Changes In Magnetic Structure Research Articles

Enhanciement of exchange bias with diluted antiferromagnets in FM/AFM bilayers

Based on Monte Carlo method, the characteristic of exchange bias is discussed in diluted antiferromagnets in FM/AFM bilayers by changing the dilute concentration. The simulation results show that the exchange bias in FM/AFM bilayers can be enhanced significantly by introducing nonmagnetic atoms into the AFM layers. And there is a maximum in the diagram of exchange bias as a function of the dilute concentration, i.e., the exchange bias at first increases then decreases with increasing concentration of dilution at the same temperature. Besides, the maximum of the exchange bias occurs at lower concentration for higher temperature. These are consistent with the results of Phys. Rev. Lett. 96 117204 (2006). The cause of these results is that the spin distribution and the magnetic domain structure change greatly when varying dilute concentration. The exchange bias is maximal when the positive and the negative magnetic domains form a connected network. In addition, at the same concentration of dilution, by comparing the results of the random and regular dilution, we find that the exchange bias are more strongly increased by regular dilution than by random dilution for the FM/AFM bilayers, which shows further that the characteristic of exchange bias in FM/AFM bilayers depends on the interface spin-microstructure.

Pressure-induced magnetic transition of layered manganite La 2−2xSr 1+2xMn 2O 7 ( x=0.315, 0.318)

We have investigated the magnetic structure under hydrostatic pressure up to 1 GPa for layered perovskite manganites La 2−2 x Sr 1+2 x Mn 2O 7 ( x=0.315 and 0.318) by magnetization measurements. For x=0.318, the magnetic structure at 4.2 K is a planar ferromagnetic structure (FM planar), whose easy axis is in the ab-plane at ambient pressure. When a hydrostatic pressure is applied, the magnetic structure changes to a uniaxial ferromagnetic structure (FM uniaxial), whose easy axis is parallel to the c-axis at a pressure between 0.5 and 1.0 GPa. For x=0.315, the magnetic structure at 4.2 K is FM uniaxial and changes to FM planar at 85 K at ambient pressure. With increasing hydrostatic pressure, FM uniaxial is more stabilized at ground state and FM planar changes to an antiferromagnetic structure at 100 K. This structure may be the same as the antiferromagnetic phase which appears for x=0.3.

Observation of magnetic domain structures in [formula omitted

We investigated ferromagnetic domain structures for La 1 - x Sr x MnO 3 ( x = 0.125 , 0.175, 0.300), as functions of x and temperature by means of low-temperature Lorentz electron microscopy. We have observed the change of magnetic domain structures accompanied with the orthorhombic (monoclinic or triclinic)-to-rhombohedral phase transitions with respect to the chemical composition. In addition, the transition of the magnetic domain structure in the x = 0.175 compound, which exhibits the orthorhombic-to-rhombohedral phase transition at ∼ 190 K , has been observed with changing in temperature.

NT-proBNP reflects right ventricular structure and function in pulmonary hypertension

The aim of the current study was to investigate whether alterations in N-terminal pro brain natriuretic peptide (NT-proBNP) reflect changes in right ventricular structure and function in pulmonary hypertension patients during treatment. The study consisted of 30 pulmonary hypertension patients; 15 newly diagnosed and 15 on long-term treatment. NT-proBNP, right heart catheterisation and cardiac magnetic resonance imaging measurements were performed, at baseline and follow-up. There were no significant differences between newly diagnosed patients and those on treatment at baseline or follow-up with respect to NT-proBNP, haemodynamics and right ventricular parameters. Relative changes in NT-proBNP during treatment were correlated to the relative changes in right ventricular end-diastolic volume index (r = 0.59), right ventricular mass index (r = 0.62) and right ventricular ejection fraction (r = -0.81). N-terminal pro brain natriuretic peptide measurements reflect changes in magnetic resonance imaging-measured right ventricular structure and function in pulmonary hypertension patients. An increase in N-terminal pro brain natriuretic peptide over time reflects right ventricular dilatation concomitant to hypertrophy and deterioration of systolic function.

Finite-temperature magnetism of tetragonal iron

Based on ab initio Monte Carlo approach, finite-temperature magnetism of tetragonal Fe is studied. It is shown that the ground state magnetic structure changes from ferromagnetic to antiferromagnetic as the structure transforms from bcc to fcc. The Curie temperature (TC) or Néel temperatures (TN) are very sensitive to the lattice distortion, which decreases from bcc to fcc and then increases over the fcc range. There are a maximum of TC at the bcc and a minimum of TN around the fcc limit due to the change of the coupling distance and symmetry consideration.

