**1. Introduction**

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The magnetic field is an efficient tool for characterizing materials but also for elaborating them with a determined magnetic and crystallographic microstructure. The main effect of a magnetic field is to align crystallites embedded in the liquid along their easy magnetization axis [1,2]. The texturing is successful when the force exerted on the crystal gives rise to a rotation thanks to the presence of a liquid around the particle. The possibility to texture under a magnetic field is nowadays known and is still studied worldwide. However, in this paper, we aim to focus the attention on the role of the overheating above the liquidus temperature where our experiments and calculations show that nuclei are surviving above this temperature [3-7]. This assumption contradicts the classical nucleation model where intrinsic nuclei are not present above the melting temperature Tm and cannot act as growth nuclei while cooling down the temperature below Tm [8-10]. Furthermore, melts can usually be supercooled below their melting or liquidus temperatures (*T*m). The degree of supercooling (Δ*T*-), measured by the difference between the onset temperature of solidification and *T*m, is affected by various factors, including the level of overheating and the time. The relations between the overheating (Δ*T*+), and the supercooling (Δ*T*-) were studied in Sn and Sn-Pb [11].The dependence of (Δ*T+*) on (Δ*T*-) is a function of the holding time. It is well known that the cooling rate plays an important role in establishing the degree of supercooling since the nucleation of solid structures and thus solidification requires a certain period of time. Reversibly, melting also takes a certain period of time to destroy the order structures. Furthermore, it is worth noting that as long as some residual solid particles (nuclei) exist above the melting temperature, the energy barrier for the nucleation of crystallization during cooling can be reduced and thus the level of supercooling will be nil. As soon as the solid structures in the melt are completely destroyed, a substantial surface energy barrier exists for the nucleation of solid particles upon cooling. Consequently, each supercooling temperature can be associated with a nucleation time. In congruent material, such as Bi, only few crystals are obtained after an overheating of 80°C [1,7]. Then, the magnetic field can act on these remaining nuclei embedded in the liquid above the melting point.

© 2012 Tournier et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Tournier et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The magnetic field was successfully used to improve texturing in crystals and alloys. The texturing of several alloys under a magnetic field was carefully studied |12]. It depends on the magnetic properties of the crystallizing nuclei and those of the melt. The mechanical force moment allowing the rotation of crystals along the direction of the magnetic field also depends on the degree of homogeneity of the magnetic field. In a homogeneous field, the texture can be induced in a magnetic isotropic crystal due to the moment of forces deriving from the demagnetizing factor and the shape anisotropy of the nucleus. However, at high temperatures and for low susceptibility, this contribution will be neglected. On the contrary, in an anisotropic crystal, the orientation arises from the anisotropic crystal characterized by an difference of magnetic susceptibility Δχ along two mutually perpendicular axes and the moment of forces can be expressed as:

$$K = \frac{\Delta \chi}{2\mu\_0} B^2 V \sin 2\alpha \tag{1}$$

Magnetic Texturing of High-Tc Superconductors 173

the values of

is the hysteresis in the sample

0

0,5

1

1,5

2

**R**

2,5

3

3,5

4

are the values of the magnetization measured in an

disordering effects [15]. As mentioned previously, the anisotropy energy must also compensate the viscous force in the surrounding liquid where the crystal is free to rotate in a liquid with low resistance. The temperature window inducing a magnetic orientation lies between 1040°C and 1060°C under atmospheric pressure of O2. Below 1040°C, a solid matrix made up of Y1Ba2Cu3O7-δ (Y123) and Y2Ba1Cu1O5 (Y211) remains throughout the annealing treatment while the melting of Y123 above 1040°C leads to a liquid containing precipitates of Y211. Grains containing inclusions of this secondary phase are partially aligned below 1040 °C with their ab-plane perpendicular to the direction of the annealing field, indicating that a partial melting of the precursor takes place during processing. Both X-ray and magnetization reveal that a weak orientation occurs. For samples prepared at temperature above 1040°C, the texturing is very efficient. The sample is cut and oriented to reveal faces perpendicular to the direction of the magnetic field. X-ray spectrum is taken as a first indication of the orientation. A measure of the orientation P00l is given in Figure 1 as a

**Figure 1.** R and P versus annealing temperature in Y1Ba2Cu3O7-<sup>δ</sup> bulk textured sample, where

performed on oriented and unoriented samples respectively [16].

0

0,2

0,4

**P**

**006**

0,6

0,8

1

P R

perpendicular field. R=∆MH//HA/∆MH┴HA and ∆M=M+-M-

the magnetization measured in an increasing and decreasing field respectively) and P00l =1-Γ where Γ=(Ihkl/I00l)o/( Ihkl/**I**00l)u. Ihkl is the intensity of the strongest forbidden non-(00l)line and I00l the intensity of a (00l) line in a X-ray diffraction spectrum. The superscripts o and u indicate that measurements were

1020 1030 1040 1050 1060 1070 1080

**Annealing temperature (K)**

In Figure 1, one can note the sharp increase of the orientation for a temperature above 1040°C while the latter gradually decreases above 1050°C. The orientation is deduced from the comparison of R, the hysteresis in sample magnetization ratio between parallel and

is the hysteresis in the sample magnetization, M+ and M-

function of annealing temperature [16].

R=∆MH//HA/∆MH┴HA (∆M=M+-M-

magnetization where M+ and M-

where α is the angle between B and the axis with maximum χ. Under real conditions, there will be a competition between magnetic orientation forces, viscous forces, convective flows in the melt and interactions between crystals or interaction with the crucible. In some cases, the orienting effect may be limited or even negligible. The window in which the orientation can be induced must be carefully found. For an alloy, Mikelson and al. found that the temperature interval in which the orienting actions of the magnetic field are effective lies below the liquidus line and to 20% of the crystallization interval.

In this article, we will focus on the texturing of high-Tc superconductors under a magnetic field where the main conditions exposed below are taken into account. The main difficulties of superconductors texturing reside in a non congruent melting of the compounds. It is the limiting factor of overheating since the phase diagram is generally complex involving a lot of transformations even above the liquidus temperature. A large overheating above the liquidus will usually lead to the formation of unwanted secondary phases during crystallization and prevent the recombination of the superconducting phase. The latter is usually taken as the critical value for the limit of overheating. The texturing under a magnetic field consist of finding the interval of overheating temperatures preventing the transformation of superconducting-phase nuclei in secondary phases and allowing a sufficient amount of liquid. Usually, the allowable overheating is a dozen of degrees above the melting temperature [7]. As will be shown below, this amount of applied overheating does not destroy the presence of nuclei.
