Cz. Kapusta

Nuclear magnetic resonance study of the RE(Co, B)5-type compounds (RE≡Y, Gd)

Abstract The nuclear magnetic resonance spin echo spectra of RECo 5 , RECo 4 B, RE 3 Co 11 B 4 , RE 2 Co 7 B 3 and RECo 3 B 2 ( RE  Y , Gd ) at 4.2 K are presented. An assignment of the resonance lines to the various cobalt sites in these compounds is proposed. The variations in the cobalt hyperfine field observed between different cobalt sites and different compounds are analysed in terms of spin and orbital contributions to the cobalt moments. The cobalt hyperfine fields at sites of the same symmetry and nearest-neighbour configuration are similar in the different compounds. The gadolinium hyperfine field in GdCo 3 B 2 is close to the free ion value. The temperature-independent Knight shift of cobalt and boron observed in YCo 3 B 2 indicates that this compound is a Pauli paramagnet.

Evidence of unquenched Re orbital magnetic moment in AA′FeReO6 double perovskites

Spin and orbital magnetic moments of rhenium in AA′FeReO6 double perovskites (A,A′=Ba, Sr, and Ca) have been directly probed employing x-ray magnetic circular dichroism at the Re L2,3 edges. A considerable orbital magnetic moment is observed in all the compounds studied, which confirm theoretical predictions of unquenched Re orbital moment despite its octahedral coordination. Relative orbital-to-spin moment ratio alters with lattice distortion from mL∕mS=−0.285to−0.337 from Ba2FeReO6 to Ca2FeReO6, respectively. Moreover, the spin moment of Re ion scales with Curie temperature, the most relevant property in spin electronics application of the compounds studied.

Magnetoresistance in FeCoZr–Al2O3 nanocomposite films containing ‘metal core–oxide shell’ nanogranules

Temperature and magnetic field dependences of electrical conductivity are systematically studied in granular films (Fe45Co45Zr10)x(Al2O3)100?x (28???x???64) containing crystalline metallic ?-FeCo-based nanoalloy cores encapsulated in an amorphous oxide shell embedded in an amorphous Al2O3 matrix. Formation of ?metallic core?oxide shell? nanogranules is confirmed by transmission electron microscopy (TEM) and HRTEM. The structure of core and shell is governed with the difference in the oxidation states of Fe and Co ions investigated with EXAFS, XANES and M?ssbauer spectroscopy. A considerable negative magnetoresistance (MR) effect of spin-dependent nature is observed in the whole range of x values. Its increase with decreasing temperature is correlated with the magnetic saturation of superparamagnetic metallic nanogranules. The enhanced MR effect in ?core?shell? granular films is related to the percolation of oxide shells and their influence through spin filtering processes. A considerable high field MR at low temperatures and the resulting deviation of MR and squared magnetization are attributed to a magnetic randomness and/or strong magnetic anisotropy of the magnetic oxide shell.

High-field magnetization measurements in Sr2CrReO6 double perovskite: Evidence for orbital contribution to the magnetization

We have synthesized a Sr2CrReO6 double perovskite sample with the expected high Curie temperature ≈ 600 K, which shows negligible impurity phases and ≈ 15% Cr/Re structural antisite disorder as shown by X-ray and neutron diffraction. High-field magnetization measurements up to 47 T show that the saturation magnetization below room temperature is much higher than the one predicted by simple models that do not take into account the Re spin-orbit coupling. The crucial role of the large Re orbital moment in the physics of Sr2CrReO6 and related compound is highlighted.

Nuclear magnetic resonance study of magnetic phase segregation in La1−xCaxMnO3

A 55Mn nuclear magnetic resonance (NMR) study of the La1−xCaxMnO3 system, 0 0.5 the spin echo spectra contain resonance lines corresponding to both the microscopic ferromagnetic and antiferromagnetic regions. A resonance line near 380 MHz, which is similar to that of the spectrum of the ferromagnetic and metallic compounds (0.2<x<0.5) is observed for all x. This indicates the presence of clusters in which the Mn magnetic moments are double exchange coupled in the whole range of doping. Their hyperfine field exhibits a much weaker temperature dependence than that of the lines of the host. This reveals the greater strength of the magnetic coupling within the clusters than in the antiferromagnetic regions and indicates a weak magnetic coupling of the clusters to the host matrix. A comparison with the NMR results on the hole doped nonstoichiometric compounds, LaMnO3+δ is made. The temperature dependence of the line intensities corresponding to the double exchange clusters and...

