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Lukas Krelle, Ryan Tan, Daria Markina, Priyanka Mondal, Kseniia Mosina, Kevin Hagmann, Regine von Klitzing, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Zdenek Sofer, Bernhard Urbaszek

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[Magnetic Correlation Spectroscopy in CrSBr](https://mdr.nims.go.jp/datasets/a0410590-7984-477a-bd4b-41cff068b941)

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Magnetic Correlation Spectroscopy in CrSBrMagnetic Correlation Spectroscopy in CrSBrLukas Krelle, Ryan Tan, Daria Markina, Priyanka Mondal, Kseniia Mosina, Kevin Hagmann,Regine von Klitzing, Kenji Watanabe, Takashi Taniguchi, Zdenek Sofer, and Bernhard Urbaszek*Cite This: ACS Nano 2025, 19, 33156−33163 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: CrSBr is an air-stable magnetic van der Waals semi-conductor with strong magnetic anisotropy, where the interaction ofexcitons with the magnetic order enables the optical identification ofdifferent magnetic phases. Here, we study the magnetic anisotropy ofmultilayer CrSBr inside a three-axis vector magnet and correlatemagnetic order and optical transitions in emission and absorption. Weidentify layer-by-layer switching of the magnetization through drasticchanges in the optical emission and absorption energy and strength asa function of the applied magnetic field. We correlate opticaltransitions in reflection spectra with photoluminescence (PL)emission using transfer-matrix analysis and find that ferromagnetic and antiferromagnetic order between layers can coexistin the same crystal. In the multipeak PL emission, the intensity of energetically lower-lying transitions reduces monotonouslywith increasing field strength, whereas energetically higher-lying transitions around the bright exciton XB brighten close to thesaturation field. Using this contrasting behavior, we can therefore correlate transitions with each other.KEYWORDS: CrSBr, magneto-optics, magnetic semiconductor, van der Waals materials, excitons, layered antiferromagnetI. INTRODUCTIONDue to a plethora of tunable magnetic intra- and interlayerinteractions,1 layered magnetic materials have motivatedresearch toward applications in data storage2,3 and spin-tronics.4,5 However, many-layered magnetic semiconductorslike chromium trihalides and related materials suffer frominstability under ambient conditions.6−9 Compared to thesematerials, the semiconducting layered antiferromagnet CrSBrexhibits a much higher stability in ambient conditions.10,11 It iswell suited for fundamental studies of magnons,12−14 exciton−phonon coupling,15−17 and exciton−photon coupling18−22 aswell as applications in magnetic devices exhibiting large negativemagnetoresistance,23,24 tunable properties with magneticfields,25 and electrostatic doping,26 as well as ion irradiation.27,28CrSBr crystals are strongly anisotropic, which results in a strongdependence of the magnetic, optical, and electronic proper-ties25,29,30 on the crystallographic axes.The intricate interplay between excitons and the magneticorder in CrSBr enables the spectroscopic identification ofdifferent magnetic phases25,26,31 as well as spatial domains.26The spectroscopic signature of excitonic transitions is notlimited to the topmost layer but also reveals information aboutdeeper-lying layers,32 which is potentially an advantagecompared to successful scanning probe techniques.33,34 Up tonow, research has mainly focused on understanding themagnetic and excitonic properties of bulk systems and 1−4-layer systems. However, the study of multilayer systems, whichexhibit complex emission properties without the influence ofstrong exciton−photon coupling, has remained elusive.In this work, we study the anisotropic magnetic properties ofCrSBr inside a vector magnet at low cryogenic temperatures. Weuse correlated spectral changes occurring with the application ofa magnetic field to investigate the origin and connections ofoptical transitions present in multilayer samples of CrSBr. Tothis end, we fabricated two multilayer samples: one (14 layers;thickness determined by atomic force microscopy = 11.3 ± 0.2nm) encapsulated in hexagonal boron nitride (hBN), andanother one as-exfoliated (10 layers; thickness determined byatomic force microscopy = 8.2 ± 0.3 nm). We performedmagnetic-field dependent photoluminescence (PL) and differ-ential reflectance contrast (DR/R) measurements at T = 4.7 Kand carefully tracked the emission energy and intensity todetermine potential emission origins. Our measurements showthat the ferromagnetic (FM) state is reached graduallythroughout the crystal as different layers switch from theantiferromagnetic (AFM) to the FM order at different fields. Wefind phases close