Nuclear Spin Relaxation of Impurities in Dilute Ferromagnetic Alloys

Kontani, Masaaki; Hioki, Tatsumi; Masuda, Yoshika

doi:10.1143/jpsj.32.416

Cited by 46 publications

(24 citation statements)

References 28 publications

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“…This effect is especially prominent and well documented for the 5d impurities in Fe [6]. (ii) For magnetic fields below 1 T there is a so far unexplained magnetic field dependence due to which the relaxation in zero field is typically 3 to 10 times faster than at high fields [8][9][10].This work is also the first experimental determination of R q in a transition metal, although the presence of this contribution to the relaxation in transition metals is meanwhile well established in the theoretical work [4,7,[11][12][13]. In Ref.…”

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confidence: 88%

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Electric Quadrupolar Contribution to the Nuclear Spin-Lattice Relaxation of Ir in Fe

Seewald

Zech

et al. 2002

Phys. Rev. Lett.

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We report on the first quantitative determination of the electric quadrupolar contribution to the nuclear spin-lattice relaxation in a transition metal. For 186 Ir and 189 Ir in Fe we have determined the magnetic and the electric quadrupolar part of the relaxation for magnetic fields between 0.01 and 2 T. The quadrupolar part gives information on the role of the orbital motion of the electrons for the relaxation process. Our results prove that the unexpected high relaxation rates in Fe and their magnetic field dependence are due to a nonorbital relaxation mechanism. DOI: 10.1103/PhysRevLett.88.057601 PACS numbers: 76.60.Es, 75.50.Bb, 76.80. +y The nuclear spin-lattice relaxation in metals arises predominantly from the magnetic hyperfine interaction between the nuclear spins and the conduction electrons. It gives information on the density of states at the Fermi energy and the magnitude of low frequency spin fluctuations [1 -3].The interpretation of the relaxation is complicated in transition metals by the fact that several relaxation mechanisms can contribute, but only the total relaxation rate R is usually measured: According to the type of the responsible hyperfine interaction, R can be subdivided into an orbital (R o ), a Fermi-contact (R c ), a spin-dipolar (R sd ), a core-polarization (R cp ), and a quadrupolar (R q ) contribution [3,4]. The first four contributions represent together the magnetic relaxation rate R m . R q is due to the electric hyperfine interaction and arises from electric field gradient fluctuations at the nuclear site.The various contributions involve quite different electronic excitations: R c , for example, is connected to spin flips of s electrons; R o and R q are connected to orbital momentum changes of p or d electrons. A separate determination would, therefore, give valuable information on the relaxation process. Such information is especially desirable since experiment and theory are in serious disagreement for several transition metal systems [5,6].In this situation, one can make use of two special features of R q : R q can be determined separately and it is so closely related to R o that R o can be estimated from R q [4,7]. This offers the unique possibility to decompose experimentally the relaxation into an orbital and a nonorbital part.This possibility is used here for the first time. It is used to study the origin of two still unsolved problems in the nuclear spin-lattice relaxation in Fe: (i) The relaxation rates are usually larger than predicted by ab initio calculations. This effect is especially prominent and well documented for the 5d impurities in Fe [6]. (ii) For magnetic fields below 1 T there is a so far unexplained magnetic field dependence due to which the relaxation in zero field is typically 3 to 10 times faster than at high fields [8][9][10].This work is also the first experimental determination of R q in a transition metal, although the presence of this contribution to the relaxation in transition metals is meanwhile well established in the theoretical wor...

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confidence: 88%

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confidence: 89%

Electric Quadrupolar Contribution to the Nuclear Spin-Lattice Relaxation of Ir in Fe

Seewald

Zech

et al. 2002

Phys. Rev. Lett.

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“…[1][2][3] The effect typically manifests itself at low applied magnetic fields by relaxation rates that are 2-10 times larger than in the high-field limit, which is essentially reached within applied fields of the order of 1 T. Since there is a close relation between the spin-lattice relaxation and low-frequency magnetic-moment fluctuations, 4,5 the lack of an explanation would point to a fundamental deficiency in our understanding of the moment fluctuations in Fe, Co, and Ni. This was the motivation to obtain more information on the effect.…”

Section: Introductionmentioning

confidence: 99%

“…1,8,[10][11][12] It had been speculated that this failure might not be due to the inadequacy of the proposed relaxation mechanisms, but due to an incomplete knowledge of the magnetization behavior, the band structure, or the spin-wave dispersion. 9,13,14 The precise data and the close examination of those mechanisms in this work show, however, that those speculations are not true.…”

