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Electron spin resonance study of a disordered Ni-Mn alloy
1429
Journal of Magnetism and Magnetic Materials 31-34 (1983) 1429-1431
ELECTRON SPIN RESONANCE STUDY OF A DISORDERED N i - M n ALLOY H. H U R D E Q U I N T ,
J.S. K O U V E L
* a n d P. M O N O D
Laboratotre de Phwtque de~ Sohds, Umver~ttb Parts-Sud, 91405 Orsav. Frame
ESR spectra were obtained for disordered Nt 74Mn26cooled to --- 4 K m zero field and 10 kOe The resonance observed after field-coohng are asymmetric with the applied field direction up to = 30 K and continue to show a macroscopic amsotrop3 field up to --- 80 K. where a trans,t~on occurs from spin-glass to ferromagneusm
In contrast to the simple ferromagnetism of atomically ordered Nl3Mn, disordered N 1 - M n alloys near this composition exhibit unusual low-temperature magnetic p r o p e m e s (e g., displaced hysteresis loops upon cooling in a field) that are charactensUc of a spin-glass state [1] A c c o r d m g to neutron dlffracUon work [2], this contrasting behavior probably arises from antiferromagneUc M n - M n nearest-neighbor exchange interactions which exist only in the &sordered alloys, where they compete with the ferromagnetic N1-NI and M n - N l interactions and produce the frustration effects considered to underhe all spin-glass states [3] Consistent with this description of &sordered N~-Mn, recent magnetic measurements [4] reveal that as the Mn concentration increases and the ferromagneUc Curie point (T~) decreases, a spln-glass-hke state emerges below a transition temperature which rises and ultimately meets T~ at 26 at% M n To obtain a dynamical characterization of &sordered N 1 - M n in this c o m p o s m o n region, we carried out an ESR study of an alloy of 26 5 at% Mn The sample was a thin rectangular polycrystalhne plate (4 x 2 5 × 0 04 m m 3) which had been encapsulated m quartz under argon, annealed 3 h at 900°C, and water-quenched Detailed ac suscepubdlty and dc magneUzation measurements on the same sample [5] delineate three temperature regimes paramagnetlc above T~ = 110 K, weakly ferromagneuc from T~ down to ---80 K, and spin-glass below = 80 K The ESR study was performed with a Varian reflection-spectrometer at fixed frequency (9.25 GHz) The sample was m o u n t e d m the microwave cawty such that the rf and dc fields are orthogonal and he in the plane of the sample, whose temperature was controlled by a hehum-flow cryostat A standard field-modulauon technique was used, so that the detected signal corres p o n d e d to the field-derivative of the absorbed power ESR spectra were obtained at various fixed temperatures upon warm-up from ---4 K w n h the field ( H ) * Permanent address
Physics Dept, University of lllmols, Chicago, IL 60680, USA Work supported in part by the US National Science Foundanon
0304-8853/83/0000
a) Hcool=+loko~
02
/I
[
I[
2---
(b) HcooI~0
I
Fig 1 ESR spectra for disordered NI74Mn26 sample cooled to 3 7 K in (a) + 10 kOe and (b) zero field Vertical scales in (a) and (b) are arbitrary but the same Marker signals at _+ H r = +331 kOe are from DPPH crystalhte ( g = 2 ) Calculated Lorentzian curves, shown dashed, are described in the text
slowly cycled between +_5 kOe, the sample having been initially cooled to ~ 4 K in zero field or in + 10 kOe Thus, our work extends beyond an earlier ESR study of disordered N13Mn, wluch was made only in the coohng-field (H~,,ol) direction [6] Fig. 1 displays the ESR spectra at 3 7 K with H varied from + 5 to - 5 kOe The spectra observed as H was raised back to + 5 kOe were nearly the same, showing httle hysteresis. For //co,, I = + 10 kOe (fig la) the resonance spectra In positive and negative H are
