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Phase separation and exchange bias effect in Ca doped EuCrO3
Author’s Accepted Manuscript Phase separation and exchange bias effect in Ca doped EuCrO3 Dongmei Deng, Xingyu Wang, Jiashun Zheng, Xiaolong Qian, Dehong Yu, Dehui Sun, Chao Jing, Bo Lu, Baojuan Kang, Shixun Cao, Jincang Zhang www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(15)30381-4 http://dx.doi.org/10.1016/j.jmmm.2015.07.075 MAGMA60440
To appear in: Journal of Magnetism and Magnetic Materials Received date: 29 December 2014 Revised date: 16 July 2015 Accepted date: 25 July 2015 Cite this article as: Dongmei Deng, Xingyu Wang, Jiashun Zheng, Xiaolong Qian, Dehong Yu, Dehui Sun, Chao Jing, Bo Lu, Baojuan Kang, Shixun Cao and Jincang Zhang, Phase separation and exchange bias effect in Ca doped EuCrO3, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.07.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Phase separation and exchange bias effect in Ca doped EuCrO3 Dongmei Deng,1,* Xingyu Wang,1 Jiashun Zheng,1 Xiaolong Qian,1 Dehong Yu,2 Dehui Sun,2 Chao Jing,1 Bo Lu,3 Baojuan Kang,1 Shixun Cao,1 and Jincang Zhang1 1
Department of Physics and Materials Genome Institute, Shanghai University, Shanghai 200444, China
2
Bragg Institute, Australian Nuclear Science and Technology Organization, Kirrawee DC NSW 2232, Australia 3
Analysis & Measurement Center and Laboratory for Microstructures of Shanghai University, Shanghai 200444, China
*Corresponding author. Email: [email protected], Tel.: +86-21-66132517
The rare-earth chromites have attracted increasing interests in recent years, as a member of a few single-phase multiferroic materials. We studied the structure and magnetic property of a series of Ca-doped EuCrO3 samples by using X-ray powder diffraction and Physical Property Measurement System. Phase separation, rotation of magnetization in M(T) curve and exchange bias effect have been identified. The Eu0.7Ca0.3CrO3 polycrystalline sample may be intrinsically phase-separated, with Cr3+-rich, Cr4+-rich canted antiferromagnetic regions surrounded by spin glass-like frustrated phase, resulting in several magnetic features including: (1) a broad and slow increase of M(T) curve with the decrease of temperature; (2) rotation of magnetization with increasing cooling field; (3) exchange bias and glassy magnetism. The rotation of magnetization is ascribed to the rotation of the moment of Cr4+-rich regions, arising from the competition between exchange coupling energy and magnetostatic energy. The exchange bias effect suggests the formation of weak ferromagnetic unidirectional anisotropy during field cooling, due to the exchange coupling among weak ferromagnetic domains and surrounding spin glass-like regions. This result helps understanding the interaction among different magnetic domains and phases in a complex system. Keywords: Phase seperation, Exchange bias
1
I.
INTRODUCTION Phase separation has been reported to play a crucial role in the physical properties of
materials, such as colossal magnetoresistance effect,1 glassy magnetic behavior,2 exchange bias (EB) effect3 etc. EB effect, characterized by a shift of the hysteresis loop [M(H)] along the magnetic field axis, can usually be observed in systems consisting of ferromagnetic (FM) -antiferromagnetic (AFM),4 FM-spin glass (SG),5 AFM-ferrimagnetic (FiM),6 or FM-FiM interfaces7, when the system is cooled with applied field through the Néel temperature (TN) of the AFM or glass temperature (TSG) of the SG, and they were attributed to a kind of unidirectional anisotropy formed at the interface of different magnetic phases. 8 In phase separated systems, EB effect were attributed to intrinsic interface exchange coupling between FM nano-droplets and surrounding AFM matrix or SG regions.3,9,10 The perovskite-related rare-earth chromites have attracted much attention in their magnetic and electronic properties in recent years.11-14 Both rare earth chromite EuCrO3 and alkaline earth chromite CaCrO3 have an orthorhombic perovskite structure with a space group Pbnm at room temperature.13-17 EuCrO3 shows a weak spontaneous ferromagnetic moment below a Néel temperature of 181 K, which is attributed to a slight canting of the antiferromagnetically aligned Cr3+ moments. The Cr3+ moments align in a way with the direction of easy magnetization along the c-axis of the orthorhombic cell, that belongs to the irreducible representation Γ4 (GxAyFz).13,14 Komarek et al.15 have demonstrated that CaCrO3 has a Γ2 (FxCyGz) magnetic structure below its magnetic ordering temperature TN = 90 K by neutron powder diffraction experiment. Considering the different types of spin ordering in EuCrO3 and CaCrO3, the doping of Ca into EuCrO3 may introduce coexistence of two types of AFM structures and interesting phenomena. In this work, we are addressing the magnetic behaviors observed in Ca-doped polycrystalline EuCrO3 samples. We demonstrate and interpret the rotation of magnetization, 2
glassy magnetism and exchange bias effect observed in Eu0.7Ca0.3CrO3 sample from the perspective of phase separation. II. EXPERIMENTAL The experimental samples were synthesized using standard solid-state sintering method in air, using dried Eu2O3 (99.99%), Cr2O3 (99.9%) and CaCO3 (99.9%). X-ray powder diffraction (XRD) (18 KWD/max-2500 diffractometer, Cu-Kα radiation) patterns of Eu1xCaxCrO3
(x = 0, 0.1, 0.3) samples were measured at room temperature, as shown in Fig. 1(a).
