Abnormal electric transport property and magnetoresistance stability of La-Sr-K-Mn-O system
Cultivating Base of Anhui Key Laboratory of Spintronics and Nano-materials Research
School of Mechanical and Electronic Engineering, Suzhou University
摘 要：
The perovskite samples La1-x (Sr1-yKy )xMnO3 (y = 0.0, 0.2, 04, 0.6, 0.8) were prepared by the solid-state reaction method with comparatively low sintering temperature and with comparatively short sintering time, and the electric transport property and temperature stability of MR of this system were studied. The q-T curves show the abnormal phenomenon that with the increase of K doping amount, resistivity increases, and the insulator-metal transition temperature decreases, which is because the influence of the occupation disorder degree of A-site ions σ2 on the electric transport property of perovskite manganites is larger than that of the radius of A-site ions<rA>. In the temperature range below 225 K, MR increases continuously with the decrease of temperature, which is the characteristic of low-field magnetoresistance; in the comparatively wide temperature range near 250 K, the MR-T curves of all the samples are comparatively flat, and the value of MR almost does not change with temperature, which shows the temperature stability of magnetoresistance, and can be explained by the competition between the low-field magnetoresistance induced by spin-dependent tunneling of surface phase and the intrinsic magnetoresistance of grain phase. The magnetoresistance value of the sample with y = 0.8 keeps at (7.92 ± 0.36) % in the very wide temperature range of 225-275 K, and this is a good reference for the preparation of this kind of sample with practical application value in the future.
作者简介：Zhen-Sheng Peng e-mail:ahpengzhsh1948@126.com;
收稿日期：1 November 2012
基金：financially supported by the National Natural Science Foundation of China (No. 19934003);the Key Program of Natural Science Foundation of Anhui Province (Nos. KJ2011A259 and KJ2013A245);the Program of Professors and Doctors' Research Startup Foundation of Suzhou College (Nos. 2011jb01 and 2011jb02);the Program of Cultivating Base of Anhui Key Laboratory of Spintronics and Nano-materials Research (No. 2012YKF09);
Abnormal electric transport property and magnetoresistance stability of La-Sr-K-Mn-O system
Abstract：
The perovskite samples La1-x (Sr1-yKy )xMnO3 (y = 0.0, 0.2, 04, 0.6, 0.8) were prepared by the solid-state reaction method with comparatively low sintering temperature and with comparatively short sintering time, and the electric transport property and temperature stability of MR of this system were studied. The q–T curves show the abnormal phenomenon that with the increase of K doping amount, resistivity increases, and the insulator–metal transition temperature decreases, which is because the influence of the occupation disorder degree of A-site ions σ2 on the electric transport property of perovskite manganites is larger than that of the radius of A-site ions<rA>. In the temperature range below 225 K, MR increases continuously with the decrease of temperature, which is the characteristic of low-field magnetoresistance; in the comparatively wide temperature range near 250 K, the MR–T curves of all the samples are comparatively flat, and the value of MR almost does not change with temperature, which shows the temperature stability of magnetoresistance, and can be explained by the competition between the low-field magnetoresistance induced by spin-dependent tunneling of surface phase and the intrinsic magnetoresistance of grain phase. The magnetoresistance value of the sample with y = 0.8 keeps at (7.92 ± 0.36) % in the very wide temperature range of 225–275 K, and this is a good reference for the preparation of this kind of sample with practical application value in the future.
Keyword：
Magnetoresistance; K doping; Disorder degree σ2 ; Electric transport property;
Received： 1 November 2012
1 Introduction
The perovskite manganite RE1-xTxMn O3with the ABO3structure(RE is rare-earth element,and T is alkaline-earth element)has become a research focus of condensed matter because of its colossal magnetoresistance(CMR)effect and its potential application value in magnetic devices.The MR–T curve of this kind of matter usually shows a sharp peak only at Curie temperature.On one hand,the temperature range of the appearance of MR peak is too narrow and usually deviates from room temperature,and on the other hand,very large applied magnetic field of several Tesla is needed,so the application of this kind of material is limited
Curie temperature of the perovskite manganite RE1-xTxMnO3is related to the average radius of A-site positive ions hrAi and the ratio of Mn3?and Mn4?.The content of A-site positive ions with different valences determines the ratio of Mn3?and Mn4?,and then affects magnetism and electric transport property.This kind of matter doped by elements with different valences at A site has obtained a lot of researches[1–3].
