We report the existence of a thermally induced sharp increase of thermoremanence around the Curie temperature of Invar-like Fe-Zr(B,Cu) soft magnetic glasses. Neutron-diffraction measurements indicate that a true enhancement of the average local magnetic moment, rather than only a change in the domain structure, occurs. Such enhancement has been tentatively attributed to the increasing volume expansion that takes place beyond the Curie temperature and reinforces ferromagnetism in some low-density clusters

Arcas, J.

Barandiarán, J. M.

Castaño, F. J.

Crespo del Arco, Patricia

Fernández Barquín, L.

Hernando Grande, Antonio

Multigner, M.

©2000 The American Physical SocietyWe report the existence of a thermally induced sharp increase of thermoremanence around the Curie temperature of Invar-like Fe-Zr(B,Cu) soft magnetic glasses. Neutron-diffraction measurements indicate that a true enhancement of the average local magnetic moment, rather than only a change in the domain structure, occurs. Such enhancement has been tentatively attributed to the increasing volume expansion that takes place beyond the Curie temperature and reinforces ferromagnetism in some low-density clusters.Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu

Docta Complutense

PHYSICAL REVIEW B 1 FEBRUARY 2000-IVOLUME 61, NUMBER 5Thermoremanence anomaly in Fe-Zr„B,Cu… Invar metallic glasses: Volume expansioninduced ferromagnetismA. Hernando, P. Crespo, F. J. Castan˜o, J. Arcas, and M. MultignerInstituto de Magnetismo Aplicado, UCM-RENFE, P.O. Box 155, 28230 Las Rozas, Madrid, SpainJ. M. Barandiara´nDepartamento de Electricidad y Electro´nica, UPV/EHU, P.O. Box 644, 48080 Bilbao, SpainL. Fernández-Barquı´nDepartamento de Materia Condensada, Universidad de Cantabria, 39005 Santander, Spain~Received 1 April 1999!We report the existence of a thermally induced sharp increase of thermoremanence around the Curie tem-perature of Invar-like Fe-Zr~B,Cu! soft magnetic glasses. Neutron-diffraction measurements indicate that a trueenhancement of the average local magnetic moment, rather than only a change in the domain structure, occurs.Such enhancement has been tentatively attributed to the increasing volume expansion that takes place beyondthe Curie temperature and reinforces ferromagnetism in some low-density clusters.n-rl-erinteepeaaagewousoacl-siupiseaxr inarieghhwas.es-Feo-ngeSetereldre-thetheesingero-.1cro-ea-dd0d byThe origin of the complex magnetic properties of irorich metallic glasses containing low concentrations of eatransition metals~Zr,Sc,Hf,Y! have attracted considerable interest during more than two decades, giving rise to sevcontroversial microscopic interpretations.1–4 In particularFexZr1002x amorphous alloys withx above 80 at.% rankamong the most extensively studied spin system,5–10 exhib-iting weak itinerant ferromagnetism, low-temperature spglass-like behavior and Invar character.Nowadays it is clear that Invar-like iron-rich FexZr1002x ,as well as Fe-Zr~B,Cu!,12 soft magnetic glasses exhibit:~i! asharp increase of the thermal expansion above the Curieperature,TC ,2,3 ~ii ! a large high-field susceptibility,5 ~iii ! adecrease in both the magnetic moment andTC with increas-ing x,9 ~iv! a large positive spontaneous volummagnetostriction,11 ~v! a positive shift inTC with the tensilestress,12 and a large but negative shift with pressure.2,3 More-over, it is well established that, as a consequence of comtive positive and negative interactions, the spontaneous mnetization is low and the effect of the field is to induce locmagnetic moment, as pointed out by the high volume mnetostriction at low fields.11 In addition, due to the sensitivityof the local magnetic moment to the interatomic distancthe spontaneous magnetization is expected to fluctuatedensity fluctuations which are always present in amorphstructures. The sign of the magnetic interactions dependthe interatomic distances giving rise to the coexistencecomplex magnetic phases and in this way both cluster13 andfrustration8 microscopic models have been proposed tocount for the overall behavior. According to the positive voume magnetostriction experimentally observed,11 it is ac-cepted that those regions or clusters with lower local denshould exhibit the higher local magnetic moment and thstronger ferromagnetic character. On the other hand, sglass-like regions with higher density will surround theferromagnetic clusters so decreasing the average local mnetic moment.In this work we present the highlights of a detailed ePRB 610163-1829/2000/61~5!