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Anomalous magnetic properties in Mn(Se, S) system
Journal of Magnetism and Magnetic Materials 483 (2019) 205–211
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Research articles
Anomalous magnetic properties in Mn(Se, S) system a,b
c
Chun-Hao Huang , Chin-Wei Wang , Chung-Chieh Chang ⁎ Gwo-Tzong Huangb, Ming-Jye Wange,b, , Maw-Kuen Wub
b,d
T b
, Yung-Chi Lee ,
a
Department of Physics, National Taiwan University, Taipei 10617, Taiwan Institute of Physics, Academia Sinica, Taipei 11529, Taiwan National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan d School of Dental Technology, College of Oral Medicine, Taipei Medical University, Taipei 110, Taiwan e Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan b c
A R T I C LE I N FO
A B S T R A C T
Keywords: MnSe MnS Magnetic susceptibility Crystal and magnetic structure
We have carried out detailed structural, magnetic and thermal properties studies of the Mn(Se, S) system. The detailed structural studies using synchrotron X-ray and neutron diffractions on MnSe confirm the coexistence of cubic and hexagonal phases at low temperature, which shows strong thermal hysteresis suggesting the first-order nature of the transition. Based on the temperature-dependent magnetic susceptibility, we confirmed that the substitution of the sulfur element to selenium critically modified the complex magnetism observed in MnSe. It was to our surprise to observe a magnetic anomaly at 16 K way below the antiferromagnetic order that sets at about 160 K in MnS. Our neutron study at low temperature did not reveal any additional magnetic structure below 16 K. However, the specific heat of MnS shows a clear T1.5 temperature dependence below 16 K, suggesting the possible emergence of a weak ferromagnetic order.
1. Introduction Transition metal chalcogenides and dichalcogenides have drawn great attention because of their important optical, electrical, transport, and a wide range of magnetic properties [1–6]. As spintronics using the quantum information of spin has attracted more and more interests [7,8], magnetic materials such as Fe-Se and Mn-Se systems have been highly investigated due to their magnetic functions in many applications like spin transistors, magnetoresistive devices, and quantum computation devices. Fe7Se8 and Fe3Se4 of NiAs-type lattice structure with ordered Fe vacancies are ferrimagnetic with Curie temperatures at 314 K and 455 K, respectively [9,10]. There exist additional merits as they can be grown epitaxially on semiconductors such as ZnSe, Si, and GaAs. In addition, MnSe and MnSe2·also show great potential for the applications in diluted magnetic semiconductors [10–13]. The unexpected discovery of superconductivity in FeSe and related compounds [14–16] has further inspired much more interests as it could provide new insight into the mechanism of the unconventional high-temperature superconductivity. More recently, pressure-induced superconductivity was first observed in CrAs system [17]. Shortly after, by introducing alkaline-metal in CrAs system, K2Cr3As3 was found to be superconducting at 6.1 K without any external pressure [18]. It has
⁎
been known that MnP, with orthorhombic Pbmn structure, shows ferromagnetic behavior between 50 K and 291 K at ambient condition [19,20]. Under an 8 G Pa pressure, MnP reveals superconductivity below 1 K [21]. MnSe exhibits similar magnetic behaviors as those observed in MnP at ambient pressure [22]. Several publications have reported the magnetic properties of MnSe [1,24,25]. However, the reported results seem not mutually consistent, especially on the details of magnetic anomalies between 100 K and 200 K. Efrem D’Sa et al. [23] studied the magnetic structure of MnSe using neutron diffraction. Their results − suggested the coexistence of main cubic phase (Fm3m symmetry) and minor hexagonal phase (P63/mmc symmetry), which takes up about 30% in volume, below 266 K. However, these results could not give a satisfactory explanation on the magnetic anomalies observed at low temperature. In addition, the origin of the partial conversion from cubic-phase to hexagonal-phase in MnSe remains unanswered. On the other hand, MnS, an isostructural compound of MnSe at room temperature, also exhibits complex magnetic phases at low temperature but without the coexistence of mixed phases in structure. A detailed study on the effects of substituting sulfur to selenium may provide more insights into the complex phases in MnSe. Furthermore, there is no existing magnetic data below 90 K [31,32] for MnS.
Corresponding author at: Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan. E-mail address: [email protected] (M.-J. Wang).
https://doi.org/10.1016/j.jmmm.2019.03.105 Received 15 February 2019; Received in revised form 22 March 2019; Accepted 25 March 2019 Available online 26 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 483 (2019) 205–211
C.-H. Huang, et al.
Therefore, we have carried out detailed studies using different tools to investigate the properties of MnSe1−xSx with 0 ≤ x ≤ 1. In this report, we present the results of a systematic investigation on the structureproperty correlation of the Mn(Se, S) series, and discuss the complex magnetism in both MnSe and MnS systems. Our experiments show, in addition to the antiferromagnetic order at ∼150 K, the presence of magnetic order below 16 K in MnS without any phase change. 2. Experimental details A series of polycrystalline MnSe1−xSx samples were synthesized by solid-state reaction method using raw materials of Mn (99.95%, AlfaAesar), Se (99.95%, Acros-Organic), and S (99.99%, Riedel-de Haën). The stoichiometric mixture of these three elements was sealed in an evacuated quartz ampoule. The mixture was slowly heated to 750 °C, annealed for several hours, and then furnace-cooled to room temperature. The structural characterization of polycrystalline samples was performed using the in-house X-ray diffractometer (Rigaku Rotaflex 18KW rotating anode diffractometer, Cu Kα radiation). The temperature dependence of X-ray powder diffraction (XRD) was performed at the BL17A beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The wavelength of the X-ray is 1.3218 Å. The Mar345 image plate, 300 mm downstream from the sample, was used for recording the two-dimensional powder diffraction patterns. Data reduction from the 2D patterns to the 1-D diffraction profiles was carried out by the Fit2D program [26]. The magnetic susceptibility measurements were performed by a QUANTUM DESIGN metal shielded 7-T VSM SQUID (superconducting quantum interference device) magnetometer with a temperature range of 2–300 K. Low-temperature heat capacity measurements down to 2 K were carried out using thermal relaxation method in a physical property measurement system (PPMS, QUANTUM DESIGN). The neutron powder diffraction (NPD) experiments were conducted on both the high-resolution powder diffractometer, Echidna [27] (Ge (3 3 5)/(3 3 1) monochromator selecting 1.622/2.4395 Å neutrons) and the high-intensity powder diffractometer, Wombat [28] (Ge (1 1 3) monochromator selecting 2.41 Å neutrons) on the OPAL reactor, ANSTO, Australia. The FullProf package was used for Rietveld refinement of the XRD and NPD data [29]. Fig. 2. The temperature dependent magnetic susceptibility of MnSe1−xSx system. (a) The data of MnSe and MnS in both warming and cooling cycles. The inset shows no hysteresis for the kink of MnS near 150 K. (b) The warming data of Se-rich samples. The anomalies near 170 K and 270 K disappear with 25% or higher sulfur substitution. (c) A sharp magnetic ordering signal arises near 16 K for S-rich samples. The inset shows the kinked structure near 150 K which shifts to high temperature as the fraction of sulfur increases.
