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the advances in the comprehension of the physical properties of manganese perovskites have been given in two main boosts .
the first one started in 1950 , when these materials were first considered by jonker and van santen .
@xcite in that period , that spanned over more than a decade , the basic mechanisms governing their magnetic and transport properties were defined , giving rise to the description of the double exchange interaction.@xcite the second one took place last decade , after the discovery of colossal magnetoresistance ( cmr)@xcite ( although a high mr had already been observed experimentally before , first by searle and wang@xcite and later by kusters _
et al._@xcite ) . during this period , in which we are still immersed
, the research activity reached a magnitude that made the topic a starring one in the community of condensed matter physics , and the current opinion about the state - of - the - art underwent several changes . in the first few years after the description of cmr
, explanations were tried on the basis of double exchange,@xcite but it was demostrated soon that such effect alone could not explain the magnitude of cmr.@xcite later , certain evidences in favor of the existence of an inhomogeneous electronic state in these materials started to appear.@xcite this tendence continues nowadays after the conclusions of many groups that found -and are still finding- that the homogeneous description of the magnetic and electronic state of manganites is almost ruled out .    in order to establish the basis for such behavior the idea of phase separation , i.e.
, the coexistence of regions of localized and itinerant carriers within the same crystallographic structure , has been proposed.@xcite in this framework , dagotto , hotta and moreo@xcite proposed in a recent review the existence of a temperature window where coexisting metallic and insulating clusters appear above the curie temperature , @xmath3 , when this transition is first - order and there is a source of disorder .
this temperature window would extend up to @xmath4 , where these clusters dissociate to form two - mn zener polarons ( or small polarons ) .
the aim of the present work is to find experimental evidence of such temperature @xmath6 . for this purpose , ultrasonic attenuation and phase velocity measurements
were carried out in la@xmath0ca@xmath1mno@xmath2 and la@xmath0sr@xmath1mno@xmath2 .
this technique is proved to be a very sensitive tool not only for studying defects and microscopic processes in solids , but also for probing systems undergoing magnetic and structural phase transitions.@xcite
samples were prepared by solid state reaction of la@xmath7o@xmath2 , cao or srco@xmath2 , mno@xmath7 and mno ( at least 99.995 @xmath8 in purity ) , which were heated in air in two steps ( 1100@xmath9c 70 hours , 1200@xmath9c 27 hours ) and pressed into disks .
the temperature was slowly ramped at 5@xmath9c / min , and cooled down to room temperature at 2@xmath9c / min .
intermediate grindings were made .
pellets were finally pressed at 7 ton/@xmath10 and annealed at 1300@xmath9c for 100 hours , with an intermediate grinding at 30 hours .
the nominal oxygen content was almost stoichiometric as determined by iodometric analysis .
x - ray powder patterns were collected at room temperature using a philips pw1710 diffractometer , working with @xmath11 radiation .
the lattice parameters , derived by rietveld analysis , are in agreement with those reported in the literature .
cylindrical pellets , with a diameter of 10.0 mm and heights 2.4 mm ( lacamno ) , and 3.1 mm ( lasrmno ) were used to perform ultrasonic measurements .
the two opposite faces of each sample were polished so that the difference between two points in each surface was not more than 1 @xmath12 m .
the ultrasonic velocity and attenuation measurements were performed on a matec-6600 series by means of the conventional pulse - echo technique .
y - cut quartz transducers of 5 mhz fundamental frequency were used for transverse ultrasonic excitation .
they were coupled to the sample surface with a grease for high temperature uses ( leak point 550 k ) .
the sample was coupled at high temperature , and then cooled until 200 k and maintained at this temperature for 15 minutes before measurementes were carried out .
the ultrasonic elapsed time for a pulse round trip ( transit time ) was obtained with the pulse - echo - overlap technique at the initial temperature .
then , time variation with temperature was automatically monitored with a sensitivity of 0.05 ns at 5 mhz .
the experiment was taken in a closed - cycle refrigerator ( janis ) and the temperature of the sample was varied at a rate of about 0.5 k / min .