/3219~4!/$15.00yal-m-ti-g-l-s,ithsonf-tysn-g--perimental study regarding the thermoremanence behavioamorphous Fe-Zr~B,Cu! which confirms the appearance ofreversible magnetic transition that occurs around their Cupoints. The nature of the transition is elucidated throuneutron-diffraction measurements.Amorphous Fe88Zr7B4Cu, Fe87.2Zr7.4B4.3Cu1.1, Fe90Zr7B3,and Fe90Zr10 were produced in the form of ribbons, witapproximate thicknesses of 20mm, using the melt spinningtechnique. The amorphous structure of all the sampleschecked by x-ray diffraction and Mo¨ssbauer spectroscopyThe room-temperature Mo¨ssbauer spectra of the samplanalyzed confirmed the absence of ferromagnetic bccgrains, at least in volume fraction above 1%. The thermmagnetic behavior was investigated in a temperature rafrom 77 to 550 K, using both a Quantum design MPMsuper conducting quantum interference device magnetomand a vibrating sample magnetometer. The magnetic-fivalues could be set to an accuracy of61026 T and the tem-perature to within61022 K.Figure 1~a! shows the thermoremanence behavior, corsponding to the sample Fe88Zr7B4Cu, after cooling in zero-field conditions from room temperature and measuringmagnetic moment on increasing the temperature. Onother hand, Fig. 1~b! shows several thermomagnetic curvobtained when cooling in a field of 100 mT and measurthe magnetic moment on increasing the temperature in zfield conditions, and with low applied fields of 0.05 and 0mT, respectively.From the measurements presented in Figs. 1~a! and 1~b!,it is clear that a sharp and reversible increase of the mascopic magnetic moment occurs in amorphous Fe88Zr7B4Cu,around its Curie point~see below!.In order to check whether this transition is a general fture in Fe-Zr~B,Cu! amorphous alloys, we have investigatethe thermomagnetic behavior in Fe87.2Zr7.4B4.3Cu1.1,Fe90Zr7B3, and Fe90Zr10 amorphous alloys. The field-coole~FC! curves~obtained with an external applied field of 10mT! and the thermoremanence measurements, performe3219 ©2000 The American Physical Societyeain-ero-eacilhubf tessliotrainxis.asal-d 2,beerhtouldatedofap-cro-usednstheetictoe.3220 PRB 61BRIEF REPORTScooling the sample down in zero-field conditions and msuring the magnetic moment with no applied field whilecreasing the temperature, are plotted in Fig. 2.The Curie temperatures of the alloys investigated westimated from the minima of thedM/dT vs T plots,14 com-puted from the low-field FC measurements. The resultstained for the alloys: Fe88Zr7B4Cu (TC5298 K),Fe87.2Zr7.4B4.3Cu1.1 (TC5314 K), Fe90Zr7B3 (TC5248 K),and Fe90Zr10 (TC5219 K), show good agreement with previously reported data.9,12 From the present results it can bconcluded that a thermally induced reversible magnetic trsition occurs for all the samples investigated.The trends of the temperature dependence of the coerfield corresponding to sample Fe88Zr7B4Cu3 are depicted inFig. 3. A clear increase above the Curie temperature, simto that observed in the rest of the samples, is exhibited.An increase of the both remanence and coercive field,been previously reported in nanocrystalline Fe91Zr7B2,15with crystalline fraction around 50%. Nonetheless osamples are amorphous within the accuracy providedx-ray and neutron~see below! diffraction as well as Mo¨ss-bauer spectroscopy. To our knowledge such increase ocoercive field aboveTC has not been published in samplwith highly diluted impurity phases.In addition, in amorphous Fe-Zr~B,Cu! alloys a largefield-induced volume expansion has been previoureported,2,3,11which led us to measure the thermal-expansFIG. 1. ~a! Thermoremanence behavior of amorphoFe88Zr7B4Cu, and ~b! thermomagnetic measurements obtainwhen cooling the sample down in an applied field of 100 mT ameasuring with different applied fields. Solid lines are shown aguide to the eye.