3. Results and discussion Fig. 1(a) displays the powder X-ray diffraction patterns of MnSe1−xSx samples. All samples are NaCl-type face-center-cubic − structure with the space group of Fm3m [23]. As sulfur substitution level increased, the diffraction peaks shift to higher angle gradually.
The lattice constant of MnSe1−xSx samples decreases almost linearly from 5.4602 Å in MnSe to 5.2223 Å in MnS, as shown in Fig. 1(b). Besides, the Mn-Se(S) bond length is surely decreased from 2.7301 Å to 2.6112 Å and the distance between Mn-Se(S) stacking layers is decreased from 1.5762 Å to 1.5076 Å. It is noted that every peak in the diffraction pattern of x = 0.75 sample has a companion peak. The Rietveld refinement result shows that the diffraction pattern can be best − fitted by two α-phases (Fm3m ) with lattice constant 5.2936 Å and 5.2498 Å in the volume ratio of 73% and 27%, respectively. Based on the linear dependence of lattice constant on S content in MnSe1−xSx system, their stoichiometry is MnSe0.3S0.7 and MnSe0.12S0.88. This observation indicates that there exists a solid solubility gap around x = 0.75 in the MnSe1−xSx system. The magnetic susceptibilities of MnSe and MnS under warming and cooling processes are shown in Fig. 2(a). The susceptibility of MnSe measured in the cooling process shows a huge irregular broad anomaly in the temperature range of 200 K to 115 K and then smoothly decreases to low temperature. The susceptibility in the warming process demonstrates a more complicated behavior. The susceptibility in the warming process demonstrates a more complicated behavior. The
Fig. 1. (a) XRD patterns of six representative MnSe1−xSx compounds (x = 0, 0.05, 0.25, 0.5, 0.75, and 1) and (b) variation of lattice constant a. 206
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broad anomaly appears at the same temperature range during cooling and warming, but the maximum shifts to a higher temperature of ∼172 K in the warming process. An additional hump emerges near 270 K. We noted that the hightemperature behavior in the warming process is similar to that reported by Efrem D'Sa et al. [23]. On the other hand, the data on MnSe reported by Peng et al. [1] did not observe the hump at 270 K, which is consistent with our data in the cooling process. There exists a clear antiferromagnetic-like transition at ∼160 K in both cooling and warming process in α-MnS, as shown in the inset of Fig. 2(a). Surprisingly, a gigantic magnetic anomaly appears below 16 K. The antiferromagnetic-like transition at high temperature was reported by Corliss et al. [32]. However, Corliss et al. did not measure the susceptibility of MnS below 50 K. Fig. 2(b) and (c) present the magnetic susceptibility of selenium-rich and sulfur-rich samples, respectively, measured under the warming process. With 5% sulfur-substitution, the anomalies are significantly modified. The magnitude of the anomaly becomes smaller and the hump peak at 270 K shifts to a lower temperature of ∼248 K. At sulfur substitution level of 25%, the hump at 270 K clearly disappears, and the broad anomaly onset at 220 K evolves to an antiferromagnetic-like transition at ∼130 K, meanwhile a weak anomaly appears at ∼16 K. As the sulfur substitution further increased, the antiferromagnetic-like transition shifts to high temperature, from ∼130 K in x = 0.25 sample to ∼160 K in α-MnS. Moreover, the anomaly below 16 K is enhanced by increasing the sulfur content. We performed the synchrotron X-ray diffraction to determine the crystal structure of α-MnSe at low temperature. Fig. 3(a) displays the diffraction pattern of α-MnSe at 300 K, which fits well to the NaCl-type − structure (α-MnSe, Fm3m symmetry). Fig. 3(b) shows the temperature evolution of the diffraction patterns from 300 K to 100 K (cooling process) and back to 300 K (warming process). The peaks associated with NiAs-type (β-MnSe with P63/mmc symmetry) structure emerge at a temperature around 140 K in the cooling process, marked as (h k l)β. Their intensities become higher at the lower temperature, indicating the increase of β-MnSe phase. For the warming process, these peaks sustain till 260 K and disappear above 270 K. This substantial thermal hysteresis on the appearance of β-MnSe is consistent with the magnetic susceptibility measurements. Fig. 3(c) shows the refinement result of Xray diffraction at 100 K. The fraction of α-MnSe and β-MnSe is about 76.6% and 23.4% respectively, which is consistent with the earlier report [23]. The lattice constants are a = 5.4416 Å for α-MnSe and a = 3.8374 Å, c = 5.9874 Å for β-MnSe. Their corresponding unit-cell volumes are 161.13 Å3 and 229.07 Å3. Because one-unit cell of α-MnSe and β-MnSe consists of 4 and 6 formulas respectively, the volume for one MnSe formula in β-MnSe is 38.18 Å3 that is slightly smaller than 40.28 Å3 in α-MnSe, which indicates the preference of β-MnSe phase at low temperature. As sample cooled, the FWHM (full-width-half-maximum) of (0 0 2) diffraction peak of α-MnSe, as shown in Fig. 3(d), increases significantly below 140 K that β-MnSe appears firstly, and the broadened peak remains in the warming process till 260 K when the βMnSe peaks disappear. This observation can be understood as the consequence of structural transformation under thermal stress at low temperature. On the other hand, we do not observe any structural change in α-MnS between 100 K and 300 K (shown in supplementary Fig. 1). Thus, the observed anomalous magnetic features in α-MnSe are the consequence of the coexistence of two phases. Fig. 4 shows the results of NPD (neutron powder diffraction) experiments. The Rietveld refinement result of neutron diffraction pattern at a temperature below the magnetic transition of α-MnSe phase is shown in Fig. 4(a). Structural peaks associated with the two polymorphs of MnSe (α- and β-phase) and the magnetic Bragg peaks are identified. These magnetic diffraction peaks are associated with α-MnSe with k = (1/2, 1/2, 1/2) and β-MnSe with k = (0, 0, 0). The magnetic structure of α-MnSe as shown in Fig. 4(b) is colinear anti-ferromagnet of which magnetic moments are perpendicular to the cubic diagonal
Fig. 3. The structural study of Mn(Se, S) using synchrotron X-ray. (a) The diffraction pattern at 300 K. The major peaks are from α-MnSe and some small peaks from impurity phases are observed. (b) The temperature evolution of diffraction patterns. The diffraction peaks of β-MnSe (NiAs-type), marked on the top of Figure, emerge at a temperature below 140 K in the cooling process and disappear above 260 K in the warming process. (c) The refinement result of diffraction pattern at 100 K. The symbols are the measured data, and the red line is the refined result. The fraction of α-MnSe and β-MnSe is 76.6% and 23.4% respectively. (d) The FWHM of (0 0 2) diffraction peak as a function of temperature, which becomes larger as the temperature decreases and shows a large thermal hysteresis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[1 1 1]. For the β-MnSe, the Mn2+ moments form ferromagnetic layers on the a-b plane which are anti-parallel with the adjacent layers along hexagonal c-axis, as shown in Fig. 4 (c). In both magnetic phases, the magnetic moments lay on the closest-packed planes which are stacking differently to form the α- and β-phase. Thus, the presence of stacking fault from thermal stress is the origin for the coexistence of α-MnSe and β-MnSe phases [30]. Utilizing the high-speed capability of the high-intensity powder 207
