Yuzhu
Song
a,
Qiang
Sun
b,
Meng
Xu
a,
Ji
Zhang
c,
Yiqing
Hao
d,
Yongqiang
Qiao
a,
Shantao
Zhang
c,
Qingzhen
Huang
e,
Xianran
Xing
a and
Jun
Chen
*a
aBeijing Advanced Innovation Center for Materials Genome Engineering, and School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China. E-mail: junchen@ustb.edu.cn
bInternational Laboratory for Quantum Functional Materials of Henan, School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China
cNational Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, College of Engineering and Applied Science & Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
dState Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China
eNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102, USA
First published on 17th September 2019
Negative thermal expansion (NTE) has been found in some magnetic functional materials due to the magnetovolume effect (MVE), in which an order-to-disorder magnetic transition occurs. Here, we report an unconventional MVE which can induce the crossover from the common positive thermal expansion of the end members of ScFe2 and TiFe2 to the NTE in the solid solution of (Sc1−xTix)Fe2 (x = 0.6, αV = −28.36 × 10−6 K−1, 125–205 K). The direct evidence of neutron powder diffraction and first-principles calculations discloses that the decrease of intrinsic magnetic moments at the Fe(2a) site results in the ferromagnetic transition from one to another, which causes a stronger negative contribution to thermal expansion than the conventional MVE. The pair distribution function analysis verifies that the decrease in the nearest distance of Fe(6h)–Fe(6h) lying in the ab plane results in the shrinkage of the a(b) axis. The analysis of anisotropic atomic displacement parameters excludes the phonon induced NTE mechanism. Furthermore, an intriguing zero thermal expansion has been found in Sc0.55Ti0.45Fe2 (10–250 K). The present study provides an approach to achieve NTE or controllable thermal expansion, which may be used to enhance other magnetic-related effects, such as the magnetocaloric and barocaloric effect.
New conceptsNegative thermal expansion (NTE) is a novel physical property of materials, which plays a critical role in understanding the nature of thermal expansion and controlling the thermal expansion property. It has been widely known that the abnormal thermal expansion of Invar alloys is a result of the magnetovolume effect (MVE). Based on the theory of MVE, there has been great progress in finding new NTE or zero thermal expansion (ZTE) materials. Here, we report an unconventional MVE, which can induce a crossover from a normal PTE of ScFe2 and TiFe2 to an intriguing NTE and ZTE in (Sc,Ti)Fe2 solid solutions. In such a case, almost no change happens in the direction order of the spin alignment, but in the magnitude of the magnetic spin, which can be regarded as an unconventional MVE. It has been known that the previous studies on MVE are all correlated to ferromagnetic-to-paramagnetic, ferromagnetic-to-antiferromagnetic or antiferromagnetic-to-paramagnetic transitions. The (Sc,Ti)Fe2 material is a first example that shows an unconventional MVE caused by the new type of ferromagnetic-to-ferromagnetic transition. |
The studies on NTE materials can be actually traced to the discovery of magnetic Invar alloys (Fe65Ni35) in 1897 by the scientist C. É. Guillaume.22 The abnormal thermal expansion of Invar alloys is a result of the magnetovolume effect (MVE), i.e., an ordered magnetic structure with larger volume transforms to a disordered magnetic configuration with smaller volume, which causes a negative contribution to thermal expansion.23 Based to the theory of MVE, there has been great progress in finding NTE materials in magnetic compounds, such as antiperovskite Mn3AN,19,21 Invar alloys,1,22,24 La(Fe,Si,Co)13,25 Mn3Ge,26 R2Fe17 (R = rare earth elements),27 and Tb(Co,Fe)2.28 It should be noted that the present NTE materials induced by MVE are due to the order-to-disorder magnetic configuration transformation, which means a change in the direction of the spin alignment. For example, the NTE of magnetic Mn3Cu1−xGexN results from the transformation of the ordered Γ5g antiferromagnetic (AFM) spin structure to the disordered paramagnetic (PM) one.29 The zero thermal expansion (ZTE) of Invar alloys is caused by the spin alignment changing from the parallel alignment (ferromagnetic) to the disordered PM state.23
It has been well known that the two conventional physical properties of ferroelectricity and ferromagnetism (FM) have comparability. NTE can be formed in strong ferroelectricity due to the decrease in the magnitude of spontaneous polarization (PS).18 In magnetics, if spontaneous magnetization (the total of intrinsic atomic magnetic moments) can be reduced with increasing temperature, NTE can occur. In such a case, no change happens in the order of spin alignment, but in the magnitude of magnetic spin, which can be regarded as an unconventional MVE (UMVE).
