6.2.2 Non-magnetic ion doping

One interesting feature of CuGeO₃ is that non-magnetic ion doping on the chain (Cu²⁺$\rightarrow$Zn²⁺) and out of the chain (Ge⁴⁺$\rightarrow$Si⁴⁺) is possible. It has been shown that these two kinds of doping lead to a Néel ordered ground state.

  

Figure: Zn-concentration (x) verses temperature (T) phase diagram of (Cu_1-xZn_x)GeO₃. Cite from Ref. [158]. The spin-glass phase (SG) was later proposed to be a Néel ordered phase.

Hase et al. measured magnetic susceptibility of the Zn-doped compounds (Cu_1-xZn_x)GeO₃, and found that (1) the spin Peierls transition disappears at $x\sim 2$ % and (2) a new cusp appears in the Zn concentration range of $2\stackrel{<}{\sim}x\stackrel{<}{\sim}8$ % [158]. The cusp temperature takes a maximum at $x\sim 4$ %. A magnetic phase diagram was obtained as shown in Fig.53.

Oseroff et al. measured the specific heat (C_p) of Zn, Ni and Mn-doped CuGeO₃ and found a peak at the cusp temperature [159]. Since a peak in the specific heat should be absent for the spin-glass transition [160], but present for Néel order, they proposed a Néel ordered ground state for the doped materials.

Recent neutron scattering measurements of single crystalline (Cu_1-xZn_x)GeO₃ (x=3.4 %) showed the existence of antiferromagnetic Bragg reflections [161]; this result directly indicates the Néel order. The size of the ordered moments was obtained as $\sim 0.2\ \mu_{\rm B}$, which is less than half of what was observed in a Néel ordered spin chain ($0.49\ \mu_{\rm B}$ for KCuF₃ [162]).

Si-doped systems Cu(Ge_1-ySi_y)O₃ were investigated by Renard et al. with susceptibility and ⁶³Cu-NMR measurements [163]. The spin Peierls transition disappeared at Si concentration $y\sim 1$ % and a Néel ground state appeared at $0.5\stackrel{<}{\sim}y\stackrel{<}{\sim}5$ %. The phase diagram for the Si-doped compounds is similar to that of the Zn-doped systems (Fig.53), with the $T_{\rm N}$-maximum concentration shifted from x=4% to y=2% for the Si doping.

Poirier et al. measured elastic constants in high magnetic fields and obtained an $H\!-\!T$ phase diagram for a Si 0.7% doped system [164]. They found that the overall structure of the $H\!-\!T$ phase diagram (Fig.54) was similar to the general phase diagram of spin Peierls systems (Fig.51), except that the spin Peierls phase (SP) is split to a SP-phase and a Néel ordered phase (AF).

  

Figure: $H\!-\!T$ phase diagram of Cu(Ge_1-ySi_y)O₃ (y=0.7%). Cite from Ref. [164].

Since a magnetic ordered phase is expected from previous measurements, $\mu$SR is a good probe for further investigations of the Zn/Si-doped systems. Previously, Tchernyshyov et al. [165] and García-Muñoz et al. [166] performed $\mu$SR measurements on Zn 4% samples and found a spin-glass-like muon spin relaxation. In the next section, more extensive $\mu$SR studies of Zn/Si-doped systems [(Cu_1-xZn_x)(Ge_1-ySi_y)O₃; x=2, 4, 8 %, and $y=2\%$], are presented in addition to the $\mu$SR results from the nominally pure CuGeO₃.