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Possible magnetic-polaron-switched positive and negative magnetoresistance in the GdSi single crystals.

Li H, Xiao Y, Schmitz B, Persson J, Schmidt W, Meuffels P, Roth G, Brückel T - Sci Rep (2012)

Bottom Line: Around T(N) the PMRV translates to negative, down to ~-10.5%.Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins.Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices.

View Article: PubMed Central - PubMed

Affiliation: Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at Institut Laue-Langevin, Boîte Postale 156, F-38042 Grenoble Cedex 9, France. h.li@fz-juelich.de

ABSTRACT
Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around T(N) the PMRV translates to negative, down to ~-10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices.

No MeSH data available.


Schematic illustration of the spin states with and without applied magnetic field, μ0H, in different temperature regimes.(a–c) At zero magnetic field. (d–f) At applied magnetic field of μ0H. When T > T1, spin moments more or less rotate (f), depending on the strength of μ0H and the size of MA, from a pure PA state (that is strictly observing the Curie-Weiss law as shown in Supplementary Fig. S4) in (c). When TN1 < T < T1, the short-range AFM spins that are attributed only to the local 4f moments appear (b) accompanied by the generations of polarized itinerate 5d spins, based on the deviation of the unit-cell volume from the Grüneisen model shown in Fig. 1b, and possible small amount of localized 5d spins according to equation (1). Applied magnetic field mainly aligns the local AFM spins (e), leading to a decrease of the electron-spin scattering and resultant the NMRV. When T < TN2, the LRO AFM state with almost equivalent AFM and FM interactions (see Supplementary Fig. S4) forms (a) with more itinerate 5d moments (based on the large PMV effect shown in Fig. 1b). Applied magnetic field mainly localizes more 5d moments by enhancing the exchange of J in equation (1), resulting in the formation of magnetic polarons and the consequent PMRV (d).
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f5: Schematic illustration of the spin states with and without applied magnetic field, μ0H, in different temperature regimes.(a–c) At zero magnetic field. (d–f) At applied magnetic field of μ0H. When T > T1, spin moments more or less rotate (f), depending on the strength of μ0H and the size of MA, from a pure PA state (that is strictly observing the Curie-Weiss law as shown in Supplementary Fig. S4) in (c). When TN1 < T < T1, the short-range AFM spins that are attributed only to the local 4f moments appear (b) accompanied by the generations of polarized itinerate 5d spins, based on the deviation of the unit-cell volume from the Grüneisen model shown in Fig. 1b, and possible small amount of localized 5d spins according to equation (1). Applied magnetic field mainly aligns the local AFM spins (e), leading to a decrease of the electron-spin scattering and resultant the NMRV. When T < TN2, the LRO AFM state with almost equivalent AFM and FM interactions (see Supplementary Fig. S4) forms (a) with more itinerate 5d moments (based on the large PMV effect shown in Fig. 1b). Applied magnetic field mainly localizes more 5d moments by enhancing the exchange of J in equation (1), resulting in the formation of magnetic polarons and the consequent PMRV (d).

Mentions: In GdSi, the conduction electrons (mainly 5d bands) are different from those responsible for the magnetism (4f component plus possible part of the 5d component). The former is normally delocalized, acting as the magnetic glue among magnetic ions (Fig. 5), and scattered by them, leading to electrical resistance. The magnetism from the 4f part is generally localized with weak interactions. Therefore, the LRO AFM state originates mainly from the isotropic RKKY interactions through conduction bands23. The interaction between localized moments, , and itinerant ones, , can generate an extraordinarily large Zeeman splitting in the mean-field approximation24where g* is the spectroscopic splitting factors for the carriers, μB is the Bohr magneton, J is the effective exchange coefficient and is the averaged local moment in the regime of band electrons. Since is usually small, and the second term could be very large (e.g., amounting to fractions of an eV in the LRO magnetic state of Eu-compounds24), in addition, J is strongly associated with applied magnetic field by virtue of modifying spin fluctuations of , the formation of magnetic polarons in the 5d bands by this splitting in the LRO AFM state of GdSi is thus possible. Therefore, when T < TN2, the modified J at 8 T drives some of the conducting moments (as foregoing remarks) to form local magnetic polarons that lead to a largely degenerate conduction (i.e., PMRV) (Figs 5a and 5b). In this case, the more extended 5d bands almost certainly offer a small FM component, which dominates the linear-magnetic-field dependence below TN2.


Possible magnetic-polaron-switched positive and negative magnetoresistance in the GdSi single crystals.

