Magnetic and electric properties of CaMn7O12 based multiferroic compounds: effect of electron doping
J. Sannigrahi, S. Chattopadhyay, D. Dutta, S. Giri, S. Majumdar
aa r X i v : . [ c ond - m a t . s t r- e l ] M a y Magnetic and electric properties of CaMn O basedmultiferroic compounds: effect of electron doping J. Sannigrahi a , S. Chattopadhyay a , D. Dutta b , S. Giri a , S.Majumdar a ∗ Department of Solid State Physics, Indian Association for the Cultivation of Science,2A & B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India b Department of Metallurgical Engineering & Materials Science, Indian Institute ofTechnology Bombay, Powai, Mumbai 400 076E-mail: ∗ [email protected] Abstract.
The mixed-valent multiferroic compound CaMn O is studied for its magneticand electric properties. The compound undergoes magnetic ordering below 90 K witha helimagnetic structure followed by a low temperature magnetic anomaly observedaround 43 K. The present study shows that the magnetic anomaly at 43 K is associatedwith thermal hysteresis indicating first order nature of the transition. The compoundalso shows field-cooled magnetic memory and relaxation below 43 K, although nozero-field-cooled memory is present. Clear magnetic hysteresis loop is present in themagnetization versus field measurements signifying the presence of some ferromagneticclusters in the system. We doped trivalent La at the cite of divalent Ca expecting toenhance the fraction of Mn ions. The La doped samples show reduced magnetization,although the temperatures associated with the magnetic anomalies remain almostunaltered. Interestingly, the spontaneous electrical polarization below 90 K increasesdrastically on La substitution. We propose that the ground state of the pure as wellas the La doped compositions contain isolated superparamagnetic like clusters, whichcan give rise to metastability in the form of field-cooled memory and relaxation. Theground state is not certainly spin glass type as it is evident from the absence of zero-field-cooled memory and frequency shift in the ac suceptibility measurements.PACS numbers: 75.85.+t, 75.60.Ej, 75.30.Kz agnetic and electric properties of CaMn O .....
1. Introduction
In recent times there have been tremendous interest in multiferroic materials fortheir versatile technological importance [1, 2, 3, 4]. A multiferroic possesses bothferroelectricity and magnetic order, and it is particularly important when the magneticand electric phenomena remain intercoupled. Such magnetoelectric (ME) effect is quitepromising for memory devices, and at the same time has fundamental importance.Ferroelectricity and magnetism in a material are generally mutually exclusive andeven when they occur together in a material they have their different sources. As aresult the ME coupling is found to be quite weak. Examples of such multiferroics arefound among transition metal oxides such BiFeO , BiMnO etc [5, 6]. However, inlast one decade several other transition metal oxide based materials were discoveredwhere the magnetic order itself is responsible for the development of ferroelectricpolarization [7]. Such materials are often referred as magnetic multiferroic and examplesinclude rare-earth manganites (TbMnO , DyMnO ,YMnO , TbMn O etc.) [8, 9],vanadates (Ni V O ) [10], chromate (CoCr O ) etc. The existence of direct couplingbetween magnetic and electric polarizations in these material are of fundamentalimportance. However, ME coupling strength is still rather weak in these materials andthe value of spontaneous electric polarization ( P ) is few order of magnitude smaller thanthe conventional proper ferroelectrics along with the low value of ferrolectric transitiontemperatures. Consequently they are not useful for practical applications.Very recently, a new magnetic multiferroic CaMn O has been reported in theliterature which show large ferroelectric polarization (2870 µ Cm − ) below the helicalmagnetic ordering temperature of 90 K [11, 12]. CaMn O belongs to the quadruple(AA ′ ) B O family of manganites and has a perovskite derived crystal structure withtrigonal space group R¯3 [13, 14]. In this structure, Mn has three crystallographicallyinequivalent sites with Wykoff positions 9e, 9d and 3b. The sites 9e and 9d are occupiedby Mn ions (three Mn-ions at each site per formula unit), and the 3b site is occupiedby one Mn ion [11, 12, 15, 16]. As evident, the compound is mixed valent with theMn and Mn ratio being 6:1. The Mn (9d) and Mn (3b) remain in a chargeordered state below 250 K [16].The compound undergoes two magnetic transitions, one at T = 90 K and the otherat T = 45 K [11, 15]. Below 90 K, the magnetic structure is helical with incommensuratemagnetic propagation vector q = (0, 1, 0.963) [12]. The magnetic structure is foundto be more complex below T and it may be represented by two coexisting propagationvectors namely, q ± = q ± (0 , , δ ), where δ ∼ c axis below 90 K [11, 12]. The magnetic nature of such electricalpolarization is also confirmed by the strong ME effect observed below 90 K, which showsabout 30% change in P under an applied magnetic field of H = 90 kOe [11, 17]. Basedon the recent density functional theory (DFT) calculation, it has been argued thatgiant P and ME in CaMn O arise from the symmetric exchange-striction in presenceof strong Dzyaloshinskii-Moriya (DM) type magnetic interaction between certain Mn agnetic and electric properties of CaMn O .....
