PLANAR ACCELERATOR STRUCTURES FOR MILLIMETER WAVELENGTHS �

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CRYSTAL-FIELD EFFECTS IN THE ELECTRON-SPIN-RESONANCE OF GD3+ AND ER3+ IN PR2CUO4

The low-temperature (T < 300 K) electron-spin-resonance (ESR) spectra of Gd3+ and Er3+ in Pr2CuO4 show symmetry properties appropriate to the crystal tetragonal symmetry. The completely resolved Gd3+ spectra allowed us to measure, at T = 2 K, the principal g values g parallel-to = 1.985(8), g perpendicular-to =2.040(8), and the crystal-field parameters [b2(0) = -399(2) X 10(-4) cm-1, b4(0) = -33.1(7) X 10(-4) cm-1, and b4(4) = 205(3) X 10(-4) cm-1]. The large broadening of the ESR lines, observed above T approximately 40 K, is due to a relaxation via the thermally populated crystal-field excited Pr levels. For Er3+ in Pr2CuO4 we observe a single ESR line corresponding to a ground-state doublet with g parallel-to = 17.94(5) and g perpendicular-to less-than-or-equal-to 0.2. The absence of any splittings of the ESR lines below the Neel temperature implies that the magnetostatic dipole field at the rare-earth-ion site due to the antiferromagnetically ordered Cu moments is < 45 Oe.44282682

Observation of an unusual ESR signal in antiferromagnetic Eu 2CuO4

We report the observation of an unusual electron spin resonance (ESR) signal in single crystals of Eu2CuO4. The signal appears to be associated with a resonance mode of the CuO2 planes, similar to the midfield and low-field absorptions we have reported previously [Phys. Rev. B 41, 1934 (1990)]. However, it is only observed when the projection of the applied dc magnetic field in the CuO2 plane is within a few degrees of the 〈110〉 crystallographic direction. Additionally the sample must be field cooled in the CuO2 plane, but with a component of the cooling field perpendicular to the 〈110〉 ESR observation direction. Both the field for resonance and the linewidth exhibit a 1/cos θ dependence, where θ is the angle of the applied dc field between the c axis and the 〈110〉 observation direction. Additional constraints for observation of the resonance are that the microwave rf magnetic field must have a component in the CuO2 plane, but perpendicular to the dc field. The signal disappears above ∼215 K, which we assume is associated with the antiferromagnetic ordering temperature

Observation of an unusual ESR signal in antiferromagnetic Eu 2CuO4

We report the observation of an unusual electron spin resonance (ESR) signal in single crystals of Eu2CuO4. The signal appears to be associated with a resonance mode of the CuO2 planes, similar to the midfield and low-field absorptions we have reported previously [Phys. Rev. B 41, 1934 (1990)]. However, it is only observed when the projection of the applied dc magnetic field in the CuO2 plane is within a few degrees of the 〈110〉 crystallographic direction. Additionally the sample must be field cooled in the CuO2 plane, but with a component of the cooling field perpendicular to the 〈110〉 ESR observation direction. Both the field for resonance and the linewidth exhibit a 1/cos θ dependence, where θ is the angle of the applied dc field between the c axis and the 〈110〉 observation direction. Additional constraints for observation of the resonance are that the microwave rf magnetic field must have a component in the CuO2 plane, but perpendicular to the dc field. The signal disappears above ∼215 K, which we assume is associated with the antiferromagnetic ordering temperature

Crystal-field effects in the electron-spin resonance of Gd3+ and Er3+ in Pr2CuO4.

The low-temperature (T&lt;300 K) electron-spin-resonance (ESR) spectra of Gd3+ and Er3+ in Pr2CuO4 show symmetry properties appropriate to the crystal tetragonal symmetry. The completely resolved Gd3+ spectra allowed us to measure, at T=2 K, the principal g values g?=1.985(8), g=2.040(8), and the crystal-field parameters [b20=-399(2)×10-4 cm-1, b40=-33.1(7)×10-4 cm-1, and b44=205(3)×10-4 cm-1]. The large broadening of the ESR lines, observed above T40 K, is due to a relaxation via the thermally populated crystal-field excited Pr levels. For Er3+ in Pr2CuO4 we observe a single ESR line corresponding to a ground-state doublet with g?=17.94(5) and g0.2. The absence of any splittings of the ESR lines below the Néel temperature implies that the magnetostatic dipole field at the rare-earth-ion site due to the antiferromagnetically ordered Cu moments is &lt;45 Oe. © 1991 The American Physical Society

Internal magnetic field in La2-xSrxCuO4:Gd observed by electron paramagnetic resonance.