Using FullProf, the structure of EuCrO3 was Rietveld refined, and confirmed to be in single phase, as shown in Fig. 1(b). The XRD patterns of Eu0.9Ca0.1CrO3 and Eu0.7Ca0.3CrO3 show only minor shifts and overlapping of diffraction peaks, compared with that of EuCrO3, indicating a single phase for both samples. EDX measurements down to a scale of 0.6 μm for the Eu0.7Ca0.3CrO3 sample did not show significant spatial fluctuation of Ca content, consistent with the conclusion of a single phase. The dc magnetization measurements were performed with a commercial Physical Property Measurement System (PPMS, Quantum Design). Temperature dependence of magnetization was obtained on cooling from 300 K to 3 K in a specified field (FCC) and on heating from 3 K in a specified field after zero field cooling (ZFC) or field cooling (FCW). All the M(H) curves were recorded after the cooling of the sample to the measuring temperature under ZFC or FC from the paramagnetic state at 300 K. III. RESULTS AND DISCUSSION A. Temperature dependence of magnetization Figure 2(a) gives the temperature dependence of magnetization M for Ca-doped samples Eu1-xCaxCrO3 (x = 0.0, 0.1, 0.3), with the inverse magnetic susceptibility χ-1(T) in the inset. χ1
behaves linearly in high-temperature region, which can be fitted by the Curie-Weiss (C-W)
equation χ-1(T) = (T - Θ) / C,
(1)
where Θ is the Weiss temperature (see Fig. 2(b)) , and C is the Curie constant expressed as C =NP2μB2/3kB 3
(2)
Where N is Avogadro constant, kB is the Boltzmann constant, B and P are the Bohr magneton and its effective number, respectively. The effective magnetic moment can be expressed as PB. The best fit gives Weiss temperature Θ = -302±4, -394±4, -321±5 and Curie constant C = 0.0217±0.0002, 0.0216±0.0002, 0.0154±0.0002 for x = 0, 0.1, 0.3 and the effective magnetic moment PB = 6.62±0.03 B, 6.45±0.02 B and 5.19±0.03 B for x = 0, 0.1, 0.3, respectively. The negative Weiss temperatures suggest that the magnetic interaction is antiferromagnetic, consistent with the previous reports of the canted antiferromagnetic (CAFM) characteristics of EuCrO3.13 The reduced Weiss temperature of 321 K for Eu0.7Ca0.3CrO3 with respect to -394 K of Eu0.9Ca0.1CrO3 can be connected to the reduced Curie constant, i.e. to the reduced magnetization of Eu0.7Ca0.3CrO3 sample, which is also clearly reflected in Fig. 2(a). Microstructures, such as phase seperation and point defects,18,19 may also affect the coupling of Cr ions. The MFC curve of the undoped EuCrO3 sample shows a paramagnetic (PM) to CAFM transition in a narrow temperature range around TN = 181 K. For low Ca content of x = 0.1, a decrease in TN is observed but the curve is rather similar to that of EuCrO3, indicating magnetic homogeneity at low temperatures. For the sample with x= 0.3, the TN cannot be obtained straightforwardly from the M(T) curves since the increase of M are obviously broadened (see Fig. 2(a)). This is attributed to the appearance of Cr4+ ions in the sample with the bivalent Ca2+ substitution of Eu3+, which brings a disruption of the long-range antiferromagnetic order of Cr3+ ions. With increasing doping level, clusters with Cr3+-O-Cr3+ ordering and different sizes may then distribute randomly in the sample. Magnetic clusters with different sizes show different PM-CAFM transition temperature TN, as observed in La0.8Ca0.2MnO3 samples, where the TN is only 143 K for clusters of 15 nm diameter, much lower than the TN of bulk La0.8Ca0.2MnO3, 216 K.20 The varying TN of Cr3+-O-Cr3+ clusters with different size may result in the obviously broad PM-CAFM transition, as indicated by the slow increase of MFC from 184 K, with the decrease of temperature, as shown in Fig. 2(a). Similar effect has also been observed in some other perovskite materials, such as La0.7Ca0.3Mn1-yTiyO3,21
La0.67Ca0.33Mn1-xGaxO3,22
La0.63Ca0.37Mn1-yFeyO3.24 4
La2/3Ca1/3Mn1-xAlxO3-δ,23
and
To study more about the magnetic property of a system with magnetic inhomogeneity, we chose the sample with nominal Ca concentration of x = 0.3 and measured its magnetism in detail. Fig. 3 shows the temperature-dependent FCC, FCW, and ZFC magnetization curves, with an applied field of 100 Oe. FCC and FCW curves almost coincide with each other for the whole measured temperature range of 3~250 K, start increasing rapidly at 184 K with decreasing temperature, while the MZFC and MFC curves show a large difference below 184 K. The increase of MFC at 184 K implies the onset of PM-CAFM transition, contributed by the Cr3+ rich clusters with a TN similar to that of EuCrO3. It is noticeable that the MFC shows a drop at about 41 K, as shown in Fig. 4(a), a magnified view of the MFCW curve in Fig. 3. For the Eu0.7Ca0.3CrO3 sample, the competition among three interactions (Cr3+- O- Cr3+, Cr4+- OCr4+, Cr3+- O- Cr4+) should be taken into account. In Cr3+-rich regions, Cr3+-O-Cr3+ is dominant (similarly to the EuCrO3 phase) and shows weak ferromagnetism (WFM) due to spin canting, with Γ4 type spin ordering of Cr.13,14 In contrast, in Cr4+-rich regions, Cr4+-OCr4+ is dominant (similarly to the CaCrO3 phase) and shows weak ferromagnetism (WFM) due to spin canting, with Γ2 type spin ordering of Cr.15,16 Accordingly, the drop in MFC can be interpreted in the following way: Cr3+-rich regions order in a Cr spin structure of Γ4 (Fz) at temperatures from well above 90 K; while Cr4+-rich regions, with a Cr spin structure of Γ2 (Fx) and TN ≤ 90 K, make weak contribution to the measured magnetization, when the cooling field is low enough and so that the magnetostatic energy of Cr4+-rich regions is lower than its anisotropy energy, since its easy axis is at an angle of 90o to that of Cr3+-rich regions. With the formation and growth of the Γ2 domains in Cr4+-rich regions, domain wall develops between neighboring, and competing Γ4 and Γ2 domains, yielding a slight suppression in the observed magnetic moment below 41 K. The cooling field dependence of the MFC(T) curve at low temperature range is shown in Figs. 4(a)-4(d). The broad cusp shifts toward lower temperature with the increase of cooling field, i.e, 41 K, 37 K, and 33 K for Hcool = 0.1 kOe, 1 kOe, and 2 kOe, respectively. This further confirms the competition between magnetostatic energy and anisotropy energy. When Hcool is large enough, the moments of Cr3+-rich and Cr4+-rich clusters are aligned parallel with Hcool, resulting in the monotonous increase of MFC with the decrease of temperature as shown in Fig. 4(d).25,26 For the sample with x = 0.1, no 5
drop has been observed, reflecting that the Cr4+-rich regions are too small to compete with the Cr3+-rich regions and the external field. The MZFC curve of the Eu0.7Ca0.3CrO3 sample exhibits a peak at 33 K and a wide shoulder at around 75 K, as illustrated in the inset of Fig. 3. The peak around Tf = 33 K indicates that there is a collective freezing of magnetic moments within the sample. Competition between Cr3+- O- Cr3+ and Cr4+- O- Cr4+ interactions might be responsible for the freezing-like effect, which is always a typical glassy magnetic behavior. The wide shoulder around 75 K could be due to the inter-cluster interactions.27,28 B. Spin glass state: time dependence of magnetization and memory effect To study more about the glassy magnetism of our sample, we measured the time dependence of magnetization and memory effect, which are commonly used to identify the existence of SG states.30,31 The measuring method is adopted from ref. 30. We recorded the magnetization as a function of time at T = 3 K in both ZFC and FC, with H = 500 Oe and tw= 2000 s, as shown in Fig. 5(a). The ZFC magnetization increases with time, while the FC magnetization decreases with time during the time span, and the relaxation for ZFC mode is more significant than that for FC mode. This is consistent with the coexistence and competition of Γ4 and Γ2 structures, indicating that the cooling field may favor long range ordering and ZFC may result in more metastable frustrated regions. Ulrich et al. studied the magnetic relaxation of single-domain ferromagnetic particle systems with different particle density, and found that the relaxation rate decays by a power law.
32
For FC mode, the
magnetic relaxation rate W is defined as, W(t) = - (d/dt)lnM(t),
(3)
W(t) = At-n,
(4)
where n depends on the particle concentration, and A is a temperature-dependent prefactor. 6
The inset of Fig. 5(a) shows the double logarithmic plot of W(t) versus t, along with best fit to Eq. (4) for T = 3 K. The value of the fitting parameter n is 0.90, which is close to 1, suggesting the co-existence of competing magnetic phases in our Eu0.7Ca0.3CrO3 sample, and the system can be seen as an assembly of interacting ferromagnetic clusters. Intercluster magnetic dipolar interaction introduces the collectivity and glassiness observed in the above relaxation experiments. Furthermore, we measured FC magnetic relaxation at different temperatures, as shown in Fig. 5(b). The exponent n obtained from the best-fit curve (see the inset of Fig. 5(b)) is 0.89, 0.67, 0.53, 0.41 for T =15K, 33 K, 90 K, and 150 K, respectively. The n decreases with the increase of temperature, indicating the weakening of magnetic dipolar interaction and glassy characteristics. The M(t) data for 33K can also be fitted to KWW stretched exponential function (t)exp(-(t/))(see the Fig. 5(c)),33 Notable deviations of the fits can be seen in the region t>4500 s, is a characteristic relaxation time and is regarded as a shape parameter. The and obtained from the fitting is 6.26 109 s and 0.4336, respectively. This result is in accordance with that of Ni50Mn34In16, where the value of varies between 0.45 and 0.7, and the existence of glasslike magnetic state has been verified. 34 Memory effect was studied using the same method used in reference 31, with an external field of 500 Oe, a temporary stop temperature of 20 K, and a time span of tw= 7200 s. As shown in Fig. 6, it is clear that the magnetization with a stop (red square) shows a typical feature of memory effect, i.e, a dip at around 20 K, reflecting the existence of SG state. As a conclusion, both the observed time dependence of magnetization and memory effect point out unambiguously the existence of SG state in the Eu0.7Ca0.3CrO3 sample.