The appearance temperature range of colossal magnetoresistance(CMR)in perovskite manganites is usually below room temperature,and the appearance of comparatively large magnetoresistance needs the magnetic field of several Tesla,so these conditions limit the practical application of magnetoresistance.For practical application it is necessary to research room-temperature low-field magnetoresistance effect and the temperature stability of magnetoresistance.In recent years,some researches in this area have been carried out at home and abroad[4–6].Wu et al.[7,8]reported the structure,magnetism,and electric transport property of the sample(1/4)Ag2O–La0.833K0.167Mn O3,and found that,its magnetoresistance(MR)value(MR???q0?qH?=q0??100%,in which q0and qHare resistivity in zero field and in applied magnetic field,respectively)in the magnetic field of 5.5 T is 64%at300 K,but deceases to 50%and 52%at 283 and at322 K,respectively.Neeraj Panwar et al.[9]reported the electric and magnetic properties of the composite samples(1-x)Pr0.67Ba0.33Mn O3:x Ag2O(x=0–30 mol%),and the magnetoresistance peak value in the applied magnetic field of 0.6 T increases from 22%of x=0 to 40%of x=30 mol%.Comparatively large magnetoresistance has been obtained with comparatively low magnetic field,but it is the extreme value in the very narrow temperature range near the Curie temperature 200 K of Pr0.67Ba0.33Mn O3,so this still has no practical application value.
La0.8Sr0.2Mn O3with the Curie temperature near room temperature as the original material was chose by Wang et al.[10],and the monovalent element K was doped at A site with the formula of La1-x(Sr1-yKy)xMn O3,in which Mn3?/Mn4?=4/1.The magnetoresistance value of the sample with y=0.8 increases slowly from 8.2%to 9.4%in the temperature range of 251–177 K,but the doping is not preferential in La1-x(Sr1-yKy)xMn O3system,because the Curie temperature of the original material is on the low side and magnetoresistance temperature range is lower than room temperature.
La0.67Sr0.33Mn O3with the Curie temperature above room temperature was chose as the original material(for the purpose that the temperature range of magnetoresistance stability is near room temperature,which will be discussed later),and the monovalent element K was doped at A site,then the matter has the formula of La1-x(Sr1-yKy)xMn O3,in which x=1/3(1?y),thus Mn3?/Mn4?can be kept at 2/1,the doping is preferential and the influence of the variation of Mn3?/Mn4?ratio induced by K doping on magnetic and electric properties is excluded.During the sample preparation by the solid-state reaction method,the sintering temperature and time were reduced appropriately,and the samples were sintered at 1200°C for 10 h(during the perovskite manganite preparation by the solid-state reaction method,the sintering temperature is usually above 1300°C,and the sintering time is usually above 24 h)[11],then the grain size of the polycrystalline samples was reduced[12],grain boundary scattering was strengthened,and comparatively large low-field magnetoresistance can be obtained.The low-field magnetoresistance effect in comparatively low temperature range and the intrinsic magnetoresistance effect inside grains in high temperature range(near Curie temperature)coexist,then the temperature stability of magnetoresistance in the comparatively wide temperature range below Curie temperature are obtained.
2 Experimental
The samples of La1-x(Sr1-yKy)xMn O3(y=0.0,0.2,0.40.6,0.8)with perovskite structure were prepared by the solid-state reaction method,and x=1/3(1?y),which can keep Mn3?/Mn4?at 2/1,and can exclude the influence of the variation of Mn3?/Mn4?ratio induced by K doping on magnetic and electric properties.La2O3of high purity(purity 99.99%,produced by Tianyi Rare Materials Limited Company in Huizhou City)was dewatered,then was matched with the chemical reagents K2CO3(purity 99.5%produced by Pengcai Fine Chemical Industry Limited Company in Langfang City),Sr CO3(purity 99.0%,produced by Kechang Fine Chemicals Company in Shanghai)and Mn O2(purity 99.0%,produced by Bailingwei Science and Technology Limited Company)in nominal composition.The system was sintered at 800°C for 24 h,then ground and was sintered at 900°C for 24 h,and further was ground and sintered at 1100°C for 24 h to get good crystallization.Finally,the system was ground and pressed into thin pellets under the pressure of 28 MPa,sintered at1200°C for 10 h,and then cut into long strips for the resistivity measurement by the four-probe method.
