Reentrant magnetic behaviour in (NiFe)25Au75 alloy

Solid State Communications, Printed in Great Britain.

0038-1098/93$6.00+.00 Pergarnon Press Ltd

Vol. 85, No. 11, pp. 911-915, 1993.

REENTRANT

MAGNETIC

BEHAVIOUR

IN (NiFe)zsAuTS ALL01

R. Ranganathan+. L.S. Vaidyanatban*, A. Chalcravarti+, G. Rangarajan. + Low Temperature Physics Section, Saha lnstitute of Nuclear Physics, l/AF Bidhannagar, Calcutta 700 064 * Low Temperature Laboratory, Department of Physics, Indian Institute of Technology, Madras 600 036 (Received 17 November 1992 by C.N.R. Rao) We report the experimental results of field cooled and zero field cooled magnetisation, hysteresis data for fee (NiFe)zsAu75 alloy in the temperature range 2 K - 350 K in an external magnetic field S mT to 5 T using SQUID magnetometer. The results suggest the reentrant spin glass like transition in low fields - 100 mT on the basis of rapid fall in the magnetisation with onset of strong irreversibility (15 K at 5 mT) and shows evidence of field - induced ferromagnetism at high fields based on Arrot’s plot. We also observed some interesting anomaly for the zero field cooled magnetisation data measured at 2 K and S K up to 5 T.

INTRODUC3ION

experiment for the alloy fee (NiFe)2sAu7s9. It may be pointed out that GT type transition can be identified using AC susceptibility experimental method whereas field cooled (FC) and zero field cooled (ZFC) magnetisation experiments are necessary to identify low temperakre AT like transition where the irreversibility is strong. This motivated us to undertake the present experiment. We chose to study the particular alloy (NiFe)zsAuTs as only limited attempt ha5 been made to investigate those spin glass& with two (or more) local moment species each of which produces a well defmed Tf when present alone. In our case the well studied systems are AuFe6, NiFe lo. The aim of this paper is to investigate experimentally the FC and ZFC magnetic behaviour, with hysteresis data for the alloy (NiFe)zJAu,s to search for spin glass like transition or AT type transition at low temperature. Also to msgnetic investigate systematically the reentrant behaviour in high magnetic field for understanding the RSG transition. It may also be mentioned that such a study has been undertaken for the fee (NiFe)zsAuTs alloy for the fust time.

A number of amorphous and crystalline materials show sequential paramagnetic - ferromagnetic - spin glass like transition i.e. so called reentrant spin glass transition (RSG)‘-‘. It has been shown theoretically4 that RSG would be characterised by the freezing of the transverse irreversibility at some magnetisation with weak temperature (below curie temperature T,) called Gabay ,Toulouse transition, To, and the freezing of longitudinal magnetisation at very low temperature at Tf (the so called de Almeida Thouless transition TAT) followed by a cross over from weak irreversibility to strong irreversibility ar TAT’. Both transition temperatures exhibit a characteristic dependence on applied field. Thus in the standard Paramagnetic - spin glass systems frozen in random spin orientations and strong irreversibility - these two aspects occur at spin freezing temperature whereas in RSG these occur at different temperatures. For example in Au - 19 at% Fe alloys T, - 170 K, TUT - 60 K, TAT - 15 K6. Focussing on the magnetic properties of SG systems, particularly RSG phenomenon is one of the most stimulating subjects in these systems’.’ and comparatively fewer investigations on field dependent phenomena on the RSG transition through magnetisation experiment have been performed As an example some of the well studied RSG systems are amorphous FeNiSiB, FeMn. AuFe, FeNiMn, FeZr, NiMn, PdFeMn CrFe, FeAl and insulator EuSrS (see for review [2]). In the RSG state the nature of the SG and ferromagnetic states seems to be not well understoods . However recent developments favour the ‘transverse spin component freezing’ approach4 based on the mean - field model. (See for a review on experimental aspects on RSG’). Recently we predicted a possible RSG like state based on the non - linear AC susceptibility

EXPERIMENTriL The sample (NiFe)zsAuTs used in our experiment is from the same batch as used in our AC susceptibility experiment’. They are single phase (fee) and homogeneous to 1 pm as confirmed by X - ray and electron micro - probe analysis. The details of the sample preparation are given in reference”. In the present experiment the sample dimensions are I = 6 mm, diameter - 2.5 mm, wt - 141.76 mg. The magnetisation measurements were made using quantum design SQUID MPMS magnetometer. The FC experiments are performed, by first applying a field (say for example 5 911

