Three-body correlation effects on the spin dynamics of double-exchange ferromagnets

We present a variational calculation of the spin wave excitation spectrum of double-exchange ferromagnets in different dimensions. Our theory recovers the random phase approximation and $1∕S$ expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three-body correlations between a Fermi sea electron-hole pair and a magnon excitation and show that they strongly affect the spin dynamics. The manifestations of these correlations are many-fold. We demonstrate that they significantly decrease the stability of the ferromagnetic phase and the magnon stiffness. We also show that the ferromagnetic state is unstable against spin wave excitations close to the Brillouin zone boundary. As a result, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness within a range of intermediate concentrations. We discuss the possible relevance of our results to experiments in colossal magnetoresistance ferromagnets.

[1]  Clarence Zener,et al.  Interaction between the d -Shells in the Transition Metals. II. Ferromagnetic Compounds of Manganese with Perovskite Structure , 1951 .

[2]  J. Igarashi.  Three-Body Problem in the One-Dimensional Hubbard Model , 1985 .

[3]  S. Mahanti,et al.  LETTER TO THE EDITOR: On the relation of the `double-exchange' model to low-temperature magnetic properties of doped manganites , 1997 .

[4]  E. Nagaev.  Colossal-magnetoresistance materials: manganites and conventional ferromagnetic semiconductors , 2001 .

[5]  Hayden,et al.  Spin Waves throughout the Brillouin Zone of a Double-Exchange Ferromagnet. , 1996, Physical review letters.

[6]  K. Kubo.  Note on the Ground States of Systems with the Strong Hund-Coupling , 1982 .

[7]  Krishnamurthy,et al.  Instability of the Nagaoka ferromagnetic state of the U= , 1990, Physical review. B, Condensed matter.

[8]  Papanicolaou,et al.  Phase separation in a t-J model. , 1990, Physical review. B, Condensed matter.

[9]  Spin wave theory of double exchange ferromagnets , 1999, Physical review letters.

[10]  D. Edwards.  Ferromagnetism and electron-phonon coupling in the manganites , 2002, cond-mat/0201558.

[11]  Takashi Hotta,et al.  Colossal Magnetoresistant Materials: The Key Role of Phase Separation , 2000, cond-mat/0012117.

[12]  W. Linden,et al.  Ferromagnetism in the Hubbard model , 1991 .

[13]  J. Igarashi,et al.  Electron Correlations in the Nagaoka State on the Square Lattice. Three-Body Correlations. , 1999 .

[14]  John B. Goodenough,et al.  Theory of the role of covalence in the perovskite-type manganites [La,M(II)]MnO3 , 1955 .

[15]  J. Bigot,et al.  Real space trajectory of the ultrafast magnetization dynamics in ferromagnetic metals. , 2005, Physical review letters.

[16]  Y. Tokura,et al.  Giant magnetoresistance of manganese oxides with a layered perovskite structure , 1996, Nature.

[17]  L. Regnault,et al.  Spin dynamics of La 0.7 Ba 0.3 MnO 3 , 2002 .

[18]  D. I. Golosov.  Effects of the on-site Coulomb repulsion in double-exchange magnets , 2004, cond-mat/0406431.

[19]  L. M. Roth.  Spin wave stability of the ferromagnetic state for a narrow s-band , 1967 .

[20]  M. Mochena,et al.  Spin-wave softening and Hund’s coupling in ferromagnetic manganites , 2005, Journal of physics. Condensed matter : an Institute of Physics journal.

[21]  M. Berciu,et al.  Spin waves in disordered III-V diluted magnetic semiconductors by a modified RPa approach , 2002, cond-mat/0204092.

[22]  A. Revcolevschi,et al.  Spin waves in the quasi-two-dimensional-ferromagnetic bilayer manganite La1.2Sr1.8Mn2O7. , 2005 .

[23]  J. Bigot,et al.  Femtosecond spectrotemporal magneto-optics. , 2004, Physical review letters.

[24]  Philip W. Anderson,et al.  Considerations on Double Exchange , 1955 .

[25]  Y. Endoh,et al.  Scaling of Spin-Wave Stiffness Constant to Curie Temperature in the Colossal Magnetoresistance Material La1-xSrxMnO3. , 1997 .

[26]  Schmitt-Rink,et al.  Many-body aspects of the optical spectra of bulk and low-dimensional doped semiconductors. , 1987, Physical review. B, Condensed matter.

[27]  E. Müller-Hartmann,et al.  Ferromagnetism and spin waves in double exchange materials , 1998 .

[28]  E. Nagaev.  MAGNON SPECTRUM AT THE NON-RKKY INDIRECT EXCHANGE IN CONDUCTING FERROMAGNETS , 1998 .

[29]  L. M. Roth.  Electron Correlation in Narrow Energy Bands. II. One Reversed Spin in an Otherwise Fully Aligned Narrow S Band , 1969 .

[30]  Xindong Wang.  Theory of spin waves in a ferromagnetic Kondo lattice model , 1998 .

[31]  L. M. Roth.  New Method for Linearizing Many-Body Equations of Motion in Statistical Mechanics , 1968 .

[32]  T. Okabe.  Spin Wave and Stability of the Double Exchange Ferromagnet , 1997 .

[33]  K. Kubo,et al.  A Quantum Theory of Double Exchange. I , 1972 .

[34]  K. Terakura,et al.  Zone Boundary Softening of the Spin-Wave Dispersion in Doped Ferromagnetic Manganites , 1999 .