Li-ion intercalation enhanced ferromagnetism in van der Waals Fe 3 GeTe 2 bilayer
Cite as: Appl. Phys. Lett.119, 012405 (2021);doi: 10.1063/5.0051882 Submitted: 28 March 2021
.
Accepted: 27 June 2021.
Published Online: 8 July 2021
XiaokunHuang,1,2,a) JinlinXu,1RenfenZeng,1QinglangJiang,1XinNie,1ChaoChen,1,2XiangpingJiang,1,2,a) and Jun-MingLiu3
AFFILIATIONS
1School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, China
2National Engineering Research Center for Domestic and Building Ceramics, Jingdezhen 333001, China
3Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
a)Authors to whom correspondence should be addressed:[email protected]and[email protected]
ABSTRACT
Recently, the issue of ferromagnetism enhancement in two-dimensional (2D) van der Waals (vdW) layered magnetic systems has been highly concerned. It is believed that ion intercalation in vdW layered ferromagnets, targeting either enhanced interlayer spin exchanges or intralayer ones, can be an efficient scheme. In this work, by means of the first-principles calculations, we investigate the Li-ion intercalation between the two monolayers of the ferromagnetic (FM) vdW Fe3GeTe2(FGT) bilayer and its impact on the ferromagnetism. It is revealed that the Li- ion intercalation provides hopping carriers between the two interfacial Te sublayers, beneficial for the enhancement of the interlayer FM cou- pling at a relatively low intercalation level. On the other hand, the Li-ion intercalation lifted Fermi level promotes the electron transfer from the minority spin channel to the majority one for the Fe-3dbands, favoring the stronger intralayer FM coupling. However, the over- intercalation generated carriers may fill up the majority spin channel, reversely leading to the reduced interlayer FM coupling. Consequently, an optimized intercalation level is expected in terms of ferromagnetism enhancement. This work not only helps to explain the recent experi- mental finding on the gate-controlled Li-ion intercalation in vdW FGT few-layers but also suggests a general scheme for ferromagnetism enhancement in 2D vdW layered ferromagnets using the ion intercalation approach.
Published under an exclusive license by AIP Publishing.https://doi.org/10.1063/5.0051882
In recent years, owing to the experimental discovery of atomically thin ferromagnetic (FM) vdW materials, such as Cr2Ge2Te6, CrI3, and Fe3GeTe2(FGT),1–6ferromagnetism has been widely accepted as a new branch of intrinsic collective properties of 2D vdW materials.7,8 This exciting breakthrough immediately gained extensive attention due to its significant importance to next-generation spintronic devices and stimulated intense works to extend knowledge and applications of 2D vdW ferromagnets.9–15Nevertheless, the reduced dimensionality, due to the weak interlayer vdW interactions, has confined the spin- ordering temperature (Curie point Tc) of a 2D vdW ferromagnet, which is somehow far below room temperature, thus severely hinder- ing practical applications. In this regard, promoting the ferromagne- tism by lifting theTcand enhancing the magnetization has been one of the most critical issues in the field of 2D vdW ferromagnets.
Considering the common layered structural feature of 2D vdW materials and evaluating those proposed schemes for magnetism enhancement, ion intercalation has emerged as a very convenient and powerful tool to modulate the properties of 2D vdW systems.16–21On
one hand, ion intercalation can provide the highest possible doping level and modulate the electronic structure of the internal region in the host without destructing structures of the monolayers.17–19On the other hand, intercalated ions can act like bridges to link the mono- layers separated by vdW gaps, effectively enhancing interlayer interac- tions.20,21Thus, it is believed that ion intercalation may be a promising scheme for ferromagnetism enhancement of 2D vdW ferromagnets.
In fact, a notable experiment tuned theTcvalue of a FGT trilayer from 100 to 300 K using the gate-controlled Li-ion intercalation, and it was proposed that the electron doping may be responsible for the increase inTc.6To study the electron doping effect, a recent theoretical work doped electrons to a monolayer FGT by using the background electron approximate method.22It was found that the intralayer magnetic frus- tration can be significantly reduced in a specific range of the electron doping level, which helps to explain the liftedTcin the Li-intercalated FGT few-layers.