- 0 0 0 0 / $ 0 3 00 © 1983 N o r t h - H o l l a n d
1430
H Hurdequmt et al / ESR rtud~ of a d:~ordered Nt
different m every respect (hne shape, field for resonance, hnewldth, amphtude) Th~s asymmetry ~s also ewdenced at lower H, where a kmk occurs at = - 0 3 kOe, which correlates closely w~th the average of the two negattve coerc~vmes of the d~splaced hysteresis loop measured under the same c o n d m o n s [5] However, at the posmve and negative fields for resonance (H~), the magnetizations are almost equal and opposite and, hence, are not &rectly responsible for the observed asymmetry The resonance spectra for H¢ool = 0 (fig. lb), compared to those for H,o,,~ = + 10 kOe, are broader, weaker m amphtude, and much more anomalous m shape, although the values of Hr are s~mflar m magmtude Moreover, the spectra are almost symmetrical w~th H, thmr small asymmetry probably d e n w n g from H o,,~ not being strictly zero Small kinks appear nearly s y m m e m tally at low H ( = _ + 0 3 kOe), which correlate fairly closely w~th the critical field for a t r a n s m o n from a spin-glass to a ferromagnetic-hke state [5] Thus, for the mare resonance at h~gher fields, the sample ~s m the latter state In fig la, the resonance spectrum measured m p o s >
Hcool=+10k0e
Mn alloy
t~ve H is compared with a calculated Lorentzlan cup,'c, adjusted to match m amphtude, peak-to-peak llnewldth (AH), and A / B ratio, where A and B are the magmtudes of the posmve and negative peaks The fit ~s ,~er,~ good except at low H where the experimental cur,~e has a p r o n o u n c e d shoulder In fig lb, a sunllar comparison is made w~th a Lorentzlan curve calculated for the same A / B as before but for AH and the a m p h t u d e adjusted to fit the upper part of the p o s m v e peak of the measured curve In this case, the calculated curve clearl> does not describe the observed resonance, whose features suggest that the zero-H~o,, t state has a more complex dynammal response After taking the ESR spectra at 3.7 K for H~,,,,t - + 10 kOe (fig la), we proceeded to make similar measurements at various fixed temperatures upon warn>up, with H always cycled between _+5 kOe F~g 2 &splays the principal features of the observed resonance spectra (posmve and negative values of H r, and AH and A / B for pOSltlVe H') as functions of temperature As evidenced m the differences between the magnudues of the p o s m v e and negative Hr, the asymmetry of the ESR spectra & m t m s h e s monotonically w~th rising temperature and &sappears at = 30 K, which Is also where the H~oormduced &splacement of the magnetic hysteresis loops ~s seen to vamsh In order to interpret the varlatmns of H,., we make use of the following expressmn for the elgenlrequency corresponding to a ferromagnetic mode of the system
o o o = T { ( H : + 4 v r M - + H a ) ( H . + HA)} ' z
(1)
28
-g o ~:24
2 2O
~ 0 4
o
o l0
t
41
0
I
oe
1
oo
J
120
e
•
I
•
[
160
I
200
T(K)
Fig 2 Vanatmns with increasing temperature of ]H,], AH and
A / B from ESR spectra for disordered N174Mn 26 sample cooled mmally t o 3 7 K m + 10 kOe Closed and open circles are for resonance m positive and negative fields, respectively