7
C. Exchange bias effect EB effect can usually be observed in heterogeneous systems containing interfaces involving SG state.35-38 Therefore, it would be interesting to explore if there is EB effect, considering the phase-separation and the existence of SG state in the Eu0.7Ca0.3CrO3 sample. To test this argument, we measured the M(H) loops at 3K with maximum measuring field |Hmmax| = 8 T after both ZFC and FC. The experimental results (see Fig. 7) show a nearly symmetric ZFC hysteresis loop and shifted FC M(H) curves (EB effect). Similar EB effect has also been observed in samples with x = 0.2 and 0.5. Since no EB effect has been observed in the single-phase EuCrO3 sample, the observed EB effect should be mainly a result of the phase separation and SG state. After field cooling to 3 K, the magnetic moments of the Γ4 domains in Eu0.7Ca0.3CrO3 are in the direction of the external cooling field, which parallel coupled with the unidirectional moments of the Γ4 domains. Therefore, during the measuring of hysteresis loops, the WFM moments of the Cr3+-rich Γ4 domains are exerted a microscopic torque to keep them in their initial direction, and thus a positive cooling field results in a negative EB; conversely, a negative cooling field results in a positive EB (see Fig. 7). For ZFC, the hysteresis loop is central symmetric since the local unidirectional moments distribute randomly in their orientations. To further reveal the origin of exchange bias in the present sample with coexisting Γ4 and Γ2 domains, the temperature dependence of exchange parameters was studied, as shown in Figs. 8(a)-8(b). For such measurements, the sample was cooled down from room temperature with an applied field Hcool = 1 T. Once the measuring temperature was reached, 8
the M(H) loop was measured between ± 8 T. The exchange-bias field HE and coercive field HC in Fig. 8 is defined as HE = (H1+H2)/2 and HC = (H2−H1)/2, respectively, where H1 and H2 are the left and right coercive fields, respectively.39 The remanence asymmetry ME and the magnetic coercivity MC are usually defined as the “vertical axis” equivalents of HE and HC, respectively.40 The temperature evolution of HE is typical for the EB systems with FM/SG interfaces,41 and both HE and ME decrease monotonously with the increase of temperature. Relatively weak EB was observed between Tf and 90 K, because of the weakening of the coupling at interfaces and the SG state, with the increase of temperature, consistent with the conclusion drawn from the data in Fig. 5. Below Tf, the colony of SG regions may be temperature dependent, have largest proportion at around 15 K as indicated by the time dependence of magnetization in Fig. 5, where the relaxation at 15 K is the largest. As a result, WFM rotation during the measurement of M(H) can drag more SG spins at around 15 K, giving rise to the maximum value of HC and MC at 15 K, a temperature lower than Tf.42 Figs. 8(c)-8(d) show the cooling field dependence of EB parameters HE, HC, ME, and MC at 3 K for Eu0.7Ca0.3CrO3, where all the parameters increase monotonously with the increasing cooling field up to 6 T. With the increase of the cooling field, more moments of WFM domains get parallel with the cooling field, and the size of the WFM domains may get larger, resulting in a relatively weak increase in exchange bias. This effect is qualitatively analogous to FM/AFM thin films, where exchange bias is inversely proportional to the thickness of the FM layer.4 IV. CONCLUSION. In the frame of a detailed study of the magnetic behavior of Eu0.7Ca0.3CrO3, the competition between Cr3+-rich Γ4 domains and Cr4+-rich Γ2 domains, and the resulting SG9
like phase in Eu0.7Ca0.3CrO3 have been confirmed by the observation of magnetic relaxation, memory effect and exchange bias effect. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 10804067, 11074163, 61171033, 11274211), the Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (Grant No. 13JC1402400), Shanghai Institute of Materials Genome Supported by the Shanghai Municipal Science and Technology Commission (Project No. 14DZ2261200). 1
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Figure captions Fig. 1. (Color online) (a) Powder X-ray diffraction patterns of Eu1-xCaxCrO3 (x = 0.0, 0.1 and 0.3) samples at room temperature. (b) Rietveld analysis of XRD on powder EuCrO3. Plus (+) symbols represent the experimental results and the red solid line is from refinement. The vertical bars at bottom represent the position of Bragg peaks for EuCrO3. The difference between experimental and calculated results is shown as cyan line between the vertical bars and the experimental results. Fig. 2. (Color online) (a) Temperature dependence of magnetization with FCW process at an applied field of 100 Oe for Eu1-xCaxCrO3 (x = 0.0, 0.1, 0.3) samples. The inset gives the temperature dependence of the reciprocal of the magnetic susceptibility χ-1. (b) The magnified view of χ-1(T) in high temperature range. The solid lines show fits to the CurieWeiss law. Fig. 3. (Color online) Temperature dependence of magnetization with ZFC, FCC and FCW processes at an applied field of 100 Oe for Eu0.7Ca0.3CrO3. The inset illustrates the 14
freezing temperature of 33 K with ZFC for Eu0.7Ca0.3CrO3. Fig. 4. (Color online) The magnified view of MFC(T) in low temperature range for Eu0.7Ca0.3CrO3, measured at different cooling field: 0.1, 1, 2 and 3 kOe. Fig. 5. (Color online) Time dependence of magnetization for Eu0.7Ca0.3CrO3 at: (a) T = 3 K of ZFC and FC processes; (b) T = 15, 33, 90, 150 K of FC process (The magnetization values have been normalized with respect to the initial magnetization obtained at time t = 0). Inset shows the relaxation rate, and the solid line is the best fit to Eq. (4); (c) the result of the fitting of KWW stretched exponential function at T=33 K. Fig. 6. (Color online) Temperature dependence of the reference magnetization Mref (solid line), the magnetization M (red square), and ∆M = M - Mref (black square). Fig. 7.(Color online) M(H) loops measured at 3 K with |Hmmax| = 8 T after ZFC and FC (Hcool = ±1 T). The inset shows the ZFC and FC M(T) curves measured on cooling. Fig. 8. (Color online) (a)-(b)Temperature dependence of HE, ME, HC and MC for Eu0.7Ca0.3CrO3 with |Hmmax| = 8 T (Hcool = 1 T); (c)-(d) Cooling field dependence of HE, ME, HC and MC for Eu0.7Ca0.3CrO3 with |Hmmax| = 8 T at 3K.
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Highlights (1) A slow increase of M(T) curve was observed in Eu0.7Ca0.3CrO3. (2) Rotation of magnetization in M(T) curve of Eu0.7Ca0.3CrO3 was observed. (3) Exchange bias effect and glassy magnetism were observed in Eu0.7Ca0.3CrO3. (4) Rotation of the moments of Cr4+-rich regions result in the rotation of magnetization in M(T) curve. (5) Spin glass-like regions contribute to the observed exchange bias effect.
