Directional Magnetic Modification of Iron Rhodium Compound by Ion Irradiation and Annealing
2019 Volume 60 Issue 3 Pages 476-478
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2019 Volume 60 Issue 3 Pages 476-478
Depth-directional magnetic modification of FeRh samples by energetic ion beam irradiation and post-annealing and its quantitative analysis are discussed. Iron-rhodium foil samples were irradiated with a 2-MeV He ion beam at various ion fluences. Post-thermal annealing at 373 K–673 K was performed for the irradiated samples. The depth-directional magnetic profiles were quantitatively estimated through the TRIM ion transport simulation, as well as previously known relations between irradiation-induced ferromagnetic moment and elastic deposition energy by ion beam irradiation. The magnetic profiles were considered to be successfully estimated because the values of the total magnetic moment measured are very consistent with the results calculated by TRIM. The effect of thermal annealing and possibility of three-dimensional magnetic modification will be also discussed.
Iron-rhodium alloy with B2 (CsCl type) crystal structure has been intensively studied because of its unique magnetic behavior of the first-order phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) near room temperature.1–3) In our previous studies, we revealed that energetic ion beam irradiation induced a FM state below room temperature, where the AFM state was originally stable.4,5) In addition, further ion irradiation of FeRh caused a structural phase transition to a high-temperature random fcc (A1) phase, which exhibited a paramagnetic (PM) nature.6) It was also revealed that the amplitude of the ion-beam-induced magnetizations significantly correlated with the density of energy deposited through elastic collisions between ions and the samples.7) Furthermore, it is worthwhile to note here that annealing of the ion-beam-irradiated FeRh samples causes a magnetic phase transition from the PM to FM and AFM states, which is behavior totally opposite that of the ion beam irradiation.8,9)
Since the deposition energy by ion beam irradiation can be estimated by the TRIM (TRansportation of Ion in Material) simulation using the Monte Carlo method, it is possible to design the magnitude of the magnetic moment induced in FeRh.10) It should be also noted that the deposition energy through the elastic collisions varies along the penetration direction, which enables us to modify the magnetic nature in the depth direction in FeRh bulk or thick film samples.
Based on the concept mentioned above, we thought that a magnetic layered structure, such as FM/PM/FM/AFM, could be produced in bulk or foil FeRh samples by designing a suitable selection of ion beam irradiation and annealing conditions. In the present study, we have tried to design the depth-directional profile and clarify the control method for depth-directional magnetic modification of FeRh by high-energy ion irradiation and post-thermal annealing.
Bulk foil samples of Fe50Rh50 about 100 µm thick were annealed at 1,073 K for 4 h in a vacuum for homogenization in the present study. It was confirmed that the unirradiated sample entirely exhibited an AFM nature at 5 K. The samples were irradiated with 2-MeV He ions in the fluence range of 1.5 × 1014∼1.5 × 1016 ions/cm2 at room temperature by using the ion beam facilities at QST-Takasaki. After ion-beam irradiation, the samples were post-annealed at temperatures ranging between 373 K and 673 K for 1 h in vacuum. Then, magnetic properties were characterized by a superconducting quantum interference device (SQUID) magnetometer.
Figure 1 shows the depth profile of the deposition energy through typical elastic collisions in the case of 2-MeV He ion irradiation for FeRh, as estimated using the TRIM code.10) This figure reveals that the accelerated He ions enter the FeRh sample and deposit small amounts of elastic collision energy in the surface region. Then, the ions gradually lose their energy, that is, their velocity, while large amounts of elastic collision energy are deposited around 2.5–3.2 µm from the surface. Finally, the ions are accumulated at region around 3.2 µm from the surface. At the same time, the relative relationship between the deposition energy by elastic collision and irradiation-induced magnetization has been already been demonstrated in previous work on FeRh thin films.7) By using these two relations, it is possible to simply estimate the depth profile of the magnetic moment for a finite irradiation ion fluence. The depth profile of the ion-beam-induced magnetization irradiated with 2-MeV He ions for a typical ion fluence of 1.5 × 1016 ions/cm2 is shown in Fig. 2. It is essential that the calculated value of the integral of the magnetization along the depth direction was set to be consistent with the magnetic moment (1.7 × 10−4 Am2 as measured by a SQUID magnetometer) divided by the surface area of the foil samples. The sharp drop observed at a depth of 0.1 µm should be responsible for the very small calculated result of almost zero deposition energy from the TRIM simulation at around 0.15 µm from the surface region (Fig. 1), which does not need to be resolved because it is a fluctuation effect. Also, the origin of the jagged line observed in the figures is due to dispersion in the TRIM simulation results. The gradual decrease in magnetization from the surface to a depth of 3.0 mm is considered to be caused by an increase in the excessive irradiation region, which means that the partial PM area grew in the FM layers. However, a layer with 0 kAm−1 of saturation magnetization can be found at around 3.0 µm deep from the surface. This simply means that such a layered region is considered to be a PM state with an A1 type crystal structure, due to the associated excessive ion beam irradiation. In addition, a sharp positive peak about 3.2 µm deep corresponded to the sharp tail of the depth profile peak in Fig. 1. In this region, the irradiation-induced FM magnetization was also considered to be realized. These facts suggest that the magnetic layered structure of FM/PM/FM/AFM can be realized near the surface of the bulk foil FeRh samples. The magnetic state in each region is schematically illustrated in Fig. 2.