Nevertheless, the underlying physics can be more complicated since the electron doping effect in the Li-intercalated FGT few-layers
depends not only on the doping level but also on the positive back- ground charges of the Li ions in the vdW gaps, which have not been explored yet. More importantly, the Li-ion intercalation modulates not only the intralayer spin exchanges but also the interlayer ones. In fact, it would be highly appreciated if the intralayer and interlayer FM cou- plings can be both enhanced ferromagnetically, while the opposite cases including the competing exchanges would be unfavored.
However, the impact of the Li-ion intercalation on the interlayer FM coupling as well as electronic structures of FGT has so far received lit- tle attention. In this work, these issues will be addressed theoretically by studying FM couplings and electronic structures of the FGT bilayer before and after Li-ion intercalation with the first-principles calculations.
We performed first-principles calculations implemented in the Viennaab initiosimulation package (VASP) code.23–25The general- ized gradient approximation (GGA) with the Perdew–Burke–
Ernzerhof parameterization is adopted for exchange and correlation functional.26We adopted the effectiveUmethod to take into account correlation effects of Fe-3delectrons,27and we checked theUdepen- dence of the magnetic properties of the FGT (see Figs. S1–S3). A small U¼0.6 eV that appropriately describes the interlayer FM coupling of the FGT is used in this work if not specifically mentioned. More com- putational details are given in thesupplementary material.
FGT is a vdW layered metallic ferromagnet.28–32The stacking sequence of the five atomic sublayers in the FGT monolayer can be written as Te-FeI-(FeIIGe)-FeI-Te, in which two inequivalent sites of Fe are denoted as FeIand FeII. As shown inFigs. 1(a)and1(b), each elemental species in a sublayer occupies one of the three inequivalent in-plane sites, which are denoted as site-A, site-B, and site-C. We investigated the Li-intercalated FGT bilayer as a representative case of the FGT few-layers. To study the spatial distribution of the intercalated
Li ions, we started with intercalating one Li ion into each in-plane unit cell of the FGT bilayer, the bilayer model with this intercalation level is denoted as Li1-FGT2. To find the best intercalation site for the Li ion, we considered various trial starting positions of the Li ion (see Fig. S4) and fully relaxed the bilayer model to reach the energy minima. It turned out that the most energetically favorable site for the Li ion is the center of the hollow site (site-A) surrounded by six interfacial Te atoms [seeFig. 1(c)]. If the Li ion occupies site-B or -C, the energy of the bilayer model will be about 0.5 eV(/in-plane unit cell) higher than occupying site-A. We performedab initiomolecular dynamic (AIMD) simulation at 300 K to evaluate the stability of the Li1-FGT2bilayer model. The simulation result indicates that the Li ions at site-A are thermally stable (see Fig. S5). If we further intercalate Li ions into the Li1-FGT2bilayer model, the interlayer FM spin configuration would be damaged. We designed the bilayer models for Lix-FGT2(x¼5/4, 4/3, and 2), and their calculated ground states are interlayer AFM states (see Fig. S6 and Table S1), suggesting that over-intercalation is detrimental to the interlayer FM coupling. Thus, the intercalation level xhigher than 1 will not be further discussed in this paper.
It was reported that the electron doping induced by Li-ion inter- calation to the host material is up to the order of 1014cm2per layer,6 which is equivalent to about one electron in every seven in-plane unit cells of the FGT. So we further studied another four different intercala- tion levels at the same order of magnitude, they are one Li ion in every three, four and nine in-plane unit cells, and two Li ions in every three in-plane unit cells, which are denoted as x¼1/3, 1/4, 1/9, and 2/3, respectively. To check whether the Li ions tend to disperse or gather, we studied two different distribution configurations of the same inter- calation level x¼1/3. As shown in Fig. S7, the ffiffiffi
p3 ffiffiffi
p3
supercell with one Li ion represents the dispersed distribution, and the 33 supercell with three neighboring Li ions represents the gathered distri- bution, respectively. The calculated energy of the dispersed distribu- tion is about 8 meV(/in-plane unit-cell) lower than that of the gathered one, indicating that the Li ions tend to disperse. So we designed the bilayer models with dispersed Li-ion distributions for x¼1/3, 1/4, 1/9, and 2/3, which are ffiffiffi
p3 ffiffiffi
p3
, 22, and 33 super- cells with one Li ion, and a ffiffiffi
p3 ffiffiffi
p3
supercell with two Li ions, respectively. Their fully relaxed structures are shown inFigs. 1(d)and S8. Their calculated structural parameters (see Fig. S9) indicate that the Li-ion intercalation slightly expands the interlayer spacing of the bilayer.
