Tuning magnetoresistance by mastered effects: chirality and trapped fields
Project Director: Dr. Anda-Elena Stanciu
Project Acronym: MR_Helix
Final registration code: PN-III-P1-1.1-PD-2019-1141
Project leader: Dr. Anda-Elena Stanciu
Mentor: Dr. Ioan Adrian Crisan
Project Category: National Projects
Start Date: 09/01/2020
End Date: 08/31/2022
Project leader: Dr. Anda-Elena Stanciu
Mentor: Dr. Ioan Adrian Crisan
Summary of the stage: The purpose of this stage consisted of preparing and studying the morpho-structural and magnetic properties of some intermetallic compounds of rare earth – transition metal (RE-TM) type, prepared as ribbons. This study provides relevant information in order to establish the appropriate systems that will be implemented as magneto-resistive structures, which will be investigated depending on their linear or helical disposal. In order to optimize their magnetic properties, intrinsic methods (elemental composition, concentration) and afterwards extrinsic methods (dimensionality, disposal, chirality) are being considered. In RE-TM systems with ferrimagnetic coupling of RE and TM sub-lattices, the control of the intrinsic magnetic properties is facilitated by the magnetism of the rare earth RE elements, determined by the 4f orbitals localized in the vicinity of the nucleus on a distance of 10% of the atomic radius.
Scientific and technical description: The scope of this stage was reached by preparing and studying magnetic properties of intermetallic ribbons of the RE-TM type for different compositions and concentrations crossing the magnetization compensation point (the concentration for which the magnetizations corresponding to the two sub-lattices, antiferromagnetically coupled, are approximately equal). Dy has speromagnetic spin structure (L ≠ 0), and the exchange interactions Fe-Dy lead to the formation of an asperomagnetic spin structure. The magnetization compensation point was estimated by considering a magnetic moment of Dy less than the theoretic one (12.5 µB) in order to take into account the non-collinearity of the spin structure. Thus, the Fe79Dy21 and Fe65Dy35 systems with a dominant Fe, respectively Dy magnetic lattice have been considered. Gd is an isotropic ion (L = 0) having ferromagnetic spin structure. The coupling between Fe and Gd is antiferromagnetic, resulting in a linear spin structure that creates a magnetization compensation point. The magnetization compensation point was estimated using the theoretical magnetic moments of the isolated atoms of Fe (2 µB) , respectively Gd (7 µB) obtained with Hund rules. The system of Fe-Gd ribbons considered in this stage is Fe70Gd30 with Gd dominant lattice. For comparison, a system of TM-TM-RE ribbons: Fe78Ni17Gd3B2 with concentration of Fe similar to RE-TM systems was prepared.
The ribbons were prepared by rapid solidification in Ar protective atmosphere. In order to ensure a good homogeneity of the samples, pre-alloys of the component elements were realized in a first step by the electric arc method. The pre-alloys were re-melted in induction and ultra-rapid cooled on a rotating Cu drum in a second step. The ultra-rapid melting promotes the attainment of room temperature phases with non-equilibrium morpho-structural properties specific exclusively to high temperatures. The preparation parameters are detailed in Table 1.
The peaks identification of the diffractograms registered on Fe65Dy35, Fe70Gd30 and Fe78Ni17Co3B2 (presented in Figure 1) was done using the DIFFRAC.EVA software . The Fe65Dy35 system presents a cubic crystalline structure (DyFe2) with constant lattice of 7.323 Å. Fe70Gd30 and Fe78Ni17Gd3B2 present both amorphous and crystalline structures. The crystalline phase identified in the case of Fe70Gd30 (GdFe2) presents a cubic structure and a lattice constant of 7.39 Å The crystalline phase identified in the case of Fe78Ni17Co3B2 (Fe0.72Ni0.28) is a solid solution with cubic structure and a lattice constant of 2.86 Å. Optical microscopy images show a rough surface of the ribbons and a texturing tendency in the longitudinal direction of the ribbons.
The magnetic characterization of the ribbons was done by SQUID (Superconducting Quantum Interference Device) magnetometry. The hysteresis curves of the Fe65Dy35, Fe79Dy21 and Fe78Ni17Co3B2 samples and the thermo-magnetic curves are illustrated in Figure 2.
