IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2502204 Inhomogeneous Ferromagnetism and Spin-Glass-Like Behavior in (Nd1−x Y x )0.7Sr0.3MnO3 With x = 0.21–0.35 T L Phan1 , V D Nguyen2, T A Ho1 , N V Khiem3 , T D Thanh2, N X Phuc2 , P D Thang4, and S C Yu1 Department of Physic, Chungbuk National University, Cheongju 361-763, Korea of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Department of Natural Science, Hongduc University, Thanhhoa, Vietnam Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam Institute The magnetic properties of polycrystalline ceramic samples (Nd1−x Y x )0.7 Sr0.3 MnO3 with x = 0.21 − 0.35 were studied by means of dc magnetization and ac susceptibility measurements Experimental results reveal a strong decrease of the ferromagnetic (FM)-paramagnetic phase-transition temperature (TC ) from 97 to 65 K as increasing x from 0.21 to 0.35, respectively There is magnetic inhomogeneity associated with short-range FM order Particularly, the samples undergo a spin-glass (SG) phase transition at the so-called blocking temperature (TB ) below TC , which shifts toward lower temperatures with increasing the applied field, Hex ; TB → Tg (the SG phase-transition temperature) as Hex → The existence of the SG behavior in these samples was also confirmed by frequency ( f ) dependences of the ac susceptibility For the in-phase/real component, χ (T ), it shows a frequencydependent peak at the SG freezing temperature (T f ); T f → Tg as f → Dynamics of this process were analyzed by means of the slowing down scaling law, τ /τ0 ∝ (T f /Tg − 1)−zv , where τ0 and zv are the characteristic time and critical exponent, respectively Fitting the experimental T f (f ) data to the scaling law gave the results of zv = 10.1–12.3 and τ0 = 10−21 –10−15 s These values are different from those expected for canonical SG systems with zv = 10 and τ0 = 10−13 s, revealing the cluster-SG behavior of (Nd1−x Y x )0.7 Sr0.3 MnO3 samples Notably, the increase in Y content leads to the shift of τ0 and zv values toward those of canonical SG systems, which is ascribed to an expansion of SG clusters Index Terms— Magnetic inhomogeneity, perovskite manganites, spin-glass (SG) behavior I I NTRODUCTION T IS known that NdMnO3 is an antiferromagnetic (AFM) insulator, and crystallized into the orthorhombic structure (space group Pbnm) At low temperatures, due to the canting and reorientation of Mn spins, the coexistence of ferromagnetic (FM) and AFM couplings leads to a non-collinear magnetic structure [1] In manganites, AFM interactions are generated from super-exchange (SE) pairs of Mn3+ -Mn3+ and Mn4+ -Mn4+ while the FM interaction is from a double-exchange (DE) pair Mn3+ Mn4+ [2] The strength of these interactions strongly depends on Mn3+ and Mn4+ concentrations, the bond length Mn-O , and the bond angle Mn–O–Mn Due to the coexistence of FM and AFM interactions, it has been suggested that intrinsic defects related to oxygen content are present in NdMnO3 [1] A dominancy of SE interactions leads to its AFM nature To widen the application range of NdMnO3 in electronic devices, a divalent alkaline-earth metal A can be doped into the Nd site to fabricate hole-doped manganites Nd1−x A x MnO3 with A = Ca, Ba, or Sr This creates more Mn4+ concentration, and results in noticeable physical effects (typically, colossal magnetoresistance and magnetocaloric effects) at phase-transition temperatures, depending on added A -dopant content (x) Various Nd1−x A x MnO3 compounds have different I Manuscript received November 13, 2013; revised January 8, 2014; accepted January 14, 2014 Date of current version June 6, 2014 Corresponding author: S C Yu (e-mail: scyu@chungbuk.ac.kr) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2300852 Jahn–Teller distortions (caused by a strong electron-phonon coupling), which change the lattice symmetry, and the structural parameters of Mn-O and Mn-O-Mn To characterize the distortion level, √ it is proposed to use the tolerance factor t = ( R A + R O )/ 2( R B + R O ), where R A and