming XRD and NPD studies reveal that field-induced FM Pnma phase is a metastable phase, which arise due to the arrest of kinetics of the first-order phase transition of FM Pnma to CO-AFM P21/m phase, below 40 K. Thus, a strong magneto-structural coupling is observed for this system. A field-temperature (H-T) phase-diagram has been constructed based on the LTHM-XRD, which matches very nicely with the reported H-T phase-diagram constructed based on magnetic measurements. Due to the occurrence of gradual growth of the re-entrant CO phase and the absence of a clear structural signature of phase-separation of CO-AFM P21/m and FM Pnma phases, the H-T minima in the phase-diagram of the present LPCMO sample has been attributed to the strengthening of AFM interaction during re-entrant CO transition and not to glass like “dynamic to frozen” transition.

Studies on magnetic field and temperature driven magneto-structural phase transition in La0.5Sr0.5MnO3+δ

Abstract The influence of magnetic field on the crystal structure of colossal magnetoresistance (CMR) material La 0.5 Sr 0.5 MnO 3+δ has been investigated. Low temperature high-magnetic field powder X-ray diffraction (XRD) measurements were carried out on La 0.5 Sr 0.5 MnO 3+δ in temperature range from 4.2 to 300 K and magnetic field range from 0 to 8 T. A first order structural phase transition from tetragonal I 4/ mcm to orthorhombic Fmmm phase, coupled with reported electronic and magnetic phase transition from ferromagnetic metal to antiferromagnetic insulator state has been started at ∼200 K and completed at ∼100 K. On the application of 8 T magnetic field this magneto-structural phase transition shifted to lower temperature (∼162 K) with phase coexistence regime down to 4.2 K. Isothermal X-ray diffraction measurements at 150 K infer the evidence of magnetic field driven first order structural phase transition from Fmmm to I 4/ mcm . These results consistent with the resistivity and magnetoresistance results; present the microscopic evidence of strong spin-lattice coupling and reveal the magnetic field driven structural phase transition as the origin of observed colossal magnetoresistance in this material.

Low-temperature high magnetic field powder x-ray diffraction setup for field-induced structural phase transition studies from 2 to 300 K and at 0 to 8-T field.

A low-temperature and high magnetic field powder x-ray diffractometer (XRD) has been developed at UGC-DAE CSR (UGC: University Grant Commission, DAE: Department of Atomic Energy, and CSR: Consortium for scientific research), Indore, India. The setup has been developed around an 18 kW rotating anode x-ray source delivering Cu-Kα x-rays coming from a vertical line source. It works in a symmetric θ-2θ parallel beam geometry. It consists of a liquid helium cryostat with an 8 T split-pair Nb-Ti superconducting magnet comprising two x-ray windows each covering an angular range of 65°. This is mounted on a non-magnetic type heavy duty goniometer equipped with all necessary motions along with data collection accessories. The incident x-ray beam has been made parallel using a parabolic multilayer mirror. The scattered x-ray is detected using a NaI detector through a 0.1° acceptance solar collimator. To control the motions of the goniometer, a computer programme has been developed. The wide-angle scattering data can be collected in a range of 2°-115° of 2θ with a resolution of ∼0.1°. The whole setup is tightly shielded for the scattered x-rays using a lead hutch. The functioning of the goniometer and the artifacts arising possibly due to the effect of stray magnetic field on the goniometer motions, on the x-ray source, and on the detector have been characterized by collecting powder XRD data of a National Institute of Standards and Technology certified standard reference material LaB6 (SRM-660b) and Si powder in zero-field and in-field conditions. Occurrence of field induced structural-phase transitions has been demonstrated on various samples like Pr0.5Sr0.5MnO3, Nd0.49Sr0.51MnO3-δ and La0.175Pr0.45Ca0.375MnO3 by collecting data in zero field cool and field cool conditions.

Magnetic anisotropy and sub-lattice magnetization study of polycrystalline magneto-electric [formula omitted

Polycrystalline magneto-electric Ga2−xFexO3 (0.8≤x≤1.2) ceramics are studied with bulk magnetization and 57Fe Mössbauer measurements. The ferrimagnetic to paramagnetic transition temperature (TC), saturation magnetization (MS) and the magnetic anisotropy values are found to vary significantly with the site-disorder of Fe3+ and Ga3+ cations. With the increase of iron content, increase of TC and MS, and decrease of magnetic anisotropy are observed. Using low temperature high magnetic field 57Fe Mössbauer measurements, individual temperature dependence of hyperfine fields and hence magnetization of the three sub-lattices are measured. It is observed that the three sub-lattices have quite a different temperature dependence resulting in the non-monotonous variation of resultant magnetization as measured by bulk magnetic measurements. Definite evidence, using 57Fe Mössbauer measurements, is submitted to show that the Fe ions at Fe2 and Ga2 sites are responsible for the observed magnetic anisotropy, site-disorder induced magnetism and thermo-magnetic irreversible features in Ga2−xFexO3.

Low temperature high magnetic field 57Fe Mössbauer study of kinetic arrest in Ta doped HfFe2

Low temperature high magnetic field 57Fe Mössbauer measurements were carried out on the inter-metallic compound Hf0.77Ta0.23Fe2 by following novel paths in H–T space. The ferromagnetic (FM) fraction at 5 K and zero magnetic field is shown to depend on the cooling field, i.e., the higher the field is, the higher the FM fraction is. Mössbauer spectra collected in the presence of a 4 T magnetic field show that the antiferromagnetic (AFM) spins are canted with respect to the applied magnetic field and hence contribute to the total bulk magnetization in this compound. The data also show an induced magnetic moment even at the 2a site of the AFM phase. Mössbauer spectra collected using the CHUF (cooling and heating in un-equal magnetic fields) protocol show a reentrant transition when the sample is cooled in zero field and measured during warming in 4 T, showing the FM state as the equilibrium state. This work is the first microscopic experimental evidence for the de-vitrification of the kinetically arrested magnetic state.

Electronic states on the surface of graphite

Abstract Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip–sample distance beyond which the effect of sample–tip interaction is negligible.

Order Parameter and Vortices in the SuperconductingQPhase ofCeCoIn5

Recently, it has been reported that the low-temperature high-magnetic field superconducting phase in CeCoIn(5) (Q phase), has spin-density wave (SDW) order that only exists within this phase. This indicates that the SDW order is the result of the development of pair density wave (PDW) order in the superconducting phase that coexists with d-wave superconductivity. Here we develop a phenomenological theory for these coexisting orders. This provides selection rules for the PDW order and further shows that the detailed structure of this order is highly constrained. We then apply our theory to the vortex phase. This reveals vortex phases in which the d-wave vortex cores exhibit charge density wave order and further reveals that the SDW order provides detailed information about the vortex phase.

Scanning Tunneling Spectroscopy of Graphene on Graphite

We report low temperature high magnetic field scanning tunneling microscopy and spectroscopy of graphene flakes on graphite that exhibit the structural and electronic properties of graphene decoupled from the substrate. Pronounced peaks in the tunneling spectra develop with increasing field revealing a Landau level sequence that provides a direct way to identify graphene and to determine the degree of its coupling to the substrate. The Fermi velocity and quasiparticle lifetime, obtained from the positions and width of the peaks, provide access to the electron-phonon and electron-electron interactions.