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Submitted on 1 Jan 1978
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EFFECT OF A MAGNETIC FIELD ON NUCLEAR
SPIN-LATTICE RELAXATION TIME IN THE
LINEAR ANTIFERROMAGNET TMMC AT LOW
TEMPERATURE
J -P. Boucher, G. Jug, F. Borsa
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au
no
8, Tome 39, aolit 1978, page C6-727E F F E C T O F A M A G N E T I C F I E L D ON NUCLEAR S P I N - L A T T I C E R E L A X A T I O N T I M E I N THE L I N E A R ANTIFERROMAGNET TMMC A T LOW TEMPERATURE
2
J -P. ~oucher+, G. J U ~ + and F. Borsa
Centre dlEtudes Nucldaires de Crenoble, Ddpartement de Recherche Fondamentale, Section de
Rdsonance Magndtique, 85 X
-
38041 Crenoble-Cedex,FranceI s t i t u t o d i F i s i c a delZ'Universitd e Unita CNSM d e l CNR, Pavia, I t a l y
Resume.- L'effet d'un champ magngtique H sur le taux de relaxation nucleaire T;' 5 basse tempdrature est Btudid dans le composg magnetique unidimensionnel (CH,),NMnCl, (TMMC). Les mesures ont et6 fai- tes sur diffgrents noyaux
-
hydrogene, azote et deuterium-
pour 2 < T < 4,2 K et 1 < H < 70 kOe. Lorsqu'on augmente H une decroissance rapide est d'abord observee, suivie d'une remontge assez sur- prenante pour H > 20 kOe.Abstract.- We present an experimental investigation of the field dependence of the nuclear spin-lat- tice relaxation rate
T;'
in the linear antiferromagnet (CH,),NMncl, (TMMC) at low temperature. T;' measurements have been performed on different nuclei-
proton, nitrogen and deuterium-
in the tem- perature interval 2-
4.2 K and for applied fields ranging from 1 to 70 kOe. After a sharp decrea- sing of T-l with H an unusual increase is observed for H > 20 kOe.Recently there has been a growing interest in the study of the properties of linear magnetic chains under the effect of an applied magnetic field. Lar- ge effects are observed experimentally which seemto be typical of the low dimensionality of the systems. One of the most striking effect has been observed on the NIel temperature TN for the 3d ordering :
T~ increases strongly with the applied magnetic field H. Since TN in quasi-Id magnets is proportional to the Id correlation length
5,
the field dependence of TN shows that at low temperature the Id spin corre- lations are very sensitive to H. In magnetic systems the nuclear spin-lattice relaxation rate T;' is de- termined among other things by the value of5
and by the lifetime of the short-range order fluctua-tions. Therefore, measurements of
seem
to be par- ticularly suitable to investigate the effect of H o n the dynamic and static correlation functions.The crystal (CH,), NMnCl, (TMMC) offers one of the best examples of a Id antiferromagnetic sys- tem. The 3d ordering temperature has recently been shown to be strongly affected by a magnetic field :
for H = 0, TN = .85 K while for H = 77 kOe, TN = 3 K /l/.
The dynamic spin correlations in W Chas been widely investigated through proton T measure-
l
ments both at high temperature /2/ and at low tem- perature 1 3 1 . At high temperature the magnetic field dependence of T;' has been shown to afford informa- tion about the diffusive character of the time de- pendent spin correlation functions.
+
E.R. CNRS no 60216At low temperatures, T measurements had been so far performed only in low magnetic field 131. For T > 4 K and when the field is parallel to the chain axis c the proton T;' data can been ex- plained qualitatively by considering that at T 20 K the electron spin system crosses over from an Heisenberg model to an XY model 141. However for T < 4 K no satisfactory explanation has yet been given.
In this paper we report recent results of
in
TMMC obtained in the temperature range 2-
4.2 K and for values of H in the range 1-
70 kOe. The T1 measurements were performed on different nu- clei : proton, nitrogen and deuterium in deuterated samples. All the measurements reported here were performed on single crystals with H ~ C . In most of the echo signal was detected rather than the free precession and the recovery of the nuclear magneti- zation was observed after a n-n/2 sequence.The proton resonance line, although inhomo- geneously broadened consists essentially of a main line which was easily observed up to 47 kOe. The va- lues of the proton T;' as a function of the field are given in figure 1, for two values of the tempe- rature T = 4.2 K and T = 2.5 K. Two relevant featu- res appear from the data in figure 1 : i) at a fi- xed temperature, the proton ~i'decreases rapidly on increasing the magnetic field. This effect can be compared to the low frequency divergence of the dynamic spin correlation functions as already repox- ted /l/, /2/ ; ii) at H = 20 kOe a drastic change in
this behavior is observed : T-l starts to increase with H. This is a complete unexpected effect since ~ - l i s usually a decreasing function of H.
Fig. I : Proton function of the the temperature
spin-lattice relaxation rate as a magnetic field H for two values of
: T = 4 . 2 K (A) and T = 2 . 5 K (0)
Because of quadrupolar interactions, the re- sonance spectrum of the deuterium is much more com- plicate. Deuterium T1 measurements were performed on
different peaks of the resonance line. The recovery of the nuclear magnetization was found to be non exponential as expected. The quantity plotted in figure 2 is an average value of T-I evaluated in the
l
time interval 0
<
T < 0 . 7 Tl. In this interval the departure from an exponential recovery was found to be less than few per cent. Whatever the peak consi- dered in the resonance spectrum, the same behavior was observed. Figure 2 displays the results for one of the peaks up to 7 0 kOe. In order to compare the results obtained for protons and for deuterium the proton data shown in figure 1 have been re-plotted in figure 2 , after having scaled the values by a factor . 1 6 . It appears clearly that proton and deu- terium T;' measurements exhibit the same basical feature.Finally identical T;' measurements were per- formed on the nitrogen nuclear resonance. At T = 2 . 5 K the resonance spectrum consists of well- separated peaks and changes little with the field. Therefore accurate T;' measurements have been ob- tained and the results for one of the peaks of the resonance line are shown on figure 2 . As for proton and deuterium a net increase of the relaxation rate is observed when the field goes up above 20 kOe.
These measurements of ~;l~erformed on dif- ferent nuclei in TMMC establishes new important re-
Fig. 2 : Spin-lattice relaxation rates for different nuclei as the function of the magnetic field H for two values of the temperature :
T 4 . 2 K, proton
(A),
deuterium (UT = 2 . 5 K, proton (O), deuterium (O), nitrogen (*)
The arrows indicate the 3d ordering for T P 2 . 5 K
sults. Since the observed behavior does not depend on the nuclear spin system, it is characteristic of the electron spin system. The application of an ex- ternal magnetic field strongly affectgthe static and the dynamic properties.Adetai1ed analysis is in progress in order to elucidate this unusual behavior observed for the T-l field dependence.
References
/ I / Borsa, F. and Boucher, J.-P., Physics Letters,
64A (1977) 256
-
Borsa, F., J.-P. Boucher and J. Villain, J. Appl. Phys.,
2
(1978) 1327/ 2 / J.-P. Boucher et al., Phys. Rev.,
12
(1976) 4098 / 3 / Hone, D . , Scherer, C. and Borsa, F., Phys. Rev.9 (1974) 965