635 640 645 650 655 660 80 85 90 95 100526 528 530 532 534 536 825 830 835 840 845 850 855 860 x =0.04 I n t e n s it y ( a . u . ) x =0.0 Mn 2p (a) (b)
Mn 3sx =0.04 x =0.0 E3s = 5.08 eVE3s = 5.19 eV (c)
B.E. (eV) O 1sx =0.0 x = (d) La 3dx =0.04
B.E. (eV)
Figure 1.
Core level x-ray photoelectron spectra of (a) Mn-2 p , (b)O-1 s , (c)Mn-3 s , and (d)La-3 d levels recorded at room temperature for CaMn O andCa . La . Mn O samples. and Mn ions [18].It is understood that the magnetic and ferroelectric anomalies in CaMn O arestrongly governed by the mixed valent nature of manganese. In the present work wecarefully examined the nature of the magnetic transitions as well as their evolutionwith changing Mn and Mn ratio. We substituted trivalent La at the divalentCa to tune the Mn /Mn ratio. Our study indicate interesting magneto-thermalirreversibility across the low temperature magnetic transition along with drastic changein the magnetic and ferroelectric properties.
2. Experimental Details
Ceramic samples of CaMn O and some La doped compositions (Ca − x La x Mn O , x = 0.04, and 0.08 ) were prepared by sol-gel method using polyethylene glycol gel. Thegel produced by the method was first heated at 250 ◦ C in the oven. The products wereheated consecutively at (i) 800 ◦ C, (ii) 925 ◦ C, and (iii) 950 ◦ C each for 16 h. Finally thepowder samples were pelletized and heated 970 ◦ C for 60 h. Room temperature powderx-ray diffraction (XRD) data of the samples were recorded using Cu K α radiation.Energy dispersive spectrometry (EDS) on the samples were performed in an FEI Quanta200 scanning electron microscope. The pure and x = 0.04 samples were also investigatedthrough core level x-ray photo-electron spectroscopy (XPS) at room temperature usingAl K α radiation on a laboratory based commercial instrument (Omicron). The magneticmeasurements were performed on a vibrating sample magnetometer from CryogenicLtd. UK, as well as on a Quantum Design SQUID magnetometer. The pyroelectriccurrent of the sample was measured using a Kithley electrometer (model 6517B) in aHelium closed cycle refrigerator. The ac dielectric measurements were performed using agnetic and electric properties of CaMn O .....
