{'fraction_non_alphanumeric': 0.04380918665368617, 'fraction_numerical': 0.026197008584830515, 'mean_word_length': 4.842812823164426, 'pattern_counts': {'":': 0, '<': 2, '<?xml version=': 0, '>': 2, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Highly sensitive spin-flop transition in antiferromagnetic van-der Waals material MPS3 (M = Ni and Mn)

Recent developments in two-dimensional (2D) magnetism have motivated the search for novel van-der Waals (vdWs) magnetic materials to explore new magnetic phenomenon in the 2D limit. Metal thiophosphates, MPX3, is a class of magnetic vdWs materials with antiferromagnetic (AFM) ordering persisting down to the atomically thin limit. The magnetism in this material family has been found to be highly dependent on the choice of transition metal M. In this work, we have synthesized the intermediate compounds Ni1-xMnxPS3 (0<x<1) and investigated their magnetic properties. Our study reveals that the variation of Ni and Mn content in Ni1-xMnxPS3 can efficiently tune the spin-flop transition, likely due to the modulation of the magnetic anisotropy. Such effective tunning offers a promising candidate to engineer 2D magnetism for future device applications.

the isothermal magnetization measured under out-of-plane magnetic field (H⊥ab) [ Fig. 2

(a)] but
is absent under in-plane field (H//ab) [ Fig. 2(b)], which is consistent with the previous reports [5,34,65,68,69]. Although the SF transition is largely explored in both bulk and atomically thin MnPS3, it has not been directly discovered in other members of this family except FePS3, in which a metamagnetic transition occurs at very high field (μ0H > 35 T) [70]. Here we report the first discovery of SF transition in NiPS3. Unlike in MnPS3 in which the SF transition occurs when H⊥ab, the metamagnetic transition in NiPS3 takes place with in-plane field, which is characterized by a clear upturn in magnetization above a critical spin-flop field μ0HSF ≈ 6 T below the Néel temperature TN ≈ 155 K, as shown in Figs. 2(d) and 2(e). The need for relatively high magnetic field could be the reason that prevented the discovery of SF transition in earlier studies [38,71].
On the other hand, the recent magneto-photoluminescence experiment implies a spin re-orientation in bulk NiPS3 under a much higher in-plane magnetic field of 15 T [32]. Such a high critical field, which is 2.5 times higher than that in our magnetization measurements, might be attributed to the nature of photoluminescence as an indirect probe.
In MnPS3 and NiPS3, the SF transition occurring under different field directions is in line with their magnetic structures: The Mn moments in MnPS3 are aligned along out-of-plane direction [58,69], while the Ni moments in NiPS3 mostly lie within the plane [31,38,71], as shown in the insets of Figs. 2b and 2c, respectively. In collinear AFM systems, a magnetic field along the easy axis exceeding a critical spin-flop field HSF forces the magnetic moments to rotate [72,73].
In such a SF state, the moments re-orient themselves to a canted configuration perpendicular to the field direction, resulting in a net moment along the easy axis [72,73]. Therefore, the SF transition behaves differently in MnPS3 and NiPS3, as illustrated in Figs. 2g and 2h. Furthermore, because the in-plane projection of Ni moments in NiPS3 forms a collinear AFM structure along the a-axis (Fig. 2c, lower inset), the SF transition is expected to show in-plane anisotropy. To examine this, we measured magnetization with the magnetic field applied along or perpendicular to the hexagonal edges of a NiPS3 single crystal, as shown in Fig. 2(e). Indeed, magnetization and SF transition are found to be dependent on in-plane field-orientations.
The scenario of SF transition in NiPS3 is also supported by the temperature dependence of magnetic susceptibility. As shown in Fig. 2(f), susceptibility measured with in-plane field (χ||) displays a low temperature upturn, which becomes more significant at higher fields (μ0H > 5 T).
A similar low temperature upturn has been observed in MnPS3 [65]. In addition, the zero-field cooling (ZFC, solid lines) and field cooling (FC, dashed lines) data display weak but clear irreversibility above 5 T, which also becomes more visible at higher fields. The development of low temperature upturn and irreversibility can be understood in terms of the ferromagnetic component from the uncompensated canted moments along the easy axis in the SF state, as illustrated in Fig. 2h. This irreversibility disappears at 140 K when applying 9 T field, coinciding well with the temperature above which the SF transition vanishes as seen in the isothermal magnetization [ Fig. 2(d), inset]. Such 140 K "disappearing temperature" is lower than the magnetic ordering temperature TN ≈ 155 K, which can be attributed to the fact that a greater field is needed for SF transition at higher temperatures, particularly when approaching TN. HSF enhancement upon increasing temperature is widely seen in other SF systems [5,[74][75][76][77][78], which can be interpreted using the molecular field theory [73,78]: In a weakly anisotropic antiferromagnet, spin-flop field HSF can be estimated by (HSF) 2 = 2K/(χꓕ -χ||), where K is the anisotropy constant, χꓕ and χ|| are the perpendicular and parallel susceptibilities, respectively [58,73,78,79]. Generally, in AFM systems the difference between χꓕ and χ|| reduces more quickly than the magnetic anisotropy constant K upon increasing temperature, leading to enhanced HSF [78].