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Fig. 4. . (a) Rietveld refined NPD results of MnSe at 50 K. The crosses represent the observed counts, and the red line is the calculated profile. The difference between the observed and calculated patterns is shown as a blue line at the bottom. The calculated positions of the reflections are shown as vertical bars. The schematic magnetic structure for (b) α-MnSe and (c) β-MnSe phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
diffractometer WOMBAT at ANSTO, we continuously collected the diffraction profiles during the cooling process and conducted proper data acquisition step-wisely with a temperature increment of 5 K in the warming process. As seen in Fig. 5(a) and (b), a significant thermal hysteresis manifests in the diffraction patterns of α-MnSe, which is consistent with the observed magnetic properties. Fig. 5(c) to (e) show the evolution of α-MnSe (2 2 0) peak under the thermal cycle. In the cooling process, the peak intensity of (2 2 0) α slightly increases at hightemperature and then abruptly decreases below 160 K, indicating the phase conversion from α-MnSe to β-MnSe. The saturated phase conversion of 23.4% is reached below 95 K. Accompanied with the α- to βMnSe phase conversion, the FWHM of (2 2 0)α gradually increases by about 50%, which is regarded as the strain broadening caused by the insertion of the β-phase domains into the α-phase matrix. On the other hand, the release of the lattice strain is accompanied by the β to α phase conversion during the warming process. The integrated intensities of (1/2 1/2 1/2)α +(0 0 1)β and (1/2 5/2 5/2)α are plotted in Fig. 5(f) and (g), respectively. Fig. 5(h) shows the thermal variation of the paramagnetic scattering background, sampled by integrating neutron intensities in the 2θ range from 14° to 19°. The decrease of the paramagnetic scattering background signifies the development of magnetic correlations in the sample. Upon cooling, the total intensities of the magnetic peaks (1/2 1/2 1/2)α and (0 0 1)β increase below ∼160 K and the (1/2 5/2 5/2)α peak appears below ∼120 K. The β-MnSe reveals the long-range magnetic order state almost right after the β-phase forms
Fig. 5. Color maps of NPD data for MnSe under (a) cooling process and (b) warming process, with intensity shown in color with the scale on the right. The integrated intensity (c) FWHM (d), and peak position (e) of the nuclear Bragg reflection (2 2 0)α. The integrated intensity of MnSe Bragg magnetic reflection (f) (1/2 1/2 1/2)α +(0 0 1)β (g) (1/2 5/2 5/2)α (h) The thermal variation of the paramagnetic scattering background, sampled by integrating neutron intensities in the 2θ range from 14° to 19°. In (c)-(h), the blue circle and red square are used for the cooling process and warming process, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and the α-phase remains paramagnetic down to ∼120 K. In the warming process (1/2 5/2 5/2)α vanishes at ∼130 K. A small difference between the magnetic ordering temperatures (TN, the onset temperature of non-zero intensity of magnetic diffraction peak) of α-MnSe in the cooling and warming processes might be due to the lag between the actual sample temperature and sensor reading in the cooling process. On the other hand, the long-range magnetic ordering in the β-MnSe persists up to 270 K that the α-phase recovers most of its full population. These results provide direct reasons for explaining the anomalous thermal behavior in the magnetic susceptibility of α-MnSe. The sulfur substituted samples, MnSe1−xSx, are isostructural with the MnSe at room temperature and they do not exhibit the phase transformation down to 5 K. The temperature dependence of neutron 208
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Fig. 6. (a) Color map of NPD data for MnSe0.8S0.2 under warming process; (b) Rietveld refined NPD results of MnSe0.8S0.2 at 50 K; (c) Variations of magnetic moment per Mn atom as a function of temperatures and the inset shows α-MnSe magnetic structure; (d) Evolution of lattice parameters of cubic phase as a function of temperature in MnSe1−xSx, x = 0, 0.2 0.5 0.7 and 1.
samples at a temperature below 100 K. Further analysis of the specific heat data shows that, below 16 K, the temperature dependence of specific heat for MnS follows T1.5 instead of T3 behavior at high temperature, as shown in Fig. 7 (b). The coincidence between the temperatures where the appearance of T1.5 dependence in specific heat and the rise of gigantic magnetic signal suggests the existence of possible ferromagnetic order at low temperature [33]. We noted that a similar abrupt increase of susceptibility at low temperature was reported in the α-MnSe under a high external magnetic field near 50 K [1]. Peng et al. attributed this anomaly to the spin-flop under high external magnetic field (> 0.5 Tesla) in antiferromagnetic order state. Our observation of the susceptibility anomaly at 16 K is unlikely due to the spin-flop mechanism because the magnetic field used in our susceptibility measurement is less than 100 Oe. Nevertheless, based on our neutron results, the moments of MnS in the layer are aligned in parallel and antiparallel with the moments in adjacent layers that give the rise of the antiferromagnetic ordering at about 150 K. We also observed a large contraction in lattice parameters below TN. Thus, it is likely that the coupling between the moments of the next-nearest-layer, which are aligned in parallel, is strong enough to provide the needed internal field for spin-flop and subsequently leads to the coexistence of two magnetic orders at low temperature. More detailed works using the single crystal to determine the magnetic coupling strength in different layers may provide more insight into the origin of the low-temperature magnetic anomaly. The origin of huge susceptibility anomaly in MnSe between 120 K and 200 K is an interesting but complicated problem. As reported by Wang et al., α-MnSe undergoes a dramatic distortion from cubic to orthorhombic structure at 30 GPa [34]. The MnSe6 octahedron is distorted in the orthorhombic phase, which results in the spin crossover of Mn2+ from high spin state to low spin state and a susceptibility drop. Based on the neutron data, the Mn2+ ions in β-phase remain at high spin state even though the α-to-β phase transformation occurs around 140 K. In addition, there is no structural change in the α-MnSe and the Mn2+ ions of α-MnSe below TN are also at high spin state. Therefore, the drop of susceptibility near 140 K in α-MnSe is not due to the spin crossover of Mn2+ ion, but instead from the antiferromagnetic ordering of β-phase.