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mT) and then it is cooled down to the required temperature, 2 K. Experiment is completed by measuring the magnetic moment of the sample as it is warmed from 2 K to 350 K. The experiments are performed for various fields i.e. 5 mT, 20 mT, 50 mT, 100 mT, 1 T, 2 T, 3 T. 4 T and 5 T. The ZFC experiments are performed by cooling the sample down to rhe lowest temperature 2 K without any field (residual field - 0.05 mT). The magnetic moment of the sample was measured as a function of temperature between 2 K to 350 K by applying the field say 5 mT and the same procedure is repeated for different fields up to 5 T. Magnetisation hysteresis experiments are performed at various temperatures i.e. 5 K, 50 K. 80 K and at 98 K. Here the sample is cooled (ZFC) to the required temperature. Then the hysteresis experiments are performed by varying the fields from 0 T - 5.5 T - 0 T -5.5 T - 0 T - 5.ST. M - H data at 2 K, 5 K are collected under ZFC condition, by varying the field from 0 T to 5.5 T. We wish to state that the time interval between successive fields is less than half a minute for the case of M - H experiment (ZFC). In the case of M - T(K) experiment, after lowering the temperature to 2 K under ZFC condition, then the field of say 5 T is applied. The stable condition of this field takes place after 3 minutes. The experiment is completed by varying the temperature up to 350 K.

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RESULTS Focussing on the present experiments the overall FC and ZFC data in the temperature range 2 K - 350 K in the field of 5 mT, 20 mT, SO mT, 100 mT are shown in figure I. The inset in figure 1 shows the FC and ZFC data for the field 100 mT at low temperature where the irreversibility starts at 5 K which we identify as TAT with a maximum in the magnetisation at 30 Kf2 K. The appearance of this. maximum is independent of FC and ZFC data. The same features have been observed for the data in the field 5 mT, 20 mT, 50 mT where the irreversibility occurs at 15 K, 12.5 K, 10 K and the maximum occurs at 70 K f10 K, 45 Kzt5 K, 36f2 K (T,,) respectively. The appearance of the maximum in the magnetisation data is the start temperature for the reentrant process when lowering the temperature and similar observation has been made in RSG FeNiMn system’. In the temperature range of there exists a mixture of ferromagnetic and T~~flflrnu SG phases. In figure 2 we have shown FC and ZFC magnetisation data in the field 1 T to 5 T in the temperature range 2 K - 350 K. It is interesting to note that there is no irreversibility as seen at the field - 100 mT and also the. disappearance of the maximum in the magnetisation data. It is clear from the figure fiat there is no appreciable difference in the FC and ZFC data at high field > 1 T. We have plotted (figure 2 inset) x-’ against T for the ZFC data which shows Curie-Weiss behaviour 8 = 195 K, - C = 3.278~10~’ emu/gOe indicating the predominant ferromagnetic exchange interaction. The magnetic hysteresis loops measured at 5 K, 50 K, 80 K, 98 K are shown in figure 3(a) for the field up to 5 T and in figure 3(b) for low field region - 60 mT. There is a weak hysteresis loop at SO K, 80 K, and at 98 K. However. no hysteresis loop was observed at 5 K. The most interesting

Figure 1

Field cooled (FC) and zero field cooled magnetisation data in the (ZFC) temperature range 2 K - 350 K in the field 5 mT, 20 mT, 50 mT, 100 mT. The inset figure in the temperature range 2 - 50 K shows the irreversibility which occurs at 5 K for the field 100 mT. Note the maximum at 30 K (for other fields see text). Some data points are not shown for clarity.

observation in our opinion is in ZFC condition the magnetisation data at 2 K, 5 K, (see figure 4) shows a peak at 40 mT, 15 mT with constant magnetisation value 2.4 emu/g. The magnetisation drops with &easing field up to 80 mT (see inset in figure 4), afterwards the magnetisation - 0.7 emu/g remains constant even up to the field of 5.5 T. However there is a slight increase of the absolute value of the magnetisation at 2 K compared to the behaviour at 5 K for the field greater than 80 mT. It may be noted that in this field region FC and ZFC data shows the drop in magnetisation ineversibility etc., whereas above this field no irreversibility occurs i,e, field > 1 T (figure 1.2). DISCUSSION The starting point of our discussion is based on our earlier reported data’. We found from the linear and non-linear AC susceptibility experiment for the (NiFe)zsAu75 alloy that PM - FM transition onset starts around 150 K and the non-linear susceptibility data shows a peak at T, - 130 K at low field - 8 Oe so also the