R B are the average radii of cations located at the A and B sites in the perovskite structure ABO3 , respectively, and R O is the radius of oxygen anion [3], [4] Comparing with La-based manganites, it has been found that the eg -electron bandwidth of Nd1−x A x MnO3 is more sensitive to changes related to the parameters t, Mn-O , and Mn-O-Mn Electrical, magnetic, and magnetotransport properties of Nd-based manganites are thus more interesting and complicated than those of La-based manganites [5]–[9] In general, FM interactions of Mn3+ -Mn4+ DE pairs in Nd1−x A x MnO3 become strongest at the doping concentration x = 0.3, corresponding to Mn3+ /Mn4+ = 7/3 [2] Among Nd0.7 A0.3 MnO3 compounds, Nd0.7 Sr0.3 MnO3 has attracted much more particular interest There is the contribution of the Nd–Mn exchange interaction [9], [10] though no Nd-related magnetic ordering has been found in NdMnO3 at temperatures down to 1.8 K [1] Depending on investigated magneticfield and temperature ranges, the NdMnO3 compound exhibits other noticeable phenomena, such as charge-orbital ordering state [11], and magnetic frustration, and spin-glassy (SG)-like behavior [8] Though Nd0.7 Sr0.3 MnO3 is known as a magneticfrustrated and disordered system [8], the FM interaction between Mn3+ and Mn4+ ions is still dominant The SG-like behavior thus appears in cluster, namely SG clusters, causing the magnetic inhomogeneity It is possible to enlarge the size of SG clusters by decreasing the parameter R A or t upon 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 2502204 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 doping a rare earth or transition-metal element into the site La/Sr or Mn, depending on its ionic radius For the substitution into the La/Sr site, the magnetic frustration (caused by the competition of FM and AFM interactions) and SG behavior become significant as R A < 1.2 Å [12] In an attempt to obtain more insight into this problem, we prepared (Nd1−x Yx )0.7 Sr0.3 MnO3 with x = 0.21–0.35, and then have studied in detail their magnetic properties based on dc magnetization and ac susceptibility measurements Our study points out that increasing Y-doping content results in the shift of the characteristic time (τ0 ) and critical exponent (zv) value toward those of canonical SG systems This is ascribed to an expansion of SG clusters II E XPERIMENTAL D ETAILS Three polycrystalline samples (Nd1−x Yx )0.7 Sr0.3 MnO3 with x = 0.21, 0.28, and 0.35 were prepared by solidstate reaction High-purity powdered precursors of Nd2 O3 , SrCO3 , Y2 O3 , and MnO2 (99.9%) were combined in nominally stoichiometrical quantities, well mixed, and pressed into pellets These pellets were then preannealed at 1150 °C for 24 h After several times of intermediate grinding, pressing and preannealing, the calcined pellets were sintered in air at 1350 °C for h The single phase in an orthorhombic structure of the final products was confirmed by an X-ray diffractometer as using Siemens D5000 Measurements of dc magnetization and ac susceptibility were performed on a physical property measurement system in the temperature range 5–300 K For the ac susceptibility, measurements were performed in an ac field Hac = Oe, and zero dc field Hdc = after the zerofield cooling Frequency ( f ) can be changed in the range 12.7–9100 Hz III R ESULTS AND D ISCUSSION Fig shows temperature dependences of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations, MZFC/FC (T ) curves, of the samples (Nd1−x Yx )0.7 Sr0.3 MnO3 in an external magnetic field Hex = 100 Oe For the MZFC (T ) curves, there are cusps peaked at the so-called blocking temperature TB ≈ 82, 68, and 53 K, corresponding to the samples with x = 0.21, 28, and 35, respectively Below TB , there is a gradual decrease of magnetization However, the decrease in magnetization at temperatures below TB does not occur for the case of the MFC (T ) curves There is a separation between MFC (T ) and MZFC (T ) curves at the reversibility temperature Tr (= 98, 80, and 69 K for x = 0.21, 28, and 