Li H, Xiao Y, Schmitz B, Persson J, Schmidt W, Meuffels P, Roth G, Brückel T - Sci Rep (2012)

Schematic illustration of the spin states with and without applied magnetic field, μ0H, in different temperature regimes.(a–c) At zero magnetic field. (d–f) At applied magnetic field of μ0H. When T > T1, spin moments more or less rotate (f), depending on the strength of μ0H and the size of MA, from a pure PA state (that is strictly observing the Curie-Weiss law as shown in Supplementary Fig. S4) in (c). When TN1 < T < T1, the short-range AFM spins that are attributed only to the local 4f moments appear (b) accompanied by the generations of polarized itinerate 5d spins, based on the deviation of the unit-cell volume from the Grüneisen model shown in Fig. 1b, and possible small amount of localized 5d spins according to equation (1). Applied magnetic field mainly aligns the local AFM spins (e), leading to a decrease of the electron-spin scattering and resultant the NMRV. When T < TN2, the LRO AFM state with almost equivalent AFM and FM interactions (see Supplementary Fig. S4) forms (a) with more itinerate 5d moments (based on the large PMV effect shown in Fig. 1b). Applied magnetic field mainly localizes more 5d moments by enhancing the exchange of J in equation (1), resulting in the formation of magnetic polarons and the consequent PMRV (d).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3475993&req=5

f5: Schematic illustration of the spin states with and without applied magnetic field, μ0H, in different temperature regimes.(a–c) At zero magnetic field. (d–f) At applied magnetic field of μ0H. When T > T1, spin moments more or less rotate (f), depending on the strength of μ0H and the size of MA, from a pure PA state (that is strictly observing the Curie-Weiss law as shown in Supplementary Fig. S4) in (c). When TN1 < T < T1, the short-range AFM spins that are attributed only to the local 4f moments appear (b) accompanied by the generations of polarized itinerate 5d spins, based on the deviation of the unit-cell volume from the Grüneisen model shown in Fig. 1b, and possible small amount of localized 5d spins according to equation (1). Applied magnetic field mainly aligns the local AFM spins (e), leading to a decrease of the electron-spin scattering and resultant the NMRV. When T < TN2, the LRO AFM state with almost equivalent AFM and FM interactions (see Supplementary Fig. S4) forms (a) with more itinerate 5d moments (based on the large PMV effect shown in Fig. 1b). Applied magnetic field mainly localizes more 5d moments by enhancing the exchange of J in equation (1), resulting in the formation of magnetic polarons and the consequent PMRV (d).
Mentions: In GdSi, the conduction electrons (mainly 5d bands) are different from those responsible for the magnetism (4f component plus possible part of the 5d component). The former is normally delocalized, acting as the magnetic glue among magnetic ions (Fig. 5), and scattered by them, leading to electrical resistance. The magnetism from the 4f part is generally localized with weak interactions. Therefore, the LRO AFM state originates mainly from the isotropic RKKY interactions through conduction bands23. The interaction between localized moments, , and itinerant ones, , can generate an extraordinarily large Zeeman splitting in the mean-field approximation24where g* is the spectroscopic splitting factors for the carriers, μB is the Bohr magneton, J is the effective exchange coefficient and is the averaged local moment in the regime of band electrons. Since is usually small, and the second term could be very large (e.g., amounting to fractions of an eV in the LRO magnetic state of Eu-compounds24), in addition, J is strongly associated with applied magnetic field by virtue of modifying spin fluctuations of , the formation of magnetic polarons in the 5d bands by this splitting in the LRO AFM state of GdSi is thus possible. Therefore, when T < TN2, the modified J at 8 T drives some of the conducting moments (as foregoing remarks) to form local magnetic polarons that lead to a largely degenerate conduction (i.e., PMRV) (Figs 5a and 5b). In this case, the more extended 5d bands almost certainly offer a small FM component, which dominates the linear-magnetic-field dependence below TN2.

Bottom Line: Around T(N) the PMRV translates to negative, down to ~-10.5%.Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins.Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices.

View Article: PubMed Central - PubMed

Affiliation: Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at Institut Laue-Langevin, Boîte Postale 156, F-38042 Grenoble Cedex 9, France. h.li@fz-juelich.de

ABSTRACT
Magnetoresistance (MR) has attracted tremendous attention for possible technological applications. Understanding the role of magnetism in manipulating MR may in turn steer the searching for new applicable MR materials. Here we show that antiferromagnetic (AFM) GdSi metal displays an anisotropic positive MR value (PMRV), up to ~415%, accompanied by a large negative thermal volume expansion (NTVE). Around T(N) the PMRV translates to negative, down to ~-10.5%. Their theory-breaking magnetic-field dependencies [PMRV: dominantly linear; negative MR value (NMRV): quadratic] and the unusual NTVE indicate that PMRV is induced by the formation of magnetic polarons in 5d bands, whereas NMRV is possibly due to abated electron-spin scattering resulting from magnetic-field-aligned local 4f spins. Our results may open up a new avenue of searching for giant MR materials by suppressing the AFM transition temperature, opposite the case in manganites, and provide a promising approach to novel magnetic and electric devices.

No MeSH data available.