Then we investigated the Li-ion intercalation modulated FM cou- plings. We started with the interlayer FM coupling. The interlayer magnetic coupling energy can be defined as DEz¼(EAFM-EFM)/u, where EAFMand EFMare the energies of the Lix-FGT2bilayer model with interlayer AFM and FM spin configurations between the two monolayers, respectively;uis the number of in-plane unit cells in the supercell. Larger positiveDEzindicates the stronger interlayer FM cou- pling. The calculatedDEzof the Lix-FGT2bilayer models are shown in Fig. 2(a). All the calculated bilayer models are FM states. When thex is relatively low (x1/3), theDEzincreases with thex, indicating the Li-ion intercalation enhanced interlayer FM coupling. As thexfurther increases, theDEzreversely decreases, indicating the over-intercalation reduced interlayer FM coupling. We double checked the DEz with changing the HubbardUparameter [seeFig. 2(b)], verifying that the non-monotonic change of theDEzaccording to thexis reliable. To check whether the increase in the interlayer distance [seeFig. 2(c)] is FIG. 1.(a) and (b) Side and top views of the pristine FGT bilayer, respectively. (c)
Side and top views of the Li1-FGT2. (d) Top views of the Li2/3-FGT2and Li1/3-FGT2. The vacuum spacer is not displayed in (a) and (c). To show the Li ions, upper monolayers of the bilayers are not displayed in the top views.
the cause of the non-monotonic behavior of theDEz, we removed Li ions from the Lix-FGT2bilayers, maintained positions of the other atoms, and calculated theDEzof the remaining FGT bilayers. TheDEz [Fig. 2(a)] slightly decreases as the x increases, indicating that the enlarged interlayer distance has minor influence on the interlayer FM coupling. Thus, the dominant cause should be the electron doping by the Li-ion intercalation.
The non-monotonic behavior of theDEzsuggests the competi- tion between at least two competing contributions of the electron dop- ing to the interlayer spin exchanges. The interlayer FM coupling of the FGT bilayer is established by the indirect spin exchanges involving not only the Fe–Te intralayer exchange channels but also the Te–Te inter- layer exchange channels. The mechanism of such multi-intermediate indirect spin exchanges,33,34 which have the same essence with the conventional double-exchange,35–37is that carriers hop between the magnetic sites with spins of incompleted-shells pointing to the same direction to reduce the energy of the system, thus establishing the FM order. In this regard, the Li-ion intercalation induced electron doping modulates two crucial factors for the interlayer indirect spin exchanges: one is the interlayer hopping carrier and the other is the spin configuration of the Fe site.
First, we checked the spatial distribution of the doping electrons.
We calculated the Li-ion intercalation induced differential electron density distribution of the Lix-FGT2bilayer model. The result of the Li1-FGT2as a representative case is shown inFig. 3(a). Apparently, doping electrons are screened by the two interfacial Te sublayers and accumulate in the vdW gap. The other four intercalation levels (x¼2/3, 1/3, 1/4, and 1/9) exhibit almost the same doping electron dis- tributions (see Fig. S10), indicating that the intercalation level has negli- gible influence on spatial distribution of the doping electrons. We also checked the distributions of the doping electrons by evaluating average changes of electron numbers in different parts of one Fe3GeTe2for- mula cell (defined as one in-plane unit cell of one monolayer) accord- ing to thex, by using the bader charge analysis [seeFig. 3(b)]. Doping
electrons per Fe3GeTe2formula cell linearly increase with thex, and they are characterized mainly by the interfacial Te site. These doping electrons offer interlayer hopping carriers for the indirect spin exchanges between the two FGT monolayers, which is beneficial for the enhancement of the interlayer FM coupling and explains the increase in theDEzat a relatively low intercalation level.
