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Nuclear spin polarization of Nd by the electronic field in the range of mK

AutorPalacios, Elias; Bartolomé, Juan; Luis, Fernando
Fecha de publicación2012
CitaciónAdvanced Seminar on "Perspectives for Neutron Science in Novel & Extreme conditions" (2012)
ResumenThe alignment of nuclear magnetic moments is difficult to observe by neutron diffraction since it requires very high magnetic fields and very low temperatures. Moreover, the nuclear magnetic moment is three orders of magnitude smaller than the electronic one. Nevertheless the electronic spins create a hyperfine magnetic field much stronger than the fields available in a laboratory. The resulting nuclear polarization leads to some interesting phenomena that can be investigated by neutron diffraction. A cause of the incoherent scattering of neutrons is the thermal distribution of different nuclear spin states. As temperature decreases, they become aligned by the electronic field and some incoherent scattering becomes gradually coherent. The spatial periodicity of the ensuing nuclear order is the same as that of the electronic moments but the physical origin of the scattering is the interaction between the neutron and nucleus. The result is that the intensity of the usually considered “magnetic” (electronic) Bragg reflections increases. New reflections also appear because those with q parallel to the magnetic moment are allowed for the nuclear scattering. 143Nd and 145Nd are especially adequate for studying this phenomenon because of the following reasons: 1) they possess high incoherent scattering lengths, 2) their nuclear magnetic moment is also relatively large, thus easily polarizable at temperatures above 100 mK, and 3) the hyperfine field can be controlled by the choice of material since the Nd electronic moment strongly depends (between 1 and 3.27 μB) on the local crystal field. In this work, we studied the nuclear magnetic polarization in single crystals of NdFeO3 and NdAlO3 by means of neutron diffraction experiments. Each crystal was rigidly attached at the desired orientation to the mixing chamber of a 3He/4He dilution cryostat. The single degree of freedom was the rotation of the full set-up around the vertical axis, which provided access to one plane of the reciprocal space. Additionally, we studied powder diffraction patterns of NdScO3, NdInO3, NdCoO3 and NdGaO3. The enhancement of the magnetic reflections below 500 mK was consistent with the polarization of nuclear spins by a hyperfine field Bhf proportional to the electronic moment μ, Bhf = Kμ, with K = 108 T/μB, very close to the theoretical calculation that gives K = 113 T/μB. As future prospects, the hyperfine polarization of other atoms can be studied, using a new especially designed set-up, which could overcome the difficulties and restrictions of the experiments reported here.
DescripciónResumen del trabajo presentado a: Advanced Seminar on "Perspectives for Neutron Science in Novel & Extreme conditions", celebrado en Zaragoza (España) del 27 al 31 de mayo de 2012.
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