diffraction in MnSe1−xSx can be exemplified by the x = 0.2 sample, as illustrated in Fig. 6(a). The signature peak of the β phase, (1 0 1)β, is not observed. The emerging peaks below 140 K are associated with the magnetic ordering of α-phase. A representative diffraction pattern below TN, as shown in Fig. 6(b), demonstrates a NaCl-type phase with a magnetic structure of k = (1/2, 1/2, 1/2). The magnetic order parameters of samples with different levels of sulfur substitution are summarized in Fig. 6(c). The magnetic ordering temperature of MnSe1−xSx is enhanced by increasing sulfur content from 130 K of x = 0.2 to 170 K of x = 1 sample. This could be attributed to the enhancement of the magnetic exchange coupling promoted by reduced Mn-Mn distance, as shown in Fig. 6(d) which plots the temperature dependent lattice parameters of all sulfur substituted samples. The decrease in the intermanganese separation increases the exchange coupling between Mn2+ and accordingly enhanced TN. Moreover, we observed a nonlinear lattice parameter change near the magnetic ordering temperature in each sample. Similar lattice distortion had been reported by Heikens et al. [30]. However, in contrast to the proposed rhombohedral distortion below TN, our results do not show any structural change. The temperature of exhibiting non-linear lattice constant change coincides with TN, suggesting that its origin might be from the coupling of magnetism and lattice. Our powder neutron diffraction results reveal that the hexagonal βphase of the NiAs-type structure no longer appears in MnSe1−xSx with x > 0.2. The substitution of selenium by smaller-sized sulfur atoms in MnSe1−xSx can generate an effectively negative pressure in the lattice to resist the thermally induced stress which is thought to be the cause of the α-to-β phase transformation. It remains a puzzle regarding the origin of magnetic transition below 16 K observed in MnSe1−xSx as our NPD did not show any new magnetic features. We performed detailed specific heat measurements on these samples in the temperature range of 2–300 K as shown in Fig. 7(a). We observe a clear transition at ∼120 K in MnSe but do not observe any anomaly around 200 K where the susceptibility sharply rises. The specific heat jump in sulfur-substituted samples shifts to high temperature, near 130 K and 150 K for x = 0.5 and x = 1 respectively. These results are consistent with both the magnetic susceptibility and neutron measurements. No obvious abnormal feature was found in all 209
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the susceptibility of the sample becomes larger at a lower temperature before the α-to-β structural transformation occurs at around 140 K. Below 140 K, the long-range antiferromagnetic order of β-MnSe and the enhancement of antiferromagnetic coupling between the short-range ferromagnetic sheets at adjacent layers due to the existence of β-MnSe reduce the susceptibility rapidly till TN (120 K) that the long-range antiferromagnetic ordering of α-MnSe sets in. In the warming process, the magnetic susceptibility increases quickly above TN of α-MnSe. However, the antiferromagnetic coupling of the short-range ferromagnetic sheets at adjacent layers does not completely vanish due to the existence of β-phase, which results in a smaller susceptibility value comparing with that in the cooling process. The magnetic susceptibility at the temperature between 120 K and 200 K is the result of the competition between the magnetic locking effect of β-phase and thermal fluctuation on the short-range ferromagnetic sheets in α-MnSe. Finally, the short-range ferromagnetic sheets disappear near 200 K where the upper temperature of anomaly appears. The above argument provides a qualitative explanation for the observed magnetic anomaly. We need further investigations to obtain the evidence for the presence of shortrange ferromagnetic sheets in α-MnSe, the coupling strength between adjacent ferromagnetic layers, and how the short-range ferromagnetic sheets interact with the β-phase. 4. Summary In summary, we report a detailed analysis of the physical properties of MnSe1−xSx. Our results provide shreds of evidence to clarify the origin for the coexistence of the α- and β-MnSe phases below 270 K, which appears as a hump in susceptibility measurement. The formation of the β-MnSe in α-MnSe most likely is due to the presence of stacking fault from thermal stress. In addition, a broad magnetic anomaly shows up between 115 K and 200 K. The detailed magnetic structure from neutron experiments suggests that the existence of two-dimensional short-range ferromagnetic sheets on (1 1 1) plane of α-MnSe below 200 K could be the reason responsible for this magnetic anomaly. These observed anomalies can be suppressed by sulfur substitution to selenium. The refinement of MnSe NPD shows that the anti-ferromagnetism is along c-axis for hexagonal type (β-phase) and along [1 1 1] direction for cubic-type (α-phase) structures. Though the NPD of MnS doesn’t show either atomic nor magnetic structural transformation below 16 K, the specific heat of MnS exhibits T1.5 dependence, which could be attributed to the presence of ferromagnetism.
Fig. 7. (a) The temperature dependence of heat capacity of MnSe1−xSx, x = 0, 0.5, and 1 measured during the cooling process. Weak peak located at ∼290 K comes from the thermal conducting grease used in the measurement. (b) At the low-temperature regime, a crossover in the temperature dependence of the heat capacity from T3 (phonon) to T1.5 in MnS at the temperature showing a huge magnetic susceptibility increase in Fig. 3(a).
In order to have a complete picture, we summarize our observations related to the large susceptibility anomaly as in the followings. 1) The antiferromagnetic system typically has a maximum susceptibility value about the order of magnitude of 10-5 emu/g-Oe, (for example, MnS in Fig. 2(c)), which is an order of magnitude smaller than the anomaly in α-MnSe. Therefore, the large susceptibility anomaly can’t be attributed to the long-range antiferromagnetic ordering of α- or β-MnSe. 2) The susceptibility of pure α-MnSe (also x = 0.05 sample) increases significantly below 200 K at which the anomaly emerges. We note that there is no long-range magnetic order at this temperature from the neutron scattering results. 3) Furthermore, the anomalous magnetic response only exists in the α-phase MnSe1−xSx that would partially transform into the β-phase at low temperature. 4) The ordered magnetic structure in both α- and β- MnSe are the stacks of alternatively ferromagnetic layers with a Se layer in between. The magnetic interaction between two adjacent ferromagnetic layers in α-phase might be weakened by the Se layer in α-phase, but not in β-phase because the Mn atom sits directly on top (or bottom) of the Mn atoms at adjacent layers. Based on the above observations, most likely the short-range twodimensional ferromagnetic sheets from below 200 K on the (1 1 1) plane of α-MnSe. These ferromagnetic sheets are more susceptible to the applied magnetic field. The susceptibility anomaly in cooling and warming between 120 K and 200 K can be understood with the following scenario. Below 200 K in the cooling process, more short-range ferromagnetic sheets form and thermal fluctuation effect reduces, and
Acknowledgements The authors acknowledge the fruitful discussion with Prof. MingHsien Lee of Tamkang University. This research is supported by the Ministry of Science and Technology Grant NSC102-2112-M-001-025MY3, MOST 103-2739-M-213-001-MY3, MOST 108-2633-M-001-001 and the Academia Sinica Thematic Research AS-106-TP-A01, Taiwan. References [1] Q. Peng, Y.J. Dong, Z.X. Deng, H.H. Kou, S. Gao, Y.D. Li, Selective synthesis and magnetic properties of alpha-MnSe and MnSe2 uniform microcrystals, J. Phys. Chem. B 106 (2002) 9261–9265. [2] A. Ennaoui, Photoelectrochemistry of highly quantum efficient single-crystalline nFeS2 (Pyrite), J. Electrochem. Soc. 133 (1986) 97. [3] H.I. Heulings, X. Huang, J. Li, T. Yuen, C. Lin, Mn-substituted inorganic-organic hybrid materials based on ZnSe: Nanostructures that may lead to magnetic semiconductors with a strong quantum confinement effect, Nano Lett. 1 (2001) 521–526. [4] X. Wu, D. Shen, Z. Zhang, J. Zhang, K. Liu, B. Li, Y. Lu, B. Yao, D. Zhao, B. Li, On the nature of the carriers in ferromagnetic FeSe, Appl. Phys. Lett. 90 (2007) 112105. [5] H.S. Jarrett, W.H. Cloud, R.J. Bouchard, S.R. Butler, C.G. Frederick, J.L. Gillson, Evidence for Itinerant-Electron Ferromagnetism, Phys. Rev. Lett. 21 (1968) 617–620. [6] J.C. Mikkelsen, A. Wold, Magnetic properties of CoAsxS2−x, J. Solid State Chem. 3 (1971) 39–48.