35, respectively) below the FM-paramagnetic phase transition temperature TC = 97, 77, and 65 K for x = 0.21, 28, and 35, respectively Here, the TC values are obtained from the minima of the dMZFC /dT (or dMFC /dT ) versus T curves, inset of Fig Experimental evidences revealed that both Tr and TB values are shifted according to a power function toward lower temperatures with increasing Hex [13], [14] Notably, TC and magnetization values gradually decrease with increasing Y-doping content It is suggested that the features of the MFC (T ) and MZFC (T ) curves shown in Fig are related to a local anisotropic field Ha generated from FM/AFM clusters (which can persist even at temperatures above the TC [15]) due to the magnetic inhomogeneity and mixed phases The energy of the anisotropic field acting on magnetic moments m of Mn ions is thus defined by E a = Ha ·m Magnetic moments can be frozen in the directions favored energetically by their local anisotropy Ha or by Fig Temperature dependences of ZFC and FC magnetizations, MZFC/FC (T ) curves, for (Nd1−x Y x )0.7 Sr0.3 MnO3 with x = 0.21, 0.28, and 0.35 in the field 100 Oe Inset: dM/dT versus T curves, and their maximum shows the TC values of the samples an external magnetic field Hex (with energy defined as E ex = Hex ·m) when the system is cooled from high temperatures in a zero or nonzero field This leads to the separation between the MZFC (T ) and MFC (T ) curves at temperatures below Tr The deviation of MFC (T ) from MZFC (T ) depends on the magnetic homogeneity, and on the Hex magnitude A large deviation is usually observed in magnetic samples having a coexistence of FM and AFM phases, and exhibiting the magnetic frustration (i.e., a strong competition between FM and AFM interactions) [8], [14], [16] At the temperature TB , E a is equal to E ex , and thus MZFC reaches the maximum Meanwhile, at temperatures T < TB , E a is higher than E ex These features are similar to those observed in some manganites and cobaltites, and an indication of coexisting short-range FM order and SG-like behavior below TB [8], [13], [14], [17] For a SG system, there is the SG-phase-transition temperature Tg (where TB → Tg as Hex → 0) Magnetic moments experience random interactions with other ones, leading to a state that is irreversible and metastable To learn about the SG behavior and spin dynamics in (Nd1−x Yx )0.7 Sr0.3 MnO3 with x = 0.21–0.35, we have measured frequency ( f ) and temperature (T ) dependences of the ac magnetic susceptibility χ(T, f ), which consists of the inphase/real, χ (T, f ), and out-of-phase/imaginary, χ (T, f ), components Among these, χ is the slope of the M(H ) curve while χ indicates dissipative processes For ferromagnets, non-zero χ values indicate irreversible domain-wall movements or absorption due to a permanent moment Because both χ and χ are very sensitive to thermodynamic phase changes (or spin dynamics), more information associated to transition temperatures is expected to be obtained from ac susceptibility measurements In Fig 2, it shows temperature dependences of the χ and χ components for the samples (Nd1−x Yx )0.7 Sr0.3 MnO3 recorded at various f values ranging from 12.7 to 9100 Hz For both components, one can see that the curves exhibit cusps at the freezing temperature (denoted as T f and T f for the χ (T ) and χ (T ) data, respectively) Similar to the cusps peaked at TB in the MZFC (T ) curves, the positions of the cusps at T f [or T f ] in the χ (T ) [or χ (T )] PHAN et al.: INHOMOGENEOUS FERROMAGNETISM AND SG-LIKE BEHAVIOR IN (Nd1−x Yx )0.7 Sr0.3 MnO3 Fig Temperature dependences of ac susceptibility components χ and χ recorded at various frequencies in the range 12.7–9100 Hz for (Nd1−x Y x )0.7 Sr0.3 MnO3 with (a) and (b) x = 0.21, (c) and (d) x = 0.28, and (e) and (f) x = 0.35 The humps marked with asterisks, besides the cusps at T f and T f , reveal magnetic inhomogeneity (meaning an coexistence of FM and SG behaviors) in the samples curves are also shifted toward lower temperatures with increasing Y-doping