Ultrasonic studies of bonding and exchange interactions in zinc(1-x)manganese(x)selenide and cadmium(1-x)manganese(x)telluride
Abstract
We have measured the velocity of 30 to 150 MHz ultrasonic waves in the diluted magnetic semiconductor Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se having x = 0.2, 0.37 and 0.53, in order to make a comparative study of tetrahedral bond weakening and a low temperature shear elastic constant minimum observed previously in this laboratory in Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te. The velocity data was used to calculate the C$\sb{11},$C$\sb{33}$,C$\sb{44}$ and C$\sb{\rm s}$ = (C$\sb{11}$-C$\sb{12})/2$ elastic constants, which were found to decrease more intensely with x than would be expected from the increase in lattice parameter with x. C$\sb{44}$ and C$\sb{\rm S}$ decreased more percentagewise than did C$\sb{11}$ and C$\sb{33}$. C$\sb{\rm ij}$(x)/C$\sb{\rm ij}$(o) reduced more with x in Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se than in Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te indicating that Mn causes more tetrahedral bond weakening in the former than the latter. We suggest that Mn 3d (t$\sb2$) orbitals hybridize more readily with Se 4p-orbitals than with Te 5p-orbitals leaving fewer p-orbitals available for tetrahedral bonding. In Zn$\sb{0.47}$Mn$\sb{0.53}$Se, C$\sb{44}$ and C$\sb{\rm S}$ each underwent a wide minimum with its lowest point near the spin glass temperature (T$\sb{\rm sg})$ which was shallower than that in Cd$\sb{0.48}$Mn$\sb{0.52}$Te. We attribute this to the dependence of the strain derivatives of the exchange energy on the type of anion. The C$\sb{44}$ minimum in Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se is deeper for x = 0.53 than for x = 0.37 and likewise at 147 MHz than at 30 MHz allowing us to deduce a magnetoelastic spin relaxation time of about 3 $\times$ $10\sp{-10}$ s. A shear wave attenuation peak, occurring primarily below T$\sb{\rm sg},$was discovered in Zn$\sb{\rm 1-x}$Mn$\sb{\rm x}$Se and subsequently also measured in Cd$\sb{\rm 1-x}$Mn$\sb{\rm x}$Te. We discuss it in terms of a summation of three non-Debye relaxation-type peaks. The activation energies of the component peaks range from 1.1 to 5.7 meV for the two compounds, when calculated using $\tau$ = $\tau\sb0\ e\sp{\rm E/k{\sb\beta}T},$ and are found to scale with the Mn-Mn exchange integral J for each compound respectively. The relaxation time $\tau\sb0$is longest for the highest temperature peak and ranges from $10\sp{-12}$ s to 5 $\times$ $10\sp{-11}$ s. Both the shear elastic constant minimum and attenuation peak are due to spin fluctuations along with the diffusivity of the spin glass transition. Since no corresponding behavior is observed for longitudinal waves, the spin-phonon coupling is explained in terms of an anisotropic exchange interaction.
Degree
Ph.D.
Advisors
Sladek, Purdue University.
Subject Area
Condensation
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