The effect of Al doping on the structure and magnetism in cobaltite CaBaCo4O7
Youming Zou, Zhe Qu, Lei Zhang, Wei Ning, Langsheng Ling, Li Pi, Yuheng Zhang
aa r X i v : . [ c ond - m a t . m t r l - s c i ] A p r The effect of Al doping on the structure and magnetismin cobaltite CaBaCo O a , Zhe Qu a, ∗ , Lei Zhang a , Wei Ning a , Langsheng Ling a , LiPi b,a , Yuheng Zhang a,b a High Magnetic Field Laboratory, Chinese Academy of Sciences,Hefei, Anhui, 230031, China b Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China, Hefei, Anhui, 230026, China
Abstract
We report the effects of Al-doping on the structure and magnetic propertiesin CaBa(Co − x Al x ) O (0 ≤ x ≤ x = 0.1. The Curie temperature and the value of themagnetization decrease with increasing Al doping level, indicating that theferrimagnetic ground state is gradually suppressed. The ground state even-tually transits into a spin-glass state for x > O , arefound to be gradually suppressed with increasing Al content and eventuallydisappear for x = 0.25. By comparing our results with other Co-site dop-ing cases, we suggest that the lattice and the spin degrees of freedom arerelatively decoupled in CaBaCo O . Keywords:
A. magnetically ordered materials, C. phase transitions
1. Introduction
In recent years, there has been an increasing interest in geometrical frus-trated magnets because of their exotic magnetic states such as spin liquids,spin ices and spin-glasses. [1, 2, 3, 4, 5, 6] In these materials, geometrical ∗ Corresponding author. Tel: +86-551-6559-5640; Fax: +86-551-6559-1149.
Email address: [email protected] (Zhe Qu)
Preprint submitted to Journal of Alloys and Compounds September 17, 2018 rustration results in highly degenerate or nearly degenerate ground states,making the system highly susceptible to external perturbations.The recently discovered ”114” cobaltites form a new class of geometricallyfrustrated magnets.[6, 7, 8, 9, 10, 11, 12] Their structure can be viewed asan alternative stacking of triangular and kagome layers build up by CoO tetrahedra. Since the strong antiferromagnetic (AFM) Co-Co interactionsare mediated by Co-O-Co superexchange pathways both in triangular and inkagome layers, they are expected to exhibit complex magnetic properties. Forexample, YBaCo O was reported to undergo various magnetic transitions,such as spin-glass (SG) transition around T f ∼
66 K, [8] long-range AFMorder below T N = 110 K, [13] and a magnetic transition with short-rangecorrelations [14, 15]Here we focus on the CaBaCo O , which has the largest orthorhombicdistortion in the series. [16] Charge ordering is observed in this material,with Co sitting on two sites (Co2, Co3 sites) and Co sitting on theother two sites (Co1, Co4 sites). [17, 18] The system does not retain adisordered ground state since the geometric frustration is partially lifted bythe large structural distortion and the charge ordering. It shows short-rangemagnetic correlations below ∼
360 K and eventually enters a ferrimagnetic(FIM) ground state below T C ∼
60 K through a first-order transition, [16, 19]whose magnetic structure is found to consist of ferromagnetic (FM) zig-zagCo chains along b -axis that are antiferromagnetically coupled with Co cations. [17] In the FIM ground state, a spin-assisted ferroelectric state hasbeen uncovered. [20]Chemical doping on cobalt sites could significantly tune the magneticproperty of the CaBaCo O . Less than 3% Zn impurities are found to inducea spectacular switching of the ground state from the FIM state to an AFMstate with T N ∼
80 K. [21] Such a spectacular switching of the ground stateis attributed to the ordered doping of Zn at Co sites in the FM zig-zagCo chains, which could possibly induce the 180 ◦ flip of the spin on theneighbor Co cations and thus create an AFM state. [21]In this work, we investigate the effect of replacing Co ions in CaBaCo O by measuring the magnetic properties of CaBa(Co − x Al x ) O with 0 ≤ x ≤ x = 0.1. The Curie temperature and the value of the magnetizationdecrease with increasing Al doping level, suggesting that the FIM groundstate is gradually suppressed. The system eventually enters a spin-glass2SG) state for x ≥ O , are found to be graduallysuppressed with increasing Al content and eventually disappear for x = 0.25.By comparing our results with other doping cases, we suggest that the latticeand the spin degrees of freedom are relatively decoupled in CaBaCo O .