where y ( = g # u / h ) is the gyromagnetlc ratio of the precessmg moments H and M. are the apphed field and magnetization m the &rectmn set by H~ool, and t/~ represents a macroscopic amsotropy field m the same direction This expression pertains to the geometry ol our ESR experiments, and the resonance con&t~on is that the spectrometer frequency ~ - ~0 at ~ - tt, At 3 7 K for H ool= + 1 0 kOe, our ESR results for the N i - M n alloy sample give H~ values of + 1 595 kOe and 2 038 kOe The corresponding M. values from magnetization data on the same sample [5] are + 2 1 4 6 and - 2 1 6 5 m e m u / c m 3 However, the corresponding It~ values cannot be derived from eq (1) without independent knowledge of the y value for the alloy Instead, we use the results for H a obtained from recent measurements of the transverse ac susceptlblhty (X J) of the same sample under identical c o n d m o n s [5] In posmve static fields, the XI data give HI +~= +396 Oe. Insertmg this H~4~ ) value and the above posmce values of H, and M: (and oa = 9 25 GHz) into eq ( 1), we calculate a Y value corresponding to g - 2 16, w'hlch 1%quite reasonable for a Nl-based alloy Substltutmg this g ~alue back into eq (1), together with the above negatme values of H r and M., we obtain HI ~= +55 Oe This small posmve HI i value agrees closely with the ~alue derived from XA m negative fields, testifying to the consistency
H Hurdequlnt et al / ESR study of a disordered N I - Mn alloy of the two types of measurements. At higher temperatures, assuming y is fairly constant, we find that the H r values shown in fig. 2 and the corresponding M. values, when inserted in eq. (1), give H~ +1 and H~A-~ values which, after merging at ~ 30 K, diminish but remain sizeable up to ~ 80 K. The variations of AH and A / B with temperature (fig. 2) also indicate changes of the dynamical behavior in th~s transitional region
References [1] J S Kouvel and C D Graham, J Phys Chem Sohds 11 (1959) 220 R B Goldfarb and C.E Patton, Phys Rev B 24 (1981) 1360
1431
[2] J W Cable and H.R Chdd, Phys Rev B 10 (1974) 4607 T J Hicks and O Moze, J Phys F 11 (1981) 2633 [3] G Toulouse, Commun. Phys 2 (1977) 115 [4] R G Altken, T D Cheung, J S Kouvel and H Hurdequmt, J Magn. Magn Mat 30 (1982)1 [5] H Hurdequlnt, F Hlppert and J S Kouvel, presented at 3IM 3 Conf, Montreal (20-23 July 1982) [6] V S Pokatdov, I M Puzel and G A Ivanov-Smolenskn, Soy Phys Sohd State 21 (1979) 97
Journal of Magnetism and Magnetic Materials 31-34 (1983) 1429-1431
ELECTRON SPIN RESONANCE STUDY OF A DISORDERED N i - M n ALLOY H. H U R D E Q U I N T ,
J.S. K O U V E L
* a n d P. M O N O D
Laboratotre de Phwtque de~ Sohds, Umver~ttb Parts-Sud, 91405 Orsav. Frame
ESR spectra were obtained for disordered Nt 74Mn26cooled to --- 4 K m zero field and 10 kOe The resonance observed after field-coohng are asymmetric with the applied field direction up to = 30 K and continue to show a macroscopic amsotrop3 field up to --- 80 K. where a trans,t~on occurs from spin-glass to ferromagneusm
In contrast to the simple ferromagnetism of atomically ordered Nl3Mn, disordered N 1 - M n alloys near this composition exhibit unusual low-temperature magnetic p r o p e m e s (e g., displaced hysteresis loops upon cooling in a field) that are charactensUc of a spin-glass state [1] A c c o r d m g to neutron dlffracUon work [2], this contrasting behavior probably arises from antiferromagneUc M n - M n nearest-neighbor exchange interactions which exist only in the &sordered alloys, where they compete with the ferromagnetic N1-NI and M n - N l interactions and produce the frustration effects considered to underhe all spin-glass states [3] Consistent with this description of &sordered N~-Mn, recent magnetic measurements [4] reveal that as the Mn concentration increases and the ferromagneUc Curie point (T~) decreases, a spln-glass-hke state emerges below a transition temperature which rises and ultimately meets T~ at 26 at% M n To obtain a dynamical characterization of &sordered N 1 - M n in this c o m p o s m o n region, we carried out an ESR study of an alloy of 26 5 at% Mn The sample was a thin rectangular polycrystalhne plate (4 x 2 5 × 0 04 m m 3) which had been encapsulated m quartz