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Field cooled (FC) and zero field cooled magnetisation data in the (ZFC) temperature range 2 K - 350 K in the field 1 T to 5 T. The ZFC data are shown with symbols and the FC data are shown as continuous lines. The inset shows the Curie-Weiss plot in the temperature range 200 K - 350 K in the field 5 mT to 5 T (ZFC). Symbols +, x represent 5 mT and 100 mT data. For other fields the same symbols are used as in the main figure.

absorption term i.e. x”(T)‘. It may be recalled that the appearance of the distinct peak in the non-linear response of magnetisation is the intrinsic feature of RSG transition and can be considered as one of the criteria for determining Tr experimentally. This is based on the experimental evidence on Ni-Mn12, PdFeMn”, amorphous FeZr14. The temperature at which the peak in non-linear response occurs is in good agreement with Mossbauer determination of the temperature at which transverse spin freezing occurs. i.e. GT like transition’4. Moreover it has been also shown on the basis of mean field theoretical considerations that this peak may be the longitudinal response to a cooperative transverse freezing”. We fmd that in the AC susceptibility experiments (third harmonic signal) the non-linear term Ix: I suggest, the present system as a possible RSG like state at T - 80 Kk 10 K measured at 8 Oe, which is sensitive to the applied field i.e. 92.8fl K at 1 Oe, 93f2.5 K at 4 Oe. We have also shown that at this temperature FM moment coexists with this SG like state on the basis of x\(T) results i.e. second harmonic signals’. Thus the

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(a). Magnetic hysteresis curve in the high field range up to 5.5 T. Figure 3(b) shows the expansion of the magnetic hysteresis curve (figure 3(a)) in the low field range up to 60 mT. Note that in figure 3(a) H is in Tesla whereas in figure 3(b) H is in (mT).

main results from our earlier study9 are that the (NiFe)25AuTs undergoes PM - FM transition at 130 K with a possible transverse spin freezing at - 93 K at 4 Ck. and the system is in the mixed phase i.e. FM moment coexists with this SG like state. However Mossbauer experiment will clarify this transition. Now we summarise the recent experimental status on RSG particularly the significance of the GT, AT transition and the influence of an external magnetic field on the RSG transition temperature. The experimental status on RSG3 clearly shows that the data can be interpreted on the basis of mean field mode14. Accordingly at T,, ferromagnetic order sets in with a magnetic domain structure. At ToT local transvetse spin components begin to freeze with weak irreversibility and at low temperature the individual spins are canted with local domain magnetisation direction i.e. freezing of the longitudinal spin components at TAT or Tr with onset of strong irreversibility. Experimentally, the RSG state is characterised by a rapid fall of the low field magnetisation at Tr below which an appreciable history - and time dependent magnetisation,

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Magnetisation against field measured in zero field cooled condition at 2 K, 5 K. The inset shows the expansion in the field up to 200 mT.

irreversibility etc. appear. In our sample this happens at 15 K, 10 K, 5 K for the field 5 mT, 50 mT, 100 mT respectively. Further in teal RSG it has been shown that the macroscopic domain structure exists both in FM and SG phase16. This provides the evidence that reentrants at low temperatures are ferromagnets on macroscopic scale whereas spin glass behaviour shows up on a microscopic scale. We now turn to the field dependence of GT, AT transition on RSG. The field dependence of GT transition is given by ToT(0) - ToT(H) - Hz [4] for the AT transition TAT(O) -TAT(H) - HU3 [5]. It has been found that close to the tricritical point Toe-TUT - H i.e. varies linerly with H”. We found that our present data in the field 5 mT to 100 mT varies as H213 with TAT(O) - 16 K. The influence of external magnetic field on RSG mostly based on Mossbauer experiments have been reported in detail for CrFe” and AuFe’* alloy whereas only limited attempt has been made in general influence of external field for magnetisation experiments (for example detailed M(H) data on FeNiMn’). It has been found that there is no change in RSG transition i.e. -ATr/H in Auoss2Feo16s 0.7, Fe6Ni72SisB)s - 0 whereas Cr7sFets - 6, (Fe.6sNi.3s),sssMn.102 - 12 show a change in Tr where ATr =Tr(H)-Tr(0). This change i.e. -ATr/H is large in comparison with mean field value” - 0.87. The mean field value is valid only at the triple point i.e. T, = TrtO). Our data shows in the field of 5 mT to 100 mT the change

Figure 5

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Arrott plot M2 vs H/M in the temperature range2K-180Kinthefield5mTto5 T. Note the linear behaviour at higher fields > 1 T as indicated.

