The influence of magnetic field orientation on creep deformation:

azimuthal angle 𝜽

In this subsection the influence of the magnetic field orientation within a plane perpendicular to the stress field is analyzed. Such perpendicular plane to the direction of the mechanical stress is characterized by the constrain 𝜙 = 0, as defined in Chapter 3. The orientation within that plane can be described by the angle 𝜃, which ranges from 𝜃 = 0 in the case that the magnetic field is applied along the width of the sample, to 𝜃 = 90 which represents a magnetic field applied along the thickness of the ribbon. Figure 5.27 depicts schematically the magnetic field orientation during the experiments described in this subsection.

Figure 5.27 Schematic view of the magnetic field orientation with respect to the mechanical stress and ribbon axes used in the experiments described in the current subsection

In order to explore the influence of the angle 𝜃, a set of creep measurements were performed keeping constant the stress, temperature, and intensity of magnetic field (^T

T_g= 0.8, 𝜎 = 15 MPa, |𝐻⃗⃗⃗| = 500 Oe) and varying such angle.

Figure 5.28 (a) displays the creep curves for this set of experiments and Figure 5.28(b) present the waiting time distributions of the creep curves shown in Figure 5.28(a). A power law regime can be appreciated for each of the experiments, as well as a higher slope for short waiting times.

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Figure 5.28 (a) Creep measurements of 2605SA1 performed at _T^T

g= 0.8, 𝜎 = 15 MPa, |𝐻⃗⃗⃗| = 500 𝑂𝑒, 𝜙 = 0) for different angles 𝜃. (b) Double logarithmic representation of the waiting time distribution calculated from the creep measurements shown in (a)

Figure 5.29 shows the waiting time distribution splitting the data between before and after the crossover time 𝑡_{𝑐𝑟𝑜𝑠𝑠}. It can be seen in Figure 5.29 (a)-(e) that there is a significant change of slope at 𝑡_{𝑐𝑟𝑜𝑠𝑠}, but the power law shape is well preserved in each of the cases. Figure 5.29(f) shows the fit of the experimental power laws. The experimental exponents before (𝜏₁) and after (𝜏₂) the crossover oscillate around the values 𝜏₁= −1.5 ± 0.2 and 𝜏₂= −0.8 ± 0.2 respectively.

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Figure 5.29 Waiting time distributions before and after the crossover time, for the creep measurements of 2605SA1 performed at _𝑇^𝑇

𝑔= 0.8, 𝜎 = 15 MPa, |𝐻⃗⃗⃗| = 500 𝑂𝑒, 𝜙 = 0) for different angles 𝜃. (a) 𝜃 = 0. (b) 𝜃 = 20°. (c) 𝜃 = 45°. (d) 𝜃 = 60°. (e) 𝜃 = 90°. (f) Best fit of the power law exponents before and after the crossover calculated from the curves (a)-(e), the coloured areas represent the intervals 𝜏₁= −1.5 ± 0.1 and 𝜏₂= −0.8 ± 0.1.

The three figures of merit, 𝑡_{𝑐𝑟𝑜𝑠𝑠}, 𝑡_𝜀̇ and 𝑊 are shown in Figure 5.30. It can be observed how all of them decrease their magnitude as the angle shifts from 𝜃 = 0 towards 𝜃 = 90°. That change in the angle 𝜃 corresponds to a rotation from the width direction towards the thickness direction of the

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sample. Since the anisotropy along the out-of-plane direction is much higher than along the sample width, a decrease in the magnetic field effect is expected as the angle approaches 𝜃 = 90°. These results are discussed further in the chapter 6.

Figure 5.30 (a) Dependence of 𝑡_{𝑐𝑟𝑜𝑠𝑠} and 𝑡_𝜀̇ with the angle 𝜃 for creep experiments performed on 2605SA1 at ,_𝑇^𝑇

𝑔= 0.8, 𝜎 = 15 MPa , |𝐻⃗⃗⃗| = 500 Oe, 𝜙 = 0).(b) 𝑊 as a function of the angle 𝜃 for creep experiments performed on 2605SA1 at , ^𝑇

𝑇_𝑔= 0.8, 𝜎 = 15 𝑀𝑃𝑎 , |𝐻⃗⃗⃗| = 500 𝑂𝑒, 𝜙 = 0

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5.2.6 The influence of magnetic field orientation on creep deformation:

polar angle 𝝓

The influence of the angle between the magnetic and stress field is analyzed in this subsection. In order to do that, the angle 𝜃 is fixed to 𝜃 = 0, and the angle 𝜙 between the stress and magnetic field is swept from experiment to experiment making use of a magnet holder feed with a couple of AlNiCo cylindrical magnets. Thus, the set of creep measurements shown below were performed keeping constant the stress, temperature, and intensity of magnetic field (^T

T_g= 0.8, 𝜎 = 15 MPa, |H⃗⃗⃗| = 120 𝑂𝑒) and varying the angle 𝜙. Figure 5.31 illustrates schematically the orientation of the magnetic field used during the experiments shown in this subsection.