Analysis of coronal magnetic fields in active regions based on detailed radio spectral data

Here we present an analysis of microwave emission spectra from flare-productive active regions (FPAR). For that we used regular observations in a wide frequency range with high spectral resolution and polarization sensitivity made with the RATAN-600 radio telescope. The multi-frequency observations make possible to detect small changes of magnetic field structures at different heights of the solar atmosphere. Observations with RATAN-600 and other large radio heliographs revealed an existence of narrow-band irregularities in the circular polarization spectra of FPARs radio emission. The new receiver complex, recently installed at RATAN-600, significantly improves its technical capabilities for the FPARs further study. Now it provides 1% (or 100 MHz) resolution in the frequency range from 6 GHz to 18 GHz in simultaneous registration. We present some observational results obtained with this new equipment.

Magnetic, specific heat and powder neutron diffraction studies of Ho [formula omitted]In

The successive magnetic transitions and magnetic structure of hexagonal Ho 2 In compound have been investigated by the magnetic, specific heat and powder X-ray and neutron diffraction measurements. Two magnetic phase transitions have been observed at the paramagnetic to ferromagnetic transition temperature T c = 85.0 ( 2 ) K and the magnetic order–order transition temperature T t = 32.5 ( 2 ) K. No antiferromagnetic reflections have been observed in the neutron diffraction patterns. The Rietveld analysis has indicated that the Ho atoms possess different moment values at two inequivalent sites: 8.8(2) μ B and 9.0(2) μ B at 2 K and 7.0(2) μ B and 7.4(2) μ B at 45 K, respectively. The moment direction from the hexagonal c -axis is 41 ( 10 ) ∘ at 2 K. At 45 K, the reliability factor takes so shallow minimum at around 26 ( 20 ) ∘ that one cannot exclude the alignment of moments along the c -axis in the higher temperature phase. A model is suggested for the magnetic structure change at T t .

Transport Property of La 0.67- xSm xSr 0.33MnO 3 at Heavy Samarium Doping (0.40 ≤ x ≤0.60)

The influence of heavy samarion (Sm) doping (0.40 ≤ x ≤ 0.60) on magnetic and electric properties of La 0.67-xSm xSr 0.33MnO 3 was investigated by measuring the magnetization-temperature ( M ∼ T) curves, magnetization-magnetic density ( M ∼ H) curves, resistivity-temperature( ρ-T) curves and magnetoresistivity-temperature( MR ∼ T) curves of the samples under different temperatures. It is found that, with the increase of Sm doping amount, the samples transform from long-range ferromagnetic order to spin-cluster glass state and anti-ferromagnetic state; and when x = 0.60, the transport property becomes abnormal under magnetic background; and the magnetic structure changes and extra magnetic coupling induced by doping leads to colossal magnetoresistance effect. The transport mechanism of metallic conduction at low temperature is mainly electron-magneton interaction and can be fitted by the formula ρ = ρ0 + AT 4.5, and the insulator-like transport mechanism on high temperature range is mainly the function of variable-range hopping and can be fitted by the formula ρ = ρ0exp( T0/ T) 1/4. In the formulas above, ρ is resistivity, T is temperature, and A, ρ0, T0 are constants.

Doping effects of hexagonal manganitesEr1−xYxMnO3with triangular spin structure

We have studied doping effects of hexagonal manganites ${\mathrm{Er}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$, which are found to have 2D triangular spin structure for all the samples studied here. Upon doping Er at the Y site, the magnetic structure changes gradually from the ${\ensuremath{\Gamma}}_{1}$ representation of $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ to the ${\ensuremath{\Gamma}}_{2}$ representation of $\mathrm{Er}\mathrm{Mn}{\mathrm{O}}_{3}$ while the ordered moment remains almost unchanged in magnitude over the whole doping range. Despite the gradual change in the magnetic structure with doping, geometrical frustration effects expected of the 2D triangular spin structure with antiferromagnetic interaction between nearest neighbors show very strong doping dependence. For example, although $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ shows all the signs of Mn moments being geometrically frustrated in the susceptibility and neutron diffraction data, such evidence of frustration effects is nearly absent in $\mathrm{Er}\mathrm{Mn}{\mathrm{O}}_{3}$. We suggest that magnetic coupling along the $c$ axis between Er and Mn moments should be responsible for the drastic doping dependence of the geometrical frustration effects.