3. Sample Characterization
The XRD patterns (not shown here) indicate that all the samples crystallize in trigonal R a h and c h decrease systematically with La doping.We performed careful EDS measurement to find out the elemental ratio of thestudied samples. The EDS shows homogeneous concentration of the constituentelements within the accuracy of the method. We carried out EDS at 8-10 differentregions of the samples with effective scanning area of 0.34 × . The meanatomic ratio (La+Ca):Mn was found to be 1:7.05 ( ± ± x =0.04 and 0.08 samples respectively. The error values indicated in the bracket are thestandard deviation of the elemental ratio measured at different parts of a particularsample. The (La+Ca):Mn values are quite close to the expected stoichiometric valueof 1:7. The atomic ratio Ca:La was found to be 1:0.05 (expected 1:0.04) and 1:0.088(expected 1:0.08) in x = 0.04 and 0.08 samples respectively. These values are quite closeto the actual stoichiometry considering the fact that the percentage of La in the dopedsamples is small.We performed core level XPS measurement on x = 0.0 and 0.04 samples at roomtemperature as shows in figs. 1. The background subtraction and peak fitting wereperformed using the software XPS-PEAK 4.1 . Fig. 1 (a) shows the Mn-2 p levels of boththe samples. Clear splitting is observed in Mn-2 p peak which corresponds to 2 p and2 p due to spin orbit coupling. The spin orbit splitting in 2 p level is found to be 11.7eV for both the samples. The Mn-2 p is broad and asymmetric in the higher bindingenergy (BE) side. Such broadness and asymmetry have often been ascribed to the mixedvalency of Mn in the sample [19]. Each 2 p doublet was fitted using three components,namely Mn , Mn and a satellite peak. The Mn-2 p peak is slightly shifted towardlower BE side in x = 0.04 sample. We have looked at the energy difference betweenO-1 s (see fig. 1 (b)) and Mn-2 p levels and it is found to be ∆ E p − s = 112.4 and112.0 eV for x = 0 and 0.04 samples respectively. The lower value of ∆ E p − s in x =0.04 may indicate the larger Mn fraction in the La-doped sample as compared to theundoped one [20].Fig. 1(c) shows XPS spectra of the exchange splitted Mn-3 s level for x = 0.0 and0.04 samples. The extend of splitting (∆ E s ) depends on the valence state of Mn ions [21]and Mn valence can be expressed by the empirical relation: v Mn = 9 . − . E s [22].We find v Mn to be 3.22 and 3.08 respectively for x = 0.0 and 0.04 samples. It is tobe noted that average Mn valence of CaMn O is expected to be 3.14 (considering 6:1ratio of the Mn and Mn ). The investigation indicates the enhancement of Mn ions in the material on La substitution. agnetic and electric properties of CaMn O .....
60 80 1000.020.03 M ( e m u / m o l ) T (K)
FC FCHZFCH H =
100 Oe
ZFCH
Figure 2.
Temperature dependence of zero-field-cooled-heating (ZFCH), field-cooling (FC) and field-cooled-heating (FCH) magnetization data of CaMn O samplemeasured in presence of 100 Oe of field. The inset shows the enlarged view of theanomaly at 90 K. Table 1.
Variation of hexagonal lattice parameters ( a h and c h ), pseudo-cubicangle( α cub ), effective paramagnetic moment( µ eff ), magnetization in 50 kOe at 5 K M K kOe ), coercive field at 5 K ( H KC ), and spontaneous polarization at 10 K ( P K )of Ca − x La x Mn O for different values of x . x a h c h α cub µ eff M K kOe H KC P K (˚A) (˚A) ( ◦ ) ( µ B /Mn) ( µ B /f.u.) (kOe) ( µ Cm − )0.0 10.63 6.31 91.18 4.69 2.99 3.1 4400.04 10.57 6.29 91.05 4.78 2.18 0.92 8780.08 10.51 6.28 90.93 4.81 1.84 0.52 1190The presence of La in x = 0.04 sample is confirmed by the observation of La-3 d doublet peaks at energies 855.3 eV (3 d ) and 838.5 eV (3 d ). Each doublet is furthersplit into two components, which occurs due to the charge transfer from the ligand 2 p level to the La 4 f level [23]. The spin-orbit splitting between 3 d and 3 d lines arefound to be close to 16.8 eV, which is expected for the La state.
4. Results O Fig. 2 shows the dc magnetization ( M ) as a function of temperature ( T ) measuredin zero-field-cooled-heating (ZFCH), field-cooled-heating (FCH) and field-cooling (FC)protocols. The ZFCH and FCH data diverges from below about T = 43 K. The signatureof the high- T magnetic transition is also evident from a kink around 90 K as shown in theinset of fig. 2. However, no signature of the emergence of thermomagnetic irreversibility agnetic and electric properties of CaMn O .....
