Millennial investors can't get enough of cannabis stocks, according to data from financial-service providers.

On Robinhood, a free-trading app popular among younger traders, three of the four most-added stocks over the past week were marijuana producers, including Aurora Cannabis (+25,051 shareholders), Cronos (+18,148 shareholders) and Aphria (+9,101 shareholders).

Aurora Cannabis last week became the most-popular stock on the app in terms of the number of investors who own shares, outpacing tech giants like Apple. Cronos was the fifth-most popular name, and Aphria was 38th.

Cannabis stocks have gained popularity among younger investors in the wake of the official legalization of recreational marijuana in Canada back in October. By Business Insider's track of data, Aurora was ranked 20th on the app on October 25, and Cronos was at seventh. Aphria, which was listed in the US on November 2, entered the top-100 during the week of December 13, ranking 90th at the time.

And millennials are not just buying individual stocks, but also snapping up exchange-traded funds that invest in weed stocks and companies who may enter the space, according to ETF Managers Group, a thematic issuer of exchange-traded funds.

In December 2017, ETF Managers Group launched an ETF under the ticker "MJ," which focuses on investing in global cannabis-related stocks. According to ETF Managers Group's CEO and founder Sam Masucci, the MJ fund invests in cannabis producers such as Cronos, Aurora, Tilray, and Canopy Growth, as well as companies that it thinks will be doing partnerships in the space, such as tobacco giant Altria.

"The average investment in MJ is smaller than we generally see with other products," Masucci told Business Insider. "This allows us to infer that it has a younger average investor audience, likely millennials."

The MJ fund, with $1 billion assets under management, has hundreds of thousands of investors, he said.