The Electronic and Magnetic Properties of FCC Iron Clusters in FCC 4D Metals

The electronic and magnetic structures of small FCC iron clusters in FCC Rh, Pd and Ag were calculated using the discrete variational method as a function of cluster size and lattice relaxation. It was found that unrelaxed iron clusters, remain ferromagnetic as the cluster sizes increase, while for relaxed clusters antiferromagnetism develops as the size increases depending on the host metal. For iron in Rh the magnetic structure changes from ferromagnetic to antiferromagnetic for clusters as small as 13 Fe atoms, whereas for Fe in Ag antiferromagnetism is exhibited for clusters of 24 Fe atoms. On the hand, for Fe in Pd the transition from ferromagnetism to antiferromagnetism occurs for clusters as large as 42 Fe atoms. The difference in the magnetic trends of these Fe clusters is related to the electronic properties of the underlying metallic matrix. The local d densities of states, the magnetic moments and hyperfine parameters are calculated in the ferromagnetic and the antiferromagnetic regions. In addition, the average local moment in iron-palladium alloys is calculated and compared to experimental results.

Mössbauer effect in f.c.c. Fe-Ni-C alloys after the ultrasonic impact surface treatment

Mossbauer spectroscopy was used for study of the f.c.c. Fe-30.5%Ni-1.5%C, Fe-30.0%Ni-1.3%C, Fe-30.1%Ni-0.44%Mn-1.22%C, Fe-30.3%Ni alloys after the heat treatment at 1373 K in vacuum and the impact ultrasonic surface treatment in vacuum. The vibration amplitude of the magnetostrictor-wave guide-sample system on the (1–3)kHz frequencies was 20 µm and 28 µm corresponding to the power of impacts of 2.4 and 4.7 J/s. In order to show the redistribution process of carbon under the treatment, the ageing of the Fe-30.0%Ni-1.3%C alloy was carried out at 773 K in vacuum. As shown, the ultrasonics of smaller power does not result in distinctive changes in the hyperfine magnetic structure and solid solution state. The increase of power of impacts does not change the phase composition of the alloys except the atomic redistribution in a solid solution varying the electron spin and charge densities on iron nuclei. The results were analyzed in comparison with the data derived after the low-frequency impact surface treatment.

Changes of magnetic domain structure induced by temperature-variation and electron-beam irradiation in Pr0.5Sr0.5CoO3

In situ observation of magnetic domain structures of Pr0.5Sr0.5CoO3 was made by Lorentz electron microscopy to reveal an unusual magnetic behavior around 120K below the Curie temperature (TC∼220K). The observations in a cooling run clearly showed that the magnetic domain structure changes below about 90K and that the transformed domain structure remains unchanged down to 20K. Furthermore, it was found that an electron-beam irradiation in an electron microscope induces a reversible transformation of the magnetic domain structure at 20K. Possible mechanisms of such magnetic-domain structural changes are discussed.

On a correlation between the electronic structure and passivability of Fe-Cr alloys

The chromium (5.8 to 30.7 wt. %) effect on the superfine magnetic structure and surrounding electrons of the 57Fe nuclei in quenched and annealed Fe-Cr alloys is studied with the use of nuclear γ-resonant (Mo ssbauer) spectroscopy. The following parameters of the Mossbauer spectra are calculated by using PC with a Normos software: the effective magnetic intensity Hef (in kOe), the isomeric (chemical) shift ° (in mm/s), the quadrupole electric splitting Δeq (in mm/s), and the area fraction S (%) of the superfine-structure subspectrum. Significant changes in the superfine magnetic structure and the electron surrounding of the 57Fe nuclei are revealed as functions of the chromium content in the alloys. The conclusion on a correlation between the electronic structure of the alloys and their passivability is drawn.

Swift heavy ion induced magnetic phase transition of Fe–Rh alloy

We report the effects of swift heavy ion irradiation on structure and magnetic properties of Fe–50at.%Rh alloys. The alloys are irradiated with 120–200MeV heavy ions (Ni, Kr or Xe) at room temperature. Before and after the irradiations, the magnetization and the lattice parameter are measured by using superconducting quantum interference device (SQUID) and X-ray diffractometer (XRD), respectively. The lattice parameter at room temperature increases by about 0.3% and antiferromagnetic–ferromagnetic transition temperature decreases below 5K by the irradiations. Effects of electronic excitation due to swift heavy ions on the change in magnetic properties and lattice structure are discussed.