30 35 40 45 500.00.10.2 M ( e m u / m o l ) T (K) H Cool =
100 Oe H Cool = H Cool = 30 kOe (a) M ( e m u / m o l ) t (s) T = 5 K H = 100 Oe (b) M ( e m u / m o l ) T (K) M stop M mem a c ’ ( a . u . ) T (K) f = 111 Hz f = 222 Hz f = 449 Hz f = 750 Hz
Figure 3. (a) Magnetization as a function of temperature measured in 100 Oe whileheating after the sample being cooled in three different fields (100 Oe, 1 kOe and 3kOe). The inset shows the magnetic relaxation curve measured at 5 K. (b) shows thefield stop field cooled memory measurement curves. Here M stop was recorded whilecooling in 1 kOe with intermediate stops. M mem is the subsequent continuous heatingcurve measured in 1 kOe. The inset of (b) shows the temperature variation of the realpart of ac susceptibility measured at different driving frequencies. is observed at this transition. The 43 K transition is more prominent and it is associatedwith the large enhancement of M . We also observe the signature of thermal hysteresisjust below T between FCH and FC data. Interestingly, FC and FCH data show ratherunusual behaviour as far as the thermal hysteresis is concerned. On cooling below T they join up at around 38 K and then separated out again to form another hysteresisloop between 38 K and 15 K. In other words, we observe two separate thermal hysteresisloops below T . The existence of thermal hysteresis between 43 and 38 K clearly indicatethe first order nature of the magnetic transition at T , which is also supported bythe structural anomaly reported around T previously [14, 17]. Notably, ac dielectricpermittivity data show an upturn below around 40 K [17], which may be connectedto the hysteresis observed in our M ( T ) data between 38 K and 15 K. A clear changein slope was reported in the P ( T ) data around 45 K followed by a flat region below35 K [11]. Such anomalies are also apparent in the M ( T ) data of our measurements.The anomalous thermal hysteresis observed in our data may indicate the existence ofmultiple first order phase transitions in the sample. agnetic and electric properties of CaMn O ..... T indicates themagneto-structural nature of the transition. First order magneto-structural couplingcan show intriguing effects as a function of T or H . There are examples, where suchtransition can give rise to metastabilty, arrested dynamics, glassyness etc [24, 25, 26]. Weinvestigated the thermo-magnetic curves of CaMn O measured in different protocols(see fig. 3). We cooled the sample down to 5 K from 200 K at different applied fields(100 Oe, 1 kOe, and 30 kOe) and measured M while heating in 100 Oe. It is clearlyevident that cooling in higher fields produces larger value of M . Field applied duringcooling is inducing some magnetic domains/clusters which are oriented favourably along H . Such favourably oriented magnetic clusters persist even when the field is loweredand the sample is heated back.The presence of thermo-magnetic irreversibility and the cooling field effectinstigated us to investigate the possible metastable character below T . The inset offig. 3(a) shows the time ( t ) variation of M in the ZFC condition at 5 K. The sample wasfirst cooled in absence of H down to 5 K and subsequently 100 Oe of field was appliedand then M was measured as a function of t . The sample shows large relaxation,where M changes by more than 8% in 3600 s. Such large relaxation is generallyobserved in disordered magnetic systems or in spin glasses. The relaxation data canbe well fitted by the Kohlrausch-Williams-Watt (KWW) stretched exponential model( ∼ exp [ − ( t/τ ) β ]) [27, 28] where τ is the characteristic relaxation time and β is the shapeparameter. Such model was widely used to analyze the data for spin glass and otherdisordered magnets. [29] The value of β was found to be 0.37 for the present CaMn O sample. The exponent β in the KWW model signifies the number of intermediate statesthrough which the system should evolve, and it approaches 1 when the number of suchintermediate states diminishes. [30] Glassy and disordered magnetic systems are foundto show β values over a wide range between 0.2 to 0.6 and the value of β for the presentcomposition falls well within the range.We performed magnetic memory experiment [31, 32] to further elucidate themetastable state. For the memory measurement, the sample was first cooled in presenceof H = 1 kOe with intermediate stops of 3600 s each at 12 K and 8 K [curve M stop in fig. 3 (b)]. At each stop, H was reduced to zero. After reaching 2 K, the samplewas heated back in 1 kOe [curve M mem in fig. 3(b)], and the signature of stops isclearly imprinted in the heating curve at respective temperatures