THE PROBLEM OF TWO MAGNETIC CORRELATION
LENGTHS IN INVAR FENI ALLOYS ABOVE Tc
REVIEW OF THE RECENT EXPERIMENTS
S.V.Grigoriev1, S.A.Klimko1, A.I.Okorokov1, V.V.Runov1, W.H.Kraan2, M.Th.Rekveldt2
1 Petersburg Nuclear Physics
Institute, Gatchina, St-Petersburg 188350, Russia
2 Interfacultair Reactor Instituut,
TU-Delft, 2629 JB Delft, The Netherlands
E-mail: grigor@rvv.lnpi.spb.su
We investigate a disordered magnetic system of the invar FeNi alloys undergoing the ferromagnetic phase transition using neutron depolarization (ND) and small angle neutron scattering (SANS) technique.
Our experiments (PNPI, Russia) [1-2] had shown a strong growth of the neutron depolarization within paramagnetic temperature range, as temperature approaches to Tc, in the samples of invar FeNi alloys: Fe75N25 (0.7 at.\% C), Fe70N30 (0.1, 0.7 at.\% C) and Fe65Ni35. We attribute the enhancement of the neutron depolarization to appearance of the large magnetic inhomogeneities with size about 104 A in the system. Besides usual critical fluctuations (with radius Rc about 100 A) are observed at the same temperatures by means of small angle neutron scattering. It was found that smaller critical correlations are well described by the Ornstein - Zernike expression while the larger correlations could be roughly modeled by the "squared Ornstein - Zernike" formula, which corresponds to inhomogeneities with relatively homogeneous magnetization inside them.
The SANS study in the wide range of the momentum transfer (GKSS, Germany) has confirmed the fact of coexistence of two length scale of correlations above Tc of invar FeNi alloy. It is found that there are two different ranges of momentum transferred where magnetic cross section above Tc is described by different laws of scattering q-n. In the ranges of q <= q0 the power n is close to 4 while in the range q >= q0 one is close to 2. The best fit of the data over the q-range by the sum of expressions of Ornstein - Zernike and "squared" Ornstein - Zernike lets us obtain both inverse correlation radius and their temperature dependencies.
The magnetization and magnetic anisotropy of FeNi alloys near Tc were investigated by means of three-dimension analysis of the neutron polarization as function of temperature and external magnetic field (IRI TUDelft, The Netherlands). The study of the magnetic anisotropy showed that the magnetic system of the alloys is easily magnetized by a weak magnetic field (1-10 Oe) in the temperature range [Tc, T0] as compared with ferromagnetic (T < Tc) and paramagnetic (T0 < T) temperature ranges. The temperature dependencies of the magnetization was interpreted using the probability function r (Tc) which describes the fluctuations of the Curie temperatures over the system. The probability function r (Tc) is well described by Gauss law besides a dispersion of the function is about 1-2 % of average Curie temperature over the system for different alloys.
We assume that an intrinsic disorder of the invar alloys leading to fluctuation of the Tc results in appearance of the second large scale above average <Tc>. We conclude that properties of the magnetic system in this range [Tc, T0] is determined by the probability function of r (Tc) and a dipol - dipol interaction between appearing magnetic inhomogeneities.