under argon, annealed 3 h at 900°C, and water-quenched Detailed ac suscepubdlty and dc magneUzation measurements on the same sample [5] delineate three temperature regimes paramagnetlc above T~ = 110 K, weakly ferromagneuc from T~ down to ---80 K, and spin-glass below = 80 K The ESR study was performed with a Varian reflection-spectrometer at fixed frequency (9.25 GHz) The sample was m o u n t e d m the microwave cawty such that the rf and dc fields are orthogonal and he in the plane of the sample, whose temperature was controlled by a hehum-flow cryostat A standard field-modulauon technique was used, so that the detected signal corres p o n d e d to the field-derivative of the absorbed power ESR spectra were obtained at various fixed temperatures upon warm-up from ---4 K w n h the field ( H ) * Permanent address
Physics Dept, University of lllmols, Chicago, IL 60680, USA Work supported in part by the US National Science Foundanon
0304-8853/83/0000
a) Hcool=+loko~
02
/I
[
I[
2---
(b) HcooI~0
I
Fig 1 ESR spectra for disordered NI74Mn26 sample cooled to 3 7 K in (a) + 10 kOe and (b) zero field Vertical scales in (a) and (b) are arbitrary but the same Marker signals at _+ H r = +331 kOe are from DPPH crystalhte ( g = 2 ) Calculated Lorentzian curves, shown dashed, are described in the text
slowly cycled between +_5 kOe, the sample having been initially cooled to ~ 4 K in zero field or in + 10 kOe Thus, our work extends beyond an earlier ESR study of disordered N13Mn, wluch was made only in the coohng-field (H~,,ol) direction [6] Fig. 1 displays the ESR spectra at 3 7 K with H varied from + 5 to - 5 kOe The spectra observed as H was raised back to + 5 kOe were nearly the same, showing httle hysteresis. For //co,, I = + 10 kOe (fig la) the resonance spectra In positive and negative H are
- 0 0 0 0 / $ 0 3 00 © 1983 N o r t h - H o l l a n d
1430
H Hurdequmt et al / ESR rtud~ of a d:~ordered Nt
different m every respect (hne shape, field for resonance, hnewldth, amphtude) Th~s asymmetry ~s also ewdenced at lower H, where a kmk occurs at = - 0 3 kOe, which correlates closely w~th the average of the two negattve coerc~vmes of the d~splaced hysteresis loop measured under the same c o n d m o n s [5] However, at the posmve and negative fields for resonance (H~), the magnetizations are almost equal and opposite and, hence, are not &rectly responsible for the observed asymmetry The resonance spectra for H¢ool = 0 (fig. lb), compared to those for H,o,,~ = + 10 kOe, are broader, weaker m amphtude, and much more anomalous m shape, although the values of Hr are s~mflar m magmtude Moreover, the spectra are almost symmetrical w~th H, thmr small asymmetry probably d e n w n g from H o,,~ not being strictly zero Small kinks appear nearly s y m m e m tally at low H ( = _ + 0 3 kOe), which correlate fairly closely w~th the critical field for a t r a n s m o n from a spin-glass to a ferromagnetic-hke state [5] Thus, for the mare resonance at h~gher fields, the sample ~s m the latter state In fig la, the resonance spectrum measured m p o s >
Hcool=+10k0e
Mn alloy
t~ve H is compared with a calculated Lorentzlan cup,'c, adjusted to match m amphtude, peak-to-peak llnewldth (AH), and A / B ratio, where A and B are the magmtudes of the posmve and negative peaks The fit ~s ,~er,~ good except at low H where the experimental cur,~e has a p r o n o u n c e d shoulder In fig lb, a sunllar comparison is made w~th a Lorentzlan curve calculated for the same A / B as before but for AH and the a m p h t u d e adjusted to fit the upper part of the p o s m v e peak of the measured curve In this case, the calculated curve clearl> does not describe the observed resonance, whose