Here, we report the crossover from positive to negative thermal expansion in (Sc,Ti)Fe2 intermetallics induced by UMVE. Intriguingly, a strong NTE property is found in the (Sc0.4Ti0.6)Fe2 solid solution. The average volume coefficient of thermal expansion (CTE) is −28.36 × 10−6 K−1 in the temperature range of 125–205 K. Furthermore, the ZTE property has been achieved in Sc0.55Ti0.45Fe2. The direct experimental evidence from neutron powder diffraction (NPD) and the pair distribution function (PDF) have revealed that the anomalous thermal expansion of (Sc,Ti)Fe2 is due to the decrease of intrinsic atomic magnetic moments, which is different from the order-to-disorder magnetic transition. The present work not only proposes a method to achieve NTE materials, but also provides a good example of ZTE materials.
Variable temperature NPD has been analyzed to study the thermal expansion property of (Sc1−xTix)Fe2 (Fig. S2, ESI†). The two end members of ScFe2 and TiFe2 exhibit normal PTE in the whole temperature range (Fig. 1 and Fig. S3, ESI†). As a general rule, PTE should be expected in any chemical compositions of (Sc,Ti)Fe2 solid solutions, when both the end members exhibit PTE. However, an intriguing PTE-to-NTE crossover is found in the solid solutions of (Sc1−xTix)Fe2. A strong NTE is found in the composition of x = 0.6. The CTE value of (Sc0.4Ti0.6)Fe2 is αV = −28.36 × 10−6 K−1 (125–205 K). As a comparison, the NTE magnitude of (Sc0.4Ti0.6)Fe2 is well approaching that of the magnetic materials of Mn3(Cu0.5Ge0.5)N (αV = −36 × 10−6 K−1, 280–365 K)32 and La(Fe,Si,Co)13 (αV = −26.1 × 10−6 1 K−1, 240–350 K).25 Furthermore, by adjusting the chemical composition of Ti in (Sc1−xTix)Fe2, an excellent ZTE property is found in Sc0.55Ti0.45Fe2 (αV = 1.24 × 10−6 K−1, 10–250 K). It should be noted that most ZTE or NTE materials resulting from electronic transitions are typified by a fairly narrow temperature range.4 It is rare that the ZTE of Sc0.55Ti0.45Fe2 is over a relatively broad temperature range in electronic-transition-type ZTE materials. Meanwhile, the macroscopic thermal expansion of (Sc,Ti)Fe2 has been investigated by thermo-dilatometer (Fig. S4, ESI†). Obviously, the thermal expansion property measured from the macroscopic dilatometer measurement is in good agreement with the result of intrinsic unit cell volume, which reveals the reliability of the PTE-to-NTE crossover in (Sc,Ti)Fe2. It should be noted that the temperature dependence of the unit cell volume was previously reported in Sc0.35Ti0.65Fe1.95.31 It has been known that the thermal expansion property of magnetic materials can be influenced by MVE. The present phenomenon of the PTE-to-NTE crossover certifies that this is a complicated magnetic property in the solid solutions of (Sc,Ti)Fe2.