PII: DOI: Reference:
S0304-8853(15)30381-4 http://dx.doi.org/10.1016/j.jmmm.2015.07.075 MAGMA60440
To appear in: Journal of Magnetism and Magnetic Materials Received date: 29 December 2014 Revised date: 16 July 2015 Accepted date: 25 July 2015 Cite this article as: Dongmei Deng, Xingyu Wang, Jiashun Zheng, Xiaolong Qian, Dehong Yu, Dehui Sun, Chao Jing, Bo Lu, Baojuan Kang, Shixun Cao and Jincang Zhang, Phase separation and exchange bias effect in Ca doped EuCrO3, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.07.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Phase separation and exchange bias effect in Ca doped EuCrO3 Dongmei Deng,1,* Xingyu Wang,1 Jiashun Zheng,1 Xiaolong Qian,1 Dehong Yu,2 Dehui Sun,2 Chao Jing,1 Bo Lu,3 Baojuan Kang,1 Shixun Cao,1 and Jincang Zhang1 1
Department of Physics and Materials Genome Institute, Shanghai University, Shanghai 200444, China
2
Bragg Institute, Australian Nuclear Science and Technology Organization, Kirrawee DC NSW 2232, Australia 3
Analysis & Measurement Center and Laboratory for Microstructures of Shanghai University, Shanghai 200444, China
*Corresponding author. Email: [email protected], Tel.: +86-21-66132517
The rare-earth chromites have attracted increasing interests in recent years, as a member of a few single-phase multiferroic materials. We studied the structure and magnetic property of a series of Ca-doped EuCrO3 samples by using X-ray powder diffraction and Physical Property Measurement System. Phase separation, rotation of magnetization in M(T) curve and exchange bias effect have been identified. The Eu0.7Ca0.3CrO3 polycrystalline sample may be intrinsically phase-separated, with Cr3+-rich, Cr4+-rich canted antiferromagnetic regions surrounded by spin glass-like frustrated phase, resulting in several magnetic features including: (1) a broad and slow increase of M(T) curve with the decrease of temperature; (2) rotation of magnetization with increasing cooling field; (3) exchange bias and glassy magnetism. The rotation of magnetization is ascribed to the rotation of the moment of Cr4+-rich regions, arising from the competition between exchange coupling energy and magnetostatic energy. The exchange bias effect suggests the formation of weak ferromagnetic unidirectional anisotropy during field cooling, due to the exchange coupling among weak ferromagnetic domains and surrounding spin glass-like regions. This result helps understanding the interaction among different magnetic domains and phases in a complex system. Keywords: Phase seperation, Exchange bias
1
I.
INTRODUCTION Phase separation has been reported to play a crucial role in the physical properties of
materials, such as colossal magnetoresistance effect,1 glassy magnetic behavior,2 exchange bias (EB) effect3 etc. EB effect, characterized by a shift of the hysteresis loop [M(H)] along the magnetic field axis, can usually be observed in systems consisting of ferromagnetic (FM) -antiferromagnetic (AFM),4 FM-spin glass (SG),5 AFM-ferrimagnetic (FiM),6 or FM-FiM interfaces7, when the system is cooled with applied field through the Néel temperature (TN) of the AFM or glass temperature (TSG) of the SG, and they were attributed to a kind of unidirectional anisotropy formed at the interface of different magnetic phases. 8 In phase separated systems, EB effect were attributed to intrinsic interface exchange coupling between FM nano-droplets and surrounding AFM matrix or SG regions.3,9,10 The perovskite-related rare-earth chromites have attracted much attention in their magnetic and electronic properties in recent years.11-14 Both rare earth chromite EuCrO3 and alkaline earth chromite CaCrO3 have an orthorhombic perovskite structure with a space group Pbnm at room temperature.13-17 EuCrO3 shows a weak spontaneous ferromagnetic moment below a Néel temperature of 181 K, which is attributed to a slight canting of the antiferromagnetically aligned Cr3+ moments. The Cr3+ moments align in a way with the direction of easy magnetization along the c-axis of the orthorhombic cell, that belongs to the irreducible representation Γ4 (GxAyFz).13,14 Komarek et al.15 have demonstrated that CaCrO3 has a Γ2 (FxCyGz) magnetic structure below its magnetic ordering temperature TN = 90 K by neutron powder diffraction experiment. Considering the different types of spin ordering in EuCrO3 and CaCrO3, the doping of Ca into EuCrO3 may introduce coexistence of two types of AFM structures and interesting phenomena. In this work, we are addressing the magnetic behaviors observed in Ca-doped polycrystalline EuCrO3 samples. We demonstrate and interpret the rotation of magnetization, 2
glassy magnetism and exchange bias effect observed in Eu0.7Ca0.3CrO3 sample from the perspective of phase separation. II. EXPERIMENTAL The experimental samples were synthesized using standard solid-state sintering method in air, using dried Eu2O3 (99.99%), Cr2O3 (99.9%) and CaCO3 (99.9%). X-ray powder diffraction (XRD) (18 KWD/max-2500 diffractometer, Cu-Kα radiation) patterns of Eu1xCaxCrO3
(x = 0, 0.1, 0.3) samples were measured at room temperature, as shown in Fig. 1(a).
