Rise and Fall of the Ferromagnetism in CrSBr Flakes by Non‐Magnetic Ion Irradiation

Abstract

The magnetic phase transition is explored in CrSBr flakes through non‐magnetic ion irradiation, revealing a novel method for magnetic control in two‐dimensional (2D) materials. The rise and fall of the ferromagnetic phase is observed in antiferromagnetic CrSBr with increasing the irradiation fluence. The irradiated CrSBr shows ferromagnetic critical temperature ranging from 110 to 84 K, well above liquid N₂ temperature. Raman spectroscopy reveals phonon softening, suggesting the formation of defects. These findings not only highlight CrSBr's potential in spintronics, but also present ion irradiation as an effective tool for tuning magnetic properties in 2D materials, opening new avenues for the development of spintronic devices based on air‐stable van der Waals semiconductors.

Introduction

Magnetic 2D materials have become a focus of materials science and engineering, due to their distinct and compelling physical and chemical properties compared to their bulk counterparts, which bring substantial potential for spintronic devices.^[1–3] Van der Waals (vdW) heterostructures have emerged as the most promising building blocks for future spintronic devices, offering advantages such as low energy consumption, rapid device operation, and high storage density.^[4–6] However, the recently discovered 2D ferromagnetic (FM) order in CrI₃^[7] and Cr₂Ge₂Te₆,^[8] despite fully satisfying the demand of spin-based applications, lacks stability and deteriorates rapidly when exposed to air. In contrast, the antiferromagnetic (AFM) 2D materials, including the well-known MPX₃ family (M = Mn, Fe, Ni; X = S, Se)^[9,10] and emerging CrSBr,^[11] exhibit much better stability under ambient conditions. Therefore, intensive research attention has been paid to manipulating the magnetic coupling in AFM 2D materials.

Particularly, the air-stable vdW semiconductor CrSBr has recently attracted significant interest, being regarded as the most promising materials due to the pronounced correlations between magnons, photons, excitons, electrons, and phonons.^[12–19] Below the Néel temperature of T_N = 132 K, CrSBr exhibits an A-type AFM arrangement comprised of the FM orders within the monolayer with the interlayer reverse spin alignment along the crystallographic c-axis.^[11] Second harmonic generation^[20] and magneto-transport measurements^[12,21] confirm the substantial intralayer FM correlations developing above the T_N at a characteristic temperature of ≈150 K. Recently, it has been demonstrated that the weak interlayer AFM coupling between the CrSBr layers is susceptible to the external stimuli, such as strain and ligand substitution,^[22–25] causing the significant change of magnetic properties. Ion irradiation is a feasible technology to perform precise structural modification through introducing intrinsic or extrinsic defects. By He ion irradiation, a switch of magnetic coupling from AFM to FM in CrSBr has been successfully achieved.^[26] The authors have performed density functional theory calculations, indicating that the formation of interstitials due to ion collisions promotes ferromagnetic ordering between layers.

In this work, we systematically investigate the effect of the non-magnetic ion irradiation to the magnetic properties of CrSBr. With increasing irradiation fluence, the transition of the magnetic ground state from AFM to FM was first observed and then the induced FM order starts to degrade. In conjunction with the change of magnetic properties, Raman spectroscopy probes the softening of the crystal lattice that is indicative of the formation of a large number of point defects. Our study reveals the potential of CrSBr in spin-based applications and also paves the way for 2D magnetic control via ion irradiation.