To reveal the reason for the PTE-to-NTE crossover in the (Sc,Ti)Fe2 solid solutions, the macroscopic magnetism has been investigated. Firstly, the temperature dependence of magnetization (M–T) at a magnetic field 0.1 T has been measured for x = 0, 0.45, 0.6, and 1.0 (Fig. 2a and Fig. S5, ESI†). According to the character of M–T curves, both the end members of ScFe2 and TiFe2 show a typical magnetic property of ferromagnetism and antiferromagnetism, respectively. However, the solid solutions of x = 0.45 and 0.6 have unambiguous two ferromagnetic transitions, which is verified by two pronounced steps before reaching TC. The first transition (FM1-to-FM2) at a lower temperature appears more sharply at x = 0.6 (Fig. 2a). The M–T curves in both FC (field cooling, cooling with magnetic field) and ZFC (zero field cooling, cooling without magnetic field) modes under 1 T for (Sc1−xTix)Fe2 (x = 0, 0.45 and 0.6) have also been measured (Fig. S6, ESI†). Clearly, the FC and ZFC curves mostly overlap for all compositions, which means little thermal hysteresis caused by the magnetic-structural interplay in (Sc1−xTix)Fe2. To study the evolution details of magnetic properties, the temperature dependence of magnetization under different magnetic fields (0.01, 0.05, 0.1, 0.5, and 1 T) was studied on (Sc0.4Ti0.6)Fe2 (Fig. S7, ESI†). The initial and the saturated magnetizations (H = 1 T) of (Sc0.4Ti0.6)Fe2 are different. With an increasing external magnetic field, the magnetization is increased in the whole temperature range, and the magnetic transition temperature is gradually enhanced to a higher temperature for both FM1–FM2 and FM2–PM transitions. This means that the ferromagnetic order is more stable in a magnetic field.
To further verify the two transitions, a set of magnetization curves (M–H) of x = 0.6 are measured at selected temperatures. As shown in Fig. 2c, the M–H curves show a typical ferromagnetic character at 3–200 K, i.e., the magnetization can reach saturation easily by applying a magnetic field. For better estimating the saturation of magnetization at a high magnetic field, the magnetization curves at various temperatures are normalized based on the data of 2 T (Fig. 2d). Obviously, the normalized magnetization curves at 3–200 K are almost saturated (parallel to the horizontal axis). This means the order of spin alignment changes a little from 3 K to 200 K. The magnetic state basically remains in the ferromagnetic order. However, the magnetization magnitude sharply reduces from 3 K to 200 K for the x = 0.6 compound (Fig. 2a). It is known that both the order of spin alignment and the local magnetic moment size can change the magnetization. Therefore, it indicates that the first transition of FM1-to-FM2 consists of two ferromagnetic states with different local moments. On the contrary, when the temperature exceeds 200 K, the spins gradually depart from the parallel alignment, resulting in the disordered spin direction. Due to the disordered spin alignment, the magnetization curves can hardly reach saturation by applying a magnetic field, which can be reflected from the inclined magnetization curves. It corresponds to the second transition of the ordered ferromagnetic state to a disordered paramagnetic one (FM2-to-PM). Fig. 2b shows the magnetization curves of different components at 5 K, which not only shows the FM and AFM character of (Sc1−xTix)Fe2 (x = 0, 0.45 and 0.6) and TiFe2 at low temperature, but also reveals the value of saturation magnetization (MS) in the order of MS (x = 0.6) > MS (x = 0.45) > MS (x = 0.0) for (Sc1−xTix)Fe2. The two ferromagnetic states in (Sc,Ti)Fe2 with different degrees of localization have also been proved in the previous macroscopic magnetic measurements.31 It should be noted that such a transition between the two ferromagnetic states is rare in magnetic materials, which is different from the order-to-disorder magnetic transition, such as in Invar alloys (FM–PM)23 and antiperovskite (AFM–PM).29
Intuitively, the temperature range of the FM1-to-FM2 transition overlaps with that of NTE at x = 0.6 (Fig. 1 and 2a), indicating that the anomalous thermal expansion property of (Sc,Ti)Fe2 is entangled with the magnetic behavior. It is well known that NPD is a powerful tool to reveal the microcosmic magnetic structure of magnetic materials. Therefore, variable temperature high-intensity NPD was carried out to study the spin–lattice coupling of (Sc,Ti)Fe2 (Fig. S8, ESI†). Firstly, the lattice parameters are extracted from the NPD data (Fig. S11, ESI†). One can see that the abnormal thermal expansion of (Sc0.4Ti0.6)Fe2 mainly comes from the shrinkage of the a(b) axis. To further study the reason for the shrinking character in the a(b) axis, it is necessary to observe the change of distance between the magnetic Fe–Fe atoms.