Using FullProf, the structure of EuCrO3 was Rietveld refined, and confirmed to be in single phase, as shown in Fig. 1(b). The XRD patterns of Eu0.9Ca0.1CrO3 and Eu0.7Ca0.3CrO3 show only minor shifts and overlapping of diffraction peaks, compared with that of EuCrO3, indicating a single phase for both samples. EDX measurements down to a scale of 0.6 μm for the Eu0.7Ca0.3CrO3 sample did not show significant spatial fluctuation of Ca content, consistent with the conclusion of a single phase. The dc magnetization measurements were performed with a commercial Physical Property Measurement System (PPMS, Quantum Design). Temperature dependence of magnetization was obtained on cooling from 300 K to 3 K in a specified field (FCC) and on heating from 3 K in a specified field after zero field cooling (ZFC) or field cooling (FCW). All the M(H) curves were recorded after the cooling of the sample to the measuring temperature under ZFC or FC from the paramagnetic state at 300 K. III. RESULTS AND DISCUSSION A. Temperature dependence of magnetization Figure 2(a) gives the temperature dependence of magnetization M for Ca-doped samples Eu1-xCaxCrO3 (x = 0.0, 0.1, 0.3), with the inverse magnetic susceptibility χ-1(T) in the inset. χ1
behaves linearly in high-temperature region, which can be fitted by the Curie-Weiss (C-W)
equation χ-1(T) = (T - Θ) / C,
(1)
where Θ is the Weiss temperature (see Fig. 2(b)) , and C is the Curie constant expressed as C =NP2μB2/3kB 3
(2)
Where N is Avogadro constant, kB is the Boltzmann constant, B and P are the Bohr magneton and its effective number, respectively. The effective magnetic moment can be expressed as PB. The best fit gives Weiss temperature Θ = -302±4, -394±4, -321±5 and Curie constant C = 0.0217±0.0002, 0.0216±0.0002, 0.0154±0.0002 for x = 0, 0.1, 0.3 and the effective magnetic moment PB = 6.62±0.03 B, 6.45±0.02 B and 5.19±0.03 B for x = 0, 0.1, 0.3, respectively. The negative Weiss temperatures suggest that the magnetic interaction is antiferromagnetic, consistent with the previous reports of the canted antiferromagnetic (CAFM) characteristics of EuCrO3.13 The reduced Weiss temperature of 321 K for Eu0.7Ca0.3CrO3 with respect to -394 K of Eu0.9Ca0.1CrO3 can be connected to the reduced Curie constant, i.e. to the reduced magnetization of Eu0.7Ca0.3CrO3 sample, which is also clearly reflected in Fig. 2(a). Microstructures, such as phase seperation and point defects,18,19 may also affect the coupling of Cr ions. The MFC curve of the undoped EuCrO3 sample shows a paramagnetic (PM) to CAFM transition in a narrow temperature range around TN = 181 K. For low Ca content of x = 0.1, a decrease in TN is observed but the curve is rather similar to that of EuCrO3, indicating magnetic homogeneity at low temperatures. For the sample with x= 0.3, the TN cannot be obtained straightforwardly from the M(T) curves since the increase of M are obviously broadened (see Fig. 2(a)). This is attributed to the appearance of Cr4+ ions in the sample with the bivalent Ca2+ substitution of Eu3+, which brings a disruption of the long-range antiferromagnetic order of Cr3+ ions. With increasing doping level, clusters with Cr3+-O-Cr3+ ordering and different sizes may then distribute randomly in the sample. Magnetic clusters with different sizes show different PM-CAFM transition temperature TN, as observed in La0.8Ca0.2MnO3 samples, where the TN is only 143 K for clusters of 15 nm diameter, much lower than the TN of bulk La0.8Ca0.2MnO3, 216 K.20 The varying TN of Cr3+-O-Cr3+ clusters with different size may result in the obviously broad PM-CAFM transition, as indicated by the slow increase of MFC from 184 K, with the decrease of temperature, as shown in Fig. 2(a). Similar effect has also been observed in some other perovskite materials, such as La0.7Ca0.3Mn1-yTiyO3,21
La0.67Ca0.33Mn1-xGaxO3,22
La0.63Ca0.37Mn1-yFeyO3.24 4
La2/3Ca1/3Mn1-xAlxO3-δ,23
and
To study more about the magnetic property of a system with magnetic inhomogeneity, we chose the sample with nominal Ca concentration of x = 0.3 and measured its magnetism in detail. Fig. 3 shows the temperature-dependent FCC, FCW, and ZFC magnetization curves, with an applied field of 100 Oe. FCC and FCW curves almost coincide with each other for the whole measured temperature range of 3~250 K, start increasing rapidly at 184 K with decreasing temperature, while the MZFC and MFC curves show a large difference below 184 K. The increase of MFC at 184 K implies the onset of PM-CAFM transition, contributed by the Cr3+ rich clusters with a TN similar to that of EuCrO3. It is noticeable that the MFC shows a drop at about 41 K, as shown in Fig. 4(a), a magnified view of the MFCW curve in Fig. 3. For the Eu0.7Ca0.3CrO3 sample, the competition among three interactions (Cr3+- O- Cr3+, Cr4+- OCr4+, Cr3+- O- Cr4+) should be taken into account. In Cr3+-rich regions, Cr3+-O-Cr3+ is dominant (similarly to the EuCrO3 phase) and shows weak ferromagnetism (WFM) due to spin canting, with Γ4 type spin ordering of Cr.13,14 In contrast, in Cr4+-rich regions, Cr4+-OCr4+ is dominant (similarly to the CaCrO3 phase) and shows weak ferromagnetism (WFM) due to spin canting, with Γ2 type spin ordering of Cr.15,16 Accordingly, the drop in MFC can be interpreted in the following way: Cr3+-rich regions order in a Cr spin structure of Γ4 (Fz) at temperatures from well above 90 K; while Cr4+-rich regions, with a Cr spin structure of Γ2 (Fx) and TN ≤ 90 K, make weak contribution to the measured magnetization, when the cooling field is low enough and so that the magnetostatic energy of Cr4+-rich regions is lower than its anisotropy energy, since its easy axis is at an angle of 90o to that of Cr3+-rich regions. With the formation and growth of the Γ2 domains in Cr4+-rich regions, domain wall develops between neighboring, and competing Γ4 and Γ2 domains, yielding a slight suppression in the observed magnetic moment below 41 K. The cooling field dependence of the MFC(T) curve at low temperature range is shown in Figs. 4(a)-4(d). The broad cusp shifts toward lower temperature with the increase of cooling field, i.e, 41 K, 37 K, and 33 K for Hcool = 0.1 kOe, 1 kOe, and 2 kOe, respectively. This further confirms the competition between magnetostatic energy and anisotropy energy. When Hcool is large enough, the moments of Cr3+-rich and Cr4+-rich clusters are aligned parallel with Hcool, resulting in the monotonous increase of MFC with the decrease of temperature as shown in Fig. 4(d).25,26 For the sample with x = 0.1, no 5
drop has been observed, reflecting that the Cr4+-rich regions are too small to compete with the Cr3+-rich regions and the external field. The MZFC curve of the Eu0.7Ca0.3CrO3 sample exhibits a peak at 33 K and a wide shoulder at around 75 K, as illustrated in the inset of Fig. 3. The peak around Tf = 33 K indicates that there is a collective freezing of magnetic moments within the sample. Competition between Cr3+- O- Cr3+ and Cr4+- O- Cr4+ interactions might be responsible for the freezing-like effect, which is always a typical glassy magnetic behavior. The wide shoulder around 75 K could be due to the inter-cluster interactions.27,28 B. Spin glass state: time dependence of magnetization and memory effect To study more about the glassy magnetism of our sample, we measured the time dependence of magnetization and memory effect, which are commonly used to identify the existence of SG states.30,31 The measuring method is adopted from ref. 30. We recorded the magnetization as a function of time at T = 3 K in both ZFC and FC, with H = 500 Oe and tw= 2000 s, as shown in Fig. 5(a). The ZFC magnetization increases with time, while the FC magnetization decreases with time during the time span, and the relaxation for ZFC mode is more significant than that for FC mode. This is consistent with the coexistence and competition of Γ4 and Γ2 structures, indicating that the cooling field may favor long range ordering and ZFC may result in more metastable frustrated regions. Ulrich et al. studied the magnetic relaxation of single-domain ferromagnetic particle systems with different particle density, and found that the relaxation rate decays by a power law.
32
For FC mode, the
magnetic relaxation rate W is defined as, W(t) = - (d/dt)lnM(t),
(3)
W(t) = At-n,
(4)
where n depends on the particle concentration, and A is a temperature-dependent prefactor. 6
The inset of Fig. 5(a) shows the double logarithmic plot of W(t) versus t, along with best fit to Eq. (4) for T = 3 K. The value of the fitting parameter n is 0.90, which is close to 1, suggesting the co-existence of competing magnetic phases in our Eu0.7Ca0.3CrO3 sample, and the system can be seen as an assembly of interacting ferromagnetic clusters. Intercluster magnetic dipolar interaction introduces the collectivity and glassiness observed in the above relaxation experiments. Furthermore, we measured FC magnetic relaxation at different temperatures, as shown in Fig. 5(b). The exponent n obtained from the best-fit curve (see the inset of Fig. 5(b)) is 0.89, 0.67, 0.53, 0.41 for T =15K, 33 K, 90 K, and 150 K, respectively. The n decreases with the increase of temperature, indicating the weakening of magnetic dipolar interaction and glassy characteristics. The M(t) data for 33K can also be fitted to KWW stretched exponential function (t)exp(-(t/))(see the Fig. 5(c)),33 Notable deviations of the fits can be seen in the region t>4500 s, is a characteristic relaxation time and is regarded as a shape parameter. The and obtained from the fitting is 6.26 109 s and 0.4336, respectively. This result is in accordance with that of Ni50Mn34In16, where the value of varies between 0.45 and 0.7, and the existence of glasslike magnetic state has been verified. 34 Memory effect was studied using the same method used in reference 31, with an external field of 500 Oe, a temporary stop temperature of 20 K, and a time span of tw= 7200 s. As shown in Fig. 6, it is clear that the magnetization with a stop (red square) shows a typical feature of memory effect, i.e, a dip at around 20 K, reflecting the existence of SG state. As a conclusion, both the observed time dependence of magnetization and memory effect point out unambiguously the existence of SG state in the Eu0.7Ca0.3CrO3 sample.