The distinct SF transitions in NiPS3 and MnPS3 due to their different magnetic structures (Figs. 2g and 2h) motivate us to further study the "mixed" compounds Ni1-xMnxPS3. As shown in been observed in Zn-substituted MnPS3. The previous study [58] on Zn substitution for Mn in MnPS3 suggests the breakdown of long-range magnetic order due to non-magnetic impurities in local substituted regions, which leads to "weakly bound" Mn moments. The polarization of these Mn moments causes the low-field nonlinear M(H). Another study [57] also propose that for MnPS3 in which the dipolar anisotropy dominates, the local dipole field is along the out-of-plane direction.
As the consequence, when Mn-magnetism is diluted by Zn substitution, the absence of the magnetic moment of one Mn site would affect the closest Mn in the neighboring layers and cause their magnetic moments to be canted, leading to an average staggered magnetic moment in a larger scale [57]. Similarly, the nonlinear M(H) at low fields in our Ni1-xMnxPS3 samples can likely be attributed to the polarization of canted moment originating from substitution. This scenario is further supported by the temperature dependence of susceptibility measurements. As shown in Fig.  4a, consistent with the nonlinear M(H) seen under H||ab in the x = 0.05 sample (Fig. 3d, inset), the in-plane susceptibility (χ||) displays a clear upturn at low temperatures that suggests the development of a ferromagnetic component due to moment canting. Likewise, close to the MnPS3 side, for x = 0.95 sample in which the nonlinear M(H) is significant (Fig. 3e, inset), a susceptibility upturn is also seen, as shown in Fig. 4c. The upturn become more obvious with increasing the Ni content to 10% (x = 0.9), as shown in Fig. 4b.
The tunable SF transition and the presence of a ferromagnetic component by Ni-Mn substitution is expected because of the distinct magnetic structures in NiPS3 and MnPS3. However, the observed high sensitivity to light substitution is surprising. For example, only 5% Ni substitution can substantially reduce HSF by half in MnPS3, which is much less than the amount of Zn needed (~20%) to reduce HSF by the same amount [58,65]. The HSF suppression by nonmagnetic Zn substitution has been ascribed to the reduction of magnetic anisotropy with magnetic dilution [58]. The Ni-Mn substitution in our Ni1-xMnxPS3, however, induces magnetic impurities.
Hence, it is necessary to consider the magnetic interaction to understand the observed sensitive doping dependence in Ni1-xMnxPS3. In MPX3 compounds, magnetism has been found to be related to a structural distortion [23,[80][81][82][83]. As illustrated in Fig. 3c, every metal atom M in MPX3 is located at the center of an octahedron formed by six X atoms. Such MX6 octahedra, however, possess a trigonal distortion that is characterized by the angle θ between the trigonal axis (perpendicular to the ab-plane) and the M-S bond. Therefore, magnetism in MPX3 can be described by an isotropic Heisenberg Hamiltonian with additional single-ion anisotropy due to the combined effect of crystal field and spin-orbit splitting: = −2 ∑ + 2 , where J and D are the exchange and crystal field parameters, respectively [37]. The relative strength of J and D leads to various magnetic structures in MPX3, so the trigonal distortion plays critical role in determining the nature of magnetic interactions in MPX3 [37,80]. In NiPS3 and MnPS3, θ has been found to be 51.05° and 51.67°, respectively [80]. Compared with θ ≈ 54.75° for the undistorted octahedra, the greater trigonal distortion in NiPS3 leads to much stronger single-ion anisotropy (0.3 meV) than that of MnPS3 (0.0086 meV), as revealed by inelastic scattering measurements [42,80,81]. This causes the Ni moments to be aligned within the basal plane perpendicular to the trigonal axis, with a small out-of-plane component likely due to weak dipolar anisotropy, as shown in the insets of Figs. 2c [37,43,82]. In contrast, the effect of crystal field and spin-orbit splitting is found to be negligible for the high spin ground state of Mn 2+ in MnPS3 [37], so the magnetism of the less distorted MnPS3 is governed by the dipolar anisotropy that leads to out-of-plane moment direction with small tilt towards a-axis [42,69,82], as shown in the inset of Fig. 2b. Therefore, the tuning of trigonal distortion by Ni-Mn substitution would significantly affect the magnetic isotropy and further efficiently modify the SF transitions. A similar mechanism has also been proposed for the suppression of HSF in MnPS3 under pressure [82].
In addition to magnetic anisotropy, magnetic exchange interaction may also play an important role. The magnetism in MPX3 is mainly mediated through superexchange interaction, thereby affected by the M-X-M bonding angle [37,81]. Additionally, the d-electron occupancy of M is important in determining the sign and the nature of the superexchange [37,80,81]. The 3d orbitals of M 2+ ion consist of high energy eg and low energy t2g groups, and their occupancies determine the strength of the exchange interactions [80,83]. This has recently been demonstrated by the inelastic neutron scattering measurements which reveals reduced exchange interactions in CoPS3 as compared to NiPS3 due to distinct occupancy of t2g orbital [80]. A similar scenario should also occur in Ni1-xMnxPS3, in which the Ni-Mn substitution modifies the magnetic exchange.
Indeed, the magnitudes of all the exchanges, except the weakest second nearest-neighbor interaction (J2), are found to systematically increase with decreasing spin on M 2+ ion and increasing M-S-M bond angles from MnPS3 to NiPS3 [80,81,83].
Among the magnetic anisotropy and magnetic exchange interactions, the former may govern the SF transitions in Ni1-xMnxPS3. The magnetic ordering temperature be related to the strength of the magnetic interaction. We have extracted TN for Ni1-xMnxPS3 from magnetic susceptibility and heat capacity measurements, as summarized in Fig. 4