Spin dynamics and magnetic ordering in mixed valence systems

Shapiro, S. M.; Möller, Hans; Axe, J. D.; Birgeneau, R. J.; Bücher, E.

doi:10.1063/1.324753

Cited by 32 publications

(3 citation statements)

References 10 publications

(9 reference statements)

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“…That the Sm3f configuration makes no contribution to the form factor is a very surprising, anomalous result. Similarly, the measured form factor in a (stoichiometric and thus mixed-valent) single crystal of TmSe agrees with the calculated Tm3+ form factor even though more than 20% of the sample is believed to be in the Tm2f configuration (Shapiro et al 1978). In CeSn3 crystals the form factor agrees with the predicted Ce3f value for temperatures greater than 40 K, but at 4.2 K there are significant discrepancies at small Q(-I A-1) which also indicate considerable anisotropy in the spatial distribution of the induced magnetisation ( figure 23).…”

Section: Magnetic Neutron Scatteringsupporting

confidence: 77%

Valence fluctuation phenomena

1981

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Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. We review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds ; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging manybody problem.

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Section: Magnetic Neutron Scatteringsupporting

confidence: 77%

Valence fluctuation phenomena

1981

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“…The small moment compared with the free ion values of 4 and 7 p, of Tm2+ and Tm3+, respectively, indicates that there is some interaction which splits the Hund's rule states. Shapiro et al [8] It is evident from the discussion above that the physical properties are strongly sample dependent. As a consequence we have undertaken a study of the solid state chemistry of TmSe in relation to its physical properties [13].…”

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confidence: 99%

Valence instabilities of Tm in its compounds and solid solutions

1979

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TmSe présente à la fois une valence non entière et un ordre antiferromagnétique, contrairement à la plupart des autres systèmes de valence intermédiaire qui sont non magnétiques à basse température. Nous présentons des mesures de constante de réseau, de susceptibilité magnétique et de résistivité effectuées sur des monocristaux de Tm^Se dans sa région de solubilité solide 0,79 <; x <, 1. Nous trouvons des corrélations directes entre les propriétés physiques, la valence et la concentration. La fraction de caractère divalent dans Tm^Se augmente avec x, l'échantillon le plus déficitaire en thulium étant essentiellement trivalent. Des monocristaux d'alliages de TmSe avec YSe ont également été préparés. Les constantes de réseau et les susceptibilités indiquent que ces matériaux ont une valence non entière pour toutes les compositions. Lorsque la concentration de thulium augmente, on passe d'une résistivité Kondo métallique à la haute résistivité caractéristique de TmSe stoechiométrique. Bien que nous trouvions les propriétés des deux configurations Tm 2 + et Tm 3 + dans les propriétés magnétiques et les propriétés de transport, le fait d'obtenir ces résultats aussi bien dans la limite diluée à une impureté que dans TmSe stoechiométrique, nous amène à conclure que tous les sites de thulium sont équivalents.

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“…Back in the 1980's, it was noted in different experimental studies that, in the TmX (X = S, Se, Te) family of compounds and alloys, the FM character generally increases when the average thulium valence becomes closer to 2+ [6]. Even in the strongly mixed-valent state of pure TmSe, a FM tendency is clearly evidenced by the metamagnetic transition occurring in a weak applied magnetic field H M = 0.4 T [18].…”

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Ferromagnetic Mixed-Valence and Kondo-Lattice State in TmTe at High Pressure

et al. 1998

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Neutron diffraction experiments on TmTe at pressures up to 7 GPa are reported. The semiconductor-to-metal transition occurring at 2 GPa in this compound is accompanied by the appearance of strong ferromagnetic exchange interactions producing a Curie temperature as high as 14 K. This behavior is characteristic for the incipient mixed-valence regime just above the transition pressure and traced back to the release of a small concentration of free charge carriers in the material. The steep decrease of both the Curie temperature and ordered magnetic moment occurring at higher pressures emphasizes the role of Kondo fluctuations, and raises the interesting possibility that a quantum critical transition to a non-magnetic ground state of mixed-valence Tm might take place around 6 GPa.In magnetic materials, the concentration of mobile charge carriers is known to strongly influence the nature and strength of exchange interactions between neighboring atoms. This is evidenced in a particularly striking manner by systems undergoing metal-insulator transitions upon doping such as Eu chalcogenides [1] and Eu hexaboride [2], or by La manganites where the existence of hole-mediated "doubleexchange" interactions is assumed to be responsible for the orders-of-magnitude response of the electrical resistivity to an applied magnetic field [3]. In the above examples, the extra carriers are usually created by substituting the rare-earth element by an ion with a different valence. This clearly produces a local charge singularity, which can have nontrivial consequences such as e.g. the formation of impurity states, bound magnetic polarons, etc. [4]. A more uniform situation may be achieved in the case where the charge carriers result from a homogeneous mixed-valence state of all magnetic atoms. This phenomenon can be viewed as the quantum admixture of two electronic configurations corresponding to different occupations n and n-1 of the rareearth 4f-shell, one electron being able to fluctuate between a localized 4f-orbital and extended 5d-6s states. Numerous examples of this behavior have been found experimentally, mostly in Ce, Sm, Eu, Tm and Yb compounds [5]. The mixed valence can occur spontaneously, or be the result of an external perturbation, typically chemical substitution or applied pressure. However, in the overwhelming majority of these systems, the valence instability is accompanied by large spin fluctuations which cause a complete suppression of the magnetic state and obscure the variation of magnetic exchange interactions. The only notable exception to date has been reported for the cubic (NaCl structure) thulium monochalcogenide family TmX (X = S, Se, Te) [6]. Among these materials, stoichiometric TmSe exhibits antiferromagnetic (AF) order below T N = 3.4 K with a valence estimated to be comprised, depending on the technique used, between 2.55 (lattice constant) and 2.72 (effective moment). This unique behavior has been claimed to result from the fact that both valence states Tm 2+ and Tm 3+ have degenerate low-lying multipl...

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Spin dynamics and magnetic ordering in mixed valence systems

Cited by 32 publications

References 10 publications

Valence fluctuation phenomena

Valence fluctuation phenomena

Valence instabilities of Tm in its compounds and solid solutions

Ferromagnetic Mixed-Valence and Kondo-Lattice State in TmTe at High Pressure

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