in the form of dips.This prominent signature of memory clearly indicate that the system evolves throughmultiple metastable states as one cools it below T .However, it should be noted that the above memory measurement was performed inthe field-cooled condition and positive signature of memory can be present in assembly ofnoninteracting super-paramagnetic particles due to their individual relaxation under anapplied field [33, 34]. An elegant way to distinguish a superparamagnet from a glassymagnetic system is the measurement of zero-field-cooled memory. In this protocol,the sample was first cooled in zero field with intermediate stops and subsequently thesample was heated back in presence of small applied field and M ( T ) data were recorded. agnetic and electric properties of CaMn O ..... x = 0.08 x = 0.04 M ( e m u / m o l ) T (K) (a) x = 0 x = . x = . x = (b) H (10 kOe) M ( B / f . u . ) Figure 4. (a) Magnetization as a function of temperature in the zero-field-cooledstate for Ca − x La x Mn O ( x = 0.0, 0.04 and 0.08) samples. (b) shows the isothermalmagnetization versus field curves at 5 K for different samples. For the present sample, although the field-cooled memory is very strong, we failed toobserve any signature of zero-field-cooled memory (not shown here). We also measuredac susceptibility of the sample around T at different driving frequencies ( f ). Thesignature of magnetic transition at T is apparent in the real part of ac susceptibility( χ ′ ac ( T )) in the form of a peak. However, we do not see any shift in the peak positionwith f (see inset of fig. 3 (b)). The negative results in zero-field-cooled memory and theabsence of f dependency of χ ′ ac ( T ) rule out the possibility of a low- T glassy magneticstate of CaMn O . O CaMn O is a mixed valent compound and it is well known that Mn valency plays acrucial role in deciding the electronic and magnetic properties of the sample. We triedto vary the Mn valency by doping La at Ca site. Since La remains in the 3+ state asopposed to 2+ state of Ca, it is expected that doping would enhance the Mn fractionat the cost of Mn . Fig. 4(a) shows a comparison of the ZFC M ( T ) data recorded forCa − x La x Mn O sample with x = 0.0, 0.04 and 0.08. The transition at T is presentin the doped samples, however they are slightly shifted to lower T with increasing x .The most important change due to doping is the drastic reduction of the value of M agnetic and electric properties of CaMn O ..... x = 0.0 x = 0.04 x = 0.08 (a) E Cool = +3.1 kV/cm J P ( n A / m ) x = 0.08 x = 0.04 x = 0.0 (b) P ( C / m ) T (K)
Figure 5. (a) and (b) respectively show the temperature dependence of pyroelectriccurrent density ( J P ) and spontaneous electrical polarization ( P ) for Ca − x La x Mn O ( x = 0.0, 0.04 and 0.08) samples. (see table 1). The peak value of M decreases from 3.33 emu/g in pure sample to 0.65emu/g for x = 0.08. Such decrease is also visible in the isothermal M versus H datadepicted in fig. 4(b). The coercivity of the magnetic hysteresis curve gets reduced withincreasing x . It indicates that the FM-like contribution present in the sample below T is getting reduced due to La doping at the Ca site.It is to be noted that all the samples ( x = 0.0, 0.04 and 0.08) show paramagneticbehaviour above about 120 K. The Curie-Weiss fitting ( χ = M/H = C/ ( T − θ ), where χ is the dc magnetic susceptibility, C = Curie constant and θ is the Weiss temperature) tothe data indicates that the effective paramagnetic moment per Mn ( µ eff /Mn) increaseswith La doping (see table 1). This is expected as La doping enhances the Mn fractionat the cost of Mn , and the former spin state has larger value of the isolated magneticmoment (4.9 µ B ) than the later (3.87 µ B ). Using these moment values along with thecalculated µ eff /Mn from Curie-Weiss law, one can estimate the effective valency of Mnin the samples. For x = 0.0, 0.04 and 0.08 samples, the effective valency of Mn wasfound to be 3.22, 3.12 and 3.09 respectively. These are close to the values expected fornominal stoichiometric compositions (3.15, 3.14 and 3.13), and they match well withthe v Mn calculated from XPS data.It was already shown in the literature that CaMn O depicts spontaneous electricalpolarization below T . We have presented here the polarization data as calculated fromthe measured pyroelectric current density ( J P ) of the samples. In order to record J P ,we used capacitor type assembly with a pair of electrodes attached to two flat surfaces agnetic and electric properties of CaMn O .....
50 100 150 200 2500.1110 ’ ( ) T (K) x = 0.0 x = 0.04 x = 0.08 f = 100 kHz x = 0.08 x = 0.04 x = 0.0 Figure 6.