to the saturation field that show a superpositionof spectral signatures of both the FM and AFM states.Received: April 1, 2025Revised: August 21, 2025Accepted: August 21, 2025Published: September 12, 2025Articlewww.acsnano.org© 2025 The Authors. Published byAmerican Chemical Society33156https://doi.org/10.1021/acsnano.5c05470ACS Nano 2025, 19, 33156−33163This article is licensed under CC-BY-NC-ND 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on September 24, 2025 at 04:48:01 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Lukas+Krelle"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ryan+Tan"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Daria+Markina"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Priyanka+Mondal"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kseniia+Mosina"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kevin+Hagmann"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Regine+von+Klitzing"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Regine+von+Klitzing"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Zdenek+Sofer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Bernhard+Urbaszek"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsnano.5c05470&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/ancac3/19/37?ref=pdfhttps://pubs.acs.org/toc/ancac3/19/37?ref=pdfhttps://pubs.acs.org/toc/ancac3/19/37?ref=pdfhttps://pubs.acs.org/toc/ancac3/19/37?ref=pdfwww.acsnano.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsnano.5c05470?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsnano.org?ref=pdfhttps://www.acsnano.org?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/Excitation-power-dependent and temperature-dependent PLmeasurements reveal several emission peaks that originate fromtrapped exciton species.II. RESULTS AND DISCUSSIONAs a first step in our detailed spectral analysis, we performedcryogenic magneto-optical spectroscopy inside a three-axisvector magnet, in which the crystallographic axes are alignedFigure 1. Experiments on CrSBr in a three-axis vector magnet. (a) CrSBr crystal structure. Crystallographic axes a, b, and c are indicated byarrows. (b) Sketch of a CrSBr sample inside the vector magnet and the orientation of laser and PL polarization with respect to crystallographicaxes. (c−e) Exemplary PL magnetic field sweeps of the encapsulated sample for the same spatial spot along the crystal a, b, and c axes,respectively, displaying magnetic anisotropy, where Ba ∥ a, Bb ∥ b, and Bc ∥ c. Dashed white arrows are a guide for the eye for the evolution of XD(at 1.324 eV at B = 0) and XB (at 1.362 eV at B = 0) transitions.Figure 2. Influence of excitation power, temperature, andmagnetic field on the optical properties of encapsulated 14-layer CrSBr. (a) Excitationpower dependence of the PL emission atB = 0 T andT = 4.7 K. (b) Temperature dependence of the PL emission atB = 0T and 10 μWexcitationpower. (c and d) DR/R and PL spectra of the 14-layer sample for the AFM (black) and FM (red) states. Emissions are as indicated in the maintext. Dashed gray and red lines indicate the energy of XB in the AFM and FM states, respectively.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.5c05470ACS Nano 2025, 19, 33156−3316333157https://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig2&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.5c05470?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aswith the magnetic field axes as in earlier seminal work.25 Parts aand b of Figure 1 display the crystal structure of CrSBr and theexperimental configuration, respectively. Unless noted differ-ently, we focus on the hBN-encapsulated 14-layer sample. Weperformed PL measurements at T = 4.7 K with an excitationenergy of 1.959 eV, above the estimated band gap of CrSBr.25,29First, we study the magnetic anisotropy of the material, which isclearly visible in the magnetic field sweeps along the crystal axesshown in Figure 1c,e for the same sample spot. At zero magneticfield, below the critical temperature, the layers show PL spectraassociated with AFM order, as spins in adjacent layers point inthe opposite direction along the crystal b axis.11,35,36 Theapplication of a magnetic field rotates the magnetization suchthat beyond a certain saturation field all layers are magnetizedalong the same direction (i.e., parallel to the applied field)corresponding to FM order. We trace the change from AFM toFM order through changes in the PL energy.25 The saturationfields along the different axes for the data shown are respectivelyBcs = ±2.05 ± 0.05 T and Bas = ±1.10 ± 0.05 T. Due to hysteresiseffects, the saturation fields along b depend on the sweepdirection with Bbs = +0.45 ± 0.01 and -0.37 ± 0.01 T. Thecontinuous energy shift