Results and Discussion

As depicted in Figure 1a,b, each layer of CrSBr is comprised of a buckled plane of CrS complexes encased by a sheet of Br atoms, interconnected to adjacent layers via weak AFM interlayer interactions. Figure 1c schematically shows the prepared CrSBr flakes and illustrates the ion irradiation process. The flakes are exfoliated by using scotch tape. Most of the flakes have a thickness from a few tens nanometers to ≈1 µm. Although most CrSBr flakes are oriented along the c-plane, their in-plane orientations are randomly distributed. A series of ion irradiations were applied to CrSBr flakes, with the fluence increasing from Sample S1–S4, to explore structural and phase transitions in CrSBr flakes. Utilizing SRIM (Stopping and Range of Ions in Matter) simulations, the times an atom is displaced from its equilibrium lattice position during irradiation (displacement per atom, DPA) was calculated. The simulation also enables the estimation of the depth profile of energy transferred from the energetic ions to the crystal matrix, thereby determining the distribution of defects within the crystal. The simulation results for DPA are presented in Figure 1d. Multi-energy irradiation (refer to Table 1) was conducted on CrSBr flakes to achieve a homogeneous distribution of defects with a penetration depth of up to 1 µm. The corresponding calculated DPA for samples S1–S4 are 0.005, 0.01, 0.02, and 0.04, respectively. For those flakes much thicker than 2 µm (the maximum penetration depth in our experiment), the bottom part is not irradiated.

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Table 1 The parameters of He irradiation for the CrSBr flake sample S1: to achieve a homogenous DPA in the top ≈1 µm, we applied multiple energies with different fluences. For samples S2–S4, the fluence is multiplied by 2, 4, and 8, respectively.

	Energy [keV]	Fluence [cm−2]
1	390	5 × 1014 cm−2
2	100	5 × 1013 cm−2
3	50	4 × 1013 cm−2
4	20	2.5 × 1013 cm−2
5	5	2.5 × 1013 cm−2

Raman spectroscopy was employed to evaluate the structural response of CrSBr to ion irradiation. Within the first Brillouin zone, it is predicted that CrSBr exhibits 18 phonon modes: 15 optical modes and 3 acoustical branches.^[27] In the backscattering measurement geometry, where the incident laser light is perpendicular to the sample surface, the room-temperature Raman spectra of CrSBr reveal merely three main optical phonons of A_g symmetry: 115 cm⁻¹ for $$A_g^1$$, 245 cm⁻¹ for $$A_g^2$$, and 345 cm⁻¹ for $$A_g^3.$$ Owing to pronounced structural anisotropy, the $$A_g^2$$ ($$A_g^1$$ and $$A_g^3$$) reach their maximum intensity when the laser's polarization direction is parallel to the a-axis (b-axis).^[28] As depicted in the Raman spectra (refer to Figure 1e,f), our CrSBr flakes can largely retain their atomic structure and still exhibit obvious anisotropy even after ion irradiation. Specifically, as irradiation fluence increases, the intensity of characteristic peaks gradually diminishes, and their center continuously shifts toward lower wavenumbers, indicating a softening of phonon modes due to defect formation.^[26,29]

To explore the impact of the irradiation on the magnetic properties, magnetization was systematically measured in the prepared thin-flake samples using SQUID-magnetometry. The field-dependent magnetization of S1 was first measured at 2 K before and after irradiation with the magnetic field applied in the ab-plane. As depicted in Figure 2a, pristine CrSBr flakes essentially inherit the magnetization behavior of bulk CrSBr, characterized by a representative spin-flip field (T_SF) of ≈0.3 T indicated by the arrows in the figure.^[11,12] Minor differences in magnetization (refer to Figure 2b, for a comparison with single crystalline CrSBr) arise from the random arrangement of CrSBr flakes in our case here. After irradiation, a ferromagnetic-like signal, concealed within the AFM ground state and saturating below 0.4 T, was detected, suggesting the partial formation of FM order.^[26] The magnetic measurements were then extended to samples irradiated with higher fluences (S2, S3, and S4). After irradiation, an integral magnetic hysteresis, centered at zero, dominates the magnetization response of all samples (refer to Figure 2c), marking a complete transition of the magnetic ground state from AFM to FM. The formation of long-range FM order suggests that the defects induced by He⁺ irradiation can significantly affect the interaction of adjacent CrSBr monolayers, ultimately promoting a phase transition from AFM to FM.^[26] Note that the magnetization becomes the most prominent for sample S3 where a notable ferromagnetic hysteresis loop can still be observed even at a higher temperature of 20 K (refer to Figure 2d), and then follow a clear decrease. We believe that the induced long-range FM order is essentially related to the intrinsic FM order within the CrSBr monolayer. During irradiation, He⁺ collisions with the crystal lattice create defects to promote the transition of interlayer interaction from AFM to FM coupling,^[26] while potentially undermining the intrinsic intralayer FM order. Consequently, at sufficiently high irradiation fluence, the regularly ordered lattice matrix can no longer be maintained, as reflected by the pronounced deterioration of the Raman signal (refer to Figure 1e,f), leading to the degradation of intralayer FM order.