-eb-n-ivearasryheyncoefficient as a function of the temperature~see Fig. 4!.These measurements were obtained by means of an sgauge cemented to the ribbon and placed parallel to its aIt is manifest, from the data shown in Fig. 4, thatexpected,2,3 there is a sharp increase of the thermexpansion coefficient in amorphous Fe88Zr7B4Cu for tem-peratures aboveTC .The increase of thermoremanence, shown in Figs. 1 anas well as the increase of the coercive field, Fig. 3, mightrelated with the existence of an impurity phase with highCurie temperature~for instance an amount below 1% weigof a-Fe! which would not be detected by Mo¨ssbauer mea-surements. In this case the softer amorphous phase shproduce a magnetic shielding of the remanence associwith the harder impurity phase. At the Curie temperaturethe amorphous matrix the magnetic shielding would dispear and the remanence of the grains should merge madaFIG. 2. FC measurements with an applied field of 100 mT~leftaxis! and thermoremanence curves obtained when coolingsample down in zero-field conditions and measuring the magnmoment on increasing the temperature~right axis!.FIG. 3. Thermal dependence of coercive field correspondingamorphous Fe88Zr7B4Cu. Solid line is shown as a guide to the eynctieaaionethorvnmioegityris,acerengontiigheeAotht ofltsted,ori-mo-er-ionro-ibit-heg-en-ticthistheag-in-pen-tede-eaitytheketsultofursin-Asck-theen-ces-r-conthePRB 61 3221BRIEF REPORTSscopically. Therefore, the increment of thermoremaneshould be due to an increase of the technical magnetizarather than to an increment of the local ferromagnetic ordIn order to elucidate this possibility it is necessary to useexperimental technique sensitive to the average local mnetic moment. Thus, we have carried out neutron-diffractexperiments at the DIB diffractometer of the ILL iGrenoble ~France!. If the origin of thermoremanencanomaly were only the existence of an impurity phasemagnetic contribution belowTC would consist in two com-ponents, one due to the amorphous phase and anothernated from the impurity phase, which should remain aboTC . Hence, the magnetic contribution to the neutrodiffraction intensity would either decrease at the Curie teperature of the matrix or remain constant, if the contributof the amorphous matrix to the diffracted intensity were nligible below TC .The values of the integrated neutron diffracted intenscorresponding to the broadS(Q) amorphous halo of theFe90Zr7B3, alloy measured at different temperatures, surpingly increase around the Curie temperature of the sampledepicted in Fig. 5.To be sure that this anomalous behavior is not an artifdata corresponding to another amorphous ferromagneticloy (Co75Si15B10), which does not exhibit the same volumsensitivity for the spontaneous magnetization, is also psented for comparison. In this case the total diffracted intsity decreases atTC indicating the disappearance of the manetic contribution. Although some structural~nuclear!component cannot be discarded in the Fe-Zr-B neutrdiffraction pattern, the significant increase of the magnesignal in the vicinity ofTC is clearly anomalous.The increase of the diffraction intensity presented in F5 indicates that the corresponding sharp increase of tmoremanence atTC , shown in Figs. 1 and 2, is due to a truenhancement of the average local magnetic moment.though the presence of some impurities cannot be ruledsuch impurities could only account for an increase ofFIG. 4. Temperature dependence of the thermal-expansionefficient in amorphous Fe88Zr7B4Cu.eonr.ng-neigi-e--n-,-ast,al-e----c.r-l-ut,etechnical magnetization and never for the enhancementhe intrinsic magnetic moment inferred from the resushown in Fig. 5.Once the nature of the transition has been elucidasome microscopic considerations regarding the possiblegin of the enhancement of the average local magneticment are outlined. The overall behavior can be well undstood by considering the increase of the volume expanscoefficient that occurs at, and above,TC . As summarized inthe introduction, low-density clusters with enhanced fermagnetic order are present and coexist with regions exhing spin-glass-like magnetic