Interacting Stresses on Plants in a Changing Climate

Abstract

Fe­ [Co¯ Fe¯], if one considers the moment of the cobalt ion being reduced due to the stabilization of the lowest Kramers doublet. [16] The slow rise of the virgin magnetization is consistent with a pinning-type magnet with no reversible region. [2] However, the S-type increase with field of the initial (virgin) magnetization after ZFC is not that expected for a random distribution of super-paramagnets, and this anomaly may be a consequence of the presence of a mean dipolar field acting on the particles, which depends on the direction and magnitude of the magnetization of surrounding particles. [18] We, therefore , think that the large coercive field reflects the ªintrinsic anisotropy of the particles enhanced by the inter-particle di-polar fieldsº. To test this hypothesis, we examine a sample (7.5 % CoFe 2 O 4 ±SiO 2 , annealed at 800 C) consisting of 3.2 nm particles with varying separation between particles. These particles are super-paramagnetic at room temperature and the ZFC±FC magnetization measurements in 5 Oe indicate a blocking temperature of 55 K (see supplementary material S2, available from the author). The super-paramag-netism is exemplified by the absence of remanance magneti-zation and coercivity above 55 K, while a wide hysteresis loop with a remanance magnetization of 50 % of that at saturation is observed and the coercivity is 13.5 kOe at 2 K (see supplementary material S3, available from the author). We also note that the remanance in this case is as expected for the model of Stoner for a random oriented polycrystalline magnetic sample. [19] Further work is in progress on samples prepared at different dilution of spinel and different annealing temperatures between 800 C and 1100 C in order to elucidate the effective dipolar field as a function of size and inter-particle distances. We can only note presently that the size of the particle , its blocking temperature and coercive field decrease on lowering both the concentration and sintering temperature. The lack of evidence for the presence of c-Fe 2 O 3 in the composite does not rule out the model of Skomski and Coey for the enhancement of magnetic hardness in composites consisting of hard and soft magnetic materials. This remains to be verified. [20] We have observed an unusually high magnetic hardness, characterized by a 20 kOe coercive field at 2 K, for CoFe 2 O 4 particles of a size of 12 nm in amorphous SiO 2 prepared by the sol±gel method and annealed at 1000 C. The coercivity reflects the intrinsic anisotropy enhanced by dipolar fields. Its high value for smaller particles tends to confirm this hypothesis. Experimental The sample was prepared by the sol±gel method as follows: Co(NO 3) 2 ´6H 2 O (2.18 g) and Fe(NO 3) 3 ´9H 2 O (6.06 g) were dissolved in 4.5 g of water acidified with nitric acid (0.03 M) as a catalyst. To this solution tetraethoxysilane (TEOS, 10.6 g), dissolved in 7 g of methanol, and formamide (2.25 g), as a modifier, were added. After gelation (approximately 2 h at 40 C) and ageing (24 h), the sample was dried at 40 C for three days in a controlled atmosphere. The sample was then heated to 300/C in vacuum (rate 5 C min ±1) followed by grinding, heating to 1000 C in air (rate 1 C min ±1) and finally, annealed for 2 h. The material was characterized by use of a Siemens D500 X-ray powder dif-fractometer employing Co Ka1 (k = 1.789 Š) radiation and a TOPCON transmission electron microscope operating at 200 kV. A Quantum Design SQUID apparatus, a PAR VSM apparatus and a home-built Faraday pendulum-type balance were used to study the magnetic properties. Fe-Mössbauer spectra were recorded at different temperatures on a home-built spectrometer. Ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF) and poly[(vinylidene fluoride)-co-trifluorethylene] [P(VDF-TrFE)], [1] are very attractive for many applications COMMUNICATIONS