Due to the constraint of crystallographic symmetry, the NPD analysis only gets the “apparent” bond length. In order to directly observe the “true” distance between the Fe–Fe atoms, experiments of PDF analysis of synchrotron radiation X-ray total scattering were carried out in NTE (Sc0.4Ti0.6)Fe2. As shown in Fig. 3a, the hexagonal model (P63/mmc) can be well reconciled with the experimental PDF data in the temperature range of 100–500 K. Temperature dependence of the pair distance of Fe–Fe obtained from PDF analysis is shown in Fig. 3c, which is similar to that obtained from the NPD data (Fig. S12, ESI†). Therefore, the local structure is consistent with the average structure in the present (Sc,Ti)Fe2 alloys. It is interesting to find that only the nearest distance of Fe(6h)–Fe(6h) (d1) lying in the ab plane shrinks in the NTE temperature range (Fig. 3b), while the other ones expand. This phenomenon may be on account of that the nearest distance of Fe(6h)–Fe(6h) controls the magnetic interaction of (Sc,Ti)Fe2. The fact that the distance of the magnetic atoms controls magnetism has been found in other magnetic materials, such as the pressure dependence of magnetism in Mn3Ga0.95N0.94.33 Furthermore, anisotropic atomic displacement parameters (ADPs) of Fe atoms are extracted from the PDF analysis (Fig. 3b and d). Obviously, the ADPs along the a(b) and c axis have little difference, which means the thermal vibration of Fe atoms is nearly isotropic. Therefore, the PDF analysis can exclude the phonon induced type NTE mechanism, and further reveal that the abnormal thermal expansion of (Sc,Ti)Fe2 is related with the magnetic interaction.
Fig. 4 shows the spin–lattice coupling effect of (Sc,Ti)Fe2, which is revealed by the variable temperature high-intensity NPD. The details of the magnetic structure analysis of (Sc,Ti)Fe2 can be found in the ESI.† For the NTE compound of x = 0.6, it has four magnetic configurations in the investigated temperatures (Fig. 4): canting, FM1, FM2, and PM. The magnetic moments of the Fe element can be calculated from the structure refinements of the NPD patterns, which are not obtained from the macroscopic magnetic measurements. These magnetic moments stand for the average atomic moments or ordered moments. As shown in Fig. 4a, the average magnetic moments of x = 0.6 are shown along the a and c axis for different Fe sites, which can be divided into four pronounced regions. In the canting region (T < 125 K), the Fe magnetic moments gradually turn from the canting spin alignment to the c-axis paralleled one with increasing temperature. It also means that the Fe moments increase to the maximum value along the c axis at 125 K, while gradually going down to zero along the a axis. The total local moments at Fe(2a) and Fe(6h) change little in the canting region (Fig. S13, ESI†). It needs to be noted that it is the first time that a canting magnetic structure has been observed in the x = 0.6 compound.31 In the FM1 region (125 K < T < 205 K), the decrease of average moments is mainly caused by the reduction in the intrinsic atom magnetic moments, since the degree of FM order has little changes (Fig. 2). Intriguingly, there is a sharp reduction in the ferromagnetic moments at Fe(2a) from 1.49 to 0.96 μB, but it remains nearly constant at Fe(6h). Therefore, the FM1-to-FM2 transition of (Sc,Ti)Fe2 is mainly attributed to the decrease of the Fe(2a) ferromagnetic moment, which is similar to the previous studies.34 In the FM2 region (205 K < T < 365 K), the average moments gradually decrease to zero when reaching TC, due to the magnetic structure configuration transition from the ordered spin (FM2) to the disordered one (PM). Such transition is often observed in ferromagnetic materials, such as Invar alloys.22
To better observe the lattice's response to the change of average magnetic moment in (Sc0.4Ti0.6)Fe2, its thermal expansion can be divided into four types on the basis of the magnitude of CTE (Fig. 4b), i.e., ZTE (αV = −0.87 × 10−6 K−1, 5–125 K), NTE (αV = −28.36 × 10−6 K−1, 125–205 K), weak PTE (αV = 15.33 × 10−6 K−1, 205–365 K), and normal PTE (αV = 43.15 × 10−6 K−1, 365–475 K). When compared with Fig. 4a and b, the four regions of thermal expansion are almost consistent with that of magnetic moments, which indicates that the thermal expansion of (Sc0.4Ti0.6)Fe2 is determined by the change of Fe magnetic moments.