of the emission peaks for sweeps alongthe magnetic hard and intermediate axis allows the assignmentof the emission peaks in the AFM and FM states, which we aimto identify. For comparison, the abrupt jumps at Bbs make peakassignments between the optical emission in the AFM and FMstates more challenging.In our PL measurements, the linear excitation polarization isaligned with the crystal a axis, while the detection polarization isaligned with the b axis (Figure 1b). The crystal axes weredetermined via the maximum and minimum PL intensity for theb and a axes, respectively. Figure S1 displays the polarizationdependence of the PL emission of encapsulated 14-layer CrSBrat B = 0 T. Due to the strong anisotropy of the material, the PLemission is highly linearly polarized, exhibiting maximumintensity along the b axis and about a factor of 200 weakerintensity along the a axis. Both samples show qualitativelysimilar PL spectra with a multitude of different emission peakswith narrow linewidths (on the order of millielectronvolts), asshown in Figure 2a,b,d for encapsulated 14-layer CrSBr and inFigure S2a,b,d for unencapsulated 10-layer CrSBr. Importantly,due to the small sample thickness, the rich spectra cannot stemfrom polaritonic states, as reported for bulk CrSBr layers,18,19but from different excitonic species, their phonon replica andtrapped states, which we aim to identify further.In Figure 2d, we mark two transitions that will be ofsignificance in our discussion: transition XD, which is thestrongest in PL, and transition XB, which is visible in PL and isthe strongest transition in terms of oscillator strength in DR/R(see below). Observing strong PL from transitions that have lowoscillator strength resembles the rich PL spectra of WSe2monolayers with several exciton species.37−39 Apart from XBand XD, we find several emission peaks between XB and XD,which we label X1−5, as well as three emissions below XD, whichwe label P1−3. In Table S1, we summarize the energies of theobserved emission peaks for the AFM and FM order.We performed excitation-power-dependent PL measure-ments, displayed in Figure 2a, to ensure that we capture alldetails of the complex spectra. Most of the emission peaks followa linear power dependence, as displayed in Figure S3. Weobserve several emission peaks, of which we highlight X2 and P1,that show signs of saturation at an excitation power of 50 μW.Temperature-dependent measurements reveal that theseemissions vanish very quickly around T = 16 K, as shown inFigure 2 b. Additionally, these measurements reveal that theemission labeled P2 is accompanied by another emission peakthat dominates over P2 at low temperatures and excitationpowers. This emission peak also vanishes around T = 16 K.These observations are characteristic for excitons in shallowtrapping potentials.40,41 As the temperature increases, thetrapped excitons gain enough thermal energy to escape thetrapping potential and the associated emission diminishes.Notably, of the multitude of emission peaks, only XB, X3, and XDremain present up toT = 100 K (see Figure S4 formeasurementsup to T = 150 K).In the unencapsulated sample, the changes from low to highexcitation power are more pronounced, as shown in Figure S2b.Here we focus on a low excitation power regime, in which bothsamples exhibit similar emission properties, as highlighted in thedirect comparison in Figure S2a. We note that the emissionpeaks in the unencapsulated sample are blueshifted by 4 meVwith respect to the encapsulated sample, most likely duedielectric screening,42,43 while relative energy splittings withinthe spectra remain unchanged.Theoretical calculations predict two conduction bandsresulting in two possible transitions to the valence band, ofwhich one is parity-allowed and one is parity-forbidden.25,29 Thenotion of parity-allowed and -forbidden transitions only strictlyholds for transitions at very specific points in k space, which isnot fully applicable to the more extended exciton states inFigure 3. Layer-by-layer magnetization switching in 10-layer CrSBr. (a) False color plot of a DR/R measurement with magnetic field appliedalong the b axis. Arrows indicate the transition fields, and numbers indicate the transitions as in part b. (b)DR/R spectra of the six different layermagnetization configurations. (c) Normalized oscillator strengths of XB,AFM and XB,FM extracted from a Lorentzian oscillator model.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.5c05470ACS Nano 2025, 19, 33156−3316333158https://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig3&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.5c05470?