Paramagnetic Domains Research Articles

Metal–insulator transition in helical SrFeO3−δ antiferromagnet

The magnetic and electrical transport properties of metallic spiral antiferromagnet SrFeO2.95 have been examined in the temperature range from 4.5–300K. A large negative magnetoresistance below 50K is observed. We find hysteresis in resistivity in 0 and 9T due to the coexistence of antiferromagnetic and paramagnetic domains in the temperature region 50–80K. The metal–insulator transition temperatures can be tuned by applying magnetic fields. The low-temperature conductivity, σ(T→0) obeys a critical law of the form σ(T→0)∝[EF(0)−EC]ν≡(H−HC)ν with ν=1, where HC is a critical field and EC the mobility edge. Such behavior is consistent with scaling theories of both localization and interaction effects. The observation of the present study indicates that the electronic states become more extended with increasing field.

Magnetic properties of the charge ordered Nd 0.75Na 0.25MnO 3

Nd 0.75Na 0.25MnO 3 polycrystalline ceramic is prepared via sol–gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.

Characterisation by EPR analysis of the magnetic phase diagram of RFe12−xMxXy compounds (R=rare earth metal, M=transition metal, X=H, C)

The compounds of formula RFe12−xMx with R=rare earth, M=d-metal and their interstitial derivatives, generally exhibit ferro- or ferrimagnetic properties. Preliminary EPR analyses performed on ErFe12−xMnxXy (x=5, 4, 3; X=H, C) and NdFe10.7V1.3Cy (y=0, 1) are reported. A direct correlation between the different types of electron resonance traces and the bulk magnetic properties is established, clearly revealing the ferromagnetic and the paramagnetic domains of the magnetic diagram of phases. No spin reorientation transition (SRT) was evidenced for any of the studied compounds.

Magnetotransport properties of SrFeO 2.95 perovskite

The resistivity, magnetoresistance, and Mössbauer effect of the metallic spiral antiferromagnet SrFeO 2.95 were examined in the temperature range 4.5–300 K. Two peaks were found in the plot of the temperature derivative of resistivity (d ρ/d T) versus temperature. The temperature of the upper peak, T N, corresponding to the onset of antiferromagnetic order, decreases monotonically from 105 to 98 K as the applied field increases from 0 to 9 T. This is due to the reduced antiferromagnetic superexchange interaction, while the temperature of the second transition, T a, which corresponds to a peak at low temperatures and coincides with the weak anomaly in χ( T) curve, remains constant upto 3.8 T and shows a rapid increase in the range from 40 to 65 K, as the magnetic field increases further. This is caused by the enhancement of the disappearance of Fe 3+ δ paramagnetic domains and also by the enhancement of the helical–conical spin transformation for the applied field greater than 3.8 T.

Long-Range Order in a Copper Borate Cu3B2O6

We measured muon spin relaxation for a copper borate Cu 3 B 2 O 6 , which has a two-dimensional network of Cu 2+ spins. It was previously reported that the ground state of this compound was a spin singlet state [G. A. Petrakovskiĭ et al. .: Phys. Solid State 412 (1999) 610]. We, however, observed an oscillation of muon spins at low temperatures, which indicates that the ground state is a long-range-ordered state. A critical slowing down of the fluctuations of electronic spins is not observed near the critical temperature, ∼11 K, suggesting that the magnetic transition is not an ordinary second-order phase transition. In addition, the coexistence of paramagnetic domains and ordered domains is observed at 6 K< T <12 K.

Spectral properties and infrared absorption in manganites

Within a recently proposed variational approach it has been shown that, in ${\mathrm{La}}_{1\ensuremath{-}x}{A}_{x}{\mathrm{MnO}}_{3}$ perovskites with $0&lt;x&lt;0.5,$ near the metal-insulator transition, the combined effect of the magnetic and electron-phonon interactions pushes the system toward a regime of two coexisting phases: a low electron density one made by itinerant large polarons forming ferromagnetic domains and a high electron density one made by localized small polarons giving rise to paramagnetic or antiferromagnetic domains depending on temperature. Employing the above-mentioned variational scheme, in this paper spectral and optical properties of manganites are derived for $x=0.3$ at different temperatures. It is found that the phase separation regime induces a robust pseudogap in the excitation spectrum of the system. Then the conductivity spectra are characterized by a transfer of spectral weight from high to low energies, as the temperature T decreases. In the metallic ferromagnetic phase, at low T two types of infrared absorption come out: a Drude term and a broad absorption band due respectively to the coherent and incoherent motion of large polarons. The obtained results turn out in good agreement with experiments.