2. Experiment
Polycrystalline samples of CaBa(Co − x Al x ) O with 0 ≤ x ≤ , BaCO ,Al O and Co O were mixed and heated at 900 o C in air to decarbonation.They are then pelletized, and then sintered at 1100 o C in air for 12 hoursand quenched to room temperature. The structure and the phase purityof the samples were checked by powder X-ray diffraction (XRD) at roomtemperature. Magnetization measurements were performed with a commer-cial superconducting quantum interference device (SQUID) magnetometer(Quantum Design MPMS 7T-XL) and a Physical Property MeasurementSystem (Quantum Design PPMS-16T) equipped with a vibrating samplemagnetometer (VSM). Since these cobaltites are sensitive to the oxygen con-tent, we have carried out iodometric titration of our samples to measure theoxygen content. The results confirm that the oxygen stoichiometry is fixedto ”O ” within the limit of accuracy of ±
3. Results and Discussion
Figure 1 displays the evolution of the lattice parameters a and b forCaBa(Co − x Al x ) O with 0 ≤ x ≤ O has an orthorhombic structure ( P bn space group).[16] Upon Al doping, the orthorhombic distortion, which can be quantifiedas D = ( b/ √ − a ) /a , [17] is significantly reduced (see Fig. 1). For x = 0.1,the heaviest doped sample retaining the orthorhombic symmetry, the valueof D is only half of that for the parent compound. When the doping levelexceeds 10%, the system shows a structure transition into a hexagonal sym-metry ( P mc space group). As a result, the splitting of the Bragg peaks3etween 2 θ ∼ ◦ - 34 ◦ , which is still evident for x = 0.1, disappears for x =0.15.The temperature dependence of the magnetization M ( T ) for CaBa(Co − x Al x ) O (0 ≤ x ≤ O , the magnetization shows a rapid increase upon cool-ing. This fact, along with the rectangle isothermal magnetization loop at 2K (shown in Fig. 3 (a)), suggests that the system enters a magnetically or-dered state. The magnetic moment, which is determined from the isothermalmagnetization measured at 2 K (shown in Fig. 3), is only ∼ µ B /f.u. under 16 T. This value is relatively small compared to a FM state and isconsistent with the FIM ground state. A clear thermal hysteresis is observedbetween FCC and FCW M ( T ) curves, indicating the first-order nature of thePM-FIM transition. All these results are consistent with previous reports.[16, 19]Upon Al substitution for Co, the transition temperature of the PM-FIM isfound to be gradually suppressed toward lower temperature; it decreases from ∼
60 K for x = 0 to ∼
30 K for x = 0.1. This is accompanied with the rapiddecrease of the magnetization and the coercive field (see Fig. 3), suggestingthat the FIM state is gradually weakened upon Al doping. Moreover, asshown in Fig. 2, while the thermal hysteresis between FCC and FCW M ( T )curves is still clear visible for x = 0.05, it could not be observed for x = 0.1,suggesting that the PM-FIM transition changes from a first-order one to asecond-order one. For x = 0.05 and 0.1 the isothermal magnetization loopsare no longer rectangular and the magnetization does not saturate even underan applied field of 16 T, hinting the possible existence of the spin cantingin these samples. With further increase of the Al content, the magneticbehaviors show drastic changes. For x > M ( H ) loops show almostlinear field dependence, hinting the absence of the FIM long-range order.Meanwhile, a sharp λ peak is observed in the FCC/FCW M ( T ) curves, whichusually means the occurrence of a spin glass state [25] or large coercivity ina long-range ordered state [26]. We further performed AC susceptibilitymeasurements on two typical samples, x = 0 and 0.2, to distinguish theorigin of the obvious irreversibility between DC magnetization curves (seeFig. 