Transverse Nuclear Relaxation in Real Ferromagnets

Barak, J.; Siegelstein, I.; Gabai, A.; Kaplan, N.

doi:10.1103/physrevb.8.5282

Cited by 27 publications

(13 citation statements)

References 14 publications

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“…The stretched exponential behavior in T 1 may be due to a spin diffusion mechanism active in a highly inhomogeneous spectrum, whereas the same behavior in T 2 of magnetic insulators has been ascribed to the effect of inhomogeneous broadening on the Suhl-Nakamura interaction. 28 The relaxation of the AF line at 260 and the FM line at 310 MHz were investigated in samples va.00 ͑at the antiferromagnetic end͒ and Ca.20 ͑macroscopically ferromagnetic͒. For va.00 at low fields good fits of both TR and LR are obtained with single exponential curves, although for LR fits of comparable quality are obtained with the multiexponential decay predicted from Redfield theory in the presence of a quadrupole splitting.…”

Section: Relaxation Of 55 Mnmentioning

confidence: 94%

Electronic phase separation in lanthanum manganites: Evidence from55MnNMR

et al. 1997

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55 Mn NMR experiments were carried out at 1.3-4.2 K on six samples of lanthanum manganites, with a hole doping concentration ranging from the antiferromagnetic-insulator to the ferromagnetic-conductor region of the phase diagram. The dependence of the resonance frequencies on external fields is either as expected for ferromagnets ͑FM͒ or for antiferromagnets ͑AF͒. No indication of a canted phase could be detected. FM and AF resonances were found to coexist in all samples and are grouped in three broad bands, whose frequencies do not depend in first order on hole concentration. Strong evidence of interaction between AF and FM regions was provided by longitudinal relaxation experiments in several samples. An explanation of this behavior is offered in terms of intrinsic phase separation of the holes into FM microdomains. We determine the 55 Mn hyperfine coupling to be isotropic in all cases, with Aϭ6.2 T/ B for the on-site term in the AF domains.

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Section: Relaxation Of 55 Mnmentioning

confidence: 94%

Electronic phase separation in lanthanum manganites: Evidence from55MnNMR

et al. 1997

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“…33,34 The SSR single exponential relaxation law is restored by fully saturating the sample magnetization in an external field of 1 T at 77.3 K and the SSR rate becomes smaller by a factor 3. The longitudinal relaxation function becomes compatible with that predicted by the master equations, as in the case of va.16 at 7 T, although a strong dependence of the SSR rate on the irradiation bandwidth indicates a relevant spin diffusion contribution to the observed relaxation, which is reduced at higher fields.…”

Section: Relaxationsmentioning

confidence: 99%

139LaNMR in lanthanum manganites: Indication of the presence of magnetic polarons from spectra and nuclear relaxations

1998

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We present 139 La NMR experiments on five powder samples of lanthanum manganites, with a Mn 4ϩ concentration ranging from the antiferromagnetic-insulator ͑AFM͒ to the ferromagnetic-conducting ͑FM͒ region of the phase diagram. We measure a positive hyperfine coupling Cϭ0.13 T/ B . A signal from nuclei in a FM environment is present at all compositions, as evidenced by a hyperfine frequency in zero-field experiments, by a positive hyperfine shift in NMR experiments below T c , and by a paramagnetic frequency shift following Curie-Weiss law. A signal from nuclei in an AFM environment is identified by a similar negative intercept Curie-Weiss law. The NMR spectra reveal a large temperature dependent fraction of static spin defects below T c in the FM domains. Nuclear relaxation indicates that the FM regions are influenced by diffusing, AFM-correlated excitations, while the AFM regions probe spin fluctuations from diffusing, FM correlated excitations. These results are interpreted in terms of electronic inhomogeneities due to the presence of a magnetic small polaron. ͓S0163-1829͑98͒04602-5͔

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“…The spin-lattice relaxation time was measured by destroying the nuclear magnetization with a comb of rf pulses and observing the nuclear magnetization recovery towards thermal equilibrium. The spin-spin relaxation time T 2 was obtained by analyzing the decay of the spin echo as a function of the separation between the rf pulses, using a model suggested for the case of EuO by Barak and co-workers [8]. Prior to the actual NMR measurements we applied magnetic fields of the order of a few tesla at low temperatures, which, in the presence of even weak magnetic anisotropy, results in a very good alignment of the crystallites.…”

Section: (Received 27 May 1998)mentioning

confidence: 99%

Low-Temperature Nuclear Magnetic Resonance Studies ofEuB6

et al. 1998

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We report the results of 153 Eu and 11 B nuclear magnetic resonance (NMR) studies of EuB 6 at temperatures above 0.04 K and in magnetic fields between 0 and 7 T. We have observed a surprising evolution of the 153 Eu NMR spectrum at very low temperatures, which we interpret as evidence for a complex electronic ground state of EuB 6 , involving the coexistence of two magnetically very similar, but electronically inequivalent, phases. The low temperature 11 B NMR spectra reveal two inequivalent boron sites with internal fields of 20.02 and 20.35 T. [S0031-9007 (98)07982-4] PACS numbers: 75.30.Kz, 76.60.CqFrom the results of early work, EuB 6 was thought to be a ferromagnetic semiconductor [1] with a Curie temperature of approximately 13 K involving the ordering of well-localized Eu 21 moments. However, this simple picture could not account for magnetic and electronic properties of this material [2][3][4]. For example, at low temperatures no insulating but rather a metallic behavior has been observed [5]. Also the magnetically ordered state of EuB 6 has proven to be difficult to understand. Previous and more recent results [6,7] on the temperature dependence of the specific heat C p ͑T ͒ indicate that two consecutive phase transitions occur with onsets at 16 and 14 K, respectively. In an effort to add microscopic information to the database concerning the low-temperature behavior of EuB 6 , we have made 153 Eu and 11 B NMR experiments on a EuB 6 sample which has been characterized by structural, thermal, transport, and optical properties [6].As the most surprising result we note an anomalous evolution of the 153 Eu NMR spectrum at low temperatures, which implies the gradual development of a new ordered phase in EuB 6 and the coexistence of two magnetically inequivalent phases in the ground state of this material. The differences appear to be minute and to consist of a rather subtle but distinct variation in the electronic environment at the Eu sites. Our 11 B NMR results are consistent with a slight distortion of the crystal structure below the Curie temperature, but we have no evidence for moment reorientation phenomena at lower temperatures [7].For our NMR experiments we used standard spin-echo techniques with external magnetic fields between 0 and 7 T, at temperatures between 0.04 and 300 K. The NMR spectra were measured at a fixed frequency by monitoring the spin-echo signal at a given frequency and varying stepwise the applied magnetic field. The spin-lattice relaxation time was measured by destroying the nuclear magnetization with a comb of rf pulses and observing the nuclear magnetization recovery towards thermal equilibrium. The spin-spin relaxation time T 2 was obtained by analyzing the decay of the spin echo as a function of the separation between the rf pulses, using a model suggested for the case of EuO by . Prior to the actual NMR measurements we applied magnetic fields of the order of a few tesla at low temperatures, which, in the presence of even weak magnetic anisotropy, results in a very good alignme...

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Transverse Nuclear Relaxation in Real Ferromagnets

Cited by 27 publications

References 14 publications

Electronic phase separation in lanthanum manganites: Evidence from55MnNMR

Electronic phase separation in lanthanum manganites: Evidence from55MnNMR

139LaNMR in lanthanum manganites: Indication of the presence of magnetic polarons from spectra and nuclear relaxations

Low-Temperature Nuclear Magnetic Resonance Studies ofEuB6

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