Second, we checked the spin-projected differential electron den- sity distribution to study the Li-ion intercalation modulated spin con- figurations of the Fe sites. The result of the Li1-FGT2 as a representative case is shown inFig. 3(c). Interestingly, although the Fe sites have not got doping electrons [Fig. 3(b)], the Li-ion intercalation causes obvious difference between their majority spin (spin up) and minority spin (spin down) channels, indicating the spin redistribution between the two spin channels. The other four intercalation levels exhibit almost the same features of spin redistributions (see Fig. S11).
Thus, the average occupation number of the spin up and spin down channels of the 3dorbitals of the Fe site increases and decreases line- arly with the x, respectively, leading to the enhanced magnetic moment [seeFig. 3(d)]. It should be noticed that the spin up channel has already more than four electrons before intercalation, while the spin redistribution further reduces the spin up holes. As the incom- pleted-shells are crucial to the interlayer indirect FM exchanges, a rel- atively high-level intercalation generated over-doping carriers may fill up the spin up channel, thus leading to the reduced interlayer FM cou- pling. Hence, the Li-ion intercalation induced interlayer hopping car- riers and spin redistribution of the Fe-3dbands are the two competing factors responsible for the non-monotonic change of theDEz.
Next, we propose a scenario with the schematic diagrams shown inFigs. 4(a)–4(d)to explain the spin redistribution mechanism and demonstrate that the spin redistribution contributes to the enhanced intralayer FM coupling. Figure 4(a) indicates the density of states (DOSs) for the Fe-3dbands and the 5pbands of the interfacial Te sub- layer in each monolayer of the pristine FGT bilayer. The doping elec- trons from the Li ions to the interfacial Te sublayers lift the Fermi level of the metallic system, while the Fe sites have no doping electrons to accordingly fill their unoccupied 3dstates above the original Fermi level [Fig. 4(b)]. To catch up with the lifted Fermi level, the spin down channel of the Fe-3d bands has to donate electrons to the spin up channel [Fig. 4(c)] and then shifts upward. Consequently, the exchange splitting between two spin channels is enhanced [Fig. 4(d)].
To confirm our scenario, we checked the difference between projected DOS (PDOS) of the pristine FGT bilayer and the Lix-FGT2bilayer in our calculations. To make a clear contrast, we show the results of the Li1-FGT2that has the highestxamong the five bilayer models. Due to the spatial inversion symmetry of the bilayer model, electronic struc- tures of its two monolayers are identical, so we just plot the PDOS of one monolayer. The Li-ion intercalation causes the PDOS of the inter- facial Te site to shift to the left, indicating the electron doping lifted Fermi level [Fig. 4(e)]. The Li-ion intercalation also causes the spin up channel of the Fe-3dbands to shift to the left, while the shift of the spin down channel is not obvious [Fig. 4(f)]. The calculated centers of the spin up and spin down channels of the Fe-3dbands for the Li1- FGT2bilayer model are 109 and 5 meV smaller than that for the pris- tine FGT bilayer, respectively, confirming the substantially enhanced exchange splitting by the Li-ion intercalation.
According to the Stoner model, larger exchange splitting corre- sponds to the stronger exchange interaction. As the spin redistribution FIG. 2.(a) CalculatedDEzaccording to the intercalation levelx. Red solid dots and
blue open circles represent the calculatedDEzof the Lix-FGT2bilayers and the FGT bilayers with Li ions being removed from the Lix-FGT2bilayers, respectively.
Red and blue lines are interpolations. (b) CalculatedDEzwithU¼0.2, 0,6, and 1.0 eV, respectively. (c) Calculated interlayer distance between the two interfacial Te sublayers.
occurs inside each monolayer, the resultant enhancement of the exchange splitting corresponds to the stronger intralayer FM coupling.
It should be noted that the spin redistribution is one of the mechanisms that enhances the intralayer FM coupling, while other previously pro- posed mechanisms, such as the suppression of magnetic frustration by the electron doping and the additional double-exchange channels
offered by intercalants,22,38may also work in Li-intercalated FGT few- layers. However, we did not quantitatively estimate the enhancement of the intralayer FM coupling, since the method to fit the intralayer FM coupling to the energy differences between different in-plane magnetic configurations is not suitable here due to the significantly reduced mag- netic moments by forced reorientation of the Fe spins.22
FIG. 3.(a) Integrals of doping electron density distributions in the ab plane for the Li1-FGT2.Dq¼qLi1-FGT2–qFGT2. (b) Average changes of electron numbers (Dn) in different parts of one formula cell according to thex. The labels “Fe3GeTe2,” “interfacial Te,” and “Fe3GeTe” represent a formula cell, the interfacial Te site, and the rest part of one for- mula cell, respectively. (c) Integrals of spin-projected differential electron density distribution in the ab plane for the Li1-FGT2. (d) Average occupation number of two spin chan- nels of 3dorbitals of one Fe site. The inset shows the average change of magnetic moment derived from 3dorbitals of one Fe site.