The microstructure of the powder samples was detected by a DX-2600 X-ray diffractometer made in Dandong of China with Ka radiation of Cu(k=0.15406 nm).Resistivity in zero field and in the magnetic field of 0.8 T was measured by the standard four-probe method,the applied magnetic field was perpendicular to the current direction and the measuring current was kept at 100 m A.
3 Results and discussion
3.1 XRD analysis
Figure 1 shows XRD patterns of the samples La1-x(Sr1-yKy)xMn O3(y=0.0,0.2,0.4,0.6,0.8).We can see that no mix peak appears,which indicates that all the samples keep in good single phase and good perovskite structure is formed.Table 1 shows the crystal cell parameters calculated by the jade 5.0 software.We can see that,all the five samples are in hex perovskite structure,and a,b,and c almost do not change with y.
3.2 Electric transport property and its abnormal feature
Figure 2 shows resistivity–temperature(q–T)curves in zero field and in applied magnetic field(B=0.8 T)of the samples La1-x(Sr1-yKy)xMn O3(y=0.0,0.2,0.4,0.6,0.8).The electric transport property has the following characters:
Fig.1 XRD patterns of La–Sr–K–Mn–O samples
Table 1 Relations between crystal cell parameters and y of La–Sr–K–Mn–O samples 下载原图
Table 1 Relations between crystal cell parameters and y of La–Sr–K–Mn–O samples
(1)K doping makes resistivity increase,and the resistivity of the sample with the largest doping amount(y=0.8)is almost one order of magnitude higher than that of the sample without doping(y=0).(2)The samples with high doping amount(y=0.6,0.8)show the phenomenon of resistivity double-peak.The resistivity peak in high temperature range is sharp,and that in low temperature range is a shoulder peak(bump)and is gentle.Other samples only show a gentle shoulder peak in low temperature range because of the limitation of the measuring temperature range of the device(the highest measuring temperature is320 K).But according to the change tendency of resistivity curves,with the increase of doping amount,the resistivity peak in high temperature range is certain to appear above320 K.(3)Both the resistivity peak in high temperature range and the resistivity shoulder peak in low temperature range shift to lower temperature with the increase of K doping amount.(4)The applied magnetic field(B=0.8 T)makes resistivity decrease,and makes the insulator–metal transition temperature shift to higher temperature.