features suggest that the zero-H~o,, t state has a more complex dynammal response After taking the ESR spectra at 3.7 K for H~,,,,t - + 10 kOe (fig la), we proceeded to make similar measurements at various fixed temperatures upon warn>up, with H always cycled between _+5 kOe F~g 2 &splays the principal features of the observed resonance spectra (posmve and negative values of H r, and AH and A / B for pOSltlVe H') as functions of temperature As evidenced m the differences between the magnudues of the p o s m v e and negative Hr, the asymmetry of the ESR spectra & m t m s h e s monotonically w~th rising temperature and &sappears at = 30 K, which Is also where the H~oormduced &splacement of the magnetic hysteresis loops ~s seen to vamsh In order to interpret the varlatmns of H,., we make use of the following expressmn for the elgenlrequency corresponding to a ferromagnetic mode of the system
o o o = T { ( H : + 4 v r M - + H a ) ( H . + HA)} ' z
(1)
28
-g o ~:24
2 2O
~ 0 4
o
o l0
t
41
0
I
oe
1
oo
J
120
e
•
I
•
[
160
I
200
T(K)
Fig 2 Vanatmns with increasing temperature of ]H,], AH and
A / B from ESR spectra for disordered N174Mn 26 sample cooled mmally t o 3 7 K m + 10 kOe Closed and open circles are for resonance m positive and negative fields, respectively
where y ( = g # u / h ) is the gyromagnetlc ratio of the precessmg moments H and M. are the apphed field and magnetization m the &rectmn set by H~ool, and t/~ represents a macroscopic amsotropy field m the same direction This expression pertains to the geometry ol our ESR experiments, and the resonance con&t~on is that the spectrometer frequency ~ - ~0 at ~ - tt, At 3 7 K for H ool= + 1 0 kOe, our ESR results for the N i - M n alloy sample give H~ values of + 1 595 kOe and 2 038 kOe The corresponding M. values from magnetization data on the same sample [5] are + 2 1 4 6 and - 2 1 6 5 m e m u / c m 3 However, the corresponding It~ values cannot be derived from eq (1) without independent knowledge of the y value for the alloy Instead, we use the results for H a obtained from recent measurements of the transverse ac susceptlblhty (X J) of the same sample under identical c o n d m o n s [5] In posmve static fields, the XI data give HI +~= +396 Oe. Insertmg this H~4~ ) value and the above posmce values of H, and M: (and oa = 9 25 GHz) into eq ( 1), we calculate a Y value corresponding to g - 2 16, w'hlch 1%quite reasonable for a Nl-based alloy Substltutmg this g ~alue back into eq (1), together with the above negatme values of H r and M., we obtain HI ~= +55 Oe This small posmve HI i value agrees closely with the ~alue derived from XA m negative fields, testifying to the consistency
H Hurdequlnt et al / ESR study of a disordered N I - Mn alloy of the two types of measurements. At higher temperatures, assuming y is fairly constant, we find that the H r values shown in fig. 2 and the corresponding M. values, when inserted in eq. (1), give H~ +1 and H~A-~ values which, after merging at ~ 30 K, diminish but remain sizeable up to ~ 80 K. The variations of AH and A / B with temperature (fig. 2) also indicate changes of the dynamical behavior in th~s transitional region
References [1] J S Kouvel and C D Graham, J Phys Chem Sohds 11 (1959) 220 R B Goldfarb and C.E Patton, Phys Rev B 24 (1981) 1360
1431
[2] J W Cable and H.R Chdd, Phys Rev B 10 (1974) 4607 T J Hicks and O Moze, J Phys F 11 (1981) 2633 [3] G Toulouse, Commun. Phys 2 (1977) 115 [4] R G Altken, T D Cheung, J S Kouvel and H Hurdequmt, J Magn. Magn Mat 30 (1982)1 [5] H Hurdequlnt, F Hlppert and J S Kouvel, presented at 3IM 3 Conf, Montreal (20-23 July 1982) [6] V S Pokatdov, I M Puzel and G A Ivanov-Smolenskn, Soy Phys Sohd State 21 (1979) 97