behavior. BelowTC the mag-netic signal of the neutron-diffraction pattern reflects tcontribution of both spin-glass-like regions and ferromanetic clusters. Therefore, the increase of the diffracted intsity at TC points out an increase of the average magnemoment of the clusters. Thus, the results presented inwork suggest that at the Curie temperature of the alloy,ferromagnetic clusters expand, so reinforcing their ferromnetic character. This effect would account for the sharpcrease of thermoremanence as well as for the thermal dedence of the magnetic contribution given by the integraneutron-diffraction intensity. Moreover, the steep enlargment of the coercive field shown in Fig. 3 reinforces the idof density fluctuations provided that only the lower denspockets are ferromagnetic. In addition, the high values ofcoercive field indicate that the average size of these pocshould be below the single-domain size. It appears difficto ascribe the increase of the coercivity to the diminutionexchange coupling between particles, provided that it occfar beyondTC .In our case, it appears more reasonable to invoke ancrease of the volume of the ferromagnetic particles aboveTCto account for the thermal dependence of the coercivity.the temperature rises, the volume of these low-density poets expands in such a way that the anisotropy energy ofsingle-domain particle increases faster than the thermalergy.A volume expansion in itinerant ferromagnets enhanhe ferromagnetic tendency.16 According to the Stoner criterion @ IN(EF).1#, the tendency to ferromagnetism in itineo-FIG. 5. Integrated intensity, obtained from neutron-diffractimeasurements, plotted versus the reduced temperature,T/TC , cor-responding to Fe90Zr7B3 ~solid circles! and Co75Si15B10 ~opencircles! amorphous alloys. Solid lines are shown as a guide toeye.thtior,inmOdeara-rre-andityduentalow-ousthein3222 PRB 61BRIEF REPORTSant systems is proportional to the product of a measure ofexchange interactionsI and the density of statesN(EF). Fur-thermore, in metallic systems where the main contributionthe exchange is the intratomic one, the exchange interactdo not depend on the interatomic distances. Howevestructural expansion will decrease the bandwidth, increasthe density of states~for details see Ref. 16!. Below TC thethermal volume expansion is counterbalanced by the volureduction due to the decrease of the magnetic moment.the other hand, aboveTC this later contribution vanishes anthe volume expansion promotes the increase of the magnordering reported here.In summary, we have shown that in the absence ofexternal magnetic field, Fe-Zr~B,Cu! itinerant ferromagnets.O.eonsagenticnxhibit a sharp reversible magnetic transition for tempetures around their Curie temperatures. This transition cosponds to an increase of the average magnetic momentcannot be attributed to the contribution of any impurphase. The effect has been tentatively explained as beingto a volume expansion that occurs in these alloys aboveTC ,in some low-density patches. Furthermore, the experimeresults reported here seem to confirm the existence of ldensity stronger ferromagnetic clusters in the amorphstructure.We thank Dr. T. Fernandez for her assistance witheutron-diffraction measurements performed at the ILLGrenoble~France!.R.J.ez-1N. Heiman and N. Kazama, Phys. Rev. B19, 1623~1979!.2K. H. J. Buschow,Magnetic Amorphous Alloys, Handbook ofMagnetic Materials, Vol. 6~Amsterdam, Elsevier, 1991!, p. 320.3K. Fukamichi, Amorphous Metallic Alloys~Butterworths, Lon-don, 1983!, p. 317.4H. Hiroyoshi and K. Fukamichi, J. Appl. Phys.53, 2226~1982!.5Z. Altounian and J. O. Stro¨m-Olsen, Phys. Rev. B27, 4149~1983!.6I. Vincze, D. Kapta´s, T. Kemény, L. F. Kiss, and J. Balogh, PhysRev. 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Physical Review B

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Thermoremanence anomaly in Fe-Zr(B,Cu) Invar metallic glasses: Volume expansion induced ferromagnetism

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Thermoremanence anomaly in Fe-Zr(B,Cu) Invar metallic glasses: Volume expansion induced ferromagnetism

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