in Tr i.e. -ATr/I-I - 0.01 which is very small. According to the classification’8 that the small or no change in -ATr/H shows that with AFM interaction, frustration enters via long-range RKKY interaction for the magnetically diluted FM system. On the other hand high value of -AT&I shows for example in CrFe. (FeNi)Mn system has short range with direct AFM interaction introduced by the Cr. Mn. This difference in ARM interactions leads to different local spin correlations which are different in different RSG systems. It may be pointed out that up to H - 3 T there is no measurable change in RSG transition in A’krs~Fei~68 18. We also found that T, - Tr / Tr(0) - 7 which is the same order of magnitude as reported for the above RSG system. We found that the maximum in the magnetisation data (i.e. 70 KflO K. 36f2 K, 30f2K at 5 mT, 50 mT, 100 mT), before the irreversibility sets in (see figure 1 inset) that there is no linear dependence of this temperature with applied field. However close to the tricritical point one would expect a linear dependence with H as shown in FeNiMn7. In order to understand the high field data we have plotted M2 against H/M (Arrot plots) at various temperatures in the range of 2 K - 180 K i.e. T>T,. Ta,, T,(onset) - 150 K in the field 5 mT to 5 T under ZFC condition (see figure 5). The data above 1 T shows parallel straight lies having positive intercepts on the M2 axis at low temperatures suggesting field induced ferromagnetism (FIFM). At low fields - 5 mT, the curve

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(NiFe)25Au75

spontaneous showing no origin bends towards magnetisation. In fact as seen from figure I the drop in magnetisation occurs in the field 5 mT - 100 mT. This shows that (NiFe)zSAuTS makes a transition from a low field spin glass state (no spontaneous moment) to a field induced ferromagnetic one at higher fields. Such t r,“df FIFM has been observed in RSG systems FeNiCr also in PdFeMn?‘. Turning to the hysteresis behaviour, it has been found for T < TG~ or local canting temperature i.e below the transverse spin component’ freezing there are strong relaxation ef%cts in magnetisation and torque signal*‘. These relaxation effects as in true SG reorganises the local spin structure on long time scale, which is not well understood’. At low temperature for different RSG systems hysteresis loops arc qualitatively different which may be due to the existence of stronger or weaker relaxation processes”. We believe that the observed M-H anomaly at 2 K, 5 K may be related to this relaxation processes as the time sequence of operation in M-H, M-T(K) experiments, as previously mentioned in the experimental section, ate different. CONCLUSION To summarise, FC and ZFC, hysteresis data in the temperature range 2 K - 350 K in the field 5 mT - 5 T, measured for the first time in fee (NiFe)25AuTS alloy using SQUID magnetometer, are reported. We observed that at low temperatures in the field range 5 mT - 100 mT (e.g. 15 K at 5 mT) strong irreversibility, drop in the magnetisation as found in other typical RSG systems. We identify this transition as Almeida - Thouless (AT) - spin freezing temperature. ?he analysis based on the Arrot plots suggest that the present system undergoes transition

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from spin glass state at low field to field induced ferromagnetism at higher field > 1 T. No hysteresis loop was observed at low temperature - 5 K even though there is a weak hysteresis loop at T < T, i.e. T - 50 K, 80 K and at 98 K, with technical saturation in the field range - 5.5 T. It may be noted in general in RSG the hysteresis loops are qualitatively different for different RSG systems depending upon the weaker or stronger relaxation effects. We found in the M-H curve measured at 2 K, 5 K in the field up to 5.5 T under ZFC condition the magnetisation drops at 40 mT, 15 mT respectively with increasing field up to - 80 mT. For further increase in the field the magnetisation remains almost constant up to 5.5 T at these temperatures. We believe that this interesting observation may be due to the d&rent time scale used in stabilising the magnetic field and sequence of operation in the data acquisition for the M-H data under ZFC at 2 K, 5 K. This may be important for systems exhibiting relaxation effects in magnetisatiou. We stress that in the absence of any such reported data in RSG systems, it is difficult to draw any conclusion for such behaviour. Finally additional measurements like dynamical susceptibility at low temperatures preferably with static field, Mossbauer experiments are required for quantitative understanding in this new RSG (NiFe)zsAu75 alloy system. We believe that the present work will assist to understand more about the (NiFe), AuFe RSG system.

ACKNOWLEDGEMENTS

We Thank Prof. A.K. Raychaudhuri for useful discussions and encouragement. We thank Dr. C. Bard for the sample.

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