Direct observation of surface and interface magnetism with the probing depth-dependent X-ray magnetic circular dichroism technique

Surface and interface magnetization of Fe/Ni/Cu(1 0 0) thin films was observed with the probing depth-dependent X-ray magnetic circular dichroism technique, in which the probing depth is controlled by changing the electron detection angle. An oscillatory behaviour of the surface magnetization dependent on the Fe thickness was directly revealed, while simple analyses indicated that the interface (bottom) Fe magnetization stays almost unchanged independent of the Fe thickness, and that the inner layers are basically nonmagnetic. These results suggest an oscillatory interlayer magnetic coupling between the surface and interface of the Fe film via the nonmagnetic inner layers. It was indicated that the observed oscillation can also be attributed to a rotatory interlayer coupling between the surface and interface. A temperature-dependent experiment indicated that the surface magnetization is reversed depending on the temperature, which might be attributed to the magnetic structural change in the inner layers.

Influence of Fast Global Variations of Solar Magnetic Fields on Space Weather in Cycle 23

It is established that the large-scale and global magnetic fields in the Sun's atmosphere do not change smoothly, and long-lasting periods of gradual variations are superseded by fast structural changes of the global magnetic field. Periods of fast global changes on the Sun are accompanied by anomalous manifestations in the interplanetary space and in the geomagnetic field. There is a regular recurrence of these periods in each cycle of solar activity, and the periods are characterized by enhanced flaring activity that reflects fast changes in magnetic structures. Is demonstrated, that the fast changes have essential influencing on a condition of space weather, as most strong geophysical disturbances are connected to sporadic phenomena on the Sun. An explanation has been offered for the origin of anomalous geomagnetic disturbances that are unidentifiable in traditionally used solar activity indices. Is shown, main physical mechanism that leads to fast variations of the magnetic fields in the Sun's atmosphere is the reconnection process.

Magnetism and magnetoresistance in Fe/Cr/V/Cr/Fe

We investigate the electric transport and the magnetic properties of Cr n V s Cr n systems with respect to the thickness of the V layer. The trilayers are grown in the (001) direction and covered with bcc Fe electrodes. For thin Cr layers we observe a quadratic decrease of the magnetic moments with decreasing distance to the V layer. In the case of thick Cr layers the magnetic structure changes to a spin-density wave. The magnetoresistances of the V-containing systems are reduced compared with Fe/Cr/Fe trilayers of the same spacer thickness, whereby this effect is most drastic for current in-plane geometry. The calculations are performed within the fully relativistic screened Korringa–Kohn–Rostoker method to obtain magnetic and electronic properties. The giant magnetoresistance is investigated within the fully relativistic Kubo–Greenwood approach.

Change of magnetoresistivity and magnetic structure of [formula omitted] by iron substitution

Alloyed ( Mn 1 - x Fe x ) Au 2 ribbons ( x = 0 –0.075) show a complex dependence of magnetization and magnetoresistivity on composition, magnetic field, and temperature. This is associated with the helimagnetism of MnAu 2 and the field-driven transition from helix to fan spin-structure. The threshold field H thr for this metamagnetic transition strongly depends on x. Neutron diffraction shows that the periodicity length of the helix increases with increasing iron contents x < 0.075 . For x = 0.05 a transition from the helical ground-state to a collinear ferromagnetic structure takes place at 150 K; for x = 0.075 only a ferromagnetic order is found.

An orbital-based scenario for the magnetic structure of neptunium compounds

In order to understand the key role of the orbital degree of freedom in the magnetic structure of some recently synthesized neptunium compounds, NpTGa5 (T = Fe, Co and Ni), we propose to discuss the magnetic phase of an effective two-orbital model, which has been constructed based on a j–j coupling scheme to explain the magnetic structure of the uranium compounds UTGa5. By analysing the model with the use of numerical techniques such as exact diagonalization, we obtain the phase diagram including several kinds of magnetic states. An orbital-based scenario is discussed to understand the change in magnetic structure among C-, A- and G-type antiferromagnetic phases, observed experimentally in NpFeGa5, NpCoGa5 and NpNiGa5.

Low-Temperature Nuclear Orientation of 144Pm in Metamagnetic (RE)NiAl4 Single Crystals

Magnetic properties of Pm ions in NdNiAl4 were investigated by low-temperature nuclear orientation of 144Pm. The observed γ-ray anisotropy as a function of external fields revealed a change in the magnetic structure of Pm ions at the metamagnetic phase transition of NdNiAl4. The extracted hyperfine field was 50(5) T which is substantially smaller than those previously reported.