5). For x = 0, the peak in χ ′ ( T ) keeps essentially unchanged fordifferent measuring frequency, agreeing with the long-range FIM state. Butfor x = 0.2, the peak in χ ′ ( T ) shifts to higher temperature with increasing4requency, characterizing the SG state. The identification of the SG statefor x > AlO . [27] All these results demonstrate that the magnetic groundstates switch from the long-range FIM ordered ground state for 0 ≤ x ≤ < x ≤ χ versus T curve, suggesting the occurrence of short-range magnetic correlations. [19]With Al substitution for Co, the temperature corresponding to the upwarddeviation from linearity and the magnitude of the upward deviation grad-ually decrease, suggesting that Al doping gradually suppresses short-rangemagnetic correlations that occurs at high temperature in the parent com-pound. For x = 0.25, the upward deviation is fully suppressed, hinting theabsence of short-range magnetic correlations at high temperatures in thissample. Since the short-range magnetic correlations should be related to thegeometrical frustration inherent to the system, the suppression of the short-range suggests that the geometry frustration might be partially released byAl doping.We have constructed the phase diagram of the CaBa(Co − x Al x ) O basedon these observations. As shown in Fig. 6, CaBaCo O enters a FIM groundstate below ∼
60 K through a first-order magnetic transition. With Al substi-tution for Co, the magnetic ordering temperature gradually decreases. Themagnetic transition retains its first-order nature for x ≤ x = 0.1. For x > sites (Co1 and Co4). While Al impuritieswill not directly block the FM Co chains along b -axis like Zn doping case, theywill weaken the interaction between Co chains. Moreover, when the Co1/Co4site is occupied by Al, the residual three Co ions could not retain their FIMconfiguration because of the triangular geometry constraint on these Co ionsand the AFM interaction between them. [18] Both effect will tend to suppressthe FIM long-range order. In addition, since geometrical frustration still5xist in doped samples and the chemical doping will inevitably introduce thedisorder, the system eventually transits into a SG state.It is interesting to compare the effect of Al doping to other Co site dopingcases. Zn, Ga, and Al substitution for Co all induce a structural transitionfrom orthorhombic to hexagonal symmetry. It can be seen that the structureis more susceptible against Al doping; while the orthorhombic symmetrysurvives up to 20% Ga doping and 15% Zn doping, [28] it becomes unstablewhen the Al content exceeds x = 0.1. This should be ascribed to the differentradius of these ions. It is known that the radius of Ga and Zn ion (6.2nm and 7.4 nm) is closed to that of the Co ion and Co (6.1 nm and 7.45nm) respectively while the radius of Al (5.35 nm) is smaller than that ofCo . [29] Compared to Al doping case, the lattice will be less affected uponGa/Zn substitution for Co. As a result, the structure of the system is moresensitive to Al impurities.On the magnetic properties, both Al and Ga doping donot result in theestablishment of an AFM state like Zn impurity. This could be understoodbecause Al and Ga impurities occupy the Co1/Co4 sites and leave the FMCo chain untouched. The Zn impurities also suppress the FIM order mosteffectively, highlighting the important role of the FM Co chains. It is in-teresting to note that while samples with different doping element exhibitdifferent structure evolutions, their magnetic properties evolve with the dop-ing level in a quite similar way. The FIM transition temperature is rapidlysuppressed to ∼
30 K by 5% chemical substitution for both Al and Ga dopedsample. And a generical formation of the SG state is observed when the dop-ing level exceeds 10% for all these doped samples. These results suggest thatwhile the structure phase transition is accompanied with the change of themagnetic ground state in Al doped CaBaCo O the lattice degree of freedomis relatively decoupled with the spin degree of freedom in CaBaCo O .