concentration in (Nd1−x Yx )0.7 Sr0.3 MnO3 , which is related to the weakening of FM interactions With f = 12.7 Hz, both T f and T f values are ∼84, 73, and 53 K for the samples with x = 0.21, 0.28, and 0.35, respectively Particularly, these temperatures shift toward high temperatures when f is increased, Fig with an enlarged view around T f The frequency dependence of the ac susceptibility demonstrates an existence of the SG behavior, where there is a strong competition of FM and AFM interactions [8], [12]–[14], [17] As mentioned, the samples go into the SG state at temperatures below the SG-phase-transition temperature Tg that T f → Tg as f → Above Tg , they usually exhibit the PM behavior However, it comes to our attention that besides the main cusps, small humps are also observed (indicated as asterisks in Fig 2), particularly for the samples with x = 0.28 and 0.35, even at temperatures above T f (or T f ) Such the features are ascribed to the magnetic inhomogeneity associated with FM/AFM clusters [8], [14], [15] Magnetic moments of Mn ions can be aligned to different anisotropic fields generated from FM/AFM clusters, leading to the appearance of humps For the χ (T ) curves, the existence of the humps and cusp is directly related to different magnetic energy dissipations These results prove the coexistence of FM and SG behaviors in the samples (Nd1−x Yx )0.7 Sr0.3 MnO3 with x = 0.21–0.35 We also measured the ac susceptibility of an additional sample x = 0.14 (not shown) However, no frequency dependence of T f and T f was observed 2502204 Fig Enlarged view of the χ (T ) data around T f at different frequencies for the samples (a) x = 0.21, (b) x = 0.28, and (c) x = 0.35 exhibiting SG-like behavior Insets: log10 (τ ) versus log10 [(T f -Tg )/Tg ] data fitted to the critical-slowing down law Dynamics of the SG behavior can be deeper understood upon analyzing frequency dependences of T f (or T f ) based on conventional critical-slowing down [14], [17], τ/τ0 ∝ (T f /Tg − 1)−zv , where τ0 and zv are the characteristic time and critical exponent, respectively In this equation, T f is understood as the frequency-dependent freezing temperature at which the maximum relaxation time τ of the system responds to the measured frequency By fitting the above equation to the T f data in the range 12.7–9100 Hz, with the Tg values selected from the T f values at the lowest frequency f = 12.7 Hz, we obtained zv = 10.1–12.3 and τ0 = 10−21 − 10−15 s, insets and their labels of Fig It appears that the obtained zv values are close to the value zv = 10 while the τ0 values are much different from τ0 = 10−13 s of canonical SG systems [17] Similar results were observed for the cases of La0.7 Ba0/3 Mn0.7 Ti0.3 O3 (with τ0 ≈ 10−16 s) [13] and Nd0.4 Gd0.3 Sr0.3 MnO3 (with τ0 ≈10−17 s) [12] It has been suggested that τ0 strongly depends on the size of SG clusters, which is usually observed in unconventional SG systems or inhomogeneous DE ferromagnets [12], [13] In other words, SG clusters coexist with the FM phase in the samples (Nd1−x Yx )0.7 Sr0.3 MnO3 with x = 0.21–0.35 Increasing the Y-doping concentration makes τ0 and zv values shifted to those of canonical SG systems due to an expansion of SG clusters To clarify the origin of the decrease in the TC and magnetization, and the SG-like behavior when Y concentration 2502204 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 in (Nd1−x Yx )0.7 Sr0.3 MnO3 is increased, let us consider the magnetic property of Nd0.7 Sr0.3 MnO3 with R A = 1.207 Å In this parent compound, the Mn3+ /Mn4+ ratio is 7/3, and the FM interaction between Mn3+ and Mn4+ ions plays a dominant role With doping and increasing Y3+ concentration into the Nd3+ site of (Nd1−x Yx )0.7 Sr0.3 MnO3 , the Mn3+ /Mn4+ ratio is unchanged while R A gradually decreases because the ionic radius of Y3+ (1.04 Å) is shorter than that of Nd3+ (1.123 Å) The strength of magnetic interactions thus only depends on the variation of R A ; definitely, this variation influences directly the bond length Mn–O and angle