SrFeO2.95: A helical antiferromagnet with large magnetoresistance

Resistivity, magnetoresistance, and M\"ossbauer effect of metallic spiral antiferromagnet ${\mathrm{SrFeO}}_{2.95}$ have been examined in the temperature range 4.5--300 K. A large negative magnetoresistance below 50 K is observed. We find hysteresis in resistivity in 0 and 9 T due to the coexistence of antiferromagnetic and paramagnetic domains in the temperature region 50--80 K. Our result shows two contributions to magnetoresistance: one below 50 K that is due to helical-conical spin transformation and another close to ${T}_{N}$ that is due to reduced spin fluctuation under magnetic field.

Coexistence of large and small polarons in manganites

The interplay of the electron-phonon interaction and of the double- and superexchange magnetic effects is analyzed in the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ perovskites with $0&lt;x&lt;0.5.$ By using an analytical variational scheme that allows us to treat the electron-phonon interaction in a fully quantum manner it is shown that in the intermediate electron-phonon coupling regime a charge-density instability occurs near the metal-insulator transition, induced by either temperature or hole doping. The system segregates in antiferromagnetic or paramagnetic small polarons and ferromagnetic large polaron domains characterized by different values of the lattice distortions that are satisfactory compared with those experimentally observed.

Observation of dia- and paramagnetic domains in beryllium and white tin by muon spin rotation spectroscopy

Dia- and paramagnetic (Condon) domains in simple metals, produced at low temperatures and in high magnetic fields via the cooperative effect of electrons in Landau orbitals, are observable by the muon spin rotation (μSR) technique. For beryllium, domains were seen for T&amp;lt;3.5 K in fields 1&amp;lt;H&amp;lt;3 T through a well resolved splitting of the muon precession frequency, the split lines reappearing in each de Haas–van Alphen (dHvA) period as the applied field H varies. The beat in the dHvA oscillations allowed varying of the amplitude χ0 of the differential susceptibility, the key parameter determining domain properties. The data points in the (B,T) plane show that the standard formula for χ0(B,T) fails to describe the phase diagram, a consequence of the nearly cylindrical Fermi surface of Be. For white tin, preliminary data indicate the presence of domains by the oscillatory behavior of the μSR linewidth at T=0.1 K in fields of H≈1 T, arising from electrons in the pocket of the Fermi surface in the sixth zone.

Diamagnetic domains in beryllium observed by muon-spin-rotation spectroscopy

Dia- and paramagnetic (Condon) domains were observed in single crystal beryllium for applied fields $\mathit{H}||[0001]$ of 1--3 T, at temperatures $T=0.1--3$ K, by muon-spin-rotation $(\ensuremath{\mu}\mathrm{SR})$ spectroscopy at PSI. On varying H, the domains with magnetization parallel and opposite to $\mathit{H}$ reappear in each de Haas--van Alphen period $\ensuremath{\Delta}H,$ detected by the doublet splitting in the precession frequency spectrum. As H varies within the ``domain section'' $\ensuremath{\delta}H$ of a dHvA cycle, the domain inductions ${B}_{1}{,B}_{2}$ stay constant, while the volume fractions of the para- and diamagnetic regions vary linearly. The beat in the susceptibility amplitude $a(B)=4\ensuremath{\pi}{(dM/dB)}_{\mathrm{max}},$ with a period of $\ensuremath{\approx}33\ensuremath{\cdot}\ensuremath{\Delta}H,$ allowed us to determine ${B}_{i}$ and $\ensuremath{\delta}H$ for different regimes. As a increases by a factor of $\ensuremath{\approx}3,$ the sections $\ensuremath{\delta}H$ grow, $|{B}_{2}\ensuremath{-}{B}_{1}\stackrel{\ensuremath{\rightarrow}}{|}\ensuremath{\Delta}H,$ and the uniform state between two domain sections shows an increasingly strong diamagnetism, $\ensuremath{-}4\ensuremath{\pi}\ensuremath{\chi}\ensuremath{\gg}1.$ The domains persist up to $T\ensuremath{\approx}3$ K and at fields down to $H\ensuremath{\approx}1$ T. On varying H, a slight ``field overheating'' was observed. The domain area in the phase diagram is much larger than expected, while other predictions of the theory proved to be qualitatively correct.

Resistance peak and giant magnetoresistance of degenerate ferromagnetic semiconductors with arbitrary spin polarization

The temperature peak of the resistance and the giant magnetoresistance of degenerate ferromagnetic semiconductors with an arbitrary degree of electron spin polarization are investigated. The spin-wave and paramagnetic domains are considered. The calculations are based on the notion of the magnetic-impurity scattering of carriers.