Depth profile of the deposition energy by elastic collisions.
Estimated magnetization depth profile for the FeRh samples irradiated at 1.5 × 1016 ions/cm2. The symbols schematically indicate the magnetic state in each region. FM: ferromagnetic, PM: paramagnetic, and AFM: antiferromagnetic.
In this way, we could define the profile that showed the relationship between the deposition energy by elastic collision through ion beam irradiation and the saturation magnetization, which is indicated in Fig. 3. These profiles were drawn to reconcile the three plots from the experimental results for the FeRh film experiments irradiated with 2.0-KeV He ions. This reasonable estimation can be also supported by other experimental evidence: the magnetization values for the samples irradiated with the ion fluences of 1.5 × 1014 ions/cm2 and 1.5 × 1015 ions/cm2 are consistent with the estimated magnetic moment from the same approach of the simulation using this profile.
Relationship between deposition energy and saturation magnetization of FeRh sample irradiated with 2-MeV He ions.
Next, let us consider what is produced by annealing the irradiated samples. According to our previous works, annealing at moderate temperature caused the ion irradiation-induced PM state to change to an FM state. In contrast, after moderately high-temperature annealing, the PM state transformed to an AFM state. By using this nature, we can expect to re-modify the magnetic depth profiles made with high-energy ion beam irradiation.
To design the magnetic depth profile, we need to know the relationship between the deposition energy by elastic collision through initial ion beam irradiation and the saturation magnetization after annealing at various temperatures. Such curves could be predicted by using our previous knowledge about the effect of annealing for the ion beam-irradiated FeRh samples with various magnetic ordering.8,9) The predicted saturation magnetization values at 5 K, as a function of energy density deposited through elastic collisions for irradiated (1.5 × 1016 ions/cm2) and post-annealed at 373 K∼673 K, are described in Fig. 4.
Saturation magnetization values at 5 K, as a function of energy density deposited through elastic collisions for as-irradiated (1.5 × 1016 ions/cm2) and post-annealed states at 373 K∼673 K.
By using this relation, we can estimate the effect of annealing on the magnetic depth profiles of the FeRh thin foils irradiated with 1.5 × 1016 ions/cm2 of 2 MeV He ions. Figure 5 shows the estimation results for the magnetic depth profile from the surface of the irradiated FeRh foils annealed at various temperatures for 1 h. Here, the inset table in Fig. 5 indicates the total magnetic moments calculated from the magnetic depth profiles. The corresponding experimental values were also determined by SQUID measurements, which are fairly consistent with the inset values. Identical results were also obtained for the samples irradiated with different ion fluences of 1.5 × 1014 ions/cm2 and 1.5 × 1015 ions/cm2. These facts suggest that the predicted curves in Fig. 5 are accurate and that the magnetic depth profile can be significantly controlled.
Estimated magnetization depth profile at 5 K for as-irradiated (1.5 × 1016 ions/cm2) and post-annealed (373 K∼673 K) FeRh.
It should be noted that we have also established a two-dimensional magnetic patterning technique using the ion microbeam irradiation technique.11) Hence, it can be said that the three-dimensional magnetic patterning would be realized by combining this technique and magnetic depth profile techniques.
An artificially magnetic layered structure in FeRh foil samples could be produced by combining energetic ion beam irradiation and post-annealing. By combining two-dimensional magnetic patterning and the technique described in the present study, three-dimensional magnetic structures having various magnetic orderings of FM, AFM, and PM can be realized. This is a very attractive technique from the viewpoint of magnetic device applications of this material.
This work was financially supported by a Grant-in-Aid for Scientific Research (C) by the Japan Society for the Promotion of Science, Kakenhi Grant 26390011. Energetic light ion beam irradiation was performed at QST-Takasaki Ion Accelerators for Advanced Radiation Application. The synchrotron radiation experiments were performed at the BL17SU and 25SU beamlines of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposals 2015A1255 and 2012B1413).