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asCrSBr.44 As proposed in other work,25,45 we refer to theemission around 1.362 eV (XB) as the parity-allowed transition.This is in accordance with DR/R measurements displayed inFigure 2c, which reveal an optical resonance with a largeoscillator strength at the same energy. This resonance isaccompanied by a much weaker resonance roughly 14 meVabove this transition (see the transition labeled X* in Figure 2c).Klein et al.29 suggest that this resonance might havecontributions from the same conduction band along the Γ−Xdirection. In the PL measurements shown, the emission of thistransition is absent due to the low excitation power.For the assignment of the so-called parity-forbidden transitionXD, we refer to work by Lin et al.15,45 This corresponds to thebrightest transition in the PL emission located around 1.324 eV,38 meV below XB, in agreement with previous reports andtheoretical predictions for the splitting of exciton energies.25,45We note that the magnitude of the band-gap energy and theorigin and magnitude of the splitting between XB and XD arediscussed in several studies.29,32,46−49Our work adds contrastingmagnetic field effects to the observed distinctions between thetwo transitions. Importantly, we do not observe any signature ofXD in DR/R, in agreement with its parity-forbidden character,i.e., predicted weak oscillator strength.To gain further insight into the origin of the complex opticalresponse, we performed magnetic field sweeps along themagnetic easy axis (for details, see Methods). For both samples,we observe several abrupt changes in the PL and DR/R atdistinctly different magnetic fields. Hence, the magnetization ofthe sample changes in discrete steps; i.e., different layers in thesample can change their spin orientation individually. We note acertain analogy to layer-by-layer switching in Fe/MgO (001)superlattices,50 albeit we use here simple PL and DR/Rmeasurements to monitor the magnetism in CrSBr. Figure 3 adisplays this layer-by-layer switching in DR/Rmeasurements forthe unencapsulated 10-layer sample. In addition to theresonance of XB,AFM, another resonance appears 15 meV red-shifted from it, which we identify as the resonance of XB in theFM state (XB,FM). The resonance of XB,FM appears several 10 mTbelow the saturation field and persists above it. For this magneticfield range, both resonances XB,AFM and XB,FM are present at thesame time; i.e., in these phases, AFM and FM order coexistwithin the sample. On the basis of our optical mappingexperiment, we conclude that the observed resonance super-position does not stem from several domains present within ouroptical spot size but from layer-specific effects. Figure S5a showsan optical micrograph image of the unencapsulated sampleincluding the 10-layer region. We mapped out a sample regionincluding 10 and 9 layers using DR/Rmeasurements at differentmagnetic fields shown in Figure S5b,c. The observed magneticdomains are much larger than the measured optical spot size(below 1 μm in diameter). Parts d−f of Figure S5 show that thechange of themagnetization state in themagnetic domain relatesto the change in the DR/R signal. We observe a discrete transferof the oscillator strength between XB,AFM and XB,FM, with eachlayer switch shown in Figure 3c. Surprisingly, the measurementsreveal an additional resonance red-shifted by 5−7 meV fromXB,AFM, as shown in Figure 3b. This resonance appears with thefirst switch of magnetization and remains present until thesystem fully transitions into the FM state, where it vanishes (seetransitions marked by arrows in Figure 3b). The origin of thisadditional resonance remains to be clarified. We use transfer-matrix analysis (for details, see Methods) to fit the spectra anddetermine the resonance energies of the oscillators present in thedifferent magnetic phases to be EB,AFM = 1.366 eV and EB,FM =1.351 eV. The encapsulated sample exhibits a similar super-position of the individual resonances, as highlighted in FigureS6. In this sample, we determine the respective resonanceenergies as EB,AFM = 1.362 eV and EB,FM = 1.347 eV.In addition to studying transitions with a high oscillatorstrength in DR/R, we now analyze themagnetic-field-dependentFigure 4. Correlated emissions in magnetic fields. (a) PL emission of the encapsulated CrSBr flake for selectedmagnetic field strengths Bb alongthe crystal b axis and spectral range displaying similar and consistent reduction of the emission intensity with the magnetic field. (b) Same aspart a but for negativemagnetic fields. (c and d)Maximum intensity of the emissionsmarked in part a for themagnetic field sweep in Figure S7b.(e and f) Maximum emission intensity in parts c and d normalized by the intensity of XD. Pointed parts of the graphs indicate regions with largeerror bars.