NiO薄膜の磁気交換誘起フォノン分裂に及ぼすひずみの影響

Strain effect on magnetic-exchange-induced phonon splitting in NiO films.

• Alireza Kashir
• Veronica Goian
• Daseob Yoon
• Byeong-Gwan Cho
• Yoon Hee Jeong
• Gil-Ho Lee
• Stanislav Kamba

抄録

パルスレーザー堆積法を用いてSrTiO(STO)とMgO基板上に様々なひずみを持つNiO薄膜を成長させた。薄膜の特性は、X線回折,原子間力顕微鏡,赤外反射分光法を用いて評価した。STO(001)基板上に成長した膜は圧縮面内ひずみを示し、膜厚を薄くするとNiOのフォノン周波数が増加することがわかった。一方、MgO(001)基板上に成長したNiO膜では、フォノン周波数の軟化を誘導する引張ひずみが検出された。面内ひずみの-0.36%(圧縮)から0.48%(引張)への変化は、原子間距離の∼1%の変化に起因して、フォノン周波数が409.6cmから377.5cmに減少することを示した。3つの異なる試料における磁気交換駆動フォノン分裂Δωは、緩和した(すなわちゼロ)ひずみ, 0.36%圧縮ひずみ及び0.48%引張ひずみで、温度の関数として測定された。以前に発表されたバルク結晶の研究と同様に、NiO緩和膜では冷却時にΔωが増加することがわかった。0.48%引張歪膜でも冷却時に分裂は増加するが、Δωは緩和膜よりも3-4cm小さくなる。フォノンの分裂は非支配的な磁気交換相互作用Jに比例するので、引張歪膜でのフォノン分裂の減少は、格子膨張に伴うJの減少によって説明される。また、冷却時のΔωの増加は、温度低下に伴うJの上昇によっても説明できる。

NiO thin films with various strains were grown on SrTiO (STO) and MgO substrates using a pulsed laser deposition technique. The films were characterized using an x-ray diffraction, atomic force microscopy, and infrared reflectance spectroscopy. The films grown on STO (001) substrate show a compressive in-plane strain which increases as the film thickness is reduced resulting in an increase of the NiO phonon frequency. On the other hand, a tensile strain was detected in the NiO film grown on MgO (001) substrate which induces a softening of the phonon frequency. Overall, the variation of in-plane strain from -0.36% (compressive) to 0.48% (tensile) yields the decrease of the phonon frequency from 409.6 cm to 377.5 cm which occurs due to the ∼1% change of interatomic distances. The magnetic exchange-driven phonon splitting Δω in three different samples, with relaxed (i.e. zero) strain, 0.36% compressive strain and 0.48% tensile strain, was measured as a function of temperature. The Δω increases on cooling in NiO relaxed film as in the previously published work on a bulk crystal. The splitting increases on cooling also in 0.48% tensile strained film, but Δω is systematically 3-4 cm smaller than in relaxed film. Since the phonon splitting is proportional to the non-dominant magnetic exchange interaction J , the reduction of phonon splitting in tensile-strained film was explained by a diminishing of J with lattice expansion. Increase of Δω on cooling can be also explained by rising of J with reduced temperature.