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Temperature-dependent measurements were conducted to investigate magnetic phase components. As depicted in Figure 3a, pristine CrSBr flakes unsurprisingly show the Néel temperature of ≈132 K (indicated by the dashed vertical line) consistent with bulk CrSBr crystals. After irradiation, a distinct hybrid phase, comprised of the AFM and FM phases, is identified in S1. It was found that low irradiation fluence only slightly weakens the AFM interaction and generates probably localized FM spin order. Figure 3b shows the temperature-dependent magnetization of S2, S3, and S4 at an applied field of 0.1 T. As expected, a continuous increase in the absolute magnetization due to the induced ferromagnetic coupling with decreasing temperature was observed, perfectly in line with our expectation of FM order in the irradiated samples. To elucidate the evolution of the magnetic phases, temperature-dependent magnetization behaviors under a small field of 0.01 and 0.05 T were further investigated. A comparison of temperature-dependent magnetization under an applied magnetic field from 0.01 to 0.1 T (refer to Figure 3b–d) revealed almost no variation in the shapes of magnetization (M) versus temperature (T) curve for S3 and S4. For S2, the initial magnetization curve resembles that of S1, which is comprised of an AFM and FM hybrid phase (refer to Figure 3c). However, with increasing field strength, the magnetization of S2 significantly increases in the FM regime, ultimately aligning with a typical FM curve (refer to Figure 3b–d). This suggests the presence of residual AFM coupling in sample S2. Thus far, it turns out that applying sufficient fluence is crucial to achieve a complete transition of magnetic coupling from AFM to FM in CrSBr.

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The critical temperature of both pristine and irradiated CrSBr was estimated by taking the first derivative of the M versus T curve. As depicted in Figure 4a, the extracted T_N of pristine CrSBr flakes is 132 K and consistent with expectations. Furthermore, with increasing irradiation fluences, the extracted Curie temperature (T_C) for S2, S3, and S4, identified as 111, 100, and 84 K, respectively, exhibits a continuous decrease. The decline in T_C implies the weakening of the FM interaction, including the induced interlayer FM coupling and inherent intralayer FM coupling, due to the progressive lattice structure degradation. Recently, several groups reported the ferromagnetic correlations above the T_N in pristine CrSBr.^[30,31] It is of substantial interest to analyze the soft ferromagnetic behavior under the applied magnetic field, as it allows the estimation of the FM correlation strength within the monolayer.^[30,31] Therefore, a magnetic critical temperature (T_M) derived from the Arrott plots^[32,33] in proximity to the paramagnetic (PM) transition was defined to measure the intralayer FM interaction. The extracted T_M in pristine CrSBr, ranging from 150–170 K,^[30,31] is significantly higher than the T_N of 132 K, manifesting the strong intralayer FM correlations. Interestingly, as depicted in Figure 5a, the high magnetization value and the S-shape in the M(H) curve in the PM regime (above T_C) were also observed in our irradiated CrSBr (sample S4). However, the extracted T_M for sample S4, ranging from 110–120 K (refer to Figure 5b) is notably decreased when compared with that of the pristine CrSBr crystal,^[30,31] indicating the significantly deteriorated intralayer FM correlations due to large-fluence irradiation.