Temperature dependence of the real part of complex dielectric permittivityfor Ca − x La x Mn O ( x = 0.0, 0.04 and 0.08) samples. The inset shows the enlargedview of the low temperature feature. of the sample using silver epoxy. The sample was first cooled down to 10 K in presenceof an electric field, E cool = 3.1 kV/cm. After reaching 10 K, E cool was set to zero and J P was measured while the sample was heated at a constant rate of 4 K/min. One cancalculate P by integrating J P ( t ). Here we assumed that P vanishes for all the samplesas soon as T goes above 120 K [35, 36]. The measured J P and P have been shown infig. 5 (a) and (b) respectively as a function of T . Clearly, a drastic increase in the valueof P is observed in the La doped samples(see table 1). The peak value of J P increasesby a factor of 2.5 in x = 0.08 sample as compared to the undoped one. Consequently, P is found to be higher in the La-doped compositions. For all the samples, P riseswith decreasing T below T and eventually saturates at low T . The magnitude of thissaturated P in our study is found to be higher in the pure CaMn O ( ∼ µ Cm − for E cool = 3.1 kV/cm) compared to the previous report on polycrystalline sample ( ∼ µ Cm − for E cool = 3.5 kV/cm) [11]. But the observed value is certainly much smallerthan the reported single crystal data ( ∼ µ Cm − for E cool = 4.4 kV/cm) [12]. Theimportant observation is that the low T saturated value of P increases drastically in theLa-doped samples and it is about three times higher in x = 0.08 sample.The electrical response of the samples were further probed by ac dielectricmeasurements as shown in fig. 6. The real part of the complex permittivity shows T -independent flat region at low T , followed by a thermally activated rise at high T . Suchactivated behaviour is quite common among ceramics and it is contributed by mobilecharge carriers in presence of grain and grain boundary. If we compare the ǫ ′ with P , this flat portion actually corresponds to the region where spontaneous polarizationis present. Very similar to P , the magnitude of ǫ ′ in the flat region increases withLa doping. Interestingly, the flat region in ǫ ′ at low- T is not exactly flat, rather itshows a shallow minimum, which was earlier reported for the pure sample [17]. It was agnetic and electric properties of CaMn O ..... T with increasing La doping. This is at parwith the polarization data, where the temperature corresponding to the appearance ofspontaneous polarization shifts to higher T with x .
5. Discussion
The multiferroic compound CaMn O is an improper ferroelectric, where spontaneouselectrical polarization occurs due to the helical spin ordering below T = 90 K [11, 12, 18].Our magnetization measurements indicate clear thermal hysteresis around T , whichsignifies a first order phase transition (FOPT). Previous x-ray and neutron diffractionstudies [14] indicated that the system undergoes structural transition around T , whichis associated with sharp change in lattice parameters. The observation of thermalhysteresis is in line with the previously reported structural anomaly. Although P develops just below T , it shows a change in slope around T , which is clearly associatedwith the FOPT occurring at this point.In our magnetic study, we observe interesting magneto-thermal anomaly, whichis reflected in the M ( T ) data measured during heating in 100 Oe after field-coolingin different H through T . The sample depicts strong signature of field stop magneticmemory below T . These observations suggest a metastable ground state of this material.Recent neutron diffraction data revealed that CaMn O has a complex magneticstructure below T with a possibility of multiple magnetic propagation vectors andit is difficult to determine the exact magnetic structure without single crystal neutrondiffraction data. The rise in M below T may be an indication of the formation ofsuperparamagnetic like clusters. On field-cooling, the number of such clusters enhancesand and they get oriented favourably along H . This is the likely origin of enhanced valueof M in the heating data if the sample is cooled in higher H . The superparamagneticnature of the sample is also supported by the observation of clear loop in the M ( H )data, which is unlikely to occur had the sample been a purely helimagnet.Superparamagnetism is generally associated with assembly of noninteractingmagnetic nanoparticles, and the field-cooled memory is observed below the blockingtemperature due to the N´eel-type relaxation superparamagntic moment of individualparticle [34, 37]. The present investigation was carried out on bulk ceramic samplewhere the particle size is too large ( ∼ µ m) to have appreciable N´eel relaxation.However, evidences for superparamagnetic clusters was reported in several bulk magneticsystems (such as manganites, nickelites etc.) [38, 39, 40, 41, 42]. Therefore, it ispossible that nanosized magnetic clusters can form in bulk CaMn O in the backdropof a helimagnetic structure and the relaxation of individual nano-cluster is primarilyresponsible for the observed