Nuclear Magnetic Relaxation in Antiferromagnetics, II
Nuclear Magnetic Relaxation in Antiferromagnetics, II
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DOI：
10.1143/ptp.16.641
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发表时间：
1956-12
影响因子：
--
通讯作者：
T. Moriya
中科院分区：
文献类型：
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作者：
T. Moriya
本文将前文中关于非磁性离子的反铁磁性体的核磁弛豫理论推广到磁性离子的情况。强超精细相互作用和四极相互作用是该问题的显着特征。在低温下采用自旋波近似，在高温下采用高斯随机调制模型，计算了线宽和弛豫速率。对几种物质的T1和T2阶数进行了估计，预测在居里点以上，由于超精细相互作用的展宽，核共振将很难被探测到，而在足够低的温度下，局部场谱的低频分量显著降低，共振将变得可探测。四极相互作用的影响也进行了讨论。在前一篇论文中，作者对反铁磁物质中非磁性原子的核磁弛豫作了理论上的处理。在这种情况下，主要的弛豫机制是由来自交换相互作用调制的电子自旋的磁偶极场提供的。用自旋波近似计算的CuC 12·2 H20中质子弛豫时间在数量级和温度依赖性上与实验符合得很好。另一方面，对于MnF_2中的氟Re，同样的机制已被证明不足以解释所报道的共振的缺失，这迫使作者期望F核与F离子上的不平衡价电子自旋之间的超精细相互作用。这种预期的自旋不平衡被认为是由于晶体中部分共价的存在以及MnF 2中每个p-离子周围的Mn + +离子的不对称排列造成的。在本文中，我们将把我们的考虑扩展到属于磁性离子的核。在这种情况下，由于超精细相互作用，原子核受到非常强的磁场的作用，该磁场是不同磁性离子的偶极场的102-103倍，其数量级为105-106高斯。因此，我们期望高的共振频率、宽的线宽和短的弛豫时间，在某些情况下甚至不可能观察到共振。然而，存在有利于观察共振的交换窄化效应。虽然涨落局部磁场非常大，但由于交换相互作用ra.pid调制，它对原子核的影响大大减弱。在频谱中
Theory of nuclear magnetic relaxation in antiferromagnetics treated in the preceding paper for non-magnetic ·ions is extended to the case of magnetic ions. Strong hyperfine interactions and quadru pole interactions are the distinctive feature of the problem. The line width and the relaxation rate are calculated by using the spin wave approximation at low temperatures and the model of Gaussian random modulation at high temperatures. Order estimations of T 1 and T 2 are made for several substances, which predict that the nuclear resonance will be difficult to detect above the Curie point because of the broadening due to the hyper fine interaction, while the resonance will become detectable at sufficiently low temperatures where the low frequency components of the local field spectra decrease remarkably. The effects of the quadrupole interaction are also discussed. In the preceding paper>, referred to below as I, the writer has presented a theoretical treatment of the nuclear magnetic relaxation of non-magnetic atoms in antiferromagnetic substances. The predominant relaxation mechanism in that case is provided by magnetic dipolar field coming from electron spins modulated by exchange interaction. The relaxation time of protons in CuC12 • 2H20 calculated by using the spin wave approximation agreed well with the experiment both in the order of magnitude and in the nature of temperature dependence. On the other hand, for fluorine re in MnF2, the same mechanism has proved to be insufficient for explaining the reported absence of the resonance, and this compelled the present author to expect a hyperfine interaction between F nuclei and the unbalanced valence electron spins on F ions. This anticipated unbalance of spins was considered to result from the existence of a partial covalency in the crystal and the asymmetric arrangement of Mn + + ions around each p- ion in MnF2• In the present article, we shall extend our consideration to the nuclei which belong to the magnetic ions. In this case a nucleus is subjected to a very strong magnetic field due to the hyperfine interaction which is 102-103 times as large as the dipolar field from different magnetic ions, and its order of magnitude is 105-106 gauss. We expect, therefore, high resonance frequencies, broad line widths, and short relaxation times, and in some cases even the impo~sibility of observing the resonance. However, there is an effect of exchange narrowing which favours the observation of the resonance. Though the fluctuating local magnetic field is very large, its effect on nuclei is much diminished by a very ra.pid modulation due to the exchange interaction. Among the frequency spectra