in fig . [ un](a ) we show ultrasonic velocity vs. temperature data for la@xmath0ca@xmath1mno@xmath2 and la@xmath0sr@xmath1mno@xmath2 , and attenuation vs. temperature data for la@xmath0ca@xmath1mno@xmath2 , obtained with 5 mhz transversal waves between 220 and 400 k. for lasrmno it was impossible to measure the attenuation because its high value allowed to obtain only two echoes at room temperature ( at higher temperatures the second echo became negligible ) .
the transit time was measurable at room temperature , and then time variations were measured with our automated improved matec system .
the influence of thermal expansion has not been taken into account , because it is negligible : if ( @xmath13@xmath14)/l=@xmath15 we have that for our sample , with @xmath14= 2.4 mm thick ( 4.8 mm microwave path ) and a velocity of about 2500 m / s , @xmath16 ( @xmath17=time ) is @xmath18 s and therefore , at t@xmath19 , the associated variation of velocity is far smaller than the one we measured .    in our original ultrasonic velocity data , an approximately linear increment with temperature
was observed at temperatures above 350 k for both samples . at this temperature range
no mechanism related with magnetic phase transitions is expected , and this variation should be attributed to another mechanism , as the produced by crystalline defects present in the material.@xcite the resonant mechanism associated to dislocation lines is well known from crystalline metals . as moreno - gobbi and eiras@xcite , working on crystalline copper , have shown , it gives a term proportional to t for velocity and proportional to t@xmath20 for attenuation .
the coefficient of proportionality depends on @xmath21 for velocity and @xmath22 for attenuation , where @xmath23 is the average free length of the segment of dislocation lines . in the present paper
we assume a background of this kind in order to take into account the variation of velocity at high temperatures .
the velocity background was fitted to the high temperature region of velocity vs. temperature curves , and then substracted from original data .
these fits gave a term @xmath24 t for lacamno and @xmath25 t for lasrmno .
the so - corrected velocity data are presented in fig .
[ un](b ) .
the differences between the numerical values of the coefficients of proportionality have their origins in the different values of @xmath23 expected for different materials.@xcite    the ultrasonic velocity for la@xmath0ca@xmath1mno@xmath2 presents two clear anomalies : an abrupt hardening at about 250 - 260 k , and another lesser at about 350 - 360 k. the first one marks the t@xmath19 of this system@xcite ( t@xmath19= 260 k ) .
the behavior is similar to that found at mhz frequencies by other authors,@xcite and at khz frequencies by cordero _
al_.@xcite the abrupt decay of the curve at t@xmath19 signals the first - order character of this transition,@xcite an interpretation that is in agreement with results obtained from other techniques.@xcite the decay in velocity is accompanied by a peak in the attenuation curve at t@xmath19 .
this peak is asymmetric , in agreement with the results of cordero _
et al._,@xcite who explain it invoking the presence of inhomogeneous phases below t@xmath19 in a similar way to relaxor ferroelectrics.@xcite    the second anomaly takes place above t@xmath19 , showing a hardening process at @xmath6 @xmath26 350 - 360 k ( fig . [ dous ] ) . in fig .
[ un ] it is also observed a very large attenuation peak at 320 k accompanying this velocity anomaly .
this was not observed by cordero _
et al . _ at khz frequencies , but is clearly observed in our mhz measurements .
the reason lies on the higher sensitivity of mhz techniques to small inhomogeneities , as magnetic clusters , due to the shortest wavelength ( approximately 0.5 mm at 5 mhz for transversal waves ) .    in figs .
[ un ] and [ dous ] it is also observed that the only noticeable anomaly in the utrasonic velocity curve of la@xmath0sr@xmath1mno@xmath2 is a hardening process beginning at about 370 k ( in the order of that of lacamno in the same temperature region ) , that is coincident with the second - order phase transition point of this compound , clearly observed by magnetization measurements ( t@xmath27 370 k).@xcite in the explored temperatures , no more transitions were detected .
based on this behavior we can perfectly associate the process of hardening observed at 370 k to this second - order phase transition .
one of the open issues in the comprehension of the physics of ferromagnetic manganese perovskites at present is to determine the nature of the magnetic phase above @xmath3 . from our ultrasonic velocity data
it seems that this phase is different in the two systems considered .
first of all , whereas lacamno displays a first - order transition at @xmath3 , lasrmno presents a second - order one .
this difference , that has already been highlighted by magnetic measurements,@xcite and attributed by mira _
et al._@xcite to a change in the crystal symmetry , from orthorhombic to rhombohedral , seems to have further implications .
