Positive temperature coefficient of magnetic anisotropy in polyvinylidene fluoride (PVDF)-based magnetic composites.
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We ascribe the enhanced magnetic anisotropy of the magnetic film at elevated temperature to the strain-induced anisotropy resulting from the anisotropic thermal expansion of the β-phase PVDF.The simulation based on modified Stoner-Wohlfarth model and the ferromagnetic resonance measurements confirms our results.The present results may help to design magnetic devices with improved thermal stability and enhanced performance.
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Affiliation: Key Laboratory of Magnetic Materials and Devices &Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo 315201, People's Republic of China.
ABSTRACT
The magnetic anisotropy is decreased with increasing temperature in normal magnetic materials, which is harmful to the thermal stability of magnetic devices. Here, we report the realization of positive temperature coefficient of magnetic anisotropy in a novel composite combining β-phase polyvinylidene fluoride (PVDF) with magnetostrictive materials (magnetostrictive film/PVDF bilayer structure). We ascribe the enhanced magnetic anisotropy of the magnetic film at elevated temperature to the strain-induced anisotropy resulting from the anisotropic thermal expansion of the β-phase PVDF. The simulation based on modified Stoner-Wohlfarth model and the ferromagnetic resonance measurements confirms our results. The positive temperature coefficient of magnetic anisotropy is estimated to be 1.1 × 10(2) J m(-3) K(-1). Preparing the composite at low temperature can enlarge the temperature range where it shows the positive temperature coefficient of magnetic anisotropy. The present results may help to design magnetic devices with improved thermal stability and enhanced performance. No MeSH data available. Related in: MedlinePlus |
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Mentions: On the other hand, we consider the contribution from the anisotropic thermal expansion of β-phase PVDF. It is a non-conjugated linear fluorinated hydrocarbon with a local polarization pointing from fluorines (F) to hydrogens (H) on each monomer (Figure 1a)3334. A tensile strain is applied along x direction during the preparation, the zigzag carbon chains are formed along the same direction3435. Therefore, the dipole chain-chain interaction along y direction is weaker than the carbon atomic interaction along x direction36, and the thermal expansion coefficient along y direction (α2 = −145 × 10−6 K−1) is larger than that along x direction (α1 = −13 × 10−6 K−1)37. An in-plane anisotropic strain can be generated in PVDF by changing temperature as shown in Figure 1a. The anisotropic strain is transferred from PVDF to the magnetostrictive film resulting in a strain-induced anisotropy. The Mr/Ms difference between two directions increases with increasing (decreasing) temperature above (below) 298 K (sample preparation temperature). Note that the easy axis changes direction around 298 K. To further understanding the phenomenon, we perform simulations based on the modified Stoner-Wohlfarth model1338. The total free energy F of CoFeB film can be written as: F = −KTcos2(θ-δ)-MHcos(θ-φ), where KT is the uniaxial magnetic anisotropy induced by the thermal field through anisotropic thermal expansion effect, δ is the angle between the average easy axis and x/y direction considering the distribution of the uniaxial magnetic anisotropy (Figure 5a), θ is the angle between the magnetization M and x direction. Thus, KT = 3εTλSEf/2(1 − ν2), where the thermal induced strain εT is εT = (α1 − α2)ΔT, λS is the magnetostriction constant of amorphous CoFeB film estimated to be 35 ppm39, Young's modulus Ef of CoFeB film is about 162 GPa39, Poisson ratio ν of CoFeB film is chosen to be 0.3, a typical value for metals40. When decreasing or increasing the temperature, effective tensile or compressive strain is generated along y direction due to the anisotropic thermal expansion of PVDF. Therefore, the initial distribution of the magnetizations is squeezed along y and x direction for decreasing and increasing temperature, respectively (Figure 5a). When ΔT = 0 K, the magnetic moments orient randomly due to the distribution of the magnetic anisotropy. Therefore, the angle δ is chosen to be 45° when ΔT = 0 K. The angle δ is estimated to decrease 5° when ΔT changes by 6 K. According to the above hypothesis, we can simulate the magnetic hysteresis loops at different temperatures as shown in Figure 5b and 5c. The simulated results match very well with the experimental observation (Figure 5d), indicating an enhanced magnetic anisotropy with increasing temperature. |
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Affiliation: Key Laboratory of Magnetic Materials and Devices &Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo 315201, People's Republic of China.
No MeSH data available.