Direct evidence for dia‐ and paramagnetic domains in beryllium

The first spectroscopic evidence for dia‐ and paramagnetic domains (Condon domains) in beryllium metal is presented. The domains, detected by the splitting of the μSR line, arise and disappear periodically in each de Haas–van Alphen cycle as the field \bf H, normal to the single crystal Be plate and parallel to its [0001] axis, is tuned near \bf H_0\approx 2.7\ T. The intensity of the lines in the doublet reflect the ratio of dia‐ to paramagnetic regions. For the difference in induction within the domains we obtain \Delta B\approx 30\mbox--40\ G in the investigated field range at T=0.8 K.

Incommensurate magnetic structure and crystal field in ErNi2B2C Studied by 166Er Mössbauer spectroscopy and specific heat measurements

166Er Mossbauer absorption spectra have been recorded in the superconductor ErNi2B2C (Tc = 10 K), in zero field and in the temperature range 1.4 K–40 K. The spectra in the magnetically ordered phase (TN ≃ 5.5 K) are characteristic of an incommensurate magnetic structure with a maximum Er3+ moment of 8.2 μB. We show that the magnetic transition is first order, with a small temperature range where magnetically ordered and paramagnetic domains coexist. The analysis of the magnetic and quadrupolar hyperfine interactions below and above TN suggests that the Er3+ ground doublet is close to |J = 15/2; Jz = ± 1/2 > and shows that there is a low lying (≃ 10 K) excited doublet. The measured Er3+ electronic relaxation rate 1/T1 shows an anomaly at 10 K (Tc suggesting that the conduction electrons that are exchange coupled to the 4f spin take part in the formation of the superconducting state. We also present specific heat data for ErxY1 − xNi2B2C and Er0.2Lu0.8Ni2B2C which also evidence a low lying doublet and from which we propose a modified (improved) Er3+ crystal field level scheme (0, 9, 62, 71, 73, 181, 212 and 216 K) where the energies of the higher levels are consistent with published neutron scattering data.

On the competition between superconductivity and magnetism in CeCu 2Si 2

Muon spin relaxation measurements in zero magnetic field on several polycrystalline samples provide evidence that superconductivity and magnetic order do not coexist on a microscopic scale but develop inhomogeneously in Ce 1+ x Cu 2+ y Si 2. The distribution of magnetic and superconducting domains depends on the temperature and sensitively on variations of the stoichiometry. Superconductivity sets in first in the paramagnetic domains and, on further cooling, reduces the magnetically ordered volume fraction. However, in all samples down to T = 60 mK considerable portions remain magnetic.

170Yb Mössbauer and neutron diffraction investigation of the antiferromagnetic phase in the Kondo lattices YbP and YbN

The Kondo lattices YbP and YbN (nonstoichiometric samples) have been investigated by 170Yb Mössbauer spectroscopy, in the temperature range 0.085 K to 80 K in zero magnetic field in YbN and with an applied magnetic field at T=0.045 K in YbP and YbN, and by neutron diffraction in YbN. In both compounds, the antiferromagnetic transition at T N≌0.7 K is first order and occurs through a progressive growth of paramagnetic domains as the temperature increases. The measured exchange interaction is much larger than k B T N in both compounds and it is found to be isotropic in YbP and anisotropic in YbN. The saturated magnetic moment in both compounds is reduced by 50% with respect to the value calculated from the crystal field and exchange interactions. These properties are interpreted in terms of the competition between the Kondo effect and the RKKY exchange interaction; using a model mean field NCA calculation to describe the magnetic phase, the Kondo temperatures T K are found to be near 5–10 K.

Magnetism and superconductivity in the 2 : 2 : 1 : 2 bismuth cuprate by dysprosium substitution ( Bi2Sr2Ca1− xDyxCu2O8+ Δ)

Magnetic and superconducting properties of the solid solution Bi 2 Sr 2[ Ca 1− x Dy x ] 1 Cu 2 O 8+ Δ are reported. It is shown that substitution of Dy 3+ for Ca 2+ as well as appropriate annealings at moderate temperatures (300°C ⩽ T ⩽ 500°C), are efficient methods to optimize the critical temperature of the 2 : 2 : 1 : 2 bismuth cuprate. Emphasis is given to the interplay between magnetism and superconductivity in these materials. Results show that the dysprosium substitution is responsible for the magnetic properties in both the normal and superconducting states but, at the same time, it provides additional electrons which change the hole carrier density. As a consequence, paramagnetic domains would grow at the expense of the superconducting regions and will suppress superconductivity at x ∼ 0.5.