Mn– O–Mn It has been found that with decreasing R A , the FM DE interaction is weakened and resultantly the AFM SE interaction is developed [18] The decreases in TC and magnetization are thus understandable Particularly, when R A is smaller than 1.2 Å, the FM and AFM interactions in samples are comparable with each other, resulting in the magnetic frustration and SG-like behaviors These phenomena happen for the cases of Nd0.4 Gd0.3 Sr0.3 MnO3 with R A = 1.198 Å [12], and our samples (Nd1−x Yx )0.7 Sr0.3 MnO3 with R A = 1.194, 1.190, and 1.186 Å for x = 0.21, 0.28, and 0.35, respectively For manganites doped by a transition metal (M, such as Fe, Cr, Ti, and so forth) into the Mn site (i.e., the B site of the perovskite structure ABO3 ) [4], [13], [17], their SG-like behavior is related to the suppression of FM DE Mn3+ -Mn4+ interactions due to the additional presence of AFM interactions caused by M-dopant ions In addition, doping M ions modifies the structural parameters of Mn–O and Mn–O–Mn , and changes the concentration of Mn3+ and Mn4+ ions These factors influence directly the FM phase of manganites IV C ONCLUSION We prepared polycrystalline samples (Nd1−x Yx )0.7 Sr0.3 MnO3 with x = 0.21–0.35 by solid-state reaction Their magnetic properties were then studied by measurements of ZFC/FC magnetizations and ac susceptibility (at various frequencies) versus temperature With increasing the Y-doping concentration (x), the TC is decreased from 97 K (for x = 0.21) to 65 K (for x = 0.35) We also found the magnetic inhomogeneity associated with the coexistence of the FM phase and SG-like behavior The samples undergo SG phase transition at temperatures below TB < TC This phenomenon was evidenced via frequency dependences of the freezing temperature T f (or T ’ f ), which were recorded from the ac susceptibility in the frequency range 12.7–9100 Hz The SG dynamics were further analyzed by conventional criticalslowing down law for the T f ( f ) data, τ /τ0 ∝ (T f /Tg -1)−zv The obtained results reveal that the values zv = 10.1–12.3 and τ0 = 10−21–10−15 s are quite different from those expected for canonical SG systems with zv = 10 and τ0 = 10−13 s We believe that this phenomenon is related to unconventional SG behaviors usually observed in inhomogeneous DE ferromagnets In other words, there exists the cluster-SG behavior in our (Nd1−x Yx )0.7 Sr0.3 MnO3 samples Increasing the Y-doping concentration leads to the expansion in size of SG clusters, and increases the τ0 and zv values, making these values shifted toward those of canonical SG systems Here, the decreases of TC and magnetization, and the appearance of the SG behavior are directly related to the decrease of R A , which weakens the FM DE interaction between Mn3+ and Mn4+ ACKNOWLEDGMENT This work was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea, under Grant 2013K000405, and the National Foundation for Science and Technology Development under Grant 103.02-2012.57 in Vietnam R EFERENCES [1] S Y Wu, C M Kuo, H Y Wang, W.-H Li, K C Lee, J W Lynn, et al., “Magnetic structure and spin reorientation of the Mn ions in NdMnO3 ,” J Appl Phys., vol 87, no 9, pp 5822–5824, May 2000 [2] A P Ramirez, “Colossal magnetoresistance,” J Phys., Condensed Matter, vol 9, no 39, 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H Hur, “Chemical pressure effects of cation size variation in layered manganites,” J Phys Soc Jpn., vol 70, no 8, pp 2448–2453, Mar 2001 ... mixed, and pressed into pellets These pellets were then preannealed at 1150 °C for 24 h After several times of intermediate grinding, pressing and preannealing, the calcined pellets were sintered in. .. of FM and SG behaviors) in the samples curves are also shifted toward lower temperatures with increasing Y-doping concentration in (Nd1−x Yx )0.7 Sr0.3 MnO3 , which is related to the weakening... parent compound, the Mn3+ /Mn4+ ratio is 7/3, and the FM interaction between Mn3+ and Mn4+ ions plays a dominant role With doping and increasing Y3+ concentration into the Nd3+ site of (Nd1−x Yx