The hysteresis curves close at different values of the applied field of the order of 104 Oe depending on the temperature. Also, the tilted shape of the hysteresis curves (similar to parallelograms shape) for the Fe65Dy35 si Fe79Dy21 ribbons suggests the non-collinearity of magnetic moments and the existence of two magnetic phases with different coercive field: 5 kOe and 28 kOe at 10 K for Fe65Dy35, and 2 kOe and 28 kOe at 10 K for Fe79Dy21. The different magnetic phases and the non-collinearity of the magnetic moments can be attributed to some morpho-structural local environments with different TM/RE concentrations. The hysteresis curves of Fe78Ni17Gd3B2 ribbons show the soft-magnetic properties of the alloy. The curves saturate in an applied field of 5kOe. The saturation magnetization is approximately 180 emu/g. The coervitive field, less than 0.1 kOe, varies insignificantly with the temperature. The hysteresis curves registered at different temperatures indicate a decay of the magnetic moment with temperature in the (10 K – 300 K) interval, for the Fe65Dy35 ribbons and an evolution with the temperature of the magnetic moment which presents a maxima at approximately 50K for the Fe79Dy21 ribbons. The temperature dependency of the net magnetic moment in RE-TM intermetallic compounds can be explained depending on the concentration and by the temperature dependency of the magnetization of RE, respectively TM sub-lattices .
The local atomic structure and the magnetic interactions in the Fe79Dy21 ((I) a) and Fe78Ni17Gd3B2 were investigated using Transmission Mössbauer Spectroscopy (TMS) at different temperatures in the (6 K – 300 K) interval. The spectra recorded at 300 K for Fe79Dy21 and Fe78Ni17Gd3B2 ribbons are illustrated in figure 3 (I) (a) and (b), respectively. The evolution of the hyperfine parameters with temperatures is shown in figure 3 (II).
The transmission Mössbauer spectra were fitted with a hyperfine field distribution whose envelope includes contributions from both amorphous and crystalline components. The spectra present wide lines that overlap at room temperature for the Fe79Dy21 ribbons, as opposed to the case of the Fe78Ni17Gd3B2 ribbons. The hyperfine field distribution is wider in Fe79Dy21 than in Fe78Ni17Gd3B2. The maximum value of the hyperfine field is lower than that of metallic Fe in Fe79Dy21, as a consequence of the polarization of the s electrons of Fe either by its own magnetic moment, or by the neighboring magnetic moments. The maximum value of the hyperfine field is larger than that of metallic Fe in Fe78Ni17Gd3B2 indicating an increased localization of the electrons around the Fe nucleus as a consequence of the change of the interatomic distance of Fe in the intermetallic compound. The IS values are reported relative to the metallic Fe. The isomer shift corresponds to amorphous Fe in Fe78Ni17Gd3B2  and has a value similar to that of metallic Fe in Fe79Dy21, specific to intermetallic alloys RE-TM . The A23 parameter demonstrates the existence of a magnetic phase oriented out of the ribbons plane.
Conclusion: RE-TM ribbons with different compositions and concentrations crossing the magnetization compensation point were prepared, and the morpho-structural, magnetic and local atomic structure properties were studied, realizing thus the scope of the stage. In order to obtain magneto-resistive structures, systems with in-plane magnetic anisotropy are sunt necesare sisteme cu anizotropie magnetica in plan. Systems with in-plane magnetic anisotropy are the most promising for their implementation in magneto-resistive devices. In the next stage the shape anisotropy will be exploited in order to induce in-plane magnetic anisotropy in wires with different compositions, diameters of µm order and lenghts of mm order.
 Stanciu, A. E., et al. (2020). Unexpected magneto-functionalities of amorphous Fe-Gd thin films crossing the magnetization compensation point. Journal of Magnetism and Magnetic Materials, 498, 166173.
 G. Xiao, C. L. Chien, J. Appl. Phys., 61 8 (1987)
 Greenwood, N. N. (2012). Mössbauer spectroscopy. Springer Science & Business Media.
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