Slow antiferromagnetic dynamics in the low temperature tetragonal phase of La_{2-x}Sr_xCuO_4 as revealed by ESR of Gd spin probes

Measuring the ESR of Gd spin probes we have studied the magnetic properties of the copper oxide planes in the low temperature tetragonal (LTT) phase of Eu doped La_{2-x}Sr_xCuO_4. The data give evidence that at particular levels of Sr and Eu doping the frequency of the antiferromagnetic fluctuations in the LTT phase dramatically decreases at low temperatures by almost three orders of magnitude. However, no static magnetic order has been found for T>8K in contrast to the observation by neutron scattering of stripe ordering of spins below 50K in a Nd doped La_{2-x}Sr_xCuO_4 single crystal. To our opinion static order in the Nd doped compound is induced due to the interaction between the Cu spins with the rare earth magnetic moments. Therefore, a really characteristic property of the magnetism in the LTT structural phase may be not static magnetic order at elevated temperatures but rather extremely slow antiferromagnetic dynamics.Comment: 12 pages RevTex, 2 EPS figures, to appear in Phys.Rev.B, Feb.,9

Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4

The ESR signal of Gd spin probes (0.5 at %) as well as the static normal state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal pronounced changes of the Cu magnetism at the structural transition from the orthorhombic to the low temperature tetragonal phase for all non-superconducting compositions. Both a jumplike decrease of \chi as well as the ESR data show an increase of the in-plane magnetic correlation length in the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and Sr concentrations in the LTT phase the correlation length increases up to more than 100 lattice constants and the fluctuation frequency of the CuO_2 spin system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4 with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the International Conference "Stripes, Lattice Instabilities and High Tc Superconductivity", (Rome, Dec. 1996

EPR study of Mn-implanted single crystal plates of TiO2 rutile

Single crystals of Mn-implanted TiO2 rutile have been investigated by electron paramagnetic resonance (EPR) technique at room temperature. We have observed an EPR signal on Mn4+ ions (S=frac(3, 2)) in the manganese-implanted single crystal TiO2 plates. Besides, weaker EPR signals due to Fe3+(S=frac(5, 2), L=0) and Cr3+(S=frac(3, 2)) ions have also been observed. Characteristic six-line splitting of the manganese EPR lines due to hyper-fine interaction with 55Mn nuclei (spin I=frac(5, 2)) has also been observed. Analysis of the EPR spectra shows that the manganese, iron and chromium ions substitute for Ti4+ ions in the TiO2 rutile host. Two structurally equivalent groups of the centers have been observed in the EPR spectra in correspondence with two octahedral positions of the Ti ions in the rutile structure. Spin Hamiltonian parameters for the crystal field of orthorhombic symmetry on the Mn4+, Fe3+ and Cr3+ centers have been obtained as result of computer modelling. © 2009 Elsevier B.V. All rights reserved

EPR study of Mn-implanted single crystal TiO2

Single crystals of manganese-implanted TiO2 rutile have been investigated by electron paramagnetic resonance (EPR) technique at room temperature. ESR spectra have been interpreted to correspond to the transitions among the spin multiplet (S=3/2) of the paramagnetic Mn4+ ion. Characteristic six-line hyper-fine splitting of the ESR spectra resulting from the spin I=5/2 of the Mn55 nucleus has been observed. Analysis of EPR spectra shows that manganese in TiO2 rutile host substitutes for Ti4+ ions. Two equivalent Mn4+ centers have been observed in the EPR spectra in correspondence with two equivalent octahedral positions of Ti ions in the rutile structure. Parameters of the crystal field of orthorhombic symmetry on the Mn4+ centers have been obtained as result of computer modelling. © 2009 IOP Publishing Ltd

FMR studies of CrO2 epitaxial thin films

Epitaxial (100) thin films of CrO2 of various thickness were fabricated by chemical vapor deposition (CVD) at atmospheric oxygen pressure onto (100) TiO2 single-crystal substrates. Ferromagnetic resonance (FMR) measurements were performed at the X-band (9.5 GHz) at room temperature. The angular dependencies of the FMR spectra in both "in-plane" and "out-of-plane" geometries were measured. The directions of easy and hard axes of magnetization were determined from the in-plane measurements, when the DC magnetic field was rotated in the film plane. It was established that, at room temperature, the easy axis of magnetization is parallel to the c-axis of the CrO2 rutile structure. Splitting of the FMR signal into surface and bulk modes was observed due to surface pinning of magnetization at interfaces of the CrO2 films. The magnetoelastic anisotropy was observed to be enhanced with decreasing film thickness. The values of the room temperature effective magnetization and parameters of the anisotropy field were obtained from analysis of the FMR data. © 2003 Elsevier B.V. All rights reserved

Strain-induced magnetic anisotropies in epitaxial CrO2 thin films probed by FMR technique

