萌妹社区

Scientists in Japan have shown that the properties of a recently discovered state of matter鈥攁 chiral spin liquid鈥攃an be controlled by applying a magnetic field1, a concept that could be harnessed for low-energy-consumption memory devices .

Many ferromagnetic materials exhibit the so-called 鈥榓nomalous Hall effect鈥� (AHE), whereby the electrons flowing through the materials experience a lateral force pushing them to one side as a result of the materials鈥� intrinsic magnetization. In materials exhibiting the 鈥榗onventional鈥� Hall effect, the lateral force is caused by an external magnetic field. The magnetic material Pr₂Ir₂O₇ is a special case because it displays AHE without possessing a uniform magnetization. This unusual behavior was recently recognized and analyzed for the first time by Shigeki Onoda from the RIKEN Advanced Science Institute, Wako, and his colleagues2. They interpreted this behavior as a good indication that Pr₂Ir₂O₇ exists as a chiral spin liquid in which effects such as AHE depend on which direction an electron is travelling.

The collaboration team of the theoretical physicist, Onoda, and colleagues at The University of Tokyo and the Tokyo Institute of Technology, Japan, and  Florida State University, USA, discovered they could control this enigmatic material by experimentally investigating how the AHE in Pr₂Ir₂O₇ depends on applied-magnetic-field strength at temperatures below 2 kelvin. They found that the sign of the voltage drop associated with the anomalous Hall effect can be reversed by a sweep cycle of a magnetic field without causing an effect called hysteresis鈥攚here the magnetization depends on the direction in which the magnetic field is swept. 鈥淢agnetization hysteresis is a main cause of heat or energy loss,鈥� explains Onoda. 鈥淭hus, our results prove that this material could function as a nonvolatile memory device.鈥�

These properties arise because every physical system organizes in a way that minimizes its energy. In some materials, two competing influences鈥攖he arrangement of the crystal lattice and the ordering of local magnetic moments or 鈥榮pins鈥� of electrons鈥攃annot balance because they cannot simultaneously attain their minimum energy state. Known as geometric frustration, this situation prevents the system from settling in a particular state, and means that the spins fail to become ordered even at absolute zero. For this reason, physicists called these materials spin liquids.

鈥淭o practically harness the effects of AHE in a chiral spin liquid, the chirality must remain finite at room temperature,鈥� says Onoda. Although a difficult challenge, the current work is a positive start.