Figure 5.31 Schematic view of the magnetic field orientation with respect to the mechanical stress and ribbon axes used in the experiments shown in the current subsection

Figure 5.32 (a) displays the creep curves for this set of experiments. Although the elastic deformation varies between each measurement, no clear change in the slope during the anelastic deformation can be appreciated from a first inspection of that figure. Figure 5.32(b) presents the waiting time distributions of the creep curves shown in Figure 5.32(a).The waiting time distribution decay in every case with a power law fashion, and a crossover can be appreciated for short waiting times.

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Figure 5.32 (a) Creep measurements of 2605SA1 performed at _𝑇^𝑇

𝑔= 0.8, 𝜎 = 15 MPa, |𝐻⃗⃗⃗| = 120 𝑂𝑒, 𝜃 = 0) for different angles 𝜙. (b) Double logarithmic representation of the waiting time distribution calculated from the creep measurements shown in (a)

Figure 5.33 shows the waiting time distribution splitting the data between before and after the crossover time 𝑡_{𝑐𝑟𝑜𝑠𝑠}, for some of the creep measurements presented in Figure 5.32. Figure 5.33(a)-(e) show a significant change of slope at 𝑡_{𝑐𝑟𝑜𝑠𝑠}, and a power law shape which is well preserved in each of the cases. Figure 5.33.(f) shows the fit of the experimental power laws. The exponents of the waiting time distributions after the crossover (𝜏₂) have all values in the range 𝜏₂= −0.8 ± 0.1.

In the case of the first regime, the experiments performed at angles between 𝜙 = −45,45 show exponents in the range 𝜏₁= −1.5 ± 0.1. However, as the magnetic field approaches a parallel orientation with respect to the stress (𝜙 = ± 90 − 80), there is a significant shift of the exponents 𝜏₁, which approach values from 𝜏₁ = −1.8 up to − 2.

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Figure 5.33 Waiting time distributions before and after the crossover time calculated from creep measurements of 2605SA1 performed at _𝑇^𝑇

𝑔= 0.8, 𝜎 = 15 𝑀𝑃𝑎, |𝐻⃗⃗⃗| = 120 𝑂𝑒, 𝜃 = 0) for different angles 𝜙 . (a) 𝜙 = −90 Oe. (b) 𝜙 =

−60°. (c) 𝜙 = −30°. (d) 𝜙 = 30°. (e) 𝜙 = 90°. (f) Power law exponent fit before and after the crossover calculated from the curves (a)-(e). Coloured areas represent the intervals 𝜏₁ = −1.5 ± 0.1 𝑎𝑛𝑑 𝜏₂= −0.8 ± 0.1.

The evolution of the three figures of merit, 𝑡_{𝑐𝑟𝑜𝑠𝑠}, 𝑡_𝜀̇ and 𝑊 with the angle 𝜙 is shown in Figure 5.34. Despite the fluctuation of the data, it can be observed in Figure 5.34.(a) that the crossover times approach minimum values when the stress and magnetic field are parallel aligned

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(|𝜙| → 90°). The rest of the values of 𝑡_{𝑐𝑟𝑜𝑠𝑠} show a small 𝜙-dependence, and fluctuate around an average value. That average value increases for the values 𝜙 = 30, −60 in which the crossover time is increased. The values of 𝑡_𝜀̇ follow a similar tendency, although in this case the dispersion in the data is bigger.

The evolution of 𝑊 shown in Figure 5.34(b) share some features with the evolution of 𝑡_{𝑐𝑟𝑜𝑠𝑠}(𝜙) shown in Figure 5.34(a). 𝑊 oscillates around a mean value for all the experimental conditions except for 𝜙 = −45, 60 in which two sharp peaks can be distinguished. Such behavior is discussed in terms of the magnetoelastic coupling and the orientation of domain walls with respect to the Shear Transformation Zones in Chapter 6.

Figure 5.34 (a) Dependence of 𝑡_{𝑐𝑟𝑜𝑠𝑠} and 𝑡_𝜀̇ with the angle 𝜙 for creep experiments performed on 2605SA1 at ,^𝑇

𝑇𝑔= 0.8, 𝜎 = 15 MPa , |𝐻⃗⃗⃗| = 120 𝑂𝑒, 𝜃 = 0). (b) 𝑊 as a function of the angle 𝜃 for creep experiments performed on 2605SA1 at ,_𝑇^𝑇

𝑔= 0.8, 𝜎 = 15 𝑀𝑃𝑎 , |𝐻⃗⃗⃗| = 120 Oe, 𝜃 = 0)

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5.3 Stress-strain measurements of magnetic