magnetoresistance , thermal expansion , magnetovolume and magnetocaloric effects experience a considerable change when moving from one compound to the other.@xcite also , calorimetric data showed evidence for an anomaly at a similar @xmath6 in those manganites la@xmath28(ca , sr)@xmath29mno@xmath2 with a first - order transformation at @xmath3,@xcite in agreement with the ultrasonic data presented here that show the existence of a transition point at @xmath30 in lacamno .
it is worth mentioning that our @xmath6 is similar to the point at which both the volume thermal expansion deviates from a gruneisen fit and the inverse magnetic susceptibility deviates from a curie - weiss fit , after results of de teresa _
_ in la@xmath0ca@xmath1mno@xmath2 ( ref .
@xcite ) . supporting our results , we have to mention that neutron diffraction studies@xcite in the paramagnetic region of la@xmath0sr@xmath1mno@xmath2 show differences with respect to la@xmath0ca@xmath1mno@xmath2 .
we want also to call the attention on the similar values of the temperatures of the second - order transitions , @xmath6 @xmath26 350 - 360 k for la@xmath0ca@xmath1mno@xmath2 , and @xmath3 @xmath26 370 k for la@xmath0sr@xmath1mno@xmath2 . in some sense
, it leads to think that the magnetic interaction between the mn atoms is of the same strength in both compounds and only the occurrence of the first - order change in lacamno is breaking the long - range order at a temperature below @xmath6 .
after rder , zhang and bishop,@xcite this might be due to lattice effects , that would decrease the t@xmath19 associated to the de coupling .
the anomaly at @xmath6 shows that the magnetic phase transition is not from ferromagnetism to a purely paramagnetic state , but some sort of magnetic structure is present .
such idea is in accordance with the theoretical results of moreo _
et al._,@xcite who have proposed , after computer simulations , that large coexisting metallic and insulating clusters of equal electronic density are generated in manganese oxides with first - order magnetic transitions .
et al._@xcite have qualified these compounds as relaxor ferromagnets , with a relaxation from one phase to the other .
the fluctuations between both phases have been seen by transmission electron microscopy by podzorov _
et al._.@xcite in such a case , after kimura _
et al._@xcite the transition would be diffuse , like the one observed by us at @xmath6 .
nowadays there is being growing experimental work that leads to assume the electronic phase - separated nature of such materials , and now it is well established that this phase separation is not due to chemical inhomogeneities or to the existence of phases with a different chemical composition.@xcite although this approach is quite recent in manganese perovskites , it had already been proposed by sears rodrguez and goodenough@xcite to explain the magnetic and electrical properties of sr - doped cobalt perovskites , thinking of a percolative transition at t@xmath19 .    in summary
, we consider that our data confirm the existence , in la@xmath0ca@xmath1mno@xmath2 , of the temperature window @xmath31 proposed by dagotto , moreo and hotta@xcite , where probably the coexistence of a phase separated regime of metallic and insulating regions could be taking place .
such window is not observed in la@xmath0sr@xmath1mno@xmath2 , a material with a conventional second - order ferromagnetic to paramagnetic phase transition at its curie temperature .
the dgi of the ministry of science of technology of spain is acknowledged for financial support under project feder mat2001 - 3749 .
fr also wants to acknowledge the fulbright foundation and mcyt of spain for financial support .                                                                                      c. a. ramos , h. r. salva , r. d. snchez , m. tovar , f. rivadulla , j. mira , j. rivas , m. a. lpez - quintela , l. e. hueso , m. saint - paul , p. lejay , and y. tokura , j. magn .
magn . mater .
* 226 - 230 * , 582 ( 2001 ) .
ultrasonic attenuation and phase velocity measurements have been carried out in the ferromagnetic perovskites la@xmath0ca@xmath1mno@xmath2 and la@xmath0sr@xmath1mno@xmath2 . 
 data show that the transition at the curie temperature , @xmath3 , changes from first- to second - order as sr replaces ca in the perovskite . 
 the compound with first - order transition shows also another transition at a temperature @xmath4 . 
 we interpret the temperature window @xmath5 as a region of coexistence of a phase separated regime of metallic and insulating regions , in the line of recent theoretical proposals .