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Research articles
Anomalous magnetic properties in Mn(Se, S) system a,b
c
Chun-Hao Huang , Chin-Wei Wang , Chung-Chieh Chang ⁎ Gwo-Tzong Huangb, Ming-Jye Wange,b, , Maw-Kuen Wub
b,d
T b
, Yung-Chi Lee ,
a
Department of Physics, National Taiwan University, Taipei 10617, Taiwan Institute of Physics, Academia Sinica, Taipei 11529, Taiwan National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan d School of Dental Technology, College of Oral Medicine, Taipei Medical University, Taipei 110, Taiwan e Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan b c
A R T I C LE I N FO
A B S T R A C T
Keywords: MnSe MnS Magnetic susceptibility Crystal and magnetic structure
We have carried out detailed structural, magnetic and thermal properties studies of the Mn(Se, S) system. The detailed structural studies using synchrotron X-ray and neutron diffractions on MnSe confirm the coexistence of cubic and hexagonal phases at low temperature, which shows strong thermal hysteresis suggesting the first-order nature of the transition. Based on the temperature-dependent magnetic susceptibility, we confirmed that the substitution of the sulfur element to selenium critically modified the complex magnetism observed in MnSe. It was to our surprise to observe a magnetic anomaly at 16 K way below the antiferromagnetic order that sets at about 160 K in MnS. Our neutron study at low temperature did not reveal any additional magnetic structure below 16 K. However, the specific heat of MnS shows a clear T1.5 temperature dependence below 16 K, suggesting the possible emergence of a weak ferromagnetic order.
1. Introduction Transition metal chalcogenides and dichalcogenides have drawn great attention because of their important optical, electrical, transport, and a wide range of magnetic properties [1–6]. As spintronics using the quantum information of spin has attracted more and more interests [7,8], magnetic materials such as Fe-Se and Mn-Se systems have been highly investigated due to their magnetic functions in many applications like spin transistors, magnetoresistive devices, and quantum computation devices. Fe7Se8 and Fe3Se4 of NiAs-type lattice structure with ordered Fe vacancies are ferrimagnetic with Curie temperatures at 314 K and 455 K, respectively [9,10]. There exist additional merits as they can be grown epitaxially on semiconductors such as ZnSe, Si, and GaAs. In addition, MnSe and MnSe2·also show great potential for the applications in diluted magnetic semiconductors [10–13]. The unexpected discovery of superconductivity in FeSe and related compounds [14–16] has further inspired much more interests as it could provide new insight into the mechanism of the unconventional high-temperature superconductivity. More recently, pressure-induced superconductivity was first observed in CrAs system [17]. Shortly after, by introducing alkaline-metal in CrAs system, K2Cr3As3 was found to be superconducting at 6.1 K without any external pressure [18]. It has
⁎
been known that MnP, with orthorhombic Pbmn structure, shows ferromagnetic behavior between 50 K and 291 K at ambient condition [19,20]. Under an 8 G Pa pressure, MnP reveals superconductivity below 1 K [21]. MnSe exhibits similar magnetic behaviors as those observed in MnP at ambient pressure [22]. Several publications have reported the magnetic properties of MnSe [1,24,25]. However, the reported results seem not mutually consistent, especially on the details of magnetic anomalies between 100 K and 200 K. Efrem D’Sa et al. [23] studied the magnetic structure of MnSe using neutron diffraction. Their results − suggested the coexistence of main cubic phase (Fm3m symmetry) and minor hexagonal phase (P63/mmc symmetry), which takes up about 30% in volume, below 266 K. However, these results could not give a satisfactory explanation on the magnetic anomalies observed at low temperature. In addition, the origin of the partial conversion from cubic-phase to hexagonal-phase in MnSe remains unanswered. On the other hand, MnS, an isostructural compound of MnSe at room temperature, also exhibits complex magnetic phases at low temperature but without the coexistence of mixed phases in structure. A detailed study on the effects of substituting sulfur to selenium may provide more insights into the complex phases in MnSe. Furthermore, there is no existing magnetic data below 90 K [31,32] for MnS.
Corresponding author at: Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan. E-mail address: [email protected] (M.-J. Wang).
https://doi.org/10.1016/j.jmmm.2019.03.105 Received 15 February 2019; Received in revised form 22 March 2019; Accepted 25 March 2019 Available online 26 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
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Therefore, we have carried out detailed studies using different tools to investigate the properties of MnSe1−xSx with 0 ≤ x ≤ 1. In this report, we present the results of a systematic investigation on the structureproperty correlation of the Mn(Se, S) series, and discuss the complex magnetism in both MnSe and MnS systems. Our experiments show, in addition to the antiferromagnetic order at ∼150 K, the presence of magnetic order below 16 K in MnS without any phase change. 2. Experimental details A series of polycrystalline MnSe1−xSx samples were synthesized by solid-state reaction method using raw materials of Mn (99.95%, AlfaAesar), Se (99.95%, Acros-Organic), and S (99.99%, Riedel-de Haën). The stoichiometric mixture of these three elements was sealed in an evacuated quartz ampoule. The mixture was slowly heated to 750 °C, annealed for several hours, and then furnace-cooled to room temperature. The structural characterization of polycrystalline samples was performed using the in-house X-ray diffractometer (Rigaku Rotaflex 18KW rotating anode diffractometer, Cu Kα radiation). The temperature dependence of X-ray powder diffraction (XRD) was performed at the BL17A beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The wavelength of the X-ray is 1.3218 Å. The Mar345 image plate, 300 mm downstream from the sample, was used for recording the two-dimensional powder diffraction patterns. Data reduction from the 2D patterns to the 1-D diffraction profiles was carried out by the Fit2D program [26]. The magnetic susceptibility measurements were performed by a QUANTUM DESIGN metal shielded 7-T VSM SQUID (superconducting quantum interference device) magnetometer with a temperature range of 2–300 K. Low-temperature heat capacity measurements down to 2 K were carried out using thermal relaxation method in a physical property measurement system (PPMS, QUANTUM DESIGN). The neutron powder diffraction (NPD) experiments were conducted on both the high-resolution powder diffractometer, Echidna [27] (Ge (3 3 5)/(3 3 1) monochromator selecting 1.622/2.4395 Å neutrons) and the high-intensity powder diffractometer, Wombat [28] (Ge (1 1 3) monochromator selecting 2.41 Å neutrons) on the OPAL reactor, ANSTO, Australia. The FullProf package was used for Rietveld refinement of the XRD and NPD data [29]. Fig. 2. The temperature dependent magnetic susceptibility of MnSe1−xSx system. (a) The data of MnSe and MnS in both warming and cooling cycles. The inset shows no hysteresis for the kink of MnS near 150 K. (b) The warming data of Se-rich samples. The anomalies near 170 K and 270 K disappear with 25% or higher sulfur substitution. (c) A sharp magnetic ordering signal arises near 16 K for S-rich samples. The inset shows the kinked structure near 150 K which shifts to high temperature as the fraction of sulfur increases.
3. Results and discussion Fig. 1(a) displays the powder X-ray diffraction patterns of MnSe1−xSx samples. All samples are NaCl-type face-center-cubic − structure with the space group of Fm3m [23]. As sulfur substitution level increased, the diffraction peaks shift to higher angle gradually.