Perovskite manganite materials of single crystal and epitaxial film only show single-peak phenomenon in resistivity curves[13].But resistivity double-peak is often observed in polycrystalline materials,such as La2/3(CaxBa1-x)1/3Mn O3[14],La0.67Sr0.08Na0.25Mn O3[15].The insulator–metal transition single-peak of perovskite manganite materials of single crystal and epitaxial film can be explained by the double-exchange model.The mechanism of the resistivity peak of polycrystalline samples in high temperature range is the same as that of perovskite manganite materials of single crystal and epitaxial film,and also comes from doubleexchange function.The average radiuses of A-site ions hrAi are calculated according to the ion radiuses of La3?,Ca2?K?(0.116,0.126,0.151 nm,respectively)and are shown in Table 2.We can see that,the increase of K doping amount makes the average radius of A-site ions increase gradually According to the influence of lattice distortion on doubleexchange,the average radius of A-site ions is larger,then the tolerance factor is larger,the bond angle of Mn3?–O–Mn4?is larger,the band width of egsingle electron becomes wider,and the double-exchange function strengthens,then the system will show that the resistivity peak temperature shifts to higher temperature and the resistivity decreases with the increase of K doping amount.However,our measured resistivity curves show the abnormal phenomenon that resistivity increases and the resistivity peak shifts to lower temperature with the increase of K doping amount.In our samples La1-x(Sr1-yKy)xMn O3,x=1/3(1?y),which keeps Mn3?/Mn4?ratio invariant,then the influence of the variation of Mn3?/Mn4?ratio on double-exchange is excluded.This abnormal phenomenon of electric transport property appears because the variation of the disorder degree of A-site positive ions r2and the variation of magnetic disorder induced by K doping affect the electric transport property.The disorder degree of A-site positive ions r2is defined as:
Fig.2 q–T curves of La–Sr–K–Mn–O samples:a x=0.0,0.2,and b x=0.4,0.6,0.8
Table 2 Relations between radius of A-site ions hrAi,disorder degree r2and y in La–Sr–K–Mn–O system 下载原图
Table 2 Relations between radius of A-site ions hrAi,disorder degree r2and y in La–Sr–K–Mn–O system
where hrAi is the average radius of A-site positive ions,riis the radius of the i-th positive ion at A site,and xiis the content of the i-th positive ion at A site.The disorder degrees of A-site positive ions are calculated according to the ion radiuses of La3?,Ca2?,K?(0.116,0.126,0.151 nm,respectively)and are shown in Table 2.We can see that,with the increase of K?doping amount,the disorder degree r2increases,and the disorder degree r2of the sample with y=0.8 is one order of magnitude larger than that of the sample with y=0.0.The increase of disorder degree will make the local distortion of the samples increase and make ferromagnetic doubleexchange function weaken,then resistivity increases,and the insulator–metal transition temperature shifts to lower temperature.In addition,the doping of K?will make the magnetic inhomogeneity of the samples increase.After the doping of K?ferromagnetic domains rich in Mn3?will form around Ca2?and at the same time,ferromagnetic domains rich in Mn4?will form around K?,then the two ferromagnetic phases coexist,and magnetic inhomogeneity appears,which affects double-exchange function,makes resistivity increase,and makes the insulator–metal transition temperature shift to lower temperature.Thus,we can reach the conclusion that the influence of the radius of A-site ions hrAi on the electric transport property of perovskite manganites is smaller than that of the disorder degree of A-site ions r2.
The sharp resistivity peak in high temperature range appears in the temperature range of paramagnetism–ferromagnetism transition,and the resistivity shoulder peak(bump)in low temperature range is far away from the temperature range of paramagnetism–ferromagnetism transition.The shoulder peak has nothing to do with the macro-magnetic property of the samples and is caused by the spin-dependent tunneling across surface phase.
3.3 Temperature stability of magnetoresistance effect
Figure 3 shows magnetoresistance–temperature(MR–T)curves of the samples,and MR is defined as:
Fig.3 MR–T curves of La–Sr–K–Mn–O samples
where q(0,T)is the resistivity in zero field,and q(H,T)is the resistivity in magnetic field.
(1)MR curves of the samples with y=0.6,0.8 show a comparatively sharp peak,the MR peak values are 18%22%,respectively,the temperatures corresponding to the peaks basically accord with those of resistivity–temperature curves,and this is the fundamental character of ferromagnetic double-exchange function in perovskite manganites.MR peak does not appear in other samples with low doping amount because of the limitation of the measuring temperature range of the device.(2)In the temperature range below 225 K,MR increases continuously with the decrease of temperature,which is the character of low-field magnetoresistance.(3)MR–T curves of all the samples are comparatively flat in a comparatively wide temperature range near 250 K,and the value of MR almost does not change with temperature,which shows the temperature stability of magnetoresistance.The MR value of the sample with y=0.8 keeps at(7.92±0.36)%in the wide temperature range of 275–225 K(shown in the inset of Fig.3),and this is in favor of the practical application of magnetoresistance.
