[Paper Review] Influence of Cu on spin-polaron tunneling in the ferromagnetic state of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) from the resistivity data

S. Sergeenkov, Hassan Bougrine+2 more|arXiv (Cornell University)|Jan 25, 1999

Magnetic and transport properties of perovskites and related materialsMaterials Science13 references19 citations

TL;DR

This study demonstrates that 4% Cu doping in La₂/₃Ca₁/₃Mn₁₋ₓCuₓO₃ induces a ~50% resistivity drop in the ferromagnetic state, attributed to a reduction in spin-polaron tunneling energy Eσ(x) ≈ Eσ(0)(1−x)⁴. The resistivity is well described by a nonthermal coherent tunneling model ρ(T,x) = ρ₀e⁻ᵞᴹ²⁽ᵀ,ˣ⁾, with magnetization M(T,x) = Mᵣ(x) + M₀(x)tanh{√[(T_C(x)/T)²−1]}, yielding quantitative fits that reveal Cu-induced suppression of polaron energy and enhanced carrier correlation length.

ABSTRACT

Nearly a 50% decrease of resistivity ρ(T,x) due to just 4% Cu doping on the Mn site of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) is observed. Attributing the observed phenomenon to the substitution induced decrease of the spin polaron energy E_s(x) below the Curie point T_C(x)=T_C(0)(1-x), all data are found to be well fitted by the nonthermal coherent tunneling expression ρ(T,x) = ρ_0*exp(-γM^2(T,x)) assuming M(T,x)=M_R(x)+M_0(x)*tanh{\sqrt{[T_C(x)/T]^2-1}} for the magnetization in the ferromagnetic state. The best fits through all the data points yield M_0(x)= \sqrt(1-x)M_0(0), M_R(x)=\sqrt(x)M_0(0), and E_s(x)=E_s(0)(1-x)^4 for the Cu induced modifications of the Mn spins dominated zero-temperature spontaneous magnetization, the residual paramagnetic contribution, and spin-polaron energy, respectively, with E_s(0)=0.12 eV.

Motivation & Objective

• To understand the origin of the dramatic resistivity drop in Cu-doped La₂/₃Ca₁/₃MnO₃ despite minimal doping.
• To determine whether the resistivity change arises from structural, electronic, or magnetic modifications due to Cu substitution on Mn sites.
• To test if the resistivity behavior can be explained by a nonthermal coherent tunneling mechanism involving spin polarons.
• To extract the doping dependence of the spin-polaron tunneling energy Eσ(x) and magnetization components from resistivity data.

Proposed method

• Resistivity ρ(T,x) was measured over 20–300 K using the four-probe method on x = 0 and x = 0.04 samples.
• The resistivity data were fitted using the nonthermal tunneling expression ρ(T,x) = ρ₀e⁻ᵞᴹ²⁽ᵀ,ˣ⁾, where M(T,x) is the magnetization in the ferromagnetic state.
• The magnetization was modeled as M(T,x) = Mᵣ(x) + M₀(x)tanh{√[(T_C(x)/T)²−1]}, representing spontaneous and residual paramagnetic contributions.
• The model parameters ρ₀, γ, M₀(x), and Mᵣ(x) were extracted via global fitting to all data points.
• The spin-polaron tunneling energy Eσ(x) was derived from the correlation length L(M) ∝ 1/M², with Eσ(x) ∝ L⁻².
• The physical consistency of the model was validated by comparing predicted resistivity at T_C and T→0 with experimental data.

Experimental results

Research questions

• RQ1 How does 4% Cu doping on Mn sites affect the resistivity of La₂/₃Ca₁/₃Mn₁₋ₓCuₓO₃ in the ferromagnetic state?
• RQ2 Can the observed resistivity drop be explained by a nonthermal coherent tunneling mechanism of spin polarons?
• RQ3 What is the doping dependence of the spin-polaron tunneling energy Eσ(x), and how does it relate to the magnetization components?
• RQ4 Why does such a small amount of Cu cause a drastic resistivity change despite no observable structural changes?
• RQ5 Is the magnetization model M(T,x) = Mᵣ(x) + M₀(x)tanh{√[(T_C(x)/T)²−1]} consistent with the resistivity data across all temperatures and doping levels?

Key findings

• A 50% decrease in resistivity is observed upon 4% Cu doping, with no significant structural changes detected via XRD or microanalysis.
• The resistivity data are well fitted by the nonthermal tunneling model ρ(T,x) = ρ₀e⁻ᵞᴹ²⁽ᵀ,ˣ⁾ with M(T,x) = Mᵣ(x) + M₀(x)tanh{√[(T_C(x)/T)²−1]}.
• The residual paramagnetic contribution Mᵣ(x) scales as √x M₀(0), and the spontaneous magnetization M₀(x) scales as √(1−x) M₀(0).
• The spin-polaron tunneling energy decreases as Eσ(x) ≈ Eσ(0)(1−x)⁴, with Eσ(0) ≈ 0.12 eV and polaron size 2R ≈ 10 Å.
• The model predicts resistivity at T_C and T→0 with high consistency, confirming self-consistency of the interpretation.
• The results are consistent with Co-doping trends in similar manganites, supporting a universal role of transition metal doping in suppressing polaron tunneling barriers.