To quantitatively describe the contribution of the magnetic moment to the thermal expansion, the spontaneous volume magnetostriction (ωm) can be calculated by subtracting thermal expansion of the nonmagnetic phase (ωnm) from the experimental (ωexp) one (Fig. 4b). It can be regarded that MVE has little influence on the thermal expansion of ScFe2 below 365 K, since its magnetization hardly changes (Fig. 2a). Therefore, the ωnm value of (Sc0.4Ti0.6)Fe2 can approximately be identical to the experimental thermal expansion of ScFe2. Intuitively, ωm almost remains constant in the ZTE region, and gently decreases in the weak PTE region, but sharply decreases in the NTE region. For explaining this phenomenon, the change of the Fe moments is combined with thermal expansion. Firstly, for better describing MVE, ΔVm and △Vnm denote the negative contribution from MVE and the positive contribution from nonmagnetic phase to thermal expansion, respectively. Thermal expansion of (Sc0.4Ti0.6)Fe2 is a result of the balance of ΔVm and ΔVnm. In the ZTE region of 5–125 K, the total local moments at Fe(2a) nearly have no change (Fig. S13, ESI†), and ZTE occurs due to little changes in both ΔVm and ΔVnm. In the NTE region, a sharp decrease of intrinsic moments at Fe(2a) makes ΔVm1 exceed ΔVnm1, so NTE occurs (125–205 K). It is not difficult to understand the weak PTE in 205–365 K, since the contribution from ΔVm2 cannot completely counteract the role of ΔVnm2.
It is to be noted that the FM2-to-PM transition in (Sc,Ti)Fe2 is a conventional order-to-disorder transition, while the FM1-to-FM2 one can be regarded as the UMVE in which an intrinsic moment decrease happens. As shown in Fig. 4b, the negative contribution to thermal expansion from the UMVE (ΔVm1) is stronger than that from the conventional MVE (CMVE) (ΔVm2). Therefore, it may be easier to induce NTE from the decrease of intrinsic moments. Similarly, the previous study has found that the decrease of intrinsic moments can enhance other magnetic effects; for example, the decrease of Mn intrinsic moments has been reported to enhance the barocaloric effect in Mn3GaN.35
To more clearly find the mechanism determining the PTE-to-NTE crossover in (Sc,Ti)Fe2, the rate of the Fe(2a) magnetic moments (MFe(2a), dM/dT) and the value of αV are compared for three representative compositions (x = 0.0, 0.45, and 0.6) as shown in Fig. 5a. Obviously, there is a strong correlation in both dM/dT and αV. For example, when MFe(2a) decreases sharply, NTE occurs (x = 0.6); however, when MFe(2a) remains almost constant, a normal PTE occurs (x = 0.0). This means that the thermal expansion can be tuned by controlling the decrease of intrinsic magnetic moment in the FM1-to-FM2 transition. For example, a ZTE property can be achieved in Sc0.55Ti0.45Fe2 (αV = 1.24 × 10−6 K−1, 10–250 K) by adjusting the chemical composition of Ti, since the MFe(2a) decreases slightly.