Epitaxial CrO2 thin films were grown onto TiO2 (1 0 0) single-crystalline substrates by chemical vapour deposition (CVD) process with use of the solid precursor CrO3. The CrO2 films with thickness of 27 and 65 nm were deposited onto TiO2 substrates pre-etched in the diluted HF. The magnetic properties of the epitaxial chromium-dioxide films have been probed by the ferromagnetic resonance (FMR) technique. Analysis of the FMR spectra shows that the magnetic behaviour of the CrO2 films results from a competition between magnetocrystalline and strain anisotropies. The thin films are heavily strained due to lattice mismatch of CrO2 epitaxial film with the TiO2 single-crystalline substrate. For the thinnest film (27 nm) the stress anisotropy dominates, and the magnetic easy axis switches from the c direction to the b direction of the rutile structure. Unusual angular dependence of the resonance signal and multiple FMR modes are observed for the film with the thickness of 65 nm, where a partial strain relaxation results in appearance of two magnetic phases with mutually perpendicular easy axes along the c and b directions. © 2005 Elsevier B.V. All rights reserved

Magnetic Resonance Study of Fe-Implanted TiO<inf>2</inf> Rutile

© 2017, Springer-Verlag Wien.Single-crystal (100) and (001) TiO2 rutile substrates have been implanted with 40 keV Fe+ at room temperature with high doses in the range of (0.5–1.5) × 1017 ions/cm2. A ferromagnetic resonance (FMR) signal has been observed for all samples with the intensity and the out-of-plane anisotropy increasing with the implantation dose. The FMR signal has been related to the formation of a percolated metal layer consisting of close-packed iron nanoparticles in the implanted region of TiO2 substrate. Electron spin resonance (ESR) signal of paramagnetic Fe3+ ions substituting Ti4+ positions in the TiO2 rutile structure has been also observed. The dependences of FMR resonance fields on the DC magnetic field orientation reveal a strong in-plane anisotropy for both (100) and (001) substrate planes. An origin of the in-plane anisotropy of FMR signal is attributed to the textured growth of the iron nanoparticles. As result of the nanoparticle growth aligned with respect to the structure of the rutile host, the in-plane magnetic anisotropy of the samples reflects the symmetry of the crystal structure of the TiO2 substrates. Crystallographic directions of the preferential growth of iron nanoparticles have been determined by computer modeling of anisotropic ESR signal of substitutional Fe3+ ions

ESR study of Co-doped TiO2 thin films

Co:TiO2 thin films prepared by reactive co-sputtering deposition were studied by electron spin resonance (ESR) technique. Magnetization measurements showed hysteretic behavior with the coercive field between 55 and 65 Oe and the saturation magnetization at room temperature ranging from 7 (2.2% Co) to 28 emu/cm3 (8.5% Co). ESR measurements at X-band (9.5 GHz) revealed an anomaly in the temperature behavior of the absorption intensity near the temperature at 60 K. This behavior is attributed to an unconventional spin-glass-like behavior, which results from competition of long-range dipole-dipole interaction and anisotropy fields in ferromagnetic Co nanoparticles. © 2002 Elsevier Science B.V. All rights reserved

High curie-temperature ferromagnetism in cobalt-implanted single-crystalline rutile

The ion implantation technique has been used to fabricate a Co-rich layer in rutile: single-crystalline TiO2 substrates were heavily irradiated by Co+ ions with energy of 40 keV. The magnetic properties of as-prepared and post-annealed samples were studied by both inductive and Faraday magnetometry as well as ferromagnetic resonance (FMR). A ferromagnetic Curie temperature as high as 700 K was measured in our samples. The analysis of the magnetic hysteresis loop, the temperature dependence of the saturation magnetization, and strong out-of-plane anisotropy of the FMR spectra allow us to suppose that the origin of the macroscopic high-temperature ferromagnetism is the exchange interaction mediated by oxygen vacancies

The magnetic anisotropy of thin epitaxial CrO2 films studied by ferromagnetic resonance

The magnetic anisotropy of thin epitaxial films of chromium dioxide (CrO2) has been studied as a function of the film thickness by the ferromagnetic resonance (FMR) technique. CrO2 films with various thicknesses in the range from 27 to 535 nm have been grown on (100)-oriented TiO2 substrates by chemical vapor deposition using CrO3 as a solid precursor. In a series of CrO2 films grown on the substrates cleaned by etching in a hydrofluoric acid solution, the FMR signal exhibits anisotropy and is strongly dependent on the film thickness. The magnetic properties of CrO2 films are determined by a competition between the magnetocrystalline and magnetoelastic anisotropy energies, the latter being related to elastic tensile stresses caused by the lattice mismatch between the film and the substrate. In the films of minimum thickness (27 nm), this strain-induced anisotropy is predominant and the easy magnetization axis switches from the [001] crystallographic direction (characteristic of the bulk magnet) to the [010] direction. © 2005 Pleiades Publishing, Inc