ESR characterization of the spin-glass Ag Mn

Journal of Magnetism and Magnehc Matermls 31-34 (1983) 1351-1352

ESR CHARACTERIZATION J.M. MACHADO

1351

OF THE SPIN-GLASS AgMn

D A S I L V A * a n d H. A B E

Institute for Sohd State Physics, Toledo 106, Japan

ESR measurements on six samples of A gMn (¢ = 2 13, 4 96, 6 35, 9 54, 13 02 and 19 29 at%) have been interpreted by means of the model of Schultz et al, our experimental results are consistent w~th the wew that the frequency of resonance ~s a linear function of the resonant field The umax~al amsotropy constant depends hnearly on the temperature K(T)/K(O)= 1- BT/Tg w~th/~ = 0 78 The umdlrectlonal amsotropy is observed below about 0 4 Tg for a field cooled specimen ( ¢ = 6 35 at%)

l. Introduction

3. Results and discussion

Most of the former ESR measurements on the spinglasses __CuMn and A g M n have been Interpreted by means of the antiferromagnetlc relation of uniaxlal crystals [1,2] Recently M o n o d et al [3] and Schultz et al. [4] determined the spectrometer frequency dependence on the resonant field for the spin-glass C uMn when the magnetization is predominantly remanent and in the paramagnetxc state, respectively Instead of the antfferromagnetic frequency dependence, they found m both cases that the frequency is nearly a hnear function of the resonant field. To interpret thew results, Schultz et al [4] obtained the frequency dependence (¢o/7) on the resonant field (H~) from a phenomenolog~cal free energy which contained magnetic remanence (M~), amsotropy and Zeeman energy It is easy to show from theLr theory that the anxsotropy constant, K, is given by K = (X(O~/71+ M~) ( ~ / ' ~ - H ~ ) which can be written as K / x = ( ¢ o / ' { ) ( ~ / ~ , - Hr) if M~=O, ( ~ o / ¥ - H~) is the shift of the resonant field

Fig. 1 shows for all the samples a plot of K / X = (¢o/'f)- (~0/7 - Hr) scaled at T/Tg = 0 5, where the temperature of the maximum susceptibility is taken from low field measurements [5], the experimental points were obtained from zero field cooled samples, keeping the sweeping condition precisely ~dentical to each other It is clear from fig 1 that the agreement between theory and experiment Js good and that the slope of K / X ~s fmrly constant down to approximately T/Tg = 0.4 The abrupt change of slope at this temperature, which Schultz et al [4] did not describe in their experiments, may be explained by an extra contribution to the anisotropy

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2. Experimental The A g M n samples (c = 2 1s, 4 96, 6 35, 9 54, 13 02 and 19 29 at%) were made by mixing the two components under vacuum better than 10 -5 Torr m a high frequency furnace The ingots obtained in this way were remelted in high vacuum m a Br]dgeman's furnace and kept at a temperature 30°C above the melting point of Ag(961°C) for 2 h, after which they were cooled at a rate of 2 ° C / r a m The homogeneity of the samples was checked by standard chemical analysis. The ESR measurements were made on a spectrometer which used low frequency modulation (130 Hz) with a conventional reflection cavity arrangement operating at - 9 3 G H z All the samples were measured after cooling in zero field and in a field of 5 3 kOe One of the samples (c = 6 35 at%) was also measured m a field ant~parallel to the coohng field * On leave of absence from the Centro de Flslca (1NIC) da Unlversldade do Porto, Portugal 0304-8853/83/0000-0000/$03.00

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© 1983 N o r t h - H o l l a n d

J M Machado da Silva and H Abe / ESR characterization of AgMn

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Fig 2 Shift (6H) of the resonant field of A_ggMn(c = 6 3s at%) when cooled through T~ m a field of 5 3 kOe (× × ) = measuring field parallel to the coohng field, (AA)= measuring field antiparallel to the coohng field The inset shows, for this sample, the behavlour of the unidirectional anlsotrop'v constant as a function of temperature constant from the fast increase at low temperature of the isothermal remanent magnetization (IRM) We attempted to check the temperature dependence of the corresponding umdtrectlonal amsotropy by measuring the shift of the resonant field, 3H = (oa/y - H~), for the sample whose c = 6 35 at%, when the field ~s parallel (SH T ) and anttparallel ( 8 H i ) to the coohng field, the

temperature dependence of 8H t and 8H+ ,s shown m fig 2 The temperature dependence of this extra contr,but,on to the amsotropy constant w,ll behave as K( 8H - S H ~ )/(SH r + S H , ) The inset of fig 2 shows lhal the unidirectional amsotropy increases very rapidly below about 0 4 Tg, while it Js neghgtble above this temperature, confirming possibly the change of slope of K / X at the same temperature From suscepttbihty results for ¢ = 4.96 and 6 3s at~ we obtained K(T), m the range of the measuring temperature K(T)/K(O) is linearly dependent on the ten> perature as K ( T ) / K ( O ) = 1-19T/Tg with / 9 = 0 7 8 + 0 08 Th~s may not differ appreciably from the /9 value (/9 = 0 67) for C u M n quoted by Schultz et al [4], since thmr Tg values, obtained at 3 kOe are - 10% smaller than those from low field measurements The K(0) values for these two A gMn samples are 205 Oe -~ e m u / g and 297 Oe z e m u / g , respectively; these values may suggest that K(0) is proportional to the square of the concentrat,on of Mn, as m C u M n [4], but more data ,s necessary to confirm this concentratmn dependence for the A gMn system Financial support to one of us (JMMS) by the Japan Society for the Promotion of Scmnce is gratefully' acknowledged References [l] J Owen, M E, Browne, V Arp and A F Kip, J Phy~ Chem Solids 2 (1957) 85 [2] D Grlfflths, Proc Phys Soc 90 (1967)707 [3] P Monod and Y Berthler, J Magn Magn Mat 15 18 (1980) 149 [4] S Sehultz, E M Gulhkson, D R Fredkln and M lovar, l Appl Phys 52 (1981)1776 [5] J M Machado da Silva, H Abe, M Mlyajlma and J M Araujo, Portugal Phys 12 (1981) 181