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Conclusion

We observe a series of changes in the magnetic ground state in van der Waals CrSBr flakes induced by ion irradiation with gradually increased fluences. From the initial effective creation to the degradation of the FM coupling, the optimal irradiation fluence (amount of defects) is determined for transforming AFM CrSBr to FM. The obtained FM CrSBr shows Curie temperature ranging from 110 to 84 K, well above the liquid N₂ temperature, opening the playground for spintronics based on air-stable CrSBr. In addition, ion implantation is a promising technology for the future vdW heterostructure-based spintronics. By applying the proper energy and fluence, we can control the induced ferromagnetism regarding the magnetization and critical temperature.

Experimental Section

Crystal Growth

CrSBr crystals were synthesized through the direct reaction from the elements. High-purity chromium (99.99%, −60 mesh, Chemsavers, USA), bromine (99.9999%, Sigma–Aldrich, Czech Republic), and sulfur (99.9999%, Stanford Materials, USA) were combined in a stoichiometric ratio within a quartz ampoule (35 × 220 mm) corresponding to 15 g of CrSBr. An excess of 0.5 g bromine was employed to enhance vapor transport. The material was pre-reacted within an ampoule utilizing a crucible furnace at 700 °C for 12 h, while the second end of the ampoule was kept below 250 °C. The heating procedure was repeated twice until the liquid bromine was fully evaporated. Subsequently, the ampoule was positioned within a horizontal two-zone furnace to facilitate crystal growth. Initially, the temperature of the growth zone was heated to 900 °C, while the source zone was heated to 700 °C for 25 h. For the growth, the thermal gradient was reversed and the source zone was heated from 900 to 940 °C and the growth zone from 850 to 800 °C over a period of 7 days. The crystals with dimensions of up to 5 mm² × 20 mm² were removed from the ampoule in an Ar glovebox.

Flake Exfoliation Procedure

First the CrSBr crystal with a suitable size was picked up and put on the scotch tape. Then a clean plastic tweezer was used to crumble CrSBr crystal into flakes and distribute them evenly in an area with the size of 5 mm × 5 mm. Finally, the clean Si wafer was put with natural SiO₂ on the scotch tape containing CrSBr flakes and pressed them together. After a few minutes, the Si was taken out from the tape and get the prepared sample (most of the adsorbed CrSBr flakes were no more than 1 um).

He Irradiation Procedure

The CrSBr flakes were exfoliated on a piece of Si with native SiO₂. He ion-beam with different energies (shown in Table 1) was homogeneously rastered over the sample surface. The resulted displacement per atom (dpa, a measure of crystal damage) was ≈0.05, 0.10, 0.15, and 0.20 for samples S1–S4.

Calculation of Magnetization

Before irradiation, the pristine prepared CrSBr flake samples (S1–S4) were performed the Field-dependent magnetization at 2 K. The maximum applied magnetic field was 5 T, which could fully polarized the CrSBr. In this case, by comparing the measured moment of pristine CrSBr flake samples with a bulk CrSBr with determined mass, it could calculate the mass of each CrSBr flake sample. And then, the “M” (emu g⁻¹) for irradiated flakes could be calculated by using the same mass (g) calculated before irradiation. Here, it was assumed no flake falling down during irradiation and measurements. The potential falling down of flakes would lead to an underestimation of the magnetization (M) after irradiation.

Acknowledgements

Ion irradiation was performed at the Ion Beam Center (IBC) of HZDR. F.L. thanks the financial support from China Scholarship Council (File No. 202108440218) for his stay in Germany. Z.S. was supported by ERC-CZ program (project LL2101) from Ministry of Education Youth and Sports (MEYS) and by the project Advanced Functional Nanorobots (Reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR).

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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