Macrospin approximation and quantum effects in models for magnetization reversal

The thermal activation of magnetization reversal in magnetic nanoparticles is controlled by the anisotropy-energy barrier. Using perturbation theory, exact diagonalization and stability analysis of the ferromagnetic spin-s Heisenberg model with coupling or single-site anisotropy, we study the effects of quantum fluctuations on the height of the energy barrier. Opposed to the classical case, there is no critical anisotropy strength discriminating between reversal via coherent rotation and via nucleation/domain-wall propagation. Quantum fluctuations are seen to lower the barrier depending on the anisotropy strength, dimensionality and system size and shape. In the weak-anisotropy limit, a macrospin model is shown to emerge as the effective low-energy theory where the microscopic spins are tightly aligned due to the ferromagnetic exchange. The calculation provides explicit expressions for the anisotropy parameter of the effective macrospin. We find a reduction of the anisotropy-energy barrier as compared to the classical high spin-s limit.Comment: 10 pages, 11 figure

Quantum magnetism in two dimensions: From semi-classical N\'eel order to magnetic disorder

This is a review of ground-state features of the s=1/2 Heisenberg antiferromagnet on two-dimensional lattices. A central issue is the interplay of lattice topology (e.g. coordination number, non-equivalent nearest-neighbor bonds, geometric frustration) and quantum fluctuations and their impact on possible long-range order. This article presents a unified summary of all 11 two-dimensional uniform Archimedean lattices which include e.g. the square, triangular and kagome lattice. We find that the ground state of the spin-1/2 Heisenberg antiferromagnet is likely to be semi-classically ordered in most cases. However, the interplay of geometric frustration and quantum fluctuations gives rise to a quantum paramagnetic ground state without semi-classical long-range order on two lattices which are precisely those among the 11 uniform Archimedean lattices with a highly degenerate ground state in the classical limit. The first one is the famous kagome lattice where many low-lying singlet excitations are known to arise in the spin gap. The second lattice is called star lattice and has a clear gap to all excitations. Modification of certain bonds leads to quantum phase transitions which are also discussed briefly. Furthermore, we discuss the magnetization process of the Heisenberg antiferromagnet on the 11 Archimedean lattices, focusing on anomalies like plateaus and a magnetization jump just below the saturation field. As an illustration we discuss the two-dimensional Shastry-Sutherland model which is used to describe SrCu2(BO3)2.Comment: This is now the complete 72-page preprint version of the 2004 review article. This version corrects two further typographic errors (three total with respect to the published version), see page 2 for detail

PROFIL WISATAWAN MUSEUM RADYA PUSTAKA SURAKARTA

Anggit Margaret, C9407031 2011. Profil Wisatawan Museum Radya Pustaka Surakarta. Program Studi Diploma III Usaha Perjalanan Wisata Fakultas Sastra Dan Seni Rupa Universitas Sebelas Maret Surakarta. Penelitian tugas akhir ini mengkaji tentang Profil Wisatawan di Museum Radya Pustaka Surakarta. Tujuan dari penelitian ini adalah untuk mengetahui dari daerah mana saja wisatawan yang berkunjung ke Museum Radya Pustaka, bagaimana ciri-ciri wisatawan yang berkunjung ke Museum Radya Pustaka serta harapan-harapan yang diinginkan wisatawan terhadap Museum Radya Pustaka. Penelitian dilakukan dengan metode kualitatif. Pengumpulan data dilakukan melalui wawancara dengan narasumber wisatawan yang berkujung di Museum Radya Pustaka Surakarta tempat penulis melakukan penelitian, serta studi pustaka dan studi dokumen guna menambah sumber data. Hasil penelitian menunjukkan bahwa (1) Sebagian besar wisatawan yang datang berasal dari Semarang sebesar 32%. (2) Mayoritas wisatawan yang berkunjung ke Museum Radya Pustaka berusia antara 17-25 tahun dan kebanyakan dari mereka adalah pelajar atau mahasiswa dengan prosentase 52%. (3) Sebagian besar wisatawan yang datang ke Museum Radya Pustaka adalah bertujuan untuk melakukan penelitian yaitu sebesar 34%. (4) Harapan wisatawan yang berkunjung terhadap kelangsungan Museum Radya Pustaka sebagian besar adalah agar ditingkatkan lagi pengelolaan dan keamanan museum, agar kejadian hilangnya benda-benda koleksi museum tidak terulang lagi dikemudian hari. Kesimpulan dari hasil penelitian ini bahwa wisatawan yang berkujung ke Museum Radya Pustaka Surakarta mayoritas berasal dari Semarang, mayoritas berusia 17-25 tahun dan kebanyakan dari mereka adalah berprofesi sebagai pelajar dan mahasiswa. Kebanyakan wisatawan yang datang bertujuan untuk melakukan penelitian, serta harapan wisatawan terhadap Museum Radya Pustaka adalah supaya lebih ditingkatkan lagi pengelolaan dan keamanan museum