It has been known that with increasing Ti concentration, the magnetic structure of (Sc,Ti)Fe2 changes from FM to AFM (Fig. 2 and Fig. S5, ESI†). Therefore, the positive magnetic exchange interaction of (Sc,Ti)Fe2 should be weakened, and the magnetic moments of Fe should decrease. However, there is an unusual phenomenon occurring in the Fe moments. The total magnetic moments for one chemical formula of (Sc,Ti)Fe2 are calculated by 3/2MFe(6h) + 1/2MFe(2a) (Fig. S15, ESI†). Obviously, the magnetic moments of (Sc,Ti)Fe2 increase after a certain content of Ti-substitution (0 ≤ x ≤ 0.6). Such an increase is in good accordance with the above macroscopic magnetic measurement (Fig. 2).
The first-principles calculations are carried out to further explain the interesting phenomenon due to the UMVE. Firstly, the schematic diagrams of the CMVE and UMVE are depicted in Fig. 5b, which clearly shows the difference between both MVEs. The cell volumes of three different magnetic configurations: FM, PM and nonmagnetic (NM) are calculated by DFT for three representative compositions of ScFe2, Sc0.75Ti0.25Fe2, and Sc0.5Ti0.5Fe2 (Table S1, ESI†). In our calculations, it is interesting to find that the volume contraction between FM and NM is larger than that of FM and PM for all compositions. It means that the reduction in the magnitude of the magnetic spin, i.e. UMVE, can induce more NTE.
Secondly, the electronic structure of ScFe2 and one Ti substitution (25% Ti) are calculated to explain the change of Fe magnetic moments by Ti substitution. Fig. 5c shows the electron density of states of (Sc1−xTix)Fe2. To study how the electron doping influences the magnetic moment, the lattice dimension of ScFe2 is kept invariant, and only one Ti atom substitution is carried out (x = 0.25, Invariant). It is found that the Fermi level moves toward higher energy, i.e., the density of states of Fe d in spin up increase (Fig. 5c). The density of states of Fe d in spin up should be all occupied, but due to the exchange effect, a small part of the electrons in the crystal go to the empty d-orbit of the adjacent atoms, which makes Fe d in spin up lose part of the electrons. The d-orbit of Sc is the emptiest, so the transfer of Fe d electrons in spin up is most obvious, which results in the decrease of Fe magnetic moment. As the Ti substitution for Sc, the d electrons increase, and the transfer of Fe d electrons in spin up becomes difficult. As a result, the Fe magnetic moment is unusually increased. On the other hand, the Ti substitution shrinks the lattice (x = 0.25, Contracted), and the Fermi level moves toward lower energy than that of the invariant lattice, resulting in the reduced magnetic moment of Fe (Fig. 5c). Comparing the above opposite effects, if the former overwhelms the latter, the magnetic moment of Fe can be unusually enhanced after a certain content of Ti-substitution (Fig. 2b and Fig. S15, ESI†).
It has been known that the previous studies on MVE are mostly correlated to the order-to-disorder magnetic transition of FM-to-PM or AFM-to-PM. For example, thermal expansion can be controlled by changing the order-to-disorder magnetic transition for the CMVE, such as Ho2(Fe,Cr)17,36 antiperovskite manganese nitrides37 and La(Fe,Si,Co)13.25 The ZTE property has also been found in the antiperovskite manganese nitrides (αV < 1.5 × 10−6 K−1, 190–272 K),38 Ho2Fe16Cr (αV = 1.3 × 10−6 K−1, 13–330 K),36 and La(Fe,Si)13 (αV = 1.5 × 10−6 K−1, 20–275 K).39 The present study on (Sc,Ti)Fe2 is the first example to show that NTE is caused by a new type of ferromagnetic transition, i.e. FM1-to-FM2.
Footnote |
† Electronic supplementary information (ESI) available: Experimental methods, computational method and model, magnetic structure analysis and supplementary figures. See DOI: 10.1039/c9mh01025d |
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