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.5c05470ACS Nano 2025, 19, 33156−3316333159https://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig4&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.5c05470?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asPL emission, which also gives additional information on excitonspecies.39 We recorded a full hysteresis loop starting in FM orderat negative magnetic field Bb and focus on the down-sweepdirection of the measurement loop shown in Figure S7a,b. Partsc and d of Figure S7 display the maximum intensity of XD and XBfor the whole magnetic field sweep including strong hysteresiseffects.The emission changes most drastically when the magneticfield reaches the saturation field and the remaining layers of thesystem switch to the FM state. For the down-sweep direction inFigure S7b, this happens at Bb < −0.44 T in the encapsulatedsample. In the unencapsulated sample, the switch happens at Bb< −0.37 T. Due to hysteresis, the FM state starts to vanish atdifferent fields of Bb < 0.38 T in the encapsulated sample and Bb< 0.36 T in the unencapsulated sample. In the FM state, theemission intensity is much weaker than that in the AFM state,and the emissions experience redshifts of 15 meV for XB and 12meV for XD. This reduction of intensity is thought to result fromthe reduced layer confinement of excitons in the FM state.51 Inthe AFM state, the antiparallel magnetization of adjacent layersdoes not allow charge transfer between layers, and thus excitonsare confined to a layer. However, the parallel magnetization ofadjacent layers in the FM state allows charge transfer, reducingthe electron−hole wave function overlap. Strikingly, all emissionpeaks in the investigated samples are weaker in the FM states,while the exact changes during the transition can differ, which wewill elaborate on further. In the FM state, the intensities of XBand XD reduce by approximately 46 ± 3% and 88 ± 1%,respectively.We now turn to a more detailed discussion of the emissionchanges. Panels a and b of Figure 4 display spectra recorded atdifferent values of Bb in a spectral range around the strongestpeak in the PL emission XD. With increasing magnetic fieldstrength, all of the emission peaks in this range decrease inintensity until either vanishing (within our detection limit) orexperiencing an energy shift at the switch to the FM phase. Amoderate reduction in intensity appears at Bb = ±0.2 T, whichlikely corresponds to a switch of an individual layer magnet-ization consistent with the observations in Figure 3. Panels c (fortransitions XD, X4, and X5) and d (for transitions P1, P2, and P3)of Figure 4 highlight the collective change, where we plot themaximum intensity of each of the emissions. We emphasize thisfurther by plotting the intensity ratios of the emissions comparedto the emission of XD in Figure 4e,f. Until fully switching to theFM phase, the intensity ratios stay almost constant. We want toemphasize that, in addition, before reaching Bbs , none of theseemissions exhibit an energy shift despite the changes in intensity.This correlated behavior suggests that these emissions stemfrom the same band transition.The emissions P1, P2, and P3 are red-shifted with respect to XDby 4.2, 7.8, and 11.9 meV, respectively, i.e., approximatelyequidistantially spaced by 4 meV. From this, one mightspeculate that the emissions P1, P2, and P3 are phonon replicasof XD. However, the intensity changes of the PL emission of P1with excitation power and temperature suggest trapped excitoncharacter, as described before. Within the resolution of theobserved PL spectra, we cannot identify the presence of P1−3 inthe FM phase. We note that, in the energy range of the P1−3,phonon replicas of XD with the Ag1 mode have been reported.15Figure 5. Anticorrelated emissions. (a) PL emission for selected magnetic field strengths and spectral range along the crystal b axis showingunexpected brightening of emissions. (b) Same as part a but for negative magnetic fields. (c) Maximum intensity of the XB and XD emissions forthemagnetic field sweep in Figure S7b. XD (XB) reduces in intensity by 9.5± 2% (4.1± 4%) forBb> 0.18T and by 12.3± 2% (8.1 ± 4.2%) forBb< −0.2 T. (d) Maximum intensity of the emissions X1, X2, and X3. Pointed parts of the graphs indicate regions with large error bars.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.5c05470ACS Nano 2025, 19, 33156−3316333160https://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?fig=fig5&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.5c05470?