The lattice constant of MnSe1−xSx samples decreases almost linearly from 5.4602 Å in MnSe to 5.2223 Å in MnS, as shown in Fig. 1(b). Besides, the Mn-Se(S) bond length is surely decreased from 2.7301 Å to 2.6112 Å and the distance between Mn-Se(S) stacking layers is decreased from 1.5762 Å to 1.5076 Å. It is noted that every peak in the diffraction pattern of x = 0.75 sample has a companion peak. The Rietveld refinement result shows that the diffraction pattern can be best − fitted by two α-phases (Fm3m ) with lattice constant 5.2936 Å and 5.2498 Å in the volume ratio of 73% and 27%, respectively. Based on the linear dependence of lattice constant on S content in MnSe1−xSx system, their stoichiometry is MnSe0.3S0.7 and MnSe0.12S0.88. This observation indicates that there exists a solid solubility gap around x = 0.75 in the MnSe1−xSx system. The magnetic susceptibilities of MnSe and MnS under warming and cooling processes are shown in Fig. 2(a). The susceptibility of MnSe measured in the cooling process shows a huge irregular broad anomaly in the temperature range of 200 K to 115 K and then smoothly decreases to low temperature. The susceptibility in the warming process demonstrates a more complicated behavior. The susceptibility in the warming process demonstrates a more complicated behavior. The
Fig. 1. (a) XRD patterns of six representative MnSe1−xSx compounds (x = 0, 0.05, 0.25, 0.5, 0.75, and 1) and (b) variation of lattice constant a. 206
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broad anomaly appears at the same temperature range during cooling and warming, but the maximum shifts to a higher temperature of ∼172 K in the warming process. An additional hump emerges near 270 K. We noted that the hightemperature behavior in the warming process is similar to that reported by Efrem D'Sa et al. [23]. On the other hand, the data on MnSe reported by Peng et al. [1] did not observe the hump at 270 K, which is consistent with our data in the cooling process. There exists a clear antiferromagnetic-like transition at ∼160 K in both cooling and warming process in α-MnS, as shown in the inset of Fig. 2(a). Surprisingly, a gigantic magnetic anomaly appears below 16 K. The antiferromagnetic-like transition at high temperature was reported by Corliss et al. [32]. However, Corliss et al. did not measure the susceptibility of MnS below 50 K. Fig. 2(b) and (c) present the magnetic susceptibility of selenium-rich and sulfur-rich samples, respectively, measured under the warming process. With 5% sulfur-substitution, the anomalies are significantly modified. The magnitude of the anomaly becomes smaller and the hump peak at 270 K shifts to a lower temperature of ∼248 K. At sulfur substitution level of 25%, the hump at 270 K clearly disappears, and the broad anomaly onset at 220 K evolves to an antiferromagnetic-like transition at ∼130 K, meanwhile a weak anomaly appears at ∼16 K. As the sulfur substitution further increased, the antiferromagnetic-like transition shifts to high temperature, from ∼130 K in x = 0.25 sample to ∼160 K in α-MnS. Moreover, the anomaly below 16 K is enhanced by increasing the sulfur content. We performed the synchrotron X-ray diffraction to determine the crystal structure of α-MnSe at low temperature. Fig. 3(a) displays the diffraction pattern of α-MnSe at 300 K, which fits well to the NaCl-type − structure (α-MnSe, Fm3m symmetry). Fig. 3(b) shows the temperature evolution of the diffraction patterns from 300 K to 100 K (cooling process) and back to 300 K (warming process). The peaks associated with NiAs-type (β-MnSe with P63/mmc symmetry) structure emerge at a temperature around 140 K in the cooling process, marked as (h k l)β. Their intensities become higher at the lower temperature, indicating the increase of β-MnSe phase. For the warming process, these peaks sustain till 260 K and disappear above 270 K. This substantial thermal hysteresis on the appearance of β-MnSe is consistent with the magnetic susceptibility measurements. Fig. 3(c) shows the refinement result of Xray diffraction at 100 K. The fraction of α-MnSe and β-MnSe is about 76.6% and 23.4% respectively, which is consistent with the earlier report [23]. The lattice constants are a = 5.4416 Å for α-MnSe and a = 3.8374 Å, c = 5.9874 Å for β-MnSe. Their corresponding unit-cell volumes are 161.13 Å3 and 229.07 Å3. Because one-unit cell of α-MnSe and β-MnSe consists of 4 and 6 formulas respectively, the volume for one MnSe formula in β-MnSe is 38.18 Å3 that is slightly smaller than 40.28 Å3 in α-MnSe, which indicates the preference of β-MnSe phase at low temperature. As sample cooled, the FWHM (full-width-half-maximum) of (0 0 2) diffraction peak of α-MnSe, as shown in Fig. 3(d), increases significantly below 140 K that β-MnSe appears firstly, and the broadened peak remains in the warming process till 260 K when the βMnSe peaks disappear. This observation can be understood as the consequence of structural transformation under thermal stress at low temperature. On the other hand, we do not observe any structural change in α-MnS between 100 K and 300 K (shown in supplementary Fig. 1). Thus, the observed anomalous magnetic features in α-MnSe are the consequence of the coexistence of two phases. Fig. 4 shows the results of NPD (neutron powder diffraction) experiments. The Rietveld refinement result of neutron diffraction pattern at a temperature below the magnetic transition of α-MnSe phase is shown in Fig. 4(a). Structural peaks associated with the two polymorphs of MnSe (α- and β-phase) and the magnetic Bragg peaks are identified. These magnetic diffraction peaks are associated with α-MnSe with k = (1/2, 1/2, 1/2) and β-MnSe with k = (0, 0, 0). The magnetic structure of α-MnSe as shown in Fig. 4(b) is colinear anti-ferromagnet of which magnetic moments are perpendicular to the cubic diagonal
Fig. 3. The structural study of Mn(Se, S) using synchrotron X-ray. (a) The diffraction pattern at 300 K. The major peaks are from α-MnSe and some small peaks from impurity phases are observed. (b) The temperature evolution of diffraction patterns. The diffraction peaks of β-MnSe (NiAs-type), marked on the top of Figure, emerge at a temperature below 140 K in the cooling process and disappear above 260 K in the warming process. (c) The refinement result of diffraction pattern at 100 K. The symbols are the measured data, and the red line is the refined result. The fraction of α-MnSe and β-MnSe is 76.6% and 23.4% respectively. (d) The FWHM of (0 0 2) diffraction peak as a function of temperature, which becomes larger as the temperature decreases and shows a large thermal hysteresis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[1 1 1]. For the β-MnSe, the Mn2+ moments form ferromagnetic layers on the a-b plane which are anti-parallel with the adjacent layers along hexagonal c-axis, as shown in Fig. 4 (c). In both magnetic phases, the magnetic moments lay on the closest-packed planes which are stacking differently to form the α- and β-phase. Thus, the presence of stacking fault from thermal stress is the origin for the coexistence of α-MnSe and β-MnSe phases [30]. Utilizing the high-speed capability of the high-intensity powder 207