Section: Introductionmentioning

confidence: 99%

Origin of the magnetic-field dependence of the nuclear spin-lattice relaxation in iron

et al. 2008

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The magnetic-field dependence of the nuclear spin-lattice relaxation at Ir impurities in Fe was measured for fields between 0 and 2 T parallel to the ͓100͔ direction. The reliability of the applied technique of nuclear magnetic resonance on oriented nuclei was demonstrated by measurements at different radio-frequency ͑rf͒ field strengths. The interpretation of the relaxation curves, which used transition rates to describe the excitation of the nuclear spins by a frequency-modulated rf field, was confirmed by model calculations. The magneticfield dependence of the so-called enhancement factor for rf fields, which is closely related to the magnetic-field dependence of the spin-lattice relaxation, was also measured. For several magnetic-field-dependent relaxation mechanisms, the form and the magnitude of the field dependence were derived. Only the relaxation via eddy-current damping and Gilbert damping could explain the observed field dependence. Using reasonable values of the damping parameters, the field dependence could perfectly be described. This relaxation mechanism is, therefore, identified as the origin of the magnetic-field dependence of the spin-lattice relaxation in Fe. The detailed theory, as well as an approximate expression, is derived, and the dependences on the wave vector, the resonance frequency, the conductivity, the temperature, and the surface conditions are discussed. The theory is related to previous attempts to understand the field dependence of the relaxation, and it is used to reinterpret previous relaxation experiments in Fe. Moreover, it is predicted that the field dependences of the relaxation in Fe and Co, on one side, and in Ni, on the other side, differ substantially, and it is suggested that the literature values of the high-field limits of the relaxation constants in Fe are slightly too large.

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“…This field dependence of the NSLR is still a controversial subject. With NMR, Kontani et al [6] observed an increase of the NSLR rate between high field and zero field for several 3d and 4d impurities in Fe. They tried to explain this field dependence as a relaxation mechanism due to electronic spin waves.…”

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Observation of a Nuclear-Magnon Contribution to the Nuclear Spin-Lattice Relaxation ofP191tin Ferromagnetic Cobalt

1997

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We report the first indication for a nuclear spin-lattice relaxation mechanism originating from the coupling of the impurity nuclei to nuclear magnons. The magnetic hyperfine interaction and the nuclear spin-lattice relaxation of 191 Pt ͑T 1͞2 2.8 d͒ in fcc Co were measured with NMR on oriented nuclei at temperatures between 8 and 14 mK in external magnetic fields B ext 0.1 10 kG. Between B ext 5 and 1 kG the relaxation rate increases by 2 orders of magnitude. This increase is attributed to the interaction of 191 Pt with nuclear magnons originating from the 59 Co nuclear spin system. [S0031-9007(97)03473-X] PACS numbers: 76.60. Es, 75.30.Ds, 75.50.Cc, 76.80. + y The nuclear spin-lattice relaxation (NSLR) of impurity nuclei in the ferromagnetic host lattices Fe, Co, and Ni has been the subject of many investigations, both experimental and theoretical [1][2][3][4][5][6][7][8][9][10][11]. Nevertheless, not all mechanisms which are responsible for the relaxation process are understood in detail.The strength of the NSLR is described by a relaxation rate,where C K is a specific constant for each impurity-host system, analogous to the Korringa constant. At present, it is an open question whether an electronic spin-wave mechanism yields an appreciable contribution to the relaxation rate. Weger [1] studied the relaxation in Fe, Co, and Ni. He proposed that there is a strong contribution to the relaxation rate which originates from the interaction of the conduction electrons with the nuclei via electronic spin waves. Moriya [2] performed calculations including d-band electrons in a tight-binding approximation. He found that the predominant NSLR mechanism comes from the fluctuation of the orbital current of the d-band electrons and that the electronic spin-wave contribution is smaller by 1 or 2 orders of magnitude. Ab initio calculations of the NSLR rates were started by Kanamori et al. [3]. In 1988, Akai [4] published such calculations for all elements as impurities in Fe (with the exception of the rare earths and actinides), in which electronic spin-wave processes were not included. A comparison of the calculated rates with experimental rates shows a unique trend. All calculated rates are too small by a factor of 2-5. This can be interpreted as an indication that an important relaxation mechanism is missing in his model [5]. At present, however, it cannot be stated whether this missing mechanism is the relaxation via electronic spin waves. As another experimental fact, it was early found that the relaxation rate depends on the external magnetic field B ext [6-11]: For high fields ͑B ext * 6 kG͒, the relaxation rate is field independent ͑R`͒; for small fields ͑B ext & 1 kG͒, the relaxation rate increases by a factor ϳ3 10. This field dependence of the NSLR is still a controversial subject. With NMR, Kontani et al. [6] observed an increase of the NSLR rate between high field and zero field for several 3d and 4d impurities in Fe. They tried to explain this field dependence as a relaxation mechanism due to electronic spin ...

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Nuclear Spin Relaxation of Impurities in Dilute Ferromagnetic Alloys

Cited by 46 publications

References 28 publications

Electric Quadrupolar Contribution to the Nuclear Spin-Lattice Relaxation of Ir in Fe

Electric Quadrupolar Contribution to the Nuclear Spin-Lattice Relaxation of Ir in Fe

Origin of the magnetic-field dependence of the nuclear spin-lattice relaxation in iron

Observation of a Nuclear-Magnon Contribution to the Nuclear Spin-Lattice Relaxation ofP191tin Ferromagnetic Cobalt

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