4. Conclusion
In summary, the effect of Al substitution for Co in CaBaCo O has beenstudied. A structural transition from the orthorhombic symmetry to thehexagonal symmetry is observed when the Al content exceeds x = 0.1. Aldoping is found to suppress the FIM ground state rapidly and eventually re-sult in a transition of the ground state into a SG state for x > O , are found to be suppressed with increasing Al content and dis-6ppear for x = 0.25. By comparing our results with other Co-site dopingcases, we suggest that the lattice and the spin degrees of freedom are rela-tively decoupled in CaBaCo O
5. Acknowledgments
This work is supported by National Natural Science Foundation of Chinaunder contracts Nos. 11004198 and 11174291. Z. Q. gratefully acknowl-edges supports from the Youth Innovation Promotion Association, ChineseAcademy of Sciences.
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(2012)18043-18050.[29] J. A. Dean, Lange’s handbook of chemistry, 15th ed., McGraw-Hill, NewYork, 1999. 9 Pbn2 ab /3 P6 mc L a tti ce p a r a m e t e r s ( n m ) Al content x I ( a r b . un it s ) q (degree) x = 0.15 x = 0.1 D i s t o r ti on D ( % ) Figure 1: (Color online) The lattice parameters a , b and the orthorhombic distortion D = ( b/ √ − a ) /a as function of the Al content x for CaBa(Co − x Al x ) O . Insets showthe enlarged XRD patterns and fitting results for x = 0.1 and 0.15. .00.40.8246 0 20 40 60 80 100234 (a)0.02 FCW FCC ZFC M ( m B / f . u . ) x = 0 (b)0.10.05 M ( - m B / f . u . ) (c)0.250.20.15 M ( - m B / f . u . ) T (K) Figure 2: (Color online)The magnetization versus the temperature under 1000 Oe forCaBa(Co − x Al x ) O with 0 ≤ x ≤ -2 -1 0 1 2-0.10.00.10.51.01.50.0 0.1 0.21.71.9 (a) x = 0 x = 0.02 x = 0.05 x = 0.1 M ( m B / f . u . ) (b) x = 0.15 x = 0.2 x = 0.25 M ( m B / f . u . ) H (T) H (T) M ( m B / f . u . ) M M T @ K ( m B / f . u . ) Al content x M . T @ K ( - m B / f . u . ) M Figure 3: (Color online)The magnetization as function of the magnetic field forCaBa(Co − x Al x ) O with 0 ≤ x ≤
100 200 300 400020406080 x = 0 x = 0.02 x = 0.05 x = 0.1 x = 0.15 x = 0.2 x = 0.25 / c ( T f . u . / m B ) T (K) Figure 4: (Color online) The reciprocal of the susceptibility as function of temperaturebetween 2 and 400 K under 0.1 T measured in FCC sequence for CaBa(Co − x Al x ) O with 0 ≤ x ≤
20 40 60 80 1 Hz 11 Hz 110 Hz 332 Hz 997 Hz c ' ( a r b . un it ) (a) x = 0(b) x = 0.2 K T (K) Figure 5: (Color online) The χ ′ ( T ) curves for CaBa(Co − x Al x ) O with x = 0 (a) and0.2 (b) at different measuring frequencies. bn2 P6 mc T ( K ) Al content x CaBa(Co x Al x ) O T C in FCC T C in FCW T f FIM SGPM
Figure 6: (Color online) The magnetic phase diagram for the CaBa(Co − x Al x ) O with 0 ≤ x ≤0.25. PM: the paramagnetic state; FIM: the ferrimagnetic state; SG: the spin-glassstate. The closed circle and the open circle represent the PM-FIM transition temperaturemeasured in FCC and FCW sequences, respectively. The filled square shows the freezingtemperature of the SG state.