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Fig. 4. . (a) Rietveld refined NPD results of MnSe at 50 K. The crosses represent the observed counts, and the red line is the calculated profile. The difference between the observed and calculated patterns is shown as a blue line at the bottom. The calculated positions of the reflections are shown as vertical bars. The schematic magnetic structure for (b) α-MnSe and (c) β-MnSe phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
diffractometer WOMBAT at ANSTO, we continuously collected the diffraction profiles during the cooling process and conducted proper data acquisition step-wisely with a temperature increment of 5 K in the warming process. As seen in Fig. 5(a) and (b), a significant thermal hysteresis manifests in the diffraction patterns of α-MnSe, which is consistent with the observed magnetic properties. Fig. 5(c) to (e) show the evolution of α-MnSe (2 2 0) peak under the thermal cycle. In the cooling process, the peak intensity of (2 2 0) α slightly increases at hightemperature and then abruptly decreases below 160 K, indicating the phase conversion from α-MnSe to β-MnSe. The saturated phase conversion of 23.4% is reached below 95 K. Accompanied with the α- to βMnSe phase conversion, the FWHM of (2 2 0)α gradually increases by about 50%, which is regarded as the strain broadening caused by the insertion of the β-phase domains into the α-phase matrix. On the other hand, the release of the lattice strain is accompanied by the β to α phase conversion during the warming process. The integrated intensities of (1/2 1/2 1/2)α +(0 0 1)β and (1/2 5/2 5/2)α are plotted in Fig. 5(f) and (g), respectively. Fig. 5(h) shows the thermal variation of the paramagnetic scattering background, sampled by integrating neutron intensities in the 2θ range from 14° to 19°. The decrease of the paramagnetic scattering background signifies the development of magnetic correlations in the sample. Upon cooling, the total intensities of the magnetic peaks (1/2 1/2 1/2)α and (0 0 1)β increase below ∼160 K and the (1/2 5/2 5/2)α peak appears below ∼120 K. The β-MnSe reveals the long-range magnetic order state almost right after the β-phase forms
Fig. 5. Color maps of NPD data for MnSe under (a) cooling process and (b) warming process, with intensity shown in color with the scale on the right. The integrated intensity (c) FWHM (d), and peak position (e) of the nuclear Bragg reflection (2 2 0)α. The integrated intensity of MnSe Bragg magnetic reflection (f) (1/2 1/2 1/2)α +(0 0 1)β (g) (1/2 5/2 5/2)α (h) The thermal variation of the paramagnetic scattering background, sampled by integrating neutron intensities in the 2θ range from 14° to 19°. In (c)-(h), the blue circle and red square are used for the cooling process and warming process, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and the α-phase remains paramagnetic down to ∼120 K. In the warming process (1/2 5/2 5/2)α vanishes at ∼130 K. A small difference between the magnetic ordering temperatures (TN, the onset temperature of non-zero intensity of magnetic diffraction peak) of α-MnSe in the cooling and warming processes might be due to the lag between the actual sample temperature and sensor reading in the cooling process. On the other hand, the long-range magnetic ordering in the β-MnSe persists up to 270 K that the α-phase recovers most of its full population. These results provide direct reasons for explaining the anomalous thermal behavior in the magnetic susceptibility of α-MnSe. The sulfur substituted samples, MnSe1−xSx, are isostructural with the MnSe at room temperature and they do not exhibit the phase transformation down to 5 K. The temperature dependence of neutron 208
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Fig. 6. (a) Color map of NPD data for MnSe0.8S0.2 under warming process; (b) Rietveld refined NPD results of MnSe0.8S0.2 at 50 K; (c) Variations of magnetic moment per Mn atom as a function of temperatures and the inset shows α-MnSe magnetic structure; (d) Evolution of lattice parameters of cubic phase as a function of temperature in MnSe1−xSx, x = 0, 0.2 0.5 0.7 and 1.
samples at a temperature below 100 K. Further analysis of the specific heat data shows that, below 16 K, the temperature dependence of specific heat for MnS follows T1.5 instead of T3 behavior at high temperature, as shown in Fig. 7 (b). The coincidence between the temperatures where the appearance of T1.5 dependence in specific heat and the rise of gigantic magnetic signal suggests the existence of possible ferromagnetic order at low temperature [33]. We noted that a similar abrupt increase of susceptibility at low temperature was reported in the α-MnSe under a high external magnetic field near 50 K [1]. Peng et al. attributed this anomaly to the spin-flop under high external magnetic field (> 0.5 Tesla) in antiferromagnetic order state. Our observation of the susceptibility anomaly at 16 K is unlikely due to the spin-flop mechanism because the magnetic field used in our susceptibility measurement is less than 100 Oe. Nevertheless, based on our neutron results, the moments of MnS in the layer are aligned in parallel and antiparallel with the moments in adjacent layers that give the rise of the antiferromagnetic ordering at about 150 K. We also observed a large contraction in lattice parameters below TN. Thus, it is likely that the coupling between the moments of the next-nearest-layer, which are aligned in parallel, is strong enough to provide the needed internal field for spin-flop and subsequently leads to the coexistence of two magnetic orders at low temperature. More detailed works using the single crystal to determine the magnetic coupling strength in different layers may provide more insight into the origin of the low-temperature magnetic anomaly. The origin of huge susceptibility anomaly in MnSe between 120 K and 200 K is an interesting but complicated problem. As reported by Wang et al., α-MnSe undergoes a dramatic distortion from cubic to orthorhombic structure at 30 GPa [34]. The MnSe6 octahedron is distorted in the orthorhombic phase, which results in the spin crossover of Mn2+ from high spin state to low spin state and a susceptibility drop. Based on the neutron data, the Mn2+ ions in β-phase remain at high spin state even though the α-to-β phase transformation occurs around 140 K. In addition, there is no structural change in the α-MnSe and the Mn2+ ions of α-MnSe below TN are also at high spin state. Therefore, the drop of susceptibility near 140 K in α-MnSe is not due to the spin crossover of Mn2+ ion, but instead from the antiferromagnetic ordering of β-phase.