We have measured the electron paramagnetic resonance (EPR) of Gd3+ in single crystals of La2-xSrxCuO4+δ, as a function of temperature T, magnetic-field angle, oxygen content, microwave frequency, and Sr concentration (0≤x≤0.024). For temperatures larger than the antiferromagnetic ordering temperature of the Cu lattice, TN, we have identified four different sites of Gd3+ as expected for twinned crystals and we are able to fit the data for all directions of the magnetic field. Samples annealed under vacuum or oxygen show only small changes on their crystal-field parameters and g values. For temperatures smaller than TN, the EPR lines are further split due to the internal magnetic field acting at the Gd site associated with the antiferromagnetic ordering of the Cu lattice. The temperature dependence of the splitting of the resonance lines allow us to calculate the moment per Cu ion. A value of ∼0.6μB per Cu ion at T=0 K is inferred, with the Cu moments lying in the bc plane about 5°from the c axis. The data only can be fit for all the directions of the applied magnetic field if other mechanisms such as a Heisenberg type of exchange between the Cu and Gd moments or a distortion of the lattice below TN are included. © 1993 The American Physical Society

Internal magnetic field in La2-xSrxCuO4:Gd observed by electron paramagnetic resonance.

We have measured the electron paramagnetic resonance (EPR) of Gd3+ in single crystals of La2-xSrxCuO4+δ, as a function of temperature T, magnetic-field angle, oxygen content, microwave frequency, and Sr concentration (0≤x≤0.024). For temperatures larger than the antiferromagnetic ordering temperature of the Cu lattice, TN, we have identified four different sites of Gd3+ as expected for twinned crystals and we are able to fit the data for all directions of the magnetic field. Samples annealed under vacuum or oxygen show only small changes on their crystal-field parameters and g values. For temperatures smaller than TN, the EPR lines are further split due to the internal magnetic field acting at the Gd site associated with the antiferromagnetic ordering of the Cu lattice. The temperature dependence of the splitting of the resonance lines allow us to calculate the moment per Cu ion. A value of ∼0.6μB per Cu ion at T=0 K is inferred, with the Cu moments lying in the bc plane about 5°from the c axis. The data only can be fit for all the directions of the applied magnetic field if other mechanisms such as a Heisenberg type of exchange between the Cu and Gd moments or a distortion of the lattice below TN are included. © 1993 The American Physical Society

Different Gd3+ sites associated with magnetic ordering and structural distortions in Eu2CuO4:Gd3+ observed via electron-paramagnetic-resonance measurements.

We have measured the EPR spectrum of dilute Gd3+ ions substituting for Eu3+ in Eu2CuO4. Each one of the resonance lines shows a splitting into two or more weaker lines below TN 215 K. The simultaneous appearance of a low-field microwave-absorption signal is attributed to the onset of long-range antiferromagnetic ordering with a weak ferromagnetic component. The splitting of the EPR spectrum is associated with different Gd sites in the magnetically ordered structure. A reduction of the local symmetry of the rare-earth sites below TN is described in terms of an internal magnetic field lying in the ab plane and additional crystal-field terms related to local crystallographic distortions. Dipolar and exchange contributions to the internal field are discussed. Possible random static displacements of the oxygen atoms in the CuO2 planes and their role in the development of the weak ferromagnetism and the presence of internal fields at the Gd sites are also analyzed. A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state. © 1991 The American Physical Society

Different Gd3+ sites associated with magnetic ordering and structural distortions in Eu2CuO4:Gd3+ observed via electron-paramagnetic-resonance measurements.

We have measured the EPR spectrum of dilute Gd3+ ions substituting for Eu3+ in Eu2CuO4. Each one of the resonance lines shows a splitting into two or more weaker lines below TN 215 K. The simultaneous appearance of a low-field microwave-absorption signal is attributed to the onset of long-range antiferromagnetic ordering with a weak ferromagnetic component. The splitting of the EPR spectrum is associated with different Gd sites in the magnetically ordered structure. A reduction of the local symmetry of the rare-earth sites below TN is described in terms of an internal magnetic field lying in the ab plane and additional crystal-field terms related to local crystallographic distortions. Dipolar and exchange contributions to the internal field are discussed. Possible random static displacements of the oxygen atoms in the CuO2 planes and their role in the development of the weak ferromagnetism and the presence of internal fields at the Gd sites are also analyzed. A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state. © 1991 The American Physical Society

INTERNAL MAGNETIC-FIELD IN LA2-XSRXCUO4/GD OBSERVED BY ELECTRON-PARAMAGNETIC RESONANCE

We have measured the electron paramagnetic resonance (EPR) of Gd3+ in single crystals of La2-xSrxCuO4+delta, as a function of temperature T, magnetic-field angle, oxygen content, microwave frequency, and Sr concentration (0 less-than-or-equal-to x less-than-or-equal-to 0. 024). For temperatures larger than the antiferromagnetic ordering temperature of the Cu lattice, T(N), we have identified four different sites of Gd3+ as expected for twinned crystals and we are able to fit the data for all directions of the magnetic field. Samples annealed under vacuum or oxygen show only small changes on their crystal-field parameters and g values. For temperatures smaller than T(N), the EPR lines are further split due to the internal magnetic field acting at the Gd site associated with the antiferromagnetic ordering of the Cu lattice. The temperature dependence of the splitting of the resonance lines allow us to calculate the moment per Cu ion. A value of approximately 0.6mu(B) per Cu ion at T = 0 K is inferred, with the Cu moments lying in the bc plane about 5-degrees from the c axis. The data only can be fit for all the directions of the applied magnetic field if other mechanisms such as a Heisenberg type of exchange between the Cu and Gd moments or a distortion of the lattice below T(N) are included.47138156816