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asAt higher excitation powers, we find signatures of theseemissions overlaid with P1−3. The attribution of X4 and X5,centered around 1.339 and 1.336 eV, respectively, needs to beclarified in further studies. At this point, we can report that X4and X5 show an evolution with magnetic field Bb that is similar tothat of XD. However, in FM order, the emission peaks of X4 andX5 are not discernible anymore.While the emission peaks between 1.31 and 1.345 eVexperience very similar emission changes in magnetic fieldscompared to XD, the emissions XB and X1−3 between 1.345 and1.37 eV behave very differently. Panels a and b of Figure 5 showspectra in this energy range for the same magnetic field values asthose in Figure 4a,b. At zero magnetic field, the emissionenergies of X1−3 are located at E1 = 1.355 eV, E2 = 1.352 eV, andE3 = 1.348 eV.Most strikingly, the intensity of XB as a function ofBb evolves very differently compared to that of XD, as highlightedin Figure 5c and compared to Figure S7c,d. As described before,the intensity of XD decreases measurably as individual layers flipthe spin orientation. In contrast, the intensity of XB reduces less,as highlighted in Figure 5c. Very surprisingly, at magnetic fieldsclose to the transition to the FM state, the intensity of XBincreases. Similar to XB, also the transitions X1, X2, and X3experience an increase in intensity close to the saturation field.Additionally, X2 and X3 initially decrease in intensity with thesingle-layer switch at Bb =±0.2 T. In contrast, at Bb =±0.2 T, thetransition X1 increases its intensity. Despite the changes, XB andX2 do not shift in energy before the transition to the FM state.However, X1 and X3 experience a slight blueshift, as shown inFigure 5a,b.Above Bbs , the entire crystal is in the FM state. As describedabove, for applied magnetic fields 0 < Bapplied < Bbs , AFM and FMorder can coexist in the crystal (Figure 3a). Similar to the DR/Rmeasurements, we observe spectral features of both orders in thePLmeasurements. For example, the emission peak of, e.g., X3,FM,is overlaid with the weakened emission peaks of X4 and X5 ofAFM order, as highlighted in Figure 4b. Similarly, the emissionpeak of XB,FM is overlaid with the emission peak of X3 displayedin Figure 5a,b. For the other emission peaks, this superposition isless visible although present.III. CONCLUSIONIn conclusion, photoluminescence and differential reflectancecontrast DR/R measurements reveal steplike changes inintensity for all observed emission peaks correlated to a layer-by-layer switching of the magnetization for fields applied alongthe magnetic easy axis. Some of these emissions around theparity-forbidden transition XD reduce monotonously withincreasing magnetic field and do not experience energy shiftsuntil the saturation field along Bb. We find several exceptionsfrom this trend between XD and XB, which do experience energyshifts and even increase in intensity close to the saturation field.Excitation-power-dependent and temperature-dependent PLmeasurements indicate the presence of trapped exciton species.Of the multitude of emission peaks, only XB, X3, and XD remainpresent up to T = 100 K. Additionally, we find that, in phasesclose to the transition to the FM state, excitonic emission andabsorption from AFM and FM order coexist.IV. METHODSA. Sample Fabrication. Bulk CrSBr crystals were fabricatedthrough chemical vapor transport.52 Nanometer-thin CrSBr and hBNflakes were mechanically exfoliated onto poly(dimethylsiloxane) andtransferred onto Si substrates with an 80-nm-thick oxide layer. Thelayer thicknesses were determined using an atomic force microscope(Oxford Instruments Cypher) equipped with AC160 cantilevers(Oxford Instruments). Atomic force microscopy yields a CrSBrthickness of 8.2 ± 0.3 nm (11.3 ± 0.2 nm) corresponding to 10 (14)layers for the unencapsulated (encapsulated) sample. Both sampleswere annealed at 200 degrees for several hours.B. Optical Spectroscopy. Optical spectroscopy was carried out ina home-built confocal setup for magneto-optical spectroscopy.39 Thesample was placed inside a closed-cycle cryostat (attocube systems,AttoDry 1000XL) equipped with a vector magnet (z axis, solenoid,maximumfield = 5T; x/y axis, split coil, maximumfield = 2 T).We usedlow-temperature piezopositioners (attocube systems, ANPx101 andANPz102) to position the sample with respect to a low-temperatureapochromatic objective. PL and DR/R measurements were performedin backscattering geometry at a sample temperature of 4.7 K. The signalwas dispersed inside a Czerny−Turner spectrograph (TeledynePrinceton Instruments, SpectraPro HRS-500) and detected by aCCD camera (Teledyne Princeton Instruments, Pylon BRexcelon100). For DR/R measurements, we used a tungsten−halogen lamp(Thorlabs, SLS201L/M) polarized along the crystal b axis by ananoparticle−film polarizer and an achromatic half-waveplate. For PLmeasurements, we used a HeNe laser (Thorlabs, HNL210LB) with itspolarization aligned along the crystal a axis, while the emission wasdetected along the b axis. PL measurements were performed atexcitation powers ranging from 0.2 to 260 μW. Magnetic-field-dependent measurements were performed by initializing the CrSBrsample in the FM state, ramping the magnet to −0.6 T (−0.5 T) for theencapsulated 14-layer (unencapsulated, 10-layer) sample, followed by asweep to 0.6 T (0.5 T) and subsequent inversion of the sweep directionto obtain a full hysteresis.C. Transfer-Matrix Analysis. For analysis of the DR/R measure-ments, we apply a transfer-matrix formalism,18,19,53 using a Lorentzianoscillator model for the dielectric constant of CrSBr:= +fi( )/jjj j22 2(1)where ωj and Γj denote the oscillator frequency and decay rate and f jdenotes the oscillator strength of the jth oscillator. We account for aconstant background permittivity ϵ∞ = 10, similar to Wang et al.,19 anduse the permittivities ϵSiOd2= 2.1,54 ϵSi = 1355 and ϵhBN = 4.6.56ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsnano.5c05470.Polarization-dependent PL measurements of the encap-sulated sample, comparative PL spectra of the twosamples and excitation-power-dependent PL of theunencapsulated sample, comparison of the DR/R andPL spectra in the AFM and FM order, excitation-power-dependent PL intensities, temperature-dependent PLmesaurements, sample image and domain mapping usingDR/Rmeasurements, transfer-matrix analysis for theDR/R spectra recorded at selected magnetic field strengths,and PL hysteresis and measurement of the PL emissionintensities of XB and XD for the full hysteresis of theencapsulated and unencapsulated samples (PDF)AUTHOR INFORMATIONCorresponding AuthorBernhard Urbaszek − Institute for Condensed Matter Physics,TU Darmstadt, D-64289 Darmstadt, Germany; orcid.org/0000-0003-0226-7983; Email: bernhard.urbaszek@pkm.tu-darmstadt.deACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.5c05470ACS Nano 2025, 19, 33156−3316333161https://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.5c05470?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsnano.5c05470/suppl_file/nn5c05470_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Bernhard+Urbaszek"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-0226-7983https://orcid.org/0000-0003-0226-7983mailto:bernhard.urbaszek@pkm.tu-darmstadt.demailto:bernhard.urbaszek@pkm.tu-darmstadt.dewww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.5c05470?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asAuthorsLukas Krelle − Institute for Condensed Matter Physics, TUDarmstadt, D-64289 Darmstadt, Germany; orcid.org/0009-0005-2981-6112Ryan Tan − Institute for Condensed Matter Physics, TUDarmstadt, D-64289 Darmstadt, GermanyDaria Markina − Institute for Condensed Matter Physics, TUDarmstadt, D-64289 Darmstadt, GermanyPriyanka Mondal − Institute for Condensed Matter Physics, TUDarmstadt, D-64289 Darmstadt, GermanyKseniia Mosina − Department of Inorganic Chemistry,University of Chemistry and Technology Prague, 16628Prague 6, Czech Republic; orcid.org/0000-0003-3570-5337Kevin Hagmann − Institute for Condensed Matter Physics, TUDarmstadt, D-64289 Darmstadt, GermanyRegine von Klitzing − Institute for Condensed Matter Physics,TU Darmstadt, D-64289 Darmstadt, GermanyKenji Watanabe − Research Center for Electronic and OpticalMaterials, National Institute for Materials Science, Tsukuba305-0044, Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − Research Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Zdenek Sofer − Department of Inorganic Chemistry, Universityof Chemistry and Technology Prague, 16628 Prague 6, CzechRepublicComplete contact information is available at:https://pubs.acs.org/10.1021/acsnano.5c05470Author ContributionsK.M. and Z.S. grew bulk CrSBr crystals. T.T. and K.W. grew bulkhBN crystals. L.K. fabricated the samples and performed opticalspectroscopy with R.T.. P.M., L.K., K.H., and R.v.K. performedand analyzed the AFM measurements. L.K., R.T., and B.U.analyzed the optical spectra. L.K., R.T., P.M., D.M., and B.U.discussed the results. B.U. suggested the experiments andsupervised the project. L.K., R.T., and B.U. wrote themanuscript. All authors contributed to the final manuscript.NotesThe authors declare no competing financial interest.ACKNOWLEDGMENTSWe thank Florian Dirnberger for fruitful discussions. K.W. andT.T. acknowledge support from the JSPS KAKENHI (Grants21H05233 and 23H02052), CREST (JPMJCR24A5), JST, andWorld Premier International Research Center Initiative, MEXT,Japan. Z.S. was supported by the ERC-CZ program (ProjectLL2101) from the Ministry of Education Youth and Sports andby the project Advanced Functional Nanorobots (Reg. 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