FIG. 4.(a)–(d) Schematic diagrams for the mechanism of the spin redistribution of the Fe-3dbands. FL refers to the Fermi level. (e) and (f) PDOS of the interfacial Te site and 3dbands of three Fe sites in one formula cell for the Li1-FGT2(x¼1) and pristine FGT (x¼0) bilayers. Fermi levels are set as 0 eV. The centers of the majority and minority spin channels of the Fe-3dbands are defined as the first moments of the PDOS.
Finally, we compare our results with the experiment findings.6 We have revealed the two mechanisms for the enhanced FM couplings in the Li-intercalated FGT bilayer, which may also be effective for thicker FGT few-layers. One is offering interlayer hopping carriers, and the other is inducing intralayer spin redistribution. At a relatively low intercalation level, the interlayer and intralayer FM couplings are both enhanced, explaining the liftedTcof the FGT few-layers. At a rel- atively high intercalation level, over-doping electrons lead to seriously reduced interlayer FM coupling. Consequently, an optimized interca- lation level is expected, which can explain the overall trend in the experiment that theTcfirst increases and then decreases as the gate- controlled Li-concentration increases. We also suggest that these two mechanisms are not only effective for the Li-intercalated FGT but also may be universally applicable to the other ion-intercalated vdW metal- lic ferromagnet, in which the intercalant offers doping electrons while the magnetic sublayers are sandwiched between two nonmagnetic sub- layers of the ferromagnet. Furthermore, two issues should be noted regarding this work. On one hand, our results are unable to explain the complex pattern of theTcas a function of the gate voltage, which may be related to the complicated diffusion process of the Li-ion inter- calation. On the other hand, this work lacks quantitative estimations of the enhanced spin exchanges, thus the Li-ion intercalation induced increase inTcremains unclear. Future investigations regarding these problems would be worthwhile.
In summary, we theoretically investigated the Li-ion intercalation in the FGT bilayer and its impact on the ferromagnetism enhancement by using the first-principles calculations. The Li-ion intercalation offers interlayer hopping carriers and induces intralayer spin redistribution, favoring the stronger interlayer and intralayer FM couplings, respectively.
On the other hand, an optimized intercalation level is expected because the over-intercalation damages the interlayer FM coupling. This work not only helps to explain the enhanced FM couplings in Li-intercalated FGT few-layers but also suggests that ion intercalation may be a general method to enhance the ferromagnetism in 2D vdW ferromagnets.
See thesupplementary materialfor computational details, theU dependence of magnetic properties of the FGT, the search for the most energetically favorable site for the Li-ion, AIMD simulation of the Li1-FGT2bilayer model, calculated structures and energies of the Lix-FGT2(x¼5/4, 4/3, and 2) bilayer models, two different Li-ion dis- tribution configurations designed for thex¼1/3, calculated structures of the Lix-FGT2(x¼1/4 and 1/9) bilayer models, calculated structural parameters of the Lix-FGT2(x¼0, 1/9, 1/4, 1/3, 2/3, and 1) bilayer models, and calculated differential electron density distributions of the Lix-FGT2(x¼2/3, 1/3, 1/4, and 1/9) bilayer models.
This work was financially supported by grants from the National Natural Science Foundation of China (Nos. 11947092, 52062018, and 51762024) and the Natural Science Foundation of Jiangxi Province (Nos. 20192BAB212002 and 20192BAB206008).
Part of the numerical calculations were carried out in the High Performance Computing Center (HPCC) of Nanjing University.
We thank Professor Weiyi Zhang for helpful discussions.
DATA AVAILABILITY
The data that support the findings of this study are available within the article and itssupplementary material.
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