7
C. Exchange bias effect EB effect can usually be observed in heterogeneous systems containing interfaces involving SG state.35-38 Therefore, it would be interesting to explore if there is EB effect, considering the phase-separation and the existence of SG state in the Eu0.7Ca0.3CrO3 sample. To test this argument, we measured the M(H) loops at 3K with maximum measuring field |Hmmax| = 8 T after both ZFC and FC. The experimental results (see Fig. 7) show a nearly symmetric ZFC hysteresis loop and shifted FC M(H) curves (EB effect). Similar EB effect has also been observed in samples with x = 0.2 and 0.5. Since no EB effect has been observed in the single-phase EuCrO3 sample, the observed EB effect should be mainly a result of the phase separation and SG state. After field cooling to 3 K, the magnetic moments of the Γ4 domains in Eu0.7Ca0.3CrO3 are in the direction of the external cooling field, which parallel coupled with the unidirectional moments of the Γ4 domains. Therefore, during the measuring of hysteresis loops, the WFM moments of the Cr3+-rich Γ4 domains are exerted a microscopic torque to keep them in their initial direction, and thus a positive cooling field results in a negative EB; conversely, a negative cooling field results in a positive EB (see Fig. 7). For ZFC, the hysteresis loop is central symmetric since the local unidirectional moments distribute randomly in their orientations. To further reveal the origin of exchange bias in the present sample with coexisting Γ4 and Γ2 domains, the temperature dependence of exchange parameters was studied, as shown in Figs. 8(a)-8(b). For such measurements, the sample was cooled down from room temperature with an applied field Hcool = 1 T. Once the measuring temperature was reached, 8
the M(H) loop was measured between ± 8 T. The exchange-bias field HE and coercive field HC in Fig. 8 is defined as HE = (H1+H2)/2 and HC = (H2−H1)/2, respectively, where H1 and H2 are the left and right coercive fields, respectively.39 The remanence asymmetry ME and the magnetic coercivity MC are usually defined as the “vertical axis” equivalents of HE and HC, respectively.40 The temperature evolution of HE is typical for the EB systems with FM/SG interfaces,41 and both HE and ME decrease monotonously with the increase of temperature. Relatively weak EB was observed between Tf and 90 K, because of the weakening of the coupling at interfaces and the SG state, with the increase of temperature, consistent with the conclusion drawn from the data in Fig. 5. Below Tf, the colony of SG regions may be temperature dependent, have largest proportion at around 15 K as indicated by the time dependence of magnetization in Fig. 5, where the relaxation at 15 K is the largest. As a result, WFM rotation during the measurement of M(H) can drag more SG spins at around 15 K, giving rise to the maximum value of HC and MC at 15 K, a temperature lower than Tf.42 Figs. 8(c)-8(d) show the cooling field dependence of EB parameters HE, HC, ME, and MC at 3 K for Eu0.7Ca0.3CrO3, where all the parameters increase monotonously with the increasing cooling field up to 6 T. With the increase of the cooling field, more moments of WFM domains get parallel with the cooling field, and the size of the WFM domains may get larger, resulting in a relatively weak increase in exchange bias. This effect is qualitatively analogous to FM/AFM thin films, where exchange bias is inversely proportional to the thickness of the FM layer.4 IV. CONCLUSION. In the frame of a detailed study of the magnetic behavior of Eu0.7Ca0.3CrO3, the competition between Cr3+-rich Γ4 domains and Cr4+-rich Γ2 domains, and the resulting SG9
like phase in Eu0.7Ca0.3CrO3 have been confirmed by the observation of magnetic relaxation, memory effect and exchange bias effect. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 10804067, 11074163, 61171033, 11274211), the Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (Grant No. 13JC1402400), Shanghai Institute of Materials Genome Supported by the Shanghai Municipal Science and Technology Commission (Project No. 14DZ2261200). 1
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Figure captions Fig. 1. (Color online) (a) Powder X-ray diffraction patterns of Eu1-xCaxCrO3 (x = 0.0, 0.1 and 0.3) samples at room temperature. (b) Rietveld analysis of XRD on powder EuCrO3. Plus (+) symbols represent the experimental results and the red solid line is from refinement. The vertical bars at bottom represent the position of Bragg peaks for EuCrO3. The difference between experimental and calculated results is shown as cyan line between the vertical bars and the experimental results. Fig. 2. (Color online) (a) Temperature dependence of magnetization with FCW process at an applied field of 100 Oe for Eu1-xCaxCrO3 (x = 0.0, 0.1, 0.3) samples. The inset gives the temperature dependence of the reciprocal of the magnetic susceptibility χ-1. (b) The magnified view of χ-1(T) in high temperature range. The solid lines show fits to the CurieWeiss law. Fig. 3. (Color online) Temperature dependence of magnetization with ZFC, FCC and FCW processes at an applied field of 100 Oe for Eu0.7Ca0.3CrO3. The inset illustrates the 14
freezing temperature of 33 K with ZFC for Eu0.7Ca0.3CrO3. Fig. 4. (Color online) The magnified view of MFC(T) in low temperature range for Eu0.7Ca0.3CrO3, measured at different cooling field: 0.1, 1, 2 and 3 kOe. Fig. 5. (Color online) Time dependence of magnetization for Eu0.7Ca0.3CrO3 at: (a) T = 3 K of ZFC and FC processes; (b) T = 15, 33, 90, 150 K of FC process (The magnetization values have been normalized with respect to the initial magnetization obtained at time t = 0). Inset shows the relaxation rate, and the solid line is the best fit to Eq. (4); (c) the result of the fitting of KWW stretched exponential function at T=33 K. Fig. 6. (Color online) Temperature dependence of the reference magnetization Mref (solid line), the magnetization M (red square), and ∆M = M - Mref (black square). Fig. 7.(Color online) M(H) loops measured at 3 K with |Hmmax| = 8 T after ZFC and FC (Hcool = ±1 T). The inset shows the ZFC and FC M(T) curves measured on cooling. Fig. 8. (Color online) (a)-(b)Temperature dependence of HE, ME, HC and MC for Eu0.7Ca0.3CrO3 with |Hmmax| = 8 T (Hcool = 1 T); (c)-(d) Cooling field dependence of HE, ME, HC and MC for Eu0.7Ca0.3CrO3 with |Hmmax| = 8 T at 3K.
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Figure 1, D. M. Deng et al.
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Figure 2, D. M. Deng et al.
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Figure 3, D. M. Deng et al.
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Figure 4, D. M. Deng et al.
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Figure 5, D. M. Deng et al.
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Figure 6, D. M. Deng et al.
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Figure 7, D. M. Deng et al.
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Figure 8, D. M. Deng et al.
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Highlights (1) A slow increase of M(T) curve was observed in Eu0.7Ca0.3CrO3. (2) Rotation of magnetization in M(T) curve of Eu0.7Ca0.3CrO3 was observed. (3) Exchange bias effect and glassy magnetism were observed in Eu0.7Ca0.3CrO3. (4) Rotation of the moments of Cr4+-rich regions result in the rotation of magnetization in M(T) curve. (5) Spin glass-like regions contribute to the observed exchange bias effect.