For polycrystalline samples,a grain system can be pided into the two parts of body phase and surface phase.The magnetoresistance peak near Curie temperature Tc in high temperature range is caused by the double-exchange function of egitinerant electron along Mn3?–O2-–Mn4?inside grains,and this belongs to intrinsic magnetoresistance Besides electrons move inside grains,itinerant electrons must run across the grain surface when they want to enter into the neighboring grain from one grain.The magnetic disorder between grain boundaries is caused by ion vacancy at the grain surface and the disorder of the spin orientations of Mn3?and Mn4?,then a scattering layer forms between neighboring magnetic grains.In zero field,because of magnetic disorder,the scattering upon electrons is large,electrons must overcome comparatively large barrier,and their probability of tunneling scattering layer is small,then the resistivity is comparatively large;in applied magnetic field,the spins of the magnetic ions in the scattering layer tend to be in parallel arrangement,the scattering upon electrons is small,electrons only need to overcome small barrier,and their probability of tunneling scattering layer is large,then the resistivity decreases,so comparatively large magnetoresistance appears in low temperature range,and it is called as low-field magnetoresistance effect.We can see that,the value of low-field magnetoresistance is connected with the size of grains,and the grains will become smaller when the sintering temperature and time of the samples are properly decreased,then the low-field magnetoresistance effect will be strengthened(the samples in our experiment were sintered at 1200°C for 10 h).
The appearance of unchanged magnetoresistance with temperature in the intermediate temperature range(the temperature stability of magnetoresistance)is the result of the competition between low-field magnetoresistance effect and intrinsic magnetoresistance effect.In high temperature range,the intrinsic magnetoresistance effect inside grains is in dominant position;in low temperature range,the lowfield magnetoresistance effect of grain boundary is in dominant position;in the intermediate temperature range,basically unchanged magnetoresistance with temperature forms because of the competition between them[16].
According to the experiments and the discussion on results,we find that the methods of the realization of temperature stability in comparatively high temperature range are as following:Curie temperature(Tc)of the perovskite original material should be high(higher than room temperature),then intrinsic magnetoresistance will appear in comparatively high temperature range;ions with different valences are doped at A site,then the disorder degree of A-site ions will increase,and the sintering temperature and time are decreased,then the size of grains will decrease,and magnetic disorder and grain boundary will increase,and these methods can lead to comparatively high low-field magnetoresistance.The competition between comparatively high low-field magnetoresistance and intrinsic magnetoresistance will lead to the stability of magnetoresistance in the intermediate temperature range,and this is a good reference for the preparation of polycrystalline perovskite samples with temperature stability of magnetoresistance and with practical application value in the future.
4 Conclusion
The original material La0.67Sr0.33MnO3was doped by monovalent K element at its A site,Mn3?/Mn4?was kept at 2/1 in the samples,and the influences on electric transport property and on magnetoresistance effect were studied in this article.The experimental results indicate that:La–Sr–K–Mn–O samples were prepared by the solid-state reaction method with comparatively low sintering temperature and with comparatively short sintering time,which makes the size of grains decrease,makes grain boundary scattering strengthen,and promotes low-field magnetoresistance effect;the abnormal phenomenon of resistivity appears in the samples with the increase of doping amount because the influence of the disorder degree of A-site ions r2and magnetic inhomogeneity on electric transport property of perovskite manganites is larger than that of the radius of A-site ions hrAi;low-field magnetoresistance effect induced by the scattering layer of grain surface and intrinsic magnetoresistance effect inside grains coexist in the system,and the competition between them two leads to basically unchanged magnetoreistance with temperature in the intermediate temperature range(temperature stability of magnetoresistance);when the applied magnetic field is 0.8 T,the magnetoresistance value of the sample with y=0.8 keeps at(7.92±0.36)%in the very wide temperature range of 225–275 K,so its temperature stability of magnetoresistance is very ideal,and this is a good reference for the preparation of polycrystalline perovskite samples with temperature stability of magnetoresistance and with practical application value in the future.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China(No.19934003),the Key Program of Natural Science Foundation of Anhui Province(Nos.KJ2011A259 and KJ2013A245),the Program of Professors and Doctors’Research Startup Foundation of Suzhou College(Nos.2011jb01 and 2011jb02),and the Program of Cultivating Base of Anhui Key Laboratory of Spintronics and Nano-materials Research(No.2012YKF09).
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