A 55Mn nuclear magnetic resonance study of mixed-valence manganites

The temperature dependence of the 55Mn nuclear magnetic resonance of some manganese perovskites of the form [La1-xAx]MnO3 (A = Ca,Sr) for x of 1/3 is reported. The 55Mn spin-echo spectra were measured in zero field and in an applied magnetic field up to 6 T for polycrystalline samples of [La0.67-yTbyCa0.33]MnO3 (y = 0,0.1,0.22), [La0.6Y0.07Ca0.33]MnO3 and [La0.7Sr0.3]MnO3. The single resonance line observed for all compounds, for temperatures between 4.2 K and 293 K, is attributed to a fast carrier motion (<10-9 s) between the Mn3+ and Mn4+ sites resulting from the double-exchange mechanism. For some of the samples the zero-field nuclear magnetic resonance signal has been detected at temperatures far above the magnetic ordering temperature, providing direct evidence of magnetic correlations whose lifetime is greater than 10-5 s. The correlations can possibly be related to magnetic polarons.

Magnetic and NMR investigations of Nd2Co14B and Y2Co14B compounds

Abstract The magnetic and zero field spin echo NMR investigations for Nd 2 Co 14 B and Y 2 Co 14 B compounds are reported and discussed. The anisotropy field and anisotropy constant K 1 derived from the experimental data for Nd 2 Co 14 B are strongly temperature dependant. In Nd 2 Co 14 B the easy direction of magnetisation deflects up to 26° from the c axis in the temperature range from about 130 K down to 4.2 K. The 59 Co NMR spectra taken at 4.2 K consist of very narrow resonance lines, well resolved, of domain wall origin for Nd 2 Co 14 B. The resonance lines were attributed to Co structural positions. The very short relaxation times T 2 were observed with quadrupole oscillations of the spin echo decays for some lines.

NMR spectroscopy in mixed valence manganites

Abstract A survey is given of the nuclear magnetic resonance (NMR) experiments in the manganese perovskites of the form RE 1 − x A x MnO 3 and REMnO 3 + δ (RE = rare earth, A = Ca, Sr, Ba). The 55 Mn, 139 La and 87 Sr spin-echo spectra in the frequency range 5–650 MHz are discussed and the microscopic information on the Mn valence states and the nature of magnetic interactions is analysed as a function of the hole doping.

MAGNETIC PROPERTIES OF (PR(CA, SR))MNO3 STUDIED BY NUCLEAR MAGNETIC RESONANCE

The valence state of Mn in magnetoresistive perovskites has been widely related to the level of alkaline metal substitution of the rare earth, by assuming that the Mn exists in 3+ or 4+ valence states. Measurements have been made on a series of samples in the system Pr1−(x+y)CaxSryMnO3. 55Mn nuclear magnetic resonance (NMR) zero field spin echo measurements on a single crystal with x=0.1 and y=0 show distinct resonances around 310 MHz (Mn4+) and 400 MHz (Mn3+) but also at 590 MHz (Mn2+). An increase in the Mn2+ resonance frequency in an applied field implies that the Mn2+ moment is antiferromagnetically coupled to the net magnetization. Spectra from powder samples are much broader. The resonance frequencies increase as the Pr content is reduced. For systems with x+y=0.4, there is little change in the NMR spectrum as the proportions of Ca and Sr are altered.

Nuclear magnetic resonance study of the Sm2Fe17Nx compounds

Abstract The 147 Sm and 149 Sm NMR spin echo spectra of Sm 2 Fe 17 N x ( x = 0, 0.4, 1.2, 2) series at 4.2 K are presented. The observed quadrupole septets are assigned to the 6c site of Sm with a different number of neighbouring N atoms. From the quadrupole splittings the values of the lattice contribution to the electric field gradient are obtained. On this basis the values of the crystal electric field coefficient A 0 2 for different numbers of N neighbours are derived.