diffraction in MnSe1−xSx can be exemplified by the x = 0.2 sample, as illustrated in Fig. 6(a). The signature peak of the β phase, (1 0 1)β, is not observed. The emerging peaks below 140 K are associated with the magnetic ordering of α-phase. A representative diffraction pattern below TN, as shown in Fig. 6(b), demonstrates a NaCl-type phase with a magnetic structure of k = (1/2, 1/2, 1/2). The magnetic order parameters of samples with different levels of sulfur substitution are summarized in Fig. 6(c). The magnetic ordering temperature of MnSe1−xSx is enhanced by increasing sulfur content from 130 K of x = 0.2 to 170 K of x = 1 sample. This could be attributed to the enhancement of the magnetic exchange coupling promoted by reduced Mn-Mn distance, as shown in Fig. 6(d) which plots the temperature dependent lattice parameters of all sulfur substituted samples. The decrease in the intermanganese separation increases the exchange coupling between Mn2+ and accordingly enhanced TN. Moreover, we observed a nonlinear lattice parameter change near the magnetic ordering temperature in each sample. Similar lattice distortion had been reported by Heikens et al. [30]. However, in contrast to the proposed rhombohedral distortion below TN, our results do not show any structural change. The temperature of exhibiting non-linear lattice constant change coincides with TN, suggesting that its origin might be from the coupling of magnetism and lattice. Our powder neutron diffraction results reveal that the hexagonal βphase of the NiAs-type structure no longer appears in MnSe1−xSx with x > 0.2. The substitution of selenium by smaller-sized sulfur atoms in MnSe1−xSx can generate an effectively negative pressure in the lattice to resist the thermally induced stress which is thought to be the cause of the α-to-β phase transformation. It remains a puzzle regarding the origin of magnetic transition below 16 K observed in MnSe1−xSx as our NPD did not show any new magnetic features. We performed detailed specific heat measurements on these samples in the temperature range of 2–300 K as shown in Fig. 7(a). We observe a clear transition at ∼120 K in MnSe but do not observe any anomaly around 200 K where the susceptibility sharply rises. The specific heat jump in sulfur-substituted samples shifts to high temperature, near 130 K and 150 K for x = 0.5 and x = 1 respectively. These results are consistent with both the magnetic susceptibility and neutron measurements. No obvious abnormal feature was found in all 209
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the susceptibility of the sample becomes larger at a lower temperature before the α-to-β structural transformation occurs at around 140 K. Below 140 K, the long-range antiferromagnetic order of β-MnSe and the enhancement of antiferromagnetic coupling between the short-range ferromagnetic sheets at adjacent layers due to the existence of β-MnSe reduce the susceptibility rapidly till TN (120 K) that the long-range antiferromagnetic ordering of α-MnSe sets in. In the warming process, the magnetic susceptibility increases quickly above TN of α-MnSe. However, the antiferromagnetic coupling of the short-range ferromagnetic sheets at adjacent layers does not completely vanish due to the existence of β-phase, which results in a smaller susceptibility value comparing with that in the cooling process. The magnetic susceptibility at the temperature between 120 K and 200 K is the result of the competition between the magnetic locking effect of β-phase and thermal fluctuation on the short-range ferromagnetic sheets in α-MnSe. Finally, the short-range ferromagnetic sheets disappear near 200 K where the upper temperature of anomaly appears. The above argument provides a qualitative explanation for the observed magnetic anomaly. We need further investigations to obtain the evidence for the presence of shortrange ferromagnetic sheets in α-MnSe, the coupling strength between adjacent ferromagnetic layers, and how the short-range ferromagnetic sheets interact with the β-phase. 4. Summary In summary, we report a detailed analysis of the physical properties of MnSe1−xSx. Our results provide shreds of evidence to clarify the origin for the coexistence of the α- and β-MnSe phases below 270 K, which appears as a hump in susceptibility measurement. The formation of the β-MnSe in α-MnSe most likely is due to the presence of stacking fault from thermal stress. In addition, a broad magnetic anomaly shows up between 115 K and 200 K. The detailed magnetic structure from neutron experiments suggests that the existence of two-dimensional short-range ferromagnetic sheets on (1 1 1) plane of α-MnSe below 200 K could be the reason responsible for this magnetic anomaly. These observed anomalies can be suppressed by sulfur substitution to selenium. The refinement of MnSe NPD shows that the anti-ferromagnetism is along c-axis for hexagonal type (β-phase) and along [1 1 1] direction for cubic-type (α-phase) structures. Though the NPD of MnS doesn’t show either atomic nor magnetic structural transformation below 16 K, the specific heat of MnS exhibits T1.5 dependence, which could be attributed to the presence of ferromagnetism.
Fig. 7. (a) The temperature dependence of heat capacity of MnSe1−xSx, x = 0, 0.5, and 1 measured during the cooling process. Weak peak located at ∼290 K comes from the thermal conducting grease used in the measurement. (b) At the low-temperature regime, a crossover in the temperature dependence of the heat capacity from T3 (phonon) to T1.5 in MnS at the temperature showing a huge magnetic susceptibility increase in Fig. 3(a).
In order to have a complete picture, we summarize our observations related to the large susceptibility anomaly as in the followings. 1) The antiferromagnetic system typically has a maximum susceptibility value about the order of magnitude of 10-5 emu/g-Oe, (for example, MnS in Fig. 2(c)), which is an order of magnitude smaller than the anomaly in α-MnSe. Therefore, the large susceptibility anomaly can’t be attributed to the long-range antiferromagnetic ordering of α- or β-MnSe. 2) The susceptibility of pure α-MnSe (also x = 0.05 sample) increases significantly below 200 K at which the anomaly emerges. We note that there is no long-range magnetic order at this temperature from the neutron scattering results. 3) Furthermore, the anomalous magnetic response only exists in the α-phase MnSe1−xSx that would partially transform into the β-phase at low temperature. 4) The ordered magnetic structure in both α- and β- MnSe are the stacks of alternatively ferromagnetic layers with a Se layer in between. The magnetic interaction between two adjacent ferromagnetic layers in α-phase might be weakened by the Se layer in α-phase, but not in β-phase because the Mn atom sits directly on top (or bottom) of the Mn atoms at adjacent layers. Based on the above observations, most likely the short-range twodimensional ferromagnetic sheets from below 200 K on the (1 1 1) plane of α-MnSe. These ferromagnetic sheets are more susceptible to the applied magnetic field. The susceptibility anomaly in cooling and warming between 120 K and 200 K can be understood with the following scenario. Below 200 K in the cooling process, more short-range ferromagnetic sheets form and thermal fluctuation effect reduces, and
Acknowledgements The authors acknowledge the fruitful discussion with Prof. MingHsien Lee of Tamkang University. This research is supported by the Ministry of Science and Technology Grant NSC102-2112-M-001-025MY3, MOST 103-2739-M-213-001-MY3, MOST 108-2633-M-001-001 and the Academia Sinica Thematic Research AS-106-TP-A01, Taiwan. References [1] Q. Peng, Y.J. Dong, Z.X. Deng, H.H. Kou, S. Gao, Y.D. Li, Selective synthesis and magnetic properties of alpha-MnSe and MnSe2 uniform microcrystals, J. Phys. Chem. B 106 (2002) 9261–9265. [2] A. Ennaoui, Photoelectrochemistry of highly quantum efficient single-crystalline nFeS2 (Pyrite), J. Electrochem. Soc. 133 (1986) 97. [3] H.I. Heulings, X. Huang, J. Li, T. Yuen, C. Lin, Mn-substituted inorganic-organic hybrid materials based on ZnSe: Nanostructures that may lead to magnetic semiconductors with a strong quantum confinement effect, Nano Lett. 1 (2001) 521–526. [4] X. Wu, D. Shen, Z. Zhang, J. Zhang, K. Liu, B. Li, Y. Lu, B. Yao, D. Zhao, B. Li, On the nature of the carriers in ferromagnetic FeSe, Appl. Phys. Lett. 90 (2007) 112105. [5] H.S. Jarrett, W.H. Cloud, R.J. Bouchard, S.R. Butler, C.G. Frederick, J.L. Gillson, Evidence for Itinerant-Electron Ferromagnetism, Phys. Rev. Lett. 21 (1968) 617–620. [6] J.C. Mikkelsen, A. Wold, Magnetic properties of CoAsxS2−x, J. Solid State Chem. 3 (1971) 39–48.
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