field-cooled memory. The complete absence of zero-field-cooled memory rules out the possibility of spin-glass or cluster-glass like ground stateindicating the lack of substantial spin-frustration among different magnetic specimens.Considering the fact that metastability occurs below T only, the formation of spin agnetic and electric properties of CaMn O ..... O contains both Mn and Mn , and near a phase boundary onecould have a chance for the formation of isolated local magnetic clusters through doubleexchange mechanism.La doping at the Ca site was performed to change the Mn and Mn ratio of thesample, and it is likely that the La would enhance the percentage of Mn ions in thesample. Our XPS investigation indicates higher Mn fraction in the x = 0.04 sample.XPS is predominantly a surface sensitive measurement, and one should be cautious inpredicting the quantitative value of Mn /Mn ratio in the bulk from the peak area ofthe XPS data. However, our investigation based on the analysis of several independentaspects of XPS data can at least provide a supportive evidence for the enhanced Mn concentration in the La-doped samples. Notably Mn /Mn ratio calculated from theeffective paramagnetic moment indicates the systematic variation of Mn-valency withLa concentration and it is in line with the XPS analysis. It is clear from the M ( T ) datathat La doping at the Ca site causes systematic decrement of M below T , although M remains almost same above T with changing x . Such observation supports theproposed scenario of superparamagnetic like clusters below T . La-doping will reducethe Mn fraction responsible for double exchange and as a result the number of suchsuperparamagnetic clusters will shrink.Contrary to the value of M , P is found to be larger in the La doped samples. We findthat the small La (8%) doping can enhance P by a factor 3. This provides a easy path toenhance the functionality of this material, and consequently important from the pointof view of applications. It is expected that the ME effect in CaMn O is connected tothe low- T spiral magnetic order, which is also present in several other multiferroics [44].However, the nature of the electric polarization is anomalous in CaMn O , where ~P lies perpendicular to the plane of the spiral magnetic structure with exceptionally largemagnitude. In addition, due to the existence of proper screw type spiral magneticstructure in CaMn O [45], a simple spin current model can not alone describe theorigin of multiferroicity [46, 44]. There are couple of different models proposed for large P in CaMn O , namely (i) ferroaxial coupling of the magnetic chirality to the structuralrotation [12], (ii) combination of exchange-striction and DM interaction [18]. At thispoint it is really difficult to comment on the mechanism behind the enhanced P on Lasubstitution at the Ca site. In the exchange-striction model based on DFT calculations,it is found that the induced P depends on the value of sin α , where α is an angle betweenMn spins with the x axis of a predefined coordinate system [18]. The optimum valueof α is found to be close to 30 ◦ , which is obtained by minimizing the energy Hamiltoniancomprising spin exchange interaction and DM interaction. It is likely that due to thereplacement of Mn by Mn , the optimum value of α would change, which may beresponsible for the drastic enhancement in P .It is to be noted that even a proper screw type magnetic structure can induceferroelectricity through the variation in the metal-ligand hybridization in presence of agnetic and electric properties of CaMn O ..... . Such scenario can not be ruled out for the observed multiferroicity inCaMn O . The metal-ligand ( p − d ) hybridization may depend upon the spin stateof Mn and hence it can contribute to the large enhancement of P in electron dopedsamples of CaMn O .In conclusion, we studied the magnetic and electric properties of CaMn O andsome of its La doped derivatives. We find that the low temperature magnetic transitionis first order in nature and the ground state is magnetically metastable. A probablescenario of the formation of superparamagnetic like spin clusters is proposed. La dopingreduces the superparamagnetic contribution at low temperature, however it enhancesthe electric polarization to large extend. Apparently, the enhanced Mn ions in thesample at least within the doping range of present investigation is somehow relatedto the enhanced polarization. This piece of information may be important for furtherunderstanding of the origin of anomalous multiferroicity in CaMn O . Lastly, theenhanced P on La doping may be important for practical applications as far as thefunctionality of the materials is concerned.
6. Acknowledgment
Unit of Nanoscience at IACS is duly acknowledged for magnetic and XPS measurements.We are thankful to the Surface Physics Division, SINP, Kolkata for EDS measurement.We also thank CSIR, India for financial support (grant number: 03(1209)/12/EMR-II).SC wishes to thank CSIR, India for his research fellowship.
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