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

The scope of the project is to tune magnetoresistive (MR) properties in soft magnetic
systems by stress anisotropy, chirality effects and, subsequently, by an indirect coupling between the
electrical resistance and the spin structure, via trapped field magnets in superconductors.

Motivation. Modern research efforts in perfecting MR-based technologies are directed towards
new materials with large spin polarization, controllable magnetization and magnetic anisotropy. The
magnetic anisotropy is a feature that confers different magnetic properties at different orientations of an
external applied magnetic field. Therefore, the key parameters to control the desired magnetic properties
for technological applications are related to components of the magnetic anisotropy: crystallinity, shape
and stress. Methods to optimize and control the magnetic anisotropy and magnetic properties can be
established via intrinsic (elemental composition, magnetostriction) and extrinsic (dimensionality,
disposal, chirality) effects. Taking into account the limited possibilities to adjust MR by elemental
composition, concentration, morpho-structural aspects and the fact that they have been already explored, extrinsic effects should be also approached.

The first objective is to tune the MR by rolling-up the ribbons in helices and to study the influence
of the electrical magneto-chiral effect and of the stress anisotropy on the magneto-resistive properties of
the ribbons. When electrical conductors are chiral, the resistance strongly depend on the current through,
on the chirality and/or on the external magnetic field, all these parameters defining the Electrical
Magneto-Chiral Anisotropy (EMCA). A current carried by a chiral conductor will generate a
magnetic field along the helicity, equivalent to a magnetization which direction depends on the direction
of the current and the handedness of the conductor. When an external magnetic field is applied, it couples
to the internal magnetization and affects the MR by its direction. Therefore, EMCA anisotropy can be
used to tune electromagnetic properties.

The second objective is to induce magnetic anisotropy in magnetic (ferrimagnetic) thin films and
curled ribbons using a magnetized superconductor acting as a trapped field magnet. In the case of thin 
films, the magnetic (ferrimagnetic) film will be deposited over a superconducting substrate with an intermediary insulating layer, whereas in the case of ribbons, they will be curled over a superconducting
wire covered with an insulator. The induced anisotropy will be evidenced via magnetic and magneto-
resistive like behaviour in both cases. To the best of the project leader knowledge, this indirect MR
approach has not been considered in previous studies. Thus, an innovative method of indirect tuning of
the spin structure (via chirality effects or trapped fields in superconductors) with influence on MR
phenomena is highlighted.

Project leader: Dr. Anda-Elena Stanciu

Mentor: Dr. Ioan Adrian Crisan

Summary of stage 1: 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.

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 [1]. 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.

Figure 1

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.

Figure 2

The hysteresis curves close at different values of the applied field of the order of 10Oe 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 Fe78Ni17Gd3Bribbons 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 [2].

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 Fe78Ni17Gd3Bribbons are illustrated in figure 3 (I) (a) and (b), respectively. The evolution of the hyperfine parameters with temperatures is shown in figure 3 (II).

Figure 3

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 [3] and has a value similar to that of metallic Fe in Fe79Dy21, specific to intermetallic alloys RE-TM [4]. 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.

References:

[1] https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/xrd-software/eva.html

[2] 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 Materials498, 166173.

[3] G. Xiao, C. L. Chien, J. Appl. Phys., 61 8 (1987)

[4] Greenwood, N. N. (2012). Mössbauer spectroscopy. Springer Science & Business Media.

Summary of stage 2:

Varying elemental composition or concentration were considered in order to optimize the magnetic properties. Afterwards extrinsic methods like changing dimensionality, disposition or chirality. In RE-TM ferimagnetic systems, control of intrinsic magnetic properties is facilitated by rare earth’s elements magnetism, which in turn is determined by 4f orbitals. Intermetalic compounds RE-TM with B in excess were prepared in order to obtain homogeneous structure systems for smoother control of magnetic properties. Micrometric wires or ribbons were prepared by ultra fast solidification. Their morpho-structural and magnetic features were studied. In-plane magnetic anisotropy is required for obtaining magneto-resistive structures. We tried to induce magnetic anisotropy in direction of current flow taking advantage of shape anisotropy. That was realised by taking into consideration intermetallic wires with composition similar to that of ribbons and with radius in the micrometer range. The wire’s shape anisotropy is higher than that of ribbon counterpart. The magneto-resistive properties of micrometer wires with linear or helical shape were studied.

We considered multi-layered superconductor/insulator/magnetic film systems in order to obtain an indirect control of the MR properties via the magnetic field trapped by the superconductor. Superconducting layers were prepared using spark-plasma sintering. Insulating layers and magnetic layer were deposited by magnetron sputtering. Morpho-structural and magnetic properties of both superconducting layers and multi-layered systems were studied.

Results:

The influence of the magneto-chiral effect on the magneto-resistive properties of magnetic microwires was studied. The MR effect is larger in helical systems than in linear counterparts possibly due to magnetostrictive effects and magneto-chiral anisotropy. Superconducting layers were prepared that can trap magnetic field of 1 T (verified by SQUID magnetometry). An insulator layer was deposited over the superconducting bulk in order to ensure the MR characterization. The uniformity of the insulating layer is supported by SEM images.

 

 

C. Locovei, N. Iacob, G. Schinteie, A. E. Stanciu, A. Leca, V. Kuncser, "Tuning the magnetic properties of amorphous Fe-Gd thin films by variation of thickness and composition." Hyperfine Interactions 242.1 (2021): 1-12; https://doi.org/10.1007/s10751-021-01763-1


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